Preparation method of surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane

Abstract

The present application discloses a method for preparing a surface-sprayed semi-permeable monovalent / polyvalent selective cation separation membrane. The method uses an aromatic cation exchange membrane as a base membrane and chloromethyl polymer and polyethyleneimine as raw materials. Through surface spraying and solvent evaporation, polyethyleneimine and chloromethyl polymer undergo a cross-linking reaction on the base membrane to form a semi-permeable surface modified layer. The surface modified layer is further cross-linked and quaternized to prepare a monovalent / polyvalent selective cation exchange membrane with a stable modified layer. The chloromethyl polymer / polyethyleneimine cross-linked layer modified on the surface of the cation exchange membrane of the present invention provides a dense pore size screening functional layer, while at the same time providing reaction sites for realizing an electrical double-layer structure. The present invention is not only simple to operate and has high monovalent / polyvalent ion selectivity, but also effectively overcomes the defect of poor stability of the modified layer of conventional monovalent / polyvalent selective cation exchange membranes.

Description

Technical Field

[0001] The present invention relates to a monovalent / multivalent selective cation exchange membrane and a preparation method thereof, and particularly relates to a preparation method of the monovalent / multivalent selective cation exchange membrane based on the principle of solvent mutual solubility by using a chloromethylated polymer material as a cross-linking and permeating polymer. Background Art

[0002] In order to meet the needs of sustainable economic and environmental development, single / multivalent ion separation has attracted great attention in the fields of resource recovery, pollutant removal and energy utilization. The process covers metal ion extraction, ion selective separation (such as Na + and Mg 2+ ) and reverse electrodialysis for energy collection / conversion. Due to its excellent screening performance and high energy efficiency, electrodialysis ion screening is a highly promising method for resource recovery and energy reuse. Monovalent cation-selective electrodialysis is widely used in seawater concentration to produce NaCl, groundwater softening to produce drinking water, salt lake lithium extraction, and heavy metal ion removal. Monovalent cation-selective permeable membranes (MCPMs) are key components that ensure the high-speed passage of monovalent cations and the effective retention of multivalent cations during the electrodialysis process.

[0003] Monovalent cation-selective permeable membranes can be prepared through covalent crosslinking (CN112739446A, 2021-04-30), surface modification (CN113019141B, 2022-08-02), polymer blending (CN114713295A, 2022-07-08), and other emerging technologies (CN113262648B, 2022-07-22). Compared to other technologies, surface modification requires only a single functional layer on the base membrane surface to achieve ion separation performance, with minimal impact on ion flux and membrane surface resistance. Therefore, surface modification is the most effective and promising technology for preparing MCPMs. Based on size sieving and the Donan effect, dense charged layers or ion channels can be constructed on the surface of ion exchange membranes through methods such as layer-by-layer self-assembly, surface spraying, electrodeposition, and interfacial polymerization. This can significantly improve the monovalent selectivity of cation exchange membranes without sacrificing membrane conductivity. However, the weak physical interaction between the selective separation layer and the base membrane cannot meet the requirements of the ion exchange membrane's operating environment, resulting in a gradual decline in membrane performance over long-term operation. How to firmly bind the selective separation layer to the base membrane surface has been one of the most vexing issues plaguing the further development and industrialization of monovalent selective ion exchange membranes. Summary of the Invention

[0004] In response to the above-mentioned technical problems existing in the prior art, the purpose of the present invention is to provide a method for preparing a surface-sprayed semi-permeable mono / multivalent selective cation separation membrane. We have designed a mono / multivalent selective cation exchange membrane with a semi-permeable double-layer surface structure. Its separation mechanism is to use a dense surface modification layer to screen ions of different valences through pore size screening and double-layer action.

