Kinetics and mechanisms of the photolytic and OH° radical induced oxidation of fluorinated aromatic compounds in aqueous solutions

Article abstract of DOI:10.1016/0045-6535(96)00011-2

Laboratory experiments with H₂O₂/UV oxidation processes and photolysis at 253.7 nm wavelength have been carried out on dilute aqueous solutions (C₀ = 0.1 to 3.0 mM) of trifluorobenzene derivatives (1,3,5-trifluorobenzene, 1,2,3 and 1,2,4-trifluorobenzene) and of α,α,α-trifluorotoluene in the presence and in the absence of dissolved oxygen. The analyses of fluoride ions content during the oxidation experiments showed that the first steps lead to the production of about 2 mol of F^-/mol of trifluorobenzene decomposed and of 1 mol of F^-/mol of trifluorotoluene decomposed. Kinetic studies lead to the determination of the quantum yield for the photolysis of 1,3,5-trifluorobenzene, 1,2,3 and 1,2,4-trifluorobenzene (Φ = 0.011, 0.010 and 0.015 respectively), and of trifluorotoluene (Φ = 0.015). The rate constants for the reaction of hydroxyl radicals with these molecules, determined under specific experimental conditions, were found to range from 3.7 10⁹ to 4.9 10⁹ M^-1.s^-1). GC/MS analyses carried out on extracts at different irradiation time (UV, H₂O₂/UV) lead to the identification of numerous by-products from trifluorobenzene and trifluorotoluene. They consist mostly in hydroxylated and dehalogenated compounds. Dimers have also been observed during photolysis. Moreover, experiments carried out under oxygen limiting conditions revealed the formation of other compounds. For each case studied, a detailed mechanism involving radical intermediates and the different reaction sequences is proposed.