Modified polyamide composite nanofiltration membrane and preparation method thereof

Abstract

The application discloses a modified polyamide composite nanofiltration membrane and a preparation method thereof, and belongs to the technical field of nanofiltration membrane preparation and ion separation. The modified polyamide composite nanofiltration membrane comprises a porous organic membrane, a MXene / modifier composite layer and a polyamide layer arranged in sequence; and the thickness of the polyamide layer is 20-50 nm. The MXene membrane is positively modified by using a modifier, a MXene / modifier composite layer is obtained, and the MXene / modifier composite layer is introduced into interfacial polymerization as an intermediate layer. The amine monomer is regulated by the tortuous nanopore channel of the MXene layer, the releasing speed of the amine monomer is slowed down, and thus an ultrathin PA layer with a Turing structure is obtained.

Description

Technical Field

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

[0002] Lithium, the lightest and most negatively charged metallic element in nature, has extremely wide applications in high-tech industries such as electronics and new energy. Therefore, it plays a significant role in driving global progress, and as a strategic resource, its global demand is soaring. Currently, most lithium compounds are extracted from salt lakes, which typically contain large amounts of magnesium ions. The hydration radii of lithium and magnesium ions are very similar, and their crystallization coefficients are also quite close, making effective separation difficult using methods such as precipitation and crystallization. This significantly increases the difficulty of extracting lithium using traditional methods such as precipitation, ion sieving, liquid-liquid extraction, electrochemical separation, and adsorption.

[0003] Membrane separation technology is highly competitive in the lithium extraction market due to its advantages of low energy consumption and high separation efficiency. Among them, nanofiltration (NF) technology has been developed, which has higher selectivity and lower operating pressure than traditional reverse osmosis (RO) technology.

[0004] Nanofiltration (NF) technology, based on the Donnan effect and size exclusion steric kinetics, can better separate multivalent ions and organic macromolecules from monovalent ions, and can be applied to the separation of magnesium and lithium ions in salt lakes. However, most commercial nanofiltration membranes have negatively charged surfaces, which is detrimental to the separation of metal cations in water. Furthermore, the density of the polyamide (PA) separation layer on the membrane surface significantly affects the permeability of pure water. Therefore, designing nanofiltration membranes with ultrathin PA layers and adjusting their surface charge to improve permeability and separation efficiency is extremely important. Summary of the Invention

[0005] The purpose of this invention is to provide a modified polyamide composite nanofiltration membrane and its preparation method, thereby overcoming the aforementioned shortcomings in the prior art. This invention effectively controls the surface charge of the nanofiltration membrane and reduces the thickness of the polyamide separation layer. The modified polyamide composite nanofiltration membrane prepared by this invention not only has significantly reduced surface negative charge but also possesses an ultrathin Turing-structured polyamide layer.

[0006] To achieve the above objectives, the present invention provides the following solution:

[0007] One of the technical solutions of the present invention is a modified polyamide composite nanofiltration membrane, comprising a porous organic membrane, an MXene / modifier composite layer and a polyamide (PA) layer arranged sequentially; the thickness of the polyamide layer is 20-50 nm.

[0008] The introduction of MXene endows the surface PA layer with a Turing structure (MXene nanosheets have tortuous nanopores; when a porous organic membrane with an MXene / modifier composite layer deposited on its surface is immersed in an aqueous solution, the tortuous nanopores of the MXene nanosheets capture amine monomers and subsequently block their release during immersion in an organic solution, resulting in a slow release of amine monomers. This slow release of amine monomers creates regular wrinkles, thus forming a Turing structure). The Turing structure effectively increases the surface area of ​​the PA layer, increases the contact area with water, and enhances the membrane's permeability.

[0009] The modifier is introduced to positively modify MXene (reduce the negative charge of the material itself). That is, the present invention introduces a positively modified MXene layer (MXene / modifier composite layer) as an intermediate layer between the porous organic membrane and the polyamide layer.

[0010] Furthermore, the polyamide layer has a Turing structure.

[0011] 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 uses a modifier to positively modify the MXene film, obtaining an MXene / modifier composite layer, which is then introduced as an intermediate layer in interfacial polymerization. The tortuous nanopores of the MXene layer regulate the release rate of amine monomers, thus obtaining an ultrathin PA layer with a Turing structure.

[0012] Furthermore, the thickness of the MXene / modifier composite layer is 30–80 nm.

