Method for preparing ultrafiltration membrane by taking polyphenol monomer as segregation agent and application of ultrafiltration membrane

Abstract

The invention discloses a preparation method of an ultrafiltration membrane using a polyphenol monomer as a segregation agent, which comprises the following steps: adding polyvinylidene fluoride and the polyphenol monomer into an organic solvent to form a membrane casting solution; after standing and defoaming, blade-coating the surface of a glass plate with the membrane casting solution; and immersing the glass plate coated with the membrane casting solution in a Tris-HCl buffer solution coagulating bath to form a membrane, and then immersing the membrane in ultrapure water for preservation to obtain the ultrafiltration membrane taking the polyphenol monomer as the segregation agent. In the preparation process, in the non-solvent induced phase separation process, polyphenol monomers rich in phenolic hydroxyl groups are subjected to surface segregation due to the hydrophilic action and are subjected to self-agglutination in an alkaline environment, so that the anti-pollution performance mechanism is enhanced. And the hydrophilicity and underwater super-oleophobic modification of the hydrophobic polyvinylidene fluoride membrane are realized by segregation enhancement caused by the self-polymerization effect. The prepared ultrafiltration membrane is endowed with surface hydrophilic-underwater superoleophobic characteristics, and also shows long-term structural stability and excellent anti-pollution capacity to different oils.

Description

Technical Field

[0001] The invention relates to a membrane preparation method, in particular to a preparation method and application of an oil-water separation ultrafiltration membrane. Background Art

[0002] Membrane fouling hinders the development of ultrafiltration membrane technology, especially in oil-water separation. Oily wastewater comes from a wide range of sources and in large quantities, and is in urgent need of effective treatment. Ultrafiltration membranes are easy to operate and have low energy consumption, but are easily blocked by oil droplets, affecting flux. The development of high-throughput, highly anti-fouling ultrafiltration membranes is crucial to improving efficiency, reducing costs and protecting water resources.

[0003] Surface segregation has become an effective method for constructing anti-fouling membrane surfaces. During the surface segregation process, polymer segregation agents with hydrophilic chains in the casting solution will migrate and enrich to the membrane surface, and the hydrophobic-hydrophobic interaction between the hydrophobic chain segments and the membrane body will play an anchoring role. The hydrophilic membrane surface thus formed can form a hydration layer and a steric hindrance effect, thereby giving the membrane a fouling resistance mechanism. The key to the surface segregation method is to design amphiphilic segregation agents in a simple and controllable manner. At present, the amphiphilic segregation agents mainly used for anti-fouling modification of membranes include polyethylene glycol (PEG) and Pluronic F127. However, due to the limited water binding capacity of these amphiphilic segregation agents, a strong fouling resistance mechanism cannot be effectively established, and the flux recovery rate (FRR) of the resulting membrane is usually low.

[0004] Polyphenol monomer is a natural organic matter extracted from plants. It is rich in phenolic hydroxyl groups such as catechol and pyrogallol, and can combine with water to form a high-density hydrogen bond network. In addition, it undergoes self-polymerization under weak alkaline conditions and has strong adhesion. Therefore, it is often used as a functional coating to modify the hydrophilicity of the membrane and improve the membrane performance. However, similar to other post-modification methods, coating polyphenol monomers on the surface of the substrate membrane will cause the membrane pores to be blocked, resulting in reduced membrane permeability. Summary of the invention

[0005] In view of the above-mentioned prior art, the present invention provides a method for preparing an ultrafiltration membrane using a polyphenol monomer as a segregation agent. In the preparation method, a polyphenol monomer rich in hydrophilic groups, such as tannic acid, proanthocyanidins and (-)-catechin esters, is used as a segregation agent and is added into a casting solution. The prepared ultrafiltration membrane has a high flux recovery rate (FRR) and a low total resistance change rate, and has an excellent anti-pollution effect.

