A modified polyamide nanofiltration membrane with surface grafted growth of ultra-smooth molecular brushes, and its preparation method and application

Abstract

The present invention relates to the technical field of sewage and wastewater treatment. The present invention discloses a modified polyamide nanofiltration membrane for sewage and wastewater treatment, and a preparation method and application thereof. The method uses a silane compound containing amino and alkoxy groups as a connecting medium between a polyamide membrane and a polydimethylsiloxane (PDMS) molecular brush, and grafts a silane compound containing amino and alkoxy groups on the surface of the polyamide membrane to form a silanol functional layer; the polyamide membrane after the reaction is immersed in a glycerol solution, washed, and dried; and then the PDMS molecular brush is in situ grown by initiating polymerization through chemical vapor deposition to prepare a modified polyamide nanofiltration membrane. The surface of the modified polyamide nanofiltration membrane has super-smooth characteristics and low friction, and has good pure water flux, inorganic salt retention, and excellent resistance to organic-inorganic composite pollution. It can be used in sewage treatment equipment, seawater desalination equipment, industrial wastewater treatment and reuse equipment and other devices.

Description

Technical Field

[0001] The present invention belongs to the technical field of wastewater and sewage treatment, and more specifically, relates to a modified polyamide nanofiltration membrane for sewage and wastewater treatment, and a preparation method and application thereof. Background Art

[0002] Sewage and wastewater treatment and resource utilization are key measures to address my country's freshwater resource crisis and a major national strategic need. Nanofiltration (NF) technology, due to its advantages such as small footprint, high effluent quality, and high treatment efficiency, is widely used in water softening, organic matter removal, heavy metal removal, wastewater treatment, and resource utilization. However, its practical application has long been constrained by membrane fouling. Although polyamide nanofiltration membranes, due to their comprehensive performance advantages and mature modification technology, account for 60%-70% of the global nanofiltration membrane market, their surface easily adsorbs and deposits colloids, organic matter, and inorganic scale in wastewater, forming a fouling layer, resulting in reduced flux and even irreversible membrane pore blockage and filter cake layer.

[0003] To effectively mitigate the critical issue of membrane fouling, the most common approach is to construct a hydrophilic antifouling buffer layer on the surface of the nanofiltration membrane to prevent contaminants from diffusing to the membrane surface. This buffer layer is rich in hydrophilic functional groups, such as carboxyl, hydroxyl, sulfonic, and amino groups. These functional groups form a unique network structure with water molecules through hydrogen bonding, subsequently forming a dense hydration layer on the membrane surface, preventing contaminants from migrating to the membrane surface and forming clusters. Furthermore, hydrophilic groups can weaken the hydrophobic interaction between contaminants and the membrane surface by enhancing surface polarity. However, the introduction of hydrophilic functional groups on the surface of nanofiltration membranes also increases the risk of membrane fouling to a certain extent. For example, hydrophilic groups (such as hydroxyl and carboxyl groups) can form hydrogen bonds with polar groups (such as amino and carboxyl groups) in pollutants to promote the adsorption of pollutants; if the hydrophilic groups are charged, they will also attract pollutants with opposite charges through electrostatic attraction; divalent cations in water (such as calcium ions, magnesium ions, etc.) are easy to promote negatively charged organic pollutants (such as humic acid) to aggregate on the surface of the hydrophilic membrane through bridging with negatively charged hydrophilic groups (such as carboxyl, sulfonic acid, phosphate, etc.), forming gel layer pollution; in high salinity conditions, the hydrogen bond network on the surface of the hydrophilic membrane will be destroyed due to the "salting out" phenomenon, resulting in a decrease in anti-pollution performance.

[0004] In light of this, researchers have proposed a low-surface-energy silicone polymer material, polydimethylsiloxane (PDMS), based on the ultra-low surface energy characteristics of superhydrophobic coatings. PDMS has a surface energy of approximately 20-22 mN / m and a low elastic modulus (approximately 0.002 GPa). Furthermore, PDMS has a low glass transition temperature (Tg), giving it high flexibility and liquid-like properties. This property makes PDMS's molecular chains significantly more flexible than more rigid fluoropolymers. Particularly in dynamic environments, the PDMS surface can reduce the adhesion of contaminants through micro-deformation.

[0005] However, in the membrane field, the application of PDMS has certain limitations: first, because PDMS molecular brushes are highly flexible and have strong hydrophobic interactions with the membrane surface, directly grafting long molecular chains to the membrane surface can easily clog the membrane pores, resulting in a significant decrease in the membrane water flux; second, the surface of the polyamide nanofiltration membrane lacks reaction sites that can form covalent bonds with the molecular chains; third, the hydrophobicity of PDMS itself will reduce the membrane flux.

[0006] Patent publication number CN117815911A discloses a method for preparing an amphiphilic ultrafiltration membrane, wherein a polyacrylonitrile ultrafiltration base membrane is first subjected to plasma treatment to form reactive groups on its surface, and then a polymerization reaction occurs on the membrane surface by vapor deposition to form polysiloxane segments. However, this method is only applied to polyacrylonitrile ultrafiltration base membranes. When this method is applied to polyamide nanofiltration membranes, which are widely used on the market, it has the following limitations: (1) The number of chemical vapor deposition reaction active sites generated by the polyamide layer after plasma treatment is small, which is not conducive to subsequent grafting, resulting in poor anti-fouling performance of the polyamide nanofiltration membrane; (2) Plasma treatment destroys the pore structure of the polyamide membrane, while reducing the mechanical strength of the base membrane, the pure water flux of the membrane, and the retention of inorganic salts; (3) The equipment required for plasma treatment is expensive, making it difficult to promote on a large scale in practical applications. Summary of the Invention

[0007] To address these existing technical issues, the present invention primarily aims to provide a method for preparing a modified polyamide nanofiltration membrane with surface-grafted ultra-smooth molecular brushes for sewage and wastewater treatment. The resulting modified polyamide nanofiltration membrane exhibits excellent pure water flux and inorganic salt retention; it also exhibits minimal flux loss and excellent flux recovery, demonstrating superior anti-fouling properties.

[0008] The second object of the present invention is to provide a modified polyamide nanofiltration membrane with ultra-smooth molecular brushes grafted onto its surface, which is obtained by the above-mentioned preparation method.

[0009] The third object of the present invention is to provide a modified polyamide nanofiltration membrane with super-smooth molecular brushes grafted onto its surface for use in treating sewage and wastewater.

