A thin-layer nanocomposite film, a preparation method and application thereof

Abstract

The application discloses a kind of thin layer nanocomposite membranes and its preparation method and application, belong to membrane separation technical field.The method first polyamino monomer and 2,4,6-triformylphloroglucinol in organic solvent are catalyzed polymerization, and covalent organic polymer nanoparticles are prepared;Subsequently, the nanoparticles are reacted with sodium alginate to obtain hydrophilic modified nanoparticles;Finally, the modified particles are added to the aqueous phase, and the organic phase is interfacially polymerized and heat treated on the substrate to obtain a thin layer nanocomposite membrane.The application constructs a hydrophilic network and a charged nanopore, significantly improves the permeation flux of the membrane, while maintaining a high rejection rate for divalent salt, achieving the synergistic optimization of high flux and high ion selectivity, and can be applied to water treatment, desalination, magnesium-lithium separation and other fields.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and more specifically, to a thin-layer nanocomposite membrane, its preparation method, and its application. Background Technology

[0002] With the continuous advancement of membrane technology, membrane separation technology is increasingly widely used in the pharmaceutical industry, mainly in areas such as drug separation and purification, wastewater treatment, and water resource recycling. Membrane separation technology, with its comprehensive advantages including high separation efficiency, low energy consumption, no phase change, no need for chemical additives, and ease of modular integration, shows broad application prospects in the separation field.

[0003] Traditional polyamide composite nanofiltration membranes face a trade-off effect in practical applications, where it is difficult to simultaneously achieve good permeability and selectivity. To overcome this bottleneck, researchers have attempted to introduce nanoparticles into the polyamide separation layer to construct thin-layer nanocomposite membranes. The introduction of nanoparticles can, to some extent, regulate the microstructure of the separation layer, enhance membrane hydrophilicity, and construct additional water molecule transport channels, thereby improving membrane flux. However, several key issues still need to be addressed in traditional nanofiller systems.

[0004] First, the interfacial compatibility between inorganic nanofillers and the polymer matrix is ​​poor, making them prone to phase separation during interfacial polymerization and leading to membrane structural defects. Second, nanofillers, due to their high surface energy, are prone to aggregation, making it difficult to uniformly disperse them in the separation layer, affecting membrane integrity and selectivity. Traditional fillers are mostly inert materials, existing only as physical fillers, lacking the ability to actively control interfacial polymerization reactions and failing to fully realize their structural optimization potential. Furthermore, while some fillers can increase flux, this comes at the cost of reduced retention rate, failing to truly overcome the trade-off between permeability and selectivity.

[0005] To address the problems of poor interfacial compatibility, easy aggregation, and membrane surface defects inherent in traditional nanofillers in thin-film and nanocomposite membranes, researchers have been dedicated to developing novel nanomaterials that possess good compatibility with polymer matrices, can precisely construct transport channels, and can endow membranes with specific functions. Commonly used nanofillers include inorganic nanoparticles, carbon nanotubes, graphene oxide, and metal-organic frameworks. The introduction of these fillers can regulate the microstructure of the separation layer, enhance the hydrophilicity of the membrane surface, and provide additional water molecule transport channels, thereby significantly improving membrane flux. However, the interfacial compatibility issues between nanofillers and the polymer matrix, filler aggregation, and the resulting membrane surface defects still limit further optimization of TFN membrane performance.

[0006] Currently, research on physically doped TFN films mainly focuses on the selection, morphology control, and dispersion strategies of nanofillers. Researchers select nanomaterials of different dimensions (such as zero-dimensional nanoparticles, one-dimensional carbon nanotubes, and two-dimensional graphene oxide) and utilize their intrinsic pore structures or surface hydrophilic groups to construct physically interpenetrating water molecule transport pathways within the polyamide layer. To improve the interfacial compatibility between the filler and the polyamide matrix, physical methods such as surfactant dispersion, ultrasonic-assisted mixing, or non-covalent modification of the filler are often employed, aiming to achieve uniform dispersion of the filler in the organic or aqueous phase without destroying its intrinsic structure. However, physical doping essentially relies on the mechanical mixing between the filler and the monomer solution, making it difficult to precisely control the positioning and arrangement of the filler at the interface of the polymerization reaction. Random distribution of the filler in the separation layer can easily lead to local structural defects or uneven cross-linking, resulting in increased permeability often accompanied by decreased selectivity.

