High-flux high-interception composite nanofiltration membrane and preparation method thereof

Abstract

The invention relates to the technical field of membrane separation, and particularly discloses a high-flux high-interception composite nanofiltration membrane and a preparation method thereof. The method comprises the following steps: dipping a porous ultrafiltration substrate in a polydopamine aqueous solution for surface modification; then loading a covalent organic framework material on the surface of the covalent organic framework material through a vacuum filtration or impregnation method to form a COFs intermediate layer; then forming a polyamide selection layer on the substrate containing the COFs intermediate layer through an oil-water-oil interfacial polymerization reaction; and finally, performing heat treatment and soaking in water to finish preparation. The COFs intermediate layer is introduced, so that the specific surface area of the membrane is remarkably increased, an extra water channel is provided, and the water flux is effectively improved while the high rejection rate of salt and the like is kept; the prepared composite nanofiltration membrane is stable in structure, simple in preparation process, good in repeatability and suitable for the fields of wastewater reuse, seawater desalination and the like.

Description

Technical Field

[0001] This invention belongs to the field of separation membrane technology, specifically relating to a high-flux, high-retention composite nanofiltration membrane and its preparation method. Background Technology

[0002] Nanofiltration membranes, as a highly efficient separation technology, play a crucial role in seawater desalination, wastewater treatment, and material separation. Currently, the most widely used commercial nanofiltration membrane is the polyamide composite nanofiltration membrane, which forms a dense polyamide separation layer through interfacial polymerization on a porous substrate. However, the inherent transport channel structure of this separation layer limits the efficient passage of water molecules, leading to a trade-off between selectivity and permeability in the membrane material. To overcome this bottleneck, existing technologies attempt to introduce nanomaterials into the polyamide layer or substrate to construct water transport channels. However, these nanofillers are prone to aggregation and have poor interfacial compatibility with the polymer substrate, often causing interfacial defects, resulting in decreased membrane selectivity and limited flux improvement.

[0003] Covalent organic frameworks (COFs), as a class of novel crystalline porous materials with regular pore structures formed by organic structural units linked by strong covalent bonds, have shown great application potential in the field of membrane separation due to their advantages such as high specific surface area, high porosity, low density, and good chemical stability. Therefore, developing a novel modification method using COFs to construct regular and stable water transport channels has significant industrial value for the preparation of polyamide composite nanofiltration membranes with both high rejection rate and high water flux. Summary of the Invention

[0004] This invention addresses the aforementioned bottlenecks by providing a high-flux, high-rejection-rate composite nanofiltration membrane and its preparation method. The preparation process involves loading COFs (Chemical Oxide-Foil) materials onto the surface of a pretreated porous polymer substrate via vacuum filtration or impregnation to form a COF interlayer. Subsequently, a dense polyamide selective separation layer is formed on the COF interlayer using an oil-water-oil interfacial polymerization method, ultimately yielding the modified polyamide composite nanofiltration membrane. This technology offers a simple, efficient, low-cost, and highly reproducible preparation method with good stability in water, significantly reducing wastewater treatment costs and possessing broad market application prospects.

[0005] Therefore, the technical solution of the present invention is as follows:

[0006] 1) Take out the ultrafiltration membrane that is soaked in ultrapure water, immerse it in an aqueous solution containing polydopamine for 5-24 hours, then pour it off and wash the membrane surface with ultrapure water.

[0007] 2) Prepare an aqueous solution of COFs by ultrasonically dispersing COFs in ultrapure water, and then coat it onto the surface of the ultrafiltration membrane treated in step 1) by vacuum filtration or impregnation.

[0008] 3) Immerse the membrane treated in step 2) in an organic phase solution containing polyacryl chloride for 1-30 min, then pour it off and remove excess organic phase solution from the surface with a rubber roller;

[0009] 4) Immerse the membrane treated in step 3) in an aqueous solution containing polyamine monomers for 1-60 seconds, then pour it off and remove excess aqueous solution from the surface with a rubber roller;

[0010] 5) Immerse the membrane treated in step 4) in an organic phase solution containing polyacrylamide chloride for 1-60 seconds, then pour it off and remove excess organic phase solution from the surface using a rubber roller.

