Oil-water separation composite membrane and preparation method thereof

Abstract

The invention discloses an oil-water separation composite membrane and a preparation method thereof, and relates to the field of oil-water separation composite membranes, the composite membrane comprises a nylon membrane, acidified carbon nanotubes are arranged on a pore skeleton of the nylon membrane, a hydrophilic oleophobic layer is deposited on the surface of the nylon membrane, and the hydrophilic oleophobic layer is formed by codeposition of polydopamine and a zwitterionic polymer; the method is a method for preparing the composite membrane. The composite membrane disclosed by the invention has a precise pore structure and excellent anti-pollution capability.

Description

Technical Field

[0001] The present invention relates to the field of oil-water separation composite membranes, and in particular to an oil-water separation composite membrane and a preparation method thereof. Background Art

[0002] For oil-containing wastewater, oil-water separation is required during its treatment, and the oil-water separation process usually uses a membrane for separation. In the process of separating oil and water using a membrane, the permeation flow process of the liquid can be regarded as capillary flow. There is an obvious trade-off effect between the permeation flux and the separation efficiency. Moreover, in the oil-water separation process, the permeation flux of the membrane is inversely proportional to the thickness of the membrane and directly proportional to the effective pore diameter of the membrane, while the separation efficiency of the membrane is inversely proportional to the effective pore diameter of the membrane. Therefore, to break the trade-off effect in the separation process and minimize the damage to the permeation flux of the composite membrane, it is necessary to further improve the separation precision of the membrane, thus requiring precise regulation of the effective pore diameter of the separation membrane. At the same time, under the condition of improving the separation precision of the membrane, the problem of membrane fouling is an unavoidable problem for all oil-water separation membranes.

[0003] Therefore, the development of high-performance oil-water separation membranes with a precise pore structure and excellent anti-fouling ability is a current research hotspot and difficulty. Summary of the Invention

[0004] The purpose of the present invention is to provide an oil-water separation composite membrane and a preparation method thereof, which have a precise pore structure and excellent anti-fouling ability, aiming at the above-mentioned existing problems.

[0005] The technical solution adopted by the present invention is as follows: An oil-water separation composite membrane includes a nylon membrane. Acidified carbon nanotubes are present on the pore framework of the nylon membrane, and a hydrophilic and oleophobic layer is deposited on the surface of the nylon membrane. The hydrophilic and oleophobic layer is co-deposited by polydopamine and zwitterionic polymer.

[0006] A preparation method of an oil-water separation composite membrane for preparing the above-mentioned oil-water separation composite membrane includes the following steps:

[0007] S1: Prepare acidified carbon nanotube powder; including steps S11 - S14;

[0008] S11: Place multi-walled carbon nanotubes in an ice-water bath environment and add a mixed solution of concentrated sulfuric acid and concentrated nitric acid, and stir to uniformly disperse the multi-walled carbon nanotubes to obtain a mixture;

[0009] S12: Acidify the mixture in step S1 in an oil bath to obtain a reaction solution;

[0010] S13: Centrifuge the reaction solution and continuously wash it with pure water to remove the residual acid;

[0011] S14: The substance obtained after the freeze-drying step S13 is used to obtain an acidified carbon nanotube powder;

[0012] S2: Adsorb or anchor the acidified carbon nanotube powder on the nylon membrane; including steps S21 - S22;

[0013] S21: Uniformly disperse the acidified carbon nanotube powder in ionized water to obtain a dispersion;

[0014] S22: Place the nylon membrane in the dispersion for ultrasonic induction treatment to complete the adsorption or anchoring of the acidified carbon nanotube powder on the nylon membrane, obtaining a semi-finished membrane, and wash and dry the semi-finished membrane;

[0015] S3: Prepare a composite membrane; Immerse the semi-finished membrane obtained in step S22 in a buffer solution containing dopamine and zwitterionic polymer, add an oxidant, and conduct a shaking reaction in a constant temperature water bath oscillator to finally obtain a composite membrane.

[0016] Furthermore, in step S11, the dosage ratio of multi-walled carbon nanotubes, concentrated sulfuric acid, and concentrated nitric acid is 1:90:30.

[0017] Furthermore, in step S11, mechanical stirring is used to disperse the carbon nanotubes during stirring, and the mechanical stirring time is 30 min.

[0018] Furthermore, in step S12, the temperature for the acidification reaction in the oil bath is 50 °C, and the reaction time is 12 h.

[0019] Furthermore, in step S21, the acidified carbon nanotube powder is dispersed by ultrasonic dispersion treatment, and the treatment time is 30 min; the concentration of the aqueous solution after dispersing the acidified carbon nanotube powder is 1 mg / ml.

