Composite membrane formed by compounding loose nanofiltration layer on ultrafiltration layer and preparation method and application of composite membrane

Abstract

The invention belongs to the technical field of membrane separation, and discloses a composite membrane formed by compounding a loose nanofiltration layer on an ultrafiltration layer and a preparation method and application of the composite membrane. Polyphenol covalent bonds and non-covalent bonds (hydrogen bonds, pi-pi accumulation) are adsorbed on an ultrafiltration membrane matrix to form functional active sites, and then a low-activity aldehyde group substance and polyphenol are subjected to a Schiff base reaction to form a covalent anchoring network; then a flexible long-chain amido substance is introduced to slowly permeate into the network to carry out Michael addition reaction or Schiff base reaction, supplementary crosslinking is carried out with residual functional groups, and finally, the flexible long-chain amido substance reacts with a low-concentration aldehyde solution for a short time to carry out secondary surface crosslinking to form a stable and loose nanofiltration layer network structure. Through step-by-step and interface regulation and control design, the process is simple, the condition is mild, stable and repeated preparation can be realized, and an innovative high-performance technical solution is provided for the practical application of the membrane material for deep treatment and resource recycling of dye wastewater.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and more specifically to a method for preparing a loose nanofiltration coating for water treatment, particularly for dye separation or removal of small molecule organic matter, and its application. Background Technology

[0002] With the continuous development of the global textile industry, water pollution generated by the industry has caused significant environmental problems. Dye wastewater, characterized by high color, complex composition, and poor biodegradability, poses a serious threat to human health and aquatic life, making it a type of hazardous industrial wastewater that is difficult to treat. Therefore, appropriate treatment of textile wastewater to reduce the discharge of such highly polluting wastewater is of great importance. Compared with traditional dye separation technologies such as chemical oxidation, physical adsorption, and biological methods, membrane separation technology has advantages such as high energy utilization, low energy consumption, simple operating conditions, low cost, and environmental friendliness, and is increasingly widely used in water treatment.

[0003] Nanofiltration membranes are of great significance for treating dye wastewater; however, when used for dye separation, nanofiltration membranes often face a trade-off between separation flux and retention rate. Loose nanofiltration membranes consist of a top separation layer and a bottom support layer, but the top separation layer is more porous and has a relatively larger pore size than that prepared by traditional interfacial polymerization. Therefore, the loose separation layer can effectively retain dye molecules, achieving dye separation, while also providing high water flux and low operating pressure.

[0004] To address this, this invention prepares a loose nanofiltration layer on a polyethersulfone ultrafiltration membrane. Utilizing the high permeability and high separation and retention characteristics of this loose nanofiltration layer, it not only maintains a retention rate of over 99% for high molecular weight pollutants (molecular weight > 400 Da) such as dyes, but also increases water flux to 2-3 times that of traditional dense nanofiltration membranes, effectively breaking the "trade-off effect" between flux and selectivity. Furthermore, the surface characteristics of this loose structure significantly reduce the irreversible adsorption of pollutants, giving the membrane excellent antifouling capabilities and stable permeability during long-term operation. Under conditions of physical cleaning only, the flux recovery rate can reach over 95%. In addition, this method is simple, operates under mild conditions, and allows for stable and repeated preparation, providing an innovative high-performance technical solution for the practical application of membrane materials in the deep treatment and resource recovery of dye wastewater.

[0005] Invention patent CN 119499874 A discloses a loose nanofiltration membrane, its preparation method, and its application. This loose nanofiltration membrane is prepared by one-step interfacial polymerization of a polyacrylamide chloride organic solution and an amine monomer aqueous solution on a porous supported membrane surface. It exhibits a rejection rate of over 95% for dyes such as methylene blue and Congo red, but the measured membrane flux is only about 10–45 L·m⁻¹. -2 ·h-1 This method is not suitable for widespread application. Another invention patent, CN 111135734 B, proposes a method for preparing a gradient-crosslinked zwitterionic modified multilayer composite nanofiltration membrane. The method involves obtaining a hydrolyzed polyacrylonitrile ultrafiltration membrane using a polyacrylonitrile ultrafiltration membrane, immersing the membrane sequentially in an ultrafiltration cup with a mixed solution of glutaraldehyde and sodium dodecyl sulfate, and then immersing it in a zwitterionic polyethyleneimine solution. The membrane is then coated with a prepared tannic acid solution through stirring, and finally pressed and compacted to obtain the gradient-crosslinked zwitterionic modified multilayer composite nanofiltration membrane. While the proposed method achieves some improvement in membrane performance, its core drawback lies in the complexity and difficulty in scaling up the process. The entire process involves multiple steps, including hydrolysis, gradient immersion, stirring and coating, and pressing and compaction, resulting in poor process repeatability and making stable industrial preparation difficult.

