A positively charged composite nanofiltration membrane and its preparation method and application

Abstract

The present invention provides a method for preparing a positively charged composite nanofiltration membrane. The method comprises the following steps: first, synthesizing quaternary ammonium salt-functionalized polyethyleneimine; then, using a graphene quantum dot nanofiltration membrane as a support, an aqueous solution of the quaternary ammonium salt-functionalized polyethyleneimine as a water phase monomer, and a trimesoyl chloride solution as an oil phase monomer, and performing oil-water interfacial polymerization to prepare the positively charged composite nanofiltration membrane. The present invention utilizes the quaternary ammonium salt-functionalized polyethyleneimine, which has active amino groups and exhibits a strong positive charge, to impart the positively charged composite nanofiltration membrane with the ability to electrostatically repel cations; and covalently bonding the active groups, such as epoxy and carboxyl groups, of the graphene quantum dot nanofiltration membrane support to the polyethyleneimine, thereby improving the stability of the positively charged composite nanofiltration membrane and significantly increasing the calcium and magnesium ion rejection rates. The membrane can be used to reduce water hardness.

Description

Technical Field

[0001] The present invention relates to the technical field of nanofiltration membranes, and in particular to a positively charged composite nanofiltration membrane and a preparation method and application thereof. Background Art

[0002] Nanofiltration is a low-energy membrane separation technology operating at pressures below 1.0 MPa. The separation mechanism of nanofiltration membranes is a synergistic effect of pore sieving and electrostatic repulsion, with a molecular weight cutoff of 200-2000. Nanofiltration membranes have a low retention rate for monovalent ions and organic compounds with a molecular weight below 200, while having a high removal rate for divalent or polyvalent ions and substances with a molecular weight greater than 200. Nanofiltration membranes are widely used in applications such as separating monovalent and divalent ions in zero-discharge wastewater, separating inorganic salts and dye molecules in printing and dyeing wastewater treatment, separating and concentrating materials of different molecular weights in biomedicine, removing hardness from industrial circulating water, and pretreatment of seawater desalination.

[0003] Prior art methods for preparing nanofiltration membranes primarily include interfacial polymerization, phase inversion, and surface grafting. Interfacial polymerization is a preferred method for industrial mass production and has been used in the production of commercial nanofiltration membranes. Interfacial polymerization nanofiltration membranes are composite membranes composed of a porous support layer and a functional layer. For example, piperazine polyamide nanofiltration membranes synthesized using interfacial polymerization have a negatively charged separation layer, making them suitable for separating monovalent and divalent anions, particularly chloride and sulfate ions. They can achieve sulfate ion rejection rates exceeding 99.0% and separation factors exceeding 200. However, negatively charged nanofiltration membranes have low cation removal capabilities, and few reports have reported calcium and magnesium ion rejection rates exceeding 95% (J. Membr. Sci., 2022, 642, 119971). Therefore, developing positively charged composite nanofiltration membranes with high calcium and magnesium ion rejection rates has significant application value. Summary of the Invention

[0004] The positively charged composite nanofiltration membrane prepared by the method provided by the present invention has a high calcium ion and magnesium ion release rate.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a method for preparing a positively charged composite nanofiltration membrane, comprising the following steps:

[0007] (1) dissolving polyethyleneimine and epoxypropyltrimethylammonium chloride in water to react and obtain an aqueous solution of polyethyleneimine functionalized with a quaternary ammonium salt;

[0008] (2) Using the graphene quantum dot nanofiltration membrane as a support, the aqueous solution of the quaternary ammonium salt functionalized polyethyleneimine obtained in the step (1) as the aqueous phase monomer solution, and the 2,3-dimethylbenzenecarboxylic acid chloride solution as the oil phase monomer solution, an interfacial polymerization reaction is carried out, and then after heat treatment, a positively charged composite nanofiltration membrane is obtained.

[0009] Preferably, the average molecular weight of the polyethyleneimine in step (1) is 500 to 50,000.

[0010] Preferably, the mass fraction of polyethyleneimine in the reaction solution in step (1) is 5% to 10%.

