A method for preparing a composite nanofiltration membrane

Abstract

The application discloses a preparation method of a composite nanofiltration membrane, and specifically comprises the following steps: (1) preparing a water-phase monomer solution; (2) preparing an organic-phase monomer solution; (3) preparing a water solution of a diamine monomer end-capped polyethylene glycol polymer; (4) preparing an alkyl sulfone lactone organic solution; (5) immersing the surface of a porous ultrafiltration base in the water-phase monomer solution; (6) immersing the surface of the porous ultrafiltration base in the organic-phase monomer solution; (7) immersing the surface of a nascent nanofiltration membrane in the water solution of the diamine monomer end-capped polyethylene glycol polymer; (8) immersing the surface of the nanofiltration membrane grafted with the diamine monomer end-capped polyethylene glycol polymer in the alkyl sulfone lactone organic solution, drying, and rinsing, and the composite nanofiltration membrane is obtained. Compared with traditional nanofiltration membranes, the water permeability, desalination rate and anti-pollution performance of the composite nanofiltration membrane are obviously improved, the composite nanofiltration membrane can be widely applied to the fields of drinking water purification, sewage treatment, seawater desalination and brackish water desalination, and has a wide industrial application prospect.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of nanofiltration membranes, in particular to a preparation method of a composite nanofiltration membrane. BACKGROUND

[0002] Most of the water resources on the earth cannot be directly utilized, and a suitable process needs to be adopted to effectively remove inorganic / organic suspended matters, colloids and various salt impurities in seawater, brackish water and sewage. Among them, the hydrodynamic size of salt and small-molecule organic matters is close to that of water molecules, which complicates the separation process based on size screening. In order to solve this problem, it is urgent to develop a high-efficiency and energy-saving separation technology to selectively intercept ions and small-molecule organic matters in water, while ensuring the rapid transmission of water, so as to realize the efficient purification of water.

[0003] The nanofiltration membrane technology is favored in the field of water deep treatment due to the advantages of small occupied area, phase change-free process, environmental friendliness, simple operation and effective interception of salt and small-molecule organic matters in water. According to the size, charge and chemical affinity of pollutants, the nanofiltration membrane can realize accurate separation of pollutants, and provide an effective solution to the increasingly serious water pollution and water shortage problems.

[0004] The polyamide nanofiltration membrane is the most widely used nanofiltration membrane in the field of water deep treatment at present due to its good stability and acceptable price. However, the traditional polyamide nanofiltration membrane also has many problems in actual use, mainly as follows: (1) low flux, the water permeability of the membrane and the solute separation efficiency are limited by the upper limit of the balance; (2) it is difficult to realize effective separation of multivalent cations in water; (3) the interaction between the membrane and the pollutants aggravates the membrane pollution, reduces the separation efficiency and increases the operation cost.

[0005] Therefore, how to develop a polyamide nanofiltration membrane with high flux, high desalination rate and strong anti-pollution ability is a problem to be solved by those skilled in the art. SUMMARY

[0006] In view of this, the purpose of the application is to provide a preparation method of a composite nanofiltration membrane with high flux, high desalination rate and strong anti-pollution ability to solve the problems in the prior art.

[0007] In order to achieve the above purpose, the application adopts the following technical scheme:

[0008] The preparation method of the composite nanofiltration membrane specifically comprises the following steps:

[0009] (1) dissolving the water-phase monomer in water to obtain a water-phase monomer solution for standby use;

[0010] (2) Dissolve the polybasic acid chloride in the organic solvent A to obtain an organic phase monomer solution, which is ready for use;

[0011] (3) Dissolve the polyethylene glycol diacrylate and excess diamine monomer in water, and react under heating. After cooling, remove the unreacted diamine monomer, and then dissolve the product in water after vacuum freeze-drying to obtain a diamine monomer-terminated polyethylene glycol polymer aqueous solution, which is ready for use;

[0012] (4) Dissolve the alkylsulfone lactone in the organic solvent B to obtain an alkylsulfone lactone organic solution, which is ready for use;

