Preparation method of positively charged acid-resistant nanofiltration membrane with high crosslinking density

Abstract

The application belongs to the technical field of membrane preparation, and particularly relates to a preparation method of a positively charged acid-resistant nanofiltration membrane with high crosslinking density, which comprises five steps of solution preparation, primary interfacial polymerization, solidification-cleaning, secondary interfacial polymerization and secondary solidification. The application constructs a high-density and strong-stable covalent crosslinking network by optimizing the active crosslinking agent introduced by the secondary interfacial polymerization and the PEI main chain, improves the membrane pore size and surface characteristics, significantly improves the initial separation precision of the membrane, and endows the membrane with excellent long-term acid resistance, effectively alleviates the metal ion pollution and membrane performance attenuation in the treatment of high-acid electroplating wastewater.

Description

Technical Field

[0001] This invention belongs to the field of membrane preparation technology, and in particular to a method for preparing a positively charged acid-resistant nanofiltration membrane with high crosslinking density. Background Technology

[0002] The rapid expansion of industries such as textiles, printing and dyeing, mining, metallurgy, and electroplating has generated large amounts of highly acidic wastewater with a pH < 2. This type of wastewater contains strong acids such as H2SO4 and HCl, as well as Cr. 3+ Ni 2+ Cu 2+ Heavy metal ions. Traditional treatment technologies such as adsorption and neutralization have drawbacks such as high energy consumption and inability to recover resources. While membrane composite (TFC) nanofiltration (NF) technology can efficiently separate pollutants, commercial polyamide (PA) membranes are prone to amide bond hydrolysis in acidic environments, leading to membrane structure damage and decreased separation performance.

[0003] Existing technologies for preparing acid-resistant nanofiltration membranes using sulfonyl chloride or cyanuric chloride as organic phase monomers have certain acid resistance properties, but they also have obvious defects: the negative charge on the membrane surface leads to low heavy metal cation rejection rate and poor antifouling performance during long-term operation; or the low reactivity of the monomers results in insufficient membrane crosslinking degree, resulting in poor water flux and rejection performance.

[0004] Hydrophilic cationic polymer polyethyleneimine (PEI) is an ideal material for preparing positively charged nanofiltration membrane functional layers. It can be prepared by interfacial polymerization, electrostatic assembly and other methods. It performs well in cation removal, but there are still shortcomings in the related preparation technology and the existing related patent technology has limitations. For example, patent CN202410046961.X uses low molecular weight amines and PEI as aqueous phase monomers and cyanuric chloride as organic phase monomers to prepare acid-resistant nanofiltration membranes. Although the acid resistance and separation performance are good, the monomer reactivity is low and the interfacial reaction is uneven, making it difficult to apply in practice. CN202010271965.X prepares composite nanofiltration membranes through interfacial polymerization of polyamine polymers and polyisocyanates. Its special cross-linking structure is beneficial to improving acid resistance and water flux, but the cross-linking structure is difficult to control and the monomer reactivity needs to be improved. CN202211410595.9 introduces a new type of polysulfonyl chloride monomer to prepare polysulfonamide acid-resistant nanofiltration membranes, which have good retention effects, but the monomer reactivity is limited and easily leads to selective layer defects. Moreover, the preparation process is complicated and costly, making it impossible to achieve commercial application.

[0005] The most commonly used method for preparing positively charged acid-resistant nanofiltration membranes by reacting PEI with traditional acid-resistant organic monomers still faces three major problems: First, the low reactivity of the monomers leads to defects in the selective layer, resulting in excessively high membrane flux but poor salt retention performance; second, the low degree of crosslinking in the selective layer results in uneven membrane morphology, large pores, and low salt retention rate; third, incomplete interfacial reactions leave a large amount of residual monomers on the membrane surface, resulting in weak positive charge, poor metal cation rejection rate, and poor antifouling properties. Currently, there are few reports on the preparation and application of positively charged PEI acid-resistant nanofiltration membranes, and developing such nanofiltration membranes with high crosslinking degree, high flux, and effective retention of multivalent metal cations remains a challenge.

