A method for preparing and applying an adjustable dual-charge polyamide nanofiltration membrane

Abstract

This invention belongs to the field of water treatment technology, specifically disclosing a method for preparing and applying an adjustable dual-charge polyamide nanofiltration membrane. The invention constructs a method for preparing an adjustable dual-charge polyamide nanofiltration membrane, which involves preparing a highly negatively charged polyamide layer through an interfacial polymerization reaction between a diamine buffer / inorganic salt solution and an oil phase, and preparing a positively charged polyamide layer through an interfacial polymerization reaction between a polyamine inorganic salt solution and an oil phase, thus producing an adjustable dual-charge polyamide nanofiltration membrane. The charge, pore size, thickness, and uniformity of both the positive and negative charged layers are adjustable. This adjustable dual-charge polyamide nanofiltration membrane can be applied in wastewater treatment, lithium extraction from salt lakes, and other fields.

Description

Technical Field

[0001] This invention belongs to the field of water treatment technology and relates to a method for preparing a polyamide composite nanofiltration membrane, and more particularly to a novel method for preparing an adjustable dual-charge polyamide nanofiltration membrane by interfacial polymerization. Background Technology

[0002] Membrane separation technology is a technique that utilizes the selectivity of membranes to separate, purify, and concentrate characteristic components of feed solutions at the molecular level. Due to its advantages such as low energy consumption, high selectivity, environmental friendliness, low operating costs, and high separation efficiency, it is widely used in seawater desalination, brackish water desalination, wastewater treatment, gas separation, and organic molecular sieving. Within the field of membrane separation, nanofiltration membrane technology is highly valued for its superior selective separation characteristics. Nanofiltration membranes are separation membranes with pore sizes between 0.5 and 2 nm, retaining molecular weights above 200 Da. Simultaneously, the membrane surface is usually charged, thus selectively retaining organic matter above 200 Da and high-valence charged ions, while exhibiting selective permeability to low-molecular-weight organic matter and low-valence ions, demonstrating excellent selective sieving performance.

[0003] The surface charge of nanofiltration membranes has a decisive influence on their separation performance. Commercially available nanofiltration membranes are usually negatively charged, exhibiting high rejection rates for anions but low rejection rates for cations. Therefore, the research and preparation of nanofiltration membranes capable of simultaneously retaining anions and cations to achieve balanced retention of ions with different charges is currently a key research focus in nanofiltration membranes. CN202210774026.6 discloses a method for preparing a double-charged layer composite membrane by interfacial polymerization and its application. A hydrophobic substrate membrane is floated in a buffer solution formed by polyphenols and polyamines to obtain the first membrane. The dried composite membrane is fixed between two reaction vessels, and the amine monomer solution and the acyl chloride monomer solution undergo interfacial polymerization to obtain the second membrane, resulting in a double-charged layer composite membrane with a negatively charged upper layer and a positively charged lower layer for selective separation. CN202310837733.X discloses a method for preparing amphoteric Janus nanofiltration membranes using a dual-nozzle electrospinning apparatus. This membrane is prepared by reacting tert-butyl hydroperoxide with 1,3,5-benzenetricarbonyl trichloride to form a positively charged active layer, and by reacting tetraethylenepentamine with 1,3,5-benzenetricarbonyl trichloride to form a negatively charged active filter layer. While existing technologies can prepare nanofiltration membranes with dual charges, they cannot achieve controllable adjustment of the charge level. Summary of the Invention

[0004] To address the shortcomings of the prior art, the present invention aims to provide a method for preparing an adjustable dual-charged polyamide nanofiltration membrane. This method not only prepares the dual-charged nanofiltration membrane but also allows for the adjustment of the charge and physical structure of the positive or negative charge layer of the dual-charged nanofiltration membrane by regulating the pH value, salt type, and concentration in the polyamine / diamine aqueous solution, thereby achieving controllable adjustment of the dual-charged nanofiltration membrane.

[0005] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0006] One aspect of the present invention provides a method for preparing an adjustable dual-charge polyamide nanofiltration membrane, comprising the following steps:

[0007] 1) Prepare a diamine buffer salt / inorganic salt aqueous phase solution (i.e., the solution contains buffer salt and / or inorganic salt), adjust the salt concentration to 0.005-0.5M and the pH value to 6-10 to obtain an interfacial polymerization aqueous phase solution;

[0008] 2) Fully wet the experimental ultrafiltration base membrane with an interfacial polymerization aqueous solution, and air dry it in the air until there are no obvious water stains on the membrane surface, thus obtaining an ultrafiltration base membrane fully wetted with an aqueous solution.

