High rejection positively charged composite nanofiltration membrane and method for making same
By forming a polyvinyl alcohol crosslinking layer and glycidyl dodecyl dimethyl ammonium chloride graft on the surface of the nanofiltration membrane base, and combining this with electrostatic repulsion, a polyamide separation layer with uniform pore size was prepared. This solved the problem of non-uniform pore size in nanofiltration membranes, improved the rejection rate of high-valence cations, and reduced operating pressure.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- 浙江美易膜科技有限公司
- Filing Date
- 2022-12-05
- Publication Date
- 2026-05-12
AI Technical Summary
The non-uniform pore size of existing nanofiltration membrane separation layers leads to a decrease in the rejection rate of high-valent cations, and increases operating pressure and cost.
A polyvinyl alcohol crosslinking layer is formed on the surface of the base film, and glycidyl dodecyl dimethyl ammonium chloride is grafted onto the surface of the base film. The quaternized polyethyleneimine is uniformly distributed in the interfacial polymerization reaction by using electrostatic repulsion to form a polyamide separation layer with uniform pore size.
It improves the retention of high-valent cations, reduces operating pressure, enhances the bonding strength between the base membrane and the separation layer, prevents the separation layer from falling off, and maintains the long-term operational stability of the nanofiltration membrane.
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Abstract
Description
Technical Field
[0001] This invention relates to the field of separation membrane composite materials technology, and in particular to a high retention charge positively charged composite nanofiltration membrane and its preparation method. Background Technology
[0002] Membrane separation technology, as a highly efficient, energy-saving, and environmentally friendly water treatment technology, has become an important means of solving water scarcity and water pollution. Nanofiltration is a membrane separation technology between ultrafiltration and reverse osmosis. Typically, nanofiltration membranes have a molecular weight cutoff of 200-1000 Da and an effective membrane pore size of 0.5-2 nm. Nanofiltration membranes exhibit good separation performance for high-valent and divalent salts and small-molecule organic matter. They also have advantages such as low operating pressure, high pure water flux, and lower cost, making them promising for applications in water softening, desalination, and wastewater treatment.
[0003] Currently, most commercially available nanofiltration membranes are negatively charged nanofiltration membranes. The retention effect of negatively charged nanofiltration membranes on positively charged metal ions mainly relies on the sieving effect of pore size. Therefore, to obtain nanofiltration membranes with high removal rates for divalent and higher valence metal ions, the pore size must be made smaller to achieve the retention of high-valence and divalent cations. However, this increases the operating pressure of the membrane, leading to increased costs in the membrane separation process. Positively charged nanofiltration membranes, on the other hand, can remove metal ions by relying on the charge repulsion on the membrane surface to prevent high-valence and divalent cations from permeating the membrane. Therefore, it is not necessary to reduce the pore size of the nanofiltration membrane, thus reducing the operating pressure. However, in the current process of preparing positively charged nanofiltration membranes through interfacial polymerization, the pore size of the resulting nanofiltration membrane separation layer is non-uniform, leading to a decrease in the retention rate of metal ions.
[0004] Chinese Patent Publication No. CN112755817 discloses a high-performance composite nanofiltration membrane, its preparation method, and its application. The composite nanofiltration membrane includes a porous ultrafiltration substrate membrane and a polyamide selective separation layer disposed on the substrate membrane. The polyamide selective separation layer is mainly formed by interfacial polymerization of polyamine monomers and polyacrylamide monomers under the control of surfactants. One of the implementation methods is to use polyethyleneimine and trimesoyl chloride to undergo interfacial polymerization on a polyethersulfone ultrafiltration substrate membrane. The resulting polyamide separation layer has poor pore size uniformity, which affects the cation rejection rate of the composite nanofiltration membrane. Summary of the Invention
[0005] To overcome the problem of decreased ion rejection rate caused by poor pore size uniformity in the separation layer of nanofiltration membranes, this invention provides a high-rejection-charged composite nanofiltration membrane and its preparation method. The composite nanofiltration membrane prepared by this invention exhibits high pore size uniformity in the separation layer and demonstrates a high rejection effect for high-valent cations.
