Method for preparing a multi-pore high-flux hydrophobic gas permeable membrane, the prepared membrane and applications thereof

Abstract

The application relates to a preparation method of a multi-level pore high-flux hydrophobic and air-permeable membrane and application thereof, and belongs to the field of hydrophobic and air-permeable membranes. The application aims to solve the problem that ammonia flux and anti-wetting property are difficult to be considered in the prior art, in particular, the problem that multi-level pore structure and hydrophobicity are difficult to be considered in the non-solvent phase inversion method (NIPS) membrane preparation. The method combines the sacrificial porogen method and the NIPS, and the specific steps include the following: polyvinylidene fluoride and amphiphilic porogen are dissolved in an organic solvent to prepare a casting solution; after film formation by blade coating, a wet membrane is obtained by phase inversion through an ethanol-based coagulation bath; after residual solvent removal by deionized water, the porogen is selectively removed by an alcohol solution, and then the finished product is obtained through deionized water cleaning and drying. The obtained membrane has a nanoscale pore diameter on the surface and a micrometer-level finger-shaped hole in the interior, and has high anti-wetting property (contact angle > 95 degrees, liquid permeation pressure reaches 180 kPa) and high flux (ammonia flux > 80 g-N / m 2 h). The method can be widely applied to ammonia nitrogen recovery in wastewater, and realizes resource conversion of ammonia nitrogen.

Description

Technical Field

[0001] This invention belongs to the field of hydrophobic and breathable membranes, specifically relating to a method for preparing a multi-level pore size high-flux hydrophobic and breathable membrane, the prepared membrane, and its application. Background Technology

[0002] Nitrogen is an essential element for life and agricultural production, but it is also a major source of eutrophication in water bodies. Traditional Haber-Bosch nitrogen fixation and activated sludge processes consume large amounts of energy and resources, leading to greenhouse gas emissions and failing to meet sustainable development requirements. Wastewater ammonia nitrogen recovery technologies, such as chemical precipitation and physical stripping, have been widely applied. Pervaporation distillation (also known as membrane stripping or membrane vapor stripping) based on hydrophobic and permeable membranes can efficiently recover ammonia nitrogen from wastewater. Hydrophobic and permeable membranes have anti-wetting properties, allowing only gases (such as ammonia) to pass through, are less susceptible to interference from complex pollutants in water bodies, and offer advantages such as ease of automation, small footprint, and simple operation. With its high selectivity and environmental friendliness, hydrophobic and permeable membrane technology meets the needs of a circular economy and sustainable development, becoming a new alternative to traditional processes.

[0003] However, existing hydrophobic and breathable membrane technologies still face two key problems that urgently need to be solved:

[0004] Firstly, commercially available hydrophobic membranes have low flux: their flux is only 10%-50% of other deammoniation membranes (such as ion exchange membranes), leading to higher application costs and making them difficult to integrate with other processes. The difficulty in increasing ammonia flux stems from the trade-off between ammonia flux and anti-wetting properties. The uniformity of pore size structure makes it impossible to achieve both simultaneously; ammonia permeability is directly proportional to membrane pore size, while anti-wetting properties are related to the maximum pore size. Under high anti-wetting conditions, membrane pore size is typically limited to within 0.22 μm, thus hindering the improvement of ammonia flux. Membrane materials with hierarchical pore structures can provide anti-wetting properties through small pores on the membrane surface while simultaneously utilizing large pores within the membrane to provide efficient mass transfer channels, thereby solving the aforementioned technical challenges.

[0005] Secondly, non-solvent phase inversion (NIPS) membrane fabrication methods struggle to achieve both hierarchical pore structures and good hydrophobicity. While hierarchical pore structure membranes can be prepared using NIPS, amphiphilic porogens facilitate rapid diffusion of water molecules in the casting solution during the phase inversion process, thus constructing macropores within the membrane. However, porogens exhibit surface segregation in the water-casting solution system, where hydrophilic segments accumulate on the membrane surface, while hydrophobic segments become entangled with polymeric membrane material segments, impairing hydrophobicity and making the membrane material susceptible to wetting. Therefore, a method for preparing NIPS-based hierarchical pore structure hydrophobic and breathable membranes that combines hierarchical pore structure with good anti-wetting properties is urgently needed. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a method for preparing a multi-level pore size high-flux hydrophobic and breathable membrane, the prepared membrane and its application. The hydrophobic membrane prepared by the non-solvent phase inversion method has both a multi-level pore size structure and hydrophobicity, which can solve the trade-off between ammonia flux and anti-wetting properties, and can be applied to ammonia nitrogen recovery from wastewater.

