Preparation method and application of a porous MXene nanoplatelet modified positively charged nanofiltration membrane

Abstract

The application discloses a preparation method and application of a porous MXene nanosheet modified positively charged nanofiltration membrane, and belongs to the technical field of nanofiltration membrane preparation and ion separation. The method comprises the following steps: depositing porous MXene nanosheet layers on the surface of a porous organic membrane substrate to obtain a porous organic membrane / porous MXene composite carrier; and preparing a polyamide layer on the surface of the porous organic membrane / porous MXene composite carrier through interfacial polymerization to obtain the positively charged nanofiltration membrane; and the reaction monomers of the interfacial polymerization are 2,3-diaminopiperazine and 1,3,5-benzene tricarbonyl chloride. The porous MXene nanosheet is used as an intermediate layer, the thickness of the polyamide layer can be effectively controlled according to the chemical characteristics of the porous MXene nanosheet, 2,3-diaminopiperazine is used as an aqueous phase monomer to positively modify the surface of the nanofiltration membrane, and finally, an ultrathin polyamide nanofiltration membrane with a positively charged surface is prepared.

Description

Technical Field

[0001] This invention relates to the fields of nanofiltration membrane preparation and ion separation technology, and in particular to a method for preparing and applying a porous MXene nanosheet-modified positively charged nanofiltration membrane. Background Technology

[0002] Lithium is a non-renewable resource, and with the explosive growth of the new energy vehicle industry, the lithium supply shortage may worsen. Developing and utilizing lithium resources in salt lakes has become crucial to ensuring supply. Currently used lithium-magnesium separation technologies, such as precipitation, extraction, adsorption, and calcination, suffer from high energy consumption, severe pollution, and high costs. Compared to these methods, membrane separation technology, which is simple to operate, has low energy consumption, and requires minimal equipment footprint, is receiving increasing attention. Nanofiltration (NF) membrane technology is commonly used in lithium-magnesium separation processes. Currently, commercially available nanofiltration membranes are mainly prepared via interfacial polymerization, with polyamide nanofiltration membranes being the most widely used. However, traditional polyamide nanofiltration membranes are hundreds of nanometers thick, significantly impacting their permeability and separation performance. Furthermore, the hydrolysis of surface acyl chloride groups during preparation makes them negatively charged, while lithium and magnesium in brine are monovalent and divalent cations. The negatively charged polyamide nanofiltration membrane exhibits poor selectivity, which is detrimental to the separation of lithium and magnesium. + / Mg 2+ Separation requires controlling the thickness of the polyamide nanofiltration membrane and modifying its surface with a positive charge. Therefore, designing and fabricating a high-performance positively charged nanofiltration membrane for lithium-magnesium separation, specifically for lithium extraction from salt lake brine, is a key research focus in this field. Summary of the Invention

[0003] The purpose of this invention is to provide a method for preparing and applying a positively charged nanofiltration membrane modified with porous MXene nanosheets, to solve the problems of traditional polyamide nanofiltration membranes with thicknesses of hundreds of nanometers, negative surface charges, difficulty in achieving the "trade-off" effect of selectivity and permeability, and difficulty in separating lithium and magnesium. This invention uses porous MXene nanosheets as an intermediate layer, and the thickness of the polyamide layer can be effectively adjusted according to the chemical characteristics of the porous MXene nanosheets (making the polyamide layer thickness 30-80 nm). Simultaneously, 2,3-diaminopiperazine is used as an aqueous monomer (amine monomer) to positively modify the surface of the nanofiltration membrane, ultimately preparing an ultrathin polyamide nanofiltration membrane with a positively charged surface.

