Method for preparing high-stability monovalent selective cation exchange membrane

Abstract

The invention discloses a preparation method of a high-stability monovalent selective cation exchange membrane, and belongs to the technical field of ion exchange membrane preparation. The method comprises the following steps: firstly, carrying out surface activation on a homogeneous cation exchange base membrane by adopting low-temperature plasma, and introducing a crosslinkable active group on the surface of the membrane; enabling the cross-linking agent and the active group of the base membrane to form a covalent cross-linking structure through an interfacial polymerization reaction to finish primary modification; carrying out secondary interfacial polymerization after cleaning and airing, and constructing a uniform and compact three-dimensional cross-linking selective separation layer on the surface of the membrane; and finally, regulating and controlling the positive charge density of the modified layer through quaternization reaction to prepare the target membrane. The prepared membrane has high monovalent cation selectivity and excellent operation stability, can tolerate high-salt, strong-acid, strong-alkali and oxidizing environments, and is suitable for the fields of lithium extraction from salt lakes, lithium extraction from waste battery lixivium, acid-base recovery, wastewater heavy metal removal and the like.

Description

Technical Field

[0001] This invention belongs to the field of ion exchange membrane material preparation and membrane separation technology, and relates to a method for preparing a highly stable monovalent selective cation exchange membrane. In particular, it relates to a method for preparing a highly stable and highly selective monovalent cation exchange membrane by coupling a covalent cross-linked structure with secondary modification. The prepared membrane material can be widely used in fields such as lithium extraction from salt lakes, lithium extraction from waste battery leachate, industrial acid and alkali recovery, and removal of calcium, magnesium and heavy metal ions from industrial wastewater. Background Technology

[0002] Monovalent selective cation exchange membranes utilize size sieving effect, charge selectivity, and Donan effect to selectively exchange monovalent cations such as H+. + Na + K + Li + This membrane material possesses preferential permeability while effectively blocking divalent and polyvalent cations and anions. It can be widely used for selective lithium extraction from salt lake brines, selective recovery of precious metal ions such as gold and silver from electronic waste liquids, selective removal of heavy metal impurities from NaOH solutions, as an ion-selective membrane to improve battery efficiency, and selective removal of Cd from wastewater. 2+ Pb 2+ Harmful ions, etc. However, there is currently a lack of large-area selective cation exchange membranes that meet practical applications. This is because selective cation exchange membranes have unsatisfactory separation performance and unstable properties, and are particularly difficult to apply to high-salt, strong acid, strong alkali, and oxidizing environments.

[0003] Monovalent selective cation exchange membranes have wide applications and have been extensively studied. Liu (Journal of Membrane Science, 733 (2025) 124291) modified commercial cation exchange membranes using an electric field-assisted secondary interfacial polymerization method based on the crosslinking reaction of polyethyleneimine (PEI) and 1,3,5-benzenetriacyl chloride (TMC). Compared with monovalent selective cation exchange membranes modified by other methods, this method significantly enhanced the selectivity of the modified membrane. Zhao (Separation and Purification Technology, 300 (2022) 121802) reported a method for preparing monovalent selective cation exchange membranes using iron ion-induced rapid co-deposition of PDA-PEI, exploring the role of Fe. 3+ The mechanism of ion-promoted PDA-PEI co-deposition significantly enhances the selective CEM's Na content. + / Mg 2+Selectivity. Zhang et al. (Membrane Science and Technology, 2021, 41(1): 57-63) used pyrrole as a modifying material and employed an impregnation surface modification method to modify the surface of ordinary homogeneous cation exchange membranes to prepare monovalent selective cation exchange membranes. They studied the influence of different preparation factors on the selective separation performance of the modified membranes, thereby optimizing the membrane preparation conditions. Although these methods can obtain monovalent selective cation exchange membranes, a common problem is that these monovalent selective cation exchange membranes all suffer from performance instability.

[0004] In recent years, numerous patents have been reported on the preparation of monovalent selective cation exchange membranes. For example, patent (CN202110534018.X) discloses a method for preparing a lithium-ion selective permeable membrane by dispersing a modified lithium-ion sieve in a sulfonated polyether ether ketone polymer matrix, coating it onto a modified filter screen, and then drying it. This method utilizes the interaction between the casting solution and the filter screen to adjust the crystallization state of the polymer, the distribution state of the modified lithium-ion sieve, and the number of anion and cation groups within the membrane, thereby improving the membrane microstructure, controlling the membrane thickness, and increasing membrane strength while maintaining high separation performance. However, the lithium-ion selective permeable membrane prepared by this method is actually a heterogeneous membrane. The modified lithium-ion sieve dispersed in the sulfonated polyether ether ketone polymer cannot form a uniform and dense lithium selective separation layer, resulting in unsatisfactory lithium-ion selective separation performance. Secondly, no covalent cross-linking structure is formed between the modified lithium-ion sieve and the polymer. In practical applications, due to the significant differences in the swelling properties of different components, the modified lithium-ion sieve may detach from the polymer matrix. Therefore, the lithium-ion selective permeable membrane prepared by this method cannot meet the requirements of practical applications.

