&EFK
United States
Environmental Protection
Agency
Municipal Environmental Research EPA-600/2-80-1 53
Laboratory August 1980
Cincinnati OH 45268
Research and Development
Selenium
Removal From
Ground Water Using
Activated Alumina
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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 development 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-600/2-80-153
August 1980
SELENIUM REMOVAL FROM GROUND
WATER USING ACTUATED ALUMINA
BY
R. Rhodes Trussell, Albert Trussell and Peter Kreft
Oames M. Montgomery, Consulting Engineers, Inc.
Pasadena, California 91101
Contract No. 68-03-1515
Project Officer
Richard Lauch
Drinking Water Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory^ 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 endorsement or recommendation for use.
ii
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FOREWORD
The U.S. Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health and
welfare of the American people. Noxious air, foul water, and spoiled land are tragic
testimonies to the deterioration of our natural environment. The complexity of that
environment and the interplay of its components require a concentrated and integrated
attack on the problem.
Research and development is that necessary first step in problem solution; it
involves defining the problem, measuring its impact, and searching for solutions. The
Municipal Environmental Research Laboratory develops new and improved technology
and systems to prevent, treat, and manage wastewater and solid and hazardous waste
pollutant discharges from municipal and community sources, to preserve and treat
public drinking water supplies, and to minimize the adverse economic, social, health,
and aesthetic effects of pollution. This publication is one of the products of that
research and provides a most vital communications link between the researcher and
the user community.
This report addresses the feasibility of removing selenium from drinking water
supplies using activated alumina. This information is helpful to determining operating
parameters for the activated alumina process, which can also remove arsenic and
fluoride from drinking water supplies.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
ill
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ABSTRACT
Selenium is a contaminant found in trace quantities in some ground- and surface-
waters in the United States. Currently, the National Drinking Water Regulations limit
total selenium to .01 mg/1. Even though the health affects of selenium in "trace"
amounts are still highly debatable, it is prudent to explore the technology of removing
selenium from drinking water.
Two species of inorganic selenium, with valence states of +4 and +6, are
typically found in selenium-contaminated waters. Se(IV) and Se(VI) act very different,
chemically. Se(IV) occurs as HSeO3~ in the pH range of 2.7 to 8.5. Se(VI) occurs as
SeOj? above pH 1.7. The valence of either of these species is thought to be determined
by the oxidation-reduction potential of a water at a certain pH. Knowing the oxidizing
state of a water, one can predict whether Se(IV) or Se(VI) should be present.
Historically, only the total selenium present in a sample has been able to be
determined by atomic adsorption spectroscopy. With the incorporation of a
fluorometric technique to determine the Se(IV) in a sample, this study was able to
differentiate between the two species of selenium.
Initial batch studies indicated that Se(IV) was preferentially adsorbed over Se(VI)
in side-by-side tests. The isotherm capacity of activated alumina for Se(IV) was
roughly three times the capacity of Se(VI). Other studies indicated that while
bicarbonate mildly interfered with Se(IV) removal, both bicarbonate and sulfate
heavily interfered with Se(VI) adsorption.
Initial column studies with a three-inch deep bed helped delimit the amounts of
NaOH and H-SCX to be used during regeneration of Se(IV)-saturated alumina. Other
items addressed in the three-inch column studies included how varied concentrations
-------
of NaOH and t^SO^ affected regeneration capabilities and how the varied concentra-
tions affected alumina degradation.
Deeper (9-inch) column studies showed that capacities for Se(IV) decreased with
increasing influent water pH. pH 5 showed the highest capacity for Se(IV) adsorption.
The kinetics of regeneration were the most important factors in determining the
capacity of activated alumina for Se(IV), with pore diffusion seeming to be the rate-
limiting step. Slow 0.5% NaOH flow rates (0.5 gpm/ft2 or less) are necessary to
effectively recover a high percentage of Se(IV) removed during a previous treatment
run. The following regeneration steps are recommended for Se(IV) removal:
1.5 lb/ft3 of 0.5% NaOH at 1/2 gpm/ft2, upflow
5 bed volumes of water at 1-2 gpm/ft , downflow
0.7 lb/ft3 of 0.25% H2SO^ or HC1 at 1 gpm/ft2, downflow
5 bed volumes of water rinse at 1-2 gpm/ft , downflow
The following bed volumes of treated water can be expected to be produced with
a Se(IV) influent concentration up to 200 ppb:
pH 5 - 1200 bed volumes
pH 6 - 900 bed volumes
pH 7 - 500 bed volumes
Selenium (VI) is much more poorly adsorbed than Se(IV). It is suspected that this
low adsorption is due to Se(VI)'s higher solubility with the oxides of alumina. Because
of this poorer adsorption, NaOH regeneration amounts and flow rates are not as
important in recovering Se(VI) removed during a treatment run. Because SO^= heavily
interferes with Se(VI) adsorption by activated alumina, only HC1 can be used as the
acid rinse during regeneration.
Capacities of activated alumina for Se(VI) are shown below at the following pH's
for an influent concentration of 50 ppb:
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pH 5 - 100 bed volumes
pH 6 - 70 bed volumes
pH 7 - 35 bed volumes
These capacities are based on a SOf concentration of 100 ppm. The following
capacities were developed for Se(VI) at different sulfate concentrations at pH 6:
SO7 = 5 ppm - 450 bed volumes
S(X = 50 ppm- 150 bed volumes
SO^ = 100 ppm - 70 bed volumes
SOJj = 500 ppm- 15 bed volumes
Variations in capacity for Se(VI) with different amounts of bicarbonate were not
as great.
Preliminary cost estimates were done to predict costs of treatment for either a
Se(IV) or a Se(VI) contaminated supply. Costs are as follows:
Se(IV) - $ 75/acre-foot
Se(Vl) - $255/acre-foot
If a mixture of the two species is present, Se(VI) concentrations will determine
the cost of treatment.
This report was submitted in fulfillment of Contract No. 68-03-1515 by
James M. Montgomery, Consulting Engineers, Inc. under the sponsorship of the U.S.
Environmental Protection Agency. This report covers the period April 1, 1979 to
March 21, 1980, and work was completed as of April 21, 1980.
vl
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CONTENTS
Foreword iii
Abstract iv
Figures ix
Tables xi
Acknowledgment xii
1. Introduction «... 1
Background 1
Selenium 1
Prior Research 7
Activated Alumina 7
Project Approach 13
2. Conclusions 13
Analysis 13
Batch Studies 15
Column Studies 18
Costs 21
3. Recommendations 23
4. Selenium Analysis Studies 25
Organic Selenium 25
Atomic Absorption Spectrophotometry 26
Colorimetric Determination of Selenium (IV) 32
Fluorometric Determination of Selenium (IV) 32
5. Selenium Batch Studies 35
Regeneration 35
Kinetics 40
pH Effects 41
Mesh Size 46
Interfering Ions 49
Selectivity Series 52
Capacities of Activated Alumina . 56
6. Selenium Column Studies 64
Introduction 64
Initial Testing 67
Experimental Summary 69
Chronological Summary of Se(IV) Removal
Tests Using 3-Inch Columns 69
Chronological Summary of Se(lV) Removal Tests
Using Nine-Inch Columns 92
Chronological Summary of Se(VI) Removal Tests
Using Nine-Inch Columns 117
vii
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CONTENTS (CONTINUED)
7. Preliminary Cost Estimate ................. 139
References -
Appendix A -
viii
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FIGURES
Number
1. Concentration - pH Diagrams for Inorganic Selenium
Species 3
2. E, - pH Diagram for Selenium • • ft
3. Pfiotograph of A.A.S 28
ft. Photograph of A.A.S 29
5. Influence of Se(VI) on Fiuorometric Determination
of Se(IV), 0-7 ppb 30
6. Influence of Se(Vl) on Fiuorometric Determination
of Se(IV), 0-70 ppb 31
7. Photograph of Fluorometer • 3ft
8. Photograph of Shaking Apparatus 36
9. Initial Preparation of Fresh Activated Alumina with NaOH 38
10. Initial Preparation of Fresh Activated Alumina with Acid
After NaOH Rinse . - 39
11. Effect of Contact Time with Activated Alumina on
Selenium Adsorption ft2
12. Effect of Initial pH on Adsorption of Selenium ft3
13. Effect on Constant pH on Adsorption of Selenium ft 5
1ft. Effect of Mesh Size on Removal of Selenium (IV) ft7
15. Effect of Mesh Size on Removal of Selenium (VI) . . . ft8
16. Influence of Various Anions on Adsorption of Selenium (IV) 50
17. Influence of Various Anions on Adsorption of Selenium (VI) 51
18. Influence of Various Cations on Adsorption of Selenium (IV) 53
19. Influence of Various Cations on Adsorption of Selenium (VI) 5ft
20. Freundiich Isotherm Plots of Se(IV) and Se(VI) Adsorption
in Deionized Water Matrix 60
21. Freundiich Isotherm Plots of Se(IV) and Se(VI) in Synthesized
Well Water Matrix 61
22. Schematic Diagram of Testing Equipment During Removal
Run and Regeneration 65
23. Photograph of Column Testing Apparatus 66
2ft. Run 1 (3-inch) - Effect of Contact Time 7ft
25. Runs 2-6 (3-inch) - Regeneration Tests 76
26. Runs 7-12 (3-inch) - Regeneration Tests 78
27. Runs 13 and 1ft (3-inch) - Regeneration Tests 82
28. Runs 15-17 (3-inch)-Regeneration Tests 8ft
29. Runs 18 and 19 (3-inch) - Regeneration Tests 88
30. Runs 20-22 (3-inch) - Regeneration Tests 90
31. Photograph of "Wastewatcher" Automatic Sampler 93
32. Runs 1-3 (9-inch) - Capacity Tests 95
ix
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FIGURES (CONTINUED)
Number Page
33. Runs 4-6 (9-inch) - Capacity Tests 99
34. Runs 7-9 (9-inch) - Capacity Tests 102
35. Runs 10-12, Capacity Tests 103
36. Neutralization of Activated Alumina by 0.05N H-SCX
and Effluent pH of Subsequent Treatment Run 106
37. Runs 13-15 (9-inch), Capacity and Saturation Tests 108
38. Elution Curves for Regeneration After Run 13 110
39. Runs 16-18 (9-inch), Capacity Tests 115
40. Runs 1-3 (Se(VI)), Initial Tests 118
41. Runs 4 and 5 (Se(VI)), Regeneration Tests 120
42. Runs 6-8, (Se(VI)), H-SO. vs. HC1 Regeneration 122
43. Runs 9-11 (Se(VI)), Regeneration Tests 125
44. Runs 12-14 (Se(VD), Regeneration Tests 126
45. Elution Curves for Regeneration After Run 14 130
46. Neutralization of Activated Alumina by 0.05N HC1 and
Effluent pH of Subsequent Treatment Run . 131
47. Runs 15-18, SO.= and HCO,~ Interferences 133
48. Se(VI) Removal vs. SO..~ ana Alkalinity Concentrations 134
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TABLES
1. Constants Used to Construct E.-pH Diagram 5
2. Verification of Reproducibility^or Regenerant Analysis 27
-3. Resuits of Selectivity Series Tests 57
4. Average RMWD Well Water Composition 68
5. Amounts of Various Reagents Added to Make Up
Synthesized Well Water 68
6. Summary of Results 70
7. Removals and Recoveries of Se(IV) for
Runs 10, 11, and 12 81
8. Removals and Recoveries of Se(IV) for
Runs 12, 13, and 14 83
9. Removals and Recoveries of Se(IV) for
Runs 1*, 15, 16, and 17 85
10. Degradation of Activated Alumina by
Various Concentrations of NaOH 86
11. Removals and Recoveries of Se(IV) for NaOH and
H2SO^ Rinses During Runs 19, 20, 21, and 22 91
12. Degradation of Activated Alumina by Various
Concentrations of H-SO^ 91
13. Summary of Removals and Recoveries of Se(IV)
During all Runs with 9-inch Columns 97
14. Comparison of Removals of Interfering
Anions and Cations During Run 12 104
15. Removals and Recoveries of Se(VI) for Runs 2 and 3 119
16. Removals and Recoveries of Se(IV) for Runs 3, 4, and 5 121
17. Removals and Recoveries of Se(VI) in Tests Using
9-inch Columns, Runs 6-18 127
18. Degradation of Activated Alumina by Varied
Concentrations of NaOH and by 0.05N HC1 Acid Rinse 137
19. Cost Estimate for Se(IV) or Se(VI) Removal Facilities 140
xi
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ACKNOWLEDGMENT
We would like to express our gratitude to Mr. Richard Lauch, Project Officer, and
Mr. Thomas Sorg of the Water Supply Research Division of the U.S. Environmental
Protection Agency in Cincinnati, Ohio, for their technical assistance and interest in
this project. We would also like to thank the following members of James M.
Montgomery, Consulting Engineers, Inc. staff for providing assistance and support
during the project:
ENGINEERING
Michael Kavanaugh
Carol Tate
LABORATORY SUPPORT AND ANALYSIS
Lawrence Y.C. Leong
Raymond G. Zehnpfennig
Douglas Peitz
Keith Mainquist
Gregg Oelker
Glynis Coulter
GRAPHICS
Larry Quay
Deborah Shibata
Susan Chapman
REPORT PRODUCTION
Rita Clark
Patricia Stewart
Roberta Bullock
Alberta Alexander
Judith Doocy
REPORT REPRODUCTION
Alfred Robinson
Jack Bencomo
Dennis McFadden
xii
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SECTION 1
INTRODUCTION
BACKGROUND
Selenium is a controversial contaminant found in trace quantities in some ground
waters and surface waters in the United States. Selenium is known to be an essential
nutrient in minute quantities, but prolonged exposure to higher concentrations has
been known to bring about respiratory disease and death. Investigators have reported
that selenium is a carcinogen and others have claimed that it has anti-carcinogenic
effects. ,
The current Federal limit for selenium in drinking water is 0.01 mg/1. While
toxicological research continues looking into the health effects of selenium, it is
prudent to investigate concurrently the technology for removing selenium from water.
Little accurate data on selenium in drinking water is currently available. As
analytical techniques for selenium become more accurate, more systems may be
affected.
SELENIUM
Unfortunately, data on the presence of selenium in drinking water supplies are
limited. The data that are available suggests that one would rarely find surface
waters containing appreciable (?-0.01 mg/1) amounts of the element . Its presence in
higher concentrations (> 0.050 mg/1) appears to be limited to ground waters. Up to
2
0.^8 mg/1 has been reportedly found in a well in Nebraska . Other known wells with
relatively high amounts of selenium have been found in Southern and Central
California, Colorado, South Dakota, and Wyoming.
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Inorganic selenium occurs in valence states of +^ and +6 in aqueous solutions. It
is assumed that their presence in natural ground water is due to seleniferous
formations within the aquifer. Selenium (IV) occurs as the anion HSeO^" in the range
of pH's from 2.7 to 8.5. Selenium (VI) occurs as the anion SeO^= above pH 1.7. The
speciation of each oxidation state is given in more detail in the concentration -pH
diagrams in Figure 1.
Selenate (Se(VI)) in the form of selenic acid (r^SeO J is comparable in strength
to sulfuric acid. In solubility, most salts of selenic acid are similar to the sulfates
of the same metals. Selenite (Se(lV)) in the form of selenious acid (H2Se03) is a weak
acid. Most selenite salts are less soluble than the corresponding selenates.
The behavior of selenium in various environments may be best determined by an
examination of the reduction-oxidation (redox) potentials for its various oxidation
states as a function of pH. The E. - pH diagram for selenium, shown in Figure 2, can
be explained as follows: Each line on the diagram represents equilibrium between the
oxidized form written above the line and the reduced form written below it. The
space between two lines is the stability field of the ion or molecule shown on the upper
side of the lower line and the lower side of the upper line. The dashed lines
represent the stability limits of HUO. The shaded area shows the normal range of
waters from pH 6.0 to 8.5, which is the region of interest in this discussion.
The upper portion of the shaded area represents a highly oxidized situation. Well
aerated surface waters having a high oxidation potential would fall in this region. So
alkaline surface waters with selenium present should show a great majority of Se(VI).
Any Se° could be oxidized to HSeO," in alkaline or mildly acidic conditions or the
HSeCL", in turn, could oxidize to SeO.=.
Selenite could be rapidly reduced to elemental selenium by mild reducing agents
such as SO-,. Selenate has been found to be more stable and there is less probability
that selenate could be reduced to selenite under mild reducing conditions, but the
diagram shows that selenate is also unstable in a reducing environment over the long
term.
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13
O
UJ
0.
(/)
o
O
O
Q£
U
0.
a
o
u
0.
(A
O
O
5
O
£E
Ul
0.
100
SE(IV)
9 10 II 12
PH
SE(VI)
8 9 10 It 12
PH
Figure 1. Concentration -pH diagrams for inorganic selenium species.
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1.6
1.2
0.8
0.4
rf
LjJ
-0.4-
-0,8-
- i.2 •
- 1.6 -
HS«04~
NORMAL EH-pH RANGE IN WATER
H2Si
HSe"
SOLID/SOLUTE INTERFACE = 10 7 M (SOLUTE)
02
, OXIDIZING
H20
. REDUCING
6 8
pH
10
12
CONSTANTS PER TABLE I-I
Figure 2. Eh - pH diagram for selenium.
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TABLE 1
CONSTANTS USED TO CONSTRUCT
Eh-pH DIAGRAM
Half -Reaction
SeO^= + 4H+ + 2e~ = H2SeO3 + H2O
SeO^= + H2O + 2e" = SeO3~ * 2OH"
H2SeO3 + 4H+ + 4e" = Se + 3 H2O
SeO3= + 3 H2O + 4e~ = Se + 6OH"
Se + 2H+ + 2e~ = H2 Se (aq)
Se + 2e~ = Se"
Acid-Base Reaction
H2 SeO3 = H+ + HSeO3"
H SeO3" = H+ + SeO3=
H2SeO^ = H+ + H SeO^~
H SeO^~ = H+ + SeO^=
H2Se(aq) = H+ + HSe"
HSe~ = H+ + Se=
Potential Source of
(Volts) of Constant
1.15
0.05
0.74
-0.37
-0.40
-0.92
pK
2.75
8.50
-3
1.66
3.89
15.0
1. Latimer, W., "Oxidation Potentials" 2nd Ed., Prentice-Hall, N.Y., 1952
2. Baes, C.F. and Mesmer, R.E.,
The Hydrolysis of Cations, Wiley-Ii
1
1
1
5
1
1
Source of
Constant
2
2
4
2
2
2,3
•
rcterscience
N.Y., 1976.
3. Sillen, L.G. and Martell, A.E., Stability Constants of Metal-Ion Complexes,
Met calf and Cooper, Ltd., London, 1964.
4. Langes' Handbook of Chemistry. 12th Ed., Editor-3. Dean, McGraw Hill CO., N.Y.
1979.
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In waters with low levels of oxygen, the oxidation potential is reduced.
Depending on the actual oxidation potential of the water, a combination of Se(IV),
Se(VI), and Se° could be present. A ground water would be expected to offer
conditions where selenium could be present as a mixture of Se(IV) and Se(VI).
There is ambiguity concerning the redox potential between Se° and H-Se. It may
be possible in a very reduced condition that selenium as H5e~ could be formed in
natural waters. However, the occurrence of free selenide (H-Se, HSe~, and Se~) in
ground waters is unlikely due to its rapid formation of precipitates of iron. Evidence
has been presented to suggest that when selenites react with ferric chloride, a very
insoluble precipitate that approximates the composition of basic ferric selenite,
is also formed. This will also happen with aluminum to a lesser
extent.
Based on the information presented, the presence of either Se(VI) or Se(lV) in a
water depends on many factors concerning the chemistry of the particular water of
interest. From the E.-pH diagram, a poorly aerated ground water could have a varied
combination of Se(lV) and Se(VI). Because of Se(IV)'s lowered solubility, any iron
present in a water would tend to form insoluble precipitates with it. Elemental
selenium presents no problem in aqueous considerations. Therefore, Se(VI) would
appear to be the most common species present in a generalized groundwater condition.
Organic selenium occurs in natural aqueous solutions as (CrO Se, (CHj Se_, and
other forms by the means of microbiological assimilation and degradation. Although
measurements are not available, organic selenium compounds are not thought to be a
large component of total selenium in groundwater. Most of the knowledge about
organic selenium has been derived from work involving synthetic organic selenium
compounds. From this work, it is known that essentially all of the synthesized
compounds contain selenium in the -2 oxidation state and that these compounds tend
to oxidize and form elemental selenium.
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PRIOR RESEARCH
In an effort to provide information explaining how the drinking water
requirement for selenium could be most effectively met, the Water Supply Research
Division of EPA conducted a research program consisting of jar test studies and bench
tests. Ferric sulfate coagulation, alum coagulation, and lime softening were studied
for Se(IV) removal. Ferric sulfate removed 85% of Se(lV) at a pH of 5.5 or less with an
initial Se(IV) concentration of 0.03 mg/1. Lime softening removed 45% with a well
water and Se(lV) concentration of 0.03 mg/1. Alum coagulation was unsuccessful,
7 5
removing only 20% at best. ' Coagulation by ferric sulfate, ferrous sulfate, alum,
and lime softening were unsuccessful for Se(VI), 10% removal- being the best.
However, ion exchange and reverse osmosis were found to both remove greater than
95% of either Se(IV) or Se(VI) from drinking water.2'5
Because coagulation and lime softening were not always effective or applicable
to small water systems, and because of the great costs associated with
demineralization by ion exchange or reverse osmosis processes, the EPA initiated
investigations of the use of activated alumina for removal of selenium. Activated
alumina has been successfully used in studies to remove phosphate, fluoride, silica, and
6789101112
arsenic from drinking water and phosphates from wastewaters. ' ' ' ' ' ' It is
currently being used in two full-scale fluoride removal facilities. Alumina is about
one-tenth the cost of most ion exchange resins and researchers have reported that it
doesn't remove sulfate, chlorides, and other anions that would compete with selenium
2
removal by anion exchange treatment.
ACTIVATED ALUMINA
Activated alumina is produced by thermal treatment of hydrated alumina, the
alumina being extracted from bauxite ore. It has a great affinity for water and is
generally used as a dessicant for drying gases and liquids. Activated alumina is
produced mainly by the Aluminum Company of America (ALCOA) and the Reynolds
Aluminum Company. The activated alumina produced by ALCOA is available in
several grades: F-l, F-5, and F-6. Type F-6 is a chromatographic alumina and costs
about eight times as much as type F-l. Type F-5 is considered a specialty product and
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is also more costly than F-l. Type F-l is the general grade of activated alumina
available and is practically 100 percent alpha alumina. It is the most inert of all the
alumina oxides. F-l is the grade used in these studies and some of the technical
aspects are shown below:
Chemical constituents: A12 0, 92.2%
Na20 0.9%
Fe2O3 0.08%
Loss on ignition 6.5%
Physical Properties: Surface Area = 250 m2/g
Loose Density = 52 lb/ft3
Packed Density = 55 lb/ft3
Specific Gravity = 3.3
Graded Mesh Ranges = 8-10,14-28,28-48,48-100
Adsorbents are defined as natural or synthetic materials of micro-cystalline
structure, whose internal pore surfaces are accessible for selective combination of
solid and solute. Usually the attractive forces are weaker and less specific than those
of chemical bonds. Its selective action is most pronounced in a monomolecular layer
next to the solid surface, but at times selectivity may persist to a height of three or
14
four molecules. Adsorption capacity of
the fluid-phase concentration of the solute.
14
four molecules. Adsorption capacity of a solid for a solute tends to increase with
As discussed by Kubli, adsorption of inorganic salts on alumina is due to
hydrolytic adsorption associated with aluminum and hydrogen ion exchange. If alumina
is treated with an aqueous acid solution, the A^O., will be charged to a hydroxyl-
bearing cation capable of binding the anions of various salts as water-insoluble salts.
If, for example, the acid is HC1, hydrogen ions from the acid react with some of the
attached hydroxide ions of the solid alumina to yield water molecules, which remain
attached to the alumina. The network of aluminum and water molecules then acquires
a positive charge. The anions of the acid become included in the solid as counter-ions,
and electroneutrality occurs. These counter-ions, since they are not built into the
network, will readily exchange and these ions are expected to be exchanged when the
8
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solid comes in contact with a less soluble anion. If Se(IV) is in solution as HSe03~,
HSeO-~will readily replace the chloride ion (CD provided that the solubility of the
chloride complex is greater than the selenite complex. The mechanism can be
explained by the following diagrams:
= Al - O x. = Al - O ^
Al-OH + H+ + Cl" —»• ^ Al- Cl + H,0
= A1-0X = Al-0 ^ *
= Al-0 = Al-0
provided that the solubility of
= Al-Ov
is lower than that of
= Al-0.
