A double-crosslinking modified MXene-based self-supporting composite membrane and a preparation method and application thereof

Abstract

The present application relates to the technical field of water treatment materials, and particularly relates to a double-crosslinking modified MXene-based self-supporting composite membrane and a preparation method and application thereof.The present application comprises the following steps: (1) preparation of functionalized modified nanosheets: dissolving an amino-containing compound in a buffer solution to form a dispersion liquid, adding MXene nanosheets to the dispersion liquid to react, and obtaining functionalized modified MXene nanosheets; (2) preparation of a composite membrane: mixing the dispersion liquid of the functionalized modified MXene nanosheets with a dispersion liquid of a hydrophilic polymer compound, taking a porous polymer filter membrane as a substrate, and performing vacuum filtration to form a membrane, thereby obtaining a double-crosslinking modified MXene-based self-supporting composite membrane.The present application provides a MXene-based self-supporting composite membrane which has superhydrophilicity, high water flux, excellent dye interception capacity and good recycling stability, and can effectively treat dye-containing organic wastewater.

Description

Technical Field

[0001] This invention relates to the field of water treatment materials technology, specifically to a double-crosslinked modified MXene-based self-supporting composite membrane, its preparation method, and its application. Background Technology

[0002] Water pollution is one of the major challenges facing the world today. With the rapid development of industries such as textiles, dyeing, papermaking, and pharmaceuticals, large amounts of organic wastewater containing dyes are directly discharged into natural water bodies or used for irrigation. Dye wastewater is characterized by its complex composition, high biotoxicity, and difficulty in degradation. Even at extremely low concentrations, it can cause serious damage to aquatic ecosystems and pose a potential threat to human health through bioaccumulation in the food chain. Therefore, developing efficient and low-cost dye wastewater treatment technologies is of great significance.

[0003] Currently, the main methods for treating dye wastewater include coagulation sedimentation, biodegradation, advanced oxidation processes, and membrane separation. Among these, coagulation sedimentation is simple to operate but has limited flocculant efficiency and presents sludge disposal issues; biodegradation is green and low-energy, but has poor adaptability to toxic dyes and a long reaction cycle; advanced oxidation processes have high degradation efficiency, but suffer from high energy consumption, high operating costs, and the need for strict control of toxic byproducts generated during the reaction. In contrast, membrane separation technology has become a research hotspot in the field of dye wastewater treatment due to its high treatment efficiency, continuous and stable operation, small footprint, ease of integration and scale-up, and potential for resource recovery and water recycling.

[0004] However, existing membrane materials generally suffer from problems such as difficulty in balancing water flux and retention rate, severe membrane fouling, and short service life, which restrict their industrial application. In particular, although pure MXene membranes have a unique layered structure and excellent mechanical strength, the tight stacking of their nanosheets results in extremely low water flux, and their strong hydrophobicity makes them susceptible to membrane fouling, making them difficult to meet practical engineering requirements. Therefore, developing novel composite membrane materials that combine high flux, high retention rate, and excellent antifouling properties is of great significance for promoting the practical application of membrane technology in dye wastewater treatment. In view of this, this invention provides a double-crosslinked modified MXene-based self-supporting composite membrane, its preparation method, and its application. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a double-crosslinked modified MXene-based self-supporting composite membrane, its preparation method, and its application. The aim is to provide an MXene-based self-supporting composite membrane that possesses superhydrophilicity, high water flux, excellent dye retention capacity, and good recycling stability, and can effectively treat dye-containing organic wastewater.

[0006] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: In a first aspect, a method for preparing a double-crosslinked modified MXene-based self-supporting composite membrane includes the following steps: (1) Preparation of functionalized modified nanosheets: An amino-containing compound is dissolved in a buffer solution with a pH of 7.5 to 9.5 to form a dispersion. MXene nanosheets are added to the dispersion to react and obtain functionalized modified MXene nanosheets. (2) Preparation of composite membrane: The dispersion of the functionalized modified MXene nanosheets is mixed with the dispersion of the hydrophilic polymer compound, and then transferred to a vacuum filtration device. The porous polymer filter membrane is used as the substrate for vacuum filtration to form a membrane. After drying, a double crosslinked modified MXene-based self-supporting composite membrane is obtained.

[0007] Based on the above technical solution, the present invention can be further improved as follows.

[0008] Furthermore, the amino-containing compound mentioned in step (1) includes at least one of dopamine hydrochloride, polyethyleneimine (PEI), norepinephrine, and serotonin; The buffer solution mentioned in step (1) includes Tris-HCl buffer, carbonate buffer, phosphate buffer, etc.; wherein, the concentration of Tris-HCl buffer is 5~20 mM and the pH is 8.0~9.0.

