The autocatalytic redox interaction between aqueous Fe(II) and Fe(III)-(oxyhydr)oxide minerals such as goethite and hematite leads to rapid recrystallization marked, in principle, by an atom exchange (AE) front, according to bulk iron isotopic tracer studies. However, direct evidence for this AE front has been elusive given the analytical challenges of mass-resolved imaging at the nanoscale on individual crystallites. We report successful isolation and characterization of the AE front in goethite microrods by 3D atom probe tomography (APT). The microrods were reacted with Fe(II) enriched in tracer 57Fe at conditions consistent with prior bulk studies. APT analyses and 3D reconstructions on cross-sections of the microrods reveal an AE front that is spatially heterogeneous, at times penetrating several nanometers into the lattice, in a manner consistent with defect-accelerated exchange. Evidence for exchange along microstructural domain boundaries was also found, suggesting another important link between exchange extent and initial defect content. The findings provide an unprecedented view into the spatial and temporal characteristics of Fe(II)-catalyzed recrystallization at the atomic scale, and substantiate speculation regarding the role of defects controlling the dynamics of electron transfer and AE interaction at this important redox interface.

It is important to understand the mechanisms controlling the removal of uranyl from solution from an environmental standpoint, particularly whether soluble Fe(II) is capable of reducing soluble U(VI) to insoluble U(IV). Experiments were performed to shed light into discrepancies of recent studies about precipitation of U-containing solids without changing oxidation states versus precipitation/reduction reactions, especially with respect to the kinetics of these reactions. To understand the atomistic mechanisms, thermodynamics, and kinetics of these redox processes, ab initio electron transfer (ET) calculations, using Marcus theory, were applied to study the reduction of U(VI) aq to U(V) aq by Fe(II) aq (the first rate-limiting ET-step). Outer-sphere (OS) and inner-sphere (IS) Fe-U complexes were modeled to represent simple species within a homogeneous environment through which ET could occur. Experiments on the chemical reduction were performed by reacting 1 mM Fe(II) aq at pH 7.2 with high (i.e., 0.16 mM) and lower (i.e., 0.02 mM) concentrations of U(VI) aq. At higher U concentration, a rapid decrease in U(VI) aq was observed within the first hour of reaction. XRD and XPS analyses of the precipitates confirmed the presence of (meta)schoepite phases, where up to ~25% of the original U was reduced to U 4+ and/or U 5+-containing phases. In contrast, at 0.02 mM U, the U(VI) aq concentration remained fairly constant for the first 3 hours of reaction and only then began to decrease due to slower precipitation kinetics. XPS spectra confirm the partial chemical reduction U associated with the precipitate (up to ~30%). Thermodynamic calculations support that the reduction of U(VI) aq to U(IV) aq by Fe(II) aq is energetically unfavorable. The batch experiments in this study show U(VI) is removed from solution by precipitation and that transitioning to a heterogeneous system in turn enables the solid U phase to be partially reduced. Ab initio ET calculations revealed that OS ET is strongly kinetically inhibited in all cases modeled. OS ET as a concerted proton-coupled ET reaction (ferrimagnetic spin configuration) is thermodynamically favorable (-35 kJ/mol), but kinetically inhibited by concurrent proton-transfer (10-19 s-1). OS ET as a sequential proton-coupled ET reaction is thermodynamically unfavorable (+102 kJ/mol) as well as kinetically inhibited, where ET is the rate-limiting step (10-12 s-1). In contrast, the reduction of U(VI) aq to U(V) aq by Fe(II) aq as an IS ET reaction is both thermodynamically favorable (-16 kJ/mol) and kinetically rapid (10 8 s-1); the IS ET rate is several orders of magnitude faster than the OS ET rate. Thus, reduction of U(VI) aq to U(V) aq by Fe(II) aq in a homogenous system could occur if an IS Fe-U complex can be achieved. However, the formation of IS Fe-U complexes in an homogeneous solution is predicted to be low; considerable thermodynamic and kinetic barriers exist to proceed from an OS ET reaction to an IS

The influence of radiation-induced (1 MeV energy H+ to ∼0.1 displacements per atom (dpa) at 450 °C), nonequilibrium point defect populations on mass transport is studied with an integrated campaign of experimental and theoretical methods. Using epitaxial thin films of hematite (α-Fe2O3) with embedded 18O tracer layers and nanoscale atom probe tomography measurements, it is shown that anion self-diffusion is enhanced by at least 2 orders of magnitude under irradiation compared to thermal diffusion alone. Complementary scanning transmission electron microscopy of vacuum-annealed and irradiated specimens reveals associated microstructural changes near the surface of the oxide films, including local phase transformation to Fe3O4 and the development of nanoscale voids from vacancy coalescence. Point defect formation and migration energies were computed from density functional theory and applied within the context of the chemical rate theory to analyze contributions from both interstitial and vacancy mechanisms to self-diffusion in thermal and irradiation conditions. Comparisons are made between calculated, literature, and newly measured self-diffusion values, revealing good agreement on the magnitude of radiation-enhanced anion diffusion. Further, the model suggests a transition from vacancy to interstitialcy mechanisms at low temperatures and high oxygen activity, providing an explanation for the varied activation energies reported from prior studies.