Method for preparing a superhydrophobic membrane by synergistically combining spray-induced nonsolvent-induced phase inversion and click chemistry

Abstract

The present invention discloses a method for synergistically preparing a superhydrophobic membrane by spray-induced nonsolvent phase inversion and click chemistry. The method uses a polymer with a polyvinylidene fluoride as the main structure as a raw material, constructs a micro-nano hierarchical structure in bulk by the spray-induced nonsolvent phase inversion method, and covalently bonds a low surface energy substance to the membrane surface with the assistance of click chemistry technology to achieve the superhydrophobicity of the membrane. The superhydrophobic membrane of the present invention can resist the wetting of liquids with different surface tensions, has good anti-wetting and anti-inorganic fouling properties as well as excellent wastewater treatment capabilities, and can be applied to fields such as membrane distillation seawater desalination, deep treatment of shale gas wastewater and clean water regeneration, etc., broadening the types and application ranges of MD membrane materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of membrane preparation, and relates to a method for synergistically preparing a superhydrophobic membrane by spray-induced nonsolvent-induced phase inversion and click chemistry. Background Art

[0002] Membrane distillation (MD) technology, as an emerging membrane technology driven by vapor pressure difference, can treat wastewater with high salt concentration, and shows great application potential in many fields such as wastewater treatment and reuse, seawater or brackish water desalination, and separation of chemical products. The MD membrane is the core of the MD technology and is a key link to reduce the energy consumption of the separation process and improve the level of material recycling. An ideal MD membrane should have sufficient hydrophobicity, sufficient porosity, excellent mechanical strength, low thermal conductivity and other advantages. However, problems such as membrane wetting, membrane fouling and instability of the functional layer are huge challenges faced by the current MD technology, which greatly hinders the large-scale application of this technology.

[0003] Based on this, many researchers have strengthened the performance of membrane materials by constructing micro-nano hierarchical structures and introducing ultra-low surface energy fluorides on the surface. The common methods for constructing micro-nano hierarchical structures mainly focus on introducing nanoparticles, electrospinning or plasma etching, etc. [Environ. Sci. Technol., 2016, 50, 12275–12282; J. Membr. Sci., 2020, 591, 117572; J. Membr. Sci., 2020, 597, 117638; Nat. Commun., 2019, 10, 1–9]. Among them, introducing nanoparticles (such as ZiO, TiO 2 and SiO 2 etc.) on the polymer-based membrane to roughen the membrane surface is the most common method for constructing micro-nano hierarchical structures. However, this construction method still has serious drawbacks. First, the nanoparticles and the surface of the base membrane are often combined by weak electrostatic adsorption forces, which cannot ensure the stable attachment of the nanoparticles on the membrane surface. Under long-term operation (scouring) or harsh operating conditions, the nanoparticles are likely to fall off the membrane surface on a large scale, resulting in the loss of MD membrane performance and posing a huge potential threat to human health and the ecological environment.

[0004] On the other hand, on the basis of constructing the micro-nano hierarchical structure, further reducing the surface energy of the membrane surface is a necessary means to improve the comprehensive performance of the MD membrane. At present, the methods for reducing the surface energy of solids mainly include coating with low surface energy fluorides and CF 4Plasma treatment, etc. [J.Membr.Sci.2020,591,117572; Environ.Sci.Technol.,2016,5012275–12282; Water Res.,2019,155,152–161]. Among them, directly coating low-surface-energy fluorides on the solid surface is the most important method to enhance the performance of MD membranes (J.Membr.Sci.2020,591,117572; Environ.Sci.Technol.,2016,50 12275–12282; J.Membr.Sci.2020,603,118031). Low-surface-energy substances are mainly polyfluoro / perfluorinated substances, including but not limited to Teflon AF1600 / 2400, FDTS, etc. The attachment of low-surface-energy fluorides is achieved by the hydrophobic-hydrophobic interaction between the membrane surface and the fluorides, thereby greatly increasing the static contact angle and greatly reducing the rolling angle. As mentioned above, the stability of MD membranes also depends on the stability of the coating. Weak non-covalent bonding cannot achieve long-term operation or operation under harsh conditions. Low-surface-energy fluorides may also be peeled off due to the shedding of nanoparticles on the membrane surface and the weak non-covalent binding between themselves and the nanoparticles, further reducing the performance of MD membranes. At the same time, the shedding of nanoparticles and fluorides will also cause serious harm to the environment and human health, and their leakage will also cause harm and risks. Summary of the Invention

[0005] Aiming at the problems of traditional membranes for membrane distillation, such as easy wetting / fouling and insufficient functional stability, the present invention provides a method for synergistically preparing a superhydrophobic membrane by spray-induced non-solvent induced phase separation and click chemistry. The method uses a polymer or blend with a polyvinylidene fluoride main structure as the raw material, constructs a micro-nano hierarchical structure in the bulk by spray-induced non-solvent induced phase separation method, and covalently bonds low-surface-energy substances to the membrane surface with the assistance of click chemistry technology, thereby realizing the superhydrophobicity of the membrane, ensuring the stability of the functions of this type of membrane, and meeting the requirements of membrane distillation applications.