[0006] For example, Wei et al. reported in a non-patent document (Catalyst-assisted secondary polymerization for fabricating positively charged polysulfonamide hollow fiber nanofiltration membrane with robust acid resistance, Journal of Membrane Science 728 (2025)124119) a method that optimizes the secondary interfacial polymerization process using the nucleophilic catalyst 4-dimethylaminopyridine (DMAP). Although this method significantly enhances the positive charge on the membrane surface and optimizes the pore structure, resulting in a membrane material with a comprehensive water flux of 13.2 L m⁻² h⁻¹ bar⁻¹ and a MgCl₂ rejection rate of 90.8%, the catalytic mechanism of DMAP used in this method is unclear, and DMAP may dissolve in a long-term acidic environment, affecting the membrane lifetime.

[0007] For example, Lai et al. reported in a non-patent document (Highly permeable benzene-trisulfonyl chloride-based polysulfonamide membranes fabricated by interfacial polymerization for acid-resistant nanofiltration, Chemical Engineering Journal 460 (2023)141708) a positively charged acid-resistant nanofiltration membrane prepared by interfacial polymerization of benzene-1,3,5-trisulfonyl chloride (BTSC) with polyethyleneimine (PEI) or piperazine (PIP). Utilizing the π-conjugated structure and high steric hindrance of the BTSC monomer and the high reactivity of the trisulfonyl chloride group, a composite membrane with a highly cross-linked thin-layer structure was obtained, exhibiting excellent water permeability, acid stability, and positive charge characteristics. However, due to the high cost of BTSC monomer and the lack of a clear equilibrium between the BTSC-controlled membrane charging mechanism and pore structure optimization, practical research is insufficient.

[0008] For example, Gu et al. reported a PEI / BCMP composite membrane prepared by gradient crosslinking in a non-patent document (Polyethyleneimine / 4,4'-Bis(chloromethyl)-1,1'-biphenyl nanofiltration membrane for metal ions removal in acidwastewater, Journal of Membrane Science 614 (2020) 118497). Benefiting from the inherent positive charge of PEI and the high acid resistance of the CN single bonds generated during the reaction, the membrane maintained a high rejection rate even after 24 hours of continuous operation under strong acid conditions. However, due to the difficulty in controlling the monomer reaction using only gradient crosslinking, the growth of the membrane structure was insufficient, the degree of crosslinking was limited, the treatment effect on high hydrochloric acid solutions was poor, and the long-term antifouling effect of the membrane was unclear, making industrial application difficult.

[0009] To address the aforementioned technical problems, this invention discloses a positively charged acid-resistant nanofiltration membrane with high crosslinking density. By optimizing the secondary interfacial polymerization process with an active crosslinking agent, the selective layer structure of conventional PEI acid-resistant nanofiltration membranes is improved, the crosslinking density is increased, and the membrane separation performance and acid resistance are enhanced. This solves the problems of low monomer reactivity and insufficient interfacial reaction in the preparation of conventional PEI acid-resistant nanofiltration membranes. At the same time, it overcomes the difficulties of poor membrane retention, weak acid resistance, and susceptibility to metal contamination in the treatment of industrial high-acid electroplating wastewater. Summary of the Invention

[0010] To address the aforementioned technical problems, this invention provides a method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density.

[0011] Specifically, this is achieved through the following technical solutions: A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Polyethyleneimine, surfactant, alkaline additive and water are mixed and stirred until completely dissolved to obtain an aqueous solution; organic monomer and organic solvent are mixed and stirred until uniform to obtain an organic solution. (2) One-time interface aggregation First, immerse the ultrafiltration membrane in an aqueous solution for 0.5 to 5 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 0.5 to 2 minutes, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane is immersed in hot water at 50-60℃ for 1-3 minutes for curing, and then washed with pure water to obtain a wet nanofiltration membrane that has completed one interfacial polymerization. (4) Secondary interface aggregation The active crosslinking agent is dissolved in water to prepare a crosslinking agent solution. The nanofiltration wet membrane is immersed in the crosslinking agent solution and reacted at a constant temperature of 20~80℃ for 10~30 min. After that, the membrane is taken out and the residual solution on the surface is drained to obtain a surface-dry wet membrane.

[0012] (5) Secondary curing The surface-dried wet membrane was dried by air drying, then washed sequentially with pure water and glycerol solution, and then dried by air drying to obtain a highly cross-linked positively charged acid-resistant nanofiltration membrane.

[0013] Furthermore, the mass fractions of each component in the aqueous solution are as follows: polyethyleneimine 3.0±1.0 wt%, surfactant 0.05±0.02 wt%, alkaline additive 0.02±0.01 wt%, and the remainder is water.

