Catalytic Reduction Of Organic Chlorinated Compounds By Manganese Dioxide Modified Pd/Fe Nanoparticles

Zero-valent iron?nZVI?application with high removal efficiency of chlorinated organic compounds for water pollution treatment,had a great application potential in the restoration of surface water and underground water pollution.Though nZVI had been widely used in reduction of many pollutants,limits still existed in practical application:on the one hand,the character of easier aggregation of nZVI influenced its a reduction reactivity;on the other hand,passivation layer,which formed on the surface active site of nanoscale zero-valent iron,would also lead to a decrease of dechlorination rate.Nanoscale Pd/Fe bimetal had a lower reductive dechlorination reaction rate and couldn't thoroughly remove the target pollutants.To solve this problem,MnO₂ was added into the bimetal system.The impacts of experimental parameters on the dechlorination of 2,4-dichlorophenoxyacetic acid?2,4-D?and 2,4-dichlorophenol?2,4-DCP?such as initial pH,palladium loading,temperature and MnO₂ amount were investigated.High performance liquid chromatography?HPLC?,transmission electron microscope?TEM?and x-ray diffraction?XRD?were used to explore the mechanism of reductive dechlorination by MnO₂ modified nanoscale Pd/Fe bimetallic particles.Results could be summarized as follows:?1?MnO₂ modified nanoscale Pd/Fe bimetallic system could effectively promote the dechlorination of 2,4-D.In the reaction,2,4-D maybe firstly reduced to intermediates 2-chlorophenoxyacetic acid?2-CPA?and then to the end product phenoxyacetic acid?PA?.And the dechlorination of 2,4-D was a pseudo-first-order reaction.The appropriate conditions for the reductive dechlorination were:manganese dioxide 1.0 g.L^-1,nano zero-valent iron 1.0 g.L^-1,initial pH 4.20,temperature 25?,palladium loading 0.100%,stirring speed 200 r.min^-1,2,4-D initial concentration 50.0mg.L^-1.After 50 min reaction,the generation ratio of PA reached up to 92.1%;the ratio was up to 99.7%after 210 min.?2?MnO₂ modified nanoscale Pd/Fe bimetallic system could effectively promote the dechlorination of 2,4-DCP.The dechlorination of 2,4-DCP was a pseudo-first-order reaction.The appropriate conditions for the reductive dechlorination were:manganese dioxide 1.0 g.L^-1,nano zero-valent iron 1.0 g.L^-1,initial pH 4.20,temperature 25?,palladium loading 0.100%,stirring speed 200r.min^-1,2,4-DCP initial concentration 20.0 mg.L^-1.After 50 min reaction,the generation ratio of phenol reached up to 63.4%;the ratio was up to 78.2%after 210min.?3?Compared with nanoscale Pd/Fe bimetallic particles,MnO₂-nPd/Fe showed a much higher efficiency and reactivity on the reductive dechlorination.Under experimental conditions,the generation rate of PA in MnO₂ 2.0 g.L^-1 system had increased 5.12 times compared with non-MnO₂ system.Higher concentration of MnO₂,lower initial pH and higher palladium loading were conducive to the reductive dechlorination.?4?MnO₂ had the character of high electrical conduction and possessed the ability of faster electron transfer.Iron oxides could be easily formed on the surface of MnO₂ particles from Fe²⁺and Fe³⁺in aqueous solution,thus the passivation layer formed on the surface of nanoscale zero-valent iron could be decrease.The decrease of passivation layer formation was conducive to the catalytic dechlorination.