[0005] The method for preparing a surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane is characterized in that an aromatic cation exchange membrane is used as a base membrane, and chloromethyl polymer and polyethyleneimine are used as raw materials. Through surface spraying and solvent evaporation processes, polyethyleneimine and chloromethyl polymer undergo a cross-linking reaction on the base membrane to form a semi-permeable surface modified layer, and the surface modified layer is further cross-linked and quaternized to prepare a monovalent / multivalent selective cation exchange membrane with a stable modified layer.

[0006] The method for preparing a surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane is characterized by comprising the following steps:

[0007] 1) Fix the unmodified aromatic cation exchange membrane on a transparent glass plate, purge the commercial membrane surface with nitrogen to remove dust and impurities, and then place the glass plate in an oven for a period of time to preheat the commercial membrane;

[0008] 2) placing the preheated and fixed cation exchange membrane obtained in step 1) on a hot plate in a fume hood, spraying a mixed solution of chloromethyl polymer and polyethyleneimine on the membrane surface, and then placing the fixed membrane in an oven to obtain a semi-permeable surface modification layer through a solvent evaporation process;

[0009] 3) immersing the cation exchange membrane having a dense semi-permeable surface modified layer modified with a chloromethyl polymer and polyethyleneimine obtained in step 2) in an aqueous solution of glutaraldehyde to crosslink unreacted amine groups, and then removing the membrane after immersion for a period of time;

[0010] 4) Immersing the cross-linked cation exchange membrane with a dense semi-permeable surface modified layer obtained in step 3) in a hexamethylenediamine solution to further quaternize the membrane to form a surface double layer structure, thereby obtaining the surface-sprayed semi-permeable mono / multivalent selective cation separation membrane.

[0011] The method for preparing a surface-sprayed semi-permeable mono / multivalent selective cation separation membrane is characterized in that the aromatic cation exchange membrane is sulfonated polyphenylene sulfone SPPSU, sulfonated polyphenylene ether SPPO or sulfonated polyetheretherketone SPEEK.

[0012] The method for preparing a surface-sprayed semi-permeable mono / multivalent selective cation separation membrane is characterized in that the chloromethyl polymer is at least one of chloromethylated polysulfone, chloromethylated polyethersulfone, chloromethylated polyphenylene ether, and chloromethylated polystyrene.

[0013] The method for preparing a surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane is characterized in that the average relative molecular weight of the polyethyleneimine used is 1800-25000.

[0014] The method for preparing a surface-sprayed semi-permeable mono / multivalent selective cation separation membrane is characterized in that the solvent used in the mixed solution of the chloromethyl polymer and polyethyleneimine is an organic solvent that can dissolve polymer materials, and is selected from at least one of N,N-dimethylformamide DMF, N,N-dimethylacetamide DMAC, N-methylpyrrolidone NMP, and dimethyl sulfoxide DMSO.

[0015] The method for preparing a surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane is characterized in that, in step 2), after the modified layer solvent is completely volatilized, the secondary spraying and secondary solvent evaporation processes are repeated.

[0016] The method for preparing a surface-sprayed semi-permeable mono / multivalent selective cation separation membrane is characterized in that the concentration of the glutaraldehyde aqueous solution in step 3) is 0.05-0.2M, preferably 0.1M, and the immersion time in the glutaraldehyde aqueous solution is 0.5-2h, preferably 1h.

[0017] The method for preparing a surface-sprayed semi-permeable mono / multivalent selective cation separation membrane is characterized in that the concentration of the hexamethylenediamine solution in step 4) is 0.05-0.2M, preferably 0.1M, and the immersion time in the hexamethylenediamine solution is 10-15 hours.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The present invention uses a surface spraying and solvent evaporation process to form a dense modified layer by cross-linking polyethyleneimine and a chloromethylated high molecular polymer. The modified layer is then further cross-linked and quaternized. Due to electrostatic repulsion and pore size screening effects, the presence of positively charged PEI / quaternary ammonium groups and a dense screening layer enables the cation exchange membrane to have certain screening properties for different metal cations. In an alkaline environment, excess aldehyde molecules undergo a Cannizzaro reaction, forming a negatively charged -COO- layer in the outermost layer. This double-layer structure induces metal cations to enter the positively charged screening layer, further improving the selectivity of MCPMs. In addition, the modified polymer material and the cation exchange membrane substrate have good solvent miscibility, allowing the modified layer to semi-permeate into the base membrane, thereby avoiding shedding and stability issues of the modified layer of surface-modified MCPMs. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the preparation process of the monovalent / multivalent selective cation separation membrane of the present invention;

