Hot-process enhanced hollow fiber microfiltration membrane based on PVDF (polyvinylidene fluoride) and preparation method of hot-process enhanced hollow fiber microfiltration membrane

Abstract

The invention relates to the field of micro-filtration membranes, in particular to a PVDF (polyvinylidene fluoride)-based hot-process enhanced hollow fiber micro-filtration membrane and a preparation method thereof. The graphene oxide doped braided tube reinforced PVDF hollow fiber membrane is prepared by adopting graphene oxide as a doping agent, glycerol triacetate as a diluent and a polyethylene glycol terephthalate braided tube as a reinforcement body and then adopting a TIPS-fiber reinforcement coupling technology. The method can be used for researching the influence of different GO doping amounts on the morphology, aperture, mechanical property and permeability of the enhanced PVDF hollow fiber membrane. The interface strength between the PVDF separation layer and the PET braided tube reinforcement is mainly researched, and a theoretical basis is provided for preparing a new generation of green, environment-friendly, high-strength and high-performance PVDF hollow fiber membrane.

Description

Technical Field

[0001] The present invention relates to the field of microfiltration membranes, and in particular to a thermally enhanced hollow fiber microfiltration membrane based on PVDF and a preparation method thereof. Background Art

[0002] At present, the membrane bioreactor (MBR) is widely used as the most common way to treat wastewater. The core part of the membrane bioreactor is the membrane material, and commonly used hollow fiber membranes such as polyvinylidene fluoride (PVDF) and polysulfone (PS) prepared by wet spinning are used. However, a large amount of solvent is used in the wet spinning process, resulting in secondary pollution in the membrane preparation process, and the treatment cost of solvent wastewater is relatively high, increasing the burden in the membrane preparation process. The thermally induced phase separation (TIPS) method, also known as the high-temperature induced phase separation method, requires a homogeneous casting solution to be prepared by selecting polymers and diluents with similar solubility parameters. Compared with the wet spinning method, the TIPS method has the advantages of adjustable pore size, stable strength, and good permeability. At present, studies have shown that the green diluent technology has a positive effect on overcoming the secondary pollution of wet spinning, and triacetin and tributyl acetylcitrate are often selected.

[0003] During the use of the MBR, the hollow fiber membrane will be affected by high-pressure water flow, aeration jitter, etc. to slow down the membrane fouling cycle. Therefore, enhanced hollow fiber membranes with good performance have been developed and prepared. At present, according to the form of the reinforcement, the enhanced membranes can be classified into three categories: porous substrate membrane (melt-stretched pores) enhanced type, continuous fiber (monofilament or multifilament fiber bundle) enhanced type, and braided tube enhanced type. Among them, the braided tube enhanced type is widely used because it is easier to control the interfacial bonding state between the separation layer and the reinforcement on the premise of cost savings. However, there are few reports on the braided tube enhanced hollow fiber membrane prepared by the TIPS method at present.

[0004] Graphene oxide (GO) is a two-dimensional layered inorganic material, which is often used for membrane matrix and membrane surface modification. Due to its rich surface functional groups and large specific surface area, it plays a positive role in the hydrophilicity (anti-fouling property) of the membrane and the permeation performance (pure water flux, retention, etc.). Research shows that after GO is introduced into the membrane matrix, strong van der Waals forces and hydrogen bond interactions will be generated between the polymer molecular chains. On the one hand, it is beneficial to construct the structure between the polymer molecular chains, and on the other hand, it will increase the dispersion of GO in the membrane matrix and solve the agglomeration problem in the inorganic particle-doped membrane. PVDF has stable C-F bonds, high mechanical strength, good chemical resistance, thermal stability, and excellent corrosion resistance, and is currently a commonly used hollow fiber membrane material. Summary of the Invention

[0005] In view of the problems existing in the background technology, a thermally enhanced hollow fiber microfiltration membrane based on PVDF and its preparation method are proposed. Using GO as a dopant, environmentally friendly triacetin as a diluent, and polyethylene terephthalate (PET) braided tube as a reinforcement, a large-flux and high-strength graphene oxide (GO)-doped braided tube-reinforced PVDF hollow fiber membrane is prepared by a green TIPS-fiber reinforcement combined technology. It can be used to study the effects of different GO doping amounts on the morphology, pore size, mechanical properties and permeability of the enhanced PVDF hollow fiber membrane. And the interfacial strength between the PVDF separation layer and the PET braided tube reinforcement is focused on, providing a theoretical basis for the preparation of a new generation of green, environmentally friendly, high-strength and high-performance PVDF hollow fiber membranes.

[0006] The present invention proposes a thermally enhanced hollow fiber microfiltration membrane based on PVDF. The membrane is a graphene oxide-doped braided tube-reinforced PVDF hollow fiber membrane prepared by using graphene oxide as a dopant, triacetin as a diluent, and a polyethylene terephthalate braided tube as a reinforcement, and then using a TIPS-fiber reinforcement combined technology.

