Selenate Reduction in an Alluvial SoilGarrison Sposito,* Andrew Yang, Rosemary H. Neal, and Adrienne Mackzum

ABSTRACTRecent studies of the mobility and solubility of Se in western San

Joaquin Valley soils suggest that this potentially hazardous elementcan be managed by controlling its oxidation-reduction reactions. Thesoluble species, SeO4, which is highly mobile and toxic, can, in prin-ciple, be reduced to SeO,, which is strongly adsorbed, or to orga-noselenium species, which may volatilize under suitable conditions.Chemical thermodynamics predicts that the reduction sequence insoils should be: NO3 — SeO4 -. MnO2 at pH >5. The objective ofthis study was to establish the position of SeO4 in the kinetic re-duction sequence for a representative western San Joaquin Valleysoil incubated in suspension with its own saturation extract. In aseries of replications of an incubation experiment, it was observedthat native NO3 (plus NO2) concentrations became undetectable after100 h in the soil suspension without O2 supply. Soluble Se, eitheradded as Na2SeO4 or indigenous to the soil, disappeared after 50 to200 h. Native soluble Mn began to rise after SO h and showed asharp increase after 100 h of incubation. Retardation of SeO4 re-duction in the presence of added NO3 was noted. The results indi-cated that, at native levels of NO3, effective microbial catalysis ofSeO4 reduction occurred in the soil under the conditions of the ex-periments, in agreement with the recent isolation of bacterial speciesthat can respire SeO4 while oxidizing organic acids typical of suboxicsoil environments.

POTENTIALLY HAZARDOUS CONCENTRATIONS ofSeO4 in the aqueous phases of soils from the west-

ern San Joaquin Valley in California have been doc-umented extensively (Fujii et al., 1988) and evaluatedin terms of irrigation water quality (Albasel et al.,G. Sposito, A. Yang, and A. Mackzum, Dep. of Soil Science, Univ.of California, Berkeley, CA 94720; and R.H. Neal, California Dep.of Pesticide Regulation, Trailer 14, Univ. of California, Riverside,CA 92521. Received 11 Dec. 1990. *Corresponding author.

Published in Soil Sci. Soc. Am. J. 55:1597-1602 (1991).

1989). Selenate poses a special hazard in the vadosezone because it is not adsorbed significantly by thealluvial soils in the region of concern (Neal and Spos-ito, 1989) and, therefore, it is readily teachable withdrainage waters (Alemi et al., 1988; Fujii et al., 1988).In a recent study of SeO4 transport through leachedcolumns of two representative agricultural soils fromthe western San Joaquin Valley, Neal and Sposito(1991) observed marked decreases in soluble Se afterdextrose starch was added to the input synthetic drain-age water. They hypothesized that, under local reduc-ing conditions in the soil columns, the presence of theadded C source effectively promoted the reduction ofSeO4 to SeO3 and/or organic forms of Se. The loss ofsoluble Se then could occur, either from the strongadsorption of SeO3 by the soils (Neal et al., 1987a,b)or from volatilization of organic Se compounds(Thompson-Eagle and Frankenberger, 1990).

These results suggested that oxidation-reduction (re-dox) reactions in soils of the western San Joaquin Val-ley can influence the mobility of Se and itsconcentration in drainage waters. The SeO4 species,which, in these soils, predominates under oxic con-ditions (Neal et al., 1987a) and is highly mobile (Nealand Sposito, 1989), gives way to SeO3 (or elementalSe and metal selenides) and organoselenium speciesunder suboxic or anoxic conditions, with a consequentloss of aqueous mobility (Neal et al., 1987a,b; Nealand Sposito, 1991). A thermodynamic picture of therelative redox lability of inorganic Se can be obtainedby poising SeO4 on the redox ladder (Scott and Mor-gan, 1990). The redox ladder is a relative electron freeenergy sequence expressed in terms of pE (minus thecommon logarithm of the aqueous electron activity[Stumm and Morgan, 1981, Ch. 7; Sposito, 1989, Ch.6]). For any chosen reduction half-reaction, a char-acteristic pE-pH relationship is derived by setting the

1598 SOIL SCI. SOC. AM. J., VOL. 55, NOVEMBER-DECEMBER 1991

activities of the oxidized and reduced species in thereaction equal to one another (see, e.g., Sposito, 1989,p. 109-112). Then, at a selected pH value, the resultingpE value represents the relative thermodynamic like-lihood of a half-reaction occurring in response to thesupply of aqueous electrons. It is well established that,in natural aqueous systems, the reduction of O2 towater resides at the top of the redox ladder, followedby the reduction of NO3, then that of MnO2 (Scott andMorgan, 1990).