[0006] Therefore, developing a method for synthesizing a loose nanofiltration layer membrane material on an ultrafiltration membrane is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0007] In view of this, the present invention provides a composite membrane with a loose nanofiltration layer on an ultrafiltration layer, its preparation method, and its application. It aims to solve the key contradiction in the preparation of traditional nanofiltration coatings where high flux and high rejection rate are difficult to achieve synergistically.

[0008] To achieve the above objectives, the present invention provides a method for composite a loose nanofiltration layer onto an ultrafiltration layer, the specific steps of which are as follows: (a) The ultrafiltration membrane is pretreated by immersing it in a hydrophilic organic solvent, and then immersed in deionized water for later use; (b) Prepare modification solution A using water-soluble polyphenols, and immerse the pretreated membrane in modification solution A; (c) Prepare modification solution B using a substance containing an aldehyde group (-CHO), and immerse the modified film obtained in step (b) in modification solution B; (d) Prepare a modification solution C using a water-soluble substance containing amino groups (-NH2), and immerse the modified membrane obtained in step (c) in the modification solution C; (e) Repeat step (c) and immerse the modified membrane obtained in step (d) in modified solution B; (f) After modification, place the film in a 40 ℃ drying oven for curing for 10 min.

[0009] Preferably, in step (a), the ultrafiltration membrane is selected from one of polyethersulfone ultrafiltration membrane, polyacrylonitrile ultrafiltration membrane, and polyvinylidene fluoride ultrafiltration membrane.

[0010] Preferably, in step (a), the hydrophilic organic solvent is an alcohol solvent, including at least one of methanol, ethanol, propanol, isopropanol, and ethylene glycol; preferably ethanol or isopropanol. The soaking temperature is room temperature, and the soaking time is 0.5 h. The main purpose of the pretreatment is to remove substances such as glycerol and surfactants from the membrane surface and to open the membrane pores. Then, soaking in deionized water for 0.5 h is to remove residual ethanol from the membrane surface.

[0011] Preferably, in step (b), the water-soluble polyphenolic substance is selected from at least one of dopamine, tea polyphenols, tannic acid, anthocyanins, catechol, and gallic acid. These water-soluble polyphenolic substances can self-polymerize and deposit under certain pH conditions, adsorb onto the membrane surface to complete the modification, and form active sites on the membrane surface, providing a platform for subsequent functionalization modification or secondary reactions.

[0012] Preferably, in step (b), the solvent of the modified solution A is deionized water, wherein the mass fraction of the water-soluble polyphenolic substance is 0.05-2%, and the pH of the modified solution A is weakly alkaline; the soaking temperature is 25-40℃, and the soaking time is 1-24h.

[0013] Preferably, in step (c), the aldehyde-containing substance is selected from at least one of terephthalaldehyde and trimesaldehyde; this aldehyde-containing substance can react with multiple phenolic hydroxyl sites of polyphenolic substances, thereby rapidly forming a three-dimensional cross-linked network. Because the benzene ring of terephthalaldehyde is rigid, the cross-linking points it forms are more fixed than those of flexible aldehydes (such as glutaraldehyde), which helps to construct a network structure with higher porosity and looser structure.

[0014] The solvent for the modified solution B is n-hexane, whose poor solvent properties and low surface energy strictly limit the reaction of the aldehyde to the outermost layer of the interface, avoiding deep diffusion. The mass fraction of the substance containing the aldehyde group is 0.05-0.2%; the immersion temperature is room temperature, and the time is 30-60 s.

