Composite nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention relates to a composite nanofiltration membrane as well as a preparation method and application thereof. The preparation method of the composite nanofiltration membrane comprises the following steps: preparing a nitrogen-doped graphene quantum dot (NG) material by taking citric acid (CA) as a carbon source material and ethylene diamine (EDA) as a nitrogen source; s2, dissolving chitosan CS in an acetic acid solution, stirring, heating and filtering to obtain a CS solution, and dispersing the carboxylated multi-walled carbon nanotube material CNTs in the CS solution to prepare an NG-CS / CNTs water-phase monomer solution; and S3, providing a CNTs / PVA base membrane, covering the surface of the CNTs / PVA base membrane with the NG-CS / CNTs aqueous-phase monomer solution, adding a TMC / n-hexane solution to carry out interfacial polymerization reaction, and washing and drying to obtain the composite nanofiltration membrane. The invention provides the composite nanofiltration membrane with efficient separation performance, and the composite nanofiltration membrane can be widely applied to the fields of separation of dyes and salts and the like.

Description

Technical Field

[0001] The invention relates to the technical field of composite nanofiltration membranes, and in particular to a composite nanofiltration membrane and a preparation method and application thereof. Background Art

[0002] The textile printing and dyeing industry consumes a lot of water, and the dye wastewater produced contains a large amount of harmful dyes and inorganic salts (such as NaCl, Na 2 SO 4 ), is one of the difficult industrial wastewaters to treat. Direct discharge of untreated dye wastewater will seriously endanger water environment safety, ecological environment and human health. The harmless treatment of dye wastewater is of great significance to environmental protection and water resource reuse. Nanofiltration membrane separation technology has the advantages of simple equipment, low energy consumption, high selectivity and environmental friendliness. It is expected to achieve efficient and selective separation of salt-containing dye wastewater, recover dyes or salts, reduce harm to the environment, and realize wastewater resource utilization or purification and recovery through reasonable design of the structure of nanofiltration membrane.

[0003] There is a "trade-off" effect between the dye retention rate and water permeation flux of composite nanofiltration membranes in separation applications. Most commercial nanofiltration membranes have low water permeation flux and require high operating pressure. The high salt retention rate will lead to a large amount of water and energy consumption, and is not conducive to the selective separation of dyes and salts. For example, NF-70 of Dow Membrane Technologies, UTC-60 of Toray and DK of General Electric cannot provide free permeation of salts, especially divalent salts (such as Na 2 SO 4 ). For example, NF-70 and UTC-60 nanofiltration membranes 2 SO 4 The salt retention rate is almost complete (>98%). In practical applications, insufficient salt penetration will not only lead to low water permeation flux caused by high osmotic pressure, but also fail to effectively recover salt in the filtrate. The "loose" nanofiltration membrane has a high retention rate for macromolecular dyes, but a low retention rate for small molecule dyes and neutral dyes, and most of them can only separate dyes and monovalent salts, and still face challenges in separating dyes and multivalent salts. Therefore, in order to effectively and sustainably treat dye wastewater, selectively separate dyes and salts, recover useful resources (dyes and salts), recycle qualified effluent, and reduce water resource and energy consumption, more exploration and research is needed on the preparation of advanced composite nanofiltration membranes.

[0004] Interfacial polymerization (IP) is the most commonly used and mature method for preparing polyamide composite nanofiltration membranes. It has the advantages of self-limiting effect, high reaction efficiency, and easy modification and modification. Composite nanofiltration membranes based on interfacial polymerization are usually composed of a thin and dense separation layer on the surface and a porous support base. The separation layer plays a vital role in the permeation flux and separation performance of the composite nanofiltration membrane. In order to prepare high-performance composite nanofiltration membranes through interfacial polymerization, researchers have proposed strategies such as surface modification, introduction of intermediate layers, addition / replacement of monomers, and change of reaction conditions (monomer concentration, reaction temperature, reaction time, etc.). Among them, the insertion of the intermediate layer and the selection of monomers will affect the diffusion of monomers in the aqueous phase and change the degree of cross-linking. The thickness, roughness, pore size, affinity and other structural properties of the subsequent separation layer and the separation performance of pollutants can be flexibly adjusted.

