Ultrafiltration membrane, its preparation method and its application in pure water treatment

Abstract

The application relates to the field of membrane separation technology, in particular to an ultrafiltration membrane, a preparation method thereof and application of the ultrafiltration membrane in pure water treatment. The preparation method of the ultrafiltration membrane is characterized by modifying graphene quantum dots and optimizing the preparation process of the ultrafiltration membrane, so that the nanometer effect of the graphene quantum dots, the biological affinity of the silk fibroin, the bulk strength of the polyimide and the thermal chemical stability of the crosslinking structure are successfully integrated, and finally the obtained ultrafiltration membrane has large flux, high retention rate, good solvent resistance and good antifouling property when used in pure water treatment.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, specifically to an ultrafiltration membrane, its preparation method, and its application in pure water treatment. Background Technology

[0002] Ultrafiltration membranes, as a highly efficient separation medium, have been widely used in water treatment, biomedicine, and the food industry. Their core performance lies in the separation layer that combines high water flux and high rejection rate. Polyimide (PI) is considered one of the ideal materials for preparing organic solvent-resistant ultrafiltration membranes due to its excellent mechanical strength, thermal stability, and chemical stability. However, pure PI membranes often face a trade-off between flux and rejection rate, and their hydrophobic surface easily leads to membrane fouling. Therefore, research on combining ultrafiltration membranes with other materials to improve their performance has become a hot topic in recent years.

[0003] For example, patent CN106215720A discloses a method for preparing an organic solvent-resistant ultrafiltration membrane using graphene quantum dots (GQDs), the prepared ultrafiltration membrane, and its application. By applying GQDs to the preparation of the ultrafiltration membrane, the tolerance and stability of the ultrafiltration membrane in organic solvents are improved, as well as the flux of the ultrafiltration membrane. However, this method has the following drawbacks: 1. Hydroxylated GQDs are easily dissociated in alkaline solutions, and aminolated GQDs may protonate in acidic solvents, both of which affect dispersion stability and may further clog pores, reducing membrane flux and batch consistency; 2. This preparation method involves phase inversion in isopropanol, which may lead to excessive swelling, causing membrane pore collapse or excessive expansion, reducing the rejection rate. The purpose of this invention is to improve upon the above technical problems and provide a high-flux, high-rejection-rate ultrafiltration membrane, its preparation method, and its application in pure water treatment. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides an ultrafiltration membrane, its preparation method, and its application in pure water treatment. The ultrafiltration membrane provided by this invention is based on polyimide and is prepared by combining it with surface-modified inorganic materials to create an organic-inorganic composite ultrafiltration membrane material. The prepared ultrafiltration membrane material exhibits a large water treatment flux and a high rejection rate. The specific technical solution of this invention is as follows:

[0005] In a first aspect, a method for preparing an ultrafiltration membrane is provided, comprising the following steps:

[0006] Step 1. Prepare porous modified graphene quantum dot particles using a modifier and graphene quantum dots as the main raw materials; the modifier is a small molecule imide substance containing both hydroxyl and phenyl groups;

[0007] Step 2: Prepare a solution by mixing the polymer, pore-forming agent, and solvent, and add the obtained porous modified graphene quantum dot particles. Stir until homogeneous, and after degassing, obtain the casting solution.

[0008] Step 3. Prepare an ultrafiltration membrane from the obtained casting solution using a phase inversion method;

[0009] Step 4. Add a crosslinking agent to the obtained ultrafiltration membrane for crosslinking modification, and the membrane is obtained.

[0010] Furthermore, the polymer is polyimide with a molecular weight range of 30,000 to 110,000 Daltons;

[0011] Furthermore, the specific steps for preparing porous modified graphene quantum dot particles are as follows:

[0012] Small molecule imide containing both hydroxyl and phenyl groups and an inert small molecule pore-forming agent are added to graphene quantum dots and stirred evenly. Then, a silane coupling agent is added to react, followed by the addition of silk fibroin dispersion and stirring to react, and then ethanol is added to precipitate. The resulting reactants are soaked in water, and after solid-liquid separation, they are dried to obtain porous graphene quantum dot particles.