[0007] To achieve the above-mentioned objectives, the technical solution adopted is as follows:

[0008] This invention discloses a method for preparing a multi-level pore size high-flux hydrophobic and breathable membrane, comprising the following steps: S1, adding polyvinylidene fluoride and an amphiphilic porogen in an organic solvent in a certain proportion, stirring to dissolve, and letting stand until the solution is free of bubbles to obtain a casting solution; S2, coating the casting solution onto a membrane preparation plate, placing it in an ethanol coagulation bath for phase inversion, and curing to obtain a wet membrane; S3, placing the cured wet membrane in deionized water to remove residual organic solvent from the membrane; S4, immersing the wet membrane treated in step S3 in an etching solution to peel off the residual amphiphilic porogen from the membrane. The etching solution is set to selectively remove the amphiphilic porogen without damaging the original structure of the membrane pores based on the difference in affinity between the membrane material and the amphiphilic porogen for the solvent; S5, immersing the membrane treated in step S4 in deionized water to remove the residual etching solution from the membrane; S6, drying the membrane treated in step S5 to obtain a multi-level pore size high-flux hydrophobic and breathable membrane.

[0009] Amphiphilic porogens possess both hydrophilic and hydrophobic segments. The hydrophilic segments have a strong affinity for water molecules, accelerating their diffusion from the coagulation bath into the casting solution, which facilitates the formation of numerous interconnected macroporous structures (such as finger pores) within the membrane. The hydrophobic segments, on the other hand, become entangled with segments of polymeric membrane materials such as polyvinylidene fluoride (PVDF), preventing the porogen from being lost due to excessive dissolution in the coagulation bath during the initial phase inversion stage. This ensures its retention time within the membrane, allowing it to stably perform its pore-forming function and guaranteeing the formation of a multi-level pore structure.

[0010] The hydrophilic segments of amphiphilic porogens accumulate on the membrane surface due to surface segregation, while the hydrophobic segments become entangled with the membrane material, resulting in porogen residues within the membrane, which impairs the membrane's hydrophobicity. Therefore, this invention addresses this issue by utilizing the difference in affinity between the membrane material (e.g., PVDF) and the porogen for the etching solution. The etching solution selectively removes the residual porogen, ultimately ensuring good hydrophobicity while preserving the multi-level pore structure.

[0011] This invention enhances pore-forming by combining an amphiphilic porogen with an ethanol coagulation bath, successfully constructing a membrane material with a significant hierarchical pore structure. Specifically, the amphiphilic porogen improves phase transformation thermodynamics and promotes macropore formation; the ethanol coagulation bath, due to its low solubility with the porogen, hinders the diffusion of the porogen into the coagulation bath, reducing its loss, ultimately resulting in nanoscale pores on the membrane surface and micrometer-scale pores inside.

[0012] Meanwhile, this invention employs a sacrificial porogen method, utilizing an etching solution (based on the difference in affinity between the membrane material and the porogen for the solvent) after film curing to selectively remove residual amphiphilic porogens within the membrane without damaging the original pore structure. This operation solves the problem of impaired membrane hydrophobicity caused by porogen surface segregation in non-solvent phase inversion (NIPS) methods. The resulting membrane not only retains its multi-level pore structure but also possesses excellent hydrophobicity (contact angle can exceed 95°).

[0013] In summary, this invention, through a combination of the sacrificial porogen method and the non-solvent phase inversion method (NIPS), achieves for the first time the preparation of a hydrophobic and breathable membrane possessing both a hierarchical pore structure and good hydrophobicity. Specifically, the amphiphilic porogen promotes the formation of a hierarchical pore structure, the ethanol coagulation bath enhances the macroporous film formation effect, and the porogen removal step eliminates hydrophobic damage. The resulting membrane has nanoscale anti-wetting micropores on its surface and micron-scale high-efficiency mass transfer macropores inside, solving the trade-off between ammonia flux and anti-wetting properties in traditional commercial membranes. This provides a high-flux, high-anti-wetting membrane material for ammonia nitrogen recovery from wastewater.

[0014] Furthermore, the etching solution is an alcohol solution, selected from isopropanol solution, n-propanol solution and methanol solution, and the immersion time is 6-10 hours.

[0015] In this invention, an alcohol etching solution selected from isopropanol solution, n-propanol solution, and methanol solution is used. With an immersion time of 6-10 hours, the difference in affinity between the membrane material and the amphiphilic porogen for the solvent can be utilized to selectively remove the residual porogen in the membrane. At the same time, while removing the porogen, the alcohol retains the multi-level pore structure of the membrane and ensures its good hydrophobicity, laying the foundation for the membrane to have both high anti-wetting properties (such as high LEP value) and high flux (such as high ammonia flux).

[0016] Furthermore, the etching solution is an isopropanol solution with a volume concentration of 35-75% (v / v).

[0017] Furthermore, in step S1, the amphiphilic porogen is a nonionic polymeric surfactant selected from one of Pluronic F127, F68, P85 and P123.

[0018] In this invention, nonionic polymeric surfactants such as Pluronic F127, F68, P85 and P123 are selected as amphiphilic porogens.

[0019] Furthermore, in step S2, the ethanol volume concentration of the ethanol coagulation bath is 15-25% (v / v), and the solidification time for the phase inversion is 15-45 min.

[0020] In this invention, a coagulation bath with an ethanol volume concentration of 15-25% (v / v) is used and the phase transformation solidification time is controlled to be 15-45 min. Because ethanol has low solubility with amphiphilic pore-forming agents, it can enhance the pore-forming effect and promote the formation of a significant multi-level structure with nanoscale pores on the membrane surface and microscale pores inside.

