Selenium Isotope Shifts during the Oxidation of Selenide-Bearing Minerals

Oxidation of selenium (Se) largely drives the mobilization of soluble selenium oxyanions from rock and soil. Isotopic fractionation of selenium (82Se/76Se) has been used to track attenuating processes like reduction of Se(IV) and Se(VI). Isotopic shifts associated with oxidative dissolution of selenide-bearing minerals are poorly understood, despite their potential importance in determining 82Se/76Se of selenium sources and causing isotopic variation in contaminated systems. We examined 82Se/76Se of dissolved Se(IV) and Se(VI) during the oxidation of ferroselite (FeSe2) and berzelianite (Cu2Se) with low and high initial hydrogen peroxide (H2O2) concentrations in solution (0.007-1 mM) and under atmospheric oxygen levels. The 82Se/76Se of Se(VI) produced by oxidation in the experiments ranged from 1.5 to 14‰ greater than the initial minerals. These isotopic shifts arise from oxidation of Se(IV), reduction of Se(IV) or Se(VI) by mineral phases, and/or isotopic exchange between Se(IV) and Se(VI). At low concentrations of H2O2, isotopic fractionation associated with the reduction of Se(IV) to Se(0) on ferroselite is apparent. At high concentrations of H2O2, this process for ferroselite and berzelianite oxidation is either absent or diluted by a larger flux of Se(IV) created by rapid mineral oxidation. As Se(VI) is more mobile than Se(IV), our results suggest that oxidative weathering of selenium-bearing minerals, previously thought to induce minimal isotopic fractionation, tends to produce an isotopically heavy selenium weathering flux.