An enhanced polyvinylidene fluoride hollow fiber nanofiltration membrane and a preparation method thereof

Abstract

The application discloses a reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and a preparation method thereof, and particularly relates to a reinforced hollow fiber composite nanofiltration membrane with sulfonated polyvinyl alcohol as a functional separation layer and a preparation method thereof. The application introduces polyvinyl alcohol into polyvinylidene fluoride to prepare a reinforced hollow fiber base membrane through phase inversion, then constructs a crosslinked sulfonated polyvinyl alcohol separation layer on the surface of the hollow fiber base membrane through surface coating and synchronous crosslinking, and finally obtains the reinforced hollow fiber composite nanofiltration membrane. The introduction of polyvinyl alcohol into the hollow fiber base membrane ensures the uniformity of the coating of the sulfonated polyvinyl alcohol, and the synchronous crosslinking of the polyvinyl alcohol in the base membrane and the surface sulfonated polyvinyl alcohol can obtain a thin and uniform defect-free separation layer. The prepared hollow fiber nanofiltration membrane has high permeation flux and sodium sulfate retention rate under a pressure of 3 bar. The preparation method of the application couples the phase inversion and coating crosslinking processes, and is simple and easy to industrialize.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high molecular separation membrane preparation, in particular to an enhanced polyvinylidene fluoride hollow fiber nanofiltration membrane and a preparation method thereof. BACKGROUND

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

[0003] At present, the roll type nanofiltration membrane occupies the dominant position in the nanofiltration membrane market due to the advantage of simple preparation process, but the configuration of the roll type nanofiltration membrane module has limitations in the internal space design and flow channel distribution, which easily leads to: 1) the space occupied by the internal components such as the spacer, the water collecting pipe and the like limits the membrane packing area, and the water flux is difficult to improve, and only the operating energy consumption can be increased by increasing the pressure compensation; 2) the spacer between the membranes hinders the fluid flow, affects the flow channel openness, causes membrane pollution during long-term operation, and causes irreversible decay of membrane performance; for example: the invention patent CN 113083033 A, compared with the hollow fiber nanofiltration membrane, has unique advantages: 1) high packing density, the three-dimensional structure makes it have a large specific surface area, and improves the separation efficiency; 2) the negative effects caused by membrane pollution can be reduced by backwashing, and it is suitable for treating feed liquid with higher solid suspended matter concentration, and reduces the necessity of pretreatment of the raw liquid; 3) the self-supporting structure reduces the demand for accessories and simplifies the processing process of the module, and reduces the cost.

[0004] Hollow fiber nanofiltration membrane has the advantages of high water production and backwashing compared with flat-rolled nanofiltration membrane, and has been applied in the fields of high-quality drinking water purification and wastewater reuse. At present, the preparation methods of hollow fiber nanofiltration membrane mainly include one-step phase inversion method for preparing hollow fiber nanofiltration membrane and interface polymerization or coating process for preparing composite nanofiltration membrane; 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 low-cut molecular weight small-pore nanofiltration membrane cannot be prepared; although the interface polymerization process can prepare high-flux and small-pore composite nanofiltration membrane, the problem is that the primary solidified hollow fiber membrane needs to be wiped off the surface floating liquid after being treated with aqueous solution of polyamine and organic solution of polyacyl chloride, and the residual amount of the surface floating liquid directly affects the interface polymerization reaction effect of the polyacyl chloride organic solution, which affects the formation of nanoscale pores. At the same time, the unstable concentration of polyamine and polyacyl chloride in industrialization causes uneven pore size formation and easy layering of the membrane, and the composite nanofiltration membrane is not resistant to chlorine and has a short service life, such as the invention patent CN 109603588 A. In addition, compared with the rolled nanofiltration membrane, the current homogeneous hollow fiber nanofiltration membrane has the problems of poor strength and easy breaking. Therefore, as the future direction of industrial and application development, it is of great significance to develop new hollow fiber nanofiltration membrane and its preparation technology from the aspects of membrane material, membrane structure and membrane preparation method.

