United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
EPA 600 2 79 181
August 1979
Research and Development
Coupled Transport
Systems for
Control of Heavy
Metal Pollutants
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further deveJopment and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution-sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-79-181
August 1979
COUPLED TRANSPORT SYSTEMS
FOR CONTROL OF
HEAVY METAL POLLUTANTS
by
W.C. Babcock
R.W. Baker
D.J. Kelly
J.C. Klelber
H.K. Lonsdale
Bend Research, Inc.
Bend, Oregon 97701
Grant No, R804682-01
Project Officer
George F. Weesner
Metals and Inorganic Chemicals Branch
Industrial Environmental Research Laboratory
Cincinnati, Ohio 45268
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
-------
DISCLAIMER
This report has been reviewed by the Industrial Environmental Research
Laboratory, Cincinnati, U.S. Environmental Protection Agency, and approved
for publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection Agency,
nor does mention of trade names or commercial products constitute endorse-
ment or recommendation for use.
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FOREWORD
When energy and material resources are extracted, processed, converted,
and used, the related pollutional impacts on our environment and even on
our health often require that new and increasingly more efficient pollution
control methods be used. The Industrial Environmental Research Laboratory-
Cincinnati (lERL-Ci) assists in developing and demonstrating new and im-
proved methodologies that will meet these needs both efficiently and
economically.
The study described in this report was undertaken to demonstrate the
applicability of a new separation method to the recovery of metal values
from plating rinse waters. The process is referred to as coupled transport.
In this brief, four-month study, it was demonstrated that coupled transport
membranes can be used to separate copper, nickel, and chromium from these
rinse waters and to "chemically pump" these metal ions against very large
concentration gradients. High recoveries of metal values can thereby be
effected, producing recycle streams sufficiently concentrated for direct
return to the plating bath. The economics of the process appear favorable
relative to existing technology.
For further information on this subject, contact the Industrial
Pollution Control Division, Metals and Inorganic Chemicals Branch.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
iii
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ABSTRACT
This report describes a process for separating and concentrating heavy
metals from electroplating rinse waters. Metal ions can be "chemically
pumped" across a coupled transport membrane against large concentration
gradients by allowing the counterflow of a coupled ion such as hydrogen ion.
The process is carried out within a microporous membrane containing
within its pores an organic, water immiscible complexing agent. The
complexing agent acts as a shuttle, picking up metal ions on one side
of the membrane, carrying them across the membrane as a complex, and
preserving electrical neutrality by carrying hydrogen ions in the opposite
direction.
The importance of coupled transport is its high selectivity and flux.
High selectivity derives from the use of specific complexing agents. High
flux is possible because these are actually liquid membranes with diffu-
sivities many times greater than those in solid membranes.
iv
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CONTENTS
Foreword iii
Abstract iv
Acknowledgement vi
1. Introduction and Summary 1
2. Principles of Coupled Transport Membranes 2
Co-transport 2
Counter Transport 4
Applicability of Coupled Transport Membranes
to Plating Wastes 5
3. Experimental 7
Distribution Coefficients 7
Membranes and Permeability Apparatus 7
Analytical Methods 9
Reagents 9
4. Results 11
Distribution Coefficients H
Demonstration of Coupled Transport H
Effect of Diluents on Flux 15
Simulation of Actual Plating Rinse Conditions 15
General Considerations 15
Chromium 20
Copper and Nickel 22
5. Economics 29
6. Future Work 31
Appendices
A. Steady State Concentration Factors 32
B. Exponential Decay 34
v
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ACKNOWLEDGMENT
Our work on coupled transport membranes began under a contract with the
Bureau of Mines (Contract No. H0252066), in which we are examining the
hydrometallurgical recovery of copper and other metals from low grade ores.
Some of the data reported in Figures 4, 5, 6b, 7, and 9 of the present
report were obtained in the course of that ongoing, parallel work.
VI
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SECTION I
INTRODUCTION AND SUMMARY
This is the final report on Phase I of American Electroplaters' Society
Project No. 43, entitled "Coupled Transport Systems for Control of Heavy
Metal Pollutants in Metal Finishing Solutions". It covers the period from
October 22, 1976 - January 21, 1977.
The goal of this program is to demonstrate the applicability of coupled
transport membranes to the processing of electroplating rinse waters. These
waters are a serious pollution problem. However, they also have an intrinsic
value in the metals they contain. Recovering these metal values and re-
cycling the rinse water has been a long standing challenge. Of considerable
promise in this regard is a new membrane process that we refer to as coupled
transport membrane processing.
In the next section, the principles of coupled transport membranes are
described. The bulk of this report is concerned with an experimental study
of these membranes and their application to plating wastes. Initially,
some range-finding studies were performed to find membranes and conditions
that would be suitable for processing dilute solutions of chromium, copper,
and nickel. The dependence of the transmembrane flux of these metal ions
on such conditions as pH and concentration was measured in laboratory perm-
eation cells. We then carried out a series of experiments in which the
conditions that would exist in a counter-current coupled transport concen-
trator were simulated. These results were used to carry out an economic
analysis of the process for chrome-plating rinse waters. The results
showed that the capital costs of such a coupled transport unit would be
substantially lower than the annual savings realized in chromium recovery.
