Pyrite Oxidation Mechanism by Oxygen in Aqueous Medium

The mechanism of the initial steps of pyrite (100) surface oxidation was investigated in detail by means of density functional theory/plane-wave calculations. Pyrite oxidation is related to many environmental and technological issues, and its mechanism has not been completely understood. A chemical picture of the pyrite oxidation process in the presence of oxygen and water was proposed in the present investigation. The reaction steps of the oxidation mechanism can be separated into two types. Type I reactions present lower activation energies and are redox processes that involve oxidation of two Fe­(II) sites on the surface to form predominantly the Fe­(III)–OH^–. This species is formed from hydrogen transfer between the adsorbed water to the adsorbed oxygen molecule on the Fe­(II) sites. Type II reactions present higher activation energies and lead to the formation of a SO bond through the hydrogen atom transference from a water molecule to the Fe­(III)–OH^– species, forming Fe­(II)–OH₂. These reactions present higher activation energies. The determinant step of this oxidation mechanism involves the formation of two adsorbed hydroxide species (OH^–) on the surface. The hydroxides in the presence of water from the bulk liquid react to form two water molecules adsorbed on the surface and the first S–O chemical bond. Parallel reactions were investigated explaining the experimental detection of the O₂^– and OOH^– species. Furthermore, the proposed mechanism explains the experimental observation that the oxygen present in the sulfate is mostly originated from water instead of an oxygen molecule. The present study strengthens the importance of the water/solid interface to understand the oxidation mechanism of pyrite in the presence of water at a molecular level.