An anti-fouling nanofiltration membrane based on a molecular stitching-interface polymerization strategy and a preparation method and application thereof

Abstract

The present application relates to a kind of anti-pollution nanofiltration membrane based on molecular stitching-interface polymerization strategy and its preparation method and application, the anti-pollution nanofiltration membrane includes porous support membrane base and active layer on it, and the active layer is two-dimensional metal organic framework-polyamide crosslinking composite structure, wherein two-dimensional metal organic framework nanosheet is stacked to form main structure, and polyamide fragment is distributed in the interlayer space of two-dimensional metal organic framework nanosheet stack formation.The preparation process is as follows: pre-deposited aqueous monomer on the surface of porous support membrane base;Two-dimensional metal organic framework nanosheet is introduced and self-assembled;Aqueous monomer spontaneously diffuses in interlayer space;Organic phase monomer is introduced and interfacial polymerization is carried out with aqueous monomer, and polyamide is generated, to obtain anti-pollution nanofiltration membrane.Compared with prior art, the present application has thinner active layer, and has high flux, high retention rate, excellent anti-pollution performance, and is suitable for drinking water purification, wastewater reuse and seawater desalination and other water environment.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of membrane separation technology, and in particular to an anti-fouling nanofiltration membrane based on a molecular stitching-interface polymerization strategy and a preparation method and application thereof. BACKGROUND

[0002] Nanofiltration membrane is a new type of separation membrane material between ultrafiltration membrane and reverse osmosis membrane, with a pore size of 0.5-2 nm, moderate molecular weight cut-off and high surface charge density, which can effectively remove organic pollutants, bacteria, viruses, heavy metal ions and emerging pollutants (such as perfluorinated compounds) in water. With the advantages of low energy consumption, high selectivity and simple process, nanofiltration membrane has been widely used in drinking water purification, wastewater reuse and seawater desalination, etc., and has important environmental and economic value.

[0003] However, the existing polyamide thin-film composite membrane (TFC) generally has a "flux-selectivity trade-off" problem: thickening the active layer is beneficial to improve the rejection rate, but it will significantly reduce the water flux; on the contrary, if high flux is pursued, the rejection performance will be easily sacrificed. At the same time, due to the uneven diffusion of monomers in the interface polymerization process, the surface of the TFC membrane is often rough, which is easy to form "pollutant traps", leading to the accumulation of organic pollutants and microorganisms on the membrane surface, and thus causing serious membrane fouling problem, shortening the service life of the membrane.

[0004] To improve the above problems, researchers have proposed the introduction of nanomaterials in the polyamide active layer to develop thin-film nanocomposite membrane (TFN). The introduction of nanomaterials improves the flux and selectivity of the membrane to some extent, making its separation performance better than that of traditional TFC membrane. However, the accumulation of nanomaterials on the membrane surface or interface often leads to an increase in surface roughness, limited improvement in hydrophilicity, and even exacerbation of organic and biological pollution. In addition, some nanomaterials have the risk of leaching or insufficient stability, which increases the risk of secondary pollution and seriously limits their popularization and application in actual water treatment.

[0005] On the other hand, there are many anti-pollution modification methods, such as hydrophilic polymer coating on the membrane surface, surface grafting of hydrophilic functional groups or introduction of antibacterial coating, etc. For example, patent CN118341269A provides a preparation method of MOF anti-pollution interlayer composite nanofiltration membrane for biological small molecule purification by sequentially loading MOF-74 modification layer and polyamide (PA) separation layer on porous polyacrylonitrile (PAN) base membrane. Although this method can improve the hydrophilicity and anti-pollution performance of the membrane surface to some extent in the short term, it still has the following shortcomings: the MOF-74 modification layer is not firmly combined with the polyamide matrix, has poor stability and is easy to fall off during long-term operation or cleaning; secondly, the increase in the thickness of the active layer will hinder the transmission of water molecules, resulting in a significant decrease in water flux (water flux is only 10.8 LMH / bar when the thickness of the PA layer is 64 nm). In addition, the release of metal ions by the MOF-74 modification layer enhances the anti-pollution property of the membrane, but this will cause more serious water pollution problems.

[0006] Therefore, the existing modification methods are more "additional" surface modification, and cannot fundamentally solve the contradiction between flux, retention and anti-pollution performance from the perspective of membrane formation mechanism. There is an urgent need for a new membrane preparation strategy that can accurately regulate the diffusion and reaction behavior of monomers during the formation of the active layer, thereby optimizing the design of the membrane structure and achieving the unification of high flux, high retention rate and strong anti-pollution ability, providing protection for the long-term stable application of nanofiltration membranes in actual water treatment. SUMMARY

[0007] The purpose of the present application is to provide an anti-pollution nanofiltration membrane based on a molecular stitching-interface polymerization strategy and its preparation method and application, which has high flux, high retention rate and strong anti-pollution ability.

[0008] The purpose of the present application can be achieved by the following technical solutions:

[0009] One of the purposes of the present application is to provide an anti-pollution nanofiltration membrane based on a molecular stitching-interface polymerization strategy, comprising a porous support membrane substrate, one side surface of the porous support membrane substrate being provided with an active layer, the active layer being a two-dimensional metal organic framework-polyamide crosslinked composite structure, the composite structure comprising two-dimensional metal organic framework nanosheets and polyamide segments, wherein the two-dimensional metal organic framework nanosheets are stacked to form a main structure, and the polyamide segments are distributed in the interlayer voids formed by the stacking of the two-dimensional metal organic framework nanosheets.

