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Interaction of selenate with a wetland sediment
Pergamon P l h S0883-2927(97)00025-5
Applied Geochemistry,Vol. 12, pp. 685-691, 1997 © 1997ElsevierScienceLtd All rightsreserved.Printedin Great Britain 0883--2927/97$17.00+ 0.00
Interaction of selenate with a wetland sediment YiQiang Zhang* and Johnnie N. Moore Department of Geology, University of Montana, Missoula, Montana 59812-1019, U. S. A. (Received 31 May 1996; accepted in revisedform 7 January 1997)
Abstract--Selenate removal from solution through various biogeochemical processes occurred in experiments with wetland sediment. The removal of selenate from solution to sediment was rapid and substantial. About 44-80% of added selenate was accumulated in sediment in a 72-h incubation. Different biogeochemicai processes competed in removing selenate from water. Reduction of selenate to elemental Se dominated removal with a reduction rate of 0.147 #g/g/h. Formation olr organically-bound S¢ (organic material Se and adsorbed organic Se) also removed Se from water; the rate of Se binding to organic material was 0.065/zg/g/h. In contrast, selenate adsorption was relatively less important due to a low adsorption rate, 0.013 #g/g/h. Volatilizationremoved little Se from water and sediment. These results show that Se reduction to the elemental form and binding of Se to organic material are the most important processes removing selenate from water and fixing it in wetland sediment. © 1997 Elsevier Science Ltd. All rights reserved
INTRODUC~ON Selenium-rich agricultural drainage has caused Se bioaccumulation in many wetlands in the Western United States (Ohlendorf, 1989; Presser et al., 1994). Selenate is the major species of Se in this drainage water (Ohlendorf, 1989; Zhang and Moore, 1996). As dissolved selenate enters the wetland, the major process removing selenate from the water column is Se accumulation in wetland sediment (Zhang and Moore, 1997a), which makes up the largest reservoir of Se in wetland systems (Weres et al., 1989). Zhang and Moore (1996) examined Se speciation and fractionation in a wetland system, and pointed out that microbial reduction of selenate to elemental Se and Se uptake by wetland organisms and incorporation of these organisms into sediments are the major processes accumulating that element in wetland sediment. Adsorption of Se (selenate, selenite and organic Se) on sediment also occurred in the wetland system, but they found that it was relatively less important in removing the element from water. Previous studies have also assessed the rates of Se adsorption, reduction, and binding to organic material in soils and sediments in laboratory experiments (Doran and Alexander, 1976; Ylaranta, 1983; Ahlrichs and Hossner, 1987; Neal et al., 1987a,b; Karlson and Frankenberger, 1989; Neal and Sposito, 1989; Oremland et al., 1989, 1990; Steinberg and Oremland, 1990). These studies showed that Se adsorption occurred in wetland soil and sediment, and the rates of adsorption on sediment and soil were much higher with selenite than with selenate (Ahlrichs and Hossner, 1987; Neal et al., 1987a,b; Neal and Sposito, 1989). Dissimilatory selenate reduction to elemental Se in wetland sediment rapidly transfers it from solution to sediment (Oremland et al., 1989, 1990;
Steinberg and Oremland, 1990). Transfer of selenate to organic-bound Se also occurs in water, soils and sediments (Doran and Alexander, 1976; Karlson and Frankenberger, 1989; Gustafsson and Johnsson, 1994). Although these studies provide valuable information on accumulation rates of Se in sediment and soil through various biogeochemicai processes, they do not elucidate how each of these processes competes for Se in water because each of these studies was focused on only one accumulation process. The objective of this study is to determine the interplay of all these mechanisms: adsorption, reduction, transfer to organic materials and volatilization to estimate the removal rate of selenate through various biogeochemical processes occurring in wetland systems.
METHODOLOGY Fine-grained surface sediment was collected from a site with relatively low Se concentration (Table 1) at Benton Lake, Montana in 1995 (Zhang and Moore, 1997a). Large organic material (large plant roots and detritus) was removed from the sample with a glass rod. The sample was then mixed
Table 1. Concentrations of Se fractions (#g/g), total C (%), water percent (%) and pH in the experimental sediment Data Soluble Se Adsorbed Se Elemental Se OM Sea Total Se pH Total C Water (%)
0.005 0.041 0.157
0.280 0.512 7.63 2.24 50.2
aSe associated with organic materials in sediment.
