A positively charged nanofiltration membrane for efficient separation of metal ions

Abstract

The application discloses a preparation method of a high-efficiency positive-charged nanofiltration membrane for separating metal ions. The method is characterized in that a polysulfone ultrafiltration membrane base film is modified by graphene and SiO2, and then polyethylene imine (PEI) modified by polyglycidylamine (PGAM) is grafted on the modified ultrafiltration membrane to successfully prepare the positive-charged nanofiltration membrane. The nanofiltration membrane combines the high strength and hydrophilic advantages of graphene and SiO2, and is endowed with positive charges on the membrane surface through the modification of polyethylene imine and polyglycidylamine. The prepared positive-charged nanofiltration membrane significantly improves the separation efficiency and durability of metal ions, and has significant application value and broad prospects in key fields such as seawater desalination, wastewater treatment and resource recovery.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of membrane separation technology, and particularly relates to a preparation method of a high-efficiency metal ion separation positively charged modified nanofiltration membrane. BACKGROUND

[0002] With the rapid development of industrialization and urbanization, the pollution problem of metal ions in water bodies is becoming increasingly serious. Heavy metal ions have characteristics such as high toxicity and difficulty in degradation, and through migration and transformation in the biosphere, they cause great harm to the ecological environment and human health. Traditional heavy metal ion removal methods mainly include chemical precipitation, ion exchange, adsorption and the like, but these methods have the disadvantages of low treatment efficiency, high cost, great influence on the environment and the like. Compared with traditional heavy metal ion removal methods, positively charged nanofiltration membranes usually have the same positive charge as heavy metal ions, which makes the charge repulsion more obvious, has a high rejection rate for multivalent cations and positively charged pollutants, and has high water permeability. By rejecting divalent or multivalent cations, heavy metals in wastewater can be efficiently removed, and trace elements beneficial to the human body are retained.

[0003] At present, researches on how to realize better separation of metal ions by positively charged nanofiltration membranes are carried out at home and abroad. For example, a positively charged nanofiltration membrane is prepared by interfacial polymerization of a quaternary ammonium salt monomer prepared from a vinyl amine salt and 2-bromoethylamine hydrobromide, and the rejection rate of the positively charged nanofiltration membrane to CaCl2 can reach 95.49%, and the flux can reach 128.34 L·m -2 ·h -1 ·MPa -1 The rejection rate of the positively charged nanofiltration membrane to MgCl2 can reach 96.26%, and the flux can reach 137.71 L·m -2 ·h -1 ·MPa -1 and the like. Patent CN115364684A provides a preparation method of a high-flux positively charged nanofiltration membrane. Through specific chemical treatment and heat treatment steps, the prepared positively charged nanofiltration membrane has a water permeation flux of 70 L / m²·h·bar, and the rejection rates of the positively charged nanofiltration membrane to various dyes and drugs are all above 90%. After 12 hours of continuous separation test, the membrane rejection rate is basically unchanged, but the water permeation flux is reduced by about 10%. The above technologies each have obvious advantages, but still have problems such as low separation efficiency and poor membrane durability.

[0004] The patent provides a method for preparing a metal ion high-efficiency separation function positive charged nanofiltration membrane, which successfully gives the membrane surface a positive charge by combining graphene, SiO2 and PEI and the like materials, significantly improves the separation efficiency and durability. The nanofiltration membrane prepared by the method has high strength, hydrophilicity, and a rejection rate of up to 97% or more for divalent metal ions, and a flux decay rate as low as 3%-8% / 12 h (the proportion of flux decay per 12 hours). The nanofiltration membrane has a wide application prospect in the fields of seawater desalination, wastewater treatment and resource recovery, and has important significance for solving the problem of metal ion pollution. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application aims to provide a PEI modified polyglycidylamine modified positive charged nanofiltration membrane based on a graphene / SiO2 modified polysulfone ultrafiltration membrane base film, which aims to improve the separation efficiency and durability of the nanofiltration membrane, and is particularly suitable for efficient separation of metal ions.

[0006] The technical scheme of the present application is as follows:

[0007] Step one: preparation of a graphene / SiO2 modified polysulfone ultrafiltration membrane base film:

[0008] (1) Mix graphene powder and mesoporous SiO2 powder in a mass ratio of 1:1-2:1, and place them in an organic solvent (such as N,N-dimethylacetamide), and disperse them by ultrasonic for 1-3 hours to form a uniform dispersion liquid.

