Liquid nitrogen frozen interlayer forward osmosis membrane and preparation method thereof

Abstract

The invention belongs to the technical field of membrane separation, and particularly relates to a liquid nitrogen freezing interlayer forward osmosis membrane and a preparation method thereof.The preparation method comprises the steps that a ZIF-8 nanometer material is adopted to construct an interlayer, and a water-phase monomer is promoted to be attached to the interlayer through the hydrogen-bond interaction formed by nitrogen atoms and hydrogen atoms in imidazole ligands in ZIF-8 and water molecules, so that the forward osmosis membrane is obtained; liquid nitrogen is applied to freeze the middle layer, so that a water phase is converted into an ice phase, and a water-phase monomer is sealed in the middle layer, so that the reaction concentration of the water-phase monomer in an interfacial polymerization process is improved, the density of a polyamide separation layer is improved, and the reverse solute interception capability of the forward osmosis membrane is improved. Moreover, after liquid nitrogen freezing, the diffusion rate of a water-phase monomer can be remarkably reduced, and the purpose of reducing the thickness of a separation layer is achieved, so that the water permeability of the forward osmosis membrane is improved, and the liquid nitrogen freezing interlayer forward osmosis membrane with excellent water permeability and reverse solute interception capability is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of membrane separation, and particularly relates to a liquid nitrogen frozen intermediate layer forward osmosis membrane and a preparation method thereof. Background Art

[0002] Forward osmosis membrane technology is a separation technology that uses the osmotic pressure difference to drive water molecules to naturally migrate from a low-concentration solution to a high-concentration solution through a semi-permeable membrane. Its core lies in the selective semi-permeable membrane, which only allows water molecules to pass through while retaining solutes, and can achieve water transfer without applying external high pressure. Therefore, it has advantages such as low energy consumption and small membrane fouling. This technology is widely used in fields such as seawater desalination, wastewater treatment, food concentration, energy generation, and medical sustained release, and is particularly suitable for the treatment of high-salt or highly polluted water bodies.

[0003] However, the separation layer of traditional forward osmosis membranes is prepared by an interfacial polymerization reaction on the surface of the support layer. Due to the presence of irregular large pores in the support layer, the aqueous monomer is easily lost from the large pores, resulting in the formation of non-selective defect holes on the separation layer, which affects the salt rejection performance of the forward osmosis membrane. Moreover, during the interfacial polymerization reaction, the diffusion behavior of the aqueous monomer is uncontrollable, leading to a too thick separation layer and reducing the water flux of the membrane.

[0004] To further optimize the separation layer structure, researchers have found that by constructing a metal-organic framework material intermediate layer between the support layer and the separation layer, effective regulation of the aqueous monomer can be achieved, significantly improving the water flux and separation performance of the forward osmosis membrane. Using a metal-organic framework material as the intermediate layer can not only utilize its high specific surface area to provide a larger attachment area for the aqueous monomer, enabling the intermediate layer to store more aqueous monomers and avoiding the formation of defects in the separation layer, but also utilize its high porosity to increase the diffusion resistance of the aqueous monomer and reduce the thickness of the separation layer.

[0005] To further increase the diffusion resistance of the aqueous monomer in the intermediate layer, the methods adopted are mostly to increase the intermediate layer loading amount and modify to reduce the pore size of the intermediate layer material. However, increasing the loading amount will lead to poor stability of the intermediate layer; although modifying the pore size of the required intermediate layer material can slow down the diffusion rate of the aqueous monomer, it also reduces the water flux of the subsequent membrane. Summary of the Invention

[0006] Aiming at the problems existing in the prior art, the present invention provides a liquid nitrogen frozen intermediate layer forward osmosis membrane and a preparation method thereof, thereby improving the water permeability and reverse solute rejection ability of the forward osmosis membrane, and synthesizing a separation layer structure with high density, thin thickness and no defects.

[0007] The present invention is realized through the following technical solutions: A preparation method of a liquid nitrogen frozen intermediate layer forward osmosis membrane, comprising the following steps: The ZIF-8 nanomaterial is ultrasonically dispersed in methanol to obtain a ZIF-8 nanomaterial dispersion; The aqueous monomer is dispersed in deionized water to obtain an aqueous solution; the organic monomer is ultrasonically dispersed in n-hexane to obtain an organic solution; The ZIF-8 nanomaterial dispersion is loaded on the surface of the nylon support layer by vacuum filtration to form a ZIF-8 intermediate layer; The aqueous solution is loaded on the ZIF-8 intermediate layer. The ZIF-8 intermediate layer loaded with the aqueous phase is treated by liquid nitrogen freezing, and then the organic solution is added. After the interfacial polymerization reaction, heat curing treatment is carried out to form a polyamide separation layer, thereby obtaining a liquid nitrogen frozen intermediate layer forward osmosis membrane.

[0008] Preferably, the specific preparation method of the ZIF-8 nanomaterial is as follows: 2-Methylimidazole and zinc nitrate are respectively dissolved in methanol. Under stirring at a speed of 600-800 rpm / min at room temperature, the zinc nitrate solution is added to the 2-methylimidazole solution to synthesize a mixed solution. After the reaction, the product is collected by centrifugation, washed and dried to obtain the ZIF-8 nanomaterial; The addition ratio of 2-methylimidazole to methanol is 10-30 g: 60-80 g; The addition ratio of zinc nitrate to methanol is 1-2 g: 5-8 g.

