1 research outputs found
Early Oxidation Processes on the Greigite Fe<sub>3</sub>S<sub>4</sub>(001) Surface by Water: A Density Functional Theory Study
Greigite
(Fe<sub>3</sub>S<sub>4</sub>), the sulfide counterpart
of the spinel-structured oxide material magnetite (Fe<sub>3</sub>O<sub>4</sub>), 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<sub>2</sub>S)
by oxygen (from H<sub>2</sub>O) in the Fe<sub>3</sub>S<sub>4</sub>(001) surface, derived from density functional theory calculations
with on-site Coulomb approach and long-range dispersion corrections
(DFT+<i>U</i>–D2). We have proposed three pathways
for the oxidation of the surface as a function of H<sub>2</sub>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<sub>2</sub>O coverage, and especially basic conditions, seem
to be essential, leading to the most favorable energetic landscape
for the oxidation of the Fe<sub>3</sub>S<sub>4</sub>(001) surface.
We have derived the thermodynamic and kinetic profile for each mechanism
and plotted the concentration of H<sub>2</sub>S and protons in aqueous
solution and thermodynamic equilibrium with the stoichiometric and
partially oxidized Fe<sub>3</sub>S<sub>4</sub>(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<sub>2</sub>S and H<sub>2</sub>O and other experimental parameters,
showing that this mineral may well be among those responsible for
the generation of AMD