Intermediate products and reductive reaction pathways in the TiO2 photocatalytic degradation of 1,1,1-trichloroethane in water

1,1,1-Trichloroethane (TCA) has been chosen for the study of the reducing versus oxidizing steps of carbon, involved in the degradation, in UV-irradiated TiO2 aqueous suspensions, of chlorinated alkanes containing two C atoms. At wavelengths > 290 nm, TCA disappearance rate was largely increased in the presence of TiO2. The corresponding apparent first-order rate constant was lower by a factor of only ca. 1.2 than that of nitrobenzene under the same conditions. Within experimental accuracy, the rate of release of Cl- ions was equivalent to the TCA disappearance rate, which illustrated the dechlorination efficiency of TiO2 photocatalysis. Indeed, monochloroacetic acid and, at very low concentrations, monochloroacetaldehyde were the only chlorinated intermediate products detected. Analyses by HPLC of aldehydes (2,4-DNPH derivatization) and carboxylic acids allowed the quantification of ten intermediate products containing these functionalities. Glycolic acid (HO-CH2-COOH) was the product that reached the highest concentration. From the practical viewpoint, it is important to emphasize that all the organic intermediate products were progressively oxidized. From the fundamental viewpoint, a discussion on the nature and formation pathways of the intermediate products brings evidence of the coexistence of reduction and oxidation steps involving the organic material; detection of cis-butenedioic acid, however at very low concentrations, indicated the existence of coupling radical reactions.