Preparation method of loose nanofiltration membrane, nanofiltration membrane and application

Abstract

The application relates to the technical field of water treatment membrane materials, and particularly discloses a preparation method of loose nanofiltration membranes, and also discloses the prepared loose nanofiltration membranes and applications thereof. The application takes high-hydrophilicity interface structure and carbon nitride with a large number of cyano and amino structures as a water-phase monomer, and a polybasic acid chloride monomer in an oil phase is subjected to a polymerization reaction at the interface, the interface polymerization process is regulated through a carbon nitride functional layer, and the loose nanofiltration membrane with high separation selectivity and high permeability for dye / salt wastewater can be obtained.

Description

Technical Field

[0001] The invention relates to the technical field of water treatment membrane materials, in particular to a method for preparing a loose nanofiltration membrane, and also to the prepared nanofiltration membrane and applications thereof. Background Art

[0002] Since inorganic salts such as sodium chloride (about 6.0 wt%) and sodium sulfate (about 5.6 wt%) are often introduced during the fabric dyeing process to promote the absorption of dyes by fibers, the coexistence of dyes and salt ions in the discharged wastewater has become a common feature of this type of wastewater. The effective separation of organic dye / inorganic salt ion mixtures is extremely important for the efficient treatment of textile printing and dyeing wastewater and the effective recovery of valuable substances in the water. Nanofiltration membranes with loose structures can selectively pass through inorganic salts while intercepting dye molecules, thereby achieving effective separation of these two types of substances. Therefore, they have broad application prospects in the treatment of printing and dyeing wastewater.

[0003] Existing nanofiltration membranes are typically prepared using a method based on the polymerization reaction between polyamine monomers in the aqueous phase and polyacyl chloride monomers in the oil phase at the interface, primarily forming an ultrathin polyamide functional layer by polymerizing trimesoyl chloride (TMC) and piperazine (PIP) on the base membrane surface. Due to the rapid reaction rate between the polyamine monomers and the polyacyl chloride monomers, precise control of the growth of the polyamide functional layer is difficult to achieve. As a result, the resulting polyamide functional layer is relatively thick and dense, with a wide pore size distribution and low water flux, making it less efficient in salt and dye separation applications.

[0004] At present, the research on loose nanofiltration membranes for the separation of dyes and salts is still in its infancy, facing problems such as high raw material costs, complex preparation processes, and separation effects that cannot meet actual application needs. There is an urgent need to develop a new loose nanofiltration membrane.

[0005] Based on the above problems, the present invention aims to develop a method for preparing a loose nanofiltration membrane to obtain a loose nanofiltration membrane with excellent performance. Summary of the Invention

[0006] The main technical problem solved by the present invention is to provide a method for preparing a loose nanofiltration membrane, the prepared loose nanofiltration membrane and the application thereof.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for preparing a loose nanofiltration membrane, comprising the following steps:

[0008] S1: mixing a nitrogen-containing precursor and an alkali and sintering the mixture to produce carbon nitride, which is then ground to obtain exfoliated nanosheet carbon nitride;

[0009] S2: dialyzing the exfoliated carbon nitride nanosheets in water to prepare a hydrophilic carbon nitride solution;

[0010] S3: The ultrafiltration support bottom membrane is infiltrated with the hydrophilic carbon nitride solution;

[0011] S4: placing the ultrafiltration support base membrane treated in step S3 into a solution containing an organic phase monomer to perform an interfacial polymerization reaction to obtain a loose nanofiltration membrane preform containing a polyamide separation layer;

[0012] S5: heat-treating the loose nanofiltration membrane preform to obtain the loose nanofiltration membrane.

[0013] As an embodiment of the present invention, the nitrogen-containing precursor is selected from at least one of cyanamide, dicyandiamide, melamine, urea, and thiourea.

[0014] As an embodiment of the present invention, the base is selected from at least one of sodium hydroxide, potassium hydroxide and calcium hydroxide.

[0015] As a specific embodiment of the present invention, the alkali is a mixture of sodium hydroxide and potassium hydroxide.

[0016] As an embodiment of the present invention, the nitrogen-containing precursor and the base are mixed in a mass ratio of (1.8-2.0):1.

[0017] As an embodiment of the present invention, the sintering temperature is 300-350° C., and the sintering time is 1-3 hours.

