INSIGHTS TO SELENIUM REDOX REACTIONS USING STABLE OXYGEN ISOTOPES

Selenium accumulation in water resources has lead to adverse biological effects in wildlife and health threats to humans, prompting a need for large-scale monitoring and bioremediation efforts. Over the past decade, stable Se isotope studies have provided valuable insights to Se redox cycling and environmental Se source identification. Now, stable oxygen isotopic analysis (¹⁸O) of Se oxyanions will provide a new approach to characterizing Se oxyanion redox transformations, with hopes that isotopic fractionations observed in the laboratory will support Se environmental forensic studies. Our work examines the usefulness of ¹⁸O to indicate specific reaction pathways through unique kinetic enrichment factors and to clarify reaction pathways through isotope tracing. Here, we report ¹⁸O kinetic isotope fractionation of selenate during reduction to selenium by green rusts, one of the few naturally-occurring abiotic reductants for selenate. Chloride green rusts formed via different synthesis routes and ferrous hydroxide produce slightly different ¹⁸O enrichments in unreacted dissolved selenate. The overall reaction can be described with a rapid initial step of selenate uptake into interlayer sites (with no fractionation) followed by a slower reduction step (with fractionation). Rayleigh fractionation behavior for unreacted dissolved selenate was observed only after reduction was complete, when all green rust was oxidized to magnetite, and not during the initial uptake step. The results suggest that ¹⁸O isotopic fractionation can indicate selenate reduction within Fe(II)-rich environments but not necessarily selenate sequestration into non-redox active layered double hydroxides. Additional studies will explore ¹⁸O isotopic fractionation of Se sorption and oxidation reactions in both biotic and abiotic systems.