Photodegradation of Dichlorophenols and Trichlorophenols in TiO2 Aqueous Suspensions: Kinetic Effect of the Positions of the Cl Atoms and Identification of the Intermediates

The photocatalytic degradation of six dichlorophenols (DCPs) and three trichlorophenols (TCPs) in TiO2 aqueous suspensions was studied. The apparent first-order rate constants k(app) of disappearance of these CPs and monochlorophenols (MCPs) were found to correlate with their Hammett constants sigma and their 1-octanol-water partition coefficients K(ow): k(app) (h-1) = -10sigma + 5.2 log K(ow) -7.5 under the conditions used (correlation coefficient, 0.987, if 2,4,6-TCP was omitted). The aromatic intermediates identified by their UV spectra (high performance liquid chromatography (HPLC) separation) and mass spectra (gas chromatography (GC) separation) corresponded to the hydroxylation of the aromatic nucleus with the following order of reactivity para > ortho >> meta. Parabenzoquinones with 0-2 Cl atoms and 0 or 1 OH group and intermediates with two aromatic or quinone rings were also detected. The temporal variations of some intermediates were determined. All of these compounds, in which the C6 ring subsisted, were very unstable, so that their maximum concentration was very low compared with the initial CP concentration. The release of chloride and the evolution of CO2 also followed apparent first-order kinetics within the first part of the degradation. The effects of the number and position of Cl atoms on these kinetics are discussed.