[0010] Preferably, the two-dimensional metal organic framework nanosheets comprise two-dimensional Cu-TCPP nanosheets.

[0011] Preferably, the thickness of the two-dimensional Cu-TCPP nanosheets is 4-5 nm.

[0012] Preferably, the lateral size of the two-dimensional Cu-TCPP nanosheet is 0.5-2.0 μm.

[0013] Preferably, the thickness of the active layer is 32.3-46.7 nm, and the average pore size of the active layer is 0.30-0.33 nm.

[0014] Preferably, the thickness of the porous support membrane substrate is 0.06-0.10 cm, and the average pore size is 0.22-0.45 μm.

[0015] Preferably, the porous support membrane substrate comprises a polyether sulfone ultrafiltration membrane.

[0016] Further preferably, the diameter of the porous support membrane substrate is 45-55 mm, and more preferably 50 mm.

[0017] The second object of the present application is to provide a preparation method of the anti-fouling nanofiltration membrane based on the molecular stitching-interface polymerization strategy, comprising the following steps:

[0018] S1, pretreatment: immersing the porous support membrane substrate in a polyvinyl alcohol solution and removing the water on the surface of the porous support membrane substrate by negative pressure suction filtration;

[0019] S2, water phase monomer pre-deposition: depositing water phase monomers on one side of the porous support membrane substrate pretreated in step S1 by negative pressure suction filtration;

[0020] S3, self-assembly of two-dimensional metal-organic framework nanosheet: self-assembling two-dimensional metal-organic framework nanosheets on the surface of the porous support membrane substrate with pre-deposited water phase monomers obtained in step S2 by negative pressure suction filtration, so that the two-dimensional metal-organic framework nanosheets are stacked to form a two-dimensional metal-organic framework body structure;

[0021] S4, water phase monomer diffusion: allowing the water phase monomers pre-deposited in step S2 to spontaneously diffuse into the interlayer space formed by the stacking of the two-dimensional metal-organic framework nanosheets in step S3 under normal pressure;

[0022] S5, interface polymerization: introducing organic phase monomers to the surface of the two-dimensional metal-organic framework body structure treated in step S4, so that the interface polymerization reaction occurs between the organic phase monomers and the water phase monomers diffused into the interlayer space, generating polyamide fragments, and obtaining the anti-fouling nanofiltration membrane.

[0023] Preferably, in step S1, the concentration of the polyvinyl alcohol solution is 0.001-0.005 wt%, the molecular weight of the polyvinyl alcohol therein is 40000-60000, and the immersion time is 1-3 min.

[0024] Preferably, the specific step of step S2 is: pouring the aqueous monomer solution on one side surface of the porous support membrane substrate pretreated in step S1, and then standing for a while, and then removing the water on the surface of the porous support membrane substrate by suction filtration under negative pressure, to complete the pre-deposition of the aqueous monomer.

[0025] Further preferably, in step S2, the concentration of the aqueous monomer solution is 1.5-2.5 wt%.

[0026] More preferably, in step S2, the concentration of the aqueous monomer solution is 2 wt%.

[0027] Further preferably, in step S2, the standing time is 30-120 s.

[0028] Further preferably, in step S2, the aqueous monomer is selected from any one of piperazine and m-phenylenediamine.

[0029] More preferably, in step S2, the aqueous monomer is piperazine.

[0030] Further preferably, in step S2, the solvent of the aqueous monomer is water.

[0031] Further preferably, in step S2, the volume of the poured aqueous monomer solution is 2-4 mL.

[0032] Preferably, the specific step of step S3 is: pouring the two-dimensional metal organic framework nanosheet solution on the surface of the porous support membrane substrate pre-deposited with the aqueous monomer in step S2, and then standing for a while, and then removing the water under negative pressure by suction filtration, to realize the self-assembly of the two-dimensional metal organic framework nanosheets.

[0033] Further preferably, in step S3, the concentration of the two-dimensional metal organic framework nanosheet solution is 0.05-0.15 mg / L.

[0034] Further preferably, in step S3, the two-dimensional metal organic framework nanosheet solution is a two-dimensional Cu-TCPP nanosheet aqueous solution.

[0035] Preferably, in step S3, the preparation method of the two-dimensional Cu-TCPP nanosheet is as follows: adding copper nitrate trihydrate, methanol and polyvinylpyrrolidone into a mixed solution of dimethylformamide (DMF) and ethanol to prepare a Cu 2+ ligand solution; adding tetrakis (4-carboxyphenyl) porphyrin (TCPP) into the mixed solution of DMF-ethanol, stirring and dissolving, and then adding the Cu 2+The ligand solution is prepared, and then a hydrothermal reaction is performed. After the reaction is completed, the reaction product is collected, and the obtained solid is washed with methanol. The washed solid is dissolved in DMF again, and then is peeled off by an ultrasonic cell disruption technique to obtain the two-dimensional Cu-TCPP nanosheet.