[0021] Figure 2 The electron microscope images and transmission electron microscopy images of the upper and lower surfaces and the cross section of the separation membrane obtained in Example 1 are characterized.

[0022] Figure 3 This is the infrared spectrum of the semi-permeable dense surface modified layer of the separation membrane obtained in Example 1;

[0023] Figure 4 a XPS spectrum of the modified layer after spraying, Figure 4 b XPS spectrum of the modified layer after cross-linking. DETAILED DESCRIPTION

[0024] The preferred embodiments of the present invention are described in detail below so that the content and features of the present invention can be easily understood by researchers in this field, thereby making a more detailed definition of the protection scope of the present invention.

[0025] Example 1

[0026] Step 1: 10g of polyphenylsulfone (MW10000, purchased from Tianjin Yanjin Technology Co., Ltd.) was dissolved in 100ml of dichloromethane in a three-necked flask at 10°C for 1.5 hours. A mixture of 25ml of dichloromethane and 4ml of chlorosulfonic acid was then added and sulfonated at 10°C for 6 hours. The mixture was washed with deionized water until neutral and then dried to obtain sulfonated polyphenylsulfone (MW11600). The sulfonated polyphenylsulfone was then prepared with dimethyl sulfoxide (DMSO) to form a 14wt% membrane-forming solution. This solution was then cast into a glass mold and dried on a 60°C hot plate to obtain a sulfonated polyphenylsulfone cation exchange membrane.

[0027] A 6cm*6cm sulfonated polyphenylsulfone cation exchange membrane was fixed on a transparent glass with adhesive tape, and the surface dust and impurities were removed by nitrogen purge. The membrane was then preheated in a 65°C oven for 5 minutes.

[0028] Step 2: 6 g of polysulfone (purchased from Tianjin Yanjin Technology Co., Ltd., MW1327.58), 4.086 g of polymethanol, 16.4 ml of trimethylsilyl chloride, and 800 ml of chloroform were placed in a three-necked flask, heated in a water bath at 50°C for 5 h, and dried to obtain chloromethylated polysulfone (MW1434.08).

[0029] Prepare the self-made chloromethylated polysulfone (MW1434.08) and polyethyleneimine (MW1800), fully disperse them in dimethyl sulfoxide (DMSO), and prepare a mixed solution of chloromethylated polysulfone and polyethyleneimine (MW1800) with a mass fraction of 1%. Figure 1 As shown, the membrane preheated in step 1 was fixed on a 65°C hot plate, 1 ml of a mixed solution of 1 wt% chloromethylated polysulfone and 1 wt% polyethyleneimine (MW1800) was sprayed on the surface of the base membrane by nitrogen, and placed in a 65°C oven to wait for the solvent to evaporate.

[0030] Step 3: After all the solvent has evaporated, repeat step 2 and spray 1 ml of a mixed solution of 1 wt% chloromethylated polysulfone (MW1434.08) and 1 wt% polyethyleneimine (MW1800) for the second time. Place the membrane in a 65°C oven and wait for the solvent to evaporate to obtain a selective cation exchange membrane with a semi-permeable modified layer on the surface.

[0031] Step 4: Soak the selective cation exchange membrane with the semipermeable surface modification layer in a 0.1M glutaraldehyde solution for 1 hour to initiate a crosslinking reaction. Then, soak it in a 0.1M hexamethylenediamine solution (30°C) for 12 hours. This results in a monovalent / polyvalent cation selective separation membrane with a semipermeable, dense surface modification layer.

