Preparation method of asymmetric hydrogel modified membrane and application of asymmetric hydrogel modified membrane in removal of ammonia nitrogen in landfill leachate

Abstract

The invention discloses a preparation method of an asymmetric hydrogel modified membrane and application of the asymmetric hydrogel modified membrane to removal of ammonia nitrogen in landfill leachate, and the preparation method comprises the following steps: soaking a cut polytetrafluoroethylene flat sheet membrane in absolute ethyl alcohol, washing with distilled water, and drying to prepare a flat sheet membrane assembly; preparing bionic biological glue, vertically immersing the dried flat sheet membrane assembly into the bionic biological glue, avoiding bubble attachment, and ensuring complete immersion; preparing hydrogel, cooling the hydrogel, uniformly covering the hydrogel on a flat sheet membrane assembly, and depositing to obtain the asymmetric hydrogel modified membrane. When the asymmetric hydrogel modified membrane is applied to removal of ammonia nitrogen in landfill leachate, the landfill leachate is prevented from being in direct contact with a polytetrafluoroethylene substrate due to the anti-wetting and anti-pollution properties of the asymmetric hydrogel modified membrane, and compared with an original polytetrafluoroethylene membrane, the asymmetric hydrogel modified membrane has the advantages that the cost is reduced; the service life of the asymmetric hydrogel modified membrane is prolonged when the asymmetric hydrogel modified membrane is used for treating ammonia nitrogen of landfill leachate, the treatment capacity of the ammonia nitrogen is improved, and the removal rate of the ammonia nitrogen reaches 90% or above within 4 h.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-concentration ammonia-nitrogen wastewater treatment. More specifically, the present invention relates to a preparation method of an asymmetric hydrogel modified membrane and its application in removing ammonia nitrogen from landfill leachate. Background Art

[0002] According to relevant data, the annual cleaning volume of urban domestic waste in China has increased from 228.018 million tons in 2018 to 263.506 million tons in 2023, with an average annual compound growth rate of approximately 3%. The gradual increase in the generation of urban waste will inevitably lead to an increase in the generation of complex landfill leachate with high ammonia-nitrogen concentration, a large amount of organic pollutants and toxic substances. In 2021, the output of landfill leachate has reached 113.63 million tons, while the treatment volume is only 50.085 million tons, less than 50% of the output. Landfill leachate is a typical high-ammonia-nitrogen wastewater. High ammonia-nitrogen concentration and poor biodegradability have always been the key points and difficulties in the landfill leachate treatment process. The high concentration of NH3-N in the leachate seriously lacks carbon sources in the biological denitrification process. The high concentration of NH3-N will reduce the activity of dehydrogenase, inhibit the activity of organisms, and is not conducive to the progress of biochemical reactions. Therefore, it is necessary to carry out deammoniation treatment on landfill leachate to reduce the load of subsequent biochemical unit treatment.

[0003] At present, the traditional methods for deammoniation of landfill leachate mainly include biological methods, stripping methods, chemical precipitation methods, and membrane contactor methods, etc. The biological method has stable treatment effect and is relatively mature, but it has disadvantages such as high infrastructure investment, large floor area, and weak shock resistance. The stripping method and chemical precipitation method have the characteristics of simple process and high treatment efficiency, but they have high treatment energy consumption, are prone to secondary pollution, have high treatment costs, and are difficult to recycle.

