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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