An asymmetric carbon nanotube hollow fiber membrane, a preparation method and application thereof

Abstract

The application discloses an asymmetric carbon nanotube hollow fiber membrane and a preparation method and application thereof, and belongs to the technical field of membrane water treatment. First, carbon nanotubes, a pore-forming agent and a polymer are added into an organic solvent to be uniformly dispersed, and then a separation layer spinning solution is obtained after defoaming; the polymer and the pore-forming agent are added into the organic solvent to be dissolved, and then a base spinning solution is obtained after defoaming; the separation layer spinning solution is used as an outer layer shell solution, the base spinning solution is used as an inner layer shell solution, and water is used as a core solution; the three-phase coaxial spinning head is spun into a coagulation bath at the same time to obtain an asymmetric hollow fiber membrane with a base of a polymer and a separation layer of carbon nanotubes / polymer. The prepared asymmetric carbon nanotube hollow fiber membrane has a thin separation layer, good electrical conductivity and excellent permeation flux, the separation layer and the base are in close contact, the mechanical strength is good, and the asymmetric carbon nanotube hollow fiber membrane can be coupled with electrochemistry, and the anti-pollution capacity of the asymmetric carbon nanotube hollow fiber membrane is significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of membrane water treatment technology, and specifically relates to an asymmetric carbon nanotube hollow fiber membrane, its preparation method, and its application. Background Technology

[0002] Membrane separation technology utilizes the selective permeability of membranes to separate, concentrate, and purify substances. It is a relatively simple and energy-efficient separation process. Currently, membrane separation technology is widely used in drinking water purification, industrial wastewater treatment, sterilization of food and beverage water, and recovery and refining of bioactive substances. It is also rapidly being extended to many fields such as textiles, power, chemicals, petroleum, biology, and pharmaceuticals. However, traditional separation membranes still have many problems due to limitations in their materials and structure, such as membrane fouling and the conflict between selectivity and permeability.

[0003] Conductive separation membranes are a novel type of separation membrane developed in recent years. They can enhance their performance by coupling electrochemical principles and achieve new functions not possessed by traditional separation membranes. Research has found that hollow fiber membranes based on carbon nanotubes have good conductivity and can achieve efficient removal of small molecule pollutants in wastewater at high flux through electro-enhanced adsorption or electro-oxidative decomposition (Environmental Science & Technology, 2014, 48, 8062-8068; ACS Applied Materials & Interfaces, 2015, 7, 14620-14627). Chinese invention patent ZL201310272800.4 discloses a method for preparing carbon nanotube hollow fiber membranes based on electrophoretic deposition, but due to the thick membrane structure, the prepared membrane has low permeate flux and very low preparation efficiency. Chinese invention patent ZL201410079152.5 discloses a method for scalable preparation of carbon nanotube hollow fiber membranes, but the energy consumption is high and the strength of the prepared membrane structure is poor, making it difficult to meet the needs of practical applications. Therefore, developing novel carbon nanotube hollow fiber membranes with excellent comprehensive performance and their efficient preparation methods is of great value. Summary of the Invention

[0004] To address the problems of poor structural strength and high energy consumption in the preparation of existing hollow carbon nanotube membranes, the present invention aims to provide an asymmetric hollow carbon nanotube membrane and its efficient preparation method. The basic concept of this invention is to design an asymmetric hollow carbon nanotube membrane, reducing the thickness of the carbon nanotube separation layer to decrease water mass transfer resistance and increase water permeation flux. A polymer identical to the substrate is added to the spinning solution of the separation layer; after phase inversion, crosslinking between carbon nanotubes and between carbon nanotubes and the substrate is achieved, resulting in high structural strength. The spinning solutions of the separation layer and the substrate are simultaneously phase-inverted to prepare a substrate-coated hollow carbon nanotube membrane in one step. The prepared asymmetric hollow carbon nanotube membrane has a thin separation layer, good conductivity, and excellent permeation flux. Furthermore, the separation layer and the substrate have close contact, good mechanical strength, and can be coupled electrochemically, significantly improving its antifouling ability.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] In a first aspect, the present invention provides a method for preparing an asymmetric carbon nanotube hollow fiber membrane, comprising the following steps:

[0007] (1) Add carbon nanotubes, pore-forming agent and polymer to organic solvent, stir at 30-80°C until carbon nanotubes are uniformly dispersed, and obtain the separation layer spinning solution after standing or vacuum degassing.

