PVA / formaldehyde / MXene composite semipermeable membrane and preparation method thereof

Abstract

The invention discloses a PVA / formaldehyde / MXene composite semipermeable membrane and a preparation method thereof, and relates to the technical field of composite semipermeable membranes, and the preparation method comprises the steps of preparation of a few-layer MXene nanosheet colloidal solution, preparation of a PVA solution, cross-linking reaction, preparation of a composite solution and membrane forming treatment. Etching an Al layer in the Ti3AlC2MAX phase by using an in-situ hydrofluoric acid (HF) etching method, and carrying out centrifugation, washing and ultrasonic treatment to obtain a few-layer MXene nanosheet colloidal solution; the PVA / formaldehyde / MXene composite semipermeable membrane is prepared by mixing the PVA / formaldehyde / MXene with a crosslinked PVA / formaldehyde solution, coating the PVA / formaldehyde / MXene composite solution on the surface of a base material, and drying to obtain the PVA / formaldehyde / MXene composite semipermeable membrane which can be applied to separation treatment of radionuclides and organic pollutants in radioactive wastewater of a nuclear power station. The problems that an existing MXene sheet is small in specific surface area, thick in interlayer spacing and incapable of being crosslinked with PVA and formaldehyde to form a film or poor in film forming effect are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of composite semi-permeable membranes, and in particular to a PVA / formaldehyde / MXene composite semi-permeable membrane, a preparation method thereof, and an application thereof. Background Art

[0002] As a clean energy source, nuclear energy is widely used globally. However, the efficient treatment of radioactive wastewater (containing radioactive nuclides such as uranium, cesium, strontium, and heavy metal ions) generated during the operation of nuclear power plants remains a major challenge. Traditional treatment methods such as chemical precipitation and ion exchange have problems such as low efficiency and high risk of secondary pollution. Membrane separation technology has become a research hotspot due to its advantages such as high efficiency, low energy consumption, and no phase change.

[0003] In membrane separation technology, porous separation membranes can effectively intercept pollutants in water with sizes larger than the membrane pores and play an increasingly important role in the field of water treatment. This technology has advantages such as energy-saving and high efficiency, no phase change, easy integration with other processes, and no secondary pollution, and has been regarded as one of the most promising water treatment technologies in the 21st century. Membrane separation technology can efficiently remove solids and suspended substances in water and achieve effects that are difficult to achieve by traditional treatment methods. For example, ultrafiltration membranes can remove macromolecular organic matter and colloidal substances in water, and reverse osmosis membranes can remove dissolved salts and inorganic substances, with very high purification capabilities. Membrane separation technology has good universality and can be applied to municipal sewage, industrial wastewater, agricultural sewage, etc. Membrane separation technology can efficiently recycle water resources and can also separate and recover valuable substances in water, such as metal ions and nutrients.

[0004] In existing membrane technologies, ultrafiltration membranes can intercept macromolecular pollutants, but have insufficient separation ability for dissolved ions; polymer-based membranes (such as polyethersulfone and polyvinylidene fluoride) have certain separation performance, but generally have problems such as low flux, poor anti-fouling performance, and insufficient interception of nanoscale pollutants; polyvinyl alcohol (PVA) has been widely used as a matrix material for separation membranes due to its hydrophilicity, film-forming property, and chemical modifiability. However, pure PVA membranes have problems such as insufficient mechanical strength and poor anti-fouling performance.

[0005] In the field of PVA membrane modification, traditional cross-linking agents (such as glutaraldehyde) can improve mechanical strength, but excessive cross-linking will sacrifice the hydrophilicity and flux of the membrane. As a low-cost cross-linking agent, formaldehyde can form a dense network by reacting with the hydroxyl groups of PVA, but its application in the MXene composite system has not been reported.

[0006] In recent years, the two-dimensional material MXene (such as Ti3C2T xDue to its unique layered structure, high specific surface area, and surface-tunable functional groups (-OH, -O, -F), it exhibits significant advantages in the fields of adsorption and catalysis. Existing research has mainly focused on the composite of MXene with materials such as graphene and carbon nanotubes, or its application in energy storage devices (such as supercapacitor electrodes). However, due to the relatively small specific surface area and thick interlayer spacing of MXene flakes, it is unable to crosslink with PVA and formaldehyde to form a film or the film-forming effect is very poor.

[0007] To solve this problem, we urgently need a new solution. Therefore, the present invention provides a PVA / formaldehyde / MXene composite semi-permeable membrane, its preparation method, and application; MXene nanosheets are crosslinked with PVA and formaldehyde to form a stable composite semi-permeable membrane. Summary of the Invention

[0008] The purpose of this part is to outline some aspects of the embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this part, as well as in the abstract and title of the specification of this application, to avoid obscuring the purpose of this part, the abstract, and the title. However, such simplifications or omissions shall not be used to limit the scope of the present invention.

[0009] In view of the problems existing in the above-mentioned prior art, the present invention is proposed.

[0010] The problem to be solved by the present invention is how to solve the problem that the specific surface area of current MXene flakes is relatively small and the interlayer spacing is thick, making it impossible to crosslink with PVA and formaldehyde to form a film or the film-forming effect is very poor.

[0011] The above technical problems are solved by the following technical solutions: The present invention provides a preparation method of a PVA / formaldehyde / MXene composite semi-permeable membrane, comprising the following steps:

[0012] (1) Preparation of few-layer MXene nanosheet colloidal solution: Mix LiF powder with HCl solution and add it to the Al layer in the Ti3AlC2 MAX phase for etching reaction. After centrifugation, washing, and ultrasonic treatment, a few-layer MXene nanosheet colloidal solution is obtained;

[0013] (2) Preparation of PVA solution: Dissolve polyvinyl alcohol (PVA) particles in water or an organic solvent to obtain a PVA solution;

[0014] (3) Crosslinking reaction: Add formaldehyde solution to the PVA solution and stir evenly to obtain a crosslinked PVA / formaldehyde solution;

[0015] (4) Preparation of composite solution: Add the few-layer MXene nanosheet colloidal solution to the crosslinked PVA / formaldehyde solution, stir and mix evenly to form a PVA / formaldehyde / MXene composite solution;

[0016] (5) Film-forming treatment: Coating the composite solution on the surface of the substrate, and peeling it off after drying to obtain a composite semi-permeable membrane.

