United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-190 Jan. 1985
Project Summary
Speciation of Selenium
in Groundwater
J. A. Oppenheimer, A. D. Eaton, and P. H. Kreft
A study was conducted to investigate
ion chromatography (1C) for determin-
ing selenium species in groundwaters.
Analytical speciation of selenium was
investigated because the removal effi-
ciencies of various processes for seleni-
um removal (activated alumina in par-
ticular) depend on which species (Se IV
or Se VI) is predominant in the ground-
waters.
In addition to investigating 1C, the
study aimed at determining a preserva-
tion technique for maintaining species
integrity and at analyzing field samples
taken from seleniferous regions to
assess which of the two selenium
species predominates.
Results showed that 1C is suitable for
selenium (VI) analysis in groundwater
samples if the sample is pretreated with
a Ba(OH)2 dose to remove the sulfate
interference. The selenium (IV) concen-
tration can then be determined as the
difference between the total selenium
concentration (as determined by
furnace atomic absorption spectros-
copy) and the selenium (VI) value as
determined by 1C. The detection limit
for selenium (VI) in groundwater
samples treated with Ba(OH)2 is approx-
imately 0.020 mg/L. 1C should there-
fore determine the predominant seleni-
um species in a groundwater if the total
selenium concentration is above 0.050
mg/L.
The analysis of field samples did not
conclusively determine the predomi-
nant selenium species because only
30% Of the collected samples had total
selenium values above 0.050 mg/L.
Neither species predominated in the
samples that were analyzed.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory. Cincinnati. OH, to announce
key findings of the research project that
is fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
The total selenium concentration in
potable water is Federally regulated not
to exceed 0.01 mg/L because of potential
health effects. Many groundwater
supplies in seleniferous areas of the
country such as South Dakota, Colorado,
Oklahoma, Kansas, Wyoming, and south-
ern California contain selenium levels
considerably above the Federal limit. An
effective removal technology is required
if these drinking water supplies are to
meet the standard.
Activated alumina was investigated as
a selenium removal technique in earlier
studies sponsored by EPA, but its removal
efficiency depends on the selenium
species present. Alumina effectively
removed selenium in the +IV valence
state (selenite) but not in the +VI valence
state (selenate). The difference is due to
the lower absorption affinity of Se(VI) and
its greater susceptibility to interferences
from other anions. Earlier investigations
of other treatment processes, including
coagulation with alum or ferric sulfate,
revealed the same poor removal of Se(Vll.
Ion exchange preferentially removes
sulfate over Se (IV) and removes sulfate
and Se (VI) equally. Reverse osmosis is
effective for removing both species of
selenium, but the operational cost is too
high for routine applications.
The predominant selenium species in
groundwater supplies is not well
characterized because the element is
regulated as total selenium, and the EPA-
approved analysis method is atomic
absorption spectroscopy (AAS), which
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cannot differentiate between species.
Determination of the predominant
selenium species in groundwater
supplies containing significant amounts
of selenium is needed to assess the
feasibility of selenium removal by activa-
ted alumina.
There is a vast range of documented
methodologies for determining Se (IV),
but none of the procedures are straight-
forward. All require organic solvent
extractions or preconcentration schemes
to lower the detection limit. Furthermore,
none of these methods is capable of
direct determination of Se (VI), which can
only be determined by the difference
between Se (IV) and total selenium con-
centrations. The 1C technique is rapid and
simple, requires no sample pretreatment
that could alter speciation, and is capable
of detecting both selenium species simul-
taneously. Refinement of the method is
required, however, to achieve adequate
sensitivity to detect selenium species at
the /ug/L level and to eliminate interfer-
ences from other anions.
This study evaluates 1C as an analytical
method for determining selenium species
in groundwaters. Optimization of
instrumental sensitivity and selectivity
for the selenium species was
investigated by varying instrumental
parameters and, if required, establishing
initial pretreatment schemes to eliminate
interferences. Sample preservation
techniques were also investigated to
develop a scheme that would preserve
the integrity of the selenium species.
Finally, selected groundwater samples
from seleniferous regions were analyzed
to determine which selenium species, if
any, predominates in the environment.
Materials and Methods
The 1C used in this study was a Dionex
Model 16* with conductivity detection.
Hardcopy output was collected on a
Pederson recorder, and peak quantifica-
tion was performed manually by measur-
ing peak heights. Peak heights were
measured manually rather than by com-
puterized peak integrations because of
the difficulty in setting adequate integra-
tion baselines for poorly resolved peaks.
Table 1 lists the standard instrumental
operating conditions and columns that
were used.