[0010] In order to achieve the above object, the present invention is implemented through the following technical solutions:

[0011] A method for preparing a modified polyamide nanofiltration membrane with ultra-smooth molecular brushes grafted onto its surface comprises the following steps:

[0012] S1. Grafting a silane compound containing amino and alkoxy groups onto the surface of a polyamide membrane to form a silanol-functionalized layer;

[0013] S2. The polyamide membrane after the reaction in step S1 is immersed in a glycerol solution, washed, and dried at 10-30 ℃;

[0014] S3. A super-smooth polydimethylsiloxane molecular brush layer is formed on the surface of the polyamide membrane in step S2 by vapor deposition to prepare a modified polyamide nanofiltration membrane.

[0015] The preparation method of this invention innovatively utilizes a silane compound containing amino and alkoxy groups as a connecting medium between the polyamide membrane and the polydimethylsiloxane molecular brush. The silane compound is grafted onto the polyamide membrane surface through a reaction. The silane compound contains amino groups that react with carboxyl groups on the polyamide membrane to form a stable amide bond, thereby forming a silanol-functionalized layer on the polyamide membrane surface. Furthermore, the silane compound contains alkoxy groups that hydrolyze to form silanol groups, which provide ample reactive sites for subsequent vapor deposition reactions. Chemical vapor deposition (CVD) is then used to initiate polymerization and in situ grow the PDMS molecular brush. This method avoids the pore clogging problem caused by direct grafting of long PDMS molecular chains, effectively preserving membrane flux. Furthermore, the method is simple, rapid, highly controllable, operates under mild reaction conditions, consumes minimal reagents, does not damage the polyamide membrane, and exhibits high reactivity, making it suitable for industrial production.

[0016] The modified polyamide nanofiltration membrane prepared by the present invention demonstrated superior anti-fouling performance compared to the original membrane in dynamic pollution experiments conducted in simulated wastewater. It also surpassed reported results for anti-fouling nanofiltration membranes prepared by hydrophilic modification, demonstrating the practicality of this method. The modified polyamide nanofiltration membrane prepared by the present invention not only exhibited good pure water flux and inorganic salt retention, but also exhibited a low flux decline rate and excellent flux recovery rate, demonstrating its excellent anti-fouling performance.

[0017] Preferably, in step S1, the molecular weight cutoff of the polyamide membrane is 150-2000 Da; further preferably, the molecular weight cutoff of the polyamide membrane is 160-1000 Da; more preferably, the molecular weight cutoff of the polyamide membrane is 170-200 Da.

[0018] Preferably, in step S1, the silane compound containing an amino group and an alkoxy group is selected from one or more of 3-aminopropyltriethoxysilane, 3-aminopropylmethyldiethoxysilane, 3-aminopropyltrimethoxysilane, and 3-aminopropylmethyldimethoxysilane. Further preferably, the silane compound containing an amino group and an alkoxy group is selected from 3-aminopropyltriethoxysilane.

[0019] Preferably, in step S1, the operation of forming the silanol functionalized layer is: immersing the polyamide membrane in a silane compound solution containing amino and alkoxy groups, so that the carboxyl groups on the surface of the polyamide membrane react with the amino groups of the silane compound, and washing to form the silanol functionalized layer.

[0020] Preferably, the concentration of the silane compound in the solution is 0.05-0.3 wt %; more preferably, the concentration is 0.06-0.15 wt %; more preferably, the concentration is 0.08-0.12 wt %; and most preferably, the concentration is 0.1 wt %. More specifically, the concentration of the silane compound can be 0.07 wt %, 0.09 wt %, 0.11 wt %, 0.13 wt %, 0.15 wt %, 0.17 wt %, 0.19 wt %, 0.21 wt %, 0.23 wt %, 0.25 wt %, 0.27 wt %, 0.29 wt %, or any range formed by the above values, such as 0.05-0.15 wt %, 0.1-0.3 wt %, etc., but the present invention is not limited thereto.

[0021] Preferably, the grafting reaction time is 30-240 minutes; more preferably, the grafting reaction time is 30-120 minutes; more preferably, the grafting reaction time is 30-60 minutes; and most preferably, the grafting reaction time is 30 minutes. More specifically, the grafting reaction time can be 60 minutes, 80 minutes, 110 minutes, 140 minutes, 170 minutes, 200 minutes, 230 minutes, or any range formed by the above values, and the present invention is not limited thereto.

[0022] Preferably, the reaction temperature of the grafting is 15-60°C; more preferably, the reaction temperature of the grafting is 20-30°C; most preferably, the reaction temperature of the grafting is 25°C.

[0023] In some embodiments, those skilled in the art can conventionally combine the concentration of the above-mentioned silane compounds, the grafting reaction time and / or the grafting reaction temperature, such as conducting the reaction under the conditions of a concentration of 0.1wt%, a grafting reaction time of 30min, and a grafting reaction temperature of 25°C; such as conducting the reaction under the conditions of a concentration of 0.15wt%, a grafting reaction time of 60min, and a grafting reaction temperature of 30°C; such as conducting the reaction under the conditions of a concentration of 0.1wt%, a grafting reaction time of 120min, and a grafting reaction temperature of 25°C, etc. The present invention is not limited thereto.

[0024] Preferably, the concentration of the silane compound in the solution is 0.08-1.5 wt %, and the grafting reaction time is 20-60 min. Further preferably, the concentration of the silane compound in the solution is 0.08-1.2 wt %, and the grafting reaction time is 30-60 min. Most preferably, the concentration of the silane compound in the solution is 1.0 wt %, and the grafting reaction time is 30 min.

[0025] Preferably, the solution of the silane compound containing amino and alkoxy groups is a buffer solution, which can be formed by adding a buffer ion pair commonly used in the art, including but not limited to phosphate buffer (PBS). More specifically, the concentration of the phosphate buffer (PBS) is 0.01-0.05M.

[0026] Preferably, in step S1, the polyamide membrane is subjected to a pretreatment and / or activation treatment before reacting with the silane compound.

[0027] More specifically, the pretreatment may be a pretreatment conventionally performed on polyamide membranes before modification in the art, so as to remove substances such as preservatives and pore retaining agents attached to the surface of the polyamide membrane.

[0028] More specifically, the activation treatment can be a conventional activation treatment in the art to activate the carboxyl groups on the surface of the polyamide membrane. More specifically, the present invention utilizes an EDC / NHS buffer solution comprising 1-ethyl-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS) to activate the polyamide membrane. In some more specific embodiments, the polyamide membrane can be immersed in the EDC / NHS buffer solution for activation.

