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 NO<sub>3</sub><sup>–</sup> and SO<sub>4</sub><sup>2–</sup>. Environmentally relevant Se­(VI) (1 mg L<sup>–1</sup>), NO<sub>3</sub><sup>–</sup> ([NO<sub>3</sub>N] = 15 mg L<sup>–1</sup>), and SO<sub>4</sub><sup>2–</sup> (1800 mg L<sup>–1</sup>) were employed to simulate mining-impacted waters. Ninety percent of Se­(VI) removal was achieved within 4–8 h in the absence of SO<sub>4</sub><sup>2–</sup> and NO<sub>3</sub><sup>–</sup>. A similar Se­(VI) removal rate was observed after 10–32 h in the presence of NO<sub>3</sub><sup>–</sup>. Dissolved Se­(VI) removal rates exhibited the highest decrease in the presence of SO<sub>4</sub><sup>2–</sup>; 90% of Se­(VI) removal was measured after 50–191 h for SO<sub>4</sub><sup>2–</sup> and after 150–194 h for SO<sub>4</sub><sup>2–</sup> plus NO<sub>3</sub><sup>–</sup> 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 [Fe<sub>3</sub>O<sub>4</sub>], 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.