[0009] (2) Add polysulfone, polyvinylidene fluoride and polyvinylpyrrolidone in a mass ratio of 5:1:1-7:1:1 into a reaction kettle, stir uniformly, and heat to 100-150 DEG C, and keep for 1-4 hours.

[0010] (3) Add N-N-methyl acetamide and inorganic salt (sodium sulfate, sodium chloride) into the reaction kettle in turn, and continue to stir for 10-30 minutes;

[0011] (4) Add N-methyl pyrrolidone and polyethylene glycol into the reaction kettle in turn, and continue to stir for 30-50 minutes;

[0012] (5) Pour the dispersion liquid obtained in step (1) into the reaction kettle, keep warm and continue to stir for 1 hour to prepare a primary casting solution;

[0013] (6) Perform vacuum degassing on the primary casting solution at a gas pressure of 1-5 MPa and a temperature of 80-180 DEG C to prepare a graphene / silica polysulfone casting solution;

[0014] (7) The casting solution is prepared into a modified polysulfone ultrafiltration membrane base membrane by phase inversion method. The casting solution after standing is poured on a glass plate, a uniform thickness membrane is prepared by a doctor blade at a constant speed, and then is placed in a pure water coagulation bath for phase inversion to obtain a modified base membrane, which is stored in deionized water for standby.

[0015] Step two: pretreatment of the modified base membrane

[0016] (1) Surface cleaning: the polysulfone ultrafiltration membrane is soaked and washed with a mild organic solvent (such as ethanol, isopropanol), and then is repeatedly washed with deionized water for 3-5 times to remove possible impurities, oil stains and residual chemicals on the surface of the membrane to ensure the cleanliness of the membrane surface.

[0017] (2) Surface activation: the membrane surface is treated with a potassium permanganate-sulfuric acid mixed solution, soaked for 1 hour, and the reaction activity groups on the surface of the polysulfone ultrafiltration membrane are increased by chemical oxidation to introduce active groups such as carboxyl and hydroxyl groups on the membrane surface.

[0018] Step three: preparation of PGAM and PEI reaction compound

[0019] (1) Reaction condition selection: PGAM and PEI are mixed in dimethyl sulfoxide (DMSO) solvent according to the molar ratio of 0.5:3-2:3, the reaction temperature is set to 40-80 ℃, the stirring speed is 300-600 rpm, and an appropriate amount of sodium hydroxide (1% of the total mass of PGAM and PEI) is added as an alkali catalyst to accelerate the ring-opening reaction of the epoxy group, and the reaction time is 1-3 hours.

[0020] (2) Product purification: after the reaction is completed, the solvent is removed by vacuum distillation at 60 ℃ and 0.08 MPa vacuum degree using a rotary evaporator. Then the crude product is dissolved in deionized water and purified by dialysis method (dialysis bag molecular weight cut-off is 500-1000 Da), deionized water is replaced every 4-6 hours, dialysis is carried out for 24-48 hours, and pure PEI-PGAM reaction compound is obtained.

[0021] Step four: grafting reaction process

[0022] (1) Grafting solution preparation: the purified PEI-PGAM reaction compound is dissolved in phosphate buffer solution (pH value is 7.5) to prepare a grafting solution with a concentration of 5%-20%.

[0023] (2) Grafting reaction operation: the pretreated polysulfone ultrafiltration membrane is soaked in the grafting solution and reacted at 60-90 ℃ for 2-10 h. During the grafting process, the amine groups in the PEI-PGAM reaction compound condense with the carboxyl and hydroxyl groups on the surface of the polysulfone ultrafiltration membrane to realize grafting.

[0024] Step five: post-treatment

[0025] After the completion of the grafting reaction, the nanofiltration membrane is taken out and rinsed with a large amount of deionized water for 10-15 minutes, 3-5 times, to remove unreacted PGAM-PEI compound and other impurities. Then the membrane is dried at a temperature of 40-60 ℃ for 3 hours, so that the grafted compound stably exists on the membrane surface, and a final positively charged nanofiltration membrane is obtained.

