Acid-resistant polyamide nanofiltration membrane, preparation method and application thereof

Abstract

The application relates to the field of membrane separation technology, and particularly relates to an acid-resistant polyamide nanofiltration membrane and a preparation method and application thereof. The acid-resistant polyamide nanofiltration membrane comprises a porous support layer and a polyamide separation layer formed on the surface of the porous support layer; wherein the polyamide separation layer is formed by interfacial polymerization of fluorine-containing aromatic polyamines and polyacyl chlorides. The interfacial polymerization of the fluorine-containing aromatic polyamines and the polyacyl chlorides can construct the polyamide separation layer containing aromatic rings, amide bonds and fluorine-containing structures on the surface of the porous support layer. The conjugate stable structure formed by the aromatic ring structure and the amide bond in the polyamide chain segment greatly increases the energy barrier of the hydrolysis reaction of the amide bond. In combination with the strong electron-withdrawing induction of the fluorine atom, the electron cloud density near the amide bond is effectively reduced, the protonation tendency in the acidic medium is weakened, the attack of protons is resisted from the root, and the chemical stability of the polyamide separation layer under acidic conditions is improved.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, specifically to an acid-resistant polyamide nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] With the development of industries such as hydrometallurgy, metal pickling, electronic etching, and chemical production, large quantities of acid-containing wastewater are continuously generated. This type of wastewater typically contains high concentrations of inorganic acids, salt ions, and metal ions. Direct discharge not only causes environmental pollution but also leads to the waste of valuable resources. Therefore, achieving efficient separation and recovery of salt ions and metal resources from acidic wastewater is of great significance.

[0003] Currently, treatment methods for acidic saline wastewater mainly include chemical precipitation, neutralization, ion exchange, electrochemical treatment, and membrane separation technology. Among these, membrane separation technology has received widespread attention in industrial wastewater treatment and resource recovery due to its advantages such as low energy consumption, high separation efficiency, and mild operating conditions. Nanofiltration membranes are pressure-driven separation membranes that fall between ultrafiltration and reverse osmosis membranes. They can selectively separate polyvalent salt ions and small molecules by utilizing pore size sieving effects and surface charge effects. Currently, composite nanofiltration membranes based on the interfacial polymerization of polyamines and polyacryl chlorides to form a polyamide (PA) selective layer have become one of the most widely used nanofiltration membrane types due to their high flux, good separation performance, and mature preparation process.

[0004] However, the amide bonds in traditional polyamide nanofiltration membranes are easily attacked by protons and hydrolyzed in strong acid environments, leading to the destruction of the polyamide network structure. This results in increased membrane flux and decreased retention performance, making it difficult to meet the requirements for long-term stable operation in strong acid systems.

[0005] To improve the acid resistance of polyamide nanofiltration membranes, existing technologies typically modify the polyamide network by introducing sulfonyl chloride monomers, triazine structures, or other rigid conjugated structures. While polysulfonamide membranes exhibit good acid resistance, the low reactivity of sulfonyl chloride monomers makes it difficult to prepare highly cross-linked separation membranes. Furthermore, the presence of triazine rings presents challenges due to insufficient monomer solubility and controllable interfacial polymerization, and may alter the membrane surface charge properties after modification, affecting ion selectivity under acidic conditions.

[0006] Therefore, there is a need to develop a polyamide nanofiltration membrane that combines high ion rejection rate, high flux, and long-term strong acid resistance to meet the application needs in the fields of acidic wastewater treatment and resource recovery. Summary of the Invention

[0007] The purpose of this invention is to provide an acid-resistant polyamide nanofiltration membrane. By conducting an interfacial polymerization reaction between a fluorinated aromatic polyamine and a polyacrylamide chloride, a polyamide separation layer is constructed on the surface of a porous support layer. While ensuring that the polyamide separation layer has a high ion rejection rate and high flux, it fundamentally resists proton attack and improves the chemical stability of the polyamide separation layer under acidic conditions.

[0008] To achieve the above objectives, the present invention provides the following technical solution: an acid-resistant polyamide nanofiltration membrane, comprising: a porous support layer, and a polyamide separation layer formed on the surface of the porous support layer; The polyamide separation layer is formed by interfacial polymerization of fluorinated aromatic polyamines and polyacrylamide chlorides.

[0009] Furthermore, the fluorinated aromatic polyamine is an aromatic polyamine containing a hexafluoroisopropyl group; The polyamide separation layer has a cross-linked network structure formed by aromatic rings, amide bonds, and hexafluoroisopropyl structures.

[0010] Furthermore, the aromatic polyamine containing the hexafluoroisopropyl structure is selected from one or more of 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane, 2,2-bis(4-aminophenyl)hexafluoropropane, and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane.

[0011] Furthermore, the polyacryl chloride is selected from one or more of isophthaloyl chloride, terephthaloyl chloride, orthophthaloyl chloride, and 1,3,5-benzenetrichlorochlorochloride.

[0012] Furthermore, the porous support layer is made of polyethersulfone or polysulfone. The molecular weight cutoff of the porous support layer is 10kDa to 100kDa.

