Construction method of cellulose acetate nanofiltration membrane with pollution resistance and high separation performance

Abstract

The invention provides a construction method of a cellulose acetate nanofiltration membrane with anti-pollution and high separation performance, and the cellulose acetate nanofiltration membrane is prepared by taking tannic acid as a modification auxiliary agent and combining layer-by-layer self-assembly and interfacial polymerization technologies. The tannic acid has catechol and galloyl, and an adhesive layer formed after oxidation auto-polymerization can be adhered to the surfaces of various substrates. Chitosan and tannic acid are deposited on the surface of a membrane by utilizing a layer-by-layer self-assembly technology to construct an adhesive layer, and the adhesive layer and amino groups of a polyamide layer obtained by reacting PEPA and TMC are subjected to Michael addition and Schiff alkali reaction, so that the binding force between the polyamide layer and a substrate layer is enhanced, and a separation layer with a stable structure is constructed. The technology is simple to operate, effectively solves the problems that the cellulose acetate membrane is unstable in structure and easy to pollute, and is not applied to the cellulose acetate membrane.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of natural polymers, also to the fields of chemistry, forestry and environmental engineering, and particularly relates to a method for constructing a cellulose acetate nanofiltration membrane with anti-pollution and high separation performance. BACKGROUND

[0002] With the acceleration of industrialization process, the wide application of food processing, chemical production and seawater desalination technology, a large amount of high-concentration salt wastewater is produced, which poses a serious threat to the environment and human health. At the same time, the vigorous development of the textile industry has also brought about the problem of water pollution. Dyeing and printing wastewater is complex in composition and contains difficult-to-degrade dyes and high-valence salt ions. In order to effectively treat dyeing and printing wastewater and obtain clean water resources, a variety of methods for treating dyeing and printing wastewater have been developed internationally, including adsorption, photocatalytic degradation and chemical degradation, biodegradation, membrane filtration. Among them, nanofiltration membrane filtration technology has outstanding advantages such as simple operation, high energy efficiency and environmental friendliness, and has been widely applied in many treatment methods.

[0003] At present, interfacial polymerization is one of the methods for preparing nanofiltration membranes. This preparation process uses a double monomer system: the water phase uses a monomer containing amino groups, such as piperazine (PIP), polyethyleneimine (PEI) or m-phenylenediamine (MPD), etc., and the organic phase uses a monomer containing acyl chloride functional groups, such as trimesoyl chloride (TMC) or the like. The organic phase and the water phase monomers undergo a polymerization reaction at the membrane interface to form a dense polyamide separation layer. According to the mechanism of interfacial polymerization reaction, the reaction monomers have a decisive influence on the structure and separation performance of the polymerization layer. Therefore, appropriate monomers must be selected to prepare a polymerization layer with excellent performance.

[0004] Polyethylene polyamine (PEPA) is a hydrophilic compound with high amino density, which is widely used in the fields of polymer materials, petroleum industry and environmental engineering due to its excellent surface activity and thermal stability. Compared with polyethyleneimine (PEI) and tetraethyl pentanediamine (TEPA), PEPA has a significant cost advantage while maintaining similar reactivity. In the field of membrane separation technology, PEPA can form a dense polyamide separation layer through interfacial polymerization with trimesoyl chloride (TMC), which makes it have important application value in the preparation of nanofiltration membranes and can effectively improve the separation performance and water flux of the membrane. The multiple active amino sites in its molecular structure provide a guarantee for realizing efficient interfacial cross-linking reaction. SUMMARY

[0005] The nanofiltration membrane prepared by the interfacial polymerization method in the prior art is easy to fall off under the scouring of water flow because of no stable chemical bond between the polymerization layer and the substrate. Inspired by the marine mussel, polydopamine (PDA) is obtained by oxidation polymerization of dopamine (DA) and is applied to surface modification, so that the polyamide layer and the substrate are tightly combined due to the adhesion property. However, DA has limitations of easy oxidation and high cost from the perspective of industrial application, which restricts its large-scale production and application.