[0009] 3) Mix polyacryl chloride and organic solvent to prepare polyacryl chloride solution. Pour the polyacryl chloride solution onto the surface of the ultrafiltration base membrane to allow the polyacryl chloride solution to fully contact the aqueous diamine phase. After the reaction is complete, wash with organic solvent and anneal to form the first layer of negatively charged polyamide nanofiltration membrane.

[0010] 4) Prepare a polyamine inorganic salt solution, adjust the inorganic salt concentration to 0.005-0.5M, and the pH value to 6-10;

[0011] 5) The negatively charged polyamide nanofiltration membrane is fully wetted by the aqueous solution of polyamine inorganic salt, and then air-dried until there are no obvious water stains on the membrane surface, thus obtaining a negatively charged bottom membrane that is fully wetted by the aqueous solution.

[0012] 6) Mix polyacryl chloride and organic solvent to prepare polyacryl chloride solution. Pour the polyacryl chloride solution onto the surface of the first layer of negatively charged polyamide nanofiltration membrane to allow the polyacryl chloride solution to fully contact the aqueous polyamine monomer. After the reaction is complete, wash with organic solvent to form the second layer of positively charged polyamide nanofiltration membrane.

[0013] 7) The prepared polyamide membrane is annealed to obtain an adjustable double-charged polyamide nanofiltration membrane with a negative lower layer and a positive upper layer.

[0014] Preferably, in step 1), the diamine is selected from at least one of piperazine, ethylenediamine, hexamethylenediamine, and thiourea.

[0015] Preferably, in step 1), the buffer salt is selected from at least one of phosphate buffer systems, carbonate buffer systems, borate buffer systems, and citrate buffer systems.

[0016] Preferably, in step 1), the inorganic salt is selected from at least one of the following systems: chloride system, sulfate system, fluoride system, bromide system, nitrate system, silicate system, tungstate system, molybdate system, permanganate system, sulfate system, sulfite system, and pyrophosphate system.

[0017] Preferably, the lower negatively charged nanofiltration membrane of the dual-charged polyamide nanofiltration membrane exhibits adjustable negative charge and physical structure according to changes in the pH value, salt type, and concentration of the diamine aqueous solution. Specifically, the higher the charge of the buffer salt / inorganic salt anions, the higher the negative charge, the thinner the membrane, and the larger the pore size of the prepared nanofiltration membrane; the lower the pH value, the higher the negative charge, the thinner the membrane, and the larger the pore size of the prepared nanofiltration membrane; and the higher the salt concentration, the higher the negative charge, the thinner the membrane, and the larger the pore size of the prepared nanofiltration membrane. Furthermore, under the pH value, salt type, and concentration conditions specified in this invention, the prepared negatively charged nanofiltration membrane exhibits good thickness uniformity.

[0018] Preferably, in step 4), the polyamine is selected from at least one of polyallylamine hydrochloride, polydiallyldimethylammonium chloride, polypropylacryloyloxyethyltrimethylammonium chloride, poly-4-vinylpyridine, and polyethyleneimine.

[0019] Preferably, in step 4), the inorganic salt in the polyamine inorganic salt water phase solution is selected from at least one of the following systems: chloride system, sulfate system, fluoride system, bromide system, nitrate system, silicate system, tungstate system, molybdate system, permanganate system, sulfate system, sulfite system, and pyrophosphate system.

[0020] Preferably, the upper positively charged nanofiltration membrane of the dual-charged polyamide nanofiltration membrane exhibits adjustable positive charge and physical structure based on changes in the type and concentration of inorganic salts in the polyamine aqueous solution. Specifically, the higher the charge of the inorganic salt anions, the lower the positive charge, the thinner the membrane, and the larger the pore size of the prepared nanofiltration membrane; conversely, the higher the inorganic salt concentration, the lower the positive charge, the thinner the membrane, and the larger the pore size of the prepared nanofiltration membrane. Furthermore, under the inorganic salt type and concentration conditions specified in this invention, the prepared positively charged nanofiltration membrane exhibits good thickness uniformity.

[0021] Preferably, in steps 3) and 6), the organic solvent is selected from at least one of hexaalkyl, heptane, pentane, Isopar G, toluene, ethyl acetate, and benzene.

[0022] Preferably, in steps 3) and 6), the polyacryl chloride is selected from at least one of 1,3,5-pyromellitic chloride, terephthaloyl chloride, isophthaloyl chloride, azeloyl chloride, adipicoyl chloride, and 2,2',4,4'-biphenyltetracarboxylic chloride; and the concentration of the polyacryl chloride in the polyacryl chloride solution is 0.25 to 7.5 mg / mL.

