Inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of wastewater treatment, and provides an inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane as well as a preparation method and application thereof. The method comprises the following steps: sequentially soaking a PVDF (Polyvinylidene Fluoride) membrane in absolute ethyl alcohol, an alkaline solution, a tannic acid solution, a FeCl3. 6H2O solution and a (NH4) 6Mo7O24. 4H2O solution, so as to obtain the inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane. The PVDF / TA / Gel composite membrane prepared by the invention has good biocompatibility, and the preparation process is simple; the oil-water emulsion separator has super-strong hydrophilicity, underwater super-oleophobicity, pollution resistance and good oil-water emulsion separation efficiency; after catalytic cleaning, excellent regeneration performance and good stability are achieved, and long-time cyclic utilization can be achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of wastewater treatment, and particularly relates to an inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane, a preparation method thereof, and an application thereof. Background Art

[0002] Currently, a large amount of oil-containing wastewater generated by oil spills and the chemical industry poses a serious threat to human health and global water resources. Therefore, effectively treating oil-containing wastewater is of great significance for alleviating water resource shortages and achieving sustainable development. Traditional separation methods, such as gravity separation, flotation, coagulation, and centrifugation, although widely used, have disadvantages such as low separation efficiency, high energy consumption, and high costs. In recent years, membrane separation technology has received extensive attention due to its advantages of high separation efficiency, low energy consumption, relatively low cost, and strong controllability. However, in the long-term oil-water separation process, membrane fouling is inevitable. Therefore, the use of hydrophilic functional materials to alleviate membrane fouling and achieve efficient oil-water separation has become the focus of research.

[0003] Currently, hydrogel modification materials are considered ideal modification materials due to their excellent hydrophilicity. In particular, inorganic hydrogel materials usually have excellent durability and can maintain their structural integrity and properties during the oil-water separation process. However, during long-term operation, membrane fouling is still inevitable due to the deformation and aggregation of oil droplets. Relying solely on hydrophilic modification cannot achieve long-term efficient operation, and even with simple hydraulic cleaning, it is difficult to remove the oil droplets inside the membrane pores.

[0004] Therefore, it is very necessary to achieve superhydrophilicity and catalytic cleaning functionality to complete the goal of long-term filtration treatment of oil-containing wastewater. Summary of the Invention

[0005] The purpose of the present invention is to provide an inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane, a preparation method thereof, and an application thereof in order to overcome the deficiencies of the prior art.

[0006] In order to achieve the above-mentioned invention purpose, the present invention provides the following technical solutions:

[0007] The present invention provides a preparation method of an inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane, comprising the following steps:

[0008] 1) Immerse the PVDF membrane in absolute ethanol to obtain a pretreated PVDF membrane;

[0009] 2) Immerse the pretreated PVDF membrane in an alkali solution to obtain an alkali-treated PVDF membrane;

[0010] 3) Immerse the alkali-treated PVDF membrane in a tannic acid solution to obtain a PVDF / TA membrane;

[0011] 4) Immerse the PVDF / TA membrane in the FeCl3·6H2O solution to obtain the PVDF / TA / Fe 3+ membrane;

[0012] 5) Immerse the PVDF / TA / Fe 3+ membrane in the (NH4)6Mo7O 24 ·4H2O solution to obtain the inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane.

[0013] Preferably, the pore size of the PVDF membrane in step 1) is 0.1 - 0.5 μm, and the soaking time is 5 - 30 min.

[0014] Preferably, the alkali solution in step 2) is sodium hydroxide solution or potassium hydroxide solution, and the concentration of the alkali solution is 1 - 5 mol / L; the soaking time is 2 - 10 min.

[0015] Preferably, the concentration of the tannic acid solution in step 3) is 30 - 120 g / L, the concentration of the FeCl3·6H2O solution in step 4) is 0.5 - 3.5 mol / L, and the concentration of the (NH4)6Mo7O 24 ·4H2O solution in step 5) is 0.05 - 2.0 mol / L.

[0016] Preferably, the soaking time in steps 3) to 5) is independently 0.5 - 4 h.

[0017] The present invention also provides the inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane prepared by the described preparation method.

[0018] The present invention also provides the application of the inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane in oily wastewater. Mix surfactant, oil and water to obtain oily wastewater, and the inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane performs oil-water filtration separation on the oily wastewater.

