Dissolved Selenium(VI) Removal by Zero-Valent Iron under Oxic Conditions: Influence of Sulfate and Nitrate.

Abstract

Dissolved Se(VI) removal by three commercially available zero-valent irons (ZVIs) was examined in oxic batch experiments under circumneutral pH conditions in the presence and absence of NO3– and SO42–. Environmentally relevant Se(VI) (1 mg L–1), NO3– ([NO3—N] = 15 mg L–1), and SO42– (1800 mg L–1) were employed to simulate mining-impacted waters. Ninety percent of Se(VI) removal was achieved within 4–8 h in the absence of SO42– and NO3–. A similar Se(VI) removal rate was observed after 10–32 h in the presence of NO3–. Dissolved Se(VI) removal rates exhibited the highest decrease in the presence of SO42–; 90% of Se(VI) removal was measured after 50–191 h for SO42– and after 150–194 h for SO42– plus NO3– depending on the ZVI tested. Despite differences in removal rates among batches and ZVI materials, Se(VI) removal consistently followed first-order reaction kinetics. Scanning electron microscopy, Raman spectroscopy, and X-ray diffraction analyses of reacted solids showed that Fe(0) present in ZVI undergoes oxidation to magnetite [Fe3O4], wüstite [FeO], lepidocrocite [γ-FeOOH], and goethite [α-FeOOH] over time. X-ray absorption near-edge structure spectroscopy indicated that Se(VI) was reduced to Se(IV) and Se(0) during removal. These results demonstrate that ZVI can be effectively used to control Se(VI) concentrations in mining-impacted waters.