Composite separation membrane as well as preparation method and application thereof

Abstract

The invention relates to the technical field of membrane separation treatment, in particular to a composite separation membrane and a preparation method and application thereof. The composite separation membrane comprises a hollow polymer base membrane and a modified layer attached to the outer surface of the hollow polymer base membrane, wherein the surface of the hollow polymer base membrane is provided with a plurality of membrane holes, and at least part of the modified layer forms recesses in the membrane holes; the modified layer comprises graphene oxide nanosheets, nanocellulose and sulfonated succinic acid; at least part of the nanocellulose is fixed between sheet layers of the graphene oxide nanosheets, and at least part of the sulfonated succinic acid is fixed between sheet layers of the graphene oxide nanosheets; the modified layer and the hollow polymer base membrane in the composite separation membrane have good bonding strength, and the composite separation membrane has the advantage of good stability, has good hydrophilicity and can effectively improve the separation efficiency.

Description

Technical Field

[0001] This application relates to the technical field of membrane separation treatment, and particularly relates to a composite separation membrane, a preparation method thereof, and an application thereof. Background Art

[0002] In order to achieve efficient separation and purification of substances in multiple fields such as water treatment, food and beverage, biopharmaceuticals, and chemical engineering, membrane separation technology has emerged. Compared with traditional separation methods such as distillation, evaporation, adsorption, extraction, and chromatographic separation, membrane separation technology has the advantages of high separation accuracy, fast filtration speed, low operating pressure, and strong adaptability.

[0003] The basic principle of membrane separation technology is to use the microporous structure to intercept suspended solids, colloids, and macromolecular pollutants, and allow small molecules such as water molecules to pass through, so as to achieve the separation and purification of substances. The membrane is the core of membrane separation technology. Currently, membrane materials such as polyvinylidene fluoride (PVDF), polyethylene (PE), and polypropylene (PP) are used more frequently. However, these polymers have low surface energy and high hydrophobicity, so they are prone to adsorb hydrophobic organic substances such as humic acid or microorganisms, which easily leads to serious membrane fouling and a significant flux decay rate (generally 50%). Currently, the main methods for hydrophilic modification of hydrophobic membrane materials are as follows: (1) introducing hydrophilic polymers such as polyvinylpyrrolidone as a modified layer to improve the hydrophilicity of the surface of hydrophobic materials. However, the above-mentioned modification method has the disadvantage that the modified layer is prone to peeling; (2) introducing materials such as titanium dioxide and carbon nanotubes to improve hydrophilicity. However, aggregates of titanium dioxide nanoparticles and carbon nanotubes are likely to cause membrane pore blockage, thereby reducing the membrane separation efficiency; (3) introducing a modified layer whose preparation raw materials include graphene oxide or graphene, organosilicon compounds, and surfactants. However, the above-mentioned modified layer also has the disadvantage of insufficient bonding strength with the base membrane and is prone to peeling. Summary of the Invention

[0004] Based on this, this application provides a composite separation membrane, a preparation method thereof, and an application thereof. The modified layer and the hollow polymer base membrane in the composite separation membrane provided by this application have good bonding strength and good stability, and the above composite separation membrane has good hydrophilicity and can effectively improve the separation efficiency.

[0005] In the first aspect of this application, a composite separation membrane is provided. The composite separation membrane includes a hollow polymer base membrane and a modified layer attached to the outer surface of the hollow polymer base membrane;

[0006] Wherein, the surface of the hollow polymer base membrane has a plurality of membrane pores, and at least part of the modified layer forms a depression in the membrane pores;

[0007] The modified layer includes graphene oxide nanosheets, nanocellulose, and sulfonated succinic acid; at least part of the nanocellulose is fixed between the layers of the graphene oxide nanosheets, and at least part of the sulfonated succinic acid is fixed between the layers of the graphene oxide nanosheets.

[0008] In one embodiment, the depth of the depression formed by the modified layer in the membrane pores is 50 nm to 200 nm.

[0009] In one embodiment, the modified layer has a two-dimensional network structure.

[0010] In one embodiment, the fixing methods of the nanocellulose include at least one of physical adsorption, coupling connection between the active sites of the nanocellulose and the graphene oxide nanosheets, coupling connection between the oxygen-containing functional groups of the nanocellulose and the graphene oxide nanosheets, and intermolecular interaction between the nanocellulose and the graphene oxide nanosheets.

[0011] In one embodiment, the fixing methods of the sulfonated succinic acid include at least one of physical adsorption, coupling connection between the active sites of the sulfonated succinic acid and the graphene oxide nanosheets, coupling connection between the oxygen-containing functional groups of the sulfonated succinic acid and the graphene oxide nanosheets, and intermolecular interaction between the sulfonated succinic acid and the graphene oxide nanosheets.

[0012] In one embodiment, the mass ratio of the graphene oxide nanosheets to the nanocellulose is (0.1 - 0.5):(0.05 - 0.2).

[0013] In one embodiment, the mass ratio of the graphene oxide nanosheets to the sulfonated succinic acid is (0.1 - 0.5):(0.2 - 0.8).

[0014] In one embodiment, the outer diameter of the hollow polymer-based membrane is 500 μm to 600 μm.

[0015] In one embodiment, the inner diameter of the hollow polymer-based membrane is 200 μm to 300 μm.

[0016] In one embodiment, the pore diameter of the membrane pores is 0.3 μm to 0.8 μm.

[0017] In one embodiment, the material of the hollow polymer-based membrane includes one or more of polyvinylidene fluoride, polypropylene, and polyacrylonitrile.

[0018] In one embodiment, the contact angle of static water on the surface of the composite separation membrane is ≤ 40°.

[0019] In one embodiment, the static adsorption amount of humic acid on the surface of the composite separation membrane is 40 mg / m 2 ~170 mg / m 2 .

[0020] In one embodiment, the rejection rate of the composite separation membrane for humic acid is 90% - 98%.

