Graphene composite nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention relates to a graphene composite nanofiltration membrane as well as a preparation method and application thereof. The graphene composite nanofiltration membrane comprises graphene oxide sheet layers, corrugated graphene balls and a substrate, the corrugated graphene balls and the graphene oxide sheet layer are jointly loaded on the surface of the substrate to form a multi-dimensional nano-channel structure. The graphene composite nanofiltration membrane provided by the invention breaks through the performance limitation of a traditional single-structure membrane, and a hot pressing process in the preparation method ensures that a functional layer is not stripped from a substrate, so that the long-acting stability of the membrane is improved; under high salt concentration, the retention rate of four salts is greatly improved, and meanwhile, high flux is kept.

Description

Technical Field

[0001] The present invention belongs to the field of nanofiltration membrane materials, and particularly relates to a graphene composite nanofiltration membrane, a preparation method thereof, and an application thereof. Background Art

[0002] With the rapid development of China's economy and industry, the application demand of nanofiltration membrane technology in the fields of high-salt wastewater treatment, hard water softening, etc. has been continuously increasing. Due to the limitations of the preparation process, traditional nanofiltration membranes generally have problems such as poor structural stability, insufficient molecular channel regulation, and low retention efficiency. Although polyacrylonitrile (PAN) has good thermal stability and chemical inertness, its insufficient mechanical properties limit its application in high-pressure separation scenarios. Graphene oxide (GO) has become a research hotspot for nanofiltration membrane modification due to its excellent hydrophilicity, high specific surface area, and mechanical strength. Existing technologies such as patent CN109046041B introduce multiple layers of GO sheets (such as a four-layer structure) through the LB self-assembly technique, which improves the separation performance of the membrane to a certain extent, but still faces bottlenecks such as single nanochannels and limited anti-pollution ability.

[0003] In recent years, researchers have optimized membrane performance by compounding nanomaterials or constructing multi-dimensional structures, but how to precisely regulate molecular-level channels and synergistically improve permeability and retention rate still poses a challenge. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a graphene composite nanofiltration membrane, a preparation method thereof, and an application thereof, so as to solve the technical problems of single molecular channels, low separation efficiency, and poor stability of existing nanofiltration membranes.

[0005] The present invention provides a graphene composite nanofiltration membrane, comprising graphene oxide sheets, wrinkled graphene spheres, and a substrate; the wrinkled graphene spheres and graphene oxide sheets are jointly loaded on the surface of the substrate to form a multi-dimensional nanochannel structure.

[0006] Preferably, the substrate is a polyacrylonitrile nanofiber membrane with a thickness of 100 - 800 μm and a porosity of 10% - 40%.

[0007] Preferably, the number of modified layers of the graphene oxide sheets is 1 - 10 layers, and the single-layer thickness is 1.0 - 10 nm; the number of modified layers of the wrinkled graphene spheres is 1 - 4 layers, and the single-layer thickness is 1.0 - 10 μm.

[0008] Preferably, the diameter of the multi-dimensional nanochannels is 0.5 - 1.2 nm.

[0009] The present invention provides a method for preparing a graphene composite nanofiltration membrane, comprising the following steps:

[0010] S1. Prepare a wrinkled graphene ball film using the Langmuir - Blodgett (LB) technique and transfer it to a substrate;

[0011] S3. Immerse and lift, and repeat this step to obtain a nanofiltration membrane modified with wrinkled graphene balls;

[0012] S3. Prepare a graphene oxide sheet film using the Langmuir - Blodgett technique and transfer it to the nanofiltration membrane modified with wrinkled graphene balls;

[0013] S4. Immerse and lift, and repeat this step to obtain a composite nanofiltration membrane modified with graphene oxide;

[0014] S5. Perform hot - pressing treatment to enhance interfacial bonding and obtain a graphene composite nanofiltration membrane.

[0015] Preferably, the Langmuir - Blodgett technique in step S1 is specifically: inject the spreading solution of wrinkled graphene balls onto the water surface, after standing, compress the barrier to form a dense wrinkled graphene ball film, and then transfer it to the substrate through a film - pulling machine; the Langmuir - Blodgett technique in step S3 is specifically: inject the spreading solution of graphene oxide onto the water surface, after standing, compress the barrier to form a dense graphene oxide film, and then transfer it to the nanofiltration membrane modified with wrinkled graphene balls through a film - pulling machine.

[0016] Preferably, the concentration of the spreading solution of wrinkled graphene balls or graphene oxide is 0.05 - 1.0 mg / mL; the spreading solvent is a mixed solvent of methanol and deionized water, and the volume ratio of methanol to deionized water is 1:3 - 1:8.