[0013] Furthermore, the porous organic membrane is one of porous polyethersulfone (PES) membrane, porous polysulfone (PSF) membrane, porous polyvinylidene fluoride (PVDF) membrane, porous polyethyleneimine (PEI) membrane, porous polyamide (PA) membrane, porous cellulose acetate (CA) membrane, and porous polyacrylonitrile (PAN) membrane; the modifier in the MXene / modifier composite layer is 3-aminopropyltriethoxysilane (APTES).

[0014] Furthermore, the pore diameter of the porous organic membrane is 0.1–0.45 μm.

[0015] The second technical solution of the present invention: a method for preparing the above-mentioned modified polyamide composite nanofiltration membrane, comprising the following steps: depositing an MXene membrane on the surface of a porous organic membrane, grafting a modifier onto the surface of the MXene membrane to obtain an MXene / modifier composite layer; preparing a polyamide layer on the surface of the MXene / modifier composite layer by interfacial polymerization to obtain the modified polyamide composite nanofiltration membrane.

[0016] Furthermore, the deposition of the MXene membrane on the surface of the porous organic membrane includes: depositing a colloidal dispersion of MXene nanosheets onto the surface of the porous organic membrane by vacuum filtration to obtain the MXene membrane.

[0017] Furthermore, the pressure of the vacuum filtration is -0.5 to -1 bar.

[0018] Furthermore, the concentration of MXene nanosheets in the colloidal dispersion of the MXene nanosheets is 0.01–0.1 mg / mL.

[0019] Furthermore, the colloidal dispersion of the MXene nanosheets is prepared by mixing lithium fluoride (LiF), titanium aluminum carbide (Ti3AlC2) and hydrochloric acid solution, heating and reacting, washing and peeling after the reaction, and ultrasonic dispersion to obtain the colloidal dispersion of the MXene nanosheets.

[0020] Furthermore, the mass-to-volume ratio of lithium fluoride, titanium aluminum carbide, and hydrochloric acid solution is 0.5–3.0 g : 1.0–5.0 g : 30–80 mL; the concentration of the hydrochloric acid solution is 9 M.

[0021] Furthermore, the heating reaction is carried out at a temperature of 25–70°C for a duration of 20–48 hours.

[0022] Furthermore, the heating reaction is carried out under stirring conditions, with the stirring speed being 20–50 r / min.

[0023] Furthermore, before depositing the MXene membrane on the surface of the porous organic membrane, a pretreatment of cleaning the porous organic membrane is also included, specifically: placing the porous organic membrane in water for ultrasonic cleaning to remove surface impurities.

[0024] Furthermore, the water is ultrapure water, the ultrasonic cleaning is performed 1 to 2 times, the duration of each ultrasonic cleaning is 10 to 60 minutes, and the frequency of the ultrasonic cleaning is 20 to 100 kHz.

[0025] Furthermore, the method for grafting the modifier onto the MXene film surface is selected from one of the following three methods:

[0026] Method 1: Coat the surface of the MXene membrane with the modifier solution and let it stand;

[0027] Method 2: Immerse the porous organic membrane with MXene film deposited on its surface in a modifier solution;

[0028] Method 3: The modifier solution is deposited on the surface of the MXene membrane by vacuum filtration. The specific operation is as follows: the porous organic membrane with the MXene membrane deposited on the surface is installed in the vacuum filtration device, the modifier solution is added under normal pressure, it is allowed to stand, and then negative pressure is applied for vacuum filtration.

[0029] The purpose of grafting modifiers onto the surface of MXene membranes is to positively modify the MXene membranes.

[0030] Furthermore, the settling time in Method 1 is 30–60 min; the soaking time in Method 2 is 30–60 min; the settling time in Method 3 is 30–60 min; and the vacuum filtration pressure is -0.5 to -1 bar.

[0031] Furthermore, the concentration of the modifier solution is 50–100 μl / mL.

[0032] Further, the preparation of the polyamide layer on the surface of the MXene / modifier composite layer by interfacial polymerization includes: firstly immersing the porous organic membrane with the MXene / modifier composite layer deposited on its surface in an aqueous phase solution and an oil phase solution in sequence, and then heating it; wherein the aqueous phase solution is an aqueous solution of amine monomers; and the oil phase solution is a hexane solution of 1,3,5-benzenetricarboxylic acid chloride (TMC).

[0033] Further, the amine monomer is one of piperazine (PIP), polyethyleneimine (PEI), m-phenylenediamine (MPD), propylenediamine (DAPE), and hexamethylenediamine (HMD), and the concentration of the amine monomer in the aqueous solution is 0.1–1 wt%; the concentration of 1,3,5-benzenetricarboxyl chloride in the oil solution is 0.05–0.2 wt%.

