High-salt-tolerance nanofiltration membrane with accurate charge distribution as well as preparation method and application of high-salt-tolerance nanofiltration membrane

Abstract

The invention discloses a high-salt-tolerance nanofiltration membrane with accurate charge distribution as well as a preparation method and application of the high-salt-tolerance nanofiltration membrane, and a high-performance membrane is prepared by reasonably designing a membrane structure on a molecular level. The polyamide membrane is subjected to post-modification through a lactone ring-opening reaction, accurate regulation and control of membrane charge distribution are achieved, the membrane surface positive charge density is enhanced, a negative charge micro-domain is introduced into a membrane structure, the ion adhesion effect is effectively utilized, synergistic improvement of permeability and selectivity is achieved, and the influence of electrostatic shielding on membrane selectivity under the high-salt condition is relieved. In the salt concentration range of 1-20 g / L, the membrane can maintain the MgCl interception performance exceeding 94.2%, the performance is superior to that of the prior art, and an efficient and economical new way is provided for extracting lithium from high-salt brine. The preparation method disclosed by the invention has high adaptability, and the separation performance of the nanofiltration membrane can be accurately regulated and controlled by adjusting the raw material proportion and the interfacial polymerization time so as to adapt to different application requirements.

Description

Technical Field

[0001] The present invention belongs to the technical field of ion separation, and relates to a highly salt-tolerant nanofiltration membrane with precise charge distribution, its preparation method and application, in particular to a nanofiltration membrane for efficient separation of magnesium and lithium under high-salt conditions. Background Art

[0002] Lithium, as a key strategic resource in the global energy transformation and high-tech industries, has become increasingly important. Lithium resources in brines account for about 65% of the global lithium reserves and have become the main source of lithium supply. Nanofiltration (NF) membrane technology is considered a key technology for extracting lithium from salt water due to its low energy consumption, economic feasibility, and environmental friendliness. In recent years, positively charged nanofiltration membranes have received extensive attention because of their high charge density, which can achieve efficient separation of magnesium and lithium through the Donnan effect. However, the structure and reactivity of the positively charged nanofiltration membrane monomers result in membranes that are usually thick and have an uneven pore size distribution. This structural feature makes the existing positively charged nanofiltration membranes vulnerable to the charge screening effect in high-salt systems, limiting their selectivity, which poses a major challenge for practical applications.

[0003] The reported strategies for solving the charge screening effect of NF membranes include process improvement and material innovation. By optimizing the cross-flow velocity and the structure of the feed spacer mesh, concentration polarization can be alleviated, and hydrodynamics can be improved, thereby alleviating the screening effect. From the perspective of materials science, it is crucial to reduce the dependence on the Donnan effect during the membrane separation process and strengthen the pore sieve to mitigate the impact of charge screening on membrane selectivity. However, the strategy of narrowing the pore size distribution to improve selectivity often comes at the cost of reducing membrane permeability.

[0004] CN119113809A discloses a preparation method of a high-performance magnesium-lithium separation membrane, which introduces strongly positively charged quaternary ammonium groups into the membrane interior by in-situ grafting to construct positively charged transport channels. Commonly used methods for enhancing surface positive charges include monomer innovation or post-treatment methods, which can increase the surface positive charge density of the membrane. This design improves the magnesium-lithium separation ability by strengthening the electrostatic repulsion of positive charges in the channels. However, this strategy of increasing the positive charge on the membrane surface has limited ability to resist the charge screening of the membrane in a high-salt environment. At the same time, the importance of the spatial distribution of charges in the nanofiltration membrane for achieving highly selective separation has also attracted people's attention. CN114452833A discloses a two-sided nanofiltration membrane with a positive-negative electroheterogeneous structure. Using graphene oxide as the functional material basis for modification, the quaternization modification and carboxylation modification of graphene oxide are realized, and two-sided nanofiltration membranes with upper and lower layer heterogeneous electricities are prepared by sequential vacuum filtration, preliminarily solving the problems of low magnesium-lithium separation efficiency and low retention ability of divalent counterions of traditional nanofiltration membranes. However, the membrane prepared by this filtration method seriously affects the service life of the membrane. Summary of the Invention