* Corresponding author. E-mail: [email protected]. 685
686
Y. Zhang and J. N. Moore
in a sealed plastic bag, with the air removed, to ensure sample homogeneity, and stored at 4°C until experimentation. Early work showed that the reduction of selenate in wetland sediment mainly occurred in the top < 1 cm of wetland sediment and diffusion of dissolved selenate in the sediment was severely limited (Tokunaga et al., 1996; Zhang and Moore, 1997b), which implies that only small amounts of wetland sediment can be used in laboratory batch experiments in order to simulate realistic homogenous reaction of added selenate with sediment. In this study, we set up two batches of the same experiment. In each batch, subsamples of 1.2+0.02 g wet sediment (about 50% water content) were placed in 50-ml sterile polypropylene centrifuge tubes at room temperature (23_+ I°C), and 0.1 ml of standard selenate made from Na2seO4 was added to the tubes. To determine selenate removal with time, samples were exposed for 1-72 h at a concentration of 1.5 and 15 #g Se/g sediment, dry weight at room temperature (23 + I°C) (a time-course study). To determine the rates of selenate removal, the tubes were exposed for 46 h to varying concentrations of selenate, ranging from 0.15 to 30~gse/g sediment dry weight at room temperature (23 + I°C) (a concentration-course study). All experiments were run in triplicate. Following the experiments, the sample in each tube was sequentially extracted based on a procedure operationally defining Se fractions (Zhang and Moore, 1996). In our early method development, we found that the extraction of soluble and adsorbed Se from wet wetland sediment can remove some undissolved organic material from sediment, which reduced the concentration of Se associated with organic material in sediment. Therefore, we set up two batches of the extraction experiment at the same time to eliminate this error (Table 2). The first batch of the experiment sequentially extracted soluble Se using 20 ml of 0.25 mol I - ~ KCI, adsorbed Se by 20 ml of 0.1 tool 1-1 K2HPO4 (Tokunaga et al., 1991) and elemental Se by using 10 ml of I mol 1-" Na2SO3 (Velinsky and Cutter, 1990). The second batch of the experiment directly extracted several Se fractions (including soluble, adsorbed, elemental and Se associated with organic material) by using 20 ml of 4--6% NaOCI (Tokunaga et al., 1991). The amount of Se associated with organic material was determined by the difference between Se in the NaOCI extraction from that found in the second batch and the sum of soluble, adsorbed and elemental Se fractions from the first batch of the extraction. After each extraction, the slurry in the tubes was centrifuged at 3000 rpm for 15 rain and decanted into a polyethylene bottle. The sample was then rinsed with Milli-Q water. The rinsed water in the tube was again centrifuged and decanted into the same polyethylene bottle. To determine the rate of Se volatilization from solution and sediment, another batch of experiments was conducted. A plastic bottle with air inlet holes and an activated C column (made by placing activated C in a 10-cm syringe) was Table 2. Sequential extraction procedure for Se in experimental sediment Experiment
Fraction
Extractant
First batch
Soluble Se
0.25 mol 1-I KCI (shaking 2 h)
First batch
Adsorbed Se
0.1 tool 1- t K2HPO4 (shaking 2 h)
First batch
Elemental Se
1 mol 1- f NazSO3 (ultrasonic bath 4 h, and shaking 2 h)
Second batch Soluble Se, Adsorbed 4-6% NaOCI (0.5 h Se, Elemental Se, and boil, two times) Se associated with organic material
attached to the tubes. Using a small vacuum (0.8 I/rain), Se gas produced in the tubes was pulled into the column and adsorbed on the activated C surface (Karlson and Frankenberger, 1990). An XAD-resin separation method developed by Fio and Fujii (1990) was used to determine Se species in soluble and adsorbed fractions in the sediment. Volatile Se in the activated C column was measured using a methanol resorption method (Karlson and Frankenberger, 1990). Selenium concentrations in all samples were determined using continuous-flow hydride-generation atomic-absorption spectrometry (Zhang and Moore, 1997a). Concentrations of soluble, adsorbed, elemental and organic material Se formed in experimental samples were calculated by subtracting concentrations found in control samples without added selenate, and their concentrations were finally reported on a 75°C dry-sediment basis. The rates of Se adsorption and reduction, binding to organic material, and volatilization in wetland sediment were estimated using the equation of the Michaelis--Menton enzyme reaction 0Vetzel and Likens, 1991): t / F f A / V m ~ + ( K t + S ~ ) / V m x . The term t is the incubation time; F is the fraction of Se adsorbed, reduced, bound to organic material and volatilized in time t; A is the concentration of added selenate; Kt is the half-saturation constant and Sn is the original Se concentration in the experimental sediment. Vmax represented an apparent maximum formation rate of adsorbed, reduced, organic material and volatile Se and is calculated from the plot of t/F against A. Linearization and standard deviation analyses of the formation of different Se fractions and species during experiments were done using StatView V. 4 (Haycock et al., 1992). Efficiency of sequential extraction and mass balance of Se in experimental samples were estimated by using the equation: Sen(%) "~-(x~SeFE)/(~SCFC+SeA)X 100, where, SeR is the recovery of Se in experimental samples; ~:SevE is the sum of different Se fractions in experimental samples with selenate added; Y-SeF¢is the sum of different Se fractions in control samples; and SeA is the concentration of selenate added in the experimental samples. The recovery of Se in all laboratory experiment samples was 93.7 +5.37% (ranging from 87.1 to 101%) in the time-course study and 92.2 + 5.36% (ranging from 86.2 to 102%) in the concentra. tion-course study. This relatively lower recovery most likely results from the fact that some small amount of Se may bind to sediment oxides that were not extracted during this study.
RESULTS The interaction between selenate and wetland sediment was expressed as selenate removal from solution through Se adsorption, reduction, organic material Se formation and volatilization (Tables 3 and 4, Figs 1 and 2). In the samples with a low concentration of selenate added (1.5 #g Se/g sediment, Fig. 1, Table 3), the concentration of adsorbed Se increased to 24 h, then slightly decreased with time. Within the adsorbed fraction, selenate and organic Se were the major Se species. Concentrations of elemental Se and organic material Se increased rapidly with time during the first 46 h, and then tended to stabilize to 72 h. During 72 h o f incubation, about 80% of the total added selenate was removed from solution. A m o n g total accumulated Se in the sediment, adsorbed selenate accounted for 15%, adsorbed selenite 3%, adsorbed organic Se 11%, elemental Se 55%, organic material Se 16%.
