Preparation method and application of in-situ growth ZIF-8 modified super-hydrophilic / underwater super-oleophobic PVDF nanofiber membrane

Abstract

The invention discloses a preparation method and application of an in-situ growth ZIF-8 modified super-hydrophilic / underwater super-oleophobic PVDF nanofiber membrane, and belongs to the technical field of membrane materials. The preparation method of the in-situ growth ZIF-8 modified super-hydrophilic / underwater super-oleophobic PVDF nanofiber membrane comprises the following steps: mixing ZIF-8 and polyvinylidene fluoride in a solvent to obtain a spinning solution; carrying out electrostatic spinning treatment by using the spinning solution, so as to obtain a PVDF-ZIF-8 nanofiber membrane; zinc nitrate, sodium formate and 2-methylimidazole are mixed in a solvent, a mixed solution is obtained, then the PVDF-ZIF-8 nanofiber membrane is immersed in the mixed solution, a reaction is conducted, and the in-situ growth ZIF-8 modified super-hydrophilic / underwater super-oleophobic PVDF nanofiber membrane is obtained. The prepared nanofiber membrane has excellent hydrophilicity, meanwhile, the membrane has the remarkable separation effect on oil-water emulsion, has the high rejection rate and the high flux recovery rate, has good acid and alkali resistance and is suitable for being used in the field of oil-water separation.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane materials, and in particular to a preparation method and application of an in-situ grown ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane. Background Art

[0002] Currently, in the field of oil-water separation technology, with the increasing demand for industrial wastewater treatment, the development of efficient, stable and environmentally friendly separation membrane materials has become a research hotspot. Traditional oil-water separation methods, such as gravity separation, flotation, centrifugation, etc., often have problems such as low efficiency, high energy consumption or limited application range. In recent years, nanofiber membranes have shown great potential in oil-water separation due to their high specific surface area, good pore structure and adjustable surface properties. Especially polyvinylidene fluoride (PVDF) nanofiber membranes have gradually become the focus of research due to their excellent mechanical strength, chemical stability and thermal stability.

[0003] However, unmodified PVDF nanofiber membranes are usually hydrophobic, which limits their direct application in oil-water separation. Especially when treating oil-in-water emulsions, it is difficult to achieve efficient separation. To solve this problem, researchers have tried to improve the hydrophilicity of PVDF membranes through surface modification techniques, such as chemical grafting, plasma treatment, etc., but these methods are often complex in operation and the modification effect is unstable.

[0004] Zeolitic imidazolate framework-8 (ZIF-8), as a porous metal-organic framework material, has been widely used in gas separation, water treatment and other fields due to its good water stability and adjustable pore size. Introducing ZIF-8 into PVDF nanofiber membranes is expected to improve the separation performance of the membranes for oil-water emulsions through its hydrophilicity and pore structure. However, how to effectively grow ZIF-8 uniformly and firmly on PVDF nanofibers while maintaining the overall performance and stability of the membrane material is still a challenge faced currently.

[0005] Therefore, there is an urgent need to develop a new and efficient method for modifying PVDF nanofiber membranes to achieve superhydrophilicity, high oil-water separation efficiency and good acid-base stability of the membrane material, so as to meet the actual needs in the field of oil-water separation. Summary of the Invention

[0006] The object of the present invention is to provide a preparation method and application of an in-situ grown ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane to solve the above problems in the background technology. The present invention first prepares a PVDF-ZIF-8 nanofiber membrane by electrospinning, and then further performs hydrophilic modification using the in-situ growth method and surface coating, successfully preparing an in-situ grown ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane. The modified nanofiber membrane has excellent hydrophilicity, and at the same time, the membrane has a significant separation effect on oil-water emulsions, with high rejection rate and high flux recovery rate, and has good acid and alkali resistance, and is suitable for the field of oil-water separation.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] One of the technical solutions of the present invention: provides a preparation method of an in-situ grown ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane, comprising the following steps:

[0009] (1) Mix ZIF-8 and polyvinylidene fluoride in a solvent to obtain a spinning solution; perform electrospinning treatment using the spinning solution to obtain a PVDF-ZIF-8 nanofiber membrane;

[0010] (2) Mix zinc nitrate, sodium formate and 2-methylimidazole in a solvent to obtain a mixed solution, and then immerse the PVDF-ZIF-8 nanofiber membrane in the mixed solution and react to obtain the in-situ grown ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane.