[0011] Preferably, the reaction temperature in step (1) is 50-90° C., and the reaction time is 5-12 h.

[0012] Preferably, the aqueous solution of quaternary ammonium salt functionalized polyethyleneimine in step (2) is diluted to a mass fraction of 0.5% to 3% before use.

[0013] Preferably, the interfacial polymerization step in step (2) comprises: first immersing the graphene quantum dot nanofiltration membrane support in an aqueous monomer solution for 1 to 5 minutes, and then immersing it in an oily monomer solution for 1 to 5 minutes.

[0014] Preferably, after the interfacial polymerization in step (2), the composite membrane obtained by the interfacial polymerization is subjected to heat treatment in an oven, the heat treatment temperature is 60 to 90° C., and the heat treatment time is 5 to 20 minutes.

[0015] The present invention also provides a positively charged composite nanofiltration membrane prepared by the preparation method described in the above technical solution.

[0016] The present invention also provides a positively charged composite nanofiltration membrane prepared by the preparation method described in the above technical solution, or the positively charged composite nanofiltration membrane has a high calcium ion and magnesium ion retention rate and can be used to reduce the hardness of water.

[0017] The invention provides a preparation method of a positively charged composite nanofiltration membrane. The method comprises the following steps: firstly, polyethyleneimine and epoxypropyltrimethylammonium chloride are reacted to synthesize polyethyleneimine with a quaternary ammonium salt group; a graphene quantum dot nanofiltration membrane is used as a support, an aqueous solution of polyethyleneimine functionalized with a quaternary ammonium salt is used as an aqueous phase monomer solution, and a trimesoyl chloride solution is used as an oil phase monomer solution; after oil-water interfacial polymerization, the composite membrane obtained by the interfacial polymerization is subjected to heat treatment to prepare the positively charged composite nanofiltration membrane. The present invention utilizes quaternary ammonium salt-functionalized polyethyleneimine, which has active amino groups and exhibits strong positive charges, to impart the positively charged composite nanofiltration membrane with the ability to electrostatically repel cations. Active groups such as epoxy and carboxyl groups of a graphene quantum dot nanofiltration membrane support are covalently linked to the polyethyleneimine, thereby improving the stability of the positively charged composite nanofiltration membrane and reducing the membrane pore size of the nanofiltration membrane, which is beneficial to improving the salt rejection rate of the nanofiltration membrane. The covalent bond between the graphene quantum dot nanofiltration membrane support and the quaternary ammonium salt-functionalized polyethyleneimine and the electrostatic repulsion of the positive charges synergistically achieve a higher calcium ion and magnesium ion removal rate.

[0018] The results of the embodiment show that the surface potential of the positively charged composite nanofiltration membrane prepared in the embodiment of the present invention is as high as 35mv, and the water permeation flux is 26.2L / (m 2 hbar), the rejection rate of magnesium chloride (0.1% aqueous solution) is 99.2%, and the rejection rate of calcium chloride (0.1% aqueous solution) is 98.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the preparation process of the positively charged composite nanofiltration membrane in the present invention. DETAILED DESCRIPTION

[0020] The present invention provides a method for preparing a positively charged composite nanofiltration membrane, comprising the following steps:

[0021] (1) dissolving polyethyleneimine and epoxypropyltrimethylammonium chloride in water to react and obtain an aqueous solution of polyethyleneimine functionalized with a quaternary ammonium salt;

[0022] (2) Using a graphene quantum dot nanofiltration membrane as a support, the aqueous solution of quaternary ammonium salt functionalized polyethyleneimine obtained in step (1) is diluted and used as an aqueous phase monomer solution; using a solution of trimesoyl chloride as an oil phase monomer solution, after interfacial polymerization reaction, the composite membrane obtained by the interfacial polymerization is subjected to heat treatment in an oven to obtain a positively charged composite nanofiltration membrane.

[0023] In the present invention, unless otherwise specified, the raw materials used are conventional commercial products in the field.

[0024] The present invention dissolves polyethyleneimine and epoxypropyltrimethylammonium chloride in water to react to obtain a quaternized polyethyleneimine solution. In the present invention, the average molecular weight of the polyethyleneimine is preferably 500 to 50,000, more preferably 2,000 to 10,000.

