EPA-902/9-74-001
Traces of Heavy Metals in Water
Removal Processes and Monitoring
Sponsored by
U.S. ENVIRONMENTAL PROTECTION AGENCY, REGION II
PRINCETON UNIVERSITY, CENTER FOR ENVIRONMENTAL STUDIES
AMERICAN INSTITUTE OF CHEMICAL ENGINEERS
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TRACES OF HEAVY METALS IN WATER
REMOVAL PROCESSES AND MONITORING
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The contents of these Proceedings
do not necessarily reflect the
views and policies of the Environ-
mental Protection Agency, nor does
mention of trade names or commercial
products constitute endorsement or
recommendation for use.
11
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TRACES OF HEAVY METALS IN WATER
REMOVAL PROCESSES AND MONITORING
Edited by J. E. Sabadell
Proceedings of a Symposium conducted by the
Center for Environmental Studies and the
Water Resources Program
Princeton University
in cooperation with the
United States Environmental Protection Agency - Eegion II
and the
American Institute of Chemical Engineers
Environmental Division
November 15-16, 1973
School of Engineering/Applied Sciences
Princeton University
Published by the United States Environmental Protection Agency, Region II
Gerald M. Hansler, PE, Regional Administrator
ENviRcryirrT/.L PROTECTION AGENCY
Library, I->~v:,ion V
1 North Wackcr Drive
Chicago, Illinois 60606
iii
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CONTENTS
Page
Preface YL
Intreduction: Overview of Drinking Water Quality Control
at the Federal Level - Robert W. Mason 1
REMOVAL PROCESSES
I. Trace Heavy Metals in Water: Removal Processes by
Ion-Exchange - C. Calmon 7
Some Examples of the Concentration of Trace Heavy
Metals with Ion-Exchange Resins - Robert E. Anderson U3
Trace Metals Removal by Activated Carbon -
Stanton B. Smith 55
II. Heavy Metals Removal by Thermal Processes -
Ronald F. Probstein 71
Freezing Concentration for Removal of Heavy
Metals from Water - Robert J. Campbell 97
III. Membrane Processes for Waste Treatment - Robert E. Lacey 105
Removal of Trace Heavy Metals from Water by
Electrodialysis - Wayne A. McRae and
Edgardo J. Parsi 143
Reverse Osmosis for the Removal of Heavy Metals
from Waste Waters - Peter C. Houle 159
Removal of Heavy Metals from Water Using Reverse
Osmosis - David H. Furukawa 179
IV. Foam and Bubble Fractionation for Removal of Traces
Metal Ions from Water - Ernesto Valdes-Krieg,
C. Judson King, and Hugo H. Sephton 189
The Adsorptive Bubble Separation Techniques -
Robert Lemlich 211
Removal of Heavy Metals by Conventional Treatment -
Gary S. Logsdon and James Symons 225
ENVIRONMENTAL FT"rs"r'nr?TOF AGENCY
iv
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CONTENTS (Continued)
Removal of Heavy Metals from Waste Water with
Starch Xanthate - R. W. Wing 257
Summary of Concluding Remarks on the Presented
Removal Processes of Traces of Heavy Metals
from Water - C. Calmon, Ronald F. Probstein,
and C. Judson King 275
MONITORING TECHNIQUES
V. A Comparative Outline of Current Methods for the
Analysis of Trace Metals in Natural Waters -
Charles J. Lancelot 285
The Use of Atomic Absorption Spectroscopy in
Analyzing for Trace Metals in the Environment -
Earl L. Henn 297
The Occurrence of Trace Metals in Surface Waters -
Robert C. Kroner 311
Ion Selective Electrode Monitoring for Traces of
Heavy Metals - Isaac Trachtenberg 323
Analytical Procedures for Trace Heavy Metals in
Water - C. Calmon 333
List of Symposium Participants 337
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PREFACE
It is by now generally accepted that the practice of dis-
posing of our industrial, domestic and agricultural wastes in air,
soil and waters is taxing our environment beyond capacity, and in
some instances with catastrophic consequences.
From a host of solutes present in our waters heavy metal
traces in their ionic state belong to the category of man-made
pollutants that can be highly toxic to humans, animals and
plants by accumulating in the organism affecting bones, skin,
organs, nervous and other systems, producing chronic diseases
and even death, as it occurred in Minamata between 1953-1960,
in Niigata in 1965, in Iraq in 1960 and in Pakistan in 1961.
The number of problems, in the water resources field, that need
to be solved are enormous. In this symposium, organized by the
Center for Environmental Studies at Princeton University, only
two questions were addressed: l) Which processes can efficiently
remove these traces of heavy metals? and 2) how can these traces,
sometimes in the part per billion level, be more accurately
measured? To partially answer these questions, a group of
scientists and engineers were invited to present their findings
on these subjects. With the sponsorship of the United States
Environmental Protection Agency - Region II, the American
Institute of Chemical Engineers, and the Water Resources Program
of Princeton University, the program was presented November 15-16,
1973 at the School of Engineering/Applied Science of Princeton
University.
J. Eleanora Sabadell
Symposium Director
VI
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OVERVIEW OF DRINKING WATER QUALITY CONTROL AT THE FEDERAL LEVEL
Robert W. Mason, Chief, Research & Development Branch
Region II, Environmental Protection Agency
The Environmental Protection Agency is primarily a regulatory agency.
All of its programs, and this includes the research and development pro-
gram, are structured so as to enable the Agency to carry out its regulatory
function of protecting the environment and abating and avoiding pollution.
Following are the six major emphases of the R&D program:
SLIDE 1
1. The development of appropriate science and technology for setting
and enforcing pollution control standards.
2. Development of the full understanding of the environmental impact
of that which we are mandated to control.
3. The knowledge of what it "costs" to meet environmental quality
standards.
4. Knowledge of the "costs" of not meeting environmental standards
(i.e., the benefits to be derived from meeting them).
5. Monitoring to meet environmental goals and
6. Establishment of the means of forecasting the long range effects.
Evaluating the costs of meeting and not meeting standards means put-
ting a dollar value not only on the labor required to institute, say, an in-
dustrial process improvement but also on the lost (or gained) human
health, enjoyment and other aspects of the quality of life.
I could spend a great deal more time discussing what is implied in
these six major areas for they contain implicitly the entire EPA Research
and Development program. This is both the in-house and extramural program
which is administered by Washington Headquarters and the four National
Environmental Research Centers at Research Triangle Park, North Carolina;
Cincinnati, Ohio; Las Vegas, Nevada; and Corvallis, Oregon. Each one of
these NERCs, as they are called, has a theme for its research. RTP is
Human Health Effects, Cincinnati is Technology and Process Development,
Las Vegas is Research Monitoring, and Corvallis is Ecology, or the ef-
fects of pollutants on the environment. All the work on drinking water
quality and on health effects related to drinking water is carried out
at the Cincinnati Laboratory. Work on recreational water is carried out
at Cincinnati and at the Narragansett Laboratory because of the Tatter's
location near marine beaches.
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The Water Supply Research Program is presently being carried out
under the authority of the Public Health Service Act (Public Law 410,
78th Congress). The Reorganization Plan of 1970 transferred the func-
tion of the Water Supply Program from the Department of Health, Education
and Welfare to the newly established Environmental Protection Agency.
Under the Public Health Service Act and the Reorganization Plan the
Environmental Protection Agency is responsible for the certification of
water supply for interstate carrier use and also has authority to con-
duct and encourage research on water purification. The Safe Drinking
Water Act is presently awaiting passage by Congress. A little later on
I'll discuss some of the provisions of this new Act which broaden the
authority of EPA.
Please let me digress for a moment. Some of you are college pro-
fessors and are undoubtedly interested in government gold. As you have
probably gathered, after having received rejection notices from one
agency or another, on some of your proposals, there is very little of
it around. However, if you have a proposal which is deemed "relevant
to our research program" and responsive to a high priority research need,
there is a chance for funding. If you are in Region II, that is in
New York, New Jersey, the Virgin Islands or Puerto Rico, you may submit
these proposals in abbreviated form as "pre-proposals" directly to me
and I will obtain an evaluation of their relevancy and responsiveness- to
our needs from our specialists. My office is in New York City at
26 Federal Plaza.
Now let us turn to Nora Sabadell's program, "Traces of Heavy Metals
in Water; Removal Processes and Monitoring". SLIDE 2 shows the Public
Health Service 1962 standards which are presently used by EPA for inter-
state carriers. I've only listed heavy metals here.
SLIDE 2
1962 Proposed
Concentration Concentration
mg/1 mg/1
Arsenic 0.05 0.1
Copper 1.0 1.0
Iron 0.3 0.3
Manganese 0.05 0.05
Zinc 5 5
Mercury - 0.002
Barium 1 1
Cadmium 0.01 0.01
Lead 0.05 0.05
Selenium 0.01 0.01
Silver 0.05 0.05
Chromium 0.05 0.05
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The proposed revisions, which have not yet been approved, are shown
in the second column. You'll notice the only difference is that arsenic
is raised to 0.1 mg/1 and a value for mercury, which was lacking in the
original 1962 figures, is included. I am sure you will agree that pub-
lishing a revised list of heavy metal standards for drinking water will
not end the matter. The actions and interactions of trace metals in
the animal organism are very complex. We know a lot about lead, but the
lead poisoning problem is still with us as higher and higher levels are
being found in human tissues. Too little can cause problems as well as
too much. Cadmium has been linked to various ailments although the only
proven pathology from too much Cadmium ingestion appears to be the
"itai-itai" or "ouch-ouch" disease in Japan. Manganese needs to be studied
further as deficiencies can cause poor growth, skeletal abnormalities and
sterility, at least in male guinea pigs. Chromium is important in small
amounts but little is known as to how it works, and so on. Work will
obviously have to be continued in the field of health effects of trace
metals. In fact the new legislation provides for review of the standards
every three years.
The present EPA water supply research program is carried out under
two broad headings: (1) Water Supply Health Effects and (2) Water Supply
Control Technology. SLIDES 3 and 4 show the detailed breakdown of
expenditures in the program.
SLIDE 3
WATER SUPPLY HEALTH EFFECTS RESEARCH
1973 1974
Establish Health Criteria for Unknown 226,000
Organic Contaminants of Drinking Water
Establish Health Criteria for Inorganic 285,000
Chemical Constituents of Drinking Water
Investigate Problems of Infectious Water 430,000
Born Disease
*Review Safety of Products Used in Water 60,000
Treatment, Storage and Distribution and
Unique Water Sources
Develop Criteria to Establish and Support 260,000
Safe Recreational Water Quality Standards
TOTAL $1,261,000 $1,568,000
EXTRAMURAL 165,000 379,000
*Unique Water Sources refer to waste water reuse.
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SLIDE 4
WATER SUPPLY CONTROL TECHNOLOGY
1973 1974
Evaluation and Improvement of Treatment $ 109,000
Processes for the Removal of Trace
Organics and Taste and Odor
Evaluation and Improvement of Treatment 16,000
Processes for the Removal of Turbidity
and Specific Particles
Evaluation and Improvement of Treatment 82,000
Processes for Reduction of Trace Metals
and Nitrate Concentrations
Evaluation and Improvement of Methods for 86,000
Killing or Inactivating Micro-organisms
in Drinking Water
Evaluation and Prevention of Chemical 113,000
Quality Deterioration During Distribution
of Drinking Water
The Study of the Behavior and Control of 65,000
Contaminants and Additives in Drinking
Water Sources During Storage
Evaluation and Control of Bacterial 112,000
Quality Deterioration of Potable Water in
Distribution Systems and Bottled Water
Supplies
TOTAL $ 583,000 $ 587,000
EXTRAMURAL 60,000 130,000
Now let us turn briefly to the implications of the new drinking water
bill. This is entitled the Safe Drinking Water Act of 1973 (Senate 433
and HR 5368). Under these new bills prime responsibility for maintaining
the quality of drinking water will remain with state and local governments.
But the Federal Government will exercise a new responsibility to set
standards and provide assistance in order to protect public water supplies
(not just supplies to interstate carriers) from contamination. Among other
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things the proposed legislation provides that EPA establish minimum federal
drinking water standards prescribing maximum limits for contaminants. EPA
is also to set standards for the operation and maintenance of drinking
water systems as well as standards for monitoring, site selection, and con-
struction of public water systems to assure safe dependable drinking water.
A few words on the definition of the term public water system: aside from
water used in interstate carriers on which there is no size limitations,
the term means any system which provides drinking water to 10 or more
premises not owned or controlled by the supplier of water or to 40 or more
individuals, any system which provides drinking water to carriers or
establishments serving travellers in interstate commerce, or any other
system which provides drinking water, if the Administrator of EPA determines
that such a system may pose an unreasonable threat to public health. The
legislation then goes on to say that EPA is to establish recommended stand-
ards to assure esthetically adequate drinking water, while the States can,
of course, establish standards which are more stringent. The States will be
primarily responsible for enforcing the standards with Federal enforcement
if the States fail to act, or in case of imminent hazard, EPA will conduct
and promote research, technical assistance, and training of personnel for
water supply occupations. EPA will conduct a rural water survey within 2.
years of enactment of the new bill. EPA will make grants for special studies
and demonstration projects with respect to water supply technology. EPA
will make grants to States to defray the cost of the State program. Citizens
will be authorized to bring injunctive suits to violators of primary drink-
ing water standards and against the Administrator for failing to perform
mandatory duties. Of particular importance to most of us is the section
in the proposed legislation which contains a specific authorization for re-
search, technical assistance, information and the training of personnel
for the current fiscal year as well as for the next two fiscal years. The
amount authorized shall not exceed 14 million dollars for Fiscal Year 1974.
$14 million is about seven times our present rate of expenditure. We don't
expect $14 million to be actually appropriated but we expect a sizeable in-
crease over present expenditures if the legislation is passed. There is
also a $2 million authorization in 1974 for special studies in demonstra-
tion project grants intended to support the demonstration of new technology
and not for use in the construction and operation of any facility unless
that facility demonstrates new technology. There are other provisions but
I think I have covered the thrust of the new legislation so far as research
is concerned.
To epitomize the new act, EPA will be given authority to set national
drinking water standards and to carry out research on the identification
and health effects of drinking water contaminants, on water treatment
technology and health effects of water reuse.
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In summary, I have told you very briefly about the general research
objectives of the Environmental Protection Agency and the allocation of
funds to drinking water research activities. I have pointed out that
EPA's responsibility for drinking water quality is at present limited to
certification of water quality on interstate carriers but that it will
probably be broadened by the enactment of the so-called Safe Drinking
Water Act of 1973.
A few words about future research - we are moving toward water reuse.
The American Water Works Association believes that "the full potential of
reclaimed water as a resource should be exploited as rapidly as specific
knowledge and technology will allow". The research tasks which need to
be carried out, which are of direct interest to this conference, are:
(1) Determine the concentrations of harmful contaminants in water and
the degree to which they can be removed.
(2) Define the testing procedures, analytical methodology, allowable
limits, and monitoring systems that should be employed with
respect to the use of reclaimed waste water for public water
supply purposes.
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TRACE HEAVY METAL REMOVAL BY ION EXCHANGE
C. Calmon, Birmingham, N.J. 08011*
Introduction
The first successful ion exchange process was water softening,
developed about sixty years after the process discovery in 1848.
Today, the major application of the ion exchangers is still water
softening for household, municipal and industrial uses. The next
major application of ion exchange is deionization which has replaced
for many requirements evaporation and distillation.
It can be said that the driving force for obtaining purer and purer
water was from the power utilities which for greater efficiency and
productivity introduced boilers operating at very high temperatures
and pressures where solids in the steam can deposit on the turbines
and walls of the boilers, resulting in heavy maintenance cost and
shutdowns. This drive resulted in the search for better ion ex-
changers to meet the needs of the industry.
The hydrometallurgical market appeared at the end of the war with
the rise of the nuclear industry. Recovery of uranium, treating
nuclear plant wastes and separation of nuclides are now basic
ion exchange processes. Other common industrial uses of ion ex-
changer for metal ion removal and recovery are copper and zinc
recovery in rayon production, chromates in blow off from cooling
waters, gold, copper, nickel, zinc, chromium, etc., from plating
rinses waters, rare earth separation, etc. Until recently the
economics of the value of the recovered metal was the dominant
factor in plant construction.
At present, government regulations for effluent discharges of heavy
metal ions into streams are becoming stricter with limits set for
many in less than ppm quantities. This may stimulate the develop-
ment of new ion exchangers or complexing resins specific for given
metallic entities because of the new commercial market that may open.
Till now most of the specific ion exchangers have been prepared
in academic laboratories or through government contracts for spe-
cific ions, e.g., ammonia, nitrate, boron, etc. What is needed
today is a systematic approach for preparing and evaluating spe-
cific ion exchangers and chelate resins for trace metal removal
*Former address Sybron Corporation, West Orange, N.J. 07052
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8
on a practical and economic basis. Most of the present ion exchangers
operate on a selective affinity so many innocuous ions are also re-
moved at the same time and therefore the volume of waste regenerant is
high and the metal ion which may have value is obtained as a mixture
requiring further purification steps.
Just as the research in the Manhattan project produced the base
for ion exchange applications to the nuclear field, a similar uni-
fied approach is essential to the progress of metal ion removal
rather than the present procedure of waiting of chance contract
proposals .
Another important outcome of the polymeric organic ion exchangers,
which will grow in importance is the development of synthetic poly-
meric adsorbents which can remove metal oxides and organics if polar
groups are present and organic pollutants if no polar groups are
present on the adsorbent.
Ion Exchange — Basic Reactions and Selectivity
The basic reactions of the available commercial resins are similar to
the behavior of strong and weak electrolytes depending on the functional
group present in the resin. Thus a strongly acidic ion exchanger con-
taining a sulfonic group in the hydrogen form will react with sodium
chloride according to equations 1 and 2.
RSC^Hj-, + Nag '< "* T?^ Nap + Kg (1)
2RS03 Nar + Ca^+~ — »(RS03)2 Cap + 2Na| (2)
Where R is the resin matrix, r on resin, s in solution. The reactions
are reversible the direction depending on the concentrations involved.
A weakly acidic ion exchanger containing a carboxylic group in the hydro-
gen form will not react with a neutral salt as the affinity for the hydro-
gen is greater than for the sodium, i.e.
RC02H + NaCl ^j£i_ RC02 Na + HC1 (3)
However, if the anion of the salt forms a weaker acid than the acidic
group on the resin exchange will take (cf . eq. 4) similarly the salt
form of the exchanger will undergo exchange with neutral salts depend-
ing on the selectivity of the resin for the specific ion in solution
(cf. eq. 5). The selectivity of the carboxylic resin for multivalent
ions is so high that frequently the resin is used to reduce the con-
centration of these ions below the solubility product of the most in-
soluble salts of the ion. Also, it must be recognized that H2C03 in
water will cause the exchanger to become in the H form due to the high
selectivity for the H ion.
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RC02H H- NaHC03 - > RC02Na + H2C03 T
2RC02Na + Ca"1"1" ~«r* (RC02)2Ca + 2Na+ (5)
The equilibrium theory applied to equation 1 can aid in predicting
the quantity an ion will be associated with the resin when in equi-
librium with a given solution of the ion. In the simplest manner
the mass action equilibrium constant can be equated to the selec-
tivity coefficient. The mass action relation of equation (1) can
be expressed
x H+
K =
or
Nat
K = / Hs+
As K has not been found to be a constant as it varies with the ionic
concentration and loading on the resin and as the knowledge of the acti-
vity coefficients of the ions in the resin is limited, the equilibrium
constant is considered as a selectivity coefficient rather than an equi-
librium constant. Table I shows the variation of the so called constant
with mole fraction of the ions in solution.
TABLE I
Observed selectivity coefficients of an 8% cross linked sulfonated
polystyrene exchanger (Data by Myers Boyd, et al, cf. Helfferic ref-
erence )
Ionic Composition Nat L| exchange Na+-H+ exchange
0% Na+
0 1.72 1.38
50 1.80 1.52
100 1.89 1.20
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As the ion exchange resins have fixed pores (macroreticular resins)
or form pores on swelling in water (gel type resins), ions or com-
plexes larger than the pores are excluded by sieve action. This
exclusion depends in the gel type resins on the cross linking of
the matrix. Table II shows the variation of capacity of polystyrene
type cation exchangers for a large organic cation.
TABLE II
Relative capacities (%) of polystyrene cation exchangers for
Dinbenzyl dimethyl ammonium ion.
%DVB 2 5 10 15
Relative capacity % 100 94 43 15
The selectivity coefficients for various metal ions will be given later.
Kinetics of Ion Exchange
The rate of exchange is favored by the following conditions:
1. The smaller the particle size
2. Efficient mixing of the exchanger with the solution
(batch process)
3. High temperature
4. High concentration of solution
5. The smaller the ion size
6. The more porous the exchanger
7. Nature of the ionic group
Rate lower for weak exchangers
8. High capacity of the exchanger
Parameters in Efficient Column Operation
Usually these will be found to be:
1. High affinity of the exchanger for an ion
2. Elevated temperatures
3. High capacity of the exchanger (volume)
4. The smaller the particle size
5. The more uniform the particles of resin
6. High porosity
7. Low flow rate
8. The lower the concentration of the ion
9. The higher the column length.
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Methods of Operation
1. Single bed or beds
2. Dual or stratified beds
3. Mixed beds
1. Batch
2. Cyclical
3. Countercurrent
U. Continuous counter current
Normal steps of a cycle
Regeneration
Rinse
Run or Exhaustion
Backwash
In mixed beds
Regeneration
Rinse
Air Mix
Rinse
Run
Separate by backwash
The Commercial Ion Exchangers
A. Forms
• The commercial ion exchangers are available in forms of
Particles, granular or beads (Diamond Shamrock, Dow,
lonac, Rohm £ Haas)
Membranes (lonac, Ionics, DuPont)
Paper-cellulosic (Whatman-Reeve Angel)
Fibers and Fabrics (Carborundum)
Foamed Resin (Scott Paper)
Tubes (Various labs)
B. Physical Structure
The standard commercial addition polymeric resins are prepared in
three porosity structures.
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12
STRUCTURE OF TYPICAL ION EXCHANGE RESINS
Gel
Polymer bead is prepared as a
suspension in water. Resulting
structure possesses varying degree
of crosslinking; thus, swelling
pores of varying sizes are formed.
The fine pores that are formed tend
to foul quickly.
Fixed pore
(macroreticular,
macroporous)
Polymer is prepared in presence or
solvent in which the monomer but not
polymer is soluble, resulting in the
formation of discrete pores in the bead
on phase separation. The pores are lar-
ger than molecular size, and to strenthen
the resin structure a greater degree of
crosslinking is employed than in gel-type
resins.
Isoporous
Final resin possesses .a more uniform
pore structure than gel-type resin.
Uniform pore size is achieved through
use of a temporary crosslinking agent
that is uniformly distributed throughout
the resin and which is destroyed in sub-
sequent processing of the resin.
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C. Variations in cross linking, particle sizes and color.
Standard Crosslinking - Crosslinking in polymers 8 to 8.5 molar
percent with major portion of the bead sizes in the -20 to +35 mesh.
Lower Crosslinking Products - Between 5 and 7% used in some opera-
tions where sturdiness not required; also for throwaway uses.
Higher Crosslinked Products - Used for higher temperatures in the
presence of low quantities of oxidizing compounds, or in mixed beds
of ultrapure water systems.
Coarse Products - Are the same as the above except the quantity
of fine particles are reduced to a minimum; these resins are
used for high flow systems, e.g. condensate demineralization.
Partially Dry Resins - The standard resins have a water retention
about 46 to 47%. These can be dried to about 25% moisture with-
out cracking or breaking of the beads when placed in water. This
makes possible cheaper freight rates and also better storage under
freezing conditions.
Stratifying Resins - For better separation of beds in layered
systems, the resins involved are varied in size, the lighter
one (or top) being finer than the one resting on the botton.
Light Colored Resins - When dyes are introduced as indicators
light colored resins are required; some prefer the light col-
ored resins for aesthetic reasons.
Trap Type - Anion exchange resins which are not readily fouled due
to their large pores. The capacity if much lower than the standard
resins.
Fine beads - In the -50 to 200 mesh.
Micro Fine Beads - In the micron size used in chromatography.
Powdered Resins - -200 mesh for particulate and ion removal.
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D. Functional Groups
The present commercial ion exchangers contain the following func-
tional groups.
Cation Exchangers Anion Exchangers
-S03H -NH2, =NH, =N
-CC^H =N+ Cl - Quaternary N
-P03H2 =N+ Cl - Pyridinium (lonac)
-P02H fS+ Cl - Sulfonium (Dow)
-SH =P+ Cl - Phosphonium
-OH
The last three are specialties of
certain labs.
Mixed group exchangers containing -SOg H and -CC^H groups or both cation
and anion exchange groups are also available.
The chelates and specific ion exchange resins will be discussed later.
Also available from various suppliers are:
1. Inorganic exchangers (BioRad, lonac, Union Carbide)
2. Electron and Redox Ion Exchangers (Diamond-Shamrock)
3. Liquid Ion Exchangers (Dow, General Mills, Rohm 6 Haas, etc.)
M-. Retardation resin (Dow)
5. Organic adsorbents (Rohm 6 Haas)
E. Types of Polymer Matrices
1. Natural polymeric materials
2. Modified natural polymeric products
3. Synthetic polymers - condensation
4. Synthetic polymers - addition
F. Grades of Ion Exchangers Depending on Use
1. Analytical
2. Chromatographic
3. Food
4. Indicator
5. Nuclear
6. Pharmaceutical
7. Household Use
8. Industrial
9. Uranium Recovery
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15
Processes Used in Combination with Ion Exchange.
Due to the presence various entities in water or waste water and the
limitation of the ion exchange process, other treatment processes
are frequently used in conjunction with it. Some of these processes
are:
1. Filtration
2. Coagulation
3. Adsorption
4. Precipitation
5. Reverse Osmosis
6. Electrodialysis
7. Degasification
8. Disinfection
9. Distillation or evaporation
10. Oxidation or reduction
Usually the combination will be determined by
1. Required purity of the water
2. The concentration and type of contaminants in the water
3. The economics involved
Also, certain entities found in water foul the ion exchangers; removal of
these are often a necessity. Therefore, pretreatment is carried out simi-
lar to other desalination processes.
Interfering Substances.
Natural (e.g. humates) and man made compounds (e.g.-sequestering agents)
can complex metal ions so that removal by ion exchange becomes difficult
or not at all. Cyanides and huinates can be destroyed by chlorination.
In determining the heavy metals, one must recognize the contribution made
by materials of construction. Ultrapure water stored in the following list
of materials were found to contain ions.
Titanium - very low in metal ions
Tin - Ca, Pb 6 Zn in ppm quantities
Glass - Boron, 20 ppb
Aluminum - Aluminum, 15 ppb
Stainless
Steel 304 - Iron, 2-18 ppb
Stainless
Steel 316 - low in metals
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16
Adsorption of metals on glass and plastics must be guarded against.
Similarly, distilled and deionized water made up for reagents may
contribute errors due to trace metals present in industrial resins.
The maximum of heavy metals in deionized water is in the 10~^ ppm.
Spectrographic analysis of freshly prepared deionized water by mixed
beds showed the following concentrations in ppb quantities.
Al 0.1 Mg 0.3
Sb < 0.5 Mn 0.05
Be <. 0.005 Hg < 1
Bi < 0.1 Mo < 0.1
B 3 Ni < 0.1
Cd < 0.1 Nb £ Q.I
Ca 1 Si ^ 0.5
Cr <. 0.1 Ag 0.01
Co < 0.1 Na 1
Cu 0.2 Sn < 0.1
Ga < 0.2 Ti < 0.1
Ge<0.5 V < 0.1
Fe 0.2 Zn < 0.1
Pb 0.1 Zr < 0.1
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IT
Forms of Heavy Metal Components in Water
The heavy metals are found in water in various forms (1) as particu-
lates or colloids as a result of
A. From natural sources
B. Reactions of the ions with other entities in the
water, (e.g., Fe2C>3 formation through the reaction
of Fe++ with oxygen).
C. From pollution.
D. From treatment chemicals (e.g., Al2C>3 or Fe2<33 after
coagulation)
E. From materials of construction (e.g. corrosion or
erosion products)
(2) As ions both cations and anions, mostly from natural sources
(e.g. As203=) or pollution (CrO^=)
(3) As complexes with natural organics, or bioorganics, and organic
pollutants.
Ion exchangers play a role in the removal of each with certain limi-
tations .
Table III is a list of all the metallic ions to which ion exchangers
have been applied in concentration, separation, or purification. Many
of these industrial processes are in existence while others have only
been utilized in analytical work. The two books on analytical proced-
ures utilizing ion exchangers should be consulted for their references
and for techniques used.
Table IV gives a list of the metallic ions often found in water for
which ion exchange processes are in use or can be utilized if need
arises.
In this paper the various ion exchange techniques applied to removal
of the various forms of the heavy metals will be discussed.
Ion Exchangers as Filters
Ion exchange beds do act as filters for particulates within certain
limits. However, it is not desirable as physical blockage of pores
of the resin can take place with a resultant drop in capacity for
ion removal and in quality of effluent and an increase in head loss.
Also, colloidal organic or bioorganics with polar groups can react
-------
18
TABLE III
Ion Exchange Utilized in Conjunction with Metal Ions
1. Actinium
and actinides
2. Alkali metals
3. Aluminum
4. Antimony
5. Arsenic
6. Americium
7. Barium
8. Beryllium
9. Bismuth
10. Boron
11. Cadmium
12. Calcium
13. Californium
14. Cerium
15. Cesium
16. Chromium
17. Cobalt
18. Columbium
19. Copper
20. Curium
21. Dysprosium
22. Erbium
23. Europium
24. Ferric ion
25. Ferrous ion
26. Godolinium
27. Gallium
28. Germanium
29. Gold
30. Hafnium
31. Holmium
32. Indium
33. Lanthanum
and lanthanides
34. Lead
35. Lithium
36. Lutetium
37. Magnesium
38. Manganese
39. Mercury
40. Molybdenum
41. Neodymum
42. Nickel
43. Niobium
44. Osmium
45. Palladium
46. Platinum
47. Plutonium
48. Polonium
49. Potassium
50. Praseodymium
51. Promethium
52. Radium
32a. Iridium
53. Rare earths
54. Rhenium
55. Rhodium
56. Rubidium
57. Ruthenium
58. Samarium
59. Scandium
60. Selenium
61. Silver
62. Sodium
63. Strontium
64. Tantalum
65. Technetium
66. Tellurium
67. Terbium
68. Thallium
69. Thorium
70. Thulium
71. Tin
72. Titanium
73. Tungsten
74. Uranium
75. Vanadium
76. Yttrium
77. Zinc
78. Zirconium
-------
19
TABLE IV
Trace Heavy Metals Removed from Water and Waste Waters
1. Aluminum
2. Arsenic
3. Barium
4. Boron
5. Cadmium
6. Chromium
7. Cobalt
8. Copper
9. Germanium
10. Gold
11. Iron
12. Lead
13. Manganese
14. Mercury
15. Nickel
16. Radioactive nuclides
17. Selenium
18. Silver
19. Uranium
20. Zinc
-------
20
irreversibly with the ion exchanger so it can be fouled. Organisms
can grow in ion exchange beds necessitating the disinfection of these.
However, when the turbidity is low ion exchange beds are used directly
without a filter. At the end of the cycle the bed is backwashed and
the fine turbidity passes with the water. If a bed becomes very fouled,
resin cleaners are available. Condensates of high pressure boilers are
purified with ion exchange beds, the main impurities removed are fine
particulates of iron and copper. In some cases very finely ground
ion exchangers are used as coatings on porous media for the removal
radioactive species in particulate form usually derived from the mat-
erials of construction.
A study was made on the degree of particulate removal for given sizes
of ion exchange particles. Fig. 1 indicates that the finer the parti-
cles, the greater the removal. But unfortunately, the finer the parti-
cles the greater the head losses (cf. Fig. 2). Another problem is
that the very fine particles can not be backwashed or, if a mixed bed,
it cannot be separated for regeneration resulting in increased costs
due to the discarding of the resins.
In several places the writer found the use of cation exchange beds
as filters for particulates coming from poor piping or from local
water supplies. The beds are only backwashed. The claim was made
that they were superior to sand filters. There is a possibility of
a charge formation on the resin component. If this be the case then
active components could be built into the future filter media. A
great deal of the effectiveness of sand filters is due to accumu-
lated "debris" during the filtration probably due to polar groups
present in the "slime".
Ion exchanger as carriers of reactants
Inorganic ion exchangers have been used now for sixty years as car-
riers of precipitated manganese dioxide for oxidizing Fe++ and Mn++
in low quantities in natural waters. The iron and manganese are
precipitated and at the end of the cycle, they are backwashed. Re-
generation is accomplished with a few ounces of KMnO^ per cubic foot
of bed.
Another example of a reactant on an ion exchanger is the regenerated
strong base anion exchanger with the sulfite ion for removal of oxy-
gen from water. The sulfite is converted to the sulfate and when the
bed is exhausted; it is regenerated with a sodium sulfite solution.
-------
TURBIDITY REMOVAL %
CD CD
CT>
CD
SO
-
-------
FIG, 2 PRESSURE DROP OF H-OH MIXED RESINS
AS A FUNCTION OF PARTICLE SIZE
90--
CM
CO
D_
Q.
CD
GO
CO
UJ
80-
70--
50
FLOW RATE
BED DEPTH
3 DETERMINATIONS
60 •- £
MEAN PARTICLE SIZE (MICRONS)
DATA BY G, P, SIMON & C, CALMON
IONAC CHEMICAL tOMPANY
ro
ro
-------
23
Thus, it is possible to utilize ion exchangers as carriers of reactants
for specific applications.
Exchange to a more insoluble salt. '
A very interesting patent for the removal of heavy metals, the sul-
fides of which are insoluble is that of Kraus and Phillips. (U.S. Patent
3,317,312, May 1967). It uses the sulfides as particles in a col-
umn through which the heavy metal ion is passed. If the metal
ion in solution has a sulfide solubility less than the metal compon-
ent of the sulfide in the column, it will exchange to form the more
insoluble sulfide in the column. Thus, silver will be exchanged
for iron when the iron is iron sulfide. Similarly, other metals
such as cadmium, Bismuth, copper, mercury and gold can be removed
in a similar manner. Table V gives the solubility of the more
common heavy metal sulfides. The iron or manganese released can
easily be removed with a manganese zeolite described above.
The particle sizes of the bed were in the range of 80-170 mean
but small beds were found to be sufficient and the capacities were
extremely high as the reaction appears to be with the bulk of the
particles and is not limited to a reaction taking place on the sur-
face of the particle.
This process is unique in that only the heavy metal ions are re-
moved without the other ions being affected. Also, the iron or
manganese released can be easily removed without producing much
regenerant wastes. Also, the heavy metal is tied to a dry product
which is insoluble. It can be discarded and if valuable can be
readily recovered. This offers the possibility of valuable metal
recovery from very dilute solutions. A cubic foot bed could treat
5 million gallons of water containing one ppm silver.
Ion Exchangers as Adsorbents.
While our past and present literature have many papers on acti-
vated carbon, activated alumina, silica and various gels as ad-
sorbents, there is a limited literature on new organic adsorbents
some containing anion exchange groups while others are polymers
without polar groups. The former has extremely large pore radii
ranging in size from 7,000 to 250,000 A° and a total ion exchange
capacity of 4 meq/g. The resin is capable of being loaded with
2 to 3% of dry weight with colloids (R203, SiC>2 and organics).
Recent work indicates that for humic acids, lignin sulfonates and
anionic detergents, weak base macroreticular resins are applicable.
For dye wastes strong base adsorbing resins appear to give best re-
sults. The latter is also good for humic acids when no detergents
are present. For plating wastes containing non ionic detergents
a macroporous adsorbing resin is recommended. Regeneration is accom-
plished with acid and caustic solutions.
-------
TABLE V
Solubility of Sulfides
Metal Sulfide Sulfide Concentration (Moles/1)
MnS
FeS
ZnS
NiS
CoS
PbS
CdS
Ag2S
CuS
HgS
3.75
6.1
3.46
1.18
3.1
1.73
1.84
6.0
3.4
4.8
9.2
4.5
X
X
X
X
X
X
X
X
X
X
X
X
10~8
10-10
10-12
10-12
10-13
10-13
1Q-14
10-15
10"1^
10-21
10-23
10-25
-------
The spent acid regenerant of such an adsorbent (Amberlite IRA-938) had
in one case the following average analysis
Solids 9.1 grams/1.
Si02 0.3 " "
R20s 1.9 " "
Organics 6.9 " "
Thus, the old view that ion exchangers will only react with ions in
solution must be modified.
Resins with Chelate Groups
Several types of chelate resins exist on the market. The best known
is Dowex A-l which contains iminodiacetic acid groups (R-CH2 N (C
A recent announcement claims the preparation of a cross-linked aliphatic
condensation resin with an amino and carboxyl groups (Seelex A-100) which
is "specifically selective for cadmium".
Some claim that the chelation of these amphoteric resins is analogous
in complexing tendency of ethylene diaminetetracetates but doubts
exist that the mechanism is similar.
It appears some of the amphoteric resins have specificities for multi-
valent ions such as Cu, Ni, Co, etc.
The Dowex chelating Resin A-l has the following selectivity sequence
Pd2+> Cu2+ -? Fe2+> Ni2+ ^ Pb2+ > Mn2+ ~? Ca2+ -7 Mg2V Na+
The chelate formation of the resin depends on the pH in the resin
phase. Copper is picked up more at a lower pH. Complexing agents
may be used as eluants.
Ion Exchange at elevated temperature and in presence of radiation
The initial ion exchangers were either synthetic sodium alumnio silicates
or natural zeolites, glauconites or clays, such as the Bentonites. With
the development of the synthetic polymeric ion exchangers the use of
the former has become very limited to a few special uses. The first
use of an exchanger for removing traces of a component in a mixture
was by Folin at Harvard in 1917 for the removal of ammonia from urine
with a synthetic sodium alumino silicate.
-------
26
Most of the natural inorganic exchangers are cation exchangers. The
only well known anion exchanger which is used for fluoride removal is
apatite which comes from animal bone.
With the need for exchangers which will withstand radiation a whole ser-
ies of amphoteric inorganics have been developed by combining group IV
oxides with the more acidic oxides of V and VI. Thus, the oxides of
Zr, Ti, Sn and Th can be combined with the acidic compounds of As, P,
Mo and W. Unfortunately, the stability of these are limited to narrow
ranges of pH. As they are resistant to radiation, they are utilized
in some of the nuclear research centers where thermal and radiation stabi-
lity is necessary.
Anion exchangers have been prepared from the oxide gels of Zr and Sn.
An alumino-silicate gel is used for removing Cs ions from waste and
then carried as a solid to a point of use.
The molecular sieves which are crystalline sodium alumino silicates,
with uniform pore diameters are used mostly as catalysts, adsorbents,
and separation of organic molecules.
No doubt specificity exists in many inorganic exchangers which will
be utilized. For instance, clinoptilolite is very specific for ammonia.
Ion Exchange Involving Selectivity.
The apparent selectivity coefficient is a good indicator of the affi-
nity or preference of an ion exchange resin for a given ion over another.
Thus, it becomes a good indicator in predicting which ion will be pre-
ferentially held by the exchanger. At this point, it must be pointed
out that the greater the affinity of an ion, the more difficult it is
to be removed through regeneration.
The selectivity of a resin depends on many parameters.
1. Solution concentration of the ion (e.g. HC03 exchange
for Cl in a anion exchange resin regenerated with NaCl).
Ratio of HCOa Ratio
to total anions capacity of the
containing only exchanger for HCO~
HCOQ and Cl
0.93 1.00
0.80 0.78
0.60 0.68
0.45 O.U1
0.33 0.29
-------
27
Softening is the removal of Ca and Mg at low concentration.
A strong NaCl reverses the process, i.e., the Ca and Mg are re-
leased and the exchanger is converted in the Na form.
2. The valence of the ion—the higher the valence the greater
the affinity (e.g. N^
-------
28
10. Precipitate formation. If a precipitate forms then a less
favored ion will be preferred (e.g., Ba++ in presence of
over Na+, or Ag+ in presence of Cl over Na+).
11. Exclusion of very large ions in resins of limited pore sizes.
This was already discussed previously.
Thus, for a large complexed metal cation a resin with a lower cross
linking or 'high fixed porosity should be used.
12. Mole fraction composition — This varies also with the ions
involved.
TABLE VI - Selectivity of Ion Exchange Resins*
Strong Acid Cation Exchanger (Styrene-DVB)
Lt exch. H exch.
Element
Li
H+
Na+
NH!;
K+
Uof
Rb+
Cs+
Mg2+
Zn2+
CQ2"1"
Cu2+
Cd2+
Ni2+
Ca2+
4
1.06
1.32
1.58
1.90
2.27
2.36
2.46
2.67
2.95
3.13
3.23
3.29
3.37
3.45
4.15
8
1.00
1.27
1.98
2.55
2.90
2.45
3.16
3.25
3.29
3.47
3.74
3.85
3.88
3.93
5.16
16
1.00
1.47
2.37
3.34
4.50
3.37
4.62
4.66
3.51
3.78
3.81
4.46
4.95
4.06
7.27
Element
Li+
H+
Na
NH£
K+
Rb+
Cs+
Mn2+
Mg2+
Fe2+
Zn++
Co++
Cu++
Cd
Ni++
4
0.9
1.0
1.3
1.6
1.75
1.9
2.0
2.2
2.4
2.4
2.6
2.65
2.7
2.8
2.85
3
0.85
1.0
1.5
1.95
2.5
2.6
2.7
2.35
2.5
2.55
2.7
2.8
2.9
2.95
3.0
12
0.81
1.0
1.7
2.3
3.05
3.1
3.2
2.5
2.6
2.7
2.8
2.9
3.1
3.3
3.1
-------
TABLE VI, Continued
Element
Sr2+
Ag+
4.70
4.73
6.56
6.71
7.47
6.51
8.51
9.91
12.4
11.5
10.1
22.9
18.0
28.5
20.8
Element
Cu +
Sr2+
3.2
3.4
3.85
5.1
5.4
6.15
5.3
3.9
4.95
7.2
7.5
8.7
9.5
4.6
6.25
9.7
10.1
11.6
Pb
Ba
*Crosslinking 4, 8, 16% based on molar cone.
Beryllium is less than magnesium. Aluminum is less than ferric ion.
Weak acid cation exchanger (carboxylic)
H^ Cu>Co^Ni^ Ca^ Mg > Na.
Weak Base anion exchanger (in Cl form).
OHVSOjJ ^CrO^T" AsO^ 7 P0| ^MoO^ 7C1~
lon Exchange Through Specificity
This is one of the most interesting aspects in heavy metal ion removal
as by means of specific resins it is possible to remove only the undes-
irable or recoverable ion and leave the innocuous ions in solution. This
reduces the cost of regenerant, volume of waste, and can treat a large
volume of water between regenerations. However, at present there are
certain undesirable aspects.
1. Very few resins are specific to a single ion.
2. The rate of exchange is reduced.
3. The resin holding a highly preferred ion is
difficult to regenerate.
In choosing a resin all the above parameters should be considered.
Table VII gives a list of known specific resins for various heavy metal
ions.
-------
30
It is this field which requires systematic studies for
1. Preparation of various resins with chemicals known to be
specific for certain metal ions.
2. Evaluation of these for capacity, rate of reaction, and
regenerant efficiency.
These resins could become important in recovering valuable metals
even when in trace concentration. In fact, the Pyridinium resin
is used to recover gold and then it is burned and the gold recov-
ered. The users do not bother to regenerate off the gold with
known solutions.
TABLE VII, Resin containing ion specificity
Element Polar Group
Arsenic C3+) Fluorone
Beryllium Phosphonic Diallyl phosphate
Bismuth Pyrogallol
Boron N-methyl glucamine, tris hydroxymethyl
amino methane
Cesium Phenolic OH + Sulfonic groups
Cobalt M-phenylene diamine, 8-hydroxyquinoline
Copper Phenolic OH + phosphonic groups,
8-hydroxy quinoline
m-phenylene diamine
imino diacetic acid
alginic acid
Germanium Phorone
Gold Pyridinium, thiourea
Iron Alginic acid
m-phenylene diamine
hydroxamic acid
phosphonous
phorone
chlorophyll
haemin deriv.
Lead Pyragallol, Phosphoric
Mercury Thiourea, thiol, iminodiacetic acid,
mercapto resins
Nickel Alginic acid, Dimethylglyoxime
Potassium Dipicrylamine
-------
TABLE VII, Continued
31
Strontium
Titanium
Uranium
Viruses
Zinc
Zirconium
Phosphorous
Chromotropic acid
Pyridinium, phosphorous ester
Metal cation proteins
Anthranilic
Phosphate ester
Removal of Specific Heavy Metals from Water
Anodizing rinse waters, the aluminum is removed with a cation exchanger
on the H cycle and the chromate in an anion exchanger.
Arsenic - has been removed as the arsenate with a weak base anion
exchanger Clonac A-260) in the Cl form.
Boron - removal from agricultural waters is possible at a cost
of 12 to 30C/1000 gallons (depending on size of plant)
with N-methylglucamine resin, (amberlite XE-243) the
regenerants are sulfuric acid and ammonia. In France,
boron was removed by a long column of weakly basic resin.
Cadmium - Claim is made that a chelate Seelex A-100 containing
an amine and carboxylic acid group is specific for cad-
mium. In plating rinse water standard cation exchange
resins are used.
Cesium - Amorphous sodium, alumino silicate gel with a ratio of
1:1:6 is used for removal from some radioactive wastes
and kept as such for long periods.
Chromium - As the chromate, it is removed with an anion exchanger.
Trivalent chromium can be removed with a cation exchanger
or it can be oxidized to the chromate and removed with
an anion exchanger.
In cooling waters, if the reject contains 100 ppm of
CrO^, it is economic to use a highly basic anion ex-
changer which preferentially removes CrO^ at a pH
of 4.5 to 5.0 as at this pH the selectivity is
Cr Oq. ^ P0^> 30147 Cl .
Polychromates appear to form
H2
R2CT207
-------
To get a neutral chromate on regeneration
a) NaOH + NaCl are used as the regenerant.
b) It is followed by NaCl.
The dosages are
a) (2.5 Ib. NaOH t 5 Ib. NaCl) as a 10% solution
b) 5 Ib. NaCl at 10% solution
Uptake of chromate is 7.7 Ib./cu.ft. Recovered chromate
on regeneraion 7.65 Ib. or 99.7%. Effluent volume of
Reg. is 6.5 gallons. Concentration of CrOii is 10% by
int. The spent regenerant is reused in the cooling
system.
Cost of regenerants are 5<:/lb. of CrO^. Cost of
Chromate per Ib. 18£, i.e. there is enough to pay
for amortization and resin replacement.
The disadvantages are
1) Low flow rate. 2 gpm/cu. ft. of resin.
2) Resin can be fouled by organics & turbidity
Copper - See Plating Rinse Waters
Iron and
Manganese - If in the divalent state, they are removed with standard
cation exchangers in the sodium form.
If the two ions are in very low concentration (.close
to one ppm) they are removed with a Mn02 carrier as
explained before.
In condensate demineralizing the iron and copper which
are either corrosion or erosion products are reduced
to low values on passing the condensate through 3 ft.
of mixed beds. Finely ground resins are used when the
particulates are to be at extremely low value as in
the case of radioactive components.
Gold - is removed today by anion exchangers, either a Pyridinium
type or Type I basic resin. These resin are excellent
when the gold is in alkaline or acid solutions. In highly
acidic solutions the thiourea resin appears to be superior.
But most gold wastes are alkaline solutions. The thiourea
resin has a good capacity for platinum and iridium.
-------
33
The capacity of the Pyridinium and Type I anion
exchangers runs from 50 to 100 troy oz. per cubic
foot of resin. Usually the resins are burned at
1000° C. and the gold recovered rather than regene-
rated.
The thiourea resin (SRAFION NMRR) appears the following capacities
at the optimum pH of 0.5.
Gold 0.85 grams/gram dry resin
Platinum 0.50 " " " "
Palladium 0.30 " " " "
Iridium 0.30 " " " "
Rhodium 0.20 " " " "
There are 400 dry grams per liter of wet resin. The pH range of
effectiveness is between C-.5 to 2.5.
If elution of the noble metals are desired a 5% aqueous solution of
thiourea to which 5 ml. of HC1 per liter of solution are added. The
metal can be recovered from solution by reduction.
Mercury - Ionic mercury as well as Methyl mercury can be re-
moved by a thiourea anion exchanger. The capacity was
about 545 mg. Hg. per gram of dry resin. Regeneration
can be accomplished with a 5% thiourea solution.
For mercury ion removal, wool may be used, the mer-
cury probably complexes with the disulfite units of
the wool. The capacity is approximately one mercury
per disulfide linkage. Resins containing thiol CSH)
groups have been prepared and applied to mercury ion
removal. No doubt, the first reaction is ion exchange
followed by complex formation.
A strong base anion exchange showed at a capacity of
four pounds of mercury per cubic foot with a solution
containing 15 ppm of ionic mercury. The effluent con-
centration was less than 0.01 ppm. The flow rate was
kept at 2 ppm/cu. ft. Regeneration is with a strong
acid and sodium chloride. About 25 Ibs. of ^SO^. is
used per cubic foot. The more dilute portion of the
regenerant waste is recycled.
A Swedish ion exchanger CQ-13) appears to be highly
selective for mercury. The chelate resin is made
from a byproduct of sulfate pulp and has a capacity
-------
Nickel
Plating Rinse
Waters
of one meq/g. If metallic mercury is present it
is oxidized with chlorine to a pH of 6 to 7. The
excess chlorine is removed with an activated carbon
filter A bed with this resin has been in operation
for nearly two years. With an extra unit as a pol-
ishing unit, the mercury is reduced to 0.1 - 0.2 ppm.
- See Plating Rinse Waters.
Platinum
Radioactive
Species
Selenium
2+
- The common ions in plating rinse waters may be Zn'
Cu2+, Ni2+, and Cr3*. These are recovered with cation
exchangers and the recovered metal ions on regene-
ration can be recycled or evaporated to the concen-
tration needed. Hundreds of such plants are oper-
ating today very efficiently.
- See Gold
In particulate form these can be removed with ion
exchange beds as described before. The ionic species
are removed with mixed beds of ion exchangers.
The beds when exhausted are encased in cement and
buried at specific storage areas. A patent is being
issued on shrinking the volume of the ion exchangers
by means of solvents so as to reduce the bulk storage
volume.
