Photodegradation of phenylurea herbicides sensitized by norfloxacin and the influence of natural organic matter

The photochemical activity of fluoroquinolone antibiotics (FQs) has gained attention due to the discovery of their phototoxicity and photocarcinogenicity in clinics. This study reveals that norfloxacin (NOR) can sensitize the photodegradation of phenylurea (PU) herbicides. This is attributed to the formation of an excited triplet of norfloxacin (3NOR*) by UV-A irradiation of its quinolone chromophore, which can further react with O2 to form singlet oxygen (1O2). The second-order rate of 3NOR* with PU ranges from 1.54 x 1010 to 2.76 x 1010 M - 1s- 1.The steady-state concentrations of 3NOR* were calculated as (4.29-31.2)x 10-16 M at 10 mu M NOR under UV365nm irradiation. Natural organic matter (NOM) inhibited the degradation of PU induced by 3NOR*. In the presence of 10 mg L-1 NOM, the pseudo-first-order rate constants (kobs,NOM) of the degradation of diuron (DIU), isoproturon (IPU), monuron (MOU), and chlorotoluron (CLU) decreased by 65%, 19%, 36%, and 62%, respec-tively. NOM mainly acts as a reductant which reacted with the radical intermediates of the PU generated by 3NOR*oxidation, thus reversing the oxidation. The inhibitory effect increases with increasing NOM concentra-tion. Results of this study underscore the role of NOR as a photosensitizer in accelerating the abatement of PU pesticides in sunlit surface waters. This study significantly advances the understandings of the behavior of NOR in aquatic environments.