[0013] Furthermore, the inert small molecule pore-forming agent is at least one of sucrose, trehalose, glucose, sodium chloride, sodium sulfate, and ammonium bicarbonate.

[0014] Furthermore, the mass ratio of the graphene quantum dots, modifier, and inert small molecule pore-forming agent is 100:(5-15):(5-10).

[0015] Furthermore, the amount of the silane coupling agent added is 2-5% of the mass of the graphene quantum dots;

[0016] Furthermore, the content of the silk fibroin dispersion is 5-15%, and the amount of silk fibroin added is 0.2-1% of the mass of the graphene quantum dots;

[0017] Furthermore, the volume of ethanol added is 2-3 times that of the reaction solution obtained in the previous step, and the precipitation time is 4-6 hours. Natural silk fibroin is water-soluble. During prolonged water immersion, silk fibroin that is not firmly cross-linked is likely to be dissolved and washed away, resulting in loss of function and possible destruction of the formed porous structure. The addition of anhydrous ethanol for precipitation causes the silk fibroin to form a water-insoluble β-sheet conformation.

[0018] Furthermore, step 3 specifically involves:

[0019] The casting solution obtained in step 2 is uniformly coated onto a nonwoven fabric to form a liquid film with a thickness of 50-300 μm. The coated liquid film is left to stand in the air for 1-100 seconds, and then immersed in a gel bath for 5-60 minutes to allow the polymer in the casting solution to undergo a gel phase transformation and precipitate onto the surface of the nonwoven fabric. The membrane is then removed, rinsed with deionized water to remove excess solution from the membrane surface, and then immersed in isopropanol solution for 1-24 hours to obtain an ultrafiltration membrane doped with graphene quantum dots.

[0020] Furthermore, in step 1, the small molecule imide containing both hydroxyl and phenyl groups is specifically at least one of N-hydroxyphthalimide (CAS: 524-38-9) and N-hydroxymethylphthalimide (CAS: 118-29-6);

[0021] Furthermore, the structural formula of the N-hydroxyphthalimide is as follows: ;

[0022] Furthermore, the structural formula of the N-hydroxymethyl phthalimide is as follows: ;

[0023] Furthermore, the pore-forming agent mentioned in step 2 is polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), water, alcohol, or inorganic salt, or any combination of two or more of the above.

[0024] Furthermore, the solvent in the casting solution in step 2 is N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), or tetrahydrofuran (THF), or other solvents capable of dissolving polyimide, or any combination of two or more of the above.

[0025] Furthermore, the silane coupling agent is at least one selected from KH550, KH792, KH560, and p-aminophenyltrimethoxysilane;

[0026] Furthermore, in step 2, the mass fraction of the polymer in the casting solution is 10-30%, the mass fraction of the pore-forming agent is 1-7%, and the mass fraction of the graphene quantum dot particles is 0.01-3%.

[0027] Furthermore, the porous modified graphene quantum dot particles have a particle size of less than or equal to 50 nm and greater than or equal to 2 nm, and a thickness of less than or equal to 2 nm.

[0028] Furthermore, the degassing described in step 2 is high-speed centrifugal degassing, with the high-speed centrifuge speed being 500-3000 rpm and the centrifugation degassing time being 1-60 min.

[0029] Furthermore, the crosslinking agent is a diamine, a diamine compound, a polyamine, a polyamine compound, or a mixture thereof;

[0030] Furthermore, the solvent in the crosslinking agent solution is water or alcohol, wherein the mass fraction of the crosslinking agent is 0.1% to 5%.

[0031] Furthermore, the specific steps of the crosslinking modification are as follows:

[0032] The ultrafiltration membrane obtained in step 3 is immersed in the crosslinking agent solution for 0.5 to 5 hours, then removed and washed several times with water.

[0033] Secondly, the present invention provides an ultrafiltration membrane prepared by the above-described preparation method.

