High chlorine-resistant hydrophilic membrane and preparation process thereof

Abstract

This application relates to the technical field of reverse osmosis membranes, and discloses a highly chlorine-resistant hydrophilic membrane and its preparation process. The hydrophilic membrane includes a membrane body and a first chlorine-resistant layer and a second chlorine-resistant layer sequentially disposed on the surface of the membrane body. The first chlorine-resistant layer uses raw material components including laccase, dopamine, 5,5-dimethylhydantoin, and a stabilizer. The concentration of the laccase is 0.1-0.3 wt%, the concentration of the dopamine is 0.8-1.2 wt%, the concentration of the 5,5-dimethylhydantoin is 0.3-0.6 wt%, and the concentration of the stabilizer is 0.2-0.4 wt%. The second chlorine-resistant layer is obtained by crosslinking a first carboxymethyl dextran and a second carboxymethyl dextran. The molecular weight of the first carboxymethyl dextran is smaller than that of the second carboxymethyl dextran, and the weight ratio of the first carboxymethyl dextran to the second carboxymethyl dextran is (1-5):1. This application can improve the durability of chlorine resistance and maintain good water flux.

Description

Technical Field

[0001] This application relates to the technical field of reverse osmosis membranes, and in particular to a highly chlorine-resistant hydrophilic membrane and its preparation process. Background Technology

[0002] A reverse osmosis membrane is an artificial semi-permeable membrane made to mimic biological semi-permeable membranes and is the core component of reverse osmosis technology. Its principle is based on the principle that, under pressure higher than the osmotic pressure of the solution, other substances cannot pass through the semi-permeable membrane, thus separating them from water. The membrane pore size of a reverse osmosis membrane is extremely small, effectively removing dissolved salts, colloids, microorganisms, organic matter, etc., from water, thereby obtaining high-quality purified water. Most commercial reverse osmosis membranes have an active layer prepared by interfacial bonding of m-phenylenediamine and trimesoyl chloride. The bottom materials are a microporous polysulfone layer for support and a non-woven fabric layer for mechanical strength. The active layer has a dense cross-linked structure, giving it extremely small pore size and excellent mechanical strength, thereby achieving desalination.

[0003] To combat membrane fouling caused by biological processes in reverse osmosis units, active chlorine disinfectants are often added to the water. Although the water undergoes dechlorination before contacting the reverse osmosis membrane, residual active chlorine can still react with the active layer of the membrane. The adverse effects of active chlorine on the polyamide active layer can easily lead to a decrease in membrane separation performance, thus significantly impacting desalination efficiency.

[0004] Currently, there are various methods to improve the chlorine resistance of reverse osmosis membranes. One method involves developing novel interfacial polymers to enhance the chlorine resistance of the active layer. However, this approach is costly, time-consuming, and challenging. A common method is surface coating modification of the reverse osmosis membrane to form a protective layer, reducing the damage caused by active chlorine. However, the stability between the protective layer and the membrane surface is poor, and it can detach under the shear force of water flow, thus easily affecting the stability of chlorine resistance. Therefore, developing membranes with durable chlorine resistance remains an important research direction. Summary of the Invention

[0005] In one aspect, this application provides a highly chlorine-resistant hydrophilic membrane.

[0006] The technical solution adopted in this application is as follows:

[0007] A highly chlorine-resistant hydrophilic membrane includes a membrane body and a first chlorine-resistant layer and a second chlorine-resistant layer sequentially disposed on the surface of the membrane body. The first chlorine-resistant layer uses raw material components including laccase, dopamine, 5,5-dimethylhydantoin, and a stabilizer. The concentration of the laccase is 0.1-0.3 wt%, the concentration of the dopamine is 0.8-1.2 wt%, the concentration of the 5,5-dimethylhydantoin is 0.3-0.6 wt%, and the concentration of the stabilizer is 0.2-0.4 wt%. The second chlorine-resistant layer is obtained by crosslinking a first carboxymethyl dextran and a second carboxymethyl dextran. The molecular weight of the first carboxymethyl dextran is smaller than that of the second carboxymethyl dextran, and the weight ratio of the first carboxymethyl dextran to the second carboxymethyl dextran is (1-5):1.

