Method for preparing cation exchange membrane from porous filter membrane based on deconstruction and remodeling

Abstract

The invention discloses a method for preparing a cation exchange membrane from a porous filter membrane based on deconstruction and remodeling and the prepared cation exchange membrane. According to the method, a polyvinylidene fluoride filter membrane is used as a substrate and is impregnated or coated with a cross-linked network precursor solution containing a sulfonated monomer, an inducer and a photoinitiator, the inducer promotes polyvinylidene fluoride crystals to be deconstructed, the precursor solution is polymerized and cured through visible / ultraviolet irradiation, and the filter membrane is compressed to increase the density of exchange sites and reduce the thickness of the membrane. And treating with a remolding solution to form a wrinkle shape on the surface of the membrane. The preparation process does not need long-time heat treatment and is low in cost. The prepared cation exchange membrane solves a plurality of problems of an existing commercial membrane, and by synchronously regulating and controlling the exchange site density and the membrane thickness, the membrane body resistance is reduced, and the ion transmission efficiency is improved; a turbulent flow field can be formed on the surface of the membrane with the fold morphology, and the anti-pollution performance is enhanced. The invention provides a new way for the preparation of the cation exchange membrane, and has important significance for the development of the fields of seawater desalination, resource recovery and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of material preparation, and particularly relates to the preparation technology of ion exchange membranes. The key is to convert porous filter membranes into cation exchange membranes by using a deconstruction-remodeling process, involving relevant cross-linking network design and Donnan dialysis and electrodialysis device technologies, which are used to improve the comprehensive performance of cation exchange membranes to meet the application requirements in the fields of seawater desalination, high-salt concentration, etc. Background Art

[0002] Cation exchange membranes play an irreplaceable role in many fields. For example, in the field of seawater desalination, it can effectively separate the salts in seawater and provide technical support for solving the shortage of fresh water resources; in high-salt concentration, it helps to increase the concentration of salt solutions and meet the requirements of industrial production for salt solutions with specific concentrations; in the processes of ammonia nitrogen recovery and lithium-magnesium separation, cation exchange membranes can selectively transport ions to achieve efficient separation and recovery of substances; in the actual field of wastewater resource recovery and treatment, it can recover useful ions in wastewater to achieve the dual purposes of resource reuse and environmental pollution reduction. In recent years, the global production capacity of cation exchange membranes has increased rapidly, and, compared with water-permeable filter membranes, such as microfiltration and ultrafiltration membranes, they are expensive. The existing high-performance cation exchange membranes in our country mainly rely on imports from countries such as the United States, Japan, and Germany.

[0003] In the actual application process, there are many problems to be solved urgently for the existing cation exchange membranes, as follows:

[0004] First, from the perspective of performance, the existing commercial cation exchange membranes are limited by the trade-off effect between ion exchange capacity and transport channel length, and there are problems of large membrane body resistance and low ion transport efficiency.

[0005] Specifically, during the ion transport process, cations jump forward along the exchange sites in the ion transport channels in the cation exchange membrane. Therefore, an ideal cation exchange membrane should simultaneously have a large number of ion exchange sites and short transport channels. During the research process, the density of exchange sites in the cation exchange membrane can be reflected by the ion exchange capacity, and the transport channel length can be macroscopically reflected by the membrane thickness. Therefore, an ideal cation exchange membrane should simultaneously have a large ion exchange capacity and a small membrane thickness. However, for existing commercial cation exchange membranes, to ensure the mechanical strength of the membrane, when increasing the ion exchange capacity, more rigid substances need to be added to ensure the mechanical strength of the membrane, which in turn leads to an increase in the membrane thickness. The pore filling method is a method for rapidly preparing a cation exchange membrane using a porous membrane as a rigid framework and fixing sulfonated functional monomers in the membrane pores with a crosslinking agent. This method can achieve high-density fixation of exchange sites in the membrane, but still cannot synchronously regulate the exchange site density and the membrane thickness. That is, the existing preparation of cation exchange membranes cannot synchronously regulate the ion exchange capacity and the membrane thickness, and thus cannot further improve the ion transport rate of the cation exchange membrane.

[0006] Second, in terms of anti-fouling performance, during the actual application of existing commercial cation exchange membranes, especially when treating actual wastewater resource recovery, the membrane surface is extremely prone to fouling, and the interior is also easily contaminated. This is because the surface of the cation exchange membrane is flat, and a turbulent flow field cannot be formed on the membrane surface, making it easy for pollutants to adhere to the membrane surface and gradually enter the membrane, which not only limits the ion transport rate but also greatly shortens the service life of the membrane.

[0007] Third, in terms of the preparation process, most of the existing processes for preparing cation exchange membranes 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 large amount of time and energy but also have high costs.

[0008] Considering these problems comprehensively, there is currently a lack of cation exchange membrane technology in the market that simultaneously has low cost, low resistance, good ion transport, and anti-fouling performance. This has brought obstacles to the development of related fields and also highlights the necessity and importance of the research of the present invention. That is, there is no existing technology that can simultaneously have a high ion exchange capacity, fast ion transport at a low concentration, and good anti-fouling effect for the preparation of low-cost cation exchange membranes. Summary of the Invention

[0009] Aiming at the deficiencies of the above-mentioned existing technologies, the purpose of the present invention is to provide a method for preparing a cation exchange membrane by deconstructing and reshaping a porous filter membrane and the cation exchange membrane obtained thereby, so as to solve the problems existing in the existing cation exchange membranes, such as large membrane body resistance, low ion transport efficiency, easy fouling and pollution, high energy consumption and high cost in the preparation process, and obtain a cation exchange membrane with high ion exchange capacity, fast ion transport, good anti-fouling effect and low cost.

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

[0011] The present invention discloses a method for preparing a cation exchange membrane by deconstructing and reshaping a porous filter membrane. The method includes the following steps: S1, using a polyvinylidene fluoride filter membrane as a substrate, impregnating it with a cross-linked network precursor solution or coating the cross-linked network precursor solution on its membrane surface, wherein the cross-linked network precursor solution includes at least one sulfonated monomer, an inducer and at least one photoinitiator; wherein, the inducer is set as a substance that can interact with the polyvinylidene fluoride crystals in the polyvinylidene fluoride filter membrane to promote the deconstruction of some polyvinylidene fluoride crystals into amorphous state; S2, irradiating the impregnated or coated polyvinylidene fluoride filter membrane under light for a time T to induce the polymerization and curing of the cross-linked network precursor solution, and compressing some deconstructed polyvinylidene fluoride filter membranes based on the cross-linked network formed by polymerization and curing, so that the membrane becomes thinner and the exchange site density increases; S3, after polymerization and curing, soaking the polyvinylidene fluoride filter membrane in a reshaping solution to reshape the membrane surface morphology; wherein, the reshaping solution is configured such that the amorphous polyvinylidene fluoride in the polyvinylidene fluoride filter membrane can be transformed from amorphous to crystalline after contacting it, and / or the cross-linked network formed by polymerization and curing can undergo non-uniform swelling after contacting it, so that the membrane surface is reshaped into a wrinkled morphology.

[0012] In the present invention, the inducer is set as trimethylolpropane triacrylate. Trimethylolpropane triacrylate contains three acrylate groups, and the oxygen atoms in the ester groups have strong polarity. During the impregnation process and after coating, it can form dipole-dipole interactions with the C-F bonds of polyvinylidene fluoride; at the same time, the small molecular structure of trimethylolpropane triacrylate can penetrate between the polyvinylidene fluoride molecular chains, weaken the van der Waals force and dipole interaction between the chains, reduce the crystallinity, and promote the dissolution of some polyvinylidene fluoride crystals, making the structure rigidity of the polyvinylidene fluoride filter membrane decrease. That is to say, based on the presence of trimethylolpropane triacrylate, it can interact with the crystals in the polyvinylidene fluoride filter membrane, so that some polyvinylidene fluoride crystals are deconstructed into an amorphous state, and further the structure rigidity of the filter membrane is reduced:

[0013] After polymerization is completed, since the membrane is immersed in the reshaping solution for reshaping, the amorphous polyvinylidene fluoride in the membrane will undergo a phase transformation when it comes into contact with the reshaping solution, that is, it transforms from an amorphous state to a crystalline state. Moreover, the phase transformation rate of polyvinylidene fluoride in different solutions is different, and wrinkles with different morphologies will be formed on the membrane surface. On the other hand, there are hydrophilic and hydrophobic phases in the cross-linked network formed by polymerization and curing. When the membrane is immersed in the reshaping solution, the cross-linked network will undergo uneven swelling, thus forming wrinkles on the membrane surface. Furthermore, the cross-linked network formed by polymerization and curing will also undergo different degrees of uneven swelling in different reshaping solutions, and the formed wrinkle morphologies are also different. That is to say, under the combined action of the phase transformation of polyvinylidene fluoride and the uneven swelling of the cross-linked network, wrinkles with different shapes and properties will be reshaped on the membrane surface.

[0014] It should be noted that these wrinkles play an important role. They can enhance the turbulence of the microfluidic field on the membrane surface, hinder the deposition of pollutants on the membrane surface, change the situation that existing commercial cation exchange membranes are prone to pollutant attachment and entry into the membrane due to the flat surface, and significantly improve the anti-fouling performance of the membrane.

[0015] In addition, the preparation method of the present invention has many advantages. The present invention uses light curing, such as visible light or ultraviolet light curing, to avoid the long-term heat treatment in the preparation process of existing cation exchange membranes. For example, the thermal polymerization method requires heat treatment at 80 °C for 3 h, and the hot rolling method requires repeated hot rolling at 105 °C for 1 h, thus reducing energy consumption. At the same time, the raw materials such as sulfonated monomers used in the present invention have no benzene ring structure, and methanol and water are used as solvents, which belongs to a green preparation method and reduces costs.

[0016] Furthermore, the reshaping solution is ultrapure water, ethanol with a concentration of 10-70 wt%, or a NaCl solution with a concentration of 0.01-0.5 mol / L. These different solutions can provide different environments for the phase transformation of polyvinylidene fluoride and the swelling of the cross-linked network, thereby precisely regulating the morphology and properties of the wrinkles on the membrane surface and optimizing the performance of the cation exchange membrane. For example, when ultrapure water is used as the reshaping solution, the phase transformation of polyvinylidene fluoride can be relatively mild, forming wrinkles with a specific morphology; while ethanol and NaCl solutions with different concentrations will affect the phase transformation rate of polyvinylidene fluoride and the swelling degree of the cross-linked network by changing the polarity and ionic strength of the system, generating differentiated wrinkle structures to meet the performance requirements of cation exchange membranes in different application scenarios.

