Preparation method of polyamide nanofiltration membrane and prepared polyamide nanofiltration membrane

Abstract

The invention discloses a preparation method of a polyamide nanofiltration membrane and the prepared polyamide nanofiltration membrane, and relates to the field of nanofiltration membranes, the preparation method comprises the following steps: a) providing an amine monomer and an aldehyde group monomer, uniformly mixing the amine monomer, the aldehyde group monomer and water, and carrying out reversible reaction to obtain a nano colloidal solution; b) providing a nanofiltration base membrane, and depositing the nano colloidal solution on the nanofiltration base membrane to obtain a membrane sheet; c) providing an acyl chloride compound, and dissolving the acyl chloride compound in an organic solvent to obtain an organic phase solution; d) pouring the organic phase solution into the membrane to carry out interfacial polymerization reaction so as to form a polyamide separation layer on the membrane; and e) carrying out heat treatment on the membrane with the polyamide separation layer to obtain the polyamide nanofiltration membrane. The polyamide nanofiltration membrane prepared by the invention has high permeation flux and high rejection rate.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane technology, and specifically to a method for preparing a polyamide nanofiltration membrane and the polyamide nanofiltration membrane obtained therefrom. Background Technology

[0002] In the field of wastewater treatment and desalination, the separation and purification of high-salinity wastewater is one of the most pressing environmental problems that need to be solved. Polyamide nanofiltration membranes, due to their economic efficiency, high cost, and environmental friendliness, have been widely used in drinking water purification, wastewater reuse, and seawater desalination. The performance of polyamide nanofiltration membranes is mainly determined by the polyamide separation layer; therefore, optimizing its structure to improve permeability and desalination rate is of great significance.

[0003] Currently, polyamide separation layers are mainly prepared through interfacial polymerization, where amine monomers in the aqueous phase and acyl chlorides in the organic phase undergo a polycondensation reaction at the interface to form a polyamide network. However, in traditional interfacial polymerization, the rapid diffusion and uncontrolled reaction of monomers can easily lead to uneven polyamide layer structure, manifested as wide pore size distribution and inconsistent crosslinking degree, which in turn affects the separation performance of polyamide nanofiltration membranes.

[0004] In existing technologies, although the polyamide layer structure can be optimized to some extent by adjusting parameters such as monomer concentration and reaction time, it is still difficult to achieve precise control of the diffusion rate and spatial distribution of amine monomers. This makes it difficult for existing polyamide nanofiltration membranes to balance permeation flux and retention rate, thus limiting their application effect in the treatment of high-salt wastewater. Summary of the Invention

[0005] This invention aims to address, to a certain extent, one of the technical problems in related technologies. To this end, this invention provides a method for preparing a polyamide nanofiltration membrane and the resulting polyamide nanofiltration membrane, which possesses both high permeation flux and high rejection rate.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a polyamide nanofiltration membrane includes the following steps: a) Provide an amine monomer and an aldehyde monomer, mix the amine monomer, the aldehyde monomer and water evenly and carry out a reversible reaction to obtain a nanocolloid solution; b) Provide a nanofiltration base membrane, and deposit the nanocolloid solution onto the nanofiltration base membrane to obtain a membrane sheet; c) Provide an acyl chloride compound, and dissolve the acyl chloride compound in an organic solvent to obtain an organic phase solution; d) Pour the organic phase solution into the membrane to carry out an interfacial polymerization reaction to form a polyamide separation layer on the membrane; e) The membrane having the polyamide separation layer is heat-treated to obtain the polyamide nanofiltration membrane.

[0007] In this application, amine monomers, aldehyde monomers, and water are mixed and subjected to a reversible reaction to obtain a nanocolloidal solution. The amine monomers and aldehyde monomers form reversibly decomposing oligomers. Compared with the prior art of directly using amine monomers and acyl chloride compounds for interfacial polymerization, the amine monomers and oligomers in the nanocolloidal solution of this application are more uniformly distributed, which is beneficial for forming a more uniform polyamide layer in subsequent reactions and avoids structural defects caused by rapid monomer diffusion and local uneven concentration in traditional interfacial polymerization. Furthermore, the aldehyde monomer, as a coupling agent, reacts with the amine monomer to form oligomers, ensuring uniform distribution and enrichment of the amine monomer at the interface. The hydrochloric acid generated in situ by the gradual reaction of the amine monomer and acyl chloride compound can promote the release of amine monomers in the oligomers. That is, as the concentration of hydrochloric acid gradually increases, the amine monomers are gradually released into the organic phase to participate in the interfacial polymerization reaction, realizing the slow diffusion of amine monomers at the interface and avoiding uneven cross-linking of the polyamide layer caused by rapid diffusion of amine monomers. The uniform degree of cross-linking gives the polyamide layer a certain degree of density to block salt ions, but it will not hinder the passage of water molecules due to excessive cross-linking. Therefore, the polyamide nanofiltration membrane has both high permeation flux and high rejection rate.

