Mechanism of Synergistic Effect of pH and Dissolved Organic Matter in Water on Photochemical Transformation Kinetics of Antibiotics

Photochemical transformation is the main way to eliminate antibiotic emerging contaminants in aquatic environments. pH and dissolved organic matter (DOM) are important for photochemical transformation of antibiotics in water, yet their synergistic mechanism remains unclear. Norfloxacin (NOR) and ciprofloxacin (CIP) were taken as the representatives of antibiotics, while Suwannee River Natural Organic Matter (SRNOM) and DOM (S-DOM) extracted from Songhua River served as representatives of DOM. Through photochemical simulation experiments under simulated sunlight irradiation and high performance liquid chromatography, the synergistic effect of pH and DOM on the photochemical transformation kinetics of antibiotics was studied, and the molecular mechanism was elucidated by exploring how different pH values influenced quantum yields of excited triplet state (³DOM^*), singlet oxygen (¹O₂), and hydroxyl radicals (•OH) generated by photosensitization of DOM and its small molecule analogues. As a result, the apparent photolysis rate constant (k_obs) of NOR at pH=9.0 was 0.39min⁻¹, which is 7.80 and 2.17 times higher than at pH=5.0 (k_obs=0.05min⁻¹) and pH=7.0 (k_obs=0.18min⁻¹), respectively. For CIP, the k_obs at pH=9.0 (k_obs=0.43min⁻¹) is 8.60 and 2.15 times higher than that at pH=5.0 (k_obs=0.05min⁻¹) and pH=7.0 (k_obs=0.20min⁻¹), respectively. Therefore, pH, which influences occurrence forms of NOR and CIP, directly affects their photochemical transformation, and that of the two antibiotics is the fastest in the form of zwitterion (pH=9.0), consistent with previous results. The k_obs of antibiotics mediated by DOM at pH=9.0 is 7.20–14.00 and 1.87–2.25 times higher than that at pH=5.0 and pH=7.0, respectively. ¹O₂ and •OH play the most significant role in promoting the photochemical transformation of antibiotics, with a contribution rate of 36.50% and 12.58% respectively, while the contribution of photochemically produced reactive intermediates (PPRIs) decreases as pH increases, with a contribution rate of 29.17% at pH=5.0, and 16.00% at pH=7.0. The photophysical properties (e.g., E₂/E₃) and functional groups (e.g., phenolic groups) can affect quantum yields of PPRIs, and pH, by influencing the dissociation degree of DOM dissociable components and the redox active functional groups of DOM non-dissociable components, changes quantum yields and indirectly impacts the photochemical transformation of antibiotics. In conclusion, the photochemical transformation of antibiotics is pH dependent, and DOM promotes the photochemical transformation of antibiotics by generating PPRIs through photosensitization. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202502100018.