[0017] Further, 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. Among them, the visible light initiator can be riboflavin, and the ultraviolet light initiator can be at least one of 2-hydroxy-2-methylpropiophenone and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone. The sulfonated monomer provides active sites for ion exchange in the cation exchange membrane, and different combinations of sulfonated monomers will affect the ion exchange capacity and other properties of the membrane. For example, 2-acrylamido-2-methylpropanesulfonic acid has strong hydrophilicity and ion exchange ability, which can increase the ion exchange capacity of the membrane; methacrylamide can adjust the structure and flexibility of the crosslinked network; 4-styrenesulfonic acid can improve the electrical properties of the membrane to a certain extent. Riboflavin as a visible light initiator, 2-hydroxy-2-methylpropiophenone and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone as ultraviolet light initiators, can efficiently initiate the polymerization and curing of the crosslinked network precursor solution under visible light or ultraviolet light irradiation, and their use avoids the high energy consumption problem brought by traditional thermal polymerization, meeting the requirements of the low energy consumption preparation of the cation exchange membrane in the present invention.

[0018] Further, before step S1, there is also a step S0 of preparing a crosslinked network precursor solution, which includes: adding the sulfonated monomer into ultrapure water and methanol (volume ratio 1:1), stirring for 10 - 20 minutes until completely dissolved; adding 0.5 - 4.0 mol% of an inducer, and continuing to stir for 10 - 20 minutes until all monomers are dissolved; under light-shielded conditions, adding 0.1 - 1.0 mol% of a photoinitiator, and stirring for 10 - 20 minutes to achieve uniform dissolution, obtaining the crosslinked network precursor solution. In this step, strictly controlling the addition sequence and stirring time of each component is to ensure that each substance can be fully dissolved and uniformly dispersed in the solution to form a stable crosslinked network precursor solution. The mixed solvent of ultrapure water and methanol can not only ensure the good solubility of the sulfonated monomer but also provide a suitable environment for the subsequent reaction; the precise dosages of the inducer and the photoinitiator have important effects on the reaction process and the performance of the final membrane. Too little may not be able to fully initiate the reaction or achieve the regulation of the membrane structure, and too much may lead to side reactions and affect the quality of the membrane.

[0019] Further, after step S3, a post-treatment step S4 is further included, which includes: soaking 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. Soaking in a 1 mol / L NaCl solution can enable the ion exchange sites in the membrane to fully exchange with sodium ions, achieving ion exchange equilibrium and improving the ion exchange stability of the membrane; at the same time, the ions in the solution interact with the membrane, which helps to repair possible microscopic defects and enhance the overall structural stability of the membrane, thereby improving the reliability and service life of the cation exchange membrane in practical applications.

[0020] Further, in the impregnation step of step S2, it includes: immersing the polyvinylidene fluoride filter membrane in the crosslinked network precursor solution, soaking it for 1 - 20 minutes under light-shielded conditions; then performing ultrasonic treatment for 5 - 20 minutes to fully fill the crosslinked network precursor solution in the membrane pores; taking out the filter membrane and scraping off the excess crosslinked network precursor solution on the surface. The light-shielded soaking is to prevent the photoinitiator from prematurely initiating the polymerization reaction during the impregnation process, affecting the penetration and effect of the crosslinked network precursor solution on the filter membrane; ultrasonic treatment can utilize the cavitation effect and mechanical vibration of ultrasonic waves to promote the more uniform and deeper filling of the crosslinked network precursor solution into the membrane pores, ensuring that the structure of the membrane is more uniform and the performance is more stable during the subsequent polymerization reaction; scraping off the excess crosslinked network precursor solution on the surface is to avoid forming an uneven polymerization layer on the membrane surface, ensuring the flatness and consistency of the membrane surface, which is beneficial to the optimization and improvement of the subsequent membrane performance.

[0021] Further, in the coating step of step S2, it includes: coating the crosslinked network precursor solution on the surface of the polyvinylidene fluoride filter membrane, standing for 1 - 10 minutes; taking out the filter membrane and scraping off the excess crosslinked network precursor solution on the surface. The purpose of standing is to allow the crosslinked network precursor solution to fully spread and penetrate on the surface of the filter membrane, making full contact with the polyvinylidene fluoride molecules on the surface of the filter membrane, creating good conditions for the subsequent polymerization reaction; scraping off the excess crosslinked network precursor solution on the surface is also to ensure the quality of the membrane surface, avoiding the formation of irregular structures by the excess precursor solution after polymerization and affecting the performance of the membrane. Compared with the impregnation step, the coating step is more suitable for application scenarios with higher requirements for membrane surface modification and relatively lower requirements for filling in the membrane pores. The two methods complement each other, providing diverse choices for the preparation of cation exchange membranes with different requirements.

[0022] Further, 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, and the crosslinking network cannot be fully formed, resulting in an unstable membrane structure, and the ion exchange capacity and mechanical properties, etc. cannot reach the expected values. If the time is too long, it may cause over-crosslinking of the crosslinking network, resulting in a decrease in the flexibility of the membrane, and even membrane aging and embrittlement phenomena, which also affect the comprehensive performance of the membrane. Therefore, the light irradiation time of 20 - 60 minutes has been verified by a large number of experiments and can ensure the preparation of a cation exchange membrane with excellent performance.

[0023] The present invention also discloses a cation exchange membrane, which is prepared according to the method disclosed in the present invention. This cation exchange membrane has many excellent properties. The thickness of its ion exchange layer is 35 - 60 nm, which is significantly lower than that of commercial membranes (100 - 600 nm). The ion exchange capacity reaches 2.8 meq / g, which is higher than that of commercial cation exchange membranes (0.9 - 2.3 meq / g). It realizes an optimized combination of high ion exchange capacity and low membrane thickness, greatly reducing the membrane body resistance and improving the ion transport efficiency. At the same time, its surface has ring-shaped micron-scale wrinkles, which not only accelerate ion transport but also significantly enhance the anti-fouling performance of the membrane. Therefore, this cation exchange membrane has greater advantages compared with traditional commercial cation exchange membranes in practical applications, especially in the fields of seawater desalination, high-salt concentration, ammonia nitrogen recovery, lithium-magnesium separation, and wastewater resource recovery, etc. It can effectively solve the problems existing in existing cation exchange membranes and has broad application prospects and great economic value.

[0024] The beneficial effects of the present invention compared with the prior art are as follows:

[0025] The cation exchange membrane prepared based on the method of the present invention can simultaneously have 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, partial deconstruction of the polyvinylidene fluoride membrane occurs, thereby reducing the structural rigidity of the polyvinylidene fluoride membrane. Since the structural rigidity of the polyvinylidene fluoride membrane has been reduced, the crosslinking network formed by the polymerization and curing reaction under visible light / ultraviolet light irradiation can compress the partially deconstructed polyvinylidene fluoride membrane, thereby making the membrane thinner and increasing the exchange site density. Subsequently, by placing it in a reshaping solution, the deconstructed polyvinylidene fluoride undergoes phase transformation, re-transforming from an amorphous state to a crystalline state, and the cured crosslinking network undergoes non-uniform swelling in the solution. The two work together to form surface wrinkles of different morphologies on the membrane surface, thereby obtaining a highly efficient ion transport cation exchange membrane prepared based on deconstruction-reshaping.

[0026] Furthermore, the ion exchange membrane prepared based on the present invention has wrinkles on its surface, enhancing the turbulence of the microfluidic field on the membrane surface, hindering the deposition of pollutants on the membrane surface, enhancing the anti-fouling performance of the membrane, and significantly improving the anti-pollution performance during operation compared to the existing commercial membranes.

[0027] The method for preparing a cation exchange membrane by deconstructing and reshaping a porous filter membrane according to the present invention will be described in detail below with reference to the embodiments shown in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a flow chart of the steps for preparing the cation exchange membrane in this study;

[0029] Figure 2 It 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 the present invention in these two aspects;

[0030] Figure 3 (a) is a scanning electron microscope image of the surface of the cation exchange membrane 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 is the NH4 + flux comparison bar chart of different cation exchange membranes at different ion concentrations, intuitively reflecting the ion transport performance advantage of the PC-4.0 membrane of the present invention;

[0032] Figure 5 It is a comparison diagram of scanning electron microscope images of the cation exchange membrane (PC-4.0 membrane) and commercial membrane in this study before and after pollution and after cleaning, showing the anti-fouling performance advantage of the membrane of the present 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) is a schematic diagram of the self-made Donnan dialysis (DD) cell structure for evaluating the ion selective permeability of the cation exchange membrane; Figure 6 (c) is a schematic diagram of the electrodialysis (ED) experimental device, respectively assisting in understanding the preparation and performance test principles;

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

[0035] Figure 8(a) is the SEM image of the cross-section of the PC membrane, showing its dense and pore-free structure; Figure 8 (b) is the SEM image of the surface of the PC membrane, highlighting the annular wrinkles; Figure 8 (c) is the AFM image of the PC membrane, reflecting the surface roughness of the membrane;

[0036] Figure 9 (a) is the ATR-FTIR spectrogram of different membranes, indicating the successful introduction of sulfonic acid groups into the PC membrane; Figure 9 (b) is the XPS spectrogram of different membranes, further confirming the existence of sulfonic acid groups; Figure 9 (c) is the high-resolution XPS spectrogram of O1s of PVDF and PC-4.0 membranes, corroborating the successful preparation of the PC membrane;

[0037] Figure 10 (a) is the graph of the ion exchange capacity and swelling ratio of different PC membranes varying with the addition amount of TMPTA; Figure 10 (b) is the Nyquist diagram of different PC membranes for analyzing the membrane resistance; Figure 10 (c) is the EIS fitting result and equivalent circuit model, showing the variation of the bulk layer resistance of the PC membrane with the addition amount of TMPTA;

[0038] Figure 11 (a) is for different PC membranes of NH4 + flux graph varying with the addition amount of TMPTA; Figure 11 (b) is the comparison graph of NH4 + flux between PC-4.0 membrane and commercial CEMs at different feed solution concentrations; Figure 11 (c) and (d) are respectively the NH4 + flux graphs of PC-4.0 membrane and commercial CEMs when treating actual domestic sewage and lemon pectin wastewater, comprehensively showing the ion transport performance of the membrane;

[0039] Figure 12 (a) is the NH4 + flux graph of PC-4.0 membrane and commercial CEMs during the electrodialysis treatment of actual domestic sewage; Figure 12 (b) is the NH4 + flux graph of PC-4.0 membrane and commercial CEMs during the electrodialysis treatment of actual lemon pectin wastewater, reflecting the ion transport performance of the membrane under the action of an electric field;

[0040] Figure 13 (a) is the comparison graph of the original surface morphologies of PC-4.0 membrane and CMVN membrane; Figure 13 (b) is the surface SEM image of PC-4.0 membrane and CMVN membrane after being contaminated; Figure 13 (c) is the surface SEM image of PC-4.0 membrane and CMVN membrane after being cleaned; Figure 13(d) EIS fitting results of PC-4.0 membrane and CMVN membrane in different states; Figure 13 (e) NH4 + flux and ion flux recovery rate diagrams of PC-4.0 membrane and CMVN membrane after simple water washing, comprehensively showing the anti-fouling performance of the membranes of the present invention. Detailed implementation manners

[0041] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention. It should be noted that, without conflict, the embodiments and features in the embodiments of this application may be combined with each other.