[0021] In the second aspect of the present application, there is provided a method for preparing the composite separation membrane according to any one of the first aspects of the present application, comprising the following steps:

[0022] Mix the graphene oxide nanosheets and the nanocellulose in a solvent, after ultrasonic treatment, add the sulfonated succinic acid for mixing reaction to prepare a casting solution;

[0023] Coat the casting solution on the surface of the hollow polymer-based membrane, apply negative pressure in the hollow cavity of the hollow polymer-based membrane, so that the casting solution forms a depression in at least part of the membrane pores of the hollow polymer-based membrane; after thermal curing, at least part of the nanocellulose is fixed between the layers of the graphene oxide nanosheets, and at least part of the sulfonated succinic acid is fixed between the layers of the graphene oxide nanosheets to form the modified layer, and the composite separation membrane is prepared.

[0024] In one embodiment, the negative pressure is -0.1 MPa to -0.05 MPa.

[0025] In one embodiment, the process parameters of the thermal curing include: the thermal curing temperature is 60°C to 80°C, and the thermal curing time is 1 h to 2 h.

[0026] In one embodiment, the process parameters of the ultrasonic treatment include: the ultrasonic time is 30 min to 60 min.

[0027] In one embodiment, the process parameters of the mixing reaction include: the mixing time is 2 h to 4 h, and the rotation speed is 500 r / min to 800 r / min.

[0028] In one embodiment, the mass ratio of the graphene oxide nanosheets to the nanocellulose is (0.1 - 0.5):(0.05 - 0.2).

[0029] In one embodiment, the mass ratio of the graphene oxide nanosheets to the sulfonated succinic acid is (0.1 - 0.5):(0.2 - 0.8).

[0030] In one embodiment, the solvent includes deionized water and an alcohol solvent with a volume ratio of (5 - 8):(2 - 5).

[0031] In one embodiment, before the step of coating the casting solution on the surface of the hollow polymer-based membrane, the method further includes: soaking, washing, and drying the hollow polymer-based membrane in an organic solvent;

[0032] Wherein, the process parameters of the soaking treatment include: the time is 10 min to 30 min.

[0033] In the third aspect of the present application, there is provided an application of the composite separation membrane according to any one of the embodiments of the first aspect of the present application in the fields of water treatment, biomedicine, food processing, or chemical purification.

[0034] The beneficial effects of the composite separation membrane provided by the present application at least include:

[0035] The composite separation membrane of the present application includes a hollow polymer-based membrane and a modified layer attached to the outer surface of the hollow polymer-based membrane. Among them, the hollow polymer-based membrane can effectively intercept organic macromolecules, thereby ensuring the interception amount of the composite separation membrane.

[0036] At least part of the modified layer forms a semi-immersed concave anchoring structure at the membrane pores of the hollow polymer-based membrane. This structure can effectively enhance the bonding strength between the modified layer and the hollow polymer membrane layer through the mechanical interlocking effect and the chemical bonding between the modified layer and the hollow polymer-based membrane, thereby improving the stability of the composite separation membrane.

[0037] In addition, at least part of the nanocellulose is fixed between the layers of the graphene oxide nanosheets. At this time, the nanocellulose can be fixed through physical adsorption, the oxygen-containing functional groups on the surface of the nanocellulose and the graphene oxide nanosheets can form chemical bonds (coupling connections) through dehydration condensation, the coupling connections between the active sites of the nanocellulose and the graphene oxide nanosheets, or the intermolecular interactions between the nanocellulose and the graphene oxide. At the same time, at least part of the sulfonated succinic acid is fixed between the layers of the graphene oxide nanosheets; at this time, the sulfonated succinic acid can be fixed through physical adsorption, the coupling connections between the active sites of the sulfonated succinic acid and the graphene oxide nanosheets, the coupling connections between the oxygen-containing functional groups of the sulfonated succinic acid and the graphene oxide nanosheets, the hydrogen bonds between the sulfonic groups of the sulfonated succinic acid and the graphene oxide nanosheets and other intermolecular interactions. Above, through the synergistic effect between the nanocellulose and the sulfonated succinic acid, a stable cross-linked structure can be formed in the modified layer, thereby ensuring the stability of the modified layer. In addition, the nanocellulose and the sulfonated succinic acid are fixed between the layers of the graphene oxide nanosheets, which can form selective water channels, thereby enabling water molecules to quickly pass through the hydrophilic modified layer and improving the water flux.

[0038] Furthermore, the introduction of the hydrophilic modification layer can reduce the adsorption of organic pollutants. At this time, the accumulation rate of organic pollutants on the surface of the composite separation membrane is significantly reduced, thereby delaying the membrane cleaning cycle, reducing the flux decay rate of the composite separation membrane, and improving the separation effect.

[0039] In summary, the modification layer in the composite separation membrane provided by this application has good bonding strength with the hollow polymer-based membrane and has the advantage of good stability. Moreover, the above composite separation membrane has good hydrophilicity, and also has good water flux and good retention capacity for organic macromolecules; it can effectively improve the separation efficiency and reduce the membrane cleaning cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a schematic structural diagram of the modification layer provided by an example of this application;

[0041] Figure 2 It is a schematic structural diagram of the hollow polymer-based membrane provided by an example of this application;

[0042] Figure 3 It is a schematic structural diagram of the composite separation membrane provided by an example of this application;

[0043] Figure 4 It is an electron microscope image of the modification layer forming a concave structure at the membrane pores of the hollow polymer-based membrane in an example of this application;

[0044] Figure 5 It is a schematic diagram of the working principle when the composite separation membrane of this application is used for water purification treatment.

[0045] In the figure, 10, hollow polymer-based membrane; 20, modification layer; 201, sulfonated succinic acid; 202, nanocellulose; 203, graphene oxide nanosheets; A, water inlet side; B, permeate side; C, water molecule flow direction. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] The following further describes the composite separation membrane of this application, its preparation method, and its application in a complete and clear manner with specific examples. This application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosed content of this application more thorough and comprehensive.

[0047] It can be understood that in this application, "flux" refers to the amount of water passing through the membrane.

[0048] It can be understood that in this application, "retention capacity" refers to the retention capacity of the membrane for organic substances.

