Double-layer blade coating aramid fiber gradient nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention discloses a double-layer blade coating aramid fiber gradient nanofiltration membrane as well as a preparation method and application thereof. The preparation method of the double-layer blade-coated aramid gradient nanofiltration membrane comprises the following steps: (1) dissolving aramid fibers in a mixed solution of dimethyl sulfoxide and a saturated potassium hydroxide aqueous solution to prepare a separation layer membrane casting solution; (2) dissolving aramid fibers in a mixed solution of dimethyl sulfoxide, polyethylene glycol and a saturated potassium hydroxide aqueous solution to prepare a permeable layer casting membrane solution; (3) covering and fixing a non-woven fabric serving as a substrate on a glass plate, firstly pouring the permeable layer membrane casting solution for blade coating, then pouring the separation layer membrane casting solution for covering and blade coating, staying in the air, and immersing in deionized water until phase inversion is finished to obtain a composite hydrogel membrane; and washing with water and drying to obtain the double-layer blade-coated aramid gradient nanofiltration membrane. The invention provides application of the double-layer blade coating aramid fiber gradient nanofiltration membrane in dye separation. The double-layer blade coating aramid fiber gradient nanofiltration membrane has high permeability and excellent separation selectivity.

Description

Technical Field

[0001] The invention relates to a double-layer scraped aramid gradient nanofiltration membrane and a preparation method and application thereof. Background Art

[0002] Nanofiltration, an emerging pressure-driven membrane separation technology, is widely used for the separation of ions, molecules, and small organic molecules. However, the stability and physical and chemical properties of traditional nanofiltration membrane materials do not support their application in complex and extreme environments. Furthermore, the limited permeation flux and membrane service life significantly hinder the development of nanofiltration membranes. Therefore, the development of nanofiltration membranes with high permeability, high separation selectivity, and stability is particularly important.

[0003] Kevlar aramid fiber exhibits excellent mechanical strength and thermal stability due to its unique molecular chain rigidity structure, and also has good chemical corrosion resistance. These properties make it a highly promising high-performance membrane material substrate. However, the separation membranes prepared from Kevlar aramid fiber in existing technologies have significant technical bottlenecks: although its dense structure can provide separation selectivity of up to 99.5% or more, the extremely low water flux caused by the excessive mass transfer resistance requires a transmembrane pressure difference to meet basic separation efficiency. This high energy consumption and low flux characteristic seriously restricts its practical application in large-scale separation scenarios such as seawater desalination and industrial wastewater treatment. The present invention breaks through the preparation paradigm of traditional single-layer homogeneous membranes and innovatively adopts a double-layer gradient scraping process. By precisely controlling the composition of the upper and lower casting liquids and the phase transformation kinetic parameters, a gradient pore structure with continuous transition characteristics is constructed in the aramid matrix. Through the synergistic effect of the surface dense selective layer and the sub-layer high-flux support layer, the "trade-off" effect of the mutual constraint between the retention rate and the permeation flux that is prevalent in the membrane separation field is effectively solved, and the synergistic improvement of the solute screening accuracy and the solvent transfer efficiency is simultaneously achieved. Summary of the Invention

[0004] The purpose of the present invention is to provide a double-layer scraped aramid gradient nanofiltration membrane with high permeability and separation selectivity, as well as a preparation method and application thereof.

[0005] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a method for preparing a double-layer scraped aramid gradient nanofiltration membrane, the preparation method comprising:

[0007] (1) Clean, dry aramid fibers are dissolved in a mixed solution of dimethyl sulfoxide and a saturated potassium hydroxide aqueous solution, and ultrasonically treated to obtain a preliminarily dissolved mixed solution A; the mixed solution A is then placed under a constant temperature heating condition of 30°C to 40°C and mechanically stirred for 12 to 15 hours until the aramid fibers are completely dissolved, thereby obtaining a uniform aramid nanofiber dispersion A; the aramid nanofiber dispersion A is then placed under a vacuum environment for degassing to obtain a separation layer casting solution (CSS); based on the total mass of the aramid fibers, dimethyl sulfoxide, and the saturated potassium hydroxide aqueous solution being 100%, the mass percentage contents of the aramid fibers, dimethyl sulfoxide, and the saturated potassium hydroxide aqueous solution are 2% to 3%, 90% to 98%, and 3% to 8%, respectively;

