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

Abstract

The invention discloses a method for preparing a cation exchange membrane by regulating and controlling a membrane casting solution based on a chelating agent. According to the method, a porous filter membrane (such as a polyvinylidene fluoride membrane) is used as a supporting layer, and the membrane is prepared through the steps of dipping or coating, ultraviolet irradiation polymerization, ultrapure water post-treatment, NaCl solution stabilization and the like in a membrane casting solution containing a specific chelating agent, a sulfonated monomer, a cross-linking agent and a photoinitiator. The chelating agent can form a hydrogen bond network with sulfonated monomers, regulate and control the viscosity and fluidity of a membrane casting solution, induce ordered arrangement of the monomers, chelate divalent cations and improve the selective separation capacity of the membrane on monovalent and divalent ions. Compared with the prior art, the problems that a commercial cation exchange membrane is slow in ion transmission, the surface appearance is difficult to control, ion permeability and selectivity are difficult to balance, and preparation is complex and expensive are solved. The prepared cation exchange membrane has the ion exchange capacity of 2.5-3.0 meq / g and the thickness of 20-30nm, and has the advantages of low cost, good ion transmission and ion selectivity.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of cation exchange membranes. Specifically, it relates to a method for preparing cation exchange membranes by regulating casting solutions with chelating agents and the cation exchange membranes prepared thereby. In particular, it relates to a preparation technology for regulating the thickness, surface morphology of cation exchange membranes and enhancing the selectivity of monovalent / divalent ions of the membranes, covering the design of the network structure and mass transfer channels of cation exchange membranes, as well as related technologies of Donnan dialysis devices using such cation exchange membranes. Background Art

[0002] Cation exchange membranes are widely used in many fields and play a key role in scenarios such as seawater desalination, high-salt concentration, ammonia nitrogen recovery, lithium-magnesium separation, etc. With the continuous growth of market demand, the global production capacity of cation exchange membranes is also increasing rapidly. However, compared with water-permeable filter membranes (such as microfiltration and ultrafiltration membranes), the price of cation exchange membranes is relatively high, and high-performance cation exchange membranes in China have long relied on imports from countries such as the United States, Japan, and Germany.

[0003] In the actual application process, many problems have emerged in existing commercial cation exchange membranes. First, the ion transport rate is relatively low. According to the Donnan dialysis ion flux calculation formula, the ion transport rate is proportional to the ion exchange capacity and inversely proportional to the membrane thickness. An ideal cation exchange membrane should have the characteristics of 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. Although the pore filling method can achieve a high-density fixation of exchange sites in the membrane, it is impossible to further reduce the membrane thickness, which leads to the inability to synchronously regulate the ion exchange capacity and the membrane thickness, and thus it is difficult to improve the ion transport rate of cation exchange membranes.

[0004] Second, it is difficult to regulate the surface morphology. The surfaces of existing commercial cation exchange membranes are smooth, which is not conducive to the capture of ions in the solution. Studies have found that wrinkles on the membrane surface can generate turbulence on the membrane surface, accelerating the migration of target ions to the membrane surface, which is thus conducive to the capture of target ions by the ion exchange membrane. The wrinkle size has a direct impact on the distribution characteristics of the flow field morphology on the membrane surface, and the ion capture efficiency of ion exchange membranes varies significantly for wrinkles with different geometric parameters. Non-optimized surface wrinkles are likely to induce hydrodynamic dead zones on the membrane surface, leading to a decrease in ion flux. Currently, most commercial cation exchange membranes have smooth surfaces without wrinkles, and it is difficult to spontaneously form wrinkles with appropriate sizes and shapes during the polymerization process. Therefore, reasonably regulating the surface morphology of cation exchange membranes is crucial for improving the ion flux.

[0005] Thirdly, there is a trade-off effect between ion permeability and selectivity. To enhance the selectivity of a cation exchange membrane for different ions, the core lies in regulating the migration rates of different ions within the transport channels. The transport of ions through a sub-nanoporous membrane can be regarded as ion diffusion driven by a chemical potential gradient, which is usually described by the Arrhenius-type equation to describe ion permeability. Here, P is the solute permeability (solute flux normalized by the driving force), A ′ is the pre-exponential factor, E a 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 can be seen that the interaction between ions and the host medium (i.e., water and pore walls) is the key mechanism inducing ion selectivity. Constructing a selective separation layer on the surface of a cation exchange membrane, such as a polyamide layer with a dense pore size or a positively charged layer, is a simple method for preparing a selective cation exchange membrane. However, while this method hinders the transmembrane transport of divalent cations, it also hinders the transport of monovalent ions to varying degrees, resulting in a decrease in the monovalent cation flux as the membrane selectivity increases, that is, there is a trade-off effect between the monovalent / divalent ion permeability and selectivity of the cation exchange membrane.

[0006] Fourthly, the preparation process of selective cation exchange membranes is complex and costly. The preparation of existing selective cation exchange membranes mainly follows two approaches: surface modification or precise design of ion transport channels. In terms of surface modification, the traditional chemical grafting method requires multiple steps of reactions such as sulfonation, quaternization, and crosslinking to regulate the surface charge density and hydrophilicity of the membrane. However, the grafting rate and uniformity are restricted by the reaction kinetic equilibrium. The increase in the grafting rate will cause the reaction time and the thickness fluctuation range of the modified layer to increase, and the batch stability to decrease. Although plasma modification can rapidly construct a nanoscale functional layer (10 - 50 nm), the equipment cost is as high as 800,000 - 1.2 million yuan per unit, and the durability of the modified layer is poor. After 10 cycles of use, the surface contact angle will rise from 35° to 68°. In terms of internal channel control, it often involves toxic and harmful chemical reagents, complex chemical reaction processes, and harsh reaction conditions, all of which significantly increase the preparation cost of selective cation exchange membranes.

[0007] In summary, there is currently no method that can simultaneously regulate the thickness, surface topology, and ion permeability-selectivity balance of cation exchange membranes, while meeting the requirements of simple preparation process and economy. It is urgent to develop a cation exchange membrane that is simultaneously low-cost, has good ion transport, and ion selectivity, which has important practical significance. Summary of the Invention

[0008] In view of this, the object of the present invention is to provide a method for preparing a cation exchange membrane by regulating a casting solution based on a chelating agent. By regulating the casting solution with a specific chelating agent, the problems of low ion transport rate, difficult surface morphology regulation, trade-off effect between ion permeability and selectivity, and high complexity and cost in the existing cation exchange membranes are synchronously solved. The thickness, surface topological structure and ion permeability-selectivity balance of the cation exchange membrane are accurately regulated, and a cation exchange membrane with an ultrathin ion exchange layer, high ion exchange capacity, good ion transport rate and high ion selectivity, and a simple and economical preparation process is prepared.