[0006] The technical solution of the present invention is as follows:

[0007] A method for synergistically preparing a superhydrophobic membrane by spray-induced non-solvent induced phase separation and click chemistry, using a polymer or blend with a polyvinylidene fluoride main structure as the raw material, and combining spray-induced non-solvent induced phase separation and click chemistry, the specific steps are as follows:

[0008] Step 1, construct a micro-nano hierarchical structure in the bulk:

[0009] (a) Using a polymer with a polyvinylidene fluoride main structure as the raw material, completely dissolve it in an organic solvent, then add water, and quickly stir to prepare a ternary casting solution of "polymer-organic solvent-water";

[0010] (b) After the casting solution stands still for defoaming, pour it onto a clean and flat glass plate, and use a scraper to scrape the casting solution into a liquid film;

[0011] (c) Use a spray gun with a nozzle specification of 0.2 - 0.5 mm to evenly spray mist or compressed air onto the liquid film prepared in (b) for 20 - 90 s;

[0012] (d) Immediately place the treated liquid film into a coagulation bath to fully complete phase inversion, and freeze-dry it to be used as a base film;

[0013] Step 2, "click chemistry" covalent bonding of the functional layer:

[0014] (a) Immerse the base film in a NaOH solution to hydroxylate the surface of the base film;

[0015] (b) Place the hydroxylated base film in a dichloromethane solution of 4-pentenoic acid, and then add the catalysts N,N'-diisopropylcarbodiimide / 4-dimethylaminopyridine (DIC / DMAP) to introduce C=C double bonds on the surface of the base film;

[0016] (c) Place the base film with introduced double bonds in a perfluorodecanethiol acetone solution, and react it under strong ultraviolet light irradiation and the action of a photoinitiator. After the reaction, rinse it thoroughly with acetone and dry it under vacuum to obtain a superhydrophobic film.

[0017] Preferably, in step 1(a), the polymer with a vinylidene fluoride main structure is selected from one or two of polyvinylidene fluoride and poly(vinylidene fluoride - hexafluoropropylene) copolymer.

[0018] Furthermore, in step 1(a), the organic solvent is selected from dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), or N-methylpyrrolidone (NMP).

[0019] Furthermore, in step 1(a), in the "polymer - organic solvent - water" ternary casting solution, the concentration of the polymer is 8 - 15 wt%, and the concentration of water is 10 wt%.

[0020] Furthermore, in step 1(c), the spray is selected from water or ethanol.

[0021] Furthermore, in step 1(c), the spray duration is 30 - 60 s.

[0022] Furthermore, in step 1(d), water is used as the coagulation bath, and the phase inversion time is more than 24 h.

[0023] Furthermore, in step 2(a), the concentration of the NaOH solution is 7.5 mol L -1 , and the soaking time is 2 - 8 hours.

[0024] Further, in step 2(b), in the dichloromethane solution of 4-pentenoic acid, the mass concentration of 4-pentenoic acid is 0.05-0.2% (w / v), and the hydroxylated base film is soaked in the dichloromethane solution of 4-pentenoic acid for 1-4 hours.

[0025] Further, in step 2(c), in the acetone solution of perfluorodecanethiol, the mass concentration of perfluorodecanethiol is 0.5-2% (w / v).

[0026] Further, in step 2(c), the ultraviolet irradiation time is 10-60 min; the ultraviolet light intensity is 500 W.

[0027] Further, in step 2(c), the photoinitiator is 2,2-dimethoxy-2-diphenylacetophenone (DMPAP).

[0028] The present invention also provides a superhydrophobic film prepared by the above method.

[0029] Further, the present invention also provides the application of the above superhydrophobic film in membrane distillation.

[0030] The application of the present invention in membrane distillation includes, but is not limited to, membrane distillation for seawater desalination, advanced treatment of shale gas wastewater and clean water regeneration, etc.

