Direct photolysis of trichloroethene in air: Effect of cocontaminants, toxicity of products, and hydrothermal treatment of products

This paper examines the practical limitations in using xenon flash lamps for the direct photolysis of organic compounds in air. Due to Cl-. atom chain reactions, higher chlorinated ethenes photolyze rapidly enough to compete with catalytic thermal oxidation. However, many common cocontaminants photooxidize slowly, limiting practical applications. The chloroethene Cl-. chains can be inhibited by some cosolutes, but can also sensitize the oxidation of some cocontaminants. Trichloroethene (TCE) photolyzes to give a >90% initial yield of dichloroacetyl chloride (DCAC), which photooxidizes further to phosgene (less than or equal to 25%), trichloroacetyl chloride (less than or equal to 2%), unidentified intermediates, and ultimately HCl and CO2. DCAC is about 40 times more toxic than TCE and requires about 100 times more exposure dose to reduce the effluent toxicity to acceptable levels. Dechlorination can be achieved by scrubbing into alkaline solution followed by high-temperature hydrolysis. The base-catalyzed hydrolysis of dichloroacetic acid proceeds with a rate constant of In k(B) [M(-1) s(-1)] = (32.6 +/- 5.7) + (129 +/- 20 kJ mol(-1))/RT measured from 90 to 220 degrees C and 0.1-1 M NaOH.