A high-flux high-salt-rejection polyamide nanofiltration membrane and a preparation method thereof

Abstract

This invention belongs to the field of membrane separation technology, specifically relating to a high-flux, high-salt polyamide nanofiltration membrane and its preparation method. The preparation method includes the following steps: S1, providing an aqueous phase solution and an oil phase solution, wherein the aqueous phase solution contains dissolved polyamines, surfactants, and hydroxyl-containing carboxylates; the oil phase solution contains dissolved polyacrylamide chlorides; S2, immersing a porous supporting membrane in the aqueous phase solution to remove excess aqueous phase solution from the surface of the porous supporting membrane, then contacting the porous supporting membrane with the oil phase solution to allow interfacial polymerization between the oil and aqueous phase solutions, obtaining a nascent composite membrane; S3, heat treatment; S4, sequentially immersing in an acid solution, pure water, an alkaline solution, and then pure water for post-treatment, obtaining the high-flux, high-salt polyamide nanofiltration membrane. This invention can improve the water permeation flux of polyamide nanofiltration membranes while simultaneously increasing their selectivity, solving the problem that existing nanofiltration membranes cannot synergistically improve water flux and retention rate, making it difficult to meet the requirements of high-demand separation applications.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology, specifically relating to a high-flux, high-salt polyamide nanofiltration membrane and its preparation method. Background Technology

[0002] Membrane separation technology, with its significant advantages such as high separation efficiency, simple operation, low energy consumption, and environmental friendliness, has been widely used in key areas such as industrial wastewater reuse, domestic sewage regeneration, deep drinking water purification, and seawater desalination. In pressure-driven membrane separation processes, based on differences in membrane pore size and molecular weight cutoff, they can be divided into four categories: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). Among them, nanofiltration membranes possess a unique pore size range of 0.5–2.0 nm and distinctive surface charge characteristics. Compared to ultrafiltration membranes, they offer higher precision in retaining monovalent / polyvalent ions; compared to reverse osmosis membranes, they can operate at lower pressures and selectively permeate monovalent salts, combining significant energy consumption advantages with process economy. Therefore, the development of high-performance nanofiltration membranes has become a research hotspot in the field of membrane materials science.

[0003] Currently, commercially available nanofiltration membranes are mainly thin-layer composite (TFC) structures, typically composed of a polyester nonwoven substrate, a polysulfone / polyethersulfone porous support layer, and a polyamide separation layer. The polyamide separation layer is the core structure determining the membrane's separation performance (permeability and selectivity). This separation layer is primarily prepared using interfacial polymerization (IP) technology, where aqueous polyamine monomers (such as piperazine) and organic polyacrylamide chloride monomers (such as trimesoyl chloride) undergo a condensation reaction at the interface of two immiscible phases. However, during this reaction, the polymerization of the acrylamide chloride monomer and the amine monomer competes with the hydrolysis of the acrylamide chloride itself, making it difficult for the prepared polyamide separation layer to simultaneously achieve high water flux and high rejection rate, resulting in a significant "permeability-selectivity" trade-off effect. This effect fundamentally limits further improvements in the performance of polyamide nanofiltration membranes and remains a core technological bottleneck that urgently needs to be addressed in this field.

[0004] To overcome these bottlenecks, researchers have conducted extensive studies on interfacial polymerization process control and membrane structure optimization. This includes introducing functional additives into the aqueous or organic phase solutions of the interfacial polymerization process, leveraging the physicochemical effects of these additives to in-situ regulate monomer diffusion and reaction kinetics, and optimizing the microstructure of the polyamide separation layer. This strategy can be directly integrated into existing production processes and has greater industrialization potential. For example, Chinese patent application CN112007525A discloses a method for preparing high-performance salt-separating nanofiltration membranes using the synergistic regulation of complexing agents and inorganic salts. While this method improves the membrane's separation performance to some extent, the improvement is still limited. The nanofiltration membrane prepared in the examples of this patent application has a maximum water flux of 166 LMH (test conditions 15.5 bar), but the water flux is only 10.7 LMH / bar. In more demanding separation applications, its performance still needs further improvement.