[0008] In addition, aldehyde monomers can also act as slow-release agents to control the diffusion rate of amine monomers, so that the slowly released amine monomers can diffuse into the organic phase at a relatively uniform rate to undergo polymerization reactions with acyl chloride compounds.

[0009] Optionally, in step a), the amine monomer is one or more of piperazine monomers, and the aldehyde monomer is one of 2,5-dihydroxyterephthalaldehyde, 2,4,6-trialdehyde phloroglucinol, or pyromellitic terephthalaldehyde.

[0010] Piperazine monomers contain two amino groups, while 2,5-dihydroxyterephthalaldehyde, 2,4,6-trialdehyde phloroglucinol, and pyromellitic terephthalaldehyde contain multiple aldehyde groups. Therefore, they all exhibit high reactivity. Taking piperazine and 2,5-dihydroxyterephthalaldehyde as an example, during the reaction, the amino group of piperazine reacts with one aldehyde group of 2,5-dihydroxyterephthalaldehyde to form an imine bond. Then, the other amino group can continue to react with other aldehyde groups, gradually forming oligomers. These oligomers form nanocolloidal particles in solution.

[0011] Optionally, in step a), the concentration of the amine monomer in the nanocolloid solution is 0.2wt%~1.0wt%, and the molar ratio of the amine monomer to the aldehyde monomer is 3:(1~2).

[0012] When the concentration of the amine monomer in the nanocolloid solution is 0.2wt%~1.0wt%, sufficient amine monomer in the nanocolloid is available to participate in the subsequent reaction with the acyl chloride compound, which is beneficial for forming a polyamide separation layer with uniform thickness and dense structure. By controlling the molar ratio of the feed, the reaction process of the amine monomer and the aldehyde monomer to form oligomers can be regulated, and the desired amount of oligomer can be obtained. When the molar ratio of the amine monomer to the aldehyde monomer is 3:(1~2), a polyamide separation layer with suitable crosslinking degree and porosity can be formed, which is beneficial for the rapid passage of water molecules and the retention of solute molecules.

[0013] Optionally, in step a), the conditions for the reversible reaction are: heating the mixture of amine monomer, aldehyde monomer and water in a water bath at 80°C, and continuously stirring the mixture for 30 min during the reaction.

[0014] Heating in a water bath at 80°C provides a suitable temperature environment for the reaction. Continuous stirring for 30 minutes ensures thorough mixing of the amine and aldehyde monomers in the reaction system, preventing local concentrations from becoming too high or too low in the nanocolloid solution.

[0015] Optionally, in step b), after depositing the nanocolloid solution onto the nanofiltration membrane, the process further includes: air-drying the nanofiltration membrane with the deposited nanocolloid solution for 2 minutes.

[0016] After 2 minutes of air drying, the water in the nanocolloid solution will evaporate naturally. Compared to rapid drying methods, such as high-temperature drying, air drying can prevent the nanocolloid particles from flowing and agglomerating when water evaporates rapidly, thus avoiding uneven distribution on the nanofiltration membrane surface and the occurrence of localized areas that are too thick or too thin.

[0017] Optionally, in step b), the nanofiltration membrane is a polyacrylonitrile membrane, and the deposition step employs a vacuum-assisted self-assembly method or a gas-liquid-liquid surface self-assembly method.

[0018] Polyacrylonitrile (PAC) membranes possess excellent stability and mechanical properties. Furthermore, PAC membranes can be prepared using various methods (such as phase inversion) to form porous structures with specific pore structures and pore size distributions, providing a support platform for nanoparticle deposition.

[0019] Optionally, in step c), the acyl chloride compound is pyromellitic acid trimethylolpropionate (PPST), and the organic solvent is n-hexane.

[0020] The acyl chloride compound is trimesoyl chloride. The three acyl chloride groups in the trimesoyl chloride molecule have extremely high reactivity, which is beneficial for improving production efficiency and shortening the nanofiltration membrane preparation cycle. Furthermore, trimesoyl chloride can react with multiple amine monomers in interfacial polymerization to form a three-dimensional network cross-linked structure, improving the retention performance of the nanofiltration membrane. Hexane has low toxicity; using hexane can reduce harm to the health of operators and is also beneficial to environmental protection. In other embodiments, fluorinated hydrocarbon solvents can also be used, as long as the selected solvent is non-toxic or slightly toxic.