[0049] As a highly efficient separation technology, hollow fiber ultrafiltration membranes are widely used in water treatment, medicine, food and beverage and other fields. The core principle is to intercept suspended solids, colloids and macromolecular pollutants through the microporous structure, while allowing water molecules to pass through. At present, the mainstream materials are mainly hydrophobic polymer polymers such as polyvinylidene fluoride (PVDF), polypropylene, and polyacrylonitrile (PAN). Although their chemical stability is excellent, the surface hydrophobicity leads to prominent membrane fouling problems, specifically manifested as: (1) Insufficient hydrophilicity: Taking traditional PVDF as an example, the static water contact angle of the PVDF membrane is as high as about 80°, which is easy to adsorb hydrophobic organic substances (such as humic acid) and microorganisms, forming an irreversible fouling layer, and the flux decay rate is significant (the total decay rate is generally about 50%). (2) Limited anti-fouling performance: The static adsorption capacity of traditional membrane materials for humic acid is as high as 295.0mg / m 2 , pollutants are easy to accumulate on the membrane surface, and frequent chemical cleaning is required, resulting in shortened membrane life and increased operating costs. (3) Low flux recovery efficiency: The flux recovery rate of conventional membranes after hydraulic backwashing is only 69%, which is difficult to effectively remove deep pollutants, and the long-term operation stability is insufficient. (4) Contradiction between retention performance and operation cycle (flux): The retention rate of traditional membranes for micro-pollutants is relatively low (such as the retention rate of humic acid is about 81%). To improve the retention rate, the pore size needs to be reduced, but this will lead to a decrease in flux and a shortening of the filtration cycle.

[0050] Graphene oxide (GO) has a high specific surface area, excellent hydrophilicity, and its surface is rich in oxygen-containing functional groups (-OH, -COOH). Therefore, GO has been tried for membrane modification. Research shows that the GO coating can reduce the membrane surface contact angle to about 40° - 50°. However, the bonding strength between the GO-containing coating and the base membrane is insufficient at present, and it is easy to peel off during long-term operation, and the large-scale preparation process is not yet mature, so the production efficiency cannot be improved.

[0051] In the first aspect of the present application, a composite separation membrane is provided, and the composite separation membrane includes a hollow polymer base membrane and a modified layer attached to the outer surface of the hollow polymer base membrane.

[0052] Among them, the surface of the hollow polymer base membrane has a plurality of membrane pores, and at least part of the modified layer forms a depression in the membrane pores.

[0053] Refer to Figure 1 , the modified layer 20 includes graphene oxide nanosheets 203, nanocellulose 202, and sulfonated succinic acid 201. At least part of the nanocellulose 202 is fixed between the layers of the graphene oxide nanosheets 203. At least part of the sulfonated succinic acid 201 is fixed between the layers of the graphene oxide nanosheets 203.

[0054] The fixing methods of the nanocellulose and the sulfonated succinic acid each independently include at least one of physical adsorption, coupling connection, and intermolecular interaction.

[0055] The composite separation membrane of the present application includes a hollow polymer-based membrane and a modified layer attached to the outer surface of the hollow polymer-based membrane. Among them, the hollow polymer-based membrane can effectively intercept organic macromolecules, thereby ensuring the interception amount of the composite separation membrane.

[0056] The modified layer includes graphene oxide nanosheets, nanocellulose, and sulfonated succinic acid. The three cooperate with each other, making the modified layer rich in hydrophilic groups such as hydroxyl groups and carboxyl groups, effectively reducing the static contact angle of the membrane surface to reduce the adsorption of hydrophobic organic pollutants, reducing the static adsorption amount of the composite separation membrane for humic acid by 60%, delaying the formation of the pollutant layer, thereby delaying the cleaning cycle of the membrane and reducing the flux decay rate of the composite separation membrane, and improving the separation effect. Moreover, due to the reduction in the adsorption amount of hydrophobic organic pollutants, the filtration cycle of the membrane is increased, the cleaning rate is reduced, and the flux recovery rate after membrane cleaning is effectively increased.

[0057] At least part of the modified layer forms a semi-immersed concave anchoring structure at the membrane pores of the hollow polymer-based membrane. This structure can effectively enhance the bonding strength between the modified layer and the hollow polymer membrane layer through the mechanical interlocking effect and the chemical bonding between the modified layer and the hollow polymer-based membrane, thereby improving the stability of the composite separation membrane. Specifically, the modified layer of the composite separation membrane of the present application has no obvious peeling under long-term high-pressure operation (>0.5 MPa) and acid-base cleaning (pH 2-12) conditions, and its stability is significantly better than the traditional GO coating technology.

[0058] In addition, at least part of the nanocellulose is fixed between the layers of the graphene oxide nanosheets. At this time, the nanocellulose can be fixed through physical adsorption, the oxygen-containing functional groups on the surface of the nanocellulose and the graphene oxide nanosheets can form chemical bonds (coupling connections) through dehydration condensation, the coupling connection between the active sites of the nanocellulose and the graphene oxide nanosheets, or the intermolecular interaction between the nanocellulose and the graphene oxide. At the same time, at least part of the sulfonated succinic acid is fixed between the layers of the graphene oxide nanosheets; at this time, the sulfonated succinic acid can be fixed through physical adsorption, the coupling connection between the active sites of the sulfonated succinic acid and the graphene oxide nanosheets, the coupling connection between the oxygen-containing functional groups of the sulfonated succinic acid and the graphene oxide nanosheets, and intermolecular interactions such as hydrogen bonds between the sulfonic groups of the sulfonated succinic acid and the graphene oxide nanosheets. Above, through the synergistic effect between the nanocellulose and the sulfonated succinic acid, a stable three-dimensional cross-linked structure can be formed in the modified layer to inhibit the swelling of the graphene oxide layers in water, thereby ensuring the stability of the modified layer. In addition, the nanocellulose and the sulfonated succinic acid are fixed between the layers of the graphene oxide nanosheets, which can form selective water channels and cooperate with the traditional mass transfer mode of the hollow polymer-based membrane to enable water molecules to quickly pass through the hydrophilic modified layer to improve the flux.

[0059] In summary, the modified layer in the composite separation membrane provided in this application has good bonding strength with the hollow polymer-based membrane and has the advantage of good stability. Moreover, the above composite separation membrane has good hydrophilicity, and also has good water flux and good retention of organic macromolecules; it can effectively improve the separation efficiency and reduce the membrane cleaning cycle.

