Nanofiltration membrane based on hydrophobic grafting interlayer modification as well as preparation method and application of nanofiltration membrane

Abstract

The invention relates to a nano-filtration membrane based on hydrophobic grafting interlayer modification and a preparation method and application thereof. The nano-filtration membrane comprises a supporting bottom membrane layer, an organic interlayer with hydrophobic groups and an active separation layer which are sequentially stacked, the active separation layer comprises an interfacial polymer formed by a water-phase monomer and an organic-phase monomer. The organic middle layer with the hydrophobic groups and the active separation layer are sequentially arranged on the supporting bottom membrane, and the introduction of the hydrophobic groups in the middle layer not only changes the surface property of the supporting bottom membrane, but also influences the interfacial polymerization process of the active separation layer, and regulates and controls the membrane surface thickness, porosity, charge property, hydrophilicity, roughness and other membrane characteristics. The water flux of the nanofiltration membrane is greatly improved while the salt rejection rate is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane separation, and in particular to a nanofiltration membrane modified by a hydrophobic grafted intermediate layer, a preparation method thereof, and an application thereof. Background Art

[0002] The nanofiltration membrane separation technology combines the sieving ability at the nanoscale and the electrostatic repulsion effect, and is widely used in the fields of brackish water desalination, hard water softening, advanced treatment of drinking water, sewage treatment and the like. Traditional nanofiltration membranes often have an inherent contradiction between ion selective retention and water flux, that is, the "Trade-off" effect, which restricts the development and application of nanofiltration membranes. In the actual application process of nanofiltration membranes, high flux corresponds to low energy consumption, which is beneficial to reducing production costs; at the same time, high retention can ensure the quality of the produced water, which is beneficial to improving production efficiency. Therefore, while ensuring the rejection rate, improving the permeation flux of nanofiltration membranes and continuously breaking through the "Trade-off" limit of nanofiltration membranes is a hot spot and a difficult point in the research of membrane separation technology.

[0003] CN117883975A discloses a high-flux salt-splitting nanofiltration membrane, which includes a support bottom membrane layer and a polyamide layer modified by grafting a polyhydroxy quaternary ammonium salt disposed on the support bottom membrane layer; the polyhydroxy quaternary ammonium salt-grafted polyamide layer includes an interfacial polymerization product of a polyhydroxy quaternary ammonium salt-grafted polyamine and a polyfunctional acyl chloride. By grafting a polyhydroxy quaternary ammonium salt onto the surface of the newly synthesized polyamide layer, the regulation of membrane characteristics such as surface hydrophilicity and charge property is realized, and surface functional modification is carried out to prepare a high-flux salt-splitting nanofiltration membrane. Preferably, the flux of the high-flux salt-splitting nanofiltration membrane is 21.11L·m -2 ·h -1 ·bar -1 , and the rejection rate of sodium sulfate is 95.15%, that is, in the preferred case, the flux of the nanofiltration membrane only reaches 21.11L·m -2 ·h -1 ·bar -1 .

[0004] CN119186261A discloses a grafted double-charged composite nanofiltration membrane, a preparation method thereof, and an application thereof. The preparation method includes the following steps: (1) grafting a positively charged aminated layer on the surface of an ultrafiltration base membrane to obtain a functionalized base membrane; (2) coating an aqueous amine solution on the surface of the functionalized base membrane obtained in step (1); (3) adding an oil phase solution of a polyfunctional acyl chloride to carry out interfacial polymerization to prepare a double-charged composite nanofiltration membrane. The prepared nanofiltration membrane can improve the removal efficiency of anions and cations in water and increase the water flux, but its water flux is only 15.3L·m -2 ·h -1 ·bar -1 .

[0005] The performance of the membrane is determined by its structure and physicochemical properties, including thickness, pore size and porosity, as well as surface charge density, hydrophilicity and roughness. In the past few decades, people have generally improved the inherent water permeability of the membrane by reducing the membrane thickness, increasing the effective surface area, increasing the membrane porosity (or internal free volume), or adding an intermediate layer. However, these methods often suffer from problems such as insufficient mechanical strength of the membrane, poor stability, and limited improvement in water flux.

[0006] Therefore, there is a need to develop a nanofiltration membrane with high flux and high salt rejection rate. Summary of the Invention

[0007] To solve the above technical problems, the present invention prepares a nanofiltration membrane with high flux and high salt rejection rate by sequentially arranging an organic intermediate layer with hydrophobic groups and an active separation layer of an interfacial polymer formed by an aqueous monomer and an organic monomer on the surface of a support bottom membrane.

[0008] To achieve this purpose, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a nanofiltration membrane modified based on a hydrophobic grafted intermediate layer, the nanofiltration membrane comprising a support bottom membrane layer, an organic intermediate layer with hydrophobic groups, and an active separation layer arranged in a stacked manner in sequence;

[0010] The active separation layer comprises an interfacial polymer formed by an aqueous monomer and an organic monomer.

[0011] In the present invention, an organic intermediate layer with hydrophobic groups and an active separation layer of an interfacial polymer formed by an aqueous monomer and an organic monomer are sequentially arranged on the support bottom membrane. The introduction of hydrophobic groups can reduce the charge property on the surface of the support bottom membrane, reduce the hydrophilicity of the support bottom membrane surface, and increase the roughness of the support bottom membrane surface. At the same time, the introduction of hydrophobic groups in the intermediate layer affects the interfacial polymerization process of the active separation layer. The intermediate layer formed by the stacking of hydrophobic groups has a beneficial effect on the storage and release of the aqueous monomer. It can make the organic intermediate layer uniformly store less aqueous monomer, and at the same time can accelerate the diffusion rate of the aqueous monomer during the subsequent interfacial reaction, inducing the organic monomer to stop reacting earlier, that is, forming a thinner active separation layer. Further, the introduction of hydrophobic groups in the intermediate layer changes the surface thickness of the nanofiltration membrane, so that the nanofiltration membrane has a higher porosity and a more uniform pore size distribution, thereby achieving a significant increase in the water flux of the nanofiltration membrane while ensuring the salt rejection rate.