[0021] Further, in step S1, the mass percentage of each component in the casting solution is as follows: polyvinylidene fluoride 18-23 wt%, amphiphilic porogen 2.5-5 wt%, and the balance is organic solvent; the organic solvent is selected from one of N,N-dimethylacetamide, dimethyl sulfoxide, and N,N-dimethylformamide.

[0022] In this invention, the casting solution is prepared in the proportion of 18-23 wt% polyvinylidene fluoride, 2.5-5 wt% amphiphilic porogen, and the balance being organic solvents such as N,N-dimethylacetamide, which ensures that all components are fully dissolved and form a stable system.

[0023] Furthermore, in step S2, the thickness of the film coated by the casting solution is 200-250 μm; in steps S3 and S5, the standing time is 16-28 h.

[0024] In this invention, the coating thickness of the casting solution is controlled at 200-250 μm, which can ensure the basic morphology and structural integrity of the membrane and provide a suitable substrate for the subsequent phase transformation to form multi-level pores. The 16-28h standing time in steps S3 and S5 can fully remove the residual organic solvent and etching solution in the membrane, avoid the residual impurities from affecting the hydrophobicity and mass transfer performance of the membrane, and ensure the stability of the membrane material performance.

[0025] Furthermore, in step S6, the drying method is vacuum oven drying or vacuum freeze drying, and the drying time is 6-10 hours.

[0026] This invention also discloses a multi-level pore size high-flux hydrophobic and breathable membrane, prepared by the method disclosed herein. It exhibits significant multi-level pore size characteristics: the membrane surface displays nanoscale pores, which act as a water-repellent barrier, giving the membrane excellent anti-wetting properties; the membrane interior has micron-scale pores, forming efficient mass transfer channels to ensure high ammonia flux; and it possesses excellent hydrophobicity, with a water contact angle on the membrane surface exceeding 95°; in wastewater ammonia nitrogen recovery applications, the ammonia flux exceeds 80 g-N / m³. 2 h, compared to commercial hydrophobic and breathable membranes (30-50 g-N / m 2 h) It improves the efficiency by 1.6-2.6 times and has outstanding anti-wetting properties. The liquid inlet pressure (LEP) reaches 180 kPa. The membrane pores can remain dry during long-term continuous operation, effectively solving the technical problem of the difficulty in balancing the ammonia flux and anti-wetting properties of existing commercial membranes.

[0027] The present invention also discloses the application of a multi-level pore size high-flux hydrophobic and breathable membrane disclosed in the present invention in the resource recovery of ammonia nitrogen from wastewater.

[0028] This invention is the first to combine an ethanol coagulation bath with an amphiphilic porogen (i.e., a Prönkel-type porogen), resulting in a technical effect that promotes the formation of a favorable finger-like pore structure within the membrane (see attached diagram). Figure 2 On the one hand, using an ethanol coagulation bath alone is a common method for technicians to prepare hydrophobic and breathable membranes. However, the ethanol coagulation bath reduces film-forming kinetics, promotes the formation of dense, sponge-like pores, and inhibits large finger-like pores, which is detrimental to air permeability. On the other hand, amphiphilic porogens (Planic-type) have high solubility in water, and some will dissolve and diffuse into the coagulation bath (water) during film formation, weakening their pore-forming effect of forming large finger-like pores. When the two are combined, the relatively low solubility of the amphiphilic porogen in ethanol can be utilized to inhibit its dissolution and diffusion into the coagulation bath, enhance the pore-forming effect, promote the formation of large finger-like pores in the membrane, and improve air permeability, producing unexpected results for technicians in the membrane manufacturing field.

[0029] The beneficial effects of this invention are as follows:

[0030] 1) This invention significantly enhances the formation of hierarchical pore structures within the membrane through the synergistic effect of an amphiphilic porogen and an ethanol coagulation bath. The amphiphilic porogen improves phase transformation thermodynamics, promoting the formation of macroporous structures within the membrane; while the ethanol coagulation bath reduces the solubility of the porogen in the coagulation bath, inhibiting its diffusion and loss into the bath, thus minimizing the weakening of the pore-forming effect. This combined effect effectively promotes the formation of large finger-like pores within the membrane (see appendix). Figure 2 This provides an efficient mass transfer channel for ammonia permeation, solving the problems of dense pore structure and insufficient flux caused by loss of pore-forming agents or improper film formation kinetics in traditional film-forming methods.

[0031] 2) To address the issue of surface segregation of amphiphilic porogens leading to hydrophilic membrane surfaces in non-solvent phase inversion methods, this invention employs an etching solution (such as an isopropanol aqueous solution) for immersion treatment after film curing. The etching solution utilizes the difference in affinity between the membrane material (such as PVDF) and the porogen for the solvent to selectively remove residual porogens from the membrane without damaging its original pore structure. This process eliminates the accumulation of hydrophilic segments of the porogen on the membrane surface, increasing the membrane contact angle to over 95° (see appendix). Figure 3 This method successfully achieves compatibility between multi-level pore structure and good hydrophobicity, overcoming the technical challenge of "multi-level pore structure and hydrophobicity being mutually exclusive" in traditional methods.