Interaction of selenate with wetland sediment
687
Table 3. Selenium~g/g) fractions and species formed in the experimentalsediment Se species and fractions
10 h
46 h
72 h
< 0.005 d
3.21 _+0.325e 0.822 _+0.059 13.6_+0.126
0.023 _+0.004 0.296_+0.025 0.094 _+0.009 8.38_+0.36
0.015 _+0.004 0.426_+0.057 0.061 _+0.012 7.19_+0.329
< 0.005 0.005-+0.004 0.093_+0.013 0.142-+0.033 0.177_+0.015 0.587_+0.178 0.270 -+0.0! 7 0.734_+0.150
0.078 -+0.013 0.338_+0.115 0.146___0.030 0.461 _+0.151 0.130_+0.004 0.698_+0.133 0.354 -t-0.017 1.50_+0.116
0.036 + 0.035 0.254+0.080 0.185-+0.031 0.628_+0.089 0.133+0.025 0.546___0.112 0.355 _+0.021 1.43-1-0.088
Elemental Se
0.249 _+0.038 0.607_+0,082
0.655 -+0.054 3.322_+0.181
0.660 -+0.091 3.817+0.191
OM Sea
0.051 _+0.006 0.187_+0.021 0.228 -+0.021 0.774-+0.196
0.169_+0.019 0.893-+0.025 0.299 _+0.019 1.5914-0.154
0.191 _+0.015 1.321 _+0.176 0.324-+0.034 1.867_+0.271
0.569_+0.054 1.529+0.232
1.178__0.055 5.712+0.166
1.206-+0.057 6.561 _+0.092
SolubleSe
(IV) Total soluble Se Adsorbed Se Se (IV) Se (VI) Organic Se Total adsorbed Se
Total OM b Se Sum Sec
Values are mean + 2S.D. aSe associated with organic materials in sediment, bThe sum of OM Se and adsorbed organic Se. ~I'be sum of adsorbed Se, elemental Se and OM Se. dThe first line shows data from the samples with selenate added (1.5 #g/g). ~'Tbesecond line shows data from the samples with selenate added (15 #g/g).
A similar trend was found in samples with higher concentrations of selenate added (15ggSe/g sediment, Table 3). With the decrease of concentrations of soluble Se, the concentration of adsorbed Se increased to 46 h, then slightly decreased with time. Within the adsorbed fraction, selenate and organic Se were also the major Se species. Concentrations of elemental Se increased rapidly with time during the first 46 h, and then stabilized to 72 h. Concentration of organic material Se increased with time during the 72-h incubation. During 72 h of incubation, about
Table 4. The rate of formation ofse species and fractions in experimental sediment Se species and fractions AdsorbedSe Selenate Selenite Organic Se ElementalSe OM Se" Total OM Seb Sum Sec Volatile Se
Apparent ;/max(#g/g/h) 0.013 0.015 0.029
44% of the total added selenate was removed from solution and added to the sediment. Among the total accumulated Se in the sediment, adsorbed selenate accounted for 10%, adsorbed selenite 4%, adsorbed organic Se 8%, elemental Se 58%, and organic material Se 20%. Concentrations of adsorbed selenate, adsorbed selenite, adsorbed organic Se, elemental Se, and organic material Se formed during the 46-h incubation increased with concentrations of added selenate (Fig. 2). The apparent rate calculated from the MichaelisMenton reaction was 0.013#g/g/h for adsorbed selenite, 0.015 pg/g/h for adsorbed selenate, 0.029 pg/ g/h for adsorbed organic Se, 0.147 gg/g/h for elemental Se, 0.036 #g/g/h for organic material Se and 0.065 #g/g/h for total organic material Se (Table 4). Volatile Se formed during the 46-h incubation also increased with concentrations of added selenate. The apparent rate was O.O0014pg/g/h. After the 46-h incubation, the rate for total Se accumulated in the sediment was 0.240 #g/g/h.
0.147
0.036 0.065 0.240 0.00014
aSe associated with organic materials in sediment, bThe sum of OM Se and adsorbed organic Se. ~l'he sum of adsorbed Se, elemental Se and OM Se.
DISCUSSION In wetlands receiving dissolved selenate, the major process removing selenate from the water is Se accumulation in wetland sediment (Weres et aL,
688
Y. Zhang and J. N. Moore
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1989; Zhang and Moore, 1996). Different biogeochemical processes compete for selenate. Se adsorption is one of these processes. Different species of dissolved Se were found to compete for adsorption sites on the sediment. This indicates that there exists a transformation process that can transfer selenate to organic Se, and a reduction process that reduced selenate to selenite. The adsorption rate was related to the formation of these Se species, it being higher for organic Se than for selenate or selenite. In comparison to total adsorbed Se found in field sediment (about 10% of total accumulated Se - - Zhang and Moore, 1996), the percentage of total adsorbed Se found in this study was higher, accounting for about 22-29%
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Interaction of selenate with wetland sediment
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Fig. 2. Relationship between increasing selenate concentration and species and fractions of Se formed in sediment. In inserted small figures, t, F and A are the incubation time, fractions of Se species and fractions formed, and concentration of selenate added, respectively. OM Se: Se associated with organic material. Error bars show +2 standard deviations.
