A positively charged nanofiltration membrane and a method for preparing the same

Abstract

A positively charged nanofiltration membrane and its preparation method are disclosed, belonging to the field of nanofiltration membrane technology, overcoming the shortcomings of existing nanofiltration membranes in terms of poor separation performance for metal cations. The preparation method of the positively charged nanofiltration membrane of this invention includes: coating a crosslinking agent onto the surface of an amine-rich nanofiltration membrane, and crosslinking it to obtain the positively charged nanofiltration membrane. This invention uses a crosslinking agent to crosslink with primary amines on the surface of the nanofiltration membrane, constructing a positively charged layer on the surface of the nanofiltration membrane, thus solving the problem that embedding amine-rich monomers into the nanofiltration membrane can only reduce electronegativity but cannot modify the membrane surface to be electronegative.

Description

Technical Field

[0001] This invention belongs to the field of nanofiltration membrane technology, specifically relating to a positively charged nanofiltration membrane and its preparation method. Background Technology

[0002] Nanofiltration is a novel membrane separation technology developed in the late 1980s, falling between reverse osmosis and ultrafiltration. Nanofiltration membranes have pore sizes of approximately 0.5–2 nm, enabling efficient separation of monovalent and divalent metal ions, while also exhibiting good removal efficiency for small molecules with molecular weights between 100 and 2000 Da. Based on its unique separation performance and environmentally friendly characteristics, nanofiltration membranes have been widely used in chemical separation, wastewater treatment, and metal recovery.

[0003] Currently, most commercially available and research-based nanofiltration membranes are negatively charged membranes. According to the Donnan effect, negatively charged nanofiltration membranes exhibit good retention of multivalent anions, but their retention performance for cations is less than satisfactory. The leachate in lithium battery recycling systems contains a wide variety and high concentration of metal ions, making it difficult for existing conventional nanofiltration membranes to achieve efficient separation. This limits the application of nanofiltration membranes in lithium battery recycling systems. In the field of high-valent metal ion recovery, positively charged nanofiltration membranes are more efficient; therefore, developing novel positively charged nanofiltration membranes is essential.

[0004] Existing techniques utilize swelling solvents and amine-containing solutes to treat nanofiltration membranes, preparing composite nanofiltration membranes that are nearly electrically neutral to achieve the separation of small organic molecules. However, this method only reduces the negative charge of the nanofiltration membrane and does not achieve a positive charge on the membrane surface, resulting in poor performance of the prepared membrane in separating metal cation systems. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the defect of poor separation performance of nanofiltration membranes for metal cations in the prior art, thereby providing a positively charged nanofiltration membrane and its preparation method.

[0006] To this end, the present invention provides the following technical solution.

[0007] In a first aspect, the present invention provides a method for preparing a positively charged nanofiltration membrane, comprising: coating a crosslinking agent onto the surface of an amine-rich nanofiltration membrane, crosslinking the agent, and obtaining a positively charged nanofiltration membrane. Specifically, the crosslinking agent is coated onto the surface of the amine-rich nanofiltration membrane on the side away from the base membrane.

[0008] Furthermore, the crosslinking agent is a compound having NHS ester groups at both ends; preferably, the compound having NHS ester groups at both ends includes one or more of bissuccinimide octanoate, bissuccinimide glutarate, and bissuccinimide malonic acid ester.

[0009] Furthermore, the crosslinking agent is coated onto the surface of the amine-rich nanofiltration membrane in a solution manner;

[0010] Preferably, the concentration of the crosslinking agent in the solution is 1–5 g / L;

[0011] Preferably, the solvent of the solution is water.

[0012] Furthermore, the cross-linking reaction time is 10s to 60s.

[0013] Furthermore, the method for preparing the amino-rich nanofiltration membrane includes:

[0014] Step 1: Mix the base membrane, polyamine, and polyacryl chloride, and react to obtain a nanofiltration membrane;

[0015] Step 2: Fix the nanofiltration membrane from Step 1 with a plate and frame, cover the surface of the nanofiltration membrane with the swelling agent aqueous solution and immerse it, then pour out the excess swelling agent aqueous solution, introduce the amine-rich monomer, and obtain the amine-rich nanofiltration membrane.