[0014] Furthermore, the mass fractions of each component in the organic phase solution are as follows: organic monomers account for 0.5 ± 0.3 wt%, and the remainder is organic solvent.

[0015] Furthermore, the surfactant is at least one of sodium dodecyl sulfate (SLS), sodium dodecyl sulfonate (SDS), and sodium dodecylbenzene sulfonate (SDBS).

[0016] Furthermore, the alkaline additive is at least one of sodium hydroxide (NaOH), potassium hydroxide (KOH), and sodium carbonate (Na2CO3).

[0017] Furthermore, the organic monomer is at least one of cyanuric chloride (CC), isocyanate, and sulfonyl chloride monomer.

[0018] Furthermore, the isocyanate monomer is at least one selected from toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), phenyl dimethylene diisocyanate (XDI), isophorone diisocyanate (IPDI), and hexamethylene diisocyanate (HDI). Furthermore, the sulfonyl chloride monomer is at least one selected from 1,3-benzenedisulfonyl chloride (BDSC), 1,3,5-benzenedisulfonyl chloride (BTSC), and 1,3,6-naphthalenetrisulfonyl chloride (NTSC).

[0019] Furthermore, the organic solvent is at least one selected from methanol (MeOH), acetone (AC), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), n-hexane (n-Hex), cyclohexane (CYH), and ethylcyclohexane (ECH).

[0020] Furthermore, the ultrafiltration base membrane is at least one of polyethersulfone ultrafiltration membrane, polyvinylidene fluoride ultrafiltration membrane, polyetheretherketone ultrafiltration membrane, and polypropylene nonwoven fabric.

[0021] Furthermore, in the first interfacial polymerization step, the ultrafiltration base membrane is first immersed in an aqueous solution and kept for 0.5 to 5 minutes, then removed and drained of residual solution on the surface; then immersed in an organic solution and kept for 0.5 to 2 minutes, then removed and dried to form a functional selective layer, thus obtaining the nascent nanofiltration membrane.

[0022] Furthermore, the active crosslinking agent is dissolved in water to form an aqueous solution, wherein the mass fraction of the active crosslinking agent is 1.0 ± 1.0 wt%.

[0023] Furthermore, the active crosslinking agent has ≥2 reactive groups, enabling high crosslinking density and efficient reaction with the primary / secondary amine groups of polyethyleneimine (PEI) to form stable chemical bonds (such as ether bonds, CN bonds, quaternary ammonium bonds), while introducing positively charged or strongly hydrophilic groups, or constructing a dense three-dimensional network.

[0024] Furthermore, the active crosslinking agent is at least one of the following: ionic epoxy compounds, multifunctional halogenated hydrocarbons, hyperbranched epoxy compounds, and naturally occurring derivatives.

[0025] Furthermore, the ionic epoxy compound includes at least one of 2,3-epoxypropyltrimethylammonium chloride (GTA), 2,3-epoxypropyltriethylammonium chloride (ETA), (3-chloro-2-hydroxypropyl)trimethylammonium chloride (CHPTAC), and methacryloyloxyethyltrimethylammonium chloride (DMC).

[0026] Furthermore, the polyfunctional halogenated hydrocarbon includes at least one of 1,2-dibromoethane (DBE), 1,3-dibromopropane (DBP), α,α'-dibromo-p-xylene (DBB), tribromomethylbenzene (TBB), 1,2,4,5-tetra(bromomethyl)benzene (TBMB), α,α'-dichloromethylbenzene (BDMC), 4,4'-dichloromethyl-1,1'-biphenyl (BCMP), 3,3',5,5'-tetrachloromethylbiphenyl (TMBP), and 1,5-dichloromethylnaphthalene (DNM).

[0027] Furthermore, the hyperbranched epoxy compound includes at least one of hyperbranched glycidyl ether (HPEE) and polyethylene glycol diglycidyl ether (EGDE).

[0028] Furthermore, the naturally occurring derivatives include at least one of quaternized chitosan (QCS), quaternized cellulose nanocrystals (Q-CNCs), quaternized starch (Q-Starch), and cationic gelatin (C-Gel).

[0029] Furthermore, the secondary curing process involves a forced-air drying temperature of 60~80℃, and the second forced-air drying process involves a forced-air drying temperature of 60~80℃.

[0030] Furthermore, the solvent in the glycerol solution is pure water, and the glycerol mass fraction is (10±5wt%).

