A method for enriching and detecting microplastics in surface water bodies

Abstract

The present invention belongs to the field of environmental protection and pollutant treatment, and relates to a method for enriching and detecting microplastics in surface water bodies. Through two-step reactions, the present invention grafts lignin with amino groups and carboxyl groups, first prepares a carbon quantum dot active site powder with a positive charge on the surface, crosslinks it with modified montmorillonite, and further prepares a carbon quantum dot active site microporous adsorbent, so that the surface of the prepared adsorbent has many functional groups, many and uniform active sites, which can accelerate the adsorption rate of microplastics in water bodies, improve the adsorption capacity of microplastics in water bodies, and realize the rapid enrichment and separation of microplastics in water bodies. Based on this enrichment and separation method, the present invention further provides a method for determining microplastics in water environments. The present invention realizes the rapid filtration, enrichment and detection of microplastics in water, with simple operation and high accuracy.

Description

Technical Field

[0001] The present invention belongs to the field of environmental protection and pollutant treatment, specifically relates to the field of microplastic treatment, and particularly relates to a method for enriching and detecting microplastics in surface water bodies. Background Art

[0002] Plastics are widely used in daily life. However, due to the high recycling cost, the recycling rate is not high. Moreover, due to their poor degradability, they exist in the environment for a long time. Plastics in the environment are further decomposed into smaller plastic fragments or particles through physical, chemical and other effects. When their diameter is less than 5 mm, they can be defined as microplastics. These plastic particles will not be degraded by the natural environment but will remain in the environment all the time and even enter organisms, bringing environmental health risks. Therefore, there is an urgent need for a method to enrich microplastics in water bodies, remove the enriched microplastics, and at the same time, an accurate, sensitive, convenient and fast method to quantitatively detect microplastics in water bodies. Transferring pollutants from the liquid phase to the solid phase through adsorption treatment has the characteristics of high efficiency and easy operation. However, for the separation and quantification of microplastics, the currently widely used method is density separation, such as soaking sediment samples with saturated sodium chloride solution, sodium polytungstate solution or seawater for separation. However, the composition of substances in actual environmental samples is relatively complex, the operation of the above methods is complicated, and the separation efficiency still needs to be further improved.

[0003] Chinese Patent with Publication No. CN105651748B discloses a method for quantitatively detecting microplastics in aquatic organisms based on fluorescence tracing technology. By selecting fluorescently labeled microplastic particles as test substances, digesting tissues with nitric acid, and making the obtained digestion solution constant volume; preparing fluorescent microplastic suspension solutions with different concentration gradients, obtaining a standard curve according to the fluorescence values measured by a fluorescence spectrometer; measuring the fluorescence value of the sample solution, and obtaining the microplastic content in the corresponding tissue sample based on the standard curve. The technical solution of this invention can effectively and accurately quantify the microplastics ingested by aquatic organisms, but this method has great limitations and complicated operations.

[0004] The Chinese patent with the publication number CN107966393B discloses a method for determining the content of microplastics in seawater and the absorption of microplastics by organisms. The method includes collecting seawater from the environment into a container; placing a viscous fluorescent powder material into the container and stirring to fluorescently label the microplastics in the seawater; adding a certain amount of seawater density solution and then stirring; removing the static precipitate at the bottom of the container and using a fluorescence spectrometer to measure the concentration of fluorescent microplastics in the seawater; placing living marine organisms into the container for cultivation for a period of time; removing the living marine organisms and using a fluorescence spectrometer to measure the concentration of fluorescent microplastics in the seawater in the container; dissecting the living marine organisms, collecting the target tissue organs, and measuring the concentration of fluorescent microplastics in each target tissue organ. Because this method does not have a directional label for microplastics, other solid particles in the seawater will also be labeled by the fluorescent powder, resulting in larger detected data.

[0005] Due to a large amount of plastic waste being discharged into rivers and discarded into the ocean, the content of microplastics in the water environment far exceeds our imagination. Moreover, microplastics are migratory, can flow with water, and can also be ingested by animals and plants in the water, seriously threatening the ecological environment and human health. Simply applying plastic-degrading bacterial communities will increase the content of microplastics in the water body and exacerbate microplastic pollution, especially for microplastics smaller than 10 μm, because they are small and light and are not easy to capture and enrich. Therefore, effectively enriching microplastics in the water body is a hot topic in the research field of removing microplastics from water bodies at present, and it is of great significance to actively explore a method for enriching and detecting microplastics in water bodies. Summary of the Invention

[0006] To solve the above problems, the purpose of the present invention is to provide a method for enriching and detecting microplastics in surface water bodies, so as to solve one or more technical problems in the prior art and at least provide a beneficial choice or create conditions.