[0007] The present invention also proposes a preparation method of a thermally enhanced hollow fiber microfiltration membrane based on PVDF, and the steps are as follows:

[0008] S1. Preparation of graphene oxide;

[0009] S2. Preparation of the enhanced hollow fiber membrane;

[0010] S21. Through the structural design of the PET braided tube, the PET fiber filaments are woven into a hollow braided tube shape;

[0011] S22. Preparation and post-treatment of the enhanced hollow fiber membrane: Graphene oxide and triacetin are mixed according to the mass ratio and subjected to a water bath treatment, and a fixed mass of PVDF resin is mixed in and then left to stand for defoaming to obtain a homogeneous casting solution; then the homogeneous casting solution is uniformly coated on the surface of the PET braided tube by a thermally induced phase separation method and a fiber reinforcement combined spinning device, and cooled to obtain a primary enhanced hollow fiber membrane; finally, the primary enhanced hollow fiber membrane is immersed in absolute ethanol to extract the triacetin diluent, washed with distilled water and dried to obtain the enhanced hollow fiber membrane.

[0012] Preferably, the proportions of PVDF resin, graphene oxide and triacetin in the homogeneous casting solution are 30%:(0.1-1%):(69-69.9%).

[0013] Preferably, the preparation method of graphene oxide is as follows: Weigh flake graphite, sodium nitrate, and concentrated sulfuric acid, and magnetically stir them in an ice-water mixture. Then, add potassium permanganate quantitatively multiple times until the low-temperature reaction stage ends; transfer to a constant-temperature water bath until the medium-temperature reaction stage ends; increase the temperature of the constant-temperature water bath, and dropwise add deionized water and hydrogen peroxide to obtain a dispersion with a bright yellow metallic luster, and continue the reaction until the high-temperature reaction stage ends; ultrasonically disperse the dispersion, add hydrochloric acid to obtain a graphene oxide dispersion, dilute it with deionized water, and take the precipitate; freeze-dry the lower-layer graphene oxide slurry of the precipitate to obtain solid graphene oxide.

[0014] Compared with the prior art, the present invention has the following beneficial technical effects: The present invention prepares graphene oxide (GO) by an improved Hummers method, and prepares a high-flux and high-strength graphene oxide (GO)-doped woven tube-reinforced polyvinylidene fluoride (PVDF) hollow fiber membrane by using a combined technology of a green diluent thermally induced phase separation (TIPS) method and fiber reinforcement. The surface of GO has abundant oxygen functional groups, which can significantly improve the permeation flux and hydrophilicity (anti-pollution property) of the PVDF hollow fiber membrane. The results show that the fiber reinforcement technology can make the breaking strength of the hollow fiber membrane reach 223.75 MPa. As the GO content increases, the water contact angle value of the reinforced PVDF hollow fiber membrane decreases from 102.04° to 89.71°, and the pure water flux increases from 920.74 L / (m2·h) to 1417.45 L / (m2·h). The filtration of membrane-activated sludge tested by an experimental MBR pilot-scale equipment shows that its rejection rate is greater than 99.50%, and the rejection rate is still greater than 98% after stable operation. Therefore, the enhanced PVDF hollow fiber membrane prepared by the green preparation process of the present invention can provide a scientific basis and technical support for replacing the currently market's hollow fiber membranes with secondary pollution. Brief Description of the Drawings

[0015] Figure 1 It is the preparation flow chart and surface morphology diagram of the PET woven tube;

[0016] Figure 2 It is the spinning flow chart;

[0017] Figure 3 It is the diagram of the bursting pressure test device;

[0018] Figure 4 It is the diagram of the pure water flux test device of the hollow fiber membrane;

[0019] Figure 5 In (a) is the phase separation schematic diagram of the PVDF hollow fiber membrane; (b) is the phase separation schematic diagram of the GO@PVDF hollow fiber membrane; (c) is the cross-sectional morphology diagram of the GO@PVDF hollow fiber membrane;

[0020] Figure 6SEM surface morphology diagram of the enhanced GO@PVDF hollow fiber membrane;

[0021] Figure 7 Infrared spectrum, XRD and Raman spectrum diagrams of the enhanced GO@PVDF hollow fiber membrane;

[0022] Figure 8 DSC curve and schematic diagram of crystallinity of the enhanced GO@PVDF hollow fiber membrane;

[0023] Figure 9 TG curve diagrams of GO and the enhanced GO@PVDF hollow fiber membrane;

[0024] Figure 10 Analysis diagrams of the surface and interfacial mechanical properties of the enhanced GO@PVDF hollow fiber membrane;

[0025] Figure 11 Static water contact angle and pore size distribution diagrams of the moderately enhanced GO@PVDF hollow fiber membrane;

[0026] Figure 12 Permeation performance diagram of the enhanced GO@PVDF hollow fiber membrane;

[0027] Figure 13 Comparison diagrams before and after the activated sludge filtration experiment of the enhanced GO@PVDF hollow fiber membrane;

[0028] Figure 14 Flux and rejection rate diagrams of the enhanced GO@PVDF hollow fiber membrane operating in the MBR device. Detailed implementation method

[0029] The present invention proposes a thermally enhanced hollow fiber microfiltration membrane based on PVDF. This membrane uses the improved Hummers method to prepare graphene oxide (GO) as a dopant, environmentally friendly triacetin as a diluent, and polyethylene terephthalate (PET) woven tube as a reinforcement, and prepares a high-flux and high-strength graphene oxide (GO) doped woven tube reinforced PVDF hollow fiber microfiltration membrane (GO@PVDF) through the green TIPS-fiber reinforcement combined technology. Next, the present invention proposes the following specific embodiments for the preparation method of the above-mentioned enhanced GO@PVDF hollow fiber membrane.

