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Selenium poisoning is a significant health problem in parts of Punjab, India, which is an area of intense agricultural productivity. To determine the complex soil dynamics that control distribution of Se in this area, we measured concentrations and δ(82/76)Se of bulk Se and individual Se pools in four soil profiles. This was compared against δ(82/76)Se of crops and groundwater used for irrigation. The isotopic composition of bulk Se and component Se pools reveal spatial heterogeneity. The bulk δ(82/76)Se show progressively lower values with increasing soil depth indicating the preferential migration of isotopically lighter Se downward through the soil profile. The δ(82/76)Se of water-soluble Se is isotopically heavier than δ(82/76)Se of adsorbed Se, suggesting Se isotope fractionation by reduction prior to scavenging by reactive minerals in the soil. The organically bound Se is isotopically lighter than water-soluble Se and correlates with the C/N ratio at different soil depths. Thus, Se immobilization by redox cycling controls the biogeochemical Se cycle in the soil. Se isotope ratios help to trace biochemical processes of Se in agricultural seleniferous soils and provide an important assessment for better soil management mitigating Se concentrations of ecotoxicological levels.

The natural abundance of stable Se isotopes may reflect sources and formation conditions of methylated Se. We aimed at (1) quantifying the degree of methylation of selenate [Se(VI)] and (hydro)selenite [Se(IV)] by the fungus Alternaria alternata at pH 4 and 7 and (2) determining the effects of these different Se sources and pH values on 82Se/76Se ratios (δ82/76Se) in methylselenides. Alternaria alternata was incubated with Se(VI) and Se(IV) in closed microcosms for 11-15 days and additionally with Se(IV) for 3-5 days at 30 °C. We determined Se concentrations and δ82/76Se values in source Se(VI) and Se(IV), media, fungi, and trapped methylselenides. In Se(VI) incubations, methylselenide volatilization ended before the 11th day, and the amounts of trapped methylselenide were not significantly different among the 11-15 day incubations. In 11-15 days, 2.9-11% of Se(VI) and 21-29% of Se(IV) were methylated, and in 3-5 days, 3-5% of Se(IV) was methylated. The initial δ82/76Se values of Se(VI) and Se(IV) were -0.69±SD 0.07‰, and -0.20±0.05‰, respectively. The δ82/76Se values of methylselenides differed significantly between Se(VI) (-3.97‰ to -3.25‰) and Se(IV) (-1.44‰ to -0.16‰) as sources after 11-15 days of incubation; pH had little influence on δ82/76Se values. Thus, the δ82/76Se values of methylselenide indicate the source species of methylselenides used in this study. The strong isotope fractionation of Se(VI) is probably attributable to the different reduction steps of Se(VI) to Se(-II) which were rate-limiting explaining the low methylation yields, but not to the methylation itself. The shorter incubation of Se(IV) for 3-5 days showed a large Se isotope fractionation of at least -6‰ before the biomethylation reaction reached its end. This initial Se isotope fractionation during methylation of Se(IV) is much larger than previously published.

We present a comprehensive set of Se concentration and isotope ratio data collected over a 3-yr period from dissolved, sediment-hosted, and organically bound Se in a Se-contaminated lake and littoral wetland. Median isotope ratios of these various pools of Se spanned a narrow isotopic range (delta80/76Se(SRM-3149)) = 1.14-2.40 per thousand). Selenium (VI) reduction in the sediments is an important process in this system, but its isotopic impact is muted by the lack of direct contact between surface waters and reduction sites within sediments. This indicates that using Se isotope data as an indicator of microbial or abiotic Se oxyanion reduction is not effective in this or other similar systems. Isotopic data suggest that most Se(IV) in the lake originates from oxidation of organically bound Se rather than directly through Se(VI) reduction. Mobilization of Se(VI) from bedrock involves only a slight isotopic shift. Temporally constant isotopic differences observed in Se(VI) from two catchment areas suggest the potential for tracing Se(VI) from different source areas. Phytoplankton isotope ratios are close to those of the water, with a small depletion in heavy isotopes (0.56 per thousand). Fish tissues nearly match the phytoplankton, being only slightly depleted in the heavier isotopes. This suggests the potential for Se isotopes as migration indicators. Volatile, presumably methylated Se was isotopically very close to median values for phytoplankton and macrophytes, indicating a lack of isotopic fractionation during methylation.

