A method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing

Abstract

This invention discloses a method for preparing a positively charged nanofiltration membrane using 3D electrostatic spray printing. The method includes the following steps: S1, uniformly mixing a polymeric membrane matrix material, a pore-forming agent, a membrane modifier, and an organic solvent, and preparing a support membrane for the nanofiltration membrane via a phase inversion method; S2, using piperazine and trimesoyl chloride as the aqueous and organic phase monomers, respectively, spraying the monomer solution onto the polymeric support membrane using 3D electrostatic spray printing to prepare a piperazine-trisoyl chloride intermediate layer; S3, using polyethyleneimine and trimesoyl chloride as the aqueous and organic phase monomers, respectively, spraying the monomer solution onto the intermediate layer using 3D electrostatic spray printing to prepare a polyethyleneimine-trisoyl chloride positively charged outer layer. After heat treatment, the positively charged nanofiltration membrane is obtained. This invention provides a simple, precise, and controllable method for preparing a positively charged nanofiltration membrane, which exhibits good selective separation ability for magnesium ions and lithium ions.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane technology, and more specifically to a method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing. Background Technology

[0002] Nanofiltration (NF) membranes are semi-permeable membranes capable of separating different substances at the molecular level. They typically consist of a porous support layer and a high-density active layer, the latter responsible for selective separation. The pore size of nanofiltration membranes ranges from approximately 0.1 to 10 nm, enabling them to effectively remove organic matter, divalent ions, and all microorganisms from water. The development of nanofiltration membrane technology began in the 1970s, initially for water treatment and seawater desalination. Over time, this technology has expanded to many other fields, including food processing, pharmaceutical manufacturing, and wastewater treatment. The unique feature of nanofiltration membranes lies in their separation mechanism; they rely not only on pore size but also on the surface charge of the membrane material and the chemical properties of the solute. Nanofiltration membranes typically have a charge load on their surface. Under the influence of charge repulsion, nanofiltration membranes have a higher separation capacity for solutes with the same surface charge as the membrane, while their separation capacity for solutes with opposite charges is weaker. Compared to reverse osmosis membranes, which only allow water molecules to pass through while retaining all solutes, the selective separation of solutes is the greatest advantage of nanofiltration membranes.

[0003] Currently, interfacial polymerization has become the main method for laboratory research and commercial production of nanofiltration membranes due to its advantages of simple process and ease of continuous production. However, problems such as uncontrollable reactions and self-termination in interfacial polymerization are difficult to solve. These problems are generated by the interfacial polymerization itself, and new nanofiltration membrane fabrication methods are needed to completely solve them. This invention proposes a method for preparing positively charged nanofiltration membranes by 3D electrostatic spraying. Using electrostatic spraying technology, aqueous and organic monomer solutions are dispersed into micron or nanometer-sized droplets, which are deposited onto the surface of a supporting membrane under the action of an electric field and undergo polymerization. Each time the two-phase solution sweeps across the surface of the supporting membrane, a loose active layer is generated. Then, the spraying is repeated multiple times by 3D printing. In this process, the active layer gradually becomes denser until the designed retention capacity is achieved. Summary of the Invention

[0004] The purpose of this invention is to propose a method for preparing a positively charged nanofiltration membrane using 3D electrostatic spray printing. Utilizing electrostatic spraying technology, aqueous and organic monomer solutions are dispersed into micron- or nanometer-sized droplets, which are then deposited onto the surface of a supporting membrane under an electric field, where a polymerization reaction occurs. Each time the two-phase solution sweeps across the supporting membrane surface, a loose active layer is formed. This process is repeated multiple times using 3D printing. During this process, the active layer gradually densifies until the designed retention capacity is achieved. This invention uses 3D electrostatic spray printing to prepare a piperazine-pyromellitic acid chloride intermediate layer, followed by the preparation of a positively charged polyethyleneimine-pyromellitic acid chloride outer layer on the intermediate layer. After post-processing, a positively charged nanofiltration membrane is obtained.

[0005] To achieve the above objectives, the present invention provides a method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing, comprising the following steps: S1. Preparation of the supporting membrane: The supporting membrane is prepared by phase inversion of the casting solution, wherein the casting solution contains a polymeric membrane matrix material, a pore-forming agent, a membrane modifier, and an organic solvent. The prepared casting solution is stirred evenly at a constant temperature and degassed, then spread evenly on a glass plate, left to stand in air for a certain period of time, and then immersed in distilled water to obtain the supporting membrane for the nanofiltration membrane. Preferably, the organic solvent in step S1 is one of dimethyl sulfoxide, acetone, cyclohexane, chloroform, N,N-dimethylformamide, N,N-dimethylacetamide, or N-methylpyrrolidone.

