Modified polyamide nanofiltration membrane with super-smooth molecular brush grafted and grown on surface as well as preparation method and application of modified polyamide nanofiltration membrane

Abstract

The invention relates to the technical field of sewage and wastewater treatment. The invention discloses a modified polyamide nanofiltration membrane for sewage and wastewater treatment as well as a preparation method and application of the modified polyamide nanofiltration membrane. According to the method, a silane compound containing an amino group and an alkoxy group is adopted as a connecting medium of a polyamide membrane and a polydimethylsiloxane (PDMS) molecular brush, and the silane compound containing the amino group and the alkoxy group is grafted on the surface of the polyamide membrane to form a silicon hydroxyl functional layer; soaking the reacted polyamide membrane in a glycerol solution, cleaning and drying; and initiating polymerization through a chemical vapor deposition method to grow a PDMS molecular brush in situ, and preparing to obtain the modified polyamide nanofiltration membrane. The surface of the modified polyamide nanofiltration membrane has a super-smooth characteristic, is low in friction force, has relatively good pure water flux and inorganic salt interception and excellent organic-inorganic combined pollution resistance, and can be applied to devices such as sewage treatment equipment, seawater desalination equipment, industrial wastewater treatment and recycling equipment and the like.

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, its preparation method and application. Background Art

[0002] Sewage and wastewater treatment and resource utilization are key measures to solve the fresh water resource crisis in China and are major strategic needs of the country. Nanofiltration technology (NF) has been widely used in the fields of water softening, organic matter removal, heavy metal removal, and sewage treatment and resource utilization due to its advantages such as small floor area, good effluent quality, and high treatment efficiency. However, its practical application has been restricted by the problem of membrane fouling for a long time. Although polyamide nanofiltration membranes occupy 60%-70% of the global nanofiltration membrane market by virtue of their comprehensive performance advantages and mature modification technologies, their surfaces are prone to adsorb and deposit colloids, organic matters, and inorganic scales in wastewater, forming a fouling layer, resulting in a decrease in flux and even causing irreversible membrane pore blockage and cake layer formation.

[0003] To effectively alleviate this key problem of membrane fouling, it is most common to construct a hydrophilic anti-fouling buffer layer on the surface of the nanofiltration membrane to block the diffusion of pollutants to the membrane surface. This buffer layer is rich in hydrophilic functional groups such as carboxyl, hydroxyl, sulfonic acid, amino, etc. These functional groups can form a unique network structure with water molecules through hydrogen bonding, and then generate a dense hydration layer on the membrane surface to block the migration of pollutants to the membrane surface and the formation of aggregates. In addition, hydrophilic groups can weaken the hydrophobic interaction between pollutants and the membrane surface by enhancing the surface polarity. However, introducing hydrophilic functional groups on the surface of the nanofiltration membrane also increases the risk of membrane fouling to a certain extent. For example, hydrophilic groups (such as hydroxyl and carboxyl) can form hydrogen bonds with polar groups (such as amino and carboxyl) 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 and magnesium ions, etc.) are prone to promote the aggregation of negatively charged organic pollutants (such as humic acid) on the hydrophilic membrane surface through bridging with negatively charged hydrophilic groups (such as carboxyl, sulfonic acid, phosphate, etc.) to form a gel layer fouling; in the case of high salinity, the hydrogen bond network on the surface of the hydrophilic membrane will be damaged due to the "salting-out" phenomenon, resulting in a decrease in anti-fouling performance.

[0004] In view of this, based on the characteristics of superhydrophobic coatings with ultra-low surface energy, researchers proposed a low-surface-energy silicone polymer material, namely polydimethylsiloxane (PDMS). The surface energy of PDMS is about 20-22 mN / m, and it has a low elastic modulus (about 0.002 GPa). At the same time, PDMS has a relatively low glass transition temperature (Tg), endowing it with high flexibility and liquid-like properties. This characteristic makes the molecular chain flexibility of PDMS significantly superior to that of more rigid fluoropolymers. Especially in a dynamic environment, the PDMS surface can reduce the attachment of pollutants through its own micro-deformation.

[0005] However, in the field of membranes, the application of PDMS has certain limitations: First, because the PDMS molecular brush has strong flexibility and a strong hydrophobic interaction with the membrane surface, directly grafting long molecular chains onto the membrane surface is likely to block the membrane pores, resulting in a significant decrease in the membrane water flux; Second, the surface of polyamide nanofiltration membranes lacks reactive sites for forming covalent bonds with molecular chains; Third, the hydrophobicity of PDMS itself will reduce the membrane flux.

[0006] Patent publication number CN117815911A discloses a preparation method of an amphiphilic ultrafiltration membrane. First, the polyacrylonitrile ultrafiltration substrate membrane is treated by plasma, and then reactive groups are formed on its surface. Subsequently, a polymerization reaction occurs on the membrane surface by vapor deposition to form polydimethylsiloxane segments. However, the object of application of this method is the polyacrylonitrile ultrafiltration substrate membrane. When this method is applied to the widely used polyamide nanofiltration membranes on the market, the following limitations will exist: (1) The number of chemically vapor deposition reactive sites generated after the polyamide layer is treated by plasma is small, which is not conducive to subsequent grafting, resulting in poor anti-fouling performance of the polyamide nanofiltration membrane; (2) Plasma treatment will damage the membrane pore structure of the polyamide and simultaneously reduce the mechanical strength of the substrate membrane, reducing the pure water flux and inorganic salt retention of the membrane; (3) The equipment required for plasma treatment is expensive and difficult to promote on a large scale in practical applications. Summary of the Invention

[0007] Aiming at the above existing technical problems, the primary objective of the present invention is to provide a preparation method of a modified polyamide nanofiltration membrane with super-smooth molecular brushes grafted and grown on the surface for sewage and wastewater treatment. The prepared modified polyamide nanofiltration membrane not only has good pure water flux and inorganic salt retention; in addition, the modified polyamide nanofiltration membrane also has a small flux decline rate and excellent flux recovery rate, indicating its excellent anti-fouling performance.