AI*CI
Since activated alumina is an amphoteric substance, it will adsorb cations at pH's
above its isoelectric point and anions in more acidic environments. This phenomona
can be explained by the net surface charge on the alumina. The isoelectric point is
defined as the particular pH where the net surface charge is zero. For type F-l
alumina, this point is at pH 9.2.
A phenomena called secondary adsorption is known to exist with alumina. This
occurs when cations bond to anions and then other anions adjacently link onto the
cations and vice versa. This effects a chain of ions. Secondary adsorption occurs as
(1) joint adsorption of anions with multivalent cations and (2) joint adsorption of
cations with multivalent anions. There appears to be no adsorption with univalent
2
anions and univalent cations . This may explain why the hardness ions, calcium and
magnesium, seem to be partially removed during a removal run at pH's in the range
of 6-8.
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Kubli developed an anion selectivity series for activated alumina based on his
work with a few anions of interest. They are, in order of decreasing preference:
OH', po^', F-, so^-, cr, No3~, cio4'
It is presumed that Kubli did his work at a normal operating pH in the range of 5
to 8 and thus the anion being removed instead of PCX would be H^PO^". Part of
the research discussed in this report involves the development of a broadened series
which includes other trace inorganics of interest, namely Se(IV), Se(VI), As(lII), As(V)
and another anion found in large amounts in every water supply, HCO7~
Kubli suggested that elution of the alumina after adsorption of the species of
interest had taken place could be done by any of three mechanisms:
(1) Alkali (OH") elution, where the heavily favored hydroxide ion displaces
all other species in or on the sites,
= Al-O^ = Al-O
Al»HSeO3 + OH" ^ Al- OH + HSeO "
(2) by contact with other anions which form less soluble basic aluminum
salts,
(3) by contact with higher concentrations of the original anion bound to the
alumina (in this case CD.
= A1-O = A1-O^
^Al'HSeO, +Cf ^ Al«Cl + HSeO"
= Al-O/ J = Al-O ^ *
The most feasible option would seem to be elution with a concentrated
Q
amount of sodium hydroxide. Ames tried NaOH and Na2CO3 as regenerants and
NaOH was found to be more efficient. Since optimum removals of anions should occur
at a pH less than 9.2, the now basic alumina must be eluted with an acid to lower
the pH. Preferably, this acid's anion will not effectively complete with the species to
10
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be adsorbed for sites on or in the alumina. Based on Kublis's results, perchloric, nitric,
hydrochloric, and sulfuric acids would be the preferred neutralizers, in decreasing
order of preference. Unfortunately, the relative costs of these acids are in the
reverse order, perchloric being most costly and sulfuric being the cheapest.
Various investigators have completed studies to determine activated
alumina's ability to remove the anions fluoride, silica, and arsenic. ''' In batch
19
tests, Choi and Chen determined that to achieve a final fluoride level of less than
1 mg/1 requires an initial fluoride concentration lower than 40 mg/1 with an adsorbent
dosage of 25 g/1. They found that the presence of other chemical species does not
seriously interfere with fluoride removal by activated alumina. pH was found to be
the most critical factor in determining the fluoride removal efficiency. The optimum
pH was around 6.
,Gupta and Chen investigated arsenic removal by activated alumina in the
batch mode. As before, pH played a major role in determining the capacity for arsenic
removal. Good removals were achieved in the pH range of 4 to 7. However, arsenic is
present in two valence states in water, +3 and +5. At pH 6.5, As(V) is present as
H2AsO ~, while at pH 9, As(III) is present as H-AsO.,. Arsenic (V) removals were on
the order of 10-20 times greater than were arsenic (III) removals under the same
testing conditions. Another difference from fluoride is that the presence of other
chemical species in the matrix reduced by as much as 80 percent the amount of
arsenic removed. As(V) adsorption was affected much more by chemical composition
than was As(III). Oxidation of As(IH) to As(V) is necessary to achieve effective arsenic
removal and chlorine has been successfully used to accomplish this.
12
Clifford and Matson determined that the optimum pH range for silica
removal by activated alumina is 8.0-8-.5. They believe that the ion Si(OH),0~ is being
removed and that this removal is optimal near the pK for ionization of Si(OHk, which
is 9.5. Varied levels of sulfate were found to have little effect on the capacity of
activated alumina for silica. However, the presence of fluoride in the water greatly
affected the capacity for silica.
11
-------
There has been some work using continuous flow activated alumina columns as a
means of determining the removal capacity of the element of interest. Rubel has
reported the optimum operating conditions for a full-scale fluoride removal facility.
He used 5 feet of alumina in a six-inch diameter column. After a treatment run was
finished, the following regeneration procedure was as follows:
2
Backwash: 2.5 bed volumes of treated water @ 9 gpm/ft , upflow
2
Regeneration: 2.5 bed volumes of 1% NaOH @ 2Yi gpm/ft , downflow
2
Intermediate Rinse: 4 bed volumes of treated water @ 5 gpm/ft , upflow
Neutralization: 3 bed volumes of 0.5% H25(X @ 2Yz gpm/ft , downflow
Neutralization is done by continually adjusting the pH of the raw water with
HLS(X until the pH of the treated water reaches that of the raw water (pH - 5.5). The
above mentioned step is the equivalent and is used to determine the actual amount of
used to neutralize the bed.
Treatment: Usable water is produced after the pH of the treated water reaches
9.0. Here, significant fluoride removal will start to occur.
With this treatment, he is able to produce 1400 bed volumes of treated water
with an average F" concentration of 0.8 mg/1, from an initial concentration of F" of
5.0 mg/1.
Q
In 1970, Bellack did some testing in the laboratory to see if activated alumina
could remove arsenic from water supplies. His brief work suggested the following
procedure:
Backwash: 15 bed volumes of tap water @ 9 gpm/ft , upflow
2
Regeneration: 4 bed volumes of 1% NaOH @ 1 gpm/ft , downflow
2
Intermediate Rinse: 8 bed volumes of distilled water @ 1 gpm/ft , downflow
2
Neutralization: 4 bed volumes of 0.1 N H2S(X @ 1 gpm/ft , downflow
Final Rinse: 4 bed volumes of distilled water Q 1 gpm/ft , downflow
Treatment: Raw water at 2 gpm/ft , downflow
12
-------
The final rinse is necessary to remove the excess 0.1 N H-SO^, from the bed.
This was not needed in Rubel's facility because the raw water's pH was adjusted with
H-SCXto perform the neutralization step and the pH was stepwise increased as the pH
of the treated water dropped. Bellack stated that with an initial concentration of 0.10
mg/1 total arsenic, approximately 900 bed volumes of water with an arsenic
concentration less than 0.01 mg/1 can be produced. However, he failed to mention
whether arsenic in the +3 or +5 valence state was present. He probably did not note
the difference in the two species and made no effort to determine the speciation.
o
Ames used fairly large (5.1 cm D x 49 cm H) columns to determine the removal
of phosphates from wastewater using activated alumina. He was able to remove
approximately 90% of all phosphates applied for 400 bed volumes. He concluded that
varied amounts of sulfate present in wastewater had little effect on phosphate
removal. pH was not selectively controlled in his experiments, but operating above pH
8.0 led to precipitation and calcium carbonate fouling of the alumina.
He prescribed the following regeneration:
8 bed volumes of 1 M NaOH @ 3 gpm/f t2
2
20 bed volumes of washwater @ 3 gpm/f t
No acid rinse was used after a caustic elution due to his apparent lack of concern
about the operating pH of the treatment cycle. It was also noted that from 1% to 5%
of the column bed was lost per elution, making it necessary to replace the activated
alumina often.
PROJECT APPROACH
The approach of this project to determine the feasibility of removing selenium
from drinking water using activated alumina was developed so that each progressive
phase contributed to the following phase. The project was divided into three phases:
I. Analytical Techniques, II. Batch Isotherm Tests, and III. Bench Scale Column
Studies. Straightforward techniques for the analysis of Se(IV), Se(VI), and organic
selenium in water and an important prerequisite as they helped facilitate full
13
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concentration on the selenium removal process itself. Adsorption isotherms were used
to determine the rough parameters for column studies. The column tests helped to
determine optimum regeneration techniques, selenium breakthrough capacities, and
interferences by other ions.
The column studies should provide relevant information to efficiently test this
process at the pilot scale. This should be the next phase in studying the activated
alumina process for seleniumn removal. A pilot plant study can produce the
engineering data required to determine the feasibility of using activated alumina to
remove selium from drinking water.
It should be noted that the columns studies contain quite a bit of information.
Initial testing done with three-inch columns of alumina may not provide the reader
with a good understanding of the actual capacity information that was developed with
the nine-inch columns. However, the short column runs did provide us with enough
information to narrow down operating parameters with regards to regenerative
processes.
To develop a clear understanding of the sequential development of the
experimental design, it is suggested that the "Summary of Results", Table 6, be
reviewed prior to reading the discussion of the results of each successive run. The
results of all the tests performed during this work are presented in this report. For a
rapid review of the data, the reader may wish to skim through the section discussing
the three-inch column work.
14
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SECTION 2
CONCLUSIONS
ANALYSIS
The Atomic Absorption Spectrophotometer (AAS) can be used to determine the
total selenium present in a sample. The method is quick, sensitive, and produces
reliable results. Selenium (IV) can be differentiated from other forms of selenium by
using a fluorometric technique. This method requires a very small sample, 10 ml, but
its limitation is the amount of time required to complete the analysis. Depending on
the amount of glassware available to the analyst, 10 samples can be analyzed in
approximately 6 hours. Although its reliability has not been proven over time, the
analysis was found to be reproducible in our experiments.
Methods evaluated for the detection of organic selenium compounds proved to be
complex and subject to interferences. Although the literature shows that these
techniques can be feasible in a research environment, they were determined unsuitable
for routine monitoring at concentrations less than 100 pg/1. Samples to be analyzed
for different species of selenium were determined as: (1) total selenium (by AAS),
(2) selenium (IV) (by f luorometry), and (3) selenium (VI) and organic selenium (by
difference).
BATCH STUDIES
As part of the overall bench scale testing program, batch tests were necessary to
determine the effect of various parameters on the adsorption of selenium by activated
alumina.
15
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Preparation (Regeneration)
Fresh activated alumina must be initially prepared for selenium adsorption, the
preparation being similar to proposed regeneration techniques to be used in column
studies. Optimum finding, were:
• 1% NaOH rinse for 50 minutes
• 5-minute deionzed water rinse
• 0.05 N HC1 rinse for 10 minutes (to be used when removing Se(Vl)) or
• 0.05 N H-SO. rinse for 10 minutes (to be used when removing Se(IV))
• 2 to 4 deionized water rinses for 5 minutes each
HLSO. worked only slightly better than HC1 in preparing the alumina for Se(IV)
removal. HC1 worked much better than H2S
-------
Mesh Sizes
Three different mesh sizes were tested as to their capability of quickly adsorbing
selenium once in contact with activated alumina. The general rule of thumb is the
smaller the mesh size, the more rapid the adsorption. This is true for both Se(IV) and
Se(VI).
Interfering Ions
Of the anions tested, bicarbonate had the most pronounced effect on selenium
(IV) adsorption. It reduced removals by approximately 10 percent. Chloride, nitrate,
and sulfate only marginally interfered with selenium (IV) removal. Both sulfate and
bicarbonate heavily interfered with selenium (VI) adsorption at concentrations greater
than 100 mg/1. Greater than 60 percent reduction in the adsorptive capacity of
activated alumina for Se(VI) was noticed with these two anions. Chloride and nitrate
had no pronounced effect.
Three cations; sodium, magnesium, and calcium were evaluated for
interferences. None of these ions in concentrations as high as 200 mg/1 negatively
effected adsorption of either selenium (IV) or (VI). Some enhancement of adsorption
was noticed at high concentrations. This is thought to be due to secondary adsorption
effects.
Selectivity Series
In equi-molar concentrations, a part of Kubli's preferred anion list for activated
alumina was verified, with additional anions of arsenic and selenium tested. Results
were (at pH 6.5):
OH" > HP0" >F~ > HAsO~ >HSeO"
Because of poor detection limits, the following species that are less preferred
than the above five anions could not be relatively listed. Based on column studies,
they are listed in assumed order:
>HCO3" >C1~ >NO
17
-------
Capacities of Activated Alumina
In side by side tests with Se(IV) and Se(VI), activated alumina exhibited a greater
capacity for Se(IV) in matrices of deionized water and a synthetic well water.
Activated alumina was found to have approximiately 3 times the capacity for Se(IV)
than for Se(VI), based on a Freundlich isotherm model.
COLUMN STUDIES
Selenium (IV)
Using a synthetic well water modeled after a known ground water with high
selenium concentrations and a 9-inch deep bed of activated alumina, the following
capacities for
meters/hour):
capacities for Se(lV) were developed at a surface loading rate of 3 gpm/ft (7.3
pH5- 1,200 bed volumes , 235 „„, Qf a™?vaffalumina
pH 6 - 900 bed volumes = 175 mg/1
pH 7 - 500 bed volumes = 100 mg/1
The above breakthrough capacities (the amount of Se(IV) alumina will adsorb
before the effluent concentration becomes greater than 0.01 mg/1) are based on an
influent concentration of 200 ppb (=0.20 rng/1), with regeneration by 0.5% NaOH at a
dose of 1.5 //NaOH/ft3 bed (24 g/1) at a flow rate of 0.5 gpm/ft2 (1.2 meters/hour). A
slower regeneration rate may increase the above capacities. The effect of actual
flow-through contact time between the NaOH and the activated alumina was the
2
greatest of any parameters tested with regards to capacity. At 0.5 gpm/ft NaOH
flow rate, roughly 90% of the Se(IV) removed during the treatment run was recovered
in the regeneration.
For lower influent concentrations of Se(IV), the actual capacity is assumed to be
a linear function of the influent concentration. Therefore, an influent Se(IV)
concentration of 50 ppb should produce a capacity of roughly 60 mg/1 at pH 5.
18
-------
Similarly, the capacities at pH 6 and pH 7 showed 45 mg/1 and 25 mg/1, respectively.
However, to reach an effluent concentration of less than 10 ppb, similar bed volumes
of treated water will be produced regardless of the influent concentration up to
200 ppb.
Neutralization of the activated alumina after NaOH rinsing can be successfully
accomplished with either F^SO^ or HC1. The following regeneration scheme was
determined for this study:
5 bed volumes 0.5% NaOH @ 0.5 gpm/ft2, up
2
5 bed volumes D.I. water (d 1 gpm/ft , up
6 bed volumes 0.05 N H2SO^ or 0.05 N HC1 @ 1 gpm/ft2, up
5 bed volumes D.I. water (d 1 gpm/ft , up
All the regeneration steps were done in the upflow mode to facilitate rapid
testing. Upflow regeneration by NaOH is recommended, but the remaining steps
should be done downflow in a full-scale removal facility.
The NaOH and H«SO^ (or HC1) tend to dissolve the activated alumina and how
fast it degrades is dependent on acid or base concentration and flow rate. Higher
concentrations will dissolve slightly more alumina and slower flow rates will dissolve
more alumina than more rapid regeneration. The above-mentioned regeneration steps
dissolved, on the average, the following percentages of a nine-inch bed (by weight):
NaOH - 0.9% per regeneration
(or HCL) - 0.08% per regeneration
Annual media replacement will depend on how often regeneration is done. pH
adjustment for removal of Se(IV) can be done with either HLSO. or HC1. HC1 was used
for these tests.
19
-------
Selenium (VI)
Using a similar synthesized water as with Se(IV) the following capacities for
Se(VI) with a nine-inch bed of activated alumina were developed at a surface loading
rate of 3 gpm/f t :
pH 5 - 100 bed volumes = 4.5 mg Se(VI) liter of activated alumina
pH 6 - 70 bed volumes =3.2 mg/1
pH 7 - 35 bed volumes = 1.6 mg/1
The above breakthrough capacities are based on an influent concentration of 50
ppb Se(VI) with regeneration by 0.5% NaOH at a dose of 1.5 //NaOH/ft3 bed at a flow
2
rate of 2 gpm/ft . This capacity is one-twelfth the capacity for Se(IV) based on
equivalent influent concentrations of Se(IV) and Se(VI).
Rate of regeneration was not as critical as with Se(IV) and a smaller NaOH dose
could be used to achieve similar breakthrough capacities. This is due to Se(YI)'s
relatively low position in activated alumina's selectivity series. Regeneration is much
less dependent on diffusion-controlled kinetics.
The other regeneration steps were the same as with Se(IV), except only HC1 can
be used to neutralize the bed. Sulfate from an H2SO^ rinse heavily interferes with
alumina's capacity for Se(VI).
Due to the noticeable difference in acid rinses, tests were done to determine the
effect of varied concentrations of two interfering anions, SCX" and HCO^".
Decreased amounts of sulfate in the water greatly increased the capacity of activated
alumina for Se(VI). The following list shows these capacities at pH 6:
SO. (ppm) Se(VI) capacity (mg/1) Bed Volumes
500 0.7 15
100 3.2 . 70
50 7.0 150
5 21.0 450
20
-------
Similar tests with bicarbonate alkalinity interference did not show as great a
difference at pH 6, as listed below:
Alkalinity
(ppm as CaCOJ Se(VI) capacity (mg/1) Bed Volumes
500 1.5 33
100 3.2 70
50 4.0 90
5 5.5 125
The above capacities are for an influent Se(VI) concentration of 50 ppb. Sulfate
tests were done with approximately 100 ppm alkalinity, while the alkalinity tests were
done with a sulfate concentration of about 100 ppm.
pH adjustment must be done with HC1. H-SO. addition would increase the
sulfate level and interfere with Se(VI) removal.
COSTS
Preliminary cost estimates were developed based on the capacities and
regeneration techniques developed during this study. Annual costs were developed for
amortized capital costs over 20 years, which included equipment, piping,
instrumentation, a small building, clear well, activated alumina, and land. Annual
operation and maintenance costs included chemicals, labor, media replacement, and
electricity.
These costs were developed to treat a water supply of 1 MGD with similar
quality to that tested in this study contaminated solely with either a Se(IV) or Se(VI)
concentration of 0.10 mg/1 (100 ppb). Costs are:
Se(IV) - 23
-------
Based on an evaluation of the equilibrium between the various oxidation states of
selenium in water, Se(VI) is expected to be predominately in well-aerated waters. In
waters with lower oxidation potentials, Se(IV) and Se(VI) would be present in varied
amounts.
22
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SECTION 3
RECOMMENDATIONS
Because the speciation of inorganic selenium in water determines the cost of
removal, a survey of known selenium-contaminated ground water sources should be
analyzed to determine the relative amounts of Se(IV) and Se(VI). Since few actual
determinations of the speciation in ground water have been done, this would be helpful
in assessing conditions.
The inability to determine the presence of organic selenium compounds in a
water sample in amounts less than 100 ppb makes this problem of great concern.
Efforts should be made to modify the techniques described in Section 4 or to develop a
new method for detecting organoselenium compounds in the microgram-per-liter
range, since its presence may hinder the removal process by activated alumina.
Perhaps a GC/MS sparge and trap method could be used.
NaOH regeneration of Se(IV)-saturated, activated alumina should be optimized to
establish actual flow-through contact time and dose of NaOH required. Work done in
this study indicates that a longer contact time may increase the amount of Se(IV)
displaced during regeneration. It is possible that smaller amounts of NaOH could be
used than were used on a day-to-day basis during these studies. Because Se(VI) is
removed much less than Se(IV) in removal runs, the regeneration did not play as large a
role in determining its capacity on activated alumina.
Based on Kubli's assumptions, chloride (CO might be able to regenerate
selenium-saturated activated alumina if present in high enough concentrations. We
recommend that some testing be done to evaluate this claim, since the cost of NaCl is
much cheaper than that of NaOH and/or HC1. Sulfate, in the form of Na2SO4, might
be a better regenerant for Se(IV) than chloride, but its interference with Se(VI)
adsorption makes it unfeasible.
23
-------
Pilot-scale testing is necessary to successfully model a full-scale selenium
removal facility. Since a number of factors (SO^2", HCO3", pH) affect the removal of
Se(IV) and Se(VI), the general water quality of the source should be evaluated. Sparse
data gathered from Ramona Municipal Water District in a series of well samples
showed that selenium concentrations varied quite a bit. Continuous pumping should be
done to verify the steady-state concentrations of Se(IV) and Se(VI) before designing
pilot-scale tests to evaluate activated alumina's feasibility.
Actual pilot testing should be done to refine the techniques developed in this
study. Removal capacities, regeneration techniques and hydraulic characteristics
should all be carefully studied to allow for an economic design of a full-scale removal
facility, should it be deemed feasible. Deeper columns may produce more bed volumes
of treated water than is shown in the 9-inch column used in this study. Determination
of the optimum method for neutralizing the alumina with acid after the caustic rinse
should be looked at closely. Since these tests showed that activated alumina is
susceptible to large amounts of dissolution during the regeneration, this should be an
item of extreme importance to evaluate in larger scale, multi-cycle tests. Headless
characteristics of various media sizes should be evaluated in order to make decisions
regarding the use of higher capacity, smaller mesh sizes of alumina.
A major item that should be investigated is the disposal of regenerant wastes. In
arid or semi-arid regions, evaporation ponds may be feasible. Otherwise, disposal may
involve a separate, on-site treatment or hauling away by contract carriers. The costs
associated with the disposal may be high.
24
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SECTION 1
SELENIUM ANALYSIS STUDIES
ORGANIC SELENIUM
Gas chromatrography research was performed on three organoselenium indicator
compounds chosen on the basis of their probability of occurrence in the soil
environment and their commercial availability. Selenium substituted amino acids are
likely to occur in a selenium contaminated soil but are likely to be quickly degraded.
The three compounds chosen were dimethylselenide, diethylselenide, and
dimethyldiselenide. Aqueous standards of these pure compounds were made up and
they were analyzed by the dynamic headspace technique using nitrogen to strip the
materials from the water. The method used was similar to that described by Bellar
for organohalides in water with the exception that the, analytical column was five
percent polyphenyl ether (five rings) on 60/80 mesh chromosorb W-HM and an FID
detector was utilized. Testing showed that these organoselenium compounds were
effectively stripped from the water and adequately captured by the adsorbent trap at
relatively high concentrations. Interferring peaks which eluted at similar retention
times to the organoselenium compounds prevented the realization of minimum
detection limits less than about 100 ug/1. Extraction of these compounds by liquid-
liquid extraction with n-pentane and analysis by electron capture detection showed
that only the dimethyldiselenide compound was detectable but the minimum detection
limit for this compound was 10-100 times better than with the dynamic headspace FID
analysis.
Because of the poor minimum detection limits for these indicator
organoselenium compounds, it appears that the technique will not be useful for
drinking water analysis until further optimization of the technique can be performed.
25
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ATOMIC ABSORPTION SPECTROPHOTOMETRY
The AAS technique remains as the most efficient and sensitive means for
determination of total selenium. In the majority of the batch scale tests done, either
selenium (IV) or selenium (VI) was tested by itself. Therefore, analysis of selenium
could be done quickly on the AAS with the assumption that the total selenium present
was either Se(IV) or Se(VI). In tests involving combined quantities of Se(IV) and Se(VI),
the AAS determined total selenium concentrations and the fluorometric technique,
described later, gave concentrations of Se(IV). Se(VI) was calculated by difference.
All total selenium analyses were done by the flameless atomic absorption
technique on a Perkin-Elmer 305B AAS with a P.E. 2200 graphite furnace utilizing
deuterium ultraviolet background correction and an electrodeless discharge selenium
lamp. Standard conditions for the furnace were: a drying cycle of 30 seconds at
105 degrees C, a charring cycle of 10 seconds at 1000 degrees C, and an atomization
cycle of 7 seconds at 2200 degrees C. Pyrolitically coated graphite tubes and the
"max. power" function were used in analyses. This enabled a much lower atomization
temperature and forced all available current into the furnace tube at a very fast rate,
which is almost equivalent to instantaneous atomization.
An equal volume of 1000 ppm Ni(NO,)2 was added to the furnace after each
sample was injected. This method is an EPA standard method.
When analyzing samples using the furnace technique, matrix interferences played
an important role in determining the selenium present. The analyses of regeneration
samples, which had high concentrations of sodium hydroxide and either sulfuric acid or
hydrochloric acid, were greatly affected by the presence of these salts in the samples.
Normally, standards of selenium were made using deionized water and concentrated
nitric acid (the normal preservative for selenium). These standards were not usable
with the regeneration samples, due to the marked difference in results when compared
with standards made up from an approximate 0.05% sodium hydroxide selenium
standard. Therefore, regeneration samples were diluted about 10:1 to 0.05% NaOH to
negate any other interferences and then were analyzed using a comparative selenium
standard made with 0.05% NaOH that was acidified with HNO3 to a pH of less than 2.
26
-------
To verify reproducibility and accuracy of these results, known amounts of
selenium were spiked into regeneration samples. The analysis of the spiked samples
showed concentrations that were equal to the sum of the known amount spiked and the
unspiked sample within 10%. This accuracy is within the limits of the furnace
technique. Table 2 shows these results.
