Loose nanofiltration membrane and preparation method thereof

Abstract

The invention discloses a loose nanofiltration membrane and a preparation method thereof, the pure water flux of the loose nanofiltration membrane is 40-60 L / (m < 2 >. H. Bar), the cutting molecular weight of the nanofiltration membrane is 500-1500 Da, and the rejection rate of the nanofiltration membrane to salt is less than 15%. The preparation method of the nanofiltration membrane comprises the following steps: mixing piperazine with a quaternary phosphorus-based eutectic solvent to obtain a first solution; trimesoyl chloride and alkane are mixed, and a second solution is obtained; enabling the first solution to be in contact with the second solution, enabling the quaternary phosphorus-based deep-eutectic solvent and alkane to be immiscible to form an interface, and enabling piperazine and trimesoyl chloride to react at the interface, so as to obtain the loose nanofiltration membrane. The loose nanofiltration membrane has high pure water flux, high micromolecular organic matter retention rate and high salt permeability.

Description

Technical Field

[0001] The present application relates to the field of membrane technology, and particularly to a loose nanofiltration membrane and a preparation method thereof. Background Art

[0002] Compared with traditional nanofiltration membranes, the separation skin layer of loose nanofiltration membranes is looser and has larger pore sizes (cut-off molecular weight 500 - 2000 Da), which can retain small molecular organic substances and allow inorganic salts to pass through. With the development of society and industry, more and more attention has been paid to efficient low-carbon production and the recycling of effective substances. Therefore, the urgent development of efficient loose nanofiltration membranes is needed. For example, as a major textile printing and dyeing country, in the wastewater discharged by the textile industry in China every year, printing and dyeing wastewater accounts for 80% of the discharge, but the recycling rate is less than 10%. In addition to organic substances such as dyes, printing and dyeing wastewater also contains a large amount of inorganic salts (such as Na 2 SO 4 , NaCl). Efficient separation of salts and dyes from printing and dyeing wastewater, realizing low-cost concentration of high-salt wastewater and high-quality recovery of waste salts, and achieving the purpose of environmental protection and resource recycling, is of extremely important significance for the sustainable development of the printing and dyeing industry and the maintenance of the ecological environment. Another example is that inorganic salts are needed in the production of antibiotics, and efficient desalination of antibiotics is also crucial, which is of great significance for the production of high-purity antibiotics.

[0003] Currently, commercially available nanofiltration membranes often have a relatively dense skin layer, with a small water flux of the membrane. While having a high retention rate for small molecular organic substances (such as dyes and antibiotics), they also have a relatively high retention rate for salts (generally greater than 30%), resulting in difficulty in separating salts and small molecular organic substances, seriously affecting the separation efficiency and resource recycling.

[0004] Currently, in the process of preparing membranes by traditional water / alkane interfacial polymerization, means such as using new reaction monomers and additives can be used to regulate the polymerization process and the structure and properties of the resulting polyamide membranes to achieve the construction of loose nanofiltration membranes. However, there are problems such as the relatively high preparation cost of new monomers, the complexity of the process caused by additives, and the disadvantage for large-scale production. In addition, the hydrolysis side reaction of acyl chloride oil-phase monomers caused by the water phase is inevitable, increasing the complexity and uncontrollability of the reaction, and is not conducive to the fine regulation of the membrane structure. Summary of the Invention

[0005] Aiming at the problems of the prior art, the present application provides a loose nanofiltration membrane and a preparation method thereof. The nanofiltration membrane has a high pure water flux and can efficiently separate small molecular organic substances (such as dyes and antibiotics) and salts at the same time.

[0006] A loose nanofiltration membrane, the pure water flux of the loose nanofiltration membrane is 40 - 60 L / (m 2(h·bar), the cut-off molecular weight of the loose nanofiltration membrane is 500-1500 Da, and the salt rejection rate of the loose nanofiltration membrane is less than 15%.

[0007] The following also provides several optional methods, which are not additional limitations to the above overall solution, but only further supplements or preferences. Without technical or logical contradictions, each optional method can be combined with the above overall solution alone, or multiple optional methods can be combined with each other.

