Modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane as well as preparation method and application thereof

Abstract

The invention discloses a modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane as well as a preparation method and application thereof. According to the method disclosed by the invention, the natural polyphenol compound tannic acid derived from plants is adopted to carry out green modification on the cellulose nanocrystals, so that the requirement of green sustainable development is met; the modified cellulose nanocrystal is used as a filler, a cellulose acetate composite membrane is constructed through a non-solvent phase separation technology, and the ultrafiltration membrane is good in hydrophilicity, high in permeability, good in interception performance on simulated protein pollutants represented by bovine serum albumin and high in flux recovery rate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of water treatment membranes, in particular to a modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane, a preparation method and application thereof. BACKGROUND

[0002] Ultrafiltration membrane separation technology has become one of the core means in the field of water treatment due to its low cost and high efficiency. With the promotion of global carbon neutralization, the demand for low-carbon and sustainable separation materials in the field of water treatment has grown rapidly. However, it is found that the traditional cellulose acetate (CA) membrane often accompanies the decrease of rejection rate while improving porosity. At the same time, the traditional CA membrane also generally has the problems of poor mechanical stability and easy pollution, which will reduce the service life of the membrane and affect the overall separation efficiency. Therefore, it is particularly important to prepare a CA membrane with excellent comprehensive performance.

[0003] In recent years, introducing hydrophilic nanofillers into CA casting solution is considered as a more effective strategy to enhance its water flux, antifouling property and pressure resistance, such as Chinese patent CN102688703A "A method for modifying cellulose acetate ultrafiltration membrane", Chinese patent CN118681407A "A cellulose acetate ultrafiltration membrane loaded with core-shell structure nanospheres and a preparation method and application thereof", Chinese patent CN110479120A "A preparation method of cellulose acetate flat plate ultrafiltration membrane", Chinese patent CN103785300A "A blending modification method of cellulose acetate ultrafiltration membrane", and Chinese patent CN102500247A "A preparation method of modified cellulose acetate ultrafiltration membrane" all involve the addition of hydrophilic nanofillers. However, the addition of inorganic hydrophilic fillers has enhanced the permeability of CA ultrafiltration membrane to some extent, but still has the following bottleneck problems: (1) poor interfacial compatibility between hydrophilic inorganic fillers and CA matrix, easy to cause phase separation, resulting in membrane structure defects and insufficient long-term stability; (2) complex modification scheme and difficult to simultaneously improve water flux, rejection rate and antifouling property, performance imbalance; (3) modification process relies on petroleum-based reagents such as silane coupling agent and high energy consumption steps, which does not meet the requirements of sustainable development.

[0004] Therefore, how to realize the uniform and stable dispersion of hydrophilic fillers between CA matrix through green process, so as to improve the comprehensive performance of CA ultrafiltration membrane, is a key technical problem to promote the application of related membrane products. SUMMARY

[0005] The present application aims to at least solve one of the technical problems in the related art to some extent.

[0006] To this end, an embodiment of the present application proposes a modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane, a preparation method and application thereof.

[0007] In a first aspect, the present application provides a method for preparing a modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane, comprising the following steps:

[0008] (1) dispersing cellulose nanocrystals in a buffer solution, adjusting the pH to weak alkaline by using an alkaline solution and fully stirring, then adding tannic acid to fully react to obtain a yellow transparent solution;

[0009] (2) dialyzing the yellow transparent solution by using a dialysis bag to remove excess tannic acid, and freeze-drying to obtain yellow modified cellulose nanocrystals;

[0010] (3) dissolving the modified cellulose nanocrystals, cellulose acetate and pore-forming agent in an organic solvent to obtain a casting solution;

[0011] (4) defoaming the casting solution and then coating it on a substrate, then immersing it in an ultrapure water coagulation bath after standing to complete phase inversion, to obtain a composite ultrafiltration membrane.

[0012] Further, the mass ratio of the tannic acid to the cellulose nanocrystals is 1:(0.5-6).

[0013] Further, in step (1), the pH is adjusted to 7.5-8.5 by using an alkaline solution.

[0014] Further, the pore-forming agent is polyvinylpyrrolidone K30.

[0015] Further, the molecular weight cut-off of the dialysis bag is 10,000 Da.

[0016] Further, in the casting solution, the mass concentration of the modified cellulose nanocrystals is 0.1%-2.5%, the mass concentration of the cellulose acetate is 10%-16%, and the mass concentration of the polyvinylpyrrolidone K30 is 0.1%-0.3%.

