Preparation method of photocatalytic nanofiltration membrane based on IL-GQDs

Abstract

The invention discloses a preparation method of a photocatalytic nanofiltration membrane based on IL-GQDs. The method comprises the following steps: pretreating bagasse, and applying the pretreated bagasse and imidazole ionic liquid (IL) to preparation of ionic liquid functionalized graphene quantum dots (IL-GQDs); the IL-GQDs are uniformly distributed on the surface of the base membrane by utilizing a vacuum-assisted low-temperature interfacial polymerization technology to form an ultrathin photocatalytic active layer; and finally, curing and cleaning to obtain the photocatalytic nanofiltration membrane based on the IL-GQDs. According to the prepared IL-GQDs, the membrane surface is endowed with high specific surface area and excellent photocatalytic activity, and meanwhile, an additional water transmission channel can be established under the condition that the pore diameter of the membrane is not changed, so that the problem of trade-off of a traditional membrane is solved. In addition, through combination of low-temperature and vacuum-assisted interfacial polymerization technologies, the membrane structure is more uniform and compact, defects on the surface of the membrane are reduced, and the stability and the anti-pollution capacity of the membrane are improved. The membrane disclosed by the invention shows relatively high water flux, salt recovery rate and long-term stability in water treatment application.

Description

Technical Field

[0001] The present invention relates to the field of membrane separation technology, and particularly to a preparation method of a photocatalytic nanofiltration membrane based on IL-GQDs. Background Art

[0002] Water pollution and freshwater resource shortage have become important bottlenecks restricting sustainable development. Membrane separation technology has received extensive attention as an effective path to solve water resource problems. Among them, nanofiltration (NF) membranes have broad application prospects in the synchronous selective separation of various impurities. However, at present, the application efficiency of NF technology is limited by the counterbalance effect between permeability and selectivity and membrane fouling. Photocatalytic self-cleaning membranes can automatically remove contaminants on the membrane surface under light irradiation, restore membrane performance, and significantly extend the service life of the membrane. Therefore, developing a photocatalytic self-cleaning NF membrane with high flux, high selectivity, and long-term high-efficiency stability has become an effective way to solve the above problems.

[0003] Interfacial polymerization (IP) is a key process for preparing the polyamide (PA) functional layer of composite NF membranes. The nanostructure and surface charge characteristics of PA membranes have been proven to play a crucial role in determining membrane separation performance and photocatalytic performance. Due to the very fast reaction rate of traditional IP processes, heat and monomer mass transfer are often uneven, resulting in unsatisfactory PA membrane performance (such as low water flux and poor selectivity). Low-temperature interfacial polymerization (LIP) technology helps to generate a stable and uniform polymerization interface by reducing the temperature of the organic solvent and slowing down its evaporation rate, optimizing the hydrophilicity and structural uniformity of the membrane. However, when preparing large-sized membranes, the uniformity of monomer distribution remains an important challenge. Vacuum-assisted interfacial polymerization (VAIP) technology realizes the uniform distribution of amine monomers on the substrate membrane through the action of vacuum, overcomes the defect of uneven monomer distribution in traditional interfacial polymerization methods, and ensures the uniformity of the membrane layer structure, but there may still be problems such as too fast reaction rate and difficulty in precisely controlling the membrane layer structure.

[0004] Graphene quantum dots (GQDs), due to their excellent photocatalytic activity, good hydrophilicity, photostability, and small size, are ideal modification materials for improving the overall performance of membranes as they are easily compatible with membrane materials. GQDs have a large specific surface area and strong photocatalytic efficiency, and can effectively degrade organic pollutants under visible light irradiation. However, how to uniformly distribute these nanomaterials in the membrane material and maintain the stability and photocatalytic efficiency of GQDs during the long-term use of the membrane material remains a challenge to be solved urgently. Summary of the Invention

[0005] To overcome the above technical problems, the object of the present invention is to provide a preparation method of an IL-GQDs-based photocatalytic nanofiltration membrane. This method effectively couples the vacuum-assisted low-temperature interfacial polymerization (VALIP) technology formed by combining the LIP technology and the VAIP technology with the IL-assisted preparation of imidazole-functionalized graphene quantum dots. The VALIP technology promotes the main enrichment of amine monomers near the reaction boundary under the surface of the substrate membrane, enabling the number of amines reacting with TMC to reach the optimum, while controlling the IP reaction rate to form a thin and dense network structure; IL improves the separation efficiency of electron-hole pairs by optimizing the energy band structure of graphene quantum dots, thereby effectively enhancing the stability and efficiency of the GQDs photocatalyst. The VALIP technology also ensures the uniform distribution and stable embedding of IL-GQDs and monomers on the surface of the substrate membrane, optimizing the microstructure of the membrane and the uniformity of the functional layer. The introduction of IL-GQDs and the organic combination of the VALIP technology achieve the effective regulation of the permeation performance and photochemical performance of the nanofiltration membrane. The prepared IL-GQDs-based photocatalytic nanofiltration (IL-GQDs-PNF) membrane has high self-cleaning performance and persistent photocatalytic activity.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A preparation method of an IL-GQDs-based photocatalytic nanofiltration membrane, comprising the following steps:

[0008] (1) First, perform high-pressure CO2 hydrothermal pretreatment on bagasse, then cool and filter to obtain pretreated bagasse;

[0009] Then, add the bagasse to the preheated imidazole-based ionic liquid, stir and dissolve to obtain a mixed solution;

[0010] After that, slowly pour the mixed solution into distilled water and stir well to obtain an IL-GQDs dispersion. Then, centrifuge the IL-GQDs dispersion to remove incompletely reacted residues, dialyze for 3 - 5 days to remove residual ionic liquids, obtain a purified IL-GQDs solution, and finally perform freeze-drying on it to obtain IL-GQDs;

[0011] (2) Place the substrate membrane in a vacuum filtration device. Then, filter and deposit 10 - 15 mL of an MPD-IL-GQDs composite aqueous solution containing IL-GQDs and m-phenylenediamine (MPD) on the surface of the substrate membrane under normal temperature vacuum (100 KPa) conditions; ensure that the surface of the substrate membrane is covered with a complete and uniform coating to obtain an amine-saturated substrate membrane;

[0012] Next, a 10 - 15 mL trimellitic acid chloride (TMC) organic phase solution at a low temperature was contacted with the amine-saturated base membrane for 30 s - 60 s to initiate interfacial polymerization, obtaining an IL-GQDs membrane. Subsequently, it was cured at 40 - 60 °C for 5 - 20 min. Finally, after washing with deionized water, an IL-GQDs-PNF membrane was obtained.

[0013] In step (1), the specific steps for the high-pressure CO2 hydrothermal pretreatment of bagasse are as follows: After drying bagasse at 40 - 50 °C to a constant weight, it was crushed to a particle size of 0.2 - 0.5 mm, and added to deionized water at a solid-liquid ratio of 1:10 - 15, and fully dispersed;

[0014] The mixture of bagasse and deionized water was transferred to a high-pressure reactor, sealed, and CO2 was introduced to 6 MPa, and the reactor was heated to 180 °C and maintained for 0.5 hours to remove part of the hemicellulose and lignin. After the reaction, the pressure was slowly reduced to room temperature to obtain pretreated bagasse.

[0015] In the said step (1), the imidazolium-based ionic liquid is any one of 1-butyl-3-methylimidazolium acetate ([BMIM][OAc]), 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM][HSO4]), 1-butyl-3-methylimidazolium dihydrogen phosphate ([BMIM][H2PO4]), 1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][CH3SO3]), 1-butyl-3-methylimidazolium p-toluenesulfonate ([BMIM][p-TSA]).

[0016] In the said step (1), the specific steps for the preheated imidazolium-based ionic liquid are as follows: The imidazolium-based ionic liquid was heated at 80 °C for 10 minutes until the solid state was completely converted into a clear liquid state, then the preheated imidazolium-based ionic liquid was obtained.

[0017] In the said step (1), the specific preparation steps of the IL-GQDs are as follows:

[0018] The pretreated bagasse was added to the preheated imidazolium-based ionic liquid at a mass ratio of 1:5 - 8, and stirred in groups for 4 - 12 hours at a temperature of 80 °C - 120 °C and under atmospheric pressure conditions for dissolution;

[0019] After dissolution is completed, the mixed solution is slowly poured into distilled water and stirred thoroughly to obtain an IL-GQDs dispersion. The IL-GQDs dispersion is centrifuged at high speed, and the supernatant is retained. After removing the unreacted impurities in the precipitate, the supernatant is filtered using a microporous filter membrane. The filtered solution is placed in a dialysis bag with a molecular weight cut-off of 500 - 1000 Da and dialyzed for 3 - 5 days, with the dialysis solution being replaced regularly every day. After dialysis, it is freeze-dried at -15 °C for 24 - 48 hours to obtain IL-GQDs.

[0020] In the step (2), the base membrane is selected from any one of polyacrylonitrile, polysulfone, or polyethersulfone ultrafiltration membranes.

[0021] In the step (2), the MPD-IL-GQDs composite aqueous solution is prepared from 1 - 2 wt% MPD, 1 - 1.5 wt% IL-GQDs, 0.1 - 0.2 wt% sodium dodecyl sulfate (SDS), 1 - 2 wt% triethylamine (TEA), and deionized water.

[0022] In the step (2), the temperature of the TMC organic solution in the low-temperature state is -20 °C - 10 °C.

[0023] In the step (2), the concentration of the TMC organic phase solution is 0.15 - 0.2 wt%, and the solvent of the TMC organic solution is any one of ethyl acetate, ethanol, n-hexane, acetone, trichloroethylene, and isopropanol.

[0024] The IL-GQDs in the IL-GQDs-PNF membrane usually have a diameter of about 5 - 10 nanometers. This size range helps to ensure its high specific surface area and excellent photocatalytic activity.

