Method for preparing a porous filtration membrane into a cation exchange membrane based on deconstruction and reshaping

Abstract

The application discloses a method for preparing a porous filter membrane into a cation exchange membrane based on deconstruction and remodeling and a prepared cation exchange membrane. The method uses a polyvinylidene fluoride filter membrane as a substrate, impregnates or coats the substrate with a cross-linking network precursor solution containing a sulfonated monomer, an inducing agent and a photoinitiator, the inducing agent promotes deconstruction of polyvinylidene fluoride crystals, visible / ultraviolet light irradiation causes the precursor solution to polymerize and solidify, and the filter membrane is compressed to increase the exchange site density and reduce the membrane thickness. Then, the membrane is treated with a remodeling solution to form a wrinkle morphology on the surface of the membrane. The preparation process does not require long-term heat treatment and has low cost. The prepared cation exchange membrane solves many problems existing in existing commercial membranes, reduces the membrane body resistance and improves the ion transmission efficiency by simultaneously controlling the exchange site density and the membrane thickness, and the wrinkle morphology on the surface of the membrane can form a turbulent flow field to enhance the anti-fouling performance. The application provides a new way for preparing a cation exchange membrane and has important significance for the development of the fields of seawater desalination and resource recovery.

Description

Technical Field

[0001] This invention belongs to the field of materials preparation technology, specifically relating to ion exchange membrane preparation technology. The focus is on using a deconstruction-reconstruction process to convert porous filter membranes into cation exchange membranes, involving related cross-linking network design and Daonan dialysis and electrodialysis device technologies, to improve the overall performance of cation exchange membranes to meet application needs in seawater desalination, high-salt concentration, and other fields. Background Technology

[0002] Cation exchange membranes play an irreplaceable role in numerous fields. For example, in seawater desalination, they enable the effective separation of salt in seawater, providing technical support for addressing freshwater shortages. In high-salt concentration, they help increase the concentration of salt solutions, meeting the industrial demand for specific concentrations. In ammonia nitrogen recovery and lithium-magnesium separation, cation exchange membranes selectively transport ions, achieving efficient separation and recovery of substances. In practical wastewater recycling, they can recover useful ions from wastewater, achieving the dual goals of resource reuse and environmental pollution reduction. In recent years, global cation exchange membrane production capacity has surged, and compared to permeable membranes, such as microfiltration and ultrafiltration membranes, they are significantly more expensive.

[0003] In practical applications, existing cation exchange membranes have many problems that urgently need to be solved, as follows:

[0004] First, from a performance perspective, existing commercial cation exchange membranes are limited by the trade-off between ion exchange capacity and transport channel length, resulting in problems such as high membrane resistance and low ion transport efficiency.

[0005] Specifically, during ion transport, cations hop along exchange sites within the ion transport channels of a cation exchange membrane. Therefore, an ideal cation exchange membrane should possess both a large number of ion exchange sites and a short transport channel. In research, the density of exchange sites in a cation exchange membrane is reflected by its ion exchange capacity, while the length of the transport channel is macroscopically reflected by the membrane thickness. Thus, an ideal cation exchange membrane should simultaneously possess a large ion exchange capacity and a small membrane thickness. However, to ensure the mechanical strength of existing commercial cation exchange membranes, increasing the ion exchange capacity requires the addition of more rigid materials, leading to an increase in membrane thickness. The pore-filling method is a rapid method for preparing cation exchange membranes that uses a porous membrane as a rigid framework and employs a crosslinking agent to fix sulfonated functional monomers within the membrane pores. This method can achieve high-density fixation of exchange sites within the membrane, but it still cannot simultaneously control the exchange site density and membrane thickness. In other words, current cation exchange membrane preparation methods cannot simultaneously control ion exchange capacity and membrane thickness, thus hindering further improvements in the ion transport rate of cation exchange membranes.

[0006] Secondly, regarding antifouling performance, existing commercial cation exchange membranes are prone to scaling on their surface and internal fouling in practical applications, especially in the treatment of actual wastewater resource recovery. This is because the flat surface of the cation exchange membrane makes it impossible to form a turbulent flow field, allowing pollutants to easily adhere to the membrane surface and gradually enter the membrane, which not only limits the ion transport rate but also greatly shortens the membrane's lifespan.

[0007] Third, in terms of preparation process, most existing cation exchange membrane preparation processes involve long-term heat treatment. For example, the thermal polymerization method requires heat treatment of the membrane at 80°C for 3 hours, and the hot rolling method requires repeated hot rolling of the membrane at 105°C for 1 hour. These methods not only consume a lot of time and energy, but also have high costs.

[0008] In light of these issues, there is currently a lack of cation exchange membrane technologies on the market that simultaneously possess low cost, low resistance, good ion transport, and antifouling properties. This hinders the development of related fields and highlights the necessity and importance of the research in this invention. Specifically, there is currently no low-cost cation exchange membrane preparation technology that can simultaneously achieve high ion exchange capacity, low-concentration rapid ion transport, and good antifouling performance. Summary of the Invention

[0009] To address the shortcomings of the existing technology, the purpose of this invention is to provide a method for preparing a cation exchange membrane from a porous filter membrane based on deconstruction and remodeling, and the cation exchange membrane obtained therefrom. This method aims to solve the problems of high membrane resistance, low ion transport efficiency, easy fouling and contamination, and high energy consumption and cost of the preparation process of existing cation exchange membranes, and to obtain a cation exchange membrane that simultaneously has high ion exchange capacity, fast ion transport, good antifouling effect and low cost.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] This invention discloses a method for preparing a cation exchange membrane from a porous filter membrane based on deconstruction and remodeling. The method includes the following steps: S1, using a polyvinylidene fluoride (PVDF) filter membrane as a substrate, impregnating it with a crosslinking network precursor solution or coating the membrane surface with the crosslinking network precursor solution, wherein the crosslinking network precursor solution includes at least one sulfonated monomer, an inducing agent, and at least one photoinitiator; wherein the inducing agent is configured to interact with the PVDF crystals in the PVDF filter membrane to induce some of the PVDF crystals to deconstruct into amorphous substances; S2, placing the impregnated or coated PVDF filter membrane on... Irradiation time T under light is used to induce polymerization and solidification of the crosslinking network precursor solution. Based on the crosslinking network formed by polymerization and solidification, the partially deconstructed polyvinylidene fluoride filter membrane is compressed to make the membrane thinner and increase the exchange site density. S3, after polymerization and solidification, the polyvinylidene fluoride filter membrane is immersed in a remodeling solution to remodel the membrane surface morphology. The remodeling solution is constructed such that the amorphous polyvinylidene fluoride in the polyvinylidene fluoride filter membrane can be transformed from amorphous to crystalline after contact, and / or the crosslinking network formed by polymerization and solidification can undergo uneven expansion after contact, so that the membrane surface is remodeled to form a wrinkled morphology.

[0012] In this invention, the inducing agent is trimethylolpropane triacrylate (TMT), which contains three acrylate groups. The oxygen atoms in the ester groups are highly polar, and during impregnation and after coating, they can form dipole-dipole interactions with the CF bonds of polyvinylidene fluoride (PVDF). Simultaneously, the small molecule structure of TMT can penetrate into the PVDF molecular chains, weakening the van der Waals forces and dipole interactions between chains, reducing crystallinity, and promoting the dissolution of some PVDF crystals, thereby reducing the structural rigidity of the PVDF filter membrane. In other words, based on the presence of TMT, it can interact with the crystals in the PVDF filter membrane, causing some PVDF crystals to decompose into an amorphous state, thus reducing the structural rigidity of the filter membrane.

[0013] After polymerization, the membrane is remodeled by immersing it in a remodeling solution. Upon contact with this solution, the amorphous polyvinylidene fluoride (PVDF) undergoes a phase inversion, transforming from amorphous to crystalline. The phase inversion 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, this cross-linked network expands unevenly, also forming wrinkles on the membrane surface. Moreover, the cross-linked network expands unevenly to varying degrees in different remodeling solutions, resulting in different wrinkle morphologies. In other words, the combined effects of PVDF phase inversion and uneven expansion of the cross-linked network remodel the membrane surface, creating wrinkles of different shapes and properties.

[0014] 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.

[0015] 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.

[0016] Furthermore, the remodeling solution consists of ultrapure water, 10-70 wt% ethanol, and 0.01-0.5 mol / L NaCl solution. These different solutions provide varying 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, ultrapure water, when used as the remodeling solution, allows for a relatively mild phase inversion of PVDF, resulting in wrinkles with specific morphologies. Conversely, different concentrations of ethanol and NaCl solutions alter the polarity and ionic strength of the system, affecting the PVDF phase inversion rate and the degree of expansion of the cross-linked network, producing differentiated wrinkle structures to meet the performance requirements of cation exchange membranes in different application scenarios.

[0017] Furthermore, the sulfonated monomer is selected from at least one of 2-acrylamido-2-methylpropanesulfonic acid, methacrylamide, and 4-styrenesulfonic acid; the photoinitiator can be a visible light initiator or an ultraviolet light initiator, wherein the visible light initiator can be riboflavin, and the ultraviolet light initiator can be at least one of 2-hydroxy-2-methylphenylacetone and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone. The sulfonated monomer provides active sites for ion exchange in the cation exchange membrane, and different combinations of sulfonated monomers affect the ion exchange capacity and other membrane properties. For example, 2-acrylamido-2-methylpropanesulfonic acid has strong hydrophilicity and ion exchange capacity, which can increase the ion exchange capacity of the membrane; methacrylamide can regulate the structure and flexibility of the crosslinked network; and 4-styrenesulfonic acid can improve the electrical properties of the membrane to a certain extent. Riboflavin, as a visible light initiator, and 2-hydroxy-2-methylphenylacetone and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone, as ultraviolet light initiators, can efficiently initiate the polymerization and curing of cross-linking network precursor liquids under visible or ultraviolet light irradiation. Moreover, their use avoids the high energy consumption problem caused by traditional thermal polymerization, which meets the requirements of the present invention for low-energy preparation of cation exchange membranes.

