D. Mas et al., INTERMEDIATE PRODUCTS AND REDUCTIVE REACTION PATHWAYS IN THE TIO2 PHOTOCATALYTIC DEGRADATION OF 1,1,1-TRICHLOROETHANE IN WATER, Research of chemical intermediates, 23(3), 1997, pp. 275-290
1,1,1-Trichloroethane (TCA) has been chosen for the study of the reduc
ing versus oxidizing steps of carbon, involved in the degradation, in
UV-irradiated TiO2 aqueous suspensions, of chlorinated alkanes contain
ing two C atoms. At wavelengths > 290 nm, TCA disappearance rate was l
argely increased in the presence of TiO2. The corresponding apparent f
irst-order rate constant was lower by a factor of only ca. 1.2 than th
at of nitrobenzene under the same conditions. Within experimental accu
racy, the rate of release of Cl- ions was equivalent to the TCA disapp
earance rate, which illustrated the dechlorination efficiency of TiO2
photocatalysis. Indeed, monochloroacetic acid and, at very low concent
rations, monochloroacetaldehyde were the only chlorinated intermediate
products detected. Analyses by HPLC of aldehydes (2,4-DNPH derivatiza
tion) and carboxylic acids allowed the quantification of ten intermedi
ate products containing these functionalities. Glycolic acid (HO-CH2-C
OOH) 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 fundament
al 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 ex
istence of coupling radical reactions.