[0032] 3) The hydrophobic and breathable membrane prepared by this invention has significant multi-level pore structure characteristics, with nanoscale anti-wetting pores on the surface and micron-level high-efficiency mass transfer pores inside (see appendix). Figure 4In terms of performance, the membrane's ammonia flux exceeds 80 g-N / m³. 2 h is a commercially available hydrophobic and breathable membrane (30-50 g-N / m 2 1.6-2.6 times (with h) Figure 5 This significantly improves ammonia nitrogen recovery efficiency; simultaneously, the membrane's anti-wetting properties are significantly enhanced, with the liquid inlet pressure (LEP value) reaching 180 kPa (with attached...). Figure 5 The membrane pores can remain dry during long-term continuous experiments, effectively solving the problems of easy wetting failure and low flux leading to high application costs in commercial membranes, and providing efficient and stable technical support for the resource recovery of ammonia nitrogen from wastewater.

[0033] The following detailed description of the preparation method of the multi-level pore size high-flux hydrophobic and breathable membrane of the present invention, the prepared membrane, and its applications are disclosed in conjunction with the embodiments shown in the accompanying drawings. Attached Figure Description

[0034] Figure 1 This is a flowchart illustrating the preparation method of the present invention;

[0035] Figure 2 To compare the membrane pore structure and nitrogen permeability of membrane materials prepared without sacrificial pore-forming agent (Comparative Example 1), without pore-forming agent (Comparative Example 2), and without ethanol coagulation bath (Comparative Example 3), (1) is an electron microscope image of the surface and cross section of the membrane, and (2) is a bar graph of nitrogen permeability.

[0036] Figure 3 To compare the differences in contact angle and surface functional groups of membrane materials prepared by the sacrificial porogen-NIPS method (Example) and membrane materials prepared without sacrificial porogen (Comparative Example 1), where (1) is an infrared spectrum and (2) is a contact angle histogram;

[0037] Figure 4 To compare the pore structure of the membrane material prepared by the sacrificial pore-forming agent-NIPS method (example) with that of the commercially available PVDF hydrophobic membrane with uniform pore structure (Comparative Example 4 and Comparative Example 5), where (1) is a surface electron microscope image of the membrane and (2) is a cross-sectional electron microscope image of the membrane;

[0038] Figure 5 To compare the ammonia recovery performance of the membrane material prepared by the sacrificial pore-forming agent-NIPS method (example) and the commercially available PVDF hydrophobic membrane with uniform pore size structure (Comparative Example 4 and Comparative Example 5), (1) is a bar chart of ammonia flux and (2) is a bar chart of wettability (LEP value). Detailed Implementation

[0039] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other.

[0040] like Figure 1 As shown, this invention discloses a method for preparing a multi-level pore size high-flux hydrophobic and breathable membrane, comprising the following steps: S1, adding polyvinylidene fluoride and an amphiphilic pore-forming agent to an organic solvent in a certain proportion, stirring to fully dissolve them, and allowing the solution to stand until no bubbles are present to obtain a casting solution; S2, coating the casting solution onto a film-forming plate to form a thin film, and then placing it in an ethanol coagulation bath for phase inversion to solidify and obtain a wet membrane; S3, placing the solidified wet membrane in deionized water to remove residual organic solvent from the membrane; S4, after... In step S3, the membrane is immersed in an etching solution to remove residual amphiphilic porogens from the membrane. The etching solution is designed to selectively remove the amphiphilic porogens from the membrane without damaging the original pore structure, based on the difference in affinity between the membrane material and the amphiphilic porogens for the solvent. In step S5, the membrane treated in step S4 is placed in deionized water to stand and remove residual etching solution from the membrane. In step S6, the membrane treated in step S5 is dried to obtain a multi-level pore size high-flux hydrophobic and breathable membrane.

[0041] Based on the disclosed steps, this invention combines the "sacrificial porogen method" with the non-solvent phase inversion method (NIPS) to achieve for the first time the preparation of a hydrophobic and breathable membrane that possesses both a hierarchical pore structure and good hydrophobicity. Specifically, the amphiphilic porogen promotes the formation of a hierarchical pore structure, the ethanol coagulation bath enhances the macroporous film formation effect, and the porogen removal step eliminates hydrophobic damage. The resulting membrane has nanoscale anti-wetting micropores on its surface and micron-scale high-efficiency mass transfer macropores inside, solving the trade-off between ammonia flux and anti-wetting properties in traditional commercial membranes. This provides a high-flux, high-anti-wetting membrane material for ammonia nitrogen recovery from wastewater.