[0007] In view of this, one of the main purposes of the present invention is to propose a method for enriching and separating microplastics in the water environment, in order to at least partially solve at least one of the above technical problems.

[0008] To solve the problem of enriching microplastics and the technical problems of generally low adsorption capacity and slow adsorption rate of enrichment materials, the present invention aims to develop an enrichment material with excellent adsorption performance and provides a method for enriching microplastics in surface water bodies, specifically as follows:

[0009] First, an adsorbent is added to the water sample of the surface water body containing microplastics for adsorption enrichment to form aggregates; then filtration is carried out to complete the enrichment of microplastics; then a digestion solution is used to dissolve and elute the microplastics on the aggregates, and finally, liquid chromatography-mass spectrometry is used to determine the concentration of microplastics in the environmental water sample.

[0010] The microplastics described above include, but are not limited to, at least one of polystyrene, polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, and polymethyl methacrylate.

[0011] The adsorbent is carbon quantum dot active site powder, and its preparation steps are specifically as follows:

[0012] S001, Stir and heat chitosan and deionized water at 100 °C for 80 - 120 min until completely dissolved to obtain a chitosan aqueous solution;

[0013] The mass concentration of the chitosan aqueous solution is 10 - 15%;

[0014] S002, Dissolve sodium hydroxide and deionized water evenly and add them, then add lignin and stir for 30 - 50 min until lignin is completely dissolved; add the chitosan aqueous solution, then add polyethyleneimine and stir evenly. React at 50 - 60 °C for 2 - 3 hours to cause a cross-linking reaction among lignin, chitosan, and polyethyleneimine to obtain a solution containing lignin grafted with amino groups; the mass concentration of the sodium hydroxide aqueous solution is 25%;

[0015] The mass ratio of polyethyleneimine, chitosan, and lignin is 3 - 4:5 - 8:50 - 60;

[0016] S003, Dissolve monochloroacetic acid in deionized water, and dropwise add the monochloroacetic acid solution to the lignin solution finally obtained in S002. Heat to 80 - 90 °C and react for 4 - 5 hours. After the reaction ends, cool down, pour out the reaction solution, adjust the pH of the solution to 5.0 ± 0.1 with 2 mol / L hydrochloric acid, filter the reaction product through a 0.5 μm microporous membrane to remove impurities, then dialyze it in deionized water with a 1000 da dialysis bag for 24 hours, and freeze-dry it for 72 hours to obtain carbon quantum dot active site powder. Among them, the mass concentration of the monochloroacetic acid solution is 60%, and the mass ratio of monochloroacetic acid to lignin is 6:50 - 60.

[0017] Through a two-step reaction, lignin was grafted with amine and carboxyl groups, and chlorine was introduced to obtain a carbon quantum dot active site powder with a positive charge on the surface. The powder has the ability to selectively adsorb microplastics, especially for microplastics with particularly small particles in water. It has a very good flocculation ability. Since the surface of suspended microplastic particles in the water body is negatively charged, when there is no carbon quantum dot active site powder adsorbent in the water, the microplastics remain stable due to mutual electrostatic repulsion. After adding the carbon quantum dot active site powder adsorbent, the surface charge of the microplastics will be neutralized, and when the surface charge of the dispersed microplastics is completely neutralized by a certain amount of carbon quantum dot active site powder adsorbent, the electrostatic repulsion is reduced to a minimum. At this time, the microplastics can gather together and form large-particle floccules. However, when an excessive amount of carbon quantum dot active site powder adsorbent is added, the microplastic particles will adsorb too much positive charge, the potential of the microplastics will reverse, and become positively charged microplastics, re-forming a stable state, and when the surface charge of the microplastics changes from negative to positive, the microplastic removal efficiency decreases while the charge is reversed. This indicates that the microplastics in the system have re-stabilized, especially those with a particle size of less than 10 μm, which are particularly prone to re-stabilization and cannot be efficiently enriched. Therefore, single charge neutralization is not effective for enriching microplastics of all particle sizes, especially small particles.

