A method for controlling the surface charge of nanofiltration membranes using dipolar solvents

Abstract

This invention relates to the field of nanofiltration membrane separation technology, and in particular to a method for controlling the surface charge of nanofiltration membranes using a dipolar solvent. The method involves using an organic phase reaction solution and an aqueous phase reaction solution as raw materials to perform an interfacial polymerization reaction on an amination- or hydroxylation-modified porous support membrane to obtain a charged nanofiltration membrane. The charge properties of the polymer surface are controlled by adding or not adding a dipolar solvent to the aqueous phase reaction solution. This invention, employing the aforementioned method for controlling the surface charge of nanofiltration membranes using a dipolar solvent, solves the problem of the difficulty in effectively controlling the surface charge of nanofiltration membranes. It provides a simple and efficient way to prepare nanofiltration membranes with different charge types and charge densities, achieving efficient and precise separation of monovalent / multivalent mixed salts.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane separation technology, and in particular to a method for controlling the surface charge of a nanofiltration membrane using a dipolar solvent. Background Technology

[0002] Separation of monovalent / polyvalent mixed salts has significant application potential in high-value ion recovery, concentrated brine reuse, chlor-alkali brine denitrification, water softening, lithium-magnesium separation, and harmful ion removal. Concentrated water from seawater desalination plants, effluent from wastewater treatment plants, and saline wastewater from mining and processing industries primarily contain salts such as NaCl and Na₂SO₄, which have low industrial application value. Therefore, separation of monovalent / divalent salts is necessary to improve the purity of the product salts. Lithium and its compounds have wide applications in refrigerants, batteries, lubricants, nuclear fusion, rocket propellants, and organic synthesis. However, the low lithium concentration and excessively high magnesium-to-lithium ratio in salt lake brines significantly increase the technical difficulty of lithium extraction from salt lakes.

[0003] Nanofiltration membrane separation technology is an emerging method for separating monovalent / polyvalent inorganic salt solutions, with broad application potential in terms of economy and operability. The separation mechanism of nanofiltration membranes is mainly based on the Donnan effect and size sieving effect, achieving high retention of polyvalent ions and high permeation of monovalent ions. The types of monovalent / polyvalent mixed salts separated by nanofiltration membranes are related to the surface charge properties (charge type and charge density) of the nanofiltration membrane; that is, the regulation of the nanofiltration membrane surface charge is crucial for achieving accurate and efficient separation of monovalent / polyvalent inorganic salts. Therefore, researchers are attempting to expand its applications by regulating the surface charge properties of nanofiltration membranes.

[0004] The regulation of the surface charge type and charge density of polyamide composite nanofiltration membranes mainly falls into two categories: (1) introducing small molecules or polymers with specific groups to participate in interfacial polymerization reactions; and (2) introducing small molecules or polymers with specific groups to undergo interfacial polymerization followed by graft modification. Positively charged nanofiltration membranes are generally prepared by interfacial polymerization of amine-rich polyvinylamine monomers and trimesoyl chloride, and can be used for the efficient removal and separation of divalent cation salts, such as calcium salts and magnesium salts. Negatively charged nanofiltration membranes are generally prepared by interfacial polymerization of piperazine and trimesoyl chloride, and can be used for the efficient removal of divalent anionic salts, such as sulfates, carbonates, and phosphates.

[0005] However, the two methods for controlling the surface charge of nanofiltration membranes mentioned above typically alter the degree of crosslinking, surface properties, and permeation selectivity of the polyamide structure, and add additional operational steps during membrane production. Therefore, there is an urgent need to develop a simple and efficient method for controlling the surface charge of nanofiltration membranes to prepare nanofiltration membranes with different charge properties and charge densities, thereby achieving efficient and precise separation of monovalent / polyvalent mixed salts. Summary of the Invention

[0006] The purpose of this invention is to provide a method for controlling the surface charge of nanofiltration membranes using dipolar solvents, which solves the problem of the difficulty in effectively controlling the surface charge of nanofiltration membranes. This method allows for the simple and efficient preparation of nanofiltration membranes with different charge types and charge densities, enabling efficient and precise separation of monovalent / multivalent mixed salts.

