Janus structure high-permeability nanofiltration membrane and preparation method thereof

Abstract

The invention belongs to the technical field of water treatment membranes, and discloses a Janus structure high-permeability nanofiltration membrane and a preparation method thereof.Green low-cost alkaline lignin with a three-dimensional dendritic structure is introduced into a water phase solution in the interfacial polymerization process, an electronegativity lignin / polyamide composite layer with a loose structure is constructed on the surface of a porous supporting layer, and the high-permeability nanofiltration membrane with the Janus structure is obtained. Then modifying the surface of the composite layer with organic amine by forming an amido bond by using the residual acyl chloride groups on the surface of the composite layer to form a positive electricity layer; then quaternization is carried out on organic amine by utilizing iodomethane, and part of amido or imino is converted into quaternary ammonium ions, so that the charge intensity of the surface of a positive electricity layer is improved, and the high-performance Janus structure nanofiltration membrane with the positive electricity on the upper layer and the negative electricity on the lower layer is obtained. The nanofiltration membrane prepared by the method can efficiently and selectively remove divalent anions and cations, and has excellent water molecule and monovalent ion permeability.

Description

Technical Field

[0001] The present invention belongs to the technical field of water treatment membranes. Specifically, it relates to a highly permeable nanofiltration membrane and a preparation method thereof. Background Art

[0002] Nanofiltration membranes are widely used in fields such as tap water purification, hard water softening, food processing, sewage treatment, and medicine, and have broad application prospects. Seeking nanofiltration membranes with high permeability and high monovalent / divalent ion separation selectivity has always been the goal pursued by membrane manufacturers and researchers. Currently, commercially available nanofiltration membranes are mainly made of polyamide materials. Generally, such nanofiltration membranes are mainly obtained by interfacial polymerization of piperazine and trimesoyl chloride, and the surface is negatively charged due to the presence of a large number of carboxyl functional groups. Therefore, polyamide nanofiltration membranes have good retention and selectivity for divalent anion salts such as sodium sulfate; however, due to electrostatic attraction, negatively charged nanofiltration membranes have weak retention for divalent cations such as calcium and magnesium. The ability of such membrane materials to remove positively charged solutes still needs to be improved in practical applications such as hard water softening and sewage treatment. In addition, permeability is also a key indicator for evaluating the performance of nanofiltration membranes in practical applications. Highly permeable nanofiltration membranes help to increase the water production in applications, thereby reducing energy consumption and improving water treatment efficiency, and obtaining highly permeable nanofiltration membranes has also been a hot topic in the membrane field.

[0003] In recent years, the development of highly permeable nanofiltration membranes that can simultaneously remove divalent anions and cations has become a research hotspot in the membrane industry and the water treatment industry. Nanofiltration membranes with a Janus double-charge structure have different charges on the upper and lower parts of their separation layer. The positively charged part of the separation layer can effectively repel divalent cations such as calcium and magnesium, and the negatively charged part can effectively remove divalent anions such as sulfate and carbonate. Therefore, Janus structure nanofiltration membranes can simultaneously remove divalent anions and cations in aqueous solutions, only retaining monovalent ions, thus having high water treatment capabilities. However, the current retention rate of Janus nanofiltration membranes for divalent ions still cannot meet the requirements. For example, a graphene-based Janus structure nanofiltration membrane prepared by Rui Wang et al. has a retention rate of less than 90% for both divalent anions and cations (Journal of Membrane Science, 2023, 667, 121191.). Ze-Lin Qiu et al. introduced polyamide-amine dendrimers into the polyamide separation layer, making the retention rate of divalent anions and cations of this nanofiltration membrane reach more than 95%, but its pure water permeability is only 10.35L m -2 h -1 bar -1(Chemical Engineering Journal, 2021, 416, 129023.) Currently, how to obtain a nanofiltration membrane with high permeability, high divalent cation and anion rejection rates, and selectivity is an important goal in the research and development of the membrane field. In addition, exploring low-cost and green membrane-making raw materials, reducing the preparation cost of nanofiltration membranes, and developing high-performance nanofiltration membrane preparation methods suitable for large-scale applications are also hotspots in current nanofiltration membrane research. Summary of the Invention