[0009] To achieve the above object of the invention, the following technical solutions are adopted:

[0010] In the first aspect of the present invention, a method for preparing a cation exchange membrane by regulating a casting solution based on a chelating agent is provided, which includes step S1 of preparing a casting solution, and step S2 of impregnating and upgrading / or coating and upgrading a porous filter membrane based on the casting solution. The casting solution includes a sulfonated monomer, an inducer, a photoinitiator, and a chelating agent. Among them, the inducer is set as a substance that can act on the molecular chain of the porous filter membrane to reduce the structural rigidity of the porous filter membrane, and the chelating agent is set as a water-soluble substance with a hydrogen bond donor so that it can form a hydrogen bond with the sulfonic acid group in the sulfonated monomer. Among them, the hydrogen bond donor carried by the chelating agent can be at least a carboxylic acid group and / or an amino group and / or a hydroxyl group.

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

[0012] Among them, the sulfonated 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, 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 carboxylate group, amino group, sulfonate group and amide group existing in the two, and the binding modes are mainly N-H···O- and O-H···O-. Numerous such hydrogen bonds are intertwined to construct a huge hydrogen bond network. For the combination of other similar chelating agents and sulfonated monomers, although the specific details of hydrogen bond formation will vary, the basic principle is the same, that is, based on the interaction of specific groups in the molecule to construct a hydrogen bond network. This interaction enables the physicochemical properties of the casting solution (such as viscosity, stability) to be affected, thus playing a key role in the preparation of ultrathin selective cation exchange membranes.

[0014] Moreover, as a relatively strong intermolecular force, hydrogen bonds can significantly enhance the intermolecular attraction. Meanwhile, the intermolecular hydrogen bond network will hinder the free movement of molecules. In the process of preparing the cation exchange membrane by the method of the present invention using EDTA-AMPS as an example, hydrogen bonds play a core role through three mechanisms: molecular crosslinking, ion screening, and mechanical enhancement, that is, a dynamic hydrogen bond network constructs ultrathin and dense sub-nanometer pores (structure construction); the interaction difference between hydrated ions and hydrogen bond sites realizes efficient monovalent / divalent ion screening (selective separation); the responsiveness adjustment of hydrogen bonds improves the anti-swelling property and mechanical strength of the membrane (performance optimization).

[0015] Specifically, a dynamic network formed based on hydrogen bonds constructs ultrathin and dense sub-nanometer pores, providing a specific channel for ion transport. When hydrated ions pass through the cation exchange membrane, they will interact with hydrogen bond sites. Due to the differences in properties such as the charge number and ionic radius of monovalent and divalent ions, the interaction between monovalent ions and hydrogen bond sites is relatively weak and they are more likely to pass through the membrane; while the interaction between divalent ions and hydrogen bond sites is stronger and they are more hindered when passing through, thus realizing efficient monovalent / divalent ion screening and further improving the ion selectivity of the membrane. In addition, hydrogen bonds are responsive. When the external environment (such as humidity, temperature, etc.) changes, hydrogen bonds can adjust their own states. When the membrane has a swelling tendency when contacting an aqueous solution, hydrogen bonds can enhance the intermolecular interaction by changing their own states and numbers, inhibit the swelling of the membrane, improve the anti-swelling property and mechanical strength of the membrane, and ensure the stability and durability of the membrane in practical applications. Moreover, the interaction of numerous hydrogen bonds makes the solution viscosity increase exponentially, which has an important impact on the fluidity of the casting solution and the subsequent film-forming process, such as controlling the penetration depth of the casting solution into the porous membrane support layer, and thus affecting the thickness and microstructure of the cation exchange membrane.

[0016] In addition, the formed hydrogen bonds can act as physical crosslinking points, exponentially increasing the solution viscosity by enhancing intermolecular interactions, hindering the free movement of molecules, and forming an intermolecular hydrogen bond network. Specifically, when using a porous filter membrane (polyvinylidene fluoride membrane, polyethersulfone membrane, cellulose acetate membrane) as the support layer, during the preparation of the ultra-thin selective cation exchange membrane of the present invention, the regulation of the chelating agent concentration has an important influence on the properties of the casting solution and the formation of the membrane structure. This is reflected in that the chelating agent can change the viscosity and fluidity of the casting solution, and the change in its concentration will affect the intrusion behavior of the casting solution into the membrane pores; by adjusting the chelating agent concentration, the intrusion depth of the casting solution into the membrane pores can be controlled, thereby regulating the thickness of the ion exchange layer; during the phase inversion process, the chelating agent concentration and physical processes act synergistically to affect the chemical stability and mechanical properties of the membrane. Not only can this control effect be verified through experimental research, 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 the membrane performance.

[0017] Meanwhile, the change in the solution properties of the casting solution will trigger differences in the properties of the microscopic regions during the photopolymerization process. These different regions further evolve during the aqueous post-treatment process, ultimately forming membrane surface wrinkles with different morphologies. The formation mechanism of the membrane surface wrinkles is closely related to the distribution of hydrophilic groups in the casting solution. For example, the sulfonic acid group in the AMPS molecule, due to its strong hydrophilicity, shows significant morphological changes in the distribution region when contacting with the aqueous solution, thus forming a wrinkled structure. Starting from the perspective of regulating the distribution of the casting solution in the membrane, the present invention realizes precise regulation of the distribution of hydrophilic groups by optimizing the intrusion process of the casting solution, thereby achieving diversified design of the membrane surface wrinkle morphology. Further utilizing the polar characteristics of hydrophilic groups (such as sulfonic acid groups), which can form hydrogen bonds with metal chelating agents. Based on this characteristic, by adjusting the type and addition amount of the metal chelating agent, the intrusion behavior of the casting solution can be effectively controlled, and thus the controllable preparation of the membrane surface wrinkle morphology can be achieved. This method provides a new idea for the microstructural design of membrane materials and at the same time broadens its potential in functional applications.

[0018] Additionally, the formation of hydrogen bonds between the added chelating agent and the sulfonated monomers can induce the ordered arrangement of the sulfonated monomers. This is mainly because there are differences in the strength and mode of hydrogen bond interactions between different types of chelating agents and sulfonated monomers. By selecting different types of chelating agents and adjusting their addition amounts, the strength and number of hydrogen bonds can be controlled, thereby affecting the arrangement mode 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 enhancing the ion transport rate. Moreover, on the other hand, the chelating agent used in the present invention has a strong chelating ability for divalent and higher-valent metal cations. Taking ethylenediaminetetraacetic acid as an example, groups such as carboxyl and amino groups in its molecule can provide lone pairs of electrons and combine with divalent cations with empty electron orbitals through coordination bonds to form stable complexes. When ions pass through the cation exchange membrane, divalent cations will be preferentially chelated, which can greatly enhance the transmembrane resistance of divalent and higher-valent metal cations, and thus improve the mono / divalent cation permeability-selectivity of the cation exchange membrane.

