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 (As₂S₃, CuS and CdS) under ultraviolet (UV) irradiation. In the dark, the aggregation rate of the arsenic sulfide (As₂S₃) particles was extremely slow. However, under UV irradiation, the growth of the As₂S₃ particles was significantly enhanced. A possible mechanism of UV-light-induced aggregation of As₂S₃ particles was proposed. The HS· and ·OH radicals formed by a series of photochemical reactions can efficiently attack the S(-II) in the As₂S₃ particle, leading to the formation of an intermediate species, [As₂S₂-S·]⁺. Then, two [As₂S₂-S·]⁺ species combine to form [As₂S₂-S-S-S₂As₂]²⁺. The formation of [As₂S₂-S-S-S₂As₂]²⁺ results in the attenuation of the electronegativity and the rapid aggregation of the sulfide particles. In addition, the small S⁰ particles generated in irradiated As₂S₃ system can efficiently coalesce into As₂S₃ 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.