Insights into generation mechanisms of halogen radicals from excited triplet state of dissolved organic matter

Triplet states of dissolved organic matter (3DOM*) can sensitize the generation of halogen radicals in marine water. The generation pathways of halogen radicals from 3DOM* is still not fully understood. In this study, the generation of halogen radicals from DOM was investigated with Suwanee River fulvic acid (SRFA) as a representative and detailed generation pathways were further revealed with anthraquinone-2-sodium sulfonate (AQ2S) as a triplet sensitizer. The results showed that in SRFA solutions with halogen ions, various halogen radicals can be generated. Among which, Br is formed by the reaction of Br− with 3AQ2S*, and Cl is produced by the reaction of Cl− with AQ2S+ that generated in the presence of dissolved oxygen (DO). Cl2− and Br2− were generated via the subsequent combination of Cl/Br with another Cl−/Br−. In solutions without DO, BrCl− is mainly generated through the combination of Br with Cl−, and BrCl− could also be generated through the combination of Cl with Br− in solutions with DO. This study provides deep insights into the generation mechanisms of different halogen radicals from 3DOM* and is helpful for understanding the photochemical processes of halogen radicals in marine waters.