[0008] (2) Clean, dry aramid fibers are dissolved in a mixed solution of dimethyl sulfoxide, polyethylene glycol, and a saturated potassium hydroxide aqueous solution, and ultrasonically treated to obtain a preliminarily dissolved mixed solution B; the mixed solution B is then placed under a constant temperature heating condition of 30°C to 40°C and mechanically stirred for 12 to 15 hours until the aramid fibers are completely dissolved, thereby obtaining a uniform aramid nanofiber dispersion B; the aramid nanofiber dispersion B is then placed under a vacuum environment for degassing treatment to obtain a permeation layer casting solution (CSP); based on the total mass of the aramid fibers, dimethyl sulfoxide, polyethylene glycol, and the saturated potassium hydroxide aqueous solution being 100%, the mass percentage contents of the aramid fibers, dimethyl sulfoxide, polyethylene glycol, and the saturated potassium hydroxide aqueous solution are 0.5% to 1%, 80% to 90%, 10% to 15%, and 3% to 8%, respectively;

[0009] (3) Using non-woven fabric as a substrate, covering and fixing it on a clean, dry glass plate, at room temperature and with an ambient humidity below 60%, first pour an appropriate amount of permeation layer casting solution (CSP) and use a scraper with a thickness of 200 μm to 300 μm to perform a scraping operation, then immediately pour an appropriate amount of separation layer casting solution (CSS) and use a scraper with a thickness of 50 μm to 400 μm to perform a covering scraping operation, after a hovering time of 8 seconds to 15 seconds, quickly and steadily immerse the glass plate in room temperature deionized water to wait for the phase inversion to end, thereby obtaining a composite hydrogel membrane; after soaking and rinsing the composite hydrogel membrane with deionized water to remove excess solvent, the composite hydrogel membrane is vacuum dried to obtain a double-layer scraped aramid gradient nanofiltration membrane.

[0010] Preferably, in step (1), based on the total mass of aramid fiber, dimethyl sulfoxide and saturated potassium hydroxide aqueous solution as 100%, the mass percentage contents of aramid fiber, dimethyl sulfoxide and saturated potassium hydroxide aqueous solution are 2-3%, 90-95% and 5-7%, more preferably 2%, 92% and 6%; in step (2), based on the total mass of aramid fiber, dimethyl sulfoxide, polyethylene glycol and saturated potassium hydroxide aqueous solution as 100%, the mass percentage contents of aramid fiber, dimethyl sulfoxide, polyethylene glycol and saturated potassium hydroxide aqueous solution are 0.8-1%, 80-85%, 10-12% and 5-7%, more preferably 1%, 83%, 10% and 6%.

[0011] Preferably, in step (1) or (2), the ultrasonic frequency, temperature and time are 20-50 kHz, 20-25° C. and 0.5-1 hour respectively.

[0012] Preferably, in step (2), the constant heating temperature and the mechanical stirring time are 35° C. and 15 hours, respectively.

[0013] Preferably, in step (3), an appropriate amount of separation layer casting solution (CSS) is poured and a doctor blade with a thickness of 250 μm to 400 μm is used for covering and scraping. Further preferably, in step (3), an appropriate amount of permeation layer casting solution (CSP) is poured and a doctor blade with a thickness of 200 μm is used for scraping, and then immediately an appropriate amount of separation layer casting solution (CSS) is poured and a doctor blade with a thickness of 250 μm is used for covering and scraping.

[0014] Preferably, in step (3), the dwell time is 10 seconds.

[0015] Preferably, in step (3), the vacuum drying conditions are: placing in a vacuum oven at 40°C to 60°C and drying for 10 to 12 hours; more preferably, the drying temperature and time are 60°C and 12 hours, respectively.

[0016] In a second aspect, the present invention provides a double-layer scraped aramid gradient nanofiltration membrane prepared according to the method described in the first aspect.

[0017] In a third aspect, the present invention provides the use of the double-layer scraped aramid gradient nanofiltration membrane described in the second aspect for separating dyes in an organic solvent environment, wherein the molecular weight of the dye is above 650.

[0018] Preferably, the molecular weight of the dye is above 690.

[0019] The organic solvent can be a polar protic solvent (such as methanol, ethanol, isopropanol, etc.), a polar aprotic solvent (such as dimethylformamide DMF), or a non-polar solvent (such as n-hexane, etc.).

