A positively charged composite nanofiltration membrane and a preparation method thereof

Abstract

The application belongs to the technical field of nanofiltration membrane modification, and provides a positively charged composite nanofiltration membrane and a preparation method thereof.The preparation method of the positively charged composite nanofiltration membrane comprises the following steps: S1: a base layer membrane is immersed in an amino-functionalized mesoporous SiO2 nanoparticle dispersion liquid, free first reaction groups on the base layer membrane react with amino groups of the mesoporous SiO2 nanoparticles, and after solidification, an amino-functionalized mesoporous SiO2 nanoparticle intermediate layer is formed; S2: the product prepared in S1 is reacted with a separation layer reaction liquid, free second reaction groups contained in the separation layer reaction liquid react with the amino groups of the mesoporous SiO2 nanoparticles, and a separation layer is formed. The mesoporous SiO2 nanoparticles are modified by surface amino-functionalization, which provides sites for chemical grafting and provides higher positive charge, improves the adhesion of the intermediate layer to the support layer and the separation layer, and improves the rejection rate of divalent cations and water permeability.

Description

Technical Field

[0001] This invention belongs to the field of nanofiltration membrane modification technology, specifically relating to a positively charged composite nanofiltration membrane and its preparation method. Background Technology

[0002] Lithium, due to its unique physical and chemical properties, is hailed as the "clean energy metal of the 21st century" and an "energy metal driving world progress." As a crucial component of lithium batteries, lithium resources are being rapidly depleted, leading to a surge in demand. Currently, over 80% of lithium resources are stored in salt lake brines, but high-Mg content... 2+ / Li + The high concentration of lithium (>20) and similar ionic hydration radii pose significant challenges to lithium extraction. Various methods exist for recovering lithium from salt lakes, including evaporation, chemical precipitation, solvent extraction, and adsorption. However, these methods generally suffer from high energy consumption, low economic efficiency, and environmental impact.

[0003] Nanofiltration membranes are an important branch of membrane separation technology, characterized by their green and low-carbon nature. Their separation mechanisms primarily include steric hindrance, the Donnan effect, and the dielectric effect, offering significant advantages in the separation and recovery of monovalent / multivalent ions. Commercial nanofiltration membranes are typically prepared through interfacial polymerization, and their surfaces are often negatively charged, particularly for Mg... 2+ / Li + The selectivity is poor. Therefore, developing positively charged nanofiltration membranes is essential for lithium extraction from salt lake brine. Chinese patent CN115709003A uses imine-bonded three-dimensional COFs as a blended aqueous monomer, and generates a polyamide separation layer through interfacial polymerization. The large number of imine structures improves the positive charge on the nanofiltration membrane surface, and the resulting positively charged composite nanofiltration membrane has high Mg content. 2+ / Li + While exhibiting good separation selectivity, the modified membrane suffers from poor stability. Chinese patent CN113289498B discloses a positively charged nanofiltration membrane and its preparation method. This method involves introducing hydrophilic surfactants such as sodium citrate and sodium camphor sulfonate, along with hydrophilic polymers like polyvinylpyrrolidone, into an aqueous solution, followed by interfacial polymerization to obtain a positively charged nanofiltration membrane. This membrane demonstrates high rejection rates for polyvalent cations, especially divalent cations, but its water flux is low. Many researchers focus on the influence of charge on the separation performance of nanofiltration membranes, often neglecting the "trade-off effect" between permeability and selectivity. Currently reported positively charged nanofiltration membranes suffer from low flux and poor antifouling performance, limiting their commercial application.

