Pda-mof / mxene modified pvdf composite nanofiltration membrane, and preparation method and application thereof

Abstract

The application provides a PDA-MOF / MXene modified PVDF composite nanofiltration membrane and a preparation method and application thereof, and relates to the technical field of membrane material preparation.A composite nanofiltration membrane is prepared by constructing an interface polymerization diffusion damping structure to regulate the interface polymerization process, so that the composite nanofiltration membrane can maintain high desalination performance while significantly improving water flux, be used for selective separation of dyes and inorganic salts in salt-containing printing and dyeing wastewater, and enhance the operation stability and adaptability of the nanofiltration membrane in a high-salt wastewater system.The application solves the problem that the water flux and desalination rate of the existing nanofiltration membrane are difficult to be considered in the treatment process of high-salt industrial wastewater, especially solves the technical defects that the diffusion behavior of the water-phase monomer on the surface and in the pores of the base membrane is difficult to be regulated in the preparation of the nanofiltration membrane by the interface polymerization method, and the polyamide separation layer is prone to be too thick or the structure is not uniform, thereby limiting the separation performance and operation stability of the membrane, and has good stability and application prospect.

Description

Technical Field

[0001] This invention relates to the field of membrane material preparation technology, and in particular to a PDA-MOF / MXene modified PVDF composite nanofiltration membrane, its preparation method and application. Background Technology

[0002] The dyeing and printing industry, as a typical high-water-consuming and high-emission industrial sector, generates wastewater characterized by high dye concentration, high inorganic salt content, complex composition, and large fluctuations in water quality. Dyeing and printing wastewater typically contains large amounts of anionic dyes, surfactants, and inorganic salts such as sodium sulfate and sodium chloride. Direct discharge without effective treatment will not only increase water color and reduce light transmittance but also pose potential threats to aquatic ecosystems and human health. Therefore, achieving efficient separation and recovery of dyes and salts from dyeing and printing wastewater is a crucial technical problem that urgently needs to be solved in the field of water treatment.

[0003] Currently, wastewater treatment technologies for dyeing and printing mainly include coagulation sedimentation, biological treatment, advanced oxidation, and membrane separation. While coagulation and biological treatment processes are effective at removing some pollutants, their adaptability to high-concentration dyes and high-salt systems is limited, and they often suffer from high reagent consumption, high sludge production, or complex treatment processes. Advanced oxidation technologies, although effective in degrading some recalcitrant organic matter, have high energy consumption and operating costs, making long-term stable application in large-scale projects difficult. In contrast, nanofiltration membrane technology, with its high retention rate of macromolecular organic matter and multivalent ions, and high permeability to some monovalent salts, shows unique advantages in dye / salt separation and reuse in dyeing and printing wastewater, and is gradually becoming a research and application hotspot.

[0004] However, existing nanofiltration membranes still generally suffer from the technical bottleneck of "difficulty in simultaneously achieving high water flux and desalination rate" in practical applications. On the one hand, traditional nanofiltration membranes often use polymers such as polyvinylidene fluoride (PVDF) as the base membrane material. Although PVDF has good acid and alkali resistance and chemical stability, its intrinsic hydrophilicity is poor, and it easily forms a dense structure during phase transformation to form the membrane, resulting in low membrane porosity and limited water flux. On the other hand, the polyamide (PA) separation layer constructed to improve the desalination rate often tends to be dense, causing the membrane to significantly sacrifice flux while achieving high retention performance, forming a typical "trade-off" phenomenon. In addition, during the preparation of nanofiltration membranes by interfacial polymerization, the diffusion behavior of aqueous monomers (such as piperazine PIP) on the surface of the base membrane has a decisive influence on the thickness, density, and defect formation of the polyamide layer. In existing technologies, the diffusion rate of PIP molecules in the pores of the base membrane is often difficult to control precisely, easily leading to an excessively thick separation layer or an uneven structure, thereby further limiting the flux improvement and long-term stable operation of the nanofiltration membrane. Although existing studies have attempted to improve membrane performance by introducing an intermediate layer or adjusting interfacial polymerization conditions, there is still a lack of a nanofiltration membrane that is structurally stable, has controllable preparation, and is suitable for high-salt dyeing and printing wastewater systems, while simultaneously improving the hydrophilicity of the base membrane, optimizing the pore structure, and regulating the interfacial polymerization kinetics.

