Crown ether grafted chitosan coating film as well as preparation method and application thereof

Abstract

The invention discloses a crown ether grafted chitosan coating film and a preparation method and application thereof.According to the coating film, chitosan serves as a base material, crown ether serves as a functional modifier, a matrix polymer serves as a coating matrix, NPOE serves as a plasticizer, and a PIM-CTS-15 coating film and a PIM-CTS-18 coating film are prepared through chitosan precursor preparation, crown ether grafting modification, film casting liquid preparation and coating forming; and the crown ether is benzo 15-crown-5 or dibenzo 18-crown-6. The invention further discloses a preparation method of the catalyst. Through the synergistic effect of specific recognition, hydrophilic regulation and structure enhancement, the coating film has high selectivity, high permeation flux and good stability on lithium ions. The membrane is simple in preparation process and easy to industrialize, can be efficiently used for lithium separation and extraction of lithium-containing waste liquid, and has remarkable economic and environmental benefits.

Description

Technical Field

[0001] This invention relates to the field of membrane preparation and application technology, specifically to a crown ether-grafted chitosan coated membrane, its preparation method, and its application. Background Technology

[0002] Lithium, as a key strategic metal, plays an irreplaceable role in lithium-ion batteries, new energy vehicles, aerospace, pharmaceuticals, and chemicals. With the rapid development of the global new energy industry, the demand for lithium resources continues to surge, while natural lithium reserves are limited. Separating and extracting lithium from secondary resources such as lithium-containing waste liquids has become an important way to alleviate lithium resource shortages.

[0003] Membrane separation technology has garnered widespread attention in the field of lithium separation and extraction due to its advantages such as ease of operation, low energy consumption, environmental friendliness, and high separation efficiency. Among these technologies, polymer-incorporated membranes (PIMs), as membrane systems synergistically constructed from a substrate, an active carrier, and toughening components, demonstrate significant application potential in energy conversion, environmental remediation, and fine chemicals. The substrate layer regulates the membrane's crystal structure, thermodynamic stability, and surface wettability; the active carrier achieves directional transport by forming coordination bonds with target metal ions; and the toughening components improve the membrane's flexibility. However, the inherent low metal ion permeability of existing polymer-incorporated membranes remains a key bottleneck restricting their engineering applications. While this limitation can be overcome through coupling and integration with electrodialysis technology, the performance of these composite membranes still requires optimization.

[0004] Chitosan, a natural polymer obtained by deacetylation of chitin, possesses excellent biocompatibility, biodegradability, and non-toxicity. The abundant amino (-NH2) and hydroxyl (-OH) groups in its molecular structure endow it with good chemical reactivity, making it an ideal carrier for chemical modification. Modifying chitosan through graft copolymerization and other methods can impart new functional properties while retaining its inherent advantages. Crown ethers, as supramolecular host compounds with macrocyclic polyether skeletons, possess unique cyclic cavity structures with tunable molecular recognition characteristics. For example, the cavity sizes of crown ethers such as dibenzo-14-crown-4, dibenzo-15-crown-5, and dibenzo-18-crown-6 match the radius of lithium ions, enabling specific recognition and complexation of lithium ions. They are ideal lithium-selective functional units, and introducing them as functional carriers into PIMs systems can significantly improve membrane separation selectivity. Matrix polymers typically possess excellent chemical stability, mechanical strength, and corrosion resistance, making them suitable as matrix materials for coating membranes.

[0005] Preparing a coated membrane by combining crown ether-grafted modified chitosan with a matrix polymer combines the advantages of all three: crown ether provides lithium-ion specific recognition sites, chitosan improves the membrane's hydrophilicity and compatibility, and the matrix polymer ensures the membrane's mechanical strength and stability. However, the existing preparation process for crown ether-modified chitosan composite membranes is not perfect, and the influence of various process parameters and operating conditions on membrane permeation performance has not been systematically studied, resulting in unclear optimal operating conditions and limiting its application in practical lithium separation. Furthermore, research on the kinetic mechanism of membrane permeation behavior and its durability is scarce, failing to provide theoretical support for the long-term stable use of the membrane.

[0006] Based on this, the present invention prepares a crown ether-grafted chitosan-coated membrane by precisely controlling the crown ether grafting process and the coating composite process. The effects of time, current, solution acidity, membrane thickness and crown ether content on membrane permeation are systematically investigated. Combined with kinetic analysis and durability testing, the optimal usage conditions are determined. The aim is to provide a high-performance crown ether-grafted chitosan-coated membrane with clearly defined applicable conditions to meet the actual needs of lithium separation and extraction from waste liquid. Summary of the Invention

[0007] The purpose of this invention is to provide a crown ether-grafted chitosan coating film to solve the above-mentioned defects.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] The present invention proposes a crown ether-grafted chitosan coating film, specifically a PIM-CTS-15 coating film or a PIM-CTS-18 coating film, which is prepared by using chitosan CTS as the base material, crown ether as the functional modifier, matrix polymer as the coating matrix, and 2-NPOE as the plasticizer. The crown ether-grafted chitosan content is 10%-50% of the coating film mass; the crown ether is dibenzo-15-crown-5 or dibenzo-18-crown-6.

[0010] Preferably, the crown ether of the PIM-CTS-15 coating is benzo15-crown-5, the crown ether of the PIM-CTS-18 coating is dibenzo18-crown-6, and the thickness of the PIM-CTS-15 coating and the PIM-CTS-18 coating is 20-80 μm.

