Electrocatalytic Reductive Dechlorination Of Polychlorinated Biphenyls By Palladium-Modified Electrode

PCBs are typical persistent organic pollutants (POPs), which are listed in Stockholm Convention as priority chemicals for eventual elimination all over the world by 2025. So it is necessarily to carry out study on the innovative, alternative cleanup technologies for PCBs waste and the remediation of water, soil, and sediment contaminated by PCBs, which will provide the significant technical support for the successful implement of Stockholm Convention. In this work, the high-performance Pd-modified electrode was prepared for PCBs dechlorination; and then the influential factors on the electrocatalytic dechlorination efficiency of PCBs were examined systematically; finally, the reaction mechanisms and pathways of the electrocatalytic dechlorination of PCBs on Pd-modified electrodes were proposed.Firstly, the preparation methods of Pd-modified electrode and the effects of cathode substrates including various metallic meshes, metallic foams, and activated carbon materials on the conversion of PCBs were evaluated. As a result, Pd-modified Ni foam electrode prepared by electroless deposition in NaCl-PdCl2 (3:1) solution shown the best dechlorination ability and catalytic stability. The characteristics of Pd catalyst on Ni foam with electroless deposition by TEM, SEM-EDX, and XRD indicated Pd particle presented micro sizes and high dispersal degree on the surface of Ni substrate, part of which took on the amorphous crystal.Then, based on the Pd-modified Ni foam electrode, the effects of pH, supporting electrolyte, PCBs initial concentration, current density, temperature, flow rate, and applied electricity mode on the dechlorination of PCBs were examined. Excessive hydrogen donor in catholyte and anolyte could enhance the reduction rate. Higher initial concentration, moderate current density, temperature, and flow rate were beneficial to improve the dechlorination rate of PCBs.Thirdly, the solvent systems included alcohol cosolvents and cationic, nonionic, anionic surfactant or hydroxypropyl-β-cyclodextrin for increasing the solubility of PCBs and promoting the dechlorination efficiency of PCBs were evaluated. Methanol was among the best cosolvents due to its low viscidity and was used in preferential concentration of 50 vol % in water for electrocatalytic dechlorination of PCBs. And PCBs solubility in aqueous solution enhanced by cationic, nonionic, anionic surfactant or hydroxypropyl-β-cyclodextrin, respectively, presented the similar electrocatalytic dechlorination efficiency. It means that the dechlorination efficiency was almost not correlated with the type of surfactants. However, cationic surfactant could lead to higher energy consumption than nonionic or anionic surfactant during electrolysis due to the effect of the positive charge in cationic surfactant.Finally, in terms of the adsorption results of PCBs on Pd-modified Ni foam electrode and the characteristics of linear-sweep voltammetry for hydrogen evolution reaction on this electrode, the hydrogen spillover mechanism was proposed for the explanation of high conversion efficiency of PCBs on Pd-modified Ni foam electrode, in which the high active hydrogen atom could transfer from Pd particle into Ni substrate and then diffuse over the surface of the electrode. In addition, based on the GC/MS analysis of intermediate products, the dechlorination pathway of PCBs was proposed which was primarily determined by the effect of steric hindrance of Cl atom in biphenyl molecule.