GO / PVDF (Polyvinylidene Fluoride) composite ultrafiltration membrane for MBR (Membrane Bioreactor) and preparation method thereof

Abstract

The invention relates to the field of composite ultrafiltration membranes, in particular to a GO / PVDF composite ultrafiltration membrane for MBR and a preparation method of the GO / PVDF composite ultrafiltration membrane. The GO / PVDF composite ultrafiltration membrane is prepared by adopting 18% of polyvinylidene fluoride as a membrane base material, 6% of polyvinylpyrrolidone as a pore-foaming agent, 0-2% of graphene oxide as a modifier and the balance of N, N-dimethylacetamide as a solvent through an impregnation phase inversion method. The GO with low mass fraction is added into the section of the membrane to form a large and thick finger-shaped pore structure, the filtration resistance of membrane pores is reduced, the filtration flux, hydrophilicity, permeability and interception performance of the membrane are improved, the method is simple to operate, the prepared finished product is excellent in performance, and the research and application expansion of a membrane modification technology are promoted.

Description

Technical Field

[0001] The invention relates to the field of ultrafiltration membranes, and in particular to a GO / PVDF composite ultrafiltration membrane for MBR and a preparation method thereof. Background Art

[0002] In recent years, researchers have conducted research on hydrophilic modification of polyvinylidene fluoride (PVDF) membranes, and expanded ion grafting modification, radiation irradiation grafting modification, interfacial polymerization chemical reaction grafting modification, and membrane substrate blending modification technology. Surface modification technology is to fix functional groups to the surface or bottom of the membrane by coating or grafting reaction, and it is difficult for the modifier to diffuse into the membrane pores and change the membrane pore structure. In the actual operation process, the modifier on the membrane surface is easy to fall off under the impact of continuous high-pressure water load, and the modification effect is less durable. Blending modification usually involves fully blending hydrophilic nanomaterials with PVDF membrane-making materials to form a membrane. Compared with surface modification technology, blending modification technology has the advantages of coordinated completion of the film-forming process and the hydrophilicity improvement process of the membrane, effective modification of the membrane pore structure, and a more durable membrane modification effect. With the in-depth research and application expansion of membrane modification technology, blending modification will become the mainstream modification technology in the industry.

[0003] Studies have found that there are a large number of oxygen-containing functional groups on the surface of graphene oxide (GO), including hydroxyl (-OH), carboxyl (-COOH), epoxy (COC), carbonyl (-C=O-), etc. GO has a high surface polarity and good hydrophilicity. It is easy to obtain a highly hydrophilic and pollution-resistant separation membrane when used for the modification of hydrophobic membranes. Some research results show that adding GO to PVDF casting solution is beneficial to improving the permeability and water flux of the composite membrane, and can even improve the retention rate. Although the performance of the membrane can be evaluated by the flux, anti-fouling performance and separation performance of the membrane, there are few studies that deeply analyze the influence of the introduction of carbon nanomaterials on membrane performance and its mechanism in combination with the morphology and performance changes of carbon nanomaterials, the morphology changes of membranes and the changes in surface properties. Summary of the invention

[0004] In view of the problems existing in the background technology, a GO / PVDF composite ultrafiltration membrane for MBR and a preparation method thereof are proposed. Self-made inorganic nanomaterial GO is used as a modifier. After being fully blended with PVDF powder, the GO / PVDF composite ultrafiltration membrane is prepared by the impregnation phase inversion method.

[0005] The present invention provides a GO / PVDF composite ultrafiltration membrane for MBR, which adopts 18% of polyvinylidene fluoride as a membrane substrate, 6% of polyvinyl pyrrolidone as a porogen, 0-2% of graphene oxide as a modifier, and the remaining amount of N, N-dimethylacetamide as a solvent, and adopts an immersion phase conversion method to prepare the GO / PVDF composite ultrafiltration membrane.

[0006] The present invention provides a method for preparing a GO / PVDF composite ultrafiltration membrane for MBR, and the steps are as follows:

[0007] S1. Prepare graphene oxide by the Hummers method;

[0008] S2. Use polyvinylidene fluoride as the membrane substrate, polyvinylpyrrolidone as the pore-forming agent, and graphene oxide as the modifier. The remaining amount is dissolved in N,N-dimethylacetamide as the solvent. Through deionized water as the coagulation bath, a GO / PVDF composite ultrafiltration membrane is prepared by the immersion phase inversion method.

