Nanofiltration membrane with controllable surface charges and preparation method thereof

Abstract

The invention discloses a surface charge controllable nanofiltration membrane and a preparation method thereof, and belongs to the technical field of water treatment. The nanofiltration membrane is prepared by taking a polysulfone ultrafiltration membrane as a base membrane, coating a membrane casting solution containing an oligomer, a reactive diluent and the like through a slit coating head, carrying out ultraviolet curing, and then alternately immersing into a polycation electrolyte solution and a polyanion electrolyte solution for coating treatment. Compared with the prior art, the nanofiltration membrane prepared by the invention has the advantages that the oligomer is crosslinked and cured through simple ultraviolet radiation curing reaction to form a stable functional layer; high-temperature heat treatment is replaced by ultraviolet curing, so that the energy consumption is low, and the preparation cost is reduced; the electrostatic adsorption coating step is simple, and is suitable for industrial production; through alternate adsorption of the polyelectrolyte coating, the method adapts to salt lake brine composed of different ions; and the selective separation of ions with different valence states is optimized. In conclusion, the prepared nanofiltration membrane with controllable surface charges has important application value in the field of lithium extraction of salt lake brine.

Description

Technical Field

[0001] The present invention belongs to the technical field of water treatment, and in particular relates to a nanofiltration membrane with controllable surface charge and a preparation method thereof. Background Art

[0002] With the continuous development of new energy technologies, lithium-ion batteries, an indispensable component, are experiencing increasing demand. Lithium, the positive electrode material for lithium-ion batteries, cannot be directly found in nature and is generally found in mineral lithium ores and salt lake brines. Most processes for extracting lithium from salt lake brines process high-calcium and high-magnesium brines. However, some specialized salt lakes, such as the Zabuye Salt Lake, have very low calcium and magnesium contents but high concentrations of high-valent anions such as carbonate and sulfate. This presents significant challenges for traditional lithium extraction methods.

[0003] Nanofiltration membrane is a new type of separation membrane with a pore size between that of reverse osmosis membrane and ultrafiltration membrane. It has a nanometer-scale pore size and multiple charges on the membrane, allowing low-molecular salts to pass through while retaining higher molecular weight organic matter and multivalent ions. It has unique separation performance and higher separation accuracy. At present, most nanofiltration composite membranes produced commercially and applied industrially are prepared by interfacial polymerization between polyamines and polyacyl chlorides. The desalination functional layer of the nanofiltration composite membrane is a polyamide material, which has many applications in lithium extraction from salt lakes. However, due to its surface charge characteristics, it cannot effectively adapt to the complex environment of high-valent anions. Therefore, for brine from salt lakes rich in high-valent anions, the lithium extraction effect is less than expected.

[0004] Therefore, in order to solve the above problems, a nanofiltration membrane with controllable surface charge is developed, which has important practical significance for expanding the industrial application of nanofiltration membrane technology in lithium extraction. Summary of the Invention

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

[0006] The purpose of the present invention can be achieved through the following technical solutions: A method for preparing a nanofiltration membrane with controllable surface charge comprises the following steps: Step S1: first, a casting solution is uniformly coated on the surface of a 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 with deionized water multiple times to obtain a primary nanofiltration membrane; Step S2: immerse the primary nanofiltration membrane obtained in step S1 in coating solution A, let it stand for 2-3 minutes, take it out to obtain a membrane, rinse the membrane with deionized water, and blow the surface of the membrane clean with an air knife; then immerse the membrane in coating solution B, let it stand for 2-3 minutes, take out the membrane and rinse it with deionized water, blow the surface of the membrane clean with an air knife, repeat the above coating operation on the membrane to form a coating, and obtain a nanofiltration membrane with controllable surface charge.

[0007] As a further technical solution, the thickness of the coating in step S1 is 0.5-10 μm.

[0008] As a further technical solution, the UV lamp irradiation time in step S1 is 5-60s.

[0009] As a further technical solution, the light intensity of the UV lamp in step S1 is 30-100 mW / cm 2 .

[0010] 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 of oligomer, 30-50 parts of active diluent, 1-5 parts of photoinitiator, 10-25 parts of additives and solvent.

[0011] As a further technical solution, the viscosity of the casting solution is 200-1000 mPa·s, and the viscosity is regulated by the amount of solvent used.

[0012] As a further technical solution, the oligomer is one of acrylate, epoxy acrylate and polyurethane acrylate.

