Methanol as a cosolvent and rate-enhancer for the oxidation kinetics of 3,3′,4,4′-tetrachlorobiphenyl decomposition in supercritical water

Experimental results and kinetic analyses are presented for the dechlorination/oxidation of the 3,3′,4,4′-tetrachlorobiphenyl (T₄CB) congener in supercritical water (SCW) at 25.3 MPa and 673-773 K. A plug-flow reactor is employed with nominal T₄CB feed concentrations of 2.97-5.48 μmol/L in methanol solutions (1.71 mmol/L) using H₂O₂ as the oxidant. Conversions vary from 20.2% (5.76 s at 673 K) to 99.5% (20.7 s at 773 K). The overall conversion is independent of the concentration of O₂ and follows apparent second order with Arrhenius parameters of A = 10^20.7±0.1 s^-1 (mol/L)^-1 and E_a = 219 ± 2 kJ/mol. Methanol functions as a rate enhancer by inducing a free-radical reaction pathway of consecutive dechlorinations leading to biphenyl, which then oxidizes to mineral products. Positively identified reaction intermediate products are lower chlorinated PCB congeners (3,3′,4- and 3,4,4′-TriCB; 3,3′-,3,4-, 3,4′-, and 4,4′-DiCB; and 3- and 4-CB), biphenyl, CO, and CO₂. Successive global reaction networks with second-order kinetics using both individual and lumped pseudo intermediates and final products are shown to represent well the main features of the reaction pathway. Conversions of the individual T₄CB in methanol are compared with those of T₄CB in Aroclor 1248/methanol solutions under similar conditions. The kinetic parameters for the two cases have very similar values, which are also close to those of Aroclor 1248, suggesting the possibility of treating this complex PCB mixture as a pseudo T₄CB. Significant thermal decomposition of T₄CB/methanol in SCW occurs at temperatures higher than 773 K (12.1% at 10.4 s).