Electrochemical oxidation of pyrite (FeS2) in aqueous electrolytes

Electrochemical oxidation of pyrite (FeS2) in aqueous electrolytes has been investigated using electrochemical techniques, in situ Fourier transform i nfrared spectroscopy (FTIR), ex situ X-ray photoelectron spectroscopy (XPS) and ion chromatography (IC). The results show that when pyrite is polarise d in the potential region of - 0.35 to 0.25 V versus SCE in 1 M HCl at swee p rates greater than or equal to 30 mV s(-1), the redox process is a revers ible electrochemical adsorption/desorption. whereas at sweep rates less tha n or equal to 10 mV s(-1) substantial oxidation of the pyrite surface occur s and becomes partially irreversible. The very limited oxidation rate of n- type pyrite over a wide range of potentials may be attributed to its semico nducting properties, or the main part of the applied potential falling acro ss the space charge region rather than the Helmholtz layer. The electrochem ical oxidation of pyrite involves a complex series and parallel reaction st eps, ultimately producing Fe2+/Fe3+ and S/S2O32-/HSO4-, depending on pH and potential. A mechanism for this process is proposed and discussed using en ergy band and molecular orbital theories. (C) 1999 Elsevier Science S.A. Al l rights reserved.