An intercalated carbon nitride modified polyamide composite nanofiltration membrane and its preparation method and application

Abstract

The present invention relates to an intercalated carbon nitride-modified polyamide composite nanofiltration membrane, its preparation method, and application. The intercalated carbon nitride-modified polyamide composite nanofiltration membrane comprises an organic ultrafiltration base membrane and a polyamide separation layer on the membrane surface. The polyamide separation layer is formed by attaching graphite-phase carbon nitride modified with Solvent Green 7 to the organic ultrafiltration base membrane via an interfacial polymerization process. Compared with existing technologies, the present invention achieves highly efficient retention of multiple ions in mixed salt solutions, maintaining higher permeation flux and retention rate during mixed salt solution treatment.

Description

Technical Field

[0001] The present invention relates to the field of novel nanofiltration membrane preparation, in particular to an intercalated carbon nitride modified polyamide composite nanofiltration membrane and a preparation method and application thereof. Background Art

[0002] Problems such as water source pollution, delayed improvements in tap water processing and equipment, corrosion of water supply networks, sediment deposition in water, and insecurity in secondary water supply have led to increasing attention for terminal water purification, leading to the emergence of water purifiers. Membrane filters are widely used in water purifiers due to their high filtration precision and excellent water quality.

[0003] Currently, there are three types of water purifiers on the market: ultrafiltration, nanofiltration, and reverse osmosis. Reverse osmosis only allows water to pass through, while all other solutes are retained. Therefore, the purified water produced does not contain minerals and is slightly acidic. Long-term consumption is harmful to human health. Ultrafiltration membranes can retain all pathogens, large proteins, pyrogens, and other impurities. When processing mixed salt solutions, nanofiltration membranes mainly utilize their selective permeability properties to separate multiple components. The pore size of nanofiltration membranes is between reverse osmosis and ultrafiltration, typically between 0.002 and 0.005 microns. They can retain large molecules and multivalent ions with a relative molecular mass of 200-2000 Daltons, while allowing monovalent ions and smaller water molecules to pass through.

[0004] Since its invention, nanofiltration water treatment technology has been widely used in the water treatment field for its advantages, including low energy consumption, excellent separation performance, high efficiency, no chemical additions, and simple operation. In multicomponent separation processes, the selectivity of nanofiltration membranes depends primarily on the chemical properties, pore size distribution, charge, and hydrophilicity of the membrane surface. Different salts have different permeation rates due to differences in ion size, charge, and hydration shell. For example, divalent ions are generally more easily retained than monovalent ions due to their larger size and generally higher charge. Operating conditions such as solution concentration, temperature, and pressure also affect separation performance. Higher concentrations may lead to concentration polarization, whereby a high concentration layer forms on the membrane surface, which can reduce the permeation rate of certain ions. Increasing temperature generally increases the hydration capacity of ions, potentially improving the permeability of small ions. Increasing pressure, on the other hand, directly promotes solution passage through the membrane, improving separation efficiency.

[0005] In practical applications, a balance must be struck between multiple performance parameters. These parameters typically include membrane retention, permeate flux, membrane stability, and operating conditions (such as pressure and temperature). Increasing retention typically results in a decrease in permeate flux (i.e., the amount of water flowing through the membrane). This is because a tighter pore structure or stronger solute-membrane interactions restrict the passage of water molecules. Increasing retention may require a more complex membrane structure, but this can reduce the membrane's mechanical stability, making it more susceptible to chemical or physical damage. Furthermore, increasing selectivity can lead to increased susceptibility to membrane surface fouling or scaling, compromising long-term performance.

[0006] Therefore, it is necessary to provide a nanofiltration membrane that can maintain higher permeation flux and retention rate during the treatment of mixed salt solutions. Summary of the Invention

[0007] The present invention aims to address the multi-component separation problem of nanofiltration membranes in mixed salt solutions by providing a polyamide composite nanofiltration membrane modified with intercalated carbon nitride, as well as its preparation method and application. Focusing on the trade-off between multi-component separation in mixed salt solutions and improving high flux and high retention, the present invention modifies the separation layer of the polyamide nanofiltration membrane at the molecular scale. The intercalated carbon nitride-modified polyamide composite nanofiltration membrane prepared by the present invention can maintain higher permeation flux and retention during the treatment of mixed salt solutions.

