Monovalent selective cation exchange membrane, preparation method thereof and electrodialysis device

Abstract

The invention discloses a monovalent selective cation exchange membrane, a preparation method thereof and an electrodialysis device, and relates to the technical field of ion exchange membranes. The preparation method of the monovalent selective cation exchange membrane comprises the following steps: soaking a cation exchange membrane loaded with a photoinitiator in a co-deposition modification solution containing dopamine, polyethyleneimine and chitosan, carrying out a co-deposition reaction under radiation initiation of ultraviolet light, taking out the cation exchange membrane after the reaction is finished to obtain a co-deposition modified membrane, and then carrying out heat treatment to obtain the monovalent selective cation exchange membrane. The monovalent selective cation exchange membrane is obtained. The ultraviolet light initiator and chitosan with certain adhesion are introduced in the co-deposition process, so that the co-deposition speed of dopamine and polyethyleneimine on the surface of the film is promoted, meanwhile, a base film and a co-deposition modified layer can be connected, the stability of the modified film is improved, the pore size screening effect is improved, and the service life of the film is prolonged. And moreover, ion exchange sites of the modified membrane are not occupied, monovalent and multivalent cations are effectively separated, and meanwhile, good ion flux is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of ion exchange membranes, and in particular to a monovalent selective cation exchange membrane and a preparation method thereof and an electrodialysis device. Background Art

[0002] As lithium is the main component of batteries, efficient acquisition and stable supply of lithium resources are the top priorities for the development of the new energy industry. Among the existing methods of obtaining lithium resources, lithium extraction from salt lakes has a natural cost advantage. However, the working conditions of different salt lakes around the world vary greatly. How to extract high-purity lithium from salt lakes with excessively high impurity content is one of the difficulties that need to be overcome in the current lithium extraction from salt lakes. As a new separation technology, membrane separation technology has been increasingly widely used in the separation industry due to its low energy consumption, easy operation, and environmental friendliness.

[0003] Electrodialysis is a membrane separation technology driven by electric field. It has the unique advantages of high selectivity, long service life and high water recovery rate. It has become one of the key methods for extracting lithium from salt lakes. In the application of electrodialysis, the performance of ion exchange membrane as the core component is the most critical. Ordinary ion exchange membranes have good separation effects between ions of different electrical properties, but cannot achieve Li + 、Na + , K + Mg 2+ , Ca 2+ Selective separation of ions of equal charge but different valence states.

[0004] The main separation mechanisms of monovalent selective ion exchange membranes include electrostatic repulsion effect, pore size screening effect and hydration energy difference. Common preparation methods for monovalent selective ion exchange membranes include polymer blending, surface modification and other methods. Polymer blending modification is to regulate the density or hydrophilicity of the overall structure of the matrix, which will result in a large membrane resistance. Surface modification is to build a positively charged modified layer on the basis of the cation exchange membrane, and separate monovalent and polyvalent ions through electrostatic repulsion and pore size screening. It is the current mainstream preparation method. Common surface modification methods include electrostatic deposition, layer-by-layer self-assembly, electrodeposition and biomimetic co-deposition. Among them, electrostatic deposition, layer-by-layer self-assembly, electrodeposition and other modification methods mostly rely on the electrostatic adsorption of the base membrane to adsorb the modified substance on the membrane surface, which has poor stability, and the adsorption amount of the modified substance is limited, and the modification effect is not good.

[0005] Bionic co-deposition is an emerging membrane surface modification method. It has developed from the mussel biomimetic deposition technology represented by dopamine. It is simple to operate and has a controllable process. It has received widespread attention in recent years. Dopamine is extracted from mussels and undergoes oxidative self-polymerization under alkaline oxygen-containing conditions. In this process, the presence of various groups such as catechol, amino, and oxidized quinone enables dopamine to be adsorbed on different substrate surfaces through hydrogen bonds, chelation, electrostatic adsorption, and covalent interactions. Taking advantage of the good adhesion of dopamine, positively charged modifying substances, such as polyethyleneimine, are mixed with dopamine and co-deposited on the membrane surface, which can promote the oxidative polymerization rate of dopamine, increase the deposition amount, uniformity, and stability of the modifying substances on the membrane surface, and improve the modification effect.

[0006] However, even though the chemical reaction between polyethyleneimine and dopamine can shorten the deposition time, the required reaction time is still relatively long, usually between 2h and 12h. In order to further shorten the reaction time, researchers added initiators during the co-deposition process. Common initiators include copper sulfate and hydrogen peroxide, ammonium persulfate, ferric chloride, etc. Although the addition of initiators increases the reaction rate, it also promotes the reaction of more reactive groups on dopamine with polyethyleneimine and its own oxidation polymerization, which weakens the connection with the base film. In addition, Cu 2+ , Fe 3+ The introduction of metal ions will also occupy the ion exchange sites of the basement membrane and reduce the ion flux.

[0007] In view of this, the present invention is proposed. Summary of the invention

[0008] The object of the present invention is to provide a monovalent selective cation exchange membrane and a preparation method thereof and an electrodialysis device.

