Preparation method of cation exchange membrane with high monovalent selectivity

Abstract

The invention relates to a preparation method of a cation exchange membrane with high monovalent selectivity. According to the method, a stable modified layer is prepared on the surface of the cation exchange membrane through secondary interfacial polymerization reaction, the surface layer of the membrane is positively charged, and a direct-current electric field is introduced to regulate and control the surface modified layer structure and surface charge of the membrane, so that the positively charged property of the surface of the modified membrane is improved, and meanwhile, the resistance of the modified membrane can be reduced; and the monovalent selective ion exchange membrane with stable performance and high selectivity is prepared. The membrane preparation method is simple to operate, industrial continuous production is easy to realize, and the prepared membrane is high in selectivity and good in stability.

Description

Technical Field

[0001] The invention belongs to the technical field of membrane separation and relates to a method for preparing a cation exchange membrane which is selective for monovalent and multivalent cations. Background Art

[0002] Electrodialysis is an electrically driven membrane separation technology. Under the action of a DC electric field, anions and cations pass through anion membranes and cation membranes respectively to achieve desalination or concentration of salt water. Ion exchange membranes are the core components of electrodialysis devices, and their performance directly determines the separation performance and energy consumption of the electrodialysis process. Conventional ion exchange membranes have good selectivity for ions with different charges (anions and cations), but the separation efficiency for ions with different valence states of the same charge is not high. If a cation exchange membrane that is selective for monovalent and polyvalent ions can be developed, it will greatly expand the application of electrodialysis technology in the fields of lithium extraction from salt lake brine, seawater softening, special ion separation, and desalination of industrial high-salt wastewater.

[0003] Interfacial polymerization is the mainstream preparation technology for commercial nanofiltration membranes, which can prepare a stable and dense surface layer. Patent CN119113809A discloses a method for preparing a high-performance magnesium-lithium separation membrane, in which a small molecule quaternary ammonium salt is mixed with an amine monomer as a solution for secondary interfacial polymerization, and a positive charge distribution surface layer is formed by secondary interfacial polymerization. This method reduces the membrane pore size and improves separation. However, there are few studies on the application of traditional interfacial polymerization technology in the field of ion exchange membrane modification.

[0004] Modification of the surface of conventional ion exchange membranes is an important method for preparing monovalent selective cation exchange membranes. Patent CN114377731A discloses a method for preparing monovalent selective cation exchange membranes by co-deposition modification of dopamine and polyethyleneimine on a cation exchange membrane pre-soaked with iron ions. The introduction of polyethyleneimine makes the membrane surface positively charged, improves the electrostatic repulsion of high-valent ions and makes the membrane monovalent selective. However, the preparation steps of this method are cumbersome, time-consuming, and Na + / Mg 2+ The selectivity coefficient only reached 5.4. Patent CN110813386A uses an alternating electric field to assemble a multilayer of a positively charged PDA-TC layer and a negatively charged PSSMA layer on the surface of a pretreated cation exchange membrane to prepare a monovalent selective cation exchange membrane. However, up to now, it is still a challenging task to prepare a monovalent selective membrane with high selectivity and good stability using a simple and scalable method. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing a high monovalent selective cation exchange membrane in view of the limitations of the current technology. The method prepares a stable modified layer on the surface of a conventional cation exchange membrane through a secondary interfacial polymerization reaction, the membrane surface is positively charged, and a direct current electric field is introduced to regulate the membrane structure and surface charge, thereby improving the positive charge of the modified membrane surface and reducing the membrane resistance of the modified layer, thereby obtaining a monovalent selective ion exchange membrane with stable performance and high selectivity. The membrane preparation method is simple to operate and easy to realize industrial continuous production, and the obtained membrane has high selectivity and good stability.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A method for preparing a high monovalent selectivity cation exchange membrane, the method comprising the following steps:

[0008] (1) dissolving an amine monomer in deionized water to obtain a first aqueous phase solution having a monomer content of 0.1 to 3% by weight; and dissolving an acyl chloride monomer in an organic solvent to obtain an organic phase solution having a monomer content of 0.05 to 0.4% by weight;

[0009] The above-mentioned amino monomer is m-phenylenediamine, piperazine and its derivatives, or polyethyleneimine;