[0007] To achieve the above objectives, the present invention provides a method for controlling the surface charge of a nanofiltration membrane using a dipolar solvent. The method involves using an organic phase reaction solution and an aqueous phase reaction solution as raw materials to perform an interfacial polymerization reaction on an amination- or hydroxylation-modified porous support membrane to obtain a charged nanofiltration membrane. The charge properties of the polymer surface are controlled by adding or not adding a dipolar solvent to the aqueous phase reaction solution.

[0008] Specifically, the steps include:

[0009] S1. The porous support membrane is pre-wetted in the wetting solution for 0.5 to 12 hours, and the surface of the pre-wetted porous support membrane is contacted with a modifier for amination or hydroxylation modification for 0.5 to 12 hours.

[0010] S2. Prepare an organic phase reaction solution containing 0.1-4% organic phase monomer, 0.1-4% polydimethylsiloxane crosslinking agent and the balance organic solvent by mass fraction;

[0011] S3. Prepare an aqueous reaction solution containing 0.1-4% by mass of aqueous monomer, 0.1-2% by mass of acid acceptor, 0.1-2% by mass of surfactant and the balance water. When adding a dipolar solvent to the aqueous reaction solution, the mass fraction of the dipolar solvent is 1-12%.

[0012] S4. The amination- or hydroxylation-modified porous support membrane is brought into contact with the organic phase reaction solution, and adsorption occurs;

[0013] S5. After discarding the excess organic phase reaction liquid and removing the residual droplets on the surface, the membrane is brought into contact with the aqueous phase reaction liquid to carry out the interfacial polymerization reaction to obtain the nascent nanofiltration membrane.

[0014] S6. The nascent nanofiltration membrane obtained in S5 is placed in a drying oven for heat treatment to prepare a charged nanofiltration membrane for the separation of monovalent / polyvalent salts.

[0015] Preferably, the wetting solution in S1 includes at least one of methanol, ethanol, ethylene glycol, glycerol, isopropanol, and acetone; the solution used for amination modification is an acidic aqueous solution of aniline as the amination modifier; and the solution used for hydroxylation modification is an alkaline aqueous solution of dopamine-Tris.

[0016] The hydroxylation or amylation of the supporting membrane provides binding sites for the subsequent attachment of the separation layer.

[0017] Preferably, the porous support membrane is one of polyimide, polytetrafluoroethylene, polyacrylonitrile, polypropylene, polyethylene, polysulfone, polyethersulfone, polyvinylidene fluoride, or alumina porous membrane.

[0018] Preferably, the organic phase monomer in S2 is one or more polyacrylamide chlorides or polyaldehydes.

[0019] Preferably, the organic phase monomer in S2 is one or more of the following: pyromellitic acid chloride, adipic acid chloride, glutaryl chloride, sebacic acid chloride, pyrophosphoryl chloride, 1,3-benzene disulfonyl chloride, 1,3,5-trialdehyde phloroglucinol, pyromellitic acid, terephthalaldehyde, 1,3,5-trialdehyde benzene, 2,4,6-trihydroxybenzene-1,3,5-tricarboxaldehyde, and 2-hydroxy-1,3,5-benzene tricarboxaldehyde.

[0020] Preferably, the polydimethylsiloxane crosslinking agent in S2 includes one or more of hydroxyl-terminated polydimethylsiloxane, hydroxyalkyl-terminated polydimethylsiloxane, methacryloyloxypropyl-terminated polydimethylsiloxane, aminopropyl-terminated polydimethylsiloxane, and cyclic polydimethylsiloxane.

[0021] Preferably, the organic solvent in S2 includes one or more of n-hexane, n-heptane, acetonitrile, ethyl acetate, toluene, and mesitylene.

[0022] Preferably, the dipolar solvent in S3 is selected from one or more of dimethylformamide, dimethyl sulfoxide, acetone, acetonitrile, dimethylacetamide, hexamethylphosphoramide, methyl ethyl ketone, and 1,3-dimethyl-2-imidazolinone.

[0023] Preferably, the aqueous monomer in S3 is a small molecule or polymer of a polyamine.

[0024] Preferably, the small molecules or polymers of the polyamine are one or more of polyethyleneimine, polyetheramine, piperazine, m-phenylenediamine, and PAMAM dendrimers with a molecular weight of 600-100,000 Da.