[0060] Nanocrystalline cellulose (CNF) is a cellulose material with a nanoscale structure extracted from natural cellulose raw materials (such as wood, cotton, bamboo, etc.) through physical, chemical or biological methods and further processed. Its diameter is usually between a few nanometers and dozens of nanometers, and its length can reach the micron level. It has many excellent properties such as high specific surface area, high crystallinity, high strength, high elastic modulus, good biocompatibility, biodegradability and renewability.

[0061] In one example, the depth of the depression formed by the modified layer in the membrane pores is 50 nm to 200 nm. It can be understood that the depth of the depression formed by the modified layer in the membrane pores can be any value selected between 50 nm and 200 nm. For example, the depth of the depression formed by the modified layer in the membrane pores includes but is not limited to 50 nm, 80 nm, 100 nm, 120 nm, 150 nm, 180 nm or 200 nm, or within the range formed by any two of the above point values as the end point values. The semi-immersed depression anchoring structure formed by the modified layer at the membrane pores of the hollow polymer-based membrane in this application can achieve the tight combination of the modified layer and the hollow polymer-based membrane.

[0062] In one example, the modified layer has a two-dimensional network structure.

[0063] In one example, the fixing methods of the nanocellulose include at least one of physical adsorption, coupling connection between the active sites of the nanocellulose and the graphene oxide nanosheets, coupling connection between the oxygen-containing functional groups of the nanocellulose and the graphene oxide nanosheets, and intermolecular interaction between the nanocellulose and the graphene oxide nanosheets.

[0064] In one example, the fixing methods of the sulfonated succinic acid include at least one of physical adsorption, coupling connection between the active sites of the sulfonated succinic acid and the graphene oxide nanosheets, coupling connection between the oxygen-containing functional groups of the sulfonated succinic acid and the graphene oxide nanosheets, and intermolecular interaction between the sulfonated succinic acid and the graphene oxide nanosheets.

[0065] It can be understood that the surface of the graphene oxide nanosheets contains oxygen-containing functional groups and active sites. At this time, it can be fixed with the nanocellulose and the sulfonated succinic acid by means of coupling connection. In addition, sulfonic acid groups of the sulfonated succinic acid can form intermolecular interactions such as hydrogen bonds with the graphene oxide nanosheets, which can further achieve fixation.

[0066] In one example, the mass ratio of the graphene oxide nanosheets to the nanocellulose is (0.1~0.5):(0.05~0.2). Limiting the mass ratio of the graphene oxide nanosheets to the nanocellulose can enable the graphene oxide nanosheets to provide hydrophilic groups, and can synergistically play the modification role of the nanographene to improve the mechanical strength of the modified layer, and form water channels between the nanocellulose layers to improve the flux. Exemplarily, the mass ratio of the graphene oxide nanosheets to the nanocellulose includes but is not limited to 0.1:0.05, 0.1:0.1, 0.15:0.1, 0.2:01, 0.3:0.1, 0.4:0.1, 0.45:0.15 or 0.5:0.2, or within the range formed by any two of the above point values as the end point values.

[0067] In one example, the mass ratio of the graphene oxide nanosheets to the sulfonated succinic acid is (0.1~0.5):(0.2~0.8). The mass ratio of the graphene oxide nanosheets to the sulfonated succinic acid includes but is not limited to 0.1:0.2, 0.2:0.3, 0.2:0.4, 0.4:0.7 or 0.5:0.8, or within the range formed by any two of the above point values as the end point values.

[0068] Hydrophilic molecules CNF and sulfonated succinic acid SSA are selected as composite crosslinking agents in the two-dimensional structure of GO to form a composite crosslinked structure, with strong structural stability of the membrane and adjustable GO layer spacing. The surface of the membrane is hydrophilically modified. That is, a dense hydrophilic layer is formed on the surface of the composite separation membrane, which can effectively reduce the adsorption of pollutants on the membrane surface and improve the anti-wetting performance of the membrane at the same time.

[0069] In one example, the outer diameter of the hollow polymer-based membrane is 500 μm to 600 μm. Exemplarily, the outer diameter of the hollow polymer-based membrane includes but is not limited to 500 μm, 520 μm, 550 μm, 580 μm or 600 μm, or within the range formed by any two of the above point values as end point values.

[0070] In one example, the inner diameter of the hollow polymer-based membrane is 200 μm to 300 μm. Exemplarily, the inner diameter of the hollow polymer-based membrane includes but is not limited to 200 μm, 210 μm, 220 μm, 230 μm, 250 μm, 270 μm, 290 μm or 300 μm, or within the range formed by any two of the above point values as end point values.

[0071] To ensure that the modified layer forms a depression at the membrane pores, it is necessary to ensure the thickness of the hollow polymer-based membrane, limit the inner and outer diameters of the hollow polymer-based membrane, which is also conducive to ensuring the water flux and ensuring the chemical and mechanical stability of the membrane.

[0072] In one example, the pore diameter of the membrane pores is 0.3 μm to 0.8 μm. It can be understood that the pore diameter of the membrane pores can be selected from any value between 0.3 μm and 0.8 μm. Exemplarily, the pore diameter of the membrane pores includes but is not limited to 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm or 0.8 μm, or within the range formed by any two of the above point values as end point values. Refer to Figure 2 , the inside of the hollow polymer-based membrane is a hollow channel, and its surface has multiple membrane pores. The hollow polymer-based membrane screens the particles flowing through it according to the size of the membrane pores, and only allows molecules of a specific size or smaller to pass through. During the membrane separation process, the raw liquid is on one side of the composite separation membrane under the action of a static pressure difference, the solvent and small molecule solutes pass through the membrane pores to become the filtrate, while the macromolecule solutes are retained by the membrane, so as to achieve the purpose of material separation and concentration. Therefore, limiting the pore diameter of the membrane pores plays an important role in ensuring the retention volume and flux of the composite separation membrane.

[0073] In one example, the material of the hollow polymer-based membrane includes one or more of polyvinylidene fluoride, polypropylene and polyacrylonitrile.

[0074] Preferably, the material of the hollow polymer-based membrane includes polyvinylidene fluoride.