[0018] Therefore, further, the carbon quantum dot active site powder and montmorillonite are compounded to obtain a carbon quantum dot active site microporous adsorbent, specifically:

[0019] S101, selecting calcium-based montmorillonite, adding the calcium-based montmorillonite to deionized water, adding dilute hydrochloric acid and stirring evenly, so that the calcium-based montmorillonite is dispersed in the dilute hydrochloric acid solution, heating to 60° C., adding dimethyldiallyl ammonium chloride, stirring fully, reacting for 1 to 2 hours, and obtaining an aminated montmorillonite suspension, adjusting the pH value to 5.0 to 5.5, adding histidine, stirring fully, and reacting for 1 to 2 hours to obtain an aminated montmorillonite microsphere suspension with a negative charge on the outside and a high density of cations on the inside; wherein the mass ratio of dimethyldiallyl ammonium chloride to calcium-based montmorillonite is 1:20; the mass ratio of the histidine to calcium-based montmorillonite is 1:4 to 5;

[0020] S102, dissolving the carbon quantum dot active site powder in a citric acid aqueous solution with a mass concentration of 8% to activate the active sites on the carbon quantum dot active site powder, and then adding the suspension of aminated montmorillonite microspheres with a negative charge on the outside and a high density of cations on the inside obtained in S101, reacting at 50-60°C for 2-3 hours to allow the carbon quantum dot active site powder and montmorillonite microspheres to fully react and cross-link, filtering, and low-temperature drying to obtain a carbon quantum dot active site microporous adsorbent.

[0021] The surface layer of the adsorbent has positively charged carbon quantum dot active sites coated with negatively charged aminated montmorillonite in the middle layer, and there are high-density cationic groups inside. When there are microplastics in the water body and the charge balance of the microplastics appears, this carbon quantum dot active site microporous adsorbent can adjust its own charge, break this balance, make the adsorption speed faster, and the enrichment effect better. This is because montmorillonite has a large number of pores and surface active sites, which can effectively adsorb and exchange cations to obtain better adsorption groups. In addition, montmorillonite can also enhance its adsorption capacity by changing its chemical properties and surface morphology. By further introducing carbon quantum dot active sites with cations, a carbon quantum dot active site microporous adsorbent is obtained, making its adsorption capacity for negatively charged microplastics stronger. And by compounding lignin carbon quantum dots with montmorillonite, the specific surface area of montmorillonite is increased, and the charge structure on the surface of montmorillonite is changed, so that the negative charge on the surface of montmorillonite and the positive charge in the inner layer can be converted with each other, thereby improving its adsorption capacity, accelerating the adsorption speed of microplastics in the water body, improving the adsorption capacity of microplastics in the water body, and realizing the rapid enrichment and separation of microplastics in the water body. The carbon quantum dot active site microporous adsorbent has high selectivity and can selectively adsorb microplastic particles without affecting substances in other environments, which can reduce the impact on the environment and the damage to other organisms.

[0022] At the same time, the process of using the carbon quantum dot active site microporous adsorbent for microplastic enrichment and separation has better environmental friendliness and biocompatibility. Therefore, the carbon quantum dot active site microporous adsorbent has high efficiency, high selectivity, good renewable property, high safety, etc.

[0023] Furthermore, a detection method for microplastics in surface water bodies is as follows:

[0024] S201, Add sodium carbonate to the water sample of the surface water body containing microplastics to adjust the pH value to 8.0 - 8.5, then add the adsorbent and conduct adsorption treatment to make the adsorbent fully contact with the microplastics, adsorb the negatively charged microplastics, and form multiple aggregation centers;

[0025] The time of the adsorption treatment is 40 - 50 minutes; the mass ratio of the adsorbent to the water sample of the surface water body containing microplastics is 0.1 - 2:100.

[0026] In a weakly alkaline environment, the carbon quantum dot active site microporous adsorbent releases negative charges, changing the charge balance of the carbon quantum dot active site microporous adsorbent. The negative charges on the surface of montmorillonite and the released negative charges repel each other, activating the high-density cationic groups inside, releasing cations, adsorbing negatively charged microplastics, and forming aggregates. Due to the excess of the carbon quantum dot active site microporous adsorbent, when the surface of the aggregate becomes positively charged due to adsorption saturation, the negative charges released by the carbon quantum dot active site microporous adsorbent in the water body and the negative charges carried by the surface of the carbon quantum dot active site microporous adsorbent itself become another adsorption center, continuing to adsorb the aggregates, making the aggregates continue to grow and then precipitate.

[0027] S202. Then, use a microporous membrane to filter the water sample of the surface water body containing aggregates to obtain the aggregates, completing the enrichment and separation of microplastics;

[0028] The microporous membrane is a glass fiber membrane, and the pore size of the microporous membrane is 0.5 - 1 μm;

[0029] S203. Use a digestion solution to dissolve and elute the microplastics on the aggregates and collect the washed liquid. Concentrate it to 1 mL, dry it with a nitrogen blower, add chromatographic grade methanol, transfer it to a brown injection vial, and make the volume up to 2 mL; use a liquid chromatography-mass spectrometry instrument to determine the content of microplastics.

