Method for preparing self-adsorbing hydrophobic gas permeable membrane, prepared membrane and applications thereof

Abstract

The application discloses a preparation method of a self-adsorptive hydrophobic and breathable membrane, the prepared membrane and application of the membrane. In view of the problems of the ammonia flux improvement of the existing hydrophobic and breathable membrane, the dependence on external additives and the difficulty in balancing adsorption and anti-wetting, the method comprises the following steps: polyvinylidene fluoride, amphiphilic pore-forming agent and organic solvent are mixed in proportion to prepare a casting solution; after the casting solution is coated into a film, the film is placed in a coagulation bath containing inorganic salt and fluoroalkylsilane (FAS) for phase inversion; the inorganic salt induces the polyvinylidene fluoride segment to change into a beta configuration through ion-dipole interaction and reduces the loss of the pore-forming agent; meanwhile, the FAS and the hydrophilic segment of the pore-forming agent are dehydrated and condensed to realize in-situ fluorination grafting; after desolventizing, de-pore-forming, desolventizing and drying, the target membrane material is finally obtained. The membrane has self-adsorption function (the proportion of beta-PVDF is greater than 90% and the adsorption capacity is greater than 35 mmol / g) without external additives, has good hydrophobicity (the contact angle is greater than 95°), the ammonia flux is more than 10 times that of a non-adsorptive membrane, and is suitable for resource recycling of ammonia-nitrogen wastewater.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of hydrophobic gas permeable membranes, and particularly relates to a preparation method of a self-adsorptive hydrophobic gas permeable membrane, the prepared membrane and application thereof. BACKGROUND

[0002] Nitrogen and its compounds are closely related to energy and water environment, and excessive discharge into natural water bodies will lead to eutrophication. In the face of resource and energy shortage, traditional Haber-Bosch artificial nitrogen fixation and activated sludge denitrification not only consume a large amount of energy and resources, but also emit greenhouse gases, which does not meet the requirements of sustainable development. In contrast, the way of recovering nitrogen resources instead of removing in wastewater meets the needs of sustainable development. Ammonia nitrogen recovery technologies in wastewater include chemical precipitation and physical stripping, among which the pervaporation based on hydrophobic gas permeable membranes only allows gas (such as ammonia) to pass through, is not disturbed by impurities, and has the advantages of automation, small occupation and easy operation. This technology efficiently recovers ammonia nitrogen, realizes the conversion from pollutants to resources, and meets the needs of circular economy and sustainable development.

[0003] However, the existing hydrophobic gas permeable membrane technology still has the following problems to be solved in the application of ammonia nitrogen recovery in wastewater:

[0004] First, the bottleneck of ammonia flux improvement: the ratio of the aerodynamic diameter of ammonia molecules to the size of the membrane pores (Knudsen number) can divide the mass transfer of ammonia molecules in the membrane pores into Knudsen diffusion, molecular diffusion and transition diffusion. The mass transfer resistance order is Knudsen diffusion < transition diffusion < molecular diffusion. Although increasing the membrane pore size can improve the ammonia flux, when the pore size of the commercial membrane increases to 0.22 μm, the ammonia mass transfer rate no longer increases significantly. Therefore, it is urgent to develop an adsorptive functional hydrophobic gas permeable membrane to break through the pore size limitation and improve the ammonia recovery efficiency.

[0005] Second, the adsorption function of the hydrophobic gas permeable membrane depends on external additives: the hydrophobic gas permeable membrane material is mostly fluorine-containing polymer, and the interaction between the material and ammonia molecules is weak, so the adsorption function usually depends on external additives (such as carbon nanomaterials, metal organic framework compounds, etc.), but such additives are unstable in combination with the membrane material and are easy to lose, resulting in the attenuation of the membrane performance. At the same time, the adsorptive material increases the cost and complexity of membrane preparation. Therefore, it is urgent to develop a self-adsorptive hydrophobic gas permeable membrane that does not depend on external additives.

[0006] Thirdly, it is difficult to balance the adsorption function and the anti-wetting property: common crystal structure types of PVDF include alpha, beta and gamma, etc. Among them, the C-F bond and C-H bond in the beta-PVDF crystal structure are arranged on both sides of the C chain, so that it has higher polarity. There is hydrogen bond interaction force between beta-PVDF and ammonia molecules, so that the beta-PVDF membrane material has adsorption function to ammonia molecules. Therefore, developing a polar hydrophobic gas permeable membrane material with high beta-PVDF content is expected to make the hydrophobic gas permeable membrane have adsorption function. However, high beta-PVDF content is prone to hydrophilicity, and hydrophilic PVDF is difficult to maintain stable anti-wetting property in the process of ammonia recovery from wastewater. Therefore, it is the key to realize efficient ammonia recovery to maintain high beta-PVDF content while ensuring that the membrane material has stable hydrophobicity. SUMMARY

[0007] The present application aims to solve the problems that the ammonia flux of the existing hydrophobic gas permeable membrane is limited, the adsorption function depends on external additives and has poor stability, and high beta-PVDF content and anti-wetting property are difficult to balance, and provides a preparation method of a self-adsorbing hydrophobic gas permeable membrane, the prepared membrane and its application. Through the method, the membrane material can have self-adsorption function without external additives, and at the same time has high ammonia flux and stable anti-wetting property.