[0006] In the present study, tannic acid is used as a modification auxiliary agent to prepare a cellulose acetate nanofiltration membrane by combining layer-by-layer self-assembly and interfacial polymerization technology. Tannic acid (TA) has catechol and galloyl groups, and the adhesive layer formed after oxidation self-polymerization can adhere to the surface of various substrates. By using layer-by-layer self-assembly technology, chitosan and TA are deposited on the membrane surface to construct an adhesive layer, and Michael addition and Schiff base reaction occur between the amino groups of the polyamide layer obtained by the reaction of PEPA and TMC, thereby enhancing the bonding force between the polyamide layer and the substrate layer and constructing a structure-stable separation layer.

[0007] The present application aims to provide a method for constructing a cellulose acetate nanofiltration membrane with anti-pollution and high separation performance.

[0008] To achieve the above-mentioned purpose, the present application adopts the following technical solutions: A method for constructing a cellulose acetate nanofiltration membrane with anti-pollution and high separation performance, comprising the following steps: (1) Place the cellulose acetate ultrafiltration base membrane in a mold, soak the membrane surface with chitosan solution for 1 hour, then pour out the chitosan solution on the mold, pour tannic acid solution into the mold, and continue to soak for 1 hour; repeat the soaking operation of first chitosan solution and then tannic acid solution for 1-5 times to complete the layer-by-layer self-assembly process; (2) Prepare a PEPA aqueous solution, pour it onto the membrane that has completed the layer-by-layer self-assembly, ensure that the membrane surface is completely soaked with the solution, after soaking for 30 minutes, pour out the remaining liquid, and after there is no residual liquid droplets on the membrane surface, wait to inject the organic phase; (3) Prepare a TMC-n-hexane solution, first, ultrasonically treat the solution for 5 minutes to ensure uniform dispersion, then heat and stir the solution at 60℃ until it is uniformly stirred, then pour the solution onto the membrane surface that has completed the PEPA aqueous solution soaking, and react for 5 minutes; (4) After the reaction is completed, pour out the remaining liquid, place the membrane in a vacuum drying oven at 60℃ for heat treatment for 5 minutes to promote further crosslinking, after heat treatment is completed, take out the composite membrane, rinse it several times with deionized water to completely remove the unreacted monomers, and obtain the cellulose acetate nanofiltration membrane.

[0009] Further, the preparation of the cellulose acetate ultrafiltration base film comprises the following steps: 15.0 g of N,N-dimethylacetamide, 5.0 g of methyl acetate and 3.0 g of N-methyl pyrrolidone solvent system are sequentially injected into 5.0 g of cellulose acetate, and after sealing, the mixture is placed in a constant-temperature magnetic stirrer for continuous stirring at 400 r / min until complete dissolution; then, the mixture is left to stand for 6 h for defoaming; after the bubbles are completely eliminated, a uniform liquid film is prepared on a glass substrate using a stainless steel scraper; after 30 s of natural solvent evaporation, the film is quickly transferred to a deionized water coagulation bath for phase inversion molding; and finally, a cellulose acetate ultrafiltration base film with a regular pore structure is obtained.

[0010] Further, the concentration of the chitosan solution is 0.1-2.1 g / L.

[0011] Further, the concentration of the tannic acid solution is 0.1-2.1 g / L.

[0012] Further, the concentration of the PEPA aqueous solution is 1-9 g / L.

[0013] Further, the concentration of the TMC-n-hexane solution is 0.1-0.9 g / L.

[0014] The present application utilizes layer-by-layer self-assembly technology to deposit tannic acid and chitosan on the film surface to construct multiple reaction sites, and through Michael addition and Schiff base reaction, the obtained polyamide layer is firmly anchored on the film surface to construct a structure-stable cellulose acetate nanofiltration membrane.

[0015] The present application has the following advantages: (1) The use of chitosan and tannic acid to construct an adhesive layer has multiple reaction sites, providing stable chemical bonds for the anchoring of the polyamide layer on the film surface.

[0016] (2) Chitosan and tannic acid contain abundant hydroxyl groups, which are easy to form hydrogen bonds with the hydroxyl groups in the cellulose acetate base layer, and the cross-linked network layer after the oxidation self-polymerization of tannic acid forms stable covalent bonds with the acetate base layer, thereby firmly adhering to the surface of the cellulose acetate membrane.

[0017] (3) The cellulose acetate nanofiltration membrane prepared by layer-by-layer self-assembly combined with interfacial polymerization has a positive charge on the membrane surface, and has advantages such as hydrophilicity, excellent rejection rate for dyes and salt ions, and good anti-fouling performance.

