Method for preparing loose nanofiltration membrane by carbon quantum dots promoting rapid deposition of polydopamine

Abstract

The present application relates to a kind of carbon quantum dots promotes the method for preparing loose nanofiltration membrane of polydopamine rapid deposition, the method includes the following steps: S1, with amino acid as carbon source, after pyrolysis, join appropriate amount of water ultrasonic 30min, obtain the dispersion of carbon quantum dots;The dispersion is dialyzed, and freeze-drying is obtained carbon quantum dots solid;S2, in the system of the mass concentration of dopamine 1-8g / L, add carbon quantum dots, obtain the dopamine solution containing carbon quantum dots;The dopamine solution containing carbon quantum dots is fully contacted with the surface of ultrafiltration membrane, under visible light irradiation, reaction 40-90min, wash, heat treatment is obtained loose nanofiltration membrane product at 40-60 ℃.The present application adds carbon quantum dots in dopamine to solve the problem of slow deposition speed and poor deposition uniformity of polydopamine, effectively prepare the loose nanofiltration membrane of high-efficiency dye-inorganic salt separation.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of loose nanofiltration membrane separation, and particularly relates to a method for preparing a loose nanofiltration membrane by using carbon quantum dots to promote rapid deposition of polydopamine. BACKGROUND

[0002] Large-volume and high-colority printing and dyeing wastewater has been the focus of industrial wastewater treatment in China. A large amount of inorganic salts in wastewater have high recycling value. Therefore, if the dyes and inorganic salts in printing and dyeing wastewater can be efficiently separated, the wastewater treatment and recycling of resources can be realized, which has important economic significance for the green development of the printing and dyeing industry.

[0003] As an efficient, environmentally friendly and energy-saving separation technology, membrane separation has a wide application in water purification and softening, wastewater treatment, industrial purification and recovery. Among them, nanofiltration has become one of the hotspots of membrane separation technology research due to its low operating pressure and high separation efficiency. In recent years, the dopamine deposition technology based on the mimicry of mussels has been applied to the preparation of loose nanofiltration membranes, realizing the effective separation of dyes and inorganic salts (high dye rejection rate and low inorganic salt rejection rate). However, the dopamine oxidation self-polymerization reaction is usually carried out in air for a long time (usually for several hours), and the uniformity of the deposited polydopamine coating is poor, and there are large polydopamine aggregate particles in the coating, which affects the compactness and stability of the polydopamine coating.

[0004] Therefore, some literatures (Angew. Chem. Int. Ed. 2016, 55, 3054-3057; Chem. Eng. J. 2020, 388, 124200; J. Colloid Interface Sci., 2018, 523, 86-97) have reported methods for promoting rapid deposition of polydopamine for preparing nanofiltration membranes. However, some prepared nanofiltration membranes are too dense to realize effective separation of dyes and inorganic salts (Angew. Chem. Int. Ed. 2016, 55); some need to add macromolecular amine monomers to avoid the precipitation of polydopamine particles due to the too fast oxidation speed (Chem. Eng. J. 2020, 388, 124200), and in addition, an extra hydrogen peroxide reagent needs to be added, increasing the economic cost. SUMMARY

[0005] The purpose of the present application is to provide a method for preparing a loose nanofiltration membrane by using carbon quantum dots to promote rapid deposition of polydopamine, which utilizes carbon quantum dots to promote the polymerization of dopamine to polydopamine, realizes uniform deposition of polydopamine in a short time, and obtains a loose nanofiltration membrane for efficient separation of dyes and inorganic salts.