[0036] Further preferably, in step S3, the two-dimensional Cu-TCPP nanosheet is prepared by the following method:

[0037] Cu is prepared by adding 24 mg of copper nitrate trihydrate, 40 μL of methanol, and 50 mg of polyvinylpyrrolidone into 120 mL of a mixed solution of dimethylformamide and ethanol (3:1 by volume). 2+ The ligand solution is prepared. Then, 44 mg of tetrakis (4-carboxyphenyl) porphyrin (TCPP) is added into 40 mL of a mixed solution of DMF-ethanol (3:1 by volume), and Cu is added dropwise after stirring for 0.5 h. 2+ The ligand solution is prepared. The mixed solution is transferred into a hydrothermal reactor, and a reaction is performed at 80 ℃ for 12 h. The obtained solid is washed with methanol three times, is dissolved in DMF again, and is peeled off by an ultrasonic cell disruption technique to obtain the nanosheet.

[0038] Further preferably, in step S3, the standing time is 30-120 s.

[0039] Further preferably, in step S3, the volume of the two-dimensional metal-organic framework nanosheet solution poured is 2-4 mL.

[0040] Preferably, in step S4, the time for the aqueous monomer diffusion is 30-120 s.

[0041] More preferably, in step S4, the time for the aqueous monomer diffusion is 60 s.

[0042] Further preferably, in step S4, the aqueous monomer diffusion refers to the diffusion of the aqueous monomer upward in the interlayer space of the stacked two-dimensional metal-organic framework nanosheet.

[0043] Preferably, in step S5, the specific steps are as follows: the organic phase monomer solution is poured onto the surface of the two-dimensional metal-organic framework nanosheet main body structure treated in step S4, and an interfacial polymerization reaction is performed. After the reaction is completed, the excess organic phase monomer solution is poured off, and then is baked and solidified to obtain the anti-pollution nanofiltration membrane.

[0044] Further preferably, in step S5, the mass concentration of the organic phase monomer solution is 0.08-0.12 wt%.

[0045] More preferably, in step S5, the mass concentration of the organic phase monomer solution is 0.1 wt%.

[0046] Further preferably, in step S5, the organic phase monomer comprises trimesoyl chloride.

[0047] Further preferably, in step S5, the solvent of the organic phase monomer solution is selected from any one of n-hexane, cyclohexane.

[0048] More preferably, in step S5, the solvent of the organic phase monomer solution is n-hexane.

[0049] Further preferably, in step S5, the interfacial polymerization reaction is performed for 40-80 s at a temperature of 50-70℃.

[0050] More preferably, in step S5, the interfacial polymerization reaction is performed for 60 s at a temperature of 60℃.

[0051] Further preferably, in step S5, the drying temperature is 50-70℃, and the drying time is 4-6 min.

[0052] More preferably, in step S5, the drying temperature is 60℃, and the drying time is 5 min.

[0053] Further preferably, in step S5, the volume of the poured organic phase monomer solution is 2-4 mL.

[0054] The third object of the present application is to provide an application of the anti-fouling nanofiltration membrane based on the molecular stitching-interfacial polymerization strategy in the field of water purification treatment.

[0055] Preferably, the anti-fouling nanofiltration membrane can be used to prepare a water treatment device.

[0056] Preferably, the anti-fouling nanofiltration membrane can be used to purify raw water to remove organic pollutants, bacteria and trace emerging pollutants.

[0057] Preferably, the preparation method of the anti-fouling nanofiltration membrane based on the molecular stitching-interfacial polymerization strategy comprises the following steps:

[0058] using a porous support membrane as a substrate;

[0059] immersing the substrate in a polyvinyl alcohol solution to enhance the adhesion between the substrate and the two-dimensional metal-organic framework nanosheet;

[0060] The water phase monomer solution is deposited on the membrane surface by suction filtration, and then the two-dimensional metal organic framework nanosheet solution is poured into the two-dimensional metal organic framework nanosheet main structure formed by suction filtration, and then the organic phase monomer solution is poured into the interface polymerization reaction, and the temperature is changed to obtain the anti-pollution nanofiltration membrane based on the molecular stitching-interfacial polymerization strategy.

[0061] The application provides an anti-pollution nanofiltration membrane based on a molecular stitching-interfacial polymerization (MS-IP) strategy and a preparation method and application thereof. The method is that a water phase monomer such as a piperazine monomer is pre-deposited on the surface of a porous support membrane substrate, and then two-dimensional metal organic framework (MOF) nanosheets such as two-dimensional Cu-TCPP metal organic nanosheets are introduced for self-assembly, and the nanosheets are stacked to form a two-dimensional sheet structure. The interlayer gap formed by the two-dimensional MOF nanosheets becomes a diffusion channel of the water phase monomer piperazine, and after the water phase monomer diffuses into the interlayer gap, the water phase monomer is subjected to an interfacial polymerization reaction with an organic phase monomer (such as trimesoyl chloride) introduced on the surface of the two-dimensional sheet layer, to generate polyamide in the form of a sheet segment. The polyamide segments are distributed in the interlayer gap of the two-dimensional sheet layer, effectively inhibiting the excessive polymerization and surface accumulation of the polyamide, and making the two-dimensional MOF sheet layer constitute the main structure of the nanofiltration membrane instead of the polyamide. At the same time, the segmented and distributed polyamide can effectively make up for the defects that may be generated in the self-assembly process of the two-dimensional sheet layer, such as interlayer collapse, irregular stacking and inter-sheet gap. Finally, the prepared anti-pollution nanofiltration membrane presents a unique structure of interweaving of the metal organic framework and the polyamide, the surface is smoother, more hydrophilic and has a stronger negative charge, and the exposed metal organic framework endows the nanofiltration membrane with excellent antibacterial performance, which can greatly improve the service life in actual drinking water treatment engineering, and has a wide application prospect.