[0005] The object of the present invention is to achieve efficient separation of Mg 2+ and Li + in a high-salinity environment, and to develop a nanofiltration membrane that creates negatively charged microdomains by precisely regulating the charge distribution on the nanofiltration membrane, so as to enhance the separation efficiency of Mg 2+ / Li + through the ion adhesion effect; achieve precise control of the pore size distribution and the spatial distribution of charges, so as to be able to utilize pore sieving and the Donnan effect simultaneously. This synergistic effect helps to reduce the influence of electrostatic shielding on the membrane selectivity, thereby improving the separation efficiency and ensuring efficient separation of magnesium and lithium ions even in a high-salt environment. The object of the present invention is achieved by the following specific technical solutions.

[0006] The primary aspect of the present invention is to provide a highly salt-resistant nanofiltration membrane with a precise charge distribution. The highly salt-resistant nanofiltration membrane comprises a polyacrylonitrile (PAN) support membrane and a polyamide layer formed on the surface of the support membrane. The highly salt-resistant nanofiltration membrane is modified by ring-opening reaction of lactone and has a membrane structure with positive charge on the upper surface and negative charge on the lower surface, and negatively charged microdomains with a gradient distribution inside the membrane.

[0007] Furthermore, the thickness of the polyamide layer is 90 - 110 nm.

[0008] Furthermore, when the salt concentration is 1 - 20 g / L, the rejection rate of the highly salt-resistant nanofiltration membrane for Mg 2+ is 94.2% - 98.6%, the water permeability coefficient is 10.2 - 19.6 L·m -2 ·h -1 ·bar -1 , and the magnesium-lithium selectivity is greater than 13.

[0009] The calculation formula for the rejection rate (R, %) is as follows:

[0010] where C p , C f are the salt concentrations of the permeate and the feed respectively.

[0011] The calculation formula for the water permeability (p, L·m -2 ·h -1 ·bar -1 ) is as follows:

[0012] where V / Δt (L·h -1 ) represents the flow rate, A (m 2 ) and ΔP (bar) are the membrane area and the transmembrane pressure respectively.

[0013] The calculation formula for the magnesium-lithium selectivity (S Li, Mg ) is as follows:

[0014] Wherein, R Li and R Mg respectively represent the rejection rates of the membrane for magnesium and lithium.

[0015] Another aspect of the present invention is to provide a method for preparing the above-mentioned highly salt-tolerant nanofiltration membrane, including the following steps: S1. Prepare a nanofiltration membrane by interfacial polymerization on a polyacrylonitrile-based membrane: Immerse the polyacrylonitrile-based membrane in an aqueous solution containing an amino monomer, take out the immersed polyacrylonitrile-based membrane, remove the excess liquid and air-dry it; coat an organic solution containing an acyl chloride monomer on the surface of the air-dried membrane, remove the excess liquid, and heat and polymerize to form a nanofiltration membrane; S2. Charge modification: Mix the modification monomer with an organic solvent and stir to dissolve it. Place the nanofiltration membrane prepared in step S1 in the modification monomer solution, continuously stir, and keep the temperature constant for a certain period of time; remove the nanofiltration membrane after the reaction, rinse it with a solvent to remove the unreacted monomers on the surface, and dry it to obtain a highly salt-tolerant nanofiltration membrane.

[0016] Further, in step S1, the amino monomer is selected from one or two of polyethyleneimine (PEI) and bipyridine diamine, and the concentration of the amino monomer is 0.5-2.0 wt%, preferably 0.8-1.2 wt%.

[0017] Further, in step S1, the acyl chloride monomer is selected from one or more of trimesoyl chloride (TMC), glutaroyl chloride (GC), sebacoyl chloride (SDC), and isophthaloyl chloride (IPC), and the concentration of the acyl chloride monomer is 0.1-0.5 wt%, preferably 0.1-0.3 wt%.

