Graft-modified membranes and methods of making, water treatment methods and systems employing the same

Abstract

A grafted modified membrane, a preparation method thereof, and a water treatment method and system using the same. The preparation method of the grafted modified membrane comprises: sealing the mesh of a membrane for filtration with vaseline, keeping 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 aqueous solution; adding an aqueous solution of a polymer containing primary amino groups to the aqueous solution, standing in the dark for 8-24 h; and cleaning the membrane surface to obtain the grafted modified membrane. The grafted modification method of the application can form a temporary protective film, and at the same time strengthen the strength of the protective film and reserve a gap for destroying the protective film, so that the service life of the membrane can be prolonged through multiple grafting and destruction operations, and the antibacterial property of the membrane surface can be enhanced and bacterial growth can be reduced due to the grafted imidazole alkyl urea.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of polymer modification and membrane filtration, and particularly relates to a grafted modified membrane sheet, a preparation method thereof, and a water treatment method and system using the same. BACKGROUND

[0002] Reverse osmosis membrane is a kind of artificial semi-permeable membrane with specific function, which is made of biological semi-permeable membrane. It is the core element to realize reverse osmosis technology. It is made of high molecular materials such as cellulose acetate, aromatic polyhydrazide and aromatic polyamide, and the surface micropore diameter is usually between 0.5 and 10 nanometers.

[0003] The application range of reverse osmosis membrane is very wide, including power, petrochemical industry, steel, electronics, medicine, food and beverage, municipal and environmental protection fields. Especially in seawater desalination, industrial pure water preparation, drinking pure water production, wastewater treatment and special separation process, it plays an important role. Reverse osmosis membrane technology has the advantages of good water quality, low energy consumption, no pollution, simple process and easy operation.

[0004] The development of reverse osmosis membrane technology has been restricted by the pollution problem of reverse osmosis membrane for a long time. In the reverse osmosis desalination process, the pollutants in the raw material liquid adhere to the surface of the reverse osmosis membrane by deposition or adsorption, which causes the decrease of water flux and the increase of cleaning frequency. In order to improve the anti-pollution performance of reverse osmosis membrane, some anti-pollution materials are usually introduced on the surface of reverse osmosis membrane. Common anti-pollution materials include polyvinyl alcohol, polyethylene glycol and its derivatives, and zwitterions. The introduction of these anti-pollution materials improves the hydrophilicity of the surface of reverse osmosis membrane, reduces the surface roughness and surface charge. Because the types of pollutants in the raw material liquid are very complex, it is generally believed that the better the hydrophilicity of the surface of reverse osmosis membrane, the closer to zero the surface charge, and the lower the roughness, the more conducive to reducing the accumulation of pollutants on the membrane surface. However, because the surface of common aromatic polyamide composite reverse osmosis membrane generally has a high density of negative charge, the traditional anti-pollution materials such as polyvinyl alcohol are generally difficult to simultaneously control the hydrophilicity, roughness, especially the surface charge of reverse osmosis membrane. Therefore, in order to balance the high density of negative charge on the surface of aromatic polyamide composite reverse osmosis membrane, a hydrophilic polymer with high cation density needs to be introduced.

[0005] PolyEthylene Imine (PEI) is a kind of water-soluble polymer containing polyamine groups. The presence of a large number of amine groups makes it have very high cationic density and strong hydrophilicity, so it is widely used in practice, such as used as a reinforcing agent in papermaking process, used as a flocculant in water treatment, and also can be used as a starting material to synthesize many chemical products with special properties, such as used in drug synthesis, printing and dyeing, etc. As a kind of hydrophilic polymer with high cationic density, polyethylene imine is introduced to the surface of aromatic polyamide composite reverse osmosis membrane as a new type of anti-pollution material.

[0006] A chemical grafting method is disclosed in the prior art: the membrane surface is activated by EDC / NHS crosslinking reaction, and PEI is grafted to the membrane surface. This method can effectively cover the carboxyl groups on the membrane surface and make the membrane surface positively charged, thereby improving the retention rate of radionuclides. However, this method may affect the permeability of the membrane, because the grafting of PEI may partially block the pores of the membrane base material.

