Experimental and Theoretical Analysis Accounting for Differences of Pyrite and Chalcopyrite Oxidative Behaviors for Prospective Environmental and Bioleaching Applications

The oxidative processes of pyrite (FeS₂) and chalcopyrite (CuFeS₂) of interest for bioleaching and/or bioremediation applications are evaluated in growing medium conditions to account for differences in their reactions mechanisms proposed with chemical and electrochemical analysis, and their electronic structures calculated with density functional theory (DFT). Electrochemical (chronoamperometry, cyclic voltammetry), spectroscopic (Raman, XPS) and microscopic techniques (SEM-EDS, AFM) are used to comprehensively characterize complex surface transformations of secondary species arising during the electrochemical oxidation of these minerals. Early oxidation steps of both sulfides involve the formation of passive polysulfide species (e.g., Fe_1-xS₂, Cu_1-xFe_1-yS₂), with the additional formation of Covelite-(CuS)-like species on a more passive chalcopyrite surface. Subsequent stages indicate the formation of semiconductive compounds including elemental sulfur (S⁰). DFT reveals that there are significant differences between pyrite and chalcopyrite densities of states (DOS), that support the fact that pyrite oxidation is more facile than chalcopyrite, as experimentally described. The DOS shows that near to the Fermi energy level of both sulfide minerals, there are few states that explain the oxidation limitations observed in the experimental region of low overpotential. At higher energies, the oxidation of pyrite is mainly due to iron species and sulfur species to a minor extent, while the chalcopyrite passivation is attributed to sulfur species and copper.