[0008] By employing the above-mentioned technical solution, dopamine is oxidized to catechol with a quinone structure through the catalytic action of laccase, triggering self-polymerization to form a polydopamine layer on the membrane surface. Furthermore, the amino groups (-NH2) present on the membrane surface undergo a Schiff base reaction with the quinone structure in dopamine, forming covalent bonds that stably connect the polydopamine layer to the membrane surface. Additionally, the carbonyl groups in the membrane can form coordination bonds with the catechol groups of dopamine, further enhancing the binding force and thus forming a stable protective layer on the membrane surface. The phenolic hydroxyl groups in the polydopamine can construct hydrophilic channels, maintaining water flux, and the stabilizer inhibits the homopolymerization of dopamine into large particles, resulting in a more uniform protective layer with good permeability.

[0009] Secondly, 5,5-dimethylhydantoin is embedded in the polydopamine layer. The imine groups (-NH-) in 5,5-dimethylhydantoin can actively capture active chlorine, generating stable N-Cl, thereby blocking the diffusion of active chlorine into the membrane and thus protecting the membrane. The second chlorine-resistant layer is obtained by cross-linking the first and second carboxymethyl dextran. Its polyhydroxy structure forms a hydration layer through cross-linking, which can effectively block ClO. - The membrane permeates through the hydroxyl groups (-OH) of the dextran, which can be oxidized to aldehydes / carboxyl groups by hypochlorous acid, further consuming the permeated free chlorine and thus giving the membrane chlorine resistance.

[0010] Optionally, the first chlorine-resistant layer is obtained through the following steps: 5,5-dimethylhydantoin, stabilizer, dopamine and laccase are added sequentially to Tris-HCl buffer, the pH of the buffer is controlled at 8-8.5, and the mixture is thoroughly mixed to obtain a reaction solution. The membrane is immersed in the reaction solution and reacted at 4°C for 110-120 min, then the temperature is raised to 25°C and reacted for 50-60 min, then the temperature is raised to 40°C and reacted for 20-30 min. After the reaction is completed, the membrane is rinsed with deionized water and dried at 50°C.

[0011] By adopting the above technical solution, laccase can be selected from spore laccase, etc. The reaction process is divided into three stages. First, slow nucleation occurs at 4℃. At low temperatures, the activity of laccase also decreases accordingly. During this process, 5,5-dimethylhydantoin can be adsorbed on the surface of the nascent polydopamine nucleus. Then, as the temperature increases, the activity of laccase increases, accelerating the formation of quinone groups. Polydopamine accumulates and extends the chain length, and the chain segments can also coat 5,5-dimethylhydantoin. As the temperature continues to increase, it promotes the nucleophilic attack of the imine groups of 5,5-dimethylhydantoin on the polydopamine quinone groups, thereby forming a cross-linked network structure and a core-shell-like structure, thereby reducing the loss of 5,5-dimethylhydantoin and improving the durability of chlorine resistance.

[0012] Optionally, the stabilizer is selected from a mixture of polyethylene glycol and cysteine, wherein the weight ratio of polyethylene glycol to cysteine ​​is (1-3):1.

[0013] By adopting the above technical solution, the steric hindrance of polyethylene glycol prevents excessive aggregation of dopamine, thereby adjusting the hydrophilicity of the coating and the dispersion stability of 5,5-dimethylhydantoin; the quinone group generated by dopamine oxidation can undergo Michael addition reaction with the thiol group of cysteine ​​to form a more stable CS bond, which significantly improves the antioxidant properties.

[0014] Optionally, the molecular weight of the first carboxymethyl dextran is 5-10 kDa, and the molecular weight of the second carboxymethyl dextran is 60-70 kDa.

[0015] By adopting the above technical solution and combining high molecular weight and low molecular weight molecules, a good balance can be achieved between water flow rate and chlorine resistance.

