The migration and bioavailability of toxic elemental arsenic (As) are influenced by the adsorption and redox processes of sulfide minerals in waters around mining areas. Pyrite is the most abundant sulfide mineral in the Earth's crust and exhibits certain photochemical activity. However, the adsorption and redox behaviors of arsenite (As(III)) on pyrite surface under solar irradiation remain unclear. Here, the interaction between As(III) and natural pyrite was investigated under light irradiation. The results indicated that solar irradiation promotes As(III) oxidation and adsorption on pyrite surface due to reactive oxygen species (ROS) intermediates. The reactions between H2O/O2 and hole–electron pairs (hvb+–ecb？) on excited pyrite and the oxidation of Fe2+ released from pyrite by dissolved O2 contributed much to the generation of OH？, O2？？ and H2O2 under light irradiation. ROS production and As(III) oxidation were accelerated by dissolved O2. An increase in pH within 5.0 to 9.0 decreased the concentration of OH？ but increased that of H2O2 and the amount of oxidized As(III). In weakly acidic and neutral environments, OH？ was mainly responsible for As(III) oxidation, while H2O2 contributed much to As(III) oxidation in weakly alkaline environments. Partial arsenate (As(V)) was adsorbed on pyrite and newly formed ferrihydrite. The present work enriches the understanding of As migration and transformation in the waters around mining areas, and provides a potential method for As(III) removal by using pyrite under solar irradiation. (C) 2021 Elsevier Ltd. All rights reserved.