[0005] To address the aforementioned issues, previous studies have attempted to intervene in the interfacial polymerization process by adjusting interfacial polymerization conditions, introducing an intermediate layer, or adding functional fillers to the base membrane. However, these methods primarily focus on improving the hydrophilicity of the base membrane or simply increasing interfacial resistance, making it difficult to precisely and stably control the diffusion behavior of aqueous monomers during interfacial polymerization. Furthermore, they still suffer from insufficient controllability of the separation layer structure, limited flux enhancement, and poor long-term operational stability.

[0006] Therefore, due to the complex and highly variable composition of dyeing and printing wastewater, the nanofiltration membrane separation process not only needs to meet the separation objective of "high dye rejection / low salt rejection," but also needs to possess a wide process window, stable and repeatable membrane fabrication consistency, and long-term operational reliability at the engineering operation level. Therefore, researching and developing a composite nanofiltration membrane preparation technology that can improve the controllability of the nanofiltration membrane interfacial polymerization process, enhance the stability of separation performance, and broaden the membrane fabrication and operation process window is of significant practical importance and has broad engineering application prospects for achieving efficient separation of dye / salt systems in the treatment of high-salt industrial wastewater such as dyeing and printing wastewater, reducing system operating energy consumption and maintenance costs, improving the long-term stable operation capability of the membrane process under complex conditions, and thus promoting the large-scale application of nanofiltration membrane technology in the field of dyeing and printing wastewater treatment and recycling. Summary of the Invention

[0007] Therefore, this invention proposes a PDA-MOF / MXene modified PVDF composite nanofiltration membrane, its preparation method, and its application.

[0008] The technical solution of this invention is implemented as follows: A method for preparing a PDA-MOF / MXene modified PVDF composite nanofiltration membrane, the specific preparation steps of which include: Preparation of S1, PDA-MOF / MXene composite nanomaterials Dopamine was added to an MXene solution, and a polydopamine layer was formed through a self-polymerization reaction. Then, metal ions and organic ligands were added to form a metal-organic framework through an in-situ reaction, thus obtaining PDA-MOF / MXene composite nanomaterials. S2. Preparation of composite base film casting solution The pore-forming agent was added to N,N-dimethylacetamide and mixed. Then, polyvinylidene fluoride (PVDF) and PDA-MOF / MXene composite nanomaterials were added and stirred to obtain the composite base film casting solution. S3. Preparation of ultrafiltration membrane The composite base membrane casting solution was subjected to phase inversion to obtain a PDA-MOF / MXene modified PVDF ultrafiltration base membrane; Preparation of S4, PDA-MOF / MXene modified PVDF composite nanofiltration membrane PDA-MOF / MXene-modified PVDF ultrafiltration membrane was subjected to interfacial polymerization to obtain a PDA-MOF / MXene-modified PVDF composite nanofiltration membrane; The loading of PDA-MOF / MXene composite nanomaterials in the PDA-MOF / MXene modified PVDF composite nanofiltration membrane is 0.5wt%-3.0wt%.

[0009] Furthermore, in step S1, the solid-liquid ratio of dopamine to MXene solution is 0.1-5:1 g / L; the MXene solution is obtained by ultrasonically dispersing MXene in water at a ratio of 0.1-5:1 g / L.

[0010] Furthermore, in step S1, the self-polymerization reaction is carried out at a pH of 7.5-9.0 for 4-12 hours; The metal-organic framework is one or more of ZIF-8, UiO-66, and MIL; The loading of the metal-organic framework in the PDA-MOF / MXene composite nanomaterial is 0.5wt%-3.0wt%.

