Preparation method and application of asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane

Abstract

The application relates to an asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane and a preparation method and application thereof. COF nanosheets are dispersed in DMF, acetone and cellulose acetate are added, a casting solution is prepared, stirring and defoaming are carried out, a scraping method is used, corresponding time is pre-volatilized, and phase inversion is carried out in deionized water to form a film. The application adopts a surface segregation technology to construct a non-uniformly distributed charged structure of a high-charge-density COF material in the membrane. The obtained asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane is applied to ion nanofiltration, has good performance, can realize nanofiltration desalination, and has great prospects in the industries of seawater desalination and wastewater treatment.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of cellulose acetate chemical utilization, and particularly relates to an asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane and a preparation method and application thereof. BACKGROUND

[0002] Nanofiltration is a membrane separation technology with a precision between ultrafiltration and reverse osmosis. Under pressure driving, nanofiltration membrane only allows water molecules to pass through efficiently, while high-valent salt ions are intercepted. Currently, ion nanofiltration technology is mainly used in the following aspects: 1) wastewater purification. Industrial and municipal wastewater contains high salt content, and direct discharge will cause serious environmental pollution and waste. Removing high-valent salt ions through nanofiltration has become an efficient water treatment process. 2) Ion resourceization. With the rapid development of new energy and other industries, the demand for lithium has increased dramatically. Separating lithium ions from impurity ions through nanofiltration technology has become a low-energy, low-cost and green strategy for lithium extraction, which has attracted widespread attention. The pore size of nanofiltration membrane is between ultrafiltration membrane and reverse osmosis membrane (0.5-2 nm), which can realize solute interception through steric hindrance effect. Nanofiltration membrane can also realize desalination through electrostatic effect, that is, through the electrostatic interaction between charged channels and ions to interfere with ion transfer and affect the desalination performance. Currently, nanofiltration technology is mainly based on non-renewable petroleum-based polymer membrane materials, which are difficult to degrade and pose challenges to the sustainability of the whole life cycle. Cellulose acetate (CA) is a bio-based membrane material based on natural cellulose, which has been widely used in commercial reverse osmosis membrane preparation, but its desalination efficiency is not good due to its lower charge than existing polyamide nanofiltration membranes, and the electrostatic desalination effect is weaker.

[0003] This study is inspired by the charged structure of mangrove salt-tolerant plant roots. The concept of asymmetrically charged nanofiltration membrane is proposed, which introduces non-uniformly distributed charged groups into cellulose acetate membrane to strengthen the role of electrostatic effect in nanofiltration desalination process and realize size screening coupled with electrostatic desalination. Ionic covalent organic framework (iCOF) is a new type of crystalline charged polymer material that can provide highly ordered one-dimensional charged nanochannels. Taking sulfonic acid type covalent organic framework (NUS-9 and NUS-10) as an example, its mass transfer channel charge density is as high as hundreds of mC / m 2 , far exceeding the reported artificial charged channels and biological charged channels, which is an ideal charged nanometer unit. In this study, charged COF nanosheets are introduced into the phase inversion process of cellulose acetate, and the non-uniform distribution of COF in the membrane is realized through surface segregation technology to construct asymmetrically charged cellulose acetate nanofiltration membrane, strengthen the surface charge density of the membrane, and improve the ion nanofiltration performance through surface electrostatic effect and size screening mechanism. The application of COF material composite biomass material in high-tech fields is expanded, but related research is rarely reported. SUMMARY

[0004] The present application aims to overcome the above-mentioned defects, and provides a preparation method of asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane, which comprises the following steps: dispersing COF nanosheets in DMF, adding acetone and cellulose acetate, preparing casting solution, stirring and defoaming, using the method of blade coating, pre-vaporizing for a corresponding time, and phase-inverting into a membrane in deionized water. The surface segregation technology is used to construct a non-uniformly distributed charged structure of COF material with high charge density in the membrane. By adjusting the COF addition amount and changing the pre-vaporization time, the threshold values of different parameters are determined, and the nanofiltration membrane with the best performance is obtained for nanofiltration desalination.