[0011] Preferably, in step (1): the mass ratio of ZIF-8 to polyvinylidene fluoride is 1:15; the concentration of ZIF-8 in the spinning solution is 1 wt%; the solvent is N, N-dimethylformamide and / or acetone.

[0012] Preferably, the conditions of the electrospinning treatment are: voltage is 13 KV, temperature is 30 °C, roller speed is 500 rpm, linear speed is 10 mm / s, flow rate is 1 mL / h, receiving distance is 11.5 cm, and spinning time is 10 h.

[0013] Preferably, in step (2): the mass ratio of zinc nitrate, sodium formate and 2-methylimidazole is 6:1.38:2.4; the solvent is methanol.

[0014] Preferably, the temperature of the reaction is 80 °C and the time is 9 h.

[0015] Preferably, after the reaction, it further includes a step of ultrasonic treatment; the time of the ultrasonic treatment is 60 s.

[0016] Preferably, after the reaction, a hydrophilic modification step is further included, and the hydrophilic modification is as follows:

[0017] Mix dopamine and polyethylenepolyamine in water at a mass ratio of 1:1, and then add Tris-HCl buffer solution to obtain a modification solution; put the product obtained in step (2) into the modification solution and perform oscillation treatment to complete the hydrophilic modification.

[0018] Technical solution two of the present invention: Provide an in-situ growth ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane obtained according to the above preparation method.

[0019] Technical solution three of the present invention: Provide an application of the above in-situ growth ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane in the field of oil-water separation.

[0020] The beneficial technical effects of the present invention are as follows:

[0021] The present invention first prepares a PVDF-ZIF-8 nanofiber membrane by electrospinning method, and then uses in-situ growth method and surface coating for further hydrophilic modification, successfully preparing an in-situ growth ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane. The modified nanofiber membrane has excellent hydrophilicity, and at the same time, the membrane has a significant separation effect on oil-water emulsion, with high rejection rate and relatively high flux recovery rate, and has good acid and alkali resistance, being suitable for the field of oil-water separation.

[0022] A hydrophilic polydopamine coating is formed on the surface of the membrane after adding dopamine and polyethylenepolyamine for hydrophilic modification, which can improve the rejection effect on oil in the oil-water emulsion, making the rejection effect significantly improved, and still maintaining a high rejection rate after recycling. The prepared membrane material has good acid and alkali resistance and still has good oleophobic performance after being soaked in acid and alkali environments for a long time. Description of the Drawings

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the following described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0024] Figure 1 For the comparison of the water contact angles of Mb-1, Mb-2, Mb-3, Mb-4, Mb-5, Mb-6, Mb-7, Mb-8.

[0025] Figure 2 For the Fourier transform infrared spectra of M1, M2, M3, M5, M7, M9, M11.

[0026] Figure 3 Fourier transform infrared spectra of M9, M9-1, M9-3, M9-5, and M9-7.

[0027] Figure 4 Comparison of underwater oil contact angles of M9-1, M9-3, M9-5, and M9-7.

[0028] Figure 5 Underwater oil contact angles of M9-3 with different oil reagents. Among them, (a), (b), (c), (d), (e), and (f) are the underwater oil contact angles of petroleum ether, chloroform, soybean oil, n-hexane, toluene, and vacuum pump oil in sequence.

[0029] Figure 6 Bar chart of water flux and soybean oil emulsion flux of M1, M2, M9, M9-1, M9-3, M9-5, and M9-7.

[0030] Figure 7 Graph of the relationship between membrane flux and filtration time of M9, M9-1, M9-3, M9-5, and M9-7.

[0031] Figure 8 Comparison chart of the flux recovery rates of two-cycle filtration of M9, M9-1, M9-3, M9-5, and M9-7.

[0032] Figure 9 Line chart of the initial filtration emulsion rejection rates of M1, M2, M9, M9-1, M9-3, M9-5, and M9-7.

[0033] Figure 10 Comparison chart of the emulsion rejection rates of two-cycle filtration of M1, M2, M9, M9-1, M9-3, M9-5, and M9-7. Detailed implementation manners

[0034] Now, various exemplary implementation manners of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention. It should be understood that the terms described in the present invention are only for describing specific implementation manners and are not used to limit the present invention.

[0035] In addition, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0036] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which this invention pertains. Although this invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of this invention.

[0037] Regarding "comprising", "including", "having", "containing", etc. used in this invention, they are all open-ended terms, meaning including but not limited to.

