Key role of hydrogen atoms in the preparation of sulfidated zero valent iron

Sulfidated zero valent iron (ZVI) is a popular material for the reductive degradation of halogenated organic pollutants. Simple and economic synthesis of this material is highly demanded. In this study, sulfidated micro/nanostructured ZVI (MNZVI) particles were prepared by simply heating MNZVI particles and sulfur elements (S⁰) in pure water (50℃). The iron oxides on the surface of MNZVI particles were conducive to sulfidation reaction, indicating the formation of iron-sulphide minerals (FeS_x) on the surface of MNZVI particles might not be from the direct reaction of Fe⁰ with S⁰ (Fe⁰ and S⁰ acted as reductant and oxidant, respectively). As an important reductant, hydrogen atom (H•) can be generated from the reduction of H⁺ by MNZVI particles and participate in the formation of FeS_x. Quenching experiment and cyclic voltammetry analysis proved the existence of H• on the surface of MNZVI particles. DFT calculation found that the potential barrier of H•/S⁰ and Fe⁰/S⁰ were 1.91 and 7.24 eV, respectively, indicating that S⁰ would preferentially react with H• instead of Fe⁰. The formed H• can quickly react with S⁰ to generate hydrogen sulfide (H₂S), which can further react with iron oxides such as α-Fe₂O₃ on the surface of MNZVI particles to form FeS_x. In addition, the H₂ partial pressure in water significantly affected the amount of H• generated, thereby affecting the sulfidation efficiency. For TCE degradation, as the sulfur loading of sulfidated MNZVI particles increased, the contribution of H• significantly decreased while the contribution of direct electron transfer increased. This study provided new insights into the synthesis mechanism of sulfidated ZVI in water.