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 center dot and center dot 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 center dot](+). Then, two [As2S2-S center dot](+) 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 S-0 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.