[0009] The mass ratio of the amino-containing compound to the MXene nanosheets in step (1) is 1:1 to 1:10; In step (1), the final concentration of MXene nanosheets in the buffer solution is 1.0~10.0 mg / mL; The reaction conditions in step (1) are: room temperature (generally 20~25℃), time of 6~48 h, and further time of 18~36 h, and stirring speed of 300~800 rpm; After the reaction in step (1) is completed, centrifuge at 5000~10000 rpm for 10~20 min, then wash with deionized water and anhydrous ethanol alternately 2~5 times, and redisperse the precipitate in deionized water and store it in a sealed container at 2~8°C.

[0010] Furthermore, the MXene nanosheets in step (1) are prepared through the following steps: Fluoride salts were dissolved in an acidic etching solution with a concentration of 6-12 M. MAX phase ceramic powder was added to the acidic etching solution for etching. After the reaction was completed, the supernatant was centrifuged and washed until the pH of the supernatant was 5-7. The multilayer product in the supernatant was ultrasonically exfoliated for 1-8 h. The precipitate was collected, dried, and then freeze-dried under vacuum for 24-72 h to obtain MXene nanosheets.

[0011] Furthermore, the fluoride salt includes at least one of LiF, NaF, and KF; The etching reaction conditions are: temperature of 30~70°C and time of 12~72 h; further, the stirring reaction temperature is 45~55°C and the reaction time is 36~60 h. The MAX phase ceramic includes at least one of Ti3AlC2, Ti2AlC, V2AlC, and Nb2AlC; The ratio of the MAX phase ceramic to the fluoride salt is 1.0~2.5:1.5~3.5; The acidic substances in the acidic etching solution include HCl, HNO3, H2SO4, etc., and further, the concentration of the acidic substances in the acidic etching solution is 8~10 M.

[0012] Furthermore, the hydrophilic polymer compound includes at least one of polyethylene glycol, polyacrylic acid, polyethyleneimine, and polydopamine; The mass ratio of the functionalized modified nanosheets to the hydrophilic polymer compound is 20:1~10.

[0013] Furthermore, the concentration of the dispersion of the hydrophilic polymer compound is 5~20 mg / mL; The concentration of the dispersion of the functionalized modified MXene nanosheets is 1.0~10.0 mg / mL; The mass ratio of the dispersion of the functionalized modified MXene nanosheets to the dispersion of the hydrophilic polymer compound is 10~30:0.5~15.

[0014] Furthermore, the porous polymer filter membrane includes at least one of polyvinylidene fluoride (PVDF) filter membrane, polyethersulfone (PES) membrane, and polytetrafluoroethylene (PTFE) membrane, wherein the pore size of the porous polymer filter membrane is 0.1~0.45μm and the diameter of the porous polymer filter membrane is 40~60 mm; The conditions for vacuum filtration film formation are: vacuum degree of 0.05~0.2 MPa; The drying conditions are: natural drying at room temperature for 12~36 hours.

[0015] Secondly, a double-crosslinked modified MXene-based self-supporting composite membrane is provided, wherein the MXene-based self-supporting composite membrane is prepared by the aforementioned preparation method; the water contact angle of the MXene-based self-supporting composite membrane is less than 20°, and the pure water flux is 1500~3000 L·m⁻¹. －2 ·h －1 ·bar －1 It has a rejection rate of more than 90% for organic dyes with a molecular weight greater than 300 Da.

[0016] Thirdly, the application of a double-crosslinked modified MXene-based self-supporting composite membrane, wherein the double-crosslinked modified MXene-based self-supporting composite membrane is used in the treatment of dye-containing organic wastewater.

[0017] Furthermore, the dye includes at least one of Congo Red, Crystal Violet, Methylene Blue, Chrome Black T, and Rhodamine B.

[0018] This invention uses a porous polymer filter membrane as a support layer and employs a vacuum-assisted self-assembly method to orderly stack and self-assemble MXene nanosheets modified with amino-containing compounds in situ on the substrate surface with hydrophilic polymer compounds to form a dense functional composite layer.

[0019] The beneficial effects of this invention are: (1) The MXene-based self-supporting composite membrane of the present invention has strong superhydrophilicity, with a water contact angle of less than 20°, effectively resisting membrane fouling; it also has high water flux, with a pure water flux of 1500~3000 L·m －2 ·h －2 ·bar －1 It significantly improves upon the performance of pure MXene membranes; it exhibits excellent dye rejection performance, with a rejection rate of over 99% for dyes such as Congo Red and Crystal Violet; it demonstrates good cycle stability, maintaining stable flux after 10 consecutive filtration cycles.