[0007] How to balance the filler loading and separation layer integrity within a physical doping framework, and achieve a synergistic improvement in high throughput and high selectivity, remains a key issue that urgently needs to be addressed in this field. Summary of the Invention

[0008] To overcome the shortcomings of the prior art, this invention provides a method for preparing a thin-layer nanocomposite membrane. Hydrophilic nanoparticles modified with sodium alginate are introduced into the interfacial polymerization process to regulate the reaction between the organic and aqueous phases. The introduction of hydrophilic sodium alginate demonstrates ultrafast water transport capabilities. The sodium alginate molecular chain contains numerous polar functional groups such as hydroxyl and carboxyl groups; introducing these groups into the membrane's separation layer significantly enhances the hydrophilicity of the membrane surface.

[0009] The preparation method provided by this invention specifically includes the following steps: S1. Polyamine monomers and 2,4,6-tricarboxymethyl phloroglucinol are mixed in an organic solvent and a catalyst is added. After polymerization, covalent organic polymer nanoparticles are obtained. S2. Covalent organic polymer nanoparticles, sodium alginate, and solvent are subjected to a hydrophilic modification reaction to obtain hydrophilic modified nanoparticles. S3. Hydrophilic modified nanoparticles are added to the aqueous phase and reacted with the organic phase on the substrate through interfacial polymerization, followed by heat treatment to prepare a thin-layer nanocomposite film.

[0010] Compared with existing technologies, this invention introduces hydrophilic modified nanoparticles into polyamide membranes to prepare thin-film nanocomposite (TFN) membranes, which can effectively overcome the long-standing "trade-off" effect of traditional polyamide membranes. Its core advantage lies in achieving synergistic optimization of the separation layer performance through the multifunctional properties of nanoparticles. After the hydrophilic nanoparticles are embedded in the polyamide separation layer, on the one hand, they can provide additional water molecule transport channels and shorten the transport path, thereby significantly increasing water flux without sacrificing the rejection rate; on the other hand, the introduction of hydrophilic nanoparticles can reduce the water contact angle on the membrane surface, enhance antifouling performance, and effectively hinder the adhesion of pollutants by forming a dense hydration layer.

[0011] In one possible implementation, in step S1, the polyamine monomer is selected from at least one of 2,5-diaminobenzenesulfonic acid, dihydroxybenzidine, p-phenylenediamine, and 1,2-dicarboxyaniline, and the molar ratio of 2,4,6-tricarboxymethyl phloroglucinol to the polyamine monomer is 1:(1-3).

[0012] Compared with the prior art, the present invention selects polyamine monomers with the above-mentioned specific structure, which can undergo efficient condensation polymerization with 2,4,6-tricarboxymethyl phloroglucinol to form covalent organic polymer nanoparticles with regular structure and uniform pores; by controlling the molar ratio of the two in the range of 1:(13), it can ensure that the monomers react fully and the polymerization is complete, avoiding monomer residue or insufficient reaction, so that the obtained nanoparticles have uniform morphology and stable structure, providing fillers with uniform structure and stable performance for subsequent hydrophilic modification and interfacial polymerization, thereby ensuring that the final thin-layer nanocomposite membrane has good hydrophilicity, structural integrity and stable separation performance.

[0013] In one possible implementation, in step S1, the organic solvent is selected from at least one of dichloromethane, mesitylene, and 1,4-dioxane; the catalyst is selected from at least one of p-toluenesulfonic acid, acetic acid, and trifluoroacetic acid, and the catalyst is added in solution form with a concentration of 19 mol / L.

