Insights into photodegradation of antibiotics in aquatic environment considering effects of dissolved organic matter and halide ions

The aquatic environment serves as a major reservoir for antibiotics, where photochemical transformation plays a crucial role in their attenuation. In estuarine waters, dissolved organic matter (DOM) and halogen ions are key factors influencing the photodegradation of antibiotics. This study selected five commonly detected antibiotics, metronidazole (MTZ), tinidazole (TNZ), ornidazole (ONZ), ciprofloxacin (CIP), and norfloxacin (NOR), as target pollutants to investigate the individual and combined effects of environmental factors, including DOM, halogen ions, salinity, and pH, on their photochemical degradation. Furthermore, the environmental persistence of these antibiotics in estuarine water was predicted. The observed photolysis rate constants (k_obs) of the five antibiotics varied with salinity and pH, directly affecting their environmental persistence. For nitroimidazole antibiotics, their second-order reaction rate constants with the triplet-excited state of dissolved organic matter (³DOM*) in artificial seawater (ASW) ((0.97–1.19) × 10⁷ M⁻¹ S⁻¹) were lower compared to those in phosphate-buffered saline (PBS, pH = 7) ((1.90–2.40) × 10⁷ M⁻¹ S⁻¹). Similarly, for NOR, its second-order reaction rate constant with singlet oxygen (¹O₂) in ASW (3.35 × 10⁶ M⁻¹ S⁻¹) was lower than in PBS (pH = 7) (11.81 × 10⁶ M⁻¹ S⁻¹). In the presence of DOM, halogen ions exhibited differential effects on the degradation rates of the target antibiotics. Compared to MTZ and TNZ, Cl⁻ and Br⁻ significantly enhanced the photodegradation of CIP and NOR, primarily due to ionic strength and specific halogen ion effects. A predictive model was employed to estimate the k_obs and photolysis half-life (t_1/2) of these antibiotics in the Yellow River estuary. Results indicated that the t_1/2 of nitroimidazole antibiotics in surface water gradually increased as river water transitioned into seawater, whereas the t_1/2 of CIP and NOR decreased due to ionic strength and halogen radical effects. This study underscores the complex interactions among environmental factors governing antibiotic photodegradation and persistence in estuarine systems, providing critical insights into their environmental fate.