Manganese oxide-mediated halogenation of carbazole under marine-related conditions

Polyhalogenated carbazoles (PHCZs) are emerging halogenated organic compounds detected in various environmental matrices, including marine environments. However, the origins of PHCZs are not yet fully understood. This study investigates the manganese oxide-mediated transformation of carbazole into PHCZs under marine-related conditions. Brominated and chlorinated carbazoles were produced through manganese oxide-mediated oxidation of carbazole in the presence of halide ions, with brominated carbazoles being the dominant species. Both reactive bromine species (RBS) and reactive chlorine species were detected in the reaction system. However, the detected RBS concentration was much lower than that of the observed brominated carbazoles under identical conditions. In reactions spiked with tert-butanol and methanol as ˙OH scavengers, RBS production decreased from 0.56 nmol L^-1 to 0.12 nmol L^-1, while the yield of 3-BCZ increased. These results suggest that PHCZ formation occurs through two primary pathways: (i) electrophilic substitution, where reactive halogen species generated by manganese oxide-mediated oxidation of halide ions react with carbazole and (ii) nucleophilic addition, where halide ions attack carbazole cations formed via manganese oxide oxidation. Our findings provide new insights into the abiotic processes contributing to the environmental load of PHCZs and suggest that manganese oxides play a critical role in the biogeochemical cycling of halogenated organic compounds in coastal and marine ecosystems.