Oxidation mechanism of the arsenopyrite surface by oxygen with and without water: Experimental and theoretical analysis

Arsenopyrite is a common mineral that is abundant in nonferrous metal tailings. Study of the atomic configuration, chemical state, and morphology of the oxidation products of the arsenopyrite surface is important for enhancing the resource utilization and environmental safety of cementitious backfill. In this paper, the chemical states of the surface atoms of arsenopyrite oxidized with and without water were analyzed by X-ray photoelectron spectroscopy (XPS). The (001) surface oxidation process of arsenopyrite by oxygen with and without water was characterized using density functional theory (DFT). Results of the XPS analysis showed that water molecules participated in the oxidation reaction and formed a large number of OH structures on the surface of arsenopyrite; the relative content of S-n(2-), As(III), and Fe(II) increased significantly. The DFT calculations revealed that oxygen molecules are more easily adsorbed on the As2-Fe2 site by chemical adsorption (Fe2 is the second ortho Fe atom of As2), and water molecules are adsorbed on the Fe2 site by physical adsorption. The oxidation process of the (001) surface by oxygen can be divided into four stages: adsorption, dissociation, bridging oxygen formation, and desorption of oxidation products; the As2 atom with three coordination sites on the surface of (001) is oxidized first. In the presence of water, the water molecule dissociates into OH and H, resulting in the hydroxylation of As and Fe atoms on the (001) surface. Calculation of the energy difference and reaction energy barrier between different reaction steps showed that the participation of water molecules reduces the reaction energy barrier and increases the reaction energy. The oxidation products are derived from the arsenic oxidation of (AsO2) and (HAsO2) types without and with water, and (HAsO2) is easily desorbed from the surface. Water molecules accelerate the surface oxidation of arsenopyrite and cause the surface to form hydroxylated oxidation products.