[0017] In a preferred embodiment of the preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane of the present invention: in step (1), the LiF powder is 4.8 g, the HCl solution is 60 mL, the concentration of the HCl solution is 9 mol / L, and the etching reaction time is 48 hours.

[0018] In a preferred embodiment of the preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane of the present invention: in step (1), the centrifugation rate is 3000 rpm - 4000 rpm; washing is performed with deionized water until the solution is neutral; ultrasonic treatment is performed at a power of 20 W - 40 W for 60 minutes.

[0019] In a preferred embodiment of the preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane of the present invention: in step (2), heating, stirring or ultrasonic method is used to promote the dissolution of PVA.

[0020] In a preferred embodiment of the preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane of the present invention: in step (2), the concentration of the PVA solution is 5% - 15%, the dissolution temperature is 80°C - 95°C, and the stirring time is 1 - 3 hours.

[0021] In a preferred embodiment of the preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane of the present invention: in step (3), the mass ratio of the formaldehyde solution to the PVA solution is 1:1 - 1:2; constant temperature stirring is performed at 50°C - 80°C under acidic conditions; the concentration of the formaldehyde solution is 37%; the cross-linking reaction time is 0.5 h - 2 h.

[0022] In a preferred embodiment of the preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane of the present invention: in step (4), the stirring temperature is 50°C - 70°C.

[0023] In a preferred embodiment of the preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane of the present invention: in step (5), the substrate is a glass sheet, and the coating method is blade coating or spin coating; the drying temperature is 60°C - 80°C, and the drying time is 12 - 24 hours.

[0024] Another object of the present invention is to overcome the deficiencies in the prior art and provide a composite semi-permeable membrane prepared by the preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane.

[0025] In a preferred embodiment of the composite semi-permeable membrane prepared by the preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane of the present invention: the composite semi-permeable membrane comprises a three-dimensional structure formed by a cross-linked PVA network, MXene nanosheets and formaldehyde, and the MXene nanosheets are uniformly dispersed in the PVA matrix.

[0026] In a preferred embodiment of the composite semi-permeable membrane prepared by the preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane of the present invention: the number of layers of the MXene nanosheets is 1-3 layers.

[0027] The beneficial effects of the present invention are as follows: The Al layer in the Ti3AlC2 MAX phase is etched by the in-situ hydrofluoric acid (HF) etching method, and few-layer MXene nanosheet colloidal solution is obtained through centrifugation, washing, and ultrasonic treatment; after mixing it with the cross-linked PVA / formaldehyde solution, a PVA / formaldehyde / MXene composite solution is obtained, and the PVA / formaldehyde / MXene composite semi-permeable membrane is obtained after coating it on the surface of the substrate and drying; it solves the problem that the specific surface area of the current MXene flakes is small and the layer spacing is thick, and it is impossible to cross-link with PVA and formaldehyde to form a membrane or the film-forming effect is very poor. Brief Description of the Drawings

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings of the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below only relate to some embodiments of the present invention and do not limit the present invention.

[0029] Figure 1 : Schematic flow chart for preparing the PVA / formaldehyde / MXene composite semi-permeable membrane;

[0030] Figure 2 : XRD patterns and Raman spectra of the MAX phase and the few-layer MXene nanosheet colloidal solution after etching;

[0031] Figure 3 : TEM morphology and electron diffraction pattern of the few-layer MXene nanosheet colloidal solution;

[0032] Figure 4 : Physical picture of the PVA / formaldehyde / MXene composite semi-permeable membrane;

[0033] Figure 5 : Fourier transform infrared spectrum of the PVA / formaldehyde / MXene composite semi-permeable membrane;

[0034] Figure 6 : Contact angle test results with a mass ratio of formaldehyde to PVA of 1:1;

[0035] Figure 7 : Contact angle test results with a mass ratio of formaldehyde to PVA of 1:1.5;

[0036] Figure 8 : Contact angle test results with a mass ratio of formaldehyde to PVA of 1:2. Detailed implementation manners

[0037] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below in conjunction with the detailed implementation manners and the accompanying drawings.

[0038] The terms used in the present invention are those general terms currently widely used in the art in consideration of the functions of the present invention, but these terms may vary according to the intentions of those of ordinary skill in the art, precedents, or new technologies in the art. In addition, specific terms may be selected by the applicant, and in such cases, their detailed meanings will be described in the detailed description of the present invention. Therefore, the terms used in the specification should not be understood as simple names, but based on the meanings of the terms and the overall description of the present invention.

[0039] Many specific details are set forth in the following description in order to fully understand the present invention, but the present invention may also be implemented in other ways different from those described herein. Those skilled in the art can make similar generalizations without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0040] Secondly, the so-called "one embodiment" or "embodiment" herein refers to specific features, structures, or characteristics that may be included in at least one implementation manner of the present invention. The appearances of "in one embodiment" in different places in this specification do not all refer to the same embodiment, nor are they separate or selectively exclusive embodiments from other embodiments.

[0041] Example 1,

[0042] This is the first embodiment of the present invention. This embodiment provides a method for preparing a PVA / formaldehyde / MXene composite semipermeable membrane:

[0043] (1) Preparation of few-layer MXene nanosheet colloidal solution: Mix LiF powder with HCl solution and add it to the Al layer in the Ti3AlC2 MAX phase for etching reaction, and obtain few-layer MXene nanosheet colloidal solution through centrifugation, washing, and ultrasonic treatment;

[0044] The specific preparation method is:

[0045] ① First, prepare MXene powder by selective etching of the Al layer in the Ti3AlC2 MAX phase.

