Preparation method of pressure-resistant high-flux polyvinylidene fluoride nanofiltration membrane
By employing phase inversion and interfacial polymerization methods to grow hydroxyl iron oxide nanoparticle layers in situ, the pressure resistance and flux issues of nanofiltration membranes were solved, achieving stable water treatment results under high pressure and expanding their application areas.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- ZHEJIANG GREEN NEW MATERIALS
- Filing Date
- 2023-12-12
- Publication Date
- 2026-07-10
AI Technical Summary
Existing commercial nanofiltration membranes have limitations in applications such as low flux, poor pressure resistance, and unstable and easily peeled separation layers, restricting their use in industries such as drinking water softening, wastewater purification, and food processing.
Polyvinylidene fluoride blend membranes were prepared by phase inversion method, and a stable separation layer was formed by in-situ growth of iron hydroxyl oxide nanoparticles combined with interfacial polymerization, thereby improving the membrane's pressure resistance and water flux.
The prepared polyvinylidene fluoride nanofiltration membrane maintains high water flux and separation layer stability under high pressure, making it suitable for different environmental conditions and broadening its application range.
Smart Images
Figure CN117563428B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of membrane separation and water purification technology, and particularly relates to a pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane and its nanofiltration process. Background Technology
[0002] Membrane separation technology, a novel separation technology that rapidly developed in the late 1960s, has attracted widespread attention due to its advantages such as high separation efficiency, low energy consumption, no phase change, energy saving, small size, and separability. It has been extensively researched and applied in fields such as chemical engineering, energy, food, medicine, and environmental protection, generating significant economic and social benefits. In water treatment systems, commonly used membrane types include microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. Ultrafiltration / microfiltration can effectively remove bacteria, viruses, and proteinaceous pollutants from water, but its retention effect on small molecule organic pollutants such as salts, heavy metal ions, and antibiotics is limited. Reverse osmosis can remove all ions and small molecules dissolved in water, producing high-purity water. To overcome osmotic pressure, reverse osmosis requires relatively high operating pressures (typically 1.0-10.0 MPa) to ensure sufficient water production. Its energy-intensive nature limits the application of reverse osmosis technology. Furthermore, the long-term health effects of drinking high-purity water produced by reverse osmosis have sparked widespread debate and concern in recent years.
[0003] Nanofiltration is a pressure-driven liquid membrane separation process that falls between ultrafiltration and reverse osmosis. It is also known as "dense ultrafiltration" and "loose reverse osmosis" membranes. Theoretically, it can efficiently retain divalent and high-valent ions, as well as low- and medium-molecular-weight organic matter, at relatively low operating pressures, while retaining some beneficial minerals. It is an important means of achieving deep water purification and ensuring drinking water safety. Currently, commercially available nanofiltration membranes are generally composite membranes composed of a support layer base membrane and a separation functional layer obtained through interfacial polymerization. The support layer membrane is typically made of hydrophilic materials such as polyethersulfone and polysulfone. However, due to the relatively poor physicochemical stability of this type of membrane support, the application range and environment of the prepared nanofiltration membrane are relatively limited. For treating wastewater containing acids, alkalis, organic solvents, or high temperatures, nanofiltration membranes obtained with this type of base membrane support layer cannot meet the corresponding requirements.
[0004] Fluorine-containing materials such as polyvinylidene fluoride (PVDF) possess excellent chemical and thermal stability, resistance to various acids, alkalis, and solvents, resistance to ultraviolet radiation, and high mechanical strength, which are crucial for the practical application of separation membranes. Furthermore, due to their abundance of good solvents, they are easily used to prepare separation membranes via simple phase inversion methods, making them one of the most widely used fluoropolymers in membrane applications. Currently, industrially produced PVDF separation membranes are primarily ultrafiltration membranes. Conventional commercial nanofiltration membranes suffer from technical problems such as low applicable flux, poor pressure resistance, and unstable separation layers that are easily peeled off.
[0005] Based on the excellent physical, chemical, and mechanical properties of polyvinylidene fluoride membrane materials, the application of nanofiltration membranes in industries such as drinking water softening, wastewater purification, and food processing can be greatly broadened and enriched. Summary of the Invention
[0006] This invention addresses the technical problems of conventional commercial nanofiltration membranes, such as low applicable flux, poor pressure resistance, and unstable and easily peeled separation layers. It proposes a method for preparing a pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane. This nanofiltration membrane has the characteristics of good stability, pressure resistance, and high water flux, and can achieve efficient and rapid purification of substances.
