Kinetics and mechanism of advanced oxidation processes (AOPs) in degradation of ciprofloxacin in water

Fluoroquinolones and their metabolites are found in surface and ground waters, indicating their ineffective removal by conventional water treatment technologies. Advanced oxidation processes (AOPs) are alternatives to traditional water treatments. They utilize free radical reactions to directly degrade fluoroquinolones. This work reports absolute rate constants for the reaction of ciprofloxacin with several free radicals, (center dot)OH, (center dot)N(3) and SO(4)(center dot-) as well as hydrated electrons. Pulsed radiolysis experiments showed that (center dot)OH, (center dot)N(3) and e(aq)(-) reacted quickly with ciprofloxacin, with bimolecular reaction rate constants of (2.15 +/- 0.10) x 10(10), (2.90 +/- 0.12) x 10(10) and (2.65 +/- 0.15) x 10(10) M(-1) s(-1), respectively. while the SO(4)(center dot-) radical appeared not to react with ciprofloxacin. Transient spectra were observed for the intermediate radicals produced by hydroxyl and azide radical reactions. Moreover, ciprofloxacin can be degraded rapidly using a typical advanced oxidation process, TiO(2) photocatalysis, with half-lives of 1.9-10.9 min depending upon pH values. Seven degradation products were elucidated by LC/MS/MS analysis, and the degradation mechanism of ciprofloxacin was also tentatively proposed by combining the experimental evidence with theoretical calculations of frontier electron densities. The calculations suggest that the addition of a hydroxyl radical to ciprofloxacin and photo-hole direct attack is two predominant reaction pathways. (C) 2009 Elsevier B.V. All rights reserved.