[0013] This application provides a method for preparing the above-mentioned acid-resistant polyamide nanofiltration membrane, comprising the following steps: S1. Dissolve a fluorinated aromatic polyamine in an alkaline aqueous solution to obtain an aqueous reaction solution; S2. Dissolve polyacryl chlorides in a hydrocarbon solvent to obtain an organic phase reaction solution; S3. Obtain a porous support membrane and immerse the porous support membrane in the aqueous reaction solution to load the fluorinated aromatic polyamine onto the surface of the porous support membrane. S4. Immerse the porous support membrane loaded with the fluorinated aromatic polyamine into the organic phase reaction solution to promote the interfacial polymerization reaction between the polyacrylamide chloride and the fluorinated aromatic polyamine, and form a polyamide separation layer on the surface of the porous support membrane. S5. The porous support membrane on which the polyamide separation layer is formed is subjected to heat treatment to obtain the acid-resistant polyamide nanofiltration membrane.

[0014] Furthermore, the content of fluorinated aromatic polyamines in the aqueous reaction solution is any value between 0.1 g / L and 10 g / L; The pH value of the aqueous reaction solution is any value between 8 and 13.

[0015] Furthermore, the content of the polyacrylamide chloride in the organic phase reaction solution is any value between 5 g / L and 20 g / L; The hydrocarbon solvent is selected from any one of n-hexane, n-heptane, cyclohexane, and isoparaffin solvents.

[0016] Furthermore, in step S4, the reaction temperature of the interfacial polymerization reaction is any value between 20℃ and 70℃; In step S5, the heat treatment temperature is any value between 20℃ and 90℃.

[0017] This application provides the application of the above-mentioned acid-resistant polyamide nanofiltration membrane in separating salt ions in acidic solutions.

[0018] The beneficial effects of this invention are as follows: In this application, by introducing fluorinated aromatic polyamines, especially aromatic polyamines containing hexafluoroisopropyl structures, and reacting them with polyacrylamide chlorides through interfacial polymerization, a polyamide separation layer containing aromatic rings, amide bonds, and fluorinated structures is constructed on the surface of the porous support layer. This results in a polyamide nanofiltration membrane that possesses both high ion rejection performance and flux, as well as excellent acid resistance, meeting the application requirements in the fields of acidic wastewater treatment and resource recovery.

[0019] The aromatic ring structure in the polyamide separation layer can form a conjugated stable structure with the amide bond in the polyamide chain segment, which can significantly increase the energy barrier for the hydrolysis reaction of the amide bond. Combined with the strong electron-withdrawing inductive effect of fluorine atoms, it can effectively reduce the electron cloud density near the amide bond, weaken the protonation tendency in acidic media, resist proton attack from the root, and hydrolytic breakage of polyamide molecular chains, thereby improving the chemical stability of the polyamide separation layer under acidic conditions.

[0020] The hexafluoroisopropyl structure is characterized by rigidity and high steric hindrance. This significant steric hindrance effect restricts the free movement of polyamide chain segments, reducing the degree of molecular chain swelling under acidic conditions. Simultaneously, the CF bond in the hexafluoroisopropyl structure possesses high bond energy, excellent chemical stability, and hydrophobicity, which reduces the penetration of acidic solutions into the polyamide separation layer. This significantly enhances the polyamide separation layer's resistance to strong acid media, thereby improving its structural stability and service life in strong acid systems while maintaining the high selective separation advantage of polyamide nanofiltration membranes.

[0021] The method for preparing acid-resistant polyamide nanofiltration membranes provided in this application is based on existing interfacial polymerization processes for polyamide nanofiltration membranes. By simply adjusting the monomer system, a polyamide nanofiltration membrane with a uniform and stable structure can be prepared, exhibiting high ion rejection rate, high flux, and long-term strong acid resistance. Furthermore, by controlling the ratio of monomers in the water and oil phases, the polymerization reaction between polyamines and polyacrylamide chloride monomers can be fully achieved at the interface, further regulating the rejection rate, flux, and acid resistance of the polyamide nanofiltration membrane to optimize its overall performance. This preparation method is simple to operate, the reaction conditions are easy to control, and the resulting polyamide nanofiltration membrane has a uniform and stable structure, showing promising prospects for industrial-scale application.

[0022] The acid-resistant polyamide nanofiltration membrane provided in this application has both excellent separation selectivity and permeation flux; and it can still maintain a stable high rejection rate and flux after being continuously immersed in 30% H2SO4 for 90 days, which has excellent long-term operational reliability. It can be applied to fields such as hydrometallurgy, metal mineral leaching, pickling waste liquid treatment, metal etching and acid resource recovery, and has good application value.