[0008] The purpose of the present invention can be achieved by the following technical solutions:

[0009] A method for preparing an intercalated carbon nitride modified polyamide composite nanofiltration membrane, the specific steps are as follows:

[0010] S1, preparing graphite phase carbon nitride;

[0011] S2, modifying the graphite phase carbon nitride obtained in step S1 to obtain a modified carbon nitride solution;

[0012] S3, mixing the modified carbon nitride solution obtained in step S2, 1,4-bis(3-aminopropyl)-piperazine and deionized water, and obtaining an interfacial polymerization aqueous phase solution after ultrasonication;

[0013] S4, mixing trimesoyl chloride and n-hexane, and obtaining an interfacial polymerization organic phase solution after ultrasonication;

[0014] S5. Immerse the base membrane in the interfacial polymerization aqueous phase solution obtained in step S3, and after drying, immerse it again in the interfacial polymerization organic phase solution obtained in step S4, and after drying, obtain an intercalated carbon nitride modified polyamide composite nanofiltration membrane.

[0015] Furthermore, in step S1, graphite phase carbon nitride is prepared by using a thermal oxidation "etching" process, and the specific steps are as follows:

[0016] Melamine is heated in a tube furnace for a period of time, cooled and ground into powder, then put into the tube furnace again, heated again for a period of time, and ground into powder to obtain graphite phase carbon nitride.

[0017] Furthermore, the heating rate of the first heating is 4-6°C / min, the first heating temperature is 500-600°C, and the first heating time is 3-5 h;

[0018] The reheating rate is 2~4℃ / min, the reheating temperature is 450~550℃, and the reheating time is 1.5~2.5h.

[0019] As a preferred technical solution, the heating rate of the first heating is 5°C / min, the first heating temperature is 550°C, and the first heating time is 4 h;

[0020] The heating rate of the reheating was 3°C / min, the reheating temperature was 500°C, and the reheating time was 2 h.

[0021] Furthermore, in step S2, the graphite phase carbon nitride obtained in step S1, solvent green 7 and deionized water are mixed and ultrasonicated to obtain a dispersion; the dispersion is centrifuged and the precipitate is discarded to obtain a modified carbon nitride solution.

[0022] Furthermore, graphite carbon nitride: solvent green 7: deionized water = 1-2 mg: 0.5-1.5 mg: 1.5-2.5 ml;

[0023] The ultrasonic frequency is 40-60 Hz, and the ultrasonic time is 1-3 h;

[0024] The centrifugal speed is 4000-6000 rpm, and the centrifugal time is 10-30 min.

[0025] As a preferred technical solution, graphite carbon nitride: solvent green 7: deionized water = 2 mg: 1 mg: 2 ml;

[0026] The ultrasonic frequency is 50 Hz, and the ultrasonic time is 2 h;

[0027] The centrifugal speed is 5000 rpm and the centrifugal time is 20 min.

[0028] Furthermore, in step S3, the volume ratio of the modified carbon nitride solution, 1,4-bis(3-aminopropyl)-piperazine and deionized water is 2.5-5:0.5-2:100;

[0029] The ultrasonic frequency is 40-60 Hz, and the ultrasonic time is 3-10 min.

[0030] As a preferred technical solution, the volume ratio of the modified carbon nitride solution, 1,4-bis(3-aminopropyl)-piperazine and deionized water is 3:2:100;

[0031] The ultrasonic frequency is 50 Hz, and the ultrasonic time is 5 min.

[0032] Furthermore, in step S4, trimesoyl chloride: n-hexane = 0.1-0.2 g: 150-250 ml;

[0033] The ultrasonic frequency is 40-60 Hz, and the ultrasonic time is 5-15 min.

[0034] As a preferred technical solution, trimesoyl chloride: n-hexane = 0.2 g: 200 ml;

[0035] The ultrasonic frequency is 50 Hz, and the ultrasonic time is 10 min.