[0006] Preferably, the polymeric membrane matrix material in step S1 is one or more of polyvinylidene fluoride, polysulfone, polyethersulfone, polyamide, cellulose acetate, polypropylene, and polyvinyl chloride. Preferably, based on the total mass of the casting solution, the mass percentage of the polymer is 18wt% to 22wt%.

[0007] Preferably, in step S1, one or more of the membrane modifier, hydrophilic agent, or surfactant are used.

[0008] S2. Preparation of piperazine-pyromellitic tricarboxylic acid intermediate layer: piperazine is used as the aqueous monomer and pyromellitic tricarboxylic acid is used as the organic monomer. The monomer solution in the atomized state is sprayed onto the above support film by 3D electrostatic spraying printing. The monomer undergoes a polymerization reaction to generate a loose piperazine-pyromellitic tricarboxylic acid loose intermediate layer. Preferably, the aqueous solvent in steps S2 and S3 is one or more of water, ethanol, and dimethyl sulfoxide.

[0009] Preferably, the ambient temperature required for the preparation of the piperazine-trimethylammonium chloride intermediate layer in step S2 is 30~40℃, the ambient humidity is 40~70%RH, the voltage is 10~15kV, the distance between the needle and the support film is 2~3cm, the needle moving speed is 40~70mm / min, the roller rotation speed is 40~70rpm, and the monomer solution injection speed is 1~3mL / h.

[0010] Preferably, in step S2, the mass ratio of the aqueous monomer to the solution is 1:99 to 5:95.

[0011] Preferably, in step S2, the number of monomer solution sprayed onto the support membrane is 5 to 40 layers.

[0012] S3. Preparation of the positively charged outer layer of polyethyleneimine-pyromellitic tricarboxylate chloride: Using polyethyleneimine as the aqueous monomer and pyromellitic tricarboxylate chloride as the organic monomer, the monomer solution in an atomized state was sprayed onto the above intermediate layer using a 3D electrostatic spraying method. The monomers underwent a polymerization reaction to generate a positively charged outer layer of polyethyleneimine-pyromellitic tricarboxylate chloride. A positively charged nanofiltration membrane was obtained after heat treatment.

[0013] Preferably, the ambient temperature required for the preparation of the positively charged outer layer of polyethyleneimine-trimethylammonium chloride in step S3 is 30~40℃, the ambient humidity is 40~70%RH, the voltage is 10~15kV, the distance between the needle and the support film is 2~3cm, the needle moving speed is 40~70mm / min, the roller rotation speed is 40~70rpm, and the monomer solution injection speed is 1~3mL / h.

[0014] Preferably, the heat treatment method in step S3 is heating at 60~80℃ for 1~10 minutes.

[0015] Preferably, in step S3, the mass ratio of aqueous monomer to solvent is 1:99 to 5:95.

[0016] Preferably, in step S3, the number of monomer solution sprayed onto the support membrane is 5 to 40 layers.

[0017] Preferably, the mass ratio of organic phase monomer to solvent in steps S2 and S3 is 1:999 to 5:995.

[0018] This invention uses a liquid-solid phase conversion method to prepare a support membrane for nanofiltration membranes. The membrane formation mechanism is as follows: a casting solution is prepared by mixing a polymer membrane matrix material, a pore-forming agent, a membrane modifier, and an organic solvent, and the support membrane for nanofiltration membranes is prepared by the liquid-solid phase conversion method.

[0019] This invention employs 3D printing to prepare nanofiltration membranes. The membrane formation mechanism is as follows: using electrostatic spraying technology, the two-phase monomer solution required to form the nanofiltration membrane separation layer is dispersed into microdroplets and sprayed onto a polymer support membrane. Organic reactants with bifunctional or trifunctional groups polymerize at the interface between two immiscible microdroplets to form a membrane. The monomer solution is swept across the support membrane surface once to form a polyamide interlayer (PA). Compared with the prior art, the present invention has the following beneficial effects: (1) Positively charged nanofiltration membranes were prepared by 3D electrostatic spray printing. By dispersing the monomer solution into microdroplets, the nanofiltration membrane generation process was made more refined. The prepared positively charged nanofiltration membranes had a smooth surface, good magnesium-lithium separation performance, and resistance to organic pollution. At the same time, they also had significant long-term separation stability. (2) When preparing positively charged nanofiltration membranes by 3D electrostatic spray printing, the amount of spray is very small and the monomer tends to form a dense separation layer, which significantly reduces the amount of monomer and solvent used, which is beneficial to reducing environmental pollution.