[0011] This invention involves ultrasonically dispersing MXene in water to form a stable dispersion. Dopamine monomer is added under weakly alkaline conditions, causing a self-polymerization reaction on the MXene surface to construct a polydopamine (PDA) coating layer. Under the adhesion and coordination of PDA, metal ions and organic ligands are introduced to undergo in-situ reactions, resulting in the in-situ loading of a metal-organic framework (MOF) onto the MXene surface, yielding a PDA-MOF / MXene composite nanomaterial. By constructing a composite functional phase with adsorption and sustained-release capabilities in-situ on a two-dimensional support surface, this phase can be simultaneously embedded into the base membrane surface and pores during subsequent phase transformation into a film.

[0012] Furthermore, in step S2, the porogen is polyvinylpyrrolidone, and the mass ratio of the porogen to N,N-dimethylacetamide is 1-10:100.

[0013] Furthermore, in step S2, the mass ratio of the polyvinylidene fluoride, PDA-MOF / MXene composite nanomaterial to N,N-dimethylacetamide is 10-20:0.05-0.6:100; The stirring is carried out at a temperature of 60-70℃ and a speed of 300-500rpm for 8-16 hours to form a uniform casting solution, followed by standing for degassing for 12-36 hours.

[0014] Furthermore, in step S3, the phase transformation specifically involves: coating a composite base membrane casting solution onto a flat glass plate to form a membrane with a wet membrane thickness of 200-400 μm, and then immersing it in deionized water or a 10%-30% v / v ethanol solution for phase transformation to obtain a PDA-MOF / MXene modified PVDF ultrafiltration base membrane.

[0015] The phase transformation of this invention fixes the spatial distribution of PDA-MOF / MXene composite nanomaterials, enabling the PDA-MOF / MXene composite nanomaterials to be distributed on the surface and within the pores of the PDA-MOF / MXene modified PVDF ultrafiltration membrane, forming a stable interfacial polymerization diffusion damping structure. This structure provides a physical and chemical basis for regulating the diffusion behavior of aqueous monomers in subsequent interfacial polymerization reactions.

[0016] Further, in step S4, the interfacial polymerization specifically involves: immersing the PDA-MOF / MXene-modified PVDF ultrafiltration membrane in a 0.05wt%-0.1wt% piperazine aqueous solution for 5-10 minutes, removing the membrane and air-drying it for 100-150 seconds, then immersing it in a hexane solution containing 0.05wt%-0.1wt% trimesoyl chloride for interfacial polymerization for 30-120 seconds, removing the membrane and drying it at 50-60℃ for 300-600 seconds, and finally storing it in deionized water to obtain the PDA-MOF / MXene-modified PVDF composite nanofiltration membrane.

[0017] The PDA-MOF / MXene modified PVDF composite nanofiltration membrane of the present invention uses piperazine as an aqueous monomer and pyromellitic trimethylol chloride as an oil monomer for interfacial polymerization to construct a polyamide separation layer on the surface of the base membrane. After drying and stabilization treatment in water, a composite nanofiltration membrane with an interfacial polymerization diffusion damping structure is obtained.

[0018] The present invention also provides a PDA-MOF / MXene modified PVDF composite nanofiltration membrane, which is prepared by any of the above preparation methods.

[0019] This invention also provides an application of PDA-MOF / MXene modified PVDF composite nanofiltration membrane in the preparation of dyeing and printing wastewater treatment materials.

[0020] Furthermore, the dyeing and printing wastewater treatment material achieves high dye retention and low salt retention by selectively separating dyes and salts in the dyeing and printing wastewater.

