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 (k = 0.0617 min-1), 60.7% greater than that of Fe(OH)2 (k = 0.0384 min-1) and 278.5% greater than that of Fe0 (k = 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.