Method for preparing cation exchange membrane based on chelating agent regulating casting solution

Abstract

The application discloses a method for preparing a cation exchange membrane based on chelating agent regulation of casting solution. The method uses a porous filter membrane (such as a polyvinylidene fluoride membrane) as a support layer, and through a casting solution containing a specific chelating agent, a sulfonated monomer, a crosslinking agent and a photoinitiator, the membrane is prepared through steps of immersion or coating, ultraviolet light irradiation polymerization, ultrapure water post-treatment and NaCl solution stabilization. The chelating agent can form a hydrogen bond network with the sulfonated monomer, regulate the viscosity and fluidity of the casting solution, induce the ordered arrangement of the monomer, and also chelate divalent cations to improve the selective separation capacity of the membrane to monovalent and divalent ions. Compared with the prior art, the application solves the problems of slow ion transmission, difficult control of surface morphology, difficult balance of ion permeability and selectivity, and complex and expensive preparation of a commercial cation exchange membrane. The prepared cation exchange membrane has an ion exchange capacity of 2.5-3.0 meq / g, a thickness of 20-30 nm, and has the advantages of low cost, good ion transmission and ion selectivity.

Description

Technical Field

[0001] This invention relates to the field of cation exchange membrane preparation technology. Specifically, it relates to a method for preparing a cation exchange membrane based on the control of the casting solution by a chelating agent, and the cation exchange membrane thereof. In particular, it relates to a preparation technology that uses a chelating agent to control the thickness and surface morphology of the cation exchange membrane and improve the selectivity of the membrane for monovalent / divalent ions. It covers the design of the cation exchange membrane network structure and mass transfer channels, as well as related technologies of the Daonan dialysis device using the cation exchange membrane. Background Technology

[0002] Cation exchange membranes have wide applications in numerous fields, playing a crucial role in seawater desalination, high-salt concentration, ammonia nitrogen recovery, and lithium-magnesium separation. With continuously growing market demand, global cation exchange membrane production capacity is also rapidly increasing. However, compared to water-permeable membranes (such as microfiltration and ultrafiltration membranes), cation exchange membranes are relatively expensive, and my country has long relied on imports of high-performance cation exchange membranes from countries such as the United States, Japan, and Germany.

[0003] In practical applications, existing commercial cation exchange membranes have revealed several problems. Firstly, their ion transport rates are relatively low. According to the Daonan dialysis ion flux calculation formula, the ion transport rate is directly proportional to the ion exchange capacity and inversely proportional to the membrane thickness. An ideal cation exchange membrane should possess both high ion exchange capacity and low membrane thickness. However, in existing commercial cation exchange membranes, the ion exchange capacity is significantly positively correlated with the membrane thickness. While the pore-filling method can achieve high-density fixation of exchange sites within the membrane, it cannot further reduce the membrane thickness. This results in the inability to simultaneously control the ion exchange capacity and membrane thickness, thus hindering the improvement of the ion transport rate of the cation exchange membrane.

[0004] Secondly, surface morphology control is difficult. Existing commercial cation exchange membranes have smooth surfaces, which is not conducive to ion capture in solution. Studies have found that wrinkles on the membrane surface can generate turbulence, accelerating the migration of target ions to the membrane surface, thus facilitating ion capture. The size of the wrinkles directly affects the distribution characteristics of the flow field morphology on the membrane surface. Wrinkles with different geometric parameters significantly affect the ion capture efficiency of the ion exchange membrane. Non-optimized surface wrinkles easily induce dead zones in membrane hydrodynamics, leading to ion flux attenuation. Currently, most commercial cation exchange membranes have smooth, wrinkle-free surfaces, and it is difficult for them to spontaneously form wrinkles of suitable size and shape during polymerization. Therefore, rationally controlling the surface morphology of cation exchange membranes is crucial for improving ion flux.

[0005] Third, there is a trade-off between ion permeability and selectivity. Improving the selectivity of cation exchange membranes for different ions hinges on controlling the migration rates of different ions within the transport channels. Ion transport through sub-nanopore membranes can be viewed as ion diffusion driven by chemical potential gradients, typically using Arrhenius-type equations. To describe ion permeability. Where P is solute permeability (solute flux normalized by driving force), and A... ′ It refers to the pre-factor, E a R is the energy barrier for solute transport, R is the universal gas constant, and T is the absolute temperature. Based on this formula and transition state theory, it is known that the interaction between ions and the host medium (i.e., water and pore walls) is the key mechanism for inducing ion selectivity. Constructing a selective separation layer on the surface of a cation exchange membrane, such as a polyamide layer with dense pores or a positively charged layer, is a simple method for preparing selective cation exchange membranes. However, while this method hinders the transmembrane transport of divalent ions, it also hinders the transport of monovalent ions to varying degrees. This causes the flux of monovalent ions to decrease as the membrane selectivity increases, indicating a trade-off between monovalent / divalent ion permeability and selectivity in cation exchange membranes.

[0006] Fourth, the preparation process of selective cation exchange membranes is complex and costly. Current selective cation exchange membranes are mainly prepared through two approaches: surface modification or precise design of ion transport channels. Regarding surface modification, traditional chemical grafting methods require multiple steps such as sulfonation, quaternization, and cross-linking to control the surface charge density and hydrophilicity. However, the grafting rate and uniformity are limited by reaction kinetics; increasing the grafting rate leads to greater fluctuations in reaction time and modified layer thickness, reducing batch stability. While plasma modification can rapidly construct nanoscale functional layers (10-50 nm), the equipment cost is as high as 800,000-1,200,000 RMB per unit, and the modified layer has poor durability; after 10 cycles, the surface contact angle will rise from 35° to 68°. In terms of internal channel control, toxic and harmful chemical agents, complex chemical reaction processes, and harsh reaction conditions are often involved, all of which significantly increase the preparation cost of selective cation exchange membranes.

[0007] In summary, there is currently no method that can simultaneously control the thickness, surface topology, and ion permeability-selectivity balance of cation exchange membranes while meeting the requirements of simple and economical preparation processes. Developing a cation exchange membrane that simultaneously possesses low cost, good ion transport, and ion selectivity is urgently needed and has significant practical implications. Summary of the Invention

[0008] In view of this, the purpose of this invention is to provide a method for preparing cation exchange membranes based on the regulation of casting solution by chelating agents. By regulating the casting solution with specific chelating agents, this method simultaneously solves the problems of low ion transport rate, difficulty in surface morphology control, trade-off effect between ion permeability and selectivity, and complex and costly preparation of existing cation exchange membranes. It achieves precise control over the thickness, surface topology, and ion permeability-selectivity balance of the cation exchange membrane, and prepares cation exchange membranes with ultrathin ion exchange layers, high ion exchange capacity, good ion transport rate, and high ion selectivity, and the preparation process is simple and economical.

[0009] To achieve the above-mentioned objectives, the technical solution adopted is as follows:

[0010] The first aspect of this invention provides a method for preparing a cation exchange membrane based on a chelating agent-controlled casting solution, comprising step S1, preparing a casting solution, and step S2, impregnating / coating a porous filter membrane with the casting solution. The casting solution comprises a sulfonated monomer, an inducing agent, a photoinitiator, and a chelating agent. The inducing agent is configured to act on the molecular chains of the porous filter membrane to reduce the structural rigidity of the porous filter membrane. The chelating agent is configured to be water-soluble and contain hydrogen bond donors, enabling it to form hydrogen bonds with the sulfonic acid groups in the sulfonated monomer. The hydrogen bond donors in the chelating agent can be at least carboxylic acid groups and / or amino and / or hydroxyl groups.