By 1980 it is estimated there will be an annual con-
sumption of 300,000 cu. ft. of ion exchange resin for
removing radioactive ionic and particulate species
from reactor cooling waters and pool storage waters.
A combination of (a) sand filter (b) activated
carbon (c) cation exchanger and (d) anion ex-
changer was used. The removal was (a) 9.5%,
(b) 43%, (c) 44.7% and (d) 99.9% respectively.
This indicates that some of the Se existed as
a colloid and as an anion. The component re-
moved by the activated carbon may be a complex.
-------
35
Silver - Silver from plating rinse solutions can be recovered
by passing the solution through a weakly acidic cation
exchanger (H form) and then through a weakly basic anion
exchanger (OH form), the silver cyanide complex is eluted
with an alkaline solution of cyanide ions. If the sil-
ver is not complexed it can easily be removed with a
cation exchanger.
Cost of Metal Ion Recovery
A theoretical calculation of the cost of metal ion recovery from sol-
ution by ion exchange has*been made by R. Kunin in the Nov. 1967
issue of Amber-Hi-Lites. Although many assumptions were made as to
the type and capacity of the exchanger, the type and quantity of re-
generant and absence of interfering ions. But two columns in Table
VIII give a key to costs which are worth reproducing:
1. Pounds of metal recovered per cu. ft. of resin, and
2. Cost of operation per pound of metal recovered.
Although the writer notes certain discrepancies in some cases involving
actual plant operations, e.g.
1. In gold recovery the resin is burned so costs
are slightly higher. The theoretical capacity
is not far from the actual.
2. In Chromium recovery while cost is close to what
is given in the table, the capacity is much higher
in cooling water system due to complex formation
on the resin.
However, it was indicated by Dr. Kunin that assumptions were made
and also six years have added more data of operation.
Under the subject of Removal of Specific Metals from Water, four
metal ions are worth discussing.
1. Boron removal at the cost indicated would be too
costly for agricultural water, unless the product
was costly. As boron is cheap and equivalent weight
is low, the use of anion exchange process would have
to involve an urgency of the production of a product.
-------
36
2. Chromium removal in cooling water systems by ion
exchange pays for itself when the concentration
reaches 100 ppm CrO^. In other words, it is a
breakeven proposition so numerous plants have ion
exchange recovery systems.
3. Cost data for mercury removal by the special resin
(Q-13) are given by AKTIEBOLAGET-BILLINGSFORS for
a 63,000 gallon per day plant with a water contain-
ing 10 ppm of Hg and 1% NaCl. The total cost is
53 cents per 1000 gallons; if credit is given for
the reused acid and mercury, there is a net profit
of 16C/1000 gallons. However, no amortization cost
for capital investments is given. It would appear
that it is another breakeven system for a company
which uses mercury in its processes.
M-. Gold at the level today makes the burning of the
resin worthwhile. If the capacity is 4 to 8 pounds
per cu. ft. then the recovered gold is worth $4-000
to $8000 while the resin is less than $100/cu. ft.
It would appear that all heavy metal ions should be checked on the degree
of recovery and operation cost of the ion exchange systems available,
especially for exchangers with the highest affinity, against the cost
of the product on the market at a period with rising prices of many
metals on the market.
Only strict laws or profits will drive industry to examine the avail-
able means of recovering wasted resources.
Comparison of Ion Exchange with Other Processes.
Ion Exchange is unique in that the lower the concentration of the metal
ion the cheaper is the cost of treatment per unit volume of water treated.
In Electrodialysis, the lower the salt concentration the greater the
resistance of the solution resulting in high power costs. In reverse
osmosis as well as in evaporation and distillation, the cost of re-
moving the water increases sharply with lower concentration of salts
if viewed from the ions removed.
Secondly, frequently the metal ions recovered may be in a concentration
which makes possible the reuse of the salt.
-------
37
TABLE VII
Ion Exchange Capacity and Cost of Ion Exchange Operation
For Metal Recovery
Cation Exchange
Anion Exchange
Metal
Form
A1203
BeO
Cd
Ce203
CsCl
CoO
Cu
Pb
LiO
Mg
MgO
Mn
Hg
Ni
Ra
Rare Earths
Ag
Sn
Zn
Capacity
Ib./cu.ft.
1.1
0.5
6.7
5.6
16.0
3.6
3.8
12.4
0.8
1.5
1.5
3.3
12
3.5
13.6
6.3
13
7.1
3.9
Cost
cents/lb.
14
30
2.3
2.7
9.4
4.2
3.9
1.2
18
10
10
4.6
13
4.3
11
2.4
1.2
2.1
38
Metal
Form
Sb
Bi
Cr203
Ga
Ge
Au
Ha
Ir
Mo
Nb
Pd
Pt
Re
Rh
Ta
Th02
W203
V205
U02
Zr
Capacity
Ib./cu. ft.
4.5
3.1
1.9
5.2
5.4
7.3
6.6
7.1
3.6
3.4
3.9
7.2
13.8
2.9
6.7
8.6
6.8
3.8
8.8
3.4
Cost
cents/lb.
6.7
9.7
16
5.8
5.6
4.1
4.9
4.2
8.4
8.8
7.8
4.2
2.2
10
4.5
3.5
4.4
7.9
3.4
8.8
-------
38
Thirdly, the volume of regenerant is limited so that, if it is a waste,
then the discarded volume is small.
Fourth, the particular metal ions may have a high selectivity coefficient
or form a complex with the polar group of the resin so that only the des-
irably metal ion is removed from solution while the innocuous ions pass
untouched or are picked up to a limited extent. Thus, a greater purity
of the metal ions may be obtained on regeneration.
The limitations of ion exchange are:
1) Fouling, especially of the anion exchanger in the presence
of certain organic complexes. Pretreatments are available
but it is an added cost.
2) Concentration of other ions may limit the capacity of the
exchanger for the heavy metal ion if they have a selectivity
close to the latter.
3) The volume of water treated is reduced as the concentration of
the ion increases.
4) Regenerant wastes can be a problem.
However, the specific type of exchanger would overcome the above
limitations and could be used for the removal of trace heavy metals
from concentrated solutions.
This type of resin would offer a minimum volume of regenerant waste
which today must be considered a liability.
For these reasons the writer believes that some systematic coordi-
nated research work should be done in this field. This will yield
the greatest economy in removal and may be a means of recovery of
scarce valuable resources.
Mr. B. Fuller defines a pollutant as a wasted resource. With highly
selective and well-defined specific ion exchangers the present heavy metal
pollutants will be recovered or recycled resources.
-------
39
ADDENDUM
Various aspects of ion exchange have not been mentioned. Therefore, these
added notes are to make the subject more complete.
1. Ion Exchange membranes used in electrodialysis will be
discussed in a later paper. E.D. units with ion exchangers
in the compartment between the membranes are in use for the
removal of trace radioactive nuclides from highly purified
waters .
2. Data for the extraction of heavy metal ions with liquid
ion exchangers are given in the analytical texts. The
following heavy metal ions have been extracted with spe-
cific liquid ion exchangers (cf. Iczedy).
1. Ac 24. Os
2. Al 25. Pd2+
3. As 26.
4. Sb 27. Po
5. Ba 28. Re7+
6. Be 29. Rn3'4
7. Bi 30. Sc
8. Cd 31. Se
9. CrO^+ 32. Sr
10. Co 33. Ta5+
11. Cu 34. TeT*
12. Ga 35. T1T+
13. Ge 36. Sn4+
14. Au3+ 37. Ti4+
15. Hf 38. Tc7+
16. Fe3+ 39.
17. In3+ 40.
18. Pb 41. V3>4>5
19. Mn2+ 42. Zn
20. Hg2+ 43. Zr
21. Mo6"1" 44. Y
22. Ni 45. Lanthanides
23. Nb5+ 46. Transuranic elements
3. The redox resins may be used for oxidizing or reducing heavy metal
ions in solution prior to analysis or ion exchange concentration.
-------
BIBLIOGRAPHY
1. Applebaum, S. B., Demineralization by Ion Exchange, Academic Press,
New York, N. Y. (1968).
2. Arden, T. V., Water Purification by Ion Exchange, Plenum Press,
New York, N. Y. (1968).
3. Calmon, C. and Kressman, R., Ion Exchangers in Organic and Bio-
chemistry , Interscience Pub. Co., New York, N. Y. (1957). Now issued
by a division of Xerox.
4. Cassidy, H. G., and Keen, K. A., Oxidation Reduction Polymers
(Redox Polymers), Interscience Publishers, New York, N. Y. (1965).
5. Dorfner, K., Ion Exchangers, Ann Arbor Science Publishers, Inc.,
Ann Arbor, Michigan.
6. Helfferich, F., Ion Exchange, McGraw Hill, New York, (1962).
7. Inczedy, J., Analytical Applications of Ion Exchangers, Pergamon
Press, New York, (1966).
8. Kitchener, J. A., Ion Exchange Resins, John Wiley and Sons, New York,
(1957).
9. Kunin, R., Ion Exchange Resins, 2nd Edition, John Wiley and Sons,
New York, (1958).
10. Nachod, F. C. and Schubert, J., Editors, Ion Exchange Technology,
Academic Press, Inc., New York, (1956).
11. Salmon, S. E., and Hale, D. K., Ion Exchange—a Laboratory Manual,
Academic Press, New York, (1956).
12. Samuelson, 0., Ion Exchangers in Analytical Chemistry, John Wiley
and Sons, New York, (1953).
13. Society of Chemical Industry, Ion Exchange and Its Applications,
London, (1955), (a collection of papers and discussions).
14. Society of Chemical Industry, Ion Exchange in the Process Industries,
London, (1970), (a collection of papers).
-------
BIBLIOGRAPHY, Continued
Specific Papers
Arsenic
Calmon, C., "Comments on Arsenic Removal", J.A.W.W.A., 65, 569
(1973).
Boron
Grinstead, R. R., and Wheaton, R. M., "Improved Resins for
the Removal of Boron from Saline Water, Exploratory Study",
OSW Report #721.
Chromium
Shepherd, C. M., and Jones, R. L., "Hexavalent Chromium, lexicological
Effects and Means for Removal from Aqueous Solutions", N.R.L. Report,
7215.
Mercury
Law, S. L., "Methyl Mercury and Inorganic Mercury Collection by a
Selective Chelating Resin", Science, 74, 285 (1971).
Colloid and Organic Removal
Kunin, R., et al., Rohm and Haas bulletins
Oehme, C., and Martinola, F., "Removal of Organic Matter from Water
by Resinous Adsorbents", Chem. and Ind., Sept. 1, 1973, pp. 823-826.
Literature may be obtained from resin manufacturers.
Lab. Products
J. T. Baker Chem. Co., Phillipsburg, N.J.
BioRad Labs., Richmond, California
Fisher Scientific Co., Fair Lawn, N. J.
Mallincrodt Chem. Works, St. Louis, Mo.
-------
BIBLIOGRAPHY, Continued
Manufacturers, U.S.A.
Diamond-Shamrock, Redwood City, California
Dow Chemical Co., Midland, Michigan
lonac Chemical Co., Birmingham, N.J.
Rohm and Haas, Philadelphia, Pa.
Manufacturers Outside U.S.A.
Ayalon, Haifa, Israel
Mitsubishi Chem. Ind., Tokyo, Japan
Naffone, (BASF Germany) (Office; New York, N.Y.)
Permutit Co., Ltd., London, England
Resindion, Milan, Italy
-------
SOME EXAMPLES OF
THE CONCENTRATION OF TRACE HEAVY METALS
WITH ION EXCHANGE RESINS
BY
ROBERT E, ANDERSON
DIAMOND SHAMROCK CHEMICAL CO,
REDWOOD CITY, CALIFORNIA
PRESENTED AT CONFERENCE ON "TRACE HEAVY METALS IN WATER,
REMOVAL PROCESSES AND MONITORING", PRINCETON UNIVERSITY,
PRINCETON, NEW JERSEY, NOVEMBER 15-16, 1973,
-------
INTRODUCTION
Most heavy metals can be reduced to low levels in solution
by the conventional techniques of precipitation and fil-
tration. However, the concentrations remaining may still
be above those that can be returned to the environment.
These trace metals can be removed from solution by ion
exchange. In some cases this will be technically or eco-
nomically unattractive because of the other materials in
the solution. However, It Is often possible to transfer
these metals selectively from a large volume of solution
to a much smaller volume by the use of resins. The original
stream, containing the bulk of the Innocuous solids, then
can be recycled or safely discarded. The heavy metals,
having been concentrated into a small volume of solution,
may be isolated by precipitation and filtration, or even
distillation.
An ion-exchange column contains a set number of ionic or
potentially ionic sites., each of which Is capable of
holding one Ionic charge of the opposite sign under the
proper conditions. The composition of the ions on the resin
is controlled by mass action. That is, at equilibrium:
resin composition = f (solution composition)
All the ions of the same charge in the solution compete for
the resin sites. If the solution contains a high ionic
background, the selective removal under practical conditions
of an ion present in very low concentration will require that
the resin show a high selectivity for that ion. This selec-
tivity may be optimized In some cases by altering the ionic
background, or even just taking advantage of the ionic back-
ground present. The selectivity between monovalent and di-
valent Ions is very dependent on total ionic concentration.
With some heavy metals high concentrations of certain anions
may actually drive the metal into the resin. A few examples
will serve to show the diversity of approaches.
-------
ZINC IN COOLING TOWER BLOW DOWN
Two problems involving low levels of zinc have been
studied in the laboratory. Different answers were
obtained for the two cases.
A blowdown stream from a cooling tower contained ap-
proximately 6 ppm zinc which originated from the
proprietary zinc-organic corrosion inhibitors used (1)
The ionic background of the solution was:
ppm as CaC03
Sodium 370
Total Hardness 84
Total Strong Acid Salts 385
Total Alkalinity 60
PH 7.5
The zinc level was to be removed from a 25 to 40 gpm
stream to a level below 1 ppm and concentrated in a
small volume for disposal.
The neutral pH and the presence of alkalinity in the
stream suggested the use of weakly-functional cation
exchangers. Such resins tend to show a higher selec-
tivity for divalent ions over monovalent ions and more
selectivity among the divalent ions than the strong-
acid resins do.
Two resins were tried. A carboxylic resin, Duolite® CC-3,
showed only a limited capacity for selective zinc removal.
A phosphonic resin, Duolite® ES-63, proved to be very se-
lective for zinc even over the other hardness ions present.
The zinc in the column effluent was below the detection
level (0.4 ppm) of the polarographic method of analysis
used. Approximately 9,000 bed volumes of solution could
be passed through the bed before zinc was detected in the
effluent. The bed was regenerated with hydrochloric acid.
The regenerant waste amounted to about six bed volumes. Thus
the zinc was concentrated by a factor of about 1,500.
® Duolite is the registered trademark of Diamond Shamrock
Chemical Company.
-------
ZINC IN KAOLIN WASH WATER
Another problem involving zinc ion occurs in the kaolin
industry. Kaolin is treated with a solution of zinc
hydrosulfite to reduce and solubilize the iron present
for color improvement. About one pound of zinc is used
per ton of kaolin processed. The annual rate of kaolin
production was roughly 4 x 106 tons in 1972. Most of
the zinc is washed out of the kaolin. The raw water has
a low solids content and is relatively soft. The used
wash water has a pH of 3-0 to 3.5, contains variable
amounts of zinc, and has a low ionJc background. This
waste currently is blended with other plant wastes, neu-
tralized with lime to a pH of 5 to 6, and ponded before
discharge. Even so, plant effluents may contain from 10
to 60 ppm of z5nc. The kaolin producers have been told
to find means of reducing the effluent zinc to well below
1 ppm.
A weak-acid exchange resin is the first choice for the
concentration of a heavy metal from dilute solution
because of the high efficiency with which it can be re-
generated. However, such a resin does not seem to be
applicable to this zinc problem. The weak-acid exchange
resins do not show a useful operating capacity until the
pH of the solution is at least 5 to 6 pH. At this pH a
portion of the zinc exists as insoluble zinc hydroxide.
This would require prefiltration of the total volume of
waste. The flow rate through the resin would have to be
relatively slow, so a large Inventory of resin would be
required. There would be a double neutralization expense
'in that the acidic waste would have to be adjusted from
3.5 to 6pH before ion exchange, but would come out of the
ion exchange at 2 to 4 pH and would have to be neutralized
again before it could be sewered.
The low solids content of the wash stream makes a con-
ventional sodium-softening cycle using a strong-acid resin
attractive for the removal of zinc In this case. A syn-
thetic waste water was made up in the laboratory to simu-
late the kaolin wash water prior to liming. The analysis was
-------
Zinc 42 ppm (as zinc)
Sulphate 90 ppm (as CaC03 )
Chloride 12 ppm (as CaC03)
Calcium 20 ppm (as CaC03)
Magnesium 24 ppm (as CaC03)
pH 3-7
This water was passed through a 2.5 cm I.D. column filled
to a depth of 92 cm with Duolite® C-20, sodium form. The
flow rate was 25 bed volumes per hour. No zinc was de-
tected in the efflue.nt (detection limit 0.4 ppm) until
after 1000 bed volumes had passed through. There was a
minor leakage of hardness before this point, but the zinc
and hardness both broke through at essentially the same
time. The capacity of the column for zinc under these
conditions was 1.3 equivalents per liter.
This cycle has several attractive features. A high flow
rate can be used, so the resin inventory is reasonable.
No pH adjustment of the solution is required prior to ion
exchange, as the softening cycle will work as well at 3 pH
as at 7- In fact, the sodium cycle exchange will give
some acid neutralization for at least part of the cycle.
Regeneration with a high level of concentrated sodium
chloride (300 g/liter as 20$ solution) elutes the zinc
well enough even with conventional downflow operation
that zinc leakage in the following run is below detection
by the analytical method used. Even at this high re-
generation level the volume of regenerant effluent is 3
volumes per volume of resin or less. If the bed treated
1000 volumes, this would represent a concentration factor
of 300 to 400 fold. The heavy metals could be precipitated
from this small volume of solution by adjusting the pH to
8.5. The residual brine could be metered back into the
untreated waste stream so that any zinc left unprecipitated
is again picked up by the ion-exchange unit.
-------
LEAD IN PLANT WASTE STREAMS
Kozak, Baczuk and Landram have reported on an investi-
gation of the removal of lead from waste streams from
an ammunition plant. (2). The plant effluents were
found to average 6.5 ppm of lead while the proposed
tentative standard was a maximum of 0.05 ppm. The lead
could be reduced to 0.1 ppm or slightly less by careful
precipitation. A number of different types of ion-ex-
change resins were investigated. The only one which gave
a satisfactory performance was the phosphonic acid resin,
Duolite® ES-63. This resin routinely gave effluent levels
of 0.01 ppm of lead or less. The resin was regenerated
with nitric acid to give a solution containing about five
percent lead, a concentration factor of over 5x10* based
on the concentration entering the ion-exchange step.
The pH of the influent stream was shown to be critical.
If the pH of the influent was above 5.2, a portion of the
lead was present as a colloid and slipped through the
bed without being exchanged. If the pH Is much below 5*
the available exchange capacity of the resin will be re-
duced. The final recommendation of the study was a
precipitation step with lime and ferric salt to remove
80$ of the lead, and Duolite® ES-63 to bring the final
discharge to below 0.05 ppm.
CHROMIUM IN COOLING TOWER SLOWDOWN
Many heavy metals form stable complex anions in solution.
Chromium is routinely concentrated by andon exchange when
present as the chrornate or dlchromate. Chromates are com-
monly/ used Inolbltors in cooling towers. The blowdown
streams from these towers may contain from several hundred
to a couple of thousand ppm of dissolved chlorides and
sulfates. These streams are essentially Innocuous except
for a low level of chromate Ion, usually less than a hundred
ppm. If the tolowdown stream Is adjusted to a pH of ap-
proximately 5 and passed through a strong-base anion-exchange
resin, the chromium Is retained and the bulk of the other ions
pass through (3), The resin Is regenerated with an alkaline
-------
salt solution. The resulting solution of chromate and
salt is concentrated enough that it can be returned to
the cooling tower.
Again in this system pH is a critical factor. If the pH
is below 4 the oxidizing power of the chromic acid starts
to attack the resin. If the pH of the stream is above 6
the ratio of chromate to dichromate in solution increases.
The resin is apparently less selective for the chromate
ion than for the dichromate ion and early leakage of
chromium occurs.
MERCURY PROM BRINE
Many of the heavy metals form anionic complexes that,
although transitory in solution, are tightly held by
strong-base ana on exchangers. This phenomenon has been
investigated for most of the elements in chloride and
several other types of backgrounds, initially by Kraus
and co-workers (4). The extent of pickup is a function
of the particular cation, the nature of the complexing
anicn, and the concentration of the anion. Two commercial
applications of this type of Ion-exchange are the removal
of trace amounts of iron from concentrated hydrochloric
acid and the recovery of low levels of uranium from
sulfuric acid leach solutions.
A strong-base anion-exchange resin shows a high affinity
for mercuric Ion over a broad range of chloride ion con-
centrations. A process for the removal of mercury frcm
the spent brine from a chlorine-caustic electrolysis cell
with a mercury cathode was developed In the early 1960's (5)
Passage of the spent brine through the resin reduced the
mercury level to below the detection limits then in use.
Since the affinity of the resin for mercury actually in-
creases as the chloride concentration decreases, it was
desirable to reduce the mercuric ion to mercurous ion on
the resin with thiosulfate in order to recover it (6).
The recovered mercury could be returned directly to the
-------
electrolysis cell. This process was evaluated at pilot-
plant scale and found to be technically feasible. Since
the only motivation for its implementation at that time
was a possible savings in mercury costs, it was not fi-
nancially attractive enough to be installed.
Understandably, there has been a renewed interest in the
use of ion-exchange resins to remove mercury from chlorine-
caustic cell wastes in the last five years. It is unlikely
that the above process would be a completely satisfactory
solution to mercury cell problems under the present standards
of discharge as monitored by newly developed analytical
methods. The pick-up of mercuric-chloride complex on a
strong-base resin is more in the nature of an adsorption
process than a true ion-exchange. An adsorption curve can
be determined showing that the amount of mercury that can
be loaded on the resin is a function of the equilibrium
concentration of mercury in solution, much as in the ad-
sorption of a solution on carbon (Fig. 1). If only trace
amounts of mercury are present in the solutions to be treated,
the resin will show a low effective capacity- and leakage is
probable. The strong-base resins would thus be excellent
for removing 99% or so of the mercury, but they alone would
not give a satisfactory answer.
A number of ion-exchange resans were recently screened for
their ability to adsorb mercury. Resins based on a phenol-
formaldehyde polymer were found to have activity (Fig. 2).
Those resins with phenolic, carboxylic or sulfonic function-
ality had a rather low adsorptive capacity. However, when
some weakly-basic groups were present, a very worthwhile
capacity was obtained. The capacity is of an adsorptive
nature, and, unlike with the strong-base resins, a chloride
background is not necessary. The mercury can be regenerated
from the resin with hydrochloric acid. These resins offer
another method of removing the major part of the soluble
mercury from an aqueous stream.
Mercury adsorption resins have been offered recently that
retain a high capacity even at very low mercury concentrations
in the contacting solution. These resins are phenol-form-
aldehyde or styrene-divinylbenzene structures containing
sulfur, probably as sulfhydro groups. These resins show
flat adsorption isotherms and should be quite effective in
-------
51
removing mercury to extremely low levels. They are
stated to be unstable to oxidative conditions. Chemical
regeneration is difficult. This type of resin will be
most useful as an expendable adsorbent to remove the
last traces of mercury after the majority of the mercury
has been removed on a regenerateable resin bed.
SUMMARY
When considering the problem of concentrating a heavy
metal from solution, a large number of interrelated factors
must be considered. A partial list will include:
the metal
initial concentration
final concentration required
oxidation state
solubility versus pH
tendency to form complex ions
the solution
volume per unit time
disposal or recycling requirements
pH
extraneous salts
concentration
nature
temperature
oxidative or reductive potential
allowable costs
capital
operating
alternatives
-------
52
Usually the most difficult step is getting the man who
understands the problem and has most of the above in-
formation, together with a man who has the necessary
specialized background and experience so that between
them they may come up with a workable answer.
1. D.G. Chamberlain, Proceed. 31st Int. Water Conf.,
Pittsburgh, Pennsylvania, Oct. 27-29, 1970, p. 151
2. M.A. Kozak, R.J. Baczuk, and G.K. Landram, SUN 143-10,
"Solventless Extruded Powder N-S General. Get The
Lead Out. Method For Removing Lead From Plant Waste
Water Streams.", August 2, 1971 (National Technical
Information Service Pub. No. AD-729 049).
3. A.W. Oberhofer, U.S. Patent 3,223,620 (Dec. 14, 1965)
4. K.A. Krauss and F. Nelson, "Proceedings of the Inter-
national Conference on the Peaceful Uses of Atomic
Energy", Vol. 7, p. 113 (1956)
5. H.G. Scholten and G.E. Prielipp, U.S. Patent 3,085,850
(1963)
6. G.E. Grain and R.H. Judice, U.S. Patent 3,213,006
(1965)
-------
53
Q Z
UJ •— «
CQ CO
Q; LU
O CXL
GO
Q h-
< LU
o: CD
•=> -\
O C2
o! m
LU
100
10
0.01
Figure 1.
Adsorption of
Mercury on
Duolite® A-109
O.I 1.0
EQUILIBRIUM CONCENTRATION, MG HG/LITER
10
-------
MERCURY ADSORBED,
MG HG/G WET RESIN
100 -
Adsorption of Mer
Phenol-FormaIdehy
cury on
de Resins
resin with st
functionality
rong-acid
resin with we
functionality
resin with su
functionality
I I I I II
vn
-p-
0.01
100
1000
EQUILIBRIUM CONCENTRATION, MG Hc/LITER
-------
-------
56
'9L-SI 'AON '
EARLY METALLURGICAL APPLICATIONS
uoq.aou.Lwid 'sa.Lpnq.s Lelu3WUOULAU3 uoj. uaq.uaQ au.q. q.B '6u.LJoq.Luow puB
recovery of gold and silver from ore pulps in the presence of cyanide. The cyanide
dissolved the metal from the ore and the activated'
the
the
aoBj.uns
-MOH •sajaL)dsowq.B pa[LOuq.uoo UL uoqjBO au,q. j.o q.uaiuq.Bauq.
A~q paq.Buj.iuL [a uo
Aside from metallurgical iises there was little interest in the adsorption of
pazLWLXBiu uau.q.L3 aq ABLU sdnoub aoBj-uns asam -suLsau j.o asBO au.q. oq. snobo[Bue aq.j.nb
tals at trace concentrations. Over the years, Hassler^' and his associates at
^uoquBO uo SUOL snouBA uoj. q.no pa>|JOM aq p[noo sauas q.uaiuaoB[ds.Lp 9q.LULj.ap B puB
Westvaco (then West Virginia Pulp & Paper Co.) conducted a number of laboratory
sq.LBS uuoj. oq. suo.iq.po Lfq.LM q.OBaJ pLnoM pus q.uasaud aq oq. punoj. auaM sadA"q. snouBA j.o
me
stiidies on, the adsorption .of various, metals and inorganic ions. Under the right
sbULdnoub aoBj.uns paq.BuabAxo '( |jT:uaquaaq.s Aq L!-6q.9p aiuos UL pai.pnq.s puB O&B s
conditions ot,pH and oxidation potential, certain metals were found to be adsorbed
AUBLU pazLuBooau auaM uoquBO paq.BALq.OB j.o saLq.uadoud a6uBqoxa UOL aqq. 'ssa[ai|q.jaAaN
very strongly and in larqe amounts. Other ions might be adsorbed either very slightly
•'sapads1 OTLUOL uoj. q.uat|uospB aALq..oaj.j.a UB SB panubooau /•[[BuauaS q.ou SL 'uoLq.ni.os
or A0_t at all. The entire ranqe of the periodic table and infinite combinations
snoanbB UKUJ. a>|L[ au.q. puB ' spirnodiuoo OLq.^mojB 'JOLOO SB Ljons 'saLq.unduiL OLUB6uo puB
of pH, oxidatiori states and dissolved salts in the medium presented a formidable
rsuodBA DLUBDUO quospB oq. Aq.LL.LqB sq.L uoj. UMOU>| L L9M M6nou.q. 'uoquBO paq.BALq.oy
task for any thorough investigation. Nevertheless, a number of systems were studied
and a general
wa s
.uo
-------
57
this point there was little interest in adsorptive removal of traces of heavy metals
except as it contributed to the efficiency of metallurgical recovery.
MERCURY REMOVAL BY ACTIVATED CARBON
In 1970, the nation became alarmed about the widespread contamination of surface
waters by mercury when mercury cell chlorine/caustic plants were found to be losing
large quantities of mercury in various forms into streams used for potable water
sources. One caustic manufacturer, Canadian Industries, Ltd., was already studying
this problem and had obtained a patent^ on carbon columns to remove mercury from
caustic. The adsorbing and filtering action of the carbon was supplemented in their
filter design by layers of metal turnings or gauze to retain free metallic mercury
by amalgamation. Westvaco and others set up column and batch contacting studies
to determine the most effective way to remove mercury in its various forms with
activated carbon. From this and other work it was learned that filtration through
combinations of coarse powdered activated carbons and cellulosic filter aids was
very effective in removing mercury from caustic down to very low levels, e.q.,<50
ppb. A number of carbon filters of various types were set up in this country and
Canada for treating caustic solutions and mercury bearing waste effluents. A paper^ '
covering field applications of this type was given by the writer at the WPCF meeting
in 1971. These filters, to our knowledge, were the first carbon installations for
the sole purpose of trace metal removal. The mercury was initially present in such
forms as metallic droplets, possibly emulsified, mercuric and/or mercurous salts
and as organic compounds such as methyl mercuric chloride or dimethyl mercury and
yet all were removed to a high degree. Thus, it was obvious that seyeral removal
mechanisms must therefore be involved.
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58
It may be well to emphasize at this point that the very reasons why carbon is
appealing for mercury removal also make it attractive for the removal of other trace
metals. These reasons are:
1) Carbon is an effective removal agent even at very low solution concentra-
tions
2) The removal action is relatively non-specific, that is, the carbon has a
capacity for the metal in several different forms
3) Carbon may be regenerated for reuse by acid washing and/or thermal treat-
ments and the metals recovered, if of value.
MECHANISMS OF REMOVAL
In order to put the phenomenon of physical adsorption into proper perspective
and to differentiate between this and "real" removal processes on activated carbon
it may be helpful to discuss some of the possible mechanisms for heavy metal removal.
1. True Adsorption - In theory only true solutions are acted upon by classical
adsorption. The dissolved adsorbate is attracted to the gross interior
surface of the carbon and establishes a dynamic equilibrium between a con-
centrated surface layer and a dilute solution in the pore space. All portions
of the surface, amounting to as much as 1200 m2/g are involved provided
the pores within which some of the surface lies are not inaccessible to
the adsorbate due to size consideration. The surface will be very non-
uniform and the more tenacious adsorption will take place in the finest
pores and most active parts of the surface resulting from unsatisfied
valence bonds, surface charges, etc. In an aqueous media the entire surface
will be saturated with water but this is easily displaced from the hydro-
phobic carbon surface except where hydrated surface groupings are attached.
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59
True adsorption will occur most efficiently for large molecules with a
minimum of surface charges and low solubility.
2. Precipitation - There is a thin line of distinction between true adsorp-
tion and precipitation on the surface. However, if a material of very low
solubility is present, a drop in temperature, the addition of other sub-
stances, etc. may cause the solution to become supersaturated. Activated
carbon can provide a potent nucleating force causing a precipitate to form
on the surface until the supersaturation is removed. Then additional
material may be removed by adsorption to reach still lower concentrations
in the surrounding solution.
3. Ion Exchange - All commercial activated carbons contain some functional
groups containing oxygen on the carbon surface. Since the basic carbon
skeleton is graphitic in nature, phenolic, carboxylic, ether, peroxide,
lactone and hydroxyl groups may exist. The total number and type will
depend on the thermal history of the adsorbent and on the oxidants to
which it has been exposed. Other groups containing sulfur and nitrogen
may also be present. Some of these groups will form salts as in the case
of ion exchange on resins. The exchangable sites act in analogous fashion
to those in ion exchange resins and various metal cations will displace
each other in a predictable fashion. Most such groups are cation acceptors
though some anion acceptors may also be present. In general, the number
of such groups is uncontrolled in commercial activated carbon but products
from certain processes will tend to have a more or less typical surface
group distribution. Such groups are fast acting and are responsible for
removal of the simple metal ions in the 1, 2 or 3+ valence states. Pur-
posely oxidized carbon will have a maximum of such activity.
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6o
4. Reduction to Metal or Oxidation to Insoluble Forms - The carbon surface
is an active catalyst for both reduction or oxidation depending upon the
surrounding medium. In the presence of dissolved oxygen, ferrous ion is
quickly oxidized to ferric ion and this is readily precipitated as Fe(OH)3
if the pH is 4 or over. Likewise, reduction can take place if reducing
agents are present. Various impurities such as elemental iron, ferrous
salts and sulfides may be present in trace amounts which can reduce metal
ions such as silver, gold or mercury to elemental form.
5. Filtration or Entrapment - A granular bed or a cake of fine carbon can act
as a very effective filter. Suspended particles will cling to the rough
carbon surfaces and coagulation of suspended or colloidal matter can be
induced by the strong surface forces surrounding particles which can remove
surface charge layers and break up emulsifier layers.
With the existence of these many surface phenomena it is very difficult to
predict what will take place in a real trace metal problem situation. With
this as background, current developments in the application of activated
carbons may be more meaningfully discussed.
REVIEW OF RECENT DEVELOPMENTS
Direct Adsorption - This classification is meant to consider simple treatment
of a stream containing metals by applying commercial activated carbons in either
granular or powdered form without prior addition of reagents to either the solution
or the carbon. A review of abstracts over the past five years reveals the removal
of the following metals in varying degrees.
Chromium as chromate at 100 ppm was 80-85% removed from electroplating waste
by coal base activated carbon according to a Japanese investigation'').
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61
Lead, Ni and Cd were adsorbed on both regular and oxidized carbons from solu-
tions of alkali metal chlorides to reduce impurities to the 2x10"^% level as
determined by a Russian group'8). Other metals were also removed including
Zn, Fe, Mn, Ca and Al.
Another Russian group'9) reported strong adsorption of Bi, Mn, Cu, Zn, Cd and
Pb from hydrochloric acid solution and a method of quantitative separation of
binary ion mixtures was proposed.
Germanium^IU' has been quantitatively adsorbed in an expanded bed of granular
wood base carbon. The adsorbed metal was quickly removed by washing with 1%
NaOH. In addition to the articles already mentioned there are a number of
references to mercury removal.
A Russian article^1) reports adsorption of mercury from waste waters on
activated charcoal followed by regeneration by heat treatment in a vacuum yielding
condensed Hg in a cooled zone. Carbons with hydrophobic surfaces were found by a
Polish researcher(12) £0 ^e superior adsorbents for mercuric chloride. Mercuric
chloride''3) was found to be well adsorbed by a number of Russian commercial carbons
and kinetics were studied by a Russian group.
The most directly applicable data for potable water treatment for Hg removal
consists of two articles by Gary Logsdon and James M. Symons at the Water Supply
Research Lab. in Cincinnati. In this work"4) which covered the comparative
effectiveness of conventional water treatment methods in removing mercury, granular
activated carbon was found to be most effective in removing organic mercury and
powdered carbon was found almost equally effective in removing both organic and
inorganic mercury. Though a variety of mechanisms of removal appear possible, such
as ion exchange or sulfide precipitation, true adsorption appeared to be the most
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62
likely mechanism in that reasonably well-defined Freundlich isotherms with slopes
less than one were obtained. Also, the column tests showed evidence of greater
loadings due to the countercurrent contacting, i.e., saturation at the high in-
fluent concentration level.
In a more recent paper^'^) Logsdon and Symons described similar work with
Ba+2 and Se+^'+^. In this case activated carbon in powdered form was poorer at
removing these elements than were coagulation or softening. Further work is planned
on cadmium, chromium and lead.
There is also work underway on the effect of powdered activated carbon in com-
bination with conventional secondary sewage treatment practices for the purification
of industrial waste. Data are being collected on the following metals, Ni , Fe, Cr,
Hg, Pb, Cu, Zn, Mn. Though high degrees of removal have been observed on certain
metals it is not clear at this time whether the carbon is mainly responsible. At
present these data are regarded as confidential, but hopefully they will be
correlated and released within a reasonable time.
Adsorption of Complexes - The adsorption of metal complexes has been known
for a number of years and the adsorption of silver and gold as cyanide complexes
has already been mentioned. Soon after EDTA came into use as a chelate for calcium
and other metals it was found that the metal chelates were easily adsorbed. It was
also found, however, that if the EDTA were preadsorbed on the carbon it unfortunately
lost most of its chelating efficiency. According to a recent reference, J. Leontiades"6'
has used the adsorption of the chromium EDTA complex to increase the sensitivity of
a radiochemical detection device. Further work on gold and silver has been reported
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63
by Taskin(17)(18) and Nizamutdinova(19). High capacities for gold are also reported
by Lodeishchikov(2°) in strong thiourea solution obtained from stripping ion exchange
resins used to pick up a variety of metals.
A highly efficient adsorption of the nitratonitrosyl and nitronitrosyl ruthenium
(21)
complexes are reported by Matsumurav ' and adsorption of yttrium in the presence
(22)
of humic acid has been reported by Shchebetkovskiiv ' Some work has been done
by Westvaco on the adsorption on metallic salts of modified soluble lignins as a
means of stripping out traces of heavy metals. This technique holds promise and
will be investigated further.
In summary, the surface has only been scratched on the possibilities of adsorp-
tion of metal complexes. The technique should certainly be tried on specific problem
metals if direct adsorption or other methods fail.
Metal Removal by Modified Carbons -
Oxidized activated carbons - In the last five years considerable work has
been done with carbons with specially prepared oxygenated surfaces. Such
carbons have been prepared by heat treating them in oxygen-containing atmos-
pheres or by exposing them in slurry form to oxidants such as nitric acid.
Apparently the ion exchange activity of the carbon can be greatly enhanced
(23)
by this means. Tarkovskaya reported in detail on a carbon oxidized in
air at 400-450°C. Dynamic sorption capacities ranging from 0.16 to 0.86
meq/g at a pH of about 4 were observed for Na+ and Fe , respectively. A
displacement series was worked out for 13 cations at pH 1-6. Selectivity
factors were also determined for various pairs of cations. Purification
processes were suggested for metal salts and hydroxides of the alkalis,
with removal of Mg, Al, Zn and Ni requiring treatment by 0.5 to 5% of the
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6k
oxidized carbon in a single operation.
Kononchulo24) used a nitric acid oxidized carbon in the sodium form to purify
the mother liquor from chlorine production and reports the capacity to be
higher than commercial carboxyl and sulfonic acid cation exchanges. Metals
removed were Mg, Ca, Cu, Pb and Fe. Another Russian investigator,
Taushkanov(9) cited earlier, also reported the purification of alkali metal
chlorides by oxidized carbon and found that oxidation greatly enhanced the
activity of the carbon. Impurities in NaCl solution consisting of £1, Ni,
Zn, Fe, Pb, Mn, Cu, Ca, Mg and Ba were reduced to 2-3 x 10~6% (20-30 ppb)
for each element.
To my knowledge, no purposely oxidized carbons are produced commercially
in this country, nor is there commercial process employing such carbons.
If purification processes involving heavy metals are going to be required
in the future perhaps oxidized carbon should be developed commercially to
compete with ion exchange resins. These are currently acid washed granular
carbons on the market but these are not in great demand. Their principal
use is in acid systems where the leaching of inorganic impurities from the
carbon would be detrimental.
Metal removal by impregnated or "loaded" carbons - Since it was learned
that metal complexes could be stronqly adsorbed on activated carbon a number
of investigators have been intrigued with the possibility of adsorbing specific
chelating or complexing agents onto carbon and thereby obtaining a specific
adsorbent for a particular metal ion.
In 1967, Egorov(25) reported on the addition of pyrocatechol and similar com-
pounds to increase the adsorption of germanium.
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65
Plyushcheva(26M'n 1969 reported on the use of carbon loaded with dimethyl -
glyoxime to remove Ni , La, Co and Fe ions. Also studied were carbons loaded
with Trilon B and citric acid. The response of the adsorption to pH changes
indicated that the sorption capacity corresponds to the stability of complexes
formed by the ions. In 1970, Evdokimov(27)pubiishec| data on the separation
of Ge^+ and As^+ on carbon treated with tartaric acid and ferric hydroxide.
Tartaric acid pushes the selectivity toward Ge whereas the Fe(OH)3 favors
As sorption. Later the same author showed that Ge adsorption is also
increased by loading citric acidv 'onto the carbon and Ge removal is in stoichio-
metric proportion to the citric acid added. With o-hydroxyquinoline it was
learned^9 'that As would only be adsorbed when Ge was present suggesting
Ge-As compound formation when the two are adsorbed together.
In 1971, Plyushcheva^ 'reported further work with dimethylglyoxime and
also hydroxyqu incline, diethyldithiocarbamate and citric acid in the ad-
sorption and purification of La, Ni , Co and Fe.
(31 )
In this country the most significant work appears to be that of R. H. Moorev ;
of Batelle Memorial Institute under the sponsorship of Office of Saline
Water of U. S. Dept. of the Interior. This work had as its objective the
removal of Cu from sea water desalination plant effluent. Eight chelating
agents and six activated carbons were evaluated. The most useful chelating
agents were: salicylaldoxime, 8-hydroxyquinoline, benzoyl acetone, dibenzoyl-
methane, anthranilic acid and mercaptobenzothiazole.
This presentation is a most significant one in that it establishes the
practicability of loading carbons with a reagent of reasonable cost and apply-
ing it in a cyclical fashion to a real problem. Both hydroxyqu incline and
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, 66 •
salicylaldoxime may be used on standard granular carbons of .several types and
used to remove copper from dilute or more concentrated solutions to levels of
10 ppb or less until an abrupt breakthrough is reached. Though only between
46 and 80% of the theoretical chelating capacity was realized it was found
that substantial amounts of "direct adsorption" on the base carbon were ob-
tained as well which were not affected by the reagent adsorbed on the carbon
surface. This "direct" adsorptive or ion exchange capacity led to combined
Cu removals as high as 187% of theory. The sorbent could be easily regenerated
by acid wash. However, the direct adsorption by carbon itself was unable
to remove low concentrations of copper which were easily, though more slowly,
taken up by the chelating reagent.
The regeneration of the saturated sorbent was found very practical though
the "working" capacity of the second and higher cycles were not as high as
the initial adsorption. Both the chelating capacity and the carbon activity
could be regained with a reasonable quantity of regenerant. A slight leak-
age of reagent in either the unreacted or metal loaded form could be contained
in a small guard bed of pure carbon downstream.
SUMMARY
From the foregoing discussion it is evident that adsorption offers many possi-
bilities for removal of traces of metals in aqueous media. Direct adsorption
offers good possibilities for certain metals where the pH and other conditions are
appropriate. Mercury, Cu, Ag and Cr are good candidates in relatively pure water.
More data are certain to be forthcoming as physical chemical sewage plants using
activated carbon columns are studied in more detail.
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67
Work on the adsorption of complexes is underway and there is a good possibility
of commercialization of this approach if cheap complexing agents, such as modified
lignins, can be developed. This approach would be especially attractive for potable
water treatment where powdered or granular activated carbon treatment facilities
are already available. Oxidized carbons pose some interesting possibilities to
adsorbent manufacturers for producing materials as effective as ion exchange resins.
Oxidized carbons are potentially much cheaper than resins and can be subjected to
very severe treatment conditions, e.g., strong oxidants, strong acids and bases, and
high temperature oxidative regeneration, which might be advantageous in case of foul-
ing. Certainly this approach should be more fully investigated for the purification
of brines, sugars, organic and inorganic chemicals.
The field of impregnated or "loaded" carbons presents infinite possibilities
in removing various ions or combinations of ions. If one commercial application such
as the purification of desalination effluent can be reduced to practice then the
stage will be set for many other applications that use the same impregnated carbon.
Experts in the field of water purification can promote this development considerably
by pointing out real trace metal problems that do not respond well to ion exchange
or other forms of treatment. Work is going on currently at North Carolina State
University under the direction of Dr. William McKean which hopefully will result
in further practical applications of this technique.
In view of the ever-widening use of activated carbon in both potable and waste
water treatment, the reliance on this all purpose purifier for removal of trace
metals most surely will increase. A challenge is presented both to water treatment
chemists and to carbon manufacturers to optimize conditions by one or more of the
aforementioned techniques to achieve high removal efficiencies of trace heavy metals
at reasonable cost.
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REFERENCES
1. Steenberg, B., "Adsorption and, Exchange of Ions on Activated Charcoal,"
Almquist and Wiksells, Uppsala (1944).
2. Hassler, J. W., "Activated Carbon," Chemical Publishing Co., New York (1963).
3. Sigworth, E. A. and Smith, S. B., "Adsorption of Inorganic Compounds by
Activated Carbon," JAWWA, 64_:386 (June, 1972).
4. Smith, S. B., Peterson, H. D. and Lewis, C. J., "Sorption of Vanadium by
Activated Carbon," Presented at the Annual Meeting of the American Institute
of Mining, Metallurgical and Petroleum Engineers, Chicago, Illinois, Feb. 14-
18, 1965.
5. MacMillan, J. B., U.S. Patent No. 3,502,434 March 24, 1970, Assigned to Canadian
Industries, Ltd.
6. Smith, S. B., et al, "Mercury Pollution Control by Activated Carbon: A Review
of Field Experience," Presented at the Annual Conference of the Water Pollution
Control Federation, San Francisco (1971).
7. Azuma, K. and Hitomi, M., (Fac. Eng. Nippon Univ., Tokyo, Japan) "Treatment
of Waste from Plating by Use of Activated Coal," Kogyo Yosui 1972 (162), 35-40,
C.A. 77 38862K (1972).
8. Taushkanov, V. P.; Kuzin, I.A.; Mironov, A. N.; Andrianov, S. F.; Mironenko, V. M.,
(USSR) "Purification of Alkali Metal Chlorides Using Active Carbons," Zh. Prikl.
Khim. (Leningrad) 1972, 45(3), 523-8, C.A. 77 9953K (1972).
9. Taushkanov, V. P.; Kuzin, I. A.; Nazarov, Yu. M.; Boganch, Ya. (Leningrad.
Technol. Institute im Lensoveta, Leningrad), "Sorption of Bismuth by Activated
Carbon SKT from Hydrochloric Acid Solutions," Izv. Vyssh. Uck. Zaved, Khim.
Khim. Tekhnol. 1968, 11(1), 30-4, C.A. 69^ 4887811179687:
10. Evdokimov, D. Ya. and Kostyuk, A. P., "Circulating Method of Adsorption of Germanium
from Solutions in a Fluidized Layer of Activated Carbon," Zh. Prikl. Khim. 39(10),
2217-22 (1966), C.A. 66 22554Y (1967).
11. Zaitsev, M. G. and Belyanskaya, A. V. (Central-Asian Scientific-Research and
Planning Institute of Nonferrous Metallurgy) "Removal of Mercury from Waste Waters,"
Otkrytiya, Izobret., Prom. Obraztsy, Tovarnye Znaki 1972, 49(11), 89, C.A. 77
79326G (19727:
12. Jodko, Czeslaw; Szopinski, Julian (Politech. Krakowska, Cracow, Pol.) "Mercury
Dichloride Adsorption on Activated Carbon," Przem. Chem. 1972, 51(9), 595-7,
C.A. 77 156747Z (1972).
13. Bakin, V. M.; Timasheva, I. A.; Denisova, G. V.; Lambrev, V. G., "Adsorption
of Mercuric Chloride from Aqueous Solutions onto Activated Carbons," Zh. Fiz.
Khim. 1971, 45(7), 1869-70, C.A. 75 122388B (1971).
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69
14. Logsdon, G. S., Symons, J. M.; "Mercury Removal by Conventional Water Treatment
Methods," Presented at the 92nd Annual Conference, American Water Works Assoc.;
Chicago, Illinois, June 4-9, 1972.
15. Logsdon, G. S. and Symons, J. M., "Removal of Trace Inorganics by Drinking
Water Treatment Unit Processes," Presented at the American Institute of Chemical
Engineers Meeting, June 3-6, Detroit, Michigan.
16. Leontiadis, J.; Dimitroulas, C. (Div. Technol. Appl., Nucl. Res. Cent. "Democritus
Aghia Paraskevi Attikis, Greece) "Adsorption of Chromium-EDTA on Carbon Active
Columns," Nucl. Res. Cent. 1971 DEMO 71/13EJO pp., C.A. 76 49777S (1972).
17. Taskin, N. I.; Romanenko, A. G.; Shevchenko, N. P.; Lebedev, K. B., "Isotherms
of Sorption of Gold and Silver from Cyanide Solutions by KAD-ground Active
Carbon," Tr. Nauchno-Issled. Proektn. Inst. Obogashch. Rud Tsvetn. Met. 1971,
No. 3, 309-13, C.A. 76 16870P (1972).
18. Taskin, N. I.; Romanenko, A. G.; Shevchenko, N. P.; Lebedev, K. B., "Effect of
Some Physicochemical Factors on Sorption of Gold by KAD-Ground Carbon," Tr.
Nauchno-Issled. Proektn. Inst. Obogashch. Rud Tsvetn. Met. 1970, No. 3, 330-38,
C.A. 76 16871Q (1972).
19. Nizamutdinova, R. A.; Chevasheva, G. L., "Use of Activated Carbon in Gold Hydro-
metallurgy," Tr. Tsent. Nauch.-Issled. Gornorazved. Inst. 1967, No. 77, 24-33,
C.A. 69_ 45364FT1968).
20. Lodeishchikov, V. V.; Panchenko, A. F., "Sorption of Gold from Acid Solutions of
Thiourea by Activated Carbon," Tsvet. Metal. 1968, 41(4). 25-7, C.A. 69 38011S
(1968).
21. Matsumura, Takashi; Ishiyama, Toshio (Radiat. Cent. Osaka Pref., Osaka, Japan)
"Adsorption Property of Nitrosylruthenium Complex on Activated Carbon," Radioiso-
topes 1970, 19(7), 326-7, C_.A^ 74 25394X (1971).
