Ozonation of norfloxacin and levofloxacin in water: Specific reaction rate constants and defluorination reaction

The degradation kinetics and mechanism of two typical fluoroquinolones (FQs), norfloxacin (NF) and levofloxacin (LOF), by ozone in water were investigated. Semi-continuous mode and competition kinetics mode experiments were conducted to determine the reaction rate constants of target FQs with ozone and OH, separately. Results indicate that both NF and LOF were highly reactive toward ozone, and the reactivity was strongly impacted by the solution pH. The specific reaction rate constants of the diprotonated, monoprotonated and deprotonated species were determined to be 7.20 × 102, 8.59 × 103, 4.54 × 105 M−1 s−1 respectively for NF and 1.30 × 103, 1.40 × 104, 1.33 × 106 M−1 s−1 respectively for LOF. The reaction rate constants of target FQs toward OH were measured to be (4.81–7.41) × 109 M−1 s−1 in the pH range of 6.3–8.3. Furthermore, NF was selected as a model compound to clarify the degradation pathways, with a particular focus on the defluorination reaction. The significant release of F− ions and the formation of three F-free organic byproducts indicated that defluorination was a prevalent pathway in ozonation of FQs, while six F-containing organic byproducts indicated that ozone also attacked the piperazinyl and quinolone moieties. Escherichia coli growth inhibition tests revealed that ozonation could effectively eliminate the antibacterial activity of target FQ solutions, and the residual antibacterial activity had a negative linear correlation with the released F− concentration.