Composite nanofiltration membrane and its preparation method and application

Abstract

The present invention relates to a composite nanofiltration membrane, a preparation method and an application thereof. The preparation method of the composite nanofiltration membrane comprises the following steps: using citric acid (CA) as a carbon source material and ethylenediamine (EDA) as a nitrogen source to prepare a nitrogen-doped graphene quantum dot (NG) material; S2, dissolving chitosan CS in an acetic acid solution, stirring, heating and filtering to obtain a CS solution, dispersing carboxylated multi-walled carbon nanotube material CNTs in the above-mentioned CS solution to prepare an NG-CS / CNTs aqueous monomer solution; S3, providing a CNTs / PVA base membrane, covering the NG-CS / CNTs aqueous monomer solution on the surface of the CNTs / PVA base membrane, adding a TMC / n-hexane solution to carry out an interfacial polymerization reaction, washing and drying to obtain the composite nanofiltration membrane. The present invention provides a composite nanofiltration membrane with high-efficiency separation performance, which can be widely used in fields such as the separation of dyes and salts.

Description

Technical Field

[0001] The present 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 large amounts of water, and the resulting dye wastewater contains large amounts of harmful dyes and inorganic salts (such as NaCl and Na2SO4), making it one of the most difficult industrial wastewaters to treat. The direct discharge of untreated dye wastewater poses a serious threat to aquatic and ecological environments, as well as human health. The harmless treatment of dye wastewater is crucial for environmental protection and water resource reuse. Nanofiltration membrane separation technology offers advantages such as simple equipment, low energy consumption, high selectivity, and environmental friendliness. Through the rational design of nanofiltration membrane structures, it is expected to achieve efficient and selective separation of salt-containing dye wastewater, recover dyes or salts, reduce environmental hazards, and simultaneously realize wastewater resource utilization or purification and recovery.

[0003] Composite nanofiltration membranes face a trade-off between dye retention and water permeate flux in separation applications. Most commercial nanofiltration membranes have low water permeate flux and require high operating pressures. High salt rejection results in significant water and energy consumption, hindering the selective separation of dyes and salts. Nanofiltration membranes such as Dow Membrane Technologies' NF-70, Toray's UTC-60, and General Electric's DK offer limited salt permeation, particularly for divalent salts such as Na₂SO₄. NF-70 and UTC-60 nanofiltration membranes, for example, offer near-complete (>98%) retention of Na₂SO₄. In practical applications, this insufficient salt permeation not only results in low water permeate flux due to high osmotic pressure, but also ineffective salt recovery from the filtrate. "Loose" nanofiltration membranes offer high retention for large-molecule dyes but low retention for small-molecule and neutral dyes. Furthermore, most nanofiltration membranes can only separate dyes from monovalent salts, and face challenges in separating dyes from 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 and energy consumption, more exploration and research are 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 cross-linking degree. It can flexibly adjust the thickness, roughness, pore size, affinity and other structural properties of the subsequent separation layer and the separation performance of pollutants.

[0005] Studies have shown that the physicochemical properties of the porous substrate 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, making it difficult to control its kinetics, which 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 while increasing the water permeation flux, it is difficult to maintain high selectivity for solutes. 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 objectives, in a first aspect, the present invention provides a method for preparing a composite nanofiltration membrane, which comprises the following steps:

[0009] S1, using citric acid (CA) as a carbon source material and ethylenediamine (EDA) as a nitrogen source, preparing nitrogen-doped graphene quantum dots (NG) material by hydrothermal synthesis of the citric acid and ethylenediamine;

[0010] 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 the mixed solution in NG aqueous phase monomer to obtain an NG-CS / CNTs aqueous phase monomer solution;

[0011] S3. Provide a CNTs / PVA base membrane, cover the surface of the CNTs / PVA base membrane with the NG-CS / CNTs aqueous monomer solution, and after standing, pour 1,3,5-benzenetricarboxylic acid chloride (TMC) / n-hexane solution on the NG-CS / CNTs impregnated membrane to perform interfacial polymerization reaction. After washing and drying, obtain the composite nanofiltration membrane.

[0012] In one implementation, in S1, the hydrothermal synthesis time is 6 to 12 hours, and the hydrothermal synthesis temperature is 120 to 200°C.

[0013] In one implementation, 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 to 0.65 wt.%.

[0014] 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.%.

[0015] 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.

[0016] 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.%.

[0017] In one implementation, in S3, the interfacial polymerization reaction lasts for 1 to 5 minutes.

[0018] 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.

[0019] 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.

[0020] In a third aspect, the present invention also provides an application of a composite nanofiltration membrane in the separation of dyes and salts.

