A nanofiltration membrane with high flux and high magnesium-lithium selectivity and a preparation method thereof

Abstract

This invention relates to a high-flux and high magnesium-lithium selective nanofiltration membrane and its preparation method. The nanofiltration membrane comprises a support layer, a polyvinyl alcohol (PVA) interlayer, and a polyamide separation membrane layer containing nanoparticles, arranged sequentially. The support layer is a polysulfone porous support layer; the nanoparticles are UiO-66-NH2. The preparation method of the PVA interlayer includes: immersing the polysulfone porous support layer in a PVA solution for soaking treatment; removing it and drying the membrane surface; then immersing it in an aldehyde solution A for soaking treatment; and then immersing it in an aldehyde solution B for soaking treatment; to obtain the PVA interlayer, the membrane is removed and washed with deionized water to remove residual aldehydes from the membrane surface. The aldehyde solution A is an alkyl dialdehyde solution, wherein the alkyl group of the alkyl dialdehyde is a straight-chain alkyl group, and the two aldehyde groups are located at the ends of the straight-chain alkyl group. This invention can improve the permeation flux of the nanofiltration membrane while ensuring the magnesium-lithium separation effect.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane preparation technology, specifically to a high-flux and high magnesium-lithium selectivity nanofiltration membrane and its preparation method. Background Technology

[0002] Nanofiltration membranes are a pressure-driven membrane separation technology that primarily uses pore size sieving, the Donnan effect, and dielectric effects to selectively separate monovalent and polyvalent ions. Currently, based on traditional polyamide nanofiltration membranes, the pore size of the polyamide separation layer is adjusted by controlling the interfacial reaction time of the polyamine and acyl chloride, solvent concentration, or adding modifiers to achieve the retention of divalent magnesium ions while allowing monovalent lithium ions to pass through, effectively separating magnesium and lithium. However, this method suffers from poor lateral permeability of both the support layer (large pore size but low porosity) and the polyamide separation layer (smaller pore size required for lithium and magnesium separation). When the solution permeates through the polyamide separation layer to the support layer, the pore size is further reduced. Because the support layer has many non-porous areas, the solution cannot pass directly through it. Instead, it needs to be transported laterally between the support layer and the polyamide separation layer to reach the porous areas of the support layer. When the lateral penetration ability of both is poor, it will inevitably affect the water flux of the nanofiltration membrane. Therefore, currently, nanofiltration membranes used for the selective separation of lithium and magnesium generally have the problem of low permeation flux. On the other hand, if the permeation flux is increased, the pore size and porosity of the polyamide separation layer need to be increased, which will inevitably affect the selectivity of magnesium and lithium. Therefore, how to improve the permeability of nanofiltration membranes without affecting the selectivity of magnesium and lithium is one of the main difficulties in promoting the application of nanofiltration membrane separation technology in the separation of magnesium and lithium. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a high-flux and high magnesium-lithium selectivity nanofiltration membrane and its preparation method, which can improve the permeation flux of the nanofiltration membrane while ensuring the magnesium-lithium separation effect.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] A high-flux and high magnesium-lithium selective nanofiltration membrane comprises a polysulfone porous support layer, a polyvinyl alcohol intermediate layer, and a polyamide separation membrane layer containing nanoparticles arranged sequentially.

[0006] The support layer is a polysulfone porous support layer; the nanoparticles are UiO-66-NH2.

[0007] The method for preparing the polyvinyl alcohol interlayer includes the following steps:

[0008] Immerse the polysulfone porous support layer in a polyvinyl alcohol solution for 1-15 minutes. After removing it and drying the membrane surface, immerse it in aldehyde solution A for 1-3 minutes, and then immerse it in aldehyde solution B for 1-10 minutes to obtain the polyvinyl alcohol intermediate layer. After removing it, wash it with deionized water to remove residual aldehydes from the membrane surface.

