Preparation method of crown ether functionalized anti-pollution nanofiltration membrane

Abstract

The invention discloses a preparation method of a crown ether functionalized anti-pollution nanofiltration membrane, which comprises the following steps: soaking a membrane of a commercial nanofiltration membrane in pure water, and coating the surface with a double-end epoxy compound ethanol solution; uniformly coating the surface of the membrane with an aza 12-crown-4 methanol solution; and drying the membrane in an atmosphere of 40-90 DEG C to obtain the crown ether functionalized anti-pollution nanofiltration membrane. The nano-filtration membrane is realized by chemically grafting aza 12-crown-4 on the membrane surface of a commercial nano-filtration membrane by utilizing a double-end epoxy compound; compared with a traditional nanofiltration membrane, the crown ether functionalized nanofiltration membrane provided by the invention has the advantages that the electronegativity of the surface is reduced, and the surface is similar to an aquatic plant molecular brush structure, so that the deposition of pollutants on the surface of the membrane can be effectively improved, and the anti-pollution performance of the membrane is enhanced; meanwhile, due to the complexing effect of 12-crown-4 molecules on lithium ions, the lithium ions can be effectively captured, concentration polarization of the lithium ions on the surface of the diaphragm is improved, and then the magnesium-lithium separation efficiency of the diaphragm is improved.

Description

Technical Field

[0001] The invention relates to the technical field of nanofiltration membrane preparation, in particular to a method for preparing a crown ether functionalized anti-pollution nanofiltration membrane. Background Art

[0002] my country's lithium reserves account for approximately 6% of the world's total, with approximately 85% contained in salt lake brines. The vast majority of this lithium-bearing salt lake brines have a high magnesium-to-lithium ratio, making lithium-magnesium separation difficult. While lithium extraction from salt lake brines has achieved initial industrial success in my country in recent years, it remains plagued by issues such as low extraction efficiency, high energy consumption, and high production costs.

[0003] Membrane separation technology has the functions of separation, concentration, purification and refinement, and is characterized by high efficiency, energy saving, environmental protection and easy control. It has been widely used in food, medicine and other fields, generating huge economic and social benefits, and has become one of the most important means in today's separation science.

[0004] Nanofiltration membranes can separate monovalent cations (Li+, Na+, etc.) and divalent cations (Mg2+, Ca2+, etc.). Therefore, nanofiltration membranes can separate Mg2+ from Li+, and after separation, lithium-rich brine with a low magnesium-to-lithium ratio can be obtained. Compared with other technologies, multi-stage membrane separation methods represented by nanofiltration membranes have the characteristics of high degree of automation, low cost, and low energy consumption. They can achieve high-quality, high-efficiency, and high-recovery separation of magnesium and lithium, and are expected to become the most effective way to solve the separation of magnesium and lithium in low-grade brine.

[0005] Defects and shortcomings of the prior art:

[0006] Nanofiltration membranes have nanoscale pores and surface charge properties. Based on size screening and Donnan exclusion mechanism, they can separate monovalent, divalent and polyvalent ions, and have significant advantages in the field of magnesium and lithium separation. In recent years, many researchers have explored the feasibility of commercial polyamide nanofiltration membranes in the separation of salt lake brine, proving that nanofiltration technology has good results in magnesium and lithium separation and can be used for the extraction of lithium resources in salt lakes. However, there are still some problems in practical applications, including: (1) the surface of nanofiltration membranes is usually negatively charged, the pore size distribution is wide, and the lithium and magnesium selectivity is low; (2) the surface charge density of the membrane is low and it is easily affected by charge shielding, making it difficult to meet the requirements of high salt resistance, anti-scaling and long-term stable operation.