[0013] (5) Submerge the surface of the porous ultrafiltration substrate with the water phase monomer solution, then pour out the water phase monomer solution and remove the droplets remaining on the surface of the porous ultrafiltration substrate to obtain a porous ultrafiltration substrate storing the water phase monomer in the pores;

[0014] (6) Submerge the porous ultrafiltration substrate storing the water phase monomer in the pores with the organic phase monomer solution, then pour out the excess organic phase monomer solution to obtain a nascent nanofiltration membrane;

[0015] (7) Submerge the surface of the nascent nanofiltration membrane with the diamine monomer-terminated polyethylene glycol polymer aqueous solution, then pour out the diamine monomer-terminated polyethylene glycol polymer aqueous solution to obtain a nanofiltration membrane grafted with the diamine monomer-terminated polyethylene glycol polymer;

[0016] (8) Submerge the surface of the nanofiltration membrane grafted with the diamine monomer-terminated polyethylene glycol polymer with the alkylsulfone lactone organic solution, then pour out the excess alkylsulfone lactone organic solution, dry and rinse to obtain the composite nanofiltration membrane.

[0017] Further, in the above step (1), the water phase monomer is a molecule having an amino group number of 2 or more, and is preferably at least one of piperazine, m-phenylenediamine and 3,5-diaminobenzoic acid; and the mass fraction of the water phase monomer solution is 0.02% to 5.0%.

[0018] Further, in the above step (2), the polybasic acid chloride is at least one of isophthaloyl chloride and trimesoyl chloride; the organic solvent A is at least one of n-hexane, cyclohexane, toluene, n-heptane and n-octane; and the mass fraction of the organic phase monomer solution is 0.01% to 0.3%.

[0019] Further, in the above step (3), the molecular weight of the polyethylene glycol diacrylate is 3000 to 20000 Da; the diamine monomer is at least one of piperazine and m-phenylenediamine; the reaction temperature is 50 to 80°C, and the reaction time is 2 to 8 h; the removal process is repeated more than four times using a membrane separation device with a molecular weight cut-off of 500 to 2000 Da; the pressure for vacuum freeze-drying is 40 to 80 kPa, the temperature is -20 to -40°C, and the time is 1 to 3 h; and the mass fraction of the diamine monomer-terminated polyethylene glycol polymer aqueous solution is 0.02% to 5.0%.

[0020] Further, in the step (4), the alkylsulfonolactone is 1,3-propane sulfonolactone; the organic solvent B is at least one of methanol, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide and acetonitrile; the mass fraction of the alkylsulfonolactone organic solution is 0.1% to 1.0%.

[0021] Further, in the step (5), the porous ultrafiltration substrate is any one of polyvinylidene fluoride ultrafiltration membrane, polysulfone ultrafiltration membrane, polyethersulfone ultrafiltration membrane, sulfonated polyethersulfone ultrafiltration membrane and polyacrylonitrile ultrafiltration membrane; the time of immersion is 0.5 to 30 min; the liquid droplets remaining on the surface of the porous ultrafiltration substrate are removed.

[0022] Further, in the step (6), the time of immersion is 5 to 300 s.

[0023] Further, in the step (7), the time of immersion is 1 to 60 min.

[0024] Further, in the step (8), the temperature of immersion is 40 to 60℃, and the time is 2 to 8 h; the temperature of drying is 30 to 80℃, and the time is 1 to 20 min.

[0025] According to the technical solution, compared with the prior art, the application has the following beneficial effects:

[0026] 1. The composite nanofiltration membrane is composed of an ultrafiltration substrate, a polyamide layer and a polymer molecular brush layer containing zwitterionic functional groups, and the preparation process is as follows: first, a diamine monomer-terminated polyethylene glycol polymer is prepared by a Michael addition method, then a nascent polyamide layer is prepared on the ultrafiltration substrate by an interfacial polymerization method, then the diamine monomer-terminated polyethylene glycol polymer is grafted on the surface of the nascent polyamide layer by a secondary interfacial polymerization method, and finally, the diamine monomer-terminated polyethylene glycol polymer is reacted with a sulfonic acid lactone or an alkyl sulfonate to form a polymer molecular brush layer containing zwitterionic functional groups, one end of which is fixed on the polyamide layer and the other end is in a free branched chain state, which can simultaneously exert the hydration layer effect and steric hindrance effect of zwitterionic polymers, thereby enhancing the repulsion of pollutants in water.

