Energetics and the Role of Defects in Fe(II)-Catalyzed Goethite Recrystallization from Molecular Simulations

Goethite is one of the most stable and common iron(III) minerals at the Earth's near surface. However, recent isotope tracer studies have suggested that goethite continuously recrystallizes in the presence of aqueous Fe(II) ions. Some of these studies indicate the presence of two regimes of atom exchange kinetics, a rapid stage assigned to reactive defect sites initially available at particle surfaces, followed by slower continuous exchange. An autocatalytic solid-state electron conduction model coupling Fe(II) oxidative adsorption to its reductive release at spatially distinct sites has been proposed, but the thermodynamic driving force has yet to be pinpointed. Here, using a novel hybrid/reactive molecular simulation method, for goethite (110) surfaces at circumneutral pH, we rigorously tested whether surface free energy minimization, including examining the role of structural defects, is sufficient to overcome the activation energy for interfacial electron transfer and conduction. The simulations quantitatively show that (i) on smooth stable surfaces the available thermal energy at dynamic equilibrium is sufficient to sustain the slow continuous regime of atom-exchange kinetics via short intrasurface electron conduction pathways of 1-2 nm (three to five Fe site hops), (ii) in this slower regime, the model converges to atom-exchange kinetics of 10(-5) Fe s(-1) cm(-2), a rate recently deduced from stochastic modeling of experimental data and linked to the reductive dissolution rate of goethite, and (iii) the driving force for smoothing of initially rough defective goethite surfaces can accelerate atom exchange to an extent quantitatively consistent with that observed in the initial rapid stage, in this case accessing conduction pathways of up to 8 nm. The findings suggest that the interaction of Fe(II) with initially defective goethite surfaces can drive, by the conduction model, atom exchange that is capable of recrystallizing the interiors of nanoscale particles and that, closer to equilibrium on smooth surfaces, slower atom exchange continues in perpetuity but likely involving only the outermost atomic layers.