Transformation of polystyrene-microplastic-derived dissolved organic matter into colloids and microparticles: Roles...

Transformations of polystyrene-microplastic-derived dissolved organic matter (PS-DOM) in aquatic systems and the implications for the fates of nanosized polystyrene (PS) were investigated focusing on the roles of microbial activity and UV irradiation. PS-DOM, composed of oxygenated and hydrophobic products and nanosized PS, were found to undergo self-assembly and aggregation to form colloids and microparticles, meaning PS-DOM can act as a precursor for the formation of particulate organic matter. Microbial activity markedly enhanced both self-assembly and aggregation, causing marked increases in the particle size and molecular weight. UV irradiation promoted PS-DOM transformations through photo-oxidation and the generation of hydrophilic products, but prolonged UV exposure caused partial breakdown of the microparticles that formed. Nanosized PS actively participated in these transformation processes and underwent reversible hydrophilic-hydrophobic conversions under UV irradiation. After 21 d of incubation under dark/UV and biotic/abiotic conditions, nanosized PS supplied approximately 20% of the total organic carbon content, indicating that transformed PS-DOM gave a stable total organic carbon content. The results indicated that self-assembly and aggregation assisted by microbial processes and UV irradiation play critical roles controlling PS-DOM transport, persistence, and environmental behaviors.