[0008] In order to achieve the above application purposes, the technical solutions adopted are as follows:

[0009] The application discloses a preparation method of a self-adsorbing hydrophobic gas permeable membrane, comprising the following steps: S1, adding polyvinylidene fluoride and amphiphilic porogen into an organic solvent according to a proportion, stirring to fully dissolve the polyvinylidene fluoride and the amphiphilic porogen, and standing until there is no air bubble in the solution to prepare a casting solution; S2, coating the casting solution prepared in the step S1 on a membrane preparation plate, and then placing it in a coagulation bath for phase inversion to solidify and form a wet membrane; the coagulation bath is an inorganic salt solution containing fluoroalkyl silane, wherein: the inorganic salt is a substance that induces the polyvinylidene fluoride segment to transform into a beta configuration through ion-dipole interaction and reduces the dissolution and diffusion of the amphiphilic porogen; the fluoroalkyl silane is a substance that dehydrates and condenses with the oxygen-containing functional groups of the hydrophilic segment of the amphiphilic porogen to realize in-situ fluorinated grafting; S3, taking out the wet membrane after solidification in the step S2, and placing it in deionized water for standing to remove the residual organic solvent in the membrane; S4, placing the membrane treated in the step S3 in a removal solution for soaking to remove the residual amphiphilic porogen in the membrane; S5, placing the membrane treated in the step S4 in deionized water again for standing to remove the residual removal solution in the membrane; and S6, vacuum drying the membrane treated in the step S5 to prepare a self-adsorbing high-flux hydrophobic gas permeable membrane.

[0010] The in-situ fluorination grafting refers to a process in which a fluorine alkyl silane (FAS) and a hydrophilic segment of an amphiphilic pore-forming agent in a coagulation bath phase inversion process of the hydrophobic gas permeable membrane chemically reacts with an oxygen-containing functional group, and a fluorine-containing group is directly covalently combined into the membrane surface and pore structure through dehydration condensation.

[0011] In the present application, the salt ions of inorganic salts induce the polyvinylidene fluoride segment to transform into a beta-polyvinylidene configuration, the polarity of the beta-polyvinylidene is used to form a hydrogen bond interaction with ammonia molecules, and an ammonia gas rapid mass transfer channel is constructed, thereby solving the problem that the ammonia flux is difficult to improve due to the upper limit of the pore size regulation of the conventional hydrophobic gas permeable membrane, and significantly improving the ammonia permeation performance.

[0012] Secondly, the conventional hydrophobic gas permeable membrane is mainly composed of fluorine-containing polymers, and such materials mostly exhibit non-polar characteristics. Ammonia molecules belong to molecules with strong polarity, and the interaction force between non-polar materials and polar ammonia molecules is extremely small, so the conventional hydrophobic gas permeable membrane has almost no adsorption function for ammonia molecules. To solve this problem, the conventional technology usually needs to add inorganic materials (such as metal organic framework compounds, carbon nanomaterials, etc.) with adsorption function for ammonia molecules to the membrane to endow the membrane with adsorption capacity. However, the introduction of such external additives not only increases the cost of membrane preparation, but also has the problem of easy loss, resulting in unstable adsorption performance of the membrane.

[0013] The present application changes the polarity of the hydrophobic gas permeable membrane from the root by innovative means, so that it is changed from non-polar to polar characteristics, and the adsorption of ammonia molecules is realized through polar-polar interaction force. Specifically, inorganic salt ions are added in the coagulation bath during membrane preparation, and the salt ions can induce the polyvinylidene fluoride (PVDF) molecular segment to transform from alpha form to beta form through ion-dipole interaction. The beta-PVDF has strong polarity, and the hydrophobic gas permeable membrane composed of the beta-PVDF has strong polarity, thereby obtaining the adsorption function for ammonia molecules. Without relying on external additives such as metal organic framework compounds and carbon nanomaterials, the adsorption function for ammonia molecules is endowed to the membrane only by the configuration transformation of polyvinylidene fluoride itself, thereby avoiding the problem of unstable performance of the membrane caused by the easy loss of external additives, reducing the cost of membrane preparation materials, and simplifying the preparation steps.

[0014] In addition, while the inorganic salt induces the beta-PVDF to form a high-polarity structure, the FAS realizes in-situ fluorination grafting through the dehydration condensation reaction with the hydrophilic segment of the amphiphilic pore-forming agent, introduces low-surface-energy fluorine-containing groups into the membrane surface and pores, solves the problem of poor hydrophilicity and wet resistance of the high-beta-PVDF membrane material, and makes the membrane have good adsorption function and stable wet resistance, thereby ensuring long-term and effective operation in the ammonia nitrogen recovery of wastewater.

[0015] In the present application, the amphiphilic porogen molecule structure contains a hydrophilic segment, which is rich in oxygen-containing functional groups (such as hydroxyl groups), and these hydroxyl groups are the key sites for in-situ fluorination grafting of FAS. In the traditional membrane preparation process, if the coagulation bath in the phase inversion stage does not contain inorganic salt, the hydrophilic segment of the porogen is easy to hydrate with water molecules, leading to its dissolution and diffusion from the membrane body to the coagulation bath and loss, resulting in a decrease in the number of residual porogens in the membrane, a shortage of hydroxyl sites available for FAS grafting, and thus affecting the fluorination grafting effect.

