Three-dimensional nanofiber tubular filter membrane for water filtration and preparation method of three-dimensional nanofiber tubular filter membrane

Abstract

The invention provides a three-dimensional nanofiber tubular filter membrane for water filtration and a preparation method thereof in order to ensure high pollutant holding capacity of a water filter membrane and enhance the mechanical strength of nanofibers. The preparation method comprises the following steps: firstly, preparing a quaternary ammonium salt chitosan / polyvinyl alcohol solution, and carrying out electrostatic spinning to obtain a two-dimensional nanofiber membrane; immersing the two-dimensional nanofiber membrane into anhydrous tert-butyl alcohol, and homogenizing to generate a nanofiber dispersion; the nanofiber dispersoid is freeze-dried in a mold to form nanofiber aerogel, and the pollutant holding amount of the filtering membrane is greatly increased through conversion from a two-dimensional structure to a three-dimensional structure; and finally, aerogel heat treatment is performed to further improve the stability, so that the mechanical strength of the filtering membrane is improved. The antibacterial filtering membrane has the advantages of being simple, practical, easy to popularize and excellent in filtering efficiency and water flux, and the selected quaternary ammonium salt chitosan has an antibacterial effect, so that the antibacterial performance of the filtering membrane is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of water treatment equipment, and relates to a three-dimensional nanofiber tubular filter membrane for water filtration and a preparation method thereof. Background Art

[0002] Membrane filters are water treatment equipment that deeply filters and purifies water quality according to the requirements of water use. The technical core of membrane filters is the filter membrane in the filtering device. However, there are many defects and challenges in current filter membranes. For example: 1) The filtration selectivity and permeability are poor, and there will be problems with insufficient filtration performance when separating impurities in water; 2) The membrane may degrade under specific environmental conditions or during long-term use, affecting its long-term use reliability; 3) The production cost of high-performance membrane materials is relatively high, lacking sufficient economy; 4) The production and disposal processes of the membrane may have an impact on the environment, resulting in environmental pollution; 5) The filter membrane does not have antibacterial properties and has no killing or inhibitory effect on harmful microorganisms such as bacteria in water.

[0003] Nanofibers have attracted attention due to their highly porous structure, narrow pore size, and high specific surface area. One of the most effective techniques for producing nanofibers is electrospinning technology, and the produced nanofibers can be used in multiple fields including water filtration. On the one hand, the pore size of nanofiber membranes is usually at the nanometer level, which can effectively intercept suspended particles and microorganisms in water, and the relatively high specific surface area of nanofiber membranes allows more pollutants to adhere to the membrane. All these make the fiber membrane have a certain dirt-holding capacity. However, as particles accumulate during the filtration process, the dirt-holding capacity may decrease. On the other hand, the small diameter of nanofibers and the porous structure of the fibers result in low overall mechanical properties of the membrane. To solve this problem, the current research focus is to first ensure a high dirt-holding capacity of the water filtration membrane, and secondly to enhance the mechanical strength of the nanofibers. Summary of the Invention

[0004] In view of the above problems, the present invention provides a three-dimensional nanofiber tubular filter membrane for water filtration and a preparation method thereof. The present invention first prepares a quaternary ammonium salt chitosan / polyvinyl alcohol (QCS / PVA) solution and obtains a two-dimensional nanofiber membrane through electrospinning; the two-dimensional nanofiber membrane is homogenized by immersion in anhydrous tert-butanol to produce a nanofiber dispersion; the nanofiber dispersion is freeze-dried in a mold to form a nanofiber aerogel. The transformation from a two-dimensional structure to a three-dimensional structure greatly improves the dirt-holding capacity of the filter membrane; finally, the aerogel is heat-treated to further enhance stability and improve the mechanical strength of the filter membrane. The present invention has the advantages of being simple and practical, easy to promote, and having excellent filtration efficiency and water flux. Moreover, the selected quaternary ammonium salt chitosan has antibacterial effects, improving the antibacterial properties of the filter membrane.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows: A preparation method of a three-dimensional nanofiber tubular filter membrane for water filtration, characterized by comprising the following steps: S1: Prepare a quaternary ammonium salt chitosan / polyvinyl alcohol (QCS / PVA) solution and obtain a two-dimensional nanofiber membrane by electrospinning; S2: Immerse the two-dimensional nanofiber membrane in anhydrous tert-butanol for homogenization to produce a nanofiber dispersion; S3: Freeze-dry the nanofiber dispersion in a mold to form a three-dimensional nanofiber aerogel; S4: Heat-treat the three-dimensional nanofiber aerogel at 150-200 °C to further improve the stability and increase the mechanical strength of the filter membrane.

