A ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds and a preparation method thereof

Abstract

The present invention discloses a ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds, its preparation method and application, belonging to the technical field of water treatment membranes. Pour an oil-phase solution onto the surface of a polysulfone membrane. After the first reaction, remove the excess oil-phase solution and dry it. Then pour an aqueous solution onto the surface of the polysulfone membrane. After the second reaction, remove the excess aqueous solution and heat to complete the interfacial polymerization reaction, and remove the unreacted substances to prepare the ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds; the oil-phase solution contains trimesic acid; the aqueous solution contains polyethyleneimine and ZIF-L nanosheets. The ZIF-L / PEI composite nanofiltration membrane is a composite nanofiltration membrane with ZIF-L doped in a polyamide separation layer having high water flux and high PFASs rejection rate, and can be used to remove PFASs in actual seawater and municipal domestic water that reach or are higher than environmental concentrations.

Description

Technical Field

[0001] The present invention belongs to the technical field of water treatment membranes, and particularly relates to a ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds, a preparation method thereof, and an application thereof. Background Art

[0002] To address the perfluoroalkyl compound crisis, a variety of remediation, separation, or degradation technologies have emerged, including adsorption, membrane filtration, bioremediation, electrocoagulation, sonolysis, photocatalysis, and electrochemical degradation. Among them, membrane filtration is a pressure-driven separation process that uses a semipermeable or porous membrane to separate solutes from solvents. Due to its simple operation and high efficiency, it has good application prospects in removing organic pollutants. Common membrane filtration technologies include microfiltration, ultrafiltration, reverse osmosis, and nanofiltration. The pore sizes of reverse osmosis membranes and nanofiltration membranes are less than 1 nanometer and 1 to 10 nanometers, respectively. Considering that the diameter of PFASs molecules is about 1 nanometer, this makes reverse osmosis membranes and nanofiltration membranes ideal choices for effectively removing PFASs from water. Reverse osmosis membranes can almost completely remove common PFASs in water, but their high operating pressure and short service life still limit their practical applications. Nanofiltration is a pressure-driven separation technology with separation performance between ultrafiltration and reverse osmosis, having advantages such as low operating pressure, no phase change, high separation efficiency, relatively low operation and maintenance costs, etc., and having more excellent water permeability compared with reverse osmosis membranes.

[0003] Patent CN 114130224 A discloses a high-flux polyamide composite nanofiltration membrane and a preparation method thereof. The high-flux polyamide composite nanofiltration membrane uses a metal-organic framework as a filler and an aqueous co-monomer, lysine as the main aqueous monomer, and trimesoyl chloride as the oil-phase monomer, and is formed by interfacial polymerization on the surface of a polyacrylonitrile ultrafiltration membrane support. Even though this invention can effectively avoid problems such as filler particle agglomeration and filler-polyamide matrix incompatibility in the preparation of conventional polyamide composite nanofiltration membranes, it uses toxic solvents such as n-hexane in the interfacial polymerization reaction; Patent CN 115463557 A discloses the synthesis of MOF-808 and its modified polyamide composite membrane. It uses the interfacial polymerization technique to composite a polyamide layer on a PMIA-based membrane and dope self-made MOF-808 in the polyamide layer to obtain a MOF-808 modified composite nanofiltration membrane. When preparing the oil-phase solution, the organic solvent used is Isopar G, which is a petroleum-derived organic solvent belonging to paint solvent oil and is likely to cause irritation to the skin and eyes of operators and may cause other health problems. Summary of the Invention

[0004] The object of the present invention is to provide a ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds, its preparation method and application, so as to solve the problems existing in the above-mentioned prior art. The present invention adopts an interfacial polymerization strategy, successively infiltrates an oil-phase solution containing trimesic acid on the surface of a polysulfone membrane and an aqueous-phase solution containing polyethyleneimine and metal-organic framework ZIF-L nanosheets, and finally undergoes an interfacial polymerization reaction by heating to obtain a ZIF-L-doped ZIF-L / PEI composite nanofiltration membrane on the polysulfone membrane. The present invention provides a simple, economical and practical preparation technology for a mixed matrix nanofiltration membrane doped with metal-organic framework materials. The whole process does not require high temperature and high pressure, and uses water and ethanol as solvents, which are non-toxic and pollution-free. It is a green synthesis process. The composite nanofiltration membrane is used for the separation of PFASs in water, and good water flux and rejection rate are obtained, which is suitable for the purification of new pollutants PFASs in urban domestic water, seawater and other water bodies.