To appreciate the position of SeO4 on the redoxladder, one may consider the following illustrative re-duction half-reactions at 298 K (Sposito, 1989, Table6.2; Sposito et al., 1986):

+ -H+(aq) + e-(aq)

h |H20(1) logK= 18.9 [1]

2H+(aq) + e-(aq)

h H2O(1) log K = 20.7 [2]

H+(aq) + e-(aq)

+ ^H2O(1) log K = 14.5 [3]

e-(aq)

= 18.2 [4]

The pE-pH relationships that correspond to these half-reactions when the oxidized and reduced species havethe same thermodynamic activity are:pE = 18.9 - 1.25 pH (N03/N20)pE = 20.7 - 2 pH (MnO2/Mn2+)pE = 14.5 - pH (SeO4/SeO3)pE = 18.2 - 1.5 pH (SeO4/HSeO3)

[5][6][7][8]

These equations lead to a redox ladder in which theordering is NO3 > SeO4 > MnO2 for any pH valuein the range 5 to 9. (For pH >7, Eq. [7] is used, where-as for pH <7, Eq. [8] is used, given that the negativelogarithm of the formation constant, pK&, of HSeO^ is7.3 [Sposito et al., 1986].) For example, at pH 8 theordering is: pE = 8.9 (NO3), 6.5 (SeO4), 4.7 (MnO2).This sequence implies that, as the pE of a soil solutiondrops below 9, enough electrons become available fordenitrification to occur; as it decreases below 6, enoughelectrons become available to reduce SeO4 to SeO3;and, as the pE declines below 4, MnO2 can be reduced.Thus, SeO4 reduction is expected to occur, on ther-modynamic grounds, between that of NO3 and MnO2in soils at pH >5. (Note: Prof. D.R. Parker [personalcommunication, 1991] has very recently informed thesenior author that the value 7.3 cited above is probablyin error. He proposes 8.5 instead, which makes the useof Eq. [7] unnecessary in the calculation of the redoxladder for pH 8. Equation [4] would then show log K

= 18.8 and Eq. [8] would accordingly have 18.8 in-stead of 18.2 as the y intercept. The value of pE at pH8 would change to 6.8. There is no qualitative effecton the redox ladder from this correction.)

Whether the reduction sequence NO3 > SeO4 >MnO2 actually is observed in soil depends on howeffectively reactions like those in Eq. [1] to [4] arecatalyzed. It is well known that microbial catalysis ofNO3 reduction (via NO3 serving as a biochemical elec-tron acceptor for bacteria that ultimately excrete, e.g.,N2O) is ubiquitous in fertile soils. Manganese oxidereduction also is catalyzed efficiently under suboxicconditions (Gotoh, 1973; Stumm and Morgan, 1981,Ch. 7). Perhaps the most favorable environment inwhich to observe catalysis of the thermodynamic re-duction sequence is in flooded soil (Turner and Pa-trick, 1968; Reddy and Patrick, 1983; Yu, 1985), whichfunctions essentially as a closed system exhibiting arelatively free diffusion of dissolved chemical speciesand a facile microbial succession in sympathy withdeclining O2 levels. Turner and Patrick (1968), in aclassic study, have demonstrated the kinetic sequenceof reduction, O2 —> NO3 —» Mn oxide —> Fe oxide,consistent with the redox Ladder, in four alluvial soilsto which rice (Oryza saliva L.) straw was added priorto waterlogging. Their results showed a decline in pEfrom about 7 to 0 over 150 h of incubation withoutO2 supply. This decline was accompanied by the suc-cessive disappearance of NO3 (after 70 h), rise of ex-changeable Mn2+, and increase of soluble Fe, similarto the thermodynamic sequence.