[0023] Preferably, in step 2), the ultrafiltration base membrane material is selected from any one of polysulfone, polyethersulfone, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene nylon, cellulose or cellulose derivatives; the pore size of the ultrafiltration base membrane is 0.01 to 2 μm.

[0024] Preferably, in steps 3) and 7), the annealing temperature is 20–80°C and the time is 1–30 min.

[0025] Preferably, in steps 3) and 6), the diamine / polyamine monomer and polyacrylamide chloride undergo interfacial polymerization for 30 to 90 seconds at an interface temperature of 20 to 35°C and a relative humidity of 50 to 100%.

[0026] Another aspect of the present invention provides an adjustable dual-charge polyamide nanofiltration membrane prepared by the above-described preparation method. The adjustable dual-charge polyamide nanofiltration membrane includes an ultrafiltration substrate membrane and a bilayer separation layer formed on the surface of the ultrafiltration substrate membrane. The upper layer of the bilayer separation layer exhibits a positive charge, and the lower layer exhibits a negative charge. Furthermore, the charge, pore size, uniformity, and thickness of the upper and lower polyamide materials are adjustable.

[0027] Another aspect of the present invention provides the application of the adjustable dual-charge polyamide nanofiltration membrane in water treatment, applying the adjustable dual-charge polyamide nanofiltration membrane to wastewater treatment, lithium extraction from salt lakes, and other fields.

[0028] By adopting the above technical solution, at least the following beneficial effects are achieved:

[0029] 1. This invention provides a method for preparing an adjustable dual-charge polyamide nanofiltration membrane, through which an adjustable dual-charge polyamide nanofiltration membrane can be obtained.

[0030] 2. This invention combines nanofiltration membrane structure and interfacial polymerization reaction control methods to prepare a dual-charged polyamide nanofiltration membrane with adjustable positive and negative charges. By adjusting the pH value, salt type and concentration of the polyamine / diamine aqueous solution, the diffusion rate of small molecule amines and the interfacial polymerization reaction rate are designed and optimized. The diffusion rate of small molecule amines is controlled by the electrostatic interaction between anions and small molecule amines, and the interfacial polymerization reaction rate is controlled by the protonation effect of small molecule amines. The interfacial polymerization reaction can be controlled to a large extent, thereby affecting the physical and chemical structure of the nanofiltration membrane, thus preparing a method for preparing an adjustable dual-charged polyamide nanofiltration membrane.

[0031] 3. The preparation method of this invention improves the selectivity of the nanofiltration membrane by controlling the charge of the first negatively charged polyamide nanofiltration membrane, thereby increasing the rejection rate of divalent anions and reducing the permeability of monovalent cations. Simultaneously, the first porous, highly negatively charged nanofiltration membrane, as an intermediate layer, provides an excellent reaction platform for the interfacial polymerization reaction of the second positively charged nanofiltration membrane.

[0032] 4. The preparation method of this invention solves the problems of complexity, high cost and unsatisfactory effect of traditional control methods. The preparation method is simple, low cost, obvious and stable control effect, easy to prepare on a large scale, and has strong industrial application value. Detailed Implementation

[0033] To facilitate understanding of the present invention and to make the above-mentioned objects, features, and advantages of the present invention more apparent, a detailed description of specific embodiments of the present invention is provided below. Many specific details are set forth in the following description to provide a thorough understanding of the present invention, and preferred embodiments are given. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention. The present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention; therefore, the present invention is not limited to the specific embodiments disclosed below.

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the invention.

[0035] While existing technologies can fabricate nanofiltration membranes with dual charges, they cannot achieve controllable adjustment of the membrane's charge and physical structure. The properties of traditional polyamide nanofiltration membranes are determined by intrinsic interfacial polymerization reactions. The interfacial polymerization reaction between trimesoyl chloride and small-molecule amines at the oil-water interface is extremely rapid, making it difficult to achieve controllable adjustment.

[0036] In view of the shortcomings of the prior art, the inventors of this case have combined nanofiltration membrane structure and interfacial polymerization reaction control methods to prepare a dual-charged polyamide nanofiltration membrane with adjustable positive and negative charges. By adjusting the pH value, salt type and concentration of the polyamine / diamine aqueous solution, the diffusion rate of small molecule amines and the interfacial polymerization reaction rate are designed and optimized. The diffusion rate of small molecule amines is controlled by the electrostatic interaction between anions and small molecule amines, and the interfacial polymerization reaction rate is controlled by the protonation effect of small molecule amines. The interfacial polymerization reaction can be controlled to a large extent, thereby affecting the physical and chemical structure of the nanofiltration membrane. The charge, pore size, thickness and thickness uniformity can be controlled and adjusted, thus preparing an adjustable dual-charged polyamide nanofiltration membrane.