[0019] Preferably, in the oily wastewater, the concentration of the surfactant is 10 - 50 mg / L, and the concentration of the oil is 1000 - 5000 ppm.

[0020] Preferably, the surfactant is one or more of sodium dodecyl sulfate, stearic acid, sodium dodecylbenzenesulfonate and Tween 80; the oil is one or more of isooctane, n-hexane, paraffin, soybean oil, hexadecane and dichloromethane.

[0021] Preferably, the inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane contaminated by oil droplets after filtration separation is placed in a persulfate solution for regeneration; the persulfate is potassium monopersulfate and / or potassium persulfate; the concentration of the persulfate solution is 0.1-0.5 mmol / L, and the regeneration time is 5-30 min.

[0022] The beneficial effects of the present invention include the following points:

[0023] 1) The PVDF / TA / Gel composite membrane prepared by the present invention has TA, a common natural plant polyphenol, which is inexpensive; the inorganic hydrogel material is two common inorganic salts, FeCl3·6H2O and (NH4)6Mo7O 24 ·4H2O, which has good biocompatibility and a simple preparation process.

[0024] 2) The PVDF / TA / Gel composite membrane prepared by the present invention has super hydrophilicity, underwater superoleophobicity, anti-pollution property, and good oil-water emulsion separation efficiency.

[0025] 3) The PVDF / TA / Gel composite membrane prepared by the present invention has excellent regeneration performance and good stability after catalytic cleaning, and can achieve long-term recycling. Description of the Drawings

[0026] Figure 1 SEM diagrams of the PVDF / TA / Gel composite membrane prepared in Example 2 and the original PVDF membrane without any treatment, where a is the PVDF / TA / Gel composite membrane of Example 2 and b is the original PVDF membrane;

[0027] Figure 2 Water contact angle diagrams of the PVDF / TA / Gel composite membrane prepared in Example 2 and the original PVDF membrane without any treatment, where a is the original PVDF membrane and b is the PVDF / TA / Gel composite membrane;

[0028] Figure 3 Flux and separation efficiency diagrams of the PVDF / TA / Gel composite membranes prepared in Examples 1-3 and the original PVDF membrane without any treatment for separating oil-containing wastewater;

[0029] Figure 4 Flux recovery rate and separation efficiency diagrams of the PVDF / TA / Gel composite membrane prepared in Example 2 for separating water-in-octane emulsion by cyclic filtration;

[0030] Figure 5 Flux recovery rate and separation efficiency diagrams of the PVDF / TA / Gel composite membrane prepared in Example 2 for separating water-in-octane emulsion after cyclic filtration and catalytic cleaning; Detailed Embodiments

[0031] The present invention provides a method for preparing an inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane, which comprises the following steps:

[0032] 1) Immerse the PVDF membrane in absolute ethanol to obtain a pretreated PVDF membrane;

[0033] 2) Immerse the pretreated PVDF membrane in an alkali solution to obtain an alkali-treated PVDF membrane;

[0034] 3) Immerse the alkali-treated PVDF membrane in a tannic acid (TA) solution to obtain a PVDF / TA membrane;

[0035] 4) Immerse the PVDF / TA membrane in an FeCl3·6H2O solution to obtain a PVDF / TA / Fe 3+ membrane;

[0036] 5) Immerse the PVDF / TA / Fe 3+ membrane in an (NH4)6Mo7O 24 ·4H2O solution to obtain an inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane (PVDF / TA / Gel composite membrane).

[0037] In the present invention, the pore size of the PVDF membrane in step 1) is preferably 0.1 - 0.5 μm, more preferably 0.2 - 0.45 μm, and even more preferably 0.22 - 0.35 μm. The soaking time is preferably 5 - 30 min, more preferably 10 - 25 min, and even more preferably 15 - 20 min.

[0038] In the present invention, the alkali solution in step 2) is preferably a sodium hydroxide solution or a potassium hydroxide solution. The concentration of the alkali solution is preferably 1 - 5 mol / L, more preferably 2 - 4 mol / L, and even more preferably 3 mol / L. The soaking time is preferably 2 - 10 min, more preferably 3 - 8 min, and even more preferably 5 - 6 min.