Masscheleyn et al. (1989) incubated suspensions ofSe-containing sediments from the Kesterson Reservoir(located in the western San Joaquin Valley) at 28 °Cwith controlled pE and pH values in order to establishthe redox conditions under which Se transformationsoccur. Their results demonstrated the oxidation se-quence; Fe oxide —> selenide —» Mn oxide —> SeO3/NH4, with the SeO3 —> Se()4 transformation occurringafter 50 hr of incubation, simultaneously with the NH4—» NO3 transformation. Selenate was detected onlywhen NO3 was present. For 6.5 < pH < 8.5, the SeO3-SeO4 transformation was nearly complete by pE = 7.6,which is approximately consistent with Eq. [7] and [8].Masscheleyn et al. (1989) concluded that pE and pHwere key factors in the biogeochemistry of Se in thesediments.

Oremland et al. (1990) monitored the disappearanceof a small quantity of H2

73SeO4 injected into San Joa-quin Valley evaporation-pond sediments that were in-cubated without O2 supply at laboratory temperature.Selenate reduction was usually complete before 20 hof incubation and appeared to coincide with denitri-fication processes, at least in the presence of 0.4 molNO3 irr3. Weres et al. (1990) have suggested that SeO4reduction follows NO3 reduction on the basis of theircolumn experiments involving the flow of SeO4-en-riched synthetic drainage waters through saturated col-umns of Kesterson Reservoir sediments. When NO3and added glucose were present initially in the per-colating solution, denitrification appeared to be oc-curring in the uppermost portion of the sedimentcolumn, with SeO4 reduction then occurring in thelower portion, such that soluble Se in the effluent de-clined sharply 120 h after SeO4-enriched solution en-

SPOSITO ET AL.: SELENATE REDUCTION IN AN ALLUVIAL SOIL 1599

tered the column. Weres et al. (1990) postulated thatNO3 poised the pE value too high for significant SeO4reduction, although they were not able to determinewhether SeO4 reduction actually began during or afterNO3 reduction.

In this study, we applied the methodology of Turnerand Patrick (1968) to investigate the position of SeO4in the kinetic reduction sequence of a representativeagricultural soil for the western San Joaquin Valley.The objectives of our study were to evaluate the appl-icability of the redox ladder to SeO4 reduction in un-contaminated soil and to provide data relevant to thescientific management of SeO4 attenuation in fieldsoils by control of oxidation-reduction conditions.

MATERIALS AND METHODSSoil Suspension

A sample collected from the 0- to 0.5-m depth of a cul-tivated Panhill (fine-silty, mixed, thermic Typic Haplargids)soil profile located northwest of the Panoche fan in the SanJoaquin Valley, California, was used in this study. In itschemical properties, this surface soil is representative of theother soils found in the area of concern about Se contami-nation of drainage waters (Neal et al., 1987a; Sposito et al.,1988). It also has a low native content of Se (Neal and Spos-ito, 1991). Some physical and chemical properties of the soilsample are listed in Table 1 (Neal et al., 1987a; Neal andSposito, 1991).

A water extract of the <2-mm size fraction of the air-dried soil sample was prepared by mixing 0.75 kg of soilwith an equal mass of deionized, distilled water and allowingthe mixture to stand about 18 h in a covered polyethylenebeaker at laboratory temperature. The mixture then was vac-uum filtered using a Buchner funnel and Whatman no. 1filter paper. The water extract was collected and stored in aTeflon bottle under refrigeration until it was used later inthe same day to prepare a soil suspension for incubation.

