A gradient cross-linked nanocomposite antifouling reverse osmosis membrane and its preparation method

Abstract

This invention discloses a gradient crosslinked nanocomposite antifouling reverse osmosis membrane and its preparation method. The reverse osmosis membrane includes a base membrane and a low-crosslinked loose layer and a high-crosslinked dense layer sequentially formed on the surface of the base membrane. The low-crosslinked loose layer is formed by crosslinking polyvinyl alcohol nanofibers and sulfonated carbon nanotubes. The high-crosslinked dense layer is formed by interfacial polymerization of acyl chloride monomers and aromatic amine monomers, and a hydrophilic metal-organic framework material is also embedded in the high-crosslinked dense layer. This reverse osmosis membrane of the present invention adopts a gradient crosslinking to construct a gradient structure of "highly crosslinked and dense surface layer + low-crosslinked and loose sublayer". The introduction of metal-organic frameworks to construct a highly crosslinked and dense surface layer can effectively improve the desalination rate, and the metal-organic framework has antibacterial activity and can inhibit biofouling. The loose polyvinyl alcohol nanofiber network and sulfonated carbon nanotubes in the sublayer can effectively promote the rapid flow of water, and the polyvinyl alcohol nanofiber network is connected to the polyamide matrix through hydrogen bonds, which is stable and efficient.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, specifically to a gradient cross-linked nanocomposite antifouling reverse osmosis membrane and its preparation method. Background Technology

[0002] Reverse osmosis (RO) technology, as one of the core membrane separation technologies in the current water treatment field, is widely used in seawater desalination, industrial pure water production, and municipal drinking water purification. However, RO membranes are highly susceptible to fouling by organic matter, colloids, microorganisms, and inorganic scaling substances in the water during actual operation, leading to problems such as decreased flux, increased energy consumption, and shortened lifespan, thus significantly increasing operating and maintenance costs. Against this backdrop, antifouling RO membranes, through surface modification (such as hydrophilic treatment and charge regulation), nanomaterial composites (such as graphene and carbon nanotubes), and biomimetic structural design, have significantly enhanced the antifouling properties, chemical stability, and desalination rate of the RO membrane surface. This effectively alleviates key bottlenecks such as flux decline, increased energy consumption, and shortened lifespan caused by RO membrane fouling. Its continuous evolution has not only driven technological innovation in fields such as seawater desalination, wastewater reuse, and high-quality drinking water production, but is also a strategic component for achieving sustainable water resource management and recycling.

[0003] However, most commercially available reverse osmosis (RO) membranes currently use aromatic polyamide separation layers, prepared through the interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC). These membranes generally exhibit an inherent "trade-off effect" between flux and desalination rate, making it difficult to achieve high water flux while maintaining a high rejection rate. Furthermore, the polyamide surface has limited hydrophilicity and a single surface charge, making it highly susceptible to adsorbing organic matter, colloids, and microorganisms during actual operation, leading to irreversible fouling and rapid flux decline. To improve performance, researchers have attempted modification strategies such as surface coating, plasma treatment, or doping with nanofillers (e.g., carbon nanotubes, metal-organic frameworks). However, these often result in non-selective defect channels within the membrane due to problems such as uneven dispersion and agglomeration of nanofillers during polymerization, or weak interfacial bonding with the polyamide matrix, leading to decreased desalination rate and even long-term stability degradation. More importantly, the polyamide layer of traditional RO membranes has a homogeneous cross-linked structure, which cannot synergistically optimize the surface density (ensuring selectivity) and sublayer porosity (promoting mass transfer), nor can it simultaneously achieve high flux, high mechanical strength, and anti-compaction capability. Therefore, there is an urgent need to develop a reverse osmosis membrane with controllable structure, stable interface, and high desalination rate, high flux, and strong anti-fouling properties. Summary of the Invention

[0004] The purpose of this invention is to provide a gradient cross-linked nanocomposite antifouling reverse osmosis membrane and its preparation method, aiming to solve the problem that traditional reverse osmosis membranes cannot simultaneously achieve high desalination rate and high water flux.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solution: This invention provides a gradient crosslinked nanocomposite antifouling reverse osmosis membrane, which includes a base membrane and a low-crosslinked loose layer and a high-crosslinked dense layer sequentially formed on the surface of the base membrane; wherein, the low-crosslinked loose layer is formed by crosslinking polyvinyl alcohol nanofibers and sulfonated carbon nanotubes; the high-crosslinked dense layer is a polyamide separation layer formed by interfacial polymerization of acyl chloride monomers and aromatic amine monomers, and a hydrophilic metal-organic framework material is also embedded in the high-crosslinked dense layer.