[0008] The second objective of the present invention is to provide a modified polyamide nanofiltration membrane with super-smooth molecular brushes grafted and grown on the surface prepared by the above preparation method.

[0009] The third object of the present invention is to provide an application of a modified polyamide nanofiltration membrane with a super-smooth molecular brush grafted on its surface in the treatment of sewage and wastewater.

[0010] In order to achieve the above object, the present invention is realized through the following technical solutions: A preparation method of a modified polyamide nanofiltration membrane with a super-smooth molecular brush grafted on its surface, comprising the following steps: S1. Graft a silane compound containing amino and alkoxy groups on the surface of the polyamide membrane to form a silicon hydroxyl-functionalized layer; S2. Immerse the polyamide membrane after the reaction in step S1 in a glycerol solution, wash it, and dry it at 10 - 30 °C; S3. Gas-phase deposit a super-smooth polydimethylsiloxane molecular brush layer on the surface of the polyamide membrane in step S2 by chemical vapor deposition to obtain the modified polyamide nanofiltration membrane.

[0011] In the preparation method of the present invention, a silane compound containing amino and alkoxy groups is innovatively used as the connection medium between the polyamide membrane and the polydimethylsiloxane molecular brush. The silane compound is grafted onto the surface of the polyamide membrane through a reaction. The silane compound contains amino groups, which can react with the carboxyl groups on the polyamide membrane to form stable amide bonds, thereby enabling the silane compound to be grafted on the surface of the polyamide membrane to form a silicon hydroxyl-functionalized layer. Further, the silane compound contains alkoxy groups, which can undergo hydrolysis to form silicon hydroxyl groups. These silicon hydroxyl groups can provide sufficient reactive sites for the subsequent chemical vapor deposition reaction. Subsequently, PDMS molecular brushes are in-situ grown by initiating polymerization through chemical vapor deposition (CVD). The method provided by the present invention avoids the problem of pore blockage caused by directly grafting long PDMS molecular chains, effectively retaining the flux of the membrane; at the same time, the method is simple and fast, has strong controllability, mild reaction conditions, small reagent consumption, and will not damage the polyamide membrane, with strong reactivity, and is suitable for industrial production.

[0012] In the dynamic fouling experiments of the modified polyamide nanofiltration membrane prepared by the present invention in simulated wastewater, it shows better antifouling performance than the original membrane, and also better than the reported results of antifouling nanofiltration membranes prepared by hydrophilic modification methods, proving the practicability of this method. The modified polyamide nanofiltration membrane prepared by the present invention not only has good pure water flux and inorganic salt retention; in addition, the modified polyamide nanofiltration membrane also has a small flux decline rate and excellent flux recovery rate, indicating its excellent antifouling performance.

[0013] Preferably, in the step S1, the molecular weight cut-off of the polyamide membrane is 150 - 2000 Da; further preferably, the molecular weight cut-off of the polyamide membrane is 160 - 1000 Da; more preferably, the molecular weight cut-off of the polyamide membrane is 170 - 200 Da.

[0014] Preferably, in the 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 3-aminopropyltriethoxysilane.

[0015] Preferably, in the step S1, the operation of forming the silicon hydroxyl functional layer is as follows: soaking the polyamide membrane in a solution of a silane compound containing an amino group and an alkoxy group, allowing the carboxyl group on the surface of the polyamide membrane to react with the amino group of the silane compound, and then washing to form the silicon hydroxyl functional layer.

[0016] Preferably, the concentration of the silane compound in the solution is 0.05-0.3 wt%; further preferably, the concentration is 0.06-0.15 wt%; more preferably, the concentration is 0.08-0.12 wt%; 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%, etc., or an interval range formed by any of the above values, such as 0.05-0.15 wt%, 0.1-0.3 wt%, etc. The present invention is not limited thereto.

[0017] Preferably, the grafting reaction time is 30-240 min; further preferably, the grafting reaction time is 30-120 min; more preferably, the grafting reaction time is 30-60 min; most preferably, the grafting reaction time is 30 min. More specifically, the grafting reaction time can be 60 min, 80 min, 110 min, 140 min, 170 min, 200 min, 230 min, etc., or an interval range formed by any of the above values. The present invention is not limited thereto.

[0018] Preferably, the grafting reaction temperature is 15-60 °C; further preferably, the grafting reaction temperature is 20-30 °C; most preferably, the grafting reaction temperature is 25 °C.

[0019] In some embodiments, those skilled in the art can routinely combine the concentration of the above-mentioned silane compounds, the grafting reaction time, and / or the grafting reaction temperature, such as reacting under the conditions of a concentration of 0.1 wt%, a grafting reaction time of 30 min, and a grafting reaction temperature of 25 °C; such as a concentration of 0.15 wt%, a grafting reaction time of 60 min, and a grafting reaction temperature of 30 °C; such as a concentration of 0.1 wt%, a grafting reaction time of 120 min, and a grafting reaction temperature of 25 °C, etc. The present invention is not limited thereto.

[0020] Preferably, the concentration of the silane compounds 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 compounds 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 compounds in the solution is 1.0 wt%, and the grafting reaction time is 30 min.

[0021] Preferably, the silane compound solution containing amino and alkoxy groups is a buffer solution, and a buffer solution can be formed by adding buffer ion pairs commonly used in the art, including but not limited to phosphate (PBS), etc. More specifically, the concentration of the phosphate (PBS) buffer solution is 0.01 - 0.05 M.

[0022] Preferably, in step S1, the polyamide membrane is also pretreated and / or activated before reacting with the silane compounds.

[0023] More specifically, the pretreatment can be a pretreatment routinely carried out before modifying the polyamide membrane in the art to remove substances such as preservatives and pore - maintaining agents attached to the surface of the polyamide membrane.

[0024] 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 uses an EDC / NHS buffer solution prepared from 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.

