A crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane, a preparation method and application thereof

Abstract

The present application relates to the technical field of nanofiltration membrane, and particularly relates to a crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane and a preparation method and application thereof. The nanofiltration membrane is prepared by attaching a carbon nanotube intermediate layer on a base membrane, then preparing a crown ether modified polyamide membrane layer on the carbon nanotube layer by an interfacial polymerization method, and further grafting positively charged molecules on the surface of the membrane layer, so that the nanofiltration membrane forms a composite functional structure of "the surface having grafted molecules with positive charge, the inside having lithium ion recognition of crown ether, and the lower part having conductivity of carbon nanotube". The positively charged layer of the nanofiltration membrane has a strong size screening effect and a Donnan exclusion effect on the divalent magnesium ions with positive charge, the crown ether provides a special channel for lithium ions, and under the electric drive, the lithium ions are transported quickly, so that the nanofiltration membrane is endowed with the characteristics of "intercepting magnesium ions and permeating lithium ions" simultaneously, thereby realizing efficient lithium-magnesium separation application.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane technology, and in particular to a crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] Nanofiltration membranes, with pore sizes between ultrafiltration and reverse osmosis membranes, can separate molecules and ions at the molecular and ionic scales due to their nanoscale pores (typically 0.5–2 nm). In high magnesium-to-lithium ratio brine lakes, nanofiltration membranes can effectively retain high-valence magnesium ions while allowing low-valence lithium ions to permeate, providing a feasible approach for lithium resource separation and extraction. For lithium-magnesium separation, using positively or negatively charged functionalized nanofiltration membranes can disrupt the hydrated ion balance of magnesium and lithium ions in water, significantly improving separation efficiency and selectivity. Crown ethers are cyclic compounds containing an ether group that possess ion recognition capabilities. By grafting or chemically modifying separation membranes with crown ethers, dedicated lithium-ion transport channels can be provided, enabling the membrane to selectively extract lithium ions from brine lakes, achieving lithium-magnesium separation. Although the energy barrier for lithium ions to lose bound water through crown ether channels is lower than that for magnesium ions, they are still subject to steric hindrance from the crown ether channels, thus requiring further improvement in lithium-magnesium separation efficiency. Summary of the Invention

[0003] In view of this, the present invention provides a crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane, its preparation method and application.

[0004] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a method for preparing a crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane, comprising the following steps: (1) Carbon nanotubes were prepared into a stable dispersion and filtered and loaded onto a substrate membrane, then dried to obtain a carbon nanotube intermediate layer; (2) Place the polyamide monomer and crown ether in water, disperse and dissolve them by ultrasonication, and prepare a mixed aqueous solution; (3) The carbon nanotube intermediate layer is first brought into contact with the mixed aqueous solution and then vacuum filtered to ensure that the aqueous solution completely wets the pores of the carbon nanotube membrane; then it is brought into contact with the organic solution to undergo an interfacial polymerization reaction and form a separation layer. (4) The surface of the separation layer is brought into contact with a solution of positively charged molecules to perform positively charged molecule grafting treatment; (5) After grafting, the membrane surface is cleaned and heat-treated to obtain the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane.

[0005] Furthermore, in step (1), the diameter of the carbon nanotube is 5~50 nm; the thickness of the intermediate layer of the carbon nanotube is 0.5~5 μm.

[0006] Further, in step (1), the base membrane is a polyethersulfone filter membrane, a polyvinylidene fluoride filter membrane, a cellulose acetate filter membrane, or a polysulfone filter membrane.

[0007] Further, in step (2), the polyamide monomer is at least one of piperazine, N-aminoethylpiperazine, 1,4-cyclohexanediamine, 1,3,5-triaminobenzene, diethylenetriamine, triethylenetetramine, aromatic diamine, and aliphatic diamine, and the concentration of the polyamide monomer ranges from 0.5 to 2 wt.%.

[0008] Further, in step (2), the crown ether includes at least one of benzo-12-crown ether-4, benzo-15-crown ether-5, benzo-18-crown 6-ether, 4'-aminobenzo-15-crown ether-5, 4'-aminobenzo-18-crown ether-6, diaminodibenzo-14-crown ether-4, diaminodibenzo-15-crown ether-5, and diaminodibenzo-18-crown ether-6, with a crown ether concentration ranging from 0.05 to 0.5 wt.%.

[0009] Furthermore, in step (2), the ultrasonic dispersion time is 0.5~1 h.

[0010] Furthermore, in step (3), the organic phase solution is prepared by dissolving trimesoyl chloride in an organic solvent.

[0011] Further, in step (3), the concentration of pyromellitic chloroformyl chloride in the organic phase solution is 0.02~0.2 wt.%.

[0012] Furthermore, in step (3), the organic solvent is at least one of n-hexane, cyclohexane, and n-heptane.

[0013] Furthermore, in step (3), the time for the interfacial polymerization reaction is 1~2 min; the temperature is 20~25 ℃.

[0014] Furthermore, in step (3), the volume ratio of the mixed aqueous solution to the organic solution is 1:0.9~1.1.

[0015] Furthermore, the volume ratio of the carbon nanotube dispersion in step (1) to the mixed aqueous solution in step (3) is 1~2:1.

[0016] Further, in step (4), the positively charged molecules in the positively charged molecule solution are at least one of polyethyleneimine, 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, 1,3-bispiperidinylpropane, 4-aminomethylpiperazine, polydiallyldimethylammonium chloride and chitosan, and the concentration of the positively charged molecules ranges from 0.1 to 2 wt.%.

[0017] Furthermore, in step (4), the time for the positively charged molecule grafting treatment is 1~10 min; the temperature is 20~25 ℃.

[0018] Furthermore, the volume ratio of the positively charged molecular solution in step (4) to the mixed aqueous phase solution in step (3) is 1:0.9~1.1.

[0019] Furthermore, in step (5), the heat treatment temperature is 50~80 ℃ and the heat treatment time is 10~30 min.

[0020] Secondly, the present invention provides a crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane prepared by the above-described preparation method.