[0016] Optionally, the second chlorine-resistant layer is obtained through the following steps: adding the first carboxymethyl dextran and the second carboxymethyl dextran to MES buffer, adding an activator, and mixing evenly to obtain a mixture; the total mass concentration of the first carboxymethyl dextran and the second carboxymethyl dextran is 2-3 wt%, and the mass concentration of the activator is 0.1-0.12 wt%; immersing the membrane loaded with the first chlorine-resistant layer into the mixture, reacting at a constant temperature, then rinsing clean, and then immersing it in a crosslinking agent solution to crosslink the first carboxymethyl dextran and the second carboxymethyl dextran, and then rinsing and drying after crosslinking.

[0017] By adopting the above technical solution, the first carboxymethyl dextran and the second carboxymethyl dextran are grafted onto the membrane surface and cross-linked by a cross-linking agent to form a second chlorine-resistant layer. The activator can be a combination of (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS).

[0018] Optionally, the crosslinking agent is selected from glutaraldehyde or genipin.

[0019] Optionally, the reaction solution may also contain 0.1-0.2 wt% of a modified filler, which is obtained by the following method: 5,5-dimethylhydantoin is dissolved in ethanol, and then nano-silica modified with an aminosilane coupling agent is added. After stirring for 6-8 hours, the mixture is filtered and dried to obtain the modified filler.

[0020] Optionally, the average particle size of the nano-silica is 10-20 nm.

[0021] Optionally, the aminosilane coupling agent is selected from 3-aminopropyltriethoxysilane.

[0022] By adopting the above technical solution, silica surface-modified with an aminosilane coupling agent exhibits better hydrophilicity and is less prone to aggregation. In a 5,5-dimethylhydantoin solution, the surface and internal pores can adsorb some 5,5-dimethylhydantoin, thus enabling the slow release of 5,5-dimethylhydantoin in the first chlorine-resistant layer, further enhancing chlorine resistance. Furthermore, the presence of amino groups on the modified silica surface allows it to interact with the carboxyl groups of dextran, thereby improving the stability of the connection between the second and first chlorine-resistant layers.

[0023] Secondly, this application provides a process for preparing a highly chlorine-resistant hydrophilic membrane.

[0024] A process for preparing a highly chlorine-resistant hydrophilic membrane includes the following steps;

[0025] (1) Pre-treat the membrane to remove impurities from its surface;

[0026] (2) 5,5-dimethylhydantoin, stabilizer, dopamine, porous filler and laccase were added to Tris-HCl buffer in sequence and mixed evenly. Then the membrane was immersed in the reaction solution and reacted at 4℃, 25℃ and 40℃ respectively. Then it was cleaned with deionized water and dried.

[0027] In summary, this application includes at least one of the following beneficial effects:

[0028] 1. The outermost layer of the membrane forms a second chlorine-resistant layer through dextran cross-linking. Its polyhydroxy structure forms a hydration layer through cross-linking, which can effectively block ClO. - The membrane permeates through the hydroxyl groups (-OH) of the dextran, which can be oxidized to aldehydes / carboxyl groups by hypochlorous acid, further consuming the permeated free chlorine and thus giving the membrane chlorine resistance.

[0029] 2. The first chlorine-resistant layer is formed by polymerizing a polydopamine layer on the membrane surface, in which 5,5-dimethylhydantoin is embedded. The polydopamine layer reacts at multiple temperature gradients, allowing the polydopamine segments to better coat the 5,5-dimethylhydantoin. The imine groups of the 5,5-dimethylhydantoin can actively capture active chlorine, blocking its damage to the membrane. In addition, the quinone groups (C=O) generated by dopamine oxidation can capture free chlorine to form inactive chloroquinone complexes, thereby consuming the active chlorine that penetrates the first chlorine-resistant layer, thus achieving the effect of protecting the membrane. Detailed Implementation

[0030] The following provides a further detailed description of this application.

[0031] The reverse osmosis membrane in this embodiment is selected from commercially available reverse osmosis membranes prepared by interfacial bonding of m-phenylenediamine and trimesoyl chloride. Its support layer consists of a non-woven fabric and a polysulfone layer, and the active layer is an amide layer formed through interfacial polymerization. In this application, by forming a first chlorine-resistant layer and a second chlorine-resistant layer on the surface of the membrane, specifically on one side of the active layer, the active chlorine is captured and consumed by the protective layer, thereby reducing the damage of the active chlorine to the active layer and giving the membrane good chlorine resistance.