[0008] The loose nanofiltration membrane provided by this application can have a high rejection rate of at least 90% for small molecule organic substances with a molecular weight above 500 Da on the premise of having a high pure water flux, and at the same time, the salt rejection rate is less than 15%, that is, it can efficiently separate small molecule organic substances and salts.

[0009] Optionally, the rejection rate of the loose nanofiltration membrane for dyes and antibiotics is greater than 90%.

[0010] The rejection rate of the loose nanofiltration membrane for dyes and antibiotics is greater than 90%, and at the same time, the salt rejection rate is less than 15%, which can effectively separate dyes and salts, and separate antibiotics and salts.

[0011] The dye is one of crystal violet, rhodamine B, acid fuchsin, congo red, methyl blue, reactive green 19, reactive red 120, reactive black 5, reactive red 24, reactive blue 19, reactive red 2, and the antibiotic is one of tetracycline, tetracycline hydrochloride, rifampicin, oxytetracycline.

[0012] This application also provides a preparation method of the loose nanofiltration membrane, including the following steps:

[0013] Mix piperazine with a quaternary phosphonium-based deep eutectic solvent to obtain a first solution;

[0014] Mix trimesoyl chloride with an alkane to obtain a second solution;

[0015] Contact the first solution and the second solution. The quaternary phosphonium-based deep eutectic solvent and the alkane are immiscible with each other to form an interface, and piperazine and trimesoyl chloride react at the interface to obtain the loose nanofiltration membrane.

[0016] Optionally, the quaternary phosphonium-based deep eutectic solvent is obtained by mixing a quaternary phosphonium-based hydrogen bond acceptor and a hydrogen bond donor. The quaternary phosphonium-based hydrogen bond acceptor is at least one of methyltriphenylphosphonium bromide, allyltriphenylphosphonium bromide, (methoxymethyl)triphenylphosphonium bromide, ethyltriphenylphosphonium bromide, propyltriphenylphosphonium bromide, benzylphenylphosphonium bromide, triphenylphosphonium chloride, allyltriphenylphosphonium chloride, (methoxymethyl)triphenylphosphonium chloride, ethyltriphenylphosphonium chloride, propyltriphenylphosphonium chloride, benzylphenylphosphonium chloride;

[0017] The hydrogen bond donor is at least one of ethylene glycol, glycerol, propylene glycol, butylene glycol, triethylene glycol, and xylitol.

[0018] The quaternary phosphonium hydrogen bond acceptor and the hydrogen bond donor are mixed and reacted under a sealed state to obtain a quaternary phosphonium deep eutectic solvent. The mixing and reaction method of the quaternary phosphonium hydrogen bond acceptor and the hydrogen bond donor is: stirring and mixing, and the temperature is 20 - 180 °C. When the quaternary phosphonium hydrogen bond acceptor and the hydrogen bond donor react to be clear and transparent, the quaternary phosphonium deep eutectic solvent is obtained.

[0019] Optionally, the quaternary phosphonium hydrogen bond acceptor is methyltriphenylphosphonium bromide, and the hydrogen bond donor is ethylene glycol.

[0020] Optionally, the molar ratio of the quaternary phosphonium hydrogen bond acceptor to the hydrogen bond donor is 1:0.01 - 1:20. Further preferably, the molar ratio of the quaternary phosphonium hydrogen bond acceptor to the hydrogen bond donor is 1:0.5 - 1:20. Most preferably, the molar ratio of the quaternary phosphonium hydrogen bond acceptor to the hydrogen bond donor is 1:1 - 1:5.

[0021] Optionally, the addition amount of piperazine in the first solution is 0.4 - 1.0 w / v%. In this application, the unit of w / v is g / mL.

[0022] Optionally, the addition amount of trimellitic trichloride in the second solution is 0.1 - 0.15 w / v%.

[0023] The ways of bringing the first solution into contact with the second solution include:

[0024] (a) Interfacial contact of the first solution with the first solution placed in the same container;

[0025] (b) After loading the first solution onto a carrier, placing the carrier adsorbed with the first solution into the second solution to form interfacial contact between the first solution and the second solution.

