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

Highlights

● Mechanisms for multiple photochemical transformation of tetracyclines were reported.

● The degradation kinetics were dependent on pH and reactivities of dissociated forms.

● Anionic forms reacted faster in the apparent photolysis and photooxidation processes.

● Different pathways and various intermediates occurred for the three reactions.

● The major by-products showed similar or more toxicities than the parent antibiotics.

Abstract

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 TCs^2–. Through competition kinetic experiments and matrix calculations, it was found that the anions HTCs^– or TCs^2– (pH ~ 8–10) were more reactive toward hydroxyl radicals (•OH), while TCs^2– (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.

Graphical abstract

Keywords

Tetracyclines / Dissociation / Photodegradation kinetics / Reactive oxygen species / Transformation products / Risks

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Acknowledgements

This work was supported by the Key Research and Development Program of Shaanxi Province (No. 2024SF-YBXM-567), the National Natural Science Foundation of China (Nos. 21976045 and 22076112), the China Scholarship Council (CSC) Scholarship (No. 202308610123), and the Shaanxi Key Laboratory of Environmental Monitoring and Forewarning of Trace Pollutants (No. SHJKFJJ202318).

Conflict of Interests

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11783-024-1899-x and is accessible for authorized users.

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