Preparation method of aramid nanofiber composite nanofiltration membrane

Abstract

The invention provides a preparation method of an aramid nanofiber composite nanofiltration membrane. The preparation method comprises the following steps: adding PVA-co-PE nanofibers into an aramid nanofiber dispersion liquid, adding an organic solution, uniformly stirring, adding concentrated hydrochloric acid, a cross-linking agent and citric acid, carrying out a cross-linking reaction, and then adopting a vacuum-assisted suction filtration method to obtain an initial composite film; then, the initial composite film is placed in a tertiary butanol aqueous solution with the first concentration for first-time solvent exchange, and then the film is placed in a tertiary butanol aqueous solution with the second concentration for second-time solvent exchange; and taking out the film, and carrying out freeze drying treatment to obtain the aramid nanofiber composite nanofiltration membrane. According to the preparation method, high mechanical strength (greater than or equal to 50 MPa), high flux (greater than or equal to 50 L m <-2 > h <-1 >) and high selectivity (the tetracycline retention rate is greater than or equal to 95%) of the composite membrane are realized, meanwhile, swelling is inhibited (the swelling rate lt is 5%), and the technical bottleneck that strength-flux-selectivity of a traditional nanofiltration membrane is difficult to synergistically improve is broken through.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of nanofiber membrane preparation, and particularly relates to a preparation method of aramid nanofiber composite nanofiltration membrane. BACKGROUND

[0002] Membrane separation technology has become one of the core technologies in the field of water treatment due to its advantages of high efficiency, low energy consumption, no secondary pollution and the like. Among them, nanofiltration membrane (pore size 1-2 nm) has unique value in the deep purification of trace pollutants due to its selective interception capability for small-molecule organic matters (200-1000 Da) and multivalent ions.

[0003] Polyvinyl alcohol-polyethylene copolymer (PVA-co-PE) is a functional polymer material copolymerized by hydrophilic PVA and hydrophobic PE. The material has the following characteristics: the hydroxyl groups (-OH) of the PVA segment endow the material with hydrophilicity and film-forming ability to form a uniform membrane structure; the PE segment provides mechanical support through hydrophobic interaction, so that the material has flexibility and high strength (tensile strength 30-50 MPa, better than pure PVA <=20 MPa); the active hydroxyl groups support chemical modification (such as crosslinking, carboxyl / sulfonic acid grafting), which can regulate the membrane surface charge and selective separation of pollutants; at the same time, the partial biodegradability of the material reduces the environmental load and highlights the environmental protection advantage.

[0004] Aramid nanofiber is considered an ideal membrane material reinforcing body due to its high strength, high chemical resistance and functional surface. However, due to the chemical inertness of aramid fiber, it is difficult to form a nanoscale pore size, and it is difficult to achieve a two-way balance between the mechanical strength and high flux of the composite membrane.

[0005] Therefore, it is necessary to design a preparation method of aramid nanofiber composite nanofiltration membrane to solve the above problems. SUMMARY

[0006] The application provides a preparation method of aramid nanofiber composite nanofiltration membrane. The preparation method covalently crosslinks aramid nanofiber (ANF) and PVA-co-PE nanofiber (NF) mixed system by glutaraldehyde / epoxy chloromethane, combines gradient solvent exchange and freeze drying, constructs strong interfacial bonding and hierarchical porous structure of ANF and PVA-co-PE, realizes high mechanical strength (>=50 MPa), high flux (>=50 L m -2 h -1 ) and high selectivity (tetracycline rejection rate >=95%) of the composite membrane, and at the same time, inhibits swelling (swelling rate <5%), breaks through the technical bottleneck that the strength, flux and selectivity of the traditional nanofiltration membrane are difficult to be improved simultaneously.

