Phototransformation of roxithromycin in the presence of dissolved organic matter: Characteriazation of the degradation products and toxicity evaluation.

Roxithromycin (ROX) is a widely used macrolide antibiotic and its environmental fate and ecotoxicity have attracted considerable attention. In this study, the phototransformation kinetics and products of ROX under the irradiation of simulated sunlight were investigated. The ecotoxicity of ROX before and after phototransformation were also examined using the bioluminescence bioassay and algae growth inhibition test. The results showed that ROX underwent direct photolysis and indirect photolysis in the presence of Suwannee River humic acid (SRHA) and Suwannee River natural organic matter (SRNOM). The kinetic rate constant of the photodegradation of ROX in the presence of 20 mg·L SRHA and SRNOM were 4.0 and 3.6 times higher than direct photolysis in the absence of dissolved organic matter (DOM). A total of 20 phototransformation products (PTPs) formed as a result of the photodegradation of ROX by simulated solar irradiation were identified, and 10 of them were reported for the first time. The PTPs were generally formed through the N-demethylation, O-demethylation or direct cleavage of the side chain, desosamine or cladinose moiety from ROX. Solutions containing ROX and its PTPs showed an increased toxicity to Vibrio fischeri, demonstrating some PTPs were more toxic to V. fischeri. On the other hand, the toxicity of ROX after irradiation to Chlorella pyrenoidosa decreased, suggesting the phototransformation of ROX in the environment may be a positive outcome in the context of the growth of green algae.