[0011] That is, in this invention, the chelating agent has strong water solubility and contains hydrogen bond donors such as carboxylic acid groups, amino groups and hydroxyl groups, which can form hydrogen bonds with the sulfonic acid groups in the sulfonated monomer.

[0012] The sulfonating monomer can be 2-acrylamido-2-methylpropanesulfonic acid or 4-styrenesulfonic acid. The chelating agent can be at least one of the following: ethylenediaminetetraacetic acid, tetrasodium glutamate diacetate, trisodium methylglycine diacetate, tetrasodium iminodisuccinate, diethylenetriaminepentaacetic acid, S,S-ethylenediaminedisuccinic acid, or hydroxyethylidene diphosphonic acid.

[0013] Specifically, taking the hydrogen bond formation process between ethylenediaminetetraacetic acid (EDTA) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as an example, it depends on the donor-acceptor matching of the carboxyl, amino, sulfonate, and amide groups present in both molecules, with the binding modes mainly being NH···O- and OH···O-. Numerous such hydrogen bonds intertwine to construct a vast hydrogen bond network. Other similar combinations of chelating agents and sulfonated monomers, while differing in specific hydrogen bond formation details, share the same fundamental principle: constructing a hydrogen bond network based on the interaction of specific groups within the molecule. This interaction allows for the influence of the physicochemical properties of the casting solution (such as viscosity and stability), thus playing a crucial role in the preparation of ultrathin selective cation exchange membranes.

[0014] Moreover, hydrogen bonds, as a strong intermolecular force, can significantly enhance the mutual attraction between molecules, while the hydrogen bond network between molecules hinders the free movement of molecules. In the process of preparing cation exchange membranes using the method of this invention, taking EDTA-AMPS as an example, hydrogen bonds play a core role through a triple mechanism of molecular cross-linking, ion screening, and mechanical enhancement. That is, the dynamic hydrogen bond network constructs ultrathin and dense sub-nanopores (structure construction); the difference in interaction between hydrated ions and hydrogen bond sites enables efficient screening of monovalent / divalent ions (selective separation); and the responsiveness of hydrogen bonds improves the membrane's swelling resistance and mechanical strength (performance optimization).

[0015] Specifically, an ultrathin and dense sub-nanopore network is constructed based on the dynamic network formed by hydrogen bonds, providing a specific channel for ion transport. Hydrated ions interact with hydrogen bond sites when passing through the cation exchange membrane. Due to differences in charge quantity, ionic radius, and other properties between monovalent and divalent ions, the interaction between monovalent ions and hydrogen bond sites is relatively weaker, allowing them to pass through the membrane more easily; while the interaction between divalent ions and hydrogen bond sites is stronger, resulting in greater resistance during passage. This achieves efficient sieving of monovalent / divalent ions, further enhancing the ion selectivity of the membrane. Furthermore, hydrogen bonds are responsive; they can adjust their state when external environmental conditions (such as humidity and temperature) change. When the membrane tends to swell in contact with aqueous solutions, hydrogen bonds can enhance intermolecular interactions by changing their state and number, inhibiting membrane swelling, improving the membrane's anti-swelling properties and mechanical strength, and ensuring the stability and durability of the membrane in practical applications. Furthermore, the interaction of numerous hydrogen bonds causes the solution viscosity to increase exponentially, which has a significant impact on the fluidity of the casting solution and the subsequent film formation process. For example, it controls the penetration depth of the casting solution into the porous membrane support layer, thereby affecting the thickness and microstructure of the cation exchange membrane.

[0016] Furthermore, the formed hydrogen bonds can act as physical cross-linking points, enhancing intermolecular interactions, hindering molecular free movement, and forming intermolecular hydrogen bond networks, thereby causing the solution viscosity to increase exponentially. Specifically, when using porous filter membranes (polyvinylidene fluoride membranes, polyethersulfone membranes, cellulose acetate membranes) as the support layer, the control of the chelating agent concentration has a significant impact on the properties of the casting solution and the formation of the membrane structure during the preparation of the ultrathin selective cation exchange membrane of this invention. This is reflected in the fact that the chelating agent can change the viscosity and fluidity of the casting solution, and its concentration changes affect the penetration behavior of the casting solution in the membrane pores; by adjusting the chelating agent concentration, the penetration depth of the casting solution into the membrane pores can be controlled, thereby adjusting the thickness of the ion exchange layer; during the phase transformation process, the chelating agent concentration works synergistically with the physical process, affecting the chemical stability and mechanical properties of the membrane. Not only can this control effect be verified through experimental studies, but in practical applications, the chelating agent concentration can be optimized according to requirements to achieve precise control of the ion exchange layer thickness and improve membrane performance.

[0017] Meanwhile, changes in the solution properties of the casting solution induce differences in the properties of micro-regions during photopolymerization. These differentially expressed regions further evolve during aqueous solution post-treatment, ultimately forming membrane wrinkles with different morphologies. The formation mechanism of membrane wrinkles is closely related to the distribution of hydrophilic groups in the casting solution. For example, the sulfonic acid groups in AMPS molecules, due to their strong hydrophilicity, exhibit significant morphological changes in their distribution areas upon contact with aqueous solution, thus forming wrinkled structures. This invention, starting from the perspective of controlling the distribution of the casting solution within the membrane, optimizes the intrusion process of the casting solution to achieve precise control over the distribution of hydrophilic groups, thereby enabling diversified design of membrane wrinkle morphologies. Furthermore, utilizing the polar properties of hydrophilic groups (such as sulfonic acid groups), they can form hydrogen bonds with metal chelating agents. Based on this characteristic, by adjusting the type and amount of metal chelating agents, the intrusion behavior of the casting solution can be effectively controlled, thereby achieving controllable preparation of membrane wrinkle morphologies. This method provides a new approach to the microstructure design of membrane materials and expands their potential in functional applications.

[0018] Furthermore, the addition of chelating agents to form hydrogen bonds with sulfonated monomers can induce an ordered arrangement of the monomers. This is mainly because different types of chelating agents have different strengths and modes of hydrogen bonding with sulfonated monomers. By selecting different types of chelating agents and adjusting their dosage, the strength and number of hydrogen bonds can be controlled, thereby affecting the arrangement of sulfonated monomers and the microstructure of the membrane. This ordered arrangement helps to construct a more regular and stable ion transport network, thus effectively improving the ion transport rate. Moreover, the chelating agents used in this invention have a strong chelating ability for divalent and high-valence metal cations. Taking ethylenediaminetetraacetic acid (EDTA) as an example, its carboxyl and amino groups can provide lone pairs of electrons, which combine with divalent cations with empty electron orbitals through coordinate bonds to form stable complexes. When ions pass through the cation exchange membrane, divalent cations are preferentially chelated, which can greatly enhance the transmembrane resistance of divalent and high-valence metal cations, thereby improving the monovalent / divalent cation permeate selectivity of the cation exchange membrane.

