Sodium Hypochlorite-Assisted Photooxidation of Salicylic Acid: Degradation Kinetics, Formation, and Ecotoxicological Assessment of Intermediates

Detailed studies were conducted on the photooxidation of salicylic acid (SA) in the presence of sodium hypochlorite (NaOCl), which is important in the context of water disinfection processes. It was shown that NaOCl alone causes slow degradation of SA (<10% after 60 min), while its combination with UV radiation significantly increases the efficiency of the process, especially at pH 7.5-10 (up to 30% degradation in 60 min). Eleven chlorinated transformation products have been identified, including 2,6-dichlorophenol and 2,4,6-trichlorophenol, which are characterized by high environmental persistence (>96 days) and the ability to travel distances exceeding 4000 km. QSAR analyses and ecotoxicological tests (Microtox^®, Daphtoxkit F^®, <i>Lemna</i> sp.) confirmed the significant toxicity of some compounds to fish, daphnia, and algae. It was found that the post-reaction mixture after the NaOCl/UV process exhibits higher toxicity than SA photolysis alone, indicating a significant contribution of chlorinated intermediates to environmental risk. The results highlight the need to develop alternative methods for removing pharmaceuticals that minimize the formation of persistent and toxic by-products, and indicate directions for further research on their monitoring in the aquatic environment.