[0005] Nanofiltration membranes with sulfonic acid type groups not only provide a large number of negatively charged groups to improve the rejection rate of multivalent anions, but also give the nanofiltration membrane good chlorine resistance. Chen Yu-hai et al. prepared nanofiltration membranes with asymmetric structure by blending polyether sulfone and sulfonated polysulfone through one-step phase inversion method; such as the invention patent CN 101979132 A, Ouyang Guihui et al. prepared hollow fiber nanofiltration membranes with sulfonated polyether sulfone coating by dry-jet wet spinning method and coating process; such as the invention patent CN 103638822 A. Compared with sulfonated polysulfone polymers, sulfonated polyvinyl alcohol has lower preparation cost. Therefore, it is necessary to develop a simple and controllable preparation of high-performance enhanced hollow fiber composite nanofiltration membrane and its preparation method. SUMMARY

[0006] The present application aims at the technical problems of low permeation flux of hollow fiber nanofiltration membrane prepared by phase inversion method, difficulty in preparing low-cut molecular weight nanofiltration membrane, poor controllability and difficulty in continuous preparation of hollow fiber nanofiltration membrane prepared by interface polymerization method, and poor strength of hollow fiber nanofiltration membrane, and proposes an enhanced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and its preparation method. The technical problem to be solved by the present application is how to realize the controllable preparation of high-strength and high-permeation-selectivity hollow fiber nanofiltration membrane by coupling phase inversion and coating cross-linking process.

[0007] The present application is realized by the following technical scheme: characterized by comprising the following steps:

[0008] S1, a certain amount of polyvinyl alcohol and polyvinylidene fluoride are dissolved in a solvent dimethylacetamide to prepare a casting solution, and a woven tube reinforced hollow fiber base film is prepared by a phase inversion spinning process;

[0009] S2, a certain amount of sulfonated polyvinyl alcohol is dissolved in pure water to prepare a coating solution A, the hollow fiber film prepared in S1 is soaked in the coating solution A, and after a certain time, it is taken out, washed with pure water and dried to prepare a hollow fiber base film loaded with sulfonated polyvinyl alcohol;

[0010] S3, a certain amount of epichlorohydrin is dissolved in sodium hydroxide solution to prepare solution B, and the hollow fiber base film loaded with sulfonated polyvinyl alcohol after soaking in the coating solution A is soaked in the solution B, and after a certain time, it is taken out and the solution B is drained;

[0011] S4, the hollow fiber base film soaked in the solution B is heat treated at a certain temperature, and after a certain time, it is taken out and soaked in pure water.

[0012] Preferably, the mass fraction of polyvinyl alcohol in the spinning casting solution for preparing the hollow fiber base film in step S1 is 2.0-4.0 wt%, and the mass fraction of polyvinylidene fluoride is 15.0-19.0 wt%.

[0013] Preferably, the sulfonated degree of the water-soluble sulfonated polyvinyl alcohol in the coating solution A in step S2 is 10%-25%, and the mass percentage concentration is 2.0-8.0 wt%.

[0014] Preferably, the soaking time of the hollow fiber base film in the coating solution A in step S2 is 30-60 min.

[0015] Preferably, the mass percentage concentration of epichlorohydrin in the solution B in step S3 is 2.0-8.0 wt%, and the concentration of sodium hydroxide solution is 1.0-1.5 mol / L.

[0016] Preferably, the soaking time of the hollow fiber base film loaded with sulfonated polyvinyl alcohol after soaking in the coating solution A in the solution B in step S3 is 10-30 min.

[0017] Preferably, the temperature for heat treating the hollow fiber base film soaked in the solution B in step S4 is 50-80℃, and the time is 3-10 min.

[0018] The beneficial effects of the present application are:

[0019] The present application aims to overcome the problems of low permeation flux of hollow fiber nanofiltration membrane prepared by phase inversion method, difficulty in preparing low molecular weight cut-off nanofiltration membrane, poor controllability and difficulty in continuous preparation of hollow fiber nanofiltration membrane prepared by interfacial polymerization method, and poor strength of hollow fiber nanofiltration membrane, and to improve the strength, high permeation selectivity and controllability of preparation of hollow fiber nanofiltration membrane, and proposes a kind of enhanced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and its preparation method. By adding polyvinyl alcohol into polyvinylidene fluoride casting solution, polyvinyl alcohol is used to inhibit polyvinylidene fluoride crystallization, and then an enhanced high porosity hollow fiber base film is prepared. Then, through surface coating of sulfonated polyvinyl alcohol functional material, crosslinking is carried out by using epoxy chloropropane, and a charged functional separation layer is constructed on the surface of the base film. Due to the introduction of polyvinyl alcohol in the hollow fiber base film, on the one hand, the uniformity of the sulfonated polyvinyl alcohol coating can be ensured, and at the same time, the polyvinyl alcohol in the base film and the surface sulfonated polyvinyl alcohol can be crosslinked synchronously, so that a thin and uniform composite separation layer without defects can be prepared, which can greatly improve the flux of the nanofiltration membrane while maintaining high selectivity. At the same time, the hollow fiber nanofiltration membrane of the present application has a higher permeation flux under low pressure driving and an energy saving effect under low pressure driving. Moreover, the preparation method of the present application couples the phase inversion and coating crosslinking process, which is simple and easy to industrialize. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below, but not limited to the protection scope of the present application.

