Preparation method for nanofiltration membrane for regulating and controlling membrane surface charges

Abstract

The present invention relates to the technical field of nanofiltration membranes, and particularly relates to a preparation method for a nanofiltration membrane for regulating and controlling membrane surface charges. The preparation method comprises: step 1: (1) sequentially adding piperazine, a fiber additive, an ionic liquid and polyvinyl pyrrolidone into deionized water to obtain a water phase; and (2) adding trimesoyl chloride into normal hexane to serve as an oil phase; step 2, dipping a base membrane in the water phase for 1-3 minutes, and removing the redundant water phase; transferring same into the oil phase and subjecting same to polymerization for 2-3 minutes, and removing the redundant solution; and curing same at 80-85ºC for 5-6 minutes, and dipping same in deionized water for 1-2 days, so as to obtain a composite base membrane; and step 3, firstly placing the composite base membrane in a sodium hypochlorite solution for a primary post-treatment, and cleaning same; and transferring same into a sodium hydrosulfide solution for a secondary post-treatment, washing same until neutral, soaking same in deionized water for 1-3 days, and drying same to obtain a nanofiltration membrane.

Description

A method for preparing a nanofiltration membrane for regulating membrane surface charge Technical Field

[0001] The present invention relates to the technical field of nanofiltration membranes, in particular to a method for preparing a nanofiltration membrane for regulating membrane surface charge. Background Art

[0002] Water separation and purification technologies not only conserve limited water resources and alleviate increasing water shortages, but also reduce wastewater discharge and water pollution. Among the many water separation methods, membrane separation technology is a green and energy-saving technology with advantages such as simple operation, high treatment efficiency, and low cost.

[0003] In membrane separation technology, nanofiltration membranes are membranes with pore sizes ranging from 1 to 100 nm that retain organic compounds and multivalent ions with molecular weights between 100 and 2000 Da. Conventional nanofiltration membranes are typically based on a porous base membrane, with a polyamide separation layer formed on its surface via interfacial polycondensation or condensation methods. The resulting composite membranes' water treatment performance is primarily determined by their effective pore size and charge density. By manipulating the reactive monomers in the aqueous or oil phase during interfacial polymerization, the salt rejection and water permeability of the polyamide separation layer can be effectively regulated. These methods include: One approach involves modifying the nanofiltration membrane with zwitterionic liquids in the aqueous phase to increase the crosslinking and charge tunability of the polyamide separation layer. However, the resulting high crosslinking increases water mass transfer resistance; another approach involves grafting nanofiltration membrane surfaces to adjust surface properties. However, surface grafting affects pore size, pore size distribution, and thickness, leading to decreased membrane permeability. Therefore, achieving high ion rejection without sacrificing water flux remains a significant challenge.

[0004] In summary, it is of great significance to improve the above defects and prepare a nanofiltration membrane for regulating the surface charge of the membrane.

[0005] Summary of the Invention

[0006] The object of the present invention is to provide a method for preparing a nanofiltration membrane for regulating the surface charge of the membrane, so as to solve the problems raised in the above background technology.

[0007] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0008] A method for preparing a nanofiltration membrane for regulating membrane surface charge comprises the following steps:

[0009] Step 1: (1) piperazine, fiber additive, ionic liquid, and polyvinyl pyrrolidone are sequentially added to deionized water to obtain an aqueous phase; (2) trimesoyl chloride is added to n-hexane as an oil phase;

[0010] Step 2: Immerse the base film in the water phase for 1 to 3 minutes and remove excess water; transfer to the oil phase for polymerization for 2 to 3 minutes and remove excess solution; cure at 80°C to 85°C for 5 to 6 minutes and immerse in deionized water for 1 to 2 days to obtain a composite base film;

[0011] Step 3: The composite base membrane is first placed in a sodium hypochlorite solution for post-treatment and cleaning; then transferred to a sodium hydrosulfide solution for secondary post-treatment, washed to neutrality, soaked in deionized water for 1 to 3 days, and dried to obtain a nanofiltration membrane.

