Free-radical-induced oxidative and reductive degradation of fluoroquinolone pharmaceuticals: kinetic studies and degradation mechanism.
Fluoroquinolones, as a class of broad-spectrum antibiotics, have been detected in both surface and ground waters, and advanced oxidation/reduction processes (AO/RPs) are currently in development to remove these and other pharmaceuticals from wastewater because currently utilized treatment methods have proven to be ineffective. This article reports the reaction kinetics of six common fluoroquinolones with hydroxyl radicals and hydrated electrons, which are the major reactive species involved in AO/RPs. The bimolecular reaction rate constants (M(-1) s(-1)) for orbifloxacin, flumequine, marbofloxacin, danofloxacin, enrofloxacin, and the model compound, 6-fluoro-4-oxo-1,4-dihydro-3-quinoline carboxylic acid, with *OH are (6.94 +/- 0.08) x 10(9), (8.26 +/- 0.28) x 10(9), (9.03 +/- 0.39) x 10(9), (6.15 +/- 0.11) x 10(9), (7.95 +/- 0.23) x 10(9), (7.65 +/- 0.20) x 10(9), and with e(aq)(-), (2.25 +/- 0.02) x 10(10), (1.83 +/- 0.01) x 10(10), (2.41 +/- 0.02) x 10(10), (1.68 +/- 0.02) x 10(10), (1.89 +/- 0.02) x 10(10), and (1.49 +/- 0.01) x 10(10). These rate constants are related to the functional groups attached to the quinolone core, particularly the steric hindrance of the piperazine ring, making it possible to obtain a preliminary estimate of the *OH rate constant of an arbitrary fluoroquinolone by observing the ring constituents. In addition, the products of gamma-irradiation degradation of fluoroquinolones were analyzed by LC-MS to elucidate the probable pathways of AO/RPs degradation. Results indicate that preliminary degradation pathways include hydroxyl radical attack on the aromatic ring with subsequent hydroxylation, the substitution of a fluorine atom with a hydroxyl group, and the removal of the piperazine-derived side chain.