[0017] Further, in step (4), the thickness of the coating is 200±5 μm.

[0018] Further, the organic solvent includes one or more of N,N-dimethylacetamide, N-methylpyrrolidone and dimethyl sulfoxide.

[0019] Further, the porosity of the composite ultrafiltration membrane is 80%-90%, the overall average pore size of the composite ultrafiltration membrane is 12-15 nm, and the contact angle of the composite ultrafiltration membrane is below 60°.

[0020] In a second aspect, the present application provides a modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane prepared by the method of the first aspect.

[0021] In a third aspect, the application provides an application of the modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane prepared by the method of the first aspect or the modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane of the second aspect in surface water treatment.

[0022] Compared with the prior art, the application has the following beneficial effects:

[0023] The introduction of the modified nanocellulose nanocrystal into the cellulose acetate ultrafiltration membrane significantly improves the porosity and pore size of the ultrafiltration membrane, and effectively improves the mechanical properties, hydrophilicity, water flux and flux recovery rate of the ultrafiltration membrane.

[0024] The method of the application is simple in process, green and sustainable in material, low in reaction condition temperature and equipment requirement, and convenient to improve on the basis of a traditional ultrafiltration membrane preparation process.

[0025] The method of the application uses the natural polyphenol compound tannic acid derived from plants to greenly modify the cellulose nanocrystal, which meets the requirements of green and sustainable development; the modified cellulose nanocrystal is used as a filler to construct a cellulose acetate composite membrane through a non-solvent phase separation technology, the ultrafiltration membrane has good hydrophilicity, high permeability, good rejection of simulated protein pollutants represented by bovine serum albumin and high flux recovery rate.

[0026] The modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane prepared by the method of the application has a water flux of 184.68 L / (m 2 ·h) or more, a bovine serum albumin rejection rate of 98.06% or more, and a water flux recovery rate of 85.21%. In the process of continuously treating surface water, the composite membrane maintains a water flux of 140.87 L / (m 2 ·h), a water flux recovery rate of 99.18%, and a total organic carbon (TOC) concentration of 4.04 mg / L in the filtrate, which meets the requirement of less than 5 mg / L of TOC in the "Drinking Water Health Standards" (GB 5749-2022). BRIEF DESCRIPTION OF DRAWINGS

[0027] The above and / or additional aspects and advantages of the application will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, in which:

[0028] Figure 1 A flow chart of the preparation method of the modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane of the application;

[0029] Figure 2 A structural formula of the cellulose nanocrystal after exposure of hydroxyl groups of the application;

[0030] Figure 3This is a structural diagram of the yellow modified cellulose nanocrystals obtained by freeze-drying according to the present invention;

[0031] Figure 4 The images are TEM images of cellulose nanocrystals before and after modification in Example 1, where (a) is cellulose nanocrystals and (b) is modified cellulose nanocrystals.

[0032] Figure 5 The Fourier transform infrared spectra of cellulose nanocrystals and tannic acid before and after modification in Example 1 are shown.

[0033] Figure 6 The dispersibility diagram of cellulose nanocrystals before and after modification at a mass concentration of 0.5% in N,N-dimethylacetamide is shown.

[0034] Figure 7 The dispersibility diagram of cellulose nanocrystals before and after modification at a mass concentration of 2.5% in N,N-dimethylacetamide is shown.

[0035] Figure 8 The diagram shows the structural characterization of the M1 film and the M1-2 film of the present invention, wherein (a) is the attenuated total reflection-Fourier transform infrared spectrum of the M1 film and the M1-2 film, and (b) is the X-ray diffraction energy spectrum of the M1 film and the M1-2 film.

[0036] Figure 9 SEM images of membranes M1, M2, M1-1, and M1-2 of this invention;

[0037] Figure 10 Flux versus time curves for dynamic circulating filtration of bovine serum albumin using M1 and M1-2 membranes;

[0038] Figure 11 The figures show the performance test results of the M1-2 membrane for treating surface water, where (a) is the time-flux curve and (b) is the three-dimensional fluorescence spectrum of the water before and after filtration. Detailed Implementation

[0039] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0040] The preparation method of the modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane proposed in this invention is described below with reference to the accompanying drawings.

