The role of aqueous free •OH in the photodegradation of polychlorinated biphenyls (PCBs) in hydrophilic and hydrophobic Regions of Yangtze River suspended particle-water system

The generation of highly reactive hydroxyl radicals (•OH) in natural water under sunlight irradiation has been demonstrated to significantly affect the degradation of compounds in water. However, studies concerning the effects of free •OH on the degradation of hydrophobic pollutants in real, complex environments remain limited. This study examined how •OH generated by a UV/H₂O₂ system affects the heterogenous photodegradation of polychlorinated biphenyls (PCBs) in a natural suspended particle-water system. The results showed that •OH effectively decomposed PCB-3, 15 and 47, but had no effect on PCB-136, 155, 206 and 209. Further experiments revealed that •OH in the aqueous phase primarily reacts with PCBs that have desorbed into the water. The short diffusion distance of •OH and the higher potential energy barriers in hydrophobic regions hindered its reaction with PCBs on particle surfaces. Lowering solution pH or adding certain cations improved PCB-3 degradation by increasing its aqueous partitioning. Conversely, the presence of humic acid (HA) could quench •OH and screen light, thereby hindering the degradation of PCB-3 on particle surface by •OH. This study provides valuable insights into the mechanisms of •OH mediated degradation of hydrophobic pollutants and may advance the development of environmental treatment technologies.