[0026] Optionally, the specific operation method of bringing the first solution into contact with the second solution is: attaching the first solution to a porous support layer by means of suction filtration, and bringing the second solution into contact with the porous support layer.

[0027] The porous support layer mainly plays a supporting role, and the porous structure can reduce the transport resistance. There is no strict limitation on the material of the porous support layer, as long as it is inert to the solvents and monomers in the interfacial polymerization reaction system, that is, it does not affect the progress of the interfacial polymerization reaction system. The porous support layer can be made of rigid or flexible materials.

[0028] Since the thickness of the nanofiltration membrane formed after the reaction of piperazine and trimesoyl chloride is relatively thin, it is usually used together with a porous support layer. That is to say, the strength support is mainly provided by the porous support layer, and the separation function effect is provided by the nanofiltration membrane.

[0029] Optionally, the porous support layer is one of a polysulfone porous membrane, a polyethersulfone porous membrane, a polyacrylonitrile porous membrane, and a polyvinylidene fluoride porous membrane.

[0030] Optionally, the reaction time of piperazine and trimesoyl chloride at the interface is 1 to 6 minutes.

[0031] The nanofiltration membrane formed by the reaction needs to be further heat-treated and cleaned. The temperature of the heat treatment is 40 to 80 °C, and the time of the heat treatment is 5 to 60 minutes.

[0032] The purpose of the heat treatment is to perform a high-temperature treatment on the nascent nanofiltration membrane formed by interfacial polymerization. Through the amidation reaction between the unreacted acyl chloride and amino groups at high temperature, the crosslinking degree and density of the polyamide separation layer are further improved.

[0033] The purpose of the cleaning is to remove residual monomers, solvents or low-molecular-weight polymers formed, reduce membrane pore blockage; it can also hydrolyze the residual acyl chloride groups to form carboxyl groups, thereby imparting a negative charge to the surface of the composite membrane. Cleaning can be carried out by conventional methods in the prior art.

[0034] Compared with the prior art, the present application has at least the following technical effects:

[0035] (1) Compared with the conventional oil / water interfacial polymerization method, the present application provides an interfacial polymerization method based on a quaternary phosphonium-based deep eutectic solvent and an alkane. The quaternary phosphonium-based deep eutectic solvent used is inexpensive, easy to obtain, and has strong solubility. In the process of interfacial polymerization of the deep eutectic solvent-alkane to form a composite membrane (the composite membrane includes a porous support layer and a nanofiltration membrane loaded thereon), on the one hand, it can expand non-water-soluble monomers and reduce side reactions such as hydrolysis of water-soluble monomers. On the other hand, the physical and chemical properties (viscosity, surface tension, etc.) of the deep eutectic solvent can be modulated to adjust the two-phase interface characteristics and reaction kinetics, thereby effectively controlling the entire interfacial polymerization process. Compared with deep eutectic solvents composed of other components (such as quaternary ammonium-based deep eutectic solvents), this quaternary phosphonium-based deep eutectic solvent can achieve a larger-scale polymerization regulation, thereby obtaining a nanofiltration membrane with a loose structure;

[0036] (2) The present application uses an interfacial polymerization method based on a quaternary phosphonium-based deep eutectic solvent and an alkane to prepare a loose nanofiltration membrane. The preparation process is simple to operate, the reaction conditions are mild, the thickness is small, and the structure is uniform, which is beneficial to improving the water permeability and small molecule / salt selectivity of the composite membrane, and has long-term stability. Description of the Drawings

[0037] Figure 1Relationship diagrams of pure water fluxes and rejection rates of Na 2 SO 4 for different deep eutectic solvents and nanofiltration membranes;

[0038] Figure 2 Relationship diagrams of pure water fluxes and rejection rates of Na 2 SO 4 for different piperazine concentrations and nanofiltration membranes;

[0039] Figure 3 Relationship diagrams of pure water fluxes and rejection rates of Na 2 SO 4 for different trimesoyl chloride concentrations and nanofiltration membranes;

[0040] Figure 4 Relationship diagram of salt rejection performance and permeation flux of the nanofiltration membrane prepared in Example 1;

[0041] Figure 5 Relationship diagram of rejection rates of different common dye molecules by the nanofiltration membrane prepared in Example 1;