[0004] Interlayer thin-film nanocomposite membranes (TFNi) with nanomaterials as the intermediate layer are an emerging type of composite membrane with great potential to overcome the permeability-selectivity upper limit of conventional thin-film composite (TFC) nanofiltration membranes. The introduction of the intermediate layer not only provides an optimized water transport pathway but also allows for the controllability of the diffusion-reaction behavior of aqueous monomers during interfacial polymerization, enabling the controllable construction of the separation layer's pore size, charge, and thickness, thereby improving the permeation and separation performance of the nanofiltration membrane. Chinese patent CN115193276A uses a positively charged polyphenol / polyethyleneimine coating as the intermediate layer on the base membrane surface. The resulting positively charged nanofiltration membrane exhibits a rejection rate of over 98% for divalent cations and good water flux, but also a relatively high lithium rejection rate. Chinese patent CN108515751A discloses a polyamide composite nanofiltration membrane with a mesoporous silica interlayer and its preparation method. The mesoporous silica interlayer is generated in situ. Results show that this nanofiltration membrane has a high sulfate rejection rate, but the flux is still relatively low. The issue of nanoparticle aggregation and the adhesion between the interlayer and the base membrane are overlooked, limiting its application in water treatment. Poor adhesion between the interlayer and the support and separation layers, leading to membrane detachment during long-term operation, and the tendency for nanoparticle aggregation, limit the development of TFNi membranes. Therefore, it is necessary to explore an effective and easily constructible method for high-performance positively charged nanofiltration membranes with an interlayer, providing new ideas for the controllable preparation of positively charged nanofiltration membranes and efficient lithium extraction from salt lake brine. Summary of the Invention

[0005] To address the problems of weak interlayer stability, nanoparticle aggregation in the interlayer, and low retention rate in existing nanofiltration membranes, this invention provides a positively charged composite nanofiltration membrane.

[0006] The positively charged composite nanofiltration membrane comprises an aminated mesoporous SiO2 nanoparticle intermediate layer, a base membrane layer, and a separation layer. The aminated mesoporous SiO2 nanoparticle intermediate layer contains free amino groups. The base membrane layer is a nanofiltration membrane containing free first reactive groups. The separation layer contains free second reactive groups. Both the first and second reactive groups form covalent bonds with the amino groups. The first and second reactive groups are independently selected from one or both of benzyl halogen groups and acyl chloride groups.

[0007] Furthermore, the base film layer is a polysulfone film grafted with benzyl chloride groups;

[0008] The separation layer is an amide film formed by the reaction of acyl chloride monomer with the intermediate layer of the aminated mesoporous SiO2 nanoparticles.

[0009] One object of the present invention is to provide the application of the positively charged composite nanofiltration membrane as described in any of the preceding claims in membrane separation technology.

[0010] One object of the present invention is to provide a method for preparing a positively charged composite nanofiltration membrane.

[0011] The preparation method of this positively charged composite nanofiltration membrane includes the following steps:

[0012] S1: The base film is immersed in the aminated mesoporous SiO2 nanoparticle dispersion. The first reactive group on the base film reacts with the amino group of the mesoporous SiO2 nanoparticles and solidifies to form an aminated mesoporous SiO2 nanoparticle intermediate layer.

[0013] S2: The product prepared in S1 is reacted with the separation layer reaction solution, wherein the free second reactive groups contained in the separation layer reaction solution react with the amino groups of the mesoporous SiO2 nanoparticles to generate a separation layer;

[0014] The first reactive group and the second reactive group are independently selected from one or both of benzyl halogen groups and acyl chloride groups.

[0015] Furthermore, the method for preparing the positively charged composite nanofiltration membrane includes step S0 before step S1 and step S3 after step S2;

[0016] S0: The first reactive group is grafted onto the nanofiltration membrane.

[0017] S3: Place the product obtained in step S2 in an oven for heat treatment for 5 to 15 minutes to obtain the positively charged composite nanofiltration membrane.

[0018] Further, step S0 includes the following steps:

[0019] S01: Add 4-chloromethylstyrene to n-hexane and sonicate until completely dissolved to obtain the modified solution;

[0020] S02: The polysulfone membrane is placed in the modified solution, nitrogen gas is passed through to remove oxygen, and after ultraviolet irradiation, the nanofiltration membrane grafted with benzyl chloride is obtained, i.e., the base membrane layer.

[0021] Furthermore, in step S01, benzophenone or benzoyl peroxide, an initiator, is added to the n-hexane. The mass concentration of the 4-chloromethylstyrene is 0.1-5 wt%, and the mass concentration of the initiator is 1-10 wt% of the amount of the 4-chloromethylstyrene.

[0022] In step S02, the ultraviolet irradiation time is 10-60 min, the ultraviolet lamp wavelength is 365 nm, and the power is 80 W.

