Pyrite Oxidation Mechanism in Aqueous Solutions: An in situ FTIR Study

FTIR spectra for the pyrite electrode/electrolyte interface at polarizations of –0.5 to 0.9 V (NHE) and pH 9.2 are obtained in situ for the first time and interpreted. A fundamentally new model for the pyrite oxidation in aqueous solutions is proposed on the basis of these data. According to the model, the oxidation occurs via an electrochemical (corrosion) mechanism. The cathodic half-reaction proceeds either on sulfur-deficient pyrite areas or FeS defects. The anodic half-reaction proceeds via a “thiosulfate” path, on areas of “common” pyrite characterized by the fixing of the Fermi level in a top portion of the valence band. The basic role of the oxidant consists of maintaining a high potential on anodic areas. The direct (chemical) oxidation of pyrite is an indirect effect and consists of an interaction with products of the anodic reaction. It is shown that the corrosion model, being free of contradictions inherent in previous models, explains some experimental facts left previously without any explanation. From this model follows an indirect mechanism of pyrite bioleaching.