[0041] like Figure 1 As shown, the preparation method of the plant-modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane of the present invention includes the following steps:

[0042] (1) dispersing cellulose nanocrystals in a buffer solution, adjusting pH to weak alkaline by using alkaline solution and fully stirring, then adding tannic acid to fully react to obtain a yellow transparent solution;

[0043] (2) dialyzing the yellow transparent solution by using a dialysis bag to remove excess tannic acid, and freeze-drying to obtain yellow modified cellulose nanocrystals;

[0044] (3) dissolving the modified cellulose nanocrystals, cellulose acetate and pore-forming agent in an organic solvent to obtain a casting solution;

[0045] (4) defoaming the casting solution and then coating it on a substrate, then standing and immersing in an ultrapure water coagulation bath to complete phase inversion to obtain a composite ultrafiltration membrane.

[0046] Step (1) is a process of modifying cellulose nanocrystals by tannic acid. First, the cellulose nanocrystals are dispersed in a buffer solution, the pH is adjusted to weak alkaline by using an alkaline solution and fully stirred to fully expose the hydroxyl groups on the surface of the cellulose nanocrystals, then tannic acid is added to fully react to obtain a yellow transparent solution.

[0047] In some embodiments, the buffer solution is N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer solution with a concentration of 0.05M. The buffer solution serves to regulate the pH value of the reaction system and maintain its stability, thereby ensuring that the reaction between tannic acid and cellulose nanocrystals can fully proceed under suitable weak alkaline conditions.

[0048] In some embodiments, the pH is adjusted to 7.5-8.5 by using an alkaline solution, wherein the alkaline solution includes one or more of NaOH solution, KOH solution and tris(hydroxymethyl)aminomethane.

[0049] In some embodiments, the stirring speed for adjusting the pH to weak alkaline by using an alkaline solution is 400-600 r / min, and the stirring time is 0.5-1.5 h. The fully stirring fully exposes the hydroxyl groups on the surface of the cellulose nanocrystals, wherein the structure of the cellulose nanocrystals after exposing the hydroxyl groups is as shown in formula (I), wherein n≥20, and the cellulose nanocrystals after exposing the hydroxyl groups are as shown in formula (II). Figure 2 Figure 2 Figure 2 It can be seen that the hydroxyl groups of the cellulose nanocrystals are exposed.

[0050] ​​In some embodiments, the reaction temperature at which the tannic acid is added and fully reacted to obtain a yellow transparent solution is 20-30 DEG C, and the reaction time is 6-8 h. The cellulose nanocrystals are modified by tannic acid at a temperature of 20-30 DEG C in the present application, so that the reaction system maintains good stability and dispersibility under mild conditions, which is conducive to the uniform adsorption and action of tannic acid on the surface of cellulose nanocrystals, avoids the agglomeration or precipitation of nanocrystals, and simplifies the operation steps and reduces energy consumption, which meets the requirements of green and low-carbon film material preparation.

[0051] In some embodiments, the mass ratio of tannic acid to cellulose nanocrystals is 1:(0.5-6). The mass ratio of tannic acid to cellulose nanocrystals is in a suitable range, so that the tannic acid can fully adsorb on the surface of cellulose nanocrystals, thereby improving the dispersibility of cellulose nanocrystals in an organic solvent and giving good surface functionalization effect, which is conducive to the structure and performance stability of the subsequent composite film. When too much tannic acid is added, the residual free tannic acid in the system can cause the viscosity of the modified product to increase and the stability to decrease, and the tannic acid is easy to migrate and precipitate in the subsequent phase inversion, resulting in defects in the film structure. When too little tannic acid is added, the surface modification of cellulose nanocrystals is not complete, which causes the cellulose nanocrystals to easily aggregate and settle in the organic system, which is not conducive to the construction of high-performance composite films.

[0052] Step (2) is a process of treating the yellow transparent solution to obtain modified cellulose nanocrystals. First, the excess tannic acid is removed by dialysis bag dialysis, and then freeze-drying is performed to obtain yellow modified cellulose nanocrystals.

[0053] In some embodiments, the molecular weight cut-off of the dialysis bag is 10,000 Da, and the dialysis time is 12-36 h.

[0054] The structural formula of the yellow modified cellulose nanocrystals obtained by freeze-drying is shown in Figure 3 As can be seen from Figure 3 the modification of cellulose nanocrystals by tannic acid is realized.