[0025] The surface of IL-GQDs is usually rich in oxidation functional groups such as carboxyl (-COOH) and hydroxyl (-OH), and at the same time, specific functional groups from imidazole-based ionic liquids (such as [BMIM]+ residues) are introduced. This functionalization not only helps to improve the dispersion of GQDs in water but also is beneficial to adjusting its energy band structure and enhancing the photocatalytic efficiency.

[0026] The size of the thin and dense membrane structure of the base membrane is 100 - 150 nm.

[0027] The beneficial effects of the present invention:

[0028] (1) Improve photocatalytic performance and separation efficiency

[0029] By adopting IL-functionalized IL-GQDs, this technical solution significantly enhances the photocatalytic performance of the membrane. The nano-scale and high specific surface area of IL-GQDs contribute to improving their photocatalytic efficiency, especially in applications such as environmental pollutant degradation and water treatment. Through the functionalization of special groups of imidazole-based ionic liquids, the dispersibility of IL-GQDs in water is enhanced, and the regulation of the energy band structure further improves the photocatalytic activity of the membrane. The VALIP technology ensures the uniform distribution and stable embedding of IL-GQDs on the surface of the base membrane, forming an ultra-thin active layer, which further enhances the light absorption rate and photocatalytic efficiency of the membrane, making it more efficient in photocatalytic reactions.

[0030] (2) Optimize the membrane structure and uniformity

[0031] The combination of VAIP method and LIP method can effectively optimize the membrane structure and uniformity. Vacuum helps to ensure the uniform distribution of amine monomers, avoiding non-uniform reactions in traditional interfacial polymerization, while low temperature slows down the polymerization reaction rate, ensuring the uniformity and fineness of the membrane layer formation. This optimized membrane structure not only enhances the denseness of the membrane, but also effectively reduces the roughness and defects on the membrane surface, thereby improving the long-term stability, photocatalytic activity and separation efficiency of the membrane. At the same time, IL-GQDs can overcome the "trade-off" problem between "water flux and selectivity" common in traditional membrane technologies by providing additional water transport channels without changing the pore size of the PA membrane, significantly increasing the water flux while maintaining high selectivity of the membrane.

[0032] (3) Enhance the stability and anti-fouling ability of the membrane

[0033] The VALIP technology combining VAIP and LIP improves the stability and anti-fouling ability of the membrane during long-term use. The low-temperature reaction controls the cross-linking degree of the membrane layer, reducing the swelling and shrinking phenomena of the membrane, while vacuum assistance ensures the uniform distribution of monomers, reducing the defects on the membrane surface, all of which contribute to improving the durability of the membrane. In addition, the introduction of IL-GQDs makes the membrane surface rich in oxidation functional groups, enhancing the anti-fouling property of the membrane, which can effectively reduce the adsorption of pollutants on the membrane surface and extend the service life of the membrane. The hydrophilicity of IL-GQDs enhances the water flux of the membrane, reduces the adsorption of pollutants, and ensures the long-term efficient operation of the membrane. Especially in practical applications, it can effectively improve the anti-fouling property and durability of the membrane.

[0034] (4) Improve the operability and production efficiency of the membrane preparation process

[0035] The combination of low temperature and vacuum-assisted interfacial polymerization technology provides higher controllability of the preparation process. During the membrane preparation process, parameters such as monomer concentration and reaction time can be precisely controlled to optimize the performance of the membrane. This method avoids the problems of uneven membrane surface and excessive membrane thickness in traditional interfacial polymerization, ensuring the consistency and stability of the membrane layer. Through this optimized preparation process, the production efficiency of the membrane can be improved, energy consumption can be reduced, and the industrial production of the membrane can be made more efficient and operational. In addition, IL not only promotes the formation of quantum dots as a solvent, but also enhances its optical properties through surface modification, so that it exhibits stronger photocatalytic activity and higher quantum efficiency (PLQY) in photocatalytic reactions. Compared with unfunctionalized quantum dots, IL-functionalized quantum dots reduce aggregation and improve dispersibility in the reaction medium, thereby improving catalytic efficiency and reactivity, providing more flexible operating space for efficient preparation of membranes.

[0036] (5) Ionic liquids are recyclable

[0037] After the synthesis of IL-GQDs is completed, absolute ethanol is added as an antisolvent to the mixture containing IL and GQDs, which can effectively recover the IL in the reaction residual liquid. Specifically, the recovered IL can be reused in the synthesis of IL-GQDs or other related nanomaterial synthesis processes after appropriate purification treatment, thereby achieving recycling. This not only effectively reduces the consumption of raw materials, but also reduces the impact on the environment during the reaction process. Since ionic liquids have good chemical stability, they can still maintain a high catalytic activity in multiple cycles, ensuring the efficiency and repeatability of the reaction. Therefore, the ionic liquid used in the present invention not only has good recoverability and regeneration, but also can maintain its performance in multiple cycles, reduce production costs, reduce environmental pollution, and meet the requirements of green and sustainable development. DETAILED DESCRIPTION

[0038] The present invention is further described in detail below in conjunction with embodiments.