[0005] Patent (CN 201210506889.1) discloses a method for preparing an ion-exchange composite membrane selective for a single cation, which involves introducing a polymer, polyethyleneimine (PEI), as a selective separation layer onto the surface of a cation exchange membrane, serving as the base membrane, using static deposition and electrodeposition methods. However, because there is no covalent cross-linking structure between the polyethyleneimine and the cation exchange membrane, the polyethyleneimine deposited solely through electrostatic adsorption is easily detached and becomes ineffective. Therefore, the ion-selective permeable membrane prepared by this method cannot meet the requirements of practical applications.

[0006] To develop monovalent selective cation exchange membranes with good selectivity and stable performance, researchers have reported the preparation of nanofiber composite membranes with varying degrees of quaternization through filling and post-quaternization. The nanofiber composite membrane with sulfonic acid groups exhibits excellent cation transport capacity, while the uniformly dispersed electrospun nanofibers with quaternary ammonium groups within the membrane demonstrate a stronger repulsion effect on higher-valence divalent cations, endowing the membrane with certain monovalent and multivalent cation selectivity. Since the ion selectivity of monovalent selective cation exchange membranes relies not only on electrostatic repulsion but also on the synergistic effect of pore size sieving, the nanofibers in the selective membrane prepared by this method cannot form a uniform and dense selective separation layer, resulting in unsatisfactory selective separation of monovalent ions. Furthermore, because the positively charged nanofibers are distributed in the main membrane, some of the positive charge cancels out the negative charge of the sulfonic acid groups in the main membrane, thereby increasing the resistance of the main membrane and affecting the transmembrane migration of positively charged ions. Therefore, the ion-selective permeable membrane prepared by this method also cannot meet the requirements of practical applications.

[0007] Based on the principle that under the excitation of an external electric field, the gas introduced into the system generates plasma through glow discharge. Accelerated by the electric field, plasma collides with the surface of polymer materials, causing the breakage of chemical bonds on the material surface and generating highly reactive free radicals. This leads to the formation of new compounds on the polymer surface, increasing the density of active groups, hydrophilicity, and antifouling properties of the membrane surface. Patent (CN201710080098.X) discloses a method for plasma-grafted modified ion exchange membranes. This method uses low-temperature plasma discharge technology to graft and polymerize amphoteric organic monomers (such as betaine sulfonate, betaine phosphate, dodecyl sulfopropyl betaine, or one or more mixtures) onto the surface of the ion exchange membrane. This simultaneously introduces acidic and basic active groups onto the ion exchange membrane surface, preparing an ion exchange membrane with high selective permeability and low resistance. While this method can improve the stability of the modified layer, it cannot form a uniform and dense modified layer, and the amphoteric organic monomer polymer cannot achieve selectivity for monovalent ions. Therefore, the ion-selective permeable membrane prepared by this method cannot meet the requirements of practical applications.

[0008] In general, existing technologies for preparing monovalent selective cation exchange membranes suffer from four common technical bottlenecks: First, the modified layer lacks stable covalent bonding with the base membrane, resulting in poor long-term operational stability and a tendency to detach and fail. Second, it is impossible to construct a uniform and dense three-dimensional cross-linked separation layer, making it difficult to simultaneously achieve high separation selectivity and high ion flux. Third, the charge density on the membrane surface cannot be precisely controlled, limiting the improvement of separation selectivity. Fourth, the modification process easily damages the properties of the base membrane, resulting in poor environmental tolerance of the membrane material and its inability to adapt to harsh industrial conditions. Therefore, developing a method for preparing monovalent selective cation exchange membranes that simultaneously possesses high selectivity, high flux, high stability, and high environmental adaptability, while also being simple and easily scalable for industrial application, is a core technical problem that urgently needs to be solved in this field. Summary of the Invention