[0020] Furthermore, in step S22, the ultrasonic induction treatment time is 1 min - 60 min, and the loading amount of carbon nanotubes and the microscopic morphology of the membrane surface are regulated by controlling the ultrasonic induction treatment time.

[0021] Furthermore, in step S22, the semi-finished membrane is washed by successively washing with ethanol and ionized water multiple times.

[0022] Furthermore, in step S22, the semi-finished membrane is dried in an oven, and the drying temperature is 60 °C.

[0023] Furthermore, in step S3, the buffer solution is an acetate solution, the concentration of the acetate solution is 50 mM, and pH = 5.00.

[0024] Further, in step S3, the concentrations of dopamine and zwitterionic polymer are 2 mg / ml and 3 mg / ml, respectively.

[0025] Further, in step S3, the oxidant is NaIO 4 , and the concentration of NaIO 4 is 2 mg / ml.

[0026] Further, in step S3, the temperature of the oscillating reaction is 50 °C and the reaction time is 30 min.

[0027] Further, in step S3, the obtained composite membrane needs to be soaked in deionized water for a sufficient time and dried at room temperature.

[0028] In summary, due to the adoption of the above technical solutions, the beneficial effects of the present invention are as follows:

[0029] 1. The present invention successfully adsorbs or anchors acidified carbon nanotubes on the pore framework of the nylon membrane by the method of ultrasonic-induced "interface sintering". The acidified carbon nanotubes will intersect with each other in the membrane pores in a manner similar to weaving a net, thereby generating many small holes in the membrane pores, and the loading amount of carbon nanotubes and the microscopic morphology of the membrane surface can be regulated by controlling the ultrasonic dispersion treatment time.

[0030] 2. The composite membrane prepared by the present invention has high chemical and physical stability. It can maintain underwater superoleophobicity not only in extreme chemical conditions such as strong acid solution (pH = 1), neutral solution (pH = 7), strong base solution (pH = 12) and high-salt aqueous solution (NaCl: 10 wt.%), but also can maintain its excellent wettability under long-term ultrasonic dispersion treatment and underwater oscillation treatment, and has a long service life.

[0031] 3. The composite membrane prepared by the present invention has superoleophobic properties for viscous oils such as diesel, soybean oil, and even heavy crude oil, has extremely low adhesion to various oil droplets, and the underwater oil contact angles are all above 160°; in addition, the hydrophilic and oleophobic layer of the composite membrane can efficiently separate surfactant-stabilized water-in-oil emulsions, with the minimum separation size reaching the nanometer level, the separation efficiency reaching more than 99.50%, and having good cyclic stability (the permeate flux recovery rate is close to 100% after 5 cycles).

[0032] 4. Due to the presence of acidified carbon nanotubes, polydopamine and zwitterionic polymer in the composite membrane of the present invention, it also has sufficient active sites to adsorb soluble dyes and heavy metal ions in wastewater, and soluble pollutants in wastewater can be quickly removed by simple filtration.

[0033] 5. The composite membrane prepared by the present invention has excellent ability to separate viscous water-in-oil emulsions and sufficient active sites to adsorb soluble dyes and heavy metal ions in wastewater, and also has great application potential in the in-situ adsorption and separation of dye emulsions, and can effectively treat complex oily sewage. Brief Description of the Drawings

[0034] The present invention will be described by way of examples and with reference to the accompanying drawings, wherein:

[0035] Figure 1 Photographs of the membranes treated with different ultrasonic times and the corresponding SEM images of their surfaces;

[0036] Figure 2 Water contact angle changes of composite membranes M1-M6, M0';

[0037] Figure 3 Contact angles of composite membranes M1-M6, M0' with oil (dichloroethane used here) underwater;

[0038] Figure 4 Underwater oil contact angles of composite membrane M4 with different oils;

[0039] Figure 5 Images of water diffusion and dynamic underwater oil adhesion tests of composite membrane M4, with water droplets and oil droplets both being 4 μL;

[0040] Figure 6 Optical images of the separation device and different emulsions, and optical microscope images of the feed solution and filtrate of soybean water-in-oil emulsion;

[0041] Figure 7 Separation flux and separation efficiency of composite membrane M4 for different water-in-oil emulsions;

[0042] Figure 8 Separation flux and rejection rate for water-in-soybean oil emulsions with different oil volume percentages;

[0043] Figure 9 Osmotic flux of surfactant-free diesel water-in-oil emulsion in 5 cycles at 0.85 bar;

[0044] Figure 10 Anti-fouling test of composite membrane M4;

[0045] Figure 11 Zeta potential trends of M4' and M4 at different pH values;

