Surface charge controllable nanofiltration membrane and preparation method thereof

Abstract

The application discloses a kind of surface charge controllable nanofiltration membrane and preparation method thereof, belong to water treatment technical field.The nanofiltration membrane with polysulfone ultrafiltration membrane as base film, by slit coating head coating casting solution containing oligomer, active diluent etc., after ultraviolet light solidification, again alternately immersed in polycation and polyanion electrolyte solution and is handled to prepare coating layer.The nanofiltration membrane prepared by the application compared with prior art, through simple through ultraviolet irradiation solidification reaction makes oligomer crosslinking solidification, forms stable functional layer;The application replaces high-temperature heat treatment with ultraviolet solidification, energy consumption is low, preparation cost reduces;Electrostatic adsorption coating step is simple, suitable for industrial production;The application is suitable for different ion composition of salt lake brine through the alternate adsorption of polyelectrolyte coating;Optimization to different valence ion selective separation.Summarized above, the surface charge controllable nanofiltration membrane prepared by the application, in salt lake brine lithium extraction field, has important application value.

Description

Technical Field

[0001] This invention belongs to the field of water treatment technology, specifically relating to a nanofiltration membrane with controllable surface charge and its preparation method. Background Technology

[0002] With the continuous development of new energy technologies, lithium-ion batteries, as an indispensable component, are experiencing increasing demand. Lithium, the cathode material of lithium-ion batteries, cannot be directly obtained from nature and is generally found in mineral lithium deposits and brine lithium deposits in salt lakes. Most lithium extraction processes from salt lake brine deal with high-calcium and high-magnesium brines. However, some special salt lakes, such as Zabuye Salt Lake, have very low calcium and magnesium content but high levels of carbonate and sulfate anions. This presents a significant challenge to traditional lithium extraction methods.

[0003] Nanofiltration membranes are a novel type of separation membrane with pore sizes between reverse osmosis and ultrafiltration membranes. They possess nanoscale pore sizes and multiple surface charges, allowing low-molecular-weight salts to pass through while retaining higher-molecular-weight organic matter and multivalent ions, resulting in unique separation performance and higher separation precision. Currently, most commercially produced and industrially applied nanofiltration composite membranes are prepared through interfacial polymerization between polyamines and polyacrylamide chlorides. The desalination functional layer of these membranes is made of polyamide-based materials, which have many applications in lithium extraction from salt lakes. However, due to their surface charge characteristics, they cannot effectively adapt to the complex environment of high-valence anions, thus their lithium extraction effect is less than expected in brine from salt lakes rich in high-valence anions.

[0004] Therefore, to address the above problems, developing a nanofiltration membrane with controllable surface charge is of great practical significance for expanding the industrial application of nanofiltration membrane technology in lithium extraction. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a nanofiltration membrane with controllable surface charge and its preparation method.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A method for preparing a nanofiltration membrane with controllable surface charge includes the following steps:

[0008] Step S1: First, the casting solution is uniformly coated onto the surface of the polysulfone-based membrane through a slit coating head to form a coating on the membrane surface; then, under nitrogen protection, the membrane surface is irradiated with an ultraviolet lamp, and then the membrane surface is rinsed multiple times with deionized water to obtain the nascent nanofiltration membrane.

[0009] Step S2: Immerse the nascent nanofiltration membrane obtained in step S1 into coating solution A, let it stand for 2-3 minutes, then remove it to obtain a membrane sheet. Rinse the membrane sheet with deionized water and clean the surface of the membrane sheet with an air knife. Then immerse the membrane sheet into coating solution B, let it stand for 2-3 minutes, then remove the membrane sheet and rinse it with deionized water. Clean the surface of the membrane sheet with an air knife. Repeat the above coating operation on the membrane sheet to form a coating and obtain a nanofiltration membrane with controllable surface charge.

[0010] As a further technical solution, the thickness of the coating in step S1 is 0.5-10μm.

[0011] As a further technical solution, the UV lamp irradiation time in step S1 is 5-60s.

[0012] As a further technical solution, the light intensity of the ultraviolet lamp in step S1 is 30-100 mW / cm². 2 .

[0013] As a further technical solution, the casting solution in step S1 is obtained by mixing the following raw materials in parts by weight: 40-45 parts oligomer, 30-50 parts reactive diluent, 1-5 parts photoinitiator, 10-25 parts additives and solvent.