[0006] Specifically, the gradient cross-linked nanocomposite antifouling reverse osmosis membrane provided by this invention employs a gradient cross-linking structure to construct a "highly cross-linked and dense surface layer + lowly cross-linked and loose sublayer". The surface layer is constructed with a metal-organic framework to create a highly cross-linked and dense surface layer, which can effectively improve the desalination rate. At the same time, the metal-organic framework material has antibacterial activity and can inhibit biofouling to a certain extent. Meanwhile, the loose sublayer of polyvinyl alcohol (PVA) nanofiber network and sulfonated carbon nanotubes (s-CNTs) can effectively promote the rapid flow of water. Furthermore, the polyvinyl alcohol nanofiber network is connected to the polyamide matrix through hydrogen bonds, which is stable and efficient.

[0007] Furthermore, a gradient cross-linked nanocomposite antifouling reverse osmosis membrane is provided: the metal-organic framework is a zirconium-based metal-organic framework (Zr-MOF). Specifically, UiO-66 can be used.

[0008] This invention also provides a method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane, the method comprising the following steps: S1. Polyvinyl alcohol is dissolved in water, and then polyvinyl alcohol nanofibers are obtained by electrospinning. The nanofibers are then dispersed in water to obtain a polyvinyl alcohol nanofiber dispersion. S2. Add sulfonated carbon nanotubes to the polyvinyl alcohol nanofiber dispersion and disperse them uniformly to obtain a polyvinyl alcohol nanofiber / sulfonated carbon nanotube dispersion. S3. Add aromatic amine monomers and hydrophilic metal-organic framework materials to water and disperse them uniformly to obtain an aqueous phase liquid; S4. Add acyl chloride monomer to an organic solvent and disperse it uniformly to obtain an oil phase liquid; S5. Coat the surface of the base film with the polyvinyl alcohol nanofiber / sulfonated carbon nanotube dispersion, let it stand and then dry it to obtain a low cross-linking loose layer. S6. Immerse the base film with a low crosslinking loose layer on the surface into the aqueous phase liquid, remove it and remove the excess aqueous phase liquid, then pour the oil phase liquid onto the surface of the base film to carry out the interfacial polymerization reaction, then remove the excess oil phase liquid, and then heat it under water / ethanol mixed steam to complete the steam-assisted gradient crosslinking. S7. The membrane after gradient cross-linking is sequentially immersed in sodium bisulfite solution and glycerol solution, then removed and dried to obtain a gradient cross-linked nanocomposite antifouling reverse osmosis membrane.

[0009] Specifically, the aromatic amine monomer in the aqueous phase can be any one of o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, and benzidine, and the concentration of the aromatic amine monomer in the aqueous phase can be 2.0–4.0 wt%; the acyl chloride monomer in the oil phase can be any one of trimesoyl chloride, isophthaloyl chloride, and terephthaloyl chloride, and the organic solvent can be any one of ethylcyclohexane, cyclohexane, n-hexane, n-heptane, and isoparaffinic solvent, and the concentration of the acyl chloride monomer in the oil phase can be 0.05–0.15 wt%.

[0010] Furthermore, a method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane: the concentration of the polyvinyl alcohol nanofiber dispersion in step S1 is 0.1-0.5 wt%.

[0011] Furthermore, a method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane: the mass ratio of sulfonated carbon nanotubes to polyvinyl alcohol in step S2 is 1:(80-120).

[0012] Furthermore, a method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane: the concentration of metal-organic framework material in the aqueous phase of step S3 is 0.05-0.15 wt%.