[0032] The electron microscope images of the upper and lower surfaces and the cross section of the separation membrane obtained in Example 1 are shown in the transmission electron microscope image. Figure 2 , Figure 2 The electron micrograph of the upper surface of the modified layer a, the electron micrograph of the lower surface of the modified layer a1, the electron micrograph of the cross-section of the modified membrane b and the transmission electron micrograph of the cross-section of the modified membrane b1.

[0033] from Figure 2 (a) and Figure 2 (a1) The electron microscope image of the modified membrane surface shows a clear difference between the upper and lower surfaces of the modified layer. Unlike the corrugated upper surface, the lower surface of the modified layer presents a spider web structure, confirming the existence of a semi-permeable surface modified layer. Figure 2 (b) and Figure 2(b1) The cross-sectional electron microscopy and projection electron microscopy images of the modified membrane clearly show the existence of the modified layer and the miscible region between the modified layer and the base membrane.

[0034] The infrared spectrum of the semi-permeable dense surface modified layer of the separation membrane obtained in Example 1 is shown in FIG. Figure 3 The XPS spectra of the modified layer after spraying obtained in step 3 of Example 1 and the modified layer after cross-linking obtained in step 4 are summarized in Figure 4 middle, Figure 4 (a) XPS spectrum corresponding to the modified layer after spraying, Figure 4 (b) XPS spectrum corresponding to the modified layer after cross-linking.

[0035] from Figure 3 The infrared spectrum and Figure 4 The XPS spectrum shows changes in the C=O and COOR bonds, confirming the presence of a double layer after quaternization. Monovalent and divalent cation selective electrodialysis experiments were conducted using this mono / multivalent selective cation exchange membrane with a semi-permeable dense surface modification layer.

[0036] According to the test method of the reference (Recovery of chemically degraded polyethyleneimine byare-modification method: prolonging the lifetime of cation exchange membranes (DOI: 10.1039 / C5RA27916J), the test system was changed to Na + / Mg 2+ is the test system, and Na + and Mg 2+ The initial concentration was 0.05 mol / L. After 1 hour, the solution in the desalination chamber was analyzed by ion chromatography for Na + and Mg 2+ The concentration of Na + Relative Mg 2+ Selective permeability value. In order to compare the effect of the modification, the same experiment of one- and two-valent selective electrodialysis of the unmodified commercial original membrane was also carried out. The results are shown in Table 1. It can be seen from Table 1 that after the spray cross-linking modification, the Na + Relative Mg 2+ The permeability selectivity increased from 0.81 to 1.70 compared with the unmodified commercial membrane.

[0037] Example 2

[0038] Take 6g of polyethersulfone (purchased from Tianjin Yanjin Technology Co., Ltd. MW1500), 4.086g of polymethanol, 16.4ml of trimethylsilyl chloride, and 800ml of chloroform and put them in a three-necked flask, heat in a water bath at 50℃ for 5h, and dry to obtain chloromethylated polyethersulfone (MW1605.5).

[0039] The mixed solution of 1 wt% chloromethylated polysulfone (MW 1434.08) and 1 wt% polyethyleneimine (MW 1800) in steps 2-3 of Example 1 was replaced with a mixed solution of 1 wt% chloromethylated polyethersulfone (MW 1605.5) and 1 wt% polyethyleneimine (MW 1800). The remaining materials, operating methods, implementation processes and other manufacturing conditions were the same as those in Example 1.

[0040] The Na + / Mg 2+ The unit selectivity of the system was tested under the same conditions as in Example 1, and the results are shown in Table 1. As can be seen from Table 1, the Na+ relative Mg2+ selectivity of the spray-crosslinked modified membrane increased from 0.81 to 1.60 compared with the unmodified commercial membrane.

[0041] Example 3

[0042] 6 g of polyphenylene ether (purchased from Tianjin Yanjin Technology Co., Ltd., MW50000), 4.086 g of polymethanol, 16.4 ml of trimethylsilyl chloride, and 800 ml of chloroform were placed in a three-necked flask, heated in a water bath at 50°C for 5 h, and dried to obtain chloromethylated polyphenylene ether (MW55000).