[0018] Preferably, the sintering treatment is carried out in a muffle furnace, and the temperature is raised to 300-350° C. at a heating rate of 4-6° C. / min, and the high-temperature sintering is carried out for 1-3 hours.

[0019] As an embodiment of the present invention, the concentration of carbon nitride in the hydrophilic carbon nitride solution is 1.0 to 3.0 mg / mL.

[0020] As an embodiment of the present invention, the dialysis treatment uses a 3500DA dialysis bag and the dialysis lasts for 12 to 48 hours.

[0021] As an embodiment of the present invention, the ultrafiltration supporting bottom membrane is a polycarbonate filter membrane, a polytetrafluoroethylene filter membrane, a polyethersulfone filter membrane, a polysulfone filter membrane or a polypropylene filter membrane.

[0022] As an embodiment of the present invention, the time of the infiltration treatment is 5 to 30 minutes.

[0023] As an embodiment of the present invention, the concentration of the organic phase monomer in the solution containing the organic phase monomer is 1 to 3 mg / mL.

[0024] As an embodiment of the present application, the organic phase monomer is selected from at least one of isophthaloyl dichloride, terephthaloyl dichloride, 1,3,5-benzene triformyl chloride, benzene tetracarboxylic dichloride, cycloalkane polyacyl chloride, and polyacyl chloride.

[0025] Preferably, the organic phase monomer is 1,3,5-benzene triformyl chloride.

[0026] As an embodiment of the present application, the solution containing the organic phase monomer is an organic solvent solution of the organic phase monomer, and the organic solvent is selected from at least one of n-hexane, n-octane, n-heptane, dichloromethane, trichloromethane, carbon tetrachloride, benzene, toluene, dimethylbenzene, pentane, and cyclohexane.

[0027] As an embodiment of the present application, the reaction temperature of the interfacial polymerization reaction is 20-30℃, the reaction time is 2-10 min, and the ambient humidity of the reaction is 45-70%.

[0028] As an embodiment of the present application, the heat treatment condition is: heating treatment at 40-70℃ for 10-60 min.

[0029] The present application also provides a loose nanofiltration membrane obtained by the preparation method.

[0030] The present application further provides an application of the loose nanofiltration membrane, preferably an application in water treatment, and further preferably an application in printing and dyeing wastewater treatment.

[0031] The preparation method of the loose nanofiltration membrane provided by the present application first prepares carbon nitride, then grinds to prepare exfoliated nanosheet carbon nitride, and then performs dialysis treatment in water to prepare a hydrophilic carbon nitride solution; the support bottom membrane of the ultrafiltration is first immersed and treated with the hydrophilic carbon nitride solution, and then is placed in a solution containing an organic phase monomer to perform interfacial polymerization reaction; the hydrophilic carbon nitride monomer itself has a high hydrophilic interface structure and a large number of cyan and amino structures, and the interfacial polymerization reaction is performed with an organic solution containing a polyacyl chloride monomer, which can effectively control the rate of the polymerization reaction and obtain the loose nanofiltration membrane.

[0032] The preparation method of the present application uses carbon nitride with a high hydrophilic interface structure and a large number of cyan and amino structures as the water phase monomer, and the polyacyl chloride monomer in the oil phase performs interfacial polymerization, and the interfacial polymerization process is controlled by the carbon nitride functional layer, so that the loose nanofiltration membrane with high separation selectivity and high permeability for dye / salt wastewater can be obtained. The loose nanofiltration membrane obtained by the method of the present application can realize effective separation of various dyes and salt ions in water under the premise that the permeation flux is increased by about 4 times. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1is a transmission electron microscope photograph of carbon nitride in the carbon nitride solution prepared in step (2) of Example 1 of the present application;

[0034] Figure 2 is a scanning electron microscope photograph of a wrinkled morphology separation selection layer formed by the hydrophilic carbon nitride polymer in the loose nanofiltration membrane prepared in Example 3 of the present application;

[0035] Figure 3 is a pure water flux detection result graph of the nanofiltration membrane prepared in Examples 1-5 and Comparative Example 1 of the present application;

[0036] Figure 4 is a rejection rate detection result graph of the nanofiltration membrane prepared in Examples 1-5 and Comparative Example 1 of the present application;

[0037] Figure 5 is a solution flux detection result graph of the nanofiltration membrane prepared in Example 3 and Comparative Example 1 of the present application for different inorganic salt aqueous solutions;