[0025] Specifically, the EDC / NHS buffer solution is an EDC / NHS morpholineethanesulfonic acid (MES) buffer solution.

[0026] More specifically, in the buffer solution, the concentration of MES 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, in the buffer solution, the concentration of NHS is 0.001 - 0.02 mol / L, and 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.

[0027] More specifically, the time of the activation treatment is 30 - 240 min; further preferably, the time of the activation treatment is 30 - 120 min; more preferably, the time of the activation treatment is 30 - 60 min; most preferably, the time of the activation treatment is 30 min. More specifically, the temperature of the activation treatment is 15 - 60 °C; further preferably, the temperature of the activation treatment is 20 - 30 °C; most preferably, the temperature of the activation treatment is 25 °C.

[0028] Preferably, in step S2, the concentration of the glycerol solution is 10 - 40 wt%; and / or the soaking time is ≥ 10 min.

[0029] Further preferably, the concentration of the glycerol solution is 15 - 25 wt%; most preferably, the concentration of the glycerol solution is 20 wt%. Under this preferred condition, glycerol can better enter the polyamide layer to form 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.

[0030] Further preferably, the soaking time is 20 - 40 min; most preferably, the soaking time is 30 min.

[0031] Preferably, in step S2, it is dried at 15 - 25 °C; further preferably, it is dried at 20 - 25 °C.

[0032] 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.

[0033] Preferably, in some more specific embodiments, step S3 includes the following operations: The polyamide membrane after being treated in step S2 is placed in a container containing a silane monomer, and a super-smooth polydimethylsiloxane molecular brush coating is formed on the surface of the polyamide membrane by chemical vapor deposition, and then it is washed to remove the residual monomers and by-products on the membrane surface to obtain a modified polyamide nanofiltration membrane.

[0034] Preferably, in the step S3, any one of the following (a) to (c) is selected: (a) The temperature of chemical vapor deposition is 30 - 50 °C; most preferably, the temperature of chemical vapor deposition is 40 °C; (b) The degree of vacuum of chemical vapor deposition is -0.1 to 0.05 Mpa; most preferably, the degree of vacuum of chemical vapor deposition is -0.1 Mpa; (c) The reaction time of chemical vapor deposition ≥ 45 s; preferably, the reaction time of chemical vapor deposition is 45 - 360 s; more preferably, the reaction time of chemical vapor deposition is 60 - 120 s; most preferably, the reaction time of chemical vapor deposition is 90 s.

[0035] Preferably, the silane monomer is selected from at least one of dichlorodimethylsilane or dimethyldiethoxysilane.

[0036] Preferably, based on the area of the polyamide membrane, the dropping amount of the silane monomer is 0.2 - 1.1 μL / cm 2 .

[0037] Furthermore, the present invention claims the modified polyamide nanofiltration membrane with super-smooth molecular brushes grown by surface grafting prepared by the above preparation method.

[0038] Furthermore, the present invention claims the application of the modified polyamide nanofiltration membrane with super-smooth molecular brushes grown by surface grafting in treating sewage and wastewater.

[0039] More specifically, the sewage and wastewater include but are not limited to humic acid, landfill leachate, bovine serum albumin, sodium alginate, etc.

[0040] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a preparation method of a modified polyamide nanofiltration membrane with super-smooth molecular brushes grown by surface grafting, using a silane compound containing amino and alkoxy groups as the connection medium between the polyamide membrane and the polydimethylsiloxane molecular brush, and then in-situ growing PDMS molecular brushes by CVD-induced polymerization. The surface of the polyamide nanofiltration membrane prepared by the present invention has a small friction force, and has excellent water flux, inorganic salt rejection rate and anti-pollution performance. Description of the Drawings

[0041] Figure 1 It is a pollution curve graph of the base membranes NF90, NF-a and the modified polyamide nanofiltration membranes prepared in Examples 1 - 3 in treating simulated wastewater containing humic acid.

[0042] Figure 2Short-term fouling curves of the base membrane NF90 and the modified polyamide nanofiltration membrane prepared in Example 2 in treating the effluent of a landfill leachate membrane bioreactor.

[0043] Figure 3 Fouling curves of the base membrane NF90, Example 2, Examples 6 - 7, and the modified polyamide nanofiltration membrane prepared in Comparative Example 3 in treating the simulated wastewater containing bovine serum albumin.

[0044] Figure 4 Fouling curve of the modified polyamide nanofiltration membrane prepared in Example 2 in the three-cycle long-term anti-fouling test in treating the effluent of a landfill leachate membrane bioreactor.

[0045] Figure 5 Summary bar chart of the water contact angles of the base membrane NF90, NF-a, and the modified polyamide nanofiltration membranes prepared in Examples 1 - 3 and Comparative Examples 3 - 4.

[0046] Figure 6 Full X-ray photoelectron spectroscopy (XPS) spectra of the base membrane NF90, NF-a, and the modified polyamide nanofiltration membranes prepared in Examples 1 - 3.

[0047] Figure 7 High-resolution O 1s X-ray photoelectron spectroscopy (XPS) spectra of the base membranes NF90 and NF-a.

[0048] Figure 8 Scanning electron microscopy (SEM) images of the base membrane NF90, NF-a, and the modified polyamide nanofiltration membranes prepared in Examples 1 - 3. Detailed implementation manners

[0049] The present invention will be further described below in conjunction with the specification drawings and specific examples, but the examples do not limit the present invention in any form. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in the technical field.