[0042] As Figure 1 shown, the present invention discloses a method for preparing a cation exchange membrane by deconstructing and reshaping a porous filter membrane. The method includes the following steps: S1, using a polyvinylidene fluoride filter membrane as a substrate, impregnating it with a crosslinked network precursor solution or coating the crosslinked network precursor solution on its membrane surface, wherein the crosslinked network precursor solution includes at least one sulfonated monomer, an inducer, and at least one photoinitiator; wherein, the inducer is set as a substance that can interact with the polyvinylidene fluoride crystals in the polyvinylidene fluoride filter membrane to promote the deconstruction of some polyvinylidene fluoride crystals into non-crystals; S2, irradiating the impregnated or coated polyvinylidene fluoride filter membrane under light for a time T to induce the polymerization and curing of the crosslinked network precursor solution, and compressing some deconstructed polyvinylidene fluoride filter membranes based on the crosslinked network formed by polymerization and curing, so that the membrane becomes thinner and the exchange site density increases; S3, after polymerization and curing, soaking the polyvinylidene fluoride filter membrane in a reshaping solution to reshape the membrane surface morphology; wherein, the reshaping solution is configured such that the amorphous polyvinylidene fluoride in the polyvinylidene fluoride filter membrane can be transformed from a non-crystal into a crystal after contacting it, and / or the crosslinked network formed by polymerization and curing can undergo uneven swelling after contacting it, so that the membrane surface is reshaped into a wrinkled morphology.

[0043] Based on the above disclosed steps, the cation exchange membrane prepared by the present invention can simultaneously have a large ion exchange capacity and a small membrane thickness, perfectly solving the problem in the prior art that it is impossible to simultaneously achieve a large ion exchange capacity and a small membrane thickness. In addition, the surface of the cation exchange membrane has wrinkles, which enhance the turbulence of the microfluidic field on the membrane surface, hinder the deposition of pollutants on the membrane surface, and thus enhance the anti-fouling performance of the membrane. Compared with the existing commercial membranes, its anti-pollution performance during operation is significantly improved.

[0044] In a preferred embodiment, the inducer is set as trimethylolpropane triacrylate. With three acrylate groups, the oxygen atoms in the ester groups form dipole-dipole interactions with the C-F bonds of polyvinylidene fluoride. The small molecule structure penetrates between the molecular chains, weakens the van der Waals force and dipole-dipole interactions, reduces the crystallinity of the polyvinylidene fluoride membrane, promotes the deconstruction of some crystals into amorphous forms, reduces the structural rigidity of the membrane, creates conditions for subsequent cross-linked network compression of the membrane, achieving membrane thinning and an increase in the density of exchange sites, and thus helps to prepare a cation exchange membrane with both a large ion exchange capacity and a small membrane thickness, solving the problems of the prior art.

[0045] In the embodiment of the present invention, the reshaping solution is ultrapure water, ethanol with a concentration of 10-70 wt%, and an NaCl solution with a concentration of 0.01-0.5 mol / L. The ultrapure water makes the phase inversion of polyvinylidene fluoride mild, forming specific wrinkles; ethanol and NaCl solutions with different concentrations change the system polarity and ionic strength, affecting the phase inversion rate of polyvinylidene fluoride and the swelling degree of the cross-linked network, generating different wrinkled structures. These wrinkles enhance the turbulence of the microfluidic field on the membrane surface, hinder the deposition of pollutants, and significantly improve the anti-fouling performance of the membrane compared with existing commercial membranes, enhancing the anti-pollution ability during operation.

[0046] In the embodiment of the present 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 the ion exchange capacity, methacrylamide regulates the cross-linked network structure and flexibility, and 4-styrenesulfonic acid improves the electrical properties of the membrane.

[0047] In the embodiment of the present invention, the photoinitiator can be an ultraviolet photoinitiator, which is at least one of 2-hydroxy-2-methylpropiophenone and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone. In another specific embodiment, the photoinitiator can be the visible light photoinitiator riboflavin. Under the irradiation of visible light or ultraviolet light, it can efficiently initiate the polymerization and curing of the cross-linked network precursor solution, avoiding the high energy consumption problem of traditional thermal polymerization, meeting the requirements of the low energy consumption preparation of the cation exchange membrane in the present invention, and reducing the production cost.

[0048] In the embodiment of the present invention, before step S1, there is also a step S0 of preparing a crosslinked network precursor solution, which includes: adding a sulfonated monomer into ultrapure water and methanol (volume ratio 1:1), stirring for 10 - 20 minutes until completely dissolved; adding 0.5 - 4.0 mol% of an inducer, and continuing to stir for 10 - 20 minutes until all monomers are dissolved; under light-shielded conditions, adding 0.1 - 1.0 mol% of a photoinitiator, and stirring for 10 - 20 minutes to achieve uniform dissolution, thus obtaining the crosslinked network precursor solution. The addition sequence and stirring time of each component are strictly controlled to ensure that substances are fully dissolved and evenly dispersed, forming a stable precursor solution, ensuring the smooth progress of subsequent reactions, laying a foundation for preparing a cation exchange membrane with excellent performance, and contributing to achieving a high ion exchange capacity and good comprehensive performance.

[0049] In the embodiment of the present invention, after step S3, there is also a post-treatment step S4, that is, soaking the membrane in a 1 mol / L NaCl solution for N hours, and the value of N can be selected from 18, 20, and 24. The selection of these time durations is determined based on a large number of experimental studies. When N is 18 hours, the ion exchange sites in the membrane can basically reach the ion exchange equilibrium state, but there are still a small number of sites that are not fully exchanged with sodium ions, which has a certain impact on the 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 in the membrane complete the exchange with sodium ions, and the ion exchange stability is significantly improved. When N reaches 24 hours, the ion exchange sites are almost completely exchanged with sodium ions, reaching the optimal ion exchange equilibrium state, which can maximize the improvement of ion exchange stability.

[0050] Meanwhile, the interaction between the ions in the solution and the membrane also varies at different time durations. At 18 hours, the repair effect of the ions on the microscopic defects of the membrane is limited; at 20 hours, the interaction between the ions and the membrane is enhanced, which can repair some microscopic defects and enhance the overall structural stability of the membrane to a certain extent; at 24 hours, the ions fully interact with the membrane, not only repairing most of the microscopic 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 the cation exchange membrane in practical applications, and achieving in-depth optimization of the membrane performance.

[0051] In step S2 of the embodiments of the present invention, the selection of the time ranges for the impregnation step and the coating step is based on sufficient experimental evidence. In the impregnation step, the polyvinylidene fluoride membrane is immersed in the crosslinked network precursor solution, and the light-shielding soaking time is controlled within 1 - 20 minutes. Experimental data shows that if the soaking time is less than 1 minute, the crosslinked network precursor solution cannot fully penetrate into the interior of the membrane, resulting in insufficient subsequent polymerization reaction and significant decreases in both the ion exchange capacity and mechanical properties of the membrane; when the soaking time exceeds 20 minutes, although the precursor solution can fully penetrate, the membrane swells excessively and the membrane structure becomes loose. During the subsequent ultrasonic treatment and photo-polymerization process, it is difficult to maintain the shape of the membrane, and local polymerization unevenness occurs, which also affects the performance of the membrane.

[0052] The ultrasonic treatment time is set to 5 - 20 minutes, which is also verified by 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 uneven membrane structure and affecting the ion transport efficiency; if the ultrasonic treatment time exceeds 20 minutes, the high-intensity mechanical vibration generated by the ultrasonic wave will damage the microstructure of the membrane and reduce the mechanical strength of the membrane, thereby affecting the comprehensive performance of the cation exchange membrane.

[0053] In the coating step, after the crosslinked network precursor solution is coated on the surface of the polyvinylidene fluoride membrane, the standing time is set to 1 - 10 minutes. Experiments have found that when the standing time is shorter than 1 minute, the precursor solution cannot fully spread and penetrate to the membrane surface, resulting in insufficient polymerization reaction and local defects on the membrane surface, affecting the performance of the membrane; while when the standing time exceeds 10 minutes, some components in the precursor solution may volatilize or react, which will also form an irregular structure on the membrane surface and reduce the quality of the membrane. These time ranges determined through a large number of experiments can ensure the preparation of cation exchange membranes with excellent performance, meet the requirements of different application scenarios, and achieve a balance of high ion exchange capacity, fast ion transport, and good mechanical properties.

[0054] In the embodiments of the present invention, in step S2, the time T is set to 20 - 60 minutes. Within this time range, the photoinitiator effectively induces the polymerization and curing of the crosslinked network precursor solution. If the time is too short, the 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 crosslinked network is over-crosslinked, the membrane flexibility decreases, and it ages and becomes brittle. Verified by a large number of experiments, this light irradiation time can ensure the preparation of cation exchange membranes with excellent performance and achieve a balance of high ion exchange capacity, fast ion transport, and good mechanical properties.