[0011] Preferably, the matrix polymer is polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP).

[0012] Preferably, a method for preparing a crown ether-grafted chitosan coating film includes the following steps:

[0013] Preparation of S1, amino crown ether:

[0014] A certain amount of benzo-15 crown-5 or dibenzo-18-crown-6 crown ether was dispersed in 200 mL of chloroform at room temperature, and 100 mL of acetic acid was added. Then, concentrated nitric acid (4.8 mL) and acetic acid (48 mL) were mixed evenly and slowly added dropwise. The reaction was carried out at a certain temperature for 12 h. After the reaction was completed, the mixture was cooled to room temperature, concentrated under reduced pressure, and the product was washed with dichloromethane and water. The organic phase was dehydrated with anhydrous CaCl2. After concentration under reduced pressure, the product was dried at 60 °C for 12 h to obtain nitro crown ether CE-NO2. The synthesized CE-NO2 was dispersed in 600 mL of ethanol, and 5% Pd / C catalyst and 150 mL of hydrazine hydrate were added and mixed. The mixture was heated under nitrogen protection at 80 °C for 5 h under reflux. After the reaction was completed, the mixture was filtered while hot, the filtrate was rotary evaporated, and the product was dried at 60 °C for 12 h to obtain amino-modified crown ether CE-15-NH2 or CE-18-NH2.

[0015] S2. Preparation of chitosan precursor:

[0016] A certain amount of chitosan and NaH were dissolved in anhydrous DMF under nitrogen protection. After the solution changed from colorless to light yellow, epichlorohydrin was added and the reaction was kept at a certain temperature for a certain time. After the reaction was completed, DMF was removed by rotary evaporation, the product was washed with deionized water, and dried under vacuum at 60°C for 6 hours to obtain the chitosan precursor material CTS-ECH.

[0017] S3. Preparation of crown ether grafted chitosan:

[0018] The chitosan precursor material CTS-ECH was dissolved in an acidic solution, and the amino crown ether prepared in step S1, plasticizer and activator were added. The mixture was reacted at 100~120℃ for 4-8 hours. After precipitation, washing and drying, crown ether-grafted chitosan was obtained.

[0019] S4. Preparation of coating film casting solution:

[0020] A certain mass of the matrix polymer is dissolved in an organic solvent, and a certain mass of the crown ether grafted chitosan prepared in step S3 is added. The mixture is stirred until it is completely dissolved and mixed evenly to obtain a homogeneous casting solution.

[0021] S5. Coating film preparation: The casting solution is coated on the surface of the support substrate, and after solvent evaporation, solidification, washing and drying, PIM-CTS-15 coating film or PIM-CTS-18 coating film is obtained.

[0022] Preferably, in step S4, the mass ratio of the matrix polymer to the crown ether-grafted chitosan is (0.8~8):1.

[0023] Preferably, in step S3, the solvent is toluene and the activator is triethylamine; in step S4, the organic solvent is THF.

[0024] Preferably, in step S2, the reaction is kept at a certain temperature for a certain period of time, with the reaction temperature being 60~80℃ and the reaction time being 48-72h.

[0025] Preferably, the supporting substrate is a polytetrafluoroethylene microporous membrane or a polyester nonwoven fabric; the coagulation molding uses deionized water as the coagulation bath at a temperature of 25-35℃; the washing is carried out until the filtrate is neutral, and the drying temperature is 60-80℃ for 2-4 hours.

[0026] Preferably, an application of a crown ether-grafted chitosan coating membrane is used to separate and extract lithium ions from waste liquid, thereby improving the efficiency of lithium ion separation and extraction.

[0027] Preferably, the specific steps are as follows: PIM-CTS-15 coated membrane or PIM-CTS-18 coated membrane is assembled into a membrane module, lithium-containing waste liquid is passed into the membrane module, and under optimal operating conditions, electrodialysis is coupled to achieve selective permeation separation and enrichment of lithium ions, thereby realizing the separation and extraction of lithium ions; the optimal operating conditions are: separation time 30-60 min, current 0.2-0.5 A, solution pH 4-6, membrane thickness 40-60 μm; and in the PIM-CTS-15 coated membrane or PIM-CTS-18 coated membrane, the amount of crown ether grafted chitosan is 10%-50% of the mass of the coated membrane.

[0028] The beneficial effects of this invention are as follows:

[0029] (1) The crown ether grafted chitosan coating of the present invention is prepared by grafting modified chitosan with crown ether and then coating it with a matrix polymer. It achieves the synergistic effect of "specific recognition-hydrophilicity regulation-structure enhancement". The resulting PIM-CTS-15 or PIM-CTS-18 coating has high selectivity for lithium ions.

[0030] (2) The preparation method of the crown ether-grafted chitosan-coated membrane of the present invention is simple and stable, with mild conditions, good repeatability, and easy to industrialize. The raw material chitosan is widely available and environmentally friendly, and the matrix polymer (such as PVDF-HFP) has excellent chemical stability, which is conducive to industrial-scale production. The prepared crown ether-grafted chitosan-coated membrane has excellent performance and high permeation efficiency in lithium separation and extraction from waste liquid (lithium permeation flux ≥ 5.0 mol / h·m). 2 It has good durability. Tests have verified that after four cycles of use, the PIM-CTS-15 coating membrane can still maintain an extraction rate of over 97% for lithium ions, and the PIM-CTS-18 coating membrane can still maintain an extraction rate of over 89% for lithium ions, demonstrating significant economic and environmental benefits.

