Nanofiltration membrane based on acid anhydride constructed gradient polyamide layer, preparation method and application thereof

Abstract

The application discloses a nanofiltration membrane based on anhydride-constructed gradient polyamide layer, a preparation method and application thereof, and belongs to the technical field of water treatment membrane material preparation. The application solves the problems of insufficient selectivity and mutual restriction between permeability and selectivity of the nanofiltration membrane in the lithium-magnesium ion separation process. The application uses an anhydride monomer to post-modify a nascent polyamide layer, and the anhydride monomer undergoes secondary amidation reaction with residual amine groups in the polyamide layer, converts part of the protonatable amine groups into carboxyl groups, and introduces a cross-linking structure. The diffusion and reaction of the anhydride monomer in the polyamide layer are synchronous, so that a continuous change of charge distribution and pore structure is formed along the film thickness direction of the polyamide layer, a polyamide layer with gradient characteristics is constructed, and the lithium-magnesium separation performance is improved. In addition, the solvent-induced polyamide layer is thinned, the mass transfer resistance is reduced, the mutual restriction between permeability and selectivity of the traditional nanofiltration membrane is relieved, and the synergistic improvement of the membrane permeation performance and the lithium-magnesium separation performance is realized.

Description

Technical Field

[0001] This invention belongs to the field of water treatment membrane material preparation technology, and relates to nanofiltration membranes based on acid anhydride-constructed gradient polyamide layers, their preparation methods, and applications. Background Technology

[0002] With the rapid development of lithium-based energy technologies, the demand for lithium resources continues to grow, and efficient and green lithium extraction technologies have become an important guarantee for meeting future energy needs. Compared with traditional separation technologies such as electrodialysis and solvent extraction, nanofiltration membrane separation has advantages such as low energy consumption, continuous process, environmental friendliness, and ease of large-scale application. However, due to the similar hydration radii and transport behaviors of lithium and magnesium ions, nanofiltration membranes still face severe challenges in achieving efficient and selective separation of these two ions.

[0003] Current research on lithium-magnesium separation nanofiltration membranes mainly focuses on controlling the membrane surface charge or optimizing the pore structure, such as enhancing the positive charge of the membrane surface, increasing the membrane pore size, or reducing the pore size distribution. These methods often only focus on the physical sieving effect of the membrane surface layer, neglecting the intrapore transport process of ions after entering the nanochannels. Because the chemical environment and spatial structure inside traditional channels are relatively uniform, the diffusion rate of lithium and magnesium ions within the pores has limited differences, restricting further improvement in separation performance.

[0004] Furthermore, existing polyamide nanofiltration membranes generally suffer from high mass transfer resistance in the selective layer and difficulty in simultaneously achieving good permeability and selectivity. Therefore, optimizing the selective layer thickness and reshaping the nanochannel structure through a solvation-induced structural expansion-functional component implantation strategy holds promise for achieving a synergistic improvement in membrane permeability and ion selectivity, providing a new technical approach for the design of high-efficiency lithium-magnesium separation nanofiltration membranes. Summary of the Invention

[0005] To address the issues of insufficient selectivity for lithium and magnesium ion separation in existing nanofiltration membranes and the mutual constraint between separation selectivity and permeability, this invention provides a nanofiltration membrane based on an anhydride-constructed gradient polyamide layer, along with its preparation method and application. This enhances the selectivity of the nanofiltration membrane for lithium and magnesium ion separation and improves membrane permeability, providing a new nanofiltration membrane construction strategy for efficient lithium resource extraction.

[0006] The technical solution of the present invention is as follows: One objective of this invention is to provide a method for preparing a nanofiltration membrane based on an anhydride-constructed gradient polyamide layer, the method comprising the following steps: Step 1: Pre-treat the ultrafiltration membrane substrate by immersing it in isopropanol solution, then wash it and store it at 4°C for later use. Step 2: Prepare an aqueous solution of polyethyleneimine and surfactant, prepare an oil solution of trimesoyl chloride (TMC) in n-hexane, and dissolve the acid anhydride monomer in an organic solvent to form a post-modification solution. Step 3: Using the pretreated ultrafiltration membrane substrate as the support layer, pour the aqueous solution onto the surface of the support layer, remove the excess aqueous solution after wetting, and then contact the support layer with the hexane oil phase solution of trimesoyl chloride to carry out the interfacial polymerization reaction to obtain the nascent polyamide nanofiltration membrane. Step 4: Immerse the nascent polyamide nanofiltration membrane in the post-modification solution to carry out the post-modification reaction, and obtain the primary modified nanofiltration membrane; Step 5: Heat and cure the primary modified nanofiltration membrane, and rinse with deionized water to obtain a nanofiltration membrane based on an anhydride-constructed gradient polyamide layer.