[0005] It is evident that the existing additive regulation technology still has limitations in improving membrane performance. The synergistic improvement in water flux and retention rate of the prepared nanofiltration membranes is insufficient, making it difficult to meet the needs of high-requirement separation applications. More efficient regulation strategies still need to be developed.

[0006] Therefore, developing a polyamide nanofiltration membrane preparation and control method that combines process simplicity and high performance, and achieving precise control of the polyamide separation layer structure, can fundamentally alleviate or even overcome the "permeability-selectivity" trade-off effect. This is of great practical significance for promoting the industrial application of high-performance nanofiltration membranes. Summary of the Invention

[0007] (a) Technical problems to be solved

[0008] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a high-flux, high-salt-separation polyamide nanofiltration membrane and its preparation method. The method involves adding specific additives to an aqueous solution and then immersing the membrane after heat treatment. This improves the water permeation flux of the polyamide nanofiltration membrane while enhancing its selectivity (increasing the rejection rate of divalent salts, decreasing the rejection rate of monovalent salts, and increasing the separation rate of monovalent and divalent salts). This solves the technical problem that existing nanofiltration membranes are unable to synergistically improve water flux and rejection rate and meet the requirements of high-demand separation applications.

[0009] (II) Technical Solution

[0010] In a first aspect, the present invention provides a method for preparing a high-flux, high-salt polyamide nanofiltration membrane, comprising:

[0011] S1. Provide an aqueous phase solution and an oil phase solution, wherein the aqueous phase solution contains dissolved polyamines, surfactants, and hydroxyl-containing carboxylates; and the oil phase solution contains dissolved polyacryl chlorides.

[0012] S2. Immerse the porous support base membrane in an aqueous solution to remove excess aqueous solution from the surface of the porous support base membrane. Then, bring the oil phase solution into contact with the porous support base membrane to allow the oil phase solution and the aqueous phase solution to undergo an interfacial polymerization reaction, thereby obtaining a nascent composite membrane.

[0013] S3. Heat treatment to obtain a heat-strengthened composite film;

[0014] S4. The membrane is sequentially immersed in acid solution, pure water, alkaline solution, and pure water for post-treatment to obtain the high-flux, high-salt polyamide nanofiltration membrane.

[0015] Optionally, in S1, the hydroxyl-containing carboxylate is at least one of sodium lactate, sodium glycolate, sodium hydroxyethylenediaminetriacetate, sodium salicylate, and sodium citrate, and its concentration in the aqueous solution is 0.01%-5wt%, preferably 0.1%-3wt.

[0016] Optionally, the surfactant is an anionic surfactant, a nonionic surfactant, or a cationic surfactant;

[0017] The anionic surfactant is selected from at least one of sodium dodecylbenzenesulfonate, sodium dodecyl sulfonate, and sodium dodecyl sulfate; the nonionic surfactant is Tween 20; and the cationic surfactant is selected from at least one of octaalkyltrimethylammonium chloride, octaalkyltrimethylammonium bromide, dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, and hexadecyltrimethylammonium bromide; the concentration of the surfactant in the aqueous solution is 0.01%-2wt%, preferably 0.1%-1wt.

[0018] Optionally, in S1, the polyamine is selected from at least one of piperazine, 2-methylpiperazine, 2,5-dimethylpiperazine, 2,6-dimethylpiperazine, 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, ethylenediamine, N,N-bis(2-aminoethyl)ethylenediamine, divinyltriamine, and polyethyleneimine; its concentration in the aqueous solution is 0.01%-2wt%; preferably 0.1-1wt%.

[0019] Optionally, in S1, the polyacryl chloride in the oil phase solution is selected from at least one of pyromellitic chloride, terephthaloyl chloride, phthaloyl chloride, pyromellitic tetracarboxylate chloride, malonyl chloride, glutaryl chloride and fumarate chloride; the concentration of the polyacryl chloride in the oil phase solution is 0.01%-2wt%; preferably 0.1%-1wt%.