[0042] In this embodiment of the invention, the amphiphilic porogen selected in step S1 is a nonionic polymeric surfactant, selected from one of Pluronic F127, F68, P85, and P123. In this embodiment, nonionic polymeric surfactants such as Pluronic F127, F68, P85, and P123 are selected as amphiphilic porogens. Their hydrophilic segments can accelerate the diffusion of water molecules during the phase transformation process, promoting the formation of a large number of interconnected macroporous structures (such as finger pores) in the membrane, laying the foundation for improving mass transfer efficiency; the hydrophobic segments can entangle with the segments of the PVDF membrane material, avoiding excessive dissolution and loss of the porogen, ensuring its stable pore-forming effect, and guaranteeing the formation of multi-level pore structures. At the same time, this type of porogen has strong pore-forming ability, and its combination with an ethanol coagulation bath can further enhance the pore-forming effect.

[0043] In a preferred embodiment, the amphiphilic porogen of step S1 is Pluronic F127.

[0044] In this embodiment of the invention, the mass percentages of each component in the casting solution in step S1 are as follows: polyvinylidene fluoride (PVDF) 18-23 wt%, amphiphilic porogen 2.5-5 wt%, and the balance being an organic solvent; the organic solvent is selected from one of N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), or N,N-dimethylformamide (DMF). In this embodiment, the casting solution is prepared according to the ratio of 18-23 wt% PVDF, 2.5-5 wt% amphiphilic porogen, and the balance being an organic solvent such as N,N-dimethylacetamide. This ensures that each component is fully dissolved and forms a stable system. The appropriate proportion of PVDF provides the basic framework for the membrane, the amphiphilic porogen effectively plays a pore-forming role, and the selected organic solvent ensures the rheological and phase transformation properties of the casting solution, laying the foundation for the subsequent formation of a membrane material with both multi-level pore structure and good hydrophobicity.

[0045] In a preferred embodiment, the mass percentages of each component in the casting solution in step S1 are as follows: 22 wt% polyvinylidene fluoride (PVDF), 3.8 wt% amphiphilic porogen (Pluronic F127), and the balance being an organic solvent (DMAc).

[0046] In this embodiment of the invention, step S2 uses a coagulation bath with an ethanol volume concentration of 15-25% (v / v), and the solidification time for phase inversion is set to 15-45 min. In this embodiment, the ethanol coagulation bath in this concentration range has a synergistic effect with the amphiphilic porogen (such as Pluronic acid).

[0047] In a preferred embodiment, step S2 uses a coagulation bath with an ethanol volume concentration of 21% (v / v) and the solidification time for phase transformation is set to 25 min.

[0048] In this embodiment of the invention, step S2 involves coating the casting solution onto a glass plate to form a thin film with a thickness of 200-250 μm. In this embodiment, this coating thickness range ensures the basic morphology and structural integrity of the film, providing a suitable substrate for subsequent phase transformation to form multi-level pores.

[0049] In a preferred embodiment, step S2 involves coating the casting solution onto a glass plate to form a film with a thickness of 210 μm.

[0050] In this embodiment of the invention, the etching solution selected in step S4 is an alcohol solution, chosen from isopropanol solution, n-propanol solution, and methanol solution, and the immersion time is 6-10 hours. In this embodiment, by using an alcohol etching solution selected from isopropanol solution, n-propanol solution, and methanol solution, and immersing for 6-10 hours, the difference in affinity between the membrane material and the amphiphilic porogen for the solvent can be utilized to selectively remove residual porogens (including hydrophilic segments enriched on the surface and hydrophobic segments entangled with the membrane material) from the membrane.

[0051] In a preferred embodiment, the etching solution in step S4 is an isopropanol solution with a volume concentration of 35-75% (v / v).

[0052] In this embodiment of the invention, the settling time for both steps S3 and S5 is set to 16-28 hours. In this embodiment, this settling time range can fully remove residual organic solvents and etching solutions from the membrane, avoiding the impact of impurities on the hydrophobicity and mass transfer performance of the membrane, and ensuring the stability of the membrane material performance.

[0053] In a preferred embodiment, the settling time for both steps S3 and S5 is set to 20 hours.

[0054] In this embodiment of the invention, the drying method in step S6 is vacuum oven drying or vacuum freeze drying, and the drying time is set to 6-10 hours. In this embodiment, using vacuum oven drying or vacuum freeze drying for 6-10 hours can avoid the collapse of the pore structure of the membrane material due to sudden temperature changes or rapid evaporation of moisture during the drying process, effectively preserve the multi-level pore size characteristics of the membrane, and ensure that the membrane surface and interior are thoroughly dried, maintain good hydrophobicity, and ensure the stability of the membrane's anti-wetting properties and mass transfer efficiency.

[0055] In a preferred embodiment, step S6 employs vacuum freeze drying, with the drying time set to 8 hours.

[0056] This invention also discloses a multi-level pore size high-flux hydrophobic and breathable membrane, prepared by the method disclosed herein. It exhibits significant multi-level pore size characteristics: the membrane surface displays nanoscale pores (down to 20 nm), which can act as a water-repellent barrier, giving the membrane excellent anti-wetting properties; the membrane interior has micron-scale pores (up to 10 μm), forming efficient mass transfer channels to ensure high ammonia flux, exhibiting excellent hydrophobicity, with a water contact angle on the membrane surface exceeding 95°; in wastewater ammonia nitrogen recovery applications, the ammonia flux exceeds 80 g-N / m³. 2 h, compared to commercial hydrophobic and breathable membranes (30-50 g-N / m 2 h) It improves the efficiency by 1.6-2.6 times and has outstanding anti-wetting properties. The liquid inlet pressure (LEP) reaches 180 kPa. The membrane pores can remain dry during long-term continuous operation, effectively solving the technical problem of the difficulty in balancing the ammonia flux and anti-wetting properties of existing commercial membranes.