[0006] Among them, in step S1, the preparation method of the quaternary ammonium salt chitosan (QCS) is as follows: Disperse chitosan in a NaBH4 / NaOH solution and stir at 100-120 °C, filter and wash, and vacuum-dry to obtain deacetylated chitosan. React deacetylated chitosan with 2,3-epoxypropyltrimethylammonium chloride, filter, purify and dry to obtain QCS; The mass ratio of chitosan to NaBH4 is 10:0.9-1.1, preferably 10:1; The mass ratio of deacetylated chitosan to 2,3-epoxypropyltrimethylammonium chloride is 1:2.5-3.2, preferably 1:2.8, and the purity of the prepared QCS is ≥98%.

[0007] Preferably, in step S1, the mass ratio of QCS to PVA in the quaternary ammonium salt chitosan / polyvinyl alcohol (QCS / PVA) solution is 1:3-7, preferably 1:5-6. More preferably, the final concentration of QCS in the QCS / PVA solution is 1-2%, and the final concentration of PVA is 6-8%.

[0008] Preferably, in step S1, the electrospinning conditions are as follows: The spinning uses a steel needle nozzle with an inner diameter of 0.4-0.7 mm, the distance between the needle tip and the collecting plate is 15-18 cm, the spinning temperature is 25-30 °C, the humidity is 50%-65% RH, the voltage at the spinneret needle tip is 25-30 kV, and the solution feeding speed is 0.3-0.5 mL / h.

[0009] In step S2 of the present invention, the electrospun nanofiber membrane is immersed in anhydrous tert-butanol. Anhydrous tert-butanol is an organic solvent and is easy to volatilize during subsequent freeze-drying, which can ensure the stability and uniformity of the fibers. Homogenization in anhydrous tert-butanol gives a nanofiber dispersion with a density of 50-100 g / L.

[0010] In step S3, the nanofiber dispersion is placed in a polyurethane plastic sleeve mold for freeze-drying. The mold includes two parts, an inner part and an outer part. The inner part is a solid cylinder with a smaller radius, and the outer part is a hollow cylinder with a larger radius. The bottom of the mold is sealed, and there is a hollow sandwich layer between its inner and outer layers. The inner diameter of the mold is 1 - 5 cm, and the outer diameter is 1.5 - 8 cm. Two cylindrical copper meshes with diameters of 1 - 5 cm and 1.5 - 8 cm respectively can be placed inside the mold to support the three-dimensional nanofiber aerogel. The pore diameter of the copper mesh is 0.07 - 0.09 mm, the wire diameter is 0.04 - 0.06 mm, the thickness is 0.1 - 0.15 mm, and the density is 8.96 - 9.65 g / cm 3 .

[0011] The freeze-drying in step S3 is preferably carried out at -35 to -55 °C for 45 to 50 h.

[0012] For the three-dimensional nanofiber tubular filter membrane prepared by the above method, a cylindrical copper mesh is placed inside the three-dimensional nanofiber tubular filter membrane to support the three-dimensional nanofiber aerogel, enhancing the overall mechanical properties. The three-dimensional nanofiber tubular filter membrane has a high porosity and a high specific surface area.

