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.
Dissolved organic matter (DOM) can affect the transformation of pollutants through photosensitization, but most current research focuses on hydrophilic pollutants, making it such that less attention is paid to hydrophobic pollutants. In this paper, the effect and action mechanism of coexisting DOM on the photodegradation of decachlorobiphenyl (PCB-209) on suspended particles collected from the Yellow River were systematically investigated in a heterogeneous system using DOM standards and model compounds. Through molecular probe experiments, mass spectrometry analysis and theoretical calculations, we found that the excited triplet state of DOM (3DOM*) could excite PCB-209 to undergo dechlorination reaction. Due to the different modes of electron transition, the presence of carbonyl groups decreased the energy of 3DOM*, whereas the electron-donating groups made the energy of 3DOM* higher. DOM containing phenolic hydroxyl groups led to a higher steady-state concentration of •OH, and DOM containing phenyl ketone structures had a stronger ability to produce •O2–. Compared with aqueous •OH, •O2– produced from hydrophobic microregions could react more readily with PCB-209. This study deepens the understanding of the role of different functional groups of DOM in the photosensitized transformation of hydrophobic compounds.
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