[0009] The present invention is achieved in that:

[0010] In a first aspect, the present invention provides a method for preparing a monovalent selective cation exchange membrane, the preparation method comprising: immersing a cation exchange membrane loaded with a photoinitiator in a co-deposition modification solution containing dopamine, polyethyleneimine and chitosan, initiating a co-deposition reaction under ultraviolet radiation, taking out the membrane after the reaction is completed to obtain a co-deposition modified membrane, and then heat treating the membrane to obtain a monovalent selective cation exchange membrane.

[0011] In an optional embodiment, the method for preparing the cation exchange membrane loaded with a photoinitiator comprises: immersing the cation exchange membrane in a solution containing a photoinitiator to adsorb the photoinitiator, and then removing excess water from the surface of the cation exchange membrane.

[0012] In an optional embodiment, the photoinitiator includes at least one of 4-azidobenzoic acid, tetrabutylammonium azide, benzophenone, benzophenone monoxide, azobisisobutylimidazoline hydrochloride, pyridinium imidazoline and isopropylthioxanthone.

[0013] In an optional embodiment, the mass concentration of the photoinitiator in the solution containing the photoinitiator is 0.001%-3%, and the immersion time is 10 min to 120 min.

[0014] In an optional embodiment, the solvent in the solution containing the photoinitiator includes at least one of methanol, deionized water and ethanol.

[0015] In an optional embodiment, the mass percentage concentration of dopamine in the co-deposition modification solution is 0.01%-4%, the mass percentage concentration of polyethyleneimine is 0.01%-4%, and the mass percentage concentration of chitosan is 0.01%-1%;

[0016] And / or, the mass ratio of the dopamine to the polyethyleneimine in the co-deposition modification solution is (0.1-5):1.

[0017] In an optional embodiment, the initiation wavelength of the ultraviolet light is 100 nm-400 nm; and the reaction time of the co-deposition reaction is 30 min to 120 min.

[0018] In an optional embodiment, the heat treatment is performed at a temperature of 60°C to 95°C, and the time is 5 min to 30 min.

[0019] In a second aspect, the present invention provides a monovalent selective cation exchange membrane, which is prepared by the method for preparing a monovalent selective cation exchange membrane as described in any one of the aforementioned embodiments.

[0020] In a third aspect, the present invention provides an electrodialysis device equipped with the monovalent selective cation exchange membrane as described in the above embodiment.

[0021] The present invention has the following beneficial effects:

[0022] The preparation method of the monovalent selective cation exchange membrane provided by the present invention is to introduce an ultraviolet light initiator and chitosan with certain adhesion during the co-deposition process, wherein, under the radiation of ultraviolet light, the photoinitiator absorbs energy and cracks to generate free radicals, and the free radicals can effectively promote the reaction of dopamine and polyethyleneimine, shorten the reaction time, and can also react with the carbon-carbon double bonds on the base film or insert into single bonds such as CC, CH, OH, NH, etc. to form a stable covalent structure, thereby enhancing the connection effect between the base film and the modified substance. Therefore, the photoinitiator is both an initiator and a cross-linking agent, and the introduction of the photoinitiator can increase the deposition amount and stability of both dopamine and polyethyleneimine on the surface of the cation exchange membrane. Moreover, the photoinitiator of the present invention has a small amount of use, rapid initiation, no pollution, and is conducive to promoting the co-deposition speed of dopamine and polyethyleneimine on the membrane surface. At the same time, it can also connect the base membrane and the co-deposition modified layer, improve the stability of the modified membrane, and improve the aperture screening effect. In addition, the photoinitiator does not contain heavy metal cations and will not occupy the ion exchange sites of the modified membrane. It effectively separates single and multivalent cations while having good ion flux. Chitosan has multiple active groups such as amino groups and hydroxyls. Under the action of the photoinitiator, it can react with dopamine and polyethyleneimine to improve stability. At the same time, it is also conducive to enhancing the surface positive charge density and improving the electrostatic repulsion of the modified membrane. The preparation method of the monovalent selective cation exchange membrane provided by the present invention is simple and fast, has repeatability, requires a small amount of reagents, and has low cost. During the preparation process, by adjusting the type and concentration of the initiator, the ratio of dopamine and polyethyleneimine and other conditions, the density and positive charge density of the modified layer can be effectively controlled, so that the obtained modified membrane has the performance of separating single and multivalent cations under different application conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0024] Figure 1 Graphs showing the morphology changes of the modified film before (A) and after (B) the co-deposition reaction of Example 1 of the present invention;

[0025] Figure 2 Graphs showing the morphology changes of the modified film before (A) and after (B) the co-deposition reaction of Example 6 of the present invention;

[0026] Figure 3 This is the surface electron microscope image of the base film of Comparative Example 1;

[0027] Figure 4This is an electron microscopic image of the surface of the modified film obtained after the co-deposition reaction in Example 1;

[0028] Figure 5 This is an electron microscopic image of the surface of the modified film obtained after the co-deposition reaction in Example 2;

[0029] Figure 6 This is an infrared comparison diagram of the co-deposited modified film of Example 1 and the base film of Comparative Example 1. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0031] The invention provides a method for preparing a monovalent selective cation exchange membrane, which comprises: immersing a cation exchange membrane loaded with a photoinitiator in a co-deposition modification solution containing dopamine, polyethyleneimine and chitosan, initiating a co-deposition reaction under ultraviolet radiation, taking out the membrane after the reaction is completed to obtain a co-deposition modified membrane, and then heat-treating the membrane to obtain a monovalent selective cation exchange membrane.