[0020] Optionally, in S1, the organic solvent of the oil phase solution is one or a combination of two or more of n-hexane, cyclohexane, n-heptane, toluene, benzene, isopar G, isopar E, isopar H, isopar L and isopar M.

[0021] Optionally, in S2, the porous support base membrane is an ultrafiltration membrane with a molecular weight cutoff of 1kDa-50kDa, and the membrane material is at least one of polysulfone, polyethersulfone, polyvinylidene fluoride, polypropylene, polyacrylonitrile, polyethylene, polystyrene, polyvinyl chloride, polyphenylene ether, polyether ether ketone, and polyimide.

[0022] Optionally, the operation in S2 is as follows: immerse the porous support base membrane in an aqueous solution for 10s-10min, remove the excess aqueous solution from the surface of the porous support base membrane to obtain a wetted porous support base membrane, and then contact the oil phase solution with the wetted porous support base membrane for 10s-10min to undergo an interfacial polymerization reaction to obtain a nascent composite membrane.

[0023] Optionally, in S3, the heat treatment conditions are: treatment in an oven at 20-120℃ for 1-30 minutes; preferably, treatment in an oven at 40-100℃ for 3-10 minutes.

[0024] Optionally, in step S4, the acid solution is one of dilute sulfuric acid, dilute hydrochloric acid, and dilute nitric acid, and the concentration of the acid solution is 0.05%-0.5wt%; the alkaline solution is one of sodium hydroxide, potassium hydroxide, and lithium hydroxide, and the concentration of the alkaline solution is 0.05%-0.5wt%.

[0025] In S4, the immersion time of the heat-strengthened composite membrane in acid solution, pure water, alkaline solution, and pure water is 30s-15min, and the immersion temperature is 20-80℃; preferably, it is 1-5min and the temperature is 25-50℃.

[0026] Secondly, the present invention relates to nanofiltration membranes prepared by the above-described preparation method and their application in removing divalent salts from water or separating monovalent / divalent salts in water.

[0027] (III) Beneficial Effects

[0028] This invention achieves a simultaneous increase in both high water flux and high salt selectivity by simultaneously introducing surfactants and hydroxyl-containing carboxylates into the aqueous phase of interfacial polymerization, combined with heat treatment and immersion treatment in acid, water, and alkali solutions. The specific technical effects are as follows:

[0029] 1. This invention introduces both surfactants and hydroxyl-containing carboxylates into an aqueous solution, creating a synergistic regulatory effect during interfacial polymerization. The surfactant primarily functions in the interface and diffusion processes, reducing interfacial tension, promoting uniform and orderly diffusion of polyamines, inhibiting acyl chloride hydrolysis, reducing membrane defects, and improving crosslinking uniformity, thus providing a structurally uniform, defect-free, and highly selective basic framework for the membrane. The hydroxyl-containing carboxylate mainly affects membrane structure, hydrophilicity, and surface charge: the -OH group provided by the hydroxyl-containing carboxylate acts as a capping group for the interfacial polymerization reaction (the -OH reacts with acyl chlorides, "occupying" some of the acyl chloride and preventing it from reacting with amines, thereby reducing the degree of crosslinking and opening larger water channels), increasing the content of unreacted acyl chlorides, reducing the degree of membrane crosslinking, and increasing flux; the introduction of carboxyl groups enhances the negative charge on the membrane surface, strengthens the Donnan repulsion effect, and improves salt separation selectivity; simultaneously, it provides a weakly alkaline environment (strong acid-weak base salts are weakly alkaline), acting as an acid-binding agent to promote more complete interfacial polymerization. The surfactant in the aqueous solution is responsible for "making the reaction more regular and the membrane more uniform", while the hydroxycarboxylate is responsible for "making the channels more reasonable, the surface more hydrophilic, and the charge more suitable". By complementing each other in multiple dimensions such as reaction kinetics, crosslinking density, microstructure and surface properties, water flux and salt selectivity are improved simultaneously, breaking through the bottleneck of the mutual restriction between "permeability and selectivity" of traditional nanofiltration membranes.