[0013] Furthermore, a method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane: in step S5, the standing time after coating the polyvinyl alcohol nanofiber / sulfonated carbon nanotube dispersion is 20-40 seconds, and the drying temperature is 55-65℃.

[0014] Furthermore, a method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane: the heating temperature in step S6 is 50-70℃ and the heating time is 1-8 minutes.

[0015] The gradient cross-linked nanocomposite antifouling reverse osmosis membrane provided by this invention has the following characteristics: ① The loose sublayer polyvinyl alcohol (PVA) nanofiber network has strong hydrophilicity, which can effectively improve water flux; ② The dense, highly cross-linked surface layer can significantly improve the desalination rate of the membrane. At the same time, the zirconium-based metal-organic framework material (Zr-MOF) embedded in the surface layer has antibacterial properties and can inhibit biofouling, while sulfonated carbon nanotubes (s-CNTs) have a strong negative charge and can effectively repel negatively charged pollutants. The synergistic effect of Zr-MOF / s-CNTs maintains the membrane's good performance while improving its service life; ③ A dense hydrogen bond network is formed between the -OH of the sublayer polyvinyl alcohol (PVA) and the -C=O and -NH- of the polyamide. This non-covalent bond force is stronger than ordinary van der Waals forces and has better stability.

[0016] The beneficial effects of this invention are: The gradient cross-linked nanocomposite antifouling reverse osmosis membrane provided by this invention has the following advantages: ① Balancing high desalination rate and high water flux: By constructing a gradient structure of "highly cross-linked and dense surface layer + lowly cross-linked and loose sublayer", hydrophilic zirconium-based metal-organic framework materials are introduced into the surface layer. Simultaneously, the hydrophilicity of PVA and s-CNTs in the sublayer helps improve water molecule transfer efficiency, resulting in a water flux significantly superior to traditional reverse osmosis membranes while maintaining a high desalination rate; ② Strong antifouling properties: The zirconium-based metal-organic framework material in the surface layer not only promotes the formation of the hydration layer through its hydrophilicity but also possesses inherent... Antibacterial activity, synergistically inhibiting biofouling; sulfonated carbon nanotubes (s-CNTs) introduced into the sublayer provide negative charges to repel pollutants; ③ Good stability and service life: the gradient crosslinking design can effectively release internal stress, avoiding the chain segment relaxation or compaction that is prone to occur in traditional homogeneous membranes under high pressure, thus improving stability; while steam-assisted post-treatment makes the crosslinking network more uniform and dense, significantly extending the service life of the membrane; ④ Significantly enhanced mechanical strength: the polyvinyl alcohol nanofiber network embedded in the sublayer interpenetrates with the polyamide matrix through hydrogen bonds, greatly improving the elongation at break and tear resistance of the membrane. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 SEM image of the gradient cross-linked nanocomposite antifouling reverse osmosis membrane prepared in Example 1 before immersion in alkaline solution; Figure 2 SEM image of the gradient cross-linked nanocomposite antifouling reverse osmosis membrane prepared in Example 1 after immersion in alkaline solution. Detailed Implementation

[0019] 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. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0020] Example 1