[0004] The technical problem to be solved by the present invention is how to realize the preparation of a high-performance Janus structure nanofiltration membrane. Focusing on the problems existing in the selective separation of divalent ions by commercial polyamide nanofiltration membranes, a Janus structure high-permeability nanofiltration membrane and its preparation method are provided. During the interfacial polymerization process, a green and low-cost alkaline lignin with a three-dimensional dendritic structure is introduced into the aqueous solution to construct a negatively charged lignin / polyamide composite layer with a loose structure on the surface of the porous support layer. Then, using the acyl chloride groups remaining on the surface of the composite layer, an organic amine is modified on the surface of the composite layer by forming an amide bond to form a positively charged layer; subsequently, the organic amine is quaternized with methyl iodide to convert some amino or imino groups into quaternary ammonium ions, thereby increasing the charge intensity on the surface of the positively charged layer, and thus obtaining a high-performance Janus structure nanofiltration membrane with a positively charged upper layer and a negatively charged lower layer. The nanofiltration membrane prepared by this method can efficiently and selectively remove divalent anions and cations, and at the same time has excellent water molecule and monovalent ion permeability.

[0005] According to one aspect of the present invention, a Janus structure high-permeability nanofiltration membrane is provided, which is obtained by the following preparation process:

[0006] (1) Prepare an aqueous solution composed of piperazine and a lignin derivative; in the aqueous solution: the concentration of piperazine is 0.5 wt% - 1.5 wt%, and the concentration of the lignin derivative is 1.5 wt% - 3 wt%;

[0007] Dissolve an organic acyl chloride in n-hexane as an oil phase solution; in the oil phase solution: the concentration of the organic acyl chloride is 0.05 wt% - 0.2 wt%;

[0008] (2) Pour the aqueous solution obtained in step (1) onto the surface of a porous ultrafiltration substrate, immerse the surface in the aqueous solution, keep it for 2 - 8 min, then pour out the remaining aqueous solution, rinse the surface of the porous ultrafiltration substrate with deionized water, and then blot the remaining moisture on the surface of the porous ultrafiltration substrate with filter paper;

[0009] (3) Pour the oil-phase solution obtained in step (1) onto the surface of the porous ultrafiltration substrate obtained in step (2) to submerge the surface with the oil-phase solution for interfacial polymerization reaction. After reacting for 0.5 - 3 min, pour out the remaining oil-phase solution, thereby forming a negatively charged nanofiltration layer on the surface of the porous ultrafiltration substrate;

[0010] (4) Then pour the organic amine solution onto the surface of the negatively charged nanofiltration layer obtained in step (3) to submerge the negatively charged nanofiltration layer. After maintaining for 2 - 6 min, pour out the remaining organic amine solution, and rinse the remaining organic amine solution on the surface of the obtained nanofiltration membrane with deionized water;

[0011] (5) Fumigate the surface of the nanofiltration membrane obtained in step (4) with methyl iodide vapor to quaternize the organic amine on the surface of the nanofiltration membrane to form a positively charged layer, thereby obtaining a Janus-structured nanofiltration membrane with a positively charged upper surface and a negatively charged lower surface.

[0012] According to one aspect of the present invention, there is provided a method for preparing a Janus-structured highly permeable nanofiltration membrane, comprising the following steps:

[0013] (1) Prepare an aqueous solution composed of piperazine and lignin derivatives; in the aqueous solution: the concentration of piperazine is 0.5 wt% - 1.5 wt%, and the concentration of lignin derivatives is 1.5 wt% - 3 wt%;

[0014] Dissolve organic acyl chloride in n-hexane as the oil-phase solution; in the oil-phase solution: the concentration of organic acyl chloride is 0.05 wt% - 0.2 wt%;

[0015] (2) Pour the aqueous solution obtained in step (1) onto the surface of the porous ultrafiltration substrate to submerge the surface with the aqueous solution. After maintaining for 2 - 8 min, pour out the remaining aqueous solution, rinse the aqueous solution on the surface of the porous ultrafiltration substrate with deionized water, and then blot the remaining moisture on the surface of the porous ultrafiltration substrate with filter paper;

[0016] (3) Pour the oil-phase solution obtained in step (1) onto the surface of the porous ultrafiltration substrate obtained in step (2) to submerge the surface with the oil-phase solution for interfacial polymerization reaction. After reacting for 0.5 - 3 min, pour out the remaining oil-phase solution, thereby forming a negatively charged nanofiltration layer on the surface of the porous ultrafiltration substrate;