[0009] Preferably, the steps of S1 are as follows: Weigh 4 g of flake graphite, 2 g of sodium nitrate, and 92 mL of concentrated sulfuric acid into a beaker. Place the beaker in an ice-water mixture and stir magnetically for 0.5 h. Then, add 4 g of potassium permanganate to the beaker at regular intervals of 15 min, for a total of three times. After adding, continue the reaction for 1 h, and the low-temperature reaction stage ends; After the low-temperature reaction stage ends, transfer the beaker to a 35 °C constant temperature water bath and continue stirring for 2 h, and the medium-temperature reaction stage ends; After the medium-temperature reaction stage ends, change the temperature of the constant temperature water bath to 95 °C, continue magnetic stirring, and slowly add 200 mL of deionized water drop by drop. After the temperature of the reaction system rises to 95 °C, wait for another 5 min and then slowly add 400 mL of deionized water and 100 mL of hydrogen peroxide. At this time, a dispersion with a bright yellow metallic luster is obtained. Continue the reaction for 15 min, and the high-temperature reaction stage ends; After the high-temperature reaction stage ends, remove the beaker from the water bath, ultrasonically disperse the obtained dispersion for 30 min, add hydrochloric acid and stir for 3 min. Finally, dilute the obtained graphene oxide dispersion with deionized water to 10 L, and pour off the supernatant by natural gravity precipitation. Repeat this step three times until the pH of the graphene oxide dispersion is neutral; Freeze-dry the lower-layer graphene oxide slurry after the third precipitation to obtain solid graphene oxide, cut it into fine powder, and place it in a desiccator for standby.

[0010] Preferably, the specific steps of S2 are as follows: First, weigh 0-2% graphene oxide and disperse it in an N,N-dimethylacetamide solution, and ultrasonically disperse it for 10 h; Secondly, mix 18% polyvinylidene fluoride powder and 6% polyvinylpyrrolidone powder evenly and add them to an N,N-dimethylacetamide solution at 70 °C. After dissolution, add the graphene oxide dispersion and continue stirring for 12 h to obtain a homogeneous and transparent casting solution; Finally, let it stand in a vacuum environment for 8 h to remove the remaining bubbles in the casting solution. Slowly spread the casting solution on a glass plate, and use a film scraper to prepare a film with a thickness of 200 μm. After staying in the air for 20 s, immerse it in deionized water at 25 °C to complete the phase inversion process.

[0011] Preferably, before film preparation, all materials need to be dried in an oven at 80 °C for 12 h to remove moisture.

[0012] Preferably, after film formation, the nascent film is rinsed with deionized water to remove residual solvents.

[0013] Compared with the prior art, the present invention has the following beneficial technical effects: After preparing graphene oxide by the improved Hummers method, a series of composite membranes containing different mass fractions of GO are prepared by the immersion phase inversion method, and the effects of the GO addition amount on the pore structure, surface characteristics, filtration performance, and anti-fouling performance of the composite membranes are investigated. The obtained effects and conclusions are as follows: (1) Since GO has extremely strong hydrophilicity, the addition of a low mass fraction of GO is more likely to form large and thick finger-like pore structures in the membrane cross-section, reducing the membrane pore filtration resistance and increasing the membrane filtration flux. GO can significantly reduce the Zeta potential on the membrane surface and improve the hydrophilicity of the ultrafiltration membrane. (2) During the filtration performance test, when the GO dosage is 1%, the composite membrane has good permeability and retention performance. The reason is that under these preparation conditions, it is easy to form a low-resistance filtration membrane pore structure, and the increase in membrane hydrophilicity leads to an increase in its effective filtration area, and the increase in the number of surface membrane pores is more conducive to the improvement of the membrane's permeability and retention performance. The method of the present invention is simple to operate, and the prepared products have excellent performance, which is conducive to promoting the research and application expansion of membrane modification technology. Description of the Drawings

[0014] Figure 1 It is a test chart of the membrane filtration performance of PVDF, GO / PVDF-0.5, GO / PVDF-1.0, GO / PVDF-1.5, and GO / PVDF-2.0 in Examples 1-5;