[0075] The material of the hollow polymer-based membrane is a semi-crystalline thermoplastic polymer. The structure of the hollow fiber membrane formed by it is as shown in Figure 2 and it is a fine hollow fiber filamentous separation membrane.

[0076] Referring to Figure 3 , in this application, the composite separation membrane includes a hollow polymer-based membrane 10 and a modified layer 20 attached to the outer surface of the hollow polymer-based membrane 10. In one example, the thickness of the modified layer 20 is 50 nm to 200 nm. Exemplarily, the thickness of the modified layer includes but is not limited to 50 nm, 80 nm, 100 nm, 120 nm, 150 nm, 180 nm or 200 nm, or within the range formed by any two of the above point values as the end point values.

[0077] In one example, the contact angle of static water on the surface of the composite separation membrane is ≤40°. The contact angle of static water on the surface of the traditional membrane is about 80°. The modified layer in this application is rich in hydrophilic groups such as hydroxyl and carboxyl groups, which can effectively reduce the contact angle of static water on the membrane surface to ≤40°, so as to reduce the adsorption tendency of hydrophobic pollutants.

[0078] In one example, the static adsorption amount of humic acid on the surface of the composite separation membrane is 40 mg / m 2 ~170 mg / m 2 . The static adsorption amount of humic acid on the surface of the composite separation membrane includes but is not limited to 50 mg / m 2 , 80 mg / m 2 , 100 mg / m 2 , 120 mg / m 2 , 140 mg / m 2 , 160 mg / m 2 or 170 mg / m 2 , or within the range formed by any two of the above point values as the end point values. The static adsorption amount of humic acid on the surface of the traditional separation membrane is about 295 mg / m 2 . The composite separation membrane of this application can effectively reduce the adsorption of organic pollutants such as humic acid, significantly slow down the accumulation rate of pollutants on the membrane surface, and thus extend the cleaning cycle of the membrane.

[0079] In one example, the retention amount of humic acid by the composite separation membrane is 90% - 98%. The modified layer in this application includes nanocellulose and sulfonated succinic acid fixed between graphene oxide sheets, which can form the nano-channel structure of the modified layer, which is easy to ensure the water flux and ensure the retention amount of organic substances such as humic acid; and the initial flux of the composite separation membrane of this application can be increased by 20% compared with the unmodified separation membrane, overcoming the contradiction between "flux-retention amount" in the traditional technology.

[0080] In addition, the composite separation membrane provided by this application also has other beneficial effects. Exemplarily, the above beneficial effects include but are not limited to: (1) Reduced operating costs: Since the composite separation membrane of this application can effectively reduce the adsorption of organic pollutants, the frequency of chemical cleaning can be effectively reduced, such as the synchronous reduction of chemical agent consumption and manual maintenance costs. At the same time, due to the slowdown of the pollution rate and the reduction of cleaning damage, the membrane replacement cycle is extended by 30% - 40%. Moreover, the service life of the composite separation membrane, that is, the filtration cycle, is extended, the water production per unit time is increased, and the unit water treatment cost is reduced. Further, the GO dosage per unit membrane area of the composite separation membrane of this application is only 0.05 - 0.2 g / m 2 , and the raw material cost accounts for a relatively low proportion. (2) Significant environmental benefits: The reduction of the chemical cleaning frequency of the composite separation membrane of this application can directly reduce the discharge of acid / alkali cleaning waste liquid and reduce the risk of secondary pollution to water bodies. Moreover, the increase in flux and the reduction of the separation operating pressure can reduce the energy consumption of the water treatment system. In addition, the rejection rate of micro-pollutants (such as humic acid and antibiotics) is increased to more than 90%, which can provide technical guarantee for drinking water safety. And the reduction of the membrane replacement frequency can reduce the generation amount of solid waste, which conforms to the concept of circular economy.

[0081] Currently, the preparation methods for the graphene oxide modified layer mainly rely on chemical modification or vacuum filtration. However, the chemical modification method requires precise control of reaction conditions and involves multiple chemical reactions, and the vacuum filtration method has special requirements for equipment and low production efficiency. Therefore, the traditional preparation methods for the graphene oxide modified layer have the disadvantages of complex processes and difficulty in realizing industrial production.

[0082] In the second aspect of this application, a preparation method for the composite separation membrane according to any one of the first aspect of this application is provided, including the following steps:

[0083] S10: Mix the graphene oxide nanosheets and the nanocellulose in a solvent, after ultrasonic treatment, add the sulfonated succinic acid for mixing reaction to prepare a casting solution.

[0084] S20: Coat the casting solution on the surface of the hollow polymer-based membrane, apply negative pressure in the hollow cavity of the hollow polymer-based membrane, so that the casting solution forms a depression in at least part of the membrane pores of the hollow polymer-based membrane; after thermal curing, fix at least part of the nanocellulose between the layers of the graphene oxide nanosheets, and fix at least part of the sulfonated succinic acid between the layers of the graphene oxide nanosheets to form the modified layer and prepare the composite separation membrane.

[0085] This application prepares a composite separation membrane by means of negative pressure and thermal curing. This preparation method does not require complex equipment, and the preparation method is simple and easy to realize industrial production.

[0086] In one example, the step of coating the casting solution on the surface of the hollow polymer-based membrane includes: immersing the hollow polymer-based membrane in the casting solution and maintaining it for 10 min to 20 min.

[0087] In this application, the negative pressure applied in the cavity of the hollow polymer-based membrane plays an important role in enabling the casting solution to penetrate to the surface of the membrane pores by suction filtration to form depressions. In one example, the negative pressure is -0.1 MPa to -0.05 MPa. Limiting the negative pressure within the above range in this application can ensure the effective infiltration of the casting solution and prevent blockage caused by infiltration of excessive pressure into the pores.

[0088] Exemplarily, the pressure of the negative pressure includes but is not limited to -0.1 MPa, -0.08 MPa, -0.06 MPa, or -0.05 MPa, or within the range formed by any two of the above point values as the end point values.