[0029] Specifically, the EDC / NHS buffer solution is an EDC / NHS morpholineethanesulfonic acid (MES) buffer solution.

[0030] More specifically, the concentration of MES in the buffer solution can be 0.001-0.02 mol / L; more specifically, the concentration of MES can be 0.008-0.012 mol / L. More specifically, the pH of the buffer solution can be 5.0-6.0; more specifically, the pH of the buffer solution can be 5.4-5.6. More specifically, the concentration of NHS in the buffer solution can be 0.001-0.02 mol / L; more specifically, the concentration of NHS can be 0.008-0.012 mol / L. More specifically, the ratio of EDC to NHS is 1:2-4:1; preferably, the ratio of EDC to NHS is 1:1.

[0031] More specifically, the activation treatment time is 30-240 minutes; further preferably, the activation treatment time is 30-120 minutes; more preferably, the activation treatment time is 30-60 minutes; most preferably, the activation treatment time is 30 minutes. More specifically, the activation treatment temperature is 15-60°C; further preferably, the activation treatment temperature is 20-30°C; most preferably, the activation treatment temperature is 25°C.

[0032] Preferably, in step S2, the concentration of the glycerol solution is 10-40 wt%; and / or the soaking time is ≥10 min.

[0033] More preferably, the concentration of the glycerol solution is 15-25 wt %; most preferably, the concentration of the glycerol solution is 20 wt %. Under these optimal conditions, glycerol can better penetrate the polyamide layer, forming a more stable hydrogen bond network to support the membrane pores, thereby improving the flux, salt rejection rate, and anti-fouling performance of the polyamide nanofiltration membrane.

[0034] More preferably, the soaking time is 20-40 min; most preferably, the soaking time is 30 min.

[0035] Preferably, in step S2, the drying is performed at 15-25°C; more preferably, the drying is performed at 20-25°C.

[0036] Preferably, in step S2, the drying time is 1-10 min; further preferably, the drying time is 1-5 min; most preferably, the drying time is 3 min.

[0037] Preferably, in some more specific embodiments, step S3 includes the following operations: the polyamide membrane treated in step S2 is placed in a container containing silane monomer, and an ultra-smooth polydimethylsiloxane molecular brush coating is formed on the surface of the polyamide membrane by vapor deposition, and the membrane is cleaned to remove residual monomers and by-products on the membrane surface to prepare a modified polyamide nanofiltration membrane.

[0038] Preferably, in step S3, any one of the following (a) to (c) is selected:

[0039] (a) the vapor deposition temperature is 30-50°C; most preferably, the vapor deposition temperature is 40°C;

[0040] (b) the vacuum degree of vapor deposition is -0.1 to 0.05 MPa; most preferably, the vacuum degree of vapor deposition is -0.1 MPa;

[0041] (c) The reaction time of vapor deposition is ≥45 s; preferably, the reaction time of vapor deposition is 45-360 s; further preferably, the reaction time of vapor deposition is 60-120 s; most preferably, the reaction time of vapor deposition is 90 s.

[0042] Preferably, the silane monomer is selected from at least one of dichlorodimethylsilane and dimethyldiethoxysilane.

[0043] Preferably, the amount of silane monomer added is 0.2-1.1 μL / cm2 based on the area of ​​the polyamide membrane. 2 .

[0044] Furthermore, the present invention seeks to protect the modified polyamide nanofiltration membrane with ultra-smooth molecular brushes grafted onto its surface, which is obtained by the above-mentioned preparation method.

[0045] Furthermore, the present invention seeks to protect the use of modified polyamide nanofiltration membranes with surface grafted super-smooth molecular brushes in treating sewage and wastewater.

[0046] More specifically, the sewage and wastewater include but are not limited to humic acid, landfill leachate, bovine serum albumin, sodium alginate, etc.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] This invention provides a method for preparing a modified polyamide nanofiltration membrane with ultra-smooth molecular brushes grafted onto its surface. A silane compound containing amino and alkoxy groups is used as a connecting medium between the polyamide membrane and the polydimethylsiloxane molecular brushes. The PDMS molecular brushes are then grown in situ via CVD-induced polymerization. The resulting polyamide nanofiltration membrane exhibits low surface friction and exhibits excellent water flux, inorganic salt rejection, and anti-fouling properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 The pollution curves of the base membranes NF90, NF-a and the modified polyamide nanofiltration membranes prepared in Examples 1 to 3 in treating simulated wastewater containing humic acid.

[0050] Figure 2 The short-term fouling curve of the base membrane NF90 and the modified polyamide nanofiltration membrane prepared in Example 2 in treating the effluent of the membrane bioreactor for landfill leachate.

[0051] Figure 3 The pollution curves of the modified polyamide nanofiltration membranes prepared by base membrane NF90, Example 2, Examples 6-7 and Comparative Example 3 in treating simulated wastewater containing bovine serum albumin are shown.

[0052] Figure 4 This is a pollution curve diagram of the three-cycle long-term anti-pollution test of the modified polyamide nanofiltration membrane prepared in Example 2 in treating the effluent of the membrane bioreactor of landfill leachate.

[0053] Figure 5 The water contact angle summary bar graph of base membranes NF90, NF-a, and modified polyamide nanofiltration membranes prepared in Examples 1-3 and Comparative Examples 3-4.

[0054] Figure 6 The full X-ray photoelectron spectroscopy (XPS) spectra of the base membranes NF90, NF-a, and the modified polyamide nanofiltration membranes prepared in Examples 1 to 3 are shown.

[0055] Figure 7 The X-ray photoelectron spectroscopy (XPS) O 1s high-resolution spectra of the base films NF90 and NF-a.

[0056] Figure 8 Scanning electron microscope (SEM) images of base membranes NF90, NF-a, and modified polyamide nanofiltration membranes prepared in Examples 1 to 3. DETAILED DESCRIPTION

[0057] The present invention will be further described below with reference to the accompanying drawings and specific examples, but the examples do not limit the present invention in any way. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0058] Example 1 Preparation of modified polyamide nanofiltration membrane with surface grafted super-smooth molecular brushes

[0059] (1) Polyamide membrane (DuPont, Fimtec TM NF90) Rinse with deionized water several times to remove preservatives and membrane pore retention agents, and then soak in deionized water for later use;

[0060] (2) Prepare a 0.1 mol / L morpholineethanesulfonic acid (MES) buffer solution with a pH of 5.5 as the activation solution. Dissolve 1-ethyl-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS) in the MES buffer solution successively. The concentrations of EDC and NHS are both 0.1 mol / L.