[0004] To achieve the above objectives, the present invention provides the following solution:

[0005] One of the technical solutions of the present invention is a method for preparing the above-mentioned positively charged nanofiltration membrane, comprising the following steps: depositing a porous MXene nanosheet layer on the surface of a porous organic membrane substrate to obtain a porous organic membrane / porous MXene composite support; preparing a polyamide (PA) layer on the surface of the porous organic membrane / porous MXene composite support by interfacial polymerization to obtain the positively charged nanofiltration membrane (abbreviated as TFN); the reactive monomers of the interfacial polymerization are 2,3-diaminopiperazine and 1,3,5-benzenetricarboxyl chloride.

[0006] Furthermore, the porous organic membrane substrate is one of a porous polyvinylidene fluoride (PVDF) membrane, a porous polyethersulfone (PES) membrane, a porous Nylon-66 membrane, a porous sulfonated polyethersulfone (SPES) membrane, a porous polysulfone (PSF) membrane, or a porous sulfonated polysulfone (SPSF) membrane.

[0007] Furthermore, the pore size of the porous organic membrane substrate is 0.22–0.45 μm.

[0008] Furthermore, the deposition of a porous MXene nanosheet layer on the surface of a porous organic membrane substrate includes: depositing porous MXene nanosheets from a porous MXene nanosheet solution onto the surface of the porous organic membrane substrate by means of vacuum filtration.

[0009] Furthermore, the concentration of the porous MXene nanosheet solution is 1–6 mg / mL.

[0010] Furthermore, the filtration pressure is -3 to -1 bar, the filtration time is 20 to 40 minutes, and the filtration mass is 0.05 to 0.5 mg (referring to the mass of porous MXene nanosheets loaded on the porous organic membrane substrate through filtration being 0.05 to 0.5 mg).

[0011] Furthermore, the method for preparing the porous MXene nanosheet solution includes: oxidizing and etching the MXene nanosheet solution with ozone, then dissolving it with acid (to dissolve the oxidation and etching products on the surface of the MXene nanosheets), centrifuging and washing, and ultrasonically dispersing to obtain the porous MXene nanosheet solution. Porous MXene nanosheets refer to in-plane porous MXene nanosheets.

[0012] MXene is a two-dimensional material, and its general formula is M. n+1 X n T x(n = 1, 2, 3, or 4), where M represents a transition metal (e.g., Ti, V, Mo, and Cr), X represents carbon and / or nitrogen, and T represents a functional group (e.g., O, F, OH, Cl). MXene materials possess high aspect ratio, good mechanical strength and flexibility, and also exhibit unique layered structural characteristics. This invention introduces hydrophilic layered MXene nanomaterials as an intermediate layer into a polyamide film. The two-dimensional layered film assembled from stacked nanosheets has the advantages of ordered and controllable interlayer channels and low transport resistance, and can serve as an interface for interfacial polymerization reactions. The tortuous nanopores of the MXene nanosheets can regulate the release rate of amine monomers, thereby controlling the thickness of the polyamide layer. However, the narrow interlayer spacing of MXene and the tortuous transport paths between multiple layers result in low membrane permeability. To address this issue, the present invention artificially creates intra-surface nanopores on MXene nanosheets. This improves the stacking problem of MXene nanosheets. Using porous MXene nanosheets as an intermediate layer can shorten the interlayer transport path and improve the membrane permeability compared to ordinary MXene nanosheets.

[0013] Furthermore, the concentration of the MXene nanosheet solution is 1–6 mg / mL.

[0014] Furthermore, the concentration of ozone is 10–50 mg / L (ozone is provided by an ozone generator), the oxidation treatment time is 1–10 min, and the acid used for acid dissolution is hydrofluoric acid.

[0015] Furthermore, in the preparation of the porous MXene nanosheet solution, the concentration of hydrofluoric acid is 1 wt%; the centrifugal washing speed is 5000-12000 rpm, and the supernatant is washed until the pH value is 6; the ultrasonic dispersion is to ultrasonically disperse the product after centrifugation and washing in water for 10-60 min.

