Intercalated carbon nitride modified polyamide composite nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention relates to an intercalated carbon nitride modified polyamide composite nanofiltration membrane and a preparation method and application thereof, and the intercalated carbon nitride modified polyamide composite nanofiltration membrane is composed of an organic ultrafiltration base membrane and a membrane surface polyamide separation layer, the membrane surface polyamide separation layer is formed by attaching graphite phase carbon nitride modified by solvent green 7 to the organic ultrafiltration base membrane through an interfacial polymerization process. Compared with the prior art, the device has the characteristic of realizing efficient interception of multiple ions in the mixed salt solution, and can keep higher permeation flux and interception rate in the treatment process of the mixed salt solution.

Description

Technical Field

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

[0002] The pollution of water sources, the lag in the improvement of tap water technology and equipment, the rust of water supply pipes, the sedimentation in water, the insecurity of secondary water supply and other issues have made the terminal purification of water receive more and more attention, and water purifiers have been applied. Membrane filter elements are widely used in water purifiers because of their high filtration accuracy and good water quality.

[0003] Currently, there are three types of water purifiers on the market: ultrafiltration, nanofiltration, and reverse osmosis. Reverse osmosis can only allow water to pass through, while all other solutes are retained. Therefore, the purified water produced does not contain minerals and is weakly acidic. Long-term drinking is not good for human health. Ultrafiltration membranes can retain all pathogens, large molecular proteins, pyrogens and other impurities. When nanofiltration membranes are used to process mixed salt solutions, they mainly use their selective permeability characteristics to separate multiple components. The pore size of nanofiltration membranes is between reverse osmosis and ultrafiltration, usually between 0.002 microns 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 field of water treatment for its advantages of low energy consumption, good separation performance, high efficiency, no chemical addition, and simple operation. In the process of multi-component separation, the selectivity of nanofiltration membranes mainly depends on the chemical properties of the membrane material, pore size distribution, charge, and hydrophilicity of the membrane surface. Different salts have different rates of permeation through the membrane due to differences in their ion size, charge, and hydration shell. For example, divalent ions are usually more easily retained than monovalent ions because they are larger in size and usually carry more charge. Operating conditions such as solution concentration, temperature, and pressure can also affect the separation effect. The higher the concentration, the more likely there is concentration polarization, that is, the solution forms a high concentration layer on the membrane surface, which may reduce the permeation rate of certain ions. Increased temperature usually increases the hydration capacity of ions and may improve the permeability of small ions. The increase in pressure can directly push the solution through the membrane and improve the separation efficiency.

[0005] In practical applications, it is necessary to balance the relationship between multiple performance parameters. These parameters usually include the membrane rejection rate, permeate flux, membrane stability, and operating conditions (such as pressure, temperature, etc.), thereby increasing the rejection rate, but usually resulting in a decrease in the permeate flux (i.e., the water flow through the membrane). This is because a tighter pore structure or a stronger solute-membrane interaction will limit the passage of water molecules. In order to increase the rejection rate, a more complex membrane structure may be required, but this may reduce the mechanical stability of the membrane, making it more susceptible to chemical or physical damage. At the same time, improving selectivity may cause the membrane surface to be more susceptible to contamination or scaling, affecting long-term operating 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 purpose of the present invention is to provide an intercalated carbon nitride modified polyamide composite nanofiltration membrane and its preparation method and application in order to solve the problem of multi-component separation of nanofiltration membrane in mixed salt solution. The present invention focuses on the multi-component separation of nanofiltration membrane in mixed salt solution and improves the trade-off effect between high flux and high retention rate to modify the separation layer of 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 rate during the treatment of mixed salt solution.

[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 ultrasonic treatment;

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

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

[0015] Further, in step S1, graphite phase carbon nitride is prepared by 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] In the above, 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 3-5h;

[0018] The heating rate of the reheating is 2 to 4°C / min, the reheating temperature is 450 to 550°C, and the reheating time is 1.5 to 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 4h;

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

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

[0022] In the above, further, graphite phase 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 to 60 Hz, and the ultrasonic time is 1 to 3 hours;

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

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

[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] Further, 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] Further, 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 = 0.2g: 200ml;

[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, the average pore size of the polyethersulfone is in the range of 0.22 μm, 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 phase solution for 2 to 5 minutes, and immersed in the interfacial polymerization organic phase 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] In the above further aspect, 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 by solvent green 7 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, and the application of the intercalated carbon nitride modified polyamide composite nanofiltration membrane in intercepting monovalent and divalent ions in a mixed salt solution.