[0039] In the present invention, the concentration of the tannic acid solution in step 3) is preferably 30 - 120 g / L, more preferably 60 - 100 g / L, and even more preferably 70 - 90 g / L. The concentration of the FeCl3·6H2O solution in step 4) is preferably 0.5 - 3.5 mol / L, more preferably 1 - 3 mol / L, and even more preferably 1.5 - 2.5 mol / L. The concentration of the (NH4)6Mo7O 24 ·4H2O solution in step 5) is preferably 0.05 - 2.0 mol / L, more preferably 0.14 - 1.5 mol / L, and even more preferably 0.3 - 1.0 mol / L.

[0040] In the present invention, the soaking time in steps 3) to 5) is independently preferably 0.5 to 4 h, more preferably 1 to 3 h, and even more preferably 1.5 to 2.5 h.

[0041] In the present invention, after the soaking in step 1) is completed, the membrane is preferably washed with water to obtain a pretreated PVDF membrane; after the soaking in step 2) is completed, the membrane is preferably washed with water to obtain an alkali-treated PVDF membrane; after the soaking in step 3) is completed, the membrane is preferably washed with water to obtain a PVDF / TA membrane; after the soaking in step 4) is completed, the membrane is preferably washed with water to obtain a PVDF / TA / Fe 3+ membrane; after the soaking in step 5) is completed, the membrane is preferably washed with water to obtain an inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane.

[0042] In the present invention, the alkali solution is an aqueous solution of an alkali, the tannic acid solution is an aqueous solution of tannic acid, the FeCl3·6H2O solution is an aqueous solution of FeCl3·6H2O, and the (NH4)6Mo7O 24 ·4H2O solution is an aqueous solution of (NH4)6Mo7O 24 ·4H2O.

[0043] The present invention also provides an inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane prepared by the preparation method described above.

[0044] The present invention also provides the application of the inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane in oily wastewater. A surfactant, oil and water are mixed to obtain oily wastewater, and the inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane is used for oil-water filtration separation of the oily wastewater.

[0045] In the present invention, in the oily wastewater, the concentration of the surfactant is preferably 10 to 50 mg / L, more preferably 20 to 40 mg / L, and even more preferably 30 mg / L; the concentration of the oil is preferably 1000 to 5000 ppm, more preferably 2000 to 4000 ppm, and even more preferably 3000 ppm.

[0046] In the present invention, the pH value of the oily wastewater is preferably 6 to 9, more preferably 7 to 8.

[0047] In the present invention, the surfactant is preferably one or more of sodium dodecyl sulfate, stearic acid, sodium dodecylbenzenesulfonate and Tween 80; the oil is preferably one or more of isooctane, n-hexane, paraffin, soybean oil, hexadecane and dichloromethane.

[0048] In the present invention, the inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane contaminated by oil droplets after filtration separation is placed in a persulfate solution for regeneration; the persulfate is preferably potassium monopersulfate and / or potassium persulfate; the concentration of the persulfate solution is preferably 0.1-0.5 mmol / L, more preferably 0.2-0.4 mmol / L, and even more preferably 0.3 mmol / L. The regeneration time is preferably 5-30 min, more preferably 10-25 min, and even more preferably 15-20 min.

[0049] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0050] In the embodiments, the PVDF membrane is a PVDF hydrophilic membrane purchased from Haining Deli New Materials Technology Co., Ltd.

[0051] Example 1

[0052] A PVDF membrane with a pore size of 0.45 μm was immersed in absolute ethanol for 10 min, and then washed with deionized water to obtain a pretreated PVDF membrane.

[0053] The pretreated PVDF membrane was immersed in a 3 mol / L sodium hydroxide solution for 5 min, and then washed with deionized water to obtain an alkali-treated PVDF membrane.

[0054] The alkali-treated PVDF membrane was immersed in a 30 g / L tannic acid solution for 1 h, and then washed with deionized water to obtain a PVDF / TA membrane.

[0055] The PVDF / TA membrane was immersed in a 1 mol / L FeCl3·6H2O solution for 1 h, and then washed with deionized water to obtain a PVDF / TA / Fe 3+ membrane.

[0056] The PVDF / TA / Fe 3+ membrane was immersed in a 0.14 mol / L (NH4)6Mo7O 24 ·4H2O solution for 1 h, and then washed with deionized water to obtain a PVDF / TA / Gel composite membrane (PVDF / T3 / Gel).

