UVC-induced degradation of cilastatin in natural water and treated wastewater

This work reports for the first time the UVC photodegradation of cilastatin, a renal dehydropeptidase inhibitor co-adminstered with the imipenem antibiotic. Initially, solutions of cilastatin at varying concentrations were prepared in ultra-pure water and the direct photolysis of cilastatin was monitored under 254-nm irradiation. Degradation was slower at higher initial cilastatin concentrations, due to absorption saturation. Of the different eluting photoproducts, only one was tentatively identified as oxidized cilastatin bearing a sulfoxide group. UV-254 photolysis occurred faster at lower pH values, because the protonated forms of the molecule (H₃A⁺, H₂A) have both higher absorption coefficients and higher photolysis quantum yields than the non-protonated ones (HA^-, A^2-). The direct photolysis of cilastatin does not involve ^•OH, as excluded by experiments in which t-butanol was added as ^•OH scavenger, whereas the presence of humic acids inhibited photolysis due to competition for radiation absorption. The same explanation partially accounts for the observation that the photolysis kinetics of cilastatin was slower in tap water, river water and treated wastewater samples compared to ultra-pure water. Moreover, the direct photolysis quantum yield was also lower in water matrices compared to ultra-pure water. Similar findings reported for triclosan and the herbicide 2-methyl-4-chlorophenoxyacetic acid in previous studies might suggest that the water matrix components could carry out either physical quenching of cilastatin's excited states or back-reduction to cilastatin of the partially oxidized degradation intermediates. Overall, the present results demonstrate that UVC irradiation is a fast and efficient process for the degradation of cilastatin in natural water and treated wastewater.