Favorable economics are also predicted for copper plating rinse waters.
Less favorable were the projected economics with nickel wastes because of
the lower nickel fluxes observed to date.
-------
SECTION II
PRINCIPLES OF COUPLED TRANSPORT MEMBRANES
In a coupled transport membrane, the flow of the permeant of interest
is coupled to the flow of some second species. Under the right conditions,
the flow of this second species can force the permeant of interest to flow
against its own concentration gradient thereby, for example, separating this
species from other species and concentrating it as well. These membranes
can thus be considered a kind of selective chemical pump.
More specifically, the coupled transport membranes being developed at
Bend Research consist of a suitable liquid complexing agent constrained by
capillary forces to the pores of a microporous membrane. Thus, these are
essentially liquid membranes. The complexing agent is specific for the
permeant of interest under certain conditions. The desired permeant is
complexed at one interface between the membrane and an external solution.
The complex diffuses across the liquid membrane to the downstream interface
where the reaction is reversed because of some appropriate change in con-
ditions. The free complexing agent then diffuses back across the membrane
where it picks up more of the desired permeant. The complexing agent thus
acts as a shuttle, carrying both the desired permeant and some second,
coupled species across the membrane.
In the work described here, the coupled species is the hydrogen ion.
This ion can either cross the membrane in the same direction as the metal
ion of interest (a case we refer to as "co-transport") or opposite to the
metal ion (referred to as "counter-transport"). Because of the requirement
to maintain electroneutrality across the membrane, the hydrogen ion must
move in the same direction as the metal ion if the metal is present in
anionic form, e.g. chromium as the chromate ion. On the other hand, with
free metal ions such as copper and nickel in solution, the flow of hydrogen
ions must be counter to the flow of metal ion. These two cases are dis-
cussed below.
CO-TRMISPORT
Consider the case in which a coupled transport membrane separates
a dilute aqueous solution of chromic acid at low pH from a less acidic
solution of sodium chromate. The membrane contains a complexing agent for
the chromate, R, which could be a water immiscible tertiary amine, for ex-
ample. These conditions are depicted schematically in Figure la. At the
left-hand membrane-solution interface, the chromate will partition into
the amine phase in the membrane where one molecule of chromate complexes
with two molecules of chromate, forming the neutral complex (RH)2Cr04. This
complex is soluble only in the amine phase. The complex will diffuse down
its concentration gradient to the opposite or downstream interface. There,
because of the higher pH, the complex dissociates, freeing the chromate to
2
-------
Low pH» dilute aoueous
2R
CpO
,,(opg)
Microporous mem-
brane containing
complex ing agent R
High pH concentrated aqueous
solution of CrOu-
K
CPO
2R
(orf.)
,,, ,
CO-TRAMSPORT
Low or moderat^ pH, dilute
solution of H ions
+ 2RH(org)
R2H
-------
the aqueous phase and regenerating the neutral amine, R. The amine then
diffuses down its own concentration gradient toward the left-hand interface
(or "feed" side of the membrane) where it combines with more chromate and
the cycle is repeated. The equilibrium existing across the aqueous-organic
phase boundaries is this:
2R(org) + 2H+(aq) + CrO^(org')^=^(RH)2Cr04(org) . (1)
At the feed interface, the hydrogen ion concentration is high and the re-
action is forced to the right. At the downstream interface, the reaction
is shifted to the left because of the higher pH. Thus, the amine is a
carrier. Hydrogen and chromate ions permeate the membrane in the same
direction, from left to right in Figure la. Note that an increase in H* con-
centration on the left-hand side of the membrane or, equivalently, a de-
crease in H+ concentration on the right-hand side will favor the left-to-
right flux of chromate. (The situation is not quite this simple because of
the equilibrium that exists in solution between CrO^ and C^Oy, but the
essential result is the same).
With a suitable pH gradient, chromate can be "chemically pumped"
against its own substantial concentration gradient. It can be shown in fact
(see Appendix A) that in the co-transport case the steady state concentration
ratio of the divalent chromium anion across the membrane depends on the
hydrogen ion concentration ratio in the following way:
(2)
where the subscripts o and £ refer to the feed and downstream solutions,
respectively (i.e. the left-hand and right-hand solutions in Figure la).
Thus, a pH difference across the membrane of two units should produce a
chromate ion concentration ratio of 10,000, with the less acidic solution
being more concentrated in chromium. The chromate ion will diffuse from the
dilute solution into the concentrated solution until the condition indicated
in equation (2) is satisfied.
COUNTER TRANSPORT
Consider next the situation depicted in Figure Ib. Here, the
metal ions exist as cations and because we are interested in copper and
nickel, we have shown them as divalent. The complexing agent in the membrane
in this case could be an oxime, for example, denoted in the Figure as RH.