[0007] Further specifying, in step 1, the ultrafiltration membrane substrate is a polysulfone (PSF) ultrafiltration membrane, a polyethersulfone (PES) ultrafiltration membrane, or a polyacrylonitrile (PAN) ultrafiltration membrane, which serves as a support layer for the interfacial polymerization reaction and is used to support the polyamide separation layer.

[0008] Further specify that the mass fraction of the isopropanol solution in step 1 is 30-50%, and the soaking time is 30 minutes.

[0009] Further specified, in step 2, the molecular weight of polyethyleneimine is 30,000~70,000, and the concentration of polyethyleneimine in the aqueous solution can be 0.3~1wt%; the surfactant is sodium dodecylbenzenesulfonate or sodium dodecyl sulfate, and the surfactant concentration in the aqueous solution is 0.05~0.2wt%; the concentration of trimesoyl chloride in the n-hexane oil phase solution is 0.5~2g / L.

[0010] Further specifying, in step 2, the anhydride monomer is at least one of succinic anhydride, maleic anhydride, phthalic anhydride, and pyromellitic dianhydride, and the concentration of the anhydride monomer in the post-modification solution is 4~16 g / L; the organic solvent is at least one of ethanol, isopropanol, ethyl acetate, methyl formate, and dimethyl sulfoxide.

[0011] Further specifying, in step 3, the wetting time of the aqueous solution on the ultrafiltration membrane substrate surface is 5–20 min, and the application volume of both the aqueous and oil phase solutions is 0.2 mL / cm³. 2 The interfacial polymerization reaction time is 30s-2min.

[0012] Further specifying, in step 3, the contact method is to uniformly pour the n-hexane oil phase solution of trimesoyl chloride onto the surface of the support layer after it has been soaked in the aqueous phase solution.

[0013] Further specifying, the post-modification reaction time in step 4 is 1~30 min, the acid anhydride gradually diffuses into the interior of the polyamide layer, and undergoes a ring-opening grafting reaction with the residual amino group, so that the polyamide layer forms a gradient structure along the thickness direction. This gradient structure is: the surface layer is dense, with small pore size and rich in carboxyl groups, gradually transitioning to the interior, which is relatively loose, with larger pore size and retains more amino groups, forming a gradient nanostructure.

[0014] Further specifying, the heating and curing temperature in step 5 is 60~80℃, and the time is 10~30min, in order to promote the full progress of the post-modification reaction and stabilize the polyamide release layer structure.

[0015] Further specifying, in step 5, the nanofiltration membrane based on the gradient polyamide layer constructed from acid anhydride has a continuously varying chemical composition and degree of crosslinking along the membrane thickness direction, with the surface layer having a high carboxyl content and crosslinking density, and the inner layer retaining more amino functional groups.

[0016] The second objective of this invention is to provide a nanofiltration membrane based on an anhydride-constructed gradient polyamide layer obtained by the above preparation method.

[0017] The third objective of this invention is to provide an application of the above-mentioned nanofiltration membrane based on anhydride-constructed gradient polyamide layer in the selective separation of lithium and magnesium ions in salt lake brine, leachate from spent lithium batteries, or other mixed systems containing lithium and magnesium ions.

[0018] The beneficial effects of this invention are as follows: (1) The modified polyamide layer of this invention synergistically regulates the nanochannel structure and chemical environment, optimizing the transport and sieving behavior of ions within the channel to improve lithium-magnesium separation performance. Specifically, this invention uses an anhydride monomer to post-modify the nascent polyamide layer. It undergoes a secondary amidation reaction with the residual amine groups in the polyamide layer, converting some protonable amine groups into carboxyl groups and introducing a cross-linking structure. The diffusion and reaction of the anhydride monomer in the polyamide layer proceed simultaneously, causing the polyamide layer to form a continuously varying charge distribution and pore structure along the film thickness direction. This constructs a gradient nanostructure that gradually transitions from a dense, small-pore, and carboxyl-rich surface region to a relatively loose, larger-pore, and more amine-retaining interior, forming a polyamide layer with both charge and pore gradients. This gradient polyamide layer achieves lithium-magnesium separation performance through the synergistic regulation of steric hindrance and the Donnan effect. + / Mg 2+ Selective transport: Surface spatial confinement structure enhances ion sieving and Mg retention 2+ And promotes Li through negatively charged regions + Transport; as ions transport into the membrane interior, the gradually increasing positively charged environment further enhances the effect on Mg. 2+ The electrostatic repulsion effect expands the Li + With Mg 2+ The migration differences enhance the membrane's affinity for Li. + / Mg 2+ Selective separation performance.

