Electrochemistry of oxidation of chalcocite in the presence and absence of microorganisms

The electrochemical behavior of the chalcocite oxidation process in the presence and absence of microorganisms was investigated using electrochemical techniques, including cyclic voltammetry, anodic polarization curves, Tafel curves, and X-ray photoelectron spectroscopy (XPS) analysis. The research results prove the stepwise dissolution mechanism of chalcocite in the presence and absence of microorganisms. The initial stage of oxidation is initiated at low redox potentials. During the initial stage, the intermediate products of Cu_xS (1 ≤ x < 2) are successively oxidized until CuS is formed. The later stage is the oxidation of the intermediate product CuS and this period requires initiation at high redox potentials owing to the formation of a passivation layer on the electrode surface, and the reaction rate of the later stage is extremely slow; in this case, it is the rate-limiting step of the whole reaction. The cyclic voltammograms show that the electric current density in the presence of microorganisms is higher than that in the absence of microorganisms, indicating that the microorganisms accelerates the dissolution rate of chalcocite. The anodic polarization curves show that the pitting potential of chalcocite is low; the potential range of the first active corrosion zone in the presence of microorganisms is much wider than that in the absence of microorganisms, indicating that the intermediate products of the sulfur film are passivating, and their effects could be reduced by the oxidation of microorganisms; in this manner, the dissolution rate of chalcocite is promoted. To identify the components of the passivation layer during the process of chalcocite dissolution in the presence and absence of microorganisms, the electrodes were detected via XPS. The XPS analysis results show that the components of the passivation layer on the electrode surface are complex, including CuS, polysulfide (S_n^2-), elemental sulfur (S⁰) and intermediate oxidation products that contain sulfate (SO₄^2-) and that CuS is the main passivating component; therefore, the oxidation of chalcocite follows the multisulfur method.