[0007] Crown ether (12-crown-4) is an organic molecule that has a unique complexing effect on lithium ions. In order to utilize its unique complexing effect, crown ether is used in the preparation process of water treatment membranes; the Chinese invention patent "A crown ether functionalized nanofiltration composite membrane and its preparation method" (application number: CN202410824498.7) uses primary amine groups and acyl chloride groups in oil phase monomers to undergo amidation reaction, and introduces crown ether into the polyamide separation layer of the nanofiltration membrane in the form of a covalent bond to prepare a nanofiltration membrane with a Li+ exclusive transport channel; in the above preparation method, the crown ether is used as a reaction monomer and participates in the interfacial polymerization process, which will cause the desalination layer to be loose, affecting the desalination efficiency and service life of the nanofiltration membrane. In addition, the Chinese invention patent "A macrocyclic molecular nanofiltration membrane for magnesium and lithium separation and its preparation method" (application number CN202410779942.8) uses physical mixing and adsorption to uniformly disperse the crown ether in molecular form on the polymer surface or embed it into its structure, and interacts with the polymer chain through non-covalent interactions such as hydrogen bonds and van der Waals forces to prepare a nanofiltration membrane that cuts off lithium and penetrates magnesium; in this method, the crown ether is bound to the desalination layer through non-covalent interactions such as hydrogen bonds and van der Waals forces, and has poor stability. The crown ether will fall off during use and its effect will disappear. Summary of the Invention

[0008] The present invention provides a method for preparing a crown ether-functionalized anti-pollution nanofiltration membrane, wherein the nanofiltration membrane is realized by chemically grafting nitrogen-12-crown-4 onto the membrane surface of a commercial nanofiltration membrane by using a double-ended epoxy compound. Compared with traditional nanofiltration membranes, the crown ether-functionalized nanofiltration membrane involved in the nanofiltration membrane of the present invention has reduced surface electronegativity and a surface structure similar to that of waterweed molecules, which can effectively improve the deposition of pollutants on the membrane surface and enhance the anti-pollution performance of the membrane. At the same time, the complexation effect of 12-crown-4 molecules on lithium ions can effectively capture lithium ions, improve lithium ion concentration polarization on the membrane surface, promote the transmission of lithium ions, and thereby improve the magnesium-lithium separation efficiency of the membrane.

[0009] In order to solve the above technical problems, a technical solution adopted by the present invention is: the present invention provides a method for preparing a crown ether functionalized anti-pollution nanofiltration membrane, comprising the following steps:

[0010] Step 1: Soak the membrane of a commercial nanofiltration membrane in pure water, remove excess water droplets on the surface using a squeeze roller, apply a double-ended epoxy compound ethanol solution on the surface, let it stand for 0.5-5 minutes, remove excess solution on the surface, and dry at 40-90°C;

[0011] Step 2: evenly apply the methanol solution of 12-crown-4-nitropropane to the surface of the membrane obtained in step 1, let it stand for 1-5 minutes, and remove the excess solution on the surface;

[0012] Step 3: drying the membrane obtained in step 2 in an atmosphere of 40-90° C. to obtain a crown ether functionalized anti-pollution nanofiltration membrane.

[0013] Furthermore, the double-ended epoxy compound in step 1 is at least one of polyethylene glycol diglycidyl ether, ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether and resorcinol diglycidyl ether.

[0014] Furthermore, the concentration of the double-terminal epoxy compound in the double-terminal epoxy compound ethanol solution in step 1 is 0.05%-1.0%.

[0015] Furthermore, the concentration of aza-12-crown-4 in the aza-12-crown-4 methanol solution in step 2 is 0.1-1.2%.

