Enhanced polyvinylidene fluoride hollow fiber nanofiltration membrane and preparation method thereof

Abstract

The invention discloses an enhanced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and a preparation method thereof, and particularly relates to an enhanced hollow fiber composite nanofiltration membrane taking sulfonated polyvinyl alcohol as a functional separation layer and a preparation method of the enhanced hollow fiber composite nanofiltration membrane. The preparation method comprises the following steps: introducing polyvinyl alcohol into polyvinylidene fluoride to prepare an enhanced hollow fiber base membrane through phase inversion, and then constructing a cross-linked sulfonated polyvinyl alcohol separation layer on the surface of the middle fiber base membrane through surface coating and synchronous cross-linking to prepare the enhanced hollow fiber composite nanofiltration membrane. Polyvinyl alcohol is introduced into a hollow fiber base membrane, the coating uniformity of sulfonated polyvinyl alcohol is ensured, meanwhile, polyvinyl alcohol in the base membrane and sulfonated polyvinyl alcohol on the surface are synchronously crosslinked, a thin, uniform and defect-free separation layer can be obtained, and the prepared hollow fiber nanofiltration membrane has high permeation flux and sodium sulfate rejection rate under the pressure of 3 bar and can be used for preparing a hollow fiber nanofiltration membrane. The preparation method disclosed by the invention is coupled with phase inversion and coating cross-linking processes, and is simple and easy to industrialize.

Description

Technical Field

[0001] The invention relates to the technical field of polymer separation membrane preparation, in particular to an enhanced polyvinylidene fluoride hollow fiber nanofiltration membrane and a preparation method thereof. Background Art

[0002] Nanofiltration membrane separation technology is a new type of green separation technology that is efficient, energy-saving, and has the outstanding characteristics of simple equipment, mild operating conditions, large processing capacity, and high separation efficiency. It can effectively remove high-valent ions and organic matter in water bodies and has a wide range of uses in water softening, drinking water purification, and industrial wastewater treatment.

[0003] At present, rolled nanofiltration membranes dominate the nanofiltration membrane market with the advantage of simple preparation process. However, the configuration of rolled nanofiltration membrane components has limitations in internal space design and flow channel distribution, which can easily lead to: 1) Because the spacers, water collection pipes and other components inside the element occupy space, the membrane filling area is limited, and the water production flux is difficult to increase. It can only be compensated by increasing pressure, which increases the operating energy consumption; 2) The spacers between the membranes hinder the flow of fluids, affect the openness of the flow channels, and cause membrane pollution during long-term operation, resulting in irreversible attenuation of membrane performance; such as: Invention patent CN 113083033 A, compared with hollow fiber nanofiltration membranes, it has unique advantages: 1) The packing density is large, and the three-dimensional structure gives it a large specific surface area, which improves the separation efficiency; 2) The negative impact of membrane pollution can be reduced by backwashing, which is suitable for treating feed liquids with higher concentrations of suspended solids, reducing the necessity of pretreatment of the raw material liquid; 3) The self-supporting structure reduces the demand for accessories and simplifies the processing of components, reducing costs.

[0004] Compared with flat rolled nanofiltration membranes, hollow fiber nanofiltration membranes have outstanding advantages such as large water production and backwashing, and have been applied in the fields of high-quality drinking water purification and wastewater reuse. At present, the preparation methods of hollow fiber nanofiltration membranes mainly include preparing hollow fiber nanofiltration membranes by phase inversion one-step method and preparing composite nanofiltration membranes by interfacial polymerization or coating process; the former process is relatively simple, but due to the one-step phase inversion process, the separation layer is relatively thick, the membrane permeation flux is small, and it is impossible to prepare small-pore nanofiltration membranes with low molecular weight cut-off; although the interfacial polymerization process can prepare high-flux and small-pore composite nanofiltration membranes, the problem is that the surface floating liquid needs to be wiped off after the initially solidified hollow fiber membrane passes through the aqueous solution of polyamine and the organic solution of polyacyl chloride. The amount of residual surface floating liquid directly affects the effect of the interfacial polymerization reaction of the polyacyl chloride organic solution, and affects the nanoscale pore size formation. At the same time, the unstable concentration of polyamine and polyacyl chloride in industrialization causes uneven membrane pore size formation and easy stratification. The composite nanofiltration membrane is not chlorine-resistant and has a short service life, such as: invention patent CN 109603588 A. In addition, compared with rolled nanofiltration membranes, current homogeneous hollow fiber nanofiltration membranes have problems such as poor strength and easy wire breakage. Therefore, as the future direction of industrial and application development, it is of great significance to develop new hollow fiber nanofiltration membranes and their preparation technologies from multiple aspects such as membrane materials, membrane structures, and membrane preparation methods.