[0007] A surface deposition method is also disclosed in the prior art: PDA is used as an intermediate layer, a polydopamine coating is first formed on the surface of the reverse osmosis membrane, and then PEI is grafted. This method can significantly improve the hydrophilicity and anti-pollution ability of the membrane, while maintaining a high water flux. However, the antibacterial property of the PDA / PEI coating can only reach about 40%, and its acid-base stability is limited, mainly suitable for extreme environments with pH=1 and pH=13.

[0008] An interfacial polymerization method is also disclosed in the prior art: by depositing a polyethylene imine (PDA) layer on a polyether sulfone (PES) ultrafiltration substrate, controlling the ammonia-induced self-assembly process, forming a high-density amine group absorption, thereby inhibiting membrane growth and forming a defect-free ultra-thin 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] Therefore, it is necessary to develop a new grafted modified membrane to solve the defects and deficiencies of the prior art. SUMMARY

[0010] Therefore, the main purpose of the present application 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 purpose, as a first aspect of the present application, a preparation method of a grafted modified membrane is provided, comprising the following steps:

[0012] Vaseline is used to close the mesh of the membrane for filtration, and the surface is cleaned to keep hydrophilic;

[0013] immersing the membrane surface in an aqueous solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride for 10-20 min;

[0014] adding N-hydroxysuccinimide to the above aqueous solution;

[0015] 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;

[0016] cleaning the membrane surface to obtain the graft-modified membrane.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] Based on the above technical solutions, 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:

[0021] 1. The present application seals the membrane mesh by 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 of the film accumulates a large amount of organic matter and dirt, the vaseline can be washed away by a high-pressure water gun because the vaseline is only combined with the membrane material by van der Waals force. Thus, the service life of the filtration membrane made of a 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;

[0022] 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 pH filtration scenarios;

[0023] 3、The present application introduces ether bond in PEI resin, on the one hand further improves the hydrophilicity of PEI grafted membrane, and on the other hand, after the treatment of Boc2O and DMAP, the stability of PEI resin grafted on the surface of the membrane is improved, when the surface dirt needs to be removed, the passing rate and filtration rate of the membrane are improved, 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;

[0024] 4、The grafting modification method of the present application can form a temporary protective film, and at the same time, the strength of the protective film is reinforced, and a gap for destroying the protective film is reserved, so that the service life thereof can be prolonged through multiple grafting and destruction operations (regeneration), the decline of desalination rate is reduced, and the antibacterial property of the membrane surface is enhanced due to the grafting of imidazole alkyl urea, and the bacterial growth is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced as follows.

[0026] Figure 1 is the infrared absorption peak spectrum of the graft-modified membrane of the present application embodiment 1;

[0027] Figure 2 is the Zeta potential change graph before and after graft modification of the present application embodiment 1 at pH = 7;

[0028] Figure 3 is the reaction schematic diagram of grafting PEI on the surface of the membrane of the present application;

[0029] Figure 4 is the flow chart of the preparation method of the present application. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present application more clear and explicit, the present application will be further described in detail below with reference to specific embodiments and drawings.

[0031] The terms used in the present application are only for the purpose of describing specific embodiments, and are not intended to limit the embodiments of the present application. The singular forms "a", "said" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0032] The EDC / NHS cross-linking reaction is used to activate the reverse osmosis membrane surface, and the PEI is chemically grafted to the membrane surface, which is a relatively mature and convenient grafting process, but this method may affect the permeability of the membrane, because the grafting of PEI may partially block the pores of the membrane base material. In addition, compared with imported membranes, existing domestic membranes also have problems such as short service life, easy breeding of microorganisms, sharp decline in desalination rate in the short term, easy blocking of flow channels by calcium and silicon compounds on the concentrated water side, and the need for frequent chemical cleaning. The inventors have found that these problems can be solved by taking some protective measures, and at the same time, the pH range of the foregoing process is expanded, and the durability is improved.

[0033] Therefore, the inventors propose a method for grafting and modifying a PEI membrane, comprising the following steps:

[0034] The mesh of the membrane used for filtration is sealed with vaseline, and the surface is cleaned and kept hydrophilic;

[0035] The membrane surface is immersed in an aqueous solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC) hydrochloride for 10-20 min;

[0036] N-hydroxysuccinimide is added, and it is slowly shaken to dissolve completely, for 10-20 min;

[0037] An aqueous solution containing a primary amino group-containing polymer is added, and the reaction is carried out under light protection for 12 h;

[0038] The membrane surface is washed with deionized water until there is no reaction residue, and a grafted membrane is obtained.

