Mechanistic Effects of Preparation Methods on the Composition, Structure, and Cr(Ⅵ) Removal Performance of 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 aerial oxidation processes affect the structural properties and Cr(Ⅵ) removal activity of S-nZVI remain unclear, thereby limiting its application in water remediation. In this study, S-nZVI was synthesized via the liquid-phase reduction method, and the effects of sulfidation approach, S/Fe molar ratio, and aerial oxidation on its composition, structure, and Cr(Ⅵ) removal efficiency were investigated. The reaction mechanism was elucidated via aqueous-phase analyses and S-nZVI material characterization before and after reaction. The results showed that Cr(Ⅵ) removal by S-nZVI followed pseudo-second-order kinetics. The rate constant (k₂) initially increased and then decreased with increasing S/Fe molar ratio, reaching a maximum of 0.556 g/(mg·min) at an S/Fe ratio 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 nZVI exhibited a removal efficiency of only 58%. The FeS_x layer in S-nZVI significantly enhanced its oxidation resistance. The reaction mechanism between S-nZVI and Cr(Ⅵ) primarily involved Cr(Ⅵ) adsorption, reduction, and (co)precipitation at the solid–liquid interface, while a minor fraction of Cr(Ⅵ) was directly reduced and precipitated in the aqueous phase. The influence of sulfidation preparation methods and oxidation processes on the composition, structure, and Cr(Ⅵ) removal efficiency of S-nZVI was revealed, providing valuable data support for the performance regulation and preparation optimization of S-nZVI.