[0006] The technical scheme adopted by the present application to solve its technical problems is:

[0007] A method for preparing a loose nanofiltration membrane by carbon quantum dots promoting rapid deposition of polydopamine, the method comprising the following steps:

[0008] S1, preparing carbon quantum dots

[0009] Pyrolysis at a temperature of 240℃±10℃ for 1h-2h, after cooling to room temperature, add appropriate amount of water and ultrasonic for 30min to obtain a dispersion of carbon quantum dots;

[0010] Remove larger particles by filtration, place the dispersion in a dialysis bag and dialyze for more than 72h, and finally freeze-dry to obtain carbon quantum dots solid;

[0011] S2, preparing a loose nanofiltration membrane by rapid deposition of polydopamine

[0012] Dissolve dopamine in a Tris buffer solution containing 20mM, pH 8.5, so that the mass concentration of dopamine is 1-8g / L; add carbon quantum dots to the system, control the mass ratio of dopamine to carbon quantum dots to be 1:1-4, and obtain a dopamine solution containing carbon quantum dots;

[0013] Sufficiently contact the dopamine solution containing carbon quantum dots with the surface of the ultrafiltration membrane, react under visible light irradiation and 100-400rpm oscillation for 40-90min, take out the obtained membrane after the reaction is completed, rinse with water, and heat treat at 40-60℃ for 10-60min to obtain a loose nanofiltration membrane product.

[0014] Generally speaking, the deposition of dopamine takes several hours or even dozens of hours, but the improvement of the present application can be completed within one hour. The present application solves the problems of slow deposition speed and poor deposition uniformity of polydopamine by adding carbon quantum dots to dopamine, and effectively prepares a loose nanofiltration membrane for efficient separation of dye-inorganic salt. Carbon quantum dots can produce a large amount of active oxygen under visible light, which promotes the polymerization of dopamine and the rapid deposition of polydopamine on the ultrafiltration membrane. The loose nanofiltration membrane prepared by the present application has a uniform polydopamine coating, can realize efficient separation of dye-inorganic salt, and has practical application value.

[0015] As a preferred, the amino acid in S1 is selected from at least one of lysine, aspartic acid, glycine or arginine; the ultrafiltration membrane in S2 is selected from one of polyacrylonitrile, polysulfone or polyethersulfone ultrafiltration membrane.

[0016] As a preferred, the mass concentration of dopamine in the solution obtained by adding Tris buffer solution in S2 is 2-5g / L.

[0017] As preferred, the mass ratio of dopamine to carbon quantum dots in S2 is 1:1-3. The addition amount of the carbon quantum dots is determined by the ratio with dopamine, and the control of the ratio is to ensure good polymerization promotion effect.

[0018] As preferred, the visible light in S2 is generated by a xenon lamp light source with a power of 300W±100w and equipped with a 420nm filter. Other light sources that can provide visible light can also be used instead.

[0019] As preferred, the oscillation speed in S2 is 150rpm.

[0020] As preferred, the deposition time in S2 is 45-60min.

[0021] As preferred, the heat treatment temperature in S2 is 40-50℃, and the heat treatment time is 20-40min.

[0022] As preferred, in S1, 1g of amino acid is used as the carbon source, pyrolyzed at 240℃ for 1h, after cooling to room temperature, 20mL of deionized water is added and ultrasonic treated for 30min to obtain a dispersion of carbon quantum dots.

[0023] As preferred, the mass ratio of dopamine to carbon quantum dots in S2 is 1:1.

[0024] The performance evaluation method of the nanofiltration membrane prepared by the carbon quantum dots promoting rapid deposition of polydopamine of the present application is as follows: the nanofiltration membrane is placed in a nanofiltration membrane performance evaluation instrument, pre-pressed at 0.6MPa for 1h before testing, then the water flux (J) and salt rejection rate (R) of the nanofiltration membrane are measured at 0.6MPa and 25℃. The calculation formula is: Wherein, A is the effective membrane area (22.5cm 2 ); t is the time required for the permeate of the collection volume V; C p and C f are the concentrations of the feed liquid and the permeate, respectively. The concentration of inorganic salt is determined by a conductivity meter, and the concentration of dye is determined by ultraviolet-visible light absorption spectrum.