[0015] Figure 2 It is a graph showing the relationship between the membrane flux and time of PVDF, GO / PVDF-0.5, GO / PVDF-1.0, GO / PVDF-1.5, and GO / PVDF-2.0 in Examples 1-5. Detailed Embodiments

[0016] Example 1, this example proposes a GO / PVDF composite ultrafiltration membrane for MBR, using 18% polyvinylidene fluoride as the membrane substrate, 6% polyvinylpyrrolidone as the pore-forming agent, 0% graphene oxide as the modifier, and the remaining amount using N,N-dimethylacetamide as the solvent, and a GO / PVDF composite ultrafiltration membrane is prepared by the immersion phase inversion method.

[0017] The specific preparation steps are as follows:

[0018] Preparation of PVDF membrane: First, a certain mass of GO was weighed according to a mass fraction of 0% with respect to PVDF and dispersed into the DMAc solution, and ultrasonic dispersion was carried out for 10 h to obtain a dispersion with better exfoliated lamellae. Second, PVDF powder (18%) and PVP powder (6%) were mixed evenly and then added to the dimethylacetamide (DMAC) solution at 70 °C. After dissolution, the GO dispersion was added and stirring was continued for 12 h to obtain a homogeneous and transparent casting solution. Finally, it was left standing in a vacuum environment for 8 h to remove the residual bubbles in the casting solution. The casting solution was slowly spread on a glass plate, and a film with a thickness of 200 μm was prepared using a film scraper. After staying in the air for 20 s, it was quickly immersed in deionized water at 25 °C to complete the phase inversion process. In addition, the as-cast membrane was rinsed with a large amount of deionized water to remove the residual solvent, and the finally obtained membrane was labeled as the PVDF membrane.

[0019] Example 2, this example presents a GO / PVDF composite ultrafiltration membrane for MBR. 18% polyvinylidene fluoride was used as the membrane substrate, 6% polyvinylpyrrolidone was used as the pore-forming agent, and 0.5% graphene oxide was used as the modifier. The remaining amount was N,N-dimethylacetamide as the solvent, and the GO / PVDF composite ultrafiltration membrane was prepared by the immersion phase inversion method.

[0020] (1) Preparation of graphene oxide (GO): Graphene oxide (GO) was prepared by the modified Hummers method, and the specific steps are as follows. 4 g of flake graphite, 2 g of sodium nitrate, and 92 mL of concentrated sulfuric acid were weighed into a beaker respectively. The beaker was placed in an ice-water mixture environment and magnetically stirred for 0.5 h. Then, 4 g of potassium permanganate was quantitatively added to the beaker every 15 min, for a total of three times. After addition, the reaction continued for 1 h and the low-temperature reaction stage ended. After the low-temperature reaction stage ended, the beaker was transferred to a 35 °C constant temperature water bath, and stirring was continued for 2 h and the medium-temperature reaction stage ended. After the medium-temperature stage reaction ended, the temperature of the constant temperature water bath was changed to 95 °C, and magnetic stirring was continued. 200 mL of deionized water was slowly added dropwise. After the temperature of the reaction system rose to 95 °C, after waiting for 5 min, 400 mL of deionized water and 100 mL of hydrogen peroxide were slowly added. At this time, a dispersion with a bright yellow metallic luster was obtained. After the reaction continued for 15 min, the high-temperature reaction stage ended. After the high-temperature reaction stage ended, the beaker was removed from the water bath, and the obtained dispersion was ultrasonically dispersed for 30 min, hydrochloric acid was added and stirred for 3 min. Finally, the obtained GO dispersion was diluted with deionized water to 10 L, and the upper clear liquid was poured off by natural gravity precipitation. This step was repeated three times and the pH of the GO dispersion was neutral. The lower GO slurry after the third precipitation was freeze-dried to obtain solid GO, which was cut into fine powder and placed in a desiccator for standby.