[0017] Preferably, the immersion speed in step S2 or step S4 is 8 - 15 mm / min, the depth below the liquid surface for a single immersion is 3 - 5 cm; the lifting speed is 0.3 - 0.8 mm / min.

[0018] Preferably, the temperature of the hot - pressing treatment in step S5 is 70 - 90 °C, and the pressure is 0.4 - 0.6 MPa.

[0019] Using the Langmuir - Blodgett technique, the specific method for preparing the composite nanofiltration membrane is as follows:

[0020] (1) Preparation of the wrinkled graphene ball spreading solution and its spreading on the water surface to form a single-particle layer: The wrinkled graphene ball spreading solution was carefully spread on the water surface through a syringe to obtain a sparse single-particle layer of wrinkled graphene balls floating on the water surface. By compressing the sliding barrier to reduce the water surface area, the wrinkled graphene balls on the water surface were compressed to form a dense film, and a film of wrinkled graphene balls arranged in a single-particle layer floating on the water surface was obtained. Using the LB technique, the dense film of wrinkled graphene balls floating on the water surface was transferred onto the substrate of an electrospun polyacrylonitrile nanofiber membrane by a film pulling machine to obtain a polyacrylonitrile nanofiltration membrane coated with wrinkled graphene balls. The above steps were repeated to obtain a polyacrylonitrile nanofiltration membrane coated with two layers of wrinkled graphene balls;

[0021] (2) Preparation of the graphene oxide spreading solution and its spreading on the water surface to form a single-molecule layer: The graphene oxide spreading solution was carefully spread on the water surface through a syringe to obtain a sparse single-molecule layer of graphene oxide floating on the water surface. By compressing the sliding barrier to reduce the water surface area, the graphene oxide on the water surface was compressed to form a dense film, and a film of graphene oxide arranged in a single-molecule layer floating on the water surface was obtained. Using the LB technique, the dense film of graphene oxide floating on the water surface was transferred onto the polyacrylonitrile nanofiltration membrane coated with two layers of wrinkled graphene balls to obtain the composite nanofiltration membrane of the present invention coated with a single layer of graphene oxide. The above steps were repeated to obtain a composite nanofiltration membrane sample containing four layers of graphene oxide;

[0022] (3) Post-treatment: Hot pressing (80 °C, 0.5 MPa) was used to enhance the interfacial bonding to ensure that the functional layer was not peeled off from the substrate, and the graphene composite nanofiltration membrane was obtained.

[0023] The present invention also provides an application of the above graphene composite nanofiltration membrane in the desalination process.

[0024] Preferably, the salt includes one or more of Na2SO4, MgSO4, NaCl, and MgCl2, and the concentration of the salt solution is 1.0 - 3.0 mg / mL.

[0025] The present invention combines lamellar graphene oxide (GO) with wrinkled graphene spheres (rGO), and uses the Langmuir-Blodgett (LB) self-assembly technique to stepwise regulate the membrane structure, solving the technical problems of single molecular channels, low separation efficiency and poor stability of existing nanofiltration membranes. Compared with the traditional single GO lamellar structure (such as the four-layer design of patent CN109046041B), the present invention forms multi-dimensional nanochannels by introducing wrinkled graphene spheres, synergistically optimizing the permeation flux and rejection performance of the membrane. At the same time, the three-dimensional structure of rGO is used to enhance the mechanical support of the substrate, significantly improving the anti-pollution ability and long-term operation stability. In particular, the present invention increases the test concentration of the salt solution to 2.0 mg / mL, breaking through the performance bottleneck of traditional membranes at high salt concentrations, and providing an efficient solution for industrial high-salt wastewater treatment.

[0026] Beneficial effects

[0027] (1) The present invention realizes the co-loading of rGO and GO through a stepwise LB technique, forms multi-dimensional nanochannels, and breaks through the performance limitations of traditional single-structure membranes;

[0028] (2) The hot pressing process in the present invention ensures that the functional layer and the substrate are not peeled off, improving the long-term stability of the membrane;

[0029] (3) The graphene composite nanofiltration membrane in the present invention has a large increase in the rejection rate of four salts at a high salt concentration (2.0 mg / mL), while maintaining a high flux;

[0030] (4) The preparation process of the present invention is simple, the conditions are mild, the thickness of the functional layer can be accurately controlled to the nanometer level, and it is suitable for large-scale production. Description of the Drawings

[0031] Figure 1 It is a comparison diagram of the nanofiltration performance of the graphene composite nanofiltration membrane prepared in Example 1 of the present invention and the prior art for salts.