[0026] Reaction mechanism

[0027] By compounding graphene and SiO2, the present application realizes the joint action of the two-dimensional structure of graphene and the porous structure of SiO2, forms a special network structure, provides more channels for water molecules to pass through, reduces the permeation resistance of the membrane, and enhances the interception capacity, mechanical strength and hydrophilicity of the membrane to different molecules. At the same time, the introduction of PEI-PGAM successfully makes the membrane surface positively charged, enhances the adsorption and separation capacity of metal ions. Through testing, the modified nanofiltration membrane has a rejection rate of more than 97% to divalent metal ions (such as copper ions, lead ions, chromium ions, etc.), a tensile strength of 5.8 MPa, which is 45% higher than that of traditional positively charged nanofiltration membranes, a contact angle of 40°, which is 10° lower than that of conventional positively charged nanofiltration membranes. In the long-term filtration experiment, the membrane flux decay rate is as low as 3%-8% / 12 h, which is about 20% lower than that of traditional membranes. The nanofiltration membrane has excellent performance in separating metal ions and has good separation performance and durability.

[0028] Advantages of the present application

[0029] (1) High separation performance: the positively charged nanofiltration membrane prepared by PEI and PGAM modification has a rejection rate of more than 97% to divalent metal ions (such as copper ions, lead ions, etc.), which significantly improves the separation efficiency of metal ions. The modification of graphene and SiO2 increases the hydrophilicity of the material, thereby improving the flux of the membrane.

[0030] (2) Excellent mechanical strength and durability: the compounding of graphene and SiO2 improves the mechanical strength of the membrane (≥5.8 MPa), and in the long-term use process, the membrane flux decay rate is as low as 3%-8% / 12 h, prolonging the service life of the membrane. DETAILED DESCRIPTION

[0031] Example 1:

[0032] (1) Graphene powder and mesoporous SiO2 powder were mixed at a mass ratio of 1:1 and ultrasonically dispersed for 2 hours to form a dispersion liquid. Polysulfone, polyvinylidene fluoride, and polyvinylpyrrolidone were added to a reaction kettle at a mass ratio of 5:1:1, heated to 120°C, and kept for 2 hours. N-N-methylacetamide and sodium chloride were added, stirred for 20 minutes, and then N-methylpyrrolidone and polyethylene glycol were added, stirred for 40 minutes. The above dispersion liquid was poured into the reaction kettle, and kept stirring for 1 hour. Vacuum degassing was performed at a gas pressure of 3 MPa and a temperature of 120°C to obtain a casting solution. A glass plate was used to scrape the film with a doctor blade, and then immersed in a coagulation bath to prepare a modified base film by phase inversion method.

[0033] (2) The polysulfone ultrafiltration membrane was soaked in ethanol and rinsed with deionized water for 3-5 times. The rinsed membrane was soaked in a mixed solution of potassium permanganate and sulfuric acid (potassium permanganate concentration of 0.1 mol / L, sulfuric acid concentration of 1 mol / L) for 1 hour.

[0034] (3) PGAM and PEI were mixed in DMSO at a molar ratio of 1:3, 1% sodium hydroxide was added, and the reaction was carried out at 60°C for 2 hours. The reaction solution was distilled under reduced pressure at 60°C and 0.08 MPa, and the graft product was obtained by dialysis for 24 hours.

[0035] (4) The purified PEI-PGAM reaction compound was dissolved in a phosphate buffer solution (pH value of 7.5) to prepare a graft solution with a concentration of 10%. The pretreated polysulfone ultrafiltration membrane was soaked in the graft solution and reacted at 75°C for 6 hours.

[0036] (5) After the grafting reaction was completed, the nanofiltration membrane was taken out and rinsed with a large amount of deionized water for 10 minutes, 3 times to remove unreacted PGAM-PEI compounds and other impurities. Then the membrane was dried at 50°C for 3 hours to make the grafted compounds stable on the membrane surface, and the final positively charged nanofiltration membrane was obtained.

[0037] (6) The positively charged nanofiltration membrane prepared in Example 1 was tested for performance, and the tensile strength of the membrane could reach 5.4 MPa, and the contact angle could reach 43°. The nanofiltration membrane had a rejection rate of 98.49% for CuCl2, a flux of 123.21 L·m -2 ·h -1 ·MPa -1 , a rejection rate of 97.25% for PbCl2, a flux of 137.70 L·m -2 ·h -1 ·MPa -1 , a rejection rate of 97.42% for CaCl2, and a flux of 132.56 L·m -2 ·h -1 ·MPa -1The rejection rate of MgCl2 can reach 97.32%, and the flux is 139.53 L·m -2 ·h -1 ·MPa -1 The rejection rate of NaCl is 71.24%, and the flux is 95.88 L·m -2 ·h -1 ·MPa -1 The membrane flux attenuation rate is 7% / 12 h.