[0021] Thirdly, the present invention provides the application of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane described in the second aspect in electrically driven lithium-magnesium separation.

[0022] Furthermore, the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane is sealed into a membrane module, with the carbon nanotube layer of the membrane as the cathode and the anode placed on the inlet side. An electric field is applied to drive lithium ions to permeate more quickly and simultaneously promote magnesium ion retention, thereby achieving efficient lithium-magnesium separation.

[0023] Furthermore, the voltage range is 0.5~2.5 V.

[0024] Compared with the prior art, the present invention has achieved the following beneficial effects: The crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane provided by this invention possesses a three-layer composite functional structure: a positively charged surface with grafted molecules, lithium-ion recognition via crown ethers, and conductivity via carbon nanotubes at the bottom. The positively charged membrane surface enables the membrane layer to readily accept positively charged divalent Mg2+. 2+ It exhibits a strong Donnan repulsion effect; the recognition of crown ethers is Li. + It provides a more relaxed microenvironment and a fast transmission channel, enabling Li to achieve quantum "jump" transmission through a "recognition-affinity-release" quantum "hop" transmission mode. + The penetration of Mg is enhanced through steric hindrance. 2+Retention. Furthermore, the conductive carbon nanotube interlayer allows it to couple with an electric field. Under the influence of the electric field, lithium ions are driven by the directional electric field force, resulting in a significant increase in migration rate within the membrane pores and at the crown ether-specific recognition sites (dominated by the electric field effect). Magnesium ions, due to their large hydration radius and mismatch with the crown ether cavity, are simultaneously affected by both the electric field effect and the membrane surface charge effect (dominated by both electric field and charge effects), thus being repelled, leading to increased magnesium ion retention. The crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane, when separating a lithium-magnesium mixed solution under electric drive, exhibits improved magnesium ion retention. 2+ The highest rejection rate was 99.98% for Li + It achieves a high negative rejection rate of -49.63% and a lithium-magnesium selectivity of up to 11411.4. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0026] Figure 1 This is a scanning electron microscope (SEM) image of the cross-sectional structure of the crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane prepared in Example 1 of this invention. Figure 2 This is a scanning electron microscope (SEM) image of the surface structure of the crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane prepared in Example 2 of this invention. Figure 3 The graph shows the electro-driven lithium-magnesium separation performance of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane prepared in Example 3 of this invention under an applied voltage gradient of 0.5~2.5 V. Figure 4 This is a schematic diagram of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane structure prepared in Example 1 of the present invention; Among them, 1. The positively charged grafted molecular layer structure on the surface of the separation layer of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane; 2. The separation layer of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane; 3. The carbon nanotube intermediate layer of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane; 4. The base layer of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane. Detailed Implementation

[0027] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, 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 invention pertains.

[0028] Nanofiltration membrane separation mainly relies on the sieving effect of membrane pores and the electrostatic repulsion effect (i.e., the Dornan effect). It can efficiently remove harmful impurities such as divalent ions, organic matter, bacteria, and viruses from water to purify water. It can also selectively separate different molecules or ions to achieve the extraction and recovery of valuable resources. It has broad development prospects in the fields of water treatment and resource utilization.

[0029] For lithium-magnesium separation, the use of positively or negatively charged functionalized nanofiltration membranes can disrupt the hydrated ion balance of magnesium and lithium ions in water, significantly improving their separation efficiency and selectivity. Currently, polyamide nanofiltration membranes reported domestically and internationally for high-efficiency lithium-magnesium separation are mainly positively charged. Compared to negatively charged nanofiltration membranes, positively charged nanofiltration membranes generally have a greater advantage in separating positively charged ions. Due to the positive charge on the membrane surface, the electrostatic repulsion generated by the Donnan effect enables the membrane to have a high rejection capacity for divalent magnesium ions. However, while the Donnan repulsion effect efficiently retains magnesium ions, it also increases the rejection capacity for lithium ions, hindering their permeation, thus requiring further improvement in the lithium-magnesium separation performance of the membrane. For lithium-magnesium separation using nanofiltration membranes, although increasing the rejection capacity of divalent magnesium ions can yield high Li-magnesium separation efficiency, this approach is not ideal. + / Mg 2+ Separation selectivity is required, but rapid lithium-ion penetration also needs to be taken into account.

[0030] Patents such as CN118831451A disclose methods for incorporating crown ether molecules into nanofiltration membranes to improve lithium-magnesium separation efficiency. Crown ethers are cyclic compounds containing ether groups and possess ion recognition capabilities. By grafting or chemically modifying the separation membrane with crown ethers, a dedicated lithium-ion transport channel can be provided, enabling the membrane to selectively extract lithium ions from brine in salt lakes, thus achieving lithium-magnesium separation. However, lithium-ion transport is hindered by steric hindrance in the pores, and the lithium-magnesium separation efficiency needs further improvement. Since positively charged ions in water are driven by an electric field to migrate directionally along the direction of the electric field, the electric field effect can drive and enhance the transport of lithium ions in the nanofiltration membrane pores, reducing the energy barrier for passing through the crown ether channels and significantly increasing its transmembrane permeation rate. Therefore, if a nanofiltration membrane is simultaneously functionalized with a positive charge and a specific lithium-ion channel is introduced to construct an electrically driven lithium-magnesium separation nanofiltration membrane, the magnesium ion retention and lithium ion permeation behavior can be simultaneously controlled / improved, which is expected to achieve efficient lithium-magnesium separation. Based on this, the present invention provides a crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane, its preparation method and application. The crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane is prepared by first attaching a conductive carbon nanotube intermediate layer to a base membrane, then preparing a crown ether-modified polyamide layer on the carbon nanotube intermediate layer by interfacial polymerization, and further performing positively charged molecule grafting treatment on the surface of the crown ether-modified polyamide layer to form a positively charged separation membrane layer. The obtained nanofiltration membrane can achieve efficient lithium-magnesium separation under electric field drive by using the carbon nanotube intermediate layer as the membrane cathode.