[0021] This embodiment 1 provides a gradient crosslinked nanocomposite antifouling reverse osmosis membrane, which includes a polysulfone-based membrane and a low-crosslinked loose layer (sublayer) and a high-crosslinked dense layer (surface layer) sequentially formed on the surface of the polysulfone-based membrane; the low-crosslinked loose layer is formed by crosslinking polyvinyl alcohol nanofibers and sulfonated carbon nanotubes; the high-crosslinked dense layer is a polyamide separation layer formed by interfacial polymerization of acyl chloride monomers and aromatic amine monomers, and the high-crosslinked dense layer also contains a hydrophilic zirconium-based metal-organic framework material (UiO-66). The preparation method of the gradient cross-linked nanocomposite antifouling reverse osmosis membrane in Example 1 above includes the following specific steps: S1. Dissolve 50.0g of polyvinyl alcohol (PVA, Mw=85000, degree of alcoholysis 98%) in 1000.0g of deionized water and stir evenly. Then, obtain polyvinyl alcohol (PVA) nanofibers by high voltage electrospinning (voltage 18 kV, receiving distance 15cm). Then, add it to deionized water to dilute to a concentration of 0.3wt% and sonicate for 30 minutes to fully disperse it, thus obtaining a polyvinyl alcohol nanofiber dispersion. S2. Add 0.5g of sulfonated carbon nanotubes (s-CNTs) to the above polyvinyl alcohol nanofiber dispersion and sonicate for 1 hour to make it uniformly dispersed, thereby obtaining a polyvinyl alcohol nanofiber / sulfonated carbon nanotube dispersion (PVA / s-CNTs dispersion). S3. Add 25.0g of m-phenylenediamine (MPD) to 975.0g of deionized water and stir to dissolve it evenly. Then add 1.0g of zirconium-based metal-organic framework material (UiO-66) and sonicate for 1 hour to ensure that the monomers in the solution are evenly dispersed to obtain an aqueous solution for later use. S4. Add 0.1g of trimesoyl chloride (TMC) to 100.0g of n-hexane solvent and sonicate for 30 minutes to disperse it evenly, to obtain an oil phase solution for later use. S5. Coat the surface of the polysulfone-based film with the above-mentioned polyvinyl alcohol nanofiber / sulfonated carbon nanotube dispersion, let it stand for 30 seconds, and then dry it in an oven at 60°C for 5 minutes to obtain a base film with a low crosslinking loose layer (PVA nanofiber / s-CNTs network layer). S6. Immerse the base film with a low crosslinking loose layer on the surface into the above aqueous solution for 20 seconds. After taking it out, use a rubber roller to gently press away the excess aqueous solution. Immediately pour the oil solution onto the surface of the base film to carry out the interfacial polymerization reaction for 30 seconds. Then blow away the excess oil solution on the film surface with a blower. Next, place the film in a sealed stainless steel reactor and introduce water / ethanol mixed steam (3:1 v / v). Control the temperature at 60°C for 5 minutes and then take it out to complete the steam-assisted gradient crosslinking, that is, to form a high crosslinking dense layer on the low crosslinking loose layer. S7. Immerse the gradient cross-linked membrane in a 0.5wt% sodium bisulfite (NaHSO3) aqueous solution and treat it at 60℃ for 10 minutes. Then take it out, rinse it clean, and put it in deionized water. Then immerse the membrane in a 5.0wt% glycerol solution for 60 seconds. After taking it out, put it in a 70℃ oven to dry for 2 minutes to obtain a gradient cross-linked nanocomposite antifouling reverse osmosis membrane.

[0022] Comparative Example 1

[0023] The difference between Comparative Example 1 and Example 1 is that in Comparative Example 1, the standing time after coating the polyvinyl alcohol nanofiber / sulfonated carbon nanotube dispersion in step S5 is 60 seconds, while the remaining steps and raw materials remain unchanged to prepare a gradient cross-linked nanocomposite antifouling reverse osmosis membrane.

[0024] Comparative Example 2

[0025] The difference between Comparative Example 2 and Example 1 is that the interfacial polymerization reaction time in step S6 of Comparative Example 2 is 60 seconds, while the remaining steps and raw materials remain unchanged, in order to prepare a gradient cross-linked nanocomposite antifouling reverse osmosis membrane.

[0026] Comparative Example 3

[0027] The difference between Comparative Example 3 and Example 1 is that the temperature of the mixed steam treatment in step S6 of Comparative Example 3 is 30°C, while the remaining steps and raw materials remain unchanged, in order to prepare a gradient cross-linked nanocomposite antifouling reverse osmosis membrane.

[0028] Comparative Example 4

[0029] The difference between Comparative Example 4 and Example 1 is that the temperature of the mixed steam treatment in step S6 of Comparative Example 4 is 80°C, while the remaining steps and raw materials remain unchanged, in order to prepare a gradient cross-linked nanocomposite antifouling reverse osmosis membrane.

[0030] Comparative Example 5

[0031] The difference between Comparative Example 5 and Example 1 is that the mixing steam treatment time in step S6 of Comparative Example 5 is 12 minutes, while the remaining steps and raw materials remain unchanged, in order to prepare a gradient cross-linked nanocomposite antifouling reverse osmosis membrane.