Again, two moles of oxime will react with one mole of the metal ion, and the
equilibrium existing at the membrane-solution interfaces is:
M"H"(aq) + 2RH(org)^=±:2H+(aq) + R2M(org) . (3)
The metal-oxime complex is again soluble only in the organic phase. At the
left-hand interface, the equilibrium is shifted to the right, because of the
-------
low concentration of H+ ions, and the metal-oxime complex partitions into
the liquid membrane. The reverse reaction occurs at the downstream or right-
hand interface. Thus, metal ions are again chemically pumped from left to
right in the figure while hydrogen ions flow the other way.
It can be shown (see Appendix A) that the steady state ratio of di-
valent metal ion concentrations across the membrane is given by:
(A)
The metal ions can again be pumped against a substantial concentration
gradient but in this case the concentrated metal ion solution will be at the
lower pH.
While we have not attempted to derive it here, it can also be shown
that the same pH conditions that favor high concentration factors across
the membrane also favor high metal ion flux across the membrane. Thus, in
the co-transport case, metal ion flux will be enhanced by increasing the
hydrogen ion concentration gradient in the same direction. In the counter-
transport case, metal ion flux is enhanced when the hydrogen ion concen-
tration gradient is increased in the opposite direction.
APPLICABILITY OF COUPLED TRANSPORT MEMBRANES TO PLATING WASTES
Coupled transport membranes could be assembled into a concentrator.
The way in which such a concentrator would be applied to the treatment of
plating wastes is illustrated in Figure 2. The rinse water containing metal
ions would be fed to one side of the membrane concentrator. The metal ions
would be transported across the membrane by the coupled transport process.
Under favorable conditions, the metal could be concentrated in the unit to
produce a solution of sufficient concentration to feed back to the plating
bath, while the rinse water, almost totally stripped of its metal values,
would be returned to the rinse bath. Alternately, a partially depleted rinse
water could be discharged.
Under favorable conditions, then, the plating and rinse operation could
be run closed-loop. No metal is wasted and no make-up water is required for
the rinse bath.
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PLATING
BATH
RINSE
BATH
r
RAG OUT
\
V
COUPLED TRANSPORT CONCENTRATOR
CONCENTRATE
DILUTE
RINSE
RINSE
EFFLUENT
T
DISCHARGE
Figure 2. Simple schematic of coupled-transport concentrator as applied to
plating rinse water effluent.
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SECTION III
EXPERIMENTAL
Two sorts of experiments were performed in this reporting period. In
the first of these, we examined suitable complexing agents for chromium,
copper, and nickel. This search was performed by measuring distribution
coefficients of the metal ions between organic liquids (known to complex
these ions) and aqueous solutions at known pH. An ideal complexing agent
is one that is highly water immiscible and which forms a very stable complex
at a given pH. The complex should also be highly soluble in the complexing
agent but insoluble in water. Finally, the complex should be unstable at
some higher or lower pH. Having identified suitable complexing agents, we
then prepared liquid membranes from them and carried out a number of perm-
eation experiments to measure the metal ion flux across the membrane and its
dependence on pH and other conditions.
The second type of experiment was designed to simulate the conditions
that would exist in an actual coupled transport membrane concentrator. The
purpose of these experiments was to obtain -data representative of what might
be achieved in practice so that an economic assessment of the process could
be made.
DISTRIBUTION COEFFICIENTS
These were obtained simply by shaking the complexing agent for two
minutes with an aqueous solution of the metal ion at a known pH in a sep-
aratory funnel at room temperature. After allowing the phases to separate,
a sample of the aqueous solution was removed and analysed for metal ion. The
concentration in the organic phase was then obtained from the known volumes
and mass balance considerations. Acid or base was then added to the aqueous
phase and the process was repeated at a new pH.
MEMBRANES AND PERMEABILITY APPARATUS
Coupled transport membranes were prepared by simply immersing a
microporous membrane in the liquid complexing agent of interest. The liquid
was rapidly sorbed into the pores bv capillary action. For our initial
studies, we chose Celanese Celgard ®2400 as the microporous membrane. It is
made of polypropylene and is highly resistant to acids.
Permeation experiments were carried out in standard two-compartment
glass cells shown in Figure 3. The membrane was mounted without a gasket
between the half cells, held together by a standard ball-joint clamp. The
volume of each half cell was 100 cnH, and the membrane area was 20 cm . The
-------
cells were supplied with stirrers to avoid concentration polarization ef-
fects. The cells were mounted in a constant temperature bath. All perm-
eation measurements were performed at 30 ± 1°C.
MEMBRAN
Permeation Cell
STIR MOTORS
an na na
r
CONSTANT TEMPERATURE BATH
Permeation Cells in Water Bath
Figure 3. Permeability apparatus for testing coupled transport
membranes.