[0020] Compared with existing technologies, this invention is based on the unique properties of aramid fiber-based membranes. Through a double-layer coating membrane-making method, two aramid fiber membranes with different characteristic properties are combined to form a gradient nanofiltration membrane. The porous support layer of the lower layer provides an efficient material transmission channel for the composite membrane, and the ultra-thin separation layer of the upper layer ensures the excellent separation performance of the composite membrane. The upper and lower layers are tightly combined through the "interfacial locking" effect, and the formed membrane has both high permeability and separation selectivity. It shows good retention performance for dye molecules under organic solvent conditions and is stable during long-term operation. Specifically, the present invention has the following advantages:

[0021] (1) The double-layer scraped aramid gradient nanofiltration membrane prepared by the present invention has significantly improved permeability to common organic solvents compared to the original aramid fiber-based membrane;

[0022] (2) The separation selectivity of the double-layer bladed aramid gradient nanofiltration membrane prepared by the present invention is comparable to that of the original aramid fiber-based membrane, and the molecular weight cut-off and pore size distribution are particularly similar;

[0023] (3) The double-layer scraped aramid gradient nanofiltration membrane prepared by the present invention uses aramid fiber as the base material and has excellent chemical stability;

[0024] (4) The preparation process of the present invention is simple, the raw material cost is low, the operation is simple and feasible, and there is low pollution, which is convenient for industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The results show the permeation separation ability of the double-layer scraped aramid gradient nanofiltration membrane series of the present invention for Congo red-ethanol solution.

[0026] Figure 2 The figure shows the retention capacity of the optimal membrane M1 of the double-layer scraped aramid gradient nanofiltration membrane series described in the present invention for dyes with different molecular weights.

[0027] Figure 3 The results of molecular weight cutoff and pore size distribution measurement of the optimal membrane M1 membrane in the double-layer scraped aramid gradient nanofiltration membrane series described in the present invention compared with the M-BASED membrane are shown.

[0028] Figure 4 The permeability of the optimal membrane M1 of the double-layer scraped aramid gradient nanofiltration membrane series described in the present invention to five common organic solvents is shown.

[0029] Figure 5 The long-term operation stability of the optimal membrane M1 membrane in the double-layer scraped aramid gradient nanofiltration membrane series of the present invention is demonstrated.

[0030] Figure 6 Scanning electron microscope images showing the surface (a) and cross-section (b) of the optimal membrane M1 of the double-layer scraped aramid gradient nanofiltration membrane series described in the present invention.

[0031] Figure 7 The thermal stability of the optimal membrane M1 membrane in the double-layer scraped aramid gradient nanofiltration membrane series described in the present invention is shown compared with the M-BASED membrane.

[0032] Figure 8 A comparison chart showing the infrared spectra of the double-layer bladed aramid gradient nanofiltration membrane series and the M-BASED membrane described in the present invention.

[0033] Figure 9 A graph showing the X-ray diffraction characterization results of the double-layer blade-coated aramid gradient nanofiltration membrane series described in the present invention. DETAILED DESCRIPTION

[0034] The present invention is further illustrated below with reference to the following examples, but the present invention is not limited to the following examples. The following examples describe in more detail a method for preparing a double-layer bladed aramid gradient nanofiltration membrane according to the present invention. These examples are provided by way of illustration and are not intended to limit the scope of the present invention. Unless otherwise noted, the experimental conditions employed in the present invention were conventional or manufacturer-recommended, and all experimental equipment, materials, and reagents used were commercially available.

[0035] The Kevlar aramid fiber used in the embodiments of the present invention was purchased from DuPont in the United States.

[0036] Example 1:

[0037] (1) Weigh 2 g of clean, dry aramid fiber in a 150 mL round-bottom flask, add saturated potassium hydroxide solution (3 g KOH + 3 g H2O) and 92 g dimethyl sulfoxide in sequence, and ultrasonicate (40 kHz) for 0.5 h at a constant temperature (25°C) to obtain a preliminarily dissolved mixed solution A; place the mixed solution A under a constant temperature heating condition of 35°C, mechanically stir for 15 h, and then degas under a vacuum environment to obtain a casting solution A, which is recorded as casting solution CSS;

[0038] 1 g of clean, dry aramid fiber was weighed into a 150 mL round-bottom flask, and saturated potassium hydroxide solution (3 g KOH + 3 g H2O), 83 g dimethyl sulfoxide, and 10 g polyethylene glycol (molecular weight, 1500) were added in sequence. The mixture was ultrasonicated (40 kHz) for 0.5 h at a constant temperature (25°C) to obtain a preliminarily dissolved mixed solution B. The mixed solution B was placed under a constant temperature heating condition of 35°C and mechanically stirred for 15 h. The mixed solution was then degassed under a vacuum environment to obtain a casting solution B, which was recorded as casting solution CSP.

