First-principles study on the effect of arsenic impurity on oxidation of pyrite surfaces

Arsenic (As) frequently exists in pyrite (FeS₂) in the form of impurities. The oxidation behavior of As in FeS₂ is important in environmental science, mineral processing, and other related fields. The adsorption behaviors of H₂O and O₂ molecules on the As-bearing pyrite (100) surface (As FeS₂(100)) are studied using the density functional theory (DFT). The results show that As prefers the S site on the pyrite (100) surface (FeS₂(100)). In the absence of O₂, an isolated H₂O molecule does not dissociate when adsorbed at an iron (Fe) site and is repelled at an As site. Furthermore, the surface area around the As atoms exhibits a hydrophobic behavior. Adsorption energy analysis reveals that the presence of As atoms is unfavorable for the adsorption of H₂O molecules on the pure FeS₂ surface, and that the adsorption of H₂O molecules on the As FeS₂(100) is physical adsorption. In the absence of H₂O, it is suggested that the O₂ molecule easily dissociates on both the pure FeS₂(100) and As FeS₂(100). The adsorption of O₂ on the As-bearing surface is weaker than that on the pure FeS₂(100). For the co-adsorption of H₂O and O₂, the adsorption energy on the As-bearing surface is more negative than that on the pure surface. This indicates that the presence of As promotes surface oxidation. Additionally, two  OH and O (AsO or SO) or  O (Fe O) species are formed on the surface of pyrite when the H₂O molecule is dissociated.