[0009] The aldehyde solution A is an alkyl dialdehyde solution, wherein the alkyl group of the alkyl dialdehyde is a straight-chain alkyl group, and the two aldehyde groups are located at the end of the straight-chain alkyl group.

[0010] A method for preparing a high-flux and high magnesium-lithium selective nanofiltration membrane includes the following steps:

[0011] Step 1: Preparation of the polyvinyl alcohol interlayer: Immerse the polysulfone porous support layer in a polyvinyl alcohol solution for 1-15 minutes. After removing it and drying the membrane surface, immerse it in aldehyde solution A for 1-3 minutes, and then immerse it in aldehyde solution B for 1-10 minutes. The polyvinyl alcohol interlayer is obtained. After removing it, wash it with deionized water to remove residual aldehydes from the membrane surface, and the polyvinyl alcohol ultrafiltration membrane is obtained. Store it in deionized water for later use.

[0012] The aldehyde solution A is an alkyl dialdehyde solution.

[0013] The alkyl dialdehyde has 7-12 carbons, with two aldehyde groups located on the terminal carbons of the straight-chain alkyl group. The aldehyde solution B is a small molecule aldehyde solution.

[0014] Step 2, Preparation of the polyamide separation membrane layer: Immerse the polyvinyl alcohol ultrafiltration membrane in an aqueous solution for 20-120 seconds. After removal, dry the membrane surface with an air gun to ensure there are no visible water droplets. Then immerse it in an organic solution for interfacial polymerization for 10-60 seconds to generate the polyamide separation membrane layer. After the reaction, heat-treat the membrane in an oven at 50-100℃ for 4-10 minutes to obtain a magnesium-lithium selective nanofiltration membrane. Store it in deionized water for later use. The aqueous solution contains multifunctional amines and nanoparticles, wherein the nanoparticles are UiO-66-NH2. The organic solution contains polyacrylamide chloride monomers.

[0015] As a further technical solution, the alkyl dialdehyde is 1,7-heptanedialdehyde, 1,8-octanedialdehyde, 1,9-nonanedialdehyde, 1,10-decanedialdehyde, 1,11-undecanedialdehyde, or 1,12-dodecanedialdehyde.

[0016] As a further technical solution, the small molecule aldehyde is formaldehyde, acetaldehyde, glyoxal, or propionaldehyde.

[0017] As a further technical solution, the concentration of the polyvinyl alcohol solution is 0.1-5 wt%.

[0018] As a further technical solution, the aldehyde solution is prepared by dissolving aldehydes in deionized water, adding an acid adjuster to adjust the pH of the system to 1-2, thereby obtaining an aldehyde solution; the concentration of aldehydes in the aldehyde solution is 0.1-2 wt%.

[0019] As a further technical solution, the pH adjuster is one or more of hydrochloric acid and sulfuric acid.

[0020] As a further technical solution, the aqueous solution is prepared by dissolving the multifunctional amine and nanoparticles in deionized water to obtain the aqueous solution.

[0021] As a further technical solution, the concentration of the multifunctional amine in the aqueous solution is 0.5-3 wt%, and the concentration of the nanoparticles is 0.01-2 wt%.

[0022] As a further technical solution, the multifunctional amine includes one or more of piperazine, m-phenylenediamine, polyethyleneimine, divinyltriethylamine, and polyimide.

[0023] As a further technical solution, the aqueous solution also contains an acid absorbent;

[0024] As a further technical solution, the concentration of the acid absorbent is 0.1-2 wt%;

[0025] As a further technical solution, the acid absorbent includes one or more of sodium bicarbonate, sodium hydroxide, triethylamine, and triethanolamine.

[0026] As a further technical solution, the aqueous solution also contains a surfactant;

[0027] As a further technical solution, the concentration of the surfactant is 0.05-1 wt%;

[0028] As a further technical solution, the surfactant includes one or more of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and benzalkonium chloride.

[0029] As a further technical solution, the organic phase solution is prepared by dissolving polyacrylamide chloride in an organic phase.

