THE GC-MS IDENTIFICATION OF SOME ALIPHATIC INTERMEDIATES FROM THE TIO2 PHOTOCATALYTIC DEGRADATION OF DIMETHOXYBENZENES IN WATER

In an attempt to answer the as yet unsolved question of the structures of the aliphatic compounds resulting from the ring opening of aromatic pollutants in water during a photocatalytic treatment, we have carried out a GC-MS study (with or without silylation) of aqueous solutions of each of the dimethoxybenzenes (DMBs) UV-irradiated in the presence of powder TiO2. The aliphatic intermediates are esters or acids which were identified either by comparison with authentic compounds (as is indicated by an asterisk in the following list) or by interpretation of the mass spectra (El and Cl). From 1,4-DMB, they are: (E)(*) and (Z)(.) H3COOC-CH double bond CH-COOCH3. From 1,3-DMB, they are: H3COOC-CH double bond CH-CO-COOH3; H3COOC-CO-CH(OCH3)-CH2OH; H3COOC-CH double bond CH-CHO; H3COOC-CH double bond CHOCH3(.) From 1,2-DMB, they are: (E,E) H3COOC- CH double bond CH-CH double bond CH-COOCH3*; HOOC-CH double bond CH-CH double bond CH-COOCH3 isomers; H3COOC-CH double bond CH-CHO; (E) HOOC-CH double bond CH-COOH*; H2COOC-CHOH-CHO; H3COOC-COOCH3*. In the case of 1,2-DMB, additional analyses showed the presence of CH,OH. Its formation was suggested to result from the cleavage of the methoxy groups of DMBs. These identifications show the complexity of the degradation pathways. The dimethyl esters of(E,E) hexadienedioic acid and of(E) and (Z) butenedioic acids could result fi om the opening of 1,2- or 1,4-DMB, respectively, before these pollutants are hydroxylated. This suggests that hydroxyl radicals are not the only oxidizing species involved. The fact that all aliphatics contain a number of C atoms less than or equal to to that in DMBs bears witness to the bond cleavages which lead to the mineralization.