[0021] 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 as 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, it can achieve effective selective separation of 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 dye and salt separation and has significant technical effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a flow chart of the steps of the method for preparing the composite nanofiltration membrane provided by the present invention;

[0023] Figure 2 yes Figure 1 Schematic diagram of the steps of the preparation method of the composite nanofiltration membrane shown;

[0024] Figure 3 Surface SEM images of the NG film (a) and NG-CS / CNTs film (b) provided by the present invention;

[0025] Figure 4 Schematic diagram comparing the separation performance of different membranes for different dyes (a) and salts (b);

[0026] Figure 5 Schematic diagram comparing the separation performance of different membranes for CR (a) and NaCl (b) in CR / NaCl mixed solution.

[0027] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to 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 descriptions of the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" described below mean 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 representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0029] See details Figure 1 , Figure 1 The present invention provides a method for preparing a composite nanofiltration membrane, which comprises the following steps:

[0030] S1, using citric acid (CA) as a carbon source material and ethylenediamine (EDA) as a nitrogen source, preparing nitrogen-doped graphene quantum dots (NG) material by hydrothermal synthesis of the citric acid and ethylenediamine;

[0031] 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 the mixed solution in NG aqueous phase monomer to obtain an NG-CS / CNTs aqueous phase monomer solution;

[0032] S3. Provide a CNTs / PVA base membrane, cover the surface of the CNTs / PVA base membrane with the NG-CS / CNTs aqueous monomer solution, and after standing, pour 1,3,5-benzenetricarboxylic acid chloride (TMC) / n-hexane solution on the NG-CS / CNTs impregnated membrane to perform interfacial polymerization reaction. After washing and drying, obtain the composite nanofiltration membrane.

[0033] The present invention prepares nitrogen-doped graphene quantum dot (NG) materials from citric acid and ethylenediamine through a hydrothermal synthesis method, and combines them with chitosan, carboxylated multi-walled carbon nanotubes (CNTs), etc. to prepare a composite nanofiltration membrane through interfacial polymerization reaction. The prepared composite nanofiltration membrane has high selectivity and permeability and can effectively separate dyes and salts.

[0034] 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.

[0035] Specifically, in S2, the aqueous monomer includes nitrogen-doped graphene quantum dots (NG) and chitosan (CS)-modified carboxylated multi-walled carbon nanotubes (CS / CNTs). The concentration of the NG solution in the aqueous monomer is 0.1-0.65 wt.%. The stirring and heating time is 2.5-3.5 hours at a temperature of 80°C. Filtration is performed by filtering undissolved particles through filter paper. The resulting CS / CNTs mixed solution has a concentration of 0.1-0.4 wt.%. Limiting the stirring and heating time, temperature, and concentration range of the CS / CNTs mixed solution helps ensure good dispersion of chitosan and carboxylated multi-walled carbon nanotubes, thereby improving the overall performance of the composite nanofiltration membrane.

[0036] Specifically, in S3, the method for preparing the CNTs / PVA-based membrane includes the following steps: uniformly dispersing the CNTs in ultrapure water with the aid of ultrasound, centrifuging, vacuum filtering the resulting CNTs dispersion onto a pure polyvinylidene fluoride (PVDF)-based membrane, removing the extracted membrane, and drying to obtain a CNTs-based membrane; immersing the CNTs-based membrane in a polyvinyl alcohol (PVA) aqueous solution for 1-2 hours, followed by drying at 50°C-70°C for 30 minutes to obtain the CNTs / PVA-based membrane. Through the steps of ultrasound-assisted dispersion, centrifugal separation, vacuum filtration, and drying, a uniform and stable CNTs-based membrane can be prepared, and further processing to obtain a CNTs / PVA-based membrane can improve the mechanical strength and stability of the composite nanofiltration membrane.

[0037] 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 duration of the interfacial polymerization reaction is 1-5 min.

[0038] Furthermore, 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 pure n-hexane solution to remove it. This step helps improve the purity and quality of the composite nanofiltration membrane, ensuring its stability and reliability during use.

[0039] The present invention further illustrates the technical method of the present invention by providing examples and comparative examples.

[0040] Example 1

[0041] See details Figure 2 , Figure 2 yes Figure 1 Schematic diagram of the steps of the preparation method of the composite nanofiltration membrane shown.

[0042] This embodiment provides a method for preparing a composite nanofiltration membrane, which specifically includes:

[0043] 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 and ethylenediamine (EDA) is used as the nitrogen source. The hydrothermal synthesis "one-step method" is adopted for preparation. The hydrothermal reaction time is 4 hours, the hydrothermal reaction temperature is 160°C, and the hydrothermal reaction equipment is a polytetrafluoroethylene reactor.