[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:
[0007] A method for preparing a high-retention-charge positively charged composite nanofiltration membrane includes the following steps:
[0008] 1) Prepare the base film by immersing it in a polyvinyl alcohol solution and allowing it to undergo a cross-linking reaction under the action of a cross-linking agent, so that the cross-linked polyvinyl alcohol adheres to the surface of the base film.
[0009] 2) The hydroxyl groups on the cross-linked polyvinyl alcohol molecules bonded to the surface of the base film undergo a ring-opening reaction with the epoxy groups on the glycidyl dodecyl dimethyl ammonium chloride, and the glycidyl dodecyl dimethyl ammonium chloride is grafted onto the surface of the base film to obtain a pretreated base film.
[0010] 3) Dissolve an acyl chloride monomer with at least two acyl chloride groups in a solvent to obtain an organic phase solution;
[0011] 4) Dissolve quaternized polyethyleneimine in water, add surfactant and acid neutralizer, disperse evenly to obtain an aqueous solution;
[0012] 5) The pretreated base membrane is first contacted with an aqueous solution, then placed in the air to stand, and then the pretreated base membrane is contacted with an organic solution to undergo an interfacial polymerization reaction to obtain a nascent composite nanofiltration membrane.
[0013] 6) Post-processing: The nascent composite nanofiltration membrane is heat-treated to obtain the final product.
[0014] In conventional composite nanofiltration membrane preparation processes, a base membrane is first immersed in an aqueous amine solution. After removal, an aqueous coating forms on the base membrane surface. Then, the nanofiltration membrane is immersed in an organic acyl chloride solution. The amine monomers diffuse from the aqueous layer into the organic acyl chloride solution, forming a polyamide separation layer on the base membrane surface. Nanofiltration membranes prepared using this conventional method have uneven pore sizes in the polyamide separation layer, affecting the retention efficiency of the composite nanofiltration membrane for high-valent cations. This uneven pore size is caused by two factors: firstly, the base membrane itself has uneven pore sizes, resulting in uneven horizontal dispersion of amine monomers on its surface; secondly, the amine monomers diffuse randomly from the aqueous phase into the organic phase, causing some areas of the nanofiltration membrane separation layer to overpolymerize while others have lower polymerization degrees. This uneven pore size leads to a decrease in the cation retention rate of the composite nanofiltration membrane. To address the aforementioned problems, this invention employs a method that involves first applying a polyvinyl alcohol crosslinking layer to the surface of a base membrane using a polyvinyl alcohol crosslinking reaction. This ensures uniform pore size on the base membrane surface and even dispersion of amine monomers. Then, the hydroxyl groups on the crosslinked polyvinyl alcohol molecules (residual hydroxyl groups from the polyvinyl alcohol crosslinking reaction) undergo a ring-opening reaction with the epoxy groups on glycidyl dodecyl dimethyl ammonium chloride, resulting in a certain amount of positive charge on the base membrane surface. After the base membrane surface is immersed in an aqueous solution, the positive charge on the base membrane surface repels the quaternized polyethyleneimine in the aqueous phase under electrostatic repulsion, causing the quaternized polyethyleneimine to aggregate on the surface of the aqueous solution. During the interfacial polymerization reaction, the quaternized polyethyleneimine aggregated on the surface of the aqueous solution can simultaneously diffuse into the organic phase solution. The resulting separation layer exhibits a relatively uniform degree of polymerization in different regions, thus forming a separation layer with uniform pore size and improving the cation retention effect of the nanofiltration membrane. It should be noted that the quaternized polyethyleneimine used in this invention is different from ordinary polyethyleneimine. Ordinary polyethyleneimine cannot form a strong electrostatic repulsion between itself and the pretreated base film of this invention, and cannot cause a large amount of polyethyleneimine to aggregate and disperse on the surface of the aqueous solution, which is not conducive to the formation of a separation layer with uniform pore size.
[0015] Preferably, the crosslinking reaction time in step 1) is 12-18 min.