[0055] The present invention also discloses a cation exchange membrane, which is prepared according to the method disclosed in the present invention. The ion exchange layer thickness of the cation exchange membrane is 35-60nm, which is significantly lower than the commercial membrane (100-600nm), and the ion exchange capacity is 2.8meq / g, which is higher than the 0.9-2.3meq / g of the commercial cation exchange membrane, achieving an optimized combination of high ion exchange capacity and low membrane thickness, greatly reducing the membrane body resistance, and improving the ion transmission efficiency. At the same time, the annular micron-scale folds on its surface not only accelerate ion transmission, but also significantly enhance the anti-fouling performance. In the fields of seawater desalination, high salt concentration, ammonia nitrogen recovery, lithium magnesium separation, and wastewater resource recovery, it has obvious advantages over traditional commercial cation exchange membranes, effectively solves the problems existing in existing cation exchange membranes, and has broad application prospects and huge economic value.

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

[0057] 1. Preparation of experimental materials

[0058] The porous polyvinylidene fluoride (PVDF) filter membrane produced by Tianjin Jinteng Experimental Equipment Co., Ltd. was selected as the base material, with a pore size of 0.22μm, a porosity of 75%, and a thickness of 110μm. This pore size is conducive to limiting the excessive penetration of macromolecules in the precursor solution and ensuring the stability of the membrane structure; the higher porosity provides sufficient space for the penetration of the cross-linked network precursor solution and promotes the formation of a uniform cross-linked structure; the appropriate thickness ensures the mechanical strength of the membrane while facilitating the subsequent deconstruction-remodeling process, which is conducive to ion transmission and membrane performance regulation.

[0059] Chemical reagents include 2-acrylamide-2-methylpropanesulfonic acid (AMPS, 98%), trimethylolpropane triacrylate (TMPTA, 95%), which are used as inducers to promote the deconstruction of polyvinylidene fluoride crystals, riboflavin (visible light initiator), 2-hydroxy-2-methylpropiophenone (ultraviolet light initiator), which is used to initiate the polymerization and curing of the cross-linked network precursor solution, 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 solvent is ultrapure water and methanol, mixed in a volume ratio of 1:1. This ratio of mixed solvent can effectively dissolve substances such as sulfonated monomers, TMPTA and photoinitiators, while 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-linked Network Precursor Solution

[0062] Taking AMPS as an example, a crosslinked network precursor solution was prepared. AMPS powder was added to a mixed solvent with a volume ratio of ultrapure water to methanol of 1:1 and stirred for 15 minutes until completely dissolved. The good solubility of the mixed solvent was utilized to ensure the uniform dispersion of the sulfonated monomer. Then, TMPTA was added, and its addition amount was 2.0 mol% of the total amount of substances in the mixed solution, and stirring was continued for 15 minutes. TMPTA interacts with PVDF crystals subsequently, prompting some crystals to be deconstructed into amorphous forms, which is a key factor in achieving the deconstruction of the membrane structure. Finally, riboflavin or 2-hydroxy-2-methylpropiophenone, as a photoinitiator, was added under light-shielded conditions, and its addition amount was 0.5 mol% of the total amount of substances in the mixed solution, and stirring for 15 minutes was carried out to achieve uniform dissolution. The photoinitiator is crucial for the preparation of the cation exchange membrane. Its addition amount affects the polymerization and curing rate of the crosslinked network precursor solution, the crosslinking degree and the microstructure of the membrane. Adding it under light-shielded conditions during the preparation of the precursor solution can prevent its premature initiation of polymerization, destroy the stability of the reaction system, and affect the crosslinking and microstructure of the membrane. Adding it under light-shielded conditions can ensure its precise function under visible light irradiation and regulate the membrane performance.

[0063] In addition to the photoinitiator, the role of TMPTA at the molecular level is also of great significance for improving the membrane performance. From the molecular level, the dipole-dipole interaction formed by the strongly polar ester oxygen atom of TMPTA and the C-F bond of PVDF changes the interaction energy between PVDF molecular chains, reducing the orderliness of the molecular chain arrangement. At the same time, small molecules of TMPTA penetrate between the molecular chains, further destroying the original crystal structure and resulting in a decrease in crystallinity. Recent research has shown that this structural change affects the free volume of PVDF, forming more channels for ion transport inside it, laying a foundation for the subsequent improvement of ion transport performance.

[0064] 3. Impregnation upgrade of the porous filter membrane

[0065] 3.1 Impregnation and ultrasonic treatment

[0066] The polyvinylidene fluoride (PVDF) filter membrane is completely immersed in the prepared cross-linked network precursor solution, and the entire immersion process is carried out in a light-proof environment, and the duration is set to 10 minutes. This is because the strong polar ester oxygen atoms contained in the TMPTA in the cross-linked network precursor solution form a dipole-dipole interaction with the CF bond of PVDF, and its small molecule structure penetrates into the PVDF molecular chain, weakening the van der Waals force and dipole interaction between the molecular chains, resulting in a decrease in the crystallinity of PVDF, partial crystal dissolution, and a decrease in the structural rigidity of the membrane. This is not only conducive to the cross-linked network precursor solution to penetrate deeper into the membrane pores, but also provides a more uniform reaction environment for the formation of the cross-linked network during subsequent light irradiation polymerization. The cross-linked network can be more closely interwoven with the PVDF molecular chains, enhancing the overall stability and mechanical strength of the membrane. At the same time, the pore structure of the PVDF membrane after deconstruction changes, making the transmission path of ions in the membrane more tortuous and orderly, which is conducive to the capture and transmission of ions, and lays the foundation for improving the ion exchange capacity and ion transmission rate of the membrane.

[0067] After soaking, the membrane was subjected to ultrasonic treatment at a frequency of 40kHz for 10 minutes. Ultrasonic treatment uses cavitation to ensure that the cross-linked network precursor solution evenly and fully fills the membrane pores. Figure 7 (a) The original staggered structure of the PVDF membrane, Figure 7 (b) The surface of the pore-filled membrane becomes smooth, and the significant effect of the precursor liquid filling the membrane pores can be directly observed. Figure 3 (a) The film treated by the present invention and Figure 3 (b) Scanning electron micrograph of the surface of a commercial cation exchange membrane, showing significant differences. After this step of treatment, the membrane structure of the present invention is more uniform, while the surface of the commercial membrane is relatively smooth and does not have the microscopic features caused by the full filling of the precursor solution and the structural changes.

[0068] 3.2 Photopolymerization

[0069] After immersion and ultrasonic treatment, carefully remove the filter membrane, carefully scrape off the excess mixed liquid on its surface, and then irradiate the filter membrane under visible light / ultraviolet light for 40 minutes. Riboflavin or 2-hydroxy-2-methylpropiophenone in the cross-linked network precursor solution triggers the polymerization and curing of the cross-linked network precursor solution under the excitation of visible light / ultraviolet light, gradually forming a cross-linked network. The contraction force generated by the polymerization and curing of the cross-linked network compresses the deconstructed PVDF filter membrane, reducing the membrane thickness and increasing the exchange site density. Figure 2As can be seen, the ion exchange capacity of the membrane prepared by the present invention reaches 2.8 meq / g, which is significantly higher than the range of 0.9 - 2.3 meq / g of commercial cation exchange membranes; the membrane thickness is between 35 - 60 nm, which is significantly lower than the thickness of commercial membranes of 100 - 600 nm. Further analysis of the data under different preparation conditions shows that when the light irradiation time is in the range of 20 - 60 minutes, as the irradiation time prolongs, the membrane thickness gradually decreases and the ion exchange capacity gradually increases. However, when the irradiation time exceeds 60 minutes, the flexibility and ion transport performance of the membrane may decline because excessive crosslinking leads to an overly dense membrane structure and blocked ion transport channels. Therefore, selecting an irradiation time of 40 minutes can optimize the ion exchange capacity and membrane thickness while ensuring that the membrane has good comprehensive performance.

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

[0071] 4.1 Remodeling treatment

[0072] After the polymerization reaction is completed, the membrane is immersed in a 50 wt% ethanol solution for remodeling, and the immersion time is 12 hours. In the ethanol solution environment, the phase transformation rate of polyvinylidene fluoride is relatively slow, and the swelling difference between the hydrophilic and hydrophobic phases of the crosslinked network is relatively large. The two cooperate with each other to form specific-shaped wrinkles on the membrane surface. As can be observed from Figure 8 (b), the surface of the membrane prepared by the present invention presents a ring-shaped micron-scale wrinkled structure. The shapes of these wrinkles are irregular and evenly distributed, with sizes between 10 - 15 μm (taking the PC-4.0 membrane as an example). The wrinkled structure greatly increases the surface area of the membrane, and more sulfonic acid groups are exposed on the membrane surface, providing more contact sites for ions and facilitating ion capture. At the same time, the wrinkles change the micro-flow field on the membrane surface, making the flow of ions on the membrane surface more disordered, reducing the ion concentration polarization phenomenon, and thus enhancing the ion transport rate. In terms of anti-fouling performance, the wrinkled structure forms a physical barrier to hinder the deposition of pollutants on the membrane surface. When pollutants come into contact with the membrane surface, they will be blocked and interfered by the wrinkles and are difficult to adhere to the membrane. Moreover, when there is water flow on the membrane surface, the wrinkled structure can enhance the turbulence degree of the water flow, making it easier for pollutants to be carried away by the water flow, further enhancing the anti-fouling performance of the membrane. Compared with the smooth structure of commercial membranes, the wrinkled structure of the membrane of the present invention has significant advantages in ion transport and anti-fouling.

[0073] 4.2 Post-treatment

[0074] The diaphragm was immersed in 1 mol / L NaCl solution for 24 h to complete the preparation of the high-quality cation exchange membrane. Immersion in 1 mol / L NaCl solution can enable the ion exchange sites in the membrane to fully exchange with sodium ions, achieve ion exchange equilibrium, and improve the ion exchange stability of the membrane. At the same time, the ions in the solution interact with the membrane, which helps to repair possible microscopic defects and enhance the overall structural stability of the membrane, thereby improving the reliability and service life of the cation exchange membrane in practical applications.