[0004] The membrane contactor method (also known as membrane absorption method, gas membrane method) works on the principle of separating the ammonia-containing wastewater and the absorbent (sulfuric acid, nitric acid, phosphoric acid, etc.) with a microporous hydrophobic membrane, and the two feed liquids flow in parallel or cross-flow. Usually, the pH value of the feed liquid is adjusted to alkaline, the content of NH3 in the solution increases, and the partial pressure difference of the volatile component on both sides of the membrane is used as the driving force to make the ammonia molecules pass through the membrane pores and enter the absorbent side, where they are quickly reacted and absorbed by the acid. Since the driving force of the membrane contactor process is the concentration gradient of the separated component on both sides of the membrane, the membrane only plays the role of providing a large gas-liquid two-phase contact area. Therefore, in the membrane contactor process, generally only mass transfer needs to be considered and heat transfer does not need to be considered. This method can efficiently remove ammonia nitrogen at normal temperature and pressure, and has the advantages of scalability, low energy consumption, high mass transfer efficiency, modular design of equipment, different ammonium salts can be generated according to different absorbents for the removed ammonia nitrogen, and no secondary pollution.

[0005] As a kind of wastewater with complex components, landfill leachate contains various organic matters, ammonia nitrogen, heavy metals, chloride inorganic salts and co-existing substances. During the process of ammonia removal by membrane absorption using traditional hydrophobic membranes, problems such as membrane wetting and membrane fouling will be faced, resulting in a decrease in membrane flux and thus a reduction in the ammonia nitrogen removal rate. Asymmetric hydrogel modified membranes have gradually become a research hotspot due to their better anti-fouling and anti-wetting properties. However, the additional layer brought by the modified composite membrane will also increase the mass transfer resistance, and there is a lack of evaluation of the application effect in treating ammonia nitrogen in actual landfill leachate. Traditional single-component hydrogel groups are prone to disadvantages such as poor pollution interception. Dopamine can adhere to the substrate in an aqueous solution environment and form a polydopamine layer, but it is usually thin, and its anti-fouling performance is not ideal when used in membrane contactor devices. Therefore, developing new flat membrane materials with pollution resistance and anti-wetting properties is a difficult problem to be solved in the treatment of ammonia nitrogen in high-concentration landfill leachate by membrane contactors. In this paper, for the first time, the practical application of a membrane contactor device with a hydrogel modified flat membrane as the core in treating ammonia nitrogen in landfill leachate is proposed, providing a new solution for removing high-concentration ammonia nitrogen in landfill leachate. Summary of the Invention

[0006] An object of the present invention is to solve at least the above problems and / or defects and provide at least the advantages described hereinafter.

[0007] To achieve these and other advantages in accordance with the present invention, a preparation method of an asymmetric hydrogel modified membrane is provided, including the following steps: Step 1: Cut the polytetrafluoroethylene flat membrane to the corresponding size, soak the cut polytetrafluoroethylene flat membrane in absolute ethanol, rinse it with distilled water and then dry it to prepare a flat membrane module; Step 2: Prepare a biomimetic bioadhesive, vertically immerse the dried flat membrane module in the biomimetic bioadhesive and avoid the attachment of air bubbles to ensure complete immersion; Step 3: Prepare a hydrogel, and evenly cover it on the polytetrafluoroethylene flat membrane after cooling. After deposition, an asymmetric hydrogel modified membrane is obtained.

[0008] Preferably, the soaking time of the polytetrafluoroethylene flat membrane in absolute ethanol is 20 min; the specific method for preparing the flat membrane module is: use the polytetrafluoroethylene flat membrane soaked in absolute ethanol and polytetrafluoroethylene flat membrane blocks to make a flat membrane module. Cut the polytetrafluoroethylene flat membrane into membrane pieces of 8.3×4.3 cm, fix the polytetrafluoroethylene flat membrane between two polytetrafluoroethylene flat membrane blocks, divide the polytetrafluoroethylene flat membrane blocks into upper and lower sides, and stick PTFE rubber strips inside the perimeter of the flat membrane module to prevent the mixing of solutions on both sides of the polytetrafluoroethylene flat membrane and ensure good sealing performance of the flat membrane module; the hydrophobic membrane surface of the polytetrafluoroethylene flat membrane faces downwards, and it is fixed with gaskets and screws to ensure the independent flow of solutions on both sides and then it can be used normally.