[0019] In addition to the properties of the chelating agent itself, factors such as temperature, chelating agent concentration, reaction time, ionic strength, and ligand structure all affect chelating ability. By optimizing these conditions, the selectivity of cation exchange membranes can be further improved.

[0020] Further, step S1 of preparing the casting solution includes:

[0021] Step S101: Add the sulfonated monomer to the dissolving solvent and stir until the sulfonated monomer is completely dissolved;

[0022] Step S102: Add a chelating agent to the solution formed in step S101 and stir until all monomers are dissolved;

[0023] Step S103: Add an inducing agent to the solution formed in step S102 and continue stirring until all monomers are dissolved;

[0024] Step S104: Add a photoinitiator to the solution formed in step S103 and stir until dissolved to obtain a casting solution.

[0025] The initiator can be trimethylolpropane triacrylate or methacrylamide. The photoinitiator can be 2-hydroxy-2-methylphenylacetone or 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone.

[0026] In this invention, photoinitiators are also indispensable in the preparation process. After absorbing ultraviolet light, the photoinitiator undergoes intramolecular electron transitions to form an excited state, which in turn generates free radicals. These free radicals initiate polymerization reactions, promoting the interconnection of monomer molecules to form a three-dimensional cross-linked network structure. This structure not only endows the film with certain mechanical strength and stability but also fixes the ordered arrangement of the sulfonated monomers.

[0027] Furthermore, the dissolving solvent was set as ultrapure water and methanol in a volume ratio of 1:1. The mixed solvent of ultrapure water and methanol ensures good solubility of the sulfonated monomer and provides a suitable environment for subsequent reactions.

[0028] Further, in step S102, the amount of the chelating agent added is 0.1-2.0 mol%; in step S103, the amount of the inducing agent added is 3.0-5.0 mol%; and in step S104, the amount of the photoinitiator added is 0.2-0.8 mol%. The precise amounts of the chelating agent, inducing agent, and photoinitiator have a significant impact on the reaction process and the final membrane performance. Too little may fail to fully initiate the reaction or achieve the regulation of the membrane structure, while too much may lead to side reactions and affect the quality of the membrane.

[0029] Furthermore, step S2, which involves impregnating / coating the porous filter membrane with the casting solution, includes:

[0030] S201 uses a porous filter membrane as a substrate and impregnates or coats it with a casting solution.

[0031] S202, the impregnated or coated porous filter membrane is irradiated under visible / ultraviolet light for a time T to induce the porous filter membrane to polymerize and solidify, and the partially deconstructed porous filter membrane is compressed based on the cross-linked network formed by polymerization and solidification, so as to make the membrane thinner and increase the density of exchange sites.

[0032] S203, after polymerization and curing, the porous filter membrane is immersed in a remodeling solution to remodel the surface morphology of the membrane; wherein, the remodeling solution is constructed such that the amorphous structure formed in the porous filter membrane due to the action of the inducing agent can be transformed from amorphous to crystalline after contact with the remodeling solution, and / or the cross-linked network formed by polymerization and curing can undergo uneven expansion after contact with it, so that the membrane surface is remodeled to form a wrinkled morphology.

[0033] The effect of the inducing agent on the molecular chains of the porous filter membrane to reduce its rigidity manifests in two ways. First, the small molecular structure of the inducing agent can penetrate between the molecular chains, weakening the van der Waals forces and thus reducing crystallinity, promoting the dissolution of crystals or some crystals, thereby reducing the membrane's rigidity. Second, the small molecular structure of the inducing agent penetrates between the molecular chains, effectively weakening the inter-chain dipole interaction and stacking effect, leading to a decrease in chain segment packing density and a looser structure. In other words, the inducing agent causes the porous filter membrane's structure to deconstruct, laying the foundation for subsequent remodeling based on a remodeling solution to form wrinkles. For crystalline or partially crystalline porous filter membranes, the former approach is primarily applicable, while for non-crystalline porous filter membranes, the latter approach is more suitable.

[0034] The following explanation uses trimethylolpropane triacrylate (TMT) as the inducing agent and polyvinylidene fluoride (PVDF) porous membrane as an example. TMT contains three acrylate groups, and the oxygen atoms in these groups are highly polar. After impregnation or coating, it can form dipole-dipole interactions with the CF bonds of PVDF. Simultaneously, the small molecule structure of TMT can penetrate between PVDF molecular chains, weakening the van der Waals forces and dipole interactions, reducing crystallinity, and causing some PVDF crystals to dissolve, thus reducing the structural rigidity of the PVDF membrane. In other words, due to the presence of TMT, it can interact with the crystals in the PVDF membrane, causing some PVDF crystals to decompose into an amorphous state, thereby reducing the structural rigidity of the membrane.

[0035] After polymerization, the membrane is remodeled by immersing it in a remodeling solution. Upon contact with the solution, the amorphous polyvinylidene fluoride (PVDF) undergoes a phase transformation, changing from amorphous to crystalline. The phase transformation rate varies in different solutions, resulting in wrinkles of different morphologies on the membrane surface. Furthermore, the cross-linked network formed by polymerization and curing contains both hydrophilic and hydrophobic phases. When the membrane is immersed in the remodeling solution, the cross-linked network expands unevenly, creating wrinkles on the membrane surface. Specifically, the differential expansion of the hydrophilic and hydrophobic phases in the cross-linked network forms ring-shaped micron-sized wrinkles, increasing the membrane's specific surface area. This, combined with the orderly arrangement of sulfonated monomers to form regular ion transport channels, effectively enhances the ion transport rate.

[0036] Furthermore, the cross-linked network formed by polymerization and curing will undergo varying degrees of uneven expansion in different remodeling solutions, resulting in different wrinkle morphologies. In other words, under the combined effect of polyvinylidene fluoride phase inversion and uneven expansion of the cross-linked network, wrinkles of different shapes and properties will be remodeled on the membrane surface.

[0037] Therefore, porous filter membranes can also be polyethersulfone porous filter membranes and cellulose acetate membranes, wherein the deconstruction process of polyethersulfone porous filter membranes depends on the polar ester group (-COO) of trimethylolpropane triacrylate. -Trimethylolpropane triacrylate (TMT) molecules interact with the sulfone groups (-SO2-) and ether bonds (-O-) of the polyethersulfone (PES) porous membrane backbone via dipole-dipole interactions. Through its small molecular structure, TMT molecules penetrate into the interchain spaces of the PES porous membrane, effectively weakening interchain dipole interactions and π-π stacking effects, leading to a decrease in chain segment packing density and a looser local structure. Since the PES porous membrane itself is an amorphous polymer, the penetration of TMT further disrupts its weakly ordered regions, enhancing molecular chain mobility. Therefore, the PES porous membrane molecular chains undergo rearrangement and reconstruction during contact with the casting solution, with TMT molecules remaining in the interchain spaces to form a dynamic cross-linked network, thus improving the processing adaptability of the remodeled membrane.