[0020] (3) When preparing positively charged nanofiltration membranes by 3D electrostatic spray printing, the monomer solution is dispersed into microdroplets and the polyamide network is formed in a confined space. Therefore, the prepared nanofiltration membrane has an ultrathin separation layer, which is beneficial to improving the permeation performance of the nanofiltration membrane.

[0021] (4) The 3D electrostatic spray printing method is flexible and can divide the nanofiltration membrane separation layer into an intermediate layer and a positively charged outer layer, which can be controlled separately. The prepared nanofiltration membrane has good magnesium-lithium separation performance.

[0022] 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

[0023] Figure 1 Fabrication process of 3D electrostatic spray printing of positively charged nanofiltration membranes Figure 2 Infrared spectrum of piperazine-pyromellitic chloride interlayer Figure 3 X-ray photoelectron spectroscopy of the piperazine-pyromellitic tricarboxylic acid interlayer Figure 4 X-ray photoelectron spectroscopy (C1s fine spectrum) of the piperazine-pyromellitic tricarboxylic acid interlayer Figure 5 Infrared spectrum of 3D electrostatic spray printed positively charged nanofiltration membrane Figure 6 X-ray photoelectron spectroscopy of 3D electrostatic spray printed positively charged nanofiltration membrane Figure 7 X-ray photoelectron spectroscopy (C1s fine spectrum) of 3D electrostatic spray-printed positively charged nanofiltration membrane Figure 8 3D electrostatic spray printing of positively charged nanofiltration membranes for the retention rate of PEG with different molecular weights Figure 9 Magnesium-Lithium Separation Coefficient of Positively Charged Nanofiltration Membranes Printed by 3D Electrostatic Spray Printing Figure 10 Pure water permeation flux of 3D electrostatic spray-printed positively charged nanofiltration membrane Figure 11 Surface Zeta potential of 3D electrostatic spray-printed positively charged nanofiltration membranes at different pH values Figure 12 3D electrostatic spray printing of positively charged nanofiltration membranes at different pH values: magnesium-lithium separation coefficient Figure 13 Salt rejection rate of intermediate layer in 3D electrostatic spray printing Figure 14 Salt rejection rate of 3D electrostatic spray-printed positively charged nanofiltration membrane Figure 15 3D electrostatic spray printing of positively charged nanofiltration membranes for different Mg 2+ / Li + Separation coefficient of mass ratio Figure 16 Heavy metal removal capability of 3D electrostatic spray printing of positively charged nanofiltration membranes Figure 17 Long-term retention capacity of MgCl2 and LiCl in 3D electrostatic spray-printed positively charged nanofiltration membranes Figure 18 Long-term magnesium-lithium separation performance of 3D electrostatic spray-printed positively charged nanofiltration membrane Figure 19 Results of 3D electrostatic spray printing of positively charged nanofiltration membranes for cyclic antifouling experiments Figure 20 Antifouling parameters of 3D electrostatic spray printed positively charged nanofiltration membranes Detailed Implementation

[0024] The following detailed description of embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0025] Example 1 This invention provides a method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing, comprising the following steps: S1. Preparation of the support membrane for 3D electrostatic spray printing of a positively charged nanofiltration membrane: 18g of polyethersulfone, 3g of polyvinylpyrrolidone, and 3g of polyethylene glycol were added to 76g of N,N-dimethylacetamide and stirred evenly at 60℃ to prepare a casting solution. After thorough degassing of the casting solution, a 300μm thick doctor blade was used to spread the casting solution evenly on a glass plate. After standing for 30s, the glass plate was immersed in distilled water for 12h to complete the phase inversion process. After washing with distilled water, the support membrane for the 3D electrostatic spray printed positively charged nanofiltration membrane was obtained.