[0018] Further, before step S1, a step S0 is included to prepare the cross-linking network precursor solution, which includes: adding the sulfonated monomer to ultrapure water and methanol (volume ratio 1:1) and stirring for 10-20 minutes until completely dissolved; adding 0.5-4.0 mol% of an initiator and continuing to stir for 10-20 minutes until all monomers are dissolved; and adding 0.1-1.0 mol% of a photoinitiator under light-protected conditions and stirring for 10-20 minutes to achieve uniform dissolution, thereby obtaining the cross-linking network precursor solution. In this step, strict control of the order of addition of each component and the stirring time is to ensure that each substance can be fully dissolved and uniformly dispersed in the solution to form a stable cross-linking network precursor solution. The mixed solvent of ultrapure water and methanol ensures good solubility of the sulfonated monomer and provides a suitable environment for subsequent reactions; the precise amount of initiator and photoinitiator has a significant impact on the reaction process and the final membrane performance. Too little may not be sufficient to initiate the reaction or achieve membrane structure regulation, while too much may lead to side reactions and affect membrane quality.

[0019] Furthermore, following step S3, a post-treatment step S4 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 any potential microscopic defects, enhancing the overall structural stability of the membrane, thereby improving the reliability and lifespan of the cation exchange membrane in practical applications.

[0020] Further, the impregnation step in step S2 includes: immersing the polyvinylidene fluoride filter membrane in the crosslinking network precursor solution for 1-20 minutes under light-protected conditions; then subjecting it to ultrasonic treatment for 5-20 minutes to ensure that the crosslinking network precursor solution fully fills the membrane pores; removing the filter membrane and scraping off excess crosslinking network precursor solution from the surface. Light-protected immersion is to prevent the photoinitiator from prematurely initiating the polymerization reaction during the impregnation process, which would affect the penetration and effectiveness of the crosslinking network precursor solution on the filter membrane; ultrasonic treatment utilizes the cavitation effect and mechanical vibration of ultrasound to promote more uniform and deeper filling of the crosslinking network precursor solution into the membrane pores, ensuring a more uniform membrane structure and more stable performance in subsequent polymerization reactions; scraping off excess crosslinking network precursor solution from the surface avoids the formation of an uneven polymer layer on the membrane surface, ensuring the smoothness and consistency of the membrane surface, which is beneficial for the subsequent optimization and improvement of membrane performance.

[0021] Furthermore, the coating step in step S2 includes: coating the crosslinking network precursor solution onto the surface of the polyvinylidene fluoride (PVDF) filter membrane and allowing it to stand for 1-10 minutes; removing the filter membrane and scraping off excess crosslinking network precursor solution from the surface. The purpose of standing is to allow the crosslinking network precursor solution to fully spread and penetrate the filter membrane surface, ensuring sufficient contact with the PVDF molecules on the membrane surface and creating favorable conditions for the subsequent polymerization reaction. Scraping off excess crosslinking network precursor solution also ensures the quality of the membrane surface, preventing excess precursor solution from forming irregular structures after polymerization, which would affect membrane performance. Compared to the impregnation step, the coating step is more suitable for applications with higher requirements for membrane surface modification and relatively lower requirements for pore filling. The two methods complement each other, providing diverse options for the preparation of cation exchange membranes with different needs.

[0022] Furthermore, in step S2, the time T is 20-60 minutes. Within this time range, the photoinitiator can effectively induce the polymerization and curing reaction of the crosslinking network precursor solution. If the time is too short, the polymerization reaction may be incomplete, the crosslinking network cannot be fully formed, resulting in an unstable membrane structure and failure to achieve the expected ion exchange capacity and mechanical properties. If the time is too long, the crosslinking network may become over-crosslinked, reducing the membrane's flexibility and even causing aging and embrittlement, which also affects the overall performance of the membrane. Therefore, the 20-60 minute irradiation time has been verified through extensive experiments and can ensure the preparation of a high-performance cation exchange membrane.

[0023] This invention also discloses a cation exchange membrane, which is prepared according to the method disclosed herein. This cation exchange membrane possesses numerous superior properties. Its ion exchange layer thickness is 35-60 nm, significantly lower than that of commercial membranes (100-600 nm), and its ion exchange capacity reaches 2.8 meq / g, higher than the 0.9-2.3 meq / g of commercial cation exchange membranes. This achieves an optimized combination of high ion exchange capacity and low membrane thickness, greatly reducing membrane bulk resistance and improving ion transport efficiency. Simultaneously, its surface has annular micron-sized folds, which not only accelerate ion transport but also significantly enhance the membrane's antifouling properties. Therefore, this cation exchange membrane has greater advantages over traditional commercial cation exchange membranes in practical applications, especially in seawater desalination, high-salt concentration, ammonia nitrogen recovery, lithium-magnesium separation, and wastewater resource recovery. It can effectively solve the problems existing in current cation exchange membranes and has broad application prospects and significant economic value.

[0024] The advantages of this invention compared to the prior art are as follows:

[0025] The cation exchange membrane prepared by the method of this invention can simultaneously possess a large ion exchange capacity and a small membrane thickness, perfectly solving the technical problem that a large ion exchange capacity and a small membrane thickness cannot be achieved simultaneously in the prior art. Specifically, by means of the interaction between trimethylolpropane triacrylate and polyvinylidene fluoride crystals, the polyvinylidene fluoride filter membrane undergoes partial deconstruction, thereby reducing the structural rigidity of the polyvinylidene fluoride filter membrane. Since the structural rigidity of the polyvinylidene fluoride filter membrane has been reduced, the cross-linked network formed by the polymerization and curing reaction under visible / ultraviolet light irradiation can compress the partially deconstructed polyvinylidene fluoride filter membrane, thereby making the membrane thinner and increasing the exchange site density. Subsequently, it is placed in a remodeling solution, causing the deconstructed polyvinylidene fluoride to undergo a phase transformation, changing from amorphous to crystalline, and the cured cross-linked network undergoes uneven expansion in the solution. The two work synergistically to form surface wrinkles of different morphologies on the membrane surface, thereby obtaining a high-efficiency ion transport cation exchange membrane prepared based on deconstruction-remodeling.

[0026] Furthermore, the ion exchange membrane prepared based on the present invention has a wrinkled surface, which enhances the microfluidic field turbulence on the membrane surface, hinders the deposition of pollutants on the membrane surface, and enhances the antifouling performance of the membrane. Compared with existing commercial membranes, it significantly improves the antifouling performance during operation.

[0027] The following describes in detail the method of preparing a cation exchange membrane from a porous filter membrane based on deconstruction and remodeling, with reference to the embodiments shown in the accompanying drawings. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating the steps involved in the preparation of the cation exchange membrane in this study.

[0029] Figure 2 This is a scatter plot comparing the ion exchange capacity and membrane thickness of the cation exchange membrane in this study with various commercial cation exchange membranes, highlighting the advantages of the membrane of this invention in these two aspects.

[0030] Figure 3 (a) is a scanning electron microscope image of the cation exchange membrane surface of the present invention; Figure 3 (b) is a scanning electron microscope image of the surface of a commercial cation exchange membrane for comparison with the membrane of the present invention;

[0031] Figure 4 NH4 under different ion concentrations using different cation exchange membranes + The flux comparison bar chart visually demonstrates the advantages of the PC-4.0 membrane ion transport performance of this invention;

[0032] Figure 5 The images show a comparison of scanning electron microscope (SEM) images of the cation exchange membrane (PC-4.0 membrane) and a commercial membrane before and after fouling and after cleaning, demonstrating the antifouling performance advantages of the membrane of this invention.

[0033] Figure 6 (a) is a schematic diagram of the preparation process of the cation exchange membrane (PC membrane) of the present invention; Figure 6 (b) Schematic diagram of the self-made Downan dialysis (DD) cell structure for evaluating the ion-selective permeability of cation exchange membranes; Figure 6 (c) is a schematic diagram of an electrodialysis (ED) experimental setup, which helps to understand the principles of preparation and performance testing.

[0034] Figure 7 (a) is a SEM image of the PVDF membrane surface, showing its interlaced structure; Figure 7 (b) is a SEM image of the pore-filled membrane surface, showing the changes in the PVDF membrane pores after they are filled; Figure 7 (c) SEM images and XRD characterization results of the PC film surface, reflecting the PC film formation process and structural changes;

[0035] Figure 8 (a) is a cross-sectional SEM image of the PC film, showing its dense, non-porous structure; Figure 8 (b) is a SEM image of the PC film surface, highlighting the annular folds; Figure 8 (c) is the AFM image of the PC film, reflecting the surface roughness of the film;

[0036] Figure 9 (a) shows the ATR-FTIR spectra of different membranes, indicating that sulfonic acid groups were successfully introduced into the PC membrane; Figure 9 (b) XPS spectra of different films, further confirming the presence of sulfonic acid groups; Figure 9 (c) High-resolution XPS spectra of O1s for PVDF and PC-4.0 films, confirming the successful preparation of the PC film;

[0037] Figure 10 (a) is a graph showing the changes in ion exchange capacity and swelling ratio of different PC membranes with the amount of TMPTA added; Figure 10 (b) Nyquist plots of different PC films, used to analyze film resistance; Figure 10 (c) The EIS fitting results and equivalent circuit model show the change in the bulk layer resistance of the PC film with the amount of TMPTA added.

[0038] Figure 11 (a) NH4 in different PC films + Graph showing flux variation with TMPTA addition amount; Figure 11 (b) NH4+ for PC-4.0 membrane and commercial CEMs at different feed solution concentrations + Flux comparison chart; Figure 11 (c) and (d) show the NH4 content of PC-4.0 membrane and commercial CEMs respectively when treating actual domestic sewage and lemon pectin wastewater.+ Flux diagrams provide a comprehensive view of the membrane's ion transport performance;

[0039] Figure 12 (a) NH4 content of PC-4.0 membrane and commercial CEMs during electrodialysis treatment of actual domestic wastewater + Flux diagram; Figure 12 (b) NH4 content of PC-4.0 membrane and commercial CEMs during electrodialysis treatment of actual lemon pectin wastewater + Flux diagrams demonstrate the ion transport performance of the membrane under the influence of an electric field.

[0040] Figure 13 (a) Comparison of the original surface morphology of PC-4.0 film and CMVN film; Figure 13 (b) SEM images of the surfaces of the PC-4.0 membrane and CMVN membrane after fouling; Figure 13 (c) SEM images of the PC-4.0 membrane and CMVN membrane after cleaning; Figure 13 (d) shows the EIS fitting results of PC-4.0 membrane and CMVN membrane under different conditions; Figure 13 (e) NH4 after simple water washing of PC-4.0 membrane and CMVN membrane + The flux and ion flux recovery rate graphs comprehensively demonstrate the antifouling performance of the membrane of this invention. Detailed Implementation

[0041] 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.

