A flat-sheet nanofiltration membrane and its preparation method

Abstract

This invention discloses a flat-sheet nanofiltration membrane and its preparation method. The flat-sheet nanofiltration membrane includes a separation layer, a flat-sheet ultrafiltration membrane, and a heavy metal and micro-pollutant removal layer. The flat-sheet ultrafiltration membrane includes a base membrane and a polyester support layer. One outer surface of the polyester support layer is closely attached to the base membrane, and the other outer surface is connected to the heavy metal and micro-pollutant removal layer. The polyester support layer and the heavy metal and micro-pollutant removal layer are connected by amide bonds. The separation layer is located on the outermost side of the flat-sheet nanofiltration membrane and is closely attached to the base membrane. Heavy metal adsorption performance tests of the flat-sheet nanofiltration membrane show that the concentration of Cd(NO3)2 permeating the membrane is less than or equal to 5 ppb, and the maximum amount of precipitated solution that can be passed through the membrane is 7.1 L. Micro-pollutant adsorption performance tests of the flat-sheet nanofiltration membrane show that the concentration of bisphenol A (BPA) permeating the membrane is less than or equal to 10 ppb, and the maximum amount of precipitated solution that can be passed through the membrane is 8.8 L.

Description

Technical Field

[0001] This invention relates to a flat-sheet nanofiltration membrane and its preparation method. Background Technology

[0002] Tap water contains ppb-level heavy metal ions and micro-pollutants. Long-term consumption of water containing these substances can lead to their accumulation in human organs, posing a significant health risk. Therefore, the lower the levels of heavy metal ions and micro-pollutants in drinking water, the better. Currently, methods for removing heavy metal ions and micro-pollutants generally include membrane methods, ion exchange, electrodialysis, and activated carbon separation. The most common method is reverse osmosis filters. While reverse osmosis removes heavy metals and micro-pollutants, it also removes beneficial minerals. Nanofiltration filters retain minerals in the water and are effective at removing heavy metals and micro-pollutants, but they cannot completely remove heavy metals. Other methods for removing heavy metals and micro-pollutants, such as ion exchange, electrodialysis, and activated carbon separation, have drawbacks such as poor removal efficiency and high cost.

[0003] Nanofiltration membranes, developed in the 1980s, are a novel type of separation membrane technology that falls between ultrafiltration and reverse osmosis. They have a molecular weight cutoff of 200–1000 Da and exhibit high retention efficiency for various organic pollutants, such as chloroform intermediates, pesticides, hormones, and small organic molecules. They are also effective at removing inorganic ions, such as calcium ions. 2+ F - Cl - SO4 2- Nanofiltration membranes can remove trace organic matter and heavy metal ions to a certain extent, making them widely adaptable to water source requirements and water purification applications. Due to their unique selective separation properties, nanofiltration has become one of the important water purification technologies. Nanofiltration has a highly efficient removal effect on trace organic matter and heavy metals, and with the decreasing cost of membrane modules, nanofiltration membranes are showing broad application prospects.

[0004] However, nanofiltration membranes face membrane fouling issues during use. Inorganic and organic matter in the water deposits at the membrane pores, leading to pore blockage, membrane surface fouling, increased operating pressure, and decreased membrane flux. Nanofiltration membrane fouling mainly includes organic fouling, inorganic fouling, colloidal particulate fouling, and biological fouling. The corresponding pollutants mainly include organic solutes, inorganic solutes, colloids, and biological solids. Among these, inorganic fouling is mainly caused by calcium in the water. 2+ Mg 2+ Al 3+ Fe 2+ Ba 2+Plasma deposits form scale on the membrane surface, causing membrane fouling. Studies have found that the higher the concentration of biodegradable organic matter in the feed water for nanofiltration membrane processes, the faster the biofilm forms. Membrane surface fouling arises from the synergistic effect of organic, inorganic, and biological contaminants. Membrane fouling severely restricts the application of nanofiltration membranes in drinking water treatment.

[0005] Therefore, how nanofiltration membranes can effectively remove heavy metals and micropollutants without reducing membrane flux due to adsorption is a problem that urgently needs to be solved in existing technologies. Summary of the Invention

[0006] To address the aforementioned technical problems, the present invention aims to provide a novel flat-sheet nanofiltration membrane with the ability to remove heavy metals and micro-pollutants, and its preparation method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] One of the technical solutions provided by the present invention is: a flat-sheet nanofiltration membrane, characterized in that the flat-sheet nanofiltration membrane includes a separation layer, a flat-sheet ultrafiltration membrane, and a heavy metal and micro-pollutant removal layer; the flat-sheet ultrafiltration membrane includes a base membrane and a polyester support layer; one outer surface of the polyester support layer is closely attached to the base membrane, and the other outer surface is connected to the heavy metal and micro-pollutant removal layer; the polyester support layer and the heavy metal and micro-pollutant removal layer are connected by amide bonds; the separation layer is located on the outermost side of the flat-sheet nanofiltration membrane and is closely attached to the base membrane.