[0023] Further, step S1 includes the following steps:

[0024] Prepare an aqueous solution of aminated mesoporous SiO2 nanoparticles with a mass-volume concentration of 0.1–5 wt%. After ultrasonic dispersion, place the base film layer in the aminated mesoporous SiO2 nanoparticle dispersion and heat for 5–20 h. After thorough rinsing, store in deionized water for later use.

[0025] Further, step S2 includes the following steps:

[0026] The product obtained in step S1 is contacted with an aqueous solution for 2 minutes; after removing excess water, it is contacted with an oil solution for 1 minute to obtain a polyamide composite layer through interfacial polymerization. The aqueous phase contains an organic amine with a concentration of 0.2 wt% to 10 wt%, and the oil phase contains an organic acyl chloride with a concentration of 0.01 wt% to 0.5 wt%. The organic amine is one or more of piperazine or polyethyleneimine, and the organic acyl chloride is one or more of trimesoyl chloride, terephthaloyl chloride, and phthaloyl chloride. The solvent is one or more of n-hexane, cyclohexane, and n-heptane.

[0027] Furthermore, the preparation method of the aqueous solution of aminated mesoporous SiO2 nanoparticles includes the following steps:

[0028] A: Cyclohexane, n-pentanol and tetraethyl orthosilicate are mixed evenly and then added dropwise to deionized water containing hexadecyltrimethylammonium bromide and urea. After pre-reaction for 20-90 minutes, the temperature is raised to 70-120°C and stirred continuously for a period of time.

[0029] B: The emulsion obtained in A is centrifuged in a centrifuge, washed repeatedly with ethanol and deionized water 3 to 6 times, and calcined at high temperature to obtain mesoporous SiO2 nanoparticles.

[0030] C: Disperse the mesoporous SiO2 nanoparticles prepared by B in an ethanol solution, add 1-10 wt% of 3-aminopropyltriethoxysilane, ultrasonically stir for 12 h, wash repeatedly with ethanol and water, repeat the above operation 3-4 times, dry, grind and store for later use.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] 1) This invention prepares a positively charged composite nanofiltration membrane by controllably constructing an intermediate layer of aminated mesoporous SiO2 nanoparticles, and adopts a different technical route from the prior art: First, benzyl chloride groups are introduced on the surface of the base membrane as a support layer using the ultraviolet grafting method. The synthesized aminated mesoporous SiO2 nanoparticles are uniformly grafted onto the surface of the support layer as an intermediate layer. The amino groups on the surface of the aminated mesoporous SiO2 nanoparticles react with benzyl chloride groups and acyl chloride groups respectively, which improves the adhesion between the intermediate layer and the support layer and the separation layer, thereby constructing a stable intermediate layer.

[0033] 2) This invention modifies the surface of mesoporous SiO2 nanoparticles with amino groups, giving them abundant amino groups, providing sites for chemical grafting and providing higher charge, while effectively inhibiting the aggregation between nanoparticles, thereby constructing a stable positively charged composite film.

[0034] 3) The introduction of the aminated mesoporous SiO2 nanoparticle intermediate layer in this invention can significantly change the surface charge of the nanofiltration membrane, improve the rejection rate of divalent cations by the nanofiltration membrane, and further enhance the Mg... 2+ / Li + Its separation selectivity far exceeds that of advanced nanofiltration membranes.

[0035] 4) This invention uses an aminated mesoporous SiO2 nanoparticle intermediate layer to regulate interfacial polymerization. The prepared positively charged composite nanofiltration membrane exhibits significant improvement in water permeability while ensuring enhanced solute rejection rate. Attached Figure Description

[0036] Figure 1 A flowchart illustrating the preparation method of the positively charged composite nanofiltration membrane provided by this invention;

[0037] Figure 2 Scanning electron microscope (SEM) image of the aminated mesoporous SiO2 nanoparticles provided by the present invention;

[0038] Figure 3 Scanning electron microscope (SEM) image of the base film of the grafted aminated mesoporous SiO2 nanoparticle intermediate layer provided by the present invention;

[0039] Figure 4 Scanning electron microscope (SEM) image of the composite nanofiltration membrane containing an aminated mesoporous SiO2 nanoparticle intermediate layer provided by the present invention. Detailed Implementation

[0040] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings, but this should not be construed as limiting the scope of the present invention.