[0007] The embodiment of the present application provides a preparation method of aramid nanofiber composite nanofiltration membrane, comprising the following steps: S1, aramid fiber is immersed in a strong alkaline solution, then dimethyl sulfoxide is added for mechanical stirring, so that the mass-volume fraction of aramid nanofiber dispersion is 0.1%-0.3%; S2, the PVA-co-PE nanofiber is added to the aramid nanofiber dispersion obtained in step S1, an organic solution is added, after uniform stirring, concentrated hydrochloric acid, a crosslinking agent and citric acid are added for crosslinking reaction, and then a vacuum assisted suction filtration method is used to obtain an initial composite membrane; S3, the initial composite membrane obtained in step S2 is placed in a first concentration of tert-butyl alcohol aqueous solution for the first solvent exchange, and then the membrane is placed in a second concentration of tert-butyl alcohol aqueous solution for the second solvent exchange; the membrane is taken out for freeze-drying treatment, and the aramid nanofiber composite nanofiltration membrane is obtained.

[0008] Further, in step S2, the mass ratio of aramid nanofiber to PVA-co-PE nanofiber is 1: (0.5-3).

[0009] Further, in step S2, the amount of citric acid is 10%-15% of the mass of PVA-co-PE nanofiber.

[0010] Further, in step S2, the volume fraction of concentrated hydrochloric acid is 0.1%-0.3%, and the volume fraction of the crosslinking agent is 2%-4%.

[0011] Further, in step S3, the first concentration is less than the second concentration.

[0012] Further, in the first solvent exchange, the volume fraction of tert-butyl alcohol aqueous solution is 40-60%, and in the second solvent exchange, the volume fraction of tert-butyl alcohol aqueous solution is 90-100%.

[0013] Further, in step S2, the crosslinking agent is one of glutaraldehyde and epichlorohydrin.

[0014] Further, in step S2, the organic solution is one of tert-butyl alcohol aqueous solution and isopropyl alcohol aqueous solution.

[0015] Further, in step S3, the freeze-drying treatment conditions are: the temperature is-80--45 DEG C, the vacuum degree is 15-20 Pa, and the freeze-drying time is 24-30 h.

[0016] Further, in step S1, the immersion time of aramid fiber in the strong alkaline solution is 10-30 s.

[0017] Compared with the prior art, the application has the following beneficial effects: (1) The preparation method of aramid nanofiber composite nanofiltration membrane provided by the application first destroys the hydrogen bonds between aramid fibers by strong alkali (KOH) etching to dissociate into nanofibers, combines the polar aprotic solvent characteristics of DMSO to enhance the peeling efficiency and inhibit agglomeration, and precisely controls the etching degree by short-term immersion (10-30 seconds) to retain the high strength characteristics of the fibers, finally obtaining ANF with high dispersity and high aspect ratio, providing a basis for mechanical enhancement of the composite membrane; then citric acid is added in the crosslinking step to graft carboxyl groups on the membrane surface through esterification reaction to enhance the electrostatic adsorption effect on antibiotics; combined with gradient solvent exchange (solvent exchange uses different proportions of tert-butyl alcohol aqueous solution) and freeze-drying, the strong interfacial bonding and hierarchical porous structure of ANF and PVA-co-PE are constructed, realizing high mechanical strength (≥50 MPa), high flux (≥50L m -2 h -1 ) and high selectivity (tetracycline rejection rate ≥95%) of the composite membrane, while inhibiting swelling (swelling rate <5%), breaking through the technical bottleneck that the strength-flux-selectivity of traditional nanofiltration membranes are difficult to be improved simultaneously.