[0006] In order to solve the above technical problems, the present invention proposes a method for preparing an ultrafiltration membrane using a polyphenol monomer as a segregation agent, comprising: adding polyvinylidene fluoride (PVDF) and polyphenol monomer powder into an organic solvent, wherein the mass percentage of the polyvinylidene fluoride is A, A=12%, the mass percentage of the polyphenol monomer is B, B=(0.1-0.4)A, and the sum of the mass percentages of the polyvinylidene fluoride, the polyphenol monomer and the organic solvent is 100%; stirring the mixture until it is completely dissolved to form a casting solution, and standing it for degassing; heating the mixture at 10 g / 100 cm 2 The casting liquid is scraped on the surface of a glass plate; the glass plate coated with the casting liquid is immersed in a Tris-HCl buffer coagulation bath to form a film, and the membrane is immersed in ultrapure water for washing to obtain an ultrafiltration membrane.

[0007] Furthermore, in the method for preparing an ultrafiltration membrane, in the casting solution, the amount of the polyphenol monomer accounts for 40% of the amount of the polyvinylidene fluoride.

[0008] The polyphenol monomer is one of tannic acid (TA), proanthocyanidin and (-)-catechin ester, preferably tannic acid (TA).

[0009] The organic solvent is one of dimethyl sulfoxide and N-methylpyrrolidone, preferably dimethyl sulfoxide.

[0010] The molar concentration of the Tris-HCl buffer is 0.05 mol / L, pH=8.5, the coagulation bath temperature is 25° C., and the film formation time is 3 h.

[0011] Compared with the prior art, the advantages of the present invention are:

[0012] In the preparation method of the present invention, polyphenol monomers are proposed as segregation agents to form a high-density hydrogen bond network to achieve membrane hydrophilic-underwater superoleophobic modification and enhance anti-pollution performance; during the preparation process, in the non-solvent induced phase separation (NIPS) process, the polyphenol monomers self-polymerize due to the influence of the alkaline environment, and at the same time, surface segregation and pore formation occur due to the hydrophilic effect, thereby improving the membrane flux and anti-pollution performance.

[0013] The ultrafiltration membrane prepared by the present invention has the ability to resist pollution of different types of oils and for a long period of time; the ultrafiltration membrane prepared by the preparation method of the present invention using polyphenol monomer as a segregation agent has a pure water flux of 97.1L m -2 h -1 bar -1 ~426.9L m -2 h -1 bar -1The total flux reduction rate is 5.71%, and the flux recovery rate is 96.8% to 100%, as shown in Figures 5(a) and 5(b), which imparts hydrophilic modification and underwater superoleophobic properties. The ultrafiltration membrane prepared by the present invention is applied to oil-water separation and has excellent anti-pollution effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A schematic diagram of the preparation process of the ultrafiltration membrane of the present invention is shown.

[0015] Figure 2 The test results of various properties of the polyphenol monomer and ultrafiltration membrane of the present invention are shown, wherein a is the Fourier transform infrared spectrum of the polyphenol monomer (TA) and PVDF / TA-40% membrane and PVDF / TA-0% membrane; b is the Fourier transform infrared spectrum of the PVDF / TA-x% membrane.

[0016] Figure 3 The surface morphology and cross-sectional morphology of the ultrafiltration membranes prepared in the embodiments and comparative examples of the present invention are shown, wherein a1-e1 are surface morphology images of each ultrafiltration membrane; a2-e2 are cross-sectional morphology images of each ultrafiltration membrane.

[0017] Figure 4 This is the surface morphology of the PVDF / TA-50% membrane prepared in Comparative Example 3;

[0018] Figure 5(a) to Figure 5(f) The pure water flux, flux recovery and total resistance change rate of the ultrafiltration membranes prepared in the embodiments of the present invention and the comparative examples are shown respectively, wherein:

[0019] FIG5( a ) is a diagram of pure water flux of each ultrafiltration membrane;

[0020] Figure 5(b) shows the flux recovery rate FRR and total resistance change rate DR of each ultrafiltration membrane t picture;

[0021] Figure 5(c) compares the long-term anti-fouling ability of ultrafiltration membranes using polyphenol segregation agents and traditional segregation agents;

[0022] Figure 5(d) is a graph showing the flux changes of PVDF / PEG-40% membrane for four oil stains;

[0023] Figure 5(e) is a graph showing the flux variation of PVDF / TA-40% membrane for four types of oil pollution;