[0005] Nanofiltration membranes with sulfonic acid groups not only provide abundant negatively charged groups to improve the retention rate of the membrane for multivalent anions, but also give the nanofiltration membrane good chlorine resistance. Chen Yuhai et al. used a blending method to blend polyethersulfone and sulfonated polysulfone, and prepared a nanofiltration membrane with an asymmetric structure by a one-step phase inversion method; such as invention patent CN 101979132 A, Ouyang Kuihui et al. prepared a hollow fiber nanofiltration membrane with a sulfonated polyethersulfone coating by dry-jet wet spinning and coating process; another example: invention patent CN 103638822 A, and compared with sulfonated polysulfone polymers, sulfonated polyvinyl alcohol has a lower preparation cost. Therefore, it is necessary to develop a high-performance enhanced hollow fiber composite nanofiltration membrane that can be prepared simply and controllably and a preparation method thereof. Summary of the invention

[0006] The present invention aims at the technical problems that hollow fiber nanofiltration membranes prepared by phase inversion method have low permeation flux and difficulty in preparing low cut-off molecular weight nanofiltration membranes, hollow fiber nanofiltration membranes prepared by interfacial polymerization method have poor controllability and difficulty in continuous preparation, and hollow fiber nanofiltration membranes have poor strength, and proposes a reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and a preparation method thereof. The technical problem to be solved by the present invention is how to achieve controllable preparation of high-strength, high-permeability selectivity hollow fiber nanofiltration membranes by coupling phase inversion and coating cross-linking processes.

[0007] The present invention is achieved by the following technical solution, which is characterized by comprising the following steps: S1, dissolving a certain amount of polyvinyl alcohol and polyvinylidene fluoride in dimethylacetamide solvent to prepare a membrane casting solution, and preparing a braided tube reinforced hollow fiber base membrane through a phase inversion spinning process; S2, at room temperature, a certain amount of sulfonated polyvinyl alcohol is dissolved in pure water to prepare a coating solution A, and the hollow fiber membrane prepared in S1 is soaked in the coating solution A, and after a certain period of time, it is taken out, washed with pure water, and dried to obtain a hollow fiber base membrane loaded with sulfonated polyvinyl alcohol; S3, at room temperature, dissolving a certain amount of epichlorohydrin in a sodium hydroxide solution to prepare a solution B, soaking the hollow fiber-based membrane loaded with sulfonated polyvinyl alcohol which has been soaked in the coating solution A in the solution B, and taking out and draining the solution B after a certain period of time; S4. The hollow fiber base membrane that has been immersed in solution B is heat-treated at a certain temperature, and after a certain period of time, it is taken out and immersed in pure water.

[0008] Preferably, the mass fraction of polyvinyl alcohol in the spinning solution for preparing the hollow fiber-based membrane in step S1 is 2.0-4.0 wt %, and the mass fraction of polyvinylidene fluoride is 15.0-19.0 wt %.

[0009] Preferably, the degree of sulfonation of the water-soluble sulfonated polyvinyl alcohol in the coating solution A described in step S2 is 10% to 25%, and the mass percentage concentration is 2.0-8.0 wt %.

[0010] Preferably, the soaking time of the hollow fiber base membrane in the coating solution A in step S2 is 30-60 min.

[0011] Preferably, the mass percentage concentration of epichlorohydrin in solution B described in step S3 is 2.0-8.0 wt%, and the concentration of sodium hydroxide solution is 1.0-1.5 mol / L.

[0012] Preferably, the immersion time of the hollow fiber-based membrane loaded with sulfonated polyvinyl alcohol after immersion in the coating solution A in step S3 in the solution B is 10-30 min.

[0013] Preferably, the heat treatment temperature of the hollow fiber-based membrane soaked in solution B in step S4 is 50-80° C. and the time is 3-10 min.

