Stable iron (Fe) oxides such as goethite can undergo extensive Fe atom exchange with aqueous Fe(II), resulting in their recrystallization. This recrystallization can affect trace metal cycling and Fe isotopic signatures in the rock record. Previous studies suggested that goethite does not structurally or morphologically change as a result of recrystallization, which has led to questions regarding the driving force and recrystallization mechanism. Here, we hypothesized that goethite transforms during Fe(II)-catalyzed recrystallization and that this transformation leads to detectable changes in its morphology and thermodynamic properties. To test this, we studied the recrystallization of nanoparticulate goethite using microscopic, potentiometric, and isotopic tracer techniques. First, to observe morphological changes, we tracked the dimensions of the nanoparticles using transmission electron microscopy (TEM) over 90 days during recrystallization. We saw that the length decreased for the first fifteen days and then increased to its original value, while the width increased steadily to produce wider recrystallized nanoparticles. These trends in size indicated preferential dissolution on the tips and growth on the edges of the nanoparticles, consistent with recent molecular dynamics studies. To test if the chemical potential energy (μ0) of goethite changes during recrystallization, we used mediated potentiometry, an electrochemical approach recently developed by our laboratory. Ongoing potentiometric experiments suggest that goethite is indeed changing as it recrystallizes. These two approaches were complemented by a radioisotopic tracer (55Fe) technique to track the atom exchange between aqueous Fe(II) and goethite. Goethite underwent 92% atom exchange in 15 days, implying near-complete recrystallization. These results provide a potential explanation for Fe(II) catalyzed recrystallization of goethite; the morphological and thermodynamic changes in goethite along with near-complete Fe atom exchange suggest that nanoparticulate goethite transforms from a less stable state to a more stable state via preferential dissolution-growth, and that recrystallization likely slows and eventually stops over time. Note: Recipient of the Student Travel Award

Morphological and thermodynamic changes in goethite during Fe(II)-catalyzed recrystallization

251st National Meeting (2016)

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