PNNL

Abstract

The autocatalytic redox interaction between aqueous Fe(II) and Fe(III)-(oxyhydr)oxide minerals such as goethite leads to rapid recrystallization marked, in principle, by an atom exchange (AE) front according to bulk iron isotopic tracer studies. However, capturing direct evidence of this AE front is challenging due to the nanoscale dimensions of individual crystallites. We report successful isolation and characterization of the exchange front in goethite microrods by three-dimensional atom probe tomography (3D 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, penetrating at times 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 of exchange extent to 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 and AE interaction at this important redox interface.