[0046] Figure 12 Adsorption performance of M4' and M4 for dyes and heavy metal ions;

[0047] Figure 13Photographs and UV-Vis absorption spectra of M4' and M4 before and after one-time adsorption of filtrate from different dye solutions at pH = 7;

[0048] Figure 14 The permeation flux and separation efficiency of the MeB solution filtered for 5 cycles at 0.85 bar;

[0049] Figure 15 The changes in the separation flux and efficiency of the composite membrane for three different dyed emulsions after five cycles;

[0050] Figure 16 Photographs of the composite membrane M4 after being rinsed with solutions of different pH values and NaCl (10% wt) solution for 72 hours;

[0051] Figure 17 The underwater oil contact angle of the composite membrane M4 after different chemical stress treatments;

[0052] Figure 18 The underwater oil contact angle, photographs and SEM images of the composite membrane M4 after 3 times of ultrasonication for 10 minutes each time;

[0053] Figure 19 The water contact angle and underwater oil contact angle and SEM images of the composite membrane M4 before and after being immersed in water for 30 days under strong shear flow at 30 °C;

[0054] Figure 20 The underwater oil contact angle and SEM images of the composite membrane M4 before and after 5 cycles of oil-water separation at 0.85 bar; Detailed implementation mode

[0055] Example 1

[0056] An oil-water separation composite membrane, including a nylon membrane, with acidified carbon nanotubes on the pore skeleton of the nylon membrane, and a hydrophilic and oleophobic layer deposited on the surface of the nylon membrane, and the hydrophilic and oleophobic layer is co-deposited by polydopamine and zwitterionic polymer.

[0057] In this example, the acidified carbon nanotubes are adsorbed or anchored on the pore skeleton of the nylon membrane, and the acidified carbon nanotubes will intersect with each other in the membrane pores in a way similar to weaving a net, thus generating many small holes in the membrane pores. The existence of these small holes achieves fine regulation of the effective pore diameter of the membrane, effectively improving the separation accuracy of the membrane, and thus reducing the trade-off effect during the separation process.

[0058] In this example, by depositing a hydrophilic and oleophobic layer on the surface of the nylon membrane, the anti-fouling ability of the membrane is effectively improved.

[0059] Example 2

[0060] A method for preparing an oil-water separation composite membrane, preparing the oil-water separation composite membrane described in Preparation Example 1, comprising the following steps:

[0061] S1: Preparation of acidified carbon nanotubes; including steps S11 - S14;

[0062] S11: Take 1 g of multi-walled carbon nanotubes and add them to a mixture of 90 ml of concentrated sulfuric acid and 30 ml of concentrated nitric acid under an ice-water bath, and mechanically stir for 30 min to uniformly disperse the multi-walled carbon nanotubes to obtain a mixture;

[0063] S12: Place the mixture in step S11 in an oil bath at 50 °C for an acidification reaction for 12 h to obtain a reaction solution;

[0064] S13: Centrifuge the reaction solution using an ultracentrifuge and continuously wash with pure water to remove residual acid;

[0065] S14: Freeze-dry the substance obtained after step S13 to obtain acidified carbon nanotube powder.

[0066] S2: Adsorbing or anchoring acidified carbon nanotube powder on a nylon membrane; including steps S21 - S22;

[0067] S21: Uniformly disperse the acidified carbon nanotube powder in ionized water to obtain a dispersion. The concentration of the dispersion is 1 mg / ml and the volume is 100 ml. The dispersion method is ultrasonic dispersion treatment for 30 min to uniformly disperse the acidified carbon nanotube powder to obtain a dispersion;

[0068] S22: Place a circular nylon membrane with a diameter of 5 cm in the dispersion for ultrasonic induction treatment for 1 min - 60 min. Among them, Test Examples 1 - 6 are 1 min (Test Example M1), 5 min (Test Example M2), 10 min (Test Example M3), 15 min (Test Example M4), 30 min (Test Example M5), 60 min (Test Example M6) respectively. Details are shown in Table 1; complete the adsorption or anchoring of the acidified carbon nanotube powder on the nylon membrane to obtain a semi-finished membrane; the method for cleaning the semi-finished membrane is to wash it twice with ethanol and ionized water in sequence, and after the cleaning is completed, dry the semi-finished membrane in an oven at a drying temperature of 60 °C.

[0069] S3: Prepare a composite membrane: Immerse the dried semi-finished membrane in step S22 in an acetate buffer solution (50 mM, pH = 5.00) containing dopamine (2 mg / ml) and zwitterionic poly(sulfobetaine methacrylate, concentration of 3 mg / ml) ml, with NaIO 4As the oxidant (1 mg / ml), and react with shaking in a constant temperature water bath oscillator at 50 °C for 30 min, and finally obtain the composite membrane; soak the just-prepared composite membrane in deionized water for a sufficient time to wash away the unreacted monomers and oligomers on the membrane surface, and dry it at room temperature for 24 hours.