[0046] At room temperature, 4.8 g of LiF powder was added to 60 mL of 9 M HCl solution, and stirred for 30 min to obtain a homogeneous solution A. Subsequently, 3 g of Ti3AlC2MAX powder was added to solution A, and then stirred at room temperature for 48 h; after stirring, centrifuged at 3500 rpm for 5 min to collect the precipitate, and impurity-free MXene powder was obtained;

[0047] ② The impurity-free MXene powder obtained by centrifugation was soaked in HCl solution for 1 h to completely remove the residual LiF; the impurity-free MXene powder after soaking was washed with deionized water until the solution was neutral, and the obtained precipitate was the multi-layer MXene powder;

[0048] ③ To obtain few-layer MXene nanosheets, the multi-layer MXene powder was dispersed in 50 mL of deionized water, centrifuged at 3500 rpm for 15 min, and the supernatant obtained after each centrifugation was collected as the few-layer MXene nanosheet colloidal solution.

[0049] After the MAX material was etched in step S1, a few-layer MXene nanosheet colloidal solution was formed, and the results of its X-ray diffraction pattern were as Figure 2 (a) shown. The (002) diffraction peak of Ti moved from 9.5° to a lower angle, fully indicating the formation of the Ti3C2T x MXene phase and the increase in the interplanar spacing of the material. At the same time, the disappearance of the strong Al diffraction peak at 2θ≈39° indicated that Al atoms were selectively etched from the Ti3AlC2MAX material, proving the formation of the MXene material. Figure 2 (b) is the Raman spectrum before and after etching. The prominent peaks in the figure are ~201 cm -1 assigned to the out-of-plane (A 1g ) vibrations of Ti, O, and C atoms, and ~723 cm -1 is another (A 1g ) vibration of C atoms. The regions around 230 - 470 cm -1 and 500 - 650 cm -1 can be assigned to the in-plane (E g ) vibrations of surface groups. The ~200 cm -1 metal-carbon vibration peak in the MAX phase weakened. After acid etching, the interlayer structure of MXene changed, and the surface might be covered with functional groups such as oxides, fluorides, or hydroxyl groups. Figure 3 Shown are the transmission electron microscope morphology and electron diffraction pattern of the prepared few-layer MXene nanosheet colloidal solution. From its morphology, it can be seen that the MXene material is a single-layer structure or a few-layer structure (the number of layers can be 1 - 3 layers), it has a relatively high specific surface area and a thin interlayer spacing; from the electron diffraction pattern, it can be obtained that it is a polycrystalline structure and there are no other impurities.

[0050] It should be noted that: the in-situ hydrofluoric acid (HF) etching method is used, and HF acid is prepared by reacting LiF with HCl for etching the Al element in the Ti3AlC2 MAX phase, so that the multi-layer MXene is dispersed into single-layer or few-layer MXene materials, ensuring that its specific surface area is relatively high and the layer spacing is relatively thin; the etching process forms layered Ti3AlC2 MXene with -O, -OH and -F at the ends; then the etching solution is centrifugally washed with water / ethanol to remove residual fluorides or other impurities. After washing, the MXene needs to be dispersed by ultrasonic treatment to prevent agglomeration. Ultrasonic treatment is carried out at 30 W for 60 min until a uniform few-layer MXene nanosheet colloidal solution is formed for standby.

[0051] (2) Preparation of PVA solution: Polyvinyl alcohol (PVA) particles are dissolved in water or an organic solvent to prepare a PVA solution;

[0052] The specific preparation method is as follows:

[0053] Weigh 3 g of polyvinyl alcohol particles with a balance, disperse them in 27 g of deionized water, and carry out oil bath magnetic stirring under heating conditions at 90 °C. Stir thoroughly for 2 h until completely dissolved to obtain a 10 wt% PVA solution.

[0054] It should be noted that: Polyvinyl alcohol (PVA) is dissolved in water or an organic solvent to prepare a PVA solution. During the dissolution process, methods such as heating, stirring, and ultrasonic treatment can be used to promote the dissolution of PVA. The concentration of the PVA solution is usually between 5% and 15%, depending on the final thickness and properties of the required film. In addition, it should be noted that PVA with different molecular weights has different solubilities, so adjustments and optimizations need to be made according to specific conditions in actual operations.

[0055] (3) Crosslinking reaction: Formaldehyde solution is added to the PVA solution and stirred evenly to obtain a crosslinked PVA / formaldehyde solution;

[0056] The specific preparation method is as follows:

[0057] The formaldehyde solution (37% formalin) is slowly added to the PVA solution obtained in step S2. The mass ratio of the formaldehyde solution to the PVA solution is 1:1 for full mixing. The composite solution is kept at a constant temperature of 60 °C with an oil bath and stirred for 1 h to allow the formaldehyde solution and the PVA solution to fully crosslink and react. During the reaction process, it can be seen that the viscosity of the solution is gradually increasing, that is, the crosslinking reaction is in progress. To promote further crosslinking reaction, a little glacial acetic acid is added dropwise to the composite solution to keep the solution acidic.

[0058] It should be noted that: The cross-linking reaction is usually carried out at a temperature of 50-80°C. Heating helps to increase the reaction rate and cross-linking degree, but too high a temperature may cause the degradation of PVA; and an acidic condition is conducive to the cross-linking reaction; the cross-linking reaction time is generally 30 minutes to 2 hours, and the specific time can be adjusted according to the cross-linking degree.