[0032] The present invention introduces ultraviolet light initiator and chitosan with certain adhesion in the co-deposition process, wherein, under the radiation of ultraviolet light, the photoinitiator absorbs energy and cracks to generate free radicals, and the free radicals can effectively promote the reaction of dopamine and polyethyleneimine, shorten the reaction time, and also can react with the carbon-carbon double bond on the base film or insert into single bonds such as CC, CH, OH, NH, etc., to form a stable covalent structure, and enhance the connection effect between the base film and the modified substance. Therefore, the photoinitiator is both an initiator and a cross-linking agent, and the introduction of the photoinitiator can improve the deposition amount and stability of both dopamine and polyethyleneimine on the surface of the cation exchange membrane. Chitosan has multiple active groups such as amino and hydroxyl groups, and while reacting with dopamine and polyethyleneimine under the action of the photoinitiator, it can also effectively enhance the positive charge density. In addition, the photoinitiator in the present invention does not contain heavy metal cations, will not occupy the ion exchange sites of the modified membrane, and effectively separates single multivalent cations while having good ion flux.

[0033] Specifically, the method for preparing the monovalent selective cation exchange membrane of the present invention comprises the following steps:

[0034] S1. Prepare a cation exchange membrane loaded with a photoinitiator.

[0035] The cation exchange membrane is immersed in a solution containing a photoinitiator to adsorb the photoinitiator, and then excess water on the surface of the cation exchange membrane is removed.

[0036] "Solution containing a photoinitiator" means that the photoinitiator is dissolved in a solvent and prepared into a solvent with a mass concentration of 0.001%-3% of the photoinitiator, wherein the photoinitiator includes but is not limited to at least one of 4-azidobenzoic acid, tetrabutylammonium azide, benzophenone, benzophenone monoxide, azobisisobutylimidazoline hydrochloride, pyridinium imidazoline and isopropylthioxanthone. The solvent includes but is not limited to at least one of methanol, deionized water and ethanol. In other embodiments of the present invention, the concentration of the photoinitiator can be, for example, any one of 0.001%, 0.003%, 0.005%, 0.008%, 0.01%, 0.03%, 0.05%, 0.08%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 2%, 3%, or any range between two of them.

[0037] In the present invention, a cation exchange membrane is used as a base membrane and is immersed in a solution containing a photoinitiator for 10 to 120 minutes. During the immersion process, the cation exchange membrane fully adsorbs the photoinitiator. Subsequently, excess surface water is removed and removed to obtain a cation exchange membrane loaded with a photoinitiator.

[0038] The loading amount of the photoinitiator in the cation exchange membrane loaded with the photoinitiator is about 0.001%-3%.

[0039] S2. Prepare a co-deposition modification solution.

[0040] Dopamine, polyethyleneimine and chitosan are added to a buffer solution to prepare a co-deposition modification solution with a mass percentage concentration of 0.01%-4% dopamine, a mass percentage concentration of 0.01%-4% polyethyleneimine, and a mass percentage concentration of 0.01%-1% chitosan. In addition, the mass ratio of dopamine to polyethyleneimine in the co-deposition modification solution also needs to meet (0.1-5):1.

[0041] The buffer solution includes but is not limited to a Tris-HCl buffer solution with a pH of 8.5. The buffer solution is used to mix dopamine, polyethyleneimine and chitosan. Dopamine has good adhesion, and polyethyleneimine is a positively charged modified substance. Polyethyleneimine and dopamine are mixed together and co-deposited on the membrane surface, which can promote the oxidative polymerization rate of dopamine, increase the deposition amount, uniformity and stability of the modified substance on the membrane surface, and improve the modification effect. Chitosan has multiple active groups such as amino groups and hydroxyl groups. When reacting with dopamine and polyethyleneimine under the action of a photoinitiator, it can also effectively enhance the positive charge density.

[0042] S3. Co-deposition reaction.

[0043] The cation exchange membrane loaded with photoinitiator in step S1 is fixed on the plate frame, and the co-deposition modified solution obtained in step S2 is poured on the cation exchange membrane loaded with photoinitiator to completely cover it, and then placed under an ultraviolet LED light source for radiation initiation, and the initiation wavelength of the ultraviolet light is 100nm-400nm; the reaction time of the co-deposition reaction is 30min-120min. After the co-deposition reaction is completed, it is taken out and the excess solution on the membrane surface is removed to obtain a co-deposition modified membrane.

[0044] S4. Heat treatment.

[0045] The co-deposition modified membrane is subjected to heat treatment at a temperature of 60° C. to 95° C. for a time of 5 min to 30 min to obtain a monovalent selective cation exchange membrane.

[0046] The monovalent selective cation exchange membrane prepared by the preparation method of the monovalent selective cation exchange membrane has good stability and screening effect, and also has good ion flux. The monovalent selective cation exchange membrane can be widely used in electrodialysis.

[0047] For example, the present invention also provides an electrodialysis device equipped with the above-mentioned valence-selective cation exchange membrane.