[0038] The preparation method of ZIF-8 in this invention is as follows: Weigh 5.866 g of zinc nitrate hexahydrate with an electronic balance into a 500 mL beaker, measure 400 mL of anhydrous methanol and pour it into the beaker, and ultrasonicate for 2 min to make it mix evenly to obtain a zinc nitrate solution; then take another beaker, measure 12.978 g of 2-methylimidazole and add 400 mL of anhydrous methanol, stir well to mix evenly, and slowly add the zinc nitrate solution to the 2-methylimidazole solution, and then stir at room temperature for 30 min in a thermostatic heating magnetic stirrer with heat collection. During the stirring process, the mixed solution gradually changes from a colorless transparent solution to a milky white turbid solution; after the stirring ends, pour the mixed solution into a centrifuge tube, centrifuge at a speed of 10000 r / s for 10 min, pour out the supernatant after centrifugation, add anhydrous methanol for washing and then centrifuge for 10 min, repeat 2 - 3 times; place the white solid substance obtained after centrifugation in a blast drying oven and dry for 1 day, then transfer it to a vacuum drying oven and dry for 2 days. After drying, grind the solid with a mortar to obtain ZIF-8, and bag it for standby.

[0039] The preparation method of the Tris-HCl buffer solution (pH = 8.5) in this invention is as follows: Weigh 6.0567 g of tris(hydroxymethyl)aminomethane (Tris) drug into a beaker, add 50 mL of distilled water to completely dissolve it, then pour it into a 500 mL volumetric flask, add 14.7 mL of 1 mol / L hydrochloric acid solution and make up the volume with distilled water to obtain the precursor of the Tris-HCl buffer solution, and then dilute it with distilled water at a volume ratio of 1:1 to obtain the Tris-HCl buffer solution (pH = 8.5).

[0040] In this invention, "room temperature" is counted as 10 - 30 °C unless otherwise specified.

[0041] All raw materials used in the following examples and comparative examples of this invention are commercially available products.

[0042] Example 1

[0043] A preparation method of an in-situ grown ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane is as follows:

[0044] (1) Preparation of 15% PVDF + 1% ZIF-8 spinning solution: Weigh 0.4667 g of ZIF-8 and put it into a 100 mL round-bottom flask. Then add 33.08 mL of N,N-dimethylformamide (DMF), and ultrasonicate for 3 min to disperse ZIF-8 in the DMF solution. After ultrasonication, the solution changes from colorless to milky white. Then slowly add 7 g of polyvinylidene fluoride (PVDF), add a magnetic stir bar, and stir in an oil bath at 60 °C for 6 h. The stirred solution changes from milky white to light yellow. Let the reaction system stand to room temperature, then slowly add 9.93 mL of acetone under stirring, and then place it in a thermostatic heating magnetic stirrer and stir at room temperature for 10 h. After stirring, ultrasonicate for 30 min and let it stand for 10 h to complete the preparation of the spinning solution;

[0045] (2) Preparation of PVDF-ZIF-8 nanofiber membrane: Spin the spinning solution prepared in step (1) through a high-voltage electrospinning machine. The electrospinning conditions are: voltage 13 KV, temperature 30 °C, roller speed 500 rpm, linear speed 10 mm / s, flow rate 1 mL / h, receiving distance 11.5 cm, electrospinning time 10 h. Use a 21-gauge needle during electrospinning and turn on the fan. After electrospinning, turn off the voltage, linear speed, and roller speed buttons, and keep the fan and temperature control buttons normally on. Let the fiber membrane stand in the instrument for 1 h and then gently remove it, and dry it in an electrothermal blast drying oven for 3 days to obtain the PVDF-ZIF-8 nanofiber membrane (denoted as M2);

[0046] (3) Preparation of PVDF-ZIF-8-in-situ grown ZIF-8 nanofiber membrane: Put 6 g of zinc nitrate hexahydrate and 1.38 g of sodium formate into a 500 mL beaker, add 180 mL of anhydrous methanol, and stir with a thermostatic heating magnetic stirrer at 25 °C for 10 min to obtain solution A; Dissolve 2.4 g of 2-methylimidazole in 180 mL of anhydrous methanol to obtain solution B; Slowly add solution B to solution A and stir for 10 min to obtain a mixed solution; Cut the PVDF-ZIF-8 nanofiber membrane in step (2) into an appropriate size and weigh it, then fix it on non-woven fabric with electrical tape and vertically immerse it in the mixed solution. Then transfer the reaction system to a stainless steel autoclave with a Teflon liner and in-situ grow at 80 °C for 9 h. After the autoclave cools for 15 h, take out the fiber membrane, ultrasonicate it with methanol for 60 s, then rinse it with methanol, and then dry it in an electrothermal blast drying oven to constant weight to obtain the PVDF-ZIF-8-in-situ grown ZIF-8 nanofiber membrane (denoted as M9).

