Influence of coexisting ions on the electron efficiency of sulfidated zerovalent iron toward Se(VI) removal

Recently, sulfidation of zerovalent iron (ZVI) has gained increasing attention due to its merits to enhance both the reactivity and electron efficiency (EE) of ZVI. While few studies have been conducted to elucidate the influence of coexisting ions on EE of sulfidated ZVI, which is important for up-scaling such a decontamination strategy. In this study, taking Se(VI) as a probe, the influence of coexisting ions, including Cl⁻, SO₄²⁻, PO₄³⁻, Ca²⁺, and Mg²⁺, on the removal capacity and EE of ZVI under aerobic conditions was investigated. Results revealed that, compared to unamended ZVI, sulfidated ZVI synthesized by ball-milling with elemental sulfur (S-ZVI^bm) could enhance EE of ZVI toward Se(VI). However, this enhancing effect is not very effective and the maximum of EE value obtained in this work is only 8.8%. Chloride with concentration range of 1–20 mM was found to exert little influence on EE of ZVI. While for SO₄²⁻ and PO₄³⁻, relative to 10 mM NaCl, their presence could depress EE of S-ZVI^bm, and this inhibiting effect became more pronounced as their concentration elevated. In contrast, Ca²⁺ or Mg²⁺ hardness ions (0.05–2 mM) increased the EE values of S-ZVI^bm by 19.2–83.3%. X-ray absorption fine structure analysis revealed that all of Fe⁰ in S-ZVI^bm was exhausted within 24 h with lepidocrocite and magnetite being the primary corrosion products. This suggested that it was the amounts of electrons accepted by Se(VI) rather than that donated by S-ZVI^bm accounted for the variation of EE. In general, these co-solutes may impose their influence on EE through affecting Se(VI) adsorption and the subsequent electron transfer from Fe⁰ core to Se(VI) at the water-particle interface.