[0038] Figure 6 is a rejection rate detection result graph of the nanofiltration membrane prepared in Example 3 and Comparative Example 1 of the present application for different inorganic salt aqueous solutions;

[0039] Figure 7 is a solution flux detection result graph of the nanofiltration membrane prepared in Example 3 and Comparative Example 1 of the present application for different dye aqueous solutions;

[0040] Figure 8 is a rejection rate detection result graph of the nanofiltration membrane prepared in Example 3 and Comparative Example 1 of the present application for different dye aqueous solutions;

[0041] Figure 9 is a water contact angle comparison graph of the nanofiltration membrane prepared in Examples 1-5 and Comparative Example 1 of the present application;

[0042] Figure 10 is a Zeta potential comparison graph of the nanofiltration membrane prepared in Example 3 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0043] The following describes in detail the embodiments of the present application, examples of which are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to explain the present application, and cannot be understood as limiting the present application.

[0044] In the following examples and comparative examples, the agents used are all commercially available products.

[0045] Example 1

[0046] This embodiment provides a loose nanofiltration membrane, which is prepared by the following steps:

[0047] (1) 6 g of melamine, 0.8 g of sodium hydroxide, and 2.24 g of potassium hydroxide were thoroughly mixed and ground in a mortar. The resulting mixture was placed in a muffle furnace and heated from room temperature to 330 ° C at a rate of 5 ° C / min and maintained for 2 h for sintering to obtain carbon nitride. After cooling to room temperature, the mixture was thoroughly ground into powder to obtain exfoliated nanosheet carbon nitride.

[0048] (2) 1.0 g of exfoliated nanosheet carbon nitride powder was dissolved in 100 ml of ultrapure water and stirred thoroughly to dissolve. The supernatant was then divided into a 3500 DA dialysis bag and dialyzed for 48 h to obtain a uniformly dispersed water-soluble carbon nitride solution, i.e., a hydrophilic carbon nitride solution. The concentration of the solution was characterized by ultraviolet absorbance test. The concentration of the carbon nitride solution was 1.0 mg / mL. The transmission electron microscope photo of the carbon nitride in the carbon nitride solution is shown in FIG. Figure 1 , carbon nitride solution can remain stable for several months without precipitation;

[0049] (3) Using a polyethersulfone filter membrane as an ultrafiltration support membrane and the hydrophilic carbon nitride solution prepared in step (2) as an aqueous phase monomer, 5 ml of the hydrophilic carbon nitride solution was dropwise added to the surface of the ultrafiltration support membrane for aqueous phase infiltration. The infiltration treatment lasted for 5 minutes, and then the liquid on the surface of the ultrafiltration support membrane was removed;

[0050] (4) The ultrafiltration support base membrane after the aqueous phase infiltration treatment was then placed in a 2 mg / mL 1,3,5-benzene trimesoyl chloride n-hexane solution to perform an interfacial polymerization reaction. The reaction temperature was 25°C, the reaction environment humidity was 60%, and the reaction lasted for 2 minutes. After that, the ultrafiltration support base membrane after the interfacial polymerization reaction was placed in the n-hexane solution and soaked for 10 seconds to remove the residual acyl chloride monomer on the surface, thereby obtaining a loose nanofiltration membrane preform;

[0051] (5) The loose nanofiltration membrane preform obtained in step (4) was heat-treated in an oven at 60° C. for 10 min to obtain a fully polymerized loose nanofiltration membrane.

[0052] Example 2

[0053] This embodiment provides a loose nanofiltration membrane, which is prepared by the following steps:

[0054] (1) 6 g of melamine, 0.8 g of sodium hydroxide, and 2.24 g of potassium hydroxide were thoroughly mixed and ground in a mortar. The resulting mixture was placed in a muffle furnace and heated from room temperature to 330 ° C at a rate of 5 ° C / min and maintained for 2 h for sintering to obtain carbon nitride. After cooling to room temperature, the mixture was thoroughly ground into powder to obtain exfoliated nanosheet carbon nitride.

[0055] (2) 1.5 g of exfoliated nanosheet carbon nitride powder was dissolved in 100 ml of ultrapure water and stirred thoroughly to dissolve. The supernatant was then divided into a 3500 DA dialysis bag and dialyzed for 48 h to obtain a uniformly dispersed water-soluble carbon nitride solution, i.e., a hydrophilic carbon nitride solution. The concentration of the solution was characterized by ultraviolet absorbance test, and the concentration of the carbon nitride solution was 1.5 mg / mL.