[0020] (2) Compared with the prior art, the MXene-based self-supporting composite membrane of the present invention has a simple preparation process, mild conditions, low cost, and is easy to scale up. Through functional modification and intercalation of hydrophilic polymer compounds, it effectively solves the technical bottleneck that traditional two-dimensional material membranes have difficulty in balancing water flux and retention rate. Attached Figure Description

[0021] Figure 1 This document presents a schematic diagram of the preparation process and morphological characterization of the double-crosslinked modified high-performance MXene-based self-supporting composite membrane in Example 1. (a) is a schematic diagram of the composite membrane preparation process; (b) is a photograph of the pure MXene membrane; (c) is a photograph of the TPM membrane; (d) is a photograph of the TPMG-1 membrane; (e) is a SEM image of the MXene membrane; (f) is a SEM image of the TPM membrane; (g) is a SEM image of the TPMG-1 membrane; and (h) is an EDS elemental distribution diagram of the TPMG-1 membrane. Figure 2 The structural characterization spectra of the composite membrane are shown; (a) is the XRD pattern of MXene, TPM, TPMG-1, TPMG-5 and TPMG-10, and (b) is the corresponding FTIR pattern. Figure 3A bar chart comparing the pure water permeation flux of MXene membrane, TPM membrane, TPMG-1 membrane, TPMG-5 membrane and TPMG-10 membrane; Figure 4 This is a comprehensive comparison chart of the filtration performance of TPMG-5 composite membrane for five dyes: CR, CV, MB, RhB, and EBT. The main chart shows the water permeation flux, and the inset in the upper left corner shows the rejection rate. Figure 5 A bar chart comparing the zeta potentials of MXene membrane, TPM membrane, TPMG-1 membrane, TPMG-5 membrane and TPMG-10 membrane; Figure 6 Comparison of water contact angles for MXene membrane, TPM membrane, TPMG-1 membrane, TPMG-5 membrane and TPMG-10 membrane; Figure 7 The images show the morphology of the TPEG-5 composite film as determined by transmission electron microscopy (TEM). Among them, (a) is a TEM image of the PEG microspheres; (b) is a TEM image of the MXene sheets; (c) is a TEM image of the composite interface; and (d) is a high-resolution TEM (HRTEM) image. Detailed Implementation

[0022] The principles and features of this invention are described below. The examples given are for illustrative purposes only and are not intended to limit the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they should be performed according to the techniques or conditions described in the literature in this field, or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased through legitimate channels.

[0023] Example 1 This embodiment provides a method for preparing a double-crosslinked modified MXene-based self-supporting composite membrane, comprising the following steps: Step 1: Dissolve 2.4 g of LiF completely in a certain amount of 9 M hydrochloric acid. Add Ti3AlC2 powder in three portions, 0.5 g of Ti3AlC2 each time. After complete addition, stir in a 50℃ water bath for 48 h. After the reaction is complete, wash the precipitate 2-3 times with 3 M HCl, centrifuge at 3500 rpm for 3 min, retain the lower precipitate, and continue until the upper liquid phase is clear. Wash several times with deionized water until the pH of the supernatant is 6. Then, collect the obtained multilayer Ti3C2T x After sonicating in an ice bath for 4 hours, centrifuge and collect the lower precipitate, then freeze-dry under vacuum for 48 hours to obtain MXene powder. Step 2: First, prepare the Tris-HCl buffer. Weigh a certain amount of Tris and dissolve it in deionized water. Adjust the pH to 8.5 with 1 M HCl to prepare a 10 mM Tris-HCl buffer solution with pH=8.5. Add a certain amount of dopamine hydrochloride to the above Tris-HCl buffer solution and stir at room temperature to form a dispersion. Under room temperature and continuous magnetic stirring (500 rpm), slowly add MXene powder to the dispersion to a final concentration of 4.0 mg / mL. Continue stirring the reaction system for 24 hours. During this process, dissolved oxygen catalyzes the oxidative self-polymerization of dopamine under alkaline conditions. The generated polydopamine (PDA) is deposited in situ and firmly coated on the surface of MXene nanosheets through covalent / non-covalent interactions. After the reaction, centrifuge the product at 7500 rpm for 15 minutes and wash it three times alternately with deionized water and anhydrous ethanol to remove unreacted monomers and free PDA polymers. Finally, the obtained PDA@MXene precipitate was redispersed in deionized water and stored in a sealed container at 4°C for later use.