[0031] Beneficial effects In this invention, an active crosslinking agent is introduced during the secondary interfacial polymerization process. This agent generates stable carbon-nitrogen single bonds (-CN-) and ether bonds (-COC-) through specific reactions between its functional groups and the amino groups of PEI, increasing the number of interfacial polymerization crosslinking points and improving the crosslinking density of the selective layer on the membrane surface. This promotes the reduction of the effective pore size within the membrane and enhances the size sieving effect. Furthermore, by further protonating the newly formed positively charged amine bonds (such as benzylamine bonds and quaternary ammonium bonds), the positive charge on the membrane surface can be regulated, which is beneficial for strengthening the Donnan effect and improving the membrane's retention performance for multivalent cations (such as Cr3+, Ni2+, Cu2+, etc.).

[0032] The introduction of the crosslinking agent in this invention can effectively consume and reduce the number of unreacted amine groups on the membrane surface, reduce the hydrophilicity of the membrane surface, thereby alleviating the swelling caused by amine matrix protonation under acidic conditions and significantly improving the acid resistance of the membrane. In addition, the regulation and balance of the membrane surface's hydrophilicity / hydrophobicity and charge also helps to reduce the contamination of the membrane surface by organic matter and metal cations.

[0033] To address the application requirements of PEI acid-resistant nanofiltration membranes, this invention provides a positively charged acid-resistant nanofiltration membrane with high crosslinking density. This membrane improves the selective layer structure of the PEI acid-resistant nanofiltration membrane by optimizing the secondary interfacial polymerization process with an active crosslinking agent, thereby increasing the crosslinking density and enhancing membrane separation performance and acid resistance. This solves problems encountered in the development of PEI acid-resistant nanofiltration membranes, such as low monomer reactivity, insufficient interfacial reaction, low degree of selective layer crosslinking, and uneven pore structure. It also addresses issues such as poor retention, severe metal contamination, and poor acid resistance in membrane materials used for treating high-acid electroplating wastewater in industrial applications.

[0034] The secondary interfacial polymerization process of this invention introduces an active crosslinking agent to construct a high-density, highly stable covalent crosslinking network with the PEI backbone, improving membrane pore size and surface properties. This significantly enhances the initial separation accuracy of the membrane (membrane surface charge ≥ +33mV under acidic conditions of pH=3, and for divalent metal ions Mg...) 2+ Ca 2+ With a rejection rate >98%, the membrane also possesses excellent long-term acid resistance and stability, resulting in a flux decline rate of <15% and a rejection rate retention rate of >96% after 70 days of rigorous operation. Furthermore, the dense, hydrophilic properties of the membrane surface and the strong positive charge distribution can significantly reduce the adsorption of positively charged pollutants on the membrane surface, thereby effectively alleviating metal ion pollution and membrane performance degradation in the treatment of high-acid electroplating wastewater.

[0035] Instruction manual illustrations Figure 1 The separation performance of the membranes prepared in Examples 1, 7, and 10 and Comparative Examples 1, 2, and 3 was evaluated. Figure 2 The surface charge of the membranes prepared in Examples 1, 7, 10 and Comparative Examples 1, 2, 3 changes with pH. Figure 3 The separation performance of membranes for heavy metal ions (200 ppm salt solution) was prepared for Examples 1, 7, 10 and Comparative Examples 1, 2, 3. Figure 4 The results show the long-term acid resistance stability test results of the membranes prepared in Examples 1, 7, 10 and Comparative Examples 1, 2, 3.

[0036] Example Example 1 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% CC and n-Hex and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 1.0 wt% GTA in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 60°C for 30 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0037] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked positively charged acid-resistant nanofiltration membrane NF-Ar-1.

[0038] Example 2 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% CC and CYH and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 0.3 wt% DBE in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at room temperature for 20 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0039] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked, positively charged, acid-resistant nanofiltration membrane NF-Ar-2.

[0040] Example 3 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% CC and ECH and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 0.6 wt% HPEE in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 70°C for 40 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0041] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked, positively charged, acid-resistant nanofiltration membrane NF-Ar-3.

[0042] Example 4 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% CC and DMF and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 0.5 wt% QCS in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 60°C for 30 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0043] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked, positively charged, acid-resistant nanofiltration membrane NF-Ar-4.

[0044] Example 5 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% MDI and AC and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 0.8 wt% GTA in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 60°C for 30 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0045] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked, positively charged, acid-resistant nanofiltration membrane NF-Ar-5.

