Correlative insights into the enhanced reactivity of sulfidated zero-valent iron from different origins for the decolorization of various azo dyes under aerobic conditions

The sulfidation of zero-valent iron (S-ZVI) has emerged as an effective strategy to enhance reactivity for contaminants removal, yet the underlying mechanisms remain incompletely understood. This study explores the enhanced reactivity of S-ZVI derived from five different commercial sources in the aerobic removal of four representative azo dyes. Sulfidation converts the passive Fe-oxide film into a conductive FeS x layer, thereby facilitating interfacial electron transfer from the Fe 0 core to adsorbed dye molecules, and promoting sustained Fe(II) regeneration for decolorization of azo dyes. It demonstrates that the rate constants of S-ZVI for azo dyes removal exhibited 1.1–2.3 fold higher than unsulfidated ZVI, with strong linear correlations across dyes and sources (R = 0.69–0.98), suggesting predictable reactivity scaling. Correlation analysis, spectroscopic evidences and chelation–quenching tests jointly confirm that low charge-transfer resistance and sustained Fe(II) release govern the enhanced reductive reactivity, whereas radical-mediated oxidation contributes only as a secondary pathway. Consequently, S-ZVI exhibited efficient azo-bond cleavage, partial mineralization, and retained over 80% of its reactivity toward azo dye after six reuse cycles under aerobic conditions. Overall, this work establishes quantitative structure–reactivity relationships linking sulfur speciation, interfacial electrochemical properties, Fe(II) dynamics, and degradation kinetics, providing a predictive framework for designing durable and high-performance ZVI-based materials for wastewater treatment.