Method for constructing a super-hydrophilic layer of polyvinylidene fluoride microfiltration membranes by co-coating

Abstract

The present application aims at overcoming the defect of hydrophobicity of the existing PVDF membrane by constructing a super-hydrophilic layer of polyvinylidene fluoride microfiltration membrane through co-coating method.The method for constructing a super-hydrophilic layer of polyvinylidene fluoride microfiltration membrane comprises the following steps: 1, soaking the polyvinylidene fluoride microfiltration membrane in anhydrous ethanol for pretreatment; 2, adding dopamine and fulvic acid into a buffer solution, mixing uniformly to obtain a mixed modification solution, immersing the pretreated PVDF membrane into the mixed modification solution, and using a gas bath constant temperature oscillator to perform surface co-coating to obtain a hydrophilic PVDF-based composite membrane; and 3, cleaning and airing the modified PVDF membrane.The present application uses a one-step method of dopamine and fulvic acid to perform surface co-coating on the PVDF-based microfiltration membrane to improve the hydrophilicity of the membrane, and the PVDF microfiltration membrane prepared by the present application has reliable quality, uniform and stable hydrophilic coating, simple operation, and easy realization of low-cost industrial production, which greatly promotes the application of high-energy-efficiency membranes in water environment remediation.

Description

Technical Field

[0001] This invention belongs to the field of polymer membrane material technology, specifically relating to a method for preparing a hydrophilic polyvinylidene fluoride microfiltration membrane. Background Technology

[0002] Oily industrial production, frequent marine oil spills, and domestic water use generate large amounts of oily wastewater annually. The direct discharge of this oily wastewater poses a significant threat to the ecological environment and human health, making its effective treatment crucial. Membrane technology, with its high selectivity, simple operation, energy efficiency, and environmental friendliness, plays a unique role in water treatment, chemical production, pharmaceuticals, and energy development. Polyvinylidene fluoride (PVDF) possesses excellent mechanical properties, chemical resistance, thermal stability, and processability, thus finding widespread application in membrane separation technology. However, the application of PVDF microfiltration membranes is greatly limited by the hydrophobic properties of the PVDF material itself. On the one hand, the hydrophobic PVDF microfiltration membrane has poor wettability, resulting in low flux. On the other hand, PVDF membranes are prone to strong non-specific adsorption with some colloids, microorganisms and other substances, causing "bioaccumulation" in the PVDF microfiltration membrane, resulting in membrane pore blockage, further reducing filtration performance, and the degree of pollution intensifies with the extension of membrane operation time. Therefore, frequent replacement or cleaning is required during long-term use to ensure filtration effect, increasing the cost of use.

[0003] To effectively improve the hydrophilicity of membrane materials, extend their service life, and reduce costs, researchers both domestically and internationally have conducted relevant studies. CN 101269302 and US 481384 utilize intermolecular crosslinking blends of PVDF with hydrophilic polymers such as sulfonated polyoxocyanate, polyether polyoxocyanate, and cellulose acetate to prepare hydrophilic microfiltration membranes, which improves the hydrophilicity of the membrane material to some extent. CN106237869A discloses a method for improving the hydrophilicity of membrane surfaces through polyphenol coating modification. CN1704152A reports a technique for hydrophilic modification of PVDF membranes by coating the surface with hydrophilic polymers such as polyethylene alcohol and chitosan. However, hydrophilic membranes prepared by blending hydrophilic polymers fall under the category of physical modification. Although simple and with good initial results, the coating and PVDF membrane are only bonded by physical adsorption (i.e., van der Waals forces), resulting in weak adhesion. During application, the hydrophilic substances on the membrane surface are easily detached, leading to a decrease in the hydrophilicity of the separation membrane, flux reduction, and performance degradation.