[0009] In this invention, the flat-sheet nanofiltration membrane also satisfies one or more of the following conditions:

[0010] a. The separation layer satisfies one or more of the following conditions:

[0011] ① The thickness of the separation layer is 10-200 nm, for example 50 nm or 100 nm;

[0012] ② The separation pore size of the separation layer is 0.5 to 5 nm, for example, 1 nm, 2 nm or 3 nm;

[0013] ③ The material of the separation layer is polyamide;

[0014] b. The main material of the base film is one or more of polysulfone, polyethersulfone, polypropylene, and polyvinyl chloride;

[0015] c. The thickness of the base film is 50–200 μm, for example, 50 μm, 100 μm or 200 μm;

[0016] d. The material of the polyester support layer is polyethylene terephthalate and / or polybutylene terephthalate;

[0017] e. The thickness of the polyester support layer is 10 to 100 μm, for example, 10 μm, 50 μm or 100 μm;

[0018] f. The thickness of the heavy metal and micro-pollutant removal layer is 50-1000 nm, for example, 100 nm, 200 nm, 300 nm or 800 nm.

[0019] g. The material of the heavy metal and micro-pollutant removal layer is a reactant of amines and β-cyclodextrin. Preferably, the amine is one or more of chitosan, polyethyleneimine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine. More preferably, the amine is polyethyleneimine or chitosan.

[0020] The second technical solution provided by the present invention is: a method for preparing a flat nanofiltration membrane, comprising the following steps: (1) preparation of a heavy metal and micro-pollutant removal layer containing amino groups: β-cyclodextrin and ethylenediaminetetraacetic acid and / or oxalic acid are added to an aqueous solution for reaction, and then amine substances are added to continue the reaction; wherein, the β-cyclodextrin and amine substances respectively satisfy one or two of the following conditions:

[0021] a. The concentration of β-cyclodextrin in the aqueous solution is 1-10 wt%;

[0022] b. The concentration of the amine substance is 1-10 wt%;

[0023] (2) Immerse the flat sheet ultrafiltration membrane containing the base membrane and the polyester support layer into the solution of the heavy metal and micro-pollutant removal layer containing amino groups, react, and then perform interfacial polymerization on the flat sheet nanofiltration membrane to form a separation layer on the base membrane; in the flat sheet ultrafiltration membrane, one outer surface of the polyester support layer is closely attached to the base membrane.

[0024] Step (2) satisfies one or two of the following conditions:

[0025] a. The reaction temperature is 60-90℃, for example, 60℃, 80℃ or 90℃;

[0026] b. The reaction time is 5-15 hours, for example, 6 hours, 8 hours or 15 hours.

[0027] Wherein, step (1) satisfies one or more of the following conditions:

[0028] a. The concentration of β-cyclodextrin in the aqueous solution is 1 wt%, 2 wt%, or 8 wt%;

[0029] b. The concentration of ethylenediaminetetraacetic acid in the aqueous solution is 0.006 wt%-0.06 wt%, for example, 0.012 wt%, 0.049 wt%, or 0.006 wt%.

[0030] c. The concentration of oxalic acid in the aqueous solution is 0.004 wt%.

[0031] d. The molar ratio of β-cyclodextrin to ethylenediaminetetraacetic acid is 42:1;

[0032] e. The molar ratio of β-cyclodextrin to oxalic acid is 42:1;

[0033] f. The reaction temperature is 40-80℃, for example, 40℃, 50℃ or 80℃;

[0034] g. The reaction time is 1-8 hours, for example, 1 hour, 6 hours or 7.5 hours;

[0035] h. The amine substance is one or more of chitosan, polyethyleneimine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine and pentaethylenehexamine, more preferably polyethyleneimine or chitosan;

[0036] i. The concentration of the amine substance is 2 wt%, 4 wt%, or 6 wt%;

[0037] j. The duration of the continued reaction is 0.5-3 hours, for example, 0.5 hours, 1 hour, or 3 hours.

[0038] In step (2), after the reaction, the flat nanofiltration membrane is washed and dried;

[0039] Preferably, the cleaning process involves immersing the flat nanofiltration membrane in pure water.

[0040] More preferably, the soaking operation satisfies one or two of the following conditions:

[0041] a. The soaking time is 5-24 hours, for example, 8 hours, 10 hours or 20 hours;

[0042] b. The soaking temperature is 20-60℃, for example, 20℃, 40℃ or 50℃.

[0043] Preferably, the drying process satisfies one or two of the following conditions:

[0044] a. The drying temperature is 20-35℃, for example, 25℃, 30℃ or 35℃;

[0045] b. The drying time is 48-72 hours, for example, 50 hours, 65 hours or 70 hours;

[0046] In step (2), the separation layer is formed by the reaction of piperazine and pyromellitic methyl chloride.

[0047] The interfacial polymerization reaction includes the following steps:

[0048] a. Place the flat sheet ultrafiltration membrane in an acrylic frame, clamp it in place, and stand it upright in a fume hood. Wait for the moisture on the membrane surface to evaporate in about 10 minutes.