[0057] Example 2

[0058] The concentration of the tannic acid solution in Example 1 was changed to 60 g / L, and other process conditions were the same as those in Example 1 to obtain a PVDF / TA / Gel composite membrane (PVDF / T6 / Gel).

[0059] The SEM diagrams of the PVDF / TA / Gel composite membrane prepared in Example 2 and the original PVDF membrane without any treatment are asFigure 1 As shown, where a is the PVDF / TA / Gel composite membrane of Example 2 and b is the original PVDF membrane. From Figure 1 it can be seen that both the PVDF membrane and the PVDF / TA / Gel composite membrane are porous structures. After being modified with TA and inorganic hydrogel, TA and inorganic hydrogel particles are loaded on the surface of the PVDF / TA / Gel composite membrane.

[0060] Example 3

[0061] The concentration of the tannic acid solution in Example 1 was changed from 30 g / L to 90 g / L, and other process conditions were the same as those in Example 1, to obtain a PVDF / TA / Gel composite membrane (PVDF / T9 / Gel).

[0062] Example 4

[0063] A PVDF membrane with a pore size of 0.35 μm was immersed in absolute ethanol for 20 min, and then it was washed with deionized water to obtain a pretreated PVDF membrane;

[0064] The pretreated PVDF membrane was immersed in a 2 mol / L potassium hydroxide solution for 8 min, and then washed with deionized water to obtain an alkali-treated PVDF membrane;

[0065] The alkali-treated PVDF membrane was immersed in a 70 g / L tannic acid solution for 1.5 h, and then washed with deionized water to obtain a PVDF / TA membrane;

[0066] The PVDF / TA membrane was immersed in a 2.5 mol / L FeCl3·6H2O solution for 1 h, and then washed with deionized water to obtain a PVDF / TA / Fe 3+ membrane;

[0067] The PVDF / TA / Fe 3+ membrane was immersed in a 1.0 mol / L (NH4)6Mo7O 24 ·4H2O solution for 0.5 h, and then washed with deionized water to obtain a PVDF / TA / Gel composite membrane.

[0068] Example 5

[0069] A PVDF membrane with a pore size of 0.22 μm was immersed in absolute ethanol for 15 min, and then it was washed with deionized water to obtain a pretreated PVDF membrane;

[0070] The pretreated PVDF membrane was immersed in a 4 mol / L sodium hydroxide solution for 3 min, and then washed with deionized water to obtain an alkali-treated PVDF membrane;

[0071] The alkali-treated PVDF membrane was immersed in a 100 g / L tannic acid solution for 2 h, and then washed with deionized water to obtain the PVDF / TA membrane;

[0072] The PVDF / TA membrane was immersed in a 1.5 mol / L FeCl3·6H2O solution for 2 h, and then washed with deionized water to obtain the PVDF / TA / Fe 3+ membrane;

[0073] The PVDF / TA / Fe 3+ membrane was immersed in a 0.3 mol / L (NH4)6Mo7O 24 ·4H2O solution for 2 h, and then washed with deionized water to obtain the PVDF / TA / Gel composite membrane.

[0074] Application Example 1 The hydrophilicity of the membrane was evaluated by contact angle measurement

[0075] The wetting performance of the membrane was tested using a water contact angle measuring instrument. The PVDF / TA / Gel composite membrane prepared in Example 2 and the original PVDF membrane without any treatment were placed on the contact angle measurement platform. Using a German KRUSS contact angle measuring instrument, a 2 μL drop of ultrapure water was slowly dropped onto the surface of the membrane sample through a needle. When the water droplet began to contact the membrane surface, the instrument recorded the static contact angle, that is, the initial contact angle between the water droplet and the membrane surface. Subsequently, the change in the contact angle over time was measured until the contact angle stabilized.

[0076] The water contact angle (WCA) diagrams of the PVDF / TA / Gel composite membrane prepared in Example 2 and the original PVDF membrane without any treatment are as Figure 2 shown, where a is the original PVDF membrane and b is the PVDF / TA / Gel composite membrane. As can be seen from Figure a, the water completely penetrated into the original PVDF membrane within 34.27 s, indicating a certain degree of hydrophilicity. As can be seen from Figure b, the water droplet completely disappeared and penetrated into the membrane within 1.50 s for the PVDF / TA / Gel composite membrane, indicating that the PVDF / TA / Gel composite membrane prepared in Example 2 has good hydrophilicity.

