Electrochemical investigation of the oxidation of pyrite in neutral solutions

Pyrite, as the most common sulfide on the Earth's surface, is of central importance in a devastating environmental issue known as acid mine drainage (AMD). Although the oxidation of pyrite under neutral conditions is related to numerous environmental issues and industrial processes, its puzzling chemistry is largely unknown. Here, we analyzed the behavior and mechanism of pyrite oxidation in neutral solutions using a combination of electrochemical techniques. The oxidation of pyrite under neutral conditions followed a two-step oxidation law, i.e., below 0.4 V oxidation of iron occurred mainly, and oxidation of sulfur occurred above 0.4 V. Below 0.7 V, a typical passivation behavior of pyrite oxidation existed and can be eliminated with anodic potential higher than 0.7 V. In situ Fourier-transformed electrochemical impedance spectroscopy showed that the surface layer formed during oxidation in neutral conditions was mainly from iron oxidation products rather than sulfur oxidation products. These results indicated that the passivation of pyrite does not originate from the surface sulfur film, but should be explained by the semiconductor properties of pyrite.