[0018] (2) The application significantly improves the comprehensive performance of the ANF / NF composite nanofiltration membrane through covalent cross-linking strategy and multi-level structure design. Interface bonding strength optimization: the interface bonding strength between ANF and PVA-co-PE matrix is improved by more than 40% through glutaraldehyde / epichlorohydrin covalent bonding, the tensile strength reaches ≥50 MPa, effectively enhancing the compression stability and service life of the membrane; high flux and high selectivity synergy: the hierarchical mass transfer channel (pore size 1-2 nm) is formed by the ANF nanofiber network and the PVA-co-PE porous matrix, the water flux is improved to ≥50L m -2 h -1 , and the rejection rate of tetracycline antibiotics is ≥95% through carboxyl functional modification to strengthen the electrostatic adsorption effect; anti-pollution and anti-swelling: the membrane surface hydrophilicity (contact angle <60°) and hierarchical pore structure synergistically inhibit the adsorption of pollutants, and the swelling rate is reduced to <5%, the long-term running flux decay rate is reduced by 60% compared with traditional membranes. Moreover, based on the chemical modifiability of the active hydroxyl group of PVA-co-PE and the surface functional groups of ANF, various functional groups (such as sulfonic acid groups and quaternary ammonium groups) can be directionally grafted, which can adapt to the high-efficiency separation needs of different pollutants (heavy metals and dyes). In summary, through the synergistic regulation of material-structure-function, the application breaks through the bottleneck that the strength-flux-selectivity of traditional nanofiltration membranes are difficult to balance, and provides high-performance membrane technology support for high-difficulty industrial wastewater treatment and resource recovery.

[0019] (3) The application synchronously improves the interfacial bonding strength of ANF and the matrix material through chemical cross-linking and structure design under the premise of ensuring the mechanical stability of ANF; optimizes the mass transfer channel structure to improve the water flux; and introduces functional groups to enhance the selective interception of antibiotics.

[0020] The above description is only a summary of the technical solutions of the present application, in order to more clearly understand the technical means of the present application, and can be implemented according to the content of the specification, and in order to make the above and other purposes, characteristics and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the present application, the drawings used in the present application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creating laborious work.

[0022] Figure 1 Optical picture of aramid nanofiber composite nanofiltration membrane prepared for example 1 of the present application.

[0023] Figure 2 Scanning electron microscope picture of aramid nanofiber composite nanofiltration membrane prepared for example 1.

[0024] Figure 3 Ultraviolet absorption visible spectrum picture of tetracycline solution through aramid nanofiber composite nanofiltration membrane prepared for example 1 at different time periods.

[0025] Figure 4 Ultraviolet absorption visible spectrum picture of tetracycline solution through aramid nanofiber composite nanofiltration membrane prepared for example 2 at different time periods.

[0026] Figure 5 Ultraviolet absorption visible spectrum picture of tetracycline solution through aramid nanofiber composite nanofiltration membrane prepared for example 3 at different time periods.

[0027] Figure 6 Ultraviolet absorption visible spectrum picture of tetracycline solution through aramid nanofiber composite nanofiltration membrane prepared for example 4 at different time periods. DETAILED DESCRIPTION

[0028] The embodiments of the technical solutions of the present application will be described in detail below with reference to the drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present application, therefore only as an example, and cannot limit the protection scope of the present application.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this application; the use of the terms "including," "comprising," "having" and "with" in the specification and claims hereof, along with their variants, are intended to be equivalent to the term "consisting of to the extent permissibly under 35 U.S.C. § 112.

[0030] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly specified.

[0031] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the application. The appearance of the phrase in various places in the specification does not necessarily all refer to the same embodiment, nor is it necessarily independent or alternative embodiments to each other. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0032] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which means that there are three cases of A alone, A and B together, and B alone. In addition, the character " / " in this paper generally represents that the front and rear associated objects are a "or" relationship.

[0033] In the description of the embodiments of the present application, the term "a plurality of" refers to two or more (including two), and similarly, "a plurality of groups" refers to two or more groups (including two groups), and "a plurality of pieces" refers to two or more pieces (including two pieces).