[0014] Compared with existing technologies, this invention, through precise control of catalyst concentration (1-9 mol / L) and the selection of acidic catalysts such as p-toluenesulfonic acid, significantly influences the crystallinity, morphology, and pore structure of the product during the solvothermal synthesis of powders. In particular, the concentration of acetic acid as a catalyst has a significant impact. Furthermore, research shows that acetic acid can reversibly catalyze the condensation reaction of aldehydes and amine monomers, and an appropriate acid concentration helps promote the reversible repair process of the polymerization reaction, enabling the orderly growth of the network structure.

[0015] In one possible implementation, the polymerization reaction parameters in step S1 are as follows: temperature is 60-120°C, and time is 68-72 h.

[0016] In one possible implementation, in step S2, the solvent is selected from at least one of water, ethanol and methanol, and the mass ratio of covalent organic polymer nanoparticles to sodium alginate is 1:(10-100).

[0017] Compared to existing technologies, sodium alginate molecules are rich in hydrophilic functional groups. As their proportion in modified nanoparticles increases, the loading density of these hydrophilic groups in the polyamide separation layer also increases, effectively reducing the water contact angle on the membrane surface. This enhanced hydrophilicity not only facilitates the rapid adsorption and diffusion of water molecules on the membrane surface but also promotes the formation of a dense hydration layer, thereby strengthening the membrane's antifouling ability. Furthermore, the improved hydrophilic environment provides a lower energy barrier for water molecule transport within the membrane, further improving the overall permeability of the membrane.

[0018] In one possible implementation, the parameters of the hydrophilic modification reaction in step S2 are as follows: temperature 58-62℃, time 46-48h.

[0019] In one possible implementation, in step S3, the aqueous phase is an aqueous solution containing piperazine, the organic phase is an organic solution containing trimesoyl chloride, and the substrate is a PES substrate.

[0020] Compared with existing technologies, this invention, by using polyethersulfone (PES) as the substrate material, offers significant advantages in the preparation of thin-film composite membranes. Its excellent physicochemical properties and structural tunability provide ideal support for the construction of high-performance separation layers. PES can be conveniently controlled in terms of surface pore size, porosity, and sponge-like or finger-like pore structure through non-solvent-induced phase separation, thereby optimizing water transport resistance and providing ideal anchoring points for the formation of the polyamide separation layer. The moderate hydrophilicity / hydrophobicity of the PES substrate surface facilitates the uniform spreading of aqueous monomers and allows for good interfacial bonding with the polyamide layer, effectively preventing the separation layer from peeling off. Furthermore, PES, as a highly commercialized engineering plastic, offers controllable costs and mature film-forming processes, facilitating the large-scale preparation of TFN membranes. In summary, PES, with its structural stability, tunability, and excellent interfacial compatibility, becomes an ideal substrate material for supporting functionalized nanoparticle / polyamide composite layers.

[0021] In one possible embodiment, the concentration of piperazine is 0.1-0.4 wt.%, the concentration of pyromellitic methyl chloride is 0.1-0.2 wt.%, and the pore size of the PES substrate is 0.1-0.5 μm; Compared with existing technologies, this invention, by controlling the concentration of PIP within the range of 0.1-0.4 wt.%, allows for a higher concentration of PIP to provide more reactive monomers to participate in interfacial polymerization, promoting the formation of a thicker and denser network structure in the polyamide layer in the early stages. This is because a high concentration of PIP accelerates the diffusion flux to the reaction zone, prolonging the film growth time. Secondly, increasing the PIP concentration affects the degree of crosslinking and microstructure of the polyamide. Studies have shown that with increasing PIP proportions, the aggregate state of polyamide may change from an amorphous state to a more regular structure, reducing the average surface roughness of the membrane and forming a more uniform and dense separation layer. However, when the PIP concentration is too high, the excessively fast reaction rate may lead to localized overpolymerization at the interface, forming defects or an overly dense barrier layer, which in turn reduces water flux.

[0022] The amount of the hydrophilic modified nanoparticles in the aqueous phase is 0.03-0.11 g / L.

