Graft modified membrane and preparation method thereof, and water treatment method and system adopting graft modified membrane

Abstract

The invention discloses a graft modified membrane and a preparation method thereof, and a water treatment method and system adopting the graft modified membrane. The preparation method of the graft modified membrane comprises the following steps: sealing meshes of the membrane for filtration by adopting vaseline, wherein the surface is clean and keeps hydrophilic; the surface of the diaphragm is immersed in an aqueous solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride for 10 min to 20 min; adding N-hydroxysuccinimide into the aqueous solution; adding an aqueous solution of a polymer containing a primary amino group into the aqueous solution, standing and reacting in a dark place for 8-24 hours; the surface of the membrane is clean, and the graft modified membrane is prepared. According to the grafting modification method, a temporary protective film can be formed, a notch for destroying the protective film is reserved while the strength of the protective film is reinforced, so that the service life of the protective film can be prolonged through multiple grafting and destroying operations, and due to the fact that imidazolidinyl urea is grafted, the antibacterial property of the surface of the film can be enhanced, and bacterium breeding can be reduced.

Description

Technical Field

[0001] The invention relates to the technical field of polymer modification and membrane filtration, and in particular to a grafted modified membrane sheet and a preparation method thereof, and a water treatment method and system using the same. Background Art

[0002] Reverse osmosis membranes are artificial semipermeable membranes with specific functions, mimicking biological semipermeable membranes. They are the core component of reverse osmosis technology. They are made of polymer materials such as cellulose acetate, aromatic polyhydrazide, and aromatic polyamide, and their surface micropores typically have diameters between 0.5 and 10 nanometers.

[0003] Reverse osmosis membranes have a wide range of applications, including power generation, petrochemicals, steel, electronics, pharmaceuticals, food and beverages, municipal administration, and environmental protection. They play a particularly important role in seawater desalination, industrial pure water production, pure drinking water production, wastewater treatment, and specialized separation processes. Reverse osmosis membrane technology offers advantages such as high water quality, low energy consumption, zero pollution, simple process flow, and easy operation.

[0004] The development of reverse osmosis membrane technology has long been hampered by the problem of membrane contamination. During the reverse osmosis desalination process, contaminants in the feed solution adhere to the membrane surface through deposition or adsorption, resulting in reduced water flux and increased cleaning frequency. To improve the membrane's anti-fouling properties, anti-fouling materials are often introduced onto the membrane surface. Common anti-fouling materials include polyvinyl alcohol (PO), polyethylene glycol (PEG), its derivatives, and zwitterions. The introduction of these anti-fouling materials increases the membrane's hydrophilicity and reduces surface roughness and charge. Because the types of contaminants in the feed solution are extremely complex, it is generally believed that the greater the hydrophilicity of the membrane surface, the closer the surface charge is to zero, and the lower the surface roughness, the more effective it is in reducing contaminant accumulation on the membrane surface. However, because common aromatic polyamide composite RO membranes typically carry a high density of negative charges on their surfaces, traditional anti-fouling materials such as PVA generally struggle to simultaneously control the membrane's hydrophilicity, roughness, and, in particular, surface charge. Therefore, to balance the high density of negative charges on the surface of aromatic polyamide composite RO membranes, hydrophilic polymers with a high cation density are needed.

[0005] Polyethylene amine (PEI) is a water-soluble polymer containing polyamine groups. The presence of a large number of amine groups gives it a high cationic density and strong hydrophilicity. As a result, it has found a wide range of practical applications, such as as a reinforcing agent in papermaking and a flocculant in water treatment. It can also be used as a starting material to synthesize many chemical products with special properties, such as those used in pharmaceutical synthesis and printing and dyeing. As a hydrophilic polymer with a high cationic density, polyethylene amine has been introduced as a new anti-fouling material onto the surface of aromatic polyamide composite reverse osmosis membranes.

[0006] Prior art discloses a chemical grafting method: PEI is grafted onto the membrane surface via an EDC / NHS cross-linking reaction to activate the membrane surface. This method effectively covers the carboxyl groups on the membrane surface and imparts a positive charge to the membrane surface, thereby improving the retention rate of radionuclides. However, this method may affect membrane permeability, as the grafted PEI may partially block the pores of the membrane substrate.

[0007] Prior art also discloses a surface deposition method: using PDA as an intermediate layer to first form a polydopamine coating on the reverse osmosis membrane surface, and then grafting PEI. This method can significantly improve the membrane's hydrophilicity and anti-fouling capabilities while maintaining high water flux. However, the antibacterial activity of the PDA / PEI coating only reaches approximately 40%, and its acid-base stability is limited, making it primarily suitable for use in extreme environments of pH 1 and pH 13.

