Mechanistic insights into UV photolysis of carbamazepine and caffeine: Active species, reaction sites, and toxicity evolution

Abstract

The pseudo-persistence of pharmaceutical and personal care products (PPCPs)in the aqueous environment may pose potential risks to human health and ecosystems. The UV disinfection in wastewater treatment plants is one of the essential processes before PPCPs enter the water environment, so it is crucial to elucidate the photolytic behavior and mechanism of PPCPs under UV radiation. In this work, carbamazepine (CBZ) and caffeine (CAF) were selected as typical pollutants to investigate the effect of water matrixes, humic acid, inorganic ions, and pH on the UV radiation performance. Hydroxyl radical (•OH) and singlet oxygen (¹O₂) were identified by quenching experiments and electron paramagnetic resonance (EPR) spectra as playing a dominant role in the degradation process. UPLC-TOF/MS was conducted to identify 13 and 14 possible intermediates of CBZ and CAF, respectively. Moreover, combining density functional theory (DFT) calculations (Frontier Molecular Orbital and Fukui index), hydroxylation, oxidation, and ring cleavage were proposed as the main degradation pathways of the contaminants, which occurred first at the C(7C), N(17 N) and O(18O) sites of CBZ and at the C(9C) site of CAF. The bio-acute toxicity experiment and the Ecological Structure-Activity Relationships (ECOSAR) program were performed to analyze and predict the toxicity of the intermediates of CBZ and CAF under UV radiation, respectively. The results showed that the acute toxicity of both solutions increased after UV radiation and followed with the combined toxicity. This work has great scientific value and practical environmental significance for evaluating the UV disinfection process and managing PPCPs in the aqueous environment.

Keywords: Degradation pathway; PPCPs; Reaction sites; Toxicity evaluation; UV radiation.