-------
The pH and metal ion concentration were periodically determined in the
course of a run by removing small samples from the cell, through sampling
ports not shown in the figure, and replacing with fresh solution at the same
pH. Metal ion fluxes were obtained from the concentration-vs.-time data
after correcting for sampling.
ANALYTICAL METHODS
Chromate concentrations have been measured spectrophotometrically
at 350 nm. The pH of the samples was first raised to 12 to insure that the
dichromate ion would not interfere.
Copper and nickel analyses have been made with a Perkin-Elmer model 290
atomic absorption spectrophotometer. With this instrument, both copper and
nickel can be measured in aqueous and organic solutions at concentrations as
low as 15 ppm with a precision of ± 2%.
REAGENTS
The complexing agents studied were Alamine 336, a tertiary amine
from General Mills; LIX 63, LIX 64N, and LIX 70, all oximes from General
Mills; and Kelex 100, a substituted quinoline from Ashland Chemical. All of
these are common industrial liquid ion exchangers. Their chemical structures
are presented in Table I.
In many permeation experiments, these complexing agents were diluted
with a compatible, inert carrier, principally to reduce their viscosity.
The diluents used were Solvesso from Exxon and kerosene. In some cases, a
small amount (2 vol%) of isohexadecyl alcohol from American Hoechst was
also added to the diluted complexing agents in order to prevent the formation
of a separate phase.(1) Apparently, this agent promotes the solubility of
the metal ion complex in the organic complexing agent.
(1) C.J. Lewis, "Liquid Ion Exchange in Hydrometallurgy", in Recent De-
velopments in Separation Science Vol. II, N. N. Li (Ed.), CRC Press,
Cleveland, Ohio 1972.
-------
Table I
COMPLEXING AGENTS FOR COPPER, NICKEL, AND
CHROMIUM
Kelex 100
(Ashland Chemical)
LIX 63
(General Mills)
OH CH2-CH3
CH--CH9-CH-CH-C-CH-(CH?)--CH
j ^ I ,, £• J
(CH2)3 N-OH
CH
LIX 64N, a mixture of 10%
LIX 63 and 90% LIX 65N
(General Mills)
LIX 70
(General Mills)
Alamine 336
(General Mills)
CH3-(CH2)
+ LIX 63
10
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SECTION IV
RESULTS
DISTRIBUTION COEFFICIENTS
The distribution coefficient is defined here as:
K _ Concentration of metal in the organic phase
Concentration of metal in the aqueous phase
Distribution coefficients for copper with several complexing agents are pre-
sented in Figure 4 as a function of the pH of the aqueous phase. The com-
plexing agents were diluted to a 10 vol% solution in kerosene in all cases.
Similar data for nickel are presented in Figure 5. Note the hysteresis
effect. The data connected by the upward-pointing arrows were taken while
nickel was being loaded into the organic phase, i.e. on increasing pH, while
the downward-pointing arrows connect data taken while nickel was being strip-
ped from the organic phase. The hysteresis effect was reflected in the perm-
eation experiments as well, as is discussed below.
DEMONSTRATION OF COUPLED TRANSPORT
The results of a coupled transport experiment with Cr03 are pre-
sented in Figure 6a. The complexing agent was Alamine 336 diluted to 40
vol% with Solvesso. The feed side of the permeation cell initially con-
tained 50 ppm Cr03 while the downstream side was buffered at either pH 7 or
11, and it initially contained no Cr03. Clearly, chromium was transported
in the direction of decreasing hydrogen ion concentration. Because the two
cell compartments are of equal volume, we can see that beyond the point
where the downstream concentration reached 25 ppm, the chromium was diffusing
uphill. The flux was higher at the higher downstream pH as is predicted from
the analysis presented in Part II. Almost complete removal of chromium from
the feed solution was observed in the course of the experiment when the
downstream pH was 11, and it is predictable that the same effect would have
occurred at a downstream pH of 7 at some longer time.
Coupled transport of copper was demonstrated using a 10% solution of
Kelex 100 in kerosene as the liquid membrane. The results are presented
in Figure 6b. The initial feed solution contained either 0 or 100 ppm
copper at pH 2.5, while the downstream solution was initially at pH 1.0
with either 3.0 or 9.3 wt% copper, the latter being an essentially satu-
rated solution of copper sulfate. In Figure 6b, the feed concentration is
plotted against time. In all cases, the copper was transported counter to
11
-------
100
10
Distribution
Coefficient
1.0
0.1
0.01
D
10% LIX 63
_L
0 1.0 2.0 3.0 4.0 5.0
pH of Aqueous Phase
6.0 7.0
^^^^
Figure 4. Distribution coefficients of Cu vs. pH for several
liquid ion exchangers dissolved in kerosene.
12
-------
10,000
1,000
100
Distribution
Coefficient
10
0.1
0.01
246
pH of Aqueous Phase
Figure 5.
Distribution coefficients for nickel vs. pH in 10% L1X 64N
in kerosene (—) and 10% Kelex 100 in kerosene ( ).