[0012] In a further embodiment, in the aqueous phase, the concentration of piperazine is 0.85wt% to 0.9wt%, the concentration of the fiber additive is 0.15wt% to 0.2wt%, the concentration of the ionic liquid is 0.4wt% to 0.5wt%, and the concentration of polyvinyl pyrrolidone is 0.05wt% to 0.06wt%; in the oil phase, the concentration of trimesoyl chloride is 0.15 to 0.2wt%.

[0013] In a further embodiment, the ionic liquid includes one or both of 1-aminoethyl-3-methylimidazolium glycine salt and 1-aminoethyl-3-methylimidazolium alanine salt.

[0014] In a further embodiment, the preparation process of the fiber additive is:

[0015] (1) 1-allyl-4-(2-hydroxyethyl)-piperazine and epichlorohydrin are mixed, potassium carbonate is added and stirred evenly, and the mixture is stirred and reacted at 80° C. to 85° C. for 8 to 10 hours, and purified to obtain epoxy-modified allylpiperazine;

[0016] (2) irradiating the polyethylene fiber with a cobalt source, placing it in a mixture of N,N-dimethylformamide containing epoxy-modified allylpiperazine, performing a grafting reaction at 50° C. to 70° C. for 1 to 3 hours under a nitrogen atmosphere, washing, and drying to obtain a grafted polyethylene fiber;

[0017] (3) placing the grafted polyethylene fiber in a 0.2 mol / L to 0.3 mol / L hydrochloric acid dioxane solution, stirring at 30° C. to 35° C. for 30 to 60 minutes, washing, and freeze-drying to obtain a fiber additive.

[0018] In a further embodiment, in the raw materials of the epoxy-modified allyl piperazine, the molar ratio of 1-allyl-4-(2-hydroxyethyl)-piperazine, epichlorohydrin and potassium carbonate is 1:(1.2-1.5):1.

[0019] In a further embodiment, the total dose of the cobalt source irradiation is 20 kGy to 30 kGy; and the concentration of the epoxy-modified allylpiperazine in the N,N-dimethylformamide mixed solution is 5 wt % to 10 wt %.

[0020] In a further embodiment, in step 3, the sodium hypochlorite solution is a sodium hypochlorite solution having a pH of 12.5 to 13.2 obtained by dissolving sodium hypochlorite in deionized water; the sodium hydrosulfide solution is a saturated sodium hydrosulfide solution obtained by dissolving sodium hydrosulfide in ethanol; the temperature of the primary post-treatment is 25° C. to 35° C., and the time is 40 to 48 hours; the temperature of the secondary post-treatment is 45° C. to 50° C., and the time is 1.5 to 2.5 hours.

[0021] In a further embodiment, the polyvinyl pyrrolidone in the aqueous phase is pre-modified, and the modification process is:

[0022] (1) L-vinylglycine and 6-(diethylamino)pyridine-3-carboxaldehyde are sequentially added to ethanol and reacted at 60°C to 65°C for 3 to 4 hours; an ethanol mixture containing an alkyl halide is added; the temperature is raised to 80 to 85°C, the reaction is carried out for 1 to 2 hours, and the mixture is purified to obtain a vinyl quaternary ammonium salt;

[0023] (2) adding polyvinyl pyrrolidone to deionized water, heating to 50° C., adding vinyl quaternary ammonium salt and vinyl imidazole, stirring, and cooling to obtain a mixture; placing the mixture in a microwave reactor, setting the microwave power to 750-800 W, and reacting for 1-2 minutes; and purifying to obtain modified polyvinyl pyrrolidone.

[0024] In a further embodiment, the molar ratio of L-vinylglycine, 6-(diethylamino)pyridine-3-carboxaldehyde, and alkyl halide is 1:1:(1-1.1); the mass ratio of polyvinylpyrrolidone to vinyl quaternary ammonium salt and vinyl imidazole is 100:(6-10):(2-4).