[0009] This invention discloses a method for preparing a highly stable monovalent selective cation exchange membrane. The method employs low-temperature plasma to activate the surface of a conventional homogeneous cation exchange membrane. Then, through interfacial polymerization, functional monomers and crosslinking agents form a covalently crosslinked structure, fixing the modified components with functional groups onto the cation exchange membrane surface. Following secondary modification and quaternization treatment, a highly stable monovalent selective cation exchange membrane is obtained. This invention solves the problem of inconsistent stability and monovalent ion selectivity in conventional monovalent selective ion exchange membranes. It can adapt to conditions such as high salt, strong acid, strong alkali, and oxidizing environments, meeting the requirements for lithium extraction from salt lakes and can be widely used for lithium extraction from waste battery leachates, acid recovery, alkali recovery, and removal of calcium, magnesium, and heavy metal impurities from wastewater.

[0010] This invention aims to provide a method for preparing highly stable monovalent selective cation exchange membranes, thereby solving the problems of easy detachment of the modified layer, poor stability, and insufficient monovalent ion selectivity in existing technologies. To achieve the above objective, this invention provides the following technical solution.

[0011] In a first aspect, the present invention provides a method for preparing a highly stable monovalent selective cation exchange membrane, comprising the following steps: S1: The homogeneous cation exchange membrane is surface activated by low-temperature plasma to form active groups on the membrane surface; S2: The membrane treated in step S1 is reacted with a crosslinking agent through interfacial polymerization to form a covalent crosslinked structure; S3: The membrane obtained in step S2 is modified a second time to form a modified layer with a three-dimensional cross-linked structure on its surface; S4: The membrane obtained in step S3 is subjected to quaternization treatment to adjust the positive charge density of the modified layer, thereby obtaining the high-stability monovalent selective cation exchange membrane.

[0012] The technical solution provided by this invention, in step S1, introduces active groups such as hydroxyl, carboxyl, and amino groups onto the membrane surface through low-temperature plasma treatment, providing reaction sites for subsequent covalent cross-linking. Compared with surface activation methods such as ultraviolet light, gamma rays, and electron beams, low-temperature plasma can achieve precise control of the chemical structure of the membrane surface under mild conditions, avoiding degradation of the substrate membrane's properties.

[0013] Step S2 utilizes interfacial polymerization to form covalent bonds between the crosslinking agent and the active groups on the membrane surface, thus stably fixing the functional components to the membrane surface. The introduction of the covalent crosslinking structure significantly enhances the bonding strength between the modified layer and the base membrane, which is key to improving membrane stability.

[0014] Step S3 involves secondary modification to construct a uniform and dense three-dimensional cross-linked structure on the membrane surface. This structure not only enhances the mechanical stability of the modified layer but also improves its ability to block high-valence cations through steric hindrance and charge synergy.

[0015] Step S4 precisely controls the positive charge density on the modified layer surface through a quaternization reaction. Increased positive charge density enhances the electrostatic repulsion against multivalent cations, further improving the selectivity for monovalent ions.

[0016] This invention achieves efficient construction and stable fixation of selective separation layers through a four-step synergistic strategy of "plasma activation-covalent crosslinking-secondary modification-quaternization regulation", solving the problem of difficulty in balancing stability and selectivity in the prior art.

[0017] Preferably, in step S1, the low-temperature plasma is used at a frequency of 5~50 kHz, an operating temperature of 5~40℃, an operating pressure of 10~1500 Pa, and a processing time of 10~60 min. Within the above parameter range, the efficient introduction of active groups on the membrane surface can be achieved while avoiding damage to the membrane structure.

[0018] Preferably, in step S1, the atmosphere used for the low-temperature plasma is selected from one or more of oxygen, ozone, nitrogen, chlorine, sulfur dioxide, and ammonia. By selecting different atmospheres, the type of active groups introduced onto the membrane surface can be controlled; for example, nitrogen-containing groups are mainly introduced under a nitrogen atmosphere, while oxygen-containing groups are mainly introduced under an oxygen atmosphere.

[0019] Preferably, in step S1, the active group includes one or more of the following: hydroxyl, carboxyl, carbonyl, aldehyde, ester, primary amine, secondary amine, imino, amide, nitrile, and quaternary ammonium groups.

[0020] Preferably, in step S2, the interfacial polymerization reaction is gas-solid interfacial polymerization or liquid-solid interfacial polymerization; the crosslinking agent is selected from one or more of dopamine, polyethyleneimine, and tannic acid, and its concentration is 0.1% to 2%. Within this concentration range, the crosslinking agent can uniformly cover the film surface and form a stable covalent crosslinked network.