[0021] (2) Preparation of GO / PVDF composite ultrafiltration membrane: First, a certain mass of GO was weighed according to a mass fraction of 0.5% with respect to PVDF and dispersed into the DMAc solution, and ultrasonic dispersion was carried out for 10 h to obtain a dispersion with better exfoliated lamellae. Second, PVDF powder (18%) and PVP powder (6%) were mixed evenly and then added to the dimethylacetamide (DMAC) solution at 70 °C. After dissolution, the GO dispersion was added and stirring was continued for 12 h to obtain a homogeneous and transparent casting solution. Finally, it was left standing for 8 h in a vacuum environment to remove the residual bubbles in the casting solution. The casting solution was slowly spread on a glass plate, and a film with a thickness of 200 μm was prepared using a film scraper. After staying in the air for 20 s, it was quickly placed into deionized water at 25 °C for immersion to complete the phase inversion process. In addition, the as-prepared membrane was rinsed with a large amount of deionized water to remove the residual solvent, and the finally obtained membrane was labeled as GO / PVDF-0.5.

[0022] Example 3. In this example, a GO / PVDF composite ultrafiltration membrane for MBR was proposed. 18% polyvinylidene fluoride was used as the membrane substrate, 6% polyvinylpyrrolidone was used as the pore-forming agent, and 1.0% graphene oxide was used as the modifier. The remaining amount was based on N,N-dimethylacetamide as the solvent, and the GO / PVDF composite ultrafiltration membrane was prepared by the immersion phase inversion method.

[0023] (1) Preparation of graphene oxide (GO): Graphene oxide (GO) was prepared by the improved Hummers method. The specific steps are as follows. 4 g of flake graphite, 2 g of sodium nitrate, and 92 mL of concentrated sulfuric acid were weighed into a beaker respectively. The beaker was placed in an ice-water mixture environment and magnetically stirred for 0.5 h. Then, 4 g of potassium permanganate was quantitatively added to the beaker every 15 min, and a total of three times were added. After adding, the reaction continued for 1 h and the low-temperature reaction stage ended. After the low-temperature reaction stage ended, the beaker was transferred to a 35 °C constant temperature water bath, and stirring was continued for 2 h and the medium-temperature reaction stage ended. After the medium-temperature stage reaction ended, the temperature of the constant temperature water bath was changed to 95 °C, and magnetic stirring was continued. 200 mL of deionized water was slowly added dropwise. After the temperature of the reaction system rose to 95 °C, after waiting for 5 min, 400 mL of deionized water and 100 mL of hydrogen peroxide were slowly added. At this time, a dispersion with a bright yellow metallic luster was obtained. After the reaction continued for 15 min, the high-temperature reaction stage ended. After the high-temperature reaction stage ended, the beaker was removed from the water bath, and the obtained dispersion was ultrasonically dispersed for 30 min, hydrochloric acid was added and stirred for 3 min. Finally, the obtained GO dispersion was diluted with deionized water to 10 L, and the upper clear liquid was poured off by natural gravity precipitation. This step was repeated three times, and the pH of the GO dispersion was neutral. The lower GO slurry after the third precipitation was freeze-dried to obtain solid GO, which was cut into fine powder and placed in a desiccator for standby.

[0024] (2) Preparation of GO / PVDF composite ultrafiltration membrane: First, a certain mass of GO was weighed according to a mass fraction of 1.0% with respect to PVDF and dispersed into the DMAc solution, and ultrasonic dispersion was carried out for 10 h to obtain a dispersion with better exfoliated lamellae. Secondly, PVDF powder (18%) and PVP powder (6%) were mixed evenly and then added to the dimethylacetamide (DMAC) solution at 70 °C. After dissolution, the GO dispersion was added and stirring was continued for 12 h to obtain a homogeneous and transparent casting solution. Finally, it was left standing for 8 h in a vacuum environment to remove the residual bubbles in the casting solution. The casting solution was slowly spread on a glass plate, and a film with a thickness of 200 μm was prepared using a film scraper. After staying in the air for 20 s, it was quickly placed in deionized water at 25 °C for immersion to complete the phase inversion process. In addition, the as-prepared membrane was rinsed with a large amount of deionized water to remove the residual solvent, and the finally obtained membrane was labeled as GO / PVDF-1.0.

[0025] Example 4. In this example, a GO / PVDF composite ultrafiltration membrane for MBR was proposed. 18% polyvinylidene fluoride was used as the membrane substrate, 6% polyvinylpyrrolidone was used as the pore-forming agent, and 1.5% graphene oxide was used as the modifier. The remaining amount was N,N-dimethylacetamide as the solvent, and the GO / PVDF composite ultrafiltration membrane was prepared by the immersion phase inversion method.