22. Shchebetkovskii, V. N.; Khoroshailov, A. G., "Behavior of Radioactive Elements
in Adsorption Systems with Humus Substances. II. Adsorption of Yttrium-91
from Aqueous Solutions by Activated Carbon in the Presence of Humic Acids,"
Radiokhimiya 1970, 12(3), 442-7, C.A. 73 136556A (1970).
23. Tarkovskaya, I. A.; Emel'yanov, V. B.; Rubanik, S. K.; Strazhesko, D. N.,
"Ion Exchange on Oxidized Carbon and Its Use," Sin. Svoistva lonoobmen, Mater.
1968, 248-55, C.A. 71 76944X (1969).
24. Kononchuk, T. I.; Tarkovskaya, I. A.; Chernenko, A. N. (Inst. Fiz. Khim. im.
Pisarzhevskogo, Kiev) "Purification of Mother Liquor for Chlorine Production
on Oxidized Carbon," lonnyi Obmen lonity 1970, 217-22, C.A. 74 143757r (1971).
25. Egorov, A. M.; Odinets, Z. K.; Evseeva, G. E. (P. Lumumba Univ., Moscow)
"Germanium Adsorption by Modified Adsorbents," Zh. Prikl. Khim. 40(2), 380-6
1967, C.A. 66 108580R (1967).
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26. Plyushcheva, S. V.; Senyavin, M. M. (Inst. Geokhim. Anal. Khim. im. Vernadskogo,
Moscow) "Statics of Metal Ion Sorption by Modified Sorbents," Zh. Fiz. Khim.
1969. 43(8), 2150-1, C.A. 71 129104G (1969).
27. Evdokimov, D. Ya.; Kogan, E. A., "Kinetics of the Separate and Mixed Adsorption
of Germanium (IV) and Arsenic (III) on Activated Carbons Saturated with Ferric
Oxide Hydrate and Tartaric Acid," Zh. Prikl. Khim. (Leningrad) 1969. 42(12),
2745-9, C.A. 72 104208B (1970).
28. Evdokimov, D. Ya.; Kogan, E. A. (Odess. Elektrotekh. Inst. Svyazi, Odessa)
"Sorption of Germanium from Solutions by Activated Carbon Saturated with Citric
Acid," Zh. Prikl. Khim. (Leningrad) 1970. 43(9), 2012-16, C.A. 74 6741S (1971).
29. Kogan, E. A.; Evdokimov, D. Ya. (Odess. Elektrotekh. Inst. Svyazi im. Popova,
Odessa) "Kinetics of the Separate and Combined Sorption of Germanium (IV) and
Arsenic (III) on Carbon Saturated with Citric Acid and 0-hydroxyquinoline,"
Ukr. Khim. Zh. (Russ. Ed.) 1972. 38(6), 541-4, C.A. 77 169139C (1972).
30. Plyushcheva, S. V.; Antonov, V. A.; Senyavin, M. M., "Removal of Colored Heavy
Metals from Lanthanum on Activated Carbon with Complexing Agents," lonoobmen.
Mater. Ikh Primen. 1968. 184-92. From Ref. Zh. Khim. 1969 Abstr. No. 16L84,
C.A. 74 25565D (197T7T
31. Moore, R. H., "Investigation of a Process for Removal of Copper from Sea Water
Desalination Plant Effluent Using Carbon Sorbates," Office of Saline Water,
Research & Development Progress Report No. 651 Contract No. 14-30-2668, U. S.
Dept. of the Interior.
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71
HEAVY METALS REMOVAL BY, THERMAL PROCESSES
Ronald F. Probstein
Department of Mechanical Engineering
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
1. Introduction
Thermal processes may be used either for the recovery of the waste-
water containing dissolved heavy metals or for the recovery or concen-
tration of the metals themselves. The two principal thermal methods
are evaporation and freezing, both of which involve phase changes and
the nonisothermal transfer of heat. In evaporation the wastewater is
either partially or wholly vaporized, with the condensed pure vapor
yielding the recovered fresh water and the non-volatile concentrate the
dissolved metals. In freezing, the waste process water is partially
frozen and the pure ice crystals formed are separated from the more con-
centrated solution and melted to yield the clean water. It should be
noted that whereas evaporation is an established commercial method in
heavy metal recovery and pollution control, as in the metal finishing
industry, freezing (as distinct from precipitation by cooling) has not
yet been developed beyond the pilot plant stage.
Among the general advantages which may be cited for thermal processes
is that they can concentrate to any desired level, without incurring the
severe economic penalties inherent in other methods whose energy require-
ments are strongly concentration dependent. By the same token such
processes will generally prove economical only when applied to relatively
concentrated wastewaters. Of course, the reduction of any trace con-
taminant for either recovery or reuse will generally involve a high
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concentration processing stage. Another advantage of such methods is
that there are generally no chemical byproducts. Finally, they are not
specific with respect to the dissolved solids, and are capable of
handling wide varieties of mixed wastewaters which may be the products
of the combining of various unsegregated wastes.
A principal disadvantage of thermal methods is that they tend to
be energy intensive because of their nonisothermal character. This is
particularly true of evaporative systems. A consequence of an in-
efficient thermal performance is the relative insensitivity of the
economics to contaminant concentration. This results from the fact that
the energy requirements are greatly in excess of the minimum thermodynamic
energy to separate out the contaminant, the value of which does increase
with contaminant concentration. It follows that any savings due to re-
duced concentrations amount to only a small reduction in the overall
cost. Other basic disadvantages with evaporation are the scale and
corrosion problems which are always an inherent byproduct of high tem-
perature operation. Corrosion is particularly acute with heavy metal
solutions such as chromic acid and copper, nickel and zinc acid solu-
tions. In freezing, although scale and corrosion problems are minimized,
there is a disadvantage of increased capital costs and mechanical com-
plexity associated with the requirements of growing, handling and wash-
ing ice crystals.
Recycling of plating chemicals by evaporative recovery is a method
presently in use in the metal finishing industry . Chromium, cyanide
and nickel plating baths are among the many solutions handled in which a
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73
wide variety of heavy metal contaminants are to be found. Shown in
Fig. 1 is a simplified flow diagram of a closed loop evaporative system
used to recover for reuse the chemicals lost in the "dragout" which is
carried on the work from tank to tank. The only chemical addition
necessary is that required to make up for the actual deposition and
1 2
accidental loss ' . It is to be noted that the plating solution is
also circulated through the thermal process as a means of minimizing
the degree of concentration required and to enable additions to the
system. A similar system has been proposed using freezing as the
3
thermal process . Other applications of thermal methods are in the
further concentration of the regenerant wastes from ion exchange or the
concentrated effluent from an electrodialysis or reverse osmosis system.
An example of concentrating a reverse osmosis effluent using both
4
evaporation and freezing may be found in the paper by Houle in this
conference. Finally, there are the applications involving the reclama-
tion to dryness of heavy metals. One such example, among many, is the
recovery of zinc chloride in vulcanized fiber operations . Freezing,
or more properly eutectic freezing , may also be used, wherein the con-
centrate is continuously frozen until the solution is saturated with
respect to the dissolved solids. By further removal of heat, ice and
precipitated metal salts (mostly in hydrated form) are continuously
removed from the liquid phase. If desired the precipitated salts can
be brought to dryness, for example, by evaporation or vacuum freeze
drying .
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*
2. Evaporation
As a consequence of the rapidly escalating costs of energy, one of
the main problems in the application of evaporation to the removal of
heavy metals from wastewaters is to reduce the system energy require-
ment and thereby the principal operating cost. Most evaporators use
heat energy, usually in the form of steam, which condenses and gives up
its latent heat of about 1,000 Btu/lb. If, as is now common in the
metal finishing industry, the evaporation process is carried out in a
single step, then at best, a pound of pure water is obtained for every
pound of steam or 1,000 Btu of energy input. Since the heating value
of fuel oil is 19,000 Btu/lb, it is clear that the amount of fuel re-
quired for a simple evaporator becomes prohibitive. In particular,
for every 20 Ibs of water evaporated more than 1 Ib of fuel oil^s
needed, assuming a 100% extraction of heat from the fuel. Evidently,
to be at all economical any evaporation process must require that a
v
major part of the latent heat of condensation of the steam, which is
used for vaporization, in turn be reused in a regenerative fashion.
Flash evaporation
9
The two principal evaporative methods are flashing and boiling .
In flash evaporation systems there is no boiling while the heat is being
* In the metal finishing industry the term evaporation is also used
to denote concentration of the processing solution by atmospheric
wet cooling towers, wherein the solution is evaporated by contact-
ing with air . Packed bed and falling film type towers are used.
In the present paper the term evaporation is reserved for the
process of both vaporizing and condensing water from a solution con-
taining non-volatile dissolved heavy metals.
-------
75
transferred to the solution, with tne vaporization takinp, place subsequent
to the heat transfer step as a result of a pressure reduction. Usually
the wastewater is heated in a separate heater to a temperature just under
the boiling point. The wastewater is then expanded into an evaporator
to a lower pressure where part of the liquid evaporates into steam. The
formation of the steam takes place within bubbles inside the liquid and
the water "flashes" into steam at the liquid-vapor interface. The steam
then condenses on an appropriate metallic heat transfer surface, giving
up its latent heat to the feed wastewater. Such flash systems only
achieve a high thermal performance when the heat is used regeneratively
by putting a large number of flash stages in series, with the waste-
water flowing from stage to stage at successively lower pressures.
Typically, 30 to 50 stages are required for a high thermal performance
necessitating multimillion gallon per day throughputs for optimum con-
figurations. For this reason we shall not consider flash systems
further in the present discussion.
Boiling
In boiling systems, boiling takes place while the heat is being
transferred. For example, in a single effect submerged tube evaporator
(see 1st effect, Fig. 2) steam is passed through tubes submerged in the
water resulting in vaporization, with the vapor condensing inside tubes
in contact with a colder environment. The thermal performance of the
system can be improved by reusing regeneratively the latent heat of con-
densation of the steam, by placing a number of effects in series as
shown in Fig. 2. In the system illustrated the vapor produced in the
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76
first effect is used as a heat source for the second effect, where it is
condensed in tubes in contact with the concentrated wastewater, thereby
producing additional vapor, but at a lower temperature and pressure.
The process is then continued in succeeding effects. In this manner the
heat released by a pound of condensing steam is reused for vaporization
of another pound of water at a lower temperature, the second pound of
steam releasing upon condensation the latent heat to a third pound of
water, and so on. Ideally, as described, in a system of n effects one
would get approximately n pounds of water evaporated per pound of steam.
Thus we may express the specific heat input, that is, the heat input
per pound of product water as
q ^ L/n , (1)
where L is the latent heat of vaporization.
The important point to be drawn from Eq. (1) is that the thermal
performance, as measured by the mass of product per unit of energy in-
put, increases in direct proportion to the number of effects n, in-
dependently of the system heat transfer characteristics. Of course,
the capital cost of the system may be expected to increase in proportion
to the number of effects, to which the heat transfer area is in turn
proportional. The lowest overall treatment cost is then obtained by
selecting that number of effects which minimizes the energy plus capital
costs.
The heat transfer area A required per unit mass of product M is
given approximately by
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77
A a, n L /2\
M * FAT ' * '
where h is the overall heat transfer coefficient and AT the total tem-
perature drop across the system. To reduce A/M for a given number of
effects, in order to reduce the capital costs, we want the highest
heat transfer coefficient and the largest temperature drop across the
system. Unfortunately, the problem of extending the temperature range
across the effects is restricted by the fact that the feed temperature
is normally fixed, requiring that the top temperature be increased.
Increasing the top temperature, however, aggravates even further the
corrosion problems usually faced when dealing with heavy metal solu-
tions such as the highly corrosive chromic acid or copper, nickel or
zinc acid plating solutions. In the metal finishing industry, this is
one of the principal reasons, apart from minimizing thermal decomposi-
tion of brighteners and cyanides, that evaporative systems are operated
under vacuum, that is, at low boiling temperatures. In addition, the
problem of carbonate or sulfate scale buildup on the heat transfer
surfaces can also be a limiting factor to the top temperature, since the
solubility limits generally have an inverse dependence on temperature in
the boiling range. Present solutions to the problem simply involve
operating at concentrations and temperatures not exceeding the solubility
limits. Research on seawater distillation systems has pointed up several
viable alternatives, including poisoning of the nucleation sites and
circulating seed solutions to provide alternative surfaces for scale
deposition.
-------
Evidently to minimize the heat transfer surface area it is always
desirable to increase the heat transfer coefficient [see Eq. (2)].
Efforts in recent years to improve heat transfer coefficients in
evaporators have centered on thin-film techniques employing both rising
and falling films. A notable example is the vertical tube, falling
film evaporator in which the vapor condenses on the outside of vertically
oriented tubes, while inside there is a thin falling film of evaporat-
9 10
ing feed ' . Heat transfer coefficients have been increased even
further by using tubes of fluted cross section, which result in a
thinning of the film on the crests, because the liquid tends to flow
from the crests to the valleys. The thin film on the convex portion
of the tube is maintained by surface tension and offers little resistance
to heat transfer. Heat transfer coefficients of 5,000 Btu/hr-ft -°F
appear to be achievable with such configurations. Discussion of the
various means for obtaining enhanced heat transfer coefficients, includ-
ing dropwise condensation and wiped film evaporators, is beyond the
scope of the present paper but should be considered when evaluating the
future of evaporative systems for heavy metal removal.
Vapor compression
With energy costs now a predominant consideration in any water
treatment system, renewed interest is being given to the vapor com-
pression evaporation method . This process employs mechanical energy
and has a higher thermal performance than boiling systems. The prin-
ciple behind the method is that the temperature driving force required
for heat transfer is obtained by mechanical compression of the vapor
instead of by heating. The vapor compression principle is most easily
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79
described with reference to Fig. 3, where a simplified flow diagram of
a submerged tube vapor compression process is pictured. In the system
illustrated, preheated feed is introduced into an evaporator say, for
example, at atmospheric pressure. A portion of the feed containing the
dissolved metals is then boiled by heat conducted through tubes in
contact with it. The saturated vapor which is produced is compressed,
thereby raising its saturation temperature. The compressed and super-
heated vapor passes through the tubes in contact with the feed, where
it is cooled and then condensed at constant pressure releasing its
latent heat to vaporize additional water. The driving temperature
difference for the heat transfer is supplied by the fact that the tem-
perature in the cooling and condensing process is greater than or equal
to the saturation temperature, which because of the increased pressure
is greater than the boiling point of the feed containing the dissolved
solids. The terminal heat exchanger, shown enclosed by the dashed
lines in Fig. 3, can be made smaller by operating the system under
vacuum, although it is then necessary to handle and compress correspond-
ingly larger vapor volumes.
The work energy input per unit mass of compressed vapor (product
water) is given approximately by [cf. Eq. (1)]
W % - L
nc
where AT includes the temperature difference required to overcome the
boiling point elevation due to the presence of the dissolved solids
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80
plus the temperature difference required for heat transfer, T is the
boiling point temperature, and n the compressor efficiency. The
corresponding heat transfer surface per unit mass of product [cf. Eq.
(2)] is
With a typical driving temperature difference of 10 F and a com-
pressor efficiency of 70% it is evident from a comparison of Eqs. (1)
and (3) that the energy consumption (measured in shaft work) is much
smaller than that for thermal distillation. Per unit of energy input,
a vapor compression system will "pump" anywhere from 5 to 10 times
the latent heat that a thermal distillation system will. At present,
work energy requirements of 50 kwh/1000 gallons of product water are
obtainable in single effect vapor compression units for sea water
desalination with outputs of about 100,000 gallons/day. This is only
somewhat more than ten times the ideal theoretical minimum work energy
which is required. Of course, a penalty is paid in vapor compression
units in the higher capital costs associated with the compressor re-
quired to handle the large volume flows of vapor and in the higher
operating costs associated with the maintenance of rotating machinery.
In the same manner as described for boiling systems, the advent
of thin-film heat transfer surfaces and improved scale control
techniques has reduced the heat transfer surface required. Comparison
of Eqs. (4) and (2) shows that the driving temperature and heat transfer
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81
coefficient affect the area requirement similarly for both processes.
Of course, in the treatment of heavy metal wastewaters the problem
of carryover of corrosive volatiles and of corrosive concentrates by
entrainment can make the corrosion problem a much more difficult one
with vapor compression systems than with thermal distillation. How-
ever, suitable equipment and compressors are available for handling
such vapors and the increased costs now appear to be outweighed by the
escalating price of energy.
As with boiling systems, vapor compression units can also be
staged to reduce the compressor work. Such staging may be advantageous
when the concentration ratio is high and the boiling point elevation
becomes large.
3. Freezing
The other thermal method applicable to heavy metal removal is
9 10
freezing ' . In freeze distillation two basic refrigeration systems
are used to partially freeze the wastewater to produce a concentrated
solution and pure ice crystals free of any contaminants. In vacuum
flash freezing the water itself is the refrigerant, while in secondary
refrigerant freezing a refrigerant that is immiscible with water is used.
Both processes are direct contact ones in that there is no heat transfer
surface interposed between the evaporating refrigerant and the process
water from which the ice is frozen.
Figure 4 is a simplified flow diagram of the vacuum freezing
process. Its relation to the vapor compression process just described
is evident. In this system the pressure is reduced below atmospheric
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82
in a freezer to which precooled feed has been introduced, thereby caus-
ing the water to vaporize and ice crystals to form. These pure ice
crystals, from which any of the dissolved metals or other contaminants
have been excluded by their highly organized structure, are separated
from the ice-concentrate slurry in a wash column. The separated ice
is then melted by direct contact condensation of the water vapor on
the ice to yield the product. The water vapor has been compressed
from its lower equilibrium vapor pressure in contact with the waste
solution to the higher value of pure water at its freezing point. Since
the process is not ideal, auxiliary refrigeration is needed to remove
the heat that enters the system because of inefficiencies, and a ter-
minal heat exchanger is required to transfer the heat from the feed to
product streams.
In the secondary refrigerant method the process description is
essentially the same, except that it is an immiscible refrigerant in-
troduced into the freezer which vaporizes at high pressures of from 1
to 5 atmospheres resulting in ice formation. The process is then as
described for vacuum freezing with the refrigerant vapor compressed and
put in direct contact with the ice where it condenses at the same time
that the ice is melted. Because of their different specific gravities,
the product water and refrigerant form stratified liquid phases which are
readily separated. Typical refrigerants are one of the fluorocarbons
(Freon) and n-butane. In the secondary refrigerant process described
3 12
by Campbell ' indirect melting is used to ensure that noncondensible
volatiles will not carry over from the freezer and compromise the
quality of the product.
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83
An advantage of direct contact freezing is that the energy consump-
tion is low in comparison with thermal evaporation. This results from
the fact that the driving potentials for freezing (and melting if
direct contact is used) are low because of the direct contact transfer
of heat. It is expected that commercial units would, on a comparable
basis, have energy consumptions less than for vapor compressor systems
since the freezing heat transfer processes are direct in comparison
with indirect for vapor compression. Perhaps of equal importance in
the removal of heavy metals, is that problems of scaling and corrosion
are much reduced in comparison with evaporation because of the low
operating temperatures. The disadvantages already noted are associated
with increased capital and maintenance costs associated with the re-
quirements of growing, handling, and washing ice crystals, along with
the mechanical vapor compression. Until recently, one of the main
difficulties with freeze systems has been the relatively large size of
the wash column and large amounts of fresh water needed to wash off the
concentrated waste from the ice crystals. These limitations have now
been overcome with the development of a pressurized wash column operat-
ing on the principle of one fluid displacing another within a porous
medium ' . In such a column, shown schematically in Fig. 5, the con-
centrate adhering to the ice by surface tension is displaced by fresh
product water from the interstices of a porous, packed bed of the ice
crystals.
The slurry of concentrated waste and ice crystals enters the bottom
of the column, which is generally cylindrical, under several atmospheres
-------
pressure and at a constant rate. The ice crystals form a moving porous
plug in the column, with the concentrate simultaneously flowing upward
through the plug and diverging outward toward an annular screen located
in the column wall about midway up its length. The pressure at the
screen is held at a value below that at the bottom of the plug so that
there is a "suction" effect there. At the same time, a small amount of
fresh wash water is supplied to the top of the ice plug at a pressure
greater than that at the screen but less than that at the bottom of
the plug. The wash water filters downward through the upwardly moving
plug and out toward the screen, displacing any waste in the interstices
and filling up the voids in the plug above the screen. The plug itself
is continuously scraped off at the top of the column along with the
entrained fresh water. The result is that, in the neighborhood of the
screen, there is a separation surface in the ice plug above which there
is only fresh water in the voids and below which there is only con-
centrated waste in the voids. Of course, the pressure difference across
the plug must be sufficient to drive it and the entrained fluid at the
constant velocity desired.
The problem of the design of efficient freezers is still the sub-
ject of development, with efforts being centered on secondary refrigerant
freezers . The greater attention to secondary refrigerant freezing
results from the fact that the higher operating pressures permit the
use of much smaller mechanical vapor compressors, since the vapor flow
volumes are from 100 to 500 times less than the water vapor volumes en-
countered in vacuum freezing. The problem in freezer design is to obtain
a sufficiently comprehensive understanding of the nucleation and growth
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85
of ice crystals in the concentrate so that equipment can be designed
with as high a rate of ice production per unit volume of freezer as
possible, to reduce the capital cost. At the same time, to minimize
the energy cost, the vaporization should take place at the lowest com-
patible driving temperature and the least expenditure of energy
associated with such operations as mixing. In addition, it is also
desirable to obtain the largest crystal sizes possible without sig-
nificantly increasing either the capital or energy charges since, in
any case, the larger the crystals the less costly will be the task
of removing the concentrated waste from them.
Eutectic freezing
Recently the secondary refrigerant freezing process has been de-
signed to carry its operation down to the eutectic point of dissolved
metallic salts . In this process the concentrated waste is con-
tinuously frozen, with the ice crystals precipitating out until the
eutectic temperature is reached, at which point the solution is
saturated with respect to the dissolved salts. The eutectic tem-
perature for sodium chloride is -6 F and is about the same value for
most inorganic salts, with very few salts having a eutectic temperature
below -10°F. The important point is that after the eutectic temperature
is reached any further removal of heat from the system leads to
crystallization of the dissolved sa3ts, generally in a hydrated form.
Continued removal of heat results both in continued ice formation and
salt precipitation with no further reduction in temperature and with
the concentration of the residual liquor remaining unchanged.
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86
Laboratory experiments have shown that the ice and salts can be
grown and nucleated as mechanically separate phases in a direct con-
tact freezer; that the ice can be handled and pumped in conventional
piping without freeze-up or clumping; and that the ice can be separated
from the salts by means of hydrocyclones. A 5,000 gallon per day
pilot plant is now installed in a mobile trailer and is presently
undergoing tests, the results of which are described more fully in the
12
paper by Campbell in this conference . A major application of this
process is envisaged for the treatment of plating wastes in the metal
finishing industry . It should be pointed out that the eutectic
freeze concentration process differs from the freezing system pictured
in Fig. 4, in that it is designed as a two stage process because con-
centration of the liquor down to the eutectic point in a single step
requires considerably more compressor work than a two-stage process.
The thermodynamic reasoning for such staging parallels that for
additional evaporators in vapor compression units.
As we have already noted, the salts will generally crystallize
out in a hydrated form, for example, sodium chloride in the dihydrate
form which is 62% NaCl by weight. If it is desired to bring the
hydrated metallic salt to dryness then additional evaporation is re-
quired. Bench scale experiments on the application of conventional
vacuum freeze drying techniques for the recovery to dryness of heavy
metals from spent plating and etching baths following freeze precipita-
tion are described in Ref. 7.
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87
4. Concluding Remarks
Thermal processes can play a major role in the removal and recovery
of heavy metals when they are present in waste streams in strong con-
centrations. It is important that the removal take place before the
wastes are discharged into receiving waters where the subsequently
diluted metals become traces which are far more difficult to remove be-
cause of the large water volumes.
Because of rapidly increasing world energy costs, standard flash
and boiling distillation systems appear less attractive at present
because of their low thermal performance and correspondingly high energy
consumption. Among the thermal processes with relatively low energy
consumption, vapor compression distillation and freezing stand out.
Vapor compression, although not a new process, has so far not been
developed for the specific purpose of handling heavy metal wastewaters.
Some development, pilot plant and demonstration work is required to
enable the method to treat reliably the highly corrosive and scale form-
ing waters with corrosive distillates using efficient high volume com-
pressors and enhanced heat transfer surfaces. At this time, freezing
also appears to be quite promising, particularly since it minimizes the
corrosion and scale problem. However, freezing is still in an embryonic
stage from a commercial viewpoint and requires much more development and
testing on heavy metal wastewaters before its potential for this applica-
tion can be properly evaluated.
-------
References
1. Culotta, J. M. and Swanton, J. F., ''Recovery of Plating Wastes:
Selection of Lowest Cost Evaporator," Plating 57, 1221-1223 (1970).
2. Culotta, J. M. and Swanton, J. F., "Controls for Plating Waste
Recovery Systems." Plating 58. 783-785 (1971).
3. Campbell, R. J. and Eimnerman, D. K., "Water Reuse in Industry.
Part 4 - Metal Finishing," Mechanical Engineering 95. No. 7, 29-32
(July 1973).
4. Houle, P. C., "Reverse Osmosis" in P_roc_._ Conf. on Traces of Heavy
Metals in Water: Removal Processes and Monitoring (J. Sabadell,
ed.), Center for Environmental Studies, Princeton University,
Princeton, N. J. (1974).
5. C. K. L., "Reclaiming Zinc From an Industrial Waste Stream,"
Environmental Science and Technology 6. 880-881 (1972).
6. Stepakoff, G. and Siegelman, D., "Application of a Eutectic Freezing
System to Industrial Waste Water Recycling," in WateReuse (L. K.
Cecil, ed.), pp. 158-171, AIChE, New York (1973).
7. Avila, A. J., Sauer, H. A., Miller, T. J. and Jaeger, R. E., "Freeze
Drying of Spent Plating and Etching Baths to Recover Metals,"
Plating 60, 239-241 (1973).
8. Ciancia, J., "New Waste Treatment Technology in the Metal Finishing
Industry." Plating 60, 1037-1042 (1973).
9. Probstein, R. F., "Desalination," American Scientist 61, 280-293 (1973)
10. Probstein, R. F., "Desalination: Some Fluid Mechanical Problems,"
Trans. ASME, J. Basic Engineering 94 (Series D), 286-313 (1972).
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89
11. Simpson, H. C. and Silver, R. S., ''Technology of Sea Water Desalina-
tion," in Desalination and Ocean Technology (S. II. Levine, ed.),
pp. 62-88, Dover, New York (1968).
12. Campbell, R. J., "Freezing" in Proc. Conf. on Traces of Heavy Metals
in Water: Removal Processes and Monitoring (J. Sabadell, ed.),
Center for Environmental Studies, Princeton University, Princeton,
W. J. (1974).
13. Probstein, R. F. and Shwartz, J., "Method of Separating Solid
Particles From a Slurry With Wash Column Separators," U.S. Patent No.
3,587,859 (1971).
14. Shwartz, J. and Probstein, R. F., "Experimental Study of Slurry
Separators For Use in Desalination," Desalination 6, 239-266 (1969).
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90
Figure Titles
Fig. 1. Closed loop chemical recovery system for plating and rinse
operations.
Fig. 2. Submerged tube multieffect evaporator.
Fig. 3. Vapor compression evaporation.
Fig. 4. Vacuum freezing.
Fig. 5. Pressurized wash column.
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91
CONCENTRATE
MAKEUP
THERMAL
PROCESS
PLATING
-1 i
FEED
DRAGOUT
PRODUCT
RINSE
DRAGOUT
FIGURE 1
-------
1st
EFFECT
2nd
EFFECT
3rd
EFFECT
STEAM
^
CONDENSATE
VAPOR
T,
HEATED
FEED
2 ' I
T2
-------
93
COMPRESSOR
SUPERHEATED
VAPOR
Psv > I atm
Tsv > 212° F
CONDENSATE TSV>TC>2I2°F
CONCENTRATE 212° F
r
SATURATED
VAPOR
EVAPORATOR /
I atm \ \
( 212° F / {
HEATED FEED
-vwwwwwv-
-/wwvwwvw-
CONCENTRATE
PRODUCT WATER
HEAT
EXCHANGER
L
WASTE
FEED
l_
FIGURE 3
-------
PRODUCT
LU
QL
h-
Z
UJ
O
z
O
O
AUXILIARY
REFRIGERATION
\
\
/ COMPRESSOR
MELTER -
CONDENSER
ICE
WASH
COLUMN
ICE-CONCENTRATE SLURRY
<
WATER
VAPOR
FREEZER
~3mm Hg
30°F
> \A
' * a A *
a
-a A
r
COOLED FEED
>—•
•WSAA/V\AAAWv\^
^VvAAAAAAA^W^
I EXCHANGER
CONCENTRATE
PRODUCT WATER
WASTE
FEED
FIGURE
-------
FRESH WATER
CONCENTRATE-
FRESH WATER
INTERFACE
SCREEN
LOWER
BOUNDARY
OF ICE PLUG
FRESH WATER + ICE
SCRAPER
STREAMLINES
CONCENTRATED
WASTE OUT
CONCENTRATED
WASTE + ICE
FIGURE 5
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96
-------
97
Princeton University Center for Environmental Studies
Environmental Protection Agency
American Institute of Chemical Engineers
Conference on
"Traces of Heavy Metals in Water: Removal Processes and Monitoring"
November 15 and 16, 1973
FREEZING CONCENTRATION FOR REMOVAL OF HEAVY METALS FROM WATER
Robert J. Campbell
Avco Systems Division
201 Lowell Street
Wilmington, MA 01887
Freezing Concentration for Removal of Heavy Metals from Water
Traces of heavy metals occur in waters which have been used in processes
wherein they are an integral part of the process, or in cases where the water
may be in contact with metallic materials containing heavy metal elements.
The alternatives facing the owner of the contaminated water are: (1) remove
the heavy metals to the degree required to permit discharge of the water from
his site or (2) remove the heavy metals and other constituents from the waste
stream to the extent necessary to permit re-use of the water within his plant.
By removing all of the contaminants from the water in equal proportions, the
freezing process permits reuse of the contaminated water within the plant.
The application of the freezing process to treatment and reuse of waste water
results directly in the removal of heavy metals which may exist only in trace
quantities. The contaminant is concentrated in a much smaller volume which
may then be re-used or disposed of as appropriate. While reductions in heavy
metal concentrations achieved by freezing may in many cases result in con-
formance with discharge standards, the maximum benefit is derived by re-
cycling the treated water to the process. In this way the monitoring and reporting
of discharges is not required, and the cost of purchasing and eventual sewering
of water is eliminated. In some cases, valuable chemicals can also be
recovered.
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The CRYSTALEX freezing process developed by Avco removes pure water
from aqueous solutions of various contaminants. It is only slightly sensitive
to the type of contaminant in that high concentrations lower the freezing point of
the solution. Corrosion of equipment is substantially reduced at the low operat-
ing temperatures characteristic of the process,, Plant size and operating
costs are functions of the quantities of water treated, and economy in the use of
water is an important part of any overall treatment system. Countercurrent
rinsing of plated parts or circuit boards is typical of such measures.
The process is not selective with respect to rejection of specific dissolved con-
taminants. All are reduced to the same degree in the product and are
concentrated in the brine. However, in some cases, one or more of the
contaminants may precipitate from solution, either as a result of
solubility reductions at low temperatures, or due to increased concentrations
of the solutions. The precipitate is then filtered from the process for disposal
or recycle as appropriate. Final disposal of the concentrate depends on several
factors. Concentrates from treatment of plating rinses may frequently
be returned to the plating bath, since they may contain valuable chemicals
and they constitute the dragout from the bath. The vastly reduced quantity
of water involved may make incineration or drying of concentrates an
attractive solution. Chemical treatment and disposal of highly concentrated
wastes may be the best solution in other cases. The high concentration
and the much smaller quantities of water involved make possible disposal
solutions which would otherwise be impractical.
Process Capabilities
The capabilities of the freezing process are best defined in terms of
purity of the recovered water, concentration of the waste, plant capacity,
power consumption and applicability to various streams. The process is
applied to the concentration of aqueous solutions having freezing points as
low as the eutectic point of sodium chloride (-6°F) although lower
temperatures can be achieved if justified by the economics. The more
usual applications are to solutions having freezing points at the brine
concentration in the range of 20 to 32 degrees fahrenheito Typical
but not exclusive ranges of applications are shown in Table 1,
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99
Table I - Applications of the Crystalex Freezing Concentration Process
FEEDS
Aqueous Solutions (Rinse Waters, Process Streams, Plant Wastes) con-
taining salts, chemicals, dissolved liquids, organic and inorganic acids. Also
applicable to certain nonaqueous process streams where crystallization provides
a positive process benefit.
CONCENTRATIONS
Limited only by freezing point depressions (20 F typical) and concentrate
viscosity
Precipitating contaminants are separated where required.
WATER QUALITY
Concentration in Product Water _ AQOB t 0 005
Concentration in Brine Solution
OPERATING EXPENSES
Electrical Power at 25°F Freezing Point
40 KWH/1000 Gallons (100,000 GPD Plants)
to
75 KWH/1, 000 Gallons (5000 GPD Plants)
Note: Energy consumption based on power generation at 10,000 btu/KWH
Heat Rate:
40 KWH/1000 Gallons = 400,000 Btu/1000 gallons 100 K GPD Plant
75 KWH/1, 000 Gallons = 750, 000 Btu/1000 gallons 5 K GPD Plant
Single Effect Evaporator 8,500,000 Btu/1000 gallons
Three Effect Evaporator 2,900,000 Btu/1000 gallons
Six Effect Evaporator 1, 500,000 Btu/1000 gallons
Cooling Water as required to dissipate electrical power.
Refrigerant make-up $0.01 to $0.10 per 1000 gallons depending on
plant size.
Plant is designed to operate unattended.
PLANT SIZES
Factory assembled and tested units from 5000 to 100,000 gallons per day.
Multiple units may be used for larger requirements. Very large plants
may be constructed on site where applications warrant.
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100
Concentration by Freezing
Separation of contaminants and concentrated salts from water by
evaporative concentration is well understood. The water phase is changed from
liquid to vapor so that it may be recovered in pure form. In an
analogous manner water which has been changed to the crystalline phase,
ice, may also be recovered as pure water.
The essential factor in recovery of pure water by freezing is that the ice
crystal, as it freezes, excludes dissolved impurities, including organics,
inorganics, and volatiles; the resulting crystals consisting of pure water.
If the brine which adheres to the crystals is removed, then the water
resulting when the crystals are melted will be pure. The insensitivity
of the process to the nature of the contaminants results in an almost
universal applicability in the concentration of solutions containing dissolved
contaminants. As the water is removed, the waste is concentrated. The
concentration can be carried to the point of saturation where the salts
precipitate out of solution. The precipitated salts can then be dried for
disposal or convenient transportation to a recycling center.
The feed, consisting of contaminated water, is pumped into the system through
a heat exchanger, where it is cooled by heat exchange with the purified
water from the melted ice (Figure 1). The cold feed enters the freezer
where it is mixed with an immiscible refrigerant. The evaporation of the
refrigerant removes heat from the feed water forming a slurry
consisting of ice and the concentrated brine. The refrigerant vapor is pumped
out of the freezer with a compressor. The slurry is pumped from the
freezer to a counterwasher where the adhered brine is washed from the ice
crystals.
The counterwasher is a simple vertical vessel with screened outlet located
midway between top and bottom. The slurry enters the bottom and forms a
porous plug, the majority of the brine flowing upward through the plug
and leaving the counterwasher through the screen. The ice plug is propelled
through the column by the force resulting from the pressure drop of the brine
flowing through the ice plug, the velocity of the brine being much greater
than that of ice. Fresh water from the previously produced product is
circulated at top of the ice plug, and a small fraction of the product (less than
five percent) flows countercurrently to the ice plug, washing away the
adhered brine.
The ice is removed from the top of the counterwasher where it is reslurried
with previously produced product. The slurry is pumped to a condenser where
the ice is melted by the release of heat from the condensing refrigerant
vapor which was evaporated to produce the ice, and which has been heated by
compression to a saturation temperature higher than the melting point of the ice,
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TO RINSE
HEAT EXCHANGER
COOLING
WATER
MELTER/
CONDENSER
I
REFRIGERANT
CONCENTRATE
SLURRY
CONTAMINATED RINSE
I
5
H
FREEZER
Figure 1 SCHEMATIC DIAGRAM OF FREEZING PROCESS CONCENTRATION AND RECOVERY SYSTEM
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102
Because of pump work, compressor work, and incomplete heat exchange a
greater amount of refrigerant is vaporized than can be condensed by melting
of the ice. A heat removal system is therefore required to maintain
thermal equilibrium. This consists of a compressor which raises
the excess vapor to a temperature and pressure that will cause it to
condense on ambient cooling water.
From the foregoing description it can be seen that a freezing process
offers several advantages for waste treatment.
1. Concentration is performed in the absence of a surface of membrane.
2. The process operates at temperatures below ambient, minimizing
corrosion of materials.
3. The process has low energy requirements.
4. There is no carryover of volatiles to the product.
Since concentration takes place by freezing out of water in direct contact with
the refrigerant there is no heat transfer surface or membrane to be fouled
by the concentrate or other contaminants. Therefore, feed pre-treatment
to prevent such scaling or other fouling is unnecessary even with waters
that present severe problems with other processes. Where heat exchange
surfaces are used they are not associated with concentration of the fluids
and therefore deposits are minimized. Suspended solids do not affect
the process and are removed only as required by the end use of the recovered
products.
Low temperature operation of the process minimizes corrosion and also
allows the use of non-metallic corrosion resistant materials for piping, pumps
and vessels. Much of the cost associated with evaporation processes is
thus eliminated.
Low energy requirements reduce operating costs and result in a drastic
reduction of cooling water requirements compared to evaporators. The low
cost holds even though electrical power is used, and a t e a fn generating equip-
ment is not required.
One impediment to the re-use of water recovered from evaporative systems is
the fact that it may contain volatile contaminants which were present in the
feed. In the freezing process volatiles are isolated from the washed
product by the tubes of the melter/condenser.
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103
Application Testing
A solution containing approximately 100 milligrams per liter of nickel,
cadmium, chromium, and zinc was tested in a 2500 gallon per day pilot
plant. The recovered water contained less than 0.5 milligram per liter
(see Table 2), yielding removal of more than 99. 5 percent of the
contaminant. The purity of the final product is controlled by the
length of the washing zone in the counterwasher, and to some extent by the
quantity of wash water used. Extremely low concentrations may be achieved
where required.
A 5000 gallon per day mobile pilot plant has also been built and at this time is
being used to treat wastes containing heavy metals as well as large
quantities of various dissolved salts which are removed by concentration
and precipitation.
TABLE 2
METAL ION SEPARATION TEST
RESULTS
Element
Nickel
Cadmium
Chromium
Zinc
NaCl
Concentration
In Feed
mg/liter
105
105
110
100
30.000
Concentration
In Product
mg/liter
0.44
0.40
0.225
0.34
120.
Percent
Removed
99.58
99.62
99.80
99.66
99.60
A cknow le dge me nt s
The processes described are currently being developed by the Systems Division
of Avco Corporation in Wilmington, Massachusetts. The waste treatment
application is an extension of a desalination process being developed by
Avco with the cooperation and support of the Office of Saline Water of the
United States Department of the Interior.
Bibliography
More detailed discussions of the freeze crystallization process and
descriptions of applications are found in the following references.
1. James H. Fraser and Wallace E. Johnson, "The Role of Freezing
Processes in Wastewater Treatment", Applications of New Concepts
of Physical-Chemical Wastewater Treatment, pp. 3G9-324,
September 18-22, 1972 Permagon Press, Inc.
-------
2. M.B,, Ziering, D0K, Emmerman, andW.E, Johnson, "The
Concentration of Industrial Wastes by Freeze Crystallizations."
Presented at AICHE 74th National Meeting, New Orleans, L.A.,
March 11-15, 1973.
3. R0 J. Campbell and D0K. Emmerman, "Water Reuse in
Industry, Part 4 - Metal Fishing," Mechanical Engineering
pp. 29-32, July 1973.
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105
MEMBRANE PROCESSES FOR WASTE TREATMENT
A Paper for the Princeton University Center for
Environmental Studies Conference on Removal of
Heavy Metals from Waste Water, November 15-16,
1973.
by
Robert E. Lacey
Senior Chemical Engineer
Southern Research Intitute
2000 Ninth Avenue South
Birmingham, Alabama 35205
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106
MEMBRANE PROCESSES FOR WASTE TREATMENT
A Paper for the Princeton University Center for
Environmental Studies Conference on Removal of
Heavy Metals from Waste Water, November 15-16,
1973.
Membrane processes, such as electrodialysis and reverse os-
mosis, have promise for use in removing heavy metals from waste-
water, and specific applications of membrane processes for the
treatment of wastewaters are discussed in the papers that follow
this presentation. In this paper the status and basic principles
of electrodialysis and reverse osmosis are given to provide back-
ground information for the papers that follow.
I. ELECTRODIALYSIS
Electrodialysis is the transport of ions through membranes
as a result of an electrical driving force. It is the underlying
principle of all electrically driven membrane processes. When
nonselective membranes that are permeable to ions (e.c[. , cello-
phane) are used, electrolytes can be separated from nonelectro-
lytes. When membranes that are more permeable to cations than
anions or vice versa (e.g.., ion-exchange membranes) are used, con-
centrations of electrolytes in solutions can be increased or
decreased. Thus, electrodialysis can accomplish practical concen-
tration or depletion of electrolytes in solution as well as the
separation of electrolytes from nonelectrolytes. Excellent
reviews in the recent literature1"3 describe the historical devel-
opment of electrodialysis, so no discussion of history is
presented here.
In any membrane process the selective membrane is the heart
of the process. Therefore, a discussion of the nature of the
selective membranes used in electrodialysis is presented first.
A. Ion-Exchange Membranes
Ion-exchange membranes are used in most electrodialytic
processes. They are ion exchangers in the form of films. There
are two basic types: cation-exchange and anion-exchange membranes,
Ion-exchange membranes are selective in that they are permeable
to cations, but not to anions, or vice versa. In Figure 1, poly-
mer chains are shown that have negatively charged groups chemi-
cally attached to them. The polymer chains are intertwined and
also crosslinked at various points. Positive ions are shown
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10?
FIXED NEGATIVELY CHARGED EXCHANGE SITE; I.E.., SOJ
MOBILE POSITIVELY CHARGED EXCHANGEABLE CATION; I.E.., Na+
POLYSTYRENE CHAIN
DIVINYLBENZENE CROSSLINK
Figure 1. Representation of an
Ion-Exchange Membrane
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108
freely dispersed in the voids between the chains. However, the
fixed negative charges on the chains repel negative ions that try
to enter the membrane, and exclude them. Thus, because of the
negative fixed charges, negative ions cannot permeate the mem-
brane, but positive ones can. If positive fixed charges are
attached to the polymer chains instead of negative fixed charges,
selectivity for negative ions is achieved. This exclusion as a
result of electrostatic repulsion is termed Donnan exclusion.
Selectivity by itself is not enough to make an ion-exchange
membrane that is practical for low-cost processing. In addition,
the resistance of the membrane to ion transfer must be low. To
decrease the resistance, the degree of crosslinking is decreased
so that the average interchain distances and the lengths of poly-
mer segments that are free to move are increased. However, if
this enlargement of void spaces between polymer chains is carried
too far, it can result in volumes in the center of the voids that
are not affected by the fixed charges on the chains (the repulsion
effect of fixed charges decreases rapidly with distance). Volumes
that are unaffected by the fixed charges result in ineffective
repulsion of the undesired ions and lowered selectivity. For this
reason, a compromise between selectivity and low resistance must
usually be made. Membranes now available combine excellent selec-
tivity with low resistance, high physical strength, and long life-
times.
There are two general types of commercially available ion-
exchange membranes: heterogenous and homogenous membranes. Het-
erogenous membranes in which ion-exchange particles are incorpo-
rated in film-forming resins are made by calendering mixtures of
ion-exchange and film-forming materials, by casting films from dis-
persions of ion-exchange materials in solutions of film-forming
materials and allowing the solvent to evaporate, and by casting
films of dispersions of ion-exchange material in partially poly-
merized film-forming polymers and completing the polymerization.
Homogenous ion-exchange membranes have been made by several
methods:
—Polymerization of mixtures of reactants that can
undergo condensation polymerization. At least one of
the reactants must contain a moiety that is anionic
or can be made to be so charged.
—Polymerization of mixtures of reactants (one of which
is anionic or cationic) that can undergo additional
polymerization.
—Graft polymerization of moieties that are anionic or
cationic (or can be made to be) into preformed films.
—Casting films from a solution of a linear film-forming
polymer and a linear polyelectrolyte, and allowing the
solvent to evaporate.
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B. Concentration and Depletion by Electrodialysis
In an electrodialysis stack used for concentration or deple-
tion of electrolytes in solution, cation-exchange membranes alter-
nate with anion-exchange membranes in a parallel array to form
thin solution-compartments (0.5 to 1.0 mm thick). The entire
assembly of membranes is held between two electrodes as shown in
Figure 2. A solution to be treated is circulated through the
solution compartments. With the application of an electrical
potential to the electrodes, all cations tend to transfer toward
the cathode and all anions tend to move toward the anode. The
ions in the even-numbered compartments transfer through the first
membranes they encounter (cations through cation-exchange mem-
branes, anions through anion-exchange membranes), but they are
blocked by the next membranes they encounter as indicated by the
arrows in the diagram. Ions in the odd-numbered compartments are
blocked in both directions. By this mechanism, ions are removed
from the solution circulating through one set of compartments
(even) and transferred to the other set of compartments (odd).
Ion depletion is accomplished for one solution, and ion concentra-
tion is accomplished for the second solution.
An electrodialysis stack is essentially a device to hold an
array of membranes parallel to each other between electrodes in
such a way that the solutions being processed are kept separated.
Figure 3 is an exploded view of part of an electromembrane
stack that shows the main components. Component 1 in Figure 3 is
one of the two end frames, each of which has provisions for hold-
ing an electrode and introducing and withdrawing the depleting,
the concentrating, and the electrode-rinse solutions. The end
frames are made relatively thick and rigid so that pressure can
be applied easily to hold the stack components together. The
inside surfaces of the electrodes are recessed, as shown, to form
an electrode-rinse compartment when an ion-exchange membrane,
component 2, is clamped in place. Components 3 and 5 are spacer
frames. Spacer frames have gaskets at the edges and ends so that
solution compartments are formed when ion-exchange membranes and
spacer frames are clamped together.
Usually the supply ducts for the various solutions are
formed by matching holes in the spacer frames, membranes, gaskets,
and end frames. Each spacer frame is provided with solution chan-
nels (E in Figure 3) that connect the solution-supply ducts
with the solution compartments. The spacer frames have mesh
spacers, or some other device, in the compartment space to support
the ion-exchange membranes to prevent collapse when there is a
differential pressure between two compartments.
An electromembrane stack usually has many repeated sections,
each consisting of components 2, 3, 4, and 5, with a second end
frame at the end.
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DESALTED PRODUCT WATER
+JUUL
CONCENTRATED BRINE
t I
_5
t t
FEED WATER
LEGEND: A- onion-permeable membrane
C = cation-permeable membrane
Figure 2. Diagram of Electrodialysis Process
SOUTHERN RESEARCH INSTITUTE
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Ill
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SOUTHERN RESEARCH INSTITUTE
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There are three basic types of electrodialysis stacks: tor-
tuous path, sheet-flow, and unit-cell stacks. In the tortuous-
path stack, the solution flow path is a long narrow channel as
illustrated in Figure 4, which makes several 180° bends between
the entrance and exit ports of a compartment. The bottom half of
the spacer gasket in Figure 4 shows the individual narrow solution
channels and the cross-straps used to promote turbulence, whereas
the individual channels have been omitted in the top half of the
figure so the flow path could be better depicted. The ratio of
channel length to width is high, usually greater than 100:1.
Spacer screens to support the membranes may or may not be used in
tortuous-path stacks.
In sheet-flow stacks, spacer screens are almost always
needed since the width of membranes is much greater than that in
the usual tortuous-path stacks. The solution flow in sheet-flow
stacks is usually in approximately a straight path from one or
more entrance ports to an equal number of exit ports as illus-
trated in Figure 3. As the solutions flow in and around the fila-
ments of the spacer screens, a mixing action is imparted to the
solutions. Thus, the spacer screens serve not only to support
the membranes but to aid in mixing the solutions and in reducing
the thicknesses of diffusional boundary layers at the surfaces of
the membranes.
Solution velocities in sheet-flow stacks are typically in
the range of 5 to 15 cm/sec, whereas the velocities in tortuous-
path stacks are usually much higher, 30 to 50 cm/sec. The drop
in hydraulic pressure through a sheet-flow stack is normally
lower than that through a tortuous-path stack because of the lower
velocities.
Unit-cell stacks were specifically developed for concentrat-
ing solutions. Each concentrating cell consists of one cation-
exchange membrane and one anion-exchange membrane sealed at the
edges to form an envelope-like bag. Many of these concentrate
cells are assembled with spacer screens between them to separate
the cells so solutions concentrated can flow between them. The
entire assembly of alternating concentrate cells and spacer
screens is held between a set of electrodes. When electric cur-
rent flows through the stack, ions flow from the external solu-
tions through the membranes to the insides of the concentrate
cells where they are trapped. Only osmotically and electro-
osmotically transferred water flows through the membranes. Thus,
the maximum degree of concentration is effected. The concentrated
solutions inside of the concentrate cells flow through small tubes
that lead from inside the concentrate cells to a plenum chamber
arranged outside the stack.
Figure 5 shows some of the details of unit-cell stacks.
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113.
n
n
Figure 4. Diagram of a Tortuous-Path Spacer
for an Electrodialysis Stack
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CONCENTRATE CELL
OUTLET FOR DILUATE
DILUATE CHAMBER
SPACER
OUTLET FOR
CONCENTRATE
CONCENTRATE
DISTRIBUTOR FOR
SOLUTION TO BE
CONCENTRATED
Figure 5. Main Structure of the Unit-Cell
Type Electrodialysis stack
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~
i\
1
1
'
f^- Static -
^ x- Zones.
Ki
i
I
| Well-
{ mixed
I zone
1
1
1
I
h-—
/I
/I
'!
J i
**<%
vv
I
\
^
*
\
\
]
-
5 a
6 6
S = Static boundary-layers
C = Cation-exchange membrane
A = Anion-exchange membrane
Figure 6. Concentration Gradients in
Electrodialysis
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C. Concentration Changes at Membrane Surfaces
The diagram in Figure 2 shows a simplified picture of the
net effects of electrical current in electrodialysis stacks, but
more complex changes in concentration actually occur, as indi-
cated on an idealized basis in Figure 6.