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

[0043] The principle of the present invention is as follows:

[0044] As a good hydrophilic material, graphite carbon nitride has a large number of free amine groups at its ends, and its triazine ring system provides additional channels for water molecules. Therefore, the introduction of graphite carbon nitride into the polyamide separation layer on the membrane surface greatly improves the hydrophilicity of the composite membrane, thereby overcoming the trade-off effect of the nanofiltration membrane.

[0045] In order to solve the multi-component separation problem of nanofiltration membrane in mixed salt solution, the present invention uses solvent green 7 to perform intercalation treatment on graphite phase carbon nitride. Solvent green 7 has a large planar aromatic structure and can be intercalated with carbon nitride through π-π bonds. Due to the presence of negatively charged groups such as sulfonates on solvent green 7, the charge distribution on the surface of the polyamide separation layer on the membrane surface is adjusted, which helps to improve the retention of different ions by the prepared intercalated carbon nitride modified polyamide composite nanofiltration membrane. In addition, the intercalation treatment also avoids 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, and the product preparation process is simple and is conducive to improving the 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 various 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 membrane surface of the organic ultrafiltration base membrane used in the present invention is formed by attaching the graphite phase carbon nitride modified by solvent green 7 to the organic ultrafiltration base membrane through the interfacial polymerization process. The present invention uses solvent green 7 to intercalate the graphite phase carbon nitride and introduces it into the polyamide membrane separation layer. From three perspectives, the interlayer spacing is adjusted to avoid the agglomeration of nanoparticles, the hydrophilicity of the membrane is improved, and the surface charge of the membrane is adjusted to enhance the dielectric repulsion. A high-efficiency polyamide nanofiltration membrane that can achieve multi-component separation in a mixed salt solution and has excellent permeation flux is prepared. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1The present invention is a flow chart for preparing a polyamide composite nanofiltration membrane modified by intercalated carbon nitride;

[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 by solvent green 7, and the polyamide membrane without adding 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 by solvent green 7, and the polyamide membrane without adding nanomaterials in two mixed salt solutions in Test Example 2;

[0054] Figure 5 The comparison of the filtration effect between the intercalated carbon nitride modified polyamide composite nanofiltration membrane and the traditional commercial model 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 in conjunction with 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] In conjunction with the accompanying drawings, some embodiments of the present invention are described in detail below. 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 efficiency separation capability 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 by solvent green 7 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, and the specific steps are as follows:

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

[0067] Melamine 5 was heated to 550°C in a tube furnace at a heating rate of 5°C / min for 4 hours. The obtained product was cooled, ground into powder, and then put into the tube furnace again, and heated to 500°C at a heating rate of 3°C / min for 2 hours. The obtained 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 are added to 100 ml of deionized water in sequence, and the mixture is evenly dispersed by ultrasonic treatment at a frequency of 50 Hz for 2 h to obtain a yellow-green dispersion; finally, the dispersion is placed in a centrifuge for centrifugal treatment at a centrifugal speed of 5000 rpm for 20 min, and the precipitate is removed to obtain a yellow-green homogeneous modified carbon nitride solution 7.

[0070] S3, preparing interfacial polymerization aqueous phase 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 uniformly mixed by ultrasonic treatment 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 is selected as the base membrane, and its average pore size is 0.22 μm and the maximum pore size is less than 0.45 μm; the surface of the base membrane is immersed in the interfacial polymerization aqueous phase solution 9 obtained in step S3, taken out after 5 minutes, and naturally air-dried until there is no liquid residue on the membrane surface. The base membrane is immersed in the interfacial polymerization organic phase solution 11 obtained in step S4 again, taken out after 1 minute, and placed in an oven for curing at 50°C for 15 minutes, and finally an intercalated carbon nitride modified polyamide composite nanofiltration membrane 12 is obtained.

[0076] The SEM electron microscope and AFM scanning images of the intercalated carbon nitride modified polyamide composite nanofiltration membrane 12 are shown in 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 traditional business model. [1-4] Including NF 90, NF 270, NF1-1812, NF 70, DF 30, NP 030.

[0079] Comparative Example 2

[0080] This comparative example provides a method for preparing a composite nanofiltration membrane that has not been modified with solvent green 7, and the specific steps are as follows:

[0081] S1. Preparation of graphite phase carbon nitride 6:

[0082] Melamine 5 was heated to 550°C in a tube furnace at a heating rate of 5°C / min for 4 hours. The obtained product was cooled, ground into powder, and then put into the tube furnace again, and heated to 500°C at a heating rate of 3°C / min for 2 hours. The obtained solid was ground into powder to obtain yellow graphite phase carbon nitride 6 powder.