[0025] Compared with the prior art, this application has the following beneficial effects:

[0026] In the scheme (1), an ionic liquid containing imidazolium cations and amino acid anions is introduced into the aqueous phase. The ionic liquid has charge properties and has adjustable charge performance when used for ion separation. By controlling its content and controlling the surface charge by controlling the pH during the water treatment process, the charge is adjustable and the separation efficiency is improved. The nanofiltration membrane of the present application is positively charged when the pH is less than 6.5±0.2 and negatively charged when the pH is greater than 6.5±0.2; thereby, the ion retention rate is improved.

[0027] (2) Since the ionic liquid contains amino groups with reactivity, it can crosslink the surface of the composite base membrane; however, it makes the surface membrane layer have a high degree of crosslinking, reduces the pore size, increases water resistance, and needs to further improve the water flux; therefore, the scheme further introduces fiber additives to increase water permeability; wherein, the fiber additive is based on polyethylene fiber, firstly grafting epoxy-modified allyl piperazine on the surface, and then the epoxy group is ring-opened in hydrochloric acid to prepare it. The introduction of polyethylene fiber can form a three-dimensional network structure in the polyamide nanofiltration membrane, increase the micropore and pore connectivity of the composite base membrane, and increase the surface roughness, so that the nanofiltration membrane has better water flux and interception separation. At the same time, its introduction increases the pressure resistance and durability of the nanofiltration membrane. And because polyethylene fiber is hydrophobic, there is a hydrophobic segment inside the composite base membrane, thereby increasing the water permeability of the nanofiltration membrane. In this scheme, to enhance the dispersion and interfacial interaction of polyethylene fibers on the composite base membrane surface, the fibers are modified to produce fiber additives containing groups such as piperazine, C-OH, and C-Cl. This effectively disperses the fiber additive by leveraging the affinity and hydrogen bonding of similar groups. Furthermore, the C-Cl residues generate sulfhydryl groups during a subsequent post-treatment with a sodium hydrosulfide solution. These sulfhydryl groups' chelating properties for metal ions enhance the retention rate.

[0028] In the (3) scheme, a small amount of polyvinyl pyrrolidone (PVP) is introduced into the aqueous phase to increase the crosslinking and stability of the composite base membrane. At the same time, its introduction can affect the interfacial polymerization process: there is a van der Waals force between polyvinyl pyrrolidone and piperazine, which slows down the diffusion of piperazine on the base membrane surface, promotes the uniform distribution of pore size, and improves water flux and retention rate. In a further scheme, polyvinyl pyrrolidone is modified with vinyl quaternary ammonium salt and vinyl imidazole. Due to the grafting of imidazole, the affinity with ionic liquid is further improved, and the uniform distribution of ionic liquid is promoted. At the same time, the introduction of quaternary ammonium salt groups has higher ionic conductivity and can synergistically increase charge stability with ionic liquids. It also further increases ion retention and water permeability.

[0029] In the (4) scheme, the composite base membrane is post-treated once in a sodium hypochlorite solution and twice in a sodium bisulfide solution. In the first treatment, the uncross-linked amino groups on the surface of the composite base membrane serve as the action sites of sodium hypochlorite chlorination, thereby inducing partial hydrolysis of the polyamide, generating loose narrow pores, and increasing the surface roughness, thereby improving the surface hydrophilicity; in the second treatment, the sites containing -Cl are converted into thiol groups under the action of the sodium bisulfide solution, thereby further improving the retention rate of metal ions. DETAILED DESCRIPTION

[0030] Based on the examples in the present invention, specific implementations are described herein, but the scope of the disclosure is not limited to these specific implementations.

