Aqueous multivariate phototransformation kinetics of dissociated tetracycline: implications for the photochemical fate in surface waters

Antibiotics are ubiquitous pollutants in aquatic systems and can exist as different dissociated species depending on the water pH. New knowledge of their multivariate photochemical behavior (i.e., the photobehavior of different ionized forms) is needed to improve our understanding on the fate and possible remediation of these pharmaceuticals in surface and waste waters. In this study, the photochemical degradation of aqueous tetracycline (TC) and its dissociated forms (TCH₂⁰, TCH⁻, and TC²⁻) was investigated. Simulated sunlight experiments and matrix calculations indicated that the three dissociated species had dissimilar photolytic kinetics and photooxidation reactivities. TC²⁻ photodegraded the fastest due to apparent photolysis with a kinetic constant of 0.938 ± 0.021 min⁻¹, followed by TCH⁻ (0.020 ± 0.005 min⁻¹) and TCH₂⁰ (0.012 ± 0.001 min⁻¹), whereas TCH⁻ was found to be the most highly reactive toward •OH (105.78 ± 3.40 M⁻¹ s⁻¹), and TC²⁻ reacted the fastest with ¹O₂ (344.96 ± 45.07 M⁻¹ s⁻¹). Water with relatively high pH (e.g., ~ 8–9) favors the dissociated forms of TCH⁻ and TC²⁻ which are most susceptible to photochemical loss processes compared to neutral TC. The calculated corresponding environmental half-lives (t₁/₂,E) in sunlit surface waters ranged from 0.05 h for pH = 9 in midsummer to 3.68 h for pH = 6 in midwinter at 45° N latitude. The process was dominated by apparent photolysis (especially in summer, 62–91%), followed by ¹O₂ and •OH oxidation. Adjusting the pH to slightly alkaline conditions prior to UV or solar UV light treatment may be an effective way of enhancing the photochemical removal of TC from contaminated water. Graphical abstract Aqueous multiple photochemical behavior of dissociated tetracycline (TCH₂⁰, TCH⁻, and TC²⁻) is first comprehensively reported on revealing the phototransformation kinetics and implications for the fate in surface waters.