[0059] The cross-linking of PVA and formaldehyde can improve the mechanical strength of the membrane, improve its thermal stability, enhance the stability and durability of the membrane, improve the anti-pollution performance of the membrane, and enhance the hydrophilicity and selectivity of the membrane. By controlling parameters such as the cross-linking reaction time and formaldehyde concentration, the optimization parameters of the PVA membrane can be precisely adjusted.

[0060] (4) Preparation of the composite solution: Add the few-layer MXene nanosheet colloidal solution to the cross-linked PVA / formaldehyde solution, and stir and mix evenly to form a PVA / formaldehyde / MXene composite solution;

[0061] The specific preparation method is as follows:

[0062] Take 5 mL of the few-layer MXene nanosheet colloidal solution prepared in step S1 and slowly add it to the PVA / formaldehyde solution prepared in step S3. Stir for 1 h under the heating condition of 60°C to mix evenly, and obtain a PVA / formaldehyde / MXene composite solution for standby.

[0063] It should be noted that: When slowly adding the few-layer MXene nanosheet colloidal solution to the PVA / formaldehyde solution prepared in step S3, heating and stirring can be used to mix it evenly, and ultrasonic assistance can be used for dispersion to increase the dispersion of MXene. The amount of the few-layer MXene nanosheet colloidal solution added is controlled according to the mass of the PVA / formaldehyde solution and determined according to the mass ratio. The mass ratio of MXene to PVA is 1:120.

[0064] The doping of the few-layer MXene nanosheet colloidal solution promotes the cross-linking of PVA and formaldehyde and reacts with them, improving the stretchability of the composite solution; at the same time, MXene nanosheets also form microchannels on the surface, improving the particle passing rate.

[0065] (5) Film-forming treatment: Coat the composite solution on the surface of the substrate, and peel it off after drying to obtain a composite semi-permeable membrane;

[0066] The specific preparation method is as follows:

[0067] Use a pipette to measure 700 μL of the PVA / formaldehyde / MXene composite solution obtained in step S4 and drop it into a glass slide mold. Place it on a 60°C hot plate and heat and dry it until the solution solidifies to form a semi-permeable membrane. After peeling the semi-permeable membrane from the mold, a PVA / formaldehyde / MXene composite semi-permeable membrane is obtained.

[0068] Figure 4 This is the optical photograph of the final PVA / formaldehyde / MXene composite semi-permeable membrane, forming a semi-permeable membrane material with uniform thickness and moderate mechanical strength.

[0069] Figure 5 This is the test result of the Fourier transform infrared (FTIR) spectrum change of the PVA / formaldehyde / MXene semi-permeable membrane. Among them, the vibration peak at 3314 cm -1 is attributed to the O-H vibration of the hydroxyl group in PVA or acetic acid, and the vibration peak at 2918 cm -1 is attributed to the C-H stretching vibration of PVA. The vibration peak at 1730 cm -1 is attributed to the C=O vibration peak of formaldehyde, and the absorption peak at 1244 cm -1 is attributed to the stretching vibration of functional groups such as C-O and C-F in the MXene material. The vibration peak at 1017 cm -1 is attributed to the C-OH bending vibration in PVA. Through FTIR spectrum testing, the PVA / formaldehyde / MXene composite semi-permeable membrane contains the active ingredients of PVA, formaldehyde, and MXene, proving the effective combination of the three materials.

[0070] It should be noted that the PVA / formaldehyde / Mxene composite solution can also be poured onto other flat molds, and the coating method can use a scraper or other tools to spread it evenly to form a film, or the spin coating method can be used to form a uniform film. The prepared semi-permeable membrane is air-dried naturally or dried in a hot air circulation oven at 60 - 80 °C for 24 hours to obtain a uniform PVA / formaldehyde / MXene composite semi-permeable membrane.

[0071] In this example, the mass ratio of the formaldehyde solution to the PVA solution is 1:1. The contact angle of the prepared composite semi-permeable membrane is tested to evaluate the hydrophilic performance of the semi-permeable membrane. The contact angle test results are as Figure 6 shown.

[0072] Example 2,

[0073] This is the second example of the present invention. This example provides another preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane:

[0074] (1) Preparation of few-layer MXene nanosheet colloidal solution: Mix LiF powder with HCl solution and then add it to the Al layer in the Ti3AlC2 MAX phase for etching reaction. After centrifugation, washing, and ultrasonic treatment, a few-layer MXene nanosheet colloidal solution is obtained;

[0075] The specific preparation method is as follows:

[0076] (1) Preparation of few-layer MXene nanosheet colloidal solution: Mix LiF powder with HCl solution, then add it to etch the Al layer in Ti3AlC2 MAX phase. After centrifugation, washing, and ultrasonic treatment, a few-layer MXene nanosheet colloidal solution is obtained;

[0077] The specific preparation method is as follows:

[0078] ② First, prepare MXene powder by selectively etching the Al layer in Ti3AlC2 MAX phase.

[0079] At room temperature, add 4.8 g of LiF powder to 60 mL of 9 M HCl solution, stir for 30 min to obtain a homogeneous solution A. Subsequently, add 3 g of Ti3AlC2 MAX powder to solution A, and then stir at room temperature for 48 h; after stirring, centrifuge at 3500 rpm for 5 min to collect the precipitate, obtaining impurity-free MXene powder;

[0080] ② Soak the centrifuged impurity-free MXene powder in HCl solution for 1 h to completely remove residual LiF; wash the soaked impurity-free MXene powder with deionized water until the solution is neutral, and the obtained precipitate is multi-layer MXene powder;

[0081] ③ To obtain few-layer MXene nanosheets, disperse the multi-layer MXene powder in 50 mL of deionized water, centrifuge at 3500 rpm for 15 min, and collect the supernatant obtained after each centrifugation, which is the few-layer MXene nanosheet colloidal solution.