[0056] (3) Using a polyethersulfone filter membrane as an ultrafiltration support membrane and the hydrophilic carbon nitride solution prepared in step (2) as an aqueous phase monomer, 5 ml of the hydrophilic carbon nitride solution was dropwise added to the surface of the ultrafiltration support membrane for aqueous phase infiltration. The infiltration treatment lasted for 5 minutes, and then the liquid on the surface of the ultrafiltration support membrane was removed;

[0057] (4) The ultrafiltration support base membrane after the aqueous phase infiltration treatment was then placed in a 2 mg / mL 1,3,5-benzene trimesoyl chloride n-hexane solution to perform an interfacial polymerization reaction. The reaction temperature was 25°C, the reaction environment humidity was 60%, and the reaction lasted for 2 minutes. After that, the ultrafiltration support base membrane after the interfacial polymerization reaction was placed in the n-hexane solution and soaked for 10 seconds to remove the residual acyl chloride monomer on the surface, thereby obtaining a loose nanofiltration membrane preform;

[0058] (5) The loose nanofiltration membrane preform obtained in step (4) was heat-treated in an oven at 60° C. for 10 min to obtain a fully polymerized loose nanofiltration membrane.

[0059] Example 3

[0060] This embodiment provides a loose nanofiltration membrane, which is prepared by the following steps:

[0061] (1) 6 g of melamine, 0.8 g of sodium hydroxide, and 2.24 g of potassium hydroxide were thoroughly mixed and ground in a mortar. The resulting mixture was placed in a muffle furnace and heated from room temperature to 330 ° C at a rate of 5 ° C / min and maintained for 2 h for sintering to obtain carbon nitride. After cooling to room temperature, the mixture was thoroughly ground into powder to obtain exfoliated nanosheet carbon nitride.

[0062] (2) 2.0 g of exfoliated nanosheet carbon nitride powder was dissolved in 100 ml of ultrapure water and stirred thoroughly to dissolve. The supernatant was then divided into a 3500 DA dialysis bag and dialyzed for 48 h to obtain a uniformly dispersed water-soluble carbon nitride solution, i.e., a hydrophilic carbon nitride solution. The concentration of the solution was characterized by ultraviolet absorbance test, and the concentration of the carbon nitride solution was 2.0 mg / mL.

[0063] (3) Using a polyethersulfone filter membrane as an ultrafiltration support membrane and the hydrophilic carbon nitride solution prepared in step (2) as an aqueous phase monomer, 5 ml of the hydrophilic carbon nitride solution was dropwise added to the surface of the ultrafiltration support membrane for aqueous phase infiltration. The infiltration treatment lasted for 5 minutes, and then the liquid on the surface of the ultrafiltration support membrane was removed;

[0064] (4) The ultrafiltration support base membrane after the aqueous phase infiltration treatment was then placed in a 2 mg / mL 1,3,5-benzene trimesoyl chloride n-hexane solution to perform an interfacial polymerization reaction. The reaction temperature was 25°C, the reaction environment humidity was 60%, and the reaction lasted for 2 minutes. After that, the ultrafiltration support base membrane after the interfacial polymerization reaction was placed in the n-hexane solution and soaked for 10 seconds to remove the residual acyl chloride monomer on the surface, thereby obtaining a loose nanofiltration membrane preform;

[0065] (5) The loose nanofiltration membrane preform obtained in step (4) was heat-treated in an oven at 60°C for 10 minutes to obtain a fully polymerized loose nanofiltration membrane. The scanning electron microscope image of the wrinkled separation selection layer formed by the polymerization of hydrophilic carbon nitride in the loose nanofiltration membrane is shown in FIG. Figure 2 , it can be seen that the hydrophilic carbon nitride prepared membrane has a smooth and wrinkled separation layer surface.

[0066] Example 4

[0067] This embodiment provides a loose nanofiltration membrane, which is prepared by the following steps:

[0068] (1) 6 g of melamine, 0.8 g of sodium hydroxide, and 2.24 g of potassium hydroxide were thoroughly mixed and ground in a mortar. The resulting mixture was placed in a muffle furnace and heated from room temperature to 330 ° C at a rate of 5 ° C / min and maintained for 2 h for sintering to obtain carbon nitride. After cooling to room temperature, the mixture was thoroughly ground into powder to obtain exfoliated nanosheet carbon nitride.

