Insight into the formation of polyhalogenated carbazoles during seawater chlorination

Although polyhalogenated carbazoles (PHCZs) have been widely detected in the marine environment, their origin is far from clear. In this study, the formation of PHCZs in the chlorination of seawater containing carbazole and its derivatives was investigated. A total of 14 PHCZs including six commonly found and eight unknown congeners were identified in the chlorination of seawater with carbazole. In addition, this study for the first time demonstrated the production of common PHCZs from the chlorination of seawater with 3-methyl carbazole and 3-formyl carbazole, especially 1,8-dibromo-3,6-dichlorocarbazole from 3-methyl carbazole. The formation of PHCZs in the reaction resulted from the halogenation of carbazole by reactive chlorine species (RCS) and mainly reactive bromine species (RBS), forming from the oxidation of bromide by RCS. Results also indicated that the reaction followed a successive halogenation pattern. A higher content of free chlorine and bromide facilitated the generation of RBS, while a higher concentration of DOC exhibited an inhibitory effect. The effects of free chlorine, bromide, DOC, and temperature on the formation of PHCZs were congener-specific. Given the wide-spread use of chlorination in seawater disinfection, seawater chlorination might be a potential source of PHCZs in the marine environment.

Automated summary

The origin of polyhalogenated carbazoles (PHCZs) in the marine environment has been unclear. This study investigated the formation of PHCZs during the chlorination of seawater containing carbazole and its derivatives. A total of 14 PHCZs were identified, including six commonly found and eight unknown congeners. The production of common PHCZs from the chlorination of seawater with 3-methyl carbazole and 3-formyl carbazole was also demonstrated for the first time. The formation of PHCZs resulted from the halogenation of carbazole by reactive chlorine species (RCS) and mainly reactive bromine species (RBS), which formed from the oxidation of bromide by RCS.