[0015] Preferably, in step (d), the amino-containing water-soluble substance is selected from at least one of polyethyleneimine (PEI, Mw=600Da) and polyethylenepolyamine; the amino-containing substance is introduced to slowly penetrate into the preliminarily fixed tannic acid-aldehyde network. The amino group reacts with the remaining aldehyde group and the quinone group of tannic acid in the network to perform supplementary cross-linking and reinforcement. The diffusion and reaction process of the amine molecules themselves disturb the network, forming larger channels.

[0016] The solvent of the modified liquid C is deionized water, wherein the mass fraction of water-soluble substances containing amino groups is 0.1-0.5%; the soaking temperature is room temperature and the soaking time is 15-30 minutes.

[0017] Preferably, in step (e), the immersion temperature is room temperature and the time is within 10 seconds. The membrane is then immersed in the modified solution B again to increase the crosslinking density of the network, thereby increasing the mechanical strength and chemical stability of the coating, making the coating less prone to swelling or damage under high pressure or harsh chemical environments. However, the immersion time must be as short as possible to prevent additional crosslinking from filling the pores, making the pores smaller or even closed, and drastically increasing the water permeation resistance.

[0018] Furthermore, all modified materials and solvents must be mixed thoroughly and stirred at low speed on a magnetic stirrer for 30 minutes.

[0019] Furthermore, the ultrafiltration membrane needs to be thoroughly cleaned with deionized water after each soaking.

[0020] The modification principle of this invention is as follows: First, polyphenols adsorb onto the ultrafiltration membrane substrate through covalent and non-covalent bonds (hydrogen bonds, π-π stacking) to form functional active sites, which can be stably fixed on the membrane surface to construct an initial template layer; then, using a low-activity aldehyde substance, a Schiff base reaction is rapidly performed with the phenolic hydroxyl groups of the polyphenols on the template surface within a very short time to form a thin and rigid covalent anchoring network, thereby locking the polyphenol layer at the interface and preventing it from falling off, while avoiding the reaction from penetrating into the interior and damaging the pore structure; then, a flexible long-chain amine substance is introduced and slowly penetrates into the interior of the network in a solvent to undergo a Michael addition reaction or a Schiff base reaction. The modified membrane is then cross-linked with residual functional groups. Finally, it is briefly immersed in a low-concentration aldehyde solution for secondary surface cross-linking, constructing covalent connections on the surface of the formed gradient network. This significantly enhances the overall mechanical strength and chemical stability of the coating, enabling it to withstand high pressure and harsh chemical environments without swelling or structural damage. At the same time, by strictly controlling the time and degree of secondary cross-linking, pore blockage and increased permeation resistance caused by excessive cross-linking are avoided to the greatest extent. Ultimately, a robust and loosely interconnected composite nanofiltration separation layer is successfully constructed, achieving synergistic optimization of high throughput, high selectivity, and long-term operational stability.

[0021] The second objective of this invention is to apply the prepared composite membrane to the advanced treatment and resource recovery of dye wastewater.

[0022] The composite membrane of the present invention can be widely used in wastewater treatment, seawater desalination, and especially has significant advantages in membrane separation fields such as dye separation or removal of small molecule organic matter.

[0023] As can be seen from the above technical solution, compared with the prior art, the technical effects achieved by the present invention are as follows: This invention, through a step-by-step, interface-controlled design, solves the key contradiction in traditional nanofiltration coating preparation where high flux and high rejection rate are difficult to achieve synergistically. Furthermore, it significantly enhances the coating's durability and separation selectivity while maintaining high flux. Compared to existing direct blending crosslinking or unidirectional deposition techniques, this invention achieves precise control over reaction sites, crosslinking density, and network structure, successfully constructing a robust yet porous composite functional layer. This provides a universally applicable innovative strategy for the manufacture of high-performance nanofiltration membranes.

[0024] The process of this invention is relatively simple and easy to implement. It does not involve expensive modifying raw materials, and the modifying material, polyphenols, is derived from plants and contains catechol and galloyl groups. It has a variety of physicochemical properties such as absorbing ultraviolet light, scavenging free radicals, chelating metal ions, and antibacterial properties. It can adhere to the surface of various substrates through covalent and / or non-covalent bonds. It uses mild and low-reactivity substances such as terephthalaldehyde, which has a large molecular size and is conducive to forming more porous structures. The modified membrane structure is relatively simple, without involving complex multilayer structures and difficult-to-control layer thicknesses. The modified membrane has significantly improved performance in all aspects, especially in high flux, high rejection rate, and antifouling properties, which are significantly better than similar products.