[0018] Further, in step S2, the modification monomer is selected from one or more of 1,4-butanesultone (BS), carboxylic acid lactone (DEO), and phosphoric acid lactone (SC), the organic solvent is ethyl acetate (EA), n-hexane or tetrahydrofuran, the concentration of the modification monomer is 0.5-2.0 wt%, the reaction temperature is 30-80 °C, and the reaction time is 30-60 min.

[0019] Another aspect of the present invention is to provide the application of the above-mentioned highly salt-tolerant nanofiltration membrane in the extraction of lithium resources from brine, and the brine contains magnesium and lithium, and the salt concentration is 1-20 g / L.

[0020] The present invention has the following beneficial technical effects: By using lactone compounds as modification monomers, the present invention realizes the construction of negatively charged microdomains inside the membrane while maintaining or enhancing the positive charge on the membrane surface. This design creates a membrane structure with a positively charged upper surface and a negatively charged lower surface, achieving precise control of the charge distribution and thus optimizing the separation performance of the membrane. The nanofiltration membrane of the present invention not only improves selectivity and permeability but also exhibits excellent anti-fouling and anti-salt properties. These characteristics make the nanofiltration membrane of the present invention particularly suitable for treating high-concentration brine. The modification strategy adopted in the present invention can be improved based on existing mature processes, and the process is simple and easy to implement, facilitating industrial production and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 FIG. is a schematic structural diagram of the high-salt-tolerant nanofiltration membrane with precise charge distribution of the present invention.

[0022] Figure 2 FIG. is the structure of the amino monomer involved in the present invention.

[0023] Figure 3 FIG. is the structure of the acyl chloride monomer involved in the present invention.

[0024] Figure 4 FIG. is the structure of the lactone monomer involved in the present invention.

[0025] Figure 5 FIG. is a comparison of the separation performance of the high-salt-tolerant nanofiltration membranes of Example 1 and Comparative Example 1.

[0026] Figure 6 FIG. is a comparison of the separation performance of the high-salt-tolerant nanofiltration membrane of Example 1 with the currently reported literature. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the specification. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention. Example 1

[0028] This example provides a nanofiltration membrane, which includes a support membrane (PAN support membrane, purchased from Amiander Membrane Technology Co., Ltd.) and a polymer layer (with a thickness of about 100 nm) disposed on the surface of the support membrane; The reaction monomers of the polymer layer include amino monomers, acyl chloride monomers, and post-modified lactones.

[0029] The nanofiltration membrane is obtained by the following preparation method, and the preparation method includes the following steps: S1 First, prepare an aqueous solution of PEI (1.0 wt%) as the aqueous phase solution; prepare a hexane solution of TMC (0.15 wt%) as the organic phase solution. Immerse the PAN membrane in the aqueous solution containing 1.0 wt% PEI for 1 minute, then remove the excess liquid, and then air-dry the membrane for 5 minutes until no visible water droplets remain. Subsequently, carefully pour the hexane solution containing TMC onto the surface for 1 minute and remove the excess liquid. Then heat the membrane in an oven at 70 °C for 5 minutes to promote the polymerization process to obtain the PA membrane.

[0030] S2 Charge modification: Using ethyl acetate as the solvent, immerse the prepared PA membrane in an ethyl acetate solution containing 1 wt% BS at 50 °C for 60 min to obtain the PA-BS-1 membrane. After the reaction, rinse the membrane with pure ethyl acetate and water to remove the unreacted monomers, and then store it in water at 4 °C for later use.

[0031] Comparative Example 1 Use the PA membrane obtained in step S1 of Example 1 as the final product. That is, the difference between Comparative Example 1 and Example 1 is that there is no charge modification process. Examples 2 - 5

[0032] The differences between Examples 2 - 5 and Example 1 lie in adjusting the concentration of the post-modification solution BS solution, controlling the mass percentages of the BS solution to be different, which are respectively: Example 2: The mass percentage of the BS solution is 0.5 wt%, denoted as PA-BS-0.5, and the rest are the same as in Example 1.