[0042] Figure 6 Relationship diagram of rejection rates of different reactive dye molecules by the nanofiltration membrane prepared in Example 1;

[0043] Figure 7 Relationship diagram of rejection rates of different antibiotics by the nanofiltration membrane prepared in Example 1;

[0044] Figure 8 Rejection diagram of a mixture of antibiotics and sodium chloride by the nanofiltration membrane prepared in Example 1 at different times;

[0045] Figure 9 Schematic diagram of the surface potential of the nanofiltration membrane prepared in Example 1. Specific Embodiments

[0046] The technical solutions described in this application will be further described below in conjunction with specific embodiments, but this application is not limited thereto.

[0047] Example 1

[0048] (1) Select methyltriphenylphosphonium bromide as the quaternary phosphonium hydrogen bond acceptor and ethylene glycol as the hydrogen bond donor, with a molar ratio of 1:4 for the two components. Weigh each component and place them in a container and seal it. Place the container at 90 °C and stir until it becomes clear and transparent to obtain a quaternary phosphonium-based deep eutectic solvent;

[0049] (2) Add piperazine to the quaternary phosphonium-based deep eutectic solvent, with the addition amount of piperazine being 1.0 w / v% of the quaternary phosphonium-based deep eutectic solvent, to obtain a first solution. Load the first solution onto the polysulfone porous support layer by suction filtration until there are no obvious liquid beads on the surface (specifically, lay the polysulfone porous support layer on the suction filtration funnel, pour the first solution into the suction filtration funnel, and perform suction filtration. The first solution remains on the polysulfone porous support layer);

[0050] (3) Add trimesoyl chloride to cyclohexane to obtain a second solution, with the addition amount of trimesoyl chloride being 0.13 w / v% of cyclohexane. Place the polysulfone porous support layer obtained in step (2) into the second solution, carry out a contact reaction for 5 min, take it out, and a loose nanofiltration membrane is in-situ generated on the polysulfone porous support layer to obtain a composite membrane;

[0051] (4) Place the composite membrane (i.e., the polysulfone porous support layer loaded with the nanofiltration membrane) in an oven at 60 °C for heat treatment for 15 min;

[0052] (5) Take it out after heat treatment, and store the obtained composite membrane in deionized water.

[0053] Examples 2 - 5

[0054] The reaction times of Examples 2 - 5 are 3, 4, 10, and 20 min, and the others are the same as those in Example 1.

[0055] Store the composite membranes prepared in Examples 1 - 5 in deionized water, and test their pure water permeability and desalination performance under the conditions of a 1.0 g / L sodium sulfate aqueous solution and an operating pressure of 0.5 MPa. The test results are shown in Table 1.

[0056] Table 1

[0057]

[0058] As shown in Table 1, the influence of different reaction times on the performance of the nanofiltration membrane is significant. The interfacial polymerization reaction time has a significant impact on the water flux and retention performance of the loose nanofiltration membrane. As the interfacial polymerization time increases, the water flux of the membrane gradually decreases, and the retention effect on crystal violet increases. When the interfacial polymerization time increases from 3 min to 5 min, the densification of the membrane increases with the increase of the reaction time. The water flux decreases from 88 L / (m 2 ·h·bar) to 48.6 L / (m 2 ·h·bar), and at the same time, the retention rate of the dye crystal violet increases from 30.1% to 99.1% respectively, while the retention rate of Na 2 SO 4 remains below 15%. Further extending the reaction time to 10 min, the increase in the membrane thickness leads to a decrease in the water flux to 25.7 L / (m 2·h·bar). Although the rejection effect of crystal violet remains unchanged, the rejection of Na 2 SO 4 significantly increases. Considering the comprehensive performance of the membrane, an interfacial polymerization time of 5 min is considered the best choice.

[0059] Examples 6 - 8 Effect of piperazine concentration on the performance of the membrane

[0060] The concentration of piperazine (i.e., PIP) is changed to 0.4 w / v%, 0.6 w / v%, 0.8 w / v%, and the others are the same as in Example 1.

