Density functional theory study of Fe(II) adsorption and oxidation on goethite surfaces

We study the interactions between Fe(II) aqua complexes and surfaces of goethite $(\ensuremath{\alpha}\text{-FeOOH})$ by means of density functional theory calculations including the so-called Hubbard $U$ correction to the exchange-correlation functional. Using a thermodynamic approach, we find that (110) and (021) surfaces in contact with aqueous solutions are almost equally stable, despite the evident needlelike shape of goethite crystals indicating substantially different reactivity of the two faces. We thus suggest that crystal anisotropy may result from different growth rates due to virtually barrierless adsorption of hydrated ions on the (021) but not on the (110) surface. No clear evidence is found for spontaneous electron transfer from an adsorbed Fe(II) hex-aqua complex to a defect-free goethite substrate. Crystal defects are thus inferred to play an important role in assisting such electron transfer processes observed in a recent experimental study. Finally, goethite surfaces are observed to enhance the partial oxidation of adsorbed aqueous Fe(II) upon reaction with molecular oxygen. We propose that this catalytic oxidation effect arises from donation of electronic charge from the bulk oxide to the oxidizing agent through shared hydroxyl ligands anchoring the Fe(II) complexes on the surface.