Anaerobic Reaction of Nanoscale Zerovalent Iron with Water: Mechanism and Kinetics

Nanoscale zerovalent iron (nZVI) is commonly used in advanced groundwater remediation processes. Here, we present a combined experimental and computational approach to elucidate the mechanism and kinetics of the reaction of nZVI with water under anaerobic conditions, which represents the basic reaction controlling the stability of nZVI in groundwater. The reaction kinetics was monitored at temperatures of 25 and 80 °C by 57Fe Mössbauer spectroscopy on frozen dispersion samples. The experimentally determined rate constant for reaction of nZVI with water at 25 °C was 1.14 × 10–3 h–1; the activation barrier measured for 60 nm sized nanoparticles (ΔG⧧298K(aq) = 26.3 kcal/mol) fits the range delineated by two limiting theoretical models from advanced quantum chemical calculations: rate-limiting activation barriers of 31.6 and 18.0 kcal/mol depending on the computational model, i.e., an iron atom and an infinite iron surface, respectively. The computations indicated a two-step reaction mechanism involving two one-electron transfer processes: the first can be described by the reaction Fe + H2O → HFeOH, which represents the rate-limiting step, and the second by HFeOH + H2O → Fe(OH)2 + H2. At 25 °C, the reaction product was identified experimentally as Fe(OH)2, which forms flat layered sheets extensively overgrowing nZVI particles. At 80 °C, ferrous hydroxide undergoes secondary anaerobic transformation to magnetite (Fe3O4).