Selenate removal by zero-valent iron under anoxic conditions: effects of nitrate and sulfate
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Batch experiments were conducted to examine aqueous Se(VI) removal by zero-valent iron (ZVI) under anoxic conditions in the presence and absence of NO3− and SO42−. Initial concentrations for Se(VI), SO4, and NO3–N of 5 mg L−1, 1800 mg L−1, and 13 mg L−1, respectively, were employed to mimic mine waters. In the control experiment, 90% Se(VI) removal occurred within 1.5 h without SO42− and NO3− (B1). This removal threshold was reached after 3 h with NO3− added (B3) and after 33 h with SO42− added (B2). Removal reached 90% after 42 h with both SO42− and NO3− added (B4). Modeled Se(VI) removal rates consistently followed first-order kinetics and revealed that the presence of SO42− and, to a lesser extent, NO3− inhibited Se(VI) removal. Increases in pH and Fe coupled with decreasing Eh are consistent with ZVI corrosion under anoxic conditions. Transmission electron microscopy, Raman spectroscopy, and X-ray diffraction revealed magnetite [Fe3O4] and lepidocrocite [γ-FeOOH] formed at ZVI surfaces during the experiments. X-ray absorption near edge structure spectroscopy indicated that Se(VI) was predominantly reduced to Se(0) (70–80%), but Se(IV) (10–13%) and Se(-II) (2–13%) were also detected at reacted ZVI surfaces. Overall, the results show that although SO42− and NO3− present in mine wastes can reduce reaction rates, Se(VI) removal by ZVI under anoxic conditions is associated with extensive reduction to insoluble Se(0).
KeywordsZero-valent iron Selenate Removal Reduction Sulfate Nitrate
The authors acknowledge funding provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) IRC program (MJH; Grant No. 184573) and Discovery Grant program (MBJL; Grant No. RGPIN-2014-06589). The authors also acknowledge the Canadian Light Source, whose operations are supported by the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research. The authors also thank Ms. Fina Nelson and Dr. Jared Robertson for assistance provided during batch and XAS experiments, respectively.
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