Preparation method of cation selective separation membrane for acid recovery

Abstract

The invention discloses a preparation method of a cation selective separation membrane for acid recovery, which comprises the following steps: by taking an intrinsic microporous polymer with strong pi-pi interaction and hydrogen bond self-assembly capability as a precursor for preparing a membrane material, constructing ion exchange sites in the membrane through grafting of sulfonated monomers; on the basis, homogeneous membrane liquid is prepared, coated and dried to form the cation selective separation membrane, and hydrogen bond self-assembly in the membrane forming process is controlled by controlling the temperature so as to construct a hydrogen ion rapid transmission channel. The preparation process of the membrane material is simple to operate by adopting a preparation route of grafting reaction, the target ion flux of the prepared membrane material meets the application requirement, high cation selectivity is finally obtained in the electrodialysis process, and large-scale industrial production and application are expected to be realized in the field of acid recovery membranes.

Description

Technical Field

[0001] The invention belongs to the field of membrane technology, and in particular relates to a method for preparing a cation selective separation membrane for acid recovery. Background Art

[0002] In a large number of chemical industrial production processes, acid is one of the basic raw materials that are inevitably used in large quantities, including steel smelting, mineral separation, organic acid production, rare earth extraction and other industries. The waste acid produced often causes serious harm to the environment. The separation of waste acid can not only achieve resource recycling, but also has important ecological and social value. Among them, the precise separation of hydrogen ions is the core issue in achieving waste acid recovery, especially the separation of ions with the same charge but different valence states, such as H + / Li + 、H + / Fe 2+ 、H + / Cu 2+ Separation of plasma systems.

[0003] Current traditional methods mainly focus on technical processes such as chemical neutralization, roasting, cooling crystallization, and solvent extraction. However, due to the technical limitations of these processes themselves, these processes have problems such as high investment costs, low recovery rates, and high energy consumption. In contrast, due to the continuous development and maturity of membrane separation technology with cation-selective separation membranes as the core, and the advantages of membrane separation technology such as high separation efficiency, low equipment cost, and small footprint, it is urgently needed for its application in many chemical separation fields. With the continuous development of various industries, higher requirements are also placed on the performance of membrane materials, including higher acid recovery purity and higher acid concentration. Further development of acid-recovery cation-selective separation membranes with better performance, industrial production, and lower cost is still needed.

[0004] Currently, commercial acid recovery devices based on ion exchange membranes are mainly concentrated in diffusion dialysis membrane devices. This process does not consume energy and can be recovered as long as there is a concentration difference between the waste acid solution and the recovered solution. However, the disadvantage of this type of device is that the recovered acid concentration is limited by the waste acid solution concentration. When the recovered concentration is close to the waste solution concentration, the diffusion process will be difficult to occur. In addition, if there are Fe 2+ 、Cu 2+ 、Li +When other ions are transported, the selective transmission between hydrogen ions and other ions is also a key performance indicator for achieving high-quality acid recovery. The main ways to achieve selectivity are: 1) to achieve selectivity of ions of different sizes by increasing density and using pore size screening. Current research based on this strategy mainly constructs ion selectivity by increasing the density of the membrane body or constructing a dense thin layer on the surface of the base membrane, such as CN112546872B, which effectively increases the density of the membrane by polymerizing a layer of pyrrole-2-carboxylic acid polymer on the surface of the cation exchange base membrane. At the same time, due to the introduction of ion exchange groups, the ionic conductivity of the membrane itself is guaranteed, thereby achieving cation selectivity. 2) Construct selectivity through electrostatic repulsion. For example, CN114377731B uses Fe 3+ Co-deposited with polyethyleneimine on the membrane surface, a cation-selective separation membrane is prepared. However, surface modification methods often increase membrane resistance, resulting in increased energy consumption during actual application. At the same time, the problem of surface layer shedding may also reduce the stability of the membrane during operation. 3) Some researchers have also constructed selectivity through the specific adsorption of ions and groups within the membrane. For example, CN1179919964A, crown ether nanomaterials are incorporated into the membrane liquid by blending to form a mixed matrix membrane. The specific effect of crown ethers and monovalent ions is utilized to prepare the membrane by casting, achieving cation selectivity. However, although this method of preparing a mixed matrix membrane by blending avoids the introduction of a surface modification layer, it increases the complexity of the process and is not suitable for the mature production process of the current membrane material production line, making it difficult to achieve large-scale preparation. In addition, the current acid recovery cation selective separation membrane still has problems such as poor selectivity, slow acid recovery efficiency (low hydrogen ion flux), and poor stability. Therefore, it is still necessary to develop an acid recovery cation selective separation membrane with better comprehensive performance to meet the needs of actual applications. Summary of the Invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the present invention utilizes the secondary interactions between microporous polymer PN molecules to regulate the self-assembly of polymer molecules, promotes the formation of ordered ion channels in the membrane, and at the same time utilizes the rich hydrogen bond network formed after membrane formation to construct hydrogen ion transition sites, thereby promoting the selective and rapid transmission of hydrogen ions. It provides a homogeneous, simple preparation method, compatible with the roll-to-roll membrane production line production process, and a preparation method for an acid recovery cation selective separation membrane with high hydrogen ion flux and excellent ion selectivity.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for preparing a cation selective separation membrane for acid recovery comprises the following steps:

[0008] Step 1: using a PN polymer having a structural formula as shown in formula (1) as a precursor for preparing a cation selective separation membrane;

[0009]

[0010] Step 2: reacting and grafting the PN polymer with the sulfonated monomer to obtain the SPN polymer with ion exchange groups;

[0011] Step 3: dissolving the SPN polymer in an organic solvent to prepare a membrane solution, pouring the membrane solution into a mold and drying it to prepare a cation selective separation membrane.

[0012] Furthermore, in step 1, the degree of polymerization m of the PN polymer is in the range of 10 2 -10 7 between.

[0013] Furthermore, in formula (1), R is derived from a bisphenol monomer, and its specific structural formula is shown in any one of formulas (2), where * represents the connection position:

[0014]

[0015] Furthermore, the PN polymer used in the present invention is prepared by reacting a bisphenol monomer (such as 1,1-bi(2-naphthol), 2,2'-dihydroxybiphenyl, 3,3,3,3-tetramethyl-1,1-spiro BI(indane)-6,6-diol) with a ketone monomer (such as isatin). The specific reaction method can be referred to the literature "One-step synthesis of hydroxyl-functionalized fully carbon main chain PIMs via a Friedel-Crafts reaction for efficient gasseparation" (Separation and Purification Technology 262(2021)118313).

[0016] Further, in step 2, the sulfonated monomer is Here, x is an integer ranging from 1 to 5.

[0017] Furthermore, in step 2, the reactive grafting site on the PN polymer is a -OH site or a -NH- site, and the molar ratio of the total number of reactive sites of the PN polymer (i.e., the total molar amount of -OH and -NH-) to the sulfonated monomer is 1:0.1-1.

[0018] Furthermore, in step 2, the reaction temperature is 50°C-120°C, the reaction time is 12h-48h, and a hydrogenation reagent (such as NaH, potassium carbonate, or sodium tert-butoxide) is added during the reaction. The molar ratio of the amount of hydrogenation reagent added to the total number of reactive sites in the PN polymer is 0.1-1:1.

[0019] Furthermore, in step 3, the organic solvent includes one of common organic solvents such as tetrahydrofuran, dimethyl sulfoxide, N,N-dimethylformamide, N-methylpyrrolidone, etc., and the drying temperature is between 30°C and 120°C.

[0020] The method for preparing a cation-selective separation membrane for acid recovery provided by the present invention is compatible with existing homogeneous membrane casting production processes and avoids the introduction of a surface modification layer, significantly improving various performance indicators of the membrane. Its beneficial effects are specifically reflected in the following aspects:

[0021] (1) Based on the inherent microporous properties of PN polymers, monomers with sulfonic acid groups are confined to the intrinsic micropores of the polymer. Selective ion transport channels are constructed using the inherent confined pores of the microporous polymer, avoiding the uncontrollable ion transport channel problem caused by the random distribution of ion exchange sites in traditional random polymers. Specifically, the grafted monomers with sulfonic acid groups first form fixed anion groups in the micropores, forming hydrogen ion binding sites, ensuring the membrane material's ability to transport hydrogen ions. In addition, the rigidity of PN as the base polymer of the membrane material also ensures excellent swelling resistance after membrane formation.