[0049] b. Pour about 30 mL of the prepared aqueous solution of piperazine into the acrylic glass frame, trying to pour it from the corner of the frame, and let it stand for 5 minutes;

[0050] c. Pour the aqueous phase into the waste liquid container, then stand the membrane upright and wait for the aqueous phase to evaporate completely, about 3 minutes.

[0051] d. Pour about 15 mL of the prepared oil phase, which is a hexane solution of trimesoyl chloride, into an acrylic glass frame. It must be poured from the corner of the acrylic glass frame. React for 30 seconds, then pour the oil phase into the waste liquid container.

[0052] e. Vigorously shake the film to make the oil phase evaporate quickly, and then place it in an oven at 80°C for 20 seconds.

[0053] The thickness of the obtained interfacial polymerization separation layer is 100 nm, and the separation pore size is 2 nm.

[0054] In this invention, the preparation method further includes cleaning, moisturizing and drying the membrane sheet forming the separation layer to obtain a flat nanofiltration membrane. The cleaning process involves immersing the membrane sheet forming the separation layer in pure water.

[0055] Preferably, the process of cleaning, moisturizing, and drying the membrane sheet that forms the separation layer satisfies one or more of the following conditions:

[0056] a. The soaking time is 5-24 hours, for example, 15 hours, 16 hours or 24 hours;

[0057] b. The soaking temperature is 20-60℃, for example, 20℃, 30℃ or 50℃;

[0058] c. In the moisturizing process, the moisturizer is one or more of glycerin, sodium lactate, potassium lactate, and sodium citrate, preferably sodium lactate or glycerin;

[0059] d. The drying temperature is 20-35℃, for example, 20℃, 25℃ or 35℃;

[0060] e. The drying time is 48-72 hours, for example, 48 hours, 60 hours or 70 hours.

[0061] The third technical solution provided by the present invention is: a flat sheet nanofiltration membrane prepared by the method described above.

[0062] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0063] The reagents and raw materials used in this invention are all commercially available.

[0064] The positive and progressive effects of this invention are as follows:

[0065] Adsorbents with heavy metal and micropollutant adsorption properties are grafted onto flat-sheet ultrafiltration membranes through a chemical reaction between amino groups and a polyester support layer. This results in a stronger bond, leading to highly efficient and stable membrane removal and a long lifespan. Heavy metal adsorption performance tests on the flat-sheet nanofiltration membrane showed that the concentration of Cd(NO3)2 permeating the membrane was less than or equal to 5 ppb, with a maximum excess volume of 7.1 L of precipitated solution. Micropollutant adsorption performance tests on the flat-sheet nanofiltration membrane showed that the concentration of bisphenol A (BPA) permeating the membrane was less than or equal to 10 ppb, with a maximum excess volume of precipitated solution of 8.8 L. Detailed Implementation

[0066] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0067] In all the following examples and comparative examples, the flat sheet ultrafiltration membranes are prepared by scraping the base membrane raw material onto a polyester support layer using a doctor blade. The polyester support layer is prepared by a melt-blown-hot rolling process.

[0068] Example 1

[0069] (1) β-Cyclodextrin and ethylenediaminetetraacetic acid were added to an aqueous solution. The concentration of β-cyclodextrin was 2 wt%, the concentration of tetraethylamine oxalate was 0.012 wt%, the molar ratio of β-cyclodextrin to ethylenediaminetetraacetic acid was 42:1, the reaction temperature was 50℃, and the reaction time was 6 h. Then, polyacetylimide was added at a concentration of 4 wt%, and the reaction was continued for 1 h.

[0070] (2) The flat ultrafiltration membrane containing a base membrane with polyethersulfone as the main material and a thickness of 100 μm and a polyester support layer with polyethylene terephthalate as the material and a thickness of 10 μm is placed in the solution of step (1), the temperature is adjusted to 80℃, and the reaction is carried out for 8 hours to form a heavy metal and micro-pollutant removal layer with a thickness of 200 nm at the bottom of the polyester support layer.

[0071] (3) The reacted flat ultrafiltration membrane was soaked in pure water at 20°C, the water was changed every hour, and the membrane was soaked for 20 hours. After taking it out, it was dried at 25°C for 70 hours. Then, an interfacial polymerization reaction was carried out in the ultrafiltration layer to form a separation layer.

[0072] The interfacial polymerization reaction includes the following steps:

[0073] a. Place the flat sheet ultrafiltration membrane in an acrylic frame, clamp it in place, and stand it upright in a fume hood. Wait for the moisture on the membrane surface to evaporate in about 10 minutes.

[0074] b. Pour about 30 mL of the prepared aqueous solution of piperazine into the acrylic glass frame, trying to pour it from the corner of the frame, and let it stand for 5 minutes;

[0075] c. Pour the aqueous phase into the waste liquid container, then stand the membrane upright and wait for the aqueous phase to evaporate completely, about 3 minutes.