[0016] Furthermore, step 1 satisfies at least one of the following conditions:

[0017] (1) Step 1 includes: immersing the base film in a polyamine aqueous solution, letting it stand for a first period of time, and removing excess solution from the surface of the base film after standing; then immersing the base film in a polyacrylamide oil solution, letting it stand for a second period of time, and then taking it out.

[0018] Preferably, the concentration of the polyamine aqueous solution is 0.1–4.0 wt%.

[0019] Preferably, the solvent for the polyacrylamide chloride oil phase solution is an organic solvent, which may be selected from at least one of isoparaffin, n-hexane, cyclohexane, n-heptane, and n-dodecane;

[0020] Preferably, the concentration of the polyacrylamide chloride oil phase solution is 0.1–1 wt%.

[0021] Preferably, the first time period is 2 to 5 minutes;

[0022] Preferably, the second segment lasts for 1 to 3 minutes;

[0023] (2) Polyamines include one or more of phenylenediamine, polyethyleneimine, ethylenediamine, and piperazine;

[0024] (3) The polyacryl chlorides include one or more of pyromellitic methyl chloride, benzoyl chloride, polysulfonyl chloride, and cycloalkane polyacryl chloride.

[0025] Furthermore, step 2 satisfies at least one of the following conditions:

[0026] (1) The swelling agent in the aqueous solution of the swelling agent includes any one or a combination of two or more of methanol, anhydrous ethanol, acetone, dimethyl sulfoxide or acetonitrile;

[0027] (2) The content of the swelling agent in the aqueous solution of the swelling agent is 1-10 g / L;

[0028] (3) The soaking time of the swelling agent aqueous solution on the nanofiltration membrane surface is 1 to 5 minutes;

[0029] (4) Introducing rich amine monomers includes: covering the swollen nanofiltration membrane surface with an aqueous solution of rich amine monomers and immersing it;

[0030] Preferably, the amine-rich monomer includes any one or a combination of two or more of ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), and tetraethylenepentamine (TEPA);

[0031] Preferably, the concentration of the aqueous solution of the amine monomer is 1–5 g / L;

[0032] Preferably, the soaking time for the swollen nanofiltration membrane surface in the aqueous solution of the amine-rich monomer is 1 to 5 minutes.

[0033] Furthermore, the method for preparing the base film includes:

[0034] Step 101: Mix the polymer with the solvent, stir, and let stand to remove bubbles to obtain the casting solution;

[0035] Step 102: Coat the casting solution onto the substrate and allow it to solidify to obtain the base film.

[0036] Furthermore, at least one of the following conditions must be met:

[0037] (1) Step 101: Prepare the casting solution at room temperature;

[0038] (2) The stirring time is 6 to 24 hours;

[0039] (3) In the casting solution, the polymer content is 10-30 wt%, and the remainder is solvent;

[0040] (4) The solvent of the casting solution includes one or more of N-dimethylformamide, N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide;

[0041] (5) The casting solution also includes hydrophilic additives;

[0042] Preferably, the hydrophilic additive accounts for 0-10 wt% of the casting solution;

[0043] Optionally, the hydrophilic additives include one or more of polyethylene glycol, lithium chloride, polyvinylpyrrolidone, and lithium nitrate;

[0044] (6) The raw material polymer is polysulfone or polyethersulfone;

[0045] (7) The thickness of the casting solution coating is 220-240 μm;

[0046] (8) After the casting solution is coated on the substrate, it is immersed in the coagulation bath for coagulation.

[0047] Preferably, the coagulation bath comprises water;

[0048] Preferably, the coagulation bath further includes additives, and optionally, the additives are one or more selected from N-methylpyrrolidone, phenol, formic acid, methanol and ethanol; preferably, the amount of additives is 0 to 5 wt% of the mass of the coagulation bath.

[0049] Preferably, the coagulation bath temperature is 10–50°C;

[0050] Preferably, the soaking time in the coagulation bath is 1 to 5 minutes.

[0051] Secondly, the present invention provides a positively charged nanofiltration membrane prepared according to the preparation method described above.

[0052] Phenylenediamine can be m-phenylenediamine and / or o-phenylenediamine.

[0053] Phthaloyl chloride can be isophthaloyl chloride and / or terephthaloyl chloride.