[0031] It should be noted that the following parts are by mass; all raw materials involved in the present invention are purchased from manufacturers without any special restrictions, including, for example, in the following examples, 1-allyl-4-(2-hydroxyethyl)-piperazine has a CAS number of 27612-67-5, epichlorohydrin has a CAS number of 106-89-8, potassium carbonate has a CAS number of 584-08-7, L-vinylglycine has a CAS number of 70982-53-5, 6-(diethylamino)pyridine-3-carboxaldehyde has a CAS number of 578726-67-7, bromooctane has a CAS number of The CAS number of the raw materials is 111-83-1, the CAS number of sodium hypochlorite is 7681-52-9, the CAS number of sodium hydrosulfide is 16721-80-5, and the ionic liquid of 1-aminoethyl-3-methylimidazolium glycine salt is a hydrochloride composed of 1-aminoethyl-3-methylimidazolium cation and glycine anion, provided by Mac Biotech. The specification of polyvinyl pyrrolidone is K-3, with an average molecular weight of 40,000. The fiber diameter of the polyethylene fiber is 30 microns, and the brand is Jitai Huahe. The base membrane is a polyethersulfone base membrane with a thickness of 100 microns and a pore size of 0.5 to 1 micron.

[0032] Example 1: A method for preparing a nanofiltration membrane for regulating membrane surface charge, comprising the following steps:

[0033] Preliminary preparation: S1: (1) 1-allyl-4-(2-hydroxyethyl)-piperazine and epichlorohydrin were mixed, potassium carbonate was added and stirred evenly, stirred and reacted at 80°C for 10 hours, purified, and epoxy-modified allyl piperazine was obtained; the molar ratio of 1-allyl-4-(2-hydroxyethyl)-piperazine, epichlorohydrin and potassium carbonate was 1:1.25:1; (2) 1 part of polyethylene fiber was irradiated with a cobalt source with a total dose of 30 kGy, and then placed in 20 parts of a 6 wt% N,N-dimethylformamide mixture containing epoxy-modified allyl piperazine, and grafted at 60°C for 3 hours under a nitrogen atmosphere, washed and dried to obtain grafted polyethylene fiber; (3) The grafted polyethylene fiber was placed in a 0.2 mol / L hydrochloric acid dioxane solution, stirred at 35°C for 60 minutes, washed, freeze-dried, and obtained a fiber additive;

[0034] S2: (1) L-vinylglycine and 6-(diethylamino)pyridine-3-carboxaldehyde were added to ethanol in sequence, and the mixture was reacted at 65°C for 3 hours; an ethanol mixture containing bromooctane was added; the mixture was heated to 82°C and reacted for 1.5 hours, and the mixture was purified to obtain a vinyl quaternary ammonium salt; the molar ratio of L-vinylglycine, 6-(diethylamino)pyridine-3-carboxaldehyde, and bromooctane was 1:1:1. (2) Polyvinylpyrrolidone was added to deionized water, the mixture was heated to 50°C, vinyl quaternary ammonium salt and vinyl imidazole were added, the mixture was stirred, and the mixture was cooled to obtain a mixture; the mixture was placed in a microwave reactor, the microwave power was set to 800W, and the mixture was reacted for 1.5 minutes; the mixture was purified to obtain a modified polyvinylpyrrolidone; the mass ratio of polyvinylpyrrolidone to vinyl quaternary ammonium salt and vinyl imidazole was 100:7:3;

[0035] Step 1: (1) 0.9 wt% of piperazine, 0.18 wt% of a fiber additive, 0.42 wt% of a 1-aminoethyl-3-methylimidazolium glycine salt ionic liquid, and 0.05 wt% of a modified polyvinyl pyrrolidone are sequentially added to deionized water to obtain an aqueous phase; (2) 0.2 wt% of trimesoyl chloride is added to n-hexane to obtain an oil phase;

[0036] Step 2: Immerse the base film in the water phase for 2 minutes and remove the excess water phase; transfer it to the oil phase for polymerization for 3 minutes and remove the excess solution; cure it at 85°C for 5 minutes and immerse it in deionized water for 2 days to obtain a composite base film;

[0037] Step 3: (1) dissolving sodium hypochlorite in deionized water to obtain a sodium hypochlorite solution with a pH of 12.9; dissolving sodium hydrosulfide in ethanol to obtain a saturated sodium hydrosulfide solution; (2) placing the composite base membrane in the sodium hypochlorite solution for a first post-treatment: the temperature is 30°C, the time is 48 hours, and washing; transferring it to the sodium hydrosulfide solution for a second post-treatment: the temperature is 50°C, the time is 2 hours, washing until neutral, placing it in deionized water for 2 days, and drying to obtain a nanofiltration membrane.