[0019] (2) In the process of post-modification of acid anhydride monomer, the present invention reduces the mass transfer resistance of polyamide layer by solvation-induced thinning and structural reshaping in post-modification solution, alleviates the problem of mutual restriction between permeability and selectivity of traditional nanofiltration membrane, and achieves synergistic improvement of membrane permeability and lithium-magnesium separation performance.

[0020] (3) Compared with the method of directly adding modified monomers to the oil phase to participate in interfacial polymerization, the present invention can avoid competitive diffusion and reaction interference of multi-component monomers in the interfacial polymerization process, realize the controllable reaction between acid anhydride monomers and residual active sites of polyamide layer, thereby more accurately adjusting the chemical composition and microstructure in the film thickness direction, and has excellent lithium-magnesium separation performance. The magnesium chloride rejection rate can reach 97.08%, the lithium chloride rejection rate can reach 21.45%, the selectivity factor can reach 26.90, and the water permeability can reach 18.24 LMH / bar. The preparation process is simple, the reaction conditions are mild, and it is easy to scale up. It has good repeatability and engineering application potential, and has good application prospects in the field of lithium-ion selective separation. Attached Figure Description

[0021] Figure 1 These are scanning electron microscope (SEM) images of the nanofiltration membrane surfaces finally obtained in Comparative Example 1 and Example 2, (a) being Comparative Example 1 and (b) being Example 2; Figure 2 These are SEM images of the nanofiltration membrane cross sections finally obtained in Comparative Example 1 and Example 2, (a) being Comparative Example 1 and (b) being Example 2; Figure 3 This is a pore size probability density distribution diagram of the polyamide nanofiltration membranes finally obtained in Examples 1-3 and Comparative Example 1; Figure 4 This is a zeta potential diagram of the polyamide nanofiltration membranes finally obtained in Examples 1-3 and Comparative Example 1 at pH=7; Figure 5 The XPS C1s fine spectra of the nanofiltration membranes finally obtained in Comparative Example 1 and Example 2 are shown in (a) for Comparative Example 1 and (b) for Example 2. Figure 6 This is a bar chart showing the pure water flux of the polyamide nanofiltration membranes finally obtained in Examples 1-3 and Comparative Example 1; Figure 7 This is a bar chart showing the retention rates of magnesium chloride and lithium chloride for the polyamide nanofiltration membranes finally obtained in Examples 1-3 and Comparative Example 1. Figure 8 This is a graph showing the separation performance of Example 2 under mixed salt conditions with different magnesium-lithium mass ratios. Detailed Implementation

[0022] 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.

[0023] 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.

[0024] 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.