[0022] (2) The abundant -OH sites and -NH- sites in the PN structure not only facilitate the modification of the base polymer, but also the π-π interactions between its molecular structures and the abundant hydrogen bonding sites promote hydrogen bond self-assembly. By controlling the membrane material forming temperature, the directional self-assembly of hydrogen bonds is regulated, the molecular structure tends to be ordered, and the formation of regular channels in the membrane is promoted, which effectively constructs the ion transport channel in the membrane and lays the foundation for the precise regulation of the membrane material performance. The intrinsically formed hydrogen bond network is more regularly oriented under the confinement of the sulfonated self-microporous polymer. It can not only maintain the original dipole-dipole interaction, but also form ion-dipole interaction with the sulfonic acid group, forming a continuous weak interaction network in the confined micropores. When hydrogen ions enter the membrane, they can produce the same weak interaction with the corresponding sites, reducing the energy barrier for hydrogen ions to enter the membrane.

[0023] (3) The functional sites of the PN polymer are modified and grafted using sulfonic acid monomers. By controlling the amount of different sulfonic acid monomers added, the microphase separation and micropore structure in the membrane are further regulated, thereby changing the distribution and size of the free volume in the membrane, and constructing the selectivity between hydrogen ions and other cations through size screening.

[0024] (4) The membrane synthesis routes designed by the present invention can be achieved through solution-to-liquid phase reactions. The homogeneous membrane liquid finally prepared reduces the membrane production process and matches the current industrial production casting production line, enabling batch roll-to-roll production. The membrane can be formed by coating and drying the production line, avoiding the introduction of surface modification methods and significantly increasing the stability of the membrane material. At the same time, the self-microporous channel construction ensures low membrane resistance, reduces the energy consumption of the membrane material during the application process of the membrane group device, and ensures high flux and high selectivity during the use of the membrane material. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a photograph of the cation selective separation membrane prepared in Example 1 of the present invention.

[0026] Figure 2 The membrane material prepared in Example 1 of the present invention is 1 HNMR result analysis.

[0027] Figure 3 This is the infrared test result analysis of the microporous polymer PN prepared in Example 1 of the present invention and the cation selective separation membranes prepared in Examples 1-4.

[0028] Figure 4 This is a diagram of the ion selectivity test device used in the present invention, wherein MCl x It is FeCl2, CuCl2 or LiCl.

[0029] Figure 5 The H of the membrane materials prepared in Examples 1-4 of the present invention and the commercial membrane CIMS + / Fe 2+ Selective test results.

[0030] Figure 6 is the H of the membrane material prepared in Examples 1-4 of the present invention + / Li + Selective test results.

[0031] Figure 7 is the H of the membrane material prepared in Examples 1-4 of the present invention + / Cu 2+ Selective test results.

[0032] Figure 8 is the H of the membrane material prepared in Examples 5-7 of the present invention + / Fe 2+ Selective test results.

[0033] Figure 9 is the H of the membrane material prepared in Examples 8-9 of the present invention + / Fe 2+Selective test results. DETAILED DESCRIPTION

[0034] The technical solutions of the present invention are further illustrated by examples below. These technical solutions are only intended to make the present invention easier to understand by researchers in the field, and are not intended to limit the scope of protection of the present invention. All variations and extensions made using the present invention are within the scope of protection of the present invention.

[0035] 1. Preparation of cation selective separation membrane

[0036] Example 1

[0037] The steps for preparing the acid recovery cation selective separation membrane provided in this embodiment are as follows:

[0038] Preparation of PN polymer: Measure 28.6g of 1,1-bi(2-naphthol) in a round-bottom three-necked flask, then add 150mL of dichloromethane, place the three-necked flask in an ice bath circulation system, insert a stirring paddle, and start stirring until all the solids are dissolved. Add 18.6g of indigo red, then add 15mL of trifluoroacetic acid dropwise, and continue stirring until all the indigo red is dissolved to form a light red uniform solution. Set the condensation circulation temperature to -10°C. After the temperature stabilizes, slowly add 150mL of trifluoromethanesulfonic acid dropwise to the three-necked flask. After reacting for 48 hours, the viscosity of the system rises sharply. Slowly pour the reactant into pure water to obtain a crude PN product. After drying this product, dissolve it in the organic solvent DMSO, and pour the dissolved solution into pure water again to precipitate. Repeat this operation three times, and then dry the precipitated polymer to obtain the final PN polymer: The degree of polymerization m is 10 2 -10 7 between.