[0043] The mixed solution of 1 wt% chloromethylated polysulfone (MW 1434.08) and 1 wt% polyethyleneimine (MW 1800) in steps 2-3 of Example 1 was replaced with a mixed solution of 1 wt% chloromethylated polyphenylene ether (MW 55000) and 1 wt% polyethyleneimine (MW 1800). The remaining materials, operating methods, implementation processes and other manufacturing conditions were the same as those in Example 1.

[0044] The Na + / Mg 2+ The unit selectivity of the system was tested under the same conditions as in Example 1, and the results are shown in Table 1. As can be seen from Table 1, the Na+ relative Mg2+ selectivity of the spray-crosslinked modified membrane increased from 0.81 to 1.88 compared with the unmodified commercial membrane.

[0045] Example 4

[0046] Take 6g of polystyrene (purchased from Tianjin Yanjin Technology Co., Ltd. MW104.149), 4.086g of polymethanol, 16.4ml of trimethylsilyl chloride, and 800ml of chloroform and put them in a three-necked flask, heat in a water bath at 50℃ for 5h, and dry to obtain chloromethylated polystyrene (MW386.956).

[0047] The mixed solution of 1 wt% chloromethylated polysulfone (MW 1434.08) and 1 wt% polyethyleneimine (MW 1800) in steps 2-3 of Example 1 was replaced with a mixed solution of 1 wt% chloromethylated polystyrene (MW 386.956) and 1 wt% polyethyleneimine (MW 1800). The remaining materials, operating methods, implementation processes and other manufacturing conditions were the same as those in Example 1.

[0048] The Na + / Mg 2+ The unit selectivity of the system was tested under the same conditions as in Example 1, and the results are shown in Table 1. As can be seen from Table 1, the Na+ relative Mg2+ selectivity of the spray-crosslinked modified membrane increased from 0.81 to 1.65 compared with the unmodified commercial membrane.

[0049] Example 5

[0050] 6 g of polyphenylene ether (MW50000), 4.086 g of polymethanol, 16.4 ml of trimethylsilyl chloride, and 800 ml of chloroform were placed in a three-necked flask, heated in a water bath at 50°C for 5 h, and dried to obtain chloromethylated polyphenylene ether (MW55000).

[0051] The mixed solution of 1 wt% chloromethylated polysulfone (MW 1434.08) and 1 wt% polyethyleneimine (MW 1800) in steps 2-3 of Example 1 was replaced with a mixed solution of 1 wt% chloromethylated polyphenylene ether (MW 55000) and 1 wt% polyethyleneimine (MW 3000). The remaining materials, operating methods, implementation processes and other manufacturing conditions were the same as those in Example 1.

[0052] The Na + / Mg 2+ The unit selectivity of the system was tested under the same conditions as in Example 1, and the results are shown in Table 1. As can be seen from Table 1, the selectivity of Na+ to Mg2+ increased from 0.81 to 1.81 after spray cross-linking modification compared with the unmodified commercial membrane.

[0053] Example 6

[0054] 6 g of polyphenylene ether (MW50000), 4.086 g of polymethanol, 16.4 ml of trimethylsilyl chloride, and 800 ml of chloroform were placed in a three-necked flask, heated in a water bath at 50°C for 5 h, and dried to obtain chloromethylated polyphenylene ether (MW55000).

[0055] The mixed solution of 1 wt% chloromethylated polysulfone (MW 1434.08) and 1 wt% polyethyleneimine (MW 1800) in steps 2-3 of Example 1 was replaced with a mixed solution of 1 wt% chloromethylated polyphenylene ether (MW 55000) and 1 wt% polyethyleneimine (MW 10000). The remaining materials, operating methods, implementation processes and other manufacturing conditions were the same as those in Example 1.