[0030] As a further technical solution, the concentration of the polyacrylamide chloride monomer in the organic phase solvent is 0.05-2 wt%.

[0031] As a further technical solution, the polyacrylamide chloride monomer includes one or more of pyromellitic trimethylolpropionate chloride, terephthaloyl chloride, cyclohexyltrimethylolpropionate chloride, and pyromellitic tetramethylolpropionate chloride.

[0032] As a further technical solution, the organic phase includes one or more of the following: n-hexane, cyclohexane, n-heptane, and isoparaffin solvents.

[0033] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0034] In the preparation of the polyamide separation layer, this invention adds UiO-66-NH2 to the polyamine solution. On the one hand, the amine groups present therein can form an active layer with a large number of amine positive groups, effectively retaining divalent magnesium ions and allowing monovalent lithium ions to pass through, thereby achieving magnesium-lithium separation and improving the selectivity of magnesium-lithium separation. On the other hand, its porous structure can provide additional transport channels for the permeation of aqueous solution, thereby increasing the permeation flux to a certain extent.

[0035] This invention incorporates a polyvinyl alcohol (PVA) interlayer between a polysulfone support layer and a polyamide separation layer. During the preparation of the PVA interlayer, aldehyde solution A and aldehyde solution B are passed sequentially. Under the influence of aldehyde solution A, near the support layer, two aldehyde groups (4-10 carbons apart) on the dialdehyde condense with different vinyl alcohol units of PVA to form rings. In addition to the small ring structure formed by the aldehyde and alcohol groups, a large ring structure can also be formed between the alkyl groups between the two aldehyde groups and the PVA chain, improving the lateral transport capacity of the interlayer. This allows the aqueous solution to be smoothly transported laterally through the interlayer to the porous region when it reaches the non-porous part of the support layer, and then through the polysulfone support layer. This reduces the lateral transport resistance of the aqueous solution within the interlayer and increases the permeation flux. Under the influence of aldehyde solution B, near the PVA separation layer, small molecule aldehydes condense with PVA to form a uniform microporous structure, which facilitates the storage and release of amine monomers. This promotes the formation of a thin and ordered PVA separation layer, enhancing its selectivity.

[0036] This invention utilizes a polyvinyl alcohol intermediate layer prepared by limiting the types and reaction times of two types of aldehyde solutions. This intermediate layer, combined with a polyamide separation layer prepared by adding UiO-66-NH2 porous nanoparticles, works synergistically to significantly improve the permeation flux while ensuring magnesium-lithium selectivity. This invention yields a nanofiltration membrane with a magnesium permeation flux of over 100 LMH, an initial magnesium rejection rate of over 98%, a lithium permeation flux of over 100 LMH, and a lithium rejection rate of less than 20%. This is the result of the combined effect of the above parameters, and the control of each parameter is indispensable. Detailed Implementation

[0037] The present invention will be further described in detail below with reference to the embodiments.

[0038] The nonwoven fabric used in the preparation of the polysulfone porous support layer was AWA KS-7910, with an air permeability of 1.3 cc / cm. 2 / sec, thickness 0.1 mm; Polysulfone used: Solvay Udel P-3500 LCD MB;

[0039] Polyvinyl alcohol: molecular weight (M) W Approximately 145,000 g / mol, Merck.

[0040] Unless otherwise specified, all raw materials used in this invention are commercially available.

[0041] Examples 1-10: Effects of Aldehyde Solution Type, Concentration, and Soaking Time on Nanofiltration Membrane Performance

[0042] A high-flux and high magnesium-lithium selective nanofiltration membrane is prepared by the following steps:

[0043] Step 1: Preparation of the polyvinyl alcohol interlayer:

[0044] Step 1-1, Preparation of polyvinyl alcohol solution: First, dissolve polyvinyl alcohol in deionized water to prepare a polyvinyl alcohol solution; the concentration of the polyvinyl alcohol is 0.2 wt%.

