Fundamental studies of electrochemically controlled surface oxidation and hydrophobicity of natural enargite

The surface oxidation and hydrophobicity of natural enargite (Cu(3)AsS(4)) and the formation of oxidation species at the mineral surface have been examined by a novel experimental approach that combines electrochemical techniques and atomic force microscopy (AFM). This approach allows for in-situ, synchronized electrochemical control and examination of the oxidative surface morphology of enargite. Combined with ex-situ cryo X-ray photoelectron spectroscopy surface analysis, the surface speciation of enargite surface oxidation has been obtained, comparing the newly fractured natural enargite surface with those that have been electrochemically oxidized at pHs 4 and 10. At pH 4, surface layer formations consisting of metal-deficient sulfide and elemental sulfur were identified, associated with a limited increase in root-mean-square (rms) roughness (1.228 to 3.143 nm) and apparent heterogeneous distribution of surface products as demonstrated by AFM imaging. A mechanism of initial rapid dissolution of Cu followed by diffusion-limited surface layer deposition was identified. At pH 10, a similar mechanism was identified although the differences between the initial and diffusion-limited phases were less definitive. Surface species were identified as copper sulfate and copper hydroxide. A significant increase in surface roughness was found as rms roughness increased from 0.795 to 9.723 nm. Dynamic (receding) contact angle measurements were obtained by a droplet evaporation method. No significant difference in the contact angle on a surface oxidized at pH 10 and the freshly polished surface was found. A significant difference was found between the polished surface and that oxidized at pH 4, with an increase in contact angle of about 13° (46° to 59°) after oxidation. Competing effects of hydrophilic (copper oxides and hydroxides) and hydrophobic (elemental sulfur) species on the mineral surface under oxidizing conditions at pH 4 and the change in surface roughness at pH 10 may contribute to the observed effects of electrochemically controlled oxidation on enargite hydrophobicity.