[0014] As a further technical solution, the viscosity of the casting solution is 200-1000 mPa·s, and the viscosity is controlled by the amount of solvent used.

[0015] As a further technical solution, the oligomer is one of acrylate, epoxy acrylate and polyurethane acrylate.

[0016] As a further technical solution, the reactive diluent is one of hydroxyethyl acrylate, vinyl monomer, methacrylate, diacrylate, triethylene glycol divinyl ether, and 4-hydroxybutyl vinyl ether.

[0017] As a further technical solution, the photoinitiator is one of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, ethyl 2,4,6-trimethylbenzoylphenylphosphine acid, phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide, and 2-isopropylthioxanthrone.

[0018] As a further technical solution, the additive is sodium styrene sulfonate and polyethylene glycol diacrylate in a mass ratio of 7-12:5.

[0019] As a further technical solution, the solvent is one of deionized water, ethanol, and methanol.

[0020] As a further technical solution, in step S2, the coating solution A is a polycationic electrolyte solution with a concentration of 0.5-10 g / L; wherein the electrolyte includes one or more of polyacrylamide hydrochloride, polydimethyldiallylammonium chloride, polyethyleneimine, methacryloyloxyethyltrimethylammonium chloride, chitosan and polyacrylamide.

[0021] As a further technical solution, in step S2, the coating solution B is a polyanionic electrolyte solution with a concentration of 0.5-10 g / L; wherein the electrolyte includes one or more of sodium polystyrene sulfonate, sodium polyacrylate, and sodium polyvinyl sulfonate.

[0022] As a further technical solution, in step S2, the coating sequence is coating solutions A, B...A, B in sequence; the coating consists of coating solutions A and B as one layer, and the number of coating layers is 1-10.

[0023] This invention uses a self-made polysulfone ultrafiltration membrane to support an oligomer casting solution. The nonwoven fabric in the polysulfone ultrafiltration membrane provides mechanical strength and stability, while the polysulfone layer provides support. The oligomer serves as the main structure of the functional layer, and UV curing is used to cross-link and solidify the oligomer, forming a stable functional layer that provides the membrane's basic separation performance. The addition of an active diluent adjusts the viscosity of the casting solution and promotes the cross-linking and solidification of the oligomer. Coating solution A is a polycationic electrolyte solution, which introduces positive charges onto the membrane surface through electrostatic adsorption, improving the cation rejection rate. Coating solution B is a polyanionic electrolyte solution, which introduces negative charges onto the membrane surface through electrostatic adsorption, improving the anion rejection rate. Through the alternating adsorption of the polyelectrolyte coatings, the surface charge properties and charge quantity of the membrane are dynamically adjusted, optimizing the selective separation of ions with different valence states.

[0024] As a further technical solution, the polysulfone-based film is prepared through the following steps:

[0025] Step B1: Slowly add polysulfone powder to N-methylpyrrolidone and stir magnetically for 4-6 hours until completely dissolved to form a transparent and homogeneous solution; then add polyethylene glycol and continue stirring for 2-3 hours, and let stand for 12-24 hours to remove bubbles to obtain the casting solution.

[0026] Step B2: Using a coating machine, uniformly coat the casting solution obtained in step B1 onto a glass plate, then immerse it in deionized water and soak it at 20°C for 1-10 minutes until the film is completely peeled off from the substrate. After removal, soak and wash it with deionized water 3-5 times to obtain a polysulfone-based film.

[0027] The beneficial effects of this invention are:

[0028] 1. Compared with the prior art, the nanofiltration membrane prepared by the present invention forms a stable functional layer by simply cross-linking and curing oligomers through an ultraviolet irradiation curing reaction.

[0029] 2. This invention uses ultraviolet curing instead of high-temperature heat treatment, resulting in lower energy consumption and reduced preparation costs; the electrostatic adsorption coating process is simple and suitable for industrial production.

[0030] 3. This invention adapts to salt lake brines with different ion compositions by alternating adsorption of polyelectrolyte coatings; and optimizes the selective separation of ions with different valence states.

