Photocatalytic mineralization of chlorinated organic pollutants in water by polyoxometallates. Determination of intermediates and final degradation products.

Homogeneous aqueous solutions of organochlorine pesticides and chlorophenols, namely, Lindane, hexachlorobenzene and 2,4-dichlorophenol (2,4DCP), undergo effective photodegradation upon photolysis with UV and near visible light in the presence of a characteristic polyoxometallate catalyst PW12O403-. These substrates remained, practically, intact (lindane, HCB) or underwent minor degradation under similar conditions in absence of catalyst. The main oxidant appears to be OH radicals formed by the reaction of the excited polyoxometallate with H2O. The system compares with the widely published TiO2. Chlorohydroquinone (CIHQ), hydroquinone (HQ), chlorobenzoquinone (CIBQ), benzoquinone (BQ), 3,5-dichlorocatechol (3,5DCC) and 4-chlorocatechol (4CC), among others, were identified as the main aromatic intermediates in the photodegradation of 2,4DCP. Acetic acid was detected as ring cleavage product. In all cases the final photodegradation leads to complete mineralization of substrates to CO2 and HCl.