[0008] (2) Add the polymer and pore-forming agent to an organic solvent, stir at 30-80°C until dissolved, and obtain the substrate spinning solution after standing or vacuum degassing.

[0009] (3) Using the separation layer spinning solution as the outer shell solution, the substrate spinning solution as the inner shell solution, and water as the core solution, the three phases are simultaneously spun into a coagulation bath at a temperature of 10-80℃ through a three-phase coaxial spinning head to obtain an asymmetric hollow fiber membrane with a polymer substrate and a carbon nanotube / polymer separation layer.

[0010] Based on the above technical solution, further, the carbon nanotubes in step (1) include untreated carbon nanotubes, carboxylated carbon nanotubes, hydroxylated carbon nanotubes, aminolated carbon nanotubes and sulfonated carbon nanotubes, and the content of carboxyl, hydroxyl, amino and sulfonic acid groups on the surface of the carbon nanotubes is 0.5 to 10%.

[0011] Based on the above technical solution, further, the carbon nanotubes mentioned in step (1) are one or a mixture of two or more types of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0012] Based on the above technical solution, further, the polymer mentioned in steps (1) and (2) is one or a mixture of two or more of polyvinylidene fluoride, polyacrylonitrile, polyethersulfone, polysulfone, and polyvinyl butyral; the organic solvent is one or a mixture of two or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, or dimethyl sulfoxide.

[0013] Based on the above technical solution, further, the pore-forming agent mentioned in steps (1) and (2) is polyethylene glycol or polyvinylpyrrolidone, preferably polyethylene glycol-400 or polyvinylpyrrolidone K30.

[0014] Based on the above technical solution, further, the mass ratio of the polymer, pore-forming agent, carbon nanotube and organic solvent in step (1) is (0.5~3):(0.1~1):1:(5~30), preferably (0.4~0.6):(0.1~1):1:(5~15).

[0015] Based on the above technical solution, further, the stirring time in step (1) is 1 to 3 days.

[0016] Based on the above technical solution, further, the mass ratio of the polymer, pore-forming agent and organic solvent in step (2) is 1:(0.1~1):(1~10), preferably 1:(0.4~0.6):1:(2~6).

[0017] Based on the above technical solution, further, the flow rate ratio among the outer shell liquid, inner shell liquid and core liquid in step (3) is 1:(1~5):(0.2~1), preferably 1:(1~1.5):(0.5~1).

[0018] Based on the above technical solution, further, the coagulation bath mentioned in step (3) is a mixture of one or two of water and organic solvents. The organic solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone or dimethyl sulfoxide. The volume ratio of water to organic solvent is 1:(0-10).

[0019] Secondly, the present invention provides an asymmetric carbon nanotube hollow fiber membrane prepared by the above preparation method.

[0020] Based on the above technical solution, the cross-section of the asymmetric carbon nanotube hollow fiber membrane is layered, the separation layer is a mixture of carbon nanotubes and polymer, and the substrate is a polymer.

[0021] Thirdly, the present invention provides the application of the above-mentioned asymmetric carbon nanotube hollow fiber membrane in wastewater treatment.

[0022] Based on the above technical solution, the asymmetric carbon nanotube hollow fiber membrane is further used for wastewater treatment by electrochemical coupling. The asymmetric carbon nanotube hollow fiber membrane is used as the working electrode, and a voltage of 0.5 to 3.0V is applied between the working electrode and the counter electrode. The filtration mode adopts dead end or cross-flow mode.

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

[0024] (1) The asymmetric carbon nanotube hollow fiber membrane prepared by the present invention has high tensile strength and good flexibility, and can be bent and twisted.

[0025] (2) The asymmetric carbon nanotube hollow fiber membrane prepared by the present invention has high permeation flux and good separation performance.

[0026] (3) The asymmetric carbon nanotube hollow fiber membrane prepared by the present invention has good conductivity, can couple electrochemical principles, strengthen the electrostatic repulsion between the membrane surface and pollutants, and significantly improve the antifouling performance of the membrane.