[0026] Preparation of the piperazine-pyromellitic tricarboxylic acid (PPT) interlayer for S2 and 3D electrostatic spray printing: 3g piperazine was added to 97g deionized water, and 0.2g PPT was added to 99.8g n-hexane. After complete dissolution, the solution was transferred to the injector, and the polymer support membrane prepared in S1 was fixed on the roller. Electrostatic spraying was performed at 14kV. The monomer solution injection rate was set to 2mL / h, the injection platform translation speed to 60mm / min, the injection platform stroke to 12cm, the roller rotation speed to 60rpm, the temperature to 30℃, and the humidity to 50%RH. Ten layers were sprayed to obtain a positively charged nanofiltration membrane piperazine-pyromellitic tricarboxylic acid (PPT) interlayer.

[0027] Preparation of the positively charged outer layer of polyethyleneimine-pyromellitic trimethylol chloride (PPTC) in S3 electrostatic spray printing: 1 g of polyethyleneimine was added to 99 g of deionized water and completely dissolved before being transferred to a sample injector. The piperazine aqueous solution in S2 was replaced with a polyethyleneimine aqueous solution, and spraying was continued for 10 more layers to obtain the positively charged outer layer of polyethyleneimine-PPTC in the nanofiltration membrane. The prepared nanofiltration membrane was placed in an oven and treated at 80°C for 5 min, then immersed in distilled water to prepare the positively charged nanofiltration membrane. The preparation process is as follows: Figure 1 As shown.

[0028] like Figure 2 As shown, the 3D electrostatic spray printing of the piperazine-pyromellitic trimethylol chloride interlayer at 1656 cm⁻¹ -1 and 1368cm -1 The appearance of an infrared absorption peak at this location corresponds to C=O and CN in the polyamide, indicating that piperazine reacted with pyromellitic trimethylol chloride. According to... Figure 3 and Figure 4 The results showed that the relative content of amide groups (-CN=O-) in the piperazine-pyromellitic chloride interlayer was 13.61%. Figure 5 In the middle, the positively charged nanofiltration membrane with the positively charged outer layer of sprayed polyethyleneimine-pyromellitic acid chloride was at 1656 cm⁻¹. -1 and 1368cm -1 An infrared absorption peak of the amide group appears at [location missing]. Meanwhile, [the text abruptly ends here]. Figure 6 and Figure 7The contents of amide groups and amino (-CN) were 13.47% and 40.52%, respectively, indicating that polyethyleneimine and trimesoyl chloride successfully reacted.

[0029] like Figure 8 As shown, polyethylene glycol (PEG) is used. M n =200Da, 300Da, 400Da, 600Da, 800Da, 1000Da, 1500Da) The molecular weight cutoff is 90%, and the molecular weight cutoff of the 3D electrostatic spray-printed positively charged nanofiltration membrane was measured to be 382.3Da.

[0030] Figure 9 and Figure 10 The magnesium-lithium separation coefficients (with simulated solutions containing MgCl2 and LiCl of 1866 ppm and 134 ppm, respectively) and pure water permeation flux of the 3D electrostatic spray-printed intermediate layer and positively charged nanofiltration membrane were measured. The magnesium-lithium separation coefficients for the intermediate layer and the positively charged nanofiltration membrane were 4.86 and 36.28, respectively, indicating that the positively charged outer layer of polyethyleneimine-pyromellitic acid chloride plays a crucial role in the magnesium-lithium separation performance of the positively charged nanofiltration membrane. The pure water permeation flux of the 3D electrostatic spray-printed positively charged nanofiltration membrane was 11.64 L·m⁻¹. -2 ·h -1 ·bar -1 .

[0031] Figure 11 The surface Zeta potential of a 3D electrostatic spray-printed positively charged nanofiltration membrane at different pH values ​​is shown. The surface Zeta potential is 12.1 mV at pH=6 and 6.7 mV at pH=7.

[0032] Figure 12 The magnesium-lithium separation coefficients of the 3D electrostatic spray-printed positively charged nanofiltration membrane at different pH values ​​are shown. The magnesium-lithium separation coefficient is 39.56 at pH=6 and 35.6 at pH=7.

[0033] Figure 13 and Figure 14 The table shows the salt rejection rates of the 3D electrostatic spray-printed intermediate layer and the positively charged nanofiltration membrane. The intermediate layer's rejection rates for Na₂SO₄, MgSO₄, MgCl₂, NaCl, and LiCl were 68.1%, 68.9%, 62.4%, 20.2%, and 16.8%, respectively. The positively charged nanofiltration membrane's rejection rates for Na₂SO₄, MgSO₄, MgCl₂, NaCl, and LiCl were 93.8%, 98.1%, 97.4%, 35.6%, and 22.1%, respectively.