[0005] To achieve the above-mentioned purpose, the present application adopts the following technical scheme:

[0006] First, COF nanosheets are dispersed in DMF, then acetone and cellulose acetate (the volume ratio of DMF to acetone is 1:2, and the mass fraction of cellulose acetate is 20wt%) are added to prepare casting solution, which is magnetically stirred at room temperature for 12 hours and defoamed for 24 hours. The casting solution is blade coated on a glass plate, pre-vaporized for 0min, 1min, 1.5min, 2min, respectively, and then phase-inverted into a membrane in deionized water. The obtained membrane is crosslinked with glutaraldehyde solution, washed clean and stored in deionized water. The obtained asymmetric covalent organic framework composite cellulose acetate membrane is used for ion nanofiltration, which can realize desalination of industrial wastewater, seawater, brackish water and the like.

[0007] Further, the COF nanosheets are sulfonated COF nanosheets NUS-9, which are prepared by liquid-liquid interfacial polymerization.

[0008] In step one, the mass fraction of COF in the casting solution is 0.005wt%-0.05wt%.

[0009] In step one, the addition amount of COF is preferably 1mg.

[0010] In step two, the pre-vaporization time is preferably 1min.

[0011] In step three, the concentration of the crosslinking glutaraldehyde solution is 4wt%, the temperature is set to 60℃, and the time length is 20min.

[0012] Compared with the prior art, the present application has the following advantages:

[0013] (1) The present application proposes a method for constructing asymmetric structure of COF in the membrane, and the surface segregation technology is used to prepare asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane, and the thickness of the membrane is microns;

[0014] (2) The asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane prepared by the application has special structure, which can improve the permeability and the rejection rate, and exhibits excellent performance in the field of ion nanofiltration desalination.

[0015] (3) The application uses DMF as the solvent, which can uniformly disperse the COF nanosheet and dissolve cellulose acetate together with acetone, thereby providing a new idea for the solvent selection of the COF composite cellulose acetate membrane. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 It is an electron microscope (SEM) image of the COF nanosheet used.

[0017] Figure 2 It is a cross-section electron microscope (SEM) image of the asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane prepared in Example 6.

[0018] Figure 3 It is an EDX-mapping-N element image of the asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane prepared in Example 6.

[0019] Figure 4 It is the rejection of the asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane prepared in Example 6 to high-concentration sodium sulfate (1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm) solution.

[0020] Figure 5 It is the rejection of the asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane prepared in Examples 5-8 to six kinds of salts, including sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride and sodium chloride. DETAILED DESCRIPTION

[0021] The application will be further described below through specific examples and drawings. The examples of the application are to better enable those skilled in the art to understand the application, and do not limit the application in any way.

[0022] The COF nanosheet used in the following examples is sulfonated COF nanosheet NUS-9, which is prepared by liquid-liquid interfacial polymerization. The specific preparation method is referred to the preparation of IPC-COF in the literature: Cao L, Wu H, Cao Y, et al. Weakly humidity-dependent proton-conducting COF membranes [J]. Advanced Materials, 2020, 32 (52): 2005565.

[0023] Example 1, preparation of asymmetrically charged covalent organic framework composite cellulose acetate membrane-1, the steps are as follows:

[0024] Step one, 0.5 mg of COF was weighed and dispersed in 2.67 ml of DMF, then acetone and cellulose acetate (DMF:acetone volume ratio of 1:2, cellulose acetate mass fraction of 20wt%) were added to prepare a casting solution. The electron microscope (SEM) image of the COF nanosheet used is as follows: Figure 1 .

[0025] Step two, magnetic stirring at room temperature for 12 hours, and standing for 24 hours to remove bubbles.

[0026] Step three, the casting solution was coated on a glass plate, pre-volatilized for 0 min, and then transferred into deionized water to form a film.

[0027] Step four: the prepared membrane was crosslinked with 4wt% glutaraldehyde solution, washed and then stored in deionized water.