The correction for matrix interferences was not necessary when concentrations
of selenium exceeded 1.0 ppm. Above this level, selenium can be detected using the
flame atomic adsorption technique and the flame technique is not subject to matrix
interferences.
At the onset of testing, it appeared as though a double peak phenomena,
presumed to be selenium (IV) and (VI), occurred when using uncoated tubes with no
"max. power" input. Further work was conducted with more concentrated standards
and the phenomena was not reproducible. It was suspected that some contamination
was present in the reagents that was responsible for some of the peaks observed.
Although this technique would have been the simplest method for differentiating Se(IV)
and Se(VI), the idea was abandoned due to the inconsistent results. See Figures 3 and 4
for photographs of the AAS and associated equipment.
TABLE 2
VERIFICATION OF REPRODUCIBILITY FOR REGENERANT ANALYSIS*
Initial Spiked
Sample Concentration Concentration Difference
Date I.D. (ppm) (ppm) (ppm)
9-14 Col. 1 0.23 0.63 0.40
Col. 2 0.28 0.70 0.42
9-26 Col. 1 0.46 0.82 0.36
Col. 2 0.44 0.83 0.39
*24 mi's of sample were added to 1 ml of 10 ppm selenium
standard to give a spiked concentration of 0.40 ppm. The
differences noted above are with 10% of this value.
27
-------
-
Figure 3. Injecting a sample into the A.A.S. furnace.
-------
-
:
Figure 4. A.A.S. strip chart recorder.
-------
100
90
80
70
6
z
D
£ 60
55
I 50
U
o
5
o
£
O
40
30
20
10
FLUOROMETER
TURNER* 110-850
PRIMARY FILTER - 369 NM
SECONDARY FILTER -522 NM
SCALE -SOX
2345
SELENIUM (IV) (Ppb )
Figure 5. Influence of selenium (VI) on fluorometric determination of
selenium (IV), 0-7 ppb.
30
-------
100
90
80
*—\
P
z
70
55 60
I
z
o
s
o
I
o
2
50
40
4+W/0 SE6+
(A)SE4+W/SE
FLUOROMETER
TURNER#IIO -850
• PRIMARY FILTER -369 NM
SECONDARY FILTER-522 NM
SCALE -3X
10 20 30 40 50
SELENIUM (IV) (ppb)
60
70
Figure 6. Influence of selenium (VI) on fluorometric determination
of selenium (IV), 0 - 70 ppb.
31
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COLORIMETRIC DETERMINATION OF SELENIUM (IV)
The Standard Methods diamino benzidine method for selenium is selective for
selenium (IV) if the permanganate oxidation and subsequent reduction steps are
omitted from the procedure. This technique has shown to produce precise results, but
the presence of selenium (VI) produces a slight interference. The accuracy of this test
with selenium (VI) present is somewhat less than that of the fluorometric technique
(discussed next). But, this technique is useful in the selective determination of
selenite. It was not used in this study because the analysis requires approximately one
liter of sample and is much more time consuming than the fluorometric technique.
FLUOROMETRIC DETERMINATION OF SELENIUM (IV)
Selenious acid (HjSeO,) reacts with 2,3 diaminonapthalene in an acid solution to
form the strongly fluorescent naptha-(2,3-d)-2-selena-l,3-diozole. Selenates (Se(VI))
are not reduced to selenites (Se(IV)) under the conditions of this analysis, hence this
method is specific only for selenium in the four-valence state.
This technique, as described by Raihle , was tested as a means to differentiate
the two forms of inorganic selenium in removal tests performed with combined
selenium (IV) and (VI) present and to determine the amount of Se(IV) present in some
well water samples that were analyzed for selenium. Plots of fluorescent intensity
versus concentration of selenium (IV) in the range of 0 to 70 ppb are linear and are
practically free from selenium (VI) interferences, as shown in Figures 5 and 6.
Standards of known concentrations of selenium (IV), as verified by the AAS, were
analyzed using this technique and the calibration curves (Figures 5 and 6) were then
drawn. Similar standards of Se(IV) were spiked with varied amounts of Se(VI) and the
results are plotted alongside the curves without Se(VI) present. The curves are
identical within experimental error.
This technique is much more convenient to use than the colorimetric technique.
10 ml of sample is required, compared to 1 liter needed to analyze samples
colorimetrically. Also, to analyze 10 samples fluorometrically requires approximately
32
-------
six hours, which is about one-quarter of the time that it takes to do 10 samples
colorimetrically. All fluorometric analyses were performed with a Turner Model 110
Filter Fiuorometer with a standard lamp (//110-850). The samples were analyzed at an
excitation frequency of 369 nm (primary filter //110-811) and had a fluorescence
emission maximum of 522 nm (secondary filter #110-822). See Figure 7 for a
photograph of the f luorometer.
33
-------
-
£
Figure 7. Fluorometer.
-------
SECTION 5
SELENIUM BATCH STUDIES
As described in the introduction, a series of bench scale adsorption isotherm
studies were undertaken to analyze the various parameters involved in the use of
activated alumina to remove selenium. Studies of regeneration techniques, kinetics,
pH effects, mesh size, selectivity effects of various anions and cations, and capacities
were performed to develop the conceptual information necessary for the efficient
design of activated alumina contactors.
All batch scale studies were performed with deionized water spiked with
selenium (IV) and/or selenium (VI) as the stock solution. No other species were added
to the matrix, except in the case of studies involving pH effects, ion interferences,
and relative selectivity.
All batch studies, with the exception of some pH tests, were performed in
250-ml ehrlenmeyer flasks with glass stoppers. Contact was achieved by mixing with
a Labllne "Junior Orbit Shaker" at 200 rpm. Control flasks containing the selenium
stock solution without activated alumina showed no loss of selenium during the tests.
It was assumed that the glassware utilized did not absorb significant amounts of
selenium within the timeframe of the tests. See Figure 8 for a photograph of the
shaking apparatus.
REGENERATION
Various regeneration techniques involving the use of acids and bases were
employed to prepare virgin activated alumina for selenium sorption. In all of the
batch studies, only virgin activated alumina were used. No actual regeneration of
selenium-saturated activated alumina was done during this phase of the study. Rather,
35
-------
•
Figure 8. Shaking apparatus.
-------
the expected regeneration techniques to be used in column studies were modeled by
preparing fresh out-of-the-can activated alumina with bases and acids to develop
optimal adsorption characteristics.
Figure 9 shows the relationship of selenium removal versus activated alumina
contact time with a one percent solution of sodium hydroxide (1% NaOH). Fresh
activated alumina was rinsed with this basic solution for various amounts of time to
test the effect of NaOH rinse time on capacity for selenium removal. This technique
has been suggested by various authors who have used activated alumina to remove the
chemically similar anions, arsenic, and fluoride ' ' ' '.
Activated alumina exhibits the highest selectivity for hydroxide (OH~) ions. This
caustic rinse should remove all other anions present in or on active sites on the
alumina and replace them with hydroxide ions.
As is seen from the graph, selenium removal efficiencies of 90 percent are
achieved with as little as five minutes of rinsing. In all the following batch scale
tests, a 50 minute rinse with 1 percent NaOH was used with fresh activated alumina to
ensure optimum removals of selenium.
Following the caustic rinse, a 5-minute rinse with 150 ml of deionized water was
found to be necessary as an intermediate step between base and acid rinses. Without
this rinse, the exothermic acid-base reaction tended to alter the activated alumina and
bind it in clumps.
The next step in the regeneration procedure is to rinse the now basic alumina bed
with an acid to lower the pH, essentially to remove the OH" ions from the sites on the
2
alumina. As previously suggested by Ball , hydrochloric acid (HC1) and sulfuric acid
(H2SO^) were utilized to compare, their abilities to prepare the activated alumina for
selenium (IV) and (VI) removal. Figure 10 plots the relationship of selenium ((IV) or
(VI)) removal versus activated alumina contact time with 0.05 N solutions of HC1 or
H2SO^. This concentration of acid was taken from Ball's work. Prior to this acid
rinse, the fresh activated alumina had been initially rinsed for 50 minutes with 1
percent NaOH, then with deionized water for 5 minutes.
37
-------
SE4+
1 40
30
10
W/H2SO4
EXPERIMENTAL CONDITIONS
ACT. ALUMINA: l.25o, 28-48 MESH
PREPARATION:
NAOH; I •/. ,TIME VARIES
DEIONIZEDH2O: 5 MIN.
HG-:O.O5N FOR 10 MIN. [Se(VI)]
H2SO4 : O.O5 N FOR 10 MIN. [S
DEIONIZED H2O: 5 MIN.
VOLUME: ISO ml
INITIAL [SE (IV)] - 33 ppb
INITIAL OPE (Vljj - 55 ppb
CONTACT TIME: 10 MIN.
pH: ^ 3.5
20 40 60 80 100 120
CONTACT TIME W/ ACT. ALUMINA (MlN.)
140
Figure 9. Initial preparation of fresh activated alumina with NaOH.
38
-------
100
90
80
SE 4+W/H2SO4
20
10
SE ' W/ M2SU4 >w
SE T W/H2S04/
EXPERIMENTAL CONDITIONS
ACT. ALUMINA: 1.25 G, 28 - 48 MESH
PREPARATION:
NAOH: I % FOR 50 MIN.
DEIONIZED H20: 5 MIN.
HC.-: TIME VARIES, 0.05N
HaSO. : TIME VARIES, 0.05N
DEIONIZED H20: 5 MIN.
VOLUME ; 150 ml
INITIAL [Se (IV)J = -~ ppb
INITIAL [Se (VI)J = — ppb
CONTACT TIME : 60 MIN.
pH ~3.5
10 20 30 40 50 60
ACID CONTACT TIME W/ACT. ALUMINA (MIN.)
70
Figure 10. Initial preparation of fresh activated alumina with acid after
NaOH rinse.
39
-------
It appears that there are no differences in the removal capabilities of selenium
(IV) with activated alumina rinsed with HC1 or H2SCV Because slightly better
removals of Se(IV) were achieved with H2SO^, this acid rinse was used in all the
following tests involving selenium (IV) removal.
Figure 10 also shows that the HC1 rinse works better in preparing activated
alumina for selenium (VI) removal than the FUSCX rinse. This is thought to be because
the sulfate ion (SO^ ) from the H-^SO^ actively competes for sites on the activated
alumina with the aqueous form (Se(k ") of selenium (VI). The chloride ion (CO does
not compete like the sulfate with selenium (VI). Therefore, an HC1 rinse was used in
all the following tests involving selenium (VI) removal.
On the basis of these experiments, a 10 minute rinse with either 0.05 N HC1 or
0.05 N HLSCX was used in all the batch scale tests from this point on. This contact
time allows for optimum selenium removals.
Following the acid rinse, the pH of the activated alumina bed was adjusted to
achieve a consistent pH of 4.0 during the removal tests, except to determine the
relative selectivity of competing ions. Depending on the amount of activated alumina
used and the type of acid rinse used, two to four 5 minute rinses with deionized water
were required to reach pH = 4.0. For most of the tests, 1.25 grams of activated
alumina were used and, with either H-SO^ or HC1 rinses, three five-minute rinses were
required. Five or more deionized water rinses did not produce any appreciable pH
change above pH = 4.0. The greatest pH change occurred within the first three
deionized water rinses.
KINETICS
The kinetics of sorption of the two selenium forms were determined to delimit
the range of flow rates suitable for the activated alumina process. This testing also
helped to determine the contact times to be used in batch scale studies. Individual
flasks, each with the same amount of prepared activated alumina, were used for each
separate time period tested, between 0 and 120 minutes.
40
-------
As seen in Figure 11 a great majority of selenium removal occurs in the first ten
minutes of contact with activated alumina. Rubel13 suggests this optimum contact
time in his work on fluoride removal. However, remoyal still seems to be taking place
for upwards of 60 minutes in the case of selenium (IV). For this reason, a 60-minute
contact time with the stock selenium solution was employed in all batch scale tests.
This ensured equilibrium between the remaining selenium in solution and the activated
alumina.
The 28-48 mesh size activated alumina was used in these tests. It was assumed
that the different mesh sizes would remove selenium at different rates, but that at
equilibium, the mesh size should not affect the total amount of selenium removed in
the batch scale.
Previous investigators have conducted isotherm tests to determine the capacity
for removal of fluoride and arsenic anions using activated alumina *'. They have
shown that contact times on the order of 48 hours are required to reach equilibrium.
Gupta and Chen used an initial concentration of 4.0 mg/1 arsenic with a dose of 2 g/1
activated alumina. Choi and Chen used an initial concentration of 25 mg/1 fluoride
with a dose of 25 g/1 activated alumina to determine kinetics. Both of these tests used
an initial concentration much greater than was used in this study. With C = 0.10 mg/i
and a dose of 8.3 g/1 activated alumina, the ratio of initial concentration of
contaminant to dose of activated alumina is 0.012 mg/g. This is nearly 100 times less
than the ratio of 1 mg/g for the fluoride study. Since the intensity of adsorption is
considered to be very dependent on the concentration of the contaminant, the results
of this study appear to be well within reason.
pH EFFECTS
The influence of pH on the sorption of selenite and selenate was investigated in
batch tests covering the range of pH values commonly occurring n\potable waters. As
a first step in determining the effect of pH on selenium adsorption by activated
alumina, the pH of the stock selenium solution was adjusted to the desired value with
NaOH or HC1 and then put in contact with the prepared activated alumina for 60
minutes. Figure 12 shows the results. As can be seen, the final pH (pHf) of the
41
-------
•••••••••••
30
20
10
EXPERIMENTAL CONDITIONS
ACT. ALUMINA: 1.25 o, 28-48 MESH
PREPARATION:
NAOH: I % FOR 5 MIN
DEIONIZED H2O: 5 MIN
HCu: 0.05 N FOR 10 MIN. [Se(VI)]
H2S04: 0.05 N FOR 10 MIN. [ SE(IV)]
DEIONIZED H2O: 3 Q 5 MIN EACH
VOLUME: ISO ML
INITIAL [SE(IV)J - 96 ppb
INITIAL [ SE (VI)] -99 ppb
CONTACT TIME: VARIES
0 20 40 60 80 100 120
SELENIUM CONTACT TIME WITH ACT. ALUMINA (MIN.)
Figure 11. Effect of contact time with activated alumina on selenium
adsorption.
42
-------
100
90
80
SE 4+
3.4 _J**^ 4.2 M 1i -it/ i
3.4
<
70
60
a
o 50
I-
30
20
10
pHF
EXPERIMENTAL CONDITIONS
ACT. ALUMINA: 1.25 G, 28 - 48 MESH
PREPARATION:
NAOH: I % FOR 50 MIN.
DEIONIZED HgO: - 5 MIN.
HCi_: 0.05N FOR 10 MIN.-SE(VI)
H2S04 : O.05 N FOR 10 MIN. - SE (IV)
DEIONIZED HgO; 5 MIN.
VOLUME: 150ml
INITIAL [Se (IV)] -58 ppb
INITIAL [SE (VI)] -53 ppb
CONTACT TIME: 60 MIN.
pH: AS NOTED: pHp - FINAL pH
PH WAS ADJUSTED WITH NAOH OR
HCu BEFORE CONTACT W/ ACT. ALUMINA
6 7
INITIAL PH
10
Figure 12. Effect of initial pH on adsorption of selenium.
43
-------
solution changed radically from the initial pH. At no time were we able to maintain a
constant pH in the solution during any of the initial pH studies, except in the range of
pH = 4.0.
The data does show a trend, though. In the small variation of pH in the selenium
(IV) tests, pH seemed to have little effect on Se(IV) adsorption. However, the slight
changes in pH noticed in the selenium (VI) tests seemed to indicate a relationship
between lower pH and increased removal. But, this difference is probably due to the
interference of OH" ions added to adjust the pH upward.
Our inability to maintain a constant pH during the 60 minutes of contact time
with the activated alumina led us to look at buffers that could maintain a constant pH
without interfering with adsorption. Different buffers comprised of pthalate, acetate,
borate, and phosphate were tried for different pH's and they all interfered with
selenium (IV) and (VI) adsorption. Their presence in the sample also interfered with
the atomic absorption spectrometry analysis of selenium.
The next approach to maintaining a constant pH during contact with the
activated alumina was to continously monitor and adjust the pH using a jar testing
apparatus to provide the mixing. The activated alumina was prepared as usual in the
ehrlenmeyer flasks, then it was transferred to 1000 ml beakers with the stock selenium
solution. The pH was monitored continously and was adjusted by the dropwise addition
of either 0.1 N NaOH or 0.1 N HC1. The pH was allowed to deviate plus or minus 0.2
pH units before any adjustment was done.
Figure 13 shows the results of the testing. Adsorption of selenium (IV) by
activated alumina was not effected by pH less than 7. At pH 8 and 9, slightly lower
removals were observed. This was probably related to the fact that the dominant
species of Se(IV) in solution changes from HSeO,~ to SeO3~ at about pH 8 and greater
amounts of NaOH were added to maintain these pH's. As can be seen, pH'had a varied
effect on selenium (VI) adsorption. Removal appears to be higher at low pH (3), drops
at slightly higher pH, then suddently increases again to reach a higher removal at
pH 7. Removal tends to drop off as pH is increased above 7. It is most likely that the
addition of NaOH caused interferences that hindered Se(VI) adsorption. Two separate
tests were done with Se(VI) and Figure 13 shows the variability.
44
-------
100
90
80
70
en
o
S, 50
40
,30
20
10
EXPERIMENTAL CONDITIONS
ACT. ALUMINA: 1.25 G, 28 -48 MESH
PREPARATION:
NAOH: I % FOR 50MIN;
DEIONIZED H2O: 5 MIN.
HCu : 0.05 N FOR 10 MIN. [ Sc(VI)]
H2S04 : 0.05 N FOR 10 MIN. [ S*(IV)J
DEIONIZED H2O: 3 <® 5 MIN. EACH
VOLUME: 150 ml
INITIAL [SE(|V)] - 97 ppb
INITIAL [SE(VI)j -99 ppb
CONTACT TIME: 60 MIN.
PH : AS NOTED
PH WAS ADJUSTED CONTINUOUSLY WITH
OR HCu
6 7
PH
Figure 13. Effect of constant pH on adsorption of selenium.
45
-------
Because activated alumina is an amphoteric material, pH should be an
important factor in determining its capacity as an adsorbent. These tests fail to show
this phenomena.
Ball's work showed a great dependence on pH for Se(lV) and Se(Vl) adsorption by
activated alumina. Adsorption increased almost linearly from pH 7 to 4, with
adsorption "peaking out" at approximately pH 3. This is the type of phenomena that
would be expected.
All further batch tests were done at pH = 4.0. No adjustment was necessary to
achieve this pH and, from this data, pH had no effect on selenium (IV) adsorption in
the batch mode. It was later determined that the pH of the activated alumina could
be adjusted by rinsing with a less concentrated acid. This was done in the selectivity
series tests and could be used to repeat the pH experiments just mentioned.
MESH SIZE
The effect of activated alumina mesh size on selenium sorption efficiency was
determined for the following medias: 14-28, 28-48, and 48-100 (arranged in order from
largest to smallest). These are three of the four mesh sizes typically produced. A
larger size, 8-10 mesh, is also available, but was not evaluated.
Figure 14 shows the relationship between selenium (IV) removal and contact time
with the three sizes of activated alumina. As expected, the smaller mesh size, 48-100,
removes selenium (IV) faster than the other two sizes due to its larger surface area per
unit weight. In actual activated alumina contacts, a 5-minute contact time through
the bed might be chosen as a median value for a cost-effective residence time.
Comparing removals of the three mesh sizes at 5 minutes contact time gives us:
14-28 - 59% Se(IV) removal
28-48 - 68% Se(lV) removal
48-100 - 81% Se(IV) removal
Figure 15 shows selenium (VI) removal versus contact time with the three sizes
of activated alumina. As expected, the smaller mesh size removes selenium (VI)
46
-------
48 -100 MESH
30
20
10
EXPERIMENTAL CONDITIONS
ACT. ALUMINA: 1.25 G, MESH VARIES
PREPARATION:
NAOH : I % FOR 50 MIN.
DEIONIZED H2O: 5 MIN.
HCu : NONE
H2SO4: 0.05 N FOR 10 MIN.
DEIONIZED H2O; 5 MIN.
VOLUME : 150ml
INITIAL [SB (IV) ] ~ 100ppb
INITIAL [SE (VI)] -NONE
CONTACT TIME : VARIES
PH : ~ 3.5
20 40 60 80 100 120 140
SELENIUM CONTACT TIME WITH ACT. ALUMINA ( MIN.)
Figure 14. Effect of mesh size on removal of selenium (IV).
47
-------
100
90
80
70
> 60
£
50
D
i
U 40
U)
30
20
10
48-100 MESH
28-48 MESH
••••••••••••••••••••••••••
28 MESH
EXPERIMENTAL CONDITIONS
ACT. ALUMINA: 1.25 G, MESH VARIES
PREPARATION:
NAOH: I %> FOR 50 MIN.
DEIONIZED HzO;5 MIN.
HCu: 0.05N~ FOR 10 MIN.
HzS04: NONE
DEIONIZED HzO; 5 MIN.
VOLUME: ISO ml
INITIAL[SE (iv)]~ NONE
INITIAL [Se (Vlj-lOOppb
CONTACT TIME: VARIES
0 20 40 60 80 100 120
SELENIUM CONTACT TIME WITH ACT. ALUMINA (MlN)
Figure 15. Effect of mesh size on removal of selenium (VI)
48
-------
faster. Again, comparing removals of the three mesh sizes at 5 minutes contact time
gives us:
28-48 - 65%
48-100 - 80%
While the smaller mesh size (48-100) provides faster removal, one must consider design
constraints in constructing a full scale removal facility. The larger mesh size would
be more difficult to fluidize during backwash, but could allow longer run lengths if
headless is involved as a limiting factor. A trade-off would have to be made between
removal efficiencies, run length, and backwash volumes in the final design of a full-
scale removal facility. The most practical size is probably a 28-48 mesh.
INTERF^ING IONS
The effect of various anions and cations on the selenium sorption capacity of
activated alumina was investigated. , Specific anions of interest were chloride,
bicarbonate, sulfate, and nitrate. Specific cations of interest were calcium,
magnesium, and sodium.
Figure 16 depicts the relationship between concentration of the various anions
and removal of selenium (IV). No great effect is seen by any of the anions introduced
in concentrations less than 50 mg/1. Greater concentrations of the anions produce a
slight decrease in efficiency of removal, with the exception of bicarbonate (HCO,~).
This ion has a more pronounced effect on activated alumina's ability to adsorb
selenium (IV), but this may be an artifact of the effect of pH on the form of Se(IV).
All anions were added as a salt of sodium (Na), because sodium is expected to have
minimal interferences with adsorption.
Figure 17 shows how the chosen anions effect the removal of selenium (VI). For
concentrations of 25 mg/1 of all the anions, an apparent increase in removal efficiency
was noted. It is difficult to say if a real trend exists at these low levels, because
certainly one would not expect enhancement of adsorption by the presence of other
49
-------
100
90
80
70
> 60
O
,*
c. 50
2
LJ
UJ
U)
40
30
20
10
&.-
PHF=6.2 '
* •
'\
S04
HC°3"
pHF = 6.5
EXPERIMENTAL CONDITIONS
ACT ALUMINA: 0.5 G, 28-48 MESH
PREPARATION:
NAOH: I % FOR 50 MIN
DEIONIZEDHgO: 5 MIN.
HCu: NONE
H2SO4: 0.05 N FOR 10 MIN.
DEIONIZED H20: 2^5 MIN EACH
VOLUME: 150 ml
INITIAL [ Se (IV)] - 100 ppb
INITIAL [ SE (VI)] -NONE
CONTACT TIME: 60 MIN.
PH : 4.0 EXCEPT AS NOTED
0 40 80 120 160 200
INTERFERING AN IONS ( mg/1)
Figvire 16. Influence of various anions on adsorption of selenium (IV).
50
-------
• ••<•
-
EXPERIMENTAL CONDITIONS
ACT. ALUMINA: 0.5 G, 28-48MESH
PREPARATION:
NAOH: I % FOR 50 WIN
DEIONIZEDHaO : 5 MIN
HCi.: 0.05 N FOR 10, MIN
HgSO^. NONE
DEIONIZED H20:3e5 MIN. EACH
VOLUME: 150ml
INITIAL CSs 0V)>NONE
INITIAL CSE (VI) I]- 100 ppb
CONTACT TIME: 60 MIN
PH : 4.0 EXCEPT AS NOTED
40 80 120 160 200
INTERFERING ANIONS ( mg/l)
Figure 17. Influence of various anions on adsorption of selenium (VI)
51
-------
anions. Perhaps the lower concentrations calculated in these flasks were due to
matrix interferences from the anions added. However, at high concentrations, sulfate
(SO^ ") and bicarbonate (HCO3~) interfered greatly. As mentioned previously, sulfate
is expected to heavily compete with selenium (VI) for sites on activated alumina since
they are chemically very similar and because of activated alumina's alumina
preference for sulfate over selenate. Other researchers have noted bicarbonate's
interference with adsorption of ions similar to selenium, but the mechanism is unclear.