[0039] The membrane is a reverse osmosis membrane, and the material of the membrane is aromatic polyamide.

[0040] The 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride is an aqueous solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride with a mass fraction of 4%-8%.

[0041] The mass ratio of N-hydroxysuccinimide to 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride is 1:2.

[0042] The primary amino group-containing polymer is PEI, and the number average molecular weight is in the range of 600-70,000.

[0043] The grafting concentration of PEI is 4%-6%, preferably 5%.

[0044] By sealing the mesh holes of the membrane with vaseline, low molecular weight PEI can be prevented from penetrating into the mesh holes and blocking the mesh holes, and a partial coverage area can be formed inside the mesh holes to form a protective film. When the surface area of the protective film accumulates a large amount of organic matter and dirt, the protective film can be washed away by a high-pressure water gun due to the van der Waals force between vaseline and the material of the membrane. Thus, the service life of the filter membrane made of a membrane body material with strong acid and alkali resistance and long service life can be extended through multiple filling and washing operations by using the temporary protective film which can be removed.

[0045] After the above steps, the method further comprises:

[0046] Mixing 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride with a mass fraction of 4% to 8% and imidazole alkyl urea aqueous solution with a mass fraction of 4% to 10%, and slowly stirring to react for 10 to 20 minutes;

[0047] Adding N-hydroxysuccinimide, slowly stirring to completely dissolve, and reacting for 10 to 20 minutes;

[0048] Then, immersing the prepared grafted membrane in the mixed solution, and standing in the dark for 8 to 24 hours, preferably 12 hours;

[0049] Removing the solution, and washing the membrane surface with deionized water until no reaction residues are left, to obtain a grafted modified membrane with imidazole alkyl urea on the surface.

[0050] By grafting imidazole alkyl urea, the antibacterial property of the membrane can be improved, and the deterioration of water quality caused by the growth of surface microorganisms can be avoided.

[0051] After sealing the holes with vaseline, the method further comprises the step of washing the membrane surface with deionized water. After the grafted membrane is prepared, the method can further comprise the step of washing the membrane with hot water at high pressure, and further using an organic solvent such as easily volatile acetone to completely dissolve the vaseline. By using the protective measure of sealing the holes with vaseline, the interior of the mesh holes can be protected from being blocked by PEI, and the through-hole rate can be improved.

[0052] After the grafted membrane is prepared, the method can further comprise the following steps:

[0053] Spraying a mixture of Boc2O (di-t-butyl carbonate) and DMAP (4-dimethylaminopyridine) on the surface of the prepared grafted membrane, and standing for a period of time.

[0054] Boc2O is di-t-butyl dicarbonate, which is a reagent widely used in organic synthesis. Its chemical formula is C 10 H 18O5, molecular weight 218.25, CAS number 24424-99-5. And DMAP (4-dimethylaminopyridine) is a highly efficient organic synthesis catalyst, widely used in acylation, esterification, polymerization and other reactions. By doing so, the active N-H component in the grafted membrane can be removed, and the acid and alkali resistance of the entire membrane can be improved to facilitate the adaptation to higher pH filtration scenarios.

[0055] In the preparation of the grafted membrane, the following steps can also be included:

[0056] Introducing an ether bond into the polymer containing primary amino groups, the specific steps are as follows: during the synthesis of PEI, first introduce an ether bond into the synthesis monomer, for example, use bisphenol A diether dianhydride (BPDEDA) to synthesize PEI, the synthesis of BPDEDA involves the condensation reaction of bisphenol A sodium salt (SDPO) and N-methyl-4-nitrophthalimide (NPI), and finally generates a dianhydride monomer containing an ether bond. The monomer is then subjected to a melt polycondensation reaction with m-phenylenediamine to generate a PEI resin.

[0057] In this way, an ether bond can be introduced into the PEI resin, which on the one hand further improves 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 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 and releasing the amide group therein, which is equivalent to regenerating the membrane and improving its use frequency and service life.