[0028] The obtained asymmetrically charged COF composite cellulose acetate nanofiltration membrane-1 was then used for nanofiltration of sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, sodium chloride and other six salts.

[0029] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in Example 1 to a sodium sulfate (1000 ppm) solution was 75.28%.

[0030] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in Example 1 to a magnesium sulfate (1000 ppm) solution was 74.21%.

[0031] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in Example 1 to a magnesium chloride (1000 ppm) solution was 74.88%.

[0032] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in Example 1 to a potassium chloride (1000 ppm) solution was 68.60%.

[0033] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in Example 1 to a lithium chloride (1000 ppm) solution was 66.13%.

[0034] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in Example 1 to a sodium chloride (1000 ppm) solution was 62.31%.

[0035] Example 2, preparation of asymmetrically charged covalent organic framework composite cellulose acetate membrane-2, the preparation process is basically the same as that of example 1, the only difference is that in step three, the pre-volatilization time is 1 min, and asymmetrically charged COF composite cellulose acetate membrane-2 is obtained.

[0036] Then the obtained asymmetrically charged COF composite cellulose acetate nanofiltration membrane-2 is used for nanofiltration of sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, sodium chloride and other six kinds of salts.

[0037] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in example 2 to sodium sulfate (1000 ppm) solution is 84.95%.

[0038] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in example 2 to magnesium sulfate (1000 ppm) solution is 82.81%.

[0039] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in example 2 to magnesium chloride (1000 ppm) solution is 78.89%.

[0040] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in example 2 to potassium chloride (1000 ppm) solution is 68.77%.

[0041] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in example 2 to lithium chloride (1000 ppm) solution is 64.16%.

[0042] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-1 prepared in example 2 to sodium chloride (1000 ppm) solution is 66.28%.

[0043] Example 3, preparation of asymmetrically charged covalent organic framework composite cellulose acetate membrane-3, the preparation process is basically the same as that of example 1, the only difference is that in step three, the pre-volatilization time is 1.5 min, and asymmetrically charged COF composite cellulose acetate membrane-3 is obtained.

[0044] Then the obtained asymmetrically charged COF composite cellulose acetate nanofiltration membrane-3 is used for nanofiltration of sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, sodium chloride and other six kinds of salts.

[0045] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-3 prepared in example 3 to sodium sulfate (1000 ppm) solution is 54.26%.

[0046] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-3 prepared in Example 3 against a magnesium sulfate (1000 ppm) solution was 50.54%.

[0047] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-3 prepared in Example 3 against a magnesium sulfate (1000 ppm) solution was 50.54%.

[0048] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-3 prepared in Example 3 against a magnesium sulfate (1000 ppm) solution was 50.54%.

[0049] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-3 prepared in Example 3 against a magnesium sulfate (1000 ppm) solution was 50.54%.

[0050] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-3 prepared in Example 3 against a magnesium sulfate (1000 ppm) solution was 50.54%.

[0051] Example 4, preparation of an asymmetrically charged covalent organic framework composite cellulose acetate membrane-4, which is prepared basically the same as in Example 1, except that in step three, the pre-volatilization time is 2 min, to obtain an asymmetrically charged COF composite cellulose acetate membrane-4.

[0052] The obtained asymmetrically charged COF composite cellulose acetate nanofiltration membrane-4 is then used for nanofiltration of sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, sodium chloride and the like.

[0053] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-4 prepared in Example 4 against a sodium sulfate (1000 ppm) solution was 39.37%.

[0054] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-4 prepared in Example 4 against a magnesium sulfate (1000 ppm) solution was 35.53%.

[0055] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-4 prepared in Example 4 against a magnesium sulfate (1000 ppm) solution was 35.53%.

[0056] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-4 prepared in Example 4 against a magnesium sulfate (1000 ppm) solution was 35.53%.

[0057] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-4 prepared in Example 4 against a magnesium sulfate (1000 ppm) solution was 35.53%.

[0058] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-4 prepared in Example 4 for a sodium chloride (1000 ppm) solution was 22.89%.

