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

Abstract

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 oxidesmediated transformation of carbazole to PHCZs under marine-related conditions. Brominated and chlorinated carbazoles were produced through manganese oxides-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 the observed brominated carbazoles under identical conditions. In reactions spiked with tertbutanol and methanol as •OH scavengers, RBS production decreased from 0.56 nmol/L to 0.12 nmol/L, 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 oxides-mediated oxidation of halide ions react with carbazole, and (ii) nucleophilic addition, where halide ions attack carbazole cations formed via manganese oxides oxidation. Our findings provide new insights into the abiotic processes contributing to the environmental load of PHCZs and suggest that manganese oxides plays a critical role in the biogeochemical cycling of halogenated organic compounds in coastal and marine ecosystems.