[0037] This invention provides a method for preparing an adjustable dual-charge polyamide nanofiltration membrane. A highly negatively charged polyamide layer is prepared via an interfacial polymerization reaction between a diamine buffer / inorganic salt solution and an oil phase, and a positively charged polyamide layer is prepared via the same interfacial polymerization reaction between a polyamine inorganic salt solution and an oil phase. The charge and physical structure of both the positive and negative layers are adjustable, exhibiting a high rejection rate for divalent anions and cations. The method for preparing the adjustable dual-charge polyamide nanofiltration membrane includes the following steps:

[0038] 1) Prepare a diamine buffer / inorganic salt water phase solution, adjust the salt concentration to 0.005-0.5M and the pH value to 6-10 to obtain an interfacial polymerization aqueous phase solution; under this pH condition, the negatively charged membrane has high electronegativity, greater porosity (the lower the pH, the larger the pore size), good selectivity, which facilitates the passage of positive ions with small charge and retains positive ions with large charge;

[0039] The diamine is selected from at least one of piperazine, ethylenediamine, hexamethylenediamine, and thiourea;

[0040] The buffer salt is selected from at least one of phosphate buffer systems, carbonate buffer systems, borate buffer systems, and citrate buffer systems;

[0041] The inorganic salt is selected from at least one of the following systems: chloride system, sulfate system, fluoride system, bromide system, nitrate system, silicate system, tungstate system, molybdate system, permanganate system, sulfate system, sulfite system, and pyrophosphate system.

[0042] 2) Fully wet the experimental ultrafiltration base membrane with an interfacial polymerization aqueous solution, and air dry it in the air until there are no obvious water stains on the membrane surface, thus obtaining an ultrafiltration base membrane fully wetted with an aqueous solution.

[0043] The ultrafiltration base membrane material is selected from any one of polysulfone, polyethersulfone, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene nylon, cellulose or cellulose derivatives; the pore size of the ultrafiltration base membrane is 0.01 to 2 μm.

[0044] 3) Mix polyacryl chloride and organic solvent to prepare polyacryl chloride solution. Pour polyacryl chloride solution onto the surface of ultrafiltration base membrane to allow the polyacryl chloride solution to fully contact the aqueous diamine phase. After full reaction, wash with organic solvent and anneal to form the first layer of negatively charged polyamide nanofiltration membrane on the front side of ultrafiltration base membrane.

[0045] The organic solvent is selected from at least one of hexaalkyl, heptane, pentane, Isopar G, toluene, ethyl acetate and benzene;

[0046] The polyacryl chloride is selected from at least one of 1,3,5-pyromellitic terephthaloyl chloride, terephthaloyl chloride, isophthaloyl chloride, azeloyl chloride, adipicoyl chloride, and 2,2',4,4'-biphenyltetracarboxylic chloride;

[0047] The concentration of polyacryl chloride in the polyacryl chloride solution is 0.25–7.5 mg / mL, and the annealing treatment temperature is 20–80 °C for 1–30 min.

[0048] The diamine and polyacrylamide chloride undergo interfacial polymerization at an interface temperature of 20–35°C and a relative humidity of 50–100% for 30–90 s.

[0049] 4) Prepare a polyamine inorganic salt solution, and adjust the concentration of the inorganic salt solution to 0.005-0.5M and the pH value to 6-10;

[0050] The polyamine is selected from at least one of polyallylamine hydrochloride, polydiallyldimethylammonium chloride, polypropylacryloyloxyethyltrimethylammonium chloride, poly-4-vinylpyridine, and polyethyleneimine.

[0051] 5) The negatively charged polyamide nanofiltration membrane is fully wetted with a polyamine inorganic salt water phase solution and air-dried until there are no obvious water stains on the membrane surface, thus obtaining an ultrafiltration substrate membrane that is fully wetted with an aqueous phase solution.

[0052] The inorganic salt in the polyamine inorganic salt phase solution is selected from at least one of the following systems: chloride system, sulfate system, fluoride system, bromide system, nitrate system, silicate system, tungstate system, molybdate system, permanganate system, sulfate system, sulfite system, and pyrophosphate system.

[0053] 6) Mix polyacryl chloride and organic solvent to prepare polyacryl chloride solution. Pour the polyacryl chloride solution onto the surface of the first layer of negatively charged polyamide nanofiltration membrane to allow the polyacryl chloride solution to fully contact the aqueous polyamine monomer. After the reaction is complete, wash with organic solvent to form the second layer of positively charged polyamide nanofiltration membrane.

[0054] The polyamine monomer and polyacrylamide chloride undergo interfacial polymerization at an interface temperature of 20–35°C and a relative humidity of 50–100% for 30–90 s.