[0038] Example 2:

[0039] (1) Mix graphene powder and mesoporous SiO2 powder in a mass ratio of 1.2:1, ultrasonic dispersion for 2.5 hours to form a dispersion liquid. Add polysulfone, polyvinylidene fluoride, and polyvinylpyrrolidone in a mass ratio of 5:1:1 into a reaction kettle, heat to 120°C and keep for 2 hours. Add N-N-methylacetamide and sodium chloride, stir for 20 minutes, then add N-methylpyrrolidone and polyethylene glycol, stir for 40 minutes. Pour the above dispersion liquid into the reaction kettle, keep stirring for 1 hour. Vacuum degassing at 3 MPa pressure and 120°C to obtain a casting solution. Use a doctor blade to coat the glass plate, then immerse it in a coagulation bath to prepare a modified base film by phase inversion method.

[0040] (2) Soak the polysulfone ultrafiltration membrane in ethanol and rinse with deionized water for 3-5 times. Soak the membrane rinsed with deionized water in a mixed solution of potassium permanganate-sulfuric acid (potassium permanganate concentration is 0.1 mol / L, sulfuric acid concentration is 1 mol / L) for 1 hour.

[0041] (3) Mix PGAM and PEI in a molar ratio of 1:3 in DMSO, add 1% sodium hydroxide, and react at 60°C for 2 hours. Distill the reacted solution at 60°C and 0.08 MPa under reduced pressure, and purify by dialysis for 24 hours to obtain the grafted product.

[0042] (4) Dissolve the purified PEI-PGAM reaction compound in a phosphate buffer solution (pH value is 7.5) to prepare a grafting solution with a concentration of 10%. Soak the pretreated polysulfone ultrafiltration membrane in the grafting solution and react at 75°C for 6 hours.

[0043] (5) After the grafting reaction is completed, take out the nanofiltration membrane, rinse it with a large amount of deionized water for 10 minutes, rinse it 3 times to remove unreacted PGAM-PEI compounds and other impurities. Then dry the membrane at 50°C for 3 hours to make the grafted compounds stable on the membrane surface, and obtain the final positively charged nanofiltration membrane.

[0044] (6) The performance of the positively charged nanofiltration membrane prepared in Example 2 was tested, and the membrane tensile strength could reach 5.8 MPa, and the contact angle could reach 44°. The rejection rate of the nanofiltration membrane to CuCl2 was 97.784%, and the flux was 123.65 L·m -2 ·h -1 ·MPa -1 The rejection rate of PbCl2 was 97.53%, and the flux was 137.91 L·m -2 ·h -1 ·MPa -1 The rejection rate of CaCl2 was 97.64%, and the flux was 94.65 L·m -2 ·h -1 ·MPa -1 The rejection rate of MgCl2 could reach 97.62%, and the flux was 139.33 L·m -2 ·h -1 ·MPa -1 The rejection rate of NaCl was 71.46%, and the flux was 95.93 L·m -2 ·h -1 ·MPa -1 The membrane flux attenuation rate was 3% / 12 h.

[0045] Example 3:

[0046] (1) Graphene powder and mesoporous SiO2 powder were mixed in a mass ratio of 1:1, ultrasonically dispersed for 2 hours to form a dispersion liquid. Poly sulfone, polyvinylidene fluoride, and polyvinylpyrrolidone were added to the reaction kettle in a mass ratio of 6:1:1, heated to 120°C and kept for 2 hours. N-N-methyl acetamide and sodium chloride were added, stirred for 20 minutes, and then N-methyl pyrrolidone and polyethylene glycol were added, stirred for 40 minutes. The above dispersion liquid was poured into the reaction kettle, and the stirring was kept for 1 hour. Vacuum degassing was carried out at a gas pressure of 3 MPa and a temperature of 120°C to obtain a casting solution. A doctor blade was used to coat the casting solution on a glass plate, and then the modified base film was prepared by immersion in a coagulation bath through phase inversion.

[0047] (2) The polysulfone ultrafiltration membrane was soaked in ethanol and rinsed with deionized water for 3-5 times. The membrane was soaked in a potassium permanganate-sulfuric acid mixed solution (potassium permanganate concentration of 0.1 mol / L, sulfuric acid concentration of 1 mol / L) for 1 hour.