[0017] (4) Then pour the organic amine solution onto the surface of the negatively charged nanofiltration layer obtained in step (3) to submerge the negatively charged nanofiltration layer. After maintaining for 2 - 6 min, pour out the remaining organic amine solution, and rinse the remaining organic amine solution on the surface with deionized water;

[0018] (5) The surface of the nanofiltration membrane with organic amine is fumigated with methyl iodide vapor to quaternize the organic amine on the surface of the nanofiltration membrane, thereby forming a positively charged layer, and thus obtaining a Janus structure nanofiltration membrane with a positively charged upper surface and a negatively charged lower surface.

[0019] In the above Janus structure highly permeable nanofiltration membrane and its preparation method:

[0020] Preferably, in step (1), the lignin derivative is one or a mixture of several of alkaline lignin, sulfonated lignin or aminated lignin.

[0021] Preferably, in step (1), the organic acyl chloride is one or a mixture of two of trimesoyl chloride or isophthaloyl chloride.

[0022] Preferably, the porous ultrafiltration substrate is a polysulfone ultrafiltration substrate, a polyethersulfone ultrafiltration substrate or a polypropylene ultrafiltration substrate.

[0023] Preferably, in step (2), the time for rinsing the surface of the porous ultrafiltration substrate with deionized water is 0.5 - 3 min.

[0024] Preferably, in step (4), the organic amine is one or a mixture of several of polyethyleneimine, triethylenetetramine or tetraethylenepentamine, and the concentration of the organic amine is 0.1 - 1 wt%.

[0025] Preferably, in step (4), the time for rinsing the surface of the nanofiltration membrane with deionized water is 0.5 - 3 min.

[0026] Preferably, in step (5), the fumigation with methyl iodide vapor is carried out at a temperature of 25 - 40 °C.

[0027] Preferably, in step (5), the density of the methyl iodide vapor is 0.2 - 0.8 mg / cm 3 , and the fumigation time of the methyl iodide vapor is 1 - 3 hours.

[0028] Due to the three-dimensional dendritic structure of alkaline lignin and the presence of a large number of hydrophilic functional groups such as alcohol hydroxyl groups, phenolic hydroxyl groups, and ether bonds, when introduced during the film-forming process, on the one hand, the presence of these hydroxyl and ether bonds increases the negative charge of the lignin / polyamide composite layer, enhancing the membrane's ability to retain divalent anions; on the other hand, the introduction of lignin increases the porosity of the negatively charged layer and enlarges its pore size, which is beneficial to the improvement of the membrane material's permeability. There will be a large number of residual acyl chloride functional groups on the surface of the newly formed lignin / polyamide layer, which can react with organic amines to form amide functional groups, thereby modifying the organic amines on the surface of the negatively charged layer. The presence of groups such as amino and imino groups in the organic amines makes the membrane surface carry a positive charge. To increase the strength of the surface positive charge, the present invention uses the method of fumigating with methyl iodide vapor to convert the amino and imino groups of the organic amines into quaternary ammonium ions with stronger positive charges, thus forming a Janus structure nanofiltration membrane with a positively charged upper layer and a negatively charged lower layer. The positive charge on the upper layer of this nanofiltration membrane has a strong electrostatic repulsive force on divalent cations, while the abundant negative charge on the lower layer has a strong electrostatic repulsion on divalent anions. Monovalent ions have a weak electrostatic interaction with the membrane surface due to their lower charge number, and monovalent ions usually have a smaller radius. Therefore, the Janus structure nanofiltration membrane can retain divalent anions and cations in aqueous solutions while selectively permeating monovalent ions.