[0013] As a further technical solution, the active diluent is one of hydroxyethyl acrylate, vinyl monomer, methacrylate, diacrylate, triethylene glycol divinyl ether and 4-hydroxybutyl vinyl ether.

[0014] As a further technical solution, the photoinitiator is one of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 2,4,6-trimethylbenzoylphenylphosphonic acid ethyl ester, phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide and 2-isopropylthioxanthone.

[0015] As a further technical solution, the additive is sodium p-styrene sulfonate and polyethylene glycol diacrylate in a mass ratio of 7-12:5.

[0016] As a further technical solution, the solvent is one of deionized water, ethanol and methanol.

[0017] As a further technical solution, the coating solution A in step S2 is a polycationic electrolyte solution with a concentration of 0.5-10 g / L; wherein the electrolyte includes one or more of polyacrylamine hydrochloride, polydimethyldiallyl ammonium chloride, polyethyleneimine, methacryloyloxyethyltrimethylammonium chloride, chitosan and polyacrylamide.

[0018] As a further technical solution, the coating solution B in step S2 is a polyanion 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.

[0019] As a further technical solution, in step S2, the coating order is coating solutions A, B...A, B in sequence; the coating is one layer of coating solutions A and B, and the number of coating layers is 1-10.

[0020] The present invention uses a self-made polysulfone ultrafiltration base membrane to carry an oligomer casting solution. The non-woven fabric in the polysulfone ultrafiltration base membrane provides a certain mechanical strength and stability, and the polysulfone layer provides a supporting and bearing function. The oligomer serves as the main structure of the functional layer, and ultraviolet curing is used to cross-link and solidify the oligomer to form a stable functional layer, which provides basic separation performance of the membrane. The addition of an active diluent can adjust the viscosity of the casting solution and promote the cross-linking and solidification of the oligomer. The coating solution A is a polycationic electrolyte solution, which can introduce positive charges on the membrane surface through electrostatic adsorption to improve the retention rate of cations. The coating solution B is a polyanionic electrolyte solution, which can introduce negative charges on the membrane surface through electrostatic adsorption to improve the retention rate of anions. Through the alternating adsorption of the polyelectrolyte coating, the charge properties and charge amount of the membrane surface are dynamically adjusted to optimize the selective separation of ions with different valence states.

[0021] As a further technical solution, the polysulfone-based membrane is prepared by the following steps: Step B1: slowly add polysulfone powder to N-methylpyrrolidone and stir magnetically for 4-6 hours until completely dissolved to form a transparent uniform solution; then add polyethylene glycol, continue stirring for 2-3 hours, and allow to stand for degassing for 12-24 hours to obtain a casting solution; Step B2: Use a coating machine to evenly coat the casting solution obtained in step B1 on a glass plate, then immerse it in deionized water at 20°C for 1-10 minutes until the membrane is completely peeled off from the substrate. After taking it out, soak it in deionized water and wash it 3-5 times to obtain a polysulfone-based membrane.

[0022] Beneficial effects of the present invention: 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 the oligomers through ultraviolet radiation curing reaction; 2. The present invention uses ultraviolet curing instead of high-temperature heat treatment, which has low energy consumption and reduced preparation costs; the electrostatic adsorption coating process is simple and suitable for industrial production; 3. The present invention adapts to salt lake brine with different ion compositions through the alternating adsorption of the polyelectrolyte coating and optimizes the selective separation of ions with different valence states.

[0023] In summary, the surface charge controllable nanofiltration membrane prepared by the present invention has important application value in the field of lithium extraction from salt lake brine. DETAILED DESCRIPTION

[0024] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0025] Example 1

[0026] Preparation of polysulfone-based membrane: Step B1, slowly adding polysulfone powder to N-methylpyrrolidone, and magnetically stirring for 6 hours until completely dissolved to form a transparent uniform solution; then adding polyethylene glycol, continuing to stir for 3 hours, and standing for degassing for 12 hours to obtain a casting solution; Step B2: Use a coating machine to evenly coat the casting solution obtained in step B1 on a glass plate, then immerse it in deionized water at 20°C for 1 minute until the membrane is completely peeled off from the substrate. After taking it out, soak it in deionized water and wash it 5 times to obtain a polysulfone-based membrane.