[0016] Furthermore, the preparation method of the MXene nanosheet solution includes: mixing lithium fluoride (LiF), titanium aluminum carbide (Ti3AlC2) and hydrochloric acid solution, heating and reacting, washing and peeling after the reaction, and ultrasonically dispersing to obtain the MXene nanosheet solution.

[0017] Furthermore, the mass-to-volume ratio of lithium fluoride, titanium aluminum carbide, and hydrochloric acid solution is 1.0–4.0 g : 1.0–3.5 g : 30–60 mL; the concentration of the hydrochloric acid solution is 9 M.

[0018] Furthermore, the heating reaction is carried out at a temperature of 50–70°C for a duration of 20–48 hours.

[0019] Furthermore, the heating reaction is carried out under stirring conditions, with the stirring speed being 20–40 rpm.

[0020] Furthermore, the specific steps for cleaning and exfoliation during the preparation of the MXene nanosheet solution are as follows: the product generated after the heating reaction is centrifuged and washed until the pH of the supernatant is 6. The ultrasonic dispersion involves ultrasonically dispersing the centrifuged and washed product in water for 1–2 hours.

[0021] Further, the preparation of the polyamide layer on the surface of the porous organic membrane / porous MXene composite carrier by interfacial polymerization includes: first, pouring an aqueous solution onto the surface of the porous organic membrane / porous MXene composite carrier, allowing it to stand, and then drying it; then, pouring an oil phase solution onto the surface of the porous organic membrane / porous MXene composite carrier, allowing it to stand, drying it, and then heating it to cure it; wherein the aqueous solution is an aqueous solution of 2,3-diaminopiperazine; and the oil phase solution is a hexane solution of 1,3,5-benzenetricarboxylic acid chloride (TMC).

[0022] 2,3-Diaminopiperazine As piperazine In addition to having a heterocyclic structure similar to piperazine, the derivative of the product also has additional amino groups, which ensure that the membrane surface is positively charged.

[0023] Further, the concentration of 2,3-diaminopiperazine in the aqueous solution is 50-60 mmol / L; the concentration of 1,3,5-benzenetricarboxyl chloride in the oil solution is 0.5-10 mmol / L; and the molar ratio of 2,3-diaminopiperazine in the aqueous solution to 1,3,5-benzenetricarboxyl chloride in the oil solution is 5-100:1.

[0024] Furthermore, the time for standing after pouring the aqueous solution onto the surface of the porous organic membrane / porous MXene composite carrier is 1–5 min; the time for standing after pouring the oil phase solution onto the surface of the porous organic membrane / porous MXene composite carrier (i.e., the time for the interfacial reaction between 1,3,5-benzenetriformyl chloride in the oil phase solution and 2,3-diaminopiperazine in the aqueous phase solution) is 1–3 min; the temperature for heating and curing is 60–80 °C, and the time is 1–5 min.

[0025] The second technical solution of the present invention: a positively charged nanofiltration membrane prepared according to the above preparation method; wherein the thickness of the polyamide layer is 30-80 nm.

[0026] Furthermore, the surface of the positively charged nanofiltration membrane carries a positive charge.

[0027] Furthermore, the thickness of the porous MXene nanosheet layer is 10–30 nm.

[0028] The third technical solution of the present invention: an application of the above-mentioned positively charged nanofiltration membrane in the separation of lithium and magnesium.

[0029] Furthermore, the positively charged nanofiltration membrane is used for the separation of lithium and magnesium during lithium extraction from salt lake brine.

[0030] Furthermore, the concentration of magnesium ions in the salt lake brine is 500–2000 ppm, and the concentration of lithium ions is 500–2000 ppm.

[0031] Furthermore, the mass ratio of magnesium to lithium in the salt lake brine is 15–35.

[0032] Furthermore, the positively charged nanofiltration membrane is used to separate lithium and magnesium using a cross-flow device.

