Environ. Sci. Techno/. 1995, 29, 2639-2646
Selenium Immobilization in Evaporation Pond Sediments by In Situ Precipitation of Ferric B R U C E A. M A N N I N G * A N D R . G. B U R A U Department of Land, Air, and Water Resources, Soils and Biogeochemistry Section, University of California, Davis, California 95616
Selenium (Se) in agricultural drainage and evaporation pond waters in the semi-arid Central Valley of California can threaten water quality and is toxic to wildlife. This laboratory study focused on the development of an in situ chemical treatment to immobilize soluble Se in drained evaporation pond sediments by amendment with ferrous iron, which forms ferric oxyhydroxide (FeOOH). Three sediment samples from Kesterson Reservoir, Merced County, CA, with elevated levels of Se were characterized by conventional extraction techniques and then subjected to in situ Fe(ll) amendment. Total Se in the three untreated samples ranged from 4 to 29 mg kg-’, the majority of which was chemically reduced and insoluble. Soluble (0.25 M KCI-extractable) selenite [Se(IV)] was >90% immobilized in sediment when the added Fe(1l) was 2 5 0 mmol of Fe(ll) kg-l sed-’ whereas immobilization of soluble selenate [Se(VI)] was strongly dependent on the initial Se(VI) concentration. Extraction of Se(lV) by 0.01 M KzHP04 decreased with increasing Fe(ll), indicating that Se(lV) immobilized by the FeOOH precipitation process was not accessible to the PO4 extractant. Exchangeable Se(VI) (0.01 M KzHP04 extractable) was partially immobilized in sediment depending on the initial Se(VI) concentration. Both Se(lV) and Se(Vl) were occluded within FeOOH produced during Fe(l1) oxidation and hydrolysis. Iron(ll) salt amendment may be a viable in situ remediation technique for alkaline sediments containing elevated levels of trace elements such as Se.
Introduction Irrigated agricultural practices coupled with unusual geochemical properties of soil parent materials in the western San Joaquin Valley, CA, have resulted in high concentrations of Se in subsurface agricultural drainage * Address correspondence to this author at USDA-ARS,US. Salinity Laboratory, 450 West Big Springs Road, Riverside, CA 92507.
0 1995 American Chemical Society
waters. Alluvial soils of this region are derived from marine shale parent materials of the Coast Range, which contain high levels of soluble salts and trace elements such as As, B, Mo, Se, V, and U ( 1 ) . Oxidation of selenide-containing minerals in these shales forms soluble Se042-[Sew)](24). In the studyarea, subsurfacedrainageof irrigated fields using perforated pipe removes shallow groundwater from the root zone, which contains S e w ) along with SO4’-, C1-, Na+, Ca2+,and Mg2+. During the period of 1980-1986, Kesterson Reservoir, a 520-ha evaporation pond system within the Kesterson National Wildlife Refuge, received saline drainage waters from the western San Joaquin Valley region with total Se concentrations ranging from 0.4 to 1.4 mg L-’ (2). Evapoconcentration, aquatic plant acquisition and deposition, and microbial reduction of SeW) and Se(IVI in the anoxic sedimentaryenvironment resulted in the accumulation of reduced, less soluble forms of Se such as elemental Se(O), inorganic selenide (e.g., FeSe, S,Se-, or HSe-), and organic selenide (RSeR) (5). Food chain transfer and biomagnification of Se in the aquatic ecosystem caused high mortality and deformity rates in bird hatchlings (€3, a discovery that resulted in the closure and draining of the reservoir in 1986. Low-lying areas at the site were subsequently filled with uncontaminatedsoil to prevent the formation of ephemeral pools with high concentrations of Se. The soils at the Kesterson site are now residual sediments maintained as an upland and represent an advanced case of Se accumulation in a semi-arid, alkaline evaporation pond system (5). Remediary techniquessuch as in situ chemical treatment and reactive chemical barriers have not been