[0021] Optionally, in step d), the conditions for the interfacial polymerization reaction are: a reaction time of 30~120s and a reaction temperature of 25℃.

[0022] The reaction time is 30–120 s, a range that matches the sustained-release rate of the amine monomer in the nanocolloid. Within this time range, the nanocolloid can stably release an appropriate amount of amine monomer, ensuring that the amine monomer maintains a suitable concentration throughout the interfacial polymerization reaction. At a mild temperature of 25°C, side reactions can be suppressed. For some acyl chloride compounds (such as trimesoyl chloride), high temperatures may cause hydrolysis, generating byproducts such as carboxylic acids and hydrogen chloride. These byproducts not only consume reactants and reduce the yield of the main reaction but may also adversely affect the structure and properties of the resulting polyamide active layer. At 25°C, the hydrolysis rate is very slow, ensuring that the interfacial polymerization reaction proceeds in the intended direction, i.e., only generating the polyamide structure.

[0023] Optionally, in step e), the heat treatment conditions are: a treatment temperature of 60°C and a treatment time of 3 min.

[0024] Heat treatment provides energy to unreacted functional groups, enabling them to continue reacting and making the polyamide nanofiltration membrane structure more stable. In addition, heat treatment can eliminate internal stress in the prepared polyamide nanofiltration membrane, reducing defects caused by stress concentration, such as cracks and pores.

[0025] Furthermore, the present invention also provides a polyamide nanofiltration membrane, which includes a nanofiltration base membrane and a polyamide separation layer formed on the nanofiltration base membrane, and the polyamide nanofiltration membrane is manufactured by the aforementioned method for preparing polyamide nanofiltration membranes. The reasoning process for the beneficial effects of the polyamide nanofiltration membrane provided by the present invention and the aforementioned method for preparing polyamide nanofiltration membranes is similar, and will not be repeated here.

[0026] These features and advantages of the present invention will be disclosed in detail in the following specific embodiments and accompanying drawings. The preferred embodiments or means of the present invention will be shown in detail in conjunction with the accompanying drawings, but are not intended to limit the technical solutions of the present invention. In addition, each of these features, elements and components appearing in the following text and drawings is a plurality of, and different symbols or numbers are used for convenience of representation, but all represent parts with the same or similar construction or function. Attached Figure Description

[0027] The present invention will be further described below with reference to the accompanying drawings: Figure 1 A scanning electron microscope (SEM) image of the polyamide nanofiltration membrane prepared in Example 2; Figure 2 The image shows a scanning electron microscope cross-section of the polyamide nanofiltration membrane prepared in Example 2. Detailed Implementation

[0028] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described are intended to explain the present invention and should not be construed as limiting the invention.

[0029] The terms "an embodiment," "example," or "trademark" used in this specification refer to a particular feature, structure, or characteristic described in connection with the embodiment itself that may be included in at least one embodiment disclosed in this patent. The phrase "in an embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment.

[0030] A method for preparing a polyamide nanofiltration membrane includes the following steps: a) Provide an amine monomer and an aldehyde monomer, mix the amine monomer, the aldehyde monomer and water evenly and carry out a reversible reaction to obtain a nanocolloid solution; b) Provide a nanofiltration base membrane, and deposit the nanocolloid solution onto the nanofiltration base membrane to obtain a membrane sheet; c) Provide an acyl chloride compound, and dissolve the acyl chloride compound in an organic solvent to obtain an organic phase solution; d) Pour the organic phase solution into the membrane to carry out an interfacial polymerization reaction to form a polyamide separation layer on the membrane; e) The membrane having the polyamide separation layer is heat-treated to obtain the polyamide nanofiltration membrane.