[0056] The Na + / Mg 2+ The unit selectivity of the system was tested under the same conditions as in Example 1, and the results are shown in Table 1. As can be seen from Table 1, the Na+ relative Mg2+ selectivity of the spray-crosslinked modified membrane increased from 0.81 to 1.93 compared with the unmodified commercial membrane.

[0057] Example 7

[0058] 6 g of polyphenylene ether (MW50000), 4.086 g of polymethanol, 16.4 ml of trimethylsilyl chloride, and 800 ml of chloroform were placed in a three-necked flask, heated in a water bath at 50°C for 5 h, and dried to obtain chloromethylated polyphenylene ether (MW55000).

[0059] The mixed solution of 1 wt% chloromethylated polysulfone (MW 1434.08) and 1 wt% polyethyleneimine (MW 1800) in steps 2-3 of Example 1 was replaced with a mixed solution of 1 wt% chloromethylated polyphenylene ether (MW 55000) and 1 wt% polyethyleneimine (MW 25000). The remaining materials, operating methods, implementation processes and other manufacturing conditions were the same as those in Example 1.

[0060] The Na + / Mg 2+ The unit selectivity of the system was tested under the same conditions as in Example 1, and the results are shown in Table 1. As can be seen from Table 1, the Na+ relative Mg2+ selectivity of the spray-crosslinked modified membrane increased from 0.81 to 2.17 compared with the unmodified commercial membrane.

[0061] Example 8

[0062] 10g of polyphenylene ether (MW 50,000) was dissolved in 100ml of dichloromethane in a three-necked flask at 10°C for 1.5 hours. A mixture of 25ml of dichloromethane and 4ml of chlorosulfonic acid was then added and sulfonated at 10°C for 6 hours. The mixture was washed with deionized water until neutral and then dried to obtain sulfonated polyphenylene ether (MW 64336). The sulfonated polyphenylene ether was then prepared with dimethyl sulfoxide (DMSO) to form a 14wt% membrane solution, cast into a glass mold, and dried on a 60°C hot plate to produce a sulfonated polyphenylene ether cation exchange membrane.

[0063] The sulfonated polyphenylene sulfone cation exchange membrane in step 1 of Example 1 was replaced with sulfonated polyphenylene ether, and the remaining materials, operating methods, implementation processes and other manufacturing conditions were the same as those in Example 1.

[0064] The Na + / Mg 2+ The unit selectivity of the system was tested under the same conditions as in Example 1, and the results are shown in Table 1. As can be seen from Table 1, the Na+ relative Mg2+ selectivity of the spray-crosslinked modified membrane increased from 0.81 to 1.72 compared with the unmodified commercial membrane.

[0065] Example 9

[0066] 10g of polyetheretherketone (PEEK) (purchased from Tianjin Yanjin Technology Co., Ltd., MW 328.31) was dissolved in 100ml of dichloromethane in a three-necked flask at 10°C for 1.5 hours. Then, 25ml of a mixture of dichloromethane and chlorosulfonic acid was added and sulfonated at 10°C for 6 hours. The mixture was washed with deionized water until neutral and then dried to obtain sulfonated PEEK (MW 409.64). The PEEK was then prepared with dimethyl sulfoxide (DMSO) to form a 14wt% membrane-forming solution. This solution was then cast into a glass mold and dried on a 60°C hot plate to produce a sulfonated PEEK cation exchange membrane.

[0067] The sulfonated polyphenylsulfone cation exchange membrane in step 1 of Example 1 was replaced with sulfonated polyetheretherketone, and the remaining materials, operating methods, implementation processes and other manufacturing conditions were the same as those in Example 1.

[0068] The Na + / Mg 2+ The unit selectivity of the system was tested under the same conditions as in Example 1, and the results are shown in Table 1. As can be seen from Table 1, the Na+ relative Mg2+ selectivity of the spray-crosslinked modified membrane increased from 0.81 to 1.65 compared with the unmodified commercial membrane.