[0025] Preferably, the surfactant in S3 is one or more of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, Tween 20, and hexadecyltrimethylammonium bromide.

[0026] Preferably, the contact time in S4 is 0.5 to 30 minutes, and the contact temperature is 10 to 60 degrees Celsius.

[0027] Preferably, the interfacial polymerization reaction time in S5 is 0.5 to 30 minutes, and the reaction temperature is 10 to 60 degrees Celsius.

[0028] Preferably, the heat treatment time in S6 is 5 to 60 minutes, and the heat treatment temperature is 50 to 100 degrees Celsius.

[0029] The mechanism of this invention: The type and density of surface charge on a nanofiltration membrane can be achieved by adding a certain amount of dipolar solvent to the aqueous reaction solution. Specifically, during interfacial polymerization, the dipolar solvent undergoes dipolar-dipole interactions with the carbonyl oxygen of polyacrylamide chlorides or polyaldehydes, generating a solvation layer around the acrylamide or aldehyde groups, thus weakening the reactivity of the acrylamide or aldehyde groups with the amine groups. After the interfacial polymerization reaction is completed, the unstable dipolar-dipole interactions terminate with the removal of the aqueous reaction solution. The solvated acrylamide or aldehyde groups are exposed to the aqueous environment. Since both acrylamide and aldehyde groups are electron-withdrawing groups, the exposed acrylamide or aldehyde groups change the surface charge of the nanofiltration membrane from positive to negative, thereby achieving the purpose of controlling the type and density of surface charge on the nanofiltration membrane.

[0030] For example, when polyvinylamine is used as the aqueous monomer and trimesoyl chloride as the organic monomer, the polyamide composite membrane prepared by interfacial polymerization is a positively charged nanofiltration membrane, which can be applied to the efficient separation of lithium salts and magnesium salts. When a certain amount of dipolar solvent (dimethyl sulfoxide) is added to the aqueous reaction solution, the polyamide composite membrane prepared by interfacial polymerization is a negatively charged nanofiltration membrane, which can be applied to the efficient separation of chloride salts and sulfates.

[0031] The beneficial effects of this invention are:

[0032] (1) This invention achieves the adjustment of the surface charge density of charged nanofiltration membrane by controlling the ratio of aqueous phase monomer to organic phase monomer, and achieves the change of the type of charge or the regulation of charge density on the surface of nanofiltration membrane by controlling the selection and addition of dipole solvent.

[0033] (2) The present invention provides a method for controlling the surface charge of nanofiltration membranes with dipolar solvents. The process is simple, the preparation conditions are mild, the application range is wide, it is easy to scale up and promote, and it is easy to realize industrial production.

[0034] (3) The present invention can effectively adjust the surface charge properties and charge density of charged nanofiltration membranes while keeping the chemical structure and cross-linking degree of nanofiltration membranes basically unchanged.

[0035] (4) The charged nanofiltration membrane prepared by the present invention can be applied to the efficient separation of mono / polyvalent cation salts and mono / polyvalent anion salts, such as the separation of lithium and magnesium in lithium extraction from salt lakes and the separation of chloride / sulfate salts.

[0036] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0037] Figure 1 This is a scanning electron microscope image of the surface of the negatively charged nanofiltration membrane prepared by adding a dipolar solvent in Example 1 of the present invention.

[0038] Figure 2This is a scanning electron microscope image of the surface of the positively charged nanofiltration membrane prepared without the addition of a dipolar solvent in Example 2 of the present invention. Detailed Implementation

[0039] The present invention will be further described below with reference to embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by one of ordinary skill in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.

[0040] The surface potential of the nanofiltration membrane was measured using a solid surface zeta potential meter. A 1 mM KCl electrolyte solution was flowed over the membrane surface. Free charged particles in the surface double electron layer moved along the direction of solution flow, generating a flow potential difference through directional charge movement, which characterizes the membrane surface charge. The zeta potential of the membrane was measured at different pH values.

[0041] The separation and selectivity parameters of a membrane mainly consist of permeate flux (P), rejection ratio (R), and selectivity (α). Permeate flux (P) refers to the volume of solution that permeates per unit time, unit pressure, and unit membrane area, and is calculated using the following formula:

[0042]

[0043] Wherein, P (unit: L·m) -2 ·h -1 V is the solution permeation flux, V (unit: L) is the volume of permeate collected within a certain time period, and A (unit: m³) is the volume of permeate collected within a certain time period. 2 ) is the effective filtration area of ​​the membrane, Δt (unit: h) is the permeation time, and ΔP (unit: bar) is the transmembrane pressure.