[0030] Example 1, Preparation method of a thermally enhanced hollow fiber microfiltration membrane based on PVDF, the steps are as follows:

[0031] (1) Preparation of graphene oxide (GO): The improved Hummers method was used to prepare graphene oxide (GO), and 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 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 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. Freeze-dry the lower-layer GO slurry after the third precipitation to obtain solid GO, cut it into fine powder, and place it in a desiccator for standby.

[0032] (2) Preparation of enhanced GO@PVDF hollow fiber membrane:

[0033] 2.1. PET braided tube structure design

[0034] Use a 24-spindle fiber braiding machine to weave PET fiber filaments into a hollow braided tube using two-dimensional three-dimensional braiding technology ( Figure 1 ). Before use, remove the sizing agent on the surface of the braided tube with a neutral surfactant (50 °C, 2 h, ultrasonic treatment) and then set it aside for standby.

[0035] 2.2. Preparation and post-treatment of enhanced GO@PVDF hollow fiber membrane

[0036] As Figure 2As shown in the figure, a GO-doped woven tube-reinforced PVDF hollow fiber membrane was prepared by using a green TIPS-fiber reinforcement combined technology. First, GO and glycerol triacetate were poured into a three-necked flask according to the mass ratio and treated in an ultrasonic water bath for 4 h to ensure the dispersion of GO. Then, a fixed mass of PVDF resin was added to the three-necked flask. After mechanical stirring in an oil bath at 175 °C for 4 h and then standing for 1 h to remove bubbles, a homogeneous casting solution was obtained. At this time, the mass fractions of PVDF, GO, and glycerol triacetate in the casting solution were 30%, 0.1%, and 69.9%, respectively. Then, the GO@PVDF homogeneous casting solution was uniformly coated on the surface of the PET woven tube by using a combined spinning device of thermally induced phase separation and fiber reinforcement. After entering the coagulation bath for cooling and forming, a primary reinforced GO@PVDF hollow fiber membrane was obtained. Finally, the primary reinforced GO@PVDF hollow fiber membrane was immersed in absolute ethanol for 24 h to extract the glycerol triacetate diluent, and then washed with distilled water for 48 h and dried to prepare a reinforced GO@PVDF hollow fiber membrane, named membrane G1.

[0037] Example 2, a preparation method of a PVDF-based thermally enhanced hollow fiber microfiltration membrane, the steps are as follows:

[0038] (1) Preparation of graphene oxide (GO): Graphene oxide (GO) was prepared by an improved Hummers method, and 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, add 4 g of potassium permanganate to the beaker at regular intervals of 15 min, for a total of three times. After adding, continue to react 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 stage reaction 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 to react 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. Freeze-dry the lower-layer GO slurry after the third precipitation to obtain solid GO, cut it into fine powder and place it in a desiccator for standby.

[0039] (2) Preparation of the enhanced GO@PVDF hollow fiber membrane:

[0040] 2.1. PET woven tube structure design

[0041] Using a 24-spindle fiber braiding machine, the PET fiber filaments are woven into a hollow braided tube by two-dimensional three-dimensional braiding technology ( Figure 1 ). Before use, it is removed the sizing agent on the surface of the braided tube with a neutral surfactant (50 °C, 2 h, ultrasonic treatment) and then reserved for use.

[0042] 2.2. Preparation and post-treatment of enhanced GO@PVDF hollow fiber membrane

[0043] As Figure 2 shown, an enhanced PVDF hollow fiber membrane doped with GO was prepared by using a green TIPS-fiber reinforcement combined technology. First, GO and glyceryl triacetate were poured into a three-necked flask according to the mass ratio and then treated in an ultrasonic water bath for 4 h to ensure the dispersion of GO. Then, a fixed mass of PVDF resin was added to the three-necked flask, and after mechanical stirring in an oil bath at 175 °C for 4 h and then standing for defoaming for 1 h, a homogeneous casting solution was obtained. At this time, the mass fractions of PVDF, GO, and glyceryl triacetate in the casting solution were 30%, 0.2%, and 69.8%, respectively. Then, the GO@PVDF homogeneous casting solution was uniformly coated on the surface of the PET braided tube by using a combined spinning device of thermally induced phase separation and fiber reinforcement, and after entering the coagulation bath for cooling and forming, a primary enhanced GO@PVDF hollow fiber membrane was obtained. Finally, the primary enhanced GO@PVDF hollow fiber membrane was immersed in absolute ethanol for 24 h to extract the glyceryl triacetate diluent, and after washing with distilled water for 48 h and then drying, an enhanced GO@PVDF hollow fiber membrane was prepared, and the membrane was named G2.

