Polyamide nanofiltration membrane, method for preparing same, and use thereof

Abstract

The application discloses a kind of polyamide nanofiltration membrane and its preparation method and application, belong to membrane separation technical field.The polyamide nanofiltration membrane is regulated interface polymerization by three-dimensional biomimetic support, realizes the accurate control of polyamide layer, breaks the trade-off limit of permeability-selectivity.The polyamide nanofiltration membrane is excellent in the wide Mg 2+ / Li + Ratio and salinity range lithium magnesium separation performance, Li + / Mg 2+ Selectivity is up to 54, Li + Permeability is significantly better than prior art.At the same time, the polyamide nanofiltration membrane is stable under the performance of 128 days continuous operation and pressure fluctuation, and resistant to acid and alkali chemical cleaning, suitable for salt lake brine environment.And, by three-stage nanofiltration series process can obtain the purity of >98% Li + Solution, and successfully prepare high-purity Li2CO3.In addition, the preparation method of the polyamide nanofiltration membrane is simple, good repeatability, suitable for actual large-scale production and application.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and in particular to a polyamide nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] Lithium (Li) is a core strategic resource supporting electric vehicles, consumer electronics, and grid-scale energy storage systems, playing an irreplaceable role in achieving the global goal of "carbon peaking and carbon neutrality." It is estimated that over 70% of the world's exploitable lithium reserves are located in salt lake brines; therefore, efficient extraction of lithium from these brines has become a crucial way to ensure the security of lithium resource supply. However, magnesium ions in salt lake brines have similar chemical properties to lithium ions, with very close hydration radii (Li... + Approximately 0.238 nm, Mg 2+ (approximately 0.428 nm), and the magnesium-to-lithium mass ratio (Mg) of most salt lake brines is... 2+ / Li + The ratio of lithium to lithium is as high as 20:1 to 100:1, making the efficient and precise separation of the two the most challenging technical problem in the lithium extraction process.

[0003] Nanofiltration membrane technology is widely considered an ideal platform for lithium-magnesium separation due to its advantages such as low energy consumption, no phase change, simple operation, and ease of scale-up. The separation mechanism of nanofiltration membranes is mainly based on the sieving effect (size repulsion) and the Donnan effect (charge repulsion). Theoretically, by reasonably controlling the pore size and surface charge characteristics of the membrane, selective separation of lithium ions and magnesium ions with different hydration radii can be achieved.

[0004] Currently, most commercially available nanofiltration membranes and polyamide nanofiltration membranes reported in the literature are prepared using interfacial polymerization, where a polymerization reaction occurs at the interface between an aqueous monomer (usually piperazine) and an organic monomer (usually trimesoyl chloride) to form an ultrathin polyamide selective layer. However, this type of nanofiltration membrane has the following technical drawbacks in lithium-magnesium separation applications:

[0005] First, there is a typical trade-off between permeability and selectivity. To improve lithium-ion permeability, it is usually necessary to reduce the thickness of the selective layer or increase the pore size, but this often leads to a decrease in magnesium ion rejection and a reduction in lithium / magnesium selectivity. Conversely, reducing the pore size or increasing the degree of cross-linking to improve selectivity will significantly reduce water flux and lithium-ion permeability. Existing nanofiltration membranes cannot simultaneously achieve high lithium permeability and high lithium / magnesium selectivity, resulting in a failure to simultaneously improve lithium recovery rate and product purity.

[0006] Second, the pore size distribution of the polyamide selective layer formed by conventional interfacial polymerization is relatively wide, resulting in insufficient sieving accuracy. The average pore size of traditional polyamide nanofiltration membranes is typically 6.5 Å to 7.5 Å, with a large geometric standard deviation. Since the difference between the hydration radius of lithium ions (approximately 2.38 Å) and the hydration radius of magnesium ions (approximately 4.28 Å) is less than 2 Å, it is difficult to achieve accurate separation of the two by relying solely on the size sieving of traditional nanofiltration membranes. This is especially true when treating brines with a high magnesium-to-lithium ratio, where even a small amount of magnesium ions remaining in the permeate can significantly reduce the purity of the lithium product.

[0007] Third, the membrane exhibits insufficient stability in high-salt, high-magnesium brine environments. Salt lake brines typically possess high ionic strength and high osmotic pressure. Under prolonged operating conditions, the polyamide selective layer is prone to swelling, densification, or peeling from the support layer, leading to membrane performance degradation. Furthermore, organic matter and colloidal particles in actual brine easily form a fouling layer on the membrane surface, and existing nanofiltration membranes have limited resistance to chemical cleaning; frequent cleaning accelerates membrane structural damage. Currently, most literature reports long-term operating test times for nanofiltration membranes typically not exceeding 200 hours, which is insufficient to meet the industrial extraction requirements for membrane lifetime (thousands of hours).