[0021] Figure 1 A schematic diagram of the hollow fiber base film of the present application is shown in the figure.

[0022] Figure 2 A schematic diagram of the hollow fiber composite nanofiltration membrane of the present application is shown in the figure. DETAILED DESCRIPTION

[0023] Reference Figures 1-2 shown in the figure: EMBODIMENT

[0024] The present embodiment provides an enhanced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and its preparation method, which is described as follows:

[0025] (1) Preparation of casting solution: polyvinylidene fluoride and polyvinyl alcohol are added to dimethylacetamide (DMAc) and stirred to dissolve uniformly, wherein the mass concentration of polyvinylidene fluoride is 19 wt%, and the mass concentration of polyvinyl alcohol is 2 wt%, and the casting solution is obtained after standing and defoaming for more than 6 hours;

[0026] (2) Preparation of hollow fiber base membrane: under the conditions of constant temperature and constant pressure at 76 °C and 0.1 MPa, the obtained casting solution is uniformly coated on the gauze tube through the spinning nozzle, and the reinforced hollow fiber base membrane is prepared through solvent evaporation, coagulation and rinsing;

[0027] (3) Preparation of sulfonated polyvinyl alcohol: 15 g of polyvinyl alcohol (PVA) and 300 mL of distilled water are added to a 500 mL three-necked flask, and stirred in a 90 °C water bath until completely dissolved. After cooling to room temperature, excess concentrated sulfuric acid is slowly added dropwise to the PVA solution in an ice water bath environment, and the process is carried out under mechanical stirring. After the addition is completed, it is placed in a 40 °C water bath, and continues to stir for 5 h. The reaction mixture is then discharged into anhydrous ethanol, and white solids are precipitated. The product is washed with anhydrous ethanol several times until the pH value reaches 6.0. The product is dried to obtain sulfonated polyvinyl alcohol with a sulfonation degree of 20%;

[0028] (4) Preparation of coating solution A: a certain amount of sulfonated polyvinyl alcohol is dissolved in pure water at room temperature to obtain a coating solution A with a mass percentage concentration of 3.0 wt%;

[0029] (5) Preparation of solution B: a certain amount of epichlorohydrin is dissolved in 0.15 mol / L sodium hydroxide solution, and stirred at 40 °C for 3 h to obtain a solution B with a mass percentage concentration of 4.0 wt%;

[0030] (6) Hollow fiber base membrane soaking coating solution A: the cleaned hollow fiber base membrane is soaked in coating solution A at room temperature and normal pressure for 45 min, then taken out, washed with pure water and dried to obtain a hollow fiber base membrane loaded with sulfonated polyvinyl alcohol;

[0031] (7) Hollow fiber base membrane soaking solution B: the hollow fiber base membrane loaded with sulfonated polyvinyl alcohol treated with solution A and washed with pure water is soaked in modified solution B for 30 min, and the modified solution B is drained;

[0032] (8) Heat treatment: the hollow fiber base membrane treated with solution B is heat treated in modified solution B at 60 °C for 5 min to obtain a hollow fiber composite nanofiltration membrane. Example

[0033] The present embodiment also provides an enhanced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and a preparation method thereof, which are described as follows:

[0034] (1) Preparation of casting solution: polyvinylidene fluoride and polyvinyl alcohol are added to dimethylacetamide (DMAc) and stirred to dissolve uniformly, wherein the mass concentration of polyvinylidene fluoride is 19 wt%, and the mass concentration of polyvinyl alcohol is 2.0 wt%. After standing for more than 6 h, a casting solution is obtained.

