Mechanistic Insights into the Selectivity of Sulfidated Zero-Valent Iron Materials in Chlorinated Ethenes Removal: A DFT Study

Abstract

Chlorinated ethenes (CEs), such as perchloroethene (PCE) and trichloroethene (TCE), are pervasive groundwater contaminants. Owing to their toxic properties, there is a considerable effort for their remediation. In this context, in situ CE chemical reduction using zero-valent iron (ZVI) materials represents a promising strategy. However, the intrinsic low electron selectivity of pristine ZVI often results in its rapid surface corrosion and passivation in subsurface environments. In the last years, sulfidation has emerged as an effective means to enhance the reactive lifetime of ZVI. Despite the efficiency of sulfidated ZVI (S-ZVI) in dechlorinating TCE and trans-1,2-dichloroethene (trans-DCE), a notably lower reactivity has been typically observed for PCE and cis-1,2-dichloroethene (cis-DCE). The mechanisms governing the variable reactivity of S-ZVI with different CEs remain poorly understood. To shed more light on the mechanisms controlling S-ZVI selectivity, we calculated the dechlorination barriers of various CEs at multiple S-ZVI surface models using density functional theory (DFT). Specifically, we focused on the electron transfer-controlled β-elimination reactions, identified as the predominant pathway for CE dechlorination with S-ZVI. Reactions of PCE, TCE, and both cis- and trans-DCE isomers were investigated at different S-ZVI surface sites, including surfaces with varying sulfur coverage.  Our calculations revealed that CE dechlorination reactions are both kinetically and thermodynamically more favorable at Fe sites compared to S sites. This finding indicates that the overall promoting effect of ZVI sulfidation on CE degradation is indirect, primarily involving the protection of the ZVI surface from corrosion in water. Sulfur coverage was identified to significantly influence the S-ZVI selectivity for individual CEs. Under low S coverage, the reactivity of Fe sites followed the order trans-DCE ≈ TCE > cis-DCE > PCE, with PCE degradation hindered by steric effects from nearby S atoms. Conversely, at high S coverage, Fe sites were sterically hindered for all CEs, and reactivity was controlled by S sites. In this scenario, energy barriers correlated with the energy of the lowest unoccupied molecular orbital (ELUMO) of CEs in the order PCE < TCE < DCE isomers. These findings demonstrate that the experimentally observed trends in S-ZVI selectivity for individual CEs can be explained by the interplay between the affinity of CEs for electron transfer and steric effects of S atoms at the ZVI surface.   Acknowledgments This work was funded by the Austrian Science Fund (FWF), project M 2892-N. The Vienna Scientific Cluster (project no. 70544) is gratefully acknowledged for providing computational resources.