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Selenium and oxygen isotope systematics can be useful tools for tracing sources and fate of Se oxyanions in water. In order to measure δ18O values of selenate, SeO42– must first be sequestered from water by precipitation as BaSeO4(s). However, other dissolved oxyanions insoluble with Ba2+ require removal. Dissolved selenate was separated from dissolved selenite, carbonate, phosphate, and arsenate by addition of Ce3+ cations that quantitatively removed these oxyanions by precipitation as insoluble Ce2(SeO3)3(s), Ce2(CO3)3(s), CePO4(s), and CeAsO4(s), respectively. δ18O-selenate (−8.19 ± 0.17 ‰) did not change after four replicates of selenite removal by Ce2(SeO3)3(s) precipitation and Ce3+ removal by cation exchange (−8.20 ± 0.14, −8.32 ± 0.09, −8.17 ± 0.13, and −8.29 ± 0.13 ‰). δ18O-selenate values (−10.86 ± 0.45 ‰) were preserved also when selenate was pre-concentrated on anion exchange resin, quantitatively retrieved by elution, and processed with Ce3+ to remove interfering oxyanions (−10.77 ± 0.07 ‰). The extraction and purification steps developed here successfully isolated dissolved selenate from interfering oxyanions while preserving δ18O-selenate values. This method should be useful for characterizing δ18O-selenate when present with the co-occurring oxyanions above in laboratory experiments and field sites with high Se concentrations, although further research is required for methods to eliminate any co-occurring sulphate. [ABSTRACT FROM AUTHOR]

The redox cycling of selenium oxyanions, elemental selenium, and selenides within water resources has implications for Se bioavailability and ecotoxicity. Dual stable isotope analysis of Se and O may provide important information for interpreting environmental Se transformations. Stable Se isotope systematics within the Se redox cycle has been well characterized within the literature, however concomitant oxygen isotope composition requires additional investigation. This study reports the O isotope fractionation of selenate ( SeO 4 2 − ) during microbial reduction by the dissimilatory Se-reducing bacterium Sulfurospirillum barnesii SES-3. Microbial reduction experiments were conducted under various conditions in order to investigate the range of 18 O enrichment factors (ε O ) in selenate. The reduction of selenate to selenite coupled to the oxidation of lactate to acetate resulted in an 18 O kinetic isotope effect with ε O values 1.5–5.8‰. Greater 18 O enrichment was observed with increasing pH, but no correlation was found between ε O and reduction rates, lactate availability, or cell density. ε O values from biotic reduction by S. barnesii here are significantly less than those observed for abiotic reduction with Fe(II)-rich minerals reported in the literature, and this difference could be explained by a diffusion limitation during enzymatic reduction. Our results expand the isotope systematics of the selenium redox cycle and suggest ε O has potential usefulness as indicators for in situ selenate reduction. [ABSTRACT FROM AUTHOR]

Immobilization of selenium oxyanions from water resources is a helpful strategy to mitigate exposure to Se at concentrations toxic to human and ecosystem health. Pyroaurite-type minerals are layered double hydroxides with high anion exchange capacity with relevance in subsurface environments and applicable to water treatment technologies. Three types of Mg(II)-Fe(III) pyroaurites and Fe(II)-Fe(III) green rusts were evaluated in kinetic studies to determine the influence of different interlayer anions on the uptake of dissolved selenate (SeO 4 2−) and selenite (HSeO 3 −, SeO 3 2−). Selenate and selenite uptake extents with pyroaurites were controlled by the identity of the interlayer anion according to the order Cl−> SO 4 2−> CO 3 2−, as predicted by known interlayer anion preferences for LDH. The kinetics of Se uptake typically expressed a rapid initial anion exchange step followed by a slow re-exchange back to solution due to competition with expelled interlayer anions. Excess dissolved interlayer anions and high pH values exacerbated this reverse exchange of Se. The results suggest Se oxyanions taken up by pyroaurites may not be firmly sequestered. Selenate uptake by redox-active green rust appears to be subject to the same interlayer anion preference as non-redox active pyroaurite, suggesting that the ability of green rust to reduce selenium oxyanions may be limited by the anion exchange process. Unlabelled Image • Se oxyanions exchange with interlayer anions of pyroaurites and green rusts. • Se exchange extent follows interlayer anion order Cl−> SO 4 2−> CO 3 2−. • Previously exchanged Se can be released back to solution. • Other anions and high pH facilitate Se re-exchange to solution. [ABSTRACT FROM AUTHOR]