[0033] The present invention discloses the following technical effects:

[0034] (1) In this invention, porous MXene nanosheets are used as an intermediate layer. The tortuous nanopores of the porous MXene nanosheets can regulate the amine monomer and slow down its release rate, thereby regulating the thickness of the polyamide layer and adjusting the thickness of the polyamide layer to 30-80 nm. Moreover, compared with ordinary MXene nanosheets, porous MXene nanosheets can shorten the interlayer transport path and improve the permeability of the membrane.

[0035] (2) Based on the Donnan balance effect of nanofiltration membrane, the present invention uses 2,3-diaminopiperazine as an aqueous monomer to prepare a novel polyamide nanofiltration membrane. By adjusting the molar ratio of aqueous monomer and oil monomer, a positively charged layer is constructed on the surface of the polyamide nanofiltration membrane, thereby improving the lithium-magnesium separation selectivity of the nanofiltration membrane.

[0036] (3) This invention uses a two-component mixed salt solution with a total concentration of 2000 ppm as the simulated saline solution (Mg). 2+ / Li + The ion separation performance of the positively charged nanofiltration membrane of the present invention was tested at a mass ratio of 20. The Mg content of the positively charged nanofiltration membrane in a mixed salt solution was measured. 2+ The retention rate can reach 98%, Li + The rejection rate was 50%, the lithium-magnesium selectivity was as high as 25%, and the prepared nanofiltration membrane had excellent water flux (54L). -1 ·m -2 ·h -1 With 4 bar, it exhibits excellent comprehensive separation performance, providing insights for the development of novel magnesium-lithium separation nanofiltration membranes.

[0037] (4) The preparation method of the present invention is simple, efficient, low-cost, and highly reproducible, making it suitable for industrial application. The positively charged nanofiltration membrane prepared by the method of the present invention exhibits good separation selectivity and flux when applied to lithium-magnesium separation, making it very suitable for large-scale industrial production and showing good industrial application potential in the field of ion separation. Attached Figure Description

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 This is a schematic diagram of the preparation process of the positively charged nanofiltration membrane of the present invention;

[0040] Figure 2 The images show SEM images of the porous MXene nanosheets, the porous Nylon-66 membrane / porous MXene composite support, and the positively charged nanofiltration membrane in Example 1. (a) is a surface SEM image of the porous MXene nanosheets, (b) is a side SEM image of the porous Nylon-66 membrane / porous MXene composite support, (c) is a horizontal SEM image of the positively charged nanofiltration membrane, and (d) is a side SEM image of the positively charged nanofiltration membrane.

[0041] Figure 3 The graph shows the surface potential data of the nanofiltration membranes in Examples 1-3 and Comparative Examples 1-4. Detailed Implementation

[0042] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0043] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0044] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0045] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This application specification and embodiments are merely exemplary.

[0046] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0047] The porous organic membrane substrate used in the following examples and comparative examples is a porous Nylon-66 membrane with a pore size of 0.22 μm. The porous Nylon-66 membrane is circular in shape with a diameter of 50 mm.

[0048] The porous polyvinylidene fluoride (PVDF) membrane, porous polyethersulfone (PES) membrane, porous sulfonated polyethersulfone (SPES) membrane, porous polysulfone (PSF) membrane, porous sulfonated polysulfone (SPSF) membrane, and porous Nylon-66 membrane with other pore sizes as defined in this invention can all replace the porous Nylon-66 membrane used in the examples without affecting the presentation of the technical effect.

[0049] The MXene nanosheet solutions used in the following examples and comparative examples are Ti3C2T x For nanosheet solutions, other well-known MXene materials in the field, such as Ti2C, Ti2N, and Mo2C, can replace Ti3C2T. x Preparing MXene nanosheet solutions does not affect the presentation of the technical effects.

[0050] All raw materials used in the following examples and comparative examples are commercially available products.

[0051] A schematic diagram of the preparation process of the positively charged nanofiltration membrane in a specific embodiment of the present invention is shown below. Figure 1 As shown.