Figure 6 shows a cation- and an anion-exchange membrane
mounted between two electrodes. A solution of an electrolyte flows
through the compartments formed by the two membranes. With the
passage of an electrical current through the system of the membranes
and solutions, anions transfer toward the cathode.
Because of the flow of solution through the center compartment
formed by the two membranes in Figure 6, there is a zone of relative-
ly well-mixed solution near the center of the compartment. The
velocity of the solution and thus the degree of mixing diminish as
the surfaces of the membranes are approached. In the figure, the
so-called Nernst idealization is used for simplification. This
assumes a completely mixed zone in the center of the compartment,
and completely static zones of solution in boundary layers adjacent
to the membranes. In the static boundary layers, ions are trans-
ferred only by electrical transfer and diffusion, but in the mixed
zone, ions transfer electrically by diffusion and by physical mixing.
Assume for convenience that the electrolyte is KC1, since the
transference numbers of K+ and Cl~ are approximately 0.50 in solution,
However, because of the selectivity of the membranes, the trans-
ference number of K+ is essentially 1.0 in the cation-exchange
membrane and 0.0 in the anion-exchange membrane. Similarly, the
transference number of Cl~ is 1.0 in the anion-exchange membrane.
Consider for now only the anion-exchange membrane and its
boundary layers. The Cl~ ions carry only 50% of the electrical
current in solution but 100% of the current in the membrane. If 1
faraday of electricty were passed through the membrane and boundary
layers, 0.5 g eq of Cl sould be transferred to or away from the
membrane surface, and 1.0 g eq of Cl~ would be transferred through
the membrane by electrical transfer. There would be a depletion of
ions at the left-hand surface of the anion-exchange membrane in
Figure 6 and a concentrations of ions at the right-hand surface.
Obviously, this transient state of affairs could not continue long
before all ions were depleted at the left surface. In steady-
state operation, concentration gradients are established in the
static boundary layers, as indicated by the dashed lines in Figure
6, so that the ions that are not electrically transferred to or
from the membrane surface are supplied by diffusion through the
boundary layers. These differences in transference numbers between
solutions and membranes are the source of the depletion and con-
centration that occurs in electrodialysis, but they can also be
a source of trouble.
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If the current density is increased through the system of
membranes and solutions in Figure 6, the rate of electrical trans-
port increases. Therefore, the rate of diffusional transport
must also increase to supply the extra ions needed at the surfaces
of the membranes. Diffusional transport can increase if the
thicknesses of the boundary layers decrease, or if the differences
in concentration at the edges of the boundary layers increase. If
the solution velocity past the membranes stays constant, the thick-
nesses of the boundary layers also remain constant and the con-
centration gradients increase. The interfacial concentrations on
the depleting sides of the membranes decrease; those on the con-
centrating sides increase.
If the current density is increased still more, a density
will be reached at which the concentrations of electrolytes at
the membrane interfaces on the depleting sides will approach zero.
At this density, usually called the limiting current density, H+
and OH~ ions from ionization of water will be transferred through
the membranes. The transfer of H+ ions through the cation-
exchange membranes is usually not particularly harmful, but trans-
fer of OH~ ions through the anion-exchange membranes can cause
increases in pH in the concentrating compartments that cause pH-
sensitive substances like CaCOa or Mg(OH)2 to precipitate in and
on the membranes. In addition, the OH~ ions can cause dimensional
changes in the anion-exchange membranes. Also, the presence of
the layers of almost pure water at the membrane surfaces causes
the resistance of the membrane cells to be high, and the energy
requirements of the process to be high.
The thicknesses of the boundary layers decrease with
increasing solution velocity. Therefore, it is important to use
the highest practicable velocities, to have equal velocities in
each solution compartment, and to have uniform velocities at all
points along the membrane surfaces (i-£., have no points of solu-
tion stagnation).
A mathematical expression has been developed1*' 5 that relates
the limiting current density to the boundary-layer thickness, the
concentration of electrolyte in the well-mixed zone between bound-
ary layers, and the transference numbers of counter-ions in the
membranes and solutions. This expression is based on the Nernst
idealization of boundary layers. A model based on the Nernst
idealization is an extremely simplified model of the actual bound-
ary layers in electrodialysis and a number of workers have dealt
with models that more.nearly resemble actual boundary layers6"8
and with hydrodynamic analyses of boundary layers.9/10
Despite the simple model provided by the Nernst idealiza-
tion, or perhaps because of it, the mathematical expression given
below is easy to use and predicts performance adequately for most
design uses.
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DF
6(t -t~)
This ratio (usually written as i/N instead of iiim/Cj-,) is
often called the "polarization parameter" and is used as a design
parameter in electrodialysis. Values of i/N have been found to
vary with solution velocity and also with the nature and shape of
the mesh-like screens or other devices used to maintain desired
spacings between the ion-exchange membranes.
D. Consequences of Operation at or Above
Limiting Current Density
There are three main consequences of operating at or above
the limiting current density. They are: scaling of anion-
exchange membranes by electrolytes that become insoluble at high
values of pH, excessively high resistances of stacks, and fouling
of membranes by organic ions. All three are deleterious to the
performance of electrodialysis stacks.
Scaling of membranes by pH-sensitive electrolytes occurs
because OH~ ions transfer through anion-exchange membranes when
the limiting current density is reached. The OH~ ions increase
the pH within the membrane and at the interface on the concentrat-
ing side so that substances such as CaCOa precipitate.
A second undesirable consequence of exceeding the limiting
current density is increased stack resistance. When the limiting
current density is reached or exceeded, the rate of dissociation
of water is increased because the H+ and OH~ ions are continuously
transferred away from the membrane interfaces, which are the loca-
tions of dissociation. An increased voltage is necessary to
induce this more rapid dissociation of water. Also, a thin film
of highly depleted solution forms at the depleting sides of mem-
branes. These films have extremely high specific resistances,
and the high resistances are in series with the normal resistances
in an electrodialysis stack. Thus, operation at or above the
limiting current density causes the stack to have resistances much
higher than normal.
Organic fouling may occur on either the cation- or anion-
exchange membranes. However, most of the trouble that has been
encountered has been fouling of anion-exchange membranes. Fouling
of anion-exchange membranes has been shown to be caused by large
organic ions becoming attached to charged groups on membranes hav-
ing the opposite charge. Since negatively charged organic sub-
stances occur more often in natural waters than positively charged
ones, anion-exchange membranes with their positive charges are
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fouled more often than cation-exchange membranes. Fouling of
membranes by organic matter has been studied by Cooke,11 Bolting,12
Olie,13 Solt,11* Mandersloot, 15 Small and Gardiner,16 and Korngold
and De Korosy.x 7
The studies of the last group were specially revealing.
They found that fouling is caused by H+ ions generated by even
minimal polarization at the surface of anion-exchange membranes.
At conditions of polarization, some of the H+ and OH~ ions from
ionization of water are present at the membrane surfaces. As
polarization increases, the concentration of H+ and OH~ ions rela-
tive to the total concentrations of ions increases. At the
depleting side of anion-exchange membranes, the OH~ ions disappear
into the membranes. This leaves H+ ions remaining in the solution.
Any large organic anions in the vicinity of the H+ ions become
associated with the H+ ions to form the acid of the organic anions.
These acids of large organic anions are usually relatively insolu-
ble, and negatively charged precipitates form on the positively
charged anion-exchange membranes. When this occurs, a bipolar
membrane "sandwich" is formed that gives rise to even faster pro-
duction of H+ ions. Thus, once the process of organic fouling is
started, it proceeds more and more rapidly with the passage of
time.
Korngold and his co-workers also found that the obvious
expedient of washing the membranes with alkali solutions while
they are in place to reverse the process of formation removed the
organic precipitates and restored the original resistances and
coulomb efficiencies of the stacks. Cleaning stacks with alkali
washes has also been found to be an effective procedure by Olie13
and by Ahlgren l 3 for restoring the performance characteristics of
electrodialysis stacks used for treating certain industrial solu-
tions. In the author's laboratory, periodic rinsing with solu-
tions of enzymes has been found to remove some types of colloidal
material.
Other findings by the Korngold group were:
—Conditions that minimize excessive polarization (e.g_. ,
high solution velocities, spacers with good mixing actionF also
minimize organic fouling.
—All of the eleven types of membranes they studied were
fouled by organic anions, but some membranes were less susceptible
to fouling than others. They also found fairly wide variations
in susceptibility to fouling between different batches of the same
type of membranes.
Membranes with glossy surfaces were less prone to foul
than ones with dull or roughened surfaces.
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—Membranes with macro-inhomogeneities were more prone to
foul than homogenous membranes.
—Operation of membrane cells under hydrodynamic conditions
that minimized polarization also minimized fouling.
Grossman and Sonin19 not only studied fouling of anion-
exchange membranes by organic matter, but also studied the fouling
and poisoning of cation-exchange membranes by hydrous ferric
oxide. Their findings on organic fouling generally confirmed the
findings of Korngold. They also found, as did De Korosy2° previ-
ously, that multivalent cations "poison" cation-exchange membranes
by entering the polymeric matrix and associating with the fixed
negative charges. In addition, they found that thin films of
hydrous oxides deposited on the cation-exchange membranes and
formed bipolar membranes. These films could be removed by rinsing
with EDTA solutions.
E. Minimizing Polarization
It was stated previously that operating conditions that
minimized polarization minimized organic fouling. Minimal polar-
ization is also desirable to prevent or minimize scaling of mem-
branes. Minimal polarization is achieved at high values of the
polarization parameter, i/N, which varies with solution velocity
and with the nature and shape of the mesh-like spacer materials
used to maintain desired spacings between membranes.
Figure 7 shows the change in the polarization parameter,
i/N, with nominal solution velocity for several commercial types
of spacers, and for an experimental thin-cell spacer just now
being commercialized. The data shown are from experiments in
which the thickness of the spacer was carefully matched to the
cell thickness. (Other data are available in the literature, but
because of a lack of correspondence of thicknesses, some of the
literature data are questionable.)
The Vexar spacer is indicated to give the best mixing action
(i^.e^. , the highest values of i/N) of the eight spacer materials
for which data are presented.
It is believed that the good mixing action results from a
semi-helical flow path of solution in which first the surface of
one membrane and then the surface of the opposite membrane is
swept by rapidly moving solution as indicated in Figure 8.
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9000
5000
4000
3000
2000
^ 1000
500
400
300
200
PRODESCO-12
100
THIN
VEXAR
TORTUOUS
PATH
2 345
10 20 30 40 50
VELOCITY, CM/SEC
100
Figure 7. Variation of Polarization Parameters
with Velocity for Different Spacers
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SOLUTION FLOW
Figure 8. Representation of Solution Flow
through Vexar Spacers
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F. Process Arrangements of Electrodialysis Stacks
Electrodialysis stacks may be used in various ways to
achieve different design goals. Mintz21 has described the pro-
cess variations in detail. Therefore, they will be discussed
only briefly here. Continuous-flow arrangements of stacks are
often used for large-scale operations in which the desired degree
of demineralization cannot be achieved in one stage. In the con-
tinuous-flow arrangement solutions flow through first-stage stacks
and directly into second-stage stacks, and from there to third
and fourt stages, if needed. This arrangement generally results
in low capital cost per unit of productivity, but the arrange-
ment is not very flexible in accomodating changes in feed con-
centrations, temperatures, or pH.
The batch-recirculation process is more flexible than the
continuous-flow process, but the capital cost per unit of produc-
tivity is usually higher. In the batch-recirculation process only
one stage of stacks is used, and the solutions are pumped from
reservoirs through the stage of stacks and back to the reservoirs.
The concentration of the depleting stream is decreased during a
batch run; that of the concentrating stream is increased.
When large changes in feed-solution concentrations are en-
countered and a continuous flow of product is required, a feed-and-
bleed process is sometimes used. In this process only one stage
of stacks is used, and solutions are pumped at high velocity through
the stage. A desired portion of the total flow is "bled" from the
effluent stream and the remainder is pumped back to the entrance
to the stack. At this point an amount of fresh feed equal to
that bled from the recirculating stream is introduced. With the
feed-and-bleed process feed streams of changing concentration can
be treated easily, but the energy requirements are high and sensitive
instrumentation is required.
Mintz21 has presented process descriptions and calculations
relating to these and other design variations.
G. Operational Limitations of Electrodialysis Processes
Excessive concentration polarization at the surfaces of the
membranes can limit the current densities that can be used in
electromembrane processing.
The degree of concentration that can be achieved is limited
by the amount of water that is transferred through the membranes
along with the ions by osmosis and electroosmosis. The flux of
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water that occurs with a flux of ions is highly dependent on the
nature of the membranes. The concomitant fluxes of water and
ions have not been studied extensively, but Lakshminarayanaiah
and Lacey22 here reported some data on the subject. In general
the number of milliliters of water transported per faraday
decreases with increases in current density, decreases with
increases in solution concentration, and decreases as the water
content of the membrane decreases.
Another limitation on the degree of concentration that can
be accomplished in some applications of electromembrane processing
is that some compounds present in the feed solutions may exceed
their maximum solubility if the feed is concentrated too much.
For example, in the electrodialytic concentration of seawater to
furnish brines for the chlor-alkali industry, the formation of
precipitates of calcium sulfate in the concentrating compartments
limited the degree of concentration that could be achieved. The
developers of this process solved the problem by developing ion-
exchange membranes that allowed the transfer of Na+ and Cl~ ions
in preference to Ca+ and SCU~2 ions.
In addition to the technical limitations above there is an
economic limitation on the degree of depletion that is feasible
in some applications of electromembrane processes. As the concen-
tration of electrolyte in the depleting stream decreases, the
electrical resistance of the solution increases, I2R losses
increase, and, at some point, an excessive amount of energy
becomes necessary to effect additional depletion of the solution
and the high costs for energy cause the process to become non-
competitive with other types of processing.
H. Applications of Electrodialysis
Electrodialysis processes are us3d:
• In the food industry-desalting of whey, and sweetening
of citrus juices.
• In the chemical industry-concentration of sea water to
recover sodium chloride, and recovery cf nickel from plating baths,
• In the pharmaceutical industry-desalting of an iron
complex.
• In the treatment of wastes-separation of pulping liquor
components for reuse, recovery of pickling acids from spent
pickle liquor, and recovery of nickel from plating wastes.
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II. REVERSE OSMOgIS
Normally, when a membrane that Is selective for a solvent
separates a concentrated solution from a dilute one, osmosis
occurs and solvent molecules transfer from the dilute solution
to the concentrated solution. However, if a hydraulic pressure
in excess of the osmotic pressure is applied to the concentrated
solution, as in Figure 9, the direction of solvent transfer is
reversed, and solvent transfers from the concentrated to the
dilute solution. The process of reverse osmosis is based on the
phenomenon of reversed transfer that occurs when a pressure
excess of the osmotic pressure is applied.
In reverse osmosis, each component of the high-pressure
solution dissolves in the membrane in accordance with an equilibrium
distribution law, and diffuses through the membrane in response to
pressure and concentration gradients. An equation has been develop-
ed23 that describes the flux of solvent:
(AP-Air)
where:
RTAx
R =
T =»
AP =
ATT =
x =
flux of solvent
diffusivity of the solvent
concentration of solvent in the membrane
partial molar volume of solvent
gas constant
temperature
difference in applied pressure
difference in osmotic pressure
thickness of membrane
Note that the net driving force for solvent transfer is the
difference between the applied pressure difference and the osmotic
pressure difference.
The flux of solute is almost unaffected by the applied
pressure and can be described as follows:
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riPressurized solution of A and B
Concentrated A
• •
Membrane
II
fl Solution of B
Figure 9. Schematic diagram of Reverse Osmosis Process
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^ Aci _ „ Ac'
} = - Dz 2 = - D2K £
2 Ax Ax
where: ; = flux of solute
D2 = diffusivity of solute
Ac2 = concentration difference within the
membrane
Aci = difference between concentrations in
the two solutions
Ax = thickness of membranes
AK = distribution coefficient for solute
between the membrane and external phases
A. Membranes
The membranes used in the reverse osmosis process are
selective for water or other substances capable of hydrogen
bonding to the membrane matrix. Water dissolves in the membrane
matrix by a hydrogen-bonding mechanism. The polymer from which
the membrane is made must be capable of hydrogen bonding to water
and still have enough of an organic nature to exclude ions. In
addition the polymer must ben an excellent film former, since with
the high transmembrane pressure used any pinholes or other imper-
fections in the films would allow passage of the concentrated
solution. Polymers of cellulose acetate, and other cellulose
esters are most often used as materials for reverse osmosis
membranes. In Figure 10, an attempt has been made to depict the
way in which water hydrogen bonds to carbonyl oxygen atoms on
cellulose acetate, and transfers from one hydrogen-bonding site
to another.
Because of the "tight" structure of the membrane matrix and
because of the retarding action of the hydrogen-bonding sites, the
resistance to water transfer is high. To achieve reasonably high
transfer rates at reasonably low pressures, two-layer membranes
have been developed. The two-layer membranes have an extremely
thin skin of dense material and a porous underlayer as shown in
the scanning electron micrograph in Figure 11. The dense "skin"
shown at the top of Figure 11 is the actual selective membrane,
and it usually is very thin (0.2 microns). The underlayer, a
part of which is shown at the bottom of Figure 11, has relatively
large interconnecting pores (0.1 to 0.3 microns diameter) that
offer little resistance to the flow of solution once it has trans-
ferred through the thin skin. The porous underlayer is usually
about 100 microns thick.
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r
C
c
c
^1
-«\_
H H 1
\0/ •
~-\
CH3 -- _
o— c -o-c^^c-
; i
HO--C 0
CH3 0.
i :c
s 0 < 0 C C
3 HO- C ,6
1 CH' °' r
° o c o c- c
HO -C- .0
CH3 oC
i C
0— C 0 -C'' C
HO C^ ,0
o.c
H20 H
H20
H20
Hit
\*\-X
•
y"3
c-o
CH3
C--0.
CH- 0
CH3
C --0-
CH2- -0
CH,
c— o.
1
CH2 0
j
2 /
/
2o S
H
H20
H- -H H
^^^'Itnpuwty
%-H • %^ •
>Ss-rt'"^ \. ^^ fl^^^^H
0 ^^^^^1
1 • _
^ 1 3 ^-^ *
H H '°=C ' Ch3
^•Q"' 0— CH2--C"' ^C— 0 -C— 0
CH3 0, ^C— OH
H ,H-°^C
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129
"TF
«*
5 pin
Figure 11. Electron micrograph of Reverse Osmosis Membrane
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130
The two-layer membranes used in many types of equipment
are often called Loeb-Sourirajan membranes after the co-discoverers
of a practical method of casting and heat-treating the membranes.
In this type of membrane the thin skin changes rather abruptly
into the porous underlayer as shown in Figure 11. Also used in
some types of equipment are hollow-fiber membranes, in which the
entire thickness of the membrane (i^. e. wall thickness of the
hollow fibers) is of the dense materTal. In addition, membranes
made of a thin layer of dense material placed on a separate porous
underlayer, and anisotropic hollow-fiber membranes are in an ad-
vanced stage of development.
Membranes made of cellulose esters which has been the most
used material, has certain limitations discussed below.
Cellulose esters hydrolyze rapidly at either high or low
values of pH. Therefore, to achieve long lifetimes of cellulose
acetate membranes the solution being treated must be maintained
within a fairly narrow range of pH (about 3 to 7.5). Another
difficulty encountered with cellulose acetate membranes is that
the porous underlayers compact slowly at the operating pressures
used and cause the thickness of the thin skin of Loeb-Sourirajan
membranes to increase and the size of the pores in the spongy
underlayer to decrease. With compaction there is a concommitant
decrease in flux. The slow decrease in flux.results in increased
processing costs, since less product is produced per unit area of
membrane.
At temperatures higher than about 50°C (122°F) the polymeric
matrix of Loeb-Sourirajan cellulose acetate membranes becomes
increasingly ordered. Indeed, the polymeric matrix of the skins
on membranes immediately after casting at low temperatures have
a low degree of ordering, and have low salt rejections. The salt
rejections are improved by holding the membranes at specified
high temperatures (in the range of 60° to 80° C) for predetermined
periods. This "annealing" process causes the skins to become less
amorphous and more ordered. However, because of the tightening
of the thin skin at temperatures above 50°C (122°F), the highest
operating temperature useable with cellulose acetate membranes is
about 120°F.
Because of the above limitations of cellulose acetate membranes,
much study has been given to the development of other polymeric
materials for making reverse osmosis membranes that do not have
these limitations. Space permits a mention of only some of the
more promising materials: the aromatic polyamides that are used
for one type of hollqw^-fiber membrane; polybenzimidazoles,2I*
poly(phenylene oxide),25 crosslinked polyvinylpyrrolidone,26
and polyethylenimine-coated membranes.27*^ The last-named appears
to be stable over a pH range of 1 to 13.
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B. Concentration Polarization
Concentration plarization occurs in reverse osmosis, as in
any process with, selective membranes. As water transfers through
the water-selective membrane solutes are left behind. The
solutes increase the concentration of the solution at the solution-
membrane interface on the high-pressure side of the membrane. A
concentration gradient is established in the boundary layer that
acts as a driving force for diffusion. The concentration increases
until at steady state the transfer' of solutes to the bulk of
solution by diffusion just equals the amount of solutes left
behind by the selective transfer of water. The back osmotic
pressure, which diminishes the driving force due to the applied
pressure, is proportional to the difference between the solute
concentrations at the interfaces of the membrane. Therefore, any
increase in concentration at the interface on the high-pressure
side diminishes the driving force for transfer of water, and is
undesirable.
To minimize concentration polarization at membrane interfaces,
high solution velocities and various types of turbulence promoters
are used to decrease the thicknesses of diffusive boundary layers.
Many of the studies of boundary-layer control in reverse osmosis
have been recently reviewed.27 Therefore, no detailed discussion
is presented here.
C. Types of Membrane Permeators
x^
The four main types of membrane permeators are, plate-and-
frame, tubular, hollow fiber, and spiral-wound types.
1. Plate-and-Frame Units
In the plate-and-frame type of construction thin plastic
plates with spiral grooves or mesh spacers are covered on both
sides with selective membranes and the membranes are cemented
at the edges to prevent leaks, as indicated in Figure 12. These
membrane-covered plates are then stacked atop each other like
a stack of dishes, and encased in a vessel that can withstand the
pressure to be applied. Each membrane covered plate is provided
with a mechanism (usually spiral grooves) to lead the permeate
to a central collecting pipe, which may also serve as a means
of indexing the plates. A number of variants of the plate-and-
frame system have been developed.
In comparison with other designs the plate-and frame design
has good volumetric efficiency and high fluxes. However, skilled
assemblers are needed when membranes must be replaced, the capital
cost per unit of productivity Is relatively high, and the probability
of leaks and other defects Is high.
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Spacer between Substrate.
/
/
Substrate
Feed
solution
- ^ Spacer
Product
Plate
No. 2
Collector
pipe
Exploded cross - section at
center with central collector
pipe shown
Figure 12. Plate-and-frame Membrane Permeator
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2. Tubular Units
Although tubular units can be made with the membranes on
the outside of porous support tubes, most tubular units have
the membranes inside the tubes. Therefore, this discussion will
be confined to tubular units with membranes inside the tubes. In
these units porous tubes that are strong enough to withstand the
operating pressures are used, since the tubes both contain the
pressure and permit transfer of water that permeates the membrane.
Tubes have been made of glass fibers bonded with thermosetting
resins, granular materials bonded with thermosetting resins, and
perforated metal.
Membranes are either cast directly on porous supports inside
the tubes, or cast separately and inserted into the tubes. The
solution to be treated is pressurized and pumped through the tubes.
Permeate transfers through the membranes, through the porous support
(if one is used) and then through the tube wall, as indicated in
Figure 13. The permeate from a number of separate tubes is usually
collected, and removed. Individual tubes, which might be only
10 to 20 ft long, may be interconnected in series, parallel, or
combinations of series-parallel arrangements.
The main advantages of tubular units are: they can handle
feed solutions with high contents of particulate matter, they can
be very easily cleaned, and they are only moderately expensive
per unit of productivity. On the other hand, membrane replace-
ment is relatively expensive, since many fittings and connections
must be dealt with in dismantling and reassembly, and there is
some risk of leaks at the many interconnections.
There are a number of designs of tubular units that differ
in the types of porous supports and turbulence promoters used.
3. Hollow-fiber Units
The basic element of these units is a fine hollow fiber (25
to 40 microns I.D. and 50 to 80 microns O.D.) of the selective
membrane material itself. Fibers as fine as this can withstand
high pressures without collapsing. A mass of these fibers are
aligned into a U-shape and the open end of the "U" is encapsulated
in a resin. Grooves are formed in the encapsulating resin to
receive a retaining ring that fits tightly in a cylindrical
pressure vessel, when the end of the U-shaped mass of fibers is
placed into the cylinder as indicated in Figure 14. The final
result is a membrane permeator that resembles a U-bend shell-
and-tube heat exchanger except that the "tubes" are extremely
fine fibers. Feed solution is introduced through a perforated tube
that extends axially near the center of the mass of fibers. Some
of the solution permeates the fibers and is withdrawn from the
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Casing
Figure 13. Diagram of a Tubular Membrane Permeator
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end of the cylindrical pressure vessel opposite to the feed-
introduction end. The concentrated brine left behind on the
outside of the fibers flows through the shell into a collector
pipe at the feed-end of the vessel.
The main advantages of the hollow-tube units is low cost
and small space requirements per unit of productivity. Because
the hollow fibers are self-supporting and can be produced cheaply,
a vast area of membrane can be encompassed in a small volume and
can be put into production at low cost. Thus, even though the
permeation rates per unit of area are low relative to that
through anisotropic membranes (perhaps 1/5Oth the rate), the
permeation rate per dollar invested might be the same, or
higher. On the other hand the mass of hollow fibers is fairly
efficient as an in-depth filter and feed solutions often must be
pre-treated to remove particulate material.
4. Spiral-wound Units
In spiral-wound modules a 4-layer sandwich comprised of a
spacer material, a membrane, a porous material to collect permeate,
and another membrane is wound spirally around a perforated center
pipe that acts as a permeate collector, as indicated in Figure 15.
One or more spiral modules are mounted in a cylindrical pressure
vessel and a number of the cylindrical pressure vessels may be
interconnected in series, parallel, or combination of series-
parallel/ as shown in Figure 16. Feedwater flows axially through
the spaces made by the spirally-wound mesh spacer. Some of the
solution permeates the membranes and enters the porous permeate
space. The permeate travels spirally inward, enters perforated
holes in the central collector pipe, and is withdrawn through
this pipe.
The capital cost of spiral-wound units per unit of productivity
is relatively low, and the volumetric and space requirements are
low. Instead of replacing membranes, pre-assembled modules are
replaced. With these units, as with the hollow-fiber units
particulate matter in the feed solutions can pose a problem,
and pretreatment to remove particulate matter may be necessary.
D. Applications for Reverse Osmosis
Reverse osmosis is suited for treating wastewaters contain-
ing heavy metals, and other papers in this symposium will deal
with that subject in detail. Among the other applications that
might be listed for reverse osmosis are:
• In the food industries - concentration of fruit
juices, coffee, tea, milk and syrups.
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• In the chemical Industry-fractionation of azeo-
tropes, and molecular weight fractionation.
• In the pharmaceutical industry - concentration of
antibiotic beers, and concentration of enzyme
solutions.
• In medicine - preparation of high-purity water
for use in artificial kidneys.
• In the treatment of wastes - treatment of dilute
solutions in the pulp industry, treatment of
drag-out rinses in the metal-plating industry,
and the recovery of protein and lactose from whey
in the cheese industry.
Obviously, the above list includes only a few of the poten-
tial uses for reverse osmosis, and other uses will be apparent.
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Epoxy
tube sheet
Porous
back-up disc
Permeate
End plate
Concentrate
Porous feed
distributor tube
0-ring seal End plate
-------
SCHEMATIC CROSS-SECTION OF A SPIRAL ELEMENT
SHOWING WATER FLOW PATTERNS
FEEDWATER
PURIFIED WATER PASSES THROUGH
MEMBRANE FROM BOTH SIDES OF
PRODUCT WATER CHANNEL.
PRODUCT WATER CHANNEL
CO
00
I I I /MEMBRANE
CONCENTRATED SOLUTION
LEAVES THROUGH
FEED SPACER
PRODUCT
\
VINYL ADHESIVE
TAPE COVERING
PRODUCT WATER FLOWS SPIRALLY IN
PRODUCT WATER CHANNEL LAST LAYER
CONTACTS HOLES IN PRODUCT TUBE FOR
EXIT TO COLLECTION SYSTEM
BRINE SEAL
o
-P
OJ
I
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Figure 16. A 100,000 gpd Spiral-wound Modular Unit
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BIBLIOGRAPHY
1. N. Lakshminarayaniah, Chem. Rev. 65, 494 (1965).
2. H. Z. Friedlander and R. N. Rickles, Anal. Chem. 37, 27A
(1966).
3. R. E. Lacey, Basis of Electromembrane Processes, in "Indus-
fTi.al Processing with Membranes" , R. E. Lacey and S. iiOeb,
Editors, Wiley-Interscience, New York (1972), Chapter 1.
4. B. A. Cooke, Electrochim. Acta 3_, 307 (1961) .
5. B. A. Cooke, Electrochim. Acta 4_, 179 (1961) .
6. G. Belfort and G. A. Guter, Desal. 5_, 267 (1968) .
7. K. S. Spiegler, Desal. 9_, 367 (1971) .
8. K. S. Spiegler, Office of Saline Water Research and Develop-
ment Report 353 (1968).
9. A. A. Sonin and R. F. Probstein, Desal. 5_, 293 (1968) .
10. A. Solan and Y. Winograd, Phys. Fluids 12^, 1372 (1969) .
11. B. A. Cooke, Proc. First International Symp. on Water
Desalination,"Washington, D. C., October 3-9, 1967 2, 219
(1967).
12. W. A. G. Holting, Milan Meeting of the Water Desalination
Working Party of the European Federation of Chem. Engr. ~,
June, 1965.
13. J. R. Olie, Milan Meeting of the Water Desalination Working
Party of the European Federation of Chem. Engr., June,1965.
14. G. S. Solt, Proc. First International Symp. on Water
Desalination, Washington, D. C .~October 3-9, 1965 2", 13
(1967).
15. W. C. B. Mandersloot, Proc. First International Symp. on
Water Desalination, Washington, D. C. , October 3-9, 1965 2,
461 (1967).
16. H. Small and R. Gardiner, Office of Saline Water Research
and Development Report 565 (1970).
17. E. Korngold, F. De Korosy, R. Rahav, and M. Taboch, Desal.
8, 195 (1970).
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18. R. A. Ahlgren, Aqua-Chem Corporation, Waukesha, Wisconsin,
private communication.
19. G. Grossman and A. Sonin, Office of Saline Water Research
and Development Report 742 (1971).
20. F. De Korosy, U. S. Office of Saline Water Research and
Development Report 380 (1968).——
21. M. S. Mintz, Ind. Eng. Chem., 55, 18 (1963).
22. R. E. Lacey, Office of Saline Water Research and Develop-
ment Report 353";(1966).
23. U. Merten, "Transport Properties of Osmotic Membranes" in
Desalination by Reverse Osmosis, U. Merten, editor, M.I.T.
Press, Cambridge, Mass.(1966), P. 15.
24. H. K. Lonsdale, "Reverse Osmosis Membrane Systems", paper
presented at the National Science Foundation Symposium on
Membranes in Separation Processes, May 8-10, 1973, Case-
Western Reserve University, Cleveland, Ohio (1973).
25. A. B. LaConti "Development and Testing of Composite Sulfonated
PPO Films", paper presented at the National Science Foundation
Symposium on Membranes in Separation Processes, May 8-10,
1973, Case-Western Reserve University, Cleveland, Ohio (1973).
26. L. T. Roselle, J. E. Cadotte, and C. V. Kopp, "NS-1 Membranes
for Reverse Osmosis", paper presented at the National Science
Foundation Symposium on Membranes in Separation Processes,
May 8-10, 1973, Case-Western Reserve University, Cleveland,
Ohio (1973).
27. H. K. Lonsdale, "Reverse Osmosis and Ultrafiltration"
Chapter VIII in Industrial Processing With Membranes,
R. Lacey and S. Loeb,editors, Wiley-Interscience, New
York (1972).
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REMOVAL OF TRACE HEAVY METAL IONS FROM WATER BY ELECTRODIALYSIS
by
Wayne A. McRae and Edgardo J. Pars!
Ionics, Inc., Watertown, Massachusetts 02172
Presented at the Conference "Traces of Heavy Metals in Water;
Removal Processes and Monitoring", Princeton University,
November 15, 1973
INTRODUCTION
Electrodialysis is a process for separating low molecular
weight electrolytes from solutions or suspensions containing essentially
unionized substances for example, water. The low molecular weight
electrolytes, which must be at least partially ionized, are caused to
pass through ion exchange membranes having a relatively low hydraulic
permeability. This process will remove traces of heavy metals ions
(as well as otherions) from water to any degree desired and generally
concentrate them in solution in a rinse stream to levels at which they
can be readily handled by conventional methods, e.g., precipitation and
ultimate disposal as the sulfides, hydroxides, carbonates, sulfates or
phosphates. In this paper such trace heavy metals are assumed to be
chromium, manganese, cobalt, copper, zinc, arsenic, cadmium, antimony,
Barium, mercury, thallium and lead
We will first discuss the nature of ion exchange membranes and the
properties which are important to electrodialysis. This will be followed
by a general description of the process and apparatus and a discussion
of the relevant economics.
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144
ION EXCHANGE MEMBRANES (Exhibit 1)
Ion exchange membranes are ion exchange resins in sheet form.
Typically they are based on crosslinked polystyrene and are therefore
chemically stable and insoluble in the usual solvents. Typical com-
mercial membranes are about 0.022 inches thick, 18 inches wide and
40 inches long and are reinforced with inert woven fabrics to provide
desirable physical properties.
There are two types of membranes. Cation exchange membranes
contain negatively charged groups such as sulfonate groups chemically
bonded to the polystyrene. The membranes are electrically neutral and
contain positively charged counterions such as sodium, calcium, hydrogen
or trace heavy metal cations equivalent to the number of fixed negatively
charged groups. When the membranes are equilibrated with water or a/queous
solutions they will absorb from 20 to 70 percent by weight of water. The
water content is fixed during manufacture and is reasonably independent
of external solution concentrations. This so-called gel water is more
or less homogeneously distributed throughout the membrane in pores which
have average diameters in the range of about 1 to 10 ^nanometers. The
average pore diameter is fixed during manufacture. These pores are thus
in the range of typical diameters of soluble molecules. Membranes having
)t
pores on the low end of the range, e.g. 1 to 2 jrfianometers, are most
advantageously used in electrodialysis.
The second classification of membranes, anion exchange membranes,
contain positively charged groups such as quaternary ammonium groups
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145
chemically bonded to the polystyrene. These membranes also contain
mobile negatively charged counter-ions such as chloride, sulfate, bi-
carbonate or trace heavy metal anions. The description of the solvent
in cation exchange membranes is entirely applicable to anion exchange
membranes.
The counterions, that is, cations in cation exchange membranes
or anions in the anion exchange membranes, are substantially dissociated
from the fixed charged groups and are free to migrate throughout the
membrane as long as electroneutrality is maintained. These mobile ions
give rise to the ion exchange properties of the membranes since they may
be exchanged for other mobile counterions including trace metal ions by
contacting the membranes with solutions of salts of such ions.
In the presence of an electrical potential gradient, electric
current is carried by the mobile counterions. The sulfonate (or quaternary
ammonium ions) which are chemically bonded to the polymer structure may
be regarded as subject to the same electrical potential gradient but are
not free to move in the gel water of the membrane and therefore do not
participate in carrying the electric current. Hence the current is
carried substantially completely by counterions. Commercial membranes
are good conductors; the specific electrical resistances generally are
2
a few hundred ohm cms and the areal resistances are about 10 ohm cm
2
or 0.01 ohm foot
Considerable frictional interaction takes place between the
counterions and gel water. Under the influence of a potential gradient,
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146
the gel water migrates in the same direction as the counterions. This
phenomenon is known as electroosmosis. Substantially all of the gel
water in a membrane participates in electroosmosis, the rate of migration
depending upon the magnitude of the potential gradient, the average pore
size of the membranes and the nature of the counterion. The specific
electroosmotic property of a membrane is expressed as the number of
milliliters of water accompanying the passage of 26.8 ampere hours (one
electrochemical equivalent). This quantity may be varied during
manufacture from about 50 milliliters to about 750 milliliters of water
per electrical equivalent.
ELECTRODIALYSIS APPARATUS FOR EEMINERALIZATION (Exhibit 2)
The basic apparatus for electrodialysis consists of a stack of
membranes between two electrodes. Plow of the process streams is con-
4
tained and directed by spacers which alternate with the membranes. The
spacers are generally about 40 mils thick. The stack is internally
manifolded by lining up apertures in the membranes and spacers. The
assembly of membranes, spacers, and electrodes is held in compression
by a pair of end plates located outboard of the electrodes and connected
by tie rods. The assembly thus resembles a plate and frame filter press
containing about 16 membranes per linear inch. The stack is typically
38 by 18 by 40 inches and contains 3000 square feet of membrane which is
•
roughly 200 square feet of membrane per cubic foot.
-------
For detnineralization (including removal of trace heavy metal
ions) the membranes in the stack are arranged alternately cation and
anion. The unit composed of a cation membrane, a spacer, an anion
membrane and another spacer is a repeating unit termed a "cell pair".
An electrodialysis stack may contain 300 or more cell pairs between a
single pair of electrodes. This stack is diagrammed in Exhibit 2.
A direct current potential of about one volt for each membrane
is applied to the stack of membranes and spacers. The current which
flows then depends on the electrical conductivity of the solution
between the membranes and is typically in the range of 20 to 150 amperes
per square foot. The current passes in series through all the membranes
and solution compartments and is therefore "used" many times before
passing out of the electrodialysis stack.
The compartments which have an anion membrane on the anode side
and a cation membrane on the cathode side are demineralizing, compartment
(heavy metal decontaminating compartments) while the alternate com-
partments are concentrating or rinse compartments. There are thus two
kinds of compartments also arranged in alternation. A solution which
we wish partially to demineralise, for example, to remove trace heavy
metal ions, is passed continuously through the demineralizing compartments
and a rinse stream is passed continuously through the concentrating com-
partments. Upon the application of the direct current, low molecular
weight electrolyte (includingirace heavy metal ions and some water) will
pass from the demineralizing compartments into the concentrating com-
-------
1U8
partments. The rinse stream may be recycled to build up the concentrator!
of heavy metal ions to a level convenient for further processing.
Since the transference number of the ions passing through the
membrane is nearly unity in the membrane but approximately 0.5 in the
solution, about half of the ions must be transferred from the bulk
solution in the deinineralizing compartment to the membrane surface by
diffusion and convection. At low current densities and high bulk con-
centrations, diffusion is adequate while at high current densities and
low bulk concentrations the only adequate process is forced convection.
This forced convection is produced by turbulence promoting obstructions
in the flow channels of the spacers. It may be further enhanced by
increasing the velocity of flow across the membrane surface.
As pointed out above the magnitude of the current will depend
upon the conductivity of the solutions in the compartments and therefore
on the salinity and viscosity of the solution. Owing to the necessity
of dissipating heat from the membranes and electrodes, the current is
generally not allowed to exceed 150 amperes per square foot though
commercial membrane applications do exist in which the current density
is about 800 amperes per square foot.
There are three possible modes of operating such an electro-
dialysis stack. In "Feed-and-Bleed", part of the product is recycled
to the feed inlet, the amount of recycle depending upon the degree of
demineralization required. This mode is useful if the required degree
of demineralization is not too high. In "Batch" operation the product
-------
is returned to a feed reservoir which is periodically charged, de-
mineralized to the required degree and then emptied. This is useful
where a low product production rate is required. In "Continuous"
operation if the degree of demineralization in one stack, for a given
current density and velocity, is less than that required, sufficient
stacks are arranged in series to meet the requirement. The degree
of decontamination of trace heavy metal ions in any application will be
approximately the same as the degree of demineralization of all electro-
lytes in that applications.
ELECTROD1ALYTIC DEMINERALIZATICN OF WATER (Exhibit 3)
Almost 400 electrodialysis plants have been sold by Ionics.
The principal commercial application of these plants has been
the demineralization of brackish water.
Details of some of these plants are given in Exhibit 3. Of
particular interest are the plants at Buckeye, Arizona (the first
municipality to have its entire water supply demineralized by electro-
dialysis) ; at Siesta Key, Florida (the largest operating municipal
electrodialysis facility in the U.S.), and at Foss Reservoir, Oklahoma
(the largest demineralization plant sold in the United States). Data
on operating costs of the Siesta Key plant are presented below.
A recent development has been the use of electrodialysis for
production of very pure water for boiler feed and electronic component
2
washing. It is now feasible to produce water containing as little as
2 to 5 ppm total dissolved solids from water containing 500 to 1000 ppm
by electrodialysis.
-------
150
COSTS OF ELECTRODIALYTIC DEMINERALIZATION (Exhibits 4 through 7)
Approximate capital cost per unit capacity is shown in Exhibit 4.
There are two major factors influencing costs: plant capacity and
number of stages. The cost per unit capacity decreases slowly as plant
capacity increases. Cost increases with the number of stages in series.
Determination of the number of stages required depends upon the desired
degree of demineralization, approximately the same as the desired degree
of decontamination of heavy metal ions. A demineralization of about 50%
per stage appears most desirable for engineering reasons. The number of
stages then depends upon the desired overall degree of demineralization.
Exhibit 4 illustrates the relationship between capacity, number of stages
++ +4- =
and investment cost. If divalent ions (Ca , Mg , SO ) predominate
in the feedwater, each stage will yield a smaller degree of demineralization.
Electrical power requirements can be calculated rather precisely
given the feedwater composition. Membrane replacement in most locations
is less than 20% per year and with favorable operation less than 10%/year.
Replacement costs can also be estimated for pretreatment equipment (if
any) to remove suspended solids, electrodes and other components.
Estimation of labor cost depends on local factors and it frequently
involves non-technological factors. Operating labor is used primarily
for monitoring of equipment and pretreatment if required. This generally
requires about one-half man-hour per day for plants below 10,000 GPD.
In large plants 2 to 4 man-hours per day may be required. Maintenance
-------
151
labor is estimated at 100 man-hours per stack year for the first stage,
75 for the second, and 50 for subsequent stages. The requirement for
maintenance labor depends strongly on the cleanliness of the feedwater.
These costs can be added to produce a total product water cost.
Exhibit 5 presents an approximation of total water costs per 1000 gallons
as a function of plant capacity and number of stages for a 500 ppm feed.
These predictions can be compared to operating costs for the Siesta
Key, Florida plant mentioned above. This plant is basically a 2 million
GPD plant for which 60% of the stacks were installed in 1969, a further
15% in 1970, and an additional 15% in 1972. The plant reduces a 1400 ppm
water to 500 ppm. Operating costs are given in Exhibit 6. The cost
estimate is based on a spot check of actual costs against the supplier's
guarantees. The former have consistently been found to be lower. The
data in the table are the guarantee figures. Total water costs are given
in Exhibit 7. Note that power costs are only about 5C out of a total of
29£ per thousand gallons for the 1400 ppm feed in this plant. If the feed
water were 500 ppm at the same degree of demineralization power costs would
be reduced to about 2C and overall costs to about 25C.
REFERENCES
1. J. R. Wilson, "Demineralization by Electrodialysis", Butterworths
Scientific Publications, London, 1960.
2. W. E. Katz, Proc. Am. Power Conf. 33, 830 (1971).
3. W. A. McRae and F. B. Leitz, "Recent Developments in Separation
Science" CRC Press, Cleveland, 1972, page 163.
-------
152
Exhibit 1
STRUCTURE AND PROPERTIES OF ION EXCHANGE MEMBRANES
Cation Exchange Membrane
Anion Exchange Membrane
("*TT fW
I
CH
CH
CH_
CH
CH,
CH
CH2 CH CH CH —
so.
CH
CH.,
Cl
CH
CH
Cl
— N (CH3)
CH
—N
Cl
Properties of Membranes
Physical Size: 0.022" x 18" x 40"
Burst Strength: > 100 psig
20 to 70% by weight
~ 2.5 meq per dry gram
10 to 100 Angstroms
Water Content:
Capacity:
Pore Size:
Specific Resistance: 100 to 300 ohm-cm
Areal Resistance: 10 ohms cm or 0.01 ohms ft
Water Transport: 50 to 750 mj6/Faraday
-------
Exhibit 2
SCHEMATIC DIAGRAM OF ELECTRODIALYTIC DEMINERALIZATION
WATER OR SALINE FEED,
RECYCLED BRINE
SALINE FEED
Feed
Haste Hoy
Cathode
ve Pole
ectifier "*
V
I
l«
1
Cathode
r ttiuent ^
h -
A ^
+
K
Cl
2
+
K .
* *
K
3
C
>
r A ^ C
-f
K
Cl~
4
i
^> v
1
1
s-
+
K •*•
5
A ' C
f . V S
+
K
Cl ~
6
I
* *^ >
L
- -
•f
K .