[0027] 2. The composite nanofiltration membrane has strong hydrophilicity, and the anti-pollution performance of the membrane is improved by forming a dense hydration layer to prevent non-specific interaction of pollutants with the membrane surface.

[0028] 3. Compared with traditional nanofiltration membranes, the water permeability, desalination rate and anti-pollution performance of the composite nanofiltration membrane are significantly improved, which can be widely applied in the fields of drinking water purification, sewage treatment, seawater desalination and brackish water desalination, and has broad industrial application prospects. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 Flow chart for synthesizing diamine monomer end-capped polyethylene glycol polymer;

[0030] Figure 2 Flow chart for synthesizing composite nanofiltration membrane;

[0031] Figure 3 Surface SEM images of nanofiltration membranes of Examples 1-2 and Comparative Examples 1-2;

[0032] Figure 4 Surface contact angles of nanofiltration membranes of Examples 1-2 and Comparative Examples 1-2;

[0033] Figure 5 Pure water fluxes of nanofiltration membranes of Examples 1-2 and Comparative Examples 1-2;

[0034] Figure 6 Desalination rates of nanofiltration membranes of Examples 1-2 and Comparative Examples 1-2;

[0035] Figure 7 Normalized fluxes of nanofiltration membranes of Examples 1-2 and Comparative Examples 1-2. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0037] In the following examples, the reagents used are commercially available, and various processes and methods not described in detail are conventional methods known in the art, and the experimental operations and experimental conditions not mentioned refer to the conventional operations and conventional conditions in the art.

[0038] Example 1

[0039] The method for preparing the composite nanofiltration membrane specifically comprises the following steps:

[0040] (1) 1.0 g of anhydrous piperazine was dissolved in 50 mL of pure water to obtain a piperazine solution in water with a mass fraction of 2.0%, which was prepared for use;

[0041] (2) 0.05 g of trimesoyl chloride was dissolved in 50 mL of n-hexane to obtain a trimesoyl chloride solution in organic phase with a mass fraction of 0.1%, which was prepared for use;

[0042] (3) 2.0 g of polyethylene glycol diacrylate with a molecular weight of 6000 Da and 2.0 g of anhydrous piperazine were dissolved in 200 mL of pure water, heated to 70°C, and reacted for 6.0 h with sufficient stirring. After the reaction was completed, the solution was cooled to room temperature. The unreacted anhydrous piperazine was removed by repeatedly screening the solution in a membrane separation device with 200 mL of pure water for four times. Then, the screened solution was vacuum freeze-dried at a pressure of 60 kPa and a temperature of -30°C for 6.0 h to obtain a piperazine-terminated polyethylene glycol polymer (as shown in FIG. 1). Finally, 1.0 g of the piperazine-terminated polyethylene glycol polymer was dissolved in 50 mL of pure water to obtain a piperazine-terminated polyethylene glycol polymer aqueous solution with a mass fraction of 2.0%, which was used for the next step; Figure 1

[0043] (4) 0.1 g of 1,3-propane sultone was dissolved in 50 mL of acetonitrile to obtain an alkane sultone organic solution with a mass fraction of 0.2%, which was used for the next step;

[0044] (5) The water phase piperazine solution was immersed on the surface of the polysulfone ultrafiltration membrane substrate for 5 min. Then, the water phase piperazine solution was poured out, and the residual droplets on the surface of the polysulfone ultrafiltration membrane substrate were blown off with compressed air to obtain a polysulfone ultrafiltration membrane substrate storing the water phase monomer in the pores;

[0045] (6) The organic phase trimesoyl chloride solution was immersed on the surface of the polysulfone ultrafiltration membrane substrate storing the water phase monomer in the pores for 1 min. Then, the excess organic phase trimesoyl chloride solution was poured out to obtain a nascent nanofiltration membrane;

[0046] (7) The piperazine-terminated polyethylene glycol polymer aqueous solution was immersed on the surface of the nascent nanofiltration membrane for 5 min. Then, the piperazine-terminated polyethylene glycol polymer aqueous solution was poured out to obtain a piperazine-terminated polyethylene glycol polymer grafted nanofiltration membrane;