[0044] Retention rate (R) refers to the degree to which the separation membrane retains inorganic salts in the solution. It is obtained from the salt concentrations of the feed solution and the permeate, and the calculation formula is as follows:

[0045]

[0046] Among them, C P (Unit: g·L) -1 C is the concentration of the permeate. f (Unit: g·L) -1 () represents the concentration of the raw material solution.

[0047] Separation factor (S) of monovalent / divalent salts 1,2 () refers to the separation membrane's ability to separate monovalent and divalent salts, calculated using the following formula:

[0048]

[0049] Among them, C 1,P (Unit: g·L) -1 ) and C 2,P (Unit: g·L) -1 C represents the concentrations of monovalent and divalent ions in the permeate, respectively. 1,f (Unit: g·L) -1 ) and C 2,f (Unit: g·L) -1 The concentrations of monovalent and divalent ions in the feed solution are respectively.

[0050] The salt concentration in the retention rate test was 0.1 g·L⁻¹. -1 The concentration of the mixed salt in the separation factor test was 0.2 g·L⁻¹. -1 The concentrations of monovalent or divalent ions were detected using inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0051] Example 1

[0052] After pre-immersing the polytetrafluoroethylene (PTFE) support membrane in ethanol for 3 hours, it was placed in a 2 g·L⁻¹ container. -1 The polytetrafluoroethylene (PTFE) membrane was hydroxylated in an alkaline aqueous solution (pH 10.0) for 8 hours, rinsed with deionized water, and dried. A heptane solution containing 2% trimesoyl chloride and 2% polydimethylsiloxane was prepared as the organic phase reaction solution. An aqueous solution containing 6% dimethyl sulfoxide, 0.1% sodium dodecyl sulfate, 0.1% sodium carbonate, and 1% polyethyleneimine (600 Da) was prepared as the aqueous phase reaction solution. The surface of the hydroxylated PTFE membrane was contacted with the organic phase reaction solution for 10 minutes. After removing excess organic phase reaction solution, it was then contacted with the aqueous phase reaction solution for 3 minutes, washed, and heat-treated in a drying oven for 10 minutes. A charged nanofiltration membrane for the separation of monovalent / polyvalent salts was obtained.

[0053] The prepared nanofiltration membrane was stored in deionized water for further testing of its separation performance for mixed salts.

[0054] Figure 1 This is a scanning electron microscope (SEM) image of the surface of the negatively charged nanofiltration membrane prepared by adding a dipolar solvent in Example 1 of this invention. Testing showed that the prepared nanofiltration membrane surface is negatively charged, with a Zeta potential of -77.0 mV at pH 6.8 and a water permeation flux of 12 L·m⁻¹. -2 ·h -1 The magnesium chloride rejection rate was 55%, the sodium sulfate rejection rate was 94%, and the sodium chloride / sodium sulfate separation factor was 15.

[0055] Example 2

[0056] The difference from Example 1 is that the aqueous reaction solution does not contain the dipolar solvent dimethyl sulfoxide.

[0057] Figure 2 This is a scanning electron microscope (SEM) image of the positively charged nanofiltration membrane prepared without the addition of a dipolar solvent in Example 2 of this invention. Testing showed that the prepared nanofiltration membrane surface is positively charged, with a Zeta potential of +7.8 mV at pH 6.8 and a water permeation flux of 10 L·m⁻¹. -2 ·h -1 The magnesium chloride rejection rate was 92%, the sodium sulfate rejection rate was 65%, and the separation factor for lithium chloride / magnesium chloride was 12.