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

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

[0021] Step S101, adding the sulfonated monomer into the dissolving solvent and stirring until the sulfonated monomer is completely dissolved;

[0022] Step S102, adding the chelating agent to the solution formed in step S101 and stirring until all monomers are dissolved;

[0023] Step S103, adding the inducer to the solution formed in step S102 and continuing to stir until all monomers are dissolved;

[0024] Step S104, adding the photoinitiator to the solution formed in step S103 and stirring until dissolved to obtain the casting solution.

[0025] Among them, the inducer can be trimethylolpropane triacrylate or methacrylamide. The photoinitiator can be 2-hydroxy-2-methylpropiophenone or 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone.

[0026] In the present invention, the photoinitiator is also indispensable in the preparation process. After the photoinitiator absorbs ultraviolet light, the electrons in the molecule transition to form an excited state, and then free radicals are generated. These free radicals initiate the polymerization reaction, prompting the monomer molecules to connect with each other 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 structure of the sulfonated monomers.

[0027] Further, the dissolving solvent is set as ultrapure water and methanol with a volume ratio of 1:1. The mixed solvent of ultrapure water and methanol can not only ensure the good solubility of the sulfonated monomer, but also provide a suitable environment for subsequent reactions.

[0028] Further, in step S102, the addition amount of the chelating agent is 0.1 - 2.0 mol%; in step S103, the addition amount of the inducer is 3.0 - 5.0 mol%; in step S104, the addition amount of the photoinitiator is 0.2 - 0.8 mol%. The precise dosages of the chelating agent, the inducer and the photoinitiator have an important influence on the reaction process and the performance of the final membrane. Too little may not be able to fully initiate the reaction or realize the regulation of the membrane structure, while too much may lead to side reactions and affect the quality of the membrane.

[0029] Further, step S2 of impregnating / upgrading or coating / upgrading the porous filter membrane based on the casting solution includes:

[0030] S201, using the porous filter membrane as a substrate, impregnating it with the casting solution or coating the casting solution on its membrane surface;

[0031] S202, irradiating the impregnated or coated porous filter membrane under visible / ultraviolet light for a time T to induce the polymerization and curing of the porous filter membrane, and compressing some of the deconstructed porous filter membrane based on the crosslinked network formed by the polymerization and curing, so that the membrane becomes thinner and the exchange site density increases;

[0032] S203, after polymerization and curing, soaking the porous filter membrane in a reshaping solution to reshape the membrane surface topography; wherein, the reshaping solution is configured such that the amorphous structure formed in the porous filter membrane due to the action of the inducer can be transformed from an amorphous state to a crystalline state after contacting the reshaping solution, and / or the crosslinked network formed by the polymerization and curing can undergo non-uniform swelling after contacting it, so that the membrane surface is reshaped to form a wrinkled topography.

[0033] Among them, the reduction of the rigidity of the porous filter membrane by the action of the inducer on the molecular chain of the porous filter membrane is reflected in two cases. In one case, the small molecular structure of the inducer can penetrate between the molecular chains of the porous filter membrane, weakening the van der Waals force between the chains, thereby reducing the crystallinity, promoting the dissolution of crystals or partial crystals, and thus reducing the rigidity of the filter membrane; in the other case, the small molecular structure of the inducer penetrates between the molecular chains of the porous filter membrane, effectively weakening the dipole interaction and packing effect between the chains, resulting in a decrease in the packing density of the chain segments and making the structure show a loosening characteristic. That is, with the help of the action of the inducer, the structure of the porous filter membrane is deconstructed, laying a foundation for the subsequent reshaping based on the reshaping solution to form wrinkles. For crystalline or partially crystalline porous filter membranes, the former case is mainly applicable, while for non-crystalline porous filter membranes, the latter case is applicable.

[0034] Taking trimethylolpropane triacrylate as the inducer and polyvinylidene fluoride porous membrane as the porous filter membrane as an example for illustration. Trimethylolpropane triacrylate contains three acrylate groups, and the oxygen atoms in the ester groups have strong polarity. After impregnation or coating, it can form dipole-dipole interactions with the C-F bonds of polyvinylidene fluoride; at the same time, the small molecule structure of trimethylolpropane triacrylate can penetrate between the molecular chains of polyvinylidene fluoride, weakening the van der Waals forces and dipole interactions between the chains, reducing the crystallinity, promoting the dissolution of some polyvinylidene fluoride crystals, and making the structure rigidity of the polyvinylidene fluoride filter membrane decrease. That is to say, due to the presence of trimethylolpropane triacrylate, it can interact with the crystals in the polyvinylidene fluoride filter membrane, so that some polyvinylidene fluoride crystals are deconstructed into an amorphous state, and then the structure rigidity of the filter membrane is reduced.

[0035] After polymerization is completed, since the membrane is immersed in the reshaping solution for reshaping, the amorphous polyvinylidene fluoride in the membrane will undergo phase transformation when contacting the reshaping solution, that is, it will transform from an amorphous state to a crystalline state, and the phase transformation rate of polyvinylidene fluoride in different solutions is different, and wrinkles with different morphologies will be formed on the membrane surface; on the other hand, because there are hydrophilic and hydrophobic phases in the cross-linked network formed by polymerization and curing, when the membrane is immersed in the reshaping solution, the cross-linked network will undergo uneven swelling, thus forming wrinkles on the membrane surface; that is, the hydrophilic and hydrophobic phases in the cross-linked network form ring-shaped micron-scale wrinkles due to differential swelling, increasing the specific surface area of the membrane, and acting together with the regular ion transport channels formed by the ordered arrangement of sulfonated monomers, effectively improving the ion transport rate.

[0036] Moreover, the cross-linked network formed by polymerization and curing will also undergo different degrees of uneven swelling in different reshaping solutions, and the formed wrinkle morphologies are also different. That is to say, under the combined action of the phase transformation of polyvinylidene fluoride and the uneven swelling of the cross-linked network, wrinkles with different shapes and properties will be reshaped on the membrane surface.