[0021] Compared with the prior art, the beneficial effects of the present invention are: This invention prepares a composite nanofiltration membrane that regulates the interfacial polymerization process by constructing an interfacial polymerization diffusion damping structure. This composite nanofiltration membrane is structurally stable, controllable in preparation, and suitable for high-salt dyeing and printing wastewater systems. This invention introduces PDA-MOF / MXene composite nanomaterials into the surface / pores of the base membrane to form an interfacial polymerization diffusion damping structure. Without significantly increasing mass transfer resistance, this structure inhibits the rapid and disordered diffusion of aqueous monomers during the interfacial polymerization process through the adsorption-slow release effect of polydopamine and metal-organic frameworks, and the extension of the diffusion path by the two-dimensional MXene sheets, thereby regulating the polyamide separation layer. The thickness and uniformity of the composite nanofiltration membrane enable it to significantly increase water flux while maintaining high desalination performance. It can be used for the selective separation of dyes and inorganic salts in saline dyeing and printing wastewater, and enhances the operational stability and adaptability of nanofiltration membranes in high-salt wastewater systems. It solves the problem that existing nanofiltration membranes often cannot achieve both high water flux and desalination rate in the treatment of high-salt industrial wastewater. In particular, it solves the technical defects of the interfacial polymerization method for preparing nanofiltration membranes, which makes it difficult to control the diffusion behavior of aqueous monomers on the surface and in the pores of the base membrane, and easily leads to excessively thick or uneven polyamide separation layers, thus limiting the membrane separation performance and operational stability. It has good stability and application prospects.

[0022] Specifically, 1. This invention introduces PDA-MOF / MXene composite nanomaterials into a PVDF base membrane and constructs an interfacial polymerization diffusion damping structure, which significantly improves the hydrophilicity, porosity, and pore structure morphology of the base membrane during the phase transformation and membrane formation process. This makes the resulting base membrane more suitable for constructing a high-performance nanofiltration separation layer, thereby effectively reducing the resistance of water molecules to transmembrane mass transfer and improving the basic permeation performance of the membrane.

[0023] 2. This invention regulates the diffusion behavior of aqueous monomers during interfacial polymerization through a diffusion damping structure, inhibiting the rapid and disordered infiltration of aqueous monomers and intervening in the formation process of the polyamide separation layer. This results in a more uniform separation layer structure with easier thickness control, significantly increasing water flux while maintaining a high desalination rate, thereby alleviating the "flux-desalination rate trade-off" problem commonly found in traditional nanofiltration membranes.

[0024] 3. The composite nanofiltration membrane prepared by this invention can be used in the treatment of dyeing and printing wastewater. It can efficiently retain dye molecules while maintaining high permeability to some inorganic salts, thus achieving effective separation of dyes and salts. It is suitable for high-salt and high-color dyeing and printing wastewater systems.

[0025] 4. The composite nanofiltration membrane prepared by this invention exhibits good stability and adaptability under continuous operation conditions, and can maintain stable separation performance under different operating pressures, salt concentrations and water quality conditions, and has the potential to be widely used in industrial wastewater treatment and reuse systems.

[0026] 5. This invention enhances the controllability of the interfacial polymerization process from the source by constructing an interfacial polymerization diffusion damping structure. This significantly reduces the sensitivity of the nanofiltration membrane separation layer structure to differences in raw materials and fluctuations in operating conditions, achieving more stable and repeatable membrane fabrication effects and operational performance in engineering applications. This technical approach helps broaden the process window for nanofiltration membrane preparation and operation, reduces the energy consumption and maintenance costs of membrane separation systems under complex operating conditions, and improves the long-term stable operation capability of membrane modules in the treatment of high-salinity industrial wastewater. It provides reliable technical support for the large-scale application and engineering promotion of nanofiltration membrane technology in industrial fields such as dyeing and printing wastewater treatment. Attached Figure Description

[0027] Figure 1 The image shows the SEM image of the PDA-MOF / MXene modified PVDF composite nanofiltration membrane from Example 1. Detailed Implementation

[0028] To better understand the technical content of this invention, specific embodiments are provided below to further illustrate the invention.