[0025] Under visible light, the present application uses carbon quantum dots to promote the polymerization of dopamine and the rapid deposition of polydopamine on the surface of the ultrafiltration membrane, so as to obtain a loose nanofiltration membrane. Compared with the prior art, the present application has the following beneficial effects:

[0026] 1) Without the need for heavy metal ions and additional reagent addition, the deposition of polydopamine is promoted by biomass carbon quantum dots, which can be completed within 40-90min, shortening the preparation time of the polydopamine deposition separation membrane;

[0027] 2) A uniform and stable polydopamine coating layer can be formed on the surface of the ultrafiltration membrane;

[0028] 3) By regulating the ratio of dopamine and carbon quantum dots, the deposition time, the separation performance of the loose nanofiltration membrane can be optimized, and the effective separation of dyes and inorganic salts can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 is the change of UV absorption of indole group with time during the polymerization of dopamine in air and the polymerization process promoted by adding carbon quantum dots;

[0030] Figure 2 is the change of fluorescence intensity at 520nm with time of 2',7'-dichlorofluorescein phosphate buffer solution and 2',7'-dichlorofluorescein phosphate buffer solution to which carbon quantum dots are added;

[0031] Figure 3 is the surface morphology diagram of the loose nanofiltration membrane prepared by the carbon quantum dots promoted rapid deposition of polydopamine described in the application;

[0032] Figure 4 is the surface morphology diagram of the nanofiltration membrane prepared by the deposition of polydopamine under the conventional air oxidation condition; DETAILED DESCRIPTION

[0033] The technical solutions of the present application will be further specifically described below through specific examples. It should be understood that the implementation of the present application is not limited to the following examples, and any form of variation and / or change made to the present application will fall within the scope of protection of the present application.

[0034] In the present application, unless specified, all parts, percentages are weight units, and the equipment and raw materials used can be purchased from the market or are commonly used in the art. The methods in the following examples are conventional methods in the art, unless otherwise specified.

[0035] The visible light in the following examples is generated by a xenon lamp light source PLS-SXE-300W produced by Beijing Po Fei Lei Company, and is equipped with a 420nm filter.

[0036] Example 1:

[0037] 1g of lysine was used as carbon source and pyrolyzed at 240℃ for 1h. After cooling to room temperature, 20mL of deionized water was added and ultrasonic treatment was performed for 30min to obtain a dispersion of carbon quantum dots. Larger particles were removed by centrifugation, the dispersion was placed in a dialysis bag and dialyzed for 72h, and finally freeze-dried to obtain carbon quantum dot solids.

[0038] Polyacrylonitrile ultrafiltration membrane was placed in a clamp for use. 2 g / L dopamine was dissolved in a Tris buffer solution containing 20 mM, pH 8.5, and carbon quantum dots were added (mass ratio of carbon quantum dots to dopamine was 1:1). 30 mL of the solution containing carbon quantum dots was poured into the clamp to make it fully contact with the surface of the ultrafiltration membrane, and reacted under visible light irradiation and oscillation conditions (150 rpm) for 60 min. After the reaction was completed, the membrane was taken out, rinsed with deionized water for 3 times, and then heat treated at 40°C for 20 min.

[0039] The nanofiltration membrane prepared by carbon quantum dots promoting rapid deposition of polydopamine had a water flux of 24.4 L m -2 h -1 , a rejection rate of 99.6% for methyl blue, and a rejection rate of 6.0% for sodium chloride.

[0040] Example 2:

[0041] 1 g of lysine was used as a carbon source and pyrolyzed at 240°C for 1 h. After cooling to room temperature, 20 mL of deionized water was added and ultrasonic treatment was performed for 30 min to obtain a dispersion of carbon quantum dots. Larger particles were removed by centrifugation, and the dispersion was placed in a dialysis bag for dialysis for 72 h. Finally, the carbon quantum dots were obtained by freeze-drying.