[0050] Example 1 Preparation of a modified polyamide nanofiltration membrane with a super-smooth molecular brush grafted on the surface (1) The polyamide membrane (DuPont, Fimtec TM NF90) was rinsed repeatedly with deionized water to remove the preservative and the pore retention agent, and then soaked in deionized water for standby; (2) A 0.1 mol / L morpholineethanesulfonic acid (MES) buffer solution with a pH of 5.5 was prepared as the activation solution. 1-Ethyl-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS) were successively dissolved in the MES buffer solution, and the prepared concentrations of EDC and NHS were both 0.1 mol / L; (3) Fix the polyamide membrane in the reaction device, ensuring that the selective layer of the membrane faces upward; pour the solution prepared in step (2) above into the reaction device, place it on a shaker, and react at a speed of 60 rpm at room temperature for 30 min. After the reaction, pour out the activation solution, wash away the remaining reactants with deionized water, and keep the membrane fixed in the reaction device; (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 oscillate the solution while dripping to fully disperse APTES to form a PBS solution of APTES with a mass fraction of 0.1%. Then quickly pour the solution into the reaction device fixed with the activated substrate membrane, place it on a shaker, and react at a speed of 60 rpm at room temperature for 30 min. After the reaction, pour out the activation solution, wash away the remaining reactants with deionized water to obtain a polyamide membrane with a silicon hydroxyl functionalized layer, labeled as NF-a; (5) Prepare a 20% glycerol aqueous solution, cut the membrane into a size of 14 cm × 7 cm, then soak it in the glycerol aqueous solution for 30 min. After soaking, wash away the glycerol on the surface with pure water, and dry the polyamide membrane with cold air from a hair dryer (20 - 25 °C) for 3 min; (6) Place the dried polyamide membrane in a petri dish with a diameter of 20 cm and a height of 2 cm, and uniformly drip dichlorodimethylsilane along the side wall into the petri dish (based on the area of the polyamide membrane, the dripping amount of dichlorodimethylsilane is 0.51 μL / cm 2 ), the reaction liquid does not directly contact the polyamide membrane, cover the petri dish lid, place the petri dish in a vacuum drying oven at 40 °C, and use a vacuum pump to perform vacuum treatment to make the vacuum degree of the vacuum drying oven reach -0.1 Mpa to initiate CVD. The reaction time is 45 s. After the reaction, take out the membrane and wash away the residual reaction monomers and by-products on the surface of the polyamide membrane with n-hexane to prepare a modified polyamide nanofiltration membrane with a super-smooth molecular brush grafted and grown on the surface.

[0051] Example 2 Preparation of a modified polyamide nanofiltration membrane with a super-smooth molecular brush grafted and grown on the surface The difference between this example and Example 1 is that in step (6), the CVD reaction time is 90 s.

[0052] Example 3 Preparation of a modified polyamide nanofiltration membrane with a super-smooth molecular brush grafted and grown on the surface The difference between this example and Example 1 is that in step (6), the CVD reaction time is 180 s.

[0053] Example 4 Preparation of a modified polyamide nanofiltration membrane with a super-smooth molecular brush grafted and grown on the surface The difference between this example and Example 2 is that in step (5), the mass fraction of the glycerol aqueous solution is 5%, and the membrane soaking time is 30 min.

[0054] Example 5 Preparation of Modified Polyamide Nanofiltration Membrane with Ultra-Smooth Molecular Brushes Grown by Surface Grafting The difference between this example and Example 2 is that in step (5), the mass fraction of the glycerol aqueous solution is 10%, and the membrane soaking time is 30 min.

[0055] Example 6 Preparation of Modified Polyamide Nanofiltration Membrane with Ultra-Smooth Molecular Brushes Grown by Surface Grafting The difference between this example and Example 2 is that in step (4), the mass fraction of APTES is 0.05%.

[0056] Example 7 Preparation of Modified Polyamide Nanofiltration Membrane with Ultra-Smooth Molecular Brushes Grown by Surface Grafting The difference between this example and Example 2 is that in step (4), the mass fraction of APTES is 0.20%.

[0057] Comparative Example 1 Comparative Example 1 is used to demonstrate that the "in-situ growth" method can effectively reduce the impact of grafting on the membrane flux. Direct grafting was carried out using amino-terminated polydimethylsiloxane (PDMS) long chains. The specific steps are as follows: (1) The polyamide membrane (DuPont, Fimtec TM NF90) was rinsed with deionized water multiple times to remove preservatives and membrane pore retention agents, and then soaked in deionized water for later use; (2) Prepare a 0.1 mol / L MES buffer solution with a pH of 5.5. Dissolve EDC and NHS in the MES buffer solution successively. The prepared concentrations of EDC and NHS are both 0.1 mol / L; (3) Fix the polyamide membrane in the reaction device, ensuring that the selective layer side of the membrane faces upward; pour the solution prepared in step (2) above into the reaction device, place it on a shaker, and react at a speed of 60 rpm at room temperature for 30 min. After the reaction, pour out the activation solution, wash away the remaining reactants with deionized water, and keep the membrane fixed in the reaction device; (4) Uniformly coat the amino-terminated PDMS with a molecular weight of 1000 Da on the membrane surface, place it on a shaker, and react at a speed of 60 rpm at room temperature for 2 h. After the reaction, wash away the remaining reactants with n-hexane to obtain the membrane grafted with PDMS molecular brushes.

[0058] Comparative Example 2 The difference between this comparative example and Comparative Example 1 is that in step (4), the amino-terminated PDMS with a molecular weight of 5000 Da was used.