[0039] Preparation of SPN polymer: Weigh 2.5g of PN polymer and place it in a single-necked round-bottom flask. Place the round-bottom flask in an oil bath and add a magnet. Add the organic solvent DMF and stir to dissolve. After dissolution is complete, add 0.3g of propane sultone and 0.13g of NaH as a hydrogen extraction agent. Raise the oil bath temperature to 70°C and react for 12 hours. Then, precipitate the reactant and dry it to obtain a crude SPN product. Dissolve the crude SPN product in the organic solvent DMF and precipitate it again. Repeat this process three times to obtain a high-purity SPN polymer.

[0040] Where y = 0.2.

[0041] Membrane Preparation: 1g of SPN polymer was weighed into a 20mL round-bottom flask. 9g of the organic solvent DMSO was added to the flask. A magnetic stirrer was added and the mixture was stirred on a magnetic stirrer until dissolved to obtain a membrane solution. The membrane solution was evenly cast onto a glass plate and dried at 70°C for 24 hours to form a membrane, resulting in a cation-selective separation membrane with high hydrogen ion selectivity.

[0042] The photo of the cation selective separation membrane prepared in this example is shown in FIG. Figure 1 As shown, 1 The results of H NMR tests are as follows Figure 2 As shown in the figure, the infrared absorption data of the film tested by Fourier infrared spectrometer is as follows Figure 3 shown.

[0043] Example 2

[0044] The steps for preparing the acid recovery cation selective separation membrane provided in this embodiment are as follows:

[0045] Preparation of PN polymer: The same preparation method as in Example 1 was used.

[0046] Preparation of SPN polymer: The same preparation method as in Example 1 was used, except that the amount of propane sultone added was adjusted to 0.7 g.

[0047] Preparation of the membrane: The same preparation method as in Example 1 was adopted.

[0048] Example 3

[0049] The steps for preparing the acid recovery cation selective separation membrane provided in this embodiment are as follows:

[0050] Preparation of PN polymer: The same preparation method as in Example 1 was used.

[0051] Preparation of SPN polymer: The same preparation method as in Example 1 was used, except that the amount of propane sultone added was adjusted to 1.3 g.

[0052] Preparation of the membrane: The same preparation method as in Example 1 was adopted.

[0053] Example 4

[0054] The steps for preparing the acid recovery cation selective separation membrane provided in this embodiment are as follows:

[0055] Preparation of PN polymer: The same preparation method as in Example 1 was used.

[0056] Preparation of SPN polymer: The same preparation method as in Example 1 was used, except that the amount of propane sultone added was adjusted to 1.9 g.

[0057] Preparation of the membrane: The same preparation method as in Example 1 was adopted.

[0058] Example 5

[0059] The steps for preparing the acid recovery cation selective separation membrane provided in this embodiment are as follows:

[0060] Preparation of PN polymer: The same preparation method as in Example 1 was used, except that the reaction time was reduced to 24 h.

[0061] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0062] Preparation of membrane: The same preparation method as in Example 1 was used, except that the drying temperature was adjusted to 60°C.

[0063] Example 6

[0064] The steps for preparing the acid recovery cation selective separation membrane provided in this embodiment are as follows:

[0065] Preparation of PN polymer: The same preparation method as in Example 5 was adopted.

[0066] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0067] Preparation of membrane: The same preparation method as in Example 1 was used, except that the drying temperature was adjusted to 50°C.

[0068] Example 7

[0069] The steps for preparing the acid recovery cation selective separation membrane provided in this embodiment are as follows:

[0070] Preparation of PN polymer: The same preparation method as in Example 5 was adopted.

[0071] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0072] Preparation of membrane: The same preparation method as in Example 1 was used, except that the drying temperature was adjusted to 40°C.

[0073] Example 8

[0074] The steps for preparing the acid recovery cation selective separation membrane provided in this embodiment are as follows:

[0075] Preparation of PN polymer: The same preparation method as in Example 1 was used, except that 28.6 g of 1,1-bi(2-naphthol) was replaced with 18.8 g of 2,2'-dihydroxybiphenyl. The structural formula of the resulting PN polymer is shown below:

[0076] The degree of polymerization m is 10 2 -10 7 between.

[0077] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0078] Preparation of the membrane: The same preparation method as in Example 1 was adopted.