[0042] like Figure 1As shown, this invention discloses a method for preparing a cation exchange membrane from a porous filter membrane based on deconstruction and remodeling. The method includes the following steps: S1, using a polyvinylidene fluoride (PVDF) filter membrane as a substrate, impregnating it with a crosslinking network precursor solution or coating the membrane surface with the crosslinking network precursor solution, wherein the crosslinking network precursor solution includes at least one sulfonated monomer, an inducing agent, and at least one photoinitiator; wherein the inducing agent is configured to interact with the PVDF crystals in the PVDF filter membrane to cause some of the PVDF crystals to deconstruct into amorphous substances; S2, applying the impregnated or coated PVDF filter membrane... Irradiation time T under light induces polymerization and solidification of the crosslinking network precursor solution. Based on the crosslinking network formed by polymerization and solidification, the partially deconstructed polyvinylidene fluoride filter membrane is compressed to make the membrane thinner and increase the exchange site density. S3, after polymerization and solidification, the polyvinylidene fluoride filter membrane is immersed in a remodeling solution to remodel the membrane surface morphology. The remodeling solution is constructed such that the amorphous polyvinylidene fluoride in the polyvinylidene fluoride filter membrane can be transformed from amorphous to crystalline upon contact, and / or the crosslinking network formed by polymerization and solidification can undergo uneven expansion upon contact, so that the membrane surface is remodeled to form a wrinkled morphology.

[0043] Based on the disclosed steps, the cation exchange membrane prepared by this invention possesses both a large ion exchange capacity and a small membrane thickness, perfectly solving the problem in existing technologies that cannot simultaneously achieve both large ion exchange capacity and small membrane thickness. Furthermore, the cation exchange membrane has wrinkles on its surface, which enhance the turbulent microfluidic field, hindering the deposition of pollutants on the membrane surface and thus enhancing the membrane's antifouling performance. Compared with existing commercial membranes, its antifouling performance during operation is significantly improved.

[0044] In a preferred embodiment, the inducing agent is trimethylolpropane triacrylate, which, through its three acrylate groups, forms a dipole-dipole interaction between the oxygen atom in the ester group and the CF bond of polyvinylidene fluoride. The small molecule structure penetrates into the molecular chain, weakening van der Waals forces and dipole interactions, reducing the crystallinity of the polyvinylidene fluoride filter membrane, causing some crystals to decompose into amorphous structures, reducing the rigidity of the filter membrane structure, and creating conditions for subsequent cross-linking network compression of the filter membrane, achieving membrane thinning and increasing the density of exchange sites. This helps to prepare a cation exchange membrane with both large ion exchange capacity and small membrane thickness, solving the existing technical problems.

[0045] In this embodiment of the invention, the remodeling solution is ultrapure water, 10-70 wt% ethanol, and 0.01-0.5 mol / L NaCl solution. Ultrapure water provides a mild phase transition for polyvinylidene fluoride (PVDF), forming specific wrinkles. Different concentrations of ethanol and NaCl solutions alter the polarity and ionic strength of the system, affecting the PVDF phase transition rate and the degree of crosslinking network expansion, resulting in differentiated wrinkled structures. These wrinkles enhance the turbulent microfluidic field on the membrane surface, hindering pollutant deposition and significantly improving the membrane's antifouling performance compared to existing commercial membranes, thus enhancing its resistance to fouling during operation.

[0046] In this embodiment of the invention, the sulfonated monomer is selected from at least one of 2-acrylamido-2-methylpropanesulfonic acid, methacrylamide, and 4-styrenesulfonic acid, providing ion exchange active sites for the cation exchange membrane. For example, 2-acrylamido-2-methylpropanesulfonic acid increases ion exchange capacity, methacrylamide regulates the crosslinking network structure and flexibility, and 4-styrenesulfonic acid improves the electrical properties of the membrane.

[0047] In this embodiment of the invention, the photoinitiator can be an ultraviolet photoinitiator, specifically at least one of 2-hydroxy-2-methylphenylacetone and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone. In another specific embodiment, the photoinitiator can be the visible light photoinitiator riboflavin. The efficient initiation of crosslinking network precursor liquid polymerization and curing under visible or ultraviolet light irradiation avoids the high energy consumption problem of traditional thermal polymerization, meeting the requirements of this invention for low-energy preparation of cation exchange membranes and reducing production costs.

[0048] In this embodiment of the invention, before step S1, a step S0 is included to prepare a cross-linking network precursor solution, which includes: adding sulfonated monomers to ultrapure water and methanol (volume ratio 1:1) and stirring for 10-20 minutes until completely dissolved; adding 0.5-4.0 mol% of an initiator and continuing to stir for 10-20 minutes until all monomers are dissolved; and adding 0.1-1.0 mol% of a photoinitiator under light-protected conditions and stirring for 10-20 minutes to achieve uniform dissolution, thereby obtaining the cross-linking network precursor solution. Strict control of the order of addition of each component and the stirring time ensures that the substances are fully dissolved and uniformly dispersed, forming a stable precursor solution. This guarantees the smooth progress of subsequent reactions, lays the foundation for preparing a high-performance cation exchange membrane, and helps to achieve high ion exchange capacity and good overall performance.

[0049] In this embodiment of the invention, after step S3, a post-treatment step S4 is included, which involves immersing the membrane in a 1 mol / L NaCl solution for N hours. The value of N can be 18, 20, or 24 hours. These durations are determined based on extensive experimental research. When N is 18 hours, the ion exchange sites within the membrane can basically reach ion exchange equilibrium, but a small number of sites still do not fully exchange with sodium ions, which has a certain impact on ion exchange stability. As the time is extended to 20 hours, the degree of ion exchange equilibrium is further improved, and most of the ion exchange sites within the membrane complete the exchange with sodium ions, significantly improving ion exchange stability. When N reaches 24 hours, the ion exchange sites are almost completely exchanged with sodium ions, reaching the optimal ion exchange equilibrium state, which maximizes ion exchange stability.

[0050] Meanwhile, the interaction between ions in the solution and the membrane varies at different time points. At 18 hours, the repair effect of ions on membrane micro-defects is limited; at 20 hours, the interaction between ions and the membrane is enhanced, repairing some micro-defects and improving the overall structural stability of the membrane to some extent; by 24 hours, ions fully interact with the membrane, not only repairing most micro-defects but also further optimizing the internal structure of the membrane, significantly enhancing the overall structural stability of the membrane, greatly improving the reliability and service life of cation exchange membranes in practical applications, and achieving deep optimization of membrane performance.

[0051] In step S2 of this embodiment, the selection of the time range for the impregnation and coating steps is based on sufficient experimental evidence. In the impregnation step, the polyvinylidene fluoride filter membrane is immersed in the crosslinking network precursor solution, and the immersion time in the dark is controlled between 1 and 20 minutes. Experimental data show that if the immersion time is less than 1 minute, the crosslinking network precursor solution cannot fully penetrate into the filter membrane, resulting in insufficient subsequent polymerization reaction, and a significant decrease in the membrane's ion exchange capacity and mechanical properties. When the immersion time exceeds 20 minutes, although the precursor solution can fully penetrate, the filter membrane swells excessively, and the membrane structure becomes loose. During subsequent ultrasonic treatment and photopolymerization, the membrane shape is difficult to maintain, and local uneven polymerization occurs, which also affects the membrane's performance.

[0052] The ultrasonic treatment time was set at 5-20 minutes, which was verified through multiple experiments. When the ultrasonic treatment time is less than 5 minutes, the precursor solution cannot be fully filled in the membrane pores, resulting in an uneven membrane structure and affecting ion transport efficiency. If the ultrasonic treatment time exceeds 20 minutes, the high-intensity mechanical vibration generated by ultrasound will damage the microstructure of the filter membrane, reduce the mechanical strength of the membrane, and thus affect the overall performance of the cation exchange membrane.

[0053] In the coating step, after coating the crosslinking network precursor solution onto the surface of the polyvinylidene fluoride (PVDF) filter membrane, the settling time was set to 1-10 minutes. Experiments showed that if the settling time was less than 1 minute, the precursor solution could not fully spread and penetrate the filter membrane surface, leading to incomplete polymerization and localized defects on the membrane surface, affecting membrane performance. Conversely, if the settling time exceeded 10 minutes, some components of the precursor solution might volatilize or react, similarly causing irregular structures to form on the membrane surface and reducing membrane quality. These time ranges, determined through extensive experiments, ensure the preparation of high-performance cation exchange membranes that meet the needs of different application scenarios, achieving a balance between high ion exchange capacity, rapid ion transport, and good mechanical properties.

[0054] In this embodiment of the invention, in step S2, the time T is set to 20-60 minutes. Within this time range, the photoinitiator effectively induces the polymerization and solidification of the crosslinking network precursor solution. If the time is too short, polymerization is incomplete, the membrane structure is unstable, and the ion exchange capacity and mechanical properties are affected; if the time is too long, the crosslinking network is over-crosslinked, the membrane flexibility decreases, and it becomes brittle due to aging. Extensive experimental verification has shown that this irradiation time can ensure the preparation of a high-performance cation exchange membrane, achieving a balance between high ion exchange capacity, rapid ion transport, and good mechanical properties.

[0055] This invention also discloses a cation exchange membrane, which is prepared according to the method disclosed herein. The ion exchange layer thickness of this cation exchange membrane is 35-60 nm, significantly shorter than that of commercial membranes (100-600 nm), and its ion exchange capacity reaches 2.8 meq / g, higher than the 0.9-2.3 meq / g of commercial cation exchange membranes. This achieves an optimized combination of high ion exchange capacity and low membrane thickness, greatly reducing the membrane's bulk resistance and improving ion transport efficiency. Simultaneously, the annular micron-sized folds on its surface not only accelerate ion transport but also significantly enhance its antifouling performance. Compared to traditional commercial cation exchange membranes, it exhibits significant advantages in fields such as seawater desalination, high-salt concentration, ammonia nitrogen recovery, lithium-magnesium separation, and wastewater resource recovery, effectively solving the problems existing in current cation exchange membranes and possessing broad application prospects and significant economic value.