[0010] The piperazine derivative is specifically p-aminobenzoylpiperazine, N-aminoethylpiperazine or sulfonated N-aminoethylpiperazine; the molecular weight of polyethyleneimine is 300 to 100000;

[0011] The above-mentioned acyl chloride monomer is isophthaloyl chloride, terephthaloyl chloride or trimesoyl chloride;

[0012] The organic solvent is n-hexane or cyclohexane;

[0013] (2) The base film is horizontally laid into the groove of the substrate, and then the prepared first aqueous phase solution is poured on the upper surface of the base film. After immersion modification for 1 to 10 minutes, the aqueous phase solution is removed; then the organic phase solution is poured on the upper film surface to perform an interfacial polymerization reaction for 0.5 to 2 minutes, and a dense modified layer is formed on the surface of the base film to obtain a first interfacial polymerization modified film;

[0014] The base membrane is a cation exchange membrane, which can be a commercially available cation exchange membrane on the market or a cation exchange membrane prepared in a laboratory.

[0015] The infiltration time of the first aqueous phase solution on the base film is preferably 3 to 5 minutes;

[0016] The material of the substrate is polytetrafluoroethylene;

[0017] (3) Suspending a cathode and an anode above and below the mold on which the primary interfacial polymerization modified membrane is fixed, the cathode is above the membrane modified layer and the anode is below; turning on the power supply, applying a DC electric field of 1000 to 4000 V / cm, and pouring a second aqueous phase solution on the membrane surface to carry out a secondary interfacial polymerization reaction for 1 to 10 minutes; turning off the power supply after the reaction is completed, pouring out the second aqueous phase solution, and washing away the unreacted second aqueous phase solution on the membrane surface with water to obtain a secondary interfacial polymerization modified membrane;

[0018] The second aqueous phase solution is an aqueous solution of amino monomers with a concentration of 0.1 to 3%.

[0019] The secondary interfacial polymerization reaction time is preferably 5 to 8 minutes;

[0020] (4) The secondary interfacial polymerization modified membrane in step (3) is removed and subjected to heat treatment at 60 to 100° C. for 5 to 15 minutes to obtain a high monovalent selectivity cation exchange composite membrane.

[0021] In the above steps (2) and (3), the amino monomers in the first aqueous phase solution and the second aqueous phase solution are the same or different; and the concentrations are the same or different.

[0022] The high-unit-valent selective cation exchange membrane prepared by the method will significantly expand the application scope of electrodialysis technology in multiple fields, including but not limited to: lithium extraction from salt lake brine, seawater desalination, special ion separation, and salt separation of industrial high-salinity wastewater. This technological breakthrough provides an efficient and environmentally friendly separation solution for related fields and has broad application prospects.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The selectivity of monovalent selective ion exchange membranes is mainly based on the electrostatic repulsion and size screening effect of the modified layer surface on ions of different valence states. The present invention utilizes secondary interfacial polymerization to modify the surface of conventional cation exchange membranes. The membrane has an amine compound as the surface, which can improve the selectivity of the membrane through electrostatic repulsion. The interfacial polymerization process is regulated by a direct current electric field to increase the positive charge on the membrane surface, thereby making the membrane have extremely high monovalent selectivity (selectivity coefficient for sodium and magnesium separation>30).

[0025] (2) From the perspective of the practical application of the electrodialysis process, the monovalent selective cation exchange membrane should not only have high selectivity, but also have as low a membrane resistance as possible to save the operating energy consumption of the process. The present invention utilizes the electric field to regulate the structure of the interfacial polymerization modified layer and reduce the density of the modified layer, so that the resistance increase of the prepared modified membrane is smaller than that of the base membrane, and the membrane resistance of the secondary interfacial polymerization modified membrane with electric field regulation is significantly lower than that of the modified membrane without electric field.

[0026] (3) The present invention uses interfacial polymerization to modify the surface of a cation exchange membrane to prepare a monovalent selective membrane, and the obtained membrane has stable performance. Moreover, the modification technology has a simple operation process and is easy to scale up industrially. The electrodialysis method based on the selective cation exchange membrane has all the advantages of simple operation, small footprint, and easy modular scale-up, and is commonly used for the separation of monovalent and polyvalent cations in salt water.