[0007] To achieve the above objectives, the present invention adopts the following technical solution:
[0008] A method for preparing a pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane includes the following steps:
[0009] (1) Add the dried polyvinylidene fluoride resin and pore-forming additive to the solvent, heat and stir, let stand to degas, and then add hydrophilic inorganic nanoparticles to obtain the polyvinylidene fluoride casting solution. The polyvinylidene fluoride blend membrane is prepared by immersion precipitation phase inversion method.
[0010] (2) The polyvinylidene fluoride blend membrane was placed in a ferric ethanol solution, impregnated and then dried to obtain a polyvinylidene fluoride blend membrane loaded with iron species.
[0011] (3) Dissolve ferric iron in deionized water to obtain ferric iron aqueous solution, then add ammonium fluoride and stir to obtain a mixed solution. Then place the polyvinylidene fluoride ultrafiltration membrane loaded with iron species in the mixed solution to grow hydroxyl iron oxide nanoparticles in situ. After the growth is completed, dry to obtain polyvinylidene fluoride blend base membrane loaded with hydroxyl iron oxide.
[0012] (4) The upper surface of the obtained polyvinylidene fluoride membrane loaded with iron hydroxyl oxide is immersed in an aqueous solution, and then immersed in an organic solution to carry out an interfacial polymerization reaction to prepare a separation layer. Finally, the membrane is placed in an oven for heating and curing to obtain a pressure-resistant high-flux polyvinylidene fluoride nanofiltration membrane.
[0013] In the phase inversion stage, water is used as the non-solvent, forming a solvent / non-solvent system together with the solvent. A blend solution of polyvinylidene fluoride (PVDF) and hydrophilic inorganic nanoparticles is used as the casting solution. PVDF blend membranes are prepared via an immersion precipitation phase inversion method. Specifically, after the casting solution enters the coagulation bath, the solvent and non-solvent diffuse through the liquid film / coagulation bath interface. When the exchange between the solvent and non-solvent reaches a certain level, the casting solution becomes a thermodynamically unstable system, leading to phase separation. After phase separation, the solvent and non-solvent further exchange, resulting in pore aggregation, interphase flow, and the formation of the polymer-rich phase as the main structure of the membrane, while the polymer-poor phase forms pores, ultimately solidifying into a membrane. In the in-situ growth stage, the PVDF blend membrane is immersed in a ferric iron / ethanol solution, allowing iron ions to adsorb onto the membrane surface and into the pores. The iron ions hydrolyze in the aqueous solution to generate ferric hydroxide, which then further nucleates and grows in a growth solution prepared from ferric iron and ammonium fluoride, ultimately resulting in a layer of iron hydroxide nanoparticles growing on the surface of the PVDF membrane. During the interfacial polymerization stage, due to the presence of the hydroxyl iron oxide nanoparticle layer, the separation layer obtained after the interfacial polymerization reaction of aqueous and organic monomers can be stably attached to the surface of the polyvinylidene fluoride membrane, thereby increasing the permeation performance of the nanofiltration membrane.
[0014] In the above steps, the purpose of adding hydrophilic inorganic nanoparticles is to improve the pressure resistance of the polyvinylidene fluoride (PVDF) membrane and simultaneously enhance its hydrophilicity. The purpose of in-situ growth of an iron hydroxyl oxide (IVO2) nanoparticle layer on the blend membrane surface is to improve the stability and permeability of the nanofiltration separation layer obtained through subsequent interfacial polymerization. More importantly, using PVDF polymers with excellent physicochemical properties as the base membrane ensures that the nanofiltration membrane maintains good anti-shrinkage capabilities under harsh environmental conditions (high temperature, acids, alkalis, etc.) and mechanical pressure generated by the components in practical applications. This prevents membrane tearing, flow rate reduction, and other phenomena, ensuring that the PVDF nanofiltration membrane maintains excellent separation performance and stability in different nanofiltration systems, thus increasing the application potential of PVDF nanofiltration membranes.
[0015] Polyvinylidene fluoride (PVDF) resin has a molecular weight of 200,000 to 700,000, and is produced in China. The molecular weight of PVDF is limited because porous membranes prepared from PVDF within this range exhibit better mechanical properties and flexibility, resulting in porous membranes with both good permeability and mechanical properties.