[0077] Application Example 2 The permeation flux and separation efficiency of different oil-in-water emulsions filtration

[0078] At room temperature, the PVDF / TA / Gel composite membrane prepared in Example 2 was placed in a cross-flow filtration membrane module to filter the oil-containing wastewater (oil-in-water emulsion). The membrane module was fixed by screws, and a peristaltic pump (Baoding Leifu BT100L) was used to transport the oil-in-water emulsion into the membrane module for oil-water separation. The effluent of the membrane module was connected by a peristaltic pump for collecting the filtrate. The oil-in-water emulsion included isooctane-in-water (2000 ppm), liquid paraffin-in-water (3500 ppm), soybean oil-in-water (4250 ppm), and cetane-in-water (1900 ppm). The concentration of the surfactant added to the oil-in-water emulsion was 25 mg / L, and the pH value of the oil-containing wastewater was 7. After filtering for 1 h, the filtration volume was recorded and the emulsion flux was calculated. The same operation was carried out with the PVDF / TA / Gel composite membranes prepared in Example 1 and Example 3 and the original PVDF membrane without any treatment.

[0079] The flux and separation efficiency diagrams of the PVDF / TA / Gel composite membranes prepared in Examples 1 to 3 and the original PVDF membrane without any treatment for separating oil-containing wastewater are as Figure 3 shown, where PVDF / T3 / Gel corresponds to Example 1, PVDF / T6 / Gel corresponds to Example 2, PVDF / T9 / Gel corresponds to Example 3, the bar graph represents the flux, and ○, □, △, ☆ represent the separation efficiency. As Figure 3 can be seen, the separation efficiency of the PVDF / TA / Gel composite membrane for various oil-in-water emulsions reached over 98.5%, which was significantly better than that of the original PVDF membrane. This indicates that the membrane modified with TA and inorganic hydrogel effectively improved the flux and separation efficiency.

[0080] Application Example 3

[0081] A cyclic filtration experiment was carried out using the PVDF / TA / Gel composite membrane prepared in Example 2. At room temperature, the PVDF / TA / Gel composite membrane prepared in Example 2 was placed in a cross-flow filtration membrane module for oil-water separation of the oil-in-water emulsion. The oil-in-water emulsion used was isooctane-in-water (2000 ppm). First, deionized water was filtered for 10 min, and its filtrate volume was recorded. Subsequently, after filtering the isooctane-in-water emulsion for 1 h, the filtrate volume was recorded. After each cycle, the contaminated membrane was removed from the membrane module, rinsed with deionized water for 5 min, placed back in the membrane module, and then filtered for 10 min. The filtrate volume was recorded, and the flux recovery rate was calculated. This cyclic process was repeated 7 times.

[0082] The flux recovery rate and separation efficiency diagrams of the PVDF / TA / Gel composite membrane prepared in Example 2 for cyclic filtration of the isooctane-in-water emulsion are as Figure 4 shown. As Figure 4It can be seen that the flux recovery rate can reach 90.6% after 7 cycles of filtration, indicating that the PVDF / TA / Gel membrane has good anti-pollution performance. However, with the long-term oil-water separation filtration, the oil droplets inside the membrane pores cannot be removed by simple hydraulic cleaning, which leads to a decrease in the flux recovery rate after the 7th cycle.

[0083] Application Example 4

[0084] The PVDF / TA / Gel composite membrane prepared in Example 2 was used for catalytic cleaning and regeneration experiments. Under room temperature conditions, the PVDF / TA / Gel composite membrane prepared in Example 2 was placed in a cross-flow filtration membrane module for oil-in-water emulsion oil-water separation. The oil-in-water emulsion used was isooctane-in-water (2000 ppm). First, deionized water was filtered for 10 min, and its filtrate volume was recorded. Subsequently, after filtering the isooctane-in-water emulsion for 1 h, the filtrate volume was recorded. After each cycle ended, the contaminated membrane was taken out from the membrane module and soaked in an aqueous solution of persulfate with a concentration of 0.25 mmol / L for 5 min, then placed in the membrane module and continued to be filtered for 10 min. The filtrate volume was recorded, and the flux recovery rate was calculated. This cyclic process was repeated 7 times.

