Passivation and sulfidation depassivation of zero-valent iron: Mechanism, properties characterization and Cr(VI)...

Zero-valent iron (ZVI) is widely used in environmental remediation owing to its strong reducing properties. However, passivation deactivation seriously limits the long-term performance. This study explored the passivation characteristics of ZVI under common conditions (air, oxygenated water, and pollutants) and elucidated the mechanism of sulfidation depassivation. Results showed that the passivation conditions determined the physicochemical properties of passivated ZVI. Passivation in air forms low-crystalline α-Fe₂O₃/γ-Fe₂O₃ layers with limited electron transfer capability (ETC). Passivation in oxygenated water leads to the formation of a porous structure dominated by γ-FeOOH, which exhibits a 100-fold increase in specific surface area and enhanced ETC due to the presence of Fe(II)-O. Passivation in Cr(VI) solution forms α-Fe₂O₃/γ-Fe₂O₃ and Cr-Fe complex layers, which almost lose ETC. Sulfidation effectively removes the passivation layer and forms an FeS layer with excellent ETC; however, its depassivation performance depends on the passivation conditions. Sulfidation of ZVI passivated in air produces highly crystalline FeS with limited thickness. Sulfidation of ZVI passivated in oxygenated water generates low-crystallinity FeS containing iron-oxides (γ-FeOOH and α-Fe₂O₃), with the smallest internal/external S/Fe molar ratio discrepancy and the strongest ETC. Sulfidation of ZVI passivated in Cr(VI) solution produces low-crystallinity FeS and Fe₃S₄, with the highest O content (up to 77.6 %) and the lowest ETC. Importantly, the Cr(VI) removal ability of passivated and sulfidation-depassivated ZVI is tightly regulated by the passivation conditions and time, and particle size. This paper proposes a sulfidation strategy to address the passivation deactivation of ZVI, thereby promoting its sustainable application in environmental remediation.