In Situ Synergistic Reduction and Oxidation of Trichloromethane Enabled by Oxalate–Sulfur Co-Modified Zero-Valent Iron

Trichloromethane (TCM), a recalcitrant emerging contaminant frequently detected in groundwater systems, exhibits limited susceptibility to conventional redox-based degradation processes─particularly those relying solely on either reductive or oxidative pathways. This study elucidates the synergistic degradation mechanism enabled by ball-milled oxalate- and sulfur-co-modified zero-valent iron (S-OA-ZVI). The S-OA-ZVI composite achieved 74.39% removal of 10 mg/L TCM within 120 h, with an observed pseudo-first-order degradation rate constant that was 13 times greater than that of unmodified ZVI. The comodification effectively mitigated surface passivation by disrupting the native iron oxide layer, promoting in situ formation of highly conductive crystalline phases: ferrous oxalate and ferrous disulfide. These phases cooperatively facilitated the generation of multiple reactive species─including adsorbed atomic hydrogen (·Hads), hydroxyl radicals (•OH), and oxalate-derived radicals (•C2O4–)─thereby enabling concurrent reductive dehalogenation and oxidative mineralization pathways for TCM transformation. Critically, the oxalate–sulfur comodification engendered a stable, self-sustaining redox-coupled interface, markedly improving the degradation efficiency of structurally persistent halogenated contaminants in complex aqueous matrices. Collectively, this work establishes oxalate–sulfur comodification as a rational and scalable materials design strategy for engineering high-performance ZVI-based remediation agents, with broad implications for the treatment of recalcitrant groundwater pollutants.