[0018] (4) The method combines layer-by-layer self-assembly technology and interfacial polymerization technology to effectively construct a stable separation layer on the surface of the cellulose acetate membrane, and obtain a cellulose acetate nanofiltration membrane with anti-pollution and high separation performance. The technology is simple to operate, effectively solves the problems of unstable structure and easy pollution of cellulose acetate membrane, and has not been applied to cellulose acetate membrane. Cellulose acetate membrane has the characteristics of high rejection, large water flux, high strength, anti-pollution, easy degradation, etc., and can be used on a large scale. The method provides a scalable and commercial solution for producing low-cost and high-performance cellulose acetate nanofiltration membranes. BRIEF DESCRIPTION OF DRAWINGS

[0019] All nanofiltration membranes (CA-NF) in the drawings are selected from nanofiltration membranes prepared by layer-by-layer self-assembly of chitosan and tannic acid (both at a concentration of 1.2 g / L) for 3 times, and interfacial polymerization of PEPA aqueous monomer at a concentration of 5 g / L and TMC-n-hexane organic monomer at a concentration of 0.5 g / L.

[0020] Figure 1 Figure 2 is an XPS spectrum of CA-UF and CA-NF, (a) C 1s, (b) O 1s, and (c) N 1s of CA-NF under high-resolution scanning; Figure 2 Figure 3 is a SEM image of CA-NF, (a) surface, (b) cross-section; Figure 3 Figure 4 is a performance of CA-NF, (a) water contact angle value and (b) surface Zeta potential at different pH values, (c) performance of CA-NF under different pressure conditions, (d) separation performance of CA-NF in four salt solutions, (e) separation effect of CA-NF on four dyes, and (f) long-term stability.

[0021] Figure 4 Figure 5 is a CA-NF membrane, (a) three-cycle anti-pollution experiment, and (b) FRR value. DETAILED DESCRIPTION

[0022] In order to make the above features and advantages of the present application more obvious and easy to understand, the following examples are given for detailed description. The method of the present application is a conventional method in the art unless otherwise specified.

[0023] Example 1 A method for constructing a cellulose acetate nanofiltration membrane with anti-pollution and high separation performance, comprising the following steps: 1. Preparation of cellulose acetate ultrafiltration base membrane Weigh 5.0 g of cellulose acetate (CA) into a 250 mL dry beaker, and then inject 15.0 g of N,N-dimethylacetamide (DMAC), 5.0 g of methyl acetate (MAC), and 3.0 g of N-methyl pyrrolidone (NMP) solvent system in sequence. After sealing the container with plastic wrap, place it in a constant-temperature magnetic stirrer and continuously stir at 400 r / min until complete dissolution. Then, let it stand for 6 h to degas. After the bubbles are completely eliminated, use a stainless steel spatula to prepare a uniform liquid film on a glass substrate. After 30 s of natural solvent evaporation, quickly transfer it to a deionized water coagulation bath for phase inversion molding, and finally obtain a cellulose acetate ultrafiltration membrane (CA-UF) with a regular pore structure.

[0024] 2. Preparation of cellulose acetate nanofiltration membrane (1) Place the prepared ultrafiltration membrane on a complete acrylic plate, ensuring that the membrane surface faces up. Then, use an acrylic frame to fix the membrane and tighten it with a dovetail clamp. Soak the membrane surface with 1.2 g / L chitosan solution for 1 hour. Then, pour off the chitosan solution on the mold, and pour 1.2 g / L tannic acid solution into it, and continue to soak for 1 hour. Repeat the above operation (i.e., first chitosan solution and then tannic acid solution) 3 times to complete the layer-by-layer self-assembly process.

[0025] (2) Prepare a 5 g / L PEPA aqueous solution and pour it onto the membrane that has completed the layer-by-layer self-assembly process, ensuring that the membrane surface is completely soaked with the solution. After soaking for 30 minutes, pour off the remaining liquid, and wait until there are no residual droplets on the membrane surface before injecting the organic phase.

[0026] (3) Prepare a 0.5 g / L TMC-hexane solution. First, ultrasonic the solution for 5 minutes to ensure uniform dispersion. Then, heat and stir the solution at 60°C until it is evenly stirred. After that, pour the solution onto the membrane surface that has been soaked in the PEPA aqueous solution and react for 5 minutes.