[0047] Example 2

[0048] The reactor will have different effects on the growth of ZIF-8 crystals on the PVDF-ZIF-8 nanofiber membrane at different cooling times. ZIF-8 crystals have unique hydrophilic properties, and growing on the surface of the PVDF-ZIF-8 nanofiber membrane can modify the membrane. However, during in-situ growth, not all of the ZIF-8 is stably attached to the surface of the nanofiber membrane. The ultrasonic treatment of the present invention can ultrasonic off the ZIF-8 crystals that are not firmly attached, making the surface of the grown nanofiber membrane more uniform and its properties more stable. To explore the effects of the reactor cooling time and ultrasonic time on the growth effect of ZIF-8 crystals on the PVDF-ZIF-8 nanofiber membrane, this example is set up.

[0049] The difference from Example 1 is only that the nanofiber membrane is prepared by modifying the reactor cooling time and ultrasonic time in step (3). The prepared fiber membranes are numbered, and the specific compositions of the fiber membranes are shown in Table 1.

[0050] Table 1 Experimental scheme of the effects of reactor cooling time and ultrasonic time on the growth of ZIF-8 crystals

[0051]

[0052]

[0053] Example 3

[0054] The in-situ growth of ZIF-8 crystals requires a certain amount of time, and the growth situation of ZIF-8 crystals will change accordingly with different growth times. To explore the effects of the growth time on the growth effect of ZIF-8 crystals, this example is set up.

[0055] The difference from Example 1 is only that the nanofiber membrane is prepared by modifying the in-situ growth time in step (3). The prepared fiber membranes are numbered, and the specific compositions of the fiber membranes are shown in Table 2.

[0056] Table 2 Experimental scheme of the effects of growth time on the growth of ZIF-8 crystals

[0057]

[0058] Example 4

[0059] The difference from Example 1 is only that after completing the preparation steps in step (3), the following hydrophilic modification steps are continued:

[0060] Weigh 0.2 g of dopamine (DA) and 0.2 g of polyethylenepolyamine (PEPA, Aladdin reagent) with an electronic balance and place them in a beaker. Add 50 mL of distilled water and dissolve them evenly. Then measure 50 mL of Tris-HCl buffer solution (pH = 8.5) and mix them evenly to obtain a modified solution (the dopamine concentration is 2 mg / mL and the polyethylenepolyamine concentration is 2 mg / mL).

[0061] Pour the modified solution into a reaction box, then immerse the PVDF-ZIF-8-in-situ-grown ZIF-8 nanofiber membrane in step (3) into the above solution. Place the reaction box in a constant temperature water bath oscillator at 25 °C and oscillate it evenly for 3 h. Then put the oscillated membrane into a box filled with distilled water and let it stand and soak. Change the water every 12 h. After soaking for 3 days, take out the membrane, place it on filter paper, fix it with a paper clip, and put it into a blast drying oven to dry to a constant weight to obtain M9-3.

[0062] On this basis, prepare the nanofiber membrane by only modifying the addition amounts of DA and PEPA and the oscillation time in the preparation process of M9-3. Number the prepared fiber membranes, and the specific modification schemes are shown in Table 3.

[0063] Table 3 Modification schemes

[0064]

[0065] Note: "-" indicates that this variable does not exist for this membrane.

[0066] Effect verification

[0067] (1) Influence of the reaction kettle cooling time and the ultrasonic time during membrane cleaning on the growth of ZIF-8 crystals and surface wettability analysis of the experiment:

[0068] Test the water contact angles of the nanofiber membranes (Example 1 and Example 2) with different reaction kettle cooling times and ultrasonic times. The test results are shown in Table 4 and Figure 1 as follows.

[0069] Table 4 Water contact angles of membrane samples with different ultrasonic times

[0070]

[0071]

[0072] Figure 1 Comparisons of the water contact angles of Mb-1, Mb-2, Mb-3, Mb-4, Mb-5, Mb-6, Mb-7, and Mb-8.

