United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati, OH 45268
 Research and Development
 EPA/600/S2-86/031  May 1986
 Project  Summary
 Selenium  Oxidation  and
 Removal  by
 Ion  Exchange

 Joan V. Boegel and Dennis A. Clifford
  Strong-base anion exchange was
evaluated as a process for the removal of
trace quantities of selenium from ground-
water. The efficiency of the process was
found to depend on the oxidation state of
selenium, with the selenate anion, Se(VI),
having  a greater affinity for the  anion-
exchange resin than selenite or biselenrte
anions, Se(IV). Bench-scale experiments
with an Ion-exchange column compared
the effluent concentration histories for
synthetic groundwater solutions con-
taminated with 100 ppb of either Se(IV)
or Se(VI). The resin bed capacity for
selenium removal was 55% greater with
Se(VI). Since Se(IV) is less preferred than
the major groundwater anion, sulfate,  it
was eluted from  the column with  a
chromatographic concentration peak 5.4
times its feed concentration. Se(VI) con-
centration in the effluent never exceeded
its feed concentration.
  An analytical method was developed to
measure the concentrations of Se(IV) and
Se(VI) (i.e., to determine the speciation of
selenium)  at  trace  concentration  in
groundwater.  This  method uses anion
exchange at pH  1.5  to  separate un-
dissociated selenious acid (Se[IVl), which
passes through the column, from selenate
and biselenate anions (Se[VI]), which are
retained by the resin.  Graphite furnace
atomic absorption spectroscopy (GFAAS)
is used to measure total selenium in an ali-
quot of the original groundwater sample
and Se(IV) in the anion-exchange column
effluent. Se(VI)  is  determined by
difference.
  Using this new analytical method as  a
tool, the oxidation of Se(IV) to SE(VI) at
trace concentration in groundwater was
studied. The study found that free chlorine
 can oxidize Se(IV) to Se(VI) in ground-
 water. In synthetic groundwater contain-
 ing sulfate, chloride, and bicarbonate at pH
 8.3, the reaction is first order in both
 Se(IV) and free chlorine concentrations,
 and it can  be described by the following
 rate expression:

        —rge =  — dCse/dt =
    (0.21  L/(min-mgCI2))(CC|)(CSe)

 The pH dependence of the reaction was
 examined in the pH range of 5 to 10. A pH
 optimum exists between  6.5 and 7.5 in
 which nearly 70% of the Se(IV) is con-
 verted to  Se(VI)  within  5  min with a
 chlorine dosage of 5 mg/L. Oxygen was
 completely ineffective as an oxidant  for
 Se(IV), and potassium permanganate and
 hydrogen peroxide were much less effec-
 tive than chlorine.
  This Project Summary  was developed
 by EPA's  Water Engineering Research
 Laboratory, Cincinnati, OH. to announce
 key findings of the research project that
 are fully documented in a separate report
 of the same title (see Project Report order-
 ing information at back).
Introduction
  The  U.S. Environmental  Protection
Agency (EPA) National Primary Drinking
Water Regulations of 1976 established
maximum contaminant levels (MCL's) for
10 inorganic contaminants: barium, cad-
mium, chromium, lead,  mercury, silver,
fluoride, nitrate,  arsenic, and selenium.
Public  drinking  water supplies  con-
taminated  with any  one of  these
substances in excess of the MCL must be

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treated  to  remove the  contaminant or
abandoned.
  The MCL for selenium was set at a very
low  level  (0.01  mg/L)  because  of
selenium's  suspected  carcinogenicity.
Selenium contamination of groundwater
is fairly common in some regions of the
United States, but its removal from these
water supplies is not  yet practiced. Thus
a continuing need exists for research to
evaluate treatment processes for selenium
removal. This  study is based  on  an
understanding of selenium chemistry and
complements  the work of previous  re-
searchers.  The study focuses on  the
following two-step process for removal of
selenium from groundwater:

  1.  Oxidation of all  aqueous  selenium
     to selenate  anion,  Se(VI), and

  2.  Strong-base anion-exchange remov-
     al of the selenate anion.

Experimental Details
  The study described in this  report in-
cluded three  phases: (1)  evaluation of
anion exchange for removal of both Se(IV)
and Se(VI) from groundwater, (2) develop-
ment of a method for the separate analysis
of Se(IV) and Se(VI) at trace concentra-
tion in groundwater, and (3) determination
of the preferred oxidant and  optimum
reaction conditions for the oxidation of
Se(IV) to Se(VI).