[0085] The flux recovery rate and separation efficiency diagram of the PVDF / TA / Gel composite membrane prepared in Example 2 after cyclic filtration catalytic cleaning for separating isooctane-in-water emulsion is as Figure 5 shown. It can be Figure 5 seen that the flux recovery rate can be as high as 98.6% after 7 cycles of filtration. This is because the iron in the inorganic hydrogel loaded on the surface of the PVDF / TA / Gel composite membrane reacts with persulfate to provide sufficient active sites. The active substances generated therefrom effectively decompose the oil droplets into water and carbon dioxide, making it easier to remove them from the membrane pores. The PVDF / TA / Gel composite membrane prepared in the present invention has good catalytic regeneration performance and is suitable for the stable separation of oil-water emulsions for a long time.

[0086] The present invention prepared a PVDF / TA / Gel composite membrane with high-efficiency oil-water separation efficiency by an immersion method. First, the PVDF membrane was modified with TA, and then two common inorganic salts, FeCl3·6H2O and (NH4)6Mo7O 24 ·4H2O, were introduced for gelation to obtain the PVDF / TA / Gel composite membrane. The inorganic hydrogel / PVDF composite membrane prepared in the present invention has good hydrophilicity and underwater superoleophobicity, and has a good separation effect on simulated oily wastewater. Through the process of coupling inorganic hydrogel superhydrophilic modification and persulfate catalytic cleaning, the contaminated membrane can generate reactive oxygen species during the catalytic process to decompose the oil on the membrane surface and inside the membrane pores, so that the cleaned membrane has good renewable performance, improves its service life, and has good application prospects in the field of oil-water separation.

[0087] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing an inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane, characterized in that: The following steps are included: 1) Soaking the PVDF membrane in anhydrous ethanol to obtain a pretreated PVDF membrane; 2) soaking the pretreated PVDF membrane in an alkaline solution to obtain an alkaline-treated PVDF membrane; 3) soaking the alkali-treated PVDF membrane in a tannic acid solution to obtain a PVDF / TA membrane; 4) Soak the PVDF / TA membrane in FeCl3·6H2O solution to obtain PVDF / TA / Fe 3+ membrane; 5) Combine PVDF / TA / Fe 3+ Film in (NH4)6Mo7O 24 ·4H2O solution to obtain an inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane.

2. The preparation method according to claim 1, characterized in that: Step 1) The pore size of the PVDF membrane is 0.1-0.5 μm, and the soaking time is 5-30 min.

3. The preparation method according to claim 1 or 2, characterized in that: Step 2) the alkaline solution is a sodium hydroxide solution or a potassium hydroxide solution, and the concentration of the alkaline solution is 1 to 5 mol / L; the soaking time is 2 to 10 minutes.

4. The preparation method according to claim 3, characterized in that: Step 3) the concentration of the tannic acid solution is 30-120 g / L, step 4) the concentration of the FeCl3·6H2O solution is 0.5-3.5 mol / L, step 5) the concentration of the (NH4)6Mo7O 24 The concentration of 4H2O solution is 0.05~2.0mol / L.

5. The preparation method according to claim 4, characterized in that: The soaking time in step 3) to step 5) is independently 0.5 to 4 hours.

6. The inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane prepared by the preparation method according to any one of claims 1 to 5.

7. Use of the inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane according to claim 6 in oily wastewater, characterized in that: The surfactant, oil and water are mixed to obtain oily wastewater, and the inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane is used to filter and separate the oily wastewater into oil and water.

8. The use according to claim 7, characterized in that: In the oily wastewater, the concentration of the surfactant is 10-50 mg / L, and the concentration of the oil is 1000-5000 ppm.

9. The use according to claim 7 or 8, characterized in that: The surfactant is one or more of sodium dodecyl sulfate, stearic acid, sodium dodecylbenzene sulfonate and Tween 80; the oil is one or more of isooctane, n-hexane, paraffin, soybean oil, hexadecane and dichloromethane.

10. The use according to claim 9, characterized in that: The inorganic hydrogel / polyvinylidene fluoride oil-water separation membrane contaminated by oil droplets after filtration separation is placed in a persulfate solution for regeneration; the persulfate is potassium peroxymonosulfate and / or potassium peroxydisulfate; the concentration of the persulfate solution is 0.1-0.5 mmol / L, and the regeneration time is 5-30 minutes.

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