Effect of solution chemistry on the reactivity and electron selectivity of zerovalent iron toward Se(VI) removal

Both the reactivity and electron selectivity (ES) of zerovalent iron (ZVI) are critical for successful application of ZVI. In this study, taking Se(VI) as a probe contaminant, the influences of pH (3.0–10.0) and coexisting ions (e.g., Ca²⁺, Mg²⁺, Cl⁻, SiO₃ ²⁻, SO₄ ²⁻, and PO₄ ³⁻) on the reactivity and ES of ZVI were comprehensively investigated. It was demonstrated that, the ES of ZVI was low in general and merely 0.9–12.1% of electrons donated by ZVI were effectively used for Se(VI) reduction under different conditions. Both Se(VI) removal rate and ES were significantly influenced by pH and both of them achieved maximum at pH 5.0, while further increasing or decreasing pH values would deteriorate the ZVI performance. Depending on the types and concentrations, coexisting ions could exert different effects on ZVI reactivity and ES, negatively, negligibly or positively. For example, relative to Cl⁻, Ca²⁺, Mg²⁺, and SiO₃ ²⁻ did not show any significant influence on Se(VI) removal rate but could enhance ES by 3.0–50.3%. The presence of SO₄ ²⁻ and PO₄ ³⁻ hindered Se(VI) removal rate during their tested concentration range, while they could enhance ES at a low concentration level and depress ES at high concentration levels. X-ray absorption fine structure analysis revealed that, pH and coexisting ions could influence the transformation of ZVI corrosion products and thereby impact the electron transfer from ZVI to Se species at the water-particle interface.