[0037] In addition, the porous filter membrane can also be a polyethersulfone porous membrane and a cellulose acetate membrane. The deconstruction process of the polyethersulfone porous membrane depends on the polar ester group (-COO of trimethylolpropane triacrylate -) The dipole-dipole interaction with the sulfone group (-SO2-) and ether bond (-O-) in the main chain of the polyethersulfone porous membrane. Trimethylolpropane triacrylate molecules penetrate between the molecular chains of the polyethersulfone porous membrane through their small molecular structure, effectively weakening the intermolecular dipole interaction and π-π stacking effect, resulting in a decrease in the chain segment packing density and a loosening of the local structure. Since the polyethersulfone porous membrane itself is an amorphous polymer, the penetration of trimethylolpropane triacrylate further destroys its weakly ordered region and enhances the molecular chain mobility. Therefore, the molecular chains of the polyethersulfone porous membrane are rearranged and reconstructed during the contact with the casting solution, and trimethylolpropane triacrylate molecules are retained in the chain gaps to form a dynamic cross-linking network, improving the processing adaptability of the reshaped membrane.

[0038] The deconstruction process of the cellulose acetate membrane depends on the formation of hydrogen bonds between the ester oxygen of trimethylolpropane triacrylate and the hydroxyl group (-OH) in the molecular chain of the cellulose acetate membrane, and at the same time, the dipole-dipole interaction occurs 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, destroying the hydrogen bond network and van der Waals forces between cellulose chains, resulting in the deconstruction of the crystalline region into an amorphous state. This deconstruction process is accompanied by a significant plasticization effect, significantly enhancing the movement ability of the molecular chains of the cellulose acetate membrane. During the contact of the cellulose acetate membrane molecular chains with the casting solution, partial rearrangement of hydrogen bonds occurs, and 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 functional modification.

[0039] After the deconstruction and polymerization of the polyethersulfone porous membrane or cellulose acetate membrane are completed, the membrane can also be immersed in the reshaping solution for reshaping, and a membrane surface reshaping process similar to that of the polyvinylidene fluoride porous membrane can occur, thus forming micron-scale wrinkles. Specifically, for the cellulose acetate membrane, the amorphous structure inside the membrane will undergo a phase transformation under the action of the reshaping solution, that is, from an amorphous state to a crystalline state, and it will also undergo uneven swelling due to the presence of hydrophilic and hydrophobic phases in the cross-linking network formed by polymerization and curing, thus forming wrinkles. For the polyethersulfone porous membrane, it will also undergo uneven swelling due to the presence of hydrophilic and hydrophobic phases in the cross-linking network formed by polymerization and curing, thus forming wrinkles.

[0040] It should be noted that these wrinkles play an important role. They can enhance the turbulence of the microfluidic field on the membrane surface, hinder the deposition of pollutants on the membrane surface, change the situation that existing commercial cation exchange membranes are prone to pollutant attachment and entry into the membrane due to the flat surface, and significantly improve the anti-fouling performance of the membrane.

[0041] In addition, the present invention also has many advantages in the preparation method. The present invention uses photo-curing, such as visible light or ultraviolet light curing, which avoids the long-term heat treatment in the preparation process of the existing cation exchange membrane. For example, the thermal polymerization method requires heat treatment at 80 °C for 3 h, and the hot rolling method requires repeated hot rolling at 105 °C for 1 h, thereby reducing energy consumption. At the same time, the raw materials such as sulfonated monomers used in the present invention have no benzene ring structure, and methanol and water are used as solvents, which belongs to a green preparation method and reduces costs.

[0042] Further, the reshaping solution is ultrapure water, 10-70 wt% ethanol, or 0.01-0.5 mol / L NaCl solution. These different solutions can provide different environments for the phase transformation of polyvinylidene fluoride and the swelling of the cross-linked network, thereby precisely regulating the morphology and properties of the wrinkles on the membrane surface and optimizing the performance of the cation exchange membrane. For example, when ultrapure water is used as the reshaping solution, the phase transformation of polyvinylidene fluoride can be relatively mild, forming wrinkles with a specific morphology; while ethanol and NaCl solutions with different concentrations will affect the phase transformation rate of polyvinylidene fluoride and the swelling degree of the cross-linked network by changing the polarity and ionic strength of the system, generating different wrinkle structures to meet the performance requirements of the cation exchange membrane in different application scenarios.

[0043] Further, after step S2, a post-treatment step S3 is also included, which includes: soaking the membrane in 1 mol / L NaCl solution for N hours. The purpose of this step is to further stabilize the performance of the cation exchange membrane. Soaking in 1 mol / L NaCl solution can enable the ion exchange sites in the membrane to fully exchange with sodium ions to achieve ion exchange equilibrium and improve the ion exchange stability of the membrane; at the same time, the ions in the solution interact with the membrane, which helps to repair possible microscopic defects and enhance the overall structural stability of the membrane, thereby improving the reliability and service life of the cation exchange membrane in practical applications. That is, soaking the membrane in NaCl solution can, on the one hand, make the ions in the membrane reach an equilibrium state and further stabilize the structure and performance of the membrane; on the other hand, the ions in the solution exchange with the active sites on the surface and inside of the membrane, making the performance of the membrane more stable and uniform, and finally successfully preparing a cation exchange membrane with excellent performance.

[0044] Compared with the traditional preparation method of cation exchange membrane, the method of the present invention avoids the use of complex, toxic and harmful chemical reagents and processes, greatly reducing the raw material cost and the cost of environmental pollution treatment. At the same time, the prepared cation exchange membrane has significantly improved key performance indicators such as ion selectivity, ion transport rate, and chemical stability. Moreover, the entire preparation process has simple steps, is easy to operate and control, is suitable for large-scale industrial production, and is expected to bring new development opportunities to related fields.

[0045] According to the method disclosed in the present invention, the porous filter membrane is one of polyvinylidene fluoride membrane, polyethersulfone membrane, and cellulose acetate membrane. As the porous filter membrane, polyvinylidene fluoride membrane, polyethersulfone membrane, and cellulose acetate membrane can cooperate with chelating agents to regulate the casting solution, helping to prepare an ultrathin selective cation exchange membrane with excellent performance to meet the requirements of various application scenarios.

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

[0047] The beneficial effects of the present invention compared with the prior art are as follows:

[0048] 1. The method of regulating the thickness and surface morphology of the cation exchange membrane with a chelating agent in the present invention realizes the rapid, simple, and precise regulation of the morphology of the cation exchange membrane compared with the prior art.

[0049] 2. The present invention selects a chelating agent as a regulator for the orderliness of the cation exchange membrane transport network, realizing the green regulation of the cation exchange membrane transport network compared with the prior art.

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

[0051] 4. The surface of the cation exchange membrane prepared by regulating with a chelating agent in the present invention has ring-shaped micron-scale wrinkles (see Appendix Figure 5 ). The wrinkles are beneficial to the capture of target ions by the cation exchange membrane and enhance the turbulence of the microfluidic field on the membrane surface, accelerating the ion exchange process and enhancing the ion transport rate. Compared with the commercial cation exchange membrane with a smooth surface, 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 the present invention has an ordered ion transport network and a higher energy barrier for high-valence cations. Compared with the existing cation exchange membranes, its selectivity for monovalent / divalent cations is increased by 2 - 20 times (see Appendix Figure 7 ).

