IDENTIFICATION OF ORGANIC-ACIDS AND OTHER INTERMEDIATES IN OXIDATIVE-DEGRADATION OF CHLORINATED ETHANES ON TIO2 SURFACES EN-ROUTE TO MINERALIZATION - A COMBINED PHOTOCATALYTIC AND RADIATION CHEMICAL STUDY

The oxidative degradation of chlorinated ethanes proceeds practically via the same mechanism in gamma-radiolysis and photocatalysis at TiO2 surfaces, respectively. C-centered radicals generated via hydroxyl radical induced C-H bond cleavage and peroxyl radicals derived therefrom after oxygen addition are the key radical intermediates in these processes. The main molecular products identified and isolated in both the gamma-radiolytic and photocatalytic experiments are organic (mostly chlorinated) acids, HCl, and CO2. Other products formed in minor yields are aldehydes and HCOOH. Photocatalytic degradation of these product acids and nonionic substrates leads eventually to complete mineralization. The results strongly suggest that the photocatalytic degradation is initiated by an oxidation of the chlorinated compounds through TiO2-surface-adsorbed hydroxyl radicals. Only some acids, like trichloroacetic acid and oxalic acid, seem to be oxidized primarily by valence band holes via a photo-Kolbe process. Several rate constants are reported on the oxidation of chlorinated ethanes and acids by free and surface-adsorbed hydroxyl radicals and on the overall photocatalytic degradation of the chlorinated compounds. The paper also includes a discussion of the material balance. The study demonstrates the value of radiation chemical investigations for the understanding of the details in the photocatalytic mineralization process of halogenated organic compounds.