Evaluation of Anion Exchange
for Removal of Selenite and
Selenate
  Ion chromatography (1C) was used to
provide a rapid determination of the posi-
tions of selenite and selenate anions in the
selectivity  sequence  of a strong-base
anion-exchange resin. A Dionex Model 16
Ion Chromatograph * (Dionex Corporation,
Sunnyvale,  California)  was  used for all 1C
work. The eluent used was the standard
carbonate buffer consisting of 0.003 N
NaHCO3 and 0.0024 N Na2C03 with pH
10.4. Standard anion  solutions were
prepared from reagent-grade sodium salts
of the anions fluoride, chloride, bromide,
nitrite, nitrate, phosphate, sulfite, sulfate,
selenite, and selenate.
  Ion-exchange column runs were used to
generate  experimental breakthrough
curves  for  selenite  and selenate in a
background of highly saline  synthetic
groundwater.  Figure   1  shows the  ap-
         Metering Pump,
           0.9 mL/min
                Ion-
              Exchange
               Column
   Feed Reservoir
       1.0L
1

\
4.5-mL Resin
5.33-mm I.D.
                           Fraction
                           Collector
Figure 1.    Experimental set-up for the ion-exchange column runs.
 'Mention of trade names or commercial products does
 not constitute endorsement or recommendation for
 use.
paratus used.  A variable-speed positive
displacement pump (Milton Roy) was used
to pump the feed water at 0.9 mL/min
from a glass reservoir bottle through a col-
umn packed with Rohm and Haas IRA-458
strong-base, acrylic anion-exchange resin.
The  column  was constructed  from
6.35-mm  (0.25-in.) O.D.  stainless steel
tubing with an I.D. of 5.33 mm (0.21 in.).
The resin bed volume was 4.5 mL and the
empty bed contact time (EBCT) was 5.0
min. Effluent from the  column was col-
lected in  glass test tubes held in the
carousel of an Eldex  universal fraction
collector.
  The multicomponent,  ion-exchange col-
umn runs  required chemical analyses for
pH, bicarbonate, chloride, sulfate, selenite,
and selenate.  Methods used for these
analyses were  pH electrode, Beckman in-
frared TOC analyzer (inorganic  carbon
channel) for bicarbonate, potentiometric
titration with  AgNO3  for chloride, ion
chromatography for sulfate, and GFAAS
for both selenite and selenate.
Development of Method for
Se(/V/Se(VV Separation and
Analysis
  The method developed to determine the
speciation  of selenium in groundwater
uses ion exchange for the separation of
Se(IV) from Se(VI) at low pH and GFAAS
for analysis of selenium in both the original
sample and the ion-exchange column ef-
fluent. Determination of optimum pH for
the ion-exchange separation involved  a
series of separation experiments at dif-
ferent pH's within the range of 0 to 2.5.
  Figure 2 shows the apparatus used for
ion-exchange separation  of Se(IV) and
                              125-mL Separatory Funnel
                              100-mL Sample
                                Gum-Rubber Tubing
                              Resin Column

                              0.8-cm I.D. x 11.5-cm Length
                               10-cm Resin Bed Depth
                              S.O-mL Resin
                           Press Fit Cap With Porous Pye Frit

                         	  Gum-Rubber Tubing

                             Flow-Restricting Capillary
                             0.20-mm I.D. x 3.5-cm Length
                                   125-mL
                              Polyethylene Bottle
                 Figure 2.    Ion-exchange apparatus for
                            separation ofSe/IV) from Se/VI).
                 Se(VI). The ion-exchange separation col-
                 umn was a glass tube filled with 5 mL
                 (wet, settled volume) of IRA-458 anion-
                 exchange resin in chloride form. Before
                 using a fresh or freshly regenerated resin