[0029] The beneficial effects of the present invention are:

[0030] The present invention discloses a Janus structure highly permeable nanofiltration membrane and its preparation method. The three-dimensional dendritic alkaline lignin is used to construct the negatively charged layer of the Janus nanofiltration membrane, reducing the compactness of the negatively charged layer and increasing the diffusion rate of monovalent salts and water molecules in the permeation channels. In addition, the introduction of alkaline lignin improves the retention ability of the nanofiltration membrane for divalent anions. By modifying the organic amines on the surface of the negatively charged layer and quaternizing them, a Janus nanofiltration membrane structure with a positively charged upper layer and a negatively charged lower layer is formed. The quaternized organic amines on the upper layer have good retention performance for divalent cations. Since the structure of the nanofiltration membrane becomes loose and monovalent salt ions have a weak electrostatic interaction with the membrane surface, monovalent ions have good permeability, which makes the nanofiltration membrane have good separation selectivity for both divalent anions and cations. In addition, oxygen-containing and nitrogen-containing functional groups such as hydroxyl groups, ether bonds in alkaline lignin molecules, and amino and imino groups in organic amines endow the membrane material with good hydrophilicity, which is beneficial to the improvement of the water permeability of the nanofiltration membrane.

[0031] The present invention discloses a Janus structure highly permeable nanofiltration membrane and its preparation method. The raw material cost is low, significantly reducing the preparation cost of the Janus structure nanofiltration membrane. The preparation process does not require special equipment, and the preparation process is easy to promote and scale up.

[0032] The Janus-structured high-permeability nanofiltration membrane disclosed by the present invention has excellent selectivity and pure water permeability for divalent anions and cations simultaneously. When using the solution composed of alkaline lignin and piperazine as the aqueous solution, trimesoyl chloride as the oil-phase solution, and polyethyleneimine (PEI) as the surface modifier, the nanofiltration membrane prepared under the optimal conditions has a separation selectivity of 18.8 for the Na2SO4 / NaCl mixture and 22.0 for the MgCl2 / NaCl mixture, and a pure water permeability of 16.3 L / m 2 h bar.

[0033] In summary, the preparation process of the present invention is simple, the raw materials are easy to obtain and the cost is low. The obtained nanofiltration membrane has good separation selectivity for divalent anions and cations, and at the same time has good permeation effects on monovalent salt ions and water molecules. This preparation method has the application prospect of large-scale production. Specific Embodiments

[0034] The following further describes the present invention in detail through specific embodiments. The following embodiments can enable those skilled in the art to understand the present invention more comprehensively, but do not limit the present invention in any way.

[0035] In the present invention, a negatively charged layer is first formed on the surface of a porous substrate by interfacial polymerization reaction, and then an organic amine is modified on the surface by using the acyl chloride functional groups remaining on the surface of the negatively charged layer to make the surface carry a certain degree of positive charge. To enhance the positive charge on the upper surface, part of the amino groups and imino groups of the organic amine are converted into quaternary ammonium ions by using methyl iodide, thereby forming a Janus-structured nanofiltration membrane with high permeability and high separation selectivity. The present invention has the characteristics of simple preparation process, strong operability, easy control of process conditions, and easy large-scale production.

[0036] Example 1

[0037] (1) Prepare the aqueous solution: The aqueous solution is prepared with piperazine and alkaline lignin. The concentration of piperazine in the aqueous solution is 1 wt%, and the concentration of alkaline lignin is 2.5 wt%.

[0038] (2) Prepare the oil-phase solution: Prepare a n-hexane solution containing 0.1 wt% trimesoyl chloride, and use it as the oil-phase solution after dissolving evenly.

[0039] (3) Prepare the Janus-structured nanofiltration membrane:

[0040] First, pour the aqueous solution on the surface of the polysulfone ultrafiltration substrate and keep it for 4 min to immerse the surface of the polysulfone ultrafiltration substrate; then pour out the remaining aqueous solution on the surface of the polysulfone ultrafiltration substrate, rinse the surface of the polysulfone ultrafiltration substrate with deionized water for 2 min, and then blot the remaining moisture on the surface of the polysulfone ultrafiltration substrate with filter paper.

[0041] Subsequently, the oil-phase solution was poured onto the surface of the polysulfone ultrafiltration substrate, and the oil-phase solution was immersed in the polysulfone ultrafiltration substrate to carry out an interfacial polymerization reaction. After maintaining for 50 s, the residual oil-phase solution on the surface was poured out, thereby forming a negatively charged nanofiltration layer on the substrate surface.

[0042] Then, the prepared 0.3 wt% polyethyleneimine (PEI) solution was poured onto the surface of the negatively charged nanofiltration layer, and the polyethyleneimine (PEI) solution was immersed in the negatively charged nanofiltration layer and maintained for 4 min, thereby modifying PEI on the surface of the obtained nanofiltration membrane. Then, the surface of the nanofiltration membrane was rinsed with deionized water for 1 min.