[0034] In the description of the embodiments of the present application, unless otherwise explicitly specified and limited, the technical terms "mounting", "connecting", "connecting", "fixing" and the like should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanical connection, or it can be electrical connection; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0035] The embodiments of the present application provide a preparation method of aramid nanofiber composite nanofiltration membrane, comprising the following steps: S1, impregnate aramid fibers in a strong alkaline solution, then add dimethyl sulfoxide for mechanical stirring to obtain aramid nanofiber dispersion with a mass-volume fraction of 0.1%-0.3%; Specifically, the strong alkaline solution is potassium hydroxide aqueous solution, the mass ratio of potassium hydroxide to aramid fiber is 3:2, the ratio of ultrapure water to dimethyl sulfoxide is 1:25, and the impregnation time of aramid fiber in the strong alkaline solution is 10-30s.

[0036] By KOH strong alkali etching to destroy the intermolecular hydrogen bond of aramid fiber to dissociate into nanofiber, combined with the polar aprotic solvent characteristics of DMSO to enhance the stripping efficiency and inhibit agglomeration, while short-time impregnation (10~30s) precisely regulates the etching degree to retain the high strength characteristics of the fiber, finally obtains ANF with high dispersity and high aspect ratio, which provides the basis for mechanical enhancement of composite film.

[0037] S2, add PVA-co-PE nanofiber to the aramid nanofiber dispersion obtained in step S1, add organic solution, stir uniformly, then add concentrated hydrochloric acid, crosslinking agent and citric acid for crosslinking reaction, and then obtain the initial composite film by vacuum assisted suction filtration; Among them, the mass ratio of aramid nanofiber to PVA-co-PE nanofiber is 1:(0.5-3).

[0038] The amount of citric acid is 10%-15% of the mass of PVA-co-PE nanofiber.

[0039] The volume fraction of concentrated hydrochloric acid is 0.1%-0.3%, and the volume fraction of crosslinking agent is 2%-4%.

[0040] The crosslinking agent is one of glutaraldehyde and epichlorohydrin.

[0041] The organic solution is one of tert-butyl alcohol aqueous solution and isopropyl alcohol aqueous solution.

[0042] In this process, citric acid is added in the crosslinking step to graft carboxyl groups on the film surface through esterification reaction to enhance the electrostatic adsorption effect on antibiotics.

[0043] The preparation process of PVA-co-PE nanofiber is: adding cellulose acetate butyrate with a mass ratio of (70~80):(20~30) and PVA-co-PE into a co-rotating twin-screw extruder, extruding, hot stretching, and curling to obtain PVA-co-PE / CAB composite fiber, then removing CAB from the PVA-co-PE / CAB composite fiber with acetone as the solvent to obtain PVA-co-PE nanofiber. Among them, the twin-screw extrusion temperature is 180~200℃, the hot stretching ratio is 3:1, and the winding speed is 5m / min.

[0044] Porous PVA-co-PE nanofiber is formed by melt blending cellulose acetate butyrate (CAB) and PVA-co-PE through double screw extrusion, heat stretching and coiling, and then selectively removing CAB with acetone, so that the molecular chain orientation and porous structure of the porous PVA-co-PE nanofiber are coordinated to improve the mechanical strength and mass transfer efficiency, thereby laying a foundation for high-throughput performance of the composite membrane.

[0045] S3, the initial composite film obtained in step S2 is placed in a first concentration of tert-butyl alcohol aqueous solution for the first solvent exchange, and then the film is placed in a second concentration of tert-butyl alcohol aqueous solution for the second solvent exchange; the film is taken out for freeze-drying treatment, and an aramid nanofiber composite nanofiltration membrane is obtained.

[0046] The first concentration is less than the second concentration. Specifically, the volume fraction of the tert-butyl alcohol aqueous solution is 40-60% during the first solvent exchange, and the volume fraction of the tert-butyl alcohol aqueous solution is 90-100% during the second solvent exchange.

[0047] The freeze-drying treatment conditions are: temperature of -80 to -45℃, vacuum degree of 15-20 Pa, and freeze-drying time of 24-30 h.