[0032] Preparation Example 1: Preparation of Modified Fillers

[0033] Nano-silica was heated at 500℃ at a rate of 2℃ / min. After heating, it was cooled to room temperature and then added to a 30% H2O2 solution (60℃, 2 hours) to increase the surface adsorption sites. The mixture was then washed with deionized water until neutral and finally vacuum dried at 100℃ for 10 hours. The average particle size of the nano-silica was 10-20 nm.

[0034] Two parts of nano-silica and 100 parts of anhydrous ethanol were added to a three-necked flask, ultrasonically dispersed, and heated to 80°C. Then, 0.1 parts of 3-aminopropyltriethoxysilane were added, and the mixture was stirred continuously for 8 hours. After the reaction was completed, the mixture was centrifuged, washed several times with deionized water, and dried to obtain nano-silica with an aminosilane coupling agent surface modification.

[0035] Add 1 part of 5,5-dimethylhydantoin to 100 parts of ethanol, heat to 60°C, and after the 5,5-dimethylhydantoin has dissolved, add 2 parts of aminosilane coupling agent surface-modified nano-silica, disperse by ultrasonication, and then stir continuously with magnetic force for 8 hours. After centrifugation, wash several times with anhydrous ethanol and vacuum dry to obtain the modified filler.

[0036] Example 1: A process for preparing a highly chlorine-resistant hydrophilic membrane, comprising the following steps:

[0037] (1) Pretreatment of the membrane: First, immerse the membrane in a 1wt% sodium hydroxide solution for 5 minutes to remove impurities such as grease from the surface of the membrane. Then, immerse the membrane in a 0.5M hydrochloric acid solution for 5 minutes to activate it. Finally, rinse with deionized water until neutral.

[0038] (2) Preparation of the first chlorine-resistant layer: Prepare Tris-HCl buffer, adjust the pH to 8.5, and then add 5,5-dimethylhydantoin, dopamine, stabilizer and laccase in sequence. Stir magnetically (300 rpm) for 10 min in the dark to obtain the reaction solution. The concentration of 5,5-dimethylhydantoin is 0.3 wt%, the concentration of dopamine is 1 wt%, the concentration of stabilizer is 0.2 wt%, the stabilizer is composed of a mixture of polyethylene glycol-400 and cysteine ​​in a weight ratio of 1:1, and the concentration of laccase is 0.1 wt%.

[0039] The membrane was immersed in the reaction solution and reacted at 4°C with shaking (80 rpm) for 2 hours. Then it was transferred to 25°C and reacted with shaking (150 rpm) for 1 hour. Next, the temperature was increased to 40°C at a rate of 1°C / min and the membrane was reacted with shaking (100 rpm) for 0.5 hours. After the reaction was completed, the membrane was rinsed with deionized water and dried in a 50°C oven to form the first chlorine-resistant layer on the surface of the membrane.

[0040] (3) Preparation of the second chlorine-resistant layer: The first carboxymethyl dextran and the second carboxymethyl dextran were dissolved in a MES buffer solution at pH 6.0, and then an activator was added and mixed evenly to obtain a mixture; the total mass concentration of the first carboxymethyl dextran and the second carboxymethyl dextran was 2 wt%, and the concentration of the activator was 0.1 wt%; the molecular weight of the first carboxymethyl dextran was 10 kDa, the molecular weight of the second carboxymethyl dextran was 70 kDa, and the weight ratio of the first carboxymethyl dextran and the second carboxymethyl dextran was 1:1; the activator was a combination of EDC and NHS, and the weight ratio of EDC and NHS was 4:1;

[0041] The membrane obtained in step (2) was immersed in the mixed solution and reacted at a constant temperature of 40°C with shaking (50 rpm) for 4 hours. After removal, it was rinsed with water to remove the physically adsorbed dextran. Then it was immersed in a 0.5 wt% glutaraldehyde solution and allowed to stand at 25°C for crosslinking reaction for 2 hours. After rinsing with water, it was vacuum dried at 40°C to obtain a high chlorine-resistant hydrophilic membrane.