[0025] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0026] Example 1 Step 1: Select a polyethersulfone ultrafiltration membrane substrate with a molecular weight cutoff of 50 kDa, cut it into a rectangular membrane sheet of 7 cm × 10 cm, soak it in a 30% isopropanol solution for 30 min to pretreat it to remove residual additives on the membrane surface, then wash it thoroughly with ultrapure water and store it in a refrigerator at 4°C for later use. Step 2: Dissolve 1.67g of polyethyleneimine aqueous solution (30% by mass) with a molecular weight of 30000 in 100mL of deionized water and dilute. Add 0.1g of sodium dodecylbenzenesulfonate and sonicate for 1h to completely dissolve the solute, obtaining a homogeneous and stable aqueous solution. Dissolve 0.1g of pyromellitic trimethylol chloride in 100mL of n-hexane and stir to dissolve to prevent water vapor from seeping in, obtaining an oil phase solution. Dissolve 0.4g of pyromellitic dianhydride in 100mL of ethyl acetate and sonicate to completely dissolve, obtaining a post-modified solution. Store the prepared solutions in a dry place at room temperature for later use. Step 3: Using the polyethersulfone ultrafiltration membrane substrate treated in Step 1 as a support layer, fix it in a polytetrafluoroethylene mold, ensuring that the membrane surface is flat and free of obvious residual water marks; weigh 25 mL of the aqueous phase solution prepared in Step 2 and pour it evenly onto the surface of the support layer, fully soak it for 10 min, remove the excess aqueous phase solution, and gently roll the surface of the polyethersulfone ultrafiltration membrane with a rubber roller to remove the residual solution on the surface until there are no visible water marks. Then, pour 25 mL of the hexane oil phase solution of trimesoyl chloride evenly onto the surface of the support layer and react for 2 min to allow the two-phase interface to polymerize and form a membrane. After the reaction, use a pipette to draw pure hexane to rinse the membrane surface, wash away unreacted monomers, and air dry naturally until there are no obvious solvent residues on the membrane surface to obtain the nascent polyamide nanofiltration membrane. Step 4: Immerse the nascent polyamide nanofiltration membrane in the post-modification solution obtained in Step 2 and perform post-modification treatment for 10 min to allow the acid anhydride to gradually diffuse into the polyamide separation layer and undergo ring-opening amidation reaction with the residual amine groups in the polyamide layer. After the reaction is completed, rinse the membrane surface with the same organic solvent ethyl acetate to remove unreacted residual monomers and air dry until the membrane surface is dry to obtain the primary modified nanofiltration membrane. Step 5: Heat the anhydride-modified nanofiltration membrane at 60°C for 10 minutes to promote the complete completion of the incomplete post-modification reaction and stabilize the polyamide separation layer structure. Then rinse with deionized water and store at 4°C to obtain the anhydride-modified nanofiltration membrane, i.e., the nanofiltration membrane based on the anhydride-constructed gradient polyamide layer, denoted as PA-SG-0.4.

[0027] Example 2 The difference between this embodiment and embodiment 1 is that the amount of pyromellitic dianhydride in step 2 is 0.8g, while the rest of the process operations and parameter settings are the same as in embodiment 1. The resulting anhydride-modified nanofiltration membrane is denoted as PA-SG-0.8.

[0028] Example 3 The difference between this embodiment and embodiment 1 is that the amount of pyromellitic dianhydride in step 2 is 1.2g, while the remaining process operations and parameter settings are the same as in embodiment 1. The resulting anhydride-modified nanofiltration membrane is denoted as PA-SG-1.2.

[0029] Comparative Example 1 The difference between this comparative example and Example 1 is that: in step 2, the post-modification solution was not prepared, step 4 was not performed, and in step 5, the nascent polyamide nanofiltration membrane was heated and cured. The remaining process operations and parameter settings are the same as in Example 1. The obtained nanofiltration membrane is referred to as PA membrane.

[0030] Comparative Example 2 The difference between this comparative example and Example 1 is that the amount of pyromellitic dianhydride in step 2 is 0.2 g, while the remaining process operations and parameter settings are the same as in Example 1. The resulting anhydride-modified nanofiltration membrane is denoted as PA-SG-0.2.

[0031] Material characterization and performance testing were performed on the nanofiltration membranes finally obtained in Examples 1-3 and Comparative Examples 1-2: (a) The surface morphology of the nanofiltration membranes in Example 2 and Comparative Example 1 was observed using a scanning electron microscope. For example... Figure 1 As shown, the PA film surface of Comparative Example 1 is relatively smooth, which is related to the large size and strong chain entanglement of polyethyleneimine molecules, resulting in slow interfacial diffusion and a relatively gradual polyamide layer formation process. In contrast, the PA-SG-0.8 film of Example 2 exhibits a large number of fine nodular structures on its surface, indicating that the post-modification with anhydride has a significant impact on the microstructure of the polyamide layer. This is because the anhydride undergoes a secondary amidation reaction with the residual amine groups, generating more amide bonds and introducing carboxyl groups. The enhanced inter-segment interactions induce local structural rearrangement, ultimately forming a nodular surface morphology.

[0032] (II) A liquid nitrogen-embrittled nanofiltration membrane was used, and the thickness variation of the polyamide layer was analyzed by SEM observation of the membrane cross-section. The results are as follows: Figure 2 As shown, the polyamide layer thickness in Comparative Example 1 was approximately 70 nm, while the polyamide layer thickness in Example 2 was reduced to approximately 35.07 nm. This is because the solvation effect of the organic solvent during the post-modification process removes the less stable fragments and residual unreacted monomers in the polyamide layer. This also indicates that the modification process does not occur on the membrane surface, but rather involves the reshaping of the polyamide layer structure through expansion-diffusion along the membrane thickness direction. Thinning the membrane layer helps to reduce mass transfer resistance and improve the membrane's permeability.

