Aquatic photo-transformation and enhanced photoinduced toxicity of ionizable tetracycline antibiotics

Most antibiotics contain ionizable groups that undergo acid-base dissociation giving rise to diverse dissociated forms in aquatic systems depending on the pH of the system. In sunlit surface waters, photochemical transformation plays a crucial role in determining the fate of antibiotics. This study presents a comprehensive examination of the photo-transformation degradation kinetics, pathways and photoinduced toxicity of three widely detected tetracyclines (TCs): tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC). Under simulated sunlight (λ > 290 nm), their apparent photolysis followed pseudo-first-order kinetics, with rate constants significantly increasing from H₂TCs⁰ to TCs²⁻. Through competition kinetic experiments and matrix calculations, it was found that the anions HTCs⁻ or TCs²⁻ (pH ∼ 8–10) were more reactive toward hydroxyl radicals (•OH), while TCs²⁻ (pH ∼ 10) reacted the fastest with singlet oxygen (¹O₂). Considering the dissociated species, the total environmental photo-transformation half-lives of TCs were determined, revealing a strong dependence on the water pH and seasonal variation in sunlight. Generally, apparent photolysis was the dominant photochemical process, followed by ¹O₂ and •OH oxidation. Different transformation pathways for the three reactions were determined based on the key photoproducts identified using HPLC-MS/MS. Toxicity tests and ECOSAR software calculations confirmed that the intermediates produced by the •OH and ¹O₂ photo-oxidation processes were more toxic than the parent compounds. These findings significantly enhance our understanding of the complex photochemical fate and associated risks of TCs in aqueous environments.