[0054] The technical solution of this invention has the following advantages:

[0055] 1. The method for preparing the positively charged nanofiltration membrane of the present invention includes: coating a crosslinking agent on the surface of an amine-rich nanofiltration membrane, crosslinking, and obtaining a positively charged nanofiltration membrane.

[0056] This invention employs a crosslinking agent to crosslink with primary amines containing short-chain amine groups on the surface of nanofiltration membranes, constructing a positively charged layer on the membrane surface. This solves the problem that embedding amine-rich monomers into nanofiltration membranes only reduces electronegativity but cannot modify the membrane surface to be electronegative. Furthermore, since the crosslinking agent can react simultaneously with multiple short-chain amine groups on the nanofiltration membrane surface, it can improve the stability of the nanofiltration membrane.

[0057] 2. The crosslinking agent of this invention is a compound with NHS ester groups at both ends. The crosslinking agent of this invention is a long-chain crosslinking agent, which can prevent it from entering the interior of the nanofiltration membrane. Therefore, the crosslinking effect occurs on the membrane surface and does not affect the internal structure of the nanofiltration membrane. This method retains the advantage of increasing flux by embedding rich amine monomers and also effectively constructs a positively charged layer on the surface of the nanofiltration membrane. The nanofiltration membrane prepared by this method can achieve good separation effect for metal cation systems.

[0058] 3. The preparation method of the amine-rich nanofiltration membrane includes: Step 1, mixing the base membrane, polyamine and polyacryl chloride and reacting; Step 2, soaking the product of Step 1 in a swelling agent and then taking it out, introducing the amine-rich monomer to obtain the amine-rich nanofiltration membrane.

[0059] The surface of the nanofiltration membrane is modified by an expansion-embedding-contraction method, in which electropositive amine-rich chain monomers are embedded into the polyamide layer and surface of the nanofiltration membrane. Embedding monomers inside the polyamide layer can effectively expand the pore size of the nanofiltration membrane and increase the permeation flux. Detailed Implementation

[0060] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0061] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0062] In the following examples and comparative examples, the polysulfone manufacturer is BASF, and the model is S6010.

[0063] Example 1

[0064] This embodiment provides a method for preparing a positively charged nanofiltration membrane, including the following steps:

[0065] Step 1: Prepare casting solution: At room temperature, mix 17g polysulfone and 83g N-N dimethylformamide, stir for 24h, and then let the solution stand to remove bubbles to obtain casting solution.

[0066] Step 2: Preparation of the base film: The nonwoven fabric is laid flat on a clean glass plate. An appropriate amount of casting solution is poured onto the surface of the nonwoven fabric, and after being quickly scraped over with a doctor blade, it is rapidly immersed in a coagulation bath to undergo phase inversion, thus obtaining the base film. The thickness of the liquid film after scraping is 230 μm. In this embodiment, the coagulation bath consists of water, the coagulation bath temperature is 20℃, and the immersion time is 3 minutes.

[0067] Step 3: Immerse the above-mentioned base membrane in a polyamine aqueous solution and let it stand for a predetermined first time. After standing, use a roller to remove excess aqueous solution from the membrane surface and then air dry for 1 minute. Then immerse the base membrane in a polyacrylamide oil solution to carry out interfacial polymerization reaction. After standing for a predetermined second time, take it out and air dry for 1 minute to obtain a nanofiltration membrane, which is then stored in deionized water.

[0068] In this embodiment, the polyamine selected is m-phenylenediamine, and the concentration of the aqueous polyamine solution is 3 wt%. The polyacrylamide selected is trimesoyl chloride, and the concentration of the oil phase solution is 0.15 wt%. The solvent for the oil phase solution is IsoparE. The first settling time is 3 min, and the second settling time is 2 min.

[0069] Step 4: Take the nanofiltration membrane prepared in Step 3, fix it with a polytetrafluoroethylene plate frame, pour the swelling agent aqueous solution onto the surface of the nanofiltration membrane for soaking, expand the inter-chain spacing of the polyamide layers of the nanofiltration membrane, after soaking, pour out the excess swelling agent aqueous solution, cover the surface of the nanofiltration membrane with the rich amine monomer aqueous solution, so that the amine monomer is embedded, after embedding, pour out the excess solution on the surface of the nanofiltration membrane, and use nitrogen to blow dry the membrane surface.

