United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/074 Aug. 1985
Project Summary
Selenium Removal from
Drinking Water by Ion
Exchange
James E. Maneval, Gerhard Klein, and Jelena Sinkovic
Strong-base anion exchangers re-
moved selenate and selenite ions from
drinking water. Because selenium
species are usually present at low con-
centrations, the efficiency of removal is
controlled by the concentration of the
common drinking water anions, the
most important being sulfate. The ion-
exchange behavior of selenate was
identical to that of sulfate; the behavior
of selenite was found to be similar to
that of nitrate. The local-equilibrium
theory for ion-exchange columns pro-
duced good results in predicting
selenium-removal capacities.
Two alternative methods of selenium
removal were also investigated. At-
tempts to find reagents, compatible
with water treatment, that were capa-
ble of reducing selenate to selenite (for
which there are selective removal
methods) were unsuccessful. Screen-
ing experiments showed that a weak-
acid cation exchanger in the ferric form
selectively removed selenite from
water containing the common
drinking-water anions.
This Project Summary was devel-
oped by EPA's Water Engineering Re-
search Laboratory, Cincinnati, OH, to
announce key findings of a research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The health effects of selenium are a
subject of current medical research. In-
stances of toxicity as well as nutritional
deficiency caused by this element have
been reported. However, as is the case
with many substances required by the
body in trace amounts, the level at
which selenium becomes toxic has yet
to be determined conclusively. Until the
role of selenium in human health has
been more clearly defined, the Federal
Government has, as a precaution, set a
maximum contaminent level of 0.01
mg/L selenium on all waters to be used
for drinking purposes. To comply with
this limit, selenium removal is required
and suitable methods should be devel-
oped to accommodate the wide varia-
tion in the chemical forms of selenium
and in the composition of the waters
containing them.
Selenium pollution is not a wide-
spread problem. The relative amount of
the element in the crust of the earth is
small, and deposits rich in selenium are
not common. Industrially, selenium is
recovered as a by-product of copper-
refining. Nevertheless, in some areas,
selenium-rich soils and deposits result
in selenium groundwater concentra-
tions several times in excess of the Fed-
eral limit. Because selenium pollution
cases are often local and relatively
minor in scale, removal processes
should be simple in design and opera-
tion as well as reliable, economic, and
specific.
Past efforts to develop selenium re-
moval methods have been moderately
successful. Conventional trace-metal re-
moval methods (such as coagulation
and precipitation with lime, alum, or
iron salts) do not always remove
enough selenium to satisfy the Federal
maximum concentration limit. This is
due, in part, to the slight solubility of
calcium, aluminum, and iron salts of
selenite and selenate, the predominant
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forms of selenium in drinking water. A
study by the U.S. Environmental Protec-
tion Agency in 1 980 concluded that acti-
vated alumina could remove selenite ef-
fectively from drinking water but that
selenate removal was limited by the
presence of other anions (most notably
sulfate). Reverse osmosis was found to
be effective but not specific for sele-
nium removal. Some studies have
shown that ion exchange is a feasible
method of selenium removal, but the
results are not a sufficient basis for the
design of a removal process.
Discussion
This study focussed on the ion-
exchange equilibrium behavior of sele-
nite and selenate in relation to common
drinking-water anions (sulfate, chloride,
bicarbonate, and nitrate). Much useful
design information can be obtained by
determining the preference of an ex-
changer for selenate and selenite rela-
tive to these anions. The specific equi-
librium model that was used was the
constant-separation-factor (CSF)
model. The utility of this model for rep-
resenting ion-exchange equilibrium be-
havior has been well established.
For Type 1 and Type 2 strong-base
exchangers, the following selectivity se-
quence (which can be quantified by the
CSF model) holds at a total concentra-
tion of 0.005 N (drinking-water concen-
tration):
> NOg > Cl >
where "A>B" signifies preference by
an exchanger for Ion A over Ion B. Since
Type 1 and Type 2 exchangers are com-
monly used in small-scale water treat-
ment (e.g., water softening), knowledge
of the position of selenite and selenate
in this sequence is needed to evaluate
the applicability of the exchangers to se-
lenium removal.