[0058] The application also discloses a grafted modified membrane prepared by the preparation method of the grafted modified membrane.

[0059] 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.

[0060] The application also discloses a water treatment method using the grafted modified membrane.

[0061] The application also discloses a water treatment system using the grafted modified membrane as a filter membrane.

[0062] 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 intended to limit the application.

[0063] Example 1: A rectangular commercial reverse osmosis membrane piece (aromatic polyamide composite reverse osmosis membrane) with a size of 13 cm x 11 cm is fixed in a mesh frame, and vaseline is scraped on the surface to penetrate into the mesh holes to close the mesh holes for filtration;

[0064] The membrane surface is washed with deionized water for 5 min to make the surface hydrophilic, and the membrane surface is then blown dry with nitrogen; 1.06 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride is added to 25 mL of deionized water, and the aqueous solution is poured onto the membrane surface and left to react in the dark for 10 min. After the reaction is complete, 0.53 g of N-hydroxysuccinimide is directly added to the aqueous solution on the membrane surface, and it is slowly shaken to completely dissolve, and then left to react in the dark for 10 min; 8.41 g of PEI containing primary amine (number average molecular weight 10,000) is added to 25 mL of aqueous solution, and then the solution is poured onto the membrane surface and slowly shaken to mix the PEI containing primary amine solution with the existing solution on the membrane surface. After 12 h of standing and reacting in the dark, the membrane surface is washed with deionized water until there is no reaction residue; 2.22 g of imidazole alkyl urea and 2.22 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride are added to 50 mL of deionized water, and after slow stirring for 10 min, 1.11 g of N-hydroxysuccinimide is added and slowly stirred to completely dissolve, and after 10 min of reaction, the above mixed solution is poured onto the membrane surface and left to react in the dark for 12 h. After the reaction is complete, the membrane surface is washed with deionized water until there is no reaction residue, and an imidazole alkyl urea grafted modified membrane piece is prepared. The grafted modified membrane piece is washed with hot water under high pressure to unblock the mesh holes.

[0065] Figure 1 The infrared absorption peak spectrum of the grafted modified membrane piece of Example 1 of the application, wherein the blue line represents the infrared curve of the test sample before grafting, and the red and green lines (superimposed to show brown) are the infrared curves of the test sample after grafting. After grafting, two infrared tests were performed, and the green and red lines basically coincide, indicating that the compound after grafting is stable. In addition, the large peak on the infrared spectrum at 3000-3500 mm -1 > 3500 indicates the presence of an amide bond.

[0066] Figure 2 The Zeta potential change graph before and after grafting modification of the membrane piece of Example 1 of the application at pH = 7, and the positive charge on the surface of the modified membrane piece proves that the grafting is successful.

[0067] The results show that the grafted PEI not only affects the surface charge of the membrane, but also significantly improves the surface properties of the membrane due to its strong hydrophilic properties. The hydrophilicity of the grafted PEI membrane is significantly enhanced compared to the original membrane, and the effect is more pronounced as the molecular weight of the PEI increases. For example, when the PEI 70,000 is grafted, the contact angle of the membrane surface is only about 30°, which is less than half of the original membrane (about 72°). Compared with the ungrafted original membrane, the flux of the modified membrane varies with the molecular weight of the PEI. When the molecular weight of the PEI is 600, the flux reduction of the grafted membrane is negligible. When the molecular weight of the PEI is between 10,000 and 70,000, the flux of the grafted membrane is reduced by about 10% to 30%. However, after using vaseline to protect the pores, the flux of the grafted membrane is reduced by 8.1% to 23.2% (different molecular weights).

[0068] Example 2: The specific steps are the same as in Example 1, except that after the grafted membrane sheet is prepared, a mixture of Boc2O (di-tert-butyl carbonate) and DMAP (4-dimethylaminopyridine) is sprayed on the surface of the grafted membrane sheet and left for a period of time. By comparing the service life, it can be found that the stable running time of the grafted membrane sheet without Boc2O and DMAP treatment is about 460 hours, while the stable running time of the grafted membrane sheet treated with Boc2O and DMAP can be extended to 523 hours. The stability of the PEI resin of the grafted membrane sheet treated with Boc2O and DMAP is greatly improved.