[0048] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0049] Example 1

[0050] This embodiment provides a monovalent selective cation exchange membrane, and the preparation method thereof comprises the following steps:

[0051] (1) Cation exchange membrane (Hangzhou Lanran Technology Co., Ltd., CT-4) is used as the base membrane. Its morphology is shown in the figure below. Figure 1 As shown in (A), the sample was immersed in an ethanol solution of 4-azidobenzoic acid having a mass percentage concentration of 0.01% for 30 minutes, and then the surface moisture was removed;

[0052] (2) adding dopamine, PEI (Mw = 7w Da) and chitosan to a Tris-HCl buffer solution of pH = 8.5, and stirring to obtain a co-deposition modified solution, wherein the mass percentage concentration of dopamine is 0.5%, the mass percentage concentration of PEI (Mw = 7w Da) is 0.5%, and the mass percentage concentration of chitosan is 0.01%;

[0053] (3) fixing the cation exchange membrane loaded with the photoinitiator in step (1) on a plate frame, pouring the above co-deposition modified solution on the cation exchange membrane loaded with the photoinitiator to completely cover it, placing it under an ultraviolet LED light source (λ=254nm) for radiation initiation, taking it out after reacting for 30 minutes, and removing the excess solution on the membrane surface;

[0054] (4) The modified membrane after co-deposition was heat treated at 80°C for 20 min to obtain a monovalent selective cation exchange membrane, the morphology of which is shown in FIG. Figure 1 As shown in (B), compared Figure 1 As can be seen in (A) and (B), the morphology has changed, indicating that dopamine and polyethyleneimine are successfully co-deposited on the membrane surface and the density of the membrane is enhanced.

[0055] Example 2

[0056] This embodiment provides a monovalent selective cation exchange membrane, and the preparation method thereof comprises the following steps:

[0057] (1) A cation exchange membrane (CT-4, Hangzhou Lanran Technology Co., Ltd.) was used as a base membrane and immersed in an ethanol solution of 0.01% benzophenone by mass for 30 min, and then the surface moisture was removed;

[0058] (2) adding dopamine, PEI (Mw = 7w Da) and chitosan to a Tris-HCl buffer solution of pH = 8.5, and stirring to obtain a co-deposition modified solution, wherein the mass percentage concentration of dopamine is 0.5%, the mass percentage concentration of PEI (Mw = 7w Da) is 0.5%, and the mass percentage concentration of chitosan is 0.01%;

[0059] (3) Fixing the cation exchange membrane loaded with photoinitiator in step (1) on a plate frame, pouring the above co-deposition modified solution on the cation exchange membrane loaded with photoinitiator to completely cover it, placing it under a UV LED light source (λ=365nm) for radiation initiation, taking it out after reacting for 30 minutes, and removing excess solution on the membrane surface;

[0060] (4) The modified membrane after co-deposition was heat treated at 80°C for 20 minutes to obtain a monovalent selective cation exchange membrane.

[0061] Example 3

[0062] This embodiment provides a monovalent selective cation exchange membrane, and the preparation method thereof comprises the following steps:

[0063] (1) A cation exchange membrane (CT-4, Hangzhou Lanran Technology Co., Ltd.) was used as a base membrane and immersed in an ethanol solution of azobisisobutylimidazoline hydrochloride with a mass percentage concentration of 0.01% for 30 min, and then the surface moisture was removed;

[0064] (2) adding dopamine, PEI (Mw = 7w Da) and chitosan to a Tris-HCl buffer solution of pH = 8.5, and stirring to obtain a co-deposition modified solution, wherein the mass percentage concentration of dopamine is 0.5%, the mass percentage concentration of PEI (Mw = 7w Da) is 0.5%, and the mass percentage concentration of chitosan is 0.01%;

[0065] (3) fixing the cation exchange membrane loaded with the photoinitiator in step (1) on a plate frame, pouring the above co-deposition modified solution on the cation exchange membrane loaded with the photoinitiator to completely cover it, placing it under an ultraviolet LED light source (λ=352nm) for radiation initiation, taking it out after reacting for 30 minutes, and removing the excess solution on the membrane surface;

[0066] (4) The modified membrane after co-deposition was heat treated at 80°C for 20 minutes to obtain a monovalent selective cation exchange membrane.

[0067] Example 4

[0068] This embodiment provides a monovalent selective cation exchange membrane, and the preparation method thereof comprises the following steps:

[0069] (1) A cation exchange membrane (CT-4, Hangzhou Lanran Technology Co., Ltd.) was used as a base membrane and immersed in an ethanol solution of 4-azidobenzoic acid with a mass percentage concentration of 0.1% for 30 minutes, and then the surface moisture was removed;

[0070] (2) adding dopamine, PEI (Mw = 7w Da) and chitosan to a Tris-HCl buffer solution of pH = 8.5, and stirring to obtain a co-deposition modified solution, wherein the mass percentage concentration of dopamine is 0.5%, the mass percentage concentration of PEI (Mw = 7w Da) is 0.5%, and the mass percentage concentration of chitosan is 0.01%;

[0071] (3) fixing the cation exchange membrane loaded with the photoinitiator in step (1) on a plate frame, pouring the above co-deposition modified solution on the cation exchange membrane loaded with the photoinitiator to completely cover it, placing it under an ultraviolet LED light source (λ=254nm) for radiation initiation, taking it out after reacting for 30 minutes, and removing the excess solution on the membrane surface;

[0072] (4) The modified membrane after co-deposition was heat treated at 80°C for 20 minutes to obtain a monovalent selective cation exchange membrane.