[0042] Polyacrylonitrile ultrafiltration membrane was placed in a clamp for use. 2 g / L dopamine was dissolved in a Tris buffer solution containing 20 mM, pH 8.5, and carbon quantum dots were added (mass ratio of carbon quantum dots to dopamine was 1:2). 30 mL of the solution containing carbon quantum dots was poured into the clamp to make it fully contact with the surface of the ultrafiltration membrane, and reacted under visible light irradiation and oscillation conditions (150 rpm) for 60 min. After the reaction was completed, the membrane was taken out, rinsed with deionized water for 3 times, and then heat treated at 40°C for 20 min.

[0043] The nanofiltration membrane prepared by carbon quantum dots promoting rapid deposition of polydopamine had a water flux of 25.1 L m -2 h -1 , a rejection rate of 98.3% for methyl blue, and a rejection rate of 4.88% for sodium chloride.

[0044] Comparative Example 1:

[0045] Polyacrylonitrile ultrafiltration membrane was placed in the clamp for use. 2 g / L dopamine was dissolved in Tris buffer solution containing 20 mM, pH 8.5. The above 30 mL solution was poured into the clamp to make it fully contact with the surface of the ultrafiltration membrane, and reacted for 60 min under the condition of oscillation (150 rpm). After the reaction was completed, the membrane was taken out, rinsed with deionized water for 3 times, and then heat treated at 40℃ for 20 min.

[0046] The separation effect of the nanofiltration membrane deposited by dopamine self-polymerization in air on the mixed solution of 0.1 g / L methylene blue and 1 g / L sodium chloride at 25℃ and 0.6 MPa was that the water flux was 117.1 L m -2 h -1 The rejection rate of methylene blue was 54.1%, and the rejection rate of sodium chloride was 3.7%.

[0047] Comparative Example 2:

[0048] Polyacrylonitrile ultrafiltration membrane was placed in the clamp for use. 2 g / L dopamine was dissolved in Tris buffer solution containing 20 mM, pH 8.5. The above 30 mL solution was poured into the clamp to make it fully contact with the surface of the ultrafiltration membrane, and reacted for 60 min under the condition of oscillation (150 rpm). After the reaction was completed, the membrane was taken out, rinsed with deionized water for 3 times, and then heat treated at 40℃ for 20 min.

[0049] The separation effect of the nanofiltration membrane deposited by dopamine self-polymerization in air on the mixed solution of 0.1 g / L methylene blue and 1 g / L sodium chloride at 25℃ and 0.6 MPa was that the water flux was 117.1 L m -2 h -1 The rejection rate of methylene blue was 54.1%, and the rejection rate of sodium chloride was 3.7%.

[0050] Table 1 Performance comparison of nanofiltration membranes prepared in Examples 1-2 and Comparative Examples 1-2

[0051]

[0052] Example 3:

[0053] 1 g of lysine was used as a carbon source and pyrolyzed at 240℃ for 1 h. After cooling to room temperature, 20 mL of deionized water was added and ultrasonicated for 30 min to obtain a dispersion of carbon quantum dots. Larger particles were removed by centrifugation, and the dispersion was placed in a dialysis bag for dialysis for 72 h. Finally, the carbon quantum dots were obtained by freeze-drying.

[0054] Polyacrylonitrile ultrafiltration membrane was placed in a clamp for use. 2 g / L dopamine was dissolved in a Tris buffer solution containing 20 mM, pH 8.5, and carbon quantum dots were added (mass ratio of carbon quantum dots to dopamine was 1:1). 30 mL of the solution containing carbon quantum dots was poured into the clamp to make it fully contact with the surface of the ultrafiltration membrane, and reacted under visible light irradiation and oscillation conditions (150 rpm) for 60 min. After the reaction was completed, the membrane was taken out, rinsed with deionized water for 3 times, and then heat treated at 50°C for 20 min.

[0055] The nanofiltration membrane prepared by carbon quantum dots promoting rapid deposition of polydopamine had a water flux of 19.6 L m -2 h -1 The rejection rate of methyl blue was 99.8%, and the rejection rate of sodium chloride was 2.94%.

