Polyhalogenated
carbazoles (PHCZs) make up a group of persistent,
bioaccumulative, and toxic contaminants and are newly identified as
chlorinated disinfection byproducts. However, the fates of these compounds
in prolonged chlorination are largely unknown, leading to a great
threat to the safety of drinking water. This study investigated the
transformation of PHCZs during prolonged chlorination by using 3,6-dichlorocarbazole
(36-CCZ) as a model congener, providing important information about
the fates of PHCZs in drinking water. The degradation kinetics of
36-CCZ showed a strong pH dependency with apparent second-order rate
constants of 1.52–5.17 M–1 s–1 at pH 6–10. The degradation rates are comparable to that
of pyrene in chlorination. Seven new chlorine-containing products P1–P7 were detected. Transformation pathways, involving
electrophilic chlorination, nucleophilic water addition, aromatic
ring opening, and HCl elimination, were proposed, and the reaction
mechanism was explored. The product evolution versus time showed the
first generation of the highly halogenated carbazoles of 1,3,6-trichlorocarbazole
(P1) and 1,3,6,8-tetrachlorocarbazole (P2), followed by the generation of hydroxylated products P3–P7. The persistence of the seven products in chlorinated water over
24 h indicated that human exposure to PHCZs and/or their transformation
products was highly possible. This study provides novel insights into
the behaviors of PHCZs in drinking water.
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