[0058] Example 3

[0059] The polyimide-based film was pre-wetted in acetone for 5 hours. Afterwards, it was placed in a 3 g·L⁻¹ container. -1 The polyimide membrane was hydroxylated in an alkaline aqueous solution (pH 11.0) for 12 hours, rinsed with deionized water, and dried. A heptane solution containing 3% trimesoyl chloride and 2% methacryloyloxypropyl dimethylsiloxane was prepared as the organic phase reaction solution. An aqueous solution containing 6% hexamethylphosphoramide, 0.1% sodium dodecyl sulfate, 0.1% sodium carbonate, and 1% polyethyleneimine (molecular weight 1800 Da) was prepared as the aqueous phase reaction solution. The surface of the hydroxylated polyimide membrane was contacted with the organic phase reaction solution for 5 minutes. After removing excess organic phase reaction solution, it was then contacted with the aqueous phase reaction solution for 5 minutes. After washing, it was heat-treated in a drying oven for 20 minutes to obtain a charged nanofiltration membrane for the separation of monovalent / polyvalent salts.

[0060] The prepared nanofiltration membrane was stored in deionized water for further testing of its separation performance for mixed salts.

[0061] Testing revealed that the prepared nanofiltration membrane had a negatively charged surface, a Zeta potential of -70.0 mV at pH 6.8, and a water permeation flux of 11 L·m⁻¹. -2 ·h -1 The magnesium chloride rejection rate was 52%, the sodium sulfate rejection rate was 95%, and the sodium chloride / sodium sulfate separation factor was 14.

[0062] Example 4

[0063] The difference from Example 3 is that the aqueous reaction solution does not contain the dipolar solvent hexamethylphosphoramide.

[0064] Testing revealed that the prepared nanofiltration membrane had a positively charged surface, a Zeta potential of +8.0 mV at pH 6.8, and a water permeation flux of 9 L·m⁻². -2 ·h -1 The magnesium chloride rejection rate was 93%, the sodium sulfate rejection rate was 62%, and the separation factor for lithium chloride / magnesium chloride was 12.

[0065] Example 5

[0066] In the experiment, the polytetrafluoroethylene film was pre-wetted in ethanol for 3 hours. Afterwards, it was placed in a 1 g·L⁻¹ container. -1 Amination modification of aniline was carried out in an acidic aqueous solution (pH 3.0) for 8 hours, followed by rinsing with deionized water and drying. A hexane solution containing 2% trimesoyl chloride and 2% hydroxyalkyl-terminated polydimethylsiloxane was prepared as the organic phase reaction solution. An aqueous solution containing 6% dimethylformamide, 0.1% sodium dodecyl sulfate, 0.1% sodium hydroxide, and 1% polyetheramine was prepared as the aqueous phase reaction solution. The surface of the amination-modified polytetrafluoroethylene membrane was contacted with the organic phase reaction solution for 5 minutes. After removing excess organic phase reaction solution, it was then contacted with the aqueous phase reaction solution for 8 minutes. After washing, it was heat-treated in a drying oven for 10 minutes to obtain a charged nanofiltration membrane for the separation of monovalent / polyvalent salts.

[0067] The prepared nanofiltration membrane was stored in deionized water for further testing of its separation performance for mixed salts.

[0068] Testing revealed that the prepared nanofiltration membrane had a negatively charged surface, a Zeta potential of -65.0 mV at pH 6.8, and a water permeation flux of 13 L·m⁻¹. -2 ·h -1 The magnesium chloride rejection rate was 48%, the sodium sulfate rejection rate was 92%, and the sodium chloride / sodium sulfate separation factor was 10.

[0069] Example 6

[0070] The difference from Example 5 is that the aqueous reaction solution does not contain the dipolar solvent dimethylformamide.

[0071] Testing revealed that the prepared nanofiltration membrane had a positively charged surface, a Zeta potential of +6.0 mV at pH 6.8, and a water permeation flux of 10 L·m⁻¹. -2 ·h -1 The magnesium chloride rejection rate was 92%, the sodium sulfate rejection rate was 50%, and the separation factor for lithium chloride / magnesium chloride was 11.

[0072] In summary, the present invention provides a method to successfully control the type and density of surface charges on nanofiltration membranes by adding a dipolar solvent to the aqueous reaction solution, thereby obtaining nanofiltration membranes with specific charge properties and achieving efficient separation of different types of monovalent / polyvalent mixed salts.

[0073] The negatively charged nanofiltration membrane obtained in Example 1 was used to design a first-stage nanofiltration process for separating NaCl / Na2SO4 mixed salt. The results showed that after separation of 5 L of NaCl / Na2SO4 mixed salt by nanofiltration, the recovery rate reached 60%, divalent ions in the permeate were concentrated by approximately 2.5 times, and the ion selectivity of the nanofiltration membrane remained relatively unchanged. This indicates that the prepared charged nanofiltration membrane has good long-term stable operating performance.