[0008] To overcome these shortcomings, researchers have explored various improvement strategies, such as introducing nanomaterials (e.g., carbon nanotubes, graphene, metal-organic frameworks) to prepare hybrid matrix membranes; constructing an intermediate layer between the support layer and the selective layer to regulate the interfacial polymerization process; and altering the surface charge properties of the membrane through surface grafting or coating. However, existing improvement schemes still suffer from the following deficiencies: nanomaterials are prone to aggregation and difficult to disperse uniformly; the intermediate layer often increases mass transfer resistance and reduces water permeability; surface modification processes are complex and have poor controllability, and the modified layer is prone to detachment during long-term operation; most methods can only optimize one aspect of performance (e.g., improving selectivity or permeability), making it difficult to achieve a synergistic improvement in permeability, selectivity, and stability.

[0009] Therefore, developing a novel nanofiltration membrane that can simultaneously achieve high permeability, high lithium / magnesium selectivity, and long-term operational stability is of great significance for the efficient and sustainable extraction of lithium resources from salt lake brine. Summary of the Invention

[0010] In view of this, the technical problem to be solved by the present invention is to provide a polyamide nanofiltration membrane, its preparation method, and its application. The polyamide nanofiltration membrane has high Li... + Permeability, high Li + / Mg 2+ Selectivity and excellent long-term operational stability.

[0011] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0012] This invention provides a polyamide nanofiltration membrane, which comprises, from bottom to top, a porous support layer, a three-dimensional biomimetic scaffold layer, and a polyamide layer;

[0013] The three-dimensional biomimetic scaffold layer is obtained by non-covalent bonding of a composite material to the surface of the porous support layer;

[0014] The composite consists of one-dimensional nanomaterials and a biomimetic adhesion coating covering its surface;

[0015] The polyamide layer is prepared by interfacial polymerization of piperazine aqueous solution and pyromellitic trimethylol chloride on the surface of a three-dimensional biomimetic scaffold layer.

[0016] The porous support layer is one or more of polyethersulfone microfiltration membrane, polyethersulfone ultrafiltration membrane, or polyamide 66 microfiltration membrane.

[0017] The three-dimensional biomimetic scaffold layer described in this invention is a biomimetic scaffold inspired by the structure of a spider web in nature. In this scaffold, one-dimensional nanomaterials in the composite form a biomimetic pull-string network, and a biomimetic adhesive coating forms biomimetic viscous droplets. The biomimetic viscous droplets coat the surface of the biomimetic pull-string network, thereby forming a biomimetic spider web structure.

[0018] The three-dimensional biomimetic scaffold layer in the polyamide nanofiltration membrane of this invention exhibits rapid adsorption capacity and high adsorption capacity for piperazine monomers in piperazine aqueous solution. Measurements using a quartz crystal microbalance showed that the adsorption rate of piperazine by the three-dimensional biomimetic scaffold layer reached 4.22 ng / cm³. 2 / s, which is 3.2 times that of bare polyethersulfone substrate, with an equilibrium adsorption capacity of approximately 2860 ng / cm³. 2 Furthermore, the adsorbed piperazine monomer is not easily eluted by deionized water, indicating a strong anchoring effect between the three-dimensional biomimetic scaffold and piperazine through hydrogen bonding and π-π interactions.

[0019] Furthermore, molecular dynamics simulations show that on biomimetic adhesive coatings such as polydopamine surfaces, the mean square displacement slope of piperazine molecules decreases from 0.324 to 0.120, and the diffusion coefficient is significantly reduced. This indicates that the three-dimensional biomimetic scaffold layer restricts the migration rate of piperazine monomers.

[0020] Furthermore, the three-dimensional biomimetic scaffold provides a three-dimensional template for the interfacial polymerization reaction, resulting in a denser polyamide layer with higher cross-linking degree and narrower pore size (approximately 5.88 Å), thereby significantly improving the Mg... 2+ The dehydration energy barrier (activation energy increased from 5.2 to 9.35 kcal / mol), while for Li + The impact was relatively small, causing the ion transport mechanism to shift from an "entropy-controlled mode" to an "enthalpy-controlled mode," significantly expanding the Li... + With Mg 2+ The difference in activation energy.

[0021] In the polyamide nanofiltration membrane of the present invention, the non-covalent bonds include, but are not limited to, hydrogen bonds, ion interactions, and van der Waals forces.