[0061] See Figure 2 As shown, with the increase in piperazine concentration, the pure water flux decreases from 70.2 L / (m 2 ·h·bar) to 48 L / (m 2 ·h·bar). A higher concentration of piperazine is beneficial to improving the denseness of the nanofiltration membrane. Although it has no significant effect on the rejection of small - molecule inorganic salts Na 2 SO 4 the rejection performance of small - molecule dyes (crystal violet (i.e., CV), rhodamine B (i.e., RdB)) is significantly improved. Therefore, when the piperazine concentration is 1%, it has optimized comprehensive performance. Under this condition, small - molecule inorganic salts can quickly pass through the membrane layer, maintaining a high transmittance, and the membrane has a high salt / dye separation ability.

[0062] Examples 9 - 13 Effect of trimesoyl chloride concentration on the performance of the membrane

[0063] The concentration of trimesoyl chloride (i.e., TMC) is changed to 0.1 w / v%, 0.11 w / v%, 0.12 w / v%, 0.14 w / v%, 0.15 w / v%, and the others are the same as in Example 1.

[0064] See Figure 3 As shown, with the increase in trimesoyl chloride concentration, the water flux of the nanofiltration membrane decreases from 65 L / (m 2 ·h·bar) to 43 L / (m 2 ·h·bar), while the rejection rate of Na 2 SO 4 remains below 15%. The rejection of small - molecule dyes crystal violet and rhodamine B increases from 20% and 30% to 99% respectively. This is because with the increase in trimesoyl chloride concentration, the increase in the denseness of the membrane surface makes the rejection rate of the membrane for dyes increase, but this also increases the transport resistance of water and inorganic salts, thereby reducing the pure water flux and increasing the salt rejection.

[0065] Comparative Examples 1 - 2

[0066] The hydrogen bond acceptors are methyltriphenylphosphonium chloride and choline chloride respectively, and the others are the same as in Example 1. The performance test of the membrane is shown in Table 2.

[0067] Table 2

[0068]

[0069] Performance test

[0070] The performance of the nanofiltration membrane prepared in Example 1 was tested. For four kinds of small molecule inorganic salts, namely Na 2 SO 4 , NaCl, MgSO 4 , MgCl 2 , separation tests were carried out to evaluate the salt rejection performance of the nanofiltration membrane. The results are shown in Figure 4 . The nanofiltration membrane shows a rejection order of Na 2 SO 4 >NaCl>MgSO 4 >MgCl 2 . Under the condition of maintaining high flux, the rejection rate of Na 2 SO 4 is less than 11%, and the rejection of NaCl is even less than 5%.

[0071] Separation tests were carried out on common dyes (including methyl orange, crystal violet, rhodamine B, acid fuchsin, congo red, methyl blue, respectively dissolved in deionized water at a concentration of 0.1 g / L as test substances) and reactive dyes commonly used in industry to evaluate the rejection performance of the nanofiltration membrane. It can be observed from Figure 5 that under the combined action of electrostatic repulsion and pore size sieving principle, the nanofiltration membrane has a high rejection rate for dyes with a molecular weight greater than 400 Da.

[0072] The rejection performance of the membrane for various reactive dyes commonly used in industry was further tested. As shown in Figure 6 , the nanofiltration membrane has a high rejection performance for reactive dyes (the rejection is greater than 95% for all), demonstrating its potential for treating industrial printing and dyeing wastewater.

[0073] In addition, antibiotics (ciprofloxacin, tetracycline, oxytetracycline, tetracycline hydrochloride, rifampicin, respectively dissolved in deionized water at a concentration of 50 ppm as test substances) were used as test substances to further evaluate the rejection performance of the nanofiltration membrane and expand the application field of loose nanofiltration membranes. As shown in Figure 7 , the experimental results show that the loose nanofiltration membrane has a good rejection effect on antibiotics with a molecular weight greater than 500 Da.

[0074] The nanofiltration membrane was compared with high-performance loose nanofiltration membranes in other literature. As shown in Table 3, due to the high NaCl / Congo red selectivity and large water flux, the nanofiltration membrane of this application has obvious advantages and has good prospects for dye / salt separation applications.