[0030] 2. This invention achieves secondary enhancement synergy by adding hydroxyl-containing carboxylates and surfactants to an aqueous solution and simultaneously subjecting the heat-treated composite membrane to a combination of acid → pure water → alkali → pure water post-treatment. The hydroxyl carboxylates in the aqueous phase introduce a large number of carboxyl groups into the membrane, laying the foundation for high hydrophilicity and high electronegativity. Acid soaking can further remove residual impurities such as unreacted monomers and oligomers, completely dissolving and washing away unreacted monomers, oligomers, and residual metal ions, clearing water channels, and converting carboxylates into -COOH, further improving hydrophilicity and flux. Alkaline post-treatment can further convert residual acyl chloride groups and incompletely hydrolyzed functional groups in the membrane into carboxyl groups (alkaline solution causes alkaline hydrolysis of residual acyl chloride groups in the polyamide separation layer, converting them into -COOH, thereby further increasing the hydrophilicity and electronegativity of the membrane surface, strengthening the Donnan electrostatic repulsion effect, and improving the salt separation selectivity of the membrane), further enhancing the surface electronegativity and hydrophilicity. Finally, while maintaining high retention of MgSO4, the permeability to NaCl is significantly optimized, achieving high salt separation and high flux. In summary, aqueous phase modification constructs a high-performance membrane structure from the source, and acid-base post-treatment further purifies the structure and enhances surface charge and hydrophilicity, achieving a dual improvement of "structural optimization + surface function enhancement", so that the final nanofiltration membrane has high water flux, high retention stability and high salt selectivity.

[0031] In summary, this invention achieves full-chain synergistic optimization in terms of interfacial reaction, crosslinking structure, microstructure, surface hydrophilicity, and charge by synergistic regulation of surfactants and hydroxycarboxylate-containing salts in the aqueous phase, combined with heat treatment and acid-base sequential post-treatment. The resulting polyamide nanofiltration membrane possesses both high water flux and high salt selectivity, effectively solving the technical bottleneck of traditional interfacial polymerized nanofiltration membranes that cannot simultaneously achieve both permeability and selectivity. Attached Figure Description

[0032] Figure 1 This is a SEM image of the surface morphology of the polyamide nanofiltration membrane obtained in Example 1.

[0033] Figure 2 This is a SEM cross-sectional morphology image of the polyamide nanofiltration membrane obtained in Example 1. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0035] Unless otherwise stated, the experimental methods used in the embodiments of this invention are performed under conventional conditions in the art or under conditions recommended by the manufacturer; the reagents, materials, or equipment used, unless otherwise specified, are commercially available. Technical contents not described in detail in this specification are all prior art known to those skilled in the art.

[0036] The method for preparing high-flux, high-salt polyamide nanofiltration membrane provided by this invention is as follows:

[0037] S1. Provide aqueous phase solution and oil phase solution.

[0038] The aqueous solution is an aqueous solution containing 0.01%-2 wt% (preferably 0.1%-1 wt%) of a polyamine, 0.01%-2 wt% (0.1%-1 wt%) of a surfactant, and 0.01%-5 wt% (0.1-3 wt%) of a hydroxyl-containing carboxylate. The polyamine is selected from at least one of piperazine, 2-methylpiperazine, 2,5-dimethylpiperazine, 2,6-dimethylpiperazine, 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, ethylenediamine, N,N-bis(2-aminoethyl)ethylenediamine, divinyltriamine, and polyethyleneimine. The surfactant is anionic, nonionic, or cationic. The anionic surfactant is selected from at least one of sodium dodecylbenzenesulfonate, sodium dodecyl sulfonate, and sodium dodecyl sulfate. The nonionic surfactant is Tween 20 or Tween 80, etc., and the cationic surfactant is selected from at least one of octaalkyltrimethylammonium chloride, octaalkyltrimethylammonium bromide, dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, and hexadecyltrimethylammonium bromide. The hydroxyl-containing carboxylate is at least one of sodium lactate, sodium glycolate, sodium hydroxyethylenediaminetriacetate, sodium salicylate, and sodium citrate.