[0059] Example 5, preparation of asymmetrically charged COF composite cellulose acetate membrane-5, the preparation process was basically the same as that of Example 1, except that in step one, the amount of COF added was 1 mg, and an asymmetrically charged COF composite cellulose acetate membrane-5 was obtained.

[0060] The obtained asymmetrically charged COF composite cellulose acetate nanofiltration membrane-5 was then used for nanofiltration of sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, sodium chloride and other six salts.

[0061] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-5 prepared in Example 5 for a sodium sulfate (1000 ppm) solution was 73.52%.

[0062] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-5 prepared in Example 5 for a magnesium sulfate (1000 ppm) solution was 78.50%.

[0063] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-5 prepared in Example 5 for a magnesium chloride (1000 ppm) solution was 78.47%.

[0064] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-5 prepared in Example 5 for a potassium chloride (1000 ppm) solution was 62.11%.

[0065] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-5 prepared in Example 5 for a lithium chloride (1000 ppm) solution was 66.93%.

[0066] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-5 prepared in Example 5 for a sodium chloride (1000 ppm) solution was 62.82%.

[0067] Example 6, preparation of asymmetrically charged COF composite cellulose acetate membrane-6, the preparation process was basically the same as that of Example 1, except that in step one, the amount of COF added was 1 mg; in step three, the pre-volatilization time was 1 min, and an asymmetrically charged COF composite cellulose acetate membrane-6 was obtained. The cross-sectional TEM image of the asymmetrically charged COF composite cellulose acetate nanofiltration membrane-6 is shown in Figure 2 , and the cross-sectional EDX-mapping-N element is shown in Figure 3 .

[0068] The obtained asymmetrically charged COF composite cellulose acetate nanofiltration membrane-6 was then used for nanofiltration of sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, sodium chloride and other six kinds of salts.

[0069] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-6 prepared in Example 6 to a sodium sulfate (1000 ppm) solution was 93.59%.

[0070] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-6 prepared in Example 6 to a magnesium sulfate (1000 ppm) solution was 92.09%.

[0071] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-6 prepared in Example 6 to a magnesium chloride (1000 ppm) solution was 90.57%.

[0072] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-6 prepared in Example 6 to a potassium chloride (1000 ppm) solution was 74.98%.

[0073] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-6 prepared in Example 6 to a lithium chloride (1000 ppm) solution was 74.87%.

[0074] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-6 prepared in Example 6 to a sodium chloride (1000 ppm) solution was 74.52%.

[0075] The rejection rates of the asymmetrically charged COF composite cellulose acetate membrane-6 prepared in Example 6 to high-concentration sodium sulfate (1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm) solutions were 93.59%, 87.78%, 86.59%, 82.46%, respectively, as shown in Figure 4

[0076] Example 7, preparation of asymmetrically charged COF composite cellulose acetate membrane-7, the preparation process was basically the same as that of Example 1, except that in step one, the amount of COF added was 1 mg; in step three, the pre-volatilization time was 1.5 min, and the asymmetrically charged COF composite cellulose acetate membrane-7 was obtained.

[0077] The obtained asymmetrically charged COF composite cellulose acetate nanofiltration membrane-7 was then used for nanofiltration of sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, sodium chloride and other six kinds of salts.

[0078] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-7 prepared in Example 7 to a sodium sulfate (1000 ppm) solution was 51.08%. ​

[0079] The asymmetric charged COF composite cellulose acetate membrane-7 prepared in Example 7 had a rejection rate of 51.94% for magnesium sulfate (1000 ppm) solution.

[0080] The asymmetric charged COF composite cellulose acetate membrane-7 prepared in Example 7 had a rejection rate of 45.79% for magnesium chloride (1000 ppm) solution.

[0081] The asymmetric charged COF composite cellulose acetate membrane-7 prepared in Example 7 had a rejection rate of 40.02% for potassium chloride (1000 ppm) solution.

[0082] The asymmetric charged COF composite cellulose acetate membrane-7 prepared in Example 7 had a rejection rate of 41.17% for lithium chloride (1000 ppm) solution.

