In the 254 nm photolysis of air-saturated aqueous solutions of tetrachloroethene, the major products (quantum yields in parentheses) are as follows: chloride ion (2.05), carbon dioxide (0.62), trichloroacetic acid (0.41), dichloroacetic acid (0.08) and hypochlorite (0.08). Trichloroacetic acid is not formed in the absence of 02; it is suppressed by the addition of hydrogen donors, such as tert-butanol and also by carbonate/bicarbonate ions. The quantum yield of dichloroacetic acid remains unaffected by 02 and hydrogen donors.
It is concluded that there are two (or three) major primary processes
From scavenging experiments with alcohols, with and without 02, t/~(1)=0.23+0.03 has been determined. Reactions ( 2) and
(3) will finally both yield dichloroacetic acid and no distinction can be made between the two routes: qb(2+3)=0.08.
In the absence of scavengers, the C1 atoms add to tetrachloroethene yielding the pentachloroethyl radical which, in the presence of 02, is converted to the corresponding peroxyl radical, the precursor of trichloroacetic acid. In the presence of 02, a short chain reaction sets in. The chain carrier is the (21 atom which is liberated in the bimolecular termination reactions of the various peroxyl radicals formed in the present system. The rate constants k(CI'+C2CL)=2.8×108 dm 3 mol -~ s -1, k(C1"+CO32-)=5.0×10 s dm 3 mol -~ s -1 and k(CI'+HCO3-)=2.2×108 dm 3 mo1-1 s -I have been arrived at in this study. Thus bicarbonate prevents the formation of trichloroacetic acid from potentially present tetrachloroethene in the UV disinfection of drinking water.