[0016] Further research into the crosslinking reaction time revealed that it simultaneously affects the adhesion strength of crosslinked polyvinyl alcohol (PVA) to the base membrane surface and the bonding strength between the base membrane and the separation layer. A crosslinking reaction time of at least 12 minutes is required for effective adhesion to the base membrane surface, thus providing stable binding sites for subsequent grafting of glycidyl dodecyl dimethyl ammonium chloride (GDC) onto the base membrane surface. However, a further problem is that the polyamide separation layer easily detaches from the base membrane surface during long-term low-pressure filtration, reducing the nanofiltration membrane's retention and separation performance. This invention effectively improves the problem of nanofiltration membrane detachment from the base membrane surface by controlling the crosslinking reaction time to less than 18 minutes. This is because controlling the crosslinking reaction time to less than 18 minutes reduces the consumption of hydroxyl groups on the PVA molecules during the crosslinking reaction. The resulting crosslinked PVA network molecules retain a large number of hydroxyl groups, which form hydrogen bonds with a large number of amino groups on the separation layer, thereby improving the bonding strength between the base membrane and the separation layer and solving the problem of the separation layer easily detaching from the base membrane surface during long-term operation of the composite nanofiltration membrane.
[0017] Preferably, the ring-opening reaction in step 2) is carried out under sodium hydroxide catalyst conditions, with the temperature raised to 170-200℃ and the reaction held at a constant temperature for 1-5 hours.
[0018] Preferably, the acyl chloride monomer in step 3) is at least one of pyromellitic tricarboxylic acid chloride and pyromellitic tetracarboxylic acid chloride.
[0019] Preferably, the mass concentration of acyl chloride monomer in the organic phase solution in step 3) is 0.01-1%.
[0020] Preferably, the mass concentration of quaternized polyethyleneimine in the aqueous solution in step 4) is 0.5-1.5%.
[0021] Preferably, the settling time in the air in step 5) is controlled to be 10-30 minutes.
[0022] A high-retention-charge positively charged composite nanofiltration membrane was prepared by the method described above.
[0023] Preferably, the nanofiltration membrane is used in the fields of hard water softening or lithium extraction from salt lakes.
[0024] The beneficial technical effects of the present invention are as follows:
[0025] 1) By adding a polyvinyl alcohol crosslinking layer and grafting glycidyl dodecyl dimethyl ammonium chloride to the surface of the base membrane, the pore size of the polyamide separation layer generated on the surface of the base membrane is more uniform, which improves the retention effect of polyvalent cations.
[0026] 2) By controlling the cross-linking reaction time of polyvinyl alcohol, the consumption of hydroxyl groups on polyvinyl alcohol molecules during the cross-linking reaction process is reduced. The resulting cross-linked polyvinyl alcohol network molecules retain a large number of hydroxyl groups. The hydroxyl groups form hydrogen bonds with a large number of amino groups on the separation layer, thereby improving the bonding strength between the base membrane and the separation layer and solving the problem that the separation layer is easy to fall off the surface of the base membrane during long-term operation of the composite nanofiltration membrane. Detailed Implementation
[0027] The present invention will be further described clearly and in detail below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.
[0028] Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available or prepared by those skilled in the art through conventional methods published in the literature; unless otherwise specified, the methods used in the embodiments of the present invention are all methods mastered by those skilled in the art.
[0029] The following examples are typical and relatively representative test records formed during the research and development of the present invention, but they do not represent all records and do not limit the scope of protection of the present invention.
[0030] Example 1
[0031] A method for preparing a high-retention-charge positively charged composite nanofiltration membrane includes the following steps:
[0032] 1) Add polyvinyl alcohol to deionized water and stir to dissolve. Let stand to remove bubbles and prepare a polyvinyl alcohol solution with a mass concentration of 1.0 wt%. Add glutaraldehyde crosslinking agent to the polyvinyl alcohol solution, with the glutaraldehyde crosslinking agent accounting for 0.3 wt% of the polyvinyl alcohol solution. Adjust the pH to 5. Put the polyethersulfone ultrafiltration membrane into the above solution and impregnate it for crosslinking reaction at a vacuum degree of -95 kPa for 17 min. After taking it out, fix the membrane on the frame and place it in an oven to bake at 50℃ for 20 min to obtain the pretreated membrane.