[0023] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0024] Figure 1 This is a surface SEM image of the acid-resistant polyamide nanofiltration membrane shown in Example 1 of the present invention; Figure 2 The surface zeta potential (Zeta potential)-pH value change curve of the acid-resistant polyamide nanofiltration membrane shown in Example 1 of this invention; Figure 3 The graph shows the retention performance and flux changes of the acid-resistant polyamide nanofiltration membrane shown in Example 1 of this invention after being immersed in a 30% H2SO4 solution for 90 days for Na2SO4 and MgSO4. Detailed Implementation

[0025] The technical solutions of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention. Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0026] A preferred embodiment of this application provides an acid-resistant polyamide nanofiltration membrane. This acid-resistant polyamide nanofiltration membrane includes a porous support layer and a polyamide separation layer formed on the surface of the porous support layer. The polyamide separation layer is formed by interfacial polymerization of a fluorinated aromatic polyamine and a polyacrylamide chloride. In this embodiment, the polyamide separation layer is formed on the surface of the porous support layer by interfacial polymerization of a fluorinated aromatic polyamine and a polyacrylamide chloride. This polyamide separation layer forms a polyamide crosslinking network through amide bonds in the aromatic structural units of the fluorinated functional groups. The fluorinated groups can block the attack of solvents and free radicals on the carbon chain backbone, thereby effectively reducing the erosion of the polyamide molecular chains in the polyamide network by acidic media, inhibiting amide bond hydrolysis, and improving the structural stability of the nanofiltration membrane in acidic environments. Simultaneously, the polyamide separation layer still uses a traditional polyamine / polyacrylamide chloride interfacial polymerization system, which can ensure the density and continuity of the polyamide selective layer, enabling the nanofiltration membrane to possess both high retention capacity, water flux, and good mechanical stability. This embodiment introduces fluorinated aromatic polyamines into the polyamide separation layer by using fluorinated aromatic polyamines, based on the traditional polyamine / polyacrylamide interfacial polymerization system. This introduces fluorinated groups and constructs a stable polyamide crosslinking network based on fluorinated aromatic ring structural units. Due to the high steric hindrance and hydrophobicity of the fluorinated groups, the attack of acidic media on the polyamide molecular chains is reduced, minimizing hydrolysis and structural damage of the polyamide separation layer in strong acid environments, thereby improving the acid resistance and long-term operational stability of the nanofiltration membrane. Simultaneously, the fluorinated aromatic polyamine can still undergo interfacial polymerization with the polyacrylamide chloride to form a continuous and dense polyamide selective layer, ensuring that the polyamide separation layer material has good ion sieving capacity and water transport performance. This results in a nanofiltration membrane with high ion rejection, high water flux, and good mechanical stability. In some embodiments, the fluorinated aromatic polyamine may be selected from any one or more of 2,2'-bis(trifluoromethyl)diaminobiphenyl (TFDB), 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether (6FODA), 4,4'-diaminooctafluorobiphenyl, and aromatic polyamines containing a hexafluoroisopropyl structure (-C(CF3)2-).

[0027] In one embodiment, the fluorinated aromatic polyamine is preferably an aromatic polyamine containing a hexafluoroisopropyl group. The polyamide separation layer formed by interfacial polymerization exhibits a cross-linked network characteristic formed by aromatic rings, amide bonds, and the hexafluoroisopropyl group. The hexafluoroisopropyl group has a significant steric hindrance effect, which can restrict the movement of polyamide chain segments and reduce the possibility of protons diffusing near the amide bonds and attacking the carbonyl structure under acidic conditions, thereby improving the hydrolysis resistance of the amide bonds. Furthermore, the fluorinated groups in the hexafluoroisopropyl group have strong hydrophobicity and chemical stability, which can reduce the degree of water absorption and swelling of the polyamide separation layer in a strong acid environment and maintain the integrity of the polyamide network structure. Therefore, by further limiting the fluorinated aromatic polyamine to an aromatic polyamine containing a hexafluoroisopropyl group, the long-term stable operation capability of the polyamide nanofiltration membrane in a strong acid environment can be significantly improved. In this embodiment or other embodiments, the aromatic polyamine containing the hexafluoroisopropyl structure is selected from any one or more of 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane (HFBAPP), 2,2-bis(4-aminophenyl)hexafluoropropane (6F-DIAMINE), and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (Bis-APAF). By limiting the specific type of aromatic polyamine containing the hexafluoroisopropyl structure, the crosslinking structure of the polyamide separation layer can be controlled according to the rigidity and fluorine content in the monomer molecule structure, further improving the hydrophobicity and chemical stability of the membrane material, achieving synergistic control of the structure and separation performance of the polyamide separation layer, thereby improving the overall performance of the nanofiltration membrane.

[0028] In one embodiment, the polyacrylamide chloride can be selected from one or more of isophthaloyl chloride (IPC), terephthaloyl chloride (TPC), phthaloyl chloride, and 1,3,5-benzenetricarboxylic chloride (TMC). By limiting the types of polyacrylamide chloride monomers, fluorinated aromatic polyamines can undergo interfacial polymerization reactions with acrylamide chloride monomers with specific functionalities, thereby adjusting the degree of crosslinking and network structure of the polyamide release layer. Specifically, the reaction of fluorinated aromatic polyamines with 1,3,5-benzenetricarboxylic chloride, which has higher functionality, can form a highly crosslinked three-dimensional polyamide network, improving the density and acid resistance of the polyamide release layer; the reaction of fluorinated aromatic polyamines with difunctional acrylamide chlorides such as isophthaloyl chloride and terephthaloyl chloride can adjust the arrangement of polyamide segments, improving the flexibility and mass transfer performance of the polyamide release layer. By rationally selecting polyacrylamide chloride monomers, a balance can be achieved between retention performance, water flux, and structural stability.