[0056] The specific operation of this invention is as follows:

[0057] 1. Preparation of experimental materials

[0058] Porous polyvinylidene fluoride (PVDF) membranes manufactured by Tianjin Jinteng Experimental Equipment Co., Ltd. were selected as the substrate material. These membranes have a pore size of 0.22 μm, a porosity of 75%, and a thickness of 110 μm. This pore size helps limit excessive permeation of macromolecules in the precursor solution, ensuring membrane structural stability. The relatively high porosity provides ample space for the cross-linked network precursor solution to permeate, promoting the formation of a uniform cross-linked structure. The suitable thickness ensures the membrane's mechanical strength while facilitating subsequent deconstruction-remodeling processes, which is beneficial for ion transport and membrane performance regulation.

[0059] Chemical reagents include 2-acrylamide-2-methylpropanesulfonic acid (AMPS, 98%), trimethylolpropane triacrylate (TMPTA, 95%), used as inducing agents to promote the deconstruction of polyvinylidene fluoride crystals; riboflavin (visible light initiator), 2-hydroxy-2-methylphenylacetone (ultraviolet light initiator), used to initiate the polymerization and curing of crosslinking network precursor solutions; hydrochloric acid (HCl, 37%, Sinopharm Group); sodium hydroxide (NaOH, AR, Sinopharm Group); phenolphthalein (pH indicator, Aladdin); sodium chloride (NaCl, analytical grade, Aladdin); ammonium chloride (NH4Cl); potassium chloride (KCl); lithium chloride (LiCl); magnesium chloride (MgCl2); and calcium chloride (CaCl2) (all analytical grade, Aladdin).

[0060] The experimental solvents were ultrapure water and methanol, mixed in a 1:1 volume ratio. This mixed solvent ratio effectively dissolves sulfonated monomers, TMPTA, and photoinitiators, while simultaneously regulating the polarity and solubility of the reaction system, promoting the interaction between TMPTA and PVDF crystals, and facilitating the penetration of the precursor solution into the membrane pores.

[0061] 2. Preparation of cross-linking network precursor solution

[0062] A cross-linking network precursor solution was prepared using AMPS as an example. AMPS powder was added to a 1:1 volume ratio of ultrapure water and methanol and stirred for 15 minutes until completely dissolved. The good solubility of the mixed solvent ensured uniform dispersion of the sulfonated monomers. Next, TMPTA was added at 2.0 mol% of the total amount in the mixed solution, and stirring continued for 15 minutes. TMPTA subsequently interacts with PVDF crystals, causing some crystals to decompose into amorphous structures, which is a key factor in achieving membrane structure deconstruction. Finally, under light-protected conditions, riboflavin or 2-hydroxy-2-methylphenylacetone, a photoinitiator, was added at 0.5 mol% of the total amount in the mixed solution, and stirred for 15 minutes to achieve uniform dissolution. Photoinitiators are crucial for cation exchange membrane preparation. Their dosage affects the polymerization and curing rate of the cross-linking network precursor solution, the degree of cross-linking of the membrane, and its microstructure. Adding the precursor solution under light-protected conditions prevents premature polymerization, which could disrupt the stability of the reaction system and affect the cross-linking and microstructure of the membrane. Adding the precursor solution under light-protected conditions ensures that it functions precisely under visible light irradiation, thus regulating membrane performance.

[0063] Besides being a photoinitiator, TMPTA plays a crucial role at the molecular level in improving membrane performance. At the molecular level, the dipole-dipole interaction between the strongly polar ester oxygen atom of TMPTA and the CF bond of PVDF alters the interaction energy between PVDF molecular chains, reducing the ordered arrangement of the molecular chains. Simultaneously, the infiltration of small TMPTA molecules into the molecular chains further disrupts the original crystal structure, leading to a decrease in crystallinity. Recent studies have shown that this structural change affects the free volume of PVDF, creating more channels for ion transport and laying the foundation for subsequent improvements in ion transport performance.

[0064] 3. Upgraded impregnation of porous filter membranes

[0065] 3.1 Impregnation and ultrasonic treatment

[0066] The polyvinylidene fluoride (PVDF) filter membrane was completely immersed in the prepared crosslinking network precursor solution. The entire immersion process was carried out in a light-protected environment for 10 minutes. This is because the strongly polar ester oxygen atoms of TMPTA in the crosslinking network precursor solution form dipole-dipole interactions with the CF bonds of PVDF, and its small molecular structure penetrates into the PVDF molecular chains, weakening the van der Waals forces and dipole interactions between the molecular chains. This leads to a decrease in the crystallinity of PVDF, partial crystal dissolution, and a reduction in the structural rigidity of the membrane. This not only facilitates deeper penetration of the crosslinking network precursor solution into the membrane pores but also provides a more uniform reaction environment for the formation of the crosslinking network during subsequent photopolymerization. The crosslinking network can more tightly intertwine with the PVDF molecular chains, enhancing the overall stability and mechanical strength of the membrane. At the same time, the changes in the PVDF membrane pore structure after deconstruction make the ion transport path within the membrane more tortuous and ordered, which is beneficial for ion capture and transport, laying the foundation for improving the membrane's ion exchange capacity and ion transport rate.

[0067] After soaking, the filter membrane was subjected to ultrasonic treatment for 10 minutes at a frequency of 40 kHz. Ultrasonic treatment utilizes cavitation to ensure that the crosslinking network precursor solution uniformly and fully fills the membrane pores. Figure 7 (a) The original interlaced structure of the PVDF membrane, to Figure 7 (b) The surface of the pore-filled membrane becomes smooth, and the significant effect of the precursor liquid filling the pores of the membrane can be visually observed. (Comparison) Figure 3 (a) The membrane after treatment according to the present invention and Figure 3 (b) Scanning electron microscope image of the surface of a commercial cation exchange membrane, showing significant differences. After this step, the membrane structure of the present invention exhibits a more uniform state, while the surface of the commercial membrane is relatively smooth and lacks the microscopic features resulting from the full filling of the precursor solution and structural changes.

[0068] 3.2 Photopolymerization

[0069] After impregnation and ultrasonic treatment, the filter membrane is carefully removed, and excess mixture on its surface is meticulously scraped off. The membrane is then irradiated under visible / ultraviolet light for 40 minutes. Riboflavin or 2-hydroxy-2-methylphenylacetone in the crosslinking network precursor solution, under visible / ultraviolet light excitation, initiates the polymerization and solidification of the precursor solution, gradually forming a crosslinked network. The shrinkage force generated by the polymerization and solidification of the crosslinked network compresses and deconstructs the PVDF filter membrane, reducing its thickness and increasing the density of exchange sites. Figure 2 As can be seen from the data, the ion exchange capacity of the membrane prepared by this invention reaches 2.8 meq / g, significantly higher than the 0.9-2.3 meq / g range of commercial cation exchange membranes; the membrane thickness is between 35-60 nm, significantly lower than the 100-600 nm thickness of commercial membranes. Further analysis of data under different preparation conditions shows that when the light irradiation time is within the range of 20-60 minutes, the membrane thickness gradually decreases and the ion exchange capacity gradually increases with the extension of the irradiation time. However, when the irradiation time exceeds 60 minutes, the flexibility and ion transport performance of the membrane may decrease. This is because excessive cross-linking leads to an overly dense membrane structure, obstructing the ion transport channels. Therefore, choosing an irradiation time of 40 minutes can optimize the ion exchange capacity and membrane thickness while ensuring good overall membrane performance.

[0070] 4. Reshaping and post-treatment of membrane surface morphology

[0071] 4.1 Reshaping Process

[0072] After the polymerization reaction was completed, the membrane was immersed in a 50 wt% ethanol solution for remodeling for 12 hours. In the ethanol solution environment, the phase inversion rate of polyvinylidene fluoride (PVDF) was relatively slow, and the hydrophilic and hydrophobic phases of the crosslinked network expanded significantly, synergistically forming wrinkles of a specific shape on the membrane surface. Figure 8 As observed in (b), the membrane surface prepared by this invention exhibits a ring-shaped micron-sized wrinkled structure. These wrinkles are irregularly shaped and uniformly distributed, with a size between 10-15 μm (taking PC-4.0 membrane as an example). The wrinkled structure greatly increases the surface area of ​​the membrane, exposing more sulfonic acid groups on the membrane surface, providing more contact sites for ions and facilitating ion capture. Simultaneously, the wrinkles alter the microfluidic field on the membrane surface, making the flow of ions on the membrane surface more turbulent, reducing ion concentration polarization, and thus improving the ion transport rate. Regarding antifouling performance, the wrinkled structure forms a physical barrier, hindering the deposition of pollutants on the membrane surface. When pollutants come into contact with the membrane surface, they are blocked and interfered with by the wrinkles, making it difficult for them to adhere to the membrane. Moreover, when water flows through the membrane surface, the wrinkled structure enhances the turbulence of the water flow, making it easier for pollutants to be carried away by the water flow, further enhancing the membrane's antifouling performance. Compared to the smooth surface structure of commercial membranes, the wrinkled structure of the membrane of this invention has significant advantages in ion transport and antifouling.

[0073] 4.2 Post-processing

[0074] The membrane was stabilized by immersing it in a 1 mol / L NaCl solution for 24 hours, thus completing the preparation of a high-quality cation exchange membrane. Immersion in the 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 the ions in the solution and the membrane helps to repair any potential microscopic defects, enhancing the overall structural stability of the membrane and thereby improving the reliability and lifespan of the cation exchange membrane in practical applications.

[0075] 5. Upgraded Coating of Porous Filter Membranes

[0076] 5.1 Coating and Settling

[0077] The prepared 5 mL crosslinking network precursor solution was uniformly coated onto the surface of the polyvinylidene fluoride (PVDF) filter membrane, ensuring complete and uniform coverage. After coating, the membrane was allowed to stand for 5 minutes. During this standing period, the TMPTA in the crosslinking network precursor solution interacted with the molecules on the PVDF membrane surface. The strongly polar ester oxygen atoms of TMPTA formed dipole-dipole interactions with the CF bonds of PVDF, allowing the small molecular structure to penetrate between the PVDF molecular chains. This altered the partial crystal structure of the membrane surface, reducing its rigidity and creating conditions for subsequent crosslinking reactions. Unlike the impregnation method, the coating method primarily acts on the membrane surface, allowing for more precise control of the membrane surface deconstruction and crosslinking effect. This results in a denser and more uniform crosslinking network on the membrane surface, which is beneficial for improving the ion exchange performance and antifouling properties of the membrane.