[0034] Figure 15 and Figure 16These are examples of 3D electrostatic spray printing of positively charged nanofiltration membranes for different Mg... 2+ / Li + Magnesium-lithium separation coefficient and heavy metal removal rate by mass ratio. For different Mg... 2+ / Li + In simulated solutions with varying mass ratios, the prepared positively charged nanofiltration membranes all exhibited magnesium-lithium separation coefficients exceeding 30. Simultaneously, the positively charged nanofiltration membranes showed good performance with respect to Cu... 2+ Ni 2+ Mn 2+ Co 2+ and Pb 2+ The removal rate of these heavy metal ions remained above 95%.

[0035] Figure 17 and Figure 18 The figures show the long-term separation results of MgCl2 and LiCl using a 3D electrostatic spray-printed positively charged nanofiltration membrane, as well as the long-term magnesium-lithium separation results of a mixed solution of MgCl2 and LiCl. As shown in the figures, after 150 hours of operation, the separation performance of MgCl2 and LiCl decreased by 1.3% and 2.3%, respectively, while the magnesium-lithium separation coefficient decreased by only 1.5%, indicating that the 3D electrostatic spray printing method has advantages in preparing nanofiltration membranes that can operate stably for a long time.

[0036] Figure 19 and Figure 20 This paper presents the results of a cyclic antifouling experiment using humic acid (HA) as a simulated pollutant, along with the calculated antifouling parameters. In actual operation of the nanofiltration membrane, although the feed solution undergoes ultrafiltration pretreatment, a small amount of organic matter still enters the nanofiltration membrane feed solution. The accumulation of organic matter over long-term operation poses a challenge to the stable operation of the nanofiltration membrane. The flux recovery rate (FRR), recoverable fouling rate (RFR), irrecoverable fouling rate (IFR), and total fouling rate (TFR) of the 3D electrostatic spray-printed positively charged nanofiltration membrane were 95.8%, 2.3%, 4.2%, and 6.5%, respectively. These results indicate that the prepared positively charged nanofiltration membrane exhibits strong resistance to organic pollution.

[0037] Example 2 This invention provides a method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing, comprising the following steps: S1. Preparation of the support membrane for 3D electrostatic spray printing of a positively charged nanofiltration membrane: 20g of polyethersulfone, 3g of polyvinylpyrrolidone, and 3g of polyethylene glycol were added to 74g of N,N-dimethylacetamide and stirred evenly at 60℃ to prepare a casting solution. After thorough degassing of the casting solution, a 300μm thick doctor blade was used to spread the casting solution evenly on a glass plate. After standing for 30s, the glass plate was immersed in distilled water for 12h to complete the phase inversion process. After washing with distilled water, the support membrane for the 3D electrostatic spray printed positively charged nanofiltration membrane was obtained.

[0038] Preparation of the piperazine-pyromellitic tricarboxylic acid (PPT) intermediate layer for S2 and 3D electrostatic spray printing: 4g piperazine was added to 96g deionized water, and 0.2g PPT was added to 99.8g n-hexane. After complete dissolution, the solution was transferred to the injector, and the polymer support membrane prepared in S1 was fixed on the roller. Electrostatic spraying was performed at 14kV. The monomer solution injection rate was set to 2mL / h, the injection platform translation speed to 60mm / min, the injection platform stroke to 12cm, the roller rotation speed to 60rpm, the temperature to 30℃, and the humidity to 50%RH. Ten layers were sprayed to obtain a positively charged nanofiltration membrane piperazine-pyromellitic tricarboxylic acid (PPT) intermediate layer.

[0039] Preparation of the positively charged outer layer of polyethyleneimine-pyromellitic trimethylol chloride (PPTC) for S3 electrostatic spraying: 1 g of polyethyleneimine was added to 99 g of deionized water and completely dissolved before being transferred to a sample injector. The piperazine aqueous solution in S2 was replaced with a polyethyleneimine aqueous solution, and spraying was continued for 10 more layers to obtain the positively charged outer layer of polyethyleneimine-PPTC in the nanofiltration membrane. The prepared nanofiltration membrane was placed in an oven and treated at 80 °C for 5 min, then immersed in distilled water to prepare the positively charged nanofiltration membrane.