\! *
^
*ci-
7
~ 1
A ' C
r v ^
_^_
K
ci -
8
"f"
K
^ *
. A
^ci-1
9
A _
+
K
ci -
10
1 1
r V >
—
p
r V >
I
i
1
V >
+
[— — — — — — — — Anode
Feed
11
f
f
Tirrelloy
Anode
To Positive Pole
of Rectifier
— > Anode Effluent
(JO
Concentrated Brine
or Waste
Qemineralized Product
Vp to 600 membranes between one set of electrodes
3.5 sq.ft. active area per membrane
D.C. potential: 1 volt per membrane
Current density: 20 to 150 amps/ft2
Separated by tortuous path spacers 0.040" thick
Turbulence promoters in spacers
Water transfer: cations - 150 ml per Faraday
anions - 100 ml per Faradav
-------
Exhibit 3
Large Electrodialysis Plants Built or Under Construction by Ionics
S ite Date of Start-up M G D
Al Khobar, Saudi Arabia 1967 0.10
Aimer, Algeria 1973 1.00
Anaconda Copper, Chile 1970 0.27
Bahrain, Arabian Gulf 1964 0.10
Bari, Italy 1970 0.53
Brindisi, Italy 1971 1.30
Buckeye, Arizona 1962 0.65
Dhahran, Saudi Arabia 1961 0.12
Foss Reservoir, Oklahoma 1974 3.00
Gillette, Wyoming 1972 1.50
Pantelleria, Italy 1973 0.27
Port Mansfield,. Texas 1965 0.27
Riyadh, Saudi Arabia 1973 0.64
Sanibel Island, Florida 1973 1.20
Siesta Key, Florida 1969 1.80
State of Kuwait, Persian Gulf 1963 0.24
U.S. Army, New Mexico 1970 0.10
Van Horn, Texas 1969 0.88
-------
155
Exhibit 4
Approximate Electrodialysis Capital Investment
As a Function of Capacity for Various Numbers of Stages
(Each Stage Removes Approximately
50% of Salts in its Feed Water)
1.2
4 6 8 10 20
40 60 80 100
.PLANT CAPACITY IN MILLIONS OF GALLONS PER DAY,
1 = One Stage, approximately 50% demineralization
2 = Two Stages, approximately 75% demineralization
3 = Three Stages, approximately 87.5% demineralization
-------
156
Exhibit 5
Total Water Costs, £ for 1000 Gallons for 500 ppm Peed
As a Function of Capacity for Various Numbers of Stages
(Each Stage Removes Approximately 50%
of the Salts in its Feed Water)
Cn
O
O
O
I-l
•o
M
J->
03
O
U
0)
-P
*
5
o
EH
60
50
40
30
20
10
0.5
6 8 10
40 60 80 100
PLANT CAPACITY IN MILLIONS OF GALLONS PER DAY
1 = One Stage, approximately 50% demineralization
2 = Two Stages, approximately 75% demlneralization
3 = Three Stages, approximately 87.5% demineralization
-------
157
Exhibit 6
Operating Costs Excluding Labor
Membrane Replacement
Electrical Power
Filters
Other Spare Parts
Acid
Miscellaneous
Total
Siesta Key, Florida
5.8
5.1
3.0
2.2
1.4
0.3
Ionics Guarantee
-------
158
-------
159
REVERSE OSMOSIS FOR THE REMOVAL OF
HEAVY METALS FROM WASTE WATER:
PRELIMINARY RESULTS
BY
PETER C, HOULE
GULF DEGREMONT, INC,
LIBERTY CORNER, N, J
PRESENTED AT:
PRINCETON UNIVERSITY
CONFERENCE ON
TRACES OF HEAVY fiFTALS IN WATER:
REMOVAL PROCESSES AND MONITORING
NOVEMBER 15 - 16, 1973
-------
160
INTRODUCTION
THE CURRENT EMPHASIS ON EFFLUENT GUIDELINES LEADING
TO "ZERO DISCHARGE" HAS STIMULATED INTEREST IN ADVANCED WASTE-
WATER TREATMENT TECHNIQUES,
CURRENT EFFLUENT GUIDELINES FOR THE METAL FINISHING
INDUSTRY HAVE BEEN SET FOR COPPER AT 0,2 MG/L, CHROMIUM 6 AT
0,05 MG/L AND NICKEL AND MANGANESE AT 1,0 MG/L,(1)
TO MEET THESE EFFLUENT STANDARDS, WATER MUST BE TREATED
TO A HIGH QUALITY WHICH OFEN MAKES THE REUSE OF WASTEWATER
ATTRACTIVE,
REVERSE OSMOSIS HAS RECEIVED WIDE ATTENTION AS A CON-
CENTRATION PROCESS WHICH IS CAPABLE OF REMOVING A WIDE VARIETY
OF DISSOLVED SALTS INCLUDING HEAVY METALS,
MOST FULL SCALE REVERSE OSMOSIS SYSTEMS AT THE PRESENT
TIME TREAT POTABLE OR BRACKISH WATER, IN THIS SERVICE THE PER-
FORMANCE AND ECONOMICS OF THE SYSTEM CAN BE PROJECTED WITH A HIGH
DEGREE OF CERTAINTY, IN WASTEWATER APPLICATIONS, HOWEVER, IT IS
NECESSARY TO TEST THE PERFORMANCE OF THE SYSTEM TO ADEQUATELY
DETERMINE MEMBRANE LIFE AND .THE QUALITY OF WATER PRODUCED,
A SUFFICIENT QUANTITY OF PRODUCT WATER SHOULD BE PRODUCED
FOR DIRECT EVALUATION IN THE PLANT MANUFACTURING PROCESSES,
-------
GULF DEGREMONT IS CURRENTLY OPERATING A 36,000 GALLON
PER DAY REVERSE OSMOSIS PILOT PLANT TO DETERMINE THE FEASIBILITY
AND ECONOMICS OF DIRECT RECYCLE OF WASTE WATERS FROM AN ELECTRONICS
MANUFACTURING PLANT,
THIS PAPER REPORTS INITIAL RESULTS OF HEAVY METAL
REMOVALS FROM THIS WASTEWATER,
-------
162
PLANT WASTE WATER TREATMENT
THE EXISTING HASTE WATER TREATMENT PLANT IS SHOWN IN
FIGURE 1, WASTE WATER, INCLUDING ION EXCHANGE REGENERANTS AND
EFFLUENTS FROM THE CHROMIUM AND CYANIDE DESTRUCTION SYSTEMS,
IS COLLECTED AND PUMPED TO A HOLDING TANK,
FROM HERE THE WATER IS PUMPED TO A LIME ADDITION SYSTEM
AND TO A SOLIDS CONTACT CLARIFIER, LIME IS ADDED TO A pH OF
APPROXIMATELY 9 - 10 AflD POLYELECTROLYTE IS FED TO THE CENTER
WELL,
SLUDGE IS BLOWN DOWN FROM THE CLARIFIER TO VACUUM
FILTERS AND THE CAKE IS HAULED TO LAND FILL,
THE CLARIFIED WATER IS THEN pH ADJUSTED TO APPROXIMATELY
6,5 TO 8,0 USING SULFURIC ACID AND IS DISCHARGED,
THE MAJOR PART OF THE HEAVY METALS ARE REMOVED AS
PRECIPITATES IN THIS PROCESS, HOWEVER, DUE TO THE PRESENCE OF
CHELATING AND COMPLEXING AGENTS, INCOMPLETE REMOVALS OCCUR,
THE PILOT PLANT RESULTS REPORTED HERE WERE OBTAINED
USING THE FINAL pH ADJUSTED EFFLUENT,
-------
163
PILOT PLANT OPERATIONS
A SCHEMATIC OF THE REVERSE OSMOSIS SYSTEM IS ALSO
SHOWN IN FIGURE 1,
THE CLARIFIED WASTE WATER WAS PUMPED TO A HOLDING
AND EQUALIZATION TANK, DURING OPERATION WASTE WATER WAS
PUMPED FROM THIS TANK THROUGH A MULTI-MEDIA PRESSURE FILTER,
THE pH WAS FURTHER ADJUSTED TO 5 - 5,5 USING SULFURIC ACID
AND POLYPHOSPHATE ADDED IN THE RANGE OF 10 MG/L TO AID IN
CONTROL OF CALCIUM PRECIPITATION,
PRIOR TO ENTERING THE REVERSE OSMOSIS SYSTEM, THE
WASTE WATER WAS PASSED THROUGH POLISHING CARTRIDGE FILTERS
FOR REMOVAL OF PARTICLES LARGER THAN 10 MICROiiS,
THE WASTE WATER WAS THEN PASSED THROUGH A REVERSE
OSMOSIS SYSTEM CONSTRUCTED WITH ROGA MODEL 4100 SPIRAL-'.OTD
MODULES,
THE SYSTEM WAS OPERATED AT 75% RECOVERY,
-------
16k
OTHER PORTIONS OF THE PILOT PLANT PROGRAM INCLUDED
THE EVALUATION OF ACTIVATED CARBON FOR REMOVAL OF TRACE ORGANIC
COMPOUNDS, DEMORALIZATION FOR THE PRODUCTION OF ULTRA-PURE
WATER AND THE EVALUATION OF FREEZE CRYSTALLIZATION AND EVAPORA-
TION FOR FINAL BRINE DISPOSAL, THESE PORTIONS ARE BEYOND THE
SCOPE OF THE PRESENT PAPER,
-------
FIGURE NO, 1
WASTE WATER TREATMENT FOR REUSE
RA\V_
"WACTE
SURGE
TANK
AnnmOH
POLYELECTROLYTE
ADDITION
SOLIDS
CONTACT
CLARIFIER
PH TRIM
PH ADJUSTMENT
WITH H2S04
*
PRESSURE
FILTRATION
U
CARTRIDGE
FILTER
UNIT
R.O. BRINE
ACTIVATED
CARBON
EVAPORATION OR
CRYSTALLIZATION
DEMINERAL-
IZER
WASTE
PRODUCT WATER
FOR REUSE
REGENERANTS
-------
166
RESULTS
THE REVERSE OSMOSIS UNIT WAS STARTED ON MONDAYS AND
ALLOWED TO RUN CONTINUOUSLY THROUGH FRIDAY, IN THE PERIOD
REPORTED HERE COMPOSITE SAMPLES OF FEED, PRODUCT AND BRINE
WERE TAKEN OVER 21-HOUR PERIODS,
DURING THIS PERIOD THE TOTAL DISSOLVED SOLIDS IN
THE REVERSE OSMOSIS FEED RANGED FROM 2000 TO 4000 MG/L,
AVERAGE, SALT REJECTIONS RANGED FROM 93 TO 95 PERCENT,
FIGURE NO, 2 ILLUSTRATES THE FLUCTUATION OF TOTAL
DISSOLVED SOLIDS BY CONDUCTIVITY IN THE REVERSE OSMOSIS FEED
AND PRODUCT WATER, THE CONCENTRATION OF DISSOLVED SOLIDS IN
THE PRODUCT WATER REMAINED RELATIVELY CONSTANT IN SPITE OF
WIDE VARIATIONS IN FEED CONCENTRATIONS,
FIGURE NO, 3 ILLUSTRATES THE CONCENTRATION OF CALCIUM
IN THE REVERSE OSMOSIS FEED FOR THE SAME PERIOD, CALCIUM CON-
CENTRATION HAS A MAJOR EFFECT ON THE DEGREE OF PRODUCT WATER
RECOVERY WHICH CAN BE OBTAINED, IN THE PRESENCE OF SULFATES
PRECIPITATION CAN OCCUR ON THE MEMBRANE SURFACES WHICH RESULTS
IN A LOSS OF FLUX,
TABLE NO, 1 ILLUSTRATES HEAVY METAL REMOVALS DURING
THIS PERIOD,
-------
FIGURE 2
WEEKLY MEAN VALUES FOR CONDUCTIVITY
REVERSE OSMOSIS FEED AND PRODUCT
FEED
PRODUCT
2500
2000
I50O --•
CONDUCTIVITY
u m h o
cr-
H
O\
-q
!000
25
-------
.50
200-
150
loo-
50
FIGURE 3
WEEKLY MEAN VALUES FOR CALCIUM
REVERSE-"OSMOSIS FEED
1.
•\
i
«i*
! !
r
"*ti
?
/
5
V/EEKS
20
25
-------
169
TABLE 1
PERCENT METAL REMOVAL
METAL AVERAGE REDUCTION - % RANGE - %
CD
CR
Cu
FE
PB
ZN
Ni
66
82
98,6
94
>99
97
15
43
73
97
92
92
14
- 83
- 90
->99
- 98
->99
- 15
-------
170
THE PERCENT REDUCTION OF COPPER, IRON, LEAD AND ZINC
CORRESPOND ROUGHLY TO THE OVERALL REDUCTION IN TOTAL DISSOLVED
SOLIDS, CADMIUM AND CHROME LEVELS INDICATE FAIR REMOVAL WHILE
NICKEL LEVELS SHOWED VERY LITTLE REMOVAL,
TABLE NO, 2 GIVES THE CONCENTRATION OF HEAVY METALS
IN THE REVERSE OSMOSIS FEED, PRODUCT AND BRINE FOR AN EIGHT WEEK
PERIOD, DAILY RESULTS WERE AVERAGED FOR ONE WEEK PERIODS, ALL
RESULTS ARE REPORTED AS MG/L.OF THE METAL, ALL ANALYSES WERE
PERFORMED BY ATOMIC ADSORPTION OR EMISSION SPECTROPHOTOMETRY,
CADMIUM LEVELS ARE QUITE LOW AND THE FAIR REDUCTIONS
EXPERIENCED MAY BE A RESULT OF THE RESOLUTION OF THE MEASURING
INSTRUMENTS,
TOTAL CHROMIUM LEVELS WERE RELATIVELY HIGHER THAN
CADMIUM AND THE PRODUCT WATER CONCENTRATIONS LIKELY REFLECT
THE LIMITATION OF THE SYSTEM, IN ALL CASES, HOWEVER, PRODUCT
WATER CONTAINED LESS THAN 0,22 MG/L,
EXCELLENT COPPER REMOVALS WERE EXPERIENCED WITH PRODUCT
WATER CONCENTRATIONS WELL BELOW THE ALLOWABLE 0,2 MG/L,
IRON REMOVALS WERE ALSO EXCELLENT WITH NO VALUES IN
EXCESS OF 0,07 MG/L, LEAD WAS FOUND ONLY INFREQUENTLY, WHEN
PRESENT HOWEVER IT WAS REMOVED TO BELOW DETECTABLE LIMITS, ZINC
REMOVALS WERE ALSO EXCELLENT, AGAIN APPROACHING THE DETECTABLE
LIMITS OF THE MEASURING LIMITS,
-------
171
NICKEL LEVELS ALTHOUGH MEETING DISCHARGE REQUIREMENTS
OF<1,0 MG/L SHOWED POOR REJECTION,
AT THE PRESENT TIME THE POSSIBILITY OF A NICKEL COMPLEX
WITH SOME OF THE ORGANIC COMPOUNDS IN THE WASTEWATER IS BEING
INVESTIGATED, NORMALLY NICKEL REJECTIONS ARE HIGHER THAN OUR
TEST RESULTS INDICATED USUALLY IN THE RANGE OF >98 PERCENT,
-------
TABLE NO, 2
METAL-ANALYSIS
FEED
PRODUCT
BRINE
FEED
PRODUCT
BRINE
CD
0,029
0,011
0,068
0,024
0,009
0,040
CR
1,20
0,22
4,95
0,01
0,15
3,26
Cu
2,29
0,07
11,20
2,51
0,04
9,86
rQTAL
FE
0,26
0,02
5,30
0,55
0,02
1,65
PB
0,00
0,00
0,00
0,00
0,00
n.oo
ZN
0,021
0,005
0,135
0,021
0,00
0,15
Ni
0,76
0,61
0,84
0,75
0,59
0,86
OJ
f-
H
-------
TABLE NO, 2 (CONT'D)
METAL ANALYSIS
TOTAL
CD CR Cu FE PB ZN Ni
FEED
PRODUCT
I
i
DRINE
!
FEED
PRODUCT
l -' r-i *• F t r-
L.-IM. UC
1
0,059
0,010
0,230
0,039
0,011
0,135
0,15
0,02
0,31
0,20
0,02
0,51
6,18
0,07
12,96
2,10
0,06
8,75
1,30
0,07
4,00
0,50
0,04
5,60
0,02
0,00
0,23
0,00
0,00
0,00
0,065
0,001
0,325
0,030
0,002
0,160
0,77
0,66
0,80
0,76
0,66
0,82
H
-3
U>
-------
TABLE NO, 2 (CONT'D)
'METAL ANALYSIS
TOTAL
CD CR Cu FE PB ZN Ni
FEED
PRODUCT
BRINE
FEED
PRODUCT
Dr-;INE
0,029
0,011
0,060
0,032
0,010
0,105
0,39
0,07
1,10
1,03
0,19
3,74
1,83
0,00
6,34
3,17
0,03
10,22
0,50
0,04
2,40
0,70
0,04
3,30
0,00
0,00
0,00
0,00
0,00
0,00
0,035
0,000
0,160
0,035
0,001
0,170
0,77
0,65
0,77
0,74
0,67
0,84
D--
H
-------
TABLE NO, 2,(coNT'D)
METAL ANALYSIS
TOTAL
CD CR Cu FE PB ZN Ni
FEED
PRODUCT
BRINE
FEED
PRODUCT
DRINE
0,021
0,012
O.OG9
0,040
0,011
0,133
0,11
0,03
0,22
0,13
0,03
0,29
3,83
0,03
12,23
5,90
0,07
12,98
2,50
0,06
5,25
1,00
0,06
3,20
0,00
0,00
0,00
0,025
0,00
0,33
0,130
0,010
0,960
0,085
0,005
0,405
0,83
0,70
1,05
0,82
0,69
0,84
-------
176
SUMMARY
AT THIS STAGE OF OUR INVESTIGATION WE HAVE DETERMINED
THAT IT IS POSSIBLE TO PRODUCE A HIGH QUALITY WATER, LOW IN
OVERALL HEAVY METAL CONTENT, FOR POSSIBLE RECYCLE IN PLANT
MANUFACTURING OPERATIONS, THE ECONOMICS OF THE REVERSE OSMOSIS
SYSTEM IS DiRECTLY RELATED TO THE PROJECTED LIFE OF THE MEMBRANES,
THIS WILL BE DETERMINED BY TESTING INDIVIDUAL MODULES AT THE
CONCLUSION OF THE STUDY,
-------
177
REFERENCES
1, REVISED EFFLUENT LIMITATION GUIDANCE FOR THE PERMIT PROGRAM,
METAL FINISHING INDUSTRY,
ENVIRONMENTAL PROTECTION AGENCY, JANUARY 1973,
2, CRUVER, J, E,, REVERSE OSMOSIS FOR WATER USE
CONFERENCE ON COMPLETE WATER USE, WASHINGTON D,C, APRIL, 1973,
-------
178
-------
179
REMOVAL OF HEAVY METALS FROM WATER
USING REVERSE OSMOSIS
by
David H. Furukawa
Fluid Sciences Division
Universal Oil Products Company
8133 Aero Drive
San Diego, California 92123
Presented at
Princeton University Conference On
"Traces of Heavy Metals in Water:
Removal Processes & Monitoring"
November 15, 1973
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180
REMOVAL OF HEAVY METALS FROM WATER
USING REVERSE OSMOSIS
by David H. Furukawa
Through the past decade, reverse osmosis has experienced a tremendous de-
velopmental effort which has taken it from a laboratory curiosity to commercial
reality. Much of this work was sponsored by the Federal Government but several
private companies were instrumental in finding commercial uses for the process
through their own developmental efforts.
Although the controversy still exists on whether the transport phenomena re-
lated to reverse osmosis is explained by diffusion theory or pore flow theory, there
is no doubt this membrane process has reached its place in industry as a unit
operation.
In spite of extensive efforts to find better membrane materials, the predominant
membrane used by the industry is still cellulose acetate or derivatives thereof. Al-
though this polymer has outstanding transport characteristics, the use of this mem-
brane is limited by several factors. Water transport changes considerably with
feed water temperature. Water permeation generally increases approximately 2.5%
per degree centigrade. The membrane is further limited by a maximum operating
temperature of approximately 110° F.
The feed solution pH must be kept between the limits of 3.0 to 7.0 pH units with
optimum being at approximately pH 5.5. Although membrane life of up to three
years has been experienced at optimum pH, membrane life time is greatly deteriorated
by high and low pH.
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181
Obviously, osmotic pressure is a primary consideration in the degree of separa-
tion possible with this membrane process. Since most commercial reverse osmosis
systems cannot be operated greater than 600 psi, concentration of inorganic ions
is limited to a value which will have an osmotic pressure less than 600.
It is well known that reverse osmosis membranes exhibit extremely high rejection
of heavy metals. The rejection of metals shown in Table 1 is a representative indica-
tion of membrane rejection. As the table shows, the rejection of heavy metals ex-
ceeds 99% in many cases. A factor which one must consider, however, in consider-
ing the use of reverse osmosis is that 100% of the fluid to be treated will come in
contact with the membrane surface. The cost of the process therefore is more de-
pendent upon quantity of water to be treated than quantity of heavy metal ions con-
tained therein. The use of reverse osmosis must be considered on an economic
basis as well as a technical separations basis. In most cases, reuse of the heavy
metals removed must have significant value before the process can be economically
used in a commercial operation. Fortunately, in many situations such by-product
recovery and reuse is possible and realistic.
Another membrane technology closely related to reverse osmosis and often
confused is ultrafiltration. The process is so named because of the use of membranes
possessing much greater porosity than reverse osmosis membranes. With ultra-
filtration membranes, the effects of osmotic pressures are minimized. The porous
structure is such that inorganic ions are free to move through the membrane barrier.
Separation of molecules is more dependent on molecular size and shape than molecular
weight or valence. Ultrafiltration can play an important part in heavy metal recovery,
particularly in those cases where the heavy metal ion can be easily complexed with
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182
an organic molecule of considerable size. The resulting organometallic compound
may be of sufficient size to be excluded from the pores of the ultrafiltration membrane.
Use of such membranes has significant value since ultrafiltration membranes generally
have water transport characteristics which far exceed any of the reverse osmosis
membranes and operate at lower pressure. An added advantage of ultrafiltration
membranes is that several are available which tolerate greater extremes in pH than
does cellulose acetate.
Considerable work has been performed (both academic and industrial) on the
(1 2)
development of reverse osmosis for heavy metal removal. '
The Fluid Sciences Division investigations with heavy metal removal began as
early as 1968 when the company, which was then known as Havens International, began
(3)
development of the process for concentration of a copper solution. v ; Even at this
early date, it was quite obvious that the process had significant potential for use in
heavy metal removal. It also became obvious that such metals must have considerable
value for reuse before companies will be willing to approve significant capital ex-
penditures for a new process. In these early tests a copper solution was concentrated
from 1,130 ppm to 10,770 ppm representing nearly a ten-fold concentration. A
significant portion of these early tests was spent in solving the problems related to
membrane surface fouling.
"Application of Reverse Osmosis to Electroplating Waste Treatment, Part I
Recovery of Nickel," by Golomb, A., Plating, October 1970
(2)
"Reverse Osmosis Treatment of Diluted Nickel Plating Solutions," by Hauck,
Andrew R. and Sourirajan, S., Journal WPCF, Volume 44, No. 7, July 1972
(3)
"Applications of Reverse Osmosis for Concentration of Copper Leaching Liquor,"
by Guy, D. B. and Lindsay, A. K., Havens Industries, San Diego, California,
February 21, 1968, Internal Report
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183
In late 1968, tests were begun in Great Britain using Fluid Sciences (formerly
(4)
Havens International) membranes to recover nickel. Water was taken from the
counter-flow rinsing tanks for concentration by reverse osmosis with product water
returned to the rinsing process. The concentrated nickel was then returned to the
plating bath. This process has since been used successfully by others. An over-
all 98% nickel recovery was achieved with 90% water recovery.
A more recent developmental effort has been initiated in use of reverse osmosis
as one step necessary to convert a pyrolytic nickel process to a hydro-metallurgic
procesSo Reverse osmosis is used for concentration of the nickel stream prior to
final recovery. This application is in a very early developmental period and com-
mercial application may not be realized for many months.
In all of the application areas investigated thus far by Fluid Sciences, the metals
recovered from the waste or process streams have considerable value in reuse. In
these situations the use of reverse osmosis pays handsome dividends in savings.
In most applications for reverse osmosis and heavy metal recovery, potential
problem areas exist. The most common phenomenon which must be considered is
concentration polarization. This phenomenon results in much higher concentration
of the metal salts at the membrane surface than would be normally found in the
bulk solution. In order to minimize these effects, it is important to establish fully
turbulent flow to reduce the boundary layer thickness.
"Nickel Recovery by Reverse Osmosis," Havens International, San Diego,
California, November 28, 1969, Internal Communication
^ ' "Reverse Osmosis for Reclamation and Reuse of Chemical and Metal Waste
Solutions," by Spatz, D. Dean, Osmonics Inc., Minneapolis, Minnesota,
December 1, 1970
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184
Membrane fouling is another problem area. Since the degree of membrane
fouling will increase as salts are carried toward the membrane by convection, it is
important to control the degree of membrane permeation as well as velocity within
the flow channeL
Other factors such as membrane compaction and system configuration have
been well discussed in literature. These considerations are common to the process
and not particularly unique to heavy metal removal.
In general, the strongest incentive to use reverse osmosis is economics. In
addition to the economics afforded by valuable by-product recovery, reverse osmosis
offers a concentration step that is uniquely more economic than conventional methods
such as evaporation. The only energy consumed by the process is pumping energy.
The thermal energy input required by evaporation is not necessary with reverse osmosis.
Because the process has not been widely used, accurate economic data on full sized
commercial systems is not available.
The greatest advances in the reverse osmosis process will probably come in the
area of membrane development. Most companies actively engaged in the manufacture
of reverse osmosis equipment are pursuing membranes made of polymers other than
cellulose acetate,, A cellulose acetate polymer, although excellent for many applica-
tions, has limitations in pH (3 to 7), temperature (approximately 110°F), and resistance
to various solvents. One of the most successful new membranes has been developed
by North Star Research & Development Institute, Minneapolis, Minnesota. ' ' The
//?\
"NS-1 Membranes for Reverse Osmosis," by Rozelle, L.T. , Cadotte, J0E., and
Kopp, C.V., North Star Research & Development Institute, Minneapolis,
Minnesota 55406
(7)
"Ultrathin Membranes for Treatment of Waste Effluents by Reverse Osmosis,"
by Rozelle, L.T0, Cadotte, J.E., Nelson, B. R., and Kopp, C0 V., North
Star Research & Development Institute, Minneapolis, Minnesota 55406
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185
North Star polymer membrane, known as the NS-1 membrane, is a non-polysaccharide
membrane. The membrane consists of a polyethylenimine-coated microporous support
(polysulfone) treated with m-tolylene-2-4-diisocyanate. This membrane has exhibited
excellent product water flux (equal or better than cellulose acetate), and has resistance
to pH from less than 1 up to 13. Typical copper ion rejection is 99. 8%. This membrane
shows great promise as a second generation membrane.
Fluid Sciences Division has undertaken further development of the NS-1 membrane
in tubular form using existing membrane support tubes. The Fluid Sciences' proprie-
tary tubular design provides an excellent substrate for this new membrane. Although
the formulations are slightly different, the basic characteristics of the tubular membrane
are very similar to the North Star formulation. Samples of this new tubular membrane,
when tested and compared to cellulose acetate, were found to possess much higher re-
jection of organic compounds as well as higher rejection of most inorganic salts.
The reverse osmosis process is now widely used in many commercial applications.
It should be strongly considered as a unit operation to be used in concert with other
established unit operations common to industry. The process, although unique and
attractive, may not solve a problem in total; however, combination with other processes
provides a most attractive package for both waste water treatment and inline processing.
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186
TABLE 1
REJECTION OF METAL SALTS BY
REVERSE OSMOSIS MEMBRANES
Typical Rejection
Percent
Iron 99
Magnesium 98
Copper 99
Nickel 99.2
Chromium, Hexavalent 97.8
Strontium 99
Cadmium 98
Silver 96
Aluminum 99
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Fluid Sciences Division
Universal Oil Products Company
8133 Aero Drive • San Diego, CA 92123
Telephone 714 • 278-7440
February 1,1 ,"±
uop
CELLULOSE ACETATE MEMBRANES AND THEIR SELECTIVITY
Membrane
Code
720
620
520
420
370
320
220
225
215
Test
Pressure
irpii
p. s.i.g.
700
600
GOO
600
600
500
150
150
150
Water Flux
at "P" and 25" C
(Mean)
G.F.S.D.
9
14
18
23
28
30
30-40
25-35
35-45
% Rejection of
0.45%
NaCl
98.5-99.5
97-98.5
95-97
89-93
80-oD
65-75
-
-
-
% Rejection of 6%
Sucrose
-
-
97-98
95-97
93-95
< 5
< 5
< 5
% Rejection of
2. 5% 6K P. E. G
Av. M.W. 6500
-
-
-
-
98-99
30-40
10-20
10
% Rejection of
2.5% 20K
P. E.G.
-
-
-
-
-
> 99
90-95
90-95
80-88
% Rejection of
2.5% P.V. P. 30K
Av. M.W. 40,000
-
-
-
-
-
95-98
95-98
90-95
Limiting Operational Conditions
Feed
Temperature
°C
15-38
38-50
15-38
38-50
15-38
38-50
15-38
38-50
15-25
25-35
15-25
25-35
15-40
40-55
15-40
40-55
15-35
35-45
Maximum
Pressure
p. s.i.g.
700
600
700
600
650
55(
600
500
600
500
600
500
250
150
250
150
200
150
Feed pH
Range
3-7
4-6
3-7
4-6
3-7
1-6
3-7
•!-6
3-7
4-6
3-7
4-6
2.5-8.0
3-7
1.5-11.0
2-10
2.5-8.0
3-7
-------
00
CO
H
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189
FOAM AND BUBBLE FRACTIONATION FOR REMOVAL OF
TRACE METAL IONS FROM WATER*
Ernesto Valdes-Krieg, C. Judson King (Speaker) and Hugo H. Sephton
Sea Water Conversion Laboratory and Department of Chemical
Engineering; University of California, Berkeley, CA 94720
Comparison of Processes
The selection of candidate separation processes for a
given application can often be based upon an identification of
certain salient features of the application, followed by
matching these features with factors which are known to
favor one separation process over others (King, 1971). The
most striking feature of the problem of trace metals removal
is the very high dilution of the substance to be removed.
Another feature is the ionic character of that substance.
Because of the high dilution one is led to seek processes
which remove the ions from the water into another phase or
stream, rather than processes which remove the water. This
places processes such as evaporation and freezing at a dis-
advantage. As we have seen in other papers presented at this
conference, these processes which concentrate the feed by
removing water should be considered for use only after the
metal ions have been brought up to a minimum level of concen-
tration by some other process. Thus processes removing the
water are suitable only for very concentrated feeds or as the
* Presented at Environmental Protection Agency Conference on
Traces of Metals in Water, Removal and Monitoring, Princeton,
N.J. November 16, 1973.
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190
last stage of a process which also uses other separations.
Among processes which remove the ions from the water,
ones will be favored which somehow derive an economic advantage
from the low level of concentration of the ions. Consider,
for example, an extraction process, such as the use of liquid
ion exchangers (Lewis, 1973; Agers and DeMent, 1972) to remove
the metal ions into an organic phase. In these processes
the distribution coefficient for the metal ion between phases
tends to be insensitive to the concentration of the ion, at
least beyond a sufficient level of dilution. Therefore the
amount of solvent circulation per unit of water feed required
to carry out the process does not change significantly as
the metal ion becomes more dilute. As a consequence the
process operating cost is relatively insensitive to the
metal ion concentration.
On the other hand, fixed-bed processes such as adsorption
and ion exchange with a solid resin do derive an advantage
from a lower concentration of the substance being removed.
The frequency of regeneration required for the solid phase and
amount of regenerant required are inversely proportional to
the feed concentration, since the solid phase will have a fixed
uptake capacity. Thus it is no accident that ion exchange
with solid resins has been one of the most successful processes
for the removal of heavy metal ions at high dilution from
water.
As a generalization of the processes removing the ions
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191
from the water, ion exchange and adsorption processes will be
relatively favored for very dilute feeds (say below 20 ppm),
whereas extraction with liquid ion exchangers is relatively
favored for higher-concentration feeds.
Another process capable of removing some metal ions -
e.g., copper — selectively from effluent waters is electro-
chemical reduction (Bennion and Newman, 1972; Posey, 1973). Here
the current requirement per unit of feed becomes less for lower
feed concentration; however competing reactions and the economics
associated with non-separative electrical potential loss
make this approach appear to be more promising for feed concen-
trations of intermediate level.
Foam and Bubble Fractionation
Foam fractionation and bubble fractionation are separation
processes based upon the selective adsorption of surface-active
species at a gas-liquid interface. By generating large quantities
of interface and removing it in an appropriate way one can
then effect a separation of species which are surface-active
from those that are not. Foam and bubble fractionation are
particularly well-suited for the removal of surface-active
substances present at high dilution, since the amount of surface
which can be generated per unit time is limited, since the
selectivity of adsorption of surface-active species is often
extremely high, and since surface-adsorption isotherms are
usually highly non-linear. These factors interact to give
a rapid increase in distribution coefficient as the level of
concentration becomes lower. Foam and bubble fractionation
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192
are capable of providing heavy metal ion removal through selec-
tive attraction of counter-ions to anionic surfactant molecules,
or by chelation or other specific interactions with surface-
active species. Consequently foam and bubble fractionation
are strong contenders for the removal of heavy metal ions
present at high dilution, say below 10 ppm. In some situations
they can be attractive for feeds of even higher concentration.
Foam fractionation for the removal and concentration of
ions from solution has received a considerable amount of atten-
tion (Rubin and Gaden, 1962; Dick and Talbot, 1971; Lemlich,
1972; Somasundaran, 1972), with the principal intended appli-
cation having been the decontamination of radioactive waste
waters. In some cases separations have also been demonstrated
on a pilot scale (Haas and Johnson, 1965; Arod, 1968).
Background of the Present Work
The work reported in this paper has been concerned with
the removal of surfactants and copper from the effluent blow-
down brine of evaporation seawater desalination plants. The
research has been carried out in connection with the develop-
ment of a process using surface-active agents to enhance the
heat-transfer coefficient and reduce the pressure drop in
vertical-tube, upflow evaporation (Sephton, 1971). Foam and
bubble fractionation are used to recover and remove the sur-
factant from the effluent brine. The removal of copper is
also of importance since copper enters the water through
corrosion and is at a level (about 0.5 ppm) high enough so
that it may be deleterious to marine life if the brine is
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193
returned directly to the ocean.
Foam and bubble fractionation experiments have been carried
out in apparatus of various sizes, including (1) a 2-1/2 in.
diameter, 6-ft high column, (2) a 1-ft square, 7-ft high column,
and (3) a 4-ft square x 9-ft high chamber. Data obtained with
the smaller column are reported here. Results for surfactant
removal with the 1-ft square column are reported elsewhere
(Valdes-Krieg, et al, -1974).
The surfactant employed has been Neodol 25-3A (Shell
Chemical Co.), which is the ammonium salt of a sulfated
primary alcohol with three ethylene oxide units and a C,, to C,_
alkyl group.
Equilibrium Data
As is noted in the paper by Lemlich at this Conference,
the surface enrichment of a surface-active species can be
estimated from surface-tension data by use of the Gibbs
adsorption isotherm, although some critical assumptions are
often necessary. Surface enrichments calculated in this way
from surface-tension data of Neodol in fresh water, normal
seawater (Bodega, California), doubly concentrated and triply
concentrated seawater are shown in Fig. 1. The surface excess
concentration, yg/cm8, is normalized by the bubble diameter,
cm, with the bubble diameter assumed to be 0.1 cm. Equilibrium
data are presented in this way since it is the ratio r/d which
is found from experimental measurements where a foanv fraction
is collected and the volumetric air flow rate is known. Since
the surface-to-volume ratio for spherical bubbles is 6/d, the
-------
Normalized Surface Concentration, F/d (/ig/cnrf
Fig.
1. Equilibrium Surface Enrichments for
Neodol Calculated from Surface-Tension D
ata.
-------
195
value of T/d may be thought of as one-sixth of the surfactant
content of the foam per unit volume of air; however the liquid
entrained with the foam is also important and sometimes
dominant. The rate of change of surface concentration with
increasing bulk concentration of the surfactant becomes less
as the bulk concentration increases and the surface becomes
more saturated, resulting in lower equilibrium distribution
coefficients between surface and bulk at higher surfactant
concentrations. Note also that the surface concentration is
greater and levels off at a lower bulk surfactant content in
seawater than in fresh water. This is the result of the ionic
content displacing the surfactant towards the interface in
seawater.
It is preferable for metals removal to limit surfactant
concentrations to those where there is still an appreciable
slope in Fig. 1, so as to allow for easier removal of the
surfactant itself by foam fractionation. In the present work
low surfactant concentrations have also been of interest
since the most economical heat-transfer enhancement effect
in seawater evaporators is realized at surfactant concentra-
tions of about 10 ppm (Sephton/ 1973). This is substantially
lower than the sodium lauryl sulfate concentrations of 200 to
400 ppm which have been used by others for copper removal
(Dick and Talbot, 1971).
Figure 2 shows results of experimental measurements of
equilibrium surface enrichments for Neodol in a synthetic
triply-concentrated seawater brine (10.5 wt % NaCl). The
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196
UJ
0 2 4 6 8 10 12 14 16
NEODOL IN SOLUTION ,/\g/cm5
Fig. 2. Equilibrium Surface Concentration of Neodol (T/d)-
in 10.5% NaCl Solution.
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197
open points were determined by sampling the foam immediately
above and the liquid immediately below an interface between
a relatively non-draining foam region and a liquid-continuous
bubble region in a foam and bubble fractionation column of
the sort shown in Figure 3. The half-shaded points were
obtained by a technique using concurrent flow of liquid and
air (liquid feed near the bottom of the column), coupled with
measurement of the depletion of surfactant content of the
liquid along the direction of flow. This latter technique
is suitable for very low surfactant concentrations where
foam coalescence is substantial.
Figure 4 shows equilibrium surface enrichments of copper
ion in otherwise pure water containing 34.8 ppm of Neodol,
for which P/d for Neodol is 1.23 yg/cm3. These data were ob-
tained by means of the concurrent flow technique. It is
apparent that substantial surface enrichments of copper are
possible, even at this rather low surfactant content.
Foam Column Operation
Figure 3 shows a schematic of a column as used for combined
foam and bubble fractionation in most of this work. Air is
supplied through a porous fused-silica diffuser plate (lOym
nominal pore size, Filtros Co.) at the bottom, in the form
of fine bubbles. Although the liquid-continuous bottom
(bubble) section has frequently been considered as a well-
mixed single stage, it has been found in the present work
that substantial separations can be accomplished by operating
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198
FOAM
LIQUID
FEED
AIR
RAFFINATE
Fig. 3. Schematic of a Simple Column for Foam and Bubble Fractionation.
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199
r/d Neodol = 1.23
QO 2.0 40 6D 8.0 10.0
LIQUID CONCENTRATION (ppm)
Fig. 4. Equilibrium Surface Concentration of Copper
in Otherwise Pure Water containing 34.8 ppm Neodol,
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200
this section with minimal axial mixing. Hence the bubble
region occupies most of the column height. The foam section
is long enough to allow the removal of entrained liquid by
drainage from the upflowing foamate product.
The liquid feed is introduced to the bubble section rather
than the foam section. Putting the feed into the foam section
can provide higher mass-transfer rates and can in some
circumstances allow simultaneous multi-stage enrichment and
stripping of surfactants and metal ions. These beneficial
aspects can be offset by the difficulty of distributing the
feed in the foam so as not to cause foam breakage, by the
inherent instability of foams - especially with low surfactant
contents - which can result in channeling, and by the much
larger column diameters which would be required by the low
liquid drainage rates typical of most foams.
Simultaneous Removal of Surfactant and Copper
Figure 5 shows the % removal of both Neodol and copper
obtained from a feed of 10.5 wt % NaCl solution containing
24 ppm Neodol and 0.6 ppm copper in solution. The experiments
were carried out with a bubble section height of 5 ft and a
foam section height of 1 ft in the 2-1/2 in. diameter column,
with about 10% of the feed removed as foamate. The removal
is shown as a function of the volumetric air-to-water feed
ratio, which was varied by changing the feed flow while
holding the air flow constant at 1500 cm3/min. The surfactant
removal capability is proportional to the rate of interface
-------
O
5
UJ
a:
100
80
60
40
FEED
Neodol: 24 ppm
Copper:0.6 ppm
ro
o
(SIMULATED DESALINATION BRINE
Fig. 5. Effect of Volumetric Air-to-Water Flow on Simultaneous
Removal of Neodol and Copper from 10.5 wt % NaCl Solution.
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202
generation, which in turn is proportional to the air flow
rate. The removal increases as G/L increases since a lower
liquid flow results in a lesser volume of liquid treated by
a given surfactant removal capacity from the air flow. The
G/L ratios used here are lower than in most previous foam
and bubble fractionation applications.
Figure 6 shows removals of Neodol and copper at a fixed
G/L, but at varying feed concentrations of Neodol. The results
differ from the corresponding point in Figure 5 because of
a different column overhead design resulting in different
foam breakage characteristics. Surfactant recoveries are
high as long as the surfactant concentration is high enough
to yield a stable foam to carry it out the top of the column.
Copper recoveries increase with increasing feed surfactant
concentrations, as would be expected since the capacity of the
surface for copper should increase as the surfactant content
increases.
It is important to note from these results that, although
the copper recoveries are substantially below 100%, the selec-
tivity of the surface for copper ion is extremely high. Sodium
is present in the solution at a concentration of 10s ppm,
and yet is not appreciably enriched in the surface phase.
Figure 7 shows the results of axial sampling of the bubble
section along its height during a run. There is a very
considerable gradient in Neodol liquid-phase concentration along
the column, which is a major contributor to the very high
recoveries of Neodol shown in Figures 5 and 6. This gradient
is the natural result of stripping of the Neodol from the
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203
100
I
UJ
or
50
i
I
_ I
I
I
I
_ I
I
NEODOL
G/L = 1.8 8
Copper in Feed: 0.6 ppm
IQ 20 30
NEODOL CONCENTRATION,ppm
Fig. 6. Simultaneous Effect of Feed Neodol Concentration
on Removal of Neodol and Copper from 10.5 wt %
NaCl Solution.
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204
II
10
8
Q.
•~> 7
6
OL
H
Id
O
Z
O
O
O
Id
2 _
I _
00
G/L = 1.88
Neodol in Fttd> 19.6 ppm
1.0 2LD 3.0 4.0
HEIGHT ABOVE BOTTOM (ft.)
Fig. 7. Axial Concentration Profile for Neodol in Bubble Section;
10.5 wt % NaCl Solution.
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205
downflowing liquid by the rising gas bubbles, and exists to
the extent that it is not eliminated by large-scale bulk
motions causing axial mixing within the bubble section. Note
also that the liquid surfactant concentration at the feed
location is substantially less than the concentration in the
feed itself. This is a result of partial axial mixing, which di-
lutes the feed with bulk liquid brought up from lower in the column.
An axial concentration profile for copper obtained in the
same run is shown in Figure 8. Again there is a substantial
drop in concentration between the feed copper content (dark
circle) and the concentration in the liquid phase at the feed
location. This is again indicative of dilution of the feed
by axial mixing with liquid from lower in the column. It is
also striking to note that the copper concentration profile
does not show the same gradient evidenced by the Neodol
concentration; in fact the copper profile is essentially flat.
Whereas much of the depletion of the water raffinate in Neodol
is accomplished by bubble fractionation in the liquid section,
the copper removal is accomplished almost solely by the en-
richment of copper into the foam occurring at the interface
between the bubble section and the foam section.
This different axial profile for copper and the lower %
removals for copper than for Neodol are accounted for by the
depletion of Neodol in the liquid phase proceeding down the
column. Lower Neodol concentrations give a lower equilibrium
distribution coefficient for the copper between surface and
bulk. The gradient in copper concentration, to the extent that
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206
0.8
~ 0.6
a
m
o
o
o
K
kl
a.
a
o
0.2
I
Q/L* 1.88
Ntodol In FMd:l9.6 Ppm
I I
0.0
1.0 2.0 3.0 4.0
HEIGHT ABOVE BOTTOM (ft.)
5.0
Fig. 8. Axial Concentration Profile for Copper in Bubble
Section; Simultaneous Removal of Neodol and Copper;
10.5 wt % NaCl Solution.
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207
it is present in Figure 8, occurs in that region of the column
where the Neodol content is still relatively high. Lower
in the bubble section the copper distribution coefficient
must be less, so that the stripping factor, 6 TG/Ld, is
reduced. This precludes effective stripping of the copper
because of the limited capacity of the upflowing surface.
Improved Method for Removing Copper
Surfactant concentrations throughout the bubble section
can be increased by feeding a surfactant-rich stream near
the bottom of the column. Then, even if the surfactant is
readily removed into the foam, there must be a significant
surfactant concentration all along the liquid because of the
necessity of an upward surfactant flux at all levels of the
column.
The results of such an operation are shown in Figure 9.
Over 99% of the copper is removed from a feed containing 1.0
ppm copper in the form of CuSOj in an otherwise pure water
stream. Neodol is fed both at the lower feed and the main
feed. A larger (4-1/2 in) diameter section is used as a
portion of the foam section to allow adequate foam drainage
at the high surfactant content. It is anticipated that the
process could work as well without the Neodol being present in
the upper feed, and that good copper recoveries could be obtained
with even smaller amounts of surfactant being present. Even
in the operation shown the surfactant concentrations are sub-
stantially lower than those which have been used elsewhere for
metals removal by foam fractionation, with the feed entering a
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208
FOAM
LIQUID FEED
2.20 gpm/ft
41.6 ppm Neodol
1.0 ppm Copptr
CONCENTRATED
SURFACTANT
0.063
1000
0
gpm/ft
ppm Neodol
ppm Copper
AIR
1.68 sofm/ft
0.35 gpm/ft2
248 ppm Neodol
6.10 ppm Copper
RAFFINATE
1.91 gpm/ft2
40.0 ppm Neodol
•"0,01 ppm Copper
Fig. 9. Improved Copper Removal
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209
foam section.
The effluent water from a process operating as shown in
Figure 9 can be sent to a second foam (bubble) column for sur-
factant removal and recovery for recycle to the first column.
Alternatively, this step can be carried in an added lower section
of the main column. The feasibility of high surfactant removals
on a large scale is shown and discussed elsewhere (Valdes-Krieg,
et al, 1974).
Acknowledgement
This work was carried out under Grant No. 14-30-2919 from
the Office of Saline Water, U. S. Dept. of the Interior. The
authors are grateful for the encouragement of Dr. Sidney Johnson
of OSW and Dr. Alan D. K. Laird of the Sea Water Conversion
Laboratory, University of California, and to Carl Freel for
technical assistance. One of the authors (E. V-K.) was supported
in part by the Consejo Nacional de Ciencia y Tecnologia of
Mexico.
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210
References
Agers, D. W. 6 DeMent, E. R., "The Evaluation of New LIX Rea-
gents for the Extraction of Copper and Suggestions for the
Design of Commercial Mixer - Settler Plants", Paper A72-87,
The Metallurgical Society of AIME, 1972.
Arod, J., "Separation of Surfactant and Metal Ions by Foaming",
in R. Lemlich, ed., Adsprptiye Bubble Separation Techniques,
Academic Press/ New York, 1972.
Bennion, D. N. & Newman, J. S., University of California,
Los Angeles and Berkeley, personal communication (1972).
Dick, W. L. 6 P. D. Talbot, Ind., Eng. Chem. Fundamentals, 10,
309 (1971).
Haas, P. A. & H. F. Johnson, AIChE Jour., U, 319 (1965).
King, C. J., Separation Processes, Ch. 14, McGraw-Hill Publ.
pa
*7
Co., New York, 19717
Lemlich, R. "Principles of Foam Fractionation", in R. Lemlich,
ed., Adsorotive Bubble Separation Techniques, Academic Press,
New York, 1972.
Lewis, C. J., "Liquid Ion Exchange in Hydrometallurgy", in
Recent Developments in Separation Science, N.N.Li, ed.,
Chemical Rubber Publ. Co., Cleveland, Ohio, 1973.
Posey, F. A., "Electrolytic Demonstration Unit for Copper Re-
moval from Distillation Plant Slowdown", ORNL-TM-4112, Oak
Ridge Nat'l. Lab., Oak Ridge, Tenn., March 1973.
Rubin, E. & E. L. Gaden, "Foam Separation", in H, M. Schoen,
ed., New Chemical Engineering Separation Techniques, Inter-
science, New York, 1962.
Sephton, H. H., "Interface Enhancement for Vertical Tube
Evaporators", ASME Publ. No. 71-HT-38, 1971; "Interface
Enhancement for Vertical Tube Evaporation of Seawater",
Proc. Fourth Int'l. Symp. on Fresh Water from the Sea,
Vol 1, 471-480, Heidelberg, 1973.
Somasundaran, P., "Foam Separation Methods", Separation &
Purification Methods, 1, (1), 117-198, 19771
Valdes-Krieg, E., C. J. King & H. H. Sephton, paper submitted
to International Journal of Desalination, 1974.
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211
THE ABSORPTIVE BUBBLE SEPARATION TECHNIQUES
(The "Adsubble" Techniques)
Foam Fractional;ion and Allied Processes
by
Robert Lemlich
Department of Chemical and Nuclear Engineering
University of Cincinnati
Cincinnati, Ohio 45221
Proceedings of the Conference on
Traces of Heavy Metals in Water: Removal Processes and Monitoring
held at the
Center for Environmental Studies
School of Engineering and Applied Science
Princeton University
Princeton, New Jersey
November 15-16, 1973
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212
INTRODUCTION
The adsorptive bubble separation techniques (which are often abbre-
viated as the adsubble techniques) are processes for partially separating
the constituents of solutions or suspensions through the adsorption or
attachment of these constituents at the surfaces of ascending bubbles
(Lemlich, 1966a, 1973). These processes remove solute or particulates
from the liquid, not vice versa as in,say, evaporation. Furthermore,
the selectivity of removal is often quite high and remains undiminished
as the removal proceeds. Therefore, some of these processes would seem
to be worth considering as possible candidates for the economical re-
moval of trace heavy metals from water. Accordingly, this paper pre-
sents an overview of various adsubble methods, with an emphasis on the
author's own work. A compilation of recent publications that report
the separation of heavy metals through the use of the adsubble methods
is also presented. For further information regarding the adsubble
methods, the reader is referred to a recent comprehensive book (Lemlich,
1972a).
MICROPARTICULATE FOAM PROCESSES
Foam fractionation is a technique for partially separating dis-
solved (or sometimes colloidal) material by adsorption at the surfaces
of bubbles that ascend through the solution to form a foam which is
then removed overhead (Karger, et al., 1967; Lemlich, 1968a/69, 1972b).
If the material to be removed (termed the "colligend") is itself sur-
face inactive, it may still be foamed off by adding a suitable sur-
face active "collector" that will combine with it. This union may
occur by counterionic attraction, formation of a complex, or otherwise.
The colligend-collector product is termed the "sublate". If the colli-
gend is ionic, the technique is sometimes called ion flotation (Sebba,
1962). However, when ion flotation gives rise to an insoluble sublate
or scum, it belongs more properly in the next section under macropartic-
ulate foam processes (Karger, et al., 1967, Pinfold, 1972a).
Foam fractionation is a partition process, at least for dissolved
material. The separation attainable in the simple mode [equilibrium
operation] is readily calculated from the solute surface excess which
is approximately the concentration of material at the bubble surface
(Brunner and Lemlich, 1963). The separation can be cascaded to give
better results by employing a countercurrent mode such as stripping op-
eration [which involves liquid entering the rising foam], enriching op-
eration [which involves collapsed foam — termed "foamate" - trickling
down as reflux through the rising foam], or combined operation [which
is stripping operation plus enriching operation] (Leonard and Lemlich,
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213
1965a; Fanlo and Lemlich, 1965). These modes are illustrated in Figure 1.
External reflux is made available by the return of externally mechanically
collapsed foam (Lemlich and Lavi, 1961). Induced internal reflux results
from instability within the rising column of foam (Lemlich, 1968b, 1972c) .
For the countercurrent modes, theoretical stages or transfer units
can be computed in terms of a downflowing stream of interstitial liquid
and an upflowing stream of interstitial liquid plus bubble surface
(Lemlich, 1968b). Foams of high liquid content are undesirable in that
they permit excessive channeling and disruption of the countercurrent
pattern of flow. HTU values of a few cm have been reported under good
conditions (Hastings, 1967; Jashnani and Lemlich, 1973). The rate of
foam overflow can be estimated from a theory for interstitial drainage
through the matrix of randomly oriented veins [called Plateau borders]
of noncircular cross section in a foam of low liquid content (Leonard
and Lemlich, 1965a,b; Fanlo and Lemlich, 1965; Shin and Lemlich, 1967,
1971; Lemlich, 1968c, Jashnani and Lemlich, in press). This drainage
is subject to both a liquid viscosity within the veins and a surface
viscosity which is the resistance to shear at their surfaces. The ef-
fective surface viscosity of some common surfactants in water at room
temperature is of the order of 10-1+ dyne sec/cm.
MACROPARTICULATE FOAM PROCESSES
If, through the addition of a precipitating agent, the colligend
is first made to precipitate, and then a collector is added so that
the precipitate can be foamed off, the process is termed precipitate
flotation of the first kind (Pinfold, 1972b) However, if there is no
separate precipitating agent and the collector is surface inactive yet
forms with the colligend a precipitate with a hydrophobic surface (so
as to make the precipitate surface active and thus removable by bubbl-
ing) , then the process is called precipitate flotation of the second
kind (Pinfold, 1972b). Precipitate flotation generally requires less
surfactant than does foam fractionation; no more than the surface of
the precipitate need be coated in the first kind, and no surfactant at
all is required in the second kind. But of course a precipitating a-
gent is required. Absorbing colloid flotation is the "piggy back" pro-
cess whereby a solute is adsorbed on a colloid which is then foamed
off (Karger, et al., 1967), perhaps with the aid of a collector.
All of the foregoing should be distinguished from the process of ore
flotation which is used in the mineral industries to separate one kind
of already solid macroparticle from another via selective wetting and
bubbling (Gaudin, 1957; Fuerstenau and Healy, 1972). Flotation is also
employed to collect macropartides on a less selective basis, as in
sewage treatment(Jenkins, et al., 1972).
-------
FOAM OVERFLOW
ELEVATED FEED
POOL FEED
^
FOAM
BREAKER
FOAM
LIQUID
T
GAS
_ REFLUX
COLLAPSED
FOAM
OVERHEAD PRODUCT
BOTTOM PRODUCT
Figure 1. Continuous foam fractionation: The solid lines show operation
in the simple mode. For the stripping mode, the pool feed is
replaced by an elevated feed [shown dashed] into the foam.
For the enriching mode some collapsed foam is returned as re-
flux [shown dashed] to re-enter near the top of the column.
For the combined mode, both stripping and enriching are employed.
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215
FOAMLESS PROCESSES
Even if no foam forms, bubbling may still produce a concentration
gradient within an elongated vertical pool by virtue of the carryup of
surface active material on the bubble surfaces (Dorman and Lemlich,
1965, Lemlich, 1972d). The gradient represents a partial separation:
stripping at the bottom of the pool where adsorption first takes place,
and enriching at the top where the escaping bubbles deposit their carry-
up. The process is termed bubble fractionation (Lemlich, 1964). It
can be employed by itself, or it can be used in conjunction with foam
fractionation as illustrated in Figure 2. Bubble fractionation can
raise a concentration up to the foaming threshold.