[0036] Furthermore, in step S5, the base membrane is made of polyethersulfone, and the average pore size of the polyethersulfone is in the range of 0.22 μm and the maximum pore size is less than 0.45 μm, and the polyethersulfone has good strength and stability.

[0037] Furthermore, in step S5, the base film is immersed in the interfacial polymerization aqueous solution for 2 to 5 minutes, and in the interfacial polymerization organic solution for 30 to 60 seconds.

[0038] Furthermore, in step S5, the base membrane is immersed in the interfacial polymerization aqueous phase solution obtained in step S3, naturally air-dried and then immersed again in the interfacial polymerization organic phase solution obtained in step S4, and placed in an oven for curing to obtain an intercalated carbon nitride modified polyamide composite nanofiltration membrane.

[0039] Furthermore, in the above, the oven temperature is 45-55°C.

[0040] The present invention also provides an intercalated carbon nitride modified polyamide composite nanofiltration membrane, which consists of an organic ultrafiltration base membrane and a polyamide separation layer on the membrane surface. The polyamide separation layer on the membrane surface is formed by graphite phase carbon nitride modified with solvent green 7 and attached to the organic ultrafiltration base membrane through an interfacial polymerization process.

[0041] In addition, the present invention also provides an application of an intercalated carbon nitride modified polyamide composite nanofiltration membrane, which is used in intercepting monovalent and divalent ions in a mixed salt solution.

[0042] Furthermore, the intercalated carbon nitride modified polyamide composite nanofiltration membrane can effectively retain monovalent and divalent ions in a mixed salt solution after stable operation under a working pressure of 0.25 MPa, and has good permeation flux.

[0043] The principles of the present invention are as follows:

[0044] Graphitic carbon nitride is a highly hydrophilic material with numerous free amino groups at its ends, and its inherent triazine ring system provides additional channels for water molecules. Therefore, the introduction of graphitic carbon nitride into the polyamide separation layer on the membrane surface significantly improves the hydrophilicity of the composite membrane, thereby overcoming the trade-off effect of nanofiltration membranes.

[0045] To address the multi-component separation problem of nanofiltration membranes in mixed salt solutions, the present invention uses Solvent Green 7 to intercalate graphite-phase carbon nitride. Solvent Green 7 has a large, planar aromatic structure that intercalates with carbon nitride via π-π bonds. The presence of negatively charged groups such as sulfonates on Solvent Green 7 modulates the charge distribution on the polyamide separation layer on the membrane surface, helping to improve the retention of different ions by the prepared intercalated carbon nitride-modified polyamide composite nanofiltration membrane. Furthermore, the intercalation treatment prevents the agglomeration of graphite-phase carbon nitride in the separation layer, thereby improving the integrity of the composite membrane.

[0046] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0047] 1. The present invention adopts a thermal oxidation "etching" process to prepare graphite phase carbon nitride, which is simple in preparation process and is conducive to improving yield.

[0048] 2. The present invention adopts interfacial polymerization to prepare the intercalated carbon nitride modified polyamide composite nanofiltration membrane. Compared with the existing commercial nanofiltration membrane, the intercalated carbon nitride modified polyamide composite nanofiltration membrane prepared by the present invention has the characteristics of achieving efficient retention of multiple ions in mixed salt solutions. The intercalated carbon nitride modified polyamide composite nanofiltration membrane prepared by the present invention can maintain higher permeation flux and retention rate during the treatment of mixed salt solutions.

[0049] 3. The polyamide separation layer on the organic ultrafiltration base membrane used in this invention is formed by attaching graphite-phase carbon nitride modified with Solvent Green 7 to the organic ultrafiltration base membrane via an interfacial polymerization process. This invention utilizes Solvent Green 7 to intercalate graphite-phase carbon nitride and introduce it into the polyamide membrane separation layer. By adjusting the interlayer spacing to prevent nanoparticle aggregation, improving membrane hydrophilicity, and regulating membrane surface charge to enhance dielectric repulsion, this invention produces a high-efficiency polyamide nanofiltration membrane capable of multicomponent separation in mixed salt solutions while exhibiting excellent permeation flux. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1The figure is a flow chart for the preparation of an intercalated carbon nitride modified polyamide composite nanofiltration membrane;

