Coupling of Fe ⁰ and Fe(OH) ₂ Enhances Carbon Tetrachloride Reduction

Fe(OH)₂ and Fe⁰ often coexist in reductive systems, yet the mechanistic significance of Fe(OH)₂ has been largely underestimated. Our results suggest that Fe(OH)₂ plays a dominant role in contaminant reduction as the key reactive mineral intermediate (RMI), while Fe⁰ appears to function primarily as an auxiliary promoter. Using carbon tetrachloride (CT) as a model contaminant, we evaluated the Fe(OH)₂ + Fe⁰ system through batch kinetics, density functional theory (DFT) calculations, and complementary spectroscopic analyses. The composite exhibited markedly higher reactivity (<i>k</i> = 0.0617 min^-1), 60.7% greater than that of Fe(OH)₂ (<i>k</i> = 0.0384 min^-1) and 278.5% greater than that of Fe⁰ (<i>k</i> = 0.0163 min^-1). DFT calculations indicated that Fe⁰ facilitated CT adsorption and interfacial electron transfer, consistent with XAFS evidence excluding direct Fe-O bonding. Surface analyses further reveal that Fe⁰ modulated the charge and structure, whereas Mössbauer data support Fe(OH)₂ likely acts as the principal electron donor. Overall, our results indicate that Fe(OH)₂ as the critical transient phase governing Fe⁰ reactivity under alkaline anaerobic conditions, with its synergy with Fe⁰, may provide mechanistic insights for designing more effective Fe-based reductants in groundwater remediation.