[0089] In one example, the process parameters of the thermal curing include: the thermal curing temperature is 60 °C to 80 °C. The thermal curing temperature includes but is not limited to 60 °C, 63 °C, 65 °C, 68 °C, 70 °C, 72 °C, 75 °C, 78 °C, or 80 °C, or within the range formed by any two of the above point values as the end point values. Defining the process parameters of the thermal curing plays an important role in ensuring that nanocellulose or sulfonated succinic acid is fixed between the layers of graphene oxide nanosheets. For example, it can ensure the full formation of hydrogen bonds, ester groups, etc. between graphene oxide and nanocellulose or sulfonated cellulose.

[0090] In one example, the process parameters of the thermal curing include: the thermal curing time is 1 h to 2 h. The thermal curing time includes but is not limited to 1 h, 1.2 h, 1.5 h, 1.8 h, or 2 h, or within the range formed by any two of the above point values as the end point values.

[0091] The electron micrograph of the modified layer forming a depression structure at the membrane pores of the hollow polymer-based membrane in this application is as Figure 4 shown. During the above preparation process, the construction of the depression structure mainly includes: during the suction filtration process, the casting solution is driven by negative pressure into the membrane pores, and the GO sheets are directionally arranged at the pore mouth edge due to capillary force; after thermal curing, a "depression" structure is formed at the pore mouth, and the bonding strength between the coating and the hollow polymer-based membrane is enhanced through the mechanical interlocking effect.

[0092] In one example, the process parameters of the ultrasonic treatment include: the ultrasonic time is 30 min to 60 min.

[0093] In one example, the process parameters of the mixing reaction include: the mixing time is 2 h to 4 h, and the rotation speed is 500 r / min to 800 r / min. Exemplarily, the rotation speed of the mixing reaction includes but is not limited to 500 r / min, 600 r / min, 700 r / min, or 800 r / min.

[0094] In one example, the mass ratio of the graphene oxide nanosheets to the nanocellulose is (0.1 - 0.5):(0.05 - 0.2).

[0095] In one example, the mass ratio of the graphene oxide nanosheets to the sulfosuccinic acid is (0.1 - 0.5):(0.2 - 0.8).

[0096] In one example, in the casting solution, the mass fraction of the graphene oxide is 0.1% to 0.5%. Specifically, the mass fraction of the graphene oxide in the casting solution includes but is not limited to 0.1%, 0.2%, 0.3%, 0.4%, or 0.5%, or within the range formed by any two of the above point values as the end values.

[0097] In one example, in the casting solution, the mass fraction of the nanocellulose is 0.05% to 0.2%. Specifically, the mass fraction of the nanocellulose in the casting solution includes but is not limited to 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, or 0.5%, or within the range formed by any two of the above point values as the end values.

[0098] In one example, in the casting solution, the mass fraction of the sulfosuccinic acid is 0.2% to 0.8%. Specifically, the mass fraction of the sulfosuccinic acid includes but is not limited to 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, or 0.8%, or within the range formed by any two of the above point values as the end values.

[0099] In one example, the solvent includes deionized water and an alcohol solvent with a volume ratio of (5 - 8):(2 - 5). Exemplarily, the alcohol solvent includes but is not limited to ethanol. Specifically, the volume ratio of the deionized water to the alcohol solvent includes but is not limited to 5:5, 6:4, 7:3, or 8:2, or within the range formed by any two of the above point values as the end values.

[0100] In one example, before the step of coating the casting solution on the surface of the hollow polymer-based membrane, it further includes: infiltrating, washing, and drying the hollow polymer-based membrane in an organic solvent.

[0101] Exemplarily, the organic solvent is an alcohol solvent. The alcohol solvent includes but is not limited to ethanol. Further, the organic solvent is ethanol with a mass concentration of 50% to 80%.

[0102] Among them, the process parameters of the infiltration treatment include: the time is 10 min to 30 min. Exemplarily, the time of the infiltration treatment includes but is not limited to 10 min, 15 min, 20 min, 25 min or 30 min, or within the range formed by any two of the above point values as the end point values.

[0103] Performing infiltration pretreatment on the hollow polymer-based membrane with an alcohol solvent can effectively reduce the surface tension of the base membrane, fully wet and open the membrane pores, so as to avoid the subsequent casting solution from blocking the pores due to capillary action; and the alcohol solvent can also dissolve the amorphous region on the surface of the base membrane, exposing more active sites on the base membrane, such as the C-F bond in the PVDF base membrane, etc., thereby enhancing the binding force with the modified layer.

[0104] In a third aspect of the present application, there is provided an application of the composite separation membrane according to any one of the examples in the first aspect of the present application in the fields of water treatment, biomedicine, food processing or chemical purification.

[0105] The composite separation membrane of the present application, as well as the composite separation membrane prepared by the above preparation method, can be used as a membrane distillation hollow fiber membrane. Specifically, referring to Figure 5 , during the separation process, water is on the feed side A, that is, the feed first penetrates through the hydrophilic modified layer 20, reaches the membrane pores of the hollow polymer-based membrane 10 and finally passes through the membrane to the permeate side B, and then completes the hydrophilic-hydrophobic pathway of water molecule transfer along the water molecule flow direction C. When the composite separation membrane of the present application is applied to water treatment, it can be applied to various fields such as industrial wastewater treatment, seawater desalination, ultra-pure water preparation, concentration and separation of aqueous solutions of volatile substances, etc.

[0106] Exemplarily, in the field of biomedicine, the hollow polymer-based membrane of the present application can be used for the separation and purification of biological macromolecules, such as proteins, DNA, vaccines, etc. In the field of food processing, the hollow polymer-based membrane of the present application can be used for separating and concentrating the nutrients in fruit juices and dairy products to improve product quality. In the field of chemical purification, the hollow polymer-based membrane of the present application can achieve precise separation of substances with different molecular weights or polarities in the production of fine chemicals.

[0107] The following specific examples are further provided to illustrate the present application in detail. It should also be understood that the following examples are only used to further illustrate the present application and cannot be construed as limiting the protection scope of the present application. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present application all fall within the protection scope of the present application. The specific process parameters and the like in the following examples are also only an example within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description in this article, and do not necessarily have to be limited to the specific values in the following examples.

