Dynamic sulfidation process of nanoscale zero-valent iron induced by sulfate-reducing bacteria and its effect on the reductive dechlorination of trichloroethylene.

Chlorinated aliphatic hydrocarbons (CAHs), particularly trichloroethylene (TCE), are pervasive groundwater contaminants with severe environmental risks, necessitating efficient remediation technologies. While the composite system of nanoscale zero-valent iron (nZVI) and non-dechlorinating bacteria, such as sulfate-reducing bacteria (SRB), exhibits synergistic capabilities for remediating TCE, the underlying dynamic processes remain elusive. Specifically, the SRB-driven biogenetic sulfidation of nZVI, along with the subsequent evolution of sulfidation products and their impacts on TCE degradation efficiency in dechlorination and mechanisms, remains inadequately understood. This study reports the dynamic sulfidation of nZVI by SRB and its effect on TCE dechlorination. Results showed that at an optimal S/Fe ratio of 0.5, the SRB-nZVI composite system achieved a 96.17 ± 1.07% TCE dechlorination over 96 h, with a reaction rate constant of 0.0358 h-1. TCE was primarily converted to acetylene (71.50 ± 1.18%) via the β-elimination pathway, as the generated FeSx layer inhibited hydrogen evolution. Product analyses across different S/Fe ratios indicated that low ratios favored reactive FeS formation due to limited biogenetic sulfide, whereas higher ratios formed chemically stable FeS2, decreasing the system reactivity. A dynamic pathway involving "metabolic drive and slow sulfur release", which promotes the nucleation of ferrous sulfide on nZVI and ultimately forms stable mackinawite (FeS) or marcasite (FeS2), accompanied by a multi-step TCE dechlorination mechanism, has been proposed. This study elucidated the efficacy, interaction processes, and mechanisms of TCE dechlorination by the SRB-nZVI composite system, providing a crucial foundation for the development of sustainable, controllable, and effective in-situ groundwater remediation technologies.