[0039] The oil phase solution is an organic solvent solution containing 0.01%-2wt% (preferably 0.1%-1wt%) of polyacryl chloride. The polyacryl chloride is selected from at least one of pyromellitic trimethylolpropionate chloride, terephthaloyl chloride, phthaloyl chloride, pyromellitic tetramethylolpropionate chloride, malonyl chloride, glutaryl chloride, and fumarate chloride. The organic solvent is one or a combination of two or more of n-hexane, cyclohexane, n-heptane, toluene, benzene, isopar G, isopar E, isopar H, isopar L, and isopar M.

[0040] S2, interfacial polymerization reaction.

[0041] A porous supporting membrane is immersed in an aqueous solution for 10-10 seconds to remove excess aqueous solution from its surface, resulting in a wetted porous supporting membrane. An oil-phase solution is then brought into contact with the wetted porous supporting membrane for 10-10 seconds to induce interfacial polymerization, yielding a nascent composite membrane. The porous supporting membrane is an ultrafiltration membrane with a molecular weight cutoff of 1-50 kDa, and its material is at least one selected from polysulfone, polyethersulfone, polyvinylidene fluoride, polypropylene, polyacrylonitrile, polyethylene, polystyrene, polyvinyl chloride, polyphenylene ether, polyetheretherketone, and polyimide.

[0042] S3, heat treatment.

[0043] The heat treatment conditions are as follows: treatment in an oven at 20-120℃ for 1-30 minutes, preferably in a hot oven at 40-100℃ for 3-10 minutes. After heat treatment, a heat-strengthened composite film is obtained.

[0044] S4, Post-processing.

[0045] The heat-strengthened composite membrane was sequentially immersed in a 0.05%-0.5wt% acid solution, pure water, a 0.05%-0.5wt% alkaline solution, and pure water to obtain the high-flux, high-salt polyamide nanofiltration membrane.

[0046] The acid solution is one of dilute sulfuric acid, dilute hydrochloric acid, and dilute nitric acid; the alkaline solution is one of sodium hydroxide, potassium hydroxide, and lithium hydroxide. The soaking time in the acid solution, pure water, alkaline solution, and pure water is 30s-15min, and the soaking temperature is 20-80℃, preferably 1-5min, and the temperature is 25-50℃.

[0047] The following description is based on preferred embodiments and comparative examples of the present invention.

[0048] Example 1

[0049] The method for preparing the polyamide nanofiltration membrane in this embodiment includes the following steps:

[0050] (1) Add 0.1 wt% piperazine, 0.2 wt% dodecyltrimethylammonium bromide, and 1 wt% sodium glycolate to water to prepare an aqueous solution. Add 0.1 wt% trimesoyl chloride to n-hexane to obtain an oil solution.

[0051] (2) After immersing the commercial polysulfone ultrafiltration membrane in an aqueous solution for 1 min, remove excess liquid from the surface of the immersed membrane; pour the organic phase solution onto the surface of the immersed membrane for 30 s to allow interfacial polymerization to occur, and obtain the nascent composite membrane.

[0052] (3) The nascent composite membrane was annealed in an 80℃ oven for 10 min to obtain a heat-strengthened composite membrane.

[0053] (4) The heat-strengthened composite membrane was sequentially immersed in 0.2wt% sulfuric acid solution, pure water, 0.2wt% sodium hydroxide solution and pure water at temperatures between 45-48℃ for 5 min each for post-treatment to obtain a high-flux high-salt polyamide nanofiltration membrane.

[0054] Microscopic morphology characterization of membrane products: The surface and cross-sectional morphology of the polyamide nanofiltration membrane prepared in Example 1 were characterized using scanning electron microscopy (SEM). The results are as follows: Figure 1 and Figure 2 As shown, the nanofiltration membrane surface forms a structurally complete, flat, highly uniform, and defect-free polyamide separation layer, and its cross-sectional structure shows that the separation layer is uniform and extremely thin. The bottom is a porous polysulfone supporting base membrane, and its surface is the polyamide separation layer, which is tightly bonded to the porous polysulfone supporting layer.