[0004] Chinese patents CN 106310965 and CN 107149881 disclose hydrophilic modified polydopamine (PDA) membranes. These methods involve first polymerizing PDA in an aqueous system to prepare PDA particles, then heterogeneously mixing the PDA particles with a PVDF solution to create a blend, followed by a secondary hydrophilic treatment to improve the membrane's hydrophilicity to some extent. However, the heterogeneous interaction between PDA particles and the PVDF solution makes complete modification difficult, and the interaction between the PDA particles and the PVDF membrane surface is weak, leading to particle desorption during filtration. Summary of the Invention

[0005] The purpose of this invention is to solve the problems of current PVDF microfiltration membranes having hydrophobic surfaces, being easily contaminated during oil-water separation, having short service life, and requiring frequent replacement of microfiltration membranes when treating oily wastewater, which significantly increases costs. The invention provides a method that introduces hydrophilic fulvic acid (FA) in situ during the self-polymerization of dopamine (DA), and the two are crosslinked in situ to hydrophilically modify the PVDF membrane, thus easily and efficiently preparing a microfiltration membrane with good stability and hydrophilicity.

[0006] The present invention utilizes a method for co-coating a superhydrophilic layer of a polyvinylidene fluoride (PVDF) microfiltration membrane, which is achieved through the following steps:

[0007] 1. Pretreatment of PVDF membrane: The polyvinylidene fluoride (PVDF) microfiltration membrane is immersed in anhydrous ethanol to fully wet it, and then rinsed with deionized water several times to obtain the pretreated PVDF membrane.

[0008] II. Preparation of DA / FA-PVDF membrane: First, prepare a Tris-HCl buffer solution. Add dopamine (DA) powder to the Tris-HCl buffer solution and stir to dissolve, obtaining a dopamine solution. Add fulvic acid (FA) powder to the Tris-HCl buffer solution and stir to dissolve, obtaining a fulvic acid solution. Mix the dopamine solution and fulvic acid solution evenly to obtain a mixed modified solution. Immerse the pretreated PVDF membrane in the mixed modified solution and transfer it to a constant temperature vibration chamber for vibration modification coating treatment at 25-40℃ to obtain a modified PVDF membrane.

[0009] III. Post-treatment of modified membrane: The modified PVDF membrane is rinsed alternately with deionized water and anhydrous ethanol. After rinsing, it is placed in the air to dry or stored in deionized water for later use, to obtain a modified polyvinylidene fluoride membrane with a dopamine / humic acid superhydrophilic layer.

[0010] In step two, the mass ratio of dopamine (DA) powder to fulvic acid (FA) powder is 1:(1-10).

[0011] The application of the modified polyvinylidene fluoride membrane with a dopamine / humic acid superhydrophilic layer in this invention is to use the modified polyvinylidene fluoride membrane with the dopamine / humic acid superhydrophilic layer for oil-water emulsion separation.

[0012] Inspired by the byssal proteins secreted by marine mussels, this invention employs physicochemical methods to enhance the chemical stability of hydrophilic substances by introducing chemical bonds, thus producing a hydrophilic microfiltration membrane with excellent overall performance. Polydopamine coating modification, as a novel and efficient surface modification technology, can firmly adhere polydopamine molecules to the material surface through numerous hydrogen bonds and non-covalent bonds such as π-π stacking. The abundant active groups (amino, imino, and phenolic hydroxyl groups) in polydopamine molecules provide excellent hydrophilicity for the coating. Furthermore, these active groups can chemically bond with hydrophilic polymers through Schiff base or Michael addition reactions, resulting in a membrane with stable hydrophilicity.

[0013] To address the problems of lengthy processes, long modification cycles, and unstable modification effects in current polydopamine coating modification methods, this invention employs a one-step modification of PVDF microfiltration membranes using dopamine and humic acid. The two react chemically (Michael addition and Schiff base reactions) to form a highly viscous hydrophilic coating, modifying the hydrophobic PVDF membrane and imparting excellent hydrophilicity, chemical stability, and mechanical stability. This invention has significant theoretical and practical value for the industrial preparation of hydrophilic microfiltration membranes, overcoming the problems of high cost, short hydrophilicity lifespan, and unsuitability for large-scale production associated with current methods for modifying the hydrophilicity of polyvinylidene fluoride membranes.