[0082] After the MAX material is etched in step S1, a few-layer MXene nanosheet colloidal solution is formed, and the results of its X-ray diffraction pattern are as Figure 2 (a) shown. The (002) diffraction peak of Ti moves from 9.5° to a lower angle, fully indicating the formation of the Ti3C2T x MXene phase and the increase in the interplanar spacing of the material. At the same time, the disappearance of the strong Al diffraction peak at 2θ≈39° indicates that Al atoms are selectively etched from the Ti3AlC2 MAX material, proving the formation of the MXene material. Figure 2 (b) is the Raman spectrum before and after etching. The prominent peaks in the figure are ~201 cm -1 assigned to the out-of-plane (A 1g ) vibrations of Ti, O, and C atoms, and ~723 cm -1 is another (A 1g ) vibration of C atoms. The regions around 230 - 470 cm -1 and 500 - 650 cm -1 can be assigned to the in-plane (E g)Vibration of surface groups. The ~200 cm in MAX phase -1 The metal-carbon vibration peak weakens. After acid etching, the interlayer structure of MXene changes, and the surface may be covered with functional groups such as oxides, fluorides, or hydroxyl groups. Figure 3 Figure 4 shows the transmission electron microscope morphology and electron diffraction pattern of the prepared few-layer MXene nanosheet colloidal solution. From its morphology, it can be seen that the MXene material is a single-layer structure or a few-layer structure (the number of layers can be 1-3 layers), with a relatively high specific surface area and a relatively thin interlayer spacing; from the electron diffraction pattern, it can be obtained that it is a polycrystalline structure and there are no other impurities.

[0083] It should be noted that: using the in-situ hydrofluoric acid (HF) etching method, HF acid is prepared by reacting LiF with HCl for etching the Al element in Ti3AlC2 MAX phase to disperse the multi-layer MXene into single-layer or MXene materials, ensuring its relatively high specific surface area and relatively thin interlayer spacing; the etched solution is centrifuged and washed with water / ethanol to remove residual fluorides or other impurities. After washing, MXene needs to be dispersed by ultrasonic treatment to prevent agglomeration. Ultrasonic treatment is carried out at 30 W for 60 min until a uniform few-layer MXene nanosheet colloidal solution is formed for standby.

[0084] (2) Preparation of PVA solution: Dissolve polyvinyl alcohol (PVA) particles in water or organic solvents to obtain a PVA solution;

[0085] The specific preparation method is as follows:

[0086] Weigh 3 g of polyvinyl alcohol particles with a balance, disperse them in 27 g of deionized water, and carry out oil bath magnetic stirring under heating conditions at 90 °C. Stir well for 2 h until completely dissolved to obtain a 10 wt% PVA solution.

[0087] It should be noted that: Dissolve polyvinyl alcohol (PVA) in water or organic solvents to obtain a PVA solution. During the dissolution process, methods such as heating, stirring, and ultrasonic treatment can be used to promote the dissolution of PVA. The concentration of the PVA solution is usually between 5% and 15%, depending on the final thickness and properties of the required film. In addition, it should be noted that PVA with different molecular weights has different solubilities, so it needs to be adjusted and optimized according to specific conditions in actual operation.

[0088] (3) Crosslinking reaction: Add formaldehyde solution to the PVA solution and stir evenly to obtain a crosslinked PVA / formaldehyde solution;

[0089] The specific preparation method is as follows:

[0090] Slowly add formaldehyde solution (37% formalin) to the PVA solution obtained in step S2, with the mass ratio of formaldehyde solution to PVA solution being 1:1.5, and mix thoroughly. Keep the composite solution at a constant temperature of 60°C using an oil bath and stir for 1 hour to allow the formaldehyde solution and the PVA solution to fully cross-link. During the reaction, it can be seen that the viscosity of the solution is gradually increasing, that is, the cross-linking reaction is in progress. In order to promote further cross-linking reaction, a small amount of glacial acetic acid is added to the composite solution to keep the solution acidic.

[0091] It should be noted that the cross-linking reaction is usually carried out at a temperature of 50 to 80°C. Heating helps to increase the reaction rate and the degree of cross-linking, but too high a temperature may cause degradation of PVA. Acidic conditions are conducive to the cross-linking reaction. The cross-linking reaction time is generally 30 minutes to 2 hours, and the specific time can be adjusted according to the degree of cross-linking.

[0092] Cross-linking of PVA and formaldehyde can improve the mechanical strength of the membrane, improve its thermal stability, improve the stability and durability of the membrane, improve the anti-fouling performance of the membrane, and enhance the hydrophilicity and selectivity of the membrane. By controlling parameters such as cross-linking reaction time and formaldehyde concentration, the optimization parameters of the PVA membrane can be accurately adjusted.

[0093] (4) Preparation of composite solution: adding the few-layer MXene nanosheet colloidal solution to the cross-linked PVA / formaldehyde solution, stirring and mixing to form a PVA / formaldehyde / MXene composite solution;

[0094] The specific preparation method is:

[0095] Take 5 mL of the few-layer MXene nanosheet colloidal solution prepared in step S1 and slowly add it to the PVA / formaldehyde solution prepared in step S3, keep stirring at 60° C. for 1 h to mix them evenly, and obtain a PVA / formaldehyde / MXene composite solution for later use.

[0096] It should be noted that when the few-layer MXene nanosheet colloidal solution is slowly added to the PVA / formaldehyde solution prepared in step S3, heating and stirring can be used to make it uniformly mixed, and ultrasonic assisted dispersion can be used to increase the dispersibility of MXene. The amount of the few-layer MXene nanosheet colloidal solution added is controlled according to the quality of the PVA / formaldehyde solution and determined according to the mass ratio. The mass ratio of MXene to PVA is 1:120.

[0097] The doping of few-layer MXene nanosheet colloidal solution promotes the cross-linking of PVA and formaldehyde and reacts with them, thereby improving the stretchability of the composite solution. At the same time, MXene nanosheets also form microchannels on the surface, thereby improving the particle pass rate.

