Early Oxidation Processes on the Greigite Fe3S4(001) Surface by Water: A Density Functional Theory Study

Greigite (Fe3S4), the sulfide counterpart of the spinel-structured oxide material magnetite (Fe3O4), is a mineral widely identified in anoxic aquatic environments and certain soils, which can be oxidized, thereby producing extremely acid solutions of sulfur-rich wastewaters, so-called acid mine drainage (AMD) or acid rock drainage (ARD). Here we report a computational study of the partial replacement of sulfur (forming H2S) by oxygen (from H2O) in the Fe3S4(001) surface, derived from density functional theory calculations with on-site Coulomb approach and long-range dispersion corrections (DFT+U-D2). We have proposed three pathways for the oxidation of the surface as a function of H2O coverage and pH. Different pathways give different intermediates, some of which are followed by a solid-state diffusion of the O atom. Low levels of H2O coverage, and especially basic conditions, seem to be essential, leading to the most favorable energetic landscape for the oxidation of the Fe3S4(001) surface. We have derived the thermodynamic and kinetic profile for each mechanism and plotted the concentration of H2S and protons in aqueous solution and thermodynamic equilibrium with the stoichiometric and partially oxidized Fe3S4(001) surface as a function of the temperature. Changes in the calculated vibrational frequencies of the adsorbed intermediates are used as a means to characterize their transformation. We have taken into account statistical entropies for H2S and H2O and other experimental parameters, showing that this mineral may well be among those responsible for the generation of AMD.