[0046] Example 6 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% TDI and CYH and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 0.2 wt% DBE in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at room temperature for 20 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0047] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked, positively charged, acid-resistant nanofiltration membrane NF-Ar-6.

[0048] Example 7 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% XDI and n-Hex and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 0.4 wt% HPEE in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 70°C for 40 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0049] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked, positively charged, acid-resistant nanofiltration membrane NF-Ar-7.

[0050] Example 8 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% HDI and n-Hex and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 1.0 wt% QCS in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 60°C for 30 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0051] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked, positively charged, acid-resistant nanofiltration membrane NF-Ar-8.

[0052] Example 9 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% BDSC and MeOH and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 1.5 wt% GTA in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 60°C for 30 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0053] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked positively charged acid-resistant nanofiltration membrane NF-Ar-9.

[0054] Example 10 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% BTSC and MeOH and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 0.4 wt% DBE in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at room temperature for 20 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0055] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked, positively charged, acid-resistant nanofiltration membrane NF-Ar-10.

[0056] Example 11 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% NTSC and MeOH and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 0.8 wt% HPEE in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 70°C for 40 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0057] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked positively charged acid-resistant nanofiltration membrane NF-Ar-11.

[0058] Example 12 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 2 wt% PEI, 0.05 wt% SDS, 0.02 wt% NaOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.5 wt% BTSC and MeOH and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 1.5 wt% QCS in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 60°C for 30 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0059] (5) Secondary curing The surface-drying wet membrane was dried by air at 70°C, then washed sequentially with pure water and glycerol solution, and then dried by air at 60°C to obtain the highly cross-linked positively charged acid-resistant nanofiltration membrane NF-Ar-12.

[0060] In Examples 1-12, the solvent used in the glycerol solution was pure water, and the glycerol mass fraction was 10 wt%.

[0061] Example 13 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 3 wt% PEI, 0.03 wt% SLS, 0.01 wt% KOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.2 wt% IPDI and DMAc and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyvinylidene fluoride ultrafiltration membrane in an aqueous solution for 0.5 min, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 0.5 min, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 50°C for 1 minute to cure it, and then washed with pure water to obtain a wet nanofiltration membrane that has completed one interfacial polymerization. (4) Secondary interface aggregation Dissolve 1 wt% DBP in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 20°C for 10 minutes, remove the membrane, drain the residual solution on the surface, and keep the membrane surface moist.

[0062] (5) Secondary curing The surface-dried wet membrane was dried by air at 60°C, washed with a solution of pure water and glycerol (5wt%), and then dried by air at 60°C to obtain the highly cross-linked positively charged acid-resistant nanofiltration membrane NF-Ar-13.

[0063] Example 14 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 4wt% PEI, 0.07wt% SDBS, 0.03wt% KOH and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.8wt% HDI and DMAc and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyetheretherketone ultrafiltration membrane in an aqueous solution for 5 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 2 minutes, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 60°C for 3 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 2 wt% CHPTAC in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 80°C for 30 minutes, remove the membrane, drain the residual solution on the surface, and keep the membrane surface moist.

[0064] (5) Secondary curing The surface-dried wet membrane was dried by air at 60°C, washed with a solution of pure water and glycerol (5wt%), and then dried by air at 60°C to obtain the highly cross-linked positively charged acid-resistant nanofiltration membrane NF-Ar-14.

[0065] Example 15 A method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density includes the following steps: (1) Solution preparation: Mix 3wt% PEI, 0.05wt% SDBS, 0.02wt% Na2CO3 and water, and stir until completely dissolved to obtain an aqueous solution; mix 0.2wt% isocyanate and DMAc and stir until homogeneous to obtain an organic solution. (2) One-time interface aggregation First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 5 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 2 minutes, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane was immersed in hot water at 55°C for 2 minutes to cure it. After being washed with pure water, a wet nanofiltration membrane that has completed one interfacial polymerization was obtained. (4) Secondary interface aggregation Dissolve 1 wt% DBB in water to prepare a crosslinking agent solution. Immerse the nanofiltration wet membrane in the crosslinking agent solution to ensure that the membrane is completely wetted. After reacting at 20°C for 10 minutes, remove the membrane, drain the residual solution from the surface, and keep the membrane surface moist.

