A method for preparing composite nanofiltration membranes by electrospinning with mixed solvents

Abstract

This invention discloses a method for preparing a composite nanofiltration membrane by electrospinning with a mixed solvent. The method includes dissolving polyacrylonitrile (PAN) in a mixed solvent of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), stirring the solution until it is completely transparent under water bath heating, and allowing it to stand to remove bubbles to obtain a PAN electrospinning solution; electrospinning the PAN solution to form a membrane, and then hot-pressing and drying it to obtain a PAN nanofiber membrane; impregnating the PAN nanofiber membrane with a piperazine (PIP) aqueous solution, tumbling it with a rubber roller, and adding a trimesoyl chloride (TMC) n-hexane solution to synthesize polyamide (PA) as the top layer to obtain a PA / PAN composite nanofiltration membrane. This composite membrane has high flux and rejection rate for both salts and dyes.

Description

Technical Field

[0001] This invention belongs to the field of composite membrane preparation, specifically relating to a method for preparing composite nanofiltration membranes by electrospinning with mixed solvents. Background Technology

[0002] Electrospinning has evolved into a powerful technology capable of producing membranes composed of nano- to micron-sized fibers, exhibiting unique membrane properties due to their extremely high surface-to-volume ratio and high porosity. However, this technology is often far from being a green and environmentally friendly process because the solvents it uses present problems such as high flammability, toxicity, difficult disposal, or energy-intensive synthesis. In fact, the most commonly used solvents in the electrospinning field are halogenated solvents (e.g., chloroform, trifluoroethanol) and toxic solvents (e.g., dimethylformamide), whose use is now restricted by the EU's REACH (Restricted Area and Chemicals Control) regulations. This is particularly important when considering the commercialization of electrospun products, as solvent choices are more limited. To make electrospinning more attractive as a commercial technology and further improve its scalability, solvent alternatives and other green routes must be established, while adhering to social and legal restrictions, especially regarding environmental and health impacts.

[0003] Compared to the extensive research on the structure regulation of phase inversion membranes, there is still a lack of systematic research on the structure regulation of nanofiber membranes in separation membranes. However, there is a relatively mature system regarding the influence of nanofiber preparation processes on the membrane structure. Unlike traditional phase inversion methods, the pores of nanofiber membranes are formed by the stacking of fibers, which solidify before being deposited onto the receiver. Therefore, the splitting process of the spinning jet determines the dimensional basis of the membrane structure.

[0004] Many factors influence the fiber structure during electrospinning, including solution properties, environmental conditions, and process parameters. Among these, solvent and polymer properties play a particularly crucial role in determining the structure and subsequent processability of electrospun fibers. The phase separation rate, determined by solvent volatility and polymer-solvent interactions, is a key factor in spinnability. Excessive phase separation can lead to nozzle clogging, hindering the spinning process; conversely, insufficient phase separation can cause the jet to struggle to overcome surface tension and split, or result in a dense film structure that fails to solidify after deposition onto the receiving device. Furthermore, the dielectric constant of the solvent also significantly affects the morphology and structure of electrospun nanofibers.

[0005] Therefore, balancing the effects of polymer-solvent interactions and solvent properties, and selecting suitable solvents to optimize electrospinning process conditions are crucial in the study of preparing polyacrylonitrile nanofiber membranes. Summary of the Invention

[0006] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0007] In view of the problems existing in the above and / or prior art, the present invention is proposed.

[0008] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing composite nanofiltration membranes by electrospinning with mixed solvents.

[0009] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for preparing a composite nanofiltration membrane by electrospinning with mixed solvents, comprising,

[0010] Polyacrylonitrile (PAN) was dissolved in a mixed solvent of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The solution was stirred until completely transparent under water bath heating conditions, and then allowed to stand to remove bubbles to obtain the PAN electrospinning solution.

[0011] PAN nanofiber membranes are obtained by electrospinning PAN solution and then hot-pressing and drying them.