[0023] Compared with existing technologies, this invention demonstrates a continuous optimization trend in the structural integrity and separation performance of the composite membrane as the doping amount of nanoparticles increases, fully reflecting the functional regulatory role of nanoparticles in the polyamide selective layer. At low doping levels, while the introduction of nanoparticles initially imparts a hydrophilic modification effect to the membrane, its contribution to the densification of the selective layer is limited due to its sparse distribution. With the gradual increase in doping amount, the nanoparticles form a denser and more uniform distribution network in the polyamide matrix, allowing their abundant hydrophilic groups and charged properties to be fully utilized. This not only promotes the cross-linking and densification of the polyamide layer but also enhances the charge density on the membrane surface, thereby achieving effective sieving of salt ions with different valence states.

[0024] The second objective of this invention is to provide a thin-layer nanocomposite membrane prepared by the aforementioned method; the separation layer of the thin-layer nanocomposite membrane contains uniformly distributed hydrophilic modified nanoparticles, forming a hydrophilic network and charged nanopores.

[0025] The third objective of this invention is to provide an application of a thin-layer nanocomposite membrane in high-throughput, high-ion-selective separation, water treatment, desalination, and magnesium-lithium separation.

[0026] Preparation and Separation Performance of SA@Tp-DHBD Thin-Layer Nanocomposite Membranes. In the construction of thin-layer composite nanofiltration membranes, how to synergistically improve permeate flux and ion selectivity remains a core research challenge. To address this issue, this study designed and prepared a sodium alginate (SA)-functionalized Tp-DHBD nanocomposite material (SA@Tp-DHBD), which was introduced into a traditional interfacial polymerization system, successfully constructing an SA@Tp-DHBD thin-layer nanocomposite membrane. Water contact angle testing results showed that as the proportion of SA in the composite system increased, the membrane surface contact angle gradually decreased from 26.5° of the pure PA membrane to 18.5°, confirming that the introduction of SA significantly improved the hydration capacity of the membrane surface, which is beneficial for reducing water molecule transport resistance. Separation performance evaluation results indicate a significant functional synergistic effect between SA and Tp-DHBD. When the ratio of SA to Tp-DHBD is 100:1 and the doping concentration is 4 mg, the composite membrane exhibits optimal separation performance: a Na2SO4 rejection rate of up to 94%, and a water flux increase of approximately 60% compared to the pure polyamide membrane, from 15.3 L·m -2 ·h -1 ·bar -1 Increased to 23.98 L·m -2 ·h -1 ·bar -1 Its performance is superior to most traditional PA films. Attached Figure Description

[0027] Figure 1 These are SEM images of Tp-DHBD and SA@Tp-DHBD obtained in Example 1 of this invention; Figure 2 The graphs show the water flux changes of the thin-layer nanocomposite membranes prepared in Examples 1-4 of this invention and the pure polyamide membrane in Comparative Example 1. Figure 3 These are contact angle variation diagrams of the thin-layer nanocomposite films prepared in Examples 1-4 of the present invention and the pure polyamide film of Comparative Example 1. Figure 4 This is a comparison chart of the performance of the thin-layer nanocomposite film prepared in Examples 1-4 of the present invention and the pure polyamide film in Comparative Example 1 under different reaction ratios. Figure 5 This is a comparison chart of the performance of different systems of the thin-layer nanocomposite film prepared by this invention. Detailed Implementation

[0028] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described in detail below. It should be noted that the following embodiments are only used to illustrate the implementation methods and typical parameters of the present invention, and are not intended to limit the parameter range described in the present invention. Reasonable variations derived therefrom are still within the protection scope of the claims of the present invention.

[0029] It should be noted that the endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0030] Unless otherwise defined, all terms, symbols, and other scientific terms used herein are intended to have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In some instances, terms having a conventional meaning are defined herein for clarification or ease of reference, and such definitions should not be construed as indicating a significant difference from conventional understanding in the art. The technical methods described or referenced herein are generally well understood by those skilled in the art and employed by conventional methods. Unless otherwise stated, the use of commercially available kits, reagents, and instruments shall be performed according to the manufacturer's instructions and parameters.