[0062] The application can accurately regulate the diffusion and reaction behavior of monomers during the formation of the active layer by adopting the MS-IP strategy, so as to realize the optimal design of the membrane structure, achieve the unification of high flux, high rejection rate and strong anti-pollution ability, and provide a strong guarantee for the long-term stable operation of the nanofiltration membrane in actual water treatment. In the application, the MS-IP strategy refers to the process that two kinds of reaction monomers (water phase and organic phase) diffuse to the interlayer gap (which is the transmission channel of water and solute) formed by the stacking of two-dimensional MOF nanosheets from different directions (organic phase: up-down, water phase: down-up), and an interfacial polymerization reaction occurs in the interlayer gap, and finally a non-continuous polyamide segment is formed between the nanosheet layers. The advantages of the MS-IP strategy used in the application mainly come from the unique MOF-polyamide cross-linked membrane surface structure, which is different from the conventional nanofiltration membrane which is mainly designed with polyamide as the surface and the main body. The membrane surface can be regarded as a mosaic structure composed of hydrophobic regions (MOF) and hydrophilic regions (polyamide), and the membrane surface with such a structure will produce a heterogeneous wetting effect, so that the overall membrane surface will be more hydrophilic, which is beneficial to the passage of water molecules to improve water flux, and can reduce the adsorption force with pollutants and enhance the anti-organic pollution ability. At the same time, the MOF itself has a large number of negative functional groups, which further strengthens the negative electric property of the membrane surface, which is beneficial to repelling negative pollutants and further improving the anti-pollution performance. In addition, the polyamide segments formed between the nanosheet layers can "sew" the two-dimensional MOF nanosheets together like "molecular threads", which significantly enhances the stability of the membrane structure, effectively prevents the nanosheets from falling off or dispersing into the water body during use, and avoids causing water pollution.

[0063] Compared with the prior art, the application has the following beneficial effects:

[0064] (1) The application provides an anti-pollution nanofiltration membrane based on a molecular sewing-interfacial polymerization strategy, a preparation method and application thereof. The anti-pollution nanofiltration membrane is composed of a porous support membrane substrate and an active layer, and the active layer is a thin layer structure composed of two-dimensional metal organic framework nanosheets and polyamide cross-linking composite. The specific structure is that the polyamide is embedded in the interlayer gap formed by the stacking of two-dimensional MOF nanosheets in the form of a discontinuous segment. The structure design makes the nanofiltration membrane have high flux, high rejection rate and excellent anti-pollution performance, effectively breaking through the technical bottleneck that the traditional nanofiltration membrane is difficult to balance between multiple performances.

[0065] (2) The molecular stitching-interfacial polymerization strategy is adopted for preparing the nanofiltration membrane, and the process comprises the following steps: pre-depositing water phase monomers on the surface of a porous support membrane, self-assembling two-dimensional MOF nanosheets to construct a main structure, spontaneously diffusing the water phase monomers in the interlayer gap, introducing organic phase monomers for interfacial polymerization to generate polyamide, and finally forming a two-dimensional MOF-polyamide cross-linked composite active layer. The preparation process precisely controls the structure of the active layer, disperses the polyamide fragments in the interlayer gap formed by the stacking of nanosheets, significantly reduces the thickness of the active layer (about 39.5 nm), and effectively fills the gaps between the MOF layers by the formation of polyamide, thereby retaining the solutes in water, ensuring high rejection rate, providing a channel for the rapid transmission of water molecules, and achieving high water flux characteristics.

[0066] (3) The molecular stitching-interfacial polymerization strategy effectively inhibits the excessive polymerization and surface accumulation of polyamide, so that the two-dimensional metal organic framework becomes the main structure of the active layer, and the polyamide only exists as dispersed fragments. This structure makes the surface of the active layer smoother, enhances the hydrophilicity, and endows it with stronger surface negative charge. The smooth membrane surface can effectively reduce the attachment area of pollutants and weaken the adsorption between the membrane surface and the pollutants; the surface negative charge can strengthen the repulsion with negatively charged pollutants in natural water bodies. At the same time, more active sites of the metal organic framework are exposed, thereby significantly improving the antibacterial and anti-pollution ability of the membrane.

[0067] (4) In the preparation process, a polyvinyl alcohol pretreatment layer is introduced on the surface of the substrate to enhance the bonding force between the substrate and the two-dimensional MOF nanosheets; in the active layer, the polyamide is stably embedded in the interlayer gap formed by the stacking of two-dimensional MOF nanosheets in the form of fragments, which play a connecting and fixing role for the two-dimensional MOF layers, thereby strengthening the interlayer bonding stability and significantly reducing the possibility of MOF shedding, avoiding the precipitation of metal ions, and further improving the stability of the two-dimensional MOF nanosheet main structure in water solution. Therefore, the nanofiltration membrane provided by the present application is not prone to interlayer peeling or functional attenuation during long-term operation or cleaning, has excellent structural stability and service life, avoids water pollution, ensures the safety of drinking water, and still maintains stable separation performance after continuous operation for 4 hours.