[0066] (5) Secondary curing The surface-dried wet membrane was dried by air drying at 60°C, washed with a solution of pure water and glycerol (15wt%), and then dried by air drying at 60°C to obtain the highly cross-linked positively charged acid-resistant nanofiltration membrane NF-Ar-15.

[0067] In some embodiments, the active crosslinking agent is an ionic epoxy compound, which may be at least one of tribromomethylbenzene (TBB), 1,2,4,5-tetra(bromomethyl)benzene (TBMB), α,α'-dichloromethylbenzene (BDMC), 4,4'-dichloromethyl-1,1'-biphenyl (BCMP), 3,3',5,5'-tetrachloromethylbiphenyl (TMBP), and 1,5-dichloromethylnaphthalene (DNM).

[0068] In some embodiments, the active crosslinking agent is a hyperbranched epoxy compound, specifically at least one of hyperbranched glycidyl ether (HPEE) and polyethylene glycol diglycidyl ether (EGDE).

[0069] In some embodiments, the active crosslinking agent is a naturally occurring derivative, specifically at least one of quaternized chitosan (QCS), quaternized cellulose nanocrystals (Q-CNCs), quaternized starch (Q-Starch), and cationic gelatin (C-Gel).

[0070] In this invention, the crosslinking agent is preferably at least one of 2,3-epoxypropyltrimethylammonium chloride (GTA), 1,2-dibromoethane (DBE), hyperbranched glycidyl ether (HPEE), and quaternized chitosan (QCS).

[0071] Comparative Example 1 (1) Mix 4wt% PEI, 0.05wt% SDS and 0.02wt% NaOH with water and stir until completely dissolved to obtain an aqueous solution; (2) Dissolve 0.5 wt% CC in ECH and stir until homogeneous to obtain an organic phase solution; (3) First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain the residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (4) The nascent nanofiltration membrane obtained in step (3) is immersed in hot water at 60°C for 2 minutes for heat treatment, and then washed with pure water to obtain a nanofiltration wet membrane that has completed one interfacial polymerization. (5) Dissolve 2.0 wt% PEI in water to prepare an aqueous solution, and then immerse the nanofiltration wet membrane obtained in step (4) into the solution to make the membrane completely wetted. React at a constant temperature of 70℃ for 40 min, then take out the membrane, drain the residual solution on the surface, and keep the membrane surface moist. (6) The surface-dry wet membrane obtained in step (5) is dried by blowing air (70°C), then washed with pure water and glycerin (10wt%) in sequence and dried again (60°C) to obtain the conventional secondary interface polymerization acid-resistant nanofiltration membrane NF-Control-1.

[0072] Comparative Example 2 (1) Mix 4wt% PEI, 0.05wt% SDS and 0.02wt% NaOH with water and stir until completely dissolved to obtain an aqueous solution; (2) Dissolve 0.5 wt% XDI in AC and stir until homogeneous to obtain an organic phase solution; (3) First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain the residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (4) The nascent nanofiltration membrane obtained in step (3) is immersed in hot water at 60°C for 2 minutes for heat treatment, and then washed with pure water to obtain a nanofiltration wet membrane that has completed one interfacial polymerization. (5) Dissolve 1.0 wt% PEI in water to prepare an aqueous solution, and then immerse the nanofiltration wet membrane obtained in step (4) into the solution to make the membrane completely wetted. React at a constant temperature of 70℃ for 40 min, then take out the membrane, drain the residual solution on the surface, and keep the membrane surface moist. (6) The surface-dry wet membrane obtained in step (5) is dried by blowing (70°C), then washed with pure water and glycerin (10wt%) in sequence and dried again (60°C) to obtain the conventional secondary interface polymerization acid-resistant nanofiltration membrane NF-Control-2.

[0073] Comparative Example 3 (1) Mix 4wt% PEI, 0.05wt% SDS and 0.02wt% NaOH with water and stir until completely dissolved to obtain an aqueous solution; (2) Dissolve 0.5 wt% BTSC in MeOH and stir until homogeneous to obtain an organic phase solution; (3) First, immerse the polyethersulfone ultrafiltration membrane in an aqueous solution for 3 minutes, then remove it and drain the residual solution on the surface; then immerse it in an organic solution for 1 minute, remove it and air dry to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (4) The nascent nanofiltration membrane obtained in step (3) is immersed in hot water at 60°C for 2 minutes for heat treatment, and then washed with pure water to obtain a nanofiltration wet membrane that has completed one interfacial polymerization. (5) Dissolve 0.5 wt% PEI in water to prepare an aqueous solution, and then immerse the nanofiltration wet membrane obtained in step (4) into the solution to completely wet the membrane. React at a constant temperature of 70℃ for 40 min, then remove the membrane, drain the residual solution on the surface, and keep the membrane surface moist. (6) The surface-dry wet membrane obtained in step (5) is dried by blowing air (70°C), then washed with pure water and glycerin (10wt%) in sequence and dried again (60°C) to obtain the conventional secondary interface polymerization acid-resistant nanofiltration membrane NF-Control-3.