[0031] In summary, the nanofiltration membrane with controllable surface charge prepared by this invention has important application value in the field of lithium extraction from salt lake brine. Detailed Implementation

[0032] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0033] Example 1

[0034] Preparation of polysulfone-based membranes:

[0035] Step B1: Slowly add polysulfone powder to N-methylpyrrolidone and stir magnetically for 6 hours until completely dissolved to form a transparent and homogeneous solution; then add polyethylene glycol, continue stirring for 3 hours, and let stand for 12 hours to remove bubbles to obtain the casting solution;

[0036] Step B2: Using a coating machine, uniformly coat the casting solution obtained in step B1 onto a glass plate, then immerse it in deionized water and soak it at 20°C for 1 minute until the film is completely peeled off from the substrate. After removal, soak and wash it 5 times with deionized water to obtain a polysulfone-based film.

[0037] Example 2

[0038] Step S1: First, the casting solution (viscosity 200 mPa·s) is uniformly coated onto the surface of the polysulfone-based membrane prepared in Example 1 using a slit coating head, forming a coating with a thickness of 1 μm on the membrane surface; then, under nitrogen protection, an ultraviolet lamp (light intensity 30 mW / cm²) is used. 2 The membrane surface was irradiated for 15 seconds, and then the membrane surface was rinsed multiple times with deionized water to obtain the nascent nanofiltration membrane.

[0039] Step S2: Immerse the nascent nanofiltration membrane obtained in step S1 into coating solution A (100 g / L sodium chloride and 5 g / L polydimethyldiallylammonium chloride, solvent is deionized water), let it stand for 2 minutes, then remove it to obtain a membrane sheet. Rinse the membrane sheet with deionized water and clean the surface of the membrane sheet with an air knife. Then immerse the membrane sheet into coating solution B (50 g / L sodium chloride and 3 g / L sodium polystyrene sulfonate, solvent is deionized water), let it stand for 2 minutes, then remove the membrane sheet and rinse it with deionized water. Clean the surface of the membrane sheet with an air knife. Repeat the coating operation on the membrane sheet more than 2 times to form a 3-layer coating and obtain a nanofiltration membrane with controllable surface charge.

[0040] The casting solution is prepared by mixing the following raw materials in parts by weight: 45g epoxy acrylate oligomer, 40g hydroxyethyl acrylate, 3g 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 7g sodium p-styrene sulfonate, 5g polyethylene glycol diacrylate and deionized water.

[0041] Example 3

[0042] Unlike Example 2, in step S2, the membrane is coated more than 5 times, with a total of 6 coating layers. The remaining steps are the same as in Example 2, resulting in a nanofiltration membrane with controllable surface charge.

[0043] Example 4

[0044] Step S1: First, the casting solution (viscosity 1000 mPa·s) is uniformly coated onto the surface of the polysulfone-based membrane prepared in Example 1 using a slit coating head, forming a coating with a thickness of 1 μm on the membrane surface; then, under nitrogen protection, an ultraviolet lamp (light intensity 100 mW / cm²) is used. 2 The membrane surface was irradiated for 15 seconds, and then the membrane surface was rinsed multiple times with deionized water to obtain the nascent nanofiltration membrane.

[0045] Step S2: Immerse the nascent nanofiltration membrane obtained in step S1 into coating solution A (100 g / L sodium chloride and 5 g / L polydimethyldiallylammonium chloride, solvent is deionized water), let it stand for 2 minutes, then remove it to obtain a membrane sheet. Rinse the membrane sheet with deionized water and clean the surface of the membrane sheet with an air knife. Then immerse the membrane sheet into coating solution B (50 g / L sodium chloride and 3 g / L sodium polystyrene sulfonate, solvent is deionized water), let it stand for 2 minutes, then remove the membrane sheet and rinse it with deionized water. Clean the surface of the membrane sheet with an air knife. Repeat the coating operation on the membrane sheet more than 2 times to form a 3-layer coating and obtain a nanofiltration membrane with controllable surface charge.

[0046] The casting solution is prepared by mixing the following raw materials in parts by weight: 40g epoxy acrylate oligomer, 40g hydroxyethyl acrylate, 3g 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 12g sodium p-styrene sulfonate, 5g polyethylene glycol diacrylate and deionized water.

