Aquatic photochemistry for different dissociation forms of cephalosporin antibiotics: Degradation kinetics, products and photo-modified toxicity

Abstract

Cephalosporin antibiotics (CFs) with ionizable groups (-COOH and -NH_n) are widely detected as emerging micropollutants that pose potential environmental risks to aquatic systems, but few studies have revealed their multivariate photochemical transformation behavior in sunlight-irradiated surface waters. In this study, the apparent photodegradation, photo-oxidation towards reactive oxygen species (ROS, •OH and ¹O₂), and photo-modified toxicity were investigated for the four ionizable CFs: cefoxitin (CFX), cephalothin (CEF), cefoperazone (CFP) and cefazolin (CFZ). Under simulated sunlight irradiation (λ > 290 nm), their multivariate photo-transformation kinetics varied as a function of pHs and the dominant protonated states of the CF in question (H₂CFs⁺, HCFs⁰ and CFs⁻). Based on competition kinetics and matrix deconvolution methods, the apparent photolytic rate constants (k_i) of different dissociation forms were found to decrease gradually from H₂CFs⁺ to CFs⁻ then to HCFs⁰, which was dominated by the changing cumulative light absorption (∑(L_λε_λ,i)) for the different dissociated forms. Interestingly, it was observed that the H₂CFs⁺ or CFs⁻ exhibited higher reactivities towards •OH, while CFs⁻ demonstrated the fastest reaction with ¹O₂. Using the theoretical derivation, the determined environmental half-lives of the CFs in sunlight-irradiated surface waters were closely dependent on the water pHs and multiple photochemical reaction types. In most cases, apparent photodegradation contributes more than ROS mediated photooxidation to the overall photo-transformation of CFs. The product identification using HPLC-MS/MS indicated that the photodegradation pathways mainly involved photoinduced hydrolysis of the β-lactam ring, cleavage of the side-chain, and decarboxylation. Based on the bioassay to Vibrio fischeri, the most CFs showed photo-enhanced toxicity, which was verified by the ECOSAR assessment, raising concerns about the formation and accumulation of more toxic intermediates. These results are of significance to better assessing the photochemical persistence and risk of the CFs in the aquatic systems and wastewater treatment.