[0083] S2, graphite phase carbon nitride 6 modification:

[0084] 100 mg of the graphite phase carbon nitride 6 obtained in step S1 is added to 100 ml of deionized water, and the mixture is evenly dispersed by ultrasonic treatment at a frequency of 50 Hz for 2 h to obtain a dispersion; finally, the dispersion is placed in a centrifuge for centrifugal treatment at a centrifugal speed of 5000 rpm for 20 min to remove the precipitate to obtain a carbon nitride solution.

[0085] S3, preparing interfacial polymerization aqueous phase solution 9:

[0086] 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 evenly by ultrasonication at a frequency of 50 Hz for 5 min to obtain an interfacial polymerization aqueous phase solution 9.

[0087] S4, preparing interfacial polymerization organic phase solution 11:

[0088] 0.2 g of trimesoyl chloride 10 was added to 200 ml of n-hexane, and the solution was uniformly mixed by ultrasonic treatment at a frequency of 50 Hz for 10 min to obtain an interfacial polymerization organic phase solution 11.

[0089] S5. Preparation of composite nanofiltration membrane without modification of solvent green 7 by interfacial polymerization:

[0090] 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 surface of the base membrane was immersed in the interfacial polymerization aqueous phase solution 9 obtained in step S3, taken out after 5 minutes, and naturally air-dried until no liquid remained on the membrane surface. The base membrane was immersed in the interfacial polymerization organic phase solution 11 obtained in step S4 again, taken out after 1 minute, and placed in an oven for curing at 50°C for 15 minutes, and finally a composite nanofiltration membrane without solvent green 7 modification was obtained.

[0091] Comparative Example 3

[0092] This comparative example provides a method for preparing a polyamide film without adding nanomaterials, and the specific steps are as follows:

[0093] S1. Preparation of interfacial polymerization aqueous solution 9:

[0094] 2 ml of 1,4-bis(3-aminopropyl)-piperazine 8 was added to 100 ml of deionized water, and the solution was mixed evenly by ultrasonication at a frequency of 50 Hz for 5 min to obtain an interfacial polymerization aqueous phase solution 9.

[0095] S2, preparing interfacial polymerization organic phase solution 11:

[0096] 0.2 g of trimesoyl chloride 10 was added to 200 ml of n-hexane, and the solution was uniformly mixed by ultrasonic treatment at a frequency of 50 Hz for 10 min to obtain an interfacial polymerization organic phase solution 11.

[0097] S3. Preparation of polyamide membrane without adding nanomaterials by interfacial polymerization method:

[0098] 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 surface of the base membrane was immersed in the interfacial polymerization aqueous phase solution 9 obtained in step S1, taken out after 5 minutes, and naturally air-dried until no liquid remained on the membrane surface. The base membrane was immersed in the interfacial polymerization organic phase solution 11 obtained in step S2 again, taken out after 1 minute, and placed in an oven for curing at 50° C. for 15 minutes, and finally a polyamide membrane without added nanomaterials was obtained.

[0099] Test Example 1

[0100] The intercalated carbon nitride modified polyamide composite nanofiltration membrane 12 prepared in Example 1, the composite nanofiltration membrane prepared in Comparative Example 2 without modification of solvent green 7, and the polyamide membrane without added nanomaterials prepared in Comparative Example 3 were subjected to cross-flow filtration experiments with different salt solutions. The treated water was three single salt solutions, 0.5g / L sodium sulfate, 0.5g / L sodium chloride, and 0.5g / L calcium chloride. Before the formal experiment, the intercalated carbon nitride modified polyamide composite nanofiltration membrane 12 was compacted by filtering for 30 minutes at an operating pressure of 0.25MPa with a water solution of the same concentration. Afterwards, filtration was started at an operating pressure of 0.2MPa for 30 minutes. The permeation flux was calculated by the volume of water 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.

[0101] 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 by Solvent Green 7, and the TFC membrane is the polyamide membrane prepared in Comparative Example 3 without adding nanomaterials.

[0102] For permeation flux, the addition of carbon nitride greatly improves the hydrophilicity of the membrane. This is because carbon nitride improves the polyamide separation layer and pore structure, and the good hydrophilicity of the particles themselves greatly promotes the penetration of water molecules. The intercalation of solvent green 7 prevents the agglomeration of carbon nitride and destroys the pores of the separation layer, thereby further improving the permeation performance. For salt retention rate, based on the Donnan effect, a certain amount of positive charge is distributed on the surface of TFC and TFN (C) membranes, and the retention rate of calcium chloride is relatively high. The intercalation of solvent green 7 introduces negatively charged groups such as sulfonic acid groups inside the pores, thereby improving the retention rate of sodium sulfate. A certain degree of reduction in the overall potential of the membrane surface weakens the rejection of the Donnan effect on the dielectric effect, thereby enhancing the dielectric rejection of different ions by the membrane.

