Mechanism of Increased Retention of Atomic Hydrogen on Moderately Sulfidated Zero-Valent Iron Surfaces

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 (∼$\frac{1}{4}$ 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.