[0009] Preferably, in the second step, the method for preparing the bionic bioadhesive is as follows: dissolve dopamine hydrochloride, polyethyleneimine, and sodium alginate in a Tris-HCl buffer solution at 25 °C, and then stir evenly at 250 rpm for 30 min.

[0010] Preferably, in the bionic bioadhesive, the mass ratio of dopamine hydrochloride, polyethyleneimine, and sodium alginate is 1:1~1.5:0.5~1. The concentration of sodium alginate in the Tris-HCl buffer solution is 2 mg / mL, the concentration of the Tris-HCl buffer solution is 50 mmol / L, and the pH value is 8.5.

[0011] Preferably, in the second step, vertically immerse the dried flat membrane module into the evenly stirred bionic bioadhesive, ensuring that the liquid always completely covers the entire flat membrane module. After direct contact for 12~24 h, air-dry for 6~12 h.

[0012] Preferably, in the third step, the method for preparing the hydrogel is as follows: mix tannic acid and polyvinyl alcohol in a mass ratio of 1:5~8 and stir evenly. The stirring speed is 250~350 rpm, the stirring temperature is 85~90 °C, and the stirring duration is 5~8 h.

[0013] Preferably, in the third step, the deposition method is as follows: after the prepared hydrogel cools down, evenly cover the hydrogel on the polytetrafluoroethylene flat membrane of the flat membrane module obtained in the second step, then soak for 5~10 min, put it into the freezer for 2 h, at room temperature for 6~12 h, repeat the freeze-thaw cycle 2~3 times, and then cover the flat membrane module with pure water for 30 min to fully swell the outer layer. After one side of the polytetrafluoroethylene flat membrane is modified with the bionic bioadhesive and the hydrogel is deposited, then complete the assembly and fixation of the polytetrafluoroethylene flat membrane with the polytetrafluoroethylene flat membrane block on the other side.

[0014] Application of an asymmetric hydrogel-modified membrane in removing ammonia nitrogen from landfill leachate. The core of the flat membrane module is an asymmetric hydrogel-modified membrane, and ammonia nitrogen in the landfill leachate is removed through the asymmetric hydrogel membrane.

[0015] Preferably, on the feed side of the asymmetric hydrogel-modified membrane flat membrane module, connect an intelligent peristaltic pump and a raw liquid tank, and on the absorption liquid side of the flat membrane module, connect another intelligent peristaltic pump and an absorption liquid tank. The raw liquid tank is filled with high-ammonia-nitrogen landfill leachate, and the high-ammonia-nitrogen landfill leachate flows through the hydrophobic surface side of the asymmetric hydrogel-modified membrane, and the absorption liquid flows through the other side of the asymmetric hydrogel-modified membrane.

[0016] Preferably, the pH value of the high-ammonia-nitrogen landfill leachate is 9.5, the flow rate of the high-ammonia-nitrogen landfill leachate is 600 mL / min, the absorbent is a 4.9% sulfuric acid solution, the flow rate of the absorbent is 500 mL / min, and the temperature on both sides of the asymmetric hydrogel modified membrane is 25 ± 2 °C at room temperature, and it can be carried out under normal pressure; ammonia nitrogen is enriched in the form of ammonium sulfate and is used in industry for the production of flame retardants, fermentation media or battery electrolytes through reverse osmosis and vacuum evaporation.

[0017] The present invention has at least the following beneficial effects: (1) The method provided by the present invention can effectively remove high-concentration ammonia nitrogen in landfill leachate, reduce the load of subsequent biochemical units, and improve the effect of biochemical reactions. The membrane contactor device with the hydrogel modified membrane as the core realizes the long-term stable operation of removing high ammonia nitrogen in landfill leachate, solves the problems of short service life of membrane materials caused by the rapid deposition of organic and inorganic pollutants on the membrane surface during the ammonia removal process by membrane contactors, and the inability to operate on a large scale for a long time. At the same time, this method can obtain different by-products according to needs, offsetting the operating cost to a certain extent, and having good application prospects.

