Selenate removal by zero-valent iron under anoxic conditions: effects of nitrate and sulfate

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).