[0073] Table 4 and Figure 1Among them, by comparing Mb-1, Mb-2 with Mb-5, M9, it can be seen that the water contact angle of the film sample grown when the reactor cooling time is 15 h is significantly lower than that of the film sample with a cooling time of 5 h. At the same time, by comparing Mb-4, Mb-5, M9, Mb-7 to Mb-9, it can be seen that the water contact angle of the film sample shows a trend of first decreasing and then increasing with the increase of the ultrasonic time. The water contact angle is the lowest at 60 s of ultrasonic time, which is 32.75°. To sum up, the hydrophilic modification effect of the film in-situ grown under the conditions of a reactor cooling time of 15 h and an ultrasonic time of 60 s is the best.

[0074] (3) Analysis of surface wettability in the experiment on the effect of growth time on the growth of ZIF-8 crystals:

[0075] The surface wettability of PVDF-ZIF-8 nanofiber membranes (Example 1, Example 3) in-situ grown for 3 h, 5 h, 7 h, 9 h, and 11 h was tested to determine the optimal growth time. By weighing the weights of the PVDF-ZIF-8 nanofiber membranes before and after in-situ growth, the growth rate was calculated according to formula (1). The test results are shown in Table 5.

[0076]

[0077] In the formula, C is the growth rate, W1 is the mass of the film after in-situ growth, g; W0 is the mass of the film before in-situ growth, g.

[0078] Table 5 Wettability of films at different growth times

[0079]

[0080] Note: "-" indicates that the film has no such variable.

[0081] Analysis of Table 5 shows that: By comparing the water contact angle data of M1 and M2, adding ZIF-8 to PVDF nanofibers (M2) by the blending method does not significantly improve the hydrophilic performance of the PVDF nanofiber membrane. By comparing the water contact angle data of M2 and M9, it can be seen that the water contact angle of the nanofiber membrane after in-situ growth for 9 h decreases significantly and can be reduced to 53.10°. After in-situ growth, the growth rate increases with the increase of the growth time. The growth rates of in-situ growth for 3 h, 5 h, 7 h, 9 h, and 11 h are 42.13%, 65.12%, 77.41%, 88.23%, and 95.65% respectively. During the in-situ growth process, if the growth time is too short, such as 3 h, the growth on the membrane surface will be uneven; if the growth time is too long, such as 11 h, flocs will be generated on the membrane surface, thereby affecting the product performance. In summary, the growth time of 9 h is the most suitable, at this time the growth rate of the membrane is the highest and the growth on the membrane surface is uniform. By comparing the data in the table, it can be seen that the water contact angle of the PVDF-ZIF-8 nanofiber membrane after in-situ growth decreases with the extension of the growth time and reaches the lowest at 9 h of growth. Among them, the water contact angle of M9 can be reduced to 32.75° after 9 h of growth, and the underwater oil contact angle can reach 110.74°. In summary, the hydrophilic modification effect of the membrane with 9 h of in-situ growth is the best.

[0082] (4) Structural characterization:

[0083] The Fourier transform infrared spectrometer was used to test the chemical structures of nanofiber membranes with different growth times (Example 1, Example 3), and the obtained Fourier infrared spectra are as Figure 2 shown.

[0084] Figure 2 The Fourier infrared spectra of M1, M2, M3, M5, M7, M9, and M11.

[0085] Figure 2 Among them, according to the analysis of the infrared spectrum of M1, it is found that there are absorption peaks at 879 cm -1 , 1170 cm -1 , and 1402 cm -1 , corresponding to the absorption peaks of the amorphous phase of PVDF, and the vibration peaks of -CF2- and -CH2- respectively. From the infrared spectrum of M9, it can be observed that there is a characteristic absorption peak of N-H at 3132 cm -1 , a characteristic absorption peak of C=N at 1617 cm -1 , 1307 cm -1 , 1148 cm -1 , and 764 cm -1The peaks at [position] are related to the planar bending of the imidazole ring. These peaks are the markers for differentiating ZIF-8 from PVDF, further indicating the presence of ZIF-8. Through comparative analysis, it can be found that as the growth time increases, the intensities of the characteristic peaks at 1307 cm -1 、1148 cm -1 and 764 cm -1 increase, and the growth rate of ZIF-8 increases. It is speculated that this may be due to the increase in the growth rate of ZIF-8 with the extension of the growth time.