[0025] This porous nanofiltration membrane not only maintains a high rejection rate for high molecular weight pollutants such as dyes while increasing water flux by 2-3 times compared to traditional dense nanofiltration membranes, but also significantly reduces irreversible adsorption of pollutants due to the surface characteristics of its porous structure. This results in excellent antifouling capabilities and stable permeability during long-term operation. The method, through stepwise, interface-controlled design, is simple, operates under mild conditions, and allows for stable and reproducible preparation. It provides an innovative, high-performance technical solution for the practical application of membrane materials in the advanced treatment and resource recovery of dye wastewater. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the reaction for preparing the modified membrane in Example 1 of the present invention. (a) Modified membrane preparation process; (b) Reaction mechanism. Figure 2 This is a SEM image of the modified membrane in Example 1 of the present invention; Figure 3 This is a water contact angle test diagram of the modified membrane in Example 1 of the present invention; Figure 4The graphs show the methylene blue dye retention performance of the modified membrane before and after modification in Example 1 of the present invention: (a) flux and retention rate performance before and after modification; (b) methylene blue filtration before and after modification. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1: A method for composited loose nanofiltration layer on ultrafiltration layer The modification principle and specific reaction of Example 1 of this invention are as follows: Figure 1 As shown, the specific preparation process is as follows: (1) The polyethersulfone ultrafiltration membrane was pretreated by immersing it in ethanol at 25°C for 0.5 h.

[0030] (2) Take out the pretreated polyethersulfone membrane, soak it in deionized water for 30 minutes, and then soak it in a tannic acid solution with pH=7.8. The pH is adjusted by Tris-HCl buffer solution with HCl at a concentration of 1.0wt% and the membrane is soaked at 25℃ for 6 hours.

[0031] (3) Take out the ultrafiltration membrane modified with tannic acid solution, wash it thoroughly with deionized water, and then soak it in a hexane solution of terephthalaldehyde with a concentration of 0.1wt% for 30s at room temperature.

[0032] (4) Take out the modified ultrafiltration membrane, rinse it with hexane and deionized water in sequence, and then soak it in an aqueous solution of polyethylene polyamine with a concentration of 0.2 wt% for 15 min at room temperature.

[0033] (5) Take out the membrane and rinse it with deionized water, then put it into the hexane solution from step (3) for 5 seconds. Finally, rinse it with hexane and deionized water and put it into an oven to dry and solidify, thus obtaining modified membrane I.

[0034] To fully understand the separation performance of the membrane before and after modification in this experiment, separation tests were conducted according to the following method: A low-pressure flat-sheet membrane experimental setup was used to measure the permeation and separation performance of PES membranes and modified membranes. The effective membrane area of ​​the membrane cell was 15.9 cm². 2100 mg of methylene blue dye was dissolved in 1000 ml of water and stirred at 700 rpm for 10 min to minimize concentration polarization. Before testing, the membrane was pre-pressurized in pure water for 30 minutes to achieve a stable pure water permeability. The permeate flow rate and dye rejection rate were calculated by measuring the volume of the solution after filtration and the absorbance before and after filtration.

[0035] The membrane permeation flux is calculated using the following formula:

[0036] Where P (L·m) -2 ·h -1 V is the osmotic flux, V (L) is the volume of the permeate, and A (m³) is the volume of the permeate. 2 ) represents the effective area of ​​the membrane during filtration, t (h) represents the filtration time, and ΔP (bar) represents the test pressure. The dye repulsion rate is calculated using the following formula:

[0037] Where R is the repulsion rate, and C... P It is the concentration of the permeate, C f This refers to the concentration of the feed solution. The concentration of the dye solution was determined using a UV-Vis spectrophotometer (UV-2550, Japan).

[0038] Example 2: A method for composited loose nanofiltration layer on ultrafiltration layer Modified materials and modification conditions: (2) Tannic acid concentration 2.0wt%, temperature 25℃, time 6h; (3) Terephthalaldehyde concentration 0.1wt%, temperature 25℃, time 30s; (4) Polyethylene polyamine concentration 0.2wt%, temperature 25℃, time 15min.

