Photocatalytic degradation of polycarboxylic benzoic acids in UV-irradiated aqueous suspensions of titania. Identification of intermediates and reaction pathway of the photomineralization of trimellitic acid (1,2,4-benzene tricarboxylic acid)

Three polycarboxylic (hemimellitic (Hem), trimellitic (Tri) and pyromelliti c (Pyro) acids, representative of industrial pollutants and of compounds fr om biomass, were degraded by heterogeneous photocatalysis. The three molecu le disappearance rates followed the order Pyre > Hem > Tri. They obeyed a L angmuir-Hinshelwood mechanism. The two competitive initial steps of attack of the molecules corresponded (i) to a hydroxylation reaction induced by ph otogenerated OH. radicals and (ii) by a decarboxylation (photo-Kolbe) react ion resulting from the direct attack of one carboxylic group by a positive photo-hole. A very careful analysis of the degradation intermediates was pe rformed using high performance liquid chromatography (HPLC) and gas chromat ograph/mass spectrometer (GC/MS), especially in the case of Tri. The loss o f several carboxyl groups leading to benzoic acid formation was observed be fore the aromatic ring opening. Several aliphatic acidic fragments were det ected, such as malonic and succinic acids. Interestingly, a condensation pr oduct was detected, which indicated that some carboxylic radicals could att ack a Tri molecule and form a Pyr molecule. However, all these acid interme diates could be photodecomposed - in agreement with previous results from t he laboratory - into CO2 with a complete carbon mass balance. A detailed de gradation pathway could be proposed. Such a reaction demonstrated that complex aromatic water pollutants, origin ating either from industry or from biomass, could be totally mineralized an d that they could produce clean water. (C) 2000 Elsevier Science B.V. All r ights reserved.