[0027] (4) The preparation method of the asymmetric carbon nanotube hollow fiber membrane prepared by the present invention is simple and efficient, does not require expensive chemical reagents and equipment, and is easy to scale up. Attached Figure Description

[0028] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the following detailed description to explain the invention, but do not constitute a limitation thereof. In the drawings:

[0029] Figure 1 This is a schematic diagram of the structure of the three-phase coaxial spinning head used in the embodiment;

[0030] Figure 2 A scanning electron microscope image of the cross-section of the asymmetric carbon nanotube hollow fiber membrane prepared in Example 1;

[0031] Figure 3 A scanning electron microscope image of the surface of the asymmetric carbon nanotube hollow fiber membrane prepared in Example 1;

[0032] Figure 4 A scanning electron microscope image of the surface of the asymmetric carbon nanotube hollow fiber membrane prepared in Example 2;

[0033] Figure 5 This is a graph showing the flux change over time when the asymmetric carbon nanotube hollow fiber membrane prepared in Example 1 is coupled with electrochemical filtration of electrophoretic coating wastewater.

[0034] Figure 6The graph shows the change in flux over time when using the asymmetric carbon nanotube hollow fiber membrane prepared in Example 1 to filter electrophoretic coating wastewater.

[0035] Figure 7 The graph shows the change in flux over time when using a commercial membrane filter to filter electrophoretic coating wastewater in Comparative Example 2. Detailed Implementation

[0036] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims.

[0037] Example 1

[0038] (1) Preparation of the separation layer spinning solution: Weigh 3g of carboxylated carbon nanotubes (carboxyl content of 2%) and 3.75g of polyvinylidene fluoride and add them to 39g of N,N-dimethylacetamide in a mass ratio of 1:1.25:13. In addition, add 1.5g of polyethylene glycol-400 as a pore-forming agent. Stir mechanically at 60℃ for 48h and degas under vacuum to obtain the separation layer spinning solution.

[0039] (2) Preparation of the base spinning solution: Weigh 9g of polyvinylidene fluoride and 3g of polyvinylpyrrolidone K30 and add them to N,N-dimethylacetamide in a mass ratio of 3:1:21. Stir mechanically at 60°C for 12h and degas under vacuum to obtain the base spinning solution.

[0040] (3) Preparation of asymmetric carbon nanotube hollow fiber membranes: A three-phase coaxial wet spinning technique was adopted, with the separation layer spinning solution as the outer shell solution, the base layer spinning solution as the inner shell solution, and water as the core solution, respectively, at 30 mL / h. -1 30mL h -1 and 20mLh -1 The flow rate was controlled, and the film was simultaneously spun into a 20°C water coagulation bath through a three-phase coaxial spinneret. After spinning, the resulting membrane was soaked in water for 24–72 hours to remove organic solvents and pore-forming agents, then naturally dried and collected.

[0041] Figure 1 This is a schematic diagram of the structure of the three-phase coaxial spinning head used in the embodiment.

[0042] Figure 2 The image shows a scanning electron microscope (SEM) image of the cross-section of the prepared carbon nanotube hollow fiber membrane. As can be seen from the image, the membrane exhibits a typical layered structure, with an inner layer of polyvinylidene fluoride (PVDF) having a macroporous structure and an outer layer of carbon nanotubes / PVDF. The inner and outer layers are tightly bonded together.

[0043] Figure 3The image shows a scanning electron microscope (SEM) image of the prepared carbon nanotube hollow fiber membrane. As can be seen from the image, the membrane surface is relatively smooth and free of defects such as cracks.

[0044] Example 2

[0045] (1) Preparation of the separation layer spinning solution: Weigh 3g of carboxylated carbon nanotubes (carboxyl content of 2.6%) and 2.25g of polyvinylidene fluoride and add them to 39g of N,N-dimethylformamide in a mass ratio of 1:0.75:13. In addition, add 1.5g of polyethylene glycol-400 as a pore-forming agent. Stir mechanically at 60℃ for 48h and degas under vacuum to obtain the separation layer spinning solution.

[0046] (2) Preparation of the base spinning solution: Weigh 9g of polyvinylidene fluoride and 3g of polyvinylpyrrolidone K30 and add them to N,N-dimethylformamide in a mass ratio of 3:1:21. Stir mechanically at 60°C for 12h and degas under vacuum to obtain the base spinning solution.

[0047] (3) Preparation of asymmetric carbon nanotube hollow fiber membranes: A three-phase coaxial wet spinning technique was adopted, with the separation layer spinning solution as the outer shell solution, the base layer spinning solution as the inner shell solution, and water as the core solution, respectively, at 30 mL / h. -1 30mL h -1 and 20mLh -1 The flow rate was controlled, and the film was simultaneously spun into a 20°C water coagulation bath through three coaxial spinning heads. After spinning, the resulting membrane was soaked in water for 24–72 hours to remove organic solvents and pore-forming agents, then naturally dried and collected.