[0031] Amine monomers and aldehyde monomers can undergo a reversible condensation reaction in aqueous solution. The aldehyde monomer acts as a chemical coupling agent, linking multiple amine monomers into oligomers, ensuring a uniform distribution of amine monomers at the nanofiltration membrane interface (avoiding aggregation). This facilitates the formation of a more uniform polyamide layer in subsequent reactions, avoiding structural defects caused by rapid diffusion and localized uneven concentrations of amine monomers in traditional interfacial polymerization. Acyl chloride compounds can dissolve in organic solvents to form a homogeneous organic phase solution. The organic solvent needs to provide a favorable dissolution environment for the acyl chloride compound to facilitate the interfacial polymerization reaction. When the organic phase solution is poured onto a membrane with a deposited nanocolloid solution, the amine monomers in the nanocolloids and the acyl chloride compounds in the organic phase undergo interfacial polymerization to generate polyamide. The oligomers can undergo reversible decomposition under certain conditions and release amine monomers, thereby controlling the diffusion rate of amine monomers and avoiding uneven polyamide layer caused by rapid diffusion of amine monomers, ultimately forming a uniform and dense polyamide separation layer. In addition, heat treatment can make the molecular chains of the polyamide separation layer more orderly, reduce defects in the separation layer, reduce the resistance of water molecules to passing through the membrane, and maintain a high porosity, which is beneficial to improving the membrane's permeability.

[0032] The mechanism of interfacial polymerization is as follows: The prepared nanocolloid solution should include amine monomers, aldehyde monomers, and oligomers of both. When the nanocolloid comes into contact with the organic phase solution, the amine monomers in the nanocolloid and the acyl chloride compounds in the organic phase begin to undergo interfacial polymerization. In the initial stage of the reaction, only a small amount of amine monomers and acyl chloride compounds near the interface react rapidly. This reaction generates polyamide and also produces hydrochloric acid in situ (acyl chloride reacts with amine to generate hydrogen chloride, which dissolves in water to form hydrochloric acid). As the reaction proceeds, the concentration of hydrochloric acid produced in situ gradually increases. As an acidic substance, hydrochloric acid affects the oligomers encapsulating the amine monomers. The oligomers undergo structural changes in an acidic environment, such as a weakening of the interactions between their molecular chains, resulting in a relatively loose structure. Furthermore, due to the loose structure of the oligomers, the originally encapsulated amine monomers can be released. Moreover, the increase in hydrochloric acid concentration is gradual, and a large amount of amine monomers will not be released in a short time. The amine monomers are gradually released and participate in the interfacial polymerization reaction, thus achieving the goal of controlling the diffusion rate of the amine monomers. This avoids the uneven cross-linking degree of the polyamide layer caused by the rapid diffusion of amine monomers. The uniform cross-linking degree gives the polyamide layer a certain degree of density to block salt ions, while preventing excessive cross-linking from hindering the passage of water molecules. Therefore, the polyamide nanofiltration membrane produced has both high permeation flux and high rejection rate.

[0033] In summary, by triggering the gradual release of amine monomers from oligomers using hydrochloric acid, the concentration and diffusion rate of amine monomers at the interface can be controlled, allowing the amine monomers to diffuse into the organic phase at a relatively stable and uniform rate to react with acyl chloride compounds.

[0034] In step a), the amine monomer is one or more of piperazine monomers, and the aldehyde monomer is one of 2,5-dihydroxyterephthalaldehyde, 2,4,6-trialdehyde phloroglucinol, or pyromellitic pyrrolizaldehyde.

[0035] In this application, piperazine monomers contain two amino groups, while 2,5-dihydroxyterephthalaldehyde, 2,4,6-trialdehyde phloroglucinol, and pyromellitic terephthalaldehyde contain multiple aldehyde groups. Therefore, they all exhibit high reactivity. Taking piperazine and 2,5-dihydroxyterephthalaldehyde as an example, during the reaction, the amino group of piperazine reacts with one aldehyde group of 2,5-dihydroxyterephthalaldehyde to form an imine bond. Then, the other amino group can continue to react with other aldehyde groups, gradually forming oligomers. The resulting oligomers form nanocolloidal particles in solution.

[0036] In step a), the concentration of the amine monomer in the nanocolloid solution is 0.2wt%~1.0wt%, and the molar ratio of the amine monomer to the aldehyde monomer is 3:(1~2).

[0037] In this application, when the concentration of the amine monomer in the nanocolloid solution is 0.2wt%~1.0wt%, sufficient amine monomer in the nanocolloid is available to participate in the subsequent reaction with the acyl chloride compound, which is beneficial for forming a polyamide separation layer with uniform thickness and dense structure. By controlling the molar ratio of the feed, the reaction process of the amine monomer and the aldehyde monomer to form oligomers can be regulated, and the desired amount of oligomer can be obtained. When the molar ratio of the amine monomer to the aldehyde monomer is 3:(1~2), a polyamide separation layer with suitable crosslinking degree and porosity can be formed, which is beneficial for achieving rapid passage of water molecules and retention of solute molecules.