[0070] Table 1: Treatment time of ultrasonic induction treatment in different test examples

[0071] Test Example M1 M2 M3 M4 M5 M6 Ultrasonic Induction Treatment Time 1 min 5 min 10 min 15 min 30 min 60 min

[0072] In order to reflect the advantages of the oil-water separation composite membrane prepared by the preparation method disclosed in this embodiment, in this embodiment, the following 8 control examples are set for comparison with the embodiment.

[0073] Control example M0':

[0074] Use the nylon membrane without any treatment as the composite membrane of this control example M0'.

[0075] Control example M0:

[0076] In the steps of the above preparation method, only step S3 is carried out, that is, directly immerse the nylon membrane without any treatment in an acetate buffer solution (50 mM, pH = 5.00) containing dopamine (2 mg / ml) and zwitterionic poly(sulfobetaine methacrylate, concentration 3 mg / ml), use NMIO4 as the oxidant (1 mg / ml), and react with shaking in a constant temperature water bath oscillator at 50 °C for 30 min, and finally obtain the composite membrane of control example M0'; soak the just-prepared composite membrane in deionized water for a sufficient time to wash away the unreacted monomers and oligomers on the membrane surface, and dry it at room temperature for 24 hours.

[0077] Control examples M1'-M6':

[0078] In the steps of the preparation method of each of the above test examples, only steps S1 - S2 are carried out, and step S3 is no longer carried out, that is, control example M1' corresponds to not carrying out step S3 in test example M1, control example M2' corresponds to not carrying out step S3 in test example M2, and so on, control example M6' corresponds to not carrying out step S3 in test example M6; the composite membranes of control examples M1'-M6' are obtained respectively.

[0079] Compare the test examples with the control examples, and conduct the following verification experiments respectively. For the convenience of description, the membranes obtained from each test example and control example are directly replaced by the labels of the test example or control example. For example, if the composite membrane is obtained from test example M1, it can be directly referred to as composite membrane M1.

[0080] Verification experiment 1: Microscopic morphology observation, pore size measurement and separation performance test.

[0081] The above composite membranes M1-M6 and composite membranes M0', M0, M1'-M6' were subjected to microscopic morphology observation, pore size measurement, and separation performance testing.

[0082] As Figure 1 shown, by observing the surface of the composite membranes M1'-M6' through an electron microscope, it can be observed that the color of the composite membranes gradually deepens with the extension of the ultrasonic time; that is, when the ultrasonic induction treatment time is short, only 1 min ( Figure 1 Figure a in Figure 1 ), only a few acidified carbon nanotubes can be observed on the surface of the composite membrane M1'; with the extension of the ultrasonic induction treatment time, when it reaches 5 min ( Figure 1 Figure b in Figure 1 ), and 10 min ( Figure 1 Figure c in

[0083] ), the number of acidified carbon nanotubes on the surfaces of the composite membranes M2' and M3' has increased to a certain extent; when the ultrasonic induction treatment time reaches 15 min (

[0084] Figure d in

[0085] Table 2: Average pore sizes of all composite membranes

[0086] Test Example Average Pore Size (nm) Control Example Average Pore Size (nm) M0' 450 M0 428.23 M1 318.25 M1' 338.34 M2 236.33 M2' 255.34 M3 139.21 M3' 158.21 M4 63.74 M4' 79.73 M5 49.91 M5' 60.92 M6 34.86 M6' 39.83

[0087] As can be seen from Table 2, by comparing the composite membranes M0', M1'-M6', it can be known that after being modified by acidified carbon nanotubes, the average pore size of the composite membrane has been greatly reduced; by comparing the composite membrane M0' and M0, and respectively comparing M1-M6 and M1'-M6', it can be known that after the composite membrane is co-deposited with polydopamine and zwitterionic polymer, the average pore size of the composite membrane further decreases, and this pore size result is consistent with the result of the above electron microscopy observation experiment.

[0088] Perform a permeation experiment on the above composite membrane to obtain the permeation flux and separation efficiency of the composite membrane, and the obtained data are shown in Table 3.