[0073] Example 5

[0074] This embodiment provides a monovalent selective cation exchange membrane, and the preparation method thereof comprises the following steps:

[0075] (1) A cation exchange membrane (CT-4, Hangzhou Lanran Technology Co., Ltd.) was used as a base membrane and immersed in an ethanol solution of 4-azidobenzoic acid with a mass percentage concentration of 0.01% for 30 minutes, and then the surface moisture was removed;

[0076] (2) adding dopamine, PEI (Mw = 7w Da) and chitosan to a Tris-HCl buffer solution of pH = 8.5, and stirring to obtain a co-deposition modified solution, wherein the mass percentage concentration of dopamine is 2%, the mass percentage concentration of PEI (Mw = 7w Da) is 2%, and the mass percentage concentration of chitosan is 0.1%;

[0077] (3) fixing the cation exchange membrane loaded with the photoinitiator in step (1) on a plate frame, pouring the above co-deposition modified solution on the cation exchange membrane loaded with the photoinitiator to completely cover it, placing it under an ultraviolet LED light source (λ=254nm) for radiation initiation, taking it out after reacting for 30 minutes, and removing the excess solution on the membrane surface;

[0078] (4) The modified membrane after co-deposition was heat treated at 80°C for 20 minutes to obtain a monovalent selective cation exchange membrane.

[0079] Example 6

[0080] This embodiment provides a monovalent selective cation exchange membrane, and the preparation method thereof comprises the following steps:

[0081] (1) Cation exchange membrane (Fuji Film Corporation, Japan, Type 12) is used as the base membrane, and its morphology is shown in the figure Figure 2 As shown in (A), the sample was immersed in a 0.01% mass percent 4-azidobenzoic acid solution for 30 minutes, and then the surface moisture was removed;

[0082] (2) adding dopamine, PEI (Mw = 7w Da) and chitosan to a Tris-HCl buffer solution of pH = 8.5, and stirring to obtain a co-deposition modified solution, wherein the mass percentage concentration of dopamine is 0.5%, the mass percentage concentration of PEI (Mw = 7w Da) is 0.5%, and the mass percentage concentration of chitosan is 0.01%;

[0083] (3) fixing the cation exchange membrane loaded with the photoinitiator in step (1) on a plate frame, pouring the above co-deposition modified solution on the cation exchange membrane loaded with the photoinitiator to completely cover it, placing it under an ultraviolet LED light source (λ=254nm) for radiation initiation, taking it out after reacting for 30 minutes, and removing the excess solution on the membrane surface;

[0084] (4) The modified film after co-deposition was heat treated at 80°C for 20 min to obtain a morphology as shown in Figure 2 The monovalent selective cation exchange membrane shown in (B) is compared Figure 2 As can be seen in (A) and (B), the morphology has changed, indicating that dopamine and polyethyleneimine are successfully co-deposited on the membrane surface and the density of the membrane is enhanced.

[0085] Example 7

[0086] This embodiment provides a monovalent selective cation exchange membrane, and the preparation method thereof comprises the following steps:

[0087] (1) A cation exchange membrane (CT-4, Hangzhou Lanran Technology Co., Ltd.) was used as a base membrane and immersed in an ethanol solution of 4-azidobenzoic acid with a mass percentage concentration of 3% for 120 min, and then the surface moisture was removed;

[0088] (2) adding dopamine, PEI (Mw = 1w Da) and chitosan to a Tris-HCl buffer solution at pH = 8.5, and stirring to obtain a co-deposition modified solution, wherein the mass percentage concentration of dopamine is 0.01%, the mass percentage concentration of PEI (Mw = 7w Da) is 0.01%, and the mass percentage concentration of chitosan is 1%;

[0089] (3) fixing the cation exchange membrane loaded with the photoinitiator in step (1) on a plate frame, pouring the above co-deposition modified solution on the cation exchange membrane loaded with the photoinitiator to completely cover it, placing it under an ultraviolet LED light source (λ=254nm) for radiation initiation, taking it out after reacting for 60 minutes, and removing the excess solution on the membrane surface;

[0090] (4) The modified membrane after co-deposition was heat treated at 95°C for 5 minutes to obtain a monovalent selective cation exchange membrane.

[0091] Example 8

[0092] This embodiment provides a monovalent selective cation exchange membrane, and the preparation method thereof comprises the following steps:

[0093] (1) A cation exchange membrane (CT-4, Hangzhou Lanran Technology Co., Ltd.) was used as a base membrane and immersed in an ethanol solution of 4-azidobenzoic acid with a mass percentage concentration of 0.01% for 30 minutes, and then the surface moisture was removed;

[0094] (2) adding dopamine, PEI (Mw = 7w Da) and chitosan to a Tris-HCl buffer solution at pH = 8.5, and stirring to obtain a co-precipitation modified solution, wherein the mass percentage concentration of dopamine is 0.4%, the mass percentage concentration of PEI (Mw = 7w Da) is 4%, and the mass percentage concentration of chitosan is 0.5%;

[0095] (3) fixing the cation exchange membrane loaded with the photoinitiator in step (1) on a plate frame, pouring the above co-deposition modified solution on the cation exchange membrane loaded with the photoinitiator to completely cover it, placing it under an ultraviolet LED light source (λ=254nm) for radiation initiation, taking it out after reacting for 120 minutes, and removing the excess solution on the membrane surface;

[0096] (4) The modified membrane after co-deposition was heat treated at 60°C for 30 minutes to obtain a monovalent selective cation exchange membrane.