[0051] Figure 2 The SEM electron microscope and AFM scanning images of the intercalated carbon nitride modified polyamide composite nanofiltration membrane;

[0052] Figure 3 The flux and retention of the intercalated carbon nitride-modified polyamide composite nanofiltration membrane, the composite nanofiltration membrane not modified with Solvent Green 7, and the polyamide membrane without added nanomaterials in three single salt solutions in Test Example 1;

[0053] Figure 4 The flux and retention of the intercalated carbon nitride-modified polyamide composite nanofiltration membrane, the composite nanofiltration membrane not modified with Solvent Green 7, and the polyamide membrane without added nanomaterials in two mixed salt solutions in Test Example 2;

[0054] Figure 5 Comparison of the filtration effects of the intercalated carbon nitride modified polyamide composite nanofiltration membrane and the traditional commercial membrane in Test Example 2;

[0055] Figure 6 Schematic diagram of the structure of intercalated carbon nitride modified polyamide composite nanofiltration membrane.

[0056] Explanation of the accompanying drawings: 1. Organic ultrafiltration base membrane, 2. Polyamide separation layer on the membrane surface, 5. Melamine, 6. Graphite phase carbon nitride, 7. Modified carbon nitride solution, 8. 1,4-bis(3-aminopropyl)-piperazine, 9. Interfacial polymerization aqueous phase solution, 10. Trimesoyl chloride, 11. Interfacial polymerization organic phase solution, 12. Intercalated carbon nitride-modified polyamide composite nanofiltration membrane. DETAILED DESCRIPTION

[0057] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0058] In the following examples, the sources of the reagents are as follows:

[0059] Polyethersulfone ultrafiltration membrane (pore size 0.22 μm, diameter 70 mm) was purchased from Haining Wanda Filter Equipment Co., Ltd.;

[0060] Sodium chloride, calcium chloride, sodium sulfate, melamine, solvent green 7, n-hexane, trimesoyl chloride, 1,4-bis(3-aminopropyl)-piperazine, bovine serum albumin, and humic acid were purchased from Sinopharm Group Chemical Reagent Co., Ltd.

[0061] The following describes some embodiments of the present invention in detail with reference to the accompanying drawings. In the absence of conflict, the following embodiments and features in the embodiments may be combined with each other.

[0062] Example 1

[0063] See also Figure 6 This embodiment provides an intercalated carbon nitride-modified polyamide composite nanofiltration membrane with high separation ability in a mixed salt solution, which is composed of an organic ultrafiltration base membrane 1 and a polyamide separation layer 2 on the membrane surface. The polyamide separation layer 2 on the membrane surface is formed by graphite phase carbon nitride modified with solvent green 7 and attached to the organic ultrafiltration base membrane 1 through an interfacial polymerization process.

[0064] In this embodiment, the organic ultrafiltration base membrane 1 uses polyethersulfone 3 as the base membrane material.

[0065] In addition, this embodiment also provides a method for preparing an intercalated carbon nitride-modified polyamide composite nanofiltration membrane, the specific steps of which are as follows:

[0066] S1. Preparation of graphite phase carbon nitride 6:

[0067] Melamine 5 was heated in a tube furnace at a heating rate of 5°C / min to 550°C for 4 hours. The resulting product was cooled, ground into powder, and then placed in a tube furnace again and heated at a heating rate of 3°C / min to 500°C for 2 hours. The resulting solid was ground into powder to obtain yellow graphite-phase carbon nitride 6 powder.

[0068] S2, graphite phase carbon nitride 6 modification:

[0069] 100 mg of graphite-phase carbon nitride 6 obtained in step S1 and 50 mg of solvent green 7 powder were added sequentially to 100 ml of deionized water, and the mixture was uniformly dispersed by ultrasonic treatment at a frequency of 50 Hz for 2 h to obtain a yellow-green dispersion. Finally, the dispersion was centrifuged at a speed of 5000 rpm for 20 min to remove the precipitate to obtain a yellow-green homogeneous modified carbon nitride solution 7.