The presence of high alkalinity in raw water will hinder the removal of both selenium
(IV) and (VI), though it will have a greater effect on selenium (VI) removal. Nitrate
and chloride exhibit little interference of adsorption.
In the next test, the cations Mg++, Ca++ and Na"1" were added to the stock
solutions of selenium (IV) and (VI) as salts of chloride (Cl) because, from the anion
interference tests, chloride seemed to exhibit less interference than the other anions.
As expected, the cations shows little tendency to inhibit selenium (IV)
adsorption. Figure 18 shows little change in the removal of selenium (IV) with varied
doses of cations. However, addition of varied amounts of cations to the selenium (VI)
stock seemed to enhance adsorption slightly. As is shown in Figure 19 removals rise
from about 90 percent with no cations present to about 98 percent with 200 mg/1 of
calcium and magnesium present. This phenomena may be an artifact of the matrix
interferences, but it may also be due to the "secondary adsorption" phenomena
explained in the introduction.
SELECTIVITY SERIES
Kubli's original selectivity series for adsorption of anions by activated alumina
included many more ions than those listed in Section 1. The complete list includes the
32 2
following anions, in order of decreasing preference: OH , PO^ , C-Qj, , F , (SO- >
(Fe(Cn)6)^~, CrO^2"), S2O32", SO^2', ((Fe(CN)6)3", Cr^of'), (NO2~, CNS"), I", Br~,
Cl", NO3", MnO^~, C10^~, CH3 COO", S2".
His method for determining the selectivity series was to introduce a solution
containing two competing anions into a very narrow column filled with activated
52
-------
100
90
80
70
J
^
> 60
I
K
I 50
2
i
UJ 40
(0
30
20
10
n
-5T-^ *-c»-H'
XM.-H-
EXPERIMENTAL CONDITIONS
ACT. ALUMINA: 0.5 G, 28-48 MESH
PREPARATION:
N*OH: 1 % FOR 50 MIN
DEIONIZEDH2O: 5 MIN
HCi_: NONE
HjjSO^ 0.05 N FOR 10 MIN.
DEIONIZED HgO: 2ft 5 MIN EACH
VOLUME: ISO ml
INITIAL CSe (IV) ] ~* 100 ppb
INITIAL CSe (vi) 3-NONE
CONTACT TIME: 60 MIN
pH: 4.0
40 80 120. 160 200
INTERFERING CATIONS (mg/1 )
Figure 18. Influence of various cations on adsorption of selenium (IV)
53
-------
100
90
80
70
Q 60
o:
>
G'
2
D
I
50
40
30
20
10
..>>er^:rrr^s^
^
NA+
-H-
EXPERIMENTAL CONDiTIONS
ACT. ALUMINA: 0.5 e, 28-48 MESH
PREPARATION:
NAOH: I % FOR 50 MIN
DEIONIZED H2O: 5 MIN
HO. : 0.05 N FOR 10 MIN.
H2SO4: NONE
DEIONIZED H2p: 3@ 5MIN. EACH
VOLUME: 150ml
INITIAL CSE (IV)>NONE
INITIAL gE (VI) J ^ 100 ppb
CONTACT TIME: 60 MIN
pH: 4.0
40 80 120 160 200
INTERFERING CATIONS ( mg/1 )
Figure 19. Influence of various cations on adsorption of selenium (VI)
54
-------
alumina which had been rinsed with 1:1 perchloric acid. The solution was "filtered"
through the column and the relative position in the column was detected by the use of
a developer which produced specific color changes depending on the species present.
The anion found in the top part of the column was determined to be the more
preferred ion.
In this study, equal molar amounts of competing species were put in solution with
identically pretreated alumina at a pH of approximately 6.5 and contacted by the
normal method. By determining the percent of each species removed, the greater
removed species was assumed to be more preferred by activated alumina. The ions of
interest were:
HSe03~ (Se(lV)), SeO^ (Se(VI)), H3AsO3 (As(III)), H2AsO^" (As(V)),
F", H2POif", S0^~, HCO3", Cf, NO3".
Two and one-half grams of activated alumina, when contacted for 1 hour with
150 mi's of 5e(IV) containing 5.0 mg/1, was found to remove approximately one-half the
selenium. This amount of alumina was chosen to do the remaining tests. 5.0 mg/1
Se(IV) corresponds to a concentration of 6.3x10" M. This molar concentration was
used for all competing species. The preparation of each batch of activated alumina
was pretreated the following way:
50 minutes, 1% NaOH, 200 ml
5 minutes, D.I. water, 200 ml
3-10 minute rinses, 0.001 N HC1, 200 ml
2-5 minute rinses, D.I. water, 200 ml
In the two competition tests with the chloride ion, 0.001 N HC1CX was used as
the acid rinse. HC1 was used .in most of the tests because it would appear that
hydrochloric acid may be the acid used in an actual treatment facility.
To reduce the number of tests, we prepared an estimated selectivity series and
tested two consecutive species in the list to verify our hypothesis, starting with the
most preferred ion. If the preferred adsorption did not occur as expected, then it
55
-------
would require only one extra test to determine the actual order of preference. Table 3
shows the results of these tests.
The data indicates that H2?O^~ and F" were the two most preferred anions.
Hydroxide (OH") was not tested as it was assumed that it was the most preferred
anion. Tests 3 and it show that As(V) is preferred over Se(IV), but is behind fluoride in
the series. It appears as though Se(VI) is removed approximately the same in all of the
tests with HCO3", SO^2~, Cl", NO3", and H3AsO3. In the nitrate-selenate test, very
little nitrate is removed and more selenate is adsorbed than nitrate. Therefore, we
can assume that Se(VI) is preferred over NO— With the arsenite-selenate tests, more
Se(VI) is removed than As(III). Since As(III) is in the unionized form, H3AsO3, at
pH 6.5, it is not expected to be removed to any great degree. Nitrate was more
preferred over arsenite.
The data concerning sulfate, bicarbonate, and chloride is hard to interpret. In
each test involving one of these species, more was detected in the treated water than
in the stock solutions. Because the detection limits are fairly high for SO.2" and Cl",
the increases may be more of a reflection on the poor detection limits than anything
2
else. From this data, it is impossible to develop a ranking between SO,, , HCO-", Cl",
2-2-
and SeO. . Where SeO^ falls in preference between these four species is
indeterminate.
Based on these tests, the following incomplete series is proposed: OH", H-PO,.",
22
F , H2AsO^~, HSeO3", (SO^ , SeO^ , -HCO3", Cl"), NO3", H3AsO3. The anions in
parenthesis represent the four species whose direct selectivity is unknown. Kubli
suggested that sulfate was preferred over chloride, but that is all we know about the
interactions of these ions. H3AsO3, being in an unionized state in water at pH's less
than nine, will be poorly adsorbed, even less than nitrate.
CAPACITIES OF ACTIVATED ALUMINA
A primary step for determining adsorption capacities of activated alumina for
selenium (IV) and (VI) is to conduct batch scale isotherm tests. These are done for
three reasons: (1) to give a general idea of how effectively activated alumina will
56
-------
TABLE 3
RESULTS OF SELECTIVITY SERIES TESTS*
Test
No.
1
2
3
4
5
6
7
Species
F"
F"
H SeO3"
• HSeO3"
F"
H Se03"
H SeO3"
H CO3"
HCO3"
*Minimum detection
Minimum detection
Initial
Concen-
tration
(mg/1)
2.0 (asP)
0.93
1.19
4.6 (asSe(lV))
3.6 (asAS(V))
3.9
1.15
4.7
6.8
4.75
6.5 (as CaCO3)
4.0
5,4
limits for Cl~ =
limits for 5
-------
TABLE 3
(CONTINUED)
Test
No.
8
9
10
11
12
13
14
Species
HC03"
secy
ay
secy
secy
cr
Secy
Secy
"f-03
H3As03
Cl"
*Minimum detection
Minimum detection
Minimum detection
Initial
Concen-
tration
(mg/1)
6.7
4.1 (asSe(VI)
5.5
4.2
4.2
2.3
4.4
0.9 (as N)
4.3
3.9 (asAS(lII)
3.8
0.9
4.0
2.5
limits for Cl" =
limits for SCy =
limits for HCO3" =
Final
Concen-
tration
(mg/1)
9.8
3.5
6.4
3.4
3.3
2.5
3.5
0.85
3.5
3.8
3.6
0.8
3.7
2.8
2.0 mg/1
2.0 mg/1
0.5 mg/1
Removal
15
19
21
20
6
19
3
5
11
8
(as CaCO3)
6.4 6.5
6.1 6.2
6.2 6.3
6.3 6.5
6.3 6.3
6.4 6.5
6.4
6.4
The anions were all added in conjunction with sodium (Na), except for As(V), (ASjO J
and N03" (KNO3).
58
-------
adsorb different forms of selenium present in water. This relatively short screening
procedure can usually show whether it is worthwhile to conduct time-consuming
column studies, (2) to predict the maximum quantity of selenium activated alumina
will adsorb, and (3) to obtain "ballpark" data to judge whether activated alumina may
be an economic way to purify a given raw water. Data from isotherms should allow a
rough estimate of the size and cost of contactor units.
Figures 20 and 21 show plots of the logarithms of the two variables frequently
calculated in isotherm data; the equilibrium concentration of the contaminant
(selenium) and amount of contaminant removed per unit weight of activated alumina.
Figure 20 is a comparison of the capacities of activated alumina for selenium (IV) and
(VI) without any matrix interferences. Either selenium (IV) or selenium (VI) only were
added to deionized water and then contacted with the activated alumina.
The activated alumina was prepared as in previous tests; for adsorption of
selenium (IV), H2S(X was used for the acid rinse, and for adsorption of selenium (VI),
HC1 was used for the acid rinse. The general trend shows that if you projected the
straight lines for selenium (IV) and selenium (VI), the "Y"-axis intercept for selenium
(IV) would be greater than that for selenium (VI). This would indicate a greater
capacity for selenium (IV) adsorption ^than for selenium (VI) adsorption by activated
alumina. Note on the plot of selenium (IV) that the sloped line degenerates into a
vertical line with decreasing x/m. This indicates that after a certain equilibrium
concentration of selenium (IV) (~3 ppb) was attained, no further removal occurred even
by adding more activated alumina. This may be an artifact of the decreased accuracy
of the AA5 to determine amounts less than 5 ppb, or it may be due to the decreased
adsorption capacity at low concentrations.
Figure 21 shows the capacity of activated alumina for removing either selenium
(IV) or selenium (VI) from a water containing a synthesized mixture of typical
constituents of a well water. The composition of this water is described in Section 6.
The resulting capacities are lower than in the test with a deionized water matrix.
59
-------
(3
2.4
2.0
3 16
1.2
0.8
0>
o
SE 44-
/ EXPERIMENTAL CONDITIONS
/ ACT. ALUMINA: WEIGHT VARIES,28-48MESH
, PREPARATION:
' NAOH: I % FOR 50 MIN
/ DEIONIZED H20: 5 MIN.
HCi.: 0.05 N FOR 10 MIN. [Se (VlQ
H2SO4: 0.05 N FOR 10 MIN. [SE (IVj]
DEIONIZED Hgp: 3 @ 5 MIN EACH
VOLUME: 150ml
INITIAL L"SE(IV) U~100ppb
INITIAL CSe (Vl)!]^ 100ppb
CONTACT TIME: 60 MIN
pH : 4.0
0.5
LOG
1.0
CE
(ppb)
1.5
2.0
Figure 20. Freundlich isotherm plots of Se (IV) and Se (VI) adsorption
in deionized water matrix.
-------
Q
til
m
K
d
(0
0
2.0-
1.6
S u
CD*
I
3
0.8
0.4
EXPERIMENTAL CONDITIONS
ACT. ALUMINA: WEIGHT VARIES, 28-48 MESH
PREPARATION:
N*OH: I •% FOR 50 MIN.
DEIONIZED H2O: 5 MIN.
HG_: 0.05N FOR 10 MIN. [Se (VI)]
H2SO4: 0.05N FOR 10 MIN. [SE(IV)]
DEIONIZED H20: 3 @ 5 MIN. EACH
VOLUME: I50ML
INITIAL [SE(IV}]~ 100 ppb
INITIAL [SE(Vli~ lOOppb
CONTACT TIME: 60 MIN.
pH JS 4.0
SE
6+
0.5
1.0
1.5
2.0
2.5
LOG Ce ppb
Figure 21. Freundlich isotherm plots of Se (IV) and Se (VI) adsorption
in synthesized well water matrix.
-------
Both of the results were approximated by Freundlich isotherms. The adsorption
values plotted were calculated according to the Freundlich equation:
x/m =KFCe1/n
or
log x/m = log Kp + (1/n) log Ce
where x/m = the amount of selenium adsorbed per unit weight of adsorbent (ug/g),
C = the concentration of selenium in the solution after 1 hour of contact time, and
n and Kp are constants. Normally, selenium concentrations and other trace amounts
of chemical species in waters are presented as ug/1 or ppb. Thus, mass units are more
convenient than mole units in the comparison of data, and their use makes no
difference in the use of the Freundlich equation except for the constant, Kp:
KF = KF . Mw
KP is on the molar basis, KP is on the mass basis, and Mw = the molecular
10
weight of the adsorbate .
Kp can be determined from the intercept of the straight line with the "y"-axis.
1/n will be the slope of the straight line. Kp is roughly an indicator of sorption
capacity and 1/n of adsorption intensity. As seen in Figure 20 neither of the lines
intersects with the "y"-axis. It is shown merely as a comparison with Figure 21. From
Figure 21, values of Kp, Kp, and 1/n are tabulated below:
KF'Gig/g) KpOumoles/g) 1/n
Se(VI) 1.41 0.069 0.31
Se(IV) 1.66 0.756 0.82
62
-------
On the basis of the selectivity series, the ions present in the synthesized well
water that could be interfering with selenium (IV) and (VI) removal are fluoride,
sulfate, and bicarbonate. The presence of SO^~ and HCO^~ may cause interferences
due to increased adsorption intensity at high concentrations.
Thus, under the same matrix conditions, we can predict the approximate
capacity of activated aluminum for selenium from the isotherms. For an initial
concentration of 100 ppb and an effluent concentration of 10 ppb (90% removal),
approximately 9.5 g of Se(IV) will be removed per gram of activated alumina.
Similarly, approximately 3 g of Se(VI) will be removed per gram of activated alumina.
63
-------
SECTION 6
SELENIUM COLUMN STUDIES
INTRODUCTION
Following the batch tests, continuous flow column studies were initiated to
analyze various operating procedures for the use of activated alumina to remove
selenium. Figure 22 shows a schematic diagram of the testing equipment. Figure 23
shows a photograph of the actual apparatus. A list of materials and instruments is in
the Appendix.
The columns were set up as pressure flow devices. The flow was controlled by
the variable speed control (SCR) for the motor. The feed was pulled out of the tank
(or tanks) through teflon tubing. The pump tubing was tygon and was connected
directly to the teflon tubing on the suction and discharge sides of the pump. Teflon
tubing then carried the stock solution into the top of the ground glass joint, which was
adapted with Swagelock fittings. At the bottom of the column, above the stop cock, a
small amount of fine stainless steel wire mesh was inserted. This was done to prevent
alumina from leaking out of the column during regeneration and the subsequent
treatment run. Once through the column, the effluent was carried via tygon tubing
into a waste trough to drain.
Caustic and acid were mixed in two-liter beakers. Since all the regeneration,
neutralization, and rinse steps were done in the upflow mode, the pump connections
were merely reversed, with the suction line being fed from either the caustic, acid, or
deionized water beaker. The eluants were collected in 2000-ml graduated cylinders to
measure the regenerant volumes.
In all of the sample containers, enough concentrated HNO, was added as a
preservative to keep the pH of the sample <2. This is an EPA recommended
-------
55 GALLON
STOCK
TANK(S)
TO SAMPLERS
AND DRAIN
MOTOR WITH
VARIABLE SPEED
CONTROL
WEIGHT
DURING REMOVAL RUN
2000 ml
GRADUATED CYLINDERS
DURING REGENERATION
2000 ml
BEAKER
(NAOH OR
ACID OR
D.I. H20)
Figure 22. Schematic diagram of testing equipment during removal run and
regeneration. (Not to scale.)
65
-------
3
3
U S ENVIRONMENTAL
PROUCtiON AGENCY
Figure 23. Column testing equipment.
-------
preservation method. Even in the regenerant samples with a high pH due to large
volumes of NaOH, not enough HNCX was added to significantly dilute the sample.
INITIAL TESTING
It was decided to test the ability of activated alumina to remove Se(lV) from
deionized water. Three different depths of alumina (3, 6, and 9 inches) were chosen to
run side-by-side as a means of comparing the effect of residence time on Se(IV)
removal. The respective empty bed contact times were .62, 1.25, and 1.87 minutes.
34, 68, and 102 grams, respectively, of activated alumina were added to achieve those
depths.
When preparing fresh out-of-the-can (virgin) activated alumina, the following
steps were followed. The alumina was added to a column partially filled with water to
prevent solidification of the material. The alumina was then backwashed at a high
enough rate (22.0 - 24.5 m^frs =9-10 gpm/ft2) to expand the bed and rinse most of
the fines out of the column. Complete regeneration of the virgin media was not
required. An initial acid rinse and subsequent deionized (D.I.) water rinse were used.
The removal run with deionized water after the acid rinse showed that no Se(lV)
appeared in the effluent after 6,200 bed volumes in the three-inch column and after
3,600 bed volumes and 2,000 bed volumes in the six-inch and nine-inch columns,
respectively. This was done at a loading rate of 3 gpm/ft (= 62 ml/min) and a Se(IV)
stock concentration of 45 ppb. It was decided that no useful breakthrough data could
be obtained from this information, so a synthetic well water was used to effectively
model a real-life situation. Previously, work had been done for the Ramona Municipal
Water District (RMWD) in San Diego County to evaluate their problem of selenium
contamination of a few wells and a synthetic water was prepared to match this
quality. Table 4 lists the average composition of the major components of those well
waters.
In order to match this quality, a combination of reagents had to be mixed with
D.I. water. Table 5 shows the different amounts of these chemicals added to 200
liters of water to achieve the above concentration. The amounts for Se(lV) and Se(VI)
are for 0.050 mg/1.
67
-------
TABLE 4
AVERAGE RMWD WELL WATER COMPOSITION
Concentration
Species _ _ (mg/1)
Sodium
Potassium 3 . 0
Caicium 90
Magnesium 40
Bicarbonate Alkalinity 335(asCaCO,)
Sulfate 105 J
Chloride 230
Nitrate 20
Fluoride 0.90
Selenium 0.005-0.050
TABLE 5
AMOUNTS OF VARIOUS REAGENTS ADDED TO MAKE UP
SYNTHESIZED WELL WATER
Reagent Weight Added (grams)
CaCl2 50.0
MgCl2 6H2O 67.0
NaHC03 113.0
KF 2H2O 0.9
Na2SO^ 31.0
KNO3 13.0
Na2Se03 (Se(IV)) 0.022
Na2SeO^ (Se(VI)) 0.024
68
-------
Freshly mixed, the synthetic solution had a pH of about 8.3. Most of the runs
were done at either pH 5, 6, or 7. Based on the batch tests, Cl~ interfered much less
than SO. = in removal of Se(VI) by activated alumina, hence concentrated HC1 was used
to adjust the pH instead of H2SO^. An analysis was made of the various stock
solutions to determine the change in alkalinity and chloride after reducing the pH.
The results were:
Alkalinity
(mg/1 as CaC03) Cl" (mg/1)
pH5 22.8 402.6
pH6 90.6 359.5
pH7 200.0 312.4
Except for varied amounts of selenium added, these were the only variables in
the Se(IV) stock solution during the testing program. In the Se(VI) tests, the matrix
remained the same, except for the tests on alkalinity and sulfate interferences.
EXPERIMENTAL SUMMARY
The remainder of this chapter will be a chronological description of each of the
column experiments that were conducted during this study. Table 6 provides a
summary of each of the experiments along with its objectives and findings so that the
reader may easily develop an overall perspective. This table will also serve as an
effective reference once the reader has reviewed the ensuing discussion.
CHRONOLOGICAL SUMMARY OF Se(IV) REMOVAL TESTS
USING 3-INCH COLUMNS
The data presented in this phase of the column studies does not draw any
conclusions as to Se(IV) capacity nor does it define optimum regeneration techniques
to efficiently recover Se(IV). The three-inch tests helped to delimit the range of
amounts of NaOH and H2SO^ in a fairly short period of time. The suggested
techniques were then tried with nine-inch deep alumina beds to verify results and
optimize operating parameters. The reader may wish to skim through this section to
get a general feeling about the process.
69
-------
Experiment
SELENIUM (IV) - 3 INCH
COLUMN
1. Effect of contact
Time
2-4. Repeatability tests
5-6. Regeneration tests:
16, 40, 80 bed
volumes of 1%
NaOH
7-9. Fresh media -
Repeatability runs
10. Regeneration tests:
1.6, 16, 80 bed
volumes of 0.5%
NaOH
11-12. Regeneration tests:
1.6, 3.2, 4.8
bed volumes of
0.5% NaOH (//I)
13-14. Regeneration tests:
1.6, 3.2, 4.8
bed volumes of
0.5% NaOH (//2)
15-17. Regeneration tests:
0.5%, 1.0%, 2.0%
NaOH
18-19. Regeneration tests:
1.6, 3.2, 4.8
bed volumes of
0.25% H
TABLE 6
SUMMARY OF RESULTS
Objective
Result
Determine minimum depth Use 3-inch bed
Verify 3" depth
Determine amount of
NaOH
Start over - prepare to
test regeneration
Determine amount of
NaOH
Fine tune NaOH require-
ment
Fine tune NaOH require-
ment
Test alumina degrada-
tion, selenium (IV)
recovery
Determine optimum
amount
20-22. Regeneration tests:
0.25%, 0.50%, 1%
Test alumina degrada-
tion
3 runs were reproducible
No apparent difference,
leakage increased
16 bed volumes of 1%
NaOH worked
Lower amount seemed
most promising
Too rapid breakthrough-
reduce stock feed -
flow rate to 3 gpm/ft
Use 4.8 bed volume
of 0.5% NaOH
Higher concentrations
dissolved media more,
didn't improve
regeneration
Use 4.8 bed volumes
of 0.25% H2SOff
Higher concentrations
of acid dissolved
media faster
70
-------
TABLE 6
Experiment
SELENIUM (IV) - 9 INCH
COLUMN
1-3. Capacity tests:
pH 5, 7.9
4. Capacity test:
pH 5, 6, 7
5-6. Capacity tests:
C. - iOO opb,
NaOH
7-12 Capacity tests:
C. = 200 ppb,
1 gpm/ft* NaOH
13-14. Capacity tests:
C. = 200 ppb,
l/2gpm/ftz
NaOH
15. Saturation test:
C. = 18 ppm
16-18. Capacity tests:
C = 200 pph,
l/2gpm/fr
NaOH
SELENIUM (VI)
1. 3-inch column
SUMMARY OF RESULTS
(CONTINUED)
Objective
Determine optimum pH
of stock
Determine optimum pH
of stock
Try to speed up rapid
breakthrough and re-
cover 100% of
selenium
Try to speed up rapid
breakthrough and re-
cover 100% qf
selenium
Try to speed up rapid
breakthrough and re-
cover 100% of
selenium
Saturate media with
Se(IV)
Try to achieve steady
state, evaluate
"worst case" capaci-
ties
Determine approximate
breakthrough
2-3. Repeatability tests Verify 9-inch depth
4-5. Regeneration tests:
0.5, 5.3, 53.0
bed volumes of
1% NaOH
Determine amount of
NaOH
Result
pH 5 best, pH 9 forms
CaCO3
pH 5 best, leakage
increased, must in-
crease stock con-
centration
Must increase stock
concentration again
Regeneration recovers
30-40%, reduce
regeneration rate
Recovery 80%
Achieved total break-
through, recovered
80-100%
Steady state not
achieved, capacity:
pH 5 - 235 mg/liter
pH6-175mg/l
pH 7 - 100 mg/1
Bed too shallow, use
9-inches
Reproducible runs, must
reduce NaOH amount
0.5 bed volumes of 1%
NaOH worked as well
as 53
71
-------
TABLE 6
Experiment
SELENIUM (VI) (Continued)
SUMMARY OF RESULTS
(CONTINUED)
Objective
6-8. Regeneration tests: Determine if H7SO.