[0055] 7) The prepared polyamide membrane is annealed to obtain an adjustable double-charged polyamide nanofiltration membrane with a negative lower layer and a positive upper layer;

[0056] The annealing process is performed at a temperature of 20–80°C for 1–30 minutes.

[0057] The prepared adjustable dual-charge polyamide nanofiltration membrane includes an ultrafiltration substrate membrane and a bilayer separation layer formed on the surface of the ultrafiltration substrate membrane. The upper layer of the bilayer separation layer exhibits positive charge and the lower layer exhibits negative charge. The charge, pore size, uniformity, and thickness of the upper and lower polyamide materials are adjustable.

[0058] The prepared adjustable dual-charge polyamide nanofiltration membrane can be applied to wastewater treatment, lithium extraction from salt lakes, and other fields.

[0059] The following are specific embodiments, including examples of the preparation of adjustable dual-charge polyamide nanofiltration membranes and a series of examples demonstrating controllability. It should be noted that, when demonstrating controllability, it is difficult to measure relevant parameters of positive and negative charge bilayer membranes, so it is demonstrated in the form of a single-layer membrane. Its control effect is also applicable to bilayer membranes.

[0060] Examples 1-4: Demonstrating the tunability of negatively charged polyamide nanofiltration membranes

[0061] Example 1

[0062] After soaking the polysulfone substrate membrane in a piperazine (2.5 mg / mL) aqueous solution for 120 s, it was air-dried until no obvious water stains were visible on the membrane surface, resulting in a polysulfone substrate membrane fully wetted by the aqueous solution. A 2 mg / mL solution of 1,3,5-trimethylpyrrolidone chloride was poured onto the wetted substrate membrane surface for interfacial polymerization at 25 °C and 80% relative humidity for 60 s. The membrane was then removed, washed with n-hexane, and placed in an oven at 60 °C for 1 min to obtain a conventional nanofiltration membrane. TEM showed a separation layer thickness of 65 nm and a carboxyl group density of 0.19 sites·nm at pH 10.5. -3 When the pH of the test aqueous solution is 7.0, the Zeta potential is -31.0mV, and the pore radius of the membrane is 0.404nm according to the pore size prediction model.

[0063] Example 2

[0064] The pH of a mixed aqueous solution of piperazine (2.5 mg / mL) and Na3PO4 (0.1 M) was adjusted to 10.0. The polysulfone substrate membrane was then soaked in this solution for 120 s and air-dried until no obvious water stains were visible on the membrane surface, resulting in a polysulfone substrate membrane fully wetted by the aqueous solution. A 2 mg / mL solution of 1,3,5-trimethylpyrrolidone chloride was poured onto the wetted substrate membrane surface for interfacial polymerization at 25 °C and 80% relative humidity for 60 s. After 60 s, the membrane was removed, washed with n-hexane, and then placed in an oven and heated at 60 °C for 1 min to obtain a negatively charged, porous nanofiltration membrane. TEM analysis showed that the nanofiltration membrane surface had a relatively smooth structure, a separation layer thickness of 35 nm, and a carboxyl group density of 0.36 sites·nm at pH 10.5. -3 When the pH of the test aqueous solution is 7.0, the Zeta potential is -51.0mV, and the pore radius of the membrane is 0.433nm according to the pore size prediction model.

[0065] Example 3

[0066] The pH of a mixed aqueous solution of piperazine (2.5 mg / mL) and Na₂HPO₄ / NaH₂PO₄ (0.1 M) was adjusted to 9.0. The polysulfone substrate membrane was then soaked in this solution for 120 s and air-dried until no obvious water stains were visible on the membrane surface, resulting in a polysulfone substrate membrane fully wetted by the aqueous solution. A 2 mg / mL solution of 1,3,5-trimethylammonium chloride was poured onto the wetted substrate membrane surface for interfacial polymerization at 25 °C and 80% relative humidity for 60 s. After 60 s, the membrane was removed, washed with n-hexane, and then placed in an oven and heated at 60 °C for 1 min to obtain a negatively charged, porous nanofiltration membrane. TEM analysis showed that the nanofiltration membrane surface had a relatively smooth structure, a separation layer thickness of 28 nm, and a carboxyl group density of 0.44 sites·nm at pH 10.5. -3 When the pH of the test aqueous solution is 7.0, the Zeta potential is -60.5mV, and the pore radius of the membrane is 0.452nm according to the pore size prediction model.