[0038] Specifically, when the molar ratio of amine monomer to aldehyde monomer is less than 3:2, there is a relative deficiency of amine monomer, resulting in excessive aldehyde monomer residue after the reaction. This residual aldehyde monomer will undergo side reactions with acyl chloride compounds in subsequent interfacial polymerization reactions, interfering with the normal polyamide formation process and leading to an uneven polyamide separation layer structure. When the molar ratio of amine monomer to aldehyde monomer is greater than 3:1, although a relative excess of amine monomer can ensure complete reaction of the aldehyde monomer, the excess amine monomer may lead to an excessively high amino content in the polyamide separation layer, resulting in an excessive positive charge on the surface of the prepared polyamide nanofiltration membrane, affecting its separation performance.

[0039] In step a), the conditions for the reversible reaction are: heating the mixture of amine monomer, aldehyde monomer and water in a water bath at 80°C, and continuously stirring the mixture for 30 min during the reaction.

[0040] In this application, a water bath heating at 80°C provides a suitable temperature environment for the reaction. Continuous stirring for 30 minutes ensures thorough mixing of the amine and aldehyde monomers in the reaction system, preventing excessively high or low local concentrations in the nanocolloid solution.

[0041] After depositing the nanocolloid solution onto the nanofiltration membrane, the process further includes: air-drying the nanofiltration membrane with the deposited nanocolloid solution for 2 minutes.

[0042] In this application, after 2 minutes of air drying, the water in the nanocolloid solution will evaporate naturally. Compared with rapid drying methods, such as high-temperature drying, air drying can prevent the nanocolloid particles from flowing and agglomerating when water evaporates rapidly, thus avoiding uneven distribution on the nanofiltration membrane surface and the occurrence of localized excessively thick or thin areas.

[0043] In step b), the nanofiltration membrane is a polyacrylonitrile membrane, and the deposition step employs a vacuum-assisted self-assembly method or a gas-liquid-liquid surface self-assembly method.

[0044] In this application, the polyacrylonitrile membrane exhibits good stability and mechanical properties. Furthermore, the polyacrylonitrile membrane can be prepared using various methods (such as phase inversion) to form a porous structure with specific pore structures and pore size distributions, providing a support platform for nanoparticle deposition. Vacuum-assisted self-assembly involves using the negative pressure generated by a vacuum to adsorb nanoparticles from a colloidal solution onto the surface of the polyacrylonitrile membrane. Specifically, the polyacrylonitrile membrane is placed in a vacuum device, and then the colloidal solution is uniformly applied to the surface of the polyacrylonitrile membrane, forming a uniform stacked structure. Then, a vacuum pump is activated, creating a negative pressure below the polyacrylonitrile membrane. Under this negative pressure, water and nanoscale particles (amine monomers, aldehyde monomers, and their oligomers) in the colloidal solution move towards the surface of the polyacrylonitrile membrane and deposit thereon. Vacuum-assisted self-assembly is preferred in this application. In other applications, the deposition step can also employ a gas-liquid-liquid surface self-assembly method. Specifically, the polyacrylonitrile membrane is immersed in the colloidal solution, and then another immiscible liquid (such as oil) is introduced onto the solution surface to form a gas-liquid-liquid surface. Nanoparticles will arrange themselves in the contact area between the gas-liquid-liquid surface and the polyacrylonitrile film under the influence of surface tension of the three phases of gas, liquid and liquid, thereby achieving deposition.

[0045] In step c), the acyl chloride compound is pyromellitic acid chloride, and the organic solvent is n-hexane.

[0046] In this application, the acyl chloride compound is trimesoyl chloride. The three acyl chloride groups in the trimesoyl chloride molecule possess extremely high reactivity, which is beneficial for improving production efficiency and shortening the nanofiltration membrane preparation cycle. Furthermore, trimesoyl chloride can react with multiple amine monomers in interfacial polymerization to form a three-dimensional network cross-linked structure, improving the retention performance of the nanofiltration membrane. Hexane has low toxicity; using hexane reduces harm to operator health and is also beneficial for environmental protection. In other applications, fluorinated hydrocarbon solvents can also be used, as long as the selected solvent is non-toxic or slightly toxic.

[0047] In step d), the conditions for the interfacial polymerization reaction are: reaction time of 30~120s and reaction temperature of 25℃.