[0055] Step (3) is a process of forming a casting solution. The modified cellulose nanocrystals, cellulose acetate, and pore-forming agent polyvinylpyrrolidone K30 are dissolved in an organic solvent to obtain a casting solution. The polyvinylpyrrolidone K30 functions as a pore former and dissolves in the coagulation bath to form a pore structure during the phase inversion process.

[0056] In some embodiments, the acetylation degree of the cellulose acetate is 39.2 wt%, and the molecular weight is 30,000.

[0057] In some embodiments, the mass concentration of the modified cellulose nanocrystal in the casting solution is 0.1% to 2.5%, the mass concentration of the cellulose acetate is 10% to 16%, and the mass concentration of the polyvinylpyrrolidone K30 is 0.1% to 0.3%. It can be understood that the mass concentration of the modified cellulose nanocrystal can be 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, or a value within a range consisting of any two of the values, the mass concentration of the cellulose acetate can be 10%, 12%, 14%, 16%, or a value within a range consisting of any two of the values, and the mass concentration of the polyvinylpyrrolidone K30 can be 0.1%, 0.2%, 0.3%, or a value within a range consisting of any two of the values.

[0058] Step (4) is a process of preparing a composite ultrafiltration membrane using the casting solution. The casting solution obtained in step (3) is first subjected to a defoaming treatment, and after defoaming, a wet membrane is obtained by using a film doctor to coat the casting solution on a glass substrate. The wet membrane is left to stand in air for a period of time, and then immersed in an ultrapure water coagulation bath to complete phase inversion, thereby obtaining a composite ultrafiltration membrane.

[0059] In some embodiments, the temperature of the defoaming treatment is 20 to 30°C, and the defoaming time is 4 to 8 h. In some embodiments, the gap height of the film doctor is 200±5 μm, and the thickness of the wet membrane obtained by using the film doctor to coat is 200±5 μm. The wet membrane is left to stand in air for 10 to 60 s, and then immersed in an ultrapure water coagulation bath to complete phase inversion, thereby obtaining a composite ultrafiltration membrane.

[0060] In some embodiments, the porosity of the composite ultrafiltration membrane is 80% to 90%, the overall average pore size of the composite ultrafiltration membrane is 12 to 15 nm, and the contact angle of the composite ultrafiltration membrane is below 60°.

[0061] The modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane of the present application is prepared by the method for preparing a modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane of the present application.

[0062] The modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane of the present application is applied in the field of surface water treatment. In some embodiments, the modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane of the present application is used to treat water to be treated containing protein.

[0063] The present application is described in detail below with reference to examples.

[0064] Example 1

[0065] (1) The cellulose nanocrystals were dispersed in 0.05 M N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer, and the pH was adjusted to 8.5 with 10 M NaOH solution. The mixture was stirred at 500 r / min for 1 h at 25 °C to fully expose the hydroxyl groups on the surface of the cellulose nanocrystals. Then, tannic acid was added at a mass ratio of 1:0.5 to the cellulose nanocrystals, and the mixture was continuously stirred at 500 r / min for 6 h at 25 °C to obtain a yellow transparent solution. Finally, the yellow transparent solution was dialyzed at 25 °C for 24 h using a dialysis bag with a molecular weight cut-off of 10,000 Da to remove unreacted tannic acid. After freeze-drying, yellow modified cellulose nanocrystals were obtained.

[0066] (2) The modified cellulose nanocrystals obtained in step (1) were prepared into a mixed solution with cellulose acetate, polyvinylpyrrolidone K30, and N,N-dimethylacetamide. In the mixed solution, the mass fraction of cellulose acetate was 14%, the mass fraction of polyvinylpyrrolidone K30 was 0.25%, the mass fraction of modified cellulose nanocrystals was 0.5%, and the mass fraction of organic solvent was 85.25%. The mixed solution was stirred at 25 °C for 6 h until the cellulose acetate was completely dissolved, and a uniform casting solution was obtained.

[0067] Subsequently, the casting solution was degassed at 25 °C for 5 h. After the degassing treatment was completed, the casting solution was coated on a glass plate using an adjustable coater at a thickness of 200 μm. The glass plate was placed in air at 25 °C for 10 s, and then the glass plate was placed in deionized water at 25 °C for phase inversion. Then, the prepared membrane was peeled off from the glass plate and transferred to new deionized water to remove residual organic solvents, and a modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane M1-1 was obtained.