[0048] Some specific examples are listed below. It should be noted that the examples described below are exemplary and are only used to explain the present application, and cannot be understood as a limitation of the present application. If the specific technology or condition is not specified in the examples, it is carried out according to the technology or condition described in the literature in the art or according to the product instruction. If the reagent or instrument is not specified by the manufacturer, it is a conventional product that can be obtained by market purchase.

[0049] I. Preparation method Example 1 Please refer to Figure 1 As shown in the above formula, the present embodiment provides a preparation method of an aramid nanofiber composite nanofiltration membrane, which comprises the following steps: S1, 0.75 g of potassium hydroxide is dissolved in 4 mL of ultrapure water to prepare a potassium hydroxide solution as an alkaline solution; the para-aramid fiber is cut into small pieces of about 3 mm, and 0.25 g of the para-aramid fiber pieces are immersed in the prepared alkaline solution for 30 s, and then 100 mL of dimethyl sulfoxide is added for mechanical stirring to obtain an aramid nanofiber dispersion with a mass-volume fraction of 0.25%.

[0050] S2, a mixture of CAB and PVA-co-PE with a mass ratio of 80:20 is added to a co-rotating twin screw extruder to extrude the blend, and then the PVA-co-PE / CAB composite fiber is obtained by heat stretching at 30℃ and coiling, and then the PVA-co-PE / CAB composite fiber is added to an acetone solution and stirred to obtain PVA-co-PE nanofiber.

[0051] 0.0125 g aramid nanofiber, 0.00625 g PVA-co-PE nanofiber were weighed according to the mass ratio of 2:1, mixed, added into the beater, 100 ml tert-butyl alcohol aqueous solution was added, and continuous beating was carried out twice. After mixing, 0.1 ml of concentrated hydrochloric acid and 2 ml of glutaraldehyde were added, and 15% of the mass of the PVA-co-PE nanofiber was added. Citric acid was used for crosslinking reaction for 2 h, and then vacuum assisted filtration method was used to obtain the initial composite film.

[0052] S3, the initial composite film was immersed in 150 ml tert-butyl alcohol aqueous solution (volume fraction 50%) for solvent exchange for 36 h, then immersed in tert-butyl alcohol (volume fraction 99%) for solvent exchange for 36 h, and finally suspended in a-80℃ refrigerator for pre-freezing for 24 h, and then placed in a freeze dryer (cold trap temperature-65℃, vacuum degree 25 KPa) for freeze drying for 24 h. The optical picture of the aramid nanofiber composite film prepared is shown in Figure 1 The thickness of the obtained composite film is 30-40 um, and the tensile strength is 65 MPa.

[0053] Figure 2 The scanning electron microscope picture of the aramid nanofiber composite nanofiltration membrane prepared in Example 1 can be seen. The aramid nanofiber and the PVA-co-PE nanofiber are intertwined with each other to form a through three-dimensional nanofiber network skeleton; the crosslinking agent (glutaraldehyde + citric acid) forms "welding points" at the fiber intersection points to enhance the structural stability; the freeze drying process forms "honeycomb-like" special-shaped holes; has a certain gradient pore structure: surface layer: "honeycomb-like" special-shaped holes, pore size about 1-5 um (reduces filtration resistance and improves flux); inner layer: dense nanopore layer, pore size about 10-50 nm, which is beneficial to intercept small molecular substances.

[0054] Comparative Example 1 Comparative Example 1 provides a preparation method of aramid nanofiber composite nanofiltration membrane. Compared with Example 1, the difference lies in that in step S3, the second solvent exchange is not carried out. The rest is basically the same as Example 1, which will not be repeated here.

[0055] Comparative Example 2 Comparative Example 2 provides a preparation method of aramid nanofiber composite nanofiltration membrane. Compared with Example 1, the difference lies in that in step S3, the first solvent exchange is not carried out. The rest is basically the same as Example 1, which will not be repeated here.