[0076] d. Pour about 15 mL of the prepared oil phase, which is a hexane solution of trimesoyl chloride, into an acrylic glass frame. It must be poured from the corner of the acrylic glass frame. React for 30 seconds, then pour the oil phase into the waste liquid container.

[0077] e. Vigorously shake the film to make the oil phase evaporate quickly, and then place it in an oven at 80°C for 20 seconds.

[0078] The thickness of the obtained interfacial polymerization separation layer is 100 nm, and the separation pore size is 2 nm.

[0079] (4) Soak the membrane obtained in step (3) in pure water at 30°C, change the water every hour, and soak for 15 hours; after cleaning, put it in an aqueous solution of sodium lactate for moisturizing, and finally dry it at 25°C for 60 hours to obtain a new type of flat sheet nanofiltration membrane with the ability to remove heavy metals and micro pollutants.

[0080] (5) A flat nanofiltration membrane with a diameter of 53 mm was placed in the test fixture and its heavy metal adsorption performance was tested. A 25 ppb cadmium nitrate (Cd(NO3)2) solution was used as the spiking solution. When the Cd(NO3)2 concentration in the solution permeating the membrane was greater than 5 ppb, the test was stopped. The final volume of the spiking solution passing through the membrane was 4.3 L. The above test shows that when the cutoff concentration of 5 ppb is reached, the volume of the spiking solution passing through the membrane can reach 4.3 L.

[0081] (6) A 53 mm diameter flat-sheet nanofiltration membrane was placed in the test fixture to test its micropollutant adsorption performance. A 50 ppb bisphenol A (BPA) solution was used as the spiking solution. The test was stopped when the BPA concentration in the solution permeating the membrane exceeded 10 ppb. The final volume of the spiking solution passing through the membrane was 5.6 L. The above test shows that when the cutoff concentration of 10 ppb is reached, the volume of the spiking solution passing through the membrane can reach 5.6 L.

[0082] Example 2

[0083] (1) β-Cyclodextrin and ethylenediaminetetraacetic acid were added to an aqueous solution. The concentration of β-cyclodextrin was 8 wt%, the concentration of tetraethylamine oxalate was 0.049 wt%, the molar ratio of β-cyclodextrin to ethylenediaminetetraacetic acid was 42:1, the reaction temperature was 80℃, and the reaction time was 1 h. Then chitosan was added at a concentration of 6 wt%, and the reaction was continued for 0.5 h.

[0084] (2) The flat ultrafiltration membrane containing a base membrane with polysulfone as the main material and a thickness of 50 μm and a polyester support layer with polyethylene terephthalate as the material and a thickness of 50 μm is placed in the solution of step (1), the temperature is adjusted to 60℃, and the reaction is carried out for 15 h to form a heavy metal and micro pollutant removal layer with a thickness of 800 nm at the bottom of the polyester support layer.

[0085] (3) The reacted flat ultrafiltration membrane was soaked in pure water at 40°C, the water was changed every hour, and the membrane was soaked for 10 hours. After taking it out, it was dried at 30°C for 65 hours. Then, an interfacial polymerization reaction was carried out in the ultrafiltration layer to form a separation layer.

[0086] The interfacial polymerization reaction includes the following steps:

[0087] a. Place the flat sheet ultrafiltration membrane in an acrylic frame, clamp it in place, and stand it upright in a fume hood. Wait for the moisture on the membrane surface to evaporate in about 10 minutes.

[0088] b. Pour about 30 mL of the prepared aqueous solution of piperazine into the acrylic glass frame, trying to pour it from the corner of the frame, and let it stand for 5 minutes;

[0089] c. Pour the aqueous phase into the waste liquid container, then stand the membrane upright and wait for the aqueous phase to evaporate completely, about 3 minutes.

[0090] d. Pour about 15 mL of the prepared oil phase, which is a hexane solution of trimesoyl chloride, into an acrylic glass frame. It must be poured from the corner of the acrylic glass frame. React for 20 seconds, then pour the oil phase into the waste liquid container.

[0091] e. Vigorously shake the film to make the oil phase evaporate quickly, and place it in an oven to react at 80°C for 20 seconds.

[0092] The thickness of the obtained interfacial polymerization separation layer is 50 nm, and the separation pore size is 3 nm.

[0093] (4) Soak the membrane obtained in step (3) in pure water at 20°C, change the water every hour, and soak for 24 hours; after cleaning, put it in an aqueous solution of glycerol for moisturizing, and finally dry it at 35°C for 48 hours to obtain a new type of flat sheet nanofiltration membrane with the ability to remove heavy metals and micro-pollutants.

[0094] (5) A flat nanofiltration membrane with a diameter of 53 mm was placed in the test fixture and its heavy metal adsorption performance was tested. A 25 ppb cadmium nitrate (Cd(NO3)2) solution was used as the spiking solution. When the concentration of Cd(NO3)2 in the solution that permeates through the membrane is greater than 5 ppb, the test is stopped. The final volume of the spiking solution passing through the membrane is 7.1 L. The above test shows that when the cutoff concentration of 5 ppb is reached, the volume of the spiking solution passing through the membrane can reach 7.1 L.