[0098] (5) Film-forming treatment: coating the composite solution on the surface of the substrate, and peeling it off after drying to obtain a composite semipermeable membrane;

[0099] The specific preparation method is as follows:

[0100] Use a pipette to measure 700 μL of the PVA / formaldehyde / MXene composite solution obtained in step S4, drop it onto a glass slide mold, and place it on a hot plate at 60 °C to heat and dry until the solution solidifies to form a semi-permeable membrane. After peeling the semi-permeable membrane from the mold, the PVA / formaldehyde / MXene composite semi-permeable membrane is obtained.

[0101] Figure 4 This is the optical photo of the final PVA / formaldehyde / MXene composite semi-permeable membrane, forming a semi-permeable membrane material with uniform thickness and moderate mechanical strength.

[0102] Figure 5 This is the test result of the Fourier transform infrared (FTIR) spectrum of the PVA / formaldehyde / MXene semi-permeable membrane; among them, the vibration peak at 3314 cm -1 belongs to the O-H vibration of the hydroxyl group in PVA or acetic acid, the vibration peak at 2918 cm -1 belongs to the C-H stretching vibration of PVA, the vibration peak at 1730 cm -1 belongs to the C=O vibration peak of formaldehyde, the absorption peak at 1244 cm -1 is the stretching vibration of functional groups such as C-O and C-F in the MXene material, and the vibration peak at 1017 cm -1 belongs to the C-OH bending vibration in PVA. Through Fourier transform infrared spectroscopy testing, the PVA / formaldehyde / MXene composite semi-permeable membrane contains the active ingredients in PVA, formaldehyde, and MXene, proving the effective combination of the three materials.

[0103] It should be noted that: the PVA / formaldehyde / Mxene composite solution can also be poured onto other flat molds, and the coating method can use a scraper or other tools to spread it evenly to form a film, or the spin coating method can be used to form a uniform film. The prepared semi-permeable membrane is naturally air-dried or dried in a hot air circulation oven at 60 - 80 °C for 24 hours to obtain a uniform PVA / formaldehyde / MXene composite semi-permeable membrane.

[0104] In this example, the mass ratio of the formaldehyde solution to the PVA solution is 1:1.5. The contact angle of the prepared composite semi-permeable membrane is tested to evaluate the hydrophilic performance of the semi-permeable membrane. The contact angle test results are as Figure 7 shown.

[0105] Example 3,

[0106] This is the third example of the present invention. This example provides another preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane:

[0107] (1) Preparation of few-layer MXene nanosheet colloidal solution: Mix LiF powder with HCl solution and then add it to etch the Al layer in Ti3AlC2 MAX phase. After centrifugation, washing, and ultrasonic treatment, a few-layer MXene nanosheet colloidal solution is obtained;

[0108] The specific preparation method is as follows:

[0109] ③ First, prepare MXene powder by selective etching of the Al layer in Ti3AlC2 MAX phase.

[0110] At room temperature, add 4.8 g of LiF powder to 60 mL of 9 M HCl solution, stir for 30 min to obtain a uniform solution A. Subsequently, add 3 g of Ti3AlC2 MAX powder to solution A and then stir at room temperature for 48 h; after stirring, centrifuge at 3500 rpm for 5 min to collect the precipitate, and obtain impurity-free MXene powder;

[0111] ② Soak the impurity-free MXene powder obtained by centrifugation in HCl solution for 1 h to completely remove residual LiF; wash the soaked impurity-free MXene powder with deionized water until the solution is neutral, and the obtained precipitate is multi-layer MXene powder;

[0112] ③ To obtain few-layer MXene nanosheets, disperse the multi-layer MXene powder in 50 mL of deionized water, centrifuge at 3500 rpm for 15 min, and collect the supernatant obtained after each centrifugation, which is the few-layer MXene nanosheet colloidal solution.

[0113] After the MAX material is etched through step S1, a few-layer MXene nanosheet colloidal solution is formed. The X-ray diffraction pattern results are as Figure 2 (a) shown. The (002) diffraction peak of Ti moves from 9.5° to a lower angle, fully indicating the formation of the Ti3C2T x MXene phase and the increase in the interplanar spacing of the material. At the same time, the disappearance of the strong Al diffraction peak at 2θ≈39° indicates that Al atoms are selectively etched from the Ti3AlC2 MAX material, proving the formation of the MXene material. Figure 2 (b) is the Raman spectrum before and after etching. The prominent peaks in the figure are ~201 cm -1 assigned to the out-of-plane (A 1g ) vibrations of Ti, O, and C atoms, and ~723 cm -1 is another (A 1g ) vibration of C atoms. The regions around 230 - 470 cm -1 and 500 - 650 cm -1 can be assigned to the in-plane (E g)Vibration of surface groups. ~200 cm in MAX phase -1 The metal-carbon vibration peak weakens. After acid etching, the interlayer structure of MXene changes, and the surface may be covered with functional groups such as oxides, fluorides, or hydroxyl groups. Figure 3 Figure 4 shows the transmission electron microscope morphology and electron diffraction pattern of the prepared few-layer MXene nanosheet colloidal solution. From its morphology, it can be seen that the MXene material is a single-layer structure or a structure (the number of layers can be 1-3 layers), with a relatively high specific surface area and a relatively thin interlayer spacing; from the electron diffraction pattern, it can be obtained that it is a polycrystalline structure and there are no other impurities.

[0114] It should be noted that: using the in-situ hydrofluoric acid (HF) etching method, HF acid is prepared by reacting LiF with HCl for etching the Al element in Ti3AlC2 MAX phase, so that the multi-layer MXene is dispersed into single-layer or MXene materials, ensuring that it has a relatively high specific surface area and a relatively thin interlayer spacing; in this etching process, layered Ti3AlC2 MXene with -O, -OH, and -F at the ends is formed; then the etching solution is centrifuged and washed with water / ethanol to remove residual fluorides or other impurities. After washing, MXene needs to be dispersed by ultrasonic treatment to prevent agglomeration. Ultrasonic treatment is carried out at 30 W for 60 min until a uniform few-layer MXene nanosheet colloidal solution is formed for standby.