[0031] (3) The application of the crown ether grafted chitosan coated membrane of the present invention systematically explored the effects of time, current, solution acidity, membrane thickness and crown ether content on membrane permeation. Combined with kinetic analysis, the control mechanism of the permeation process was clarified, and the long-term stability of the membrane was verified through durability testing. Finally, the optimal use conditions were determined, providing precise parameter guidance for the actual application of the membrane and avoiding the decrease in separation efficiency caused by improper use conditions. Attached Figure Description

[0032] Figure 1 This is a process flow diagram for preparing the crown ether-grafted chitosan coating membrane of the present invention;

[0033] Figure 2 The following are FTIR analysis diagrams of the crown ether grafted chitosan-coated membrane of the present invention (a is PIM-CTS-15 membrane, b is PIM-CTS-18 membrane).

[0034] Figure 3 SEM images of the crown ether-grafted chitosan-coated membrane of the present invention (a is PIM-CTS-15 membrane, b is PIM-CTS-18 membrane).

[0035] Figure 4 The chemical stability analysis diagrams of the crown ether grafted chitosan-coated membrane of the present invention are shown (a is the PIM-CTS-15 membrane, b is the PIM-CTS-18 membrane).

[0036] Figure 5 This is a TGA image of the crown ether-grafted chitosan-coated membrane of the present invention;

[0037] Figure 6 IV curve of the crown ether-grafted chitosan-coated membrane of the present invention;

[0038] Figure 7 This is a diagram of the separation experimental apparatus for the crown ether-grafted chitosan-coated membrane of the present invention;

[0039] Figure 8 The diagram shows the effect of the energizing time on the permeation of the crown ether grafted chitosan-coated membrane of the present invention (a is the PIM-CTS-15 membrane, b is the PIM-CTS-18 membrane).

[0040] Figure 9 The diagram shows the effect of solution pH on permeation of the crown ether grafted chitosan coated membrane of the present invention (a is PIM-CTS-15 membrane, b is PIM-CTS-18 membrane).

[0041] Figure 10 The diagram shows the effect of the thickness and porosity of the crown ether grafted chitosan coating membrane of the present invention on the permeation rate (a is PIM-CTS-15 membrane, b is PIM-CTS-18 membrane).

[0042] Figure 11 The diagram shows the effect of CTS-CE content on permeation in the crown ether grafted chitosan coated membrane of the present invention (a is PIM-CTS-15 membrane, b is PIM-CTS-18 membrane).

[0043] Figure 12 The diagram shows the permeability selectivity analysis of the crown ether-grafted chitosan-coated membrane of the present invention (a is the PIM-CTS-15 membrane, b is the PIM-CTS-18 membrane).

[0044] Figure 13 The diagram shows the recyclability analysis of the crown ether grafted chitosan coating membrane of the present invention (a is PIM-CTS-15 membrane, b is PIM-CTS-18 membrane).

[0045] Figure 14 The diagram shows the application effect of the crown ether-grafted chitosan coating membrane of the present invention in lithium extraction from fly ash acid leaching solution. Detailed Implementation

[0046] The present invention will be further described below with reference to the embodiments. It should be noted that these are merely examples and descriptions of the inventive concept. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the inventive concept or exceed the scope defined in the claims, they should all be considered to fall within the protection scope of the present invention.

[0047] Example 1:

[0048] Figure 1 This is a flow chart illustrating the preparation process of the crown ether-grafted chitosan-coated membrane of the present invention. Figure 1 As shown, a method for preparing a crown ether-grafted chitosan-coated membrane according to the present invention includes the following steps:

[0049] Preparation of S1, amino crown ether:

[0050] 20 g of benzo-15-crown-5 was dispersed in 200 mL of chloroform at room temperature, and 100 mL of acetic acid was added. Then, concentrated nitric acid (4.8 mL) and acetic acid (48 mL) were mixed evenly and slowly added dropwise for 30 min, followed by stirring for 1 h. The reaction was carried out at 70 °C for 12 h. After the reaction was completed, the mixture was cooled to room temperature and concentrated under reduced pressure. The product was washed with dichloromethane and water, and the organic phase was dehydrated with anhydrous CaCl2. After concentration under reduced pressure, the product was dried at 60 °C for 12 h to obtain a pale yellow solid (CE-15-NO2). The synthesized 4′-nitrobenzo-15-crown-5 (20 g) was dispersed in 600 mL of ethanol and mixed with 5% Pd / C (4 g) catalyst and hydrazine hydrate (150 mL). The mixture was heated under nitrogen protection at 80 °C for 5 h under reflux. After the reaction was completed, the mixture was filtered while hot, the filtrate was rotary evaporated, and the product was dried at 60 °C for 12 h to obtain a yellow product (CE-15-NH2).

[0051] S2. Preparation of chitosan precursor:

[0052] 40 g of chitosan and 17.8 g of NaH were dissolved in 400 ml of anhydrous DMF and reacted at 60 °C under a nitrogen atmosphere. After the solution changed from colorless to light yellow, 19.4 ml of epichlorohydrin was added, and the reaction was continued at 60 °C for 72 h. After the reaction was completed, DMF was removed by rotary evaporation, and the product was washed several times with deionized water. The product was then vacuum dried at 60 °C for 6 h to obtain the chitosan precursor material CTS-ECH.