[0012] As a preferred technical solution of the present invention, the material of the supporting bottom film layer includes any one or a combination of at least two of polyethersulfone, polysulfone, polyvinylidene fluoride or polytetrafluoroethylene. Typical but non-limiting combinations include: a combination of polyethersulfone and polysulfone, a combination of polyethersulfone and polyvinylidene fluoride, a combination of polyethersulfone and polytetrafluoroethylene, a combination of polysulfone and polyvinylidene fluoride, a combination of polysulfone and polytetrafluoroethylene, a combination of polyvinylidene fluoride and polytetrafluoroethylene, a combination of polyethersulfone, polysulfone and polyvinylidene fluoride, a combination of polyethersulfone, polysulfone and polytetrafluoroethylene, a combination of polyethersulfone, polyvinylidene fluoride and polytetrafluoroethylene, a combination of polysulfone, polyvinylidene fluoride and polytetrafluoroethylene, and a combination of polyethersulfone, polysulfone, polyvinylidene fluoride and polytetrafluoroethylene.

[0013] Preferably, the supporting bottom film layer has amino groups.

[0014] The supporting bottom film layer of the present invention has amino groups, which can react with the carboxyl groups on the organic intermediate layer with hydrophobic groups, so that the organic intermediate layer with hydrophobic groups is grafted on the supporting bottom film.

[0015] Preferably, the hydrophobic group includes phenyl and / or alkyl.

[0016] In the present invention, an organic intermediate layer with hydrophobic groups is provided on the supporting bottom film. The hydrophobic groups can increase the chemical stability and thermal stability of the supporting bottom film. At the same time, the introduction of hydrophobic groups will affect the pore size and distribution of the supporting bottom film, thereby optimizing the porosity and pore size uniformity of the nanofiltration membrane. Further, the introduction of hydrophobic groups makes the aqueous monomers more likely to be enriched at the interface during the formation process of the active separation layer, thereby increasing the collision probability between monomers and promoting the interfacial polymerization reaction. At the same time, the hydrophobic groups also participate in the formation process of the interfacial polymer. The hydrophobic groups can be embedded in the polymer chain to form a specific cross-linked structure or microphase separation structure. For example, for hydrophobic groups related to phenyl, such as p-aminobenzoic acid, the π-π interaction between benzene rings will induce it to form a funnel-shaped ordered arrangement; hydrophobic groups related to alkyl will aggregate through van der Waals forces and hydrophobic effects. These structures will increase the separation performance and mechanical properties of the nanofiltration membrane to extend the service life of the nanofiltration membrane.

[0017] Preferably, the aqueous monomers include any one or a combination of at least two of piperazine, m-phenylenediamine or polyethyleneimine. Typical but non-limiting combinations include: a combination of piperazine and m-phenylenediamine, a combination of piperazine and polyethyleneimine, a combination of m-phenylenediamine and polyethyleneimine, and a combination of piperazine, m-phenylenediamine and polyethyleneimine.

[0018] Preferably, the organic phase monomer includes any one or a combination of at least two of trimesoyl chloride, terephthaloyl chloride, or phthaloyl chloride. Typical but non-limiting combinations include: a combination of trimesoyl chloride and terephthaloyl chloride, a combination of trimesoyl chloride and phthaloyl chloride, a combination of terephthaloyl chloride and phthaloyl chloride, and a combination of trimesoyl chloride, terephthaloyl chloride, and phthaloyl chloride.

[0019] Preferably, the mass ratio of the aqueous phase monomer to the organic phase monomer is (0.5 - 5):1. For example, it can be 0.5:1, 1:1, 2:1, 3:1, 4:1, or 5:1, but is not limited to the listed values. Other unlisted values within the above numerical range are equally applicable.

[0020] As a preferred technical solution of the present invention, the thickness of the supporting bottom film layer is 100 - 300 μm. For example, it can be 100 μm, 150 μm, 200 μm, 250 μm, or 300 μm, but is not limited to the listed values. Other unlisted values within the above numerical range are equally applicable.

[0021] Preferably, the thickness of the organic intermediate layer with a hydrophobic group is 30 - 100 nm. For example, it can be 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, or 100 nm, but is not limited to the listed values. Other unlisted values within the above numerical range are equally applicable.

[0022] Preferably, the thickness of the active separation layer is 10 - 30 nm. For example, it can be 10 nm, 15 nm, 20 nm, 25 nm, or 30 nm, but is not limited to the listed values. Other unlisted values within the above numerical range are equally applicable.

[0023] In the present invention, the boundary between the supporting bottom film layer and the organic intermediate layer with a hydrophobic group is located at the position where the organic intermediate layer with a hydrophobic group starts to graft on the surface of the supporting bottom film layer. The boundary between the organic intermediate layer with a hydrophobic group and the active separation layer is located at the position where the active separation layer starts to form, that is, the position where a chemical reaction occurs and causes the separation layer material to deposit on the surface of the organic intermediate layer. The above boundaries can be determined by observing the microscopic structure of the cross-section of the nanofiltration membrane.

[0024] In a second aspect, the present invention provides a method for preparing a nanofiltration membrane modified based on a hydrophobic grafted intermediate layer according to the first aspect. The preparation method includes the following steps:

[0025] (1) Immerse the supporting bottom film with amino groups in a grafting solution containing a hydrophobic substance to obtain a membrane containing an organic intermediate layer;

[0026] (2) performing an interfacial polymerization reaction of aqueous phase monomers and organic phase monomers on the surface of the membrane containing the organic intermediate layer to prepare an active separation layer, thereby obtaining a membrane containing the active separation layer;

[0027] (3) Heat-treating the membrane containing the active separation layer to obtain the nanofiltration membrane.

[0028] The present invention first grafts a hydrophobic substance having a hydrophobic group onto a support base membrane having an amine group to obtain a membrane containing an organic intermediate layer, wherein the introduction of the hydrophobic group can reduce the charge on the surface of the support base membrane, reduce the hydrophilicity on the surface of the support base membrane, and increase the roughness of the surface of the support base membrane. Then, an interfacial polymerization reaction of an aqueous phase monomer and an organic phase monomer is carried out on the surface of the membrane containing the organic intermediate layer to prepare an active separation layer. The introduction of the hydrophobic group in the intermediate layer affects the process of interfacial polymerization of the active separation layer and changes the surface thickness of the nanofiltration membrane, so that the nanofiltration membrane has a higher porosity and a more uniform pore size distribution. Thirdly, the membrane containing the active separation layer is heat-treated. The heat treatment process can remove the residual organic solvent in the membrane containing the active separation layer to improve the purity of the nanofiltration membrane and increase the service life of the nanofiltration membrane. At the same time, the heat treatment can enhance the thermal stability and chemical stability of the membrane, so that the nanofiltration membrane can maintain good performance during long-term use and is not easily affected by environmental factors and degraded or failed. Furthermore, the heat treatment can also change the internal structure and performance of the membrane, making the voids inside the nanofiltration membrane more uniform and the pore size more consistent, so as to improve the water flux of the nanofiltration membrane.