[0032] Figure 2 It is the nanofiltration performance and flux of the nanofiltration membrane prepared in Example 1 of the present invention for NaCl.

[0033] Figure 3 It is the nanofiltration performance and flux of the nanofiltration membrane prepared in Example 1 of the present invention for Na2SO4.

[0034] Figure 4 It is the nanofiltration performance and flux of the nanofiltration membrane prepared in Example 1 of the present invention for MgCl2.

[0035] Figure 5 It is the nanofiltration performance and flux of the nanofiltration membrane prepared in Example 1 of the present invention for MgSO4.

[0036] Figure 6This is a comparison chart of the nanofiltration performance of the nanofiltration membrane prepared in Example 1 of the present invention for different salts. Detailed implementation manners

[0037] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0038] Example 1

[0039] The preparation of the graphene composite nanofiltration membrane in this example includes the following steps:

[0040] Step 1: First, prepare 3.0 mL of an aqueous solution of wrinkled graphene balls (rGO) with a concentration of 1.0 mg / mL, and then add 15.0 mL of methanol to the above solution and stir for 20 min to obtain a graphene oxide spreading solution with a final concentration of 0.167 mg / mL.

[0041] Step 2: Take 10.0 mL of the above rGO spreading solution and carefully spread it drop by drop on the water surface in an LB water tank with a water tank area of 10 * 20 cm. Let it stand for 20 minutes. After the methanol on the water surface has completely volatilized, move the slide barrier at a speed of 0.8 cm / min. When the distance to the slide barrier is 1 cm, stop moving and let it stand to wait for the self-assembly of a monolayer rGO film to form.

[0042] Step 3: Cut the polyacrylonitrile nanofiltration membrane into specimens with a size of 5 cm * 2.5 cm. By controlling the parameters of the film pulling machine, set the dipping speed to 10 mm / min and dip the polyacrylonitrile nanofibers in the LB water tank. The length immersed below the liquid surface is 4.0 cm. After waiting for the liquid surface to stabilize for 20 minutes, lift the polyacrylonitrile nanofibers at a lifting speed of 0.5 mm / min to transfer the monolayer rGO on the water surface to the surface of the polyacrylonitrile nanofiber membrane.

[0043] Step 4: Repeat Step 3 to obtain a nanofiltration membrane coated with two layers of rGO, named PAN-2rGO.

[0044] Step 5: Take 10.0 mL of the above GO spreading solution and carefully spread it drop by drop on the water surface in an LB water tank with a water tank area of 10 * 20 cm. Let it stand for 20 minutes. After the methanol on the water surface has completely volatilized, move the slide barrier at a speed of 0.8 cm / min. When the distance to the slide barrier is 1 cm, stop moving and let it stand to wait for the self-assembly of a monolayer GO film to form.

[0045] Step 6: By controlling the parameters of the film drawing machine, set the impregnation speed to 10 mm / min, immerse the blank polyacrylonitrile nanofiltration membrane in the LB water tank, with the length immersed below the liquid surface being 4.0 cm. After waiting for the liquid surface to stabilize for 20 minutes, lift the nanofiltration membrane at a lifting speed of 0.5 mm / min, and transfer the single-molecule layer of GO on the water surface to the polyacrylonitrile blank membrane respectively to obtain a polyacrylonitrile nanofiltration membrane covered with a single layer of GO.

[0046] Step 7: Repeat Step 6 three times to obtain a nanofiltration membrane coated with four layers of GO, named PAN-4GO.

[0047] Step 8: By controlling the parameters of the film drawing machine, set the impregnation speed to 10 mm / min, and immerse the rGO-coated nanofiltration membrane (PAN-2rGO) obtained in Step 4 in the LB water tank respectively, with the depth immersed below the liquid surface being 4.0 cm. After waiting for the liquid surface to stabilize for 20 minutes, lift the nanofiltration membrane at a lifting speed of 0.5 mm / min, and transfer the single-molecule layer of GO on the water surface to the surface of the PAN-2rGO nanofiltration membrane respectively to obtain a PAN-2rGO nanofiltration membrane covered with a single layer of GO.

[0048] Step 9: Repeat Step 8 three times to obtain a PAN-2rGO nanofiltration membrane coated with four layers of GO, named PAN-2rGO-4GO, which is the graphene composite nanofiltration membrane sample of the present invention.