[0056] Example 4:

[0057] 1 g of lysine was used as a carbon source and pyrolyzed at 240°C for 1 h. After cooling to room temperature, 20 mL of deionized water was added and ultrasonic treatment was performed for 30 min to obtain a dispersion of carbon quantum dots. Larger particles were removed by centrifugation, and the dispersion was placed in a dialysis bag for dialysis for 72 h. Finally, the carbon quantum dots were obtained by freeze-drying.

[0058] Polyacrylonitrile ultrafiltration membrane was placed in a clamp for use. 2 g / L dopamine was dissolved in a Tris buffer solution containing 20 mM, pH 8.5, and carbon quantum dots were added (mass ratio of carbon quantum dots to dopamine was 1:1). 30 mL of the solution containing carbon quantum dots was poured into the clamp to make it fully contact with the surface of the ultrafiltration membrane, and reacted under visible light irradiation and oscillation conditions (150 rpm) for 45 min. After the reaction was completed, the membrane was taken out, rinsed with deionized water for 3 times, and then heat treated at 40°C for 20 min.

[0059] The nanofiltration membrane prepared by carbon quantum dots promoting rapid deposition of polydopamine had a water flux of 28.6 L m -2 h -1 The rejection rate of methyl blue was 96.3%, and the rejection rate of sodium chloride was 3.21%.

[0060] Example 5:

[0061] A dispersion of carbon quantum dots was obtained by pyrolysis of 1 g of lysine at 240 °C for 1 h, after cooling to room temperature, 20 mL of deionized water was added and ultrasonic for 30 min. Larger particles were removed by centrifugation, the dispersion was placed in a dialysis bag for dialysis for 72 h, and finally freeze-dried to obtain carbon quantum dot solids.

[0062] A polyacrylonitrile ultrafiltration membrane was placed in a clamp for use. 2 g / L of dopamine was dissolved in a Tris buffer solution containing 20 mM, pH 8.5, and carbon quantum dots were added (mass ratio of carbon quantum dots to dopamine was 1:1). 30 mL of solution containing carbon quantum dots was poured into the clamp to make it fully contact with the surface of the ultrafiltration membrane, and reacted under visible light irradiation and oscillation conditions (150 rpm) for 60 min. After the reaction was completed, the membrane was taken out, rinsed with deionized water for 3 times, and then heat treated at 50 °C for 45 min.

[0063] The nanofiltration membrane prepared by carbon quantum dots promoting rapid deposition of polydopamine had a water flux of 17.6 L m -2 h -1 , a rejection rate of 99.9% for methylene blue, and a rejection rate of 9.98% for sodium chloride.

[0064] Example 6:

[0065] A dispersion of carbon quantum dots was obtained by pyrolysis of 1 g of lysine at 240 °C for 1 h, after cooling to room temperature, 20 mL of deionized water was added and ultrasonic for 30 min. Larger particles were removed by centrifugation, the dispersion was placed in a dialysis bag for dialysis for 72 h, and finally freeze-dried to obtain carbon quantum dot solids.

[0066] A polyacrylonitrile ultrafiltration membrane was placed in a clamp for use. 2 g / L of dopamine was dissolved in a Tris buffer solution containing 20 mM, pH 8.5, and carbon quantum dots were added (mass ratio of carbon quantum dots to dopamine was 1:1). 30 mL of solution containing carbon quantum dots was poured into the clamp to make it fully contact with the surface of the ultrafiltration membrane, and reacted under visible light irradiation and oscillation conditions (150 rpm) for 60 min. After the reaction was completed, the membrane was taken out, rinsed with deionized water for 3 times, and then heat treated at 50 °C for 45 min.

[0067] The nanofiltration membrane prepared by carbon quantum dots promoting rapid deposition of polydopamine had a water flux of 17.6 L m -2 h -1 , a rejection rate of 99.9% for methylene blue, and a rejection rate of 9.98% for sodium chloride.