[0031] This invention provides a method for preparing a crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane, comprising the following steps: (1) Carbon nanotubes were prepared into a stable dispersion and filtered and loaded onto a substrate membrane, then dried to obtain a carbon nanotube intermediate layer; (2) Place the polyamide monomer and crown ether in water, disperse and dissolve them by ultrasonication, and prepare a mixed aqueous solution; (3) The carbon nanotube intermediate layer is first brought into contact with the mixed aqueous solution and then vacuum filtered to ensure that the aqueous solution completely wets the pores of the carbon nanotube membrane; then it is brought into contact with the organic solution to undergo an interfacial polymerization reaction and form a separation layer. (4) The surface of the separation layer is brought into contact with a solution of positively charged molecules to perform positively charged molecule grafting treatment; (5) After grafting, the membrane surface is cleaned and heat-treated to obtain the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane.

[0032] Further, in step (1), the diameter of the carbon nanotubes is 5~50 nm; the thickness of the carbon nanotube intermediate layer is 0.5~5 μm. The diameter of the carbon nanotubes can be any value between 5~50 nm, such as 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, etc.; the thickness of the carbon nanotube intermediate layer can be any value between 0.5~5 μm, such as 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, etc. When the diameter of the carbon nanotubes is 5~50 nm, the membrane pores are more regular and the lithium and magnesium ion sieving selectivity is better; the larger the diameter, the higher the membrane water permeation flux, and excessively large diameter will destroy the polyamide cross-linking structure, leading to increased membrane defects and decreased separation performance.

[0033] Further, in step (1), the base membrane is one of polyethersulfone filter membrane, polyvinylidene fluoride filter membrane, cellulose acetate filter membrane or polysulfone filter membrane.

[0034] Further, in step (2), the polyamide monomer is at least one selected from piperazine, N-aminoethylpiperazine, 1,4-cyclohexanediamine, 1,3,5-triaminobenzene, diethylenetriamine, triethylenetetramine, aromatic diamine, and aliphatic diamine, and the concentration of the polyamide monomer ranges from 0.5 to 2 wt.%. The concentration of the polyamide monomer can be any value between 0.5 and 2 wt.%, such as 0.6 wt.%, 0.7 wt.%, 0.8 wt.%, 0.9 wt.%, 1.0 wt.%, 1.1 wt.%, 1.2 wt.%, 1.3 wt.%, 1.4 wt.%, 1.5 wt.%, 1.6 wt.%, 1.7 wt.%, 1.8 wt.%, 1.9 wt.%, etc. The higher the concentration of polyamide monomer, the more complete the interfacial polymerization and cross-linking, and the better the membrane density and magnesium ion retention effect. If the concentration is too low, membrane structure defects and unstable separation performance are likely to occur. If the concentration is too high, the separation layer will be too thick and the pores will shrink, resulting in a significant decrease in permeation flux.

[0035] Further, in step (2), the crown ether includes at least one of benzo-12-crown ether-4, benzo-15-crown ether-5, benzo-18-crown 6-ether, 4'-aminobenzo-15-crown ether-5, 4'-aminobenzo-18-crown ether-6, diaminodibenzo-14-crown ether-4, diaminodibenzo-15-crown ether-5, and diaminodibenzo-18-crown ether-6, with a crown ether concentration ranging from 0.05 to 0.5 wt.%. The crown ether concentration can be any value between 0.05 and 0.5 wt.%, such as 0.1 wt.%, 0.15 wt.%, 0.2 wt.%, 0.25 wt.%, 0.3 wt.%, 0.35 wt.%, 0.4 wt.%, 0.45 wt.%, etc. Increased crown ether concentration increases the number of lithium-ion specific coordination recognition sites, significantly improving the lithium-magnesium separation selectivity; if the concentration is too low, there are insufficient effective binding sites and poor separation selectivity; if the concentration is too high, molecular aggregation and blockage of membrane pores are likely to occur, resulting in reduced permeate flux and decreased overall membrane separation performance.

[0036] Furthermore, in step (2), the ultrasonic dispersion time is 0.5 to 1 h. The ultrasonic dispersion time can be any value between 0.5 and 1 h, such as 0.6 h, 0.7 h, 0.8 h, 0.9 h, etc.

[0037] Furthermore, in step (3), the organic phase solution is prepared by dissolving trimesoyl chloride in an organic solvent.

[0038] Further, in step (3), the concentration of pyromellitic chloride in the organic phase solution is 0.02~0.2 wt.%. The concentration of pyromellitic chloride in the organic phase solution can be any value between 0.02 and 0.2 wt.%, for example, 0.03 wt.%, 0.04 wt.%, 0.05 wt.%, 0.06 wt.%, 0.07 wt.%, 0.08 wt.%, 0.09 wt.%, 0.1 wt.%, 0.11 wt.%, 0.12 wt.%, 0.13 wt.%, 0.14 wt.%, 0.15 wt.%, 0.16 wt.%, 0.17 wt.%, 0.18 wt.%, 0.19 wt.%, etc. Tristyrene chloride is a highly active trifunctional acyl chloride monomer that can rapidly undergo interfacial condensation with aqueous amines to construct a rigid, highly cross-linked three-dimensional polyamide network separation layer. This layer has a dense and stable structure, which can firmly anchor crown ethers and positively charged functional groups and precisely control the lithium-magnesium sieve channels. Within the range of 0.02 to 0.2 wt.%, the membrane cross-linking density increases with increasing concentration, and the magnesium ion retention and lithium-magnesium separation selectivity are simultaneously improved. If the concentration is too low, insufficient cross-linking can easily lead to membrane defects, while if the concentration is too high, the membrane pores will shrink excessively and the water permeation flux will decrease significantly.

[0039] Furthermore, in step (3), the organic solvent is at least one of n-hexane, cyclohexane, and n-heptane.