[0067] Example 4

[0068] The pH of a mixed aqueous solution of piperazine (2.5 mg / mL) and Na₂HPO₄ / NaH₂PO₄ (0.1 M) was adjusted to 6.5. The polysulfone substrate membrane was then soaked in this solution for 120 s and air-dried until no obvious water stains were visible on the membrane surface, resulting in a polysulfone substrate membrane fully wetted by the aqueous solution. A 2 mg / mL solution of 1,3,5-trimethylammonium chloride was poured onto the wetted substrate membrane surface for interfacial polymerization at 25 °C and 80% relative humidity for 60 s. After 60 s, the membrane was removed, washed with n-hexane, and then placed in an oven and heated at 60 °C for 1 min to obtain a negatively charged, porous nanofiltration membrane. TEM analysis showed that the nanofiltration membrane surface had a relatively smooth structure, a separation layer thickness of 12 nm, and a carboxyl group density of 1.02 sites·nm at pH 10.5. -3 When the pH of the test aqueous solution is 7.0, the Zeta potential is -68.9mV, and the pore radius of the membrane is 0.496nm according to the pore size prediction model.

[0069] Examples 5-7: Demonstrating the tunability of positively charged polyamide nanofiltration membranes

[0070] Example 5

[0071] After immersing the polysulfone substrate membrane in a polyethyleneimine (5.0 mg / mL) solution for 120 s, it was air-dried until no obvious water stains were visible on the membrane surface, resulting in a polysulfone substrate membrane fully wetted by the aqueous solution. A 2 mg / mL solution of 1,3,5-trimethylammonium chloride was poured onto the wetted substrate membrane surface for interfacial polymerization at 25 °C and 80% relative humidity. After 60 s, the membrane was removed, washed with n-hexane, and then placed in an oven and heated at 60 °C for 1 min to obtain a conventional positively charged nanofiltration membrane. TEM analysis showed that the separation layer thickness was 52 nm, the zeta potential was 35.2 mV at a pH of 7.0, and the pore radius of the membrane was 0.53 nm according to the pore size prediction model.

[0072] Example 6

[0073] Add NaCl (0.1M) to a 5.0 mg / mL aqueous solution of polyethyleneimine, soak the polysulfone substrate membrane for 120 s, and then air-dry it until there are no obvious water stains on the membrane surface, thus obtaining a polysulfone substrate membrane fully wetted by the aqueous solution. Pour a 2 mg / mL solution of 1,3,5-trimethylammonium chloride onto the wetted substrate membrane surface and carry out an interfacial polymerization reaction at 25 °C and 80% relative humidity. After 60 s, remove the membrane, wash it with n-hexane, and place it in an oven to heat it at 60 °C for 1 min to obtain a weakly positively charged nanofiltration membrane. TEM analysis showed that the separation layer thickness was 46 nm, the zeta potential was 28.3 mV when the pH of the aqueous solution was 7.0, and the pore radius of the membrane was 0.57 nm according to the pore size prediction model.

[0074] Example 7

[0075] Add Na3PO4 (0.1M) to a 5.0 mg / mL aqueous solution of polyethyleneimine, soak the polysulfone substrate membrane for 120 s, and then air-dry it until there are no obvious water stains on the membrane surface, thus obtaining a polysulfone substrate membrane fully wetted by the aqueous solution. Pour a 2 mg / mL solution of 1,3,5-trimethylammonium chloride onto the wetted substrate membrane surface and carry out an interfacial polymerization reaction at 25 °C and 80% relative humidity. After 60 s, remove the membrane, wash it with n-hexane, and place it in an oven to heat it at 60 °C for 1 min to obtain a weakly positively charged nanofiltration membrane. TEM analysis showed that the separation layer thickness was 37 nm, the zeta potential was 18.7 mV when the pH of the aqueous solution was 7.0, and the pore radius of the membrane was 0.61 nm according to the pore size prediction model.

[0076] Example 8: Preparation of tunable dual-charge polyamide nanofiltration membrane