[0089] Table 3: Permeation Flux and Separation Efficiency of Composite Membrane

[0090]

[0091] As can be seen from Table 3, the pure water flux of the nylon membrane (M0') without any treatment is 8818±231L·m -2 ·h -1 . Compared with M0', the permeation flux of the composite membranes (M1'-M6') after being treated by ultrasonic induction of acidified carbon nanotubes gradually decreases with the prolongation of the ultrasonic treatment time. At the same time, after co-depositing polydopamine and zwitterionic polymer, the permeation flux of all membranes further decreases, from 7943±235L·m -2 ·h -1 (M0) gradually decreases to 1375±118L·m -2 ·h -1 (M6), indicating that the pore size has a significant impact on the pure water permeation flux of the membrane, that is, when the pore size decreases, the separation accuracy of the membrane improves, but the permeation flux will decrease. Therefore, the ultrasonic induction treatment time is preferably 15 min to make the separation accuracy and permeation flux reach a more suitable regulation.

[0092] The result of the permeation flux of the diesel water-in-oil emulsion is similar to that of the pure water flux. The oil-water permeation flux of the composite membranes M1-M6 gradually decreases with the prolongation of the ultrasonic treatment time; but for the small-sized oil-in-water emulsion, its separation efficiency gradually increases with the prolongation of the ultrasonic treatment time, from 95.22% (M1) to 99.99% (M6), because the surface wettability and pore size of the membrane also have an important impact on the oil-water separation efficiency. From the above data, it can be seen that a suitable pore structure and strong wettability are beneficial to the separation of water-in-oil emulsions. In addition, the excellent surface pore size, roughness and surface wettability of the composite membrane M4 are more suitable for separating water-in-oil emulsions, so the composite membrane M4 is selected for subsequent performance evaluation.

[0093] Verification Experiment 2: Wettability Test.

[0094] The wetting performance of composite membranes M4, M4' and untreated nylon membrane (M0') was tested and compared to obtain the water absorption data of the three membranes, as shown in Table 4.

[0095] Table 4: Water absorption of three composite membranes

[0096] M0' M4' M4 <![CDATA[Water absorption rate (mg / m 2 )]]> 118±1.42 120±1.22 226.29±1.52

[0097] As can be seen from Table 4, the water absorption rates of composite membrane M4' (the composite membrane only attached or anchored with acidified carbon nanotubes) and untreated nylon membrane (M0') are both about 120 mg / m 2 . Composite membrane M4 is also co-deposited with polydopamine and zwitterionic polymer, and its water absorption rate is as high as 226.29 ± 1.52 mg / m 2 ; This phenomenon may be attributed to the strong hydration ability after the co-deposition of polydopamine and zwitterionic polymer, which can adsorb a large number of water molecules and form a uniform hydration layer on the membrane surface to prevent oil stains from contacting the surface of the composite membrane, thus solving the problem of oil stain pollution on the membrane.

[0098] Furthermore, the wettability of composite membranes M0', M1 - M6 was tested. As Figure 2 shown, the initial water contact angle of composite membrane M4 is only 26 ± 0.89°, and a 4 μL water droplet can quickly spread and penetrate the entire membrane within 1.96 s. Compared with the initial water contact angle of M0' reaching nearly 37.5°, this excellent hydrophilicity is beneficial for the composite membrane to have high oleophobicity in the underwater environment. Figure 3 Shown is the contact angle of underwater oil (dichloroethane used here). It can be found that the underwater oil contact angles of composite membranes M1 - M6 are all greater than 160°, while the underwater oil contact angle of M0' is less than 160°. This indicates that the composite membranes M1 - M6 co-deposited with polydopamine and zwitterionic polymer have high underwater oleophobicity. To further confirm the universality of this underwater oleophobic performance for different types of oil stains, as Figure 4 shown, the underwater oil contact angles of M4 for various oils (dichloroethane, hexadecane, diesel, soybean oil and crude oil) were measured. It can be clearly found that even in the face of high-viscosity soybean oil and crude oil, its underwater oil contact angle is above 165°, proving that after co-depositing polydopamine and zwitterionic polymer, the composite membrane prepared in the present invention has super oleophobic ability underwater. In addition, taking dichloroethane oil droplets as an example, the underwater anti-oil adhesion performance of composite membrane M4 was also tested. As Figure 5 shown, when dichloroethane is forced to contact the surface of M4 membrane underwater, the oil droplet can be easily pulled away from the membrane surface. During this process, no obvious force between the oil droplet and the membrane surface is found, and at the same time, the oil droplet hardly shows any deformation phenomenon, indicating that the composite membrane prepared in the present invention has extremely low adhesion to oil stains.