[0078] 5.2 Light Irradiation and Post-processing

[0079] After settling, the filter membrane was removed, excess mixture was scraped off, and then it was irradiated under visible / ultraviolet light for 40 minutes to initiate the polymerization reaction. After polymerization, the membrane was remodeled by immersing it in a 50wt% ethanol solution (the same remodeling treatment described above) for 12 hours. Finally, the membrane was immersed in a 1mol / L NaCl solution for 24 hours to complete the preparation of the cation exchange membrane. The membrane prepared using this coating method also exhibits a ring-shaped micron-scale wrinkled structure on its surface, demonstrating high-efficiency ion transport and antifouling properties. Figures 2-5 A comparison of various performance data shows that its performance advantages are consistent with those of the membrane prepared by the impregnation method, further verifying the reliability and effectiveness of the preparation method of the present invention.

[0080] 6. Membrane performance testing and analysis

[0081] 6.1 Microstructure Characterization

[0082] The surface and cross-sectional morphology of the film were observed using a field emission scanning electron microscope (Hitachi Regulus 8100, Japan) under different voltage conditions. Figure 8 (a) SEM image of the cross-section of the PC film and Figure 8 (b) The microstructural features of the PC membrane are clearly visible in the SEM image. The unique annular micron-sized wrinkled structure and dense, non-porous cross-sectional structure of the membrane surface of this invention contrast sharply with commercial cation exchange membranes. Commercial membranes have relatively smooth surfaces with almost no obvious wrinkles. This structural difference results in commercial membranes being inferior to the membrane of this invention in terms of ion transport and antifouling performance. The smooth surface of commercial membranes is not conducive to ion capture and transport and makes them prone to contaminant adhesion.

[0083] The chemical valence states of the membrane samples were recorded using X-ray photoelectron spectroscopy (Thermos Scientific K-Alpha, USA). Figure 9 (b) XPS spectra of different films revealed an S2p characteristic peak in the PC film, providing strong evidence of the successful introduction of functional sulfonic acid groups. The functional groups on the film surface were characterized using an attenuated total reflectance infrared spectroscopy (Thermos Fisher Scientific Nicoleti S20, USA). Figure 9 (a) The ATR-FTIR spectrum shows that the PC film has a symmetric stretching vibration absorption peak of sulfonic acid groups at 1035 cm⁻¹, which further confirms the presence of sulfonic acid groups in the membrane structure, indicating that the present invention has successfully introduced functional sulfonic acid groups into the membrane.

[0084] 6.2 Physical and chemical performance testing

[0085] The water contact angle was measured using a contact angle meter (SDC-100, Shengding, China) to characterize the hydrophilicity of the membrane surface. Electrochemical impedance spectroscopy (EIS) measurements were performed using a self-made four-electrode system with 0.10M NaCl electrolyte. During the measurement, a 3mA AC current was applied within a frequency range of 10³–10⁻² Hz, and then... Figure 10 (b) Nyquist plots and equivalent circuit models are used to analyze electrochemical parameters such as membrane resistance, providing a deeper understanding of the membrane's electrical properties.

[0086] In determining the swelling rate and water absorption rate, the values ​​are calculated by measuring the area difference and weight difference of the wet film and the dry film, respectively.

[0087] The formulas for calculating swelling rate and water absorption rate are as follows:

[0088] ;

[0089] ;

[0090] in, and These are the areas of the wet film and the dry film, respectively (cm²). 2 ); and These are the weights (g) of the wet film and the dry film, respectively.

[0091] Swelling rate is one of the core indicators for evaluating the stability of ion exchange membranes. PVDF-based membranes exhibit weak swelling in water. When the TMPTA addition is below 1.0 mol%, TMPTA disrupts the PVDF structure in the membrane, weakening the interactions between molecular chains and increasing the distance between them, leading to an increase in the swelling rate. At this stage, the cross-linked network within the membrane has not yet fully formed a stable structure and cannot effectively restrict the movement of PVDF molecular chains, allowing water molecules to more easily enter the membrane and cause swelling. As the TMPTA addition continues to increase, the cross-linked network formed in the system gradually stabilizes. This network restricts the movement of PVDF molecular chains, partially compensating for the swelling trend caused by TMPTA's disruption of PVDF, causing the PC membrane's swelling rate to gradually decrease from 42.5% to 24.8%. When the TMPTA addition exceeds 4.0 mol%, PVDF will exhibit significant structural defects due to excessive deconstruction. The cross-linked network becomes too dense, causing stress concentration within the membrane, leading to a decrease in the overall performance and stability of the membrane, and potentially causing abnormal changes in the swelling rate.

[0092] Ion exchange capacity (IEC) was evaluated using a titration method. Specifically, the cation exchange membrane (2 cm × 2 cm) was first soaked in 1 M HCl solution for 24 hours to convert it to H₂O. + The membrane was then thoroughly cleaned to remove any residual HCl solution. After cleaning, the membrane was immersed in 1M NaCl solution for 24 hours. The solution permeated through the membrane was then titrated with 0.01M NaOH. The titration endpoint was reached when the solution did not fade within 30 seconds. The volume of NaOH consumed was recorded, and the IEC value was calculated. The formula for calculating IEC is as follows:

[0093] ;

[0094] in, It is the ion exchange capacity (meq·g) -1 ), This is the volume (mL) of NaOH consumed. It is the concentration of NaOH (mol·L⁻¹) -1 ), This is the weight (g) of the dry film.

[0095] from Figure 10As shown in (a), the IEC values ​​of the PC membrane vary with different TMPTA additions, indicating that the ion exchange capacity of the membrane prepared in this invention reaches 2.8 meq / g, which is higher than that of commercial cation exchange membranes. Furthermore, EIS analysis reveals that the membrane of this invention has a low resistivity. These optimized physicochemical properties make the membrane more efficient in ion transport.

[0096] 6.3 Ion transport performance test

[0097] like Figure 6 As shown in (b), when evaluating the ion-selective permeability of the membrane using a self-made Daonan dialysis (DD) cell, the specific "DD conditions" were as follows: the experimental setup consisted of two compartments, each with a capacity of 100 mL and an effective working membrane area of ​​4.15 cm². The solution temperature was controlled at 25 °C. Different salt solutions (NH₄Cl, LiCl, MgCl₂, CaCl₂) were selected as feed solutions, and NaCl solution was used as the driving solution. The stirring device was turned on, and the stirring speed was set to 500 rpm to ensure thorough mixing of the solution, reduce concentration polarization, and ensure the accuracy of the experimental results. During the experiment, 0.5 mL samples were precisely taken from the feed and receiving compartments every 30 minutes, and the concentration of the target cation was measured using a UV spectrophotometer (model 722, Shanghai Jingke Tianmei Scientific Instruments Co., Ltd.). Relevant data on the permeability of different ions to the membrane were obtained, and the selective permeability of the cation exchange membrane to various ions was analyzed.

[0098] Ion flux was measured using electrodialysis (ED) experiments. For example... Figure 6 (c) The experimental setup shown consists of an ED stack composed of six compartments with an effective membrane area of ​​4 cm² arranged in an orderly manner. The experiment was conducted at a constant current density of 10 mA / cm², with the solution circulating at a rate of 150 mL / min to ensure relative stability of the ion concentration in the solution. The formula for calculating the ion flux is as follows:

[0099] ;

[0100] in, It is ion flux (mol·m -2 ·h -1 ), It is the effective membrane area (m²) 2 ), and The initial ion concentration (mg·L) of the feed / dilute side. -1 ) and initial solution volume (L), and It is the ion concentration (mg·L) after running for t (h). -1 ) and solution volume (L), It is the relative molecular mass of the ion (g·mol).-1 ).

[0101] from Figure 11 NH4 in different PC films in (a) + Flux variation curves with TMPTA addition amount, and Figure 11 (b) Comparison of NH4+ between PC-4.0 membranes and commercial cation exchange membranes (CEMs) at different feed solution concentrations + Flux data show that as the amount of TMPTA added increases, the NH4 content of the PC membrane decreases. + Flux increased, and the PC-4.0 membrane under different ion concentration conditions for NH4 + The flux is superior to that of commercial membranes. In practical application simulation tests, actual domestic wastewater (containing NH4+) was used. + The content was 2.5 mM) and the lemon pectin wastewater (NH4) + (Content of 50mM) on NH4 in PC-4.0 membrane and commercial membrane + Flux was tested. Real-world domestic wastewater contains a large number of impurity cations, such as Ca2+. 2+ Mg 2+ Na + These impurity cations will react with NH4 + Competition for transmembrane transport channels, occupation of some ion exchange sites, and interference with NH4+ + Exercise increases NH4 + Transmembrane transport resistance leads to NH4 in the PC-4.0 membrane. + The flux decreased significantly. In actual pectin wastewater treatment processes, in addition to interference from impurity cations, a large amount of organic matter may also adhere to the membrane surface or inside the membrane pores, clogging the pores, reducing the effective cross-sectional area of ​​ion transport channels, altering the membrane surface properties and microstructure, affecting the activity of ion exchange sites, and reducing the NH4+ flux of the PC-4.0 membrane. + Flux from 2.55 mol·m -2 ·h -1 Decreased to 0.90 mol·m -2 ·h -1 Even in complex real-world wastewater environments, the PC-4.0 membrane demonstrates excellent performance in treating domestic sewage and lemon pectin wastewater, with low NH4 content. + The flux is still superior to that of commercial membranes.

[0102] The effectiveness of PC-4.0 membranes and commercial membranes in treating actual domestic sewage and lemon pectin wastewater was compared and evaluated using an ED (Enhanced Electrostatics) device. Figure 12 Experimental data show that, under the influence of an electric field, the NH4 content of all tested membranes... + Flux rates have all increased. When treating actual domestic wastewater, the NH4+ flux of the PC-4.0 membrane has improved. + Flux from 0.09 mol·m-2 ·h -1 Rapidly increased to 0.55 mol·m -2 ·h -1 The efficiency was increased by 6.0 times; when treating lemon pectin wastewater, the NH4 content of the PC-4.0 membrane was significantly reduced. + Flux from 0.90 mol·m -2 ·h -1 Increased to 1.62 mol·m -2 ·h -1 This represents a 1.8-fold increase. Similarly, when treating these two actual wastewaters, the NH4+ of the PC-4.0 membrane... + The fluxes were all higher than those of commercial membranes. This indicates that the PC-4.0 membrane can transport NH4 more efficiently under electric field assistance. + This further demonstrates its high efficiency and superiority in actual wastewater treatment, providing strong experimental evidence for its wide application in seawater desalination, high-salt concentration, ammonia nitrogen recovery, lithium and magnesium separation, and actual wastewater resource recycling.