[0022] Preferably, the thickness of the three-dimensional biomimetic scaffold layer is 10~50nm;

[0023] The three-dimensional biomimetic scaffold layer has an open porous structure with a porosity greater than 70%.

[0024] Preferably, the one-dimensional nanomaterial of the present invention is selected from carbon nanotubes, cellulose nanocrystals or cellulose nanofibers;

[0025] Preferably, the biomimetic adhesive coating is selected from polydopamine, polyphenol-polyethyleneimine oxidative co-crosslinking polymer, or polyphenol-metal ion coordination complex;

[0026] More preferably, the polyphenols in the polyphenol-polyethyleneimine oxidative cocrosslinking polymer or the polyphenol-metal ion coordination complex are independently selected from one or more of catechol, tannic acid, gallic acid, and tea polyphenols; wherein, the types of polyphenols in the polyphenol-polyethyleneimine oxidative cocrosslinking polymer or the polyphenol-metal ion coordination complex may be the same or different.

[0027] More preferably, the metal in the polyphenol-metal ion coordination complex is selected from one or more of iron, copper, zinc, and nickel.

[0028] In a further preferred embodiment of the present invention, the composite consists of carbon nanotubes and polydopamine coated on their surface. The polydopamine forms a polydopamine layer on the surface of the carbon nanotubes.

[0029] More preferably, the carbon nanotubes are selected from multi-walled carbon nanotubes.

[0030] More preferably, the thickness of the polydopamine layer is 3-5 nm.

[0031] Preferably, in this invention, the loading amount of the composite on the surface of the porous support layer is 2~20 μg / cm². 2 More preferably 5~10 μg / cm 2 .

[0032] Preferably, the thickness of the polyamide layer in this invention is 20~60 nm;

[0033] Preferably, the average pore size of the polyamide layer is 5.5~6.2 Å;

[0034] The geometric standard deviation of the average aperture is less than 1.2.

[0035] Preferably, the degree of cross-linking of the network structure of the polyamide layer is 50% to 70%.

[0036] The molecular weight cutoff of the polyamide layer is 150-200 Da;

[0037] The surface water contact angle of the polyamide layer is 40°~60°;

[0038] The polyamide layer has a zeta potential of -10 to -20 mV at a pH value of 7 to 10.

[0039] More preferably, the concentration of the piperazine aqueous solution is 0.02 wt% to 0.1 wt%; more preferably, it is 0.05 wt%.

[0040] More preferably, the concentration of the organic solution of pyromellitic methyl chloride is 0.05 wt% to 0.2 wt%; more preferably 0.1 wt%.

[0041] More preferably, the organic solution of the pyromellitic trimethylolpropionate chloride is selected from n-hexane, cyclohexane, or Isopar G (i.e., isoparaffin solvent oil G).

[0042] In the above-mentioned polyamide nanofiltration membrane, the average pore size of the polyethersulfone microfiltration membrane is preferably 0.2~0.3 μm;

[0043] The average pore size of the polyamide 66 microfiltration membrane is preferably 0.2~0.3 μm;

[0044] The preferred molecular weight cutoff of the polyethersulfone ultrafiltration membrane is 50 to 150 kDa.

[0045] The thickness of the porous support layer is preferably 80μm~200μm;

[0046] The porosity of the porous support layer is preferably 60% to 85%. This invention also provides a method for preparing the above-mentioned polyamide nanofiltration membrane, comprising the following steps:

[0047] (1) A composite dispersion is coated or vacuum filtered and deposited on the surface of the porous support layer, and then heat-treated to obtain membrane S;

[0048] (2) The piperazine aqueous solution and the organic solution of pyromellitic methyl chloride are mixed and subjected to interfacial polymerization on the surface of the membrane S, and then cured to obtain the polyamide nanofiltration membrane.

[0049] The preparation method of the present invention prepares a high-performance polyamide nanofiltration membrane by controlling the adsorption, diffusion and reaction kinetics of monomers during the interfacial polymerization process through the three-dimensional biomimetic scaffold layer.

[0050] The structure of the membrane S in step (1) is a porous support layer and the three-dimensional biomimetic scaffold layer disposed thereon;

[0051] The interfacial polymerization reaction in step (2) is carried out on the surface of the membrane S with the above structure.

[0052] Preferably, the solvent of the composite dispersion is water;

[0053] Preferably, the concentration of the complex dispersion is 0.01 ~ 0.5 mg / mL.

[0054] Preferably, the coating method is selected from spraying, dripping, or dipping.

[0055] The above preparation method uses heat treatment to form a three-dimensional porous network structure in the three-dimensional biomimetic scaffold layer, and the interface between the scaffold layer and the porous support layer is stably bonded.