A soil suspension was prepared immediately before eachreplicate incubation experiment. Fifty grams of the wet soilremaining from the water extraction process (and storedbriefly under refrigeration) were placed in a 250-mL poly-ethylene beaker and mixed with ^100 mL of the aqueoussoil extract. This mixture was decanted into the reactionvessel of the incubation apparatus, with quantitative transferensured by rinsing the polyethylene beaker with additionalextract. The extract also was used to bring the suspensionvolume to 400 mL in the reaction vessel at the beginning ofthe incubation period (1:8 soil/solution ratio by mass). Prep-aration of the suspension with a water extract of the soil,instead of with distilled, deionized water, was believed toprovide the aqueous medium more conducive to microbialinoculation and catalysis of reduction reactions.

Incubation ExperimentsIncubation Apparatus

A modified version of the incubation apparatus describedby Turner and Patrick (1968) was constructed for use in these

TUBE FOR VOLUMEMEASUREMENT

SYRINGE(ARGON)

-SEPTUM

-SAMPLING TUBE

3-WAY VALVE

.pH METER

REDOX ELECTRODE

SUSPENSION

MAGNETICSTIRRER

Fig. 1. Diagram of the incubation apparatus.

experiments. A diagram of the apparatus is shown in Fig. 1.Its principal components are: (i) a 400-mL glass reactionvessel that contains the soil suspension, magnetic stir bar,sampling tube, and Orion combination redox electrode (Or-ion Research, Boston, MA); and (ii) a sampling system com-prising a three-way valve to permit the introduction of Argas equal in volume to that of the sample withdrawn undervacuum. The entire assembly is placed in a temperature-controlled room at 23 ± 2 °C.

ProcedureApproximately 20 replications of the incubation experi-

ment were performed. In these experiments, a fresh soil sus-pension was prepared as described above, with 2 g ofdextrose starch (Dextran, J.T. Baker, Phillipsburg, NJ) and0.15 mL of 5 mol nr3 Na2SeO4 being added just before (< 10min) the reaction vessel was filled with soil extract solutionand sealed. The SeO4 added created an initial soluble-Seconcentration near 2 mmol m~3, which is representative ofthe aqueous phase in contaminated western San JoaquinValley soils (Fujii et al., 1988). The electrode potential inthe suspension was monitored continuously to provide aqualitative indicator of O2 depletion and to detect any failure

Table 1. Selected physical and chemical properties of the Panhill soil sample.Particle-size analysis!

Sand

369

Silt

308

Clay——— gkg-' ——

323

CaC03t Cf

4.3 ± 0.1 7.1 ± 0.3

Cation-exchangecapacity}:

mot. kg~'0.14 ± 0.01

pH(saturated paste)f

7.7

Electricalconductivity

(extract)t

dSm-'1.7 ± 0.2

t Data from Neal and Sposito (1990)j Data from Neal et al. (1987a)

1600 SOIL SCI. SOC. AM. J., VOL. 55, NOVEMBER-DECEMBER 1991

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TIME (HOURS)Fig. 2. Time variation of soluble Se, (NO3 + NO2) concentration

(H- 10), and pH (-*- 16) in the Panhill soil suspension with nostarch added (baseline experiment). The pE and concentration ofMn in the suspension also did not vary significantly from theirinitial values (2.9 ± 0.8 and 0.05 ± 0.02 mmol nr3, respectively.

of the sealing of the reaction vessel during the course ofincubation. Elapsed time was measured from the momentthat starch was added and continued for up to 250 h, duringwhich pE values dropped from about 3.5 to —7.5 (data notshown).

Aliquots of 4 mL were withdrawn periodically from thesporadically stirred suspension in sequential groups of threefor individual NO3, Mn, or Se analysis. The pH value ineach aliquot was measured immediately, using an Orion mi-cro-combination electrode, after which the aliquot was cen-trifuged for 10 min and filtered quickly (< 30 s) through a0,45-Mm membrane filter. Aliquots designated for individualNO3 or Mn analysis were acidified to pH <2 with H2SO4,whereas those designated for Se analysis were acidified topH <2 with HC1, then stored in a refrigerator.

In one baseline experiment, no starch was added, with theresult that no significant changes in the concentrations ofNO3, Se, or Mn, and in pE or pH were observed during a200-h incubation (Fig. 2). The lack of trends in Fig. 2 in-dicates also that aliquot removal did not affect the monitoredconcentrations. In another experiment, no Na2SeO4 wasadded, permitting observation of the effect of incubation onthe low native concentration of Se in the soil suspension. Ina third experiment, KNO3 was added to augment the nativeconcentration of NO3 in the soil suspension and thus observethe effect of this augmentation on the rate of soluble Sereduction.