[0029] Unless otherwise specified, the experimental methods used in the embodiments of this invention are all conventional methods.

[0030] Unless otherwise specified, all materials and reagents used in the embodiments of this invention are commercially available.

[0031] The metal-organic framework of the present invention is one or more of ZIF-8, UiO-66 and MIL, and the metal-organic framework used in the embodiments of the present invention is ZIF-8.

[0032] Example 1 Preparation of S1, PDA-MOF / MXene composite nanomaterials Under conditions of pH 8.0±0.5, 5g of dopamine was added to 1L of MXene solution (prepared by ultrasonically dispersing 5g of MXene in 1L of pure water) for a self-polymerization reaction for 10h, forming a polydopamine (PDA) coating layer on the MXene surface. Subsequently, zinc nitrate hexahydrate was added to the system as a metal ion source and 2-methylimidazole as an organic ligand, and the reaction was continued at room temperature for 4h, allowing ZIF-8 to grow in situ on the PDA-coated MXene surface, resulting in a PDA-MOF / MXene composite nanomaterial. The loading of the metal-organic framework ZIF-8 in the PDA-MOF / MXene composite nanomaterial was 1.5wt%. S2. Preparation of composite base film casting solution 3 g of polyvinylpyrrolidone was added to 100 g of N,N-dimethylacetamide and mixed. 15 g of polyvinylidene fluoride and 0.225 g of PDA-MOF / MXene composite nanomaterials were added. The mixture was stirred at 60 °C and 400 rpm for 12 h to form a uniform casting solution. After standing for 24 h to remove bubbles, the composite base film casting solution was obtained. S3. Preparation of ultrafiltration membrane A composite base membrane casting solution was coated onto a flat glass plate to form a wet membrane with a thickness of 300 μm. Then, the flat glass plate and the wet membrane were immersed together in deionized water for phase inversion to obtain a PDA-MOF / MXene modified PVDF ultrafiltration base membrane. Preparation of S4, PDA-MOF / MXene modified PVDF composite nanofiltration membrane The PDA-MOF / MXene modified PVDF ultrafiltration membrane was immersed in a 0.1 wt% piperazine aqueous solution for 10 min, then removed and air-dried for 150 s. Next, it was immersed in a hexane solution containing 0.1 wt% trimesoyl chloride for interfacial polymerization for 60 s. The membrane was then removed and dried at 50 °C for 10 min, and finally stored in deionized water to obtain the PDA-MOF / MXene modified PVDF composite nanofiltration membrane, wherein the loading of PDA-MOF / MXene composite nanomaterials in the composite nanofiltration membrane was 1.5 wt%.

[0033] Example 2 Based on Example 1, the amount of PDA-MOF / MXene composite nanomaterial added was changed so that the loading of PDA-MOF / MXene composite nanomaterial in the composite nanofiltration membrane was 0.5 wt%, while other aspects remained the same as in Example 1.

[0034] Example 3 Based on Example 1, the amount of PDA-MOF / MXene composite nanomaterial added was changed so that the loading of PDA-MOF / MXene composite nanomaterial in the composite nanofiltration membrane was 3.0 wt%, while the rest was the same as in Example 1.

[0035] Comparative Example 1 The difference from Example 1 is that PDA-MOF / MXene composite nanomaterials are not added; otherwise, they are the same as in Example 1.

[0036] Comparative Example 2 The difference from Example 1 is that the loading of PDA-MOF / MXene composite nanomaterials in the composite nanofiltration membrane is 4 wt%, while the rest is the same as in Example 1.

[0037] Comparative Example 3 The difference from Example 1 is that the concentrations of the piperazine aqueous solution and the hexane solution of trimesoyl chloride were both 0.5 wt% during the interfacial polymerization reaction, while the rest was the same as in Example 1.

[0038] Test case The PDA-MOF / MXene modified PVDF composite nanofiltration membranes prepared in Examples 1-3 and the nanofiltration membranes prepared in Comparative Examples 1-3 were subjected to performance tests. The test indicators included: membrane permeability test, membrane desalination rate test and membrane stability test.

