Photodegradation of dichlorophenols and trichlorophenols in TiO 2 aqueous suspensions: kinetic effects of the positions of the Cl atoms and identification of the intermediates

Abstract

The photocatalytic degradation of six dichlorophenols (DCPs) and three trichlorophenols (TCPs) in TiO 2 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 λ and their 1-octanol-water partition coefficients K ow: k aw (h −1)= −10σ+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. Para-benzoquinones 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 CO 2 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 fkinetics are discussed.