[0075] The NaCl / Congo red selectivity S is calculated using the following formula:

[0076] S = (1 - R NaCl ) / (1 - R N )

[0077] In the formula: R NaCl and R N respectively represent the rejection rates of sodium chloride (NaCl) and dye (or antibiotic).

[0078] Table 3

[0079]

[0080] Remarks: In Table 3, a: dye / NaCl mixed system; b: dye / NaCl single system;

[0081] The sources of the membranes are as follows:

[0082] 1. Bai, Y.; Liu, B.; Li, J.; Li, M.; Yao, Z.; Dong, L.; Rao, D.; Zhang, P.; Cao, X.; Villalobos, L.F.; Zhang, C.; An, Q.-F.; Elimelech, M., Microstructure optimization of bioderived polyester nanofilms for antibiotic desalination via nanofiltration. Sci. Adv. 2023, 9, eadg6134.

[0083] 2. Sun, W.; Zhang, N.; Li, Q.; Li, X.; Chen, S.; Zong, L.; Baikeli, Y.; Lv, E.; Deng, H.; Zhang, X.; Baqiah, H., Bioinspired lignin-based loose nanofiltration membrane with excellent acid, fouling, and chlorine resistances toward dye / salt separation. J. Membr. Sci. 2023, 670, 121372.

[0084] 3.Wang,Y.;Zhang,Y.;Zuo,H.;Wang,X.;Hao,Y.;Liu,M.;Gong,G.;Hu,Y.,Enhancing the permeability and selectivity of graphene oxide membranethroughpolyhedral oligomeric silsesquioxane intercalation for textile wastewatertreatment.Desalination 2024,587,117933.

[0085] 4.Lin,W.-T.;Fu,P.;Li,W.-L.;Yu,Y.-H.;Zhang,Z.-L.;Fan,H.-Y.;Huang,X.-J.;Xu,Z.-K.;Wan,L.-S.,Cross-linked g-C3N4 nanofibers enablethermal stablecomposite membranes for high-performance loosenanofiltration.Chem.Eng.J.2024,494,153197.

[0086] 5.Li,Q.;Liao,Z.;Fang,X.;Xie,J.;Ni,L.;Wang,D.;Qi,J.;Sun,X.;Wang,L.;Li,J.,Tannic acid assisted interfacial polymerization based loosethin-filmcomposite NF membrane for dye / salt separation.Desalination 2020,479,114343.

[0087] 6. Ding, L.; Huang, Y.; Shi, L.; Chen, Y.; Cheng, D.; Zhan, H.; Li, S.-L.; Wu, C., Tris-phenol phosphine oxide-based polyester loose nanofiltration membranes with a three-dimensional structure for efficient dye / salt separation. Sep. Purif. Technol. 2025, 354, 128962.

[0088] 7. Wu, B.; Li, S.-L.; Ullah, N.; Duan, Q.; Wang, M.; Yu, N.; Gong, G.; Hu, Y., Highly hydrophilic loose nanofiltration membrane based on a novel diaminobenzamide monomer for efficient dye / salt separation. Sep. Purif. Technol. 2025, 360, 131005.

[0089] 8. Fan, H.; Gu, J.; Meng, H.; Knebel, A.; Caro, J., High-Flux Membranes based on the covalent organic framework COF-LZU1 for selective dye separation by nanofiltration. Angew. Chem. Int. Ed. 2018, 57, 4083-4087.

[0090] 9. Zhang, Y.; Guo, J.; Han, G.; Bai, Y.; Ge, Q.; Ma, J.; Lau, C.H.; Shao, L., Molecularly soldered covalent organic frameworks for ultrafast precision sieving. Sci. Adv. 2021, 7, eabe8706.

[0091] 10. Guo, B.-B.; Liu, C.; Zhu, C.-Y.; Xin, J.-H.; Zhang, C.; Yang, H.-C.; Xu, Z.-K., Double charge flips of polyamide membrane by ionic liquid-decoupled bulk and interfacial diffusion for on-demand nanofiltration. Nat. Commun. 2024, 15, 2282.