[0016] However, after introducing inorganic salt into the coagulation bath in the present application, the salt ions can significantly inhibit the hydration of the hydrophilic segment of the porogen by interacting with it, hindering its dissolution and diffusion process into the coagulation bath. This effect allows more amphiphilic porogens to remain in the membrane, thereby providing more abundant hydroxyl grafting sites for FAS. More low-surface-energy fluorine-containing groups are successfully introduced into the membrane surface and pore structure, ensuring the hydrophobicity and wet resistance of the membrane.

[0017] Further, the inorganic salt is selected from one of potassium chloride, calcium chloride, magnesium chloride and sodium chloride; and the fluoroalkyl silane is selected from one of perfluorodecyltriethoxysilane, perfluorodecyltrichlorosilane, perfluorooctyltriethoxysilane, perfluorooctylmethyldimethoxysilane, perfluorooctyltrichlorosilane, perfluorohexyltriethoxysilane and perfluorohexyltrichlorosilane.

[0018] Further, the inorganic salt is sodium chloride, and its concentration in the coagulation bath is 100-300 g / L; the fluoroalkyl silane is perfluorodecyltriethoxysilane, and its dispersion mass fraction in the coagulation bath is 0.1-1 wt%; and the phase inversion solidification time of the coagulation bath is 15-45 min.

[0019] In the present application, the concentration of sodium chloride in the coagulation bath is controlled at 100-300 g / L, which can efficiently induce the transformation of polyvinylidene fluoride to β form through ion-dipole interaction, giving the membrane self-adsorption properties, while inhibiting the loss of amphiphilic porogens to retain sufficient grafting sites; perfluorodecyltriethoxysilane is dispersed at 0.1-1 wt%, which can uniformly introduce fluorine-containing groups through in-situ fluorination grafting, significantly enhancing the hydrophobicity of the membrane and reducing the risk of wetting; and the phase inversion solidification time of 15-45 min ensures the stable formation of the membrane structure, and the synergistic effect of the three ultimately realizes the effective improvement of ammonia flux.

[0020] Further, in the casting solution, the mass percentage of polyvinylidene fluoride is 18-23wt%, the mass percentage of the amphiphilic pore-forming agent is 2.5-5wt%, and the rest is an organic solvent; the amphiphilic pore-forming agent is a non-ionic high molecular surfactant selected from one of Pluronic F127, F68, P85 and P123; and the organic solvent is selected from one of N,N-dimethylacetamide, dimethyl sulfoxide and N,N-dimethylformamide. This ratio of the casting solution can realize the full dissolution of each component to form a stable system, and lay a good foundation for the pore construction of the membrane and subsequent functional modification.

[0021] Further, in step S2, the thickness of the thin film formed by the casting solution is 200-250μm. In this thickness range, the phase inversion reaction can be fully carried out to form a stable wet film, providing a suitable substrate thickness for the subsequent beta-polyvinylidene fluoride configuration transformation and in-situ fluorination grafting, and ensuring the mechanical properties and gas permeation mass transfer efficiency of the membrane.

[0022] Further, in steps S3 and S5, the standing time of the membrane in deionized water is 16-28h. In this standing time range, the residual solvent and removal solution can be fully removed to avoid impurities interfering with the membrane structure and performance, and to ensure the stable quality of the final membrane product.

[0023] Further, the removal solution is isopropanol, ethanol or dilute sulfuric acid solution with a volume concentration of 35-75%(v / v), and the soaking time is 6-10h. Isopropanol, ethanol or dilute sulfuric acid solution with a volume concentration of 35-75% as the removal solution can efficiently remove the residual amphiphilic pore-forming agent in the membrane after 6-10h of soaking, without damaging the pore structure and grafted functional groups of the membrane, and ensuring the gas permeation performance and hydrophobic stability of the membrane.

[0024] Further, in step S6, the vacuum drying method is vacuum oven drying or vacuum freeze drying, and the drying time is 6-10h. Using vacuum oven drying or vacuum freeze drying and a drying time of 6-10h can fully remove the water in the membrane without damaging the membrane structure, ensuring the hydrophobic performance and pore structure stability of the membrane, and improving the quality of the final product.

[0025] The application also discloses a self-adsorbing hydrophobic gas-permeable membrane prepared by the preparation method. The beta-polyvinylidene fluoride configuration forms a high-polarity feature, which has a strong adsorption effect on ammonia molecules; at the same time, the in-situ fluorination grafting modification realizes good hydrophobicity, which can reduce the risk of wetting and ensure long-term stable operation. Based on the ammonia molecule adsorption function, the gas permeation performance of the membrane is optimized, and the ammonia flux is significantly improved, which is suitable for the ammonia-nitrogen wastewater resource recovery scene.

[0026] The application also discloses application of the self-adsorptive hydrophobic gas-permeable membrane to ammonia-nitrogen wastewater resource recovery.

[0027] The application has the following advantages compared with the prior art:

[0028] 1) The PVDF segment is induced to transform into a beta crystal form by inorganic salt, the hydrogen bond interaction between the polarity of the beta-PVDF and ammonia molecules is utilized to construct an efficient ammonia mass transfer channel, the problem of difficult improvement of ammonia flux caused by the upper limit of pore size regulation of the traditional hydrophobic gas-permeable membrane is solved, and the ammonia permeation performance is significantly improved.