690
Y. Zhang and J. N. Moore
In comparison with Se adsorption, microbial reduction of selenate and selenite to the element is the dominant Se immobilization process. The reduction rate of Se to elemental form was much higher than any other process, and higher than the sum o f all other processes in the 46-h experiment. The amount of elemental Se formed after the 72-h incubation accounted for 55-58% of the total accumulated in the sediment samples. A similar result was also found by Velinsky and Cutter (1991 ), Weres et aL (1989) and Zhang and Moore (1996), who all found that elemental Se was the dominant fraction in wetland sediment. Part of the Se removed from solution can be attributed to uptake by microorganisms and binding to organic material through transformation in the sediment. This process proceeds rapidly and removes significant amounts of Se from water to sediment. A study by Foda et al. (1983) showed that Se uptake by microorganisms is quite rapid, and it can be incorporated into amino acids and proteins. Zhang and Moore (1996) also found that organic material Se accounted for about 38% of the total in sediment. These studies suggest that organic material Se formed during this experimental study could result from Se uptake by microorganisms and transformation to organically-bound selenide in the sediment. In comparison to 38% of the total accumulated Se found in field surface sediment (Zhang and Moore, 1996), organic material Se in this study was lower, accounting for 16-20% of the total accumulated. This indicates that about half of the organic Se is directly from the interaction of selenate with wetland sediment and another half may be directly from the deposition of dead organisms containing Se (such as wetland plants). The incorporation o f Se into organic material is also seen in the formation of adsorbed organic Se in this study. Selenium bound to organic material can also be volatile. The amount of volatile Se formed during the experiments increased with concentrations of added selenate. Because the volatilization rate was very low (it only removes 0.1% of the total accumulated Se), volatilization was not important in removing Se from the system.
CONCLUSION Results from this study show that removal of selenate from solution to sediment was rapid and substantial. About 44-80% of added selenate was accumulated in sediment in a 72-h incubation. Different biogeochemical processes controlled selehate removal from water. Reduction of selenate to elemental Se dominated removal processes. Formation of organically-bound Se (organic material Se and adsorbed organic S¢) also removed it from water, but the rate was about half of the reduction rate. In contrast, selenate adsorption was relatively less
important and Se volatilization was not important in removing it from the system. Acknowledgements---We thank Steve Martin and Jim McCollum from Benton Lake National Wildlife Refuge. Special thanks to David Nimick for helpful discussions. Funding for this study was provided by the U.S. Fish and Wildlife Service and the Department of the Interior's National Irrigation Water Quality Program. Thanks to the M,J. Murdock Charitable Trust for funding a major laboratory upgrade to aid in analyses. Editorial handling: R. Fuge.
REFERENCES Ahlrichs J. S. and Hossner L. R. (1987) Selenate and selenite mobility in overburden by saturated flow. Journal of Environmental Quality 16, 95--98. Doran J. W. and AlexanderM. (1976) Microbial formation of volatile selenium compounds in soil. Journal of the Soil Science Society of America 40, 687-690. Fio J. L. and Fujii R. (1990) Selenium speciation methods and application to soil saturation extracts. Journal of the Soil Science Society of America 54, 363-369. Foda A., Vandermeulen J. and Wrench J. J. (1983) Uptake and conversion of selenium by a marine bacterium. Canadian Journal of Fisheries and Aquatic Sciences 40(Suppl. 2), 215-220. Gustafsson J. P. and Johnsson L. (1994) The association between selenium and humic substance in forested ecosystem - - laboratory evidence. Applied Organometallic Chemistry 8, 141-147. Haycock K. A., Roth J., Gagnon J., Finzer W. F. and Soper C. (1992) StatView. Abacus Concepts, Inc., Berkeley, CA. Karison U. and Frankenberger W. T. (1989) Accelerated rates of selenium volatilization from California soils. Journal of the Soil Science Society of America 53, 749-753. Karlson U. and Frankenberger W. T. (1990) Volatilization of selenium from agricultural evaporation pond sediments. The Science of the Total Environment92, 41-54. Neal R. H., Sposito G., Hoitzclaw K. M. and Traina S. J. (1987) Selenite adsorption on alluvial soils: I. Soil composition and pH effects. Journal of the Soil Science Society of America 51, 1161-1165. Neal R. H., Sposito G., Holtzclaw K. M. and Traina S. J. (1987) Selenite adsorption on alluvial soils: II. Solution composition effects. Journal of the Soil Science Society of America 51, 1165-1169. Neal R. H. and Sposito G. (1989) Selenate adsorption on alluvial soils. Journalof the Soil Science Society of America 53, 70-74. Ohlendorf H. M. (1989) Bioaccumniation and effects of selenium in wildlife. In Selenium in Agriculture and the Environment, ed. L. W. Jacobs, pp. 133-177. ASA and SSSA, Madison, WI. Oremland R. S., Hollibaugh J. T., Maest A. S., Presser T. S., Miller L. G. and Culbertson C. W. (1989) Selenate reduction to elemental selenium by anaerobic bacteria in sediments and culture: Biogeochemical significance of a novel, sulfate-independent respiration. Applied Environmental Microbiology 55, 2333-2343. Oremland R. S., Steinberg N. A., Maest A. S., Miller L. G. and HoUibaugh J. T. (1990) Measurement of in situ rates of selenate removal by dissimilatory bacterial reduction in sediments. Environmental Science and Technology 24, 1157-1164. Presser T. S., Sylvester M. A. and Low W. H. (1994) Bioaccumulation of selenium from natural geologic
Interaction of selenate with wetland sediment sources in western states and its potential consequences. Environmental Management lg, 423-436. Steinberg N. A. and Oremland R. S. (1990) Dissimilatory selenate reduction potentials in a diversity of sediment types. Applied Environmental Microbiology 56, 3550-3557. Tokunaga T. K., Lipton D. S., Benson S. M., Yee A. W., Oldfather J. M., Duckart E. C., Johannis P. W. and Halvorsen K. E. (1991 ) Soil selenium fractionation, depth profiles and time trends in a vegetated site at Kesterson Reservoir. Water, Air, and Soil Pollution 57]58, 31-4 1. Tokunaga T. K., Picketing I. J. and Brown G. E. J. (1996) Selenium transformation in ponded sediment. Journal of the Soil Science Society of America 60, 790-791. Velinsky D. J. and Cutter G. A. (1990) Determination of elemental selenium and pyrite-selenium in sediments. Analytica Chimica Acta 235, 419-425. Velinsky D. J. and Cutter G. A. (1991) Geochemistry of selenium in a coastal salt marsh. Geochimicaet Cosmochimica Acta 55, 179-191.