[0024] Figure 5(f) shows the flux recovery rate FRR and total resistance change rate DR of the ultrafiltration membrane with polyphenol segregation agent and traditional segregation agent t Retention rate graph. DETAILED DESCRIPTION

[0025] The design concept of the method for preparing ultrafiltration membrane using polyphenol monomer as segregation agent proposed in the present invention is that, inspired by the ability of polyphenol monomer to form a high-density hydrogen bond network, polyphenol monomer is used as a new type of surface segregation agent for preparing ultrafiltration membrane for oil-water separation. Polyphenol monomer is added to the casting solution, Tris-HCl buffer is used as a coagulation bath, and a hydration layer is formed on the membrane surface to strengthen the pollution resistance mechanism. In the non-solvent induced phase separation (NIPS) process, polyphenol monomer rich in phenolic hydroxyl groups undergoes surface segregation due to hydrophilic effect, and self-polymerizes in an alkaline environment, thereby enhancing the anti-pollution performance mechanism. The segregation enhancement caused by self-polymerization realizes the hydrophilicity and underwater superoleophobic modification of the hydrophobic polyvinylidene fluoride membrane. The water flux of the ultrafiltration membrane prepared by the method of the present invention can be as high as 426.91L m -2 h -1 bar -1 The total flux decline rate was 5.71%, and the flux recovery rate was 100%. In addition, the membrane also showed long-term structural stability and excellent anti-fouling ability to different oils.

[0026] The method for preparing an ultrafiltration membrane using a polyphenol monomer as a segregation agent proposed by the present invention comprises the following steps: adding polyvinylidene fluoride and polyphenol monomer powders into an organic solvent, wherein the mass percentage of the polyvinylidene fluoride is 12%, the mass percentage of the polyphenol monomer is 10-40%, preferably 40%, of the polyvinylidene fluoride, and the sum of the mass percentages of the polyvinylidene fluoride, the polyphenol monomer and the organic solvent is 100%; stirring the mixture until it is completely dissolved to form a casting liquid, wherein the stirring conditions are: a stirring temperature of 80°C, a stirring speed of 700 rpm, and a stirring time of 80 min; standing for degassing, wherein the degassing conditions are: placing the casting liquid at 60°C for 40 min to remove bubbles; and heating the casting liquid at 10 g / 100 cm 2 The casting liquid is scraped on the surface of a glass plate; the glass plate coated with the casting liquid is immersed in a Tris-HCl buffer coagulation bath to form a film, the coagulation bath temperature is 25° C., and the film formation time is 3 hours; the membrane is immersed in ultrapure water for washing to obtain an ultrafiltration membrane.

[0027] In the present invention, the polyphenol monomer is one of tannic acid, proanthocyanidin and (-)-catechin ester. The organic solvent is one of dimethyl sulfoxide and N-methylpyrrolidone, and the organic solvent includes but is not limited to dimethyl sulfoxide.

[0028] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention in any way.

[0029] Example 1

[0030] An oil-water separation ultrafiltration membrane was prepared using a non-solvent induced phase separation method. Figure 1As shown in a, the preparation steps of the ultrafiltration membrane are as follows:

[0031] Step 1) 1.2 g of polyvinylidene fluoride (PVDF) particles and 0.12 g of tannic acid (TA) powder were dissolved in 8.68 g of dimethyl sulfoxide (DMSO) solution, and stirred continuously at 80° C. and 700 rpm for 80 min until they were completely dissolved to form a casting solution.

[0032] Step 2) Place the casting solution in an oven at 60°C for 40 minutes to remove bubbles, and then quickly pour the casting solution on the top of a glass plate with an area of ​​100 cm 2 , use a steel scraper to evenly apply the solution from top to bottom to completely cover the surface of the glass plate.

[0033] Step 3) The glass plate was immersed in a 25°C Tris-HCl buffer coagulation bath with a pH of 8.5 for 3 hours to form a membrane, and then the membrane was peeled off the glass plate. The obtained membrane was immersed in ultrapure water for 24 hours, during which the water was changed three times to obtain an oil-water separation ultrafiltration membrane, which is referred to as a PVDF / TA-10% membrane. Figure 3 b1 and b2 in the figure show the surface morphology and cross-sectional morphology of the PVDF / TA-10% membrane, respectively.