[0005] Studies have shown that the physicochemical properties of the porous substrate will affect the formation of the surface separation layer. A porous support layer with low hydrophilicity will lead to a low permeation diffusion rate of the amine monomer, resulting in defects in the formed separation layer and reducing the separation performance of the composite nanofiltration membrane. In addition, the reaction rate of the commonly used piperazine monomer and trimesoyl chloride is fast, which is difficult to control kinetically, is not conducive to the formation of a thin and dense separation layer, and the water permeation flux is low (<10Lm -2 h -1 bar -1 ), has limited application in the selective separation of dyes and salts in textile dye wastewater.

[0006] Therefore, the existing technology needs to be improved. Summary of the invention

[0007] The water permeation flux of the nanofiltration membrane in the prior art is low, and it is difficult to maintain high selectivity for solutes while increasing the water permeation flux. Although the existing loose nanofiltration membrane has a good retention effect on large molecular solutes, it has a poor retention effect on small molecular weight solutes. Therefore, it is necessary to provide a composite nanofiltration membrane and its preparation method and application to solve the above problems.

[0008] To achieve the above object, in a first aspect, the present invention provides a method for preparing a composite nanofiltration membrane, which comprises the following steps: S1. Using citric acid (CA) as a carbon source material and ethylenediamine (EDA) as a nitrogen source, the citric acid and ethylenediamine are prepared by a hydrothermal synthesis method to obtain a nitrogen-doped graphene quantum dot (NG) material; S2, dissolving chitosan CS in acetic acid solution, stirring, heating and filtering to obtain a CS solution, dispersing carboxylated multi-walled carbon nanotube material CNTs in the CS solution to obtain a CS / CNTs mixed solution, stirring and dispersing in NG aqueous phase monomer to obtain a NG-CS / CNTs aqueous phase monomer solution; S3, providing a CNTs / PVA base membrane, covering the surface of the CNTs / PVA base membrane with the NG-CS / CNTs aqueous monomer solution, pouring 1,3,5-benzenetricarboxylic acid chloride (TMC) / n-hexane solution on the NG-CS / CNTs impregnated membrane after standing, performing an interfacial polymerization reaction, and obtaining the composite nanofiltration membrane after washing and drying.

[0009] In one implementation, in S1, the hydrothermal synthesis time is 6 to 12 hours, and the hydrothermal synthesis temperature is 120 to 200°C.

[0010] In one implementation, in S2, the aqueous phase monomer includes nitrogen-doped graphene quantum dot (NG) material and chitosan (CS)-modified carboxylated multi-walled carbon nanotubes (CS / CNTs), and the concentration of the NG solution in the aqueous phase monomer is 0.1-0.65 wt.%.

[0011] In one implementation, in S2, the stirring and heating time is 2.5-3.5 h, the stirring and heating temperature is 80° C., the filtration is to filter the undissolved particles through filter paper, and the concentration of the prepared CS / CNTs mixed solution is 0.1-0.4 wt.%.

[0012] In one implementation, in S3, the specific steps of the preparation method of the CNTs / PVA base film include: uniformly dispersing the CNTs in ultrapure water with the assistance of ultrasound, centrifuging, vacuum filtering the obtained CNTs dispersion on a pure polyvinylidene fluoride (PVDF) base film, removing the extracted film, and drying to obtain a CNTs base film; immersing the CNTs base film in a polyvinyl alcohol (PVA) aqueous solution for 1 to 2 hours, and then drying at 50°C to 70°C for 30 minutes to obtain the CNTs / PVA base film.

[0013] In one implementation, in S3, the concentration of the PVA solution is 1.0-2.0 wt.%; the concentration of the TMC solution is 0.1-0.3 wt.%.

[0014] In one implementation, in S3, the interfacial polymerization reaction time is 1 to 5 minutes.

[0015] In one implementation, in S3, before the interfacial polymerization reaction is carried out, the excess aqueous monomer solution is poured out and the residual solution is removed with a rubber roller; after the interfacial polymerization reaction is carried out, the unreacted TMC is removed by washing with a pure n-hexane solution.