13
-------
SO
Downstream CrO ,0
Concentration
(ppm)
20
10
Downstream pH 11
pH 7
Initial conditions: 50 ppm
CrO, in feed, 0 ppm downstrean
20 30 MO
Time (min)
50
60
Figure 6a. Downstream chromate concentration vs. time.
120
100
80
Feed
Concentration 60
(ppm)
Initial conditions: feed pH
2.5, Cu concentration 0 or 100
ppm; downstream pH 1.0, Cu
concentration 3.0% or 9.3%.
1000
3000
Time (min)
Figure 6b. Concentration of copper in the feed vs. time.
-------
the hydrogen ion concentration gradient. In these experiments, the steady
state concentration factor was attained from both directions, i.e. both the
0 ppm and the 100 ppm copper feed solutions reached a final concentration of
about 20 ppm when the downstream compartment was 3.0 wt% Cu. The steady
state copper concentration factors were in the range 1500-2000 in these
two cases.
A series of steady state concentration factors for copper were measured
as a function of the pH difference across the membrane. The results are
presented in Figure 7. Note that at a pH difference of less than 2 units,
copper was concentrated against its own concentration gradient by about
4000-fold. In fact, the slope of the line in this essentially log-log plot
is 2, which is just the value predicted by equation (4).
EFFECT OF DILUENTS ON FLUX
The diluent added to the complexing agent can be expected to have
two effects on the flux of metal ions across coupled transport membranes.
First, because the diluent displaces some of the complexing agent, less of
the metal ion complex will be formed, and we could expect a lower metal flux
as a result. Opposing this effect, however, is the fact that the diluents
are lower molecular weight compounds and hence are much more fluid at room
temperature than the complexing agents. The reduced viscosity of the liquid
membrane in the presence of diluent should enhance the diffusivity of the
metal ion complex (according to the Stokes-Einstein equation) and hence in-
crease the flux. These two effects thus run counter to one another.
The effect of the concentration of complexing agent in the diluent on
the flux of metal was measured. Typical results for chromium are presented
in Figure 8. The complexing agent was Alamine 336 and the diluent was
Solvesso. The feed solution contained 5000 ppm chromic acid, while the
downstream solution was maintained essentially free of chromium and at pH 7
throughout the experiment. A maximum occurred in the flux at about 40%
Alamine, apparently the result of the opposing effects described above.
Because of this maximum, we carried out our further chromium experiments at
this concentration of complexing agent.
Similar data are plotted in Figures 9 and 10 for copper and nickel,
respectively. The liquid membrane used in the copper experiments was Kelex
100 diluted with kerosene; for the nickel experiments, the Kelex was
diluted with Solvesso. Flux maxima occurred in both cases, and we again
carried out our further experiments at the complexing agent concentrations
at which the maxima occurred.
It is noteworthy that the maximum flux in the chromium experiments was
3-4 times the maximum observed with either copper or nickel.
SIMULATION OF ACTUAL PLATING RINSE CONDITIONS
General Considerations. in Figure 2 we indicated in a general
way how a coupled transport membrane concentrator might be applied to the
treatment of plating rinse waters. An ideal system would be one that could
be run closed-loop, i.e. with no discharge of metal ions and with complete
15
-------
recycle of the purified rinse water. This ideal situation will generally
not prevail in a coupled transport membrane concentrator, because there is
always a flow of coupled ion across the membrane along with the flow of
metal. In co-transport, the co-permeating species (or another ion used to
•p
o
C
o
•H
•M
•P
C
0)
O
C
O
o
0)
ex
(X
o
o
5000
2000
1000
500
200
100
50
20
10
o.o
0.5
O Feed solution maintained
at pH 2.5
A Downstream solution
maintained at pH 1.0
1.0
1.5
2-0
ApH
U£SJ.J.
Figure 7. Steady state copper concentration factor vs. pH cifference across
the membrane.
16
-------
Flux
yg
2
cm -min
20 40 60 80
% Alamine.in Solvesso
Figure 8. Cr03 flux vs. concentration of Alamine 336 in
Solvesso.
17
-------
Copper2Flux
(jjg/cm -min)
20 40 60
% Kelex 100
80 100
Figure 9. Copper flux vs. concentration of Kelex 100 in kerosene.
18
-------
12
10
(yg Ni/cm -min) 6
o
o
0 20
60
Kelex 100
80 100
Figure 10. Ni flux vs. concentration of Kelex 100 in Solvesso.
19
-------
neutralize it) would add to the concentrated stream. Before this stream
could be fed back to the plating bath, this contaminant would have to be
removed, in general. In counter-transport, the coupled ion contaminates
the rinse water feed from which the metal ion was removed. These contam-
inants may be simple to remove, but it should be recognized that in general
only one of the two streams emanating from the concentrator could be reused
without some further treatment. In this section, we describe a series of
experiments designed to simulate the conditions that might exist in such a
concentrator.
