Photodegradation of PCB-209 on suspended particles: Discrepancy in mechanism of direct dechlorination and active species-mediated indirect dechlorination reactions

While the reaction mechanism of direct photochemical dechlorination has been extensively studied, significant knowledge gaps remain regarding the specific roles of reactive species in governing dechlorination pathways during indirect photodegradation. By integrating mass spectrometry analysis, probe experiments and theoretical calculations, we propose for the first time an indirect dechlorination reaction mechanism for PCB-209 involving active species commonly found in DOM photosensitive systems, which differs significantly from that of direct photodegradation. Due to the lower bond energy of the C Cl bond at the position opposite the carbon bridge, PCB-209 primarily yielded PCB-208 in direct dechlorination systems. However, our study revealed that active species ( 3 DOM*, •O 2 − and 1 O 2 ) can specifically weaken the meta-positioned C Cl bond at the carbon bridge through electron transfer and cycloaddition mechanisms, thereby fundamentally altering the regioselectivity of dechlorination. Remarkably, our results confirm the indirect degradation of PCB-209 through DOM photosensitization to form dechlorination products dominated by PCB-207, a previously unrecognized transformation pathway in natural photochemical processes. This study refines the mechanism of indirect dechlorination mediated by reactive species and provides a predictive framework for the environmental fate of halogenated pollutants in real complex environmental systems.