[0024] Step 3: The above PDA@MXene dispersion was mixed with pre-dissolved PEG aqueous solution (10 mg / mL) at a mass ratio of 20:0 (i.e., pure PDA@MXene, control group TPM). The mixture was ultrasonically stirred for 1 hour. Then, a membrane was formed using vacuum-assisted self-assembly technology. The mixture was transferred to a vacuum filtration apparatus, and filtration was performed using a PVDF filter membrane with a pore size of 0.22 μm and a diameter of 50 mm as the substrate, under a vacuum of 0.1 MPa. During filtration, the nanomaterials were orderly stacked and self-assembled on the substrate surface, forming a dense wet membrane. After filtration, the substrate with the wet membrane was carefully transferred to a clean petri dish and allowed to air dry at room temperature for 24 hours. Once the membrane was completely dry, a self-supporting composite membrane was obtained.

[0025] Figure 1 This is a schematic diagram of the preparation process and morphology characterization diagram of the composite membrane in this embodiment; Figure 1 (a) is a schematic diagram of the preparation process of the composite membrane in this embodiment, which fully demonstrates the complete preparation path from the MAX phase precursor, to the removal of the aluminum atom layer by LiF / HCl etching, to the MXene nanosheets obtained by ice bath ultrasonic exfoliation, to the PDA@MXene functionalized nanosheets formed by dopamine alkaline oxidation self-polymerization, and finally to the self-assembly on the PVDF substrate by vacuum filtration after mixing with a hydrophilic polymer compound. Figure 1 (b), (c) and (d) are photographs of the pure MXene membrane, TPM membrane and TPMG-1 membrane, respectively. All three membranes are black discs with smooth and dense surfaces and no obvious defects. Figure 1Images (e), (f), and (g) are corresponding scanning electron microscope (SEM) images, with a scale bar of 500 nm. The pure MXene film exhibits a typical multilayer sheet-like tightly stacked morphology with narrow interlayer gaps. After being coated with PDA, the surface of the TPM film nanosheets is significantly roughened, and fine particles can be observed uniformly attached to the sheet surface, with some gap expansion between the layers. In the TPMG-1 film, a large number of spherical PEG particles with a diameter of approximately 200–500 nm are embedded between the sheet layers, and the interlayer channels are significantly enlarged. Figure 1 Image (h) shows the EDS elemental distribution of the TPMG-1 film. C, Ti, O, and N elements are uniformly distributed on the film surface. The N element signal is specific to the amino and indole ring nitrogen atoms in the PDA. This uniform distribution directly proves that the PDA is uniformly coated on the MXene nanosheets. No significant phase separation of the four elements is observed, verifying the uniformity of the spatial distribution of each component in the composite film. After etching the MXene nanosheets using a LiF / HCl system, the Al atomic layer in Ti3AlC2 was selectively removed, forming a multilayer Ti3C2T with abundant surface end groups. x Structure. Under alkaline conditions, the catechol groups in the dopamine monomer undergo self-polymerization driven by dissolved oxygen oxidation to generate PDA with strong adhesion. Its catechol groups form a stable chemical bond with the Ti-OH end groups on the MXene surface through hydrogen bonds and coordination bonds, while the amino groups undergo electrostatic adsorption with the negative potential surface of MXene. This dual binding mechanism realizes the efficient in-situ functionalization of the nanosheet surface.

[0026] Example 2 The difference between this embodiment and embodiment 1 is that in step 3), the mass ratio of PDA@MXene to PEG aqueous solution is 20:1 (i.e., TPMG-1 formulation). The remaining steps and parameters are the same as in embodiment 1, and a TPMG-1 composite membrane is obtained.

[0027] Figure 2 (a) shows the XRD pattern. The (002) diffraction peak of TPMG-1 appears at approximately 6° 2θ, indicating that the layered crystal structure of MXene remains intact after PDA coating and PEG introduction; the (002) peak shifts slightly towards the lower angle direction, indicating that the interlayer spacing is expanded by the insertion of a small amount of PEG, and the interlayer channels are initially regulated. Figure 2 (b) shows the FTIR spectrum, with TPMG-1 in the range of 3200–3500 cm⁻¹. －1 The O-H stretching vibration peak at 1400–1600 cm⁻¹ is significantly broadened and enhanced, indicating that PDA coating introduces abundant hydrophilic functional groups; －1The presence of characteristic absorptions attributable to PDA benzene ring skeletal vibrations and C-N stretching vibrations confirms the successful coating of PDA on the MXene surface. The absence of additional characteristic peaks for PEG indicates that PEG exists in the interlayer space via physical intercalation, without altering the membrane's main chemical composition. XRD results confirm that a small amount of PEG intercalation slightly widens the interlayer spacing of TPMG-1, providing wider mass transfer channels for water molecules. FTIR results confirm that the phenolic hydroxyl and amino groups introduced by PDA coating are the chemical basis for the significant improvement in membrane hydrophilicity. These two characterizations together validate the successful implementation of PDA coating and PEG intercalation in TPMG-1.