[0008] Prior art also discloses an interfacial polymerization method: by depositing a polyethyleneimine (PDA) layer on a polyethersulfone (PES) ultrafiltration substrate, the ammonia-induced self-assembly process is controlled to form a high-density amine group absorption, thereby inhibiting membrane growth and forming a defect-free ultrathin PA nanofilm. This method can enhance the structural stability of the PA nanofilm, but may require complex process conditions to ensure the uniformity and stability of the membrane.

[0009] It can be seen that it is necessary to develop a new grafted modified membrane to solve the defects and shortcomings of the existing technology. Summary of the Invention

[0010] In view of this, the main purpose of the present invention is to provide a grafted modified membrane and a preparation method, in order to at least partially solve the above technical problems.

[0011] In order to achieve the above-mentioned object, as a first aspect of the present invention, a method for preparing a grafted modified membrane is proposed, comprising the following steps: Use vaseline to seal the mesh of the filtration membrane to keep the surface clean and hydrophilic; immersing the membrane surface in an aqueous solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride for 10-20 min; adding N-hydroxysuccinimide to the above aqueous solution; adding an aqueous solution of a polymer containing primary amino groups to the above aqueous solution, and allowing the solution to stand and react in the dark for 8-24 h; cleaning the membrane surface to obtain the graft-modified membrane.

[0012] As a second aspect of the present application, a graft-modified membrane prepared according to the preparation method of the graft-modified membrane as described above is also provided.

[0013] As a third aspect of the present application, a water treatment method using the graft-modified membrane as described above for reverse osmosis filtration is also provided.

[0014] As a fourth aspect of the present application, a water treatment system using the graft-modified membrane as described above as a filtration membrane is also provided.

[0015] Based on the above technical solution, the graft-modified membrane and the preparation method of the present application have at least one of the following beneficial effects relative to the prior art: 1. The present application seals the membrane mesh with vaseline, which can prevent PEI from penetrating into the mesh and blocking the mesh, and can form a partial coverage area inside the mesh to form a protective film. When the surface accumulates a large amount of organic matter and dirt, the vaseline can be washed away by a high-pressure water gun because it is only combined with the membrane material by van der Waals force. Thus, the service life of the filtration membrane made of the membrane body material with strong acid and alkali resistance and long service life can be extended by filling and washing away the temporary protective film repeatedly; 2. The present application can remove the active N-H component in the graft membrane by spraying a mixture of Boc2O (di-tert-butyl carbonate) and DMAP (4-dimethylaminopyridine) on the surface of the prepared graft membrane, thereby improving the acid and alkali resistance of the entire membrane and adapting to higher filtration scenarios; 3. The present application introduces an ether bond into the PEI resin, which further improves the hydrophilicity of the PEI graft membrane. After treatment with Boc2O and DMAP, the stability of the PEI resin grafted on the surface of the membrane is improved. When it is necessary to remove surface dirt and improve the passing rate and filtration rate of the membrane, strong HI acid can be used to break the ether bond, thereby destroying the corresponding protecting group and releasing the amide group, which is equivalent to regenerating the membrane and improving its use frequency and service life; 4. The grafting modification method of the present invention can form a temporary protective film, and while strengthening the strength of the protective film, it also reserves a gap for destroying it, so that its life can be extended through multiple grafting and destruction operations (regeneration), reducing the attenuation of the desalination rate. In addition, due to the grafting of imidazolidinyl urea, the antibacterial property of the membrane surface can be enhanced, reducing bacterial growth. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments.

[0017] Figure 1 is the infrared absorption peak spectrum of the grafted modified membrane of Example 1 of the present invention; Figure 2 This is a graph showing the change in Zeta potential before and after grafting modification at pH = 7 in Example 1 of the present invention; Figure 3 This is a schematic diagram of the reaction of grafting PEI onto the surface of the membrane of the present invention; Figure 4 It is a flow chart of the preparation method of the present invention. DETAILED DESCRIPTION

[0018] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0019] The terms used in the present invention are only for the purpose of describing specific embodiments and are not intended to limit the embodiments of the present invention. The singular forms "a", "an", "the" and "the" used in the embodiments of the present invention and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0020] Chemically grafting PEI onto the reverse osmosis membrane surface via an EDC / NHS cross-linking reaction to activate the membrane surface is a relatively mature and convenient grafting process. However, this method may affect membrane permeability because the grafted PEI may partially block the pores of the membrane substrate. Furthermore, compared to imported membranes, some existing domestically produced membranes also suffer from issues such as shorter lifespans, increased microbial growth, short-term desalination rate drops, and the blockage of the flow channel by crystallization of calcium and silicon compounds on the concentrate side, necessitating frequent chemical cleaning. The inventors, after in-depth research, have discovered that these issues can be addressed through protective measures, while also expanding the pH range of the aforementioned process and improving its durability.