Chemical AnalysesNitrate-N was determined in the acidified, filtered ali-

quots by quantitative reduction to NO2 followed by color-imetric quantisation on a Lachat autoanalyzer (LachatInstruments, Milwaukee, WI). Nitrite present in an aliquotis included in this determination, but can be accounted forby a separate colorimetric measurement that does not in-clude the reduction step (passage through a Cu-Cd column).

Manganese and Se were determined by inductively cou-pled plasma (ICP) spectrometry on a Perkin-Elmer Plasma40 (Perkin-Elmer, Nqrwalk, CT), with hydride generationused in the Se determination. Selenium standards were pre-pared with SeO2; separate experiments comparing SeO2 stan-dards with Na2SeO4 standards demonstrated no quantitativedifferences in the accuracy of quantitation by ICP (data notshown).

RESULTS AND DISCUSSIONFigure 3 shows a representative plot of the time var-

iation of soluble (NO3 + NO2), Mn, and Se concen-

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trations in aliquots of the Panhill soil suspensioncollected over a 250-h incubation period. These plotsare consistent with the graphs of decreasing NO3-Nand increasing exchangeable Mn published by Turnerand Patrick (1968) in their study of the kinetic reduc-tion sequence in four alluvial soils under conditionslike those in our experiments. Native concentrationsof NO3-N in the Panhill soil extract (=1.5 mol nr3)and the added SeO4 (=2 mmol m~3) were depleted after100 h of incubation. After 50 h of incubation, the con-centration of soluble Mn began to increase signifi-cantly, and after 100 h it rose sharply. Althoughchanges in pH and organic-acid concentrations maycontribute to these trends, they are not inconsistentwith the thermodynamic reduction sequence for NO3,Mn, and Se as illustrated in Eq. [5] to [8].

Figure 4 is a plot of the time variation of the soluble(NO3 + NO2), Mn, and Se concentrations in aliquotsof the Panhill soil suspension, as observed in an ex-periment involving native concentrations of both NO3and soluble Se. The data indicate that native solubleSe (=0.07 mmol nr3 initially) declined to undetectablelevels after 150 h. The time variation of the (NO3 +NO2) and Mn concentrations is quite similar to thatin Fig. 3. The time trends for soluble Se in severalincubation experiments are shown in Fig. 5, where itis apparent that undetectsible levels were observed af-ter 50 to 250 h. Evidently, this decrease in soluble Seaccompanying SeO4 reduction can be observed in asuboxic soil suspension because the reduction prod-

SPOSITO ET AL.: SELENATE REDUCTION IN AN ALLUVIAL SOIL 1601

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Fig. 5. Time variation of soluble Se (added as Na2SeO4 plus native)during several replicate incubation experiments with starch added.Asterisks denote native soluble-Se measurements (X 10) in anexperiment without added SeO4.

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Fig. 6. Composite time variation of pH during six incubation ex-periments with added starch.

ucts from SeO4 are likely to be either SeO3 (Oremlandet al., 1990), which is adsorbed strongly by the Panhillsoil particles (Neal et al., 1987a,b), or volatile Se spe-cies (Thompson-Eagle and Frankenberger, 1990) atleast at pE >3 (Masscheleyn et al., 1989), which thencan be lost into the headspace of the reaction vessel.

Figure 6 is a plot of pH vs. elapsed time for a com-posite of six incubation experiments. The monotonicdecrease in suspension pH observed is the opposite ofthe trend expected from reduction half-reactions alone(see Eq. [l]-[4]), which almost always consume pro-tons (Sposito, 1989, Ch. 6). It is likely in our experi-ments that net proton production occurred duringincubation because of the copious formation of or-ganic acids from degradation of the starch added tothe soil suspension; e.g., via the reaction:2 CH20(aq) = C2H3Oi(aq) + H+(aq)

logK= 11.2 [9]for the formation of acetic acid from a generic dis-solved carbohydrate, CH2O (Sposito, 1989, Table 6.2).Note that no change in suspension pH was observedduring incubation in the absence of added starch (Fig.2).

