Sulfidation represents a promising approach to increase the reactivity, selectivity, and longevity of zero-valent iron (ZVI) in groundwater remediation applications. Recent studies suggest that reductive reactions mediated via adsorbed H* may dominate the degradation of prominent contaminants, such as chlorinated ethenes, on sulfidated ZVI (S-ZVI) surfaces with moderate S coverage, challenging the initially proposed major role of direct electron transfer. This study employs density functional theory to investigate how S coverage and surface corrosion influence H* formation, stability, mobility, and recombination at S-ZVI surfaces at atomic resolution. Our calculations reveal that sulfidation suppresses water adsorption and H* formation via water dissociation, while also weakening H* adsorption affinity on ZVI. However, as surface oxidation also hinders H* adsorption and promotes H* recombination, S-ZVI with moderate (∼14 monolayer) S coverage retains more reduced Fe sites, which are favorable for H* adsorption, compared to the corroded ZVI surface. Adsorbed H* at the reduced Fe sites exhibits restricted mobility near S atoms, limiting H* recombination and increasing its availability for contaminant degradation. These findings provide a fundamental mechanistic understanding of increased H* retention at S-ZVI surfaces with moderate S coverage, with implications for the role of H*-mediated reactions in these systems.

Regional Centre of Advanced Technologies and Materials Czech Advanced Technology and Research Institute Palacký University Olomouc Šlechtitelů 27 779 00 Olomouc Czech Republic

University of Natural Resources and Life Sciences Vienna Department of Ecosystem Management Climate and Biodiversity Institute of Soil Research Peter Jordan Straße 82 1190 Vienna Austria

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