Photocatalytic degradation of chlorobenzoic isomers in aqueous suspensions of neat and modified titania

The three isomers of chlorobenzoic acid (CBA) have been degraded using photocatalysis. The three pollutants disappeared from water in the following order: 3-CBA < 2-CBA < 4-CBA, the para position being the most reactive. The zeroth kinetic order was interpreted by a Langmuir-Hinshelwood mechanism involving a saturation of the adsorption sites. The chemisorption of CBA molecules involves the carboxylic group linked to the surface with the aromatic ring possibly oriented perpendicularly to the surface. The initial step of CBA disappearance was the decarboxylation of the molecule (photo-Kolbe reaction) with an initial rate of CO2 formation equal to that of the disappearance of CBA. The mass balance could be established for all the final products. All the intermediate products found correspond to successive hydroxylations of the corresponding chlorophenol obtained after the initial photo-Kolbe reaction. They all disappeared within less than 2 h of UV irradiation. In 2-CBA degradation, a transient condensation product (4'-chloro,3,4-dihydroxy-(1,1')biphenyl) could be identified before its degradation. Modification of titania by doping with Cr3+ ions or by depositing 1 wt.% Pt was detrimental for the activity, the best catalyst remaining neat titania. The present study illustrates the difference in the kinetics of photodegradation of various isomers for one compound. (C) 1998 Elsevier Science S.A. All rights reserved.