[0075] 5. Coating upgrade of the porous filter membrane

[0076] 5.1 Coating and standing

[0077] The above-prepared 5 mL crosslinked network precursor solution was evenly coated on the surface of the polyvinylidene fluoride filter membrane to ensure that the precursor solution fully and evenly covered the membrane surface. After coating, the filter membrane was allowed to stand for 5 min. During the standing process, TMPTA in the crosslinked network precursor solution interacted with the molecules on the surface of the PVDF membrane. The strong polar ester oxygen atoms of TMPTA formed dipole-dipole interactions with the C-F bonds of PVDF, and the small molecular structure penetrated between the PVDF molecular chains, causing some crystal structures on the membrane surface to change and the structural rigidity to decrease, creating conditions for subsequent crosslinking reactions. Different from the impregnation treatment method, the coating method mainly acts on the membrane surface and can more precisely control the degree of surface deconstruction and crosslinking effect of the membrane, making the crosslinked network formed on the membrane surface denser and more uniform, which is beneficial to improving the ion exchange performance and anti-fouling performance of the membrane surface.

[0078] 5.2 Light irradiation and post-treatment

[0079] After standing, the filter membrane was taken out, the excess mixed solution on the surface was scraped off, and then it was placed under visible light / ultraviolet light for irradiation for 40 minutes to initiate the polymerization reaction. After polymerization, the membrane was immersed in the same 50 wt% ethanol solution as the above reshaping treatment for reshaping for 12 hours. Finally, the membrane was immersed in 1 mol / L NaCl solution for 24 h to complete the preparation of the cation exchange membrane. The membrane prepared by this coating treatment method also formed a ring-shaped micron-scale wrinkled structure on the membrane surface and had high ion transport and anti-fouling performance. From Figures 2 - 5 the comparison of various performance data, it can be seen that the performance advantages shown are the same as those of the membrane prepared by the impregnation treatment 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 Microstructural characterization

[0082] The surface and cross-sectional morphologies of the membranes were scanned and observed using a field emission scanning electron microscope (Hitachi Regulus 8100, Japan) under different voltage conditions. From Figure 8 (a) The SEM image of the cross-section of the PC membrane and Figure 8 (b) the SEM image of the surface of the PC membrane, the microstructural features of the membrane can be clearly observed. The unique ring-shaped micron-scale wrinkled structure on the surface of the membrane of the present invention and the dense and pore-free cross-sectional structure are in sharp contrast to commercial cation exchange membranes. The surface of the commercial membrane is relatively smooth and there are almost no obvious wrinkled structures. This structural difference results in the commercial membrane being inferior to the membrane of the present invention in terms of ion transport and anti-fouling performance. The smooth surface of the commercial membrane is not conducive to the capture and transport of ions and is prone to the attachment of pollutants.

[0083] The chemical valence states of the membrane samples were recorded using an X-ray photoelectron spectrometer (Thermo Scientific K-Alpha, USA). From Figure 9 (b) the XPS spectra of different membranes, it was found that the PC membrane showed an S2p characteristic peak, which is strong evidence for the successful introduction of functional sulfonic acid groups. An attenuated total reflection infrared spectrometer (Thermo Fisher Scientific Nicolet iS20, USA) was used to characterize the functional groups on the membrane surface. From Figure 9 (a) the ATR-FTIR spectrum, it was seen that the PC membrane showed a symmetric stretching vibration absorption peak of sulfonic acid groups at 1035 cm-1, further confirming the presence of sulfonic acid groups in the membrane structure, indicating that the present invention successfully introduced functional sulfonic acid groups into the membrane.

[0084] 6.2 Physicochemical property tests

[0085] The water contact angle was measured using a contact angle measuring instrument (SDC-100, Shengding, China) to characterize the hydrophilicity of the membrane surface. A self-made four-electrode system was used to measure the electrochemical impedance spectroscopy (EIS) with 0.10 M NaCl electrolyte. During the measurement, an alternating current of 3 mA was applied in the frequency range of 103 - 10 -2 Hz, and then the electrochemical parameters such as membrane resistance were analyzed according to Figure 10 (b) the Nyquist plot and the equivalent circuit model to deeply understand the electrical properties of the membrane.

[0086] In the determination of the swelling ratio and water absorption rate, they were calculated by measuring the area difference and weight difference between the wet membrane and the dry membrane, respectively.

[0087] The calculation formulas for the swelling ratio and water absorption rate are as follows:

[0088]

[0089]

[0090] Among them, A wet and A dry are the areas of the wet film and the dry film (cm 2 ), respectively; M wet and M dry are the weights of the wet film and the dry film (g), respectively.

[0091] The swelling ratio is one of the core indicators for evaluating the stability of ion exchange membranes. The PVDF-based membrane shows weak swelling in water. When the addition amount of TMPTA is less than 1.0 mol%, TMPTA has a destructive effect on the PVDF structure in the base membrane, weakening the intermolecular interaction, increasing the distance between molecular chains, and resulting in an increase in the swelling ratio. At this time, the cross-linking network in the membrane has not yet formed a completely stable structure and cannot effectively restrict the movement of PVDF molecular chains, making it easier for water molecules to enter the interior of the membrane and cause swelling. As the addition amount of TMPTA continues to increase, the cross-linking network formed in the system gradually becomes stable. The cross-linking network restricts the movement of PVDF molecular chains and partially compensates for the swelling trend caused by the destruction of PVDF by TMPTA, so that the swelling ratio of the PC membrane gradually drops from 42.5% to 24.8%. When the addition amount of TMPTA exceeds 4.0 mol%, PVDF will have noticeable structural defects due to excessive deconstruction, the cross-linking network is too dense, stress concentration occurs inside the membrane, resulting in a decline in the overall performance and stability of the membrane, and the swelling ratio may show abnormal changes.

[0092] The ion exchange capacity (IEC) is evaluated by titration method. The specific operation is to soak the cation exchange membrane (with a size of 2 cm × 2 cm) in 1 M HCl solution for 24 h to convert it into the H + form, and then carefully wash it to remove the residual HCl solution. After washing, soak the membrane in 1 M NaCl solution for another 24 h, and then titrate the solution passing through the membrane with 0.01 M NaOH. When it does not fade within 30 seconds, it is regarded as the titration end point. At this time, record the volume of NaOH consumed and calculate the IEC value. The calculation formula of IEC is as follows:

[0093]

[0094] Among them, IEC is the ion exchange capacity (meq·g -1 ), V NaOH is the volume of NaOH consumed (mL), C NaOH is the concentration of NaOH (mol·L -1 ), and M dry is the weight of the dry film (g).

[0095] From Figure 10(a) It can be seen from the change of IEC value of PC membrane under different TMPTA addition amounts 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. At the same time, through EIS analysis, it is obtained that the membrane resistance of this invention is lower. These optimized physical and chemical properties make the membrane perform more efficiently during ion transport.

[0096] 6.3 Ion transport performance test

[0097] As Figure 6 (b) shown, when evaluating the ion selective permeability of the membrane using a self-made Donnan dialysis (DD) cell, the "DD conditions" are specifically as follows: The experimental device consists of two compartments with a capacity of 100 mL each and an effective working membrane area of 4.15 cm 2 . The solution temperature is controlled at 25 °C. Different salt solutions (NH4Cl, LiCl, MgCl2, CaCl2) are selected as the feed solution, and the NaCl solution is used as the driving solution. The stirring device is turned on, and the stirring speed is set at 500 rpm to fully mix the solution, reduce the concentration polarization phenomenon, and ensure the accuracy of the experimental results. During the experiment, every 30 minutes, 0.5 mL of samples are accurately taken from the feed chamber and the receiving chamber respectively, and the concentration of the target cation in them is measured using an ultraviolet spectrophotometer (model 722, Shanghai Jingke Tianmei Scientific Instrument Co., Ltd.) to obtain the relevant data of different ions passing through the membrane, and then analyze the ion selective permeability of the cation exchange membrane to various ions.

[0098] The ion flux is tested through an electrodialysis (ED) experiment. As Figure 6 (c) shows, the ED stack consists of six compartments with an effective membrane area of 4 cm 2 arranged in an orderly manner. The experiment is carried out at a constant current density of 10 mA / cm 2 , and the solution circulates at a rate of 150 mL / min to ensure the relative stability of the ion concentration in the solution. The calculation formula for ion flux is as follows:

[0099]

[0100] Among them, J s is the ion flux (mol·m -2 ·h -1 ), A is the effective membrane area (m 2 ), C o and V o are the initial ion concentration (mg·L -1 ) and the initial solution volume (L) of the feed / dilute side, C t and V t are the ion concentration (mg·L -1 ) and the solution volume (L) after running for t (h), Ms is the relative molecular mass of the ion (g·mol -1 ).

[0101] From Figure 11 (a), the variation curves of NH4 + flux with the addition amount of TMPTA for different PC membranes, and Figure 11 (b) comparing the NH4 + flux data of PC-4.0 membrane and commercial cation exchange membranes (CEMs) at different feed solution concentrations, it can be seen that as the addition amount of TMPTA increases, the NH4 + flux of PC membranes increases, and the NH4 + flux of PC-4.0 membrane is better than that of commercial membranes under different ion concentration conditions. In the simulation test of actual application scenarios, the NH4 + flux of PC-4.0 membrane and commercial membranes was tested using actual domestic sewage (with NH4 + content of 2.5 mM) and lemon pectin wastewater (NH4 + content of 50 mM). A large number of impurity cations are contained in actual domestic sewage, such as Ca 2+ , Mg 2+ , Na + , etc. These impurity cations will compete with NH4 + for transmembrane transport channels, occupy some ion exchange sites, interfere with the movement of NH4 + , increase the transmembrane transport resistance of NH4 + , and cause a significant decrease in the NH4 + flux of PC-4.0 membrane. During the actual pectin wastewater treatment process, in addition to the interference of impurity cations, a large number of organic substances may also adhere to the membrane surface or within the membrane pores, block the membrane pores, reduce the effective cross-sectional area of the ion transport channels, change the membrane surface properties and microstructure, affect the activity of ion exchange sites, and cause the NH4 + flux of PC-4.0 membrane to decrease from 2.55 mol·m -2 ·h -1 to 0.90 mol·m -2 ·h -1 . Even facing a complex actual wastewater environment, when treating actual domestic sewage and lemon pectin wastewater, the NH4 + flux of PC-4.0 membrane is still better than that of commercial membranes.