[0026] (1) Preparation of graphene oxide (GO): Graphene oxide (GO) was prepared by the improved Hummers method, and the specific steps are as follows. 4 g of flake graphite, 2 g of sodium nitrate, and 92 mL of concentrated sulfuric acid were weighed into a beaker respectively. The beaker was placed in an ice-water mixture environment and magnetically stirred for 0.5 h. Then, 4 g of potassium permanganate was quantitatively added to the beaker every 15 min for a total of three times. After adding, the reaction continued for 1 h and the low-temperature reaction stage ended. After the low-temperature reaction stage ended, the beaker was transferred to a 35 °C constant temperature water bath, and stirring was continued for 2 h and the medium-temperature reaction stage ended. After the medium-temperature stage reaction ended, the temperature of the constant temperature water bath was changed to 95 °C, and magnetic stirring was continued. 200 mL of deionized water was slowly added dropwise. After the temperature of the reaction system rose to 95 °C, after waiting for another 5 min, 400 mL of deionized water and 100 mL of hydrogen peroxide were slowly added. At this time, a dispersion with a bright yellow metallic luster was obtained, and the high-temperature reaction stage ended after continuing the reaction for 15 min. After the high-temperature reaction stage ended, the beaker was removed from the water bath, and the obtained dispersion was ultrasonically dispersed for 30 min, hydrochloric acid was added and stirred for 3 min. Finally, the obtained GO dispersion was diluted with deionized water to 10 L, and the upper clear liquid was poured off by natural gravity precipitation. This step was repeated three times and the pH of the GO dispersion was neutral. The lower GO slurry after the third precipitation was freeze-dried to obtain solid GO, which was cut into fine powder and placed in a desiccator for standby.

[0027] (2) Preparation of GO / PVDF composite ultrafiltration membrane: First, a certain mass of GO was weighed according to 1.5% of the mass fraction of PVDF and dispersed in DMAc solution, and ultrasonic dispersion was carried out for 10 h to obtain a dispersion with better exfoliated lamellae. Secondly, PVDF powder (18%) and PVP powder (6%) were mixed evenly and then added to the dimethylacetamide (DMAC) solution at 70 °C. After dissolution, the GO dispersion was added and stirring was continued for 12 h to obtain a homogeneous and transparent casting solution. Finally, it was left standing in a vacuum environment for 8 h to remove the residual bubbles in the casting solution. The casting solution was slowly spread on a glass plate, and a film with a thickness of 200 μm was prepared using a film scraper. After staying in the air for 20 s, it was quickly placed in deionized water at 25 °C for immersion to complete the phase inversion process. In addition, the as - cast membrane was rinsed with a large amount of deionized water to remove the residual solvent, and the finally obtained membrane was labeled as GO / PVDF - 1.5.

[0028] Example 5. In this example, a GO / PVDF composite ultrafiltration membrane for MBR was proposed. 18% polyvinylidene fluoride was used as the membrane substrate, 6% polyvinylpyrrolidone was used as the pore - forming agent, 2% graphene oxide was used as the modifier, and the remaining amount was based on N, N - dimethylacetamide as the solvent. The GO / PVDF composite ultrafiltration membrane was prepared by the immersion phase inversion method.

[0029] (1) Preparation of graphene oxide (GO): Graphene oxide (GO) was prepared by the improved Hummers method. The specific steps are as follows. Weigh 4 g of flake graphite, 2 g of sodium nitrate, and 92 mL of concentrated sulfuric acid into a beaker. Place the beaker in an ice - water mixture environment and stir magnetically for 0.5 h. Then, 4 g of potassium permanganate was quantitatively added to the beaker every 15 min, for a total of three times. After adding, continue the reaction for 1 h and the low - temperature reaction stage ends. After the low - temperature reaction stage ends, transfer the beaker to a 35 °C constant - temperature water bath and continue stirring for 2 h, and the medium - temperature reaction stage ends. After the medium - temperature reaction stage ends, change the temperature of the constant - temperature water bath to 95 °C, continue magnetic stirring, and slowly add 200 mL of deionized water drop by drop. After the temperature of the reaction system rises to 95 °C, wait for another 5 min and then continue to slowly add 400 mL of deionized water and 100 mL of hydrogen peroxide. At this time, a dispersion with a bright yellow metallic luster is obtained. After continuing the reaction for 15 min, the high - temperature reaction stage ends. After the high - temperature reaction stage ends, remove the beaker from the water bath, ultrasonically disperse the obtained dispersion for 30 min, add hydrochloric acid and stir for 3 min. Finally, dilute the obtained GO dispersion with deionized water to 10 L, and pour off the supernatant by natural gravity precipitation. Repeat this step three times until the pH of the GO dispersion is neutral. The lower - layer GO slurry after the third precipitation was freeze - dried to obtain solid GO, which was cut into fine powder and placed in a desiccator for standby.