[0039] The present invention provides a method for preparing a carbon quantum dot photocatalytic multi-separation layer composite nanofiltration membrane, comprising the following steps:

[0040] Step 1: Dry the bagasse at 40-50°C to constant weight, crush it to a particle size of 0.2-0.5 mm, add it to deionized water at a solid-liquid ratio of 1:15, and fully disperse it. Then, transfer the mixture of bagasse and deionized water to a high-pressure reactor, seal it, introduce CO2 to 6MPa, and heat the reactor to 180°C for 0.5 hours to remove some hemicellulose and lignin. After the reaction is completed, slowly reduce the pressure to room temperature to obtain a pretreated bagasse sample

[0041] One of 1-butyl-3-methylimidazolium acetate (BMIMOAc), 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM][HSO4]), 1-butyl-3-methylimidazolium dihydrogen phosphate ([BMIM][H2PO4]), 1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][CH3SO3]), and 1-butyl-3-methylimidazolium p-toluenesulfonate ([BMIM][p-TSA]) is used as a reaction medium and a functionalized molecule for preparing graphene quantum dots. The imidazolium-based ionic liquid is heated at 80° C. for 10 minutes until the solid state is completely converted into a clear liquid state, thereby obtaining a preheated imidazolium-based ionic liquid.

[0042] The pretreated bagasse was added to the preheated imidazole ionic liquid at a mass ratio of 1:5, and stirred for 4, 8 and 12 hours respectively at 80°C, 100°C and 120°C and normal pressure to fully dissolve it. Stir at a suitable temperature to fully dissolve it. After that, the mixed solution was slowly poured into distilled water and stirred thoroughly, and then the mixed solution was slowly poured into distilled water and stirred thoroughly to precipitate IL-GQDs to obtain an IL-GQDs dispersion. The IL-GQDs dispersion was then centrifuged to remove the incompletely reacted residue, dialyzed for 3-5 days to remove the residual ionic liquid, and a purified IL-GQDs solution was obtained, which was finally freeze-dried to obtain IL-GQDs.

[0043] Step 2, using an ultrafiltration membrane prepared from one of polyacrylonitrile, polysulfone or polyethersulfone ultrafiltration membranes as a raw material as a base membrane, and placing the base membrane in a vacuum filtration device. Next, 10 mL of MPD-IL-GQDs composite aqueous phase solution containing IL-GQDs solution and MPD is filtered and deposited on the surface of the base membrane under vacuum (100 KPa) at room temperature; ensuring that the surface of the base membrane is fully and evenly covered with a coating to obtain an amine-saturated base membrane; then, 10 mL of 0.15 wt% TMC organic solution in a low temperature state (-20 ° C, -10 ° C, 0 ° C, 10 ° C) is contacted with the amine-saturated base membrane for 30s to 60s to initiate interfacial polymerization to obtain a photocatalytic intermediate membrane based on IL-GQDs. It is then cured at 40-60 ° C for 5.0-20 min. Finally, after washing with deionized water, an IL-GQDs-PNF membrane is obtained.

[0044] The MPD-IL-GQDs composite aqueous solution was prepared from 2 wt % MPD, 1.2 wt % IL-GQDs, 0.2 wt % SDS, 1 wt % TEA and deionized water.

[0045] Example 1: Preparation of IL-GQDs and standard preparation process of nanofiltration membrane

[0046] (1) After drying the bagasse at 45°C to constant weight, crush it to a particle size of 0.35 mm, add it into deionized water at a solid-liquid ratio of 1:15, and fully disperse it. Transfer the mixture to a high-pressure reactor, seal it, introduce CO2 to 6MPa, heat it to 180°C, and maintain it for 0.5 hours to remove some hemicellulose and lignin. After the reaction is completed, slowly reduce the pressure to room temperature to obtain a pretreated bagasse sample.

[0047] 1-Butyl-3-methylimidazolium acetate (BMIMOAc) was heated at 80°C for 10 minutes until the solid was completely converted into a clear liquid to obtain a preheated imidazole-based ionic liquid. The pretreated bagasse was added to the preheated BMIMOAc at a mass ratio of 1:5, and stirred for 8 hours at a temperature of 100°C and normal pressure to fully dissolve it. After that, the mixed solution was slowly poured into distilled water and stirred thoroughly to obtain an ionic liquid-functionalized graphene quantum dot (IL-GQDs) dispersion, and then the IL-GQDs dispersion was centrifuged to remove the incompletely reacted residue, dialyzed for 3-5 days to remove the residual ionic liquid, and a purified IL-GQDs solution was obtained, which was finally freeze-dried to obtain IL-GQDs.