[0047] (8) The sulfonic acid lactone organic solution was immersed on the surface of the piperazine-terminated polyethylene glycol polymer grafted nanofiltration membrane at 50°C for 4.0 h to perform a quaternary amination reaction. Then, the excess sulfonic acid lactone organic solution was poured out and dried at 40°C for 5 min. Finally, the composite nanofiltration membrane (as shown in FIG. 2) was obtained by thoroughly rinsing the membrane with pure water. Figure 2

[0048] Example 2

[0049] The preparation method of the composite nanofiltration membrane specifically includes the following steps:

[0050] (1) 0.5 g of anhydrous piperazine was dissolved in 50 mL of pure water to obtain a water phase piperazine solution with a mass fraction of 1.0%, which was used for the next step;

[0051] ​​(2) 0.025 g of trimesoyl chloride was dissolved in 50 mL of n-heptane to obtain an organic phase trimesoyl chloride solution with a mass fraction of 0.05%, which was prepared for use;

[0052] (3) 2.0 g of polyethylene glycol diacrylate with a molecular weight of 20,000 Da and 2.0 g of m-phenylenediamine were dissolved in 200 mL of pure water, and the solution was heated to 60°C and stirred for 4.0 h. After the reaction was completed, the solution was cooled to room temperature. The unreacted m-phenylenediamine was removed by repeatedly screening the solution in a membrane separation device with 200 mL of pure water four times. Then, the screened solution was vacuum freeze-dried at a pressure of 60 kPa and a temperature of -20°C for 12.0 h to obtain m-phenylenediamine-terminated polyethylene glycol polymers (as shown in Figure 1 Finally, 0.5 g of the m-phenylenediamine-terminated polyethylene glycol polymers was dissolved in 50 mL of pure water to obtain an aqueous solution of the m-phenylenediamine-terminated polyethylene glycol polymers with a mass fraction of 1.0%, which was prepared for use;

[0053] (4) 0.25 g of 1,3-propane sultone was dissolved in 50 mL of acetonitrile to obtain an alkane sultone organic solution with a mass fraction of 0.5%, which was prepared for use;

[0054] (5) The aqueous piperazine solution was immersed on the surface of the polyethersulfone ultrafiltration membrane substrate for 10 min. Then, the aqueous piperazine solution was poured out, and the residual droplets on the surface of the polyethersulfone ultrafiltration membrane substrate were blown off with compressed air to obtain a polyethersulfone ultrafiltration membrane substrate for storing the aqueous phase monomer in the pores;

[0055] (6) The organic phase trimesoyl chloride solution was immersed on the surface of the polyethersulfone ultrafiltration membrane substrate for storing the aqueous phase monomer for 2 min. Then, the excess organic phase trimesoyl chloride solution was poured out to obtain a nascent nanofiltration membrane;

[0056] (7) The aqueous solution of the m-phenylenediamine-terminated polyethylene glycol polymers was immersed on the surface of the nascent nanofiltration membrane for 10 min. Then, the aqueous solution of the m-phenylenediamine-terminated polyethylene glycol polymers was poured out to obtain a nanofiltration membrane grafted with the m-phenylenediamine-terminated polyethylene glycol polymers;

[0057] (8) The alkane sultone organic solution was immersed on the surface of the nanofiltration membrane grafted with the m-phenylenediamine-terminated polyethylene glycol polymers at 50°C for 2.0 h to perform a quaternary amination reaction. Then, the excess alkane sultone organic solution was poured out, and the solution was dried at 60°C for 2 min. Finally, the solution was thoroughly washed with pure water to obtain a composite nanofiltration membrane (as shown in Figure 2

[0058] Comparative Example 1

[0059] The preparation method of the unmodified nanofiltration membrane specifically includes the following steps:

[0060] ​(1) 1.0 g of anhydrous piperazine was dissolved in 50 mL of pure water to obtain a water phase piperazine solution with a mass fraction of 2.0%, which was prepared for use;