[0028] Example 3 The difference between this embodiment and embodiment 1 is that in step 3), the mass ratio of PDA@MXene to PEG aqueous solution is 20:5 (i.e., TPMG-5 formulation). The remaining steps and parameters are the same as in embodiment 1, and a TPMG-5 composite membrane is obtained.

[0029] Example 4 The difference between this embodiment and embodiment 1 is that in step 3), the mass ratio of PDA@MXene to PEG aqueous solution is 20:10 (i.e., TPMG-10 formulation). The remaining steps and parameters are the same as in embodiment 1, and a TPMG-10 composite membrane is obtained.

[0030] Example 5 This embodiment provides a method for preparing a PEI@MXene / PEG composite membrane (TPEG-5) by replacing dopamine hydrochloride with polyethyleneimine (PEI), achieving a water flux of up to 2600 L·m -2 ·h -1 ·bar -1 Among the TPMG series, the TPMG-5 composite membrane exhibited the best performance. When used to filter five dye solutions (CR, CV, MB, RhB, and EBT) at a concentration of 20 mg / L, the TPMG-5 membrane showed retention rates exceeding 99% for CR and CV, approximately 98% for MB, approximately 94% for EBT, and approximately 88% for RhB, demonstrating broad applicability to a variety of dyes. This also verifies the universality of the technical route of this invention for other amino-containing hydrophilic polymers. The preparation method of the PEI@MXene / PEG composite membrane includes the following steps: Step 1 is the same as in Example 1, and MXene powder is obtained.

[0031] Step 2: Branched polyethyleneimine (bPEI) was dissolved in 10 mM Tris-HCl buffer (pH 8.5) to prepare a PEI solution with a concentration of 2.0 mg / mL. MXene powder was added to bring the final concentration to 4.0 mg / mL, and the reaction was carried out with magnetic stirring at room temperature for 12 h. The abundant primary, secondary, and tertiary amine groups on the PEI molecular chain formed a strong coating layer with the -OH and -F end groups on the MXene surface through hydrogen bonding and electrostatic interactions. After the reaction was completed, the mixture was centrifuged at 7500 rpm for 15 min, washed three times with deionized water, and the resulting PEI@MXene precipitate was redispersed in deionized water and stored at 4 °C for later use.

[0032] Step 3: Mix PEI@MXene dispersion with 10 mg / mL PEG aqueous solution at a mass ratio of 20:5, ultrasonically stir for 1 h, and then filter the mixture under vacuum of 0.1 MPa onto a PVDF substrate with a pore size of 0.22 μm and a diameter of 50 mm to form a membrane. Dry at room temperature for 24 h to obtain TPEG-5 composite membrane.

[0033] Figure 7 Transmission electron microscopy (TEM) morphology characterization of the TPEG-5 composite film; Figure 7 In the middle (a), a TEM image of PEG microspheres shows that spherical particles with a diameter of about 200~500 nm are densely aggregated. The microspheres have smooth surfaces and uniform morphology, indicating that PEG exists in the composite film as an independent spherical microphase. Figure 7 In (b), it can be clearly observed that MXene exhibits a typical two-dimensional lamellar morphology with translucent edges; Figure 7 In (c), the composite interface between PEG spherical microspheres and MXene nanosheets can be observed simultaneously. The PEG microspheres are tightly attached to the surface and interlayer region of the MXene sheets. The two phases are tightly bonded at the interface, and no obvious detachment or separation phenomenon is observed. Figure 7 (d) is a high-resolution TEM (HRTEM) image, which clearly shows the lattice fringes of MXene nanosheets. The measured interlayer spacing is about 1.517 nm, which is significantly larger than the theoretical interlayer spacing of pure MXene, confirming that PEI coating and PEG intercalation together achieve effective control of the interlayer spacing.

[0034] TEM characterization of the TPEG-5 membrane provided a direct, multi-scale view of the microstructure assembly mechanism of the composite membrane. PEI, through its abundant primary, secondary, and tertiary amine groups, forms multi-point hydrogen bonds and electrostatic anchors with the -OH and -F end groups on the MXene surface, creating a functionalized coating layer on the nanosheet surface. This endows the nanosheets with hydrophilicity and positive charge properties, confirming the synergistic structural regulation effect of the PEI / PEG dual-component system in opening interlayer channels. This provides solid microstructural evidence for the composite membrane's ability to achieve both high permeation flux and high rejection rate.

[0035] Comparative Example 1 This comparative example provides a pure MXene membrane. The difference from Example 1 is that in step 3), only the MXene dispersion is directly filtered to form a membrane without PDA modification or the addition of PEG. The remaining steps and parameters are the same as in step 1) of Example 1, resulting in a pure MXene membrane.