[0048] (2) Take a polysulfone ultrafiltration membrane as a base membrane and place it in a vacuum filtration device. 10mL of the MPD-IL-GQDs composite aqueous solution containing IL-GQDs solution and MPD was filtered and deposited onto the surface of the base membrane under vacuum (100KPa) at room temperature to ensure that the surface of the base membrane was fully and evenly covered to obtain an amine-saturated base membrane. Subsequently, 10mL of 0.15wt% TMC organic solution cooled to -20°C was contacted with the amine-saturated base membrane for 60 seconds to induce an interfacial polymerization reaction. Thereafter, the membrane was cured at 50°C for 10 minutes. Finally, it was thoroughly rinsed with deionized water to obtain an IL-GQDs-PNF membrane.

[0049] Example 2: Preparation of IL-GQDs by long-term stirring at low temperature combined with lower temperature interfacial polymerization

[0050] (1) Treat bagasse in the same pretreatment method as in Example 1. Heat BMIMOAc at 80°C for 10 minutes to obtain a preheated imidazole ionic liquid. Add the pretreated bagasse to BMIMOAc at a mass ratio of 1:5, and stir for 12 hours at 80°C and normal pressure to fully dissolve it. Subsequent operations are the same as in Example 1, and IL-GQDs are finally obtained.

[0051] (2) Take the polysulfone ultrafiltration membrane as the base membrane and place it in a vacuum filtration device. Filter and deposit 10 mL of the MPD-IL-GQDs composite aqueous solution containing the IL-GQDs solution and MPD onto the surface of the base membrane under normal temperature and vacuum conditions to obtain an amine-saturated base membrane. Subsequently, bring 10 mL of a 0.15 wt% TMC organic solution cooled to -10°C into contact with the amine-saturated base membrane for 50 seconds to initiate an interfacial polymerization reaction. After that, cure the membrane at 45°C for 15 minutes. Finally, thoroughly wash it with deionized water to obtain the IL-GQDs-PNF membrane.

[0052] Example 3: Preparation of the combination of IL-GQDs by high-temperature long-time stirring and interfacial polymerization at medium temperature

[0053] (1) Pretreat the bagasse according to the method of Example 1. Heat BMIMOAc at 80°C for 10 minutes. Add the pretreated bagasse to BMIMOAc at a mass ratio of 1:5, and stir at 120°C under normal pressure for 4 hours to fully dissolve it. The subsequent operations are the same as those in Example 1, and finally, IL-GQDs are obtained.

[0054] (2) Take the polysulfone ultrafiltration membrane as the base membrane and place it in a vacuum filtration device. Filter 10 mL of the MPD-IL-GQDs composite aqueous solution containing the IL-GQDs solution and MPD onto the base membrane under normal temperature and vacuum conditions to obtain an amine-saturated base membrane. Subsequently, bring 10 mL of a 0.15 wt% TMC organic solution cooled to 0°C into contact with the amine-saturated base membrane for 40 seconds to initiate an interfacial polymerization reaction. After that, cure the membrane at 60°C for 20 minutes. Finally, thoroughly wash it with deionized water to obtain the IL-GQDs-PNF membrane.

[0055] Example 4: Preparation of the combination of IL-GQDs by medium-temperature medium-time stirring and interfacial polymerization at a relatively high temperature

[0056] (1) Pretreat the bagasse according to the method of Example 1. Heat BMIMOAc at 80°C for 10 minutes. Add the pretreated bagasse to BMIMOAc at a mass ratio of 1:5, and stir at 100°C under normal pressure for 12 hours to fully dissolve it. The subsequent operations are the same as those in Example 1, and finally, IL-GQDs are obtained.

[0057] (2) Take the polysulfone ultrafiltration membrane as the base membrane and place it in a vacuum filtration device. Filter 10 mL of the MPD-IL-GQDs composite aqueous solution onto the base membrane under normal temperature and vacuum conditions to obtain an amine-saturated base membrane. Subsequently, bring 10 mL of a 0.15 wt% TMC organic solution cooled to 10°C into contact with the amine-saturated base membrane for 30 seconds to initiate an interfacial polymerization reaction. After that, cure the membrane at 50°C for 10 minutes. Finally, thoroughly wash it with deionized water to obtain the IL-GQDs-PNF membrane.

[0058] Example 5: Preparation of IL-GQDs by Stirring at High Temperature for a Long Time and Combining with Interfacial Polymerization at Low Temperature

[0059] (1) The dried bagasse was pretreated according to the method of Example 1. BMIMOAc was heated at 80 °C for 10 minutes. The pretreated bagasse was added to BMIMOAc at a mass ratio of 1:5, and stirred at 120 °C under normal pressure for 8 hours to dissolve it completely. After dissolution, post-treatment was carried out according to the previous method, and finally IL-GQDs were obtained.

[0060] (2) A polysulfone ultrafiltration membrane was taken as the base membrane and placed in a vacuum filtration device. 10 mL of MPD-IL-GQDs composite aqueous solution was filtered onto the base membrane under normal temperature and vacuum conditions to obtain an amine-saturated base membrane. Subsequently, 10 mL of 0.15 wt% TMC organic solution cooled to -20 °C was contacted with the amine-saturated base membrane for 60 seconds to initiate an interfacial polymerization reaction. After that, the membrane was cured at 50 °C for 10 minutes. Finally, it was thoroughly washed with deionized water to obtain the IL-GQDs-PNF membrane.