[0032] Performance testing: Performance tests were conducted on the reverse osmosis membranes of Examples 1, 1-5, and commercial reverse osmosis membranes described above, including membrane surface properties, membrane surface structure stability, and antifouling tests: (1) Static contact angle tests were performed on the diaphragm before and after rinsing. The results are shown in Table 1 below: Table 1 shows the contact angles of each reverse osmosis membrane before and after water flushing.

[0033] As can be seen from the test results in Table 1, the initial static contact angle of the commercial reverse osmosis membrane was 24.83°. After rinsing with a large amount of deionized water, the static contact angle increased to 40.31°, indicating that the coating on the membrane surface peeled off after rinsing. In contrast, the static contact angle of each membrane in Example 1 and Comparative Examples 1 to 5 changed by less than 2° before and after rinsing, indicating that the prepared nanocomposite antifouling reverse osmosis membrane has good physical stability.

[0034] (2) Performance testing and stability testing: Test conditions: According to the national standard GB / T 32373-2025, on a cross-flow test bench, using 2000 ppm NaCl as the test solution, the membrane was run stably for 30 minutes under the test conditions of 225 psi operating pressure, 25℃ temperature, and pH value of 6.5-7.5, and then the membrane permeate flow rate (m0) and desalination rate were tested. Then, bovine serum albumin was selected as an organic pollutant and added to the test solution at a concentration of 50 ppm. Under the same test conditions, the membrane was run for 180 minutes, and the membrane permeate flow rate (m1) was tested. Finally, the membrane was circulated and cleaned with sodium hydroxide solution (1000 ppm) for 30 minutes, and the membrane permeate flow rate (m2) was tested. The membrane's antifouling performance was assessed by calculating the flux loss rate and flux recovery rate. The less flux decline after operation, the better the antifouling performance. Flux loss rate = (1-m... l / m0)×100%, flux recovery rate = m2 / m0×100%, the test results are shown in Table 2 below.

[0035] Table 2 shows the antifouling performance of each reverse osmosis membrane.

[0036] As can be seen from the test results in Table 2, compared with the initial performance of commercial reverse osmosis membranes, Example 1, and Comparative Examples 1-5, the flux of each membrane is greater than 50 LMH, and the desalination rate is generally greater than 99.40%. In addition, the flux of Comparative Example 1 is lower due to the long soaking time of the polyvinyl alcohol nanofiber / sulfonated carbon nanotube dispersion. In Comparative Example 2, the long interfacial polymerization time leads to excessive cross-linking, and its flux is also lower than that of Example 1, but its desalination rate can reach 99.70%. In Comparative Example 3, the low steam temperature leads to insufficient cross-linking of the membrane surface, resulting in a higher flux and a lower desalination rate. In Comparative Example 4, the high steam temperature causes the metal-organic framework to collapse, affecting the membrane flux and desalination rate. In Comparative Example 5, the excessively long steam treatment time causes the sublayer to become denser, and the flux and desalination rate change accordingly.

[0037] It can be seen that the membrane flux changed to varying degrees after organic fouling due to its structure. The flux of the commercial reverse osmosis membrane decreased by 39.56%, while the flux decrease of Examples 1, Comparative Examples 1 and 2 was less than 10%, which is due to their denser membrane structure. The flux decrease of Comparative Examples 3-5 was greater than 10%, because the membrane structure changed and the degree of cross-linking was lower. After cleaning, the membrane flux recovered somewhat. The commercial reverse osmosis membrane recovered 64.44% of its flux, while the flux recovery of Examples 1 and Comparative Examples 1-5 was significantly higher than that of the commercial reverse osmosis membrane. Moreover, except for Comparative Example 3, the flux recovery rate of the other reverse osmosis membranes was higher than 90.00%, and the overall change rate is consistent with the influence of membrane structure. In summary, this invention demonstrates that the gradient cross-linked nanocomposite antifouling reverse osmosis membrane has good antifouling performance and performance stability.