[0034] Furthermore, the soaking time in the aqueous solution is 3–5 min, and the soaking time in the oil solution is 0.5–1 min.

[0035] Furthermore, after grafting the modifier onto the MXene membrane surface to obtain the MXene / modifier composite layer, the process also includes drying and pre-wetting operations.

[0036] Furthermore, the pre-wetting involves immersing the porous organic membrane with the MXene / modifier composite layer deposited on its surface in ultrapure water for 0.5 to 1 hour.

[0037] Furthermore, the prewetting is performed under vacuum conditions. The purpose is to remove dissolved oxygen from the water to reduce the oxidation of MXene.

[0038] Furthermore, the drying process includes natural ventilation drying, room temperature vacuum drying, or heated vacuum drying, with the heating temperature ranging from 25 to 80°C; the drying time is 20 to 60 minutes. The purpose of drying is to ensure that the MXene nanosheets adhere firmly to the surface of the porous organic membrane without detaching.

[0039] The third technical solution of the present invention: an application of the above-mentioned modified polyamide nanofiltration membrane in the extraction and separation of lithium.

[0040] Furthermore, the modified polyamide nanofiltration membrane is used for the separation of lithium and magnesium during lithium extraction from salt lakes.

[0041] The present invention discloses the following technical effects:

[0042] (1) This invention utilizes the MXene intermediate layer to regulate the diffusion rate of amine monomers in the aqueous phase during interfacial polymerization, thereby preparing an ultrathin PA layer with a Turing structure, which has a thickness of only 20-50 nm.

[0043] (2) This invention positively modifies the surface charge of the MXene interlayer, effectively reducing the inherent negative charge of the material and the negatively charged groups generated by the hydrolysis of excessive acyl chloride groups during interfacial polymerization. In water treatment ion separation systems, cation separation is mainly dominated by the Donnan effect, i.e., electrostatic interaction. When the nanofiltration membrane is negatively charged, it attracts positively charged metal ions, making them difficult to retain. However, by reducing the negative charge through positive modification, or even achieving a positive charge, electrostatic repulsion plays a significant role, effectively blocking the invasion of cations. When the positively modified nanofiltration membrane is used for the separation of magnesium and lithium ions, magnesium ions, with their higher valence state and higher charge energy, are more easily retained and can be effectively separated from lithium ions.

[0044] (3) The preparation method of the present invention is simple, the manufacturing cost is low, and it has extremely broad application prospects. Attached Figure Description

[0045] 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.

[0046] Figure 1 The image shows a side view of the intermediate MXene / APTES composite layer obtained in step (5) of Example 1.

[0047] Figure 2The image shows the SEM image (side view) of the PA layer obtained in step (7) of Example 1.

[0048] Figure 3 The image shows the SEM (horizontal plane) of the Turing structure of the PA layer obtained in step (7) of Example 1.

[0049] Figure 4 The image shows the SEM image (side view) of the PA layer obtained in Comparative Example 1 without an intermediate layer.

[0050] Figure 5 The image shows the SEM image (horizontal plane) of the PA layer obtained in Comparative Example 1 without an intermediate layer.

[0051] Figure 6 The graph shows the surface potential data of the nanofiltration membranes finally prepared in Example 1 (after positive charge modification), Comparative Example 1 (without MXene / APTES composite layer), and Comparative Example 2 (before positive charge modification).

[0052] Figure 7 The graph shows the separation performance data of the nanofiltration membranes finally prepared in Example 1 (after positive charge modification), Comparative Example 1 (without MXene / APTES composite layer), and Comparative Example 2 (before positive charge modification).

[0053] Figure 8 The Li / Mg separation coefficients of the nanofiltration membranes finally prepared in Example 1 (after positive charge modification), Comparative Example 1 (without MXene / APTES composite layer), and Comparative Example 2 (before positive charge modification) are given. Detailed Implementation

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] The porous organic membrane used in the specific embodiments of this invention is a commercially available porous polyethersulfone (PES) membrane with a pore diameter of 0.1 μm and a circular shape with a diameter of 50 mm. Porous polysulfone (PSF) membranes, porous polyvinylidene fluoride (PVDF) membranes, porous polyethyleneimine (PEI) membranes, porous polyamide (PA) membranes, porous cellulose acetate (CA) membranes, porous polyvinyl acrylonitrile (PAN) membranes, and PES membranes with other pore sizes as defined in this invention can all replace the porous PES membrane used in the specific embodiments without affecting the technical effect.