[0033] 2) Add glycidyl dodecyl dimethyl ammonium chloride to deionized water, stir to dissolve, and prepare a 2.0 wt% glycidyl dodecyl dimethyl ammonium chloride solution. Heat to 200°C, add sodium hydroxide catalyst (0.2 wt% of the glycidyl dodecyl dimethyl ammonium chloride solution), immerse the pretreated base film in the above solution and react at a constant temperature for 1 h. After removing the base film, place it in an oven and dry at 50°C for 30 min to obtain the pretreated base film.
[0034] 3) Dissolve the trimesoyl chloride monomer in n-hexane solvent to prepare a trimesoyl chloride solution with a mass concentration of 0.5%;
[0035] 4) Dissolve quaternized polyethyleneimine in water, add the surfactant sodium dodecyl sulfate and the acid neutralizer triethylamine, disperse evenly to obtain an aqueous solution. The mass concentration of quaternized polyethyleneimine in the aqueous solution is 1.0%, the mass concentration of sodium dodecyl sulfate is 0.05%, and the mass concentration of triethylamine is 1.5%.
[0036] 5) Immerse the pretreated base membrane in the aqueous solution for 1 min, remove it and let it stand in the air for 25 min. Then immerse the pretreated base membrane in the organic solution and carry out interfacial polymerization reaction at room temperature for 30 s to obtain the nascent composite nanofiltration membrane; 6) Post-treatment: Place the nascent composite nanofiltration membrane in an oven and dry it at 60℃ for 15 min to obtain the positively charged composite nanofiltration membrane with an ultrathin separation layer.
[0037] Comparative Example 1
[0038] The difference between Comparative Example 1 and Example 1 above is that steps 1 and 2 are omitted, that is, the polyethersulfone ultrafiltration membrane is not modified.
[0039] Comparative Example 2
[0040] The difference between Comparative Example 2 and Example 1 above is that step 2 is omitted, namely, the surface modification treatment of the base film by non-epoxypropyldodecyldimethylammonium chloride.
[0041] To investigate the performance differences between the composite nanofiltration membrane prepared in Example 1 of this invention and the composite nanofiltration membranes of Comparative Examples 1 and 2, a cross-flow experimental setup was used to test the rejection rates of magnesium sulfate, calcium chloride, and lithium chloride, as well as the pure water flux, of the two composite nanofiltration membranes. Test conditions: ambient temperature 25℃, test pressure 0.3 MPa, and concentrations of magnesium chloride, calcium chloride, and lithium chloride all 5000 ppm. The test results are shown in the table below:
[0042]
[0043] The test results above show that the composite nanofiltration membrane prepared by the present invention has a rejection rate of over 90% for divalent Mg2+ and Ca2+, and a rejection rate of approximately 12% for monovalent Li+, proving that the composite nanofiltration membrane of the present invention has a high rejection rate for divalent cations.
[0044] Example 2
[0045] A method for preparing a high-retention-charge positively charged composite nanofiltration membrane includes the following steps:
[0046] 1) Add polyvinyl alcohol to deionized water and stir to dissolve. Let stand to remove bubbles and prepare a polyvinyl alcohol solution with a mass concentration of 1.0 wt%. Add glutaraldehyde crosslinking agent to the polyvinyl alcohol solution, with the glutaraldehyde crosslinking agent accounting for 0.3 wt% of the polyvinyl alcohol solution. Adjust the pH to 5. Place the polyethersulfone ultrafiltration membrane into the above solution and impregnate it under a vacuum of -95 kPa for 13 min for crosslinking reaction. After taking it out, fix the membrane on the frame and place it in an oven to bake at 50°C for 20 min to obtain the pretreated membrane.