applied to sediments at the Kesterson site but have been investigated elsewhere in soils contaminated with metals (7) and mine wastes (8). In situ treatment with soluble iron results in the precipitation of poorly crystalline ferric oxyhydroxide (hereafter referred to as FeOOH) (9). The formation of FeOOH can incorporate other metals and metalloids by coprecipitationor adsorption, thus minimizing the potential for contaminant transport into groundwater. This laboratory-scale study investigated the attenuation of soluble Se in Se-contaminatedsediment samplesby in situ treatment with ferrous iron. Adsorption of Se on FeOOH depends on pH and the chemical speciation of Se. Seleniteexhibits a strong affinity for the surface of crystalline a-FeOOH (goethite)below pH 8 (10-12) and adsorbs on the goethite surface by a ligand exchange mechanism (13,141. The S e w ) species exhibits weaker electrostatic attraction for positively charged surfaces below pH 4.5 (10)and is the most mobile form of Se in neutral soils and alkaline sediments (15,1€3. Working with alluvial soils from the western San JoaquinValley, CA, Neal and Sposito ( 1 7 ) concluded that SeW) adsorption behavior was similar to S 0 2 - and that S e w ) was not removed from the soil solution by soil acidification. The objectives of this study were to determine ambient physical and chemical properties of Kesterson sediments that irduence Se solubility and to investigate in situ precipitation of FeOOH as a remediary measure to immobilize soluble Se. The experimental approach was to allow in situ Fe(I1)oxidation to form FeOOH in sediments.
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This process increases the ion adsorption capacity of sediment by increasing the amount of reactive FeOOH surface. Hydrolysis of Fe3+cations generates H+, which increases positive surface charge on oxide surfaces in the sediment system and neutralizes sediment alkalinity. Iron(I1)oxidationmay also couple with chemical reduction of S e w ) to Se(IV), which is more strongly adsorbed on iron(II1) oxide.
Materials and Methods Chemicalsand Glassware. All chemicals used in this study were reagent grade. Distilled water was deionized by Coming Megapure System DF deionizing cartridges. Laboratory glassware was washed by soaking for 0.5 h in hot MICRO soap solution. Articles were then scrubbed, thoroughly rinsed with distilledwater, and soaked for 2 h in 5% sulfuric acid solution. Glassware was then soaked and rinsed with distilled deionized (DI)water, dried, and stored under protective cover for later use. Kesterson Sediment Sampling. In May 1990, grab samples of surfacial sediment (0- 10 cm) from Kesterson Reservoir were collected 30 m west from the levee road in evaporation ponds 7 and 9 and 50 m south of the U.S. Bureau of Reclamation service road in pond 2. Sediment was stored refrigerated in 2-Lpolyethylene bottles. Aliquots of air-dried sediment sieved to < 2 mm were subjected to particle size analysisby a wet sieve method (18). Sediment used in all other analyses and experiments was air-dried, sieved to less than 500pm (coarsesand fraction)to promote sample homogeneity, and stored at 5 "C. Iron, Manganese, and Aluminum Oxides. Iron, Mn, and Al in '500 pm fractions of sedimentswere determined by sequential partial dissolution. For all extraction steps, 1 g of sediment was shaken with 100 mL of extractant in 250-mLpolycarbonate centrifugebottles on areciprocating shaker and centrifuged (4000g) for 15 min, and the supernatantswere then filtered. Organic matter-associated Fe, Mn, and Al were extracted with 0.1 M sodium pyrophosphate (N&P207) for 12 h (19). Amorphous iron, aluminum, and manganese oxides were then extracted from the residual sediment with 0.2 M ammonium oxalate buffer [(NH&C204/H2C204,pH 2.751 for 12 h in the dark (19). Finally, crystalline forms were extracted from sediment samples by reductive dissolution with sodium dithionite (Na2S204) and chelation with