[0016] The coating according to the present invention has the following characteristics:

[0017] 1. The 12-crown-4 molecules grafted onto the membrane surface have excellent complexing effects on lithium ions, can capture lithium ions, enhance lithium ion concentration polarization on the membrane surface, facilitate lithium ion transport across the membrane, and thus improve magnesium-lithium separation efficiency;

[0018] 2. After the membrane surface is grafted with double-ended epoxy compounds and crown ethers, a waterweed-like molecular brush structure will be formed. This structure can effectively reduce the deposition of pollutants on the membrane surface, slow down the adhesion of pollutants, improve the membrane's anti-pollution performance, and extend the product's service life;

[0019] 3. After the diepoxide reacts, a large number of hydrophilic hydroxyl groups are generated, which is beneficial to improve the hydrophilicity of the membrane surface and reduce the adsorption of pollutants;

[0020] 4. The nanofiltration membrane modification method of the present invention is achieved through intermolecular chemical grafting. The grafted material is stable and not easy to fall off, thus ensuring long-term stable operation.

[0021] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic flow chart of the method for preparing a nanofiltration membrane of the present invention;

[0023] Figure 2 This is the SEM morphology of the membrane surface before functionalization of the membrane of the present invention;

[0024] Figure 3 This is the SEM morphology of the membrane surface after functionalization of the membrane of the present invention. DETAILED DESCRIPTION

[0025] The following describes the specific embodiments of the present invention through specific examples. Those skilled in the art will readily understand the advantages and benefits of the present invention from the disclosure herein. The present invention may also be implemented in various other forms, i.e., various modifications and variations are possible without departing from the scope of the present invention.

[0026] Comparative Example 1:

[0027] 1. A commercial nanofiltration membrane prepared by Entai Environmental Technology (Changzhou) Co., Ltd. (hereinafter referred to as "Entai Technology") was soaked in pure water and set aside as comparative sample 1.

[0028] Example 1:

[0029] Step 1: Soak a commercial nanofiltration membrane produced by Entai Technology in pure water, remove excess water droplets on the surface using a squeeze roller, and coat the surface with 10 mL of a 0.15% concentration of polyethylene glycol diglycidyl ether ethanol solution. After standing for 1 minute, remove excess surface solution and dry at 60°C.

[0030] Step 2: evenly apply 10 mL of a 0.5% concentration of aza-12-crown-4 methanol solution to the surface of the membrane obtained in step 1, let it stand for 2 minutes, and remove excess solution from the surface;

[0031] Step 3: drying the membrane obtained in step 2 in an atmosphere of 60° C. to obtain a crown ether-polyethylene glycol diglycidyl ether grafted functionalized anti-pollution nanofiltration membrane.

[0032] Example 2:

[0033] The 0.15% concentration of polyethylene glycol diglycidyl ether ethanol solution in step 1 of Example 1 was changed to a 0.5% concentration of polyethylene glycol diglycidyl ether ethanol solution, and the other steps and raw materials remained unchanged to prepare a crown ether-polyethylene glycol diglycidyl ether grafted functionalized anti-pollution nanofiltration membrane;

[0034] Example 3:

[0035] The polyethylene glycol diglycidyl ether in step 1 of Example 1 was changed to ethylene glycol diglycidyl ether, and the other steps and raw materials remained unchanged to prepare a crown ether-ethylene glycol diglycidyl ether grafted functionalized anti-pollution nanofiltration membrane;

[0036] Example 4:

[0037] The 0.5% concentration of aza-12-crown-4 methanol solution in step 2 of Example 1 was replaced with a 0.25% concentration of aza-12-crown-4 methanol solution, and the other steps and raw materials remained unchanged to prepare a crown ether-polyethylene glycol diglycidyl ether grafted functionalized anti-fouling nanofiltration membrane.

[0038] Performance Testing

[0039] The anti-pollution nanofiltration membranes modified from the above examples and comparative examples were subjected to relevant performance tests, including anti-pollution performance test, magnesium-lithium separation efficiency test, membrane physical and chemical property analysis, etc.

