An XPS investigation of surface species formed by electrochemically induced surface oxidation of enargite in the oxidative potential range

Oxidation of the surface of natural enargite (Cu3AsS4) under potentiostatic control and the formation of oxidation species at the mineral surface has been investigated at selected applied potentials in the oxidative range. Potentials at which oxidation reactions were found to occur were identified by cyclic voltammetry as +347, +516, +705, +869 and +1100 mV SHE (versus Standard Hydrogen Electrode). The mineral surface was oxidized by application of these potentials in pH 10 buffer solution, and then analyzed by X-ray photoelectron spectroscopy (XPS). Liquid nitrogen cooling was used during XPS analysis to minimize the effects of exposure to X-rays, thermal degradation, and ultra-high vacuum conditions. The surface speciation of electrochemically oxidized natural enargite obtained for these anodic potentials at pH 10 demonstrated a progressively increasing level of oxidation as applied potential increases. Surface layer deposition was linked to potential, with limited evidence of oxidation products at the surface found after treatment at applied potentials of +347 and +516 mV. At these potentials no evidence of Cu(II) compounds was found, while a decrease in the proportion of copper at the surface suggests dissolution as the primary reaction mechanism. At a treatment potential of +705 mV, Cu(II) species identified as CuSO4 and Cu(OH)(2) were found, although the presence of arsenic oxides or sulfides was not found. After treatment at +869 and +1100 mV significant evidence of oxidation was found, with Cu(II) species of CuSO4 and Cu(OH)(2) found. Additional sulfur and arsenic species of CuS and As2O3 were identified that were not present after treatment at lower potentials. Comparison of the XPS findings with previously published proposed reaction mechanisms for similar treatment potentials showed that they did not account for all species identified in the XPS data. Analysis of buffer solutions post-treatment by ICP (Inductively Coupled Plasma spectroscopy) showed a pattern of change in concentrations of Cu, As and S characterized by a step-change increase in dissolution between the +516 and +705 mV treatment conditions, which correlates with the formation of Cu(II) identified on the mineral surface. (C) 2013 Elsevier Ltd. All rights reserved.