[0079] Example 9

[0080] The steps for preparing the acid recovery cation selective separation membrane provided in this embodiment are as follows:

[0081] Preparation of PN polymer: The same preparation method as in Example 1 was used, except that 28.6 g of 1,1-bi(2-naphthol) was replaced with 30.8 g of 3,3,3,3-tetramethyl-1,1-spiro-BI(indane)-6,6-diol. The structural formula of the resulting PN polymer is shown below:

[0082] The degree of polymerization m is 10 2 -10 7 between.

[0083] Preparation of SPN polymer: The same preparation method as in Example 1 was used.

[0084] Preparation of the membrane: The same preparation method as in Example 1 was adopted.

[0085] 2. Membrane flux and ion selectivity test

[0086] Use Figure 4 The device shown is at 10mAcm -2 The H of the cation selective separation membrane obtained in Examples 1 to 4 was tested under the current density + / Fe 2+ Ion selectivity (commercial monovalent / divalent cation selective separation membrane CIMS tested under the same conditions was used as a control), H + / Li + Ion selectivity and H + / Cu 2+ Ion selectivity, and H of the cation selective separation membranes obtained in Examples 5 to 9 + / Fe 2+ Ion selectivity. The specific method is:

[0087] The device is divided into four chambers by three membranes: (cathode) polar chamber | concentration chamber | desalination chamber | polar chamber (anode). The membranes between the polar chamber and the concentration chamber, and between the polar chamber and the desalination chamber are commercial common anion exchange membranes AGU, whose main purpose is to separate the polar chamber solution from the internal chamber solution. The membrane to be tested is between the desalination chamber and the concentration chamber. 0.3M sodium sulfate solution circulates in the polar chamber, 0.01M hydrochloric acid solution circulates in the concentration chamber, and 1M hydrochloric acid and 0.267M MCl circulate in the desalination chamber. X (M is the corresponding metal ion for ion selectivity test, and x is the valence state of the metal ion). After the device circulates the corresponding solution, voltage is applied on both sides. Under the action of the electric field, the cations in the desalination chamber migrate from the desalination chamber to the concentration chamber. Due to the ion selectivity of the membrane, hydrogen ions pass through in large quantities while other ions pass through in small quantities. Finally, the hydrogen ion concentration and metal ion concentration in the concentration chamber are measured, and the ion flux is calculated based on the concentration:

[0088]

[0089] Where: C t is the ion concentration after the concentration chamber has been running for t hours, C0 is the initial ion concentration of the concentration chamber, and A m is the effective area of the membrane in the test, t is the operating time, V is the final volume of the concentration chamber, and the final selectivity P is calculated by the following formula:

[0090]

[0091] in, is the initial hydrogen ion concentration in the desalination chamber, and is the concentration of the corresponding metal ion M, and x is the valence of the metal ion.

[0092] The H of the cation selective separation membrane obtained in Examples 1 to 4 + / Fe 2+ Ion selectivity, H + / Li + Ion selectivity and H + / Cu 2+ The ion selectivity test results are as follows Figure 5 、 Figure 6 、 Figure 7 As shown in the figure, it can be seen that compared with the commercial membrane CIMS, the H + / Fe 2+The selectivity exceeds that of commercial membranes by an order of magnitude, which proves the effectiveness of the design concept of the cation selective separation membrane of the present invention. With the increase of the amount of sulfonated monomer grafting, the flux of hydrogen ions gradually increases, further verifying the promoting effect of the introduction of sulfonated monomers on hydrogen ion transmission. The reason why the selectivity of the membrane material prepared in Example 4 is lower than that of the commercial membrane CIMS is precisely because the excessive addition of sulfonic acid groups allows other ions besides hydrogen ions to be transmitted through the excessive sulfonic acid group sites even without the assistance of the hydrogen bond network, which also proves the effectiveness of the sulfonic acid group in promoting ion transmission. The above results verify the promoting effect of the present invention on the selectivity of hydrogen ion flux based on the self-microporous polymer confinement construction of ion channels and the promotion of hydrogen ion transmission by hydrogen bond networks, and prove the effectiveness of the technical route proposed by the present invention.