Selenium stable isotope ratios are known to shift in predictable ways during various microbial, chemical, and biological processes, and can be used to better understand Se cycling in contaminated environments. In this study we used Se stable isotopes to discern the mechanisms controlling the transformation of oxidized, aqueous forms of Se to reduced, insoluble forms in sediments of Se-affected environments. We measured 80Se/76Se in surface waters, shallow ground waters, evaporites, digested plants and sediments, and sequential extracts from several sites where agricultural drainage water is processed in the San Joaquin Valley of California. Selenium isotope analyses of samples obtained from the Tulare Lake Drainage District flow-through wetland reveal small isotopic contrasts (mean difference 0.7%) between surface water and reduced Se species in the underlying sediments. Selenium in aquatic macrophytes was very similar isotopically to the NaOH and Na2SO3 sediment extracts designed to recover soluble organic Se and Se(0), respectively. For the integrated on-farm drainage management sites, evaporite salts were slightly (approximately 0.6%) enriched in the heavier isotope relative to the inferred parent waters, whereas surface soils were slightly (approximately 1.4%) depleted. Bacterial or chemical reduction of Se(VI) or Se(IV) may be occurring at these sites, but the small isotopic contrasts suggest that other, less isotopically fractionating mechanisms are responsible for accumulation of reduced forms in the sediments. These findings provide evidence that Se assimilation by plants and algae followed by deposition and mineralization is the dominant transformation pathway responsible for accumulation of reduced forms of Se in the wetland sediments.

The sequential extraction (SEP) of selenium to determine the isotopic composition of different Se pools in soils is an important tool for tracking biogeochemical cycling of Se in soils. An appropriate Se extraction method for the stable Se isotope analysis was established and tested for different Se pools in soils in terms of reproducibility and recovery. Soil samples (n = 4) from a seleniferous field site in Punjab, India and two standard reference materials were chosen because of their relatively high total Se concentration of 2 to 4.5 mg kg-1. Additionally, a soil sample with low Se content was utilized to verify the procedure and reproducibility. The isotopic mass-balance calculation gives consistent δ82Se values for the sum of the extracted pools, relative to δ82Se of the bulk soils. For instance, in one studied soil (soil-1), the sum of δ82Se of all individual Se-pools (δ82Semass-balance = 3.35‰) is in good agreement with the analysis of δ82Se of the bulk soil (δ82Sebulk of 3.56‰). We observed up to a 2‰ range in δ82Se among the different Se soil-pools (soil-3). Our Se isotopic results of Se soil pools provide more detailed observations of the Se linked processes in soils compared to bulk δ82Se. Thus, quantitative Se isotopic analysis of different Se pools can be an important tool for predicting the environmental cycling of selenium in soils or sediments. [ABSTRACT FROM AUTHOR]

Measurements of selenium (Se) isotope fractionation began about 1960, with work by Roy Krouse and Harry Thode on Se reduction. Se isotope research to date has confirmed these measurements, developed new measurement techniques, and investigated Se isotope fractionation during several other critical processes. As the geochemistry of Se is complex, current understanding of Se isotope systematics involves studies of many reactions, and synthesis of these results. Fractionation during dissimilatory Se reduction by pure bacterial cultures varies according to experimental conditions, but bacterial reduction in sediment slurries from three wetlands induces consistent 80Se/76Se fractionations of about 2.8‰ and 5.6‰ for Se(VI) reduction and Se(IV) reduction, respectively. Assimilation by higher plants, Se(IV) oxidation, sorption, and biological volatilization induce little or no isotopic fractionation. Preliminary experiments suggest that fractionation of about 1.0‰ accompanies assimilation of Se by algae. Thus, significant fractionation occurs in only a few reactions and if fractionation is observed in nature, it provides evidence that those reactions occur.Understanding of the systematics of other “heavy stable isotopes”, including those of calcium, iron, copper, molybdenum, chromium, and zinc, is currently being developed. Mass-dependent fractionation of these and other elements should provide a new set of tools for low-temperature biogeochemical applications, such as detecting redox reactions or distinguishing between potential contaminant sources. Some general conclusions from the Se isotopes work can be extended to the other heavy stable isotopes to aid in developing them and assessing their likely geochemical applicability. [Copyright &y& Elsevier]