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Weres O., Jaouni A. R. and Tsao L. (1989) The distribution, speciation and geochemical cycling of selenium in a sedimentary environment, Kesterson Reservoir, California, U. S. A Applied Geochemistry 4, 543-563. Wetzel R. G. and Likens G. E. (1991) Decomposition: relative bacterial heterotrophic activity on soluble organic matter. In Limnological Analyses, eds. R. G. Wetzel and G. E. Likens, pp. 271-279. Springer-Verlag, New York. Ylaranta T. (1983) Sorption of selenite and selenate in the soil. Annales AgriculturaeFenniae22, 29-39. Zhang Y. Q. and Moore J. N. (1996) Selenium speciation and fractionation in a wetland system. Environmental Science and Technology 30, 2613--2619. Zhang Y. Q. and Moore J. N. (1997a) Controls of selenium distribution in wetland sediment, Benton Lake, Montana. Water, Air, and Soil Pollution97, 323-340. Zhang Y. Q. and Moore J. N. (1997b) Reduction potential of selenate in wetland sediment. Journal of Environmental Quality 26, 910-916.
Applied Geochemistry,Vol. 12, pp. 685-691, 1997 © 1997ElsevierScienceLtd All rightsreserved.Printedin Great Britain 0883--2927/97$17.00+ 0.00
Interaction of selenate with a wetland sediment YiQiang Zhang* and Johnnie N. Moore Department of Geology, University of Montana, Missoula, Montana 59812-1019, U. S. A. (Received 31 May 1996; accepted in revisedform 7 January 1997)
Abstract--Selenate removal from solution through various biogeochemical processes occurred in experiments with wetland sediment. The removal of selenate from solution to sediment was rapid and substantial. About 44-80% of added selenate was accumulated in sediment in a 72-h incubation. Different biogeochemicai processes competed in removing selenate from water. Reduction of selenate to elemental Se dominated removal with a reduction rate of 0.147 #g/g/h. Formation olr organically-bound S¢ (organic material Se and adsorbed organic Se) also removed Se from water; the rate of Se binding to organic material was 0.065/zg/g/h. In contrast, selenate adsorption was relatively less important due to a low adsorption rate, 0.013 #g/g/h. Volatilizationremoved little Se from water and sediment. These results show that Se reduction to the elemental form and binding of Se to organic material are the most important processes removing selenate from water and fixing it in wetland sediment. © 1997 Elsevier Science Ltd. All rights reserved
INTRODUC~ON Selenium-rich agricultural drainage has caused Se bioaccumulation in many wetlands in the Western United States (Ohlendorf, 1989; Presser et al., 1994). Selenate is the major species of Se in this drainage water (Ohlendorf, 1989; Zhang and Moore, 1996). As dissolved selenate enters the wetland, the major process removing selenate from the water column is Se accumulation in wetland sediment (Zhang and Moore, 1997a), which makes up the largest reservoir of Se in wetland systems (Weres et al., 1989). Zhang and Moore (1996) examined Se speciation and fractionation in a wetland system, and pointed out that microbial reduction of selenate to elemental Se and Se uptake by wetland organisms and incorporation of these organisms into sediments are the major processes accumulating that element in wetland sediment. Adsorption of Se (selenate, selenite and organic Se) on sediment also occurred in the wetland system, but they found that it was relatively less important in removing the element from water. Previous studies have also assessed the rates of Se adsorption, reduction, and binding to organic material in soils and sediments in laboratory experiments (Doran and Alexander, 1976; Ylaranta, 1983; Ahlrichs and Hossner, 1987; Neal et al., 1987a,b; Karlson and Frankenberger, 1989; Neal and Sposito, 1989; Oremland et al., 1989, 1990; Steinberg and Oremland, 1990). These studies showed that Se adsorption occurred in wetland soil and sediment, and the rates of adsorption on sediment and soil were much higher with selenite than with selenate (Ahlrichs and Hossner, 1987; Neal et al., 1987a,b; Neal and Sposito, 1989). Dissimilatory selenate reduction to elemental Se in wetland sediment rapidly transfers it from solution to sediment (Oremland et al., 1989, 1990;
Steinberg and Oremland, 1990). Transfer of selenate to organic-bound Se also occurs in water, soils and sediments (Doran and Alexander, 1976; Karlson and Frankenberger, 1989; Gustafsson and Johnsson, 1994). Although these studies provide valuable information on accumulation rates of Se in sediment and soil through various biogeochemicai processes, they do not elucidate how each of these processes competes for Se in water because each of these studies was focused on only one accumulation process. The objective of this study is to determine the interplay of all these mechanisms: adsorption, reduction, transfer to organic materials and volatilization to estimate the removal rate of selenate through various biogeochemical processes occurring in wetland systems.