Selenite and selanate standards of 25
and 50 ug/L were analyzed by 1C under
standard operating conditions to
determine retention times and
Table 1. Standard Instrumental Operating Conditions
Instrument Parameter
Operating Condition
Columns
Eluent
Eluent Flow Rate
Sample Loop Size
Detector Sensitivity
Recorder Sensitivity
4- x S-mm Fast-Run (ASS) Pre-Column (P/N 030831)
4- x 250-mm Fast-Run (AS3) Separator Column
(P/N 030831)
Fiber Suppressor Column (P/N 35350)
0.003 M NaHC03/0.0024 M NaiCOa
2.07 mL/min
600 (iL
0.3 umho/full scale
100 mV/full scale
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
sensitivities. A mixed 25 ug/L selenite,
25 ug/L selenate standard was made up
fresh each analysis day and run by 1C to
determine any any significant change in
peak height for either species. The peak
height values obtained for the selenite
and selenate standards over the duration
of the project were recorded.
Due to slight instrument variations, the
peak heights for the selenite a nd selenate
changed slightly from day to day.
Evidence of contamination or degenera-
tion of a stock would appear as a steadily
increasing or decreasing peak height
value more than 2 o- away from the mean.
No such evidence was observed during
the duration of the project.
The selenite and selenate stock
standards were prepared with reagent
grade Na2Se03 and Na2Se04 and initially
verified by graphite furnace AAS. The
barium hydroxide used in sample
pretreatment and the tested preserva-
tives were all reagent grade.
Resolution Study
Table 2 presents retention times,
instrumental sensitivities, and detection
limits for selenite, selenate, and some of
the commomn anions that should be
present in groundwaters at significant
levels. The selenite and selenate detec-
tion limits of 8 ug/L in deionized water
are only marginally acceptable in light of
EPA's current maximum contaminant
level (MCL) of 0.01 mg/L for total
selenium, but investigation of concentra-
tion methods such as column loading
could lead to adequate detection limits.
Most groundwaters containing
selenium levels above 10 ug/L also
contain sulfate levels ranging from 100 to
1000 mg/L. This indicates a probable
interference problem between the
selenate and sulfate peaks, which was
verified by running mixed selenate-
sulfate standards. A 10-//g/L selenate
and 100-mg/L sulfate standard produced
a detectable selenate peak, but resolution
was too poor for reliable quantification. A
similar interference problem was
observed for selenite in the presence of
chloride.
Instrumental parameters were varied
as a means of achieving adequate
resolution between the selenate and
sulfate peaks. To increase column
efficiency, the eluent flow rate was
reduced from 2.1 mL/min to 1.3 mL/min,
but this had no effect on resolution. Use.
of two ASS separator columns connected
in series to increase column capacity also
had no impact on resolution. Use of a
weaker eluent to improve resolution was
not investigated because the selenate
and sulfate are such similar species that
they both would be affected to the same
degree.
Chemical pretreatment, by adding
Ba(OH)2, was investigated as a means to
eliminate the sulfate interference. Adding
a barium salt should precipitate sulfate
without precipitating selenate because of
the much lower solubility product of
BaSO4 and the much lower concentration
of selenate in groundwaters. To
empirically test the efficacy of barium
addition, a set of selenate-sulfate
standards were analyzed with and
without the added barium. Stoichiometric
addition of barium to match the sulfate
concentration effectively improved
resolution between a 25 ug/L selenate
and 100 mg/L sulfate standard to the
point where the selenate could be
quantified. Barium addition in excess of
the sulfate concentration resulted in loss
of some of the selenate; this indicates
that barium addition is effective only
when the dose is stoichiometrically
matched to the sulfate concentration. The
speciation was not altered after barium.
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Table 2. Retention Times, Sensitivities, and Detection Limits for Selenite, Selenate, and
Other Common Anions
Anion
Selenite
Selenate
Chloride
Nitrate
Sulfate
Retention Time
(minutes)
4.6
11.8
2.4
6.6
8.4
Sensitivity
(limho/ppb)
0.0013
0.0013
0.012
0.012
0.002
Detection Limit
(ppb)
8
8
1
1
5
as hydroxide, was added to a mixed
selenite-selenate standard and the pH
increased from 6 to 12.
Preservation Study
The optimum preservation method for
maintaining selenium speciation is not
known. Numerous studies have been
published but the results are contradic-
tory; many studies used unrealistically
high selenium species concentrations
that were orders of magnitude above
those found in the environment. A
preservation study was run to find an
adequate method to maintain
speciation—one that wouldn't interfere
with the 1C analysis.