[0070] S3. Prepare interfacial polymerization aqueous solution 9:

[0071] 3 ml of the modified carbon nitride solution 7 obtained in step S2 and 2 ml of 1,4-bis(3-aminopropyl)-piperazine 8 were added to 100 ml of deionized water, and the solution was uniformly mixed by ultrasonication at a frequency of 50 Hz for 5 min to obtain an interfacial polymerization aqueous phase solution 9.

[0072] S4, preparing interfacial polymerization organic phase solution 11:

[0073] 0.2 g of trimesoyl chloride 10 was added to 200 ml of n-hexane, and the solution was mixed evenly by ultrasonication at a frequency of 50 Hz for 10 min to obtain an interfacial polymerization organic phase solution 11.

[0074] S5. Preparation of intercalated carbon nitride modified polyamide composite nanofiltration membrane 12 by interfacial polymerization method:

[0075] Polyethersulfone 3 was selected as the base membrane, with an average pore size of 0.22 μm and a maximum pore size of less than 0.45 μm. The base membrane surface was immersed in the interfacial polymerization aqueous phase solution 9 obtained in step S3, removed after 5 minutes, and naturally air-dried until no liquid remained on the membrane surface. The base membrane was again immersed in the interfacial polymerization organic phase solution 11 obtained in step S4, removed after 1 minute, and cured in an oven at 50°C for 15 minutes, ultimately obtaining an intercalated carbon nitride-modified polyamide composite nanofiltration membrane 12.

[0076] The SEM electron microscope and AFM scanning images of the intercalated carbon nitride modified polyamide composite nanofiltration membrane 12 are as follows Figure 2 As shown, the addition of nanomaterials significantly reduced the surface roughness of the composite nanofiltration membrane 12, and the modification of solvent green 7 avoided the agglomeration of CNs and further reduced the roughness. The improvement of the roughness significantly improved the anti-fouling performance of the membrane.

[0077] Comparative Example 1

[0078] This comparative example provides a conventional commercial membrane in the prior art. [1-4] , including NF 90, NF 270, NF1-1812, NF70, DF 30, NP 030.

[0079] References to traditional commercial membranes are as follows:

[0080] [1] H. Al-Zoubi, W. Omar, Rejection of salt mixtures from high salineby nanofiltration membranes, Korean Journal of Chemical Engineering, Vol.26(2009) 799-805.

[0081] [2] HM Krieg, SJ Modise, K. Keizer, HWJP Neomagus, Saltrejection in nanofiltration for single and binary salt mixtures in view of sulphate removal, Desalination, Vol.171 (2005) 205-215.

[0082] [3] J. Tanninen, M. Mänttäri, M. Nyström, Effect of salt mixture concentration on fractionation with NF membranes, Journal of MembraneScience, Vol.283 (2006) 57-64.

[0083] [4] F. Wu, X. Jiao, Y. Guo, C. Wang, Y. Xue, Study on the Retention of Inorganic Salts by Domestic Nanofiltration Membrane, Technology of WaterTreatment, Vol.44 (2018) 41-44. DOI:10.16796 / j.cnki.1000-3770.2018.12.008.

[0084] Comparative Example 2

[0085] This comparative example provides a method for preparing a composite nanofiltration membrane that has not been modified with Solvent Green 7. The specific steps are as follows:

[0086] S1. Preparation of graphite phase carbon nitride 6:

[0087] Melamine 5 was heated in a tube furnace at a heating rate of 5°C / min to 550°C for 4 hours. The resulting product was cooled, ground into powder, and then placed in a tube furnace again and heated at a heating rate of 3°C / min to 500°C for 2 hours. The resulting solid was ground into powder to obtain yellow graphite-phase carbon nitride 6 powder.

[0088] S2, graphite phase carbon nitride 6 modification:

[0089] 100 mg of the graphite-phase carbon nitride 6 obtained in step S1 was added to 100 ml of deionized water, and the mixture was uniformly dispersed by ultrasonic treatment at a frequency of 50 Hz for 2 h to obtain a dispersion. Finally, the dispersion was placed in a centrifuge for centrifugation at a speed of 5000 rpm for 20 min to remove the precipitate to obtain a carbon nitride solution.

