  We study the interactions between Fe(II) aqua complexes and surfaces of
goethite (alpha-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.