[0027] (4) Separation membranes prepared by traditional interfacial polymerization technology are mostly used in ultrafiltration and nanofiltration processes, and their application in ion exchange membrane surface modification is still relatively limited. The present invention applies the interfacial polymerization process to the surface modification of ion exchange membranes and develops a simple, easy and stable method for preparing a monovalent selective cation exchange membrane. DETAILED DESCRIPTION

[0028] The present invention is further described in detail below by way of examples.

[0029] The base membrane described in the present invention is a cation exchange membrane, which is a commercial membrane commonly sold in the industry; but it is not limited thereto.

[0030] The device for treating saline wastewater with electrodialysis is composed of a desalination chamber, a concentration chamber and an electrode chamber. The performance of the modified selective cation exchange membrane is tested on the electrodialysis device. 0.1M NaCl and 0.1M MgCl 2 The mixed solution is placed in the desalination chamber, and 0.1M NaCl solution is added to the concentration chamber. 2 SO 4 The solution is polar water, and a DC power supply is used to separate monovalent and polyvalent salt ions in salt water.

[0031] The selectivity of the prepared selective cation exchange membrane is expressed by the selectivity coefficient P, and the specific calculation method is:

[0032] J=(C t -C 0 )·V / (A·t), where J represents the ion flux, C 0 and C t represent the ion concentration of the concentration chamber at the initial moment and time t respectively (mol / L), A represents the effective area of ​​the membrane, and V represents the volume of the solution in the concentration chamber (L).

[0033] P=J Na + ·C Mg 2+ / (J Mg 2+ ·C Na + ), where P represents the selective separation coefficient, J Na+ and J Mg 2+ represents the ion flux of monovalent cations and divalent cations (mol·cm -2 ·s -1 ), C Na + and C Mg 2+ Represent the concentrations of monovalent cations and divalent cations, respectively (mol / L).

[0034] Unless otherwise specified, the examples used Fujifilm Type 2 cation exchange membrane as the base membrane, and its membrane resistance was 8.0 Ω·cm 2 , membrane thickness 160 microns. In the electrodialysis device, 0.1M NaCl and 0.1MM MgCl 2 The mixed solution was treated and tested to obtain its Na + / Mg 2+ The selectivity coefficient is 1.17.

[0035] In this embodiment, Na + / Mg 2+ The selectivity of the membrane is characterized by separation as an example. The application of the monovalent selective cation exchange membrane prepared by the present invention is not limited to Na + / Mg 2+ Separation, Li + / Mg 2+ Separation.

[0036] Embodiment 1:

[0037] (1) dissolving polyethyleneimine with a molecular weight of 600 in deionized water to obtain a first aqueous phase solution with a monomer content of 0.35% by mass; dissolving trimesoyl chloride in n-hexane to obtain an organic phase solution with a monomer content of 0.08% by mass; dissolving polyethyleneimine with a molecular weight of 70,000 in deionized water to obtain a second aqueous phase solution with a monomer content of 0.35% by mass;

[0038] (2) Fixing the base membrane on a mold (the mold is composed of two polytetrafluoroethylene plates with a mold size of 8×8 cm and a thickness of 1 cm, wherein the upper plate has a through groove with a size of 6×6 cm in the middle, and the base membrane is fixed between the two plates, and the through groove is used to hold the reaction solution for single-sided membrane modification) to make the membrane surface flat, pouring the prepared first aqueous phase solution on the surface of the base membrane, reacting for 5 minutes, pouring out the first aqueous phase solution and removing the residual solution on the surface of the base membrane with a silicone roller; then pouring the prepared organic phase solution on the surface of the membrane for a first interfacial polymerization reaction, the reaction time is 1 minute, and a single dense modified layer is formed on the surface of the base membrane to obtain a first interfacial polymerization modified membrane;

