New insight into the formation of polyhalogenated carbazoles: Aqueous chlorination of residual carbazole under bromide condition in drinking water

Although polyhalogenated carbazoles (PHCZs) have been increasingly detected in aquatic environments, studies on their source and formation are extremely scarce. After investigation of PHCZs and carbazole in drinking water, an overlooked formation route of chloro/bromo-CZs was now proposed as aqueous chlorination of residual carbazole under bromide condition. Four groups of PHCZ, including mono-, di-, tri-, and tetra-halogenated groups, were identified in chlorinated drinking water, and the mechanism of the formation of the four groups of PHCZs was proposed. Twenty-six PHCZ congeners, including chloro-, bromo-, and chlorobromo-mixed species, were thoroughly studied and it was revealed for the first time the four groups of PHCZs shifted from chlorinated to being more brominated with increasing bromide level. The speciation of the four groups of PHCZs was modeled. It was revealed that the shift of PHCZs from chlorinated to being more brominated was caused by the reactivity differences between HOCl and HOBr in reactions forming PHCZs. Furthermore, this study revealed that the species numbers and concentrations of PHCZs formed in chlorination were much higher than those in chloramination, and the formation of PHCZs was decreased by ∼50% with an increase of pH from 6.0 to 9.0. This study of the variety of PHCZs in chlorinated water is an important step toward revealing the occurrence of PHCZs in aquatic environments.