[0016] In a second aspect, the present invention further provides a composite nanofiltration membrane, which is prepared by any of the above-mentioned methods for preparing the composite nanofiltration membrane.

[0017] In a third aspect, the present invention also provides an application of a composite nanofiltration membrane in the separation of dyes and salts.

[0018] Beneficial effects: In the present invention, polyvinyl alcohol-modified carboxylated multi-walled carbon nanotube material CNTs / PVA is used as the middle layer, NG and CS / CNTs are used together as the aqueous phase monomers, and the addition amount of CS / CNTs is changed to optimize the separation performance of the NG-CS / CNTs membrane, so that its retention rate for single charged organic dye molecules reaches more than 90%, and at the same time, effective selective separation of dye / salt (CR / NaCl) mixed solutions can be achieved. The present invention provides a composite nanofiltration membrane with high-efficiency separation performance, which can be widely used in the fields of separation of dyes and salts, and has significant technical effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a flow chart of the steps of the method for preparing the composite nanofiltration membrane provided by the present invention; Figure 2 yes Figure 1 Schematic diagram of the steps of the preparation method of the composite nanofiltration membrane shown; Figure 3 Surface SEM images of the NG film (a) and the NG-CS / CNTs film (b) provided by the present invention; Figure 4 Schematic diagram comparing the separation performance of different membranes for different dyes (a) and salts (b); Figure 5 Schematic diagram of the separation performance comparison of different membranes for CR (a) and NaCl (b) in CR / NaCl mixed solution.

[0020] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the description of the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" described below means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms is not necessarily for the same embodiment or example. Moreover, the technical features involved in each embodiment of the present invention can be combined with each other as long as they do not conflict with each other.

[0022] Read more Figure 1 , Figure 1 The present invention provides a method for preparing a composite nanofiltration membrane, which comprises the following steps: S1. Using citric acid (CA) as a carbon source material and ethylenediamine (EDA) as a nitrogen source, the citric acid and ethylenediamine are prepared by a hydrothermal synthesis method to obtain a nitrogen-doped graphene quantum dot (NG) material; S2, dissolving chitosan CS in acetic acid solution, stirring, heating and filtering to obtain a CS solution, dispersing carboxylated multi-walled carbon nanotube material CNTs in the CS solution to obtain a CS / CNTs mixed solution, stirring and dispersing in NG aqueous phase monomer to obtain a NG-CS / CNTs aqueous phase monomer solution; S3, providing a CNTs / PVA base membrane, covering the surface of the CNTs / PVA base membrane with the NG-CS / CNTs aqueous monomer solution, pouring 1,3,5-benzenetricarboxylic acid chloride (TMC) / n-hexane solution on the NG-CS / CNTs impregnated membrane after standing, performing an interfacial polymerization reaction, and obtaining the composite nanofiltration membrane after washing and drying.

[0023] The present invention prepares nitrogen-doped graphene quantum dot (NG) materials from citric acid and ethylenediamine by a hydrothermal synthesis method, combines the materials with chitosan, carboxylated multi-walled carbon nanotubes (CNTs), etc., and prepares a composite nanofiltration membrane by an interfacial polymerization reaction. The prepared composite nanofiltration membrane has high selectivity and permeability and can effectively separate dyes and salts.

[0024] Specifically, in S1, the hydrothermal synthesis time is 6 to 12 hours, and the hydrothermal synthesis temperature is 120 to 200° C. The preparation conditions of the NG material are further optimized to ensure its good performance and stability.

[0025] Specifically, in S2, the aqueous phase monomer includes nitrogen-doped graphene quantum dots (NG) material and chitosan (CS) modified carboxylated multi-walled carbon nanotubes (CS / CNTs), and the concentration of the NG solution in the aqueous phase monomer is 0.1-0.65wt.%. The stirring and heating time is 2.5-3.5h, the stirring and heating temperature is 80°C, and the filtration is to filter the undissolved particles through filter paper, and the concentration of the configured CS / CNTs mixed solution is 0.1-0.4wt.%. By limiting the stirring and heating time, temperature and concentration range of the CS / CNTs mixed solution, it helps to ensure the good dispersion of chitosan and carboxylated multi-walled carbon nanotubes, thereby improving the overall performance of the composite nanofiltration membrane.