[0013] The present invention also discloses the application of the three-dimensional nanofiber tubular filter membrane in water filtration. The nanofiber aerogel has a filtration efficiency of up to 97.34%, a water flux of not less than 2500 L / m 2 ·h, and an antibacterial rate of higher than 98%.

[0014] The reaction principle of QCS and PVA in the present invention generally involves physical and chemical interactions between the two substances: the positively charged quaternary ammonium salt groups on QCS can undergo electrostatic interactions with groups such as hydroxyl groups and anionic substances on PVA; hydrogen bond interactions are formed between the large number of hydroxyl groups on PVA molecules and the amino groups of QCS; during co-blending spinning, the molecular chains of the two will also penetrate each other to form a uniform mixture.

[0015] The ammonium group (-NH3) on the chitosan quaternary ammonium salt molecular chain +) makes it positively charged, and chitosan quaternary ammonium salt can be adsorbed to the negatively charged surface of bacteria through electrostatic interaction, destroying the integrity of the cells, increasing the permeability of the cell membrane, and leaking the cell contents, thereby inhibiting the growth and reproduction of bacteria. Therefore, the two-dimensional nanofibers containing quaternary ammonium salt chitosan have antibacterial properties. When the two-dimensional nanofiber membrane is innovatively prepared into a three-dimensional form, its overall specific surface area is significantly improved, the adsorption efficiency and the pollutant holding capacity are significantly enhanced, and the material is given higher durability, so that it can also ensure a stable pollutant filtration effect during long-term use. Secondly, previous studies have shown that heat treatment can promote fiber bonding and thus improve mechanical strength. Therefore, the application of heat-treated three-dimensional structured nanofiber aerogels in the field of water filtration can successfully achieve the purpose of removing solid particles, suspended matter and other impurities in water, thereby purifying the water quality.

[0016] Compared with the prior art, the present invention has the following beneficial effects: First, the three-dimensional nanofiber aerogel prepared by the present invention has a unique microscopic cell structure with small pores and large specific surface area, which gives it excellent physical properties: compared with a single nanofiber membrane, the three-dimensional structure of the nanofiber aerogel not only greatly improves the porosity of the material, but also forms abundant micro-pores on its surface. These microporous channels provide more passage paths for water molecules. Therefore, nanofiber aerogels show significant advantages in the water treatment process. They have both efficient filtering capabilities, which can effectively remove impurities and pollutants in water, and can ensure a high water flux, allowing water to pass quickly. The excellent filtration efficiency and water flux of aerogels make them ideal filtration materials for a variety of high-performance water treatment applications.

[0017] Secondly, QCS and PVA are selected as the matrix, and the matrix solution can form continuous and stable nanofibers during the electrospinning process. The combination of PVA and QCS can produce a synergistic effect. On the one hand, PVA can improve the fiber-forming property and mechanical strength of QCS, so that the nanofiber membrane has better physical properties and stability; on the other hand, the antibacterial property of QCS can make up for the lack of antibacterial property of PVA, so that the composite nanofiber membrane shows better performance in terms of antibacterial property.