[0040] Example 3 This invention provides a method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing, comprising the following steps: S1. Preparation of the support membrane for 3D electrostatic spray printing of a positively charged nanofiltration membrane: 20g of polyethersulfone, 3g of polyvinylpyrrolidone, and 3g of polyethylene glycol were added to 74g of N,N-dimethylacetamide and stirred evenly at 60℃ to prepare a casting solution. After thorough degassing of the casting solution, a 300μm thick doctor blade was used to spread the casting solution evenly on a glass plate. After standing for 30s, the glass plate was immersed in distilled water for 12h to complete the phase inversion process. After washing with distilled water, the support membrane for the 3D electrostatic spray printed positively charged nanofiltration membrane was obtained.

[0041] Preparation of the piperazine-pyromellitic tricarboxylic acid (PPT) intermediate layer for S2 and 3D electrostatic spray printing: 4 g of piperazine was added to 96 g of deionized water, and 0.2 g of PPT was added to 99.8 g of n-hexane. After complete dissolution, the solution was transferred to the injector, and the polymer support membrane prepared in S1 was fixed on the roller. Electrostatic spraying was performed at 13 kV. The monomer solution injection rate was set to 2 mL / h, the injection platform translation speed to 60 mm / min, the injection platform stroke to 12 cm, the roller rotation speed to 60 rpm, the temperature to 30℃, and the humidity to 50% RH. Ten layers were sprayed to obtain a positively charged nanofiltration membrane piperazine-pyromellitic tricarboxylic acid (PPT) intermediate layer.

[0042] Preparation of the positively charged outer layer of polyethyleneimine-pyromellitic trimethylol chloride (PPTC) for S3 electrostatic spraying: 1 g of polyethyleneimine was added to 99 g of deionized water and completely dissolved before being transferred to a sample injector. The piperazine aqueous solution in S2 was replaced with a polyethyleneimine aqueous solution, and spraying was continued for 10 more layers to obtain the positively charged outer layer of polyethyleneimine-PPTC in the nanofiltration membrane. The prepared nanofiltration membrane was placed in an oven and treated at 80 °C for 5 min, then immersed in distilled water to prepare the positively charged nanofiltration membrane.

[0043] Example 4 This invention provides a method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing, comprising the following steps: S1. Preparation of the support membrane for 3D electrostatic spray printing of a positively charged nanofiltration membrane: 20g of polyethersulfone, 3g of polyvinylpyrrolidone, and 3g of polyethylene glycol were added to 74g of N,N-dimethylacetamide and stirred evenly at 60℃ to prepare a casting solution. After thorough degassing of the casting solution, a 300μm thick doctor blade was used to spread the casting solution evenly on a glass plate. After standing for 30s, the glass plate was immersed in distilled water for 12h to complete the phase inversion process. After washing with distilled water, the support membrane for the 3D electrostatic spray printed positively charged nanofiltration membrane was obtained.

[0044] Preparation of the piperazine-pyromellitic tricarboxylic acid (PPT) interlayer for S2 and 3D electrostatic spray printing: 4g piperazine was added to 96g deionized water, and 0.2g PPT was added to 99.8g n-hexane. After complete dissolution, the solution was transferred to the injector, and the polymer support membrane prepared in S1 was fixed on the roller. Electrostatic spraying was performed at 13kV. The monomer solution injection rate was set to 2mL / h, the injection platform translation speed to 60mm / min, the injection platform stroke to 12cm, the roller rotation speed to 60rpm, the temperature to 30℃, and the humidity to 60%RH. Ten layers were sprayed to obtain a positively charged nanofiltration membrane piperazine-pyromellitic tricarboxylic acid (PPT) interlayer.

[0045] Preparation of the positively charged outer layer of polyethyleneimine-pyromellitic trimethylol chloride (PPTC) for S3 electrostatic spraying: 1 g of polyethyleneimine was added to 99 g of deionized water and completely dissolved before being transferred to a sample injector. The piperazine aqueous solution in S2 was replaced with a polyethyleneimine aqueous solution, and spraying was continued for 10 more layers to obtain the positively charged outer layer of polyethyleneimine-PPTC in the nanofiltration membrane. The prepared nanofiltration membrane was placed in an oven and treated at 80 °C for 5 min, then immersed in distilled water to prepare the positively charged nanofiltration membrane.