[0036] Comparative Example 2 This comparative example provides a pure PDA@MXene (TPM) membrane. The difference from Example 1 is that in step 3), only PDA@MXene is used to directly filter and form a membrane without adding PEG. The remaining steps and parameters are the same as in step 1) of Example 1, resulting in a pure PDA@MXene membrane.

[0037] Test case (1) Pure water flux detection: The permeation performance of the membrane was investigated under a certain pressure using a vacuum filtration device. Different solvent systems with pure water were used as feed solutions to evaluate the membrane's separation performance. The volume V (L) of permeate through the membrane over time was measured, and the permeation flux J (L·m³) was calculated using the equation J = V / (A×t×ΔP). -2 ·h -1 ·bar -1 ): Where A is the effective membrane area (m²) 2 ), t represents the permeation time (h), and P is the transmembrane pressure (bar).

[0038] Figure 3 A bar chart comparing pure water flux; by Figure 3 It can be seen that the pure water flux of TPMG-5 is approximately 2600 L·m －2 ·h －1 ·bar －1 The PDA@MXene to PEG ratio of 20:5 was found to be the best among the TPMG series, indicating that the high-purity water flux of TPMG-5 is due to the PDA coating layer expanding the interlayer spacing of MXene and introducing hydrophilic functional groups. At the same time, an appropriate amount of PEG further optimizes the interlayer channel size. PDA and PEG form a double cross-linked network through phenolic hydroxyl-ether hydrogen bonds, which builds efficient water mass transfer channels while maintaining the stability of the membrane structure, achieving a balance between permeation flux and structural integrity.

[0039] (2) Applicability in multiple dye systems: This embodiment examines the applicability of the TPMG-5 composite membrane in various dye systems. The TPMG-5 composite membrane was used to filter five dye solutions (CR, CV, MB, RhB, and EBT) at a concentration of 20 mg / L, and filtration tests were conducted at an operating pressure of 0.1 MPa.

[0040] Figure 4 This is a comparative graph showing the filtration performance of the TPMG-5 composite membrane for five dyes: Congo Red (CR), Crystal Violet (CV), Methylene Blue (MB), Rhodamine B (RhB), and Chrome Black T (EBT). The bars in the main graph represent water flux, and the inset in the upper left corner shows the corresponding rejection rate. The TPMG-5 membrane exhibits excellent filtration performance for all five dyes, with the EBT system showing the highest water flux at approximately 2100 L·m⁻¹. －2 ·h －1 ·bar －1 The MB system has a capacity of approximately 980 L·m⁻¹. －2 ·h －1 ·bar －1 The CR, CV, and RhB system has a chromatogram of approximately 780–860 L·m⁻¹. －2 ·h －1 ·bar －1 Regarding retention rates, the TPMG-5 membrane achieved over 99% retention for CR and CV, approximately 98% for MB, approximately 94% for EBT, and approximately 88% for RhB, demonstrating that the differences in retention performance for different dyes are closely related to the dye molecular size and charge properties. CR and CV have large molecular weights, resulting in significant steric hindrance retention effects; EBT and MB have moderate molecular weights, and their retention relies on a synergistic mechanism of membrane pore sieving and electrostatic repulsion; RhB has an amphoteric structure, partially offsetting the contribution of electrostatic retention, leading to a slightly lower retention rate. These results demonstrate that the double crosslinking modification strategy of TPMG-5 achieves broad applicability for the treatment of various dye wastewaters.

[0041] (2) Zeta potential: The stability of colloidal solutions was characterized and the electrostatic charge of substances was qualitatively analyzed using a zeta potential analyzer. Electrophoretic light scattering (ELS) can effectively measure the electrophoretic mobility and charge (zeta potential) of molecules or particles in a dispersion system. During the test, the MXene composite solution was diluted with water to 0.05 mg / mL. The test conditions were: temperature 25℃, pre-stabilization time 120 s, 10 runs per test, and 3 cycles per sample.

[0042] Figure 5The chart shows a comparison of zeta potentials. The zeta potential of TPMG-10 is approximately +12 mV, indicating a positively charged membrane surface. This demonstrates that even with the highest PEG addition, the positive charge characteristic imparted by the PDA coating layer is maintained, and the large amount of PEG did not alter the surface charge polarity. The positive charge on the TPMG-10 membrane surface originates from the protonated amino groups in the PDA covering the original negatively charged surface of MXene, achieving a charge reversal. Despite the highest PEG content in the series, TPMG-10 maintained a stable positive zeta potential, indicating that the double cross-linked network structure has a good stabilizing effect on surface charge. PEG, as an electrically neutral polymer, is intercalated between layers and does not participate in the regulation of the membrane surface charge.

