A two-dimensional positively charged sub-nanometer channel membrane for monovalent cation separation and its preparation method and application

Abstract

The application provides a preparation method of a two-dimensional positively charged sub-nanometer channel film for monovalent cation separation. The application provides Co-Al hydrotalcite nanosheets with two-dimensional positive charges, constructs a two-dimensional nanosheet film through layer-by-layer self-assembly, constructs a positively charged confined channel to separate positively charged monovalent ions, and has excellent performance. On the basis of the confined ion transmission channel, the channel wall is modified with positive charges and hydrogen bonds, the effect of this micro-chemical environment is amplified, lithium, sodium and potassium ions enter the channel after partial dehydration, due to the different hydration radii of the three ions, the charge density is different, and then the channel has obvious differences in resistance contrast, thereby excellent separation effect is achieved; self-supporting film formation can be realized, the mechanical flexibility is good, the ion permeability is good; and the interlayer spacing of the film remains unchanged after entering water, which is the same as in the dry state, and there is no problem of expansion of the two-dimensional channel film, and the film has good stability and is suitable for practical application.

Description

Technical Field

[0001] This invention belongs to the field of ion separation technology, specifically relating to a two-dimensional positively charged subnanochannel thin film for monovalent cation separation, its preparation method, and its application. Background Technology

[0002] Membrane separation technology plays an increasingly important role in many crucial applications such as seawater desalination and purification, lithium extraction from salt lakes, and wastewater treatment. Among all separation systems, monovalent cations (K+) are crucial. + Na + Li + Separating ions with the same valence state and similar sub-nanometer ion size (such as hydrated K+) is particularly difficult. + radius is Na+ hydrate + for Hydrated Li + for ), and the angstrom-level differences between these monovalent ions. This makes separating monovalent cations one of the most challenging tasks. Despite these difficulties, separating K... + Na + and Li + This is of great significance. For example, in the practical application of membrane separation technology for extracting lithium from salt lakes, its main principle is to separate lithium from Li... + Na + K + Mg 2+ Ca 2+ Li was separated from the mixture + Currently, Li + / Mg 2+ and Li + / Ca 2+ The actual binary ion selectivity has reached a high level of over 1000, while K + / Li + and Na + / Li + The selectivity still needs to be improved. Summary of the Invention

[0003] In view of this, the technical problem to be solved by the present invention is to provide a two-dimensional positively charged subnanometer channel thin film for monovalent cation separation, its preparation method and application. The thin film provided by the present invention has excellent monovalent cation separation capability, K + / Li + and Na + / Li + It has advantages such as high selectivity, fast penetration rate, low cost, and no pollution.

[0004] This invention provides a method for preparing a two-dimensional positively charged subnanometer channel thin film for monovalent cation separation, comprising the following steps:

[0005] A) Carbonate-type Co-Al hydrotalcite nanoparticles undergo an ion exchange reaction with chloride salts under weak acid conditions to obtain chloride-type Co-Al hydrotalcite nanoparticles.

[0006] B) The chloride ion-type Co-Al hydrotalcite nanoparticles were exfoliated to obtain positively charged nanosheets.

[0007] C) The positively charged nanosheets are self-assembled on the surface of a porous base membrane to obtain a two-dimensional positively charged subnanochannel thin film for the separation of monovalent cations.

[0008] Preferably, the preparation method of the carbonate-type Co-Al hydrotalcite nanoparticles includes the following steps:

[0009] CoCl2·6H2O, AlCl3·6H2O, urea, and deionized water were mixed and hydrolyzed to obtain carbonate-type Co-Al hydrotalcite nanoparticles.

[0010] Preferably, the chloride salt is selected from sodium chloride.

[0011] Preferably, the pH of the weak acid condition is 2.26 to 3.

[0012] Preferably, the stripping is performed in formamide.

[0013] Preferably, the porous base membrane is selected from Nylon base membrane, polyethersulfone porous base membrane or polysulfone porous base membrane.