[0038] The deconstruction process of cellulose acetate membranes relies on the formation of hydrogen bonds between the ester oxygen group of trimethylolpropane triacrylate and the hydroxyl group (-OH) in the cellulose acetate membrane molecular chain, while simultaneously engaging in dipole-dipole interactions with the carbonyl group (C=O) of the acetyl group (-OAc). Trimethylolpropane triacrylate molecules penetrate into the crystalline region of the cellulose acetate membrane through their small molecular structure, disrupting the hydrogen bond network and van der Waals forces between cellulose chains, leading to the deconstruction of the crystalline region into an amorphous state. This deconstruction process is accompanied by a significant plasticizing effect, significantly enhancing the mobility of the cellulose acetate membrane molecular chains. During contact with the casting solution, the hydrogen bonds of the cellulose acetate membrane molecular chains undergo partial rearrangement, while the presence of trimethylolpropane triacrylate inhibits complete crystallization, promoting the formation of a dynamic hydrogen bond cross-linking network between molecular chains, providing a structural basis for subsequent functionalization modifications.

[0039] After the polyethersulfone porous membrane or cellulose acetate membrane has been deconstructed and polymerized, it can be immersed in a remodeling solution for remodeling, undergoing a membrane surface remodeling process similar to that of the polyvinylidene fluoride porous membrane, thus forming micron-level wrinkles. Specifically, for the cellulose acetate membrane, its amorphous structure undergoes a phase transformation under the action of the remodeling solution, that is, it transforms from amorphous to crystalline. Moreover, due to the presence of hydrophilic and hydrophobic phases in the crosslinked network formed by polymerization and solidification, uneven expansion occurs, resulting in wrinkles. Similarly, for the polyethersulfone porous membrane, the presence of hydrophilic and hydrophobic phases in the crosslinked network formed by polymerization and solidification leads to uneven expansion, resulting in wrinkles.

[0040] It is worth noting that these wrinkles play an important role. They can enhance the microfluidic turbulence on the membrane surface, prevent the deposition of pollutants on the membrane surface, change the situation in existing commercial cation exchange membranes where pollutants are easily attached to and enter the membrane due to the flat surface, and significantly improve the membrane's antifouling performance.

[0041] Furthermore, this invention offers numerous advantages in its preparation method. It employs photocuring, such as visible or ultraviolet light curing, avoiding the prolonged heat treatment required in existing cation exchange membrane preparation processes. For instance, thermal polymerization requires heat treatment at 80°C for 3 hours, and hot rolling requires repeated hot rolling at 105°C for 1 hour, thus reducing energy consumption. Simultaneously, the sulfonated monomers and other raw materials used in this invention lack benzene ring structures, and methanol and water are used as solvents, making it a green preparation method that reduces costs.

[0042] Furthermore, the remodeling solution is ultrapure water, 10-70 wt% ethanol, or 0.01-0.5 mol / L NaCl solution. These different solutions provide different environments for the phase inversion of polyvinylidene fluoride (PVDF) and the expansion of the cross-linked network, thereby precisely controlling the morphology and properties of the membrane surface wrinkles and optimizing the performance of the cation exchange membrane. For example, when ultrapure water is used as the remodeling solution, the phase inversion of PVDF is relatively mild, forming wrinkles with specific morphologies; while different concentrations of ethanol and NaCl solutions will affect the phase inversion rate of PVDF and the degree of expansion of the cross-linked network by changing the polarity and ionic strength of the system, producing differentiated wrinkle structures to meet the performance requirements of cation exchange membranes in different application scenarios.

[0043] Furthermore, following step S2, a post-processing step S3 is included, which involves immersing the membrane in a 1 mol / L NaCl solution for N hours. The purpose of this step is to further stabilize the performance of the cation exchange membrane. Immersion in a 1 mol / L NaCl solution allows the ion exchange sites within the membrane to fully exchange with sodium ions, achieving ion exchange equilibrium and improving the membrane's ion exchange stability. Simultaneously, the interaction between ions in the solution and the membrane helps repair potential microscopic defects, enhancing the overall structural stability of the membrane and thus improving the reliability and lifespan of the cation exchange membrane in practical applications. In other words, immersing the membrane in NaCl solution, on the one hand, allows the ions in the membrane to reach an equilibrium state, further stabilizing the membrane's structure and performance; on the other hand, the exchange of ions in the solution with the active sites on the membrane surface and inside makes the membrane's performance more stable and uniform, ultimately successfully producing a high-performance cation exchange membrane.

[0044] Compared with traditional cation exchange membrane preparation methods, the method of this invention avoids the use of complex and toxic chemical reagents and processes, significantly reducing raw material costs and environmental pollution control costs. Simultaneously, the prepared cation exchange membrane exhibits significant improvements in key performance indicators such as ion selectivity, ion transport rate, and chemical stability. Furthermore, the entire preparation process is simple, easy to operate and control, suitable for large-scale industrial production, and is expected to bring new development opportunities to related fields.

[0045] According to the method disclosed in this invention, the porous filter membrane is one of polyvinylidene fluoride (PVDF) membrane, polyethersulfone (PES) membrane, or cellulose acetate membrane. PVDF membrane, PES membrane, and cellulose acetate membrane, as porous filter membranes, can be used in conjunction with chelating agents to regulate the casting solution, facilitating the preparation of high-performance ultrathin selective cation exchange membranes to meet the needs of various application scenarios.

[0046] The second aspect of the present invention also discloses a cation exchange membrane, which is prepared by the method disclosed in the first aspect of the present invention.

[0047] The advantages of this invention compared to the prior art are as follows:

[0048] 1. The present invention uses a chelating agent to regulate the thickness and surface morphology of cation exchange membranes, which, compared with the prior art, achieves rapid, simple and precise control of the morphology of cation exchange membranes.

[0049] 2. This invention selects chelating agents as regulators of the orderliness of cation exchange membrane transport networks, achieving green regulation of cation exchange membrane transport networks compared to existing technologies.

[0050] 3. The chelating agent-regulated selective ion exchange membrane prepared in this invention has an ultrathin ion exchange layer and high ion exchange capacity. Compared with the prior art, the ion exchange layer thickness is 20-30 nm, which is much lower than that of commercial membranes (100-600 nm), and the ion exchange capacity reaches 2.5-3.0 meq / g, which is higher than that of commercial cation exchange membranes (0.9-2.3 meq / g). This significantly improves the ion exchange capacity of the cation exchange membrane and reduces the membrane thickness (see appendix). Figure 4 ).

[0051] 4. The cation exchange membrane prepared by this invention utilizes a chelating agent to regulate the surface of the membrane, which exhibits annular micron-sized folds (see appendix). Figure 5 The wrinkles facilitate the capture of target ions by the cation exchange membrane and enhance the microfluidic field turbulence on the membrane surface, accelerating the ion exchange process and increasing the ion transport rate. Compared with commercially available cation exchange membranes with smooth surfaces, its ion transport rate is increased by 2-3 times (see appendix). Figure 6 ).

[0052] 5. The chelating agent-regulated selective ion exchange membrane prepared in this invention has an ordered ion transport network and a higher energy barrier for high-valence cations. Compared with existing cation exchange membranes, its selectivity for monovalent / divalent cations is improved by 2-20 times (see appendix). Figure 7 ).