[0031] The abbreviations and corresponding names in this embodiment are as follows: SA: Sodium alginate; PIP: Piperazine; TMC: Tristyroyl chloride.

[0032] Example 1 This embodiment provides a thin-layer nanocomposite film, which is prepared by the following method: Preparation of S1 and Tp-DHBD powders: Accurately weigh 97 mg of 2,5-dihydroxybenzidine and 63 mg of 2,4,6-tricarboxymethylresorcinol, and dissolve them in a mixed solvent of 1.5 mL of 1,4-dioxane and 1.5 mL of mesitylene. Sonicate the mixture for 3–5 minutes to ensure complete dissolution and uniform dispersion of the monomers. Then, add dropwise 0.5 mL of 6 mol / L acetic acid solution as a catalyst to promote the Schiff base reaction. Transfer the mixture to a hydrothermal reactor, seal it, and place it in a 120 °C oven for 72 h. After the reaction, wash the resulting reddish-brown solid product three times with tetrahydrofuran to remove unreacted monomers and residual impurities. Finally, dry it in a 60 °C oven for 12 h to obtain pure covalent organic polymer nanoparticles Tp-DHBD.

[0033] S2, Tp-DHBD powder hydrophilic modification: A certain amount of Tp-DHBD was added to a 100 mL round-bottom flask containing 20 mL of deionized water. After stirring for 30 min, the mass ratio of SA to Tp-DHBD was controlled at 10:1. The mixture was heated and stirred in an oil bath at 60 °C for 48 h. After the reaction was completed, the product was cooled to room temperature, and unreacted SA was removed by centrifugation (8000 rpm, 10 min). The obtained product was dried in a 60 °C oven for 12 h to obtain hydrophilic modified nanoparticles SA@Tp-DHBD.

[0034] S3. Prepare 35 mL of 0.4 wt.% PIP aqueous solution, add 4 mg of hydrophilic modified nanoparticles SA@Tp-DHBD, sonicate for 5 min, and place in a 35 ℃ water bath for later use. Prepare 0.1 wt.% TMC organic phase solution for later use. Pour 10 mL of 0.4 wt.% PIP aqueous solution containing SA@Tp-DHBD onto a 0.1 μm PES substrate, let stand for 2 min, then remove the surface solution and allow the membrane to air dry vertically to remove residual solution. Then add 10 mL of 0.1 wt.% TMC / n-hexane solution and react for 1 min for interfacial polymerization. After the reaction, thoroughly wash the membrane surface with n-hexane to remove unreacted monomers, and transfer the membrane to a 60 ℃ oven for heat treatment for 5 min to obtain a thin-layer nanocomposite membrane. Pre-pressurize in a cross-flow apparatus for 1 h, collect 2 mL of filtrate from the outlet using a graduated cylinder and time the process to calculate the water flux.

[0035] Example 2 This embodiment provides a thin-layer nanocomposite film. The only difference from Embodiment 1 is that the mass ratio of SA to Tp-DHBD in step S2 of this embodiment is 40:1. The rest is the same as in Embodiment 1 and will not be repeated here.

[0036] Example 3 This embodiment provides a thin-layer nanocomposite film. The only difference from Embodiment 1 is that the mass ratio of SA to Tp-DHBD in step S2 of this embodiment is 80:1. The rest is the same as in Embodiment 1 and will not be repeated here.

[0037] Example 4 This embodiment provides a thin-layer nanocomposite film. The only difference from Embodiment 1 is that the mass ratio of SA to Tp-DHBD in step S2 of this embodiment is 100:1. The rest is the same as in Embodiment 1 and will not be repeated here.