[0011] 6) Place the membrane treated in step 5) into an oven and treat it at 25-60 °C for 1-30 min, then soak it in ultrapure water for 5-24 h to obtain polyamide composite nanofiltration membranes modified with different concentrations of COFs.

[0012] Furthermore, the ultrafiltration membrane mentioned in step 1) is any one of polysulfone, polyethersulfone, and polyvinylidene fluoride.

[0013] Furthermore, the ultrafiltration membrane mentioned in step 1) is any one of a flat sheet membrane, a spiral wound membrane, and a hollow fiber membrane.

[0014] Further, the COFs mentioned in step 2) are one or more of the COFs formed by the monomer pyromellitic methyl ether and its derivatives and polyamine monomers, with a content of 0.01-1 g / L.

[0015] Further, the organic solvent in step 3) is one or a mixture of several of n-hexane, m-xylene and isoalkanes, and the solute is trimesoyl chloride with a solute content of 0.01-1 w / v.

[0016] Further, the polyamine monomer in step 4) is one or a mixture of several of m-phenylenediamine, p-phenylenediamine and piperazine, with a content of 1-10 w / v.

[0017] Furthermore, the aqueous solution described in step 4) contains 0.1-10 w / v% of a catalyst, which is one or both of triethylamine and 4-dimethylaminopyridine.

[0018] The present invention prepares a high-flux, high-retention composite nanofiltration membrane using the above method, and the technical effects achieved are as follows:

[0019] 1. Due to the large specific surface area, high porosity, and ordered and adjustable pore structure of COFs materials, the specific surface area of ​​COFs-modified polyamide composite nanofiltration membranes increases. While maintaining high salt rejection performance, the water permeability is effectively improved, showing good industrialization prospects.

[0020] 2. The present invention obtains the modified polyamide composite nanofiltration membrane through vacuum filtration / impregnation and interfacial polymerization. The preparation process is simple, requires no post-processing, is easy to mass-produce, helps to save industrial costs and time, and effectively improves the yield.

[0021] 3. In this invention, COFs are fixed between the organic phases of polydopamine and polyacrylamide chloride through cross-linking as an interlayer, making the functional layer less prone to detachment and exhibiting good stability. Attached Figure Description

[0022] Figure 1 These are test graphs showing the permeation and retention performance of the composite nanofiltration membranes prepared in Examples 1-5 of this invention at 0.2 MPa for a 1 g / L Na2SO4 solution;

[0023] Figure 2 This is a scanning electron microscope image of the composite nanofiltration membrane prepared in Example 4 of the present invention. Detailed Implementation

[0024] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0025] Example 1

[0026] 1) Take out the polysulfone ultrafiltration membrane that is soaked in ultrapure water, immerse it in an aqueous solution containing polydopamine for 24 hours, then pour it off and wash the membrane surface with ultrapure water.

[0027] 2) Prepare an aqueous solution of COFs by ultrasonically dispersing 0.2 mg COFs in 10 mL of ultrapure water, and then coat it onto the surface of the ultrafiltration membrane treated in step 1) by vacuum filtration.

[0028] 3) Immerse the membrane treated in step 2) in isoalkanes containing 0.15 w / v% pyromellitic chloride for 5 min, then pour it off and remove excess organic phase solution from the surface with a rubber roller.

[0029] 4) Immerse the membrane treated in step 3) in an aqueous solution containing 2 w / v% piperazine and 2 w / v% triethylamine for 10 s, then pour it off and remove excess aqueous solution from the surface with a rubber roller;

[0030] 5) Immerse the membrane treated in step 4) in isoalkanes containing 0.15 w / v% trimesoyl chloride for 10 s, then pour it off and remove excess organic phase solution from the surface with a rubber roller.

[0031] 6) After polymerizing the membrane treated in step 5) at 25 °C for 1 min, it was immersed in ultrapure water to obtain a polyamide composite nanofiltration membrane PA-COFs-1 with a concentration of 0.02 g / L COFs modified by the "oil-water-oil" interface polymerization.