[0014] Compared with the prior art, the present invention has the following advantages:

[0015] (1) The present invention uses a one-step method to prepare hydrophilic modified PVDF microfiltration membranes. The raw materials are easy to obtain, the operation in the preparation process is simple and the steps are concise. There is no complicated post-processing. It has strong universality and can realize low-cost industrial production.

[0016] (2) The preparation process of the polyvinylidene fluoride modified membrane prepared by the present invention is simple, and the hydrophilic modification effect is obvious. The membrane surface changes from hydrophobic (129.8°) to strong hydrophilic (32.3°).

[0017] (3) The surface coating of the membrane prepared by the present invention contains a large number of hydrophilic groups, including hydrophilic dopamine and humic acid. The pure water flux is increased by 15 times, the water-in-oil emulsion flux is increased by 5 times, the rejection rate is above 99%, the self-cleaning ability is significantly improved, and the flux can be restored to 84% of the initial value after water washing.

[0018] (4) The membrane prepared by the present invention has good resistance to oil pollution and high reusability. After filtering the emulsion 10 times, the oil rejection rate is still as high as 98%.

[0019] (5) The present invention utilizes a one-step co-coating method of hydrophilic DA and FA to modify PVDF base film, which can maintain the long-term stability and chemical stability of the coating, avoid damage to PVDF film in strong acid environment and strong alkaline solution, increase its service life in wastewater treatment process, make hydrophilic PVDF film have a wide range of application environments, and broaden its application scope and application cost. Attached Figure Description

[0020] Figure 1 These are SEM images of the PVDF base film and the DA / FA-PVDF composite film in the embodiments;

[0021] Figure 2 These are test graphs showing the flux and rejection rate of the oil-in-water emulsion (toluene) of the PVDF base membrane and the DA / FA composite membrane in the embodiments;

[0022] Figure 3 These are water contact angle (WCA) test diagrams of the PVDF base membrane and the DA / FA composite membrane in the embodiments;

[0023] Figure 4 These are underwater oil contact angle (UOCA) test diagrams of the PVDF base film and the DA / FA composite film in the embodiments;

[0024] Figure 5 These are photographs of the underwater oil adhesion experiment of the PVDF base film and the DA / FA composite film in the examples. Detailed Implementation

[0025] Specific Implementation Method 1: This implementation method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane through co-coating is carried out according to the following steps:

[0026] 1. Pretreatment of PVDF membrane: The polyvinylidene fluoride (PVDF) microfiltration membrane is immersed in anhydrous ethanol to fully wet it, and then rinsed with deionized water several times to obtain the pretreated PVDF membrane.

[0027] II. Preparation of DA / FA-PVDF membrane: First, prepare a Tris-HCl buffer solution. Add dopamine (DA) powder to the Tris-HCl buffer solution and stir to dissolve, obtaining a dopamine solution. Add fulvic acid (FA) powder to the Tris-HCl buffer solution and stir to dissolve, obtaining a fulvic acid solution. Mix the dopamine solution and fulvic acid solution evenly to obtain a mixed modified solution. Immerse the pretreated PVDF membrane in the mixed modified solution and transfer it to a constant temperature vibration chamber for vibration modification coating treatment at 25-40℃ to obtain a modified PVDF membrane.

[0028] III. Post-treatment of modified membrane: The modified PVDF membrane is rinsed alternately with deionized water and anhydrous ethanol. After rinsing, it is placed in the air to dry or stored in deionized water for later use, to obtain a modified polyvinylidene fluoride membrane with a dopamine / humic acid superhydrophilic layer.

[0029] In step two, the mass ratio of dopamine (DA) powder to fulvic acid (FA) powder is 1:(1-10).

[0030] In this embodiment, hydrophilic polyphenolic fulvic acid is introduced in situ into the dopamine self-polymerization coating process. Under mild reaction conditions, the polyphenols or oxidized quinones in fulvic acid are cross-linked in situ with the functional end groups (amino groups) of polydopamine to prepare a polydopamine / fulvic acid biomimetic composite functional coating. This coating can improve the antifouling ability and cycle stability of PVDF microfiltration membranes, and the hydrophilic modification effect is obvious, resulting in a significant improvement in the physicochemical stability of the coating.