[0046] Example 5 This invention provides a method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing, comprising the following steps: S1. Preparation of the support membrane for 3D electrostatic spray printing of a positively charged nanofiltration membrane: 20g of polyethersulfone, 3g of polyvinylpyrrolidone, and 3g of polyethylene glycol were added to 74g of N,N-dimethylacetamide and stirred evenly at 60℃ to prepare a casting solution. After thorough degassing of the casting solution, a 300μm thick doctor blade was used to spread the casting solution evenly on a glass plate. After standing for 30s, the glass plate was immersed in distilled water for 12h to complete the phase inversion process. After washing with distilled water, the support membrane for the 3D electrostatic spray printed positively charged nanofiltration membrane was obtained.

[0047] Preparation of the piperazine-pyromellitic tricarboxylic acid (PPT) intermediate layer for S2 and 3D electrostatic spray printing: 4 g of piperazine was added to 96 g of deionized water, and 0.2 g of PPT was added to 99.8 g of n-hexane. After complete dissolution, the solution was transferred to the injector, and the polymer support membrane prepared in S1 was fixed on the roller. Electrostatic spraying was performed at 13 kV. The monomer solution injection rate was set to 2 mL / h, the injection platform translation speed to 60 mm / min, the injection platform stroke to 12 cm, the roller rotation speed to 60 rpm, the temperature to 35℃, and the humidity to 60% RH. Ten layers were sprayed to obtain a positively charged nanofiltration membrane piperazine-pyromellitic tricarboxylic acid (PPT) intermediate layer.

[0048] Preparation of the positively charged outer layer of polyethyleneimine-pyromellitic trimethylol chloride (PPTC) for S3 electrostatic spraying: 1 g of polyethyleneimine was added to 99 g of deionized water and completely dissolved before being transferred to a sample injector. The piperazine aqueous solution in S2 was replaced with a polyethyleneimine aqueous solution, and spraying was continued for 10 more layers to obtain the positively charged outer layer of polyethyleneimine-PPTC in the nanofiltration membrane. The prepared nanofiltration membrane was placed in an oven and treated at 80 °C for 5 min, then immersed in distilled water to prepare the positively charged nanofiltration membrane.

[0049] Example 6 This invention provides a method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing, comprising the following steps: S1. Preparation of the support membrane for 3D electrostatic spray printing of a positively charged nanofiltration membrane: 20g of polyethersulfone, 3g of polyvinylpyrrolidone, and 3g of polyethylene glycol were added to 74g of N,N-dimethylacetamide and stirred evenly at 60℃ to prepare a casting solution. After thorough degassing of the casting solution, a 300μm thick doctor blade was used to spread the casting solution evenly on a glass plate. After standing for 30s, the glass plate was immersed in distilled water for 12h to complete the phase inversion process. After washing with distilled water, the support membrane for the 3D electrostatic spray printed positively charged nanofiltration membrane was obtained.

[0050] Preparation of the piperazine-pyromellitic tricarboxylic acid (PPT) intermediate layer for S2 and 3D electrostatic spray printing: 4 g of piperazine was added to 96 g of deionized water, and 0.2 g of PPT was added to 99.8 g of n-hexane. After complete dissolution, the solution was transferred to the injector, and the polymer support membrane prepared in S1 was fixed on the roller. Electrostatic spraying was performed at 13 kV. The monomer solution injection rate was set to 2 mL / h, the injection platform translation speed to 60 mm / min, the injection platform stroke to 12 cm, the roller rotation speed to 60 rpm, the temperature to 35℃, and the humidity to 60% RH. Ten layers were sprayed to obtain a positively charged nanofiltration membrane piperazine-pyromellitic tricarboxylic acid (PPT) intermediate layer.

[0051] Preparation of the positively charged outer layer of polyethyleneimine-pyromellitic trimethylol chloride (PPTC) for S3 electrostatic spraying: 1 g of polyethyleneimine was added to 99 g of deionized water and completely dissolved before being transferred to a sample injector. The piperazine aqueous solution in S2 was replaced with a polyethyleneimine aqueous solution, and spraying was continued for 10 layers to obtain the positively charged outer layer of polyethyleneimine-PPTC in the nanofiltration membrane. The prepared nanofiltration membrane was placed in an oven and treated at 80 °C for 7 min, then immersed in distilled water to prepare the positively charged nanofiltration membrane.