[0061] (2) 0.05 g of trimesoyl chloride was dissolved in 50 mL of n-hexane to obtain an organic phase trimesoyl chloride solution with a mass fraction of 0.1%, which was prepared for use;

[0062] (3) The water phase piperazine solution was immersed on the surface of the polysulfone ultrafiltration membrane substrate for 5 min, then the water phase piperazine solution was poured out, and the residual droplets on the surface of the polysulfone ultrafiltration membrane substrate were gently blown away by compressed air, thereby obtaining a polysulfone ultrafiltration membrane substrate storing water phase monomers in pores;

[0063] (4) The organic phase trimesoyl chloride solution was immersed on the surface of the polysulfone ultrafiltration membrane substrate storing water phase monomers in pores for 1 min, then the excess organic phase trimesoyl chloride solution was poured out, and dried at 40°C for 5 min, finally, the unmodified nanofiltration membrane was obtained by fully rinsing with pure water.

[0064] Comparative Example 2

[0065] The preparation method of the un-quaternized grafted nanofiltration membrane specifically includes the following steps:

[0066] (1) 1.0 g of anhydrous piperazine was dissolved in 50 mL of pure water to obtain a water phase piperazine solution with a mass fraction of 2.0%, which was prepared for use;

[0067] (2) 0.05 g of trimesoyl chloride was dissolved in 50 mL of n-hexane to obtain an organic phase trimesoyl chloride solution with a mass fraction of 0.1%, which was prepared for use;

[0068] (3) 2.0 g of polyethylene glycol diacrylate with a molecular weight of 6000 Da and 2.0 g of anhydrous piperazine were dissolved in 200 mL of pure water, and the solution was heated to 70°C and stirred for 6.0 h. After the reaction was completed, the solution was cooled to room temperature. The unreacted anhydrous piperazine was removed by repeatedly screening the solution four times in a membrane separation device using 200 mL of pure water with a molecular weight of 2000 Da. Then, the screened solution was vacuum freeze-dried at a pressure of 60 kPa and a temperature of -30°C for 6.0 h to obtain a piperazine-terminated polyethylene glycol polymer. Finally, 1.0 g of the piperazine-terminated polyethylene glycol polymer was dissolved in 50 mL of pure water to obtain a piperazine-terminated polyethylene glycol polymer aqueous solution with a mass fraction of 2.0%, which was prepared for use;

[0069] (4) The water phase piperazine solution was immersed on the surface of the polysulfone ultrafiltration membrane substrate for 5 min, then the water phase piperazine solution was poured out, and the residual droplets on the surface of the polysulfone ultrafiltration membrane substrate were gently blown away by compressed air, thereby obtaining a polysulfone ultrafiltration membrane substrate storing water phase monomers in pores;

[0070] (5) Immerse the surface of the polysulfone ultrafiltration membrane substrate containing the aqueous monomer in the pores of the organic phase pyromellitic chloride solution for 1 min, and then pour out the excess organic phase pyromellitic chloride solution to obtain the nascent nanofiltration membrane.

[0071] (6) Immerse the surface of the nascent nanofiltration membrane in the piperazine-terminated polyethylene glycol polymer aqueous solution for 5 minutes, then pour out the piperazine-terminated polyethylene glycol polymer aqueous solution, dry it at 40°C for 5 minutes, and finally rinse it thoroughly with pure water to obtain the unquaternized grafted nanofiltration membrane.

[0072] Performance testing

[0073] 1. Membrane characterization

[0074] Nanofiltration membranes prepared in Examples 1-2 and Comparative Examples 1-2 were characterized by SEM and contact angle, respectively. The results are as follows: Figures 3-4 As shown.

[0075] Depend on Figure 3 It can be seen that the surface nodules of the unmodified nanofiltration membrane prepared in Comparative Example 1 are relatively small, while the surface nodules of the unquaternized grafted nanofiltration membrane prepared in Comparative Example 2 are larger. The surface nodules of the composite nanofiltration membranes prepared in Examples 1 and 2 are further increased, which can increase the effective filtration area of ​​the nanofiltration membrane surface.