[0056] Preferably, the heat treatment temperature is 50℃~80℃.

[0057] The preferred heat treatment time is 5 to 20 minutes.

[0058] The interfacial polymerization reaction is carried out at room temperature, preferably between 20°C and 35°C.

[0059] The time for the interfacial polymerization reaction is 0.5 to 3 minutes.

[0060] Preferably, the curing temperature is 50℃~80℃.

[0061] In the above preparation method, step (2) specifically includes the following steps:

[0062] 1) After immersing membrane S in an aqueous solution containing piperazine, remove it and remove excess water to obtain membrane S1;

[0063] 2) Immerse membrane S1 in an organic solution of pyromellitic methyl chloride and carry out an interfacial polymerization reaction on the surface of membrane S1 to obtain the polyamide nanofiltration membrane.

[0064] The membrane S1 is obtained by adsorbing piperazine aqueous solution onto membrane S;

[0065] The piperazine aqueous solution and the organic solution of pyromellitic trimethylol chloride undergo an interfacial polymerization reaction on the surface of membrane S1 to generate a polyamide layer, which together with membrane S1 constitutes the polyamide nanofiltration membrane of the present invention.

[0066] After the above-mentioned interfacial polymerization reaction is completed, an organic solvent washing post-treatment is also included, the purpose of which is to terminate the reaction and remove unreacted trimesoyl chloride monomer.

[0067] After the above-mentioned organic solvent rinsing and treatment is completed, the polyamide nanofiltration membrane can be soaked to remove residual solvent or unreacted monomers, so as to further improve the hydrophilicity and storage stability of the membrane.

[0068] The solvent used for soaking is selected from deionized water or a 10wt%~30wt% aqueous solution of glycerol.

[0069] In some specific embodiments of the present invention, the preparation method of the complex in the preparation method includes the following steps:

[0070] Multi-walled carbon nanotubes, a dispersant, dopamine hydrochloride, and Tris-HCl buffer were mixed and reacted to obtain the complex, which is the PDA-CNT complex.

[0071] The preferred mass ratio of the multi-walled carbon nanotubes to the dispersant is 1 (5~20).

[0072] The multi-walled carbon nanotubes have an outer diameter of 8-15 nm and a length of 0.5-2 μm.

[0073] The dispersant is selected from sodium dodecylbenzenesulfonate or sodium dodecyl sulfate (SDS).

[0074] The preferred mass ratio of dopamine hydrochloride to multi-walled carbon nanotubes is (0.5~2):1; more preferably, it is 1:1.

[0075] The concentration of the Tris-HCl buffer solution is preferably 0.05 ~ 0.2 M.

[0076] The reaction temperature is 30℃~50℃;

[0077] The reaction time is 24-48 hours;

[0078] The pH value of the reaction is 7.5~8.5.

[0079] The reaction mainly involves the oxidative self-polymerization of dopamine hydrochloride on the surface of carbon nanotubes to form a polydopamine coating layer.

[0080] The reaction process includes post-treatment steps such as centrifugation and washing.

[0081] The preferred centrifugal speed is 7500-8500 rpm;

[0082] The solvent used for washing is water.

[0083] The present invention also provides the application of the above-described polyamide nanofiltration membrane or the polyamide nanofiltration membrane prepared by the above preparation method in lithium-magnesium separation.

[0084] The lithium-magnesium separation includes, but is not limited to, the extraction of lithium from salt lake brine or brine with a high magnesium-to-lithium ratio.

[0085] In some specific embodiments of the present invention, the polyamide nanofiltration membrane exhibits excellent separation and purification effects, good performance stability, and good acid and alkali resistance in lithium extraction from salt lake brine.