[0018] (2) In the present invention, a bionic bioadhesive and a hydrogel are successively used to modify one side of a polytetrafluoroethylene flat membrane to obtain a PDA / PEI / SA intermediate layer and a hydrogel layer in sequence, thereby obtaining an asymmetric hydrogel modified membrane. Through the bifunctional layer modified by the hydrogel in the present invention, the liquid inlet capillary pressure at the membrane-liquid interface is greatly increased, and organic and inorganic pollutants in the landfill leachate need to penetrate more membrane pores, improving the anti-wetting and anti-pollution capabilities of the membrane. Through the membrane contactor technology of the asymmetric hydrogel modified membrane material, the ammonia nitrogen removal rate reaches more than 90% within 4 hours, and after 13 cycles, the membrane flux is still 94.8% and 96.67% of the initial value, almost the same as the original membrane flux, verifying the feasibility of this method.

[0019] Other advantages, objectives and features of the present invention will be partially reflected by the following description, and partially will also be understood by those skilled in the art through the research and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is the structural formula of the materials prepared in each step of the present invention; Figure 2 It is the schematic diagram of the membrane contactor device in the application example of the present invention; Figure 3 It is the SEM image of the original polytetrafluoroethylene (PTFE) membrane in Example 1 of the present invention; Figure 4 It is the SEM image of the finally prepared asymmetric hydrogel modified membrane in Example 1 of the present invention; Figure 5Data graph of ammonia nitrogen in landfill leachate treated by unmodified polytetrafluoroethylene membrane, which is Comparative Example 1 of the present invention; Figure 6 Data graph of ammonia nitrogen in landfill leachate treated by the asymmetric hydrogel modified membrane of Example 2 of the present invention; Figure 7 Data graph of high ammonia nitrogen concentration in landfill leachate treated by the asymmetric hydrogel modified membrane of Example 3 of the present invention. Detailed implementation manners

[0021] The present invention will be further described in detail below with reference to the accompanying drawings, so that those skilled in the art can implement it according to the text of the specification.

[0022] It should be understood that terms such as "having", "comprising" and "including" used herein do not exclude the presence or addition of one or more other elements or their combinations.

[0023] Example 1 A preparation method of an asymmetric hydrogel modified membrane, comprising the following steps: Step 1: Cut a polytetrafluoroethylene (PTFE) flat membrane of 8.3×4.3 cm and soak it in absolute ethanol for 20 min, then rinse it with distilled water and dry it. Use the PTFE membrane block and the PTFE flat membrane to make a flat membrane module, and stick PTFE rubber strips inside the four sides of the flat membrane module to prevent the mixing of the solutions on both sides and ensure good sealing performance. The hydrophobic membrane surface of the PTFE flat membrane faces downwards, and it is fixed with gaskets and screws, and left overnight.

[0024] Step 2: Measure 20 mL of Tris-HCl buffer solution (50 mmol / L, pH 8.5), 40 mg of hydrochloric acid dopamine (PDA), 40 mL of polyethyleneimine (PEI), and 20 mg of sodium alginate (SA). Stir magnetically at 250 r / min at 25 °C for 30 min to obtain a biomimetic bioadhesive (PDA / PEI / SA) reagent, and immerse the flat membrane module vertically into the evenly stirred biomimetic bioadhesive, ensuring that the liquid always completely covers the entire flat membrane module. After direct contact for 24 h, it is naturally dried for 12 h. After drying, a relatively thin PDA / PEI / SA intermediate layer is formed on the surface of the PTFE flat membrane, and the PTFE flat membrane still maintains its fibrous structure.

[0025] Step 3: Weigh 2 g of polyvinyl alcohol and 0.6 g of tannic acid, put them into a magnetic heating stirrer, with a stirring speed of 300 r / min, a stirring temperature of 90 °C, and a stirring duration of 6 h to obtain a polyvinyl alcohol and tannic acid hydrogel. Then, completely and evenly cover the polytetrafluoroethylene flat membrane with the hydrogel, soak it for 10 min, put it into the freezer for 2 h, store it at room temperature for 12 h, repeat the freeze-thaw process 2 times, and then cover the polytetrafluoroethylene flat membrane with pure water for 30 min. After the outermost layer is fully swollen, a hydrogel modified membrane is obtained and stored in pure water.