[0068] Example 2

[0069] (1) The cellulose nanocrystals were dispersed in 0.05 M N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer, and the pH was adjusted to 8.5 with 10 M NaOH solution. The mixture was stirred at 500 r / min for 1 h at 25 °C to fully expose the hydroxyl groups on the surface of the cellulose nanocrystals. Then, tannic acid was added at a mass ratio of 1:4 to the cellulose nanocrystals, and the mixture was continuously stirred at 500 r / min for 6 h at 25 °C to obtain a yellow transparent solution. Finally, the yellow transparent solution was dialyzed at 25 °C for 24 h using a dialysis bag with a molecular weight cut-off of 10,000 Da to remove unreacted tannic acid. After freeze-drying, yellow modified cellulose nanocrystals were obtained.

[0070] (2) The modified cellulose nanocrystal obtained in step (1) is prepared into a mixed solution with cellulose acetate, polyvinylpyrrolidone K30 and N,N-dimethylacetamide, wherein the mass fraction of cellulose acetate in the mixed solution is 14%, the mass fraction of polyvinylpyrrolidone K30 is 0.25%, the mass fraction of modified cellulose nanocrystal is 2%, and the mass fraction of organic solvent is 83.75%. The mixed solution is stirred at 25°C for 6h until the cellulose acetate is completely dissolved, and a casting solution in a uniform state is obtained.

[0071] Subsequently, the casting solution is defoamed at 20°C for 6h, and after the defoaming treatment is completed, the casting solution is coated on a glass plate with a thickness of 200μm by using an adjustable coater, the glass plate is placed in air at 25°C for 10s, and then the glass plate is placed in deionized water at 25°C for phase inversion. Then, the prepared membrane is peeled off from the glass plate and transferred to new deionized water to remove residual organic solvents, and a modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane M1-2 is obtained.

[0072] Example 3

[0073] (1) The cellulose nanocrystal is dispersed in 0.05M N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer, and the pH is adjusted to 8.5 with 10M NaOH, and the solution is stirred at 500r / min at 25°C for 1h to fully expose the hydroxyl groups on the surface of the cellulose nanocrystal. Subsequently, tannic acid is added at a mass ratio of 1:6 with respect to the cellulose nanocrystal, and the solution is continuously stirred at 500r / min at 25°C for 6h to obtain a yellow transparent solution. Finally, the yellow transparent solution is dialyzed in a dialysis bag with a molecular weight cut-off of 10000Da at 25°C for 24h to remove unreacted tannic acid, and the yellow modified cellulose nanocrystal is obtained after freeze-drying.

[0074] (2) The modified cellulose nanocrystal obtained in step (1) is prepared into a mixed solution with cellulose acetate, polyvinylpyrrolidone K30 and N,N-dimethylacetamide, wherein the mass fraction of cellulose acetate in the mixed solution is 14%, the mass fraction of polyvinylpyrrolidone K30 is 0.25%, the mass fraction of modified cellulose nanocrystal is 2.5%, and the mass fraction of organic solvent is 83.25%. The mixed solution is stirred at 25°C for 6h until the cellulose acetate is completely dissolved, and a casting solution in a uniform state is obtained.

[0075] Subsequently, the casting solution was degassed at 30°C for 4h, and after the degassing treatment was completed, the casting solution was coated on a glass plate using an adjustable coater at a thickness of 200pm, the glass plate was left to stand in air at 25°C for 10s, and then the glass plate was placed in deionized water at 25°C to perform phase inversion. Then, the prepared membrane was peeled off from the glass plate and transferred to new deionized water to remove residual organic solvents, thereby obtaining a modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane M1-3.

[0076] Comparative Example 1

[0077] A casting solution was prepared using cellulose acetate, polyvinylpyrrolidone K30, and N,N-dimethylacetamide, and the mass fraction of cellulose acetate in the casting solution was 14%, the mass fraction of polyvinylpyrrolidone K30 was 0.25%, and the mass fraction of the organic solvent was 85.75%. The mixed solution was stirred at 25°C at a speed of 500r / min for 6h until the cellulose acetate was completely dissolved, and a uniform casting solution was obtained.

[0078] Subsequently, the casting solution was degassed at 25°C for 5h, and after the degassing treatment was completed, the casting solution was coated on a glass plate using an adjustable coater at a thickness of 200pm, the glass plate was left to stand in air at 25°C for 10s, and then the glass plate was placed in deionized water at 25°C to perform phase inversion. Then, the prepared membrane was peeled off from the glass plate and transferred to new deionized water to remove residual organic solvents, thereby obtaining a cellulose acetate ultrafiltration membrane M1.