[0052] Example 1

[0053] A method for preparing a positively charged nanofiltration membrane, comprising the following steps:

[0054] (1) Preparation of MXene nanosheet solution: 2.5g LiF was mixed with 50mL 9M hydrochloric acid solution, and 2g Ti3AlC2 powder was added. The mixture was heated and stirred at 50℃ for 24h (rotation speed of 25rpm). The product generated after the reaction was centrifuged and washed at 6000rpm, and rinsed with deionized water for 5 minutes per cycle until the pH of the supernatant reached 6 (i.e., washing and peeling). Then, the centrifuged and washed product was placed in water and ultrasonically dispersed for 1h under an argon protective atmosphere to obtain Ti3C2T with a concentration of 2mg / mL. x Nanosheet solution.

[0055] (2) Preparation of porous MXene nanosheet solution: Adjust the current and oxygen flow rate of the ozone generator to control the ozone concentration to 20 mg / L. Ozone is used to oxidize and etch the 2 mg / mL MXene nanosheet solution for 1 min through the aeration tube. Add 2 mL of 1 wt% hydrofluoric acid to the 20 mL solution after oxidation and etching to dissolve the oxidation and etching products on the surface of the MXene nanosheets to obtain a mixed solution. Centrifuge and wash the mixed solution at 6000 rpm (rinse with deionized water for 5 minutes per cycle until the pH of the supernatant reaches 6). Then, put the product obtained by centrifugation and washing into water and ultrasonically disperse it for 30 min under argon protection to obtain a porous MXene nanosheet solution with a concentration of 2 mg / mL.

[0056] (3) Preparation of porous Nylon-66 membrane / porous MXene composite support: The porous Nylon-66 membrane was installed on a vacuum filtration device, and 0.15 mL of porous MXene nanosheet solution with a concentration of 2 mg / mL was added. Vacuum filtration was performed (filtration pressure was -2 bar, filtration time was 30 min, and filtration mass was 0.3 mg, that is, the mass of porous MXene nanosheets loaded on the porous Nylon-66 membrane substrate was 0.3 mg through filtration). The porous MXene nanosheets were deposited on the surface of the porous Nylon-66 membrane to form a two-dimensional porous MXene nanosheet membrane layer, and the porous Nylon-66 membrane / porous MXene composite support was obtained.

[0057] (4) Preparation of polyamide layer via interfacial polymerization: 2 mL of an aqueous solution of 2,3-diaminopiperazine (50 mmol / L) was poured onto the surface of the porous Nylon-66 membrane / porous MXene composite support. The solution was allowed to stand for 3 min to fully wet the porous MXene nanosheets. Excess solution was then poured off and the membrane was dried. Next, 2 mL of a hexane solution of 1.67 mmol / L trimesoyl chloride (1.67 mmol / L) was poured onto the porous Nylon-66 membrane / porous MXene composite support. The surface of the on-66 membrane / porous MXene composite support (the molar ratio of 2,3-diaminopiperazine in the aqueous solution and 1,3,5-benzenetricarboxyl chloride in the oil solution is 30:1) is uniformly covered with the porous MXene nanosheet membrane. After standing for 3 minutes, the excess solution on the membrane surface is poured off, and the membrane is allowed to air dry naturally. Then, it is heated and cured at 60°C for 5 minutes to form a polyamide layer, thus obtaining a positively charged nanofiltration membrane modified with porous MXene nanosheets.

[0058] The porous MXene nanosheets obtained after centrifugation in step (2) of this embodiment, the porous Nylon-66 membrane / porous MXene composite support prepared in step (3), and the positively charged nanofiltration membrane, the final product of step (4), were subjected to SEM characterization. The SEM images of the porous MXene nanosheets, the porous Nylon-66 membrane / porous MXene composite support, and the positively charged nanofiltration membrane are shown below. Figure 2 As shown, (a) is a surface SEM image of porous MXene nanosheets, (b) is a side SEM image of the porous Nylon-66 membrane / porous MXene composite support, (c) is a horizontal SEM image of the positively charged nanofiltration membrane, and (d) is a side SEM image of the positively charged nanofiltration membrane. From (a), it can be seen that the surface of the porous MXene nanosheets is smooth and contains a large number of nanopores; from (b), it can be seen that the thickness of the porous MXene nanosheets is 20 nm; from (c), it can be seen that the surface of the positively charged nanofiltration membrane is smooth; and from (d), it can be seen that the thickness of the PA layer of the positively charged nanofiltration membrane is 35 nm.