[0076] Depend on Figure 4 It can be seen that the surface contact angle of the composite nanofiltration membrane in Example 2 is 18°. ° Compared with the polyamide composite nanofiltration membrane in Comparative Example 1, it reduced by 30%. ° .

[0077] 2. Water treatment efficiency test

[0078] Nanofiltration membranes prepared in Examples 1-2 and Comparative Examples 1-2 were subjected to water treatment performance tests, including water permeability tests, desalination rate tests, and antifouling performance tests.

[0079] The test conditions for water treatment efficiency testing are as follows:

[0080] (1) When conducting the water permeability (pure water flux) test, the operating pressure is 0.6 MPa; the cross-flow velocity is 10 cm / s; and the water temperature is 25℃.

[0081] (2) Pure water flux (PWF) refers to the volume (V) of pure water passing through a unit membrane area (A) per unit time (t) under a unit operating pressure (S), with units of L·m -2 ·h -1 ·bar -1The desalination rate (PWF) is used to measure the water permeability of nanofiltration membranes, and its calculation formula is: PWF = V / (A·t·S). During the desalination rate test, the feed solution concentrations were: 1000 mg / L NaCl solution, 1000 mg / L Na₂SO₄ solution, 1000 mg / L MgCl₂ solution, and 1000 mg / L MgSO₄ solution; the operating pressure was 0.6 MPa; the cross-flow velocity was 10 cm / s; the solution pH was 7.0; and the water temperature was 25℃.

[0082] Desalination rate (R) refers to the ratio of the concentration of the solute in the feed solution (C) under a certain operating pressure. f ) and the concentration of solute in the permeate (C) p The ratio of the difference between the concentrations of inorganic salt ions (C%) and the solute concentration in the feed solution is used to evaluate the removal capacity of nanofiltration membranes for inorganic salt ions. The calculation formula is: R(%) = (C / C) * ... f -C p ) / C f ×100%.

[0083] (3) When conducting the antifouling performance (normalized flux) test, the feed concentrations were: bovine serum albumin 50 mg / L, sodium chloride 1000 mg / L; and the initial flux was 70 L·m⁻¹. -2 ·h -1 Cross-flow velocity 10 cm / s; solution pH 7.0; water temperature 25℃. Hydraulic cleaning conditions: cross-flow velocity 20 cm / s; water temperature 25℃; duration 10 min.

[0084] Normalized flux (N) f ) refers to the membrane flux (J) during the fouling process. f The ratio of N to the initial membrane flux (J0) is used to evaluate the membrane's antifouling performance. The formula for calculating N is: f =J f / J0.

[0085] Test results are as follows Figures 5-7 As shown. By Figure 5 It can be seen that the pure water flux of the composite nanofiltration membrane in Example 2 is 15.6 L·m. -2 ·h -1 ·bar -1 Compared with the polyamide composite nanofiltration membrane in Comparative Example 1, it improved by 35.7%.

[0086] Depend on Figure 6 As can be seen, the composite nanofiltration membrane in Example 2 achieved a Na2SO4 desalination rate of 99.2% and a MgCl2 desalination rate of 95.2%.

[0087] Depend on Figure 7It can be seen that compared with Comparative Example 1-2, the anti-fouling performance of the composite nanofiltration membrane of Example 1-2 is obviously improved, and the final normalized fluxes thereof are 85.7% and 90.0%, respectively, while the final normalized fluxes of the nanofiltration membrane of Comparative Example 1-2 are only 50.0% and 61.0%. In addition, after simple hydraulic cleaning, the normalized flux of the composite nanofiltration membrane of Example 1-2 is almost completely restored, while the normalized flux of the nanofiltration membrane of Comparative Example 1-2 is only restored to 57.6% and 71.0%.