[0055] Example 2

[0056] The method for preparing the polyamide nanofiltration membrane in this embodiment includes the following steps:

[0057] (1) Add 0.5 wt% piperazine, 0.2 wt% dodecyltrimethylammonium bromide, and 1 wt% sodium glycolate to water to prepare an aqueous solution. Add 0.1 wt% trimesoyl chloride to n-hexane to obtain an oil solution.

[0058] For the remaining steps and conditions, please refer to Example 1.

[0059] Example 3

[0060] The method for preparing the polyamide nanofiltration membrane in this embodiment includes the following steps:

[0061] (1) An aqueous solution was prepared by adding 0.1 wt% 2-methylpiperazine, 0.2 wt% dodecyltrimethylammonium bromide, and 1 wt% sodium glycolate to water. An oil solution was prepared by adding 0.1 wt% trimesoyl chloride to n-hexane.

[0062] For the remaining steps and conditions, please refer to Example 1.

[0063] Example 4

[0064] The method for preparing the polyamide nanofiltration membrane in this embodiment includes the following steps:

[0065] (1) Add 0.1 wt% piperazine, 0.2 wt% sodium dodecylbenzenesulfonate, and 1 wt% sodium glycolate to water to prepare an aqueous solution. Add 0.1 wt% trimesoyl chloride to n-hexane to obtain an oil solution.

[0066] For the remaining steps and conditions, please refer to Example 1.

[0067] Example 5

[0068] The method for preparing the polyamide nanofiltration membrane in this embodiment includes the following steps:

[0069] (1) Add 0.1 wt% piperazine, 0.2 wt% dodecyltrimethylammonium bromide, and 1 wt% sodium hydroxyethylenediaminetriacetate to water to prepare an aqueous solution. Add 0.1 wt% trimesoyl chloride to n-hexane to obtain an oil solution.

[0070] For the remaining steps and conditions, please refer to Example 1.

[0071] Example 6

[0072] The method for preparing the polyamide nanofiltration membrane in this embodiment includes the following steps:

[0073] (1) Add 0.1 wt% piperazine, 0.2 wt% dodecyltrimethylammonium bromide, and 1 wt% sodium glycolate to water to prepare an aqueous solution. Add 0.5 wt% trimesoyl chloride to n-hexane to obtain an oil solution.

[0074] For the remaining steps and conditions, please refer to Example 1.

[0075] Example 7

[0076] The method for preparing the polyamide nanofiltration membrane in this embodiment includes the following steps:

[0077] (1) Add 0.1 wt% piperazine, 0.2 wt% dodecyltrimethylammonium bromide, and 0.5 wt% sodium lactate to water to prepare an aqueous solution. Add 0.1 wt% trimesoyl chloride to n-hexane to obtain an oil solution.

[0078] For the remaining steps and conditions, please refer to Example 1.

[0079] Example 8

[0080] The method for preparing the polyamide nanofiltration membrane in this embodiment includes the following steps:

[0081] (1) Add 0.1 wt% piperazine, 0.2 wt% Tween 20, and 2 wt% sodium salicylate to water to prepare an aqueous solution. Add 0.1 wt% trimesoyl chloride to n-hexane to obtain an oil solution.

[0082] For the remaining steps and conditions, please refer to Example 1.

[0083] Example 9

[0084] The method for preparing the polyamide nanofiltration membrane in this embodiment includes the following steps:

[0085] (1) Add 0.1 wt% piperazine, 0.2 wt% hexadecyltrimethylammonium bromide, and 3 wt% sodium citrate to water to prepare an aqueous solution. Add 0.1 wt% trimesoyl chloride to n-hexane to obtain an oil solution.

[0086] For the remaining steps and conditions, please refer to Example 1.

[0087] Example 10

[0088] The method for preparing the polyamide nanofiltration membrane in this embodiment includes the following steps:

[0089] (1) Add 0.1 wt% piperazine, 0.2 wt% hexadecyltrimethylammonium bromide, and 0.1 wt% sodium hydroxyethylenediaminetriacetate (lower concentration) to water to prepare an aqueous solution. Add 0.1 wt% trimesoyl chloride to n-hexane to obtain an oil solution.