[0014] Preferably, step C) includes the following steps:

[0015] The solution of the positively charged nanosheets was placed on the surface of a porous membrane and then filtered to self-assemble into a membrane. [Co] 0.8 Al 0.2 [(OH)2](Cl] - ) 0.2 The mH2O membrane naturally detached from the substrate, yielding a two-dimensional positively charged subnanochannel membrane for the separation of monovalent cations.

[0016] The present invention also provides a two-dimensional positively charged subnanochannel thin film for monovalent cation separation prepared by the above preparation method, characterized in that the interlayer spacing of the two-dimensional positively charged subnanochannel thin film is 0.75±0.008nm, the interlayer channel height is 0.27±0.008nm, and the thickness of the two-dimensional positively charged subnanochannel thin film is 2.5~4μm.

[0017] The present invention also provides an application of the above-mentioned thin film in the selective separation of lithium ions in a salt solution system of monovalent cations.

[0018] Preferably, the salt solution system is selected from salt lakes, seawater, or brackish water.

[0019] Compared with existing technologies, this invention provides a method for preparing a two-dimensional positively charged subnanochannel film for monovalent cation separation, comprising the following steps: A) Carbonate-type Co-Al hydrotalcite nanoparticles undergo an ion exchange reaction with chloride salts under weakly acidic conditions to obtain chloride-type Co-Al hydrotalcite nanoparticles; B) The chloride-type Co-Al hydrotalcite nanoparticles are exfoliated to obtain positively charged nanosheets; C) The positively charged nanosheets are self-assembled on the surface of a porous substrate to form a film, thereby obtaining a two-dimensional positively charged subnanochannel film for monovalent cation separation. This invention provides two-dimensional positively charged Co-Al hydrotalcite nanosheets, which construct a two-dimensional nanosheet film through layer-by-layer self-assembly, thereby constructing a positively charged confined channel for separating positively charged monovalent ions, exhibiting excellent performance. Furthermore, based on the confined ion transport channel, positive charges and hydrogen bonds are modified on the channel walls to amplify the effect of this microchemical environment. + Na + K + After partial dehydration, the three types of ions enter the channel. Due to the different hydration radii of the three ions, their charge densities differ, resulting in significant differences in channel resistance and achieving excellent separation performance. The membrane can form a self-supporting film with good mechanical flexibility and good ion permeability. Furthermore, the interlayer spacing remains unchanged after entering water, the same as in the dry state, and there is no problem of two-dimensional channel membrane expansion. It has good stability and is suitable for practical applications. Attached Figure Description

[0020] Figure 1 SEM image of the chloride ion-type Co-Al hydrotalcite nanoparticles prepared in Example 1;

[0021] Figure 2 X-ray diffraction pattern of the two-dimensional positively charged subnanochannel thin film prepared in Example 1;

[0022] Figure 3 A cross-sectional SEM image of the two-dimensional positively charged subnanochannel thin film prepared in Example 2;

[0023] Figure 4 A photograph of the two-dimensional positively charged subnanochannel thin film prepared in Example 1;

[0024] Figure 5 Example 1 shows the relationship between the permeation rate of monovalent metal cations through a two-dimensional positively charged subnanometer channel membrane and the diameter of hydrated ions.

[0025] Figure 6 Example 1: Monovalent metal cation (K) + Na + Li + The permeation rate and ideal selectivity (single-ion system) of a two-dimensional positively charged sub-nanometer channel membrane;

[0026] Figure 7 Example 2: Monovalent metal cation (K) + Li + The permeation rate and actual selectivity of two-dimensional positively charged subnanometer channel films (binary ion system);

[0027] Figure 8 Example 2: Monovalent metal cation (Na) + Li + The permeation rate and actual selectivity (binary ion system) of a two-dimensional positively charged subnanometer channel membrane. Detailed Implementation

[0028] This invention provides a method for preparing a two-dimensional positively charged subnanometer channel thin film for monovalent cation separation, comprising the following steps:

[0029] A) Carbonate-type Co-Al hydrotalcite nanoparticles undergo an ion exchange reaction with chloride salts under weak acid conditions to obtain chloride-type Co-Al hydrotalcite nanoparticles.