[0048] In this application, the reaction time is 30–120 s, a range that matches the sustained-release rate of the amine monomer in the nanocolloid. Within this time range, the nanocolloid can stably release an appropriate amount of amine monomer, ensuring that the amine monomer maintains a suitable concentration throughout the interfacial polymerization reaction. Under mild conditions of 25°C, side reactions can be suppressed. For some acyl chloride compounds (such as trimesoyl chloride), high temperatures may cause hydrolysis, generating byproducts such as carboxylic acids and hydrogen chloride. These byproducts not only consume reactants and reduce the yield of the main reaction but may also adversely affect the structure and properties of the resulting polyamide active layer. At 25°C, the hydrolysis rate is very slow, ensuring that the interfacial polymerization reaction proceeds in the intended direction, i.e., only generating the polyamide structure.

[0049] In step e), the heat treatment conditions are: a treatment temperature of 60°C and a treatment time of 3 minutes.

[0050] In this application, some unreacted functional groups exist during the interfacial polymerization process to form the active layer of the nanofiltration membrane. Heat treatment can provide energy to these unreacted functional groups, enabling them to continue reacting and making the polyamide nanofiltration membrane structure more stable. In addition, heat treatment can also eliminate the internal stress of the prepared polyamide nanofiltration membrane and reduce defects caused by stress concentration, such as cracks and pores.

[0051] In addition, this application also provides a polyamide nanofiltration membrane, which includes a nanofiltration base membrane and a polyamide separation layer formed on the nanofiltration base membrane, and the polyamide nanofiltration membrane is made by the aforementioned method for preparing polyamide nanofiltration membranes.

[0052] Example 1: 0.2 g of piperazine monomer (PIP) and 0.16 g of trialdehyde resorcinol (TP) were weighed and added to 50 ml of water to make the molar ratio of piperazine monomer to trialdehyde resorcinol 3:1. The mixture of piperazine monomer, trialdehyde resorcinol and water was heated in a water bath at 80 °C, and the mixture was continuously stirred for 30 min during the reaction to prepare a nanocolloidal solution containing PIP-TP oligomers. 5 ml of the nanocolloidal solution was deposited onto a polyacrylonitrile membrane using a vacuum-assisted self-assembly method and air-dried for 2 min. 0.037 g of trimesoyl chloride (TMC) was weighed and added to 50 ml of n-hexane. 10 ml of the hexane solution containing TMC was poured onto the polyacrylonitrile membrane and reacted for 60 s, forming a polyamide separation layer at the interface. The polyacrylonitrile membrane with the polyamide separation layer was placed in a forced-air drying oven and heat-treated at 60 °C for 3 min to obtain a polyamide nanofiltration membrane.

[0053] Example 2: This embodiment prepared as follows: Figure 1 and Figure 2 The polyamide nanofiltration membrane shown in this embodiment differs from that in Example 1 in that: 0.2g of piperazine monomer (PIP) and 0.32g of trialdehyde phloroglucinol (TP) are weighed and placed in 50ml of water to make the molar ratio of piperazine monomer to trialdehyde phloroglucinol 3:2. The remaining steps in this embodiment are basically the same as those in Example 1, and will not be repeated here.

[0054] Example 3: 0.1 g of piperazine monomer (PIP) and 0.08 g of trialdehyde resorcinol (TP) were weighed and added to 50 ml of water to make the mass fraction of PIP in water 0.2 wt%, and the molar ratio of piperazine monomer to trialdehyde resorcinol 3:1. The mixture of piperazine monomer, trialdehyde resorcinol and water was heated in a water bath at 80 °C, and the mixture was continuously stirred for 30 min during the reaction to obtain a nanocolloidal solution containing PIP-TP oligomers. 5 ml of the nanocolloidal solution was deposited onto a polyacrylonitrile membrane by vacuum-assisted self-assembly and air-dried for 2 min. 0.037 g of trimesoyl chloride (TMC) was weighed and added to 50 ml of n-hexane. 10 ml of the hexane solution containing TMC was poured onto the polyacrylonitrile membrane and reacted for 60 s, forming a polyamide separation layer at the interface. A polyacrylonitrile membrane with a polyamide separation layer is placed in a forced-air drying oven and heat-treated at a constant temperature of 60°C for 3 minutes to obtain a polyamide nanofiltration membrane.

[0055] Example 4: The difference between this embodiment and Embodiment 3 is that 0.2g of piperazine monomer (PIP) is weighed and placed in 50ml of water. The remaining steps in this embodiment are basically the same as in Embodiment 3, and will not be repeated here.