[0086] The Fourier transform infrared spectrometer was used to test the chemical structure of the membranes (Example 4) coated by oscillation for different times, and the obtained Fourier transform infrared spectra are as Figure 3 shown.

[0087] Figure 3 are the Fourier transform infrared spectra of M9, M9-1, M9-3, M9-5, and M9-7.

[0088] Figure 3 Among them, through comparative analysis, it can be found that as the oscillation coating time increases, the characteristic peak at 1580 cm -1 is the bending vibration absorption peak of the N-H bond, and the characteristic peak at 2350 cm -1 is the stretching vibration absorption peak of the ammonium salt, which can prove that DA and PEPA are successfully coated on M9. As time goes by, the intensities of the corresponding characteristic peaks increase. It is speculated that the extension of the oscillation time helps to improve the coating effect of DA and PEPA. Through Figure 3 it can be confirmed that after the introduction of DA and PEPA, the infrared spectrum changes insignificantly, indicating that the introduction of DA and PEPA will not have an excessive impact on the product.

[0089] (5) Hydrophilic modification experiment - Surface wettability analysis:

[0090] The surface wettability of the nanofiber membranes in Example 4 was tested to evaluate the modification effect, and the underwater oil contact angle was measured. The underwater oil contact angle of the membrane was measured by fixing the membrane in a distilled water medium and discharging the oil reagent onto the surface of the membrane underwater. A water contact angle tester (SCI4000) was used to record the process of the oil reagent in contact with the membrane to be tested, and the measurement was calculated through the contact angle analysis software. When the oil reagent was toluene, the test data are shown in Figure 4 and Table 6.

[0091] Table 6 Wettability of membranes under different oscillation times

[0092]

[0093] Note: "-" indicates that this variable does not exist for this membrane.

[0094] Analysis of the data in Table 6 shows that when hydrophilic modification was carried out by adding 2 mg / mL dopamine, although the water contact angle showed a trend of first decreasing and then increasing with the increase of the oscillation time, and the membrane sample oscillated for 3 h reached 18.35°. Under the condition of hydrophilic modification with 2 mg / mL dopamine and 2 mg / mL polyethylenepolyamine, the water contact angle of the membrane first decreased and then increased with the prolongation of the oscillation duration. When the oscillation time reached 3 h, the water contact angle reached the minimum value of 4.97°, and the water droplet completely disappeared 0.5 s after dropping, and its hydrophilicity was significantly better than that of the membrane modified only with 2 mg / mL dopamine (M9-a, M9-b and M9-c). The underwater oil contact angle of the membrane sample showed a trend of first increasing and then decreasing with the increase of the oscillation time, and the underwater oil contact angle was the highest at 166.12° when the oscillation time was 3 h.

[0095] Figure 4 Comparison of the underwater oil contact angles of M9-1, M9-3, M9-5, and M9-7.

[0096] In addition to testing the underwater oil contact angle of toluene on the membrane sample, the underwater oil contact angles of soybean oil, n-hexane, vacuum pump oil, and chloroform were also tested, and the test results are shown in Table 7 and Figure 5 as follows.

[0097] Table 7 Underwater oil contact angle of the modified membrane (M9-3)

[0098]

[0099] Analysis of the data in Table 7 shows that the M9-3 membrane sample has good oil repellency performance, and the underwater oil contact angles of the measured oils are all above 160°. Among them, the oil repellency effect of vacuum pump oil is the best, and the underwater oil contact angle reaches 166.47°.

[0100] Figure 5 Test diagram of the underwater oil contact angle of M9-3 in different oil reagents. Among them, (a), (b), (c), (d), (e), and (f) are the underwater oil contact angles of petroleum ether, chloroform, soybean oil, n-hexane, toluene, and vacuum pump oil in turn.

[0101] (6) Analysis of acid and alkali resistance performance of hydrophilic modification experiment:

[0102] The acid and alkali resistance performance of the M9-3 membrane sample was tested, and the change in the underwater oil contact angle of toluene was analyzed to intuitively illustrate the acid and alkali resistance performance of the modified PVDF nanofiber membrane. The test results are shown in Table 8.

[0103] Table 8 Underwater oil contact angle of toluene of M9-3 at different pH values and different time periods

[0104]

[0105]

[0106] As can be seen from Table 8, after M9-3 was immersed in solutions with different pH values for a series of times, the underwater oil contact angle of toluene measured was basically unchanged. Even after soaking for 336 h (14 days), the membrane still had good oil-repellent performance (the underwater oil contact angle was greater than 165.14°), indicating its good acid and alkali resistance. In summary, the M9-3 membrane sample has good acid and alkali resistance.

