Electrochemical and spectroscopic analysis of enargite (Cu3AsS4) dissolution mechanism in sulfuric acid solution

Enargite (Cu3AsS4) is a valuable but arsenic bearing copper resource, which is associated with common copper sulfides. The dissolution mechanism of enargite electrodes in 0.1 mol/L sulfuric acid, at room temperature was investigated by electrochemical and spectroscopic analysis methods. Cyclic voltammetry (CV), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and Mott-Schottky methods were applied to systematically investigate the electrochemical dissolution of enargite. Spectroscopic analysis methods including X-ray photoelectron spectroscopy (XPS) with cold stage, Raman spectroscopy and Synchrotron X-ray absorption near edge structure (XANES) spectroscopy were used to analyze the product layers on the electrode surface after electrochemical dissolution. The enargite electrode demonstrated an apparent passive potential region from 450 to 750 mV (Ag/AgCl. sat). Results of XPS and Raman suggested an As-sulfide surface lay of As(1-y-z)S4 was formed on the electrode surface, which accounted for the decrease in current density in the passive potential region. A trans-passive region with a small potential window between 750 mV and 900 mV was observed which might be related to the partial dissolution of the passive surface film. It is demonstrated that the trans-passive dissolution of enargite might be linked to the oxidation of sulfur, from the sulfide or polysulfide in the passive film to elemental sulfur. Moreover, copper was found to be more preferentially dissolved than arsenic in the passive and trans-passive regions. At the active dissolution region above 900 mV, the previously formed passive film got completely dissolved accompanying with the oxidation of enargite surface to produce elemental sulfur, which has been confirmed by Raman spectroscopy and sulfur K-edge XANES spectroscopy.