PCB/MeOH Decomposition Networks in Supercritical Water

摘要

The results of this study demonstrate that the use of methanol as a second solvent to deliver water-insoluble chlorinated compounds provides the benefit of a significantly enhanced reaction rate. The experimental results show that the presence of methanol in the reactor significantly accelerates the rate of T4CB disappearance through a free radical dechlorination step. The reaction pathway of T_4CB, in the presence of methanol, proceeds through consecutive dechlorinations to biphenyl, which undergoes oxidation resulting in products of which CO and CO_2 are observed. Competing phenomena of production and decomposition of the reaction intermediates occur in the SCWO process. Cl-abstraction from PCB molecules by hydrogen species, produced through methanol oxidation, avoids the reported formation of PCDDs/Fs. These compounds, which are more harmful than PCBs and form through OH radical addition mechanisms in the absence of methanol, are not detected in our experiments within the analytical limits (～0.1 ppb). In the absence of methanol, PCBs undergo parallel, competing reaction pathways through hydroxylated compounds, chlorinated benzene compounds, and dimers as primary SCWO products. A global reaction network of coupled reactions with second order kinetics is shown to capture the detailed step mechanism of successive dechlorination of PCB congeners followed by ring-opening oxidation of biphenyl leading to mineral products. Successively simplified global reaction networks are shown to represent the main features of this reaction process by using lumped pseudo reaction products.