Redox-Driven Fe Atom Exchange at the Magnetite–Water Interface: Insights from ⁵⁷Fe–Mössbauer Spectroscopy and First-Principles Molecular Dynamics Simulations

Magnetite (Fe₃O₄) plays a pivotal role in the biogeochemical cycling of iron (Fe) and contaminant fate through interfacial electron transfer processes. While aqueous Fe(II)-magnetite interactions drive these environmentally critical redox reactions, the atomistic mechanisms governing electron transfer and isotopic exchange between aqueous Fe(II) and structural Fe(III) in magnetite during recrystallization remain elusive. Herein, this study combined Fe isotopic tracer experiments, ⁵⁷Fe-Mössbauer spectroscopy and molecular dynamics simulations to investigate the structural controls on Fe(II)-catalyzed magnetite recrystallization under circumneutral conditions. The results demonstrate that Fe(II)_aq exchanges with structural Fe(III) in magnetite, with the extent increasing for smaller particle sizes and nonstoichiometric compositions. Mössbauer spectra demonstrate preferential Fe atom exchange at octahedral sites. Molecular simulations further identify the most stable surface configurations of Fe(II) on magnetite: a monodentate mononuclear inner-sphere complex at tetrahedral sites versus a tridentate inner-sphere complex (stabilized by cation bridging) at octahedral sites. Calculated desorption free energies (0.37 eV vs 0.67 eV) confirm stronger Fe(II) retention at octahedral sites. These mechanistic insights advance the fundamental understanding of Fe(II)-catalyzed mineral transformations and inform the rational design of magnetite-based remediation strategies.