[0033] (III) Using polyethylene glycol molecules of different molecular weights (200~1000 Da), the pore size probability density distribution of the nanofiltration membrane was fitted through a retention experiment. For example... Figure 3 As shown, compared to Comparative Example 1, the average membrane pore diameter of Examples 1-3 decreased from 0.480 nm to 0.427 nm, and the pore size distribution was more uniform, indicating that the selective layer pore structure can be controlled by the anhydride post-modification.

[0034] (iv) The Zeta potential of the polyamide nanofiltration membranes obtained in Examples 1-3 and Comparative Example 1 at pH=7 was determined using a solid surface charge analyzer. Figure 4 As shown, compared to Comparative Example 1, the Zeta potential on the polyamide membrane surface gradually decreased with the increase of anhydride concentration in Examples 1-3, indicating that the negative charge on the membrane surface continuously increased. This is because the anhydride monomer undergoes a secondary amidation reaction with the residual amine groups in the polyamide layer, which introduces carboxyl groups while reducing some protonable amine groups, thereby increasing the negative charge density on the membrane surface.

[0035] (v) The fine C 1s spectra of the nanofiltration membranes finally obtained in Comparative Example 1 and Example 2 were analyzed using X-ray photoelectron spectroscopy. For example... Figure 5 As shown, compared to Comparative Example 1, the peak area ratio corresponding to carboxyl groups (OC=O) in the nanofiltration membrane of Example 2 significantly increased, while the peak area ratio corresponding to CN bonds decreased. This is consistent with the amine group consumption and carboxyl group introduction in the reaction mechanism, and also consistent with... Figure 4 The results for the zeta potential were consistent, further confirming that the post-modification with anhydride modifies the surface chemical composition and charge properties of the polyamide layer.

[0036] (vi) The water permeability of the polyamide nanofiltration membranes finally obtained in Examples 1-3 and Comparative Examples 1-2 was determined using a cross-flow filtration device. For example... Figure 6 As shown, the pure water flux of the nanofiltration membranes in Examples 1-3 significantly increased from 4.527 LMH / bar in Comparative Example 1 to 20.75 LMH / bar, indicating that the polyamide layer thickness was significantly reduced during the anhydride post-modification process, thus lowering the resistance to water molecule transport. However, with increasing anhydride concentration, the permeability of the membranes in Examples 1-3 gradually decreased to 11.38 LMH / bar. This is due to the increased cross-linking degree of the polyamide layer, further reduction in pore size, and increased water transport resistance. Comparative Example 2, due to its lower anhydride concentration, achieved a higher flux of 24.43 LMH / bar.

[0037] (VII) Determine the separation performance of the polyamide nanofiltration membranes finally obtained in Examples 1-3 and Comparative Examples 1-2 for LiCl and MgCl2 single salt solutions with a concentration of 1000 mg / L. Figure 7 As shown, the nanofiltration membrane in Comparative Example 1 exhibited rejection rates of 56.67% for LiCl and 98.14% for MgCl2. Although it showed high rejection capacity for divalent ions, its lithium-magnesium separation selectivity was low. Comparative Example 2, despite introducing 0.2 wt% acid anhydride for modification, suffered from insufficient modification, failing to construct an ideal gradient polyamide layer. This resulted in a significant decrease in MgCl2 rejection rate while the LiCl rejection rate remained high, making effective lithium-magnesium separation difficult and indicating poor modification effect. In Examples 1-3, with increasing anhydride modification concentration, the LiCl rejection rate further decreased from 32.42% to 21.45% and 18.18%. This was mainly due to the introduction of carboxyl groups weakening the positive charge on the membrane surface, making LiCl more susceptible to oxidation. + The electrostatic repulsion weakens, and although the membrane pore size decreases, it is still within the Li... +The LiCl rejection rate generally decreased due to the easily permeable range. On the other hand, the MgCl2 rejection rate gradually decreased from 97.72% to 97.08% and 93.71%. At lower anhydride concentrations, the membrane maintained strong positive charge, with the Donnan effect dominating. Simultaneously, the reduced pore size further enhanced steric hindrance, resulting in a relatively stable MgCl2 rejection rate. However, as the anhydride concentration continued to increase, the positive charge on the membrane surface weakened further, and the Donnan effect significantly decreased. This effect outweighed the increased steric hindrance caused by pore size reduction, leading to a decrease in the MgCl2 rejection rate. In summary, Example 2 exhibited good permeability and lithium-magnesium selectivity, achieving a pure water flux of 18.24 LMH / bar, a magnesium chloride rejection rate of 97.08%, and a lithium chloride rejection rate of 21.45%, resulting in a calculated lithium-magnesium separation selectivity factor of 26.90.