[0057] The present invention also discloses the application of a multi-level pore size high-flux hydrophobic and breathable membrane disclosed in the present invention in the resource recovery of ammonia nitrogen from wastewater.

[0058] To verify the performance of the multi-level pore size high-flux hydrophobic and breathable membrane prepared by the present invention, the following examples and several comparative examples are used for comparison and illustration. The examples are prepared by using a sacrificial pore-forming agent combined with NIPS. Comparative examples 1 to 3 are control groups without sacrificial pore-forming agent, without pore-forming agent, and without ethanol coagulation bath, respectively. Comparative examples 4 and 5 are commercially available uniform pore size hydrophobic and breathable membranes. By comparing and analyzing the structure and performance of the membranes, the advantages of the present invention are highlighted.

[0059] Example

[0060] I. Preparation of Casting Solution

[0061] Polyvinylidene fluoride (PVDF) powder (molecular weight 600 kDa) and an amphiphilic porogen (Pluronic F127) were added to the organic solvent N,N-dimethylacetamide (DMAc) in a certain proportion: PVDF accounted for 22 wt% of the total mass of the casting solution, Pluronic F127 accounted for 3.8 wt%, and the balance was DMAc. The mixture was then heated and stirred until the PVDF and the amphiphilic porogen were completely dissolved. After dissolution, the solution was allowed to stand until no bubbles were present, thus obtaining a uniform casting solution.

[0062] II. Preparation of hydrophobic and breathable membranes

[0063] The above casting solution was uniformly coated onto a glass plate to form a film with a thickness of 210 μm. The coated film was then transferred to an ethanol coagulation bath for phase inversion curing: the volume concentration of the ethanol coagulation bath was 21% (v / v), and the curing time was 25 min, resulting in a wet film.

[0064] III. Post-processing

[0065] 1) Residual solvent removal: After curing, the wet film is removed from the ethanol coagulation bath and placed in deionized water for 20 hours to completely remove the residual organic solvent (DMAc) in the film.

[0066] 2) Removal of pore-forming agent: The membrane that has been treated with deionized water is transferred to an isopropanol aqueous solution for immersion in an isopropanol solution with a volume concentration of 50% (v / v) for 6 hours to selectively remove residual Pluronic F127 pore-forming agent from the membrane.

[0067] 3) Residual removal agent removal: After the pore-forming agent has been removed, the membrane is placed in deionized water and left to stand for 20 hours to remove the residual isopropanol solution in the membrane.

[0068] 4) Drying: The membrane after the above treatment is placed in a vacuum freeze dryer and dried for 8 hours to finally obtain a high-flux hydrophobic and breathable membrane with a multi-level pore structure.

[0069] Comparative Example 1

[0070] I. Preparation of Casting Solution

[0071] Polyvinylidene fluoride (PVDF) powder (molecular weight 600 kDa) and an amphiphilic porogen (Pluronic F127) were added to the organic solvent N,N-dimethylacetamide (DMAc) in a specific ratio: PVDF accounted for 22 wt% of the total mass of the casting solution, Pluronic F127 accounted for 3.8 wt%, and the balance was DMAc. The mixture was heated and stirred until the PVDF and porogen were completely dissolved. After dissolution, the solution was allowed to stand until no bubbles were present, resulting in a uniform casting solution.

[0072] II. Preparation of hydrophobic and breathable membranes

[0073] The above casting solution was uniformly coated onto a glass plate to form a film with a thickness of 210 μm. The coated film was then transferred to a deionized water solution for phase inversion and curing for 25 min to obtain a wet film.

[0074] III. Post-processing

[0075] The cured wet membrane was removed from the deionized water coagulation bath and placed in deionized water for 20 hours to remove residual organic solvent (DMAc) from the membrane. The membrane was then placed in a vacuum freeze dryer and dried for 8 hours to finally obtain the hydrophobic and breathable membrane of Comparative Example 1.

[0076] Comparative Example 2

[0077] I. Preparation of Casting Solution

[0078] Polyvinylidene fluoride (PVDF) powder (molecular weight 600 kDa) was added to the organic solvent N,N-dimethylacetamide (DMAc) in a certain proportion, wherein PVDF accounted for 22 wt% of the total mass of the casting solution, and the balance was DMAc. The mixture was heated and stirred until the PVDF was completely dissolved. After dissolution, the solution was allowed to stand until no bubbles were added, thus obtaining a uniform casting solution (without the addition of pore-forming agents).

[0079] II. Preparation of hydrophobic and breathable membranes

[0080] The above casting solution was uniformly coated onto a glass plate to form a film with a thickness of 210 μm. The coated film was then transferred to an ethanol coagulation bath for phase inversion curing: the volume concentration of the ethanol coagulation bath was 21% (v / v), and the curing time was 25 min, resulting in a wet film.