[0047] 2) Add glycidyl dodecyl dimethyl ammonium chloride to deionized water, stir to dissolve, and prepare a 2.0 wt% glycidyl dodecyl dimethyl ammonium chloride solution. Heat to 170°C, add sodium hydroxide catalyst, the sodium hydroxide catalyst accounts for 0.2 wt% of the glycidyl dodecyl dimethyl ammonium chloride solution, immerse the pretreated base film in the above solution and react at a constant temperature for 5 h. After taking out the base film, place it in an oven and dry at 50°C for 30 min to obtain the pretreated base film.
[0048] 3) Dissolve the trimesoyl chloride monomer in n-hexane solvent to prepare a trimesoyl chloride solution with a mass concentration of 0.1%;
[0049] 4) Dissolve quaternized polyethyleneimine in water, add the surfactant sodium dodecyl sulfate and the acid neutralizer triethylamine, disperse evenly to obtain an aqueous solution. The mass concentration of quaternized polyethyleneimine in the aqueous solution is 0.8%, the mass concentration of sodium dodecyl sulfate is 0.05%, and the mass concentration of triethylamine is 1.5%.
[0050] 5) Immerse the pretreated base membrane in the aqueous solution for 1 min, remove it and let it stand in the air for 12 min. Then immerse the pretreated base membrane in the organic solution and carry out interfacial polymerization reaction at room temperature for 30 s to obtain the nascent composite nanofiltration membrane; 6) Post-treatment: Place the nascent composite nanofiltration membrane in an oven and dry it at 60℃ for 15 min to obtain the positively charged composite nanofiltration membrane with an ultrathin separation layer.
[0051] Example 3
[0052] A method for preparing a high-retention-charge positively charged composite nanofiltration membrane includes the following steps:
[0053] 1) Add polyvinyl alcohol to deionized water and stir to dissolve. Let stand to remove bubbles and prepare a polyvinyl alcohol solution with a mass concentration of 1.0 wt%. Add glutaraldehyde crosslinking agent to the polyvinyl alcohol solution, with the glutaraldehyde crosslinking agent accounting for 0.3 wt% of the polyvinyl alcohol solution. Adjust the pH to 5. Place the polyethersulfone ultrafiltration membrane into the above solution and impregnate it under a vacuum of -95 kPa for 18 min for crosslinking reaction. After taking it out, fix the membrane on the frame and place it in an oven to bake at 50°C for 20 min to obtain the pretreated membrane.
[0054] 2) Add glycidyl dodecyl dimethyl ammonium chloride to deionized water, stir to dissolve, and prepare a 2.0 wt% glycidyl dodecyl dimethyl ammonium chloride solution. Heat to 180°C, add sodium hydroxide catalyst, the sodium hydroxide catalyst accounts for 0.2 wt% of the glycidyl dodecyl dimethyl ammonium chloride solution, immerse the pretreated base film in the above solution and react at a constant temperature for 4 hours. After taking out the base film, place it in an oven and dry at 50°C for 30 minutes to obtain the pretreated base film.
[0055] 3) Dissolve the trimesoyl chloride monomer in n-hexane solvent to prepare a 1% trimesoyl chloride solution;
[0056] 4) Dissolve quaternized polyethyleneimine in water, add the surfactant sodium dodecyl sulfate and the acid neutralizer triethylamine, disperse evenly to obtain an aqueous solution. The mass concentration of quaternized polyethyleneimine in the aqueous solution is 1.5%, the mass concentration of sodium dodecyl sulfate is 0.05%, and the mass concentration of triethylamine is 1.5%.
[0057] 5) Immerse the pretreated base membrane in the aqueous solution for 1 min, remove it and let it stand in the air for 30 min. Then immerse the pretreated base membrane in the organic solution and carry out interfacial polymerization reaction at room temperature for 30 s to obtain the nascent composite nanofiltration membrane; 6) Post-treatment: Place the nascent composite nanofiltration membrane in an oven and dry it at 60℃ for 15 min to obtain the positively charged composite nanofiltration membrane with an ultrathin separation layer.