Bubble fractionation can be analyzed in a simplified manner in
terms of the adsorbed carryup which promotes the concentration gradient,
and the dispersion in the liquid which diminishes the gradient (Lemlich,
1966b/67) . A more detailed analysis takes into consideration the vari-
ation in the rate of adsorption around the leading surface of the bubble
and the behavior of the turbulently discharging wake (Cannon and Lemlich,
1972). Either approach shows that the degree of separation increases
as the pool is vertically elongated, and (unfortunately) decreases as
the pool is widened (Harper and Lemlich, 1966; Shah and Lemlich, 1970).
If a layer of some immiscible liquid is placed on top of the main
liquid to trap the adsorbed carryup, the process is called solvent sub-
lation (Sebba, 1962; Karger , 1972 ). In general, the degree of en-
trapment exceeds the equilibrium solubility in the upper liquid.
There is theoretically no lower concentration limit for the oper-
ability of either solvent sublation or bubble fractionation.
CAUTION AND COMMENT
The bursting bubbles from any adsubble process may throw off an
aerosol of extremely fine droplets that can carry appreciable quanti-
ties of adsorbed material. With certain systems, this material can be
noxious, toxic, or even pathogenic. So, to avoid converting a problem
in water pollution into a problem in air pollution, it may be necessary
to remove these airborne particles by some suitable means. However, a
simpler approach might be just to recycle the air (or other gas) back
to the blower for rebubbling through the liquid pool.
Recycling the gas would also allow one to incorporate a suitable
organic vapor into the gas stream without loosing any of this vapor,
except by slight solution in the liquid or through other minor losses.
-------
FEED
1
GAS
LIQUID
T
GAS
(a)
RICH
LIQUID
LEAN
LIQUID
FEED
LIQUID
LEAN
LIQUID
RICH
FOAM
ro
Figure 2. Continuous bubble fractionation: (a) alone; (b) in conjunction with foam fractionation.
-------
217
The presence of certain such vapors reportedly increases the selectivity
of adsorption from the liquid (Maas, 1969, 1970).
It is interesting to note that the production of very fine air-
borne droplets with material preferentially adsorbed at their surfaces
seems to occur on a large scale at the surface of the sea in the pro-
cess of forming the marine aerosol. The phenomenon itself, as well as
its effect on the atmosphere and elsewhere, is a subject of continuing
study by a number of investigators. The interested reader might con-
sult the proceedings of a recent conference on sea-air chemistry which
are published in volume 77, issue 27, of the J. Geophs. Res. (Oceans and
Atomospheres), 1972. [A contribution of a general nature by the present
author is also included (Lemlich, 1972f)]. Other related papers appear
in later issues of the said journal.
SYSTEMS SEPARATED
A wide variety of substances, including ions of heavy metals, have
been separated by means of the adsubble techniques. For a compilation
of recently reported separations, the reader is referred to Lemlich
(1972e). Earlier separations are compiled in Lemlich (196£d, 1972a),
and in Karger and DeVivo (1968) . Still earlier work is listed in Rubin
and Gaden (1962).
The remainder of the present paper covers reports of heavy-metal
separations that have chiefly appeared quite recently. These are not
included in any of the aforementioned compilations.
These recent reports are listed b'elow in alphabetical order by
the chemical symbols of the heavy metals involved, with some miscellany
at the end. As a further guide to the reader, the citations include
the complete titles and, in certain cases, some additional information
in brackets.
Ag - Charewicz and Walkowiak (1972), Grieves and Bhattacharyya (1972).
**
Au - Charewicz (1973a), Charewicz and NiemLec (1969b), Charewicz and
Walkowiak (1972).
Cd - Huang (1973?), Shimoiizaka, et al. (1970, 1972a), Takahashi, et
al. (1971).
Ce - Kepak and Kriva (1972), Pustovalov and Pushkarev (1972), Robert-
son and Vermeulen (1969).
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218
Co - Charewicz and Walkowiak (1972), Jurkiewicz and Waksmundzki (1973),
Waksmundzki and Jurkiewicz (1973).
Cr - Grieves and Bhattacharyya (1972), Grieves, et al. (1973).
Cu - Huang (1973?), Kim and Zeitlin (1972), Pearson and Shirley
(1973), Takahashi, et al. (1972).
Fe - Horizons (1971), Shimoiizaka, et al. (1972b).
Hg - Nanjo, et al. (1971, 1972).
Mn - Horizons (1971).
Mo - Charewicz (1973a,b), Charewicz and Walkowiak (1972).
Nd - Robertson and Vermeulen (1969) .
Ni - Charewicz and Walkowiak (1972) , Jurkiewicz and Waksmundski
(1973), Pearson and Shirley (1973),
Waksmundski and Jurkiewicz (1973), Zhidrova, et al. (1972).
Pb - Huang (1973?)
Pd - Charewicz and Walkowiak (1972) , Walkowiak and Bartecki (1973a).
Pm - Kepak and Kriva (1972).
Pt - Charewicz and Walkowiak (1972) , Walkowiak and Bartecki (1973b).
Re - Charewicz (1973a,b), Charewicz and Niemiec(1969a), Charewicz
and Walkowiak (1972).
Ru - Kepak and Kriva (1970).
Sm - Robertson and Vermeulen (1969) .
U - Shakir (1973a,b).
V - Charewicz (1972).
Y - Pustovalov and Pushkarev (1972).
Zn - Kim and Zeitlin (1972).
Radioactive - Koyanaka and Tsutsui (1970).
Various - Gibb (1970), Kuz'kin, et al. (1971), Matsuzaki and Zeitlin
(1973), Skrylev and Amanov (1972, 1973), Pushkarev, et al. (1969).
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219
LITERATURE CITED
(with titles*)
Brunner, C.A., and Lemlich, R., Foam fractionation: Standard separator
and refluxing column, Ind. Eng. Chem. Fundam. 2_, 297-300 (1963).
Cannon, K.D., and Lemlich, R., A theoretical study of bubble fractiona-
tion, Chem. Eng. Prog. Symp. Ser. £8(124), 180-184 (1972).
Charewicz, W., Ion flotation studies on vanadium (V) solutions [to
determine the isoelectric point for V03 ^ V02 ], Roczniki Chemii,
Ann. Soc. Chim. Polonorum 46_, 1979-1986 (1972).
Charewicz, W., "Flotation of Anions using Cationic Surfactants" [in
particular, using cetyldiethylammonium hydrochloride to remove
molybdates, perrhenates, and chloroaurates,in the presence of Cl ,
N03, and S0f;~ ], Communication no. 11, Institute of Inorganic
Chemistry and Metallurgy of Rare Elements, Technical University
of Wroclaw, Wroclaw, Poland (1973a).
Charewicz, W., Flotation of phosphorous (V), molybdenum (VI), and
rhenium (VII) oxyanions [with surfactant of the (R)ttN+ type],
J. Appl. Chem. Biotechnol 23, in press (1973b).
Charewicz, W., and Niemiec, J., Flotation of anions using cationic sur-
factants: II. Flotation of perrhenates [with cetyldiethylammonium
hydrochloride] Nukleonika JL4, 607-617 (1969a).
Charewicz, W., and Niemiec, J., Flotation of anions using catonic sur-
factants: III. Flotation of chloroaurates [with cetyldiethylammoniuin
hydrochloride], Nukleonika 14, 799-806 (1969b).
Charewicz, W., and Walkowiak, W., Selective floatation of inorganic
ions [via various complexes], Separation Sci. 7_, 631-646 (1972).
Fanlo, S. and Lemlich, R., Predicting the performance of foam fraction-
ation columns, A.I.Ch.E. - I. Chem. E. (London) Symp. Ser. 9_,
75-78, 85-86 (1965).
Fuerstenau, D.W., and Healy, T.W., Principles of mineral flotation,
pp. 91-131, in "Adsorptive Bubble Separation Techniques", Lemlich,
R., ed., Academic Press, New York (1972).
Gaudin, A.M. "Flotation," 2nd edition, McGraw-Hill, New York (1957).
Gibb, T.R.P., Jr., "Enrichment of Metals in Sea Water by Foam Production"
[without the need for additives], Report on N.S.F. grant GP-18623,
Tufts University, Medford, Massachusetts (1970).
Grieves, R.B., and Bhattacharyya, D., Foam separation of anions:
Stoichiometry [using a cationic collector] , Separation Sci. 1_, 115-
129 (1972).
Grieves, R.B., Bhattacharyya, D., and Ghosal, J.K., Ion flotation of
chromium (VI) species: pH, ionic strength, mixing time, and tem-
perature [with ethylhexadecyldimethylammonium bromide as collector],
Separation Sci. 8., 501-510 (1973).
*The words in brackets are not part of the titles but are simply
furnished by the present writer as additional information.
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220
Harper, D.O., and Lemlich, R., Direct visual observation of nonfearning
adsorptive bubble fractionation, A.I.Ch.E.J. 12, 1220-1221 (1966).
Hastings, K., "A Study of Continuous Foam Fractionation," Ph.D. disser-
tation, Michigan State University, East Lansing, Mich. (1967).
Horizons, Inc., "Foam Separation of Acid Mine Drainage" [reportedly
economically unfeasible], Report on project 14010 FUI, E.P.A. con-
tract 14-12-876, Horizons, Inc., Cleveland, Ohio (1971).
Huang, R.C.H., "The removal of lead, cadmium and copper ions from
aqueous solutions using foam fractionation" [with sodium dodecylbenzene
sulfonate as collector], Ph.D. dissertation, University of Ottawa,
Ottawa, Canada (1973?).
Jashnani, I.L., and Lemlich, R., Transfer units in foam fractionation,
Ind. Eng. Chem. Proc. Design Devel. 12_, 312-321 (1973).
Jashnani, I.L., and Lemlich, R., Foam drainage, surface viscosity, and
bubble size bias, J. Colloid Interface Sci., in press.
Jenkins, D., Scherfig, J., and Eckhoff, D.W., Application of adsorptive
bubble separation techniques to wastewater treatment, pp. 219-242,
in "Adsorptive Bubble Separation Techniques," Lemlich, R., ed.,
Academic Press, New York (1972).
Jurkiewicz, K., and Waksmundzki, A., Flotation of rhodanate complexes
of cobalt and nickel ions, Roczn. Chem. 47_, 1457-1465 (1973).
Karger, B.L., Solvent sublation, pp. 145-156 in "Adsorptive Bubble Sepa-
ration Techniques," Lemlich, R., ed., Academic Press, New York (1972).
Karger, B.L., and DeVivo, General survey of adsorptive bubble separation
processes, Separation Sci. 3., 393-424 (1968).
Karger, B.L. Grieves, R.B., Lemlich, R., Rubin, A.J., and Sebba, F.,
Nomenclature recommendations for adsorptive bubble separation
methods, Sep. Sci. _2, 401-404 (1967).
Kepak, F., and Kriva, J., The foam separation of radioruthenium [with
gelatin and dodecylamine], Separation Sci. _5> 385-391 (1970).
Kepak, F., and Kriva, J., Ionic and colloidal flotation of lt+l+Ce (III)
and ltt7Pm (III) [with anionic, cationic, or nonionic collectors]
Separation Sci. T_, 433-440 (1972).
Kim, Y.S., and Zeitlin, H., The separation of zinc and copper from sea-
water by adsorption colloid flotation [utilizing ferric hydroxide
and dodecylamine], Separation Sci. ]_, 1-12 (1972).
Koyanaka, Y., and Tsutsui, T., Treatment of the radioactive sludge pro-
duced by precipitate flotation [in order to dewater the sludge; see
Nuclear Sci. Abstracts, 25, 2081 (1971) for abstract no. 21153],
Annu. Report Res. Reactor Inst., Kyoto University 3., 147-152 (1970).
Kuz'kin, S.F., and Gholman, A.M., "Flotatsya lonov i Molyekow" [A book
in Russian entitled "Flotation of Ions and Molecules"], Atomizdat,
Moscow (1971)
Lemlich, R., Progress Report, Research Grant WP-00161, submitted to U.S.
Public Health Service (1964).
Lemlich, R. , Adsubble methods, Chem. Eng. T5_ (21), 7 (1966a) .
Lemlich, R., A theoretical approach to non-foaming adsorptive bubble
fractionation, A.I.Ch.E. 12_, 802-804 (1966b) . Errata in 13., 1017
(1967).
Lemlich, R., Foam Fractionation, Chem. Eng. 75 (27), 95-102 (1968a).
Errata in 76(6), 5 (1969).
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221
Lemlich, R., Principles of foam fractionation, pp. 1-56 in "Progress
in Separation and Purification," vol. 1, Perry, E.S., ed., Wiley
(Interscience), New York (1968b).
Lemlich, R., Foam overflow rate: Comparison of theory with experiment,
Chem. Eng. Sci. 23, 932-933 (1968c) .
Lemlich, R., Adsorptive bubble separation methods, Ind. Eng. Chem. 60
(10), 16-29 (1968d).
Lemlich, R., ed., "Adsorptive Bubble Separation Technique", Academic
Press, New York (1972a).
Lemlich, R., Principles of foam fractionation and drainage, pp. 33-51
in "Adsorptive Bubble Separation Techniques," Lemlich, R., ed.,
Academic Press, New York (1972b) .
Lemlich, R., Some physical aspects of foam, J. Cosmet. Chem. 23,
299-311 (1972c).
Lemlich, R., Bubble fractionation, pp. 133-143 in "Adsorptive Bubble
Separation Techniques," Lemlich, R., ed., Academic Press, New York
(1972d).
Lemlich, R., Adsubble methods, pp. 113-127 in "Recent Developments in
Separation Science, " vol. 1, Li, N.N., ed., Chemical Rubber(CRC)
Cleveland (1972e).
Lemlich, R., Adsubble processes: Foam fractionation and bubble frac-
tionation, J. Geophys. Res. (Oceans and Atmospheres) 77, 5204-
5210 (1972f).
Lemlich, R., Adsorptive - bubble separation methods, section 17, pp.
29-34 in "Chemical Engineers Handbook," 5th edition, Perry, R.H.,
and Chilton, C.H., eds., McGraw-Hill, New York (1973).
Lemlich, R., and Lavi, E., Foam fractionation with reflux, Science 134,
191 (1961).
Leonard, R.A., and Lemlich, R., A study of interstitial liquid flow in
foam. Part I: Theoretical model, and application to foam frac-
tionation, A.I.Ch.E.J. J_l, 18-25 (1965a).
Leonard, R.A., and Lemlich, R. , A study of interstitial liquid flow in
foam. Part II: Experimental verification and observation. A.I.
Ch.E.J. 11, 25-29 (1965b).
Maas, K., A new type of adsubble methods: Booster bubble fractionation,
hastened and improved bubble fractionatian of low-foaming solu-
tions, Separation Sci. 4-, 457-465 (1969).
Maas, K. , Kugelschaum - Chromatographie [in German, with English ab-
stract] , Fette Seifen Anstrichm. 72, 1032-1037 (1970).
Matsuzaki, C., and Zeitlin, H., The separation of collectors used as
coprecipitants of trace elements in seawater by adsorption colloid
flotation [a screening of 9 surfactants and 6 collectors (copre-
cipitants)] , Separation Sci. 8^, 185-192 (1973).
Nanjo, M., Usui, S., and Shimoiizaka, J., The removal of mercury from
waste water by xanthates and aerofloats, first report: Fundamental
studies on the reactions of mercury ion with xanthates and aero-
floats [in Japanese, with English summary], J. Min. Assoc. Jap. 87,
755-760 (1971).
Nanjo, M. , Usui, S., and Shimoiizaka, J., Study on the removal of mer-
cury from waste water by xanthate and aerofloat, second report:
Removal of Hg2 by flotation method [in Japanese, with English
summary], J. Min. Assoc. Jap. 88, 95-98 (1972).
Pearson, D., and Shirley, J.M., Precipitate floation in the treatment
of metal-bearing effluents [from metal-finishing operations], J.
Appl. Chem. Biotechnol _23_, 101-109 (1973).
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222
Pinfold, T.A., Ion flotation, pp. 53-73 in "Adsorptive Bubble Separation
Techniques," Lemlich, R., ed., Academic Press, New York (1972a).
Pinfold, T.A., Precipitate flotation, pp. 75-90 in "Adsorptive Bubble
Separation Techniques," Lemlich, R., ed., Academic Press, New York
(1972b).
Pushkarev, V.V., Egorov, Yu.V.,and Khrustalev, B.K., "Osvetlenie i
Dezaktivatsiya Stochnykh vod Pennoi Flotatsiei" [A book in Russian
entitled "Clarification and Deactivation of Waste Waters by Froth
Flotation," see Nuclear Sci. Abstracts, 25, 399 (1971) for ab-
stract no. 4141 in English] Atomizdat, Moscow (1969).
Pustovalov, N.N., and Pushkarev, V.V., Extraction of Y-91 and Ce-144
from aqueous solutions by foam flotation [in Russian], Atomnaya
Energia 32, 435 (1972).
Robertson, G. H., and Vermeulen, T., "Foam Fractionation of Rare-Earth
Elements" [via EDTA chelates and a cationic surfactant], Report
UCRL-19525 on A.E.G. contract W-7405-eng-48, Lawrence Radiation
Laboratory, University of California, Berkeley, California (1969).
Rubin, E., and Gaden, E.L., Jr., Foam separation, pp. 319-385 in "New
Chemical Engineering Separation Techniques," Schoen, E.M., ed.,
Wiley (Interscience), New York (1962).
Sebba, F., "Ion Flotation," American Elsevier, New York (1962).
Shah, G.N., and Lemlich, R., Separation of dyes in nonfearning adsorptive
bubble columns, Ind. Eng. Chem. Fundam. 9_, 350-355 (1970).
Shakir, K., Separation of U(VI) from carbonate solutions by ion flota-
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Shakir, K., Studies on low gas-flow rate foam separation of U(VI) from
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Shih, F.S., and Lemlich, R., A study of interstitial liquid flow in
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223
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REMOVAL OF HEAVY METALS BY CONVENTIONAL TREATMENT
by Gary S. Logsdon and James M. Symons, respectively, Research Sanitary Engineer
and Chief, Standards Attainment Branch, Water Supply Research Laboratory,
National Environmental Research Center, U.S. Environmental Protection Agency (EPA)
Cincinnati, Ohio.
INTRODUCTION
As a regulatory agency, the U. S. Environmental Protection Agency (EPA)
sets Drinking Water Standards for a number of substances in water, including
certain inorganic constituents (1). Another function of the Agency is to
determine the efficacy of present water treatment technology to meet these
Standards. Knowing whether methods now used by the water utility industry
can remove environmental levels of inorganic contaminants is necessary and
for substances not removed or reduced to the limit of the Standards, new
technology needs to be developed.
For these reasons, a program of research on the removal of trace inorganic
substances by water treatment processes has been underway for over 2 years
at the Water Supply Research Laboratory of EPA's National Environmental Research
Center in Cincinnati. Constituents in the proposed EPA Drinking Water Standards
that have been or are being studied are mercury, barium, arsenic, and selenium.
Among those considered for possible future study are lead, cadmium, and
chromium.
This paper summarizes the results obtained thus far: information on
removing methyl mercury, inorganic mercury, barium, selenate, selenite, arsenite,
and arsenate. Treatment processes studied in the laboratory are iron coagulation,
aluminum coagulation, lime softening, excess lime softening, and activated
carbon adsorption. Although no single treatment process has been found
effective for every contaminant studied, recommendations based upon the
laboratory results are made for the treatment processes likely to be most
effective for removing each of the contaminants.
REVIEW OF LITERATURE
A number of studies of trace element removal have been conducted. Barnett
et al. reported that barium concentration of 20 to 50 yg/1 were reduced very
little or not at all by alum coagulation at Denver's water plants (2). None
of the other elements studied at Denver has yet been investigated in the
program of studies reported herein.
Decontamination of radioactive waters was studied at Oak Ridge in the
early 1950's (3). In that work, removals of a mixture of 140Ba and 140La by
coagulation ranged from 1 to 84 percent and averaged from 44 to 59 percent.
Chemical doses (ferric sulfate, ferric chloride, or alum) ranged from 17 to
102 mg/1. Activated silica and lime or soda ash were also added. Ninety
percent removals could be obtained by lime softening. Ion exchange experiments
with greensand showed 96.3 percent removal of the barium-lanthanum mixture.
Presented at the Conference on Traces of Heavy Metals in Water: Removal Processes
and Monitoring, Princeton University, Princeton, New Jersey, Nov. 15-16, 1973.
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226
Actual chemical concentrations were not stated. Barium-140 has a specific
activity of 7.3x10 Ci/g, so it would be detectable in microgram or nanogram
per liter concentrations. Thus, there could have been much less than 1 mg/1
of barium in the water tested at Oak Ridge.
Cherkinskii et al. reported results of pilot plant studies of alum
coagulation, sedimentation, and filtration for removal of chemical
substances (4). Arsenic originally present at concentrations of 0.5 to 1.0 mg/1
was removed 50 to 60 percent. For concentrations of 2 to 10 mg/1 of
selenium, removals ranged from 50 to 70 percent. The final concentrations
reported in this work, however, exceeded the proposed USEPA Drinking Water
Standard of 0.1 mg/1 for arsenic and 0.01 mg/1 for selenium.
Removal of arsenic by coagulation was studied by Gulledge and O'Connor
at the University of Illinois (5). When the initial arsenate concentration
was 0.05 mg/1, removals in excess of 90 percent could be attained, depending
on coagulant dose and pH. Coagulation with ferric sulfate was more effective
than coagulation with alum at concentrations of up to 50 mg/1 for either
coagulant. Also at Illinois, Ebersole (6) found that mercuric chloride at
5.8 yg/1 or 50 pg/1 could be removed from water by alum or iron coagulation (5).
Removals generally ranged from 30 to 70 percent. Ferric sulfate was more
effective than was alum when both were added at a nominal dose of 20 mg/1.
Laboratory and field studies on arsenic removal from drinking water in
Taiwan were described by Shen (7). The arsenic content of some well waters
there was reported to be in the range of 0.6 to 2.0 mg/1. High arsenic
concentrations in drinking water had been associated with the occurrence of
black-foot disease, so Sh'en conducted a long and thorough investigation of
arsenic removal by coagulation.
Although arsenic could be removed by coagulation, treatment of the
groundwater with an oxidant was necessary to obtain a residual arsenic
concentration of 0.05 mg/1 or less. For example, with 1 mg/1 of arsenic in
the raw water, coagulation with 50 mg/1 of ferric chloride gave 90 percent
removal. Coagulation with the same dose of ferric chloride after oxidation
with 20 mg/1 of chlorine resulted in 98.7 percent arsenic removal. In this
case, the initial arsenic content was 0.8 mg/1; the residual arsenic was
0.01 mg/1; the chlorine residual was 1.0 mg/1. This example indicates that
the oxidation state of the arsenic may have an important bearing on its removal
by various unit processes.
Angino et al. (8) reported an arsenic removal of 87 percent (with the
reduction of a 3.1 mg/1 initial arsenic concentration to 0.4 yg/1) at the
Lawrence, Kansas, water plant, which uses the cold lime softening process.
In laboratory work, they'obtained 85 percent removal with cold lime softening
when the original arsenic content was 0.2 mg/1.
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227
Linstedt et al. (9) studied removal of cadmium, chrome, silver, and
selenium in advanced waste treatment processes. A small pilot plant was
employed to treat 105 gallon (398 1) batches of secondary sewage effluent.
They reported that lime coagulation to pH 11 removed only 16.2 percent
of the selenium added as selenite. The initial selenium concentration was
< 12.8 yg/1 in this study. On the other hand, about 35 percent of the
selenite was removed by activated carbon, and this was attributed to
interaction with organic matter, followed by carbon adsorption of the
organics. Finally, removal by cation and anion exchange exceeded 99 percent.
EXPERIMENTAL METHODS
Experiments described in this report were performed in the laboratory on
a jar-test apparatus. The methods used have been described by Logsdon and
Symons (10). Waters used in the work were raw Ohio River water;raw well water
from Glendale, Ohio; Cincinnati tap water; and a Midwestern groundwater
containing barium. Except for the barium-laden water, waters were dosed
with the contaminant to be studied, given 2 minutes of rapid mix after
addition of the treatment chemical and 20 minutes of slow mix for coagulation
tests, or they were given 3 minutes of rapid mix and 30 minutes of slow mix
for softening. One hour of settling was used for all tests. Settled jar-test
samples from barium, arsenic, and selenium experiments were centrifuged to
attain the final clarification normally accomplished by filtration in a water
works. In tests of mercury removal by coagulation, clarification was very
good (most samples below 1.0 turbidity unit (T.U.) and all below 5 T.U.) and
samples were not centrifuged. Clarification by centrifuge was used in soften:
tests involving mercury and in all powdered activated carbon adsorption tests.
Analyses were made for pH, turbidity, alkalinity, and in some cases, hardness,
as well as for contaminant concentration.
Two methods were used for metals analysis. An atomic absorption
spectrophotometer was used for analysis of nonradioactive contaminants in
significant portions of the mercury, barium, and arsenic work, and occasionall
in the selenium experiments. Methods have been described by Kopp et al. (11),
McFarren (12), and Caldwell et al. (13). In some experiments, radiotracers
were used along with stable carriers. The tracers were Hg as methyl mercur
and as mercuric nitrate, 'As as arsenate, ^^As as arsenite, ?%e as selenite
and selenate, and 133Ba as barium chloride. Radioactivity was measured using
a shielded Nal (Tl) crystal and a single channel analyzer. Samples having a
volume of 100 ml were counted in 1-quart, plastic-lined, disposable paper
containers. When radiotracers were used, the initial contaminant concentratio
was determined by adding the radioisotope, plus a known volume of stock carrie
solution, to the water being treated and calculating the initial metal
concentration. Removal percentage was the percentage of reduction of
radioactivity.
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To check the analytical work and to verify the removal of
contaminants by coagulation of softening, material balances were performed.
These, although desirable, were done only occasionally for experiments not
employing radiotracers because of the large amount of extra work involved-
Because analysis was so S1™ple, however, balances were done in almost all
raaiotracer tests. A balance involved calculation of the amount of contaminant
or radioactivity in the untreated water vs. the sum of contaminant or radioactivity
in settled water and in the recovered sludge.
Material balance results were considered acceptable for atomic absorption
data and very good for tracer experiments. In the latter, tracers balanced
to within - 3 percent in 63 percent of the tests and to within - 5 percent in
80 percent. Twenty-two of 365 experiments had divergences in excess of 10
percent. Results of the material balances confirmed the quality of the
analyses and demonstrated the fate of contaminants in the treatment processes.
RESULTS AND DISCUSSION
Mercury
The results of mercury removal studies of the Water Supply Research
Laboratory, presented in more detail elsewhere by Logsdon and Symons (10),
are summarized here. In coagulation experiments involving Ohio River water,
initial mercury concentration did not appear to be related to removal of either
methyl mercury or inorganic mercury in the 2 to 16 pg/1 concentration range.
Raw water turbidity was considerably more important, as can be seen in Figures
1 and 2. Because mercury also could be removed by removal of turbidity in the
absence of coagulants, adsorption on natural turbidity was thought to be an
important factor in mercury removal by coagulation. When coagulant doses were
selected to give good clarification in the treatment process, ferric sulfate
was more effective than alum -- often giving 50 percent removal of inorganic mercury.
Methyl mercury removals were low for ferric sulfate or alum, usually 30 percent
or less.
Mercury removal by lime softening was tested using Glendale, Ohio, well
water, with about 300 mg/1 hardness as calcium carbonate. Figure 3, which also
shows the relationship of pH and hardness, shows that softening the water to
pH 9.5 removed 30 to 40 percent of the inorganic mercury. This could be
increased to 50 percent by coagulating the softened water with ferric sulfate,
a step that might be done to remove the pinpoint calcium carbonate that failed
to settle. Excess lime softening to pH 10.6 to 11 resulted in formation of
a voluminous magnesium hydroxide floe, good clarification, and 70 percent
removal of inorganic mercury. Removal of methyl mercury by softening could not
be detected at either pH range.
Preliminary experiments with mercury removal by ion exchange have been
carried out. As much as 98 percent of the mercury added to distilled water as
CH 203HgCl and Hg(NO,)? and buffered to desired pH ranges could be removed
by decontamination columns containing IR 120 and IRA 458 (Rhom and Haas)*
ion exchange resins. These columns are normally used to remove excessive
radioactivity from solutions at the conclusion of experiments.
*Mention of commercial products does not imply endorsement by the U.S.
Environmental Protection Agency.
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100
80
60
20 TO 30 MG/L ALUM
O INORGANIC MERCURY
@ METHYL MERCURY
METHYL MERCURY
L_
3.0 10 30
TURBIDITY OF UNTREATED WATER, TU
100
Figure 1. Turbidity vs. Mercury Removal by Alum Coagulation.
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230
100
80
20 TO 30 MG/L FERRIC SULFATE
D INORGANIC MERCURY
H METHYL MERCURY
D
60
20
D
1,0
-s-
INORGANIC
MERCURY
3,0 10 30
TURBIDITY OF UNTREATED HATER, TU
100
Figure 2. Turbidity vs. Mercury Removal by Iron Coagulation.
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231
100
80
g 60
20
0
O INORGANIC MERCURY
9 METHYL MERCURY
9 10
pH OF TREATED WATER
•O-
11
250
200
150
100
50
0
Figure 3. Mercury Removal by Lime Softening.
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232
In another exploratory group of tests, the use of ActiveX (J.M. Huber Corp.)
heavy metal ion exchanger (a synthetic silica material) to remove mercury was
studied. This powdered-type material is added in much the same way as powdered
activated carbon. Preliminary results indicate inorganic mercury can be removed,
but efficiency for methyl mercury removal is poor. Inorganic mercury was
reduced from 10 to 5 yg/1 (50 percent) by 80 mg/1 of treatment chemical. Further
investigations are needed before firm statements can be made on this process.
Mercury removal by activated carbon was evaluated using granular carbon in
columns and powdered carbon in jar tests. Both methyl mercury and inorganic
mercury were adsorbed on the carbon. This process has been discussed by
Sigworth and Smith (14). In an experiment to learn how activated carbon
might improve mercury removal in a plant treating surface water, powdered
activated carbon was added to Ohio River water in various doses. The water
was stirred for 10 minutes and then 30 mg/1 alum was added. The results
(Figure 4) show that 40 percent of the inorganic mercury could be removed by
coagulation alone, and that increasing amounts of removal occurred as the activated
carbon dose increased. Because the activated carbon doses applied are much
larger than those normally used in water plants, the doses typically applied
for taste and odor control would not be very effective for mercury removal.
Nevertheless, for control of mercury during times of known pollution incidents,
high-dose activated carbon treatment should be effective.
Barium
Barium is not often found in waters in excess of the 1 mg/1 limit of the
existing and proposed Drinking Water Standards, possibly because of the low
solubility of barium sulfate. However, barium has been detected in certain
groundwaters in the Midwest in concentrations of up to 10 mg/1 (15,16). Samples
of groundwater containing barium were obtained from Illinois, so that treatment
experiments could be conducted. The sulfate concentrations in raw water
samples were less than 1 mg/1.
Coagulation with aluminum sulfate and also with ferric sulfate was expected
to remove barium effectively. The sulfate added in the treatment process was
expected to form insoluble barium sulfate because the treatment process would
cause the solubility product for this compound to be exceeded. The anticipated
results were not achieved, however (Figure 5)5 removal did not exceed 30
percent when the initial barium content was in the 7 to 8 mg/1 range.
A possible reason for poor barium removal was supersaturation of barium
sulfate. If a nonequilibrium condition existed, perhaps, with the passage of
time, the distribution of dissolved and precipitated barium would tend toward
equilibrium. A series of experiments was set up in which water was treated
with either 100 mg/1 of ferric sulfate or 100 mg/1 of alum'. Settled waters
were withdrawn and kept in test jars. At time intervals of 3, 6, 12, and 24
hours after the first treatment, a second stage of coagulation was carried out.
In the second treatment, the coagulant dose was 20 mg/1 of the same chemical
used for the first stage of the treatment.
*Mention of commercial products does not imply endorsement by the U.S.
Environmental Protection Agency.
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233
ca
UJ
o
UJ
100
80
60
20
O
10
INITIAL INORGANIC M-ERCURY CONCENTRATION = 9.3 /JG/L
30 MG/L ALUM
20 30 40 50 60
POWDERED ACTIVATED CARBON ADDED MG/L
70
Figure 4. Inorganic Mercury Removal by Powdered Activated Carbon
and Alum Coagulation.
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23k
EAR i UM 7 TO 8.6 MG/L
H 7.5 TO 8.0
20
§
UJ
CL-
BARIUM 7 TO-8.6 MG/L
H 7.5 TO 8.0
40 60 80
COAGULANT DOSE, MG/L
Figure S. Barium Removal by Coagulation.
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235
The results of barium removal by two-stage coagulation (Figure 6) give
strong support for the suggestion that barium sulfate was supersaturated after
the first coagulation. Better barium removals occurred when greater time was
allowed for the supersaturated barium sulfate to form solid crystals that could
be removed by the second coagulation step. This result does not give much
encouragement to the design engineer, however, because two-step coagulation would
be most costly than treatment in a single stage and providing sufficient
volume to store 24 hours of accumulated treated water between the two treatment
steps would add further capital expense.
Barium removal by lime softening was also studied, using water
containing 7 to 8 mg/1 of barium. As the pH of the treated water increased
above 9, barium removal increased (Figure 7) and barium removals exceeded 90
percent in the range from pH 10 to pH 11. Maximum removal was nearly 98 percent.
Precipitation of barium carbonate is the mechanism suggested for barium
removal by softening. On the basis of the raw water barium concentration and the
alkalinity, adding lime to convert bicarbonates to carbonates could cause the
solubility product for barium carbonate to be exceeded. The tendency for lime
softening to cause the barium carbonate solubility product to be exceeded would
be greater as the lime dose increased, and this would be reflected with
increasing barium removals as pH increased to the 10 to 11 range. Further
increases of pH, >11, resulted in a sharp decline in barium removal. This
may have been because barium carbonate was converted to the more soluble barium
hydroxide.
On the basis of laboratory results, the most effective conventional
water treatment process should be lime softening in a pH range of 10 to 11.
This process in the laboratory gave removals as high as 98 percent. It
should provide satisfactory treatment for waters containing up to 10 mg/1
of barium.
A limited amount of other data is available. Samples of groundwater
obtained before and after treatment in a full-scale ion exchange softening
plant showed barium reduction from 11.7 to 0.18 mg/1, a 98 percent removal.
The sodium concentration of the softened water was 125 mg/1. Because excess
sodium, as well as excess barium, can be a health hazard (17), some persons
whose doctors have prescribed a low-sodium diet find it necessary to drink
bottled water rather than sodium-cycle ion-exchange-softened water. Sodium-
cycle ion exchange for barium removal must be evaluated in this context. The
high reduction by ion exchange also agrees with Oak Ridge data.
Further results of laboratory jar tests with the ActiveX resin and
stable barium plus 133ga indicate that removals of 80 percent or more can be
obtained when initial barium concentrations are in the 3 to 10 mg/1 range
and the ActiveX dose is about 400 mg/1. Both the ion exchange and heavy
metal resin techniques merit further study. Finally, barium added as stable
carrier and radiotracer was not removed in jar tests with powdered
activated carbon. Thus, the most effective methods fcr control of barium would
be ion exchange and lime softening; then coagulation; with activated carbon,
the least effective.
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236
100
80
BARIUM CONCENTRATION BEFORE
START OF FIRST COAGULATIOM
WAS 7 TO 8.6 MG/L
O FERRIC SULFATE
D ALUM
„„ J 1 1
5
12 18
HOURS ELAPSED BETWEEN FIRST COAGULATION
AND SECOND COAGULATION
Figure 6. Effect of Time Interval on Barium Removal
by Two-Stage Coagulation.
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237
100
80
60
20
BARIUM REMOVED
HARDNESS
250
200
150
100
50
7 TO 8 HG/L BARIUM
10
PH OF TREATED WATER
11
12
Figure 7. Barium Removal by Lime Softening.
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238
Arsenic
Because Gulledge and O'Connor (4) thoroughly investigated the effect
of pH, the emphasis in this present research was on the effect of coagulant
dose on the removal of various concentrations of arsenic from surface waters
(Ohio River water). Results are shown in Figure 8 for coagulation experiments
in the pH range of 6.4 to 7.5. Initial arsenic concentrations varied from
0.10 to 20 mg/1.
The preferred type of treatment was related to initial arsenic concentrations,
Below 1.0 mg/1, coagulation with 30 mg/1 of either alum or ferric sulfate gave
removals of 90 percent or more and resulted in a water than would meet the
proposed Drinking Water Standards of 0.10 mg/1. When arsenicv concentrations
exceeded 1 mg/1, ferric sulfate began to perform better than alum in terms of
actual pounds of treatment chemical used. For instance, at 2 mg/1 arsenic^
removals with 30 mg/1 of ferric'sulfate and 100 mg/1 of alum were comparable.
At the highest arsenic^ levels studied, ferric sulfate was considerably more
effective than alum.
Treatment of Glendale, Ohio, well water to which arsenate had been added
was also investigated. The results (Figure 9) show that when initial arsenic
concentration was 0.4 mg/1, removals by lime softening to pH 9 to 10 ranged
from 40 to 70 percent and increased with pH. Better arsenic^ reductions were
obtained when lime softening was followed by iron coagulation as a secondary .
step. This agrees with the findings of Angino et al. (8).
In work done for this report, arsenic removals were best with excess
lime softening at pH 10.6 to 11.4. Data in Figure 8 indicate that 95 percent
removal could be accomplished with excess lime softening up to an arsenic
concentration of 12 mg/1. These results strongly suggest that municipal water
plants should be able to control arsenic present in the pentavalent state.
. . Ill
Arsenic
Studies on the behavior of arsenite (arsenic ) were also carried out.
Arsenic in groundwater would probably be trivalent, so Glendale well water was
ased. The tracer was ^As in the form of sodium arsenite. Radioactive decay
calculations were made to correct for the effects of the 1.1 day half-life.
To minimize the oxidation of arsenic to arsenic by dissolved oxygen
in the test water, nitrogen gas was bubbled through the test waters to purge
the, dissolved oxygen. Dissolved oxygen concentrations of the test water were
generally below 0.5 mg/1 when the arsenic*** was added. Dissolved oxygen
concentrations at the end of experiments were typically about 3 mg/1 when
this analysis was performed. Under such conditions, oxidation of arsenic111
to arsenic^ should not have occurred to a significant extent except perhaps at the
nigh pH values associated with excess lime softening (18).
Initially dramatic differences in arsenic removal were observed when
some waters spiked with arsenite were chlorinated before coagulation and
others were not. To investigate the effect of chlorine contact time on
arsenic removal, experiments were performed on waters containing 0.3 mg/1 of
and treated with 30 mg/1 of alum. Arsenic^ removals in
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239
100.
60
20
EXCESS LIME
M 10.9-11.3
WELL WATER
LIME SOFTENING
O
D
OHIO RIVER
COAGULATION
FERRIC SULFATE
ALUM
0,10
0.30
1,0
3.0
10
ORIGINAL ARSENICV CONCENTRATION, MG/L
30
Figure 8. Effect of Arsenic Concentration and Treatment Process on Removal.
-------
100
80
60
20
J I
10
pH OF TREATED WATER
11
250
200
150
100
50
f
v
Figure 9. Arsenic Removal by Lime Softening.
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unchlorinated samples were about 10 percent. The contact time between
addition of chlorine (sodium hypochlorite) and the beginning of rapid mix
of the coagulant varied from 5 to 120 minutes. Arsenic removals for all
contact times from the minimum to the maximum ranged between 50 and 60 percent.
After this study, a standard chlorine contact time of 10 minutes was used
in subsequent chlorination tests.
Several variables were considered in the arsenite removal by coagulation
experiments: one of these was pH. Arsenite removal by alum and by iron
coagulation did not show great variation with pH. Maximum arsenite removals
occurred from pH 6 to pH 8 (Figure 10). Experiments dealing with chlorination
of arsenical were also performed. Chlorine residual measurements were made
at the end of such tests to ascertain that chlorine demand had been satisfied
while still leaving a residual in the treated water. Chlorination produced
arsenic removal results similar to those found by Gulledge and O'Connor in
studies of arsenic" (5). Arsenic removals decreased above pH 7 with alum
coagulation. A decreasing trend also was seen for arsenic removal above pH 8
with ferric sulfate coagulation.
Studies of arsenic removal by ferric sulfate showed that removals
depend on coagulant dose and initial arsenite concentration. Removals increased
with higher coagulant doses, but decreased when coagulant dose was held constant
and initial arsenite concentration was increased (Figure 11). The most
significant factor in the treatment was chlorination, however, Chlorijiated
water samples dosed with arsenic^ showed removals almost identical to
earlier results obtained in research with arsenic^' (Figure 8).
Arsenic removal by alum was also investigated. Alum was considerably
less effective for arsenite removal than ferric sulfate when samples were
not chlorinated (Figure 12). For example, results for coagulation with
30 mg/1 of alum were generally 20 percent of less compared with removals in
the 40 to 60 percent range for 30 mg/1 of ferric sulfate. Again, though,
dramatic increases in arsenic removal were observed when waters spiked with
arsenite were chlorinated before coagulation. Then the results obtained were
similar to previous results with arsenic^ (Figure 8).
During precipitative softening, unchlorinated arsenic removal was poor
when the pH of the treated water was below 10.5. As the pH approached 11
arsenic removal increased sharply and then leveled off at 70 percent (Figure 13).
When waters containing arsenic were chlorinated and then softened, the
results were nearly identical to the previous results obtained for arsenic^
removal by softening. In both cases, maximum removal occurred at pH values
of 10.7 or above.
Effects of arsenic concentration on removal by softening are shown
in Figure 14. Slight decreases in removal of unchlorinated arsenic**1 occurred
as initial concentrations of arsenic increased from 2 to 10 mg/1. The
best removals of unchlorinated arsenic*** did not exceed 80 percent, however.
In contrast to this removals of chlorinated arsenic111 and-arsenic
(for comparison) by lime softening to pH values between 10.9 and 11.3 were
95 percent or higher for arsenic concentrations from 0.1 to 10 mg/1.
-------
ALUM COAGULATION
100
80
UJ
g 60
UJ
a;
Lul
c-j
ce:
20
O
O
CHLORINATED
Q
ARSENIC111 0.3 MG/L
ALUM 30 MG/L
O
UNCHLORINATED
7
pH OF TREATED WATER
FERRIC SULFATE COAGULATION
100
80
60
^ '-10
UJ
GO
20
•o-
O
CHLORINATED
UNCHLORINATED
ARSENIC111 0.3 MG/L
FERRIC SULFATE 30 MG/L
pH OF TREATED WATER
Figure 10. Effect of pH and Chlorination on Arsenic Removal by Coagulation.
-------
PERCENT ARSENIC REMOVED
•n
H-
(Jq
O
Ni
O
cn
CD
00
O
C3
CD
H
IT.
O
3
H-
O
C
2
O
X
o
I
o
73
3
6
P
\—i
cr
6
n
o
M
C
•r—»
P
rt
H-
O
i— < CD
co
m
ti
m
O
ro
CD
CD
-------
100
o
sr
LU
a;
oo
LU
a.
80
60
20
100 MG/L ALUM H 6.8-7.0
60 MG/L ALUM w i ft 7
pn o.o—f.
30 MG/L ALUM
© AS !! CHLORINATED
A ASV UNCHLORINATED
O AS111 UNCHLORINATED
30 MG/L ALUM pH 7.3-7.5
o-
-o-
-o
0.1 0.3 1.0 3,0
INITIAL ARSENIC CONCENTRATION, MG/L
10.0
Figure 12. Arsenic Rer.ioval by Alum Coagulation.
-------
100
oo
rr-
UJ
UJ
CL.
60
'10
20
D ASVT
@ AS111 CHLORINATTD
O AS111 NOT CHLORINATED
AS 0.4 MG/L
3=0
9 10
pH OF TREATED WATER
11
12
Figure ]3. Arsenic Removal by Lime Softening.
-------
80
60^
ASV AND CHLORINATED AS111
,H 10.9 - 11.3
AS111 UNCHLORINATED
pH 11.2
D
© AS111 CHLORINATED
O AS111 UNCHLORINATED
D AS111 UNCHLORINATED
A ASV
_L
0,1
0.3 1,0 3.0
INITIAL ARSENIC CONCENTRATION, MG/L
10
30
Figure 14. Arsenic Removal by Excess Lime Softening.
-------
Removing arsenic by powdered activated carbon was attempted. The
arsenic concentration used for these studies was 1.0 mg/1. Carbon doses
varied from 5 to 300 mg/1. Experiments were conducted at pH values ranging
from 6.1 to 8.2. The removals of arsenite were poor; all were less than 10
percent. Because removing arsenic by activated carbon was also less than
4 percent for the various conditions investigated, no carbon adsorption tests
involving chlorination were carried out.
The most significant finding in the research on arsenite removal was
that for treatment by coagulation or softening, removal efficiencies will
nearly always be significantly improved if the arsenite is oxidized to
arsenate before removal is attempted. Although other forms of oxidation,
such as treatment with ozone or potassium permanganate, were not attempted,
they should also be effective.
This agrees with the work Shen, who found that higher removal efficiencies
could be obtained by treatment of groundwater with potassium permanganate or
chlorine before coagulation was initiated (6). The groundwater studied in
his work exhibited a chlorine demand. All of the chlorine demand of a water
should be satisfied to ensure that all the arsenite present in an arsenic-
bearing groundwater has been oxidized to the more readily removed arsenate
form. When this has been accomplished, arsenic removal should prove less
difficult.
Selenium
The behavior of two forms of selenium, selenite (Se ) and selenate (Se ),
has been studied. Results of selenate removal can be easily summarized --
they were uniformly poor. Selenate removals by coagulation with iron or
alum (up to 100 mg/1 of coagulant), by softening from pH 9 to 10.8, or by
treatment with up to 100 mg/1 of powdered activated carbon were less than
10 percent for initial selenate concentrations of 0.1 mg/1 as selenium.
Coagulation tests were carried out with Ohio River water and also with Glendale
well water. This work was done with 75Se and stable carrier. Selenate
removal was observed only in the cation-anion exchange column, and this technique
is the one studied thus far that merits further investigation.
Because an existing problem of selenium in groundwater had been
identified in the Midwest, removal of selenite was investigated primarily
with Glendale well water, rather than with Ohio River water. Because
substances such as iron and sulfur are found in the reduced state in
groundwater, expecting selenium in groundwater to be in an oxidation state
no higher than selenium seems logical.
Literature had shown that the reaction of selenium with hydrous
oxides of iron was quite dependent upon pH (19-22). The reactions of
selenite, either coprecipitation with or sorption by hydrous ferric oxides,
decreases greatly when pH exceeds 8. Therefore, studies of selenite
included pH as a variable.
-------
The results of coagulation experiments are shown in Figures 15 and 16.
At a constant ferric sulfate dose and selenium concentration, removal
improves as pH decreases, with smaller removal increments as pH decreases
from 6 to 5 (Figure 15). Removals also increase with higher coagulant dose,
but at low pH values, the improvement that resulted from adding more ferric
sulfate was not as great as that obtained at pH 7 or above. Results of alum
coagulation (Figure 16) indicate the trends are the same as those observed
for iron coagulation. Removals with alum, however, were generally lower
than removals with ferric sulfate.
A limited study of coagulation of surface water spiked with selenite
was undertaken, and results were similar to those obtained with well water.
Selenite removals from Ohio River water with 25 to 100 mg/1 of ferric
sulfate fell within the limits shown in Figure 15 for pH 7.0 to 7.5.
Selenite removals from river water with alum were somewhat higher than
removals from well water using alum.
When either selenite or selenate was added to Ohio River water samples
with turbidities of up to 65 T.U.s, removals by stirring overnight and
then centrifuging for clarification did not exceed 6 percent. Thus sorption
of selenium onto natural turbidity at pH 7 to 8 does not seem very likely.
In jar tests with up to 100 mg/1 of powdered activated carbon, removals
of selenium were less than 4 percent for selenite. Glendale well water was
used in carbon adsorption tests in contrast to the coagulated and settled
secondary sewage effluent used by Linstedt et al. (9). The finding of poor
selenite removal in a water free of organics adds strength to the suggestion
that selenite adsorption by activated carbon in the Linstedt et al. research
was related to the organic content of the test water.
Removal of selenite by softening was attempted. A slightly increasing
trend for removal with increasing pH was observed (Figure 17). Nevertheless,
removals did not exceed 40 to 50 percent and generally were lower, so lime
softening or excess lime softening would not be recommended for selenium
removal.
-------
100
60
Rf
20
INITIAL SELENIUMIV = 0.09 TO 0.10
FERRIC SULFATE DOSE
O 25 MG/L
D 60 MG/L
A 100 MG/L
pH OF TREATED WATER
Figure 15. Effect of pH on Selenium Removal by Iron Coagulation.
-------
250
oo
O 25 MG/L
D 40 MG/L
A 100 MG/L
O 200 MG/L
20 -
pH OF TREATED WATER
Figure 16. Effect of pll on Selenium Removal by Mum Coagulation.
-------
251
100
S
80
60
UJ
UJ
D_
20
O SELENIUMIV 0.10 - 0.11 MG/L
D SELENIUMIV 0.3 MG/L
10
pH OF TREATED WATER
11
Figure 17. Selenium Removal by Lime Softening.
-------
252
SUMMARY
The results of research for the removal of inorganic contaminants are
summarized in Table I. The data upon which Table I is based are almost
entirely the results of bench-scale work. Engineering designs should not
be undertaken until these data are verified by pilot plant tests. This work
will be useful in selecting processes which might be most fruitfully
investigated on a larger scale, however.
Three of the processes listed in Table I were found to be most effective
for removal of one or more of the trace inorganics studied: activated carbon,
for organic and inorganic mercury; ferric sulfate coagulation, for removal
of seleniumlV, arsenic1^, and arsenic^; excess lime softening, for
inorganic mercury, barium, arsenical, and arsenicv. Note that because
efficacy of treatment techniques is categorized in removal ranges, e.g.,
60 to 90 percent for good removal, more than one treatment method may be
designated as the best means for removal of a given contaminant.