[0038] (3) Investigation of membrane surface stability: The gradient cross-linked nanocomposite antifouling reverse osmosis membrane obtained in Example 1 was immersed in a 4.0 wt% sodium hydroxide solution for 150 hours. The stability of the reverse osmosis membrane was determined by observing the morphology of the membrane surface before and after immersion. The SEM image of the reverse osmosis membrane before immersion in the alkaline solution (sodium hydroxide solution) is shown below. Figure 1 As shown, the SEM image of the reverse osmosis membrane after immersion in alkaline solution is as follows. Figure 2 As shown, through comparison Figure 1 and Figure 2 It can be observed that the surface morphology of the membrane did not change significantly before and after soaking, which indicates that the nanocomposite antifouling reverse osmosis membrane prepared by gradient crosslinking in this invention has good chemical stability.

[0039] The above-described preferred embodiments of the present invention are for illustrative purposes only and are not intended to limit the scope of the invention. Any obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A gradient cross-linked nanocomposite antifouling reverse osmosis membrane, characterized in that, The reverse osmosis membrane includes a base membrane and a low-crosslinked loose layer and a high-crosslinked dense layer sequentially formed on the surface of the base membrane; The low-crosslinked loose layer is formed by crosslinking polyvinyl alcohol nanofibers and sulfonated carbon nanotubes; the high-crosslinked dense layer is a polyamide separation layer formed by interfacial polymerization of acyl chloride monomers and aromatic amine monomers; and the high-crosslinked dense layer also contains a hydrophilic metal-organic framework material.

2. The gradient cross-linked nanocomposite antifouling reverse osmosis membrane according to claim 1, characterized in that, The metal-organic framework is a zirconium-based metal-organic framework.

3. The method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane according to claim 1 or 2, characterized in that, The method includes the following steps: S1. Polyvinyl alcohol is dissolved in water, and then polyvinyl alcohol nanofibers are obtained by electrospinning. The nanofibers are then dispersed in water to obtain a polyvinyl alcohol nanofiber dispersion. S2. Add sulfonated carbon nanotubes to the polyvinyl alcohol nanofiber dispersion and disperse them uniformly to obtain a polyvinyl alcohol nanofiber / sulfonated carbon nanotube dispersion. S3. Add aromatic amine monomers and hydrophilic metal-organic framework materials to water and disperse them uniformly to obtain an aqueous phase liquid; S4. Add acyl chloride monomer to an organic solvent and disperse it uniformly to obtain an oil phase liquid; S5. Coat the surface of the base film with the polyvinyl alcohol nanofiber / sulfonated carbon nanotube dispersion, let it stand and then dry it to obtain a low cross-linking loose layer. S6. Immerse the base film with a low crosslinking loose layer on the surface into the aqueous phase liquid, remove it and remove the excess aqueous phase liquid, then pour the oil phase liquid onto the surface of the base film to carry out the interfacial polymerization reaction, then remove the excess oil phase liquid, and then heat it under water / ethanol mixed steam to complete the steam-assisted gradient crosslinking. S7. The membrane after gradient cross-linking is sequentially immersed in sodium bisulfite solution and glycerol solution, then removed and dried to obtain a gradient cross-linked nanocomposite antifouling reverse osmosis membrane.

4. The method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane according to claim 3, characterized in that, The concentration of the polyvinyl alcohol nanofiber dispersion in step S1 is 0.1 to 0.5 wt%.

5. The method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane according to claim 3, characterized in that, The mass ratio of sulfonated carbon nanotubes to polyvinyl alcohol in step S2 is 1:(80-120).

6. The method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane according to claim 3, characterized in that, The concentration of the metal-organic framework material in the aqueous phase of step S3 is 0.05–0.15 wt%.

7. The method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane according to claim 3, characterized in that, In step S5, the standing time after coating the polyvinyl alcohol nanofiber / sulfonated carbon nanotube dispersion is 20-40 seconds, and the drying temperature is 55-65℃.

8. The method for preparing a gradient cross-linked nanocomposite antifouling reverse osmosis membrane according to claim 3, characterized in that, The heating temperature in step S6 is 50-70°C and the heating time is 1-8 minutes.

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