[0038] Example 5 This embodiment provides a thin-layer nanocomposite membrane. The only difference from Example 1 is that, in the preparation process of the thin-layer nanocomposite membrane in this embodiment, the specific steps of step S1 are as follows: 56.4 mg of 2,5-diaminobenzenesulfonic acid and 63 mg of 2,4,6-tricarboxymethyl phloroglucinol are dissolved in a mixed solvent of 1.5 mL of 1,4-dioxane and 1.5 mL of mesitylene. The mixture is ultrasonically treated for 3-5 minutes to ensure complete dissolution and uniform dispersion of the monomers. Subsequently, 0.5 mL of 6 mol / L acetic acid solution is added dropwise as a catalyst to promote the Schiff base reaction. The above mixture is transferred to a hydrothermal reactor, sealed, and placed in a 120 °C oven for a constant temperature reaction for 72 h. After the reaction, the resulting reddish-brown solid product is washed three times with tetrahydrofuran to remove unreacted monomers and residual impurities. Finally, it is dried in a 60 °C oven for 12 h to obtain pure covalent organic polymer nanoparticles Tp-DHBD.

[0039] Example 6 This embodiment provides a thin-layer nanocomposite membrane. The only difference from Example 1 is that in the preparation process of the thin-layer nanocomposite membrane in this embodiment, in step S1, 45.9 mg of 2-nitro-1,4-phenylenediamine and 63 mg of 2,4,6-tricarboxymethyl phloroglucinol are dissolved in a mixed solvent of 1.5 mL of 1,4-dioxane and 1.5 mL of mesitylene. The mixture is ultrasonically treated for 3-5 minutes to ensure that the monomers are fully dissolved and uniformly dispersed. Subsequently, 0.5 mL of 6 mol / L acetic acid solution is added dropwise as a catalyst to promote the Schiff base reaction. The above mixture is transferred to a hydrothermal reactor, sealed, and placed in an oven at 120 °C for 72 h for constant temperature reaction. After the reaction, the resulting reddish-brown solid product is washed three times with tetrahydrofuran to remove unreacted monomers and residual impurities. Finally, it is dried in an oven at 60 °C for 12 h to obtain pure covalent organic polymer nanoparticles Tp-DHBD.

[0040] Example 7 This embodiment provides a thin-layer nanocomposite film. The only difference from Embodiment 1 is that in the preparation process of the thin-layer nanocomposite film in this embodiment, the PIP concentration in step S3 is 0.2 wt.%, and the rest is the same as in Embodiment 1, which will not be repeated here.

[0041] Example 8 This embodiment provides a thin-layer nanocomposite membrane. The only difference from Embodiment 1 is that in the TFN membrane preparation process of this embodiment, the TMC concentration in step S2 is 0.2 wt.%, and the rest is the same as in Embodiment 1, which will not be repeated here.

[0042] Example 9 This embodiment provides a thin-layer nanocomposite film. The only difference from Embodiment 1 is that in the TFN film preparation process of this embodiment, the doping amount of SA@Tp-DHBD in the interfacial polymerization process in step S3 is 2 mg. The rest is the same as in Embodiment 1, and will not be repeated here.

[0043] Comparative Example 1 This comparative example provides a pure polyamide (PA) film.

[0044] Figure 1 The images are SEM images of Tp-DHBD and SA@Tp-DHBD prepared in Example 1. Figure 2 The graphs show the water flux changes of the thin-layer nanocomposite membranes prepared in Examples 1-4 and the pure polyamide membrane in Comparative Example 1. Figure 3 These are contact angle variation diagrams of the thin-layer nanocomposite films prepared in Examples 1-4 and the pure polyamide film of Comparative Example 1; Figure 4 This is a comparison chart showing the performance of the thin-layer nanocomposite films prepared in Examples 1-4 of the present invention and the pure polyamide film of Comparative Example 1 under different reaction ratios.