[0061] The effects of the present invention are further illustrated by the following experiments.

[0062] 1. Nanofiltration membrane separation performance

[0063] Under the operating pressure of 0.6 MPa and the condition of 20 °C, a 2 g / L magnesium sulfate solution was used to measure the flux and rejection rate of the nanofiltration membrane. The flux and rejection rate results of the nanofiltration membranes prepared in Examples 1 to 5 are shown in Table 1.

[0064] Table 1 Flux and rejection rate of the nanofiltration membranes prepared in the examples for filtering magnesium sulfate solution

[0065]

[0066] 2. Membrane anti-fouling performance

[0067] Under visible light with an illumination intensity of 1000 lx and an operating pressure of 0.6 MPa, a 10 mg / L bovine serum albumin (BSA) solution was used for a 24-hour membrane fouling experiment. After that, a membrane cleaning experiment was carried out using deionized water, and the flux of the nanofiltration membrane after cleaning was measured using deionized water, and the flux decay rate of the nanofiltration membrane was calculated.

[0068] Table 2 Performance of the nanofiltration membranes prepared in the examples in membrane fouling and membrane cleaning experiments

[0069]

[0070] 3. Photocatalytic self-cleaning performance

[0071] Photocatalytic degradation rate

[0072] To evaluate the photocatalytic degradation ability of the IL-GQDs-PNF membrane, an organic pollutant solution of methyl orange with a concentration of 50 mg / L was first prepared. The IL-GQDs-PNF membrane was immersed in the pollutant solution and stirred for 30 minutes under darkroom conditions to reach the adsorption equilibrium state. Subsequently, it was subjected to light treatment under visible light with a wavelength range of 400 - 700 nm, and the light intensity was set to 100 mW / cm 2 , and the irradiation time was 1 hour. During the light irradiation process, samples were taken every 15 minutes, and the change in the concentration of methyl orange in the solution was measured using a UV-visible spectrophotometer to calculate the degradation efficiency. C0 is the initial pollutant concentration, and C t is the pollutant concentration after the light irradiation time t.

[0073]

[0074] Self-cleaning efficiency test

[0075] To evaluate the anti-pollution performance and flux recovery ability of the IL-GQDs-PNF membrane during the photocatalytic self-cleaning process, the IL-GQDs-PNF membrane was first contaminated with BSA, and the pure water flux (J initial ) after contamination was recorded. Then, under visible light with a wavelength range of 400 - 700 nm and a light intensity set to 100 mW / cm 2 , the membrane was subjected to 1 hour of self-cleaning light treatment. After the self-cleaning treatment, the pure water flux (J recovered ) was measured again, and the flux recovery rate (%) was calculated using the following formula

[0076]

[0077] Photocatalytic stability test

[0078] To evaluate the photocatalytic stability of the IL-GQDs-PNF membrane during long-term use, multiple cyclic photocatalytic degradation experiments were conducted. After each cycle, the membrane was re-immersed in a new BSA solution, subjected to light treatment under visible light conditions, and the change in the degradation efficiency of the membrane was measured. By comparing the degradation efficiencies after different numbers of cycles, the photocatalytic stability of the membrane and the retention of its activity during long-term use were evaluated.

[0079] Table 3 Performance of the nanofiltration membrane prepared in the examples in the membrane pollution and membrane cleaning experiments

[0080]

[0081] In summary, the photocatalytic nanofiltration membrane prepared by the present invention based on IL-GQDs and VALIP technology exhibits excellent performance in terms of pure water flux, salt rejection rate, anti-fouling performance, and photocatalytic activity, significantly superior to the comparative membrane. In particular, Examples 3 and 5, combining high-temperature preparation and low-temperature interfacial polymerization conditions, further enhance the overall performance of the membrane, demonstrating the synergistic effect of the introduction of IL-GQDs and VALIP technology. This membrane has the ability to resist membrane fouling, improve photocatalytic degradation efficiency, and achieve self-cleaning function, greatly expanding the application potential of nanofiltration membranes in the field of water treatment, and possessing significant research and application value.

[0082] In addition, the VALIP technology can generate an ultrathin separation layer with uniform pore size on the membrane surface, significantly enhancing the rejection effect of the substances to be separated. Through vacuum assistance and low-temperature interfacial polymerization, the VALIP technology ensures the uniform distribution and stable embedding of IL-GQDs and polymerization monomers on the membrane surface, improves the surface properties of the membrane, enhances the flux and rejection rate, while increasing the membrane preparation efficiency and broadening the application scope of nanofiltration membranes. The membrane of the present invention significantly enhances the hydrophilicity and anti-fouling ability, improving the overall performance and service life.