[0044] Example 3. A preparation method of a thermally enhanced hollow fiber microfiltration membrane based on PVDF, the steps are as follows:

[0045] (1) Preparation of graphene oxide (GO): The improved Hummers method was used to prepare graphene oxide (GO), and 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 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 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. Freeze-dry the lower-layer GO slurry after the third precipitation to obtain solid GO, cut it into fine powder, and place it in a desiccator for standby.

[0046] (2) Preparation of enhanced GO@PVDF hollow fiber membrane:

[0047] 2.1. PET braided tube structure design

[0048] Weave PET fiber filaments into a hollow braided tube using a 24-spindle fiber braiding machine with two-dimensional three-dimensional braiding technology ( Figure 1 ). Before use, remove the sizing agent on the surface of the braided tube with a neutral surfactant (50 °C, 2 h, ultrasonic treatment) and then set it aside for standby.

[0049] 2.2. Preparation and post-treatment of enhanced GO@PVDF hollow fiber membrane

[0050] As Figure 2As shown, a GO-doped woven tube-reinforced PVDF hollow fiber membrane was prepared using the green TIPS-fiber reinforcement combined technology. First, GO and glycerol triacetate were poured into a three-necked flask according to the mass ratio and treated in an ultrasonic water bath for 4 h to ensure the dispersion of GO. Then, a fixed mass of PVDF resin was added to the three-necked flask. After mechanical stirring in an oil bath at 175 °C for 4 h and standing for 1 h to remove bubbles, a homogeneous casting solution was obtained. At this time, the mass fractions of PVDF, GO, and glycerol triacetate in the casting solution were 30%, 0.5%, and 69.5%, respectively. Then, the GO@PVDF homogeneous casting solution was uniformly coated on the surface of the PET woven tube using a combined spinning device of thermally induced phase separation and fiber reinforcement. After entering the coagulation bath for cooling and forming, a primary reinforced GO@PVDF hollow fiber membrane was obtained. Finally, the primary reinforced GO@PVDF hollow fiber membrane was immersed in absolute ethanol for 24 h to extract the glycerol triacetate diluent, and then washed with distilled water for 48 h and dried to prepare the reinforced GO@PVDF hollow fiber membrane, named membrane G3.

[0051] Example 4, a preparation method of a thermally enhanced PVDF-based hollow fiber microfiltration membrane, the steps are as follows:

[0052] (1) Preparation of graphene oxide (GO): Graphene oxide (GO) was prepared using 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, add 4 g of potassium permanganate to the beaker at regular intervals of 15 min, for a total of three times. After adding, continue to react 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 stage reaction 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 reaction system temperature 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 to react 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. Freeze-dry the lower-layer GO slurry after the third precipitation to obtain solid GO, cut it into fine powder, and place it in a desiccator for standby.

[0053] (2) Preparation of the enhanced GO@PVDF hollow fiber membrane:

[0054] 2.1. PET woven tube structure design

[0055] Using a 24-spindle fiber braiding machine, the PET fiber filaments were woven into a hollow braided tube using two-dimensional three-dimensional braiding technology ( Figure 1 ). Before use, it was removed the sizing agent on the surface of the braided tube with a neutral surfactant (50 °C, 2 h, ultrasonic treatment) and then reserved for use.

[0056] 2.2 Preparation and post-treatment of enhanced GO@PVDF hollow fiber membranes

[0057] As Figure 2 shown, an enhanced PVDF hollow fiber membrane doped with GO was prepared using a green TIPS-fiber reinforcement combined technology. First, GO and glycerol triacetate were poured into a three-necked flask according to the mass ratio and treated in an ultrasonic water bath for 4 h to ensure the dispersion of GO. Then, a fixed mass of PVDF resin was added to the three-necked flask, mechanically stirred in an oil bath at 175 °C for 4 h, and then left to stand for 1 h to defoam to obtain a homogeneous casting solution. At this time, the mass fractions of PVDF, GO, and glycerol triacetate in the casting solution were 30%, 1%, and 69% respectively. Then, the GO@PVDF homogeneous casting solution was uniformly coated on the surface of the PET braided tube using a combined spinning device of thermally induced phase separation and fiber reinforcement, and cooled and formed in a coagulation bath to obtain a primary enhanced GO@PVDF hollow fiber membrane. Finally, the primary enhanced GO@PVDF hollow fiber membrane was immersed in absolute ethanol for 24 h to extract the glycerol triacetate diluent, and then washed with distilled water for 48 h and dried to prepare an enhanced GO@PVDF hollow fiber membrane, named membrane G4.

[0058] It should be further noted that the braiding process parameters of the PET braided tube structure design in the above examples are shown in Table 1.

[0059]

[0060] Table 1 Braided tube parameters

[0061] It should be further noted that the GO doping amount in the casting solution and the spinning parameters for the preparation and post-treatment of the enhanced GO@PVDF hollow fiber membranes are shown in Table 2 and Table 3 respectively.

[0062]

[0063] Table 2 Casting solutions with different GO doping amounts

[0064]

[0065] Table 3 Spinning parameters of enhanced GO@PVDF hollow fiber membranes

[0066] Next, the performance tests of G1, G2, G3, and G4 were carried out, and the performance test and characterization methods are as follows:

[0067] 1. Observe the inner and outer surfaces and cross-section photos of the hollow fiber membrane using a desktop scanning electron microscope.