[0090] S3. Prepare interfacial polymerization aqueous solution 9:

[0091] 3 ml of the carbon nitride solution obtained in step S2 and 2 ml of 1,4-bis(3-aminopropyl)-piperazine 8 were added to 100 ml of deionized water, and the solution was mixed uniformly by ultrasonication at a frequency of 50 Hz for 5 min to obtain an interfacial polymerization aqueous phase solution 9.

[0092] S4, preparing interfacial polymerization organic phase solution 11:

[0093] 0.2 g of trimesoyl chloride 10 was added to 200 ml of n-hexane, and the solution was mixed evenly by ultrasonication at a frequency of 50 Hz for 10 min to obtain an interfacial polymerization organic phase solution 11.

[0094] S5. Preparation of composite nanofiltration membrane without modification of solvent green 7 by interfacial polymerization:

[0095] Polyethersulfone 3 was selected as the base membrane, with an average pore size of 0.22 μm and a maximum pore size of less than 0.45 μm. The base membrane surface was immersed in the interfacial polymerization aqueous phase solution 9 obtained in step S3, removed after 5 minutes, and naturally air-dried until no liquid remained on the membrane surface. The base membrane was again immersed in the interfacial polymerization organic phase solution 11 obtained in step S4, removed after 1 minute, and cured in an oven at 50°C for 15 minutes, ultimately obtaining a composite nanofiltration membrane that had not been modified with Solvent Green 7.

[0096] Comparative Example 3

[0097] This comparative example provides a method for preparing a polyamide film without adding nanomaterials, and the specific steps are as follows:

[0098] S1. Preparation of interfacial polymerization aqueous solution 9:

[0099] 2 ml of 1,4-bis(3-aminopropyl)-piperazine 8 was added to 100 ml of deionized water, and the solution was mixed uniformly by ultrasonication at a frequency of 50 Hz for 5 min to obtain an interfacial polymerization aqueous phase solution 9.

[0100] S2. Prepare interfacial polymerization organic phase solution 11:

[0101] 0.2 g of trimesoyl chloride 10 was added to 200 ml of n-hexane, and the solution was mixed evenly by ultrasonication at a frequency of 50 Hz for 10 min to obtain an interfacial polymerization organic phase solution 11.

[0102] S3. Preparation of polyamide membrane without adding nanomaterials by interfacial polymerization:

[0103] Polyethersulfone 3 was selected as the base membrane, with an average pore size of 0.22 μm and a maximum pore size of less than 0.45 μm. The base membrane surface was immersed in the interfacial polymerization aqueous phase solution 9 obtained in step S1, removed after 5 minutes, and naturally air-dried until no liquid remained on the membrane surface. The base membrane was again immersed in the interfacial polymerization organic phase solution 11 obtained in step S2, removed after 1 minute, and placed in an oven to cure at 50°C for 15 minutes, ultimately obtaining a polyamide membrane without the addition of nanomaterials.

[0104] Test Example 1

[0105] The intercalated carbon nitride-modified polyamide composite nanofiltration membrane 12 prepared in Example 1, the composite nanofiltration membrane prepared in Comparative Example 2 without solvent green 7 modification, and the polyamide membrane prepared in Comparative Example 3 without the addition of nanomaterials were subjected to cross-flow filtration experiments with different salt solutions. The treated water was three single salt solutions: 0.5 g / L sodium sulfate, 0.5 g / L sodium chloride, and 0.5 g / L calcium chloride. Before the formal experiment, the intercalated carbon nitride-modified polyamide composite nanofiltration membrane 12 was compacted by filtering with water solutions of the same concentration at an operating pressure of 0.25 MPa for 30 minutes. Filtration was then started at an operating pressure of 0.2 MPa for 30 minutes. The permeation flux was calculated by the volume of water effluent per unit time; the salt interception rate was calculated by collecting the influent and effluent water solutions before and after filtration, detecting the ion concentration in the water sample by inductively coupled plasma emission spectroscopy, and finally calculating the actual retention rate.