[0034] Figure 1 Figure b shows that in the process of non-solvent induced phase separation (NIPS), polyphenol monomers are surface segregated due to their rich phenolic hydroxyl groups and hydrophilicity. At the same time, under the influence of a slightly alkaline environment, polyphenol monomers undergo self-polymerization to generate nanospheres attached to the membrane surface, achieving hydrophilicity and underwater superoleophobic modification.

[0035] Example 2

[0036] An oil-water separation ultrafiltration membrane was prepared using a non-solvent induced phase separation method. The preparation process of Example 2 was basically the same as that of Example 1, except that in step 1), the amount of TA powder was changed from 0.12 g to 0.24 g, and the amount of dimethyl sulfoxide solution was changed from 8.68 g to 8.56 g. The ultrafiltration membrane finally prepared was referred to as PVDF / TA-20% membrane. Figure 3 Figures c1 and c2 show the surface morphology and cross-sectional morphology of the PVDF / TA-20% membrane, respectively.

[0037] Example 3

[0038] An oil-water separation ultrafiltration membrane was prepared using a non-solvent induced phase separation method. The preparation process of Example 3 was basically the same as that of Example 1, except that in step 1), the amount of TA powder was changed from 0.12 g to 0.36 g, and the amount of dimethyl sulfoxide solution was changed from 8.68 g to 8.44 g. The ultrafiltration membrane finally prepared was referred to as PVDF / TA-30% membrane. Figure 3 Figures d1 and d2 show the surface morphology and cross-sectional morphology of the PVDF / TA-30% membrane, respectively.

[0039] Example 4

[0040] An oil-water separation ultrafiltration membrane was prepared using a non-solvent induced phase separation method. The preparation process of Example 3 was basically the same as that of Example 1, with the only difference being that in step 1), the amount of TA powder was changed from 0.12 g to 0.48 g, and the amount of dimethyl sulfoxide solution was changed from 8.68 g to 8.32 g. The ultrafiltration membrane finally prepared was referred to as PVDF / TA-40% membrane. Figure 3 Figures e1 and e2 show the surface morphology and cross-sectional morphology of the PVDF / TA-30% membrane, respectively.

[0041] Comparative Example 1

[0042] An oil-water separation ultrafiltration membrane was prepared using a non-solvent induced phase separation method. The preparation process of Comparative Example 1 was basically the same as that of Example 1, except that in step 1), when preparing the casting solution, no TA powder was added, and only 1.2 g of polyvinylidene fluoride (PVDF) particles were dissolved in 8.8 g of dimethyl sulfoxide solution. The ultrafiltration membrane finally prepared was referred to as PVDF / TA-0% membrane. Figure 3 a1 and a2 show the surface morphology and cross-sectional morphology of the PVDF / TA-0% membrane, respectively. It can be seen that the surface of the PVDF / TA-0% membrane is relatively flat and there are almost no membrane pores, and its cross-sectional morphology presents a typical finger-like pore structure.

[0043] Comparative Example 2

[0044] An oil-water separation ultrafiltration membrane was prepared using a non-solvent induced phase separation method. The preparation process of Comparative Example 2 was basically the same as that of Example 1, except that in step 1), when preparing the casting solution, the segregation agent used was changed from 0.12g of TA powder to 0.48g of polyethylene glycol (PEG), that is, 1.2g of PVDF particles and 0.48g of PEG were dissolved in 8.32g of dimethyl sulfoxide solution. The ultrafiltration membrane finally prepared was referred to as PVDF / PEG-40% membrane.

[0045] Comparative Example 3

[0046] An oil-water separation ultrafiltration membrane was prepared using a non-solvent induced phase separation method. The preparation process of Comparative Example 3 was basically the same as that of Example 1, except that in step 1), the amount of TA powder was changed from 0.12 g to 0.6 g, and the amount of dimethyl sulfoxide solution was changed from 8.68 g to 8.2 g. The ultrafiltration membrane finally prepared was referred to as PVDF / TA-50% membrane. Figure 4 The surface morphology of the PVDF / TA-50% membrane is shown.

