Methanol as a Cosolvent and Rate-Enhancer for the Oxidation Kinetics of 3,3‘,4,4‘-Tetrachlorobiphenyl Decomposition in Supercritical Water

Abstract

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 = 1020.7±0.1 s-1 (mol/L)-1 and Ea = 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).