Dechlorination-Oxypyrolysis Coupled Treatment of Polychlorinated Biphenyls: Reaction Process, Degradation Pathway, and Cost Analysis

The high-concentration desorbed gas containing polychlorinated biphenyls (PCBs) generated during the thermal desorption treatment of PCB-contaminated soil poses a serious threat to both the environment and human health. We developed the multifunctional catalyst CuO@HZSM-5@Meso-SiO2 that integrates Si–OH, Si–OH-Al, and CuO species for the PCBs dechlorination-oxypyrolysis reaction. CuO@HZSM-5@Meso-SiO2 exhibits 99.03% conversion for 2,2′-dichlorobiphenyl (2,2′-DCB) at 400 °C and 10,000 h–1. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and gas chromatography–mass spectrometry (GC-MS) analyses demonstrate that the Si–OH groups break the C–Cl bonds in 2,2′-DCB to form a biphenyl. The subsequent cracking of biphenyl over the Si–OH-Al sites produces benzene and phenol, which are ultimately converted into CO2 and H2O over CuO nanoparticles. The degradation of PCBs by the CuO@HZSM-5@Meso-SiO2 catalyst was successfully extended to 2,6-dichlorobiphenyl, 2,2′,3,5′-tetrachlorobiphenyl, and 2,2′,3,4,5,6′-hexachlorobiphenyl. Laboratory-scale treatment of actual PCB-contaminated soil via the thermal desorption-dechlorination-oxypyrolysis process exhibited that both treated off-gas and condensate met national emission standards. Energy consumption and operational costs were evaluated for the treatment of PCB-contaminated soil at a processing capacity of 5 t/h. Heat recovered from the off-gases of the reactor and the thermal desorption combustion chamber was utilized to preheat the incoming air for the treatment system, thereby achieving a 16% reduction in the total energy consumption during soil remediation. The overall operation cost was reduced to as low as 30.4 $/tsoil, which is markedly lower than the costs associated with traditional remediation approaches. Taken together, these results demonstrate a practical catalytic route to eliminate PCBs pollutants from thermal desorbed gas through sequential dechlorination, molecular fragmentation, and complete oxidation. The energy integration and low unit cost further support the potential scalability of this approach for the on-site remediation of PCB-contaminated soils.