[0056] Comparative Example 3 Comparative Example 3 provides a preparation method of aramid nanofiber composite nanofiltration membrane, which is different from Example 1 in that no citric acid is added in step S2. The rest is substantially the same as Example 1, which is not described here.

[0057] The composite nanofiltration membranes prepared in Example 1 and Comparative Examples 1-3 are tested for performance, and the test results are shown in the following table.

[0058] As can be seen from the above table, the tensile strength, pure water flux and Na2SO4 rejection rate of the aramid nanofiber composite nanofiltration membrane obtained in Example 1 are better than those of Comparative Examples 1-3. It is shown that the synergistic effect of double solvent exchange and citric acid crosslinking enables Example 1 to achieve high strength, high rejection rate and high stability at the same time, breaking through the performance bottleneck of traditional nanofiltration membranes.

[0059] Comparative Example 1 does not perform the second solvent exchange, and the residual moisture forms destructive ice crystals, resulting in a 51% decrease in flux and a 26% decrease in strength.

[0060] Comparative Example 2 does not perform the first solvent exchange, which causes the densification of the membrane structure, a 77% drop in flux and a 51% decrease in strength, indicating that gradient solvent exchange is the key to maintaining the nano-porous structure.

[0061] Comparative Example 3 does not add citric acid, and the PVA-co-PE swells due to no crosslinking, resulting in a 72% drop in strength and a 64% drop in rejection rate, indicating that citric acid builds a three-dimensional network through esterification, which cooperates with glutaraldehyde to solve the strength-selectivity contradiction of the composite membrane.

[0062] Examples 2-4 and Comparative Examples 4-5 Examples 2-4 and Comparative Examples 4-5 provide a preparation method of aramid nanofiber composite nanofiltration membrane, which is different from Example 1 in that the mass ratio of aramid nanofiber to PVA-co-PE nanofiber in step S2 is changed, as shown in the following table. The rest is substantially the same as Example 1, which is not described here.

[0063] Test method: I. Hydrophilicity test of membrane material Contact angle, also known as wetting angle, is an important parameter for testing the hydrophilicity and hydrophobicity of the surface of a material. In this test, a JC-2000C1 contact angle tester is used to characterize the hydrophilicity and hydrophobicity of the membrane surface. The larger the contact angle, the worse the hydrophilicity of the sample surface, and the smaller the contact angle, the better the hydrophilicity of the membrane sample.

[0064] The test process of the hydrophilicity test is specifically as follows: the sample is cut into a 3*3 cm rectangle, deionized water is ultrasonically cleaned for 15 min, and then transferred to a 60°C oven for drying to obtain a sample to be tested, which is ready for use; the ambient temperature is kept at 25°C, 10 μL of deionized water is taken out with a dry and clean pipette gun and dropped on the surface of the laid sample to be tested, and immediately after the dropping is completed, the image is quickly saved by using the image capture function of the instrument, and then the contact angle data of the sample to be tested are obtained by calculating the three-point method. The contact angle is measured at three random points on the surface of the sample to be tested, and finally the average value of three measurements is selected.

[0065] II. Membrane separation performance test The test process of the separation performance test is specifically as follows: after the membrane sample is washed several times with pure water, it is placed in a test cell with a size of 5 cm*5 cm for separation performance test. To ensure stable performance of the membrane sample, the sample to be tested is first pre-pressed at 0.6 MPa for 10 min before each test to achieve stable flux, the effective filtration area A of the sample is 25 cm 2 , the circulation flow rate is 5 LPM, and the test temperature is 25±0.5°C. The rejection rate is used as a parameter to evaluate the separation performance of the membrane sample.

[0066] In this experiment, the conductivity of the feed liquid and the raw material liquid is tested with 100 mg / L of Na2SO4 and NaCl, and the corresponding concentration is calculated according to the standard (concentration-conductivity) curve; 100 mg / L of tetracycline is used as an antibiotic solution to test the antibiotic separation performance of the membrane.