[0039] (3) Anode and cathode electrodes of the same size as the mold (8×8 cm) are fixed on the upper and lower parts of the mold fixed with the primary interfacial polymerization modified membrane, specifically, the cathode is above the membrane modified layer, the anode is below, and the distance between the two electrodes is 2.5 cm. Turn on the power, apply a DC electric field of 1000 V / cm, and pour a second aqueous phase solution (different from the first aqueous phase solution, the first aqueous phase solution is a polyethyleneimine aqueous solution with a molecular weight of 600, while the second aqueous phase solution is a polyethyleneimine aqueous solution with a molecular weight of 70,000) on the surface of the modified membrane to carry out a secondary interfacial polymerization reaction. The reaction time is 8 minutes. After the reaction is completed, turn off the power, pour out the second aqueous phase solution, and rinse off the unreacted second aqueous phase solution on the membrane surface with water;

[0040] (4) The modified membrane in step (3) is removed and heat treated at 60° C. for 10 minutes to obtain a monovalent selective cation exchange composite membrane.

[0041] After testing, the membrane resistance is 8.0Ω·cm of the base film. 2 Improved to 8.63Ω·cm for selective cation exchange membrane 2 . In an electrodialysis unit, 0.1 M NaCl and 0.1 M MgCl 2 The mixed solution was used as the raw material for the selectivity coefficient test. Under the same conditions, the Na of Fujifilm Type 2 cation exchange membrane was tested. + / Mg 2+ The selectivity coefficient of the monovalent selective ion exchange membrane prepared in this embodiment is 1.17. + / Mg 2+ The selectivity coefficient is 23.50.

[0042] Embodiment 2:

[0043] The membrane was prepared by the method of Example 1, the only difference being that in step (2), the base membrane was replaced with a CSE cation exchange membrane produced by Astom Company, the thickness of which was 160 μm. The remaining materials and operating methods were the same, and the monovalent selective cation exchange membrane was prepared.

[0044] After testing, the performance of the prepared ion exchange membrane is as follows: the membrane resistance is 1.8Ω·cm 2 Improved to 2.43Ω·cm for selective cation exchange membrane 2 The monovalent selective ion exchange membrane prepared was tested in an electrodialysis device, and its Na + / Mg 2+ The selectivity coefficient is 16.10. Under the same conditions, the Na + / Mg 2+ The selectivity coefficient is 1.11.

[0045] Embodiment 3:

[0046] The membrane was prepared by the method of Example 1, the only difference being that in step (2), the molecular weight of polyethyleneimine in the first aqueous phase solution was changed from 600 to 70000. The remaining materials and operating methods were the same to prepare the monovalent selective cation exchange membrane.

[0047] After testing, the performance of the prepared ion exchange membrane is as follows: the membrane resistance is 8.0Ω·cm 2 Improved to 8.41Ω·cm for selective cation exchange membrane 2 The monovalent selective ion exchange membrane prepared was tested in an electrodialysis device, and its Na + / Mg 2+ The selectivity coefficient is 15.20.

[0048] Embodiment 4:

[0049] The membrane was prepared by the method of Example 1, the only difference being that in step (2), the polyethyleneimine in the first aqueous phase solution was replaced with piperazine. The remaining materials and operating methods were the same to prepare the monovalent selective cation exchange membrane.

[0050] After testing, the performance of the prepared ion exchange membrane is as follows: the membrane resistance is 8.0Ω·cm 2 Improved to 8.39Ω·cm for selective cation exchange membrane 2 The monovalent selective ion exchange membrane prepared was tested in an electrodialysis device, and its Na + / Mg 2+ The selectivity coefficient is 20.80.

[0051] Embodiment 5:

[0052] The membrane was prepared by the method of Example 1, the only difference being that in step (2), the concentration of polyethyleneimine with a molecular weight of 600 in the first aqueous phase solution was changed to 0.5%. The remaining materials and operating methods were the same to prepare the monovalent selective cation exchange membrane.

[0053] After testing, the performance of the prepared ion exchange membrane is as follows: the membrane resistance is 8.0Ω·cm 2 Improved to 8.82Ω·cm for selective cation exchange membrane 2 The monovalent selective ion exchange membrane prepared was tested in an electrodialysis device, and its Na + / Mg 2+ The selectivity coefficient is 10.62.

[0054] Embodiment 6:

[0055] The membrane was prepared by the method of Example 1, the only difference being that the soaking time of the second aqueous phase solution in step (3) was changed to 5 minutes. The remaining materials and operating methods were the same to prepare the monovalent selective cation exchange membrane.