[0068] (5) Compared with the prior art, the anti-pollution nanofiltration membrane has a thinner active layer, a smoother surface, stronger hydrophilicity, and stronger negative charge; it exhibits the lowest pollution adsorption and the highest flux recovery rate for organic pollutants (BSA anti-bovine serum albumin) and bacteria (E. coli anti-Escherichia coli); the water flux reaches 25.6 L·m -2 ·h -1 ·bar -1, Na2SO4 rejection rate is 99.2%, and the removal efficiency of small molecule persistent pollutants such as perfluoroalkyl substances (Per- and polyfluoroalkyl substances, PFAS) is excellent, and the rejection rate of difficult-to-remove small molecule substances (such as PFBA) is higher than 89%.

[0069] (6) The anti-pollution nanofiltration membrane of the application is suitable for complex water environments such as drinking water purification, wastewater reuse and seawater desalination, and has good application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0070] Figure 1 Transmission electron microscope (TEM) image of the two-dimensional Cu-TCPP nanosheet in Example 1.

[0071] Figure 2 Atomic force microscope (AFM) image of the two-dimensional Cu-TCPP nanosheet in Example 1.

[0072] Figure 3 Schematic diagram of the membrane structure of Example 1 and Comparative Examples 1-3.

[0073] Figure 4 Cross-section transmission electron microscope (TEM) photos of Example 1 and Comparative Examples 1-3.

[0074] Figure 5 Time-of-flight secondary ion mass spectrometry (TOF-SIMS) characterization results of the nanofiltration membrane prepared by the molecular stitching-interface polymerization strategy in Example 1. Figure 5 A and B are three-dimensional structure characterization diagrams of the membrane, wherein A represents polyamide distribution, and B represents Cu-TCPP distribution. Figure 5 C and D are surface structure characterization diagrams of the membrane, wherein C represents polyamide distribution, and D represents Cu-TCPP distribution.

[0075] Figure 6 Water flux comparison data of Example 1 and Comparative Examples 1-3.

[0076] Figure 7 Average pore size comparison data of Example 1 and Comparative Examples 1-3.

[0077] Figure 8 Rejection rate comparison data of Example 1 and Comparative Examples 1-3 on five kinds of inorganic salt solutions.

[0078] Figure 9 PFAS rejection comparison data of Example 1 and Comparative Examples 1-3.

[0079] Figure 10Anti-bovine serum albumin contamination comparison data for Example 1 and Comparative Examples 1-3.

[0080] Figure 11 Anti-E. coli contamination comparison data for Example 1 and Comparative Examples 1-3. DETAILED DESCRIPTION

[0081] The application will be described in greater detail with reference to the accompanying drawings and specific embodiments. The embodiments are implemented on the premise of the technical solutions of the application, and detailed implementation methods and specific operation processes are given, but the protection scope of the application is not limited to the following embodiments.

[0082] Unless otherwise specified, the reagents, methods, instruments and equipment used in the application are conventional reagents, methods, instruments and equipment in the art. Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0083] Example 1

[0084] This embodiment provides an anti-fouling nanofiltration membrane based on a molecular stitching-interface polymerization strategy, and the preparation method is as follows: a 0.22 um microfiltration substrate (purchased from Haining Wanda Filter Material, 0.22 um microporous circular filter membrane, diameter 50 mm, material polyether sulfone, thickness about 0.1 cm) is soaked in a 0.005 wt% PVA (polyvinyl alcohol, molecular weight 47000) solution for 3 min, and the water on the surface of the base film is removed by suction filtration. 3 mL of 2.00%wt piperazine (PIP) aqueous solution is introduced onto the surface of the base film, and after standing for one minute, the water on the surface of the film is removed by suction filtration. Keep the suction filtration device in a negative pressure state, and evenly pour 3 mL (concentration 0.1 mg / L) of two-dimensional Cu-TCPP nanosheet aqueous solution onto the surface of the film. The nanosheet solution is independently prepared in the laboratory, and the synthesis process is as follows: 24 mg of copper nitrate trihydrate, 40 μL of methanol and 50 mg of polyvinylpyrrolidone are added to 120 mL of a mixed solution of dimethylformamide and ethanol (volume ratio 3:1) to prepare Cu 2+ ligand solution. Then, 44 mg of tetrakis (4-carboxyphenyl) porphyrin (TCPP) is added to 40 mL of DMF-ethanol (volume ratio 3:1) mixed solution, and after stirring for 0.5 h, Cu 2+Ligand solution. The mixed solution was transferred into a hydrothermal reactor and reacted at 80 °C for 12 h. The obtained solid after centrifugation was washed with methanol for three times, dissolved in DMF again, and subjected to ultrasonic cell disruption to exfoliate the nanosheets. Finally, the Cu-TCPP nanosheets were dispersed in 100 mL of deionized water. After removing the water on the surface of the membrane by negative pressure, the valve was opened to restore the normal pressure state of the filtration device, and the PIP was allowed to diffuse between the layers of the nanosheets for 60 s. 3 mL of 0.10 wt% TMC (trimesoyl chloride) / n-hexane solution was poured onto the surface of the membrane, and after 60 s of reaction, the excess solution was poured out, and the membrane was placed in a 60 °C oven for 5 min to obtain the anti-fouling nanofiltration membrane based on the molecular stitching-interfacial polymerization strategy.

