Polysulfides do not cause passivation: Results from the dissolution of pyrite and implications for other sulfide minerals

One of the most prevalent suppositions in hydrometallurgy and geochemistry is that sulfur, polysulfides or metal-deficient layers are responsible for the passivation of the dissolution of mineral sulfides. Hence, the purpose of this work was to determine whether the dominant paradigm, that polysulfides are the cause of passivation, is true. The effect of polysulfides on the rate of dissolution of pyrite was studied specifically under conditions in which passivation and pseudo-passivation were not observed. The argument of this work is that if polysulfides are present on the surface and no passivation is observed, then polysulfides are not the primary passivating agent as previously claimed. In order to do this, pyrite was oxidized electrochemically at different potentials (anodic dissolution) under conditions in which no passivation was observed. The pyrite samples were then analysed by X-ray electron spectroscopy (XPS) and Raman spectroscopy (RS). The thickness of the polysulfide layer was estimated by two methods (i) the gold sputtering technique in conjunction with the XPS analysis, and (ii) a comparison of relative peak heights on the RS. Both techniques showed that the polysulfide thickness increased with potential. However, at the same time the electrical current increased exponentially with potential, indicating that polysulfides are not the causative agent for passivation. Samples were also obtained from experiments in which pyrite was dissolved by ferric ions (oxidative dissolution) and in the presence of bacteria (bacterial dissolution). The thickness of the polysulfide layer was similar in both cases to that of the anodic dissolution experiments, suggesting that neither ferric ions nor bacteria affected these surface coatings. Finally, it is argued that mineral sulfides experience pseudo-passivation, and a more probable explanation for this pseudo-passivation is one based on semiconductor electrochemistry.