Co-carrier effect of polyethylene microplastics in the heterogenous phototransformation of o,p’-DDT in suspended particulate matter-Yangtze River water system

Microplastics are widespread in aquatic environments and, due to their small size and similar density to water, often coexist with suspended particulate matter (SPM). Both microplastics and SPM act as carriers, playing key roles in the migration and transformation of organic contaminants. However, most studies have focused on isolated systems, necessitating an investigation into how co-existing microplastics affect the phototransformation behavior of hydrophobic pollutants on carrier surfaces. In this study, a model system was constructed with the most commonly detected polyethylene (PE) microplastics, dichlorodiphenyltrichloroethane (o,p’-DDT), and Yangtze River SPM. The results showed that the introduction of PE into the SPM system notably accelerated the photodegradation of o,p’-DDT, with a synergistic promotion between PE and SPM. Utilizing chemical probes, quenchers, and electrochemical experiments, we found that PE facilitated the electron transfer in SPM, thereby enhancing the generation of reactive oxygen species (ROS) and ultimately, leading to the synergistic effect. By combining liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry, we identified 17 intermediates and proposed three main transformation pathways: elimination, hydroxylation, and dechlorination. The potential mechanisms by which ROS induced these reaction pathways were further validated by density functional theory calculations. Moreover, PE formed interaction products with o,p’-DDT. Initial environmental risk assessment using VEGA and EPI Suite software indicated the products became less toxic, but were more likely to remain in water. This study deepened the understanding of the photochemical effects of microplastics and provided new insights into the environmental fate and risk of organic pollutants.