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,  column, an HCI solution of the desired
  separation pH was passed through the col-
  umn at about 10 mL/min to adjust column
  pH. After each separation test, the column
  was regenerated with 1.0  N NaCI or 0.5
  N HCI.
    For each experiment, three 5-mL ion-
  exchange columns were adjusted to the
  specified pH, and test solutions (adjusted
  to the same specified pH by dropwise ad-
  dition of HCI) were passed through each
  column. The three test solutions, desig-
  nated A, B, and C, were standard solutions
  of selenate or  selenite  or both in  a
  383-mg/L IDS  synthetic  groundwater
  containing 1 meq/L (58.5  mg/L) sodium
  chloride, 1 meq/L (72 mg/L) sodium sul-
  fate,  and 3 meq/L  (252 mg/L)  sodium
  bicarbonate. Test solution A contained 100
  ppb Se(IV), B contained 100 ppb Se(VI),
  and C contained 50 ppb Se(IV) and 50 ppb
  Se(VI). Column effluents were collected,
  preserved with 5 mL concentrated nitric
  acid per 100 mL of sample, and analyzed
  for selenium by GFAAS.
    A Perkin Elmer model 5000 atomic ab-
  sorption  spectrophotometer equipped
  with  a  graphite furnace,  a  model 400
  graphite furnace programmer, and Zeeman
  background correction was used for all
  selenium analyses. A selenium electrode-
  less discharge lamp was the light source.
  Pyrolytically coated graphite tubes with
  Lvov platforms were used, and all sample
  injections were 20 uL followed by 20 ^L
  of NiNO3 matrix modifier (1000 ppm Ni).

  Study of Se(IV) Oxidation
   The test solution for all  oxidation ex-
  periments was the previously described
  383 mg/L TDS synthetic  groundwater,
  with an initial pH of 8.3 and spiked with
  100 ppb Se(IV). The Se(IV) stock solution
  was made up from sodium selenite (99%)
 from  Aldrich  Chemical Company.  Rea-
  gents used for oxidation experiments were
 sodium hypochlorite solution prepared by
 dilution  of Clorox bleach,  oxygen gas,
 hydrogen  peroxide solution prepared by
 dilution  of 30% H202, potassium per-
 manganate, and ammonium hydroxide. All
 chemicals were  reagent  grade  unless
 otherwise noted.
   For all oxidation reaction experiments in-
 volving  liquid-form  oxidizing  agents,
  100-mL aliquots of the test  solution were
 placed in 250-mL Pyrex Erlenmeyer flasks
 and stirred with Teflon-coated magnetic
 stirring bars on a six-position stirring plate.
 A  pre-measured aliquot of the  oxidant
 solution was injected into each test solu-
 tion flask with a graduated  glass pipette.
 Quenching agents were added at predeter-
 mined times in the same manner. The new
 ion-exchange separation/GFAAS method
 was used to determine the concentration
 of Se(IV) remaining in solution after each
 oxidation experiment.
   In the test of oxygen as an  oxidizing
 agent for Se(IV), pure oxygen at a supply
 pressure of 20 psi was bubbled into 200
 mL of the test solution in an open 500-mL
 Erlenmeyer flask through a porous gas-
 diffusing stone made of fused crystalline
 alumina with an average pore size of 60 p.
   The temperature was between 21 ° and
 24.5°C for all tests. For the study of pH
 dependence of selenium oxidation by free
 chlorine, the pH of the test solution was
 adjusted in the range of 5 to 10 by the
 dropwise addition of either NaOH or HCI.

 Results and Discussion

 Evaluation of Anion-Exchange
 Removal of Selenite and
 Selenate
   Figure 3 shows the multicomponent ion
 chromatogram for a dilute aqueous mix-
 ture of anions, including selenate and
selenite. The anion corresponding to each
peak is noted on this chromatogram, along
with its observed retention time. The least
preferred anion has the shortest retention
time and  appears first  in the effluent,
whereas the most preferred anion has the
longest retention time and is eluted last.
The selectivity  sequence  from  most
preferred to least preferred at the eluent
pH of 10.5 is as follows:
  The most-preferred-ion  status  of
selenate is significant in consideration of
an ion-exchange process to remove trace
amounts of selenium from groundwater
containing much larger concentrations of
sulfate. Based on this chromatogram, and
regardless of their relative concentrations,
selenate is expected to break through after
sulfate with no chromatographic peak elu-
tion of selenate. Furthermore, the break-
through  capacity  of the ion-exchange
column for selenate is expected to be
much greater than it would be if the major
 I

 I
                              Time, Minutes
Figure 3.   Ion chromatogram for selenium species in the presence of common anions. Eluent -
           0.003 M /VaHCOa/O.0024 M NaiCOa. pH = 10.5.