[0053] The following describes in detail the method of regulating the morphology and ion selectivity of cation exchange membranes based on chelating agents, with reference to the embodiments shown in the accompanying drawings. Attached Figure Description

[0054] Figure 1This is a flowchart illustrating the main process steps for preparing the cation exchange membrane using the method of the present invention.

[0055] Figure 2 This is a flowchart of step S1 in the method of the present invention;

[0056] Figure 3 This is a flowchart of step S2 in the method of the present invention;

[0057] Figure 4 The following are structural and performance test diagrams related to the cation exchange membrane of the present invention: (a) is a cross-sectional SEM image of the membrane, showing the microstructure of the membrane and the thickness of each layer, with the inset in the upper right corner being a magnified view; (b) is a bar chart showing the change of ion exchange capacity (IEC) with EDTA content, reflecting the ion exchange capacity of the membrane under different EDTA contents.

[0058] Figure 5 The images show SEM images and elemental distribution maps of different cation exchange membranes (PEC-0.5, PEC-1.0, PEC-1.5, PEC-2.0) of the present invention. The main image in each sub-image is an SEM image showing the microstructure of the membrane surface. The upper right corner shows the distribution map of element F, and the lower right corner shows the distribution map of element S, which are used to analyze the distribution of elements on the membrane surface.

[0059] Figure 6 NH4+ at different concentrations using different membranes (CMI-7000S, FKS-130, CMVN, EPC-1.0) + Flux bar chart comparing NH4+ concentrations of 2.5 mM, 25 mM, and 50 mM for different membranes. + Flux differences, where the concentrations corresponding to each membrane from left to right are 2.5 mM, 25 mM, and 50 mM (EPC-1.0 is the cation exchange membrane of this invention);

[0060] Figure 7 NH4 for different membranes (CMVN, FKS-130, CMI-7000S, EPC-1.0) + Flux and NH4 + / Mg 2+ Selective comparison bar chart, where the left bar for each membrane represents NH4. + Flux, the right column represents NH4 + / Mg 2+ Selectivity, used to evaluate the different membranes for NH4 + Transmission capability and NH4 + and Mg 2+ The selectivity difference (EPC-1.0 is the cation exchange membrane of the present invention). Detailed Implementation

[0061] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other.

[0062] like Figure 1 As shown, this invention discloses a method for preparing cation exchange membranes based on chelating agents regulating casting solutions, comprising: step S1, preparing a casting solution, and step S2, impregnating / coating a porous filter membrane with the casting solution. The casting solution includes a sulfonated monomer, an inducing agent, a photoinitiator, and a chelating agent. The inducing agent is configured to act on the molecular chains of the porous filter membrane to reduce the rigidity of the membrane structure. The chelating agent is configured to be water-soluble and contain hydrogen bond donors, enabling it to form hydrogen bonds with the sulfonic acid groups in the sulfonated monomer. The hydrogen bond donors in the chelating agent can be at least carboxylic acid groups and / or amino and / or hydroxyl groups.

[0063] Step S1 of preparing the casting solution includes:

[0064] Step S101: Add the sulfonated monomer to the dissolving solvent and stir until the sulfonated monomer is completely dissolved.

[0065] Step S102: Add a chelating agent to the solution formed in step S101 and stir until all monomers are dissolved; the amount of chelating agent added is 0.1-2.0 mol%, which can be any value including both ends in the range of 0.1-2.0 mol%.

[0066] Step S103: Add an inducing agent to the solution formed in step S102, and continue stirring until all monomers are dissolved; the amount of inducing agent added is 3.0-5.0 mol%, which can be any value including both ends of the range of 3.0-5.0 mol%, preferably 4.0 mol%.

[0067] Step S104: Add photoinitiator to the solution formed in step S103 and stir until dissolved to obtain casting solution; the amount of photoinitiator added is 0.2-0.8 mol%, which can be any value including both ends of the range of 0.2-0.8 mol%, preferably 0.5 mol%.

[0068] The chelating agent can be at least one of the following: ethylenediaminetetraacetic acid, tetrasodium glutamate diacetate, trisodium methylglycine diacetate, tetrasodium iminodisuccinate, diethylenetriaminepentaacetic acid, S,S-ethylenediaminedisuccinic acid, or hydroxyethylidene diphosphonic acid. The sulfonating monomer can be 2-acrylamido-2-methylpropanesulfonic acid or 4-styrenesulfonic acid. The initiator can be trimethylolpropane triacrylate or methacrylamide. The photoinitiator can be 2-hydroxy-2-methylphenylacetone or 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone.

[0069] In addition, the so-called dissolving solvent is set as ultrapure water and methanol with a volume ratio of 1:1.

[0070] Specifically, firstly, sulfonated monomer powders (2-acrylamido-2-methylpropanesulfonic acid, 4-styrenesulfonic acid) are added to ultrapure water and methanol (volume ratio 1:1) and stirred for 10-20 minutes until completely dissolved. Then, a certain amount (0.1-2.0 mol%) of chelating agent is added to the solution, and stirring continues for 10-20 minutes until all monomers dissolve. Next, a certain amount (4.0 mol%) of initiator (trimethylolpropane triacrylate, methacrylamide) is added to the solution, and stirring continues for 10-20 minutes until all monomers dissolve. Then, under light-protected conditions, photoinitiators (2-hydroxy-2-methylphenylacetone, 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone) (0.5 mol%) are added to the mixed solution, and stirring is carried out for 10-20 minutes to achieve uniform dissolution.

[0071] By precisely controlling the raw material ratio and stirring time, the components are fully dissolved and uniformly mixed. This not only creates favorable conditions for the subsequent impregnation or coating of the casting solution onto the porous membrane, as well as the photopolymerization reaction, but also helps to prepare cation exchange membranes with ultra-thin ion exchange layers, high ion exchange capacity, fast ion transport rate, and excellent selectivity for monovalent / divalent ions, meeting the needs of many practical applications such as seawater desalination and lithium extraction from salt lake brine.

[0072] Step S2, which involves impregnating / or coating the porous filter membrane with a casting solution, includes:

[0073] S201 uses a porous filter membrane as a substrate, and impregnates or coats the membrane surface with a casting solution; wherein the porous filter membrane can be any one of polyvinylidene fluoride membrane, polyethersulfone membrane, or cellulose acetate membrane.

[0074] S202, the impregnated or coated porous filter membrane is irradiated under visible / ultraviolet light for a time T to induce the porous filter membrane to polymerize and solidify, and the partially deconstructed porous filter membrane is compressed based on the cross-linked network formed by polymerization and solidification, so as to make the membrane thinner and increase the density of exchange sites.

[0075] S203, after polymerization and curing, the porous filter membrane is immersed in a remodeling solution to remodel the surface morphology of the membrane; wherein, the remodeling solution is constructed such that the amorphous structure in the porous filter membrane can be transformed from amorphous to crystalline after contact with the remodeling solution, and / or the cross-linked network formed by polymerization and curing can undergo uneven expansion after contact with it, so that the membrane surface is remodeled to form a wrinkled morphology.

[0076] Following step S2, a post-processing step S3 is also included, which includes immersing the membrane in a 1 mol / L NaCl solution for N hours; this allows the ions inside the membrane to reach equilibrium, stabilizes the structure and performance of the membrane, promotes the exchange of solution ions with the active sites on the membrane surface and inside, and makes the membrane performance more stable and uniform; wherein the time N is 24 hours.