[0049] Example 2

[0050] Prepare four salt solutions:

[0051] Accurately measure 1.0 g of four salt solutions of N a2 SO4, MgSO4, NaCl, and MgCl2 respectively, stir continuously until completely dissolved, add them to a 500 mL volumetric flask for constant volume respectively, and store at room temperature for standby. The solution concentrations of the four salts are all 2.0 mg / ml.

[0052] Test the nanofiltration performance of the nanofiltration membrane for salts at room temperature. The specific nanofiltration experiment steps are as follows:

[0053] Step 1: Pre-pressurization: Fix the sample on the filtration device, keep the peristaltic pump rotation speed at 5 rpm, and press the membrane with deionized water until the water flux reaches a stable state.

[0054] Step 2: Measurement of pure water flux: Test the flux J of deionized water, that is, record the volume of the permeated liquid, and terminate the test after stabilization.

[0055] Step 3: Measurement of feed liquid retention: Replace it with the salt solution and test again, and take samples of both the permeated solution and the feed solution for testing.

[0056] Step 4: The retention performance of the nanofiltration membrane is measured by the rejection rate R, and the calculation formula is as follows:

[0057] R = (1 - C p / Cf)

[0058] In the above formula, R is the rejection rate (%) of the GO / PAN nanofiltration membrane for the four salts, C p is the solute concentration in the permeate, and C f is the solute concentration in the four salts.

[0059] Step 5: Determination of C p and C f for the four salts: The conductivities of the solutions permeated by the four salt solutions and the original salt solutions are measured by a DDS-11C conductivity meter, and then the rejection rates of the four salts are calculated through the formula.

[0060] Figures 2 to 5 The nanofiltration performance and flux of PAN, PAN-2rGO, PAN-4GO, and PAN-2rGO-4GO nanofiltration membranes for NaCl, Na2SO4, MgCl2, and MgSO4 are shown in turn. Figure 6 is a comparison chart of the nanofiltration performance of the nanofiltration membrane for the four salts. It can be seen from Figure 6 that among these four salts, the desalination rate of the nanofiltration membrane with different layers of graphene oxide for Na2SO4 is the highest. When the graphene oxide reaches four layers, the rejection rate of Na2SO4 reaches more than 80%; in addition, the rejection rate of the composite nanofiltration membrane (PAN-2rGO-4GO) of the present invention for the four salts is significantly better than that of other samples, and the rejection rate for Na2SO4 is the highest, reaching more than 90%. Compared with the membrane containing only graphene oxide (PAN-4GO), the composite structure introducing wrinkled graphene spheres (rGO) further improves the retention performance, indicating that the design of the multi-dimensional nanochannel effectively enhances the molecular sieving effect.

[0061] At the same time, the nanofiltration performance of the nanofiltration membrane of the present invention for salts is compared and analyzed with the prior art. It can be seen from Figure 1 that the fluxes of the composite nanofiltration membrane prepared by the present invention for the four salts Na2SO4, MgSO4, NaCl, and MgCl2 are much higher than those in the relevant literature, but the rejection rates for the four salts are comparable to those in the literature; compared with Patent CN109046041B, the fluxes are comparable, but the rejection rates for the four salts are greatly improved, and they remain stable at a high salt concentration (2.0 mg / mL). This breakthrough not only verifies the synergistic effect of the rGO / GO composite structure, but also provides an efficient solution with high flux, high rejection rate and long-term stability for industrial high-salt wastewater treatment.

[0062] Example 3

[0063] An aqueous solution of bovine serum albumin (BSA) was used as a simulated dye system to evaluate a series of anti-fouling performance indicators of the nanofiltration membrane.

[0064] The specific steps for preparing the BSA solution are as follows: Accurately weigh 1000 mg of BSA, then add deionized water, stir continuously until dissolved, and then place the obtained solution in a 1000 mL volumetric flask, make up the volume to obtain a BSA aqueous solution with a concentration of 1000 mg / L, and store it at room temperature for later use.

[0065] The main steps for evaluating the anti-fouling performance of the composite nanofiltration membrane are as follows:

[0066] (1) Pre-pressurization: Fix the sample on the filtration device, keep the peristaltic pump speed at 5 r / min, and press the membrane with deionized water for 20 min until the water flux reaches a stable state.

[0067] (2) Pure water flux measurement: Measure the flux J of deionized water, that is, record the volume of the permeated liquid, and measure the water flux J w1 , and terminate the test until it is stable.

[0068] (3) BSA aqueous solution measurement: Replace the original deionized water with the BSA aqueous solution for testing, and measure the flux of this solution as J p .