[0044] S2. Preparation of CS / CNTs materials Figure 1 As shown in (b), chitosan (CS) was dissolved in a 1% (v / v) acetic acid solution and heated at 80°C with continuous stirring for 3 hours. Undissolved particles were then filtered using filter paper. Carboxylated multi-walled carbon nanotubes (CNTs) were dispersed in the CS solution to create 0.1–0.4 wt.% CS / CNTs mixed solutions. A fixed volume of NG aqueous monomer was then stirred and dispersed to create an NG-CS / CNTs aqueous monomer solution. The selected NG aqueous monomer concentration was 0.35 wt.%.

[0045] S3, CNTs / PVA film preparation Figure 1 As shown in (c):

[0046] The CNTs were uniformly dispersed in ultrapure water with the aid of ultrasound, centrifuged, and the obtained CNT dispersion was vacuum filtered on a pure polyvinylidene fluoride (PVDF) based membrane. The membrane was removed and dried to obtain a CNT based membrane.

[0047] 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.%;

[0048] A wet CNTs / PVA membrane was assembled on a clean blind flange assembly, with the CNTs / PVA layer placed on top. Subsequently, 30 mL of the NG-CS / CNTs aqueous monomer solution was applied to the surface of the CNTs / PVA membrane and allowed to stand for 30 minutes. The excess aqueous monomer solution was then poured out, and the remaining solution was removed with a rubber roller. After removal of the remaining solution, a TMC / n-hexane solution was poured onto the NG-CS / CNTs-impregnated membrane for interfacial polymerization (IP). The TMC / n-hexane solution concentration was 0.1 wt.%, and the interfacial polymerization reaction time was 1 minute. After the reaction, the membrane was washed with pure n-hexane solution to remove any unreacted TMC. Finally, the membrane was dried at 60°C for 15 minutes to obtain the prepared composite nanofiltration membrane (labeled as the NG-CS / CNTs membrane).

[0049] Comparative Example 1

[0050] Unlike Example 1, this comparative example did not add CS / CNTs material, that is, the aqueous monomer was only NG solution, and the resulting composite membrane was an NG membrane. The other steps were the same as in Example 1 and will not be repeated here.

[0051] Comparative Example 2

[0052] Different from Example 1, this comparative example provides a composite nanofiltration membrane prepared on a CNTs / PVA base film with PIP as the 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 C4H 10 N2, and TMC interfacial polymerization reaction, can form an ultra-thin separation layer. Other steps are the same as in Example 1 and are not repeated here.

[0053] See Figure 3 , Figure 3 The surface SEM images of the NG membrane (a) and 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 by SEM analysis. Figure 3 As shown. The surface of pure NG membrane has 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 the NG-CS / CNTs composite film ( Figure 3 b) in the above example.

[0054] 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 began, the prepared composite nanofiltration membrane was pre-pressed with ultrapure water at 4 bar pressure for at least 30 minutes.

[0055] In this specific example, five dyes of different molecular weights (MB, BG, RB, CR, and MO; concentrations of 10 mg / L) and four typical inorganic salts (Na2SO4, NaCl, MgSO4, and MgCl2; concentrations of 1.0 g / L) were selected as target pollutants to test the separation performance of a composite nanofiltration membrane. Salt and dye concentrations were measured using a conductivity meter (Mettler Toledo, SevenEasy) and a UV spectrophotometer (UV-2550, Shimadzu), respectively. The membrane's water permeation flux and solute retention were calculated according to Equations 1.1 and 1.2.

[0056] Among them, the permeation flux (F, Lm -2 h -1 bar -1 ) is calculated using the following formula 1.1:

[0057]

[0058] 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.

[0059] The solute retention rate (R, %) is calculated according to the following formula 1.2:

[0060]

[0061] Among them C i Expressed as the initial concentration of the feed solution, C f is the concentration of the filtrate.

[0062] Using a dye / salt binary mixed solution (CR / NaCl, 10 / 1000 mg / L) as the feed solution and operating at a pressure of 4 bar for 24 hours, the permeability and retention performance of the NG-CS / CNTs membrane were tested to investigate the selective separation and stability of the composite membrane.

[0063] Result analysis:

[0064] The separation results of composite membrane for single component dye or salt 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 dye flux 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) Compared with the PA-NF membrane, the NG-CS / CNTs membrane showed a slight increase in the rejection rate of MO, CR, and BG (92.01-99.33%), with a slight decrease in the rejection rate of MB and RB.

[0065] The order of salt rejection of PA-NF membranes is Na2SO4 (86.22%) > MgSO4 (63.90%) > MgCl2 (45.97%) > NaCl (37.12%); the salt flux is 2.88~4.42Lm -2 h -1 bar -1 ( Figure 4 b). The water permeation flux of NG-CS / CNTs membrane when filtering MgSO4 is the largest (~13.67Lm -2 h -1 bar -1 ) is about 3.73 times that of PA-NF membrane; but the rejection rate of MgSO4 is reduced to 30.83%; the rejection rate of Na2SO4 is about 34.56% ( Figure 3 b) The high permeability of the composite nanofiltration membrane provided by the present invention to salt is beneficial for selectively separating dye and salt in a dye / salt mixture, and can be applied to the treatment of actual dye wastewater.