[0059] Example 2

[0060] Same as Example 1, except that in step (4), the concentration of the aqueous solution is 50 mmol / L and the concentration of the oil solution is 10 mmol / L (the molar ratio of 2,3-diaminopiperazine in the aqueous solution to 1,3,5-benzenetricarboxyl chloride in the oil solution is 5:1). The thickness of the PA layer of the positively charged nanofiltration membrane prepared in this example is 30 nm.

[0061] Example 3

[0062] Same as Example 1, except that in step (4), the concentration of the aqueous solution is 50 mmol / L and the concentration of the oil solution is 0.5 mmol / L (the molar ratio of 2,3-diaminopiperazine in the aqueous solution to 1,3,5-benzenetricarboxyl chloride in the oil solution is 100:1). The thickness of the PA layer of the positively charged nanofiltration membrane prepared in this example is 75 nm.

[0063] Comparative Example 1

[0064] A method for preparing a nanofiltration membrane includes the following steps:

[0065] Two mL of a 50 mmol / L aqueous solution of piperazine (aqueous phase) was poured onto the surface of a porous Nylon-66 membrane substrate. The solution was allowed to stand for 3 minutes to fully wet the substrate. Excess solution was then poured off and the substrate was dried. Next, two mL of a 1.67 mmol / L hexane solution of trimesoyl chloride (oil phase) was poured onto the substrate, ensuring even coverage. The substrate was allowed to stand for 3 minutes, then excess solution was poured off. The membrane was allowed to air dry naturally, and then heated at 60°C for 5 minutes to cure, forming a polyamide layer and obtaining the nanofiltration membrane. The thickness of the PA layer in the nanofiltration membrane prepared in this comparative example was 80 nm.

[0066] Comparative Example 2

[0067] A method for preparing a nanofiltration membrane includes the following steps:

[0068] Two mL of a 50 mmol / L aqueous solution of 1,4-diaminopiperazine (aqueous phase solution) was poured onto the surface of a porous Nylon-66 membrane substrate. The solution was allowed to stand for 3 minutes to fully wet the substrate. Excess solution was then poured off and the substrate was dried. Next, two mL of a 1.67 mmol / L hexane solution of trimesoyl chloride (oil phase solution) was poured onto the substrate, ensuring even coverage. The substrate was allowed to stand for 3 minutes, then excess solution was poured off. The membrane was allowed to air dry naturally and then cured at 60°C for 5 minutes to form a polyamide layer, thus obtaining the nanofiltration membrane. The thickness of the PA layer in the nanofiltration membrane prepared in this comparative example was 78 nm.

[0069] Comparative Example 3

[0070] A method for preparing a nanofiltration membrane includes the following steps:

[0071] Two mL of a 50 mmol / L aqueous solution of 2,3-diaminopiperazine (aqueous phase solution) was poured onto the surface of the Nylon-66 membrane substrate. The solution was allowed to stand for 3 minutes to fully wet the Nylon-66 membrane substrate. Excess solution was then poured off and the substrate was dried. Next, two mL of a 1.67 mmol / L hexane solution of trimesoyl chloride (oil phase solution) was poured onto the Nylon-66 membrane substrate, ensuring even coverage. The solution was allowed to stand for 3 minutes, after which excess solution was poured off, and the membrane was allowed to air dry. Finally, it was heated at 60°C for 5 minutes to cure the polyamide layer, thus obtaining the nanofiltration membrane. The thickness of the PA layer in the nanofiltration membrane prepared in this comparative example was 75 nm.