[0083] Uniform distribution of graphene quantum dots (GQDs):

[0084] During the preparation process, ion-functionalized graphene quantum dots (IL-GQDs) can be uniformly distributed on the membrane surface. Graphene quantum dots have a larger specific activation and excellent photocatalytic activity, which can significantly enhance the photocatalytic performance and hydrophilicity of the membrane. Through low-temperature interfacial polymerization (LIP) and vacuum-assisted interfacial polymerization (VAIP) technologies, it can ensure the uniform distribution of IL-GQDs on the membrane surface, avoiding the aggregation phenomenon of IL-GQDs that may occur in traditional methods. This uniform distribution not only improves the performance of the membrane but also enhances the long-term use stability and photocatalytic efficiency of the membrane.

[0085] Hydrophilicity and self-cleaning property of the membrane surface:

[0086] The addition of IL-GQDs can significantly improve the hydrophilicity of the membrane, increase the high-efficiency water power of the membrane, and reduce the resistance of water molecule flow on the membrane surface. In addition, the photocatalytic self-cleaning membrane can effectively degrade the surface tension of the membrane under light irradiation, thereby restoring the performance of the membrane and extending the duration of the membrane. This self-cleaning property makes the membrane have higher stability and performance in practical applications.

[0087] Thin and dense polyamide network structure:

[0088] The Vacuum-Assisted Low-Temperature Interfacial Polymerization (VALIP) technique combines the advantages of low-temperature interfacial polymerization and vacuum-assisted techniques, effectively optimizing the membrane preparation process, especially in addressing the membrane surface non-uniformity caused by differences in monomer diffusion rates in traditional interfacial polymerization. Low-temperature interfacial polymerization reduces the reaction temperature, slows down the diffusion rates of aqueous and oil-phase monomers, thereby reducing the self-limiting effect caused by overly fast reaction rates in the traditional interfacial polymerization process, ensuring uniform deposition of the polymer on the membrane surface, and avoiding problems such as uneven membrane layer thickness or local voids. Vacuum-assisted interfacial polymerization enables the uniform distribution of aqueous amine monomers on the surface of the substrate membrane through external vacuum action, overcoming the defect of uneven monomer distribution in traditional methods and further improving the uniformity of the membrane layer;

[0089] This combined technique not only optimizes the membrane structure but also improves the mechanical strength and photocatalytic performance of the membrane. The thin and dense membrane layer structure effectively reduces water flux loss and significantly enhances the membrane's selective separation ability. This uniform and dense structure endows the membrane with stronger anti-fouling and self-cleaning capabilities, while also enhancing the membrane's stability and efficiency during long-term use. In the application of photocatalytic degradation of organic pollutants, this membrane can provide higher photocatalytic reaction efficiency and more durable performance, thus meeting the requirements of efficient water treatment and pollutant separation. Therefore, vacuum-assisted low-temperature interfacial polymerization not only effectively enhances the surface structure of the membrane but also gives the membrane significant advantages in practical applications, such as higher water flux, separation efficiency, mechanical strength, and stability.

[0090] Enhancement of photocatalytic efficiency:

[0091] IL enhances the photocatalytic activity of IL-GQDs by optimizing the energy band structure of graphene quantum dots. This means that the membrane can rapidly degrade organic molecules in water under the lamp, further improving the membrane's photocatalytic activity.

[0092] Long-term stability:

[0093] The surface structure of IL-GQDs provides operational stability. Ionic liquids can not only improve the dispersibility of graphene quantum dots but also enhance their stability in the membrane, thereby ensuring that the membrane maintains a high photocatalytic efficiency and self-cleaning ability during long-term use.

[0094] In summary, the surface structure characteristics of the photocatalytic nanofiltration membrane, such as the uniform dispersion of graphene quantum dots, enhanced photocatalytic activity, improved hydrophilicity and self-cleaning ability, and the thin and dense membrane structure, are all key factors for enhancing the membrane's permeation performance and photocatalytic effect. These characteristics enable the prepared membrane to not only have high water adsorption and high selectivity but also maintain excellent performance during long-term use.

[0095] The present invention is not limited to the above embodiments. Based on the technical solutions disclosed in the present invention, those skilled in the art can, according to the disclosed technical content, replace and deform some of the technical features, and these replacements and deformations are all within the protection scope of the present invention.

Claims

1. A preparation method of an IL-GQDs-based photocatalytic nanofiltration membrane, characterized in that, The steps include: Step (1): pre-treating bagasse with high-pressure CO2 hydrothermal treatment, followed by cooling and filtering to obtain pre-treated bagasse; Adding the bagasse to the preheated imidazole-based ionic liquid, stirring and dissolving, to obtain a mixed solution; The mixed solution is slowly poured into distilled water and fully stirred to obtain an ionic liquid functionalized graphene quantum dot (IL-GQDs) dispersion, and then the IL-GQDs dispersion is centrifuged to remove incompletely reacted residues, dialyzed to obtain a purified IL-GQDs solution, and the purified IL-GQDs solution is freeze-dried to obtain IL-GQDs; Step (2): placing the basement membrane in a vacuum filtration device, and then filtering and depositing the MPD-IL-GQDs composite aqueous solution containing IL-GQDs and metaphenylenediamine (MPD) onto the surface of the basement membrane under vacuum conditions at room temperature to obtain an amine-saturated basement membrane; A low-temperature trimesoyl chloride (TMC) organic phase solution is brought into contact with an amine-saturated base film to obtain an IL-GQDs film, which is then cured to obtain an IL-GQDs-based photocatalytic nanofiltration membrane.