[0016] Preferably, in step (1), the pore-forming additive is at least one of polyvinylpyrrolidone and polyethylene glycol, the solvent is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, 1-methyl-2-pyrrolidone, and chloroform, and the hydrophilic inorganic nanoparticles are at least one of silicon dioxide, zinc oxide, iron oxide, and titanium dioxide with a size of 10 to 100 nanometers.
[0017] Preferably, in step (1), the mass fraction of polyvinylidene fluoride casting solution is 16-30%, and the mass ratio of hydrophilic inorganic nanoparticles to polyvinylidene fluoride is 5-30%.
[0018] Preferably, in step (2), the concentration of the ferric iron-ethanol solution is 1~4 mol / L and the soaking time is 10~60 min.
[0019] Preferably, in step (3), the concentration of the ferric aqueous solution is 0.1~1 mol / L, the in-situ growth temperature is 30~80℃, the in-situ growth time is 1~10h, the concentration of ammonium fluoride is 0.16~1.62 mol / L, and the mixture is stirred for 1~3 hours after the ammonium fluoride is added.
[0020] The concentration of ferric iron (Fe3+) in the in-situ growth stage is 0.1–1 mol / L, the in-situ growth temperature is 30–80 °C, and the in-situ growth time is 1–3 h. This is because a concentration of Fe3+ below 0.1 mol / L, a growth temperature below 30 °C, and a growth time below 1 h are unfavorable for the nucleation and growth of iron hydroxide crystals. Conversely, a concentration of iron salt solution above 1 mol / L, a growth temperature above 80 °C, and a growth time above 3 h can lead to the aggregation and accumulation of iron hydroxide nanoparticles, and the higher growth temperature also results in energy waste. It is understandable that the concentration of Fe3+ solution in the in-situ growth stage can also be 0.2 mol / L, 0.4 mol / L, 0.6 mol / L, 0.8 mol / L, or any value within the above range; the in-situ growth temperature can also be 40 °C, 50 °C, 60 °C, or any value within the above range; and the in-situ growth time can also be 1.5 h, 2 h, or any value within the above range. Those skilled in the art can adjust these values according to the actual reaction conditions.
[0021] Preferably, in steps (2) and (3), the ferric iron is at least one of ferric chloride, ferric sulfate, and ferric nitrate.
[0022] Preferably, in step (4), the mass concentration of the aqueous phase during the interfacial polymerization process is 0.1~3%, the soaking time of the aqueous phase is 2~20 minutes, the mass concentration of the organic phase is 0.01~0.2%, the soaking time of the organic phase is 1~5 minutes, and the heating and curing time is 1~10 minutes.
[0023] The mass concentrations of the aqueous and organic phases are limited because interfacial polymerization of the monomers in both phases must occur within the aforementioned suitable ranges to obtain a complete, defect-free separation layer of appropriate thickness. Too much or too little aqueous or organic phase will result in an excessively thick separation layer or surface defects, thus reducing the performance of the nanofiltration membrane. It is understood that the concentrations of the aqueous and organic phases can also be any values within the aforementioned range, and those skilled in the art can adjust them according to the actual reaction conditions.
[0024] Preferably, in step (4), the aqueous solute is any one of piperazine, m-phenylenediamine, p-phenylenediamine, ethylenediamine, and polyethyleneimine; the organic solute is any one of trimesoyl chloride and isophthaloyl chloride; and the solvent is at least one of n-hexane and xylene.
[0025] The selection of solvents, inorganic nanoparticles, ferric iron, aqueous monomers, and organic monomers is not limited to those listed in the above embodiments, and may also include other substances that can be reasonably selected and adjusted by those skilled in the art based on common sense.
[0026] A pressure-resistant high-flux polyvinylidene fluoride nanofiltration membrane, the thickness of which is 160~450μm, and which is one of the following forms: flat sheet membrane, hollow fiber membrane and tubular membrane, having a composite structure of dense separation layer and asymmetric porous support layer.
[0027] The thickness of the pressure-resistant high-flux polyvinylidene fluoride (PVDF) nanofiltration membrane is obtained using a thickness gauge. It is understood that the thickness of the pressure-resistant high-flux PVDF nanofiltration membrane can also be 180 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, or any value within the above range. This invention limits the thickness of the PVDF nanofiltration membrane to 160~450 μm because when the membrane thickness is below 160 μm, the pressure resistance is poor; when the thickness is above 450 μm, the membrane resistance increases, which is not conducive to large-scale water purification and reduces the water treatment capacity.