[0030] The digestion solution is a mixed solution obtained by mixing tetrahydrofuran and dichloromethane in a mass ratio of 1:3;

[0031] The use of this method for quantitatively detecting microplastics in water bodies based on liquid chromatography-mass spectrometry in monitoring the environmental pollution of microplastics in surface water bodies.

[0032] The adsorption of the carbon quantum dot active site microporous adsorbent includes three stages: rapid adsorption in the early stage, slow adsorption in the middle stage, and equilibrium adsorption in the later stage. The first 10 minutes is the period of rapid adsorption. Due to the action of charges, the carbon quantum dot active site microporous adsorbent has a strong adsorption capacity for microplastics, and the adsorption speed is very fast. The adsorption types are mainly surface adsorption and diffusion. At the same time, the positively charged carbon quantum dots play a bridging role in the adsorption process. In the stage of slow adsorption in the middle from 10 to 20 minutes, at this time, the microplastics undergo internal diffusion in the lamellar structure of the carbon quantum dot active site microporous adsorbent, diffusing from the surface to the deep pores. The charges in the lamellar structure are released, breaking the charge balance of the system again, resulting in the third stage of equilibrium adsorption with a strong adsorption capacity and a very fast adsorption speed in the later stage, enabling the rapid enrichment of microplastics in the water body to be completed.

[0033] The carbon quantum dot active site microporous adsorbent can interact with microplastics through charge adsorption and hydrogen bonding. The hydroxyl groups, carboxyl groups, and amino groups on the surface of the carbon quantum dot active site microporous adsorbent increase the active sites for the interaction between microplastics and the adsorbent. When one end of the carbon quantum dot adsorbs a certain microplastic particle, and the other end adsorbs another microplastic particle, a "microplastic-carbon quantum dot-microplastic" flocculent can be formed. At the same time, the positively charged carbon quantum dot plays a bridging role during the adsorption process, causing the flocculent to continuously bridge and form aggregates, which then precipitate, completing rapid enrichment and separation, and realizing the enrichment of microplastics in surface water bodies.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] 1) The present invention provides a method for the enrichment and separation of microplastics in surface water bodies. Through two-step reactions, lignin is grafted with amino groups and carboxyl groups, and chlorine elements are introduced to obtain a carbon quantum dot active site powder with a positive charge on the surface. This powder has the ability to selectively flocculate microplastics, especially having very good flocculation ability for microplastics with extremely small particles in water bodies. In order to overcome the re-stabilization phenomenon of microplastics, the carbon quantum dot active site powder and modified montmorillonite are crosslinked to obtain a carbon quantum dot active site microporous adsorbent, making the prepared carbon quantum dot active site microporous adsorbent have many surface functional groups and active sites, which can enhance the interaction with microplastics. At the same time, the negative charge on the surface and the positive charge in the inner layer of the montmorillonite can be converted with each other, thereby improving its adsorption ability, accelerating the adsorption rate of microplastics in water bodies, increasing the adsorption ability of microplastics in water bodies, and realizing the rapid enrichment and separation of microplastics in water bodies. The carbon quantum dot active site microporous adsorbent has a high degree of selectivity and can selectively adsorb microplastic particles without affecting substances in other environments, which can reduce the impact on the environment and the damage to other organisms.

[0036] 2) The present invention uses dimethyldiallylammonium chloride to increase the interlayer space of the calcium-based montmorillonite sheet structure, increase the wrinkles inside the montmorillonite, increase the pore structure, and increase the specific surface area. Adding histidine changes the charge distribution inside and outside the montmorillonite. And through the synergistic effect of the carbon quantum dot active site powder, not only the microporous structure on the surface is increased, the specific surface area is further enlarged, but also the adsorption active sites are increased, increasing the adsorption content of microplastics, thereby improving the removal rate; the adsorption effect on microplastics is very effective within a short period of time. It verifies the feasibility and application prospects of its use as an adsorption material in the treatment of microplastic pollution in water, providing new ideas for the treatment of microplastics in water.

