Selenium sorption and isotope fractionation: Iron(III) oxides versus iron(II) sulfides

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 ε^82/76Se, were always less than 1‰ in the experiments with iron oxides (mean ε^82/76Se: 0.2‰). Fractionations were significantly higher in the experiments with iron sulfides, with ε^82/76Se values of up to ~10‰ in the Se(IV)-pyrite system, and a mean ε^82/76Se 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‰).