[0018] Finally, aerogels have a unique three-dimensional network structure, which endows them with extremely abundant pores and a high specific surface area, enabling them to effectively adsorb and accommodate more contaminants. Moreover, due to the increased surface area, the contaminant adsorption capacity of aerogels is significantly improved, allowing them to adsorb more pollutants and enhancing their application effects under different environmental conditions. The high adsorption capacity of aerogels not only improves their water treatment efficiency but also delays the rate of contaminant saturation of the material, thereby significantly extending its service life, reducing the need for frequent material replacement, and further improving economic efficiency and sustainability. By heat-treating the aerogel and combining the three-dimensional nanofiber aerogel with a copper mesh, the overall mechanical properties of the prepared filter mesh are enhanced, enabling it to be applicable to practical scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic structural diagram of the three-dimensional nanofiber tubular filter membrane of the present invention; in the figure, 1, copper mesh, 2, three-dimensional nanofiber aerogel filling matrix; Figure 2 is a structural sectional view of the AA position of the three-dimensional nanofiber tubular filter membrane ( Figure 1 ) of the present invention; in the figure, 1, copper mesh, 2, three-dimensional nanofiber aerogel filling matrix; Figure 3 is an SEM image of the unheated QCS / PVA nanofiber aerogel; Figure 4 are the fiber diameters (a) of the unheated QCS / PVA nanofiber aerogel and the fiber diameters (b) of the heated QCS / PVA nanofiber aerogel; Figure 5 are the filtration efficiencies of the QCS / PVA nanofiber aerogels generated under different conditions in Examples 1-3; Figure 6 are the pure water fluxes of the QCS / PVA nanofiber aerogels generated under different conditions in Examples 1-3; Figure 7 are the antibacterial effects (a) and antibacterial rates (b) of the QCS / PVA nanofiber aerogel (NF-A2) and the control group without adding aerogel; DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] The technical solutions of the present invention will be further described in detail below in conjunction with the drawings and specific embodiments. The following examples and drawings are used to illustrate the present invention but are not used to limit the scope of the present invention.

[0021] The mold used in this embodiment includes two parts, an inner part and an outer part. The inner part is a solid cylinder with a smaller radius, and the outer part is a hollow cylinder with a larger radius. The bottom of the mold is blocked, and there is a hollow interlayer between the inner and outer layers. The inner diameter of the mold is 3 cm and the outer diameter is 4.5 cm. Two layers of cylindrical copper mesh with diameters of 3 cm and 4.5 cm can be placed inside the mold to support the three-dimensional nanofiber aerogel. The copper mesh has a pore size of about 0.08 mm, a wire diameter of about 0.05 mm, a thickness of 0.125 mm, and a density of about 9.30 g / cm 3 .

[0022] The polyvinyl alcohol (PVA) 1799 used in this example has an alcoholysis degree of 98-99% mol / mol; manufacturer: Aladdin (China).

[0023] The chitosan used in this example has a deacetylation degree of ≥95% and a viscosity of 100-200 mpa.s. The manufacturer is Shanghai MacLean.

[0024] The preparation method of QCS in this embodiment is as follows: (1) Preparation of deacetylated chitosan Chitosan was dispersed in a 10% (w / w) NaOH solution containing 1% NaBH4 (antioxidant) to prepare 200 mL of a 10% (w / w) chitosan / NaOH solution. After stirring at 110 °C for 5 h, the solution was filtered with filter paper and washed with distilled water until neutral. The obtained material was filtered by suction, first rinsed with a methanol solution and then with acetone, and then vacuum dried at 70 °C overnight to obtain 17.8 g of deacetylated chitosan.

[0025] (2) QCS preparation Deacetylated chitosan was dispersed in 85° C. distilled water to prepare 60 mL of a 10% (w / w) deacetylated chitosan solution.

[0026] 2,3-Epoxypropyltrimethylammonium chloride (21.3 mL, 111 mmol) was added to the deacetylated chitosan solution in portions (7.1 mL each) every 2 h. After 10 h of reaction, the clear light yellow reaction solution was poured into 4°C cold pure acetone (200 mL) while stirring and refrigerated overnight. The next day, the acetone was decanted and the remaining gel-like product was dissolved in pure methanol (100 mL). The solution was precipitated with 4:1 acetone-ethanol (250 mL), the white product was collected by filtration, and then further purified by washing with hot ethanol for 24 h through a Soxhlet extractor. The final product was dried at 70°C overnight to obtain 10.2 g of quaternary ammonium chitosan with a purity of 98% and a quaternization degree of 1.0.