[0031] Compared with the prior art, the present invention has the following remarkable advantages:

[0032] (1) The present invention abandons the traditional method of incorporating nanoparticles, and uses the self-performance of the polymer to construct a stable micro-nano hierarchical structure in the bulk; in addition, a low surface energy substance is covalently bonded to the membrane surface, which improves the performance of the membrane material while greatly reducing the environmental risk;

[0033] (2) The preparation method of the present invention is simple, low in cost, and easy to realize mass production;

[0034] (3) The superhydrophobic film of the present invention has good anti-wetting / anti-fouling performance and stability;

[0035] (4) The superhydrophobic film can stably treat seawater or shale gas wastewater in the application of membrane distillation to realize the regeneration of clean water.

[0036] In summary, the superhydrophobic membrane of the present invention uses a polymer with a polyvinylidene fluoride as the main structure as the raw material, constructs a stable micro-nano hierarchical structure through spray-induced nonsolvent phase inversion method in bulk, and covalently bonds low surface energy fluorides on the membrane surface with click chemistry technology, thereby realizing the superhydrophobicity of the membrane and ensuring the stability of the functions of this type of membrane. The superhydrophobic membrane prepared by the present invention exhibits excellent anti-wetting / anti-fouling and mechanical stability, and can be effectively applied in seawater desalination, advanced treatment of shale gas wastewater and clean water regeneration, broadening the types and scope of application of MD membrane materials. Brief Description of the Drawings

[0037] Figure 1 SEM images of the surfaces of the base membranes M1-M4 and the comparative membranes M1'-M4' prepared in Examples 1, 2, 3, 4 and Comparative Examples;

[0038] Figure 2 SEM images of the surfaces of the superhydrophobic membranes Omni-1-Omni-4 prepared in Examples 5, 6, 7, 8;

[0039] Figure 3 Contact angle / rolling angle and LEP results of the superhydrophobic membranes Omni-1-Omni-4 in Example 9;

[0040] Figure 4 Contact angle / rolling angle and LEP results of the comparative membranes M1-M4 and M1'-M4' in Example 9;

[0041] Figure 5 Anti-wetting performance of the superhydrophobic membranes Omni-1-Omni-4 in Example 12;

[0042] Figure 6 Anti-gypsum scale performance of the superhydrophobic membrane Omni-1 in Example 12, (a) shows the flux change; (b) shows the conductivity change;

[0043] Figure 7 Seawater desalination performance of the superhydrophobic membranes Omni-1-Omni-4 in Example 13;

[0044] Figure 8 Treatment performance of the superhydrophobic membranes Omni-1-Omni-4 on shale gas wastewater in Example 14. Detailed Description of the Invention

[0045] The present invention will be further described in detail below in conjunction with specific embodiments and the accompanying drawings. The following embodiments are only used to illustrate the present invention, but not to limit the protection scope of the present invention. Unless otherwise specified, the embodiments are carried out under conventional experimental conditions. In addition, for those skilled in the art, without departing from the essence and scope of the present invention, various modifications or improvements to the material components and ratios in these embodiments all fall within the scope of protection required by the present invention.

[0046] Example 1

[0047] Preparation of the base film M1

[0048] Using polyvinylidene fluoride as the raw material and NMP as the organic solvent, a "PVDF / NMP / H 2 O" ternary system casting solution was prepared, and its mass ratio was controlled to be 8 / 82 / 10. After the casting solution was left standing for defoaming, it was poured onto a clean and flat glass plate and scraped into a liquid film with a thickness of 20 ± 5 μm using a scraper. Compressed air was evenly sprayed onto the prepared liquid film using a spray gun with a nozzle specification of 0.5 mm for 20 seconds. Subsequently, the treated liquid film was immediately placed in a coagulation bath (water) for 24 hours to fully complete phase inversion, and then freeze-dried to thoroughly dry it, obtaining the base film M1.

[0049] Comparative Example 1

[0050] Using polyvinylidene fluoride as the raw material and NMP as the organic solvent, a "PVDF / NMP / H 2 O" ternary system casting solution was prepared, and its mass ratio was controlled to be 8 / 82 / 10. After the casting solution was left standing for defoaming, it was poured onto a clean and flat glass plate and scraped into a liquid film with a thickness of 20 ± 5 μm using a scraper. Subsequently, the treated liquid film was immediately placed in a coagulation bath (water) for 24 hours to fully complete phase inversion, and then freeze-dried to thoroughly dry it, obtaining the base film M1'.