[0115] (2) Preparation of PVA solution: Dissolve polyvinyl alcohol (PVA) particles in water or organic solvents to obtain a PVA solution;

[0116] The specific preparation method is as follows:

[0117] Weigh 3 g of polyvinyl alcohol particles with a balance, disperse them in 27 g of deionized water, and perform oil bath magnetic stirring under heating conditions at 90 °C. Stir thoroughly for 2 h until completely dissolved to obtain a 10 wt% PVA solution.

[0118] It should be noted that: Dissolve polyvinyl alcohol (PVA) in water or organic solvents to obtain a PVA solution. During the dissolution process, methods such as heating, stirring, and ultrasonic treatment can be used to promote the dissolution of PVA. The concentration of the PVA solution is usually between 5% and 15%, specifically depending on the final thickness and properties of the required film. In addition, it should be noted that PVA with different molecular weights has different solubilities, so in actual operation, it needs to be adjusted and optimized according to specific conditions.

[0119] (3) Crosslinking reaction: Add formaldehyde solution to the PVA solution and stir evenly to obtain a crosslinked PVA / formaldehyde solution;

[0120] The specific preparation method is as follows:

[0121] Slowly add formaldehyde solution (37% formalin) to the PVA solution obtained in step S2, with the mass ratio of formaldehyde solution to PVA solution being 1:2, and mix thoroughly. Use an oil bath to maintain a constant temperature of 60°C and stir for 1 hour to allow the formaldehyde solution and PVA solution to fully cross-link. During the reaction, it can be seen that the viscosity of the solution is gradually increasing, that is, the cross-linking reaction is in progress. In order to promote further cross-linking reaction, a small amount of glacial acetic acid is added to the composite solution to keep the solution acidic.

[0122] It should be noted that the cross-linking reaction is usually carried out at a temperature of 50 to 80°C. Heating helps to increase the reaction rate and the degree of cross-linking, but too high a temperature may cause degradation of PVA. Acidic conditions are conducive to the cross-linking reaction. The cross-linking reaction time is generally 30 minutes to 2 hours, and the specific time can be adjusted according to the degree of cross-linking.

[0123] Cross-linking of PVA and formaldehyde can improve the mechanical strength of the membrane, improve its thermal stability, improve the stability and durability of the membrane, improve the anti-fouling performance of the membrane, and enhance the hydrophilicity and selectivity of the membrane. By controlling parameters such as cross-linking reaction time and formaldehyde concentration, the optimization parameters of the PVA membrane can be accurately adjusted.

[0124] (4) Preparation of composite solution: adding the few-layer MXene nanosheet colloidal solution to the cross-linked PVA / formaldehyde solution, stirring and mixing to form a PVA / formaldehyde / MXene composite solution;

[0125] The specific preparation method is:

[0126] Take 5 mL of the few-layer MXene nanosheet colloidal solution prepared in step S1 and slowly add it to the PVA / formaldehyde solution prepared in step S3, keep stirring at 60° C. for 1 h to mix them evenly, and obtain a PVA / formaldehyde / MXene composite solution for later use.

[0127] It should be noted that when the few-layer MXene nanosheet colloidal solution is slowly added to the PVA / formaldehyde solution prepared in step S3, heating and stirring can be used to make it uniformly mixed, and ultrasonic assisted dispersion can be used to increase the dispersibility of MXene. The amount of the few-layer MXene nanosheet colloidal solution added is controlled according to the quality of the PVA / formaldehyde solution and determined according to the mass ratio. The mass ratio of MXene to PVA is 1:120.

[0128] The doping of few-layer MXene nanosheet colloidal solution promotes the cross-linking of PVA and formaldehyde and reacts with them, thereby improving the stretchability of the composite solution. At the same time, MXene nanosheets also form microchannels on the surface, thereby improving the particle pass rate.

[0129] (5) Film-forming treatment: coating the composite solution on the surface of the substrate, and peeling it off after drying to obtain a composite semipermeable membrane;

[0130] The specific preparation method is as follows:

[0131] Use a pipette to measure 700 μL of the PVA / formaldehyde / MXene composite solution obtained in step S4, drop it onto a glass slide mold, and place it on a hot plate at 60 °C to heat and dry until the solution solidifies to form a semi-permeable membrane. After peeling the semi-permeable membrane from the mold, the PVA / formaldehyde / MXene composite semi-permeable membrane is obtained.

[0132] Figure 4 is the optical photo of the final PVA / formaldehyde / MXene composite semi-permeable membrane, forming a semi-permeable membrane material with uniform thickness and moderate mechanical strength.

[0133] Figure 5 is the test result of the Fourier transform infrared (FTIR) spectrum of the PVA / formaldehyde / MXene semi-permeable membrane; among them, the vibration peak at 3314 cm -1 belongs to the O-H vibration of the hydroxyl group in PVA or acetic acid, and the vibration peak at 2918 cm -1 belongs to the C-H stretching vibration of PVA, the vibration peak at 1730 cm -1 belongs to the C=O vibration peak of formaldehyde, the absorption peak at 1244 cm -1 is the stretching vibration of functional groups such as C-O and C-F in the MXene material, and the vibration peak at 1017 cm -1 belongs to the C-OH bending vibration in PVA. Through FTIR spectrum testing, the PVA / formaldehyde / MXene composite semi-permeable membrane contains the active ingredients of PVA, formaldehyde, and MXene, proving the effective compounding of the three materials.