[0039] At room temperature and with an ambient humidity below 60%, a non-woven fabric was used as a substrate and covered on a clean and dry glass plate. First, an appropriate amount of casting liquid CSP was poured, and a stainless steel scraper with a thickness of 200 μm was used to scrape the film. Then, an appropriate amount of casting liquid CSS was immediately poured, and a stainless steel scraper with a thickness of 250 μm was used to cover the film. After a 10-second air time, the glass plate was quickly and steadily immersed in room temperature deionized water to wait for the phase inversion to complete, thereby obtaining a composite hydrogel membrane. After soaking and rinsing with deionized water to remove excess solvent, the composite hydrogel membrane was placed in a vacuum drying oven at 60°C and dried for 12 hours. Finally, a double-layer scraped aramid gradient nanofiltration membrane that can be used directly was obtained and named M1.

[0040] (2) Membrane performance evaluation: The membrane to be tested was loaded into a cross-flow filtration device, where the effective test area of the membrane was 7.065 cm 2 The test temperature was room temperature, the test pressure was 4 bar, and the raw material liquid used was 50 ppm Congo red-ethanol solution (Congo red molecular weight is 696.663). The permeability coefficient and retention rate of the membrane to be tested were measured. The results showed that the permeability coefficient of the M1 membrane to Congo red-ethanol solution was 14.375 L m -2 h -1 bar -1 The interception rate of Congo Red is 96.44%. Figure 1 .

[0041] Example 2:

[0042] The thickness of the stainless steel blade used for coating the casting solution CSS in Example 1 was changed to 300 μm. The other steps remained unchanged, and the prepared membrane was named M2. The membrane's permeability and separation performance were measured using the performance evaluation method of Example 1. The results showed that the permeability coefficient of the M2 membrane to Congo red-ethanol solution was 11.16 L m -2 h -1 bar -1 The interception rate of Congo Red is 97.63%. Figure 1 .

[0043] Example 3:

[0044] The thickness of the stainless steel blade used for coating the casting solution CSP in Example 1 was changed to 300 μm, and the thickness of the stainless steel blade used for coating the casting solution CSS was changed to 350 μm. All other steps remained unchanged, and the resulting membrane was named M3. The membrane's permeability and separation performance were measured using the performance evaluation method of Example 1. The results showed that the permeability coefficient of the M3 membrane to Congo red-ethanol solution was 12.265 L m -2 h -1 bar -1 The interception rate of Congo Red is 96.53%. Figure 1 .

[0045] Example 4:

[0046] The thickness of the stainless steel blade used for coating the casting solution CSP in Example 1 was changed to 300 μm, and the thickness of the stainless steel blade used for coating the casting solution CSS was changed to 400 μm. Other steps remained unchanged, and the prepared membrane was named M4. The permeation separation performance of the membrane was measured using the performance evaluation method of Example 1. The results showed that the permeability coefficient of the M4 membrane to Congo red-ethanol solution was 10.15 L m -2 h -1 bar -1 The interception rate of Congo Red is 98.365%. Figure 1 .

[0047] Example 5:

[0048] The stainless steel blade used for coating the casting solution CSS in Example 1 was modified to a thickness of 200 μm. The casting solution CSP was omitted, and only the casting solution CSS was used to prepare the membrane as the base membrane for comparison of the performance of the gradient nanofiltration membrane. All other steps remained unchanged, and the prepared membrane was named M-BASED. The results showed that the permeability coefficient of the M-BASED membrane to the Congo red-ethanol solution was 1.075 L / m -2 h -1 bar -1 The interception rate of Congo Red is 99.87%. Figure 1 .

[0049] Example 6:

[0050] The nanofiltration membranes prepared according to Example 1 and Example 5 were respectively selected, and the performance evaluation method of Example 1 was used to test the retention capacity of the M1 membrane for five dyes with different molecular weights (methyl orange -MO, molecular weight of 327.334, chrome black T-EBT, molecular weight of 461.38, Congo red -CR, molecular weight of 696.663, erythrosine -EB, molecular weight of 835.892, and rose Bengal -BR, molecular weight of 1017.636). The results showed that the retention rate of the M1 membrane for MO was 38.66%; the retention rate for EBT was 47.13%; the retention rate for CR was 96.44%; the retention rate for EB was 95.47%; and the retention rate for BR was 95.13%. Compared with the M-BASED membrane, the retention performance of the M1 membrane for dyes was slightly reduced. See the attached for details. Figure 2 .

