Fundamental Studies of Electrochemically Controlled Surface Oxidation and Hydrophobicity of Natural Enargite

Abstract

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.

Investigation of the surface species formed on enargite in electrochemically controlled oxidising environments and in the presence of flotation collectors

“…Electrochemical oxidation of enargite at low pH is typified by Cu dissolution and the formation of native sulfur, while at high pH sulfur is expected to oxidize to polysulfide (Sn ) . In a previous study of enargite oxidation at pH 10 and 4, and +869 mV (Plackowski et al 2013) the presence of elemental sulfur at pH 4 was confirmed by comparing the S 2p photoelectron spectra obtained at below -130 °C with that resulting from the same surface after warming to 30 °C. No evidence of S 0 was found at pH 10.…”

“…Typical cyclic polarization curve for natural enargite at pH 10 obtained using a scan rate of 1 mV/s. The polarization was initiated at 0 mV, returned to the cathodic scan at 1100 mV, switched to the anodic direction at -690 mV, and terminated at 0 mV (Plackowski et al 2013) Figure 10. Eh-pH diagrams for the Cu-As-S-H2O, As-Cu-S-H2O and S-As-Cu-H2O systems at 278K.…”

“…Small differences between the CV results in this work and the published data are likely due to variation in sample composition, including type and concentrations of impurities present, sample preparation, and experimental technique. switched to the anodic direction at -690 mV, and terminated at 0 mV (Plackowski et al 2013). …”

“…In a previous study (Plackowski et al, 2013b), a detailed investigation of the enargite surface subjected to oxidation at applied potentials of +869 mV (SHE) at pH 10, and + 610 mV at pH 4, was carried out. Changes is surface morphology (roughness and distribution of surface products), hydrophobicity and surface speciation were investigated.…”

“…The complete XPS data is contained in Table 1. In a previous publication (Plackowski et al 2013), consideration was given to the possible mechanisms that might exist at each of the applied potentials identified in the CV data, based on the work of , who proposed a number of reaction mechanisms for a system at pH 10 for each of the CV peaks at +228, +548, +728, +826 and +1074 mV, identified in their work. proposed the reactions corresponding to the five peaks on the anodic scan shown in Figure 2 as follows:…”