Reactive oxygen species (ROS)-mediated photodegradation mechanism of 2,4′-DDT in the irradiated Pearl River suspended particulate matter-water system

o,p’-dichlorodiphenyltrichloroethane (2,4′-DDT) is a typical persistent organic pollutant (POP) characterized by environmental persistence and acute toxicity. Its pronounced hydrophobicity drives preferential adsorption onto suspended particulate matter (SPM) in aquatic systems. This study systematically investigated the photochemical transformation mechanism of 2,4′-DDT in the Pearl River SPM-water system. Environmental factor analysis revealed negligible effects from pH and cations on 2,4′-DDT photodegradation, while NO 3 − and HCO 3 − inhibited degradation via competitive UV absorption and reactive species quenching. Reaction mechanisms were probed through H 2 O 2 -enhanced hydrophilic reactive oxygen species (ROS) generation, revealing dual photosensitizing and ROS-quenching roles of dissolved organic matter (DOM) in SPM. Hydrophobicity-dependent degradation patterns were revealed through quenching experiments and comparative POPs studies, demonstrating that hydrophilic-region ROS (particularly •OH) contributed dominantly to 2,4′-DDT degradation compared to hydrophobic-region ROS. The LC-MS and GC-MS analyses identified three transformation pathways: dechlorination, elimination, and hydroxylation. Theoretical calculations confirmed C-Cl bond cleavage and elimination feasibility, with •OH attacking benzene-ring carbons at minimal energies, explaining the formation of hydroxylated products. Notably, toxicity assessments revealed enhanced hazards of dechlorination and elimination photoproducts to humans and aquatic organisms than the parent compound.