In order for a thermodynamic reduction sequenceto coincide with a kinetic reduction sequence in aflooded soil or in a soil suspension, effective catalysisand coupling of reduction and oxidation half-reactionsmust occur (Stumm and Morgan, 1981, Ch. 7). Theseconditions are often met for NO3 and Mn reduction

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soluble-Se concentrations during an incubation experiment withstarch and KNO3 added.

(Stumm and Morgan, 1981, Ch. 7; Yu, 1985, Ch. 4),and our study, along with the results of Masscheleynet al. (1989) and Oremland et al. (1990), suggest thatthey can exist as well for SeO4. Macy et al. (1989)recently have provided direct evidence for microbialcatalysis of SeO4 reduction by isolating a Pseudomonasspecies that respires SeO4 to SeO3 according to thereaction:

C2H3O2(aq) + H+(aq) + 4 SeO^aq) = 2 CO2(g)+ 4 SeOi-(aq) + 2 H2O(1) log K = 106.4 [10]

Elemental Se also was produced by this species andanother unidentified bacterial species isolated by Macyet al. (1989) in pure culture. Similar results with abacterial strain ("SeS") that respires SeO4 while grow-ing on acetate have been reported by Oremland et al.(1989).

Figure 7 is a plot of the time variation of soluble Seobserved in the incubation experiment in which KNO3was added to the soil suspension to give an initial NO3concentration near 2.5 mol rir3. The data show thatno significant change in the soluble-Se concentrationoccurred until after 50 h had elapsed, by which timethe concentration of NO3 + NO2 had dropped to theinitial native level (1.5 mol nr3). Thereafter, both SeO4and NO3 concentrations declined to undetectable val-ues at =100 h elapsed incubation time. Thus, it ispossible that the excess NO3 was exerting a retardingeffect on SeO4 reduction. Weres et al. (1990) noted anincrease in soluble Se in the effluent from their suboxicsediment columns that accompanied an increase inadded initial NO3 concentration in the percolatingdrainage water. Oremland et al. (1989, 1990) andSteinberg and Oremland (1990) have shown that SeO4reduction in sediments is inhibited significantly byNO3 at concentrations >10 mol nr3. This effect maybe chemical, i.e., the poising of the pE value at a highlevel because of an abundance of NO3 (Weres et al.,1990), or biochemical, via direct inhibition of respi-ration in SeO4 reducers (Oremland et al., 1989). Macy(1990) has shown recently that the SeO4-reducingPseudomonas species identified by Macy et al. (1989)utilizes a separate SeO4 reductase enzyme to catalyzethe reaction in Eq. [10], but that the synthesis of thisenzyme is indeed inhibited by the presence of NO3.

1602 SOIL SCI. SOC. AM. J., VOL. 55, NOVEMBER-DECEMBER 1991

CONCLUSIONSThermodynamic calculations predict that SeO4

should be reduced after NO3, but before MnO2, as thepE value decreases in a soil at pH >5. This sequencewas followed for kinetic reduction processes in a 1:8(w/w) suspension of Panhill soil in its own water ex-tract, incubated without O2 supply for up to 250 h.Nitrate (plus NO2) concentrations dropped from 1.5mol nr3 to undetectable values after about 100 h, whilesoluble-Se concentrations declined to zero after 50 to200 h. Manganese concentrations increased after 50 h,with the sharpest rise occurring after the depletion ofNO3. Some retardation of SeO4 reduction in the pres-ence of added NO3 was observed. These results suggestthat management of oxidation-reduction conditions infield soils may be an effective means of controllingaqueous SeO4 mobility in the western San JoaquinValley.

ACKNOWLEDGMENTSThis research was supported in part by the University of

California Salinity/Drainage Task Force. Gratitude is ex-pressed to Ms. Terri DeLuca for her excellent typing of themanuscript and to Mr. Frank Murillo for preparation of thefigures.