[0040] Further, in step (3), the interfacial polymerization reaction takes 1-2 min and the temperature is 20-25 ℃. The interfacial polymerization reaction time can be any value between 1 and 2 min, such as 1.1 min, 1.2 min, 1.3 min, 1.4 min, 1.5 min, 1.6 min, 1.7 min, 1.8 min, 1.9 min, etc. The temperature can be any value between 20 and 25 ℃, such as 21 ℃, 22 ℃, 23 ℃, 24 ℃, etc. The interfacial polymerization reaction is a rapid condensation reaction between the aqueous phase amine monomer and the oil phase acyl chloride monomer at the oil-water interface. The amine group and the acyl chloride group generate amide bonds through the condensation reaction, forming an ultra-thin and dense polyamide separation layer in situ.

[0041] Further, in step (3), the volume ratio of the mixed aqueous solution to the organic solution is 1:0.9~1.1. The volume ratio of the mixed aqueous solution to the organic solution can be any ratio between 1:0.9 and 1.1, such as 1:0.95, 1:1, 1:1.05, etc.

[0042] Further, in step (4), the positively charged molecules in the positively charged molecule solution are at least one of polyethyleneimine, 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, 1,3-bispiperidinylpropane, 4-aminomethylpiperazine, polydiallyldimethylammonium chloride, and chitosan, and the concentration of the positively charged molecules ranges from 0.1 to 2 wt.%. The concentration of the positively charged molecules can be any value between 0.1 and 2 wt.%, for example, 0.2 wt.%, 0.3 wt.%, 0.4 wt.%, 0.5 wt.%, 0.6 wt.%, 0.7 wt.%, 0.8 wt.%, 0.9 wt.%, 1 wt.%, 1.1 wt.%, 1.2 wt.%, 1.3 wt.%, 1.4 wt.%, 1.5 wt.%, 1.6 wt.%, 1.7 wt.%, 1.8 wt.%, 1.9 wt.%, etc. As the concentration of positively charged molecules increases, the number of positively charged sites on the membrane surface continues to increase, and the selectivity of lithium and magnesium ion separation and the electrostatic antifouling ability are improved simultaneously. If the concentration is too low, the grafting is insufficient and the charge regulation effect is weak. If the concentration is too high, the molecules are prone to stacking and agglomeration, clogging the membrane pores, resulting in a significant decrease in membrane permeation flux and a deterioration in separation stability.

[0043] Further, in step (4), the time for the positively charged molecule grafting treatment is 1~10 min; the temperature is 20~25 ℃. The time for the positively charged molecule grafting treatment can be any value between 1 and 10 min, such as 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, etc. The temperature can be any value between 20~25 ℃, such as 21 ℃, 22 ℃, 23 ℃, 24 ℃, etc. Positively charged molecule grafting can impart a stable positively charged surface to the film, and efficiently retain divalent Mg through electrostatic repulsion. 2+ Promote monovalent Li + It prioritizes permeation, significantly improving the selectivity of lithium-magnesium separation, while strengthening interfacial bonding, regulating membrane pore structure, and simultaneously improving membrane hydrophilicity and antifouling stability.

[0044] Furthermore, the volume ratio of the positively charged molecular solution in step (4) to the mixed aqueous solution in step (3) is 1:0.9~1.1. The volume ratio of the positively charged molecular solution in step (4) to the mixed aqueous solution in step (3) can be any ratio between 1:0.9 and 1.1, such as 1:0.95, 1:1, 1:1.05, etc.

[0045] Further, in step (5), the heat treatment temperature is 50~80 ℃, and the heat treatment time is 10~30 min. The heat treatment temperature can be any value between 50~80 ℃, such as 55 ℃, 60 ℃, 65 ℃, 70 ℃, 75 ℃, etc. The heat treatment time can be any value between 10~30 min, such as 15 min, 20 min, 25 min. Heat treatment can promote the deep cross-linking and curing of the polyamide separation layer, regulate the membrane pore structure and interface bonding stability. Appropriate temperature can improve the membrane density. Too high a temperature will cause excessive shrinkage of the membrane pores and decrease the permeate flux. Too low a temperature will result in insufficient cross-linking, which can easily lead to structural defects and reduce ion retention and separation performance.

[0046] This invention provides a crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane prepared by the above-described preparation method.

[0047] The crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane provided by this invention achieves narrower pore size nanofiltration membrane layers by copolymerizing crown ether and monomer molecules on the carbon nanotube interlayer, resulting in a tight bond between the nanofiltration membrane layer and the carbon nanotube layer, exhibiting good stability and pressure resistance. Further, positively charged molecules are grafted onto the surface of the nanofiltration membrane layer, forming a three-layer composite functional structure: a positively charged surface with grafted molecules, lithium-ion recognition via the crown ether interior, and conductivity of the carbon nanotubes at the bottom. The positively charged membrane surface enables the membrane layer to resist positively charged divalent Mg2+. 2+ It exhibits a strong Donnan repulsion effect; the recognition of crown ethers is Li. + It provides a more relaxed microenvironment and a fast transmission channel, enabling Li to achieve quantum "jump" transmission through a "recognition-affinity-release" quantum "hop" transmission mode. + The penetration of Mg is enhanced through steric hindrance. 2+ Retention. The nanofiltration membrane described above has the characteristic of simultaneously "intercepting magnesium ions and allowing lithium ions to pass through".

[0048] This invention provides the application of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane described above in electrically driven lithium-magnesium separation.

[0049] Furthermore, the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane is sealed into a membrane module, with the carbon nanotube layer of the membrane as the cathode and the anode placed on the inlet side. An electric field is applied to drive lithium ions to permeate more quickly and simultaneously promote magnesium ion retention, thereby achieving efficient lithium-magnesium separation.

[0050] Furthermore, the voltage range is 0.5~2.5 V. The voltage range can be any value between 0.5 and 2.5 V, such as 0.6 V, 0.7 V, 0.8 V, 0.9 V, 1 V, 1.1 V, 1.2 V, 1.3 V, 1.4 V, 1.5 V, 1.6 V, 1.7 V, 1.8 V, 1.9 V, 2 V, 2.1 V, 2.2 V, 2.3 V, 2.4 V, etc.