The mobility and bioavailability of Se depend on its redox state, and reduction of Se oxyanions to less mobile, reduced species controls transport of this potentially toxic element in the environment. Stable isotope fractionation of Se is currently being developed as an indicator of Se immobilization through reduction. In this study, Se isotope fractionation resulting from reduction of Se(VI) and Se(IV) oxyanions by natural microbial consortia was measured in sediment slurry experiments under nearly natural conditions, with no substrate added. Experiments were conducted with a wide range of initial Se concentrations and with sediment and water from three locations with contrasting environmental settings. The products of Se(VI) and Se(IV) reduction were enriched in the lighter isotopes relative to the reactants. Shifts of −2.6‰ to −3.1‰ and −5.5‰ to −5.7‰, respectively, were observed in the 80Se/76Se ratio. These isotopic fractionations did not depend significantly on initial Se concentrations, which were varied from 22 μg/l to 8 mg/l, or on geochemical differences among the sediments. These results provide estimates of Se isotope fractionation in organic-rich wetland environments but may not be appropriate for substrate-poor aquifers and marine sediments. [Copyright &y& Elsevier]

Adsorption plays an important role in the biogeochemical cycling of selenium (Se) in natural environments and Se isotope fractionation during adsorption is a significant but poorly studied part of Se isotope system. This paper examined Se(IV) and Se(VI) adsorption onto four naturally occurring metal oxides (hematite, manganese dioxide (β-MnO 2), and α and γ-alumina oxides) and revealed Se isotope fractionation as a function of the absorbents used, Se species adsorbed, exposure time, and pH. The results show that Se(IV)/(VI) adsorption was initially rapid and was accompanied with kinetic isotope fractionations as large as 3‰, but slowed down as adsorption equilibrium was approached, eventually approaching isotopic equilibrium. The Se(IV) adsorption onto the studied metal oxides took at least 12 h to reach adsorption equilibrium, longer than Se(VI) adsorption (<60 min). At adsorption equilibrium, Se(IV) adsorption onto Fe and Mn oxides induces significant isotope fractionation, with lighter Se isotopes preferentially adsorbed, whereas Se(IV) adsorption onto Al oxides causes only small fractionation: Δ82/76Se dissolved-adsorbed = δ82/76Se dissolved − δ82/76Se adsorbed is 0.87 ± 0.12 ‰ for hematite, 1.24 ± 0.05 ‰ for β-MnO 2 , 0.08 ± 0.10 ‰ for α-alumina, and 0.05 ± 0.09 ‰ for γ-alumina at pH 5. In contrast to Se(IV) adsorption, Se(VI) adsorption does not induce detectable Se isotope fractionation. The contrasting Se isotope fractionation between Se(IV) and Se(VI) adsorption is likely related to the mechanism of adsorption onto metal oxides, which causes a structural difference between dissolved and adsorbed Se(IV)/Se(VI). In addition, pH had a strong influence on Se isotope fractionation during Se(IV) adsorption onto β-MnO 2 : Δ82/76Se dissolved-adsorbed varied from 1.24‰ to −0.08‰ as pH increased from 5 to 8. However, there was little pH effect on Se isotope fractionation during adsorption onto Fe (consistently at 0.7–0.9‰) and Al oxide (consistently at ∼0‰). Our findings show that, beside abiotic and biotic reduction, Se(IV) adsorption onto Fe-Mn oxides is potentially another important process that can induce Se isotope fractionation in Earth's surface environments. This moves an important step toward correctly reconstructing the Se isotopic composition of seawater using Fe-Mn nodules or crusts. [ABSTRACT FROM AUTHOR]