[0034] Thirdly, the present invention provides an application of the ultrafiltration membrane prepared by the above preparation method in pure water treatment.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] 1. The present invention provides abundant and rapid transport channels for water molecules due to the pores in the modified graphene quantum dot particles and the excellent hydrophilicity of silk fibroin and GQDs themselves, thereby improving the flux of the ultrafiltration membrane; through the nano-sieving effect of graphene quantum dots, and through the interface that is firmly bonded to the polyimide matrix by silane coupling agents and imide modifiers, the membrane is ensured to have a dense separation layer and high rejection rate.

[0037] 2. Through the interaction of diamine crosslinking agents with polyimide, a stable three-dimensional crosslinking network is formed between polymer molecular chains, which greatly limits the swelling and damage of polymer chains by solvent molecules. The π-π conjugation between the benzene ring structure of the modifier and the PI matrix, as well as the bridging of the silane coupling agent, make the GQDs firmly bonded to the PI matrix and not easy to fall off under solvent immersion, avoiding defects caused by additive detachment. The ultrafiltration membrane prepared in the end has extremely high flux and rejection rate retention after immersion in strong polar organic solvents such as DMF, showing excellent long-term stability and is suitable for separation in harsh organic solvent systems.

[0038] 3. By introducing silk fibroin and GQDs, the membrane surface is endowed with extreme hydrophilicity, forming a strong hydration layer that can effectively block the adsorption of hydrophobic pollutants such as proteins and oils on the membrane surface. In addition, as a protein, silk fibroin itself can minimize the non-specific adsorption of ultrafiltration membranes with other proteins, thereby increasing antifouling performance.

[0039] 4. This invention modifies graphene quantum dots by using small-molecule imides containing both hydroxyl and phenyl groups. The added silk fibroin can quench excess active groups generated by silane coupling agents, improving the dispersibility of graphene quantum dots in the system and increasing the compatibility between graphene quantum dots and polyimide system. This results in the final ultrafiltration membrane having better consistency and higher stability. Detailed Implementation

[0040] The present invention will be further explained below through specific comparative examples and embodiments.

[0041] Examples 1-4 and Comparative Examples 1-4

[0042] Preparation method of ultrafiltration membrane:

[0043] Step 1. Add small molecule imide containing hydroxyl and phenyl groups and an inert small molecule pore-forming agent to graphene quantum dots (GQDs, particle size 5-50 nm) in a mixed solvent of ethanol / water (volume ratio 4:1) at pH=5. Stir magnetically at 50°C for 2 hours. Then, add silane coupling agent dropwise and continue the reaction at 50°C for 6 hours. Then, add silk fibroin dispersion and adjust the pH to 8.0 with 5wt% NaOH solution. Stir and react at room temperature for 12 hours. Then, add ethanol twice the volume of the reaction solution obtained in the previous step to precipitate for 5 hours. After centrifugation, soak the precipitate in water, centrifuge again, and freeze dry to obtain porous graphene quantum dot particles.

[0044] Step 2: Prepare a solution of polyimide (30,000-110,000 Daltons), pore-forming agent, and N-methylpyrrolidone. Stir mechanically at 60°C for 10 hours. Add the obtained porous modified graphene quantum dot particles and stir evenly. Then centrifuge at 2000 rpm for 30 minutes to remove bubbles and obtain the casting solution.

[0045] Step 3. The casting solution obtained in Step 2 is uniformly coated onto the nonwoven fabric to form a liquid film. The coated liquid film is left to stand in the air for 30 seconds, and then kept in a deionized water coagulation bath at 25°C for 15 minutes to precipitate a film on the surface of the nonwoven fabric. After that, the film is taken out, rinsed with deionized water to remove excess solution from the surface of the film, and then the film is immersed in isopropanol solution for 12 hours.

[0046] Step 4. Immerse the above phase conversion membrane in a 2% (w / w) ethylenediamine / isopropanol crosslinking agent solution for 2 hours for crosslinking. After removal, rinse the membrane surface with a large amount of deionized water and air dry at room temperature to obtain the ultrafiltration membrane.