[0033] Example 3: The mass percentage of the BS solution is 0.75 wt%, denoted as PA-BS-0.75, and the rest are the same as in Example 1.

[0034] Example 4: The mass percentage of the BS solution is 1.5 wt%, denoted as PA-BS-1.5, and the rest are the same as in Example 1.

[0035] Example 5: The mass percentage of the BS solution is 2.0 wt%, denoted as PA-BS-2, and the rest are the same as in Example 1. Example 6

[0036] This example provides a nanofiltration membrane, which includes a support membrane (PAN support membrane, purchased from Amiand Membrane Technology Co., Ltd.) and a polymer layer (with a thickness of about 110 nm) disposed on the surface of the support membrane; The reaction monomers of the polymer layer include amino-blocking monomers, acyl chloride monomers, and post-modified lactones.

[0037] The nanofiltration membrane is obtained by the following preparation method, and the preparation method includes the following steps: S1 First, prepare an aqueous solution of PEI (1.0 wt%) as the aqueous phase solution; prepare a hexane solution of TMC (0.2 wt%) as the organic phase solution. Immerse the PAN membrane in the aqueous solution containing 1.0 wt% PEI for 1 minute, then remove the excess liquid, and then air-dry the membrane for 5 minutes until no visible water droplets remain. Subsequently, carefully pour the hexane solution containing TMC onto the surface for 1 minute and remove the excess liquid. Then heat the membrane in an oven at 70 °C for 5 minutes to promote the polymerization process to obtain the PA membrane.

[0038] S2 Charge modification: Using tetrahydrofuran as the solvent, immerse the prepared PA membrane in a tetrahydrofuran solution containing 1 wt% DEO at 50 °C for 40 min to obtain the PA-DEO-1 membrane. After the reaction, rinse the membrane with pure tetrahydrofuran and water to remove the unreacted monomers, and then store it in water at 4 °C for standby. Examples 7 - 10

[0039] The difference between Examples 7 - 10 and Example 6 lies in adjusting the concentration of the post-modification solution DEO solution, controlling the mass percentage of the DEO solution to be different, which are respectively: Example 7: The mass percentage of the DEO solution is 0.5 wt%, denoted as PA-DEO-0.5, and the rest are the same as Example 6.

[0040] Example 8: The mass percentage of the DEO solution is 0.75 wt%, denoted as PA-DEO-0.75, and the rest are the same as Example 6.

[0041] Example 9: The mass percentage of the DEO solution is 1.5 wt%, denoted as PA-DEO-1.5, and the rest are the same as Example 6.

[0042] Example 10: The mass percentage of the DEO solution is 2.0 wt%, denoted as PA-DEO-2, and the rest are the same as Example 6. Example 11

[0043] This example provides a nanofiltration membrane, which includes a support membrane (PAN support membrane, purchased from Amiand Membrane Technology Co., Ltd.) and a polymer layer (with a thickness of about 90 nm) disposed on the surface of the support membrane; The reaction monomers of the polymer layer include amino-blocking monomers, acyl chloride monomers, and post-modified lactones.

[0044] The nanofiltration membrane is obtained by the following preparation method, and the preparation method includes the following steps: S1 First, prepare an aqueous solution of PEI (1.0 wt%) as the aqueous phase solution; prepare a hexane solution of TMC (0.2 wt%) as the organic phase solution. Immerse the PAN membrane in the aqueous solution containing 1.0 wt% PEI for 1 minute, then remove the excess liquid, and then air-dry the membrane for 5 minutes until no visible water droplets remain. Subsequently, carefully pour the hexane solution containing TMC onto the surface for 1 minute and remove the excess liquid. Then heat the membrane in an oven at 70 °C for 5 minutes to promote the polymerization process to obtain the PA membrane.