[0009] Preferably, in the ZIF-8 nanomaterial dispersion, when the volume of methanol is 100-120 mL, the mass of the ZIF-8 nanomaterial is 0.001-0.003 g.

[0010] Preferably, the aqueous monomer is m-phenylenediamine, and the mass percentage concentration of m-phenylenediamine in the aqueous solution is 2 wt%-3 wt%.

[0011] Preferably, the organic monomer is trimesoyl chloride, and the mass concentration of trimesoyl chloride in the organic solution is 0.1 wt%-0.5 wt%.

[0012] Preferably, the volume of liquid nitrogen is 5-15 ml, and the liquid nitrogen freezing time is 10-30 s.

[0013] Preferably, the interfacial polymerization reaction time is 1-2 min.

[0014] Preferably, the heat curing treatment time is 1-5 min under vacuum at a temperature of 50-80 °C.

[0015] Preferably, the power of the ultrasonic dispersion is 160W - 180W, the time is 30min - 40min, the pressure of the vacuum filtration is 0.07 - 0.09MPa, and the time is 30 - 50s.

[0016] A liquid nitrogen frozen intermediate layer forward osmosis membrane, characterized in that it is prepared by the method for preparing a liquid nitrogen frozen intermediate layer forward osmosis membrane described above.

[0017] Compared with the prior art, the present invention has the following beneficial technical effects: In the preparation scheme of the liquid nitrogen frozen intermediate layer forward osmosis membrane of the present invention, ZIF-8 nanomaterials are used as the intermediate layer, and the liquid nitrogen frozen intermediate layer is used to regulate the attachment and diffusion behavior of the aqueous monomers, improve the separation layer structure, and significantly enhance the water permeability and reverse solute rejection performance of the forward osmosis membrane. As a typical metal-organic framework nanomaterial, ZIF-8 has the characteristics of strong thermal stability, high specific surface area, rich and tiny pores. Its high specific surface area provides a larger attachment area for the aqueous monomers, and the abundant angstrom-level pores hinder the loss of the aqueous monomers, which is beneficial for the intermediate layer to retain more aqueous monomers, increase the reaction concentration of the aqueous monomers in the subsequent interfacial polymerization reaction, improve the density of the polyamide separation layer, and enhance the reverse solute rejection ability of the forward osmosis membrane. In addition, the nitrogen and hydrogen atoms in the imidazole ligand of ZIF-8 can form hydrogen bonds with water molecules, which promotes the adsorption of the aqueous monomers while also inhibiting the diffusion of the aqueous monomers.

[0018] After the aqueous monomers are fully attached to the ZIF-8 intermediate layer, liquid nitrogen is added to freeze the intermediate layer, so that the remaining aqueous phase in the intermediate layer is transformed into an ice phase, increasing the diffusion resistance of the aqueous monomers, accelerating the self-termination of the interfacial polymerization reaction, reducing the thickness of the separation layer, and improving the water permeability of the forward osmosis membrane. In addition, during the interfacial polymerization process, the liquid nitrogen remaining on the intermediate layer will volatilize rapidly due to the increase in temperature, forming nano-scale bubbles and escaping upward, so that a rich nano-bubble-like structure is formed on the polyamide separation layer. During the operation of the membrane, it provides a larger contact area and a shorter transport path for water molecules, further improving the water permeability of the forward osmosis membrane. Compared with the traditional thin film composite membrane, the water permeability and reverse solute rejection performance of the liquid nitrogen frozen intermediate layer forward osmosis membrane are greatly improved, providing a feasible solution to the technical problems such as poor water permeability and ion rejection ability caused by the thick separation layer and the existence of non-selective pores in the existing forward osmosis membrane. Description of the Drawings

[0019] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can be obtained based on these drawings.

[0020] Figure 1 It is a schematic flow chart of the preparation method of the liquid nitrogen frozen intermediate layer forward osmosis membrane provided by the present invention; Figure 2 It is a surface morphology diagram of a traditional polyamide forward osmosis membrane in the prior art of forward osmosis membrane preparation (Comparative Example 1); Figure 3 It is a cross-sectional morphology diagram of a traditional polyamide forward osmosis membrane in the prior art of forward osmosis membrane preparation (Comparative Example 1); Figure 4 It is a surface morphology diagram of the liquid nitrogen frozen intermediate layer forward osmosis membrane in the embodiment of the present invention (Example 5); Figure 5 It is a cross-sectional morphology diagram of the liquid nitrogen frozen intermediate layer forward osmosis membrane in the embodiment of the present invention (Example 5). Detailed Embodiments

[0021] The technical solutions of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than 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 efforts shall fall within the protection scope of the present invention.

[0022] To enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than 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 efforts shall belong to the protection scope of the present invention.

[0023] To improve the water permeability and reverse solute rejection ability of forward osmosis membranes, it is crucial to synthesize a separation layer structure with high density, thin thickness, and no defects. Therefore, choosing a suitable intermediate layer material is the key to achieving the above separation layer structure. As a classic metal-organic framework nanomaterial, ZIF-8 has the characteristics of a high specific surface area and abundant pores. Using ZIF-8 as the intermediate layer material not only prevents the loss of aqueous monomers but also increases the attachment area of aqueous monomers, promoting more aqueous monomers to attach to the intermediate layer. Moreover, the nitrogen and hydrogen atoms in the imidazole ligand of ZIF-8 can form hydrogen bonds with water molecules, which not only promotes the adsorption of aqueous monomers but also inhibits the diffusion of aqueous monomers, enhancing the cross-linking degree of the internal structure of the separation layer and improving the density of the separation layer. However, there is an obvious upper limit to the effect of regulating the attachment and diffusion behavior of aqueous monomers by simply increasing the loading amount of ZIF-8 nanomaterials. Beyond the upper limit, the attachment amount of aqueous monomers does not increase significantly with the increase in the ZIF-8 loading amount. On the contrary, when the ZIF-8 loading amount is too large, it is not conducive to the formation of the subsequent polyamide separation layer.