[0070] In this embodiment, methanol was selected as the swelling agent. The swelling agent concentration in the aqueous solution was 4 g / L, and the soaking time was 2 min. Diethylenetriamine was selected as the rich amine monomer, and the concentration of the rich amine monomer in the aqueous solution was 3 g / L, with a reaction time of 3 min.

[0071] Step 5: Pour the crosslinking agent aqueous solution onto the surface of the nanofiltration membrane until the membrane surface is completely covered, and crosslink with the amine chains embedded on the membrane surface. After reacting for a period of time, pour out the excess crosslinking agent aqueous solution and use nitrogen gas to dry the membrane surface to obtain a positively charged nanofiltration membrane.

[0072] In this embodiment, the crosslinking agent is bis(succinimide) octanoate. The concentration of the crosslinking agent in the aqueous solution is 3 g / L, and the crosslinking reaction time is 45 s.

[0073] Example 2

[0074] This embodiment provides a method for preparing a positively charged nanofiltration membrane, including the following steps:

[0075] Step 1: Prepare casting solution: At room temperature, mix 18g polysulfone, 80g N-methylpyrrolidone and 2g hydrophilic additive lithium chloride, stir for 24h and then let the solution stand to remove bubbles to obtain casting solution.

[0076] Step 2: Preparation of the base film: The nonwoven fabric is laid flat on a clean glass plate. An appropriate amount of casting solution is poured onto the surface of the nonwoven fabric, and after being quickly scraped over with a doctor blade, it is rapidly immersed in a coagulation bath for phase inversion to obtain the base film. The thickness of the liquid film after scraping is 230 μm. The coagulation bath composition contains 2 wt% of the additive N-methylpyrrolidone, with the balance being water. The coagulation bath temperature is 50℃, and the immersion time is 1 min.

[0077] Step 3: Immerse the above-mentioned base membrane in a polyamine aqueous solution and let it stand for a predetermined first time. After standing, use a roller to remove excess aqueous solution from the membrane surface and then air dry for 1 minute. Then immerse the base membrane in a polyacrylamide oil solution to carry out interfacial polymerization reaction. After standing for a predetermined second time, take it out and air dry for 1 minute to obtain a nanofiltration membrane, which is then stored in deionized water.

[0078] In this embodiment, piperazine was selected as the polyamine, and the concentration of the polyamine in the aqueous solution was 3.5 wt%. Isophenylenesulfonyl chloride was selected as the polyacrylamide chloride, and the concentration of the polyacrylamide chloride in the oil solution was 0.2 wt%. The solvent for the oil solution was n-hexane. The first settling time was 5 min, and the second settling time was 3 min.

[0079] Step 4: Take the nanofiltration membrane prepared in Step 3, fix it with a polytetrafluoroethylene plate frame, pour the swelling agent aqueous solution onto the surface of the nanofiltration membrane for soaking, expand the inter-chain spacing of the polyamide layers of the nanofiltration membrane, after soaking, pour out the excess swelling agent aqueous solution, cover the surface of the nanofiltration membrane with the rich amine monomer aqueous solution, so that the amine monomer is embedded, after embedding, pour out the excess solution on the surface of the nanofiltration membrane, and use nitrogen to blow dry the membrane surface.

[0080] In this embodiment, anhydrous ethanol was selected as the swelling agent. The swelling agent concentration in the aqueous solution was 1 g / L, and the soaking time was 5 min. Ethylenediamine was selected as the rich amine monomer. The concentration of the rich amine monomer in the aqueous solution was 5 g / L, and the reaction time was 1 min.

[0081] Step 5: Pour the crosslinking agent aqueous solution onto the surface of the nanofiltration membrane until the membrane surface is completely covered, and crosslink with the amine chains embedded on the membrane surface. After reacting for a period of time, pour out the excess crosslinking agent aqueous solution and use nitrogen gas to dry the membrane surface to obtain a positively charged nanofiltration membrane.

[0082] In this embodiment, the crosslinking agent is bis(succinimide) malonate. The concentration of the crosslinking agent in the aqueous solution is 5 g / L, and the crosslinking reaction time is 10 s.