Batch equilibrium experiments were
conducted to determine the necessary
parameters for the CSF model. The re-
sults lead to the following selectivity se-
quence:
SO? = SeOj > NOa >
> C\~ >HCC>3
Because selenate is identical to sulfate,
selenate removal can be predicted read-
ily from sulfate removal. Because sul-
fate concentrations are normally much
higher than selenate concentrations in
drinking water, selenate removal will be
governed by sulfate removal.
Selenite, on the other hand, is less
preferred by the exchangers than both
sulfate and nitrate. Thus, both sulfate
and nitrate will limit selenite removal
efficiency by strong-base resins.
To confirm the results of the equilib-
rium experiments, laboratory column
runs were carried out with Dowex 2x8*
(chosen to represent the strong-
base resins). Feeds contained sulfate,
chloride and bicarbonate in addition to
selenate. Predictions of effluent-
concentration histories, using the local-
equilibrium theory for ion-exchange
columns, compared well with experi-
mental results.
Reduction of selenate to selenite with
mild reducing agents was unsuccessful
due to low concentrations of selenate
encountered in drinking water. Though
both the Zn - 2n + + and S03 - SOJ
redox couples could theoretically ac-
complish the transformation, kinetic-
ally, they could not. The only reagent
examined that could achieve the reduc-
tion was 4N HCI. Even this required 30
minutes boiling time for complete re-
duction. This cannot be considered a
practical method.
Finally, by taking advantage of the ex-
tremely low solubility of ferric selenite,
a novel method of selenite removal was
developed. Ferric ion was placed on a
weak-acid cation exchanger and the
resin loaded into a column. A solution
of sulfate, chloride, and selenite was
passed through the bed. However,
breakthrough of selenite occurred at 16
bed volumes (more than an order of
magnitude lower than might be ex-
pected for ion exchange), and no effort
was made to intensively investigate this
process.
Conclusions
1) Ion exchange is capable of remov-
ing selenium anions (selenate and sele-
nite) from drinking water. The effective-
ness of this method depends on the
composition of the water. Specifically,
for the strong-base resin Dowex 2x8 at
representative drinking-water concen-
trations (around 0.005 N),
• selenate removal is limited only by
the presence of sulfate, as the ion
exchange behavior of selenate was
found to be identical to that of sul-
fate, and
• selenite removal is limited by the
presence of the sulfate and nitrate
anions, which the resins prefer to
selenite.
2) Reducing agents suitable for water
treatment do not homogeneously re-
duce selenate to selenite. Under ambi-
ent temperature conditions, neither sul-
fite ion nor zinc metal achieved any
reduction of low selenate concentra-
tions. Boiling 4N hydrochloric acid was
the only agent examined that accom-
plished the transformation.
3) Local-equilibrium analysis of
fixed-bed ion-exchange columns aided
in the interpretation of experimental re-
sults and was effective in predicting se-
lenium removal capacities. It provided a
theoretical basis on which the relative
merits of the ion-exchange process
could be evaluated.
4) A weak-acid cation exchanger in
the ferric form selectively removed sele-
nate from water that also contained sul-
fate and chloride. Only preliminary
screening experiments were carried out
to test this method.
The full report was submitted in fulfill-
ment of Agreement CR-810254 by the
Water Thermal and Chemical Technol-
ogy Center, Richmond, CA, under the
sponsorship of the U.S. Environmental
Protection Agency.
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
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J. E. Maneval, G. Klein, and J. Sinkovic are with Water Thermal and Chemical
Technology Center, Richmond, CA 94804.
Richard P. Lauch is the EPA Project Officer (see below).
The complete report, entitled "Selenium Removal from Drinking Water by Ion
Exchange," (Order No. PB 85-216 653/AS; Cost: $10.00, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
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EPA
PERMIT No G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-85/074
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