[0108] Example 1

[0109] (1) Pretreatment of hollow polymer-based membrane: Take a hollow fiber ultrafiltration membrane (hollow polymer-based membrane, made of PVDF). Among them, the outer diameter of the hollow fiber ultrafiltration membrane is 500 μm to 600 μm, the inner diameter of the hollow fiber ultrafiltration membrane is 200 μm to 300 μm, and the pore diameter of the membrane pores on the surface of the hollow fiber ultrafiltration membrane is 0.3 μm to 0.8 μm. Immerse the hollow fiber ultrafiltration membrane in an ethanol solution with a mass concentration of 70% and wet it for 20 min to reduce the surface tension of the hollow fiber ultrafiltration membrane, dissolve the amorphous region of the hollow fiber ultrafiltration membrane, and expose active sites such as C-F bonds.

[0110] (2) Preparation of casting solution: Graphene oxide nanosheets and nanocellulose are mixed in a solvent (water: ethanol = 7:3, v / v). After ultrasonic treatment for 50 min, sulfosuccinic acid is added and mixed for reaction for 2 h to 4 h to prepare a casting solution. Among them, the mass concentration of graphene oxide in the casting solution is 0.3%, the mass concentration of nanocellulose is 0.1%, and the mass concentration of sulfosuccinic acid is 0.5%.

[0111] (3) Preparation of modified layer by negative pressure - suction filtration: Immerse the pretreated hollow fiber ultrafiltration membrane in the casting solution for 15 min, apply a negative pressure of -0.08 MPa to the hollow inner cavity of the hollow fiber ultrafiltration membrane, and use the suction filtration effect to form a depression (depth about 80 nm to 120 nm) in the membrane pores of the hollow fiber ultrafiltration membrane; after thermal curing at 70 °C for 1.5 h, at least part of the nanocellulose is fixed between the layers of graphene oxide nanosheets, and at least part of the sulfosuccinic acid is fixed between the layers of graphene oxide nanosheets to form a modified layer and prepare a composite separation membrane.

[0112] Comparative Example 1

[0113] Comparative Example 1 only selects the hollow fiber ultrafiltration membrane of Example 1.

[0114] Comparative Example 2

[0115] Comparative Example 2 is basically the same as Example 1. The main difference is that in the preparation steps of the composite separation membrane of Comparative Example 2, nanocellulose is not included.

[0116] (1) Pretreatment of hollow polymer-based membrane: Take a hollow fiber ultrafiltration membrane (hollow polymer-based membrane, made of PVDF). Among them, the outer diameter of the hollow fiber ultrafiltration membrane is 500 μm to 600 μm, the inner diameter of the hollow fiber ultrafiltration membrane is 200 μm to 300 μm, and the pore diameter of the membrane pores on the surface of the hollow fiber ultrafiltration membrane is 0.3 μm to 0.8 μm. Immerse the hollow fiber ultrafiltration membrane in an ethanol solution with a mass concentration of 70% and wet it for 20 min to reduce the surface tension of the hollow fiber ultrafiltration membrane, dissolve the amorphous region of the hollow fiber ultrafiltration membrane, and expose active sites such as C-F bonds.

[0117] (2) Preparation of the casting solution: Graphene oxide nanosheets are mixed in a solvent (water: ethanol = 7:3, v / v). After ultrasonic treatment for 50 min, sulfosuccinic acid is added and mixed for reaction for 2 h to 4 h to prepare the casting solution. Among them, the mass concentration of graphene oxide in the casting solution is 0.3%, and the mass concentration of sulfosuccinic acid is 0.5%.

[0118] (3) Preparation of the modified layer by negative pressure - suction filtration: The pretreated hollow fiber ultrafiltration membrane is immersed in the casting solution for 15 min, and a negative pressure of -0.08 MPa is applied to the hollow inner cavity of the hollow fiber ultrafiltration membrane. The casting solution forms a depression (depth of about 80 nm to 120 nm) in the membrane pores of the hollow fiber ultrafiltration membrane by the suction filtration effect; after heat curing at 70 °C for 1.5 h, at least part of the sulfosuccinic acid is fixed between the layers of the graphene oxide nanosheets to form a modified layer, and a composite separation membrane is prepared.

[0119] Comparative Example 3

[0120] Comparative Example 3 is basically the same as Example 1. The main difference is that in the preparation steps of the composite separation membrane of Comparative Example 3, sulfosuccinic acid is not included.

[0121] (1) Pretreatment of the hollow polymer-based membrane: Take a hollow fiber ultrafiltration membrane (hollow polymer-based membrane, PVDF material). Among them, the outer diameter of the hollow fiber ultrafiltration membrane is 500 μm to 600 μm, the inner diameter of the hollow fiber ultrafiltration membrane is 200 μm to 300 μm, and the pore diameter of the membrane pores on the surface of the hollow fiber ultrafiltration membrane is 0.3 μm to 0.8 μm. The hollow fiber ultrafiltration membrane is immersed in an ethanol solution with a mass concentration of 70% and wetted for 20 min to reduce the surface tension of the hollow fiber ultrafiltration membrane and dissolve the amorphous region of the hollow fiber ultrafiltration membrane to expose active sites such as C-F bonds.

[0122] (2) Preparation of the casting solution: Graphene oxide nanosheets and nanocellulose are mixed in a solvent (water: ethanol = 7:3, v / v). After ultrasonic treatment for 50 min, the casting solution is prepared. Among them, the mass concentration of graphene oxide in the casting solution is 0.3%, and the mass concentration of nanocellulose is 0.1%.

[0123] (3) Preparation of the modified layer by negative pressure - suction filtration: The pretreated hollow fiber ultrafiltration membrane is immersed in the casting solution for 15 min, and a negative pressure of -0.08 MPa is applied to the hollow inner cavity of the hollow fiber ultrafiltration membrane. The casting solution forms a depression (depth of about 80 nm to 120 nm) in the membrane pores of the hollow fiber ultrafiltration membrane by the suction filtration effect; after heat curing at 70 °C for 1.5 h, at least part of the nanocellulose is fixed between the layers of the graphene oxide nanosheets to form a modified layer, and a composite separation membrane is prepared.