[0041] This invention modifies the surface of mesoporous SiO2 nanoparticles with amino groups, providing sites for chemical grafting and higher charge, improving the adhesion between the intermediate layer, support layer, and separation layer, while effectively inhibiting the aggregation between nanoparticles and enhancing the retention rate and water permeability of the nanofiltration membrane for divalent cations.

[0042] The positively charged composite nanofiltration membrane provided by this invention comprises an aminated mesoporous SiO2 nanoparticle intermediate layer, a base film layer, and a separation layer. The aminated mesoporous SiO2 nanoparticle intermediate layer contains free amino groups. The base film layer is a nanofiltration membrane containing free first reactive groups, and the separation layer contains free second reactive groups. Both the first and second reactive groups form covalent bonds with amino groups, and the first and second reactive groups are independently selected from one or both of benzyl halogen groups and acyl chloride groups.

[0043] In one embodiment, the base film layer is a polysulfone film grafted with benzyl chloride groups;

[0044] The separation layer is an amide film formed by the reaction of acyl chloride monomer and aminated mesoporous SiO2 nanoparticles as an intermediate layer.

[0045] One object of the present invention is to provide a method for preparing a positively charged composite nanofiltration membrane.

[0046] Refer to the instruction manual Figure 1 The method for preparing the positively charged composite nanofiltration membrane provided by the present invention includes the following steps:

[0047] S1: The base film is immersed in the aminated mesoporous SiO2 nanoparticle dispersion. The first reactive group on the base film reacts with the amino group of the mesoporous SiO2 nanoparticles and is solidified to form an aminated mesoporous SiO2 nanoparticle intermediate layer.

[0048] S2: The product prepared in S1 is reacted with the separation layer reaction solution. The free second reactive groups contained in the separation layer reaction solution react with the amino groups of the mesoporous SiO2 nanoparticles to generate the separation layer.

[0049] The first reactive group and the second reactive group are independently selected from one or both of the benzyl halogen group and the acyl chloride group.

[0050] Aminated mesoporous SiO2 nanoparticles carry a positive charge, which can prevent particle aggregation when constructing a stable intermediate layer. At the same time, due to their mesoporous channels, they improve water permeability while ensuring retention rate.

[0051] In one embodiment, the method for preparing a positively charged composite nanofiltration membrane includes step S0 prior to step S1: grafting a first reactive group onto the nanofiltration membrane.

[0052] Specifically, step S0 includes the following steps:

[0053] S01: Add 4-chloromethylstyrene to n-hexane and sonicate until completely dissolved to obtain the modified solution;

[0054] S02: The polysulfone membrane is placed in the modification solution, nitrogen gas is passed through to remove oxygen, and after ultraviolet irradiation, a nanofiltration membrane grafted with benzyl chloride is obtained, namely the base membrane layer.

[0055] Specifically, in step S01, an initiator, benzophenone or benzoyl peroxide is added to the n-hexane, the mass concentration of 4-chloromethylstyrene is 0.1-5 wt%, most preferably 1-3 wt%, and the mass concentration of the initiator is 1-10 wt% of the amount of 4-chloromethylstyrene.

[0056] In step S02, the ultraviolet irradiation time is 10-60 min, preferably 10-20 min, the ultraviolet lamp wavelength is 365 nm, and the power is 80 W.

[0057] Grafting benzyl chloride onto the polysulfone film increases the sites for chemical reactions, preparing for the further construction of a stable aminated mesoporous SiO2 nanoparticle intermediate layer.

[0058] In one embodiment, step S1 includes the following steps:

[0059] Prepare an aqueous solution of aminated mesoporous SiO2 nanoparticles with a mass-volume concentration of 0.1–5 wt%. After ultrasonic dispersion, place the base film layer in the aminated mesoporous SiO2 nanoparticle dispersion and heat for 5–20 h. After thorough rinsing, store in deionized water for later use.

