Electrochemical and surface analytical studies of enargite in acid solution

The electrochemical oxidation and reduction of the surface of natural enargite (Cu3AsS4) was investigated in 0.1 M HCl solution using cyclic voltammetry and chronoamperometry. Surface analysis by ex situ X-ray photoelectron spectroscopy (XPS) and in situ Raman spectroscopy, together with aqueous phase analysis by inductively coupled plasma-atomic emission spectrometry (ICP-AES), were used to aid in the interpretation of the electrochemical behavior of this complex system. XPS analyses on enargite oxidized at potentials >0.2 V (SCE) detected the presence of Cull surface species, with associated sulfate and chloride. At potentials <0.6 V (SCE), oxidation of arsenic was detected with possible minor amounts of As-V at higher potentials. Analysis of the electrolyte in contact with enargite showed that the amount of dissolved sulfur increased under reducing potentials due to H2S formation and at oxidizing potentials due to the formation of sulfoxy species. The amount of dissolved copper increased at potentials >0.2 V (SCE), whereas dissolved arsenic concentrations were negligible over the entire potential range investigated. Raman spectroscopy provided additional evidence of elemental sulfur formation at oxidizing potentials at which sulfate forming reactions occurred in parallel. The sulfur was responsible for an active-passive transition observed at ca. 0.3 V (SCE) in voltammograms. (C) 2004 The Electrochemical Society.