[0039] 1. Membrane permeability test: A flat sheet membrane testing device was used, with deionized water as the medium. The membrane module was pre-pressed for 20 minutes at a constant temperature of 25℃ and a constant operating pressure of 4 bar. The effective membrane area was 50 cm². 2 After the flux stabilizes, record the outflow volume and calculate the water flux according to the flux formula.

[0040] The formula for calculating water flux is: Where J is the membrane water flux, in L·m -2 ·h -1 V represents the volume of liquid permeating per unit time, in liters (L); A represents the effective area of ​​the membrane, in square meters (m²). 2 t represents the test time, in hours (h).

[0041] 2. Membrane desalination rate test: Under the conditions of 25℃ and 4 bar, after pre-pressurization for 20 min, 1000 ppm of Na2SO4 solution was introduced, and the Na2SO4 rejection rate was calculated according to the rejection rate formula.

[0042] The retention rate formula is: Where R is the retention rate, in %; C pC represents the solute concentration in the permeate, expressed in ppm. f This represents the solute concentration in the feed solution, expressed in ppm.

[0043] 3. Membrane stability test: Using 1000 ppm Na2SO4 as the feed solution, the pressure was increased from 0.2 MPa to 1.0 MPa, and the flux and salt rejection rate were tested under pressure gradient conditions to evaluate the membrane stability.

[0044] The test results are shown in Table 1-2.

[0045] Table 1

[0046] Table 2

[0047] As shown in Tables 1 and 2, under the same test conditions, Comparative Example 1, without the addition of PDA-MOF / MXene composite nanomaterials (i.e., without constructing an interfacial polymerization diffusion damping structure), exhibited a high desalination rate but a significantly low water flux. Comparative Example 2, with 4.0 wt% PDA-MOF / MXene composite nanomaterials added to vinylidene fluoride (PVDF), excessively constructed a diffusion damping structure. While this significantly improved water flux, the desalination rate decreased markedly with increasing operating pressure, resulting in insufficient membrane separation stability. Comparative Example 3, with 0.5 wt% concentrations of both piperazine aqueous solution and trimesoyl chloride n-hexane solution during the interfacial polymerization reaction, altered the interfacial polymerization conditions. Lacking structural control over the diffusion behavior of monomers in the aqueous phase, it was difficult to obtain a nanofiltration membrane that simultaneously achieves high water flux, desalination rate, and operational stability. Compared to Comparative Examples 1-3, the composite nanofiltration membranes with interfacial polymerization diffusion damping structures prepared in Examples 1-3 of this invention exhibit superior overall performance in terms of water flux, desalination rate, and pressure stability. This indicates that the constructed diffusion damping structure can effectively regulate the interfacial polymerization process and improve the overall separation performance of the nanofiltration membrane. By constructing an interfacial polymerization diffusion damping structure, this invention can improve the stability and repeatability of nanofiltration membrane separation performance without significantly increasing separation resistance. This helps to broaden the process window for nanofiltration membrane preparation and operation, providing reliable support for its long-term stable operation and engineering application in the treatment of high-salinity industrial wastewater.

[0048] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a PDA-MOF / MXene modified PVDF composite nanofiltration membrane, characterized in that, The specific preparation steps include: Preparation of S1, PDA-MOF / MXene composite nanomaterials Dopamine was added to an MXene solution, and a polydopamine PDA layer was formed through a self-polymerization reaction. Then, metal ions and organic ligands were added to form a metal-organic framework through an in-situ reaction, thus obtaining a PDA-MOF / MXene composite nanomaterial. S2. Preparation of composite base film casting solution The pore-forming agent was added to N,N-dimethylacetamide and mixed. Then, polyvinylidene fluoride (PVDF) and PDA-MOF / MXene composite nanomaterials were added and stirred to obtain the composite base film casting solution. S3. Preparation of ultrafiltration membrane The composite base membrane casting solution was subjected to phase inversion to obtain a PDA-MOF / MXene modified PVDF ultrafiltration base membrane; Preparation of S4, PDA-MOF / MXene modified PVDF composite nanofiltration membrane PDA-MOF / MXene-modified PVDF ultrafiltration membrane was subjected to interfacial polymerization to obtain a PDA-MOF / MXene-modified PVDF composite nanofiltration membrane; The loading of PDA-MOF / MXene composite nanomaterials in the PDA-MOF / MXene modified PVDF composite nanofiltration membrane is 0.5wt%-3.0wt%.