[0029] As a preferred technical solution of the present invention, the preparation method further comprises pre-treating the supporting base film.

[0030] Preferably, the pre-treatment includes a cleaning treatment and a surface amination treatment performed sequentially.

[0031] Preferably, the cleaning agent of the cleaning treatment comprises any one of isopropyl alcohol, ethanol, acetone or dimethylformamide, or a combination of at least two of them, wherein typical but non-limiting combinations include: a combination of isopropyl alcohol and ethanol, a combination of isopropyl alcohol and acetone, a combination of isopropyl alcohol and dimethylformamide, a combination of ethanol and acetone, a combination of ethanol and dimethylformamide, a combination of acetone and dimethylformamide, a combination of isopropyl alcohol, ethanol and acetone, a combination of isopropyl alcohol, ethanol and dimethylformamide, a combination of isopropyl alcohol, acetone and dimethylformamide, a combination of ethanol, acetone and dimethylformamide, a combination of isopropyl alcohol, ethanol, acetone and dimethylformamide.

[0032] Preferably, the volume concentration of the cleaning agent is 20-40%, for example, 20%, 25%, 30%, 35% or 40%, but is not limited to the listed values, and other values not listed within the above numerical range are also applicable.

[0033] Preferably, the steps of the surface amination treatment include: immersing the cleaned support bottom film in an amination solution.

[0034] In the present invention, the support bottom film is subjected to surface amination treatment to make the surface of the support bottom film have amino groups, and then the support bottom film with amino groups is immersed in a grafting solution containing a hydrophobic substance. Among them, the hydrophobic substance has a carboxyl group, and the carboxyl group in the hydrophobic substance undergoes a polycondensation reaction with the amino group on the support bottom film, so that the hydrophobic substance can be grafted on the support bottom film.

[0035] Preferably, the aminating agent for the surface amination treatment includes any one or a combination of at least two of diethylenetriamine, dopamine or ethylenediamine. Among them, typical but non-limiting combinations include: a combination of diethylenetriamine and dopamine, a combination of diethylenetriamine and ethylenediamine, a combination of dopamine and ethylenediamine, and a combination of diethylenetriamine, dopamine and ethylenediamine.

[0036] Preferably, the volume concentration of the aminating agent in the amination solution is 5-20%, for example, it can be 5%, 10%, 15% or 20%, but is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable.

[0037] Preferably, the temperature of the surface amination treatment is 80-100 °C, for example, it can be 80 °C, 85 °C, 90 °C, 95 °C or 100 °C, but is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable.

[0038] Preferably, the time of the surface amination treatment is 35-50 h, for example, it can be 35 h, 40 h, 45 h or 50 h, but is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable.

[0039] As a preferred technical solution of the present invention, the grafting solution containing a hydrophobic substance includes a buffer solution and a hydrophobic substance.

[0040] Preferably, the solute of the buffer solution includes a grafting catalyst and a buffer agent.

[0041] Preferably, the solvent of the buffer solution includes ultrapure water.

[0042] The ultrapure water used in the present invention, also known as UP water, refers to water with a resistivity of 18 MΩ·cm (25 °C). In this kind of water, except for water molecules (H2O), there are almost no impurities, no bacteria, viruses, organic substances such as chlorinated dioxins, and of course no mineral trace elements required by the human body, that is, water that has almost removed all atoms except oxygen and hydrogen.

[0043] Preferably, the grafting catalyst comprises 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and / or N-hydroxysuccinimide.

[0044] Preferably, the buffer comprises morpholineethanesulfonic acid and / or sodium acetate.

[0045] Preferably, the hydrophobic substance has both a hydrophobic group and a carboxyl group.

[0046] Preferably, the hydrophobic substance comprises p-aminobenzoic acid and / or 6-aminohexanoic acid.

[0047] Preferably, the concentration of the hydrophobic substance in the grafting solution containing the hydrophobic substance is 0.5-1.5 mg / mL. For example, it can be 0.5 mg / mL, 0.7 mg / mL, 1 mg / mL, 1.3 mg / mL, or 1.5 mg / mL. However, it is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable.

[0048] Preferably, the molar ratio of the carboxyl group of the hydrophobic substance to the amino group on the supported bottom membrane after surface amination treatment in the grafting solution containing the hydrophobic substance is (10-20):1. For example, it can be 10:1, 12:1, 14:1, 16:1, 18:1, or 20:1. However, it is not limited to the listed values, and other unlisted values within the above numerical range are equally applicable.

[0049] By defining the molar ratio of the carboxyl group of the hydrophobic substance to the amino group on the supported bottom membrane as (10-20):1, the present invention can enable the carboxyl group of the hydrophobic substance to effectively react with the amino group on the supported bottom membrane, form stable chemical bonds, and thus achieve a good grafting effect. This helps to ensure the uniform distribution and firm attachment of the grafting layer on the supported bottom membrane, improve the overall stability and durability of the nanofiltration membrane, and extend the service life of the nanofiltration membrane. If the molar ratio of the carboxyl group of the hydrophobic substance to the amino group on the supported bottom membrane is less than 10:1, it will cause some amino groups to fail to react with the carboxyl group, resulting in un-grafted areas, which will affect the uniformity and integrity of the grafting layer and thus reduce the performance of the nanofiltration membrane. In addition, the unreacted amino groups may also become potential active sites, triggering other unnecessary chemical reactions. If the molar ratio of the carboxyl group of the hydrophobic substance to the amino group on the supported bottom membrane is greater than 20:1, it will cause over-grafting, which will increase the thickness and density of the grafting layer, thus hindering the penetration and transmission of water molecules and reducing the permeation flux of the nanofiltration membrane. At the same time, excessive grafting may lead to instability in the internal structure of the grafting layer, prone to problems such as cracks and detachment, which will seriously affect the service life and performance stability of the nanofiltration membrane. Further, using too much hydrophobic substance not only increases the raw material cost but also may increase the energy consumption and waste emissions during the treatment process, causing adverse effects on the environment.