[0027] (4) After the reaction is complete, pour off the remaining liquid and place the membrane in a vacuum drying oven at 60°C for heat treatment for 5 minutes to promote further cross-linking. After heat treatment, remove the composite membrane and rinse it several times with deionized water to completely remove unreacted monomers. After this series of treatments, the final cellulose acetate composite nanofiltration membrane (CA-NF) is obtained.

[0028] Figure 1 XPS spectra of (a) CA-UF and CA-NF, (b) C1s, (c) O 1s, and (d) N 1s of CA-NF under high-resolution scanning The functional group characteristics of the membrane surface were explored by XPS test. The test results are shown in Figure 1 As can be seen from Figure 1 , both CA-UF and CA-NF contain C, N and O elements. The elemental analysis results show that the contents of carbon (C), nitrogen (N) and oxygen (O) in the CA-NF membrane are 70.1%, 14.5% and 15.3%, respectively, while the corresponding element contents of the CA-UF membrane are 62.9%, 1.9% and 35%, respectively. By comparing the element contents of CA-UF and CA-NF, we found that the N / O ratio changed significantly, which strongly confirmed the generation of amide bond (O=C-N), thus indicating that the interfacial polymerization reaction was successful. Further, we analyzed the C, O and N high-resolution XPS spectra of CA-NF in detail to characterize the functional group composition of the material surface (as shown in Figure 1 b-d). The C 1s spectrum analysis shows four characteristic peaks, which may correspond to C-C (284.4 eV), C-O (286.4 eV), C=N / C=O (288.3 eV) and C-N (285.9 eV), respectively. In the O 1s spectrum, we observed two peak values, which may be C-O (531.7 eV) and C=O (533.1 eV), respectively. In the N 1s spectrum, we found two peak values, which correspond to C-N (400 eV) and C=N (402 eV), respectively. The presence of C=O bond indicates that TA has undergone oxidation on the membrane surface and self-assembled. In the O 1s spectrum, the proportion of C=O bond is as high as 35.15%, which includes the C=O bond of oxidized TA and the C=O bond of amide group. The discovery of C=N bond indicates that Michael addition reaction occurs between TA and PEPA on the membrane surface. The detection of C-N bond further confirms the formation of amide group, which provides strong evidence for the success of interfacial polymerization reaction.

[0029] Figure 2 SEM images of CA-NF, (a) surface, (b) cross-section; From the surface morphology (as shown in Figure 2 a), the nanofiltration membrane surface presents a uniform and complete amide layer structure, and the nodular structure produced by the typical "spine" is clearly visible. When magnified to 1 micrometer, the amide layer structure is dense and defect-free, which endows it with excellent filtration performance. From the cross-sectional morphology (as shown in Figure 2 b), the finger-like pore structure is still complete and has not changed significantly, and there is an obvious polyamide layer on the uppermost layer, with a thickness of 164 nm.

[0030] Figure 3The (a) CA-UF and CA-NF water contact angle values and (b) surface Zeta potential under different pH conditions; (c) CA-NF performance under different pressure conditions, (d) CA-NF separation performance in four salt solutions, (e) CA-NF separation effect on four dyes, and (f) long-term stability.

[0031] The rejection rates of the nanofiltration membrane to NaCl, MgCl2, MgSO4, Na2SO4 four salt solutions were 85%, 93.8%, 92%, 88% respectively, and the corresponding permeate fluxes were 11.2 L / m²h, 9.5 L / m²h, 9.3 L / m²h, 10 L / m²h respectively. Under the same operating conditions, the rejection order of the nanofiltration membrane to the four salt solutions was MgCl2>MgSO4>Na2SO4>NaCl. Under the same operating conditions, the rejection rates of the nanofiltration membrane to BR, MB, CR, MO were 99.8%, 99.3%, 99.2%, 97.1% respectively, and the permeate fluxes were 8.5 L / m²h, 7.5 L / m²h, 9.2 L / m²h, 8.7 L / m²h respectively.