[0030] B) The chloride ion-type Co-Al hydrotalcite nanoparticles were exfoliated to obtain positively charged nanosheets.

[0031] C) The positively charged nanosheets are self-assembled on the surface of a porous base membrane to obtain a two-dimensional positively charged subnanochannel thin film for the separation of monovalent cations.

[0032] This invention first involves subjecting carbonate-type Co-Al hydrotalcite nanoparticles to an ion exchange reaction. The preparation method of the carbonate-type Co-Al hydrotalcite nanoparticles includes the following steps:

[0033] CoCl2·6H2O, AlCl3·6H2O, urea, and deionized water were mixed and hydrolyzed to obtain carbonate-type Co-Al hydrotalcite nanoparticles.

[0034] The concentration of CoCl2·6H2O was 0.004–0.016 mol·L. -1 It can be 0.004, 0.005, 0.006, 0.008, 0.010, 0.012, 0.014, 0.016, or 0.004–0.016 mol·L⁻¹. -1Any value between.

[0035] The concentration of AlCl3·6H2O was 0.001–0.004 mol·L. -1 It can be 0.001, 0.002, 0.003, 0.004, or 0.001–0.004 mol·L⁻¹. -1 Any value between.

[0036] The concentration of urea is 0.01–0.04 mol·L⁻¹. -1 It can be 0.01, 0.02, 0.03, 0.04, or 0.01–0.04 mol·L⁻¹. -1 Any value between.

[0037] The hydrolysis reaction temperature is 85–95°C, which can be any value between 85, 90, 95, or 85–95°C; the reaction time is 24–48 hours, and nitrogen protection is maintained throughout the process.

[0038] After the hydrolysis reaction was completed, the hydrolysis product was washed and dried to obtain carbonate-type Co-Al hydrotalcite nanoparticles. The size of the carbonate-type Co-Al hydrotalcite nanoparticles was 5 ± 0.5 μm.

[0039] Then, carbonate-type Co-Al hydrotalcite nanoparticles, chloride salts, acids, and water are mixed to obtain a mixed solution, which undergoes an ion exchange reaction under nitrogen protection.

[0040] In the mixed solution, the concentration of the chloride salt is 5.8–48.75 wt%, and can be any value between 5.8 wt%, 6 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 48.75%, or 5.8–48.75 wt%. The chloride salt is selected from sodium chloride.

[0041] The concentration of the carbonate-type CoAl-hydrotalcite nanoparticles is 0.1–0.2 wt%.

[0042] The acid is used to adjust the pH of the mixed solution, which is between 2.26 and 3. In this invention, the acid is preferably hydrochloric acid. The concentration of the hydrochloric acid is 0.001–0.0054 mol·L⁻¹. -1Upon addition of hydrochloric acid, the number of protons in the solution increases, reacting with carbonate ions to form bicarbonate ions, resulting in a change in charge. To compensate for this charge, the chloride ions added to the solution are absorbed into the interlayer space of the hydrotalcite. Furthermore, both chloride and bicarbonate ions are monovalent anions, making their exchange much easier than the direct exchange between chloride and carbonate ions. Because a large amount of sodium chloride is added to the solution, the abundant chloride ions shift the equilibrium towards the chloride ion form, enhancing the intercalation and deintercalation of carbonate and bicarbonate ions. The protons produced by the addition of HCl may further protonate bicarbonate ions, forming carbon dioxide, which is then expelled from the interlayer space of the hydrotalcite. This strengthens the binding between the hydrotalcite and chloride ions in the solution.

[0043] In this invention, the ion exchange reaction is carried out at room temperature for 12 to 24 hours under nitrogen protection throughout the process. The chloride ion-type Co-Al hydrotalcite nanoparticles have a regular hexagonal sheet structure and are uniform in size, with a size of 5 ± 0.5 μm.

[0044] Next, the chloride ion-type Co-Al hydrotalcite nanoparticles were exfoliated to obtain positively charged nanosheets.