-------
anion,  sulfate,  were  preferred  over
selenate. Selenite's low position in the
selectivity sequence suggests that ion ex-
change is unfavorable for the removal of
selenium in the +4 oxidation state.
  Experimental breakthrough curves for
selenite and selenate in a highly mineral-
ized (712 mg/L TDS) synthetic ground-
water verified the predictions of the ion
chromatogram. Rgure 4, the breakthrough
curve for Se(IV), shows selenium concen-
tration reaching the 10 ppb MCL at about
152 bed volumes (BV's) throughput and
peaking to  540 ppb (5.4 times the feed
concentration) at 237 BV's. The column
is completely exhausted at 265  BV's,
where the  most-preferred sulfate anion
has  reached its  influent concentration.
Figure 5, the breakthrough  curve for
Se(VI),  shows selenium concentration
reaching the MCL at 235  BV's with a
gradual, presumably  mass-transfer-
controlled increase to feed concentration
at about 385 BV's. As expected from the
1C selectivity sequence,  selenate broke
through after sulfate, and the Se(VI) con-
centration in the effluent never exceeded
the feed concentration of 100 ppb. A very
significant  improvement (55%  in  this
case) exists in ion-exchange removal of
selenium when it is in the selenate rather
than the selenite form. In less  saline
groundwaters with lower sulfate concen-
trations (e.g., 50 mg/L sulfate), run lengths
exceeding   1000  BV's  to  Se(VI)
breakthrough may be expected.

Method for Sef/V)/SefVI)
Separation and Analysis
  The  basis for the ion-exchange separ-
ation of Se(IV) and Se(VI) is the difference
in acid strength of selenious (Se[IV]> and
selenic (Se[VI](  acids. The dissociation
equations and constants for the two acids
are as follows:
Se(IV)    Selenious Acid
  H2Se03 = H+ + HSe03-
  HSe03- = H+ + Se03=
                                                  Time. Mrs.
Se(VI)    Selenic Acid
  H2Se04 = H+ + HSe04-
  HSe04~  = H+ + Se04=
pK, = 2.55
pK2 = 8.15


pK, = -3.0
pK2 = 1.66
Below its pK1  of 2.55,  the weaker
selenious acid exists predominantly as the
undissociated acid that has no charge and
therefore no electrostatic attraction to the
positively charged quarternary ammonium
sites  of  a strong-base  anion-exchange
resin. Selenic acid is a much stronger acid,
comparable to sulfuric acid. Above a pH
of -3, Se(VI) exists predominantly as a

                                    4
              I
              I
              6
              I

              §
               o

               Q.
;o.o


8.0


 6.0


 4.0


 2.0
                                 JO
                                —i—
                                             20 .
                                                          30
                                                     40
                             SeflVlRun
         Resin: IRA-458
         EBCT: 5 minutes
         TDS: 712 mg/L
 Ion

~SeflVJ
 cr
 HCOa
                                           Effluent   Feed
                                           Cone.     Cone.
100 ppb
3.0 meq/L
3.0 meq/L
4.0 meq/L
8.3
                               600
                         50   100   150 200 250  300  350  400  450  500  550

                                        Bed Volumes Throughout, BV
              400


              300


              200


              700


              0
                                     \
                                     vj
                                     a
              Figure 4.
        Breakthrough curves for Run No. 1 —synthetic groundwater feed contaminated with
        lOOppbSef/V).
                                                                      100 ppb
                                                                      3.0 meq/L
                                                                      3.0 meq/L
                                                                      4.0 meq/L
                                                                      8.3
         Resin: IRA -458
         EBCT: 5 minutes
         TDS: 712 mg/L
              Figure 5.
        50   100  150  200 250 300  350 400  450  500  550