[0021] Therefore, the present inventors proposed a method for PEI grafting modified membrane, comprising the following steps: Vaseline is used to seal the mesh of the filtration membrane to keep the surface clean and hydrophilic; Immersing the membrane surface in an aqueous solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC) hydrochloride for 10-20 minutes; Add N-hydroxysuccinimide and shake slowly to dissolve it completely for 10-20 minutes; Add an aqueous solution of a polymer containing primary amino groups and allow to react in the dark for 12 h; The surface of the membrane was rinsed with deionized water until no reaction residue remained, thereby obtaining a grafted membrane; Wherein, the membrane is a reverse osmosis membrane, and the material of the membrane is aromatic polyamide.

[0022] Wherein, the 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride is a 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride aqueous solution with a mass fraction of 4% to 8%.

[0023] Wherein, the mass ratio of the N-hydroxysuccinimide to 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride is 1:2.

[0024] Wherein, the polymer containing primary amino groups is PEI, and the number average molecular weight is in the range of 600 to 70,000.

[0025] Wherein, the grafting concentration of the PEI is 4% to 6%, preferably 5%.

[0026] By sealing the membrane mesh with petroleum jelly, low-molecular-weight PEI is prevented from infiltrating and clogging the mesh. It also forms a partial covering area within the mesh, forming a protective film. When organic matter and dirt accumulate on the surface, the petroleum jelly is only bonded to the membrane material by van der Waals forces, allowing it to be washed away with a high-pressure water gun. This disposable temporary protective film can be used to extend the service life of the filter membrane, which is made of a membrane material with strong acid and alkali resistance and a long lifespan, through multiple filling and washing operations.

[0027] The above steps further include: Mix 4% to 8% by mass of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and 4% to 10% by mass of an imidazolidinyl urea aqueous solution, and slowly stir to allow the two to react for 10 to 20 minutes; Add N-hydroxysuccinimide, stir slowly to dissolve it completely, and react for 10-20 minutes; Then, the surface of the prepared grafted membrane is immersed in the above-mentioned mixed solution and allowed to stand in the dark for 8 to 24 hours, preferably 12 hours; The solution was removed, and the membrane surface was rinsed with deionized water until no reaction residues were left, thereby obtaining a grafted modified membrane sheet with imidazolidinyl urea grafted on the surface.

[0028] By grafting imidazolidinyl urea, the antibacterial property of the membrane can be improved, avoiding the deterioration of water quality caused by the growth of surface microorganisms.

[0029] After sealing the pores with vaseline, the process further includes washing the membrane surface with deionized water. After the grafted membrane is obtained, the process further includes high-pressure rinsing of the membrane with hot water. Furthermore, the membrane may be washed with an organic solvent, such as volatile acetone, to completely dissolve the vaseline. The protective measure of sealing the pores with vaseline protects the mesh from being blocked by PEI, thereby increasing the through-hole ratio.

[0030] After the grafted membrane is prepared, the following steps may be further included: A mixture of Boc2O (di-tert-butyl carbonate) and DMAP (4-dimethylaminopyridine) was sprayed onto the surface of the grafted membrane and allowed to stand for a period of time.

[0031] Among them, Boc2O is di-t-butyl carbonate (Di-t-butyl dicarbonate), which is a reagent widely used in organic synthesis. Its chemical formula is C 10 H 18 O5 has a molecular weight of 218.25 and a CAS number of 24424-99-5. DMAP (4-dimethylaminopyridine) is a highly effective organic synthesis catalyst, widely used in reactions such as acylation, esterification, and polymerization. This treatment removes the active NH components in the grafted membrane, improving the membrane's acid and alkali resistance, making it suitable for filtration applications with higher pH values.

[0032] The preparation of the grafted membrane may further include the following steps: The ether bond is introduced into the polymer containing primary amino groups. Specifically, during the synthesis of PEI, the ether bond is first introduced into the synthetic monomer. For example, bisphenol A diether dianhydride (BPDEDA) is used to synthesize PEI. The synthesis of BPDEDA involves a condensation reaction between bisphenol A sodium salt (SDPO) and N-methyl-4-nitrophthalimide (NPI), ultimately producing a dianhydride monomer containing an ether bond. This monomer is then subjected to a melt polycondensation reaction with m-phenylenediamine to produce the PEI resin.