[0060] In one embodiment, step S2 includes the following steps:

[0061] The product obtained in step S1 is contacted with an aqueous solution for 2 minutes; after removing excess water, it is contacted with an oil solution for 1 minute to obtain a polyamide composite layer through interfacial polymerization. The aqueous phase contains an organic amine with a concentration of 0.2 wt% to 10 wt%, and the oil phase contains an organic acyl chloride with a concentration of 0.01 wt% to 0.5 wt%. The organic amine is one or more of piperazine or polyethyleneimine, and the organic acyl chloride is one or more of trimesoyl chloride, terephthaloyl chloride, and phthaloyl chloride, with trimesoyl chloride being the most preferred. The solvent is one or more of n-hexane, cyclohexane, and n-heptane, with n-hexane being the most preferred.

[0062] In step S2, a polyamide composite layer is prepared by surface interfacial polymerization reaction.

[0063] In one embodiment, the method for preparing the positively charged composite nanofiltration membrane further includes step S3 after step S2:

[0064] The product obtained in step S2 is placed in an oven for heat treatment for 5 to 15 minutes to obtain the positively charged composite nanofiltration membrane.

[0065] In one embodiment, the preparation method of aminated mesoporous SiO2 nanoparticles includes the following steps:

[0066] A: Mix cyclohexane, n-pentanol and tetraethyl orthosilicate in a volume ratio of 20:1:2 until homogeneous, then add dropwise to deionized water containing 3.25-9.75 wt% hexadecyltrimethylammonium bromide and 0.02% urea. After pre-reaction for 20-90 min, raise the temperature to 70-120℃ and stir continuously for a period of time.

[0067] B: The emulsion obtained in A is centrifuged in a centrifuge, washed repeatedly with ethanol and deionized water 3 to 6 times, and calcined at high temperature to obtain mesoporous SiO2 nanoparticles.

[0068] C: Disperse the mesoporous SiO2 nanoparticles prepared by B in an ethanol solution, add 1-10 wt% of 3-aminopropyltriethoxysilane (ATPES), ultrasonically stir for 12 h, wash repeatedly with ethanol and water, repeat the above operation 3-4 times, dry, grind and store for later use.

[0069] In one embodiment, the method for preparing a positively charged composite nanofiltration membrane includes the following steps:

[0070] 1) Prepare a 1 wt% 4-chloromethylstyrene modified solution, including 0.2 wt% benzophenone as an initiator and n-hexane as a solvent, and sonicate until completely dissolved to obtain the modified solution.

[0071] 2) Place the polysulfone membrane in the modification solution prepared in step 1), purge with nitrogen for 10 min to remove oxygen; turn on the ultraviolet lamp for 10 min to obtain the modified nanofiltration membrane, thoroughly clean the modified nanofiltration membrane with n-hexane to remove ungrafted monomers and homopolymers on the surface of the modified nanofiltration membrane, air dry and weigh, and soak in deionized water for later use.

[0072] 3) Prepare aqueous solutions of aminated mesoporous SiO2 nanoparticles with a mass-volume concentration of 0.1wt% to 2wt%, respectively. After ultrasonic dispersion, place the modified nanofiltration membrane prepared in step 2) into the aminated mesoporous SiO2 nanoparticle dispersion and heat for 5h to 20h. After thorough rinsing, store in deionized water for later use. Figure 3 Scanning electron microscope (SEM) image of the base film of the grafted aminated mesoporous SiO2 nanoparticle intermediate layer provided by the present invention.

[0073] 4) Take out the membrane prepared in step 3), remove excess water, pour in an organic amine aqueous solution with a concentration of 0.5 wt% and contact it for 2 min. After removing excess water, contact it with a pyromellitic trichlorohexane solution with a concentration of 0.1 wt% for 1 min to form a polyamide composite layer through interfacial polymerization reaction.

[0074] 5) The above membrane was placed in an oven for heat treatment for 10 minutes to obtain a positively charged composite nanofiltration membrane containing aminated mesoporous SiO2 nanoparticles as the intermediate layer. Figure 4 Scanning electron microscope image of the composite nanofiltration membrane containing an aminated mesoporous SiO2 nanoparticle intermediate layer provided by the present invention.

[0075] Examples 1-9 are provided according to the preparation method, and the above examples have the following differences:

[0076]

[0077]

[0078] The present invention also provides Comparative Example 1, which, compared with Example 1, does not involve benzyl chloride grafting modification of the polysulfone-based film and does not involve grafting aminated mesoporous SiO2 nanoparticles as an intermediate layer, and only includes steps 4) and 5).