[0083] The asymmetric charged COF composite cellulose acetate membrane-7 prepared in Example 7 had a rejection rate of 39.28% for sodium chloride (1000 ppm) solution.

[0084] Example 8: Preparation of asymmetric charged covalent organic framework composite cellulose acetate membrane-8. The preparation process is basically the same as that in Example 1, except that: in step one, the amount of COF added is 1 mg; and in step three, the pre-evaporation time is 2 min, to obtain asymmetric charged COF composite cellulose acetate membrane-8.

[0085] The obtained asymmetric charged COF composite cellulose acetate nanofiltration membrane-8 was then used for nanofiltration of six salts, including sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, and sodium chloride.

[0086] The asymmetric charged COF composite cellulose acetate membrane-8 prepared in Example 8 had a rejection rate of 28.21% for sodium sulfate (1000 ppm) solution.

[0087] The asymmetric charged COF composite cellulose acetate membrane-8 prepared in Example 8 had a rejection rate of 25.67% for magnesium sulfate (1000 ppm) solution.

[0088] The asymmetric charged COF composite cellulose acetate membrane-8 prepared in Example 8 had a rejection rate of 28.16% for magnesium chloride (1000 ppm) solution.

[0089] The asymmetric charged COF composite cellulose acetate membrane-8 prepared in Example 8 had a rejection rate of 24.43% for potassium chloride (1000 ppm) solution.

[0090] The asymmetric charged COF composite cellulose acetate membrane-8 prepared in Example 8 had a rejection rate of 21.56% for lithium chloride (1000 ppm) solution.

[0091] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-8 prepared in Example 8 to a sodium chloride (1000 ppm) solution was 23.06%.

[0092] Figure 5 The rejection rate change graph of the asymmetrically charged COF composite cellulose acetate membranes 5-8 to six kinds of salts such as sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, and sodium chloride (1000 ppm).

[0093] Example 9, preparation of asymmetrically charged COF composite cellulose acetate membrane-9, the preparation process is basically the same as that of Example 1, the only difference is that in step one, the amount of COF added is 2 mg, and the asymmetrically charged COF composite cellulose acetate membrane-9 is obtained.

[0094] Then the obtained asymmetrically charged COF composite cellulose acetate nanofiltration membrane-9 is used for nanofiltration of six kinds of salts such as sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, and sodium chloride.

[0095] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-9 prepared in Example 9 to a sodium sulfate (1000 ppm) solution was 71.39%.

[0096] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-9 prepared in Example 9 to a magnesium sulfate (1000 ppm) solution was 69.12%.

[0097] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-9 prepared in Example 9 to a magnesium chloride (1000 ppm) solution was 70.52%.

[0098] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-9 prepared in Example 9 to a potassium chloride (1000 ppm) solution was 54.35%.

[0099] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-9 prepared in Example 9 to a lithium chloride (1000 ppm) solution was 63.35%.

[0100] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-9 prepared in Example 9 to a sodium chloride (1000 ppm) solution was 57.40%.

[0101] Example 10, preparation of asymmetrically charged COF composite cellulose acetate membrane-10, the preparation process is basically the same as that of Example 1, the only difference is that in step one, the amount of COF added is 2 mg; in step three, the pre-volatilization time is 1 min, and the asymmetrically charged COF composite cellulose acetate membrane-10 is obtained.

[0102] The obtained asymmetrically charged COF composite cellulose acetate nanofiltration membrane-10 was then used for nanofiltration of sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, sodium chloride and other six kinds of salts.

[0103] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-10 prepared in Example 10 to a sodium sulfate (1000 ppm) solution was 76.72%.

[0104] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-10 prepared in Example 10 to a magnesium sulfate (1000 ppm) solution was 75.95%.

[0105] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-10 prepared in Example 10 to a magnesium chloride (1000 ppm) solution was 75.36%.

[0106] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-10 prepared in Example 10 to a potassium chloride (1000 ppm) solution was 63.78%.

[0107] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-10 prepared in Example 10 to a lithium chloride (1000 ppm) solution was 62.46%.

