Oxygen isotope effects in selenate during selenium redox cycling

Abstract

Selenium contamination of environmental systems poses a threat to ecosystems and human health. Of the four valence states of selenium, the oxyanions selenate and selenite are the most mobile, and developing a complete understanding of the mechanisms of selenium cycling and selenium oxyanion immobilization is paramount in assessing the efficacy of natural attenuation and in developing efficient engineered methods for selenium removal. Compound specific isotope analysis is a tool that shows promise for identifying mechanisms of transport and transformation of environmental contaminants, and has been applied to some extent using selenium isotopes to examine selenium cycling. This work develops the applicability of oxygen stable isotope analysis for the purpose of identifying and examining selenium oxyanion transformations and immobilization in natural systems. Oxygen isotopes may be used in conjunction with selenium isotopes to provide a more complete picture of selenium cycling. The immobilization of selenate via reduction by the layered double hydroxide (LDH) chloride green rust was found to cause a fractionation in the oxygen isotope of selenate, resulting in enrichment of the heavier isotope (22.7%0) much larger than the one identified for selenium isotopes for reduction by green rust. The biotic reduction of selenate by the bacterium Sulfurospirillum barnesii resulted in enrichment of the heavy oxygen isotope (1.5-5.8%0) that was distinct from the value found for reduction by green rust, indicating that these isotopes may be useful for distinguishing between the two dominant methods of selenate reduction in natural systems. These experiments also suggest that mass transfer prior to reduction may have a muting effect on fractionation of the oxygen isotope. Kinetic questions raised by the isotope work with chloride green rust were addressed through sorption experiments of selenate and selenite by the non-reducing LDH pyroaurite. It was found that the affinity and capacity of the LDH for selenate was affected by the interlayer anion preference of the LDH; selenate exchanges rapidly with chloride but not as rapidly with sulfate, and very little exchange occurs when carbonate is the interlayer anion. This mass transfer trend extended to selenate reaction with different types of green rust, and may impact isotope fractionation. The impact of mass transfer on oxygen isotope fractionation was also examined for surface photocatalytic reduction with TiO₂, which was found to cause an enrichment of 23.7%0 in the oxygen isotope. Further work to establish the impact and trend associated with mass transfer prior to reduction examined electrochemical reduction (no mass transfer) and enzymatic reduction (no mass transfer); this work is ongoing. Fungal uptake of selenate was examined, but was found to be too low to be measureable using the current method of oxygen isotope analysis. Microbial oxidation of selenite to selenate (a re-mobilization process) was also examined; no oxidation was observed but some reduction did occur. It appears that oxygen isotope analysis is a useful tool for both identifying and dissecting selenate reduction processes, but is less applicable at this time for uptake processes which operate at selenate uptake concentrations below the processing limit.