[0061] (3) Fix the polyamide membrane in the reaction device, and ensure that the selective layer of the membrane faces upward; pour the solution prepared in step (2) into the reaction device, place it on a shaker, and react at room temperature at a speed of 60 rpm for 30 min. After the reaction is completed, pour out the activation solution, wash away the remaining reactants with deionized water, and keep the membrane fixed in the reaction device;

[0062] (4) Prepare a 0.01 mol / L PBS buffer solution with a pH of 7.4, slowly drip 3-aminopropyltriethoxysilane (APTES) into the solution, and shake the solution while dripping to fully disperse the APTES to form a PBS solution with a mass fraction of 0.1% APTES. Then quickly pour the solution into the reaction device fixed with the activated base membrane, place it on a shaker, and react at room temperature at a speed of 60 rpm for 30 min. After the reaction is completed, pour off the activation solution and wash away the remaining reactants with deionized water to obtain a polyamide membrane with a silanol functional layer, which is marked as NF-a.

[0063] (5) Prepare a 20% glycerol aqueous solution, cut the membrane into a size of 14 cm × 7 cm, and then soak it in the glycerol aqueous solution for 30 min. After soaking, wash off the glycerol on the surface with pure water, and dry the polyamide membrane with cold air (20-25°C) using a hair dryer for 3 min.

[0064] (6) Place the dried polyamide membrane in a culture dish with a diameter of 20 cm and a height of 2 cm, and evenly drip dichlorodimethylsilane into the culture dish along the side wall (the amount of dichlorodimethylsilane dripped is 0.51 μL / cm based on the area of ​​the polyamide membrane). 2 ), the reaction liquid did not directly contact the polyamide membrane, the culture dish was covered with a lid, and the culture dish was placed in a vacuum drying oven at 40°C. A vacuum pump was used to evacuate the vacuum oven to -0.1 MPa to initiate CVD. The reaction time was 45 s. After the reaction was completed, the membrane was taken out and the residual reaction monomers and by-products on the surface of the polyamide membrane were washed with n-hexane to prepare a modified polyamide nanofiltration membrane with ultra-smooth molecular brushes grafted onto the surface.

[0065] Example 2 Preparation of modified polyamide nanofiltration membrane with surface grafted super-smooth molecular brushes

[0066] The difference between this embodiment and embodiment 1 is that in step (6), the CVD reaction time is 90 s.

[0067] Example 3 Preparation of modified polyamide nanofiltration membrane with surface grafted super-smooth molecular brushes

[0068] The difference between this embodiment and embodiment 1 is that in step (6), the CVD reaction time is 180 s.

[0069] Example 4 Preparation of modified polyamide nanofiltration membrane with super-smooth molecular brushes grafted onto its surface

[0070] The difference between this embodiment and embodiment 2 is that in step (5), the mass fraction of the glycerol aqueous solution is 5%, and the membrane soaking time is 30 min.

[0071] Example 5 Preparation of modified polyamide nanofiltration membrane with surface grafted super-smooth molecular brushes

[0072] The difference between this embodiment and embodiment 2 is that in step (5), the mass fraction of the glycerol aqueous solution is 10%, and the membrane soaking time is 30 min.

[0073] Example 6 Preparation of modified polyamide nanofiltration membrane with surface grafted super-smooth molecular brushes

[0074] The difference between this embodiment and embodiment 2 is that in step (4), the mass fraction of APTES is 0.05%.

[0075] Example 7 Preparation of modified polyamide nanofiltration membrane with surface grafted super-smooth molecular brushes

[0076] The difference between this embodiment and embodiment 2 is that in step (4), the mass fraction of APTES is 0.20%.

[0077] Comparative Example 1

[0078] Comparative Example 1 is used to demonstrate that the "in situ growth" method can effectively reduce the impact of grafting on membrane flux. Direct grafting was performed using amino-terminated polydimethylsiloxane (PDMS) long chains. The specific steps are as follows:

[0079] (1) Polyamide membrane (DuPont, Fimtec TM NF90) Rinse with deionized water several times to remove preservatives and membrane pore retention agents, and then soak in deionized water for later use;

[0080] (2) Prepare a 0.1 mol / L MES buffer solution with a pH of 5.5, and dissolve EDC and NHS in the MES buffer solution successively. The concentrations of EDC and NHS are both 0.1 mol / L.

[0081] (3) Fix the polyamide membrane in the reaction device, and ensure that the selective layer of the membrane faces upward; pour the solution prepared in step (2) into the reaction device, place it on a shaker, and react at room temperature at a speed of 60 rpm for 30 min. After the reaction is completed, pour out the activation solution, wash away the remaining reactants with deionized water, and keep the membrane fixed in the reaction device;

[0082] (4) The amino-terminated PDMS with a molecular weight of 1000 Da was evenly coated on the membrane surface, placed on a shaker, and reacted at room temperature at a speed of 60 rpm for 2 h. After the reaction, the remaining reactants were washed away with n-hexane to obtain a membrane grafted with PDMS molecular brushes.

[0083] Comparative Example 2

[0084] The difference between this comparative example and comparative example 1 is that in step (4), the molecular weight of the amino-terminated PDMS used is 5000 Da.

[0085] Comparative Example 3

[0086] Comparative Example 3 demonstrates the significant advantages of the present invention in increasing the grafting density of polydimethylsiloxane (PDMS) molecular brushes by introducing active groups through chemical grafting, while also ensuring the structural integrity of the polyamide membrane and improving the anti-fouling properties of the polyamide membrane. Comparative Example 3 employed a plasma cleaning method to graft the PDMS molecular brushes onto the polyamide membrane. The specific steps are as follows:

[0087] (1) Polyamide membrane (DuPont, Fimtec TM NF90) Rinse with deionized water several times to remove preservatives and membrane pore retention agents, and then soak in deionized water for later use;

[0088] (2) Prepare a 20% glycerol aqueous solution, cut the base film into a size of 14 cm × 7 cm, and then soak it in the glycerol aqueous solution for 30 min. After soaking, wash off the glycerol on the surface with pure water, and dry the film with cold air from a hair dryer for 3 min.