H2SO/f vs. HC1
9. Regeneration test:
pH5, 7
10-11. Regeneration tests:
1/2 gpm/ft^ NaOH
12-13. Regeneration tests:
2 gpm/fr NaOH
14. Capacity test:
PH5,7
15-16. Sulfate inter-
ference
17-18. Alkalinity inter-
ference
rinse interferes
with Se(VI) removal
Determine optimum pH
Effect of slower re-
generation on re-
covery of Se(VI)
Effect of faster re-
generation on re-
covery of Se(VI)
Run another cycle,
verify previous
results
Determine effect of
SOj,-~ concen-
tration on Se(VI)
capacity
Determine effect of
HCO," concen-
tration on Se(VI)
capacity
Result
Use HC1 rinse,
interferes
pH 5 best, 1
NaOH recovered 100%
of Se(VI)
No increased recovery
versus 1 gpm/ft
No decreasecLrecovery
vs 1 gpm/ft
Approximate capacity:
pH 5 - 4.5 mg/liter
pH - 1.5 mg/liter
^~ heavily inter-
feres, greater capacity
with reduced sulf ate
Not as interfering as
SO.=, slight im-
provement with reduced
HCO3
72
-------
After terminating the first try at selenium removal in a deionized water matrix,
the 3 inch, 6 inch, and 9 inch columns were all regenerated as follows:
20 bed volumes of D.I. water at 9 gpm/ft , upflow
2
80 bed volumes of 1 percent NaOH at 3 gpm/ft , upflow
2
20 bed volumes of D.I. water at 6 gpm/ft , upflow
2
20 bed volumes of 0.05N H2SO^ at 3 gpm/ft , upflow
20 bed volumes of D.I. water at 6 gpm/ft , upflow
A liberal amount of NaOH was used to ensure optimum recoveries of selenium
removed during the previous run. Comparisons of Se(IV) removed during a run versus
Se(IV) recovered in the regenerant were not conducted until after run 6.
Run No. 1; 'Effect of Contact Time
Run 1 was done under operating conditions similar to the initial removal run,
except that the synthesized well water was used and pH was adjusted to 6.5. As is
seen in Figure 24, the three-inch column produced water with less than 10 ppb Se(IV)
for 600 bed volumes, the six-inch column produced 750 bed volumes, and the nine-inch
column produced 1,100 bed volumes. Due to the length of the exhaustion cycle,
operation was not continuous. Later on automatic samplers were used so that
breakthrough curves could be monitored through the night. In run 1, two overnight
shutdowns were required. The results of this can be seen most dramatically in the
three-inch column data. The discontinuities were just after 1,000 and 2,150 bed
volumes. Upon restarting in the morning, the effluent concentration markedly
decreased from the night before, and thereafter, the slope of the original breakthrough
curve was resumed. This would suggest that the adsorption process is "slow" and may
be diffusion limited. Notice also that there seem to be plateaus in effluent concentra-
tions. Refer to the discussion after runs 13 and 14 of the Se(IV) nine-inch column
experiments for a more detailed explanation of this phenomena.
On the basis of these tests, three-inch columns were chosen to do some
preliminary testing regarding various parameters of interest. The presupposition was
that the length of a three-inch column run would be six to seven hours, whereas a
73
-------
40
30
UI
20
M
U)
10
9"
STOCK-45
pH-6.5
500
1000
1500 2000
BED VOLUMES
2500
3000
Figure 24. Run 1 -Effect of contact time on Se(IV) removal.
-------
nine-inch run would last about 30 hours. Also, the three-inch columns seemed to
provide enough contact time to achieve quality goals at the beginning of the run.
Information pertaining to amount and concentration of NaOH, rate at which NaOH is
applied, amount and concentration of H2SO^, rate at which H2SO^ is applied and
degradation of the activated alumina by various amounts and concentrations of NaOH
and HUSO,, can be quickly determined using these short columns. Understandably,
£ *r»
capacity information that is generated should not be taken as complete in light of the
fact that the deeper columns were able to produce acceptable water for more bed
volumes. For all subsequent tests, the removal run was completed during the day, the
columns were regenerated, and then sat overnight until the following removal run was
started the next morning.
Run Nos. 2-fr; Repeatability Tests
Figure 25 shows these runs. The media from the three columns of run 1 was
mixed together and three inches were put into each column. Prior to each run, the
columns were regenerated as described before run 1. The operating pH was 6.5.
The breakthrough curve for each run is actually a composite of the three
columns' data. Since the three columns' effluent qualities were practically identical,
the data was averaged and the resultant graph was plotted.
From these tests, it appeared that the three columns would, produce long enough
runs of high quality water to continue testing with them. The overall removal was still
as good as seen in the three-inch column from run 1. A slight decrease in removal was
noted from run 3 to run 4.
Runs Nos. 5 and 6; ReReneration Tests, Varied Amounts of NaOH
These runs were done to test the effects of large differences in volume of 1%
NaOH in the regeneration. The remaining steps in the regeneration were kept the
same. The surface loading rate was increased to 6 gpm/ft because the preceding runs
lasted on the order of 10 to 12 hours and we wished to decrease the duration of the
runs to six or seven hours. At a stock pH of 6.2, run 5 shows a definite difference
75
-------
20 -
ft
20 -I RUN 5
STOCK =60 ppb
• 16 B. V. I %
• 40B. V. 1 %
A 80 B. V. I %
a-
LL
UJ
<> 5 "I
lit
W «
20 -
15 -
10 -
5 -
RUN 6
STOCK =60 ppb
• 16 B. V. I %
* 40 B. V. 1 %
4 80 B. V. I % NAOH
l
200
400 600
BED VOLUMES
T
800
1000
1200
Figure 25. Runs 2-6, Regeneration Tests.
76
-------
between the three columns. The leakages, defined as the amount of Se(IV) present in
the column effluent immediately upon the start of a removal run, were 3, 2, and 0 ppb
for columns 1, 2, and 3. 600, 800, and 1,000 bed volumes of treated water were
produced with a concentration less than 0.010 mg/1. After another similar regenera-
tion, run 6 failed to show the same difference. Figure 25 shows that the effluent
concentrations for run 6 were much closer to each other, all having an initial leakage
of 2 or 3 ppb and 700-800 bed volumes until a 0.010 mg/1 breakthrough occurred.
Run Nos. 7-9; Fresh Media, Repeatability Runs
After run 6, it was decided to remove the old media and start with a fresh batch.
Because the history of each column varied quite a bit, it was difficult to predict how
each compared with the others. A main point that should be brought out here is that
the proces^ shows a lot of hysteresis and the history of each bed's exhaustion and
regeneration plays an important role in determining how it will operate in a removal
run. Inefficient regeneration in one column and not the others will allow more
selenium to remain on the alumina, thus diminishing its capacity for removal. Even in
side-be-side comparisons, this may lead to inaccurate conclusions.
Although experiments 2 through 6 develop useful data on relative performance,
they also demonstrated that the experimental conditions used did not result in
reproducible data and did not allow reasonable projections of adsorption capacity. As
a result, several changes were made in the operating conditions to match them to
conditions shown earlier to be suitable for fluoride removal in large scale facilities.
29
These conditions were summarized by Trussell following an extensive review of data
from the literature and the field. It was presumed that the nature of the process of
adsorption/elution of Se(IV) and Se(VI) on activated alumina did'not necessite operating
conditions different from fluoride.
Runs 7-9 (Figure 26) represent three consecutive runs done with initially fresh
activated alumina in 3-inch columns. Preceding run 7, only a 0.05N H-SCX rinse for
10 bed volumes at 3 gpm/ft followed by 20 bed volumes of D.I. water rinse at
6 gpm/ft2 was done. Both of these rinses were upflow. Preceding runs 8 and 9, the
following regeneration took place:
77
-------
RUN 7-VIRGIN MEDIA
20-
15-
10-
5-
RUN 10
STOCK = 50 ppb
S
• 1.6 B. v. 0.5 % NAOH
• 16 B. V. 0.5 % NxOH
* 80 B. V. 0.5 %
fa 2°-l
RUN II
STOCK =50 ppb
in
15-
10-
5-
• 1.6 B. V. 0.5 %
• 3.2 B. V. 0.5 %
A 4.8 B. V. 0.5 % NAOH
20-
15-
10-
5-
RUN 12
STOCK = 50 ppb
• 1.6 B. V. 0.5 %
• 3.2 B. V. 0.5 % NAOH
* 4.8 B. V. 0.5 %
600
BED VOLUMES
800
1000
1200
Figure 26. Runs 7-12, Regeneration Tests.
78
-------
10 bed volumes of D.I. water at 9 gpm/ft , up
16 bed volumes of 1 percent NaOH at 3 gpm/ft , up
20 bed volumes of D.I. water at 6 gpm/ft , up
2
8 bed volumes of 0.05 H2SO^ at 3 gpm/ft , up
10 bed volumes of D.I. water at 6 gpm/ft , up
2
The treatment run was operated at 6 gpm/ft . Seen from the figure, run 7 is
much poorer than runs 8 and 9. Again, the runs are composites of the three columns
effluent curves for each run. Only slight differences were found between any of the
columns in any run. 8 and 9 show a breakthrough capacity of 1,100 bed volumes. The
following chart shows the amounts of Se(lV) removed during the run and recovered
during the following regeneration. Calculation of the removal during the run was done
by integrating the area above the breakthrough curve using the stock concentration of
65 ppb. Regenerant amounts were determined by knowing the volume of regenerant
and analyzing the regenerant for Se(IV).
Se(IV) Recovered
Se(IV) Removed During Subsequent %
Run During Run (mg) Regeneration (mg) Recovery
7 2.87 1.50 52
8 3.25 1.48 46
9 3.20
Approximately 50 percent of the selenium removed is recovered by the
regeneration. If continued like this for many repetitive cycles, the percent recovery
should approach 100 as the Se(IV) removed during each run grows progressively less.
Run No. 10; Regeneration Test. Varied Amounts of NaOH (//I)
Following run 9, it was decided to test various amounts of 0.5 percent NaOH on
the three columns. 0.5% NaOH was chosen because a review of the current operating
procedure at a full-scale fluoride removal facility used this concentration. It was
assumed that each column had undergone the same history since using the virgin
alumina in run 7. Column 1 was given 1.6 bed volumes of 0.5 percent NaOH at
3 gpm/ft , upflow, and Columns 2 and 3 were given 16 and 80 bed volumes,
respectively at the same concentration and rate. Recoveries were as follows in mg:
79
-------
Column 1 - 0.32 = 10 percent recovery
Column 2 - 1.82 = 57 percent recovery
Column 3-2.4 = 75 percent recovery
2
As Figure 26 depicts, run 10 at a surface loading rate 6 gpm/ft showed obvious
differences. The leakage for columns 1, 2, and 3 was 4, 3, and 2 ppb respectively.
Removals of Se(lV) during the run were 1.48 mg, 1.72 mg, and 1.86 mg with break-
through occurring at 250, 400, and 650 bed volumes. In looking at the removals of
Se(IV) during regeneration, column 2 used ten times as much regenerant as column 1,
but recovered only six times as much selenium. Column 3 used fifty times as much
regenerant, but recovered only 7.5 times as much selenium. This fact led us to believe
that the optimum recovery of Se(IV) would fail in the range of 1.6 to 16 bed volumes of
0.5 percent NaOH, with a better chance of the range being 1.6 to 8 bed volumes.
Hence, runs 11 and 12 were done with 1.6, 3.2, and 4.8 bed volumes of 0.5 percent
2
NaOH at 3 gpm/ft as the regenerant. The other steps in the regeneration remained
the same as described in run 7.
Runs Nos. 11-12; Regeneration Tests; Small Amounts of NaOH
Figure 26 shows runs 11 and 12. Run 11 shows leakage in the range of 6 to 8 ppb
and a very rapid breakthrough to 10 ppb. Run 12 had leakage from 0 to 1 ppb and the
change from the previous run is probably due to lesser amounts of selenium present to
be regenerated. However, the slope of the breakthrough curve increased rapidly,
resulting in a very sharp decrease in adsorption capacity. Table 7 shows the removals
and recoveries for the two runs.
As a matter of reference, 1.6, 3.2, and 4.8 bed volumes of 0.5% NaOH
correspond to the following doses, respectively:
grams NaOH -
8.0, 16.0, and JM ,iter act. alumina = 0.5, 1.0, 1.5 lb/ft3
Pounds of regenerant per cubic foot of bed and grams per liter are typical ways to
discuss regerant doses in ion exchange literature.
80
-------
TABLE 7
REMOVALS AND RECOVERIES OF Se(IV)
FOR RUNS 10, 11, AND 12
Se(IV) Recovered Amount of 0.5%
Se(IV) Removed in Subsequent NaOH Used (Bed %
Run During Run (mg) Regeneration (mg) Volumes) Recovery
1.48 0.15 1.6 10
10 1.72 0.20 3.2 12
1.86 0.27 4.8 15
0.15
0.20
0.27
0.15
0.19
0.25
1.6
3.2
4.8
1.6
3.2
4.8
1.36 0.15 1.6 11
11 1.41 0.19 3.2 13
17
1.05
12 1.21
1.18
Run Nos. 13-14; Regeneration Tests, Smaller Amounts of NaOH (//2)
Due to the relatively poor breakthrough capacity exhibited by the previous runs,
an adjustment had to be made. The empty bed residence time in the activated alumina
during the previous runs was 0.31 minute (38.7 ml/124 ml/min.). This was determined
to be too short. This prompted us to return to a lower surface loading rate of
3 gpm/ft (=7.3 meters/hour). This doubled the previous residence time and hopefully
would show more promising results.
Runs 13 and 14 were conducted under .the same regenerative scheme as runs
2
11 and 12, the only difference being the treatment rate of 3 gpm/ft . Figure 27 shows
leakage was less than the other two runs and that the breakthrough capacity increased
to 600-700 bed bolumes for run 13, but dropped to 300-400 bed volumes for run 14. It
is evident that the larger the amount of 0.5 percent NaOH used, the less leakage and a
greater capacity in the following runs were produced. Table 8 shows recoveries and.
removals for runs 12, 13, and 14.
Because of the results of runs 13 and 14, 4.8 bed volumes of 0.5 percent NaOH at
3 gpm/ft2 was adopted as the regenerant dose. This is equivalent to 24.0 g/1 or 1.5
///ft3.
81
-------
30
25-
20
15
10
5
3
U.
U.
U
u
tO
30 i
25
20
15
10
RUN 13
STOCK = 50 ppb
1.6 B. V. 0.5 %
3.2 B. V. 0.5 %
1.6 B. v. 0.5
RUN 14
STOCK =50 ppb
3.2 B. V. 0.5 % N*OH
200
400 600 800
BED VOLUMES
1000
1200
Figure 27. Runs 13 and 14, Regeneration Tests.
82
-------
TABLE 8
REMOVALS AND RECOVERIES OF Se(lV)
FOR RUNS 12, 13, AND 14
Se(IV) Removed
Run During Run (mg)
12
13
14
1.05
1.21
1.18
1.87
1.96
2.03
1.06
1.17
1.24
Se(IV) Recovered
in Subsequent
Regeneration (mg)
Amount of 0.5%
NaOH Used (Bed
Volumes)
0.04
0.09
0.15
0.07
0.13
0.22
1.6
3.2
4.8
1.6
3.2
4.8
Recovery
4
7
13
4
7
11
Runs Nos. 15-17; Regeneration Tests; 0.5%, 1.0%, 2.0% NaOH
i
Runs 15, 16, and 17 were done with a 1.5 ///ft NaOH dose, but the concen-
trations were varied to see if they had any bearing on regeneration efficiency or
activated alumina degradation. The other regeneration steps were kept the same as in
previous runs. For 0.5 percent NaOH, regeneration time was 3 minutes. Respective
times for 1.0 percent and 2.0 percent were 1.5 and 0.75 minute. Figure 28 shows
virtually no difference in any of the breakthrough curves between the three columns
for any given run. The breakthrough capacity for both runs 15 and 16 were 500 bed
volumes, while it increased to 800 bed volumes in run 17. This is probably due to the
decreased stock concentration of 35 ppb Se(IV) for this run. Table 9 shows removal
and recoveries of Se(IV) in runs 14, 15, 16, and 17.
Under these conditions, it appears as though 0.30 mg - 0.35 mg of Se(IV) can be
recovered during regeneration. Upon a number of repetitive cycles, this would be
expected as the equilibrium amount of Se(IV) that could be removed during a removal
run. The difference between the different concentrations of NaOH in recovering
Se(IV) is very small.
83
-------
201
15
10-
5
0'
20-
15
RUN IS
STOCK - 50 ppb
• 4.8 B.V. 0.5 %
• 2.4 B. V. 1.0 %
A 1.2 B. V. 2.0 %
I
ft
RUN 16
STOCK = 55 ppb
• 4.8 B. V. 0.5 %
• 2.4 B. V. 1.0 %
* 1.2 B. V, 2.0 %
20 i
15
10-
5 -
RUN 17
STOCK = 35 ppb
•4^'
• 4.8 B. V. 0.5
• 2.4 B. V. 1.0 %
A 1.2 B. V. 2.0 % NAOH
200
400 600 800
BED VOLUMES
1000
1200
Figure 28. Runs 15 - 17, Regeneration Tests.
84
-------
Table 9 shows that increasing concentrations of NaOH regenerant have little
effect on the ability to remove Se(IV), providing that the dose of NaOH per volume of
activated alumina is kept the same. Quite possibly the effect of increasing the driving
force for elution (increased concentration) is negated by the decreased flow through
time in the bed. If the concentration of NaOH is doubled while keeping the dose the
same, it will take only one-half the time to complete this. Actual duration of contact
between the NaOH and the activated alumina may be very important in determining
optimum regenerative techniques.
TABLE 9
REMOVALS AND RECOVERIES OF Se(lV)
FOR RUNS 14, 15, 16, AND 17
Se(IV) Recovered
Se(IV) Removed in Subsequent %
Run During Run (mg) Regeneration (mg) % NaOH Recovery
1.06 0.32 0.5 30
14 1.17 0.33 1.0 28
1.24 0.34 2.0 27
1.31 0.32 0.5 24
15 1.31 0.34 1.0 26
1.32 0.35 2.0 27
1.47 0.31 0.5 21
16 1.45 0.23 1.0 16
1.45 0.32 2.0 22
1.28
17 1.29
1.28
Table 10 depicts the comparative amounts of aluminum removed during
regeneration under the above described conditions, 1.5 //NaOH/ft^, operating with
0.5 percent, 1.0 percent, and 2.0 percent NaOH. This amount was present in a
combined sample containing the initial backwash, the NaOH rinse and the subsequent
deionized water rinse. Samples were analyzed for aluminum on the flame A AS and the
percent of activated alumina removed was calculated by converting from Al to A1-OV
85
-------
Table 10 gives an indication that higher concentrations of NaOH were able to
dissolve activated alumina faster while keeping the dose the same. This will be
verified in future tests. From this data the following numbers of cycles could be
completed at each concentration of NaOH before all of the alumina would be
dissolved:
0.5% NaOH - 910 cycles
1.0% NaOH - 770 cycles
2.0% NaOH - 625 cycles
TABLE 10
DEGRADATION OF ACTIVATED ALUMINA BY
VARIOUS CONCENTRATIONS OF NaOH
Al Recovered in Subsequent Regeneration ((mg)
(% by Weight of 3-inch column in parenthesis)
Run 0.5% NaOH 1.0% NaOH 2.0% NaOH
14 18.6 (0.09%) 21.0 (0.10%) 26.5 (0.13%)
15 24.5 (0.12%) 32.0 (0.16%) 39.3 (0.20%)
16 22.0 (0.11%) 26.8 (0.13%) 32.6 (0.16%)
Operating conditions:
10 bed volumes of D.I. water at 9 gpm/ft , up
1.5 //Na^H at 3 gpm/ft2 w/0.5%, 1.0%, 2.0% NaOH, up
ft
15 bed volumes of D.I. water at 6 gpm/ft , up
On another basis, if a column was regenerated once every day, after one year the
following percentages of original weight would need to be replaced.
0.5% NaOH - 40%
1.0% NaOH - 47%
2.0% NaOH - 58%
86
-------
Remember that these figures are only for the dissolution of activated alumina by
NaOH. Additional degradation is expected by the acid rinse. The above differences
could mean quite a bit of extra money being spent on replacing media on an annual
basis. Currently, type F-l, 28-48 mesh is selling for $0.60/lb. For the above figures,
the following annual costs for replacement of media could be expected for an
activated alumina contactor with a volume of 250 ft , with regeneration every day.
This size contactor was taken from a typical fluoride removal facility with a capacity
of 0.7 mgd.
0.5% NaOH - $3,300
1.0% NaOH - $3,900
2.0% NaOH - $4,800
Run Nos. 18-19; Regeneration Tests; Varied Amounts of H,
The next step was to determine the optimum acid rinse for a given dose of
i
NaOH. The limits set for testing were again taken from a review of operating
procedures at a full-scale fluoride removal facility. 1.6, 3.2, and 4.8 bed volumes of
0.05N H2SO^ (= 0.25% H2SO^) were evaluated on their ability to reduce the pH of the
bed and its effect on Se(IV) adsorption. The regeneration prior to the acid rinse was as
follows:
10 bed volumes D.I. water at 9 gpm/ft , up
4.8 bed volumes of 0.5% NaOH at 3 gpm/ft2, up
10 bed volumes of D.I. water at 6 gpm/ft , up
Figure 29 shows that there is no discernable difference in the breakthrough
curves for the different acid rinses. However, the pH of the activated alumina was
noted after the final rinse (which consisted of 5 bed volumes of D.I. water at
6 gpm/ft2). For 1.6 bed volumes of 0.5N HjSO^, the pH was 10.0-10.5. For 3.2 bed
volumes, the pH was 9.0-9.5, and for 4.8 bed volumes, the pH was 5.5-6.5.
Note that the bed volumes until breakthrough increased from 200 in run 18 to
900-1,000 in run 19. This may be partially due to the change in stock concentration
from 60 ppb to 40 ppb.
87
-------
30 n
25 -
20-
15-
10-
RUN 18
STOCK = 60 ppb
• 1.6 B. V. 0.25 % H2SO4
• 3.2 B. V. 0.25 % H2S04
* 4.8 B. V. 0.25 °/0 H2S04
h
Z
y 0
UJ 30-i
u
-------
Run Nos. 20-22; Regeneration Tests, 0.25%, 0.50%. 1.0%
From the above data, it was decided to use the equivalent of 4.8 bed volumes of
0.05N H2SO^ for the acid rinse. This is a dose of 0.75 #H2SO^/ft3 bed. Runs 20
through 22 are very similar to the varied concentrations of NaOH runs, 15 through 17.
Q
Keeping the other regeneration steps constant, 0.75 ///ft H2SO^ was applied at 0.05 N,
0.1 ON, 0.20N H2SO^ at 3 gpm/ft2. The amount of time it took to do this was 3.0, 1.5,
and 0.75 minutes, respectively. No real difference was expected in the breakthrough
curves. Regeneration following runs 19, 20, and 21 was as follows:
2
10 bed volumes D.I. water at 9 gpm/ft , up
4.8 bed volumes of 0.5 percent NaOH at 3 gpm/ft2, up
•\
10 bed volumes of D.I. water at 6 gpm/ft , up
4.8, 2.4, and 1.2 bed volumes of 0.05N, 0.10N, 0.20N HjSCL at 3 gpm/ft2, up
10 bed volumes of D.I. water at 6 gpm/ft , up
Table 11 gives the removals and recoveries of Se(IV) for the NaOH and
rinses for runs 19 through 22 (Figures 29 and 30). Caustic rinses contained the initial
backwashes and intermediate rinses. Acid rinses included the final rinses.
Notice that the acid rinse removed in the range of 7%-13% of the total Se(IV)
recovered during regeneration. There should be some removal expected from the acid
rinse because it is very concentrated compared to the Se(IV) concentration on or in
the alumina, even though Se(IV) is preferred over SO** in the selectivity series.
Figure 30 shows that the bed volumes to breakthrough vary from 200 to 400. The
leakage is still significant and reduces the volume capacity until breakthrough.
Table 12 shows the amount of dissolved aluminum present in the acid rinse
portion of the regeneration for runs 19 through 21. No regeneration was done after
run 22, therefore, no data is available. As before, the aluminum was analyzed by the
flame AAS and the present of total weight involved the conversion of AI to A17OV
Table 12 shows aluminum has a moderately higher solubility in the more
concentrated acid. Although the differences aren't that great, there is a trend that
89
-------
20-
15-
10-
5
RUN 20
STOCK=60ppb
• 4.8 B. V. 0.25 % H2S04
• 2.4 B.V. 0.50 % H2S04
A 1.2 B. V. 1.0% H2S04
5
G!
U.