[0026] Specifically, in S3, the specific steps of the preparation method of the CNTs / PVA base film include: uniformly dispersing the CNTs in ultrapure water with the assistance of ultrasound, centrifugal separation, vacuum filtering the obtained CNTs dispersion on a pure polyvinylidene fluoride (PVDF) base film, removing the drawn film, and drying to obtain a CNTs base film; immersing the CNTs base film in a polyvinyl alcohol (PVA) aqueous solution for 1 to 2 hours, and then drying at 50°C to 70°C for 30 minutes to obtain the CNTs / PVA base film. Through the steps of ultrasound-assisted dispersion, centrifugal separation, vacuum filtration and drying, a uniform and stable CNTs base film can be prepared, and a CNTs / PVA base film can be obtained by further processing, thereby improving the mechanical strength and stability of the composite nanofiltration membrane.

[0027] Furthermore, the concentration of the PVA solution is 1.0-2.0 wt.%, the concentration of the TMC solution is 0.1-0.3 wt.%, and the time of the interfacial polymerization reaction is 1-5 min.

[0028] In addition, in S3, before the interfacial polymerization reaction, the excess aqueous monomer solution is poured out and the residual solution is removed with a rubber roller; after the interfacial polymerization reaction, the unreacted TMC is washed with a pure n-hexane solution. This step helps to improve the purity and quality of the composite nanofiltration membrane and ensure its stability and reliability during use.

[0029] The present invention further illustrates the technical method of the present invention by providing embodiments and comparative examples.

[0030] Embodiment 1, Read more Figure 2 , Figure 2 yes Figure 1 Schematic diagram of the steps of the method for preparing the composite nanofiltration membrane.

[0031] This embodiment provides a method for preparing a composite nanofiltration membrane, which specifically includes: The general preparation method of S1 and NG materials is as follows Figure 1 As shown in (a): citric acid (CA) is used as the carbon source material, ethylenediamine (EDA) is used as the nitrogen source, and the hydrothermal synthesis "one-step method" is adopted for preparation, wherein the hydrothermal reaction time is 4h, the hydrothermal reaction temperature is 160°C, and the hydrothermal reaction equipment is a polytetrafluoroethylene reactor; S2. Preparation of CS / CNTs materials Figure 1As shown in (b): Chitosan (CS) was dissolved in 1% (v / v) acetic acid solution, heated at 80°C for 3 hours with continuous stirring, and then filtered with filter paper to remove undissolved particles. Carboxylated multi-walled carbon nanotubes (CNTs) were dispersed in the above CS solution to prepare 0.1~0.4wt.% CS / CNTs mixed solutions. Stir and disperse in NG aqueous phase monomers at a fixed volume to prepare NG-CS / CNTs aqueous phase monomer solutions. Among them, the selected NG aqueous phase monomer concentration was 0.35wt.%.

[0032] S3, CNTs / PVA film preparation Figure 1 As shown in (c): The CNTs are uniformly dispersed in ultrapure water with the aid of ultrasound, and centrifuged. The obtained CNTs dispersion is vacuum filtered on a pure polyvinylidene fluoride (PVDF) base membrane, and the drawn membrane is removed and dried to obtain a CNTs base membrane. The CNTs-based film was immersed in a polyvinyl alcohol (PVA) aqueous solution for 1 hour, and dried at 60° C. for 30 minutes to obtain a CNTs / PVA-based film; wherein the concentration of the PVA was 1.0 wt.%; The wet CNTs / PVA membrane was assembled on a clean blind flange device, and the CNTs / PVA layer was placed on the top. Subsequently, 30 mL of NG-CS / CNTs aqueous monomer solution was covered on the surface of the CNTs / PVA membrane and allowed to stand for 30 minutes. Subsequently, the excess aqueous monomer solution was poured out and the residual solution was removed with a rubber roller. After removing the residual solution, the TMC / n-hexane solution was poured on the NG-CS / CNTs impregnated membrane for interfacial polymerization (IP) reaction. Among them, the selected TMC / n-hexane solution concentration was 0.1wt.%, and the interfacial polymerization reaction time was 1min. After the reaction was completed, it was washed with pure n-hexane solution to remove the unreacted TMC. Finally, the prepared composite nanofiltration membrane (labeled as NG-CS / CNTs membrane) was obtained by drying at 60°C for 15min.