[0050] Preferably, the soaking time in step (1) is 2 - 10 h. For example, it can be 2 h, 4 h, 6 h, 8 h or 10 h, but is not limited to the listed values. Other unlisted values within the above numerical range are equally applicable.

[0051] By defining the soaking time in step (1) of the present invention as 2 - 10 h, sufficient time can be provided for the chemical reaction between amino groups and carboxyl groups to form chemical bonds, thereby achieving grafting, which helps to ensure the uniformity and integrity of the grafted layer. At the same time, by controlling the soaking time, the grafted layer can have an appropriate thickness and density, which helps to maintain the high permeation flux and selectivity of the nanofiltration membrane. If the soaking time is less than 2 h, the short soaking time will lead to insufficient chemical reaction between amino groups and carboxyl groups, forming ungrafted areas, which will affect the uniformity and integrity of the grafted layer, and further reduce the performance of the nanofiltration membrane. At the same time, the short soaking time will result in insufficient thickness of the grafted layer, thus affecting the service life of the nanofiltration membrane; if the soaking time is greater than 10 h, the long soaking time will lead to over - grafting, increasing the thickness and density of the grafted layer, which will hinder the penetration and transmission of water molecules, reducing the permeation flux of the nanofiltration membrane. At the same time, excessive grafting may lead to instability of the internal structure of the grafted layer, prone to problems such as cracks and peeling, which will seriously affect the service life and performance stability of the nanofiltration membrane. Further, the too - long treatment time will also reduce production efficiency.

[0052] As a preferred technical solution of the present invention, the interfacial polymerization reaction includes: first wetting the membrane containing the organic intermediate layer with an aqueous phase containing an aqueous phase monomer, then removing the excess liquid, and then wetting the membrane containing the organic intermediate layer with an organic phase containing an organic phase monomer, and then removing the excess liquid to obtain the membrane containing the active separation layer.

[0053] In the present invention, the excess liquid refers to the excessive liquid that fails to penetrate and be fixed inside the membrane structure successfully but remains on the membrane surface or in the membrane pores. Among them, a roller is used to remove the excess liquid on the surface after the first wetting, and the excess liquid after the second wetting can be directly poured out.

[0054] Preferably, the mass concentration of the aqueous phase monomer in the aqueous phase is 0.1 - 1%. For example, it can be 0.1%, 0.2%, 0.4%, 0.6%, 0.8% or 1%, but is not limited to the listed values. Other unlisted values within the above numerical range are equally applicable.

[0055] Preferably, the temperature of the aqueous phase is 20 - 25°C. For example, it can be 20°C, 21°C, 22°C, 23°C, 24°C or 25°C, but is not limited to the listed values. Other unlisted values within the above numerical range are equally applicable.

[0056] Preferably, the time of the first infiltration is 60 - 120 s, for example, it can be 60 s, 70 s, 80 s, 90 s, 100 s, 110 s or 120 s, but not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0057] Preferably, the mass concentration of the organic phase monomer in the organic phase is 0.05 - 0.5%, for example, it can be 0.05%, 0.1%, 0.2%, 0.3%, 0.4% or 0.5%, but not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0058] Preferably, the temperature of the organic phase is 20 - 25 °C, for example, it can be 20 °C, 21 °C, 22 °C, 23 °C, 24 °C or 25 °C, but not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0059] Preferably, the organic solvent in the organic phase includes any one or a combination of at least two of n - hexane, cyclohexane or n - heptane. Typical but non - limiting combinations include: the combination of n - hexane and cyclohexane, the combination of n - hexane and n - heptane, the combination of cyclohexane and n - heptane, the combination of n - hexane, cyclohexane and n - heptane.

[0060] Preferably, the time of the second infiltration is 15 - 60 s, for example, it can be 15 s, 20 s, 30 s, 40 s, 50 s or 60 s, but not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0061] Preferably, the volume ratio of the aqueous phase of the aqueous - phase monomer to the organic phase of the organic - phase monomer containing the organic - phase monomer is (0.8 - 1.2):1, for example, it can be 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, but not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0062] As a preferred technical solution of the present invention, the temperature of the heat treatment is 50 - 80 °C, for example, it can be 50 °C, 60 °C, 70 °C or 80 °C, but not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0063] Preferably, the time of the heat treatment is 2 - 10 min, for example, it can be 2 min, 4 min, 6 min, 8 min or 10 min, but not limited to the listed values, and other unlisted values within the above value range are equally applicable.

[0064] As a preferred technical solution of the present invention, the method includes the following steps:

[0065] (1) Clean the support bottom film with a cleaning agent, and then wash it with pure water and set it aside for later use;

[0066] (2) At 80 - 100 °C, immerse the support bottom film in an aminating agent for 35 - 50 h to perform surface amination treatment on the support bottom film;

[0067] (3) Immerse the support bottom film after surface amination treatment in a grafting solution containing 0.5 - 1.5 mg / mL of hydrophobic substances for 2 - 10 h to obtain a film containing an organic intermediate layer;

[0068] (4) Immerse the film containing the organic intermediate layer in an aqueous phase with a mass concentration of the aqueous monomer of 0.1 - 1% for 60 - 120 s, then remove the excess liquid. Then, immerse the film containing the organic intermediate layer in an organic phase with a mass concentration of the organic monomer of 0.05 - 0.5% for 15 - 60 s, and then remove the excess liquid to obtain a film containing an active separation layer;

[0069] (5) At 50 - 80 °C, heat - treat the film containing the active separation layer for 2 - 10 min to obtain the nanofiltration membrane.

[0070] The pure water used in the present invention refers to H2O without impurities, which is the water filtered by a pure water machine (RO machine, model Milli - Q Advantage).

[0071] In a third aspect, the present invention provides an application of the nanofiltration membrane modified based on a hydrophobic grafted intermediate layer according to the first aspect in the desalination of brackish water, softening of hard water, drinking water treatment, or sewage treatment.