[0030] (2) Preparation of GO / PVDF composite ultrafiltration membrane: First, a certain mass of GO was weighed according to 2.0% of the mass fraction of PVDF and dispersed into the DMAc solution, and ultrasonic dispersion was carried out for 10 h to obtain a dispersion with better lamellar exfoliation. Second, PVDF powder (18%) and PVP powder (6%) were mixed evenly and then added to the dimethylacetamide (DMAC) solution at 70 °C. After dissolution, the GO dispersion was added and stirring was continued for 12 h to obtain a homogeneous and transparent casting solution. Finally, it was left standing in a vacuum environment for 8 h to remove the residual bubbles in the casting solution. The casting solution was slowly spread on a glass plate, and a film with a thickness of 200 μm was prepared using a film scraper. After staying in the air for 20 s, it was quickly placed into deionized water at 25 °C for soaking to complete the phase inversion process. In addition, the as-prepared membrane was rinsed with a large amount of deionized water to remove the residual solvent, and the finally obtained membrane was labeled as GO / PVDF-2.0.

[0031] The GO / PVDF composite ultrafiltration membranes in Examples 1-5 of the present application were characterized and performance-tested as follows:

[0032] The hydrophilicity of the composite membrane was characterized using a static contact angle measuring instrument (CA, DSA30). The porosity ε (%) of the membrane was defined as the ratio of the membrane pore volume to the total volume of the filter membrane and could be measured by the gravimetric method. The calculation formula is shown in Equation (1).

[0033]

[0034] In Equation (1), m1 is the mass of the wet membrane; m2 is the mass of the dry membrane; ρ w is the density of pure water, with a value of 0.998 g / cm 3 ; ρ p is the density of the composite membrane. Since the amount of GO used is small and the mass fraction is low, ρ p can be approximated as the density of PVDF, with a value of 1.765 g / cm 3 .

[0035] The average pore size (γ m ) of the composite membrane can be calculated by the Guerout-Elford-Ferry formula. The calculation formula is shown in Equation (2).

[0036]

[0037] In Equation (2), ε is the porosity; η is the viscosity of water, with a value of 8.9×10 -4 Pa·S; l is the membrane layer thickness; Q is the volume of water filtered per unit time (m 3 / s); A is the effective area of the membrane (m 2 ); ΔP is the operating pressure, with a value of 0.1 MPa.

[0038] The calculation formula for the pure water flux of the membrane is shown in Equation (3).

[0039]

[0040] In Equation (3), J is the flux of the membrane (L·m -2 ·h -1 ); Q is the volume of the membrane effluent (L); t is the filtration time (h); A is the effective area of the membrane sheet (m 2 ). Replace pure water with a 0.1 g / L BSA buffer solution (PBS, pH = 7.4), and measure the flux of the membrane sheet by the same method as J p .

[0041] The rejection rate of the membrane sheet sample can be calculated from the change in the mass concentration of BSA before and after filtration, and measured at 280 nm using an ultraviolet spectrophotometer (model UV-1700, SHIMADZU). The calculation formula for the rejection rate is shown in Equation (4).

[0042]

[0043] In Equation (4), R (%) is the rejection rate; C p , C f are the mass concentrations of BSA in the filtrate and the stock solution, respectively.

[0044] Rinse the membrane sheet sample contaminated with BSA three times with pure water, and then test the pure water flux, denoted as J w2 . Denote the initial flux of the membrane as J w1 , therefore, the flux recovery rate FRR (%) can be calculated from Equation (5).

