Photodegradation of octachlorodibenzo-p-dioxin (OCDD) on suspended particles from pearl river: Kinetics and transformation mechanism.

Octachlorodibenzo-p-dioxin (OCDD) is widely present in suspended particles and sediments, and some of its transformation products may pose potential threats to the environment and human health. In this work, the photodegradation of OCDD on suspended particles from the Pearl River was systematically investigated by conducting water quality parameter experiments, mass spectrometry analysis, theoretical calculations and toxicity prediction. The experimental results showed that neutral conditions were favorable for OCDD photodegradation, and the presence of K and Mg significantly promoted the reaction rate. The photodegradation process was driven by reactive oxygen species (ROS) such as hydroxyl radicals (·OH), singlet oxygen (O), and superoxide anions (·O), primarily generated from suspended particles, with ·O being the predominant species. A variety of degradation products, including hydroxylated, dechlorinated, and ether bond-cleaved products, were identified by GC-MS and LC-HRMS, from which the reaction pathways were proposed and corroborated by theoretical calculations. Toxicity assessments indicated that bioaccumulation potential and mutagenicity of OCDD degradation products were generally reduced, yet developmental toxicity remained. This study deepens our understanding on the phototransformation mechanisms of OCDD on suspended particles and offers insights into the natural photochemical behavior of hydrophobic compounds.