[0102] Using an ED device, the treatment effects of PC-4.0 membrane and commercial membranes on actual domestic sewage and lemon pectin wastewater were compared and evaluated. From Figure 12 the experimental data, it can be seen that under the action of an electric field, the NH4 +The fluxes have all increased. When treating actual domestic sewage, the NH4 + flux of the PC-4.0 membrane increased rapidly from 0.09 mol·m -2 ·h -1 to 0.55 mol·m -2 ·h -1 , a 6.0-fold increase; when treating lemon pectin wastewater, the NH4 + flux of the PC-4.0 membrane increased from 0.90 mol·m -2 ·h -1 to 1.62 mol·m -2 ·h -1 , a 1.8-fold increase. Similarly, when treating these two actual wastewaters, the NH4 + flux of the PC-4.0 membrane is higher than that of the commercial membrane. This indicates that under the assistance of an electric field, the PC-4.0 membrane can more effectively transport NH4 + , further demonstrating its high efficiency and superiority in actual wastewater treatment, providing strong experimental evidence for its wide application in fields such as seawater desalination, high-salt concentration, ammonia nitrogen recovery, lithium-magnesium separation, and actual wastewater resource recovery and treatment.

[0103] 6.4 Fouling Resistance Test

[0104] Using actual lemon pectin wastewater as the feed solution, the fouling resistance of the membrane of the present invention (PC-4.0 membrane) and the commercial membrane (CMVN) was compared under DD conditions. It can be visually observed from Figure 13 (a) that the surface of the PC-4.0 membrane has a unique annular wrinkled structure, while the surface of the commercial CMVN membrane is relatively smooth.

[0105] When the two membranes were operated in the actual pectin wastewater environment for 24 hours, it can be clearly seen from Figure 13 (b) that a large amount of obvious pollutant deposition appeared on the surface of the CMVN membrane, while no obvious pollutants were attached to the surface of the PC-4.0 membrane. This is because the wrinkled structure of the PC-4.0 membrane forms a physical barrier, increasing the difficulty of pollutant attachment, and the enhanced turbulent micro-flow field on the membrane surface due to the wrinkled structure makes it difficult for pollutants to stay on the membrane surface.

[0106] After rinsing the fouled membrane with pure water for 1 minute, it can be seen from Figure 13 (c) that a small amount of pollutants still remained on the surface of the CMVN membrane. The electrochemical impedance spectroscopy (EIS) technique was used to comprehensively characterize the electrochemical properties of the PC-4.0 membrane and the CMVN membrane before and after fouling and after cleaning. It can be seen from Figure 13 (d) that compared with the original unfouled membrane, the resistance of the two fouled membranes (including the membrane body resistance R m and the fouling resistance R p, the diffusion boundary layer resistance R DBL and the electric double layer resistance R EDL ) both show a systematic increase.

[0107] For the PC-4.0 membrane, the fouling resistance R Fouling accounts for 12.7% of the total impedance, with a value of 0.9 Ω·cm 2 , and its membrane bulk resistance R m increases from 2.8 Ω·cm 2 to 3.5 Ω·cm 2 , with an increase of 25.0%. This is because pollutants attach to the membrane surface and enter the membrane interior, blocking some ion transport channels, increasing the resistance of ion transport in the membrane. At the same time, pollutants may interact with active sites such as sulfonic acid groups in the membrane, changing the microscopic conductive environment of the membrane and resulting in an increase in the membrane bulk resistance. The fouling on the membrane surface and inside also weakens the negative charge on the membrane surface and slows down the charge transfer response speed, thus increasing R EDL by 10 times.

[0108] However, the fouling degree of the CMVN membrane is more serious. R m increases from 4.3 Ω·cm 2 to 8.9 Ω·cm 2 , an increase of 107.0%, and R EDL increases from 0.4 Ω·cm 2 to 1.9 Ω·cm 2 , significantly higher than that of the PC-4.0 membrane (R EDL = 1.3 Ω·cm 2 ). This is because the CMVN membrane surface is smooth and lacks a wrinkled structure like the PC-4.0 membrane to hinder pollutant deposition. Pollutants are more likely to attach to the membrane surface in large quantities and penetrate into the membrane interior, causing greater damage to the membrane microstructure, resulting in serious obstruction of ion transport channels and a significant increase in membrane resistance.

[0109] Subsequently, the fouled membranes were simply rinsed with pure water for 1 minute, and their EIS and NH4 + flux were tested, and the results are as shown in Figure 13 (e). After physical cleaning, the membrane bulk resistance of the PC-4.0 membrane decreases from 3.5 Ω·cm 2 to 3.1 Ω·cm 2 , and the membrane bulk resistance of the CMVN membrane decreases from 8.9 Ω·cm 2 to 6.3 Ω·cm 2Physical cleaning can effectively remove the dirt on the cation exchange membrane because the cation exchange membrane has strong negative charge, weak binding force with dirt and poor stability. However, the intrinsic resistance of the CMVN membrane after physical cleaning increased by 46.5% compared with the uncontaminated CMVN membrane and 10.7% higher than the PC-4.0 membrane. In addition, the NH4 + The flux recovery rate was 75.1%, while the NH4 + The flux recovery rate is as high as 83.9%. The comparative analysis of the above indicators fully proves that PC-4.0 membrane has stronger anti-fouling performance than commercial CMVN membrane. In practical applications, it can better maintain the performance stability of the membrane, reduce performance degradation and frequent cleaning and maintenance caused by pollution, and provide strong performance guarantee for its wide 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" structural wrinkles promote ion transport. By building corresponding models, the transport process of ions on the membrane surface and inside is simulated, and how the wrinkle structure affects the ion movement trajectory, velocity distribution, and concentration distribution is analyzed.

[0112] The simulation results show that annular micron-scale folds with specific sizes and shapes can significantly enhance the turbulence of the microfluidic field on the membrane surface. When ions are transmitted on the membrane surface, they are more easily captured by the folds. Under the effect of the enhanced turbulence of the microfluidic field on the membrane surface, the ion transmission rate is effectively improved. For example, when ions encounter folds, their movement direction will change, increasing the chance of contact with the active sites on the membrane surface, thereby improving the ion exchange efficiency. At the same time, the fold structure also affects the concentration distribution of ions inside the membrane, making the distribution of ions in the membrane more uniform and reducing the ion concentration polarization phenomenon.

[0113] Based on the simulation results, the size, shape and distribution density of the folds can be further optimized in the subsequent membrane structure design. For example, by adjusting the concentration of the post-treatment solution, immersion time and other parameters during the preparation process, the formation process of the folds can be precisely controlled to obtain a membrane surface structure that is more conducive to ion transport. In addition, the simulation results can also provide theoretical guidance for the research and development of new membrane materials, and select more suitable raw materials and inducers according to the optimization requirements of ion transport to further improve the performance of the membrane.

[0114] 7. Results and Discussion

[0115] 7.1 Analysis of membrane preparation process

[0116] In the process of preparing the cation exchange membrane of the present invention, Figure 7(a) It can be seen that the microporous PVDF membrane used as the substrate exhibits an interlaced structure. The abundant pores inside it provide sufficient space for the filling of the crosslinked network precursor solution and offer a rigid framework support for subsequent reactions.

[0117] In the pore filling step, comparing Figure 7 (a) and Figure 7 (b), it can be clearly observed that significant changes have occurred in the PVDF membrane. The originally pore-covered membrane surface has become smooth and flat, and the pores have completely disappeared, indicating that the crosslinked network precursor solution has successfully penetrated into the interior of the PVDF membrane and completed the filling of the pores.

[0118] Trimethylolpropane triacrylate (TMPTA) in the crosslinked network precursor solution plays a key role as an inducer. Due to the strong polar ester oxygen atoms it contains, TMPTA forms dipole-dipole interactions with the C-F bonds of PVDF, and its small molecular structure can penetrate between PVDF molecular chains, weakening the van der Waals forces and dipole interactions between the chains, thereby causing partial deconstruction of PVDF. It can be seen from the XRD test results that the full width at half maximum of the PVDF characteristic peak (2θ = 20.2°) increased significantly from 0.72 to 4.62 after being impregnated with the crosslinked network precursor solution for 10 min, confirming the decrease in the crystallinity of PVDF. This change in the PVDF structure results in a decrease in the membrane thickness, shortening the transmembrane path of ions during their transport in the membrane and reducing the resistance to ion transport.

[0119] When the membrane after pore filling is irradiated with visible / ultraviolet light, the photoinitiator in the crosslinked network precursor solution is activated, initiating the polymerization and curing of the crosslinked network precursor solution to form a crosslinked network. Thereafter, the membrane is immersed in an aqueous solution for reshaping treatment. As shown in Figure 7 (c), PVDF will undergo partial recrystallization, and the full width at half maximum of its XRD characteristic peak decreases from 4.62 to 3.34. At the same time, the crosslinked network with alternating hydrophilic and hydrophobic regions in the pore-filled membrane will undergo non-uniform swelling in water. It is the synergistic effect of the recrystallization process of PVDF and the non-uniform swelling process of the crosslinked network that enables the membrane to be reshaped in the aqueous solution, finally forming a unique annular fold structure on the membrane surface.

[0120] This series of closely linked and interlocking change processes change the structure of the membrane at the microscopic level and then present different performance characteristics at the macroscopic level. From the initial change in the PVDF membrane structure, to the formation of the crosslinked network, and then to the appearance of annular folds on the membrane surface, each step lays a solid foundation for the preparation of a cation exchange membrane with high ion transport and anti-fouling properties.

[0121] 7.2 Microstructural and Chemical Analysis of the Membrane

[0122] The surface and cross-sectional microstructures of the prepared PC membranes were characterized by SEM and AFM. As can be clearly observed from Figure 8 (a) and Figure 8 (b), compared with the porous PVDF membrane, the highly developed pores in the PVDF membrane were successfully filled with the crosslinked network precursor solution. After a series of preparation processes, the PC membranes presented a dense, complete and pore-free structure on both the membrane surface and cross-section, which was extremely beneficial for the penetration of cations and provided a structural basis for efficient ion transport. In addition, in Figure 8 (b), annular wrinkles appeared on the surfaces of the PC series membranes. Among them, the wrinkle size of the PC-4.0 membrane remained within the range of 10 - 15 μm. For the other PC membranes (PC-0.5, PC-1.0, PC-2.0), although the wrinkle size was not explicitly mentioned, similar wrinkle structures were presented.

