| In recent years,with the diversified development of industry and irrational human activities,heavy metals such as mercury,lead,cadmium and chromium are arbitrarily discharged from agriculture,metallurgy and batteries,which makes environmental problems more serious.Since these heavy metals are not biodegradable,they are easy to transfer and accumulate in crops,fish and other substances,and eventually reach the human body through the food chain.When the concentration of heavy metals exceeds a certain threshold,it will cause serious damage to the human body,including respiratory,digestive,nervous system,etc.In severe cases,it may endanger life.For example,prolonged exposure to mercury can lead to dizziness,fatigue,anorexia and weakness;Excessive lead can affect the composition of digestive enzymes in the human gut and reduce the amount of various protein enzymes such as transferrin in the liver.In view of the serious harm of heavy metal pollution,many international environmental protection organizations have made strict regulations on the content of heavy metals in order to avoid irreversible negative effects.Metal-organic skeletons(MOFs)have been shown to be excellent adsorbent materials with excellent heavy metal removal properties in wastewater treatment.Compared with traditional adsorbent materials,MOFs stand out among adsorbents due to their advantages of large surface area,high and adjustable porosity,open metal site,adjustable structural framework and a large number of portable functional groups.However,the consumption,depletion,or blockage of MOFs in reactor pipelines and the long and complex recovery process severely limit their practical application.Therefore,the construction of new MOFs composite materials with high usability has become a research hotspot of two-dimensional(2D)MOFs fiber membranes.Based on this,we designed and developed three kinds of metal-organic frame composite membranes for adsorption and removal of heavy metals in water.The design idea is as follows:1.Carboxylated Zr-MOFs with high specific surface area and porous structure were prepared by using ligands(1,2,4,5-p Henyl-tetracarboxylic acid)containing rich carboxyl functional groups and zirconium tetracarboxylic acid by condensation reflux method,and then Zr-MOFs were spun into nanofiber membranes by electrospinning method.One-dimensional PAN/MOFs nanofiber membranes with large surface area were prepared successfully.The adsorption performance of Pb(II)was compared between Zr-MOFs powder and PAN/MOFs nanofiber membrane.The adsorption experiment showed that the maximum adsorption capacity of lead from PAN/MOFs ENMs was 427.35 mg/g,and the adsorption process was chemical monomolecular layer adsorption.The study of FT-IR and XPS shows that the adsorption mechanism is due to the coordination between Pb and carboxyl group in adsorbed materials.2.Since electrostatic co-spinning would limit the spinning content of MOFs,thus affecting the adsorption performance,Zr-MOFs were in-situ grown on the composite fiber membrane of polyacrylonitrile and chitosan(PAN/CS)by aminosylated ligand(aminosylated terep Hthalic acid)and zirconium tetrachloride,and the crystal shape of MOFs was adjusted by trifluoroacetic acid.PAN/CS/Ui O-66(PCU)ENMs were successfully prepared.The results showed that PCU ENMs still had effective adsorption properties for Cr(VI)even at extremely high ionic strength.At 298 K,the maximum adsorption capacity of PPU-2 ENMs reached 571.4 mg/g,respectively.The adsorption process is chemical monolayer adsorption.In addition,the selective adsorption performance of Cr(VI)by PPU-2 ENMs is good.FT-IR and XPS analysis showed that the adsorption of Cr(VI)was due to the reduction and adsorption of electron-rich nitrogen groups.3.Since MOFs grow in situ on the membrane,it is easy to cause uneven growth or partial agglomeration of MOFs.Therefore,ligands(1,3,5-homop Henic acid)were spun into polyacrylonitrile(PAN)and montautonite(MMT)nanofibers in advance to provide seeds for the growth of MOFs.Then Fe-MOFs were grown in situ on the surface of the fibers,and PMM mixed matrix films were prepared successfully.On this basis,the adsorption capacity of Pb was investigated.A series of experimental results show that the adsorption capacity of the metal-organic framework compound for Pb(II)reaches674.1 mg/g,and its adsorption behavior on Pb(II)satisfies the quasi-second-order kinetics and Langmuir isothermal adsorption model,showing a spontaneous endothermic property of chemical monolayer.FT-IR and EDS analysis showed that the synergistic interaction between the carboxyl group on PMM MMMs and Si-O-Si’π-π’ in montauto soil improved the adsorption performance of Pb(Ⅱ). |
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