Mixed selenite-selenate standards (25
ug/L each) were stored in 125-mL and
500-mL glass and polyethylene
containers. A variety of preservatives
were added and monitored for a 3-week
period: reducing agents were tested to
see whether they would prevent
oxidation of Selenite to selenate because
groundwater samples originate from a
much more anoxic environment;
although pH reduction could cause
species conversion, it was evaluated
because it is the standard preservation
technique for total selenium; a biological
inhibitor was investigated to determine
whether biological activity in a field
sample could be removing selenite or
selenate.
Perchloric acid addition was eliminated
because the perchlorate peak
overshadowed both the selenate and
selenite peaks. The integrity of the mixed
selenite-selenate standards was
maintained over the 3-week period for all
of the preservation conditions except for
nitric acid addition, which converted all
the selenate to selenite.
Field Sample Data
Twenty-one field samples from Kansas,
South Dakota, Colorado, and Oklahoma
were received. Since all the preservation
methods appeared comparable, the field
samples were collected in polyethylene
containers. An aliquot was transferred to
a 60-ml polyethylene bottle and acidified
to a pH <2 with nitric acid for total
selenium analysis by graphite furnace
AAS. The remainder of the sample was
refrigerated before analysis and then
analyzed for pH, alkalinity, sulfate, chlo-
ride, selenate, and selenite (Table 3).
The analytical results are given in
Table 4. The total selenium concentra-
tions in these samples ranged from 620
//g/L to <1/yg/L, but 60% of the samples
were below 20/ug/Land 70% were below
50 //g/L. Only 8 of the 21 samples had
total selenium concentrations high
enough to be detected by ion
chromatography. Only one of these eight
samples gave a resolvable peak for
selenite as well as selenate. The
summation of the selenite and selenate
concentrations obtained by 1C (141 /ug/L)
was in excellent agreement with the total
selenium value of 150 /ug/L obtained by
graphite furnace AAS. Selenate was
detected in two other samples, but
selenite could only be determined as the
difference in value between the total
selenium and selenate since the chloride
peak interfered. Selenate was not
detected in the remaining samples
because the concentration was probably
below the matrix detection limit. In the
three samples containing selenate/
selenite, the distribution was 50%
selenate/50% selenite for two samples
and 70% selenite/30% selenate for the
third.
Conclusions and
Recommendation
IC with conductivity detection lacks
sufficient sensitivity and resolution to
Table 3. Analytical Methods Used in Analysis of Field Samples
determine selenite and selenate at levels
near the current EPA MCL of 0.01 mg/L.
Although the instrument detection limit
is 8 A/g/L for each species, the matrix
detection limit is higher because of
interferences from sulfate and chloride.
Dosing samples with Ba(OH)2 stoichio-
. metrically matched to the sulfate concen-
tration effectively removes the sulfate
interference and allows quantification of
selenate, but the selenate-matrix-depend-
ent detection limit is somewhere between
15 and 25 A
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Table 4. Analytical Results for Field Samples
Total
Sample Name
A(KS)»
B(KS)
C(KS)
D(KS)
E(KS>
F(KS)
G(KS)
H(KS)
KCO)
J(CO)
K(CO)
L(CO)
M(COJ
N(COl
OfCOJ
P(OKJ
CtfOK)
R(OK)
SfOK)
T(SD)
UtSDjd
Selenium
(M/L)
150
80
30
60
10
<1
16
12
620
60
19
10
17
10
15
10
17
8
70
19
30
Sulfate
(mg/L)
440
200
129
320
133
172
480
79
990
485
168
166
173
178
162
77
53
81
145
177
870
Chloride
(mg/L)
19
37
57
119
25
482
18
28
191
110
49
61
58
46
65
165
5.9
95
160
11
254
Alkalinity
(mg/L CaCO3 )
270
230
270
320
40
240
300
290
290
310
320
165
280
220
220
250
320
340
380
320
210
pH
(Units)
7.7
7.9
7.7
7.3
7.4
7.9
7.6
7.8
7.6
7.4
7.9
c
7.8
7.9
7.7
8.0
8.8
7.6
7.7
7.9
7.7
Se/VI)
(H9/L)
35
44
ND
ND
NA
NA
ND
NA
342
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
Se(IV)
(V9/L)
106
NDb
ND
ND
NA
NA
ND
NA
ND
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
NA
a KS =Kansas
CO =Colorado
OK =0klahoma
SD =South Dakota
b ND = Not Detected
\/A = Not Analyzed
c Insufficient sample for analysis
d Traces of chlorine in raw sample
J. A. Oppenheimer, A. D. Eaton, and P. H. Kreft are with James M. Montgomery.
Consulting Engineers. Inc., Pasadena, CA 91101.
Richard Lauch is the EPA Project Officer (see below).
The complete report, entitled "Speciation of Selenium in Groundwater," (Order
No. PB 85-125 979; Cost: $8.50, 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
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