[0072] The nanofiltration membrane modified based on a hydrophobic grafted intermediate layer provided by the present invention has a high salt rejection rate and a high water flux. It has significant advantages such as high efficiency, energy conservation, environmental protection, good effluent water quality, simple operation, and strong pollution resistance in the desalination of brackish water, softening of hard water, drinking water treatment, or sewage treatment. These advantages make the nanofiltration membrane have broad application prospects and market value in the desalination of brackish water, softening of hard water, drinking water treatment, or sewage treatment.

[0073] Compared with the prior art, the present invention has at least the following beneficial effects:

[0074] (1) The preparation method of the nanofiltration membrane modified based on a hydrophobic grafted intermediate layer provided by the present invention uses surface chemical grafting technology and classical interfacial polymerization to synthesize the required nanofiltration membrane. The whole operation process is simple, and at the same time, it has good compatibility with the existing membrane preparation process;

[0075] (2) The nanofiltration membrane modified by a hydrophobic grafted intermediate layer provided by the present invention is obtained by sequentially disposing an organic intermediate layer with a hydrophobic group and an active separation layer on a support substrate membrane with an amino group. The obtained nanofiltration membrane has a high water flux and a high salt rejection rate. The water flux can reach more than 202 LMH, the sodium sulfate rejection rate can reach more than 97.3%, and the separation factor of Na2SO4 / NaCl can reach more than 31. It has good application prospects in brackish water desalination, hard water softening, drinking water treatment, or sewage treatment. Description of the Drawings

[0076] Figure 1 It is the change diagram of XPS obtained by argon ion (Ar + ) sputtering etching of the nanofiltration membrane obtained in Example 3 of the present invention as the etching time changes.

[0077] Figure 2 It is the TEM diagram of the nanofiltration membrane provided in Example 3 of the present invention. Detailed Description of the Invention

[0078] The technical solution of the present invention will be further described below in conjunction with the drawings and through specific embodiments. However, the following examples are only simple examples of the present invention and do not represent or limit the scope of the claimed protection of the present invention. The scope of protection of the present invention shall be subject to the claims.

[0079] Example 1

[0080] This example provides a nanofiltration membrane modified by a hydrophobic grafted intermediate layer. The nanofiltration membrane includes a polyethersulfone membrane with an amino group (brand: PES-UP150) having a thickness of 200 μm, an organic intermediate layer containing a benzene ring having a thickness of 70 nm, and an active separation layer having a thickness of 20 nm, which are sequentially stacked. The active separation layer includes an interfacial polymer formed by piperazine and trimesoyl chloride in a mass ratio of 2:1.

[0081] This example also provides a preparation method for the nanofiltration membrane modified by a hydrophobic grafted intermediate layer. The preparation method includes the following steps:

[0082] (1) Clean the support substrate membrane with isopropanol with a volume concentration of 30%, and then wash it with pure water and set it aside;

[0083] (2) Immerse the support substrate membrane in an aqueous solution of diethylenetriamine with a volume concentration of 10% at 90 °C for 40 h to perform surface amination treatment on the support substrate membrane so that the support substrate membrane has an amino group;

[0084] (3) The supported bottom membrane after surface amination treatment was immersed in a grafting solution containing 1 mg / mL of p-aminobenzoic acid for 6 h to obtain a membrane with an organic intermediate layer. The preparation method of the grafting solution containing p-aminobenzoic acid was as follows: 500 mg of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and 333.3 mg of N-hydroxysuccinimide were respectively dissolved in 500 mL of morpholineethanesulfonic acid buffer (0.1 M, pH = 5.0), then 500 mg of p-aminobenzoic acid was added, and after stirring, it was placed in a refrigerator at 4 °C for 4 h for standby. The molar ratio of the carboxyl group in p-aminobenzoic acid to the amino group on the supported bottom membrane after surface amination treatment was 15:1;

[0085] (4) The membrane with an organic intermediate layer was infiltrated with water containing 0.5% piperazine by mass for 90 s, and the excess liquid was removed using a roller. Then, the membrane with an organic intermediate layer was infiltrated with an organic phase containing 0.3% trimesoyl chloride by mass and n-hexane as the solvent for 30 s, and then the excess liquid was poured out to obtain a membrane with an active separation layer. Among them, the volume ratio of the aqueous phase containing piperazine to the organic phase containing trimesoyl chloride was 1:1;

[0086] (5) At 60 °C, the membrane with an active separation layer was heat-treated for 5 min to obtain the nanofiltration membrane.

[0087] Example 2

[0088] This example provides a nanofiltration membrane modified based on a hydrophobic grafted intermediate layer. The nanofiltration membrane includes a polyethersulfone membrane (grade PES-UP150) with amino groups and a thickness of 100 μm, an organic intermediate layer containing alkyl groups and a thickness of 100 nm, and an active separation layer with a thickness of 30 nm, which are stacked in sequence. The active separation layer includes an interfacial polymer formed by piperazine and terephthaloyl chloride with a mass ratio of 0.5:1.

[0089] This example also provides a preparation method of the nanofiltration membrane modified based on a hydrophobic grafted intermediate layer. The preparation method includes the following steps:

[0090] (1) The supported bottom membrane was cleaned with ethanol with a volume concentration of 20%, and then washed with pure water and reserved;

[0091] (2) At 80 °C, the supported bottom membrane was immersed in an aqueous dopamine solution with a volume concentration of 5% for 50 h to perform surface amination treatment on the supported bottom membrane so that the supported bottom membrane had amino groups;

[0092] (3) The supported bottom membrane after surface amination treatment was immersed in a grafting solution containing 0.5 mg / mL 6 - aminohexanoic acid for 10 h to obtain a membrane with an organic intermediate layer. The preparation method of the grafting solution containing 6 - aminohexanoic acid is as follows: Dissolve 500 mg of 1 - (3 - dimethylaminopropyl) - 3 - ethylcarbodiimide hydrochloride in 500 mL of morpholineethanesulfonic acid buffer (0.1 M, pH = 5.0), then add 250 mg of 6 - aminohexanoic acid, stir and refrigerate at 4 °C in a refrigerator for 4 h for later use. The molar ratio of the carboxyl group in 6 - aminohexanoic acid to the amino group on the supported bottom membrane after surface amination treatment is 10:1;

[0093] (4) The membrane with an organic intermediate layer was infiltrated with water containing 0.1% piperazine for 120 s, and the excess liquid was removed using a roller. Then, the membrane with an organic intermediate layer was infiltrated with an organic phase containing 0.01% terephthaloyl chloride and cyclohexane as the solvent for 60 s, and then the excess liquid was poured off to obtain a membrane with an active separation layer. Among them, the volume ratio of the aqueous phase containing piperazine to the organic phase containing terephthaloyl chloride is 0.8:1;

[0094] (5) At 80 °C, the membrane with an active separation layer was heat - treated for 2 min to obtain the nanofiltration membrane.