[0081] III. Post-processing

[0082] The cured wet membrane was removed from the ethanol coagulation bath and placed in deionized water for 20 hours to remove residual organic solvent (DMAc) from the membrane. The membrane was then placed in a vacuum freeze dryer and dried for 8 hours to finally obtain the hydrophobic and breathable membrane of Comparative Example 2.

[0083] Comparative Example 3

[0084] I. Preparation of Casting Solution

[0085] Polyvinylidene fluoride (PVDF) powder (molecular weight 600 kDa) and an amphiphilic porogen (Pluronic F127) were added to the organic solvent N,N-dimethylacetamide (DMAc) in a specific ratio: PVDF accounted for 22 wt% of the total mass of the casting solution, Pluronic F127 accounted for 3.8 wt%, and the balance was DMAc. The mixture was heated and stirred until the PVDF and porogen were completely dissolved. After dissolution, the solution was allowed to stand until no bubbles were present, resulting in a uniform casting solution.

[0086] II. Preparation of hydrophobic and breathable membranes

[0087] The above casting solution was uniformly coated onto a glass plate to form a film with a thickness of 210 μm. The coated film was then transferred to a deionized water solution for phase inversion and curing for 25 min to obtain a wet film.

[0088] III. Post-processing

[0089] 1) Residual solvent removal: After curing, the wet film is removed from the deionized water coagulation bath and placed in deionized water for 20 hours to remove residual organic solvent (DMAc) in the film.

[0090] 2) Pore-forming agent removal: The membrane, after being left to stand in deionized water, was transferred to an isopropanol aqueous solution with a volume concentration of 50% (v / v) and a soaking time of 6 hours to remove residual Pluronic F127 pore-forming agent from the membrane.

[0091] 3) Residual removal agent removal: After the pore-forming agent has been removed, the membrane is placed in deionized water and left to stand for 20 hours to remove the residual isopropanol solution in the membrane.

[0092] 4) Drying: The membrane after the above treatment was placed in a vacuum freeze dryer and dried for 8 hours to finally obtain the hydrophobic and breathable membrane of Comparative Example 3.

[0093] Comparative Example 4 and Comparative Example 5 are commercial hydrophobic and breathable membranes with nominal pore sizes of 0.22 μm and 0.1 μm, respectively.

[0094] Figure 2 The pore structure and nitrogen permeability of membrane materials prepared without sacrificial porogens (Comparative Example 1), without porogens (Comparative Example 2), and without ethanol coagulation bath (Comparative Example 3) were compared: Figure 2 As shown in (1), the membrane of Comparative Example 1 contains a large number of vertical finger-shaped macropores and has a relatively higher surface porosity, while the proportion of finger-shaped pores in the membranes of Comparative Example 2 and Comparative Example 3 is significantly lower; Figure 2 As shown in (2), the nitrogen permeability of Comparative Example 1 is much higher than that of Comparative Examples 2 and 3. This result indicates that the membrane fabrication method described in this invention can effectively promote the formation of a multi-level pore structure within the membrane, thereby obtaining a membrane material with good air permeability.

[0095] Figure 3 The contact angle and differences in membrane surface functional groups (Fourier transform infrared spectroscopy) were compared between membrane materials prepared by the sacrificial porogen-NIPS method (Example) and membrane materials prepared without sacrificial porogen (Comparative Example 1): For example... Figure 3 As shown in (1), compared with Comparative Example 1, the infrared spectrum curve of the embodiment does not contain the characteristic peak of the COC functional group representing Pluronic F127, indicating that the embodiment successfully removed the pore-forming agent from the film through the etching process; Figure 3 (2) As shown, the membrane surface contact angle of the embodiment is as high as 95°, which is 80% higher than that of Comparative Example 1. This shows that the membrane preparation method of the embodiment can produce a hydrophobic and breathable membrane with good hydrophobicity.

[0096] Figure 4 The pore structures of membrane materials prepared by the sacrificial pore-forming agent-NIPS method (examples) and commercially available uniform PVDF hydrophobic membranes (Comparative Examples 4 and 5) were compared (observed by scanning electron microscopy): For example... Figure 4 As shown in (1), the surface pore size of the membrane in the embodiment is significantly smaller than that in Comparative Examples 4 and 5; in contrast, as Figure 4 (2) As shown, the internal pore size of the membrane in the example is significantly higher than that in Comparative Examples 4 and 5. This comparison shows that the hydrophobic and breathable membrane prepared in the example is different from the commercially available hydrophobic and breathable membranes with uniform pore size in Comparative Examples 4 and 5, and has significant multi-level pore size structure characteristics.