[0074] The surface properties of the films prepared in the examples and comparative examples were tested, and the results are as follows: Table 1. Characterization of membrane surface properties

[0075] The data in the table show that using an active crosslinking agent to optimize secondary interfacial polymerization can effectively reduce the functional layer of the membrane, increase the crosslinking density, reduce the pore size and porosity, which is beneficial to improving the membrane's acid resistance; at the same time, it can maintain good hydrophilicity and positive charge, which is beneficial to enhancing the membrane's selectivity and antifouling properties.

[0076] (2) Membrane separation performance The nanofiltration membranes prepared in Examples 1, 7, and 10, and Comparative Examples 1, 2, and 3 were evaluated for membrane separation performance under the following conditions: operation at 150 psi for 30 min, with a feed solution concentration of 2350 μs / cm. 2 The results of MgCl2, MgSO4, Na2SO4 and NaCl solutions are as follows: Figure 1 As shown.

[0077] from Figure 1 As can be seen, due to the high crosslinking density and small membrane pore size of each embodiment, the membrane flux is moderate and the salt rejection rate is high; while the comparative examples, due to their larger membrane pore size and greater hydrophilicity, exhibit high flux and low rejection.

[0078] (3) Membrane surface charge characteristics and heavy metal ion removal performance The surface charge of the nanofiltration membranes prepared in Examples 1, 7, and 10, and Comparative Examples 1, 2, and 3 was tested and recorded as a function of ambient pH. Simulated electroplating wastewater (containing Cr) was also used. 3+ Ni 2+ Cu 2+ Mn 2+ Zn 2+ The removal performance of the membrane for heavy metal ions was evaluated using a mixed solution (200 ppm each). The results are as follows: Figure 2 , 3 As shown.

[0079] from Figure 2 As can be seen, the surface charge of all nanofiltration membranes changed from positive to negative as pH increased. However, the membranes in each example showed a slower potential decrease under alkaline conditions and could maintain a high positive charge over a wide pH range. In contrast, the comparative example, due to its low crosslinking density and high amine exposure, exhibited the strongest positive charge under acidic conditions but showed significant protonation under alkaline conditions.

[0080] from Figure 3 As can be seen, thanks to the high cross-linking density and small pore characteristics of the membrane surface, the membrane flux of each embodiment is slightly lower, but the rejection rate of heavy metal ions is very high, making it suitable for the treatment of heavy metal wastewater with high requirements.

[0081] (4) Acid resistance of the membrane Nanofiltration membranes prepared in Examples 1, 7, and 10 and Comparative Examples 1, 2, and 3 were used to treat simulated high-acid electroplating wastewater (pH=2.0, containing Cr). 3+ Ni 2+ Cu 2+ The acid resistance stability of the membrane was evaluated by long-term stability tests (200 ppm each). The results are as follows: Figure 4 As shown.

[0082] from Figure 4 As can be seen, the performance degradation of each embodiment is relatively small, and it still retains a high performance of up to 20 L·m⁻² after 70 days of long-term treatment. -2 · h -1 ·bar -1The high flux and approximately 95% heavy metal ion rejection rate are attributed to the high crosslinking density and stable chemical bonds (quaternary ammonium bonds, ether bonds) in each embodiment, which effectively enhance the acid resistance and antifouling properties of the membrane.

[0083] It should be noted that the above embodiments and test examples are only for further elaboration and understanding of the technical solutions of the present invention, and should not be construed as further limiting the technical solutions of the present invention. Inventions that do not highlight substantive features or make significant progress by those skilled in the art still fall within the protection scope of the present invention.

