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 pyromellitic (Pyro) acids, representative of industrial pollutants and of compounds from biomass, were degraded by heterogeneous photocatalysis. The three molecule disappearance rates followed the order Pyre > Hem > Tri. They obeyed a Langmuir-Hinshelwood mechanism. The two competitive initial steps of attack of the molecules corresponded (i) to a hydroxylation reaction induced by photogenerated OH(.) radicals and (ii) by a decarboxylation (photo-Kolbe) reaction resulting from the direct attack of one carboxylic group by a positive photo-hole. A very careful analysis of the degradation intermediates was performed using high performance liquid chromatography (HPLC) and gas chromatograph/mass spectrometer (GC/MS), especially in the case of Tri. The loss of several carboxyl groups leading to benzoic acid formation was observed before the aromatic ring opening. Several aliphatic acidic fragments were detected, such as malonic and succinic acids. Interestingly, a condensation product was detected, which indicated that some carboxylic radicals could attack a Tri molecule and form a Pyr molecule. However, all these acid intermediates could be photodecomposed - in agreement with previous results from the laboratory - into CO(2) with a complete carbon mass balance. A detailed degradation pathway could be proposed. Such a reaction demonstrated that complex aromatic water pollutants, originating either from industry or from biomass, could be totally mineralized and that they could produce clean water. (C) 2000 Elsevier Science B.V. All rights reserved.