No single treatment process was the most effective or preferred method
for all of the contaminants tested so far (See Table I). Therefore, no
one treatment technique can be recommended as being always preferred. The
design engineer must carefully consider the water to be treated, its chemical
content, and any characteristics about its source that might suggest the
possibility of contamination with trace inorganics. For instance, the natural
occurrence of arsenic, selenium, and barium in certain geographical areas
of the country should influence the design of facilities in those areas.
Because of differences in the treatability of selenium ^ and selenium
treatment should be attempted before oxidation.
Agreement of this work with previous research is generally good. With
one exception, there were not serious contradictions of previous findings.
The very negligible removal of selenite by activated carbon adsorption
contrasted with the findings of Linstedt et al. (9). They explained the
somewhat unexpected adsorption of inorganic selenium by suggesting that the
selenium was first associated with organic matter that was subsequently
adsorbed on carbon. This explanation appears reasonable because in this
work with low-organic-content well water, selenium adsorption was quite poor.
The differences in selenium removal in wastewater and in well water call
attention to the necessity of doing water treatment studies with waters
similar to potable water sources rather than with waste waters.
Of the substances studied thus far (with the exception of selenium ),
if the limits in the Drinking Water Standards are not exceeded by a factor
of more than 3 to 5, some conventional treatment method exists that should
be adequate to reduce the concentrations below the limits set in the
proposed Drinking Water Standards.
-------
253
TABLE 1. SUMMARY OF JAR-TEST RESULTS OF TREATMENT PROCESSES TO REMOVE
TRACE METALS FROM DRINKING WATER
Trace
Metal
Mercury-
Organic
CH3HgCl
Mercury-
Inorganic
HgCl2
Ba++
Selenium-
Inorganic
Se
Selenium-
Inorganic
Se+6
Arsenic-
Inorganic
Arsenic-
Inorganic
As+5
Alum
Poor*
Poor
Poor
Poor
pH <7
Poor
Poor
Good to
very good
pH *(5)
Ferric
Sulfate
Poor
Fair
Poor
Fair to
good
pH <7
Poor
Fair to
Good**
Good to**
very good
pH 8(5)
Lime Excess Lime
pH 9.5-10 pH 10.6-11
Poor Poor
Fair Good**
Good Good to
very good**
Zeolite field data -
Poor Fair
Poor Poor
Poor Good**
Very
Good good**
Activated
Carbon
Good**
Good
Poor
very good**
Poor
Poor
Poor
Poor
*Key- Poor = 0 - 30% removal
Fair = 30 - 60% removal
Good = 60 - 90% removal
Very good = Above 90% removal
**Best treatment technique.
-------
Acknowledgment
The authors wish to acknowledge the laboratory assistance of Bradford
Smith and Charles Donovan and the participation of Thomas Sorg in portions
of this work.
Figures 1,2,3,4,8,9,15,16 and 17 appeared in "Removal of Trace Inorganics
by Drinking Water Treatment Unit Processes," by G. S. Logsdon and J.M. Symons,
published in Water 1975. These figures are reproduced with the permission
of the American Institute of Chemical Engineers.
-------
255
REFERENCES
1. U. S. Environmental Protection Agency, Drinking Water Standards.
2. Barnett, P.R., M.W. Skougstad, and K.J. Miller, Journal AWWA, 6^, 60 (1969)
3. Anon., Report of the Joint Program of Studies on the Decontamination of
Radioactive Waters, ORNL-2557 TID-4500 (14th Edition).
4. Cherkinskii, S.N., L.N. Gavrilevskaya, V.P. Laskina and M.N. Rubleva,
Gigiena i Sanitariya; trans; Hygiene and Sanitation, 35, 157 (1970).
5. Gulledge, J.H., and J.T. O'Connor, Journal AWWA, 65, 548 (1973).
6, Ebersole, G., presented at the 92nd Annual Conference, AWWA, Chicago
Illinois (June 1972) .
7. Shen, Y.S., Journal AWWA, 65, 543 (1973).
8. Angino, E.E., L.M. Magnuson, T.C. Waugh, O.K. Galle and J. Bredfeldt,
Science, 168, 389 (1970).
9. Linstedt, K.D., C.P. Houck, and J. T. O'Connor, Journal WPCF, 43., 1507
(1971).
10. Logsdon, G.S. and J.M. Symons, Journal AWWA, 65, 554 (1973).
11. Kopp, J.F., M.C. Longbottom and L.B. Lobring, Journal AWWA, 64_, 20 (1972).
12. McFarren, E.F., Journal AWWA, 6£, 28 (1972).
13. Caldwell, J.S., R.J. Lishka, and E. F. McFarren, Journal AWWA
6Ss 731 (1973). '~
14. Sigworth, E.A. and S. B. Smith, Journal AWWA, 64, 386 (1972).
15. Dietrick, L.V., Review of Water Quality Data from Community Water
Supply Systems in Ohio, Ohio EPA.
16. Private Communication, Dorothy Bennett, Division of Public Water
Supplies, Illinois Environmental Protection Agency, Springfield, 111.
(January 16, 1974).
17. Anon., Food and Nutrition Board - NAS-NRC, Sodium-Restricted Diets.
Publication 325, National Research Council, Washington, D. C. (1954).
18. Ferguson, J.F. and J. Gavis, Water Research, 6^ 1259 (Nov. 1972).
19. Allaway, W.H., Trace Substances in Environmental Health II.
Proc. Second Annual Conference, 181 (1968).
20. Howard, J.H., III, Trace Substances in Environmental Health V.,
Proc. Fifth Annual Conference, 485 (1971).
-------
256
21. Plotnikov, V.I., Zhurnal Neorganicheskoi Chimmi, III, 1761 (1958);
trans; Journal of Inorganic Chemistry, USSR, 3, 56 (1958).
22. Hingston, F.J., A.M. Posner, and J.P. Quirk, Advances in Chemistry
Series No. 79, 82 (1968).
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257
9-17-73
REMOVAL OF HEAVY METALS FROM WASTEWATER
WITH STARCH XANTHATE
R. E. Wing
Research Chemist
Northern Regional Research Laboratory
Agricultural Research Service
U.S. Department of Agriculture
Peoria, Illinois 61604
Presented at a conference on
"Traces of Heavy Metals in Water:
Removal and Monitoring"
at
Princeton University
School of Engineering and Applied Science
Princeton, New Jersey
Session IV
Foam Fractionation, Solvent Extraction
and Other Processes
November 16, 1973
-------
258
Introduction
Worldwide concern developed a few years ago when people became aware
of the effect of discharging industrial effluents containing heavy
metals into waterways. This led to the formation of a set of strict
discharge limits for heavy metals that must he met before industrial
effluents can be discharged. Several methods have been developed
and are being used to treat these industrial effluents contaminated
with heavy metals (Figure 1). Each of these methods has advantages
but most have disadvantages requiring special modifications before
use by certain industries.
The method developed at the Northern Regional Research Laboratory in
Peoria offers industry a tool to remove heavy metals in most cases
to concentrations below discharge limits.
Description of Method
Our method involves the sequential addition of starch xanthatc and
a cationic polymer [e.g., poly(vinylbenzyltrimethylammonium chloride)
(PVBTMAC) or polyethylenimine (PHI)] to a heavy metal solution to'
precipitate both polymers and the heavy metals as a cohesive floe
(Figure 2). Some of the xanthate groups on the starch backbone
form heavy metal salts, while others react with the polycation to
form an insoluble polyelectrolyte complex.
The overall process (Figure 3) involves precipitation, separation
and recovery. Once the precipitate forms it settles rapidly and
separation may be achieved by filtration, centrifugation or decantation.
The resulting sludge can then be treated with acid or incinerated
to recover the metal.
Variables Investigated
Several variables were evaluated to optimize heavy metal removal.
Mercury was selected as the metal used in this evaluation; however
most heavy metals give similar results.
A. End-Point Determination. Solutions containing mercury and cationic
polymer were titrated with \\ solutions of starch xanthate. The
amount of xanthate required to give maximum precipitation of the
cationic polymer and mercury was determined by streaming current
measurements. Figure 4 shows a typical titration curve for this
precipitation. Figure 5 shows how critical the amount of starch
xanthate added is for the optimal removal of mercury. An excess
of starch xanthate causes an imbalance between the polyelectrolytes
and some, mercury xanthate complex redissolves.
B« Concentration of Cationic Polymer. The optimum amount of cationic
polymer was determined for various concentrations of mercury. For
-------
259
a 100 ppm solution of mercury, 0.46 g/1. PVBTMAC should be used to
obtain a minimum residual mercury concentration,
C. Rate of Starch Xanthate Addition. The slower the starch xanthate
is added th~e more effectively it complexes with mercury. Therefore
complexation is probably not an instantaneous reaction. Slow
precipitation does produce fewer and larger particles, which are
more easily filtered.
D. Presence of Salt (NaCl). The method is effective if the salt
(NaCl) concentration is below 3%. Increasing salt concentrations
above 31 slightly lowers the effectiveness of the method.
E. Presence of Seguestrants. The presence of selected sequestrants
at the 0.1 g/T7 level does not affect removal of mercury (Table I).
F. Effect of Initial pH. Treatment of mercury solutions having
initial pH's of 3-11 result in low residual mercury concentrations.
Solutions with pH's of less than 3 cause rapid decomposition of the
starch xanthate.
G• Other Variables. 1. Degree of substitution (DS) of_ starch xanthate--
Starch xanthates having DS from 0.11 to 0.40 were effective for
removing mercury. Smaller addition volumes were required as the DS
of the starch xanthate increased.
2. Order of addition of the polymers--The starch xanthate and cationic
polymer carTbe added in either order for effective removal.
3. Omitting cationic polymer--Starch xanthate can react with mercury
in the absence of cationic polymer to form a precipitate. However,
this method was only effective for high mercury concentrations (100 mg/1.)
and still left residual mercury above 100 ^ug/1.
Residual Contaminants
Any process for removing heavy metals from water should not add other
noxious substances. Contaminants that might be introduced by treatment
of water with starch xanthate and a cationic polymer include: small
ionic species (Cl from the cationic polymer; Na , OH and CS3= from
the xanthate), small nonionic species (CS2 and COS from the xanthate),
and the polyelectrolytes themselves. The residual concentrations found
[chloride, 200 mg/1. and sulfur (reported as sulfate), 36 mg/1.] and
the BOD5 (0-12.5 mg/1.) were well below established limits. Therefore,
precipitation of heavy metals with starch xanthate and cationic poly-
electrolytes does not appear, in itself, to produce legally unacceptable
concentrations of these residual contaminants for which limits have
been established.
-------
260
Removal of_ Other Heavy Metals
Tables 2 and 3 show the effectiveness of removal of in metals tested
using the starch xanthate-cationic polyner method. For comparison
similar metal solutions were treated with sodium hydroxide to precipi-
tate the metals as their hydroxides. The tables also list the
solubility product constants for the metal ethyl xanthates. The
constants are useful in determining how effectively the metal will
be removed with the starch system.
Scale-Up
The method has been scaled-up to 25 gallons of metal solution. Table 4
shows the concentrations of residual metal.
Testing of_ Other Cationics
Several cationic polymers have been evaluated. Table 5 compares the
effectiveness of several polymers ranging in price from SO.13 to $1.50
per pound.
Advantages of the Method
Several important advantages of the starch xanthate-PVBT^IAC method are
apparent. Metal recovery for reuse is possible with mild acid treatment
of the complex. The volume of sludge to be treated for recovery of
the metal or to be disposed of is small compared to that obtained from
several other processes. The physical nature of the sludge is not
gelatinous as with usual basic precipitation and the floe settles faster
and can be removed easier. Some methods require the removal of suspended
solids before treatment; however, this is not necessary with the xanthate
method. The starch xanthate method is effective over a pH range of 3-11.
Additional Information
In the previous discussion it was shown that the fairly expensive
cationic polymer was necessary for effective removal. Recently, the
process has been modified to eliminate the cationic polymer and utilize
only starch xanthate to effect heavy metal removal.
The starch xanthate in this modified process is water insoluble.
Figure 6 gives a scheme for the preparation and use of this product.
Table 6 shows the effectiveness of solid starch xanthate in removing
heavy metals from water. The solid starch xanthate is basic so the
pi I of the solution will increase. Effective removal is always obtained
in the pH range of acceptable discharge. A contact time of as little
as 5 min is sufficient for good removal. For example, a solution con-
taining 31,770 ug/1. Cu+2 was reduced to a residual level of 22 ug/1.
Cu+2 with 5 min contact time and to a level of 20 ug/1. with 120 min.
Low residual metal concentrations were obtained with varous PS solid
-------
261
starch xanthates; however the higher the DS, the greater the metal
binding capacity assuming one metal ion per one xanthate group.
Metal release from the product using nitric acid is effective and
the metal is recovered as a concentrated metal solution. The xanthate
is oxidized by the acid to sulfate and the insoluble starch is
recovered for rexanthation and reuse.
Bibliography
"Recovering Heavy Metals," Agricultural Research, 21(9), 3-4 (1P73).
"Mercury Removal from Wastewater with a Starch Xanthate-Cationic
Polymer Complex," C. L. Swanson, R. E. Wing, W. M. Doane and
C. R. Russell, Environ. Sci. Techno 1., 7(7), 614 (1973).
"Removal of Heavy Metal Ions from Wastewater with a Starch Xanthate-
Cationic Polymer Complex," R. E. Wing, C. L. Swanson, W. M. Doane
and C. R. Russell, J. Water Pollut. Contr. Fed, (submitted).
"Insoluble Starch Xanthate: Preparation and Use in Heavy Metal
Recovery," R. E. Wing (correspondence aid).
-------
262
TABLE 1.--Effect of Sequestrants on Residual
Mercury Level and Starch Xanthate
Requirement1
Sequestrant
Diglycolate
Nitrilotriacetic acid
Polyphosphate
Citrate
Control
Residual
Mercury
(MB/1.)
4.37
3.86
10.3
in. 33
6.1
Xanthate
Requirement
(meq X 10Z)
5.61
5.19
3.36
4.36
6.53
Meq of starch xanthate [II w/v, 0.23
degree of substitution (DS)] required to precip-
itate 2.5 X 10"z i*i (5 mg) mercury(II) and
4.74 X 10" meq poly(vinylbenzyltrimethyl-
anmonium chloride) (PVBTMAC) from 50 ml of
solution.
2
In each 50-ml test solution 5 ng of
sequestrant was used. The control contained
none.
-------
TABLE 2.--Heavy Metal Removal from Solutions of Individual Metals by Treatment with Starch
Xanthate-PVBTMAC1 (Final pH 7) vs. Sodium Hydroxide (pH 7 and 9)
Metal
Cadmium(II)
Chromium (I I I)
Copper(II)
Iron(II)
Iron(III)
Lead(II)
Manganese (II)
Mercury (I I)
Nickel (II)
Silver (I)
Zinc(II)
7 Starch-Xan3
Treatment (ml)
Xan-PVBTMAC „ 4.65
NaOH (pH 7K)
NaOH (pH 9)
Xan-PVBTMAC 3.45
NaOH (pH 7)
NaOH (pH 9)
Xan-PVBTMAC 4.50
NaOH (pH 7)
NaOH (pH 9)
Xan-PVBTMAC 3.40
NaOH (pH 7)
NaOH (pH 9)
Xan-PVBTMAC 3.70
NaOH (pH 7)
NaOH (pH 9)
Xan-PVBTMAC 4.60
NaOH (pH 7)
NaOH (pH 9)
Xan-PVBTMAC 4.10
NaOH (pH 7)
NaOH (pH 9)
Xan-PVBTMAC 4.70
NaOH (pH 7)
NaOH (pH 9)
Xan-PVBTMAC 4.15
NaOH (pH 7)
NaOH (pH 9)
Xan-PVBTMAC 4.52
NaOH (pH 7)
NaOH (pH 9)
Xan-PVBTMAC 4.10
NaOH (pH 7)
NaOH (pH 9)
Initial pH
3.16
2.9
2.9
4.56
3.0
3.0
3.06
2.9
2.9
3.76
2.6
2.6
3.65
3.65
3.65
3. 16
3.0
3.0
3.26
3.0
3.0
3.2
3.2
3.2
3.36
3.2
3.2
3.2
3.2
3.2
3.66
3.3
3.3
Initial Concn.
fcgA.)
56,200
56,200
56,200
26,000
26,000
26,000
31,770
31,770
31,770
27,920
27,920
27,920
27,920
27,920
27,920
103,600
103,600
103,600
27,470
27,470
27,470
100,000
100,000
100,000
29,350
29,350
29,350
53,935
53,935
53,935
32,680
32,680
32,680
Residual Illinois pH, Optimum
Concn. Limit'* for Hydroxide
(ug/1.) (ug/1) Precipitation
8 50 6.77
29,600
4,500 ,
52 1,000 5.3'
3,800
416 ,
12 20 5.3'
1,100
24 ?
2,980 1,000 5.5
2,920
14,500 ,
890 1,000 2.0
146
15 ?
8 100 6.0
34,500
8 7
10,700 1,000 8.5
10,700
9,600 7
3.8 0.5 7.3'
>10,000
8,140 q
275 1,000 6.7y
5,820
4,350 „
3 5 9.0y
15,800
14,800 7
3,300 1,000 7.0'
6,900
1,010
Ksp, Metal
Ethyl Xanthateb
2.6 X 10"14
—
-?n
5.2 X 10 ^
-8
8.0 X 10 8
—
-17
2 X 10 1
+ 7
10 i
-W
1.7 X 10 i8
-12
1.4 X 10 *•*
-1Q
5 X 10 ly
-Q
4.9 X 10 s
PVBTMAC at 1 percent concn.
Stock metal solution (2.5 ml) + distilled water (47.5 ml).
Starch xanthate (xan) (DS 0.23) required to reach maximum precipitation of stock metal solution (2.5 ml) + distilled
water (47.5 ml) at pH 7, after PVBTMAC (1 ml) is added.
State of Illinois discharge limits for public and food processing waters [Illinois Pollution Control Board, Newsletter
No. 44, p. 7 (Mar. 1972)].
r '
Kakovsky, I. A., "Physicochemical Properties of Some Flotation Reagents and Their Salts with Ions of Heavy Non-Ferrous
Metals." Proc. Int. Congr. Surface Activ. 2nd, 4, 225 (1957).
Increase in pH over stock metal solution (2.5 ml) + distilled water (47.5 ml) so final solution after treatment wouH have
a pH of 7.0.
Dean, J. G., et al. , "Removing Heavy Metals from Waste Water." Environ. Sci. Techno 1 . , 6(6), 518 (1972).
8
pH of 7 or 9.
pH 7 or pH 9 = addition of 0. IN NaOH to stock metal solution (2.5 ml) + distilled water (47.5 ml) to reach final
Diehl, H. , and Smith, G. F., "Quantitative Analysis." John Wiley § Sons, Inc., New York, N.Y. (1952).
-------
TABLE 3.--Heavy Metal Removal From Solutions Containing a Mixture of Them by Treatment with Starch Xanthate-PVBTMAC
(Final pH of 7) vs. Sodium Hydroxide (pH 7 and 9)
Sample1 Oig/1.)
Iron(II
Treatment Cadmium(II) Chromium(III) Copper(II) and III) Lead(II) Manganese(II) Mercury(II) Nickel(II) Silver(I) Zinc(II)
None
Xan-FVBTMAC2
NaOH (pH 7)3
NaOH (pH 9)
Illinois
limit4
5,620
3
2,978
390
50
2,600
21
42
31
1,000
3,177
16
151
20
20
5,484
990
79
34
1,000
10,360
8
180
8
100
2,747
1,510
1,508
833
1,000
10,000
3.
4,580
4,275
0.
8
5
2,935
57
1,610
681
1,000
5,394
5
93
62
5
3,268
319
1,800
53
1,000
Sample contains 0.25 ml of all stock solutions of metals and 47.5 ml distilled water.
2 Starch xanthate (0.23 DS) and PVBTMAC (1 ml).
3 Samples were treated with 0.1N NaOH to attain pH 7 or 9.
4 Table 2, footnote 4.
-------
265
TABLE 4.--Heavy Metal Removal from 25 Gallons of Solution by Treatment
with Starch Xanthate-Polyethylenimine (PEI)
Metal (ppb)
Treatment tf* E75 C?2S*2 Pb^if* Ni*2Xg^
None . 5,620 2,600 3,177 5,484 10,360 10,000 2,935 5,394
St-xan-PEI1 27 22 3 100 8 2 77 3
Illinois limit 50 1,000 . 20 1,000 100 0.5 1,000 5
1 Starch xanthate (0.23 DS) - 79.6 g (d.b.) and PEI Olontrek 1000) - 18.9 g
(d.b.).
-------
TABLE 5.--Evaluation of Several Cationic Polymers with Starch Xanthate for
Removing Heavy Metals
Treatment
None
St-xan- PVBTMAC2
St-xan-PEI3
St-xan-CPA4
St-xan-CSt-15
St-xan-CSt- 26
St-xan-AEWF7
Illinois limit
Cd+2
3,620
7
35
10
30
83
26
50
Cr+3
2,600
0
11
0
44
11
0
1,000
Cu+2
3,177
3
2
1
6
15
6
20
Fe+2
5,484
66
42
50
583
92
100
1,000
Metal
Pb+2
10,360
8
8
8
8
8
8
100
(ppb)1
Mn+2
2,747
1,179
83
1,093
1,083
833
1,083
1,000
V2
10,000
3.6
Tr
Tr
Tr
Tr
Tr
0.5
XT.+2
Ni
2,935
45
135
61
338
354
69
1,000
A/1
5,394
3
3
5
5
8
13
5
Zn+2
3,268
73
67
77
1,037
210
73
1,000
Metal concentration determined by atomic absorption with a Varian Techtron AA120
spectrophotometer.
2 PVBTMAC, 0.2 g/1.; st-xan, 0.78 g/1.
3 PEI, 0.2 g/1.; st-xan, 0.84 g/1.
CPA (a commercial cationic polyacrylamide, Reten 220), 0.2 g/1.; st-xan, 0.34 g/1.
5 CSt-1 (a commercial cationic starch, "Cato 8"), 0.4 g/1.; st-xan, 0.26 g/1.
6 CSt-2 (a commercial cationic starch, "Cato 15"), 0.4 g/1.; st-xan, 0.28 g/1.
7 AEWF (aminoethylated wheat flour), 0.4 g/1.; st-xan, 0.34 g/1.
ro
ON
a\
-------
267
TABLE 6.--Removal of Heavy Metals with Insoluble
Starch Xanthate
Metal
Cu*2
Ni+2
Cd+2
Pb+2
Cr+3
Ag+
Zn+2
Fe+2
Mn+2
Hg+2
i
Initial1
Concn.
(ug/1.)
31,770
29,350
56,200
103,600
26,000
53,935
32,680
27,920
27,470
100,000
pH
(Initial)
3.4
3.2
3.0
3.1
3.2
3.1
3.1
3.0
3.3
3.1
7
pHZ
(2 h)
6.4
7.7
6.8
7.3
6.5
7.2
7.5
6.4
9.0
4.2
Residual
Metal
Oug/10
7
19
9
25
3
245
46
0
1,628
3
Illinois
Discharge
Limit
Oug/1.)
20
1,000
50
100
1,000
5
1,000
1,000
1,000
0.5
50-ml sample.
Stir solution 2 h before filtering.
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268
Common Methods Used for
Heavy Metal Removal by Industry
1. Chemical Precipitation
a. Lime
b. Alum
c. Iron salts
2. Chemical Treatment
a. Oxidation
b. Reduction
3. Ion Exchange
4. Ultrafiltration
5. Electrochemical
6. Evaporative Recovery
Figure 1. Common methods used for heavy metal removal by industry.
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269
Starch Xanthate • Cationic Polymer • Metal Complex Formation
s
\\ Q
Starch Xanthate *' Na(
1 [~
0 0—
/„
NR3CIG
Cationic Polymer @T
f mfiiYftfl A t\ \ I*li
(PVBTMAC)
3'
i
S
\\ /^ .A
f\^v w "" ^
1 ( n
0 0
\ ^ I
Starch Xanthate • Cationic Polymer • Metal Complex
Figure 2. Starch xanthate-cationic polymer-metal complex.
-------
2?0
Polyelectrolyte Complex Method
« *• • n • , Heavy Metal ,
Catiomc Polymer + +
starch_0__s- Na
Polyelectrolyte Metal Complex Effluent
Tiltration
Separation
or
Centrifugation
or
Decantation
Effluent Complex
V
Metal Recovery
Figure 3. Polyelectrolyte complex method.
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271
End-Point Determination (SCDj for Residual Metal
30
25
20
10
CO
-10-
NaCI-1% N
0
2345
Xanthate (ml)
6
Figu.re U. Typical streaming current detector (SOD) curves from
titration of mercury(Il) (5 mg) and poly(vinylbenzyltrimethylammoriium
chloride) (PVBWAC) (10 mg) with starch xanthate [0.2^ degree of
substitution (DS), 1$ (w/v)l in the presence (lower curve) and
absence (upper curves) of sodium chloride [1$ (w/v)].
-------
272
Relationships Between Volume of Starch Xanthate Added,
SCO Reading and Residual Mercury Concentration
490
V
70
^60,
_5i
£50
CJ
cu
=E 40
CO
a
•1 30
Q>
oc
20
10
n
1
-«
i
V
-
—
A
-
**A A Xanthate /
/
i
i
1
1
J
V Residual Mercurv^/ _
i i i i i
8
7
6
5
4
3
2
1
-_15 _1Q -5 0 5 10
SCO Reading (Arbitrary units]
Figure 5. Relationships between volume of starch xanthate added,
SCD reading, and residual mercury concentration. Fifty-milliliter
samples containing mercury(ll) (5 mg) and PVBTMA.C (10 mg) were
treated with starch xanthate [0.23 DS, 1% (w/v)].
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273
Preparation and Use of Insoluble Starch Xanthate
Highly crosslinked starch + NaOH + CS2
H20
r
Insoluble starch xanthate (Solid)
Heavy metal effluent
Separation (Stir-filter)
r
Insoluble metal starch xanthate + Clean effluent
HN03
r
Insoluble starch + H2S04 + Metal ions
Figure 6. Insoluble starch xanthate method.
-------
-------
275
SUMMAEY OF CONCLUDING REMAEKS
PRESENTED AT THE SESSIONS
ON THE
REMOVAL PROCESSES OF
TRACES OF HEAVY METALS FROM WATER
C. Calmon
R. F. Probstein
C. J. King
-------
C. Calmon
The symposium on "Traces of Heavy Metals in Water" has shown
the economic and technical limitations in the removal processes.
The symposium should be repeated in a few years to note if progress
was made along the two parameters mentioned above. However, the speakers
must take into consideration the following:
1. Potable water is delivered to a household after collection
and treatment at a cost of about 50^/1000 gallons, the volume being
equivalent to 8000 pounds.
2. Tests must be run in accordance with standard water treat-
ment procedures.
3. Solubility -or loss of an absorbent or chemical additives can
add high costs and may also prove to be toxic in itself.
It was quite evident from the data presented that there is no one
favored method or one which is applicable to all ions or all concentration
of ions present in the solution in question.
It is essential that each situation requires a total system approach
which may include (1) process modification, (2) segregation of wastes,
or (3) a combination of treatment methods.
Today, three processes appear to have merit because they are well
established and their limitations and economics can be readily cal-
culated; namely
1. evaporative processes where ionic concentrations are high.
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277
2. ion exchange where ionic concentrations can reach to about
2000 ppm; and
3. chemical precipitation where the solubility product of the
product formed is extremely low.
Evaporative processes have the disadvantage of high cost per unit
volume of liquid treated but has the advantages of (1) not being
affected by ionic concentration, (2) the final product is dry or in
a concentration which can be reused, and (3) equipment and waste heat
are frequently available.
Ion exchange has the advantage that the cost per unit volume of
water treated is a function of the ionic concentration, the lower the
concentration, the lower the costs. Aero ion exchange resins frequently
have high selectivities for specific ions so that the resin can treat
very large quantities of water per volume of exchange. However, the
problem of regenerant waste in a solution strength of 5 to 15% can
be a problem. Many inocuous ions present in the water may reduce the
capacity of the resin as well as the purity of the recovered product
and also some ions can foul the exchange leavages, with concentrations
above permissible limits, may often appear during the operation.
Ion exchange today is applied successfully to many heavy metal
wastes because the economics favor it, e.g., gold, geranium, chromium,
copper and zinc recovery, the problem is how to get industries to use
ion exchange to systems which show no profit in concentrating cheap
ionic species. A solution to this problem would be the development of
highly specific ion exchanges. This work should be carried out in a
well-coordinated and planned program funded by EPA in place of the present
-------
method of solicited or unsolicited contract proposals for a specific
ion. If zero discharge is ever to be attained economically, specific
ion exchanges appear to be the most promising.
Chemical precipitation and coagulation must also be considered as
these can be directed to the ion in question. If the solubility product
is exceeded then the effluent from this treatment may be followed by
a selective ion exchange resin.
Several new developments in polymeric resins should be watched,
e.g., polymeric absorbents and macroporous resins capable of absorbing
colloidal and complexed materials, thus making resins capable of
removing trace heavy metal ions in solution or existing in colloidal
or complexed forms.
-------
279
R. F. Probstein
The most salient point to emerge from the papers on the use of thermal
processes, reverse osmosis and electrodialysis for heavy metal removal is
that insufficient effort has been devoted to the development of these
methods for the specific purposes of heavy metals removal. Despite the
fact that they could prove in many instances to be the only economic
means of meeting upcoming EPA standards, they remain untried and un-
developed for a wide variety of situations. From the statements made
at the Conference this would appear to stem in part from the equipment
manufacturers' unwillingness on economic grounds to make the necessary
R&D investments. It would seem appropriate, therefore, that until condi-
tions change, EPA itself should assume the obligations for any R&D fund-
ing, with the public the recipient of both the developments and the
cleaner water. The funds cited by Mason as now being set aside by EPA
for process technology development, pilot plant and demonstration test-
ing would seem to fall far short of what is needed to meet the Agency's
strict discharge and receiving water standards on the time scale en-
visaged.
Despite the fact that electrodialysis has been used commercially
for well over a decade to separate out low molecular weight ionic
species from brackish waters, no detailed examples were cited of its
application to heavy metals removal. Prototype electrodialysis systems
for treating electrodialysis systems have been discussed briefly in the
2
literature (see, e.g., Ciancia ) but so far the process has been little
studied over a wide range of conditions of heavy metal removal. How-
ever, its potential and advantages are obvious, particularly in that
range of dissolved metal concentrations too high to make ion exchange
-------
280
economical and too low for optimum reverse osmosis performance.
Reverse osmosis has fared somewhat better in the attention it has
3
received, as evidenced by the work reported by Houle . Some bench
scale studies on the rejection efficiencies for a variety of trace
level heavy metal contaminants have also been recently reported by
4
Mixon . At this time, however, it is far from possible to characterize
specifically reverse osmosis heavy metal rejection capabilities as a
function of concentration, membrane types, system flow characteristics
and fouling tendencies associated with the feed.
In my own paper I pointed out that evaporation processes
presently used for heavy metals separation suffer from their energy
intensiveness. Vapor compression evaporation is the one method offer-
ing a distinct advantage in energy savings but it has had little
testing on a variety of heavy metal distillates. Freezing, on the
other hand, which appears to combine both energy savings and the
advantages of limited corrosion is still in an embryonic stage, prin-
cipally because of the limited funds available to mount a major develop-
ment effort.
It has long been recognized that for the most part the problem
of separating out metal impurities from water in the laboratory is
not difficult, what is difficult is to carry out these separations
economically and on a large scale. To do this requires process develop-
ment not only in the laboratory and on bench scale but up through and
including demonstration systems. From the papers presented at this
Conference it would appear that many promising methods for heavy metals
-------
281
removal have not advanced beyond the laboratory stage because of in-
sufficient funding and/or lack of incentive to move ahead.
References
1. Mason, R. W., "Overview on Heavy Metals Pollution Control at the
Federal Level," (this Conference).
2. Ciancia, J., "New Waste Treatment Technology in the Metal Finishing
Industry," Plating 60. 1037-1042 (1973).
3. Houle, P. C., "Reverse Osmosis" (discussant, this Conference).
4. Mixon, F. 0., "The Removal of Toxic Metals from Water by Reverse
Osmosis," Office of Saline Water R&D Progress Report No. 889,
Sept. 1973.
5. Probstein, R. F., "Heavy Metals Removal by Thermal Processes,"
(this Conference).
-------
282
C. J. King
It is apparent that the development of processes for metals
removal is a separation problem of considerable challenge. The
specifications which are put forward for desired purities are
such that one often has to reduce concentrations by many orders
of magnitude. As we have seen this is no easy task, nor should
it be expected to be.
In view of the difficulties of economical metals removal,
more attention should be devoted to the modification of the pro-
cesses themselves which are sources of metal-bearing effluents.
One would expect that process modification would often be less
expensive than treatment of the original effluent.
It is apparent that there are today no "plug-in" proven
processes. Because of this, a very considerable amount of
research and development will be required if we are to deal
effectively with the problem. This will require funding on a
large scale and will require broad technical capabilities which
are not now present in most of the companies who are confronted
directly with the need for metals removal. It does not appear
that a means of funding or coordinating this research presently
exists, and it would therefore be a most appropriate government
program.
With respect to the attractiveness of different processes
which can be considered for metals removal, we have seen that there
is no clear-cut forerunner. We have also seen that different
-------
283
processes become most competitive at different levels of metals
concentration in the stream to be treated and for different
effluent specifications. In addition to those processes discussed
specifically at this Conference, electrochemical and liquid-ion-
exchange processes deserve consideration for feeds of intermediate
concentration ranges. Another important question is what should
be the eventual destiny of the recovered metals. Often the amount
involved is trivial in terms of economic worth. Recycle to an
appropriate point in the main process will be the answer in some
cases, but that will not be possible when the metallic species
are not principal participants, products or side-products of the
main process.
-------
284
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285
A COMPARATIVE OUTLINE OF CURRENT METHODS FOR THE ANALYSIS OF TRACE
METALS IN NATURAL WATERS
Charles J. Lancelot, Ph.D., Manager
Environmental Research Laboratories
Industrial Chemical Division
FMC Co'rporation
Princeton, New Jersey 085^0
The mounting pressure for a cleaner environment has spawned explosive
growth in the many aspects of environmental science. One of the many
consequences of this enhanced scientific growth has been a demand for
increasingly sensitive methods for the accurate monitoring of extreme-
ly low levels of environmental pollutants. This paper gives a brief
preview of one particular aspect of the subject: a comparative out-
line of the current techniques available for the determination of
trace levels of heavy metals in natural waters. Three general areas
of methodology will be discussed and compared: 1) Atomic absorption
spectroscopy, 2) Neutron activation, and 3) Polarographic methods.
Preceding these discussions, some aspects of the crucial but little
understood problem of sample preservation, storage, and transfer
will be outlined.
SAMPLE PRESERVATION, STORAGE, AND TRANSFER
As the subsequent discussion will show, we are well into the era of
routine analytical capability in the part-per-billion and sub-part-
per-billion range for most aquatic contaminants, including most heavy
metals. Such extreme sensitivity, however, has exposed a new and im-
portant problem which must be dealt with before any natural water
sample can be reliably analyzed in the first place. The problem is
simply that, when dealing with water characterizations in the part-
-------
286
per-billion and sub-part-per-billion range, previously negligible
physical, chemical, and biological changes which occur in a natural
water.sample during storage and transfer now assume major signifi-
cance. In many cases, unless special precautions are followed
(e.g., proper chemical stabilization or choice of sampling and
storage container material), there may be little or no resemblance
between the sample taken and the sample analyzed. Obviously there
can be no use for today's ultrasensitive analytical techniques
if there is no equally precise capability in sampling storage and
transfer. Unfortunately, progress in this important area has not
kept up with the rapid development of increasingly sensitive an-
alytical capability, and proper sample preservation is still
largely a matter of trial-and-error. A few typical difficulties
and suggested procedures are illustrative.
Adsorption of desired elements on sampler or transfer container
walls may give significantly lower analyses; conversely, leeching
of undesired contaminants from container walls may introduce false
positive results or give significantly higher analyses if the ele-
ment was already present in the sample. Scrupulous pre-washing of
all containers is absolutely essential, and special washes with
pure solutions of desired elements may also be required.
Although this paper is limited to a discussion of heavy metals, it
is useful at this point to use information collected for phosphorus
to illustrate the problem of adsorption and desorption, since it is
one of the very few environmentally significant elements which have
been systematically studied in this respect. Solutions containing
<10-20 ppb phosphorus (typical for many lakes and streams) have
-------
287
been shown to undergo significant loss of orthophosphate while
standing in Pyrex containers for 3-5 days (1), due to adsorption
on the glass. This could only be eliminated by the use of poly-
carbonate containers which had been pre-washed with a solution of
orthophosphate (1). In our laboratory, we have been able to store
natural water samples for up to 4 days, but no longer, in 1-quart
size PVC containers without significant drop in the phosphorus level.
Biological processes in natural water samples are also a major
cause -of significant changes in sample composition when working in
the part-per-billion-and-less range. Addition of a stabilizer to
kill or severely retard biological systems is necessary, provided
the stabilizer itself does not introduce additional problems. The
choice of stabilizer depends on many unfortunately not always known,
sample parameters. The most common, effective means of stabilization
is acidification to pH=2-3 with diluted nitric acid. Where a mercury
or chloride anaylsis is not desired or conditions and methods don't
otherwise preclude it, preservation with ^4 ppm mercuric chloride
is also very effective. Refrigeration, if used as a preservation
technique where it is necessary that no chemical preservative of
any sort be added, must be done very carefully so as to avoid
freezing the sample, since this can cause the irreversible precipita-
tion of many natural water components. The chief disadvantage with
acidification as a preservation technique is that it can cause the
desorption of contaminants from the container walls. Acid pre-
washing of containers can minimize this. Sometimes, the problems
that arise with stabilizers are altogether unexpected; we found that
acid stabilizing of water samples kept in PVC containers often caused
slow leaching of the plasticizer from the container walls within
-------
288
2-3 days. The plasticizer was an organic phosphate ester, and the
error in the resulting phosphate analyses is obvious!
It should be re-emphasized that all of the above processes re-
present very small changes in an absolute sense, but such small
changes become extremely significant when analyzing in the range of
parts per billion and less. The only really reliable approach,
then, is to analyze water samples immediately after they have been
taken, or as soon as possible thereafter, preferably within the
same day. Spiking control samples at the time of collection and
ascertaining the degree of analytical "recovery" of the spike
often serves to indicate the presence or absence of storage problems
CRITERIA FOR THE SELECTION OF TRACE ANALYTICAL TECHNIQUES
Several methods are available today for the ultrasensitive analysis
of heavy metals in natural waters. The most widely used are atomic
absorption methods including both flame and the new flameless tech-
niques, polarographic methods, including anodic stripping and pulse
techniques, neutron activation, spark source mass spectrometry, and
specific ion electrodes. The first two seem to enjoy the most wide-
spread use, and are the subjects of the following brief discussions,
although some comparisons have been made with neutron activation
analysis. These techniques have been reviewed in depth elsewhere
(2-4), and excellent bibliographies are available. Consequently,
the present discussion assumes basic familiarity with the methods
discussed and is limited to a practical comparison between the
methods, particularly with respect to cost, sensitivity, ease of
set-up and operation, and principal advantages and drawbacks.
-------
289
The factors to be considered in selecting an analytical method for
trace metal analysis may be listed as follows (2):
(a) the analytical operations should be reasonably simple in
their execution;
(b) the hardware requirements should be met with relatively in-
expensive intruments;
(c) sample manipulation and treatment should be minimal;
(d) simultaneous or rapid sequential multielement determinations
should be possible;
(e) the power of detection of the technique should be in the low
ppb and sub-ppb levels;
(f) the technique should be highly selective (relatively free from
interferences).
In general, the typical environmental analytical laboratory will be
handling a large number of parameters for a large number of water
samples routinely, and usually with non-professional operators.
Consequently, speed, simplicity, and reliability with minimum
special effort are indispensable. Pre-concentration of the sample
to be able to operate in the sub-ppb range is usually no longer
necessary with some of the refined methods now available, as seen
below.
ADVANTAGE/DISADVANTAGE COMPARISON
Table I compares the chief advantages and disadvantages of the two
techniques for atomic absorption spectroscopy for neutron activation
analysis, and for polarographic methods.
-------
290
Method
Atomic Absorption
Flame
Flameless
TABLE I
Advantages
fast; good precision;
inexpensive
can achieve several
orders of magnitude
sensitivity increase
over flame methods
Disadvantages
can't do many elements
simultaneously
limited to single ele-
ment analysis; not as
fast as flame methods
Neutron Activation high sensitivity and pre- usually the most ex-
cision; does many elements pensive method, but
simultaneously; can ana- seldom most sensitive
lyze refractory materials,
such as ceramics
high sensitivity and
precision; does several
elements simultaneously:
very inexpensive
Polarographic high sensitivity and can become very tedious
operation settling up
for trace analysis;
may be subject to com-
plications due to un-
wanted solution
equilibria, e.g. con-
plexation.
The additional cost-range comparisons given in Table II show at once
that neutron activation analysis, with its extremely high equipment
cost, cannot be considered truly competitive as a trace analytical
technique for routine use with natural water samples.
TABLE II
COST COMPARISON OF INSTRUMENTATION
Equipment
Atomic Absorption Spectrophotometer (Flame)
Range of Cost
$4,000 to $15,000
Atomic Absorption Spectrophotometer (adaptable $9,000 to $20,000
for flameless)
Neutron Activation Analyzer
Polarographic Analyzer
over $100,000
$2,000 to $3,000
Its real utility is in its applicability to the analysis of intract-
ible or refractory materials such as ceramics, rocks, etc., since a
-------
291
solid sample can be readily bombarded with neutrons and counted.
Other points in Table I warrant further, brief discussion. While
it is felt that, in general, spectroscopic techniques require some-
what less skilled technical personnel and involves considerably leas
set-up than electrochemical (polarographic) methods. (2-4), this
is offset by the fact that equipment costs for the electrochemical
methods are significantly lower than for atomic absorption spectro-
scopy, while sensitivities and detection limits are quite comparable,
as is shown in Table III. More importantly, however, atomic absorpti
spectroscopy has still not been demonstrated to be conveniently and
practically adaptable to the routine, simultaneous analysis of severa
elements in a given sample, and the hollow cathode lamp must be
changed over for each new element. Instrumentation for simultaneous
multi-element analysis by atomic absorption spectroscopy is not yet
available (3a). In contrast, polarographic methods are by nature
perfectly suited to the rapid, simultaneous determination of several
trace metals in a given water sample.
Finally, all methods for trace metal analysis are affected to varying
degrees by so-called "matrix" effects: the effect of the surrounding
sample environment both liquid and gaseous, upon the interaction be-
tween the desired element and the detection system. Means for
minimizing matrix effects are discussed in detail elsewhere (2,5),
but one particular solution effect which could seriously affect
trace metal determinations by polarographic techniques warrants
specific discussion here. The potential at which a given metal
oxidation/reduction will occur is, of course, affected by any solution
equilibria in which the metal is involved in the water sample.
-------
292
Many natural waters today may contain any of several chelating
agents, such as NTA (nitrilotriacettc acid), which will shift the
redox potential of a metal ion through complexation. Thus, the
position of a complexed metal wave or peak in a polarogram could
easily be mistaken for this or another, uncomplexed metal. In fact
complexed and uncomplexed trace metals were distinguished and de-
termined in secondary sewage effluent by an anodic stripping
polarographic technique (5).
DETECTION LIMITS
Table III compares the detection limits for atomic absorption,
neutron activation, and polarographic techniques. For the ele-
ments listed, neutron activation analysis gives no improvement
in sensitivity over the other methods, and in most cases is consider-
ably less sensitive, precluding its routine application to aquatic
TABLE III
DETECTION LIMITS, PARTS PER BILLION
Element Atomic Absorption Neutron Activation Folarographlc
Flame
Ag
As
Cd
Cu
Fe
Hg
Mn
Pb
Se
Sn
Zn
3a
250
0.6
3
5
200
3
20
480
30
2
10b
50
1
5
4
5
10
5
Flame less
0.04d
20
0.02
1.4
0.6
20, 1C
0.1
1.0
20
12
0.016
2
5
200
4
50,000
5
0.1
40,000
400
10,000
400
0.1
20
5(.2)®
5(.2)e
100
100
5(.5)e
.10
5
5(De
o
Detection Limits reported by Varian Techtron.
Detection Limits reported by EPA using Instrumentation Laboratories
Model IL-153 instrument. Cf. "Methods for Chemical Analysis of Water
and Wastes, 1971, U.S. Environmental Protection Agency.
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293
Detection Limit for Hg using chemical digestion/reduction techniques
Detection Limits reported by Varian Techtron for carbon rod atomizer
eAnodic stripping techniques; e.g. DPAS.
trace metal analysis. Atomic absorption and polarographic methods
exhibit very comparable, frequently very low, detection limits.
Recent advances in both spectroscopic and electrochemical methods
have extended detection limits for most heavy metals to truly low
levels, in many cases under one ppb. Among such techniques are the
flameless variant of atomic absorption (2,3a)*, and differential
pulse anodic stripping polarography (DPAS) (4b). Table III shows
that each of these methods is capable of extending the detection
*In flameless atomic absorption procedures, the sample is injected
into or onto a small electrically heated furnace or filament rather
than aspirated into a flame to effect atomization. Consequently,
the entire sample is atomized directly into the light path. Since
the furnace or filament is heated in stages to evaporate solvent,
ash organic matter and then atomize the element,several injections
can be made into or onto the furnace or filament and solvent removed
prior to the atomization stage, in effect carrying out a rapid,
convenient concentration step right in the apparatus.
tAnodic stripping polarography differs from direct polarography in
that in the former method, a trace metal is first plated out onto
a mercury electrode (hanging drop), and is then re-oxidized into
solution. Current/potential curves are measured during the oxida-
tion stage and the element determined using calibration curves.
Dilute samples can be analyzed because the desired element is first
"pre-concentrated" in the mercury drop. Differential pulse anodic
stripping (DPAS) polarography is a refinement which further in-
creases sensitivity by minimizing capacitance charging currents.
This is achieved by applying a pulse technique in the re-oxidation
stage. Cf. ref. 4b.
-------
limit downward by up to an order of magnitude or more compared to
standard flame atomic absorption methods on the one hand and direct
conventional or direct pulse polarography on the other. Essentially,
both these variants have extended the detection limits of their
respective methods into the sub-ppb range for many heavy metals
while at the same time eliminating pre-concentration as a necessary
preliminary step. Of course, it must be remembered that such
ultra-sensitivity entails some additional cost, additional operator
skill, and much additional attention to the problems of sample
storage and transfer.
Finally, analysis for mercury should be discussed separately both
because of its increasing ecological significance in the public eye
and because it cannot be handled well by. any of the methods discussed
here. Table III shows the relative insensitivity of conventional
flame atomic absorption analysis of mercury, due to its volatility.
It cannot, of course, be determined polarographically since these
methods employ mercury cathodes. Instead, satisfactory sensitivity
for mercury is achieved by so-called "cold-vapor" atomization
techniques, whereby the sample is first oxidatively digested
(usually with permanganate/sulfuric acid) to convert all mercury
compounds to inorganic salts, and the mercury is then vaporized
monatomically into a gas cell in the radiation path by means of
chemical reduction (usually with stannous chloride). Table III
shows that the detection limit for mercury is improved at least
tenfold over conventional flame atomic absorption procedures by the
use of such "cold-vapor" chemical atomization techniques. One of
the many types of apparatus for "cold-vapor" mercury detection is
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295
shown on Figure 1 (6).
FIGURE 1
flushing .. J f— m-
— • i — i-
gas
"1
7
'_7
I (
\ T
s
water
trap
/-
V / \
/ (} /^SN
r ,i
c'
\
i
drying
agent
'i
H
I
F
f---
L _ __.
flow-t
absr rpt
] cell
\V_ -^/
Hg
treatment
and
volatilization
Apparatus for "cold-vapor" (flameless) atomic absorption
determination of mercury
ACKNOWLEDGEMENT
The author wishes to thank Dr. Joseph Dziedzic of the FMC Corporation
Princeton, N.J., for the many discussions and useful information
which he provided, and essentially which constitute this paper.
-------
296
REFERENCES
1) "The Analyst", Vol. 97, pg. 903-908, 1972, J.C. Ryden, J.K.
Syers and R.P. Harris.
2) C.E. Chakrabartl, Canadian Research and Development, Sept.-Oct.,
17 (1973).
3) a) W.J. Price, "Analytical Atomic Absorption Spectroscopy",
Heyden, London, Eng., 1972.
b) W. Slavin, "Atomic Absorption Spectroscopy", Interscience,
New York, N.Y., 1968.
4) a) J. G. Osteryoung and R.A. Osteryoung, American Laboratory,
4_, (7), page no. 8, (1972).
b) H. Siegerman and G. O'Dom, ibid., 4_ (6), page no. 59 (1972).
5) M.E. Binder, W.R. Matson, and R.A. Jordan, Environmental Science
and Technology,, 4_, 520 (1970.
6) Brooks and Wolfram, Chem. and Eng. News, 48, 37 (1970) -
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297
THE USE OF ATOMIC ABSORPTION SPECTROSCOPY IN ANALYZING
FOR TRACE METALS IN THE ENVIRONMENT
By
Earl L. Henn
Calgon Corporation
Pittsburgh, Pennsylvania
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298
During the last ten years a revolution has been occurring
in the field of trace metal analysis. That revolution has been
the advent of atomic absorption spectroscopy. Ten years ago
chemists were spending large amounts of time in analyzing for
trace metal by colorimetric methods . The addition of many
chemical reagents and the use of tedious extraction techniques
were usually necessary to measure low levels of metallic
constituents in samples brought to them. Quite often the
results from these colorimetric and wet chemical analyses had
to be taken with a grain of salt because of the many interferences
in such methods. Atomic absorption spectroscopy has changed all
that and has provided the modern day analyst with a rapid, sensitive,
specific, and highly reliable method of analyzing for trace
metals in the environment just at the time that such a method is
direly needed. The atomic absorption technique of metals analysis
is not only widely accepted but is generally the preferred method
of metals analysis in education, government and industry and has
been endorsed by the American Society for Testing and Materials^
and the Environmental Protection Agency .