[0077] The remodeling solution is ultrapure water, or 10-70 wt% ethanol, or 0.01-0.5 mol / L NaCl solution.

[0078] In step S2, there are two operation methods: impregnation and coating. Impregnation involves immersing the porous membrane in a casting solution under light-protected conditions. This allows the casting solution to fully penetrate the membrane pores. Excess casting solution is then scraped off to ensure a smooth membrane surface, preventing excess solution from affecting membrane performance and morphology. This ensures the subsequent photopolymerization reaction occurs in a homogeneous system, resulting in a stable and uniform membrane structure after polymerization. Coating, on the other hand, involves applying the casting solution to the surface of the porous membrane and allowing it to stand, allowing the solution to distribute evenly across the membrane surface. Compared to impregnation, coating focuses more on forming a uniform liquid film on the membrane surface, which is beneficial for the subsequent photopolymerization reaction to build a uniform cross-linked network on the membrane surface. Scraping off excess casting solution also ensures the uniformity and cleanliness of the membrane surface, preventing performance differences due to uneven distribution of the casting solution and ensuring the final cation exchange membrane has stable and consistent performance.

[0079] Specifically, the impregnation upgrade of porous filter membranes can be performed as follows: Immerse the porous filter membrane (polyvinylidene fluoride membrane, polyethersulfone membrane, cellulose acetate membrane) in the above mixed solution for 1-20 minutes in a light-protected environment. Hydrogen bonds form between the chelating agent and the sulfonated monomer, altering the viscosity and flowability of the casting solution and controlling the depth of penetration into the pores. Remove the filter membrane, scrape off excess mixed solution from the surface, and then irradiate it under ultraviolet light for 20-60 minutes to initiate a polymerization reaction and form a cross-linked network. After polymerization, the membrane is immersed in ultrapure water for post-treatment. The cross-linked network within the membrane contains both hydrophilic and hydrophobic phases; different types and amounts of chelating agents will result in varying degrees of uneven expansion, leading to different wrinkle morphologies. Finally, the membrane is stabilized by immersing it in a 1 mol / L NaCl solution for 24 hours to complete the preparation of a high-quality selective cation exchange membrane.

[0080] For the coating upgrade of porous filter membranes, the procedure is as follows: A quantitative amount (1-10 mL) of casting solution is coated onto the surface of the porous membrane (polyvinylidene fluoride membrane, polyethersulfone membrane, cellulose acetate membrane), and allowed to stand for 1-8 minutes. The filter membrane is then removed, excess mixture is scraped off, and then irradiated under ultraviolet light for 20-60 minutes to initiate the polymerization reaction. After polymerization, the membrane is immersed in ultrapure water for post-treatment. Finally, the membrane is immersed in a 1 mol / L NaCl solution for 24 hours to complete the preparation of a high-quality cation exchange membrane.

[0081] Regardless of the upgrade method, in step S2, the irradiation time T is 20-60 minutes. During this period of irradiation, the photoinitiator contained in the membrane effectively absorbs light energy to generate free radicals, initiating a polymerization reaction. This causes the monomers in the casting solution to crosslink and form a stable three-dimensional network structure. This structure determines the basic properties of the membrane, such as ion exchange capacity and ion transport channels, and is crucial for preparing a cation exchange membrane that meets the requirements.

[0082] This invention focuses on the preparation of ultrathin selective cation exchange membranes. The method is green and simple, meets the needs of industrial production, and endows the membrane with excellent performance through a variety of synergistic mechanisms.

[0083] In the preparation process, the interaction between the chelating agent and the sulfonated monomer is crucial. Common chelating agents, such as ethylenediaminetetraacetic acid (EDTA) and tetrasodium glutamate diacetate (TGA), are highly water-soluble, and their carboxylic acid groups, amino groups, and hydroxyl groups act as hydrogen bond donors. Sulfonated monomers, such as 2-acrylamido-2-methylpropanesulfonic acid (2-acrylamido-2-methylpropanesulfonic acid) and 4-styrenesulfonic acid (4-styrenesulfonic acid), contain sulfonic acid groups. Taking EDTA and AMPS as examples, they form hydrogen bonds primarily through the donor-acceptor relationship between the carboxylic acid, amino, sulfonic acid, and amide groups, following two main modes: NH···O- and OH···O-. Numerous such hydrogen bonds intertwine to construct a vast hydrogen bond network. While the specific details of hydrogen bond formation may differ in other similar combinations of chelating agents and sulfonated monomers, the basic principle remains the same: constructing a hydrogen bond network based on the interaction of specific groups within the molecule.

[0084] This hydrogen-bonded network significantly influences the properties of the casting solution. It greatly enhances the intermolecular forces in the casting solution, restricting molecular movement and increasing viscosity and reducing fluidity, while simultaneously improving stability. These changes further affect the permeation behavior of the casting solution in porous supported membranes. By adjusting the chelating agent concentration, the depth of the casting solution's penetration into the membrane pores can be precisely controlled, thereby accurately regulating the thickness of the ion exchange layer. Moreover, the hydrogen-bonded network also induces the ordered arrangement of sulfonated monomers. Different types of chelating agents form hydrogen bonds with sulfonated monomers with varying strengths and numbers; changing the amount of chelating agent added also alters the hydrogen bond configuration, thus affecting the arrangement of sulfonated monomers and the membrane's microstructure. The ordered arrangement of sulfonated monomers lays the foundation for constructing a regular and stable ion transport network, effectively improving the ion transport rate.

[0085] Furthermore, the unique structure of chelating agents endows them with the ability to chelate divalent cations. Taking ethylenediaminetetraacetic acid (EDTA) as an example, its carboxyl and amino groups can donate lone pairs of electrons, which combine with divalent cations with empty electron orbitals through coordination bonds to form stable complexes. When ions pass through a cation exchange membrane, divalent cations are preferentially chelated, leading to a significant increase in transmembrane resistance, while monovalent cations pass relatively more easily. This significantly improves the membrane's monovalent / divalent ion selectivity, playing a crucial role in practical applications such as seawater desalination and lithium extraction from salt lake brine.

[0086] In the preparation of ultrathin selective cation exchange membranes, porous membranes such as polyvinylidene fluoride (PVDF), polyethersulfone (PES), or cellulose acetate membranes are selected as the support layer, making the chelating agent concentration a crucial factor. Changes in chelating agent concentration alter the viscosity and flowability of the casting solution. Higher concentrations enhance intermolecular interactions, increasing the viscosity and decreasing the flowability of the casting solution; conversely, lower concentrations have the opposite effect. This concentration difference also influences the penetration behavior of the casting solution within the membrane pores, thus affecting the thickness of the ion exchange layer. High concentrations of chelating agent result in high viscosity and poor flowability of the casting solution, making it difficult for the solution to penetrate deep into the membrane pores, resulting in a thinner ion exchange layer. Conversely, low concentrations of chelating agent result in low viscosity and good flowability, allowing the solution to penetrate deeper into the membrane pores, leading to a thicker ion exchange layer. Therefore, precise control of the ion exchange layer thickness can be achieved by accurately adjusting the chelating agent concentration. During phase transformation, the chelating agent concentration works synergistically with the physical processes, affecting the chemical stability and mechanical properties of the membrane. An appropriate concentration of chelating agent can promote the formation of a more stable chemical structure in the membrane, enhancing its chemical stability. Simultaneously, it also affects the membrane's microstructure and molecular arrangement. High concentrations result in a denser membrane structure, increasing strength and hardness, but reducing flexibility; conversely, low concentrations lead to a relatively looser membrane structure, providing better flexibility, but potentially decreasing strength. Experiments can directly verify the control effect of chelating agent concentration on the membrane preparation process and performance. In practical applications, the chelating agent concentration needs to be optimized according to different requirements.