[0012] PAN nanofiber membranes were impregnated with piperazine (PIP) aqueous solution, dried by rubber rollers, and then a solution of trimesoyl chloride (TMC) in n-hexane was added dropwise to synthesize polyamide (PA) as the top layer, thus obtaining a PA / PAN composite nanofiltration membrane.

[0013] As a preferred embodiment of the preparation method described in this invention, the polyacrylonitrile (PAN) is dissolved in a mixed solvent of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), wherein the mass fraction of the mixed solvent is 10-15 wt%.

[0014] As a preferred embodiment of the preparation method described in this invention, the N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) mixed solvents are wherein the proportion of DMSO is 0, 10, 20, 30, 40, 50, 60, and 80 wt.%.

[0015] In a preferred embodiment of the preparation method described in this invention, the DMSO content is 30 wt.%.

[0016] In a preferred embodiment of the preparation method described in this invention, the solution is stirred until completely transparent under water bath heating conditions, wherein the water bath heating temperature is 60-70°C.

[0017] As a preferred embodiment of the preparation method described in this invention, the PAN solution is electrospun to form a film, wherein the electrospinning process parameters are: liquid feed rate 0.5-2 mL / h, applied voltage 15-25 kV, receiving distance 8-15 cm, and roller speed 100-200 r / min.

[0018] In a preferred embodiment of the preparation method described in this invention, the PAN nanofiber membrane is obtained by hot pressing and drying, wherein the hot pressing temperature is 100-120°C.

[0019] In a preferred embodiment of the preparation method described in this invention, the mass fraction of the trimesoyl chloride (TMC) is 0.1–0.2 wt%, and the mass fraction of piperazine (PIP) is 1–2 wt%.

[0020] As a preferred embodiment of the preparation method described in this invention, the PAN nanofiber membrane is impregnated with a piperazine (PIP) aqueous solution for 2-3 minutes; the addition of a trimesoyl chloride (TMC) hexane solution comprises adding 2 mL of 0.1 wt% TMC and reacting for 2 minutes.

[0021] As a preferred embodiment of the preparation method described in this invention, after the synthesized polyamide PA is used as the top layer, the method further includes washing the surface of the PA / PAN composite nanofiltration membrane with n-hexane, and then placing it in deionized water for washing 3 to 4 times.

[0022] Beneficial effects of this invention:

[0023] (1) By comprehensively comparing the difference in solubility parameters between commonly used strongly polar aprotic solvents and polyacrylonitrile, as well as the dielectric constant and saturated vapor pressure of the solvents, this invention selects DMF and DMSO as the solvent system for preparing polyacrylonitrile spinning solution; to a certain extent, the emission of DMF is reduced, and DMSO green solvent is selected for doping.

[0024] (2) The solubility of the polymer by the mixed solvent and the changes in the dielectric constant and saturated vapor pressure of the solvent have a phased effect on the structure of the nanofiber membrane. By adjusting the ratio of DMF / DMSO solvent and coordinating hot pressing post-treatment, as the DMSO content increases, the effective fusion degree between fibers increases, which improves the elongation at break of the base film and reduces the average pore size of the film surface. At the same time, as the DMSO content increases, the spinnability of the polymer decreases. The optimal spinning range is 10-40 wt% DMSO content.

[0025] (3) The present invention uses interfacial polymerization to prepare PA / PAN composite membrane on PAN membrane, which achieves the best effect with DMSO content of 30%wt, and has high throughput and rejection rate for salt and dye. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0027] Figure 1 This is a schematic diagram of the process for preparing the PAN / PA composite nanofiltration membrane in an embodiment of the present invention.

[0028] Figure 2 This is a viscosity diagram of the PAN solution in an embodiment of the present invention.

[0029] Figure 3 This is a pure water flux diagram of the PAN-based membrane in Example 1 of the present invention.

[0030] Figure 4 This is a pure water flux diagram of the PAN / PA composite nanofiltration membrane in Example 1 of the present invention.

[0031] Figure 5 This is a diagram of the water contact angle of the PAN-based membrane in Embodiment 1 of the present invention.