[0029] In one embodiment, the porous support layer is made of polyethersulfone or polysulfone, and the molecular weight cutoff of the porous support layer is preferably 10 kDa to 100 kDa. By limiting the material and molecular weight cutoff range of the porous support layer, it is ensured that the porous support layer can provide stable mechanical support for the polyamide separation layer while reducing the resistance to water molecule transport, providing a stable mass transfer channel for the polyamide separation layer, thereby ensuring that the nanofiltration membrane has good chemical stability and mechanical strength, high permeation flux, and preventing the nanofiltration membrane from structurally deforming, cracking, or detaching under high-pressure filtration or acidic environments.

[0030] In one embodiment, a method for preparing the above-mentioned acid-resistant polyamide nanofiltration membrane is provided. The preparation method includes the following steps: S1. Dissolve a fluorinated aromatic polyamine in an alkaline aqueous solution to obtain an aqueous reaction solution; S2. Dissolve polyacryl chlorides in a hydrocarbon solvent to obtain an organic phase reaction solution; S3. Obtain a porous support membrane and immerse the porous support membrane in the aqueous reaction solution to load the fluorinated aromatic polyamine onto the surface of the porous support membrane. S4. Immerse the porous support membrane loaded with the fluorinated aromatic polyamine into the organic phase reaction solution to promote the interfacial polymerization reaction between the polyacrylamide chloride and the fluorinated aromatic polyamine, and form a polyamide separation layer on the surface of the porous support membrane. S5. The porous support membrane on which the polyamide separation layer is formed is subjected to heat treatment to obtain the acid-resistant polyamide nanofiltration membrane.

[0031] The preparation method provided in this embodiment involves preparing a fluorinated aromatic polyamine as an aqueous reaction solution and a polyacrylamide chloride as an organic reaction solution, and then sequentially loading them onto the surface of a porous support membrane. This allows the two monomers to polymerize at the oil-water interface, forming a continuous, dense, and uniform polyamide separation layer. This effectively controls the formation process of the polyamide layer, ensuring the uniform introduction of fluorinated structures into the polymer network and improving the acid resistance of the polyamide separation layer. Heat treatment further promotes the reaction of unreacted groups, perfecting the polyamide crosslinking network structure and enhancing the stability of the polyamide separation layer. Furthermore, this preparation method is based on the mature interfacial polymerization process of existing polyamide nanofiltration membranes. Membrane structure optimization is achieved simply by adjusting the interfacial polymerization monomer system, without the need for complex preparation equipment or special processing steps. It offers advantages such as simple process, high controllability, and ease of industrial-scale production.

[0032] In one embodiment, the content of fluorinated aromatic polyamines in the aqueous reaction solution is any value ranging from 0.1 g / L to 10 g / L; the pH value of the aqueous reaction solution is any value ranging from 8 to 13. By limiting the concentration range of fluorinated aromatic polyamines in the aqueous reaction solution and its pH range, the interfacial polymerization rate and membrane structure can be controlled. A suitable concentration of fluorinated aromatic polyamines ensures that the monomers are fully adsorbed onto the porous support membrane surface and react fully with the polyacrylamide chlorides. An alkaline environment promotes the maintenance of high reactivity of the amine groups and neutralizes the acidic byproducts generated during interfacial polymerization, which is beneficial for forming a polyamide separation layer with a complete structure and fewer defects.

[0033] In one embodiment, the content of polyacrylamide chloride in the organic phase reaction solution is any value ranging from 5 g / L to 20 g / L. A suitable concentration of polyacrylamide chloride can fully react with the fluorinated aromatic polyamine in the aqueous phase to form a polyamide separation layer with suitable thickness and cross-linking degree, thus achieving a balance between retention performance and water flux. In this embodiment or other embodiments, the hydrocarbon solvent can be selected from any one of n-hexane, n-heptane, cyclohexane, and isoparaffinic solvents such as isododecane (Isopar E). These hydrocarbon solvents have good chemical inertness, which can avoid side reactions with the reactant monomers, and at the same time, help maintain a stable oil-water interface environment and improve film uniformity.

[0034] In one embodiment, in step S4, the reaction temperature of the interfacial polymerization reaction is any value between 20°C and 70°C. A suitable interfacial polymerization temperature can promote the diffusion and amidation reaction of fluorinated aromatic polyamines and polyacrylamide chlorides, that is, promote the diffusion of aqueous amine monomers to the oil-water interface, improve polymerization efficiency, and form a dense and uniform separation layer. Furthermore, the ambient humidity is preferably 25% to 80% RH. In this embodiment or other embodiments, in step S5, the heat treatment temperature is any value between 20°C and 90°C. Suitable heat treatment conditions can promote further arrangement and crosslinking of polymer segments, improve the stability of the membrane structure, reduce defect generation, and thus further enhance the ion rejection capacity, flux, and acid resistance of the nanofiltration membrane.

