Levofloxacin oxidation by ozone and hydroxyl radicals: Kinetic study, transformation products and toxicity
Résumé
This work was carried out to investigate the fate of the antibiotic levofloxacin upon oxidation with ozone and hydroxyl radicals. A kinetic study was conducted at 20 degrees C for each oxidant. Ozonation experiments were performed using a competitive kinetic method with carbamazepin as competitor. Significant levofloxacin removal was observed during ozonation and a rate constant value of 6.0 x 10(4) M-1 s(-1) was obtained at pH 7.2. An H2O2/UV system was used for the formation of hydroxyl radicals HO.. The rate constant of HO was determined in the presence of a high H2O2 concentration. The kinetic expressions yielded a k(Ho/LVF) value of 4.5 x 10(9) M-1 s(-1) at pH 6.0 and 5.2 x 10(9) M-1 s(-1) at pH 7.2. These results were used to develop a model to predict the efficacy of the ozonation process and pharmaceutical removal was estimated under different ozonation conditions (i.e. oxidant concentrations and contact times). The results showed that levofloxacin was completely degraded by molecular ozone during ozonation of water and that hydroxyl radicals had no effect in real waters conditions. Moreover, LC/MS/MS and toxicity assays using Lumistox test were performed to identify ozonation transformation products. Under these conditions, four transformation products were observed and their chemical structures were proposed. The results showed an increase in toxicity during ozonation, even after degradation of all of the observed transformation products. The formation of other transformation products not identified under our experimental conditions could be responsible for the observed toxicity. These products might be ozone-resistant and more toxic to Vibrio fisheri than levofloxacin