[0053] The method for regulating the morphology and ion selectivity of the cation exchange membrane based on a chelating agent in the present invention will be detailedly disclosed below in combination with the examples shown in the accompanying drawings. Description of the Drawings

[0054] Figure 1It is the main process step diagram of the cation exchange membrane prepared by the method of the present invention;

[0055] Figure 2 It is the flow chart of step S1 in the method of the present invention;

[0056] Figure 3 It is the flow chart of step S2 in the method of the present invention;

[0057] Figure 4 It is the structure and performance test diagram related to the cation exchange membrane of the present invention. (a) is the cross-sectional SEM image of the membrane, showing the microstructure of the membrane and the thickness of each layer. The inset in the upper right corner is a partial enlarged view; (b) is the bar chart of the ion exchange capacity (IEC) varying with the content of EDTA, reflecting the ion exchange capacity of the membrane under different EDTA contents;

[0058] Figure 5 It is the SEM image and element distribution mapping diagram of different cation exchange membranes (PEC-0.5, PEC-1.0, PEC-1.5, PEC-2.0) of the present invention. In each sub-diagram, the main diagram is the SEM image, showing the surface microstructure of the membrane. The upper right corner is the distribution mapping diagram of element F, and the lower right corner is the distribution mapping diagram of element S, which is used to analyze the distribution of elements on the membrane surface;

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

[0060] Figure 7 It is the NH4 + flux and NH4 + / Mg 2+ selectivity comparison bar chart of different membranes, where the left column of each membrane represents the NH4 + flux, and the right column represents the NH4 + / Mg 2+ selectivity, which is used to evaluate the transport ability of different membranes to NH4 + and the selectivity difference between NH4 + and Mg 2+ (EPC-1.0 is the cation exchange membrane of the present invention). Specific embodiments

[0061] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention. It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments may be combined with each other.

[0062] As Figure 1 shown, the present invention discloses a method for preparing a cation exchange membrane by regulating a casting solution based on a chelating agent, including: Step S1, preparing a casting solution, and Step S2, impregnating / upgrading and / or coating / upgrading a porous filter membrane based on the casting solution. The casting solution includes a sulfonated monomer, an inducer, a photoinitiator, and a chelating agent. Among them, the inducer is set as a substance that can act on the molecular chain of the porous filter membrane to reduce the structural rigidity of the porous filter membrane, and the chelating agent is set as a water-soluble substance with a hydrogen bond donor so that it can form a hydrogen bond with the sulfonic acid group in the sulfonated monomer. Among them, the hydrogen bond donor carried by the chelating agent can be at least a carboxylic acid group and / or an amino group and / or a hydroxyl group.

[0063] The step S1 of preparing the casting solution includes:

[0064] Step S101, adding the sulfonated monomer into a dissolving solvent and stirring until the sulfonated monomer is completely dissolved.

[0065] Step S102, adding the chelating agent to the solution formed in Step S101 and stirring until all monomers are dissolved; the addition amount of the chelating agent is 0.1-2.0 mol%, and can be any value including both endpoints in the range of 0.1-2.0 mol%.

[0066] Step S103, adding the inducer to the solution formed in Step S102 and continuing to stir until all monomers are dissolved; the addition amount of the inducer is 3.0-5.0 mol%, and can be any value including both endpoints in the range of 3.0-5.0 mol%, preferably 4.0 mol%.

[0067] Step S104, adding the photoinitiator to the solution formed in Step S103 and stirring until dissolved to obtain the casting solution; the addition amount of the photoinitiator is 0.2-0.8 mol%, and can be any value including both endpoints in the range of 0.2-0.8 mol%, preferably 0.5 mol%.

[0068] Among them, 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, hydroxyethylethylenediphosphonic acid. The sulfonated monomer can be 2-acrylamido-2-methylpropanesulfonic acid or 4-styrenesulfonic acid. The inducer can be trimethylolpropane triacrylate or methacrylamide. The photoinitiator can be 2-hydroxy-2-methylpropiophenone or 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone.

[0069] In addition, the dissolution solvent is set to ultrapure water and methanol with a volume ratio of 1:1.

[0070] Specifically, first, the sulfonated monomer (2-acrylamido-2-methylpropanesulfonic acid, 4-styrenesulfonic acid) powder is added to ultrapure water and methanol (volume ratio 1:1), and stirred for 10 - 20 minutes until completely dissolved. Subsequently, a certain amount (0.1 - 2.0 mol%) of the chelating agent is added to the solution, and stirring is continued for 10 - 20 minutes until all monomers are dissolved. Then, a certain amount (4.0 mol%) of the inducer (trimethylolpropane triacrylate, methacrylamide) is added to the solution, and stirring is continued for 10 - 20 minutes until all monomers are dissolved. Next, under light-shielded conditions, a photoinitiator (2-hydroxy-2-methylpropiophenone, 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone) (0.5 mol%) is added to the mixed solution, and stirred for 10 - 20 minutes to achieve uniform dissolution.

[0071] Through precise control of the raw material ratio and stirring time, the full dissolution and uniform mixing of each component are ensured. This not only creates good conditions for the subsequent impregnation or coating of the casting solution on the porous membrane and the photopolymerization reaction, but also helps to prepare a cation exchange membrane with an ultrathin ion exchange layer, high ion exchange capacity, fast ion transport rate, and excellent monovalent / divalent ion selectivity, meeting the requirements of many practical applications such as seawater desalination and lithium extraction from salt lake brine.

[0072] The step S2 of upgrading the porous filter membrane by impregnation / coating with the casting solution includes:

[0073] S201, using the porous filter membrane as a substrate, impregnating it with the casting solution or coating the casting solution on its membrane surface; among them, the porous filter membrane can be any one of a polyvinylidene fluoride membrane, a polyethersulfone membrane, and a cellulose acetate membrane.

[0074] S202, irradiating the impregnated or coated porous filter membrane with visible / ultraviolet light for a time T to induce polymerization and curing of the porous filter membrane, and compressing and partially deconstructing the porous filter membrane based on the crosslinked network formed by polymerization and curing, so that the membrane becomes thinner and the exchange site density increases.

[0075] In S203, after polymerization and curing, the porous filter membrane is immersed in a reshaping solution to reshape the membrane surface morphology. Among them, the reshaping solution is configured such that the amorphous structure in the porous filter membrane can be transformed from a non-crystal to a crystal after contacting the reshaping solution, and / or the cross-linked network formed by polymerization and curing can undergo uneven swelling after contacting it, so that the membrane surface is reshaped to form a wrinkled morphology.

