Degradation of Carbamazepine Under Solar Irradiation in the Presence of Chlorine in the Discharged Secondary Effluent: Efficiency, Influences, Degradation Pathways and Ecotoxicity

Abstract

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 obvious during disinfection alone or sunlight irridiation. 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, CBZ generating acridine under the action of ·Cl and ·OH was the main degradation pathway, and the decarboxylation process was the rate-limiting step. In addition, the ecotoxicity of CBZ and its transformation products were evaluated by bioassay and predictions (quantitative structure-activity relationship). Then, the reasons for the elevated ecotoxicity of CBZ degradation under chlorine/solar irradiation and the measures to control the ecotoxicity were discussed.

Keywords: carbamazepine, ecotoxicity, chlorine, solar irradiation, quantum chemical calculation