[0032] Figure 6 This is a tensile strength diagram of the PAN base film in Embodiment 1 of the present invention.

[0033] Figure 7 This is a scanning electron microscope image of the PAN base film in an embodiment of the present invention.

[0034] Figure 8 The diagram below shows a cross-flow filtration device in an embodiment of the present invention, wherein: 1. jacketed constant temperature raw material tank, 2. vacuum pump, 3. shut-off valve, 4. pressure gauge, 5. flow meter, 6. membrane module, 7. weighing device, and 8. constant temperature water bath. Detailed Implementation

[0035] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0036] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0037] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0038] In this invention, polyacrylonitrile (PA) is an Aladdin reagent; N,N-dimethylformamide (DMF) is a Lingfeng Chemical reagent; N,N-dimethylacetamide (DMAc) is a Lingfeng Chemical reagent; dimethyl sulfoxide (DMSO) is a Lingfeng Chemical reagent; anhydrous piperazine (PIP) is a Sinopharm reagent; trimesoyl chloride (TMC) is an Aladdin reagent; n-hexane is an Aladdin reagent; sodium sulfate is a Sinopharm reagent; and Victoria Blue B is a Shanghai Maclean reagent.

[0039] Example 1

[0040] (1) Prepare a PAN solution with a solid content of 10 wt%:

[0041] After drying PAN overnight in a vacuum drying oven at 80°C, 4g of PAN was dissolved in 36g of DMF solvent and stirred for 4h in a water bath at 70°C to obtain a 10wt% PAN electrospinning solution. After standing for 12h to remove bubbles, a clear and transparent orange-yellow solution was obtained.

[0042] (2) The PAN electrospinning solution prepared in step (1) was used for electrospinning. A stainless steel roller was used as the receiving device. The spinning solution feed rate was 0.9 mL / h, the voltage was 25 kV, the receiving distance was 12 cm, the roller speed was 100 r / min, and the spinning was carried out for 8 hours. The membrane was cut into 10×10 cm pieces and then subjected to repeated hot pressing at 100℃ for 3 times to obtain the PAN membrane.

[0043] (3) The PAN prepared in step (2) is immersed in 2wt% PIP solution for 2 min and then dried by rubber roller; 2 mL of 0.1wt% TMC is added and reacted for 2 min to carry out interfacial polymerization; the surface is washed twice with n-hexane and then naturally dried in air for 10 min before being placed in deionized water.

[0044] (4) After washing the PA / PAN composite membrane (60nm:110μm) prepared in step (3) three times with pure water, the membrane separation performance includes pure water permeability (PWP) and rejection rate.

[0045] See the schematic diagram of the PAN / PA composite nanofiltration membrane preparation process. Figure 1 A self-made cross-flow filtration device is used (see...) Figure 8 The separation performance was measured using [a specific method / mechanism]. All experiments were conducted at 0.4 MPa and 25 °C. The effective area of ​​the membrane was 4.52 × 10⁻⁴ m². 2Before each test, each membrane was pretreated with deionized water for at least 30 minutes to ensure performance stability. The concentration of salt ions in the permeate and feed solutions was determined using a conductivity meter. The concentration of VB dye in the feed and permeate was measured using a UV-Vis spectrophotometer at an absorption wavelength of 664 nm.

[0046] It is used in salt / water separation systems at a concentration of 1000 ppm sodium sulfate (Na2SO4). It is used in dye / water separation systems for Victoria Blue (MW 506.08) at a concentration of 100 ppm.

[0047] Example 2

[0048] After drying PAN overnight in a vacuum drying oven at 80°C, 4g of PAN was dissolved in a mixed solvent of 32.4g DMF and 3.6g DMSO (i.e., DMSO accounting for 10wt%) and stirred for 4h in a water bath at 70°C to obtain a PAN electrospinning solution with a concentration of 10wt%. After standing for 12h to remove bubbles, a clear and transparent orange-yellow solution was obtained.

[0049] The specific operations of steps (2), (3), and (4) are the same as those in Example 1.

