Improvement Of 2,4-D Dechlorination By Pd/Fe Nanoparticles

The dechlorination of chlorinated compounds in water by nano scale zero-valent iron?nZVI?has been considered the most effective way with a great application prospect.However,nZVI would aggregate due to its huge specific surface area,and the new generated ferrous oxides would form a passivation layer on nZVI surface.All that would lead to the decreasing of reactivity.In addition,the nano size of nZVI is also a challenge for recycling.In this paper,nano Fe₃O₄ was used to prepare Fe₃O₄-Pd/Fe nanocomposites in order to improve the reactivity of Pd/Fe and make it easier to recycle,and on the other hand,sodium citrate?Cit?was added into the solution continually to prevent the formation of passivation layer.The effects of experimental parameters on the dechlorination of 2,4-dichlorophenoxyacetic?2,4-D?such as initial pH,palladium loading and temperature were investigated.Transmission electron microscope?TEM?,Scanning electron microscoped?SEM?,X-ray photoelectron spectroscopy?XPS?and X-ray diffraction?XRD?were employed to explore the improvement mechanism of Cit and nano Fe₃O₄.And High performance liquid chromatography?HPLC?was applied to determine and analyze the removal process of 2,4-D.Results can be summarized as follows:?1?Fe₃O₄-Pd/Fe nanocomposites and Cit modified nano Pd/Fe can markedly promote the dechlorination of 2,4-D.In the reaction,2,4-D may be firstly reduced to intermediates 2-chlorophenoxyacetic acid?2-CPA?,and then dechlorinated to the end product phenoxyacetic acid?PA?.And the dechlorination of 2,4-D was a pseudo first order reaction.?2?Compared to Pd/Fe nanoparticles,Fe₃O₄-Pd/Fe nanocomposites showed a much higher efficiency on the dechlorination of 2,4-D.Fe₃O₄ acted as a medium to promote the electron transfer between nZVI and 2,4-D.It can also attract magnetic substances in the system and make the passivation layer much thinner.After three times recycling,Fe₃O₄-Pd/Fe nanocomposites maintained a high reactivity.The removal rate reached 100.0%,83%and 60.3%in the three reactions,respectively.?3?The higher Pd loading,temperature,mass ratio of Fe₃O₄ to Fe,lower stirring speed,initial concentration of 2,4-D and the near neutral pH favored the removal of 2,4-D.The appropriate experimental parameters for the dechlorination were:C_Fee 1.0 g.L^-1,Fe₃O₄:Fe 1:1,pH 7.00,temperature 25.0?,Pd loading 0.150%,string speed 200 r.min^-11 and the initial concentration of 2,4-D 40.0 mg.L^-1.?4?There was a complexation between the Cit added into the solution continually and the Fe²⁺generated by nZVI in the reaction.Thus the formation of passivation layer would be prevented,and the reactivity of Pd/Fe nanoparticles would be maintained at a high level.The higher Pd loading,temperature,the lower pH were conducive to the dechlorination of 2,4-D by Cit improved Pd/Fe nanoparticles.Under the suitable condition:C_Fee 1.0 g.L^-1,C_Citit 50.0 mmol.L^-1,pH 6.00,temperature 25.0?,Pd loading 0.300%and stirring speed 200 r.min^-1,20.0 mg.L^-11 2,4-D could be converted completely to phenoxyacetic acid?PA?after 210 min.According to Arrehenius Equation,the apparent activation energy 57.82 kJ.mol^-11 and pre-exponential factor 1.10×10⁸ min^-11 were obtained in the dechlorination of 2,4-D by Cit modified Pd/Fe nanoparticles.