[0021] Preferably, in step S3, the modifier used in the secondary modification is selected from one or more of pyromellitic methyl chloride, glutaraldehyde, ethylene oxide, crown ether, and silane coupling agent, and its concentration is 0.01%~1.5%. The secondary modification can further crosslink unreacted functional groups to form a three-dimensional structure, thereby improving the density and stability of the modified layer.

[0022] Preferably, in step S4, the reagent used for the quaternization treatment is selected from one or more of iodomethane, iodoethane, bromomethane, and bromoethane, with a concentration of 0.1~0.5 mol / L and a treatment time of 10~60 min. Quaternization treatment introduces a high density of positive charges onto the surface of the modified layer, enhancing the electrostatic repulsion against polyvalent cations.

[0023] Preferably, a washing and drying step is included between steps S2 and S3: soaking in deionized water for 5-30 minutes, followed by blowing with compressed air to dry. This treatment removes unreacted monomers and avoids interference with subsequent reactions.

[0024] Secondly, the present invention provides a highly stable monovalent selective cation exchange membrane, which is prepared by the above method.

[0025] Thirdly, the present invention provides the use of the above-mentioned high-stability monovalent selective cation exchange membrane for lithium extraction from salt lakes, lithium extraction from waste battery leachate, acid recovery, alkali recovery, or removal of calcium, magnesium and heavy metal ions from wastewater.

[0026] Compared with the prior art, the present invention has the following beneficial technical effects: (1) By low-temperature plasma surface activation treatment, active groups can be precisely introduced under mild conditions, providing ideal reaction sites for subsequent covalent crosslinking and avoiding the deterioration of the substrate film's properties.

[0027] (2) A covalent cross-linked structure is formed through interfacial polymerization reaction, which enables chemical bonding between the modified layer and the base film, significantly improving the bonding stability of the modified layer and solving the problem of easy detachment of conventional electrostatic adsorption modification.

[0028] (3) By constructing a three-dimensional cross-linked structure through secondary modification, the density and mechanical stability of the modified layer are further enhanced, and the ability to block high-valence cations is improved through the steric hindrance effect.

[0029] (4) The positive charge density on the surface of the modified layer is precisely controlled by quaternization treatment, and the selectivity for multivalent cations is enhanced by electrostatic repulsion, without significantly increasing the membrane resistance.

[0030] (5) The monovalent selective cation exchange membrane prepared by the present invention exhibits excellent stability in high salt, strong acid, strong alkali and oxidizing environments, and is suitable for a variety of harsh application scenarios such as lithium extraction from salt lakes, lithium extraction from waste battery leachate, acid / alkali recovery, and removal of calcium, magnesium and heavy metal ions from wastewater. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the preparation method of the present invention.

[0032] Figure 2 The images show SEM images of the base film and the surface of the N2 and O2 plasma-modified films, where a is the base film, b is the N2 plasma-modified film, and c is the O2 plasma-modified film.

[0033] Figure 3 The images show the C1s XPS spectra of the base film and the N2 and O2 plasma modified film surfaces, where a is the base film, b is the N2 plasma modified film, and c is the O2 plasma modified film.

[0034] Figure 4 The images show the AFM images of the base film and the surface of the N2 and O2 plasma modified films, where a is the base film, b is the N2 plasma modified film, and c is the O2 plasma modified film.

[0035] Figure 5 SEM images of the base membrane and the surface of the monovalent selective cation exchange membrane prepared in this invention are shown, where a is the base membrane and b is the monovalent selective cation exchange membrane. Detailed Implementation

[0036] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the protection scope of the present invention.

[0037] Example 1 This embodiment employs low-temperature plasma to activate the surface of a conventional homogeneous cation exchange membrane, forming crosslinkable active groups on the surface of the base membrane. The low-temperature plasma operates at a frequency of 20 kHz, a temperature of 30°C, a pressure of 600 Pa, in a nitrogen atmosphere, and for 30 minutes, thereby obtaining an N2 low-temperature plasma-modified membrane. The C1s XPS spectra of the base membrane and the N2 low-temperature plasma-treated membrane show characteristic peaks on the base membrane: 284.7 eV corresponds to CC, 285.3 eV to CH, 286.6 eV to CN (quaternary ammonium group), 287.8 eV to C=O, and 291.4 eV to C=C. Based on the photoemission spectroscopy, different forms of matter, such as N2, are detected by the N2 plasma. + N2 * , N, N + These substances, including electrons, activate the membrane surface, leading to the breaking of C-C or CH bonds. The base membrane surface reacts with N2 plasma to form new functional groups. The SEM images, C1s XPS spectra, and AFM images of the (a) base membrane and (b) N2 plasma-modified membrane surfaces used in this embodiment are shown in Figures (2-4). It can be inferred that the new active groups formed on the surface of the ordinary homogeneous cation exchange membrane after N2 low-temperature plasma treatment are mainly primary amines (NH2), secondary amines (-NH-), imino groups (-C=NH), amide groups (-CONH2), nitrile groups (-C≡N), and quaternary ammonium groups (-N...). + The study used a combination of active groups, including R3, with varying proportions of different groups. Measurements were performed using a simulated solution from salt lake brine.