[0027] Example 2

[0028] Step S1: First, the casting solution (viscosity of 200 mPa·s) is evenly coated on the surface of the polysulfone-based membrane prepared in Example 1 through a slit coating head to form a coating with a thickness of 1 μm on the membrane surface; then, under nitrogen protection, an ultraviolet lamp (light intensity of 30 mW / cm 2 ) irradiate the membrane surface for 15 seconds, and then rinse the membrane surface with deionized water several times to obtain a primary nanofiltration membrane; Step S2, immersing the primary nanofiltration membrane obtained in step S1 in coating solution A (100 g / L sodium chloride and 5 g / L polydimethyldiallyl ammonium chloride, the solvent is deionized water), letting it stand for 2 minutes, taking it out to obtain a membrane, rinsing the membrane with deionized water, and blowing the surface of the membrane clean with an air knife; then immersing the membrane in coating solution B (50 g / L sodium chloride and 3 g / L sodium polystyrene sulfonate, the solvent is deionized water), letting it stand for 2 minutes, taking it out and rinsing the membrane with deionized water, blowing the surface of the membrane clean with an air knife, repeating the coating operation on the membrane more than twice to form three layers of coating, and obtaining a nanofiltration membrane with controllable surface charge; 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-styrenesulfonate, 5g polyethylene glycol diacrylate and deionized water.

[0029] Example 3

[0030] The difference from Example 2 is that in step S2, the membrane is coated for more than 5 times, and the number of coating layers is 6. The remaining steps are the same as those in Example 2, and a nanofiltration membrane with controllable surface charge is obtained.

[0031] Example 4

[0032] Step S1: First, the casting solution (viscosity of 1000 mPa·s) is evenly coated on the surface of the polysulfone-based membrane prepared in Example 1 through a slit coating head to form a coating with a thickness of 1 μm on the membrane surface; then, under nitrogen protection, an ultraviolet lamp (light intensity of 100 mW / cm 2 ) irradiate the membrane surface for 15 seconds, and then rinse the membrane surface with deionized water several times to obtain a primary nanofiltration membrane; Step S2, immersing the primary nanofiltration membrane obtained in step S1 in coating solution A (100 g / L sodium chloride and 5 g / L polydimethyldiallyl ammonium chloride, the solvent is deionized water), letting it stand for 2 minutes, taking it out to obtain a membrane, rinsing the membrane with deionized water, and blowing the surface of the membrane clean with an air knife; then immersing the membrane in coating solution B (50 g / L sodium chloride and 3 g / L sodium polystyrene sulfonate, the solvent is deionized water), letting it stand for 2 minutes, taking it out and rinsing the membrane with deionized water, blowing the surface of the membrane clean with an air knife, repeating the coating operation on the membrane more than twice to form three layers of coating, and obtaining a nanofiltration membrane with controllable surface charge; 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-styrenesulfonate, 5g polyethylene glycol diacrylate and deionized water.

[0033] Example 5

[0034] The difference from Example 4 is that in step S2, the membrane is coated for more than 5 times, and the number of coating layers is 6. The remaining steps are the same as those in Example 4, and a nanofiltration membrane with controllable surface charge is obtained.

[0035] Example 6

[0036] Step S1: First, the casting solution (viscosity of 1000 mPa·s) is evenly coated on the surface of the polysulfone-based membrane prepared in Example 1 through a slit coating head to form a coating with a thickness of 1 μm on the membrane surface; then, under nitrogen protection, an ultraviolet lamp (light intensity of 100 mW / cm 2 ) irradiate the membrane surface for 15 seconds, and then rinse the membrane surface with deionized water several times to obtain a primary nanofiltration membrane; Step S2, immersing the primary nanofiltration membrane obtained in step S1 in coating solution A (100 g / L sodium chloride and 5 g / L polydimethyldiallyl ammonium chloride, the solvent is deionized water), letting it stand for 3 minutes, taking it out to obtain a membrane, rinsing the membrane with deionized water, and blowing the surface of the membrane clean with an air knife; then immersing the membrane in coating solution B (50 g / L sodium chloride and 3 g / L sodium polystyrene sulfonate, the solvent is deionized water), letting it stand for 3 minutes, taking it out and rinsing the membrane with deionized water, blowing the surface of the membrane clean with an air knife, repeating the coating operation on the membrane more than twice to form three layers of coating, and obtaining a nanofiltration membrane with controllable surface charge; The casting solution is prepared by mixing the following raw materials in parts by weight: 45g of polyurethane acrylate oligomer, 40g of methacrylate, 3g of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 7g of sodium p-styrenesulfonate, 5g of polyethylene glycol diacrylate and deionized water.