[0060] The MXene nanosheet colloidal dispersion used in the specific embodiments of this invention is Ti3C2T. x For nanosheet colloidal dispersions, other well-known MXene materials in the field, such as Ti2C, Ti2N, and Mo2C, can replace Ti3C2T. x The preparation of MXene nanosheet colloidal dispersions does not affect the presentation of technical effects.

[0061] In the specific embodiments of the present invention, the amine monomers used in the interfacial polymerization process are piperazine (PIP) and ethyleneimine (PEI). The m-phenylenediamine (MPD), propylenediamine (DAPE), and hexamethylenediamine (HMD) specified in the present invention can all replace PIP or PEI used in the specific embodiments without affecting the presentation of the technical effect.

[0062] Example 1

[0063] A method for preparing a modified polyamide nanofiltration membrane (a nanofiltration membrane with an ultrathin PA layer of Turing structure) includes the following steps:

[0064] (1) Preparation of MXene nanosheet colloidal dispersion: 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 30h (30r / min). The resulting solution was repeatedly washed with deionized water by centrifugation until the pH reached 6 (i.e., washing and peeling). The product was then redispersed in water and ultrasonically dispersed in an argon atmosphere for 20min (50kHz) to obtain Ti3C2T with a concentration of 0.1mg / mL. x Nanosheet colloidal dispersion.

[0065] (2) Cleaning pretreatment of porous organic membrane: Soak the porous PES membrane in 100mL of ultrapure water, and then place it in an ultrasonic cleaner at a frequency of 100kHz for 30min, and repeat once.

[0066] (3) The porous PES membrane cleaned in step (2) is installed on a vacuum filtration device, and 10 mL of MXene nanosheet colloidal dispersion with a concentration of 0.1 mg / mL is added under a negative pressure of 1 bar to deposit it on the surface of the porous PES membrane to form a two-dimensional MXene membrane.

[0067] (4) The porous PES membrane with MXene deposited on its surface from step (3) is still installed in the vacuum filtration device. 10 mL of APTES solution with a concentration of 100 μl / mL is added under normal pressure and allowed to stand for 30 min to allow it to react and graft with MXene. Then, the solution on the membrane is removed by vacuum filtration under a negative pressure of 1 bar, and the membrane surface is gently rinsed with ultrapure water to remove unreacted APTES, thus obtaining the MXene / APTES composite layer and completing the positive charge modification.

[0068] (5) The porous PES membrane with the MXene / APTES composite layer deposited on its surface in step (4) was placed in a vacuum drying oven at 60°C for 30 minutes to dry and strengthen it, ensuring that the MXene nanosheets adhered firmly to the PES membrane surface without falling off. After cooling to room temperature, the MXene / APTES composite layer was observed using a scanning electron microscope. The SEM image (side view) is shown below. Figure 1 As shown, by Figure 1 It can be seen that the thickness of the MXene / APTES composite layer is about 40.5 nm.

[0069] (6) The porous PES membrane with the MXene / APTES composite layer deposited on its surface after stabilization in step (5) is pre-wetted by immersing it in 50 mL of ultrapure water for 1 h. The pre-wetting is carried out under vacuum conditions to remove dissolved oxygen in the water and thus reduce the oxidation of MXene.

[0070] (7) After drying the surface moisture of the pre-wetted membrane in step (6), perform interfacial polymerization. The specific steps are as follows: prepare a 0.5 wt% PIP aqueous solution (aqueous phase solution) and a 0.15 wt% TMC hexane solution (oil phase solution). First, soak the membrane in the aqueous phase solution for 5 min to allow the membrane to fully absorb the amine monomer, and remove excess liquid from the surface using a blower. Then, soak the membrane in the oil phase solution for 1 min to allow the acyl chloride groups in the TMC to fully react with the amine groups on the amine monomer, forming a PA film on the surface of the MXene / APTES composite layer. Then, rinse the excess TMC with hexane and place it in an oven at 60°C for 5 min to make the interfacial polymerization reaction more thorough and the bonding between the PA layer and the MXene / APTES composite layer stronger, thus obtaining a modified polyamide nanofiltration membrane. This modified polyamide nanofiltration membrane includes a porous organic membrane, an MXene / APTES composite layer, and a PA layer arranged sequentially. The SEM image of the PA layer is shown below. Figure 2 (side view) and Figure 3 As shown in the (horizontal plane), by Figure 2 It can be seen that the thickness of the PA layer is approximately 26.1 nm; from Figure 3 It can be seen that the PA layer has a Turing structure.

