Photochemical oxidation and dissolution of arsenopyrite in acidic solutions

Abstract

Arsenopyrite (FeAsS) is the most common iron sulfide mineral in the nature, and its oxidation causes arsenic (As) release and thus environmental pollution. Sunlight participates in the oxidative dissolution of sulfide minerals in supergene environments, affecting the release of toxic elements. In this work, we investigated the oxidative dissolution process of arsenopyrite in acidic solution under ultraviolet (UV) irradiation. The effects of pH, oxygen partial pressure and waveband of incident light on the dissolution and release of As were further investigated. The results showed that UV irradiation can promote the oxidative dissolution of arsenopyrite and the release of As. As(V), Fe(II) and SO42− were the major forms of the released As, Fe and S, respectively. Reactive oxygen species (ROS) including O2−/HO2, OH and H2O2 were generated through the oxidation of As(III) by oxygen under far-UV irradiation, photo-Fenton reaction of Fe(II)/FeOH2+ and conversion of OH/O2−, which play important roles in arsenopyrite oxidation. A small amount of OH and H2O2 was generated from the decomposition of H2O induced by Fe(III) at the sulfur-deficient sites on arsenopyrite surface. Oxygen accelerated the formation of ROS, which facilitated the oxidative dissolution of arsenopyrite and the release of As. At initial pH 3.0, the released As mainly existed in the form of ions in the solution. With initial pH increasing to 5.0, the formation of ferric arsenate precipitate and goethite contributed to the adsorption and fixation of released As. This work reveals the release mechanism of As in the photochemical oxidation process of arsenopyrite, and expands the understanding about the geochemical cycling of As.