[0095] Example 3

[0096] This example provides a nanofiltration membrane modified based on a hydrophobic grafted intermediate layer. The nanofiltration membrane includes a polyethersulfone membrane with amino groups (grade PES - UP150) having a thickness of 300 μm, an organic intermediate layer containing benzene rings with a thickness of 30 nm, and an active separation layer with a thickness of 10 nm, which are stacked in sequence. The active separation layer includes an interfacial polymer formed by piperazine and phthaloyl chloride with a mass ratio of 5:1. XPS test and TEM test were carried out on the nanofiltration membrane, and the test results are respectively as Figure 1 and Figure 2 shown.

[0097] This example also provides a preparation method of the nanofiltration membrane modified based on a hydrophobic grafted intermediate layer. The preparation method includes the following steps:

[0098] (1) The supported bottom membrane was cleaned with acetone with a volume concentration of 40%, and then washed with pure water and reserved for later use;

[0099] (2) At 100 °C, the supported bottom membrane was immersed in an aqueous solution of ethylenediamine with a volume concentration of 20% for 35 h to perform surface amination treatment on the supported bottom membrane so that the supported bottom membrane has amino groups;

[0100] (3) The supported bottom membrane after surface amination treatment was immersed in a grafting solution containing 1.5 mg / mL of p-aminobenzoic acid for 2 h to obtain a membrane with an organic intermediate layer. The preparation method of the grafting solution containing hydrophobic substances was as follows: 333.3 mg of N-hydroxysuccinimide was dissolved in 500 mL of morpholineethanesulfonic acid buffer (0.1 M, pH = 5.0), then 750 mg of p-aminobenzoic acid was added. After stirring, it was placed in a refrigerator at 4 °C for 4 h for standby. The molar ratio of the carboxyl group in p-aminobenzoic acid to the amino group on the supported bottom membrane after surface amination treatment was 20:1;

[0101] (4) The membrane with an organic intermediate layer was infiltrated with an aqueous phase containing 1% by mass of piperazine for 60 s, and the excess liquid was removed using a roller. Then, the membrane with an organic intermediate layer was infiltrated with an organic phase containing 0.5% by mass of phthaloyl chloride and n-heptane as the solvent for 15 s, and then the excess liquid was poured off to obtain a membrane with an active separation layer. The volume ratio of the aqueous phase containing piperazine to the organic phase containing phthaloyl chloride was 1.2:1;

[0102] (5) The membrane with an active separation layer was heat-treated at 50 °C for 10 min to obtain the nanofiltration membrane.

[0103] Figure 1 The XPS spectrum of the nanofiltration membrane is shown. Through XPS depth profiling, the longitudinal structure of the nanofiltration membrane at a certain depth can be explored. Each layer was etched for 30.888 s, and the theoretical depth was 10 nm. The depth profiling of O1s was subjected to peak fitting. An acyl chloride group and a carboxyl group were detected on the surface, indicating that the surface of the nanofiltration membrane was a typical polyamide active layer; when etching the first layer, new groups O=S and C-O were detected, and at the same time, the carboxyl group disappeared because the active separation layer of the nanofiltration membrane had been penetrated, that is, the thickness of the active separation layer ≤ 10 nm. When etching the third layer, an amide group from the organic intermediate layer and S=O and C-O from the polyethersulfone membrane were detected, indicating that the organic intermediate layer was partially embedded in the pores of the polyethersulfone membrane; when etching to the fourth layer, it was found that the content of O-C decreased and the amide group disappeared, indicating that the organic intermediate layer began to disappear, that is, the thickness of the organic intermediate layer was between 30 - 40 nm.

[0104] Figure 2 The TEM image of the nanofiltration membrane is shown. It can be seen from the figure that the thickness of the active separation layer of the nanofiltration membrane is 10 nm.

[0105] Example 4

[0106] This example provides a nanofiltration membrane modified with a hydrophobic grafted intermediate layer. The difference from Example 1 is only that, except that the molar ratio of the carboxyl group in p-aminobenzoic acid to the amino group on the supported bottom membrane after surface amination treatment is 5:1, the rest are the same as in Example 1.

[0107] Example 5

[0108] This example provides a nanofiltration membrane modified based on a hydrophobic grafted intermediate layer. The difference from Example 1 is only that, except that the molar ratio of the carboxyl group in p-aminobenzoic acid to the amino group on the supported bottom membrane after surface amination treatment is 25:1, the rest are the same as in Example 1.

[0109] Example 6

[0110] This example provides a nanofiltration membrane modified based on a hydrophobic grafted intermediate layer. The difference from Example 1 is only that, except that the supported bottom membrane after surface amination treatment is immersed in a grafting solution containing 1 mg / mL p-aminobenzoic acid for 0.5 h, the rest are the same as in Example 1.

[0111] Example 7

[0112] This example provides a nanofiltration membrane modified based on a hydrophobic grafted intermediate layer. The difference from Example 1 is only that, except that the supported bottom membrane after surface amination treatment is immersed in a grafting solution containing 1 mg / mL p-aminobenzoic acid for 15 h, the rest are the same as in Example 1.

[0113] Comparative Example 1

[0114] This comparative example provides a nanofiltration membrane. The difference from Example 1 is only that, except that the nanofiltration membrane does not contain an organic intermediate layer with a hydrophobic group, that is, step (3) is not included in the preparation method, the rest are the same as in Example 1.

[0115] Comparative Example 2

[0116] This comparative example provides a nanofiltration membrane. The difference from Example 1 is only that, except that step (2) is not carried out, and the supported bottom membrane after cleaning treatment is directly immersed in a grafting solution containing p-aminobenzoic acid without surface amination treatment, that is, there is no amino group on the supported bottom membrane, the rest are the same as in Example 1.