[0024] To further regulate the attachment and diffusion behavior of aqueous monomers, a membrane preparation method of introducing a liquid nitrogen frozen ZIF-8 intermediate layer is proposed. After the aqueous monomers are loaded onto the ZIF-8 intermediate layer, liquid nitrogen is added to freeze the ZIF-8 intermediate layer, causing the aqueous phase remaining in the interlayer of the intermediate layer to rapidly cool and turn into an ice phase, sealing the aqueous monomers in the intermediate layer to increase the diffusion resistance of the aqueous monomers, reduce the thickness of the separation layer, and improve the water permeability of the forward osmosis membrane. Moreover, the more liquid nitrogen and the longer the freezing time, the deeper the frozen layer, the slower the ice layer dissolves, the stronger the inhibitory effect on the diffusion behavior of aqueous monomers, and the smaller the thickness of the separation layer, thereby improving the water permeability of the forward osmosis membrane. In addition, during the interfacial polymerization process, the liquid nitrogen remaining on the intermediate layer will volatilize rapidly due to the increase in temperature, forming nanoscale bubbles and escaping upward, resulting in a rich nanobubble-like structure on the polyamide separation layer. During the operation of the membrane, it provides a larger contact area and a shorter transmission path for water molecules, further improving the water permeability of the forward osmosis membrane. Furthermore, partial freezing of the aqueous phase at the interface causes MPD to aggregate in the unfrozen area, increasing the equilibrium concentration, thereby forming a denser polyamide separation layer, which is beneficial to improving the reverse solute rejection of the forward osmosis membrane.

[0025] It should be noted that the process equipment or devices not specifically specified in the following examples are all conventional equipment or devices in the art, unless otherwise stated.

[0026] Please refer to Figure 1 , a method for preparing a liquid nitrogen frozen intermediate layer forward osmosis membrane provided by the present invention, includes the following steps: Step 1: Dissolve 10 - 30 g of 2-methylimidazole (2-Hmim) and 1 - 2 g of zinc nitrate (Zn(NO3)2·H2O) in 60 - 80 g and 5 - 8 g of methanol respectively. While vigorously stirring at a speed of 600 - 800 rpm / min at room temperature, slowly add the zinc nitrate solution to the 2-methylimidazole solution to synthesize a mixed solution. After stirring for 12 - 24 hours, centrifuge at a high speed of 5000 - 10000 rpm / min for 20 - 40 minutes to collect the product. Wash the product several times with methanol, and then place the product in a vacuum dryer at 80 - 100 °C for 12 - 24 hours to obtain zinc 2-methylimidazolate (ZIF-8) nanomaterials.

[0027] Step 2: Disperse 0.001 - 0.003 g of ZIF-8 nanomaterials into 100 - 120 ml of methanol by stirring and then ultrasonic treatment to obtain a ZIF-8 nanomaterial dispersion.

[0028] Step 3: Use m-phenylenediamine (MPD) as the aqueous-phase monomer, dissolve it in deionized water by stirring to obtain an aqueous-phase solution; the mass percentage concentration of m-phenylenediamine in the prepared aqueous-phase solution is 2 - 3 wt%.

[0029] Disperse trimesoyl chloride (TMC) into n-hexane by ultrasonic treatment to obtain an organic-phase solution; the mass concentration of trimesoyl chloride in the organic-phase solution is 0.1 - 0.5 wt%; the power of ultrasonic dispersion is 160 W - 180 W, and the time is 30 min - 40 min; Step 4: Load the ZIF-8 nanomaterial dispersion on the surface of the nylon support layer in the form of vacuum filtration to form a ZIF-8 intermediate layer. The pressure of the vacuum filtration is 0.07 - 0.09 MPa, and the time is 30 - 50 s. After adding the aqueous-phase solution and soaking for 2 - 3 min, remove the excess solution, and load the aqueous-phase solution on the ZIF-8 intermediate layer; then add liquid nitrogen to freeze the ZIF-8 intermediate layer loaded with the aqueous-phase monomer, so that the aqueous phase turns into an ice phase. The volume of the liquid nitrogen used is 5 - 15 ml, and the liquid nitrogen freezing time is 10 - 30 s; after the freezing is completed, immediately add the organic-phase solution and carry out an interfacial polymerization reaction for 1 - 2 min, then remove the excess solution, and place it in a vacuum oven at 50 - 80 °C for heat curing treatment for 1 - 5 min to form a polyamide separation layer, and obtain a liquid nitrogen frozen intermediate layer forward osmosis membrane.

[0030] The effective area of the nylon support layer is 15 cm 2 -20 cm 2 , and the pore size is 0.1 - 0.25 μm.

[0031] ZIF-8 has characteristics such as abundant angstrom-level pores and high specific surface area. While filling the large pores of the nylon support layer, it can also promote the attachment of aqueous monomers. When the loading amount of ZIF-8 nanomaterials is too small, they cannot be evenly distributed on the surface of the nylon support layer, resulting in the subsidence of aqueous monomers and defects in the polyamide separation layer; when the loading amount of ZIF-8 nanomaterials is too large, the nanomaterials will agglomerate, affecting the formation of the subsequent separation layer. Therefore, an appropriate loading amount of ZIF-8 nanomaterials can fully fill the large pores on the nylon support layer and provide favorable conditions for the subsequent interfacial polymerization reaction.