[0051] Example 2

[0052] Preparation of the base film M2

[0053] Using polyvinylidene fluoride as the raw material and DMF as the organic solvent, a "PVDF / DMF / H 2 O" ternary system casting solution was prepared, and its mass ratio was controlled to be 8 / 82 / 10. After the casting solution was left standing for defoaming, it was poured onto a clean and flat glass plate and scraped into a liquid film with a thickness of 20 ± 5 μm using a scraper. A water mist was evenly sprayed onto the prepared liquid film using a spray gun with a nozzle specification of 0.5 mm for 20 seconds. Subsequently, the treated liquid film was immediately placed in a coagulation bath (water) for 24 hours to fully complete phase inversion, and then freeze-dried to thoroughly dry it, obtaining the base film M2.

[0054] Comparative Example 2

[0055] Using polyvinylidene fluoride as the raw material and DMF as the organic solvent, a "PVDF / DMF / H 2 O" ternary system casting solution was prepared, and its mass ratio was controlled to be 8 / 82 / 10. After the casting solution was left standing for defoaming, it was poured onto a clean and flat glass plate and scraped into a liquid film with a thickness of 20 ± 5 μm using a scraper. Subsequently, the treated liquid film was immediately placed in a coagulation bath (water) for 24 hours to fully complete phase inversion, and then freeze-dried to thoroughly dry it, obtaining the base film M2'.

[0056] Example 3

[0057] Preparation of the base film M3

[0058] Using polyvinylidene fluoride as the raw material and DMSO as the organic solvent, a "PVDF / DMSO / H 2 O" ternary system casting solution was prepared, and its mass ratio was controlled to be 15 / 75 / 10. After the casting solution was left standing for defoaming, it was poured onto a clean and flat glass plate and scraped into a liquid film with a thickness of 20 ± 5 μm using a scraper. Ethanol was evenly sprayed onto the prepared liquid film using a spray gun with a nozzle specification of 0.5 mm for 90 seconds. Subsequently, the treated liquid film was immediately placed in a coagulation bath (water) for 24 hours to fully complete phase inversion, and then freeze-dried to thoroughly dry it, obtaining the base film M3.

[0059] Comparative Example 3

[0060] Using polyvinylidene fluoride as the raw material and DMSO as the organic solvent, a "PVDF / DMSO / H 2 O" ternary system casting solution was prepared, and its mass ratio was controlled to be 15 / 75 / 10. After the casting solution was left standing for defoaming, it was poured onto a clean and flat glass plate and scraped into a liquid film with a thickness of 20 ± 5 μm using a scraper. Subsequently, the treated liquid film was immediately placed in a coagulation bath (water) for 24 hours to fully complete phase inversion, and then freeze-dried to thoroughly dry it for standby, obtaining the base film M4'.

[0061] Example 4

[0062] Preparation of the base film M4

[0063] Using polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - HPF) as the raw material and NMP as the organic solvent, a "PVDF - HPF / NMP / H 2Prepare a casting solution of the "PVDF-HPF / NMP / H₂O" ternary system and control its mass ratio to be 8 / 82 / 10. After the casting solution is left standing to remove bubbles, pour it onto a clean and flat glass plate and use a scraper to scrape it into a liquid film with a thickness of 20 ± 5 μm. Use a spray gun with a nozzle specification of 0.5 mm to evenly spray water mist on the prepared liquid film for 60 seconds. Subsequently, immediately place the treated liquid film into a coagulation bath (water) for 24 hours to fully complete phase inversion, and use freeze-drying to thoroughly dry it to obtain the base film M4.

[0064] Comparative Example 4

[0065] Using polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HPF) as the raw material and NMP as the organic solvent, prepare a "PVDF-HPF / NMP / H₂O" ternary system casting solution and control its mass ratio to be 8 / 82 / 10. After the casting solution is left standing to remove bubbles, pour it onto a clean and flat glass plate and use a scraper to scrape it into a liquid film with a thickness of 20 ± 5 μm. Subsequently, immediately place the treated liquid film into a coagulation bath (water) for 24 hours to fully complete phase inversion, and use freeze-drying to thoroughly dry it for standby to obtain the base film M4'. 2 Prepare a casting solution of the "PVDF-HPF / NMP / H₂O" ternary system and control its mass ratio to be 8 / 82 / 10. After the casting solution is left standing to remove bubbles, pour it onto a clean and flat glass plate and use a scraper to scrape it into a liquid film with a thickness of 20 ± 5 μm. Subsequently, immediately place the treated liquid film into a coagulation bath (water) for 24 hours to fully complete phase inversion, and use freeze-drying to thoroughly dry it for standby to obtain the base film M4'.