[0059] Comparative Example 3 Comparative Example 3 demonstrates the significant advantages of the present invention in improving the grafting density of polydimethylsiloxane (PDMS) molecular brushes by introducing active groups using the chemical grafting method, while ensuring the structural integrity of the polyamide membrane and enhancing the anti-fouling performance of the polyamide membrane. In Comparative Example 3, the method of plasma cleaning was adopted to graft PDMS molecular brushes on the polyamide membrane. The specific steps are as follows: (1) The polyamide membrane (DuPont, Fimtec TM NF90) was rinsed with deionized water multiple times to remove preservatives and pore retention agents, and then soaked in deionized water for later use; (2) A glycerol aqueous solution with a mass fraction of 20% was prepared. The base membrane was cut into a size of 14 cm × 7 cm, and then soaked in the glycerol aqueous solution for 30 min. After soaking, the surface glycerol was washed off with pure water, and the membrane was dried with a hair dryer on the cold wind for 3 min; (3) The dried membrane was subjected to air plasma treatment with a power of 100 W for 60 s to generate reactive groups on the surface of the base membrane; (4) The treated membrane was placed in a petri dish with a diameter of 20 cm and a height of 2 cm. 50 μL of dichlorodimethylsilane was uniformly dropped along the side wall into the petri dish. The reaction liquid did not directly contact the membrane. The petri dish cover was covered, and the petri dish was placed in a vacuum drying oven at 40 °C. A vacuum pump was used for vacuum treatment to make the vacuum degree of the vacuum drying oven reach -0.1 Mpa to initiate chemical vapor deposition. The reaction time was 90 s respectively. After the reaction ended, the membrane was taken out, and the residual reaction monomers and by-products on the membrane surface were washed off with n-hexane to obtain a modified polyamide membrane.

[0060] Comparative Example 4 The difference between this comparative example and Comparative Example 3 is that the power of plasma treatment is 200 W and the time is 300 s.

[0061] Comparative Example 5 The difference between this comparative example and Example 2 is that in step (5), the drying method of the membrane is: the membrane after washing off the surface glycerol with pure water was dried in an oven at 60 °C for 10 min.

[0062] Comparative Example 6 The difference between this comparative example and Example 2 is that in step (5), instead of soaking with a glycerol aqueous solution, the surface of the membrane was directly blown with a blower for 3 min until dry.

[0063] Test Example The following will conduct performance analysis experiments on the modified polyamide nanofiltration membranes prepared in each example and comparative example.

[0064] I. Experimental Object Modified polyamide nanofiltration membranes obtained by the preparation methods of each example and comparative example.

[0065] II. Experimental methods The modified polyamide nanofiltration membranes prepared in each example and comparative example were tested by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), determination of hydrophilic and hydrophobic properties, pure water flux, and anti-fouling performance during the filtration of simulated wastewater containing humic acid and bovine serum albumin and the effluent of a membrane bioreactor for landfill leachate. Among them, (1) Determination of pure water flux: In this experiment, a cross-flow filtration device was used for determination. The effective area of the membrane was 16 cm 2 , the 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 was stable, the water permeation flux of the membrane was recorded J , and the calculation formula is:

[0066] Among them, V represents the volume of leachate (L), A represents the effective membrane area (m 2 ), represents the filtration duration (h).

[0067] (2) Determination of membrane rejection rate: The rejection rate of the modified polyamide nanofiltration membrane to salts in 2000 mg / L MgCl2 and Na2SO4 solutions was determined. The membrane rejection rate was calculated according to the following formula:

[0068] Among them, C F and C P represent the conductivity of the concentrate and the permeate, respectively.

[0069] (3) Anti-fouling performance test: The humic acid (HA) simulated wastewater simulated the composition of actual landfill leachate and was prepared from 6 g / L NaCl, 0.15 g / L CaCl2 and 0.1 g / L humic acid, and the pH was adjusted to 7.0. Before the test, the modified polyamide nanofiltration membrane was pre-pressed with 30 bar ultrapure water overnight, and the initial flux was adjusted with 6 g / L NaCl solution J from 0 to 30 L·m -2 ·h -1 . The fouling test lasted for 24 hours. During this period, the permeate was refluxed every hour to keep the total osmotic pressure of the feed liquid constant and maintain the membrane surface flow rate at 0.44 m·s -1And at a temperature of 25 ± 3°C. After the test, the membrane was rinsed with ultrapure water for 30 minutes, and the flux after cleaning was tested with a NaCl solution having the same conductivity. J 2, and then the flux recovery ratio (FRR) was evaluated. The flux decline ratio (FDR) and the flux recovery ratio (FRR) were calculated respectively through the initial flux ( J 0), the flux after fouling ( J t ), and the flux after cleaning ( J 2) using the following formulas:

[0070]

[0071] Where, J 0 and J t are the initial flux and the final flux during the test respectively, J 2 is the NaCl flux after physical cleaning after fouling filtration.

[0072] Bovine serum albumin (BSA) simulated wastewater was used to evaluate the treatment effect of wastewater treatment processes and equipment on protein-containing wastewater, which was prepared from 2 g / L NaCl, 0.15 g / L CaCl2 and 0.1 g / L bovine serum albumin. Before the test, the modified polyamide nanofiltration membrane was pre-pressed with ultrapure water at 30 bar overnight, and the initial flux was adjusted to 30 L·m -2 ·h -1 using a 2 g / L NaCl solution. The fouling test lasted for 6 hours, during which the permeate was refluxed every hour to keep the total osmotic pressure of the feed liquid constant and maintain a transmembrane flow rate of 0.44 m·s -1 and a temperature of 25 ± 3°C. After the test, the membrane was rinsed with ultrapure water for 30 minutes, and the flux recovery rate (FRR) of the membrane was evaluated with a NaCl solution having the same conductivity.

[0073] The actual wastewater was taken from the effluent of a membrane bioreactor of a landfill leachate treatment plant in Guangzhou. The water sample was stored at 4°C. The pretreatment included filtration through a 0.22 μ μm polyethersulfone microfiltration membrane to remove suspended particulate matter, and the metal ion and organic matter concentrations were detected by inductively coupled plasma optical emission spectrometry (ICP-OES) and total organic carbon (TOC) analyzer. During the test, the membrane was first pre-pressed with a sodium chloride solution having the same conductivity as the actual water and the initial flux was adjusted to 30 ± 2 L·m -2 ·h -1, using 4.5 L of wastewater as the raw material liquid, continuously filtered until the cumulative permeation 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 with a NaCl solution having the same conductivity.