METHODOLOGY Fine-grained surface sediment was collected from a site with relatively low Se concentration (Table 1) at Benton Lake, Montana in 1995 (Zhang and Moore, 1997a). Large organic material (large plant roots and detritus) was removed from the sample with a glass rod. The sample was then mixed
Table 1. Concentrations of Se fractions (#g/g), total C (%), water percent (%) and pH in the experimental sediment Data Soluble Se Adsorbed Se Elemental Se OM Sea Total Se pH Total C Water (%)
0.005 0.041 0.157
0.280 0.512 7.63 2.24 50.2
aSe associated with organic materials in sediment.
* Corresponding author. E-mail: [email protected]. 685
686
Y. Zhang and J. N. Moore
in a sealed plastic bag, with the air removed, to ensure sample homogeneity, and stored at 4°C until experimentation. Early work showed that the reduction of selenate in wetland sediment mainly occurred in the top < 1 cm of wetland sediment and diffusion of dissolved selenate in the sediment was severely limited (Tokunaga et al., 1996; Zhang and Moore, 1997b), which implies that only small amounts of wetland sediment can be used in laboratory batch experiments in order to simulate realistic homogenous reaction of added selenate with sediment. In this study, we set up two batches of the same experiment. In each batch, subsamples of 1.2+0.02 g wet sediment (about 50% water content) were placed in 50-ml sterile polypropylene centrifuge tubes at room temperature (23_+ I°C), and 0.1 ml of standard selenate made from Na2seO4 was added to the tubes. To determine selenate removal with time, samples were exposed for 1-72 h at a concentration of 1.5 and 15 #g Se/g sediment, dry weight at room temperature (23 + I°C) (a time-course study). To determine the rates of selenate removal, the tubes were exposed for 46 h to varying concentrations of selenate, ranging from 0.15 to 30~gse/g sediment dry weight at room temperature (23 + I°C) (a concentration-course study). All experiments were run in triplicate. Following the experiments, the sample in each tube was sequentially extracted based on a procedure operationally defining Se fractions (Zhang and Moore, 1996). In our early method development, we found that the extraction of soluble and adsorbed Se from wet wetland sediment can remove some undissolved organic material from sediment, which reduced the concentration of Se associated with organic material in sediment. Therefore, we set up two batches of the extraction experiment at the same time to eliminate this error (Table 2). The first batch of the experiment sequentially extracted soluble Se using 20 ml of 0.25 mol I - ~ KCI, adsorbed Se by 20 ml of 0.1 tool 1-1 K2HPO4 (Tokunaga et al., 1991) and elemental Se by using 10 ml of I mol 1-" Na2SO3 (Velinsky and Cutter, 1990). The second batch of the experiment directly extracted several Se fractions (including soluble, adsorbed, elemental and Se associated with organic material) by using 20 ml of 4--6% NaOCI (Tokunaga et al., 1991). The amount of Se associated with organic material was determined by the difference between Se in the NaOCI extraction from that found in the second batch and the sum of soluble, adsorbed and elemental Se fractions from the first batch of the extraction. After each extraction, the slurry in the tubes was centrifuged at 3000 rpm for 15 rain and decanted into a polyethylene bottle. The sample was then rinsed with Milli-Q water. The rinsed water in the tube was again centrifuged and decanted into the same polyethylene bottle. To determine the rate of Se volatilization from solution and sediment, another batch of experiments was conducted. A plastic bottle with air inlet holes and an activated C column (made by placing activated C in a 10-cm syringe) was Table 2. Sequential extraction procedure for Se in experimental sediment Experiment
Fraction
Extractant
First batch
Soluble Se
0.25 mol 1-I KCI (shaking 2 h)
First batch
Adsorbed Se
0.1 tool 1- t K2HPO4 (shaking 2 h)
First batch
Elemental Se
1 mol 1- f NazSO3 (ultrasonic bath 4 h, and shaking 2 h)
Second batch Soluble Se, Adsorbed 4-6% NaOCI (0.5 h Se, Elemental Se, and boil, two times) Se associated with organic material
