Phototransformation of roxithromycin in the presence of dissolved organic matter: Characteriazation of the degradation products and toxicity evaluation.

Roxithromycin (ROX) is a widely used macrolide antibiotic and its environmental fate and ecotoxicity have attracted considerable attention. In this study, the phototransformation kinetics and products of ROX under the irradiation of simulated sunlight were investigated. The ecotoxicity of ROX before and after phototransformation were also examined using the bioluminescence bioassay and algae growth inhibition test. The results showed that ROX underwent direct photolysis and indirect photolysis in the presence of Suwannee River humic acid (SRHA) and Suwannee River natural organic matter (SRNOM). The kinetic rate constant of the photodegradation of ROX in the presence of 20 mg·L-1 SRHA and SRNOM were 4.0 and 3.6 times higher than direct photolysis in the absence of dissolved organic matter (DOM). A total of 20 phototransformation products (PTPs) formed as a result of the photodegradation of ROX by simulated solar irradiation were identified, and 10 of them were reported for the first time. The PTPs were generally formed through the N-demethylation, O-demethylation or direct cleavage of the side chain, desosamine or cladinose moiety from ROX. Solutions containing ROX and its PTPs showed an increased toxicity to Vibrio fischeri, demonstrating some PTPs were more toxic to V. fischeri. On the other hand, the toxicity of ROX after irradiation to Chlorella pyrenoidosa decreased, suggesting the phototransformation of ROX in the environment may be a positive outcome in the context of the growth of green algae.

Degradation of typical macrolide antibiotic roxithromycin by hydroxyl radical: kinetics, products, and toxicity assessment.

The degradation of roxithromycin (ROX) by hydroxyl radical (·OH) generated by UV/H2O2 was systematically investigated in terms of degradation kinetics, effects of water chemistry parameters, oxidation products, as well as toxicity evaluation. The degradation of ROX by UV/H2O2 with varying light irradiation intensity, initial ROX concentration, and H2O2 concentration in pure water and wastewater all followed pseudo-first-order kinetics. The second-order rate constant for reaction between ROX and ·OH is 5.68 ± 0.34 × 109/M/s. The degradation rate of ROX increased with the pH; for instance, the apparent degradation rates were 0.0162 and 0.0309/min for pH 4 and pH 9, respectively. The presence of natural organic matter (NOM) at its concentrations up to 10 mg C/L did not significantly affect the removal of ROX. NO3- and NO2- anions inhibited the degradation of ROX due to the consumption of ·OH in reactions with these ions. Fe3+, Cu2+, and Mg2+ cations inhibited the degradation of ROX, probably because of the formation of ROX-metal chelates. A total of ten degradation products were tentatively identified by HPLC/LTQ-Orbitrap XL MS, which mainly derived from the attack on the oxygen linking the lactone ring and the cladinose moiety, tertiary amine and oxime side chain moiety by ·OH. The toxicity evaluation revealed that UV/H2O2 treatment of ROX induced the toxicity to bioluminescent bacteria increased.