[0099] In the present invention, highly viscous oils (heavy crude oil, light crude oil, and soybean oil) were also used to test the anti-sticking oil fouling performance of the composite membrane. First, the composite membrane M4 was pre-wetted with pure water, and it was found that a dense hydration layer would appear on the surface of the composite membrane. Subsequently, some viscous crude oil was dropped onto the surface of the composite membrane M4 that had been pre-wetted with water. Finally, the entire membrane was placed in water, and the crude oil would quickly (less than 0.1 s) fall off the membrane surface and float on the water surface. Similar results were also observed in the immersion cleaning tests with light crude oil and soybean oil. When the membrane pre-wetted with water came into contact with light crude oil and soybean oil, the oil fouling was isolated by the hydration layer on the membrane surface. When it was immersed underwater, the oil fouling on the membrane surface would automatically fall off and float on the water surface, confirming the universality of the powerful anti-sticking oil fouling performance of the composite membrane prepared in this patent.

[0100] At the same time, the same experiments were also carried out on the original nylon membrane (M0') and the membrane with only attached or anchored acidified carbon nanotubes. Although a hydration layer would also appear on the membrane surface after being pre-wetted with water, obvious adhesion phenomena of both heavy crude oil and light crude oil or soybean oil would occur on the membrane surface, and even shaking and cleaning underwater could not completely remove these oil foulings from the membrane surface.

[0101] In summary, due to the co-deposition of polydopamine and zwitterionic polymer in the present invention, it has a powerful hydration ability. It can adsorb a large number of water molecules and form a uniform hydration layer on the membrane surface to prevent oil fouling from contacting the surface of the composite membrane, thus solving the problem of the membrane being contaminated by oil fouling, and at the same time, it also has strong oil repellency.

[0102] Verification experiment 3: Oil-water separation test.

[0103] Using the composite membrane M4, separation tests were carried out on a series of high-viscosity oil-in-water emulsions (diesel oil, soybean oil, and crude oil) with micro-scale (nanometer scale) sizes respectively. The test results Figure 6 are shown as follows. Before filtration, it was found that the emulsion was milky white with a large number of oil droplets (the diameter of the oil droplets was about 200 nm). After filtration using the composite membrane M4, the filtrate became clear and transparent, and there was no visible oil droplet residue, directly verifying that the composite membrane M4 can efficiently separate even high-viscosity nanometer-scale oil-in-water emulsions. As Figure 7 shown, for the oil-in-water emulsions without surfactants, the water permeation fluxes of the composite membrane were 1690 ± 51 L·m -2 ·h -1 、1650 ± 53 L·m -2 ·h -1 and 1639 ± 50 L·m -2 ·h -1, meanwhile, the separation rates are all higher than 99.90%; for those emulsions containing surfactants, the separation fluxes decrease slightly, being 1290±46L·m -2 ·h -1 、1269±48L·m -2 ·h -1 and 1180±49L·m -2 ·h -1 , and the separation efficiency shows no obvious change and is still higher than 99.90%.

[0104] The present invention also tested the filtration efficiency of soybean oil-in-water emulsions with different oil volume contents (1%, 3%, 5%, 7% and 10%). As Figure 8 shown, it can be found that the separation fluxes and separation efficiencies of the composite membranes for emulsions with different oil concentrations remain unchanged, further proving that the composite membranes prepared by this patent have excellent emulsion separation performance.

[0105] Verification experiment 4: Cycling performance test.

[0106] To study the cycling performance of the composite membranes prepared by the present invention, we carried out a cycling experiment on the separation of soybean oil emulsion using composite membrane M4. The results are as Figure 9 shown. In each cycle, the initial permeation flux is about 1700L·m -2 ·h -1 , and then the flux decreases slightly within 10 min because even under stirring conditions, a small amount of soybean oil will adhere to the membrane surface during dead-end filtration; after each cycle, the permeation flux can be completely restored by simple washing. To further study the antifouling performance of the composite membrane, the total fouling ratio (Rt), reversible fouling ratio (Rr), irreversible fouling ratio (Rir) and flux recovery rate (FRR) of the composite membrane during the separation process were calculated ( Figure 10 shown). It can be found that the total fouling ratio (Rt) and reversible fouling ratio (Rr) of the composite membrane are about 6.11%, and the irreversible fouling ratio (Rir) is close to 0. In addition, the flux recovery rate (FRR) is close to 100% after 5 cycles, indicating that the composite membrane prepared by the present invention has extremely strong antifouling ability and excellent long-term cycling performance.

[0107] Verification experiment 5: Dye and heavy metal ion adsorption test.