[0072] Comparative Example 4

[0073] A method for preparing a nanofiltration membrane, comprising the following steps:

[0074] (1) Preparation of MXene nanosheet solution: 2.5g LiF was mixed with 50mL 9M hydrochloric acid solution, and 2g Ti3AlC2 powder was added. The mixture was heated and stirred at 50℃ for 24h (rotation speed of 25rpm). The product generated after the reaction was centrifuged and washed at 6000rpm, and rinsed with deionized water for 5 minutes per cycle until the pH of the supernatant reached 6 (i.e., washing and peeling). Then, the centrifuged and washed product was placed in water and ultrasonically dispersed for 1h under an argon protective atmosphere to obtain Ti3C2T with a concentration of 2mg / mL. x Nanosheet solution.

[0075] (2) Preparation of porous Nylon-66 membrane / MXene composite support: The porous Nylon-66 membrane was installed on a vacuum filtration device, and 0.15 mL of MXene nanosheet solution with a concentration of 2 mg / mL was added. Vacuum filtration was performed (filtration pressure of -2 bar, filtration time of 30 min, and filtration mass of 0.3 mg) to deposit MXene nanosheets on the surface of the porous Nylon-66 membrane, forming a two-dimensional MXene nanosheet membrane layer, and the porous Nylon-66 membrane / MXene composite support was obtained.

[0076] (3) Preparation of polyamide layer via interfacial polymerization: 2 mL of a 50 mmol / L aqueous solution of 2,3-diaminopiperazine (aqueous phase solution) was poured onto the surface of the porous Nylon-66 membrane / MXene composite support. The solution was allowed to stand for 3 min to fully wet the MXene nanosheet membrane. Excess solution was then poured off and the membrane was dried. Next, 2 mL of a 10 mmol / L hexane solution of trimesoyl chloride (oil phase solution) was poured onto the surface of the porous Nylon-66 membrane / MXene composite support, ensuring uniform coverage of the MXene nanosheet membrane. After standing for 3 min, excess solution was poured off, and the membrane was allowed to air dry naturally. Finally, it was heated at 60°C for 5 min to cure, forming a polyamide layer and obtaining the MXene nanosheet-modified nanofiltration membrane. The thickness of the PA layer in the nanofiltration membrane prepared in this comparative example was 50 nm.

[0077] Effect verification

[0078] 1. Surface potential test

[0079] The surface potential of the nanofiltration membranes finally prepared in Examples 1-3 and Comparative Examples 1-4 was tested, and the results are as follows: Figure 3 As shown, by Figure 3 It can be seen that the nanofiltration membrane obtained by using 2,3-diaminopiperazine as an element in an aqueous solution carries a positive charge on its surface.

[0080] 2. Ion separation performance test

[0081] The nanofiltration membranes prepared in Examples 1-3 and Comparative Examples 1-4 were used for lithium-magnesium separation. The separation performance of the nanofiltration membranes was tested using a cross-flow filtration device at a transmembrane pressure of 4 bar and an operating temperature of 25°C. The membrane permeation flux was tested using a 2000 ppm MgCl2 solution, a 2000 ppm LiCl solution, and pure water as feed solutions. A mixed solution of lithium (LiCl) and magnesium (MgCl2) (with MgCl2 concentration of 1857 ppm, LiCl concentration of 143 ppm, and LiCl concentration of 143 ppm) was also tested. + / Mg 2+ The inorganic salt rejection rate and separation factor (S) of the membrane were tested using a mass ratio of 1:20 as the feed solution. Mg，Li The test results are shown in Table 1.

[0082] The membrane permeation flux is calculated using equation (1):

[0083]

[0084] In the formula, J(L·m -2 ·h -1 ·bar -1V(L) is the permeation flux of the membrane, V(L) is the total volume of liquid permeating through the membrane, and A(m) is the permeation flux of the membrane. 2 ) represents the effective filtration area, t(h) represents the filtration time, and P(bar) represents the filtration pressure.