[0027] Example 1: Preparation of three-dimensional nanofiber tubular filtration membrane (1) Preparation of QCS / PVA nanofiber membrane by electrospinning: According to the volume ratio, a QCS / PVA solution was prepared from 30% QCS solution (QCS dissolved in deionized water with a concentration of 6.5 wt%) and 70% PVA solution (PVA dissolved in deionized water at 80 °C with a concentration of 9 wt%). Electrospinning was carried out on aluminized paper, and the electrospinning conditions were as follows: a steel needle nozzle with an inner diameter of 0.4 - 0.7 mm was used for spinning, the distance between the needle tip and the collection plate was 15 - 18 cm, the spinning temperature was 25 - 30 °C, the humidity was 50% - 65% RH, the voltage at the spinneret needle tip was 30 kV, and the solution feeding rate was 0.5 mL / h; (2) The electrospun nanofibers were immersed in anhydrous tert-butanol and homogenized (using an IKA T25 homogenizer) to obtain a nanofiber dispersion with a density of 100 g / L; (3) The nanofiber dispersion was freeze-dried at -40 °C for 48 h to form a nanofiber aerogel in a mold; (4) The stability of the freeze-dried aerogel was improved by heat treatment at 160 °C for 10 min, and the resulting aerogel was named NF-A1.

[0028] The structural schematic diagram of the three-dimensional nanofiber tubular filtration membrane of the present invention is as shown in Figure 1 、 2 shown. It includes a copper mesh 1 and a three-dimensional nanofiber aerogel filling matrix 2. A cylindrical copper mesh is placed inside the three-dimensional nanofiber tubular filtration membrane to support the three-dimensional nanofiber aerogel.

[0029] Among them, the SEM image of the unheated QCS / PVA nanofiber aerogel is as shown in Figure 3 shown; the fiber diameter (a) of the unheated QCS / PVA nanofiber aerogel and the fiber diameter (b) of the heated QCS / PVA nanofiber aerogel are as shown in Figure 4 shown. It can be seen from Figure 4 that after the nanofibers undergo the heat treatment process, their fiber diameters show a certain degree of increase. Before heating, the fiber diameters were generally distributed around 0.55 μm, and after heating, the fiber diameters were generally distributed around 0.7 μm.

[0030] Example 2: Preparation of three-dimensional nanofiber tubular filtration membrane (1) Preparation of QCS / PVA nanofiber membrane by electrospinning: According to the volume ratio, a QCS / PVA solution was prepared from 30% QCS solution (dissolving QCS in deionized water with a concentration of 5 wt%) and 70% PVA solution (dissolving PVA in deionized water at 80 °C with a concentration of 11 wt%). Electrospinning was carried out on aluminized paper, and the electrospinning conditions were as follows: A steel needle nozzle with an inner diameter of 0.4 - 0.7 mm was used for spinning, the distance between the needle tip and the collecting plate was 15 - 18 cm, the spinning temperature was 25 - 30 °C, the humidity was 50% - 65% RH, the voltage at the spinneret needle tip was 30 kV, and the solution feeding rate was 0.5 mL / h; (2) The electrospun nanofibers were immersed in anhydrous tert-butanol and homogenized (using an IKA T25 homogenizer) to obtain a nanofiber dispersion with a density of 100 g / L; (3) The nanofiber dispersion was freeze-dried at -40 °C for 48 h to form a nanofiber aerogel in a mold; (4) The stability of the freeze-dried aerogel was improved by heat treatment at 160 °C for 10 min, and the resulting aerogel was named NF-A2.