[0097] Comparative Example 1

[0098] This comparative example provides a cation exchange base membrane (Hangzhou Lanran Technology Co., Ltd., CT-4) that has not been processed by the preparation method provided by the present invention.

[0099] Comparative Example 2

[0100] This comparative example is basically the same as Example 1, except that 4-azidobenzoic acid is not added in this comparative example, and specifically comprises the following steps:

[0101] (1) adding dopamine, PEI (Mw = 7w Da) and chitosan to a Tris-HCl buffer solution of pH = 8.5, and stirring to obtain a co-deposition modified solution, wherein the mass percentage concentration of dopamine is 0.5%, the mass percentage concentration of PEI (Mw = 7w Da) is 0.5%, and the mass percentage concentration of chitosan is 0.01%;

[0102] (2) A cation exchange membrane (CT-4, Hangzhou Lanran Technology Co., Ltd.) was used as a base membrane and fixed on a plate frame. The above co-deposition modification solution was poured onto the cation exchange membrane loaded with a photoinitiator to completely cover it, and the co-deposition reaction was carried out for 30 minutes;

[0103] (3) The modified membrane after co-deposition was heat treated at 80°C for 20 minutes to obtain a monovalent selective cation exchange membrane.

[0104] Comparative Example 3

[0105] This comparative example is basically the same as Example 5, except that chitosan is not added in this comparative example, and specifically comprises the following steps:

[0106] (1) A cation exchange membrane (CT-4, Hangzhou Lanran Technology Co., Ltd.) was used as a base membrane and immersed in a 0.01% mass percent 4-azidobenzoic acid solution for 30 min, and then the surface moisture was removed;

[0107] (2) adding dopamine and PEI (Mw = 7w Da) to a Tris-HCl buffer solution at pH = 8.5, and stirring the solution to obtain a co-deposition modification solution, wherein the mass percentage concentration of dopamine is 2%, and the mass percentage concentration of PEI (Mw = 7w Da) is 2%;

[0108] (3) fixing the cation exchange membrane loaded with the photoinitiator in step (1) on a plate frame, pouring the above co-deposition modified solution on the cation exchange membrane loaded with the photoinitiator to completely cover it, placing it under an ultraviolet LED light source (λ=254nm) for radiation initiation, taking it out after reacting for 30 minutes, and removing the excess solution on the membrane surface;

[0109] (4) The modified membrane after co-deposition was heat treated at 80°C for 20 minutes to obtain a monovalent selective cation exchange membrane.

[0110] Comparative Example 4

[0111] This comparative example is basically the same as Example 1, except that the initiator added in this comparative example is FeCl3, and specifically comprises the following steps:

[0112] (1) A cation exchange membrane (CT-4, Hangzhou Lanran Technology Co., Ltd.) was used as a base membrane and immersed in a 0.01% FeCl3 solution for 30 min, and then the surface moisture was removed;

[0113] (2) adding dopamine, PEI (Mw = 7w Da) and chitosan to a Tris-HCl buffer solution of pH = 8.5, and stirring to obtain a co-deposition modified solution, wherein the mass percentage concentration of dopamine is 2%, the mass percentage concentration of PEI (Mw = 7w Da) is 2%, and the mass percentage concentration of chitosan is 0.1%;

[0114] (3) fixing the cation exchange membrane loaded with the photoinitiator in step (1) on a plate frame, pouring the above co-deposition modified solution on the cation exchange membrane loaded with the photoinitiator to completely cover it, reacting for 30 minutes, taking it out, and removing the excess solution on the membrane surface;

[0115] (4) The modified membrane after co-deposition is heat treated at 80°C to obtain a monovalent selective cation exchange membrane.

[0116] Comparative Example 5

[0117] This comparative example is basically the same as Example 1, except that no photoinitiator is added in this comparative example, but radiation initiation is still carried out under ultraviolet light conditions, specifically comprising the following steps:

[0118] (1) Cation exchange membrane (Hangzhou Lanran Technology Co., Ltd., CT-4) as base membrane;

[0119] (2) adding dopamine, PEI (Mw = 7w Da) and chitosan to a Tris-HCl buffer solution of pH = 8.5, and stirring to obtain a co-deposition modified solution, wherein the mass percentage concentration of dopamine is 0.5%, the mass percentage concentration of PEI (Mw = 7w Da) is 0.5%, and the mass percentage concentration of chitosan is 0.01%;

[0120] (3) Fixing the cation exchange membrane in step (1) on a plate frame, pouring the above co-deposition modified solution on the cation exchange membrane to completely cover it, placing it under an ultraviolet LED light source (λ=254nm) for radiation initiation, taking it out after reacting for 30 minutes, and removing excess solution on the membrane surface;

[0121] (4) The modified membrane after co-deposition was heat treated at 80°C for 20 minutes to obtain a monovalent selective cation exchange membrane.