[0045] Steps 1-2: Preparation of aldehyde solutions: First, dissolve 1,6-hexanedialdehyde in deionized water and adjust the pH of the system to 1-2 with hydrochloric acid to obtain aldehyde solution A; then dissolve acetaldehyde in deionized water and adjust the pH of the system to 1-2 with hydrochloric acid to obtain aldehyde solution B.

[0046] Steps 1-3: Reaction: Immerse the polysulfone porous support layer in a polyvinyl alcohol solution for 8 minutes. After removing it and drying the membrane surface, immerse it in aldehyde solution A and then in aldehyde solution B for further treatment. This yields a polyvinyl alcohol intermediate layer. After removing it, wash it with deionized water to remove residual aldehydes from the membrane surface, obtaining a polyvinyl alcohol ultrafiltration membrane. Store the membrane in deionized water for later use.

[0047] Step 2: Preparation of the polyamide separation membrane layer:

[0048] Step 2-1, Preparation of aqueous solution: Dissolve the multifunctional amine, acid absorbent triethylamine, surfactant sodium dodecyl sulfate, and nanoparticles UiO-66-NH2 in deionized water to prepare an aqueous solution;

[0049] In the aqueous solution, the concentration of the multifunctional amine is 1 wt%, the concentration of the acid absorbent is 1 wt%, the concentration of the surfactant is 0.1 wt%, and the concentration of the nanoparticles is 0.25 wt%.

[0050] Step 2-2, Preparation of the organic phase solution: Dissolve trimesoyl chloride in the organic phase n-hexane to prepare the organic phase solution.

[0051] The concentration of pyromellitic acid chloride in the organic phase solution is 0.1 wt%.

[0052] Steps 2-3: Interfacial polymerization reaction: Immerse the polyvinyl alcohol ultrafiltration membrane in an aqueous solution for 60 seconds. After removal, dry the membrane surface with an air gun to ensure that there are no visible water droplets. Then immerse it in an organic solution for interfacial polymerization for 30 seconds to generate a polyamide separation membrane layer. After the reaction is completed, heat-treat it in an oven at 70°C for 5 minutes to obtain a high-flux and high magnesium-lithium selectivity nanofiltration membrane.

[0053] The types, concentrations, and soaking times of the aldehyde solutions in Examples 1-10 are shown in Table 1;

[0054] Table 1

[0055]

[0056] Examples 11-15: Effects of polyvinyl alcohol concentration and soaking time on nanofiltration membrane performance

[0057] A high-flux and high magnesium-lithium selective nanofiltration membrane is prepared by the following steps:

[0058] Step 1: Preparation of the polyvinyl alcohol interlayer:

[0059] Step 1-1, Preparation of polyvinyl alcohol solution: First, dissolve polyvinyl alcohol in deionized water to prepare a polyvinyl alcohol solution;

[0060] Steps 1-2: Preparation of aldehyde solutions: First, dissolve 1,10-decanoic acid in deionized water and adjust the pH of the system to 1-2 with hydrochloric acid to obtain aldehyde solution A; then dissolve acetaldehyde in deionized water and adjust the pH of the system to 1-2 with hydrochloric acid to obtain aldehyde solution B.

[0061] In aldehyde solution A, the concentration of 1,10-decanoic acid is 19.0 mmol / L; in aldehyde solution B, the concentration of acetaldehyde is 110.5 mmol / L.

[0062] Steps 1-3, Reaction: Immerse the polysulfone porous support layer in a polyvinyl alcohol solution for several minutes. After removing it and drying the membrane surface, immerse it in aldehyde solution A for 1.5 minutes, then immerse it in aldehyde solution B for 3 minutes. The resulting polyvinyl alcohol intermediate layer is then removed and washed with deionized water to remove residual aldehydes from the membrane surface, yielding a polyvinyl alcohol ultrafiltration membrane. Store the membrane in deionized water for later use.

