Photochemical degradation of polyhalogenated carbazoles in hexane by sunlight

Polyhalogenated carbazoles (PHCZs) are a class of halogenated dibenzopyrrole, which have been increasingly detected in the environment and found to be bioaccumulative and potentially toxic. However, their environmental transformation potential is largely unknown. In this study, UV absorption spectra of carbazole (CZ) and 10 PHCZs were obtained with wavelength range 290-400 nm, and three peaks were identified in most cases with the highest occurring around 300 nm. Hexane solutions of CZ, 10 individual PHCZs, and a sediment extract containing nine other PHCZs were separately irradiated under natural sunlight in order to investigate their photodegradation kinetics and pathways. The pseudo-first-order reaction rate constants (k) of these PHCZs varied from 0.183 h^-1 to 2.394 h^-1, and increased exponentially with increasing numbers of chlorines and bromines in PHCZ molecules. Contribution to ln k from each bromine atom is more than doubling of that from each chlorine atom. Stepwise reduction debromination was confirmed to be one of the photodegradation mechanisms for both brominated and mixed halogenated (containing both bromine and chlorine) carbazoles. Only sporadic dechlorinated products were found during the photolysis of chlorinated carbazoles. By adopting a simplified kinetic approach, we estimated that dehalogenation contributed approximately 20% to 51% of the total loss of the parent PHCZs.