[0085] The inorganic salt rejection rate is calculated using formula (2):

[0086]

[0087] Where R represents the inorganic salt rejection rate, and C0 and C1 represent the conductivity of the feed solution and the permeate solution, respectively.

[0088] The separation factor of nanofiltration membrane for a mixed solution of MgCl2 and LiCl is calculated using equation (3):

[0089]

[0090] In the formula, C Mg,p and C Li,p These represent the Mg in the osmotic solution, respectively. 2+ and Li + The concentration of C Mg,f and C Li,f These represent the Mg content in the feed solution. 2+ and Li + The concentration.

[0091] Table 1

[0092]

[0093] Table 1 shows that using porous MXene nanosheets as the intermediate layer and 2,3-diaminopiperazine as the aqueous monomer to prepare a composite polyamide nanofiltration membrane effectively improves the Li + / Mg 2+ Separation performance.

[0094] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing a positively charged nanofiltration membrane, characterized in that, Includes the following steps: A porous MXene nanosheet layer is deposited on the surface of a porous organic membrane substrate to obtain a porous organic membrane / porous MXene composite support; a polyamide layer is prepared on the surface of the porous organic membrane / porous MXene composite support by interfacial polymerization to obtain the positively charged nanofiltration membrane; the reactive monomers of the interfacial polymerization are 2,3-diaminopiperazine and 1,3,5-benzenetricarboxyl chloride; The deposition of a porous MXene nanosheet layer on the surface of a porous organic membrane substrate includes: depositing porous MXene nanosheets from a porous MXene nanosheet solution onto the surface of a porous organic membrane substrate by means of vacuum filtration; The method for preparing the porous MXene nanosheet solution includes: oxidizing and etching the MXene nanosheet solution with ozone, then dissolving it with acid, washing it by centrifugation, and dispersing it by ultrasonication to obtain the porous MXene nanosheet solution. The concentration of ozone is 10–50 mg / L, and the oxidation etching time is 1–10 min; the acid used for acid dissolution is hydrofluoric acid. The preparation of the polyamide layer on the surface of the porous organic membrane / porous MXene composite carrier by interfacial polymerization includes: first, pouring an aqueous solution onto the surface of the porous organic membrane / porous MXene composite carrier, allowing it to stand, and then drying it; then, pouring an oil phase solution onto the surface of the porous organic membrane / porous MXene composite carrier, allowing it to stand, drying it, and then heating it to cure it; wherein the aqueous solution is an aqueous solution of 2,3-diaminopiperazine; and the oil phase solution is a hexane solution of 1,3,5-benzenetriformyl chloride. The concentration of 2,3-diaminopiperazine in the aqueous phase solution is 50–60 mmol / L; the concentration of 1,3,5-benzenetricarboxyl chloride in the oil phase solution is 0.5–10 mmol / L; and the molar ratio of 2,3-diaminopiperazine in the aqueous phase solution to 1,3,5-benzenetricarboxyl chloride in the oil phase solution is 5–100:

1.

2. The method for preparing a positively charged nanofiltration membrane as described in claim 1, characterized in that, The preparation method of the MXene nanosheet solution includes: mixing lithium fluoride, titanium aluminum carbide and hydrochloric acid solution, heating and reacting, washing and peeling after the reaction, and ultrasonically dispersing to obtain the MXene nanosheet solution.

3. A positively charged nanofiltration membrane prepared by the method according to any one of claims 1-2, characterized in that, The thickness of the polyamide layer is 30–80 nm.

4. The application of a positively charged nanofiltration membrane as described in claim 3 in the separation of lithium and magnesium.

5. The application as described in claim 4, characterized in that, The positively charged nanofiltration membrane is used to separate lithium and magnesium during lithium extraction from salt lake brine.

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