[0083] Example 3

[0084] This embodiment provides a method for preparing a positively charged nanofiltration membrane, including the following steps:

[0085] Step 1: Prepare casting solution: At room temperature, mix 16g polysulfone, 79g N-methylpyrrolidone and 5g hydrophilic additive polyethylene glycol, stir for 24h, and then let the solution stand to remove bubbles to obtain casting solution.

[0086] Step 2: Preparation of the base film: The nonwoven fabric is laid flat on a clean glass plate. An appropriate amount of casting solution is poured onto the surface of the nonwoven fabric, and after being quickly scraped over with a doctor blade, it is rapidly immersed in a coagulation bath for phase inversion to obtain the base film. The thickness of the liquid film after scraping is 230 μm. The coagulation bath composition is 1 wt% methanol (additive), with the remainder being water. The coagulation bath temperature is 30℃, and the immersion time is 2 min.

[0087] Step 3: Immerse the above-mentioned base membrane in a polyamine aqueous solution and let it stand for a predetermined first time. After standing, use a roller to remove excess aqueous solution from the membrane surface and then air dry for 1 minute. Then immerse the base membrane in a polyacrylamide oil solution to carry out interfacial polymerization reaction. After standing for a predetermined second time, take it out and air dry for 1 minute to obtain a nanofiltration membrane, which is then stored in deionized water.

[0088] In this embodiment, o-phenylenediamine was selected as the polyamine, and the concentration of the aqueous polyamine solution was 2.5 wt%. Terephthalic acid chloride was selected as the polyacrylamide, and the concentration of the oil solution of the polyacrylamide chloride was 0.15%. The solvent for the oil solution of the polyacrylamide chloride was Isopar G. The first settling time was 2 min, and the second settling time was 1 min.

[0089] Step 4: Take the nanofiltration membrane prepared in Step 3, fix it with a polytetrafluoroethylene plate frame, pour the swelling agent aqueous solution onto the surface of the nanofiltration membrane for soaking, expand the inter-chain spacing of the polyamide layers of the nanofiltration membrane, after soaking, pour out the excess swelling agent aqueous solution, cover the surface of the nanofiltration membrane with the rich amine monomer aqueous solution, so that the amine monomer is embedded, after embedding, pour out the excess solution on the surface of the nanofiltration membrane, and use nitrogen to blow dry the membrane surface.

[0090] In this embodiment, acetone was selected as the swelling agent. The swelling agent concentration in the aqueous solution was 10 g / L, and the soaking time was 1 min. Tetraethylenepentamine was selected as the rich amine monomer, with a concentration of 1 g / L, and the reaction time was 5 min.

[0091] Step 5: Pour the crosslinking agent aqueous solution onto the surface of the nanofiltration membrane until the membrane surface is completely covered, and crosslink with the amine chains embedded on the membrane surface. After reacting for a period of time, pour out the excess crosslinking agent aqueous solution and use nitrogen gas to dry the membrane surface to obtain a positively charged nanofiltration membrane.

[0092] In this embodiment, bis(succinimide) glutarate was selected as the crosslinking agent. The concentration of the crosslinking agent in the aqueous solution was 1 g / L, and the crosslinking reaction time was 60 s.

[0093] Comparative Example 1

[0094] This comparative example provides a method for preparing a nanofiltration membrane, which is basically the same as that in Example 1, except that step 5 is not included and no crosslinking agent is added for crosslinking.

[0095] Comparative Example 2

[0096] This comparative example provides a method for preparing a nanofiltration membrane, which is basically the same as that in Example 1, except that step 5 is: rinse with a swelling agent for 20 seconds, and then react at 50°C for 5 minutes.

[0097] Test case

[0098] (1) The separation performance of the products prepared in the test examples and comparative examples for metal cations was tested by using a 2000 ppm MgCl2 aqueous solution as the raw material and calculating the MgCl2 content using the conductivity values ​​of the raw material and the permeate. 2+ The retention rate was calculated using a conductivity meter to measure the conductivity of the feed solution and the permeate. The data were then substituted into the following formula.

[0099] Mg 2+ Retention rate R = (1 - C2 / C1) × 100%

[0100] In the formula, C1 is the conductivity of the feed liquid; C2 is the conductivity of the permeate.