[0107] (7) Analysis of the flux in the hydrophilic modification experiment:

[0108] The permeability of the membrane can be characterized by its flux test. By performing water flux and soybean oil emulsion (1 g / L) flux tests on M1, M2, M9, M9-1, M9-3, M9-5, and M9-7, the flux was calculated using formula (2), and the test results are as Figure 6 shown.

[0109]

[0110] In the formula, J is the flux, L / (m 2 ·h); V is the volume of the liquid permeating through the membrane, L; A is the effective area of the membrane, m 2 ; t is the sampling time, h.

[0111] Figure 6 It is a column chart of the water flux and soybean oil emulsion flux of M1, M2, M9, M9-1, M9-3, M9-5, and M9-7.

[0112] As Figure 6 can be seen, after in-situ growth of ZIF-8, the pore size of the PVDF-ZIF-8 nanofiber membrane becomes smaller, resulting in the water flux of the M9 membrane sample being lower than that of M2 without growth. After hydrophilic modification with 2 mg / mL dopamine and 2 mg / mL polyethylenepolyamine, the water flux of the membrane sample shows a trend of first increasing and then decreasing with the increase of the oscillation coating time. The water flux reaches the maximum of 1679.37 L / (m 2 ·h) at an oscillation time of 3 h. In summary, under the condition of hydrophilic modification with 2 mg / mL dopamine and 2 mg / mL polyethylenepolyamine, the membrane at an oscillation coating time of 3 h has a relatively high water flux, as Figure 6 shown.

[0113] (8) Analysis of the anti-pollution performance in the hydrophilic modification experiment:

[0114] To further study the anti-fouling performance of the membrane, two-cycle filtration was carried out using an oil-in-water emulsion (soybean oil emulsion, 1 g / L) as the pollutant. The relationship between the membrane flux and time is as Figure 7As shown, the water flux recovery rate (FRR) is as Figure 8 shown. The water flux recovery rate is calculated by formula (3).

[0115]

[0116] In the formula, FRR is the membrane water flux recovery rate, %; J W1 is the initial membrane water flux, L / (m 2 ·h); J W2 is the water flux after the membrane is subjected to emulsion filtration measurement, L / (m 2 ·h).

[0117] Figure 7 are the diagrams of the relationship between the membrane fluxes of M9, M9-1, M9-3, M9-5, and M9-7 and the filtration time.

[0118] Figure 8 are the comparison diagrams of the two-cycle filtration flux recovery rates (FRR1, FRR2) of M9, M9-1, M9-3, M9-5, and M9-7.

[0119] Figure 8 In

[0120] , FRR1 and FRR2 respectively represent the pure water flux recovery rates of the membrane after the first and second filtration of the emulsion. Figure 8 It can be seen from

[0121] that when filtering the emulsion once and then filtering pure water again, all membranes show flux attenuation, which is caused by oil fouling adhering to the membrane pores or the membrane surface. After simple cleaning, the pure water flux can be restored to different proportions of its initial value. After filtering the emulsion twice, the FRR1 and FRR2 values of membrane M9-3 are much higher than those of membrane M9, indicating that the anti-fouling performance of the membrane after 3 h of oscillating coating with hydrophilic modification is significantly enhanced. In addition, the difference between the water flux recovery rate FRR1 after the first cycle filtration and the water flux recovery rate FRR2 after the second cycle filtration (FRR1 - FRR2) of membrane M9-3 is 5.4%, and the difference between the water flux recovery rate FRR1 after the first cycle filtration and the water flux recovery rate FRR2 after the second cycle filtration (FRR1 - FRR2) of membrane M9 is 8.51%. The decrease value of the water flux recovery rate of M9 after two-cycle filtration is greater than that of M9-3, which further indicates that the anti-fouling performance of the membrane after 3 h of oscillating coating with hydrophilic modification is enhanced.

[0122] By testing and calculating the oil-water emulsion separation rejection rates of M1, M2, M9, M9-1, M9-3, M9-5, and M9-7, the concentrations of the emulsion permeate of the membrane samples are obtained, and the rejection rate is calculated by formula (4). The test results are as Figure 9 and Figure 10 shown.

