Mechanism of Preparation Method Effects on the Composition, Structure, and Cr(Ⅵ) Removal Efficiency S-nZVI

Sulfidized nanoscale zero-valent iron (S-nZVI) has received significant attention as a highly reactive and selective environmental remediation material. However, the mechanisms by which different preparation methods and air oxidation processes affect the structural properties and Cr(Ⅵ) removal activity of S-nZVI remain unclear, limiting its application in water remediation. This study synthesized S-nZVI through the liquid-phase reduction method and systematically investigated the effects of sulfidation approach, S/Fe molar ratio, and aerial oxidation on its composition, structure, and efficiency for Cr(Ⅵ) removal. The reaction mechanism was elucidated via aqueous-phase analyses and S-nZVI material characterization before and after reaction. The results showed that the removal of Cr(Ⅵ) by S-nZVI conformed to pseudo-second-order kinetics. The rate constant (k₂) initially increased and then decreased as the S/Fe molar ratio increased, reaching a maximum of 0.556 g/(mg·min) at an S/Fe of 0.55. This optimal performance was mainly attributed to a larger surface area and higher content of reduced sulfur species (S²⁻, S₂²⁻, and S_n²⁻), among which S₂²⁻ played a pivotal role in promoting electron transfer and enhancing electron selectivity. After 14 days of aerial oxidation, S-nZVI maintained a high Cr(Ⅵ) removal efficiency of 73%-100%, whereas the removal efficiency of Cr(Ⅵ) by nZVI dropped to 58%. The FeS_x layer in S-nZVI significantly enhanced its oxidation resistance. The reaction mechanism between S-nZVI and Cr(Ⅵ) mainly involved Cr(Ⅵ) adsorption, reduction, and (co)precipitation at the solid-liquid interface, with a minor fraction of Cr(Ⅵ) being directly reduced and precipitated in the aqueous phase. This study systematically reveals the influence of sulfidation preparation methods and oxidation processes on the composition, structure, and Cr(Ⅵ) removal efficiency of S-nZVI, providing valuable data support for the performance regulation and preparation optimization of S-nZVI.