[0052] 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 preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing, characterized in that, Includes the following steps: Preparation of S1 Support Membrane: The support membrane was prepared by phase inversion method using a casting solution containing a polymeric membrane matrix material, a pore-forming agent, a membrane modifier, and an organic solvent. The prepared casting solution was stirred evenly at a constant temperature and degassed. It was then spread evenly on a glass plate, allowed to stand in air for a certain period of time, and then immersed in distilled water to obtain the support membrane for the nanofiltration membrane. S2. Preparation of piperazine-pyromellitic tricarboxylic acid intermediate layer: piperazine is used as the aqueous monomer and pyromellitic tricarboxylic acid is used as the organic monomer. The monomer solution in the atomized state is sprayed onto the above support film by 3D electrostatic spraying printing. The monomer undergoes a polymerization reaction to generate a loose piperazine-pyromellitic tricarboxylic acid loose intermediate layer. S3. Preparation of positively charged nanofiltration membrane: Using polyethyleneimine as the aqueous monomer and trimesoyl chloride as the organic monomer, the monomer solution in an atomized state was sprayed onto the above intermediate layer using a 3D electrostatic spraying method. The monomers underwent a polymerization reaction to generate a positively charged outer layer of polyethyleneimine-trisoyl chloride. After heat treatment, the positively charged nanofiltration membrane was obtained.

2. The method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing according to claim 1, characterized in that: Aqueous and oil-phase monomer solutions are placed in an injection apparatus connected to a high-voltage positive electrode, while the supporting membrane is placed on a roller connected to a negative electrode. Using electrostatic spraying technology, the aqueous and organic phase monomer solutions are dispersed into micron- or nanometer-sized droplets, which are deposited onto the surface of the supporting membrane under an electric field and undergo polymerization. Each time the two phase solutions sweep across the supporting membrane surface, a nanoscale active layer with retention capacity is generated. By horizontally moving the injection apparatus and rotating the roller, the spray is repeatedly applied to the supporting membrane surface, achieving three-dimensional growth of the nanofiltration membrane active layer using a 3D printing method. During this process, the active layer gradually densifies until the designed retention capacity is achieved. The types and amounts of monomers injected into the injection apparatus can be adjusted as needed, and the membrane fabrication process is precise, continuous, and efficient.

3. The method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing according to claim 1, characterized in that: In step S1, the polymeric membrane matrix material is one or more of polyvinylidene fluoride, polysulfone, polyethersulfone, polyamide, cellulose acetate, polypropylene, and polyvinyl chloride. Preferably, based on the total mass of the casting solution, the mass percentage of the polymer is 18wt% to 22wt%.

4. The method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing according to claim 1, characterized in that: In step S1, the membrane modifier is one or more of a hydrophilic agent or a surfactant.

5. The method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing according to claim 1, characterized in that: The organic solvent in step S1 is one of dimethyl sulfoxide, acetone, cyclohexane, chloroform, N,N-dimethylformamide, N,N-dimethylacetamide, or N-methylpyrrolidone.

6. The method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing according to claim 1, characterized in that: In steps S2 and S3, the aqueous solvent is one or more of water, ethanol, and dimethyl sulfoxide.

7. The method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing according to claim 1, characterized in that: The required ambient temperature for the preparation of the piperazine-trimethylammonium chloride intermediate layer in step S2 is 30~40℃, the ambient humidity is 40~70%RH, the voltage is 10~15kV, the distance between the needle and the support film is 2~3cm, the needle moving speed is 40~70mm / min, the roller rotation speed is 40~70rpm, and the monomer solution injection speed is 1~3mL / h.

8. The method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing according to claim 1, characterized in that: The preparation of the positively charged outer layer of polyethyleneimine-trimethylammonium chloride in step S3 requires an ambient temperature of 30-40℃, an ambient humidity of 40-70%RH, a voltage of 10-15kV, a distance of 2-3cm between the needle and the support film, a needle moving speed of 40-70mm / min, a roller rotation speed of 40-70rpm, and a monomer solution injection speed of 1-3mL / h.

9. The method for preparing a positively charged nanofiltration membrane by 3D electrostatic spray printing according to claim 1, characterized in that: In step S3, the heat treatment method is to heat at 60~80℃ for 1~10 min; in step S2, the mass ratio of aqueous monomer to solution is 1:99~5:95; the number of monomer solution sprayed on the support membrane is 5~40 layers; and the mass ratio of organic monomer to solvent is 1:999~5:995.

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