[0101] (2) The permeation flux of the products prepared in the test examples and comparative examples was tested using the following method:

[0102] After rinsing with deionized water, the sample was placed in a flat-sheet membrane testing device. The feed solution was a 2000 ppm MgCl2 aqueous solution. The nanofiltration membrane was pre-pressed at 15.5 bar for 30 minutes at 25°C. After stabilization, timed sampling was performed. The data were then substituted into the following formula for calculation:

[0103] Permeation flux P = V / (S×t)

[0104] In the formula, V is the permeate volume (L), and S is the membrane area (m²). 2 ), where t is the sampling time (h).

[0105] The test results are shown in Table 1.

[0106] Table 1. Performance of the membranes prepared in the examples and comparative examples.

[0107] Example 1 143.32 98.72 Example 2 138.34 98.14 Example 3 145.23 97.23 Comparative Example 1 147.89 65.42 Comparative Example 2 109.28 57.32

[0108] The results in the table above show that the positively charged nanofiltration membranes prepared in this invention all exhibit good desalination effects, with retention rates all above 97%, and are effective against Mg2+. 2+The high rejection rate is mainly attributed to the Donan effect, which enhances the positive charge on the membrane surface. Furthermore, the membrane operates stably at 15.5 bar for extended periods, demonstrating the excellent stability of positively charged nanofiltration membranes prepared using the crosslinking method.

[0109] Because Comparative Example 1 only modified the nanofiltration membrane surface through an expansion-intercalation-contraction process without further cross-linking, a partially negatively charged region existed on the nanofiltration membrane surface, leading to the presence of Mg... 2+ The reduced retention efficiency, coupled with the compromised membrane stability, also contributed to the decreased retention rate. In Comparative Example 2, the modification only reduced the electronegativity of the nanofiltration membrane surface; the membrane surface remained negatively charged, resulting in ineffective Mg retention. 2+ The heating process during the reaction caused the cross-linking reaction of the polyamide to proceed further, which resulted in a decrease in the water flux of the membrane.

[0110] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a positively charged nanofiltration membrane, characterized in that, include: A crosslinking agent is coated onto the surface of an amine-rich nanofiltration membrane, and crosslinking is performed to obtain a positively charged nanofiltration membrane. The crosslinking agent is a compound with NHS ester groups at both ends; The method for preparing the amino-rich nanofiltration membrane includes: Step 1: Mix the base membrane, polyamine, and polyacryl chloride, and react to obtain a nanofiltration membrane; Step 2: Fix the nanofiltration membrane from Step 1 with a plate and frame, cover the surface of the nanofiltration membrane with a swelling agent aqueous solution and immerse it, then pour out the excess swelling agent aqueous solution, introduce a rich amine monomer, and obtain a rich amine nanofiltration membrane. The compounds having NHS ester groups at both ends include one or more of bis(succinimide) octanoate, bis(succinimide) glutarate, and bis(succinimide) malonate. The amine-rich monomer includes any one or a combination of two or more of ethylenediamine, diethylenetriamine, triethylenetetramine, and tetraethylenepentamine.

2. The method for preparing a positively charged nanofiltration membrane according to claim 1, characterized in that, The crosslinking agent is coated onto the surface of the amine-rich nanofiltration membrane in a solution manner.

3. The method for preparing a positively charged nanofiltration membrane according to claim 2, characterized in that, The concentration of the crosslinking agent in the solution is 1~5 g / L.

4. The method for preparing a positively charged nanofiltration membrane according to claim 2, characterized in that, The solvent for the solution is water.

5. The method for preparing a positively charged nanofiltration membrane according to claim 1, characterized in that, The cross-linking reaction time is 10s~60s.

6. The method for preparing a positively charged nanofiltration membrane according to claim 1, characterized in that, Step 1 satisfies at least one of the following conditions: (1) Step 1 includes: immersing the base film in a polyamine aqueous solution, letting it stand for a first period of time, and removing excess solution from the surface of the base film after standing; then immersing the base film in a polyacrylamide oil solution, letting it stand for a second period of time, and then taking it out. (2) Polyamines include one or more of phenylenediamine, polyethyleneimine, ethylenediamine, and piperazine; (3) The polyacryl chlorides include one or more of pyromellitic methyl chloride, benzoyl chloride, polysulfonyl chloride, and cycloalkane polyacryl chloride.