[0038] Example 2 In this embodiment, the atmosphere used for the low-temperature plasma is oxygen, and other conditions are the same as in Example 1, thereby obtaining an O2 low-temperature plasma modified film. The free radicals generated by the O2 low-temperature plasma, such as O... + or O2 +It can also react with the activated membrane surface to generate new polar group base films. After O2 low-temperature plasma treatment, the intensity and position of characteristic peaks changed. The intensity of the CH characteristic peak at 285.3 eV decreased significantly, while the intensity of carbon-oxygen bonds such as CO (286.4 eV) and C=O (287.8 eV) increased. In addition, new characteristic peaks at 289.3 and 289.6 eV appeared, possibly due to the formation of ester / carboxyl O=CO and carbonate O=C(-O)2. During the O2 plasma treatment of the ion exchange membrane, the CC or CH bonds on the activated membrane surface are easily broken and react with the free radicals generated by the O2 plasma to generate ester / carboxyl groups or further oxidize to generate carbonate groups. The SEM images, C1s XPS spectra, and AFM spectra of the (a) base membrane and (c) O2 low-temperature plasma modified membrane surface used in this embodiment are shown in Figures (2-4). It can be inferred that the new active groups formed on the surface of the ordinary homogeneous cation exchange membrane after O2 low-temperature plasma treatment are mainly combinations of active groups such as hydroxyl (-OH), carboxyl (-COOH), carbonyl (C=O), aldehyde (-CHO), and ester (-COOR), with different groups having different proportions. Measurements were performed using a simulated solution of salt lake brine.

[0039] Example 3 This embodiment provides a method for preparing a highly stable monovalent selective cation exchange membrane. Using the base membrane with active groups obtained in Example 1, a covalent cross-linked structure is formed with a cross-linking agent through interfacial polymerization. A mixed solution of 1.5% dopamine and polyethyleneimine (Mw = 25,000 Da) was used as the cross-linking agent and modifier. The pH was adjusted to 8.5 using a buffer solution, and the membrane was immersed on one side for 24 hours at room temperature to obtain a modified membrane with functional groups. The membrane was then washed three times with deionized water, purged with compressed air, and dried. After washing and drying, a 0.8% glutaraldehyde solution was used as the modifier, and the reaction was continued at 55°C for 60 minutes to obtain a secondary modified membrane. After washing and drying, a 0.5 mol / L iodomethane solution was used for quaternization treatment at room temperature for 60 minutes, thereby preparing a highly stable monovalent selective cation exchange membrane. SEM images of the modified membrane surface are shown below. Figure 5 The modified membrane was measured using a simulated solution of salt lake brine. Three consecutive measurements were taken, with a 10-day interval between each measurement. The membrane was then immersed in simulated brine at pH 3 to assess its stability.

[0040] Example 4 This embodiment provides a method for preparing a highly stable monovalent selective cation exchange membrane. The base membrane with active groups obtained in Example 2 is used to form a covalently cross-linked structure with a cross-linking agent through interfacial polymerization. Other conditions are the same as in Example 3, thereby preparing a highly stable monovalent selective cation exchange membrane. Measurements are performed using a simulated solution of brine from a salt lake.

[0041] Comparative Example 1 This comparative example uses ultraviolet (UV) irradiation to activate the surface of the ion exchange membrane. 254 nm UV irradiation directly breaks the covalent bonds in organic molecules through high-energy photons, decomposing them into ions, free atoms, or excited molecules. Simultaneously, ozone is introduced, and the UV light further decomposes O3 into active oxygen atoms, forming a continuous photosensitive oxidation reaction chain, thereby introducing active groups onto the surface of the ion exchange membrane. Other conditions are the same as in Example 3, thus preparing a highly stable monovalent selective cation exchange membrane. Measurements were performed using a simulated solution from salt lake brine.