UJ
111
0)
20-
15-
10-
5-
RUN 21
STOCK = 60 ppb
• 4.8 B. V. 0.25 % H2SO4
• 2.4 B. V. 0.50 % H2S04
A |.2 B. V. 1.0 % H2S04
20-
15-
10
5H
RUN 22
STOCK =60 ppb
• 4.8 B.V. 0.25 % H2S04
• 2.4 B. V. 0.50 % H2S04
A 1.2 B. V, 1.0 °/0 H2S04
200 400 600
BED VOLUMES
800
1000
Figure 30. Runs 20 - 22, regeneration tests.
90
-------
would suggest using a less concentrated acid to prevent rapid dissolution of the
alumina. In comparison with Table 10, approximately one-third to one-fourth of the
amount removed from the bed during the caustic rinse is removed in the acid rinse.
This will be verified in deeper column tests.
Run
19
20
21
22
TABLE 11
REMOVALS AND RECOVERIES OF Se(IV) FOR NaOH AND H9SO,.
RINSES DURING RUNS 19, 20, 21, AND 22 *
Removal of
Se(IV)
During Run
(mg)
1.58
1.59
1.68
1.43
1.45
1.42
1.42
1.44
1.41
1.56
1.57
1.56
Recovery of Se(IV) During
Subsequent Concentration
Regeneration (mg) of H2SOj,
NaOH
0.44
0.46
0.44
0.42
0.45
0.44
0.45
0.43
0.44
*•"
H2S04
0.035
0.054
0.037
0.062
0.070
0.054
0.056
0.066
0.041
—
Used
0.05N (0.25%)
0.10N (0.5%)
0.20N (1.0%)
0.05N
0.10N
0.20N
0.05N
0.10N
0.20N
"•
% Total
Se(IV)
Recovery
30
32
28
34
36
35
36
34
35
—
TABLE 12
DEGRADATION OF ACTIVATED ALUMINA BY VARIOUS
CONCENTRATIONS OF r
Run
Al Recovered in Subsequent Regeneration (mg)
(% by Weight of 3-inch bed in Parenthesis)
0.25%
19
20
21
5.2
6.9
6.1
(0.03%)
(0.03%)
(0.03%)
0.50%
7.2 (0.04%)
5.2 (0.03%)
7.6 (0.04%)
1.0%
8.9 (0.04%)
10.8 (0.05%)
8.9 (0.04%)
91
-------
CHRONOLOGICAL SUMMARY OF Se(IV) REMOVAL
TESTS USING NINE-INCH COLUMNS
In reviewing the three-inch column data, a couple of key points were noted.
When doing countercurrent regeneration (where the direction of regenerant flow is
opposite to that of the treatment cycle), it is important to keep the particles of the
alumina in the same place during regeneration. The strongest portion of regenerant
will react with the least saturated (in terms of Se(IV)) amount of alumina. At the top
of the bed, the weakened NaOH will still be able to regenerate the more saturated
alumina. In ion exchange processes, co-current regeneration sometimes presents
leakage problems, but countercurrent processes minimize leakage and the use of
regenerant. Therefore, a high rate backwash prior to the regeneration was excluded
from the regeneration program. This backwash mixed the bed thoroughly and altered
the distribution of alumina that was present during the treatment run. Also, the rates
at which the intermediate and final D.I. rinses were done were lowered. Since those
2
high rates (6 gpm/ft ) caused some expansion of the bed and mixing, it was thought
that reducing the rate would increase the rinsing efficiency.
The majority of the information we hoped to obtain related to the capacity of
the activated alumina for Se(IV) under varying conditions of pH and regenerant flow
rate. Since these tests with 9-inch columns were expected to last from 36 to 48 hours
2
at a treatment flow rate of 3 gpm/ft , and each column was going to be operated at a
different pH, each column had to have its own separate tank of stock solution. Slight
variations in the synthetic well water composition could occur and the Se(IV)
concentration could greatly vary. However, extra care was taken to ensure that each
container received the same amounts of each constituent. After the mixing and pH
adjustment, the solutions were allowed to sit for 24 hours to ensure that bubbles were
removed from the water and did not enter the columns.
Since the 9-inch column runs would last overnight, a method was needed to
collect samples during the period when no one was attending the columns. We
obtained three automatic samplers, called "Wastewatchers" that took a sample every
hour up to 24 hours. See Figure 31 for a photograph of a "Wastewatcher." The
effluent drain lines were put into 250 ml beakers in the drain trough and allowed to fill
92
-------
oo
Figure 31. Automatic sampler,
-------
up and overflow. The sampler lines were put in the beakers, also. Every hour the
sampler would take a 200 ml sample, then purge the line of remaining water by reverse
pumping back into the beaker. Since the beakers were just larger than the volume of
samples taken, and the detention time in the beaker was approximately 4 minutes
samples were fairly discrete.
. . , (250ml)
4 minutes =
(62 ml/min)
The "Wastewatcher" sample containers were analyzed for their ability to collect
selenium samples and not effect the actual concentration of selenium present.
Previously, known standards of selenium were stored with HN(X preservation in glass
bottles and in "Qubetainers." The glass was able to store samples of selenium with ppb
concentrations up to two weeks without any noticeable change, while samples stored in
Qubetainers were reproducible for one week. The conventional polyethylene sample
bottles from the "Wastewatcher" showed no measurable change in selenium concen-
trations in the ppb range for up to 48 hours. After this time, a loss of selenium was
noticed. During the course of the 9-inch column runs, any samples collected in the
"Wastewatcher" containers were analyzed within 48 hours after taking them. From
time to time during the testing, duplicate samples were taken in the "Qubetainers,"
glass bottles, and the "Wastewatcher" sample bottles. No difference was noticeable
between the three containers as long as they were analyzed within 48 hours after
taking the sample.
Run Nos. 1-3; Capacity Tests at pH 5, 7 and 9
Runs 1 and 2 were conducted at stock pH's of 5, 7, and 9. During the initial part
of run 3, there was a considerable amount of calcium carbonate build-up in the pH 9
column. The pH 9 run was discontinued after two hours during this run. These tests
were done to check the varied capacities for Se(IV) over a wide pH range.
Run 1 used virgin media that had been acid rinsed prior to use. Figure 32 shows
that the pH 7 column had the greatest capacity for Se(IV) in run 1, but dropped
considerably in run 2. The breakthrough capacities for a stock concentration of 50 ppb
Se(IV) after run 2 are estimated to be: pH 5 - 1500 bed volumes, pH 7-900 bed
94
-------
30 -
20 -i
10 -
RUN I
VIRGIN MEDIA
PH9
/ STOCK = 68 ppb
pHS
STOCK = 67 ppb
PH7
STOCK =67 ppb
30 -
20
u
(0
RUN 2
.X PH9
/ STOCK = 51 ppb
/"" PH7
STOCK = 52 ppb
PH5
STOCK= 50
30 -
20 -
10 -
RUN 3
_«-- PH7
^.^-* STO(
.^"•-"
STOCK =49 ppb
PH5
STOCK = 50 ppb
.l I
200 400 600 800 1000 1200
BED VOLUMES
1400
I
1600
Figure 32. Run 1-3, Capacity Tests
95
-------
volumes, and pH 9 - 400 bed volumes. This equates to the following capacities for
Se(IV) in terms of milligrams of Se(IV) per liter of activated alumina (mg/1): 67.5 mg/J,
40.5 mg/1, and 18 mg/1.
Based on the results from the 3-inch column studies, the regeneration after
runs 1 and 2 was:
5 bed volumes of 0.5% NaOH at 3 gpm/ft2, up
2
10 bed volumes of D.I. water at 3 gpm/ft , up
2
5 bed volumes of 0.05N H-SCX at 3 gpm/ft , up
2
5 bed volumes of D.I. water at 3 gpm/ft , up
It is evident that the capacity for Se(IV) at pH 5 and 7 decreased in both cases
and leakage of Se(IV) was 5 ppb for pH 7 during run 3. At pH 7, the breakthrough
capacity dropped to 300 bed volumes (13.5 mg/1). Since the pH 5 run was terminated
prior to 10 ppb breakthrough, its capacity can only be estimated as 1300 bed volumes
(58.5 mg/1).
Table 13 includes a list of the removals and recoveries of Se(lV) during all the
9-inch column tests. This table will be referred to often in the remainder of this
section. For runs 1 through 3, the amount of Se(IV) recovered during regeneration
stays the same; approximately 1.9 mg for pH 5 and 2.9 mg for pH 7. Notice that the
total amount of Se(IV) removed during each run steadily decreased. The recoveries
ranged from 20 to 40 percent; obviously not as much was recovered as was removed.
Run No. 4; Capacity Test, pH 5, 6, and 7
Figure 33 shows run 4. Following run 3, the media used for the pH 9 runs was
discarded and replaced with virgin media, which was rinsed with acid only. The
breakthrough curve for pH 6 is longer than pH 5 or pH 7, approximately 1400 bed
volumes (77 mg/1 capacity for Se(IV) with C. = 60 ppb). Capacity for pH 5 was
* ^
1100 bed volumes (60.5 mg/1) and 900 bed volumes for pH 7 (49.5 mg/1). The larger
breakthrough capacity for pH 6 is' attributed to the initial high capacity of the virgin
media.
96
-------
TABLE 13
SUMMARY OF REMOVALS AND RECOVERIES OF Se(lV)
DURING ALL RUNS WITH 9-INCH COLUMNS
(Regeneration with 5 bed volumes of
0.5% NaOH at noted flow rate)
Run
3
4
Se(IV)
Stock
Con. (ppb)
68
67
67
50
52
51
50
49
61
62
60
112
113
115
110
110
111
210
212
205
211
209
206
190
195
Stock
pH
5
7
9
5
7
9
5
7
5
6
7
5
6
7
5
6
7
5
6
7
5
6
7
5
6
Se(IV)
Removed
During
Run
(mg)
9.3
11.8
6.9
8.7
6.7
4.4
6.3
4.5
7.3
7.5
7.0
9.3
9.3
9.1
12.3
11.8
11.9
17.7
17.0
16.7
22.6
22.2
21.8
21.3
21.0
Se(IV)
Recovered
During
Subsequent
Regeneration
(mg)
1.8
2.9
2.4
2.1
2.9
3.5
1.8
2.8
2.7
2.3
2.7
10.9*
9.1
8.9
8.8
9.0
5.9
7.7
7.5
8.0
6.8
8.8
8.7
8.1
7.1
NaOH
Flow Rale
(gpm/ft2)
3
3
3
3
1
1
1
1
1
Recovery
19
25
34
24
44
79
29
62
37
31
39
117
99
97
72
76
50
43
44
48
30
40
40
38
34
97
-------
TABLE 13
(CONTINUED)
(Regeneration with 5 bed volumes of
0.5% NaOH at noted flow rate)
Run
10
11
12
13
14
15
16
17
18
Se(lV)
Stock
Con. (ppb)
215
204
205
201
195
202
205
205
200
201
198
205
18200
18000
18000
200
204
210
185
195
192
200
204
197
Stock
PH
5
6
5
6
5
6
5
6
7
5
6
7
5
6
7
5
6
7
5
6
7
5
6
7
Se(IV)
Removed
During
Run
(mg)
26.3
24.8
25.1
24.0
23.1
23.7
23.8
23.5
14.9
29.5
28.6
28.6
1391.0
1195.0
891.0
12.4
4.9
-14.5
16.4
16.2
13.3
19.1
19.2
16.9
Se(IV)
Recovered
During
Subsequent
Regeneration
(mg)
7.9
8.0
9.2
10.2
9.0
9.1
19.7
19.3
13.1
20.5
19.9
21.3
1154.0
1086.0
892.0
128.9
112.6
91.3
48.6
42.1
36.9
35.2
31.4
27.8
NaOH
Flow Rale
(gpm/ftT
1
1
1
.5
.5
.5
.5
.5
.5
%
Recovery
30
32
37
43
37
43
83
82
88
70
70
74
82
91
100
— „
—
~
— —
__
—
— —
—
__
*Run #5 was completed as usual, but the columns were allowed to sit for 1 week prior
to regeneration. The high figures would appear to be due to diffusion from the media
into the surrounding water during this "rest period.
98
-------
20-
15-
10-
RUN 4
pH7
STOCK = 60 ppb
pH5
STOCK=61 ppb
»..p—*"
/ pH 6 - VIRGIN MEDIA
/ STOCK = 62 ppb
3
20-i
15 -\
u.
Ill
u
(A
RUN 5
• pK 5, STOCK =112 ppb
• PH7, STOCK = 115 ppb
A pH6, STOCK = 113ppb
20-
15 -
10-
RUN 6
PH 7
STOCK=IIIppb
V-T
pH 6
STOCK = MO ppb
pH 5
STOCK = IIOppb
200
400 600
BED VOLUMES
800
1000
I
1200
Figure 33. Runs 4-6, Capacity Tests.
99
-------
2
Run Nos. 5 and 6; Capacity Tests. C^ = 100 ppb, 1 gpm/ft- NaQH
From runs 1 through 4, we observed two important points:
1) With the NaOH being applied at 3 gpm/ft2, approximately 30% of the
selenium was being recovered. If left in this cycle, the breakthrough
curves would have continued to get worse, and the leakage would have
increased until some equilibrium was established between the amount
adsorbed during the run and the amount eluted during regeneration. To
increase the amount of Se(IV) eluted, the flow rate was decreased to
1 gpm/ft . If diffusion plays a large role in the adsorption process, then
lowering the flow rate threefold should recover at least three times the
amount of Se(IV). For more discussion on the diffusion problem, reference
is made to the section following runs 13-14.
2) In an effort to increase the slope of the breakthrough curve from shallow
to steeper, the stock concentration was increased to about 100 ppb
(=0.10 mg/1). Expectedly, the amount of Se(IV) adsorbed should increase
due to the increased "driving force." But by increasing the stock
concentration, perhaps once the alumina had adsorbed as much as it could,
a rapid breakthrough up to the influent concentration would be noticed.
Based on the above discussion, runs 5 and 6 were conducted with a stock
concentration of Se(IV) of roughly 100 ppb and the subsequent regenerations with
1.5 ///ft3 of NaOH at a flow rate of 1 gpm/ft . This lowered flow rate increased the
flow-through time of the caustic in the bed from 10 minutes to 30 minutes.
Figure 33 shows runs 5 and 6. Because of the increased stock concentration and
2
the fact that the previous regeneration was done at 3 gpm/ft , the resultant
breakthrough pH 5 drifted slowly up, with no breakthrough to 10 ppb. After run 5, the
columns were not regenerated immediately. The columns sat unregenerated for one
week before elution. As seen in Table 13, the amounts recovered from the columns
were at least equal to the amounts removed during run.5. The long rest period helped
to diffuse some 5e(IV) out of the alumina into the surrounding fluid.
100
-------
The following run (Figure 33) shows that the leakage dropped to zero for all the
pH's and that breakthrough bed volumes for pH 6 and 7 were 900-1000 (94.5-105 mg/1).
Since the run was stopped prior to breakthrough with pH 5, breakthrough can only be
estimated to occur at 1300 bed volumes (136.5 mg/1). Note that the amount removed
during treatment increased significantly from run 5. The following regeneration
2
recovered at least three times what the 3 gpm/ft regeneration recovered in runs 1
through 4.
Run Nos. 7-12; Capacity Tests. C. a 200 ppb. 1 gpm/ft- NaOH
Because the 100 ppb stock concentration didn't appear that it would rapidly
produce a saturation of the alumina with Se(IV), and therefore not produce the desired
rapid breakthrough, it was decided to jump to a 200 ppb stock solution. It was
expected that the capacity of the alumina for Se(IV) would increase again, perhaps
double from the removals seen with C^ = 100 ppb and four times as much as with
C. = 50 ppb. We were still trying to quickly fill up the sites on and in the alumina to
achieve rapid breakthrough.
The initial run (#7) in this series with C. = 200 ppb showed poor breakthrough
characteristics, similar to what happened after we changed the stock from 50 ppb to
100 ppb in run 5. Figure 34 shows that during run 7, the pH 7 column exhibited no
capacity to achieve an effluent concentration of less than 10 ppb. The breakthrough
capacities for pH 5 and pH 6 were, respectively, 600 bed volumes (123 mg/1) and
500 bed volumes (102.5 mg/1).
Run 8 shows a greatly improved breakthrough curves, with leakage being 5 ppb,
2 ppb and 0 ppb for pH 7, 6, and 5, respectively. The breakthrough bed volumes
increased to 500 (102.5 mg/1), 900 (184.5 mg/1), and 1100 (225.5 mg/1), respectively.
The total amount of-SeilYl_removed increased from run 7 to about 22 mg.
After run 8, one of the motors broke down and we decided to continue testing
pH 5 and 6, as they seemed to produce the optimum removals. Subsequent runs 9
through 12 were run under the same condition. See also Figure 35. For these four
runs, the average breakthrough capacity for pH 5 was 900 bed volumes (175.5 mg/1),
and for pH 6 was 500 (97.5 mg/1).
101
-------
20 -
15
10
5
RUN 7
pH6
STOCK = 2l2ppb
pH 7
STOCK=205ppb
pH 5
STOCK =210 ppb
20 -
I
ft
^ 15
§
3 I0
LL
U.
5
u
10
RUN 8
PH 7
S TOOK = 206 ppb .f
--jr.---'
PH 6
STOCK = 209 ppb
pH 5
STOCK =211 ppb
20 -
15 -
10 -
5 -
RUN 9
PH 6 .
STOCK = 195 ppb /
pH 5
STOCK = 190 ppb
200
600
BED VOLUMES
800
1000
1200
Figure 34. Runs 7-9, Capacity Tests.
102
-------
pHS, STOCK = 2l5ppb
OpH6, STOCK = 204ppb
I
Dj
h
1
u.
U.
U
III
U)
20
15 •
10-
RUN II
pH6, STOCK = 201 ppb
,
5 J
pH5, STOCK = 205 ppb
^
20
15
RUN 12
PH6, STOCK=202ppb
pH5, STOCK=l95ppb
200
400 600
BED VOLUMES
800
1000
1200
Figure 35. Runs 10 - 12, Capacity Tests.
103
-------
Notice that for runs 9-12, there is a general trend of increased leakage and
lower breakthrough capacity. If you look at the recoveries during these runs, the
percent recovered averages approximately 37%. The amount recovered has not
increased from the 100 ppb stock tests by the same factor that the amount removed
has. The decreasing breakthrough capacities attest to the fact that the poor
recoveries are affecting how the alumina performs in the following run. The slower
2
regeneration rate (1 gpm/ft ) was not slow enough.
During run 12, composite samples were taken of the first 500 bed volumes of the
effluent for the three columns. These composites were analyzed for bicarbonate
alkalinity, chloride, sulfate, nitrate, fluoride, and hardness. These were compared
with the stock concentrations of each column. Results are listed in Table 14.
TABLE 14
COMPARISON OF REMOVALS OF INTERFERING
ANIONS AND CATIONS DURING RUN 12
pH5
pH6
pH7
Species
Alkalinity
(mg/1) (as
CaCO.,
Chloride
(mg/1)
Sulfate
(mg/1)
Nitrate (as N)
(mg/1)
Fluoride
(mg/1)
Calcium
(mg/1)
Magnesium
(mg/1)
Influent Effluent Influent Effluent Influent Effluent
22.8
402
98
4.5
0.85
46.3
26.0
420
97.3
3.3
0.15
82.4
42.3
90.6
359
96.6
4.8
0.90
91.4
42.5
92.0
361
109.3
2.9
0.20
40.5
200.0
312
102.6
4.5
0.89
86.2
43.6
190.4
311
102.6
3.2
0.40
80.1
11.9
104
-------
As seen, the levels of chloride remain fairly constant, except there is a slight
rise at pH 5. Alkalinity and sulfate vary, with some levels increasing through the bed,
others decreasing and some staying the same. This variation is not thought to be
significant. However, decreases in nitrate and fluoride are seen in each run. Because
of its low rank in the selectivity series, nitrate wasn't expected to be removed.
However, because each column shows some removal and nitrate's detection limit very
low (0.01 mg/1), the results show that activated alumina is removing some NO"! . The
removal of fluoride is expected and greater removals were achieved at lower pH. The
hardness ions, calcium and magnesium, decrease in each of the columns. This
decrease, noted by other authors using activated alumina to remove fluoride, is
thought to be due to secondary adsorption. The extremely low magnesium concen-
tration at pH 7 is unexpected and may be due to poor analytical technique.
During runs 9-12, the acid rinse was analyzed more closely. Its objective is to
reduce the pH of the bed to an operating level that will not reduce the capacity of the
media for Se(IV). Rubel discovered that removal of fluoride begins to occur at pH's
less than 10. His neutralization mode involved adjusting the pH of the raw water to be
treated in steps to bring the pH of the treated water down to the pH of the raw water.
In our system, we had no easy way to continuously adjust the pH of the stock feed
solution. We tried to neutralize the bed with 0.05N H2SO^ until the pH of the bed
dropped to around 7. In determining when this occurs, some very interesting facts
came to light. Figure 36 shows plots of averages of the acid rinse waters' pH just out
of the top of the bed versus the bed volumes of acid applied at 1 gpm/ft2 (= 20 ml/min)
for these four runs. Actually, pH was measured out of the teflon tube that fed the
2000 ml graduated cylinders to collect the regenerant. The pH out of the top of the
bed was then back-calculated by subtracting the volume of the column and tubing
above the top of the media. As can be seen, the pH decreased considerably near 5 bed
volumes. While slowly decreasing for the first few bed volumes, the pH suddenly drops
6 or 7 pH units in 1 bed volume or less. At this point, the acid had neutralized most of
the hydroxide ions from the caustic rinse. The pH seemed to level off at 3.5, and since
the pH of the acid was 1.8, some neutralization of the acid must still have been taking
place. To ensure that the pH of the bed dropped to at least 7, all ensuring runs were
conducted with 6 bed volumes (700 ml) of 0.05N (0.25%) H2SO, at 1 gpm/ft2.
This was also done in the upflow mode. It would not be necessary to do
105
-------
Q
Ul
m
u.
o
H
U
O
U
K
U.
O
s
10-
8-
6-
PH 6 COLUMN
pH 5 COLUMN
NOTE; PH OF 0.05 N H2SO4 =1.8
1 BED VOLUME = 116 ml
2468
BED VOLUMES OF ACID APPLIED
10
LJ
7-
6-
5-
S 4-
H
3-
2-
PH 6 COLUMN
PH 5 COLUMN
200
400 600
BED VOLUMES
800
1000
Figure 36. Neutralization of activated alumina by 0.05N H2SO4 and
effluent pH of subsequent treatment run.
106
-------
this in a large-scale removal facility. As a matter of fact, co-current acid rinsing
would probably make the neutralization step more efficient by decreasing the chances
for channelization. Up flow acid rinsing in these tests was done to keep the
regeneration steps as simple as possible, without a lot of changes in tubing and pump
arrangements.
Figure 36 also shows a plot of average effluent pH versus treated water bed
volumes during the four runs, 9-12. The pH started to approach that of the stock
solution immediately and levelled off at 300 bed volumes for pH 6 and 400 bed volumes
for pH 5.
For a more economical use of the acid, this approach is suggested. Run 0.05N
H-SCX through the bed until the pH of the water at the exit of the bed is 10. Then
start the treatment run with the pH adjusted raw water and let it bring the treated
water pH down the rest of the way. The amount of time to bring the pH down will be
shprt compared to the removal runs' length, A short operating time at a pH higher
than the optimum, but below 10 will probably not affect the capacity of the alumina
for Se(IV) to any great degree.
Run Nos. 13 and 14; Capacity Tests, C. = 200 ppb, 0.5 gpm/ft-
Run 13, shown in Figure 37 was conducted the same as runs 9-12, except that the
broken motor had been replaced and testing of three pH's was resumed. However, the
regeneration after run 13 was done at a 0.5% NaOH flow rate 0.5 gpm/ft2. As
explained previously, a slower regeneration was expected to significantly increase the
amount of Se(IV) recoverable during regeneration. This regeneration took 60 minutes
to accomplish, compared to 30 minutes for 1 gpm/ft and 10 minutes for 3 gpm/ft2.
Unexplainedly, the breakthrough curves for run 13 improved from run 12. 5 ppb,
2 ppb, and 0 ppb leakage were present in the pH 7, 6 and 5 columns respectively. The
breakthrough volumes concurrently increased to 600 bed volumes (120 mg/1), 800 bed
volumes (160 mg/1), and 1100 bed volumes (220 mg/1).
The amount of Se(IV) recovered in the regeneration after run 13 was more than
twice as much as what was recovered during any of the 1 gpm/ft regenerations.
107
-------
RUN 13
I
2
u.
bl
u
(fl
pH 7
STOCK = 200 ppb
pH6
STOCK *205ppb
STOCK* 205 ppb
20
15
10 •
5
RUN 14
StTO
TOCK-205ppb
•'STOCK «i98Ppb
pH 5
STOCK = 201 ppb
RUN IS
LL
UJ
ui
tn
• pH 5, STOCK - 18.2 ppb
• pH 6, STOCK = 18.0 ppb
A PH 7, STOCK - 18.0 ppb
200
400 600 800
BED VOLUMES
1000
1200
Figure 37. Runs 13 - 15, Capacity Tests and Saturation Test.