[0063] Step 2: Preparation of the polyamide separation membrane layer:

[0064] Step 2-1, Preparation of aqueous solution: Dissolve the multifunctional amine, acid absorbent triethylamine, surfactant sodium dodecyl sulfate, and nanoparticles UiO-66-NH2 in deionized water to prepare an aqueous solution;

[0065] In the aqueous solution, the concentration of the multifunctional amine is 1 wt%, the concentration of the acid absorbent is 1 wt%, the concentration of the surfactant is 0.1 wt%, and the concentration of the nanoparticles is 0.25 wt%.

[0066] Step 2-2, Preparation of the organic phase solution: Dissolve trimesoyl chloride in the organic phase n-hexane to prepare the organic phase solution.

[0067] The concentration of pyromellitic acid chloride in the organic phase solution is 0.1 wt%.

[0068] Steps 2-3: Interfacial polymerization reaction: Immerse the polyvinyl alcohol ultrafiltration membrane in an aqueous solution for 60 seconds. After removal, dry the membrane surface with an air gun to ensure that there are no visible water droplets. Then immerse it in an organic solution for interfacial polymerization for 30 seconds to generate a polyamide separation membrane layer. After the reaction is completed, heat-treat it in an oven at 70°C for 5 minutes to obtain a high-flux and high magnesium-lithium selectivity nanofiltration membrane.

[0069] The concentration of polyvinyl alcohol used in Examples 11-15 and the soaking time in the polyvinyl alcohol solution are shown in Table 2.

[0070] Table 2

[0071]

[0072] Examples 16-21: The effect of nanoparticle concentration on nanofiltration membrane performance

[0073] A high-flux and high magnesium-lithium selective nanofiltration membrane is prepared by the following steps:

[0074] Step 1: Preparation of the polyvinyl alcohol interlayer:

[0075] Step 1-1, Preparation of polyvinyl alcohol solution: First, dissolve polyvinyl alcohol in deionized water to prepare a polyvinyl alcohol solution; the concentration of the polyvinyl alcohol is 0.25 wt%.

[0076] Steps 1-2: Preparation of aldehyde solutions: First, dissolve 1,10-decanoic acid in deionized water and adjust the pH of the system to 1-2 with hydrochloric acid to obtain aldehyde solution A; then dissolve acetaldehyde in deionized water and adjust the pH of the system to 1-2 with hydrochloric acid to obtain aldehyde solution B.

[0077] In aldehyde solution A, the concentration of 1,10-decanoic acid is 19.0 mmol / L; in aldehyde solution B, the concentration of acetaldehyde is 105.5 mmol / L.

[0078] Steps 1-3, Reaction: Immerse the polysulfone porous support layer in a polyvinyl alcohol solution for 8 minutes. After removing it and drying the membrane surface, immerse it in aldehyde solution A for 1.5 minutes, and then immerse it in aldehyde solution B for 3 minutes. A polyvinyl alcohol intermediate layer is obtained. After removing it, wash it with deionized water to remove residual aldehydes from the membrane surface, and obtain a polyvinyl alcohol ultrafiltration membrane. Store it in deionized water for later use.

[0079] Step 2: Preparation of the polyamide separation membrane layer:

[0080] Step 2-1, Preparation of aqueous solution: Dissolve the multifunctional amine, acid absorbent triethylamine, surfactant sodium dodecyl sulfate, and nanoparticles UiO-66-NH2 in deionized water to prepare an aqueous solution;

[0081] In the aqueous solution, the concentration of the multifunctional amine is 1 wt%, the concentration of the acid absorbent is 1 wt%, and the concentration of the surfactant is 0.1 wt%.

[0082] Step 2-2, Preparation of the organic phase solution: Dissolve trimesoyl chloride in the organic phase n-hexane to prepare the organic phase solution.

[0083] The concentration of pyromellitic acid chloride in the organic phase solution is 0.1 wt%.

