PHOTOLYSIS (LAMBDA=254 NM) OF TETRACHLOROETHENE IN AQUEOUS-SOLUTIONS

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 O-2; 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 O-2 and hydrogen donors. It is concluded that there are two (or three) major primary processes (1) CCl2=CCl2+hv-->CCl2=CCl.+Cl-. (2) CCl2=CCl2+hv+H2O-->CCl2(OH)-CCl2H (3) CCl2=CCl2+hv-->CCl2=CCl++Cl- From scavenging experiments with alcohols, with and without O-2, phi(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: phi(2+3)=0.08. In the absence of scavengers, the Cl atoms add to tetrachloroethene yielding the pentachloroethyl radical which, in the presence of O-2, is converted to the corresponding peroxyl radical, the precursor of trichloroacetic acid. In the presence of O-2, a short chain reaction sets in. The chain carrier is the Cl atom which is liberated in the bimolecular termination reactions of the various peroxyl radicals formed in the present system. The rate constants k(Cl-.+C2Cl4)=2.8X10(8) dm(3) mol(-1) s(-1), k(Cl-.+CO32-)=5.0x10(8) dm(3) mol(-1) s(-1) and k(Cl-.+HCO3-)=2.2x10(8) dm(3) mol(-1) s(-1) 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.