[0079] Comparative Example 2

[0080] A casting solution was prepared using cellulose acetate, polyvinylpyrrolidone K30, cellulose nanocrystals, and N,N-dimethylacetamide, and the mass fraction of cellulose acetate in the casting solution was 14%, the mass fraction of cellulose nanocrystals was 2%, the mass fraction of polyvinylpyrrolidone K30 was 0.25%, and the mass fraction of the organic solvent was 83.75%. The mixed solution was stirred at 25°C at a speed of 500r / min for 6h until the cellulose acetate was completely dissolved, and the casting solution reached a uniform state.

[0081] Subsequently, the casting solution was degassed at 25°C for 5h, and after the degassing treatment was completed, the casting solution was coated on a glass plate using an adjustable coater at a thickness of 200pm, the glass plate was left to stand in air at 25°C for 10s, and then the glass plate was placed in deionized water at 25°C to perform phase inversion. Then, the prepared membrane was peeled off from the glass plate and transferred to new deionized water to remove residual organic solvents, thereby obtaining a cellulose acetate ultrafiltration membrane M2.

[0082] Comparative Example 3

[0083] The difference from Example 2 is that the mass fraction of modified cellulose nanocrystals is 0.02%, and other processes are the same as Example 2. The prepared film is recorded as M3.

[0084] Comparative Example 4

[0085] The difference from Example 2 is that the mass fraction of modified cellulose nanocrystals is 4%, and other processes are the same as Example 2. The prepared film is recorded as M4.

[0086] Test Example 1

[0087] The modified cellulose nanocrystals prepared in Example 1 are respectively subjected to transmission electron microscope test and infrared spectrum test, and the test results are respectively shown in Figure 4 and Figure 5 .

[0088] According to (a) and (b) in Figure 4 , it can be seen that the modified cellulose nanocrystals are rod-like structures with obvious coating. According to Figure 5 , it can be seen that the modified cellulose nanocrystals have new characteristic peaks at 1613 cm -1 , 1531 cm -1 and 1715 cm -1 , which correspond to the vibration of benzene ring skeleton and the stretching vibration of carbonyl group in tannic acid, respectively, which indicates that the cellulose nanocrystals are successfully modified by tannic acid.

[0089] The modified cellulose nanocrystals prepared in Example 1 are subjected to dispersion experiment test, specifically, the modified cellulose nanocrystals and cellulose nanocrystals are respectively dissolved in N,N-dimethylacetamide to form solutions with a mass concentration of 0.5%, and the dispersion results are shown in Figure 6 ; the modified cellulose nanocrystals and cellulose nanocrystals are respectively dissolved in N,N-dimethylacetamide to form solutions with a mass concentration of 2.5%, and the dispersion results are shown in Figure 7 .

[0090] As can be seen from Figure 6 , the modified cellulose nanocrystals with a mass fraction of 0.5% form a transparent and stable dispersion liquid in N,N-dimethylacetamide, while the unmodified cellulose nanocrystals with the same concentration show obvious turbidity and poor dispersion of the system; Figure 7Further, it is shown that when the mass fraction is increased to 2.5%, the modified cellulose nanocrystals can still form a uniform and stable colloidal dispersion in N,N-dimethylacetamide, while the unmodified cellulose nanocrystals quickly settle and are difficult to maintain dispersion stability. The above results show that the cellulose nanocrystals modified by tannic acid have good dispersibility in organic solvents such as N,N-dimethylacetamide, and can still maintain system stability under high concentration conditions, which is conducive to the uniform preparation of subsequent composite casting solution and the regulation of membrane structure.

[0091] Test Example 2

[0092] The cellulose acetate membrane M1 of Comparative Example 1 and the modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane M1-2 of Example 2 were subjected to attenuated total reflection-Fourier transform infrared spectroscopy test and X-ray diffraction spectrum test, respectively, and the results are shown in Figure 8 .

[0093] As shown in (a) of Figure 8 , the M1-2 membrane has a new absorption peak at 1508 cm -1 , which corresponds to the stretching vibration of the benzene ring skeleton. The wide and strong characteristic absorption peak appearing at 3400 cm -1 is related to the stretching vibration of the phenolic hydroxyl group of tannic acid. In addition, the X-ray diffractometer analysis results, as shown in (b) of Figure 8 , the M1-2 membrane has a new sharp characteristic peak at 22.9°, which corresponds to the (002) crystal plane of CNC crystals. These results show that the modified cellulose nanocrystals have been successfully incorporated into the modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane.