[0042] Comparative Example 2 This comparative example uses gamma-ray irradiation to activate the surface of the ion exchange membrane. The high energy of gamma rays induces chemical grafting or structural modification of the ion exchange membrane, improving its ion selectivity, stability, and antifouling ability. With the irradiation dose controlled at 50 kGy, the ambient atmosphere at N2, and the temperature at 30°C, free radicals are generated during gamma-ray irradiation, initiating polymer chain breakage or cross-linking reactions and causing surface oxidation to generate carbonyl groups, thereby introducing active groups onto the cation exchange membrane surface. Other conditions are the same as in Example 3, thus preparing a highly stable monovalent selective cation exchange membrane. Measurements were performed using a simulated solution of brine from a salt lake.

[0043] Comparative Example 3 This comparative example utilizes a high-energy electron beam to activate the surface of an ion-exchange membrane. Surface activation with a high-energy electron beam introduces specific functional groups onto the membrane surface, significantly altering its physical and chemical properties. The activation is performed under controlled conditions of an accelerating voltage of 2 MeV, a beam current density of 10 mA / cm², an irradiation dose of 50 Gy, and a protected N₂ atmosphere. This triggers a series of complex chemical reactions on the ion-exchange membrane surface, such as free radical generation, bond breaking, and recombination, introducing oxygen- and nitrogen-containing polar groups, such as -COOH, -OH, and -CONH₂, thereby enhancing the membrane surface activity. Other conditions are the same as in Example 3, thus preparing a highly stable monovalent selective cation exchange membrane. Measurements were performed using a simulated solution of brine from a salt lake.

[0044] Comparative Example 4 This comparative example uses a conventional homogeneous cation exchange membrane as the base membrane without surface activation treatment. Other conditions are the same as in Example 3, thus preparing a monovalent selective cation exchange membrane. Measurements were performed using a simulated solution of brine from a salt lake. Three consecutive measurements were taken, with each measurement spaced 10 days apart. The modified membrane was then immersed in simulated brine at pH 3 to assess the stability of the modified membrane's performance.

[0045] Comparative Example 5 Comparative Example 5 uses similar operating conditions to Example 3 to prepare a highly stable monovalent selective cation exchange membrane. The difference between Comparative Example 5 and Example 3 is that the primary modified membrane obtained through the same steps uses a 0.8% glutaraldehyde solution as the modifier, and undergoes a secondary modification at 55°C for 60 minutes. However, the resulting modified membrane does not undergo quaternization treatment. Measurements were performed using a simulated solution from salt lake brine.

[0046] Comparative Example 6 This comparative example uses similar operating conditions to Example 3 to prepare a highly stable monovalent selective cation exchange membrane. The base membrane with active groups obtained in Example 1 forms a covalently cross-linked structure with a cross-linking agent via interfacial polymerization. A mixed solution of 1.5% dopamine and polyethyleneimine (Mw = 25,000 Da) was used as the cross-linking agent and modifier. The pH was adjusted to 8.5 with a buffer solution, and the membrane was immersed on one side for 24 hours at room temperature to obtain a modified membrane with functional groups. No secondary modification or quaternization treatment was performed. Measurements were conducted using a simulated solution from salt lake brine.

[0047] Table 1. Experimental Results of Examples and Comparative Examples

[0048] As can be seen from Table 1: As shown in Examples 1 and 2, simply using low-temperature plasma to activate the surface of a conventional homogeneous cation exchange membrane, while producing active groups on the membrane surface, does not enable the membrane to achieve selective permeability to monovalent cations. Comparing Example 1 (using an N2 atmosphere) with Example 2 (using an O2 atmosphere), it is evident that treatment under an N2 atmosphere slightly affects the Li and Mg ion flux, suggesting that low-temperature plasma treatment under an N2 atmosphere can generate amine-containing active groups on the membrane surface.

[0049] As shown in Example 3, a conventional homogeneous cation exchange membrane was surface activated by low-temperature plasma, followed by surface modification and quaternization treatment. Three consecutive measurements demonstrated that this invention can prepare highly stable monovalent selective cations with a Li / Mg selective separation coefficient >15 and a lithium-ion flux of approximately 1.438 mol / (m²). 2 The flux of magnesium ions (·h) is approximately 0.688 mol / (m2 The performance parameters remained essentially unchanged after three measurements, indicating that the prepared monovalent selective cation has good stability.