[0032] The performance was evaluated by long-term stability experiment, and the results are shown in Figure 3 After 360 minutes of operation, the water flux decreased slightly from 9.5 L / m²h to 9.3 L / m²h, only decreased by 0.2 L / m²h. At the same time, the rejection rate increased slightly from 93.8% to 94.1%. The experimental results fully demonstrated that the cellulose acetate nanofiltration membrane exhibited excellent long-term stability performance.

[0033] Figure 4 CA-NF membrane (a) three-cycle antifouling experiment; (b) FRR value.

[0034] In order to evaluate the antifouling performance of the nanofiltration membrane, we used a three-cycle test experiment. In the experiment, 1.0 g / L BSA solution was used as the fouling solution, and 1.0 g / L MgCl2 solution was used as the test solution. The permeate flux of the nanofiltration membrane was measured as a function of time, and the test results are shown in Figure 4 (a). The experimental results showed that after the first fouling liquid was contaminated and backwashed, the permeate flux remained almost unchanged. Until the end of the second cycle, the permeate flux decreased slightly from 9.5 L / m²h to 9.3 L / m²h. Even after completing the three-cycle test, the permeate flux remained stable at 9.3 L / m²h. We further calculated the FRR value of the nanofiltration membrane, and the calculation results are shown in Figure 4 (b). Generally, the higher the FRR value, the stronger the antifouling performance of the membrane. In this experiment, the FRR values of the three cycles were 100%, 97%, and 100% respectively.

[0035] The above description is only the preferred embodiment of the present application, and any equivalent change and modification made according to the scope of the present application should be included in the scope of the present application.

Claims

1. A method for constructing a cellulose acetate nanofiltration membrane with antifouling properties and high separation performance, characterized in that, Includes the following steps: (1) Lay the cellulose acetate ultrafiltration membrane flat in the mold, soak the membrane surface with chitosan solution for 1 hour, then pour off the chitosan solution on the mold, pour in the tannic acid solution, and continue soaking for 1 hour; repeat the soaking operation of chitosan solution followed by tannic acid solution 1-5 times to complete the layer-by-layer self-assembly process. (2) Prepare an aqueous PEPA solution and pour it onto the membrane that has completed layer-by-layer self-assembly. Ensure that the membrane surface is completely immersed in the solution. After soaking for 30 minutes, pour off the remaining liquid. Once there are no residual droplets on the membrane surface, wait for the organic phase to be injected. (3) Prepare TMC-n-hexane solution. First, sonicate the solution for 5 minutes to ensure uniform dispersion. Then, heat and stir the solution at 60°C until it is uniform. After that, pour the solution onto the membrane surface that has been soaked in PEPA aqueous solution and react for 5 minutes. (4) After the reaction is complete, the remaining liquid is poured off and the membrane is placed in a vacuum drying oven at 60°C for heat treatment for 5 minutes to promote further cross-linking. After the heat treatment is complete, the composite membrane is taken out and rinsed several times with deionized water to completely remove unreacted monomers, and the cellulose acetate nanofiltration membrane is obtained.

2. The method according to claim 1, characterized in that, The preparation of cellulose acetate ultrafiltration membrane includes the following steps: A solvent system consisting of 15.0 g N,N-dimethylacetamide, 5.0 g methyl acetate, and 3.0 g N-methylpyrrolidone was sequentially added to 5.0 g cellulose acetate. After sealing, the mixture was placed in a constant-temperature magnetic stirrer and stirred continuously at 400 r / min until completely dissolved. The mixture was then allowed to stand for 6 h to remove bubbles. After the bubbles were completely eliminated, a uniform liquid film was prepared on a glass substrate using a stainless steel scraper. After the solvent evaporated naturally for 30 s, the film was quickly transferred to a deionized water coagulation bath for phase inversion and molding, finally obtaining a cellulose acetate ultrafiltration membrane with a regular pore structure.

3. The method according to claim 1, characterized in that, The concentration of the chitosan solution is 0.1-2.1 g / L.

4. The method according to claim 1, characterized in that, The concentration of the tannic acid solution is 0.1-2.1 g / L.

5. The method according to claim 1, characterized in that, The concentration of the PEPA aqueous solution is 1-9 g / L.

6. The method according to claim 1, characterized in that, The concentration of the TMC-n-hexane solution is 0.1-0.9 g / L.

7. The cellulose acetate nanofiltration membrane with antifouling properties and high separation performance obtained by the method according to any one of claims 1-6.

Images