[0106] The experimental results are as follows Figure 3 As shown, in the retention experiment of a single salt solution, the TFN (S) membrane is the intercalated carbon nitride-modified polyamide composite nanofiltration membrane 12 prepared in Example 1, the TFN (C) membrane is the composite nanofiltration membrane prepared in Comparative Example 2 without being modified with Solvent Green 7, and the TFC membrane is the polyamide membrane prepared in Comparative Example 3 without adding nanomaterials.

[0107] Regarding permeate flux, the addition of carbon nitride significantly improves the membrane's hydrophilicity. This is because carbon nitride improves the polyamide separation layer and pore structure. Combined with the inherent hydrophilicity of the particles, this significantly facilitates the permeation of water molecules. The intercalation of Solvent Green 7 prevents carbon nitride from agglomerating and disrupting the separation layer pores, further enhancing permeability. Regarding salt rejection, TFC and TFN(C) membranes have a certain amount of positive charge distributed on their surfaces due to the Donan effect, resulting in a high rejection of calcium chloride. The intercalation of Solvent Green 7 introduces negatively charged groups, such as sulfonic acid groups, into the pores, thereby improving the rejection of sodium sulfate. This reduction in the overall membrane surface potential weakens the Donan effect's repulsion of the dielectric effect, thereby enhancing the membrane's dielectric rejection of different ions.

[0108] Test Example 2:

[0109] Cross-flow filtration experiments with different salt solutions were conducted using the intercalated carbon nitride-modified polyamide composite nanofiltration membrane 12 prepared in Example 1, the conventional commercial membrane prepared in Comparative Example 1, the composite nanofiltration membrane unmodified with Solvent Green 7 prepared in Comparative Example 2, and the polyamide membrane without nanomaterials added in Comparative Example 3. The treated water consisted of two mixed salt solutions: sodium sulfate-sodium chloride and calcium chloride-sodium chloride solutions, each with a salt concentration of 0.5 g / L. Before the formal experiment, the intercalated carbon nitride-modified polyamide composite nanofiltration membrane 12 prepared in Example 1 was compacted by filtering the solution at an operating pressure of 0.25 MPa for 30 minutes. Filtration was then commenced at an operating pressure of 0.2 MPa for 30 minutes. The permeate flux was calculated based on the volume of effluent per unit time. The salt rejection rate was calculated by collecting the influent and effluent water samples before and after filtration, measuring the ion concentration in the water samples by inductively coupled plasma emission spectroscopy, and finally calculating the actual retention rate.

[0110] The experimental results are as follows Figure 4 As shown, in the retention experiment of mixed salt solution, the intercalated carbon nitride modified polyamide composite nanofiltration membrane 12 prepared in Example 1 has a significantly improved retention rate for sodium chloride. The main reason is that: carboxyl groups and amino groups exist simultaneously on the surface of the polyamide separation layer formed by interfacial polymerization, that is, there are both negatively charged and positively charged regions. The overall charge of the membrane surface depends on the ratio of positively charged and negatively charged regions. The ions in the solution will be adsorbed and complexed with the oppositely charged groups on the membrane surface, thereby changing the charge of the membrane surface to a certain extent. The special charge distribution on the membrane surface enhances the retention of different ions.

[0111] Under the same operating pressure and similar salt solution concentration, the filtration effect of TFN (S) and various commercial nanofiltration membranes in mixed salt solution was compared. The results are as follows: Figure 5As shown in Figure 2, the rejection rate of different ions is increased by more than 50% compared with traditional commercial membranes, and the permeation flux is more than 1.5 times that of commercial membranes, reaching 50.76 Lm −2 h −1 .

[0112] Therefore, the intercalated carbon nitride modified polyamide composite nanofiltration membrane prepared by the present invention can maintain higher permeation flux and retention rate during the treatment of mixed salt solutions.

[0113] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention, without departing from the scope of the present invention, should be within the scope of protection of the present invention.