[0071] Example 2

[0072] A method for preparing a modified polyamide nanofiltration membrane (a nanofiltration membrane with an ultrathin PA layer of Turing structure) includes the following steps:

[0073] (1) Preparation of MXene nanosheet colloidal dispersion: 2.5 g LiF was mixed with 50 mL of 9 M hydrochloric acid solution, and 2 g Ti3AlC2 powder was added. The mixture was heated and stirred at 50 °C for 30 h (30 r / min). The resulting solution was repeatedly washed with deionized water by centrifugation until the pH reached 6 (i.e., washing and peeling). The product was then redispersed in water and sonicated in an argon atmosphere for 20 min (50 kHz) to obtain Ti3C2T with a concentration of 0.1 mg / mL. x Nanosheet colloidal dispersion.

[0074] (2) Cleaning pretreatment of porous organic membrane: Soak the porous PES membrane in 100mL of ultrapure water, and then place it in an ultrasonic cleaner at a frequency of 100kHz for 30min, and repeat once.

[0075] (3) The porous PES membrane cleaned in step (2) is installed on a vacuum filtration device, and 10 mL of MXene nanosheet colloidal dispersion with a concentration of 0.1 mg / mL is added under a negative pressure of 1 bar to deposit it on the surface of the porous PES membrane to form a two-dimensional MXene membrane.

[0076] (4) Remove the porous PES membrane with MXene deposited on its surface from the filtration device in step (3) and immerse it in 10 mL of APTES solution with a concentration of 100 μl / mL. Let it stand for 30 min to allow it to react and graft with MXene. After the reaction is complete, pour off the excess solution and rinse the membrane surface slightly with ultrapure water to remove unreacted APTES, thus obtaining the MXene / APTES composite layer and completing the positive charge modification.

[0077] (5) The porous PES membrane with the MXene / APTES composite layer deposited on its surface in step (4) was placed in a vacuum drying oven at 60°C for 30 min to dry and strengthen it, so that the MXene nanosheets could be firmly attached to the surface of the PES membrane without falling off. After cooling to room temperature, the MXene / APTES composite layer was observed using a scanning electron microscope, and it was found that the thickness of the MXene / APTES composite layer was about 34 nm.

[0078] (6) The porous PES membrane with the MXene / APTES composite layer deposited on its surface after stabilization in step (5) is pre-wetted by immersing it in 50 mL of ultrapure water for 1 h. The pre-wetting is carried out under vacuum conditions.

[0079] (7) After drying the surface moisture of the pre-wetted membrane in step (6), perform interfacial polymerization. The specific steps are as follows: prepare a 0.5 wt% PEI aqueous solution (aqueous phase) and a 0.15 wt% TMC hexane solution (oil phase). First, soak the membrane in the PEI aqueous solution for 5 min to allow the membrane to fully absorb the amine monomer, and remove excess liquid from the surface using a blower. Then, soak the membrane in the oil phase solution for 1 min to allow the acyl chloride groups in the TMC to fully react with the amine groups on the amine monomer, forming a PA film on the surface of the MXene / APTES composite layer. Then, rinse the excess TMC with hexane and place it in an oven at 60°C for 10 min to make the interfacial polymerization reaction more thorough and the bonding between the PA layer and the MXene / APTES composite layer stronger, thus obtaining a modified polyamide nanofiltration membrane. This modified polyamide nanofiltration membrane includes a porous organic membrane, an MXene / APTES composite layer, and a PA layer arranged sequentially. SEM observation of the PA layer revealed that the PA layer has a thickness of approximately 27.8 nm and exhibits a Turing structure.

[0080] The only difference between this embodiment and Embodiment 1 is that the method of grafting APTES onto the MXene membrane for positive charge modification in step (4) is different, the monomers contained in the aqueous solution in step (7) are different, and the heating time is different.

[0081] Example 3

[0082] A method for preparing a modified polyamide nanofiltration membrane (a nanofiltration membrane with an ultrathin PA layer of Turing structure) includes the following steps:

[0083] (1) Preparation of MXene nanosheet colloidal dispersion: 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 30h (30r / min). The resulting solution was repeatedly washed with deionized water by centrifugation until the pH reached 6 (i.e., washing and peeling). The product was then redispersed in water and ultrasonically dispersed in an argon atmosphere for 20min (50kHz) to obtain Ti3C2T with a concentration of 0.1mg / mL. x Nanosheet colloidal dispersion.

[0084] (2) Cleaning pretreatment of porous organic membrane: Soak the porous PES membrane in 100mL of ultrapure water, and then place it in an ultrasonic cleaner at a frequency of 100kHz for 30min, and repeat once.