[0053] S3. Preparation of crown ether grafted chitosan:

[0054] 12 g of chitosan precursor material CTS-ECH, 2.4 mL of triethylamine and 14.3 g of CE-15-NH2 were dissolved in 150 mL of toluene and reacted at 115 °C for 5 h under a nitrogen atmosphere. The product was obtained by filtration, washed with toluene and ethanol in sequence, and then dried under vacuum at 80 °C for 8 h to obtain crown ether grafted chitosan CTS-15.

[0055] S4. Preparation of coating film casting solution:

[0056] 0.6 g PVDF-HFP, 0.3 g CTS-15, and 0.1 g plasticizer (2-NPOE) were dissolved together in 15 mL THF and heated and stirred at 50 °C for 12 h to completely dissolve and mix evenly, thus obtaining a homogeneous casting solution.

[0057] S5. Preparation of coating film:

[0058] The casting solution prepared in step S4 was ultrasonically degassed for 15 min, and then coated onto the surface of the polytetrafluoroethylene substrate. After the solvent evaporated naturally and the substrate was dried, it was transferred to a 30°C deionized water coagulation bath and allowed to stand for 3 h to form. The membrane was washed until the filtrate was neutral and then vacuum dried at 70°C for 3 h to obtain the PIM-CTS-15 coated membrane. The content of crown ether grafted chitosan in the prepared coated membrane was 30%.

[0059] Using dibenzo-18-crown-6 as the raw material crown ether, and repeating the above experimental steps S1-S5, a PIM-CTS-18 polymer-coated film can be prepared.

[0060] Example 2:

[0061] The preparation method of the crown ether grafted chitosan coating film of the present invention is basically the same as that in Example 1, except that:

[0062] In step S2, the reaction is carried out at the same temperature condition of 80°C for 48 hours.

[0063] In step S3, the reaction is carried out at 100°C for 8 hours under a nitrogen atmosphere.

[0064] In step S4, 0.8 g of PVDF-HFP, 0.1 g of CTS-15, and 0.1 g of plasticizer (2-NPOE) were dissolved together in 15 mL of THF and heated and stirred at 50 °C for 12 h to completely dissolve and mix evenly, thus obtaining a homogeneous casting solution.

[0065] In step S5, the membrane is transferred to a 25°C deionized water coagulation bath and allowed to stand for 3 hours to form. The membrane is washed until the filtrate is neutral and then vacuum dried at 60°C for 4 hours to obtain a PIM-CTS-15 coated membrane. The content of crown ether grafted chitosan in the prepared coated membrane is 10%.

[0066] By repeating the above experimental steps using dibenzo-18-crown-6 as a functional modifier, PIM-CTS-18 coated films can be prepared.

[0067] Example 3:

[0068] The preparation method of the crown ether grafted chitosan coating film of the present invention is basically the same as that in Example 1, except that:

[0069] In step S2, the reaction is carried out at the same temperature condition of 70°C for 60 hours.

[0070] In step S3, the reaction is carried out at 120°C for 4 hours under a nitrogen atmosphere.

[0071] In step S4, 0.7 g of PVDF-HFP, 0.2 g of CTS-15, and 0.1 g of plasticizer (2-NPOE) were dissolved together in 15 mL of THF and heated and stirred at 50 °C for 12 h to completely dissolve and mix evenly, thus obtaining a homogeneous casting solution.

[0072] In step S5, the membrane is transferred to a 35°C deionized water coagulation bath and allowed to stand for 2 hours to form. The membrane is washed until the filtrate is neutral and then vacuum dried at 80°C for 2 hours to obtain a PIM-CTS-15 coated membrane. The content of crown ether grafted chitosan in the prepared coated membrane is 20%.

[0073] By repeating the above experimental steps using dibenzo-18-crown-6 as a functional modifier, PIM-CTS-18 coated films can be prepared.

[0074] Example 4:

[0075] The preparation method of the crown ether grafted chitosan coating film of the present invention is basically the same as that in Example 1, except that:

[0076] In step S2, the reaction is carried out at the same temperature condition of 65°C for 70 hours.

[0077] In step S3, the reaction is carried out at 110°C for 6 hours under a nitrogen atmosphere.

[0078] In step S4, 0.5 g of PVDF-HFP, 0.4 g of CTS-15, and 0.1 g of plasticizer (2-NPOE) were dissolved together in 15 mL of THF and heated and stirred at 50 °C for 12 h to completely dissolve and mix evenly, thus obtaining a homogeneous casting solution.

[0079] In step S5, the membrane is transferred to a 30°C deionized water coagulation bath and allowed to stand for 3 hours to form. The membrane is washed until the filtrate is neutral and then vacuum dried at 70°C for 3.5 hours to obtain a PIM-CTS-15 coated membrane. The content of crown ether grafted chitosan in the prepared coated membrane is 40%.

[0080] By repeating the above experimental steps using dibenzo-18-crown-6 as a functional modifier, PIM-CTS-18 coated films can be prepared.

[0081] Example 5:

[0082] The preparation method of the crown ether grafted chitosan coating film of the present invention is basically the same as that in Example 1, except that:

[0083] In step S2, the reaction is carried out at the same temperature condition of 75°C for 52 hours.

[0084] In step S3, the reaction is carried out at 105°C for 6 hours under a nitrogen atmosphere.

[0085] In step S4, 0.4 g of PVDF-HFP, 0.5 g of CTS-15, and 0.1 g of plasticizer (2-NPOE) were dissolved together in 15 mL of THF and heated and stirred at 50 °C for 12 h to completely dissolve and mix evenly, thus obtaining a homogeneous casting solution.