[0031] The modified PVDF-based microfiltration membrane of this embodiment exhibits significantly improved hydrophilicity, reliable quality, uniform and stable hydrophilic coating, simple operation, and ease of use for low-cost industrial production, greatly promoting the application of high-efficiency membranes in water environment remediation.

[0032] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that in step one, the polyvinylidene fluoride microfiltration membrane is immersed in anhydrous ethanol for 1 to 1.5 hours.

[0033] Specific Implementation Method 3: This implementation method differs from Specific Implementation Method 1 or 2 in that the pH of the Tris-HCl buffer solution prepared in step 2 is 8 to 8.5.

[0034] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the volume ratio of dopamine solution and humic acid solution in step two is 1:1.

[0035] Specific Implementation Method 5: This implementation method differs from Specific Implementation Method 4 in that the concentration of dopamine solution in step 2 is 3-5 g / L.

[0036] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that the oscillation modification coating treatment is performed at 25-35°C in step two.

[0037] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that the rotation speed is controlled at 40 to 160 r / min during the oscillation modification coating process in step two.

[0038] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that the time for the oscillation modification coating treatment in step two is 2h to 12h.

[0039] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that the mass ratio of dopamine (DA) powder to fulvic acid (FA) powder in step two is 1:(4-6).

[0040] Specific Implementation Method 10: This implementation method differs from Specific Implementation Methods 1 to 9 in that in step 3, the modified PVDF membrane is rinsed three times each with deionized water and anhydrous ethanol alternately.

[0041] Example (Implementation Case) 1: This example demonstrates a method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane using dopamine, implemented according to the following steps:

[0042] 1. PVDF membrane pretreatment: The commercially available polyvinylidene fluoride (PVDF) microfiltration membrane was immersed in anhydrous ethanol for 1 hour to fully wet its pores and surface, so that the membrane surface and pore walls could fully contact the modified material during the coating process. It was then rinsed multiple times with deionized water to remove residual anhydrous ethanol from the surface and remove impurities such as emamectin chloride additives, dimethylformamide solvents or polyethylene glycol added during the production of the microfiltration membrane, as well as dust, etc. The pretreated PVDF membrane was obtained and stored in deionized water for later use.

[0043] II. Preparation of DA / FA-PVDF membrane: First, prepare a Tris-HCl buffer solution with pH=8.5. Add 0.2g of dopamine (DA) powder to 100mL of Tris-HCl buffer solution, stir to dissolve, pour into a petri dish and mix well to obtain a modified solution. Immerse the pretreated PVDF membrane in the mixed modified solution, transfer to a constant temperature shaking chamber, set the rotation speed to 60r / min at room temperature, and shake for 360min to obtain the modified PVDF membrane.

[0044] III. Post-treatment of modified membrane: The modified PVDF membrane is rinsed alternately with deionized water and anhydrous ethanol to remove residual unreacted substances on the membrane. Finally, it is dried and stored to obtain a modified polyvinylidene fluoride membrane with a dopamine layer.

[0045] Example 2: The method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane using fulvic acid in this example is implemented according to the following steps:

[0046] 1. PVDF membrane pretreatment: The commercially available polyvinylidene fluoride (PVDF) microfiltration membrane was immersed in anhydrous ethanol for 1 hour to fully wet its pores and surface, so that the membrane surface and pore walls could fully contact the modified material during the coating process. It was then rinsed multiple times with deionized water to remove residual anhydrous ethanol from the surface and remove impurities such as emamectin chloride additives, dimethylformamide solvents or polyethylene glycol added during the production of the microfiltration membrane, as well as dust, etc. The pretreated PVDF membrane was obtained and stored in deionized water for later use.

[0047] II. Preparation of DA / FA-PVDF membrane: First, prepare a Tris-HCl buffer solution with pH=8.5. Add 0.2g of fulvic acid (FA) powder to 100mL of Tris-HCl buffer solution, stir to dissolve, pour into a petri dish and mix well to obtain a modified solution. Immerse the pretreated PVDF membrane in the mixed modified solution, transfer to a constant temperature shaking chamber, set the rotation speed to 60r / min at room temperature, and shake for 360min to obtain the modified PVDF membrane.