7. The method for preparing a positively charged nanofiltration membrane according to claim 6, characterized in that, The concentration of the polyamine aqueous solution is 0.1~4.0 wt%.

8. The method for preparing a positively charged nanofiltration membrane according to claim 6, characterized in that, The solvent for the polyacrylamide chloride oil phase solution is an organic solvent.

9. The method for preparing a positively charged nanofiltration membrane according to claim 8, characterized in that, The organic solvent includes at least one of isoalkanes, n-hexane, cyclohexane, n-heptane, and n-dodecane.

10. The method for preparing a positively charged nanofiltration membrane according to claim 6, characterized in that, The concentration of the polyacrylamide chloride oil phase solution is 0.1~1wt%.

11. The method for preparing a positively charged nanofiltration membrane according to claim 6, characterized in that, The first period lasts 2-5 minutes.

12. The method for preparing a positively charged nanofiltration membrane according to claim 6, characterized in that, The second segment lasts 1 to 3 minutes.

13. The method for preparing a positively charged nanofiltration membrane according to claim 1, characterized in that, Step 2 satisfies at least one of the following conditions: (1) The swelling agent in the aqueous solution of the swelling agent includes any one or a combination of two or more of methanol, anhydrous ethanol, acetone, dimethyl sulfoxide or acetonitrile; (2) The content of the swelling agent in the aqueous solution of the swelling agent is 1~10g / L; (3) The soaking time of the swelling agent aqueous solution on the nanofiltration membrane surface is 1~5 min; (4) Introducing rich amine monomers includes: covering the swollen nanofiltration membrane surface with an aqueous solution of rich amine monomers and immersing it.

14. The method for preparing a positively charged nanofiltration membrane according to claim 13, characterized in that, The concentration of the aqueous solution of the amine monomer is 1~5 g / L.

15. The method for preparing a positively charged nanofiltration membrane according to claim 13, characterized in that, The immersion time for the swollen nanofiltration membrane surface in the aqueous solution of amine-rich monomer is 1-5 minutes.

16. The method for preparing a positively charged nanofiltration membrane according to claim 1, characterized in that, The method for preparing the base film includes: Step 101: Mix the polymer with the solvent, stir, and let stand to remove bubbles to obtain the casting solution; Step 102: Coat the casting solution onto the substrate and allow it to solidify to obtain the base film.

17. The method for preparing a positively charged nanofiltration membrane according to claim 16, characterized in that, At least one of the following conditions must be met: (1) Step 101: Prepare the casting solution at room temperature; (2) The stirring time is 6~24h; (3) In the casting solution, the polymer content is 10~30 wt%, and the balance is solvent; (4) The solvent of the casting solution includes one or more of N-dimethylformamide, N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; (5) The casting solution also includes hydrophilic additives; (6) The polymer is polysulfone or polyethersulfone; (7) The thickness of the casting solution coating is 220~240 μm; (8) After the casting solution is coated on the substrate, it is immersed in the coagulation bath for coagulation.

18. The method for preparing a positively charged nanofiltration membrane according to claim 17, characterized in that, The hydrophilic additive accounts for 0-10 wt% of the casting solution.

19. The method for preparing a positively charged nanofiltration membrane according to claim 17, characterized in that, The hydrophilic additives include one or more of polyethylene glycol, lithium chloride, polyvinylpyrrolidone, and lithium nitrate.

20. The method for preparing a positively charged nanofiltration membrane according to claim 17, characterized in that, The coagulation bath comprises water.

21. The method for preparing a positively charged nanofiltration membrane according to claim 20, characterized in that, The coagulation bath also includes additives.

22. The method for preparing a positively charged nanofiltration membrane according to claim 21, characterized in that, The additive is one or more of N-methylpyrrolidone, phenol, formic acid, methanol, and ethanol.

23. The method for preparing a positively charged nanofiltration membrane according to claim 21, characterized in that, The amount of the additive is 0-5 wt% of the mass of the coagulation bath.

24. The method for preparing a positively charged nanofiltration membrane according to claim 17, characterized in that, The coagulation bath temperature is 10~50℃.

25. The method for preparing a positively charged nanofiltration membrane according to claim 17, characterized in that, The soaking time in the coagulation bath is 1 to 5 minutes.

26. A positively charged nanofiltration membrane prepared by the preparation method according to any one of claims 1-25.