[0086] Compared with existing technologies, the polyamide nanofiltration membrane provided by this invention comprises, from bottom to top, a porous support layer, a three-dimensional biomimetic scaffold layer, and a polyamide layer. The three-dimensional biomimetic scaffold layer is obtained by non-covalent bonding of a composite material to the surface of the porous support layer. The composite material consists of one-dimensional nanomaterials and a biomimetic adhesive coating on its surface. The polyamide layer is prepared by interfacial polymerization of a piperazine aqueous solution and a trimesoyl chloride organic solution on the surface of the three-dimensional biomimetic scaffold layer. The porous support layer is one or more of polyethersulfone microfiltration membranes, polyethersulfone ultrafiltration membranes, or polyamide 66 microfiltration membranes. The polyamide nanofiltration membrane achieves precise control of the polyamide layer structure through interfacial polymerization regulated by the three-dimensional biomimetic scaffold, overcoming the trade-off limitations of permeability and selectivity. The polyamide nanofiltration membrane has a wide Mg... 2+ / Li + It exhibits excellent lithium-magnesium separation performance within the range of specific gravity (10~100) and salinity (1~6 g / L). + / Mg 2+ Selectivity can reach up to 54, Li + The permeability is significantly superior to existing technologies. Furthermore, the polyamide nanofiltration membrane maintains stable performance under 128 days of continuous operation and pressure fluctuations, and is resistant to acid and alkali chemical cleaning, making it suitable for practical salt lake brine environments. A three-stage nanofiltration process can yield Li with a purity >98%. + The solution was successfully prepared and high-purity Li2CO3 was successfully obtained, verifying its industrial feasibility. Furthermore, the preparation method of the polyamide nanofiltration membrane is simple, reproducible, and suitable for large-scale practical applications. Attached Figure Description

[0087] Figure 1 The images shown are TEM images of the PDA-CNT complex obtained in Example 1, where a is a low-magnification TEM image, b is a high-resolution TEM image, c is a high-magnification TEM morphology image of the elemental plane scanning region, and d is an elemental plane scanning (EDS-Mapping) image of the selected region in image c.

[0088] Figure 2 AFM images comparing the surface morphology of the three-dimensional biomimetic scaffold substrate obtained in Example 1 (right image) and the pure PES (pure polyethersulfone) substrate (left image);

[0089] Figure 3 This is a QCM-D adsorption curve of piperazine monomer on the three-dimensional biomimetic scaffold substrate and the pure PES substrate in Example 1;

[0090] Figure 4 The Zeta potential (Figure a), MWCO (molecular weight cutoff) diagram (Figure b), and pore size distribution diagram (Figure c) of the MC0 and MC5 membranes obtained in Example 1 and Comparative Example 1, respectively.

[0091] Figure 5 The graph shows a comparison of the magnesium-lithium separation performance of the MC0 and MC5 membranes obtained in Example 1 and Comparative Example 1, respectively.

[0092] Figure 6 This is a graph showing the long-term performance changes of the biomimetic spider web structure high-selectivity polyamide nanofiltration membrane obtained in Example 1.

[0093] Figure 7 This is a comparison of lithium recovery rate and lithium purity in a multi-stage lithium extraction process between the biomimetic spider web structure high-selectivity polyamide nanofiltration membrane obtained in Example 1 and other commercial nanofiltration membranes and nanofiltration membranes reported in the literature. Detailed Implementation

[0094] To further illustrate the present invention, the following detailed description of the polyamide nanofiltration membrane, its preparation method, and its application, in conjunction with embodiments, is provided.

[0095] I. Preparation of Nanofiltration Membranes - Example 1

[0096] Preparation of the PDA-CNT complex: 100 mg of multi-walled carbon nanotubes (MWCNTs, outer diameter 8–15 nm, length 0.5–2 μm) and 1000 mg of sodium dodecylbenzenesulfonate were dispersed in 1 L of deionized water. The mixture was sonicated for 4 hours and centrifuged at 10,000 rpm for 1 hour to remove large aggregates. 100 mg of dopamine hydrochloride was added to the supernatant, and the mixture was stirred for 1 hour. Then, 100 mL of Tris-HCl buffer (0.1 M, pH 7.5) was added, and the mixture was reacted at 40 °C for 36 hours. After the reaction was completed, the mixture was centrifuged at 8,000 rpm for 30 minutes to obtain the PDA-CNT complex.

[0097] Construction of the biomimetic scaffold: The PDA-CNT composite was redispersed in water, and after adjusting the concentration, it was vacuum filtered onto the surface of a polyethersulfone (PES) microfiltration membrane (pore size 0.22 μm) with a loading rate controlled at 5 μg / cm³. 2 Then heat-treat at 60℃ for 10 minutes.

[0098] Interfacial polymerization: The PES membrane with a biomimetic scaffold was immersed in a 0.05 wt% piperazine aqueous solution for 3 minutes. Excess solution on the surface was removed with a rubber roller, and then it was contacted with a 0.1 wt% hexane solution of trimesoyl chloride for 1 minute to carry out interfacial polymerization. After the reaction, it was rinsed with hexane and cured at 60°C for 10 minutes to obtain the finished nanofiltration membrane (denoted as MC5).