108
-------
Obviously, the slower rate had quite an effect. Subsequent run l(f showed a vastly
improved breakthrough curve, with no leakage for any of the columns and
breakthrough bed volumes of 700 (136.5 mg/1), 1000 (195 mg/1), and 1*00 (273 mg/1) for
pH 7, 6, and 5, respectively. The regeneration after run 1* collected more Se(IV) than
after run 13, but the percent recovery decreased due to the increased amount of Se(IV)
removed in run 1* as compared to that of run 13.
The regenerant after run 13 was collected differently than after the previous
runs. 100 ml aliquots of the regenerant after the NaOH rinse were collected to
develop an elution curve. This curve, plotted as a percent of Se(IV) recovered during
NaOH regeneration versus bed volumes, will tell a lot about whether kinetics play a
large role in adsorption. If the curve has a very sharp peak, with very little "tail,"
then diffusion may not play a key role in regeneration. However, a curve with a long
"tail" usually indicates that the regeneration is not as rapid and is controlled by
diffusion.
Figure 38 plots data for the regeneration after run 13 for the three columns.
The curves are corrected for the actual bed volumes of NaOH rinse and the deionized
water rinse afterwards. The initial 200 ml of the regenerant collected was that water
which was left over in the column when the treatment run was stopped. It had very
little Se(IV) present. That volume was discarded and, as explained before, 100 ml
aliquots were collected up to 1100 ml (9.5 bed volumes).
Note that the highest peak occurs earlier with increasing pH. This is probably
due to the lower pH columns requiring more NaOH to initially bring the pH up to a
level where OH" ions are readily adsorbed by the media. As can be seen, the curves
have long, drawn-out tails, suggesting that Se(IV) adsorption is diffusion-limited.
Based on these results so far, a discussion of the diffusion phenomena is
presented in order to more clearly define the resistances to efficient mass-transfer in
the adsorption process.
20
As explained by Weber , there are essentially three consecutive steps in the
adsorption of materials from solution by porous adsorbents. Listed in order of
occurrence, they are:
109
-------
30n
20-
10-
Z
o
UJ
UJ
o
ui
K
X
9
O
z
£
a
a
UJ
(E
Id
30 -i
20
10
o
£ o
u
(A
U.
O
30 i
20
REGENERATION CONDITIONS FOR ALL COLUMNS:
5 BED VOLUMES 0.5% NAOH AT 1/2 gpm/ft2
S BED VOLUMES D. I. RINSE AT I gpm/ft2
PH 7 COLUMN
pH 6 COLUMN
PH 5 COLUMN
0! 23456789 10
BED VOLUMES OF REGENERANT DURING N*OH AND SUBSEQUENT D. I. RINSES
Figure 38. Elution curves for Regeneration after run 13.
110
-------
1) Transport of the adsorbate through a surface film to the exterior of the
adsorbent ("film diffusion")
2) Diffusion of the adsorbate within the pores of the adsorbent ("pore
diffusion")
3) Adsorption of the solute on the interior surfaces bounding the pore and
capillary spaces of the adsorbent.
After step 1, there will be a small amount of adsorption that occurs on the
exterior surface of the adsorbent after transport across the surface film.
Investigations have suggested that the adsorption process itself (in activated
alumina's case, the exchange of anions discussed in Chapter 1) is probably not rate-
determining, and that a much slower process must control the overall rate of uptake.
•v
\
Film diffusion describes the resistance to mass transfer in the region separating
a turbulent bulk solution and a solid surface. Many theories have been postulated to
explain this phenomena, but boundary layer theory seems to be the most plausible.
Since boundary layer theory accounts for a velocity distribution and is more
realistic than theories which assume a laminar film surrounding the particle, it seems
to be the best answer within today's knowledge.
Based on experimental evidence outlined by Weber, the rate of uptake of a solute
by many porous adsorbents is governed by "intraparticle transport", the rate of
transfer of adsorbed materials from the exterior sites of an adsorbent to surfaces
bounding inner pore spaces. This was outlined in 2 as pore diffusion. The rate for
diffusion processes can be expressed by Pick's first law,
° - _ D _
- ui ax '
111
-------
where Dj is the diffusion coefficient
F° is the mass transport through a unit cross-section in unit
time (f Jux),
C is the mass concentration of diffusing substance, and
x is the space coordinate in the direction of diffusion, so
*\ >-»
-£-TT becomes the concentration gradient.
When this diffusion is accompanied by a surface process such as adsorption, then
Pick's law must be modified to include a term for adsorption.
In relating the above discussion to our results, the effective removal of Se(IV) by
activated alumina and its subsequent elution by NaOH seem to be controlled by the
pore diffusion process. Figure 24 shows "plateaus" in the breakthrough curves and also
increased removal for a short time after the columns had "rested." This would imply
that significant film diffusion is occurring, with its co-current small amount of
adsorption on the exterior surfaces. But when the concentration gradient between the
surface and the interior of the alumina becomes sufficient, rapid particle diffusion
depletes the surface layer, promoting a period of time during the run when the rate of
increase in effluent concentration versus time decreases, i.e., the curve flattens out.
If this column is allowed to rest overnight, the pore diffusion occurs deeper into the
particles of alumina until a "semi-equilibrium" state is reached between the selenium
molecules and the activated alumina. Restarting the column in the morning will
repeat the above described process, but initially the alumina will be able to remove
more Se(IV) due to the availability of sites within the pores closer to the surface of the
alumina.
During elution of the Se(IV) saturated column with NaOH, a great difference in
recovery was noted in the three different flow rates through the bed. By slowing down
the NaOH flow within the bed while keeping the dosage of NaOH the same, the OH"
ions have more time to diffuse into the pores and exchange with the Se(IV) molecules.
112
-------
2 2
In comparing the 1 gpm/ft and 1/2 gpm/ft rates, more than twice the Se(IV) was
recovered in the slower regeneration. This implies that an even slower rate may
2
recover even more Se(IV) than 1/2 gpm/ft . An optimum has not yet been found.
Based on the results of the batch kinetic tests, a smaller mesh (48-100) of
activated alumina should appreciably improve the diffusion kinetics of a continuous
flow column. The smaller particles have a larger total surface area per unit weight,
therefore increased adsorption at the surface should occur. By adsorbing the Se(IV)
more rapidly at the surface, the alumina should exhibit higher capacities. - It would
also make available more inner sites per unit weight. Unfortunately, 48-100 mesh is
very fine and dissolution during regeneration would be greater. In addition to this,
headless during a treatment run would increase and the particles would be subject to
washing out of the bed during backwash, if it is employed.
To effectively test the optimum dose and flow-through contact time of NaOH to
regenerate Se(IV)-saturated alumina, one must continue decreasing the flow rate until
no appreciable change in recovery is noted. Then, using this flow rate, vary the
amount of NaOH applied (///ft ) until the maximum amount of Se(IV) is recovered.
Both variables, of course, affect each other. It may be, that by using a slower flow
rate (increasing the flow-through contact time of the NaOH), a much smaller amount
of NaOH can eventually be used as the optimum regenerant dose. After diffusion was
understood to be more significant than batch experiments indicated, time restrictions
on this project forced us to delay this type of testing. We recommend that these tests
be done in the future to completely investigate the diffusion phenomena and to further
optimize NaOH flow rate and dose. An attempt was made to determine the operating
characteristics of the columns in a "worst case" condition; as described in run 15.
Run 15: Saturation Test, C. = 18 ppm
The remaining tests for Se(IV) were done to approximate the "worst case"
condition, when the alumina has filled all its sites with Se(IV). The alumina was
saturated with Se(IV) in run 15 with a stock concentration of approximately 18 mg/1.
Figure 37 shows a fairly rapid breakthrough from 0 to 18 ppm for all the columns, with
pH 7 breaking through faster than pH 6 and pH 5. Table 13 shows that almost
113
-------
•1,400 mg of Se(IV) were removed at pH 5, with 1,200 mg removed at pH 6, and 900 mg
at pH 7. The following regeneration recovered 82, 91, and 100 percent of the Se(IV)
removed in the pH 5, 6, and 7 columns, respectively. From the regeneration data, the
o
1/2 gpm/ft NaOH regeneration would not elute all of the Se(lV) adsorbed during the
previous saturation run. This would lead one to believe that the steady-state capacity
for Se(IV) for the pH 5 and pH 6 columns was less than shown for run 14, with
1/2 gpm/ft2 NaOH elution.
Run Nos. 16-18; Capacity Tests, GI = 200 ppb. 1/2 gpm/ft2 NaOH
After the saturation test, runs 16-18 show an approach to a steady-state
condition where all of the Se(IV) would be recovered in regeneration that was adsorbed
during the treatment run. The data in Table 13 show that the amount of Se(IV)
removed during the treatment run increases as the alumina approaches an equilibrium
between the Se(IV) removed and the Se(IV) recovered. Notice that for the 3 runs
(Figure 39) following the saturation run, more Se(IV) is recovered in the regeneration
than was removed in the previous treatment run. The differences in the two figures
decreases as the cycles are continued, with run 18 suggesting that the amount
removable and recoverable in a steady-state condition is probably 20 to 25 mg Se(IV).
The breakthrough curves for the 3 different pH's are very different, with pH 5
producing more treated water with Se(IV) < 0.010 mg/1. The breakthrough for runs 16,
17, and 18 suggest that they will get progressively more bed volumes of treated water
from each pH condition until steady-state is observed. Quite possibly, an even slower
2
(< 1/2 gpm/ft ) regeneration rate may be needed to remove enough Se(IV) to
effectively prepare the alumina for a long enough removal run with sufficient
breakthrough capacity.
Due to the time constraints on the project, it was necessary to initiate Se(VI)
removal testing to get a good grip on the parameters of interest concerning Se(VI).
Therefore, no further testing was done on Se(lV).
In conclusion, the ultimate capacity of activated alumina for Se(IV) will be a
function of many variables: pH, dosage of NaOH, and NaOH regeneration flow rate.
Depending on factors such as the cost of pH adjustment, the cost of chemicals, and the
114
-------
500-
400-
300-
200-
100-
RUN 16
pB 7
STOCK =210 ppb
pH 6
STOCK = 204 ppb
PH5
STOCK =200 ppb
RUN 17
3TOCK =
192 ppb
J?H 6
STOCK = 195 ppb
— — ••-*-
pH5
STOCK= 185 ppb
50-
40-
30-
20-
10
RUN 18
,..-••**
*****
TOCK= 197 ppb
••*
PH6
STOCKs 204 ppb
oH 5
STOCK = 200 ppb
200
400 600
BED VOLUMES
800
1000
1200
Figure 39. Runs 16 - 18, Capacity Tests.
115
-------
influent Se(IV) concentration, an optimum operating plan may be different for two
different sources of water. The kinetics of regeneration play the biggest role of any
of the above-mentioned parameters in determining the ultimate capacity of activated
alumina for Se(IV).
Based on data gathered during this phase of work, the following breakthrough
capacities are estimated for an influent Se(IV) concentration of 200 ppb, with 0.5%
2 3
NaOH regeneration at 1/2 gpm/ft (a dose of 1.5 //NaOH/ft ) and similar water quality
to that tested in this study.
pH 5 - 1,200 bed volumes = 235 liter ot
pH 6 - 900 bed volumes = 175 mg/1
pH 7 - 500 bed volumes = 100 mg/1
Work at lower concentrations suggested that similar breakthrough bed volumes
for each pH could be achieved. This implies a linear relationship between adsorptive
capacity and concentration. Some isotherm models predict a linear relationship
between capacity and concentration, qe a c, for very low amounts of adsorption. It is
assumed that the levels of adsorption discussed in this report are low compared with
adsorption of organics by activated carbon. We estimate that with lower concen-
trations of Se(lV) in the influent, capacities will be reduced. The following table
predicts capacities for influent concentrations of Se(IV) of 50 and 100 ppb at the three
pH's tested.
50 ppb Se(IV) 100 ppb Se(IV)
pH5 60 mg/1 120 mg/1
pH 6 4 5 mg/1 90 mg/1
pH 7 25 mg/1 50 mg/1
These capacities are all based on a 9-inch bed. Increasing the bed depth will
probably increase the breakthrough capacity in bed volumes and should minimize
leakage. Remember from Figure 24 that initial testing with 3", 6", 9" depths produced
116
-------
respectively, 600, 700, and 1100 bed volumes of treated water within Se(IV)
concentration less than 0.01 mg/1 at pH 6.5. This trend indicates that increasing the
depth will increase the number of bed volumes produced prior to breakthrough. There
will be some optimum level when increasing the depth provides no better removal.
Pilot testing should verify this.
CHRONOLOGICAL SUMMARY OF Se(VI) REMOVAL TESTS
USING 9-INCH COLUMNS
Run No. 1; Initial Test. 3-Inch Column
Prior to testing Se(IV) removal with 9-inch columns, and after testing Se(IV)
removal with 3-inch columns, preliminary studies looked at rough estimates of Se(VI)
removal. (Runs 1 through 5). Figure 40 (run 1) shows that the breakthrough was very
rapid with a 3-inch column and the slope of the breakthrough curve was much steeper
than with Se(IV). Since the run lasted only one-half an hour until breakthrough, it was
immediately decided to try a deeper column of 9-inches.
Run Nos. 2 and 3; 9-lnch Columns, Repeatability Tests
Runs 2 and 3 (Figure 40) were done at pH 6.1 with a feed concentration of about
50 ppb Se(VI). The regeneration was as follows:
2
10 bed volumes D.I. water at 9 gpm/ft , upflow
27 bed volumes of 1% NaOH at 3 gpm/ft2, upflow
10 bed volumes of D.I. water at 6 gpm/ft , upflow
2
16 bed volumes of 0.05N HC1 at 3 gpm/ft , upflow
10 bed volumes of D.I. water at 6 gpm/ft , upflow
Remember that we had done no Se(IV) testing with 9-inch columns and had not
optimized regeneration yet. However, HC1 was used as the acid rinse because data
from the batch tests showed HC1 worked better for Se(VI) removal than H-SO..
Table 15 shows removals and recoveries for runs 2 and 3.
117
-------
a
5
gj
a
u.
LI
70-
60-
50-
40-
30-
20-
10-
0
70-
60-
50-
40 -
• - _ _ *
. ^^^
/
J RUN 1
j • COL. I 1 VIRGIN MEDIA (3 INCHES)
I • COL. 2 j
I STOCK= 67 ppb
/
-/
to*«-£»<4
A^^^
n RUN 2
// «COL. l] VIRGIN MEDIA (9 INCHES)
// .COL. 2J
' 1 STOCK= 50 ppb
20-
10-
0
70-
60-
50-
40-
30-
20-
10-
0
RUN 3
• COL. I
• COL. 2
STOCK =65 ppb
ISO 200 250
BED VOLUMES
300
350
400
Figure 40. Runs 1 -3, Initial Tests.
118
-------
It was apparent that the liberal amount of NaOH used to regenerate was
sufficient to recover all of the Se(VI) removed during the previous run.
TABLE 15
REMOVALS AND RECOVERIES OF Se(VI) FOR RUNS 2 AND 3
Se(VI) Se(VI Recovered
Removed During Subsequent
During Regeneration %
Run pH Run (mg) (mg) Recovery
6.1 0.28 0.35 100
6.1 0.29 0.31 100
6.2 0.68
6.2 0.65
Run Nos. fr and 5; Regeneration Tests, Varied Amounts of NaOH
The regenerations after runs 3, 4, and 5 were done to delimit the amount of
NaOH necessary to efficiently regenerate the column. 0.5, 5.3, and 53.0 bed volumes
of 1 percent NaOH were tested to see if there were differences in recoveries. The
2
flow rate was 3 gpm/ft and the other regeneration steps were kept the same as
before. Table 16 shows the removals and recoveries for runs 3, 4, and 5. Runs 4 and 5
are shown in Figure 41.
The data from these runs indicate that small amounts of NaOH (.5 bed volumes
of 1 percent NaOH = 0.3 •• 3 can recover as much Se(VI) as the amount recovered
using 100 times the applied dose of NaOH. Se(VI) is not as preferred by activated
alumina as Se(IV) and, therefore, is much easier to recover during regeneration. Also,
note that the treatment runs lasted on the order of one-tenth as long as the Se(IV)
runs. This shortened run length limited the role that diffusion could play in adsorbing
more selenium.
119
-------
ft
V_X
I
t
UJ
III
in
70
60
50
40
30
ZO-
IC-
CD
RUN 4
70
60
50-|
40
30
20
10-
0
RUN 5
• 0.5 B. V. I %
• 5.3 B. V. I %
A 53.4 B. V. 1 %
STOCK-52 ppb
• 0.5 B. V. I %
• 5.3 B. V. I %
A 53.4 B. V. I %
STOCK=50 ppb
50 100
BED VOLUMES
150
200
Figure 41. Runs 4 and 5, Regeneration Tests.
120
-------
Run
3
4
TABLE 16
REMOVALS AND RECOVERIES OF Se(IV)
FOR RUNS 3, 4, AND 5
PH
6.2
6.2
6.2
6.2
6.2
6.2
6.2
6.2
Se(VI)
Removed
During Run
(mg)
.68
.65
.53
.49
.51
.47
.47
.49
Se( VI) Recovered
During
Subsequent
Regeneration
(mg)
.72
.70
.50
.52
.48
.50
.45
.46
Bed Volumes
of
1% NaOH
5.3
53.0
0.5
5.3
53.0
0.5
5.3
53.0
%
Recovery
100
100
94
100
94
100
96
94
Run Nos. 6-8; Regeneration Tests.
Versus HC1
Following run 5, Se(VI) testing was delayed until after the completion of Se(IV)
testing. With the knowledge gained about Se(IV), we were able to quickly determine
the important parameters concerning Se(VI) removal and regeneration. Based on Se(IV)
data, the following regeneration scheme was adhered to throughout the remaining
Se(VI) testing:
5 bed volumes of 0.5% NaOH, flow rate varied, up
5 bed volumes of D.I. water at 1 gpm/ft , up
6 bed volumes of 0.05N H2SO^ or HC1 at 1 gpm/ft2, up
5 bed volumes of D.I. water at 1 gpm/ft , up
Runs 6 through 8 (Figure 42) were regenerated prior to the treatment runs with
H9SO.. as the acid rinse. As we noticed in the batch tests, a sulfuric acid rinse
£ *r ^
interferred with activated alumina's ability to adsorb Se(VI). Because SO. and
SeO,~2 are relatively close in the selectivity series for adsorption by activated
29
alumina, the much greater concentrations of SO^~ (compared to SeO^) increased
121
-------
RUN 6
• PH 5, STOCK = 46 ppb
• pH 6, STOCK = 49ppb
* pH 7, STOCK =48 ppb
VIRGIN MEDIA
u
tf)
SO-i
40-
S| 30-
fc 20
10
RUN 7
• pH 6, STOCK=50ppb
• pH 7, STOCK= 50 ppb
1001
RUNS
• pH 5, STOCK= 60 ppb
• pH7, STOCK =59 ppb
100 ISO
BED VOLUMES
200
Figure 42. Runs 6-8, H2SO. versus HCL regeneration.
122
-------
its adsorption capacity in relation to Se(\~ . Therefore, the competition presented by
*\ "
SOj. ions increases the difficulty of Se(VI) adsorption. The synthesized well water
quality remained the same as in the Se(lV) removal runs.
The graphs indicate that pretreatment with HjSO^ will not produce any
breakthrough capacity for Se(VI) at pH 5, 6, or 7. The alumina does adsorb some
Se(VI), as shown in Table 17, but it didn't remove enough to bring the concentration
lower than 10 ppb. Therefore, the regeneration following run 8 and all subsequent
regenerations were done with a 0.05N HC1 rinse. Chloride should not interfere with
Se(VI) adsorption. During the regeneration after run 6, the motor for the pH 5 column
broke down. No replacement was readily available and a new motor was not obtained
until run 15 was initiated. pH 5 and pH 7 were done on runs 8 through 1*, while the
data for pH 6 was estimated by interpolation.
Run No. 9; Capacity Test. pH 5 and 7
The regeneration following runs 8 and 9 was done with the NaOH surface loading
2 2
rate of 1 gpm/ft . It appeared as though the 1 gpm/ft rate wasn't recovering all of
the Se(VI). As with Se(IV), the amounts removed were calculated by integrating the
area above the breakthrough curve and below the stock concentration level. Figure 43
shows run 9 and it is apparent that the concentration of Se(VI) increases above the
stock concentration for a short period of time after breakthrough. This means that a
more selective species is being adsorbed on the alumina causing the Se(VI) previously
adsorbed to be desorbed, thus increasing the concentration of Se(VI) in the effluent. It
is thought that SO£ is causing this. In this case, after breakthrough has occurred,
more Se(VI) is coming off than is being put on the column. This requires subtracting
the amount desorbed after breakthrough from the amount adsorbed prior to break-
through. The amounts of Se(VI) removed during the run shown in Table 17 reflect this
calculation. From the graph, 100 bed volumes of treated water with an Se(VI)
concentration less than 0.01 mg/1 are produced at pH 5. This is equivalent to
4.5 milligrams of Se(VI) per liter of activated alumina. For pH 7, 35 bed volumes, or
1.6 mg/1 Se(VI) were adsorbed.
123
-------
Run Nos. 10 and 11: Regeneration Tests. 1/2 gpm/ft2 NaOH
o
Because the regeneration at 1 gpm/ft didn't appear to recover all of the Se(VI)
removed during runs 6 through 9, the regenerations after runs 10 and 11 (Figure 43)
2
were done at 1/2 gpm/ft . Table 17 shows that the amount of Se(VI) in the regenerant
did not increase from the previous runs. Testing of Se(IV) regeneration showed that a
decreased regeneration rate increased the amount recovered in the eluant. This was
not found in runs 10 and 11.
o
Run Nos. 12 and 13; Regeneration Tests. 2 gpm/ft NaOH
To verify that the kinetics of regeneration did not play as big a role as they did
2
with Se(IV), regenerations after runs 12 and 13 (Figure 44) were done at 2 gpm/ft . As
seen in Table 17, no change in the amount of Se(VI) recovered was noticed, but the
apparent percentage of recovery still indicated that only 50 to 60 percent of the Se(VI)
removed during a run was being recovered in the regeneration.
Run No. 14; Capacity Test, ph 5 and 7
We decided to collect all the effluent from both columns during run 14
(Figure 44). This represented 160 bed volumes or 18.6 liters of water for each column.
The Se(VI) concentration in the composite samples for each column were: pH 5 -
45 ppb, pH 7 - 55 ppb. These amounts, subtracted from the initial concentration of
Se(VI) in the stock solution and multiplied by the volume of effluent, should give the
total amount of Se(VI) remaining on the alumina after termination of the run. The
resultant amounts were: pH 5 - 0.26 mg, pH 7 - 0.09 mg. Table 17 shows that these
amounts correspond to the amounts recovered in the regeneration after run 14.
The previous method for calculating the amounts of Se(VI) removed during the
run was subject to some error, due to the small number of sample points on the
breakthrough curve. Apparently, right after breakthrough, the curve goes much higher
than the sample points indicate. There must be a very sharp peak immediately after
breakthrough which made the integration by averaging the values of consecutive points
inaccurate. Therefore, the amounts listed as Se(VI) removed during run for runs 6-13
124
-------
100-
80-
60-
40-
20-
0
100-
2T
'8 8
s^x
H
U 60 H
u 40H
Ul
(A
RUN 9
A
r
pHS
PH7
STOCKS: 58 ppb
20-
0
[00
80
60-
40-
20-
RUN 10
PH7
STOCK =57 ppb
pKS
STOCK = 55 ppb
RUN II
/ PH7
/ STOCK = 60ppb
PH5
STOCK = 60 ppb
-I 1 —
50 100
BED VOLUMES
ISO
—1
200
Figure 43. Runs 9 - 11, Regeneration Tests.
125
-------
too-i
PH5
STOCK = 60ppb
[00-i
80-
50
100
BED VOLUMES
ISO
200
Figure 44. Runs 12 - 14, Regeneration Tests.
