Study On Structure Regulation And Nanofiltration Performance Of Two-dimensional Layered Membrane Channels

Nanofiltration separation membranes are widely used in the field of water treatment due to their advantages of high separation accuracy,convenient operation,and no phase change.The pore size of nanofiltration membrane is about 0.5-2 nm,which can be used to remove small molecular weight organics and high-value salts,so it can be used for softening of drinking water,further treatment of sewage,recycling and treatment of dye waste liquid.Because the separation layer of the traditional polymer nanofiltration membrane is composed of small molecular aromatic rings,the length of the chain segment between the cross-linking points is short,and the formed filtration pore size is small,resulting in high operating pressure and low water flux in the membrane.In recent years,new 2D materials have emerged continuously,and their functional properties have received extensive attention from the successful exfoliation of graphene to the design and synthesis of framework materials.Two-dimensional materials have high diameter-to-thickness ratios,which are beneficial to assemble to form ordered interlayer or in-plane channel structures,and this feature provides the possibility to fabricate high-performance nanofiltration membranes.In this paper,aiming at the major challenges in the development of nanofiltration membrane materials in aqueous systems,that is,the development of ion-layered two-dimensional nanofiltration membranes with high performance and high water flux at low pressure,various approaches are proposed to construct ordered ion channels in the membrane.Constructing vertical mass transfer channels via porous,negatively charged 2D metal-organic frameworks（MOFs）to achieve excellent water flux.Based on the covalent doping of 2D MOF nanosheets and non-porous,negatively charged graphene oxide（GO）to construct interlayer ion transport channels,GO can change the microstructure of the separation membrane and endow the composite membrane with additional selective transport channels.Based on the doping of porous covalent organic frameworks（COFs）and GO,and utilizing the modularity of COFs,the precise construction of in-plane ion channels is realized.The effects of different two-dimensional materials and assembly methods on the structural properties of the channel were explored and compared,and a coordinated regulation method of the chemical microenvironment of the channel in the membrane was established.Based on the precise regulation of ion channels,the structure-activity relationship between channel structure and mass transfer performance was revealed,in order to provide a theoretical basis for the development of high-performance nanofiltration membranes.In addition,the interception behavior of a series of layered two-dimensional nanofiltration membranes for dye molecules with different surface properties and anions of different valences was discussed.The main research results are as follows:（1）Ultrathin two-dimensional Zn（BDC）nanosheets were successfully synthesized by two different synthesis strategies,"top-down"and"bottom-up".The morphology of nanosheets was studied by characterization methods such as TEM and AFM,and Zn（BDC）layered films were prepared by a simple vacuum filtration method.For layered 2D separation membranes,the interaction and ordered stacking of nanosheets are crucial to improve the separation performance.In this paper,the precise control of the thickness of the 2D MOF layered film is achieved by changing the suspension concentration of nanosheets.By introducing a cross-linking agent,the nanosheet layered film is more stable,which is mainly due to the large number of hydroxyl groups in the long chain of polyvinyl alcohol（PVA）,which can generate hydrogen bonds with the unreacted carboxyl groups around the Zn（BDC）nanosheets.Adjusting the amount of PVA added can effectively control the pore size and specific surface area of the membrane,thereby achieving excellent dye/ion removal performance,enabling it to achieve ultra-high water flux and have good retention performance.When the colloidal concentration of 2D Zn（BDC）nanosheets was 1 mg m L^-1and the concentration of PVA was 2 mg m L^-1,the as-prepared layered Zn（BDC）nanofiltration membrane could achieve～99.9%rejection of direct red 23（DR23）and～99.8%retention of reactive black 5（RB5）dye,with permeation fluxes of335.0 L m^-2h^-1bar^-1and 362.5 L m^-2h^-1bar^-1.Compared with traditional commercial nanofiltration membranes,the flux is increased by an order of magnitude,and it has excellent long-term stability and anti-fouling performance.（2）Aminated 2D Zn（BDC）nanosheets were directly synthesized using an improved three-layer synthesis method,using rigid metal-organic framework nanosheets（Zn（BDC）-NH₂）as the support between the GO sheets,via The layered Zn（BDC）-NH₂/GO composite nanofiltration membrane was prepared by pressure-driven self-assembly method.When the concentrations of GO and Zn（BDC）-NH₂are both 1 mg m L^-1,while ensuring the high rejection rate of SO₄^2-（～90.0%）,the rejection rate of iodide ions is only 10.6%,indicating that the composite membrane is effective for a Divalent anions are selectively separable.Therefore,it is believed that the electrostatic interaction on the surface of the membrane and in the pores plays an important role in the removal of dianions.Under the conditions of different initial concentrations of filtrate and different p H,the composite membrane can still maintain good separation performance.Zn（BDC）-NH₂was inserted into the GO layer,and the water flux was as high as 331.1 L m^-2h^-1bar^-1,which was much higher than that of the pure GO membrane.The results of FT-IR and XPS analysis showed that GO-COOH interacted with-NH₂in Zn（BDC）-NH₂to form hydrogen bonds and amide bonds.The presence of hydrogen bonds and amide bonds makes Zn（BDC）-NH₂uniformly dispersed in the film,providing additional molecular transport channels.In addition,the novel Zn（BDC）-NH₂/GO composite nanofiltration membrane has good anti-fouling properties and excellent long-term stability,and it can maintain high-efficiency separation capability throughout continuous operation for up to 7 days.（3）Using a"top-down"synthesis method,ultrathin 2D TAPB-DVA-COF nanosheets were obtained by ultrasonic exfoliation in dichloromethane.A series of 2D TAPB-DVA-COF/GO/PAN composite nanofiltration membranes were prepared by pressure-assisted self-assembly method using 2D TAPB-DVA-COF nanosheets as graphene oxide（GO）interlayer nano-supports.The 2D TAPB-DVA-COF nanosheets were modified with allyltrimethylammonium chloride,and the electrostatic force between the nanosheets and GO was enhanced by introducing cationic groups,in order to improve the stability of the composite membrane.The effects of 2D TAPB-DVA-COF/GO concentration and composite ratio on nanofiltration performance and flux were investigated subsequently.Under the low pressure operation of 2.5 bar,when the concentration of 2D TAPB-DVA-COF is 0.25 mg m L^-1and the concentration of GO is 0.33mg m L^-1,the composite nanofiltration membrane can achieve a rejection rate of 93.6%for SO₄^2-,and for I^-rate is only 14.5%.The factors influencing the performance of the 2D TAPB-DVA-COF/GO/PAN composite membrane were further analyzed,and the mechanism of the enhanced separation performance was expounded.