[0043] Subsequently, at 35 °C, the surface of the nanofiltration membrane was fumigated with methyl iodide vapor with a density of 0.37 mg / cm 3 to cause the quaternization of PEI. The fumigation time was 2 hours, and then a positively charged layer was formed on the surface of the nanofiltration membrane, obtaining a Janus-structured nanofiltration membrane with a positively charged upper surface and a negatively charged lower surface.

[0044] Subsequently, the prepared Janus-structured nanofiltration membrane was immersed in deionized water for storage.

[0045] Example 2

[0046] The Janus-structured highly permeable nanofiltration membrane was prepared according to the method of Example 1, except that the concentration of piperazine was 0.5 wt%.

[0047] Example 3

[0048] The Janus-structured highly permeable nanofiltration membrane was prepared according to the method of Example 1, except that the concentration of piperazine was 1.5 wt%.

[0049] Example 4

[0050] The Janus-structured highly permeable nanofiltration membrane was prepared according to the method of Example 1, except that the concentration of alkaline lignin was 1.5 wt%.

[0051] Example 5

[0052] The Janus-structured highly permeable nanofiltration membrane was prepared according to the method of Example 1, except that the concentration of alkaline lignin was 3 wt%.

[0053] Example 6

[0054] The Janus-structured highly permeable nanofiltration membrane was prepared according to the method of Example 1, except that the contact time between the aqueous phase and the polysulfone ultrafiltration support layer was 2 min.

[0055] Example 7

[0056] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the contact time between the aqueous phase and the polysulfone ultrafiltration support layer is 8 min.

[0057] Example 8

[0058] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that after the aqueous phase is adsorbed on the substrate surface, the time for rinsing the surface with deionized water is 0.5 min.

[0059] Example 9

[0060] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that after the aqueous phase is adsorbed on the substrate surface, the time for rinsing the surface with deionized water is 3 min.

[0061] Example 10

[0062] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the oil phase reaction time is 0.5 min.

[0063] Example 11

[0064] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the oil phase reaction time is 3 min.

[0065] Example 12

[0066] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the concentration of the polyethyleneimine solution used is 0.1 wt%.

[0067] Example 13

[0068] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the concentration of the polyethyleneimine solution used is 1 wt%.

[0069] Example 14

[0070] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the retention time of the polyethyleneimine solution used on the negatively charged layer surface is 2 min.

[0071] Example 15

[0072] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the retention time of the polyethyleneimine solution used on the negatively charged layer surface is 6 min.

[0073] Example 16

[0074] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that after polyethyleneimine is retained on the membrane surface for a certain period of time, the rinsing time with deionized water is 0.5 min.

[0075] Example 17

[0076] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that after polyethyleneimine is retained on the membrane surface for a certain period of time, the rinsing time with deionized water is 3 min.

[0077] Example 18

[0078] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the fumigation time with methyl iodide used is 1 h.

[0079] Example 19

[0080] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the fumigation time with methyl iodide used is 3 h.

[0081] Example 20

[0082] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the concentration of methyl iodide fumigation used is 0.2 mg / cm 3 。

[0083] Example 21

[0084] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the concentration of methyl iodide fumigation used is 0.8 mg / cm 3 。

[0085] Example 22

[0086] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the lignin derivative used is sulfonated lignin.

[0087] Example 23

[0088] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the lignin derivative used is aminated lignin.

[0089] Example 24

[0090] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, with the only difference being that the organic acyl chloride used is isophthaloyl chloride.

[0091] Example 25

[0092] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, except that triethylenetetramine is used instead of polyethyleneimine.

[0093] Example 26

[0094] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, except that tetraethylenepentamine is used instead of polyethyleneimine.

[0095] Example 27

[0096] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, except that the porous ultrafiltration substrate used is a polyethersulfone ultrafiltration substrate.

[0097] Example 28

[0098] Prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, except that the porous ultrafiltration substrate used is a propylene ultrafiltration substrate.

[0099] In addition, to confirm the necessity of the existence of key preparation parameters, the present invention is further illustrated by specific comparative examples below. The following comparative examples can enable those skilled in the art to more comprehensively understand the present invention.