[0035] One embodiment provides an application of the above-mentioned acid-resistant polyamide nanofiltration membrane in separating salt ions in acidic solutions, specifically for the selective separation of cations and / or anions in acidic solutions. This nanofiltration membrane has a fluorinated aromatic polyamide cross-linked network structure, which maintains good structural stability in strongly acidic environments. Simultaneously, utilizing the size sieving effect and surface charge of the nanofiltration membrane, it achieves efficient retention of monovalent or polyvalent salts in acidic solutions. This makes it suitable for industrial applications such as hydrometallurgy, pickling, and metal etching that generate acidic, salt-containing wastewater, improving the efficiency of acidic wastewater treatment and achieving salt ion separation and resource recovery.

[0036] Example 1

[0037] S1. Under the conditions of relative humidity of 60% and temperature of 25℃, 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane is dissolved in NaOH aqueous solution with pH value of 12, stirred evenly, and an aqueous solution with a content of 3g / L of 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane is prepared.

[0038] S2. Dissolve 1,3,5-benzenetricarboxyl chloride in n-hexane solution, stir evenly, and prepare an organic phase reaction solution with a content of 10 g / L of 1,3,5-benzenetricarboxyl chloride. S3. Obtain a polysulfone ultrafiltration membrane as a porous support membrane; and immerse the polysulfone ultrafiltration membrane in an aqueous reaction solution at a temperature of 25°C, so that 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane is loaded onto the surface of the polysulfone ultrafiltration membrane. After immersion for 60 seconds, purge the surface of the polysulfone ultrafiltration membrane with nitrogen gas to remove excess aqueous solution until no obvious droplets are visible.

[0039] S4. The polysulfone ultrafiltration membrane loaded with 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane is immersed in the organic phase reaction solution, and interfacial polymerization is carried out at a temperature of 60°C. This promotes the amidation reaction between 1,3,5-benzenetricarboxyl chloride and 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane at the oil-water interface. After 60 seconds of polymerization, a polyamide separation layer is formed on the surface of the polysulfone ultrafiltration membrane.

[0040] S5. Place the polysulfone ultrafiltration membrane with a polyamide separation layer on its surface in an oven and heat-treat it at 60°C. After heat treatment for 5 minutes, an acid-resistant polyamide nanofiltration membrane is obtained. Immerse the acid-resistant polyamide nanofiltration membrane in deionized water and store it in a refrigerator at 4°C for later use.

[0041] The surface morphology of the acid-resistant polyamide nanofiltration membrane prepared in Example 1 was characterized by scanning electron microscopy (SEM) to observe its surface microstructure. The results are as follows: Figure 1 As shown. Simultaneously, the surface zeta potential of the acid-resistant polyamide nanofiltration membrane prepared in Example 1 was tested, and the results are as follows. Figure 2 As shown.

[0042] Depend on Figure 1 It can be seen that the surface of the acid-resistant polyamide nanofiltration membrane is generally smooth, with no obvious cracks, pores, or large-area defects observed, and the membrane surface structure is continuous and uniform. This indicates that a dense and complete polyamide separation layer can be formed through the interfacial polymerization reaction of fluorinated aromatic polyamines and polyacrylamide chlorides. At the same time, the uniform surface morphology indicates that the polyamide network has good film-forming properties and structural stability, which is beneficial to ensuring that the acid-resistant polyamide nanofiltration membrane material maintains stable ion selectivity and separation performance during filtration.

[0043] Depend on Figure 2 It is known that the isoelectric point of the acid-resistant polyamide nanofiltration membrane is approximately 2.71. When the pH value is higher than the isoelectric point, the membrane surface exhibits negative charge, and the zeta potential gradually decreases with increasing pH value, indicating that the negative charge on the surface of the acid-resistant polyamide nanofiltration membrane gradually increases. This demonstrates that the polyamide separation layer prepared in this embodiment has stable surface charge characteristics under different pH environments. The strong surface negative charge can enhance the selective retention of polyvalent anions by the membrane through the Donnan Exclusion effect, which is beneficial to improving the separation performance of the acid-resistant polyamide nanofiltration membrane for inorganic salts such as sulfates. At the same time, the introduction of fluorinated aromatic polyamines improves the acid resistance stability of the polyamide network while retaining the charge regulation ability of the membrane surface, giving the obtained nanofiltration membrane both acid resistance and high salt selectivity.

[0044] Example 2

[0045] The difference between this embodiment and Example 1 is that in step S1, 2,2-bis(4-aminophenyl)hexafluoropropane is used instead of 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane. Specifically, 2,2-bis(4-aminophenyl)hexafluoropropane is dissolved in an aqueous NaOH solution with a pH of 12 to prepare an aqueous solution with a 2,2-bis(4-aminophenyl)hexafluoropropane content of 3 g / L.

[0046] Subsequently, an interfacial polymerization reaction and post-treatment were carried out according to the preparation method described in Example 1 to prepare an acid-resistant polyamide nanofiltration membrane; the acid-resistant polyamide nanofiltration membrane was then immersed in deionized water and stored in a refrigerator at 4°C for later use.

[0047] Example 3

[0048] The difference between this embodiment and Example 1 is that in step S1, 4,4'-diaminooctafluorobiphenyl is used instead of 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane. Specifically, 4,4'-diaminooctafluorobiphenyl is dissolved in an aqueous NaOH solution with a pH of 12 to prepare an aqueous solution with a 4,4'-diaminooctafluorobiphenyl content of 3 g / L.