[0029] 2) The adsorption function of the membrane to ammonia molecules is endowed by the crystal structure transformation of PVDF itself instead of relying on external additives such as carbon nanomaterials and metal organic framework compounds, so that the problem of performance attenuation of the membrane caused by easy loss of the external additives is avoided. Meanwhile, the preparation steps are simplified, and the cost of the membrane material is reduced.

[0030] 3) The inorganic salt induces the beta-PVDF to form a high-polarity structure, and the fluoralkylsilane is in-situ fluorinated and grafted by dehydration condensation reaction with the hydrophilic segment of the amphiphilic pore-forming agent to introduce low-surface-energy fluorine-containing groups into the membrane surface and pore structure, so that the problems of high hydrophilicity and poor wet resistance of the high-beta-PVDF membrane material are solved. The preparation method makes the membrane have good adsorption function and stable wet resistance, and ensures long-term stable operation of the membrane material in ammonia-nitrogen wastewater recovery.

[0031] The self-adsorptive hydrophobic gas-permeable membrane preparation method, the prepared membrane and the application thereof will be disclosed in detail below in combination with the embodiments shown in the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 It is a preparation method flow chart of the application;

[0033] Figure 2 It is a reaction principle diagram of in-situ fluorination grafting of the application;

[0034] Figure 3 It is the structure and performance characterization of the embodiment and comparative example 1, wherein Figure 3 (1) It is the X-ray diffraction (XRD) spectrum of the embodiment and comparative example 1; Figure 3 (2) It is the infrared spectrum (IR) spectrum of the embodiment and comparative example 1; Figure 3 (3) It is the crystallinity and the proportion of the polar crystal form columnar diagram of the embodiment and comparative example 1; Figure 3 (4) It is the ammonia adsorption capacity and ammonia mass transfer coefficient columnar diagram of the embodiment and comparative example 1;

[0035] Figure 4 It is the surface functional group and contact angle characterization of the embodiment and comparative example 2, wherein Figure 4(1) is an X-ray photoelectron spectroscopy (XPS) C-1s spectrum of the example; Figure 4 (2) is an X-ray photoelectron spectroscopy (XPS) C-1s spectrum of Comparative Example 2; Figure 4 (3) is a contact angle histogram of the example and Comparative Example 2.

[0036] Figure 5 A comparison histogram of ammonia nitrogen flux of the example and Comparative Examples 1, 3 and 4. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application. It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0038] As shown in Figure 1 The present application discloses a method for preparing a self-adsorptive hydrophobic and breathable membrane, comprising the following steps: S1, adding polyvinylidene fluoride and amphiphilic porogen into an organic solvent in a certain proportion, stirring to fully dissolve the polyvinylidene fluoride and the amphiphilic porogen, and standing until there is no air bubble in the solution to prepare a casting solution; S2, coating the casting solution prepared in step S1 into a thin film on a film preparation plate, and then placing it in a coagulation bath for phase inversion to solidify and form a wet film; the coagulation bath is an inorganic salt solution in which fluoralkylsilane is dispersed, wherein: the inorganic salt is configured to induce the polyvinylidene fluoride segment to transform into a β-polyvinylidene fluoride configuration through ion-dipole interaction between its salt ions and the dipole of the polyvinylidene fluoride molecular chain, and to dehydrate the hydrophilic segment of the amphiphilic porogen to reduce its dissolution and diffusion into the coagulation bath; the fluoralkylsilane is configured to hydrolyze and then undergo a dehydration condensation reaction with the oxygen-containing functional groups of the hydrophilic segment of the amphiphilic porogen in the casting solution to realize in-situ fluorinated grafting; S3, taking out the wet film solidified in step S2, placing it in deionized water for standing to remove the residual organic solvent in the film; S4, placing the film treated in step S3 in a removal solution that can selectively remove the amphiphilic porogen for soaking to remove the residual amphiphilic porogen in the film; S5, placing the film treated in step S4 in deionized water again for standing to remove the residual removal solution in the film; S6, vacuum drying the film treated in step S5 to prepare a self-adsorptive high-flux hydrophobic and breathable membrane.

[0039] Based on the above-disclosed steps, this invention induces a configurational transformation of polyvinylidene fluoride (PVDF) segments to β-PVDF using inorganic salts. The polarity of β-PVDF allows for hydrogen bonding interactions with ammonia molecules, constructing a highly efficient ammonia mass transfer channel. This solves the problem of limited ammonia flux improvement in traditional hydrophobic and permeable membranes due to the upper limit of pore size control, significantly improving ammonia permeation performance. Secondly, it eliminates the need for exogenous additives such as carbon nanomaterials or metal-organic frameworks, instead endowing the membrane with ammonia adsorption capabilities through the transformation of PVDF's own crystal structure, avoiding the performance degradation caused by the easy loss of exogenous additives. Simultaneously, it simplifies the preparation steps and reduces the cost of membrane materials. Finally, inorganic salts induce the formation of a highly polar structure in β-PVDF, while fluoroalkylsilanes achieve in-situ fluorination grafting through dehydration condensation with the hydrophilic segments of amphiphilic porogens. This introduces low surface energy fluorinated groups into the membrane surface and pore structure, solving the problems of poor hydrophilicity and anti-wetting properties in high β-PVDF membrane materials. This preparation method enables the membrane to possess both excellent adsorption function and stable anti-wetting properties, ensuring the long-term stable operation of the membrane material in wastewater ammonia nitrogen recovery.