[0026] The structural formulas of the materials used in each step of this example are as Figure 1 shown. The membrane contactor device in this example is as Figure 2 shown. The SEM image of the polytetrafluoroethylene flat membrane obtained in Step 1 is as Figure 3 shown. The original PTFE flat membrane shows a synergistic construction of a fibrous crack topology and a spherulite cluster structure. The microcrack network formed during the fibrillation process exhibits topological connectivity in the range of 10 - 50 μm, and the inter-fiber pore network effectively constitutes the transmembrane mass transfer path for water vapor molecules. The SEM image of the hydrogel modified membrane obtained in Step 3 is as Figure 4 shown. The grains at the top of the membrane surface disappear, and the membrane surface shows a flat, smooth, complete and dense functional layer.

[0027] Example 2 Build a membrane contactor device as Figure 2 shown. The feed side of the flat membrane module equipped with an asymmetric hydrogel modified membrane (prepared in Example 1) is connected to an intelligent peristaltic pump and a raw material liquid tank, and the absorption liquid side of the flat membrane module is connected to another intelligent peristaltic pump and an absorption liquid tank. The raw material liquid tank is filled with high ammonia-nitrogen landfill leachate. The high ammonia-nitrogen landfill leachate flows through the hydrophobic surface side of the asymmetric hydrogel modified membrane, and the absorption liquid flows through the other side of the asymmetric hydrogel modified membrane.

[0028] The temperatures on both sides of the polytetrafluoroethylene flat membrane are both at room temperature (29 °C). An ammonia-nitrogen removal experiment is carried out using a flat membrane contactor device with a hydrogel modified membrane as the core to treat the ammonia-nitrogen in the landfill leachate.

[0029] The results show that as Figure 6 shown, after each 4 h of operation, the ammonia-nitrogen concentration can be reduced within 150 mg / L. After the first batch of cycles ends, the ammonia removal rate is greater than 95%, and the ammonia transmembrane flux is 61.64 (g / m 2·h). After the two batches of experiments within 24 h, the ammonia transmembrane flux was still 97% and 95.8% of the initial transmembrane flux, and the removal rate only decreased by 0.75% and 2.04%. Compared with the conditions of the unmodified membrane during the same period, the removal rate and ammonia transmembrane flux were greatly improved and almost maintained the initial effect. After the 13th batch of the experiment, the ammonia removal rate only decreased by 4.5% compared with the first batch, and the flux slightly decayed but was still 94.8% of the initial flux. It could still reduce the ammonia nitrogen to within 150 mg / L, indicating that the hydrogel-modified membrane greatly improved the anti-pollution and anti-wetting properties, and the ammonia removal effect remained stable during long-term operation, almost maintaining the initial level. Example 3 Keep the remaining conditions the same as in Example 2, adjust the pH value of the raw material liquid to 10.5, and use a flat membrane contactor device with a hydrogel-modified membrane as the core to remove the higher ammonia nitrogen concentration in the landfill leachate.

[0030] The results show that, as Figure 7 shown, the device can reduce the ammonia nitrogen concentration to within 150 mg / L every time it runs for 5 h. After the first batch of cycles ended, the ammonia removal rate was 97%, and the ammonia transmembrane flux was 64.72 g / m 2 ·h. Similarly, consistent with Example 2 above, after the device ran for 13 batches, the ammonia nitrogen removal rate and ammonia transmembrane flux hardly decayed, and the ammonia nitrogen removal rate was greater than 90%. The ammonia nitrogen removal rate of the 13th batch only decreased by 2.29% compared with the first batch, and the ammonia transmembrane flux was 95.67% of the first batch, indicating that this modification method is still effective at a higher ammonia nitrogen concentration, and the modified membrane has strong anti-pollution performance.