[0123] ATR-FTIR was used to evaluate the chemical structure of the PC membranes. Comparing the spectra of the PVDF membrane and the PC series membranes in Figure 9 (a), an obvious absorption peak attributed to the symmetric stretching vibration of the sulfonic acid group appeared at 1035 cm-1 in the PC series membranes. The appearance of this characteristic peak clearly indicated that the functional sulfonic acid group had successfully participated in the membrane structure. In addition, the characteristic peak at 1550 cm-1 was attributed to the stretching vibration of the N-H group, and the sharp absorption peak near 1150 cm-1 was attributed to the stretching vibration of the -CF group of the PVDF substrate, and the transmittance of this peak decreased due to pore filling and subsequent reactions, further corroborating the structural changes during the membrane preparation process.

[0124] XPS spectra were used to characterize the surface chemistry of the prepared PC membranes. As can be seen from Figure 9 (b), compared with the PVDF membrane, an S2p characteristic peak appeared at 168.2 in the spectrum of the prepared PC membrane, while the F1s characteristic peak at 688.1 disappeared. Figure 9 (c) shows the high-resolution O1s spectra of the PVDF membrane and the PC-4.0 membrane. The deconvolution of the O1s peak of the PC-4.0 membrane showed characteristic peaks of the sulfonic acid group compared with the PVDF membrane. Combining the analysis results of ATR-FTIR and XPS spectra fully indicated that the polymer electrolyte containing sulfonic acid groups had successfully filled the porous PVDF substrate, strongly proving the successful preparation of the cation exchange membrane (PC membrane) in this study.

[0125] 7.3 Analysis of the physical and chemical properties of the membrane

[0126] The ion exchange capacity (IEC), as a key parameter to measure the ion transport through the cation exchange membrane, directly reflects the number of free charge sites in the membrane and plays a decisive role in the ion transport performance of the membrane. According to Figure 10From the data change trend in (a), it can be seen that when the addition amount of TMPTA is gradually increased from 0.5 mol% to 4.0 mol%, the IEC value of the PC membrane correspondingly increases from 2.20 meq / g to 2.97 meq / g. This is because TMPTA can promote more sulfonated monomers to firmly bind inside the membrane, thereby significantly increasing the number of active sites available for ion exchange in the membrane, providing a richer channel for ion transport, and effectively enhancing the ion exchange ability of the membrane.

[0127] The swelling ratio is one of the core indicators for evaluating the stability of ion exchange membranes. In the present invention, the PVDF-based membrane used as the substrate shows extremely weak swelling in water. When the addition amount of TMPTA is less than 1.0 mol%, TMPTA will damage the PVDF structure in the base membrane, resulting in a decrease in its stability and a weakening of the intermolecular interaction, thus leading to an increase in the swelling ratio. As the addition amount of TMPTA continues to increase, the cross-linked network formed in the system gradually becomes stable. This stable cross-linked structure can partially compensate for the swelling trend caused by the damage of TMPTA to PVDF, making the swelling ratio of the PC membrane gradually drop from 42.5% to 24.8%. It is worthy of particular attention that once the addition amount of TMPTA exceeds 4.0 mol%, PVDF will have noticeable structural defects due to excessive deconstruction, seriously affecting the overall performance and stability of the PC membrane.

[0128] With the help of electrochemical impedance spectroscopy (EIS) technology, the resistance characteristics of the PC series membranes can be deeply analyzed. Observing Figure 10 the Nyquist plots of different PC membranes in (b), it can be found that there is an obvious negative correlation between the membrane resistance (characterized by the intersection point Z' of the Nyquist plot and the x-axis) and the addition amount of TMPTA. As the addition amount of TMPTA continuously increases, the membrane resistance gradually decreases from the initial 39.3 to 37.1, 37.0, and finally to 36.2. By constructing and analyzing the equivalent circuit model, the key electrochemical indicators related to the membrane bulk layer, double-layer, and diffusion boundary layer can be accurately identified. Further analyzing Figure 10 (c), it can be seen that as the addition amount of TMPTA increases, the resistance of the PC membrane bulk layer significantly decreases from 11.1 Ω·cm 2 to 2.8 Ω·cm 2 . This significant decrease in membrane resistance is mainly attributed to the successful introduction of more charged sulfonated monomers, which is highly consistent with the previous IEC test results. More charged sulfonated monomers not only increase the ion exchange capacity but also provide a smoother channel for the transport of charged ions in the membrane, greatly facilitating the rapid transmembrane transport of charged ions, significantly enhancing the ion transport efficiency of the membrane, and thus showing more excellent performance in actual application scenarios.

[0129] 7.4 Ion transport performance analysis of the membrane

[0130] With the help of Donnan dialysis experiments, the present invention deeply studies the relationship between the addition amount of TMPTA and the NH4 + flux of the membrane. It can be clearly observed from the data change trend of Figure 11 (a) that with the gradual increase of the addition amount of TMPTA, the NH4 + flux of the PC membrane increases steadily from 0.68 mol·m -2 ·h -1 to 1.63 mol·m -2 ·h -1 . This significant improvement is mainly attributed to two major advantages of the PC-4.0 membrane: a relatively high ion exchange capacity (IEC) and a relatively low membrane resistance. The relatively high IEC provides abundant active sites for ion transport, while the relatively low membrane resistance greatly reduces the hindrance during the ion transmembrane transport process. The two work together to greatly promote the efficient transport of NH4 + .

[0131] To comprehensively evaluate the performance advantages of the PC-4.0 membrane in different ion concentration environments, the present invention selects three different NH4 + inlet concentrations of 2.5 mM, 25 mM, and 50 mM, and conducts comparative experiments on commercial membranes and PC-4.0 membranes. As can be seen from Figure 11 (b), for all the membranes participating in the test, their NH4 + flux will increase with the increase of the NH4 + concentration in the feed solution. Taking the PC-4.0 membrane as an example, when the NH4 + concentration in the feed solution increases from 2.5 mM to 50 mM, its NH4 + flux increases significantly, from 0.18 mol·m -2 ·h -1 sharply rising to 2.55 mol·m -2 ·h -1 , with an increase amplitude of up to 14 times. This is because the significant increase in the NH4 + concentration in the feed solution effectively increases the ion concentration gradient on both sides of the membrane. According to the basic principle of ion diffusion, the increase in the concentration gradient can enhance the transmembrane driving force of ions, making it easier for NH4 + to cross the membrane for transport under the action of an electric field. Under different concentration conditions, the NH4 + flux of the PC-4.0 membrane is always better than that of commercial membranes, 2-4 times higher than that of commercial membranes. This is mainly due to the fact that the PC-4.0 membrane has a larger IEC value and a smaller membrane resistance compared with commercial membranes, thus showing more excellent performance in ion transport.

[0132] In the simulation test of actual application scenarios, the present invention uses actual domestic sewage (where the NH4 + content is 2.5 mM) and lemon pectin wastewater (NH4 + content is 50 mM) to test the NH4 + flux of the PC-4.0 membrane and commercial membranes. The results show that compared with the test using pure NH4Cl solution, during the test with actual domestic sewage, the NH4 + flux of all membranes decreased significantly, and the NH4 + flux of the PC-4.0 membrane even decreased by half. This is because actual domestic sewage contains a large number of impurity cations, such as Ca 2+ , Mg 2+ , Na + , etc. These impurity cations will compete with NH4 + for transmembrane transport channels, seriously hindering the normal transmembrane transport of NH4 + . During the actual pectin wastewater treatment process, the phenomenon of decreased NH4 + flux also occurred. The NH4 + flux of the PC-4.0 membrane decreased from 2.55 mol·m -2 ·h -1 to 0.90 mol·m -2 ·h -1 . Different from actual domestic sewage, during the actual pectin wastewater treatment process, in addition to the interference of impurity cations, a large number of organic substances may also adhere to the membrane surface or within the membrane pores. These organic substances will not only block the membrane pores, reducing the effective cross-sectional area of the ion transport channels, but may also interact with the membrane material, changing the surface properties and microstructure of the membrane, further increasing the difficulty of NH4 + transmembrane transport. Even facing such a complex actual wastewater environment, when treating actual domestic sewage and lemon pectin wastewater, the NH4 + flux of the PC-4.0 membrane is still superior to that of commercial membranes, fully demonstrating the excellent ion transport performance of the PC-4.0 membrane in a complex ion environment.

[0133] The present invention further uses an ED device to comparatively evaluate the effects of the PC-4.0 membrane and commercial membranes in treating actual domestic sewage and lemon pectin wastewater. It can be clearly seen from the Figure 12 experimental data that under the action of the electric field condition, the NH4 + flux of all tested membranes has increased. When treating actual domestic sewage, the PC-4.0 membrane shows strong treatment ability, and its NH4 + flux rapidly increases from 0.09 mol·m -2 ·h -1 to 0.55 mol·m -2·h -1 , increased by 6.0 times; when treating lemon pectin wastewater, the NH4 + flux of the PC-4.0 membrane also increased from 0.90 mol·m -2 ·h -1 to 1.62 mol·m -2 ·h -1 , an increase of 1.8 times. Similarly, when treating these two kinds of actual wastewater, the NH4 + flux of the PC-4.0 membrane was higher than that of the commercial membrane. This indicates that under the assistance of an electric field, the PC-4.0 membrane can transport NH4 + more effectively, further proving its high efficiency and superiority in the treatment of actual wastewater, and providing strong experimental evidence for its wide application in the fields of seawater desalination, high-salt concentration, ammonia nitrogen recovery, lithium-magnesium separation, and the recovery and treatment of actual wastewater resources.

[0134] Antifouling performance analysis of the 7.5 membrane

[0135] To deeply explore the difference in the antifouling performance between the PC-4.0 membrane prepared by the present invention and the commercial membrane (CMVN) in practical applications, in this study, actual lemon pectin wastewater was used as the feed solution, and the antifouling performance of the PC-4.0 membrane and the commercial membrane (CMVN) was comparatively analyzed under the conditions of Donnan dialysis (DD).

[0136] From Figure 13 (a), it can be visually observed that the surface of the PC-4.0 membrane has a unique annular wrinkled structure, while the surface of the commercial CMVN membrane is relatively smooth. After the two membranes operate in the actual pectin wastewater environment for 24 hours, it can be clearly seen from Figure 13 (b) that a large amount of obvious pollutant deposition appears on the surface of the CMVN membrane, while no obvious pollutants are attached to the surface of the PC-4.0 membrane. After rinsing the polluted membrane with pure water for 1 minute, it can be seen from Figure 13 (c) that a small amount of pollutants still remain on the surface of the CMVN membrane.