[0028] Preferably, the pressure tolerance range is 0~20 bar, and the pure water flux is 10~45 L / (m³). 2 (·h·bar), the retention rate of divalent salts is greater than 96%, and the retention rate of monovalent salts is 10-80%.
[0029] By adopting the above technical solution, the present invention has the following beneficial effects:
[0030] 1. This invention provides a polyvinylidene fluoride composite nanofiltration membrane prepared by phase inversion, in-situ growth, and interfacial polymerization. The preparation process is simple, efficient, and easy to operate.
[0031] 2. The polyvinylidene fluoride composite nanofiltration membrane prepared by the above method has excellent pressure resistance and high water flux. At the same time, due to the presence of the hydroxyl iron oxide nanoparticle layer, the separation layer after interfacial polymerization can be stably attached to the surface of the polyvinylidene fluoride membrane, avoiding the detachment of the separation layer.
[0032] 3. The polyvinylidene fluoride nanofiltration membrane obtained by the above method can withstand a pressure range of up to 20 bar and a pure water flux of up to 45 L / (m³). 2 (·h·bar), providing effective methodological guidance for the preparation of nanofiltration membranes. Attached Figure Description
[0033] The present invention will now be further described with reference to the accompanying drawings.
[0034] Figure 1 The image shows a front electron microscope image of the polyvinylidene fluoride blend film prepared in Example 1 with a silica content of 10% of the total solids.
[0035] Figure 2 This is an electron microscope image of the polyvinylidene fluoride nanofiltration membrane prepared in Example 1, on which hydroxyl iron oxide nanoparticles are loaded.
[0036] Figure 3 The image shown is a front electron microscope image of the polyvinylidene fluoride nanofiltration membrane prepared in Example 1. Detailed Implementation
[0037] To provide a clearer and more detailed description of the preparation method of the pressure-resistant high-flux polyvinylidene fluoride nanofiltration membrane provided in the embodiments of the present invention, the following description will be based on specific embodiments.
[0038] Example 1
[0039] Polyvinylidene fluoride (PVDF) blend membranes were prepared using a phase inversion method. Specifically, 23 g of PVDF polymer, 3 g of polyvinylpyrrolidone (PVP), and 71.45 g of N,N-dimethylformamide (NDM) were added to a three-necked flask. The mixture was stirred at 70 °C for 8 h to obtain a homogeneous and clear PVDF casting solution. Subsequently, 2.55 g of hydrophilic silica (approximately 25 nm in size) was added, and stirring was continued for 3 h to ensure uniform dispersion of the silica particles. After standing for 12 h to remove bubbles, the PVDF blend membrane was obtained by immersion precipitation phase inversion using water as a non-solvent coagulation bath. The front electron microscope image of the PVDF blend membrane is shown below. Figure 1 As shown.
[0040] A polyvinylidene fluoride (PVDF) membrane loaded with iron hydroxyl oxide was prepared using an in-situ growth method. Specifically, 27 g of ferric chloride was dissolved in 50 mL of ethanol to obtain a 2 mol / L ferric chloride / ethanol solution. The obtained PVDF blend membrane was immersed in the ferric chloride / ethanol solution for 30 min, and then dried in an oven at 60 °C for 1 h to obtain a PVDF membrane loaded with iron species. Alternatively, 6.8 g of ferric chloride was dissolved in 50 mL of deionized water to obtain a 0.5 mol / L ferric chloride solution. 2.3 g of ammonium fluoride was added, and the mixture was stirred for 3 h. The PVDF membrane loaded with iron species was then placed in the above growth solution and reacted at 60 °C for 5 h. After the reaction, the membrane was washed three times with deionized water and then freeze-dried to obtain a PVDF membrane with an iron hydroxyl oxide nanoparticle layer on its surface. The electron micrograph is shown in Figure 2.
[0041] The upper surface of the polyvinylidene fluoride membrane with the hydroxyl oxide nanoparticle layer obtained above was immersed in a 2% piperazine aqueous solution for 10 min, and then the front side of the membrane was immersed in a 0.2% trimesoyl chloride-n-hexane solution for 2 min to carry out interfacial polymerization. Finally, the membrane was placed in an oven for heating and curing for 2 min to obtain the polyvinylidene fluoride nanofiltration membrane. The electron micrograph is shown below. Figure 3 As shown, its water flux is 11 L / (m²). 2 (·h·bar), with a pressure resistance of 10 bar.