[0099] Performance Testing: Membrane separation performance was tested using a cross-flow filtration system. Test conditions: temperature 25±0.5℃, cross-flow velocity 21.3 cm / s, operating pressure 5 bar, feed solution was a mixed solution of MgCl2 and LiCl, total salinity was 2 g / L, and Mg... 2+ / Li + The mass ratio was 40:1; permeate was collected per unit time, and the membrane water permeability was measured; the Mg content in the feed solution and permeate was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). 2+ With Li + Concentration, calculate the effect of the membrane on Mg 2+ With Li + Retention rate and Li + / Mg 2+ Selectivity.

[0100] Examples 2-4

[0101] The method used was essentially the same as in Example 1, except that the PDA-CNT loading amounts were 2.5, 10, and 20 μg / cm³, respectively. 2 These are denoted as MC2.5, MC10, and MC20, respectively. Comparative Example 1

[0102] The results were basically the same as in Example 1, except that no biomimetic scaffold was constructed, and interfacial polymerization was performed directly on the PES microfiltration membrane to obtain the control membrane MC0.

[0103] Examples 5-7

[0104] Similar to Example 1, except that during the performance test, the total salinity of the mixed MgCl2 and LiCl solutions was 1 g / L, 4 g / L, and 6 g / L, respectively.

[0105] Examples 8-11

[0106] Similar to Example 1, except that during performance testing, the Mg in the mixed solution of MgCl2 and LiCl was... 2+ / Li + The mass ratios are 10, 20, 60, and 100.

[0107] Example 12

[0108] Similar to Example 1, except that during performance testing, a biomimetic spider web structure high-selectivity polyamide nanofiltration membrane that had been running continuously for a long period of 128 days was selected for the selective separation of magnesium and lithium.

[0109] Example 13

[0110] Similar to Example 1, the difference is that during performance testing, lithium recovery rate and lithium purity were measured under conditions of with and without cycling. In the non-cycling mode, the results were obtained through experimental testing, which was performed one stage after another. In the cycling mode, the lithium recovery rate and lithium purity were calculated by numerical simulation after the system reached a steady state.

[0111] II. Membrane Structure and Characterization

[0112] (1) High-resolution transmission electron microscopy analysis: The PDA-CNT complex in Example 1 above was tested using a Talos F200X high-resolution transmission electron microscope at the Physical and Chemical Experiment Center of the University of Science and Technology of China. Figure 1 As shown, the results indicate that a distinct core-shell structure is formed on the surface of multi-walled carbon nanotubes, with a thin and continuous amorphous polydopamine coating (approximately 3–5 nm) uniformly coating the periphery of the highly crystalline multi-walled carbon nanotube core. The nitrogen signal in the elemental distribution diagram further confirms that the amine-rich polydopamine layer has been successfully anchored or functionalized on the carbon nanotube surface.

[0113] (2) Atomic force microscopy analysis: The three-dimensional biomimetic scaffold substrate constructed in Example 1 above (obtained by loading a PDA-CNT composite onto the surface of a polyethersulfone (PES) microfiltration membrane) and the pure PES substrate were characterized using a Bruker Dimension Icon scanning probe microscope at the Physical and Chemical Experiment Center of the University of Science and Technology of China. Figure 2 As shown, the results indicate that the pure PES substrate surface exhibits a typical macroporous structure of PES microfiltration membranes, with dramatic surface undulations and obvious dark pores. In contrast, the constructed three-dimensional biomimetic scaffold substrate surface displays an extremely dense, porous, and interwoven three-dimensional nanocomposite network. This highly interconnected scaffold structure provides a uniform and confined reaction space for the subsequent interfacial polymerization process, serving as the physical basis for achieving precise ion sieving.

[0114] (3) Analysis using a dissipative quartz crystal microbalance: The adsorption and diffusion of piperazine monomers on the three-dimensional biomimetic scaffold substrate constructed in Example 1 and the pure PES substrate were determined using a dissipative quartz crystal microbalance (QCM-D). Figure 3 As shown, the results indicate that the adsorption capacity of piperazine monomers on the PES substrate coated with PDA-CNTs is increased by orders of magnitude compared with the bare PES substrate, confirming that the three-dimensional biomimetic scaffold can effectively capture monomers in solution and pre-enrich them at the interface, laying the foundation for subsequent controlled polymerization.

[0115] (4) Surface charge property analysis: The surface charge properties of the MC5 film (bionic film) and MC0 film (control film) obtained in Example 1 and Comparative Example 1, respectively, were analyzed. The specific characterization experiments were conducted using an Anton Paar SurPASS 3 solid surface zeta potential analyzer at the Chemical Experiment Teaching Center of the University of Science and Technology of China. The results showed that both films were negatively charged (e.g., ...). Figure 4 (As shown in Figure a), but the MC5 membrane exhibits slightly weaker electronegativity across the entire pH range than the MC0 membrane. This may be due to the higher degree of crosslinking reducing residual unreacted carboxyl groups, and the introduction of some amino groups by polydopamine, which has a neutralizing effect.