[0076] After step S2, there is also a post-treatment step S3, which includes: immersing the membrane in a 1 mol / L NaCl solution for N hours; this can make the ions in the membrane reach an equilibrium state, stabilize the structure and performance of the membrane, promote the exchange of solution ions with the active sites on the membrane surface and inside, and make the membrane performance more stable and uniform; where the time N is 24 hours.

[0077] Among them, the reshaping solution is ultrapure water, or ethanol with a concentration of 10 - 70 wt%, or a NaCl solution with a concentration of 0.01 - 0.5 mol / L.

[0078] In step S2, there are two operation methods: impregnation and coating. The impregnation operation is to put the porous filter membrane into the casting solution and soak it in a light-shielded environment. The purpose is to let the casting solution fully penetrate into the pores of the filter membrane, and then scrape off the excess casting solution on the surface to ensure the smoothness of the membrane surface, avoid the influence of the excess casting solution on the performance and morphology of the membrane, and ensure that the subsequent photopolymerization reaction proceeds in a uniform system, making the structure of the polymerized membrane stable and the performance uniform. The coating operation is to coat the casting solution on the surface of the porous filter membrane and let it stand still to make the casting solution evenly distributed on the membrane surface. Compared with the impregnation operation, the coating operation focuses more on forming a uniform liquid film on the membrane surface, which is beneficial to constructing a uniform cross-linked network on the membrane surface during the subsequent photopolymerization reaction. Scratching off the excess casting solution is also to ensure the uniformity and cleanliness of the membrane surface, avoid differences in membrane performance caused by uneven distribution of the casting solution on the surface, and ensure that the finally prepared cation exchange membrane has stable and consistent performance.

[0079] Specifically, for the impregnation upgrade of the porous filter membrane, the operation can be: immerse the porous filter membrane (polyvinylidene fluoride membrane, polyethersulfone membrane, cellulose acetate membrane) in the above-mentioned mixed solution and soak it in a light-shielded environment for 1 - 20 minutes. Hydrogen bonds are formed between the chelating agent and the sulfonated monomer, changing the viscosity and fluidity of the casting solution and controlling the depth of penetration of the casting solution into the pores. Take out the filter membrane, scrape off the excess mixed solution on the surface, and then irradiate it with ultraviolet light for 20 - 60 minutes to initiate the polymerization reaction and form a cross-linked network. After the polymerization is completed, soak the membrane in ultrapure water for post-treatment. There are hydrophilic and hydrophobic phases in the cross-linked network in the membrane. Different types and addition amounts of chelating agents contained in them will cause different degrees of uneven swelling, and the formed wrinkled morphologies will also be different. Finally, soak the membrane sheet in a 1 mol / L NaCl solution for 24 h to complete the preparation of a high-quality selective cation exchange membrane.

[0080] For the coating upgrade of the porous filter membrane, the operation can be as follows: Quantitatively coat (1 - 10 mL) the casting solution on the surface of the porous membrane (polyvinylidene fluoride membrane, polyethersulfone membrane, cellulose acetate membrane), and let it stand for 1 - 8 minutes. Take out the filter membrane, scrape off the excess mixed solution on the surface, and then place it under ultraviolet light for irradiation for 20 - 60 minutes to initiate the polymerization reaction. After the polymerization is completed, soak the membrane in ultrapure water for post-treatment. Finally, soak the membrane sheet in 1 mol / L NaCl solution for 24 hours to complete the preparation of a high-quality cation exchange membrane.

[0081] Regardless of the specific upgrade operation method, in step S2, the irradiation time T is 20 - 60 minutes. Under the illumination during this period, the photoinitiator contained in the membrane effectively absorbs light energy to generate free radicals, initiating the polymerization reaction, causing 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, ion transport channels, etc., and is crucial for preparing a cation exchange membrane that meets the requirements.

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

[0083] During the preparation process, the interaction between the chelating agent and the sulfonated monomer is extremely crucial. Common chelating agents, such as ethylenediaminetetraacetic acid and tetrasodium glutamate diacetate, have strong water solubility, and the carboxylic acid groups, amino groups, and hydroxyl groups carried in the molecules are all hydrogen bond donors. Sulfonated monomers such as 2-acrylamide-2-methylpropanesulfonic acid and 4-styrenesulfonic acid contain sulfonic acid groups. Taking EDTA and AMPS as examples, they form hydrogen bonds according to the donor-acceptor matching relationship between the carboxylate group, amino group, sulfonate group, and amide group, mainly in the two modes of N-H···O- and O-H···O-. Numerous such hydrogen bonds are intertwined to form a large hydrogen bond network. For other similar combinations of chelating agents and sulfonated monomers, although the specific details of hydrogen bond formation will vary, the basic principle is the same, that is, a hydrogen bond network is constructed based on the interaction of specific groups in the molecules.

[0084] This hydrogen bond network has a significant impact on the properties of the casting solution. It greatly enhances the intermolecular forces in the casting solution, restricts the free movement of molecules, increases the viscosity of the casting solution and makes its fluidity worse, while improving the stability of the casting solution. These property changes further affect the penetration behavior of the casting solution in the porous support membrane. By adjusting the concentration of the chelating agent, the depth of the casting solution invading the membrane pores can be precisely controlled, thereby accurately regulating the thickness of the ion exchange layer. Moreover, the hydrogen bond network induces the ordered arrangement of sulfonated monomers. Different types of chelating agents have different strengths and numbers of hydrogen bonds formed with sulfonated monomers, and changing the addition amount of the chelating agent will also change the hydrogen bond situation, thereby affecting the arrangement mode of sulfonated monomers and the microstructure of the membrane. 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] In addition, the unique structure of the chelating agent endows it with the ability to chelate divalent cations. Taking ethylenediaminetetraacetic acid as an example, groups such as carboxyl and amino groups in its molecule can provide lone pairs of electrons and bind to divalent cations with empty electron orbits through coordination bonds to form stable complexes. When ions pass through the cation exchange membrane, divalent cations will be preferentially chelated, resulting in a significant increase in their transmembrane resistance, while monovalent cations are relatively easier to pass through, significantly improving the selectivity of the membrane for monovalent / divalent ions, which plays a key role in practical application scenarios such as seawater desalination and lithium extraction from salt lake brines.