[0089] (3) The dried film was treated with air plasma at a power of 100 W for 60 s to generate reactive groups on the surface of the base film;

[0090] (4) The treated membrane was placed in a culture dish with a diameter of 20 cm and a height of 2 cm. 50 μL of dichlorodimethylsilane was evenly added to the culture dish along the side wall. The reaction liquid did not directly contact the membrane. The culture dish was covered with a lid and placed in a vacuum drying oven at 40°C. A vacuum pump was used to evacuate the vacuum oven to -0.1 MPa to induce chemical vapor deposition. The reaction time was 90 s. After the reaction was completed, the membrane was taken out and the residual reaction monomers and by-products on the membrane surface were washed away with n-hexane to prepare a modified polyamide membrane.

[0091] Comparative Example 4

[0092] The difference between this comparative example and comparative example 3 is that the power of the plasma treatment is 200 W and the time is 300 s.

[0093] Comparative Example 5

[0094] The difference between this comparative example and Example 2 is that in step (5), the membrane is dried by washing the glycerol on the surface with pure water and then drying the membrane in an oven at 60°C for 10 min.

[0095] Comparative Example 6

[0096] The difference between this comparative example and Example 2 is that in step (5), the glycerol aqueous solution is not used for soaking, and the membrane surface is directly blown for 3 minutes using an air pump until it is dry.

[0097] Test Case

[0098] The following is a performance analysis experiment of the modified polyamide nanofiltration membranes prepared in each embodiment and comparative example.

[0099] 1. Experimental Subjects

[0100] Modified polyamide nanofiltration membranes were obtained using the preparation methods of the various embodiments and comparative examples.

[0101] 2. Experimental Methods

[0102] The modified polyamide nanofiltration membranes prepared in the examples and comparative examples were subjected to scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) tests, hydrophilicity and hydrophobicity measurements, pure water flux, and anti-fouling performance during the filtration of simulated wastewater containing humic acid and bovine serum albumin, as well as the effluent from a membrane bioreactor of landfill leachate, using existing technologies.

[0103] (1) Pure water flux measurement:

[0104] This experiment uses a cross-flow filtration device for measurement, and the effective area of ​​the membrane is 16 cm 2The filtration temperature was controlled at 25°C, the test pressure was 6 bar, and the membrane surface flow rate was controlled at 0.1 m / s. The membrane was pre-pressed with pure water, and after the flux stabilized, the membrane permeability was recorded. J , the calculation formula is:

[0105]

[0106] in, V represents the volume of leachate (L), A Indicates the effective membrane area (m 2 ), Indicates the filtering time (h).

[0107] (2) Determination of membrane retention rate:

[0108] The retention rate of the modified polyamide nanofiltration membrane for 2000 mg / L MgCl2 and Na2SO4 solutions was determined. The retention rate of the membrane was calculated using the following formula:

[0109]

[0110] Among them, C F with C P Represent the conductivity of the concentrate and leachate respectively.

[0111] (3) Anti-pollution performance test:

[0112] Humic acid (HA) simulated wastewater simulates the composition of actual landfill leachate and is prepared with 6 g / L NaCl, 0.15 g / L CaCl2, and 0.1 g / L humic acid, with a pH adjusted to 7.0. Before testing, the modified polyamide nanofiltration membrane was pre-pressurized with 30 bar ultrapure water overnight and the initial flux was adjusted with 6 g / L NaCl solution. J 0 to 30 L·m -2 ·h -1 The pollution test lasted for 24 hours, during which the leachate was refluxed every hour to keep the overall osmotic pressure of the raw liquid constant and the membrane surface flow rate at 0.44 m·s -1 and temperature 25±3℃. After the test, rinse the membrane with ultrapure water for 30 minutes and test the flux after cleaning with NaCl solution with the same conductivity. J 2, and then evaluate the flux recovery ratio (FRR). The flux decline ratio (FDR) and flux recovery rate (FRR) are respectively calculated by the initial flux ( J 0), post-pollution flux ( J t ) and post-cleaning flux ( J 2) Calculation: The formula is as follows:

[0113]

[0114]

[0115] in, J 0 and J t are the initial flux and final flux during the test, J 2 is the NaCl flux after physical cleaning after contamination filtration.

[0116] Bovine serum albumin (BSA) simulated wastewater was used to evaluate the treatment effects of wastewater treatment processes and equipment on protein-containing wastewater. It was prepared with 2 g / L NaCl, 0.15 g / L CaCl2, and 0.1 g / L BSA. Before testing, the modified polyamide nanofiltration membrane was pre-pressurized with 30 bar ultrapure water overnight, and the initial flux was adjusted to 30 L·m with 2 g / L NaCl solution. -2 ·h -1 The contamination test lasted for 6 hours, during which the leachate was refluxed every hour to keep the overall osmotic pressure of the raw liquid constant and the transmembrane flow rate at 0.44 m·s. -1 and temperature 25 ± 3 ° C. After the test, the membrane was rinsed with ultrapure water for 30 min, and the flux recovery rate (FRR) of the membrane was evaluated with NaCl solution with the same conductivity.

[0117] The actual wastewater was taken from the leachate membrane bioreactor of a waste incineration plant in Guangzhou. The water sample was stored at 4 °C and the pretreatment included 0.22 μ m polyethersulfone microfiltration membrane was used to remove suspended particulate matter, and the concentrations of metal ions and organic matter were measured by inductively coupled plasma optical emission spectrometry (ICP-OES) and total organic carbon (TOC) analyzer. During the test, sodium chloride solution with the same conductivity as actual water was used for pre-pressurization and the initial flux was adjusted to 30 ± 2 L·m -2 ·h -1 4.5 L of wastewater was used as the feed solution and continuous filtration was performed until the cumulative permeate volume reached 3 L or the FDR exceeded 80%. After the test, the membrane was rinsed with ultrapure water for 30 minutes, and the flux recovery rate (FRR) was evaluated using a NaCl solution with the same conductivity.