[0066] As Figure 1 shown, the surface morphologies of the prepared base films M1 - M4 and the comparative films M1' - M4' were characterized by field emission scanning electron microscopy. Figure 1 It shows that the surfaces of the M1 - M4 base films exhibit a rough micro-nano hierarchical structure and a bicontinuous network structure; while the surfaces of the comparative base films M1' - M4' are relatively smooth and without a micro-nano hierarchical structure.

[0067] Example 5

[0068] Preparation of the fully hydrophobic film Omni-1

[0069] On the basis of the base film M1, immerse the base film M1 in a 7.5 mol / L NaOH solution for 2 hours to make the surface of the base film M1 carry hydroxyl groups. Subsequently, place the base film M1 in a 0.05% (w / v) 4-pentenoic acid dichloromethane solution for 1 hour. Immediately afterwards, place the base film M1 into a 0.5% (w / v) perfluorodecanethiol acetone solution and irradiate it with strong ultraviolet light (500 W) for 10 min under a DIC / DMAP catalytic system; thoroughly rinse the prepared film with acetone and dry it under vacuum to obtain the fully hydrophobic film Omni-1. -1 On the basis of the base film M1, immerse the base film M1 in a 7.5 mol / L NaOH solution for 2 hours to make the surface of the base film M1 carry hydroxyl groups. Subsequently, place the base film M1 in a 0.05% (w / v) 4-pentenoic acid dichloromethane solution for 1 hour. Immediately afterwards, place the base film M1 into a 0.5% (w / v) perfluorodecanethiol acetone solution and irradiate it with strong ultraviolet light (500 W) for 10 min under a DIC / DMAP catalytic system; thoroughly rinse the prepared film with acetone and dry it under vacuum to obtain the fully hydrophobic film Omni-1.

[0070] Example 6

[0071] Preparation of the fully hydrophobic film Omni-2

[0072] On the basis of the base film M1, immerse the base film M1 in a 7.5 mol / L -1In an 8-hour period in an NaOH solution, the surface of the base film M1 was made to carry hydroxyl groups. Subsequently, the base film M1 was placed in a 0.2% (w / v) 4-pentenoic acid dichloromethane solution for 4 hours. Immediately afterwards, the treated base film M1 was placed into a 2.0% (w / v) perfluorodecanethiol acetone solution, and under a DIC / DMAP catalytic system, irradiated with strong ultraviolet light (500 W) for 60 minutes; the prepared film was thoroughly rinsed with acetone and dried in vacuo to obtain the superhydrophobic film Omni-2.

[0073] Example 7

[0074] Preparation of the superhydrophobic film Omni-3

[0075] Based on the base film M3, the base film M3 was immersed in a 7.5 mol L -1 NaOH solution for 4 hours to make the surface of the base film M3 carry hydroxyl groups. Subsequently, the base film M3 was placed in a 0.2% (w / v) 4-pentenoic acid dichloromethane solution for 1 hour. Immediately afterwards, the treated base film M3 was placed into a 0.5% (w / v) perfluorodecanethiol acetone solution, and under a DIC / DMAP catalytic system, irradiated with strong ultraviolet light (500 W) for 30 minutes; the prepared film was thoroughly rinsed with acetone and dried in vacuo to obtain the superhydrophobic film Omni-3.

[0076] Example 8

[0077] Preparation of the superhydrophobic film Omni-4

[0078] Based on the base film M4, the base film M4 was immersed in a 7.5 mol L -1 NaOH solution for 4 hours to make the surface of the base film M3 carry hydroxyl groups. Subsequently, the base film M4 was placed in a 0.2% (w / v) 4-pentenoic acid dichloromethane solution for 1 hour. Immediately afterwards, the treated base film M3 was placed into a 1.0% (w / v) perfluorodecanethiol acetone solution, and under a DIC / DMAP catalytic system, irradiated with strong ultraviolet light (500 W) for 30 minutes; the prepared film was thoroughly rinsed with acetone and dried in vacuo to obtain the superhydrophobic film Omni-4.

[0079] As Figure 2 shown, compared with the surface morphologies of the base films M1 - M4, there was not much change in the surfaces of the prepared superhydrophobic films Omni-1 - Omni-4, indicating that the introduction of the low surface energy fluoride caused less change to the film surface morphology.