[0108] By measuring the surface Zeta potential of the composite membrane M4 and composite membrane M4' prepared by the present invention, it is found that ( Figure 11As shown, where the A-MWCNTs@Nylon composite membrane is composite membrane M4', and the A-MWCNTs@Nylon@PDA / PSBMA composite membrane is composite membrane M4. The surface of the nylon membrane (M4') that only adsorbs or anchors acidified carbon nanotubes is always negatively charged, and the electronegativity of the membrane surface becomes stronger as the pH value increases. In contrast, for the composite membrane M4 modified by the co-deposition of polydopamine and zwitterionic polymer, in acidic conditions, due to the protonation of amino groups on the membrane surface playing a dominant role, the Zeta potential of composite membrane M4 is positive. On the contrary, in alkaline conditions, the Zeta potential of composite membrane M4 is negative. Therefore, it can be determined that the composite membrane M4 modified by the co-deposition of polydopamine and zwitterionic polymer can adapt to both acidic and alkaline environments.

[0109] Meanwhile, as Figure 12 shown, the adsorption capacities of composite membranes M4' and M4 for heavy metal ions (Cu 2+ , Cr 6+ ) and dyes (cationic dye: methylene blue MeB and anionic dye: congo red CR) were investigated through a simple and rapid filtration process. The adsorption capacities of composite membrane M4' for Cu 2+ and Cr 6 + are 59.32 mg / g and 68.14 mg / g respectively, while the adsorption capacities of composite membrane M4 for Cu 2+ and Cr 6+ are 84.53 mg / g and 93.34 mg / g respectively. This is because there are ion exchange, chelation, and electrostatic interactions between the functional groups on the composite membrane surface and Cu 2+ , Cr 6+ , resulting in high adsorption capacities.

[0110] The UV spectra and color changes of the dye solutions (10 ppm, 100 mL) and the filtrates are as Figure 13 shown. It can be found that the characteristic absorption peaks of CR and MeB are greatly reduced. Meanwhile, the filtrate also becomes clear and transparent, indicating that the composite membrane prepared in this patent also has a strong adsorption capacity for dyes during the filtration process. In addition, the adsorption capacities of the composite membrane for the cationic dye MeB (composite membrane M4', 129.23 mg / g; composite membrane M4, 138.27 mg / g) are much higher than those for the anionic dye CR (composite membrane M4', 13.34 mg / g; composite membrane M4, 58.28 mg / g). This is because the dye adsorption process mainly relies on electrostatic adsorption and is greatly affected by the total charge on the surface of the adsorption material. In addition, due to the co-deposition of polydopamine and zwitterionic polymer on composite membrane M4, the amino and sulfonic acid functional groups on its surface have extremely strong hydration effects, resulting in a stronger adsorption capacity for anionic dyes than composite membrane M4'.

[0111] The present invention evaluates the cycling ability of the composite membrane for dye adsorption capacity. For each cycle, a MeB (100 mL) solution was poured onto the surface of the composite membrane M4 prepared by the present invention for filtration, and the adsorption capacity was quickly restored by washing with a small amount of HCl solution (pH = 1) or ethanol within 30 seconds. Figure 14 It shows that the separation flux and efficiency of each cycle hardly change; after 5 cycles, the separation efficiency can still be maintained above 99.90%.

[0112] In this verification experiment 5, three types of dyed water-in-oil emulsions were also prepared, including a water-in-diesel emulsion containing MeB (20 ppm), a water-in-soybean oil emulsion containing MeB (20 ppm), and a water-in-dilute crude oil emulsion containing MeB (20 ppm). The application potential of the composite membrane prepared by the present invention in the in-situ adsorption and separation of complex dyed emulsions was verified by simple filtration. Figure 15 The experimental results of five-cycle tests of the composite membrane M4 prepared by the present invention on different dyed emulsions are shown. It can be found that the separation flux of the composite membrane hardly changes during 5 cycles, the flux recovery rate is higher than 99%, and at the same time, after 5 cycles, our oil-water separation efficiency is still higher than 99.90%, and the separation efficiency for dyes decreases slightly but is still higher than 99.70%. This further proves that the composite membrane prepared by the present invention can effectively treat complex oily sewage.

[0113] Verification experiment 6: Stability performance test.