[0117] Flux (J) and rejection rate (R) are two important parameters of the nanofiltration membrane performance. In the present invention, the prepared nanofiltration membrane is tested by a pressure-driven cross-flow nanofiltration instrument, and the test conditions are: 2000 ppm sodium sulfate solution, and the test pressure and temperature are 0.4 MPa and 25 °C respectively.

[0118] The membrane flux calculation formula is as follows:

[0119]

[0120] Among them, J is the membrane flux (LMH), S is the effective test area of the membrane (m 2 ), t is the operation time (s), and V is the volume of the permeated liquid (L) within time t.

[0121] The test results of the membrane flux are shown in Table 1.

[0122] The calculation formula for the membrane rejection rate is as follows:

[0123]

[0124] Among them, C0 and C p are the sodium sulfate concentrations of the feed liquid and the permeate liquid respectively. The sodium sulfate concentration is obtained by measuring with a conductivity meter.

[0125] The calculation formula for the separation factor of Na2SO4 / NaCl is ɑ = 1 - (R NaCl / R Na2SO4 ).

[0126] The test results of the sodium sulfate rejection rate and the separation factor of Na2SO4 / NaCl are shown in Table 2.

[0127] Table 1

[0128] Membrane flux (LMH) Example 1 255 Example 2 202 Example 3 232 Example 4 139 Example 5 152 Example 6 200 Example 7 180 Comparative Example 1 125 Comparative Example 2 68

[0129] Table 2

[0130]

[0131]

[0132] It can be seen from the test results that:

[0133] (1) It can be seen from Examples 1 to 3 that by designing the structure of the nanofiltration membrane in the present invention, an organic intermediate layer with a hydrophobic group and an active separation layer are sequentially arranged on the support bottom membrane, obtaining a nanofiltration membrane with high water flux and high salt rejection rate, wherein the water flux can reach more than 202 LMH, the sodium sulfate rejection rate can reach more than 97.3%, and the separation factor of Na2SO4 / NaCl can reach more than 31.

[0134] (2) It can be seen from Example 1 and Examples 4 - 5 that in Example 1, the molar ratio of the carboxyl group in p - aminobenzoic acid added to the amino group on the supported bottom membrane after surface amination treatment is 15:1. The water flux of the nanofiltration membrane prepared is 255 LMH, and the sodium sulfate rejection rate is 97.3%; while in Example 4, the molar ratio of the carboxyl group in p - aminobenzoic acid added to the amino group on the supported bottom membrane after surface amination treatment is 5:1. The water flux of the nanofiltration membrane prepared is 139 LMH, and the sodium sulfate rejection rate is 98.3%. In Example 5, the molar ratio of the carboxyl group in p - aminobenzoic acid added to the amino group on the supported bottom membrane after surface amination treatment is 25:1. The water flux of the nanofiltration membrane prepared is 152 LMH, and the sodium sulfate rejection rate is 98.3%. Thus, it shows that by defining the molar ratio of the carboxyl group of the hydrophobic substance to the amino group on the supported bottom membrane as (10 - 20):1 in the present invention, the carboxyl group of the hydrophobic substance can react effectively with the amino group on the supported bottom membrane to form stable chemical bonds, thereby achieving a good grafting effect. This helps to ensure the uniform distribution and firm attachment of the grafting layer on the supported bottom membrane, so as to improve the overall stability and water flux of the nanofiltration membrane while ensuring the salt rejection rate.

[0135] (3) It can be seen from Example 1 and Examples 6 - 7 that in Example 1, the supported bottom membrane after surface amination treatment is soaked in the grafting solution containing 1 mg / mL p - aminobenzoic acid for 6 h. The water flux of the nanofiltration membrane prepared is 255 LMH, and the sodium sulfate rejection rate is 97.3%; while in Example 6, the supported bottom membrane after surface amination treatment is soaked in the grafting solution containing 1 mg / mL p - aminobenzoic acid for 0.5 h. The water flux of the nanofiltration membrane prepared is 200 LMH, and the sodium sulfate rejection rate is 97.5%. In Example 7, the supported bottom membrane after surface amination treatment is soaked in the grafting solution containing 1 mg / mL p - aminobenzoic acid for 15 h. The water flux of the nanofiltration membrane prepared is 180 LMH, and the sodium sulfate rejection rate is 98.1%. Thus, it shows that by defining the soaking time of the supported bottom membrane after surface amination treatment in the hydrophobic substance as 2 - 10 h in the present invention, the amino group on the supported bottom membrane and the carboxyl group in the hydrophobic substance have enough time to react chemically to form chemical bonds, thereby achieving grafting. This helps to ensure the uniformity and integrity of the grafting layer. At the same time, by controlling the soaking time, the grafting layer can have an appropriate thickness and density, which helps to maintain the high permeation flux and selectivity of the nanofiltration membrane while ensuring the salt rejection rate.

[0136] (4) It can be seen from Example 1 and Comparative Examples 1-2 that the nanofiltration membrane of the present invention is obtained by grafting an organic intermediate layer with a hydrophobic group on a supporting bottom membrane having an amino group, and then grafting an active separation layer. The introduction of the hydrophobic group can reduce the charge property on the surface of the supporting bottom membrane, lower the hydrophilicity of the surface of the supporting bottom membrane, and increase the roughness of the surface of the supporting bottom membrane. Then, by grafting the active separation layer, the introduction of the hydrophobic group in the intermediate layer affects the interfacial polymerization process of the active separation layer, changing the surface thickness of the nanofiltration membrane, so that the nanofiltration membrane has a higher porosity and a more uniform pore size distribution, to greatly improve the water flux of the nanofiltration membrane while ensuring the salt rejection rate.

[0137] In summary, by sequentially arranging an organic intermediate layer with a hydrophobic group and an active separation layer on the supporting bottom membrane, the obtained nanofiltration membrane of the present invention has a high water flux and a high salt rejection rate. The water flux can reach more than 202 LMH, the sodium sulfate rejection rate can reach more than 97.3%, and the separation factor of Na2SO4 / NaCl can reach more than 31, having good application prospects in brackish water desalination, hard water softening, drinking water treatment or sewage treatment. Further, the present invention uses surface chemical grafting technology and classical interfacial polymerization to synthesize the required nanofiltration membrane. The whole operation process is simple and has good compatibility with the existing membrane preparation process.