[0080] Example 9

[0081] The contact angles / rolling angles and liquid entry pressures (LEP) of the base membranes M1 - M4, M1' - M4' and the omniphobic membranes Omni-1 - Omni-4 were measured using a contact angle meter and a liquid entry pressure tester respectively. Specifically, the contact angles of liquids with different surface tensions (such as water, SDS solution, dichloromethane, mineral oil, ethanol) on the membrane surface were tested respectively; the rolling angles of water on different omniphobic membrane surfaces were tested. Using 3.5% NaCl solution as the stock solution, it was measured with an LEP test device. The permeate side was a certain volume of deionized water, and a conductivity meter was used to monitor the conductivity of the permeate side. During the measurement, the inlet pressure was slowly adjusted until the conductivity at the permeate side increased sharply. The pressure at this time was the LEP value.

[0082] Figure 3 Shows the contact angles / rolling angles and liquid entry pressures of the omniphobic membranes Omni-1 - Omni-4. It can be seen from the figure that all omniphobic membranes have large water contact angles (>160a), small rolling angles, and a slipping phenomenon. The liquid entry pressure is greater than 1.6 bar, showing good anti-wetting properties. On the other hand, the omniphobic membranes Omni-1 - Omni-4 have a certain resistance to liquids with different surface tensions (such as water, ethylene glycol, 140 mM SDS solution, mineral oil, and ethanol), ensuring that they are not wetted by the above liquids, reflecting omniphobicity.

[0083] From Figure 4 It can be seen that the base membranes M1 - M4 exhibit relatively high water contact angles, small rolling angles, and high liquid entry pressures. However, compared with the omniphobic membranes Omni-1 - Omni-4, the performance of water contact angle, rolling angle, and liquid entry pressure has all decreased, and they cannot resist the wetting of mineral oil and ethanol. For the comparative membranes M1' - M4', they show the worst water contact angle and liquid entry pressure, and water droplets cannot roll on the membrane surface (rolling angle > 90°). In addition, they can be easily wetted by 140 mM SDS solution, mineral oil, and ethanol. In summary, the omniphobic membranes Omni-1 - Omni-4 have excellent properties.

[0084] Example 10

[0085] The mechanical stability of the omniphobic membranes Omni-1 - Omni-4

[0086] The mechanical stability of the omniphobic membranes was evaluated by two methods: friction cycle experiment and ultrasonic treatment.

[0087] Friction cycle experiment: Fix the membrane sample on the glass slide with double-sided tape, ensuring that the functional layer of the membrane faces upward. Then, place the glass slide face down on an A4 paper and put a 200g weight on the glass slide. Push the glass slide forward by 10 cm and then backward by 10 cm to return to the origin. This is called one cycle. Measure the contact angle and sliding angle of each membrane sample after each cycle for 5, 10, 20, and 30 cycles respectively. Ultrasonic treatment: Place the superhydrophobic membrane in an ultrasonic cleaner and ultrasonicate it for 0, 90, 180, and 270 min respectively under the operating conditions of 100W / 40Hz. After drying, measure the contact angle and sliding angle of each membrane sample.

[0088] As shown in Table 1 and Table 2, for the superhydrophobic membranes Omni-1 to Omni-4, under the friction cycle experiment and ultrasonic treatment, the hydrophobicity of the membrane surface hardly changed, indicating that the superhydrophobic membranes have strong mechanical stability. In contrast, for the superhydrophobic membrane (PVDF-E, J.Membr.Sci., 2021.644, 120124) prepared by conventional methods, the surface hydrophobicity showed a significant decrease in contact angle under both the friction cycle experiment and ultrasonic treatment, indicating that the traditional PVDF-E membrane has weak mechanical stability.

[0089] Table 1 Contact angle of water on superhydrophobic membranes Omni-1 to Omni-4 before and after friction experiment

[0090]

[0091] Note: Reference 1: Li, H.J., Feng, H., Li, M.*, Zhang, X.* Engineering a covalently constructed superomniphobic membrane for robust membrane distillation. J.Membr.Sci., 2021.120124.

[0092] Table 2 Contact angle of water on superhydrophobic membranes Omni-1 to Omni-4 before and after ultrasonic treatment

[0093]

[0094] Note: Reference 1: Li, H.J., Feng, H., Li, M.*, Zhang, X.* Engineering a covalently constructed superomniphobic membrane for robust membrane distillation. J.Membr.Sci., 2021.120124.

[0095] Example 11

[0096] Application of fully hydrophobic membranes Omni-1 to Omni-4 in membrane distillation

[0097] A 3.5 wt% NaCl solution was used as the feed solution (hot side) in the membrane distillation process, and pure water was used as the permeate (cold side). The hot side and the cold side were stably controlled at 60 ± 0.5 and 20 ± 0.5 °C, respectively. The cross-flow velocities of the liquids on the hot side and the cold side were controlled at 200 and 100 mL min -1 , respectively. An analytical balance and a conductivity meter were used to record the changes in the weight and conductivity of the permeate, respectively, so as to deduce the basic membrane distillation performance of each membrane. The corresponding experiments were carried out using fully hydrophobic membranes Omni-1 to Omni-4, respectively.