[0025] In the present invention, the mass fraction of epoxypropyltrimethylammonium chloride in the mixed solution is 5% to 10%, more preferably 5%.

[0026] In the present invention, the reaction temperature is preferably 50-90°C; the reaction time is preferably 5-12 hours. The present invention controls the reaction temperature and time within the above ranges to promote the complete reaction of polyethyleneimine and glycidyltrimethylammonium chloride to synthesize polyethyleneimine with a quaternary ammonium salt group.

[0027] In the present invention, the aqueous solution of the quaternary ammonium salt functionalized polyethyleneimine is diluted to a mass fraction of 0.5% to 3%, more preferably 1% to 2%.

[0028] The invention uses a graphene quantum dot nanofiltration membrane as a support, uses the quaternary ammonium salt functionalized polyethyleneimine as an aqueous monomer solution, uses trimesoyl chloride as an oily monomer solution, carries out an interfacial polymerization reaction, and then obtains a positively charged composite nanofiltration membrane after heat treatment.

[0029] In the present invention, the preparation method of the graphene quantum dot nanofiltration membrane support preferably includes the following steps: first, graphene quantum dots are prepared by citric acid thermal decomposition method, and then an aqueous solution of graphene quantum dots with a mass fraction of 1% is used as an aqueous monomer solution, a n-heptane solution of trimesoyl chloride with a mass fraction of 0.2% is used as an oily monomer solution, and a polysulfone or polyacrylonitrile ultrafiltration membrane is used as a support, and the polysulfone or polyacrylonitrile ultrafiltration membrane support is first immersed in the aqueous monomer solution for 2 minutes, and then immersed in the oily monomer solution for 2 minutes to perform interfacial polymerization, and then the support after interfacial polymerization is heat-treated in an oven at 90°C for 5 minutes to prepare a graphene quantum dot nanofiltration membrane (see previous reports of this research group, J. Membr. Sci., 2019, 572, 504, Journal of Chemical Industry and Engineering, 2021, 76 (6), 3390-3398)

[0030] In the present invention, the interfacial polymerization step preferably includes: first immersing the graphene quantum dot nanofiltration membrane support in an aqueous monomer solution of quaternary ammonium salt functionalized polyethyleneimine for 1 to 5 minutes, and then immersing it in an oily monomer solution of trimesoyl chloride for 1 to 5 minutes.

[0031] In the present invention, the trimesoyl chloride solution is preferably a trimesoyl chloride n-heptane solution with a mass fraction of 0.01% to 0.3%, more preferably a trimesoyl chloride n-heptane solution with a mass fraction of 0.1% to 0.3%.

[0032] In the present invention, after the interfacial polymerization, the composite membrane obtained by the interfacial polymerization is subjected to an oven heat treatment, and the heat treatment temperature is preferably 50 to 90°C, more preferably 60 to 70°C.

[0033] In the present invention, the heat treatment time is preferably 5 to 20 minutes. The present invention controls the temperature and time of the heat treatment within the above range to promote the reaction between the graphene quantum dot nanofiltration membrane support and the quaternary ammonium salt functionalized polyethyleneimine, thereby improving the structural stability of the positively charged composite nanofiltration membrane.

[0034] The preparation method of the positively charged composite nanofiltration membrane provided by the present invention has simple operation, mild reaction conditions and is suitable for large-scale production.

[0035] The present invention also provides a positively charged composite nanofiltration membrane prepared by the preparation method described in the above technical solution.

[0036] The present invention also provides a positively charged composite nanofiltration membrane prepared by the preparation method described in the above technical solution, or the positively charged composite nanofiltration membrane has a high calcium ion and magnesium ion retention rate and can be used to reduce water hardness.