[0039] The modified membrane preparation and testing methods are the same as in Example 1, resulting in modified membrane II.

[0040] Example 3: A method for composited loose nanofiltration layer on ultrafiltration layer Modified materials and modification conditions: (2) Tannic acid concentration 1.0wt%, temperature 40℃, time 6h; (3) Terephthalaldehyde concentration 0.1wt%, temperature 25℃, time 30s; (4) Polyethylene polyamine concentration 0.2wt%, temperature 25℃, time 15min.

[0041] The modified membrane preparation and testing methods are the same as in Example 1, resulting in modified membrane III.

[0042] Example 4: A method for composited loose nanofiltration layer on ultrafiltration layer Modified materials and modification conditions: (2) Tannic acid concentration 1.0wt%, temperature 25℃, time 12h; (3) Terephthalaldehyde concentration 0.1wt%, temperature 25℃, time 30s; (4) Polyethylene polyamine concentration 0.2wt%, temperature 25℃, time 15min.

[0043] The modified membrane preparation and testing methods are the same as in Example 1, resulting in modified membrane IV.

[0044] Example 5: A method for composited loose nanofiltration layer on ultrafiltration layer Modified materials and modification conditions: (2) Tannic acid concentration 1.0wt%, temperature 25℃, time 6h; (3) Terephthalaldehyde concentration 0.1wt%, temperature 25℃, time 60s; (4) Polyethylene polyamine concentration 0.2wt%, temperature 25℃, time 15min.

[0045] The modified membrane preparation and testing methods are the same as in Example 1, resulting in modified membrane V.

[0046] Example 6: A method for composited loose nanofiltration layer on ultrafiltration layer Modified materials and modification conditions: (2) Tannic acid concentration 1.0wt%, temperature 25℃, time 6h; (3) Terephthalaldehyde concentration 0.1wt%, temperature 25℃, time 30s; (4) Polyethylene polyamine concentration 0.2wt%, temperature 25℃, time 30min.

[0047] The modified membrane preparation and testing methods are the same as in Example 1, resulting in modified membrane VI.

[0048] Example 7: A method for composited loose nanofiltration layer on ultrafiltration layer Modified materials and modification conditions: (2) Dopamine concentration 1.0wt%, temperature 25℃, time 6h; (3) Terephthalaldehyde concentration 0.1wt%, temperature 25℃, time 30s; (4) Polyethylene polyamine concentration 0.2wt%, temperature 25℃, time 15min.

[0049] The modified membrane preparation and testing methods are the same as in Example 1, resulting in modified membrane VII.

[0050] Example 8: A method for composited loose nanofiltration layer on ultrafiltration layer Modified materials and modification conditions: (2) Tea polyphenol concentration 1.0wt%, temperature 25℃, time 6h; (3) Terephthalaldehyde concentration 0.1wt%, temperature 25℃, time 30s; (4) Polyethylene polyamine concentration 0.2wt%, temperature 25℃, time 15min.

[0051] The modified membrane preparation and testing methods are the same as in Example 1, resulting in modified membrane VIII.

[0052] Example 9: A method for composited loose nanofiltration layer on ultrafiltration layer Modified materials and modification conditions: (2) Tannic acid concentration 1.0wt%, temperature 25℃, time 6h; (3) Trimethylbenzaldehyde concentration 0.1wt%, temperature 25℃, time 30s; (4) Polyethylene polyamine concentration 0.2wt%, temperature 25℃, time 15min.

[0053] The modified membrane preparation and testing methods are the same as in Example 1, resulting in modified membrane IX.

[0054] Example 10: A method for composited loose nanofiltration layer on ultrafiltration layer Modified materials and modification conditions: (2) Tannic acid concentration 1.0wt%, temperature 25℃, time 6h; (3) Terephthalaldehyde concentration 0.1wt%, temperature 25℃, time 30s; (4) Polyethyleneimine concentration 0.2wt%, temperature 25℃, time 15min.

[0055] The modified membrane preparation and testing methods are the same as in Example 1, resulting in modified membrane X.

[0056] The modified membrane materials prepared in each embodiment were subjected to performance tests according to the test methods described above. The performance test results are shown in Table 1.