[0098] Comparative Example 11

[0099] A 3.5 wt% NaCl solution was used as the feed solution (hot side) in the membrane distillation process, and pure water was used as the permeate (cold side). The hot side and the cold side were stably controlled at 60 ± 0.5 and 20 ± 0.5 °C, respectively. The cross-flow velocities of the liquids on the hot side and the cold side were controlled at 200 and 100 mL min -1 , respectively. An analytical balance and a conductivity meter were used to record the changes in the weight and conductivity of the permeate, respectively, so as to deduce the basic membrane distillation performance of each membrane. The corresponding experiments were carried out using base membranes M1 to M4, respectively.

[0100] As can be seen from Tables 3 and 4, the water fluxes of the fully hydrophobic membranes Omni-1 to Omni-4 in the MD process were slightly lower than those of their corresponding base membranes M1 to M4. This was mainly because the click chemistry step in the preparation of the fully hydrophobic membranes introduced low-surface-energy substances, which reduced the pore structure of the membranes. On the other hand, the NaCl rejection capabilities of the fully hydrophobic membranes Omni-1 to Omni-4 were slightly stronger than those of the base membranes M1 to M4. This was mainly because the introduction of low-surface-energy fluorides improved the anti-wetting property and the liquid inlet pressure of the membranes.

[0101] Table 3 Basic MD performance of fully hydrophobic membranes Omni-1 to Omni-4

[0102]

[0103] Table 4 Basic MD performance of base membranes M1 to M4

[0104]

[0105] Example 12

[0106] Anti-wetting and anti-fouling properties of fully hydrophobic membranes Omni-1 to Omni-4

[0107] Based on the operation process of Example 10, the anti-wetting and anti-fouling properties of the superhydrophobic membrane were tested. Specifically, for the anti-wetting property operation: First, a 3.5 wt% NaCl solution was selected as the stock solution. Ultra-pure water equal to the permeation amount was replenished to the stock solution every two hours to ensure that the volume of the stock solution remained unchanged. At the same time, a certain concentration of SDS surfactant was added to the stock solution to reduce the surface tension of the stock solution. The above steps were repeated until there were obvious changes in the water flux and the conductivity on the permeate side.

[0108] In the anti-fouling experiment, the stock solution was a mixture of 20 mM Na 2 SO 4 and 20 mM CaCl 2 to evaluate the resistance of the membrane to gypsum scale and silica scale. Based on the operation process of Example 10, 800 mL of the mixed solution was selected as the stock solution. When 400 mL of pure water was collected on the permeate side (corresponding to a water recovery rate of 50%), the membrane distillation process was stopped. The changes in the flux and conductivity of each membrane were observed and recorded.

[0109] Figure 5 It reflects the anti-wetting situation of the superhydrophobic membranes Omni-1 to Omni-4. It was found that all four membranes could resist solutions with an SDS concentration higher than 0.4 mM, which was better than the MD membranes reported in other literatures [J. Membr. Sci. 2020, 591, 117572; Environ. Sci. Technol., 2016, 50 12275–12282; Nat. Commun., 2019, 10, 1–9]. Figure 6 It reflects the anti-fouling performance of the superhydrophobic membranes Omni-1 to Omni-4. It was found that although the fluxes of the four membranes decreased to varying degrees, the conductivity on the permeate side could still be maintained at a low value, ensuring the cleanliness of the permeate.

[0110] Example 13

[0111] Application of the superhydrophobic membranes Omni-1 to Omni-4 in seawater desalination

[0112] Based on the operation process of Example 10, 800 mL of simulated seawater (the composition of the wastewater is listed in Table 5) was selected as the stock solution to investigate the seawater desalination ability of the superhydrophobic membranes Omni-1 to Omni-4. During the experiment, the changes in the flux and the conductivity on the permeate side during the membrane distillation process were monitored in real time. When the water recovery rate reached 50%, the experiment was stopped.

[0113] Table 5 Composition of the simulated seawater

[0114]

[0115] Figure 7Desalination performance of the fully hydrophobic membranes Omni-1 to Omni-4. As can be seen from the figure, a recovery rate of approximately 50% can be achieved with all four membranes. Although the flux decreases, the conductivity of the permeate always remains at a low level, indicating that all four membranes can achieve the regeneration of clean water.

