Evolvement of carbamazepine under solar irradiation in the presence of chlorine: Efficiency, influences, degradation pathways and ecotoxicity

Carbamazepine (CBZ) and residual chlorine coexist in wastewater discharged from wastewater treatment plants (WWTPs), while the mechanism of their photochemical transformation in the natural aquatic environment is still unclear. In this study, an approach combining experiment and quantum chemical calculation was used to explore the degradation mechanism of CBZ under chlorine/solar irradiation. The degradation of CBZ was not evident during disinfection alone or sunlight irradiation. However, CBZ was rapidly degraded under chlorine/solar irradiation (kobs = 0.463 min−1 at pH = 8.0). Quenching and probe experiments showed that the efficient removal of CBZ under chlorine/solar irradiation was credited with the production of ·Cl, ·OH, and O3, in which ·Cl (61.95%) and ·OH (33.14%) contributed significantly. The reaction mechanism of CBZ degradation was postulated, in conjunction with the identification of degradation products by HPLC/MS analysis and density functional theory. Among them, the primary degradation way was acridine production under the action of ·Cl and ·OH, and the decarboxylation process was the rate-limiting step. In addition, bioassay results indicated that the luminescent bacteria toxicity and genotoxicity of CBZ after chlorine/solar irradiation increased. Based on the identified transformation products and the quantitative structure-activity relationship, the elevated ecotoxicity was attributed to the TP209 and TP179 generated by pathway 1, and then controlling the decarboxylation process by adjusting pH and temperature to reduce the high toxic degradation products were proposed for ecological safety.