[0086] In step S5, the membrane is transferred to a 35°C deionized water coagulation bath and allowed to stand for 2 hours to form. The membrane is washed until the filtrate is neutral and then vacuum dried at 60°C for 4 hours to obtain a PIM-CTS-15 coated membrane. The content of crown ether grafted chitosan in the prepared coated membrane is 50%.

[0087] By repeating the above experimental steps using dibenzo-18-crown-6 as a functional modifier, PIM-CTS-18 coated films can be prepared.

[0088] The PIM-CTS-15 and PIM-CTS-18 coated films prepared in Examples 1-5 had thicknesses ranging from 20 to 80 μm. Testing of the prepared PIM-CTS-15 and PIM-CTS-18 coated films revealed that they both achieved a synergistic effect of "specific recognition-hydrophilicity regulation-structure enhancement," exhibiting excellent performance and varying degrees of high selectivity for lithium ions.

[0089] The PIM-CTS-15 or PIM-CTS-18 coated films prepared in Example 1 are described in detail below:

[0090] Figure 2 The images show the FTIR analysis of the crown ether-grafted chitosan-coated membranes of this invention. Image a shows the PIM-CTS-15 membrane, and image b shows the PIM-CTS-18 membrane. Figure 2 It can be seen that at 1402 cm -1 At a wavenumber of 1279 cm⁻¹, poly(vinylidene fluoride-co-hexafluoropropylene) exhibits a significant characteristic peak, attributed to the deformation vibration of the -CF₂ group. In the synthesized CTS-ECH-18 material, the peak at 1279 cm⁻¹ is a characteristic peak of the CN vibration, while in the CTS-ECH-15 and CTS-ECH-18 materials, the peak at 1279 cm⁻¹ is also significant. -1 1515 cm -1 Characteristic peaks for CN and the benzene ring in crown ethers appeared at 1594 cm⁻¹. -1 The peaks are NH stretching peaks, indicating the successful synthesis of PIM-CTS-15 and PIM-CTS-18 polymer-coated films.

[0091] Figure 3 These are SEM images of the crown ether-grafted chitosan-coated membranes of the present invention. Image a shows the PIM-CTS-15 membrane, and image b shows the PIM-CTS-18 membrane. Figure 3As can be seen, the surface is free of pores and cracks, which indicates that the prepared film is very complete and uniform, without obvious defects.

[0092] Figure 4 This is a chemical stability analysis diagram of the crown ether-grafted chitosan-coated membrane of the present invention. a is the PIM-CTS-15 membrane, and b is the PIM-CTS-18 membrane. Figure 4 It can be seen that the crystal structure of the PIM-CTS-15 coating membrane was not damaged in acidic solutions with pH 2, 4, and 6, or in alkaline solutions with pH 8, 10, and 12, indicating that the PIM-CTS-15 coating membrane has good chemical and structural stability.

[0093] Figure 5 This is a TGA image of the crown ether-grafted chitosan-coated membrane of the present invention. Figure 5 It can be seen that the thermal decomposition of the PIM-CTS-15 coated membrane can be divided into three stages. In the first stage, the membrane material exhibits a mass loss of no more than 10% in the temperature range of 25-250℃, which is mainly attributed to the removal of residual solvent inside the membrane. In the second and third stages, in the temperature ranges of 250-320℃ and 320-480℃, the membrane material experiences a significant mass decrease, which is mainly attributed to the thermal depolymerization process of its polymer backbone and subsequent decomposition reaction.

[0094] Figure 6 This is the IV curve of the crown ether-grafted chitosan-coated membrane of the present invention. Figure 6 It is evident that the ohmic resistance of the prepared PIM-CTS-15 membrane is lower than that of the PIM-CTS-18 membrane. This difference stems from the varying matching degree between the crown ether ring size and the ions to be separated. This host-guest chemical interaction enhances the ion hopping transport mechanism within the membrane, resulting in a higher ion transport number for the PIM-CTS-15 membrane. Nonlinear regression analysis of the IV curves further reveals that the current response of both coated membranes exhibits a typical three-stage characteristic: in the low-field region of 0-25 V, the current maintains a linear relationship with the voltage (ohmic region); when the voltage rises to the 25-38 V range, the current growth slows down and a plateau effect forms (transition region); above 38 V, the ion migration rate is limited by membrane interface polarization, entering the limiting diffusion control region. This phenomenon indicates that in actual electrodialysis operations, the operating current density should be strictly controlled below the limiting current threshold to avoid membrane performance degradation caused by concentration polarization.

[0095] The PIM-CTS-15 and PIM-CTS-18 coated membranes prepared in Example 1 were applied to the separation of lithium from fly ash acid leaching solution. The specific steps are as follows: The PIM-CTS-15 or PIM-CTS-18 coated membranes were assembled into a membrane module; the lithium-containing waste liquid was passed through the membrane module; and the membrane separation device was configured as follows: Figure 7As shown, under certain operating conditions, coupled electrodialysis enables the selective permeation separation and enrichment of lithium ions, thereby achieving the separation and extraction of lithium ions.

[0096] Based on a comprehensive consideration of the current-voltage curves under different ion concentrations, a solution containing 0.05 mol / L Li was prepared. + Mg 2+ Ca 2+ K + The effects of different conditions on ion transport performance were investigated in a mixed solution of ion transport, including energizing time, solution pH, membrane thickness, CTS-CE content, membrane permeation selectivity, recyclability, and its application in lithium separation in simulated fly ash acid leaching solution.

