ISOTOPE AND ELECTRON EXCHANGE DYNAMICS IN AQUEOUS REDOX TRANSFORMATIONS OF IRON

Ferrous iron catalyzed transformation of Fe(III)-oxides and oxyhydroxides is a key part of iron flux at subsurface redox transitions, but mechanisms remain poorly understood. For example, experiments designed to study iron atom exchange dynamics between an aqueous ⁵⁷Fe(II) pool and a solid phase ⁵⁶Fe(III) pool in the form of acicular goethite nanorod and larger microrod crystallites implicate interfacial Fe(II)_(aq)-Fe(III)_(goe) electron exchange reactions remotely linked by a net bulk electrical current flow. Complete isotopic exchange in both goethite forms without mineralogy, crystallinity, crystal size or shape change occurs within 30 days at pH 7.5. Indistinguishable exchange rates for nanorods and microrods are consistent with a relatively facile bulk electron conduction step. Notwithstanding, fundamental thermodynamic and molecular-level understanding of this mechanism is so far lacking. The net bulk current requires free energy input to overcome the intrinsically high electrical resistivity of the solid phase (~10⁵-10⁶ Ω•m). It also implies preferential Fe(II) adsorption and electron injection sites, and a separate site type with the opposite preference. Comparison of the estimated free energy driving force available to the system from Gibbs’ entropy of (ideal) mixing to the estimated heat dissipation rate in the solid shows that the entropic driving force is sufficient to sustain the observed exchange behavior. Empirical potential molecular dynamics simulations were performed to assess the free energies and activation energies for elementary processes of Fe(II) adsorption, interfacial electron exchange, and subsurface electron migration for common low-index goethite surfaces. The findings show strong inner-sphere Fe(II) adsorption, mildly thermodynamically uphill electron injectivity, and select pathways for electron migration into goethite interiors, with substantial differences in energetic requirements between rod prismatic (side) faces and rod terminal (end) faces. The collective findings are generally consistent with the bulk current mechanism. Further investigation of this mechanism will have important implications for understanding aqueous iron redox transformations, and closely interlinked subsurface biogeochemical processes.