[0042] Example 2: The process steps and components of this example are the same as those of Example 1. The difference is that in the reaction solution of step (2), the concentration of 5,5-dimethylhydantoin is 0.6wt%, the concentration of dopamine is 1.2wt%, the concentration of stabilizer is 0.4wt%, and the concentration of laccase is 0.3wt%.

[0043] Example 3: The process steps and components of this example are the same as those of Example 1. The difference is that in step (2), the stabilizer is composed of polyethylene glycol-400 and cysteine ​​in a weight ratio of 3:1.

[0044] Example 4: The process steps and components of this example are the same as those of Example 1. The difference is that in step (3), the weight ratio of the first carboxymethyl dextran to the second carboxymethyl dextran is 5:1.

[0045] Example 5: The process steps and components of this example are the same as those of Example 1. The difference is that in step (2), 0.1 wt% of the modified filler from Preparation Example 1 is added.

[0046] Comparative Example 1

[0047] The process steps and components of this comparative example and Example 1 are the same, except that the membrane does not have a second chlorine-resistant layer loaded.

[0048] Comparative Example 2

[0049] The process steps and components of this comparative example and Example 1 are the same. The difference is that the first chlorine-resistant layer is not loaded, and the membrane in step (1) is directly loaded with the second chlorine-resistant layer in step (3).

[0050] Comparative Example 3

[0051] The process steps and components of this comparative example and Example 1 are the same, except that no stabilizer is added in step (2).

[0052] Comparative Example 4

[0053] The process steps and components of this comparative example and Example 1 are the same. The difference is that in step (2), after the membrane is immersed in the reaction solution, it is shaken (150 rpm) at 25°C for 3 hours. Then, the temperature is increased to 40°C at a rate of 1°C / min and shaken (100 rpm) for 0.5 hours. After the reaction is completed, it is rinsed with deionized water and dried in an oven at 50°C.

[0054] Comparative Example 5

[0055] The process steps and components of this comparative example and Example 1 are the same, except that in step (3), an equal amount of the first carboxymethyl dextran is used to replace the second carboxymethyl dextran.

[0056] Comparative Example 6

[0057] The process steps and components of this comparative example and Example 1 are the same, except that in step (3), an equal amount of the second carboxymethyl dextran is used to replace the first carboxymethyl dextran.

[0058] Performance testing:

[0059] The reverse osmosis membrane prepared above was tested for water flux and desalination rate in an aqueous solution of 5000 mg / L NaCl at 25 °C, pH 7.5-8, and operating pressure of 1.55 MPa.

[0060] Chlorine resistance test:

[0061] The membrane was continuously run under 800 ppm active chlorine conditions for 96 hours. After each test, it was cleaned with deionized water, and the desalination rate was tested again after cleaning.

[0062] Table 1 Examples 1-5 and Comparative Examples 1-6

[0063]

[0064] By comparing the data of Example 1, Comparative Example 1, and Comparative Example 2, it can be seen that the membrane of Example 1 can maintain a good water flux, and after the chlorine resistance test, the desalination rate can maintain a high level. This indicates that the first chlorine-resistant layer and the second chlorine-resistant layer can work together to block and consume active chlorine before the membrane, thereby reducing the oxidation of the membrane by active chlorine and improving the chlorine resistance of the prepared hydrophilic membrane.

[0065] A comparison of Example 1 and Comparative Example 3 shows that the stabilizer mainly restricts the polydopamine layer from forming uniform large particles through steric hindrance, thereby maintaining good water flux. Furthermore, the stable CS bonds formed by combining with cysteine ​​significantly enhance antioxidant properties, thus improving the stability of the polydopamine layer. Comparing Example 1 and Comparative Example 4, it can be seen that controlling the polydopamine reaction through multiple temperature steps allows for better encapsulation of 5,5-dimethylhydantoin, improving chlorine resistance durability, and the resulting chain segments better maintain water flow channels. Comparing Example 1, Comparative Examples 5 and 6, the second chlorine-resistant layer formed by low molecular weight carboxymethyl dextran has less impact on water flux but also lower chlorine resistance. While the second chlorine-resistant layer formed by high molecular weight carboxymethyl dextran has better chlorine resistance, it has a greater impact on water flux.

