PCB destruction in subcritical and supercritical water--evaluation of PCDF formation and initial steps of degradation mechanisms.

The reduction of polychlorinated biphenyl (PCB) emissions to the environment are contemporary issues of global efforts, and possible destruction technologies have to be selected and evaluated for PCB remediation. In this study PCB destruction in subcritical and supercritical water were assessed under oxidative conditions and nonoxidative, alkaline conditions. In both cases PCBs could be destroyed by more than 99%. The formation of polychlorinated dibenzofurans (PCDFs) during PCB destruction was evaluated. Under both oxidative and nonoxidative treatments of sub- and supercritical conditions, the formation of PCDFs was observed. The PCDFs formed in the early stages of PCB destruction resulted in up to 47-fold increase in terms of toxic equivalency (TEQ) compared to the initial PCB mixture. However, the PCDFs were destroyed together with the PCBs under more severe conditions i.e., at higher temperature or prolonged residence time. The mechanism of PCDF formation and the initial step of PCB degradation was evaluated. Our laboratory-scale investigation indicates that PCB destruction under supercritical water conditions is feasible, but because of the PCDF formation potential, in particular the high ratio of toxic 2,3,7,8-substituted congeners, conditions have to be carefully selected.