[0045] S2 During the membrane functionalization process, hexane is used as the solvent. Immerse the prepared PA membrane in a hexane solution containing 1 wt% SC at 40 °C for 30 min to obtain the PA-SC-1 membrane. After the reaction, rinse the membrane with pure hexane and water to remove the unreacted monomers, and then store it in water at 4 °C for later use. Examples 12 - 15

[0046] The differences between Examples 12 - 15 and Example 10 lie in adjusting the concentration of the modified liquid SC solution, controlling different mass percentages of the SC solution, which are respectively: Example 12: The mass percentage of the SC solution is 0.5 wt%, denoted as PA-SC-0.5, and the rest are the same as Example 10.

[0047] Example 13: The mass percentage of the SC solution is 0.75 wt%, denoted as PA-SC-0.75, and the rest are the same as Example 10.

[0048] Example 14: The mass percentage of the SC solution is 1.5 wt%, denoted as PA-SC-1.5, and the rest are the same as Example 10.

[0049] Example 15: The mass percentage of the SC solution is 2.0 wt%, denoted as PA-SC-2, and the rest are the same as Example 10.

[0050] Performance testing Perform the following tests on the nanofiltration membranes of Examples 1 - 15 and Comparative Example 1: (1) Water permeability: Evaluate the separation performance of the membrane using a cross-flow filtration system equipped with three filtration units. First, compact the membrane with pure water at 6 bar for 1 h. Subsequently, measure the water permeability of the membrane under the conditions of a pressure of 4 bar and a flow rate of 50 L h –1 .

[0051] (2) Retention performance: After testing the water flux, maintain the temperature of the feed solution (2000 ppm) at 25 °C and test the salt retention performance of the membrane. The concentration is determined by a conductivity meter or ICP-OES.

[0052] (3) Selectivity: The selectivity performance of the membrane was calculated based on the retention of MgCl2 and LiCl. The retention was tested by cross-flow for the membrane performance.

[0053] The test results are shown in Table 1.

[0054] Table 1 Performance test results of the nanofiltration membranes of Examples 1-15 and Comparative Example 1

[0055] Analyzing the data in Table 1, it can be seen that the water permeability of the nanofiltration membrane obtained in the present invention is between 10.2-19.6 L·m -2 ·h -1 ·bar -1 and the retention rate is between 94.2%-98.6%; for the nanofiltration membrane obtained in the present invention, a negatively charged microdomain was constructed in the membrane by ring-opening reaction to regulate the charge distribution and pore size of the membrane; the preparation method of the nanofiltration membrane is simple, has good reproducibility, has good universality, and according to different application requirements, the functional groups and contents of the modifier can be controlled to obtain nanofiltration membranes with different separation performances.

[0056] From Figure 5 it can be seen that the retention performance of Example 1 in MgCl2 solutions with different concentrations is excellent. As the salt concentration increased from 1 g / L to 20 g / L, the retention rate of the PA-BS-1 membrane for Mg²⁺ only slightly decreased from 98.43% to 96.21%, and the performance attenuation amplitude was only 2.2%. In contrast, for the unmodified PA membrane under the same conditions, the retention rate significantly decreased from 94.2% to 85.0%, and the performance attenuation rate reached 9.7%, showing an obvious deterioration trend.

[0057] From Figure 6It can be seen that in Example 1, the selectivity performance in the feed liquid with a magnesium-lithium ratio of 20 is significantly better than that of similar materials reported in the existing literature, demonstrating excellent separation performance. The summarized references are as follows: Desalination 2019, 449, 57-68. Desalination 2015, 369, 26-36. Chemical Engineering Journal 2022, 438, 135658. Journal of Membrane Science 2022, 663, 121063. Journal of Membrane Science 2023, 668, 121251. Journal of Membrane Science 2023, 672, 121468. Separation and Purification Technology 2021, 258, 118042. Separation and Purification Technology 2006, 49, 230-236. Separation and Purification Technology 2021, 270, 118796. Journal of Membrane Science 2021, 620, 118862. Journal of Membrane Science 2020, 603, 117997. ACS Applied Materials & Interfaces 2022, 14, 32420-32432. Separation and Purification Technology 2023, 308, 122968. Advanced Membranes 2023, 3, 100065. Journal of Membrane Science 2022, 659, 120809. Desalination 2023, 565, 116814. Journal of Membrane Science 2022, 644, 119942. Journal of Membrane Science 2023, 688, 122133。