[0093] The H of the cation selective separation membrane obtained in Examples 5 to 7 + / Fe 2+ Ion selectivity such as Figure 8 As shown in the figure, it can be seen that the selectivity test results of Examples 5-7 show that as the film forming temperature gradually decreases, the selectivity of the membrane shows a certain upward trend. By regulating the molecular motion state by temperature, the secondary interactions in the membrane are further regulated, including π-π interactions and hydrogen bond self-assembly, which is conducive to the adjustment of the molecular structure in the membrane to order during film formation. Based on the characteristics of the intrinsic micropores of the microporous polymer, it is conducive to reducing the discrete degree of the micropore size distribution in the membrane, making the pore size tend to be consistent. Under this effect, the energy barrier for large-sized ions to enter the pore channel increases significantly, and it is difficult for them to enter the membrane for transmission, thereby forming hydrogen ion selectivity. In contrast, hydrogen ions are easier to enter the membrane due to their relatively smaller size. At this time, due to the rich hydrogen bond network formed in the membrane, the transmission of hydrogen ions in the membrane is significantly promoted, and finally high selectivity and high flux performance as shown in the test results are obtained.

[0094] The H of the cation selective separation membrane obtained in Examples 8 to 9 + / Fe 2+ Ion selectivity such as Figure 9 The selectivity of the cation selective separation membrane obtained in Example 8 is lower than that of the membrane material prepared in Example 1. This is mainly because the weaker self-microporous properties of 2,2'-dihydroxybiphenyl and the weaker π-π interaction make the internal pore distribution more disordered after membrane formation, resulting in its final selectivity being lower than that of Example 1. The selectivity of the membrane material prepared in Example 9 is higher than that of Example 1 because the bisphenol monomer used in Example 9 retains the characteristics of hydrogen bond self-assembly and self-microporous properties while having stronger pore dispersion properties, making the internal pores more evenly dispersed, constructing a relatively higher density of pores, significantly increasing the hydrogen ion flux, and achieving higher selectivity.

[0095] Analysis of the nuclear magnetic resonance (NMR) results of Example 1 and the infrared (IR) results of Examples 1-4 verified the successful synthesis of the proposed target membrane material structure, demonstrating the feasibility of the proposed technical route. The initial scale-up preparation of the membrane material also demonstrated the potential for industrial production. The prepared membrane is a completely homogeneous membrane material, avoiding the problems of poor stability, numerous defects, and low ion flux associated with traditional surface-modified membranes and mixed-matrix membranes, thus verifying the advanced membrane-making process of the present invention. The examples show that the SPN polymer prepared by reactive grafting of sulfonated monomers onto the microporous PN polymer inherently possesses sulfonate groups, which impart ion exchange functionality to the polymer, significantly promoting ion transport and improving the efficiency of the acid recovery process. In summary, the method for preparing an acid-recovery cation-selective separation membrane provided by the present invention is simple in process, superior in performance, and readily scalable. It has excellent results in addressing current key issues in the acid recovery field, has broad application needs and potential, and is of great significance to environmental protection and resource recovery.

Claims

1. A method for preparing a cation selective separation membrane for acid recovery, characterized in that: The steps include: Step 1: using a PN polymer having a structural formula as shown in formula (1) as a precursor for preparing a cation selective separation membrane; Step 2: reacting and grafting the PN polymer with the sulfonated monomer to obtain the SPN polymer with ion exchange groups; Step 3: dissolving the SPN polymer in an organic solvent to prepare a membrane solution, pouring the membrane solution into a mold and drying it to prepare a cation selective separation membrane.

2. The preparation method according to claim 1, wherein: In step 1, the degree of polymerization m of the PN polymer is in the range of 10 2 -10 7 between.

3. The preparation method according to claim 1, wherein: In formula (1), R is derived from a bisphenol monomer.

4. The preparation method according to claim 1 or 3, characterized in that In formula (1), the structural formula of R is shown in any one of formulas (2), where * represents the connection position:

5. The preparation method according to claim 1, wherein: In step 2, the sulfonated monomer is Here, x is an integer ranging from 1 to 5.

6. The preparation method according to claim 1, wherein: In step 2, the reactive grafting site on the PN polymer is a -OH site or a -NH- site, and the molar ratio of the total reactive sites of the PN polymer to the sulfonated monomer is 1:0.1-1.

7. The preparation method according to claim 1 or 6, characterized in that: In step 2, the reaction temperature of the grafting reaction is 50° C.-120° C., the reaction time is 12 h-48 h, a hydrogenation reagent is added during the reaction, and the molar ratio of the amount of the hydrogenation reagent added to the total number of reaction sites of the PN polymer is 0.1-1:

1.

8. The preparation method according to claim 1, wherein: In step 3, the drying temperature is between 30°C and 120°C.

9. A cation selective separation membrane for acid recovery prepared by the preparation method according to any one of claims 1 to 8.

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