[0069] (4) Clean the nanofiltration membrane sample: Pour out the BSA feed liquid, and then add deionized water and clean for 15 min.

[0070] (5) Measure the water flux for the second time: Pour out the solution after washing, add deionized water again, and measure the secondary pure water flux J w2 .

[0071] Four parameters for evaluating the anti-fouling index of the GO / PAN nanofiber membrane are introduced, namely the flux recovery rate (FRR) after water cleaning, the flux decay rate (R t ), the reversible flux decay rate (R r ), and the irreversible flux decay rate (R ir ). According to the calculation formula:

[0072] FRR (%) = J w2 / J w1 × 100%

[0073] R t (%) = (1 - J p / J w1 ) × 100%

[0074] R r (%) = (J w2 - J p ) / J w1 × 100%

[0075] R ir (%) = (1 - J w2 / J w1 ) × 100%

[0076] Table 1. Anti-pollution index of nanofiltration membrane

[0077]

[0078] As can be seen from Table 1, the graphene composite nanofiltration membrane prepared by the present invention has excellent anti-pollution performance and can be practically applied to the desalination treatment of wastewater.

Claims

1. A graphene composite nanofiltration membrane, characterized in that, The graphene composite nanofiltration membrane includes graphene oxide sheets, wrinkled graphene spheres and a substrate; the wrinkled graphene spheres and graphene oxide sheets are jointly loaded on the surface of the substrate to form a multi-dimensional nanochannel structure.

2. The graphene composite nanofiltration membrane according to claim 1, wherein, The substrate is a polyacrylonitrile nanofiber membrane with a thickness of 100 - 800 μm and a porosity of 10% - 40%.

3. The graphene composite nanofiltration membrane according to claim 1, wherein The number of modified layers of the graphene oxide sheets is 1 - 10 layers, and the single-layer thickness is 1.0 - 10 nm; the number of modified layers of the wrinkled graphene spheres is 1 - 4 layers, and the single-layer thickness is 1.0 - 10 μm.

4. The graphene composite nanofiltration membrane according to claim 1, wherein The diameter of the multi-dimensional nanochannels is 0.5 - 1.2 nm.

5. A method for preparing a graphene composite nanofiltration membrane, comprising the following steps: S1. Prepare a wrinkled graphene sphere thin film by Langmuir - Blodgett technology and transfer it onto the substrate; S2. Then impregnate and lift, and repeat this step to obtain a nanofiltration membrane modified with wrinkled graphene spheres; S3. Prepare a graphene oxide sheet thin film by Langmuir - Blodgett technology and transfer it onto the nanofiltration membrane modified with wrinkled graphene spheres; S4. Then impregnate and lift, and repeat this step to obtain a composite nanofiltration membrane modified with graphene oxide; S5. Perform hot pressing treatment to enhance the interfacial bonding to obtain the graphene composite nanofiltration membrane.

6. The preparation method of the graphene composite nanofiltration membrane according to claim 5, characterized in that, The Langmuir - Blodgett technology in step S1 is specifically as follows: Inject the spreading solution of wrinkled graphene spheres onto the water surface, after standing, compress the barrier to form a dense wrinkled graphene sphere thin film, and then transfer it onto the substrate through a film pulling machine; the Langmuir - Blodgett technology in step S3 is specifically as follows: Inject the spreading solution of graphene oxide onto the water surface, after standing, compress the barrier to form a dense graphene oxide thin film, and then transfer it onto the nanofiltration membrane modified with wrinkled graphene spheres through a film pulling machine.

7. The preparation method of the graphene composite nanofiltration membrane according to claim 6, characterized in that, The concentration of the spreading solution of wrinkled graphene spheres or graphene oxide spreading solution is 0.05 - 1.0 mg / mL; the spreading solvent is a mixed solvent of methanol and deionized water, and the volume ratio of methanol to deionized water is 1:3 - 1:

8.

8. The preparation method of the graphene composite nanofiltration membrane according to claim 5, characterized in that, The impregnation speed in step S2 or step S4 is 8 - 15 mm / min, and the depth below the liquid surface for a single impregnation is 3 - 5 cm; the lifting speed is 0.3 - 0.8 mm / min.

9. The preparation method of the graphene composite nanofiltration membrane according to claim 5, wherein, The temperature of the hot pressing treatment in step S5 is 70 - 90 °C, and the pressure is 0.4 - 0.6 MPa.

10. An application of the graphene composite nanofiltration membrane according to any one of claims 1 - 4 in a desalination process.

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