[0040] a. Water treatment performance test conditions: According to the test platform required by the national standard GB / T 34242-2017, 100 ppm of bovine serum albumin was added to a 500 ppm CaCl2 solution, mixed thoroughly, and used as the test solution. Under the test conditions of 0.48 MPa operating pressure, 25±1°C, and a pH value of 7.0±0.5, the membrane's water production (m0) and desalination efficiency were tested. After 60 hours of continuous operation, the membrane's water production (m1) was tested. The membrane was then rinsed with hydrochloric acid solution (pH=2) and sodium hydroxide solution (pH=12), and the membrane's water production (m2) was tested. The membrane's fouling resistance was assessed by calculating the flux loss rate and flux recovery rate. The lower the flux attenuation after operation, the better the fouling resistance. Flux loss rate = (1-m1 / m0) × 100%, flux recovery rate = m2 / m0 × 100%. The results are shown in Table 1.

[0041] Table 1 Performance data of nanofiltration membranes obtained by modification of Examples and Comparative Examples

[0042]

[0043] According to the data analysis in Table 1, the initial water flux of Entai Technology's commercial nanofiltration membrane (Comparative Example 1) is 26.01GFD, which is slightly higher than the anti-pollution nanofiltration membrane prepared by the present invention. The present invention is functionalized on the basis of commercial nanofiltration membrane, resulting in a decrease in water production; after 24 hours of continuous pollution test, the water flux of Entai Technology's commercial nanofiltration membrane (Comparative Example 1) decayed to 22.88GFD. Compared with the initial data, the water flux of the membrane decayed by 12.03%; after pollution, the flux attenuation rate of the anti-pollution nanofiltration membrane prepared by the present invention was below 9.0%, indicating that the functionalized anti-pollution nanofiltration membrane of the present invention has excellent anti-pollution performance; the excellent anti-pollution performance is the bionic water grass structure of the epoxy compound, which can form a layer of molecular brushes on the surface of the membrane, effectively slowing down the deposition of pollutants; at the same time, a large number of hydrophilic hydroxyl groups are generated after the diepoxide compound is grafted, which enhances the hydrophilicity of the membrane surface and improves the anti-pollution performance of the membrane.

[0044] An in-depth comparison was made between the flux loss rate of each membrane after 60 hours of operation and the flux recovery rate after cleaning; after chemical cleaning, the water flux of Entai Technology's commercial nanofiltration membrane recovered to 24.18GFD. Compared with the initial flux data, the flux recovery rate after cleaning was 92.96%; after chemical cleaning, the functionalized anti-pollution nanofiltration membrane of the present invention maintained a water flux recovery rate of more than 95%. This phenomenon is that the bionic water grass structure of the epoxy compound slows down the deposition of pollutants while effectively blocking the adsorption of pollutants to the polyamide layer. The pollutants are only deposited on the surface of the molecular brushes. The pollutants can be efficiently washed away during chemical cleaning, so that the membrane exhibits excellent water flux cleaning recovery.

[0045] b. Magnesium-lithium separation efficiency test

[0046] Usually, brine with a magnesium to lithium ratio of 30:1 (about 6200 mg / L) is used as the influent water, and the magnesium and lithium concentrations of the produced water under a pressure of 2.0 MPa are measured to evaluate the magnesium and lithium separation performance of the nanofiltration membrane. The magnesium and lithium separation factor (β) of the nanofiltration membrane is calculated according to the following formula:

[0047] β=[c(Mg 2+ ) / c(Li + )] 产水 / [c(Mg 2+ ) / c(Li + )] 进水

[0048] Where, c(Mg 2+ )、c(Li + ) are Mg in the influent or produced water respectively 2+ 、Li + The mass concentration of Li + First pass through the nanofiltration membrane into the produced water, β is small enough, Li + It can be enriched in the produced water; the smaller the magnesium-lithium separation factor β, the main analytical indicator of the nanofiltration membrane, the higher the magnesium-lithium separation efficiency of the nanofiltration membrane.