[0100] Comparative Example 1

[0101] To confirm the role of piperazine in the preparation of the Janus-structured highly permeable nanofiltration membrane, prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, except that piperazine is not added during the preparation process.

[0102] Comparative Example 2

[0103] To confirm the role of lignin derivatives in the preparation of the Janus-structured highly permeable nanofiltration membrane, prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, except that no lignin derivatives such as alkaline lignin are added during the preparation process.

[0104] Comparative Example 3

[0105] To confirm the role of organic amines in the preparation of the Janus-structured highly permeable nanofiltration membrane, prepare the Janus-structured highly permeable nanofiltration membrane according to the method of Example 1, except that no organic amines such as polyethyleneimine are added during the preparation process.

[0106] Comparative Example 4

[0107] To verify the effect of methyl iodide vapor fumigation in the preparation of Janus-structured highly permeable nanofiltration membranes, Janus-structured highly permeable nanofiltration membranes were prepared according to the method of Example 1, with the only difference being that methyl iodide was not added during the preparation process.

[0108] For the separation performance of the Janus-structured highly permeable nanofiltration membranes prepared in the above examples and comparative examples, the following methods were used for testing: The permeability of the nanofiltration membrane was tested with deionized water as the feed water, in a cross-flow filtration mode at 25 °C. After pre-pressing at 0.6 MPa for 30 minutes, the produced water was collected for 15 minutes, and the permeability (F) of the nanofiltration membrane was calculated based on the volume of the produced water and the effective membrane area.

[0109] The separation selectivity was tested in the following way: A mixed solution containing 1 g / L sodium chloride and 1 g / L sodium sulfate, and a mixed solution containing 1 g / L sodium chloride and 1 g / L magnesium chloride were used as the test solutions respectively. The separation selectivity of the obtained Janus-structured nanofiltration membrane was tested at 0.6 Mpa. The temperature of the test solution was 25 °C. After pre-pressing at 0.6 Mpa for 30 minutes, the produced water was collected for 15 minutes. Subsequently, the ion concentrations in the produced water and the feed water were analyzed and tested using ion chromatography, and the desalination rate (R) and selectivity (S) of the nanofiltration membrane were calculated based on the concentration values. The average value of three consecutive tests was taken as the final result.

[0110] The formula for calculating the water flux is as follows:

[0111]

[0112] Where F represents the permeability (L m -2 h -1 bar -1 ), V represents the volume of the collected produced water (L), A is the effective membrane area (m 2 ), t is the produced water collection time (h), and △P represents the transmembrane pressure difference (bar).

[0113] The formula for calculating the desalination rate is as follows:

[0114]

[0115] Where R represents the desalination rate (%), C1 represents the ion concentration in the feed water (g / L), and C2 represents the ion concentration in the produced water (g / L).

[0116] The formula for calculating the selectivity is as follows:

[0117]

[0118] Where S represents the selectivity, R1 represents the rejection rate of monovalent ions (%), and R2 represents the rejection rate of divalent ions (%).

[0119] (1) Test results of the examples are as follows:

[0120] Table 1 Permeability, desalination rate and selectivity of Janus - structured nanofiltration membranes under different preparation conditions

[0121]

[0122]

[0123] Note: a indicates that the desalination rate of the measured NaCl uses a mixed solution composed of 1 g / L NaCl and 1 g / L Na2SO4 as the test solution; b indicates that the desalination rate of the measured NaCl uses a mixed solution composed of 1 g / L NaCl and 1 g / L MgCl2 as the test solution.

[0124] (2) Test results of the comparative examples are as follows:

[0125] Table 2 Permeability, desalination rate and selectivity of Janus - structured nanofiltration membranes under different comparison conditions

[0126]

[0127]

[0128] The above results of the examples and comparative examples show that by using the preparation method combining interfacial polymerization and surface modification proposed in the present invention, Janus - structured nanofiltration membranes with good permeability, desalination rate and ion selectivity can be prepared. This preparation method is simple, fast and suitable for large - scale application. In addition, the above comparative examples show that the introduction of piperazine, lignin derivatives, organic amines and methyl iodide fumigation during the preparation process all have obvious effects on the separation and selection performance of the Janus - structured nanofiltration membrane, which are the keys to successfully preparing the Janus - structured nanofiltration membrane. The optimization of these preparation conditions plays an important role in improving the performance of the nanofiltration membrane.