[0037] 3) The method for determining the content of microplastics in surface water provided by the present invention can directly enrich and detect the content of microplastics in various surface water environments, provide accurate and effective data references for microplastics in the real environment, and more provide a basis for environmental protection and the treatment of microplastics. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 , the enrichment and separation curve diagram of microplastics with the change of adsorption time of the adsorbents prepared in Example 1, Comparative Example 1 and Comparative Example 4;

[0039] Figure 2 , the electron micrograph of the carbon quantum dot active site microporous adsorbent prepared in Example 1;

[0040] Figure 3 , the electron micrograph of the carbon quantum dot active site microporous adsorbent prepared in Example 1 after adsorbing microplastics. DETAILED DESCRIPTION OF THE INVENTION

[0041] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content described in the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0042] Example 1

[0043] A method for enriching and detecting microplastics in surface water. First, an adsorbent needs to be prepared. The specific preparation steps of the carbon quantum dot active site powder are as follows:

[0044] Dissolve 10 parts of chitosan and 90 parts of deionized water by stirring and heating at 100 °C for 120 min until completely dissolved to obtain a 10% chitosan aqueous solution;

[0045] Dissolve 10 parts of sodium hydroxide and 40 parts of deionized water evenly and add them. Add 50 parts of lignin and stir for 30 min until the lignin is completely dissolved; add 50 parts of the chitosan aqueous solution, then add 4 parts of polyethyleneimine, stir evenly, and react at 60 °C for 3 hours to cause cross-linking reactions among lignin, chitosan and polyethyleneimine to obtain a solution containing lignin grafted with amino groups;

[0046] 6 parts of monochloroacetic acid were dissolved in 10 parts of deionized water, and the monochloroacetic acid solution was added dropwise to the lignin solution obtained, and the temperature was raised to 90°C for reaction for 5 hours. After the reaction was completed, the reaction solution was cooled, and the reaction solution was poured out. The pH value of the solution was adjusted to 5.0 with 2 mol / L hydrochloric acid. The reaction product was filtered with a 0.5 μm microporous membrane to remove impurities, and then dialyzed in deionized water with a 1000 da dialysis bag for 24 hours, and freeze-dried for 72 hours to obtain a carbon quantum dot active site powder;

[0047] Further, a carbon quantum dot active site microporous adsorbent is prepared, specifically:

[0048] 50 parts of calcium montmorillonite were selected, and the calcium montmorillonite was added to 50 parts of deionized water, and 10 parts of 5% citric acid aqueous solution were added and stirred evenly to disperse the calcium montmorillonite in the dilute hydrochloric acid solution, and the temperature was raised to 60°C, and 2.5 parts of dimethyldiallyl ammonium chloride were added, and the mixture was stirred fully and reacted for 2 hours to obtain an aminated montmorillonite suspension, and the pH value was adjusted to 5.2, and 10 parts of histidine were added, and the mixture was stirred fully and reacted for 2 hours to obtain an aminated montmorillonite microsphere suspension with a negative charge on the outside and a high density of cations on the inside;

[0049] Dissolve 10 parts of carbon quantum dot active site powder in 10 parts of 5% citric acid aqueous solution to activate the active sites on the carbon quantum dot active site powder, then add it to a suspension of aminated montmorillonite microspheres with negative charges on the outside and high-density cations on the inside, react at 60° C. for 3 hours to allow the carbon quantum dot active site powder and montmorillonite microspheres to fully react and cross-link, filter, and dry at low temperature to obtain a carbon quantum dot active site microporous adsorbent;

[0050] Finally, a method for enrichment and detection of microplastics in surface water is provided, specifically:

[0051] Add 0.1 parts of carbon quantum dot active site microporous adsorbent to 100 parts of surface water samples containing microplastics, then add sodium carbonate aqueous solution to adjust the pH value to 8.0, and perform adsorption treatment for 50 minutes to allow the adsorbent to fully contact the microplastics and adsorb negatively charged microplastics to form multiple aggregate centers; then use a 0.5 μm microporous membrane to filter the surface water sample containing aggregates to obtain aggregates, thereby completing the enrichment and separation of the microplastics;

[0052] A digestion solution was obtained by mixing 1 part of tetrahydrofuran and 3 parts of dichloromethane. The digestion solution was used to dissolve the microplastics on the eluted aggregates, and the washed liquid was collected and concentrated to 1 mL. The liquid was blown dry with a nitrogen blower, chromatographic grade methanol was added, and the volume was transferred to a brown injection bottle and fixed to 2 mL. The content of microplastics was determined by liquid chromatography-mass spectrometry.