[0118] 3. Experimental Results

[0119] Table 1

[0120]

[0121] Table 1 shows the pure water permeation flux and inorganic salt rejection of modified polyamide nanofiltration membranes prepared using NF90, NF-a, Examples 1-7, and Comparative Examples 1-6. The results show that regardless of the method used to graft the PDMS molecular brushes, the flux of the modified membranes decreased significantly. The pure water permeation fluxes of the original NF90 membrane and the membrane grafted with the silane coupling agent NF-a were 7.70 and 6.50 L·m, respectively. -2 ·h -1 bar -1 When the CVD reaction time is 45 s, 90 s, and 180 s, the corresponding membrane pure water permeation fluxes of Examples 1 to 3 are 4.15, 3.90, and 3.25 L·m -2 ·h -1 bar -1 This is because as the CVD reaction time increases, the PDMS layer thickness gradually increases, which increases the resistance to water molecule diffusion, resulting in a gradual decrease in the pure water permeation flux. At the same time, the presence of the PDMS layer reduces the membrane pore size. While maintaining a high sodium sulfate rejection rate, the membranes of Examples 1-3 have increased magnesium chloride rejection rates to 97.12%, 96.38%, and 96.69%, respectively, compared to 94.78% for the base membrane NF90 and 94.93% for the silane coupling agent-modified membrane.

[0122] Example 2, Comparative Example 5, and Comparative Example 6 investigated the effects of different drying methods on the pure water permeation flux of the final prepared PDMS molecular brush modified membrane. Among them, the pure water flux of the membranes prepared in Comparative Examples 5 and 6 decreased most seriously, with the pure water permeation flux of Comparative Example 6 being only 0.57 L·m -2 ·h -1 bar -1 , while Comparative Example 5 even failed to produce water under the test conditions. This is primarily because the hydrogen bond network between the polyamide and solvent molecules is destroyed during the drying process, weakening the solvent's support for the membrane pores. Heating accelerates the thermal motion between molecules, leading to softening of the polyamide layer and further collapse of the membrane pores. Therefore, the flux of Comparative Example 5 is lower than that of Comparative Example 6, which was dried at room temperature.

[0123] In order to reduce the impact of drying on the polyamide layer, glycerol is introduced in the present invention, which can form a more stable and strong hydrogen bond network with the polyamide layer to support the membrane pores. Therefore, the membrane pores are not easy to collapse during the membrane drying process, thereby maintaining the pure water permeation flux of the prepared membrane.

[0124] The protective effect of glycerol aqueous solution on membrane pores was compared in Example 4, Example 5 and Example 2. The pure water permeation flux of the membranes finally prepared were 3.20, 3.60 and 3.90 L·m -2 ·h -1 bar-1 , which indicates that with the increase of the concentration of glycerol aqueous solution, more glycerol molecules can enter the polyamide layer and form a more stable hydrogen bond network to support the membrane pores.

[0125] Examples 6 and 7 are membranes prepared at different silane coupling agent APTES grafting concentrations, and their pure water permeation fluxes are 5.25 and 2.60 L·m -2 ·h -1 bar -1 , sodium sulfate rejection rates were 98.23% and 99.01%, respectively, and magnesium chloride rejection rates were 93.64% and 95.36%, respectively. In conjunction with Example 2, it can be seen that with the increase in APTES grafting concentration, the pure water permeation flux of the final membrane gradually decreases. This is mainly because the sites on the membrane surface for CVD reaction increase, ultimately forming a denser PDMS molecular brush layer, which increases the transmission resistance of water molecules.

[0126] In this study, PDMS molecular chains with molecular weights of 1000 Da and 5000 Da were grafted onto the membrane surface using a direct grafting method (Comparative Examples 1 and 2). Test results showed that the pure water permeation fluxes of the resulting PDMS modified membranes were 0.60 and 0.34 L·m -2 ·h -1 bar -1 , which is significantly lower than the modified membrane prepared by grafting PDMS brushes using the CVD method. Analysis of the reason is that the long-chain PDMS molecules have low surface energy and can easily penetrate into the membrane pores during the direct grafting process, causing the membrane pores to be blocked, thereby causing a significant decrease in the permeation flux. In contrast, the CVD method grafts PDMS molecular brushes through surface growth, which can effectively avoid the negative impact of long-chain PDMS on the membrane pore structure, thereby better maintaining the permeation performance of the membrane. Therefore, the PDMS modified membrane prepared by the direct grafting method has limited value in practical applications due to its low permeation flux.

[0127] In order to demonstrate the advantages of the present invention over the existing method of using plasma to treat the membrane to generate active groups and then grafting PDMS molecular brushes by CVD in terms of grafting efficiency and sustainability, the present application designed experiments of Comparative Examples 3 and 4. Similarly, the polyamide membrane was plasma treated and then grafted with PDMS molecular brushes by CVD. The pure water permeation flux and inorganic retention performance of the final prepared membrane were tested. From the results, the final membrane flux of Comparative Example 3 was 4.84 L·m -2 ·h -1 bar -1 , although higher than 3.90 L·m -2 ·h -1 bar -1However, the inorganic salt rejection rate of Comparative Example 3 did not improve compared with the base membrane NF90. The sodium sulfate rejection rate dropped from 99.26% to 97.16%, and the magnesium chloride rejection rate dropped from 94.78% to 92.45%. This may be due to the damage of the polyamide selective layer by the plasma treatment. Continuing to increase the plasma treatment power and treatment time, the flux of the membrane prepared by Comparative Example 4 dropped significantly to 2.18 L·m -2 ·h -1 bar -1 , the sodium sulfate retention rate dropped to 96.34%, and the magnesium chloride retention rate dropped to 90.12%. The decrease in flux may be due to the destruction of the cross-linking structure in the polyamide layer by plasma, causing the membrane pores to collapse, resulting in the obstruction of water molecule transmission, and the decrease in the inorganic salt retention rate may be because the plasma treatment destroyed the microstructure of the membrane pores, causing the membrane pores in some areas of the polyamide layer to expand, resulting in the penetration of inorganic salts through the selective layer. The changes in pure water flux and inorganic salt retention rate indicate that plasma treatment has limitations in polyamide membrane modification. In addition, the water contact angles of comparative examples 3 to 4 will be tested below to evaluate the efficiency of the plasma method for grafting PDMS molecular brushes.

[0128] Table 2

[0129]

[0130] Table 2 shows the FDR and FRR of the polyamide nanofiltration membranes prepared by NF90, NF-a, Examples 1-3, and Examples 6-7 in the filtration tests of simulated wastewater containing humic acid, simulated wastewater containing bovine serum albumin, and actual wastewater leachate. Figure 1 For the treatment of simulated wastewater containing humic acid, the fouling curves of NF90, NF-a, and the modified polyamide nanofiltration membranes prepared in Examples 1 to 3 were 24.73%, 27.87%, 25.23%, 12.16%, and 18.19%, respectively, and the FRRs were 82.66%, 82.74%, 83.86%, 92.07%, and 86.68%, respectively.