sodium citrate (Na3CsH507) (20). Iron, Al, and Mn in extractswere then determined by sequential inductively coupled plasma emission spectrometry using wavelengths of 259.940, 257.610, and 396.152 nm for Fe, Mn, and Al, respectively. Soluble Salts, pH, &, EC,and Totd Organic Carbon. Major ions, pH, and electrical conductivity (EC) were measured in 1:lO sediment:DI water (1:lO sed:DI) extracts produced by shaking 20 g of sediment with 200 mL of DI water in 250-mL polycarbonate centrifuge bottles for 4 h. The bottles were centrifuged (4000g, 10 min), and the supernatants were filtered into acid-washed polyethylene bottles and stored at 5 "C until analysis. Major cations were determined by air-acetylene flame atomic emission (Na and K) and atomic absorption (Ca and Mg) spectrophotometry on a Perkin Elmer Model 2100 spectrophotometer. Major anions (Cl-, S042-,NO3-, and HP042-)were determined by a Dionex ion chromatograph and EC by a Jenway 4010 conductivitymeter. Redox potential (Eh) was determined in saturated sediment pastes by a platinum electrode with an Ag/AgCl reference electrode. The pH 2640 rn ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL, 29, NO. 10, 1995
values of sediment pastes and extracts were determined with anAg/AgClcombination glass electrode. Total organic carbon in 0.5-g aliquots of sediment was analyzed by a LECO carbon analyzer using 0.2-g samples of sucrose to calibrate the instrument. Selenium Analysis. In all experiments, Se was determined by flow-throughhydride generation atomic absorption spectrophotometry (HGAAS)using established methods (2,5,21). Analyses were performed on a Perkin-Elmer Model 2100 spectrophotometer at a wavelength of 196.0 nrn with an electrodelessdischargelamp operatingat 4.5-5 W. Hydrogen selenide (HZSe)was stripped from solution with& (flowrate 45 mL min-1) after reaction of Se(W with sodium borohydride (NaBHd using a Varian vapor generation accessory (VGA 76) and atomized by a quartz tube heated with an acetylene flame. Reagent and sample flow rates were 1 mL min-' (0.16M NaBH4),1 mL min-I (10 M HCl), and 7 mL min-l (sample). The sensitivity of the technique was 0.5 pg L-' Se(W in sample solutions. Speciation of Se was determined by subdividing sample solutions into selective treatments for Se(W and total Se [Se(N+VI)].Selenite was determined by acidifying aliquots of sample to 4 M HCl and analyzing directly by HGAAS within 1-2 h. Solutionsof Na2Se03and Na2SeO4were used as standards. Preliminary workverified that S e w ) did not react with NaBH4 to form H*Se, and thus S e w ) was not measured by HGAAS (data not shown). To determine Se(N+VI),5-mL sample aliquots solutions and 5 mL of concentrated HCl were transferredto 50-mLgraduated glass digestion tubes along with 0.2 mL of 2% ammonium persulfate (NH4S20a).The tubes were heated in a digestion block at 100 "C for 30 min to first oxidize all Se to SeW) followed by quantitative reduction of S e w ) to Se(W. The digest was brought to volume with 4 M HC1 for HGAAS analysis. Total Se was then determined by HGAAS and S e w ) was calculated as the difference between the Se(N+VI)and the Se(IV) analyses. Selenium Sequential Extraction. Selenium speciation in Kesterson sediments was determined by a sequential extraction scheme developed by Lipton (22)and outlined in Tokunaga et al. (23). The method operationally defines Se fractions in soil or sediment by the following extraction sequence: (1)soluble Se (0.25MKCl); (2) adsorbed Se (0.01 M K2HP04);(3)carbonate-associated Se (1.0M NaC2H302, pH 5);(4)reducibleoxide-associated Se (0.1M NH20HeHCl); (5)organic Se (5%NaOCl);and (6)total residual Se (HN03/ HC104). Three grams of sediment and 0.25 g of silica gel (105-420 mm, added to minimize caking) were weighed into 40-mL polycarbonate centrifuge tubes. Each extraction step employed 30 mL of extractant, shaking,centrifugation (12500g), and filtering. Only total Se was measured in extracts by selective hydride