[0079] The separation performance of the positively charged composite nanofiltration membranes prepared in Examples 1-4 and Comparative Example 1 was tested using a single-component salt solution of 1.0 g / L MgCl2 and 1.0 g / L LiCl as the filter medium. The test results are shown in Table 1. A mixed solution of 2.0 g / L MgCl2 and 0.1 g / L LiCl was used as the filter medium to systematically test the magnesium-lithium separation performance of the composite nanofiltration membranes prepared in Examples 1-4 and Comparative Example 1 for mixed salt solutions. The test results are shown in Table 2.

[0080] The separation performance of the positively charged composite nanofiltration membranes prepared in Examples 5-8 was tested. A single-component salt solution of MgCl2 and LiCl with a concentration of 1.0 g / L and a concentration of 1.0 g / L was used as the filter medium. The test results are shown in Table 3.

[0081] The separation performance of the positively charged composite nanofiltration membrane prepared in Example 9 was tested. The magnesium-lithium separation factor was 44.38, and the salt solution flux was 20.96 LMH / bar.

[0082] The separation performance of the positively charged composite nanofiltration membrane prepared in Example 3 was tested. High-concentration simulated salt lake brine with a salt concentration of 120 g / L containing Na2SO4, MgSO4, MgCl2, NaCl and LiCl was used as the filter medium. The retention performance of the positively charged composite nanofiltration membrane in Example 3 for salt in the simulated brine was systematically tested. The test results are shown in Table 4.

[0083] Test conditions:

[0084] In this invention, the separation performance of the nanofiltration membrane is measured using a cross-flow filtration device. To ensure a stable water flux, the nanofiltration membrane is first pre-pressurized at 0.8 MPa for 1 hour, and then the permeate volume is collected at regular intervals at 0.8 MPa. The formula for calculating the pure water flux is as follows:

[0085]

[0086] Where P represents the pure water flux, the unit is L·m -2 ·h -1 ·bar -1 ΔV represents the volume of filtrate during the filtration time, in liters (L); A represents the filtration area, in square meters (m²). 2 Δt represents the filtration time in hours (h); Δp represents the applied pressure in bars.

[0087] The retention rate test method is as follows: A single-component salt solution of 1.0 g / L MgCl2 and 1.0 g / L LiCl is used to test the Mg content of the NF membrane. 2+ Li + Retention rate was tested; using Mg 2+ / Li + The effect of a simulated saline solution composed of MgCl2 and LiCl in a 20 ratio on the Mg content of the NF membrane. 2+ Li + The retention rate was tested; or a high concentration of simulated salt lake brine (120 g / L) containing Na₂SO₄, MgSO₄, MgCl₂, NaCl, and LiCl was used to test the Mg retention rate of the NF membrane. 2+ Li + The rejection rate was tested. Before the experiment, the membrane was pre-pressurized for 1 hour at a pressure of 0.8 MPa and a flow rate of 3 LPM. After the flux stabilized, the separation performance was tested under the same conditions. The formula for calculating the rejection rate is as follows:

[0088]

[0089] Where R represents the retention rate (%); C f C represents the concentration of the solute in the feed solution. p Represents the concentration of the solute in the permeate (g·L) -1 For a single-component salt solution, the concentration of the single electrolyte in the solution is linearly related to the conductivity of the solution; therefore, C p / C f The conductivity of the permeate and feed liquid is measured separately using a conductivity meter, and then equivalent calculations are performed using the ratio of conductivity. In the selectivity test of magnesium-lithium mixed salts, Mg... 2+ and Li + The concentration was determined by inductively coupled plasma optical emission spectrometry.

[0090] Lithium-magnesium separation factor refers to the Li in the permeate liquid. + and Mg 2+ The concentration ratio of Li in the feed solution + and Mg 2+ The ratio of the concentrations is calculated using the following formula:

[0091]

[0092] Among them, S Li,M g Represents the lithium-magnesium separation factor, C Li,p Li permeates the liquid + Concentration, g / L; C Mg,p Refers to the permeation of Mg in the liquid 2+ Concentration, g / L; C Li,f Li in the feed liquid + Concentration, g / L; C Mg,f Mg in the feed liquid 2+ The concentration, g / L.