[0069]

Claims

1. A method for preparing a surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane, characterized in that: Using aromatic cation exchange membrane as the base membrane, chloromethyl polymer and polyethyleneimine as raw materials, through surface spraying and solvent evaporation process, polyethyleneimine and chloromethyl polymer undergo cross-linking reaction on the base membrane to form a semi-permeable surface modification layer, and the surface modification layer is further cross-linked and quaternized to prepare a mono / multivalent selective cation exchange membrane with a stable modification layer.

2. The method for preparing a surface-sprayed semi-permeable mono / multivalent selective cation separation membrane according to claim 1, characterized in that: The following steps are involved: 1) Fix the unmodified aromatic cation exchange membrane on a transparent glass plate, purge the commercial membrane surface with nitrogen to remove dust and impurities, and then place the glass plate in an oven for a period of time to preheat the commercial membrane; 2) placing the preheated and fixed cation exchange membrane obtained in step 1) on a hot plate in a fume hood, spraying a mixed solution of chloromethyl polymer and polyethyleneimine on the membrane surface, and then placing the fixed membrane in an oven to obtain a semi-permeable surface modification layer through a solvent evaporation process; 3) immersing the cation exchange membrane having a dense semi-permeable surface modified layer modified with a chloromethyl polymer and polyethyleneimine obtained in step 2) in an aqueous solution of glutaraldehyde to crosslink unreacted amine groups, and then removing the membrane after immersion for a period of time; 4) Immersing the cross-linked cation exchange membrane with a dense semi-permeable surface modified layer obtained in step 3) in a hexamethylenediamine solution to further quaternize the membrane to form a surface double layer structure, thereby obtaining the surface-sprayed semi-permeable mono / multivalent selective cation separation membrane.

3. The method for preparing a surface-sprayed semi-permeable mono / multivalent selective cation separation membrane according to claim 2, wherein: The aromatic cation exchange membrane is sulfonated polyphenylene sulfone SPPSU, sulfonated polyphenylene ether SPPO or sulfonated polyetheretherketone SPEEK.

4. The method for preparing a surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane according to claim 2, wherein: The chloromethyl polymer is at least one of chloromethylated polysulfone, chloromethylated polyethersulfone, chloromethylated polyphenylene ether, and chloromethylated polystyrene.

5. The method for preparing a surface-sprayed semi-permeable mono / multivalent selective cation separation membrane according to claim 2, wherein: The average relative molecular weight of the polyethyleneimine used is 1800-25000.

6. The method for preparing a surface-sprayed semi-permeable mono / multivalent selective cation separation membrane according to claim 2, wherein: The solvent used in the mixed solution of the chloromethyl polymer and polyethyleneimine is an organic solvent capable of dissolving the polymer material, and is selected from at least one of N,N-dimethylformamide DMF, N,N-dimethylacetamide DMAC, N-methylpyrrolidone NMP, and dimethyl sulfoxide DMSO.

7. The method for preparing a surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane according to claim 2, wherein: Step 2) After the modified layer solvent has completely evaporated, repeat the secondary spraying and secondary solvent evaporation process.

8. The method for preparing a surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane according to claim 2, wherein: In step 3), the concentration of the glutaraldehyde aqueous solution is 0.05-0.2 M, and the immersion time in the glutaraldehyde aqueous solution is 0.5-2 h.

9. The method for preparing a surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane according to claim 8, characterized in that: In step 3), the concentration of the glutaraldehyde aqueous solution is 0.1 M, and the immersion time in the glutaraldehyde aqueous solution is 1 h.

10. The method for preparing a surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane according to claim 2, wherein: In step 4), the concentration of the hexamethylenediamine solution is 0.05-0.2 M, and the immersion time in the hexamethylenediamine solution is 10-15 hours.

11. The method for preparing a surface-sprayed semi-permeable monovalent / multivalent selective cation separation membrane according to claim 10, characterized in that: The concentration of the hexamethylenediamine solution in step 4) is 0.1M.

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