[0031] Comparative Example 1 At a raw material liquid pH value of 9.5, a feed liquid flow rate of 600 mL / min, an absorbent liquid of 4.9% sulfuric acid solution, an absorbent liquid flow rate of 500 mL / min, and the temperature on both sides of the PTFE flat membrane at room temperature (25°C), a flat membrane contactor device with a PTFE hydrophobic membrane that was neither immersed in the bionic bioadhesive nor covered with a hydrogel as the core was used to remove ammonia nitrogen from the landfill leachate.

[0032] The results show that, as Figure 5As shown, when using an unmodified polytetrafluoroethylene membrane, after the first four batches of cycles were completed, the ammonia removal rate decreased by only 4.42% compared to the first batch. The ammonia transmembrane flux was still 86.31%, 85.01%, and 82.37% of the initial transmembrane flux after the second, third, and fourth experiments, respectively. This indicates that without modifying the polytetrafluoroethylene flat membrane, pollutants continuously deposit on the membrane surface but do not completely block the membrane pores. Both the ammonia removal rate and the ammonia transmembrane flux decreased slightly, and the ultimate performance of the membrane was greater than 24 h. The ammonia removal rate and the ammonia transmembrane flux severely decayed during the fifth batch of cycles. After the experiment ended, the ammonia removal rate was only 46.91%. After 30 h, the transmembrane flux was only 19.49 (g / m 2 ·h), which was 42.62% of the initial ammonia transmembrane flux. This shows that with an increase in the number of cycles and without modifying the membrane, pollutants deposit on the membrane surface and almost completely block the membrane pores, causing irreversible pollution, greatly reducing the removal rate and the ammonia transmembrane flux. This cycling method is not conducive to long-term stable operation and causes significant damage to the membrane.

[0033] The equipment quantity and treatment scale described here are used to simplify the description of the present invention. Applications, modifications, and variations of the present invention will be obvious to those skilled in the art.

[0034] Although the embodiments of the present invention have been disclosed as above, it is not limited to the applications listed in the specification and embodiments. It can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily achieved. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and the illustrated and described examples here.

Claims

1. A preparation method of an asymmetric hydrogel modified membrane, characterized in that It includes the following steps: Step 1: Cut the polytetrafluoroethylene flat membrane to the corresponding size. Immerse the cut polytetrafluoroethylene flat membrane in absolute ethanol, rinse it with distilled water and then dry it to prepare a flat membrane module; Step 2: Prepare a biomimetic bioadhesive. Vertically immerse the dried flat membrane module in the biomimetic bioadhesive and avoid air bubbles adhering, ensuring complete immersion; Step 3: Prepare a hydrogel. After cooling, evenly cover it on the polytetrafluoroethylene flat membrane. After deposition, an asymmetric hydrogel modified membrane is obtained.

2. The preparation method of the asymmetric hydrogel modified membrane according to claim 1, characterized in that In the said Step 1, the immersion time of the polytetrafluoroethylene flat membrane in absolute ethanol is 20 min; the specific method for preparing the flat membrane module is: use the polytetrafluoroethylene flat membrane soaked in absolute ethanol and polytetrafluoroethylene flat membrane blocks to make a flat membrane module. Cut the polytetrafluoroethylene flat membrane into membrane pieces of 8.3×4.3 cm. Fix the polytetrafluoroethylene flat membrane between two polytetrafluoroethylene flat membrane blocks. Divide the polytetrafluoroethylene flat membrane blocks into upper and lower sides. Stick PTFE rubber strips inside the perimeter of the flat membrane module to prevent the mixing of the solutions on both sides of the polytetrafluoroethylene flat membrane and ensure good sealing performance of the flat membrane module; the hydrophobic membrane surface of the polytetrafluoroethylene flat membrane faces downwards and fix it with gaskets and screws to ensure the independent flow of the solutions on both sides and then it can be used normally.