Chromium. In Figure 11, we present a diagram showing how the
concentrator might operate on a chrome plating rinse bath. The following
assumptions have been made: the plating bath contains 25 wt% Cr03v2) and
there is a constant dragout rate equal to 2% of the flow rate of water
through the rinse baths, producing a Cr03 concentration in the first rinse
bath of 5000 ppm.(3) This solution would be fed to one end of the mem-
brane concentrator, and depleted of chromium as it passed through the con-
centrator from left-to-right in the figure. To drive the chromate ion
across the membrane against its concentration gradient, we must either
acidify this feed stream or make the downstream side of the membrane basic.
Because we want to reuse the treated rinse water, we do not want to con-
taminate it with acid and hence we chose to add sufficient base to the
downstream side of the membrane to neutralize the chromic acid that is
transported across the membrane. The most favorable conditions will be
achieved when the feed and downstream solutions flow counter to one another
so that high concentration gradients are established everywhere.
The concentrated downstream solution emerging from the concentrator
thus contains Na2CrO^. If we assume that a 50-fold concentration factor
is achieved in the concentrator, the emerging concentrate stream will con-
tain 41% Na2CrO^, which has the same chromium content as the plating bath.
However, the chromium would have to be recovered from this stream as CrOg
before it could be returned to the plating bath. This might be achieved,
for example, by the addition of I^SO^ followed by fractional crystal-
lization. (4) in this case, NaHS04 would become a by-product.
To simulate the changing concentrations that would exist in an oper-
ating concentrator, we arbitrarily divided the length of the concentrator
into four zones of decreasing chromium concentration. Within each zone, we
assumed a certain CrOg concentration and calculated the concentrations of
and NaOH that would exist within that zone. These concentrations are
(2) A. Logazzo, Metal Finishing Guidebook and Directory, Metals and Plastics
Publications, Inc. Hackensack, N.J. 1976.
(3) E. McGuire, Crown City Plating Co., El Monte, CA, personal communication.
(4) J.R. Partington, General and Inorganic Chemistry, MacMillan, London
1954.
20
-------
CrO
PLAT IN6
BATH
2S% CrO.
RINSB CYCLE
5000 ppm CrO
i !
>
'^s—^^^-^N.^^^^yO^s^^y
< 100 ppm Cr03
COUPLED TRANSPORT CONCENTRATOR
IT
*
NdOH
NaHSO,
Figure 11. Schematic representation of coupled transport concentrator as
applied to chromium plating wastes.
-------
summarized in Table II.
TABLE II
ZONAL CONCENTRATIONS IN A SIMULATED COUPLED TRANSPORT MEMBRANE CONCENTRATOR
FOR CHROMIUM
Zone 12 34
Feed Cr03 Concentration (ppm)
Downstream NaOH Concentration (M)
Downstream Na2Cr04 Concentration (wt%)
5000
0.0
41
2500
2.5
20
1000
4.0
8
100
5.0
0.0
A series of experiments was then performed in permeation cells under
each of these four sets of conditions, and the chromium flux was measured.
The liquid membrane consisted of 40% Alamine 336 in Solvesso. The results
are presented in Figure 12. Particularly noteworthy are the very high
chromium fluxes over most of the concentration range and the linear de-
pendence of flux on the concentration of Cr03 in the feed.
Copper and Nickel. The analogous situation that would occur
in the treatment of rinse waters from the acid plating of copper or nickel is
shown in Figure 13. Typical compositions were again taken from Reference
3. Here hydrogen ion will flow counter to the metal ions, acidifying the
depleted rinse water. This water could thus not be fed directly back to
the rinse bath, as in the case of chrome plating rinse waters, but it might
be reused after neutralization in an ion exchange column, for example. The
concentrated downstream solution emerging from the concentrator could be
fed directly back into the plating bath.
The concentrations of metals and acids in the hypothetical zones of the
concentrator are presented in Table III. (For copper we arbitrarily se-
lected five zones instead of four). These were calculated using the same
considerations applied to the chromium case, above. In the nickel case, we
have assumed conditions typical of a Watts bath. The boric acid present in
these baths is used as a mild buffering agent and it would not significantly
affect the coupled transport process.
The simulation experiments with copper were carried out using Kelex 100
as the complexing agent diluted to 10% in kerosene. This choice was based
on the fact that Kelex complexes effectively with copper at the pH at which
the concentrator would be operated (i.e. pH approximately 1), whereas the
other complexing agents do not.
The results for copper are presented in Figure 14. Again, the copper
flux is plotted against feed copper concentration when the composition of
the solutions is that shown in Table IIIA. These data produced the dashed
line. The copper fluxes in this case were less than Iug/cm2-hr, well below
the data in Figure 9 for the same Kelex concentration. However, this flux
was markedly enhanced when the pH of the feed solution was increased. The
solid line in Figure 14 represents comparable data obtained when 0.03 M/liter
22
-------
180-
160-
140-
120-
Flux2
(yg CrO /cm -min)
O
100 _
1000
2000 3000 4000 5000
Feed Concentration (|ig Cr03/ml)
Figure 12. Cr03 flux vs. feed concentration. All other
conditions simulate those in a coupled transport
concentrator. (See Table II).
