Phototransformation of selected human-used macrolides in surface water: kinetics, model predictions and degradation pathways.

The phototransformation of clarithromycin and roxithromycin, two human-used macrolide (MLs) antibiotics was investigated in surface waters. Photolysis kinetic data suggest that degradation in water would occur via the direct photolysis of the Fe(III)-MLs complexes. Hydroxyl radicals, singlet oxygen and other photooxidants generated from nitrate ions and from excited chromophores present in humic acids appeared to have only a very limited impact on the overall degradation of MLs under the adopted UV-vis irradiation conditions. A photolysis model applied to the Fe(III)-clarithromycin complex in river water showed that a half-life of 40 days was predicted under clear-sky irradiation in November, 26 days in February, and 10 in May. Direct photolysis could have a limited impact on the environmental concentrations of MLs in rivers, due to a too short water residence time but might be important in shallow lakes and lagoons. Photoinduced degradation of MLs mainly implied changes in the structure of the aglycone, probably leading to their detoxification because the pseudoerythromycin derivatives have very little antimicrobial activity.