[0033] Thus, the ether bond can be introduced into the PEI resin, on the one hand to further improve the hydrophilicity of the PEI grafted membrane, and on the other hand, after the treatment of Boc2O+DMAP, the stability of the PEI resin grafted on the surface of the membrane is improved. When it is necessary to remove the surface scale and improve the passing rate and filtration rate of the membrane, the ether bond can be destroyed by strong HI acid, thereby destroying the corresponding protecting group, releasing the amide group therein, which is equivalent to regenerating the membrane, improving the use frequency and service life thereof.

[0034] The application further discloses a grafted modified membrane prepared by the preparation method of the grafted modified membrane.

[0035] The initial permeation flux and salt retention rate of the grafted modified membrane are greater than or equal to 60 L / (m 2 ·h) and 95%, respectively.

[0036] The application further discloses a water treatment method for reverse osmosis filtration by using the grafted modified membrane.

[0037] The application further discloses a water treatment system using the grafted modified membrane as a filtration membrane.

[0038] The application will be further described and illustrated by specific examples. It should be noted that the following examples are only illustrative and are not used to limit the application.

[0039] Example 1: A rectangular commercial reverse osmosis membrane (aromatic polyamide composite reverse osmosis membrane) with a size of 13 cm*11 cm is fixed in a mesh frame, vaseline is scraped on the surface to make it penetrate into the mesh hole, and the mesh hole for filtration is closed; The membrane surface was rinsed with deionized water for 5 minutes to make it hydrophilic, then purged with nitrogen until dry. 1.06 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride was added to 25 mL of deionized water, poured onto the membrane surface, and allowed to react in the dark for 10 minutes. After the reaction was complete, 0.53 g of N-hydroxysuccinimide was added directly to the aqueous solution on the membrane surface and gently shaken to dissolve it completely. The solution was then allowed to react in the dark for 10 minutes. 8.41 g of PEI (number-average molecular weight 10,000) containing primary amines was added to 25 mL of aqueous solution, poured onto the membrane surface, and gently shaken to mix the PEI solution with the existing solution on the membrane surface. After allowing the reaction to stand in the dark for 12 hours, rinse the membrane surface with deionized water until no reaction residue remains. Add 2.22 g of imidazolidinyl urea and 2.22 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride to 50 mL of deionized water and slowly stir for 10 minutes. Then, add 1.11 g of N-hydroxysuccinimide and slowly stir until completely dissolved. After 10 minutes of reaction, pour the mixed solution onto the membrane surface and allow it to stand in the dark for 12 hours. After the reaction is complete, rinse the membrane surface with deionized water until no reaction residue remains, thereby producing an imidazolidinyl urea grafted membrane. The grafted membrane is then rinsed with hot water at high pressure to clear the mesh.

[0040] Figure 1 This is the infrared absorption peak spectrum of the grafted modified membrane of Example 1 of the present invention, where the blue line represents the infrared curve of the test sample before grafting, and the red and green lines (brown after superposition) are the infrared curves of the test sample after grafting. Two infrared tests were conducted after grafting, and the green and red lines basically overlapped, indicating that the compound after grafting is stable. In addition, the infrared spectrum at 3000-3500 mm -1 The large peak indicated the presence of an amide bond.

[0041] Figure 2 This is a diagram showing the Zeta potential change before and after grafting modification at pH = 7 in Example 1 of the present invention. The positive charge on the surface of the modified membrane proves that the grafting is successful.

[0042] Results show that grafting PEI not only affects the membrane surface charge but also significantly improves its surface properties due to its strong hydrophilicity. The hydrophilicity of the grafted PEI membrane is significantly enhanced compared to the original membrane, and the effect becomes more pronounced with increasing molecular weight. For example, when grafted with 70,000 PEI, the contact angle of the membrane surface is only approximately 30°, less than half that of the original membrane (approximately 72°). Compared to the ungrafted original membrane, the flux of the modified membrane varies depending on the PEI molecular weight. When the PEI molecular weight is 600, the flux reduction of the grafted membrane is negligible. However, when the PEI molecular weight ranges from 10,000 to 70,000, the flux of the grafted membrane decreases by approximately 10% to 30%. Furthermore, when the mesh is protected with petroleum jelly, the flux of the grafted membrane decreases by 8.1% to 23.2% (depending on the molecular weight).