[0050] Example 3

[0051] After drying PAN overnight in a vacuum drying oven at 80°C, 4g of PAN was dissolved in a mixed solvent of 28.8g DMF and 7.2g DMSO (i.e., DMSO accounting for 20wt%) and stirred for 4h in a water bath at 70°C to obtain a PAN electrospinning solution with a concentration of 10wt%. After standing for 12h to remove bubbles, a clear and transparent orange-yellow solution was obtained.

[0052] The specific operations of steps (2), (3), and (4) are the same as those in Example 1.

[0053] Example 4

[0054] After drying PAN overnight in a vacuum drying oven at 80°C, 4g of PAN was dissolved in a mixed solvent of 25.2g DMF and 10.8g DMSO (i.e., DMSO accounting for 30wt%) and stirred for 4h in a water bath at 70°C to obtain a PAN electrospinning solution with a concentration of 10wt%. After standing for 12h to remove bubbles, a clear and transparent orange-yellow solution was obtained.

[0055] The specific operations of steps (2), (3), and (4) are the same as those in Example 1.

[0056] Example 5

[0057] After drying PAN overnight in a vacuum drying oven at 80°C, 4g of PAN was dissolved in a mixed solvent of 21.6g DMF and 14.4g DMSO (i.e., DMSO content of 40wt%) and stirred for 4h in a water bath at 70°C to obtain a PAN electrospinning solution with a concentration of 10wt%. After standing for 12h to remove bubbles, a clear and transparent orange-yellow solution was obtained.

[0058] The specific operations of steps (2), (3), and (4) are the same as those in Example 1.

[0059] The above embodiments are shown in Table 1.

[0060] Table 1

[0061]

[0062] DMSO was chosen for its superior solubility in PAN. A smaller difference in polymer-solvent solubility parameters indicates better solubility of the solvent for that polymer. Simultaneously, the solvent's dielectric constant determines its ability to carry charge under an electrostatic field and the splitting stability of the jet, thus affecting fiber uniformity. The solvent's saturated vapor pressure determines its evaporation rate and the stage at which the fiber is fully cured. Therefore, DMF and DMSO, which have relatively large differences in polymer solubility, dielectric constant, and saturated vapor pressure, were chosen as the spinning solution solvents to effectively control the morphology of the nanofiber support layer.

[0063] The solubility of polymers in mixed solvents, as well as changes in the dielectric constant and saturated vapor pressure of the solvents, have a phased impact on the nanofiber membrane structure. By adjusting the DMF / DMSO solvent ratio and coordinating hot-pressing post-treatment, the effective fusion between fibers increases with increasing DMSO content, thereby improving the elongation at break of the base membrane and reducing the average pore size of the membrane surface. Thin-layer composite nanofiltration membranes were prepared, and the performance of the base membrane and composite membrane before and after interfacial polymerization based on different solvent systems was compared. Interfacial polymerization improved the solvent resistance of the base membrane while reducing the permeability of the composite membrane.

[0064] After drying PAN overnight in a vacuum drying oven at 80℃, 4g of PAN was dissolved in 36g of DMF + DMSO solvent and stirred in a water bath at 70℃ for 4 hours to obtain a 10wt% PAN electrospinning solution. The DMSO concentrations were 0wt%, 10wt%, 20wt%, 30wt%, 40wt%, 50wt%, 60wt%, and 80wt%. The viscosity of the different spinning solutions was measured using a viscometer (NDJ-8S, Shanghai Fangrui Co., Ltd.). (See [reference needed]). Figure 2 The data were obtained through an average of at least three experiments. It can be seen that as the proportion of DMSO increases, the viscosity of the PAN solution increases, and the diameter of the nanofibers obtained by spinning increases due to the increased difficulty and slowness of jet extension; the membrane pore size also increases.

[0065] See the pure water flux diagram for PAN-based membranes. Figure 3 See the pure water flux diagram for the PAN / PA composite nanofiltration membrane. Figure 4 , Figure 3 This indicates that the pore size of the base film first decreases and then increases. Figure 4 The results show that after interfacial polymerization, the flux increases with the increase of DMSO content. However, when the DMSO content is more than 50% wt, the selective layer is difficult to form on the base film, resulting in defects in the film and a significant increase in flux.