[0095] (6) A 53 mm diameter flat-sheet nanofiltration membrane was placed in the test fixture to test its micropollutant adsorption performance. A 50 ppb bisphenol A (BPA) solution was used as the spiking solution. The test was stopped when the BPA concentration in the solution permeating the membrane exceeded 10 ppb. The final volume of the spiking solution passing through the membrane was 8.8 L. The above tests show that when the cutoff concentration of 10 ppb is reached, the volume of the spiking solution passing through the membrane can reach 8.8 L.

[0096] Example 3

[0097] (1) β-Cyclodextrin and ethylenediaminetetraacetic acid were added to an aqueous solution. The concentration of β-cyclodextrin was 1 wt%, the concentration of tetraethylamine oxalate was 0.006 wt%, the molar ratio of β-cyclodextrin to ethylenediaminetetraacetic acid was 42:1, the reaction temperature was 40℃, and the reaction time was 7.5 h. Then chitosan was added at a concentration of 2 wt%, and the reaction was continued for 3 h.

[0098] (2) The flat ultrafiltration membrane containing a polyester support layer with polysulfone as the main material and a thickness of 200 μm and polybutylene terephthalate as the material and a thickness of 100 μm is placed in the solution of step (1), the temperature is adjusted to 90℃, and the reaction is carried out for 6 hours to form a heavy metal and micro-pollutant removal layer with a thickness of 100 nm at the bottom of the polyester support layer.

[0099] (3) The reacted flat sheet ultrafiltration membrane is immersed in 50°C pure water, the water is changed every hour, and it is soaked for 8 hours. After taking it out, it is dried at 35°C for 50 hours, and then an interfacial polymerization reaction is carried out in the ultrafiltration layer to form a separation layer.

[0100] The interfacial polymerization reaction includes the following steps:

[0101] a. Place the flat sheet ultrafiltration membrane in an acrylic frame, clamp it in place, and stand it upright in a fume hood. Wait for the moisture on the membrane surface to evaporate in about 10 minutes.

[0102] b. Pour about 30 mL of the prepared aqueous solution of piperazine into the acrylic glass frame, trying to pour it from the corner of the frame, and let it stand for 5 minutes;

[0103] c. Pour the aqueous phase into the waste liquid container, then stand the membrane upright and wait for the aqueous phase to evaporate completely, about 3 minutes.

[0104] d. Pour about 15 mL of the prepared oil phase, which is a hexane solution of trimesoyl chloride, into an acrylic glass frame. It must be poured from the corner of the acrylic glass frame. React for 40 seconds, then pour the oil phase into the waste liquid container.

[0105] e. Vigorously shake the film to make the oil phase evaporate quickly, and then place it in an oven at 80°C for 20 seconds.

[0106] The thickness of the obtained interfacial polymerization separation layer is 100 nm, and the separation pore size is 1 nm.

[0107] (4) Soak the membrane obtained in step (3) in pure water at 50°C, change the water every hour, and soak for 16 hours; after cleaning, put it in an aqueous solution of glycerol for moisturizing, and finally dry it at 20°C for 70 hours to obtain a new type of flat sheet nanofiltration membrane with the ability to remove heavy metals and micro-pollutants.

[0108] (5) A flat nanofiltration membrane with a diameter of 53 mm was placed in the test fixture and its heavy metal adsorption performance was tested. A 25 ppb cadmium nitrate (Cd(NO3)2) solution was used as the spiking solution. When the concentration of Cd(NO3)2 in the solution that permeates through the membrane is greater than 5 ppb, the test is stopped. The final volume of the spiking solution passing through the membrane is 2.9 L. The above test shows that when the cutoff concentration of 5 ppb is reached, the volume of the spiking solution passing through the membrane can reach 2.9 L.

[0109] (6) A 53 mm diameter flat-sheet nanofiltration membrane was placed in the test fixture to test its micropollutant adsorption performance. A 50 ppb bisphenol A (BPA) solution was used as the spiking solution. The test was stopped when the BPA concentration in the solution permeating the membrane exceeded 10 ppb. The final volume of the spiking solution passing through the membrane was 3.5 L. The above test results show that when the cutoff concentration of 10 ppb is reached, the volume of the spiking solution passing through the membrane can reach 3.5 L.

[0110] Example 4

[0111] (1) β-Cyclodextrin and oxalic acid were added to an aqueous solution. The concentration of β-cyclodextrin was 2 wt%, the concentration of oxalic acid was 0.004 wt%, the molar ratio of β-cyclodextrin to oxalic acid was 42:1, the reaction temperature was 50℃, and the reaction time was 6 h. Then, polyacetylimide was added at a concentration of 4 wt%, and the reaction was continued for 1 h.