[0103] 6.4 Antifouling performance test

[0104] Using actual lemon pectin wastewater as the feed solution, the antifouling performance of the membrane of this invention (PC-4.0 membrane) and a commercial membrane (CMVN) was compared under DD conditions. Figure 13 As can be visually observed in (a), the PC-4.0 membrane surface has a unique annular folded structure, while the surface of the commercial CMVN membrane is relatively smooth.

[0105] After both membranes had been running in a real pectin wastewater environment for 24 hours, through Figure 13 (b) It is evident that a large amount of contaminant deposition occurred on the surface of the CMVN membrane, while no significant contaminant adhesion was observed on the surface of the PC-4.0 membrane. This is because the pleated structure of the PC-4.0 membrane forms a physical barrier, increasing the difficulty of contaminant adhesion, and the enhanced microfluidic field turbulence on the membrane surface due to the pleated structure makes it difficult for contaminants to remain on the membrane surface.

[0106] After rinsing the contaminated membrane with pure water for 1 minute, from... Figure 13 (c) As can be seen, a small amount of contaminants still remain on the surface of the CMVN membrane. Electrochemical impedance spectroscopy (EIS) was used to comprehensively characterize the electrochemical performance of the PC-4.0 membrane and the CMVN membrane before and after fouling, as well as after cleaning. Figure 13 (d) It can be seen that, compared with the uncontaminated original membrane, the resistance of the two contaminated membranes (including the membrane bulk resistance R) is significantly different. m Pollution resistance R p Diffusion boundary layer resistance R DBL and double layer resistance R EDL All showed a systematic increase.

[0107] For PC-4.0 membrane, the contamination resistance R Fouling It accounts for 12.7% of the total impedance, with a value of 0.9 Ω·cm. 2 Its membrane bulk resistance R m From 2.8Ω·cm 2 Increased to 3.5Ω·cm 2 The increase reached 25.0%. This is because contaminants adhere to the membrane surface and penetrate the membrane interior, blocking some ion transport channels and increasing the resistance to ion transport within the membrane. Simultaneously, contaminants may interact with active sites such as sulfonic acid groups within the membrane, altering the membrane's microscopic conductive environment and leading to an increase in the membrane's bulk resistance. Surface and internal fouling also weakens the negative charge on the membrane surface, slowing down the charge transfer response rate, thus increasing R... EDL It increased tenfold.

[0108] The contamination level of CMVN membranes is even more severe, R m From 4.3Ω·cm 2 Increased to 8.9Ω·cm 2 It increased by 107.0%, R EDL From 0.4Ω·cm 2 Increased to 1.9Ω·cm 2 It is significantly higher than that of PC-4.0 film (R EDL =1.3Ω·cm 2 This is because the surface of the CMVN membrane is smooth and lacks the wrinkled structure of the PC-4.0 membrane to prevent pollutant deposition. Pollutants are more likely to adhere to the membrane surface and penetrate deep into the membrane, causing greater damage to the membrane's microstructure, resulting in severe obstruction of ion transport channels and a significant increase in membrane resistance.

[0109] Subsequently, the fouled membrane was simply rinsed with pure water for 1 minute, and its EIS and NH4 were tested. + Flux, results as follows Figure 13 As shown in (e). After physical cleaning, the bulk resistivity of the PC-4.0 film decreased from 3.5 Ω·cm. 2 Reduced to 3.1 Ω·cm 2 The bulk resistivity of the CMVN film is 8.9 Ω·cm. 2 Reduced to 6.3 Ω·cm 2 Physical cleaning effectively removes fouling from cation exchange membranes because cation exchange membranes have strong negative charges, resulting in weaker binding to fouling and lower stability. However, after physical cleaning, the inherent resistance of the CMVN membrane increased by 46.5% compared to the uncontaminated CMVN membrane, and by 10.7% compared to the PC-4.0 membrane. Furthermore, the NH4+ of the cleaned CMVN membrane... +The flux recovery rate was 75.1%, while the NH4 of the PC-4.0 membrane... + The flux recovery rate reached as high as 83.9%. The comprehensive comparative analysis of the above indicators fully demonstrates that the PC-4.0 membrane has stronger antifouling performance than commercial CMVN membranes. In practical applications, it can better maintain membrane performance stability, reduce performance degradation caused by fouling and the need for frequent cleaning and maintenance, and provide strong performance assurance for its widespread application in complex wastewater treatment and other fields.

[0110] 6.5 Computational Fluid Dynamics (CFD) Simulation

[0111] Computational fluid dynamics (CFD) simulations are used to deeply analyze the mechanism by which different Turing structure folds promote ion transport. By constructing corresponding models, the transport process of ions on the membrane surface and inside is simulated, and the influence of fold structures on ion trajectory, velocity distribution, and concentration distribution is analyzed.

[0112] Simulation results show that annular micron-sized folds of specific size and shape can significantly enhance the turbulent microfluidic field at the membrane surface. Ions are more easily captured by the folds during transport on the membrane surface, and the enhanced turbulent microfluidic field effectively improves the ion transport rate. For example, when ions encounter folds, their direction of motion changes, increasing the chance of contact with active sites on the membrane surface, thereby improving ion exchange efficiency. Simultaneously, the fold structure also affects the concentration distribution of ions within the membrane, making the ion distribution more uniform and reducing ion concentration polarization.

[0113] Based on the simulation results, the size, shape, and distribution density of wrinkles can be further optimized in subsequent membrane structure design. For example, by adjusting parameters such as the concentration of the post-treatment solution and the soaking time during the preparation process, the wrinkle formation process can be precisely controlled to obtain a membrane surface structure more conducive to ion transport. Furthermore, the simulation results can provide theoretical guidance for the development of novel membrane materials, allowing for the selection of more suitable raw materials and inducers based on the optimization requirements of ion transport, thereby further improving membrane performance.

[0114] 7. Results and Discussion

[0115] 7.1 Analysis of the membrane preparation process

[0116] In the process of preparing the cation exchange membrane in this invention, from Figure 7 (a) It can be seen that the microporous PVDF membrane as the substrate exhibits an interlaced structure, and its abundant pores provide sufficient space for the filling of the crosslinking network precursor liquid, providing a rigid framework support for subsequent reactions.

[0117] In the hole filling stage, comparison Figure 7 (a) and Figure 7(b) It can be clearly observed that the PVDF membrane has undergone significant changes. The membrane surface, which was originally full of pores, has become smooth and flat, and the pores have completely disappeared. This proves that the cross-linking network precursor liquid has successfully penetrated into the interior of the PVDF membrane and completed the filling of the pores.

[0118] Trimethylolpropane triacrylate (TMPTA) in the crosslinking network precursor solution plays a crucial role as an inducing agent. TMPTA, with its strongly polar ester oxygen atom, forms dipole-dipole interactions with the CF bonds of PVDF. Furthermore, its small molecular structure allows it to penetrate between PVDF molecular chains, weakening interchain van der Waals forces and dipole interactions, thus leading to partial destructive changes in PVDF. XRD results show that the full width at half maximum (FWHM) of the PVDF characteristic peak (2θ = 20.2°) significantly increased from 0.72 to 4.62 after 10 min of immersion in the crosslinking network precursor solution, confirming a decrease in PVDF crystallinity. This change in PVDF structure results in a reduction in film thickness, shortening the transmembrane path for ions and lowering the resistance to ion transport.

[0119] When the membrane with filled pores is exposed to visible / ultraviolet light, the photoinitiator in the crosslinking network precursor solution is activated, initiating polymerization and solidification of the precursor solution to form a crosslinked network. Subsequently, the membrane is immersed in an aqueous solution for reshaping treatment, such as... Figure 7 As shown in (c), PVDF undergoes partial recrystallization, with its XRD characteristic peak's full width at half maximum (FWHM) decreasing from 4.62 to 3.34. Simultaneously, the cross-linked network of alternating hydrophilic and hydrophobic regions in the pore-filled membrane undergoes uneven expansion in water. It is the synergistic effect of the PVDF recrystallization process and the uneven expansion of the cross-linked network that enables the membrane to be reshaped in aqueous solution, ultimately forming a unique annular wrinkled structure on the membrane surface.

[0120] This series of closely linked and interconnected changes alters the membrane structure at the microscopic level, thereby exhibiting different performance characteristics at the macroscopic level. From the initial alteration of the PVDF membrane structure to the formation of the cross-linked network and the appearance of annular folds on the membrane surface, each step lays a solid foundation for the preparation of cation exchange membranes with high-efficiency ion transport and antifouling properties.

[0121] 7.2 Microstructure and Chemical Analysis of the Membrane

[0122] The surface and cross-sectional microstructure of the prepared PC film were characterized using SEM and AFM. Figure 8 (a) and Figure 8(b) It can be clearly observed that, compared to the porous PVDF membrane, the highly developed pores within the PVDF membrane are successfully filled by the cross-linked network precursor solution. After a series of preparation processes, the PC membrane exhibits a dense, complete, and non-porous structure on both its surface and cross-section. This structure greatly facilitates cation permeation, providing a structural basis for efficient ion transport. Furthermore, in Figure 8 In (b), the surface of the PC series membranes all exhibits annular folds. Among them, the fold size of the PC-4.0 membrane is maintained in the range of 10-15μm. For other PC membranes (PC-0.5, PC-1.0, PC-2.0), although the fold size is not explicitly mentioned, they all exhibit similar fold structures.

[0123] The chemical structure of the PC film was evaluated using ATR-FTIR. (Comparison) Figure 9 (a) The spectra of PVDF membrane and PC series membrane show that the PC series membrane has a lower spectral density at 1035 cm⁻¹. -1 A distinct absorption peak at 1550 cm⁻¹ appeared, attributed to the symmetric stretching vibration of the sulfonic acid group. The appearance of this characteristic peak clearly indicates that the functional sulfonic acid group has successfully participated in the membrane structure. Furthermore, at 1550 cm⁻¹… -1 The characteristic peak at 1150 cm⁻¹ is attributed to the stretching vibration of the NH group. -1 The sharp absorption peak in the vicinity is attributed to the stretching vibration of the -CF group in the PVDF substrate, and the transmittance of this peak is weakened due to pore filling and subsequent reactions, further confirming the structural changes during membrane preparation.