[0048] (3) PGAM and PEI were mixed in a molar ratio of 1.2:3 in DMSO, 1.5% sodium hydroxide was added, and the reaction was carried out at 60°C for 2 hours. The reaction solution was distilled under reduced pressure at 60°C and 0.08 MPa, and the graft product was obtained by dialysis for 24 hours.

[0049] (4) The purified PEI-PGAM reaction compound was dissolved in a phosphate buffer solution (pH 7.5) to prepare a grafting solution with a concentration of 15%. The pretreated polysulfone ultrafiltration membrane was immersed in the grafting solution and reacted at 80 °C for 7 hours.

[0050] (5) After the completion of the grafting reaction, the nanofiltration membrane was taken out and washed with a large amount of deionized water for 10 minutes, 3 times to remove unreacted PGAM-PEI compounds and other impurities. Then the membrane was dried at 50 °C for 3 hours to make the grafted compounds stably exist on the membrane surface, obtaining the final positively charged nanofiltration membrane.

[0051] (6) The performance of the positively charged nanofiltration membrane prepared in Example 3 was tested, and the tensile strength of the membrane could reach 5.8 MPa, and the contact angle could reach 38°. The nanofiltration membrane had a rejection rate of 98.36% for CuCl2, a flux of 124.19 L·m -2 ·h -1 ·MPa -1 , a rejection rate of 97.25% for PbCl2, a flux of 137.78 L·m -2 ·h -1 ·MPa -1 , a rejection rate of 96.98% for CaCl2, a flux of 94.78 L·m -2 ·h -1 ·MPa -1 , a rejection rate of 97.24% for MgCl2, a flux of 139.54 L·m -2 ·h -1 ·MPa -1 , a rejection rate of 72.15% for NaCl, a flux of 96.22 L·m -2 ·h -1 ·MPa -1 , and a membrane flux decay rate of 4% / 12 h.

[0052] Example 4:

[0053] (1) Graphene powder and mesoporous SiO2 powder were mixed at a mass ratio of 1:1 and ultrasonically dispersed for 2 hours to form a dispersion liquid. Polysulfone, polyvinylidene fluoride, and polyvinylpyrrolidone were added to the reaction kettle at a mass ratio of 5:1:1, heated to 120 °C and kept for 2 hours. N-N-methyl acetamide and sodium chloride were added, stirred for 20 minutes, and then N-methyl pyrrolidone and polyethylene glycol were added, stirred for 40 minutes. The above dispersion liquid was poured into the reaction kettle and kept stirring for 1 hour. Vacuum degassing was carried out at a pressure of 3 MPa and a temperature of 120 °C to obtain a casting solution. A glass plate was used to scrape the membrane with a doctor blade, and then immersed in a coagulation bath to prepare a modified base membrane by phase inversion method.

[0054] (2) Soak the polysulfone ultrafiltration membrane in ethanol and rinse with deionized water for 3-5 times. Soak the rinsed membrane in a mixed solution of potassium permanganate and sulfuric acid (potassium permanganate concentration of 0.1 mol / L, sulfuric acid concentration of 1 mol / L) for 1 hour.

[0055] (3) Mix PGAM and PEI in a molar ratio of 0.8:3 in DMSO, add 1% sodium hydroxide, and react at 60°C for 2 hours. Distill the reacted solution at 60°C and 0.08 MPa under reduced pressure, and purify by dialysis for 24 hours to obtain the grafted product.

[0056] (4) Dissolve the purified PEI-PGAM reaction compound in a phosphate buffer solution (pH 7.5) to prepare a grafting solution with a concentration of 10%. Soak the pretreated polysulfone ultrafiltration membrane in the grafting solution and react at 75°C for 6 hours.

[0057] (5) After the grafting reaction is completed, take out the nanofiltration membrane, rinse it with a large amount of deionized water for 10 minutes, rinse it 3 times to remove unreacted PGAM-PEI compounds and other impurities. Then dry the membrane at 50°C for 3 hours to make the grafted compounds stable on the membrane surface to obtain the final positively charged nanofiltration membrane.