[0112] (2) The flat ultrafiltration membrane containing a base membrane with polyethersulfone as the main material and a thickness of 100 μm and a polyester support layer with polyethylene terephthalate as the material and a thickness of 10 μm is placed in the solution of step (1), the temperature is adjusted to 80℃, and the reaction is carried out for 8 hours to form a heavy metal and micro-pollutant removal layer with a thickness of 300 nm at the bottom of the polyester support layer.

[0113] (3) The reacted flat ultrafiltration membrane was soaked in pure water at 20°C, the water was changed every hour, and the membrane was soaked for 20 hours. After taking it out, it was dried at 25°C for 70 hours. Then, an interfacial polymerization reaction was carried out in the ultrafiltration layer to form a separation layer.

[0114] The interfacial polymerization reaction includes the following steps:

[0115] a. Place the flat sheet ultrafiltration membrane in an acrylic frame, clamp it in place, and stand it upright in a fume hood. Wait for the moisture on the membrane surface to evaporate in about 10 minutes.

[0116] b. Pour about 30 mL of the prepared aqueous solution of piperazine into the acrylic glass frame, trying to pour it from the corner of the frame, and let it stand for 5 minutes;

[0117] c. Pour the aqueous phase into the waste liquid container, then stand the membrane upright and wait for the aqueous phase to evaporate completely, about 3 minutes.

[0118] d. Pour about 15 mL of the prepared oil phase, which is a hexane solution of trimesoyl chloride, into an acrylic glass frame. It must be poured from the corner of the acrylic glass frame. React for 30 seconds, then pour the oil phase into the waste liquid container.

[0119] e. Vigorously shake the film to make the oil phase evaporate quickly, and then place it in an oven at 80°C for 20 seconds.

[0120] The thickness of the obtained interfacial polymerization separation layer is 100 nm, and the separation pore size is 2 nm.

[0121] (4) Soak the membrane obtained in step (3) in pure water at 30°C, change the water every hour, and soak for 15 hours; after cleaning, put it in an aqueous solution of sodium lactate for moisturizing, and finally dry it at 25°C for 60 hours to obtain a new type of flat sheet nanofiltration membrane with the ability to remove heavy metals and micro pollutants.

[0122] (5) Take a flat nanofiltration membrane with a diameter of 53 mm and put it into the test fixture to test its heavy metal adsorption performance. Use a 25 ppb cadmium nitrate (Cd(NO3)2) solution as the spiking solution. When the Cd(NO3)2 concentration in the solution that permeates through the membrane is greater than 5 ppb, the test is stopped. The final volume of the spiking solution passing through the membrane is 3.5 L. The above test shows that when the cutoff concentration of 5 ppb is reached, the volume of the spiking solution passing through the membrane can reach 3.5 L.

[0123] (6) A 53 mm diameter flat-sheet nanofiltration membrane was placed in the test fixture to test its micropollutant adsorption performance. A 50 ppb bisphenol A (BPA) solution was used as the spiking solution. The test was stopped when the BPA concentration in the solution permeating the membrane exceeded 10 ppb. The final volume of the spiking solution passing through the membrane was 4.1 L. The above test shows that when the cutoff concentration of 10 ppb is reached, the volume of the spiking solution passing through the membrane can reach 4.1 L.

[0124] Comparative Example 1

[0125] (1) An interfacial polymerization reaction occurs on the surface of the flat sheet ultrafiltration membrane to form a separation layer;

[0126] The interfacial polymerization reaction includes the following steps:

[0127] a. Place the flat sheet ultrafiltration membrane in an acrylic frame, clamp it in place, and stand it upright in a fume hood. Wait for the moisture on the membrane surface to evaporate in about 10 minutes.

[0128] b. Pour about 30 mL of the prepared aqueous solution of piperazine into the acrylic glass frame, trying to pour it from the corner of the frame, and let it stand for 5 minutes;

[0129] c. Pour the aqueous phase into the waste liquid container, then stand the membrane upright and wait for the aqueous phase to evaporate completely, about 3 minutes.

[0130] d. Pour about 15 mL of the prepared oil phase, which is a hexane solution of trimesoyl chloride, into an acrylic glass frame. It must be poured from the corner of the acrylic glass frame. React for 30 seconds, then pour the oil phase into the waste liquid container.

[0131] e. Vigorously shake the film to make the oil phase evaporate quickly, and then place it in an oven at 80°C for 20 seconds.

[0132] The obtained interfacial polymerization separation layer has a thickness of 100 nm and a separation pore size of 2 nm.

[0133] (2) The membrane obtained in step (1) is soaked in pure water at 30°C, the water is changed every hour, and the soaking time is 15 hours. After cleaning, it is moisturized in an aqueous solution of sodium lactate and finally dried at 25°C for 60 hours to obtain a new type of flat sheet nanofiltration membrane with partial removal performance but a removal rate of less than 80%.