[0131] Pollution testing results using simulated humic acid wastewater showed that the modified polyamide nanofiltration membrane prepared in Example 2 exhibited the best anti-pollution performance, with a lower FDR and higher FRR than other membranes. This is primarily due to the appropriate length of the PDMS molecular brushes grafted using the method in Example 2, which not only weakened the interaction between the polyamide nanofiltration membrane and pollutants but also prevented chain entanglement, which could lead to decreased fluidity.

[0132] In contrast, the shorter CVD reaction time in Example 1 resulted in insufficient length of the PDMS molecular brushes grown on the surface, resulting in uneven coating coverage. Furthermore, the shorter chains are more rigid than the longer chains, leading to poor molecular chain mobility and a reduced ability to release pollutants. In Example 3, the longer CVD reaction time resulted in a dense PDMS molecular brush coating growing on the membrane surface. However, the overly long molecular chains entangled with each other, resulting in poor molecular chain mobility and a loss of pollutant release.

[0133] Figure 2 Figure 2 shows the short-term fouling curves of the base membrane NF90 and the modified polyamide nanofiltration membrane prepared in Example 2 in the ultrafiltration effluent of a landfill leachate MBR. When treating 4.5 L of actual wastewater, the modified polyamide nanofiltration membrane prepared in Example 2 achieved a cumulative effluent volume of 3 L, with FDR and FRR values ​​of 28.65% and 90.05%, respectively. In contrast, when the NF90 membrane achieved an FDR of 81.76%, the cumulative effluent volume was only 2.4 L, and after physical cleaning with ultrapure water, the FRR value was only 61.68%. This demonstrates that the modified polyamide nanofiltration membrane prepared in Example 2 exhibits significant anti-fouling performance advantages over the commercial nanofiltration membrane NF90 in actual water treatment. Actual water contains higher levels of organic matter and divalent salts than the humic acid simulated wastewater. Divalent salt ions can accelerate the migration of pollutants to the membrane surface through bridging reactions. Furthermore, some divalent salt ions (such as calcium and magnesium ions) easily bind to SO4 during the concentration process. 2- 、CO3 2- PO4 2- Isoanions induce irreversible inorganic scaling of the membrane, leading to a significant decrease in water flux. The highly flexible PDMS molecular brushes grafted onto the membrane surface promote the release of pollutants under cross-flow, delaying the formation of inorganic scaling and significantly improving the membrane's anti-fouling properties.

[0134] Figure 3 The following graphs show the pollution curves of modified polyamide nanofiltration membranes prepared using base membrane NF90, Example 2, Examples 6-7, and Comparative Example 3 in simulated wastewater containing bovine serum albumin. The final measured FDRs were 35.39%, 14.52%, 22.67%, 12.41%, and 26.87%, respectively, and the FRRs were 85.24%, 96.17%, 93.25%, 95.85%, and 90.13%, respectively. Comparing the anti-pollution test results of base membrane NF90, Example 2, Example 6, and Example 7, it is clear that the modified polyamide nanofiltration membranes prepared using Examples 2 and 7 exhibit the best anti-pollution performance. This is primarily due to the higher concentration of the grafted silane coupling agent APTES, which provides more CVD reaction sites on the membrane surface, ultimately resulting in a more uniform molecular brush coating and, therefore, better anti-pollution performance.

[0135] Comparing the anti-fouling test results of the base membrane NF90, Example 2, and Comparative Example 3, the PDMS-modified polyamide membrane prepared in Example 2 using a chemically grafted aminosilane coupling agent combined with CVD exhibits significantly superior performance to the modified polyamide nanofiltration membrane in Comparative Example 3, which was modified using the prior art method of plasma treatment combined with CVD. This is partly because the polyamide membrane primarily contains amide bonds, where the lone electron pairs of nitrogen atoms form resonance structures with carbonyl groups, dispersing the electron cloud density and reducing susceptibility to plasma attack. Furthermore, the high polarity of the polyamide membrane makes plasma treatment more likely to introduce oxygen-containing groups (such as carbonyl and carboxyl groups) onto the polyamide surface, rather than polar groups (such as hydroxyl groups, which can be used in CVD reactions). This makes the practical application value of the prior art method of modifying polyamide membranes using plasma treatment combined with CVD limited.

[0136] Based on the results in Table 1 and Table 2, from the values ​​of FDR and FRR, there is no significant difference in the anti-pollution performance between Example 2 and Example 7, but the pure water permeation flux of Example 2 is significantly higher than that of Example 7. Therefore, Example 2 is selected as the optimal condition for preparing the PDMS molecular brush modified membrane.

[0137] Table 3

[0138]

[0139] Furthermore, the modified polyamide nanofiltration membrane prepared in Example 2 was used to conduct a long-term multi-cycle actual water filtration test to evaluate the sustainable use of the modified membrane. The FDR and FRR values ​​of each cycle are shown in Table 3, and the pollution curve is shown in Figure 4 The results show that membrane fouling gradually worsened from cycles one to three, with FDR values ​​of 29.08%, 44.44%, and 68.25%, and FRR values ​​of 93.41%, 75.09%, and 74.48%, respectively. The significant decrease in membrane flux in cycles two and three is due, on the one hand, to the fact that as pollutant concentration in the wastewater gradually increased, the pollutant release efficiency of the PDMS molecular brushes was lower than the pollutant migration efficiency to the membrane surface, leading to continuous accumulation of pollutants and exacerbating concentration polarization. On the other hand, as concentration progressed, the osmotic pressure of the solution gradually increased, reducing the driving force for water transport. After simple physical cleaning of the membranes at the end of cycles two and three, the flux recovery rate was significantly lower than that after cleaning in cycle one. This is primarily due to the high concentration of divalent inorganic salts in the actual water during the later stages of concentration, exceeding the critical concentration for crystallization. Furthermore, the pollutant release rate of the PDMS molecular brushes was lower than the migration rate of the crystals to the membrane surface, ultimately leading to irreversible inorganic fouling on the membrane surface and reducing membrane flux.