[0084] Steps 2-3: Interfacial polymerization reaction: Immerse the polyvinyl alcohol ultrafiltration membrane in an aqueous solution for 60 seconds. After removal, dry the membrane surface with an air gun to ensure that there are no visible water droplets. Then immerse it in an organic solution for interfacial polymerization for 30 seconds to generate a polyamide separation membrane layer. After the reaction is completed, heat-treat it in an oven at 70°C for 5 minutes to obtain a high-flux and high magnesium-lithium selectivity nanofiltration membrane.

[0085] The concentrations of nanoparticles in Examples 16-21 are shown in Table 3;

[0086] Table 3

[0087]

[0088] Comparative Example 1

[0089] Similar to Example 22, except that the polyamide separation layer is prepared directly on the polysulfone porous support layer, without preparing the polyvinyl alcohol intermediate layer.

[0090] Comparative Example 2

[0091] Similar to Example 22, except that no nanoparticles are added to the aqueous solution.

[0092] Example 1: Performance Test

[0093] The filtration performance of the nanofiltration membranes prepared in each embodiment and comparative example was tested. After the first test, water treatment was carried out to extract lithium from the magnesium-lithium aqueous solution. After continuous extraction for 30 days, the filtration performance was tested again. The results are shown in Table 4.

[0094] The membrane filtration performance was tested under the following conditions: 2000 ppm magnesium chloride solution and 2000 ppm lithium chloride solution, temperature 25°C, pH=7, and test pressure 70 psi.

[0095]

[0096] The data in Table 4 shows that:

[0097] 1) A comparison of the data from Example 19 and Comparative Example 1 shows that when a polyacrylol intermediate layer prepared by aldehyde solution A and aldehyde solution B is provided between the polysulfone porous support layer and the polyamide separation membrane layer, its flux can be significantly improved.

[0098] A comparison of the data from Examples 1-6 shows that during the preparation of the polyethylene interlayer, before soaking with small molecule aldehydes, dialdehydes with 4-10 carbon atoms as intervening are first used for soaking. As the number of carbon atoms between the dialdehydes increases, the permeation flux increases. This may be because the alkyl group between the two aldehyde groups of the dialdehyde forms a macrocyclic structure with the polyvinyl alcohol, which improves the lateral transport capacity of the polyvinyl alcohol and reduces its lateral transport resistance, thereby increasing the permeation flux.

[0099] 2) As can be seen from the comparison of Examples 4 and 7-8, in the preparation process of polyvinyl alcohol intermediate layer, compared with Example 4, increasing the soaking time in aldehyde solution A and shortening the soaking time in aldehyde solution B will improve the permeation flux, but the magnesium-lithium selectivity of nanofiltration membrane will be slightly reduced and the stability will be seriously reduced.

[0100] 3) A comparison of Examples 4 and 9-10 shows that during the preparation of polyvinyl alcohol, compared to Example 4, Example 9 increases the concentration of aldehyde solution A, resulting in increased permeation flux but decreased stability. In Example 10, decreasing the concentration of aldehyde solution A decreases the permeation flux, but the difference in stability is not significant.

[0101] 4) As can be seen from the comparison of Examples 11-13, during the preparation of polyvinyl alcohol, as the concentration of the polyvinyl alcohol solution increases, its permeation flux increases, but the stability of magnesium-lithium selectivity decreases.

[0102] 5) As can be seen from the comparison of Examples 12 and 14-15, during the preparation of polyvinyl alcohol, as the soaking time in the polyvinyl alcohol solution is extended, its permeation flux increases, but the stability of magnesium-lithium selectivity decreases.

[0103] 6) As can be seen from the comparison of Examples 16-21 and Comparative Example 2, the permeation flux decreases as the concentration of nanoparticles increases, but the magnesium-lithium selectivity increases. Among them, when the polyvinyl alcohol interlayer is present, Comparative Example 2, which does not add nanoparticles, has the highest permeation flux and the lowest magnesium-lithium selectivity.