[0077] This embodiment describes the preparation of an adjustable dual-charge polyamide nanofiltration membrane. The preparation parameters for the negatively charged porous nanofiltration membrane are the same as in Example 3, and the preparation parameters for the positively charged nanofiltration membrane are the same as in Example 7. The specific process is as follows: The pH of the mixed aqueous solution of piperazine (2.5 mg / mL) and Na2HPO4 / NaH2PO4 (total salt concentration 0.1 M, initial molar ratio of Na2HPO4 to NaH2PO4 1:1) is adjusted to 9.0. After soaking the polysulfone substrate membrane in this solution for 120 s, it is air-dried until there are no obvious water stains on the membrane surface, thus obtaining a polysulfone substrate membrane fully wetted by the aqueous solution. A 2 mg / mL solution of 1,3,5-trimethylammonium chloride is poured onto the surface of the wetted substrate membrane to carry out an interfacial polymerization reaction at a reaction temperature of 25°C and a relative humidity of 80% for 60 minutes. After 60 seconds, the membrane was removed, washed with hexane, and placed in an oven at 60°C for 1 minute to obtain a negatively charged porous nanofiltration membrane. Na3PO4 (0.1M) was added to a 5.0 mg / mL aqueous solution of polyethyleneimine to soak the negatively charged porous composite nanofiltration membrane for 120 seconds, and then air-dried until there were no obvious water stains on the membrane surface, resulting in a negatively charged substrate membrane fully wetted by the aqueous solution. A 2 mg / mL solution of 1,3,5-trimethylammonium chloride was poured onto the surface of the wetted negatively charged substrate membrane to carry out an interfacial polymerization reaction at 25°C and 80% relative humidity. After 60 seconds, the membrane was removed, washed with hexane, and placed in an oven at 60°C for 1 minute to obtain an adjustable dual-charge polyamide nanofiltration membrane with a positively charged upper layer and a negatively charged lower layer.

[0078] The dual-charge nanofiltration membrane prepared in this embodiment was tested using an aqueous solution cross-flow method at a temperature of 25°C. The membrane was first refluxed at 4 bar for 30 min, followed by testing at 2 bar, yielding a flux of 26.3 L·h. -1 ·m -2 ·bar -1 The following cross-flow tests were conducted using 1000 ppm Na₂SO₄ aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 95.1%; 1000 ppm MgSO₄ aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 95.7%; 1000 ppm MgCl₂ aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 97.2%; 1000 ppm CaCl₂ aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 96.1%; 1000 ppm NaCl aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 30.1%; and 1000 ppm LiCl aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 35.4%.

[0079] Cross-flow tests were performed on the nanofiltration membrane prepared in Example 3 at a temperature of 25°C. The membrane was first refluxed at 4 bar for 30 min, followed by testing at 2 bar. The flux was 82.3 L·h. -1 ·m -2 ·bar -1 The following cross-flow tests were conducted using 1000 ppm Na₂SO₄ aqueous solution at 25°C and 2 bar pressure, yielding a rejection rate of 96.1%; 1000 ppm MgSO₄ aqueous solution at 25°C and 2 bar pressure, yielding a rejection rate of 82.7%; 1000 ppm MgCl₂ aqueous solution at 25°C and 2 bar pressure, yielding a rejection rate of 20.3%; 1000 ppm CaCl₂ aqueous solution at 25°C and 2 bar pressure, yielding a rejection rate of 15.2%; 1000 ppm NaCl aqueous solution at 25°C and 2 bar pressure, yielding a rejection rate of 11.1%; and 1000 ppm LiCl aqueous solution at 25°C and 2 bar pressure, yielding a rejection rate of 15.6%.

[0080] Cross-flow tests were performed on the nanofiltration membrane prepared in Example 7 at a temperature of 25°C. The membrane was first refluxed at 4 bar for 30 min, followed by testing at 2 bar. The flux was 36.1 L·h. -1 ·m -2 ·bar -1 The following cross-flow tests were conducted using 1000 ppm Na₂SO₄ aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 36.1%; 1000 ppm MgSO₄ aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 43.5%; 1000 ppm MgCl₂ aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 97.3%; 1000 ppm CaCl₂ aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 96.9%; 1000 ppm NaCl aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 46.1%; and 1000 ppm LiCl aqueous solution at 25°C and 2 bar pressure, yielding a retention rate of 44.6%.

[0081] In conclusion, the following conclusions can be drawn:

[0082] The adjustable dual-charge polyamide nanofiltration membrane prepared by this invention exhibits a positively charged upper layer and a negatively charged lower layer, and the charge, pore size, uniformity, and thickness of the upper and lower polyamide materials are adjustable. The dual-charge polyamide nanofiltration membrane shows high rejection rates for Na₂SO₄, MgSO₄, MgCl₂, and CaCl₂.