[0114] To test the stability of the composite membrane prepared by the present invention, the composite membrane M4 was selected as the test membrane and immersed in a strong acid solution (pH = 1), a neutral solution (pH = 7), a strong base solution (pH = 12), and a high-salt aqueous solution (NaCl: 10 wt.%) for 72 hours. From Figure 16 it can be clearly seen that all the solutions are clear and the composite membrane M4 does not show obvious fading. At the same time, the underwater oil contact angle of the composite membrane M4 stored in each solution for 72 hours was also tested, and the results are as Figure 17 shown. The composite membrane still maintains underwater superoleophobicity under acidic, neutral, alkaline, and high-salt conditions, confirming the strong chemical stability of the composite membrane prepared by the present invention. This strong chemical stability may be due to the higher degree of oxidation of the co-deposited polydopamine and zwitterionic polymer, which can form more stable covalent bonds rather than non-covalent bonds with the membrane surface during the co-deposition process. In addition to the super strong chemical stability, the composite membrane also exhibits good physical stability. After ultrasonic treatment for 60 min, as Figure 18As shown, there is no obvious peeling phenomenon of the carbon nanotubes and polymer coatings on the surface and in the pores of the composite film M4, and at the same time, the composite film M4 still maintains underwater superoleophobicity. In addition, the present invention also tested the wettability of the composite film M4 after being shaken in water at 30 °C for 30 days and passing through 5 cycles of oil / water separation at 0.85 bar to evaluate the physical stability ( Figure 19 and Figure 20 ), and it can be found that the composite film prepared by the present invention still maintains superhydrophilicity and underwater superoleophobicity, and the hydrophilic and oleophobic layer co-deposited with acidified carbon nanotubes, polydopamine and zwitterionic polymer is still stably fixed on the surface of the composite film. The above results indicate that the composite film prepared by this patent has good environmental and mechanical durability.

[0115] The present invention is not limited to the specific embodiments described above. The present invention extends to any new feature or any new combination disclosed in this specification, as well as any new combination of the steps of any new method or process disclosed.

Claims

1. An oil-water separation composite membrane, characterized in that: The invention comprises a nylon membrane, wherein the pore skeleton of the nylon membrane is provided with acidified carbon nanotubes, and a hydrophilic and oleophobic layer is deposited on the surface of the nylon membrane, wherein the hydrophilic and oleophobic layer is formed by co-deposition of polydopamine and zwitterionic polymer.

2. A method for preparing an oil-water separation composite membrane, comprising preparing the oil-water separation composite membrane according to claim 1, characterized in that: The following steps are involved: S1: preparing acidified carbon nanotube powder; including steps S11 to S14; S11: adding a mixed solution of concentrated sulfuric acid and concentrated nitric acid to the multi-walled carbon nanotubes in an ice water bath, and stirring to uniformly disperse the multi-walled carbon nanotubes to obtain a mixture; S12: placing the mixture in step S11 in an oil bath for acidification reaction to obtain a reaction solution; S13: centrifuging the reaction solution and continuously washing it with pure water to remove residual acid; S14: freeze-drying the substance obtained after step S13 to obtain acidified carbon nanotube powder; S2: adsorbing or anchoring the acidified carbon nanotube powder on the nylon membrane; including steps S21 to S22; S21: uniformly dispersing the acidified carbon nanotube powder in ionized water to obtain a dispersion; S22: placing the nylon membrane in a dispersion liquid for ultrasonic induction treatment to complete adsorption or anchoring of the acidified carbon nanotube powder on the nylon membrane to obtain a semi-finished membrane, and washing and drying the semi-finished membrane; S3: preparing a composite membrane; immersing the semi-finished membrane obtained in step S22 in a buffer solution containing dopamine and zwitterionic polymers, adding an oxidant, and performing an oscillation reaction in a constant temperature water bath oscillator to finally obtain a composite membrane.

3. The preparation method according to claim 2, characterized in that: In step S11, the usage ratio of multi-walled carbon nanotubes, concentrated sulfuric acid and concentrated nitric acid is 1:90:

30.

4. The preparation method according to claim 2, characterized in that: In step S12, the temperature of the acidification reaction in the oil bath is 50° C., and the reaction time is 12 h.

5. The preparation method according to claim 2, characterized in that: In step S21, the acidified carbon nanotube powder is dispersed by ultrasonic dispersion treatment for 30 minutes; the concentration of the aqueous solution after the acidified carbon nanotube powder is dispersed is 1 mg / ml.

6. The preparation method according to claim 2, characterized in that: In step S22, the semi-finished film is cleaned by washing it with ethanol and ionized water in sequence for multiple times; or / and drying the semi-finished film in an oven at a drying temperature of 60°C.

7. The preparation method according to claim 2, characterized in that: In step S3, the buffer solution is an acetate solution, the concentration of the acetate solution is 50 mM, and the pH is 5.

00.

8. The preparation method according to claim 2, characterized in that: In step S3, the concentrations of dopamine and zwitterionic polymer are 2 mg / ml and 3 mg / ml respectively; or / and the oxidant is NaIO4, and the concentration of NaIO4 is 2 mg / ml.

9. The preparation method according to claim 2, characterized in that: In step S3, the shaking reaction temperature is 50° C. and the reaction time is 30 min.

10. The preparation method according to claim 2, characterized in that: In step S3, the obtained composite membrane needs to be soaked in deionized water for a sufficient time and dried at room temperature.

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