[0032] Using liquid nitrogen to freeze the ZIF-8 intermediate layer loaded with aqueous monomers can rapidly cool the aqueous phase in the intermediate layer to the ice phase. During the subsequent interfacial polymerization reaction, the aqueous monomers sealed in the ice phase can continue to diffuse into the organic phase only after the ice phase dissolves, thereby slowing down the diffusion rate of the aqueous monomers. Moreover, the longer the liquid nitrogen freezing time, the deeper the frozen layer, the slower the ice layer dissolves, the stronger the inhibitory effect on the diffusion behavior of the aqueous monomers, and the smaller the thickness of the separation layer, thereby improving the water permeability of the forward osmosis membrane. In addition, during the interfacial polymerization process, the residual liquid nitrogen on the intermediate layer will evaporate rapidly due to the increase in temperature, forming nanoscale bubbles and escaping upward, resulting in a rich nanobubble-like structure on the polyamide separation layer. During the membrane operation, it provides a larger contact area and a shorter transport path for water molecules, further improving the water permeability of the forward osmosis membrane.

[0033] Using ZIF-8 nanomaterials as the intermediate layer has advantages such as simple operation and low cost. Moreover, as a 3D MOFs nanomaterial, ZIF-8 has a high specific surface area and abundant angstrom-level pores, which can provide a larger attachment area and diffusion resistance for aqueous monomers, helping to extend the residence time of aqueous monomers in the intermediate layer and avoid the loss of aqueous monomers.

[0034] In the embodiments of the present invention, the test conditions for the prepared membrane are as follows: using deionized water as the feed solution and 1 mol / L NaCl solution as the draw solution, controlling the rotation speed of the peristaltic pump at 350 r / min at room temperature, and testing the performance of the prepared forward osmosis membrane under the condition that the flow rates of the feed solution and the draw solution are kept consistent. The water flux J V and the reverse salt flux J S , are calculated according to the following formulas respectively:

[0035]

[0036] In the formula, △V represents the permeation volume (L), and Aeff represents the effective membrane area (m 2), Ct represents the salt concentration (g / L) on the raw water side at time t, Vt represents the volume (L) on the raw water side at time t, and △t represents the osmosis time (h).

[0037] In the preparation method of a liquid nitrogen frozen intermediate layer forward osmosis membrane proposed by an embodiment of the present invention, by means of vacuum filtration, a ZIF-8 nanomaterial dispersion is loaded on the surface of a nylon support layer to form a ZIF-8 intermediate layer. Then, an aqueous solution is added to the ZIF-8 intermediate layer, and an aqueous monomer is loaded on the intermediate layer. Liquid nitrogen is added to freeze the intermediate layer loaded with the aqueous monomer, so that the aqueous phase in the intermediate layer rapidly cools down and turns into an ice phase. After the freezing time ends, an organic phase solution is immediately added to carry out an interfacial polymerization reaction, and finally a polyamide separation layer is obtained after thermal curing. Compared with traditional forward osmosis membranes, the liquid nitrogen frozen intermediate layer forward osmosis membrane introduces ZIF-8 as the intermediate layer, which fills the irregular large pores on the nylon support layer while providing a larger attachment area for the aqueous monomer, promoting more aqueous monomers to attach to the intermediate layer, avoiding the formation of non-selective defects in the polyamide separation layer, improving the density of the separation layer, and improving the reverse solute rejection performance of the membrane. Moreover, by using liquid nitrogen to freeze the ZIF-8 intermediate layer loaded with the aqueous monomer, the residual aqueous phase between the layers of the intermediate layer is turned into an ice phase, and the aqueous monomer is sealed in the intermediate layer, thereby slowing down the diffusion rate of the aqueous monomer in the subsequent interfacial polymerization process, reducing the thickness of the polyamide separation layer, and further improving the water permeability of the membrane. In addition, during the interfacial polymerization process, the liquid nitrogen remaining on the intermediate layer will volatilize rapidly due to the increase in temperature, forming nanoscale bubbles and escaping upward, so that a rich nanobubble-like structure is formed on the polyamide separation layer, providing a larger contact area and a shorter transmission path for water molecules during the operation of the membrane, which can further improve the water permeability of the membrane. A liquid nitrogen frozen intermediate layer forward osmosis membrane with excellent water permeability and reverse solute rejection ability is prepared.

[0038] In the following examples, unless otherwise specified, all raw materials are obtained from commercial sources or prepared by conventional methods in the art.

[0039] In the following, m-phenylenediamine: with a purity of 99.5%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; zinc nitrate hexahydrate: with a purity of 98%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; 2-methylimidazole: with a purity of 98%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; trimesoyl chloride: with a purity of 98%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; n-hexane: with an analytical purity, purchased from Guangdong Guanghua Sci-Tech Co., Ltd.; methanol: with an analytical purity, purchased from Tianjin Tianli Chemical Reagent Co., Ltd. Example 1 The preparation method of a liquid nitrogen frozen intermediate layer forward osmosis membrane provided by the present invention is specifically as follows: Step 1: Dissolve 20 g of 2-methylimidazole (2-Hmim) and 1.5 g of zinc nitrate (Zn(NO3)2·H2O) in 70 g and 6 g of methanol respectively. While vigorously stirring at a speed of 700 rpm / min at room temperature, slowly add the zinc nitrate solution to the 2-methylimidazole solution to synthesize a mixed solution. After stirring for 16 hours, centrifuge at a high speed of 7000 rpm / min for 30 minutes to collect the product. Wash the product several times with methanol, and then place the product in a vacuum dryer at 90 °C for 18 hours to obtain ZIF-8 nanomaterials.