Claims

1. A method for preparing a positively charged, acid-resistant nanofiltration membrane with high cross-linking density, characterized in that, Includes the following steps: (1) Solution preparation: Polyethyleneimine, surfactant, alkaline additive and water are mixed and stirred until completely dissolved to obtain an aqueous solution; organic monomer and organic solvent are mixed and stirred until uniform to obtain an organic solution. (2) One-time interface aggregation First, immerse the ultrafiltration membrane in an aqueous solution for 0.5 to 5 minutes, then remove it and drain off any residual solution on the surface; then immerse it in an organic solution for 0.5 to 2 minutes, remove it and air dry it to form a functional selective layer, thus obtaining the nascent nanofiltration membrane. (3) Curing-cleaning The nascent nanofiltration membrane is immersed in hot water at 50-60℃ for 1-3 minutes for curing, and then washed with pure water to obtain a wet nanofiltration membrane that has completed one interfacial polymerization. (4) Secondary interface aggregation An active crosslinking agent is dissolved in water to prepare a crosslinking agent solution. The nanofiltration wet membrane is immersed in the crosslinking agent solution and reacted at a constant temperature of 20~80℃ for 10~30 min. After that, the membrane is removed and the residual solution on the surface is drained to obtain a surface-dried wet membrane. The active crosslinking agent has ≥2 reactive groups and includes positively charged groups or strong hydrophilic groups. (5) Secondary curing The surface-dried wet membrane was dried by air drying, then washed sequentially with pure water and glycerol solution, and then dried by air drying to obtain a highly cross-linked positively charged acid-resistant nanofiltration membrane.

2. The method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density as described in claim 1, characterized in that, The surfactant is at least one of sodium dodecyl sulfate, sodium dodecyl sulfonate, and sodium dodecylbenzene sulfonate.

3. The method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density as described in claim 1, characterized in that, The alkaline additive is at least one of sodium hydroxide, potassium hydroxide, and sodium carbonate.

4. The method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density as described in claim 1, characterized in that, The organic monomer is at least one of cyanuric chloride, isocyanate, and sulfonyl chloride monomer.

5. The method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density as described in claim 1, characterized in that, The organic solvent is at least one of methanol, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, n-hexane, cyclohexane, and ethylcyclohexane.

6. The method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density as described in claim 4, characterized in that, The isocyanate monomer is at least one of toluene diisocyanate, diphenylmethane diisocyanate, phenyl dimethylene diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate.

7. The method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density as described in claim 4, characterized in that, The sulfonyl chloride monomer is at least one of 1,3-benzenedisulfonyl chloride, 1,3,5-benzenetrisulfonyl chloride, and 1,3,6-naphthalenetrisulfonyl chloride.

8. The method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density as described in claim 1, characterized in that, The ultrafiltration membrane is at least one of polyethersulfone ultrafiltration membrane, polyvinylidene fluoride ultrafiltration membrane, polyetheretherketone ultrafiltration membrane, and polypropylene nonwoven fabric.

9. The method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density as described in claim 1, characterized in that, The active crosslinking agent is at least one of the following: ionic epoxy compound, multifunctional halogenated hydrocarbon, hyperbranched epoxy compound, and naturally occurring derivative.

10. The method for preparing a positively charged, acid-resistant nanofiltration membrane with high crosslinking density as described in claim 9, characterized in that, The ionic epoxy compound includes at least one of 2,3-epoxypropyltrimethylammonium chloride, 2,3-epoxypropyltriethylammonium chloride, (3-chloro-2-hydroxypropyl)trimethylammonium chloride, and methacryloyloxyethyltrimethylammonium chloride; the polyfunctional halogenated hydrocarbon includes 1,2-dibromoethane, 1,3-dibromopropane, α,α'-dibromo-p-xylene, tribromomethylbenzene, 1,2,4,5-tetra(bromomethyl)benzene, and α,α'- At least one of dichloromethylbenzene, 4,4'-dichloromethyl-1,1'-biphenyl, 3,3',5,5'-tetrachloromethylbiphenyl, and 1,5-dichloromethylnaphthalene; the hyperbranched epoxy compound includes at least one of hyperbranched glycidyl ether and polyethylene glycol diglycidyl ether; the naturally occurring derivative includes at least one of quaternized chitosan, quaternized cellulose nanocrystals, quaternized starch, and cationic gelatin; preferably, the crosslinking agent selected in the embodiments of the present invention is at least one of 2,3-epoxypropyltrimethylammonium chloride, 1,2-dibromoethane, hyperbranched glycidyl ether, and quaternized chitosan.

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