[0088] The above description of disclosed embodiments enables one of ordinary skill in the art to make or use the application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other embodiments without departing from the spirit or scope of the application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a composite nanofiltration membrane, characterized by, Specifically comprising the following steps: (1) dissolving water phase monomer in water to obtain water phase monomer solution, ready for use; The water phase monomer is at least one of piperazine, m-phenylenediamine and 3,5-diaminobenzoic acid; (2) dissolving polybasic acid chloride in organic solvent A to obtain organic phase monomer solution, ready for use; (3) dissolving polyethylene glycol diacrylate and excess diamine monomer in water, warming reaction, cooling, removing unreacted diamine monomer, dissolving in water after vacuum freeze-drying to obtain diamine monomer end-capped polyethylene glycol polymer aqueous solution, ready for use; (4) dissolving alkylsulfonolactone in organic solvent B to obtain alkylsulfonolactone organic solution, ready for use; (5) immersing the surface of the porous ultrafiltration substrate in the water phase monomer solution, then pouring out the water phase monomer solution and removing the liquid drops remaining on the surface of the porous ultrafiltration substrate to obtain the porous ultrafiltration substrate storing water phase monomer in pores; (6) immersing the surface of the porous ultrafiltration substrate storing water phase monomer in pores in the organic phase monomer solution, then pouring out the excess organic phase monomer solution to obtain nascent nanofiltration membrane; (7) immersing the surface of the nascent nanofiltration membrane in the diamine monomer end-capped polyethylene glycol polymer aqueous solution, then pouring out the diamine monomer end-capped polyethylene glycol polymer aqueous solution to obtain nanofiltration membrane grafted with diamine monomer end-capped polyethylene glycol polymer; (8) immersing the surface of the nanofiltration membrane grafted with diamine monomer end-capped polyethylene glycol polymer in the alkylsulfonolactone organic solution, then pouring out the excess alkylsulfonolactone organic solution, drying and rinsing to obtain the composite nanofiltration membrane.

2. The method of claim 1, wherein the composite nanofiltration membrane is prepared by the steps of: In step (1), the mass fraction of the water phase monomer solution is 0.02% to 5.0%.

3. The method of claim 1, wherein the composite nanofiltration membrane is prepared by the steps of: In step (2), the polybasic acid chloride is at least one of isophthaloyl chloride and trimesoyl chloride; the organic solvent A is at least one of n-hexane, cyclohexane, toluene, n-heptane and n-octane; the mass fraction of the organic phase monomer solution is 0.01% to 0.3%.

4. The method of claim 1, wherein the composite nanofiltration membrane is prepared by the steps of: In step (3), the molecular weight of the polyethylene glycol diacrylate is 3000 to 20000 Da; the diamine monomer is at least one of piperazine and m-phenylenediamine; the reaction temperature is 50 to 80℃, and the reaction time is 2 to 8h; the removal process is repeated more than four times in a membrane separation device with a molecular weight cut-off of 500 to 2000 Da; the vacuum freeze-drying is performed at a pressure of 40 to 80 kPa and a temperature of-20 to-40℃ for 1 to 3h; the mass fraction of the diamine monomer end-capped polyethylene glycol polymer aqueous solution is 0.02% to 5.0%.

5. The method of claim 1, wherein the composite nanofiltration membrane is prepared by the steps of: In step (4), the alkylsulfonolactone is 1,3-propane sulfonolactone; the organic solvent B is at least one of methanol, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide and acetonitrile; the mass fraction of the alkylsulfonolactone organic solution is 0.1% to 1.0%.

6. The method of claim 1, wherein the composite nanofiltration membrane is prepared by the steps of: In step (5), the porous ultrafiltration substrate is any one of polyvinylidene fluoride ultrafiltration membrane, polysulfone ultrafiltration membrane, polyethersulfone ultrafiltration membrane, sulfonated polyethersulfone ultrafiltration membrane and polyacrylonitrile ultrafiltration membrane; the immersion time is 0.5 to 30 min; the drying is performed until no liquid drops are left on the surface.

7. The method of claim 1, wherein the composite nanofiltration membrane is prepared by the steps of: In step (6), the immersion time is 5 to 300 s.

8. The method of claim 1, wherein the composite nanofiltration membrane is prepared by the steps of: In step (7), the time of immersion is 1-60 min.

9. The method of claim 1, wherein the composite nanofiltration membrane is prepared by the steps of: In step (8), the temperature of immersion is 40-60℃, and the time is 2-8 h.

10. The method of claim 1, wherein the composite nanofiltration membrane is prepared by the steps of: In step (8), the temperature of drying is 30-80℃, and the time is 1-20 min.

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