23
-------
P LATINO
BATH
CYCLE
drag out __
5* Cu, 0.5 M H2SOM,
or
25% NiCl,,19.5% NisO
N3
5% Cu
0.5 M H2
25% NiCl
19.5% ItiS'
**
I
I
_> _
1000 c»>» «« ,
0.01 M~H2SOi, or
5000 ppm NiCl2
3900 ppm NiSOi,
800 ppm K3B04
COUPLED TRAIMSPOR1 CONCENTRATOR
^CU++,yi+^
^H?
Concen-
Dilute
Discharge
Figure 13. Schematic representation of coupled transport concentrator
as applied to nickel or copper plating wastes.
-------
Table III
ZONAL CONCENTRATIONS IN A SIMULATED COUPLED TRANSPORT MEMBRANE
CONCENTRATOR FOR COPPER AND NICKEL
Zone
A. Copper
Feed Cu (ppm)
Feed H2SO, (M)
Downstream Cu (wt%)
Downstream H SO, (M)
Zone
Feed N1C12 (ppm)
Feed NiSO^ (ppm)
Feed HC1 (M)
Feed H-BO- (ppm)
Feed H2S04 (M)
Downstream NiCl2 (wt%)
Downstream NiSO^ (wt%)
Downstream HC1 (M)
Downstream 113803 (wt%)
Downstream H2SOA (M)
1000
0.01
5
0.5
B.
1
5000
3900
0
800
0
25
19.5
0
4
0.0
500 250 100
0.0175 0.0213 0.0235
2.5 1.25 0.5
0.875 1.06 1.18
Nickel
2 3
2500 1000
1950 780
0.017 0.026
800 800
0.009 0.014
12.5 5
9.75 1.95
0.96 1.38
4 4
0.44 0.70
50
0.0250
0.0
1.25
4
100
78
0.033
800
0.0174
0
0
1.72
4
0.81
25
-------
Flux,2
(yg Cu/cm -min)
250 500 750 1000
Feed Concentration (yg Cu/ml)
Figure 14. Cu flux vs. feed concentration. Dashed curve:
conditions simulate those in a coupled transport
concentrator. (See Table IIIA) Solid curve:
0.03 M/liter NaOH added to feed solution.
26
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of NaOH was added to each of the feed solutions. (This is just the amount
of base needed to neutralize the acid permeating counter to the copper.)
Nickel removal from rinse waters is similar in principle to copper re-
moval. However, the hysteresis observed in the distribution coefficient
measurements with both Kelex 100 and LIX 64N adds some complexity. Large
pH gradients would be required to drive the nickel across the membrane, and,
as can be seen from Table II1B, such large pH gradients would not normally
occur in the concentrator. Furthermore, addition of base to the feed solu-
tion to increase the pH gradient could not be tolerated because nickel
precipitates above pH 6, where the concentrator would operate. The much
narrower pH differences existing under these simulated conditions produced
much lower nickel fluxes than we observed under more favorable conditions.
The nickel fluxes are plotted against feed concentration in Figure 15.
27
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1.2
1.0
0.8
Flux2 0.6
(pg Ni/cm -min)
0.4
0.2
J_
500 1000 1500 2000
Feed Concentration (pg Ni/ml)
Figure 15. Ni flux vs. feed concentration. All other conditions
simulate those in a coupled transport concentrator.
(See Table IIIB).
28
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SECTION V
ECONOMICS
Using the flux data generated under these simulated processing con-
ditions, we are in a position to crudely evaluate the economic feasibility
of the coupled membrane transport process. We consider here only chromium
for the present, which, because of the large observed fluxes, is the most
favorable case.
We begin by calculating the membrane area required to process a given
volume of rinse water per unit time. An effective membrane permeability
can be obtained from Figure 12. The flux-vs.-concentration curve in Figure
12 can be fitted by a straight line of slope k, where
, _ 170 yg/cm2-min . _„. 3, 2 .
k = ° — = 0.034 cm /cm -mm.
5000 yg/cm
Thus, k is a kind of "clearance rate", expressing the volume of solution
that is depleted of chromium per unit area per unit time. As shown in
Appendix B, the concentration of chromium, C, in a volume, V, of solution
in contact with a unit area, A, of membrane will decay according to a first-
order rate law:
C = C exp(-kAt/V) ,
o
where CQ is the initial concentration of chromium in the rinse water. If
we assume that CQ = 5000 ppm and if we further assume that we want to reduce
the concentration to 100 ppm for return to the rinse bath, we have that
C/CQ = 0.02 from which we have that V/At = 8.7 x 10~3 cm3/cm2-min. This,
then, is the volume of rinse water that can be treated per minute by 1 cvr
of membrane.