[0048] Figure 4 The image shows a scanning electron microscope (SEM) image of the prepared carbon nanotube hollow fiber membrane. As can be seen from the image, compared with the carbon nanotube hollow fiber membrane in Example 1, the surface of the membrane prepared in this example is rougher and has a large number of concave pore structures.

[0049] Example 3

[0050] (1) Preparation of spinning solution for separation layer: Weigh 10g of carboxylated multi-walled carbon nanotube powder (carboxyl content is 2.6%), 10g of polyethersulfone powder and 5g of polyvinylpyrrolidone K30 powder and add them to 75g of N-methylpyrrolidone. Stir at 60℃ for 24h. After vacuum degassing, a viscous spinning solution with a carbon nanotube mass fraction of 10% is obtained, wherein the mass ratio of carbon nanotubes to polyethersulfone is 1:1.

[0051] (2) Preparation of the base spinning solution: Weigh 5g of polyethersulfone powder and 1g of polyvinylpyrrolidone K30 powder and add them to 20g of N-methylpyrrolidone. Stir at 60°C for 24h to fully dissolve. Then, degas under vacuum or by standing to obtain the base spinning solution.

[0052] (3) Preparation of asymmetric carbon nanotube hollow fiber membranes: A three-phase coaxial dry-wet spinning technique was adopted, with the separation layer spinning solution as the outer shell solution, the base layer spinning solution as the inner shell solution, and water as the core solution, respectively, at 20 mL / h. -1 30mL h -1 and 20mL h -1 The flow rate was simultaneously spun into a 20°C water coagulation bath through a three-phase coaxial spinneret. After spinning, the prepared asymmetric carbon nanotube hollow fiber membrane was immersed in water for 24–72 hours to remove organic solvents and pore-forming agents, then naturally dried and collected.

[0053] The prepared membrane has a uniform appearance, no obvious surface defects, and good hydrophilicity.

[0054] Example 4

[0055] (1) Preparation of the separation layer spinning solution: Weigh 10g of carboxylated multi-walled carbon nanotube powder (carboxyl content is 2%) and 5g of polyvinylpyrrolidone K30 powder and add them to 75g of N-methylpyrrolidone. Stir at room temperature for 12h, then add 10g of polyacrylonitrile powder and stir at 80℃ for 24h. After vacuum degassing, the separation layer spinning solution is obtained.

[0056] (2) Preparation of the base spinning solution: Weigh 5g of polyacrylonitrile powder and 2.5g of polyvinylpyrrolidone K30 powder and add them to 20g of N-methylpyrrolidone. Stir at 60℃ for 24h to fully dissolve. Then, degas under vacuum or by standing to obtain the base spinning solution;

[0057] (3) Preparation of asymmetric carbon nanotube hollow fiber membranes: Three-phase coaxial wet spinning technology was adopted, with the separation layer spinning solution as the outer shell solution, the base layer spinning solution as the inner shell solution, and water as the core solution, respectively, at 20 mL / h. -1 30mL h -1 and 20mL h -1 The flow rate was controlled, and the mixture of water and N-methylpyrrolidone was simultaneously spun into a coagulation bath at 60°C through a three-phase coaxial spinneret, with water comprising 40% of the volume. After spinning, the resulting asymmetric carbon nanotube hollow fiber membrane was immersed in water for 24–72 hours to remove organic solvents and pore-forming agents, and then naturally dried and collected.

[0058] Scanning electron microscopy revealed that the prepared asymmetric carbon nanotube hollow fiber membrane had a uniform appearance, no obvious surface defects, and good hydrophilicity.

[0059] Application examples

[0060] Using the asymmetric carbon nanotube hollow fiber membrane prepared in Example 1 as the anode and a titanium mesh as the cathode, with the voltage between the anode and cathode set to 1.5V, a cross-flow mode was used to filter electrophoretic coating wastewater generated by an automotive company, and the transmembrane pressure difference was set to 0.25 bar. The results are as follows: Figure 5 As shown, during the 26-hour operation, the membrane permeation rate increased from the initial 284 L / m³. -2 h -1 bar -1 Gradually decreased to 130L m -2 h -1 bar -1 The decline rate was approximately 54%.