[0049] Subsequently, an interfacial polymerization reaction and post-treatment were carried out according to the preparation method described in Example 1 to prepare an acid-resistant polyamide nanofiltration membrane; the acid-resistant polyamide nanofiltration membrane was then immersed in deionized water and stored in a refrigerator at 4°C for later use.

[0050] Example 4

[0051] The difference between this embodiment and Example 1 is that in step S1, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane is used instead of 2,2-bis[4-(4-aminophenoxyphenyl)]hexafluoropropane. Specifically, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane is dissolved in an aqueous NaOH solution with a pH of 12 to prepare an aqueous solution containing 3 g / L of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane.

[0052] Subsequently, an interfacial polymerization reaction and post-treatment were carried out according to the preparation method described in Example 1 to prepare an acid-resistant polyamide nanofiltration membrane; the acid-resistant polyamide nanofiltration membrane was then immersed in deionized water and stored in a refrigerator at 4°C for later use.

[0053] Example 5

[0054] The difference between this embodiment and Example 1 is that in step S1, 2,2'-bis(trifluoromethyl)diaminobiphenyl is used instead of 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane. Specifically, 2,2'-bis(trifluoromethyl)diaminobiphenyl is dissolved in an aqueous NaOH solution with a pH of 12 to prepare an aqueous solution with a 2,2'-bis(trifluoromethyl)diaminobiphenyl content of 3 g / L.

[0055] Subsequently, an interfacial polymerization reaction and post-treatment were carried out according to the preparation method described in Example 1 to prepare an acid-resistant polyamide nanofiltration membrane; the acid-resistant polyamide nanofiltration membrane was then immersed in deionized water and stored in a refrigerator at 4°C for later use.

[0056] Example 6

[0057] The difference between this embodiment and Example 1 is that in step S1, 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether is used instead of 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane. Specifically, 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether is dissolved in an aqueous NaOH solution with a pH of 12 to prepare an aqueous solution with a content of 3 g / L of 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether.

[0058] Subsequently, an interfacial polymerization reaction and post-treatment were carried out according to the preparation method described in Example 1 to prepare an acid-resistant polyamide nanofiltration membrane; the acid-resistant polyamide nanofiltration membrane was then immersed in deionized water and stored in a refrigerator at 4°C for later use.

[0059] Example 7

[0060] The difference between this embodiment and Example 4 is as follows: In step S1, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane is dissolved in an aqueous NaOH solution with a pH of 12 to prepare an aqueous solution containing 4 g / L of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane. In step S2, 1,3,5-benzenetricarboxyl chloride is dissolved in a hexane solution to prepare an organic reaction solution containing 15 g / L of 1,3,5-benzenetricarboxyl chloride.

[0061] Subsequently, an interfacial polymerization reaction and post-treatment were carried out according to the preparation method described in Example 1 to prepare an acid-resistant polyamide nanofiltration membrane; the acid-resistant polyamide nanofiltration membrane was then immersed in deionized water and stored in a refrigerator at 4°C for later use.

[0062] Example 8

[0063] The difference between this embodiment and Example 1 is that in step S4, the immersion time of the polysulfone ultrafiltration membrane loaded with 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane in the organic phase reaction solution is adjusted to 120 s, so that the reaction time of the interfacial polymerization reaction is 120 s. The remaining steps and process conditions are the same as in Example 1. Finally, an acid-resistant polyamide nanofiltration membrane is prepared, and this acid-resistant polyamide nanofiltration membrane is immersed in deionized water and stored in a refrigerator at 4°C for later use.

[0064] Example 9

[0065] The difference between this embodiment and Example 1 is that in step S4, when the polysulfone ultrafiltration membrane loaded with 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane is immersed in the organic phase reaction solution, the reaction temperature of the interfacial polymerization reaction is adjusted to 45°C. The remaining steps and process conditions are the same as in Example 1. Finally, an acid-resistant polyamide nanofiltration membrane is prepared, and this acid-resistant polyamide nanofiltration membrane is immersed in deionized water and stored in a refrigerator at 4°C for later use.

[0066] Example 10

[0067] The difference between this embodiment and Embodiment 1 is that in step S5, the heat treatment time is adjusted to 10 minutes. All other preparation steps and process conditions are the same as in Embodiment 1. Finally, an acid-resistant polyamide nanofiltration membrane is prepared and immersed in deionized water, then stored in a 4°C refrigerator for later use.

[0068] Example 11

[0069] The difference between this embodiment and Example 1 is that in step S2, terephthaloyl chloride (TPC) is used instead of 1,3,5-tribenzoyl chloride (TMC). Specifically, terephthaloyl chloride is dissolved in n-hexane solution to prepare an organic phase reaction solution with a terephthaloyl chloride content of 10 g / L.

[0070] Subsequently, an interfacial polymerization reaction and post-treatment were carried out according to the preparation method described in Example 1 to prepare an acid-resistant polyamide nanofiltration membrane; the acid-resistant polyamide nanofiltration membrane was then immersed in deionized water and stored in a refrigerator at 4°C for later use.