[0108] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-10 prepared in Example 10 to a sodium chloride (1000 ppm) solution was 62.51%.

[0109] Example 11, preparation of asymmetrically charged covalent organic framework composite cellulose acetate membrane-11, the preparation process is basically the same as that of Example 1, except that in step one, the amount of COF added is 2 mg; in step three, the pre-volatilization time is 1.5 min, and the asymmetrically charged COF composite cellulose acetate membrane-11 is obtained.

[0110] The obtained asymmetrically charged COF composite cellulose acetate nanofiltration membrane-11 was then used for nanofiltration of sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, sodium chloride and other six kinds of salts.

[0111] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-11 prepared in Example 11 to a sodium sulfate (1000 ppm) solution was 59.08%.

[0112] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-11 prepared in Example 11 to a magnesium sulfate (1000 ppm) solution was 57.42%.

[0113] The asymmetrically charged COF composite cellulose acetate membrane-11 prepared in Example 11 had a rejection rate of 46.90% for a magnesium chloride (1000 ppm) solution.

[0114] The asymmetrically charged COF composite cellulose acetate membrane-11 prepared in Example 11 had a rejection rate of 44.01% for a potassium chloride (1000 ppm) solution.

[0115] The asymmetrically charged COF composite cellulose acetate membrane-11 prepared in Example 11 had a rejection rate of 37.62% for a lithium chloride (1000 ppm) solution.

[0116] The asymmetrically charged COF composite cellulose acetate membrane-11 prepared in Example 11 had a rejection rate of 40.21% for a sodium chloride (1000 ppm) solution.

[0117] Example 12, preparation of an asymmetrically charged COF composite cellulose acetate membrane-12, was prepared in substantially the same manner as in Example 1, except that in Step 1, the amount of COF added was changed to 2 mg; and in Step 3, the pre-volatilization time was changed to 2 min, to obtain an asymmetrically charged COF composite cellulose acetate membrane-12.

[0118] The asymmetrically charged COF composite cellulose acetate nanofiltration membrane-12 thus obtained was then used for nanofiltration of sodium sulfate, magnesium sulfate, magnesium chloride, potassium chloride, lithium chloride, and sodium chloride.

[0119] The asymmetrically charged COF composite cellulose acetate membrane-12 prepared in Example 12 had a rejection rate of 40.98% for a sodium sulfate (1000 ppm) solution.

[0120] The asymmetrically charged COF composite cellulose acetate membrane-12 prepared in Example 12 had a rejection rate of 36.70% for a magnesium sulfate (1000 ppm) solution.

[0121] The asymmetrically charged COF composite cellulose acetate membrane-12 prepared in Example 12 had a rejection rate of 32.57% for a magnesium chloride (1000 ppm) solution.

[0122] The asymmetrically charged COF composite cellulose acetate membrane-12 prepared in Example 12 had a rejection rate of 30.99% for a potassium chloride (1000 ppm) solution.

[0123] The asymmetrically charged COF composite cellulose acetate membrane-12 prepared in Example 12 had a rejection rate of 26.83% for a lithium chloride (1000 ppm) solution.

[0124] The rejection rate of the asymmetrically charged COF composite cellulose acetate membrane-12 prepared in Example 12 to a sodium chloride (1000 ppm) solution was 27.25%.

[0125] The comparison of the rejection performance of the membranes prepared in the various embodiments of the present application to six kinds of salts is shown in Table 1.

[0126] Table 1

[0127]

[0128]

[0129] Through the above examples, the present application obtains corresponding experimental results.