126
-------
TABLE 17
REMOVALS AND RECOVERIES OF SE(VI)
IN TESTS USING 9-INCH COLUMNS, RUNS 6-18
Run
6*
7*
8*
9
10
11
12
13
14
15
16
pH
5
6
7
6
7
5
7
5
7
5
7
5
7
5
7
5
7
5
7
6(S04"2 =5)
6(S04'2 = 50)
6(S0^2 = 500)
6(50 -2 = 5)
6(S04'2 = 50)
6(SO ,'2 = 500)
Se(VI;
Removed
During
Run (mg)
0.22
0.09
0.01
0.30
0.21
0.33
0.17
0.40
0.16
0.45
0.15
0.43
0.10
O.*15
0.45
0.11
0.26
0.09
2.50
0.73
0.06
2.45
0.69
0.05
Se(VI)
Recovered
During
Subsequent
Regeneration
(mg)
0.17
0.11
0.02
0.13
0.09
0.19
,0.20
0.25
0.12
0.24
0.11
0.24
0.12
0.23
0.12
0.23
0.10
0.24
0.09
2.15
0.68
0.04
2.13
0.63
0.04
NaOH
Flow Rale
(gpm/ft^)
1
1
1
1
.5
.5
2
-
2
.5
.5
.5
Recovery
77
100
100
43
43
58
100
63
75
53
73
56
100
52
80
51
91
92
100
85
93
67
87
91
80
127
-------
Run
17
18
pH
6(Alk = 5)
6(Alk = 50)
6(Alk = 500)
6(Alk = 5)
6(Alk = 50)
6(Alk = 500)
TABLE 17
(CONTINUED)
Se(VI)
Removed
During
Run (mg)
0.52
0.32
0.18
0.57
0.27
0.14
Se(VI)
Recovered
During
Subsequent
Regeneration
(mg)
0.48
0.28
0.15
0.53
0.27
0.14
NaOH
Flow Rale
.5
.5
Recovery
92
88
83
93
89
100
*Prior to these runs, 0.05N H-SCX was used as the acid rinse. The removals of Se(VI)
were resultantly low.
are probably in error to some degree, and it is likely that 100 percent recovery by the
regenerations may have been accomplished. The remaining four runs (15-18) had the
effluent samples analyzed every hour as before, but all of the effluent was collected
and analyzed, as per run 14.
Notice for runs 9 through 24 that leakage is present in all the runs. This leakage
didnt affect the breakthrough volumes of any run since the effluent concentration of
Se(VI) never got above 10 ppb until the rapid breakthrough. We assumed that all the
Se(VI) was being removed during regeneration, so there should not be any present as-
leakage. Leakage was not present in runs 1 through 5 when the HC1 acid rinse was
initially used to treat the virgin media. It is possible that the FUSO^. rinses prior to
runs 6 through 8 affected the performance of the following runs.
Based on the data acquired in runs 9 through 14, the following breakthrough
capacities are estimated at pH 5, 6, and 7: 100 bed volumes, 65 bed volumes, and
128
-------
30 bed volumes, given the water quality explained in Table 4 and an initial
concentration of 50 ppb Se(VI). These volumes correspond to the following capacities:
pH 5 - 4.5 mg/1, pH 6 - 3.0 mg/1, and pH 7 - 1.5 mg/1.
Similar to the data presented in Figure 38 for Se(IV), an elution curve was
prepared for the regeneration after run 14. Because the kinetics of the regeneration
did not play an important role in the recovery of Se(VI), it was assumed that the
elution curve would show a sharp peak with very little tail. Compared with Se(IV),
Se(VI) is not adsorbed nearly as well, therefore it makes sense that it should be easier
to desorb.
Figure 45 shows that the elution curves for the pH 5 and 7 columns are almost
identical in shape, with no relevant elution occurring after 2.5 bed volumes. The sharp
peak occurred almost instantaneously with the first amount of NaOH that was applied.
From this graph, it is apparent that OH" ions are much preferred over Se(VI) ions and
that the rate of application of the NaOH is not important. Probably much less NaOH
could be used to effectively regenerate Se(VI), based on these results and earlier
2
testing done with 0.5, 5.3, and 53.0 bed volumes of NaOH at 3 gpm/ft . To ascertain
what minimum dose of NaOH would be appropriate to regenerate these columns, side-
by-side tests with decreasing amounts of NaOH applied should be done. When less than
100% recovery is noted, it will indicate that the minimum dose has been used.
Because HC1 would be the required acid for neutralization of the alumina in a
full-scale removal facility removing a combination of Se(IV) and Se(VI), in order to
»
optimize removals, its use had to be compared to that of H2SO^. A pH breakthrough
diagram was constructed, similar to Figure 36 for 0.05N HC1 during the regeneration,
after run 14. Figure 46 shows the results. The amounts of HC1 required to reduce the
O
pH of the alumina after the 1/2 gpm/ft NaOH regeneration are virtually identical to
those for H2SO^. We expect no decrease in the capacity of activated alumina for
Se(IV) due to the use of HC1. Cl" is lower on the selectivity series than SO^ and using
this as the acid rinse may enhance Se(IV) adsorption.
129
-------
70
60
50-
Z 40
O
P
S 30
§
S 20
a
9 10
o
1 »
73
REGENERATION CONDITIONS FOR BOTH COLUMNS:
5 BED VOLUMES 0.5% NAOH AT 1/2 gpm/ft2
5 BED VOLUMES D. I. RINSE AT I gpm/ft2
pH 7 COLUMN
a 70
ui
8 60
HI
K
^. 50
40-
30
20
10
pH 5 COLUMN
01 234 5 6 78 9 10
BED VOLUMES OF REGENERANT DURING N*OH AND SUBSEQUENT D. I. RINSES
Figure 45. Elution curves for regeneration after run 14,
130
-------
a
m
u.
o
h
Z
el
s
£
li.
O
12
10
4-
2-
PH 5 COLUMN
PH 7 COLUMN
NOTE: pH OF 0,05 N HCu =1.8
I BED VOLUME =116ml
-» I I T 1 ,
345678
BED VOLUMES OF ACID APPLIED
T
9
T"
10
a
PH 7 COLUMN
T
400 600
BED VOLUMES
800
1000
Figure 46. Neutralization of activated alumina by 0.05 HCL and subsequent
treatment run.
131
-------
Run Nos. 15 and 16; Sulfate Interferences
Because it was noted that sulfate and bicarbonate interfered with Si VI)
adsorption in batch tests and that rinsing with PUSCX prior to a treatment run causes
poor removal of Se(VI), it was decided to test varied levels of SCL~ and HCO^" for a
few runs.
Runs 15 and 16 were done with sulfate concentrations of 5, 50, and 500 ppm
2
S
-------
80-
60-
40 •
20-
0-
RUN 15 -^
\ .^".*. ' ""X^
V** / STOCK
SOOppm SOj /
STOCK- SOppb 1
1
••* 1 =
' 5 ppm SO4
MfcJ— *• " STOCK" 55 ppb
m
nS04
- 59 ppb
— — '
80
60
40 -
RUN 16
I \
X
'•*.« 50ppmS04
STOCKs 57 ppb
J..A 500 PP» S04
J STOCK- 60ppb
• 5 ppm ALK, STOCK -54 ppb
• 50 ppm ALK, STOCK -49 ppb
500 ppm ALK, STOCK - 49 ppb
80
60
40-
20-
0
RUN
nr-f^i
• /
+ mm**mJ
• 5 ppm ALK, STOCK-52 ppb
• 50 ppm ALK, STOCK-48 ppb
A 500 ppm ALK, STOCK - 49 ppb
100
200 300
BED VOLUMES
400
Figure 47. Runs 15 - 18, SO4 and HC03 interferences.
500
133
-------
100
8
10
8 i.o
ui
U) 0.5
O.I
—-so4-
(ALK = 100 ppm
ALKALINITY
(AS
ppm )
10
50
100
500
1,000
5,000 10,000
RATIO OFSO~4 OR ALKALINITY TO Se(VI)
SO4- ALK
OR ALL CONCENTRATIONS IN ppm
St(VI)
Figure 48. Se(IV) removal versus SOj and alkalinity concentrations,
134
-------
all of the Se(VI) adsorbed. Diffusion allowed some of the Se(VI) to penetrate far
enough into the bed to escape desorption by the NaOH. The shorter runs did not allow
enough time for significant diffusion to occur, and all of the adsorbed Se(VI) was
recovered.
Run Nos. 17 and 18; Alkalinity Interferences
The final series of tests were done to test alkalinity's effect on Se(VI) adsorption.
The other parameters in the synthetic water were kept as before (SO^ = 100 ppm) and
only the alkalinity was varied to achieve 5, 50, and 500 ppm alkalinity (as CaCOJ at
pH 6. This required adding varied amounts of NaHCO3 to each solution, as the amount
of buffering capacity each system had would vary, and different amounts of HC1 must
be added to achieve the desired alkalinity at pH 6. For 100 liters of water, the
following amounts of NaHCO, and HC1 were added:
5 ppm - .33 meq/1 HCO" = 2.8 grams NaHCO-
.23 meq/1 H+ = 3.9 ml of 6N HC1
50 ppm - 2.72 meq/1 HCO^ = 22.9 grams of NaHCO3
1.72 meq/i H+ = 28.7 ml of 6N HC1
500 ppm - * 33 meq/1 HCO^ = 277.2 grams of NaHCO3
23 meq/1 H"1" = 383.3 ml of 6N HC1
The results are shown in runs 17 and 18 (Figure 47). The effect of varied HC
-------
100 bed volumes are produced. The effect of reducing the alkalinity from 100 ppm to
50 ppm was barely noticeable, comparing results obtained in the previous runs.
Figure 48 shows the relationship of alkalinity to Se(VI) capacity. Compared with the
plot for sulfate, its effects are not as great.
In summary, Se(VI), being lower in the selectivity series, is more susceptible to
interferences with adsorption. For the same operating conditions and water composi-
tion, Se(VI) was removed about one-tenth as much as Se(IV). Based on data gathered
during Se(VI) testing, the following breakthrough capacities are estimated for an
influent concentration of 50 ppb Se(VI), with 0.5% NaOH applied as the regenerant at
2 gpm/ft (a dose of 1.5 #NaOH/ft ), a neutralization step using HC1 as the acid, and
similar water quality to that tested in this study.
PH 5 - 100 bed volumes = *.J liter to alumina
pH 6 - 70 bed volumes = 3.2 mg/1
pH 7 - 35 bed volumes = 1.6 mg/1
These capacities are 13 times less than Se(IV) adsorption under similar condi-
tions. If sulfate concentrations in the water are less than those used in this testing,
(<100 ppm), increased capacity can be expected. If sulfates were present at 50 ppm,
then the above capacities could be doubled. Alkalinity does not play as important a
role in interferring with Se(VI) adsorption.
It is also predicted that for increased Se(VI) initial concentrations, the capacity
for Se(VI) will linearly increase, as explained in the summary of the Se(IV) testing.
However, the actual number of bed volumes of treated water with Se(VI) concentration
less than 0.01 mg/1 would remain the same.
Based on the results of these tests, Se(VI) would be the limiting factor in
determining the capacity of activated alumina for a mixture of Se(IV) and Se(VI). If
concentrations of Se(VI) exceeded 0.01 mg/1 in the influent to a full-scale removal
facility, then the run length would be limited to the breakthrough capacity of Se(VI).
136
-------
Regeneration would therefore be required more often, with resultant increased costs
due to chemicals and replacement of bed media.
One last test was performed on the activated alumina after the runs had been
completed for approximately 1 week. The remaining media in the columns was mixed
together, then 9-inches was put into the three columns. Another test was done with
different concentrations of NaOH to see if the reduced regeneration rate and the
different concentrations effected the amount of alumina dissolved. Since the alumina
had been previously filled with Se(VI), the differences in this test of desorption were
not considered.
The three columns were dosed equally with 1.5 #/ft NaOH at a flow rate of
2
1/2 gpm/ft , the only difference being the concentrations of NaOH, 2%, 1%, and 0.5%.
They were then rinsed with 5 bed volumes of D.I. water. Following this was a
D.05N HC1 rinse (6 bed volumes at 1 gpm/ft2) and subsequent 5 bed volume rinse with
D.I. water. The results are listed in Table 18.
TABLE 18
DEGRADATION OF ACTIVATED ALUMINA BY VARIED
CONCENTRATIONS OF NaOH AND
BY 0.05N HC1 ACID RINSE
Al recovered during Caustic
and Subsequent
D.I. Rinse (mg)
(% by weight of 9 -inch column
_ in parentheses) _
0.5% NaOH
1.0% NaOH
2.0% NaOH
(0.86%)
480 (0.87%)
489 (0.89%)
Al Recovered During
0.05N HC1
Rinse and Sub-
sequent D.I.
Rinse (mg)
48 (0.09%)
41 (0.07%)
38 (0.07%)
137
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The table shows that the decreased regeneration rate had a large effect on
dissolution of alumina when compared with Table 10. The differences noted between
the 3 different concentrations of NaOH are not as great as with the previous test,
however there is a slight increase in amount dissolved as concentration is increased.
The amount of alumina dissolved during the acid rinse increased a little also, from
0.03% with 3 gpm/ft2 rate to 0.08% with 1 gpm/ft2.
Based on these results, the following numbers of cycles could be completed at
each concentration of NaOH before all of the activated alumina would be dissolved
(included in loss due to acid rinse, also):
0.5% NaOH - 106 cycles
1.0% NaOH - 105 cycles
2.0% NaOH - 103 cycles
If this regeneration were done once a day, the media would have to be replaced
3.5 times a year. If it were possible to regenerate less often, then the resultant loss
of media would be less.
This information would have to be evaluated more closely in pilot studies. This
work indicates that increasing the amount of flow through contact time of NaOH in
2 2
the bed fr.om 10 minutes (3 gpm/ft ) to 60 minutes (1/2 gpm/ft ) will greatly increase
the amount of alumina dissolved. The associated replacement costs for this could be
very expensive.
138
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SECTION 7
PRELIMINARY COST ESTIMATE
Based on the capacities and regeneration techniques developed during the column
studies, separate cost estimates have been prepared for the removal of either Se(IV) or
Se(VI) from a ground water similar in chemical composition to the water used during
these tests. Main design criteria used to develop the costs include:
Average Flow - 1 mgd
Se(IV) or Se(VI) concentration - 0.10 mg/1
Capacity of Activated Alumina at pH6:
Se(IV) - 90 mg/1
Se(VI) - 7 mg/1
Minimum Duration of Treatment Run - 2k hours
Regeneration:
NaOH - minimum flow through time with 0.5% NaOH of 1.5 hours
@ 1 gpm/ft2
H-SO^ or HC1 - same volume of NaOH with a concentration
of 0.25% at 2 gpm/ft2
The system considered would be very basic, with no automatic control systems.
Regeneration would be done manually. Capital costs include all manufactured
equipment, activated alumina, piping and valves, electrical and instrumentation, a
small building for chemical storage and other operations, and contingencies. A small
clearweil was provided to supply sufficient regeneration water. Land acquisition is
included.
Annual operation and mantenance costs include:
139
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chemicals for regeneration and pH adjustment
labor @ $15,000/yr
media replacement (9 0.5% per regeneration
electrical energy (d 7£/kw-hr
Current costs used were:
50% NaOH = $175/ton
28% HC1 = $65/ton
100% H2SOlt = $60/ton
Activated alumina = $0.60/lb
To develop costs on a cents per 1000 gallons produced (£/1000 gal) or dollars per
acre-foot ($/ac-ft.) basis, estimated capital costs were amortized over the life of the
equipment, estimated to be 20 years. An annual interest rate was assumed to be 10%.
Table 19 lists these estimated costs.
As can be seen, ground water contaminated with only Se(VI) will cost much more
to treat than will water with Se(IV) present. If there is an appreciable amount of
Se(VI) (>10 ppb) present in a water with Se(IV), then the removal of selenium will be
dependent on how well Se(Vl) can be removed during treatment.
TABLE 19
COST ESTIMATE FOR Se(lV) OR Se(VI) REMOVAL FACILITIES
Cost
Item Se(IV) Se(VI)
Amortized Capital Costs ($/yr.) $27,000 $115,000
Operation and Maintenance ($/yr.) 53,000 1^5,000
Total Annual Costs ($/yr.) $80,000 $ 260,000
Cost to Treat 100% Se(IV) = 23
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Because the reduction of Se(VI) to Se(IV) requires very vigorous reduction, sulfur
dioxide or other mild reducing agents could not be used for this purpose. It would be
virtually impossible to chemically pretreat a drinking water supply to reduce Se(VI) to
Se(IV).
The estimated costs, prepared for this report, are based solely on nine-inch deep
columns of alumina. It is quite possible that deeper beds may show a greater capacity
for both species of inorganic selenium. To develop truly accurate cost estimates,
capacities would have to be developed from pilot scale testing. This testing would also
develop other engineering parameters that would allow the rapid determination of the
feasibility for using activated alumina to remove selenium from drinking water
supplies.
141
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REFERENCES
1. Committee on Medical and Biologic Effects of Environmental Pollutants,
Selenium, National Academy of Sciences, Washington, D.C. (1976) 203 pp.
2. Ball, R. "Removal of Selenium from Drinking Water Supply Using Activated
Alumina." Senior Thesis, University of Cincinnati, Ohio, 1977. 62 pp.
3. Anderson, M.S., Lakin, H.W., Beeson, K.C., Smith, F.F., and Thacker, E.
Selenium in Agriculture, Agriculture Handbook #200, U.S. Dept. of Agriculture,
1961,65pp.
4. Klayman, D.L. and Gunther, W.H.H. Organic Selenium Compounds; Their
Chemistry and Biology. Wiley-Interscience, New York, New York, 1973, 1188 pp.
5. Sorg, T.3., and Logsdon, G.S. "Treatment Technology to Meet the Interim
Primary Drinking Water Regulations for Inorganics: Part 2." 3. AWWA. 3uly,
1978, pp. 379-396.
6. Gupta, S.K. and Chen, K.Y. "Arsenic Removal by Adsorption." 3. WPCF.
March, 1978, pp. 493-506.
7. Wu, Y.C. "Activated Alumina Removes Fluoride Ions From Water." Water and
Sewage Works 3. 3une, 1978, pp. 76-82.
8. Ames, L.L. and Dean, R.D. "Phosphorus Removal From Effluents in Alumina
Columns." 3. WPCF. May, 1970, pp. R161-R172.
9. Bellack, E. "Arsenic Removal from Potable Water." 3. AWWA, 3uly 1971, pp 454-
458.
10. Choi, W.W. and Chen, 3.Y. "The Removal of Fluoride from Waters by Adsorption"
3. AWWA October, 1979, pp. 562-570.
11. Clifford, Dennis, Assoc. Prof, of Civil Engineering, Univ. of Houston, Texas
Personal Communication, 1980.
12. . Clifford, D., Matson, 3., and Kennedy, R. "Activated Alumina: Rediscovered
'Adsorbent1 for Fluoride, Humic Acids, and Silica" University of Houston, Texas,
1979.
142
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13. Rubel, F. and Woosley, R.D. "Removal of Excess Fluoride from Drinking Water."
EPA 570/9-78-001, U.S. Environmental Protection Agency, Washington, D.C.
1978. 16pp.
14. Perry, R.H. and Chilton, C.H. "Adsorption and Ion Exchange" Chemical
Engineers' Handbook, 5th Edition, McGraw-Hill Book Co., New York, N.Y.
pp 16-1 to 16-10.
15. Bellar, T.A. and Lichtenberg, 3.3. "Determining Volatile Organics at Microgram-
Per-Litre Levels by Gas Chromatography." J.AWWA, Dec., 1974 p 739.
16. Raihle, J.A. "Fluorometric Determination of Selenium in Effluent Streams with
2,3 - Diaminonapthelene." Environmental Science and Technology. Vol. 6:7,
pp. 621-622, 1972.
17. Martin, T.D., Kopp, J.F., and Ediger, R.D. "Determining Selenium in Water,
Wastewater, Sediment, and Sludge by Flameless Atomic Absorption
Spectroscopy." Atomic Absorption Newsletter. Vol. 14:5 pp. 861-868, 1975.
18. APHA-AWWA-WPCF. Standard Methods for the Examination of Water and
Wastewater, 14th Edition. American Public Health Association, Washington,
D.C., 1975, pp. 237-242.
19. Zingaro, R.A. and Cooper, W.C. Selenium. Van Nostrand Reinhold Co., New
York, New York, 1974 835 pp.
20. Weber, W.3., Physicochemical Processes for Water Quality Control. Wiley-
Interscience, New York, New York, 1972. 640 pp.
21. National Research Council, Drinking Water and Health, National Academy of
Sciences, Washington, D.C. (1977) 939 pp.
22. U.S. Environmental Protection Agency, Office of Water Supply, National Interim
Primary Drinking Water Regulations (1977), EPA-570/9-76-003.
23. Hadjimarkos, t».M. "Selenium in Relation to Dental Cavies." Food and Cosmetics
Toxicology 11(6), 1083-1095 (1973).
24. Lakin, H.W. "Selenium in Our Environment." Trace Elements in the Environment
(1972), pp. 96-111.
25. Kubli, H. "On the Separation of Anions by Adsorption on Alumina" Helvetica
Chemica Acta. Vol. 30, No. 2 , 1947, pp. 453-463.
26. Geering, H.R., Gary, E.E., Jones L.H.P., and Allaway, W.H. Solubility and Redox
Criteria for the Possible Forms of Selenium in Soils. Soil Sci. Soc. Amer. Proc.
32: 35-40, 1968.
143
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27. Stumm, W. and Morgan, 3.3. Aquatic Chemistry; An Introduction Emphasizing
Chemical Equilibria in Natural Waters, Wiley-Interscience, Ne,w York, 1970 593
pp.
28. EPA "Estimating Water Treatment Costs: Volume 2, Cost Curves Applicble for
1 to 200 mgd Treatment Plants." EPA-600/2-79-1626, U.S. Environmental
Protection Agency, Cincinnati, Ohio 1979, 506 pp.
29. Trussell, R.R., "A Summary of Fluoride Removal Technlogy" presented at the
National Defluoridation Meeting, November 8, 1977, Dallas, Texas.
144
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APPENDIX A
MATERIALS AND INSTRUMENTS
All of the analytical instruments that were used have been described in
Chapter 4, except for an Orion pH meter. A list of materials includes:
1. Three Cole-Palmer peristaltic pumps, model #7015.
2. Three variable-speed motors with speed controls, 1/30 hp with an output
gear ratio of 12:1 and a maximum speed of 290 rpm, distributed by Minarik
Electric Company, Los Angeles.
3. Three automatic samplers, Wastewatcher II, with 24-200 ml conventional
polyethylene sample bottles each, manufactured by Raymond Jensen
Company, Los Angeles.
it. One six paddle stirrer, manufactured by Phipps and Bird, Inc., Richmond,
Virginia.
5. Three 15-inch glass columns of 1-inch inside diameter. Fitted with ground
glass joints with ball and socket clamps and stopcocks.
6. Three conventional, nonlinear polyethylene tanks, 55 each.
7. Tygon tubing, 3/16 inch I.D., 3/8 inch O.D.
8. Teflon tubing, 3/16 inch I.D., I/* inch O.D.
9. Swagelock fittings, 1/4-inch.
145
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10. Polyethylene sample containers (1- and 4-liter), "Qubetainers"
manufactured by Hedwin Corporation, Laporte, Indiana
11. ALCOA activated alumina, type F-l, 14-28, 28-48, and 48-100 mesh.
12. Reagent grade concentrated HC1, H-SO^,, and HNO, and 50 percent
NaOH.
146
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO
EPA-600/2-80-153
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
SELENIUM REMOVAL FROM GROUND WATER USING
ACTIVATED ALUMINA
5. REPORT DATE
August 1980 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
R. Rhodes Trussell, Albert Trussell, Peter Kreft
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
James M. Montgomery Consulting Engineers, Inc.
555 East Walnut Street
Pasadena, California 91101
10. PROGRAM ELEMENT NO.
C110 1CC824 Task 50
11. CONTRACT/GRANT NO.
68-03-1515
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory—Cin.,OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Research 8/78 - 6/80
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Richard P. Lauch (513) 684-7467
Laboratory studies were performed to determine optimum conditions for
using activated alumina to remove selenium from drinking water supplies. Column
tests showed that the capacity of alumina for Se IV decreased as influent pH
increased. Best removal of Se IV occurred at pH 5 where 1200 bed volumes of water,
with influent concentration of 0.2 mg/L, were treated before breakthrough (Se
cone. >0.01 mg/L) occurred. Optimum regenerant was 1.5 lbs/ft3 of 0.5% NaOH at
1/2 gpra/ft* upflow and 0.7 lb/ft3 of 0.25% H,S04 at 1 gpm/ft^ downflow. The
capacity of alumina for Se VI decreased as either pH or sulfate concentration
increased. Removal costs would be three to four times higher if Se VI is the
predominant form of selenium present in water supply.
Tests showed that the flourometric technique can be used to determine Se IV
concentrations. Atomic Absorption Spectroscopy remains the best method for total
selenium and Se VI plus organic selenium concentratiions can be determined by
difference.
Information from this project should be useful to research and design
engineers, and state personnel concerned with the removal of selenium from ground
water.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Potable Water
Water Treatment
Adsorption
Selenium Removal
Activated Alumina
13B
21. NO. OF PAGES
159
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
Unclassified
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (R.v. 4-77)
147
U.S. GOVERNMENT PRINTING OFFICE: 1980--657-165/0104
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