Visible-light photocatalysis accelerates As(III) release and oxidation from arsenic-containing sludge

Arsenic containing sludge, a product of the treatment of acid smelting wastewater, is susceptible to temperature, pH, co-existing salt ions and organic matter, which might lead to the release of arsenic ions into the environment. Here, we studied the effect of visible light on the dissolution and oxidation of arsenic sulfide sludge (ASS) sampled from a smelting plant. Results show that by exposure to visible light, both the release of As(III) ions from ASS and the oxidation of As(III) into As(V) were markedly accelerated. Electron paramagnetic resonance (EPR) and free radical quenching experiments revealed that ASS acts as a semiconductor photocatalyst to produce hydroxide and superoxide free radicals under visible light. At pH 7 and 11, both the dissolution and the oxidation of the sludge are directly accelerated by O2¯. At pH 3, the dissolution of the sludge is promoted by both O2¯ and OH, while the oxidation of As(III) is mainly controlled by OH. In addition, the solid phase of ASS was transformed to sulfur (S8) which favored the aggregation and precipitation of the sludge. The transformation was affected by the generation of intermediate sulfur species and sulfur-containing free radicals, as determined by ion chromatography and low-temperature EPR, respectively. A photocatalytic oxidation-based model is proposed to underpin the As(III) release and oxidation behavior of ASS under visible light conditions. This study helps to predict the fate of ASS deposited in the environment in a range of natural and engineered settings.