[0116] (5) The retention of neutral substances of different molecular weights by the MC5 membrane (bionic membrane) and MC0 membrane (control membrane) obtained in Example 1 and Comparative Example 1, respectively, was determined using a cross-flow filtration device: The results showed that the retention curve of the MC5 membrane for neutral small organic molecules shifted significantly to the left, indicating that it has a smaller molecular weight cutoff (181 Da, e.g., 181 Da). Figure 4 As shown in Figure b). Through calculation and fitting, the average effective pore size decreased from 6.44 Å for the MC0 membrane to 5.88 Å for the MC5 membrane (as shown in Figure b). Figure 4 (as shown in Figure c).

[0117] III. Magnesium-Lithium Separation Performance Test

[0118] The biomimetic spider web structure high-selectivity polyamide nanofiltration membranes prepared in Examples 1-11 were applied to the precise separation of magnesium and lithium.

[0119] Membrane performance was tested using a cross-flow filtration system. Test conditions: temperature 25±0.5℃, cross-flow velocity 21.3 cm / s, operating pressure 5 bar, feed solution was a mixed solution of MgCl2 and LiCl, total salinity 1~6 g / L, Mg... 2+ / Li + The mass ratio is 10~100.

[0120] The results showed that the water permeability of the MC5 membrane was 17.4 ± 0.3 L / m³. 2 / h / bar, Li + / Mg 2+ The selectivity ranged from 39.4 to 54.0, with a Mg²⁺ rejection rate >95% and a negative Li⁺ rejection rate (-112% to -15%), indicating that the concentration on the product water side was higher than that on the feed water side. This is due to the presence of Li⁺. + Different mass transfer mechanisms lead to this, which indicates that Li + Through side enrichment (e.g.) Figure 5 Furthermore, during 128 days of continuous operation, Mg 2+The retention rate remained stable at 96.7±1.1%, and the water permeability remained stable at 11.1±0.8 L / m³. 2 / h / bar. After 7 pressure cycles (2-6 bar), performance showed no degradation. After sequential washing with 0.025 wt% SDS (containing 0.1 wt% NaOH), 0.2 wt% HCl, and 0.5 wt% H3PO4, water permeability was restored (e.g., Figure 6 ).

[0121] IV. Multi-stage lithium extraction efficiency

[0122] The lithium recovery rate and lithium purity of the biomimetic spider web structure high-selectivity polyamide nanofiltration membrane prepared in Example 13 were determined under conditions with and without circulation.

[0123] Test conditions in non-circulation mode: initial feed solution 28 L, Mg 2+ / Li + With a mass ratio of 40:1 and a total salinity of 6 g / L, the water recovery rate per stage is approximately 60%. The first-stage permeate is used as the feed for the second stage, and so on. The third-stage permeate contains Li... + With a purity of 98.1%, the permeate was concentrated by rotary evaporation, and Na₂CO₃ was added to obtain solid Li₂CO₃. Its XRD pattern was consistent with the standard card PDF#80-1307, showing no MgCO₃ impurity peaks, indicating that the Mg impurity was ultimately removed. 2+ and Li + Highly efficient separation.

[0124] Furthermore, when using a recirculation mode (the concentrate from the later stage is returned to the feed from the previous stage), the numerical simulation results show that Li + Purity > 99%, total recovery rate > 96% (e.g.) Figure 7(ideal region), indicating that the polyamide nanofiltration membrane with biomimetic spider web structure described in this invention has excellent lithium-magnesium separation performance. In the figure, the circulating and non-circulating membranes used are those corresponding to Example 13. The commercial membrane (XN45) was purchased from Mann+Hummel GmbH, Germany (TRISEP-XN45 type nanofiltration membrane), the commercial membrane (DK) was purchased from Veolia Water Technologies, France (Desal-DK type nanofiltration membrane), and the commercial membrane (NF270) was purchased from DuPont, USA (FilmTec-NF270 type nanofiltration membrane). The literature-reported membranes (PIP-PEI) and (TFC-PEI) are both from the literature Li, Y., Wang, M., Xiang, X., Zhao, YJ & Peng, ZJ Separation performance and fouling analyses of nanofiltration membrane for lithium extraction from salt lake brine. Journal of Water Process Engineering 54, 104009 (2023).