[0043] Example 2: The specific steps were the same as in Example 1, except that after the grafted membrane was prepared, a mixture of Boc2O (di-tert-butyl carbonate) and DMAP (4-dimethylaminopyridine) was sprayed onto the surface of the grafted membrane and allowed to stand for a period of time. Comparison of service life revealed that the grafted membrane without Boc2O and DMAP treatment had a stable operating time of approximately 460 hours, while the grafted membrane treated with Boc2O and DMAP had a stable operating time of 523 hours. This significantly improved the stability of the PEI resin in the grafted membrane treated with Boc2O and DMAP.

[0044] Example 3: The specific steps are the same as those in Example 1, differing only in that ether bonds are introduced into the PEI containing primary amino groups. The PEI is synthesized using bisphenol A diether dianhydride (BPDEDA). Comparing the service life reveals that the stable operation duration of the grafted membrane does not change much. However, after a period of time, when the surface accumulates a lot of scale, the grafted membrane surface can be cleaned with periodic acid (HI) and then flushed with a high-pressure water jet to remove the grafted PEI, revealing the original aromatic polyamide material. At this point, the grafting steps of Example 1 can be repeated to re-graft and restore the filtration function, thereby also extending the service life of the grafted membrane.

[0045] Example 4: The specific steps are the same as those in Example 1, except that ether bonds are introduced into the PEI containing primary amino groups. The PEI is synthesized using bisphenol A diether dianhydride (BPDEDA). After the grafted membrane is prepared, a mixture of Boc2O (di-tert-butyl carbonate) and DMAP (4-dimethylaminopyridine) is sprayed onto the surface of the grafted membrane and allowed to stand for a period of time. Experimental verification also achieved good results.

[0046] A series of experiments have verified that the grafting modification method of the present invention can form a temporary protective film, and while strengthening the protective film, it also reserves a gap for its destruction, so that its life can be extended through multiple grafting and destruction operations (regeneration), reducing the attenuation of the desalination rate. In addition, due to the grafting of imidazolidinyl urea, the antibacterial property of the membrane surface can be enhanced, reducing bacterial growth.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a grafted modified membrane, characterized in that: The steps include: Use vaseline to seal the mesh of the filtration membrane to keep the surface clean and hydrophilic; Immersing the membrane surface in an aqueous solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride for 10 to 20 minutes; Adding N-hydroxysuccinimide to the above aqueous solution; Add an aqueous solution of a polymer containing primary amino groups to the above aqueous solution, and allow to react in the dark for 8 to 24 hours; The surface of the membrane is cleaned to obtain the grafted modified membrane.

2. The preparation method according to claim 1, characterized in that The material of the diaphragm is aromatic polyamide; and / or The 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride is a 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride aqueous solution with a mass fraction of 4% to 8%; and / or The mass ratio of the N-hydroxysuccinimide to 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride is 1:

2.

3. The preparation method according to claim 1, characterized in that The polymer containing primary amino groups is polyethyleneimine, and the number average molecular weight is in the range of 600 to 70,000; and / or The grafting concentration of the polyethyleneimine is 4-6 wt %.

4. The preparation method according to claim 3, characterized in that The polymer containing primary amino groups is polyethyleneimine containing ether bonds.

5. The preparation method according to claim 1, characterized in that After the grafted modified membrane is obtained, the following treatments are performed: Mix 4% to 8% by mass of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and 4% to 10% by mass of an imidazolidinyl urea aqueous solution, and slowly stir to allow the two to react for 10 to 20 minutes; Add N-hydroxysuccinimide and react for 10-20 min; The surface of the grafted modified membrane was immersed in the above-mentioned mixed solution and allowed to react for 12 h in the dark. The liquid is removed, and the surface of the grafted modified membrane is rinsed with deionized water until no reaction residue is left, thereby obtaining a grafted modified membrane with imidazolidinyl urea grafted on the surface.

6. The preparation method according to claim 1, characterized in that After the graft modified membrane is prepared, the method further includes the step of using hot water and high pressure to wash the graft modified membrane.

7. The preparation method according to claim 6, characterized in that After the grafted modified membrane is prepared, the method further includes the step of washing the grafted modified membrane with an organic solvent to completely dissolve the vaseline.

8. The preparation method according to claim 1, characterized in that After the graft modified membrane is obtained, the following treatment is further performed: Spray a mixture of di-tert-butyl carbonate and 4-dimethylaminopyridine onto the surface of the prepared graft-modified membrane and let it stand for a period of time.

9. A grafted modified membrane prepared according to the method for preparing a grafted modified membrane according to any one of claims 1 to 8.

10. A water treatment method comprising performing reverse osmosis filtration using the grafted modified membrane according to claim 9.

11. A water treatment system using the grafted modified membrane according to claim 9 as a filter membrane.

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