[0068] 2. Test the characteristic functional groups and crystalline structures on the membrane surface using a Fourier transform infrared spectrometer.

[0069] 3. Characterize the crystal form and crystal structure inside the membrane using an XRD diffractometer.

[0070] 4. Test the characteristic peaks of the membrane using a Raman spectrometer.

[0071] 5. Characterize the thermal properties (melting point, decomposition temperature, etc.) of the membrane using a comprehensive thermal analyzer.

[0072] 6. The mechanical properties of the membrane mainly include the membrane breaking strength and elongation at break. Use an electronic tensile testing machine to test the mechanical properties of the membrane.

[0073] 7. Characterization of interfacial bonding strength: The interfacial bonding strength between the support and the separation layer of the enhanced hollow fiber membrane can reflect its service life. Adopt the ultrasonic oscillation method to put the membrane sample into an ultrasonic cleaner. Measure the pure water flux of the membrane before ultrasonic oscillation (take the average of 3 times), the test frequency is 40 kHz, and the time is 30 min. Retest the pure water flux of the membrane after ultrasonic oscillation (take the average of 3 times).

[0074] 8. Membrane burst pressure: The pressure resistance of the enhanced hollow fiber membrane is often characterized by the burst pressure. Use a self-made laboratory pressure testing device ( Figure 3 ) to characterize the membrane burst pressure. The pressure range is 0.01 - 0.3 MPa, and the rate is 0.01 MPa / min. As the pressure increases, the evenly attached small bubbles on the membrane surface are gradually replaced by large bubbles. When the large bubbles appear significantly, it indicates that the membrane is damaged. At this time, the displayed pressure is the burst pressure, also known as the burst strength.

[0075] 9. Permeation performance

[0076] The pore size distribution is an important factor for characterizing the pore size and retention of the membrane. Use a bubble point pore size analyzer to measure the average pore size and its distribution of the hollow fiber membrane. Before the test, fully wet the membrane with a wetting liquid, gradually apply gas (N2) pressure to the membrane sample, and obtain the pressure-flow change curve. Through software analysis and calculation, obtain the membrane pore size and distribution.

[0077] The porosity refers to the volume occupied by the membrane pores in the membrane matrix. Immerse the membrane sample in n-butanol for 12 h, and measure the membrane porosity by the weighing method. Wipe the membrane sample with filter paper to remove the n-butanol solution on the membrane surface and weigh (wet weight), and then dry it in an oven at 60 °C for 12 h and weigh (dry weight). The porosity is shown in Equation (1).

[0078]

[0079] Where ε is the porosity, %; m1 is the wet weight, g; m2 is the dry weight, g; ρ is the density of n-butanol, g / cm 3 ; D and d are the outer diameter and inner diameter of the membrane, cm; l is the length of the membrane, cm.

[0080] The flux of the hollow fiber membrane was measured using a self-made flux test device in the laboratory. The device is as Figure 4 shown. The measurement was carried out at 25 °C under a working pressure of 0.1 MPa. The membrane flux was calculated by Equation (2).

[0081]

[0082] Where J is the pure water flux, L / (m 2 ·h); V is the filtrate volume, L; S is the membrane area, m 2 ; t is the filtration time, h.

[0083] Using Figure 4 the self-made pure water flux device shown, the activated sludge suspension was filtered to characterize the anti-fouling performance of the membrane. Among them, the initial concentration of the activated sludge suspension was 10 g / L and the turbidity was 3048 NTU. The turbidity of the effluent was measured using a turbidimeter every 10 min / time. After converting the turbidity to concentration, the rejection rate was calculated using Equation (3).

[0084]

[0085] Where R is the rejection rate; C f and C p are the concentrations of the stock solution and the filtrate, g / L, respectively.

[0086] The activated sludge suspension was filtered for 60 min at 0.1 MPa, followed by a physical cleaning, and then filtered for another 30 min, followed by a second physical cleaning. The specific steps were as follows: first, wipe with absorbent cotton, and then rinse repeatedly with deionized water for 5 min. Then, measure its pure water flux again to evaluate its compressive resistance and durability. Among them, the flux recovery rate (FRR) was calculated by Equation (4).

[0087]

[0088] Where J i is the initial flux of the activated sludge, L / (m 2 ·h); J r is the initial flux of the activated sludge after physical cleaning, L / (m 2 ·h).

[0089] After the above tests, the results obtained are as follows:

[0090] I. Influence of GO on the membrane morphology:

[0091] Figure 5 The phase separation process and cross-sectional SEM morphology of the enhanced GO@PVDF hollow fiber membrane. As can be observed from Figure 5 (a) and (b), after introducing 1.0% mass fraction of GO into the PVDF casting solution system, an increase in the number of spherulites and a decrease in the size (spherulite diameter) of the cross-section of the hollow fiber membrane can be observed. This is because GO has good hydrophilicity, which accelerates the exchange between the diluent (solvent) and the coagulation bath (non-solvent), enabling the membrane to obtain a more stable spherulite arrangement structure and allowing the regulation of the membrane pore size according to this change. As shown in Figure 5 (c), the cross-sectional SEM images of the enhanced GO@PVDF hollow fiber membrane at different magnifications. From G1-G4 (×50), it can be observed that the enhanced GO@PVDF hollow fiber membrane consists of a dense separation layer, a porous (spherulite structure) transition layer, an interfacial bonding layer, and a support body, and a good interfacial bonding layer is formed between the support body and the transition layer, indicating that the TIPS-fiber reinforcement combined technology can effectively improve the mechanical properties of the membrane. From G1-G4 (×1000), it can be seen that the dense separation layer is less than 10 nm, indicating that a relatively small resistance is generated during the filtration process, and the porous transition layer forms a spherulite structure, indicating that the green diluent triacetin is a good solvent for PVDF at high temperatures. From G1-G4 (×5000), it can be seen that as the GO content increases, the size of the spherulite structure first increases and then decreases (instantaneous phase separation gradually appears). This is because after introducing GO into the casting solution system, although it will increase the hydrophilicity of the system, at the same time, due to its unique two-dimensional layered structure and large specific surface area, it will increase the viscosity of the system, thus delaying the phase separation process between the solvent and the non-solvent. Therefore, the spherulite structure first increases. As the GO content increases, the number of hydrophilic functional groups increases, accelerating the exchange between the non-solvent and the solvent, so the phenomenon of spherulite reduction appears again. In summary, the introduction of an appropriate amount of GO into the membrane system has a positive impact on the structure and performance.

[0092] Figure 6 The surface SEM morphology of the enhanced GO@PVDF hollow fiber membrane. As can be observed from the figure, as the GO content increases, the surface of the hollow fiber membrane first becomes dense and then loose. When 1.0% mass fraction of GO is introduced, the dense cortex is torn (spindle-shaped pore structure), and the spherulites are exposed in the spindle-shaped pores. The above phenomena also confirm that a small amount of GO delays the phase separation process between the solvent and the non-solvent, so the cortex shows a dense trend. As the GO content increases, the number of hydrophilic functional groups increases, accelerating the exchange between the non-solvent and the solvent, so that the spherulite crystallization has formed before the cortex is formed, so the loose trend appears again.

[0093] II. Influence of GO on the membrane structure and performance:

[0094] As Figure 7(a) FTIR spectra of the enhanced GO@PVDF hollow fiber membranes. Characteristic peaks of PVDF can be observed at 1400 cm-1 (C-H) and 1175 cm -1 (C-F). In addition, the stretching vibration peak of C-OH of GO is located near 1400 cm-1. However, the stretching vibration peak of -OH of GO is not observed at 3000 - 3400 cm -1 . This may be because most of the GO is wrapped in the membrane matrix during the preparation of the hollow fiber membranes. The surface of GO contains abundant functional groups (such as hydroxyl groups and carboxyl groups). Such functional groups will undergo partial reduction when the temperature is higher than 150 °C. After a small amount of GO moves to the membrane surface, partial reduction of GO to form RGO occurs under the action of the spinning temperature of 175 °C. Therefore, the stretching vibration peak here is not observed. In addition, from Figure 2 the physical diagram of the spinning process, it can be observed that the enhanced GO@PVDF hollow fiber membranes are partially black (the color of RGO), showing a difference from the yellowish-brown color of GO. This is due to the mechanical stirring during the preparation of the casting solution and the shear force during the spinning process under the condition of 175 °C, objectively indicating the formation of RGO. It can also be seen from the FTIR spectra that the introduction of GO has little effect on the α-phase and β-phase in PVDF, which may also be caused by the partial reduction of GO. Therefore, XRD is used to further verify the influence of GO on the membrane structure stability. As Figure 7 (b) shows, characteristic peaks of 2θ = 17.6°, 18.2° and 20.8° can be observed in the XRD spectra of the GO@PVDF hollow fiber membranes. It is known from the literature that the characteristic peaks represent the (100), (020) and (110) crystal planes of the α-phase and β-phase respectively. As the doping amount of GO increases, the peak intensity of the β-phase weakens, indicating that a larger doping amount of GO will affect the regular arrangement of the membrane structure. Similarly, the Raman spectra also show changes in the carbon chain structure. Generally, in the displacement range of 1000 - 2000 cm -1 , there is no obvious Raman peak in PVDF, and most of the peaks are concentrated below 1000 cm -1 . While GO has two characteristic peaks, the D peak and the G peak, located at 1350 cm -1 and 1602 cm -1 respectively. It can be seen from Figure 7 (c) that the changes in the D peak and G peak of GO in the enhanced GO@PVDF hollow fiber membranes are not obvious, only showing a slight right shift. Although the data indicate that GO is successfully introduced into the PVDF film-forming system, it also shows that the relatively high spinning temperature causes partial reduction of GO, resulting in the above phenomenon.

[0095] III. Thermal performance analysis:

[0096] As Figure 8(a) is the DSC heating curve of the enhanced GO@PVDF hollow fiber membrane. As shown in the figure, with the increase in the GO content, the melting peak temperature is basically around 173.5 °C, and there is no obvious shift. This further indicates that when GO is doped in PVDF, the two have good dispersibility and do not significantly affect the compatibility of the system. Figure 8 (b) is the enthalpy change crystallinity. It can be observed from the figure that with the increase in the GO doping amount, the melting peak area increases, and the crystallinity increases from 31.58% to 44.87%. This shows that GO plays a role of heterogeneous nucleation on PVDF, thereby promoting the crystallization of PVDF and improving the thermal stability of the enhanced GO@PVDF hollow fiber membrane.