[0051] Example 7:

[0052] The double-layer scraped aramid gradient nanofiltration membrane prepared according to Example 1 and Example 5 was selected, and the molecular weight cutoff of the M-BASED membrane and the M1 membrane was tested using the performance evaluation method of Example 1. In particular, the solute used in the test was polyethylene glycol (molecular weight 200, 400, 600, 800, 1000, 1500), the solvent was water, and the solute concentration was 100 ppm. In addition, based on the results of the molecular weight cutoff determination, the pore size distribution of the membrane was calculated by the transfer method. The results showed that the molecular weight cutoff of the M1 membrane was 725 g / mol, which was only 2% higher than the molecular weight cutoff of the M-BASED membrane (710 g / mol), and the separation performance was good; the pore size distribution results showed that the pore size distribution of the M1 membrane was almost the same as that of the M-BASED membrane. See the attached for details. Figure 3 .

[0053] Example 8:

[0054] The double-layer bladed aramid gradient nanofiltration membranes prepared according to Examples 1 and 5 were selected and the permeability coefficients of the M-BASED membrane and the M1 membrane to three types of solvents (polar protic solvents methanol, ethanol, and isopropanol; polar aprotic solvent dimethylformamide (DMF); and nonpolar solvent n-hexane) were tested using the performance evaluation method of Example 1. The results showed that the permeability coefficient of the M1 membrane to methanol was 17.197 L m -2 h -1 bar -1 ; The permeability coefficient to ethanol is 14.718 L m -2 h -1 bar -1 ; The permeability coefficient to isopropyl alcohol is 14.165 L m -2 h -1 bar -1 ; The permeability coefficient to DMF is 15.627 L m -2 h -1 bar -1 ; The permeability coefficient to n-hexane is 23.289 L m -2 h -1 bar -1 , which is significantly improved compared to M-BASED membrane. Figure 4 .

[0055] Example 9:

[0056] The double-layer bladed aramid gradient nanofiltration membrane prepared according to Example 1 was selected and the long-term operational stability of the M1 membrane was tested using the performance evaluation method of Example 1. The results showed that the permeability of the M1 membrane continued to increase within 10 hours of continuous operation and then stabilized, with no significant change within 50 hours; the retention performance decreased slightly within 10 hours and then stabilized, with no significant change within 50 hours. For details, see the attached Figure 5 .

[0057] Example 10:

[0058] The double-layer bladed aramid gradient nanofiltration membrane prepared according to Example 1 was characterized using cold-field scanning electron microscopy (FE-SEM). (a) The dense structure of the membrane surface ensures the separation selectivity of the composite membrane. (b) The double-layer structure of the membrane cross-section is clearly layered and tightly integrated, confirming the successful implementation of the theory. Figure 6 .

[0059] Example 11:

[0060] The double-layer bladed aramid gradient nanofiltration membranes prepared according to Examples 1 and 5 were selected and characterized by TGA thermal stability analysis of the membrane pyrolysis process. The pyrolysis curves of the M1 membrane and the M-BASED membrane were basically similar, but the pyrolysis rate fluctuations were completely different. Figure 7 .

[0061] Example 12:

[0062] The double-layer blade-coated aramid gradient nanofiltration membranes prepared according to Examples 1, 2, 3, 4, and 5 were selected and characterized by Fourier transform infrared spectroscopy. The basically identical absorption peak curves proved the similarity of the surface structure of the M1-M4 membrane and the M-BASED membrane, while the M1-M4 membrane had a higher absorption peak at 2800 cm -1 The significant absorption peak is due to the PEG dissolution being blocked and remaining in the membrane matrix. Figure 8 .

[0063] Example 13:

[0064] The double-layer blade-coated aramid gradient nanofiltration membranes prepared according to Examples 1, 2, 3, and 4 were selected and characterized by X-ray diffraction. The results confirmed the structural similarity of the M1-M4 membranes. Figure 9 .