[0050] Comparing Example 4 and Example 3, it was found that the monovalent selective cations obtained by low-temperature plasma surface treatment in O2 atmosphere followed by surface modification and quaternization treatment have similar properties to the monovalent selective cation exchange membrane obtained by N2 atmosphere treatment. Because O2 has strong oxidizing properties, it causes the formation of more oxygen-containing active groups on the surface of ordinary homogeneous cation exchange membranes, thus affecting their surface modification effect. The high-stability monovalent selective cation exchange membrane obtained in Example 4 has a Li / Mg selective separation coefficient of 13.17 and a lithium-ion flux of approximately 1.382 mol / (m²). 2 The flux of magnesium ions (·h) is approximately 0.863 mol / (m 2 ·h). This means that the monovalent selective cation exchange membrane obtained by low-temperature plasma surface treatment in an N2 atmosphere is superior to the selective ion exchange membrane obtained in an O2 atmosphere.

[0051] Comparative Examples 1, 2, and 3 show that ordinary homogeneous cation exchange membranes can be surface-activated by γ-ray, high-energy electron beam, and ultraviolet light + ozone irradiation, followed by surface modification and quaternization treatment. This process yields three types of monovalent selective cation exchange membranes. However, their key performance indicators, such as the Li / Mg selective separation coefficient, lithium-ion flux, and magnesium-ion flux, are slightly inferior to those of the monovalent selective cation exchange membranes obtained by surface modification using low-temperature plasma. Specifically, the monovalent selectivity of the γ-ray surface-activated membrane is 12.9, and the lithium-ion flux is approximately 1.166 mol / (m²). 2 The flux of magnesium ions (·h) and magnesium ions is approximately 0.731 mol / (m 2 •h); The monovalent selectivity of surface activation treatment with high-energy electron beam is 14.3, and the lithium-ion flux is approximately 0.949 mol / (m 2 The flux of magnesium ions (·h) is approximately 0.554 mol / (m) 2 While exhibiting high monovalent ion selectivity, the ion flux of the monovalent selective cation exchange membrane obtained by surface modification with ultraviolet light and ozone irradiation is relatively low. The monovalent selective cation exchange membrane has a monovalent selectivity of 11.3, a lithium ion flux of approximately 1.482 mol / (m²·h), and a magnesium ion flux of approximately 0.859 mol / (m²·h). 2(h). Compared to low-temperature plasma surface treatment, these methods cannot adjust the atmosphere to obtain suitable surface-active groups, and different ion exchange membrane surface activation methods result in significant differences in the microstructure of the ion exchange membrane surface. In addition, based on the principle of high-energy irradiation, high-energy electron beams are used, but the high irradiation energy can easily lead to the breakage of the polymer backbone, resulting in the deterioration of the base membrane properties.

[0052] Comparing Comparative Example 4 and Example 3, it can be seen that a monovalent selective cation exchange membrane was prepared by using a common homogeneous cation exchange membrane as the base membrane without surface activation treatment, while other conditions were the same as in Example 3. The monovalent selectivity prepared in this way was 13.26, and the lithium-ion flux was approximately 1.439 mol / (m²). 2 The flux of magnesium ions (·h) and magnesium ions is approximately 0.715 mol / (m 2 (h). Although the initial measurement results showed that the modified membrane obtained by this method had performance similar to that of Example 3, its key technical indicators decreased significantly with the increase of the number of tests, which is presumably caused by the instability of the selective separation layer.

[0053] Comparing Comparative Example 5 with Example 3, it can be seen that the primary modified membrane obtained in Comparative Example 5, after the same steps, uses 0.8% glutaraldehyde solution as a modifier and reacts at 55°C for 60 min for secondary modification, but the resulting modified membrane does not undergo quaternization treatment. The selective cation exchange membrane prepared in this way exhibits a valence selectivity of 7.31 and a lithium-ion flux of approximately 0.976 mol / (m²). 2 The flux of magnesium ions (·h) is approximately 1.112 mol / (m 2 This result indicates that quaternization treatment can regulate the charge on the surface of the modified membrane, significantly improving the performance of monovalent selective ion exchange membranes.

[0054] Comparing Comparative Example 6 with Example 3, it can be seen that Comparative Example 6 uses 1.5% dopamine and polyethyleneimine as crosslinking agent and modifier, and obtains a modified membrane with functional groups by immersion on one side for 24 hours at room temperature, but does not undergo secondary modification or quaternization treatment. The monovalent selectivity of the prepared cation exchange membrane is 3.76, and the lithium-ion flux is approximately 0.761 mol / (m²). 2 The flux of Li / Mg ions was approximately 1.541 mol / (m²·h). This result indicates that no three-dimensional cross-linking was formed on the surface of the modified membrane, and therefore there was no obvious selectivity for Li / Mg ions.