[0005] One of the technical solutions provided by the present invention:

[0006] A preparation method of a ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds, comprising the following steps: pouring an oil-phase solution onto the surface of a polysulfone membrane, after the initial reaction, removing the excess oil-phase solution, drying, and then pouring the aqueous-phase solution onto the surface of the polysulfone membrane. After the second reaction, the excess aqueous-phase solution is removed, and heating is carried out to complete the interfacial polymerization reaction, and the ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds is prepared;

[0007] The oil-phase solution contains trimesic acid;

[0008] The aqueous-phase solution contains polyethyleneimine and ZIF-L nanosheets.

[0009] The further preparation method includes: fixing the polysulfone membrane in a polytetrafluoroethylene frame, pouring the prepared oil-phase solution onto the upper surface of the membrane, removing the excess oil-phase solution after reacting for 5 minutes, and drying at 30 °C for 15 minutes. Then, pour the aqueous-phase solution onto the surface of the polysulfone membrane, remove the excess aqueous-phase solution after reacting for 3 minutes, continue to react at 90 °C for 15 minutes to complete the interfacial polymerization, and soak it in deionized water for 24 h to remove unreacted substances, thus obtaining the ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds.

[0010] Preferably, the preparation method of the oil-phase solution is: dissolving trimesic acid and sodium dodecyl sulfate in an equal-volume mixed solution of ethanol and deionized water according to a mass ratio of 1:0.15 to prepare the oil-phase solution.

[0011] Preferably, the aqueous solution is prepared by adding polyethyleneimine and sodium dodecyl sulfate to water in a mass ratio of 4:1, and then adding ZIF-L nanosheets and mixing well to obtain the aqueous solution.

[0012] Preferably, the addition amount of the ZIF-L nanosheets accounts for 1-10 (wt.)% of the mass of the polyethyleneimine.

[0013] Preferably, the addition amount of the ZIF-L nanosheets accounts for 5 (wt.)% of the mass of the polyethyleneimine.

[0014] More preferably, the method for preparing the solution required for interfacial polymerization is as follows: Add 0.6 g of polyethyleneimine (Ethylene imine polymer, PEI) and 0.15 g of sodium dodecyl sulfate to 100 mL of deionized water, then add ZIF-L nanosheets and ultrasonicate to obtain an aqueous solution; Dissolve 1.0 g of trimesic acid and 0.15 g of sodium dodecyl sulfate in a mixed solution of ethanol and deionized water with equal volumes to obtain an oil phase solution.

[0015] The polyamide nanofiltration membrane is a high-performance nanofiltration membrane prepared by the interfacial polymerization method. The interfacial polymerization (IP) method uses a porous support layer as the base membrane, dissolves two reactive monomers in the aqueous phase and the organic phase respectively, and a polymerization reaction occurs at the interface of the two immiscible phases to form an ultrathin active separation layer. The difference in the aqueous phase monomers during the interfacial polymerization process directly determines the overall performance of the membrane.