[0066] See the diagram of the water contact angle of the PAN-based membrane. Figure 5 See the tensile strength diagram of the PAN base film. Figure 6 , Figure 5 This indicates that as the proportion of DMSO increases, the hydrophilicity of the membrane gradually increases. Figure 6 This indicates that increasing the DMSO content has little impact on the mechanical properties of the base film. See the scanning electron microscope image of the PAN base film. Figure 7 .

[0067] Comparative Example 1

[0068] Replace the 2% wtPIP solution in the first step of Example 1 with a 1% wt% PIP solution to wet the PAN film surface for 2 minutes, then roll it dry with a rubber roller; add 2 mL of 0.1 wt% TMC, react for 2 minutes to carry out interfacial polymerization; wash the surface twice with n-hexane, air dry for 10 minutes, and then place it in deionized water.

[0069] It is applied to a salt / water separation system at a concentration of 1000 ppm sodium sulfate (Na2SO4).

[0070] Comparative Example 2

[0071] Replace the 2% wt% PIP solution in step 2 with a 1% wt% PIP solution to wet the PAN film surface for 2 min, then roll it dry with a rubber roller; add 2 mL of 0.1 wt% TMC, react for 2 min to carry out interfacial polymerization; wash the surface twice with n-hexane, air dry for 10 min, and then place it in deionized water.

[0072] It is applied to a salt / water separation system at a concentration of 1000 ppm sodium sulfate (Na2SO4).

[0073] Comparative Example 3

[0074] Replace the 2% wt% PIP solution in step 3 with a 1% wt% PIP solution to wet the PAN film surface for 2 min, and then roll it dry with a rubber roller; add 2 mL of 0.1 wt% TMC, react for 2 min, and carry out interfacial polymerization; wash the surface twice with n-hexane, let it air dry for 10 min, and then put it into deionized water.

[0075] It is applied to a salt / water separation system at a concentration of 1000 ppm sodium sulfate (Na2SO4).

[0076] Comparative Example 4

[0077] Replace the 2% wt% PIP solution in step 4 with a 1% wt% PIP solution to wet the PAN film surface for 2 min, then roll it dry with a rubber roller; add 2 mL of 0.1 wt% TMC, react for 2 min to carry out interfacial polymerization; wash the surface twice with n-hexane, air dry for 10 min, and then place it in deionized water.

[0078] It is applied to a salt / water separation system at a concentration of 1000 ppm sodium sulfate (Na2SO4).

[0079] Comparative Example 5

[0080] Replace the 2% wt% PIP solution in step 5 with a 1% wt% PIP solution to wet the PAN film surface for 2 min, then roll it dry with a rubber roller; add 2 mL of 0.1 wt% TMC, react for 2 min to carry out interfacial polymerization; wash the surface twice with n-hexane, air dry for 10 min, and then place it in deionized water.

[0081] It is applied to a salt / water separation system at a concentration of 1000 ppm sodium sulfate (Na2SO4).

[0082] Table 2

[0083] membrane <![CDATA[Salt flux L / (m 2 hbar)]]> Salt rejection rate (%) Comparative Example 1 15.2 15.8 Comparative Example 2 18.6 9.6 Comparative Example 3 15.3 20.1 Comparative Example 4 13.8 12.6 Comparative Example 5 14.6 13.1

[0084] Comparative Example 6

[0085] After drying PAN overnight in a vacuum drying oven at 80°C, 4g of PAN was dissolved in a mixed solvent of 25.2g DMF and 10.8g NMP (i.e., NMP content of 30wt%) and stirred for 4h in a water bath at 70°C to obtain a PAN electrospinning solution with a concentration of 10wt%. After standing for 12h to remove bubbles, a clear and transparent orange-yellow solution was obtained.

[0086] The specific operations of steps (2), (3), and (4) are the same as those in Example 1.