[0069] (2) 2.5 g of exfoliated nanosheet carbon nitride powder was dissolved in 100 ml of ultrapure water and stirred thoroughly to dissolve. The supernatant was then divided into a 3500 DA dialysis bag and dialyzed for 48 h to obtain a uniformly dispersed water-soluble carbon nitride solution, i.e., a hydrophilic carbon nitride solution. The concentration of the solution was characterized by ultraviolet absorbance test, and the concentration of the carbon nitride solution was 2.5 mg / mL.

[0070] (3) Using a polyethersulfone filter membrane as an ultrafiltration support membrane and the hydrophilic carbon nitride solution prepared in step (2) as an aqueous phase monomer, 5 ml of the hydrophilic carbon nitride solution was dropwise added to the surface of the ultrafiltration support membrane for aqueous phase infiltration. The infiltration treatment lasted for 5 minutes, and then the liquid on the surface of the ultrafiltration support membrane was removed;

[0071] (4) The ultrafiltration support base membrane after the aqueous phase infiltration treatment was then placed in a 2 mg / mL 1,3,5-benzene trimesoyl chloride n-hexane solution to perform an interfacial polymerization reaction. The reaction temperature was 25°C, the reaction environment humidity was 60%, and the reaction lasted for 2 minutes. After that, the ultrafiltration support base membrane after the interfacial polymerization reaction was placed in the n-hexane solution and soaked for 10 seconds to remove the residual acyl chloride monomer on the surface, thereby obtaining a loose nanofiltration membrane preform;

[0072] (5) The loose nanofiltration membrane preform obtained in step (4) was heat-treated in an oven at 60° C. for 10 min to obtain a fully polymerized loose nanofiltration membrane.

[0073] Example 5

[0074] This embodiment provides a loose nanofiltration membrane, which is prepared by the following steps:

[0075] (1) 6 g of melamine, 0.8 g of sodium hydroxide, and 2.24 g of potassium hydroxide were thoroughly mixed and ground in a mortar. The resulting mixture was placed in a muffle furnace and heated from room temperature to 330 ° C at a rate of 5 ° C / min and maintained for 2 h for sintering to obtain carbon nitride. After cooling to room temperature, the mixture was thoroughly ground into powder to obtain exfoliated nanosheet carbon nitride.

[0076] (2) 3.0 g of exfoliated nanosheet carbon nitride powder was dissolved in 100 ml of ultrapure water and stirred thoroughly to dissolve. The supernatant was then divided into a 3500 DA dialysis bag and dialyzed for 48 h to obtain a uniformly dispersed water-soluble carbon nitride solution, i.e., a hydrophilic carbon nitride solution. The concentration of the solution was characterized by ultraviolet absorbance test, and the concentration of the carbon nitride solution was 3.0 mg / mL.

[0077] (3) Using a polyethersulfone filter membrane as an ultrafiltration support membrane and the hydrophilic carbon nitride solution prepared in step (2) as an aqueous phase monomer, 5 ml of the hydrophilic carbon nitride solution was dropwise added to the surface of the ultrafiltration support membrane for aqueous phase infiltration. The infiltration treatment lasted for 5 minutes, and then the liquid on the surface of the ultrafiltration support membrane was removed;

[0078] (4) The ultrafiltration support base membrane after the aqueous phase infiltration treatment was then placed in a 2 mg / mL 1,3,5-benzene trimesoyl chloride n-hexane solution to perform an interfacial polymerization reaction. The reaction temperature was 25°C, the reaction environment humidity was 60%, and the reaction lasted for 2 minutes. After that, the ultrafiltration support base membrane after the interfacial polymerization reaction was placed in the n-hexane solution and soaked for 10 seconds to remove the residual acyl chloride monomer on the surface, thereby obtaining a loose nanofiltration membrane preform;

[0079] (5) The loose nanofiltration membrane preform obtained in step (4) was heat-treated in an oven at 60° C. for 10 min to obtain a fully polymerized loose nanofiltration membrane.