[0085] Comparative Example 1

[0086] The 0.22 um microfiltration substrate was soaked in a 0.005 wt% PVA solution for 3 min, and the water on the surface of the base membrane was removed by filtration. 3 mL of 0.20 wt% piperazine aqueous solution was poured onto the surface of the base membrane, and after the water on the surface of the membrane was removed by negative pressure, it was left for 60 s. 3 mL of 0.10 wt% TMC / n-hexane solution was poured onto the surface of the membrane, and after 60 s of reaction, the excess solution was poured out, and the membrane was placed in a 60 °C oven for 5 min.

[0087] Comparative Example 2

[0088] The 0.22 um microfiltration substrate was soaked in a 0.005 wt% PVA solution for 3 min, and the water on the surface of the base membrane was removed by filtration. 3 mL of 0.20 wt% PIP aqueous solution was poured onto the surface of the base membrane, and after the water on the surface of the membrane was removed by negative pressure, it was left for 60 s. 3 mL of 0.10 wt% TMC / n-hexane solution was poured onto the surface of the membrane, and after 60 s of reaction, the excess solution was poured out, and the membrane was placed in a 60 °C oven for 5 min.

[0089] Comparative Example 3

[0090] The 0.22 um microfiltration substrate was soaked in a 0.005 wt% PVA solution for 3 min, and the water on the surface of the base membrane was removed by filtration. 3 mL of 0.20 wt% PIP aqueous solution was poured onto the surface of the base membrane, and after the water on the surface of the membrane was removed by negative pressure, it was left for 60 s. 3 mL of 0.10 wt% TMC / n-hexane solution was poured onto the surface of the membrane, and after 60 s of reaction, the excess solution was poured out, and the membrane was placed in a 60 °C oven for 5 min.

[0091] Figure 1 For the transmission electron microscopy (TEM) image of the two-dimensional Cu-TCPP nanosheets in Example 1, it can be clearly demonstrated that it has a typical nanosheet structure, and the lateral size distribution range of the Cu-TCPP nanosheets is 0.5-2.0 pm. Figure 2 For the atomic force microscopy (AFM) image thereof, it can be determined from the image that the longitudinal size (thickness) of the prepared Cu-TCPP nanosheets is 4-5 nm.

[0092] Figure 3 For the schematic diagram of the membrane structure of Example 1 and Comparative Examples 1-3, in the above examples and comparative examples, Example 1 is a composite nanofiltration membrane prepared by using the molecular stitching-interfacial polymerization strategy, and the active layer structure thereof is: two-dimensional MOF nanosheets are stacked to form the main structure of the active layer, and the polyamide is distributed in the interlayer space in the form of fragments; Comparative Example 1 is a TFC membrane (thin-film composite membrane) prepared by using the traditional interfacial polymerization strategy without introducing Cu-TCPP material, and the obtained active layer structure is a polyamide membrane layer; Comparative Example 2 is a TFNi membrane (thin-film nanocomposite membrane with MOF interlayer) in which Cu-TCPP is deposited first and then polyamide is formed, and the obtained active layer structure is: a vertical structure of two-dimensional MOF nanosheet layer + polyamide layer; Comparative Example 3 is a TFNa membrane (thin-film nanocomposite membrane with MOF in active layer) in which polyamide is used as the main structure of the active layer, and the two-dimensional MOF nanosheets are wrapped by polyamide.

[0093] The cross-sectional TEM photos of Example 1 and Comparative Examples 1-3 are shown in Figure 4 It can be seen that the thickness of the polyamide active layer of Comparative Example 1 is about 69.1 nm; the thicknesses of the active layers of Comparative Examples 2 and 3 are 49.0 nm and 47.9 nm, respectively. In contrast, the thickness of the active layer of Example 1 is significantly reduced to 39.5 nm, indicating that the polyamide is formed between the two-dimensional membrane layers formed by stacking the two-dimensional MOF nanosheets, thereby forming a thinner and more uniform active layer structure. This structural feature is conducive to realizing the separation performance of high flux and high selectivity.

[0094] Figure 5 For the time-of-flight secondary ion mass spectrometry (TOF-SIMS) characterization results of the composite nanofiltration membrane of Example 1 prepared by using the molecular stitching-interfacial polymerization strategy, among them, Figure 5Fig. 3A and Fig. 3B are three-dimensional structure characterization diagrams of the membrane, and the results show that the polyamide is distributed throughout the two-dimensional MOF nanosheet layer; Figure 5 Fig. 3C and Fig. 3D are surface structure characterization diagrams of the membrane, and it can be seen that the distribution intensity of the polyamide is similar to that of the MOF, indicating that the MOF and the polyamide exist on the surface of the membrane at the same time, and a structure in which the MOF is wrapped by the polyamide is not formed. Figure 5 The characterization results of Fig. 3A to Fig. 3D further confirm that the polyamide is distributed between the two-dimensional membrane layers in the composite nanofiltration membrane prepared in Example 1.