[0014] The beneficial effects of the present invention are: In order to overcome the problems of low permeation flux and difficulty in preparing low molecular weight nanofiltration membranes in the preparation of hollow fiber nanofiltration membranes by phase inversion method, poor controllability and difficulty in continuous preparation in the preparation of hollow fiber nanofiltration membranes by interfacial polymerization method, and poor strength of hollow fiber nanofiltration membranes, the present invention improves the strength, high permeation selectivity and preparation controllability of hollow fiber nanofiltration membranes, and proposes an enhanced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and a preparation method thereof. By adding polyvinyl alcohol to polyvinylidene fluoride casting solution, polyvinyl alcohol is used to inhibit the crystallization of polyvinylidene fluoride, and an enhanced high-porosity hollow fiber base membrane is prepared, and then a sulfonated polyvinyl alcohol functional material is coated on the surface, cross-linked by epichlorohydrin, and a charged functional separation layer is constructed on the surface of the base membrane. Since polyvinyl alcohol is introduced into the hollow fiber base membrane, the coating uniformity of sulfonated polyvinyl alcohol can be ensured on the one hand, and the polyvinyl alcohol in the base membrane and the sulfonated polyvinyl alcohol on the surface can be cross-linked synchronously, so that a thin and uniform defect-free composite separation layer can be prepared, and the flux of the nanofiltration membrane is greatly improved while maintaining high selectivity; at the same time, the hollow fiber nanofiltration membrane of the present invention has both a high permeation flux driven by low pressure and an energy-saving effect driven by low pressure at a pressure of 3.0 bar. The preparation method of the present invention couples phase transformation and coating cross-linking processes, which is simple and easy to industrialize. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art, but this does not limit the protection scope of the present invention.

[0016] Figure 1 It is a schematic diagram of the hollow fiber-based membrane of the present invention; Figure 2 It is a schematic diagram of the hollow fiber composite nanofiltration membrane of the present invention; DETAILED DESCRIPTION

[0017] See also Figure 1 to Figure 2 As shown: Example

[0018] This embodiment provides a reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and a preparation method thereof, which is described as follows: (1) preparing a casting solution: adding polyvinylidene fluoride and polyvinyl alcohol into dimethylacetamide (DMAc), stirring and dissolving the mixture uniformly, wherein the mass concentration of polyvinylidene fluoride is 19 wt % and the mass concentration of polyvinyl alcohol is 2 wt %, and standing and degassing for more than 6 h to obtain a casting solution; (2) Preparation of hollow fiber-based membrane: Under constant temperature and pressure conditions of 76 °C and 0.1 MPa, the obtained casting solution is uniformly coated on the woven tube through a spinning nozzle, and the enhanced hollow fiber-based membrane is obtained through solvent evaporation, coagulation, and rinsing; (3) Preparation of sulfonated polyvinyl alcohol: Add 15 g of polyvinyl alcohol (PVA) and 300 mL of distilled water into a 500 mL three-necked flask and stir in a 90 °C water bath until completely dissolved. After cooling to room temperature, slowly add excess concentrated sulfuric acid drop by drop into the PVA solution in an ice water bath. This process needs to be carried out under mechanical stirring. After the addition is completed, place it in a 40 °C water bath and continue stirring. After 5 hours, discharge the reaction mixture into anhydrous ethanol to precipitate a white solid, which is then washed with anhydrous ethanol several times until the pH value reaches 6.0 and the washing is stopped. The product is dried to obtain a sulfonated polyvinyl alcohol with a sulfonation degree of 20%; (4) Preparation of coating solution A: dissolving a certain amount of sulfonated polyvinyl alcohol in pure water and stirring at room temperature for 2 h to obtain a coating solution A with a mass percentage concentration of 3.0 wt%; (5) Preparation of solution B: Dissolve a certain amount of epichlorohydrin in 0.15 mol / L sodium hydroxide solution and stir at 40°C for 3 h to obtain a solution B with a mass percentage concentration of 4.0 wt%; (6) Soaking the hollow fiber-based membrane in coating solution A: Soak the cleaned hollow fiber-based membrane in coating solution A at room temperature and pressure for 45 min, take it out, wash it with pure water, and air-dry it to obtain a hollow fiber-based membrane loaded with sulfonated polyvinyl alcohol; (7) Soaking the hollow fiber-based membrane in solution B: Soak the hollow fiber-based membrane loaded with sulfonated polyvinyl alcohol treated with solution A and washed with pure water in modified solution B for 30 min, and drain the modified solution B; (8) Heat treatment: The hollow fiber base membrane treated with the modified solution B was heat treated at 60°C for 5 min to obtain a hollow fiber composite nanofiltration membrane. Example