[0035] (2) Preparation of hollow fiber base membrane: under the conditions of constant temperature and constant pressure at 76 ℃ and 0.1 MPa, the obtained casting solution is uniformly coated on the hook-knitted tube through a spinning nozzle, and a reinforced hollow fiber base membrane is prepared through solvent evaporation, coagulation and rinsing;

[0036] (3) Preparation of sulfonated polyvinyl alcohol: 20 g of polyvinyl alcohol (PVA) and 300 mL of distilled water are added to a 500 mL three-necked flask, and stirring is performed in a 90 °C water bath until complete dissolution. After cooling to room temperature, an excess of concentrated sulfuric acid is slowly added dropwise to the PVA solution in an ice-water bath environment, and the process is performed under mechanical stirring. After the addition is completed, it is placed in a 40 °C water bath, and stirring is continued. After 5 h, the reaction mixture is discharged into anhydrous ethanol, and white solids are precipitated. The product is washed with anhydrous ethanol several times until the pH value reaches 6.0, and then the product is dried to obtain sulfonated polyvinyl alcohol with a sulfonation degree of 15%;

[0037] (4) Preparation of coating solution A: a certain amount of sulfonated polyvinyl alcohol is dissolved in pure water at room temperature to obtain a coating solution A with a mass percentage concentration of 3.0 wt%;

[0038] (5) Preparation of solution B: a certain amount of epichlorohydrin is dissolved in a 0.15 mol / L sodium hydroxide solution to obtain a solution B with a mass percentage concentration of 4.0 wt% after stirring at 40 °C for 3 h;

[0039] (6) Immersion of hollow fiber base membrane in coating solution A: the cleaned hollow fiber base membrane is immersed in coating solution A at room temperature and normal pressure for 60 min, and then taken out, washed with pure water and dried to obtain a hollow fiber base membrane loaded with sulfonated polyvinyl alcohol;

[0040] (7) Immersion of hollow fiber base membrane in solution B: the hollow fiber base membrane loaded with sulfonated polyvinyl alcohol, which has been treated with solution A and washed with pure water, is immersed in modified solution B for 15 min, and the modified solution B is drained;

[0041] (8) Heat treatment: the hollow fiber base membrane treated with solution B is heat treated at 60 ℃ for 10 min using modified solution B to obtain a hollow fiber composite nanofiltration membrane.

[0042] Comparative Example 1:

[0043] The present comparative example provides a reinforced polyvinylidene fluoride hollow fiber composite nanofiltration membrane and a preparation method thereof, and the difference from Example 1 is that:

[0044] Step (3) is omitted;

[0045] (1) Preparing casting solution: polyvinylidene fluoride and polyvinyl alcohol were added into dimethylacetamide (DMAc) and stirred to be uniformly dissolved, wherein the mass concentration of polyvinylidene fluoride was 19 wt%, the mass concentration of polyvinyl alcohol was 2 wt%, and the casting solution was obtained after standing and defoaming for more than 6 h.

[0046] (2) Preparing hollow fiber base membrane: the obtained casting solution was uniformly coated on a crochet tube through a spinning nozzle under the conditions of constant temperature and constant pressure at 76 ℃ and 0.1 MPa, and a reinforced hollow fiber base membrane was prepared through solvent evaporation, coagulation and rinsing.

[0047] (3) Preparing coating solution A: a certain amount of polyvinyl alcohol was dissolved in pure water at room temperature to obtain coating solution A with a mass percentage of 3.0 wt% after stirring for 2 h.

[0048] (4) Preparing solution B: a certain amount of epichlorohydrin was dissolved in 0.15 mol / L sodium hydroxide solution to obtain solution B with a mass percentage of 4.0 wt% after stirring and reacting at 40 ℃ for 3 h.

[0049] (5) Immersing the hollow fiber base membrane in coating solution A: the cleaned hollow fiber base membrane was immersed in coating solution A at room temperature and normal pressure for 45 min, and then taken out, washed with pure water and dried to obtain a hollow fiber base membrane loaded with polyvinyl alcohol.

[0050] (6) Immersing the hollow fiber base membrane in solution B: the hollow fiber base membrane loaded with polyvinyl alcohol treated by solution A and washed with pure water was immersed in modified solution B for 30 min, and then the modified solution B was drained.

[0051] (7) Heat treatment: the hollow fiber base membrane treated by solution B was heat treated in modified solution B at 60 ℃ for 5 min to obtain a hollow fiber composite nanofiltration membrane.

[0052] Test example:

[0053] Test example 1:

[0054] In this test example, the hollow fiber base membrane and the prepared hollow fiber composite nanofiltration membrane of example 1 were analyzed by scanning electron microscopy, and it was found that, compared with the hollow fiber base membrane ( Figure 1 ), the surface of the prepared reinforced hollow fiber composite nanofiltration membrane ( Figure 1 ) was more dense.