generation after the 1.0 M NaC2H302extraction because Se speciation was altered in the 0.1 M NH20H-HCl,NaOC1, andHN03:HC104extractions, and thus the original valences of Se extracted were unknown. Method for Total Se in Sediments. A total of 500 mg of sediment was weighed into graduated glass digestion tubes, mixed with 5 mL each of concentrated HN03 and HC104acids, and allowed to stand for 1 h. The tubes were then heated in a digestion block at 150 "C for 1 h followed by slowly raising the temperature to 220 "C over 1.5h. The digestion was terminated when the digestate volume was .C 1mL. A 20-mL sample of 6 M HC1 was then added to the residne, and the mixture was heated at 95 "C for 15 min to
reduce SeW) to Se(W. The mixture was then diluted in the digestion tube to an appropriate volume for Se analysis. Determination of Elemental Se. A parallel extraction method, developed by Weres (3,was used in this study that operationallydefines Se(0). One sediment aliquot was extracted with 0.3 M sodium sulfide (NaZS), a thiophylic reagent that affects the dissolution of elemental Se as well as all aqueous-solubleforms of Se,while another sediment aliquot was extracted with a solution designed to match the pH and ionic strength of the Na2Ssolution, but which did not extract elemental Se. Elemental Se was then determined as the difference in total Se in the two extracts. The appropriate extractant was added to the sediment in 40-mL polycarbonate Oak Ridge-type centrifuge tubes, shaken for 10 min, and centrifuged (4000g), and the supernatants were transferred to polyethylene bottles containing 0.2 mL of 1.0 M NaOH. While gently being swirled, two 1-mL portions of 30% H202 were added to both sample extracts to oxidize Se to S e w ) and to destroy excess sulfide. One milliliter of 6 M HC1 was then added to the extracts to precipitate organic colloids, and the mixtures were then filtered and diluted in small volumetric containers. The solutions were then treatedwith 2% K&08 in 6 M HCl to determine total dissolved Se. The extract solutions then were analyzed by HGAAs. Experiments with Se(VI) Solutions. Typical S e w ) concentrations in drainage water entering Kesterson Reservoir were between 0.4 and 1.4 mg L-' (21,and thus a starting concentration of 0.5 mg L-' [6.3 x M Sew)] was chosen for a test solution. In centrifugetubes, solutions of 30 mL of total volume containing 6.3 x M Se(VI) in 0.1 M NaC104 with varying Fe(I1) concentrations lo-'.' M Fe) were shakenfor 28 d. In order to induce FeOOH precipitation, microliter quantities of 1 M NaHC03 were pipetted into the tubes daily. The quantity of NaHC03 added was 3 mol of NaHC03/mol of Fe, equivalent to the expected proton production during Fe(II1) hydrolysis:
After the reaction was complete, the tubes were briefly centrifuged at low speed (1300g)to minimize compaction of FeOOH particles. Supernatant solutions were filtered (0.2 pm), the FeOOH particles were rinsed once with DI water, and the rinseate was added to the solution to create extract 1. Extract 1wasthenanalyzedforSe(ni3,Se(IV+VI), Fe(II), and pH. The FeOOH particles were then resuspended in 30 mL of 0.01 M KzHPO~(pH 8). After shaking for 20 h, the tubes were briefly centrifuged, and the solution was filtered (0.2 pm) to create extract 2 (PO4-desorbable Se) and analyzed for Se(W and Se(IV+VI). The FeOOH particles were then dissolved in 4 M HC104/2M HC1. After 2 h, the mixtureswere centrifuged (12500g,10min) to create extract 3 in which Se(W and Se(IV+VI)were determined as above. Crystalline goethite was prepared by titration of 0.45 M Fe(N0313 solution with KOH to pH > 12.5 and reacting for 7 d. The solid material was washed with DI water and stored as a suspension. Aliquots of FeOOH suspension produced from both Fe(N03)3and FeS04 were dried and ground in a mortar and pestle, and the identity of both solids was found to be goethite by X-ray diffraction (XRD) analysis of random powder mounts with Cu-K, radiation.