[0031] Example 3: Preparation of three-dimensional nanofiber tubular filter membrane (1) Preparation of QCS / PVA nanofiber membrane by electrospinning: According to the volume ratio, a QCS / PVA solution was prepared from 30% QCS solution (dissolving QCS in deionized water with a concentration of 5 wt%) and 70% PVA solution (dissolving PVA in deionized water at 80 °C with a concentration of 11 wt%). Electrospinning was carried out on aluminized paper, and the electrospinning conditions were as follows: A steel needle nozzle with an inner diameter of 0.4 - 0.7 mm was used for spinning, the distance between the needle tip and the collecting plate was 15 - 18 cm, the spinning temperature was 25 - 30 °C, the humidity was 50% - 65% RH, the voltage at the spinneret needle tip was 30 kV, and the solution feeding rate was 0.5 mL / h; (2) The electrospun nanofibers were immersed in anhydrous tert-butanol and homogenized (using an IKA T25 homogenizer) to obtain a nanofiber dispersion with a density of 50 g / L; (3) The nanofiber dispersion was freeze-dried at -40 °C for 48 h to form a nanofiber aerogel in a mold; (4) The stability of the freeze-dried aerogel was improved by heat treatment at 200 °C for 10 min, and the resulting aerogel was named NF-A3.

[0032] Red pigment nanoparticles (PNPs) with a concentration of 66.7 μL / L were dispersed in pure water. The particle size distribution of PNPs was analyzed using a Malvern laser particle size analyzer (Mastersizer 2000). At a preset pressure of 0.1 MPa, the separation performance of the nanofiber membrane for PNPs was studied. The filtration efficiency was calculated using the following equation:

[0033] where, C f is the concentration of the feed solution (μL / L), C P is the permeate solution concentration (μL / L) calculated from the spectrogram measured by a UV-visible spectrophotometer (UV-2550, Shimadzu, China).

[0034] The filtration efficiencies of the QCS / PVA nanofiber aerogels generated under different conditions in Examples 1-3 are as Figure 5 shown. It can be seen from the figure that the NF-A2 nanofiber aerogel has a filtration efficiency of up to 97.34%, and the filtration efficiencies of the NF-A1 and NF-A3 aerogels are 78.65% and 76.82% respectively. This confirms that when the concentrations of PVA and QCS and the heat treatment temperature vary within the optional range, the three-dimensional nanostructure can maintain a high filtration efficiency.

[0035] The pure water flux of the aerogel was tested using cross-flow filtration (SF-SA Membrane Separation Technology Co., Ltd.) after pre-pressing the aerogel at a preset pressure of 0.1 MPa for 10 min. The pure water flux of the nanofiber membrane was calculated using the flow equation:

[0036] where J is the pure water flux (L·m -2 ·h -1 ), Q is the volume of the test solution passing through the material (L), A is the effective area of the test material (m 2 ), and T is the sampling time (h).

[0037] The test results of the water flux of the filter membrane are as Figure 6 shown. The water flux of the aerogel filter membrane began to decline in the first 0.75 days. All the aerogel nanofiber membranes still had a water flux of not less than 2500 L / m 2 ·h -1 after 3 days of filtration. And the water flux of NF-A2 could be maintained stable and at a relatively high value for a long time.

[0038] The NF-A2 aerogel was selected for the antibacterial experiment. First, a single colony of Staphylococcus aureus and Escherichia coli was separately picked and amplified in 20 mL of LB liquid medium for 12 h, and then the bacterial suspension was diluted to 10 6 CFU / mL for subsequent use. 5 mg of the NF-A2 aerogel was taken and broken up and dispersed in 1 mL of the diluted bacterial suspension, and incubated in a shaker at 37 °C for 24 h. The bacterial solution without the aerogel was used as the control group. Subsequently, 0.1 mL of the bacterial culture was placed on an agar plate and incubated overnight at 37 °C. Finally, an automatic colony counter (Shinesosupre G9) was used to measure the number of colonies, and the bacterial survival rate was calculated. The results of the antibacterial experiment are as Figure 7 (a) and (b) show that, compared with the control group, the aerogel in the experimental group has obvious antibacterial properties. Its antibacterial rate against Staphylococcus aureus reaches 99%, and the antibacterial rate against Escherichia coli reaches 100%.

