Investigating the oxidation mechanism of facet-dependent pyrite: implications for the environment and sulfur evolution†

The oxidation of pyrite (FeS₂) not only adversely affects the environment, but also plays a critical role in the geochemical evolution of Fe and S elements. However, the oxidation rate of FeS₂ is often controlled by its exposed crystal facets. Herein, the oxidation behaviors and mechanisms of naturally existing FeS₂(100) and FeS₂(210) crystals are investigated. The adsorption models of O₂ on FeS₂(100) and FeS₂(210) facets are established, additionally, their corresponding surface energies, O₂ adsorption sites and energies are also obtained using Density Functional Theory (DFT) calculations. These results suggest that the FeS₂(210) facet more readily reacts with O₂ because it has more unsaturated coordination of Fe atoms compared with the FeS₂(100) facet. Moreover, electrochemical results such as EIS, Tafel and CV curves further prove that FeS₂(210) possesses a higher oxidation rate than that of FeS₂(100). The results of chemical oxidation experiments and XPS analyses show that FeS₂(210) can produce more total Fe, SO₄²⁻ and H⁺ than FeS₂(100). Furthermore, various intermediate S species such as SO₃²⁻, S₂O₃²⁻, S₃O₆²⁻, S₄O₆²⁻ and S₅O₆²⁻ are also detected. This work can provide a basis for understanding the oxidation mechanism of facet-dependent FeS₂ and the geochemical evolution of Fe and S elements.