[0061] Comparative Example 1

[0062] The asymmetric carbon nanotube hollow fiber membrane prepared in Example 1 was used to filter electrophoretic coating wastewater from an automotive company (same as the application example) in a cross-flow mode, with the transmembrane pressure difference set at 0.25 bar. The results are as follows... Figure 6 As shown, during the 26-hour operation, the membrane permeation rate increased from the initial 278 L / m³. -2 h -1 bar -1 Gradually decreased to 90 L m -2 h -1 bar -1 The decrease rate was approximately 68%, significantly higher than the flux decrease rate in the application example.

[0063] Comparative Example 2

[0064] A commercially purchased separation membrane was used to filter electrophoretic coating wastewater from an automotive company in a cross-flow mode (same as the application example), with the transmembrane pressure difference set at 0.25 bar. The results are as follows... Figure 7 As shown, during the 26-hour operation, the membrane permeation rate increased from the initial 91 μL / m³. -2 h -1 bar -1 Gradually decreased to 25L m -2 h -1 bar -1 The decrease rate was approximately 73%, significantly higher than the flux decrease rate in the application example.

[0065] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing an asymmetric carbon nanotube hollow fiber membrane, characterized by, The preparation method comprises the following steps: (1) adding carbon nanotubes, pore-forming agent and high molecular polymer into organic solvent, stirring at a temperature of 30-80℃ until the carbon nanotubes are uniformly dispersed, and then standing or vacuum degassing to obtain a separation layer spinning solution; (2) adding high molecular polymer and pore-forming agent into organic solvent, stirring at a temperature of 30-80℃ until the high molecular polymer and pore-forming agent are dissolved, and then standing or vacuum degassing to obtain a base spinning solution; (3) using the separation layer spinning solution as outer shell liquid, the base spinning solution as inner shell liquid, and water as core liquid, the three liquids are simultaneously spun into a coagulation bath with a temperature of 10-80℃ through a three-phase coaxial spinning head to obtain an asymmetric hollow fiber membrane with a base of high molecular polymer and a separation layer of carbon nanotubes / high molecular polymer.

2. The production method according to claim 1, characterized by, The carbon nanotubes in step (1) include untreated carbon nanotubes, carboxylated carbon nanotubes, hydroxylated carbon nanotubes, aminated carbon nanotubes and sulfonated carbon nanotubes, and the content of carboxyl, hydroxyl, amino and sulfonic acid groups on the surface of the carbon nanotubes is 0.5-10%; the type of the carbon nanotubes is one or a mixture of two or more of single-walled carbon nanotubes, double-walled carbon nanotubes or multi-walled carbon nanotubes.

3. The production method according to claim 1, characterized by, The high molecular polymer in steps (1) and (2) is one or a mixture of two or more of polyvinylidene fluoride, polyacrylonitrile, polyethersulfone, polysulfone or polyvinyl butyral; the organic solvent is one or a mixture of two or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone or dimethyl sulfoxide; and the pore-forming agent is polyethylene glycol or polyvinylpyrrolidone.

4. The method of claim 1, wherein, The mass ratio of the high molecular polymer, pore-forming agent, carbon nanotubes and organic solvent in step (1) is (0.5-3):(0.1-1):1:(5-30).

5. The preparation method according to claim 1, characterized in that, The mass ratio of the high molecular polymer, pore-forming agent and organic solvent in step (2) is 1:(0.1-1):(1-10).

6. The method of claim 1, wherein, The flow rate ratio among the outer shell liquid, inner shell liquid and core liquid in step (3) is 1:(1-5):(0.2-1).

7. The preparation method according to claim 1, characterized in that, The coagulation bath in step (3) is a mixture of water and one or two organic solvents, the organic solvent is one or a mixture of two or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone or dimethyl sulfoxide, and the volume ratio of water to organic solvent is 1:(0-10).

8. The asymmetric carbon nanotube hollow fiber membrane prepared by the preparation method in any one of claims 1-7.

9. The use of the asymmetric carbon nanotube hollow fiber membrane in claim 8 in wastewater treatment.

10. Use according to claim 9, characterized in that, The asymmetric carbon nanotube hollow fiber membrane is used for wastewater treatment through electrochemical coupling, a voltage of 0.5-3.0V is applied between the working electrode of the asymmetric carbon nanotube hollow fiber membrane and a counter electrode, and the filtration mode adopts dead-end or cross-flow mode.

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