[0040] like Figure 2 As shown in this embodiment of the invention, the specific principle of in-situ fluorination grafting is as follows: Fluoroalkylsilane (FAS) molecules in the coagulation bath contain methoxy (-OCH3) groups, which undergo hydrolysis in the presence of water. The reaction formula is: ≡Si-OCH3 + H2O → ≡Si-OH + CH3OH (where ≡Si represents the silicon atom portion in the FAS molecule). This reaction converts the methoxy group in the FAS molecule into a silanol group (-Si-OH). Meanwhile, the Pranic-type amphiphilic porogen molecule (composed of PEO and PPO blocks) contains a hydroxyl (-OH) group. The silanol group (-Si-OH) generated from the hydrolysis of FAS undergoes a dehydration condensation reaction with the hydroxyl group (-OH) in the porogen molecule. The reaction formula is: ≡Si-OH + HO-PEO / PPO → ≡Si-O-PEO / PPO + H2O. Through this reaction, the FAS molecule is successfully grafted onto the porogen molecule. The grafting reaction allows FAS molecules to bind to porogen molecules via Si-OC covalent bonds, thereby introducing fluorine-containing groups into the membrane structure. This not only enhances the membrane's hydrophobicity (contact angle >95°) but also works synergistically with the β-PVDF structure induced by salt ions, giving the membrane its own adsorption function (β-PVDF content exceeds 90%, adsorption capacity >35 mmol / g) and high ammonia flux (more than 10 times that of non-adsorbent membranes and 1.5-2.5 times that of commercial membranes).

[0041] In a preferred embodiment, the inorganic salt is selected from one of potassium chloride, calcium chloride, magnesium chloride and sodium chloride; the fluoroalkylsilane is selected from one of perfluorodecyltriethoxysilane, perfluorodecyltrichlorosilane, perfluorooctyltriethoxysilane, perfluorooctylmethyldimethoxysilane, perfluorooctyltrichlorosilane, perfluorohexyltriethoxysilane and perfluorohexyltrichlorosilane.

[0042] In a preferred embodiment, the inorganic salt is sodium chloride, the concentration of which in the coagulation bath is 100-300 g / L; the fluoroalkylsilane is perfluorodecyltriethoxysilane, the mass fraction of which in the coagulation bath is 0.1-1 wt%; the phase inversion solidification time of the coagulation bath is 15-45 min. In this embodiment, the concentration of sodium chloride in the coagulation bath is controlled at 100-300 g / L, which can efficiently induce the polyvinylidene fluoride to transform into β form through ion-dipole interaction, endow the membrane with self-adsorption performance, and inhibit the loss of amphiphilic porogen to retain sufficient grafting sites; the perfluorodecyltriethoxysilane is dispersed at 0.1-1 wt%, which can introduce fluorine-containing groups uniformly through in-situ fluorination grafting, significantly enhance the hydrophobicity of the membrane and reduce the risk of wetting; and the phase inversion solidification time of 15-45 min guarantees the stable formation of the membrane structure, and the synergistic effect of the three finally realizes the effective improvement of ammonia flux.

[0043] In a preferred embodiment, in the casting solution, the mass fraction of polyvinylidene fluoride is 18-23 wt%, the mass fraction of amphiphilic porogen is 2.5-5 wt%, and the rest is organic solvent; the amphiphilic porogen is a non-ionic high molecular surfactant selected from one of pluronic F127, F68, P85 and P123; and the organic solvent is selected from one of N,N-dimethylacetamide, dimethyl sulfoxide and N,N-dimethylformamide. In this embodiment, this ratio of the casting solution can realize the sufficient dissolution of each component to form a stable system, laying a good foundation for the pore construction of the membrane and subsequent functional modification.

[0044] In a preferred embodiment, in step S2, the casting solution is scraped to form a thin film with a thickness of 200-250 μm. In this embodiment, within this thickness range, the phase inversion reaction can be guaranteed to proceed sufficiently, forming a wet membrane with stable structure, providing a suitable substrate thickness for the subsequent β-polyvinylidene fluoride configuration transformation and in-situ fluorination grafting, and guaranteeing the mechanical properties and gas permeation mass transfer efficiency of the membrane.

[0045] In a preferred embodiment, in steps S3 and S5, the standing time of the membrane in deionized water is 16-28 h. In this embodiment, within this standing time range, the residual solvent and stripping solution can be removed sufficiently, avoiding impurities from interfering with the structure and performance of the membrane, and guaranteeing the stable quality of the final membrane product.

[0046] In a preferred embodiment, the removing solution is isopropyl alcohol, ethanol or dilute sulfuric acid aqueous solution with a volume concentration of 35-75% (v / v), and the soaking time is 6-10 h. In this embodiment, the isopropyl alcohol, ethanol or dilute sulfuric acid aqueous solution with a volume concentration of 35-75% is used as the removing solution, and the amphiphilic pore-forming agent remaining in the membrane can be removed efficiently through 6-10 h of soaking, without damaging the pore structure and grafted functional groups of the membrane, thereby ensuring the air permeability and hydrophobic stability of the membrane.