[0085] (3) The porous PES membrane cleaned in step (2) is installed on a vacuum filtration device, and 10 mL of MXene nanosheet colloidal dispersion with a concentration of 0.1 mg / mL is added under a negative pressure of 0.1 bar to deposit it on the surface of the porous PES membrane to form a two-dimensional MXene membrane.

[0086] (4) The porous PES membrane with MXene deposited on its surface from step (3) is still installed in the vacuum filtration device. 10 mL of APTES solution with a concentration of 100 μl / mL is added under normal pressure and allowed to stand for 30 min to allow it to react and graft with MXene. Then, the solution on the membrane is removed by vacuum filtration under a negative pressure of 1 bar, and the membrane surface is gently rinsed with ultrapure water to remove unreacted APTES, thus obtaining the MXene / APTES composite layer and completing the positive charge modification.

[0087] (5) The porous PES membrane with the MXene / APTES composite layer deposited on its surface in step (4) was placed in a vacuum chamber at room temperature for 2 hours to dry and strengthen it, so that the MXene nanosheets could be firmly attached to the surface of the PES membrane without falling off. After drying, the MXene / APTES composite layer was observed using a scanning electron microscope, and it was found that the thickness of the MXene / APTES composite layer was about 42 nm.

[0088] (6) The porous PES membrane with the MXene / APTES composite layer deposited on its surface after stabilization in step (5) is pre-wetted by immersing it in 50 mL of ultrapure water for 1 h. The pre-wetting is carried out under vacuum conditions.

[0089] (7) After the membrane in step (6) is wetted, the surface moisture is dried and the interfacial polymerization reaction is carried out. The specific steps are as follows: Prepare a 0.2 wt% PIP aqueous solution (aqueous phase solution) and a 0.15 wt% TMC hexane solution (oil phase solution). First, soak the membrane in the aqueous phase solution for 5 min to allow the membrane to fully absorb the amine monomer, and remove excess liquid from the surface with a blower. Then, soak the membrane in the oil phase solution for 1 min to allow the acyl chloride groups in TMC to fully react with the amine groups on the amine monomer, forming a PA film on the surface of the MXene / APTES composite layer. Then, rinse the excess TMC with hexane and place it in an oven at 60°C for 5 min to make the interfacial polymerization reaction more thorough and the bonding between the PA layer and the MXene / APTES composite layer stronger, thus obtaining a modified polyamide nanofiltration membrane. The modified polyamide nanofiltration membrane includes a porous organic membrane, an MXene / APTES composite layer and a PA layer arranged sequentially. SEM observation of the PA layer revealed that the PA layer has a thickness of approximately 24.2 nm and exhibits a Turing structure.

[0090] The only difference between this embodiment and Embodiment 1 is that the drying method for the porous PES membrane with the MXene / APTES composite layer deposited on the surface is different in step (5), and the concentration of the aqueous solution is different in step (7).

[0091] Comparative Example 1

[0092] A method for preparing a polyamide nanofiltration membrane, comprising the following steps:

[0093] (1) Cleaning pretreatment of porous organic membrane: Soak the porous PES membrane in 100mL of ultrapure water, and then place it in an ultrasonic cleaner at a frequency of 100kHz for 30min, and repeat once.

[0094] (2) The porous PES membrane cleaned in step (1) is pre-wetted by immersing it in 50 mL of ultrapure water for 1 h. The pre-wetting is carried out under vacuum conditions.

[0095] (3) After drying the surface moisture of the pre-wetted porous PES membrane, interfacial polymerization was carried out. The specific steps were as follows: a 0.5 wt% PIP aqueous solution (aqueous phase solution) and a 0.15 wt% TMC hexane solution (oil phase solution) were prepared. The membrane was first soaked in the aqueous phase solution for 5 min to allow it to fully absorb the amine monomer, and excess liquid on the surface was removed by a blower. Then, the membrane was soaked in the oil phase solution for 1 min to allow the acyl chloride groups in the TMC to fully react with the amine groups on the amine monomer, forming a PA film on the surface of the porous PES membrane. Then, excess TMC was rinsed with hexane and heated in an oven at 60°C for 5 min to make the interfacial polymerization reaction more thorough and the bonding between the PA layer and the porous PES membrane stronger, resulting in a polyamide nanofiltration membrane. This polyamide nanofiltration membrane consists of a porous organic membrane and a PA layer arranged sequentially. The PA layer was observed by SEM, and its SEM image is shown below. Figure 4 (side view) and Figure 5 As shown in the (horizontal plane), by Figure 4 It can be seen that the thickness of the PA layer is approximately 300 nm; from Figure 5 It can be seen that the PA layer does not have a Turing structure.