[0019] This embodiment also provides a reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and a preparation method thereof, which are described as follows: (1) preparing a casting solution: adding polyvinylidene fluoride and polyvinyl alcohol into dimethylacetamide (DMAc) and stirring to dissolve evenly, wherein the mass concentration of polyvinylidene fluoride is 19 wt %, and the mass concentration of polyvinyl alcohol is 2.0 wt %, and standing to degas for more than 6 h to obtain a casting solution; (2) Preparation of hollow fiber-based membrane: Under constant temperature and pressure conditions of 76 °C and 0.1 MPa, the obtained casting solution is uniformly coated on the woven tube through a spinning nozzle, and the enhanced hollow fiber-based membrane is obtained through solvent evaporation, coagulation, and rinsing; (3) Preparation of sulfonated polyvinyl alcohol: Add 20 g of polyvinyl alcohol (PVA) and 300 mL of distilled water into a 500 mL three-necked flask and stir in a 90 °C water bath until completely dissolved. After cooling to room temperature, slowly add excess concentrated sulfuric acid drop by drop into the PVA solution in an ice water bath. This process needs to be carried out under mechanical stirring. After the addition is completed, place it in a 40 °C water bath and continue stirring. After 5 hours, discharge the reaction mixture into anhydrous ethanol to precipitate a white solid, which is washed with anhydrous ethanol several times until the pH value reaches 6.0 and then stop washing. Dry the product to obtain sulfonated polyvinyl alcohol with a sulfonation degree of 15%; (4) Preparing coating solution A: dissolving a certain amount of sulfonated polyvinyl alcohol in pure water and stirring at room temperature for 2 h to obtain a coating solution A with a mass percentage concentration of 3.0 wt%; (5) Preparation of solution B: Dissolve a certain amount of epichlorohydrin in 0.15 mol / L sodium hydroxide solution and stir at 40°C for 3 h to obtain a solution B with a mass percentage concentration of 4.0 wt%; (6) Soaking the hollow fiber-based membrane in coating solution A: Soak the cleaned hollow fiber-based membrane in coating solution A at room temperature and pressure for 60 min, take it out, wash it with pure water, and air-dry it to obtain a hollow fiber-based membrane loaded with sulfonated polyvinyl alcohol; (7) Soaking the hollow fiber-based membrane in solution B: Soak the hollow fiber-based membrane loaded with sulfonated polyvinyl alcohol treated with solution A and washed with pure water in modified solution B for 15 min, and drain the modified solution B; (8) Heat treatment: The hollow fiber base membrane treated with the modified solution B was heat treated at 60°C for 10 min to obtain a hollow fiber composite nanofiltration membrane.

[0020] Comparative Example 1: This comparative example provides a reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and a preparation method thereof, which is different from Example 1 in that: Omit step (3); (1) Preparation of casting solution: Add polyvinylidene fluoride and polyvinyl alcohol into dimethylacetamide (DMAc), stir and dissolve evenly, wherein the mass concentration of polyvinylidene fluoride is 19 wt %, and the mass concentration of polyvinyl alcohol is 2 wt %. Let stand and degas for more than 6 h to obtain casting solution.

[0021] (2) Preparation of hollow fiber-based membrane: Under constant temperature and pressure conditions of 76 °C and 0.1 MPa, the obtained casting solution is evenly coated on the woven tube through a spinning nozzle, and the enhanced hollow fiber-based membrane is obtained through solvent evaporation, coagulation and rinsing.

[0022] (3) Preparation of coating solution A: Dissolve a certain amount of polyvinyl alcohol in pure water and stir at room temperature for 2 h to obtain a coating solution A with a mass percentage concentration of 3.0 wt%.

[0023] (4) Preparation of solution B: Dissolve a certain amount of epichlorohydrin in 0.15 mol / L sodium hydroxide solution and stir the mixture at 40°C for 3 h to obtain a solution B with a mass percentage concentration of 4.0 wt%.

[0024] (5) Soaking the hollow fiber-based membrane in coating solution A: Soak the cleaned hollow fiber-based membrane in coating solution A at room temperature and pressure for 45 minutes, take it out, wash it with pure water, and air-dry it to obtain a hollow fiber-based membrane loaded with polyvinyl alcohol.

[0025] (6) Soaking the hollow fiber-based membrane in solution B: The hollow fiber-based membrane loaded with polyvinyl alcohol, which had been treated with solution A and washed with pure water, was soaked in modified solution B for 30 min, and the modified solution B was drained.

[0026] (7) Heat treatment: The hollow fiber base membrane treated with the modified solution B was heat treated at 60°C for 5 min to obtain a hollow fiber composite nanofiltration membrane.

[0027] Test example: Test Example 1: In this test example, the hollow fiber base membrane of Example 1 and the prepared hollow fiber composite nanofiltration membrane were analyzed by scanning electron microscope and found that compared with the hollow fiber base membrane ( Figure 1 ), the reinforced hollow fiber composite nanofiltration membrane prepared by the present invention ( Figure 1 ) surface is more dense.