[0134] (3) Take a flat nanofiltration membrane with a diameter of 53 mm and put it into the test fixture to test its heavy metal adsorption performance. Use a 25 ppb cadmium nitrate (Cd(NO3)2) solution as the spiking solution. When the Cd(NO3)2 concentration in the solution that permeates through the membrane is greater than 5 ppb, the test is stopped. Finally, the amount of water passing through the membrane in the spiking solution is 0 L. The Cd(NO3)2 concentration in the solution that permeates through the membrane is always maintained at 8 ppb.

[0135] (4) Take a flat nanofiltration membrane with a diameter of 53 mm and put it into the test fixture to test its micro-pollutant adsorption performance. Use a 50 ppb bisphenol A (BPA) solution as the spiking solution. When the BPA concentration in the solution that permeates through the membrane is greater than 10 ppb, the test is stopped. Finally, the amount of water in the spiking solution that permeates through the membrane is 0 L. The BPA concentration in the solution that permeates through the membrane is always maintained at 23 ppb.

Claims

1. A method for preparing a flat-sheet nanofiltration membrane, characterized in that, Includes the following steps: (1) Preparation of a heavy metal and micro-pollutant removal layer containing amino groups: β-cyclodextrin is added to an aqueous solution and reacted with ethylenediaminetetraacetic acid and / or oxalic acid, followed by the addition of amines and the reaction is continued; wherein the concentration of β-cyclodextrin in the aqueous solution is 1-10 wt%; and the concentration of the amines is 1-10 wt%. (2) Immerse the flat sheet ultrafiltration membrane containing the base membrane and the polyester support layer into the solution of the heavy metal and micro-pollutant removal layer containing amino groups, react, and then perform interfacial polymerization on the flat sheet nanofiltration membrane to form a separation layer on the base membrane; in the flat sheet ultrafiltration membrane, one outer surface of the polyester support layer is closely attached to the base membrane; the material of the polyester support layer is polyethylene terephthalate and / or polybutylene terephthalate.

2. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (2): the reaction temperature is 60-90℃.

3. The method for preparing a flat-sheet nanofiltration membrane as described in claim 2, characterized in that, In step (2): the reaction temperature is 60℃, 80℃ or 90℃.

4. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (2): the reaction time is 5-15 hours.

5. The method for preparing a flat-sheet nanofiltration membrane as described in claim 4, characterized in that, In step (2): the reaction time is 6h, 8h or 15h.

6. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (1): the concentration of β-cyclodextrin in the aqueous solution is 1 wt%, 2 wt%, or 8 wt%.

7. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (1): the concentration of ethylenediaminetetraacetic acid in the aqueous solution is 0.006wt%-0.06wt%.

8. The method for preparing a flat-sheet nanofiltration membrane as described in claim 7, characterized in that, In step (1): the concentration of ethylenediaminetetraacetic acid in the aqueous solution is 0.012wt%, 0.049wt%, or 0.006wt%.

9. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (1): the concentration of oxalic acid in the aqueous solution is 0.004 wt%.

10. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (1): the molar ratio of β-cyclodextrin to ethylenediaminetetraacetic acid is 42:

1.

11. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (1): the molar ratio of β-cyclodextrin to oxalic acid is 42:

1.

12. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (1): the temperature of the reaction is 40-80℃.

13. The method for preparing a flat-sheet nanofiltration membrane as described in claim 12, characterized in that, In step (1): the reaction temperature is 40℃, 50℃ or 80℃.

14. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (1): the reaction time is 1-8 hours.

15. The method for preparing a flat-sheet nanofiltration membrane as described in claim 14, characterized in that, In step (1): the reaction time is 1h, 6h or 7.5h.

16. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (1): the amine substance is one or more of chitosan, polyethyleneimine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine and pentaethylenehexamine.

17. The method for preparing a flat-sheet nanofiltration membrane as described in claim 16, characterized in that, In step (1): the amine substance is polyethyleneimine or chitosan.

18. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (1): the concentration of the amine substance is 2wt%, 4wt% or 6wt%.

19. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (1): the duration of the continued reaction is 0.5-3 hours.

20. The method for preparing a flat-sheet nanofiltration membrane as described in claim 19, characterized in that, In step (1): the duration of the continued reaction is 0.5h, 1h or 3h.

21. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (2), after the reaction, the flat nanofiltration membrane is washed and dried.

22. The method for preparing a flat-sheet nanofiltration membrane as described in claim 21, characterized in that, The cleaning process involves immersing the flat nanofiltration membrane in pure water.

23. The method for preparing a flat-sheet nanofiltration membrane as described in claim 22, characterized in that, The soaking time is 5-24 hours.

24. The method for preparing a flat-sheet nanofiltration membrane as described in claim 23, characterized in that, The soaking time is 8 hours, 10 hours, or 20 hours.

25. The method for preparing a flat-sheet nanofiltration membrane as described in claim 22, characterized in that, The soaking temperature is 20-60℃.

26. The method for preparing a flat-sheet nanofiltration membrane as described in claim 25, characterized in that, The soaking temperature is 20°C, 40°C, or 50°C.