[0116] Example 14

[0117] Application of the fully hydrophobic membranes Omni-1 to Omni-4 in shale gas wastewater treatment

[0118] Based on the operation process of Example 10, 800 mL of multi-component shale gas simulated wastewater (the wastewater composition is listed in Table 6) was selected as the stock solution to investigate the ability of the fully hydrophobic membranes Omni-1 to Omni-4 to regenerate clean water from shale gas wastewater under complex components. During the experiment, the flux and the change of the conductivity on the permeate side during the membrane distillation process were monitored in real time. When the water recovery rate reached 50%, the experiment was stopped.

[0119] Table 6 Composition of the simulated shale gas wastewater

[0120]

[0121]

[0122] Figure 8 Shows the treatment performance of the fully hydrophobic membranes Omni-1 to Omni-4 for shale gas wastewater. As can be seen from the figure, when treating shale gas wastewater with complex multi-components, although the flux decreases, the membrane performance is not lost (the conductivity is within an acceptable range) and a water recovery rate of 50% can be finally achieved.

Claims

1. Method for preparing a superhydrophobic membrane by synergistically combining spray-induced nonsolvent-induced phase inversion and click chemistry, characterized in that, the specific steps are as follows: Step 1, constructing a micro-nano hierarchical structure in the bulk: (a) Using a polymer with a polyvinylidene fluoride main structure as a raw material, completely dissolving it in an organic solvent, and then adding water, and rapidly stirring and configuring it into a ternary casting solution of "polymer-organic solvent-water"; (b) After the casting solution is left to stand and defoamed, pour it onto a clean and flat glass plate, and use a scraper to scrape the casting solution into a liquid film; (c) Use a spray gun with a nozzle specification of 0.2-0.5 mm to evenly spray the spray or compressed air onto the liquid film prepared in (b) for 20-90 s; (d) Immediately place the treated liquid film into a coagulation bath, fully complete the phase inversion, and freeze-dry it to be used as a base film; Step 2, covalently bonding a functional layer by "click chemistry": (a) Immerse the base film in a NaOH solution to hydroxylate the surface of the base film; (b) Place the surface-hydroxylated base film in a dichloromethane solution of 4-pentenoic acid, and then add a catalyst N,N'-diisopropylcarbodiimide / 4-dimethylaminopyridine to introduce a C=C double bond on the surface of the base film; (c) Place the base film with the introduced double bond in a perfluorodecanethiol acetone solution, and react under strong ultraviolet light irradiation and the action of a photoinitiator. After the reaction, rinse it thoroughly with acetone and dry it under vacuum to obtain a superhydrophobic membrane.

2. The method according to claim 1, characterized in that, in step 1(a), the polymer with a polyvinylidene fluoride main structure is selected from one or two of polyvinylidene fluoride and polyvinylidene fluoride-hexafluoropropylene copolymer; the organic solvent is selected from dimethyl sulfoxide, N,N-dimethylformamide or N-methylpyrrolidone; in the ternary casting solution of "polymer-organic solvent-water", the concentration of the polymer is 8-15 wt%, and the concentration of water is 10 wt%.

3. The method according to claim 1, characterized in that, in step 1(c), the spray is selected from water or ethanol; the spray duration is 30-60 s; in step 1(d), water is used as the coagulation bath, and the phase inversion time is more than 24 h.

4. The method according to claim 1, characterized in that, In step 2(a), the concentration of the NaOH solution is 7.5 mol / L -1 , and the soaking time is 2 to 8 hours.

5. The method according to claim 1, characterized in that, in step 2(b), in the dichloromethane solution of 4-pentenoic acid, the mass concentration of 4-pentenoic acid is 0.05-0.2%, and the soaking time of the hydroxylated base film in the dichloromethane solution of 4-pentenoic acid is 1-4 hours.

6. The method according to claim 1, characterized in that, in step 2(c), in the perfluorodecanethiol acetone solution, the mass concentration of perfluorodecanethiol is 0.5-2%.

7. The method according to claim 1, characterized in that, in step 2(c), the ultraviolet irradiation time is 10-60 min; the ultraviolet light intensity is 500 W; the photoinitiator is 2,2-dimethoxy-2-diphenylacetophenone.

8. A superhydrophobic membrane prepared by the method according to any one of claims 1-7.

9. Application of the superhydrophobic membrane according to claim 8 in membrane distillation.

10. The application according to claim 9, characterized in that, The specific application method is for membrane distillation seawater desalination or advanced treatment of shale gas wastewater and clean water regeneration.

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