[0097] (1) Effect of energizing time on membrane permeation.

[0098] Figure 8 This diagram illustrates the effect of energizing time on permeation in the crown ether-grafted chitosan-coated membrane of the present invention. a represents the PIM-CTS-15 membrane, and b represents the PIM-CTS-18 membrane. Figure 7 It can be seen that within the time range of 0-50 min, the permeation of metal ions by both the PIM-CTS-15 and PIM-CTS-18 coated membranes gradually increases with time. After 30 min, the permeation of the PIM-CTS-15 coated membrane remains essentially unchanged with increasing time. After 40 min, the permeation of the PIM-CTS-18 coated membrane remains essentially unchanged with increasing time. Considering all factors, 40 min is the optimal permeation time. The PIM-CTS-15 and PIM-CTS-18 coated membranes exhibit the best permeation time for Li... + The permeability was 1.67 mol·h⁻¹. -1 ·m -2 and 0.78 mol·h -1 ·m -2 .

[0099] (2) Effect of solution pH on membrane permeation.

[0100] Changes in pH can affect the charge distribution on the adsorbent surface, the activity of functional groups, and the form of ions in the acid leaching solution, thus influencing its ability to adsorb metal ions from the solution. The prepared mixed solution contains Li. + Mg 2+ Ca 2+ K + The concentrations were all 0.05 mol / L, and the pH of the solutions was adjusted to 1.0-8.0 for each measurement.

[0101] Figure 9This diagram illustrates the effect of solution pH on permeation of the crown ether-grafted chitosan-coated membrane of the present invention. a represents the PIM-CTS-15 membrane, and b represents the PIM-CTS-18 membrane. Figure 8 It can be seen that the ion permeation capacity of both PIM-CTS-15 and PIM-CTS-18 coated membranes gradually decreases with increasing pH, and the membrane materials exhibit the best ion permeation effect at pH 1.0. The PIM-CTS-15 coated membrane... + Mg 2+ Ca 2+ K + The permeability was 4.38 mol·h⁻¹. -1 ·m -2 3.18 mol·h -1 ·m -2 1.19 mol·h -1 ·m -2 2.96 mol·h -1 ·m -2 PIM-CTS-18 coating film for Li + Mg 2+ Ca 2+ K + The permeability was 4.24 mol·h⁻¹. -1 ·m -2 3.04 mol·h -1 ·m -2 2.64 mol·h -1 ·m -2 3.27 mol·h -1 ·m -2 .

[0102] (3) The effect of membrane thickness on membrane permeate.

[0103] PIM-CTS-15 and PIM-CTS-18 coated membranes of different thicknesses were prepared for electrodialysis experiments. The effect of the thickness of the PIM-CTS-15 and PIM-CTS-18 coated membranes on ion permeation was investigated. The PIM-CTS-15 and PIM-CTS-18 coated membranes were cut into 20*20 mm pieces, dried in an oven at 80℃ for 24 h, and then weighed. The membranes were then immersed in deionized water for 24 h, and the surface moisture was wiped dry. The mass and dimensions of the membranes were recorded, and the porosity was calculated.

[0104] Figure 10 This diagram illustrates the effect of the thickness and porosity of the crown ether-grafted chitosan-coated membrane on permeation volume according to the present invention. a represents the PIM-CTS-15 membrane, and b represents the PIM-CTS-18 membrane. Figure 9It can be seen that the permeation capacity of both PIM-CTS-15 and PIM-CTS-18 coated membranes increases with increasing membrane thickness. However, when the membrane thickness exceeds 56 μm, the permeation capacity of both PIM-CTS-15 and PIM-CTS-18 coated membranes does not change significantly with increasing membrane thickness. The PIM-CTS-15 coated membrane exhibits relatively high permeation capacity for Li... + Mg 2+ Ca 2+ K + The permeability was 4.46 mol·h⁻¹. -1 ·m -2 5.70 mol·h -1 ·m -2 2.77 mol·h -1 ·m -2 2.15 mol·h -1 ·m -2 PIM-CTS-18 coating film for Li + Mg 2+ Ca 2+ K + The permeability was 4.40 mol·h⁻¹. -1 ·m -2 3.31 mol·h -1 ·m -2 1.47 mol·h -1 ·m -2 3.79 mol·h -1 ·m -2 Membrane porosity is also a key factor in evaluating membrane performance. As membrane thickness increases, so does membrane porosity. When the membrane thickness reaches 76 μm, the porosity increases significantly. Considering all factors, a membrane thickness of 66 μm was chosen.

[0105] (4) Effect of CTS-CE content on membrane permeation.

[0106] The content of CTS-CE has a significant impact on the membrane's ability to permeate ions, and it is directly related to the membrane's flexibility and pore structure characteristics.

[0107] Figure 11 This diagram illustrates the effect of CTS-CE content on permeation in the crown ether-grafted chitosan-coated membrane of the present invention. a represents the PIM-CTS-15 membrane, and b represents the PIM-CTS-18 membrane. Figure 10It can be seen that as the CTS-CE content increases, the membrane's ion permeation gradually increases, but when a certain proportion is reached, the permeation no longer shows a significant increase. The membrane porosity also gradually increases with increasing CTS-CE content. When the CTS-CE content is 40%, due to the low content of the substrate components, the membrane porosity increases significantly after membrane formation. Therefore, a CTS-CE content of 40% results in optimal permeation. The PIM-CTS-15 coated membrane exhibits good permeation for Li... + Mg 2+ Ca 2+ K + The permeability was 4.75 mol·h⁻¹. -1 ·m -2 1.82 mol·h -1 ·m -2 1.35 mol·h -1 ·m -2 2.10 mol·h -1 ·m -2 PIM-CTS-18 coating film for Li + Mg 2+ Ca 2+ K + The permeability was 4.48 mol·h⁻¹. -1 ·m -2 3.81 mol·h -1 ·m -2 2.16 mol·h -1 ·m -2 3.19 mol·h -1 ·m -2 .