[0045] The present invention does not impose any particular limitation on the exfoliation method; any exfoliation method known to those skilled in the art is acceptable. In the present invention, exfoliation is preferably carried out in formamide. The concentration of the chloride ion type Co-Al hydrotalcite nanoparticles in formamide is 1 g / L, the reaction time is 24–48 h, and the thickness of the positively charged nanosheets is 0.8 ± 0.05 nm.

[0046] Then, the dispersion of positively charged nanosheets is filtered through a porous membrane, whereby the positively charged nanosheets self-assemble into a membrane on the surface of the porous membrane. The porous membrane is selected from Nylon, polyethersulfone, or polysulfone porous membranes, and the pore size of the porous membrane is 0.05 μm or 1 μm. Vacuum filtration is preferably used, and the filtration time is preferably 12–18 h. The membrane is then washed for 12–18 h. Finally, after being placed in air, the two-dimensional positively charged subnanometer channel film automatically separates from the substrate, yielding a self-supporting [Co] membrane. 0.8 Al 0.2 [(OH)2](Cl] - ) 0.2 ·mH2O membrane, namely a two-dimensional positively charged subnanometer channel membrane used for the separation of monovalent cations.

[0047] The two-dimensional positively charged subnanochannel thin film prepared by this invention has the advantages of simple processing and high K0 + / Li + and Na + / Li +It has advantages such as high selectivity, fast penetration rate, low cost, and no pollution.

[0048] The present invention also provides a two-dimensional positively charged subnanometer channel film for monovalent cation separation prepared by the above-described preparation method. The film is composed of ordered stacks of chloride-type Co-Al hydrotalcite nanosheets, with adjacent stacked chloride-type Co-Al hydrotalcite nanosheets connected by hydrogen bonds, thus providing a channel structure for rapid ion transport.

[0049] The interlayer spacing of the two-dimensional positively charged subnanochannel film is 0.75±0.008nm, the interlayer channel height is 0.27±0.008nm, and the thickness of the two-dimensional positively charged subnanochannel film is 2.5~4μm.

[0050] The present invention also provides an application of the above-mentioned thin film in the selective separation of lithium ions in a salt solution system of monovalent cations.

[0051] The salt solution system is selected from salt lakes, seawater, or brackish water. The monovalent cation includes Li. + K + Na + .

[0052] The separation principle of this invention is as follows: Co-Al layered double hydroxide (TLH) nanosheets self-assemble through layer-by-layer stacking, resulting in a self-supporting membrane with uniform sub-nanometer interlayer channels. This provides a confined environment for ion transport. At the microscopic confinement scale, modification of the chemical environment (hydrogen bonding and electrostatic interactions) plays a crucial role in ion transport. Furthermore, the positively charged surface of the Co-Al TDH nanosheets exhibits electrostatic repulsion with similarly positively charged potassium, sodium, and lithium ions. In addition, hydrated potassium, sodium, and lithium ions also interact with the Co-Al TDH channels through hydrogen bonding. Combined, these effects lead to partial dehydration of potassium, sodium, and lithium ions entering the channels. The combined effect of electrostatic interactions and hydrogen bonding results in resistance to the transport of these three ions within the channels. However, the interaction energy between potassium and sodium ions and the channels is smaller than that of lithium ions, resulting in faster transport rates for potassium and sodium ions and slower transport rates for lithium ions. This leads to the membrane's excellent ability to separate monovalent cations. Furthermore, immersing the membrane in an aqueous solution causes almost no change in the interlayer spacing, and there is no problem of expansion of the two-dimensional channel membrane, making it suitable for practical applications.

[0053] The two-dimensional positively charged subnanochannel membrane prepared by this invention has obvious advantages in the selective separation of lithium ions in monovalent cation salt solution systems, such as fast permeation rate, high selectivity and simple preparation method. The two-dimensional positively charged subnanochannel membrane prepared by this invention has precisely adjustable size of two-dimensional subnanochannels between membrane layers, and has good selectivity for monovalent cations, which is beneficial for selective lithium extraction in complex salt water environments.