[0097] The rich surface functional groups of GO make its heat resistance poor. As can be observed from Figure 9 (a), when the temperature is higher than 100 °C, GO undergoes partial degradation (weight loss of about 15%). When the temperature rises to 200 °C, the weight loss of GO reaches 65%. This degradation is mainly due to the decomposition of the surface functional groups of GO and water loss. When the temperature rises to 300 °C, the weight loss rate significantly decreases, and at this time, the C—C bond begins to decompose and degradation occurs. As can be observed from Figure 9 (b), the weight loss process of the thermal weight loss curve of the enhanced GO@PVDF hollow fiber membrane mainly occurs at 450 - 500 °C. Since the temperature during the membrane preparation process is 175 °C, part of the GO in the hollow fiber membrane is reduced to RGO with a lower mass. The reduced RGO has higher thermal stability than GO and is not easily decomposed. Therefore, the prepared GO@PVDF hollow fiber membrane has high thermal stability, indicating that the introduction of an appropriate amount of GO into the membrane system improves the membrane stability.

[0098] IV. Analysis of the mechanical properties of the membrane surface and interface:

[0099] As Figure 10 (a) shows, for the mechanical properties (tensile strength and elongation at break) of the enhanced GO@PVDF hollow fiber membrane, with the increase in the GO doping amount, its tensile strength and elongation at break gradually increase. This is because the introduction of GO into the PVDF membrane system improves the mechanical properties of the membrane. As known from the DSC test results, GO has good compatibility with PVDF, generating strong interactions between the PVDF molecular chains, so that the molecular chains still have high entanglement force under the action of external tensile force. Therefore, both the tensile strength and elongation at break of the membrane are improved, indicating that GO can be used as a good nano-filler under certain conditions. In addition, as known from the data in the previous sections, GO will be reduced to RGO during the membrane preparation process. The two-dimensional structure of RGO has a larger specific surface area, so it can increase the interaction with the PVDF matrix, thereby improving the mechanical properties of the membrane. The interfacial bonding state of the enhanced GO@PVDF hollow fiber membrane can be characterized by the ultrasonic oscillation method. As Figure 10As shown in Fig. (b), the water flux growth rates of G1, G2, G3, and G4 before and after ultrasonic oscillation of the enhanced GO@PVDF hollow fiber membrane are 39.29%, 27.17%, 25.44%, and 27.45% respectively, showing a decreasing trend, indicating that an appropriate amount of GO can improve the interfacial bonding strength of the enhanced hollow fiber membrane. As Figure 10 As shown in Fig. (c), the burst pressures (strengths) of G1, G2, G3, and G4 are 0.22 MPa, 0.26 MPa, 0.39 MPa, and 0.35 MPa respectively, also indicating that an appropriate amount of GO can increase the stability of the enhanced hollow fiber membrane. When the GO mass fraction is 1.0% (G4), the burst pressure decreases slightly because the skin layer becomes a spindle-shaped structure, reducing the resistance to gas passage, so the phenomenon of reduced burst pressure appears.

[0100] V. Pore size distribution and static water contact angle:

[0101] The main indicators of the membrane are porosity and pore size distribution. Figure 11 Fig. (a) shows the porosity and average pore size of the membrane. As the GO content increases, the average pore size increases. When the GO mass fraction is 1.0%, the average pore size of the membrane reaches 1.49 μm (microfiltration membrane). The porosity changes slightly and remains between 31% - 33%. This is mainly because after the introduction of two-dimensional layered structure GO into the system, it will increase the macroscopic size between membrane pores, showing a certain "bridging" effect. As Figure 11 As shown in Fig. (b), it can be observed that the static water contact angle of the membrane decreases with the increase of GO content, and the water contact angle value decreases from 102.04° to 89.71°. As known from the previous text, although GO is partially reduced, there are still hydrophilic functional groups retained, but the appearance of RGO still makes the water contact angle value of the TIPS method GO@PVDF hollow fiber membrane greater than that of the wet spinning GO@PVDF hollow fiber membrane. From Figure 11 It can be observed from Fig. (c) that the membrane pore size distribution range is 0.5 - 1.5 μm. As the GO doping amount increases, the membrane pore size distribution becomes wider. This is mainly because the addition of GO improves the hydrophilicity of the casting solution and accelerates the double diffusion process with the non-solvent (water), thus forming more through holes and resulting in a larger pore size distribution of the membrane. The bubble point pressure and maximum pore size data of the membrane are shown in Table 4. In summary, GO improves the hydrophilicity and pore size of the membrane to a certain extent.