[0040] Step 2: Disperse 0.003 g of ZIF-8 nanomaterials into 100 ml of methanol by stirring and then ultrasonic dispersion to obtain a ZIF-8 nanomaterial dispersion.

[0041] Step 3: Use m-phenylenediamine (MPD) as the aqueous-phase monomer, and dissolve it in deionized water by stirring to obtain an aqueous-phase solution; the mass percentage concentration of m-phenylenediamine in the prepared aqueous-phase solution is 2 wt%.

[0042] Disperse trimesoyl chloride (TMC) into n-hexane by ultrasonic dispersion to obtain an organic-phase solution; the mass concentration of trimesoyl chloride in the organic-phase solution is 0.3 wt%; the power of ultrasonic dispersion is 160 W and the time is 30 min; Step 4: Load the ZIF-8 nanomaterial dispersion on the surface of the nylon support layer in the form of vacuum filtration to form a ZIF-8 intermediate layer. The pressure of vacuum filtration is 0.08 MPa and the time is 40 s. After adding the aqueous-phase solution and soaking for 2 min, remove the excess solution, and load the aqueous-phase solution on the ZIF-8 intermediate layer; then add liquid nitrogen to freeze the ZIF-8 intermediate layer loaded with the aqueous-phase monomer, so that the aqueous phase is transformed into an ice phase. The volume of liquid nitrogen used is 5 ml and the liquid nitrogen freezing time is 10 s; after the freezing is completed, immediately add the organic-phase solution and carry out an interfacial polymerization reaction for 1 min, then remove the excess solution, and place it in a vacuum oven at 60 °C for heat curing treatment for 2 min to form a polyamide separation layer, and obtain a liquid nitrogen frozen intermediate layer forward osmosis membrane. The effective area of the nylon support layer is 18 cm 2 , and the pore size is 0.22 μm.

[0043] Evaluate the prepared forward osmosis membrane. The feed solution is deionized water, and the draw solution is 1 mol / L sodium chloride solution. Test for 30 min at room temperature. The average water flux of the forward osmosis membrane is 25.01 LMH, and the reverse salt flux is 0.52 gMH.

[0044] Comparative Example 1: Without adding the ZIF-8 nanomaterial dispersion liquid and without using the liquid nitrogen freezing technology, a nylon membrane with a pore size of 0.22 μm is used as the support layer. At room temperature, the support layer is immersed in an aqueous solution with a mass percentage concentration of 2 wt% for 2 min, and then an organic phase solution with a mass percentage concentration of 0.3 wt% is poured onto the surface of the support layer. After 1 min, the excess organic phase solution is poured off, and finally, it is thermally cured at 60 °C for 2 min. The remaining preparation methods are the same as those in Example 1; The prepared forward osmosis membrane was evaluated. The feed solution was deionized water, and the draw solution was 1 mol / L sodium chloride solution. It was tested at room temperature for 30 min; the water flux of the prepared forward osmosis membrane was 13.85 (LMH), and the reverse salt flux was 1.38 (gMH).

[0045] Comparative Example 2: Compared with Comparative Example 1, the difference in the preparation method of the forward osmosis membrane in Comparative Example 2 of the present invention is that 0.001 g of ZIF-8 nanomaterials are dispersed in 100 mL of methanol, the liquid nitrogen freezing technology is not used, and a ZIF-8 intermediate layer is formed on the surface of the nylon support layer, and then the aqueous phase solution and the organic phase solution are sequentially loaded for interfacial polymerization reaction. The mass of the ZIF-8 nanomaterials is 0.001 g, and they are uniformly dispersed in methanol by vigorous stirring and ultrasonic treatment. Similarly, the prepared forward osmosis membrane was tested. The average water flux of the forward osmosis membrane was 15.46 LMH, and the reverse salt flux was 1.07 gMH.

[0046] Comparative Example 3: Compared with Comparative Example 1, the difference in the preparation method of the forward osmosis membrane in Comparative Example 3 of the present invention is that the ZIF-8 nanomaterials are dispersed in 100 mL of methanol and loaded on the surface of the nylon support layer to form a ZIF-8 intermediate layer, and then the aqueous phase solution and the organic phase solution are sequentially loaded for interfacial polymerization reaction. The mass of the ZIF-8 nanomaterials is 0.003 g, and they are uniformly dispersed in methanol by vigorous stirring and ultrasonic treatment. Similarly, the prepared forward osmosis membrane was tested. The average water flux of the forward osmosis membrane was 21.59 LMH, and the reverse salt flux was 0.52 gMH.

[0047] Example 2 of the present invention: In Example 2 of the present invention, compared with Example 1, the preparation method is only different in that the mass of the ZIF-8 nanomaterials is 0.003 g, and they are uniformly dispersed in methanol by vigorous stirring and ultrasonic treatment; the amount of liquid nitrogen used is 5 ml, and the freezing time is 20 s. Similarly, the prepared forward osmosis membrane was tested. The average water flux of the forward osmosis membrane was 27.91 LMH, and the reverse salt flux was 0.58 gMH.