[0051] The crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane described above can be coupled with an electric field due to its conductive carbon nanotube interlayer. Under the action of the electric field, lithium ions are driven by the directional electric field force, and their migration rate within the membrane pores and at the crown ether-specific recognition sites is greatly increased (dominated by the electric field effect). Magnesium ions, due to their large hydration radius and mismatch with the crown ether cavity, are simultaneously affected by the electric field effect and the membrane surface charge effect (dominated by both electric field and charge effects), thus being repelled, leading to increased magnesium ion retention. The synergistic effect of the electric field and the selectivity of the membrane material can simultaneously promote lithium ion permeation and magnesium ion retention, significantly improving lithium-magnesium separation performance.

[0052] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0053] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art. The reagents and raw materials used in this invention are readily available through conventional means, and unless otherwise specified, they are used in accordance with conventional methods in the art or product instructions. Similarly, unless otherwise specified, the test methods of this invention are performed in accordance with conventional methods in the art or industry-standard methods or practices. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to the methods of this invention. The preferred embodiments and materials described herein are for illustrative purposes only.

[0054] Example 1 A method for preparing a crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane, specifically implemented according to the following steps: (1) Weigh 0.1 mg of carbon nanotubes with a diameter of 10 nm and put them into pure water to prepare a stable dispersion of carbon nanotubes with a concentration of 0.1 mg / mL. Then take 50 mL of the stable dispersion of carbon nanotubes and load the carbon nanotubes onto a 0.45 μm cellulose acetate filter membrane by vacuum filtration. Dry at 80 °C for 20 min to obtain the carbon nanotube intermediate layer.

[0055] (2) Take 0.6 g of 1,4-cyclohexanediamine and 0.05 g of benzo-12-crown ether-4 in pure water, and disperse them by ultrasonication for 0.5 h to prepare a mixed aqueous solution with a concentration of 0.6 wt.% 1,4-cyclohexanediamine and 0.05 wt.% benzo-12-crown ether-4.

[0056] (3) Dissolve 0.05 g of trimesoyl chloride in n-hexane to obtain a n-hexane solution of trimesoyl chloride with a concentration of 0.05 wt.%, which is the organic phase solution.

[0057] (4) Take 30 mL of mixed aqueous solution to completely wet the carbon nanotube intermediate layer, remove excess liquid on the surface by vacuum-assisted filtration and keep the aqueous solution completely wetted in the pores of the carbon nanotube layer; then take 30 mL of organic solution to contact the surface of the carbon nanotube membrane, and conduct an interfacial polymerization reaction on the surface of the carbon nanotube membrane at 20 °C for 1 min to form a separation layer.

[0058] (5) Take 0.1 g of polyethyleneimine in pure water to prepare a polyethyleneimine solution with a concentration of 0.1 wt.%.

[0059] (6) After the interfacial polymerization reaction, remove the unreacted organic phase solution on the surface of the separation layer, and then add 30 mL of polyethyleneimine solution to perform positively charged molecular grafting treatment on the surface of the separation layer for 10 min at a temperature of 20 °C.

[0060] (7) The membrane surface was washed with pure water and then heat-treated at 80 °C for 5 min to obtain the composite nanofiltration membrane.

[0061] Figure 1 This is a scanning electron microscope (SEM) image of the cross-sectional structure of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane prepared in Example 1 of this invention. Figure 4 This is a schematic diagram of the crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane structure prepared in Example 1 of the present invention. Figure 1 , 4 As shown, the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane includes a positively charged grafted molecular layer structure 1 on the surface of the separation layer of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane, a separation layer 2 of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane, a carbon nanotube intermediate layer 3 of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane, and a base layer 4 of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane.

[0062] The electro-driven lithium-magnesium separation application of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane is implemented according to the following steps: The nanofiltration membrane prepared above was sealed into a membrane module, with the carbon nanotube layer of the membrane as the cathode and a titanium mesh placed on the inlet side as the anode. Power was supplied via a DC regulated voltage, using Mg...2+ / Li + Using a mixed solution of 20 g / L as the stock solution, the separation performance of the nanofiltration membrane prepared above was tested at a pressure of 3 bar. Without applied voltage, the pure water permeability of the membrane was 7.48 L·m⁻¹. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 99.93%, the lithium ion rejection rate was 34.25%, and the selectivity was 1044.94. When a voltage of 1.5 V was applied, the pure water permeability of the membrane was 8.28 L·m⁻¹. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 99.94%, the lithium ion rejection rate was -8.25%, and the selectivity was 1801.56. When a voltage of 2.5 V was applied, the pure water permeability of the membrane was 7.64 L·m³. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 92.52%, the lithium ion rejection rate was -14.53%, and the selectivity was 14.56%.

[0063] Example 2 A method for preparing a crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane, specifically implemented according to the following steps: (1) Weigh 0.2 mg of carbon nanotubes with a diameter of 15 nm and put them into pure water to prepare a stable dispersion of carbon nanotubes with a concentration of 0.2 mg / mL. Then take 20 mL of the stable dispersion of carbon nanotubes and load the carbon nanotubes onto a 0.22 μm polyvinylidene fluoride filter membrane by vacuum filtration. Dry at 60 °C for 60 min to obtain the carbon nanotube intermediate layer.

[0064] (2) Take 0.8 g of N-aminoethylpiperazine and 0.1 g of benzo-15-crown ether-5 in pure water and disperse them by ultrasonication for 0.75 h to prepare a mixed aqueous solution with a concentration of 0.8 wt.% N-aminoethylpiperazine and 0.1 wt.% benzo-15-crown ether-5.

[0065] (3) Dissolve 0.1 g of pyromellitic chloride in n-hexane to obtain a n-hexane solution of pyromellitic chloride with a concentration of 0.1 wt.%, which is the organic phase solution.

[0066] (4) Take 20 mL of mixed aqueous solution to completely wet the carbon nanotube intermediate layer, remove excess liquid on the surface by vacuum-assisted filtration and keep the aqueous solution completely wetted in the pores of the carbon nanotube layer; then take 20 mL of organic solution to contact the surface of the carbon nanotube membrane, and conduct an interfacial polymerization reaction on the surface of the carbon nanotube membrane at 22 °C for 2 min to form a separation layer.