[0048] III. Post-treatment of modified membrane: The modified PVDF membrane is rinsed alternately with deionized water and anhydrous ethanol to remove residual unreacted substances on the membrane. Finally, it is dried and stored to obtain a modified polyvinylidene fluoride membrane with a small amount of humic acid layer.

[0049] Example 3: This example describes a method for constructing a superhydrophilic layer of a polyvinylidene fluoride (PVDF) microfiltration membrane through co-coating, implemented according to the following steps:

[0050] 1. PVDF membrane pretreatment: The commercially available polyvinylidene fluoride (PVDF) microfiltration membrane was immersed in anhydrous ethanol for 1 hour to fully wet its pores and surface, so that the membrane surface and pore walls could fully contact the modified material during the coating process. It was then rinsed multiple times with deionized water to remove residual anhydrous ethanol from the surface and remove impurities such as emamectin chloride additives, dimethylformamide solvents or polyethylene glycol added during the production of the microfiltration membrane, as well as dust, etc. The pretreated PVDF membrane was obtained and stored in deionized water for later use.

[0051] II. Preparation of DA / FA-PVDF membrane: First, prepare a Tris-HCl buffer solution with pH=8.5. Add 0.2g of dopamine (DA) powder to 50mL of Tris-HCl buffer solution and stir to dissolve, obtaining a dopamine solution. Add 0.05g of fulvic acid (FA) powder to 50mL of Tris-HCl buffer solution and stir to dissolve, obtaining a fulvic acid solution. Pour the dopamine solution and fulvic acid solution into a petri dish and mix evenly to obtain a mixed modification solution. Immerse the pretreated PVDF membrane in the mixed modification solution and transfer it to a constant temperature vibration chamber. At room temperature, the rotation speed is set to 60r / min, and the vibration modification time is 360min, to obtain the modified PVDF membrane.

[0052] III. Post-treatment of modified membrane: The modified PVDF membrane is rinsed alternately with deionized water and anhydrous ethanol to remove residual unreacted substances on the membrane. Finally, it is dried and stored to obtain a modified polyvinylidene fluoride membrane with a dopamine / humic acid superhydrophilic layer (hydrophilic DA / FA-PVDF composite membrane).

[0053] Example 4: This example describes a method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane through co-coating, implemented according to the following steps:

[0054] 1. PVDF membrane pretreatment: The commercially available polyvinylidene fluoride (PVDF) microfiltration membrane was immersed in anhydrous ethanol for 1 hour to fully wet its pores and surface, so that the membrane surface and pore walls could fully contact the modified material during the coating process. It was then rinsed multiple times with deionized water to remove residual anhydrous ethanol from the surface and remove impurities such as emamectin chloride additives, dimethylformamide solvents or polyethylene glycol added during the production of the microfiltration membrane, as well as dust, etc. The pretreated PVDF membrane was obtained and stored in deionized water for later use.

[0055] II. Preparation of DA / FA-PVDF membrane: First, prepare a Tris-HCl buffer solution with pH=8.5. Add 0.2g of dopamine (DA) powder to 50mL of Tris-HCl buffer solution and stir to dissolve, obtaining a dopamine solution. Add 0.2g of fulvic acid (FA) powder to 50mL of Tris-HCl buffer solution and stir to dissolve, obtaining a fulvic acid solution. Pour the dopamine solution and fulvic acid solution into a petri dish and mix evenly to obtain a mixed modification solution. Immerse the pretreated PVDF membrane in the mixed modification solution and transfer it to a constant temperature vibration chamber. At room temperature, the rotation speed is set to 60r / min, and the vibration modification time is 360min, obtaining the modified PVDF membrane.