[0054] This invention constructs confined ion transport channels by preparing self-assembled Co-Al layered double hydroxide (TLD) films with two-dimensional positively charged sub-nanometer channels and modifying them with a positively charged chemical environment rich in hydrogen bonds. This confined channel leads to partial dehydration of ions. After entering the interlayer channels, the dehydrated ions encounter transport resistance due to the aforementioned chemical environment. Since the interaction energy between potassium and sodium ions and the channels is smaller than that of lithium ions, potassium and sodium ions are transported faster, while lithium ions are transported very slowly, resulting in excellent monovalent cation separation capabilities. Furthermore, the method provided by this invention enables self-supporting film formation, resulting in films with excellent mechanical flexibility. Immersion in aqueous solutions causes almost no change in interlayer spacing, eliminating the problem of two-dimensional channel film expansion, making it suitable for practical applications.

[0055] To further understand the present invention, the following embodiments illustrate the two-dimensional positively charged subnanochannel thin film for monovalent cation separation provided by the present invention, its preparation method, and its application. The scope of protection of the present invention is not limited by the following embodiments.

[0056] Example 1

[0057] 1.5226 g (6.4 mmol) of CoCl₂·6H₂O, 0.3862 g (1.6 mmol) of AlCl₃·6H₂O, and 0.961 g (16 mmol) of urea were weighed into a 500 mL three-necked flask equipped with a reflux condenser. 400 mL of deionized water was added to dissolve all the precipitate. The mixture was reacted at 90 °C for 24 h under nitrogen protection. As the reaction proceeded, more and more pink crystalline particles precipitated. After the reaction was complete, the product was thoroughly washed with deionized water and finally with ethanol. The filtered product was dried in a 60 °C oven for 5 h to obtain carbonate-type Co-Al hydrotalcite nanoparticles.

[0058] Mix 0.4 g of the above product, 140 g (2.4 mol) of sodium chloride, and 110 μL of concentrated hydrochloric acid (molar concentration 12 mol·L⁻¹). -1 The product was added to a 500 mL round-bottom flask containing 400 mL of deionized water, at pH 2.26–3, and reacted at room temperature for 24 h under nitrogen protection. The resulting product was thoroughly washed with deionized water, and finally washed with ethanol. The filtered product was dried in a vacuum oven at 60 °C for 5 h to obtain chloride ion-type Co-Al hydrotalcite nanoparticles.

[0059] 0.025 g of chloride-ion type Co-Al hydrotalcite nanoparticles and 25 mL of formamide were placed in a 50 mL round-bottom flask. The mixture was vigorously stirred at room temperature under nitrogen protection and subjected to alternating sonication for 48 h to obtain a pink, semi-transparent colloidal suspension. Further processing was performed by centrifugation at 2000 rpm for 10 minutes. The supernatant was extracted to obtain a uniform nanosheet dispersion.

[0060] The obtained uniform nanosheet dispersion was filtered through a Φ50 mm nylon filter membrane with a pore size of 20 μm to remove impurities that could not be removed by centrifugation. Then, using a Φ50 mm nylon filter membrane with a pore size of 0.05 μm as the substrate membrane, the nanosheets were self-assembled into a membrane by vacuum filtration. Vacuum filtration was performed for 12 hours, followed by washing with water for 12 hours. Finally, after being placed in air for 3 hours, the two-dimensional positively charged subnanochannel membrane automatically separated from the substrate.

[0061] Then, its monovalent cation separation performance was tested. For the single-system diffusion dialysis test, the feed side was 25 mL of 0.1 mol·L⁻¹ solution. -1 Potassium chloride, sodium chloride, and lithium chloride solutions were used, with 25 mL of deionized water as the permeameter. Concentration-driven diffusion dialysis tests were performed for 16 min each, with the permeameter's conductivity changes monitored simultaneously. After the tests, the permeameter solution was subjected to ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) to determine its concentration, and then the permeation rate was calculated to derive K0. + / Li + and Na + / Li + Selectivity. The potassium ion diffusion rate was found to be 0.33 mol·h⁻¹. -1 ·m -2 The sodium ion permeation rate is 0.22 mol·h⁻¹. -1 ·m -2 The lithium-ion permeation rate is 0.0014 mol·h⁻¹. -1 ·m -2 The prepared two-dimensional positively charged subnanometer channel film for K + / Li + The selectivity was 23.7 for Na. + / Li + The selectivity is 15.0.