[0067] (5) Take 0.25 g of polyethyleneimine in pure water to prepare a polyethyleneimine solution with a concentration of 0.25 wt.%.

[0068] (6) After the interfacial polymerization reaction, remove the unreacted organic phase solution on the surface of the separation layer, and then add 20 mL of polyethyleneimine solution to perform positively charged molecular grafting treatment on the surface of the separation layer for 5 min at a temperature of 20 °C.

[0069] (7) The membrane surface was washed with pure water and then heat-treated at 50 °C for 20 min to obtain the composite nanofiltration membrane.

[0070] Figure 2 This is a scanning electron microscope (SEM) image of the surface structure of the crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane prepared in Example 2 of this invention. Figure 2 As shown, the prepared crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane has a uniform, dense, and interwoven morphology on its surface, and the membrane structure is continuous and regular with no obvious agglomeration or pore defects.

[0071] The electro-driven lithium-magnesium separation application of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane is implemented according to the following steps: The nanofiltration membrane prepared above was sealed into a membrane module, with the carbon nanotube layer of the membrane as the cathode and a titanium mesh placed on the inlet side as the anode. Power was supplied via a DC regulated voltage, using Mg... 2+ / Li + Using a mixed solution of 20 g / L as the stock solution, the separation performance of the nanofiltration membrane prepared above was tested at a pressure of 3 bar. Without applied voltage, the pure water permeability of the membrane was 7.94 L·m⁻¹. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 99.48%, the lithium ion rejection rate was 26.80%, and the selectivity was 143.28. When a voltage of 1.5 V was applied, the pure water permeability of the membrane was 8.45 L·m³. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 99.97%, the lithium ion rejection rate was -49.34%, and the selectivity was 5129.49%. At an applied voltage of 2.5 V, the membrane's pure water permeability was 7.77 L·m³. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 88.92%, the lithium ion rejection rate was -76.01%, and the selectivity was 15.89%.

[0072] Example 3 A method for preparing a crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane, specifically implemented according to the following steps: (1) Weigh 0.5 mg of carbon nanotubes with a diameter of 20 nm and put them into pure water to prepare a stable dispersion of carbon nanotubes with a concentration of 0.5 mg / mL. Then take 10 mL of the stable dispersion of carbon nanotubes and load the carbon nanotubes onto a 0.45 μm polyethersulfone filter membrane by vacuum filtration. Dry at 60 °C for 30 min to obtain the carbon nanotube intermediate layer.

[0073] (2) Take 1 g of piperazine and 0.5 g of benzo-18-crown 6-ether in pure water and disperse them by ultrasonication for 1 h to prepare a mixed aqueous solution with a concentration of 1 wt.% piperazine and 0.5 wt.% benzo-18-crown 6-ether.

[0074] (3) Dissolve 0.2 g of pyromellitic chloride in n-hexane to obtain a n-hexane solution of pyromellitic chloride with a concentration of 0.2 wt.%, which is the organic phase solution.

[0075] (4) Take 10 mL of mixed aqueous solution to completely wet the carbon nanotube intermediate layer, remove excess liquid on the surface by vacuum-assisted filtration and keep the aqueous solution completely wetted in the pores of the carbon nanotube layer; then take 10 mL of organic solution to contact the carbon nanotube membrane surface, and conduct interfacial polymerization reaction on the carbon nanotube membrane surface at 25 °C for 1.5 min to form a separation layer.

[0076] (5) Take 0.5 g of polyethyleneimine in pure water to prepare a polyethyleneimine solution with a concentration of 0.5 wt.%.

[0077] (6) After the interfacial polymerization reaction, remove the unreacted organic phase solution on the surface of the separation layer, and then add 10 mL of polyethyleneimine solution to perform positively charged molecular grafting treatment on the surface of the separation layer for 2 min at a temperature of 20 °C.

[0078] (7) The membrane surface was washed with pure water and then heat-treated at 60 °C for 10 min to obtain the composite nanofiltration membrane.

[0079] Figure 3 This image shows the electro-driven lithium-magnesium separation performance of the crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane prepared in Example 3 of this invention under an applied voltage gradient of 0.5–2.5 V. (Source: [Insert image here]) Figure 3 As shown, increasing the applied voltage can significantly increase Mg 2+ Retention rate, enhanced Li + Preferred permeation behavior, the membrane permeation rate first increases and then decreases, achieving optimal lithium-magnesium separation effect under 1.5 V conditions.

[0080] The electro-driven lithium-magnesium separation application of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane is implemented according to the following steps: The nanofiltration membrane prepared above was sealed into a membrane module, with the carbon nanotube layer of the membrane as the cathode and a titanium mesh placed on the inlet side as the anode. Power was supplied via a DC regulated voltage, using Mg... 2+ / Li + Using a mixed solution of 20 g / L as the stock solution, the separation performance of the nanofiltration membrane prepared above was tested at a pressure of 3 bar. Without applied voltage, the pure water permeability of the membrane was 8.06 L·m⁻¹. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 99.23%, the lithium ion rejection rate was 10.48%, and the selectivity was 115.83. When a voltage of 1.5 V was applied, the pure water permeability of the membrane was 8.50 L·m³. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 99.98%, the lithium ion rejection rate was -49.63%, and the selectivity was 11411.4. When a voltage of 2.5 V was applied, the pure water permeability of the membrane was 8.27 L·m³. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 87.15%, the lithium ion rejection rate was -72.86%, and the selectivity was 14.10.

[0081] Comparative Example 1 A method for preparing a carbon nanotube-modified positively charged polyamide nanofiltration membrane, specifically implemented according to the following steps: (1) Weigh 0.5 mg of carbon nanotubes with a diameter of 20 nm and put them into pure water to prepare a stable dispersion of carbon nanotubes with a concentration of 0.5 mg / mL.

[0082] (2) Take 1 g of piperazine and 10 mL of carbon nanotube stable dispersion in pure water, and ultrasonically disperse for 1 h to prepare a mixed aqueous solution with a concentration of 1 wt.% piperazine and 0.005 wt.% carbon nanotube.

