Thermally induced formation of PCDD and PCDF from tri- and tetrachlorobenzene in dielectric fluids

The thermal behavior of dilute (5000-50 ppm) solutions of tri- and tetrachlorobenzene in mineral oil, silicone oil, and tetrachloroethylene (TCE) has been investigated. Although pyrolyses or combustion of neat chlorobenzenes readily forms significant yields of PCDDs/PCDFs, corresponding treatment of dilute solutions was generally non-productive. This result is consistent with the proposition that, under the conditions studied, the rate-limiting step for PCDD/PCDF formation is bimolecular in chlorobenzene. In contrast, earlier studies of PCBs implied a rate-limiting step that was unimolecular in chloroaromatics. Combustion of dilute solutions of chlorobenzenes in tetrachloroethylene did produce detectable concentrations of PCDF; this result reflects combination of chlorobenzene and TCE- or TCE-derived species.