[0090] For the remaining steps and conditions, please refer to Example 1.

[0091] Comparative Example 1

[0092] The only difference between this comparative example and Example 1 is that "dodecyltrimethylammonium bromide and sodium glycolate" were not added to the aqueous solution.

[0093] For the remaining steps and conditions, please refer to Example 1.

[0094] Comparative Example 2

[0095] The only difference between this comparative example and Example 1 is that sodium glycolate was not added to the aqueous solution.

[0096] For the remaining steps and conditions, please refer to Example 1.

[0097] Comparative Example 3

[0098] The only difference between this comparative example and Example 1 is that "dodecyltrimethylammonium bromide" was not added to the aqueous solution.

[0099] For the remaining steps and conditions, please refer to Example 1.

[0100] Comparative Example 4

[0101] The only difference between this comparative example and Example 1 is that step (4) is not performed after heat treatment, that is, the composite film heat-treated in step (3) is the final product.

[0102] Separation performance testing: Under room temperature conditions, the separation performance of the polyamide nanofiltration membranes prepared in Examples 1-10 and Comparative Examples 1-4 was evaluated using a cross-flow filtration mode. The test indicators included the membrane's water flux, divalent salt rejection rate, and mono / divalent salt separation efficiency. The test conditions were uniformly set as follows: feed solution was pure water or a 2000 ppm magnesium sulfate or sodium chloride aqueous solution; operating pressure (gauge pressure) was 0.5 MPa; feed solution temperature was 25°C; and feed solution pH was 7.0.

[0103] Membrane water flux (J, unit: L·m) -2 ·h -1 LMH (Less than LMH) is calculated using the following formula:

[0104] J = V / (A × t)

[0105] In the formula, V is the permeate volume (L), and A is the effective filtration area of ​​the membrane (m²). 2), where t is the collection time of the permeate (h).

[0106] The retention rate (R) is calculated using the following formula:

[0107] R = (1 - Cp / Cf) × 100%

[0108] In the formula, Cp and Cf are the concentrations (ppm) of the solute in the permeate and feed liquid, respectively.

[0109] The separation factor (α) is calculated using the following formula:

[0110] α = (1 - Ra) / (1 - Rb)

[0111] In the formula, Ra is the sodium chloride rejection rate and Rb is the magnesium sulfate rejection rate.

[0112] Detailed performance test data for the examples and comparative examples are listed in Table 1.

[0113] Table 1. Statistical table of nanofiltration membrane performance test results for Examples 1-10 and Comparative Examples 1-4

[0114]

[0115] Based on the performance test data in Table 1, the technical effects of the present invention are clearly verified. Specifically, compared with the comparative examples that lack certain technical features, the composite nanofiltration membranes prepared in the various embodiments of the present invention not only have significantly higher water flux but also excellent salt separation efficiency for monovalent and divalent salts. This demonstrates that the composite nanofiltration membrane preparation process provided by the present invention can synergistically and significantly improve the pure water flux and separation factor of polyamide nanofiltration membranes, resulting in polyamide nanofiltration membranes with both high flux and high salt separation performance.