[0042] Example 2
[0043] The preparation method of the polyvinylidene fluoride blend membrane is the same as that in Example 1, except that the mass of polyvinylidene fluoride is 26g, the mass of N,N-dimethylformamide is 62.86g, and the mass of silica is 11.14g.
[0044] The preparation method of polyvinylidene fluoride membrane loaded with iron hydroxyl oxide is the same as that in Example 1, except that the concentration of ferric chloride ethanol solution is 4 mol / L, the impregnation time is 60 min, the concentration of ferric chloride in the growth stage is 1 mol / L, the amount of ammonium fluoride is 4.6 g, the growth temperature is 70 °C, and the reaction time is 8 hours.
[0045] The preparation method of the nanofiltration separation layer is the same as in Example 1, except that xylene is used as the organic solvent, and the water flux of the obtained polyvinylidene fluoride nanofiltration membrane is 8 L / (m²). 2 (·h·bar), with a pressure resistance of 20 bar.
[0046] Example 3
[0047] The preparation method of the polyvinylidene fluoride blend membrane is the same as that in Example 1, except that the added inorganic nanoparticles are 40 nm zinc oxide and the polymer solvent is N,N-dimethylacetamide.
[0048] The preparation method of the polyvinylidene fluoride membrane loaded with ferric hydroxyoxide is the same as that in Example 1, except that the ferric trivalent used is ferric sulfate.
[0049] The preparation method of the nanofiltration separation layer is the same as in Example 1, except that the water phase concentration is 1% and the organic phase concentration is 0.1%, and the water flux of the obtained polyvinylidene fluoride nanofiltration membrane is 15 L / (m²). 2 (·h·bar), with a pressure resistance of 9 bar.
[0050] Example 4
[0051] The preparation method of the polyvinylidene fluoride blend membrane is the same as that in Example 2, except that the hydrophilic inorganic nanoparticles used are 50nm-sized iron oxide.
[0052] The preparation method of the polyvinylidene fluoride membrane loaded with iron hydroxyoxide is the same as in Example 2.
[0053] The preparation method of the nanofiltration separation layer is the same as in Example 2, except that the water phase solute is polyethyleneimine, and the water flux of the obtained polyvinylidene fluoride nanofiltration membrane is 12 L / (m²). 2 (·h·bar), with a pressure resistance of 20 bar.
[0054] Example 5
[0055] The preparation method of the polyvinylidene fluoride blend membrane is the same as that in Example 1, except that the mass of polyvinylidene fluoride is 20g, the mass of N,N-dimethylformamide is 75g, and the mass of silica is 5g.
[0056] The preparation method of polyvinylidene fluoride membrane loaded with iron hydroxyl oxide is the same as that in Example 1, except that the iron chloride concentration is 1 mol / L in the impregnation stage and 0.1 mol / L in the growth stage.
[0057] The preparation method of the nanofiltration separation layer is the same as in Example 1, except that the aqueous phase monomer is m-phenylenediamine and the organic phase solute is isophthaloyl chloride, resulting in a polyvinylidene fluoride nanofiltration membrane with a water flux of 40 L / (m). 2 (·h·bar), with a pressure resistance of 8 bar.
[0058] Example 6
[0059] The preparation method of the polyvinylidene fluoride blend membrane is the same as that in Example 1, except that the mass of polyvinylidene fluoride is 18g, the mass of N,N-dimethylformamide is 80g, and the mass of silica is 2g.
[0060] The method for preparing the polyvinylidene fluoride membrane loaded with iron hydroxyoxide is the same as in Example 5.
[0061] The preparation method of the nanofiltration separation layer is the same as in Example 1, except that the mass concentration of the aqueous phase is 0.1% and the mass concentration of the organic phase is 0.01%, resulting in a polyvinylidene fluoride nanofiltration membrane with a water flux of 45 L / (m²). 2 (·h·bar), with a pressure resistance of 6 bar.
[0062] The above are merely specific embodiments of the present invention, but the technical features of the present invention are not limited thereto. Any simple changes, equivalent substitutions, or modifications made based on the present invention to solve essentially the same technical problems and achieve essentially the same technical effects are all covered within the protection scope of the present invention.