Treatment of 1:lO s d D I Water Extracts with Solid FeOOH and Pe(1I). A total of 20 ,uL of FeOOH suspension was added to 20 mL of 1:lO sed:DI water extracts of Kesterson Reservoir sediment making the final FeOOH suspension density 28 mg of FeOOH L-' [0.31 mM Fe(II1)I. The suspension pH was adjusted with HCl and NaOH treatments to give a pH range from 2.5 to 11. The tubes were shaken for 4 h and centrifuged (12500g, 10 min), and the supernatantsolutionwas filtered through 0.5-pm syringe filters. Selenite and S e O were then determined by HGAAS. In another experiment,ponds 7 and 9 1:lO sedDI water extractsand a 0.78pM S e O standard solutionwere treated with Fe(I1) to give a range of starting concentrations from 0 to 50 mM Fe(I1). The treated sediment extracts were shakenfor 5 days while the pH of a separate reference Fe(1I) solution was monitored. After the pH of the reference Fe(11) solution had stabilized at approximately 3.0 (5 d), the tubes were centrifuged, filtered, and analyzed for Se(W and S e w ) by HGAAS. In Situ Precipitation of FeOOH in Sediments. The 500-pmfractions of Kesterson Reservoir sedimentsamples were amended with a range of FeS04treatments. In this study, FeS04 was used to treat sediments due to the predominantly sulfate-dominated anion chemistry of the sediments. Also, Fe(I1) remains soluble in alkaline conditions longer than Fe(III1, which would increase the depth of percolation if applied in the field. Aliquots of sediment (25 g) were placed in separate polyethylene film plastic bags and amended with FeS04so that the Fe(I1) concentration was varied from 0 to 0.45 mol of Fe(I1) kg-' sed-' with a final water content of 0.44 g of H20 (g of sed)-'. The sediment:Fe(II)solution pastes were then thoroughly mixed by hand and allowed to air dry to promote oxidation. The sediments were mixed at least twice a day, and the bags were covered but exposed to air throughout the experiment to ensure access to atmospheric oxygen and to minimize the formation of reducing microsites in the pastes. After 14 d, the samples had completely dried, and iron oxide formation was visibly evident. The sediments were then subjected to the first two steps of the sequential extraction scheme to determine soluble Se(W and SeW) (0.25 M KC1-extractable) and adsorbed Se(W and S e w ) (0.01M K2HP04-extractable)as outlined above. The pH of 1:lO sedDI water extracts of Fe(I1)-treatedsediments were measured to evaluate acidification from Fe(II1)hydrolysis. The identity of the iron(II1)oxide formed after oxidation of Fe(I1) from FeS04 in a separate, clean system was determined by XRD. The oxide was formed from a 27 mM Fe(I1) solution, which was allowed to oxidize for 21 d. The FeOOH precipitation was visually evident, the suspension was filtered, the filter paper dried, and the oxide ground with a mortar and pestle prior to mountingfor XRD analysis. The purpose was to compare XRD data from the iron(II1) oxideformed from FeS04 with a synthetic goethite prepared from Fe(N03)3by an established method (11).
Results and Discussion Characterizationof Kesterson Sediments. Selected characteristics of the Kesterson sediment samples are listed in Table 1. The particle size distribution of the three samples classified them as loams (ponds 2 and 9) and a sandy loam (pond 7). All samples contained moderate amounts of clay (15-25%), and the sand size fractions were the largest. The mineral soils underlying Kesterson Reservoir were classified before becoming reservoir sedimentsas Turlock sandy loam VOL. 29, NO. 10. 1995 I ENVIRONMENTAL SCIENCE & TECHNOLOGY
Physical and Chemical Characteristics of 5 was likely due to coprecipitation of S e w ) with FeOOH or encapsulation between sediment particles cemented by the FeOOH precipitate. Precipitation of ferric oxide has been shown to concentrate metals from mine tailing leachate by coprecipitation (8).