[0045] The applicant conducted performance tests on the thin-layer nanocomposite films prepared in Examples 1-4 and the pure polyamide film of Comparative Example 1. The specific test items and test steps are as follows: (1) Water flux measurement: The thin-layer nanocomposite membranes prepared in Examples 1-4 and the pure polyamide membrane of Comparative Example 1 were placed in a self-made cross-flow apparatus in the laboratory for water flux measurement. Preliminary data testing was performed first. After pre-pressurizing with deionized water at a pressure of 5 bar for 1 hour, 2 mL of filtrate was collected from the outlet using a graduated cylinder to calculate the water flux. The water flux of the COF membrane prepared in Example 1 was measured to be 23.98 L·m. -2 ·h -1 ·bar -1 .

[0046] Test results as follows Figure 2 As shown, from Figure 2 It can be seen that the TFN film prepared by the method of the present invention increases in size with the increase of the ratio of SA to Tp-DHBD, from 10:1 to 17 L·m -2 ·h -1 ·bar -1 The ratio of horizontal to vertical displacement increased to 100:1, resulting in 23.98 L·m. -2 ·h -1 ·bar -1 The values ​​are approximately 15.3 L·m, which is higher than that of pure PA membranes. -2 ·h -1 ·bar -1 .

[0047] from Figure 3 It can be seen that the enhanced hydrophilicity of the TFN membrane prepared by the method of this invention facilitates the adsorption and spreading of water molecules on the membrane surface, reduces the energy barrier required for water molecules to enter the membrane pores, and thus promotes the preferential permeation of water molecules. Simultaneously, the hydrophilic surface can form a stable hydration layer, which to some extent inhibits the adsorption and deposition of hydrophobic pollutants on the membrane surface, improving the membrane's antifouling ability. When the ratio of SA to Tp-DHBD is 100:1, the membrane surface hydrophilicity reaches its optimum, a result that highly matches its optimal water flux performance, further confirming the positive correlation between membrane hydrophilicity and water transport efficiency.

[0048] (2) Retention test of salt molecules. The salt molecules selected for the test are MgCl2, Na2SO4, MgSO4 and NaCl.

[0049] Weigh the corresponding salts and prepare 1000 ppm aqueous solutions of MgCl2, Na2SO4, MgSO4, and NaCl, respectively. For the retention experiment, first introduce the MgCl2 solution into the system and circulate for 20 min to stabilize the membrane performance. Then, collect the filtrate from the outlet using a graduated cylinder, and simultaneously take a sample of the original solution. Measure the conductivity values ​​of both using a conductivity meter. Finally, calculate the retention rate of each salt based on the change in conductivity.

[0050] The testing and calculation of Na2SO4, MgSO4 and Na2Cl are the same as those for MgCl2.

[0051] from Figure 4 As can be seen, the thin-film nanocomposite membrane prepared in Example 1 exhibits a Na₂SO₄ rejection rate of 97.5%, a MgCl₂ rejection rate of 76.9%, a MgSO₄ rejection rate of 93.4%, and a NaCl rejection rate of 27%. A sufficient number of nanoparticles can form a denser network of charge potential points and a more compact cross-linked network within the polyamide layer, effectively improving the membrane's sieving capacity. The increased doping amount did not cause a decrease in the rejection rate, indicating that within the selected doping range, the SA@Tp-DHBD nanoparticles possess good dispersibility and interfacial compatibility in the polyamide matrix, and the integrity of the selective layer is not compromised due to agglomeration or other issues.

[0052] Test results as follows Figure 5 As shown, Figure 5 This is a comparison chart of the performance of different systems of the thin-layer nanocomposite film prepared by this invention. Figure 5It can be seen that the flux increase of the physically mixed membrane in the four different systems (SA@Tp-DHBD, SA, Tp-DHBD, and SA+Tp-DHBD physical mixture) is also lower than that of the composite membrane. This indicates that although both the hydrophilicity of SA and the pore structure of Tp-DHBD are introduced, effective synergy between the two is not achieved. This can be attributed to the lack of effective interfacial interaction between the two components in the physically mixed system. During the physical mixing process, SA and Tp-DHBD exist only in the interfacial polymerization layer in a simple blending form, and they fail to form a tightly bound composite structure through hydrogen bonding and other interactions.