[0116] 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 the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features. These modifications or substitutions, or combinations of technical features in the above embodiments that do not conflict with each other, can be made in accordance with the manner described in the embodiments. These modifications, substitutions or combinations do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a high-flux, high-salt polyamide nanofiltration membrane, characterized in that, Includes the following steps: S1. An aqueous phase solution and an oil phase solution are provided, wherein the aqueous phase solution contains a polyamine, a surfactant, and a hydroxyl-containing carboxylate; the oil phase solution contains a polyacrylamide chloride; wherein the polyamine is selected from at least one of piperazine, 2-methylpiperazine, 2,5-dimethylpiperazine, 2,6-dimethylpiperazine, 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, ethylenediamine, N,N-bis(2-aminoethyl)ethylenediamine, divinyltriamine, and polyethyleneimine; S2. Immerse the porous support base membrane in an aqueous solution to remove excess aqueous solution from the surface of the porous support base membrane. Then, bring the oil phase solution into contact with the porous support base membrane to allow the oil phase solution and the aqueous phase solution to undergo an interfacial polymerization reaction, thereby obtaining a nascent composite membrane. S3. Heat treatment to obtain a heat-strengthened composite film; S4. The membrane is sequentially immersed in acid solution, pure water, alkaline solution, and pure water for post-treatment to obtain the high-flux, high-salt polyamide nanofiltration membrane. The acid solution is one of dilute sulfuric acid, dilute hydrochloric acid, or dilute nitric acid, and the concentration of the acid solution is 0.05%-0.5wt%. The alkaline solution is one of sodium hydroxide, potassium hydroxide, and lithium hydroxide, and the concentration of the alkaline solution is 0.05%-0.5wt%. S4 includes: immersing the heat-strengthened composite membrane in acid solution, pure water, alkaline solution, and pure water for 30s-15min respectively, with an immersion temperature of 20-80℃.

2. The preparation method according to claim 1, characterized in that, In S1, the hydroxyl-containing carboxylate is at least one of sodium lactate, sodium glycolate, sodium hydroxyethylenediaminetriacetate, sodium salicylate, and sodium citrate, and its concentration in the aqueous solution is 0.01%-5wt%.

3. The preparation method according to claim 1, characterized in that, In S1, the surfactant is an anionic surfactant, a nonionic surfactant, or a cationic surfactant; the concentration of the surfactant in the aqueous solution is 0.01%-2wt%.

4. The preparation method according to claim 3, characterized in that, In S1, the anionic surfactant is selected from at least one of sodium dodecylbenzenesulfonate, sodium dodecyl sulfonate, and sodium dodecyl sulfate; the nonionic surfactant is Tween 20; and the cationic surfactant is selected from at least one of octaalkyltrimethylammonium chloride, octaalkyltrimethylammonium bromide, dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, and hexadecyltrimethylammonium bromide.

5. The preparation method according to claim 1, characterized in that, In S1, the concentration of polyamines in the aqueous solution is 0.01%-2wt%.

6. The preparation method according to claim 1, characterized in that, In S1, the polyacryl chloride in the oil phase solution is selected from at least one of pyromellitic methyl methacrylate (PMMA), terephthaloyl chloride (TBMA), phthaloyl chloride (DMA), pyromellitic methyl methacrylate (TMM), malonyl chloride (MA), glutaryl chloride (GA), and fumarate chloride (FMA); the concentration of the polyacryl chloride in the oil phase solution is 0.01%-2 wt%. The organic solvent of the oil phase solution is one or a combination of two or more of the following: n-hexane, cyclohexane, n-heptane, toluene, benzene, isopar G, isopar E, isopar H, isopar L, and isopar M.

7. The preparation method according to claim 1, characterized in that, In S2, the porous support base membrane is an ultrafiltration membrane with a molecular weight cutoff of 1kDa-50kDa, and the membrane material is at least one of polysulfone, polyethersulfone, polyvinylidene fluoride, polypropylene, polyacrylonitrile, polyethylene, polystyrene, polyvinyl chloride, polyphenylene ether, polyether ether ketone and polyimide.

8. The preparation method according to claim 1, characterized in that, The operation in S2 is as follows: immerse the porous support base membrane in an aqueous solution for 10s-10min, remove the excess aqueous solution from the surface of the porous support base membrane to obtain a wetted porous support base membrane, and then contact the oil phase solution with the wetted porous support base membrane for 10s-10min to undergo an interfacial polymerization reaction to obtain a nascent composite membrane. In S3, the heat treatment conditions are: treatment in an oven at 20-120℃ for 1-30 minutes.

9. A nanofiltration membrane, characterized in that, Prepared using the preparation method described in any one of claims 1-8.

10. The application of the nanofiltration membrane according to claim 9 in removing divalent salts from water or in the separation of monovalent / divalent salts in water.

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