Oxidation mechanism of arsenopyrite under alkaline conditions: Experimental and theoretical analyses

In this study, arsenopyrite leaching test was conducted by sodium hydroxide solution with pH = 10, 12, and 13.5. The electrochemical oxidation behavior of arsenopyrite in sodium hydroxide solution at pH = 10, 12, and 13.5 was analyzed by cyclic voltammetry and electrochemical impedance spectroscopy. The elemental composition and chemical state of arsenopyrite after surface oxidation was investigated by X-ray photoelectron spectroscopy. The hydroxylation and electronic configuration of surface atoms of arsenopyrite were calculated by density functional theory. The concentration of Fe3+ in the solution was found to increase by one order of magnitude with increasing pH of the sodium hydroxide solution. The total As content in the solution also increases with increase in pH value as a result of sharp increase in the ratio of As(III)/As(V). In sodium hydroxide solution with pH = 10, the surface of arsenopyrite undergoes two oxidation stages. At pH = 12 and 13.5, the second stage (oxidation of the S-rich zone) disappears. The hydroxylation of Fe and As atoms on the arsenopyrite surface is the first step in the oxidation process. As atoms are then oxidized to As(III) and enter the solution while Fe and S atoms form Fe(OH)(3) and S-rich zone covering the arsenopyrite surface. Finally, As(III) in the solution is oxidized to As(V), and the S-rich zone is oxidized to SO42-. The oxidation dissolution rate of arsenopyrite increases sharply with increasing pH. The research results have important theoretical significance for the back filling process of tailings containing arsenopyrite.