[0080] Comparative Example 1

[0081] This comparative example provides a nanofiltration membrane, and the specific preparation steps are as follows:

[0082] Step 1: Use piperazine as the aqueous phase monomer and 1,3,5-trimethylbenzyl chloride as the organic phase monomer, dissolve them in water and n-hexane respectively to prepare a piperazine aqueous solution with a concentration of 4.0 mg / mL and a 1,3,5-trimethylbenzyl chloride organic solution with a concentration of 2 mg / mL, respectively, and set aside;

[0083] Step 2: Using a polyethersulfone filter membrane as an ultrafiltration support membrane, 5 ml of a piperazine aqueous solution was added dropwise to the surface of the ultrafiltration support membrane for aqueous infiltration for 5 minutes, after which the liquid on the surface of the ultrafiltration support membrane was removed;

[0084] Step 3: Immersing the ultrafiltration support base membrane after the aqueous phase infiltration treatment in a 1,3,5-benzene trimesoyl chloride organic solution for interfacial polymerization reaction at a reaction temperature of 25° C. and a reaction ambient humidity of 60% for 2 minutes, then immersing the ultrafiltration support base membrane after the interfacial polymerization reaction in a n-hexane solution for 10 seconds to remove residual acyl chloride monomers on the surface, thereby obtaining a nanofiltration membrane preform;

[0085] Step 4: Heat-treat the nanofiltration membrane preform in an oven at 60° C. for 10 minutes to obtain a fully polymerized nanofiltration membrane.

[0086] The performance of the nanofiltration membranes prepared in the above embodiments and comparative examples was tested, as follows.

[0087] (1) The nanofiltration membranes prepared in Examples 1-5 and Comparative Example 1 were respectively applied to deionized water solutions to test their performance.

[0088] The test process is as follows: 2L deionized water solution is used as the test liquid, and the separation performance of the nanofiltration membrane is tested by cross-flow filtration at 4 bar. The test liquid temperature is 25°C. After pre-pressurization at 4 bar for 30 minutes, the produced water is collected for 30 minutes, and the pure water flux of the nanofiltration membrane is calculated based on the volume of the output water.

[0089] The pure water flux test results of the nanofiltration membranes prepared in Examples 1-5 and Comparative Example 1 are shown in FIG. Figure 3 .from Figure 3 It can be seen that the pure water flux of the nanofiltration membrane prepared in Comparative Example 1 is only 10 Lm-2h-1bar-1, while the pure water flux of the loose nanofiltration membrane provided by the embodiment of the present invention can reach 3-8 times that of the nanofiltration membrane prepared in Comparative Example 1.

[0090] (2) The nanofiltration membranes prepared in Examples 1-5 and Comparative Example 1 were respectively applied to a mixed aqueous solution of sodium chloride and methyl blue to test their performance.

[0091] The test process is as follows: a mixed solution containing 1000ppm sodium chloride and 200ppm methyl blue is prepared as the dye / salt mixed test solution, and the separation performance of the nanofiltration membrane is tested at 4bar by cross-flow filtration. The test solution temperature is 25°C, and the produced water is collected for 30 minutes after pre-pressurization at 4bar. The retention rate of the nanofiltration membrane for salt ions is calculated based on the conductivity values ​​of the produced water and the influent water, and the retention rate of the nanofiltration membrane for dyes is calculated based on the absorbance values ​​of the produced water and the influent water.

[0092] The rejection rate test results of the nanofiltration membranes prepared in Examples 1-5 and Comparative Example 1 are shown in FIG. Figure 4 .from Figure 4 As can be seen, the nanofiltration membrane prepared in Example 3, for example, can retain nearly 100% of methyl blue and allow nearly 90% of sodium chloride to pass through. In contrast, the nanofiltration membrane in Comparative Example 1, while retaining nearly 100% of methyl blue, allows 70% of salt ions to pass through. This demonstrates that the loose nanofiltration membrane prepared in Example 3 of the present invention can better separate dye molecules and salt ions.

[0093] (3) The nanofiltration membranes prepared in Example 3 and Comparative Example 1 were respectively applied to different inorganic salt aqueous solutions to test their performance.

[0094] The retention rate test process is as follows: prepare aqueous solutions of sodium chloride, magnesium chloride, sodium sulfate, and magnesium sulfate, each with a concentration of 1000 ppm, as test solutions; use cross-flow filtration to test the separation performance of the nanofiltration membrane at 4 bar, with the test solution temperature at 25°C. After pre-pressurization at 4 bar for 30 minutes, collect the produced water for 30 minutes, and calculate the retention rate of the nanofiltration membrane for salt ions based on the conductivity values ​​of the produced water and the influent water.