[0047] In order to clearly distinguish the difference between this embodiment and the comparative example, the casting solution formula and coagulation bath required for the ultrafiltration membrane prepared in the above embodiment and the comparative example are listed in Table 1.

[0048] Table 1 Ultrafiltration membranes prepared in various embodiments and comparative examples and the required casting solution formula and coagulation bath composition

[0049]

[0050] The performance of the oil-water separation ultrafiltration membranes prepared in the examples and comparative examples is evaluated by characterization and testing.

[0051] 1) The PVDF / TA-40% membrane prepared in Example 4 and the PVDF / TA-0% membrane prepared in Comparative Example 1 were subjected to infrared spectroscopy test. The results are as follows: Figure 2 As shown in b.

[0052] Fourier transform infrared spectroscopy (FT-IR) was used to demonstrate the successful introduction of polyphenol monomers. The FT-IR spectra of the prepared membrane PVDF / TA-40% membrane and polyphenol monomer TA are shown in Figure 2. Figure 2 The comparison results show that the PVDF / TA-40% membrane has a wavelength of 3100-3600 cm -1 and 1500-1750cm -1 There are broad peaks at 1400cm -1 and 1170cm -1 The peaks at represent the CH2 and CF2 groups of PVDF. Figure 2 In figure b, with the increase of polyphenol monomer content (PVDF / TA-0%, PVDF / TA-10%, PVDF / TA-20%, PVDF / TA-30%, PVDF / TA-40%), the corresponding OH and C=C peaks also gradually enhanced, proving the successful introduction of polyphenol monomers.

[0053] 2) The surface morphology of the oil-water separation ultrafiltration membranes prepared in the above examples and comparative examples was observed using a scanning electron microscope. Figure 3 and Figure 4 shown.

[0054] Scanning electron microscopy (SEM) was used to observe the surface morphology of the membrane. Figure 3 As shown in the figure, the surface of the PVDF / TA-0% membrane is relatively flat and almost has no membrane pores, and its cross-sectional morphology shows a typical finger-like pore structure. Figure 3As the content of b1, c1, d1, and e1 in the figure gradually increases, the number of holes on the membrane surface gradually increases and becomes rougher. This phenomenon proves that the polyphenol monomer has a pore-forming effect. The finger-like pores in the membrane cross section gradually decrease, while the sponge pores increase. This is because the increase in the viscosity of the system makes the film-forming process mainly dominated by the kinetic characteristics, which leads to the slowing of the phase separation process and the formation of sponge-like pores. However, if the polyphenol monomer content is greater than 40%, the pore size of the prepared membrane is too large, resulting in defects in the membrane, such as Figure 4 As shown, the oil-in-water emulsion cannot be effectively retained. Therefore, in the preparation method of the present invention, the content of polyphenol monomer is limited to 0% to 40%.

[0055] 3) The pure water flux results of the oil-water separation ultrafiltration membranes prepared in the comparative examples and examples are shown in FIG5( a ).

[0056] The pure water flux of the PVDF / TA-0% membrane prepared in Comparative Example 1 was about 28 L m -2 h -1 bar -1 This is because pure PVDF membrane does not contain segregation agent, and the membrane surface is dense and has no holes. When the polyphenol monomer content increases to 10%, the pure water flux increases to 97.1L m -2 h -1 bar -1 As the content of polyphenol monomers increases, the flux gradually increases. The water permeability of the PVDF / TA-40% membrane prepared in Example 4 is 426.9 L m -2 h -1 bar -1 This is because the polyphenol monomer acts as a segregation agent, which plays a pore-forming role and leads to an increase in the pure water flux.

[0057] 4) The anti-pollution effect of the oil-water separation ultrafiltration membrane prepared in the above comparative examples and examples and the ultrafiltration membrane prepared in the examples have long-term structural stability.

[0058] 1) Flux recovery rate (FRR) and total resistance change rate (DR) of the oil-water separation ultrafiltration membranes prepared in the comparative examples and examples t ), as shown in Figure 5(b).