[0102]

[0103] Table 4 Bubble point pressure and maximum pore size of the enhanced GO@PVDF hollow fiber membrane

[0104] VI. Permeation performance:

[0105] From Figure 12(a) It can be observed that the pure water flux of the membrane and the rejection rate of activated sludge change significantly with the addition of GO doping. The stable fluxes of G1, G2, G3, and G4 membranes are 205.85 L / (m 2 ·h), 512.99 L / (m 2 ·h), 663.63 L / (m 2 ·h), and 501.31 L / (m 2 ·h) respectively, showing an increasing and stable trend. Generally, the pure water flux is mainly related to the hydrophilicity and pore structure of the membrane. As mentioned above, the addition of GO improves the hydrophilicity and pore structure of the membrane, resulting in the above phenomenon. The activated sludge filtration experiment can be used to evaluate the periodic performance of the membrane in the MBR system. As Figure 12 (b) shows, after secondary cleaning, the pure water flux recovered to 500 L / (m 2 ·h), and the higher the GO content, the better the flux recovery. This indicates that the addition of GO can well improve the pressure resistance and operation stability of the membrane. As Figure 12 (c) shows, with the extension of the filtration time, the permeation flux of the membrane decreases significantly. After physical cleaning, the flux recovery rate increases with the increase of the GO content. When the GO mass fraction is 1%, the flux recovery rate reaches 80.28%, indicating that the addition of GO has good anti-fouling performance for activated sludge. The reason is that the hydrophilic additive GO endows the enhanced GO@PVDF hollow fiber membrane with certain hydrophilicity, and the hydrophilicity of the membrane can, to a certain extent, hinder the irreversible adsorption and deposition of activated sludge aggregates on the membrane surface. It can also be observed from Figure 13 (c) that the sludge deposited on the surfaces of the enhanced hollow fiber membranes G3 and G4 is significantly less than that of G1 and G2.

[0106] Figure 13(a) shows the comparison between the activated sludge stock solution and the permeate. It can be observed that the feed liquid becomes transparent after filtration; Figure 13 (b) Comparing the feed liquid before and after under the electron microscope, it is found that there are basically no impurities after filtration. This indicates that the enhanced GO@PVDF hollow fiber membrane has potential application value in the MBR system.

[0107] VII. MBR Performance Analysis:

[0108] Through the above data comparison, when the GO doping amount is 0.5%, the anti-fouling performance and mechanical properties of the membrane are better improved. Therefore, G3 is selected to make the membrane module in the MBR device for characterization. It can be observed from Figure 14 (a) that after 24 h of continuous operation, the membrane is backwashed and it is found that the flux recovers. With the extension of the operation time, the rejection rate increases and remains above 98% all the time, indicating that the membrane has excellent filtration performance and also proving the feasibility of the enhanced GO@PVDF hollow fiber membrane in treating domestic sewage by MBR.

[0109] The embodiments of the present invention have been described in detail with reference to the accompanying drawings. However, the present invention is not limited thereto, and 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.

Claims

1. A PVDF-based thermally enhanced hollow fiber microfiltration membrane, characterized in that, It is a graphene oxide doped woven tube reinforced PVDF hollow fiber membrane prepared by using graphene oxide as a dopant, triacetin as a diluent, and a polyethylene terephthalate woven tube as a reinforcement, and then adopting the TIPS-fiber reinforcement combined technology.

2. The preparation method of the PVDF-based thermal enhanced hollow fiber microfiltration membrane according to claim 1, characterized in that, The steps are as follows: S1. Preparation of graphene oxide; S2. Preparation of the reinforced hollow fiber membrane; S21. Through the PET woven tube structure design, the PET filaments are woven into a hollow woven tube shape; S22. Preparation and post-treatment of the reinforced hollow fiber membrane: Graphene oxide and triacetin are mixed according to the mass ratio and treated in a water bath. After mixing a fixed mass of PVDF resin, it is left standing to defoam to obtain a homogeneous casting solution; then the homogeneous casting solution is uniformly coated on the surface of the PET woven tube by using a thermally induced phase separation method and a fiber reinforcement combined spinning device, and cooled to obtain a primary reinforced hollow fiber membrane; finally, the primary reinforced hollow fiber membrane is immersed in absolute ethanol to extract the triacetin diluent, washed with distilled water, and dried to obtain the reinforced hollow fiber membrane.

3. The preparation method of the PVDF-based thermally enhanced hollow fiber microfiltration membrane according to claim 2, wherein The proportions of PVDF resin, graphene oxide, and triacetin in the homogeneous casting solution are 30%:(0.1-1%):(69-69.9%).

4. The preparation method of the PVDF-based thermally enhanced hollow fiber microfiltration membrane according to claim 2, wherein, The preparation method of graphene oxide is as follows: Weigh flake graphite, sodium nitrate, and concentrated sulfuric acid, and stir magnetically in an ice-water mixture environment. Then, potassium permanganate is quantitatively added multiple times until the end of the low-temperature reaction stage; transfer to a constant-temperature water bath environment until the end of the medium-temperature reaction stage; increase the temperature of the constant-temperature water bath, and dropwise add deionized water and hydrogen peroxide to obtain a dispersion with a bright yellow metallic luster, and continue to react until the end of the high-temperature reaction stage; after ultrasonic dispersion of the dispersion, add hydrochloric acid to obtain a graphene oxide dispersion, dilute with deionized water, and take the precipitate; the lower-layer graphene oxide slurry of the precipitate is freeze-dried to obtain solid graphene oxide.

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