[0097] Figure 5 The performance of membrane materials prepared by the sacrificial pore-forming agent-NIPS method (examples) and commercially available uniform-pore-structure PVDF hydrophobic membranes (Comparative Examples 4 and 5) in ammonia recovery applications was compared: Figure 5 As shown in (1), the ammonia flux in the example is as high as 80 g / Nm³. 2 h was significantly higher than that of Comparative Examples 4 and 5, indicating that the hydrophobic and breathable membrane prepared in the examples has higher ammonia permeability; Figure 5 (2) As shown, the liquid inlet pressure (LEP value) of the embodiment is 180 kPa, which is significantly higher than that of Comparative Example 4 and Comparative Example 5, indicating that the hydrophobic and breathable membrane prepared in the embodiment has higher anti-wetting performance.

[0098] Compared with previous technologies, the present invention has the following advantages:

[0099] 1. The amphiphilic porogen and ethanol solution coagulation bath used in this invention enhance pore formation, imparting a good hierarchical pore structure to the membrane material. Specifically, the porogen can improve phase transformation thermodynamics, thus promoting macropore formation. However, some porogens dissolve and diffuse into the coagulation bath during phase transformation, weakening the pore-forming effect. This invention adds ethanol to the coagulation bath, which has relatively low solubility with the porogen, hindering the aforementioned dissolution and diffusion process, thereby reducing porogen loss and enhancing pore formation. (See appendix) Figure 2 ).

[0100] 2. After the film is cured, the pore-forming agent within the film is removed by a simple immersion treatment with an etching solution, ultimately obtaining a hydrophobic and breathable membrane with a hierarchical pore structure and good hydrophobicity. The specific principle is as follows: After curing, the pore-forming agent remains within the film. Simultaneously, due to surface segregation, its internal hydrophilic segments arrange themselves on the film surface, making the film material hydrophilic. The etching solution utilizes the difference in solvent affinity between the film material and the pore-forming agent to selectively remove it from the film without damaging the original pore structure, thus ultimately producing a hydrophobic and breathable membrane with a hierarchical pore structure and good hydrophobicity. (See attached diagram) Figure 3 ).

[0101] 3. Results shown in the attached figure. The hydrophobic and breathable membrane material prepared by this method has a multi-level pore structure (see attached figure). Figure 4 It also exhibits good hydrophobicity (contact angle greater than 95°). Figure 3 Ammonia flux exceeds 80 g-N / m³ 2h, higher than commercially available breathable and hydrophobic membrane materials (30-50 g-N / m). 2 1.6-2.6 times (with h) Figure 5 Its anti-wetting performance is also superior to commercially available breathable and hydrophobic membrane materials (LEP value reaches 180 kPa, and the membrane pores remain dry during long-term continuous experiments).

[0102] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a multi-level pore size high-flux hydrophobic and breathable membrane, characterized in that, Includes the following steps: S1, add polyvinylidene fluoride and amphiphilic porogen to an organic solvent in proportion, stir to dissolve, and let stand until the solution is free of bubbles to obtain casting solution; S2, the casting solution is scraped onto the film-forming plate and placed in an ethanol coagulation bath for phase inversion and solidification to obtain a wet film; S3, place the cured wet film in deionized water to stand and remove residual organic solvents from the film; S4, Immerse the wet film after step S3 in an etching solution to remove the residual amphiphilic pore-forming agent in the film. The etching solution is set to selectively remove the amphiphilic pore-forming agent without damaging the film pore structure based on the difference in affinity between the film material and the pore-forming agent for the solvent. S5, Immerse the membrane treated in step S4 in deionized water to remove residual etching solution from the membrane; S6. After drying the membrane treated in step S5, the multi-level pore size high-flux hydrophobic and breathable membrane is obtained. The amphiphilic porogen mentioned in step S1 is a nonionic polymeric surfactant selected from one of Pluronic F127, F68, P85 and P123; The etching solution mentioned in steps S4 and S5 is an alcohol solution, selected from isopropanol solution, n-propanol solution and methanol solution.

2. The preparation method according to claim 1, characterized in that, The immersion time in the etching solution is 6-10 hours.

3. The preparation method according to claim 2, characterized in that, The etching solution is an isopropanol solution with a volume concentration of 35-75% (v / v).

4. The preparation method according to claim 1, characterized in that, The ethanol volume concentration of the ethanol coagulation bath in step S2 is 15-25% (v / v), and the solidification time of the phase inversion is 15-45 min.

5. The preparation method according to claim 1, characterized in that, In step S1, the mass percentage of each component in the casting solution is as follows: polyvinylidene fluoride 18-23 wt%, amphiphilic porogen 2.5-5 wt%, and the balance is an organic solvent; the organic solvent is selected from one of N,N-dimethylacetamide, dimethyl sulfoxide, or N,N-dimethylformamide.

6. The preparation method according to claim 5, characterized in that, The thickness of the film formed by the casting solution in step S2 is 200-250 μm; the standing time in steps S3 and S5 is 16-28 h.

7. The preparation method according to claim 6, characterized in that, The drying method in step S6 is vacuum oven drying or vacuum freeze drying, and the drying time is 6-10 hours.

8. A multi-level pore size high-flux hydrophobic and breathable membrane, characterized in that, It is prepared by any one of claims 1-7.

9. The application of a multi-level pore size high-flux hydrophobic and breathable membrane according to claim 8 in the resource recovery of ammonia nitrogen from wastewater.

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