* Pond2 * Pond9 -2 -1 log Fe(I1) Added (rnolkg)
FIGURE 7. Selenate [Se(Vl)] in 0.25 M KCI extracts of Kesterson Reservoir sediment after in situ treatment with FeS04. Data have been normalized where 6 and Care Se concentrations before and after Fe(ll)treatment, respectively.
t- pnd7sDjimmt *-
log Fe(I1) added (molkg) FIGURE 8. Selenite [Se(lV)] in 0.01 M kHPO4 extracts of Kesterson Reservoir sediment after in situ treatment with FeSO,. Data have been normalized where 6 and Care Se concentrations before and after Fe(ll) treatment, respectively.
Phosphate-extractableSe(IV)data, when plotted against the amount of added Fe(I1) (Figure 81, showed that Se(IV) bound in sedimentliron oxide particles was more resistant to extraction. Selenium in the 0.01 M K2HP04 extract was >90% Se(IV),and thus S e w ) data are not shown. Increases in phosphate-extractableSe(IV)at the low Fe(I1)treatment (Figure 8) suggested that Se(W was removed from the soluble Se fraction by adsorption when Fe(I1) treatment was below mol of Fe(I1)kg-l. However, decreases in phosphate-extractable Se(IV) at Fe(I1) treatments above mol of Fe(I1)kg-l were an indication that mechanisms other than adsorption were controlling Se(IV) solubility. Using a value for the maximum surface coverage for HP042on iron oxide of 2000 mmol of P kg-l (Tmax)(301,it was estimated that increased adsorption capacity in the Fe(I1)amended sediment resulting from FeOOH would be saturated with the HP04*-anion, and thus HP042-would be expected to desorb most adsorbed Se(IV). Selenite was adsorbed by fresh FeOOH surfaces when Fe(I1) added was 57.1 mmol of Fe(I1) kg-l and probably occluded during larger Fe(I1) treatments where Se(IV)was not on a particle surface site that was accessible to the HP0d2- anion.
Conclusions The major chemical characteristics of Se in sediment samples collected from three evaporation ponds at Kester-
son Reservoir indicated that Se was primarily in reduced, insoluble forms that represented a temporary sink for Se. Reduced Se may eventually oxidize to form soluble Se(IV) and S e w ) as the site is managed as an upland environment. Kesterson Reservoir sediments were alkaline, saline, and slightly reducing with thin layers of evaporite material overlying and interspersed with the mineral soils of the Turlock soil series. The effects of FeS04 amendment on soluble Se in the presence and absence of sediment particles indicated that Se(W was readily adsorbed on the FeOOH surface. The attenuation of S e w ) by FeOOH depended on the initial S e w ) concentration. When Fe(I1)was reacted in situ with sediment, S e w ) was removed from the soluble fraction by coprecipitationwithFeOOH andlor physical encapsulation between sedimentparticles followingFe(II1)hydrolysis and FeOOH precipitation. During FeOOH precipitation, both Se(W and Se(VI) were bound in sediment particles at sites that were not available for displacementby HP02- anions when the Fe(I1) treatment was greater than mol of Fe(I1) kg-'. The incorporation of FeOOH as an in situ precipitate from Fe(I1) oxidation would be effective in attenuating soluble Se(IV)produced followingthe oxidation of reduced forms of Se. Practical problems with this remediary approach need to be addressed on a field scale and may include (a) the long-term stability of bound Se, (b) the potential displacementof soluble Se ahead of any solution in which Fe(I1) was introduced, and (c) determining costeffective application schemes.
Acknowledgments This work was supported in part by a National Institute of Environmental Health Sciences Graduate Training Grant.
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Received for review March 1 , 1995. Revised manuscript received June 14, 1995.Accepted June 14, 1995.' ES950141E Abstract published in Advance ACS Abstracts, August 1, 1995.