[0049] Table 2 Functionalized nanofiltration membrane magnesium and lithium separation data

[0050]

[0051]

[0052] According to the data in Table 2, the nanofiltration membrane of the comparative example has a magnesium ion removal rate of 95.79%, and a lithium ion permeability of 94.39% (94.39% = 100% - 5.61%), showing excellent magnesium ion removal effect and good lithium ion permeability effect; after the nanofiltration membrane is functionalized by the method of the present invention, the magnesium ion removal rate of the functionalized nanofiltration membrane in the embodiment is increased to more than 96.4%, and the lithium ion removal rate is negative, indicating that the lithium ions pass through the membrane well and are concentrated; analyzing the magnesium-lithium separation factor β value, the separation factor of the comparative example is 0.045, and the separation factor β of the functionalized nanofiltration membrane is ≤0.035. After functionalization, the magnesium-lithium separation effect of the membrane is significantly improved; the improvement of the magnesium ion removal efficiency and the magnesium-lithium separation effect of the functionalized nanofiltration membrane of the present invention is attributed to the introduction of 12-crown-4 molecules on the surface of the functionalized membrane, which can effectively capture and complex lithium ions, increase the concentration of lithium ions on the membrane surface, and thus improve the permeability of lithium ions.

[0053] C. Analysis of the physicochemical properties of the membrane

[0054] The functionalized nanofiltration membrane was subjected to scanning electron microscopy (SEM), Zeta potential test, and contact angle test, as shown in Table 3, to further analyze the microscopic changes before and after functionalization.

[0055] Table 3 Zeta potential and contact angle data of membranes before and after functionalization

[0056]

[0057]

[0058] like Figure 2 and Figure 3 As shown in the figure, through the comparison of SEM images, the convex structure on the surface of the membrane is slightly reduced after functionalization, which is due to the formation of a functional layer after the grafting of crown ether and diepoxide, which covers some of the protrusions on the surface; the surface potential of the nanofiltration membrane is tested in the pH range of 6.5-7.0. The negative potential value of the surface before functionalization is 42.3mV, and the negative potential of the surface is significantly reduced (below 27mV) after crown ether functionalization. This is caused by the reaction of diepoxide with surface carboxyl group (-COOH) during the functionalization process; the contact angle data show that the contact angle value of the membrane surface is reduced by about 20° after functionalization. This change is caused by the generation of a large number of hydroxyl groups (-OH) after the reaction of crown ether with epoxy group; the reduction of Zeta potential and contact angle is beneficial to improving the anti-pollution performance of the membrane, which is consistent with the change law of the above anti-pollution data.

[0059] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structures made using the contents of the present invention's description and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for preparing a crown ether functionalized anti-pollution nanofiltration membrane, characterized in that: The steps include: Step 1: Soak the membrane of a commercial nanofiltration membrane in pure water, remove excess water droplets on the surface using a squeeze roller, apply a double-ended epoxy compound ethanol solution on the surface, let it stand for 0.5-5 minutes, remove excess solution on the surface, and dry at 40-90°C; Step 2: evenly apply the methanol solution of 12-crown-4-nitropropane to the surface of the membrane obtained in step 1, let it stand for 1-5 minutes, and remove the excess solution on the surface; Step 3: drying the membrane obtained in step 2 in an atmosphere of 40-90° C. to obtain a crown ether functionalized anti-pollution nanofiltration membrane.

2. The method for preparing a crown ether functionalized anti-pollution nanofiltration membrane according to claim 1, wherein: The double-ended epoxy compound in step 1 is at least one of polyethylene glycol diglycidyl ether, ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether and resorcinol diglycidyl ether.

3. The method for preparing a crown ether functionalized anti-pollution nanofiltration membrane according to claim 1, wherein: The concentration of the double-terminal epoxy compound in the double-terminal epoxy compound ethanol solution in step 1 is 0.05%-1.0%.

4. The method for preparing a crown ether functionalized anti-pollution nanofiltration membrane according to claim 1, wherein: The concentration of aza-12-crown-4 in the aza-12-crown-4 methanol solution in step 2 is 0.1-1.2%.

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