[0016] Polyethyleneimine is a water-soluble cationic long-chain polymer with a large number of reactive amino groups in its structure, which can participate in the interfacial polymerization process as a reactive monomer. Due to its excellent hydrophilicity, high charge density and high reactivity, it has become one of the most important positively charged nanofiltration membrane materials. Compared with other small molecule amine substances commonly used in current research (such as piperazine and m-phenylenediamine, etc.), its transfer rate from the aqueous phase interface to the organic phase interface is slower, and the interfacial polymerization process that occurs is easier to control; A large number of amine groups in the PEI molecule endow it with strong positive electricity; At the same time, the amine group has antibacterial properties, which can endow the PEI nanofiltration membrane with antibacterial effects; The macromolecular structure makes the pore size of the nanofiltration membrane prepared by it relatively loose, and the interception performance has a large adjustable range. In short, due to the unique physical and chemical structure of PEI, the nanofiltration membrane prepared by interfacial polymerization has many excellent properties that are lacking in current commercial nanofiltration membranes.

[0017] More preferably, the molecular weight of the polyethyleneimine is 70000 MW.

[0018] More preferably, the addition amount range of the ZIF-L nanosheets is 0.006~0.06 g, and preferably 0.03 g.

[0019] Preferably, the synthesis steps of the ZIF-L nanosheets are as follows: An aqueous solution of cobalt nitrate is added dropwise to an aqueous solution of dimethylimidazole, and the reaction is stirred at room temperature. The resulting reaction solution is centrifuged and dried under vacuum to obtain the ZIF-L nanosheets.

[0020] More preferably, the specific steps are as follows: 1.143 g of cobalt nitrate hexahydrate and 2.626 g of dimethylimidazole are respectively dissolved in 40 mL of deionized water. The aqueous solution of cobalt nitrate is added dropwise to the aqueous solution of dimethylimidazole, and the mixture is stirred at room temperature for 4 h to end the reaction. The reaction solution is centrifuged, and the solid is taken out and dried under vacuum at 80 °C to obtain ZIF-L nanosheets. The synthesis vessel of the ZIF-L nanosheets is a glass beaker, and the vessel used for dropwise addition of the aqueous solution of cobalt nitrate is a glass separatory funnel.

[0021] Metal-Organic Framework (MOF) is a class of inorganic hybrid materials formed by the coordination of metal ions and organic ligands, which has the advantages of controllable pore size and stable performance. As an important member of the MOF family, ZIF-L has a cavity size of about 1.16 nm and a pore size of about 0.34 nm. It is a porous material with a three-dimensional network structure. The nanofiltration membrane modified with this material has the following advantages: (1) It has a porous structure with a high specific surface area, and a higher porosity can provide more water channels for water treatment, thereby improving the water flux of the modified membrane; (2) After the membrane is modified with ZIF-L, the pore size can be reduced to the nanometer level. It can not only improve the salt rejection rate and the separation efficiency of organic pollutants of the membrane, but also does not affect the water flux of the membrane, and can achieve a double improvement in the rejection rate and the water flux; (3) It has the characteristics of being resistant to organic solvents, acids, and alkalis. The modified separation membrane has good tolerance to organic solvents, acids, and alkalis, broadening the application range of the modified separation membrane.

[0022] The second technical solution provided by the present invention:

[0023] A ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds prepared by the above preparation method.

[0024] The third technical solution provided by the present invention:

[0025] An application of the above ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds in removing PFASs from seawater and domestic water.

[0026] The separation operation steps of the ZIF-L / PEI composite nanofiltration membrane for PFASs in water are as follows: Fix the ZIF-L / PEI composite nanofiltration membrane on a circulating cross-flow nanofiltration membrane test device, pre-compress it at 3 bar for 1 h. After circulating and filtering the feed test solution at a cross-flow rate of 20 L / h for 1 h, take a water sample, weigh it to obtain the water output within a fixed time, measure the water flux of the membrane, and collect the water sample at the outlet end to determine the PFASs content and obtain the PFASs rejection rate. The feed test solution is a solution of perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorocaproic acid (PFHxA), perfluorhexane sulfonic acid (PFHxS), and perfluorononanoic acid (PFNA) with a concentration in the range of 10 - 200 ng / mL.