[0037] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0038] Figure 1 The present invention provides a schematic diagram of a process for preparing a positively charged composite nanofiltration membrane, comprising the following steps: reacting polyethyleneimine and epoxypropyltrimethylammonium chloride in water to obtain an aqueous solution of polyethyleneimine functionalized with a quaternary ammonium salt; using a graphene quantum dot nanofiltration membrane as a support, the aqueous solution of polyethyleneimine functionalized with a quaternary ammonium salt as an aqueous phase monomer solution, and an n-heptane solution of trimesoyl chloride as an oil phase monomer solution, performing an interfacial polymerization reaction, and then heat-treating the composite membrane in an oven to obtain a positively charged composite nanofiltration membrane.

[0039] Example 1

[0040] The preparation method of the positively charged composite nanofiltration membrane comprises the following steps:

[0041] (1) dissolving 5 g of polyethyleneimine (average molecular weight 600) and 5 g of propylene oxide trimethylammonium chloride in 100 g of water, heating the mixture at 70° C. for 6 hours to obtain an aqueous solution of quaternary ammonium salt functionalized polyethyleneimine;

[0042] (2) diluting the aqueous solution of quaternary ammonium salt functionalized polyethyleneimine obtained in step (1) to a mass fraction of 1%; immersing the graphene quantum dot nanofiltration membrane support body in the aqueous solution of quaternary ammonium salt functionalized polyethyleneimine for 2 minutes, and then immersing it in a 0.2% mass fraction of trimesoyl chloride n-heptane oil phase solution for 2 minutes to perform interfacial polymerization reaction, and then heat-treating the composite membrane obtained by the interfacial polymerization in an oven at 60° C. for 10 minutes to prepare a positively charged composite nanofiltration membrane;

[0043] The preparation steps of the graphene quantum dot nanofiltration membrane support are as follows: first, graphene quantum dots are prepared by a citric acid pyrolysis method, and then a graphene quantum dot aqueous solution with a mass fraction of 1% is used as an aqueous phase monomer solution, a 0.2% mass fraction of trimesoyl chloride n-heptane solution is used as an oil phase monomer solution, and a polysulfone or polyacrylonitrile ultrafiltration membrane is used as a support. The polysulfone or polyacrylonitrile ultrafiltration membrane support is first immersed in the aqueous phase monomer solution for 2 minutes, and then immersed in the oil phase monomer solution for 2 minutes to perform interfacial polymerization, and then the support after interfacial polymerization is heat-treated in an oven at 90°C for 5 minutes to prepare a graphene quantum dot nanofiltration membrane (for specific steps, see the literature, J. Membr. Sci., 2019, 572, 504, Journal of Chemical Industry and Engineering, 2021, 76 (6), 3390-3398);

[0044] The surface potential of the positively charged composite nanofiltration membrane prepared in Example 1 was tested using a Zeta potential analyzer and was found to be 34 mV, indicating that the positively charged composite nanofiltration membrane prepared in Example 1 had strong positive electrical properties.

[0045] The water permeability of the positively charged composite nanofiltration membrane was measured using a cross-flow device, and the permeation flux of the nanofiltration membrane was calculated using formula (1):

[0046]

[0047] In formula (1): J is the pure water flux, L / (m 2 hbar); V is the permeate volume, unit L; A is the membrane surface area, unit m 2 ; Δt is the penetration time, unit is h; P is the operating pressure, unit is bar.

[0048] A 0.1% aqueous solution of magnesium chloride and calcium chloride was used as the raw material for nanofiltration membrane separation testing to measure the inorganic salt retention rate of the nanofiltration membrane. The inorganic salt concentration was measured by a conductivity meter. The salt retention rate of the nanofiltration membrane was calculated using formula (2):

[0049]

[0050] In formula (2): R is the inorganic salt retention rate, %; C pis the inorganic salt concentration of the filtrate, in g / L; C f is the inorganic salt concentration of the raw material solution, unit is g / L.

[0051] According to the above method, the water permeation flux of the positively charged composite nanofiltration membrane prepared in Example 1 was measured to be 21.9 L / (m 2 hbar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 99.6%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 97.6%.

[0052] Example 2

[0053] The method of Example 2 is the same as that of Example 1, except that the aqueous monomer solution is immersed for 1 minute, the oil phase monomer solution is immersed for 1 minute, the composite membrane obtained by interfacial polymerization is heat-treated in an oven at 70°C for 5 minutes to prepare a positively charged composite nanofiltration membrane.