[0087] Comparative Example 7

[0088] After drying PAN overnight in a vacuum drying oven at 80°C, 4g of PAN was dissolved in a mixed solvent of 25.2g DMF and 10.8g DMAc (i.e., DMAc content of 30wt%) and stirred for 4h in a water bath at 70°C to obtain a PAN electrospinning solution with a concentration of 10wt%. After standing for 12h to remove bubbles, a clear and transparent orange-yellow solution was obtained.

[0089] The specific operations of steps (2), (3), and (4) are the same as those in Example 1.

[0090] Comparative Example 8

[0091] After drying PAN overnight in a vacuum drying oven at 80°C, 4g of PAN was dissolved in a mixed solvent of 25.2g NMP and 10.8g DMSO (30wt%) and stirred for 4h in a water bath at 70°C to obtain a 10wt% PAN electrospinning solution. After standing for 12h to remove bubbles, a clear and transparent orange-yellow solution was obtained.

[0092] The specific operations of steps (2), (3), and (4) are the same as those in Example 1.

[0093] Comparative Example 9

[0094] After drying PAN overnight in a vacuum drying oven at 80°C, 4g of PAN was dissolved in a mixed solvent of 25.2g DMAc and 10.8g DMSO (i.e., DMSO accounting for 30wt%) and stirred for 4h in a water bath at 70°C to obtain a PAN electrospinning solution with a concentration of 10wt%. After standing for 12h to remove bubbles, a clear and transparent orange-yellow solution was obtained.

[0095] The specific operations of steps (2), (3), and (4) are the same as those in Example 1.

[0096] Table 3

[0097] membrane <![CDATA[Salt flux L / (m 2 h bar)]]> Salt rejection rate (%) Comparative Example 6 0.6 15.8 Comparative Example 7 10.8 42.4 Comparative Example 8 0.1 23.7 Comparative Example 9 9.8 36.2

[0098] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the present invention.

Claims

1. A method for preparing composite nanofiltration membranes by electrospinning with mixed solvents, characterized in that: include, Polyacrylonitrile (PAN) was dissolved in a mixed solvent of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The solution was stirred until completely transparent under water bath heating, and then allowed to stand to remove bubbles, yielding the PAN electrospinning solution. The mass fraction of PAN in the mixed solvent was 10-15 wt%, and the DMSO content in the N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) mixed solvent was 30 wt%. PAN solution was electrospun into a film, which was then dried by hot pressing to obtain a PAN nanofiber membrane. The electrospinning process parameters were as follows: liquid feed rate 0.5~2 mL / h, applied voltage 15~25 kV, receiving distance 8~15 cm, and roller speed 100~200 r / min. PAN nanofiber membranes were impregnated with piperazine (PIP) aqueous solution, dried by rubber rollers, and then a solution of trimesoyl chloride (TMC) in n-hexane was added dropwise to synthesize polyamide (PA) as the top layer, thus obtaining a PA / PAN composite nanofiltration membrane.

2. The preparation method according to claim 1, characterized in that: The solution is stirred until completely transparent under water bath heating conditions, wherein the water bath heating temperature is 60~70℃.

3. The preparation method according to claim 1, characterized in that: The PAN nanofiber membrane is obtained by hot pressing and drying, wherein the hot pressing temperature is 100~120℃.

4. The preparation method according to claim 1, characterized in that: The content of pyromellitic methyl chloride (TMC) is 0.1-0.2 wt%, and the content of piperazine (PIP) is 1-2 wt%.

5. The preparation method according to claim 1, characterized in that: The PAN nanofiber membrane is impregnated with piperazine (PIP) aqueous solution for 2-3 minutes; the addition of trimesoyl chloride (TMC) in hexane solution includes adding 2 mL of 0.1 wt% TMC and reacting for 2 minutes.

6. The preparation method according to claim 1, characterized in that: After the synthesized polyamide PA is used as the top layer, the process further includes cleaning the surface of the PA / PAN composite nanofiltration membrane with n-hexane, and then immersing it in deionized water for washing 3-4 times.

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