[0104] In summary, this invention optimizes the type, soaking time, and concentration of the aldehyde solution, the concentration and soaking time of the polyvinyl alcohol, and the concentration of nano-ions during the preparation of the polyamide separation layer, resulting in the following technical solution: Polyvinyl alcohol concentration 0.1-0.3 wt%, soaking in the polyvinyl alcohol solution for 6-8 min; aldehyde solution A uses alkyl dialdehydes with 7-12 carbons and the aldehyde group located at the end, with a concentration of 15-25 mmol / L, and soaking for 1.5 min; aldehyde solution B uses small molecule aldehydes, with a concentration of 100-130 mmol / L, and soaking for 3 min; the concentration of nanoparticles is selected as 2-3 wt%. Using this technical solution, nanofiltration membranes with magnesium permeation flux exceeding 100 LMH and initial magnesium rejection exceeding 98%, lithium permeation flux exceeding 100 LMH and lithium rejection below 20% can be obtained, exhibiting good stability. Specifically, the membranes are prepared by soaking in a polyvinyl alcohol solution (0.2-0.25 wt%) for 8 min, using terminal dialdehydes (8 carbon atoms apart) in aldehyde solution A (18-21 mmol / L alkyl dialdehyde concentration) for 1.5 min, and using small molecule aldehydes (110-120 mmol / L) in aldehyde solution B for 3 min. A nanofiltration membrane with a nanoparticle concentration of 2.5 wt% exhibits optimal performance in all aspects.

[0105] The technical effects of the present invention are the result of the combined effect of the above parameters. The parameters interact and influence each other, and any change in any parameter will affect the performance of the nanofiltration membrane.

[0106] The embodiments described above are merely preferred embodiments of the present invention, and not an exhaustive list of all possible implementations of the present invention. Any obvious modifications made by those skilled in the art without departing from the principles and spirit of the present invention should be considered to be included within the scope of protection of the claims of the present invention.

Claims

1. A high-flux and high magnesium-lithium selective nanofiltration membrane, characterized in that, It includes a support layer, a polyvinyl alcohol intermediate layer, and a polyamide separation membrane layer containing nanoparticles arranged sequentially; The support layer is a polysulfone porous support layer; the nanoparticles are UiO-66-NH2. The method for preparing the polyvinyl alcohol interlayer includes the following steps: The polysulfone porous support layer is immersed in a polyvinyl alcohol solution for 1-15 minutes. After removing it and drying the surface moisture, it is then immersed in aldehyde solution A for 1-3 minutes, and then immersed in aldehyde solution B for 1-10 minutes. After obtaining the polyvinyl alcohol intermediate layer, remove it and wash it with deionized water to remove residual aldehydes from the membrane surface. The aldehyde solution A is an alkyl dialdehyde solution, wherein the alkyl group of the alkyl dialdehyde is a straight-chain alkyl group, and the two aldehyde groups are located at the end of the straight-chain alkyl group; The alkyl dialdehyde has 7-12 carbon atoms, and the aldehyde solution B is a small molecule aldehyde solution, wherein the small molecule aldehyde is formaldehyde, acetaldehyde, glyoxal, or propionaldehyde.