[0047] Example 5

[0048] Unlike Example 4, in step S2, the membrane is coated more than 5 times, with a total of 6 coating layers. The remaining steps are the same as in Example 4, resulting in a nanofiltration membrane with controllable surface charge.

[0049] Example 6

[0050] Step S1: First, the casting solution (viscosity 1000 mPa·s) is uniformly coated onto the surface of the polysulfone-based membrane prepared in Example 1 using a slit coating head, forming a coating with a thickness of 1 μm on the membrane surface; then, under nitrogen protection, an ultraviolet lamp (light intensity 100 mW / cm²) is used. 2 The membrane surface was irradiated for 15 seconds, and then the membrane surface was rinsed multiple times with deionized water to obtain the nascent nanofiltration membrane.

[0051] Step S2: Immerse the nascent nanofiltration membrane obtained in step S1 into coating solution A (100 g / L sodium chloride and 5 g / L polydimethyldiallylammonium chloride, solvent is deionized water), let it stand for 3 minutes, then remove it to obtain a membrane sheet. Rinse the membrane sheet with deionized water and clean the surface of the membrane sheet with an air knife. Then immerse the membrane sheet into coating solution B (50 g / L sodium chloride and 3 g / L sodium polystyrene sulfonate, solvent is deionized water), let it stand for 3 minutes, then remove the membrane sheet and rinse it with deionized water. Clean the surface of the membrane sheet with an air knife. Repeat the coating operation on the membrane sheet more than 2 times to form a 3-layer coating and obtain a nanofiltration membrane with controllable surface charge.

[0052] The casting solution is prepared by mixing the following raw materials in parts by weight: 45g polyurethane acrylate oligomer, 40g methacrylate, 3g 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 7g sodium p-styrene sulfonate, 5g polyethylene glycol diacrylate and deionized water.

[0053] Comparative Example 1

[0054] Step 1: Immerse the surface of the polysulfone-based membrane prepared in Example 1 into a 2wt% piperazine aqueous solution. After standing for 1 minute, use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The pressure is 30 PSI to obtain a pre-treated polysulfone-based membrane.

[0055] Step 2: Dip the surface of the pre-treated polysulfone membrane obtained in Step 1 into a hexane solution containing 0.2 wt% trimesoyl chloride. After standing for 15 seconds, place the polysulfone ultrafiltration membrane in a 60°C oven and dry for 1.5 min to obtain a nanofiltration membrane. Store the nanofiltration membrane in deionized water for subsequent testing.

[0056] The membrane was evaluated using cross-flow filtration. The test solution was a diluted solution from Zabuye Salt Lake (Zabuye Salt Lake solution: deionized water = 1:3, volume ratio), with a pH of 9-10. The water flux and ion rejection of the membrane were tested at 300 PSI. The main components of the solution are shown in Table 1.

[0057] Table 1

[0058] <![CDATA[Li + / ppm]]> <![CDATA[Na + / ppm]]> <![CDATA[K + / ppm]]> <![CDATA[Cl - / ppm]]> <![CDATA[SO4 2- / ppm]]> <![CDATA[CO3 2- / ppm]]> 201.42 24240.25 6854.13 34027.90 2442.94 4999.67

[0059] The salt separation performance of nanofiltration membranes in Examples 2, 3, 4, 5, 6 and Comparative Example 1 was evaluated, and the salt rejection rate and water flux were measured.

[0060] Salt rejection rate R: Under certain test conditions, the concentration of ion A in the influent (C f ) and the concentration of ion A in the product water (C p The difference between (1-C) and (2-C) is then divided by the concentration of ions A in the influent: R = (1-C) p / C f )*100%.

[0061] Water flux: Under certain test conditions, the amount of water produced per unit membrane area per unit time (GFD);

[0062] The measurement results are shown in Table 2:

[0063] Table 2

[0064] Flux (GFD) <![CDATA[Li + / Retention Rate]]> <![CDATA[Na + / Retention Rate]]> <![CDATA[K + / Retention Rate]]> <![CDATA[Cl - / Retention Rate]]> <![CDATA[SO4 2- / Retention Rate]]> <![CDATA[CO3 2- / Retention Rate]]> Comparative Example 1 15.53 22.13% 10.25% 3.31% -8.79% 91.52% 89.18% Example 2 18.88 15.21% 8.22% 2.65% -11.44% 93.30% 93.17% Example 3 16.32 15.87% 8.39% 2.71% -10.96% 95.72% 96.21% Example 4 17.05 14.12% 8.09% 2.53% -11.82% 94.46% 94.65% Example 5 16.17 14.83% 7.77% 2.58% -11.46% 97.08% 96.89% Example 6 19.02 14.89% 8.14% 2.77% -11.29% 92.99% 92.38%