[0095] The anti-pollution nanofiltration membranes prepared in Example 1 and Comparative Examples 1 to 3 were washed with deionized water and then subjected to performance test: a custom-made cross-flow membrane module was used, the temperature of the water solution was controlled at 25°C by using a constant-temperature cooling operation system under the condition of an operating pressure of 4 bar, cross-flow filtration was carried out, and the water flux and the rejection ability of the inorganic salt ions of the anti-pollution nanofiltration membranes prepared in Example 1 and Comparative Examples 1 to 3 were tested. In addition, the anti-pollution ability of the membranes was tested by using a configured bovine serum albumin water solution and an Escherichia coli water solution as the water solution.

[0096] The pure water permeation flux results of Example 1 and Comparative Examples 1 to 3 are shown in Table 1. Figure 6 As can be seen, the water flux of Example 1 reaches 25.6 L·m -2 ·h -1 ·bar -1 , while the water fluxes of Comparative Example 1, Comparative Example 2 and Comparative Example 3 are 15.0 L·m -2 ·h -1 ·bar -1 , respectively. It can be seen that the nanofiltration membrane prepared by the molecular stitching-interfacial polymerization strategy of the present application can maintain a high pure water flux, which is also the advantage of the MOF-PA crosslinking structure, that is, the water flux is greatly improved while maintaining a high rejection rate. The rejection rates of all the membranes to sodium sulfate are maintained at a high level (>98%), indicating that the charge density and selectivity of the membranes are good, and completely meet the needs of drinking water nanofiltration applications.

[0097] The rejection results of neutral organic solutes were analyzed to analyze the pore size change of the membranes, as shown in Table 2. Figure 7 By adjusting the concentration of the water phase monomer, the average pore size of Example 1 (0.328 nm) and Comparative Examples 1 to 3 (0.332 nm, 0.322 nm, 0.313 nm, respectively) was kept highly similar, which excluded the interference of pore size difference on the rejection rate, anti-pollution property and stability of the membranes, and ensured that the performance difference between different membranes was only due to the structure design of the active layer.

[0098] The inorganic salt rejection rates of Example 1 and Comparative Examples 1-3 are shown in Figure 8 (Fig. Na2SO4 is sodium sulfate, MgSO4 is magnesium sulfate, MgCl2 is magnesium chloride, CaCl2 is calcium chloride, and NaCl is sodium chloride). The ionic strength of each inorganic salt test solution was adjusted to 10 mmol / L, and the pH was maintained at 7 by adding NaHCO3 (sodium bicarbonate). The results show that the rejection rates of Example 1 for calcium and magnesium ions are lower than those of Comparative Examples 1-3, and in combination with the similar average pore size, it is speculated that Example 1 has a higher negative charge density on the surface, which is beneficial to the removal of negative pollutants in water, and at the same time can enhance the anti-pollution performance of the membrane surface (most of the solutes in natural water bodies are negatively charged).

[0099] The PFAS rejection rates of Example 1 and Comparative Examples 1-3 are shown in Figure 9 (Fig. Na2SO4 is sodium sulfate, MgSO4 is magnesium sulfate, MgCl2 is magnesium chloride, CaCl2 is calcium chloride, and NaCl is sodium chloride). The ionic strength of each inorganic salt test solution was adjusted to 10 mmol / L, and the pH was maintained at 7 by adding NaHCO3 (sodium bicarbonate). The results show that the rejection rates of Example 1 for calcium and magnesium ions are lower than those of Comparative Examples 1-3, and in combination with the similar average pore size, it is speculated that Example 1 has a higher negative charge density on the surface, which is beneficial to the removal of negative pollutants in water, and at the same time can enhance the anti-pollution performance of the membrane surface (most of the solutes in natural water bodies are negatively charged).

[0100] The anti-pollution performance of Example 1 and Comparative Examples 1-3 is shown in Figure 10- Figure 11 (Fig. DI water is pure water). Bovine serum albumin and E. coli were used as target pollutants, and the flux decay was tested under constant pressure cross-flow filtration conditions. After continuous operation for 4 hours, the flux decline rate of Example 1 (bovine serum albumin pollution was 16.3%, and E. coli pollution was 41.4%) was significantly lower than that of Comparative Examples 1-3 (bovine serum albumin was 37.1%, 27.6%, and 46.9%, respectively, and E. coli pollution was 59.3%, 45.0%, and 56.9%, respectively). At the same time, after flushing with pure water, the flux recovery rate of Example 1 (bovine serum albumin pollution was 96.1%, and E. coli pollution was 94.2%) was also higher than that of Comparative Examples 1-3 (bovine serum albumin pollution was 66.9, 79.0%, and 58.8%, respectively, and E. coli pollution was 72.3%, 83.2%, and 82.5%, respectively). The results show that Example 1 has stronger anti-organic and biological pollution ability.

[0101] The foregoing description of the embodiments has been presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the application to the precise form disclosed. Modifications and variations are possible in light of the above teachings or can be acquired from practice of the application. As well, the description is presented in the context of the preferred embodiments as a number of alternatives. It is not intended to limit the application to the precise form described.

Claims

1. An anti-fouling nanofiltration membrane based on a molecular stitching- interfacial polymerization strategy, characterized in that, The anti-pollution nanofiltration membrane comprises a porous support membrane substrate, and an active layer arranged on one side surface of the porous support membrane substrate, wherein the active layer is a two-dimensional metal organic framework-polyamide cross-linked composite structure, and the composite structure comprises two-dimensional metal organic framework nanosheets and polyamide segments, wherein the two-dimensional metal organic framework nanosheets are stacked to form a main body structure, and the polyamide segments are distributed in interlayer voids formed by the two-dimensional metal organic framework nanosheets.