[0054] The same method as in Example 1 was used to test the performance. The surface potential of the positively charged composite nanofiltration membrane prepared in Example 2 was 31 mv, and the water permeation flux was 25.6 L / (m 2 h bar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 99.3%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 98.2%.

[0055] Example 3

[0056] The method of Example 3 is the same as that of Example 1, except that the aqueous monomer solution is immersed for 5 minutes, the oil phase monomer solution is immersed for 1 minute, the composite membrane obtained by interfacial polymerization is heat-treated in an oven at 80°C for 8 minutes to prepare a positively charged composite nanofiltration membrane.

[0057] The same method as in Example 1 was used to test the performance. The surface potential of the positively charged composite nanofiltration membrane prepared in Example 3 was 35 mv, and the water permeation flux was 21.2 L / (m 2 h bar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 99.4%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 97.8%.

[0058] Example 4

[0059] The method of Example 4 is the same as that of Example 1, except that the mass fraction of the aqueous phase monomer solution is 2.0%, and the mass fraction of the oil phase monomer solution is 0.2%.

[0060] The same method as in Example 1 was used to test the performance. The surface potential of the positively charged composite nanofiltration membrane prepared in Example 4 was 28 mV, and the water permeation flux was 26.2 L / (m 2hbar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 99.1%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 97.4%.

[0061] Example 5

[0062] The method of Example 5 is the same as that of Example 1, except that the mass fraction of the aqueous phase monomer solution is 3.0%, and the mass fraction of the oil phase monomer solution is 0.2%.

[0063] The same method as in Example 1 was used to test the performance. The surface potential of the positively charged composite nanofiltration membrane prepared in Example 5 was 36 mv, and the water permeation flux was 26.2 L / (m 2 hbar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 98.7%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 97.1%.

[0064] Example 6

[0065] The method of Example 6 is the same as that of Example 1, except that the mass fraction of the aqueous phase monomer solution is 2.0%, and the mass fraction of the oil phase monomer solution is 0.3%.

[0066] The same method as in Example 1 was used to test the performance. The surface potential of the positively charged composite nanofiltration membrane prepared in Example 6 was 26 mV, and the water permeation flux was 22.2 L / (m 2 hbar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 99.4%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 97.6%.

[0067] Example 7

[0068] The method of Example 7 is the same as that of Example 1, except that the average molecular weight of the polyethyleneimine used is 1800.

[0069] The performance test was carried out in the same manner as in Example 1. The surface potential of the positively charged composite nanofiltration membrane prepared in Example 7 was 31.3 mv, and the water permeation flux was 20.2 L / (m 2 hbar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 99.3%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 97.0%.

[0070] Example 8

[0071] The method of Example 8 is the same as that of Example 1, except that the average molecular weight of the polyethyleneimine used is 50,000.

[0072] The same method as in Example 1 was used to test the performance. The surface potential of the positively charged composite nanofiltration membrane prepared in Example 8 was 34.6 mv, and the water permeation flux was 22.2 L / (m 2 hbar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 99.3%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 97.5%.

[0073] Example 9

[0074] The method of Example 9 is the same as that of Example 1, except that the average molecular weight of the polyethyleneimine used is 1800, the mass fraction of the aqueous phase monomer solution is 2.0%, and the mass fraction of the oil phase monomer solution is 0.2%.

[0075] The same method as in Example 1 was used to test the performance. The surface potential of the positively charged composite nanofiltration membrane prepared in Example 9 was 31 mV, and the water permeation flux was 26.2 L / (m 2 hbar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 99.1%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 97.4%.

[0076] Example 10

[0077] The method of Example 10 is the same as that of Example 1, except that the average molecular weight of the polyethyleneimine used is 1800, and the reaction temperature for the synthesis of the quaternary ammonium salt functionalized polyethyleneimine is 90°C.

[0078] The performance test was carried out in the same manner as in Example 1. The surface potential of the positively charged composite nanofiltration membrane prepared in Example 10 was 32.1 mV, and the water permeation flux was 24.6 / (m 2 hbar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 99.4%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 98.2%.

