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

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 SO₄ ^2-. Environmentally relevant Se(VI) (1 mg L^-1), NO₃ ^- ([NO₃-N] = 15 mg L^-1), and SO₄ ^2- (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 SO₄ ^2- 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 SO₄ ^2-; 90% of Se(VI) removal was measured after 50-191 h for SO₄ ^2- and after 150-194 h for SO₄ ^2- 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.