[0053] Example 2

[0054] A method for enriching and detecting microplastics in surface water. First, an adsorbent is prepared. The steps for preparing the carbon quantum dot active site powder are as follows:

[0055] 15 parts of chitosan and 85 parts of deionized water were stirred and heated at 100°C for 100 min until completely dissolved to obtain a 15% chitosan aqueous solution;

[0056] Dissolve 10 parts of sodium hydroxide and 40 parts of deionized water and add them uniformly, add 50 parts of lignin, stir for 50 minutes, until the lignin is completely dissolved; add 50 parts of chitosan aqueous solution, then add 3 parts of polyethyleneimine, stir uniformly, react at 60°C for 2 hours to allow lignin, chitosan and polyethyleneimine to undergo cross-linking reaction, and obtain a solution containing lignin grafted with amine groups;

[0057] 6 parts of monochloroacetic acid were dissolved in 10 parts of deionized water, and the monochloroacetic acid solution was added dropwise to the lignin solution obtained, and the temperature was raised to 90°C for reaction for 4 hours. After the reaction was completed, the reaction solution was cooled, and the reaction solution was poured out. The pH value of the solution was adjusted to 5.1 with 2 mol / L hydrochloric acid. The reaction product was filtered with a 0.5 μm microporous membrane to remove impurities, and then dialyzed in deionized water with a 1000 da dialysis bag for 24 hours, and freeze-dried for 72 hours to obtain a carbon quantum dot active site powder;

[0058] Further, a carbon quantum dot active site microporous adsorbent is prepared, specifically:

[0059] 50 parts of calcium montmorillonite were selected, and the calcium montmorillonite was added to 50 parts of deionized water, and 10 parts of 5% citric acid aqueous solution were added and stirred evenly to disperse the calcium montmorillonite in the dilute hydrochloric acid solution, and the temperature was raised to 60°C, and 2.5 parts of dimethyldiallyl ammonium chloride were added, and the mixture was stirred fully and reacted for 2 hours to obtain an aminated montmorillonite suspension, and the pH value was adjusted to 5.5, and 12 parts of histidine were added, and the mixture was stirred fully and reacted for 2 hours to obtain an aminated montmorillonite microsphere suspension with a negative charge on the outside and a high density of cations on the inside;

[0060] Dissolve 10 parts of carbon quantum dot active site powder in 10 parts of 5% citric acid aqueous solution to activate the active sites on the carbon quantum dot active site powder, then add it to a suspension of aminated montmorillonite microspheres with negative charges on the outside and high-density cations on the inside, react at 50°C for 3 hours to allow the carbon quantum dot active site powder and montmorillonite microspheres to fully react and cross-link, filter, and dry at low temperature to obtain a carbon quantum dot active site microporous adsorbent;

[0061] Example 3

[0062] A method for enriching and detecting microplastics in surface water. First, an adsorbent is prepared. The steps for preparing the carbon quantum dot active site powder are as follows:

[0063] 12 parts of chitosan and 88 parts of deionized water were stirred and heated at 100°C for 80 min until completely dissolved to obtain a 12% chitosan aqueous solution;

[0064] Dissolve 10 parts of sodium hydroxide and 40 parts of deionized water and add them uniformly, add 60 parts of lignin, stir for 30 minutes, until the lignin is completely dissolved; add 50 parts of chitosan aqueous solution, then add 3 parts of polyethyleneimine, stir uniformly, react at 60°C for 3 hours to allow lignin, chitosan and polyethyleneimine to undergo cross-linking reaction, and obtain a solution containing lignin grafted with amine groups;

[0065] Dissolve 6 parts of monochloroacetic acid in 10 parts of deionized water, add the monochloroacetic acid solution drop by drop into the lignin solution, heat to 90°C and react for 5 hours, cool after the reaction, pour out the reaction solution, adjust the solution pH to 4.9 with 2 mol / L hydrochloric acid, filter the reaction product with a 0.5 μm microporous membrane to remove impurities, and then dialyze in deionized water with a 1000 da dialysis bag for 24 hours, and freeze-dry for 72 hours to obtain carbon quantum dot active site powder;

[0066] Further, a carbon quantum dot active site microporous adsorbent is prepared, specifically:

[0067] 50 parts of calcium montmorillonite were selected, and the calcium montmorillonite was added to 50 parts of deionized water, and 10 parts of 5% citric acid aqueous solution were added and stirred evenly to disperse the calcium montmorillonite in the dilute hydrochloric acid solution, and the temperature was raised to 60°C, and 2.5 parts of dimethyldiallyl ammonium chloride were added, and the mixture was stirred fully and reacted for 2 hours to obtain an aminated montmorillonite suspension, and the pH value was adjusted to 5.3, and 10 parts of histidine were added, and the mixture was stirred fully and reacted for 2 hours to obtain an aminated montmorillonite microsphere suspension with a negative charge on the outside and a high density of cations on the inside;