[0093] Test results:

[0094] Table 1: Separation performance test results

[0095]

[0096] As shown in Table 1, the positively charged composite nanofiltration membranes prepared in Examples 1-4 have a pure water flux of over 20 LMH / bar. Furthermore, the water flux of the positively charged composite nanofiltration membrane continuously increases with the increase of the concentration of aminated mesoporous SiO2 nanoparticles, and the rejection rate of MgCl2 also increases accordingly. When the concentration increases to 1 wt%, the water flux reaches its maximum value of 25.53 LMH / bar, with a MgCl2 rejection rate of 97.5% and a LiCl rejection rate of 23.5%.

[0097] Table 2: Separation performance test results

[0098]

[0099] As shown in Table 2, the flux of the positively charged composite nanofiltration membranes prepared in Examples 1-4 is above 20 LMH / bar. Furthermore, with the increase of the concentration of aminated mesoporous SiO2 nanoparticles, the flux, MgCl2 rejection rate, LiCl rejection rate, and magnesium-lithium separation factor of the positively charged composite nanofiltration membranes show a trend of first increasing and then decreasing. This is because the increasing number of nanoparticles bound to the membrane surface enhances hydrophilicity and positive charge, leading to an increase in flux and rejection rate. However, when the concentration of aminated mesoporous SiO2 nanoparticles increases to a certain value, particle aggregation affects the membrane flux and salt rejection. When the concentration increases to 1 wt%, the water flux, MgCl2 rejection rate, and LiCl rejection rate all reach their maximum values: flux 22.14 LMH / bar, MgCl2 rejection rate 98.2%, LiCl rejection rate 24.9%, and magnesium-lithium separation factor 41.6.

[0100] Table 3: Separation performance test results

[0101]

[0102] As shown in Table 3, the flux initially increases and then decreases with increasing reaction time. During the initial increase, the flux increases due to the increasing number of aminated mesoporous SiO2 nanoparticles on the membrane surface and their enhanced hydrophilicity. When the reaction time reaches 17 hours, the flux decreases, as the prolonged reaction time leads to the deposition of aminated mesoporous SiO2 nanoparticles on the base membrane surface, increasing mass transfer resistance. However, with further increases in reaction time, the deposition of aminated mesoporous SiO2 nanoparticles on the base membrane surface continues to increase the rejection rates of MgCl2 and LiCl.

[0103] Table 4: Results of Retention Performance Tests

[0104]

[0105] The retention performance of the positively charged composite nanofiltration membrane prepared in Example 3 was systematically tested using high-concentration simulated salt lake brine as the filtration medium. Comparison of Tables 1 and 4 shows that the positively charged composite nanofiltration membrane prepared in Example 3 exhibits good separation performance for chlorate and sulfate under different application scenarios.

[0106] The positively charged composite nanofiltration membrane provided by this invention has strong adhesion between the intermediate layer, the support layer, and the separation layer, and features high retention rate and water permeability for divalent cations, making it suitable for the field of membrane separation technology.

[0107] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope of the claims of the present invention.

Claims

1. A positively charged composite nanofiltration membrane, comprising a base membrane layer and a separation layer, characterized in that, It also includes an aminated mesoporous SiO2 nanoparticle intermediate layer, which contains free amino groups; the base film layer is a nanofiltration membrane containing free first reactive groups; the separation layer contains free second reactive groups; both the first and second reactive groups form covalent bonds with the amino groups; and the first and second reactive groups are independently selected from one or both of benzyl halogen groups and acyl chloride groups. The base film layer is a polysulfone film grafted with benzyl chloride groups; The separation layer is an amide film formed by the reaction of acyl chloride monomer with the intermediate layer of the aminated mesoporous SiO2 nanoparticles; The method for preparing the aminated mesoporous SiO2 nanoparticle intermediate layer is as follows: The base film layer is immersed in an aminated mesoporous SiO2 nanoparticle dispersion. The first reactive group on the base film layer reacts with the amino group of the mesoporous SiO2 nanoparticles, and after solidification, an aminated mesoporous SiO2 nanoparticle intermediate layer is formed.