Claims

1. A method for preparing an intercalated carbon nitride modified polyamide composite nanofiltration membrane, characterized in that: The specific steps are as follows: S1. Preparation of graphite phase carbon nitride (6); S2, mixing the graphite phase carbon nitride (6), solvent green 7 and deionized water obtained in step S1, and obtaining a dispersion after ultrasonic treatment. The dispersion is centrifuged and the precipitate is discarded to obtain a modified carbon nitride solution (7); S3, mixing the modified carbon nitride solution (7) obtained in step S2, 1,4-bis(3-aminopropyl)-piperazine (8) and deionized water, and obtaining an interfacial polymerization aqueous phase solution (9) after ultrasonic treatment; S4, mixing trimesoyl chloride (10) and n-hexane, and obtaining an interfacial polymerization organic phase solution (11) after ultrasonication; S5. Immerse the base membrane in the interfacial polymerization aqueous phase solution (9) obtained in step S3, and after drying, immerse it again in the interfacial polymerization organic phase solution (11) obtained in step S4, and after drying, obtain an intercalated carbon nitride modified polyamide composite nanofiltration membrane (12).

2. The method for preparing a carbon nitride-modified polyamide composite nanofiltration membrane according to claim 1, characterized in that: In step S1, graphite phase carbon nitride (6) is prepared by thermal oxidation "etching" process, and the specific steps are as follows: Melamine (5) is heated in a tube furnace for a period of time, cooled and ground into powder, and then put into the tube furnace again, heated again for a period of time, and ground into powder to obtain graphite phase carbon nitride (6).

3. The method for preparing a carbon nitride-modified polyamide composite nanofiltration membrane according to claim 2, characterized in that: The first heating rate is 4-6°C / min, the first heating temperature is 500-600°C, and the first heating time is 3-5 h; The reheating rate is 2~4℃ / min, the reheating temperature is 450~550℃, and the reheating time is 1.5~2.5h.

4. The method for preparing a carbon nitride-intercalated polyamide composite nanofiltration membrane according to claim 1, characterized in that: In step S2, graphite carbon nitride (6): solvent green 7: deionized water = 1-2 mg: 0.5-1.5 mg: 1.5-2.5 ml; The ultrasound frequency is 40-60 Hz, and the ultrasound time is 1-3 h; The centrifugal speed is 4000~6000 rpm and the centrifugation time is 10~30 min.

5. The method for preparing a carbon nitride-intercalated polyamide composite nanofiltration membrane according to claim 1, characterized in that: In step S3, the volume ratio of the modified carbon nitride solution (7), 1,4-bis(3-aminopropyl)-piperazine (8) and deionized water is 2.5-5:0.5-2:100; The ultrasonic frequency was 40-60 Hz, and the ultrasonic time was 3-10 min.

6. The method for preparing an intercalated carbon nitride modified polyamide composite nanofiltration membrane according to claim 1, characterized in that: In step S4, trimesoyl chloride (10): n-hexane = 0.1-0.2 g: 150-250 ml; The ultrasonic frequency was 40-60 Hz, and the ultrasonic time was 5-15 min.

7. The method for preparing a carbon nitride-intercalated polyamide composite nanofiltration membrane according to claim 1, characterized in that: In step S5, the base film material is polyethersulfone (3); In step S5, the base film is immersed in the interfacial polymerization aqueous phase solution (9) for 2 to 5 minutes and in the interfacial polymerization organic phase solution (11) for 30 to 60 seconds; In step S5, the base membrane is immersed in the interfacial polymerization aqueous phase solution (9) obtained in step S3, and after natural air drying, it is immersed again in the interfacial polymerization organic phase solution (11) obtained in step S4, and placed in an oven for curing to obtain an intercalated carbon nitride modified polyamide composite nanofiltration membrane (12).

8. An intercalated carbon nitride modified polyamide composite nanofiltration membrane obtained by the preparation method according to any one of claims 1 to 7, characterized in that: The invention comprises an organic ultrafiltration base membrane (1) and a polyamide separation layer (2) on the membrane surface, wherein the polyamide separation layer (2) on the membrane surface is formed by graphite phase carbon nitride modified with solvent green 7 and attached to the organic ultrafiltration base membrane (1) through an interfacial polymerization process.

9. An application of an intercalated carbon nitride modified polyamide composite nanofiltration membrane, characterized in that: Use of an intercalated carbon nitride modified polyamide composite nanofiltration membrane (12) obtained by the preparation method as described in any one of claims 1 to 7 for intercepting monovalent and divalent ions in a mixed salt solution.

Images