[0068] 12 parts of carbon quantum dot active site powder were dissolved in 10 parts of 5% citric acid aqueous solution to activate the active sites on the carbon quantum dot active site powder, and then added to a suspension of aminated montmorillonite microspheres with negative charges on the outside and high density of cations on the inside, reacted at 60°C for 3 hours to allow the carbon quantum dot active site powder and montmorillonite microspheres to fully react and cross-link, filtered, and low-temperature dried to obtain a carbon quantum dot active site microporous adsorbent;

[0069] Comparative Example 1: An equal amount of carbon quantum dot active site powder was used to replace the carbon quantum dot active site microporous adsorbent for adsorption enrichment, and the rest was the same as in Example 1;

[0070] Comparative Example 2: A product obtained by cross-linking carbon quantum dot active site powder and ordinary calcium-based montmorillonite was used as an adsorbent, and the other conditions were the same as those in Example 1;

[0071] Comparative Example 3: Without the cross-linked carbon quantum dot active site powder, the modified calcium-based montmorillonite was directly used as the adsorbent, and the carbon quantum dot active site microporous adsorbent was equivalently replaced for adsorption enrichment, and the others were the same as in Example 1;

[0072] Comparative Example 4: The adsorbent of a magnetic porous graphene prepared in Example 1 of the method for removing microplastics in water based on graphene materials disclosed in the Chinese patent with the publication number CN111569829B was equivalently replaced for the carbon quantum dot active site microporous adsorbent for adsorption enrichment, and the others were the same as in Example 1;

[0073] Performance test

[0074] 1. The specific surface area of the adsorbent was measured using a JW-BK200C specific surface area and pore size analyzer. The adsorbent sample was first degassed under vacuum at 100 °C for 8 hours, and then an N2 adsorption-desorption experiment was carried out under liquid nitrogen environment. The adsorbent samples in Examples 1-3 and Comparative Examples 1-4 were tested to obtain the specific surface area (m 2 / g) and porosity (%) of the adsorbent. The specific data are shown in Table 1. From this, it can be seen that the adsorbents involved in Examples 1-3 have a larger specific surface area and porosity, which have a more significant promoting effect on the adsorption of microplastics.

[0075] Table 1 Performance characterization data of the adsorbent

[0076]

[0077] 2. Scanning electron microscopy

[0078] The morphology of the sample was observed using a Nova NanoSEM450 field emission scanning electron microscope of FEI Company. Before the test, the carbon quantum dot active site microporous adsorbent sample in Example 1 was evenly dispersed on the conductive adhesive, and the parameters of the scanning electron microscope were adjusted. The acceleration voltage was set, the detection current was 10 mA, the resolution was 1.7 nm, and the magnification was 500 times; the morphology is shown in Figure 2 .

[0079] A polystyrene microplastic dispersion with a concentration of 1.0 g / kg and an average particle size of 0.1-1 μm was prepared. The solvent was ethanol and distilled water with a volume ratio of 1:1. During the preparation process, it was ultrasonically dispersed until uniform. 1 kg of the polystyrene microplastic dispersion was taken, and 1 g of the carbon quantum dot active site microporous adsorbent in Example 1 was placed. After adsorption for 50 minutes, the obtained aggregates were filtered and dried, and then evenly dispersed on the conductive adhesive. The parameters of the scanning electron microscope were adjusted. The acceleration voltage was set, the detection current was 10 mA, the resolution was 1.7 nm, and the magnification was 300 times; the morphology is shown in Figure 3

[0080] From Figure 2It can be seen that the carbon quantum dot active site microporous adsorbent has a spherical microporous structure with relatively uniform size, a three-dimensional porous structure, and many fine pore structures on the spherical body. This special morphological structure has a high specific surface area and a large number of active sites, and can selectively and efficiently adsorb tiny microplastic particles in water. From Figure 3 It can be seen that many irregular spherical particles are evenly adsorbed on the aggregates obtained after adsorption equilibrium in water. This indicates that a large number of active sites are evenly dispersed on the surface of the spherical body of the carbon quantum dot active site microporous adsorbent, and it has a very good adsorption and enrichment effect on microplastics.

