The effect of temperature on the kinetics of the ferric-ferrous redox couple on pyrite

This article presents two methods for the study of ferric-ferrous reaction kinetics on the surface of an electro-active electrode. The working electrode in this study was a high quality bulk pyrite mineral, and the experimental results have been collected in a 0.5 M sulfuric acid solution within a temperature range of 22 to 65 °C. Two electrochemical methods, based on mixed potential theory, are introduced to isolate the Fe(II)/Fe(III) reaction from the other interfering reactions of pyrite oxidation and oxygen reduction, and to study its kinetics. The two methods use chronoamperometry or cyclic voltammetry for the kinetics analysis. By subtracting the current density of pyrite oxidation in an Fe-free acidic solution from that in an Fe-containing solution the Fe(II)/Fe(III) kinetics were isolated and studied.Tafel behavior was established by both chronoamperometry and cyclic voltammetry to analyze the kinetics of the Fe(II)/Fe(III) redox reactions. The results showed that the exchange current densities increased slightly with temperature, however they remained in the order of 10−5 A cm−2. The activation energy for Fe(II) oxidation and Fe(III) reduction on the pyrite surfaces was calculated to be 102.0 and 32.7 kJ mol−1, respectively. Anodic transfer coefficients also increased by increasing temperature from 0.24 to 0.39. Electrochemical impedance spectroscopy measurements showed a typical two-step pyrite oxidation process. The EIS measurement under different temperature showed that the predominant reaction in an Fe-free solution is pyrite oxidation, which has a large Rct value, starting from 4200 Ω at 22 °C and decreasing to 1490 Ω at 65 °C. Meanwhile the Rct values in an Fe-containing solution reduces from 1760 to 511 Ω by increasing the temperature from 22 °C to 65 °C, as the Fe(II)/Fe(III) redox reaction is the predominant reaction. The diffusion coefficients of Fe(II) during pyrite polarization was related to the Warburg impedance and calculated to be 1.9×10−6 cm2 S−1 at 22 °C and increases to 7.1×10−6 cm2 S−1 at 65 °C.