Chemical oxidation of selenite to selenate: Evaluation of reactive oxygen species and O transfer pathways

Abstract

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 (HSeO3−, SeO32−) and selenate (SeO42−). Whereas the oxidation of selenides to selenium oxyanions is possible in the presence of O2, 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 (O3) 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 H2O2. Hydroxyl radicals were generated by photocatalytic decomposition of H2O2 and oxidized selenite completely within six hours. Singlet oxygen, superoxide, and peroxynitrite were not observed to oxidize selenite. By using selenite and H2O 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.