[0056] III. Post-treatment of modified membrane: The modified PVDF membrane is rinsed alternately with deionized water and anhydrous ethanol to remove residual unreacted substances on the membrane. Finally, it is dried and stored to obtain a modified polyvinylidene fluoride membrane with a dopamine / humic acid superhydrophilic layer.

[0057] Example 5: This example differs from Example 1 in that step two first prepares a Tris-HCl buffer solution with pH=8.5. 0.2g of dopamine (DA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, obtaining a dopamine solution. 0.4g of fulvic acid (FA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, obtaining a fulvic acid solution. The dopamine solution and fulvic acid solution are poured into a petri dish and mixed thoroughly to obtain a mixed modification solution. The pretreated PVDF membrane is immersed in the mixed modification solution and transferred to a constant temperature vibration chamber. At room temperature, the rotation speed is set to 60r / min, and the vibration modification time is 360min, obtaining the modified PVDF membrane.

[0058] Example 6: This example differs from Example 1 in that step 2 first prepares a Tris-HCl buffer solution with pH=8.5. 0.2g of dopamine (DA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, obtaining a dopamine solution. 0.8g of fulvic acid (FA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, obtaining a fulvic acid solution. The dopamine solution and fulvic acid solution are poured into a petri dish and mixed thoroughly to obtain a mixed modification solution. The pretreated PVDF membrane is immersed in the mixed modification solution and transferred to a constant temperature vibration chamber. At room temperature, the rotation speed is set to 60r / min, and the vibration modification time is 360min, obtaining a modified PVDF membrane.

[0059] Example 7: This example differs from Example 1 in that step 2 first prepares a Tris-HCl buffer solution with pH=8.5. 0.2g of dopamine (DA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, obtaining a dopamine solution. 1.2g of fulvic acid (FA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, obtaining a fulvic acid solution. The dopamine solution and fulvic acid solution are poured into a petri dish and mixed thoroughly to obtain a mixed modification solution. The pretreated PVDF membrane is immersed in the mixed modification solution and transferred to a constant temperature vibration chamber. At room temperature, the rotation speed is set to 60r / min, and the vibration modification time is 360min, obtaining a modified PVDF membrane.

[0060] Example 8: This example differs from Example 1 in that step two involves first preparing a Tris-HCl buffer solution with pH=8.5. 0.2g of dopamine (DA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, yielding a dopamine solution. 0.8g of fulvic acid (FA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, yielding a fulvic acid solution. The dopamine and fulvic acid solutions are poured into a petri dish and mixed thoroughly to obtain a mixed modification solution. The pretreated PVDF membrane is immersed in the mixed modification solution and transferred to a constant temperature vibration chamber. At room temperature, the rotation speed is set to 100r / min, and the vibration modification time is 360min, yielding a modified PVDF membrane.

[0061] Example 9: This example differs from Example 1 in that step two involves first preparing a Tris-HCl buffer solution with pH = 8.5. 0.2g of dopamine (DA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, yielding a dopamine solution. 0.8g of fulvic acid (FA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, yielding a fulvic acid solution. The dopamine and fulvic acid solutions are poured into a petri dish and mixed thoroughly to obtain a mixed modification solution. The pretreated PVDF membrane is immersed in the mixed modification solution and transferred to a constant temperature vibration chamber. At room temperature, the rotation speed is set to 60r / min, and the vibration modification time is 240min, yielding a modified PVDF membrane.

[0062] Example 10: This example differs from Example 1 in that step two involves first preparing a Tris-HCl buffer solution with pH=8.5. 0.2g of dopamine (DA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, yielding a dopamine solution. 0.8g of fulvic acid (FA) powder is added to 50mL of the Tris-HCl buffer solution and stirred to dissolve, yielding a fulvic acid solution. The dopamine and fulvic acid solutions are poured into a petri dish and mixed thoroughly to obtain a mixed modification solution. The pretreated PVDF membrane is immersed in the mixed modification solution and transferred to a constant temperature vibration chamber. At room temperature, the rotation speed is set to 60r / min, and the vibration modification time is 480min, yielding a modified PVDF membrane.

[0063] Table 1 compares the flux and rejection rate of the modified membrane.