[0140] Figure 5The water contact angle summary bar graph of the base membrane NF90, NF-a and the modified polyamide nanofiltration membrane prepared in Examples 1 to 3 and Comparative Examples 3 to 4. Figure 5 The results show that the original membrane NF90 has the lowest water contact angle of 23.2°, indicating that NF90 is the most hydrophilic. This is mainly because the polyamide membrane contains a large number of hydrophilic carboxyl groups. After grafting APTES, the water contact angle increased significantly to 55.8°. This is mainly because the grafting of APTES consumes the hydrophilic carboxyl groups on the membrane surface, and the hydrophilicity of silanol groups is not as good as that of carboxyl groups, which reduces the hydrophilicity of the membrane surface. After CVD grafting PDMS, the contact angle increased significantly again, and with the increase of CVD reaction time, the water contact angle of the membrane showed a trend of gradual increase. The water contact angles of Example 1, Example 2, and Example 3 were 90.6°, 95.5°, and 97.2°, respectively. This is mainly because the longer the reaction time, the longer the PDMS molecular chain, the more hydrophobic the methyl group, and the gradually increased hydrophobicity. The water contact angle results reflect that the molecular brush was successfully grafted on the membrane surface by the CVD method, and the molecular chain length can be controlled by controlling the reaction time. Comparative Examples 3 and 4 are the water contact angles of the membranes that were CVD-grafted with PDMS molecular brushes after plasma pretreatment, which are 44.5° and 65°, respectively. The results show that the contact angles of Comparative Examples 3 and 4 are significantly lower than those of Example 2. This shows that the plasma pretreatment method cannot effectively generate CVD reaction active sites on the polyamide membrane. Although the power and reaction time of the plasma treatment are increased, the contact angle of the membrane prepared in Comparative Example 4 only rises to 65°. And from Table 1, it can be seen that high-power plasma treatment and longer reaction time will damage the polyamide layer, which is reflected in a significant decrease in flux and a decrease in salt retention rate. In summary, the method of introducing active sites by chemically grafting silane coupling agent APTES in the present invention has significant advantages. On the one hand, chemical grafting can stably introduce active groups, and on the other hand, this method will not reduce the separation performance of the base membrane.

[0141] Figure 6 The full X-ray photoelectron spectrum of the base membranes NF90, NF-a, and the membranes prepared in Examples 1 to 3 can be seen from the full spectrum that the surface of the modified polyamide nanofiltration membranes prepared in Examples 1 to 3 has been covered by a dense layer of molecular brushes, which is reflected in the fact that the N 1s characteristic peak belonging to polyamide has almost disappeared. Compared with the base membranes NF90 and NF-a, the characteristic peak Si 2p of the silicon element contained in the PDMS molecular brush appears in the full spectrum, which indicates the successful growth of the PDMS molecular brush. Furthermore, in order to verify the successful grafting of APTES, the O 1s high-resolution spectra of NF90 and NF-a were subjected to peak fitting processing, and the results are shown in Figure 2. Figure 7As shown in the figure, the “*O-Si” absorption peak appears in the O 1s characteristic peak of NF-a, which is not found in the base film NF90. This peak is mainly derived from the silicon element of APTES, which indicates the successful grafting of APTES.

[0142] Figure 8 The following are SEM images of the base membrane NF90, the silane-modified membrane NF-a, and the modified polyamide nanofiltration membranes prepared in Examples 1-3. As can be seen, the surface morphology of the NF-a membrane is not significantly different from that of the base membrane NF90. This indicates that the grafting process primarily involves the reaction of APTES with the carboxyl groups on the membrane surface, and that APTES does not self-polymerize to form a hydrogel layer on the membrane surface. The surface morphology of the modified polyamide nanofiltration membranes prepared in Examples 1, 2, and 3 differs significantly from that of the base membrane NF90. The membrane surface is covered with a smooth and dense PDMS molecular brush layer, with the polyamide layer structure almost invisible. Furthermore, the smoothness of the membrane surface decreases with increasing reaction time, indicating the uniform growth of the molecular brushes and the increasing thickness of the molecular brush layer with increasing reaction time.

[0143] The foregoing examples are merely illustrative, serving to illustrate some of the features of the method of the present invention. The appended claims are intended to claim the widest possible scope that can be envisioned, and the embodiments presented herein are demonstrated by the applicant's actual experimental results. Therefore, the applicant intends that the appended claims are not limited by the selection of examples illustrating the features of the present invention. Some numerical ranges used in the claims also include subranges therein, and variations in these ranges should also be interpreted as being covered by the appended claims where possible.

Claims

1. A method for preparing a modified polyamide nanofiltration membrane with ultra-smooth molecular brushes grafted onto its surface, characterized in that: The steps include: S1. Grafting a silane compound containing amino and alkoxy groups onto the surface of a polyamide membrane to form a silanol-functionalized layer; S2. The polyamide membrane after the reaction in step S1 is immersed in a glycerol solution, washed, and dried at 10-30 ℃; S3. A super-smooth polydimethylsiloxane molecular brush layer is formed on the surface of the polyamide membrane in step S2 by vapor deposition to prepare a modified polyamide nanofiltration membrane.

2. The preparation method according to claim 1, characterized in that In step S1, the silane compound containing an amino group and an alkoxy group is selected from one or more of 3-aminopropyltriethoxysilane, 3-aminopropylmethyldiethoxysilane, 3-aminopropyltrimethoxysilane, and 3-aminopropylmethyldimethoxysilane.

3. The preparation method according to claim 1, characterized in that: In step S1, the operation of forming the silanol functionalized layer is as follows: immersing the polyamide membrane in a silane compound solution containing amino and alkoxy groups, so that the carboxyl groups on the surface of the polyamide membrane react with the amino groups of the silane compound, and then washing to form the silanol functionalized layer.

4. The preparation method according to claim 3, characterized in that The concentration of the silane compound in the solution is 0.05-0.3wt%.

5. The preparation method according to claim 3 or 4, characterized in that: The grafting reaction time is 20-240 min.

6. The preparation method according to claim 5, characterized in that: The concentration of the silane compound in the solution is 0.08-1.5wt%, and the grafting reaction time is 20-60min.

7. The preparation method according to claim 1, characterized in that: In step S2, the concentration of the glycerol solution is 10-40 wt%; and / or the soaking time is ≥10 min.

8. The preparation method according to claim 1, characterized in that: In step S3, any one of the following (a) to (c) is selected: (a) The temperature of vapor deposition is 30-50°C; (b) Vacuum degree of vapor deposition is -0.1 to 0.05 MPa; (c) The reaction time of vapor deposition is ≥45 s.

9. A modified polyamide nanofiltration membrane with ultra-smooth molecular brushes grafted onto its surface, obtained by the preparation method according to any one of claims 1 to 8.

10. Use of the modified polyamide nanofiltration membrane with super-smooth molecular brushes grafted onto its surface as claimed in claim 9 in treating sewage and wastewater.

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