27. The method for preparing a flat-sheet nanofiltration membrane as described in claim 21, characterized in that, The drying temperature is 20-35℃.

28. The method for preparing a flat-sheet nanofiltration membrane as described in claim 27, characterized in that, The drying temperature is 25°C, 30°C, or 35°C.

29. The method for preparing a flat-sheet nanofiltration membrane as described in claim 21, characterized in that, The drying time is 48-72 hours.

30. The method for preparing a flat-sheet nanofiltration membrane as described in claim 29, characterized in that, The drying time is 50h, 65h, or 70h.

31. The method for preparing a flat-sheet nanofiltration membrane as described in claim 1, characterized in that, In step (2), the separation layer is formed by the reaction of piperazine and pyromellitic acid chloride, and the process also includes cleaning, moisturizing and drying the membrane sheet that forms the separation layer to obtain a flat nanofiltration membrane. The cleaning process involves immersing the membrane sheet that forms the separation layer in pure water.

32. The method for preparing a flat-sheet nanofiltration membrane as described in claim 31, characterized in that, The soaking time is 5-24 hours.

33. The method for preparing a flat-sheet nanofiltration membrane as described in claim 32, characterized in that, The soaking time is 15h, 16h, or 24h.

34. The method for preparing a flat-sheet nanofiltration membrane as described in claim 31, characterized in that, The soaking temperature is 20-60℃.

35. The method for preparing a flat-sheet nanofiltration membrane as described in claim 34, characterized in that, The soaking temperature is 20°C, 30°C, or 50°C.

36. The method for preparing a flat-sheet nanofiltration membrane as described in claim 31, characterized in that, In the moisturizing process, the moisturizer is one or more of glycerin, sodium lactate, potassium lactate, and sodium citrate.

37. The method for preparing a flat-sheet nanofiltration membrane as described in claim 36, characterized in that, In the moisturizing process, the moisturizer is sodium lactate or glycerin.

38. The method for preparing a flat-sheet nanofiltration membrane as described in claim 31, characterized in that, The drying temperature is 20-35℃.

39. The method for preparing a flat-sheet nanofiltration membrane as described in claim 38, characterized in that, The drying temperature is 20°C, 25°C, or 35°C.

40. The method for preparing a flat-sheet nanofiltration membrane as described in claim 31, characterized in that, The drying time is 48-72 hours.

41. The method for preparing a flat-sheet nanofiltration membrane as described in claim 40, characterized in that, The drying time is 48h, 60h or 70h.

42. A flat sheet nanofiltration membrane prepared by any one of claims 1-41.

43. The flat-sheet nanofiltration membrane as described in claim 42, characterized in that, The flat-sheet nanofiltration membrane includes a separation layer, a flat-sheet ultrafiltration membrane, and a heavy metal and micro-pollutant removal layer. The flat-sheet ultrafiltration membrane includes a base membrane and a polyester support layer. One outer surface of the polyester support layer is in close contact with the base membrane, and the other outer surface is connected to the heavy metal and micro-pollutant removal layer. The polyester support layer and the heavy metal and micro-pollutant removal layer are connected by amide bonds. The separation layer is located on the outermost side of the flat-sheet nanofiltration membrane and is in close contact with the base membrane.

44. The flat-sheet nanofiltration membrane as described in claim 43, characterized in that, The thickness of the separation layer is 10~200nm.

45. The flat-sheet nanofiltration membrane as described in claim 44, characterized in that, The thickness of the separation layer is 50 nm or 100 nm.

46. ​​The flat-sheet nanofiltration membrane as described in claim 43, characterized in that, The separation pore size of the separation layer is 0.5~5nm.

47. The flat-sheet nanofiltration membrane as described in claim 46, characterized in that, The separation pore size of the separation layer is 1 nm, 2 nm or 3 nm.

48. The flat-sheet nanofiltration membrane as described in claim 43, characterized in that, The material of the separation layer is polyamide.

49. The flat-sheet nanofiltration membrane as described in claim 43, characterized in that, The base film is made of one or more of the following materials: polysulfone, polyethersulfone, polypropylene, and polyvinyl chloride.

50. The flat-sheet nanofiltration membrane as described in claim 43, characterized in that, The thickness of the base film is 50~200μm.

51. The flat-sheet nanofiltration membrane as described in claim 50, characterized in that, The thickness of the base film is 50 μm, 100 μm or 200 μm.

52. The flat-sheet nanofiltration membrane as described in claim 43, characterized in that, The thickness of the polyester support layer is 10~100μm.

53. The flat-sheet nanofiltration membrane as described in claim 52, characterized in that, The thickness of the polyester support layer is 10μm, 50μm or 100μm.

54. The flat-sheet nanofiltration membrane as described in claim 43, characterized in that, The thickness of the heavy metal and micro-pollutant removal layer is 50~1000 nm.

55. The flat-sheet nanofiltration membrane as described in claim 54, characterized in that, The thickness of the heavy metal and micro-pollutant removal layer is 100nm, 200nm, 300nm or 800nm.