[0055] A comprehensive comparison of Examples 1-4 and Comparative Examples 1-6 reveals that low-temperature plasma surface activation treatment (Examples 1-2) can precisely introduce active groups onto the membrane surface without damaging the base membrane structure, which is superior to activation methods such as ultraviolet light, gamma rays, and electron beams. The formation of a covalent cross-linked structure through interfacial polymerization (Examples 3-4) significantly improves the bonding stability of the modified layer, solving the problem of easy detachment of the modified layer in Comparative Example 4. A comparison between the secondary modification to construct a three-dimensional cross-linked structure (Examples 3-4) and Comparative Example 6 shows that a three-dimensional structure is a necessary condition for forming a dense selective layer and achieving high selectivity. A comparison between quaternization treatment (Examples 3-4) and Comparative Example 5 shows that the regulation of positive charge density has a significant effect on improving the selectivity of monovalent ions. The membrane prepared by this invention (Example 3) exhibits excellent long-term stability in high-salt and strong-acid environments, with small fluctuations in three measurements, while the performance of Comparative Example 4 significantly degraded under the same conditions.

[0056] In summary, this invention successfully prepared a monovalent selective cation exchange membrane with both high selectivity and high stability through a four-step synergistic strategy of "plasma activation-covalent crosslinking-secondary modification-quaternization regulation", solving the long-standing problem in existing technologies that it is difficult to balance stability and selectivity.

[0057] Although embodiments of the present invention have been shown and described above, it is understood that these embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions, and alterations to the above embodiments within the scope of the present invention without departing from its principles and spirit. The scope of protection of the present invention is defined by the claims and their equivalents.

Claims

1. A method for preparing a highly stable monovalent selective cation exchange membrane, characterized in that, Includes the following steps: S1: The homogeneous cation exchange membrane is surface activated by low-temperature plasma to form active groups on the membrane surface; S2: The membrane treated in step S1 is reacted with a crosslinking agent through interfacial polymerization to form a covalent crosslinked structure; S3: The membrane obtained in step S2 is modified a second time to form a modified layer with a three-dimensional cross-linked structure on its surface; S4: The membrane obtained in step S3 is subjected to quaternization treatment to adjust the positive charge density of the modified layer, thereby obtaining the high-stability monovalent selective cation exchange membrane.

2. The method according to claim 1, characterized in that, In step S1, the low-temperature plasma is used at a frequency of 5~50 kHz, a working temperature of 5~40℃, a working pressure of 10~1500 Pa, and a processing time of 10~60 min.

3. The method according to claim 1 or 2, characterized in that, In step S1, the atmosphere used for the low-temperature plasma is selected from one or more of oxygen, ozone, nitrogen, chlorine, sulfur dioxide, and ammonia.

4. The method according to claim 1, characterized in that, In step S1, the active group includes one or more of the following: hydroxyl, carboxyl, carbonyl, aldehyde, ester, primary amine, secondary amine, imino, amide, nitrile, and quaternary ammonium groups.

5. The method according to claim 1, characterized in that, In step S2, the interfacial polymerization reaction is gas-solid interfacial polymerization or liquid-solid interfacial polymerization; the crosslinking agent is selected from one or more of dopamine, polyethyleneimine, and tannic acid, and its concentration is 0.1% to 2%.

6. The method according to claim 1, characterized in that, In step S3, the modifier used in the secondary modification is selected from one or more of pyromellitic methyl chloride, glutaraldehyde, ethylene oxide, crown ether, and silane coupling agent, and its concentration is 0.01%~1.5%.

7. The method according to claim 1, characterized in that, In step S4, the reagent used in the quaternization treatment is selected from one or more of iodomethane, iodoethane, bromomethane, and bromoethane, with a concentration of 0.1~0.5 mol / L and a treatment time of 10~60 min.

8. The method according to claim 1, characterized in that, Between steps S2 and S3, there is also a washing and drying step: soaking in deionized water for 5 to 30 minutes, and then blowing and drying with compressed air.

9. A highly stable monovalent selective cation exchange membrane, prepared by the method described in any one of claims 1-8.

10. The use of the high-stability monovalent selective cation exchange membrane according to claim 9, for lithium extraction from salt lakes, lithium extraction from waste battery leachate, acid recovery, alkali recovery, or removal of calcium, magnesium, and heavy metal ions from wastewater.

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