2. The anti-fouling nanofiltration membrane based on molecular stitching- interfacial polymerization strategy according to claim 1, characterized in that, The two-dimensional metal organic framework nanosheets comprise two-dimensional Cu-TCPP nanosheets, the thickness of the two-dimensional Cu-TCPP nanosheets is 4-5 nm, and the lateral size is 0.5-2.0 μm; the thickness of the active layer is 32.3-46.7 nm, and the average pore size of the active layer is 0.30-0.33 nm. 3.The anti-fouling nanofiltration membrane based on molecular stitching-interface polymerization strategy according to claim 1, characterized in that, The thickness of the porous support membrane substrate is 0.06-0.10 cm, the average pore size is 0.22-0.45 μm, and the porous support membrane substrate comprises a polyether sulfone ultrafiltration membrane.

4. A method for preparing an anti-fouling nanofiltration membrane based on the molecular stitching-interface polymerization strategy according to any one of claims 1-3, characterized in that, The method comprises the following steps: S1, pretreatment: immersing the porous support membrane substrate in a polyvinyl alcohol solution, and removing water on the surface of the porous support membrane substrate by negative pressure suction filtration; S2, water phase monomer pre-deposition: depositing water phase monomers on one side surface of the porous support membrane substrate pretreated in step S1 by negative pressure suction filtration; S3, self-assembly of two-dimensional metal organic framework nanosheets: self-assembling two-dimensional metal organic framework nanosheets on the surface of the porous support membrane substrate pre-deposited with water phase monomers obtained in step S2 by negative pressure suction filtration, so that the two-dimensional metal organic framework nanosheets are stacked to form a two-dimensional metal organic framework main body structure; S4, water phase monomer diffusion: spontaneously diffusing the water phase monomers pre-deposited in step S2 into interlayer voids formed by stacking of the two-dimensional metal organic framework nanosheets in step S3 under normal pressure; S5, interfacial polymerization: introducing organic phase monomers to the surface of the two-dimensional metal organic framework main body structure treated in step S4, so that the organic phase monomers and the water phase monomers diffused into the interlayer voids are subjected to interfacial polymerization reaction to generate polyamide segments, thereby obtaining the anti-pollution nanofiltration membrane.

5. The method for preparing anti-fouling nanofiltration membrane based on molecular sewing-interface polymerization strategy according to claim 4, characterized in that, In step S1, the concentration of the polyvinyl alcohol solution is 0.001-0.005 wt%, the molecular weight of the polyvinyl alcohol is 40000-60000, and the immersion time is 1-3 min.

6. The method for preparing anti-fouling nanofiltration membrane based on molecular sewing-interface polymerization strategy according to claim 4, characterized in that, The specific steps of step S2 are as follows: pouring a water phase monomer solution on one side surface of the porous support membrane substrate pretreated in step S1, standing, and then removing water on the surface of the porous support membrane substrate by suction filtration under negative pressure to complete the pre-deposition of the water phase monomers; the concentration of the water phase monomer solution is 1.5-2.5 wt%, the standing time is 30-120 s, and the water phase monomer is selected from any one of piperazine and m-phenylenediamine, and the solvent of the water phase monomer is water.

7. The method for preparing anti-fouling nanofiltration membrane based on molecular sewing-interface polymerization strategy according to claim 4, characterized in that, The specific step of step S3 is: pouring the two-dimensional metal organic framework nanosheet solution into the surface of the porous support membrane substrate pre-deposited with the aqueous phase monomer in step S2, standing, and then removing water under negative pressure by suction filtration to realize self-assembly of the two-dimensional metal organic framework nanosheet; the two-dimensional metal organic framework nanosheet solution is a two-dimensional Cu-TCPP nanosheet aqueous solution, and the concentration is 0.05-0.15 mg / L; and the standing time is 30-120 s.

8. The method for preparing anti-fouling nanofiltration membrane based on molecular sewing-interface polymerization strategy according to claim 4, characterized in that, In step S4, the diffusion time of the aqueous phase monomer is 30-120 s.

9. The method for preparing anti-fouling nanofiltration membrane based on molecular sewing-interface polymerization strategy according to claim 4, characterized in that, The specific step of step S5 is: pouring the organic phase monomer solution into the surface of the two-dimensional metal organic framework nanosheet main structure treated in step S4 and performing interfacial polymerization reaction, pouring off the excess organic phase monomer solution after the reaction is completed, and then drying and solidifying to obtain the anti-pollution nanofiltration membrane; the organic phase monomer includes trimesoyl chloride, the solvent of the organic phase monomer solution is selected from any one of n-hexane and cyclohexane, the mass concentration of the organic phase monomer solution is 0.08-0.12 wt%, the interfacial polymerization reaction time is 40-80 s, the reaction temperature is 50-70 ℃, the drying temperature is 50-70 ℃, and the drying time is 4-6 min.

10. Application of the anti-pollution nanofiltration membrane based on the molecular stitching-interfacial polymerization strategy according to any one of claims 1-3 in the field of water purification treatment.

Images