[0048] Example 3 of the present invention: In Example 3 of the present invention, compared with Example 1, the preparation method is only different in that the mass of the ZIF-8 nanomaterial is 0.003 g respectively, and it is uniformly dispersed in methanol by vigorous stirring and ultrasonic treatment; the liquid nitrogen dosage is 5 ml, and the freezing time is 30 s. Similarly, the prepared forward osmosis membrane is tested, and the average water flux of the forward osmosis membrane is 28.44 LMH, and the reverse salt flux is 0.32 gMH.

[0049] Example 4 of the present invention: In Example 4 of the present invention, compared with Example 1, the preparation method is only different in that the mass of the ZIF-8 nanomaterial is 0.003 g respectively, and it is uniformly dispersed in methanol by vigorous stirring and ultrasonic treatment; the liquid nitrogen dosage is 8 ml, and the freezing time is 10 s. Similarly, the prepared forward osmosis membrane is tested, and the average water flux of the forward osmosis membrane is 24.76 LMH, and the reverse salt flux is 0.35 gMH.

[0050] Example 5 of the present invention: In Example 5 of the present invention, compared with Example 1, the preparation method is only different in that the mass of the ZIF-8 nanomaterial is 0.003 g respectively, and it is uniformly dispersed in methanol by vigorous stirring and ultrasonic treatment; the liquid nitrogen dosage is 8 ml, and the freezing time is 20 s. Similarly, the prepared forward osmosis membrane is tested, and the average water flux of the forward osmosis membrane is 31.46 LMH, and the reverse salt flux is 0.19 gMH.

[0051] Example 6 of the present invention: In Example 6 of the present invention, compared with Example 1, the preparation method is only different in that the mass of the ZIF-8 nanomaterial is 0.003 g respectively, and it is uniformly dispersed in methanol by vigorous stirring and ultrasonic treatment; the liquid nitrogen dosage is 8 ml, and the freezing time is 30 s. Similarly, the prepared forward osmosis membrane is tested, and the average water flux of the forward osmosis membrane is 26.31 LMH, and the reverse salt flux is 0.28 gMH.

[0052] Example 7 of the present invention: In Example 7 of the present invention, compared with Example 1, the preparation method is only different in that the mass of the ZIF-8 nanomaterial is 0.003 g respectively, and it is uniformly dispersed in methanol by vigorous stirring and ultrasonic treatment; the liquid nitrogen dosage is 10 ml, and the freezing time is 10 s. Similarly, the prepared forward osmosis membrane is tested, and the average water flux of the forward osmosis membrane is 27.83 LMH, and the reverse salt flux is 0.45 gMH.

[0053] Example 8 of the present invention: Example 8 of the present invention. The preparation method is different from that of Example 1 only in that the masses of the ZIF-8 nanomaterials are 0.003 g respectively, which are uniformly dispersed in methanol by vigorous stirring and ultrasonic treatment; the amount of liquid nitrogen used is 10 ml, and the freezing time is 20 s. Similarly, the prepared forward osmosis membrane is tested, and the average water flux of the forward osmosis membrane is 25.19 LMH, and the reverse salt flux is 0.73 gMH.

[0054] Example 9 of the present invention: Example 9 of the present invention. The preparation method is different from that of Example 1 only in that the masses of the ZIF-8 nanomaterials are 0.003 g respectively, which are uniformly dispersed in methanol by vigorous stirring and ultrasonic treatment; the amount of liquid nitrogen used is 10 ml, and the freezing time is 30 s. Similarly, the prepared forward osmosis membrane is tested, and the average water flux of the forward osmosis membrane is 22.76 LMH, and the reverse salt flux is 0.81 gMH.

[0055] Example 10: Step 1: Dissolve 10 g of 2-methylimidazole (2-Hmim) and 1 g of zinc nitrate (Zn(NO3)2·H2O) in 60 g and 5 g of methanol respectively. While vigorously stirring at a speed of 600 rpm / min at room temperature, slowly add the zinc nitrate solution to the 2-methylimidazole solution to synthesize a mixed solution. After stirring for 12 hours, centrifuge at a high speed of 5000 rpm / min for 20 minutes to collect the product. Wash the product with methanol several times, and then place the product in a vacuum dryer at 80 °C for 12 hours to obtain the ZIF-8 nanomaterial.

[0056] Step 2: Prepare a ZIF-8 nanomaterial dispersion by dispersing 0.001 g of the ZIF-8 nanomaterial in 100 ml of methanol by stirring and then ultrasonic treatment.

[0057] Step 3: Use m-phenylenediamine (MPD) as the aqueous phase monomer, and dissolve it in deionized water by stirring to prepare an aqueous phase solution; the mass percentage concentration of m-phenylenediamine in the prepared aqueous phase solution is 2.5 wt%.

[0058] Disperse trimesoyl chloride (TMC) in n-hexane by ultrasonic treatment to prepare an organic phase solution; the mass concentration of trimesoyl chloride in the organic phase solution is 0.1 wt%; the power of ultrasonic dispersion is 170 W and the time is 35 min; Step 4: Load the ZIF-8 nanomaterial dispersion on the surface of the nylon support layer by vacuum filtration to form a ZIF-8 intermediate layer. The pressure of vacuum filtration is 0.07 MPa and the time is 30 s. After adding the aqueous solution and soaking for 3 min, remove the excess solution, and load the aqueous solution on the ZIF-8 intermediate layer. Subsequently, add liquid nitrogen to freeze the ZIF-8 intermediate layer loaded with the aqueous monomer, converting the aqueous phase into an ice phase. The volume of liquid nitrogen used is 5 ml and the freezing time is 10 s. After freezing, immediately add the organic phase solution and carry out an interfacial polymerization reaction for 1 min, then remove the excess solution, and place it in a vacuum oven at 50 °C for heat curing treatment for 1 min to form a polyamide separation layer, obtaining a liquid nitrogen frozen intermediate layer forward osmosis membrane.