[0129] Although the preferred embodiments of the present invention are described above, the present invention is not limited to the above - mentioned specific embodiments. The above - mentioned specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many specific transformations in form without departing from the spirit of the invention and the scope protected by the claims. All of these fall within the protection scope of the present invention.

Claims

1. A method for preparing a Janus structure high permeability nanofiltration membrane, characterized in that: The steps include: (1) preparing an aqueous solution consisting of piperazine and a lignin derivative; in the aqueous solution, the concentration of piperazine is 0.5wt%-1.5wt%, and the concentration of the lignin derivative is 1.5wt%-3wt%; Dissolving an organic acid chloride in n-hexane as an oil phase solution; in the oil phase solution, the concentration of the organic acid chloride is 0.05wt%-0.2wt%; (2) pouring the aqueous solution obtained in step (1) onto the surface of the porous ultrafiltration substrate so that the surface is immersed in the aqueous solution, maintaining the aqueous solution for 2-8 minutes, pouring out the remaining aqueous solution, and rinsing the aqueous solution on the surface of the porous ultrafiltration substrate with deionized water, and then absorbing the remaining water on the surface of the porous ultrafiltration substrate with filter paper; (3) pouring the oil phase solution obtained in step (1) onto the surface of the porous ultrafiltration substrate obtained in step (2), so that the surface is immersed in the oil phase solution to carry out an interfacial polymerization reaction, reacting for 0.5-3 minutes and then pouring off the remaining oil phase solution, thereby forming a nanofiltration negative charge layer on the surface of the porous ultrafiltration substrate; (4) pouring the organic amine solution onto the surface of the nanofiltration negative layer obtained in step (3) to immerse the organic amine solution into the nanofiltration negative layer, keeping the solution for 2-6 minutes, then pouring out the remaining organic amine solution, and rinsing the remaining organic amine solution on the surface of the nanofiltration membrane with deionized water; (5) fumigating the surface of the nanofiltration membrane obtained in step (4) with iodomethane vapor to quaternize the organic amine on the surface of the nanofiltration membrane to form a positively charged layer, thereby obtaining a Janus structure nanofiltration membrane with a positively charged upper surface and a negatively charged lower surface.

2. The method for preparing a Janus structure high permeability nanofiltration membrane according to claim 1, characterized in that: In step (1), the lignin derivative is one or a mixture of alkaline lignin, sulfonated lignin or aminated lignin.

3. The method for preparing a Janus structure high permeability nanofiltration membrane according to claim 1, characterized in that: In step (1), the organic acid chloride is one of trimesoyl chloride and isophthaloyl chloride or a mixture of the two.

4. The method for preparing a Janus structure high permeability nanofiltration membrane according to claim 1, characterized in that: The porous ultrafiltration substrate is a polysulfone ultrafiltration substrate, a polyethersulfone ultrafiltration substrate and a polypropylene ultrafiltration substrate.

5. The method for preparing a Janus structure high permeability nanofiltration membrane according to claim 1, characterized in that: In step (2), the time for washing the surface of the porous ultrafiltration substrate with deionized water is 0.5-3 minutes.

6. The method for preparing a Janus structure high permeability nanofiltration membrane according to claim 1, characterized in that: In step (4), the organic amine is one or a mixture of polyethyleneimine, triethylenetetramine or tetraethylenepentamine, and the concentration of the organic amine is 0.1-1wt%.

7. The method for preparing a Janus structure high permeability nanofiltration membrane according to claim 1, characterized in that: In step (4), the time for washing the surface of the nanofiltration membrane with deionized water is 0.5-3 minutes.

8. The method for preparing a Janus structure high permeability nanofiltration membrane according to claim 1, characterized in that: In step (5), iodomethane vapor fumigation is performed at a temperature of 25-40°C.

9. The method for preparing a Janus structure high permeability nanofiltration membrane according to claim 1, characterized in that: In step (5), the density of methyl iodide vapor is 0.2-0.8 mg / cm 3 The fumigation time of iodine methyl vapor is 1-3 hours.

10. A Janus structure high permeability nanofiltration membrane, characterized in that: The method is obtained by the preparation method described in any one of claims 1 to 9.

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