[0096] The difference between this comparative example and Example 1 is that the introduction of MXene and the modification of MXene are omitted, that is, the introduction of the positively modified MXene intermediate layer is omitted.

[0097] Comparative Example 2

[0098] A method for preparing a polyamide nanofiltration membrane, comprising the following steps:

[0099] (1) Preparation of MXene nanosheet colloidal dispersion: 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 30h (30r / min). The resulting solution was repeatedly washed with deionized water by centrifugation until the pH reached 6 (i.e., washing and peeling). The product was then redispersed in water and ultrasonically dispersed in an argon atmosphere for 20min (50kHz) to obtain Ti3C2T with a concentration of 0.1mg / mL. x Nanosheet colloidal dispersion.

[0100] (2) Cleaning pretreatment of porous organic membrane: Soak the porous PES membrane in 100mL of ultrapure water, and then place it in an ultrasonic cleaner at a frequency of 100kHz for 30min, and repeat once.

[0101] (3) The porous PES membrane cleaned in step (2) is installed on a vacuum filtration device, and 10 mL of MXene nanosheet colloidal dispersion with a concentration of 0.1 mg / mL is added under a negative pressure of 0.1 bar to deposit it on the surface of the porous PES membrane to form a two-dimensional MXene membrane.

[0102] (4) The porous PES membrane with MXene film deposited on its surface in step (3) was placed in a vacuum drying oven at 60°C for 30 min to dry and strengthen it, so that the MXene nanosheets could be firmly attached to the surface of the PES membrane without falling off. After cooling to room temperature, the MXene layer was observed using a scanning electron microscope, and it was found that the thickness of the MXene layer was about 37 nm.

[0103] (5) The porous PES membrane with MXene film deposited on its surface after stabilization in step (4) is pre-wetted by immersing it in 50 mL of ultrapure water for 1 h. The pre-wetting is carried out under vacuum conditions.

[0104] (6) After drying the surface moisture of the pre-wetted membrane in step (5), perform interfacial polymerization. The specific steps are as follows: prepare a 0.5 wt% PIP aqueous solution (aqueous phase solution) and a 0.15 wt% TMC hexane solution (oil phase solution). First, soak the membrane in the aqueous phase solution for 5 min to allow the membrane to fully absorb the amine monomer, and remove excess liquid from the surface using a blower. Then, soak the membrane in the oil phase solution for 1 min to allow the acyl chloride groups in the TMC to fully react with the amine groups on the amine monomer, forming a PA film on the MXene membrane surface. Then, rinse the excess TMC with hexane and place it in a 60°C oven for 5 min to make the interfacial polymerization reaction more thorough and the bonding between the PA layer and the MXene layer stronger, thus obtaining a polyamide nanofiltration membrane. This polyamide nanofiltration membrane includes a porous organic membrane, an MXene layer, and a PA layer arranged sequentially. SEM observation of the PA layer revealed that the thickness of the PA layer is about 29.1 nm, and the PA layer has a Turing structure.

[0105] The only difference between this comparative example and Example 1 is that the step of grafting APTES onto the surface of the MXene film for positive electrical modification is omitted.

[0106] Effect verification

[0107] 1. Surface potential test

[0108] The surface potential of the nanofiltration membranes finally prepared in Example 1 (with positively charged modification), Comparative Example 1 (without MXene / APTES composite layer), and Comparative Example 2 (without positively charged modification) was tested, and the results are as follows: Figure 6 As shown, by Figure 6 It can be seen that grafting MXene with a modifier significantly reduces the negative charge on the membrane surface.

[0109] 2. Ion separation performance test

[0110] Use a filter area of ​​3.14 cm² 2 The cross-flow filtration equipment was used to measure the NF performance of the membranes at 5 bar at room temperature. Before testing, all membranes were compressed at 5 bar for at least 30 minutes to reach a stable state before testing.

[0111] The membrane permeation flux is calculated using equation (1):

[0112]

[0113] In the formula, J(L·m -2 ·h -1 ·bar -1 V(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.

[0114] The inorganic salt rejection rate is calculated using formula (2):

[0115]

[0116] Where R represents the inorganic salt rejection rate, and C0 and C1 represent the conductivity of the feed solution and the permeate solution, respectively.

[0117] A 1000 ppm MgCl2 solution was prepared and used as the feed solution to test the permeation flux and inorganic salt rejection rate.