[0064] The experimental conditions were as follows: the ratio of oil phase (toluene) to water phase was 1:99, Tween 80 was used as the surfactant, and Oil Red was used as the dye. The mixed emulsion was stirred at 1000 r / min until stable. Using a sand core apparatus, the pure water flux, oil-in-water emulsion flux, and rejection rate of the membrane were measured at room temperature and under a negative pressure of 0.1 MPa.

[0065] Table 2 compares the water contact angle (WCA) and underwater oil contact angle (UOCA) of the modified membrane.

[0066] Based on the analysis of the data in Tables 1 and 2, it was found that using DA and FA to hydrophilically modify the surface of the hydrophobic PVDF membrane had a very significant effect. The modified composite membrane successfully became superhydrophilic and superoleophobic underwater, exhibiting good flux and retention rate in the separation of oil-in-water emulsions, and significantly improving its antifouling ability. The optimal modification conditions for the DA / FA co-coated membrane were a DA to FA feed ratio of 2 / 8, a rotation speed of 60 r / min, and a reaction time of 360 min.

[0067] Table 1 Comparison of flux and rejection rate of modified membranes

[0068]

[0069] Table 2 Comparison of Water Contact Angle (WCA) and Underwater Oil Contact Angle (UOCA) of Modified Films

[0070]

[0071]

Claims

1. A method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane by co-coating, characterized in that... The method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane by co-coating is implemented according to the following steps:

1. Pretreatment of PVDF membrane: The polyvinylidene fluoride microfiltration membrane is soaked in anhydrous ethanol to fully wet it, and then rinsed with deionized water several times to obtain the pretreated PVDF membrane. II. Preparation of DA / FA-PVDF membrane: First, prepare a Tris-HCl buffer solution, add dopamine powder to the Tris-HCl buffer solution, stir to dissolve, and obtain a dopamine solution; Fulvic acid powder was added to Tris-HCl buffer solution and stirred to dissolve, thus obtaining fulvic acid solution. Dopamine solution and fulvic acid solution were mixed evenly to obtain mixed modified solution. The pretreated PVDF membrane was immersed in the mixed modified solution and transferred to a constant temperature vibration chamber for vibration modification coating treatment at 25-40℃ to obtain modified PVDF membrane. III. Post-treatment of modified membrane: The modified PVDF membrane is rinsed alternately with deionized water and anhydrous ethanol. After rinsing, it is placed in the air to dry or stored in deionized water for later use, to obtain a modified polyvinylidene fluoride membrane with a dopamine / humic acid superhydrophilic layer. In step two, the mass ratio of dopamine powder to humic acid powder is 1:(1-10).

2. The method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane by co-coating according to claim 1, characterized in that... In step one, the polyvinylidene fluoride microfiltration membrane is immersed in anhydrous ethanol for 1 to 1.5 hours.

3. The method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane by co-coating according to claim 1, characterized in that... In step two, the pH of the Tris-HCl buffer solution is prepared to be 8-8.

5.

4. The method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane by co-coating according to claim 1, characterized in that... In step two, the volume ratio of the dopamine solution to the fulvic acid solution is 1:

1.

5. The method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane by co-coating according to claim 4, characterized in that... In step two, the concentration of the dopamine solution is 3–5 g / L.

6. The method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane by co-coating according to claim 1, characterized in that... In step two, an oscillation modification coating treatment is performed at 25–35°C.

7. The method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane by co-coating according to claim 1, characterized in that... In step two, the rotation speed is controlled at 40–160 r / min during the oscillation modification coating process.

8. The method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane by co-coating according to claim 1, characterized in that... The oscillation modification coating treatment in step two takes 2 to 12 hours.

9. The method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane by co-coating according to claim 1, characterized in that... In step two, the mass ratio of dopamine powder to humic acid powder is 1:(4-6).

10. The method for constructing a superhydrophilic layer of a polyvinylidene fluoride microfiltration membrane by co-coating according to claim 1, characterized in that... In step three, the modified PVDF membrane is rinsed three times each with deionized water and anhydrous ethanol.

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