[0027] Advantages of the present invention:

[0028] In the process of preparing the ZIF-L / PEI composite nanofiltration membrane of the present invention, an ethanol solution is used to prepare the oil-phase solution, which is different from the traditional interfacial polymerization reaction that uses toxic solvents such as n-hexane to prepare the oil-phase solution. In addition, the synthesis of ZIF-L nanosheets only requires water as a solvent at room temperature, while the synthesis of most MOFs requires organic solvents and conditions such as high temperature and high pressure. The preparation method of the ZIF-L / PEI composite nanofiltration membrane provided by the present invention improves the environmental compatibility of the membrane. The ZIF-L / PEI composite nanofiltration membrane is a composite nanofiltration membrane with a polyamide separation layer doped with ZIF-L having high water flux and high PFASs rejection rate, and can be used to remove PFASs in actual seawater and municipal domestic water that reach or are higher than the environmental concentration.

[0029] The present invention adds ZIF-L nano-fillers, which improves the performance of the membrane by increasing surface hydrophobicity and increasing water channels. Compared with the nanofiltration membrane without ZIF-L added, the membrane water flux increases significantly from 20.46 L•m -2 •h -1 •bar -1 to 47.56 L•m -2 •h -1 •bar -1 , which is increased by 2.3 times. At the same time, the rejection rates of the ZIF-L / PEI composite nanofiltration membrane provided by the present invention for representative PFASs such as PFOA and PFOS reach 97.75% and 97.85% respectively, and the rejection rates for short-chain PFASs (PFHxA, PFHxS) also exceed 80%.

[0030] The rejection rate of the ZIF-L / PEI composite nanofiltration membrane prepared by the present invention does not decrease with the increase of the PFASs concentration, inorganic salt ion concentration, and valence state in water. Higher PFASs concentration, ionic strength, and cation valence can all improve the removal rate of PFASs in polluted water.

[0031] The ZIF-L / PEI composite nanofiltration membrane provided by the present invention has a removal rate of more than 80% for representative PFASs in seawater and urban domestic water, and has a good separation effect on PFASs in actual water bodies. Description of the Drawings

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0033] Figure 1 It is a scanning electron microscope characterization diagram of the composite nanofiltration membrane prepared in Example 1 and doped with different proportions of ZIF-L. Among them, a), b), c), d) and e) respectively represent the composite nanofiltration membranes with ZIF-L doping ratios of 0%, 1%, 3%, 5% and 10%;

[0034] Figure 2 It is the contact angle test result of the composite nanofiltration membrane prepared in Example 1 and doped with different proportions of ZIF-L;

[0035] Figure 3 It is the determination result of the water flux and the rejection rate of PFASs of the composite nanofiltration membrane prepared in Example 1 and doped with different proportions of ZIF-L;

[0036] Figure 4 It is the influence of feed pressure, feed concentration and inorganic salt ions on the rejection rate of PFASs in water bodies. Among them, a) is the influence of feed pressure, b) is the influence of inorganic salt ions, and c) is the influence of feed concentration;

[0037] Figure 5 It is the change of water flux with filtration time in different PFASs / HA pollution systems. Among them, a) is the change of water flux of PFHxA, and b) is the change of water flux of PFHxS;

[0038] Figure 6 It is the determination result of the rejection rate of PFHxA and PFHxS in different water bodies by the composite nanofiltration membrane M-5 prepared in Example 1. Detailed Embodiments

[0039] Now, various exemplary embodiments of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics and implementation schemes of the present invention.

[0040] It should be understood that the terms described in the present invention are only for describing specific embodiments and are not intended to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0041] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0042] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific embodiments of the description of the present invention, which are obvious to those skilled in the art. Other embodiments obtained from the description of the present invention are obvious to those skilled in the art. The description and examples of this application are merely exemplary.

[0043] Regarding the use of "comprising", "including", "having", "containing", etc. herein, they are all open-ended terms, meaning including but not limited to.

[0044] The room temperature in the present invention refers to 25 ± 2 °C.