[0079] Example 11

[0080] The method of Example 11 is the same as that of Example 1, except that the average molecular weight of the polyethyleneimine used is 1800; the reaction temperature for the synthesis of quaternary ammonium salt functionalized polyethyleneimine is 80° C., and the reaction time is 8 hours; the mass fraction of the oil phase monomer solution used in the preparation of the positively charged composite nanofiltration membrane is 0.3%. The performance test was carried out using the same method as in Example 1, and the surface potential of the positively charged composite nanofiltration membrane prepared in Example 11 was 29.2 mv, and the water permeation flux was 22.6 L / (m 2hbar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 98.9%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 97.2%.

[0081] Example 12

[0082] The method of Example 12 is the same as that of Example 1, except that the average molecular weight of the polyethyleneimine used is 10,000; the reaction temperature for the synthesis of quaternary ammonium salt functionalized polyethyleneimine is 90°C, and the reaction time is 10 hours; the mass fraction of the aqueous monomer solution used for the preparation of the positively charged composite nanofiltration membrane is 0.5%.

[0083] The performance test was carried out in the same manner as in Example 1. The surface potential of the positively charged composite nanofiltration membrane prepared in Example 12 was 34.5 mv, and the water permeation flux was 22.6 L / (m 2 hbar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 99.2%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 98.5%.

[0084] In summary, the surface potential of the positively charged composite nanofiltration membrane prepared in the embodiment of the present invention is as high as 35mv, and the water permeation flux is 26.2L / (m 2 hbar), the retention rate of magnesium chloride (0.1% by mass aqueous solution) is 99.2%, and the retention rate of calcium chloride (0.1% by mass aqueous solution) is 98.5%.

[0085] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a positively charged composite nanofiltration membrane, comprising the following steps: (1) Polyethyleneimine and epoxypropyltrimethylammonium chloride are dissolved in water to react to obtain an aqueous solution of quaternary ammonium salt-functionalized polyethyleneimine; (2) Using the graphene quantum dot nanofiltration membrane as a support, using the aqueous solution of the quaternary ammonium salt functionalized polyethyleneimine obtained in step (1) as the aqueous phase monomer solution, and using the 2,3-dimethylbenzenecarboxylic acid chloride solution as the oil phase monomer solution, performing an interfacial polymerization reaction, and then heat-treating the composite membrane obtained by the interfacial polymerization to obtain a positively charged composite nanofiltration membrane; The aqueous solution of the quaternary ammonium salt functionalized polyethyleneimine is diluted to a mass fraction of the aqueous monomer solution of 0.5% to 3% before use; The interfacial polymerization step comprises: first immersing the graphene quantum dot nanofiltration membrane in an aqueous monomer solution for 1 to 5 minutes, and then immersing it in an oily monomer solution for 1 to 5 minutes; After the interfacial polymerization is completed, the composite film obtained by the interfacial polymerization is placed in an oven, the heat treatment temperature is 60-90° C., and the heat treatment time is 5-20 minutes.

2. The method for preparing the positively charged composite nanofiltration membrane according to claim 1, characterized in that: The average molecular weight of the polyethyleneimine in step (1) is 500-50,000.

3. The method for preparing a positively charged composite nanofiltration membrane according to claim 1, wherein: The mass fraction of polyethyleneimine in the reaction system solution in step (1) is 5% to 10%.

4. The method for preparing a positively charged composite nanofiltration membrane according to claim 1, wherein: The reaction temperature of step (1) is 50-90° C., and the reaction time is 5-12 h.

5. The method for preparing a positively charged composite nanofiltration membrane according to claim 1, wherein: In the step (2), trimesoyl chloride is dissolved in n-heptane, n-hexane or isoparaffin to prepare an oil phase monomer solution; the mass fraction of the trimesoyl chloride solution is 0.01% to 1%.

6. A positively charged composite nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 5.

7. Use of the positively charged composite nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 5 or the positively charged composite nanofiltration membrane according to claim 6 in reducing water hardness.

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