Mechanism of Pyrite Oxidation in Aqueous Mixtures

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Abstract
The rate of pyrite oxidation in aqueous mixtures was studied at 25 °C for periods up to 3 wk. Dissolved plus citrate‐dithionite‐extractable Fe was used to measure the extent of reaction. Pyriteoxidation rate was independent of time (zero order) in the presence of NaClO4, CaCO3, and NaHCO3. Oxidation rate was a linear function of substrate concentration. Oxidation rates increased with pH. Solutions of Cl−, SO2−4, and diethylenetriamine pentaacetic acid (DTPA) inhibited pyrite oxidation. The data are consistent with a surface‐controlled reaction with Fe3+ as the rate‐controlling oxidant. The effect of OH−, Cl−, SO2−4, and DTPA on oxidation rates is explained by the relative reactivity of each of these in ligand exchange during an inner‐sphere electron transfer. It is suggested that OH− is the rate‐determining bridging ligand between Fe3+ and the pyrite surface. This reaction is compatible with recently proposed surface‐reaction mechanisms.

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