[0092] 11. Zhao, L.-L.; Cao, X.-L.; Luo, C.; Wang, Q.; Lu, T.-D.; Tang, M.-J.; Sun, S.-P.; Xing, W., Locking patterned carbon nanotube cages by nanofibrous mats to construct cucurbituril[n]-based ultra-permselective dye / salt separation membranes. Nano Lett. 2023, 23, 4167 - 4175.

[0093] Using a mixed solution of 50 pm rifampicin and 1 g / L NaCl as the feed solution, the nanofiltration membrane was placed in a cross-flow device for long-term cyclic testing. After a long-term test of up to 120 h, the performance of the membrane did not show a significant decline. While maintaining low NaCl rejection, it has a high water flux and rifampicin antibiotic rejection, as Figure 8 shown. The nanofiltration membrane provided by this application exhibits excellent antibiotic rejection and permeation flux performance, and also has great application potential in antibiotic desalination.

[0094] See Figure 9 shown. The surface charge of the nanofiltration membrane prepared in Example 1 was studied by surface streaming potential in the pH range of 3 - 11. The unreacted acyl chloride functional groups in trimesoyl chloride during the interfacial polymerization process will hydrolyze into carboxyl groups, so the membrane is negatively charged. Under neutral conditions (pH = 7), the surface potential value of the nanofiltration membrane reaches -39.5 mV, which is more negative than the surface potential value (∼ -20 mV) of the nanofiltration membrane obtained by traditional interfacial polymerization.

[0095] This indicates that the low eutectic solvent with a higher viscosity can effectively inhibit the diffusion of piperazine and reduce the cross-linking degree of the membrane, so more unreacted acyl chloride groups hydrolyze to produce carboxyl groups. At the same time, the water contact angle of the nanofiltration membrane is 61°, indicating that the membrane has good hydrophilicity, which is beneficial to increasing the water flux and reducing membrane fouling.

[0096] Effect of the Composition of Deep Eutectic Solvents on Interfacial Polymerization Reaction

[0097] The compositions of the hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) of different deep eutectic solvents are shown in Table 4, the structural formulas of the hydrogen bond acceptor and hydrogen bond donor are shown in Table 5, and the viscosities of each solvent are shown in Table 6.

[0098] Table 4

[0099] Name of deep eutectic solvent DES-1 DES-2 DES-3 Hydrogen bond acceptor HBA MTPPB MTPPCl ChCl Hydrogen bond donor HBD EG EG EG

[0100] Table 5

[0101]

[0102] Table 6

[0103] Solvent Water DES-1 DES-2 DES-3 Viscosity (η / mP·s) 0.92 111.4 101.5 20.0

[0104] As shown in Table 6, with the increase of the viscosity of DES, the diffusion rate of piperazine gradually decreases, and DES-1 has the highest viscosity, so piperazine has the lowest diffusion rate in DES-1. There is a strong interaction between DES and piperazine, and the DES solution has a large viscosity, which can reduce the monomer diffusion rate and effectively regulate the interfacial polymerization reaction.

[0105] As shown in Table 7, compared with water, the surface tension of DES is smaller. Therefore, using DES as the monomer solvent significantly reduces the interfacial tension between the DES and alkane phases, can significantly inhibit the Marangoni effect at the interface, helps the interfacial stability and the cross-interfacial transport of monomers, and is beneficial to the formation of a nanofiltration membrane with a uniform structure at the interface. By changing the composition of the deep eutectic solvent, the monomer diffusion rate and the spatial distribution of monomers at the interface can be regulated, and it is expected to obtain nanofiltration membranes with different performances.

[0106] Table 7

[0107] Cyclohexane Water DES-1 DES-2 DES-3 Air 24.1 70.9 53.1 53.1 52.3 Cyclohexane - 45.5 14.6 15.1 19.2