[0047] In a preferred embodiment, in step S6, the vacuum drying method is vacuum oven drying or vacuum freeze drying, and the drying time is 6-10 h. In this embodiment, the vacuum oven drying or vacuum freeze drying method is used for 6-10 h, which can remove the water in the membrane sufficiently without damaging the structure of the membrane, thereby ensuring the hydrophobic performance and stable pore structure of the membrane and improving the quality of the final product.

[0048] The application also discloses a self-adsorptive hydrophobic air-permeable membrane prepared by the preparation method.

[0049] The application also discloses an application of the self-adsorptive hydrophobic air-permeable membrane in the resource recycling of ammonia-nitrogen wastewater.

[0050] To verify the effectiveness of the self-adsorptive hydrophobic air-permeable membrane preparation method and determine the influence of the key preparation conditions on the performance of the membrane, the specific preparation process of the examples and comparative examples is described in detail below, and the structure and performance of the membrane material under different process parameters are compared and analyzed to comprehensively evaluate the advantages of the technical solution of the application.

[0051] Examples

[0052] I. Casting solution preparation: polyvinylidene fluoride (PVDF) powder with a molecular weight of 600 kDa and amphiphilic pore-forming agent (Pluronic F127) are added to an organic solvent N,N-dimethylacetamide (DMAc) in a certain proportion, wherein the PVDF accounts for 22 wt% of the total mass of the casting solution, the Pluronic F127 accounts for 3.8 wt%, and the rest is DMAc. Then the mixed system is heated and stirred until the PVDF and Pluronic F127 are completely dissolved. After the dissolution is completed, the solution is left to be free of air bubbles, and a uniform casting solution is prepared.

[0053] II. Preparation of the hydrophobic and gas permeable membrane: The casting solution was cast onto a glass plate to form a film with a thickness of 210 μm, and then placed in a sodium chloride solution containing perfluorodecyltriethoxysilane (FAS) for phase inversion and solidification, and FAS was grafted in situ at the same time. The dispersion ratio of FAS in the solution was 0.5 wt%, the concentration of sodium chloride was 210 g / L, and the solidification time was 30 min, to obtain a wet film.

[0054] III. Post-treatment of the hydrophobic and gas permeable membrane: After the wet film was taken out, it was first placed in deionized water for 20 h to remove the residual solvent (DMAc) in the film; then soaked in isopropanol with a volume concentration of 50% (v / v) for 6 h to remove the residual pore-forming agent (Pluronic F127) in the film; again placed in deionized water for 20 h to remove the residual isopropanol; and finally vacuum freeze-dried for 8 h to obtain the self-adsorbing high-flux hydrophobic and gas permeable membrane.

[0055] Comparative Example 1

[0056] I. Preparation of the casting solution: Polyvinylidene fluoride (PVDF) powder with a molecular weight of 600 kDa was added to an organic solvent N,N-dimethylacetamide (DMAc) in a certain proportion, wherein the PVDF accounted for 20 wt% of the total mass of the casting solution. Then the mixed system was heated and stirred until the PVDF was completely dissolved, and the solution was left to stand until there were no bubbles, to obtain a uniform casting solution.

[0057] II. Preparation of the hydrophobic and gas permeable membrane: The casting solution was cast onto a glass plate to form a film with a thickness of 350 μm, and then placed in an ethanol solution with a concentration of 15% (v / v) for phase inversion and solidification, and the solidification time was 25 min, to obtain a wet film.

[0058] III. Post-treatment: After the wet film was taken out, it was first placed in deionized water for 20 h to remove the residual solvent (DMAc); then dried by vacuum freeze-drying for 8 h to obtain the hydrophobic and gas permeable membrane.

[0059] Comparative Example 2

[0060] I. Preparation of the casting solution: Polyvinylidene fluoride (PVDF) powder with a molecular weight of 600 kDa and the amphiphilic pore-forming agent Pluronic F127 were added to an organic solvent N,N-dimethylacetamide (DMAc) in a certain proportion, wherein the PVDF accounted for 22 wt% of the total mass of the casting solution, the Pluronic F127 accounted for 3.8 wt%, and the rest was DMAc. Then the mixed system was heated and stirred until the PVDF and Pluronic F127 were completely dissolved, and the solution was left to stand until there were no bubbles, to obtain a uniform casting solution.

[0061] II. Preparation of hydrophobic and breathable membrane: The above casting solution was coated onto a glass plate to form a thin film with a thickness of 210 μm. Then, it was placed in a sodium chloride inorganic salt solution with a mass concentration of 210 g / L for phase inversion and curing. The curing time was 25 min to obtain a wet film.

[0062] III. Post-treatment: After the cured wet membrane is taken out, it is first placed in deionized water and left to stand for 20 hours to remove residual solvent (DMAc); then it is soaked in 50% (v / v) isopropanol for 6 hours to remove residual pore-forming agent (Pluronic F127) from the membrane; it is then placed in deionized water again and left to stand for 20 hours to remove residual isopropanol; finally, it is freeze-dried under vacuum for 8 hours to obtain the hydrophobic and breathable membrane.