[0124] The surface chemistry of the prepared PC film was characterized using XPS spectroscopy. Figure 9 (b) It can be seen that, compared with the PVDF film, the prepared PC film spectrum shows an S2p characteristic peak at 168.2, while the F1s characteristic peak at 688.1 disappears. Figure 9 (c) High-resolution O1s spectra of the PVDF and PC-4.0 membranes are shown. The deconvolution of the O1s peak of the PC-4.0 membrane reveals characteristic peaks of sulfonic acid groups compared to the PVDF membrane. The combined analysis results of ATR-FTIR and XPS spectra strongly demonstrate that the polymer electrolyte containing sulfonic acid groups successfully filled the porous PVDF substrate, powerfully proving the successful preparation of the cation exchange membrane (PC membrane) in this study.

[0125] 7.3 Physicochemical Properties Analysis of the Membrane

[0126] Ion exchange capacity (IEC), a key parameter for measuring ion transport across cation exchange membranes, directly reflects the number of free charge sites within the membrane and plays a decisive role in the membrane's ion transport performance. Based on... Figure 10As shown in the data trend in (a), when the TMPTA addition amount gradually increased from 0.5 mol% to 4.0 mol%, the IEC value of the PC membrane increased from 2.20 meq / g to 2.97 meq / g. This is because TMPTA can promote the stable binding of more sulfonated monomers inside the membrane, thereby significantly increasing the number of active sites available for ion exchange within the membrane, providing richer channels for ion transport, and effectively enhancing the ion exchange capacity of the membrane.

[0127] Swelling rate is one of the core indicators for evaluating the stability of ion exchange membranes. In this invention, the swelling phenomenon of the PVDF base membrane in water is extremely weak. When the TMPTA addition is below 1.0 mol%, TMPTA will damage the PVDF structure in the base membrane, causing its stability to decrease and the intermolecular interactions to weaken, thus leading to an increase in swelling rate. As the TMPTA addition continues to increase, the cross-linked network formed in the system gradually stabilizes. This stable cross-linked structure can partially compensate for the swelling trend caused by TMPTA damage to PVDF, causing the swelling rate of the PC membrane to gradually decrease from 42.5% to 24.8%. It is worth noting that once the TMPTA addition exceeds 4.0 mol%, PVDF will exhibit significant structural defects due to excessive deconstruction, seriously affecting the overall performance and stability of the PC membrane.

[0128] Electrochemical impedance spectroscopy (EIS) allows for in-depth analysis of the resistive properties of PC series films. (Observation) Figure 10 (b) Nyquist plots of different PC films reveal a clear negative correlation between membrane resistance (characterized by the intersection of the Nyquist plot and the x-axis, Z') and the amount of TMPTA added. With increasing TMPTA addition, the membrane resistance gradually decreases from an initial 39.3 to 37.1, 37.0, and finally 36.2. Through the construction and analysis of the equivalent circuit model, key electrochemical indicators related to the bulk layer, electrical double layer, and diffusion boundary layer can be accurately identified. Further analysis... Figure 10 (c) It can be seen that as the amount of TMPTA added increases, the bulk resistivity of the PC film decreases from 11.1 Ω·cm. 2 Significantly reduced to 2.8 Ω·cm 2 This significant reduction in membrane resistance is primarily attributed to the successful introduction of more charged sulfonated monomers, a result highly consistent with previous IEC test results. The increased charged sulfonated monomers not only enhance ion exchange capacity but also provide a smoother channel for charged ion transport within the membrane, greatly facilitating rapid cross-membrane transport and significantly improving the membrane's ion transport efficiency, thus exhibiting superior performance in practical applications.

[0129] 7.4 Analysis of the ion transport performance of the membrane

[0130] This invention utilizes the Dornan dialysis experiment to investigate the relationship between the amount of TMPTA added and membrane NH4. + The relationship between fluxes was studied in depth. Figure 11 (a) The data trend clearly shows that with the gradual increase of TMPTA addition, the NH4 content of the PC membrane decreases. + Flux from 0.68 mol·m -2 ·h -1 Steadily increased to 1.63 mol·m -2 ·h -1 This significant improvement is primarily attributed to two major advantages of the PC-4.0 membrane: a high ion exchange capacity (IEC) and low membrane resistance. The high IEC provides abundant active sites for ion transport, while the low membrane resistance greatly reduces the obstacles to ion transport across the membrane. These two factors work synergistically to significantly promote NH4+ ion exchange. + High-efficiency transmission.

[0131] To comprehensively evaluate the performance advantages of the PC-4.0 membrane under different ion concentration environments, this invention selected three different NH4 concentrations: 2.5 mM, 25 mM, and 50 mM. + Feed water concentration was compared between commercial membranes and PC-4.0 membranes. Figure 11 (b) It can be seen that all the membranes involved in the test have NH4 + The flux will vary with the amount of NH4 in the feed solution. + The concentration increases with increasing NH4+. Taking the PC-4.0 membrane as an example, when the feed solution contains NH4+... + When the concentration increases from 2.5 mM to 50 mM, its NH4+... + The flux increased significantly, from 0.18 mol·m -2 ·h -1 The concentration increased dramatically to 2.55 mol·m⁻¹ -2 ·h -1 The increase was as high as 14 times. This is because of the NH4+ in the feed solution. + The significant increase in NH4+ concentration effectively increased the ion concentration gradient across the membrane. Based on the fundamental principles of ion diffusion, an increased concentration gradient enhances the transmembrane driving force of ions, allowing NH4+ to diffuse across the membrane. + Under the influence of an electric field, NH4+ is more easily transported across the membrane. Under different concentration conditions, the NH4+ transport across the PC-4.0 membrane... + The flux is consistently superior to commercial membranes, exceeding them by 2-4 times. This is mainly due to the PC-4.0 membrane having a higher IEC value and lower membrane resistance compared to commercial membranes, thus exhibiting superior performance in ion transport.

[0132] In practical application scenario simulation tests, this invention uses actual domestic sewage (containing NH4) + The content was 2.5 mM) and the lemon pectin wastewater (NH4) + (Content of 50mM), NH4 content of PC-4.0 membrane and commercial membrane + Flux was tested. Results showed that, compared to tests using pure NH4Cl solution, the NH4 flux of all membranes was significantly reduced in actual domestic wastewater tests. + The flux of all membranes decreased significantly, particularly the NH4+ flux of the PC-4.0 membrane. + The flux decreased by half. This is because actual domestic sewage contains a large number of impurity cations, such as Ca. 2+ Mg 2+ Na + These impurity cations will react with NH4 + Competition for transmembrane transport channels severely hinders NH4. + Normal transmembrane transport. In actual pectin wastewater treatment processes, NH4 also appears. + The phenomenon of decreased flux, NH4 in PC-4.0 membrane + Flux from 2.55 mol·m -2 ·h -1 Decreased to 0.90 mol·m -2 ·h -1 Unlike actual domestic sewage, in the actual treatment of pectin wastewater, in addition to interference from impurity cations, a large amount of organic matter may also adhere to the membrane surface or inside the membrane pores. This organic matter not only clogs the membrane pores and reduces the effective cross-sectional area of ​​the ion transport channels, but may also interact with the membrane material, altering the membrane's surface properties and microstructure, further increasing the NH4+ content. + The difficulty of transmembrane transport. Even in such a complex real-world wastewater environment, the PC-4.0 membrane, when treating domestic sewage and lemon pectin wastewater, exhibits low NH4 content. + The flux is still superior to commercial membranes, fully demonstrating the excellent ion transport performance of the PC-4.0 membrane in complex ion environments.

[0133] This invention further utilizes an ED device to compare and evaluate the effectiveness of PC-4.0 membranes and commercial membranes in treating actual domestic sewage and lemon pectin wastewater. Figure 12 The experimental data clearly show that, under the influence of an electric field, the NH4 content of all tested membranes... + Flux rates have all increased. In treating actual domestic wastewater, the PC-4.0 membrane demonstrated strong treatment capabilities, particularly in its NH4+ treatment. + Flux from 0.09 mol·m -2 ·h -1 Rapidly increased to 0.55 mol·m -2·h -1 The efficiency was increased by 6.0 times; when treating lemon pectin wastewater, the NH4 content of the PC-4.0 membrane was significantly reduced. + The flux also increased from 0.90 mol·m -2 ·h -1 Increased to 1.62 mol·m -2 ·h -1 This represents a 1.8-fold increase. Similarly, when treating these two actual wastewaters, the NH4+ of the PC-4.0 membrane... + The fluxes were all higher than those of commercial membranes. This indicates that the PC-4.0 membrane can transport NH4 more efficiently under electric field assistance. + This further demonstrates its high efficiency and superiority in actual wastewater treatment, providing strong experimental evidence for its wide application in seawater desalination, high-salt concentration, ammonia nitrogen recovery, lithium and magnesium separation, and actual wastewater resource recycling.

[0134] 7.5 Analysis of the antifouling performance of the membrane

[0135] To further investigate the differences in antifouling performance between the PC-4.0 membrane prepared in this invention and the commercial membrane (CMVN) in practical applications, this study used actual lemon pectin wastewater as the feed solution and conducted a comparative analysis of the antifouling performance of the PC-4.0 membrane and the commercial membrane (CMVN) under Downan dialysis (DD) conditions.

[0136] from Figure 13 As can be visually observed in (a), the PC-4.0 membrane surface has a unique annular pleated structure, while the commercial CMVN membrane surface is relatively smooth. After both membranes have been running in a real pectin wastewater environment for 24 hours, the flow rate of the membranes... Figure 13 (b) It is clearly visible that a large amount of contaminant deposition appeared on the surface of the CMVN membrane, while no obvious contaminant adhesion was observed on the surface of the PC-4.0 membrane. After rinsing the fouled membrane with pure water for 1 minute, from Figure 13 (c) It can be seen that a small amount of contaminants still remain on the surface of the CMVN membrane.

