Selenium contamination of water resources occurs worldwide, with the more well-known example being the widespread selenium occurrence within irrigation canals, lakes and wastewaters in the western U.S. Selenium is usually found within shales, mostly in the form of reduced selenides as well as elemental Se0 metal. In contrast downstream waters and soils contain the more thermodynamically favorable selenium compounds as the oxyanions selenite (SeO32-) and selenate (SeO42-). Although selenite could possibly be formed through oxidation of selenides in the presence of O2, the actual selenate formation process, mechanism, and conditions of how selenate acquires its oxygen atoms are not fully understood. The goal of this study was to investigate selenite oxidation kinetics and reaction pathways by employing a novel stable oxygen isotopic approach. Selenite oxidation by possible reactive oxygen species was investigated within batch reactors containing 5 mM selenite at circumneutral pH. Complete selenite oxidation to selenate by ozone (O3) and hypochlorite (OCl-, as a positive control) was observed to be almost instantaneous, while 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. By using selenite with varying d18O isotopic compositions (induced by automatic scrambling selenite-O with O-H2O), it was possible to trace sources of O for selenate formation. For all four oxidants, selenate formation was suggested to occur by inheriting three O atoms from selenite and its fourth O from the oxidant. Other reactive oxygen species examined that did not oxidize selenite were superoxide and peroxynitrite.

Chemical oxidation of selenite to selenate by reactive oxygen species

254th National Meeting (2017)

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