[0074] III. Experimental Results Table 1

[0075] Table 1 shows the pure water permeation flux and inorganic salt rejection rates of the modified polyamide nanofiltration membranes prepared from NF90, NF-a, Examples 1-7, and Comparative Examples 1-6. From the results, it can be seen that regardless of the method used to graft the PDMS molecular brush, the flux of the modified membranes decreased significantly. The pure water permeation fluxes of the original membrane NF90 and the membrane grafted with the silane coupling agent NF-a were 7.70 and 6.50 L·m -2 ·h -1 ·bar -1 , respectively. When the CVD reaction time was 45 s, 90 s, and 180 s, the pure water permeation fluxes of the membranes of Examples 1-3 were 4.15, 3.90, and 3.25 L·m -2 ·h -1 ·bar -1 , respectively. This is because as the CVD reaction time increases, the thickness of the PDMS layer gradually increases, which makes the resistance to water molecule diffusion greater, so the pure water permeation flux gradually decreases. At the same time, the presence of the PDMS layer reduces the pore size of the membrane. On the premise of maintaining a high rejection rate of sodium sulfate, the rejection rates of magnesium chloride for the membranes of Examples 1-3 increased to 97.12%, 96.38%, and 96.69% compared with 94.78% of the base membrane NF90 and 94.93% of the silane coupling agent modified membrane, respectively.

[0076] Examples 2, Comparative Example 5, and Comparative Example 6 explored the influence of different drying methods on the pure water permeation flux of the finally prepared PDMS molecular brush modified membrane. Among them, the pure water fluxes of the membranes prepared in Comparative Example 5 and Comparative Example 6 decreased the most severely. The pure water permeation flux of Comparative Example 6 was only 0.57 L·m -2 ·h -1 ·bar -1 , and in the test conditions, water did not flow out even in Comparative Example 5. This is mainly because during the drying process of the polyamide membrane, the hydrogen bond network between the polyamide and the solvent molecules is destroyed, weakening the support of the solvent for the membrane pores, and heating will accelerate the thermal motion between molecules, leading to the softening of the polyamide layer, thus further causing the collapse of the membrane pores. Therefore, the flux of Comparative Example 5 is lower than that of Comparative Example 6 dried at room temperature.

[0077] To reduce the impact of drying on the polyamide layer, glycerol is introduced in the present invention. It can form a more stable and firm hydrogen bond network with the polyamide layer to support the membrane pores. Therefore, during the membrane drying process, the membrane pores are not prone to collapse, and thus the pure water permeation flux of the prepared membrane can be maintained.

[0078] Examples 4, 5 and 2 compared the protective effect of glycerol aqueous solution on the membrane pores. The pure water permeation fluxes of the finally prepared membranes were 3.20, 3.60 and 3.90 L·m -2 ·h -1 ·bar -1 respectively. This shows that with the increase in the concentration of glycerol aqueous solution, more glycerol molecules can enter the polyamide layer to form a more stable hydrogen bond network to support the membrane pores.

[0079] Examples 6 - 7 are membranes prepared at different grafting concentrations of silane coupling agent APTES. Their pure water permeation fluxes are 5.25 and 2.60 L·m -2 ·h -1 ·bar -1 respectively, the sodium sulfate rejection rates are 98.23% and 99.01%, and the magnesium chloride rejection rates are 93.64% and 95.36%. Combining with Example 2, it can be seen that with the increase in the APTES grafting concentration, the pure water permeation flux of the final membrane gradually decreases. This is mainly because the number of sites for CVD reaction on the membrane surface increases, and finally a denser PDMS molecular brush layer is formed, increasing the transport resistance of water molecules.

[0080] In this study, PDMS molecular chains with molecular weights of 1000 Da and 5000 Da were grafted onto the membrane surface by the direct grafting method (Comparative Examples 1 - 2). The test results show that the pure water permeation fluxes of the obtained PDMS modified membranes are 0.60 and 0.34 L·m -2 ·h -1 ·bar -1 respectively, which are significantly lower than those of the modified membranes prepared by grafting PDMS brushes using the CVD method. Analyzing the reasons, it is mainly because long-chain PDMS molecules have a low surface energy and are prone to penetrate into the membrane pores during the direct grafting process, resulting in membrane pore blockage and thus 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, and thus better maintains the permeation performance of the membrane. Therefore, the PDMS modified membranes prepared by the direct grafting method have limited value in practical applications due to their too low permeation flux.

[0081] To demonstrate the advantages of the present invention over the existing method of generating active groups by plasma treating the membrane and then grafting PDMS molecular brushes by CVD in terms of grafting efficiency and sustainability, Experiments 3 to 4 of the comparative examples were designed in this application. The polyamide membrane was also treated by plasma and then PDMS molecular brushes were grafted by CVD, and the pure water permeation flux and inorganic retention performance of the finally 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 -1 of the membrane prepared in Example 2, the inorganic salt retention rate of Comparative Example 3 did not increase compared with the base membrane NF90. The sodium sulfate retention rate decreased from 99.26% to 97.16%, and the magnesium chloride retention rate decreased from 94.78% to 92.45%. This may be due to the destructive effect of plasma treatment on the polyamide selective layer. Continuing to increase the power and treatment time of plasma treatment, the membrane flux prepared in Comparative Example 4 decreased significantly to 2.18 L·m -2 ·h -1 ·bar -1 , the sodium sulfate retention rate decreased to 96.34%, and the magnesium chloride retention rate decreased to 90.12%. The decrease in flux may be due to the fact that plasma destroyed the cross-linked structure within the polyamide layer, causing the membrane pores to collapse and resulting in hindered water molecule transmission. The decrease in the inorganic salt retention rate may be because plasma treatment destroyed the microscopic structure of the membrane pores, causing the membrane pores in some regions of the polyamide layer to expand, resulting in the permeation of inorganic salts through the selective layer. The changes in pure water flux and inorganic salt retention rate indicate the limitations of plasma treatment in modifying polyamide membranes. In addition, the water contact angles of Comparative Examples 3 to 4 will be tested below to evaluate the efficiency of grafting PDMS molecular brushes by the plasma method.

