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► We evaluate the photodegradability of the sewage-borne antibiotic azithromycin in natural and artificial surface water. ► Azithromycin undergoes mainly indirect photolysis. ► Several photoproducts are identified by LC-QqToF-MS, some of which appear to be more persistent than the parent compound. ► Photolysis of azithromycin is considered as a potential natural attenuation process in surface waters.
This article describes the photolysis of azithromycin, a macrolide antibiotic with reported occurrence in environmental waters, under simulated solar radiation. The photodegradation followed first-order reaction kinetics in five matrices examined. In HPLC water, the degradation rate was the slowest (half-life: 20
h), whereas in artificial freshwater supplemented with nitrate (5
mg
L
−1) or humic acids (0.5
mg
L
−1) the degradation of azithromycin was enhanced by factors of 5 and 16, respectively, which indicated the role of indirect photolysis involving the formation of highly reactive species. Following chromatographic separation on a UPLC system, the characterization of the transformation products was accomplished using high-resolution QqToF-MS analysis. The presence of seven photoproducts was observed and their formation was postulated to originate from (bis)-N-demethylation in the desosamine sugar, O-demethylation in the cladinose sugar, combinations thereof, as well as from hydrolytic cleavages of the desosamine and/or cladinose residue. Two of these photoproducts could also be detected in natural photodegradation process in river water which was spiked with azithromycin.