[0122] Comparative Example 6

[0123] This comparative example is substantially the same as Example 1, except that the photoinitiator in this comparative example is camphorquinone and the initiation wavelength is 500 nm.

[0124] Experimental Example 1

[0125] The monovalent selective cation exchange membranes obtained in Examples 1 and 2 and the cation exchange membrane obtained in Comparative Example 1 were scanned by electron microscope. Figure 3 and Figure 4 It can be seen that the base membrane provided in Comparative Example 1 has more holes, while the monovalent selective cation exchange membrane provided in Example 1 has an obvious dense co-deposition modified layer formed on its surface. Figure 5 It can be seen that the morphology of the monovalent selective cation exchange membrane of Example 2 is different from that of the monovalent selective cation exchange membrane of Example 1. The reason for this is that the initiators used in Examples 1 and 2 are different.

[0126] The monovalent selective cation exchange membrane obtained in Example 1 and the cation exchange membrane obtained in Comparative Example 1 were subjected to infrared detection. Figure 6It can be seen that compared with the base film of Comparative Example 1, the co-deposition modified film of Example 1 has a -1 -3600cm -1 The bending vibration peak about NH in the region is enhanced, which is because PDA and PEI have abundant NH groups. -1 On the left and right are the absorption peaks of the quinone structure formed by the oxidation of PDA and the C=N bond generated by the reaction of PDA and PEI, which indicates that PDA and PEI are successfully co-deposited and modified on the membrane surface.

[0127] Experimental Example 2

[0128] The monovalent selective cation exchange membranes prepared in Examples 1-8 and the cation exchange membranes in Comparative Examples 1-6 were tested for membrane resistance and selectivity.

[0129] The membrane resistance test method includes: immersing the cation exchange membrane in a 0.5 mol / L NaCl solution for more than 30 minutes, placing it in a four-chamber tank, and testing its transmembrane voltage (V) under constant current conditions to calculate the membrane resistance (M), the unit is Ω·cm 2 , the calculation formula is:

[0130]

[0131] A m To test the effective membrane area, the unit is m 2 .

[0132] V1 is the transmembrane voltage of the modified membrane, in V, and V0 is the transmembrane voltage of the blank solution, in V.

[0133] I is the test current, unit is A.

[0134] Li-Mg selectivity coefficient The test method includes: the cation exchange membrane to be tested and two anion exchange membranes (Hangzhou Lanran Technology Co., Ltd., ATG-10) are arranged alternately to form a four-chamber electrodialysis. The cathode chamber is a 0.5 mol / L NaCl solution, the concentrated chamber is a mixed solution of LiCl and MgCl2, the concentrations of LiCl and MgCl2 are 0.1 mol / L and 0.5 mol / L respectively (simulating the lithium-magnesium ratio in brine), the dilute chamber is a 0.01 mol / L LiCl solution, and the applied current is 10 mA / cm 2 , calculate the Li and Mg concentrations after 2 h.

[0135]

[0136] J i is the ion flux, in mol / m 2 ·h, t is the test time, the unit is h.

[0137] C t Li is the Li concentration in the concentration chamber after 2 hours, in mol / L, C0 Li It is the Li concentration in the concentration chamber before power is applied, in mol / L.

[0138] C t Mg is the Mg concentration in the concentration chamber after 2 hours, in mol / L, C0 Mg is the Mg concentration in the concentration chamber before power is applied, in mol / L;

[0139] C Li + is the initial Li concentration in the freshwater chamber, in mol / L, C Mg 2+ is the initial Mg concentration in the freshwater chamber, in mol / L.

[0140] The test results are shown in Table 1 and Table 2:

[0141] Table 1. Membrane resistance and selectivity test results of modified membranes of Examples 1-8 and Comparative Examples 1-6

[0142]

[0143] Table 2. Li and Mg ion flux results of Example 1 and Comparative Example 4 during selectivity test

[0144] Membrane samples <![CDATA[Li flux / 10 -6 mol / m 2 s]]> <![CDATA[Mg flux / 10 -6 mol / m 2 s]]> Example 1 0.164 0.062 Comparative Example 4 0.126 0.06

[0145] It can be seen from Table 1 above that: from Examples 1-3, it can be seen that different types of photoinitiators promote the reaction to different degrees, and the membrane surface resistance and selectivity of the modified membranes obtained are different, but all perform well; from Examples 1 and Examples 4-5, it can be seen that as the concentrations of the photoinitiator and the co-deposited modifying substance increase, more modifying substances are co-deposited on the membrane surface, the density of the obtained interfacial polymerization modified membrane is enhanced, and the membrane surface resistance and selectivity of the obtained modified membrane are higher; from Examples 1 and 6, it can be seen that different cation exchange membranes are used as base membranes, and the modification method is also applicable;

[0146] From Examples 1-8 and Comparative Examples 1-2, it can be seen that the modified membrane obtained by using a photoinitiator to promote the co-deposition of dopamine, polyethyleneimine and chitosan exhibits good monovalent selectivity under the action of electrostatic repulsion and pore size screening. And the separation effect is significantly higher than that of the base membrane and the co-deposition modified membrane without a photoinitiator; from Example 1 and Comparative Example 3, it can be seen that the introduction of chitosan can improve the monovalent selectivity of the modified membrane to a certain extent; from Example 1 and Comparative Example 4, it can be seen that compared with Fe 3+Initiator, the ion exchange sites of the modified membrane prepared by the photoinitiator are not occupied, the ion flux is higher, and it is more conducive to industrial application. It can be seen from Example 1 and Comparative Example 5 that without the addition of photoinitiator, the modification effect is not obvious even under ultraviolet irradiation. It can be seen from Example 1 and Comparative Example 6 that visible light initiator is not applicable in this scheme. The reason for analysis may be that it is difficult to control the light conditions.