[0068] Example 7:

[0069] After pyrolysis at 240℃ for 1h with 1g arginine as carbon source, the sample was cooled to room temperature, 20mL deionized water was added and ultrasonic treatment was performed for 30min to obtain a dispersion of carbon quantum dots. Larger particles were removed by centrifugation, and the dispersion was dialyzed in a dialysis bag for 72h. Finally, the carbon quantum dots were obtained by freeze-drying.

[0070] A polyacrylonitrile ultrafiltration membrane was placed in a clamp for use. 2g / L dopamine was dissolved in 30mL Tris buffer solution containing 20mM, pH 8.5, and carbon quantum dots were added (the mass ratio of carbon quantum dots to dopamine was 1:3). The solution containing carbon quantum dots was poured into the clamp to make it fully contact with the surface of the ultrafiltration membrane, and the reaction was carried out under visible light irradiation and oscillation conditions (150rpm) for 45min. After the reaction was completed, the membrane was taken out, washed with deionized water for 3 times, and then heat-treated at 50℃ for 60min.

[0071] The nanofiltration membrane prepared by carbon quantum dots promoting rapid deposition of polydopamine had a water flux of 30.89L m -2 h -1 , a rejection rate of 97.6% for methyl blue, and a rejection rate of 8.54% for sodium chloride.

[0072] The following two solutions were prepared respectively: (1) 2g / L dopamine was dissolved in 30mL Tris buffer solution containing 20mM, pH 8.5; (2) 2g / L dopamine was dissolved in 30mL Tris buffer solution containing 20mM, pH 8.5, and carbon quantum dots prepared in Example 1 were added (the mass ratio of carbon quantum dots to dopamine was 1:1). The change of the ultraviolet absorption of indole group with time in the above two solutions was measured as shown in Figure 1 . According to Figure 1 , it can be seen that the addition of carbon quantum dots promotes the rapid increase of the ultraviolet absorption intensity of indole group, indicating that under the same conditions, carbon quantum dots can promote the polymerization of dopamine to form polydopamine more than air.

[0073] Test for proving the production of active oxygen by carbon quantum dots

[0074] 2mL of 2',7'-dichlorofluorescein phosphate buffer solution at 10μg / mL was prepared, then 1mg of carbon quantum dots was added, and the fluorescence intensity of the solution at 520nm was measured after visible light irradiation for 1-5min, respectively. The 2',7'-dichlorofluorescein phosphate buffer solution without the addition of carbon quantum dots was used as a blank control sample. The change of fluorescence intensity is shown in Figure 2 .

[0075] from Figure 2 It is known that 2',7'-dichlorodihydrofluorescein can be oxidized by reactive oxygen species, and the oxidation product fluoresces at 520 nm. Compared with the blank control, the solution with added carbon quantum dots has a higher fluorescence intensity, and the fluorescence intensity increases rapidly over time, indicating that carbon quantum dots can generate a large amount of reactive oxygen species. Figure 1 and Figure 2 It is known that carbon quantum dots promote the rapid polymerization of dopamine because they generate a large amount of reactive oxygen species.

[0076] The surface morphology of the loose nanofiltration membrane (prepared in Example 1) prepared by carbon quantum dot-assisted rapid deposition of polydopamine is shown in the figure. Figure 3 The surface morphology of nanofiltration membranes prepared by polydopamine deposition under conventional air oxidation conditions (Comparative Example 1) is shown in the figure. Figure 4 .according to Figure 3 and Figure 4 It is known that carbon quantum dots can promote the rapid polymerization of dopamine, and the resulting polydopamine can be uniformly covered on the surface of polyacrylonitrile. In contrast, polydopamine obtained by air oxidation has a slow polymerization rate and is prone to forming large polydopamine particles, which is not conducive to forming a uniform nanofiltration membrane surface.