3. The preparation method of the asymmetric hydrogel modified membrane according to claim 1, characterized in that In the said Step 2, the method for preparing the biomimetic bioadhesive is: dissolve dopamine hydrochloride, polyethyleneimine and sodium alginate in Tris-HCl buffer solution at 25 °C, and then stir evenly at 250 rpm for 30 min.

4. The preparation method of the asymmetric hydrogel modified membrane according to claim 3, characterized in that, In the said biomimetic bioadhesive, the mass ratio of dopamine hydrochloride, polyethyleneimine and sodium alginate is 1:1~1.5:0.5~1. The concentration of sodium alginate in Tris-HCl buffer solution is 2 mg / mL, the concentration of Tris-HCl buffer solution is 50 mmol / L, and the pH value is 8.

5.

5. The preparation method of the asymmetric hydrogel modified membrane according to claim 1, characterized in that, In the said Step 2, vertically immerse the dried flat membrane module in the evenly stirred biomimetic bioadhesive, ensure that the liquid always completely covers the entire flat membrane module, and let it dry naturally for 6~12 h after direct contact for 12~24 h.

6. The preparation method of the asymmetric hydrogel modified membrane according to claim 1, characterized in that, In the said Step 3, the method for preparing the hydrogel is: mix tannic acid and polyvinyl alcohol in a mass ratio of 1:5~8 and stir evenly. The stirring speed is 250~350 rpm, the stirring temperature is 85~90 °C, and the stirring duration is 5~8 h.

7. The preparation method of the asymmetric hydrogel modified membrane according to claim 1, characterized in that, In the said Step 3, the deposition method is: after the prepared hydrogel cools, evenly cover the hydrogel on the polytetrafluoroethylene flat membrane of the flat membrane module obtained in Step 2, then soak for 5~10 min, put it into the freezer for 2 h, at room temperature for 6~12 h, repeat the freeze-thaw process 2~3 times, and then cover the flat membrane module with pure water for 30 min to make the outer layer swell sufficiently.

8. Application of an asymmetric hydrogel modified membrane in removing ammonia nitrogen from landfill leachate, wherein the asymmetric hydrogel modified membrane is prepared by the preparation method of the asymmetric hydrogel modified membrane according to any one of claims 1-7, and is characterized in that, The core of the flat membrane module is an asymmetric hydrogel modified membrane, which removes ammonia nitrogen in the landfill leachate through the asymmetric hydrogel membrane.

9. Use of the asymmetric hydrogel modified membrane according to claim 8 in removing ammonia nitrogen from landfill leachate, characterized in that, The feed side of the flat membrane module of the asymmetric hydrogel modified membrane is connected to an intelligent peristaltic pump and a raw material liquid tank, and the absorption liquid side of the flat membrane module is connected to another intelligent peristaltic pump and an absorption liquid tank. The raw material liquid tank is filled with high ammonia nitrogen landfill leachate. The high ammonia nitrogen landfill leachate flows through the hydrophobic surface side of the asymmetric hydrogel modified membrane, and the absorption liquid flows through the other side of the asymmetric hydrogel modified membrane.

10. Use of the asymmetric hydrogel modified membrane according to claim 9 in removing ammonia nitrogen from landfill leachate, characterized in that, The pH value of the high ammonia nitrogen landfill leachate is 9.5, the flow rate of the high ammonia nitrogen landfill leachate is 600 mL / min, the absorption liquid is a 4.9% sulfuric acid solution, the flow rate of the absorption liquid is 500 mL / min, and the temperature on both sides of the asymmetric hydrogel modified membrane is 25 ± 2 °C under room temperature conditions, and it can be carried out under normal pressure; ammonia nitrogen is enriched in the form of ammonium sulfate and is used in industry for the production of flame retardants, fermentation media or battery electrolytes through reverse osmosis and vacuum evaporation.

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