[0059] The effective area of the nylon support layer is 15 cm 2 , and the pore size is 0.1 μm.

[0060] Example 11 Step 1: Dissolve 30 g of 2-methylimidazole (2-Hmim) and 2 g of zinc nitrate (Zn(NO3)2·H2O) in 80 g and 8 g of methanol respectively. While vigorously stirring at a speed of 800 rpm / min at room temperature, slowly add the zinc nitrate solution to the 2-methylimidazole solution to synthesize a mixed solution. After stirring for 24 hours, centrifuge at a high speed of 10000 rpm / min for 40 minutes to collect the product. Wash the product several times with methanol, and then place the product in a vacuum dryer at 100 °C for 24 hours to obtain the ZIF-8 nanomaterial.

[0061] Step 2: Disperse 0.003 g of the ZIF-8 nanomaterial into 120 ml of methanol by stirring and then ultrasonic treatment to prepare a ZIF-8 nanomaterial dispersion.

[0062] Step 3: Use m-phenylenediamine (MPD) as the aqueous monomer, dissolve it in deionized water by stirring to prepare an aqueous solution; the mass percentage concentration of m-phenylenediamine in the prepared aqueous solution is 3 wt%.

[0063] Disperse trimesoyl chloride (TMC) into n-hexane by ultrasonic treatment to prepare an organic phase solution; the mass concentration of trimesoyl chloride in the organic phase solution is 0.5 wt%; the power of ultrasonic dispersion is 180 W and the time is 40 min; Step 4: Load the ZIF-8 nanomaterial dispersion on the surface of the nylon support layer by vacuum filtration to form a ZIF-8 intermediate layer. The pressure of vacuum filtration is 0.09 MPa and the time is 50 s. After adding the aqueous solution and soaking for 2 min, remove the excess solution, and load the aqueous solution on the ZIF-8 intermediate layer. Subsequently, add liquid nitrogen to freeze the ZIF-8 intermediate layer loaded with the aqueous monomer, converting the aqueous phase into an ice phase. The volume of liquid nitrogen used is 15 ml and the liquid nitrogen freezing time is 30 s. After freezing, immediately add the organic phase solution and carry out an interfacial polymerization reaction for 2 min. Then remove the excess solution and place it in a vacuum oven at 80 °C for heat curing treatment for 5 min to form a polyamide separation layer, obtaining a liquid nitrogen frozen intermediate layer forward osmosis membrane.

[0064] The effective area of the nylon support layer is 20 cm 2 , and the pore size is 0.25 μm.

[0065] The detailed comparison of various test data of the forward osmosis membranes in Comparative Example 1 and Examples 1-9 is shown in Table 1.

[0066] Table 1. Comparison of various test data of the forward osmosis membranes of the embodiments of the present invention

[0067] As can be seen from Table 1, constructing a ZIF-8 intermediate layer and using liquid nitrogen to regulate the diffusion behavior of the aqueous monomer can effectively improve the water permeability and reverse solute rejection ability of the forward osmosis membrane. Among them, the performance improvement of the liquid nitrogen frozen intermediate layer forward osmosis membrane prepared under the condition of a freezing time of 20 s is the most significant. Comparing Figure 2 and Figure 4 , it is found that the surface convex structure of the forward osmosis membrane obtained by constructing a liquid nitrogen frozen intermediate layer increases; comparing Figure 3 and Figure 5 , it is found that the thickness of the separation layer of the forward osmosis membrane obtained by constructing a liquid nitrogen frozen intermediate layer decreases.

[0068] In summary, the present invention provides a preparation scheme for a liquid nitrogen frozen intermediate layer forward osmosis membrane. By constructing a ZIF-8 intermediate layer and using liquid nitrogen to regulate the attachment and diffusion behavior of aqueous monomers, the structure of the separation layer is improved, not only reducing the reverse salt flux of the forward osmosis membrane, but also enhancing the water flux of the membrane. Specifically, in the technical solution of the embodiment of the present invention, a ZIF-8 intermediate layer is constructed. By utilizing the characteristics of high specific surface area and tiny pores of ZIF-8, a larger attachment area is provided for aqueous monomers. Moreover, the nitrogen atoms and hydrogen atoms in the imidazole ligand of ZIF-8 can form hydrogen bonds with water molecules, which not only promotes the adsorption of aqueous monomers but also inhibits the diffusion of aqueous monomers, prompting more aqueous monomers to attach to the intermediate layer. Then, the intermediate layer is frozen with liquid nitrogen, converting the residual aqueous phase between the layers of the intermediate layer into an ice phase, encapsulating the aqueous monomers in the intermediate layer to increase the reaction concentration of aqueous monomers in the subsequent interfacial polymerization process, avoiding the formation of defects in the polyamide separation layer, improving the density of the separation layer, enhancing the reverse solute rejection ability of the forward osmosis membrane, and reducing the reverse salt flux. The aqueous monomers encapsulated in the ice phase can only continue to diffuse into the organic phase after the ice phase dissolves. Moreover, the longer the freezing time and the deeper the freezing layer, the slower the ice layer dissolves and the slower the diffusion of aqueous monomers, thereby reducing the thickness of the separation layer, improving the hydrophilicity of the forward osmosis membrane, and enhancing the water flux. In addition, during the interfacial polymerization process, the liquid nitrogen remaining on the intermediate layer will volatilize rapidly due to the increase in temperature, forming nanoscale bubbles that escape upward, resulting in a rich nanobubble-like structure on the separation layer, providing a larger contact area and a shorter transmission path for water molecules during the operation of the membrane, and further enhancing the water flux of the membrane. A liquid nitrogen frozen intermediate layer forward osmosis membrane with high water permeability and strong reverse solute rejection performance is prepared.