[0056] After testing, the performance of the prepared ion exchange membrane is as follows: the membrane resistance is 8.0Ω·cm 2 Improved to 8.31Ω·cm for selective cation exchange membrane 2 The monovalent selective ion exchange membrane prepared was tested in an electrodialysis device, and its Na + / Mg 2+ The selectivity coefficient is 20.10.

[0057] Embodiment 7:

[0058] The membrane was prepared by the method of Example 1, the only difference being that the field strength of the electric field in step (3) was changed to 3000 V / cm. The other materials and operating methods were the same, thereby obtaining the monovalent selective cation exchange membrane.

[0059] After testing, the performance of the prepared ion exchange membrane is as follows: the membrane resistance is 8.0Ω·cm 2 Improved to 8.40Ω·cm for selective cation exchange membrane 2 The monovalent selective ion exchange membrane prepared was tested in an electrodialysis device, and its Na + / Mg 2+ The selectivity coefficient is 31.50.

[0060] Matters not covered by the present invention are known technologies.

Claims

1. A method for preparing a high monovalent selectivity cation exchange membrane, characterized in that: The method comprises the following steps: (1) dissolving an amine monomer in deionized water to obtain a first aqueous phase solution having a monomer content of 0.1 to 3% by weight; and dissolving an acyl chloride monomer in an organic solvent to obtain an organic phase solution having a monomer content of 0.05 to 0.4% by weight; The above-mentioned amino monomer is m-phenylenediamine, piperazine and its derivatives, or polyethyleneimine; (2) The base film is horizontally laid into the groove of the substrate, and then the prepared first aqueous phase solution is poured on the upper surface of the base film. After immersion modification for 1 to 10 minutes, the aqueous phase solution is removed; then the organic phase solution is poured on the upper film surface to perform an interfacial polymerization reaction for 0.5 to 2 minutes, and a dense modified layer is formed on the surface of the base film to obtain a first interfacial polymerization modified film; The base membrane is a cation exchange membrane; (3) Suspending a cathode and an anode above and below the mold on which the primary interfacial polymerization modified membrane is fixed, the cathode is above the membrane modified layer and the anode is below; turning on the power supply, applying a DC electric field of 1000 to 4000 V / cm, and pouring a second aqueous phase solution on the membrane surface to carry out a secondary interfacial polymerization reaction for 1 to 10 minutes; turning off the power supply after the reaction is completed, pouring out the second aqueous phase solution, and washing away the unreacted second aqueous phase solution on the membrane surface with water to obtain a secondary interfacial polymerization modified membrane; The second aqueous phase solution is an aqueous solution of amino monomers with a concentration of 0.1 to 3%. (4) The secondary interfacial polymerization modified membrane in step (3) is removed and subjected to heat treatment at 60 to 100° C. for 5 to 15 minutes to obtain a high monovalent selectivity cation exchange composite membrane.

2. The method for preparing a high monovalent selectivity cation exchange membrane according to claim 1, characterized in that: The piperazine derivative is specifically p-aminobenzoylpiperazine, N-aminoethylpiperazine or sulfonated N-aminoethylpiperazine; the molecular weight of polyethyleneimine is 300-100000.

3. The method for preparing a high monovalent selectivity cation exchange membrane according to claim 1, characterized in that: The acyl chloride monomer is isophthaloyl chloride, terephthaloyl chloride or trimesoyl chloride; the organic solvent is n-hexane or cyclohexane.

4. The method for preparing a high monovalent selectivity cation exchange membrane according to claim 1, characterized in that: The first aqueous phase solution infiltrates the base film for 3 to 5 minutes; the secondary interface polymerization reaction time is 5 to 8 minutes.

5. The method for preparing a high monovalent selectivity cation exchange membrane according to claim 1, characterized in that: The material of the substrate is polytetrafluoroethylene.

6. The method for preparing a high monovalent selectivity cation exchange membrane according to claim 1, characterized in that: The amino monomers in the first aqueous phase solution and the second aqueous phase solution may be the same or different; and the concentrations may be the same or different.

7. Application of the high monovalent selective cation exchange membrane prepared by the method of claim 1, characterized in that: Used for lithium extraction from salt lake brine, seawater desalination, special ion separation or salt separation of industrial high-salt wastewater.