Naturally occurring selenium is usually found in shales, mostly in the form of reduced selenides or elemental selenium metal. In contrast, downstream oxic waters and soils contain the oxidized forms of selenium as the oxyanions selenite (HSeO 3 −, SeO 3 2−) and selenate (SeO 4 2−). Whereas the oxidation of selenides to selenium oxyanions is possible in the presence of O 2 , the actual mechanisms of oxidation, and selenate formation in particular, are not fully understood. In this work, reactive oxygen species were evaluated for selenite oxidation within batch reactors at circumneutral pH. Complete selenite oxidation to selenate by ozone (O 3) and hypochlorite (OCl−, as a positive control) occurred within minutes and seconds, respectively. Partial oxidation of selenite to selenate by hydrogen peroxide required two weeks reaction at 2 M H 2 O 2. Hydroxyl radicals were generated by photocatalytic decomposition of H 2 O 2 and oxidized selenite completely within six hours. Singlet oxygen, superoxide, and peroxynitrite were not observed to oxidize selenite. By using selenite and H 2 O with varying δ18O isotopic compositions in oxidation experiments, it was possible to infer the two different sources of O during selenate formation. Selenate inherits three O from selenite and is suggested to acquire the fourth O via O transfer from the oxidants studied here. [Display omitted] • Pathways of selenite oxidation to selenate were investigated. • Oxidants included ozone, hypochlorite, hydroxyl radicals, and hydrogen peroxide. • Se(VI) forms by incorporating one O from the oxidant into three O from Se(IV). [ABSTRACT FROM AUTHOR]

Adsorption of selenium oxyanions (selenite and selenate) on soil minerals can mitigate Se ecotoxicity effects by removing Se from water resources. Se oxyanion surface complexation on Fe oxides have been investigated for decades using spectroscopic and surface complex modeling. Here, insights on the Fe-O-Se bonding process on goethite and hematite were gained via integrating our observed batch Se adsorption isotherms, H+ coadsorption with Se, X-ray absorption spectroscopy (XAS)-derived surface complex geometry, surface complexation modeling, and 18O tracing across oxide and aqueous phases Adsorption extents of selenite and selenate on goethite and hematite decreased as pH increased from 3.0 to 7.0 and follows a corresponding decrease in positive zeta potential which offers less electrostatic attraction. Surface complexation geometry was determined with XAS results that indicated the formation of inner-sphere binuclear bidentate selenite complexes and outer-sphere selenate complexes on both oxides within the range of experimental conditions. These surface complexes were sufficient to model Se adsorption isotherms and H+ coadsorption on both oxides at pH 3.0, 5.0, and 7.0 using the Charge Distribution Multi-site Complexation (CD-MUSIC) model. Inner-sphere bidentate selenite complex formation requires release of two O, and by tracing 18O release from 18O-enriched oxides to isotopically normal water, it was found that O release from the oxide surface accounted for 22% and 7% of total adsorption-induced O release for goethite and hematite, respectively, but only at pH 3.0. The remaining O release is assumed to be from adsorbed selenite. Selenate at pH 3.0 also induced a slight increase in 18O enrichment in bulk solution, which is unexpected for selenate's outer-sphere configuration that should produce no O ligand exchange, but instead highlights some spontaneous adsorption-induced oxide O release. Image 1 • Selenite formed inner-sphere, bidentate binuclear complexes on goethite and hematite. • Selenate formed outer-sphere complexes on goethite and hematite. • CD-MUSIC modeling fits Se and H+ adsorption isotherms with these complexes at varying pH. • Some sorption-induced O release from oxide surface hydroxyl groups occurs only at acidic pH. [ABSTRACT FROM AUTHOR]