[0033] Comparative Example 1 The difference from Example 1 is that no CS / CNTs material is added in this comparative example, that is, the aqueous phase monomer is only NG solution, and the obtained composite membrane is NG membrane. The other steps are the same as those in Example 1 and will not be repeated here.

[0034] Comparative Example 2 Different from Example 1, this comparative example provides a composite nanofiltration membrane prepared on a CNTs / PVA base film with PIP as an aqueous monomer solution (with a concentration of 0.35 wt.%), which is labeled as PA-NF membrane. PIP is piperazine, an organic compound with a chemical formula of C 4 H 10 N2 , and TMC undergoes interfacial polymerization reaction to form an ultra-thin separation layer. The other steps are the same as those in Example 1 and will not be described again.

[0035] See also Figure 3 , Figure 3 The surface SEM images of the NG membrane (a) and the NG-CS / CNTs membrane (b) provided by the present invention are shown in Figure 2. Before the specific test, the surface of the pure NG membrane and the NG-CS / CNTs composite nanofiltration membrane were characterized and analyzed by SEM. Figure 3 As shown. The surface of pure NG membrane presents a nodular structure ( Figure 3 a in the figure), the surface is relatively rough; after adding CS / CNTs, fuzzy and staggered CS / CNTs were observed on the surface of NG-CS / CNTs composite film ( Figure 3 b) in the above.

[0036] The present invention also conducted a separation performance test of the composite nanofiltration membrane using a low-pressure cross-flow flat membrane test device. The effective membrane area of ​​the unit membrane pool of the flat membrane device is 9.62 cm 2 The experimental operating pressure was set to 4 bar (0.4 MPa). Before the filtration experiment officially started, the prepared composite nanofiltration membrane was pre-pressed with ultrapure water at 4 bar for at least 30 minutes.

[0037] In this specific example, five dyes with different molecular weights (MB, BG, RB, CR, and MO; concentration is 10 mg / L) and four typical inorganic salts (Na 2 SO 4 , NaCl, MgSO 4 and MgCl 2 ; concentration of 1.0 g / L) as the target pollutant to test the separation performance of the composite nanofiltration membrane. The concentrations of salt and dye were measured by a conductivity meter (Mettler Toledo, SevenEasy) and an ultraviolet spectrophotometer (UV-2550, Shimadzu), respectively. The calculation of the water permeation flux and the solute retention rate of the membrane is shown in formulas 1.1 and 1.2.

[0038] Among them, the permeation flux (F, Lm -2 h -1 bar -1 ) is calculated by the following formula 1.1: In the above formula Expressed as the filtrate volume, is the effective area of ​​the composite nanofiltration membrane, is the relative time of filtering, is the relative pressure applied.

[0039] The retention rate of the solute (R, %) is calculated according to the following formula 1.2: Among them C i Expressed as the initial concentration of the feed solution, C f is the concentration of the filtrate.

[0040] Using dye / salt binary mixed solution (CR / NaCl, 10 / 1000 mg / L) as the feed solution and operating at 4 bar pressure for 24 h, the permeability and retention performance of the NG-CS / CNTs membrane were tested to investigate the selective separation and stability of the composite membrane.

[0041] Result analysis: The separation results of composite membranes for single component dyes or salts are as follows: Figure 4 As shown, Figure 4 This is a schematic diagram comparing the separation performance of different membranes for different dyes (a) and salts (b). Figure 4 The results show that the pure water permeation flux of the PA-NF membrane provided in Comparative Example 2 is about 6.42±0.37Lm -2 h -1 bar -1 , the flux of dye is 4.12~6.26Lm -2 h -1 bar -1 ; The flux of the NG-CS / CNTs membrane to the dye provided in Example 1 (12.21~14.63Lm -2 h -1 bar -1 ) is more than twice that of PA-NF membrane (2.10~3.07 times) ( Figure 4 (a) in the figure). Compared with the PA-NF membrane, the rejection rate of the NG-CS / CNTs membrane for MO, CR and BG did not decrease, but increased slightly (92.01~99.33%); the rejection rate for MB and RB decreased slightly.

