Ab initio simulations have been carried out to identify, at the atomic level, a solid-state transformation pathway from ferrihydrite to goethite. The energy barriers calculated suggest a solid-state pathway exists, providing first theoretical support for the formation of goethite precursor particles within ferrihydrite aggregates that then self-assemble into goethite crystallites by oriented aggregation, a competitive transformation process to dissolution/reprecipitation. Starting from the two most recent and still debated ferrihydrite structures proposed by Michel et al. in 2007 and Manceau et al. in 2014, we have calculated the energy barriers for self-diffusion of Fe(III) along various channels, including counter diffusion of charge balancing protons where necessary, that yield goethite. While the transformation from the Manceau model to goethite is straightforward, we found that the transition from the Michel model to goethite has a more complex reaction coordinate that would require passing through an intermediate orthorhombic phase that possesses a stoichiometry and energy similar to the Michel model, while at the same time being structurally very close to goethite. As a companion investigation, we have also determined the energy barriers for solid-state transformation between the Michel and Manceau ferrihydrite models. The information gathered supports the hypothesis that aging of ferrihydrite within aggregates entails a spontaneous internal atomic reorganization towards more stable goethite, mediated by a kinetically slow counter diffusion of iron and protons through the ferrihydrite lattice.

First principles simulation of solid-state transformation of ferrihydrite to goethite

253rd National Meeting (2017)

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