                          Bed Volume Throughput, BV
        Breakthrough curves for Run No. 2—synthetic groundwater feed contaminated with
        lOOppbSefVI).
              negatively charged ion, and Se(VI) anions
              will be attracted and held by strong-base
              anion-exchange resins. Because of their
              acid dissociation equilibria,  most Se (IV)
                                     will pass through an ion-exchange column,
                                     and most Se (VI) will be retained in the
                                     same column at any pH  between  -3
                                     (pK, of selenic acid) and + 2.55 (pK2 of   t

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selenious acid). Based on acid dissociation
equilibria alone, the optimum separation of
Se(IV) and Se(VI) would be expected at
the midpoint of this range, pH 0.225.
  Table 1  presents results of separation
experiments at five pH's: 0.6, 1.0,1.1,1.5,
and 2.1. Figure 6 plots the percentage of
recovery of selenium in  the column ef-
fluent (i.e., passage of selenium through
the column) as a function of separation pH
for test solutions A and B, prepared with
pure Se(IV) and pure Se(VI), respectively.
The  experimental  passage is compared
with that  predicted by the acid dissocia-
tion constants for Se(IV)  and Se(VI). The
best correlation of experimental results to
predicted  %  recovery occurred at a pH
equal to or greater than 1.5, and the max-
imum separation  of Se(IV) from Se(VI)
was observed at pH 1.5.
  At  lower pH, a very significant, unex-
pected recovery of Se(VI) occurred in the
column effluent. This passage of Se(VI) at
low pH could have resulted either from
reduction  of  Se(VI)  to Se(IV)  in  the
presence  of strong HCI, or unfavorable
competition between the low concentra-
tions of Se(VI) anions and the very high
chloride concentration,  e.g. 8900 mg/L
Cr at pH 0.6.
  Based on the pH screening experiments,
an optimum  separation  pH of  1.5 was
identified, and the following procedure
was adopted for the analytical technique:

  1.  Collect a 200-mL water sample.

  2.  Oivide'the sample into two  100-ml
     aliquots  —  one (A)  for total  Se
     analysis and the other (B) for Se (IV)
     analysis.

  3.  Adjust pH of sample B to 1.5 by drop-
     wise addition of 1.5 N hydrochloric
     acid.
  4.  Pass acidified sample  B through  a
     small (5-mL)  column  of chloride-
     form, strong-base anion-exchange
      resin at a flowrate  of 1.5 mL/min.
      Collect the effluent and then pass 25
      ml of deionized water through the
     same column at 10 mL/min and col-
     lect the rinse effluent. Theoretical-
     ly, 91.8% of the Se(IV) and 0% of
     the Se(VI) from the original sample
     will be recovered in the effluent and
     rinse.

  5.  Preserve all samples with 5 ml con-
     centrated nitric acid per 100  ml of
     sample.

  6.  Analyze the original water sample. A,
     and the IX column effluent and rinse,
     B, for Se by GFAAS.
Table 1.  Results of pH Screening Experiments
                                                % Selenium Recovered (% Passage)H
PH
0.6
0.6
0.6
1.0
1.0
1.0
1.1
1.1
1.1
1.5
1.5
1.5
1.5
1.5
1.5
2.1
2.1
2.1
Flow Hate
(mL/min)
10.3
9.9
8.5
1.5
1.5
1.4
9.1
11.0
9.1
11.6
11.9
12.6
1.5
1.6
1.7
11.0
11.0
11.O
Test
Solution"
Se(IV)
Se(VI)
SellWVI)
Se(IV)
Se(Vt)
Se(IV/VI)
Se(IV)
SetVI)
Se(IV/VI>
SetIV)
SetVI)
Se(IV/V/)
Se(IV)
SetVI)
SetlV/VI)
SetIV)
SetVI)
SellV/VI)
Predicted
96.91
.02
48.45
95.31
<0.01
47.65
94.64
<0.01
47.32
89.98
<0.01
44.99
89.98
<0.01
44.99
72.3
<0.01
36.15
Experimental
78.7
66.9
72.9
97.4
40.5
70.2
95.5
13.1
48.6
93.5
8.5
54.9
85.6
2.0
52.0
70.3
6.8
36.5
 *AII test solutions were prepared with synthetic groundwater containing 3 meq/L NaHCOg,
  1 meq/L NaCI, and 1 meq/L Na2SO4. Test solution Se(IV) contained 100 ppb Se(IV); Se(VI)
  contained 100 ppb Se(VI); and SetlV/VI) contained 50 ppb Se(IV) and 50 ppb Se(VI).