2. A method for preparing a high-flux and high magnesium-lithium selective nanofiltration membrane, characterized in that, Includes the following steps: Step 1: Preparation of polyvinyl alcohol intermediate layer: Immerse the polysulfone porous support layer in polyvinyl alcohol solution for 1-15 min, remove it and blow dry the surface moisture of the film, then immerse it in aldehyde solution A for 1-3 min, and then immerse it in aldehyde solution B for 1-10 min. The polyvinyl alcohol intermediate layer was obtained, and after being removed, it was washed with deionized water to remove residual aldehydes on the membrane surface, thus obtaining a polyvinyl alcohol ultrafiltration membrane, which was stored in deionized water for later use. The aldehyde solution A is an alkyl dialdehyde solution, wherein the alkyl group of the alkyl dialdehyde is a straight-chain alkyl group, and the two aldehyde groups are located at the end of the straight-chain alkyl group; The alkyl dialdehyde has 7-12 carbons, and the aldehyde solution B is a small molecule aldehyde solution, wherein the small molecule aldehyde is formaldehyde, acetaldehyde, glyoxal, or propionaldehyde. Step 2, Preparation of the polyamide separation membrane layer: Immerse the polyvinyl alcohol ultrafiltration membrane in an aqueous solution for 20-120 seconds. After removal, dry the membrane surface with an air gun to ensure there are no visible water droplets. Then immerse it in an organic solution for interfacial polymerization for 10-60 seconds to generate the polyamide separation membrane layer. After the reaction, heat-treat the membrane in an oven at 50-100℃ for 4-10 minutes to obtain a magnesium-lithium selective nanofiltration membrane. Store the membrane in deionized water for later use. The aqueous solution contains multifunctional amines and nanoparticles, wherein the nanoparticles are UiO-66-NH2. The organic solution contains polyacrylamide chloride monomers.

3. The method for preparing a high-flux and high magnesium-lithium selective nanofiltration membrane according to claim 2, characterized in that, The alkyl dialdehyde is 1,7-heptanedialdehyde, 1,8-octanedialdehyde, 1,9-nonanedialdehyde, 1,10-decanedialdehyde, 1,11-undecanedialdehyde, or 1,12-dodecanedialdehyde.

4. The method for preparing a high-flux and high magnesium-lithium selective nanofiltration membrane according to claim 2, characterized in that, The concentration of the polyvinyl alcohol solution is 0.1-5 wt%.

5. The method for preparing a high-flux and high magnesium-lithium selective nanofiltration membrane according to claim 2, characterized in that, Preparation of aldehyde solution A or aldehyde solution B: Dissolve the aldehyde in deionized water, add an acid adjuster to adjust the pH of the system to 1-2, and obtain the aldehyde solution; The concentration of aldehydes in the aldehyde solution is 0.1-2 wt%; The pH adjuster is one or more of hydrochloric acid and sulfuric acid.

6. The method for preparing a high-flux and high magnesium-lithium selective nanofiltration membrane according to claim 2, characterized in that, Preparation of the aqueous solution: The multifunctional amine and nanoparticles are dissolved in deionized water to prepare the aqueous solution; In the aqueous solution, the concentration of the multifunctional amine is 0.5-3 wt%, and the concentration of the nanoparticles is 0.01-2 wt%. The multifunctional amine includes one or more of piperazine, m-phenylenediamine, polyethyleneimine, divinyltriethylamine, and polyimide.

7. The method for preparing a high-flux and high magnesium-lithium selective nanofiltration membrane according to claim 2, characterized in that, The aqueous solution also contains an acid absorbent; The concentration of the acid absorbent is 0.1-2 wt%; The acid absorbent includes one or more of sodium bicarbonate, sodium hydroxide, triethylamine, and triethanolamine.

8. The method for preparing a high-flux and high magnesium-lithium selective nanofiltration membrane according to claim 2, characterized in that, The aqueous solution also contains surfactants; The concentration of the surfactant is 0.05-1 wt%; The surfactant includes one or more of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and benzalkonium chloride.

9. The method for preparing a high-flux and high magnesium-lithium selective nanofiltration membrane according to claim 2, characterized in that, Preparation of the organic phase solution: Dissolve polyacryl chloride in the organic phase to prepare the organic phase solution; The concentration of the polyacrylamide chloride monomer in the organic phase solvent is 0.05-2 wt%. The polyacryl chloride monomer includes one or more of pyromellitic trichloroethylene chloride, terephthaloyl chloride, cyclohexyltrichloroethylene chloride, and pyromellitic tetrachloroethylene chloride. The organic phase includes one or more of the following: n-hexane, cyclohexane, n-heptane, and isoparaffin solvents.

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