[0045] In view of the increasing pollution of the new pollutant PFASs in the water environment, the present invention intends to adopt an interfacial polymerization strategy to synthesize a ZIF-L / PEI composite membrane with excellent performance and ecological friendliness for separating PFASs in water. By characterizing the microstructure and composition of the prepared membrane, the dominant role of the ZIF-L-doped polyamide layer in PFASs removal was determined. In addition, the present invention also demonstrated that the ZIF-L / PEI composite nanofiltration membrane provided by the present invention has a good separation effect on PFASs in water by selecting several representative PFASs components with different head chemical groups and carbon chain lengths.

[0046] A preparation method of a ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds provided by an embodiment of the present invention includes the following steps: (1) synthesis of ZIF-L nanosheets; (2) preparation of an interfacial polymerization solution; (3) preparation of a ZIF-L / PEI composite nanofiltration membrane. In addition, the embodiment of the present invention also verifies the separation effect of the ZIF-L / PEI composite nanofiltration membrane on PFAS in water bodies.

[0047] In the embodiment of the present invention, the polysulfone membrane used is an industrial-grade polysulfone membrane; the polytetrafluoroethylene frame is a square frame with an inner side of 20 cm × 20 cm and an outer side of 23 cm × 23 cm; the equipment used for drying is a precision forced-air drying oven; the equipment used for the interfacial polymerization reaction is a precision forced-air drying oven.

[0048] Example 1 A preparation method of a ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds

[0049] (1) Dissolve 1.143 g of cobalt nitrate hexahydrate and 2.626 g of dimethylimidazole in 40 mL of deionized water respectively. Gradually add the obtained cobalt nitrate aqueous solution dropwise to the dimethylimidazole aqueous solution, stir at room temperature for 4 h. After the reaction ends, centrifuge, take out the solid, and dry it in vacuo at 80 °C to obtain ZIF-L nanosheets.

[0050] (2) Prepare 5 portions of aqueous solutions. The preparation method is as follows: Add 0.6 g of polyethyleneimine and 0.15 g of sodium dodecyl sulfate to 100 mL of deionized water; add 0.006 g, 0.018 g, 0.03 g, and 0.06 g of ZIF-L nanosheets to four of them respectively, and ultrasonicate to obtain aqueous solutions doped with ZIF-L at different mass ratios (1%, 3%, 5%, and 10%), and do not add ZIF-L nanosheets to the other portion.

[0051] (3) Prepare 5 portions of organic phase solutions: Dissolve 1.0 g of trimesic acid and 0.15 g of sodium dodecyl sulfate in a mixed solution of ethanol and deionized water with equal volumes to obtain an organic phase solution.

[0052] (4) Fix the polysulfone membrane in the polytetrafluoroethylene frame, pour the prepared oil-phase solution onto the upper surface of the membrane, remove the excess oil-phase solution after reacting for 5 minutes, dry it at 30 °C for 15 minutes, then pour the water-phase solution onto the membrane surface, remove the excess water-phase solution after 3 minutes, and continue to react at 90 °C for 15 minutes to complete the interfacial polymerization. Finally, place the polysulfone membrane in deionized water and soak it for 24 h to remove unreacted substances, thus obtaining the ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds, which are denoted as M-0 (without adding ZIF-L nanosheets), M-1 (ZIF-L nanosheets accounting for 1% of the mass of polyethyleneimine), M-3 (ZIF-L nanosheets accounting for 3% of the mass of polyethyleneimine), M-5 (ZIF-L nanosheets accounting for 5% of the mass of polyethyleneimine), and M-10 (ZIF-L nanosheets accounting for 10% of the mass of polyethyleneimine) according to the mass ratio of ZIF-L nanosheets added in step (2).

[0053] The surfaces of the obtained nanofiltration membranes with different doping ratios of ZIF-L were characterized by a scanning electron microscope and a contact angle tester. The obtained SEM images are as Figure 1 shown, where a), b), c), d), and e) represent the ZIF-L / PEI composite nanofiltration membranes with ZIF-L doping mass ratios of 0%, 1%, 3%, 5%, and 10% respectively; the contact angle data are as Figure 2 shown.