[0108] (5) The permeation selectivity of the prepared crown ether grafted chitosan membrane.

[0109] Under optimal experimental conditions, this invention systematically investigated the effects of PIM-CTS-15 and PIM-CTS-18 coatings on Li. + With interfering ions (Mg 2+ Ca 2+ Selective separation performance of K⁺).

[0110] The experiment used 0.05 mol / L Mg 2+ Ca 2+ Electrodialysis tests were performed with K⁺ solution. Figure 11 This is a permeability selectivity analysis diagram of the crown ether-grafted chitosan-coated membrane of the present invention. a is the PIM-CTS-15 membrane, and b is the PIM-CTS-18 membrane. Figure 12 It can be seen that the two coatings affect Li + / Mg2+ Li + / Ca 2+ and Li + Both / K⁺ exhibited good separation selectivity. Among them, Li + / Mg 2+ The selectivity is optimal, Li + / K + Secondly, Li + / Ca 2+ Relatively low. Furthermore, the PIM-CTS-15 coating exhibits superior selectivity for Li. + / Mg 2+ Li + / Ca 2+ and Li + / K + The selectivity coefficients reached 2.61, 3.53 and 2.27 respectively, all of which were higher than the corresponding values ​​of PIM-CTS-18 coating film.

[0111] (6) The recyclability of the prepared crown ether grafted chitosan membrane.

[0112] Membrane recyclability is a prerequisite for determining whether a membrane can be mass-produced and used, and it is also an important indicator of membrane stability. Used membranes were immersed in a 1 mol / L hydrochloric acid solution for 12 hours, then immersed in deionized water for 12 hours, and finally air-dried before the next experiment. The membrane was subjected to the same experimental procedure repeatedly, and the results are as follows: Figure 13 As shown, after four cycles of use, the extraction rate of lithium ions by the PIM-CTS-15 coated membrane remained above 97%; the extraction rate of lithium ions by the PIM-CTS-18 coated membrane remained above 89%, indicating that the membranes have good stability in extracting lithium ions.

[0113] As can be seen from the above, the optimal operating conditions for PIM-CTS-15 and PIM-CTS-18 coated membranes in the separation of lithium from fly ash acid leaching solution are: separation time 30-60 min, current 0.2-0.5 A, solution pH 4-6, and membrane thickness 40-60 μm; and in PIM-CTS-15 or PIM-CTS-18 coated membranes, the amount of crown ether grafted is 10%-15% of the chitosan mass.

[0114] A simulated acid leaching solution for fly ash was prepared using lithium chloride, calcium chloride, potassium chloride, and magnesium sulfate hexahydrate. The application of polymer-coated films PIM-CTS-15 and PIM-CTS-18 in the simulated acid leaching solution was investigated. Figure 14 This image shows the application effect of the crown ether-grafted chitosan-coated membrane of the present invention in lithium extraction from fly ash acid leaching solution. Figure 13It was found that in the simulated acid leaching solution, the polymer-coated membranes PIM-CTS-15 and PIM-CTS-18 exhibited the highest permeability for calcium ions, followed by magnesium ions, potassium ions, and lithium ions. This is because the lithium ion content in the simulated leachate was only 0.04 mol·h⁻¹. -1 ·m -2 Excessive concentration of interfering ions can affect the penetration of lithium ions.

[0115] The crown ether-grafted chitosan-coated membrane of the present invention is prepared by grafting modified chitosan with crown ether and then coating it with a matrix polymer. It achieves the synergistic effect of "specific recognition-hydrophilicity regulation-structure enhancement". The resulting PIM-CTS-15 or PIM-CTS-18 coated membrane has high selectivity for lithium ions, with lithium / sodium and lithium / magnesium selectivity coefficients ≥100, which is significantly better than existing composite membrane materials.

[0116] The method for preparing the crown ether-grafted chitosan-coated membrane of the present invention is simple, stable, mild, reproducible, and easy to industrialize. The chitosan used as the raw material is widely available and environmentally friendly, and the matrix polymer (such as PVDF-HFP) exhibits excellent chemical stability, which is beneficial for industrial-scale production. The prepared crown ether-grafted chitosan-coated membrane possesses excellent performance, exhibiting high permeation efficiency (lithium permeation flux ≥ 5.0 mol / h·m³) in lithium separation and extraction from waste liquid. 2 It has good durability. Tests have verified that after four cycles of use, the extraction rate of lithium ions by the PIM-CTS-15 coating membrane can still remain above 97%, and the extraction rate of lithium ions by the PIM-CTS-18 coating membrane can still remain above 89%, demonstrating significant economic and environmental benefits.

[0117] The application of the crown ether-grafted chitosan-coated membrane of this invention systematically investigated the effects of time, current, solution acidity, membrane thickness, and crown ether content on membrane permeation. Combined with kinetic analysis, the control mechanism of the permeation process was clarified, and the long-term stability of the membrane was verified through durability testing. Finally, the optimal operating conditions were determined, providing precise parameter guidance for the practical application of the membrane and avoiding the decrease in separation efficiency caused by improper operating conditions.

