Oxidation of Aroclor 1248 in supercritical water: A global kinetic study

Supercritical water oxidation of extracted contaminants is the second step of a two-stage supercritical fluid technology proposed to remediate soils and sediments contaminated with polychlorinated biphenyls (PCBs) and/or polyaromatic hydrocarbons. In connection with the second step, the supercritical water oxidation rate of Aroclor 1248 (A1248), a mixture of approximately 76 PCB congeners, is investigated at 25.3 MPa and temperatures of 723, 748, 773, and 823 K. The reactions are conducted in an isothermal, isobaric plug-flow tubular reactor, and GC/ECD, GC/FID, GC/TCD, and GC/MS chromatographic methods are employed for product analysis. Experiments are conducted at a nominal A1248 feed concentration of 5.75×10^-5 mol/L (reaction conditions) using a methanol solution of 5.245 g/L (5245 ppm) and H₂O₂ as an initial oxidant (providing approximately 20 mol % excess of O₂). Molar global conversion of A1248 varies from 36.06% (for residence time equal to 6.29 s at 723 K) to 99.95% (54.4 s at 823 K). The overall conversion follows apparent second order, and the rate constant calculated from the data leads to Arrhenius parameters of frequency factor A = 10^17.0±0.1 s^-1 (mol/L)^-1 and energy of activation E_a = 186±2 kJ/mol (44.43±0.51 kcal/mol). The congener specific analysis indicates a buildup of intermediate congener byproducts, which also undergo oxidation decomposition. The identified reaction products are mainly biphenyl, low-chlorinated PCB congeners such as 2-chlorobiphenyl and 2,2′-dichlorobiphenyl, CO, and CO₂.