Instrumental Principle
Atomic absorption is based on the fact that metal atoms,
reduced to their "ground" state, will absorb light of the same
wavelength that would normally be emitted by the same metal atoms
when in an excited state. Schematic diagrams of the two different
types of atomic absorption spectrophotometers available on the
market are shown in Figures 1 and 2. A beam of light,
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299
characteristic of the element to be analyzed, is modulated by a
chopper and then passed through a flame or other device designed
to provide sufficient thermal energy to dissociate the metal atoms
in the sample from their various compounds and reduce them to the
ground state. The ground state atoms then absorb the modulated
light from the source. The amount of light absorbed is a measure
of the concentration of the element of interest in the solution
being analyzed. Using this principle, as many as 70 different
elements can be analyzed.
Advantages of Atomic Absorption
There are numerous advantages to the environmentalist in
using atomic absorption for detecting low levels of metallic
elements. First of all, atomic absorption is a highly specific
method of analysis. In colorimetric techniques it is usually
very difficult, if not impossible, to find a reagent which is
specific to a certain metal to the exclusion of all others.
Consequently, numerous other metals usually interfere. Also,
flame emission analysis is subject to numerous spectral interferences
which cause spuriously high results when certain interfering
metals are present. None of these problems are encountered in
atomic absorption. Spectral interferences are virtually non-
existent in atomic absorption and the method is highly specific
for each metal analyzed.
Another advantage which atomic absorption offers is that it
is relatively free from chemical interferences. Such interferences
do exist but they are relatively few in number and, when they
do occur, can usually be obviated with relative ease.
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300
Another important factor in the rapid acceptance of
atomic absorption is that it is fast and economical. Today,
many analyses which used to take hours to perform can be done
by atomic absorption in a matter of seconds. In addition, the
cost of equipping a laboratory to perform atomic absorption
analysis is moderate. Most commercially available instruments
cost somewhere in the neighborhood of $10,000 and some single
beam instruments can be obtained for as little as $5000. In
a laboratory which performs many metals analyses, an atomic
absorption spectrophotometer will easily pay for itself in a short
period of time.
Finally, probably the biggest advantage which atomic
absorption offers the environmentalist is its high sensitivity
for most elements. In our laboratories, we calculated the
detection limits for 24 different elements using a Perkin-Elmer
Model 403 Atomic Absorption Spectrophotometer. (The detection
limit is defined as the concentration of an element in water
solution which gives a signal twice the size of the peak-to-peak
variability of the background.) These detection limits are
listed in the attached table. Note that a majority of the
detection limits are in the low ppb range while the detectability
for three of the elements (calcium, magnesium and beryllium) is
less than one ppb. While these detection limits cannot always
be attained when analyzing real samples, they give some indication
as to the applicability of atomic absorption to trace metal
analysis.
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301
In addition to the" high sensitivity attainable by direct
aspiration of a sample into the flame, numerous methods have
been devised for increasing the sensitivity of atomic absorption.
Chelation of the metal ions in solution and extraction of the
resultant complex into an organic solvent can increase the
sensitivity of atomic absorption by one or two orders of
magnitude ^ '.
Other methods of increasing the sensitivity of atomic
absorption measurements have been introduced in recent years.
The most successful of these have been the tantalum boat
and Delves cup technique. These methods are based on the
fact that some metals are sufficiently volatile to be vaporized
at the temperature of the air-acetylene flame and involve direct
introduction of the sample (after drying) into the flame on
a tantalum strip or nickel cup. This technique, however, is
limited to only those metals which will vaporize at or near
2000°C. The only metals of concern to environmentalists which
fit into this category are lead, cadmium, zinc, silver,
arsenic and selenium.
Flameless Techniques
Enormous strides have been made in recent years in improving
the sensitivity of atomic absorption measurements by using
electrically heated graphite tubes or similar devices for atomi-
zation of samples(6). These flameless atomization devices provide
great increases in sensitivity resulting in detection limits
two or three orders of magnitude below those achieved with most
flame techniques because all of the sample is atomized (compared
with only 5-10% when flame methods are used) and the residence
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302
time of the ground state atoms in the light beam from the source
is greatly increased. In addition to increased sensitivity, only
a very small amount of sample is required using these devices
and the flameless methods can be used for analysis of all but
the most refractory metals.
Another flameless technique which has been extensively
used by environmentalists for the last five years is the "cold
vapor" atomic absorption analysis of mercury . This type of
analysis is different from most atomic absorption methods in
that chemical reagents rather than thermal energy are used to
ceduce the mercury to the elemental ground state. In the method,
mercuric ions in solution are reduced to elemental mercury by
the action of stannous ion. The mercury vapor is then pumped
into a quartz absorption cell positioned in the light path of
an atomic absorption spectrophotometer. The amount of light
Absorbed at the resonance wavelength of mercury is a measure
of the concentration of mercuric ion in solution. Using this
technique, mercuric ion can be measured down to concentrations
of a fraction of a ppb.
Applications
Atomic absorption has found many different applications in
the field of environmental analysis. The most immediate and
obvious application is to the analysis of water. This includes
/ o q "\0)
natural waters, saline waters and industrial wastewaterv ' ' .
The most extensive application of atomic absorption to the analysis
of natural waters has been the analysis of mercury^ '. The
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303
concern in recent years over contamination of the environment with
mercury as a byproduct of various industrial processes resulted
in an urgent need for analysis of natural waters for mercury
content. The cold vapor atomic absorption method of analyzing
for mercury provided the sensitivity and reliability needed to
meet the crisis. Almost every metal of environmental concern
found in natural waters can and is routinely analyzed by atomic
absorption methods in laboratories throughout the nation. This
includes seawater as well as freshwater.
Another important application of atomic absorption methods
is to the analysis of industrial wastewaters ^ . As is well
known, the main source of pollution in our lakes and streams is
the effluent from industrial processes. In order for industry
to monitor the quality of a wastewater being dumped into a
waterway and for government to enforce quality standards an accurate
means of analyzing wastes is required. The specificity and
freedom from interference of atomic absorption permits this
technique to play a major role in regulating the quality of
industrial waste effluents.
Many times pollutants are lodged in the sediments found
at the bottom of lakes and streams. Atomic absorption has also
/1 n \
been applied to this type of materialv;. In most cases
where metal analyses are desired, the metals are leached from
the sediment with acids and determined by atomic absorption.
Even various marine organisms have yielded to the versatile
analytical technique of atomic absorption. Methods of analyzing
for mercury in fish by atomic absorption are well established^ '
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304
and the analysis of cadmium in marine organisms by this technique
(14)
has been reported . In most cases these analyses are performed
by homogenizing the flesh of the organism and chemically oxidizing
the organic material by means of acid reflux. The resulting
solution is then analyzed for mercury using the cold vapor method
and for other metals either by direct aspiration or flameless
methods.
Some of the pollutants in the atmosphere end up being
precipitated in rain or snow. Flameless atomic absorption
techniques have proved their worth in analyzing for these trace
contaminants
Atomic absorption has been used to monitor air pollution
as well as water pollution^16). In analyzing the metal content
of particulate matter in the air, the air is usually drawn
through a filter. The filter is then either burned or washed
with acid solution. The particulate matter that had been
trapped is then solubilized and analyzed by atomic absorption.
This technique has been found very successful in monitoring
the amount of lead in airborne particles ' '.
Another place where lead in the environment has created
a problem is in household paints. Lead poisoning in young
children who have chewed on the walls and woodwork of old
houses that have been painted with lead-based paint is fairly
common(18). As a result, the federal government has laid down
stringent regulations governing the amount of lead permitted in
f19 20)
household coatings v ' . Atomic absorption is the only
method that provides sufficient sensitivity to meet the
analytical requirements of these regulations. In analyzing for
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305
lead in paint, either the paint is ashed and the residue
dissolved in acid solution(21) or else the Delves cup method
is used whereby the paint is suspended in an appropriate solvent
and a portion of the slurry is transferred to a nickel cup.
After drying, the cup is inserted into the air-acetylene flame
(? 2}
where the lead is volatile zed and measured by atomic absorption v ' .
Similar methods are used in the field of biochemistry in
diagnosing and treating victims of environmental pollution. The
Delves cup method has been used quite extensively in clinical
(5)
laboratories for the last few years to determine lead in blood
The new flameless techniques have also begun to be used in
clinical applications for determining concentrations of toxic
metals due to the high sensitivity attainable and small sample
(23)
size requirements of these methods
Besides the many direct applications of atomic absorption
to environmental problems there are many indirect applications
as well. The use of atomic absorption as a quality control tool
in the food industry is one such application '. The analysis
of coal, oil, rubber and plastics provides a means for evaluating
the amount of toxic metal contaminants introduced to the environmer
by these common substances.
In light of the fact that atomic absorption methods are
continuing to grow in scope and application there is no
doubt that this analytical technique will continue to play a
large role in monitoring the level of contaminants in our
environment and in pollution abatement.
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306
Detection Limits for Various Metals by
Atomic Absorption With the Perkin-Elmer
Model 403 Atomic Absorption Spectrophotometer
Element Detection Limit, mg/1
Aluminum 0.04
Antimony 0.06
Arsenic 0.6
Barium 0.06
Beryllium 0.0009
Boron 2.6
Cadmium 0.002
Calcium 0.0004
Chromium 0.007
Cobalt 0.016
Copper 0.003
Iron 0.003
Lead 0.025
Magnesium 0.0003
Manganese 0.003
Molybdenum 0.014
Nickel 0.006
Selenium 0.6
Silicon 0.08
Silver 0.002
Tin 0.12
Titanium 0.06
Vanadium 0.002
Zinc 0.002
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307
LAMP
CHOPPER
FLAME
MONOCHROMATOR
DETECTOR+AC
ELECTRONICS
SINGLE BEAM AC SYSTEM OF ATOMIC ABSORPTION
FIGURE 1
REFERENCE BEAM
SAMPLE BEAM
MONOCHROMATOR
DETECTOR+AC
ELECTRONICS
BEAM RECOMBINED
(Half Silvered Mirror)
DOUBLE BEAM AC SYSTEM OF ATOMIC ABSORPTION
FIGURE 2
-------
308
REFERENCES
(1) American Society for Testing and Materials, 1972 Annual
Book of ASTM Standards, Part 23, Philadelphia, Pa.,
D2576, Pg. 692 (1972).
(2) Environmental Protection Agency, Methods for Chemical Analysis
of Water and Wastes, Cincinnati, Ohio, pg. 83 (1971).
(3) Sachdev, Sham L. and West, Philip W. - Environmental
Science and Technology, Vol. 4, No. 9, Pg. 749 (Sept. 1970).
(4) Kahn, H.L.; Peterson, G.E. and Schallis, J.E. - Atomic
Absorption Newsletter, Vol. 7, pg. 35 (1968).
(5) Delves, H.T. - Analyst, Vol. 95, pg. 431 (1970).
(6) Kahn, Herbert - Industrial Research, Feb. 1973, pg. 37.
(7) Hatch, N.R. and Ott, W.L. - Analytical Chemistry, Vol. 40,
pg. 2085 (1968).
(8) Kuwata, K; Hisatomi, K. and Hasegawa, T. - Atomic
Absorption Newsletter, Vol. 10, pg. Ill (1971).
(9) Orren, M.J. - J. S. Afr. Chem. Inst., Vol. 24, pg. 96 (1971).
(10) Talalaev, B.M. and Mironova, O.N. - Zh. Anal. Khim,
Vol. 25, pg. 1317 (1970).
(11) Cranston, R.E. and Buckley, D.E. - Environmental Science and
Technology, Vol. 6, Pg. 274 (1972).
(12) Delfino, J.J.; Bartleson, G.C. and Lee, G.F. - Environmental
Science and Technology, Vol. 3, pg. 1189 (1969).
(13) Munns, R.K. and Holland, D.C. - J. Assoc. Offie. Anal.
Chemists , Vol. 54, pg. 202 (1971).
(14) Eisler, R. - J. Fish, Res. Bd. Canada,, Vol. 28, pg. 1225 (1971)
(15) Murozumi, M.; Nakamura, S. and Patterson, C.C. - Bunseki
Kagaku, Vol. 19, pg. 1057 (1970).
(16) Hwang, Jae Young, - Analytical Chemistry, Vol. 44, No. 14,
pg. 30A (Dec. 1972).
(17) Kettner, H. - Journal of Water, Earth and Air Hygiene, Vol. 29,
pg. 55 (1969).
(18) Smith, H.D. - Arch. Environ. Health, Vol. 8, pg. 256 (1964).
(19) Federal Register, Vol. 37, No. 155, Aug. 10, 1972.
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309
(20) Chemical and Engineering News, Aug. 21, 1972, pg. 13.
(21) Searle B.; Chan, W.; Jensen, C. and Davidow, B. -
Atomic Absorption' Newsletter, Vol. 8, pg. 126 (1969).
(22) Henn, E.L. - Atomic Absorption Newsletter, Vol. 12, No. 5
Pg. 109 (1973).
(23) Glenn, M.; Savory, J; Hart, L.; Glenn, T. and
Winefordner, J. - Anal. Chim. Acta, Vol. 57, Pg. 263 (1971)
(24) Meredith, M.K.; Baldwin, S. and Andreasen, A.A.- J. Assoc.
Offic. Anal. Chemists, Vol. 53, pg. 12 (1970).
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310
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311
THE OCCURRENCE OF TRACE METALS IN SURFACE WATERS
Robert C. Kroner
The spectrographic measurement of trace metals in surface waters
of the United States was performed routinely by the Water Pollution
Surveillance System of the USPHS from the years 1960 to about 1968.
The system, which is no longer operated, was established in 1957, in
cooperation with industry, state, and local authorities and other
Federal agencies with related responsibilities for the collection and
dessimination of basic data on water quality. The Surveillance System
at its peak consisted of about 130 sampling points located on major
waterways throughout the United States. Local personnel at the sampling
sites collected and mailed samples to Cincinnati where they were analyzed,
In addition to trace metals, data was also collected for physical,
chemical, biologic, microbiologic and radiologic parameters.
The information and statistical observations given in this paper
are taken from the analysis of approximately 1500 samples collected over
a five-year period. The analyses were performed on a direct reading
emission spectrograph using a rotating disc according to the method
described by Kopp and Kroner.
TRACE ELEMENTS
The source and significance of dissolved minerals are very important
in evaluating water quality. Indeed, the spectrographic analysis of
water commonly reveals the presence of a surprisingly large number of
elements, most in only trace amounts.
The significance of minor elements in human metabolism is not under-
stood completely, but much progress has been made by biochemists,
-------
312
physicians, and public health scientists. For instance, cobalt, copper,
zinc and certain other metals are believed to be important catalysts in
the biosynthesis of amino acids, whereas certain forms of arsenic, lead
and cadmium are known to be significantly toxic.
(2)
The 1968 Water Quality Criteria report discusses the significance
of a number of trace metals and attempts to set limiting standards as they
affect public water supplies, aquatic life, agricultural uses and industry.
Table 1 shows the criteria for trace elements in drinking water supplies.
With regard to agriculture, variations and interactions of soils,
plants, water and climate preclude the establishment of a single set of
criteria to evaluate all water quality characteristics. Toxic limits
for trace elements which would be generally applicable to all soils and
all crops are not easily defined. In general, trace element tolerances
for irrigation waters are much higher than for other farmstead oses.
Industrial uses present much the same problem, inasmuch as water
that meets the standard for the textile industry may not be acceptable
to the food canning industry, and so on. However, industrial requirements
are not nearly as stringent as those for public water supplies. In the
majority of instances, a water that meets the criteria for public water
supplies, as shown in Table 1, will also be acceptable for fish and aquatic
life, livestock and other agricultural uses as well as for industrial uses.
In general, the criteria for public water supplies are much the same as
(3)
those given in the 1962 revised USPHS Drinking Water Standards.
Among the listed materials that fall into the trace metal category
and that may be measured spectrographically are arsenic (As), barium (Ba),
cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), lead (Pb), manganese
-------
313
(Mn), silver (Ag), and zinc (Zn). Other trace metals also occur with
varying frequencies in river waters and may be of significance, even
though their roles in physiologic processes are not completely understood.
For this reason, in addition to those already listed, boron (B), molybdenum
(Mo), aluminum (Al), beryllium (Be), nickel (Ni), cobalt (Co), vanadium (V)
and strontium (Sr) were also routinely monitored.
The aforementioned trace elements, along with many others, originate
from a variety of sources, but can be classified into three principal
groups:
1. Elements contributed by soluble materials chemically
weathered from soil and rocks.
2. Elements that are concentrated selectively by vegetation
and find their way to surface waters after decay and
runoff.
3, Industrial sources, especially those devoted to mining,
alloying, and cleaning and plating of metals.
Although the first two sources tend to provide normal or expected
trace-element levels, it is with the last source that our main interest
lies.
Figure 1 shows the percent frequency of detection for trace elements
in waters of the United States as determined from the analysis of over
1500 surveillance system samples. Barium, boron, and strontium are found
in measurable concentrations in all samples; iron, zinc, copper, and
manganese occur between 60 and 80 percent of the Lime; and aluminum,
molybdenum, chromium, and lead, between 20 and 30 percent. Beryllium,
vanadium, cadmium, and cobalt, however, are rarely found, and their
-------
presence would indicate an unusual source of pollution. From experience,
it is possible in many instances to predict beforehand what will be found
in a particular stream. Trace-element levels in some streams remain
remarkably consistent; others fluctuate considerably.
Figure 1 shows only the frequency of detection of the nineteen elements
included in the analytical program. The mean concentrations and ranges of
concentrations of the metals are shown in Table II. Elements such as zinc,
boron, iron, molybdenum, manganese, aluminum and strontium are observed
occasionally at concentrations exceeding 1 mg/liter. Other elements,
including cadmium, arsenic, copper, nickel, cobalt, lead, chromium and
barium, are observed with lesser frequency and at approximating one tenth
the concentrations found for the former group. Beryllium and silver, two
metals that are rarely seen, occur at concentrations generally below
1 yg/liter.
An example of trace metal concentrations in spectrographically "clean"
and "dirty" streams is shown in Table 3.
The St. Marys River is a stream that carries a heavy traffic of
shipping but little or no industrial discharges. The Cuyahoga River
receives numerous industrial discharges especially from the metals
industries as it flows through the city of Cleveland. By comparison, the
concentrations in the Cuyahoga River are many times higher for most metals.
Zinc, boron, manganese, aluminum, lead, chromium and nickel are increased
by large factors. In industrial streams such as the Cuyahoga, these
metals slowly precipitate onto the stream bottoms, where they serve as a
constant source of metals for re-solution into the flowing supernatants.
Further, if dredging of the stream is required, the bottom sediments are
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315
extremely high in trace metals and disposal of the sediment creates
another problem.
When the Surveillance System initiated the routine measurement of
these trace elements, it was concerned primarily with elements in
solution because any suspended material would be removed before it
reached the consumer. While the philosophy of this approach is valid,
it does not measure the total trace element load in the stream. It should
be emphasized at this point, and strongly so, that not enough is known
about the trace elements associated with river-borne suspended matter.
Water quality criteria selected for trace elements in solution ignore or
overlook the possibility of trace metals in the sediment load. Current
laboratory practices are to analyze both the dissolved, as well as the
suspended, fraction.
When natural water samples are filtered through a micropore filter
and the suspended and dissolved fractions analyzed separately, the data
obtained shows, in a general way, the solubilities of the metal salts.
A review of the data indicates that some metals occur much more frequently
in the suspended matters than in solution. This is particularly true of
iron, aluminum, manganese, nickel and lead. Conversely, strontium, boron,
copper and cobalt are more likely to be found in solution. Tables 4 and 5
show the relationship between metals in solution and metals in suspension.
An analytical difficulty arises when one needs to discriminate between
naturally-occurring and industrially-sourced metals in suspension. Types
of clay consist of aluminum and iron, and dissolved metals can and do,
adsorb onto the clay particles. An additional analytical burden is placed
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316
upon the analyst when it is necessary to distinguish between the sources
of metals in natural waters.
SUMMARY
This paper has reviewed sources and concentrations of trace metals
in surface waters of the United States. The mean concentrations of metals
in natural waters have been discussed and the difference between a
spectrographically "clean" and "dirty" stream is shown.
-------
Co
Cd
V
Be
Ag
Ni
Pb
Cr
Mo
Al
Mn
Cu
Zn
Fe
Sr
B
Bo
0 10 20 30 40 50 60 70 80 90 100
Figure I.- Frequency of detection for trace elements in waters of the United States.
(1500 samples)
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318
TABLE 1
SURFACE WATER CRITERIA FOR TRACE ELEMENTS
Metal
IN PUBLIC WATER SUPPLIES
Permissible
Criteria, mg/1
+6
Arsenic
Barium
Boron
Cadmium
Chromium'
Copper
Iron (filterable)
Lead
Manganese (filterable)
Selenium
Silver
Zinc
0.05
1.0
1.0
0.01
0.05
1.0
0.3
0.05
0.05
0.01
0.05
5
Desirable
Criteria, mg/1
Absent
11
Virtually Absent
Absent
ti
M
ii
Virtually Absent
Absent - The most sensitive analytical procedure in Standard Methods
(or other approved procedures) does not show the presence of the
subject constituent.
Virtually Absent - This terminology implies that the substance is
present in very low concentrations and is used where the substance
is not objectionable in these barely detectable concentrations.
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319
TABLE 2
SUMMARY OF TRACE ELEMENTS IN WATERS OF THE UNITED STATES*
Element
Zinc
Cadmium
Arsenic
Boron
Phosphorus
Iron
Molybdenum
Manganese
Aluminum
Beryllium
Copper
Silver
Nickel
Cobalt
Lead
Chromium
Vanadium
Barium
Strontium
Frequency
Of Detection, %
76.5
2.5
5.5
98.0
47.4
75.6
32.7
51.4
31.2
5.4
74.4
6.6
16.2
2.8
19.3
24.5
3.4
99.4
99.6
Observed
Positive Values, yg/1
Max.
1183
120
336
5000
5040
4600
1500
3230
2760
1
280
38
130
48
140
112
300
340
5000
Mean
64
9.5
64
101
120
52
68
58
74
0.19
15
2.6
19
17
23
9.7
40
43
217
*Based on 1500 samples.
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320
TABLE 3
COMPARISON OF TRACE METAL CONCENTRATIONS IN
POLLUTED AND UNPOLLUTED RIVERS*
Cuyahoga River St. Marys River
(Cleveland) (Sault Ste. Marie)
Metal Freq. % X, yg/1 Freq. % X, Hg/1
Zn 100 423 100 46
B 100 302 100 10
Fe 69 59 80 24
Mn 88 285 60 2
Al 19 27 71 6
Cu 31 9 100 6
Ba 100 50 100 11
Sr 100 148 100 15
Pb 12 28 53 5
Cr 19 11 20 3
Ni 69 70 20 11
Mo 12 27 33 9
Cd 19 64 0 0
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TABLE 4
Mean Concentration and Percent Occurrence of Trace Elements*
Element
Strontium
Boron
Barium
Zinc
Copper
Aluminum
Manganese
Iron
DISSOLVED
Range of
Positive
Values, yg/l
4-520
2-750
5-195
6-97
3-280
1-1,875
1-3,230
2-144
Percent
Occurrence
100
100
100
91
80
25
56
70
Mean
yg/l
104
72
43
63
24
85
330
19
SUSPENDED
Range of
Positive
Values, yg/l
2-3
3-108
1-65
5-151
3-66
3-1,440
2-442
7->2500
Percent
Occurrence
17
86
97
90
85
87
98
100
Mean,
yg/l
2
16
11
30
10
316
100
>395
ro
H
*At seven selected stations.
-------
TABLE 5
Concentration Ranges and Percent Occurrence for Some Lesser Elements*
Element
Vanadium
Cadmium
Silver
Cobalt
Chromium
Beryllium
Nickel
Lead
DISSOI
Range of Positive
Values, yg/1
15-54
3-11
0.7-3.0
13-48
2-25
0.03-3.2
3-86
6-205
,VED
Percent
Occurrence
3
4
4
12
23
30
35
25
SUSPENDED
Range of Positive
Values, yg/1
12, 13
6, 16, 35
0.5-2.5
8-13
3-13
0.04-2.35
5-900
10-625
Percent
Occurrence
2
3
5
8
38
39
20
35
ro
*At seven selected stations.
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323
ION SELECTIVE ELECTRODE MONITORING
FOR TRACES OF HEAVY METALS
by
Isaac Trachtenberg
Texas Instruments Incorporated
Dallas, Texas 75222
Ion-selective electrodes can be most readily used to moni-
tor relative changes in concentration of traces of heavy met-
als in a variety of aqueous solutions. Note the emphasis on
changes in concentrations. Absolute concentrations may be
determined only after very careful electrode calibrations. If
very accurate analytical results are required, samples must
still be taken to the laboratory for careful work with either
ion-selective electrodes or other, more precise analytical
techniques. Automated monitoring with ion-selective electrodes
will not replace the analyst, but can relieve him of much of
the monotony of analyzing routine samples that are usually
within specifications. The ion-selective electrode signals
any significant deviation from a preset condition, allowing
the operator to take a sample and the analyst to determine
very carefully the exact concentration of the particular ionic
species involved. An added advantage of ion-selective electrode
monitoring of process streams is that it can also signal the
operator or the control computer that a change or an upset has
occurred in the process so that the various non-continuous
monitors can be integrated and appropriate actions taken.
It may be convenient to think of a three-light system in
connection with continuous automated monitoring with ion-selec-
tive electrodes. If the concentration of the species to be
-------
monitored is at its designed level and well within the pre-
scribed limits, then the light as controlled by the electrode
is green and there is no need to sample the stream. If signi-
ficant changes in concentration occur (>25%), the light changes
to amber, signaling the operator to check the process and take
samples for laboratory analyses. If the concentration continues
to change and reaches a level where there is significant proba-
bility that the process is grossly out of specification and that
damage to personnel, equipment, or the environment can occur,
then a red light is turned on, alarms are sounded, and the pro-
cess may be automatically shut down.
With such a three-light system samples are taken for lab-
oratory analysis only when the ion-selective electrodes indi-
cate there has been a deviation from the normally specified
process concentrations. In addition, signals from the ion-
selective electrodes can be used to adjust other operating
parameters such as flows, temperature, and concentration of
reactants.
Ion-selective electrodes have been applied to monitoring
heavy metal ions in saline and brackish waters (1,2). Appli-
cations of these electrodes to several non-heavy metal ionic
species have been described by Light (3) and Jasinski and
Trachtenberg (4). Orion Research Incorporated, Cambridge,
Mass., has published several booklets that list a large number
of commercially available electrodes, how they operate, inter-
ferences, limits of detection, and an extensive bibliography,
These are available from OrionVs Technical Service Department (5).
Ion-selective electrodes in combination with a suitable
reference electrode yield a potential response which is Nernstian
with respect to the selected ion. The potential developed be-
tween the ion-selective electrode and the references is propor-
tional to the logarithm of the concentration of the selected
-------
325
ion (C ), provided the ionic strength of the solution does not
s
change (constant activity coefficient). Equation (1) describes
this relation:
o nF ^ s
where E is a constant depending on the electrode system, R is
the usual gas constant, T is absolute temperature, F the Faraday,
and n the number of electrons involved in either the surface
adsorption process on the electrode or the number of charges
transferred per ion. The quantity n is usually an integer;
however, in instances of certain adsorption processes or mixed
reactions the value of n need not be a whole integer. For a
one-charge process at 25°C, 2.3RT/nF is approximately 59 millivol
and the ion-selective electrode has a response similar to that
of a pH electrode, approximately 60 millivolts per tenfold
change in concentration. Actually, ion-selective electrodes
measure activities rather than concentration; however, in the
presence of large, unchanging amounts of indifferent electro-
lytes the ionic strength of the solution is fixed, the acti-
vity coefficient becomes constant, and the potential of the
electrode is proportional to the logarithm of the concentration.
Because of the logarithmic relation between potential and
concentration, ion-selective electrodes exhibit relative rather
than absolute error. It is usually fairly easy to obtain the
potential of any given solution to within ±1 mV. For a one-
charge process this represents a ±4% relative error. This re-
lative error is the same at 0.1 ppm as at 100 ppm of the se-
lected ion. For a two-charge process the relative error be-
comes about ±8%. (Because relative rather than absolute error
is indicated, more precise results require additional labora-
tory work. )
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326
Ion-selective electrodes, like pH electrodes, require
periodic calibration. This is similar to adjusting zero and
100% on a variety of other instruments. This operation is
necessary to compensate for changes in the electrode which
change the EQ term. The slope remains constant, but as the
electrode ages, random, slow drifts, occur which must be com-
pensated for periodically.
The slow drifts, at least for the solid state electrodes,
are usually the result of some slow, irreversible adsorption or
chemical process occurring on the surface of the electrode.
These processes can almost totally destroy the activity of the
electrode. Usually, activity can be restored by etching and
reactivation of the electrode or by a mechanical grinding
followed by chemical reactivation of the fresh surface. It is
important to note that these electrodes are active and do under-
go certain side reactions, albeit very slow, and thus under
prolonged continuous use may require periodic activation. The
time between reactivation should be of the order of several
months to a year for most practical electrodes.
Ion-selective electrodes are subject to a number of inter-
ferences; therefore, one must know at least qualitatively the
composition of the solutions being monitored.
The electrodes can be operated in one of several modes,
depending on the composition of the stream to be monitored:
(1) The electrodes are directly immersed in the stream to
be measured. This mode of operation is most practical when
there is a large excess of indifferent electrolyte such as NaCl
in saline waters.
(2) A small side stream is taken from the main process
stream, and reagents are added to this sample stream before it
contacts the ion-selective electrode. This reagent usually
adjusts pH and ionic strength.
-------
-------
328
TABLE I
Analyses by Cu-Au2S3 Electrodes of Brines From
OSW Desalting Plant, Freeport, Texas
(Samples Taken 10/2/72)
Sample #1 Sample #2 Sample #3
pH (as received) 8.0 6.3 7.0
Cu+ (ppm)
PH 6-° N.D.* 0.025 0.63
„ +2 .
Cu (ppm)
PH 2-0 N.D.* 0.45 1.44
+2
Cu (ppm)
pH 6.0 (AA**) 0.001 0.080 1.02
S0~ (ppm) 2360 2550 5700
*N.D. = Not Detectable
**AA = Extraction followed by atomic absorption analyses
A Cu -As-S- (1,6) , ion-selective electrode (ISE) was used
+2
to monitor the samples for Cu . The atomic absorption (AA)
results were obtained by extracting the sample at pH 6.0. The
S0~ analyses were performed by titrating with Bad, and using
an Fe-1173 electrode (7,8,9) to indicate the end point. The
pH of the solutions was adjusted by adding dilute
The results presented in Table I indicate there is very
little copper (<1 ppb) of any kind in the incoming raw seawater
(Sample fl) . As the brine proceeds through the process, it
begins to pick up copper in various forms. The results for
Sample #2 indicate that about 0.025 ppm Cu is present (ISE
at pH 6) and about 0.425 ppm is present as either a soluble
complex or particulate copper (ISE at pH 2 minus ISE at pH 6) .
The AA results indicate that at pH 6.0, total soluble copper
-------
329
had increased to 1.02 ppm. The SO. results indicate a reduc-
tion in brine volume of 55%. The copper concentration in the
brine should increase by 223% if there were no additional
pick-up during the distillation portion of the process. These
results indicate that there has been additional copper disolu-
tion, but no significant increase in copper particulate (0.24
ppm copper particulate). In fact, it appears that some of the
particulate copper was dissolved during the heat exchanging
portion of the process.
This is but one illustration of how ion-selective elec-
trodes can be used to monitor a process. In this case two
reasons existed for monitoring: (1) to limit the amount of
copper in the final brine discharge, and (2) to monitor the
rate of corrosion of the heat exchangers. It should be noted
that the accuracy of these laboratory measurements is much
better than the results to be expected from a field operation.
Ion-selective electrodes can also be used in laboratories
to replace certain existing methods. Previously, the standard
method for determining total copper in th^ese brines was to add
excess HF to the sample and determine the total copper concen-
tration spectrophotometrically. The HF was added to complex
iron and dissolve a variety of siliceous materials which ad-
sorb copper. The standard procedure was carried out with the
replacement of the spectrophotometer by an ion-selective elec-
trode as the copper indicator. When HF was added, the copper
concentration increased; however, as the CaF2 precipitated, the
copper began to decrease, indicating significant adsorption or
coprecipitation of copper. Thus, an ion-selective electrode
can be used to measure a variety of kinetic parameters associ-
ated with removal of free ions from solution.
Finally, certain precautions should be taken in using ion-
selective electrodes. In general, these precautions are very
similar to those required for in-line process monitoring of pH
-------
330
or conductivity. The temperature must be known and compen-
sated; the electrode must be kept clean of foreign matter;
single point calibration should be performed frequently; and
multipoint recalibration should be accomplished periodically.
The frequency with which these operations should be carried
out is greatly dependent on the nature of the operation, the
rate of change of the electrode, the rate of change of the
process, and the desired accuracy.
Ion-selective electrodes can be used to continuously
monitor a wide variety of aqueous process streams. However,
they are just monitors. The cannot be used to replace the
analyst in the laboratory, but can be a significant aid in
reducing the number of routine analyses required. They are
best used to indicate significant concentration changes
(>25%) on a rapid, real-time basis. With precise calibration,
they can be used to indicate changes as small as a few per-
cent (<5%) . Effective use of these electrodes requires quali-
tative information about the streams to be monitored. Suc-
cessful applications of these electrodes have been described
previously (3,5).
REFERENCES
Trachtenberg, I., Ion-Selective Electrochemical Sensors
Final Report OSW Contract #14-01-0001-1737 Feb. 1973.
Rice, G., Trachtenberg, I., and Jasinski, R., The Elec-
trochemical Society, Extended Abstracts 73-2, 708, 1973.
Light, T.S., Ion-Selective Electrodes, Richard A. Durst
Editor National Bureau of Standards Special Publication
314, 1969 p.349.
Jasinski, R., and Trachtenberg, I., Anal. Chem. 45, 1277,
(1973).
Orion Research Incorporated, 11 Blackstone St., Cambridge,
Mass 02139.
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331
6. Jasinski, R., Trachtenberg, I., and Rice, G., J. Elec-
trochem. Soc., 121, (March 1974).
7. Baker, C., and Trachtenberg, I., J. Electrochem. Soc.,
118, 571, (1971).
8. Jasinski, R., and Trachtenberg, I., Anal. Chem., 44, 2373,
(1972).
9. Jasinski, R., and Trachtenberg, I., Anal. Chem., 45^ 1277,
(1973).
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332
-------
333
ANALYTICAL PROCEDURES FOR
TRACE HEAVY METALS IN WATER
C. Calmon, Birmingham, N. J.*
It was pointed out by the writer at this conference that trace
heavy metal ions in water can exist in three forms, (1) as parti-
culates , (2) as ions, and (3) as complexed with organic entities.
Most of the writer's experience has been with ultrapure water sys-
tems which require water with resistances of 10 to 18 megohm-cm and
with very low particulate counts. For example, the specifications
for water used in oncethrough boilers (super critical pressures)
of many modern plants are:
Impurity
Total Dissolved Solids
Silica
Iron
Copper
Calcium and Magnesium
Oxygen
Resistance
Max. Limit (PPB)
<^ 50
<10 - 20 (some)
< 5 - 10 (some)
•< 2
0
-=. 5 - 7 (some)
>10 megohm-cm (at 25° C.)
Particulates
Two approaches are used for particulates.
(1) Membrane filter, and
(2) Particle Counters
Membrane filters are applied (1) in a static or (2) dynamic way. In the
static procedure water is passed through a membrane of given porosity and
the rate of flow is determined. This gives the rate of particulate depos-
ition which is a measure of the concentration. This method is frequently
referred to as the "Silting Index".
In the dynamic method the membrane tape continuously passes at a controlled
speed and with a constant flow of water passing through the tape. The
color and intensity of the deposits give an indication of the type and con-
centration of particulates. Both white and black filters are available.
Such filters can detect particulate solids in the range of 2 to 4 ppb.
The accumulated deposits can be analyzed by various chemical or spectre-
graphic methods.
^Formerly Sybron Corp., West Orange, N.J. 07052
-------
334
As these water purification systems use ion exchangers frequently
some of the particulates may be sloughed or broken off from the
resin particles. If present, these will also deposit on the mem-
brane filters, and therefore must be distinguished from the metal
deposits. The resin particles are insoluble in acid or alkali
solutions.
A particle counter can give an indication of the size and concen-
tration of particulates at various points of a system so the
source of fouling can be traced.
Total c . Concentration
To determine the solid concentration in the ppb range, a large volume
of water is evaporated with an IR lamp as the heat source in a closed
system. The water of a given volume is introduced into a silicon dish
at the same rate as the evaporation rate. The dish rests on a balance
so the weight of the dish and the dish with the solids can be read with-
out transfer.
Ion exchangers are used for concentrating trace quantities of ions
when existing in concentration below the limit of a specific method
of analysis. The ion is eluted or the resin can be burned and then
treated chemically as required for the specific analysis. Cu, Pb,
Zn, Co, Ni, Ag, Sr, etc. have been concentrated from natural waters.
To determine the total ionic (cations) concentration in ppb quanti-
ties, the water is passed over a cation exchanger in the H form and
the conductivity of the effluent is determined. The acid makes
the conductivity more sensitive to cations in the range of a few
ppb's. This holds if the anion present forms a strong acid with
the released hydrogen ion.
Breakthrough of Multivalent Ions
Low cross linked resins show wide changes in volume depending on
the valence of the ion. The higher the valance the lower the vol-
ume of the resin. Thus, in a softener when calcium and/or magnes-
ium breakthrough the resin in contact with the effluent begins to
shrink and the shrinkage is detected photoelectrically so regene-
ration of the resin can take place.
If a specific resin of low cross linking were available, the change
in volume could be an indication of the breakthrough of a specific
multivalent ion.
-------
335
Miscellaneous uses of Ion Exchangers for analysis which may be used
as monitoring techniques.
(1) Ion Exchange papers with which rapid qualitative analysis
of trace amounts of metallic ions can be made are available.
(2) Many spot tests (microchemical detection) with ion exchange
beads are also available. Nearly colorless ion exchange
beads are bound with reagents which on reacting with a
given metal ion produce a distinct color or decolization.
Microchemical detection tests for the following metal ion
tests are available — the reagents, type of exchanger as
well as the color are well covered by Iczedy (cf . biblio-
graphy)
1. Al 6. CrO^ 11. Ga 16. Se4+ 21. Zr4+
2. Ba 7. Co 12. Ge 17.
3. Bi 8. Cu 13. Hg2+ 18. Ti4+
4. Cs 9. In3+ 14. Mo05 19. T1+
5. Cr 10. Fe3+ 15. Ni 20. VOg
(3) Systems of Chromatographic separation of many heavy metal ions
can be accomplished with ion exchangers. The ion exchange texts,
especially the analytical books, given in the bibliography have
detailed chapters on these separations and analyses.
Precautions
Several precautions must be mentioned.
(l) Many materials of construction will introduce heavy metal ions.
(2) Glass and plastic containers may adsorb some heavy metal ions.
(3) Minute trace of heavy metal ions are found in some of the purest
waters.
(M-) Ion exchangers should be well purified before being used for
analytical work.
(5) The resins themselves have traces of solubility when the strong
acid resin is in the H form and the anion exchanger in the OH form.
-------
336
-------
337
11/15/73
Princeton University
Center for Environmental Studies
CONFERENCE
"Traces of Heavy Metals in Water: Removal Processes and Monitoring"
LIST OF PARTICIPANTS
Robert C. Ahlert
Dept. of Chemical & Biochemical
Engineering
Rutgers University
College of Engineering
Busch Campus
New Brunswick, New Jersey 08903
Osman M. Aly
Campbell Soup Co.
Campbell Place
Camden, New Jersey 08101
Robert E. Anderson
Nopco Chemical Division
Diamond Shamrock Chemical Co.
P. 0. Box 829
Redwood City, California 94064
Robert C. Axtmann
Chemical Engineering
A302 Engineering Quadrangle
Princeton University
Princeton, New Jersey
Bruce H. Birnbaum
Naval Ship Engineering Center
3700 East-West Highway
Hyattsville, Maryland 20782
Sanford Blair
Pennutit
Ridge Road
Momaouth Junction, New Jersey
William Blankenship
E.P.A. Enforcement
6th & Walnut
Philadelphia, Pennsylvania
E. L. Bourodimos
Rutgers University
College of Engineering
New Brunswick, New Jersey 08903
C. Calmon
Sybron Corporation
One Cherry Hill - Suite 724
Cherry Hill, New Jersey 08034
R. J. Campbell
Senior Consultant
Avert - Systems Division
Wall Street
Wilmington, Massachusetts
Larry Cecil
AIChE - Environmental Division
Consulting Chemical Engineer
418 Lincoln Building
Champaign, Illinois 61820
Paul Cheremisinoff
Newark College of Engineering
323 High Street
Newark, New Jersey
Robert Cleary
Civil & Geological Engineering
E324 Engineering Quadrangle
Princeton University
Princeton, New Jersey
Edgar Cloeren
Ionics Inc. - 65 Grove Street
Watertown, Massachusetts 02172
H. C. Colby, Jr. '44
American Littoral Society
99 Cloverdale
New Shrewsbury, New Jersey 07724
Stephen A. Costa, P.E.
Suffolk County Dept. of Env. Control
1324 Motor Parkway
Hauppauge, New York 11787
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338
Burton Davidson
Dept. of Chemical & Biomedical
Engineering
College of Engineering
Busch Campus
New Brunswick, New Jersey 03903
W. Drobot
Engelhard Industries Division
Systems Dept.
205 Grant Avenue
East Newark, New Jersey 07029
Floyd R. Duncan
Pennsylvania Electric Co.
1001 Broad Street
Johnstown, Pa. 15907
Arnold Freiberger
E.P.A. - Region II
26 Federal Plaza
New York, New York 10007
David H. Furukawa
Fluid Sciences Division
8133 Aero Drive
San Diego, California 92123
Thomas A. Gallagher, P.E.
F11C Corporation
P. 0. Box 8
Princeton, New Jersey 08540
L. W. Gendvil
AMAX Case Metals Research
& Development, Inc.
Irvin Classman
Aerospace & Mechanical Sciences
& Center for Env. Studies
D329 Engineering Quadrangle
Princeton University
Princeton, New Jersey
Mervin K. Goss
Director, Air & Water Resources
J. T. Baker Chatnical Co.
600 No. Broad Street
Phillipsburg, N. J. 08865
Joseph P. Gould
Metcalf & Eddy, Inc.
Statler Building
Boston, Massachusetts
02116
Michael S. Gould
63 Princeton Arms East
Cranbury, New Jersey 08512
Wra. G. Gray
Water Resources Program
Civil & Geological Engineering
Princeton University
Princeton, New Jersey
Harry P. Gregor
Columbia University
356 Engineering Terrace
New York, New York 10027
William A. Hall
Wesleyan University
Davison House
327 High Street
liiddletown, Conn. 06457
Tom Hardin
N. J. State Dept. of Env. protection
209 E. State Street
Trenton, New Jersey 08609
Earl Henn
Calgon Corporation
Hall Laboratories Division
Pittsburgh, Pa.
Wm. T. Herrick
R. J. Martin Cons. Eng.
P. 0. Box 387
Vestol, New York 13305
Dwight Hlustich
U. S. Environmental Protection Agency
Region II
6th & Walnut Streets
Philadelphia, Pa. 19106
P. C. Houla
Gulf Degremont, Inc.
Liberty Corner, New Jersey
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339
Malcolm Howie Robert Kroner
N. J. State Dept. of Env. Pro. 1014 Broadway
209 E. State Street Environmental Protection Agency
Trenton, New Jersey 08609 Cincinnatti, Ohio 45202
Peter Huck Robert E. Lacey
Waste Water Technology Center Southern Research Institute
Canada Center for Inland Waters U. S. Dept. of Agriculture
Box 5050 Birmingham, Alabama
Burlington, Ontario
CANADA Bernard Lalli
Calgon Corporation
Harry H. Hughes
N. J. Dept. of Snv. Protection Charles Lancelot
141 Oliver Avenue F11C Corporation
Trenton, New Jersey 08618 Box 8
Princeton, New Jersey 08540
Robert G. Jahn
Dean, School of Engineering & Robert Lemlich
Applied Science Dept. of Chemical & Nuclear Eng.
C230 Engineering Quadrangle University of Cincinnatti
Princeton University Cincinnatti, Ohio 45221
Princeton, New Jersey
Norman N. Li
Haig Kasabach Esso Research & Engineering Co.
N. J. State Dept. of Env. Pro. P. 0. Box 45
209 E. State Street Linden, New Jersey 07036
Trenton, New Jersey 08609
John W. Liskowitz
K. B. Keating Newark College of Engineering
E. I. du Pont de Nemours & Co. 323 High Street
Du Pont Experimental Station Newark, New Jersey
Bldg. 304
Wilmington, Delaware 19898 Gary S. Logsdon
U. $. Environmental Protection Agency
David Kennedy NERC
Environmental Contol System Dev. 4676 Columbia Parkway
5000 Richmond Street Cincinnatti, Ohio 45268
Philadelphia, Pa. 19137
L. B. Luttinger
C. Judson King, Chairman Permutit R & D
Dept. of Chemical Engineering Ridge Road
University of California Monmouth Jet., New Jersey 08852
Berkeley, California
W. K. Mammel
Rod Knight Western Electric Co.
AMF Inc. p. 0. Box 900
689 Hope Street Princeton, New Jersey 08540
Stamford, Conn. 06907
Aldo Marletti, P.E.
Stuart Korchin Suffolk County Dept. of Environ-
Rutgers University mental Control
Dept. of Chemical and Biochemical 1324 Motor Parkway
Engineering Hauppauge, New York 11787
New Brunswick, New Jersey
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340
Robert W. Mason
Chief Research & Development
Representative
E.P.A. - Region II
Room 845
New York, New York 10007
George E. Mattingly, Jr.
Civil & Geological Engineering
E328 Engineering Quadrangle
Princeton University
Princeton, New Jersey
Richard R. Metcalf
Onondaga Co.
Div. of Drainage & Sanitation
650 Hiawatha Blvd. W.
Syracuse, New York 13204
R. Michalek
Engelhard Industries Division
Systems Dept.
205 Grant Avenue
East Newark, New Jersey 07029
John Mikes, ION EXCHANGE JOURNAL
71 Grace Avenue
Great Neck, New York 11021
Joseph Mikulka
N. J. State Dept. of Env. Pro.
209 E. State Street
Trenton, New Jersey 08609
Andrew Mills
Dames & Moore
14 Commerce Drive
Cranford, New Jersey 07016
A. B. Mindler
International Hydronics Corp.
P.O. Box 910 - R, D. 4
Princeton, New Jeraey 08540
Ralph Moore
T. E. L. Labs
Du Pont Chambers Works
Deep Water, New Jersey 08023
James G. Nalven
Woodward-Moorhouse & Associates
1373 Broad Street
Clifton, New Jersey 07012
Luke R. Ocone
Pennwalt Corp.
Three - Parkway
Philadelphia, Pa.
19102
Randy R. Ott
Onodaga Co.
Div. of Drainage & Sanitation
650 Hiawatha Blvd. W.
Syracuse, New York 13204
Edgardo Parsi
Director of Research
Ionics Inc.
65 Grove Street
Watertown, Massachusetts 02172
George F. Pinder
Civil & Geological Engineering
E332 Engineering Quadrangle
Princeton University
Princeton, New Jersey
James B. Platz
E. I. du Pont de Nemours & Co.,,Inc.
10th & Market Streets
Wilmington, Delaware 19890
Ronald F. Probstein
Dept. of Mechanical Engineering
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
John Reiss, Jr.
Woodward-Moorhouse & A&BOC.
1373 Broad Street
Clifton, New Jersey 07012
Steve Reznek
Center for Environmental Studies
D334 Engineering Quadrangle
Princeton University
Princeton, New Jersey
Josefina E. Sabadell
Chemical Engineering
A325 Engineering Quadrangle
Princeton University
Princeton, New Jersey
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341
Dudley A. Saville
Chemical Engineering
A323 Engineering Quadrangle
Princeton University
Princeton, New Jersey
Robert H. Schaefer
Civil & Geological Engineering
E208 Engineering Quadrangle
Princeton University
Princeton, New Jersey
Robert D. Schwartz
Esso Research & Engineering Co.
Corporate Research Labs
Linden, New Jersey 07036
G. H. Singhal
Esso Research ft Engineering Co.
P. 0. Box 45
Linden, Hew Jersey 07036
Mr. B. R. Smith
c/o S. B. Smith
Chemical Division
Westvaco Corporation
Covington, Virginia 24426
S. B. Smith
Chemical Division
Westvaco Corporation
Covington, Virginia 24426
Thomas G. Spiro
Department of Chemisty
127 Frick Chem Lab
Princeton University
Princeton, New Jersey
Ernest K. Tanzer
Kodak Park Division
Rochester, New York
Isaac Trachtenberg
Texas Instruments
Dallas, Texas
Hugo Vanderheide
Matheson Gas Products
P. 0. Box 85
East Rutherford, New Jersey
John Wagner
Gilbert Associates
Reading, Pa.
Leonard Weisler
Eastman Kodak Co.
Paper Service Div.
Kodak Park Div.
Rochester, New York
B-36
14650
John II. Williams
E.I. du Pont de Nemours
Bldg. 304, Experimental Station
Wilmington, Delaware 19398
R. E. Wing
Northern Regional Research Lab
ARS USDA
1815 N. University Street
Peoria, Illinois 61604
Douglas Wright
N. J. State Dept. of Env. Protection
209 E. State Street
Trenton, New Jersey 08609
Tsi Shan Yu
U. S. Naval Ship Research &
Development Center
Annapolis, Maryland 21402
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342
11/16/73
Princeton University
Center for Environmental Studies
CONFERENCE
"Traces of Heavy Metals in Water: Removal Processes and Monitoring"
ADDENDUM TO
LIST OF PARTICIPANTS
CORRECTION: D. Hlustich is in Region III, Enforcement Division.
ADDITIONS:
D. Adrian
Civil & Geological Engineering
E222 Engineering Quadrangle
Princeton University
Denyse Reid
Princeton Environmental
Commission
Stanley E. Jaros
Esso Research & Engineering Co.
Linden, N. J.
R. J. Mannella
Radio Corporation of America
Bldg. 202-2
Camden, N. J. 08101
Frank S. Model
Celanese Research Co.
Summit, N. J.
Harry Nissen
U.S. Metals Refining Co.
400 Middlesex Ave.
Carteret, N. J. 07008
Dr. John E. Poist
Celanese Research Co.
Summit, N. J.
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