[0124] Test Example

[0125] The ultrafiltration membranes of the examples and comparative examples were contaminated with a 500 ppm humic acid solution for 2 h, and the membrane filtration performance was tested. In addition, the membranes were operated for 100 h under high pressure (>0.5 Mpa), acidic (pH = 2), and alkaline (pH = 12) conditions respectively, and the bonding strength between the modified layer and the hollow fiber ultrafiltration membrane was tested. The corresponding test results are shown in Table 1.

[0126] Table 1 Comparison of membrane filtration performance

[0127]

[0128] As can be seen from Table 1, the composite separation membrane provided by the examples of the present application has more excellent membrane filtration performance, and the bonding strength between the modified layer and the hollow polymer-based membrane is relatively high, with excellent stability, which can effectively improve the water production efficiency and reduce the operating cost.

[0129] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0130] The above-described embodiments only represent several implementation manners of the present application, which are convenient for understanding the technical solutions of the present application specifically and in detail, but should not be construed as a limitation on the protection scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several deformations and improvements can be made, and these all belong to the protection scope of the present application. It should be understood that the technical solutions obtained by those skilled in the art through logical analysis, reasoning or limited experiments based on the technical solutions provided by the present application are all within the protection scope of the appended claims of the present application. Therefore, the protection scope of the patent of the present application should be subject to the content of the appended claims, and the specification can be used to interpret the content of the claims.

Claims

1. A composite separation membrane, characterized in that: The composite separation membrane comprises a hollow polymer-based membrane and a modified layer attached to the outer surface of the hollow polymer-based membrane; Wherein, the surface of the hollow polymer-based membrane has a plurality of membrane pores, and at least part of the modified layer forms a depression in the membrane pores; The modified layer comprises graphene oxide nanosheets, nanocellulose and sulfonated succinic acid; at least part of the nanocellulose is fixed between the sheets of the graphene oxide nanosheets, and at least part of the sulfonated succinic acid is fixed between the sheets of the graphene oxide nanosheets.

2. The composite separation membrane according to claim 1, characterized in that The depth of the depression formed by the modified layer in the membrane pore is 50nm~200nm; And / or, the modified layer has a two-dimensional network structure; And / or, the fixing method of the nanocellulose includes at least one of: physical adsorption, coupling connection between the nanocellulose and the active sites of the graphene oxide nanosheets, coupling connection between the nanocellulose and the oxygen-containing functional groups of the graphene oxide nanosheets, and intermolecular interaction between the nanocellulose and the graphene oxide nanosheets; And / or, the fixation method of the sulfonated succinic acid includes at least one of: physical adsorption, coupling connection between the sulfonated succinic acid and the active sites of the graphene oxide nanosheets, coupling connection between the sulfonated succinic acid and the oxygen-containing functional groups of the graphene oxide nanosheets, and intermolecular interaction between the sulfonated succinic acid and the graphene oxide nanosheets.

3. The composite separation membrane according to claim 1, characterized in that The mass ratio of the graphene oxide nanosheets to the nanocellulose is (0.1-0.5): (0.05-0.2); And / or, the mass ratio of the graphene oxide nanosheets to the sulfonated succinic acid is (0.1-0.5):(0.2-0.8).

4. The composite separation membrane according to any one of claims 1 to 3, characterized in that The hollow polymer-based membrane has one or more of the following characteristics: (1) The outer diameter of the hollow polymer-based membrane is 500 μm to 600 μm; (2) The inner diameter of the hollow polymer-based membrane is 200 μm to 300 μm; (3) The pore size of the membrane pore is 0.3 μm to 0.8 μm; (4) The material of the hollow polymer-based membrane includes one or more of polyvinylidene fluoride, polypropylene and polyacrylonitrile.

5. The composite separation membrane according to any one of claims 1 to 3, characterized in that The composite separation membrane has one or more of the following characteristics: (1) The contact angle of static water on the surface of the composite separation membrane is ≤40°; (2) The static adsorption amount of humic acid on the surface of the composite separation membrane is 40 mg / m 2 ~170mg / m 2 ; (3) The retention capacity of humic acid by the composite separation membrane is 90% to 98%.

6. A method for preparing a composite separation membrane according to any one of claims 1 to 5, characterized in that: The following steps are involved: The graphene oxide nanosheets and the nanocellulose are mixed in a solvent, and after ultrasonic treatment, the sulfonated succinic acid is added to carry out a mixing reaction to prepare a casting solution; The casting liquid is coated on the surface of the hollow polymer-based membrane, and negative pressure is applied in the hollow cavity of the hollow polymer-based membrane to form a depression in at least part of the membrane pores of the hollow polymer-based membrane by the casting liquid; after thermal curing, at least part of the nanocellulose is fixed between the layers of the graphene oxide nanosheets, and at least part of the sulfonated succinic acid is fixed between the layers of the graphene oxide nanosheets to form the modified layer, thereby preparing the composite separation membrane.

7. The method for preparing a composite separation membrane according to claim 6, characterized in that: The negative pressure is -0.1MPa~-0.05MPa; And / or, the process parameters of the thermal curing include: a thermal curing temperature of 60° C. to 80° C. and a thermal curing time of 1 h to 2 h.

8. The method for preparing a composite separation membrane according to claim 6, characterized in that: The preparation method has one or more of the following characteristics: (1) The process parameters of the ultrasonic treatment include: ultrasonic time is 30min~60min; (2) The process parameters of the mixing reaction include: mixing time of 2h~4h, rotation speed of 500r / min~800r / min; (3) The mass ratio of the graphene oxide nanosheets to the nanocellulose is (0.1-0.5): (0.05-0.2); (4) The mass ratio of the graphene oxide nanosheets to the sulfonated succinic acid is (0.1-0.5): (0.2-0.8); (5) The solvent includes deionized water and an alcohol solvent in a volume ratio of (5-8): (2-5).

9. The method for preparing a composite separation membrane according to any one of claims 6 to 8, characterized in that: Before the step of coating the casting liquid on the surface of the hollow polymer base membrane, the method further includes: soaking, washing and drying the hollow polymer base membrane in an organic solvent; The process parameters of the infiltration treatment include: time is 10min~30min.

10. Use of the composite separation membrane according to any one of claims 1 to 5 in the fields of water treatment, biomedicine, food processing or chemical purification.

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