+ The desired selenium passages are Se(IV)  = 100%, Se(VI) = 0%, and SetlV/VI) = 50%.
    it
     t. CD
    -S! ^.
     0) _„
    CO C
          100


           90

           80


           70

           60


           50

           40

           30


           20


           10
                  Optimum Separation at pH 1.5
Predicted
SeflV)
Experimental
SeflV}
Experimental
SefVH
                       0.5
                                 1.0
                                           1.5

                                           pH
                                                    2.0
  2.5      3.0
Figure 6.
           SeflV) and Se(VI) passage through ion-exchange separation columns as a function
           ofpH.

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  7. Calculate the concentrations of total
     Se, Se(IV), and Se(VI) as follows:
     Total Se(mg/L) is determined direct-
     ly  by GFAAS analysis of sample A.
     Se(IV)(mg/L) =  (1/0.91 8) x (total mg
     Se measured in the combined IX col-
     umn effluent and rinse from sample
     B)/(0.100 L original sample B). Se(VI)
     concentration is determined by dif-
     ference — that  is, total Se concen-
     tration minus Se(IV) concentration.

  The accuracy of this technique with the
separation pH of 1.5 is verified in Table 2,
which shows a  slight tendency toward
Se(VI)  leakage. This leakage results  in
slightly high values for the reported Se(IV)
concentration. Nevertheless, the method
proved to be of adequate precision and ac-
curacy for this research.


Se(/V) Oxidation  Study
  Experiments proved ammonia to be a
satisfactory quenching  agent for the ox-
idation of Se(IV) by free chlorine in syn-
thetic groundwater. When added to a test
solution at a dosage 10 times the chlorine
dosage, ammonia completely and instan-
taneously destroyed the free chlorine. The
product of this reaction was chloramine,
which  had no tendency to react  with
either Se(IV) or Se(VI), and did not inter-
fere with the ion-exchange separation/
GFAAS  technique used to  monitor the
progress of the oxidation reaction.
  With ammonia as a quenching agent,
kinetic data were obtained for the oxida-
tion of Se(IV) by free chlorine. The results
are plotted in Figure 7. The experimental
rate expression  for  the  reaction is  as
follows:
         -rse = -
    (0.21  L/(min-mg CI2))(CC|)(CSe),


where the chlorine concentration is given
in mg/L and the Se(IV) concentration,
CSe, is arbitrary.
  The pH dependence of the oxidation
was examined in the range of pH 5 to 10.
Results are plotted  in Figure 8. The op-
timum pH is clearly between 6.5 and 7.5.
The reaction becomes very slow at both
extremes, with  less than  10% of the
Se(IV) converted to Se(VI) in 5 min at pH
5 and at pH 10.
  Experiments were designed to compare
the effectiveness of three other oxidizing
agents commonly used in water treatment
— oxygen, permanganate, and hydrogen
peroxide — with that of chlorine.  The
dosages of permanganate and hydrogen
l*g Se in
Se(IV)
10.0
0.0
5.0
1.0
2.0
Sample
Se(VI)
0.0
10.0
5.0
2.0
1.0
Total
M0 Se(IV)
Passage
9.27
0.36
4.57
1.04
1.82
pg Se(IV)
Reported
10.1
0.4
5.0
1.1
2.0
Apparent*
Se(IV)
Recovery<%)
101
100
110
100
Apparent*
Se(VI)
Leakagel%l
4
0
5
0
*The apparent values are based on the observed total Se passage and a calculation that
 presumes 91.8% Se(IV) passage.
                          Chlorine Dosage

