Exploring the aqueous photodegradation of three ionisable macrolide antibiotics: Kinetics, intermediates and photoinduced toxicity

Three macrolide antibiotics (MLs, i.e., roxithromycin, clarithromycin and spiramycin) were investigated to reveal the aqueous photochemistry of the respective neutral and dissociated species (i.e., H2MLs+, HMLs0 and MLs−), including their susceptibility to direct photolysis as well as the hydroxyl radical (•OH) and singlet oxygen (1O2) mediated photooxidation. Under simulated sunlight (λ > 290 nm), no obvious or slow photodegradation of the three MLs were observed in pure water, while they photodegraded rapidly under shorter wavelength irradiation (λ > 200 nm). It was further found that the dependence of the kinetics on pH was attributed to the different reactivities of the dissociation species. The rate constants and cumulative light absorption increased with the order H2MLs+ < HMLs0 < MLs−. Based on competition kinetic experiments and matrix calculations, MLs− was differentiated to be more highly reactive towards •OH/1O2. The corresponding environmental half-lives were evaluated considering the reactivities and proportions of the speciated forms at different pH, indicating that 1O2 oxidation (t1O2,E = 1.83–2.53 h) contributed more than •OH oxidation (t•OH,E = 33.17–787.49 h) to the ML phototransformation in sunlit surface waters. In general, these reactions preserved the core backbone structures of the parent ML molecules and gave rise to intermediates that displayed higher toxicity to Vibrio fischeri than the parent molecules, hence demonstrating photo-modified toxicity. These results are of importance towards the goal of assessing the persistence of MLs during wastewater treatment using UV-light tertiary treatment processes, as well as in sunlit surface waters.