[0074] Therefore, the present invention provides a raw material, method, and application for controlling the surface charge of a nanofiltration membrane using a dipolar solvent with the above-mentioned structure. The method is simple, the conditions are mild, the applicability is wide, and it is easy to scale up and realize industrial production. The charged nanofiltration membrane after charge control has a strong separation layer, no loss of permeation flux and separation performance, and good long-term operational stability.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for controlling the surface charge of a nanofiltration membrane using a dipolar solvent, characterized in that: Charged nanofiltration membranes were prepared by interfacial polymerization of amination- or hydroxylation-modified porous support membranes using organic and aqueous reaction solutions as raw materials. The charge properties of the polymer surface were controlled by adding or not adding a dipolar solvent to the aqueous reaction solution. Specifically, the steps include: S1. Pre-wet the porous support membrane in the wetting solution for 0.5 to 12 hours, and then contact the surface of the pre-wetted porous support membrane with the modifier for amination or hydroxylation modification for 0.5 to 12 hours. S2. Prepare an organic phase reaction solution containing 0.1-4% organic phase monomer, 0.1-4% polydimethylsiloxane crosslinking agent and the balance organic solvent by mass fraction; The organic phase monomer is one or more of the following: pyromellitic chlorohydrin, adipyl chloride, glutaryl chloride, sebacyl chloride, pyrophosphoryl chloride, 1,3-benzenedisulfonyl chloride, 1,3,5-trialdehyde phloroglucinol, pyromellitic chlorohydrin, terephthalaldehyde, and 2-hydroxy-1,3,5-benzenedialdehyde. S3. Prepare an aqueous reaction solution containing 0.1-4% by mass of aqueous monomer, 0.1-2% by mass of acid acceptor, 0.1-2% by mass of surfactant and the balance water. When adding a dipolar solvent to the aqueous reaction solution, the mass fraction of the dipolar solvent is 1-12%. The dipolar solvent is selected from one of dimethylformamide, dimethyl sulfoxide, and hexamethylphosphoramide; The aqueous phase monomer is one or more of polyethyleneimine, polyetheramine, piperazine, m-phenylenediamine, and PAMAM dendrimers with a monomer content of 600-100000 Da; S4. The amination- or hydroxylation-modified porous support membrane is brought into contact with the organic phase reaction solution, and adsorption occurs; S5. After discarding the excess organic phase reaction liquid and removing the residual droplets on the surface, the membrane is brought into contact with the aqueous phase reaction liquid to carry out the interfacial polymerization reaction to obtain the nascent nanofiltration membrane. S6. The nascent nanofiltration membrane obtained in S5 is placed in a drying oven for heat treatment to prepare a charged nanofiltration membrane for the separation of monovalent / polyvalent salts.

2. The method for controlling the surface charge of a nanofiltration membrane using a dipolar solvent according to claim 1, characterized in that: The wetting solution in S1 includes at least one of methanol, ethanol, ethylene glycol, glycerol, isopropanol, and acetone; the modifier includes an alkaline aqueous solution of dopamine-Tris or an acidic aqueous solution of aniline.

3. The method for controlling the surface charge of a nanofiltration membrane using a dipolar solvent according to claim 1, characterized in that: The polydimethylsiloxane crosslinking agent in S2 includes one or more of hydroxyl-terminated polydimethylsiloxane, hydroxyalkyl-terminated polydimethylsiloxane, methacryloyloxypropyl-terminated polydimethylsiloxane, aminopropyl-terminated polydimethylsiloxane, and cyclic polydimethylsiloxane.

4. The method for controlling the surface charge of a nanofiltration membrane using a dipolar solvent according to claim 1, characterized in that: The organic solvent in S2 includes one or more of the following: n-hexane, n-heptane, acetonitrile, ethyl acetate, toluene, and mesitylene.

5. A method for controlling the surface charge of a nanofiltration membrane using a dipolar solvent according to claim 1, characterized in that: The surfactant in S3 is one or more of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, Tween 20, and hexadecyltrimethylammonium bromide.

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