Light-source-dependent effects of main water constituents on photodegradation of phenicol antibiotics: mechanism and kinetics.

Thiamphenicol and florfenicol are two phenicol antibiotics commonly used in aquaculture. Photodegradation experiments on these phenicols were performed in aqueous solutions under irradiation of different light sources. We found under UV-vis irradiation (lamda >200 nm) they photodegraded the fastest in seawater, followed by pure water and freshwater, whereas under solar or simulated sunlight (lamda >290 nm), they photodegraded in freshwater only. The effects of Cl- (the dominant seawater constituent), humic acids (HA, main constituents in freshwater) and other water constituents on the photodegradation of the antibiotics as a function of different light sources were studied so as to interpret the light-source-dependent effects of different waters. Under UV-vis irradiation, Cl- was found to promote singlet oxygen ((1)O2) formation and accelerated the photodegradation of phenicols, whereas the phenicols did not photolyze under simulated solar irradiation, irrespective of Cl-. In contrast, the presence of HA inhibited phenicol photolysis under UV-vis irradiation through competitive photoabsorption, but HA photosensitized degradation under simulated solar irradiation. Under UV-vis irradiation, the wavelength-averaged (200-290 nm) quantum yields for thiamphenicol and florfenicol in pure water were 0.022 +/- 0.001 and 0.029 +/- 0.001, respectively. Their solar photolytic half-lives in freshwater were 186 +/- 17 h and 99 +/- 16 h, respectively. UV-vis photodegradation intermediates were identified by HPLC-MS/MS, and degradation pathways were proposed. These involve photoinduced hydrolysis, dechlorination, self-sensitized photo-oxidation processvia (1)O2, and chlorination. These results are of importance toward the goal of assessing the persistence of phenicols in wastewater treatment and the environment.