[0039] Generally speaking, the three-dimensional nanofiber structure has higher water flux and filtration efficiency, and maintains good durability. This shows that the three-dimensional aerogel structure enables the filter membrane to maintain a high dirt-holding capacity while allowing water to pass through, and the material also has obvious antibacterial properties, proving that the material has good application prospects.

[0040] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can be modified and changed. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a three-dimensional nanofiber tubular filtration membrane for water filtration, characterized in that, It includes the following steps: S1: Prepare a quaternary ammonium salt chitosan / polyvinyl alcohol solution and obtain a two-dimensional nanofiber membrane through electrospinning; S2: Immerse the two-dimensional nanofiber membrane in anhydrous tert-butanol for homogenization to produce a nanofiber dispersion; S3: Freeze-dry the nanofiber dispersion in a mold to form a three-dimensional nanofiber aerogel; a cylindrical copper mesh is placed inside the mold to support the three-dimensional nanofiber aerogel; S4: Heat-treat the three-dimensional nanofiber aerogel at 150-200 °C to obtain a three-dimensional nanofiber tubular filter membrane.

2. The preparation method according to claim 1, characterized in that, In step S1, the preparation method of the quaternary ammonium salt chitosan is as follows: disperse chitosan in a NaBH4 / NaOH solution and stir at 100-120 °C, filter and wash, then vacuum-dry to obtain deacetylated chitosan, and react the deacetylated chitosan with 2,3-epoxypropyltrimethylammonium chloride, filter, purify and dry to obtain the quaternary ammonium salt chitosan.

3. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of quaternary ammonium salt chitosan to polyvinyl alcohol in the quaternary ammonium salt chitosan / polyvinyl alcohol solution is 1:3-7.

4. The preparation method according to claim 3, characterized in that, The final concentration of quaternary ammonium salt chitosan in the quaternary ammonium salt chitosan / polyvinyl alcohol solution is 1-2%, and the final concentration of polyvinyl alcohol is 6-8%.

5. The preparation method according to claim 1, characterized in that, In step S1, the electrospinning conditions are as follows: use a steel needle nozzle with an inner diameter of 0.4-0.7 mm for spinning, the distance between the needle tip and the collection plate is 15-18 cm, the spinning temperature is 25-30 °C, the humidity is 50%-65% RH, the voltage at the spinneret needle tip is 25-30 kV, and the solution feeding rate is 0.3-0.5 mL / h.

6. The preparation method according to claim 1, characterized in that, In step S2, homogenize in anhydrous tert-butanol to obtain a nanofiber dispersion with a density of 50-100 g / L.

7. The preparation method according to claim 1, characterized in that, In step S3, put the nanofiber dispersion into a polyurethane plastic sleeve mold for freeze-drying. The mold includes an inner part and an outer part. The inner part is a solid cylinder, and the outer part is a hollow cylinder. The bottom of the mold is sealed, and there is a hollow sandwich between its inner and outer layers. The inner diameter of the mold is 1-5 cm, and the outer diameter is 1.5-8 cm; two layers of cylindrical copper meshes with diameters of 1-5 cm and 1.5-8 cm respectively can be placed inside the mold to support the three-dimensional nanofiber aerogel; The aperture of the copper mesh is 0.07~0.09 mm, the wire diameter is 0.04~0.06 mm, the thickness is 0.1~0.15 mm, and the density is 8.96~9.65 g / cm 3 .

8. The preparation method according to claim 1, characterized in that, The freeze-drying in step S3 is as follows: freeze-dry at -35--55 °C for 45-50 h.

9. The three-dimensional nanofiber tubular filtration membrane prepared by the method according to any one of claims 1-8, characterized in that, It includes a copper mesh and a three-dimensional nanofiber aerogel filling matrix, and a cylindrical copper mesh is placed inside the three-dimensional nanofiber tubular filter membrane to support the three-dimensional nanofiber aerogel.

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