[0056] Example 5: The difference between this embodiment and Embodiment 3 is that 0.3g of piperazine monomer (PIP) is weighed and placed in 50ml of water. The remaining steps in this embodiment are basically the same as in Embodiment 3, and will not be repeated here.

[0057] Example 6: The difference between this embodiment and Embodiment 3 is that 0.4g of piperazine monomer (PIP) is weighed and placed in 50ml of water. The remaining steps in this embodiment are basically the same as in Embodiment 3, and will not be repeated here.

[0058] Example 7: The difference between this embodiment and Embodiment 3 is that 0.5g of piperazine monomer (PIP) is weighed and placed in 50ml of water. The remaining steps in this embodiment are basically the same as in Embodiment 3, and will not be repeated here.

[0059] Example 8: 0.2 g of piperazine monomer (PIP) and 0.16 g of trialdehyde resorcinol (TP) were weighed and added to 50 ml of water to make the molar ratio of piperazine monomer to trialdehyde resorcinol 3:1. The mixture of piperazine monomer, trialdehyde resorcinol and water was heated in a water bath at 80 °C, and the mixture was continuously stirred for 30 min during the reaction to obtain a nanocolloidal solution containing PIP-TP oligomers. 5 ml of the nanocolloidal solution was deposited onto a polyacrylonitrile membrane using a vacuum-assisted self-assembly method and air-dried for 2 min. 0.037 g of trimesoyl chloride (TMC) was weighed and added to 50 ml of n-hexane. 10 ml of the hexane solution containing TMC was poured onto the polyacrylonitrile membrane and reacted for 30 s, forming a polyamide separation layer at the interface. The polyacrylonitrile membrane with the polyamide separation layer was placed in a forced-air drying oven and heat-treated at 60 °C for 3 min to obtain a polyamide nanofiltration membrane.

[0060] Example 9: The difference between this embodiment and Example 8 is that 10 ml of hexane solution containing TMC is poured onto a polyacrylonitrile membrane and reacted for 60 seconds. The remaining steps in this embodiment are basically the same as in Example 8, and will not be repeated here.

[0061] Example 10: The difference between this embodiment and Example 8 is that 10 ml of hexane solution containing TMC is poured onto a polyacrylonitrile membrane and reacted for 90 seconds. The remaining steps in this embodiment are basically the same as in Example 8, and will not be repeated here.

[0062] Example 11: The difference between this embodiment and Example 8 is that 10 ml of hexane solution containing TMC is poured onto a polyacrylonitrile membrane and reacted for 120 seconds. The remaining steps in this embodiment are basically the same as in Example 8, and will not be repeated here.

[0063] Test example: This experiment used a dead-end vacuum filtration device pressurized by a nitrogen cylinder. The test solution was a 2000 ppm sodium sulfate solution. The permeation flux and inorganic salt molecule rejection rate of the prepared polyamide nanofiltration membrane were measured to evaluate the membrane's filtration performance. The effective area of ​​the filtration unit was 15.9 cm², and the operating pressure range provided by the filtration device was 3–6 bar. The obtained permeation flux values ​​were the average of three replicate experiments on three different samples. Permeation flux (L·m³) -2 ·h -1 ·bar -1 Calculate using the following formula: Where P, V, A, Δt, and ΔP represent the permeation flux, permeation volume, effective membrane area, test time, and operating pressure, respectively.

[0064] The formula for calculating the retention rate is as follows: Where R represents the rejection rate (%), and Cp and Cf represent the concentrations (ppm) of the permeate and feed liquid, respectively.

[0065] Tests were conducted on Examples 1 to 11, and the results are shown in Tables 1 to 3.

[0066] Table 1: Separation performance of nanofiltration membranes prepared in Examples 1 and 2 for 2000 ppm sodium sulfate solution. Table 1 shows that the polyamide nanofiltration membrane prepared in Example 1 has higher performance than the polyamide nanofiltration membrane prepared in Example 2, with a permeation flux of 28.4 L·m⁻¹. -2 ·h -1 ·bar -1 The sodium sulfate rejection rate was 98.6%. The increased TP introduced in Example 2 resulted in more defects in the nanofiltration membrane prepared in Example 2 compared to Example 1, and the permeation flux increased while the rejection rate decreased in Example 2.