[0067] The calculation method of the rejection rate is as follows: R=(1-C p / C f )×100%。

[0068] In formula I, R is the rejection rate, %; C p is the permeate concentration, mg·L -1 ; C f is the raw material liquid concentration, mg·L -1 .

[0069] The performance of the aramid nanofiber composite nanofiltration membrane prepared in Test Examples 1-4 and Comparative Examples 4-5 is shown in the following table.

[0070] As shown in the above table, by balancing the hydrophilicity and optimizing the structural design, Example 1 achieves a high flux (85 L m -2 h -1), high rejection rate (tetracycline 95%, Na2SO463.9%) and high strength (65 MPa), and the comprehensive performance is optimal. Example 2 has a slightly reduced hydrophilicity and a slightly adjusted structure, and the tetracycline rejection rate is reduced to 82%, but still maintains a relatively high strength (54 MPa).

[0071] Examples 3 and 4 have further weakened hydrophilicity and loose structure, and the rejection rate (tetracycline 80%→61%, Na2SO433.7%→26.9%) and the strength (49→32 MPa) are significantly reduced, indicating that excessive sacrifice of hydrophilicity and structural tightness will weaken the performance of the membrane.

[0072] In Comparative Example 4, although the rejection rate is high, the flux is extremely low, and the applicability is limited.

[0073] In Comparative Example 5, the flux is outstanding but the rejection rate and the strength are insufficient, and it is difficult to meet the actual demand.

[0074] Figures 3 to 6 The UV absorption visible spectrum of the tetracycline solution through the aramid nanofiber composite nanofiltration membrane prepared in Examples 1-4 at different time periods is shown in the following table. Filtrate 10min~60min respectively represents the filtration time, and in the ultraviolet spectrum test of the purified solution, the characteristic peak is significantly reduced, which proves that the aramid nanofiber composite nanofiltration membrane has excellent purification effect on the simulated antibiotic.

[0075] Examples 5-6 and Comparative Examples 6-7 Examples 5-6 and Comparative Examples 6-7 provide a preparation method of an aramid nanofiber composite nanofiltration membrane. Compared with Example 1, the difference is that the amount of citric acid added in step S2 is changed, as shown in the following table. The rest is basically the same as Example 1, which will not be repeated here.

[0076] It can be known from the experiment that when the amount of citric acid added is less than 10% of the mass of PVA-co-PE nanofiber, the swelling rate increases dramatically due to insufficient crosslinking; when the amount of citric acid added is more than 15% of the mass of PVA-co-PE nanofiber, the excess citric acid forms a brittle crosslinking point (strength decreases) and the residual acid catalyzes hydrolysis (swelling rate rises); when the amount of citric acid added is 15% of the mass of PVA-co-PE nanofiber, the comprehensive performance of the aramid nanofiber composite nanofiltration membrane prepared is optimal.

[0077] In summary, the application significantly improves the comprehensive performance of ANF / NF composite nanofiltration membranes through covalent cross-linking strategy and multi-level structure design. The interface bonding strength between ANF and PVA-co-PE matrix is increased by more than 40% through glutaraldehyde / epichlorohydrin covalent bonding, the tensile strength is ≥50 MPa, which effectively enhances the compression stability and service life of the membrane; high flux and high selectivity synergy: ANF nanofiber network and PVA-co-PE porous matrix form a hierarchical mass transfer channel (pore size 1~2 nm), water flux is increased to ≥50 L m -2 h -1 At the same time, the electrostatic adsorption effect is strengthened through carboxyl functional modification, and the rejection rate of tetracycline antibiotics is ≥95%; the anti-pollution and anti-swelling properties are enhanced, the membrane surface hydrophilicity (contact angle <60°) and hierarchical pore structure synergistically inhibit the adsorption of pollutants, the swelling rate is reduced to <5%, and the long-term running flux decay rate is reduced by 60% compared with traditional membranes; based on the chemical modifiability of PVA-co-PE active hydroxyl and ANF surface functional groups, various functional groups (such as sulfonic acid groups, quaternary ammonium groups) can be directionally grafted, which meets the high-efficiency separation needs of different pollutants (heavy metals, dyes). In summary, through the synergistic regulation of material-structure-function, the application breaks through the bottleneck of the balance between "strength-flux-selectivity" of traditional nanofiltration membranes, and provides high-performance membrane technology support for high-difficulty industrial wastewater treatment and resource recovery.