[0058] (6) The positively charged nanofiltration membrane prepared in Example 4 was tested for performance, and the membrane tensile strength could reach 5.8 MPa, and the contact angle could reach 42°. The nanofiltration membrane had a rejection rate of 97.12% for CuCl2, a flux of 122.32 L·m -2 ·h -1 ·MPa -1 , a rejection rate of 97.55% for PbCl2, a flux of 136.98 L·m -2 ·h -1 ·MPa -1 , a rejection rate of 97.32% for CaCl2, a flux of 93.88 L·m -2 ·h -1 ·MPa -1 , a rejection rate of 97.12% for MgCl2, a flux of 138.83 L·m -2 ·h -1 ·MPa -1 , a rejection rate of 70.11% for NaCl, a flux of 95.82 L·m -2 ·h -1 ·MPa -1 , and a membrane flux decay rate of 6% / 12 h.

Claims

1. A method for preparing a positively charged nanofiltration membrane for efficient separation of metal ions, characterized by the following steps: Step 1: Preparation of graphene / SiO2 modified polysulfone ultrafiltration membrane base membrane: (1) Mix graphene powder and mesoporous SiO2 powder in a mass ratio of 1:1-2:1 in an organic solvent, and disperse by ultrasonic for 1-3 hours to form a uniform dispersion liquid; (2) Add polysulfone, polyvinylidene fluoride, and polyvinylpyrrolidone in a mass ratio of 5:1:1-7:1:1 into a reaction kettle, stir uniformly, heat to 100-150 ℃, and keep for 1-4 hours; (3) Add any one of N,N-dimethylacetamide, N,N-dimethylformamide, and N-methyl pyrrolidone, and one of sodium sulfate and sodium chloride into the reaction kettle, and continue stirring for 10-30 minutes; (4) Add N-methyl pyrrolidone and polyethylene glycol into the reaction kettle, and continue stirring for 30-50 minutes; (5) Pour the uniform dispersion liquid obtained in (1) into the reaction kettle, keep and continue stirring for 1 hour to prepare a primary casting solution; (6) Perform vacuum degassing of the primary casting solution at a pressure of 1-5 MPa and a temperature of 80-180 ℃ to prepare a graphene / silica polysulfone casting solution; (7) Prepare the modified polysulfone ultrafiltration membrane base membrane by phase inversion method; Step 2: Pretreatment of the modified base membrane: (1) Surface cleaning: soak and rinse the modified polysulfone ultrafiltration membrane base membrane with one of ethanol and isopropanol, and then rinse repeatedly with deionized water for 3-5 times to remove possible impurities, oil stains, and residual chemicals on the membrane surface and ensure the cleanliness of the membrane surface; (2) Surface activation: treat the membrane surface with a potassium permanganate-sulfuric acid mixed solution, and soak for 1 hour; Step 3: Preparation of PGAM and PEI reaction compound: (1) Reaction condition selection: mix PGAM and PEI in a molar ratio of 0.5:3-2:3 in dimethyl sulfoxide (DMSO) solvent, set the reaction temperature to 40-80 ℃, the stirring speed to 300-600 rpm, and add an appropriate amount of sodium hydroxide as an alkali catalyst, and the reaction time is 1-3 hours; (2) Product purification: after the reaction is completed, remove the solvent by vacuum distillation at 60 ℃ and 0.08 MPa vacuum degree using a rotary evaporator; then dissolve the crude product in deionized water, and purify by dialysis, with a dialysis bag molecular weight cutoff of 500-1000 Da, deionized water changed every 4-6 hours, and dialysis for 24-48 hours to obtain pure PEI-PGAM reaction compound; Step 4: Grafting reaction process: (1) Grafting solution preparation: dissolve the purified PEI-PGAM reaction compound in a phosphate buffer solution with a pH value of 7.5 to prepare a grafting solution with a concentration of 5%-20%; (2) Grafting reaction operation: soak the pretreated polysulfone ultrafiltration membrane base membrane in the grafting solution, and react at 60-90 ℃ for 2-10 h; Step 5: Post-treatment: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ After the completion of the grafting reaction, the nanofiltration membrane is taken out and washed with a large amount of deionized water for 10-15 minutes, 3-5 times, to remove unreacted PGAM-PEI compound and other impurities; then the membrane is dried at a temperature of 40-60 ℃ for 3 hours, so that the grafted compound stably exists on the membrane surface, to obtain a final positively charged nanofiltration membrane.

2. The positively charged nanofiltration membrane prepared by the method of claim 1, wherein, The nanofiltration membrane has a retention rate of divalent metal ions of ≥97%, a tensile strength of ≥5.8 Mpa, and a membrane flux decay rate of as low as 3%-8% after 12 hours of continuous operation.