[0086] When preparing an ultra-thin selective cation exchange membrane, porous membranes such as polyvinylidene fluoride membranes, polyethersulfone membranes or cellulose acetate membranes are selected as the support layer, and at this time, the concentration of the chelating agent becomes a key factor. The change in the concentration of the chelating agent will change the viscosity and fluidity of the casting solution. When the concentration is high, the intermolecular interaction is enhanced, the viscosity of the casting solution increases and the fluidity becomes worse; when the concentration is low, the situation is opposite. This concentration difference will also affect the invasion behavior of the casting solution in the membrane pores, and then affect the thickness of the ion exchange layer. A high concentration of chelating agent makes the casting solution have high viscosity and poor fluidity, and it is difficult for the casting solution to penetrate deep into the membrane pores, and finally the formed ion exchange layer is relatively thin; a low concentration of chelating agent makes the casting solution have low viscosity and good fluidity, and the casting solution can penetrate deeper into the membrane pores, and the ion exchange layer becomes thicker accordingly. Therefore, by precisely adjusting the concentration of the chelating agent, precise control of the thickness of the ion exchange layer can be achieved. During the phase inversion process, the concentration of the chelating agent and the physical process act synergistically to affect the chemical stability and mechanical properties of the membrane. An appropriate concentration of the chelating agent can promote the formation of a more stable chemical structure of the membrane and enhance the chemical stability of the membrane; at the same time, it will also affect the microstructure and molecular arrangement of the membrane. A high concentration makes the membrane structure dense, with improved strength and hardness but reduced flexibility, while a low concentration makes the membrane structure relatively loose, with better flexibility but possibly reduced strength. The control effect of the chelating agent concentration on the membrane preparation process and performance can be intuitively verified through experiments. In practical applications, it is necessary to optimize the chelating agent concentration according to different requirements.

[0087] In the EDTA-AMPS cation exchange membrane system, hydrogen bonds also exhibit more unique effects. The dynamic network formed by them constructs ultrathin and dense sub-nanopores, providing specific channels for ion transport. When hydrated ions pass through the cation exchange membrane, they interact with hydrogen bond sites. Due to the differences in properties such as the charge number and ionic radius between monovalent and divalent ions, the interaction between monovalent ions and hydrogen bond sites is relatively weak, making it easier for them to pass through the membrane; while the interaction between divalent ions and hydrogen bond sites is stronger, and they encounter greater obstacles when passing through, thus achieving efficient monovalent / divalent ion sieving and further enhancing the ion selectivity of the membrane. In addition, hydrogen bonds are responsive. When the external environment (such as humidity, temperature, etc.) changes, hydrogen bonds can adjust their own states. When the membrane has a swelling tendency upon contact with an aqueous solution, hydrogen bonds can enhance the intermolecular interaction by changing their own states and numbers, inhibit the swelling of the membrane, improve the anti-swelling property and mechanical strength of the membrane, and ensure the stability and durability of the membrane in practical applications. Moreover, the interaction of numerous hydrogen bonds causes the solution viscosity to increase exponentially, which has an important impact on the fluidity of the casting solution and the subsequent film-forming 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 will result in differences in the properties of microscopic regions during the photopolymerization process. During the aqueous solution post-treatment, due to the different distributions of hydrophilic groups (such as the sulfonic acid group in the AMPS molecule) in the casting solution, the regions containing sulfonic acid groups will absorb a large amount of water and swell, while the surrounding regions with weaker hydrophilicity or hydrophobicity have a smaller degree of swelling. This swelling difference in different regions will generate internal stress. When the internal stress reaches a certain level, it will cause wrinkles to form on the membrane surface. The present invention starts from regulating the distribution of the casting solution in the membrane, and precisely regulates the distribution of hydrophilic groups by optimizing the process of the casting solution penetrating into the pores of the porous filter membrane support layer. Utilizing the characteristic that hydrophilic groups form hydrogen bonds with metal chelating agents, the types and addition amounts of metal chelating agents are adjusted, thereby controlling the penetration behavior of the casting solution and achieving the controllable preparation of the membrane surface wrinkled morphology. This innovative method provides a new idea for the microscopic structure design of membrane materials and broadens the potential of membranes in functional applications.

[0089] Photoinitiators are also indispensable in the preparation process. After absorbing ultraviolet light, the electrons in the photoinitiator molecules transition within the molecule to form an excited state, and then free radicals are generated. These free radicals initiate the polymerization reaction, prompting monomer molecules to connect with each other 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 structure of the sulfonated monomers. In the post-treatment stage of the membrane, the hydrophilic and hydrophobic phases in the cross-linked network form ring-shaped micron-scale wrinkles due to differential swelling, increasing the specific surface area of the membrane. Acting together with the regular ion transport channels formed by the ordered arrangement of sulfonated monomers, the ion transport rate is effectively improved. Finally, the membrane is immersed in an NaCl solution. On the one hand, it can make the ions in the membrane reach an equilibrium state, further stabilizing the structure and performance of the membrane; on the other hand, the ions in the solution exchange with the active sites on the surface and inside of the membrane, making the performance of the membrane more stable and uniform, and finally successfully preparing a cation exchange membrane with excellent performance.

[0090] Compared with the traditional methods for preparing cation exchange membranes, the method of the present invention avoids the use of complex, toxic, and harmful chemical reagents and processes, significantly reducing the raw material cost and the cost of environmental pollution control. At the same time, the prepared cation exchange membrane has remarkable improvements in key performance indicators such as ion selectivity, ion transport rate, and chemical stability. Moreover, the entire preparation process has simple steps, is easy to operate and control, is suitable for large-scale industrial production, and is expected to bring new development opportunities to related fields.

[0091] In the embodiments of the present invention, the porous filter membrane is one of a polyvinylidene fluoride membrane, a polyethersulfone membrane, and a cellulose acetate membrane. As porous filter membranes, the polyvinylidene fluoride membrane, the polyethersulfone membrane, and the cellulose acetate membrane can cooperate with chelating agents to regulate the casting solution, helping to prepare an ultrathin selective cation exchange membrane with excellent performance to meet the requirements of various application scenarios.

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

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

[0094] I. Preparation of the casting solution: First, add sulfonated monomer powders such as 2-acrylamido-2-methylpropanesulfonic acid and 4-styrenesulfonic acid into a mixed solution of ultrapure water and methanol with a volume ratio of 1:1, and stir for 10 - 20 minutes to completely dissolve them, providing pure raw materials for subsequent reactions. Then add 0.1 - 2.0 mol% of chelating agents such as ethylenediaminetetraacetic acid and sodium glutamate diacetate, and continuously stir for 10 - 20 minutes. The chelating agents form a hydrogen bond network with the sulfonated monomers, affecting the properties of the casting solution and regulating the morphology and ion selectivity of the membrane. Next, add 4.0 mol% of crosslinking agents such as trimethylolpropane triacrylate and methacrylamide, and stir for 10 - 20 minutes to promote the connection of monomers and construct a stable membrane structure. Finally, add 0.5 mol% of photoinitiators such as 2-hydroxy-2-methylpropiophenone and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone in a light-shielded environment, and stir for 10 - 20 minutes to dissolve them evenly, completing the preparation of the casting solution. Precise control of the raw material ratio and stirring time ensures the full dissolution and uniform mixing of each component, laying the foundation for subsequent steps.