[0078] It should be noted that the application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and the same effect as the technical idea and the same effect as the embodiments within the scope of the technical solutions of the application are also included in the technical scope of the application. In addition, within the scope of the main idea of the application, various modifications of the embodiments that can be thought of by those skilled in the art, and other ways constructed by combining part of the components of the embodiments are also included in the scope of the application.

Claims

1. A method for preparing aramid nanofiber composite nanofiltration membrane, characterized in that, The method comprises the following steps: S1, aramid fiber is immersed in a strong alkaline solution, dimethyl sulfoxide is added, and mechanical stirring is carried out, so as to obtain an aramid nanofiber dispersion liquid with a mass-volume fraction of 0.1%-0.3%; S2, PVA-co-PE nanofiber is added to the aramid nanofiber dispersion liquid obtained in step S1, an organic solution is added, stirring is uniformly carried out, concentrated hydrochloric acid, a crosslinking agent and citric acid are added for crosslinking reaction, and then a vacuum-assisted suction filtration method is used to obtain an initial composite film; S3, the initial composite film obtained in step S2 is placed in a first concentration of tert-butyl alcohol aqueous solution for the first solvent exchange, and then the film is placed in a second concentration of tert-butyl alcohol aqueous solution for the second solvent exchange; the film is taken out and subjected to freeze-drying treatment, so as to obtain an aramid nanofiber composite nanofiltration membrane.

2. The method for preparing the aramid nanofiber composite nanofiltration membrane according to claim 1, characterized in that, In step S2, the mass ratio of the aramid nanofiber to the PVA-co-PE nanofiber is 1: (0.5-3).

3. The method for preparing the aramid nanofiber composite nanofiltration membrane according to claim 1, characterized in that, In step S2, the amount of citric acid is 10%-15% of the mass of the PVA-co-PE nanofiber.

4. The method for preparing the aramid nanofiber composite nanofiltration membrane according to claim 1, characterized in that, In step S2, the volume fraction of the concentrated hydrochloric acid is 0.1%-0.3%, and the volume fraction of the crosslinking agent is 2%-4%.

5. The method for preparing the aramid nanofiber composite nanofiltration membrane according to claim 1, characterized in that, In step S3, the first concentration is less than the second concentration.

6. The method for preparing the aramid nanofiber composite nanofiltration membrane according to claim 5, characterized in that, In the first solvent exchange, the volume fraction of the tert-butyl alcohol aqueous solution is 40-60%, and in the second solvent exchange, the volume fraction of the tert-butyl alcohol aqueous solution is 90-100%.

7. The method for preparing the aramid nanofiber composite nanofiltration membrane according to claim 1, characterized in that, In step S2, the crosslinking agent is one of glutaraldehyde and epichlorohydrin.

8. The method for preparing the aramid nanofiber composite nanofiltration membrane according to claim 1, characterized in that, In step S2, the organic solution is one of tert-butyl alcohol aqueous solution and isopropyl alcohol aqueous solution.

9. The method for preparing the aramid nanofiber composite nanofiltration membrane according to claim 1, characterized in that, In step S3, the freeze-drying treatment conditions are as follows: the temperature is-80--45℃, the vacuum degree is 15-20 Pa, and the freeze-drying time is 24-30 h.

10. The method for preparing the aramid nanofiber composite nanofiltration membrane according to claim 1, characterized in that, In step S1, the immersion time of aramid fiber in the strong alkaline solution is 10-30 s.

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