[0138] The applicant declares that the above description is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A nanofiltration membrane modified by a hydrophobic grafted intermediate layer, characterized in that, The nanofiltration membrane includes a support bottom membrane layer, an organic intermediate layer with hydrophobic groups, and an active separation layer that are stacked in sequence; The active separation layer includes an interfacial polymer formed by an aqueous monomer and an organic monomer.

2. The nanofiltration membrane according to claim 1, wherein The material of the support bottom membrane layer includes any one or a combination of at least two of polyethersulfone, polysulfone, polyvinylidene fluoride, or polytetrafluoroethylene; Preferably, the support bottom membrane layer has amino groups; Preferably, the hydrophobic groups include phenyl and / or alkyl; Preferably, the aqueous monomer includes any one or a combination of at least two of piperazine, m-phenylenediamine, or polyethyleneimine; Preferably, the organic monomer includes any one or a combination of at least two of trimesoyl chloride, terephthaloyl chloride, or phthaloyl chloride; Preferably, the mass ratio of the aqueous monomer to the organic monomer is (0.5 - 5):

1.

3. The nanofiltration membrane according to claim 1 or 2, characterized in that, The thickness of the support bottom membrane layer is 100 - 300 μm; Preferably, the thickness of the organic intermediate layer with hydrophobic groups is 30 - 100 nm; Preferably, the thickness of the active separation layer is 10 - 30 nm.

4. A method for preparing a nanofiltration membrane modified based on a hydrophobic grafted intermediate layer according to any one of claims 1-3, characterized in that, The preparation method includes the following steps: (1) Immerse the support bottom membrane with amino groups in a grafting solution containing a hydrophobic substance to obtain a membrane with an organic intermediate layer; (2) Perform an interfacial polymerization reaction of an aqueous monomer and an organic monomer on the surface of the membrane with an organic intermediate layer to prepare an active separation layer, obtaining a membrane with an active separation layer; (3) Heat-treat the membrane with an active separation layer to obtain the nanofiltration membrane.

5. The preparation method according to claim 4, characterized in that, The preparation method also includes pre-treating the support bottom membrane; Preferably, the pre-treatment includes a cleaning treatment and a surface amination treatment performed in sequence; Preferably, the cleaning agent for the cleaning treatment includes any one or a combination of at least two of isopropanol, ethanol, acetone, or dimethylformamide; Preferably, the steps of the surface amination treatment include: immersing the support bottom membrane after the cleaning treatment in an amination solution; Preferably, the aminating agent for the surface amination treatment includes any one or a combination of at least two of diethylenetriamine, dopamine, or ethylenediamine; Preferably, the temperature of the surface amination treatment is 80 - 100 °C; Preferably, the time of the surface amination treatment is 35 - 50 h.

6. The preparation method according to claim 4 or 5, characterized in that The grafting solution containing a hydrophobic substance includes a buffer solution and a hydrophobic substance; Preferably, the solute of the buffer solution includes a grafting catalyst and a buffer; Preferably, the grafting catalyst includes 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and / or N-hydroxysuccinimide; Preferably, the buffer includes morpholineethanesulfonic acid and / or sodium acetate; Preferably, the hydrophobic substance includes p-aminobenzoic acid and / or 6-aminohexanoic acid; Preferably, the concentration of the hydrophobic substance in the grafting solution containing a hydrophobic substance is 0.5 - 1.5 mg / mL; Preferably, the molar ratio of the carboxyl group of the hydrophobic substance to the amino group on the support bottom membrane after the surface amination treatment in the grafting solution containing a hydrophobic substance is (10 - 20):1; Preferably, the soaking time in step (1) is 2 - 10 h.

7. The preparation method according to any one of claims 4 to 6, characterized in that The interfacial polymerization reaction includes: first wetting the membrane containing the organic intermediate layer with an aqueous phase containing an aqueous phase monomer, then removing the excess liquid, and then wetting the membrane containing the organic intermediate layer with an organic phase containing an organic phase monomer, and then removing the excess liquid to obtain the membrane containing the active separation layer; Preferably, the mass concentration of the aqueous phase monomer in the aqueous phase is 0.1-1%; Preferably, the time of the first wetting is 60-120 s; Preferably, the mass concentration of the organic phase monomer in the organic phase is 0.05-0.5%; Preferably, the organic solvent in the organic phase includes any one or a combination of at least two of n-hexane, cyclohexane or n-heptane; Preferably, the time of the second wetting is 15-60 s; Preferably, the volume ratio of the aqueous phase containing the aqueous phase monomer to the organic phase containing the organic phase monomer is (0.8-1.2):1; 8. The preparation method according to any one of claims 4-7, characterized in that, The temperature of the heat treatment is 50-80 °C; Preferably, the time of the heat treatment is 2-10 min; 9. The preparation method according to any one of claims 4-8, characterized in that The method includes the following steps: (1) Cleaning the support bottom membrane with a cleaning agent, and then washing it with pure water and leaving it for standby; (2) At 80-100 °C, immersing the support bottom membrane in an aminating agent for 35-50 h to perform surface amination treatment on the support bottom membrane; (3) Immersing the support bottom membrane after surface amination treatment in a grafting solution containing 0.5-1.5 mg / mL of a hydrophobic substance for 2-10 h to obtain a membrane containing an organic intermediate layer; (4) Wetting the membrane containing the organic intermediate layer with an aqueous phase with a mass concentration of the aqueous phase monomer of 0.1-1% for 60-120 s, then removing the excess liquid, and then wetting the membrane containing the organic intermediate layer with an organic phase with a mass concentration of the organic phase monomer of 0.05-0.5% for 15-60 s, and then removing the excess liquid to obtain a membrane containing an active separation layer; (5) At 50-80 °C, heat-treating the membrane containing the active separation layer for 2-10 min to obtain the nanofiltration membrane.

10. Application of the nanofiltration membrane modified based on a hydrophobic grafted intermediate layer according to any one of claims 1-3 in desalination of brackish water, softening of hard water, drinking water treatment or sewage treatment.

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