[0032] 7) The salt-blocking performance of the membrane obtained in step 6) was tested using a cross-flow apparatus. The test was conducted at 0.2 MPa with a 1 g / L Na₂SO₄ aqueous solution, and the water flux was 120 L × m. 2 ×h -1 ×MPa -1 The rejection rate for Na2SO4 was 97%.

[0033] Example 2

[0034] 1) Take out the polysulfone ultrafiltration membrane that is soaked in ultrapure water, immerse it in an aqueous solution containing polydopamine for 24 hours, then pour it off and wash the membrane surface with ultrapure water.

[0035] 2) Prepare an aqueous solution of COFs by ultrasonically dispersing 0.4 mg COFs in 10 mL of ultrapure water, and then coat the surface of the ultrafiltration membrane treated in step 1) by vacuum filtration.

[0036] 3) Immerse the membrane treated in step 2) in isoalkanes containing 0.15 w / v% pyromellitic chloride for 5 min, then pour it off and remove excess organic phase solution from the surface with a rubber roller.

[0037] 4) Immerse the membrane treated in step 3) in an aqueous solution containing 2 w / v% piperazine and 2 w / v% triethylamine for 10 s, then pour it off and remove excess aqueous solution from the surface with a rubber roller;

[0038] 5) Immerse the membrane treated in step 4) in isoalkanes containing 0.15 w / v% trimesoyl chloride for 10 s, then pour it off and remove excess organic phase solution from the surface with a rubber roller.

[0039] 6) After polymerizing the membrane treated in step 5) at 25 °C for 1 min, it was immersed in ultrapure water to obtain a polyamide composite nanofiltration membrane PA-COFs-2 with a concentration of 0.04 g / L COFs modified by the "oil-water-oil" interface polymerization.

[0040] 7) The salt-blocking performance of the membrane obtained in step 6) was tested using a cross-flow apparatus. The test was conducted at 0.2 MPa with a 1 g / L Na₂SO₄ aqueous solution, and the water flux was 125 L × m. 2 ×h -1 ×MPa -1 The rejection rate for Na2SO4 was 97%.

[0041] Example 3

[0042] 1) Take out the polysulfone ultrafiltration membrane that is soaked in ultrapure water, immerse it in an aqueous solution containing polydopamine for 24 hours, then pour it off and wash the membrane surface with ultrapure water.

[0043] 2) Prepare an aqueous solution of COFs by ultrasonically dispersing 0.6 mg COFs in 10 mL of ultrapure water, and then coat the surface of the ultrafiltration membrane treated in step 1) by vacuum filtration.

[0044] 3) Immerse the membrane treated in step 2) in isoalkanes containing 0.15 w / v% pyromellitic chloride for 5 min, then pour it off and remove excess organic phase solution from the surface with a rubber roller.

[0045] 4) Immerse the membrane treated in step 3) in an aqueous solution containing 2 w / v% piperazine and 2 w / v% triethylamine for 10 s, then pour it off and remove excess aqueous solution from the surface with a rubber roller;

[0046] 5) Immerse the membrane treated in step 4) in isoalkanes containing 0.15 w / v% trimesoyl chloride for 10 s, then pour it off and remove excess organic phase solution from the surface with a rubber roller.

[0047] 6) After polymerizing the membrane treated in step 5) at 25 °C for 1 min, it was immersed in ultrapure water to obtain a polyamide composite nanofiltration membrane PA-COFs-3 with a concentration of 0.06 g / L COFs modified by the "oil-water-oil" interface polymerization.

[0048] 7) The salt-blocking performance of the membrane obtained in step 6) was tested using a cross-flow apparatus. The test was conducted at 0.2 MPa with a 1 g / L Na₂SO₄ aqueous solution, and the water flux was 127 L × m. 2 ×h -1 ×MPa -1 The rejection rate for Na2SO4 was 97%.