[0066] 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 the following table. 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 during 24 h of continuous operation, and the average water permeation fluxes are 14.79±0.60 and 6.52±0.36 Lm, respectively. -2 h -1 bar -1 ( Figure 5 (a) The average NaCl rejection of the NG-CS / CNTs membrane was 16.74 ± 2.70% (excluding the first rejection after each flush), which was lower than that of the PA-NF membrane (19.85 ± 3.22%). The high CR dye rejection and low NaCl rejection of the NG-CS / CNTs membrane demonstrate that the NG-CS / CNTs membrane provided in Example 1 can effectively separate CR / NaCl mixed solutions. Furthermore, the higher water permeation flux of the NG-CS / CNTs membrane provided in Example 1 compared to the PA-NF membrane in Comparative Example 2 makes it more suitable for selective separation of dyes and salts.

[0067] 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 properties. In the present invention, the separation performance of the NG-CS / CNTs membrane is optimized by using NG and CS / CNTs as aqueous phase monomers and changing the addition amount of CS / CNTs. While maintaining the dye rejection rate (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.

[0068] The improved water permeation flux of the composite nanofiltration membrane provided by the present invention is primarily due to the CS / CNTs material, a nano-additive for the aqueous phase monomers. The unique tubular structure of the CNTs provides additional transport channels for water molecules, reducing mass transfer resistance. Furthermore, the NG-CS / CNTs composite aqueous phase monomers diffuse more slowly into the organic phase than pure NG monomers. Given the same interfacial polymerization reaction time, the NG-CS / CNTs aqueous phase monomers tend to form a thinner separation layer, shortening the water transport path and promoting improved water permeation flux. The NG-CS / CNTs composite nanofiltration membrane exhibits a retention rate exceeding 90% for selected single charged organic dye molecules and can achieve effective selective separation of dye / salt (CR / NaCl) mixed solutions.

[0069] In general, the present invention uses polyvinyl alcohol-modified carboxylated multi-walled carbon nanotube material CNTs / PVA as the middle layer, uses NG and CS / CNTs together as the 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 fields such as dye and salt separation, and has significant technical effects.

[0070] 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 description 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, preparing nitrogen-doped graphene quantum dots (NG) material by hydrothermal synthesis of the citric acid and ethylenediamine; 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 the mixed solution in NG aqueous phase monomer to obtain an 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, and allowing it to stand to obtain an NG-CS / CNTs impregnated membrane, pouring a 1,3,5-benzenetricarboxylic acid chloride (TMC) / n-hexane solution on the NG-CS / CNTs impregnated membrane to perform an interfacial polymerization reaction, and washing and drying to obtain the composite nanofiltration membrane; The preparation method of the CNTs / PVA-based membrane is as follows: the CNTs are uniformly dispersed in ultrapure water with the assistance of ultrasound, centrifuged, the obtained CNTs dispersion is vacuum-filtered on a pure polyvinylidene fluoride (PVDF)-based membrane, the extracted membrane is removed, and dried to obtain a CNTs-based membrane; the CNTs-based membrane is immersed in a polyvinyl alcohol (PVA) aqueous solution for 1 to 2 hours, and then dried at 50°C to 70°C for 30 minutes to obtain the CNTs / PVA-based membrane.

2. The method for preparing a composite nanofiltration membrane according to claim 1, wherein 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 a composite nanofiltration membrane according to claim 1, wherein: 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 graphene quantum dots material in the NG-CS / CNTs aqueous phase monomer solution is 0.1 to 0.65 wt.%.

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

5. The method for preparing a composite nanofiltration membrane according to claim 1, wherein: In S3, the concentration of the PVA aqueous solution is 1.0-2.0 wt.%; the concentration of the TMC / n-hexane solution is 0.1-0.3 wt.%.

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

7. The method for preparing a composite nanofiltration membrane according to claim 1, wherein: In S3, before the interfacial polymerization reaction, the method further includes: pouring out the excess aqueous monomer solution and removing the residual solution with a rubber roller; and after the interfacial polymerization reaction, the method further includes: washing with a pure n-hexane solution to remove unreacted TMC.

8. A composite nanofiltration membrane, characterized in that The composite nanofiltration membrane is prepared by the preparation method of any one of claims 1 to 7.

9. Use of the composite nanofiltration membrane according to claim 8 in the separation of dyes and salts.

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