[0042] Among them, the order of the salt rejection rate of PA-NF membrane is Na 2 SO 4 (86.22%)>MgSO 4 (63.90%)>MgCl 2 (45.97%)>NaCl (37.12%); the flux to salt is 2.88~4.42Lm -2 h -1 bar -1 ( Figure 4 (b) NG-CS / CNTs membrane filtration of MgSO 4 The water permeation flux is the largest (~13.67Lm-2 h -1 bar -1 ) is about 3.73 times that of PA-NF membrane; but for MgSO 4 The retention rate of Na 2 SO 4 The interception rate is about 34.56% ( Figure 3 b). The high permeability of the composite nanofiltration membrane provided by the present invention to salt is conducive to the selective separation of dye and salt in a dye / salt mixture, and can be applied to the treatment of actual dye wastewater.

[0043] The results of the stability and selective separation performance of NG-CS / CNTs membrane for CR / NaCl mixed solution in 24 h continuous operation are shown in Figure 5 As shown, Figure 5 This is a schematic diagram comparing the separation performance of different membranes for CR (a) and NaCl (b) in a CR / NaCl mixed solution. Figure 5 It can be seen that the NG-CS / CNTs membrane and PA-NF membrane can maintain a rejection rate of more than 99% for CR dye in 24 h of continuous operation, and the average water permeation flux is 14.79±0.60 and 6.52±0.36 Lm, respectively. -2 h -1 bar -1 ( Figure 5 a). The average rejection rate of NG-CS / CNTs membrane for NaCl is 16.74±2.70% (the first rejection rate after each flushing is not calculated), which is lower than that of PA-NF membrane (19.85±3.22%). The high rejection rate of NG-CS / CNTs membrane for CR dye and the low rejection rate for NaCl indicate that the NG-CS / CNTs membrane provided in Example 1 can effectively separate the CR / NaCl mixed solution; and the higher water permeation flux of the NG-CS / CNTs membrane provided in Example 1 than that of the PA-NF membrane in Comparative Example 2 is more conducive to the selective separation of dyes and salts.

[0044] This is because carbon nanotubes have a unique hollow tubular structure, high specific surface area, excellent mechanical properties and chemical properties, and are easy to be hydrophilically modified or specifically functionalized. Carbon nanotubes and their modified or derived nanomaterials can be used as film-forming materials, nanofillers or intermediate layers in composite nanofiltration membranes to improve their structure and performance. Their special hollow tubular structure can give them excellent water transmission performance. In the present invention, NG and CS / CNTs are used as aqueous phase monomers, and the addition amount of CS / CNTs is changed to optimize the separation performance of the NG-CS / CNTs membrane. While maintaining the retention rate of the dye (MO: 97.49%), the pure water permeation flux of the NG-CS / CNTs membrane can reach 14.94Lm -2 h-1 bar -1 , which is about 2~3 times that of pure NG membrane and PA-NF membrane.

[0045] The improvement of water permeation flux of the composite nanofiltration membrane provided by the present invention is mainly due to the fact that CS / CNTs materials, as nano-additives for water phase monomers, can provide additional transport channels for water molecules through the unique tubular structure of CNTs, reducing mass transfer resistance; and the diffusion rate of NG-CS / CNTs composite water phase monomers to the organic phase is slower than that of pure NG monomers. Within the same interfacial polymerization reaction time, NG-CS / CNTs water phase monomers tend to form a thinner separation layer, shortening the water transmission path, which is conducive to the improvement of water permeation flux. The retention rate of NG-CS / CNTs composite nanofiltration membranes for selected single charged organic dye molecules is above 90%; effective selective separation can be achieved for dye / salt (CR / NaCl) mixed solutions.