Abstract: Sorption and reduction are important processes influencing the environmental mobility and cycling of Se. In this study, we determined the rates of reaction and isotopic fractionations of Se(IV) and Se(VI) during sorption to iron oxides (2-line ferrihydrite, hematite and goethite) and iron sulfides (mackinawite and pyrite) at pH7 and room temperature (22±2°C). More than 80% of aqueous Se(IV) was removed from solution in the presence of the mineral phases, except for hematite where only 40% of aqueous Se(IV) was sorbed. In contrast, less than 20% of aqueous Se(VI) was removed in the mineral suspensions, except for 2-line ferrihydrite where approximately 50% removal was observed. While XANES spectra revealed no change in Se oxidation state when Se(IV) and Se(VI) sorbed to iron oxides, they showed evidence of reduction in the presence of iron sulfides. Selenium isotopic fractionations, expressed as ε82/76Se, were always less than 1‰ in the experiments with iron oxides (mean ε82/76Se: 0.2‰). Fractionations were significantly higher in the experiments with iron sulfides, with ε82/76Se values of up to ~10‰ in the Se(IV)-pyrite system, and a mean ε82/76Se value of 2.3‰ for all sorption experiments with iron sulfides combined. The larger fractionations in the experiments with iron sulfides reflect the chemical reduction of Se(IV) and Se(VI). The highest isotope fractionation observed in the Se(IV)–FeS2 system (9.7‰) is comparable to that previously reported for Se(VI) reduction by green rust (11.1‰). [Copyright &y& Elsevier]

Abstract: Selenium (Se) is an essential trace element, which, with multiple oxidation states and six stable isotopes, has been suggested as a potentially powerful paleoenvironmental proxy. In this study, bulk Se concentrations and isotopic compositions were analyzed in a suite of about 120 samples of fine-grained marine sedimentary rocks and sediments spanning the entire Phanerozoic. While the Se concentrations vary greatly (0.22–72ppm), the δ82/76Se values fall in a fairly narrow range from −1 to +1‰ (relative to NIST SRM3149), with the exception of laminated black shales from the New Albany Shale formation (Late Devonian), which have δ82/76Se values of up to +2.20‰. Black Sea sediments (Holocene) and sedimentary rocks from the Alum Shale formation (Late Cambrian) have Se to total organic carbon ratios (Se/TOC) and δ82/76Se values close to those found in modern marine plankton (1.72±0.15×10−6 mol/mol and 0.42±0.22‰). For the other sedimentary sequences and sediments, the Se/TOC ratios show Se enrichment relative to modern marine plankton. Additional input of isotopically light terrigenous Se may explain the Se/TOC and δ82/76Se data measured in recent Arabian Sea sediments (Pleistocene). The very high Se concentrations in sedimentary sequences that include the Cenomanian–Turonian Oceanic Anoxic Event (OAE) 2 may reflect an enhanced input of volcanogenic Se to the oceans. As the latter has an isotopic composition not greatly different from marine plankton, the volcanogenic source does not impart a distinct signature to the sedimentary Se isotope record. The lowest average δ82/76Se values are observed in the OAE2 samples from Demerara Rise and Cape Verde Basin cores (δ82/76Se=−0.14±0.45‰) and could reflect fractionation associated with microbial or chemical reduction of Se oxyanions in the euxinic water column. In contrast, a limiting availability of seawater Se during periods of increased organic matter production and burial may be responsible for the elevated δ82/76Se values and low Se/TOC ratios in the black shales of the New Albany Shale formation. Overall, our results indicate that to unlock the full proxy potential of marine sedimentary Se records, we need to gain a much more detailed understanding of the sources, chemical speciation, isotopic fractionations and cycling of Se in the marine environment. [Copyright &y& Elsevier]