| Polyvinylidene fluoride(PVDF)is the most widely used membrane material due to its excellent chemical stability,outstanding mechanical strength and good film forming properties.However,due to its hydrophobic property,PVDF membrane has a poor wettability of aqueous solution when it is used as a support layer for interfacial polymerization,which is not conducive to the preparation of high performance polyamide layer on its surface.In recent years,Metal Organic Frameworks(MOFs)is widely used in membrane modification researches.This is mainly attributed to its advantages of high porosity,good adsorption affinity and good compatibility with polymers.Therefore,the application of MOFs to modify PVDF membrane makes it possible to prepare high performance polyamide layer by interfacial polymerization on its surface.With the PVDF flat membrane as the base membrane,a uniform and dense zeolitic imidazole framework-8(ZIF-8)sub-layer was synthesized in-situ on the surface of PVDF membrane via a contra-diffusion method.The optimal conditions for the preparation of ZIF-8/PVDF composite membranes was further studied by changing the solvents of the two monomer solutions used to synthesize ZIF-8 and the towards of the PVDF membrane.Then the high performance composite nanofiltration(NF)membrane was prepared by interfacial polymerization(IP)process.Scanning electron microscope(SEM),X-ray diffraction(XRD)patterns,Fourier transform infrared spectroscopy(FTIR)and atomic force microscopy(AFM)were applied to characterize the chemical compositions,structures and morphologies of the prepared ZIF-8/PVDF composite membranes and the composite NF membranes.The effects of different conditions on the synthesis of ZIF-8 as well as the influence of ZIF-8sub-layer on the structure and properties of polyamide layer was further investigated.Results showed that when the solvent of Zn(NO3)2·6H2O was water,the solvent of Hmim was methanol,and the membrane was facing the methanol solution of Hmim,the ZIF-8 sub-layer growing in situ on the PVDF membrane surface was more uniform and dense.The existence of the ZIF-8 sub-layer improved the interfacial compatibility between the polyamide layer and the PVDF membrane,and improved the desalination property of the polyamide layer.Under the pressure of 0.6 MPa,the PA/m4 composite NF membrane exhibited a high rejections for four kinds of salt(MgSO4of 89.84%,Na2SO4of 86.11%,NaCl of 83.07%and MgCl2of 80.01%).On the basis of the above investigation,the concentration of two monomer solutions of ZIF-8 was further optimized,a more uniform and dense sub-layer of ZIF-8 nanocrystalline with smaller particle size was synthesized on the PVDF membrane via an in-situ contra-diffusion method,and the IP process was further optimized to successfully prepare high performance PA-1 and PA-2 composite NF membranes.SEM,XRD patterns,FTIR,AFM,water contact angle,X-ray photoelectron spectroscopy(XPS)and solid surface zeta potential were applied to measure the chemical compositions,structures and morphologies of the prepared ZIF-8/PVDF composite membranes and the composite NF membranes.The effect of the ZIF-8 sub-layer on the IP process,structure and performance of the composite NF membranes were investigated systematically.Results indicated that the existence of ZIF-8 sub-layer not only improved the compatibility of polyamide layer and PVDF support membrane,but also increased the crosslinking degree of the polyamide.Under the pressure of 0.6 MPa,the composite NF membrane exhibited a high flux of24.05 L m-2h-1,together with high rejections for four kinds of salt(MgSO4of 97.34%,Na2SO4of 93.57%,NaCl of 89.31%and MgCl2of 85.16%)and excellent anti-fouling property. |
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