[0062] The thin film prepared in this embodiment was characterized. Figure 1 This is a SEM image of the chloride ion-type Co-Al hydrotalcite nanoparticles prepared in this embodiment; the image shows a regular hexagonal sheet-like structure with uniform size, approximately 4 μm.

[0063] Figure 2 This is the X-ray diffraction pattern of the two-dimensional positively charged subnanometer channel film prepared in this embodiment. Typical layered structure characteristic peaks can be observed in the image, indicating that the chloride ion-type Co-Al hydrotalcite nanosheets are orderly stacked, and the film possesses a channel structure for rapid ion transport. Using the Bragg equation, the interlayer spacing of the film in the dry state can be calculated to be 0.753 nm, and the interlayer spacing in the wet state is 0.757 nm. This demonstrates that the film exhibits almost no expansion when immersed in aqueous solution.

[0064] Figure 4 This is a photograph of the two-dimensional positively charged subnanochannel thin film prepared in this embodiment; it shows the excellent bending resistance of the film, which did not break even after being bent 180°, indicating the good toughness and strength of the film.

[0065] Figure 5 This embodiment shows the relationship between the permeation rate of monovalent metal cations through a two-dimensional positively charged subnanometer channel membrane and the diameter of hydrated ions; the permeation rate order is: K + Na + >Li + .

[0066] Figure 6 This embodiment is a monovalent metal cation (K). + Na + Li + The permeation rate and ideal selectivity (single-ion system) of a two-dimensional positively charged subnanometer channel membrane were used to study K... + / Li + The ideal selectivity is 23.7 for Na. + / Li + The ideal selectivity is 15.0.

[0067] Other characterization results of the thin film material prepared in this embodiment are similar to those in Example 2.

[0068] Example 2

[0069] The only difference from Example 1 is that 0.03g of chloride-type Co-Al hydrotalcite nanoparticles and 30mL of formamide were placed in a 50mL round-bottom flask, stirred vigorously at room temperature under nitrogen protection, and sonicated alternately for 48h to obtain a pink semi-transparent colloidal suspension. The rest of the preparation method is the same as in Example 1.

[0070] Then, its monovalent cation separation performance was tested. For the single-system diffusion dialysis test, the test method was the same as in Example 1. The prepared two-dimensional positively charged subnanochannel film was effective against K... + / Li + The selectivity was 23.6 for Na. + / Li +The selectivity was 14.1. For the diffusion dialysis test of the binary system, a mixed chloride solution of 0.05M KCl + 0.05M LiCl and 0.05M NaCl + 0.05M LiCl was used as the feed solution, and deionized water was used as the permeation chamber solution. K was measured at 20℃ and 40℃. + / Li + Na + / Li + Actual ion permeability. The prepared two-dimensional positively charged subnanochannel film exhibits K+ permeability at 20℃. + / Li + The actual selectivity is 5.7 for Na. + / Li + The actual selectivity is 4. For K at 40℃ + / Li + The actual selectivity is 5.1 for Na. + / Li + The actual selectivity is 3.8.

[0071] Figure 3 This is a cross-sectional SEM image of the two-dimensional positively charged subnanometer channel film prepared in this embodiment. The image further confirms the orderliness of the channel, and the chloride ion-type Co-Al hydrotalcite nanosheets can be observed to exhibit an ordered stacked layered structure. The thickness of the film can also be determined to be 3 ± 0.4 μm.

[0072] Figure 7 This embodiment is a monovalent metal cation (K). + Li + The permeation rate and actual selectivity (binary ion system) of a two-dimensional positively charged subnanometer channel membrane for K+ at 20℃ were analyzed. + / Li + The actual selectivity is 5.7 for K at 40℃. + / Li + The actual selectivity is 5.1.