[0063] Figure 3 The crystal structure and performance differences of the membrane material prepared using a salt coagulation bath (sodium chloride 210 g / L) (Example) and the membrane material prepared without using a salt coagulation bath (Comparative Example 1) were compared: X-ray diffraction results ( Figure 3 (1) The diffraction peak of the film in the example is located around 20.8°, indicating that its internal structure is mainly polar β-PVDF crystal structure, which is significantly different from the crystal structure of Comparative Example 1; Infrared spectroscopy analysis ( Figure 3 (2) This further confirms that the relative peak values ​​of the characteristic peaks representing β-PVDF in the examples are significantly higher than those in Comparative Example 1, thus confirming the dominant position of the β-PVDF crystal form; Figure 3 As shown in (3), the crystallinity of both is similar (approximately 60%), but the polar β-PVDF crystal form accounts for more than 90% in the example, indicating that the introduction of the salt coagulation bath gives the hydrophobic and breathable membrane a high polarity characteristic; performance test results ( Figure 3 (4) shows that the ammonia adsorption capacity of the membrane in the example is 40 mmol / g and the ammonia mass transfer coefficient is 3 × 10⁻⁶. -5 The m / s values ​​were significantly higher than those of Comparative Example 1, indicating that the hydrophobic and breathable membrane with high polarity characteristics achieved higher ammonia adsorption capacity and better ammonia mass transfer efficiency through the polarity of β-PVDF.

[0064] Figure 4 The differences in surface functional groups and contact angle between membrane materials prepared using in-situ FAS grafting fluorination (FAS dispersion mass concentration 0.5 wt%) (Example) and membrane materials prepared without in-situ FAS grafting fluorination (Comparative Example 2) were compared: Figure 4 (1) and Figure 4 (2) As shown, the presence of C-Si bonds was detected on the membrane surface in the embodiment, indicating that FAS has been successfully grafted onto the membrane using this film-forming method; Figure 4 (3) As shown, the contact angle of the membrane in the example reached 96°, which is significantly higher than that of 75° in Comparative Example 2. This result shows that the hydrophobic and breathable membrane prepared in the example has good hydrophobicity.

[0065] Figure 5 The ammonia flux difference of the self-adsorption hydrophobic and breathable film (example), the non-adsorption hydrophobic and breathable film (comparative example 1) and the commercial hydrophobic and breathable film (comparative examples 3 and 4) is compared: as shown in Figure 5 , the ammonia nitrogen flux of the film material prepared in the example reaches 70 g-N / m 2 h, which is 10 times of the non-adsorption comparative example 1 (7 g-N / m 2 h) and 1.5-2.5 times of the commercial hydrophobic and breathable film (comparative examples 3 and 4), which shows that the self-adsorption hydrophobic and breathable film prepared in the example has higher ammonia flux.

[0066] The beneficial results of the present application compared with the prior art are as follows:

[0067] 1) The present application realizes the transformation of the PVDF chain segment configuration in the film to β-PVDF by introducing an inorganic salt solution into the coagulation bath and coupling FAS in-situ grafting. Compared with the traditional technology, the induction of salt ions makes the hydrophobic and breathable film transform from the conventional non-polar to polar, thereby giving the film the adsorption function of ammonia gas without adding any external additives; at the same time, the presence of salt ions also strengthens the in-situ fluorination grafting effect of FAS, successfully preparing a polar hydrophobic and breathable film with self-adsorption function.

[0068] 2) The results of the drawings further verify the effectiveness of the method: the prepared hydrophobic and breathable film material has high polarity characteristics, with a β-PVDF ratio of more than 90%, and an ammonia molecule adsorption capacity of more than 35 mmol / g (Appendix Figure 3 ); the hydrophobicity of the film is good, with a contact angle of more than 95° (Appendix Figure 4 ); in terms of ammonia flux, the ammonia flux of the film is more than 10 times of the non-adsorption film and more than 1.5-2.5 times of the commercial hydrophobic and breathable film (Appendix Figure 5 ), showing excellent ammonia adsorption performance and permeation efficiency.

[0069] In this technical field, the conventional scheme of "polyvinylidene fluoride and pluronics amphiphilic porogen together form a casting solution" is to prepare a hydrophilic and water-permeable membrane, mainly applied to water-permeable membrane separation processes such as ultrafiltration and nanofiltration. In this type of application scenario, the function of the membrane focuses on the selective permeation of water-phase substances, and the membrane surface does not need to be made hydrophobic by fluorination treatment, so "in-situ fluorination grafting of pluronics amphiphilic porogen as FAS grafting site" has not become a research direction in this field.

[0070] Meanwhile, the conventional scheme has a technical obstacle difficult to break through. Specifically, the hydrophilic segment of the Pluronic type amphiphilic porogen is rich in hydroxyl groups. During the phase inversion stage of film formation, these hydrophilic segments are prone to hydration with water molecules in the coagulation bath, leading to the dissolution and diffusion of the porogen from the film body to the coagulation bath and a large amount of loss. This makes the available hydroxyl grafting sites for the in-situ fluorinated grafting of FAS in the conventional scheme severely insufficient, and the grafting effect extremely poor. It is this difference in application orientation and the technical prejudice formed by the technical obstacle that leads other researchers to fail to think of this technical path.