[0049] Example 4

[0050] 1) Take out the polysulfone ultrafiltration membrane that is soaked in ultrapure water, immerse it in an aqueous solution containing polydopamine for 24 hours, then pour it off and wash the membrane surface with ultrapure water.

[0051] 2) Prepare an aqueous solution of COFs by ultrasonically dispersing 0.8 mg COFs in 10 mL of ultrapure water, and then coat the surface of the ultrafiltration membrane treated in step 1) by vacuum filtration.

[0052] 3) Immerse the membrane treated in step 2) in isoalkanes containing 0.15 w / v% pyromellitic chloride for 5 min, then pour it off and remove excess organic phase solution from the surface with a rubber roller.

[0053] 4) Immerse the membrane treated in step 3) in an aqueous solution containing 2 w / v% piperazine and 2 w / v% triethylamine for 10 s, then pour it off and remove excess aqueous solution from the surface with a rubber roller;

[0054] 5) Immerse the membrane treated in step 4) in isoalkanes containing 0.15 w / v% trimesoyl chloride for 10 s, then pour it off and remove excess organic phase solution from the surface with a rubber roller.

[0055] 6) After polymerizing the membrane treated in step 5) at 25 °C for 1 min, it was immersed in ultrapure water to obtain a polyamide composite nanofiltration membrane PA-COFs-4 with a concentration of 0.08 g / L COFs modified by the "oil-water-oil" interface polymerization.

[0056] 7) The salt-blocking performance of the membrane obtained in step 6) was tested using a cross-flow apparatus. The test was conducted at 0.2 MPa with a 1 g / L Na₂SO₄ aqueous solution, and the water flux was 158 L × m. 2 ×h -1 ×MPa -1 The rejection rate for Na2SO4 was 98%.

[0057] Comparative Example 1

[0058] 1) Remove the polysulfone ultrafiltration membrane that has been soaked in ultrapure water and remove excess water from its surface with a rubber roller;

[0059] 2) Immerse the membrane treated in step 1) in an aqueous solution containing 2 w / v% piperazine and 2 w / v% triethylamine for 5 min, then pour it off and remove excess aqueous solution from the surface with a rubber roller;

[0060] 3) Immerse the membrane treated in step 2) in isoalkanes containing 0.15 w / v% trimesoyl chloride for 10 s, then pour it off and remove excess organic phase solution from the surface with a rubber roller.

[0061] 4) After polymerizing the membrane treated in step 3) at 25 °C for 1 min, immerse it in ultrapure water to obtain a polyamide composite nanofiltration membrane PA formed by interfacial polymerization;

[0062] 5) The salt-blocking performance of the membrane obtained in step 4) was tested using a cross-flow apparatus. The test was conducted at 0.2 MPa with a 1 g / L Na₂SO₄ aqueous solution, and the water flux was 80 L × m. 2 ×h -1 ×MPa -1 The rejection rate for Na2SO4 was 97%.

[0063] A comparison of Example 4 and Comparative Example 1 shows that the modified polyamide composite nanofiltration membrane PA-COFs-4, formed by "oil-water-oil" interfacial polymerization, has a significantly higher water flux than the PA membrane, while its salt rejection performance remains almost unchanged. This fully demonstrates that COFs materials play an effective role in constructing high-performance composite nanofiltration membranes and solves the trade-off problem between selectivity and permeability in the field of separation membranes.

[0064] Example 5

[0065] 1) Take out the polysulfone ultrafiltration membrane that is soaked in ultrapure water, immerse it in an aqueous solution containing polydopamine for 24 hours, then pour it off and wash the membrane surface with ultrapure water.

[0066] 2) Prepare an aqueous solution of COFs by ultrasonically dispersing 1.0 mg COFs in 10 mL of ultrapure water, and then coat the surface of the ultrafiltration membrane treated in step 1) by vacuum filtration.

[0067] 3) Immerse the membrane treated in step 2) in isoalkanes containing 0.15 w / v% pyromellitic chloride for 5 min, then pour it off and remove excess organic phase solution from the surface with a rubber roller.