[0125] In summary, this invention successfully prepared a biomimetic spiderweb-structured polyamide nanofiltration membrane with a dense sub-nanoscale sieve network by constructing a three-dimensional biomimetic scaffold composed of carbon nanotubes and polydopamine on the surface of a porous support layer, and by utilizing this scaffold to regulate the interfacial polymerization reaction between piperazine and trimesoyl chloride. The average pore size of this nanofiltration membrane can be precisely controlled within the range of 5.5 Å to 6.2 Å, with a crosslinking degree of 50% to 70%, while maintaining a high water permeability (≥17 L / m³). 2 While achieving a high lithium / magnesium selectivity (approximately 54 / h / bar), this method also achieves excellent lithium / magnesium selectivity. Validated on various substrates, this preparation method demonstrates broad substrate adaptability. After pressure cycling, long-term continuous operation, and chemical cleaning tests, the membrane exhibits excellent mechanical stability and chemical durability (no significant performance degradation after >3000h of continuous operation). Using a three-stage nanofiltration process combined with cyclic operation, a lithium solution with a purity >99% can be obtained from high magnesium-to-lithium ratio brine, and further high-purity lithium carbonate products can be prepared. This invention overcomes the technical bottleneck of the mutual constraint between permeability and selectivity in traditional nanofiltration membranes, providing an efficient, stable, and scalable technical solution for lithium extraction from unconventional water resources such as salt lake brines, and has significant industrial application value in the fields of membrane separation and resource recovery.

[0126] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A polyamide nanofiltration membrane, characterized in that, From bottom to top, it includes a porous support layer, a three-dimensional biomimetic scaffold layer, and a polyamide layer; The three-dimensional biomimetic scaffold layer is obtained by non-covalent bonding of a composite material to the surface of the porous support layer; The composite consists of one-dimensional nanomaterials and a biomimetic adhesion coating covering its surface; The polyamide layer is prepared by interfacial polymerization of piperazine aqueous solution and pyromellitic trimethylol chloride on the surface of a three-dimensional biomimetic scaffold layer. The porous support layer is one or more of polyethersulfone microfiltration membrane, polyethersulfone ultrafiltration membrane, or polyamide 66 microfiltration membrane.

2. The polyamide nanofiltration membrane according to claim 1, characterized in that, The thickness of the three-dimensional biomimetic scaffold layer is 10~50nm; The three-dimensional biomimetic scaffold layer has an open porous structure with a porosity greater than 70%.

3. The polyamide nanofiltration membrane according to claim 1 or 2, characterized in that, The one-dimensional nanomaterial is selected from carbon nanotubes, cellulose nanocrystals, or cellulose nanofibers. The biomimetic adhesion coating is selected from polydopamine, polyphenol-polyethyleneimine oxidative co-crosslinking polymer, or polyphenol-metal ion coordination complex; The polyphenols in the polyphenol-polyethyleneimine oxidative co-crosslinking polymer or the polyphenol-metal ion coordination complex are independently selected from one or more of catechol, tannic acid, gallic acid, and tea polyphenols; The metal in the polyphenol-metal ion coordination complex is selected from one or more of iron, copper, zinc, and nickel.

4. The polyamide nanofiltration membrane according to claim 3, characterized in that, The composite consists of carbon nanotubes and polydopamine coated on their surface.

5. The polyamide nanofiltration membrane according to claim 1, characterized in that, The composite is loaded at a rate of 2~20 μg / cm² on the surface of the porous support layer. 2 .

6. The polyamide nanofiltration membrane according to claim 1, characterized in that, The thickness of the polyamide layer is 20~60nm; The polyamide layer has an average pore size of 5.5~6.2 Å; The degree of cross-linking of the network structure of the polyamide layer is 50%~70%.

7. The method for preparing the polyamide nanofiltration membrane according to any one of claims 1-6, characterized in that, Includes the following steps: (1) A composite dispersion is coated or vacuum filtered and deposited on the surface of the porous support layer, and then heat-treated to obtain membrane S; (2) The piperazine aqueous solution and the organic solution of pyromellitic methyl chloride are mixed and subjected to interfacial polymerization on the surface of the membrane S, and then cured to obtain the polyamide nanofiltration membrane.

8. The preparation method according to claim 7, characterized in that, The solvent for the composite dispersion is water; The concentration of the complex dispersion is 0.01 ~ 0.5 mg / mL.

9. The preparation method according to claim 7, characterized in that, The coating method is selected from spraying, dripping, or dipping. The heat treatment temperature is 50℃~80℃; The curing temperature is 50℃~80℃.

10. The application of the polyamide nanofiltration membrane according to any one of claims 1-6 or the polyamide nanofiltration membrane prepared by the preparation method according to any one of claims 7-9 in lithium-magnesium separation.

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