[0071] The present application breaks through the above-mentioned limitations through innovative design. By introducing inorganic salt into the coagulation bath, the interaction between salt ions and the hydrophilic segment of the porogen is utilized to significantly inhibit the hydration of the hydrophilic segment, hinder the dissolution and diffusion of the porogen to the coagulation bath, reduce the loss, and retain more porogen in the film, thereby providing sufficient FAS grafting sites and effectively promoting and enhancing the in-situ fluorinated grafting effect. The Pluronic type amphiphilic porogen is successfully used as the FAS grafting site for in-situ fluorinated grafting.

[0072] The grafting principle is that the hydrophilic functional groups (such as hydroxyl groups) of the amphiphilic porogen can serve as grafting sites for FAS. The silane groups in FAS are first hydrolyzed to form active silanol groups, which undergo dehydration condensation reaction with the hydroxyl groups to achieve covalent bonding.

[0073] In addition, the present application also solves the adsorption function defect of conventional hydrophobic and breathable films. Conventional hydrophobic and breathable films are mostly composed of non-polar fluorine-containing polymers, which have very weak interaction with polar ammonia molecules and almost no adsorption function. Therefore, external additives (such as metal organic framework compounds and porous carbon materials) are needed, but these additives increase the cost and are prone to loss, resulting in unstable performance. In the present application, inorganic salt ions are added to the coagulation bath to induce the conversion of polyvinylidene fluoride molecular segments from alpha form to beta form through ion-dipole interaction. Beta-polyvinylidene fluoride has strong polarity, which enables the film to have strong polarity characteristics and achieve adsorption of ammonia molecules through polarity-polarity interaction, without the need for external additives to endow the film with adsorption function.

[0074] Finally, it should be pointed out that the above examples are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the purpose and scope of the present application, which should be covered in the scope of the claims of the present application.

Claims

1. A method for producing a self-adsorbing hydrophobic gas permeable membrane, characterized by, The method comprises the following steps: S1, polyvinylidene fluoride and amphiphilic porogen are added into an organic solvent in a certain proportion, stirring to fully dissolve the polyvinylidene fluoride and amphiphilic porogen, and standing until the solution is free of bubbles to prepare a casting solution; S2, the casting solution prepared in step S1 is coated on a film preparation plate, and then placed in a coagulation bath for phase inversion to solidify and form a wet film; the coagulation bath is an inorganic salt solution containing a fluoroalkyl silane, wherein: the inorganic salt is configured to induce the polyvinylidene fluoride segment to transform into a β configuration through ion-dipole interaction and reduce the dissolution and diffusion of the porogen; the fluoroalkyl silane is configured to undergo a dehydration condensation reaction with the oxygen-containing functional groups of the hydrophilic segment of the amphiphilic porogen to achieve in-situ fluorinated grafting; S3, the wet film after solidification in step S2 is taken out and placed in deionized water for standing to remove the residual organic solvent in the film; S4, the film treated in step S3 is immersed in a removal solution to remove the residual amphiphilic porogen in the film; S5, the film treated in step S4 is again placed in deionized water for standing to remove the residual removal solution in the film; S6, the film treated in step S5 is vacuum dried to prepare a self-adsorbing high-flux hydrophobic and air-permeable film; The amphiphilic porogen is a non-ionic high molecular surfactant selected from one of Pluronic F127, F68, P85 and P123.

2. The production method according to claim 1, characterized by, The inorganic salt is selected from one of potassium chloride, calcium chloride, magnesium chloride and sodium chloride; the fluoroalkyl silane is selected from one of perfluorodecyltriethoxysilane, perfluorodecyltrichlorosilane, perfluorooctyltriethoxysilane, perfluorooctylmethyldimethoxysilane, perfluorooctyltrichlorosilane, perfluorohexyltriethoxysilane and perfluorohexyltrichlorosilane.

3. The production method according to claim 2, characterized by, The inorganic salt is sodium chloride, the concentration of which in the coagulation bath is 100-300 g / L; the fluoroalkyl silane is perfluorodecyltriethoxysilane, the dispersion mass fraction of which in the coagulation bath is 0.1-1 wt%; the phase inversion and solidification time of the coagulation bath is 15-45 min.

4. The method of claim 1, wherein, The mass fraction of polyvinylidene fluoride in the casting solution is 18-23 wt%, the mass fraction of amphiphilic porogen is 2.5-5 wt%, and the rest is an organic solvent; the organic solvent is selected from one of N,N-dimethylacetamide, dimethyl sulfoxide and N,N-dimethylformamide.

5. The preparation method according to claim 4, characterized in that, In step S2, the casting solution is coated into a thin film with a thickness of 200-250 μm.

6. The preparation method according to claim 5, characterized in that, In steps S3 and S5, the standing time of the film in deionized water is 16-28 h.

7. The preparation method according to claim 6, characterized in that, The removal solution is isopropanol, ethanol or dilute sulfuric acid solution with a volume concentration of 35-75% (v / v), and the immersion time is 6-10 h.

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

9. A self-adsorbing, hydrophobic, gas permeable membrane, characterized in that, Prepared by the preparation method of any one of claims 1-8.

10. The self-adsorbing hydrophobic and air-permeable film according to claim 9 is used in the resource recovery of ammonia-nitrogen wastewater.

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