2. The preparation method of a photocatalytic nanofiltration membrane based on IL-GQDs according to claim 1, characterized in that, In step (1), the specific steps of high-pressure CO2 hydrothermal pretreatment of bagasse are as follows: After drying the bagasse at 40-50°C to constant weight, crush it to a particle size of 0.2-0.5 mm, add it into deionized water at a solid-liquid ratio of 1:10-15, and fully disperse it; The mixture of bagasse and deionized water is transferred to a high-pressure reactor, which is sealed and introduced with CO2 to 6-8MPa, and the reactor is heated to 180°C-200°C and maintained for 0.5-1 hour. After the reaction is completed, the pressure is slowly reduced to room temperature to obtain pretreated bagasse.

3. The preparation method of a photocatalytic nanofiltration membrane based on IL-GQDs according to claim 1, characterized in that, In the step (1), the imidazolyl ionic liquid is any one of 1-butyl-3-methylimidazolium acetate ([BMIM][OAc]), 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM][HSO4]), 1-butyl-3-methylimidazolium dihydrogen phosphate ([BMIM][H2PO4]), 1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][CH3SO3]), and 1-butyl-3-methylimidazolium p-toluenesulfonate ([BMIM][p-TSA]).

4. The preparation method of a photocatalytic nanofiltration membrane based on IL-GQDs according to claim 1, characterized in that, In the step (1), the specific steps of preheating the imidazolyl ionic liquid are: heating the imidazolyl ionic liquid at 80°C-100°C for 10-15 minutes until the solid state is completely converted into a clear liquid state, thereby obtaining the preheated imidazolyl ionic liquid.

5. The preparation method of a photocatalytic nanofiltration membrane based on IL-GQDs according to claim 1, characterized in that, In the step (1), the specific preparation steps of the IL-GQDs powder are as follows: The pretreated bagasse is added to the preheated imidazole ionic liquid at a mass ratio of 1:5-8, and the mixture is stirred for 4-12 hours in groups at a temperature of 80°C-120°C and normal pressure to dissolve the mixture; After dissolution is completed, the mixed solution is slowly poured into distilled water and stirred thoroughly to obtain an IL-GQDs dispersion. The IL-GQDs dispersion is centrifuged at high speed, and the supernatant is retained. After removing the unreacted impurities in the precipitate, the supernatant is filtered using a microporous filter membrane. The filtered solution is placed in a dialysis bag with a molecular weight cut-off of 500 - 1000 Da and dialyzed for 3 - 5 days, with the dialysis solution replaced regularly every day. After dialysis, it is freeze-dried at -15 °C for 24 - 48 hours to obtain IL-GQDs.

6. The preparation method of a photocatalytic nanofiltration membrane based on IL-GQDs according to claim 1, characterized in that, In the step (2), the base membrane is selected from any one of polyacrylonitrile, polysulfone, or polyethersulfone ultrafiltration membranes.

7. The preparation method of a photocatalytic nanofiltration membrane based on IL-GQDs according to claim 1, characterized in that In the step (2), the MPD-IL-GQDs composite aqueous solution is prepared from 1 - 2 wt% MPD, 1 - 1.5 wt% IL-GQDs, 0.1 - 0.2 wt% sodium dodecyl sulfate (SDS), 1 - 2 wt% triethylamine (TEA), and deionized water.

8. The preparation method of a photocatalytic nanofiltration membrane based on IL-GQDs according to claim 1, characterized in that, In the step (2), the temperature of the TMC organic solution in the low-temperature state is -20 °C - 10 °C; The volume ratio of the trimesoyl chloride (TMC) organic phase solution to the MPD-IL-GQDs composite aqueous solution is 1:

1.

9. The preparation method of a photocatalytic nanofiltration membrane based on IL-GQDs according to claim 1, characterized in that, In the step (2), the concentration of the TMC organic phase solution is 0.15 - 0.2 wt%, and the solvent of the TMC organic solution is any one of ethyl acetate, ethanol, n-hexane, acetone, trichloroethylene, and isopropanol.

10. Photocatalytic nanofiltration membrane based on IL-GQDs, characterized in that, The diameter of the ionic liquid-functionalized graphene quantum dots (IL-GQDs) is 5 - 10 nanometers; The surface of IL-GQDs is rich in oxidized functional groups such as carboxyl (–COOH) and hydroxyl (–OH), and specific functional groups from imidazole-based ionic liquids are introduced simultaneously; The size of the membrane structure of the base membrane is 100 - 150 nm.

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