Claims

1. A method for preparing a double-layer blade-coated aramid gradient nanofiltration membrane, characterized in that: The preparation method is: (1) Clean, dry aramid fibers are dissolved in a mixed solution of dimethyl sulfoxide and a saturated potassium hydroxide aqueous solution, and ultrasonically treated to obtain a preliminarily dissolved mixed solution A; the mixed solution A is then placed under a constant temperature heating condition of 30°C to 40°C and mechanically stirred for 12 to 15 hours until the aramid fibers are completely dissolved, thereby obtaining a uniform aramid nanofiber dispersion A; the aramid nanofiber dispersion A is then placed under a vacuum environment for degassing to obtain a separation layer casting solution; based on the total mass of the aramid fibers, dimethyl sulfoxide, and the saturated potassium hydroxide aqueous solution being 100%, the mass percentage contents of the aramid fibers, dimethyl sulfoxide, and the saturated potassium hydroxide aqueous solution are 2% to 3%, 90% to 98%, and 3% to 8%, respectively; (2) The clean, dry aramid fiber is dissolved in a mixed solution of dimethyl sulfoxide, polyethylene glycol and a saturated potassium hydroxide aqueous solution, and ultrasonically treated to obtain a preliminarily dissolved mixed solution B; the mixed solution B is then placed under a constant temperature heating condition of 30°C to 40°C and mechanically stirred for 12 to 15 hours, and the aramid fiber is completely dissolved to obtain a uniform aramid nanofiber dispersion B; the aramid nanofiber dispersion B is then placed under a vacuum environment for degassing to obtain a permeation layer casting solution; based on the total mass of the aramid fiber, dimethyl sulfoxide, polyethylene glycol and saturated potassium hydroxide aqueous solution as 100%, the mass percentage contents of the aramid fiber, dimethyl sulfoxide, polyethylene glycol and saturated potassium hydroxide aqueous solution are 0.5% to 1%, 80% to 90%, 10% to 15% and 3% to 8% respectively; (3) Using non-woven fabric as a substrate, covering and fixing it on a clean, dry glass plate, at room temperature and with an ambient humidity below 60%, first pouring an appropriate amount of permeation layer casting liquid and using a scraper with a thickness of 200 μm to 300 μm to perform a scraping operation, then immediately pouring an appropriate amount of separation layer casting liquid and using a scraper with a thickness of 50 μm to 400 μm to perform a covering scraping operation, after a hovering time of 8 seconds to 15 seconds, the glass plate is quickly and steadily immersed in room temperature deionized water to wait for the phase conversion to be completed, thereby obtaining a composite hydrogel membrane; after soaking and rinsing the composite hydrogel membrane with deionized water to remove excess solvent, the composite hydrogel membrane is vacuum dried to obtain a double-layer scraped aramid gradient nanofiltration membrane.

2. The preparation method according to claim 1, wherein: In step (1), based on the total mass of aramid fiber, dimethyl sulfoxide and saturated potassium hydroxide aqueous solution as 100%, the mass percentage contents of aramid fiber, dimethyl sulfoxide and saturated potassium hydroxide aqueous solution are 2-3%, 90-95% and 5-7%, preferably 2%, 92% and 6%; in step (2), based on the total mass of aramid fiber, dimethyl sulfoxide, polyethylene glycol and saturated potassium hydroxide aqueous solution as 100%, the mass percentage contents of aramid fiber, dimethyl sulfoxide, polyethylene glycol and saturated potassium hydroxide aqueous solution are 0.8-1%, 80-85%, 10-12% and 5-7%, preferably 1%, 83%, 10% and 6%.

3. The preparation method according to claim 1, wherein: In step (1) or (2), the ultrasonic frequency, temperature, and time are 20-50 kHz, 20-25° C., and 0.5-1 hour, respectively.

4. The preparation method according to claim 1, wherein: In step (2), the constant temperature heating temperature and the mechanical stirring time are 35° C. and 15 hours, respectively.

5. The preparation method according to claim 1, wherein: In step (3), pour an appropriate amount of separation layer casting liquid and use a scraper with a thickness of 250 μm to 400 μm to perform covering scraping operation.

6. The preparation method according to claim 5, wherein: In step (3), pour an appropriate amount of permeation layer casting solution and use a scraper with a thickness of 200 μm to perform a scraping operation, and then immediately pour an appropriate amount of separation layer casting solution and use a scraper with a thickness of 250 μm to perform a covering scraping operation.

7. The preparation method according to claim 1, wherein: In step (3), the hovering time is 10 seconds.

8. A double-layer blade-coated aramid gradient nanofiltration membrane prepared according to the preparation method according to any one of claims 1 to 7. 9 . Use of the double-layer scraped aramid gradient nanofiltration membrane according to claim 8 for separating dyes in an organic solvent environment, wherein the molecular weight of the dye is above 650.

10. The use according to claim 9, characterized in that: The molecular weight of the dye is above 690.

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