[0038] Example 2: A method for preparing a nanofiltration membrane for regulating membrane surface charge, comprising the following steps:

[0039] Preliminary preparation: S1: (1) 1-allyl-4-(2-hydroxyethyl)-piperazine and epichlorohydrin were mixed, potassium carbonate was added and stirred evenly, stirred and reacted at 80°C for 10 hours, purified, and epoxy-modified allyl piperazine was obtained; the molar ratio of 1-allyl-4-(2-hydroxyethyl)-piperazine, epichlorohydrin and potassium carbonate was 1:1.25:1; (2) 1 part of polyethylene fiber was irradiated with a cobalt source with a total dose of 30 kGy, and then placed in 20 parts of a 5 wt% N,N-dimethylformamide mixed solution containing epoxy-modified allyl piperazine, and grafted at 60°C for 3 hours under a nitrogen atmosphere, washed and dried to obtain grafted polyethylene fiber; (3) The grafted polyethylene fiber was placed in a 0.2 mol / L hydrochloric acid dioxane solution, stirred at 35°C for 60 minutes, washed, freeze-dried, and obtained a fiber additive;

[0040] S2: (1) L-vinylglycine and 6-(diethylamino)pyridine-3-carboxaldehyde were added to ethanol in sequence, and the mixture was reacted at 65°C for 3 hours; an ethanol mixture containing bromooctane was added; the mixture was heated to 82°C and reacted for 1.5 hours, and the mixture was purified to obtain a vinyl quaternary ammonium salt; the molar ratio of L-vinylglycine, 6-(diethylamino)pyridine-3-carboxaldehyde, and bromooctane was 1:1:1. (2) Polyvinylpyrrolidone was added to deionized water, the mixture was heated to 50°C, vinyl quaternary ammonium salt and vinyl imidazole were added, the mixture was stirred, and the mixture was cooled to obtain a mixture; the mixture was placed in a microwave reactor, the microwave power was set to 800W, and the mixture was reacted for 1.5 minutes; the mixture was purified to obtain a modified polyvinylpyrrolidone; the mass ratio of polyvinylpyrrolidone to vinyl quaternary ammonium salt and vinyl imidazole was 100:7:3;

[0041] Step 1: (1) 0.85 wt% of piperazine, 0.15 wt% of a fiber additive, 0.5 wt% of a 1-aminoethyl-3-methylimidazolium glycine salt ionic liquid, and 0.05 wt% of a modified polyvinyl pyrrolidone are sequentially added to deionized water to obtain an aqueous phase; (2) 0.15 wt% of trimesoyl chloride is added to n-hexane to obtain an oil phase;

[0042] Step 2: Immerse the base film in the water phase for 1 minute and remove the excess water phase; transfer it to the oil phase for polymerization for 3 minutes and remove the excess solution; cure it at 85°C for 6 minutes and immerse it in deionized water for 2 days to obtain a composite base film;

[0043] Step 3: (1) dissolving sodium hypochlorite in deionized water to obtain a sodium hypochlorite solution with a pH of 12.9; dissolving sodium hydrosulfide in ethanol to obtain a saturated sodium hydrosulfide solution; (2) placing the composite base membrane in the sodium hypochlorite solution for a post-treatment at a temperature of 30°C for 48 hours and washing; transferring it to a sodium hydrosulfide solution for a second post-treatment at a temperature of 45°C for 1.5 hours, washing until neutral, soaking it in deionized water for 2 days, and drying it to obtain a nanofiltration membrane.