[0069] In the present invention, if there is no special explanation, all the embodiments and preferred implementation methods mentioned in this article can be combined with each other to form a new technical solution.

[0070] In the present invention, if there is no special explanation, all the technical features and preferred features mentioned in this article can be combined with each other to form a new technical solution.

[0071] In the present invention, if there is no special explanation, the percentage (%) or part refers to the weight percentage or weight part relative to the composition.

[0072] In the present invention, if there is no special explanation, the various components or their preferred components involved can be combined with each other to form a new technical solution.

[0073] In the present invention, unless otherwise stated, the numerical range "a~b" represents an abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "6~22" means that all real numbers between "6~22" have been fully listed in this article, and "6~22" is only an abbreviated representation of these numerical combinations.

[0074] The "scope" disclosed in the present invention can be in the form of one or more lower limits and one or more upper limits, respectively.

[0075] In the present invention, the term "and / or" as used herein refers to any combination and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0076] In the present invention, unless otherwise specified, each reaction or operation step can be carried out sequentially or in order. Preferably, the reaction method herein is carried out sequentially.

[0077] Unless otherwise specified, the professional and scientific terms used herein have the same meanings as those familiar to persons skilled in the art. In addition, any methods or materials similar or equivalent to the described content can also be applied to the present invention.

[0078] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. Generally, the components of the present invention described and shown in the accompanying drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

Claims

1. A method for preparing a liquid nitrogen frozen intermediate layer forward osmosis membrane, characterized in that, It includes the following steps: Ultrasonically disperse ZIF-8 nanomaterials in methanol to obtain a ZIF-8 nanomaterial dispersion; Disperse the aqueous monomer in deionized water to obtain an aqueous solution; ultrasonically disperse the organic monomer in n-hexane to obtain an organic solution; Load the ZIF-8 nanomaterial dispersion on the surface of the nylon support layer by vacuum filtration to form a ZIF-8 intermediate layer; Load the aqueous solution on the ZIF-8 intermediate layer, freeze the ZIF-8 intermediate layer loaded with the aqueous phase by liquid nitrogen, then add the organic solution, carry out an interfacial polymerization reaction, and then carry out a thermal curing treatment to form a polyamide separation layer, thereby obtaining a liquid nitrogen frozen intermediate layer forward osmosis membrane.

2. The preparation method of a liquid nitrogen frozen intermediate layer forward osmosis membrane according to claim 1, wherein, The specific preparation method of the ZIF-8 nanomaterials is as follows: Dissolve 2-methylimidazole and zinc nitrate in methanol respectively. Under stirring at a speed of 600-800 rpm / min at room temperature, add the zinc nitrate solution to the 2-methylimidazole solution to synthesize a mixed solution. After the reaction, collect the product by centrifugation, wash and dry to obtain ZIF-8 nanomaterials; The addition ratio of 2-methylimidazole to methanol is 10-30 g: 60-80 g; The addition ratio of zinc nitrate to methanol is 1-2 g: 5-8 g.

3. The preparation method of a liquid nitrogen frozen intermediate layer forward osmosis membrane according to claim 1, characterized in that, In the ZIF-8 nanomaterial dispersion, when the volume of methanol is 100-120 mL, the mass of the ZIF-8 nanomaterials is 0.001-0.003 g.

4. The preparation method of a liquid nitrogen frozen intermediate layer forward osmosis membrane according to claim 1, characterized in that, The aqueous monomer is m-phenylenediamine, and the mass percentage concentration of m-phenylenediamine in the aqueous solution is 2 wt%-3 wt%.

5. The preparation method of a liquid nitrogen frozen intermediate layer forward osmosis membrane according to claim 1, characterized in that, The organic monomer uses trimesoyl chloride, and the mass concentration of trimesoyl chloride in the organic solution is 0.1 wt%-0.5 wt%.

6. The preparation method of a liquid nitrogen frozen intermediate layer forward osmosis membrane according to claim 1, wherein, The liquid nitrogen freezing time is 10-30 s.

7. The preparation method of a liquid nitrogen frozen intermediate layer forward osmosis membrane according to claim 1, characterized in that, The interfacial polymerization reaction time is 1-2 min.

8. A method for preparing a liquid nitrogen frozen intermediate layer forward osmosis membrane according to claim 1, characterized in that, The thermal curing treatment time is 1-5 min under vacuum at a temperature of 50-80 °C.

9. The preparation method of a liquid nitrogen frozen intermediate layer forward osmosis membrane according to claim 1, wherein, The power of the ultrasonic dispersion is 160 W-180 W, the time is 30 min-40 min, the pressure of the vacuum filtration is 0.07-0.09 MPa, and the time is 30-50 s.

10. A liquid nitrogen frozen intermediate layer forward osmosis membrane, characterized in that, It is obtained by the method for preparing a liquid nitrogen frozen intermediate layer forward osmosis membrane according to any one of claims 1 to 9.

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