[0043] (3) Water contact angle: The hydrophilicity of MXene composite membranes was analyzed using a water contact angle test. The specific procedure was as follows: a rectangular piece of MXene composite membrane was cut, fixed to a horizontally positioned glass plate, and placed on the sample stage of a contact angle meter. A water droplet was expelled from the universal injection needle and applied to the membrane surface. The accurate value of the water contact angle was determined using the θ / 2 analysis method.

[0044] Figure 6 The figure shows a comparison of the water contact angles of various membrane samples. As can be seen, the water contact angle of the pure MXene membrane (M) is approximately 92°, exhibiting significant hydrophobicity. However, the water contact angle of the TPM membrane coated with PDA drops sharply to approximately 19°. The water contact angles of the TPMG-1, TPMG-5, and TPMG-10 membranes, after adding different proportions of PEG, further decrease to the range of 10°–15°, all exhibiting superhydrophilic properties. The difference in water contact angle between the pure MXene membrane and the functionalized composite membrane is approximately 77°–82°, a significant difference. The fundamental reason for the hydrophobicity of the pure MXene membrane can be attributed to Ti3C2T… xThe surface of the nanosheets is dominated by -F and -O end groups, which are weakly polar and lack strongly hydrophilic functional groups. Furthermore, the unmodified multilayered sheets are tightly stacked by van der Waals forces, forming a dense layered structure that further hinders the rapid wetting and spreading of water molecules on the membrane surface. In contrast, PDA coating on the MXene nanosheets introduces a large number of phenolic hydroxyl groups (-OH) and amino groups (-NH2). These strongly polar functional groups significantly increase the surface free energy of the membrane, allowing water droplets to spread rapidly upon contact, with the water contact angle dropping sharply from 92° to approximately 19°. The further introduction of PEG provides flexible hydrophilic segments rich in ether bonds (C-O-C). The oxygen atoms in these ether bonds can form hydrogen bonds with water molecules, and the hydrogen bond crosslinking between the PEG segments and the phenolic hydroxyl groups of PDA further enhances the affinity of the membrane surface for water molecules, reducing the water contact angle of the TPMG series membranes to 10°–15°. Water contact angle data show that the unfunctionalized pure MXene membrane has a large interfacial mass transfer resistance between water molecules and the membrane surface due to its high hydrophobicity, which is not conducive to achieving efficient water permeation. This verifies the necessity and effectiveness of the dual modification strategy of PDA functionalization coating and PEG hydrophilic intercalation described in this invention in improving membrane wettability.

[0045] (5) Cyclic performance: Cyclic stability tests were performed on the samples using a vacuum filtration apparatus. The composite membranes were fixed onto the apparatus, and a solvent was used as the feed liquid. Ten consecutive cycles were conducted under a certain pressure. In each cycle, an equal volume of deionized water or organic solvent was added, and the volume V (L) of permeate through the membrane over time was measured. The permeation flux J (L·m³) was then calculated using the equation J = V / (A × t × ΔP). -2 ·h -1 ·bar -1 After each cycle, the next cycle begins. After 10 cycles, the feed solution is replaced with dye, the flux is measured, and the result is calculated using the formula R=(1−C). P The dye rejection rate was measured to evaluate the differences in water flux stability and final separation performance between the two membranes after long-term operation. The composite membrane prepared in this invention maintained a rejection rate of over 96% and a flux recovery rate of over 90% after ten cycles of testing, demonstrating excellent operational stability.

[0046] In summary, the MXene-based self-supporting composite membrane of this invention uses a porous polymer filter membrane as the support layer. Functionalized MXene nanosheets, modified in situ with amino-containing compounds, and hydrophilic polymer compounds are stacked in an orderly manner on the substrate surface using a vacuum-assisted self-assembly method to form a dense functional composite layer. The MXene nanosheets are obtained using the MAX phase as a precursor through in-situ etching, interlayer exfoliation, and freeze-drying. The functionalized layer is formed by the in-situ deposition of polymers generated from the oxidative self-polymerization of amino monomers under alkaline oxygen conditions, which are then firmly encapsulated on the nanosheet surface. The introduction of hydrophilic polymer compounds further constructs a multi-layered chemical cross-linking network within the membrane layer and between the layers, significantly enhancing the membrane's structural stability and mechanical strength. This effectively inhibits the swelling, shedding, and aggregation of MXene sheets during water treatment, while also greatly improving the membrane's surface hydrophilicity and antifouling properties. The MXene-based self-supporting composite membrane prepared by this invention has superhydrophilicity, high water flux, excellent dye retention capacity and good recycling stability. It can effectively treat dye-containing organic wastewater and has broad application prospects in the fields of industrial printing and dyeing wastewater treatment and water resource reuse.