[0123]

[0124] where R is the rejection rate, %; C p is the concentration of the permeate, mg / L; C f is the concentration of the original emulsion, mg / L.

[0125] Figure 9 is the broken line graph of the rejection rate of the primary filtration emulsion of M1, M2, M9, M9-1, M9-3, M9-5, and M9-7.

[0126] Figure 10 is the comparison graph of the rejection rate of the two-cycle filtration emulsion of M1, M2, M9, M9-1, M9-3, M9-5, and M9-7.

[0127] Figure 10 In which, R1 and R2 respectively represent the rejection rate of oil when filtering the oil-water emulsion (soybean oil emulsion, 1 g / L) for the first and second times.

[0128] It can be seen from Figure 9 that adding 1% ZIF-8 and in-situ growth for 9 h to the spinning solution can increase the rejection rate of the membrane. A hydrophilic polydopamine coating is formed on the surface of the membrane after hydrophilic modification with 2 mg / mL dopamine and 2 mg / mL polyethylenepolyamine, which greatly enhances the hydrophilic performance of the membrane, so that the oil in the emulsion can be well retained, resulting in a significant improvement in the retention effect. As the coating time prolongs, the retention efficiency of the membrane first increases and then decreases, reaching the peak value (at this time, a superhydrophilic / underwater superoleophobic membrane is formed) at 3 hours of oscillation, which is 96.92%. At the same time, it can be seen from Figure 10 that after the second cycle of filtering the emulsion, the rejection rates of the un-hydrophilic-coated modified membranes (M1, M2, and M9) decrease significantly, which may be caused by the membrane being contaminated during the first emulsion filtration. However, the rejection rates of the membranes (M9-1 to M9-7) after hydrophilic coating modification do not change significantly and can still maintain a relatively high rejection rate equivalent to that of the first cycle. Combining Figure 8 it can be known that under the condition of hydrophilic modification with 2 mg / mL dopamine and 2 mg / mL polyethylenepolyamine, the membrane at 3 h of oscillation time has a high rejection rate and a relatively high flux recovery rate, which further indicates that the membrane has a good oil-water separation effect and is suitable for the field of oil-water separation.

[0129] The above-described embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A preparation method of an in-situ grown ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane, characterized in that, It includes the following steps: (1) Mix ZIF-8 and polyvinylidene fluoride in a solvent to obtain a spinning solution; perform electrospinning treatment using the spinning solution to obtain a PVDF-ZIF-8 nanofiber membrane; (2) Mix zinc nitrate, sodium formate, and 2-methylimidazole in a solvent to obtain a mixed solution, then immerse the PVDF-ZIF-8 nanofiber membrane in the mixed solution and react to obtain the in-situ grown ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane.

2. The preparation method according to claim 1, characterized in that, In step (1): The mass ratio of ZIF-8 to polyvinylidene fluoride is 1:15; the concentration of ZIF-8 in the spinning solution is 1 wt%; the solvent is N,N-dimethylformamide and / or acetone.

3. The preparation method according to claim 1, characterized in that, The conditions for the electrospinning treatment are: voltage is 13 KV, temperature is 30 °C, roller rotation speed is 500 rpm, linear speed is 10 mm / s, flow rate is 1 mL / h, receiving distance is 11.5 cm, and electrospinning time is 10 h.

4. The preparation method according to claim 1, wherein, In step (2): The mass ratio of zinc nitrate, sodium formate, and 2-methylimidazole is 6:1.38:2.4; the solvent is methanol.

5. The preparation method according to claim 1, characterized in that, The temperature of the reaction is 80 °C and the time is 9 h.

6. The preparation method according to claim 1, wherein After the reaction, it also includes a step of ultrasonic treatment; the time of the ultrasonic treatment is 60 s.

7. The preparation method according to claim 1, characterized in that, After the reaction, it also includes a step of hydrophilic modification, and the hydrophilic modification is as follows: Mix dopamine and polyethylenepolyamine in a mass ratio of 1:1 in water, then add Tris-HCl buffer solution to obtain a modified solution; put the product prepared in step (2) into the modified solution and perform oscillation treatment to complete the hydrophilic modification.

8. An in-situ grown ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane obtained by the preparation method according to any one of claims 1-7.

9. An application of the in-situ grown ZIF-8 modified superhydrophilic / underwater superoleophobic PVDF nanofiber membrane according to claim 8 in the field of oil-water separation.

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