[0095] II. Treatment of the porous filter membrane: Select a polyvinylidene fluoride membrane, a polyethersulfone membrane, or a cellulose acetate membrane as the porous filter membrane, and there are two operation methods: impregnation and coating.

[0096] Impregnation operation: Immerse the porous filter membrane in the casting solution and soak it for 1 - 20 minutes in the dark. During this period, the hydrogen bond network formed by the chelating agent and the sulfonated monomer regulates the viscosity and fluidity of the casting solution, thereby controlling the depth of the casting solution invading the pores of the filter membrane. After soaking, take out the filter membrane and scrape off the excess casting solution on the surface to ensure the smoothness of the membrane surface and prevent it from affecting the subsequent photopolymerization reaction and the performance of the membrane. Refer to Figure 1 to understand its position and role in the preparation process.

[0097] Or, coating operation: Coat the casting solution on the surface of the porous filter membrane and let it stand for 1 - 8 minutes to make the casting solution evenly distributed on the membrane surface, creating favorable conditions for constructing a uniform crosslinked network in the subsequent photopolymerization reaction. Then scrape off the excess casting solution on the surface to ensure the uniformity and cleanliness of the membrane surface, Figure 1 which also shows the sequence and importance of the coating operation in the whole process.

[0098] III. Photopolymerization reaction: Place the impregnated or coated porous filter membrane under ultraviolet light irradiation for 20 - 60 minutes. After the photoinitiator absorbs light energy, the intramolecular electron transition forms an excited state and generates free radicals, initiating the polymerization reaction and causing the monomers in the casting solution to crosslink to form a stable three-dimensional network structure. From Figure 4 (a) The cross-sectional SEM image of the membrane can visually show the microscopic structure of the membrane, which is closely related to the crosslinked network formed by the photopolymerization reaction; Figure 4(b) The variation of the ion exchange capacity (IEC) with the EDTA content shown reflects the influence of the photopolymerization reaction conditions on the membrane properties. This structure plays a decisive role in the basic properties of the membrane such as the ion exchange capacity and ion transport channels.

[0099] IV. Further treatment: Immerse the polymerized porous filter membrane in ultrapure water for further treatment. Due to the different distributions of hydrophilic groups (such as the sulfonic acid group in the AMPS molecule) in the casting solution, the regions containing sulfonic acid groups absorb a large amount of water and swell, while the surrounding regions with weaker hydrophilicity or hydrophobicity have a smaller degree of swelling. This swelling difference generates internal stress. When the internal stress reaches a certain level, micron-scale ring-shaped wrinkles form on the membrane surface. From Figure 5 the SEM images of different cation exchange membranes (PEC-0.5, PEC-1.0, PEC-1.5, PEC-2.0), the wrinkled morphology on the membrane surface can be clearly observed. These wrinkles are beneficial for the cation exchange membrane to capture target ions, enhance the turbulent state of the microfluidic field on the membrane surface, and accelerate the ion exchange process.

[0100] V. Stabilization treatment (i.e., post-treatment): Immerse the further treated porous filter membrane in a 1 mol / L NaCl solution for 24 hours. This operation can make the ions in the membrane reach an equilibrium state, stabilize the structure and properties of the membrane, promote the exchange of solution ions with the active sites on the membrane surface and inside, and make the membrane properties more stable and uniform.

[0101] VI. Performance of the cation exchange membrane: The cation exchange membrane prepared through the above steps has excellent performance, with an ion exchange capacity of 2.5 - 3.0 meq / g and a thickness of 20 - 30 nm. From Figure 6 the NH4 + flux column charts of different membranes (CMI-7000S, FKS-130, CMVN, EPC-1.0) at different concentrations, and Figure 7 the NH4 + flux and NH4 + / Mg 2+ selectivity comparison column charts of different membranes (CMVN, FKS-130, CMI-7000S, EPC-1.0), it can be seen that the cation exchange membrane (EPC-1.0) prepared by the present invention has a significant improvement in ion transport rate and ion selectivity compared with other commercial membranes, and can meet the requirements of many practical applications such as seawater desalination and lithium extraction from salt lake brine.

[0102] Among them, CMI-7000S, FKS-130, CMVN, and EPC-1.0 mentioned in the specification and the specification drawings are all commercial membranes existing in the current market, while PEC represents the 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 rather than to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A method for preparing a cation exchange membrane based on regulating a casting solution with a chelating agent, comprising step S1, preparing a casting solution, and step S2, performing an immersion upgrade / or coating upgrade on a porous filter membrane based on the casting solution, characterized in that: The casting solution includes a sulfonated monomer, an inducer, a photoinitiator, and a chelating agent, wherein the inducer is configured as a substance that can act on the molecular chains of the porous filter membrane to reduce the rigidity of the porous filter membrane structure, and the chelating agent is configured as a water-soluble substance with a hydrogen bond donor so that it can form a hydrogen bond with the sulfonic acid group in the sulfonated monomer.

2. The method according to claim 1, characterized in that The hydrogen bond donor carried by the chelating agent can be at least a carboxylic acid group and / or an amino group and / or a hydroxyl group.

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

4. The method according to claim 3, characterized in that 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-ethylenediamine disuccinic acid, hydroxyethylidene diphosphonic acid; The sulfonated monomer can be 2-acrylamide-2-methylpropanesulfonic acid or 4-styrenesulfonic acid; The inducing agent can be trimethylolpropane triacrylate or methacrylamide; The photoinitiator can be 2-hydroxy-2-methylpropiophenone or 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone.

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

1.

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

7. The method according to any one of claims 1 to 6, characterized in that The step S2 of performing immersion upgrading / or coating upgrading on the porous filter membrane based on the casting solution comprises: S201, using a porous filter membrane as a substrate, and impregnating it with a casting solution or coating the casting solution on the membrane surface; S202, irradiating the impregnated or coated porous filter membrane under visible / ultraviolet light for a time T to induce polymerization and curing of the porous filter membrane, and compressing the partially deconstructed porous filter membrane based on the cross-linked network formed by the polymerization and curing, so that the membrane becomes thinner and the exchange site density increases; S203, after polymerization and curing, immersing the porous filter membrane in a reshaping solution to reshape the membrane surface morphology; wherein the reshaping solution is constructed so that the amorphous structure formed by the inducing agent in the porous filter membrane can be transformed from amorphous to crystalline after contacting the reshaping solution, and / or the cross-linked network formed by polymerization and curing can expand unevenly after contacting it, so that the membrane surface is reshaped to form a wrinkled morphology.

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

9. The method according to claim 7, characterized in that: After step S2, a post-treatment step S3 is also included, which includes: soaking the membrane in a 1 mol / L NaCl solution for N hours.

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

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