[0044] Example 3: A method for preparing a nanofiltration membrane for regulating membrane surface charge, comprising the following steps:

[0045] Preliminary preparation: S1: (1) 1-allyl-4-(2-hydroxyethyl)-piperazine and epichlorohydrin were mixed, potassium carbonate was added and stirred evenly, stirred and reacted at 80°C for 10 hours, purified, and epoxy-modified allyl piperazine was obtained; the molar ratio of 1-allyl-4-(2-hydroxyethyl)-piperazine, epichlorohydrin and potassium carbonate was 1:1.25:1; (2) 1 part of polyethylene fiber was irradiated with a cobalt source with a total dose of 30 kGy, and then placed in 20 parts of a 10 wt% N,N-dimethylformamide mixture containing epoxy-modified allyl piperazine, and grafted at 60°C for 3 hours under a nitrogen atmosphere, washed and dried to obtain grafted polyethylene fiber; (3) The grafted polyethylene fiber was placed in a 0.2 mol / L hydrochloric acid dioxane solution, stirred at 35°C for 60 minutes, washed, freeze-dried, and obtained a fiber additive;

[0046] S2: (1) L-vinylglycine and 6-(diethylamino)pyridine-3-carboxaldehyde were added to ethanol in sequence, and the mixture was reacted at 65°C for 3 hours; an ethanol mixture containing bromooctane was added; the mixture was heated to 82°C and reacted for 1.5 hours, and the mixture was purified to obtain a vinyl quaternary ammonium salt; the molar ratio of L-vinylglycine, 6-(diethylamino)pyridine-3-carboxaldehyde, and bromooctane was 1:1:1. (2) Polyvinylpyrrolidone was added to deionized water, the mixture was heated to 50°C, vinyl quaternary ammonium salt and vinyl imidazole were added, the mixture was stirred, and the mixture was cooled to obtain a mixture; the mixture was placed in a microwave reactor, the microwave power was set to 800W, and the mixture was reacted for 1.5 minutes; the mixture was purified to obtain a modified polyvinylpyrrolidone; the mass ratio of polyvinylpyrrolidone to vinyl quaternary ammonium salt and vinyl imidazole was 100:7:3;

[0047] Step 1: (1) 0.9 wt% of piperazine, 0.2 wt% of a fiber additive, 0.4 wt% of a 1-aminoethyl-3-methylimidazolium glycine salt ionic liquid, and 0.06 wt% of a modified polyvinyl pyrrolidone are sequentially added to deionized water to obtain an aqueous phase; (2) 0.2 wt% of trimesoyl chloride is added to n-hexane to obtain an oil phase;

[0048] Step 2: Immerse the base film in the water phase for 2 minutes and remove the excess water phase; transfer it to the oil phase for polymerization for 2 minutes and remove the excess solution; cure it at 85°C for 5 minutes and immerse it in deionized water for 2 days to obtain a composite base film;

[0049] Step 3: (1) dissolving sodium hypochlorite in deionized water to obtain a sodium hypochlorite solution with a pH of 12.9; dissolving sodium hydrosulfide in ethanol to obtain a saturated sodium hydrosulfide solution; (2) placing the composite base membrane in the sodium hypochlorite solution for a first post-treatment: the temperature is 30°C, the time is 48 hours, and washing; transferring it to the sodium hydrosulfide solution for a second post-treatment: the temperature is 45°C, the time is 2.5 hours, washing until neutral, placing it in deionized water for immersion for 2 days, and drying to obtain a nanofiltration membrane.

[0050] Comparative Example 1: The modified polyvinyl pyrrolidone was replaced with polyvinyl pyrrolidone, and the rest was the same as in Example 1; the specific changes were as follows: Step 1: (1) 0.9 wt% of piperazine, 0.18 wt% of fiber additive, 0.42 wt% of 1-aminoethyl-3-methylimidazolium glycine salt ionic liquid, and 0.05 wt% of polyvinyl pyrrolidone were added to deionized water in sequence to obtain an aqueous phase; (2) 0.2 wt% of trimesoyl chloride was added to n-hexane as an oil phase.

[0051] Comparative Example 2: No fiber additive is introduced, and the rest is the same as Example 1; the specific changes are as follows: Step 1: (1) 1 wt% of piperazine, 0.5 wt% of 1-aminoethyl-3-methylimidazolium glycine salt ionic liquid, and 0.05 wt% of modified polyvinyl pyrrolidone are added to deionized water in sequence to obtain an aqueous phase; (2) 0.2 wt% of trimesoyl chloride is added to n-hexane as an oil phase.

