A review of the rate of pyrite oxidation in aqueous systems at low temperature

Abstract

The thermodynamic stability of pyrite is limited to a narrow range of Eh conditions, but pyrite occurs in almost any geological environment under earth surface conditions. When mine wastes or pyritic shales are exposed to the atmosphere and meteoric water, however, the weathering of pyrite takes place at such a rate that it can cause pollution and geotechnical problems within a few years. Recent work on pyrite oxidation has been aimed at determining which chemical properties of the geological environment affect the rate of pyrite oxidation most and to what extent. Some investigators have leached samples of ground pyrite in the laboratory under a range of experimental conditions. They observed that the rate of pyrite oxidation depends on the surface area of the sample, on the initial concentration of oxidising agent and only slightly on the initial pH of the leaching medium. Results were expressed as a rate law which relates these experimental factors to the rate of pyrite oxidation. There is some disagreement between authors about the rate laws obtained. This may be due to changes in the properties of the pyrite surface during the experiments or to unforeseen reactions that are part of the chemical pathway for pyrite oxidation. Mechanisms have been proposed for pyrite oxidation according to theoretical considerations about chemical reactions between solute and solid, magnetic properties of substances and molecular orbital theory.