Abstract

A natural enargite (Cu3AsS4) electrode has been studied by cyclic voltammetry (CV) and XPS, Cyclic voltammetry was carried out in a preparation chamber filled with Ar gas and coupled to the spectrometer in order to analyse by XPS, quasi-irt situ, the electrochemically modified electrode surface. By CV, different potentials of oxidation and reduction were applied to the electrode in a disodium tetraborate decahydrate electrolyte solution of pH 9.2 at room temperature. Cycling in the anodic direction, XPS analysis has shown that at +500 mV the electrode surface is oxidized to CuO, CuSO4 and As2Ox (x = 3; 5) and the corresponding hydroxides and polysulphide are formed, whereas at lower oxidation potentials the Cu 2p and As 3d photoelectron peaks do not alter. However, with increasing oxidation potential up to +200 mV a new contribution in the S 2p signal and Cu loss indicate the formation of non-stoichiometric (metal-deficient) enargite and polysulphides in the electrode surface. Cycling then from +500 mV in the cathodic direction until -800 mV, we observe with increasing reduction potential a reverse process in the S 2p signal and Cn concentration. Furthermore, even at +200 mV the reduction from Cu(II) to Cu(I) occurs and no sulphate species are observed any more. These observations indicate that the copper concentration at the electrode surface is a sweep-direction-independent function of applied potential and that the oxide species formed at the electrode surface are only stable by application of high oxidation potentials. For comparison, the surfaces of the fractured mineral and the original polished electrode surface were also studied. Copyright (C) 2000 John Wiley & Sons, Ltd.