[0052] Comparative Example 3: A sodium hydroxide solution with a pH of 13 was used for the first treatment, and the rest was the same as in Example 1; the specific changes were as follows: Step 3: (1) sodium hydroxide was dissolved in deionized water to obtain a sodium hydroxide solution with a pH of 13; sodium hydrosulfide was dissolved in ethanol to obtain a saturated sodium hydrosulfide solution; (2) the composite base membrane was first placed in a sodium hydroxide solution for a first post-treatment: the temperature was 30°C, the time was 48 hours, and it was washed; it was transferred to a sodium hydrosulfide solution for a second post-treatment: the temperature was 50°C, the time was 2 hours, it was washed to neutrality, it was placed in deionized water for immersion for 2 days, and it was dried to obtain a nanofiltration membrane.

[0053] Comparative Example 4: No secondary post-treatment was performed, and the rest was the same as Example 1; specifically, the following changes were made: Step 3: (1) sodium hypochlorite was dissolved in deionized water to obtain a sodium hypochlorite solution with a pH of 12.9; (2) the composite base membrane was first placed in the sodium hypochlorite solution for a post-treatment at a temperature of 30°C for 48 hours, washed until neutral, soaked in deionized water for 2 days, and dried to obtain a nanofiltration membrane.

[0054] Performance test: The nanofiltration membranes of Examples 1 to 3 and Comparative Examples 1 to 4 were subjected to performance tests. The retention rate and water flux were tested using a feed solution containing 1000 mg / L Na2SO4 and 1000 mg / L MgCl2. The test pressure was 0.5 MPa and the pre-pressing time was 30 minutes. The data obtained are shown below:

[0055] Conclusion: The data from Examples 1 to 3 show that the scheme effectively enhances ion retention by synergistically regulating the charge of the nanofiltration membrane through the ionic liquid and modified polyvinyl pyrrolidone, and effectively improves the surface roughness and groups through two surface treatments. The introduction of fiber additives effectively ensures water flux. The data from Comparative Examples 1 to 4 show that further modification of polyvinyl pyrrolidone can improve the retention rate; fiber additives can increase the retention rate of the nanofiltration membrane while ensuring water flux; and compared with sodium hydroxide treatment, sodium hypochlorite treatment can increase surface active sites and increase the retention rate. Without secondary treatment, the surface roughness decreases and the absence of thiol groups leads to a decrease in retention rate and flux.

[0056] Finally, it should be noted that the above is only a preferred embodiment of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a nanofiltration membrane for regulating the surface charge of the membrane, characterized in that: The following steps are involved: Step 1: (1) piperazine, fiber additive, ionic liquid, and polyvinyl pyrrolidone are sequentially added to deionized water to obtain an aqueous phase; (2) trimesoyl chloride is added to n-hexane as an oil phase; Step 2: Immerse the base film in the water phase for 1 to 3 minutes, remove the excess water phase; transfer to the oil phase for polymerization for 2 to 3 minutes, remove the excess solution; cure at 80°C to 85°C for 5 to 6 minutes, and immerse in deionized water for 1 to 2 days to obtain a composite base film; Step 3: The composite base membrane is first placed in a sodium hypochlorite solution for post-treatment and cleaning; transferred to a sodium hydrosulfide solution for secondary post-treatment, washed to neutrality, soaked in deionized water for 1 to 3 days, and dried to obtain a nanofiltration membrane.

2. The method for preparing a nanofiltration membrane for regulating membrane surface charge according to claim 1, characterized in that: In the aqueous phase, the concentration of piperazine is 0.85wt% to 0.9wt%, the concentration of the fiber additive is 0.15wt% to 0.2wt%, the concentration of the ionic liquid is 0.4wt% to 0.5wt%, and the concentration of polyvinyl pyrrolidone is 0.05wt% to 0.06wt%; in the oil phase, the concentration of trimesoyl chloride is 0.15 to 0.2wt%.