[0054] Through Figure 1 and Figure 2It can be seen that the surface morphology of the M-0 membrane without the incorporation of ZIF-L is granular, without obvious defects, and the contact angle is 66.32°; after the incorporation of 1% mass ratio (0.006 g) of ZIF-L nanosheets, the surface of the M-1 membrane is denser and smoother, without obvious defects, and the contact angle is 65.86°; after the incorporation of 3% mass ratio (0.018 g) of ZIF-L nanosheets, the surface of the M-3 membrane is also denser and smoother, without obvious defects, and the contact angle increases to 67.58°; after the incorporation of 5% mass ratio (0.03 g) of ZIF-L nanosheets, the surface of the M-5 membrane is also denser and smoother, without obvious defects, and the contact angle further increases to 69.17°; after the incorporation of 10% mass ratio (0.06 g) of ZIF-L nanosheets, obvious defects appear on the surface of the M-10 membrane, and the contact angle is 73.22°, which leads to a significant increase in water flux and a substantial decrease in interception rate. From the characterization results of the above different membranes, as the incorporation amount of ZIF-L increases, the contact angle becomes larger, reflecting the further increase in the hydrophobicity of the membrane. Due to the loose and porous nature of MOF itself, additional water channels are increased, resulting in the continuous increase in water flux. However, when the addition amount of ZIF-L is too high (10% mass ratio), it will affect the polymerization reaction of polyethyleneimine and trimesic acid, resulting in obvious defects on the membrane surface, thus leading to the decrease in the interception rate of PFASs.

[0055] Application Example 1

[0056] Feed solutions with concentrations of 200 ng / mL of PFOA, PFOS, PFHxA, PFHxS, and PFNA were respectively prepared. The nanofiltration membrane prepared in Example 1 was fixed on a circulating crossflow test device, pre-compressed at 3 bar for 1 h, and the crossflow velocity was 20 L / h. After the feed test solution was circulated and filtered for 1 h, a water sample was taken, weighed to obtain the water output within a fixed time, the water flux of the membrane was measured. At the same time, the water sample at the outlet end was collected, the PFASs content was measured, and the rejection rates of the composite nanofiltration membrane for PFHxA, PFHxS, PFOA, PFOS, and PFNA were obtained.

[0057] The water fluxes and PFASs rejection rates of the composite nanofiltration membranes doped with different ratios of ZIF-L prepared in Example 1 are as Figure 3 shown. From Figure 3 it can be seen that the water fluxes of M-0, M-1, M-3, M-5, and M-10 are 20.46, 25.34, 33.30, 47.56, and 74.73 L·m -2 ·h -1 ·bar -1For the rejection rates of PFHxA, PFHxS, PFOA, PFOS, and PFNA, those of M-0 were 77.60%, 85.29%, 97.38%, 99.43%, and 99.54% respectively; those of M-1 were 80.56%, 83.01%, 98.15%, 99.10%, and 98.95% respectively; those of M-3 were 79.48%, 84.20%, 95.55%, 99.21%, and 99.40% respectively; those of M-5 were 81.52%, 87.85%, 95.85%, 98.47%, and 98.42% respectively; those of M-10 were 65.33%, 70.32%, 73.20%, 75.26%, and 85.55% respectively. The above results show that considering both the water flux of the nanofiltration membrane and the rejection rate of PFASs, the performance of the M-5 membrane doped with 5% by mass (0.03 g) of ZIF-L nanosheets is optimal. It can still maintain high efficiency in rejecting PFHxA, PFHxS, PFOA, PFOS, and PFNA while maintaining a relatively high water flux.