[0037] Comparative Example 1 Step 1: Immerse the surface of the polysulfone-based membrane prepared in Example 1 in a 2 wt% piperazine aqueous solution, let it stand for 1 minute, and then use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane to a pressure of 30 PSI to obtain a preliminarily treated polysulfone-based membrane; Step 2: Dip-coat the surface of the preliminarily treated polysulfone-based membrane obtained in step 1 with a n-hexane solution containing 0.2 wt% trimesoyl chloride. After standing for 15 seconds, place the polysulfone ultrafiltration-based membrane in a 60°C oven and dry it for 1.5 minutes to obtain a nanofiltration membrane, which is then stored in deionized water for subsequent testing.

[0038] The membrane was evaluated by cross-flow filtration. The test solution was a dilution of Zabuye Salt Lake solution (Zabuye Salt Lake solution: deionized water = 1:3, volume ratio), with a pH of 9-10. The membrane's water flux and ion retention were tested at a pressure of 300 PSI. The main components of the solution are shown in Table 1: Table 1 <![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 The salt separation performance of the nanofiltration membranes of Examples 2, 3, 4, 5, 6 and Comparative Example 1 was evaluated to determine the salt rejection rate and water flux; Salt retention rate R: Under certain test conditions, the concentration of ion A in the influent (C f ) and the concentration of ion A in the produced water (C p ) divided by the concentration of ion A in the influent: R = (1-C p / C f )*100%.

[0039] Water flux: Under certain test conditions, the water production per unit membrane area per unit time (GFD); The measured results are shown in Table 2: Table 2 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% As can be seen from Table 2, the nanofiltration membrane prepared in the embodiment of the present invention has a lower retention rate for lithium ions than the comparative example, and a higher retention rate for high-valent anions than the comparative example, that is, it has a higher separation efficiency and a high 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.

[0040] Throughout the specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these 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 any one or more embodiments or examples.

[0041] The above contents are merely examples and explanations of the present invention. Any modifications or additions made by those skilled in the art to the described specific embodiments, or replacements made in a similar manner, shall fall within the scope of protection of the present invention.

Claims

1. A method for preparing a nanofiltration membrane with controllable surface charge, characterized in that: The following steps are involved: Step S1: first, applying a casting solution to the surface of a polysulfone-based membrane to form a coating on the membrane surface; Then, under nitrogen protection, the membrane surface is irradiated with an ultraviolet lamp to rinse the membrane surface, thereby obtaining a primary nanofiltration membrane; Step S2: immerse the primary nanofiltration membrane obtained in step S1 in coating solution A, let it stand, take it out to obtain a membrane, rinse and purge the membrane; then immerse the membrane in coating solution B, let it stand, take out the membrane, rinse and purge it, repeat the above coating operation on the membrane to form a coating, and obtain a nanofiltration membrane with controllable surface charge.

2. The method for preparing a nanofiltration membrane with controllable surface charge according to claim 1, wherein: 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, wherein: The UV lamp irradiation time in step S1 is 5-60s; the light intensity is 30-100 mW / cm 2 .

4. The method for preparing a nanofiltration membrane with controllable surface charge according to claim 1, wherein: In step S1, the casting solution is prepared by mixing the following raw materials in parts by weight: 40-45 parts of oligomer, 30-50 parts of reactive diluent, 1-5 parts of photoinitiator, 10-25 parts of additives and solvent.

5. 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 regulated by the amount of solvent used.

6. The method for preparing a nanofiltration membrane with controllable surface charge according to claim 4, 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, phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide and 2-isopropylthioxanthone; the additive is sodium p-styrene sulfonate and polyethylene glycol diacrylate in a mass ratio of 7-12:5; and the solvent is one of deionized water, ethanol and methanol.

7. The method for preparing a nanofiltration membrane with controllable surface charge according to claim 1, characterized in that: 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 polyacrylamine hydrochloride, polydimethyldiallyl ammonium chloride, polyethyleneimine, methacryloyloxyethyltrimethylammonium chloride, chitosan and polyacrylamide.

8. The method for preparing a nanofiltration membrane with controllable surface charge according to claim 1, characterized in that: In step S2, the coating solution B is a polyanion 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.

9. 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 solution A and B constitute one layer, and the number of coating layers is 1-10.

10. A nanofiltration membrane with controllable surface charge, characterized in that: Prepared according to the method according to any one of claims 1 to 9.