                            1 mg/L C/2
                            2 mg/L C/2
                            5 mg/L C/2
        Symbol

          O
          A
          D
                                                          10      20       30       40      50      60
Figure 7.    Kinetics of Se(IV) oxidation by free chlorine at pH 8.3 in simulated groundwater.
peroxide were chosen to be equivalent to
2 mg/L free chlorine, a dosage that pro-
duced easily measurable Set IV) remaining
after 5- and 30-min contact times. Table
3  lists  the experimental  results.  No
quenching other than pH reduction to 1.5
before separation was used  in the  non-
chlorine oxidation experiments. Thus the
nonchlorine oxidations indicated in Table
3 may be greater than actually achieved
in the indicated contact time. Oxygen ap-
pears to be completely ineffective in the
oxidation of Se(IV). Neither permanganate
nor hydrogen peroxide is  as effective as
chlorine at near-neutral pH.
Conclusions and
Recommendations

  The two-step process of oxidation with
free chlorine followed by anion exchange
appears to be technically  feasible and
should  be considered  for  removal of
selenium to produce potable water from
selenium-contaminated groundwater. Fur-
ther research and development is recom-
mended to develop the optimum process
and to adapt the process  for optimum
operation  at specific sites.
  Some problems that must be addressed
in the next phase of process development
                                    6

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       100
                                                   2 ppm C/s
                                                   NH3 Quench
                                                   After 5 min
                                             Simulated Groundwater
                                              (Before pH Adjusted)
                                               HCOs       3 meq/L
                                               Cr         1 meq/L
                                               SOl'       1 meq/L
                                                                 I
                                                        10
                                                                 11
           56789

                                      pH

Figure 8.    Effect of pH on Se(IV) oxidation at 23°C in pH-adjusted, simulated groundwater.
Table 3.  Comparison of Oxidizing Agents*
Oxidizing                     Se(IV)
 Agent   pH   Time(min)  Remaining 1%)
02
KMnO4
KMnO4
H202
H202
NaOCI
NaOCI
NaOCI
8.3
7.5
7.5
7.5
7.5
7.5
8.3
8.3
60
5
30
5
30
5
5
30
101.9
72.1
63.5
97.4
89.8
32.0
63.5
7.0
*AII oxidant concentrations are equivalent to
 2.0 ppm chlorine — that is, a  22-fold
 stoichiometric excess for the 100 ppb Se(IV)
 initially present.
                                         and Se(VI) in actual groundwaters that are
                                         contaminated with selenium. However, the
                                         analytical technique should be refined by
                                         optimizing the acid used and the sample
                                         flow rate.
                                           The full  report was submitted in fulfill-
                                         ment of Cooperative Research Agreement
                                         No.  CR-807939  by the  University of
                                         Houston-University Park under the spon-
                                         sorship of  the U.S. Environmental Protec-
                                         tion Agency.
include determining the effects of tem-
perature  and  the  presence  of  other
chlorine demands such as TOC, Fe2+,
Mn2+, and  S2~  on  oxidation  kinetics,
countering the effects of residual chlorine
from the oxidation process on the anion-
exchange resin, determining the optimum
anion-exchange resin type and empty bed
contact time (EBCT), developing a process
control scheme that includes some means
(possibly pH  changes)  of detecting
selenium breakthrough, and disposing of
spent regenerant.
  The analytical  method  developed  to
determine the speciation of selenium in
groundwater proved to be a useful tool for
the study of Se(IV) oxidation and is recom-
mended for measurement of both Se(IV)
                                                                                •&U. S. GOVERNMENT PRINTING OFFICE:1986/646-l 16/20846

-------
      Joan V. Boegelis now with Metcalf and Eddy. Inc., Boston. MA 02114; and Dennis
        Clifford is with the University of Houston-University Park, Houston, TX 77004.
      Thomas J. Sorg is the EPA Project Officer (see below).
      The complete report, entitled "Selenium Oxidation andlRemoval by Ion Exchange,"
        (Order No. PB 86-171 428/AS; Cost: $11.95,  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
Official Business
Penalty for Private Use $300

EPA/600/S2-86/031
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