[0118] The separation performance test results of Example 1, Comparative Example 1, and Comparative Example 2 are as follows: Figure 7 As shown, by Figure 7 It can be seen that the permeation fluxes of Example 1, Comparative Example 1, and Comparative Example 2 are 25.8, 15.2, and 22.9 L·m, respectively. -2 ·h -1 ·bar -1 The MgCl2 rejection rates were 92%, 40%, and 76%, respectively, indicating that the nanofiltration membrane with the introduction of the MXene / APTES composite layer effectively improved the membrane's permeation flux and ion separation performance.

[0119] In addition, a mixed solution of lithium (LiCl) and magnesium (MgCl2) was prepared (with a concentration of 1856.5 ppm for MgCl2 and 143.5 ppm for LiCl) and the separation coefficient was tested.

[0120] The separation coefficient of the nanofiltration membrane for the mixed solution of MgCl2 and LiCl is calculated using equation (3):

[0121]

[0122] 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.

[0123] The Li / Mg separation coefficients of Example 1, Comparative Example 1, and Comparative Example 2 are as follows: Figure 8 As shown, by Figure 8 It can be seen that the Li / Mg separation coefficients of Example 1, Comparative Example 1 and Comparative Example 2 are 20, 3.2 and 6, respectively. The nanofiltration membrane after introducing the MXene / APTES composite layer effectively improved the Li / Mg separation performance of the membrane.

[0124] 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 modified polyamide composite nanofiltration membrane, characterized in that, It comprises a porous organic membrane, an MXene / modifier composite layer, and a polyamide layer arranged sequentially; the thickness of the polyamide layer is 20–50 nm. The modifier in the MXene / modifier composite layer is 3-aminopropyltriethoxysilane; The polyamide layer has a Turing structure; The preparation steps of the modified polyamide composite nanofiltration membrane include: depositing an MXene membrane on the surface of a porous organic membrane, grafting a modifier onto the surface of the MXene membrane to obtain an MXene / modifier composite layer; and preparing a polyamide layer on the surface of the MXene / modifier composite layer by interfacial polymerization to obtain the modified polyamide composite nanofiltration membrane. The method for grafting the modifier onto the MXene film surface is selected from one of the following three methods: Method 1: Coat the surface of the MXene membrane with the modifier solution and let it stand; Method 2: Immerse the porous organic membrane with MXene film deposited on its surface in a modifier solution; Method 3: The modifier solution is deposited on the surface of the MXene membrane by vacuum filtration.

2. The modified polyamide composite nanofiltration membrane as described in claim 1, characterized in that, The porous organic membrane is one of the following: porous polyethersulfone membrane, porous polysulfone membrane, porous polyvinylidene fluoride membrane, porous polyethyleneimine membrane, porous polyamide membrane, porous cellulose acetate membrane, and porous polyacrylonitrile membrane.

3. The modified polyamide composite nanofiltration membrane as described in claim 1, characterized in that, The deposition of MXene membrane on the surface of porous organic membrane includes: depositing a colloidal dispersion of MXene nanosheets onto the surface of porous organic membrane by vacuum filtration to obtain an MXene membrane.

4. The modified polyamide composite nanofiltration membrane as described in claim 3, characterized in that, The colloidal dispersion of the MXene nanosheets is prepared by mixing lithium fluoride, titanium aluminum carbide and hydrochloric acid solution, heating and reacting, washing and peeling after the reaction, and ultrasonic dispersion to obtain the colloidal dispersion of the MXene nanosheets.

5. The modified polyamide composite nanofiltration membrane as described in claim 3, characterized in that, The preparation of a polyamide layer on the surface of the MXene / modifier composite layer by interfacial polymerization includes: firstly, immersing a porous organic membrane with the MXene / modifier composite layer deposited on its surface in an aqueous solution and an oil solution in sequence, and then heating it; wherein the aqueous solution is an aqueous solution of an amine monomer; and the oil solution is a hexane solution of 1,3,5-benzenetricarboxylic acid chloride.

6. The modified polyamide composite nanofiltration membrane as described in claim 5, characterized in that, The amine monomer is one of piperazine, polyethyleneimine, m-phenylenediamine, propylenediamine, and hexamethylenediamine, and the concentration of the amine monomer in the aqueous solution is 0.1-1 wt%; the concentration of 1,3,5-benzenetricarboxyl chloride in the oil solution is 0.05-0.2 wt%.

7. The modified polyamide composite nanofiltration membrane as described in claim 3, characterized in that, After grafting the modifier onto the MXene membrane surface to obtain the MXene / modifier composite layer, the process also includes drying and pre-wetting treatment.

8. The application of a modified polyamide composite nanofiltration membrane as described in any one of claims 1-7 in the extraction and separation of lithium.

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