[0108] Using DES with different compositions as the solvent for piperazine to prepare nanofiltration membranes, the nanofiltration membranes prepared using DES-1, DES-2, and DES-3 as solvents are named PA-1, PA-2, and PA-3 respectively (except for the different selection of solvents, the other conditions are the same as in Example 1), and their performances are tested as Figure 1 shown. The high viscosity of DES-1 significantly reduces the diffusion rate of piperazine in the DES phase, while the small interfacial tension is conducive to the diffusion of reaction monomers at the interface. Therefore, a polyamide network with a loose but uniform structure can be prepared in the DES-1 / alkane system, and the pure water flux of the prepared PA-1 membrane is as high as 48 L / (m 2 ·h·bar), Na2 SO 4 The interception is only 9%, which is a loose nanofiltration membrane. When piperazine is dissolved by DES-3 formed by ChCl and EG for interfacial polymerization, the viscosity of DES-3 is small, resulting in an increased diffusion rate of piperazine, increasing the membrane density and forming a dense nanofiltration membrane. Therefore, the prepared PA-3 membrane has a small water flux (20.6 L / (m 2 ·h·bar)), and has a Na interception as high as 90.6% 2 SO 4 interception. The viscosity of DES-2 is between the two, so the flux and interception of the membrane are also between the two. In the above embodiments, DES-1 is used as the deep eutectic solvent to participate in the reaction.

[0109] The above embodiments only represent several implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the patent scope. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several deformations and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. A loose nanofiltration membrane, characterized in that: The pure water flux of the loose nanofiltration membrane is 40 to 60 L / (m 2 ·h·bar), the cut-off molecular weight of the loose nanofiltration membrane is 500-1500Da, and the retention rate of the loose nanofiltration membrane for salt is less than 15%.

2. The loose nanofiltration membrane according to claim 1, characterized in that The loose nanofiltration membrane has a retention rate of more than 90% for dyes and antibiotics.

3. A method for preparing a loose nanofiltration membrane as claimed in claim 1 or 2, characterized in that: The steps include: mixing piperazine with a quaternary phosphorus-based deep eutectic solvent to obtain a first solution; mixing trimesoyl chloride and an alkane to obtain a second solution; The first solution and the second solution are brought into contact with each other, the quaternary phosphorus-based low eutectic solvent and the alkane are immiscible with each other to form an interface, and the piperazine and trimesoyl chloride react at the interface to obtain the loose nanofiltration membrane.

4. The method for preparing a loose nanofiltration membrane according to claim 3, characterized in that: The quaternary phosphorus-based deep eutectic solvent is obtained by mixing a quaternary phosphorus-based hydrogen bond acceptor and a hydrogen bond donor, wherein the quaternary phosphorus-based hydrogen bond acceptor is at least one of methyl triphenyl phosphonium bromide, allyl triphenyl phosphonium bromide, (methoxymethyl) triphenyl phosphonium bromide, ethyl triphenyl phosphonium bromide, propyl triphenyl phosphonium bromide, benzyl phenyl phosphonium bromide, triphenyl phosphonium chloride, allyl triphenyl phosphonium chloride, (methoxymethyl) triphenyl phosphonium chloride, ethyl triphenyl phosphonium chloride, propyl triphenyl phosphonium chloride, and benzyl phenyl phosphonium chloride; The hydrogen bond donor is at least one of ethylene glycol, glycerol, propylene glycol, butylene glycol, triethylene glycol and xylitol.

5. The method for preparing a loose nanofiltration membrane according to claim 3, characterized in that: The quaternary phosphorus-based hydrogen bond acceptor is methyltriphenylphosphonium bromide, and the hydrogen bond donor is ethylene glycol.

6. The method for preparing a loose nanofiltration membrane according to claim 4 or 5, characterized in that: The molar ratio of the quaternary phosphorus-based hydrogen bond acceptor to the hydrogen bond donor is 1:0.01 to 1:

20.

7. The method for preparing a loose nanofiltration membrane according to claim 3, characterized in that: The amount of piperazine added to the first solution is 0.4-1.0 w / v%.

8. The method for preparing a loose nanofiltration membrane according to claim 3, characterized in that: The amount of trimesoyl chloride added to the second solution is 0.1-0.15 w / v%.

9. The method for preparing a loose nanofiltration membrane according to claim 3, characterized in that: The specific operation method of contacting the first solution with the second solution is: the first solution is attached to the porous support layer by suction filtration, and the second solution is contacted with the porous support layer.

10. The method for preparing a loose nanofiltration membrane according to claim 3, characterized in that: The reaction time of piperazine and trimesoyl chloride at the interface is 1 to 6 minutes.

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