[0059] FRR and DR of the PVDF / TA-10% membrane prepared in Example 1 t The FRR of the membrane increased and the DRt decreased with the increase of the polyphenol monomer content. When the polyphenol monomer content was 40%, the antifouling performance of the membrane was the best. The FRR of the PVDF / TA-40% membrane prepared in Example 4 was 100%, and the DRt was 11.5%. tThe FRR and DRt of Comparative Example 2 were 73.9% and 39.4%, respectively, when polyethylene glycol was used as a segregation agent, showing a poor anti-pollution effect.

[0060] 2) As shown in Figure 5(c), in order to further compare the anti-fouling performance of the two segregation agents (polyphenol monomer and polyethylene glycol), a long-term anti-fouling performance test was performed. -2 h - 1 bar -1 Compared with the initial flux, the initial flux of the PVDF / TA-40% membrane prepared in Example 4 is higher. And even after three pump oil cycles, the FRR of Example 4 can still be maintained at 100%.

[0061] 3) Excellent anti-pollution ability against different oils. As shown in FIG. 5(d), FIG. 5(e) and FIG. 5(f), vacuum pump oil, hexadecane oil, soybean oil and silicone oil emulsions were used to compare and prove the anti-pollution versatility of the PVDF / TA-40% membrane prepared in Example 4. The PVDF / TA-40% membrane showed excellent interception and anti-fouling performance against these four pollutants, proving the versatility of the membrane. The anti-pollution effect of the PVDF / PEG-40% membrane prepared in Comparative Example 2 was poor. As for the separation mechanism, the polyphenol monomer is rich in phenolic hydroxyl groups, and self-polymerization in an alkaline environment enhances the surface segregation process. The large amount of phenolic hydroxyl groups on the membrane surface enhances the interaction between water molecules and the surface, allowing the membrane to form a dense hydration layer on the surface. This strong physical barrier enhances the membrane's anti-pollution mechanism.

[0062] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments, which are merely illustrative rather than restrictive. Under the guidance of the present invention, ordinary technicians in this field can make many improvements and changes without departing from the purpose of the present invention, which are all within the protection of the present invention.

Claims

1. A method for preparing an ultrafiltration membrane using polyphenol monomers as a segregation agent, characterized in that it comprises: Adding polyvinylidene fluoride and polyphenol monomer powder into an organic solvent, wherein the mass percentage of the polyvinylidene fluoride is A, A=12%, the mass percentage of the polyphenol monomer is B, B=(0.1-0.4)A, and the sum of the mass percentages of the polyvinylidene fluoride, the polyphenol monomer and the organic solvent is 100%; stirring the mixture until it is completely dissolved to form a casting solution, and standing it for degassing; heating the mixture at 10 g / 100 cm 2 The casting liquid is scraped on the surface of a glass plate; the glass plate coated with the casting liquid is immersed in a Tris-HCl buffer coagulation bath to form a film, and the membrane is immersed in ultrapure water for washing to obtain an ultrafiltration membrane.

2. The method for preparing an ultrafiltration membrane according to claim 1, characterized in that: B = 0.4A.

3. The method for preparing an ultrafiltration membrane according to claim 1, characterized in that: In the casting solution, the polyphenol monomer is one of tannic acid, proanthocyanidin and (-)-catechin ester.

4. The method for preparing an ultrafiltration membrane according to claim 1, characterized in that: The organic solvent is one of dimethyl sulfoxide and N-methylpyrrolidone.

5. The method for preparing an ultrafiltration membrane according to claim 1, characterized in that: The molar concentration of the Tris-HCl buffer is 0.05 mol / L, pH=8.5, the coagulation bath temperature is 25° C., and the film formation time is 3 h.

6. An ultrafiltration membrane using polyphenol monomer as a segregation agent, characterized in that: The ultrafiltration membrane is prepared by the method for preparing an ultrafiltration membrane according to any one of claims 1 to 5, wherein the pure water flux of the ultrafiltration membrane is 97.1 L m -2 h -1 bar -1 ~426.9L m -2 h -1 bar -1 , the flux recovery rate is 96.8% to 100%.

7. An application of an ultrafiltration membrane, characterized in that: The ultrafiltration membrane is the ultrafiltration membrane using polyphenol monomer as a segregation agent as described in claim 6, and the ultrafiltration membrane is applied to oil-water separation.

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