[0047] Table 1. Substances and proportions for Examples and Comparative Examples

[0048] Types of modifiers Types of inert small molecule pore-forming agents m graphene quantum dots; m modifier; m inert small molecule pore-forming agent Silane coupling agent mSilane coupling agent: mGraphene quantum dot mass Silk fibroin dispersion concentration m-Silk Fiber Protein: m-Graphene Quantum Dot Mass Step 2: Types of pore-forming agents Step 2: Polyimide: Pore-forming agent: Porous modified graphene quantum dot particles: Solvent Step 3: Coating thickness Example 1 N-hydroxyphthalimide sucrose 100：10：8 KH550 3:100 1wt% 0.5:100 PVP K30 18：3：0.15：78.85 150μm Example 2 N-hydroxymethylphthalimide Sodium chloride 100：15：10 KH792 4:100 1wt% 0.2:100 PEG-400 20：1：0.5：78.5 200μm Example 3 N-hydroxyphthalimide Trehalose 100：5：5 KH560 2：100 5wt% 1:100 PVP K30 15:5:0.04:79.96 100μm Example 4 N-hydroxymethylphthalimide ammonium bicarbonate 100:12:9 p-Aminophenyltrimethoxysilane 5:100 2wt% 0.8:100 PEG-1000 22:2:1:75 250 μm Comparative Example 1 N-hydroxysuccinimide Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Comparative Example 2 N,N-Dimethylformamide Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Comparative Example 3 Same as Example 1 PVP K30 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Comparative Example 4 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 0 - Same as Example 1 Same as Example 1 Same as Example 1

[0049] Comparative Example 5

[0050] The difference from Example 1 is that no inert small molecule pore-forming agent is added.

[0051] Performance testing

[0052] The performance testing methods for the ultrafiltration membranes obtained in the examples and comparative examples are as follows:

[0053] 1. Pure water flux (PWF): The membrane flux after stabilization is tested using deionized water at a pressure of 0.2 MPa, in units of L / (m²·h·MPa);

[0054] 2. Bovine serum albumin rejection (BSA Rejection): The rejection performance of a 500 mg / L bovine serum albumin solution was tested at a pressure of 0.2 MPa.

[0055] 3. Solvent stability: After immersing the membrane in N,N-dimethylformamide (DMF) for 7 days, the changes in flux and rejection rate were tested, and the changes in flux decay rate and rejection rate were used for evaluation.

[0056] 4. Antifouling performance: After fouling the membrane with 500 mg / L BSA solution for 1 hour, its flux recovery rate (FRR) was measured.

[0057] Table 2. Performance test results of ultrafiltration membranes obtained from the examples and comparative examples.

[0058] sample Pure water flux (LMH / MPa) BSA Retention Rate (%) Flux decay rate (%) after 7-day DMF immersion Changes in DMF retention rate after 7 days of soaking (percentage points) Flux recovery rate (FRR) (%) Example 1 385 98.5 -5.2 +0.3 92.1 Example 2 350 97.8 -6.1 +0.1 90.5 Example 3 420 96.0 -7.5 -0.5 88.3 Example 4 310 99.0 -4.8 +0.2 93.5 Comparative Example 1 280 95.2 -18.5 -3.1 85.0 Comparative Example 2 150 90.5 -35.2 -8.7 72.3 Comparative Example 3 320 97.5 -15.2 -2.0 86.1 Comparative Example 4 295 97.0 -20.1 -3.5 80.5 Comparative Example 5 220 98.1 -9.5 -1.2 84.8