Claims
1. A method for preparing a pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane, characterized in that, The process includes the following steps: (1) Adding dried polyvinylidene fluoride resin and pore-forming additives to a solvent, heating and stirring, allowing to stand and degas, and then adding hydrophilic inorganic nanoparticles to obtain a polyvinylidene fluoride blend membrane by immersion precipitation phase inversion method; (2) Placing the polyvinylidene fluoride blend membrane in a ferric iron-ethanol solution, immersing it and then drying it to obtain a polyvinylidene fluoride blend membrane loaded with iron species; (3) Dissolving ferric iron in deionized water to obtain a ferric iron aqueous solution, then adding ammonium fluoride and stirring to obtain a mixed solution, and then placing the polyvinylidene fluoride blend membrane loaded with iron species in the mixed solution for in-situ growth of hydroxyl iron oxide nanoparticles, and drying it after growth to obtain a polyvinylidene fluoride blend base membrane loaded with hydroxyl iron oxide; (4) Immersing the upper surface of the obtained polyvinylidene fluoride blend base membrane loaded with hydroxyl iron oxide in an aqueous solution, and then immersing it in an organic solution to carry out an interfacial polymerization reaction to prepare a separation layer, and finally placing the membrane in an oven for heating and curing to obtain a pressure-resistant high-flux polyvinylidene fluoride nanofiltration membrane.
2. The method for preparing a pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane according to claim 1, characterized in that: In step (1), the pore-forming additive is at least one of polyvinylpyrrolidone and polyethylene glycol, the solvent is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, 1-methyl-2-pyrrolidone and chloroform, and the hydrophilic inorganic nanoparticles are at least one of silicon dioxide, zinc oxide, iron oxide and titanium dioxide with a size of 10~100 nanometers.
3. The method for preparing a pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane according to claim 1, characterized in that: In step (1), the mass fraction of polyvinylidene fluoride in the polyvinylidene fluoride casting solution is 16-30%, and the mass ratio of hydrophilic inorganic nanoparticles to polyvinylidene fluoride is 5-30%.
4. The method for preparing a pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane according to claim 1, characterized in that: In step (2), the concentration of the ferric iron-ethanol solution is 1~4 mol / L, and the immersion time is 10~60 min.
5. The method for preparing a pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane according to claim 1, characterized in that: In step (3), the concentration of the ferric aqueous solution is 0.1~1 mol / L, the in-situ growth temperature is 30~80℃, the in-situ growth time is 1~10h, the concentration of ammonium fluoride is 0.16~1.62 mol / L, and the mixture is stirred for 1~3 hours after the ammonium fluoride is added.
6. The method for preparing a pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane according to claim 1, characterized in that: In steps (2) and (3), the ferric iron is at least one of ferric chloride, ferric sulfate, and ferric nitrate.
7. The method for preparing a pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane according to claim 1, characterized in that: In step (4), the mass concentration of aqueous monomers in the aqueous solution during interfacial polymerization is 0.1-3%, the soaking time in the aqueous solution is 2-20 minutes, the mass concentration of organic monomers in the organic solution is 0.01-0.2%, the soaking time in the organic solution is 1-5 minutes, and the heating curing time is 1-10 minutes.
8. The method for preparing a pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane according to claim 1, characterized in that: In step (4), the aqueous phase monomer is any one of piperazine, m-phenylenediamine, p-phenylenediamine, ethylenediamine, and polyethyleneimine; the organic phase monomer is any one of pyromellitic methyl chloride and isophthalic acid methyl chloride; and the solvent is at least one of n-hexane and xylene.
9. A pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane obtained by the preparation method according to any one of claims 1-8, characterized in that: The pressure-resistant high-flux polyvinylidene fluoride nanofiltration membrane has a thickness of 160~450μm and is in the form of a flat sheet membrane, hollow fiber membrane, or tubular membrane, and has a composite structure of a dense separation layer and an asymmetric porous support layer.
10. The pressure-resistant, high-flux polyvinylidene fluoride nanofiltration membrane according to claim 9, characterized in that: Pressure withstand range: 0~20 bar; pure water flux: 10~45 L / (m²) 2 (·h·bar), the retention rate of divalent salts is greater than 96%, and the retention rate of monovalent salts is 10-80%.