[0053] The results show that the thin-layer nanocomposite membrane of this invention exhibits excellent performance. A sodium alginate-modified Tp-DHBD composite nanomaterial (SA@Tp-DHBD) was prepared and introduced as a functional nanoparticle into the interfacial polymerization system to prepare an SA@Tp-DHBD thin-film nanostructured composite nanofiltration membrane. The introduction of SA effectively improves the hydrophilicity of the membrane surface and reduces the resistance to water molecule transport. Tp-DHBD, with its inherent charged sites, enhances the electrostatic repulsion of high-valence anions, while its regular rigid pore structure provides additional selective transport pathways for water molecules. This study provides a feasible strategy and experimental basis for the rational design of high-performance TFN nanofiltration membranes, demonstrating the effectiveness of the synergistic introduction of hydrophilic modification and rigid pore materials.

[0054] While the disclosure is as stated above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of this disclosure, and all such changes and modifications will fall within the protection scope of this invention.

Claims

1. A method for preparing a thin-layer nanocomposite film, characterized in that, Includes the following steps: S1. Polyamine monomers and 2,4,6-tricarboxymethyl phloroglucinol are mixed in an organic solvent and a catalyst is added. After polymerization, covalent organic polymer nanoparticles are obtained. S2. Covalent organic polymer nanoparticles, sodium alginate, and solvent are subjected to a hydrophilic modification reaction to obtain hydrophilic modified nanoparticles. S3. Hydrophilic modified nanoparticles are added to the aqueous phase and reacted with the organic phase on the substrate through interfacial polymerization, followed by heat treatment to prepare a thin-layer nanocomposite film.

2. The preparation method according to claim 1, characterized in that, In step S1, the polyamine monomer is selected from at least one of 2,5-diaminobenzenesulfonic acid, dihydroxybenzidine, p-phenylenediamine, and 1,2-dicarboxyaniline, and the molar ratio of 2,4,6-tricarboxymethyl phloroglucinol to the polyamine monomer is 1:(1-3).

3. The preparation method according to claim 1, characterized in that, In step S1, the organic solvent is selected from at least one of dichloromethane, mesitylene, and 1,4-dioxane; the catalyst is selected from at least one of p-toluenesulfonic acid, acetic acid, and trifluoroacetic acid, and the catalyst is added in solution form with a concentration of 19 mol / L.

4. The preparation method according to claim 1, characterized in that, In step S1, the polymerization reaction parameters are as follows: temperature is 60-120℃, and time is 68-72 h.

5. The preparation method according to claim 1, characterized in that, In step S2, the solvent is selected from at least one of water, ethanol and methanol, and the mass ratio of covalent organic polymer nanoparticles to sodium alginate is 1:(10-100).

6. The preparation method according to claim 1, characterized in that, In step S2, the parameters for the hydrophilic modification reaction are as follows: temperature is 58-62℃, and time is 46-48h.

7. The preparation method according to claim 1, characterized in that, In step S3, the aqueous phase is an aqueous solution containing piperazine, the organic phase is an organic solution containing trimesoyl chloride, and the substrate is a PES substrate.

8. The preparation method according to claim 7, characterized in that, The concentration of piperazine is 0.1-0.4 wt.%, the concentration of pyromellitic methyl chloride is 0.1-0.2 wt.%, and the pore size of the PES substrate is 0.1-0.5 μm; The amount of the hydrophilic modified nanoparticles in the aqueous phase is 0.03-0.11 g / L.

9. A thin-layer nanocomposite film, characterized in that, The thin-layer nanocomposite membrane is prepared by any one of the preparation methods described in claims 1-8; the separation layer of the thin-layer nanocomposite membrane is uniformly distributed with hydrophilic modified nanoparticles, forming a hydrophilic network and charged nanopores.

10. The application of the thin-layer nanocomposite membrane according to claim 9 in high-throughput high ion-selective separation, water treatment, desalination, and magnesium-lithium separation.

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