Research On The Structure And Properties Of Positively Charged Composite Nanofiltration Membrane Based On Modified Polyvinylidene Fluoride

Polyvinylidene fluoride（PVDF）polymer membrane material has excellent physical and chemical properties and good filtration performance,so it is widely used in the field of water treatment.Due to the low surface energy and strong hydrophobicity of PVDF membrane,the membrane is easy to be polluted,resulting in reduced service life and increased operating costs.Researchers have carried out surface modification of PVDF membrane by various methods,and surface coating modification has attracted much attention due to its advantages of simple operation and remarkable effect.The separation performance of nanofiltration（NF）membrane is between ultrafiltration and reverse osmosis,which can effectively realize the separation of high and low molecular weight substances and high and low univalent ions.Composite NF membrane is usually composed of substrate and nanofiltration functional layer,the separation performance of composite NF membrane is determined by the two parts.The surface modified PVDF membrane is used as the nanofiltration substrate,and the performance of the NF membrane is improved through the construction and regulation of the surface functional separation layer,the composite NF membrane with high ion separation efficiency will be obtained.In this paper,PVDF was used as the substrate,which was cross-linked by polyvinyl alcohol（PVA）and glutaraldehyde（GA）to coat the surface of the substrate.Then,the modified PVDF membrane was used as the support layer,and the interfacial polymerization of water-phase monomer polyethyleneimine（PEI）and oil-phase monomer terephthaloyl chloride（TPC）was carried out on the surface of the support layer to form a positively charged functional separation layer.The results are as follows:（1）Firstly,a series of surface modified PVDF composite membranes were prepared by the cross-linking reaction of PVA and GA with PVDF prepared by phase inversion method.The effects of PVA concentration,cross-linking reaction time and reaction temperature on the structure and performance of the membranes were studied.The results of FTIR,SEM,EDS and AFM show that the composite film is formed by the acetalization of PVA and GA on the surface of PVDF.The surface of PVDF film is covered by cross-linked products,and the surface roughness decreases significantly.After cross-linked coating,the surface hydrophilicity of the film is significantly enhanced,and the surface water contact angle of the membrane decreases from 91°to 12°.The cross-linking products make the surface and internal structure of the membrane densified,and the BSA rejection rate of the membrane increases from 68.4%to 92.4%of the substrate.The results show that the modified membrane with good surface hydrophilicity and retention performance.The optimal preparation conditions of cross-linking modification are determined as follows:PVA concentration is 2.0 wt.%;cross-linking reaction time is 10 min;reaction temperature is 55℃.（2）The PVDF modified composite membrane with the best surface hydrophilicity and retention performance was used as the support layer,PEI was used as the water phase monomer to carry out interfacial polymerization with oil phase monomer TPC on the surface of PVDF modified support layer.The effects of molecular weight of PEI in water phase,concentration of PEI in water phase,concentration of TPC in oil phase and interfacial polymerization time on the structure and properties of NF membrane were studied.The composite films were characterized by FTIR,EDS,SEM and AFM,the results show that polyamide is obtained by interfacial polymerization of PEI and TPC;there is a thin and dense separation layer of functional layer on the surface of PVDF modified support layer.Zeta potential was used to test the charge of the composite membrane,under the optimal preparation conditions,the prepared NF membrane has an isoelectric point of8.4 and shows positive charge at p H=7.Nanofiltration device was used to test the filtration performance of the membrane,the results show that the molecular weight of NF membrane and the rejection rate of Mg Cl₂ increase with the increase of PEI molecular weight;with the increase of PEI concentration,the water flux of NF membrane decreases and the rejection rate of Mg Cl₂increases;with the increase of TPC concentration,the rejection rate of NF membrane increases and the water flux decreases;with the extension of interfacial polymerization time,the permeation flux of NF membrane decreases first and then decreases.The retention of Mg Cl₂ increases slowly,then tend to be constant,and then decreased slightly.The results show that the optimal preparation conditions are as follows:molecular weight of PEI is 70000,monomer concentration of PEI-70000is 0.8 wt%,monomer concentration of TPC oil phase is 0.10 wt%,interfacial polymerization time is 180 s;the molecular weight cut-off of NF membrane is about 674 Da.The rejection order of NF membrane for inorganic salts is:Mg Cl₂（92.7%）>Mg SO₄（80.2%）>Na₂SO₄（76.3%）>Na Cl（39.2%）.The positively charged NF membrane has a good retention effect on Mg²⁺and a broad application prospect in the field of ion separation and purification.