[0137] This study used electrochemical impedance spectroscopy (EIS) to comprehensively characterize the electrochemical performance of PC-4.0 and CMVN membranes before and after fouling and after cleaning. Figure 13 (d) It can be seen that, compared with the uncontaminated original membrane, the resistance of the two contaminated membranes (including the membrane bulk resistance R) is significantly different. m Pollution resistance R p Diffusion boundary layer resistance R DBL and double layer resistance R EDL All showed a systematic increase. Among them, the contamination resistance R of the contaminated PC-4.0 membrane increased. Fouling It accounts for 12.7% of the total impedance, with a value of 0.9 Ω·cm. 2Its membrane bulk resistance R m From 2.8Ω·cm 2 Increased to 3.5Ω·cm 2 The increase reached 25.0%, indicating that internal fouling occurred in the PC-4.0 membrane, and that internal fouling was the main factor leading to its performance changes. Meanwhile, R... EDL The increase was 10-fold, mainly due to contaminants adhering to the membrane surface and internal fouling weakening the negative charge on the membrane surface, thus slowing down the charge transfer response. It is worth noting that the RVN of the CMVN membrane and the PC-4.0 membrane... Fouling Both are 0.9Ω·cm 2 However, the R of the CMVN film m The increase was even more significant, from 4.3 Ω·cm 2 Increased to 8.9Ω·cm 2 The percentage increased by 107.0%, clearly indicating that the internal fouling of the CMVN membrane is more severe. Similarly, the R0 of the CMVN membrane... EDL From 0.4Ω·cm 2 Increased to 1.9Ω·cm 2 It is significantly higher than that of PC-4.0 film (R EDL =1.3Ω·cm 2 This is a comprehensive manifestation of the more severe surface and internal fouling of the CMVN membrane. Therefore, although both membranes experienced surface and internal fouling, the PC-4.0 membrane, compared to the CMVN membrane, exhibited primarily surface fouling, making it difficult for contaminants to penetrate deep into the membrane.

[0138] Subsequently, the fouled membrane was simply rinsed with pure water for 1 minute, and its EIS and NH4 were tested. + Flux, results as follows Figure 13 As shown in (e). After physical cleaning, the bulk resistivity of the PC-4.0 film decreased from 3.5 Ω·cm. 2 Reduced to 3.1 Ω·cm 2 The bulk resistivity of the CMVN film is 8.9 Ω·cm. 2 Reduced to 6.3 Ω·cm 2 Physical cleaning effectively removes fouling from cation exchange membranes because cation exchange membranes have strong negative charges, resulting in weaker binding to fouling and lower stability. However, after physical cleaning, the inherent resistance of the CMVN membrane increased by 46.5% compared to the uncontaminated CMVN membrane, and by 10.7% compared to the PC-4.0 membrane. Furthermore, the NH4+ of the cleaned CMVN membrane... + The flux recovery rate was 75.1%, while the NH4 of the PC-4.0 membrane... +The flux recovery rate reached as high as 83.9%. The comprehensive comparative analysis of the above indicators fully demonstrates that the PC-4.0 membrane has stronger antifouling performance than commercial CMVN membranes. In practical applications, it can better maintain membrane performance stability, reduce performance degradation caused by fouling and the need for frequent cleaning and maintenance, and provide strong performance assurance for its widespread application in complex wastewater treatment and other fields.

[0139] 8. Conclusion

[0140] This invention successfully prepared a high-efficiency ion-transporting, antifouling cation exchange membrane using a deconstruction-reconstruction method. The method uses a polyvinylidene fluoride (PVDF) membrane as a substrate, inducing the deconstruction of the PVDF membrane crystals through trimethylolpropane triacrylate (TMT), followed by visible / ultraviolet (UV) light curing and solution treatment to induce cross-linking network reconstruction, forming a ring-shaped micron-scale wrinkled structure on the membrane surface. The prepared cation exchange membrane exhibits a high-density thin-film structure, with an ion exchange layer thickness of 35-60 nm, lower than commercial membranes, and an ion exchange capacity of 2.8 meq / g, higher than commercial cation exchange membranes. Compared with commercial cation exchange membranes, the ion transport rate of this membrane is increased by 2-4 times, and its antifouling performance is significantly enhanced. Furthermore, this invention employs visible / ultraviolet light polymerization technology, avoiding the high energy consumption problem of traditional heat treatment, and selects a benzene ring-free material as the functional monomer, achieving green preparation of the cation exchange membrane. This membrane has broad application prospects in seawater desalination, high-salt concentration, ammonia nitrogen recovery, lithium-magnesium separation, actual wastewater resource recovery and treatment, and the Daonan dialysis unit and electrodialysis unit.

[0141] The advantages of this invention compared to the prior art are as follows:

[0142] 1. Advantages of the Preparation Process: Existing cation exchange membrane preparation processes often involve prolonged heat treatment. For example, thermal polymerization requires heat treatment at 80℃ for 3 hours, and hot rolling requires repeated hot rolling at 105℃ for 1 hour. These processes are time-consuming and energy-intensive. This invention employs a visible / ultraviolet light polymerization coupled with a deconstruction-reconstruction design method for the membrane microstructure, avoiding prolonged high-temperature treatment. With the help of a photoinitiator, the crosslinking network precursor solution can complete the polymerization and curing process within just 20-60 minutes, significantly shortening the preparation time and reducing energy consumption. This enables low-energy and rapid preparation of cation exchange membranes, aligning with the concept of sustainable development, and helping to reduce production costs and improve production efficiency.

[0143] 2. Environmental Advantages: This invention selects benzene-free compounds as functional monomers, such as 2-acrylamido-2-methylpropanesulfonic acid and methacrylamide, and uses methanol and water as solvents. In contrast, existing technologies may use substances containing benzene rings or other environmentally unfriendly materials. The raw material selection of this invention avoids the use of harmful substances, reducing environmental pollution at the source and achieving green preparation of cation exchange membranes, which is conducive to promoting the green development of the industry.

[0144] 3. Performance Advantages: Existing commercial cation exchange membranes are limited by the trade-off between ion exchange capacity and transport channel length, resulting in high membrane resistance and low ion transport rates. Furthermore, increasing ion exchange capacity to ensure mechanical strength leads to increased membrane thickness. The cation exchange membrane prepared in this invention features a high-density thin-film structure with an ion exchange layer thickness of 35-60 nm, lower than commercial membranes (100-600 nm), and an ion exchange capacity of 2.8 meq / g, higher than the 0.9-2.3 meq / g of commercial cation exchange membranes. This significantly improves the ion exchange capacity and reduces membrane thickness, effectively lowering membrane resistance and improving ion transport efficiency, resulting in superior performance in various ion exchange applications.

[0145] 4. Antifouling Advantages: Existing commercial cation exchange membranes have smooth surfaces, making it impossible to form a turbulent flow field on the membrane surface, resulting in limited ion transport rates and susceptibility to fouling. The cation exchange membrane prepared in this invention has annular micron-sized folds on its surface. These folds facilitate the capture of target ions and enhance the turbulent microflow field on the membrane surface, accelerating the ion exchange process. Compared to commercially available cation exchange membranes with smooth surfaces, its ion transport rate is increased by 2-4 times. Simultaneously, these folds also hinder the deposition of contaminants on the membrane surface, enhancing the membrane's antifouling performance. This significantly improves the membrane's antifouling ability in practical applications, reduces the frequency of membrane cleaning and replacement, lowers operating costs, and improves the operational stability and reliability of the equipment.

[0146] 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 from a porous filter membrane based on deconstruction and remodeling, characterized in that, The method includes the following steps: S1, using a polyvinylidene fluoride (PVDF) filter membrane as a substrate, impregnating it with a crosslinking network precursor solution or coating the membrane surface with the crosslinking network precursor solution, wherein the crosslinking network precursor solution includes at least one sulfonated monomer, an inducing agent, and at least one photoinitiator; wherein the inducing agent is configured to interact with PVDF crystals in the PVDF filter membrane to cause some of the PVDF crystals to decompose into amorphous substances. S2, the impregnated or coated polyvinylidene fluoride filter membrane is irradiated under visible / ultraviolet light for a time T to induce the polymerization and solidification of the crosslinking network precursor liquid, and the polyvinylidene fluoride filter membrane partially deconstructed based on the crosslinking network formed by polymerization and solidification is compressed to make the membrane thinner and increase the exchange site density. S3. After polymerization and curing, the polyvinylidene fluoride 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 polyvinylidene fluoride in the polyvinylidene fluoride filter membrane can be transformed from amorphous to crystalline after contact with it, 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. The inducing agent is set as trimethylolpropane triacrylate; The sulfonating monomer is selected from at least one of 2-acrylamido-2-methylpropanesulfonic acid and 4-styrenesulfonic acid.

2. The method according to claim 1, characterized in that, The remodeling solution is one of ultrapure water, 10-70 wt% ethanol, or 0.01-0.5 mol / L NaCl solution.

3. The method according to claim 2, characterized in that, The photoinitiator is a visible light initiator or an ultraviolet light initiator, wherein the visible light initiator is riboflavin, and the ultraviolet light initiator is at least one of 2-hydroxy-2-methylphenylacetone and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone.

4. The method according to any one of claims 1-3, characterized in that, Prior to step S1, a step S0 is included to prepare a cross-linked network precursor solution, which includes: Add the sulfonated monomer to ultrapure water and methanol, and stir for 10-20 minutes until completely dissolved; Add 0.5-4.0 mol% of the inducing agent and continue stirring for 10-20 minutes until all monomers are dissolved; Under light-protected conditions, add 0.1-1.0 mol% of photoinitiator and stir for 10-20 minutes to achieve uniform dissolution, thereby obtaining the crosslinking network precursor solution.

5. The method according to claim 4, characterized in that, Following step S3, a post-processing step S4 is also included, which includes immersing the membrane in a 1 mol / L NaCl solution for N hours.

6. The method according to claim 4, characterized in that, The impregnation step in step S2 includes: Immerse the polyvinylidene fluoride filter membrane in the cross-linking network precursor solution and soak it for 1-20 minutes under light-protected conditions; Then, ultrasonic treatment is performed for 5-20 minutes to ensure that the AC network precursor solution is fully filled in the membrane pores; Remove the filter membrane and scrape off any excess cross-linking network precursor solution from its surface.

7. The method according to claim 4, characterized in that, The coating step in step S2 includes: The cross-linking network precursor solution was coated onto the surface of the polyvinylidene fluoride filter membrane and allowed to stand for 1-10 minutes. Remove the filter membrane and scrape off any excess cross-linking network precursor solution from its surface.

8. The method according to claim 1, characterized in that, The time T is 20-60 minutes.

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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