[0087] In the EDTA-AMPS cation exchange membrane system, hydrogen bonds exhibit even more unique roles. The dynamic network they form constructs ultrathin and dense sub-nanopores, providing specific channels for ion transport. Hydrated ions interact with hydrogen bond sites as they pass through the cation exchange membrane. Due to differences in charge quantity and ionic radius between monovalent and divalent ions, the interaction between monovalent ions and hydrogen bond sites is relatively weaker, allowing them to pass through the membrane more easily; while the interaction between divalent ions and hydrogen bond sites is stronger, resulting in greater resistance during passage. This achieves efficient sieving of monovalent / divalent ions, further enhancing the membrane's ion selectivity. Furthermore, hydrogen bonds are responsive; they can adjust their state when external environmental conditions (such as humidity and temperature) change. When the membrane tends to swell in contact with aqueous solutions, hydrogen bonds can enhance intermolecular interactions by changing their state and number, inhibiting membrane swelling, improving the membrane's anti-swelling properties and mechanical strength, and ensuring the membrane's stability and durability in practical applications. Furthermore, the interaction of numerous hydrogen bonds causes the solution viscosity to increase exponentially, which has a significant impact on the fluidity of the casting solution and the subsequent film formation process. For example, it controls the penetration depth of the casting solution into the porous membrane support layer, thereby affecting the thickness and microstructure of the cation exchange membrane.

[0088] Changes in the properties of the casting solution can lead to differences in the microscopic properties of different regions during photopolymerization. During aqueous post-treatment, due to the varying distribution of hydrophilic groups (such as sulfonic acid groups in AMPS molecules) in the casting solution, regions containing sulfonic acid groups absorb a large amount of water and swell, while surrounding less hydrophilic or hydrophobic regions swell less. This difference in expansion between regions generates internal stress, which, when it reaches a certain level, leads to wrinkles on the membrane surface. This invention addresses this by controlling the distribution of the casting solution within the membrane. By optimizing the process of the casting solution penetrating the pores of the porous filter membrane support layer, the distribution of hydrophilic groups is precisely controlled. Utilizing the hydrogen bond formation between hydrophilic groups and metal chelating agents, the type and amount of metal chelating agents are adjusted to control the penetration behavior of the casting solution, achieving controllable preparation of membrane wrinkle morphology. This innovative method provides a completely new approach to the microstructure design of membrane materials, broadening the potential of membranes in functional applications.

[0089] Photoinitiators are also indispensable in the preparation process. After absorbing ultraviolet light, the photoinitiator undergoes intramolecular electron transitions to form an excited state, which in turn generates free radicals. These free radicals initiate polymerization reactions, promoting the interconnection of monomer molecules to form a three-dimensional cross-linked network structure. This structure not only endows the membrane with certain mechanical strength and stability but also fixes the ordered arrangement of the sulfonated monomers. In the post-treatment stage of the membrane, the hydrophilic and hydrophobic phases in the cross-linked network expand differentially, forming ring-shaped micron-sized wrinkles, increasing the specific surface area of ​​the membrane. This, combined with the regular ion transport channels formed by the ordered arrangement of the sulfonated monomers, effectively enhances the ion transport rate. Finally, immersing the membrane in a NaCl solution allows the ions in the membrane to reach equilibrium, further stabilizing the membrane's structure and performance. Furthermore, the ions in the solution exchange with the active sites on the membrane surface and inside, making the membrane's performance more stable and uniform, ultimately successfully producing a high-performance cation exchange membrane.

[0090] Compared with traditional cation exchange membrane preparation methods, the method of this invention avoids the use of complex and toxic chemical reagents and processes, significantly reducing raw material costs and environmental pollution control costs. Simultaneously, the prepared cation exchange membrane exhibits significant improvements in key performance indicators such as ion selectivity, ion transport rate, and chemical stability. Furthermore, the entire preparation process is simple, easy to operate and control, suitable for large-scale industrial production, and is expected to bring new development opportunities to related fields.

[0091] In this embodiment of the invention, the porous filter membrane is one of polyvinylidene fluoride (PVDF) membrane, polyethersulfone (PES) membrane, or cellulose acetate membrane. As porous filter membranes, PVDF membranes, PES membranes, and cellulose acetate membranes can be used in conjunction with chelating agents to regulate the casting solution, facilitating the preparation of high-performance ultrathin selective cation exchange membranes to meet the needs of various application scenarios.

[0092] The present invention also discloses a cation exchange membrane, which is prepared by the method disclosed in the present invention. The cation exchange membrane has an ion exchange capacity of 2.5-3.0 meq / g and a thickness of 20-30 nm.

[0093] This invention focuses on the preparation and performance improvement of cation exchange membranes based on chelating agents, and the specific operations are as follows:

[0094] I. Preparation of Casting Solution: First, sulfonated monomer powders such as 2-acrylamido-2-methylpropanesulfonic acid and 4-styrenesulfonic acid are added to a 1:1 volume ratio mixture of ultrapure water and methanol, and stirred for 10-20 minutes until completely dissolved, providing pure raw materials for subsequent reactions. Next, 0.1-2.0 mol% of chelating agents such as ethylenediaminetetraacetic acid and tetrasodium glutamate diacetate are added, and stirring is continued for 10-20 minutes. The chelating agents form a hydrogen bond network with the sulfonated monomers, affecting the properties of the casting solution and controlling the morphology and ion selectivity of the membrane. Then, 4.0 mol% of crosslinking agents such as trimethylolpropane triacrylate and methacrylamide are added, and stirring is continued for 10-20 minutes to promote monomer interconnection and construct a stable membrane structure. Finally, under light-protected conditions, 0.5 mol% of photoinitiators such as 2-hydroxy-2-methylphenylacetone and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone are added, and stirring is continued for 10-20 minutes until uniformly dissolved, completing the preparation of the casting solution. Precise control of raw material ratios and mixing time ensures that all components are fully dissolved and evenly mixed, laying the foundation for subsequent steps.

[0095] II. Porous Filter Membrane Treatment: Polyvinylidene fluoride membrane, polyethersulfone membrane or cellulose acetate membrane are selected as porous filter membranes, and there are two operation methods: impregnation and coating.

[0096] Impregnation process: Immerse the porous filter membrane in the casting solution for 1-20 minutes under light-protected conditions. During this time, the hydrogen bond network formed by the chelating agent and the sulfonated monomer regulates the viscosity and flowability of the casting solution, thereby controlling the depth to which the casting solution penetrates the filter membrane pores. After immersion, remove the filter membrane and scrape off excess casting solution to ensure a smooth membrane surface, preventing interference with subsequent photopolymerization reactions and membrane performance. (Refer to...) Figure 1 Its position and role in the preparation process can be understood.