3. The method for preparing a nanofiltration membrane for regulating membrane surface charge according to claim 1, characterized in that: The ionic liquid includes one or two of 1-aminoethyl-3-methylimidazolium glycine salt and 1-aminoethyl-3-methylimidazolium alanine salt.

4. The method for preparing a nanofiltration membrane for regulating membrane surface charge according to claim 1, characterized in that: The preparation process of the fiber additive is as follows: (1) 1-allyl-4-(2-hydroxyethyl)-piperazine and epichlorohydrin are mixed, potassium carbonate is added and stirred evenly, and the mixture is stirred and reacted at 80° C. to 85° C. for 8 to 10 hours, and purified to obtain epoxy-modified allyl piperazine; (2) irradiating the polyethylene fiber with a cobalt source, placing it in a mixed solution of N,N-dimethylformamide containing epoxy-modified allyl piperazine, performing a grafting reaction at 50° C. to 70° C. for 1 to 3 hours under a nitrogen atmosphere, washing, and drying to obtain a grafted polyethylene fiber; (3) placing the grafted polyethylene fiber in a 0.2 mol / L to 0.3 mol / L hydrochloric acid dioxane solution, stirring at 30° C. to 35° C. for 30 to 60 minutes, washing, and freeze-drying to obtain a fiber additive.

5. The method for preparing a nanofiltration membrane for regulating membrane surface charge according to claim 4, characterized in that: In the raw materials of the epoxy-modified allyl piperazine, the molar ratio of 1-allyl-4-(2-hydroxyethyl)-piperazine, epichlorohydrin and potassium carbonate is 1:(1.2-1.5):

1.

6. The method for preparing a nanofiltration membrane for regulating membrane surface charge according to claim 4, characterized in that: The total dose of the cobalt source irradiation is 20 kGy to 30 kGy; the concentration of the epoxy-modified allyl piperazine in the N,N-dimethylformamide mixed solution is 5 wt % to 10 wt %.

7. The method for preparing a nanofiltration membrane for regulating membrane surface charge according to claim 1, characterized in that: In step 3, the sodium hypochlorite solution is a sodium hypochlorite solution with a pH of 12.5 to 13.2 obtained by dissolving sodium hypochlorite in deionized water; the sodium hydrosulfide solution is a saturated sodium hydrosulfide solution obtained by dissolving sodium hydrosulfide in ethanol; the temperature of the primary post-treatment is 25° C. to 35° C., and the time is 40 to 48 hours; the temperature of the secondary post-treatment is 45° C. to 50° C., and the time is 1.5 to 2.5 hours.

8. The method for preparing a nanofiltration membrane for regulating membrane surface charge according to claim 1, characterized in that: The polyvinyl pyrrolidone in the aqueous phase is modified in advance, and the modification process is as follows: (1) L-vinylglycine and 6-(diethylamino)pyridine-3-carboxaldehyde are added to ethanol in sequence, and reacted at 60° C. to 65° C. for 3 to 4 hours; an ethanol mixed solution containing a halide is added; the temperature is raised to 80 to 85° C., the mixture is reacted for 1 to 2 hours, and the mixture is purified to obtain a vinyl quaternary ammonium salt; (2) adding polyvinyl pyrrolidone to deionized water, heating to 50° C., adding vinyl quaternary ammonium salt and vinyl imidazole, stirring, and cooling to obtain a mixture; placing the mixture in a microwave reactor, setting the microwave power to 750 to 800 W, and reacting for 1 to 2 minutes; and purifying to obtain modified polyvinyl pyrrolidone.

9. The method for preparing a nanofiltration membrane for regulating membrane surface charge according to claim 8, characterized in that: The molar ratio of L-vinylglycine, 6-(diethylamino)pyridine-3-carboxaldehyde and alkyl halide is 1:1:(1-1.1); the mass ratio of polyvinylpyrrolidone, vinyl quaternary ammonium salt and vinylimidazole is 100:(6-10):(2-4).

10. The nanofiltration membrane prepared by the method for preparing a nanofiltration membrane for regulating membrane surface charge according to claim 1.

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