2. The method for preparing a PDA-MOF / MXene modified PVDF composite nanofiltration membrane as described in claim 1, characterized in that, In step S1, the solid-liquid ratio of dopamine to MXene solution is 0.1-5:1 g / L; the MXene solution is obtained by ultrasonically dispersing MXene in water at a ratio of 0.1-5:1 g / L.

3. The method for preparing a PDA-MOF / MXene modified PVDF composite nanofiltration membrane as described in claim 1, characterized in that, In step S1, the self-polymerization reaction is carried out at a pH of 7.5-9.0 for 4-12 hours. The metal-organic framework is one or more of ZIF-8, UiO-66, and MIL; The loading of the metal-organic framework in the PDA-MOF / MXene composite nanomaterial is 0.5wt%-3.0wt%.

4. The method for preparing a PDA-MOF / MXene modified PVDF composite nanofiltration membrane as described in claim 1, characterized in that, In step S2, the porogen is polyvinylpyrrolidone, and the mass ratio of the porogen to N,N-dimethylacetamide is 1-10:

100.

5. The method for preparing a PDA-MOF / MXene modified PVDF composite nanofiltration membrane as described in claim 1, characterized in that, In step S2, the mass ratio of polyvinylidene fluoride, PDA-MOF / MXene composite nanomaterial to N,N-dimethylacetamide is 10-20:0.05-0.6:100; The stirring is carried out at a temperature of 60-70℃ and a speed of 300-500rpm for 8-16 hours to form a uniform casting solution, followed by standing for degassing for 12-36 hours.

6. The method for preparing a PDA-MOF / MXene modified PVDF composite nanofiltration membrane as described in claim 1, characterized in that, In step S3, the phase transformation specifically involves: coating a composite base membrane casting solution onto a flat glass plate to form a membrane with a wet membrane thickness of 200-400 μm, and then immersing it in deionized water or a 10%-30% v / v ethanol solution for phase transformation to obtain a PDA-MOF / MXene modified PVDF ultrafiltration base membrane.

7. The method for preparing a PDA-MOF / MXene modified PVDF composite nanofiltration membrane as described in claim 1, characterized in that, In step S4, the interfacial polymerization specifically involves immersing the PDA-MOF / MXene-modified PVDF ultrafiltration membrane in a 0.05wt%-0.1wt% piperazine aqueous solution for 5-10 minutes, removing the membrane and air-drying it for 100-150 seconds, then immersing it in a hexane solution containing 0.05wt%-0.1wt% trimesoyl chloride for interfacial polymerization for 30-120 seconds, removing the membrane and drying it at 50-60℃ for 300-600 seconds, and finally storing it in deionized water to obtain the PDA-MOF / MXene-modified PVDF composite nanofiltration membrane.

8. A PDA-MOF / MXene modified PVDF composite nanofiltration membrane, characterized in that, It is prepared by the preparation method according to any one of claims 1-7.

9. The application of the PDA-MOF / MXene modified PVDF composite nanofiltration membrane as described in claim 8 in the preparation of dyeing and printing wastewater treatment materials.

10. The application as described in claim 9, characterized in that, The dyeing and printing wastewater treatment material selectively separates dyes and salts in dyeing and printing wastewater, achieving high dye retention and low salt retention.

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