[0130] According to the data in Table 1, it can be seen that: during the preparation of the asymmetrically charged COF composite cellulose acetate membrane of the present application, 1. The change of the pre-volatilization time can affect the asymmetrically charged structure of the membrane. With the increase of the time, the water permeability increases, the rejection rate first increases and then decreases, and there is a threshold for the effect of the pre-volatilization time on the membrane structure; 2. As can be seen from Examples 1, 5 and 9, the change of the COF addition amount can change the charge of the asymmetrically charged membrane, thereby strengthening the electrostatic effect of the nanofiltration process. With the increase of the COF addition amount, the water permeability and the rejection rate of the six kinds of salts increase first and then decrease; 3. The formation of the asymmetrically charged structure can not only improve the water permeability, but also improve the rejection rate of each salt. The asymmetrically charged COF composite cellulose acetate membrane with the best performance is Example 6, and the rejection performance of the six kinds of salts reaches the highest, among which sodium sulfate > magnesium sulfate > magnesium chloride > potassium chloride > lithium chloride > sodium chloride.

[0131] From Figure 1 It can be seen that the COF nanosheet added to each membrane has a regular shape and a size of several microns, which is beneficial to the construction of the asymmetrically charged structure.

[0132] From Figure 2 and Figure 3 It can be seen that in Example 6, the casting solution is pre-volatilized for 1 min after being scraped, which constructs an asymmetric physical structure. The COF nanosheet forms a charge-rich layer on the upper part of the membrane through the surface segregation technology, thereby forming a complete and continuous asymmetrically charged structure. The membrane has a smooth and flat surface morphology, and the thickness is about 120 microns. In addition, from Figure 3 It can be seen that the characteristic element N of COF is enriched in the upper part of the membrane, which confirms the successful construction of the asymmetrically charged COF composite cellulose acetate membrane.

[0133] From Figure 4 It can be seen that the increase of the concentration of sodium sulfate presents a stable state, and the rejection rate of 4000 ppm of sodium sulfate can reach 86.87%.

[0134] By Figure 5 It can be seen that the asymmetrically charged COF composite cellulose acetate membranes prepared in Examples 5-8 were tested for rejection of six common salts (1000 ppm) and the results showed different degrees of rejection of the six salts, with Na2SO4 > MgSO4 > MgCl2 > KCl > LiCl > NaCl. The membrane of Example 6 had the best performance, with a maximum rejection rate of 95.41% for Na2SO4.

[0135] It should be understood that the embodiments and examples discussed herein are merely illustrative and that modifications or variations can be made by those skilled in the art, and all such modifications and variations are intended to be within the scope of the appended claims.

Claims

1. A method for preparing asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membranes, characterized in that, The COF nanosheet is dispersed in N,N-dimethylformamide, then acetone and cellulose acetate are added to prepare a casting solution, the casting solution is stirred magnetically at room temperature for 12 hours, and then is left to stand for 24 hours to remove bubbles, then is scraped to form an asymmetrically charged structure, and then is placed in deionized water to be phase-inverted to form a membrane, the membrane is crosslinked in a glutaraldehyde solution for 20 minutes, and then is stored in deionized water, and the pre-volatilization time is 1 minute, 1.5 minutes or 2 minutes.

2. The method of claim 1, wherein the asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane is prepared by the steps of: The COF nanosheet is sulfonated COF nanosheet NUS-9.

3. The method of claim 1, wherein the asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane is prepared by the steps of: The cellulose acetate has an acetyl group content of 39.8 wt% and a hydroxyl group content of 3.5 wt%.

4. The method of claim 1, wherein the asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane is prepared by the steps of: The mass fraction of cellulose acetate in the casting solution is 20 wt%.

5. The method for preparing the asymmetric charged covalent organic framework composite cellulose acetate nanofiltration membrane according to claim 1, characterized in that, The volume ratio of N,N-dimethylformamide to acetone is 1:

2.

6. The method of making asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membranes according to claim 1, wherein, The mass fraction of COF in the casting solution is 0.005 wt%-0.05 wt%.

7. The method of making asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membranes according to claim 1, wherein, The concentration of the glutaraldehyde solution is 4 wt%, the temperature is 60℃, and the time is 20 minutes.

8. A non-symmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane, characterized in that, Prepared according to any one of claims 1-7.

9. Use of the asymmetrically charged covalent organic framework composite cellulose acetate nanofiltration membrane according to claim 8 in nanofiltration desalination of seawater, wastewater or brackish water.

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