[0038] (viii) Further testing was conducted on the mixed salt separation performance of the nanofiltration membrane from Example 2. The test conditions were a mixed solution of LiCl and MgCl2, with a total salt concentration of 2000 mg / L and a MgCl2 concentration of 1000 mg / L. 2+ With Li + The mass ratios were 10:1, 20:1, 50:1, and 100:1. The results are as follows: Figure 8 As shown, compared to the single-salt system, the nanofiltration membrane of Example 2 exhibits better performance in the mixed-salt system for Mg. 2+ Higher retention rate, more effective against Li + The rejection rate is even lower, and even negative rejection occurs. The separation factor can reach up to 82.17, which fully demonstrates the excellent selectivity of the membrane in magnesium-lithium separation.

[0039] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a nanofiltration membrane based on a gradient polyamide layer constructed from acid anhydrides, characterized in that, The method includes the following steps: Step 1: Pre-treat the ultrafiltration membrane substrate by immersing it in isopropanol solution, then wash it and store it at 4°C for later use. Step 2: Prepare an aqueous solution of polyethyleneimine and surfactant, prepare an oil solution of pyromellitic methyl methacrylate in hexane, and dissolve the acid anhydride monomer in an organic solvent to form a post-modification solution. Step 3: Using the pretreated ultrafiltration membrane substrate as the support layer, pour the aqueous solution onto the surface of the support layer, remove the excess aqueous solution after wetting, and then contact the support layer with the hexane oil phase solution of trimesoyl chloride to carry out the interfacial polymerization reaction to obtain the nascent polyamide nanofiltration membrane. Step 4: Immerse the nascent polyamide nanofiltration membrane in a post-modification solution to carry out a post-modification reaction, thereby obtaining a primary modified nanofiltration membrane; Step 5: Heat and cure the primary modified nanofiltration membrane, and rinse with deionized water to obtain a nanofiltration membrane based on an anhydride-constructed gradient polyamide layer.

2. The preparation method according to claim 1, characterized in that, In step 1, the ultrafiltration membrane substrate is a polysulfone ultrafiltration membrane, a polyethersulfone ultrafiltration membrane, or a polyacrylonitrile ultrafiltration membrane.

3. The preparation method according to claim 1, characterized in that, In step 1, the mass fraction of the isopropanol solution is 30-50%, and the pretreatment soaking time is 30 min.

4. The preparation method according to claim 1, characterized in that, In step 2, the molecular weight of polyethyleneimine is 30,000~70,000, and the concentration of polyethyleneimine in the aqueous solution is 0.3~1wt%; the surfactant is sodium dodecylbenzenesulfonate or sodium dodecyl sulfate, and the concentration of surfactant in the aqueous solution is 0.05~0.2wt%; the concentration of trimesoyl chloride in the n-hexane oil solution is 0.5~2g / L.

5. The preparation method according to claim 1, characterized in that, In step 2, the anhydride monomer is at least one of succinic anhydride, maleic anhydride, phthalic anhydride, and pyromellitic dianhydride, and the concentration of the anhydride monomer in the post-modification solution is 4~16 g / L.

6. The preparation method according to claim 1, characterized in that, In step 2, the organic solvent is at least one of ethanol, isopropanol, ethyl acetate, methyl formate, and dimethyl sulfoxide.

7. The preparation method according to claim 1, characterized in that, In step 3, the wetting time of the aqueous solution on the ultrafiltration membrane substrate surface is 5–20 min, and the application rate of both the aqueous and oil phase solutions is 0.2 mL / cm³. 2 The interfacial polymerization reaction time is 30s-2min.

8. The preparation method according to claim 1, characterized in that, In step 5, the heating and curing temperature is 60~80℃ and the time is 10~30min.

9. A nanofiltration membrane based on an anhydride-constructed gradient polyamide layer, prepared by the method according to any one of claims 1-8.

10. An application of the nanofiltration membrane based on an anhydride-constructed gradient polyamide layer as described in claim 9, characterized in that, This nanofiltration membrane is used for the selective separation of lithium and magnesium ions in salt lake brine, leachate from spent lithium batteries, and mixed systems containing lithium and magnesium ions other than salt lake brine and leachate from spent lithium batteries.

Images