[0028] Test Example 2: This test example tests the performance of the hollow fiber nanofiltration membranes prepared in Example 1, Example 2, and Comparative Example 1, and the test method is as follows: The present invention evaluates the separation performance (salt rejection (R) and water flux (J)) of the hollow fiber nanofiltration membrane as follows: A cross-flow experiment was conducted with 1000 ppm NaCl and Na2SO4 aqueous solution as feed solution at 0.3 MPa, 25 °C, and pH = 7.0 ± 0.2. The specific steps are as follows: (1) With pure water as the feed liquid, the pure water flux of the hollow nanofiltration membrane was measured at a pressure of 0.3 MPa and 25 ± 1°C. The flux (J) reflects the permeability of the membrane, J = V / (A·t). Where V is the permeate volume on the permeation side (L); A is the effective area of ​​the membrane (m2); and t is the permeation time (h).

[0029] (2) Maintaining the operating pressure unchanged, replace the pure water with 1000 ppm NaCl and Na2SO4. After running for 30 min, measure the desalination rate (R) of the hollow nanofiltration membrane, which reflects the separation performance of the membrane. R = (1-Cp / Cf) × 100%. In the formula, Cf and Cp are the concentrations of the solute components in the feed liquid and the permeate, respectively.

[0030] Table 1: Separation performance of hollow fiber nanofiltration membranes membrane Water flux (L / m2·h) Na2SO4 desalination rate (%) NaCl desalination rate (%) Hollow fiber membrane 754 / / Hollow fiber composite nanofiltration membrane prepared in Example 1 68.6 98.2 39.0 Hollow fiber composite nanofiltration membrane prepared in Example 2 64.5 97.5 33.5 Hollow fiber composite nanofiltration membrane prepared in Comparative Example 1 58.8 82.8 35.6 From the data in Table 1, it can be found that the hollow nanofiltration composite nanofiltration membrane of the present invention exhibits high pure water flux and Na2SO4 desalination rate under a pressure of 3 bar, and a low NaCl desalination rate; compared with the hollow fiber nanofiltration membrane prepared in the comparative example, the sulfonated polyvinyl alcohol introduced in the present invention has a higher divalent salt removal rate.

[0031] The above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that are not conceived through creative work should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope defined in the claims.

Claims

1. A reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and a preparation method thereof, characterized in that: The steps include: S1, dissolving a certain amount of polyvinyl alcohol and polyvinylidene fluoride in dimethylacetamide solvent to prepare a membrane casting solution, and preparing a braided tube reinforced hollow fiber base membrane through a phase inversion spinning process; S2, at room temperature, a certain amount of sulfonated polyvinyl alcohol is dissolved in pure water to prepare a coating solution A, and the hollow fiber membrane prepared in S1 is soaked in the coating solution A, and after a certain period of time, it is taken out, washed with pure water, and dried to obtain a hollow fiber base membrane loaded with sulfonated polyvinyl alcohol; S3, at room temperature, dissolving a certain amount of epichlorohydrin in a sodium hydroxide solution to prepare a solution B, soaking the hollow fiber-based membrane loaded with sulfonated polyvinyl alcohol which has been soaked in the coating solution A in the solution B, and taking out and draining the solution B after a certain period of time; S4. The hollow fiber base membrane that has been immersed in solution B is heat-treated at a certain temperature, and after a certain period of time, it is taken out and immersed in pure water.

2. The method for preparing a polyvinylidene fluoride hollow fiber nanofiltration membrane with a braided tube according to claim 1, characterized in that: The mass percentage concentration of polyvinyl alcohol in the casting solution is 2.0-4.0 wt %.

3. The reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and the preparation method thereof according to claim 1, characterized in that: The sulfonation degree of the sulfonated polyvinyl alcohol is 10% to 25%.

4. The reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and the preparation method thereof according to claim 1, characterized in that: The mass percentage concentration of sulfonated polyvinyl alcohol in the coating solution A is 2.0-8.0 wt %, and the immersion time of the hollow fiber-based membrane in the coating solution A is 30-60 min.

5. The reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and the preparation method thereof according to claim 1, characterized in that: The hollow fiber-based membrane is immersed in the coating solution A for 30-60 minutes.

6. The reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and the preparation method thereof according to claim 1, characterized in that: The mass percentage concentration of epichlorohydrin in the solution B is 2.0-8.0 wt %, and the concentration of the sodium hydroxide solution is 1.0-1.5 mol / L.

7. The reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and the preparation method thereof according to claim 1, characterized in that: The immersion time of the hollow fiber-based membrane loaded with sulfonated polyvinyl alcohol after being immersed in the coating solution A in the solution B is 10-30 min.

8. The reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and the preparation method thereof according to claim 1, characterized in that: The heat treatment temperature is 50-80 ℃, and the heat treatment time is 3-10 min.

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