[0066] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A highly chlorine-resistant hydrophilic membrane, characterized in that: The membrane comprises a first chlorine-resistant layer and a second chlorine-resistant layer sequentially disposed on the surface of the membrane. The first chlorine-resistant layer uses raw material components including laccase, dopamine, 5,5-dimethylhydantoin, and a stabilizer. The concentration of the laccase is 0.1-0.3 wt%, the concentration of the dopamine is 0.8-1.2 wt%, the concentration of the 5,5-dimethylhydantoin is 0.3-0.6 wt%, and the concentration of the stabilizer is 0.2-0.4 wt%. The second chlorine-resistant layer is obtained by crosslinking a first carboxymethyl dextran and a second carboxymethyl dextran. The molecular weight of the first carboxymethyl dextran is smaller than that of the second carboxymethyl dextran, and the weight ratio of the first carboxymethyl dextran to the second carboxymethyl dextran is (1-5):

1.

2. The highly chlorine-resistant hydrophilic membrane according to claim 1, characterized in that: The first chlorine-resistant layer is obtained through the following steps: 5,5-dimethylhydantoin, stabilizer, dopamine and laccase are added sequentially to Tris-HCl buffer, the pH of the buffer is controlled at 8-8.5, and the mixture is mixed evenly to obtain a reaction solution. The membrane is immersed in the reaction solution and reacted at 4°C for 110-120 min, then the temperature is raised to 25°C and reacted for 50-60 min, and then the temperature is raised to 40°C and reacted for 20-30 min. After the reaction is completed, the membrane is rinsed with deionized water and dried at 50°C.

3. The highly chlorine-resistant hydrophilic membrane according to claim 1, characterized in that: The stabilizer is selected from a mixture of polyethylene glycol and cysteine, wherein the weight ratio of polyethylene glycol to cysteine ​​is (1-3):

1.

4. The highly chlorine-resistant hydrophilic membrane according to claim 2, characterized in that: The molecular weight of the first carboxymethyl dextran is 5-10 kDa, and the molecular weight of the second carboxymethyl dextran is 60-70 kDa.

5. The highly chlorine-resistant hydrophilic membrane according to claim 4, characterized in that: The second chlorine-resistant layer is obtained through the following steps: adding the first carboxymethyl dextran and the second carboxymethyl dextran to MES buffer, adding an activator, and mixing evenly to obtain a mixture; the total mass concentration of the first carboxymethyl dextran and the second carboxymethyl dextran is 2-3 wt%, and the mass concentration of the activator is 0.1-0.12 wt%; immersing the membrane loaded with the first chlorine-resistant layer into the mixture, reacting at a constant temperature, then rinsing clean, and then immersing it in a crosslinking agent solution to crosslink the first carboxymethyl dextran and the second carboxymethyl dextran, and then rinsing and drying after crosslinking.

6. The highly chlorine-resistant hydrophilic membrane according to claim 5, characterized in that: The crosslinking agent is selected from glutaraldehyde or genipin.

7. The highly chlorine-resistant hydrophilic membrane according to claim 5, characterized in that: The reaction solution also contains 0.1-0.2 wt% of modified filler, which is obtained by the following method: 5,5-dimethylhydantoin is dissolved in ethanol, and then nano-silica modified with aminosilane coupling agent is added. After stirring for 6-8 hours, the mixture is filtered and dried to obtain the modified filler.

8. The highly chlorine-resistant hydrophilic membrane according to claim 7, characterized in that: The average particle size of the nano-silica is 10-20 nm.

9. The highly chlorine-resistant hydrophilic membrane according to claim 7, characterized in that: The aminosilane coupling agent is selected from 3-aminopropyltriethoxysilane.

10. The preparation process of a highly chlorine-resistant hydrophilic membrane according to any one of claims 1-9, characterized in that: Includes the following steps; (1) Pre-treat the membrane to remove impurities from its surface; (2) The first chlorine-resistant layer is loaded onto the surface of the membrane to obtain the first modified membrane; (3) The second chlorine-resistant layer is loaded onto the surface of the first chlorine-resistant layer to obtain the highly chlorine-resistant hydrophilic membrane.

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