[0082] Table 2

[0083] Table 2 shows the FDR and FRR of the polyamide nanofiltration membranes prepared from NF90, NF-a, Examples 1 to 3, and Examples 6 to 7 in the filtration tests of simulated wastewater containing humic acid, simulated wastewater containing bovine serum albumin, and actual landfill leachate wastewater. Figure 1 For the treatment of simulated wastewater containing humic acid, the fouling curves of the modified polyamide nanofiltration membranes prepared from NF90, NF-a, and Examples 1 to 3, with their FDR being 24.73%, 27.87%, 25.23%, 12.16%, and 18.19% respectively, and their FRR being 82.66%, 82.74%, 83.86%, 92.07%, and 86.68% respectively.

[0084] The pollution test results of humic acid simulated wastewater show that the modified polyamide nanofiltration membrane prepared in Example 2 has the best anti-pollution performance, with its FDR lower than that of other membranes and its FRR higher than that of other membranes. This is mainly because the PDMS molecular brush grafted by the method in Example 2 has an appropriate length, which on the one hand weakens the interaction between the polyamide nanofiltration membrane and pollutants, and on the other hand avoids the entanglement of chains resulting in a decrease in fluidity.

[0085] In contrast, in Example 1, the CVD reaction time is shorter, the length of the PDMS molecular brush grown on the surface is insufficient, and the coating coverage is uneven. At the same time, the rigidity of the short chain is greater than that of the long chain, and the fluidity of the molecular chain is poor, resulting in a weakened ability of the coating to release pollutants. In Example 3, the CVD reaction time is longer, and although a dense PDMS molecular brush coating grows on the membrane surface, the overly long molecular chains will entangle with each other, leading to a poor flow of the molecular chains and losing the function of pollutant release.

[0086] Figure 2 It is the short-term pollution curve graph of the base membrane NF90 and the modified polyamide nanofiltration membrane prepared in Example 2 in the MBR ultrafiltrate of landfill leachate. During the process of treating 4.5 L of actual wastewater, finally, the cumulative effluent volume of the modified polyamide nanofiltration membrane prepared in Example 2 reached 3 L, and the FDR and FRR values were 28.65% and 90.05% respectively; while when the FDR of the NF90 membrane reached 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 shows that the modified polyamide nanofiltration membrane prepared in Example 2 has significant anti-pollution performance advantages compared with the commercial nanofiltration membrane NF90 in the actual water treatment process. The content of organic matter and divalent salts in actual water is higher than that in humic acid simulated wastewater. On the one hand, divalent salt ions can accelerate the migration of pollutants to the membrane surface through bridging, and secondly, some divalent salt ions (such as calcium and magnesium ions) are prone to combine with anions such as SO4 2- 、CO3 2- 、PO4 2- etc. to cause irreversible inorganic fouling of the membrane, resulting in a significant decrease in water flux. The PDMS molecular brush grafted on the membrane surface has strong flexibility, which promotes the release of pollutants under the cross-flow action, delays the formation of inorganic fouling, and significantly improves the anti-pollution performance of the membrane.

[0087] Figure 3Pollution curves of the modified polyamide nanofiltration membranes prepared from the base membrane NF90, Example 2, Examples 6-7, and Comparative Example 3 in simulated wastewater containing bovine serum albumin. The finally measured FDR values are 35.39%, 14.52%, 22.67%, 12.41%, and 26.87% respectively, and the FRR values are 85.24%, 96.17%, 93.25%, 95.85%, and 90.13% respectively. Comparing the anti-pollution test results of the base membrane NF90, Example 2, Example 6, and Example 7, it can be seen that the modified polyamide nanofiltration membranes prepared in Example 2 and Example 7 have the best anti-pollution performance. This is mainly because the concentration of the grafted silane coupling agent APTES is relatively high, and there are more CVD reaction sites on the membrane surface, so that a more uniform molecular brush coating can be finally prepared, and thus the anti-pollution performance is better.

[0088] Comparing the anti-pollution test results of the base membrane NF90, Example 2, and Comparative Example 3 shows that the performance of the PDMS-modified polyamide membrane prepared by chemically grafting an amino silane coupling agent combined with CVD in Example 2 is significantly higher than that of the modified polyamide nanofiltration membrane modified by the existing technology, i.e., plasma treatment combined with CVD, in Comparative Example 3. On the one hand, this is because the polyamide membrane mainly contains amide bonds, and the lone pair electrons of the nitrogen atoms in it form a resonance structure with the carbonyl group, dispersing the electron cloud density and reducing the sensitivity to plasma attack. On the other hand, it is because the polyamide membrane has a relatively strong polarity, making plasma treatment more likely to introduce oxygen-containing groups (such as carbonyl groups and carboxyl groups) on the polyamide surface rather than some polar groups (such as hydroxyl groups that can be used for CVD reactions). This makes the application value of the method of modifying polyamide membranes by the existing technical scheme, i.e., plasma treatment combined with CVD, limited in practice.

[0089] Based on the results of Table 1 and Table 2, in terms of the values of FDR and FRR, the difference in anti-pollution performance between Example 2 and Example 7 is not significant, but the pure water permeation flux of Example 2 is significantly higher than that of Example 7. Therefore, Example 2 is selected as the best condition for preparing the PDMS molecular brush modified membrane.