[0095] The solution flux test process is as follows: prepare aqueous solutions of sodium chloride, magnesium chloride, sodium sulfate, and magnesium sulfate, each with a concentration of 1000 ppm, as test solutions; use cross-flow filtration to test the separation performance of the nanofiltration membrane at 4 bar, the test liquid temperature is 25°C, and after pre-pressurization at 4 bar for 30 minutes, collect the produced water for 30 minutes, and calculate the solution flux of the nanofiltration membrane based on the volume of the effluent.

[0096] The results of the solution flux test of the nanofiltration membrane prepared in Example 3 and Comparative Example 1 for different inorganic salt aqueous solutions are shown in FIG. Figure 5 , the retention rate test results of different inorganic salt solutions are shown in the figure Figure 6 .

[0097] from Figure 5 and Figure 6 It can be seen that the nanofiltration membrane prepared in Example 3 has a flux of 40 Lm-2h-1bar-1 for sodium chloride solution and a rejection rate of 11%. The nanofiltration membrane prepared in Comparative Example 1 has a flux of 10 Lm-2h-1bar-1 for sodium chloride solution and a rejection rate of 305. The nanofiltration membrane prepared in Example 3 has a flux of 36 Lm-2h-1bar-1 for sodium sulfate solution and a rejection rate of 32%. The nanofiltration membrane prepared in Comparative Example 1 has a flux of 9 Lm-2h-1bar-1 for sodium sulfate solution and a rejection rate of 98%. This shows that the loose nanofiltration membrane prepared in Example 3 has good permeability to salt ions.

[0098] (4) The nanofiltration membranes prepared in Example 3 and Comparative Example 1 were applied to different dye aqueous solutions to test their performance.

[0099] The retention rate test process is as follows: prepare aqueous solutions of sunset yellow, rhodamine B, Congo red, brilliant green SF and methyl blue, each with a concentration of 100 ppm, as test solutions; use cross-flow filtration to test the separation performance of the nanofiltration membrane at 4 bar, the test liquid temperature is 25°C, and after pre-pressurization at 4 bar for 30 minutes, collect the produced water for 30 minutes, and calculate the retention rate of the nanofiltration membrane for the dye based on the absorbance values ​​of the produced water and the influent.

[0100] The solution flux test process is as follows: prepare aqueous solutions of sunset yellow, rhodamine B, Congo red, brilliant green SF and methyl blue, each with a concentration of 100 ppm, as test solutions; use cross-flow filtration to test the separation performance of the nanofiltration membrane at 4 bar, the test liquid temperature is 25°C, and after pre-pressurization at 4 bar for 30 minutes, collect the produced water for 30 minutes, and calculate the solution flux of the nanofiltration membrane based on the volume of the effluent.

[0101] The results of the solution flux test of the nanofiltration membrane prepared in Example 3 and Comparative Example 1 for different dye aqueous solutions are shown in FIG. Figure 7 , the retention rate test results of different dye aqueous solutions are shown in the figure Figure 8 .

[0102] from Figure 7 and Figure 8 It can be seen that the methyl blue solution flux obtained by the nanofiltration membrane prepared in Example 3 of the present invention is 32 Lm-2h-1bar-1, and the retention rate is 99%. The methyl blue solution flux obtained by the nanofiltration membrane prepared in Comparative Example 1 is 8 Lm-2h-1bar-1, and the retention rate is 99%. This shows that the loose nanofiltration membrane prepared in Example 3 has good dye retention performance and can maintain a high flux.

[0103] (5) The surface contact angles of the nanofiltration membranes prepared in Examples 1-5 and Comparative Example 1 were measured using an OCA25 video contact angle meter produced by Dataphysics of Germany.

[0104] The test procedure involves using ultrapure water as the test fluid. A nanofiltration membrane is placed on the instrument's sample stage, ensuring it lies flat. A 1μL drop of water is suspended at the tip of a needle. The sample stage is raised to allow the nanofiltration membrane surface to contact the suspended drop, and the membrane is then removed to complete the droplet transfer. When the droplet reaches equilibrium on the nanofiltration membrane surface, the contact angle is measured using optical projection. The test environment is 25°C.