The amount of chromic acid recovered from this rinse water is 4900
yg/cm2. At a price of 60c?/lb (or 13c/gram) of chromic acid, the chromic
acid recovered has a value of $26 per ft2 of membrane per year. This is
well below the expected capital cost of a coupled transport membrane con-
centrating unit. We believe that these units could be constructed for a
cost comparable to that of current reverse osmosis units, which are com-
parable in complexity. These units sell for approximately $10/ft2 of
membrane. Thus, the expected payback time for the membrane concentrator,
ignoring operating costs, could be less than 6 months.
29
-------
Under favorable circumstances where complete recycle was practiced,
there would be an additional saving in the cost of make-up water for the
rinse bath. This could amount to a savings of 1000 gallons of water per
ft^ of membrane per year, or approximately $l/ft^ of membrane per year.
In spite of the lower fluxes observed for copper and the lower cost of
this metal, the economic picture for copper is also quite favorable. Only
for nickel is the present economic projection unfavorable, because of the
low fluxes observed to date.
30
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SECTION VI
FUTURE WORK
The work we proposed to do in Phase I of this project is now essentially
completed.
In Phase II, we propose to design and construct a 50 ft^ mobile mem-
brane concentrator for use in the field. The membranes would be prepared
in the form of small, microporous polysulfone hollow fibers. These would
be assembled into a multiple tube-in-shell configuration and ultimately
field tested on plating rinse waters. The unit will be designed for con-
tinuous, unattended operation.
31
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Appendix A
Steady State Concentration Factors
First, consider the reaction
CrO^ + 2H+ + 2R:5=
for which there is an equilibrium constant:
Let us denote the two sides of the membrane by the subscripts o and $,.
lere will be no flux of CrO-r across the membrane when |(RH),,CrO, | =
^ 4 2 4 |o
, !• «J
Fer
RH)
Under these conditions, it follows that _Rj = [Rn- Thus, because the
equilibrium constant must hold at both membrane-solution interfaces, a
steady state is established when
or
M
(This development, of course, strictly applies only at pH > 7. Below this PH
the chromate-dichromate equilibrium becomes important).
Next, consider the reaction
M"*""1" + 2RH , WR M + 2H+
32
-------
K
eq
As above, there, wilj, b,e zero flux when
conditions |RH
Thus,
=r
the
t
or
, and under these
33
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Appendix B
Exponential Decay of CrO in a Concentrator
The removal of CrO« from the feed follows first order kinetics:
dt
where k is the slope of the flux-vs. -concentration curve, A is the membrane
area, and V is the volume of the feed solution. Set fcrOo] at t=0 equal to
C and [CrO,,"| at time t equal to C. We can then integrate:
C dC
or ln(C/C ) = -kAt/V
and C/C = exp (-kAt/V) .
34
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TECHNICAL REPORT DATA
ff lease read Instructions on the reverse before completing)
REPORT NO.
EPA-600/2-79-181
TITLE AND SUBTITLE
COUPLED TRANSPORT SYSTEMS FOR CONTROL OF HEAVY METAL
POLLUTANTS
. AUTHOR(S) ~~~~
W. C. Babcock, R. ¥. Baker, D. J. Kelly, J. C. Kleiber
3. RECIPIENT'S ACCESSION NO.
5. REPORT DATE
August 1979 issuing date
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
i. PERFORMING ORGANIZATION NAME.AND ADDRESS
Bend Research, Inc.
Bend, Oregon 97701
10. PROGRAM ELEMENT NO.
1BB610
11. CONTRACT/GRANT NO.
R80^682-01
12. SPONSORING AGENCY NAME AND ADDRESS
Industrial Environmental Research Laboratory
Office of Research and Development
U. S. Environmental Protection Agency
Cincinnati, OH ^5268
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/12
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report describes a process for separating and concentrating heavy metals
from electroplating rinse waters. Metal ions can be "chemically pumped" across a
coupled transport membrane against large concentration gradients by allowing the
counterflow of a coupled ion such as hydrogen ion. The process is carried out within
a microporous membrane containing within its pores an organic, water immiscible
complexing agent. The complexing agent acts as a shuttle, picking up metal ions on
one side of the membrane, carrying them across the membrane as a complex, and
preserving electrical neutrality by carrying hydrogen ions in the opposite direction.
The importance of coupled transport is its high selectivity and flux. High
selectivity derives from the rise of specific complexing agents. High flux is
possible because these are actually liquid membranes with diffusivities many times
greater than those in solid membranes.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Electroplating, Industrial Waste Treatment,
Water Pollution, Membranes, Osmosis, Copper,
Nickel, Chromium
Liquid Membranes
Metal Ion Separation
Membrane Transport
13B
18. DISTRIBUTION STATEMENT
Release to Public
EPA Fo,m 2220-1 (Rev. 4-77) PREV.OUS ED.T.ON is OBSOLETE
Unclassified
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
35
-65T-060/5389
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