[0083] (3) Dissolve 0.2 g of pyromellitic chloride in n-hexane to obtain a n-hexane solution of pyromellitic chloride with a concentration of 0.2 wt.%, which is the organic phase solution.

[0084] (4) Take 10 mL of mixed aqueous solution to completely wet the 0.45 μm polyethersulfone filter membrane, and remove excess liquid from the surface by vacuum-assisted filtration; then take 10 mL of organic phase solution to contact the surface of the polyethersulfone filter membrane, and conduct an interfacial polymerization reaction on the surface of the polyethersulfone filter membrane at 25 °C for 1.5 min to form a separation layer.

[0085] (5) Take 0.5 g of polyethyleneimine in pure water to prepare a polyethyleneimine solution with a concentration of 0.5 wt.%.

[0086] (6) After the interfacial polymerization reaction, remove the unreacted organic phase solution on the surface of the separation layer, and then add 10 mL of polyethyleneimine solution to perform positively charged molecular grafting treatment on the surface of the separation layer for 2 min at a temperature of 20 °C.

[0087] (7) The surface of the membrane was washed with pure water and heat-treated at 60 °C for 10 min to obtain the carbon nanotube modified positively charged polyamide nanofiltration membrane.

[0088] The application of the carbon nanotube-modified positively charged polyamide nanofiltration membrane for lithium-magnesium separation is implemented according to the following steps: The nanofiltration membrane prepared above was sealed into a membrane module, and Mg was used. 2+ / Li + A mixed solution of 20 g / L was used as the stock solution, and the separation performance of the nanofiltration membrane prepared above was tested at a pressure of 3 bar. The pure water permeability of the membrane was 6.49 L·m³. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 99.46%, the lithium ion rejection rate was 27.79%, and the selectivity was 133.72, which was significantly lower than the performance in Example 3. Furthermore, unlike Example 3, in Comparative Example 1, carbon nanotubes were added to an aqueous solution for interfacial polymerization to form a separation layer. The prepared nanofiltration membrane did not have a carbon nanotube interlayer; therefore, no electric field was applied to test the performance in the implementation of Comparative Example 1.

[0089] Comparative Example 2 A method for preparing a positively charged polyamide / carbon nanotube nanofiltration membrane, specifically implemented according to the following steps: (1) Weigh 0.5 mg of carbon nanotubes with a diameter of 20 nm and put them into pure water to prepare a stable dispersion of carbon nanotubes with a concentration of 0.5 mg / mL. Then take 10 mL of the stable dispersion of carbon nanotubes and load the carbon nanotubes onto a 0.45 μm polyethersulfone filter membrane by vacuum filtration. Dry at 60 °C for 30 min to obtain the carbon nanotube intermediate layer.

[0090] (2) Take 1 g of piperazine in pure water, and dissolve it by ultrasonication for 1 h to prepare a 1 wt.% piperazine aqueous solution.

[0091] (3) Dissolve 0.2 g of pyromellitic chloride in n-hexane to obtain a n-hexane solution of pyromellitic chloride with a concentration of 0.2 wt.%, which is the organic phase solution.

[0092] (4) Take 10 mL of aqueous solution to completely wet the carbon nanotube intermediate layer, remove excess liquid on the surface by vacuum-assisted filtration and keep the aqueous solution completely wetted in the pores of the carbon nanotube layer; then take 10 mL of organic solution to contact the surface of the carbon nanotube membrane, and conduct an interfacial polymerization reaction on the surface of the carbon nanotube membrane at 25 °C for 1.5 min to form a separation layer.

[0093] (5) Take 0.5 g of polyethyleneimine in pure water to prepare a polyethyleneimine solution with a concentration of 0.5 wt.%.

[0094] (6) After the interfacial polymerization reaction, remove the unreacted organic phase solution on the surface of the separation layer, and then add 10 mL of polyethyleneimine solution to perform positively charged molecular grafting treatment on the surface of the separation layer for 2 min at a temperature of 20 °C.

[0095] (7) The membrane surface was washed with pure water and then heat-treated at 60 °C for 10 min to obtain the composite nanofiltration membrane.

[0096] The application of the above-mentioned positively charged polyamide / carbon nanotube nanofiltration membrane for lithium-magnesium separation is implemented according to the following steps: The nanofiltration membrane prepared above was sealed into a membrane module, with the carbon nanotube layer of the membrane as the cathode and a titanium mesh placed on the inlet side as the anode. Power was supplied via a DC regulated voltage, using Mg... 2+ / Li + Using a mixed solution of 20 g / L as the stock solution, the separation performance of the nanofiltration membrane prepared above was tested at a pressure of 3 bar. Without applied voltage, the pure water permeability of the membrane was 8.03 L·m⁻¹. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 99.22%, the lithium ion rejection rate was 14.24%, and the selectivity was 116.25. When a voltage of 1.5 V was applied, the pure water permeability of the membrane was 8.49 L·m³. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 99.36%, the lithium ion rejection rate was -6.63%, and the selectivity was 166.61. When a voltage of 2.5 V was applied, the pure water permeability of the membrane was 8.29 L·m³. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 89.84%, the lithium ion rejection rate was -28.25%, and the selectivity was 12.62, which was significantly lower than the performance in Example 3.

[0097] Comparative Example 3 A method for preparing a crown ether modified polyamide / carbon nanotube nanofiltration membrane, specifically implemented according to the following steps: (1) Weigh 0.5 mg of carbon nanotubes with a diameter of 20 nm and put them into pure water to prepare a stable dispersion of carbon nanotubes with a concentration of 0.5 mg / mL. Then take 10 mL of the stable dispersion of carbon nanotubes and load the carbon nanotubes onto a 0.45 μm polyethersulfone filter membrane by vacuum filtration. Dry at 60 °C for 30 min to obtain the carbon nanotube intermediate layer.

[0098] (2) Take 1 g of piperazine and 0.5 g of benzo-18-crown 6-ether in pure water and disperse them by ultrasonication for 1 h to prepare a mixed aqueous solution with a concentration of 1 wt.% piperazine and 0.5 wt.% benzo-18-crown 6-ether.