[0045]

[0046] It should be understood that the above test methods and test equipment are common methods for evaluating the relevant performance of polyvinylidene fluoride (PVDF) membranes in this industry, and are only a means to characterize or evaluate the technical solutions and technical effects of the present invention. Other test methods and test equipment can also be used, which will not affect the final results.

[0047] The test results of the surface characteristics of the GO / PVDF composite ultrafiltration membrane in the present invention are as follows:

[0048] Hydrophilicity directly affects the water flux and anti-fouling performance of the composite membrane, which can be characterized by the static contact angle. The results are shown in Table 1. The increase in the mass fraction of GO is beneficial to improving the hydrophilicity of the membrane surface. The contact angle decreases from 73.1° without addition to 65.5° when the addition amount is 1%, because GO has good hydrophilicity and GO with a lower mass fraction is more easily dispersed uniformly in the casting solution. When the addition amount of GO exceeds 1%, the increase in the viscosity of the casting solution will cause GO to accumulate on the membrane substrate or surface, resulting in the generation of membrane structure defects. In addition, the membrane porosity and membrane pore size also show a trend of first increasing and then decreasing with the increase in the mass fraction of GO, which is consistent with the SEM analysis.

[0049] Table 1 Contact Angle and Membrane Pore Parameters of the Composite Membrane

[0050]

[0051] Performance Test Results of GO / PVDF Composite Ultrafiltration Membrane:

[0052] Flux and rejection rate are important indicators of membrane filtration performance. The filtration performance of the composite membrane was evaluated through a self-made experimental system, and the experimental results are as Figure 1 . From Figure 1 , it can be seen that whether it is the pure water flux (J w1 ), filtration flux (J p ), recovery flux (J w2 ), or rejection rate (R%), the experimental values of the composite membrane are all higher than those of the pure PVDF membrane. In addition, when the dosage of GO is 1%, the composite membrane has better comprehensive performance because the addition of GO with a low mass fraction is easy to form a low-resistance membrane pore structure, and the number of pores and pore sizes on the membrane surface increases, which is the main reason for the increase in membrane flux. Through SEM surface morphology analysis, it can be seen that the addition of GO is beneficial to improving the surface roughness of the membrane, making it smoother, and pollutants are not easy to stay, which is beneficial to the improvement of membrane anti-fouling performance. Generally speaking, when the membrane pore size becomes larger and the water flux increases, the rejection rate of the membrane will decrease, but this law was not found in this experiment, which may be due to the strong adsorption effect of carbon nanomaterials. Under the action of external pressure filtration, the BSA solution filters from the surface of the membrane filament to the internal channel of the membrane filament and is easily adsorbed by GO embedded in the separation layer or inserted inside the finger-like pores and cannot pass through the membrane, resulting in a decrease in the mass concentration of BSA in the permeate.

[0053] The experiment studied the relationship curve of the water flux changing with time within 3 cleaning cycles of the composite membrane (see Figure 2 ), and controlled the concentration polarization phenomenon during the filtration process by rapidly stirring the BSA solution. Figure 2The results show that the attenuation of water flux during the experiment is mainly due to the fact that during the filtration process, proteins are extremely likely to adsorb on the surface of the membrane filaments, causing blockage of the surface membrane pores and easily entering the interior of the membrane layer to block the finger-shaped pores, and it is very difficult to completely remove them through hydraulic cleaning. Therefore, it is also very difficult for the flux to recover to the initial level during the subsequent cyclic tests. Within 3 cleaning cycles, the average flux attenuation rates of the composite membrane are 68.7%, 63.7%, 61.0%, 64.5%, and 64.8%. Comparatively speaking, when the GO addition amount is 1%, the flux attenuation rate of the composite membrane is the smallest, and the final stable filtration flux is the highest. Continuing to increase the GO dosage, the hydrophilicity of the composite membrane surface becomes worse, and the anti-fouling property decreases, which is consistent with the results of the static contact angle analysis. It shows that appropriate addition of GO helps to improve the anti-fouling performance of the composite membrane. The improvement of the anti-fouling performance of the composite membrane mainly stems from the increased hydrophilicity of the composite membrane surface. BSA is a hydrophobic protein. The hydrophilic functional groups on the surface of GO can be closely connected with water molecules in the solution, forming a strong affinity, thereby blocking the channel for BSA molecules to diffuse and advance towards the membrane surface, reducing the probability of their contact and migration, and reducing the risk of membrane fouling.

