UV-Light-Induced Aggregation of Arsenic and Metal Sulfide Particles in Acidic Wastewater: The Role of Free Radicals.

The removal of arsenic and metals by sulfide (S(-II)) from acidic wastewater is an efficient method. However, the small sulfide particles formed in such a process make solid-liquid separation difficult, which greatly hinders its application. This study investigated the aggregation behavior of different sulfide particles (AsS, CuS and CdS) under ultraviolet (UV) irradiation. In the dark, the aggregation rate of the arsenic sulfide (AsS) particles was extremely slow. However, under UV irradiation, the growth of the AsS particles was significantly enhanced. A possible mechanism of UV-light-induced aggregation of AsS particles was proposed. The HS· and ·OH radicals formed by a series of photochemical reactions can efficiently attack the S(-II) in the AsS particle, leading to the formation of an intermediate species, [AsS-S·]. Then, two [AsS-S·] species combine to form [AsS-S-S-SAs]. The formation of [AsS-S-S-SAs] results in the attenuation of the electronegativity and the rapid aggregation of the sulfide particles. In addition, the small S particles generated in irradiated AsS system can efficiently coalesce into AsS particles. The CuS and CdS particles should have similar aggregation mechanisms. This study proposed a potential method for sulfide particle aggregation and provided a theoretical foundation for the development and application of UV-light-induced sulfide particle aggregation technology.