[0047] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a double-crosslinked modified MXene-based self-supporting composite membrane, characterized in that, Includes the following steps: (1) Preparation of functionalized modified nanosheets: An amino-containing compound is dissolved in a buffer solution with a pH of 7.5 to 9.5 to form a dispersion. MXene nanosheets are added to the dispersion to react and obtain functionalized modified MXene nanosheets. (2) Preparation of composite membrane: The dispersion of the functionalized modified MXene nanosheets is mixed with the dispersion of the hydrophilic polymer compound, and then transferred to a vacuum filtration device. The porous polymer filter membrane is used as the substrate for vacuum filtration to form a membrane. After drying, a double crosslinked modified MXene-based self-supporting composite membrane is obtained.

2. The method for preparing a double-crosslinked modified MXene-based self-supporting composite membrane according to claim 1, characterized in that, The amino-containing compound mentioned in step (1) includes at least one of dopamine hydrochloride, polyethyleneimine, norepinephrine, and serotonin; The buffer solution mentioned in step (1) includes Tris-HCl buffer, carbonate buffer, and phosphate buffer; The mass ratio of the amino-containing compound to the MXene nanosheets in step (1) is 1:1 to 1:10; In step (1), the final concentration of MXene nanosheets in the buffer solution is 1.0~10.0 mg / mL; The reaction conditions in step (1) are: room temperature, time of 6 to 48 hours, and stirring speed of 300 to 800 rpm.

3. The method for preparing a double-crosslinked modified MXene-based self-supporting composite membrane according to claim 1, characterized in that, In step (1), MXene nanosheets are prepared through the following steps: Fluoride salts were dissolved in an acidic etching solution with a concentration of 6-12 M. MAX phase ceramic powder was added to the acidic etching solution for etching. After the reaction was completed, the supernatant was centrifuged and washed until the pH of the supernatant was 5-7. The multilayer products in the supernatant were ultrasonically exfoliated, the precipitate was collected and dried to obtain MXene nanosheets.

4. The method for preparing a double-crosslinked modified MXene-based self-supporting composite membrane according to claim 3, characterized in that, The fluoride salt includes at least one of LiF, NaF, and KF; The etching reaction conditions are: temperature 30~70°C, time 12~72 h; The MAX phase ceramic includes at least one of Ti3AlC2, Ti2AlC, V2AlC, and Nb2AlC; The ratio of the MAX phase ceramic to the fluoride salt is 1.0~2.5:1.5~3.

5.

5. The method for preparing a double-crosslinked modified MXene-based self-supporting composite membrane according to any one of claims 1 to 4, characterized in that, The hydrophilic polymer compound includes at least one of polyethylene glycol, polyacrylic acid, polyethyleneimine, and polydopamine; The mass ratio of the functionalized modified nanosheets to the hydrophilic polymer compound is 20:1~10.

6. The method for preparing a double-crosslinked modified MXene-based self-supporting composite membrane according to claim 5, characterized in that, The concentration of the dispersion of the hydrophilic polymer compound is 5~20 mg / mL; The concentration of the dispersion of the functionalized modified MXene nanosheets is 1.0~10.0 mg / mL; The mass ratio of the dispersion of the functionalized modified MXene nanosheets to the dispersion of the hydrophilic polymer compound is 10~30:0.5~15.

7. The method for preparing a double-crosslinked modified MXene-based self-supporting composite membrane according to claim 5, characterized in that, The porous polymer filter membrane includes at least one of polyvinylidene fluoride filter membrane, polyethersulfone membrane, and polytetrafluoroethylene membrane, and the pore size of the porous polymer filter membrane is 0.1~0.45μm; The conditions for vacuum filtration film formation are: vacuum degree of 0.05~0.2 MPa; The drying conditions are: natural drying at room temperature for 12~36 hours.

8. A double-crosslinked modified MXene-based self-supporting composite membrane, characterized in that, The MXene-based self-supporting composite membrane is prepared by the preparation method according to any one of claims 1 to 7.

9. An application of a double-crosslinked modified MXene-based self-supporting composite membrane, characterized in that, The double-crosslinked modified MXene-based self-supporting composite membrane of claim 8 is used in the treatment of dye-containing organic wastewater.

10. The application of the double-crosslinked modified MXene-based self-supporting composite membrane according to claim 9, characterized in that, The dyes include at least one of Congo Red, Crystal Violet, Methylene Blue, Chrome Black T, and Rhodamine B.

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