[0134] It should be noted that: The PVA / formaldehyde / Mxene composite solution can also be poured onto other flat molds, and its coating method can use a spatula or other tools to spread evenly to form a film, or the spin coating method can be used to form a uniform film. The prepared semi-permeable membrane is air-dried naturally or dried in a hot air circulation oven at 60 - 80 °C for 24 hours to obtain a uniform PVA / formaldehyde / MXene composite semi-permeable membrane.

[0135] In this embodiment, the mass ratio of the formaldehyde solution to the PVA solution is 1:2. The contact angle of the prepared composite semi-permeable membrane is tested to evaluate the hydrophilic property of the semi-permeable membrane. The contact angle test results are as Figure 8 shown.

[0136] The composite semipermeable membranes with different mass ratios of formaldehyde solution to PVA solution were tested by Fourier transform infrared spectroscopy (FTIR). The test results were basically consistent, which proved that PVA, formaldehyde, and MXene could crosslink. Moreover, semipermeable membranes with uniform thicknesses could be formed at all three ratios, indicating that good crosslinking effects could be achieved at all three ratios. Finally, the contact angles of the semipermeable membranes with three different ratios were measured. It was found that the hydrophilicity was the best when the mass ratio of formaldehyde solution to PVA solution was 1:2, that is, it was the optimal preparation ratio among the three preparation ratios.

[0137] The composite semipermeable membrane prepared in this invention contains a three-dimensional structure formed by crosslinked PVA network, MXene nanosheets, and formaldehyde. Usually, PVA is used to prepare the flexible substrate of the semipermeable membrane. However, due to the poor water resistance of PVA, crosslinking with formaldehyde can improve the water resistance and strength of PVA while retaining its semipermeable membrane properties. In this invention, in order to further improve the properties of the composite semipermeable membrane, the two-dimensional material MXene was added, which can improve the tensile strength and durability of the semipermeable membrane. Since MXene is a hydrophilic material, its incorporation can promote the adsorption of the semipermeable membrane. Due to the layered characteristics of MXene, small molecules or ions can be screened through the interlayer channels, thus improving the separation efficiency. After incorporating MXene, its high specific surface area and abundant surface functional groups (-OH, -F, etc.) can capture radioactive ions through adsorption, thus realizing the selective separation of radionuclides. At the same time, as a two-dimensional material, MXene has certain antioxidant and radiation-resistant functions, which can make up for the deficiencies of PVA, so that the composite membrane can maintain its performance in a radiation environment. Therefore, the composite semipermeable membrane of this invention can be applied to the separation and treatment of radionuclides and organic pollutants in radioactive wastewater from nuclear power plants.

[0138] Finally, it should be pointed out that the methods and devices described in detail above are only examples, and those skilled in the art can modify these examples in different ways as long as they do not depart from the scope of this invention.

[0139] Importantly, the above examples are only used to illustrate the technical solutions of this invention and not to limit them. Although this invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of this invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of this invention, and they should all be covered within the scope of the claims of this invention.

Claims

1. A preparation method of a PVA / formaldehyde / MXene composite semipermeable membrane, characterized in that: It includes the following steps: (1) Preparation of few-layer MXene nanosheet colloidal solution: Mix LiF powder with HCl solution, then add it to the Al layer in Ti3AlC2 MAX phase for etching reaction. After centrifugation, washing, and ultrasonic treatment, a few-layer MXene nanosheet colloidal solution is obtained; (2) Preparation of PVA solution: Dissolve polyvinyl alcohol (PVA) particles in water or organic solvents to obtain a PVA solution; (3) Crosslinking reaction: Add formaldehyde solution to the PVA solution and stir evenly to obtain a crosslinked PVA / formaldehyde solution; (4) Preparation of composite solution: Add the few-layer MXene nanosheet colloidal solution to the crosslinked PVA / formaldehyde solution, stir and mix evenly to form a PVA / formaldehyde / MXene composite solution; (5) Film-forming treatment: Coating the composite solution on the surface of the substrate, and peeling it off after drying to obtain a composite semi-permeable membrane.

2. The preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane according to claim 1, characterized in that: In step (1), the LiF powder is 4.8 g, the HCl solution is 60 mL, the concentration of the HCl solution is 9 mol / L, and the etching reaction time is 48 hours.

3. The preparation method of the PVA / formaldehyde / MXene composite semipermeable membrane according to claim 1, wherein: In step (1), the centrifugation rate is 3000 rpm - 4000 rpm; washing is carried out with deionized water until the solution is neutral; ultrasonic treatment is carried out at a power of 20 W - 40 W for 60 minutes.

4. The preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane according to claim 1, wherein: In step (2), methods such as heating, stirring, or ultrasonic treatment are used to promote the dissolution of PVA.

5. The preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane according to claim 1, characterized in that: In step (2), the concentration of the PVA solution is 5% - 15%, the dissolution temperature is 80°C - 95°C, and the stirring time is 1 - 3 hours.

6. The preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane according to claim 1, characterized in that: In step (3), the mass ratio of the formaldehyde solution to the PVA solution is 1:1 - 1:2; constant temperature stirring is carried out at 50°C - 80°C under acidic conditions; the concentration of the formaldehyde solution is 37%; the crosslinking reaction time is 0.5 h - 2 h.

7. The preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane according to claim 1, wherein: In step (4), the stirring temperature is 50°C - 70°C.

8. The preparation method of the PVA / formaldehyde / MXene composite semi-permeable membrane according to claim 1, characterized in that: In step (5), the substrate is a glass slide, and the coating method is blade coating or spin coating; the drying temperature is 60°C - 80°C, and the drying time is 12 - 24 hours.

9. A composite semi-permeable membrane prepared by the method for preparing a PVA / formaldehyde / MXene composite semi-permeable membrane according to any one of claims 1 - 8.

10. The composite semipermeable membrane according to claim 9, wherein: The composite semi-permeable membrane comprises a three-dimensional structure formed by a crosslinked PVA network, MXene nanosheets, and formaldehyde.

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