2. The method for preparing a positively charged composite nanofiltration membrane according to claim 1, characterized in that, Includes the following steps: S0: Grafting the first reactive group onto the nanofiltration membrane to form the base membrane layer; S1: The base film layer is immersed in an aminated mesoporous SiO2 nanoparticle dispersion. The first reactive group free in the base film layer reacts with the amino group of the mesoporous SiO2 nanoparticles, and after solidification, an aminated mesoporous SiO2 nanoparticle intermediate layer is generated. S2: The product prepared in S1 is reacted with the separation layer reaction solution, wherein the free second reactive groups contained in the separation layer reaction solution react with the amino groups of the mesoporous SiO2 nanoparticles to generate a separation layer; Wherein, the first reactive group and the second reactive group are independently selected from one or both of benzyl halogen groups and acyl chloride groups; S3: Place the product obtained in step S2 in an oven for heat treatment for 5~15 min to obtain the positively charged composite nanofiltration membrane.

3. The method for preparing the positively charged composite nanofiltration membrane as described in claim 2, characterized in that, Step S0 includes the following steps: S01: Add 4-chloromethylstyrene to n-hexane and sonicate until completely dissolved to obtain the modified solution; S02: The polysulfone membrane is placed in the modified solution, nitrogen gas is passed through to remove oxygen, and after ultraviolet irradiation, the nanofiltration membrane grafted with benzyl chloride is obtained, namely the base membrane layer.

4. The method for preparing the positively charged composite nanofiltration membrane as described in claim 3, characterized in that, In step S01, an initiator, benzophenone, or benzoyl peroxide was added to the n-hexane. The mass concentration of the 4-chloromethylstyrene was 0.1-5 wt%, and the initiator was 1-10 wt% of the mass concentration of the 4-chloromethylstyrene (VBC). In step S02, the ultraviolet irradiation time is 10~60 min, the ultraviolet lamp wavelength is 365 nm, and the power is 80 W.

5. The method for preparing the positively charged composite nanofiltration membrane as described in claim 2, characterized in that, Step S1 includes the following steps: Prepare an aqueous solution of aminated mesoporous SiO2 nanoparticles with a mass-volume concentration of 0.1~5 wt%. After ultrasonic dispersion, place the base film layer in the aminated mesoporous SiO2 nanoparticle dispersion and heat for 5~20 h. After thorough rinsing, store in deionized water for later use.

6. The method for preparing the positively charged composite nanofiltration membrane as described in claim 2, characterized in that, Step S2 includes the following steps: The product obtained in step S1 is contacted with an aqueous solution for 2 min; after removing excess water, it is contacted with an oil solution for 1 min to obtain a polyamide composite layer through interfacial polymerization. The aqueous phase contains an organic amine with a concentration of 0.2 wt% to 10 wt%, and the oil phase contains an organic acyl chloride with a concentration of 0.01 wt% to 0.5 wt%. The organic amine is one or more of piperazine or polyethyleneimine, and the organic acyl chloride is one or more of trimesoyl chloride, terephthaloyl chloride, and phthaloyl chloride. The solvent is one or more of n-hexane, cyclohexane, and n-heptane.

7. The method for preparing the positively charged composite nanofiltration membrane as described in claim 2, characterized in that, The preparation method of the aqueous solution of aminated mesoporous SiO2 nanoparticles includes the following steps: A: Cyclohexane, n-pentanol and tetraethyl orthosilicate are mixed evenly and then added dropwise to deionized water containing hexadecyltrimethylammonium bromide and urea. After pre-reaction for 20-90 min, the temperature is raised to 70-120℃ and stirred continuously for a period of time. B: The emulsion obtained in A is centrifuged, washed repeatedly with ethanol and deionized water 3 to 6 times, and calcined at high temperature to obtain mesoporous SiO2 nanoparticles. C: Disperse the mesoporous SiO2 nanoparticles prepared by B in an ethanol solution, add 1-10 wt% of 3-aminopropyltriethoxysilane, ultrasonically stir for 12 h, wash repeatedly with ethanol and water, repeat the above operation 3-4 times, dry, grind and store for later use.

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