[0099] (3) Dissolve 0.2 g of pyromellitic chloride in n-hexane to obtain a n-hexane solution of pyromellitic chloride with a concentration of 0.2 wt.%, which is the organic phase solution.

[0100] (4) Take 10 mL of mixed aqueous solution to completely wet the carbon nanotube intermediate layer, remove excess liquid on the surface by vacuum-assisted filtration and keep the aqueous solution completely wetted in the pores of the carbon nanotube layer; then take 10 mL of organic solution to contact the carbon nanotube membrane surface, and conduct interfacial polymerization reaction on the carbon nanotube membrane surface at 25 °C for 1.5 min to form a separation layer.

[0101] (5) The membrane surface is washed with pure water and then heat-treated at 60 °C for 10 min to obtain the composite nanofiltration membrane.

[0102] The above-mentioned application of crown ether modified polyamide / carbon nanotube nanofiltration membrane for lithium-magnesium separation is implemented according to the following steps: The nanofiltration membrane prepared above was sealed into a membrane module, with the carbon nanotube layer of the membrane as the cathode and a titanium mesh placed on the inlet side as the anode. Power was supplied via a DC regulated voltage, using Mg... 2+ / Li + Using a mixed solution of 20 g / L as the stock solution, the separation performance of the nanofiltration membrane prepared above was tested at a pressure of 3 bar. Without applied voltage, the pure water permeability of the membrane was 8.19 L·m⁻¹. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 97.06%, the lithium ion rejection rate was 8.91%, and the selectivity was 30.98. When a voltage of 1.5 V was applied, the pure water permeability of the membrane was 8.61 L·m³. -2 ·h -1 ·bar -1 The magnesium ion rejection rate was 98.81%, the lithium ion rejection rate was -55.55%, and the selectivity was 130.71. When a voltage of 2.5 V was applied, the membrane's pure water permeability was 8.45 L·m³. -2 ·h-1 ·bar -1 The magnesium ion rejection rate was 80.96%, the lithium ion rejection rate was -78.78%, and the selectivity was 9.39, which was significantly lower than the performance in Example 3.

[0103] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. The application of a crown ether-modified positively charged polyamide / carbon nanotube nanofiltration membrane in electrically driven lithium-magnesium separation, characterized in that, The specific steps are as follows: the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane is sealed into the membrane module, the carbon nanotube layer of the membrane is used as the cathode, and the anode is placed on the water inlet side. An electric field is applied to drive lithium ions to permeate faster through electricity, while promoting the retention of magnesium ions, so as to achieve efficient lithium and magnesium separation. The preparation method of the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane includes the following steps: (1) Carbon nanotubes were prepared into a stable dispersion and filtered and loaded onto a substrate membrane, then dried to obtain a carbon nanotube intermediate layer; (2) Place the polyamide monomer and crown ether in water, disperse and dissolve them by ultrasonication, and prepare a mixed aqueous solution; (3) The carbon nanotube intermediate layer is first brought into contact with the mixed aqueous solution and then vacuum filtered to ensure that the aqueous solution completely wets the pores of the carbon nanotube membrane; then it is brought into contact with the organic solution to undergo an interfacial polymerization reaction and form a separation layer. (4) The surface of the separation layer is brought into contact with a solution of positively charged molecules to perform a positively charged molecule grafting treatment; (5) After grafting, the membrane surface is cleaned and heat-treated to obtain the crown ether modified positively charged polyamide / carbon nanotube nanofiltration membrane. In step (2), the crown ether comprises at least one of benzo-12-crown ether-4, benzo-15-crown ether-5, benzo-18-crown 6-ether, 4'-aminobenzo-15-crown ether-5, 4'-aminobenzo-18-crown ether-6, diaminodibenzo-14-crown ether-4, diaminodibenzo-15-crown ether-5, and diaminodibenzo-18-crown ether-6, with a crown ether concentration ranging from 0.05 to 0.5 wt.%. In step (4), the positively charged molecules in the positively charged molecule solution are at least one of polyethyleneimine, 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, 1,3-bispiperidinylpropane, 4-aminomethylpiperazine, polydiallyldimethylammonium chloride and chitosan, and the concentration of positively charged molecules ranges from 0.1 to 2 wt.%.

2. The application as described in claim 1, characterized in that, In step (1), the diameter of the carbon nanotube is 5~50nm; the thickness of the intermediate layer of the carbon nanotube is 0.5~5 μm; or, the base membrane is a polyethersulfone filter membrane, a polyvinylidene fluoride filter membrane, a cellulose acetate filter membrane, or a polysulfone filter membrane.

3. The application as described in claim 1, characterized in that, In step (2), the polyamide monomer is at least one of piperazine, N-aminoethylpiperazine, 1,4-cyclohexanediamine, 1,3,5-triaminobenzene, diethylenetriamine, triethylenetetramine, aromatic diamine, and aliphatic diamine, and the concentration of the polyamide monomer ranges from 0.5 to 2 wt.%.

4. The application as described in claim 1, characterized in that, In step (3), the organic phase solution is prepared by dissolving trimesoyl chloride in an organic solvent; the concentration of trimesoyl chloride in the organic phase solution is 0.02~0.2 wt.%; or, in step (3), the organic solvent is at least one of n-hexane, cyclohexane, and n-heptane; or, in step (3), the interfacial polymerization reaction time is 1~2 min and the temperature is 20~25 ℃; or, in step (3), the volume ratio of the mixed aqueous phase solution to the organic phase solution is 1:0.9~1.

1.

5. The application as described in claim 1, characterized in that, In step (4), the time for the positively charged molecule grafting treatment is 1~10 min, and the temperature is 20~25 ℃.

6. The application as described in claim 1, characterized in that, The volume ratio of the positively charged molecular solution in step (4) to the mixed aqueous solution in step (3) is 1:0.9~1.

1.

7. The application as described in claim 1, characterized in that, In step (5), the heat treatment temperature is 50~80 ℃ and the heat treatment time is 10~30 min.

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