Early Oxidation Processes on the Greigite Fe3S4(001) Surface by Water: A Density Functional Theory Study
Greigite (Fe 3 S 4 ), the sulfide counterpart of the spinelstructured oxide material magnetite (Fe 3 O 4 ), 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 H 2 S) by oxygen (from H 2 O) in the Fe 3 S 4 (001) surface, derived
... ) 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 H 2 O 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 H 2 O coverage, and especially basic conditions, seem to be essential, leading to the most favorable energetic landscape for the oxidation of the Fe 3 S 4 (001) surface. We have derived the thermodynamic and kinetic profile for each mechanism and plotted the concentration of H 2 S and protons in aqueous solution and thermodynamic equilibrium with the stoichiometric and partially oxidized Fe 3 S 4 (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 H 2 S and H 2 O and other experimental parameters, showing that this mineral may well be among those responsible for the generation of AMD.