attached to the tubes. Using a small vacuum (0.8 I/rain), Se gas produced in the tubes was pulled into the column and adsorbed on the activated C surface (Karlson and Frankenberger, 1990). An XAD-resin separation method developed by Fio and Fujii (1990) was used to determine Se species in soluble and adsorbed fractions in the sediment. Volatile Se in the activated C column was measured using a methanol resorption method (Karlson and Frankenberger, 1990). Selenium concentrations in all samples were determined using continuous-flow hydride-generation atomic-absorption spectrometry (Zhang and Moore, 1997a). Concentrations of soluble, adsorbed, elemental and organic material Se formed in experimental samples were calculated by subtracting concentrations found in control samples without added selenate, and their concentrations were finally reported on a 75°C dry-sediment basis. The rates of Se adsorption and reduction, binding to organic material, and volatilization in wetland sediment were estimated using the equation of the Michaelis--Menton enzyme reaction 0Vetzel and Likens, 1991): t / F f A / V m ~ + ( K t + S ~ ) / V m x . The term t is the incubation time; F is the fraction of Se adsorbed, reduced, bound to organic material and volatilized in time t; A is the concentration of added selenate; Kt is the half-saturation constant and Sn is the original Se concentration in the experimental sediment. Vmax represented an apparent maximum formation rate of adsorbed, reduced, organic material and volatile Se and is calculated from the plot of t/F against A. Linearization and standard deviation analyses of the formation of different Se fractions and species during experiments were done using StatView V. 4 (Haycock et al., 1992). Efficiency of sequential extraction and mass balance of Se in experimental samples were estimated by using the equation: Sen(%) "~-(x~SeFE)/(~SCFC+SeA)X 100, where, SeR is the recovery of Se in experimental samples; ~:SevE is the sum of different Se fractions in experimental samples with selenate added; Y-SeF¢is the sum of different Se fractions in control samples; and SeA is the concentration of selenate added in the experimental samples. The recovery of Se in all laboratory experiment samples was 93.7 +5.37% (ranging from 87.1 to 101%) in the time-course study and 92.2 + 5.36% (ranging from 86.2 to 102%) in the concentra. tion-course study. This relatively lower recovery most likely results from the fact that some small amount of Se may bind to sediment oxides that were not extracted during this study.
RESULTS The interaction between selenate and wetland sediment was expressed as selenate removal from solution through Se adsorption, reduction, organic material Se formation and volatilization (Tables 3 and 4, Figs 1 and 2). In the samples with a low concentration of selenate added (1.5 #g Se/g sediment, Fig. 1, Table 3), the concentration of adsorbed Se increased to 24 h, then slightly decreased with time. Within the adsorbed fraction, selenate and organic Se were the major Se species. Concentrations of elemental Se and organic material Se increased rapidly with time during the first 46 h, and then tended to stabilize to 72 h. During 72 h o f incubation, about 80% of the total added selenate was removed from solution. A m o n g total accumulated Se in the sediment, adsorbed selenate accounted for 15%, adsorbed selenite 3%, adsorbed organic Se 11%, elemental Se 55%, organic material Se 16%.
Interaction of selenate with wetland sediment
687
Table 3. Selenium~g/g) fractions and species formed in the experimentalsediment Se species and fractions
10 h
46 h
72 h
< 0.005 d
3.21 _+0.325e 0.822 _+0.059 13.6_+0.126
0.023 _+0.004 0.296_+0.025 0.094 _+0.009 8.38_+0.36
0.015 _+0.004 0.426_+0.057 0.061 _+0.012 7.19_+0.329
< 0.005 0.005-+0.004 0.093_+0.013 0.142-+0.033 0.177_+0.015 0.587_+0.178 0.270 -+0.0! 7 0.734_+0.150
0.078 -+0.013 0.338_+0.115 0.146___0.030 0.461 _+0.151 0.130_+0.004 0.698_+0.133 0.354 -t-0.017 1.50_+0.116
0.036 + 0.035 0.254+0.080 0.185-+0.031 0.628_+0.089 0.133+0.025 0.546___0.112 0.355 _+0.021 1.43-1-0.088
Elemental Se
0.249 _+0.038 0.607_+0,082
0.655 -+0.054 3.322_+0.181
0.660 -+0.091 3.817+0.191
OM Sea
0.051 _+0.006 0.187_+0.021 0.228 -+0.021 0.774-+0.196
0.169_+0.019 0.893-+0.025 0.299 _+0.019 1.5914-0.154
0.191 _+0.015 1.321 _+0.176 0.324-+0.034 1.867_+0.271
0.569_+0.054 1.529+0.232
1.178__0.055 5.712+0.166
1.206-+0.057 6.561 _+0.092
SolubleSe
(IV) Total soluble Se Adsorbed Se Se (IV) Se (VI) Organic Se Total adsorbed Se
Total OM b Se Sum Sec
Values are mean + 2S.D. aSe associated with organic materials in sediment, bThe sum of OM Se and adsorbed organic Se. ~I'be sum of adsorbed Se, elemental Se and OM Se. dThe first line shows data from the samples with selenate added (1.5 #g/g). ~'Tbesecond line shows data from the samples with selenate added (15 #g/g).