Claims

1. A method for preparing an adjustable dual-charge polyamide nanofiltration membrane, characterized in that, Includes the following steps: 1) Prepare a diamine buffer / inorganic water phase solution containing buffer salts and / or inorganic salts, adjust the salt concentration to 0.005–0.5 M and the pH value to 6–10 to obtain an interfacial polymerization aqueous phase solution; wherein, the diamine is selected from at least one of piperazine, ethylenediamine, hexamethylenediamine, and thiourea, the buffer salt is selected from at least one of phosphate buffer systems, carbonate buffer systems, borate buffer systems, and citrate buffer systems, and the inorganic salt is selected from at least one of chloride systems, sulfate systems, fluoride systems, bromide systems, nitrate systems, silicate systems, tungstate systems, molybdate systems, permanganate systems, sulfite systems, and pyrophosphate systems; 2) Fully wet the experimental ultrafiltration base membrane with an interfacial polymerization aqueous solution, and air dry it in the air until there are no obvious water stains on the membrane surface, thus obtaining an ultrafiltration base membrane fully wetted with an aqueous solution. 3) Mix polyacryl chloride and organic solvent to prepare polyacryl chloride solution. Pour the polyacryl chloride solution onto the surface of the ultrafiltration base membrane to allow the polyacryl chloride solution to fully contact the aqueous diamine phase. After the reaction is complete, wash with organic solvent and anneal to form the first layer of negatively charged polyamide nanofiltration membrane. 4) Prepare a polyamine inorganic salt solution, adjusting the inorganic salt concentration to 0.005–0.5 M and the pH value to 6–10; wherein the polyamine is selected from at least one of polyallylamine hydrochloride, polydiallyldimethylammonium chloride, polypropylacryloyloxyethyltrimethylammonium chloride, poly-4-vinylpyridine, and polyethyleneimine, and the inorganic salt in the polyamine inorganic salt solution is selected from at least one of chloride system, sulfate system, fluoride system, bromide system, nitrate system, silicate system, tungstate system, molybdate system, permanganate system, sulfite system, and pyrophosphate system; 5) The negatively charged polyamide nanofiltration membrane is fully wetted by the aqueous solution of polyamine inorganic salt, and then air-dried until there are no obvious water stains on the membrane surface, thus obtaining a negatively charged bottom membrane that is fully wetted by the aqueous solution. 6) Mix polyacryl chloride and organic solvent to prepare polyacryl chloride solution. Pour the polyacryl chloride solution onto the negatively charged substrate membrane surface that is fully wetted by the aqueous solution, so that the polyacryl chloride solution can fully contact the aqueous polyamine monomer. After the reaction is complete, wash with organic solution to form a second layer of positively charged polyamide nanofiltration membrane. 7) The prepared polyamide nanofiltration membrane is annealed to obtain an adjustable double-charged polyamide nanofiltration membrane with a negative lower layer and a positive upper layer; In steps 3) and 7), the annealing treatment is carried out at a temperature of 20–80°C for 1–30 min; in steps 3) and 6), the diamine or polyamine monomer and the polyacrylamide chloride undergo interfacial polymerization at an interface temperature of 20–35°C and a relative humidity of 50–100% for 30–90 s.

2. The preparation method according to claim 1, characterized in that, The lower negatively charged nanofiltration membrane of the dual-charged polyamide nanofiltration membrane can exhibit adjustable negative charge and physical structure according to changes in the pH value, salt type and concentration of the diamine aqueous solution.

3. The preparation method according to claim 1, characterized in that, The upper positively charged nanofiltration membrane of the dual-charged polyamide nanofiltration membrane can exhibit adjustable positive charge and physical structure according to the changes in the type and concentration of inorganic salts in the polyamine aqueous solution.

4. The preparation method according to claim 1, characterized in that, In steps 3) and 6), the organic solvent is selected from at least one of hexaane, heptane, pentane, Isopar G, toluene, ethyl acetate and benzene.

5. The preparation method according to claim 1, characterized in that, In steps 3) and 6), the polyacryl chloride is selected from at least one of 1,3,5-pyromellitic chloride, terephthaloyl chloride, isophthaloyl chloride, azeloyl chloride, adipicoyl chloride, and 2,2',4,4'-biphenyltetracarboxylic chloride; the concentration of the polyacryl chloride in the polyacryl chloride solution is 0.25 to 7.5 mg / mL.

6. The preparation method according to claim 1, characterized in that, In step 2), the ultrafiltration base membrane material is selected from any one of polysulfone, polyethersulfone, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, nylon, cellulose or cellulose derivatives; the pore size of the ultrafiltration base membrane is 0.01 to 2 μm.

7. An adjustable dual-charge polyamide nanofiltration membrane prepared by any one of the preparation methods described in claims 1-6, characterized in that, The adjustable dual-charge polyamide nanofiltration membrane includes an ultrafiltration base membrane and a bilayer separation layer formed on the surface of the ultrafiltration base membrane. The upper layer of the bilayer separation layer exhibits positive charge, and the lower layer exhibits negative charge. The charge, pore size, uniformity, and thickness of the upper and lower polyamide materials are adjustable.

8. The application of the adjustable dual-charge polyamide nanofiltration membrane as described in claim 7 in water treatment, wherein the adjustable dual-charge polyamide nanofiltration membrane is applied to wastewater treatment or lithium extraction from salt lakes.