[0118] The above is an exemplary description of the invention. Obviously, the specific implementation of the invention is not limited to the above-described manner. Any non-substantial improvement made using the inventive concept and technical solution of the invention, or the direct application of the inventive concept and technical solution to other situations without modification, is within the protection scope of the invention.

Claims

1. A crown ether grafted chitosan coated film, characterized by, Specifically, the PIM-CTS-15 coating film or the PIM-CTS-18 coating film is prepared by taking chitosan CTS as a base material, taking crown ether as a functional modifier, taking a base polymer as a coating matrix, and taking 2-NPOE as a plasticizer, wherein the content of the crown ether grafted chitosan is 10%-50% of the mass of the coating film; the crown ether is benzo 15-crown-5 or diphenyl 18-crown-6.

2. The crown ether grafted chitosan coating film according to claim 1, characterized in that, The crown ether of the PIM-CTS-15 coating film is benzo 15-crown-5, the crown ether of the PIM-CTS-18 coating film is diphenyl 18-crown-6, and the thickness of the PIM-CTS-15 coating film and the PIM-CTS-18 coating film is 20-80 μm.

3. The crown ether grafted chitosan coating film according to claim 1, characterized in that, The base polymer is PVDF-HFP.

4. A method for preparing the crown ether grafted chitosan coated film according to any one of claims 1-3, characterized in that, The method comprises the following steps: S1. Preparation of amino crown ether A certain amount of crown ether is dissolved in chloroform, acetic acid is added, then a mixture of concentrated nitric acid and acetic acid is slowly added dropwise, and the reaction is kept for 12 hours. After the reaction is completed, the solution is cooled to room temperature, concentrated under reduced pressure, and the product is washed with dichloromethane and water, respectively, and dried at 60°C for 12 hours to obtain a light yellow solid nitro crown ether CE-NO2. The synthesized CE-NO2 is dispersed in ethanol and mixed with Pd / C catalyst and hydrazine hydrate, heated to reflux under nitrogen protection for a certain period of time, filtered while hot after the reaction is completed, and the filtrate is rotary evaporated. The product is dried at 60°C for 12 hours to obtain a yellow amino-modified crown ether CE-15-NH2 or CE-18-NH2. S2. Preparation of chitosan precursor A certain amount of chitosan and NaH is dissolved in anhydrous DMF under nitrogen protection. After the solution changes from colorless to light yellow, epichlorohydrin is added, and the reaction is kept for a certain period of time. After the reaction is completed, the DMF is removed by rotary evaporation, the product is washed with deionized water, and dried at 60°C under vacuum for 6 hours to obtain a chitosan precursor material CTS-ECH. S3. Preparation of crown ether grafted chitosan The chitosan precursor material CTS-ECH is dissolved in an acidic solution, the amino crown ether prepared in step S1, a plasticizer and an activator are added, and the reaction is kept at 100-120°C for 4-8 hours. After precipitation, washing and drying, the crown ether grafted chitosan is obtained. S4. Preparation of coating film casting solution The base polymer is dissolved in an organic solvent, the crown ether grafted chitosan prepared in step S3 is added, and stirring is performed until complete dissolution and uniform mixing to obtain a homogeneous casting solution. S5. Preparation of coating film: The casting solution is coated on the surface of a support substrate, and after solvent evaporation, coagulation molding, washing and drying, the PIM-CTS-15 coating film or the PIM-CTS-18 coating film is obtained.

5. The method for preparing a crown ether grafted chitosan coating film according to claim 4, characterized in that, In step S4, the mass ratio of the base polymer to the crown ether grafted chitosan is (0.8-8):

1.

6. The method for preparing a crown ether grafted chitosan coating film according to claim 4, characterized in that, In step S3, the acidic solution is a toluene solution, and the activator is triethylamine. In step S4, the organic solvent is THF.

7. The method for preparing a crown ether-grafted chitosan-coated membrane according to claim 4, characterized in that, In step S1, the reaction is kept for a certain period of time, the reaction temperature is 60-80°C, and the reaction time is 48-72 hours.

8. The method for preparing a crown ether-grafted chitosan-coated membrane according to claim 4, characterized in that, In step S5, the support base material is a polytetrafluoroethylene microporous membrane or a polyester non-woven fabric; coagulation molding uses deionized water as a coagulation bath, the temperature is 25-35 DEG C; washing is performed until the filtrate is neutral, the drying temperature is 60-80 DEG C, and the time is 2-4 h.

9. Use of the crown ether grafted chitosan coating film according to any one of claims 1 to 3, characterized in that, The crown ether grafted chitosan coated membrane is applied to separation and extraction of lithium ions in waste liquid, and separation and extraction efficiency of lithium ions is improved.

10. The use of a crown ether grafted chitosan coating film according to claim 9, characterized in that, The specific steps are as follows: The PIM-CTS-15 coated membrane or the PIM-CTS-18 coated membrane is assembled into a membrane assembly, lithium-containing waste liquid is introduced into the membrane assembly, under optimal use conditions, selective permeation separation and enrichment of lithium ions are realized by coupling electrodialysis, so that separation and extraction of lithium ions are realized. The optimal use conditions are as follows: separation time is 30-60 min, current size is 0.2-0.5 A, solution pH value is 4-6, and membrane thickness is 40-60 mu m.

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