[0058] Application Example 2

[0059] Taking PFHxA and PFHxS with shorter carbon chains and more difficult to intercept as examples, and using the M-5 membrane prepared in Example 1 as the test object, the effects of feed pressure, feed concentration, and inorganic salt ions on the rejection rate of PFASs were investigated respectively. Without specific preconditions, the set operating pressure was 3 bar and the initial concentration of PFASs was 200 ng / mL. Before the experiment, the composite nanofiltration membrane was pre-pressed for 1 hour. After the feed test solution was circulated and filtered at a cross-flow velocity of 20 L / h for 1 h, water samples were taken to measure the water flux or the content of PFASs.

[0060] The contents of PFHxA and PFHxS at the water outlet end of the circulating cross-flow nanofiltration membrane test equipment were measured at operating pressures of 1, 2, 3, 4, and 5 bar respectively to determine the effect of feed pressure on the rejection rate. As shown in Figure 4 a) in the figure, it can be seen from the figure that the increase in operating pressure corresponds to the acceleration of the water treatment speed. At 1, 2, and 3 bar, the interception rates of PFHxA and PFHxS both exceeded 80%; however, the rejection rates of PFHxA and PFHxS at 4 bar and 5 bar were significantly lower than those at other pressures. For example, after working at 5 bar for 1 h, the rejection rates of PFHxA and PFHxS decreased to 47.23% and 62.1% respectively. The reason is that PFASs molecules are more likely to penetrate the membrane pores at higher pressures. Therefore, it is crucial to select an appropriate operating pressure to achieve excellent membrane operation performance. In addition, concentration polarization (CP), especially aggravated at high transmembrane pressures, can also reduce the rejection rate of PFASs in water.

[0061] The feed solution was prepared by using KCl, NaCl, MgCl2 and CaCl2 solutions with a concentration of 3 mM as diluents for PFASs to determine the influence of inorganic salt ions on the rejection efficiency of PFHxA and PFHxS. Figure 4 In b), the results of the influence of inorganic salt ions on the rejection rate are shown. The results indicate that inorganic salt ions improve the rejection rate of PFASs, and the influence of divalent ions is more significant than that of monovalent ions. The reason is that the increase in ionic strength and cation valence can delay the migration of PFASs in the porous medium, thereby improving the rejection rate of PFASs in the salt solution, especially the salt solution of high-valence cations. In addition, Ca 2+ and Mg 2+ can act as a bridging agent during the aggregation of PFASs into micelles, forming non-monomer entities in the solution, increasing the possibility of generating larger micelles, and hindering the transmembrane transport of PFASs.

[0062] Figure 4 In c), the results of the influence of the concentrations of PFHxA and PFHxS in the feed solution on the rejection rate are shown. It can be seen from the figure that as the concentration of the feed solution increases from 10 ng / mL to 200 ng / mL, the rejection rates of PFHxA and PFHxS increase significantly. Specifically, the rejection rate of PFHxA increases from 73.00% to 81.52%, and the rejection rate of PFHxS increases from 73.68% to 87.85%. This result is attributed to the fact that a higher pollutant concentration will cause its deposition on the membrane surface, forming a thicker cake layer, further hindering the transport of pollutants through the membrane. In addition, low concentrations of PFASs usually exist in the solution in the form of single molecules, showing characteristics similar to those of an ideal solution. When the critical micelle concentration is reached, surfactant monomers begin to aggregate into micelles. This process destroys the concentration polarization layer that is stable in the colloid, promotes the retention of PFASs, and at the same time, larger aggregates increase the molecular size, thereby improving the rejection efficiency.

[0063] Application Example 3

[0064] Humic acid (HA) is representative of natural organic fouling agents. In this invention, it was used to evaluate the anti-fouling effect of the composite nanofiltration membrane prepared in Example 1.