[0090] Table 3

[0091] Furthermore, the modified polyamide nanofiltration membrane prepared in Example 2 was subjected to a long-term multi-cycle actual water filtration test to evaluate the sustainable usability of the modified membrane. The FDR and FRR values of each cycle are shown in Table 3, and the pollution curve is as Figure 4As shown. The results show that membrane fouling gradually worsens from cycle 1 to cycle 3. The FDR values for each cycle are 29.08%, 44.44%, and 68.25% respectively, and the FRR values are 93.41%, 75.09%, and 74.48% respectively. The reasons for the significant decline in membrane flux in cycle 2 and cycle 3 are as follows. On the one hand, as the concentration of pollutants in the wastewater gradually increases, the pollutant release efficiency of the PDMS molecular brush is lower than the migration efficiency of pollutants to the membrane surface, resulting in the continuous accumulation of pollutants and exacerbating the concentration polarization phenomenon. On the other hand, as the concentration continues, the osmotic pressure of the solution gradually increases, reducing the driving force for water transport. After simple physical cleaning of the membranes at the end of cycle 2 and cycle 3, the flux recovery rate is significantly lower than that after cleaning in cycle 1. This is mainly because the concentration of divalent inorganic salts in the actual water is relatively high in the later stage of concentration, exceeding the critical concentration for crystal formation. At the same time, the pollutant release rate of the PDMS molecular brush is lower than the migration rate of crystals to the membrane surface, ultimately resulting in irreversible inorganic scaling on the membrane surface and reducing the membrane flux.

[0092] Figure 5 Bar chart summarizing the water contact angles of the base membrane NF90, NF-a, and the modified polyamide nanofiltration membranes prepared in Examples 1 to 3 and Comparative Examples 3 to 4. From Figure 5The 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 increases 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, resulting in a decrease in the hydrophilicity of the membrane surface. After CVD grafting of PDMS, the contact angle increases significantly again, and with the increase of the CVD reaction time, the water contact angle of the membrane shows a gradually increasing trend. The water contact angles of Example 1, Example 2, and Example 3 are 90.6°, 95.5°, and 97.2° respectively. This is mainly because the longer the reaction time, the longer the PDMS molecular chain obtained, and the number of hydrophobic methyl groups also increases, resulting in a gradual increase in hydrophobicity. The results of the water contact angle reflect that molecular brushes have been successfully grafted onto the membrane surface by the CVD method, and the molecular chain length can be controlled by controlling the reaction time. Comparative Examples 3-4 are the water contact angles of the membranes grafted with PDMS molecular brushes by CVD after plasma pretreatment, which are 44.5° and 65° respectively. The results show that the contact angles of Comparative Examples 3-4 are significantly lower than those of Example 2. This shows that the method of plasma pretreatment cannot effectively generate CVD reaction active sites on the polyamide membrane. Although the power and reaction time of 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, manifested as a significant decrease in flux and salt rejection rate. In summary, the method of introducing active sites by chemically grafting the 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.

[0093] Figure 6 Figure 4 shows the full X-ray photoelectron spectroscopy spectra of the base membranes NF90, NF-a, and the membranes prepared in Examples 1-3. It can be seen from the full spectra that the surfaces of the modified polyamide nanofiltration membranes prepared in Examples 1-3 are covered by a dense layer of molecular brush layer. It is reflected that the characteristic peak of N 1s belonging to polyamide has almost disappeared, while compared with the base membranes NF90 and NF-a, the characteristic peak Si 2p of silicon element contained in the PDMS molecular brush appears in the full spectra, which indicates the successful growth of the PDMS molecular brush. Further, 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 treatment, and the results are as Figure 7 shown. It can be seen from the figure that the "O-Si" absorption peak that does not exist in the base membrane NF90 appears in the O 1s characteristic peak of NF-a, which mainly comes from the silicon element of APTES, indicating the successful grafting of APTES.

[0094] Figure 8 SEM images of the base membrane NF90, the silane-modified membrane NF-a, and the modified polyamide nanofiltration membranes prepared in Examples 1 to 3. As can be seen from the figures, there is no obvious difference in the surface morphology between membrane NF-a and the base membrane NF90, indicating that the grafting process mainly involves the reaction of APTES with the carboxyl groups on the membrane surface, and APTES does not self-polymerize on the membrane surface to form a hydrogel layer. There are significant differences in the surface morphology structures of the modified polyamide nanofiltration membranes prepared in Examples 1, 2, and 3 compared with the base membrane NF90. A smooth and dense PDMS molecular brush layer covers the membrane surface, and the structure of the polyamide layer can hardly be observed. Moreover, the smoothness of the membrane surface gradually decreases with the increase in the reaction time, indicating the uniform growth of the molecular brush and the increase in the thickness of the molecular brush layer with the increase in the reaction time.

[0095] The foregoing examples are illustrative only and are used to explain some of the features of the method of the present invention. The appended claims are intended to claim the broadest scope conceivable, and the examples presented herein are supported by the applicant's actual test results. Therefore, the applicant's intention is that the appended claims should not be limited by the selection of examples that illustrate the features of the present invention. Some of the numerical ranges used in the claims also include sub-ranges within them, and variations within these ranges should also be interpreted as being covered by the appended claims whenever possible.

Claims

1. A preparation method of a modified polyamide nanofiltration membrane with a super-smooth molecular brush grafted on the surface, characterized in that, It includes the following steps: S1. Graft a silane compound containing amino and alkoxy groups on the surface of the polyamide membrane to form a silicon hydroxyl-functionalized layer; S2. Immerse the polyamide membrane after the reaction in step S1 in a glycerol solution, wash it, and dry it at 10 - 30 °C; S3. Gas-phase deposit a super-smooth polydimethylsiloxane molecular brush layer on the surface of the polyamide membrane in step S2 by gas-phase deposition method to prepare a modified polyamide nanofiltration membrane.

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

3. The preparation method according to claim 1, wherein, In the step S1, the operation of forming the silicon hydroxyl-functionalized layer is as follows: Immerse the polyamide membrane in a solution of a silane compound 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, wash it, and form a silicon hydroxyl-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 - 0.3 wt%.

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 - 1.5 wt%, and the grafting reaction time is 20 - 60 min.

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

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

9. A modified polyamide nanofiltration membrane with a super-smooth molecular brush grafted and grown on the surface prepared by the preparation method according to any one of claims 1 - 8.

10. Application of the modified polyamide nanofiltration membrane with a super-smooth molecular brush grafted and grown on the surface according to claim 9 in treating sewage and wastewater.

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