[0073] Figure 8 This embodiment uses a monovalent metal cation (Na) + Li + The permeation rate and actual selectivity (binary ion system) of a two-dimensional positively charged subnanometer channel membrane for Na+ at 20℃ were analyzed. + / Li + The actual selectivity is 4, for Na at 40℃. + / Li + The actual selectivity is 3.8.

[0074] Other characterization results of the thin film material prepared in this embodiment are similar to those in Example 1.

[0075] Example 3

[0076] The only difference in Example 1 is that 0.04g of chloride-type Co-Al hydrotalcite nanoparticles and 40mL of formamide were placed in a 50mL round-bottom flask and stirred vigorously at room temperature under nitrogen protection. The mixture was then subjected to alternating sonication for 48h to obtain a pink, semi-transparent colloidal suspension. All other preparation methods were the same as in Example 1.

[0077] Then, its monovalent cation separation performance was tested. For the single-system diffusion dialysis test, the test method was the same as in Example 1. The prepared two-dimensional positively charged subnanochannel film was effective against K... + / Li + The selectivity was 9.74 for Na. + / Li + The selectivity was 6.18. For the diffusion dialysis test of the binary system, a mixed chloride solution of 0.05M KCl + 0.05M LiCl and 0.05M NaCl + 0.05M LiCl was used as the feed solution, and deionized water was used as the permeation chamber solution. K was measured. + / Li + Na + / Li + Actual ion permeability. The prepared two-dimensional positively charged subnanometer channel film for K... + / Li + The actual selectivity is 4.02 for Na. + / Li + The actual selectivity is 2.66.

[0078] The characterization results of the thin film material prepared in this embodiment are similar to those in Embodiment 1 and Embodiment 2.

[0079] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a two-dimensional positively charged subnanometer channel thin film for monovalent cation separation, characterized in that, Includes the following steps: A) Carbonate-type Co-Al hydrotalcite nanoparticles undergo an ion exchange reaction with chloride salts under weak acid conditions to obtain chloride-type Co-Al hydrotalcite nanoparticles. B) The chloride ion-type Co-Al hydrotalcite nanoparticles were exfoliated to obtain positively charged nanosheets; C) After placing the solution of the positively charged nanosheets on the surface of a porous membrane, the solution is filtered to self-assemble into a membrane. [Co] 0.8 Al 0.2 [(OH)2](Cl] - ) 0.2 The mH2O membrane naturally detached from the substrate, yielding a two-dimensional positively charged subnanochannel membrane for the separation of monovalent cations.

2. The preparation method according to claim 1, characterized in that, The preparation method of the carbonate-type Co-Al hydrotalcite nanoparticles includes the following steps: CoCl2·6H2O, AlCl3·6H2O, urea, and deionized water were mixed and hydrolyzed to obtain carbonate-type Co-Al hydrotalcite nanoparticles.

3. The preparation method according to claim 1, characterized in that, The chloride salt is selected from sodium chloride.

4. The preparation method according to claim 1, characterized in that, The pH of the weakly acidic conditions is 2.26~3.

5. The preparation method according to claim 1, characterized in that, The stripping is performed in formamide.

6. The preparation method according to claim 1, characterized in that, The porous base membrane is selected from Nylon base membrane, polyethersulfone porous base membrane or polysulfone porous base membrane.

7. A two-dimensional positively charged subnanochannel thin film for monovalent cation separation prepared by the preparation method according to any one of claims 1 to 6, characterized in that, The interlayer spacing of the two-dimensional positively charged subnanochannel film is 0.75±0.008nm, and the interlayer channel height is 0.27±0.008nm; the thickness of the two-dimensional positively charged subnanochannel film is 2.5~4μm.

8. The application of the thin film as described in claim 7 in the selective separation of lithium ions in a salt solution system of a monovalent cation.

9. The application according to claim 8, characterized in that, The salt solution system is selected from salt lakes, seawater, or brackish water.

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