[0097] Alternatively, a coating process can be used: The casting solution is coated onto the surface of the porous filter membrane and allowed to stand for 1-8 minutes to allow for uniform distribution of the solution on the membrane surface. This creates favorable conditions for the subsequent photopolymerization reaction to build a uniform cross-linked network. Afterward, excess casting solution is scraped off to ensure a clean and uniform membrane surface. Figure 1 It also demonstrates the sequence and importance of the coating operation in the entire process.

[0098] III. Photopolymerization Reaction: The impregnated or coated porous filter membrane is irradiated under ultraviolet light for 20-60 minutes. After absorbing light energy, the photoinitiator undergoes intramolecular electron transitions to form excited states and generate free radicals, initiating a polymerization reaction that crosslinks the monomers in the casting solution to form a stable three-dimensional network structure. From Figure 4 (a) The cross-sectional SEM image of the membrane shows the microstructure of the membrane, which is closely related to the cross-linked network formed by the photopolymerization reaction. Figure 4(b) The variation of ion exchange capacity (IEC) with EDTA content shows the influence of photopolymerization reaction conditions on membrane performance. This structure plays a decisive role in the basic properties of the membrane, such as ion exchange capacity and ion transport channels.

[0099] IV. Further Processing: The polymerized porous membrane is then immersed in ultrapure water for further processing. Due to the varying distribution of hydrophilic groups (such as sulfonic acid groups in AMPS molecules) in the casting solution, areas containing sulfonic acid groups absorb a large amount of water and swell, while surrounding areas with weaker hydrophilicity or hydrophobicity swell less. This difference in swelling generates internal stress. When the internal stress reaches a certain level, annular micron-sized wrinkles form on the membrane surface. Figure 5 In the SEM images of different cation exchange membranes (PEC-0.5, PEC-1.0, PEC-1.5, PEC-2.0), the wrinkled morphology of the membrane surface can be clearly observed. These wrinkles are beneficial for the cation exchange membrane to capture target ions, enhance the microfluidic field turbulence on the membrane surface, and accelerate the ion exchange process.

[0100] V. Stabilization Treatment (i.e., Post-treatment): The post-treated porous membrane is immersed in a 1 mol / L NaCl solution for 24 hours to stabilize it. This operation allows the ions inside the membrane to reach equilibrium, stabilizes the membrane's structure and performance, promotes the exchange of solution ions with the membrane surface and internal active sites, and makes the membrane performance more stable and uniform.

[0101] VI. Cation Exchange Membrane Performance: The cation exchange membrane prepared through the above steps exhibits excellent performance, with an ion exchange capacity of 2.5-3.0 meq / g and a thickness of 20-30 nm. Figure 6 NH4+ concentrations at different membrane concentrations (CMI-7000S, FKS-130, CMVN, EPC-1.0) + Flux bar chart, and Figure 7 NH4 content of different membranes (CMVN, FKS-130, CMI-7000S, EPC-1.0) + Flux and NH4 + / Mg 2+ As can be seen from the selective comparison bar chart, the cation exchange membrane (EPC-1.0) prepared in this invention has significantly improved ion transport rate and ion selectivity compared with other commercial membranes, and can meet the needs of many practical applications such as seawater desalination and lithium extraction from salt lake brine.

[0102] In this specification and the accompanying drawings, CMI-7000S, FKS-130, CMVN, and EPC-1.0 are all commercially available membranes on the market, while PEC represents a membrane prepared according to the method of the present invention.

[0103] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a cation exchange membrane based on a chelating agent-controlled casting solution, comprising step S1, preparing a casting solution, and step S2, impregnating or coating a porous filter membrane with the casting solution, characterized in that... The casting solution includes sulfonated monomers, inducing agents, photoinitiators, and chelating agents. The inducing agent is a substance that can act on the molecular chains of the porous filter membrane to reduce the rigidity of the porous filter membrane structure. The chelating agent is a water-soluble substance with hydrogen bond donors so that it can form hydrogen bonds with the sulfonic acid groups in the sulfonated monomer. The chelating agent is at least one of the following: ethylenediaminetetraacetic acid, tetrasodium glutamate diacetate, trisodium methylglycine diacetate, tetrasodium iminodisuccinate, diethylenetriaminepentaacetic acid, S,S-ethylenediaminedisuccinic acid, and hydroxyethylidene diphosphonic acid. The inducing agent is trimethylolpropane triacrylate or methacrylamide; Step S2, which involves impregnating or coating the porous filter membrane with a casting solution, includes: S201 uses a porous filter membrane as a substrate and impregnates or coats it with a casting solution. S202, the impregnated or coated porous filter membrane is irradiated under visible / ultraviolet light for a time T to induce the porous filter membrane to polymerize and solidify, and the partially deconstructed porous filter membrane is compressed based on the cross-linked network formed by polymerization and solidification, so as to make the membrane thinner and increase the density of exchange sites. S203, after polymerization and curing, the porous filter membrane is immersed in a remodeling solution to remodel the surface morphology of the membrane. The remodeling solution is configured such that the amorphous structure formed in the porous filter membrane due to the action of the inducing agent can be transformed from amorphous to crystalline after contact with the remodeling solution, and / or the cross-linked network formed by polymerization and curing can undergo uneven expansion after contact with the remodeling solution, so that the membrane surface is remodeled to form a wrinkled morphology.

2. The method according to claim 1, characterized in that, The hydrogen bond donors in the chelating agent are at least carboxylic acid groups and / or amino and / or hydroxyl groups.

3. The method according to claim 2, characterized in that, Step S1 of preparing the casting solution includes: Step S101: Add the sulfonated monomer to the dissolving solvent and stir until the sulfonated monomer is completely dissolved; Step S102: Add a chelating agent to the solution formed in step S101 and stir until all components are dissolved; Step S103: Add an inducing agent to the solution formed in step S102 and continue stirring until all components are dissolved; Step S104: Add a photoinitiator to the solution formed in step S103 and stir until dissolved to obtain a casting solution.

4. The method according to claim 3, characterized in that, The sulfonated monomer is 2-acrylamido-2-methylpropanesulfonic acid or 4-styrenesulfonic acid; the photoinitiator is 2-hydroxy-2-methylphenylacetone or 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone.

5. The method according to claim 3, characterized in that, The dissolving solvent is set as ultrapure water and methanol in a volume ratio of 1:

1.

6. The method according to claim 3, characterized in that, In step S102, the amount of the chelating agent added is 0.1-2.0 mol% In step S103, the amount of the inducing agent added is 3.0-5.0 mol% In step S104, the amount of photoinitiator added is 0.2-0.8 mol.

7. The method according to claim 6, characterized in that, The remodeling solution is ultrapure water, 10-70 wt% ethanol, or 0.01-0.5 mol / L NaCl solution.

8. The method according to claim 6, characterized in that, Following step S2, a post-processing step S3 is also included, which includes immersing the membrane in a 1 mol / L NaCl solution.

9. A cation exchange membrane, characterized in that, The cation exchange membrane is prepared by the method according to any one of claims 1-8.

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