[0077] In summary, the addition of carbon quantum dots promotes the polymerization and rapid deposition of dopamine, forming a loose nanofiltration membrane within 45-60 minutes. This membrane exhibits high dye rejection and low sodium chloride rejection, enabling the separation of dyes and inorganic salts. Under the same conditions, the polydopamine nanofiltration membrane deposited by oxidation in air shows a lower dye rejection rate (only 75.4%), far below the performance requirements for dye-inorganic salt separation. The ratio of carbon quantum dots to dopamine significantly affects the performance of the prepared nanofiltration membrane. Excessive carbon quantum dots can act as nanofillers, reducing the density of the polydopamine deposition layer and decreasing the dye rejection rate. Furthermore, heat treatment conditions also significantly impact the prepared nanofiltration membrane. Increasing the heat treatment temperature and time improves the dye rejection rate but reduces the water flux. Therefore, the nanofiltration membrane with the optimal overall performance is achieved when the mass ratio of carbon quantum dots to dopamine is in the range of 1:1-2, the heat treatment temperature is 40℃, and the treatment time is 20 minutes.

[0078] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since it corresponds to the method disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to in the method section.

[0079] The method for preparing the loose nanofiltration membrane by using the carbon quantum dots to promote the rapid deposition of polydopamine is described in detail. The principles and implementation manners of the present application are described by using specific examples. The above description of the examples is only used to help understand the method of the present application and its core idea. It should be pointed out that the ordinary skilled in the art can make some improvements and modifications to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the protection scope of the claims of the present application.

Claims

1. A method for preparing a loose nanofiltration membrane by carbon quantum dots promoting rapid deposition of polydopamine, characterized in that The method comprises the following steps: S1, preparing carbon quantum dots The amino acid is used as a carbon source, pyrolysis is performed at a temperature of 240℃±10℃ for 1h-2h, after cooling to room temperature, a proper amount of water is added and ultrasonic treatment is performed for 30min to obtain a dispersion liquid of carbon quantum dots; Large particles are removed by filtration, the dispersion liquid is placed in a dialysis bag for dialysis for more than 72h, and finally freeze-drying is performed to obtain carbon quantum dot solids; S2, preparing a loose nanofiltration membrane through rapid deposition of polydopamine Dopamine is dissolved in a Tris buffer solution containing 20mM and having a pH of 8.5, so that the mass concentration of dopamine is 1-8g / L; carbon quantum dots are added to the system, and the mass ratio of dopamine to carbon quantum dots is controlled to be 1:1-4 to obtain a dopamine solution containing carbon quantum dots; The dopamine solution containing carbon quantum dots is fully contacted with the surface of an ultrafiltration membrane, reaction is performed under visible light irradiation and 100-400rpm oscillation for 40-90min, after the reaction is completed, the obtained membrane is taken out, washed with water, and then heat treatment is performed at 40-60℃ for 10-60min to obtain a loose nanofiltration membrane product.

2. The method of claim 1, wherein: The amino acid in S1 is at least one selected from lysine, aspartic acid, glycine or arginine; and the ultrafiltration membrane in S2 is one selected from polyacrylonitrile, polysulfone or polyethersulfone ultrafiltration membranes.

3. The method of claim 1, wherein: The mass concentration of dopamine in the solution obtained by adding the Tris buffer solution in S2 is 2-5g / L.

4. The method of claim 1, wherein: The mass ratio of dopamine to carbon quantum dots in S2 is 1:1-3.

5. The method of claim 1, wherein: The visible light in S2 is generated by a xenon lamp light source, the power is 300W±100w, and a 420nm filter is provided.

6. The method of claim 1, wherein: The oscillation speed in S2 is 150rpm.

7. The method of claim 1, wherein: The deposition time in S2 is 45-60min.

8. The method of claim 1, wherein: The heat treatment temperature in S2 is 40-50℃, and the heat treatment time is 20-40min.

9. The method of claim 1, wherein: In S1, 1g of amino acid is used as a carbon source, pyrolysis is performed at 240℃ for 1h, after cooling to room temperature, 20mL of deionized water is added and ultrasonic treatment is performed for 30min to obtain a dispersion liquid of carbon quantum dots.

10. The method of claim 1, wherein: The mass ratio of dopamine to carbon quantum dots in S2 is 1:1.

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