[0058] Comparative Example 3
[0059] The difference between this comparative example and Example 3 is that the impregnation crosslinking reaction time in step 1 is adjusted to 19 min.
[0060] To examine the difference in operational stability between the composite nanofiltration membrane prepared in Example 3 of this invention and the composite nanofiltration membrane in Comparative Example 2, the two composite nanofiltration membranes were first run at 0.3 MPa for 5 h, 10 h, 20 h, and 40 h, respectively. Then, the retention rate of the nanofiltration membranes for magnesium chloride (magnesium chloride concentration 5000 ppm) was tested. The test results are shown in the table below:
[0061]
[0062] Comparing the above test results, after prolonged operation at 0.3 MPa pressure, the magnesium chloride rejection rate of the composite nanofiltration membrane in Example 3 remained essentially stable, while the flux of the composite nanofiltration membrane in Comparative Example 2 showed a significant downward trend after 20 hours of pressure operation. This is because the present invention controls the crosslinking reaction time of polyvinyl alcohol, reducing the consumption of hydroxyl groups on the polyvinyl alcohol molecules during the crosslinking reaction. The resulting crosslinked polyvinyl alcohol network molecules retain a large number of hydroxyl groups, which form hydrogen bonds with a large number of amino groups on the separation layer, thereby improving the bonding strength between the base membrane and the separation layer. The separation layer of the composite nanofiltration membrane is less likely to detach from the base membrane, maintaining the stability of the flux performance of the composite nanofiltration membrane.
[0063] Example 4
[0064] A method for preparing a high-retention-charge positively charged composite nanofiltration membrane includes the following steps:
[0065] 1) Add polyvinyl alcohol to deionized water and stir to dissolve. Let stand to remove bubbles and prepare a polyvinyl alcohol solution with a mass concentration of 1.0 wt%. Add glutaraldehyde crosslinking agent to the polyvinyl alcohol solution, with the glutaraldehyde crosslinking agent accounting for 0.3 wt% of the polyvinyl alcohol solution. Adjust the pH to 5. Place the polyethersulfone ultrafiltration membrane into the above solution and impregnate it under a vacuum of -95 kPa for 12 min for crosslinking reaction. After taking it out, fix the membrane on the frame and place it in an oven to bake at 50°C for 20 min to obtain the pretreated membrane.
[0066] 2) Add glycidyl dodecyl dimethyl ammonium chloride to deionized water, stir to dissolve, and prepare a 2.0 wt% glycidyl dodecyl dimethyl ammonium chloride solution. Heat to 175°C, add sodium hydroxide catalyst (0.2 wt% of the glycidyl dodecyl dimethyl ammonium chloride solution), immerse the pretreated base film in the above solution and react at a constant temperature for 2 hours. After removing the base film, place it in an oven and dry at 50°C for 30 minutes to obtain the pretreated base film.
[0067] 3) Dissolve the trimesoyl chloride monomer in n-hexane to prepare a trimesoyl chloride solution with a mass concentration of 0.01%; 4) Dissolve the quaternized polyethyleneimine in water, add the surfactant sodium dodecyl sulfate and the acid neutralizer triethylamine, disperse evenly to obtain an aqueous solution, in which the mass concentration of quaternized polyethyleneimine is 0.5%, the mass concentration of sodium dodecyl sulfate is 0.05%, and the mass concentration of triethylamine is 1.5%;
[0068] 5) Immerse the pretreated base membrane in the aqueous solution for 1 minute, remove it and let it stand in the air for 10 minutes. Then immerse the pretreated base membrane in the organic solution and carry out interfacial polymerization reaction at room temperature for 30 seconds to obtain the nascent composite nanofiltration membrane; 6) Post-treatment: Place the nascent composite nanofiltration membrane in an oven and dry it at 60°C for 15 minutes to obtain the positively charged composite nanofiltration membrane with an ultrathin separation layer.
[0069] Comparative Example 4
[0070] The difference between this comparative example and Example 4 is that the impregnation crosslinking reaction time in step 1 is adjusted to 11 min.