[0059] As shown in Table 2, the ultrafiltration membrane prepared by this invention has high water treatment flux, high rejection rate, and good solvent resistance and antifouling properties. In Comparative Example 1, the modifier used is N-hydroxysuccinimide (containing hydroxyl groups but no phenyl groups). The graphene quantum dot particles modified by it cannot form effective π-π conjugation with the PI matrix, resulting in poor compatibility and uneven dispersion of GQDs in the system. Ultimately, this leads to a decrease in flux, rejection rate, solvent resistance, and antifouling properties. In Comparative Example 2, no modifier was added, and the GQDs severely agglomerated in the PI, forming defects. Its performance was the worst, especially its solvent resistance deteriorated sharply; Comparative Example 3 used PVP K30 pore-forming agent. Due to the long molecular chain of PVP and the possibility of partial bonding with the silane coupling agent through its lactam group, it was difficult to be completely washed out, resulting in incomplete pore formation and low throughput; Comparative Example 4 did not add silk fibroin, and the FRR was only 80.5%, with a significant decrease in antifouling performance. At the same time, its solvent resistance also deteriorated, indicating that the cross-linking network formed by silk fibroin and silane coupling agent further consolidated the entire modified layer. The introduction of silk fibroin greatly improved the surface hydrophilicity and antifouling performance of the membrane and contributed to the solvent resistance stability; Comparative Example 5 did not add pore-forming agent and had no pore-forming step. The silk fibroin layer was relatively dense. Although the rejection rate was acceptable, the throughput was significantly lower than that of Example 1.

Claims

1. A method for preparing an ultrafiltration membrane, characterized in that, Includes the following steps: Step 1: Prepare porous modified graphene quantum dot particles using a modifier and graphene quantum dots as the main raw materials; the modifier is a small molecule imide substance containing both hydroxyl and phenyl groups; Step 2: Prepare a solution by mixing the polymer, pore-forming agent, and solvent, and add the obtained porous modified graphene quantum dot particles. Stir until homogeneous, and after degassing, obtain the casting solution. Step 3: Prepare an ultrafiltration membrane from the obtained casting solution using the phase inversion method; Step 4: Add a crosslinking agent to the obtained ultrafiltration membrane for crosslinking modification, and the membrane is obtained. The specific steps for preparing porous modified graphene quantum dot particles in step 1 are as follows: Small molecule imide containing both hydroxyl and phenyl groups and an inert small molecule pore-forming agent are added to graphene quantum dots and stirred evenly. Then, a silane coupling agent is added to react, followed by the addition of silk fibroin dispersion and stirring to react, and then ethanol is added to precipitate. The resulting reactants are soaked in water, and after solid-liquid separation, they are dried to obtain porous graphene quantum dot particles.

2. The method for preparing the ultrafiltration membrane according to claim 1, characterized in that, The polymer is polyimide, and the molecular weight of the polyimide is in the range of 30,000 to 110,000 Daltons.

3. The method for preparing the ultrafiltration membrane according to claim 1, characterized in that, The inert small molecule pore-forming agent is at least one of sucrose, trehalose, glucose, sodium chloride, sodium sulfate, and ammonium bicarbonate.

4. The method for preparing the ultrafiltration membrane according to claim 1, characterized in that, The mass ratio of the graphene quantum dots, modifier, and inert small molecule pore-forming agent is 100:(5-15):(5-10). The amount of the silane coupling agent added is 2-5% of the mass of the graphene quantum dots; The content of the silk fibroin dispersion is 5-15%, and the amount of silk fibroin added is 0.2-1% of the mass of the graphene quantum dots.

5. The method for preparing an ultrafiltration membrane according to claim 1, characterized in that, The small molecule imide containing both hydroxyl and phenyl groups is at least one of N-hydroxyphthalimide and N-hydroxymethylphthalimide.

6. The method for preparing an ultrafiltration membrane according to claim 1, characterized in that, The silane coupling agent is at least one of KH550, KH792, KH560, and p-aminophenyltrimethoxysilane.

7. The method for preparing an ultrafiltration membrane according to claim 1, characterized in that, In step 2, the mass fraction of the polymer in the casting solution is 10-30%, the mass fraction of the pore-forming agent is 1-7%, and the mass fraction of the porous modified graphene quantum dot particles is 0.01-3%.

8. An ultrafiltration membrane prepared by the method according to any one of claims 1-7.

9. The application of an ultrafiltration membrane prepared by the method according to any one of claims 1-7 in pure water treatment.