[0094] Test Example 3

[0095] The four membranes obtained from Comparative Example 1, Comparative Example 2, Example 1 and Example 2 were subjected to morphological characterization SEM test, and the cross-sectional SEM images of the obtained membranes correspond to (a), (b), (c) and (d) in Figure 9 .

[0096] As can be seen from Figure 9 , the M1 membrane of Comparative Example 1 shows a typical dense finger-like pore structure, the pore channel is short and the distribution is uneven, and the porosity of the membrane is 76.73%; the M2 membrane of Comparative Example 2 has a prolonged finger pore layer but a defective sponge layer, and the porosity of the membrane is 82.15%; the M1-1 membrane of Example 1 has increased finger pore length and thinned sponge layer, but the distribution is not too uniform, and the porosity of the membrane is 85.35%; the M1-2 membrane of Example 2 shows a uniform sponge-like pore structure, the finger pores extend to the bottom of the membrane, and the sponge layer is very thin, and the porosity is 88.55%.

[0097] The four membranes obtained from Comparative Example 1, Comparative Example 2, Example 1 and Example 2 were subjected to water contact angle test, and the test results are shown in Table 1.

[0098] Table 1:

[0099]

[0100] As can be seen from Table 1, the water contact angles of M1-1 membrane and M1-2 are smaller than those of M1 membrane and M2 membrane, which shows that the introduction of modified cellulose nanocrystals enhances the hydrophilicity of the membrane surface, which is caused by the enrichment of hydrophilic modified cellulose nanocrystals on the surface by surface segregation during phase inversion.

[0101] Test Example 4

[0102] The M1-1 membrane prepared in Example 1 and the M1 membrane prepared in Comparative Example 1 were subjected to tensile property test and pressure resistance test, and the test results are shown in Table 2.

[0103] Table 2:

[0104]

[0105] The mechanical properties of the ultrafiltration membrane were evaluated by tensile property test and pressure resistance test, and the results showed that the tensile stress of the M1-1 membrane prepared in Example 1 reached 1.79 MPa, which was increased by 81.48% compared with the M1 prepared in Comparative Example 1, and the modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane had good mechanical properties.

[0106] Test Example 5

[0107] The membranes prepared in Comparative Example 1, Comparative Example 3, Comparative Example 4 and Example 3 were subjected to filtration performance test, and the test method was as follows: the membrane was pre-pressed for 30 min under an operating pressure of 0.15 MPa, and then the operating pressure was reduced to 0.1 MPa, and the water flux of the membrane was measured. Similarly, according to the same test procedure, the rejection rate of the membrane to 1 g / L bovine serum albumin contaminant model was measured. The test results are shown in Table 3.

[0108] Table 3:

[0109]

[0110] As can be seen from Table 3, the water fluxes of M1-1 membrane, M1-2 membrane and M1-3 membrane are significantly improved compared with M1 membrane, and the bovine serum albumin rejection rates of M1-1 membrane, M1-2 membrane and M1-3 membrane are higher than that of M1 membrane.

[0111] The water fluxes of the M1-1 film, the M1-2 film and the M1-3 film are all greater than those of the M3 and M4 films, indicating that too much or too little modified cellulose nanocrystal is not conducive to the improvement of the film performance, and when the amount of the modified cellulose nanocrystal is moderate, it helps to form a continuous and uniform pore network in the film structure, improve the water molecule transmission efficiency, and at the same time avoid the insufficient modification caused by too little filler or the pore blockage and structure densification caused by too much filler.

[0112] Test Example 6

[0113] The static adsorption capacity of bovine serum albumin of the films prepared in the Comparative Example 1, the Example 1, the Example 2 and the Example 3 was tested, and the dynamic circulation filtration test was performed on the films prepared in the Comparative Example 1 and the Example 2, and the test results are shown in Table 4 and Figure 10

[0114] Table 4:

[0115]

[0116] According to Table 4, the static adsorption capacity of the Examples 1 to 3 is significantly lower than that of the Comparative Example 1, indicating that the introduction of the modified cellulose nanocrystal can effectively reduce the adsorption of the protein pollutants on the film surface.