[0071] To examine the performance difference between the composite nanofiltration membrane prepared in Example 4 and the composite nanofiltration membrane in Comparative Example 3, a cross-flow experimental setup was used to test the retention rates of magnesium chloride, calcium chloride, and lithium chloride in the two composite nanofiltration membranes. The concentrations of magnesium chloride, calcium chloride, and lithium chloride were all 5000 ppm. The test results are shown in the table below:
[0072] <![CDATA[MgCl2(%)]]> <![CDATA[CaCl2(%)]]> LiCl (%) Example 4 94.3 91.6 11.3 Comparative Example 4 91.2 87.7 8.5
[0073] The above tests show that if the polyvinyl alcohol crosslinking time is too short, less crosslinked polyvinyl alcohol will adhere to the base membrane surface and cannot completely cover the base membrane surface, which will result in uneven pore size of the subsequent polyamide separation layer and a reduced rejection rate of high-valent cations by the nanofiltration membrane.
[0074] The embodiments described above are merely examples illustrating several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection claimed by the present invention.
Claims
1. A method for preparing a high-retention-charge positively charged composite nanofiltration membrane, characterized in that, Includes the following steps: 1) Prepare the base film by immersing it in a polyvinyl alcohol solution and allowing it to undergo a cross-linking reaction under the action of a cross-linking agent, so that the cross-linked polyvinyl alcohol adheres to the surface of the base film. 2) The hydroxyl groups on the cross-linked polyvinyl alcohol molecules bonded to the surface of the base film undergo a ring-opening reaction with the epoxy groups on the glycidyl dodecyl dimethyl ammonium chloride, and the glycidyl dodecyl dimethyl ammonium chloride is grafted onto the surface of the base film to obtain a pretreated base film. 3) Dissolve an acyl chloride monomer with at least two acyl chloride groups in a solvent to obtain an organic phase solution; 4) Dissolve quaternized polyethyleneimine in water, add surfactant and acid neutralizer, disperse evenly to obtain an aqueous solution; 5) The pretreated base membrane is first contacted with an aqueous solution, then placed in air to stand, and then the pretreated base membrane is contacted with an organic solution to undergo an interfacial polymerization reaction to obtain a nascent composite nanofiltration membrane. 6) Post-processing: The nascent composite nanofiltration membrane is heat-treated to obtain the final product.
2. The method for preparing a high-retention-charge positively charged composite nanofiltration membrane according to claim 1, characterized in that, The cross-linking reaction time in step 1) is 12-18 min.
3. The method for preparing a high-retention-charge positively charged composite nanofiltration membrane according to claim 1, characterized in that, The ring-opening reaction process in step 2) is carried out under sodium hydroxide catalyst conditions, with the temperature raised to 170-200℃ and the reaction held at a constant temperature for 1-5 hours.
4. The method for preparing a high-retention-charge positively charged composite nanofiltration membrane according to claim 1, characterized in that, In step 3), the acyl chloride monomer is at least one of pyromellitic tricarboxylic acid chloride and pyromellitic tetracarboxylic acid chloride.
5. The method for preparing a high-retention-charge positively charged composite nanofiltration membrane according to claim 1, characterized in that, In step 3), the mass concentration of acyl chloride monomer in the organic phase solution is 0.01-1%.
6. The method for preparing a high-retention-charge positively charged composite nanofiltration membrane according to claim 1, characterized in that, In step 4), the mass concentration of quaternized polyethyleneimine in the aqueous solution is 0.5-1.5%.
7. The method for preparing a high-retention-charge positively charged composite nanofiltration membrane according to claim 1, characterized in that, In step 5), the time for standing in the air is controlled to be 10-30 minutes.
8. A high-retention-charge positive composite nanofiltration membrane, characterized in that, Prepared by the method described in any one of claims 1-7.
9. The high-retention-charge positively charged composite nanofiltration membrane according to claim 8, characterized in that, The nanofiltration membrane is used in the fields of hard water softening or lithium extraction from salt lakes.