[0067] Table 2: Separation performance of nanofiltration membranes prepared in Examples 3 to 7 for 2000 ppm sodium sulfate solution Table 2 shows that, among Examples 3 to 7, the polyamide nanofiltration membrane prepared in Example 4 has the best performance, with a permeation flux of 28.4 L·m⁻¹. -2 ·h -1 ·bar -1The sodium sulfate rejection rate was 98.6%. In Example 3, the PIP concentration was too low, resulting in an incomplete polyamide separation layer. In Example 7, the PIP concentration was too high, leading to excessive cross-linking of the polyamide separation layer, which significantly reduced the permeation flux. Therefore, it can be seen that nanofiltration membranes prepared with different amine monomer concentrations all have certain retention properties, but the performance is best when the amine monomer concentration is 0.4 wt%.

[0068] Table 3: Separation performance of nanofiltration membranes prepared in Examples 8 to 11 for 2000 ppm sodium sulfate solution As shown in Table 3, among Examples 8 to 11, Example 9 exhibits the best performance, with a permeation flux of 28.4 L·m⁻¹. -2 ·h -1 ·bar -1 The sodium sulfate rejection rate was 98.6%. In Example 8, the reaction time was shorter, resulting in more defects in the polyamide layer. In Example 11, as the reaction time increased, the degree of cross-linking of the polyamide layer also increased, leading to a decrease in permeation flux. Therefore, the optimal performance was achieved when the reaction time between the amine monomer and the acyl chloride compound was 60 seconds.

[0069] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that the present invention includes, but is not limited to, the contents described in the accompanying drawings and the specific embodiments above. Any modifications that do not depart from the functional and structural principles of the present invention will be included within the scope of the claims.

Claims

1. A method for preparing a polyamide nanofiltration membrane, characterized in that, Includes the following steps: a) Provide an amine monomer and an aldehyde monomer, mix the amine monomer, the aldehyde monomer and water evenly and carry out a reversible reaction to obtain a nanocolloid solution; b) Provide a nanofiltration base membrane, and deposit the nanocolloid solution onto the nanofiltration base membrane to obtain a membrane sheet; c) Provide an acyl chloride compound, and dissolve the acyl chloride compound in an organic solvent to obtain an organic phase solution; d) Pour the organic phase solution into the membrane to carry out an interfacial polymerization reaction to form a polyamide separation layer on the membrane; e) The membrane having the polyamide separation layer is heat-treated to obtain the polyamide nanofiltration membrane.

2. The method for preparing a polyamide nanofiltration membrane according to claim 1, characterized in that, In step a), the amine monomer is one or more of piperazine monomers, and the aldehyde monomer is one of 2,5-dihydroxyterephthalaldehyde, 2,4,6-trialdehyde phloroglucinol, or pyromellitic pyrrolizaldehyde.

3. The method for preparing a polyamide nanofiltration membrane according to claim 1, characterized in that, In step a), the concentration of the amine monomer in the nanocolloid solution is 0.2wt%~1.0wt%, and the molar ratio of the amine monomer to the aldehyde monomer is 3:(1~2).

4. The method for preparing a polyamide nanofiltration membrane according to claim 1, characterized in that, In step a), the conditions for the reversible reaction are: heating the mixture of amine monomer, aldehyde monomer and water in a water bath at 80°C, and continuously stirring the mixture for 30 min during the reaction.

5. A method for preparing a polyamide nanofiltration membrane according to any one of claims 1-4, characterized in that, In step b), after depositing the nanocolloid solution onto the nanofiltration membrane, the process further includes: air-drying the nanofiltration membrane with the deposited nanocolloid solution for 2 minutes.

6. The method for preparing a polyamide nanofiltration membrane according to claim 5, characterized in that, In step b), the nanofiltration membrane is a polyacrylonitrile membrane, and the deposition step employs a vacuum-assisted self-assembly method or a gas-liquid-liquid surface self-assembly method.

7. A method for preparing a polyamide nanofiltration membrane according to any one of claims 1-4, characterized in that, In step c), the acyl chloride compound is pyromellitic acid chloride, and the organic solvent is n-hexane.

8. A method for preparing a polyamide nanofiltration membrane according to any one of claims 1-4, characterized in that, In step d), the conditions for the interfacial polymerization reaction are: reaction time of 30~120s and reaction temperature of 25℃.

9. A method for preparing a polyamide nanofiltration membrane according to any one of claims 1-4, characterized in that, In step e), the heat treatment conditions are: a treatment temperature of 60°C and a treatment time of 3 minutes.

10. A polyamide nanofiltration membrane, characterized in that, The polyamide nanofiltration membrane includes a nanofiltration base membrane and a polyamide separation layer formed on the nanofiltration base membrane, and the polyamide nanofiltration membrane is made by the method for preparing the polyamide nanofiltration membrane according to any one of claims 1 to 9.

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