[0065] As shown in Table 2, the nanofiltration membrane prepared in the embodiments of the present invention has a lower rejection rate for lithium ions than the comparative example, but a higher rejection rate for high-valence anions than the comparative example, which means it has higher separation efficiency and higher water flux. Therefore, the nanofiltration membrane with controllable surface charge prepared in the present invention has important application value in the field of lithium extraction from salt lake brine.

[0066] In the description of this specification, the references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0067] The above description is merely an example and illustration of the present invention. Any modifications or additions made by those skilled in the art to the specific embodiments described, or any substitutions made in a similar manner, should fall within the protection scope of the present invention.

Claims

1. A method for preparing a nanofiltration membrane with controllable surface charge, characterized in that, Includes the following steps: Step S1: First, the casting solution is coated onto the surface of the polysulfone-based membrane to form a coating on the membrane surface; Then, under nitrogen protection, the membrane surface is irradiated with ultraviolet light and washed to obtain the nascent nanofiltration membrane. Step S2: Immerse the nascent nanofiltration membrane obtained in step S1 into coating solution A, let it stand, take it out to obtain a membrane sheet, rinse and blow the membrane sheet clean; then immerse the membrane sheet into coating solution B, let it stand, take it out to rinse and blow the membrane sheet clean, repeat the above coating operation on the membrane sheet to form a coating, and obtain a nanofiltration membrane with controllable surface charge. In step S1, the casting solution is prepared by mixing the following raw materials in parts by weight: 40-45 parts oligomer, 30-50 parts reactive diluent, 1-5 parts photoinitiator, 10-25 parts additives and solvent. The additives are sodium p-styrene sulfonate and polyethylene glycol diacrylate, with a mass ratio of 7-12:

5. In step S2, the coating solution A is a polycationic electrolyte solution with a concentration of 0.5-10 g / L; the electrolyte includes one or more of polyacrylamide hydrochloride, polydimethyldiallylammonium chloride, polyethyleneimine, methacryloyloxyethyltrimethylammonium chloride, chitosan, and polyacrylamide. In step S2, the coating solution B is a polyanionic electrolyte solution with a concentration of 0.5-10 g / L; the electrolyte includes one or more of sodium polystyrene sulfonate, sodium polyacrylate, and sodium polyvinyl sulfonate.

2. The method for preparing a nanofiltration membrane with controllable surface charge according to claim 1, characterized in that, The thickness of the coating in step S1 is 0.5-10 μm.

3. The method for preparing a nanofiltration membrane with controllable surface charge according to claim 1, characterized in that, In step S1, the UV lamp irradiation time is 5-60 seconds; the light intensity is 30-100 mW / cm². 2 .

4. The method for preparing a nanofiltration membrane with controllable surface charge according to claim 1, characterized in that, The viscosity of the casting solution is 200-1000 mPa·s, and the viscosity is adjusted by the amount of solvent used.

5. The method for preparing a nanofiltration membrane with controllable surface charge according to claim 1, characterized in that, The oligomer is one of acrylate, epoxy acrylate, and polyurethane acrylate; the reactive diluent is one of hydroxyethyl acrylate, vinyl monomer, methacrylate, diacrylate, triethylene glycol divinyl ether, and 4-hydroxybutyl vinyl ether; the photoinitiator is one of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, ethyl 2,4,6-trimethylbenzoylphenylphosphonate, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, and 2-isopropylthioxanthrone; the solvent is one of deionized water, ethanol, and methanol.

6. The method for preparing a nanofiltration membrane with controllable surface charge according to claim 1, characterized in that, In step S2, the coating sequence is coating solutions A, B...A, B in sequence; the coating consists of coating solutions A and B as one layer, and the number of coating layers is 1-10.

7. A nanofiltration membrane with controllable surface charge, characterized in that, Prepared according to the method according to any one of claims 1-6.