[0103] Test Example 2:

[0104] The intercalated carbon nitride modified polyamide composite nanofiltration membrane 12 prepared in Example 1, the traditional commercial model obtained in Comparative Example 1, the composite nanofiltration membrane prepared in Comparative Example 2 without solvent green 7 modification, and the polyamide membrane without nanomaterial added prepared in Comparative Example 3 were subjected to cross-flow filtration experiments with different salt solutions. The treated water was two mixed salt solutions, namely sodium sulfate-sodium chloride and calcium chloride-sodium chloride solutions, and the salt concentration of each solution was 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 for 30 minutes at an operating pressure of 0.25 MPa with a water solution of the same concentration. Afterwards, filtration was started at an operating pressure of 0.2 MPa, and the filtration time was 30 minutes. The permeation flux was calculated by the volume of water 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.

[0105] The experimental results are as follows Figure 4 As shown, in the interception experiment of mixed salt solution, the intercalated carbon nitride modified polyamide composite nanofiltration membrane 12 prepared in Example 1 has a significantly improved interception rate for sodium chloride. The main reason is that the surface of the polyamide separation layer formed by interfacial polymerization has both carboxyl and amino groups, that is, there are both negatively charged and positively charged areas, and the overall charge of the final membrane surface depends on the ratio of positively charged and negatively charged areas, and 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 interception of different ions.

[0106] Under the same operating pressure and similar salt solution concentration, the filtration performance of TFN(S) and various commercial nanofiltration membranes in mixed salt solution was compared. The results are as follows: Figure 5As shown in the figure, the rejection rate of different ions is increased by more than 50% compared with the traditional commercial membrane, and the permeation flux is more than 1.5 times higher than the commercial membrane, reaching 50.76L·m -2 h -1 .

[0107] 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 solution.

[0108] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and 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.

[0109] References are as follows:

[0110] [1] H.Al-Zoubi, W.Omar, Rejection of salt mixtures from high salinebynanofiltration membranes, Korean Journal of Chemical Engineering, Vol. 26 (2009) 799-805.

[0111] [2]HMKrieg,SJModise,K.Keizer,HWJPNeomagus,Salt rejectioninnanofiltration for single and binary salt mixtures in view of sulphateremoval,Desalination,Vol.171(2005)205-215.

[0112] [3] J. Tanninen, M. M. Effect of salt mixture concentration onfractionation with NF membranes, Journal of Membrane Science, Vol. 283 (2006) 57-64.

[0113] [4]F.Wu,X.Jiao,Y.Guo,C.Wang,Y.Xue,Study on the Retention ofInorganicSalts by Domestic Nanofiltration Membrane,Technology of WaterTreatment,Vol.44(2018)41-44.DOI:10.16796 / j.cnki.1000-3770.2018.12.008。

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, preparing graphite phase carbon nitride (6); S2, modifying the graphite phase carbon nitride (6) obtained in step S1 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 ultrasonic treatment; S5, immersing the base membrane into the interfacial polymerization aqueous phase solution (9) obtained in step S3, and after drying, immersing it again into the interfacial polymerization organic phase solution (11) obtained in step S4, and after drying, obtaining the 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, 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 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 3-5h; The heating rate of the reheating is 2 to 4°C / min, the reheating temperature is 450 to 550°C, and the reheating time is 1.5 to 2.5h.

4. The method for preparing a carbon nitride-modified polyamide composite nanofiltration membrane according to claim 1, characterized in that: In step S2, the graphite phase carbon nitride (6), solvent green 7 and deionized water obtained in step S1 are mixed and ultrasonicated to obtain a dispersion; the dispersion is centrifuged and the precipitate is discarded to obtain a modified carbon nitride solution (7).

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

6. The method for preparing a carbon nitride-modified 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 is 40-60 Hz, and the ultrasonic time is 3-10 min.

7. The method for preparing a 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 is 40-60 Hz, and the ultrasonic time is 5-15 min.

8. The method for preparing a carbon nitride-modified 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 immersed 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, naturally air-dried and then 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).

9. An intercalated carbon nitride modified polyamide composite nanofiltration membrane obtained by the preparation method as claimed in any one of claims 1 to 8, characterized in that: The invention comprises 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.

10. An application of an intercalated carbon nitride modified polyamide composite nanofiltration membrane, characterized in that: Use of the intercalated carbon nitride modified polyamide composite nanofiltration membrane (12) obtained by the preparation method as described in any one of claims 1 to 8 in intercepting monovalent and divalent ions in a mixed salt solution.

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