[0071] The acid-resistant polyamide nanofiltration membranes prepared in Examples 1-11 were tested for their retention and permeation properties. The test conditions were as follows: operating temperature was 25℃, operating pressure was 15 bar, and Na2SO4 solution and MgSO4 solution with a concentration of 2000ppm were used as test solutions to test the ion rejection rate and flux of the acid-resistant polyamide nanofiltration membranes. The test results are shown in Table 1.

[0072] Table 1. Ion rejection rate and flux test results of acid-resistant polyamide nanofiltration membranes prepared in Examples 1-11

[0073] As shown in Table 1, interfacial polymerization of fluorinated aromatic polyamines and polyacrylamide chlorides can form polyamide nanofiltration membranes with certain salt separation performance. Among them, the polyamide nanofiltration membrane obtained by using an aromatic polyamine containing a hexafluoroisopropyl structure as the aqueous monomer exhibits superior retention performance.

[0074] In Example 1, 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane was used as the aqueous monomer. The nanofiltration membrane prepared exhibited excellent comprehensive performance, showing high rejection rates for both Na₂SO₄ and MgSO₄ while maintaining high flux. This is because the 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane molecule contains a hexafluoroisopropyl group and a flexible phenyl ether linker segment. The hexafluoroisopropyl group has a significant steric hindrance effect, which can restrict the free rotation of polyamide segments, improve the stability of the polyamide network structure, and reduce the possibility of amide bond hydrolysis under acidic conditions. Meanwhile, the phenyl ether segment can enhance the diffusion ability of the aqueous monomer at the oil-water interface, allowing it to fully react with the 1,3,5-phenyltricarboxylic acid chloride monomer, thereby forming a more uniform and dense polyamide separation layer, which is beneficial to improving the rejection performance of the polyamide nanofiltration membrane.

[0075] Comparing the polyamide nanofiltration membranes prepared in Example 1 with those prepared in Examples 2-6, the different fluorinated aromatic diamine structures significantly affect the performance of the prepared polyamide nanofiltration membranes. In Example 4, although 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane also contains a hexafluoroisopropyl structure, the presence of ortho-hydroxyl groups in its molecular structure may lead to side reactions with acyl chloride groups during interfacial polymerization, such as the formation of ester bonds. This affects the regularity of the polyamide network and may cause structural defects within the membrane layer, resulting in significantly lower retention performance compared to Example 12,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane. Compared to monomers such as 2,2-bis(4-aminophenyl)hexafluoropropane, 2,2'-bis(trifluoromethyl)diaminobiphenyl, and 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether, the 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane used in Example 1 has a longer molecular chain structure and a flexible ether bond structure, which can improve monomer diffusion behavior and enhance the crosslinking integrity of the polyamide network during interfacial polymerization. Therefore, the membrane material prepared using 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane exhibits a better balance between flux and retention performance.

[0076] Compared with the polyamide nanofiltration membrane prepared in Example 1, the polyamide nanofiltration membrane prepared in Example 3 showed a significant decrease in ion rejection rate and flux, indicating a marked decline in the performance of the polyamide nanofiltration membrane prepared in Example 3. This may be because the benzene ring in the 4,4'-diaminooctafluorobiphenyl structure has a high degree of fluorine substitution, which reduces the electron cloud density of the amino group, thereby reducing its nucleophilic reactivity and limiting the interfacial polymerization reaction with TMC. This results in insufficient crosslinking of the formed polyamide network, reduced membrane density, and thus affects the rejection performance.

[0077] Compared to the polyamide nanofiltration membrane prepared in Example 4, the polyamide nanofiltration membrane prepared in Example 7 by increasing the concentrations of monomers in the aqueous phase and acyl chloride monomers in the organic phase exhibits superior retention performance. This demonstrates that increasing the concentrations of monomers in the aqueous phase and acyl chloride monomers in the organic phase can enhance the degree of interfacial polymerization, thereby increasing the crosslinking density of the polyamide separation membrane layer and thus improving retention performance. However, excessively high monomer concentrations may also lead to increased membrane thickness or increased mass transfer resistance; therefore, a balance needs to be struck between retention performance and flux.

[0078] Comparing the polyamide nanofiltration membranes prepared in Examples 8-10 with those prepared in Example 1, it is evident that the interfacial polymerization reaction time, reaction temperature, and heat treatment conditions all affect the structural properties of the polyamide separation layer, thereby influencing the retention and flux performance of the polyamide nanofiltration membrane. When the interfacial polymerization time or heat treatment time is prolonged, the degree of cross-linking of the polyamide network increases, which is beneficial for enhancing the structural stability of the polyamide separation layer; however, excessive reaction may lead to thickening of the polyamide separation layer, reducing the flux of the polyamide nanofiltration membrane. Therefore, by optimizing the interfacial polymerization conditions, a membrane structure with both high retention rate and high flux can be obtained.

[0079] Comparing the polyamide nanofiltration membrane prepared in Example 11 with that prepared in Example 1, it can be seen that when terephthaloyl chloride is used instead of 1,3,5-benzoic acid chloride as the organic phase monomer, the membrane's retention performance is significantly reduced. This is because 1,3,5-benzoic acid chloride has three acyl chloride reaction sites, enabling the formation of a three-dimensional polyamide network with a higher degree of crosslinking; while terephthaloyl chloride only has two acyl chloride reaction sites, resulting in a lower degree of crosslinking in the polymer network, leading to a decrease in the density of the formed polyamide separation layer structure.