[0068] 4) Immerse the membrane treated in step 3) in an aqueous solution containing 2 w / v% piperazine and 2 w / v% triethylamine for 10 s, then pour it off and remove excess aqueous solution from the surface with a rubber roller;

[0069] 5) Immerse the membrane treated in step 4) in isoalkanes containing 0.15 w / v% trimesoyl chloride for 10 s, then pour it off and remove excess organic phase solution from the surface with a rubber roller.

[0070] 6) After polymerizing the membrane treated in step 5) at 25 °C for 1 min, it was immersed in ultrapure water to obtain a polyamide composite nanofiltration membrane PA-COFs-5 with a concentration of 0.1 g / L COFs modified by the "oil-water-oil" interface polymerization.

[0071] 7) The salt-blocking performance of the membrane obtained in step 6) was tested using a cross-flow apparatus. The test was conducted at 0.2 MPa with a 1 g / L Na₂SO₄ aqueous solution, and the water flux was 170 L × m. 2 ×h -1 ×MPa -1 The rejection rate for Na2SO4 was 87%.

[0072] As shown in Examples 1-5, the polyamide composite nanofiltration membrane PA-COFs-4 exhibits optimal performance when the COF concentration is 0.08 g / L, as detailed below:

[0073] Table 1

[0074] COF concentration (g / L) 0.02 0.04 0.06 0.08 0.10 <![CDATA[Flux (L×m -2 ×h -1 ×MPa -1 )]]> 120 125 127 158 170 Withholding (%) 97 97 97 98 87

[0075] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and improvements made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a high-flux, high-retention composite nanofiltration membrane, characterized in that... Includes the following steps: 1) Take out the ultrafiltration membrane that is soaked in ultrapure water, immerse it in an aqueous solution containing polydopamine for 5-24 hours, then pour it out and rinse the membrane surface with ultrapure water. 2) Prepare an aqueous solution of COFs by ultrasonically dispersing COFs in ultrapure water, and then uniformly coat the surface of the ultrafiltration membrane treated in step 1) by vacuum filtration or impregnation. 3) Immerse the membrane treated in step 2) in an organic phase solution containing polyacryl chloride for 1-30 min, then pour it off and remove excess organic phase solution from the surface with a rubber roller; 4) Immerse the membrane treated in step 3) in an aqueous solution containing polyamine monomers for 1-60 s, then pour it off and remove excess aqueous solution from the surface with a rubber roller; 5) Immerse the membrane treated in step 4) in an organic phase solution containing polyacryl chloride for 1-60 s, then pour it off and remove excess organic phase solution from the surface with a rubber roller; 6) Place the membrane treated in step 5) into an oven and treat it at 25-60 °C for 1-30 min, then soak it in ultrapure water for 5-24 h to obtain a polyamide nanofiltration membrane or a polyamide composite nanofiltration membrane modified with different concentrations of COFs.

2. The method according to claim 1, characterized in that: The ultrafiltration membrane mentioned in step 1) is any one of polysulfone, polyethersulfone, and polyvinylidene fluoride.

3. The method according to claim 1, characterized in that: Step 1) The ultrafiltration membrane can be any one of a flat sheet membrane, a spiral wound membrane, or a hollow fiber membrane.

4. The method according to claim 1, characterized in that: Step 2) The COFs mentioned are one or a mixture of several of the COFs formed by the monomer pyromellitic methyl ether and its derivatives and polyamine monomers, with a content of 0.01-1 g / L.

5. The method according to claim 1, characterized in that: The solvent of the organic phase solution in step 3) is one or a mixture of several of n-hexane, m-xylene and isoalkanes, and the solute is trimesoyl chloride with a content of 0.01-1 w / v.

6. The method according to claim 1, characterized in that: Step 4) The polyamine monomer is one or a mixture of several of m-phenylenediamine, p-phenylenediamine and piperazine, with a content of 1-10 w / v.

7. The method according to claim 1, characterized in that: The aqueous solution in step 4) contains 0.1-10 w / v% of a catalyst, which is one or both of triethylamine and 4-dimethylaminopyridine.

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