[0105] Figure 9 The water contact angle comparison diagram of the nanofiltration membranes prepared in Examples 1 to 5 and Comparative Example 1 is shown. Figure 9This shows that the nanofiltration membranes provided by Examples 1 to 5 of the present invention have strong hydrophilicity, which is better than the nanofiltration membrane provided by Comparative Example 1.

[0106] (6) The zeta potential of the nanofiltration membrane surfaces prepared in Example 3 and Comparative Example 1 was tested using an Austrian Anton Paar surpass3 solid surface zeta potential tester.

[0107] The test procedure involved using a 74.5 ppm (0.001 mMol / L) potassium chloride solution as the test solution. The nanofiltration membrane was cut into two 1 x 2 cm rectangular pieces and fixed to the instrument's sample stage. The solution's pH was adjusted to 3, 4, 5, 6, and 9, and the zeta potential of the nanofiltration membrane surface was measured using streaming potential and streaming current measurements. The test environment was maintained at 25°C.

[0108] Figure 10 This is a comparison chart of the Zeta potential of the nanofiltration membranes prepared in Example 3 and Comparative Example 1. Figure 10 It shows that the nanofiltration membrane prepared in Example 3 has stronger electronegativity than the nanofiltration membrane provided in Comparative Example 1.

[0109] In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" 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 expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and the features of different embodiments or examples without contradiction.

[0110] Although the above embodiments have been shown and described, it is understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. Changes, modifications, substitutions and variations of the above embodiments by those skilled in the art are all within the scope of protection of the present invention.

Claims

1. A method for preparing a loose nanofiltration membrane, characterized in that: The following steps are involved: S1: mixing a nitrogen-containing precursor and an alkali and sintering the mixture to obtain carbon nitride, which is then ground to obtain exfoliated nanosheet carbon nitride; the nitrogen-containing precursor is selected from at least one of cyanamide, dicyandiamide, melamine, urea, and thiourea; S2: dialyzing the exfoliated carbon nitride nanosheets in water to prepare a hydrophilic carbon nitride solution; S3: The ultrafiltration support bottom membrane is infiltrated with the hydrophilic carbon nitride solution; S4: placing the ultrafiltration support base membrane treated in step S3 into a solution containing an organic phase monomer to perform an interfacial polymerization reaction to obtain a loose nanofiltration membrane preform containing a polyamide separation layer; S5: heat-treating the loose nanofiltration membrane preform to obtain the loose nanofiltration membrane.

2. The preparation method according to claim 1, characterized in that The alkali is selected from at least one of sodium hydroxide, potassium hydroxide and calcium hydroxide.

3. The preparation method according to claim 1 or 2, characterized in that The nitrogen-containing precursor and the base are mixed in a mass ratio of (1.8-2.0):

1.

4. The preparation method according to claim 1, characterized in that The sintering temperature is 300-350° C., and the sintering time is 1-3 hours.

5. The preparation method according to claim 4, characterized in that The sintering process is carried out in a muffle furnace, and the temperature is raised to 300-350° C. at a heating rate of 4-6° C. / min, and the high-temperature sintering is carried out for 1-3 hours.

6. The preparation method according to claim 1, characterized in that The concentration of carbon nitride in the hydrophilic carbon nitride solution is 1.0-3.0 mg / mL.

7. The preparation method according to claim 1 or 6, characterized in that The dialysis treatment uses a 3500DA dialysis bag and the dialysis is performed for 12 to 48 hours.

8. The preparation method according to claim 1, characterized in that The time of the infiltration treatment is 5 to 30 minutes.

9. The preparation method according to claim 1, characterized in that The concentration of the organic phase monomer in the solution containing the organic phase monomer is 1 to 3 mg / mL; and / or, the organic phase monomer is at least one selected from isophthaloyl chloride, terephthaloyl chloride, 1,3,5-trimesoyl chloride, pyromellitoyl chloride, cycloalkane polyacyl chloride, and polysulfonyl chloride; And / or, the reaction temperature of the interfacial polymerization reaction is 20-30° C., the reaction time is 2-10 min, and the reaction environment humidity is 45-70%.

10. The preparation method according to claim 9, characterized in that The organic phase monomer is 1,3,5-trimethylbenzenecarboxylic acid chloride.

11. A loose nanofiltration membrane, characterized in that: The method is prepared according to any one of claims 1 to 10.

12. Use of the loose nanofiltration membrane according to claim 11 in the treatment of printing and dyeing wastewater.

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