[0147] In summary, the preparation method of the monovalent selective cation exchange membrane provided by the present invention introduces an ultraviolet light initiator and chitosan with certain adhesion during the co-deposition process, wherein, under the irradiation of ultraviolet light, the photoinitiator absorbs energy and cleaves to generate free radicals, and the free radicals can effectively promote the reaction of dopamine and polyethyleneimine, shorten the reaction time, and can also react with carbon-carbon double bonds or insert into single bonds such as CC, CH, OH, NH, etc., to form a stable covalent structure, and enhance the connection effect between the base membrane and the modified substance. Therefore, the photoinitiator is both an initiator and a cross-linking agent, and the introduction of the photoinitiator can increase the deposition amount and stability of both dopamine and polyethyleneimine on the surface of the cation exchange membrane. Moreover, the photoinitiator of the present invention has a small amount of use, rapid initiation, no pollution, and is conducive to promoting the co-deposition speed of dopamine and polyethyleneimine on the membrane surface. At the same time, it can also connect the base membrane and the co-deposition modified layer, improve the stability of the modified membrane, and improve the aperture screening effect. In addition, the photoinitiator does not contain heavy metal cations and will not occupy the ion exchange sites of the modified membrane. It effectively separates single and multivalent cations while having good ion flux. Chitosan has multiple active groups such as amino groups and hydroxyls. Under the action of the photoinitiator, it can react with dopamine and polyethyleneimine to improve stability. At the same time, it is also conducive to enhancing the surface positive charge density and improving the electrostatic repulsion of the modified membrane. The preparation method of the monovalent selective cation exchange membrane provided by the present invention is simple and fast, has repeatability, requires a small amount of reagents, and has low cost. During the preparation process, by adjusting the type and concentration of the initiator, the ratio of dopamine and polyethyleneimine and other conditions, the density and positive charge density of the modified layer can be effectively controlled, so that the obtained modified membrane has the performance of separating single and multivalent cations under different application conditions.

[0148] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a monovalent selective cation exchange membrane, characterized in that: The preparation method comprises: immersing a cation exchange membrane loaded with a photoinitiator in a co-deposition modification solution containing dopamine, polyethyleneimine and chitosan, initiating a co-deposition reaction under ultraviolet radiation, taking out the membrane after the reaction is completed to obtain a co-deposition modified membrane, and then heat treating the membrane to obtain a monovalent selective cation exchange membrane.

2. The method for preparing a monovalent selective cation exchange membrane according to claim 1, characterized in that: The method for preparing the cation exchange membrane loaded with a photoinitiator comprises: immersing the cation exchange membrane in a solution containing a photoinitiator to adsorb the photoinitiator, and then removing excess water on the surface of the cation exchange membrane.

3. The method for preparing a monovalent selective cation exchange membrane according to claim 2, characterized in that: The photoinitiator includes at least one of 4-azidobenzoic acid, tetrabutylammonium azide, benzophenone, benzophenone monoxide, azobisisobutylimidazoline hydrochloride, pyridinium imidazoline and isopropylthioxanthone.

4. The method for preparing a monovalent selective cation exchange membrane according to claim 2, characterized in that: The mass concentration of the photoinitiator in the solution containing the photoinitiator is 0.001%-3%, and the soaking time is 10min-120min.

5. The method for preparing a monovalent selective cation exchange membrane according to claim 2, characterized in that: The solvent in the solution containing the photoinitiator includes at least one of methanol, deionized water and ethanol.

6. The method for preparing a monovalent selective cation exchange membrane according to claim 1, characterized in that: The mass percentage concentration of dopamine in the co-deposition modification solution is 0.01%-4%, the mass percentage concentration of polyethyleneimine is 0.01%-4%, and the mass percentage concentration of chitosan is 0.01%-1%; And / or, the mass ratio of the dopamine to the polyethyleneimine in the co-deposition modification solution is (0.1-5):

1.

7. The method for preparing a monovalent selective cation exchange membrane according to claim 1, characterized in that: The initiation wavelength of the ultraviolet light is 100nm-400nm; the reaction time of the co-deposition reaction is 30min-120min.

8. The method for preparing a monovalent selective cation exchange membrane according to claim 1, characterized in that: The temperature of the heat treatment is 60°C-95°C, and the time is 5min-30min.

9. A monovalent selective cation exchange membrane, characterized in that: The membrane is prepared by the method for preparing a monovalent selective cation exchange membrane as described in any one of claims 1 to 8.

10. An electrodialysis device, characterized in that: It is equipped with the monovalent selective cation exchange membrane as claimed in claim 9.

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