[0046] In general, the present invention uses polyvinyl alcohol-modified carboxylated multi-walled carbon nanotube material CNTs / PVA as an intermediate layer, uses NG and CS / CNTs together as aqueous phase monomers, and changes the addition amount of CS / CNTs to optimize the separation performance of the NG-CS / CNTs membrane, so that its retention rate for single charged organic dye molecules reaches more than 90%, and at the same time, it can effectively and selectively separate dye / salt (CR / NaCl) mixed solutions. The present invention provides a composite nanofiltration membrane with high-efficiency separation performance, which can be widely used in the fields of separation of dyes and salts, and has significant technical effects.

[0047] The above description is only a preferred embodiment of the present invention, and does not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for preparing a composite nanofiltration membrane, characterized in that: The following steps are involved: S1. Using citric acid (CA) as a carbon source material and ethylenediamine (EDA) as a nitrogen source, the citric acid and ethylenediamine are prepared by a hydrothermal synthesis method to obtain a nitrogen-doped graphene quantum dot (NG) material; S2, dissolving chitosan CS in acetic acid solution, stirring, heating and filtering to obtain a CS solution, dispersing carboxylated multi-walled carbon nanotube material CNTs in the CS solution to obtain a CS / CNTs mixed solution, stirring and dispersing in NG aqueous phase monomer to obtain a NG-CS / CNTs aqueous phase monomer solution; S3, providing a CNTs / PVA base membrane, covering the surface of the CNTs / PVA base membrane with the NG-CS / CNTs aqueous monomer solution, pouring 1,3,5-benzenetricarboxylic acid chloride (TMC) / n-hexane solution on the NG-CS / CNTs impregnated membrane after standing, performing an interfacial polymerization reaction, and obtaining the composite nanofiltration membrane after washing and drying.

2. The method for preparing a composite nanofiltration membrane according to claim 1, characterized in that: In S1, the hydrothermal synthesis time is 6 to 12 hours, and the hydrothermal synthesis temperature is 120 to 200°C.

3. The method for preparing the composite nanofiltration membrane according to claim 1, characterized in that: In S2, the aqueous phase monomer includes nitrogen-doped graphene quantum dot (NG) material and chitosan (CS)-modified carboxylated multi-walled carbon nanotubes (CS / CNTs), and the concentration of the NG solution in the aqueous phase monomer is 0.1-0.65 wt.%.

4. The method for preparing the composite nanofiltration membrane according to claim 1, characterized in that: In S2, the stirring and heating time is 2.5-3.5 hours, the stirring and heating temperature is 80°C, and the filtration is to filter the undissolved particles through filter paper, and the concentration of the prepared CS / CNTs mixed solution is 0.1-0.4wt.%.

5. The method for preparing the composite nanofiltration membrane according to claim 1, characterized in that: In S3, the specific steps of the preparation method of the CNTs / PVA base film include: uniformly dispersing the CNTs in ultrapure water with the assistance of ultrasound, centrifuging, vacuum filtering the obtained CNTs dispersion on a pure polyvinylidene fluoride (PVDF) base film, removing the drawn film, and drying to obtain a CNTs base film; immersing the CNTs base film in a polyvinyl alcohol (PVA) aqueous solution for 1 to 2 hours, and then drying at 50°C to 70°C for 30 minutes to obtain the CNTs / PVA base film.

6. The method for preparing the composite nanofiltration membrane according to claim 5, characterized in that: In S3, the concentration of the PVA solution is 1.0-2.0 wt.%; the concentration of the TMC solution is 0.1-0.3 wt.%.

7. The method for preparing a composite nanofiltration membrane according to claim 1, characterized in that: In S3, the interfacial polymerization reaction time is 1 to 5 minutes.

8. The method for preparing a composite nanofiltration membrane according to claim 1, characterized in that: In S3, before the interfacial polymerization reaction is carried out, the excess aqueous monomer solution is poured out and the residual solution is removed with a rubber roller; after the interfacial polymerization reaction is carried out, the unreacted TMC is removed by washing with a pure n-hexane solution.

9. A composite nanofiltration membrane, characterized in that: The composite nanofiltration membrane is prepared by the method for preparing the composite nanofiltration membrane according to any one of claims 1 to 8.

10. Use of the composite nanofiltration membrane according to claim 9 in the separation of dyes and salts.

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