[0057] Figure 1 The steps and reaction mechanism diagram for preparing the modified membrane in Example 1 of this invention are shown. Figure 2 The modified membrane is shown in SEM images. The surface is covered with a large number of white granular / clustered protrusions of varying sizes. There are no obvious large-pore cracks or defects. The overall surface has a microscopic roughness. The white protrusions are distributed with local aggregation and overall dispersion. Irregularly arranged nanoscale pores are interspersed between these protrusions, forming the porous channels of the nanofiltration layer. The substrate of the polyethersulfone ultrafiltration membrane does not show obvious macropore exposure, indicating that the nanofiltration layer has been uniformly covered on the surface of the ultrafiltration membrane.

[0058] Figure 3 The water contact angle test results of the modified membrane are shown. The test results show that the modified membrane has a certain degree of hydrophilicity, which contributes to the improvement of separation flux.

[0059] Figure 4 The study demonstrated the methylene blue dye rejection performance of the modified membrane before and after modification. The performance test results showed that the modified membrane achieved a significant improvement in rejection rate compared to the original membrane, and the rejection flux was 2-3 times that of the traditional dense nanofiltration membrane.

[0060] Table 1 Coating performance test results

[0061] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method of compounding a loose nanofiltration layer on an ultrafiltration layer, characterized in that, The specific steps are as follows: (a) the ultrafiltration membrane is immersed in a hydrophilic organic solvent for pretreatment; (b) a modifying solution A is prepared using a water-soluble polyphenol substance, and the pretreated membrane is immersed in the modifying solution A; (c) a modifying solution B is prepared using a substance containing an aldehyde group, and the modified membrane obtained in step (b) is immersed in the modifying solution B; (d) a modifying solution C is prepared using a water-soluble substance containing an amino group, and the modified membrane obtained in step (c) is immersed in the modifying solution C; (e) finally, the modified membrane obtained in step (d) is immersed in the modifying solution B; (f) after the modification is completed, the membrane is placed in a 40 ℃ air drying oven for solidification for 10 min.

2. The method of claim 1, wherein the loose nanofiltration layer is formed on the ultrafiltration layer. In step (a), the ultrafiltration membrane is selected from one of polyether sulfone ultrafiltration membrane, polyacrylonitrile ultrafiltration membrane, and polyvinylidene fluoride ultrafiltration membrane.

3. The method of claim 1, wherein the loose nanofiltration layer is formed on the ultrafiltration layer. In step (a), the hydrophilic organic solvent is an alcohol solvent, including at least one of methanol, ethanol, propanol, isopropanol, and ethylene glycol; the immersion temperature is room temperature, and the time is 0.5 h.

4. The method of claim 1, wherein the loose nanofiltration layer is formed on the ultrafiltration layer. In step (b), the water-soluble polyphenol substance is selected from at least one of dopamine, tea polyphenol, tannic acid, anthocyanin, catechol, and gallic acid.

5. The method of claim 1, wherein the loose nanofiltration layer is formed on the ultrafiltration layer. In step (b), the solvent of the modifying solution A is deionized water, the mass fraction of the water-soluble polyphenol substance is 0.05-2%, the pH of the modifying solution A is weakly alkaline; the immersion temperature is 25-40 ℃, and the time is 1-24 h.

6. The method of claim 1, wherein the loose nanofiltration layer is formed on the ultrafiltration layer. In step (c), the substance containing an aldehyde group is selected from at least one of p-phenylenediformde and triformylphenyl; the solvent of the modifying solution B is n-hexane, the mass fraction of the substance containing an aldehyde group is 0.05-0.2%; the immersion temperature is room temperature, and the time is 30-60 s.

7. The method of claim 1, wherein the loose nanofiltration layer is formed on the ultrafiltration layer. In step (d), the water-soluble substance containing an amino group is selected from at least one of polyethyleneimine and polyethylene polyamine; the solvent of the modifying solution C is deionized water, the mass fraction of the water-soluble substance containing an amino group is 0.1-0.5%; the immersion temperature is room temperature, and the time is 15-30 min.

8. The method of claim 1, wherein the loose nanofiltration layer is formed on the ultrafiltration layer. In step (e), the immersion temperature is room temperature, and the time is within 10 s.

9. The composite membrane prepared by the method according to any one of claims 1-8.

10. The application of the composite membrane according to claim 9 in the advanced treatment and resource recovery of dye wastewater.

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