A similar trend was found in samples with higher concentrations of selenate added (15ggSe/g sediment, Table 3). With the decrease of concentrations of soluble Se, the concentration of adsorbed Se increased to 46 h, then slightly decreased with time. Within the adsorbed fraction, selenate and organic Se were also the major Se species. Concentrations of elemental Se increased rapidly with time during the first 46 h, and then stabilized to 72 h. Concentration of organic material Se increased with time during the 72-h incubation. During 72 h of incubation, about
Table 4. The rate of formation ofse species and fractions in experimental sediment Se species and fractions AdsorbedSe Selenate Selenite Organic Se ElementalSe OM Se" Total OM Seb Sum Sec Volatile Se
Apparent ;/max(#g/g/h) 0.013 0.015 0.029
44% of the total added selenate was removed from solution and added to the sediment. Among the total accumulated Se in the sediment, adsorbed selenate accounted for 10%, adsorbed selenite 4%, adsorbed organic Se 8%, elemental Se 58%, and organic material Se 20%. Concentrations of adsorbed selenate, adsorbed selenite, adsorbed organic Se, elemental Se, and organic material Se formed during the 46-h incubation increased with concentrations of added selenate (Fig. 2). The apparent rate calculated from the MichaelisMenton reaction was 0.013#g/g/h for adsorbed selenite, 0.015 pg/g/h for adsorbed selenate, 0.029 pg/ g/h for adsorbed organic Se, 0.147 gg/g/h for elemental Se, 0.036 #g/g/h for organic material Se and 0.065 #g/g/h for total organic material Se (Table 4). Volatile Se formed during the 46-h incubation also increased with concentrations of added selenate. The apparent rate was O.O0014pg/g/h. After the 46-h incubation, the rate for total Se accumulated in the sediment was 0.240 #g/g/h.
0.147
0.036 0.065 0.240 0.00014
aSe associated with organic materials in sediment, bThe sum of OM Se and adsorbed organic Se. ~l'he sum of adsorbed Se, elemental Se and OM Se.
DISCUSSION In wetlands receiving dissolved selenate, the major process removing selenate from the water is Se accumulation in wetland sediment (Weres et aL,
688
Y. Zhang and J. N. Moore
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1989; Zhang and Moore, 1996). Different biogeochemical processes compete for selenate. Se adsorption is one of these processes. Different species of dissolved Se were found to compete for adsorption sites on the sediment. This indicates that there exists a transformation process that can transfer selenate to organic Se, and a reduction process that reduced selenate to selenite. The adsorption rate was related to the formation of these Se species, it being higher for organic Se than for selenate or selenite. In comparison to total adsorbed Se found in field sediment (about 10% of total accumulated Se - - Zhang and Moore, 1996), the percentage of total adsorbed Se found in this study was higher, accounting for about 22-29%
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Interaction of selenate with wetland sediment
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Fig. 2. Relationship between increasing selenate concentration and species and fractions of Se formed in sediment. In inserted small figures, t, F and A are the incubation time, fractions of Se species and fractions formed, and concentration of selenate added, respectively. OM Se: Se associated with organic material. Error bars show +2 standard deviations.
690
Y. Zhang and J. N. Moore
In comparison with Se adsorption, microbial reduction of selenate and selenite to the element is the dominant Se immobilization process. The reduction rate of Se to elemental form was much higher than any other process, and higher than the sum o f all other processes in the 46-h experiment. The amount of elemental Se formed after the 72-h incubation accounted for 55-58% of the total accumulated in the sediment samples. A similar result was also found by Velinsky and Cutter (1991 ), Weres et aL (1989) and Zhang and Moore (1996), who all found that elemental Se was the dominant fraction in wetland sediment. Part of the Se removed from solution can be attributed to uptake by microorganisms and binding to organic material through transformation in the sediment. This process proceeds rapidly and removes significant amounts of Se from water to sediment. A study by Foda et al. (1983) showed that Se uptake by microorganisms is quite rapid, and it can be incorporated into amino acids and proteins. Zhang and Moore (1996) also found that organic material Se accounted for about 38% of the total in sediment. These studies suggest that organic material Se formed during this experimental study could result from Se uptake by microorganisms and transformation to organically-bound selenide in the sediment. In comparison to 38% of the total accumulated Se found in field surface sediment (Zhang and Moore, 1996), organic material Se in this study was lower, accounting for 16-20% of the total accumulated. This indicates that about half of the organic Se is directly from the interaction of selenate with wetland sediment and another half may be directly from the deposition of dead organisms containing Se (such as wetland plants). The incorporation o f Se into organic material is also seen in the formation of adsorbed organic Se in this study. Selenium bound to organic material can also be volatile. The amount of volatile Se formed during the experiments increased with concentrations of added selenate. Because the volatilization rate was very low (it only removes 0.1% of the total accumulated Se), volatilization was not important in removing Se from the system.
CONCLUSION Results from this study show that removal of selenate from solution to sediment was rapid and substantial. About 44-80% of added selenate was accumulated in sediment in a 72-h incubation. Different biogeochemical processes controlled selehate removal from water. Reduction of selenate to elemental Se dominated removal processes. Formation of organically-bound Se (organic material Se and adsorbed organic S¢) also removed it from water, but the rate was about half of the reduction rate. In contrast, selenate adsorption was relatively less
important and Se volatilization was not important in removing it from the system. Acknowledgements---We thank Steve Martin and Jim McCollum from Benton Lake National Wildlife Refuge. Special thanks to David Nimick for helpful discussions. Funding for this study was provided by the U.S. Fish and Wildlife Service and the Department of the Interior's National Irrigation Water Quality Program. Thanks to the M,J. Murdock Charitable Trust for funding a major laboratory upgrade to aid in analyses. Editorial handling: R. Fuge.
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