[0137] In this study, electrochemical impedance spectroscopy (EIS) technology was used to comprehensively characterize the electrochemical properties of the PC-4.0 membrane and the CMVN membrane before and after pollution and after cleaning. It can be seen from Figure 13 (d) that compared with the original non-polluted membrane, the resistance of the two polluted membranes (including the membrane body resistance R m , the pollution resistance R p , the diffusion boundary layer resistance R DBL and the double-layer capacitance resistance R EDL ) all showed a systematic increase. Among them, the pollution resistance R Fouling of the polluted PC-4.0 membrane accounted for 12.7% of the total impedance, with a value of 0.9 Ω·cm 2, its membrane body resistance R m increased from 2.8 Ω·cm 2 to 3.5 Ω·cm 2 , with an increase rate of 25.0%, indicating that the PC-4.0 membrane was contaminated internally, and the internal contamination was the main factor causing its performance change. At the same time, R EDL increased by 10 times, mainly because the pollutants adhered to the membrane surface and the internal contamination weakened the negative charge on the membrane surface, thus leading to a slowdown in the charge transfer response speed. It should be noted that the R Fouling of both the CMVN membrane and the PC-4.0 membrane was 0.9 Ω·cm 2 , but the increase rate of the R m of the CMVN membrane was more significant, increasing from 4.3 Ω·cm 2 to 8.9 Ω·cm 2 , an increase of 107.0%, fully indicating that the degree of internal contamination of the CMVN membrane was more serious. Similarly, the R EDL of the CMVN membrane increased from 0.4 Ω·cm 2 to 1.9 Ω·cm 2 , significantly higher than that of the PC-4.0 membrane (R EDL = 1.3 Ω·cm 2 ), which was a comprehensive manifestation of more serious surface and internal contamination of the CMVN membrane. Thus, it can be seen that although both membranes experienced surface and internal contamination, compared with the CMVN membrane, the contamination of the PC-4.0 membrane was mainly concentrated on the surface, making it difficult for pollutants to penetrate deep into the membrane interior.

[0138] Subsequently, the contaminated membranes were simply rinsed with pure water for 1 minute, and their EIS and NH4 + flux were tested, and the results are shown in Figure 13 (e). After physical cleaning, the body resistance of the PC-4.0 membrane decreased from 3.5 Ω·cm 2 to 3.1 Ω·cm 2 , and the body resistance of the CMVN membrane decreased from 8.9 Ω·cm 2 to 6.3 Ω·cm 2 . Physical cleaning can effectively remove the dirt on the cation exchange membrane because the cation exchange membrane has strong negative charge, weak binding force with dirt, and poor stability. However, after physical cleaning, the intrinsic resistance of the CMVN membrane increased by 46.5% compared with the uncontaminated CMVN membrane, 10.7% higher than that of the PC-4.0 membrane. In addition, the NH4 + flux recovery rate of the CMVN membrane after cleaning was 75.1%, while that of the PC-4.0 membrane was NH4 +The flux recovery rate is as high as 83.9%. The comparative analysis of the above indicators fully proves that PC-4.0 membrane has stronger anti-fouling performance than commercial CMVN membrane. In practical applications, it can better maintain the performance stability of the membrane, reduce performance degradation and frequent cleaning and maintenance caused by pollution, and provide strong performance guarantee for its wide application in complex wastewater treatment and other fields.

[0139] 8. Conclusion

[0140] The present invention successfully prepares a high-efficiency ion-transfer anti-fouling cation exchange membrane by a deconstruction-remodeling method. The method uses a polyvinylidene fluoride filter membrane as a substrate, induces the deconstruction of the polyvinylidene fluoride filter membrane crystal by trimethylolpropane triacrylate, and coordinates visible light / ultraviolet light curing and solution treatment to induce cross-linking network remodeling to form a ring-shaped micron-level wrinkled structure on the membrane surface. The prepared cation exchange membrane has a high-density film structure, the thickness of the ion exchange layer is 35-60nm, which is lower than the commercial membrane, and the ion exchange capacity is 2.8meq / g, which is higher than the commercial cation exchange membrane. Compared with the commercial cation exchange membrane, the ion transfer rate of the membrane is increased by 2-4 times, and the anti-fouling performance is significantly enhanced. At the same time, the present invention adopts visible light / ultraviolet light polymerization technology to avoid the high energy consumption problem of traditional heat treatment, selects non-benzene ring structure materials as functional monomers, and realizes the green preparation of cation exchange membranes. The membrane has broad application prospects in the fields of seawater desalination, high salt concentration, ammonia nitrogen recovery, lithium-magnesium separation, actual wastewater resource recovery and treatment, Daonan dialysis device and electrodialysis device.

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

[0142] 1. Advantages of the preparation process: The existing preparation process of cation exchange membranes often involves long-term heat treatment. For example, the thermal polymerization method requires the membrane to be heat treated at 80°C for 3 hours, and the hot rolling method requires the membrane to be repeatedly hot rolled at 105°C for 1 hour. The process takes a long time and has high energy consumption. The present invention adopts a deconstruction-remodeling design method of the visible light / ultraviolet light polymerization coupling membrane microstructure to avoid long-term high-temperature treatment. With the help of the photoinitiator, the cross-linked network precursor solution can complete the polymerization and curing process in just 20-60 minutes, which greatly shortens the preparation time and reduces energy consumption, realizing low-energy and rapid preparation of cation exchange membranes, which is in line with the concept of sustainable development, helps to reduce production costs, and improve production efficiency.

[0143] 2. Environmental advantages: The present invention selects substances without benzene ring structure as functional monomers, such as 2-acrylamide-2-methylpropanesulfonic acid, methacrylamide, etc., and uses methanol and water as solvents. In contrast, the prior art may use substances containing benzene rings that are not conducive to environmental protection. The raw material selection of the present invention avoids the use of harmful substances, reduces environmental pollution from the source, realizes the green preparation of cation exchange membranes, and 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 effect between ion exchange capacity and transport channel length, resulting in problems such as high membrane body resistance and low ion transport rate. Moreover, when increasing the ion exchange capacity to ensure mechanical strength, the membrane thickness will increase. The cation exchange membrane prepared by the present invention has a thin-film high-density structure. The thickness of the ion exchange layer is 35 - 60 nm, which is lower than that of commercial membranes (100 - 600 nm), and the ion exchange capacity reaches 2.8 meq / g, which is higher than 0.9 - 2.3 meq / g of commercial cation exchange membranes. It significantly improves the ion exchange capacity of the cation exchange membrane and reduces the membrane thickness, effectively reducing the membrane resistance and improving the ion transport efficiency, and performing more excellently in various ion exchange application scenarios.

[0145] 4. Anti-fouling advantages: The surfaces of existing commercial cation exchange membranes are flat, unable to form a turbulent flow field on the membrane surface, resulting in limited ion transport rate and easy fouling. The cation exchange membrane prepared by the present invention has ring-shaped micron-scale wrinkles on its surface. The wrinkles are beneficial to the capture of target ions of the cation exchange membrane and enhance the turbulence of the micro-flow field on the membrane surface, accelerating the ion exchange process. Compared with the commercial cation exchange membrane with a smooth surface, its ion transport rate is increased by 2 - 4 times. At the same time, these wrinkles can also hinder the deposition of pollutants on the membrane surface, enhance the anti-fouling performance of the membrane, significantly improve the anti-pollution ability of the membrane in the actual application process, reduce the frequency of membrane cleaning and replacement, lower the operation cost, and improve the operation 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 them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the purpose and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A method for preparing a porous filter membrane into a cation exchange membrane based on deconstruction and remodeling, characterized in that: The method comprises the following steps: S1, using a polyvinylidene fluoride filter membrane as a substrate, impregnating it with a cross-linked network precursor solution or coating the cross-linked network precursor solution on the surface of the membrane, wherein the cross-linked network precursor solution comprises at least one sulfonated monomer, an inducer and at least one photoinitiator; wherein the inducer is configured to be a substance capable of interacting with polyvinylidene fluoride crystals in the polyvinylidene fluoride filter membrane to cause part of the polyvinylidene fluoride crystals to decompose into amorphous substances; S2, irradiating the impregnated or coated polyvinylidene fluoride filter membrane under visible / ultraviolet light for a time T to induce polymerization and curing of the cross-linked network precursor solution, and compressing the partially deconstructed polyvinylidene fluoride filter membrane based on the cross-linked network formed by the polymerization and curing, so that the membrane becomes thinner and the exchange site density increases; S3, after polymerization and curing, immersing the polyvinylidene fluoride filter membrane in a reshaping solution to reshape the membrane surface morphology; wherein the reshaping solution is constructed so 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 expand unevenly after contact with it, so that the membrane surface is reshaped to form a wrinkled morphology.

2. The method according to claim 1, characterized in that The inducing agent is trimethylolpropane triacrylate.

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

4. The method according to claim 3, characterized in that The sulfonated monomer is selected from at least one of 2-acrylamide-2-methylpropanesulfonic acid, methacrylamide and 4-styrenesulfonic acid; The photoinitiator is 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-methylpropiophenone and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone.

5. The method according to any one of claims 1 to 4, characterized in that Before step S1, the method further includes step S0 of preparing 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 inducer and continue stirring for 10-20 minutes until all the monomers are dissolved; Under light-proof conditions, 0.1-1.0 mol % of a photoinitiator is added, and the mixture is stirred for 10-20 minutes to achieve uniform dissolution, thereby obtaining the cross-linked network precursor solution.

6. The method according to claim 5, characterized in that After step S3, a post-treatment step S4 is also included, which includes: soaking the membrane in a 1 mol / L NaCl solution for N hours.

7. The method according to claim 5, characterized in that In the impregnation step in step S2, it includes: Immerse the polyvinylidene fluoride filter membrane in the cross-linked network precursor solution for 1-20 minutes in a light-proof condition; Then, ultrasonic treatment is performed for 5-20 minutes to allow the AC network precursor solution to fully fill the membrane pores; Take out the filter membrane and scrape off the excess cross-linking network precursor solution on the surface.

8. The method according to claim 5, characterized in that In the coating step in step S2, it includes: Coat the cross-linked network precursor solution on the surface of the polyvinylidene fluoride filter membrane and let it stand for 1-10 minutes; Take out the filter membrane and scrape off the excess cross-linking network precursor solution on the surface.

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

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

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