[0080] The polyamide nanofiltration membrane prepared in Example 1 was immersed in a 30% H2SO4 solution for 90 days. Samples were taken every 15 days, and after thorough washing with deionized water until neutral, its retention capacity and flux for 2000 ppm Na2SO4 and MgSO4 solutions were tested at 25°C and 15 bar. The test results are shown in Table 2. Figure 3 As shown.

[0081] Table 2. Retention performance and flux variation of the polyamide nanofiltration membrane prepared in Example 1 for Na2SO4 and MgSO4 over 90 days.

[0082] From Table 2 and Figure 3 It can be seen that the acid-resistant polyamide nanofiltration membrane prepared in Example 1, after long-term immersion in 30% H2SO4 solution, still maintains a high rejection rate for Na2SO4 and MgSO4. After immersion for 90 days, the rejection rate remains above 98%, and the flux does not decrease significantly. This indicates that the polyamide nanofiltration membrane has excellent acid resistance.

[0083] The above results demonstrate that by introducing fluorinated aromatic polyamines into the polyamide separation layer, this application enables the resulting polyamide network to simultaneously contain aromatic ring structures, amide bonds, and fluorinated structures. The rigid aromatic structure enhances the chemical stability of the polyamide network and reduces the damaging effects of acidic environments on amide bonds. Simultaneously, the steric hindrance provided by the hexafluoroisopropyl structure restricts polymer chain movement and hydrophobic properties, improving the membrane's resistance to swelling. This allows the resulting polyamide nanofiltration membrane to maintain stable separation performance even under strong acid conditions. Therefore, the acid-resistant polyamide nanofiltration membrane provided by this application balances high retention capacity, good water flux, and long-term acid resistance, showing promising application prospects in acidic wastewater treatment, hydrometallurgy, metal pickling, and resource recovery.

[0084] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0085] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. An acid-resistant polyamide nanofiltration membrane, characterized in that: include: A porous support layer, and a polyamide separation layer formed on the surface of the porous support layer; The polyamide separation layer is formed by interfacial polymerization of fluorinated aromatic polyamines and polyacrylamide chlorides.

2. The acid-resistant polyamide nanofiltration membrane as described in claim 1, characterized in that, The fluorinated aromatic polyamine is an aromatic polyamine containing a hexafluoroisopropyl group; The polyamide separation layer has a cross-linked network structure formed by aromatic rings, amide bonds, and hexafluoroisopropyl structures.

3. The acid-resistant polyamide nanofiltration membrane as described in claim 2, characterized in that, The aromatic polyamine containing the hexafluoroisopropyl structure is selected from one or more of 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane, 2,2-bis(4-aminophenyl)hexafluoropropane, and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane.

4. The acid-resistant polyamide nanofiltration membrane as described in claim 1, characterized in that, The polyacryl chloride is selected from one or more of isophthaloyl chloride, terephthaloyl chloride, orthophthaloyl chloride, and 1,3,5-benzenetricarboxyl chloride.

5. The acid-resistant polyamide nanofiltration membrane as described in claim 1, characterized in that, The porous support layer is made of polyethersulfone or polysulfone. The molecular weight cutoff of the porous support layer is 10kDa to 100kDa.

6. The method for preparing the acid-resistant polyamide nanofiltration membrane according to any one of claims 1-5, characterized in that, Includes the following steps: S1. Dissolve a fluorinated aromatic polyamine in an alkaline aqueous solution to obtain an aqueous reaction solution; S2. Dissolve polyacryl chlorides in a hydrocarbon solvent to obtain an organic phase reaction solution; S3. Obtain a porous support membrane and immerse the porous support membrane in the aqueous reaction solution to load the fluorinated aromatic polyamine onto the surface of the porous support membrane. S4. Immerse the porous support membrane loaded with the fluorinated aromatic polyamine into the organic phase reaction solution to promote the interfacial polymerization reaction between the polyacrylamide chloride and the fluorinated aromatic polyamine, and form a polyamide separation layer on the surface of the porous support membrane. S5. The porous support membrane on which the polyamide separation layer is formed is subjected to heat treatment to obtain the acid-resistant polyamide nanofiltration membrane.

7. The preparation method according to claim 6, characterized in that, The content of fluorinated aromatic polyamines in the aqueous reaction solution is any value between 0.1 g / L and 10 g / L; The pH value of the aqueous reaction solution is any value between 8 and 13.

8. The preparation method according to claim 6, characterized in that, The content of the polyacrylamide chloride in the organic phase reaction solution is any value between 5 g / L and 20 g / L; The hydrocarbon solvent is selected from any one of n-hexane, n-heptane, cyclohexane, and isoparaffin solvents.

9. The preparation method according to claim 6, characterized in that, In step S4, the reaction temperature of the interfacial polymerization reaction is any value between 20℃ and 70℃; In step S5, the heat treatment temperature is any value between 20℃ and 90℃.

10. The application of the acid-resistant polyamide nanofiltration membrane according to any one of claims 1-5 in the separation of salt ions in acidic solutions.

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