[0054] The embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited thereto. Various changes can be made without departing from the spirit of the present invention within the scope of knowledge possessed by those skilled in the art to which the present invention pertains.

Claims

1. A GO / PVDF composite ultrafiltration membrane for MBR, characterized in that, Using 18% polyvinylidene fluoride as the membrane substrate, 6% polyvinylpyrrolidone as the pore former, 0 - 2% graphene oxide as the modifier, and the remaining amount with N,N - dimethylacetamide as the solvent, a GO / PVDF composite ultrafiltration membrane was prepared by the immersion precipitation phase inversion method.

2. The preparation method of the GO / PVDF composite ultrafiltration membrane for MBR according to claim 1, wherein, The steps are as follows: S1. Prepare graphene oxide by the Hummers method; S2. Using polyvinylidene fluoride as the membrane substrate, polyvinylpyrrolidone as the pore former, graphene oxide as the modifier, and the remaining amount with N,N - dimethylacetamide as the solvent, a GO / PVDF composite ultrafiltration membrane was prepared by using deionized water as the coagulation bath and the immersion precipitation phase inversion method.

3. The preparation method of the GO / PVDF composite ultrafiltration membrane for MBR according to claim 2, characterized in that, The steps of S1 are as follows: Weigh 4 g of flake graphite, 2 g of sodium nitrate, and 92 mL of concentrated sulfuric acid into a beaker. Place the beaker in an ice - water mixture and stir magnetically for 0.5 h. Then, add 4 g of potassium permanganate to the beaker at regular intervals of 15 min for a total of three times. After adding, continue the reaction for 1 h and the low - temperature reaction stage ends; After the low - temperature reaction stage ends, transfer the beaker to a 35 °C constant - temperature water bath and continue stirring for 2 h, then the medium - temperature reaction stage ends; After the medium - temperature reaction stage ends, change the temperature of the constant - temperature water bath to 95 °C, continue magnetic stirring, and slowly add 200 mL of deionized water drop by drop. After the temperature of the reaction system rises to 95 °C, wait for another 5 min and then slowly add 400 mL of deionized water and 100 mL of hydrogen peroxide. At this time, a dispersion with a bright yellow metallic luster is obtained. Continue the reaction for 15 min and the high - temperature reaction stage ends; After the high - temperature reaction stage ends, remove the beaker from the water bath, ultrasonically disperse the obtained dispersion for 30 min, add hydrochloric acid and stir for 3 min. Finally, dilute the obtained graphene oxide dispersion with deionized water to 10 L, pour off the supernatant by natural gravity precipitation, and repeat this step three times until the pH of the graphene oxide dispersion is neutral; Freeze - dry the lower - layer graphene oxide slurry after the third precipitation to obtain solid graphene oxide, cut it into fine powder and place it in a desiccator for standby.

4. The preparation method of the GO / PVDF composite ultrafiltration membrane for MBR according to claim 2, wherein, The specific steps of S2 are as follows: First, disperse 0 - 2% graphene oxide into an N,N - dimethylacetamide solution and ultrasonically disperse it for 10 h; Second, mix 18% polyvinylidene fluoride powder and 6% polyvinylpyrrolidone powder evenly and add them to an N,N - dimethylacetamide solution at 70 °C. After dissolution, add the graphene oxide dispersion and continuously stir for 12 h to obtain a homogeneous and transparent casting solution; Finally, stand still in a vacuum environment for 8 h to remove the residual bubbles in the casting solution. Slowly spread the casting solution on a glass plate, use a film - scraping device to prepare a film with a thickness of 200 μm, stay in the air for 20 s and then immerse it in deionized water at 25 °C to complete the phase inversion process.

5. The preparation method of the GO / PVDF composite ultrafiltration membrane for MBR according to claim 4, characterized in that, Before membrane preparation, all materials need to be dried in an oven at 80 °C for 12 h to remove moisture.

6. The preparation method of the GO / PVDF composite ultrafiltration membrane for MBR according to claim 4, wherein, After membrane preparation, rinse the as - cast membrane with deionized water to remove the residual solvent.

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