[0117] According to the dynamic circulation filtration test results shown in Figure 10 , after 5 filtration cycles, the flux recovery rate of the M1-2 film of the Example 2 is 85.21%, which is significantly higher than that of the M1 film of the Comparative Example 1 (44.48%), further verifying the effectiveness of the modified cellulose nanocrystal in inhibiting the pollution of the film.

[0118] Test Example 7

[0119] The performance test of the film prepared in the Example 2 on the treated surface water was performed, wherein the surface water was river water, and the test method was as follows: the surface water sample was coarsely filtered through a qualitative filter paper to remove suspended particulate matter, and the obtained filtrate was used for the circulation filtration test. The membrane piece was pre-pressed for 30 min under an operating pressure of 0.15 MPa, and then the operating pressure was reduced to 0.1 MPa, and the water flux of the membrane piece was measured. Similarly, according to the same test steps, the TOC rejection rate of the membrane piece was measured. The test results are shown in Figure 11 . As shown in (a) of Figure 11 , after 3 circulation filtration cycles, the water flux of the M1-2 is basically stable at 140.87 L·m -2 ·h -1 , and the flux recovery rate reaches 99.18%, indicating that it has excellent antifouling property, permeability and pressure resistance when treating real surface water. In addition, Figure 11 ​The three-dimensional fluorescence spectrum analysis of the middle (b) shows that the fluorescence intensity of the humic-like acid and protein region in the water body is significantly reduced after being filtered by the M1-2 composite membrane, which confirms the effective removal ability of the membrane to organic matter. In addition, the TOC concentration of the filtrate is 4.04 mg / L, which meets the requirements of the "Drinking Water Health Standards" (GB5749-2022) of China on TOC.

[0120] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms can be directed to different embodiments or examples. Moreover, the described specific features, structures, materials or characteristics can be combined in any suitable manner in any one or more embodiments or examples. In addition, the person skilled in the art can combine and combine the different embodiments or examples described in the present specification and the features of the different embodiments or examples without contradiction.

[0121] In addition, the terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, for example, two, three, etc., unless otherwise specifically limited.

[0122] Although the embodiments of the present application have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, replacements and variations can be made to these embodiments without departing from the principles and purposes of the present application, and the scope of the present application is defined by the claims and their equivalents.

Claims

1. A method for preparing a modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane, characterized in that, The method comprises the following steps: (1) dispersing cellulose nanocrystals in a buffer solution, adjusting pH to weak alkaline by using an alkaline solution and fully stirring, then adding tannic acid and fully reacting to obtain a yellow transparent solution; (2) removing excess tannic acid by using a dialysis bag to dialyze the yellow transparent solution, and freeze-drying to obtain yellow modified cellulose nanocrystals; (3) dissolving the modified cellulose nanocrystals, cellulose acetate and pore-forming agent in an organic solvent to obtain a casting solution; (4) after defoaming the casting solution, coating it on a substrate, standing and then immersing it in an ultrapure water coagulation bath to complete phase inversion, thereby obtaining a composite ultrafiltration membrane.

2. The method of claim 1, wherein, The mass ratio of the tannic acid to the cellulose nanocrystals is 1:(0.5-6).

3. The method of claim 1, wherein, In the step (1), the pH is adjusted to 7.5-8.5 by using an alkaline solution.

4. The method of claim 1, wherein, The pore-forming agent is polyvinylpyrrolidone K30; and / or, the dialysis bag has a molecular weight cut-off of 10,000 Da.

5. The method of claim 4, wherein, In the casting solution, the mass concentration of the modified cellulose nanocrystals is 0.1%-2.5%, the mass concentration of the cellulose acetate is 10%-16%, and the mass concentration of the polyvinylpyrrolidone K30 is 0.1%-0.3%.

6. The method of claim 1, wherein, In the step (4), the thickness of the coating is 200±5 μm.

7. The method of claim 1, wherein, The organic solvent comprises one or more of N,N-dimethylacetamide, N-methylpyrrolidone and dimethyl sulfoxide.

8. The method of claim 1, wherein, The porosity of the composite ultrafiltration membrane is 80%-90%, the overall average pore size of the composite ultrafiltration membrane is 12-15 nm, and the contact angle of the composite ultrafiltration membrane is below 60°.

9. A modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane, characterized in that, Prepared by the method of any one of claims 1-8.

10. The application of the modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane prepared by the method of any one of claims 1-8 or the modified cellulose nanocrystal / cellulose acetate composite ultrafiltration membrane of claim 9 in surface water treatment.

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