[0055] Test example 2:

[0056] In this test example, the performance of the hollow fiber nanofiltration membranes prepared in example 1, example 2 and comparative example 1 was detected, and the detection method was as follows:

[0057] The separation performance (desalination rate (R) and water flux (J)) of the hollow fiber nanofiltration membrane of the present application was evaluated as follows:

[0058] A cross-flow experiment was adopted to test the 1000 ppm NaCl and Na2SO4 aqueous solution as the feed liquid under the conditions of 0.3 MPa, 25℃, and pH = 7.0 ± 0.2. The specific steps were as follows:

[0059] (1) The pure water flux of the hollow nanofiltration membrane was determined under the conditions of a pressure of 0.3 MPa and 25 ± 1℃ with pure water as the feed liquid, as the flux (J), reflecting the permeation performance of the membrane, J = V / (A·t). In the formula, V is the permeation liquid volume (L) on the permeation side; A is the effective area of the membrane (m2); and t is the permeation time (h).

[0060] (2) The desalination rate (R) of the hollow nanofiltration membrane was determined after 30 min of operation with 1000 ppm NaCl and Na2SO4 instead of pure water under the condition of maintaining the operating pressure unchanged, reflecting the separation performance of the membrane, R = (1-Cp / Cf) × 100%. In the formula, Cf and Cp are the concentrations of the solute component in the feed liquid and the permeation liquid, respectively.

[0061] Table 1: Separation performance of the hollow fiber nanofiltration membrane

[0062] membrane water flux (L / m2-h) Na2SO4desalination rate (%) NaCl desalination rate (%) hollow fiber-based 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

[0063] From the data in Table 1, it can be found that the hollow nanofiltration composite nanofiltration membrane of the present application exhibits high pure water flux, Na2SO4 desalination rate, and low NaCl desalination rate under a pressure of 3 bar; compared with the hollow fiber nanofiltration membrane prepared in the comparative example, the present application has higher divalent salt removal rate due to the introduction of sulfonated polyvinyl alcohol.

[0064] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, and any change or replacement without creative labor should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be limited by the protection scope defined in the claims.

Claims

1. A method for preparing an enhanced polyvinylidene fluoride hollow fiber composite nanofiltration membrane, characterized by: The method comprises the following steps: S1, a certain amount of polyvinyl alcohol and polyvinylidene fluoride are dissolved in a solvent dimethylacetamide to prepare a casting solution, and a braided tube reinforced hollow fiber base film is prepared by a phase inversion spinning process; S2, a certain amount of sulfonated polyvinyl alcohol is dissolved in pure water at room temperature to prepare a coating solution A, the hollow fiber film prepared in S1 is soaked in the coating solution A for a certain time, then taken out, washed with pure water and dried to prepare a hollow fiber base film loaded with sulfonated polyvinyl alcohol; the sulfonation degree of the sulfonated polyvinyl alcohol is 10%-25%; the mass percentage concentration of the sulfonated polyvinyl alcohol in the coating solution A is 2.0-8.0 wt%, and the soaking time of the hollow fiber base film in the coating solution A is 30-60 min; S3, a certain amount of epichlorohydrin is dissolved in a sodium hydroxide solution at room temperature to prepare a solution B, the hollow fiber base film loaded with sulfonated polyvinyl alcohol and treated by the coating solution A is soaked in the solution B for a certain time, then taken out and drained; S4, the hollow fiber base film treated by the solution B is subjected to heat treatment at a certain temperature for a certain time, then taken out and soaked in pure water.

2. The preparation method of the enhanced polyvinylidene fluoride hollow fiber composite nanofiltration membrane 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 preparation method of the enhanced polyvinylidene fluoride hollow fiber composite nanofiltration membrane 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.

4. The method of claim 1, wherein the enhanced polyvinylidene fluoride hollow fiber composite nanofiltration membrane is prepared by the steps of: a) providing a polyvinylidene fluoride hollow fiber membrane; b) coating the polyvinylidene fluoride hollow fiber membrane with a coating solution comprising a polymeric additive; and c) drying the polyvinylidene fluoride hollow fiber membrane. The soaking time of the hollow fiber base film loaded with sulfonated polyvinyl alcohol and treated by the coating solution A in the solution B is 10-30 min.

5. The method of claim 1, wherein the enhanced polyvinylidene fluoride hollow fiber composite nanofiltration membrane is prepared by the steps of: a) providing a polyvinylidene fluoride hollow fiber membrane; b) coating the polyvinylidene fluoride hollow fiber membrane with a coating solution comprising a polymeric additive; and c) drying the polyvinylidene fluoride hollow fiber membrane. The heat treatment temperature is 50-80 ℃, and the heat treatment time is 3-10 min.

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