[0058] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention. The protection scope of the present invention is defined by the claims and their equivalent technical solutions.

Claims

1. Highly salt-tolerant nanofiltration membrane with precise charge distribution, characterized in that, The high salt-resistant nanofiltration membrane comprises a polyacrylonitrile support membrane and a polyamide layer formed on the surface of the support membrane. The high salt-resistant nanofiltration membrane is modified by a lactone ring-opening reaction and has a membrane structure with a positive upper surface and a negative lower surface, and has gradient distributed negative charge microdomains inside the membrane.

2. The high salt-resistant nanofiltration membrane according to claim 1, characterized in that: The thickness of the polyamide layer is 90-110 nm.

3. The high salt-resistant nanofiltration membrane according to claim 1, characterized in that: When the salt concentration is 1-20 g / L, the high salt-tolerant nanofiltration membrane has a good effect on Mg 2+ The interception rate is 94.2%~98.6%, and the water permeability coefficient is 10.2~19.6 L·m -2 ·h -1 bar -1 , magnesium-lithium selectivity is greater than 13.

4. The method for preparing a highly salt-resistant nanofiltration membrane according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1. Preparing a nanofiltration membrane on a polyacrylonitrile-based membrane by interfacial polymerization: immersing the polyacrylonitrile-based membrane in an aqueous solution containing an amino monomer, taking out the immersed polyacrylonitrile-based membrane, removing excess liquid and air-drying; coating an organic solution containing an acyl chloride monomer on the surface of the air-dried membrane, removing excess liquid, and heating to polymerize to form a nanofiltration membrane; S2. Charge modification: Mix the modified monomer with an organic solvent and stir to dissolve, place the nanofiltration membrane prepared in step S1 in the modified monomer solution, stir continuously, and keep the reaction at a constant temperature for a period of time; remove the nanofiltration membrane after the reaction is completed, rinse with a solvent to remove the unreacted monomer on the surface, and dry to obtain a highly salt-resistant nanofiltration membrane.

5. The preparation method according to claim 4, characterized in that: In step S1, the amino monomer is selected from one or both of polyethyleneimine and bipyridine diamine, and the concentration of the amino monomer is 0.5-2.0 wt%.

6. The preparation method according to claim 5, characterized in that: The concentration of the amino monomer is 0.8~1.2 wt%.

7. The preparation method according to claim 4, characterized in that: In step S1, the acyl chloride monomer is selected from one or more of trimesoyl chloride, glutaryl chloride, sebacoyl chloride and isophthaloyl chloride, and the concentration of the acyl chloride monomer is 0.1-0.5 wt %.

8. The preparation method according to claim 7, characterized in that: The concentration of the acyl chloride monomer is 0.1~0.3 wt%.

9. The preparation method according to claim 4, characterized in that: In step S2, the modified monomer is selected from one or more of 1,4-butane sultone, carboxylic acid lactone and phosphate lactone, the organic solvent is ethyl acetate, n-hexane or tetrahydrofuran, the concentration of the modified monomer is 0.5-2.0 wt%, the reaction temperature is 30-80°C, and the reaction time is 30-60 min.

10. The use of the high salt-resistant nanofiltration membrane according to any one of claims 1 to 3 in the extraction of lithium brine resources, characterized in that: The brine contains magnesium and lithium, and the salt concentration is 1-20 g / L.

Images