[0069] Example 3: The specific steps are the same as in Example 1, except that the PEI containing primary amino groups has an ether bond, which is synthesized by using bisphenol A type diether dianhydride (BPDEDA). By comparing the service life, it can be found that the stable running time of the grafted membrane sheet changes little, but after a period of time, when the surface area of the scale is relatively large, by using high iodine acid HI to clean the surface of the grafted membrane sheet and then using a high-pressure water gun to flush, the grafted PEI can be washed away, exposing the original aromatic polyamide material. At this time, the grafting step of Example 1 can be repeated to regraft to restore its filtering function, thereby also prolonging the service life of the grafted membrane sheet.

[0070] Example 4: The specific steps are the same as in Example 1, except that the PEI containing primary amino groups has an ether bond, which is synthesized by using bisphenol A type diether dianhydride (BPDEDA), and after the grafted membrane sheet is prepared, a mixture of Boc2O (di-tert-butyl carbonate) and DMAP (4-dimethylaminopyridine) is sprayed on the surface of the grafted membrane sheet and left for a period of time. The experiment also achieved good results.

[0071] Through a series of experiments, it is verified that the grafting modification method can form a temporary protective film, and the strength of the protective film is reinforced while a gap for destroying the protective film is reserved, so that the service life of the protective film can be prolonged through multiple grafting and destroying operations (regeneration), the attenuation of the desalination rate is reduced, and the antibacterial property of the film surface is enhanced due to the grafted imidazole alkyl urea, and the bacterial breeding is reduced.

[0072] The above merely describes preferred embodiments of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a graft-modified membrane sheet, characterized by, The method comprises the following steps: The mesh holes of the membrane for filtration are sealed with vaseline, the surface of the membrane is cleaned, and the membrane is kept hydrophilic; wherein the material of the membrane is aromatic polyamide; wherein the mesh holes of the membrane are sealed with vaseline, which can prevent PEI from penetrating into the mesh holes and blocking the mesh holes, and can form a partial covering area inside the mesh holes and form a protective film; The surface of the membrane is immersed in an aqueous solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride for 10-20 min; N-hydroxysuccinimide is added to the aqueous solution; An aqueous solution of a polymer containing primary amino groups is added to the aqueous solution, and the reaction is carried out in the dark for 8-24 h; wherein the polymer containing primary amino groups is polyethyleneimine containing ether bonds; A mixture of di-tert-butyl carbonate and 4-dimethylaminopyridine is sprayed on the surface of the prepared membrane, and the membrane is left to stand for a period of time; the surface of the membrane is cleaned, and the grafted modified membrane is prepared; The grafted modified membrane is washed with hot water under high pressure.

2. The production method according to claim 1, characterized by, The 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride is an aqueous solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride with a mass fraction of 4%-8%; and / or The mass ratio of N-hydroxysuccinimide to 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride is 1:

2.

3. The production method according to claim 1, characterized by, The number average molecular weight of the polyethyleneimine containing ether bonds is in the range of 600-70,000; and / or The grafting concentration of the polyethyleneimine containing ether bonds is 4-6 wt%.

4. The method of claim 1, wherein, After the aqueous solution of the polymer containing primary amino groups is added, the following treatment is carried out before the mixture of di-tert-butyl carbonate and 4-dimethylaminopyridine is sprayed: An aqueous solution of imidazole alkyl urea with a mass fraction of 4%-10% is mixed with the 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride aqueous solution with a mass fraction of 4%-8%, and the two are slowly stirred to react for 10-20 min; N-hydroxysuccinimide is added, and the reaction is carried out for 10-20 min; The surface of the prepared grafted modified membrane is immersed in the above mixture, and the reaction is carried out in the dark for 12 h; The liquid is removed, the surface of the grafted modified membrane is washed with deionized water until there is no reaction residue, and the grafted modified membrane with imidazole alkyl urea grafted on the surface is prepared.

5. A grafted modified membrane prepared by the method according to any one of claims 1-4.

6. A water treatment method using the grafted modified membrane according to claim 5 for reverse osmosis filtration.

7. A water treatment system using the grafted modified membrane according to claim 5 as a filtration membrane.

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