[0065] Prepare 0.5 g / L HA + 200 ng / mL PFHxA solution, 0.5 g / L HA + 200 ng / mL PFHxS solution, 200 ng / mL PFHxA solution, and 200 ng / mL PFHxS solution as feed solutions respectively. Fix the M-5 membrane prepared in Example 1 on a circulating cross-flow test device, pre-compress it at 3 bar for 1 h, circulate and filter the feed test solution at a cross-flow velocity of 20 L / h for 1 h for pre-operation, and then conduct long-term circulating filtration for 16 h. Take a water sample to measure the water flux of the membrane. Subsequently, disassemble the membrane sheet, clean the surface with deionized water, reinstall it on the circulating cross-flow test device, adjust the pressure to 3 bar, conduct circulating filtration for 16 h at a cross-flow velocity of 20 L / h, and take a water sample again to measure the water flux. The experimental results are shown in Figure 5 the variation of water flux with filtration time in different PFASs / HA pollution systems. Among them, a) is the variation of the water flux of PFHxA, and b) is the variation of the water flux of PFHxS. The experimental results show that in the single PFASs system, the fouling degree of PFHxA and PFHxS on the membrane is relatively light, and the water flux reduction rates are 5.34% and 4.75% respectively. In contrast, the fouling of the PFASs + HA system is more serious, and the water flux reduction rates of the PFHxA + HA and PFHxS + HA systems are 61.70% and 58.28% respectively. After flushing the composite nanofiltration membrane with deionized water and then conducting nanofiltration performance testing, the water flux recovery rate of the severely fouled PFASs + HA system is higher than 88%. The experimental results show that the anti-fouling problem of the composite nanofiltration membrane provided by the present invention can be effectively solved by implementing the flushing operation.

[0066] Application Example 4

[0067] In the present invention, deionized water, urban domestic water, and seawater are used as diluents of PFASs to prepare feed solutions respectively, for measuring the separation effects of PFHxA and PFHxS in different water bodies by the composite nanofiltration membrane prepared in Example 1. Fix the M-5 membrane prepared in Example 1 on a circulating cross-flow nanofiltration membrane test device, pre-compress it at 3 bar for 1 h, circulate and filter at a cross-flow velocity of 20 L / h for 1 h, and then take a water sample to measure the PFASs content. The test results are shown in Figure 6 , and the results show that the rejection rates of PFHxA and PFHxS in different water bodies are both greater than 80%, and the rejection rates of PFHxA and PFHxS in seawater are significantly higher than those in urban domestic water and deionized water, which is consistent with the results of inorganic salt ions in the previous examples.

[0068] The embodiments described above are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. Application of a ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds in removing PFASs from seawater and domestic water, characterized in that, The preparation method of the ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds comprises the following steps: Pour the oil-phase solution onto the surface of the polysulfone membrane. After the initial reaction, remove the excess oil-phase solution and dry it. Then pour the water-phase solution onto the surface of the polysulfone membrane. After the second reaction, remove the excess water-phase solution and heat to complete the interfacial polymerization reaction to prepare the ZIF-L / PEI composite nanofiltration membrane for separating perfluoroalkyl compounds; the oil-phase solution contains trimesic acid; the water-phase solution contains polyethyleneimine and ZIF-L nanosheets; The preparation method of the water-phase solution is as follows: Add polyethyleneimine and sodium dodecyl sulfate into water according to a mass ratio of 4:1, and then add ZIF-L nanosheets and mix evenly to obtain the water-phase solution; The addition amount of the ZIF-L nanosheets accounts for 5 wt% of the mass of the polyethyleneimine; Dissolve trimesic acid and sodium dodecyl sulfate in an equal-volume mixed solution of ethanol and deionized water according to a mass ratio of 1:0.15 to prepare the oil-phase solution.

2. The application according to claim 1, wherein The preparation method of the ZIF-L nanosheets is as follows: Dropwise add an aqueous solution of cobalt nitrate into an aqueous solution of dimethylimidazole, stir and react at room temperature, centrifuge the obtained reaction solution and dry it under vacuum to prepare the ZIF-L nanosheets.

3. The application according to claim 1, characterized in that The temperature of the drying is 30 °C and the time is 15 min.

4. The application according to claim 1, characterized in that, The temperature of the heating is 90 °C and the time is 15 min.

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