[0081] 3. Adsorption effect test of the adsorbent

[0082] Three kinds of microplastic materials of polypropylene, polystyrene, and polyvinyl chloride were prepared by the experiment and crushed into two different particle sizes of microplastic particles. The particle size ranges included 0.01 - 10 μm and 10 - 100 μm, each accounting for half. They were added to deionized water to prepare a simulated microplastic-containing water sample with a microplastic concentration of 1.0 mg / L and a water temperature of 27 °C;

[0083] 1 g of the adsorbents of Example 1, Comparative Example 1, and Comparative Example 4 were respectively added to 1 kg of the microplastic-containing water sample and adsorbed for 50 minutes. The change curve of the adsorption amount of the adsorbent for microplastics with time was recorded, as shown in Figure 1 . The horizontal axis is the adsorption time, and the vertical axis is the adsorption amount of the adsorbent for microplastics. From Figure 1 It can be seen that the adsorption amount of the carbon quantum dot active site microporous adsorbent for microplastics can basically reach the saturated adsorption amount of 641 mg / g in about 40 minutes. Therefore, the carbon quantum dot active site microporous adsorbent has very excellent adsorption ability for microplastics of various particle sizes, especially significantly improving the capture ability for 0.01 - 10 μm, achieving a very good adsorption effect. For Comparative Example 1, because of its small specific surface area and limited microporous structure, although it can adsorb microplastics, its adsorption amount is limited, only playing a tiny flocculation role. For Comparative Example 4, its adsorption amount is about 219 mg / g. Analyzing the reason, it may be that its capture ability for 0.01 - 10 μm is relatively poor, and for 10 - 100 μm microplastics, because of their large particles, when the adsorption equilibrium is reached, it is easy to hinder its capture of microplastics of other particle sizes. For the carbon quantum dot active site microporous adsorbent of Example 1, because of its convertible charge property, when the adsorption reaches a bottleneck, the preferentially adsorbed small particle microplastics enter the micropores due to the charge effect, changing the charge balance of the carbon quantum dot active site microporous adsorbent, disrupting the charge balance of the re-stabilization phenomenon. The negative charge on the surface of montmorillonite and the positive charge in the inner layer can be mutually converted, thereby improving its adsorption ability, accelerating the adsorption rate of microplastics in water, improving the adsorption ability of microplastics in water, and realizing the rapid enrichment and separation of microplastics in water.

[0084] Test the self-made microplastic particles with a particle size of less than 1 μm: polypropylene microplastics, polystyrene microplastics, and polyvinyl chloride microplastics. Add them to deionized water to form a simulated microplastic-containing water sample with a microplastic concentration of 0.5 mg / kg and a water temperature of 27°C. Add 1 g of the adsorbents in Examples 1-3 and Comparative Examples 1-4 to 1 kg of the microplastic-containing water sample respectively, and measure their removal rates, as shown in Table 2;

[0085] Table 2 Record Table of the Adsorption Effect of the Adsorbent

[0086]

[0087] 4. Determination of Microplastics in Actual Surface Water Bodies

[0088] First, pass 5 kg of different water samples collected from actual environmental water bodies through a glass fiber membrane with a pore size of 200 μm to remove the interference of large particle impurities in the water body. Then, use the separation and enrichment method in Example 1 of the present invention for enrichment and determination, which mainly includes: adding an aqueous sodium carbonate solution to the 5 kg environmental water sample to adjust the pH value to 8.0, adding 5 g of the carbon quantum dot active site microporous adsorbent, and shaking it vigorously for 50 minutes to make them react fully. After the full reaction, aggregates are formed. Filter the obtained liquid through a glass fiber membrane, then dissolve and elute it with a digestion solution, collect the washing solution, concentrate it to 1 mL, dry it with a nitrogen blower, add chromatographic grade methanol, transfer it to a brown injection vial, and make the volume up to 2 mL; use a liquid chromatography-mass spectrometry instrument to measure the content of microplastics. The results are shown in Table 3. By combining the separation and enrichment method with a high enrichment multiple in the present invention, the accurate mass concentration of microplastics in the water environment can be determined.

[0089] Table 3. Determination Results of Microplastics in Actual Surface Water Bodies

[0090]

[0091] The above is only used to illustrate the technical solution of the present invention and not to limit it. Any equivalent modifications and changes made by those of ordinary skill in the art to the technical solution of the present invention still fall within the scope covered by the present invention as long as they do not depart from the overall concept of the present invention.

Claims

1. A method for enriching and detecting microplastics in surface water, characterized in that: First, add adsorbent to the water sample of the surface water containing microplastics for adsorption enrichment to form aggregates; then filter to complete the enrichment of microplastics; then use digestion solution to dissolve and elute the microplastics on the aggregates, and finally determine the concentration of microplastics in the environmental water sample by liquid chromatography-mass spectrometry; specifically: S201, adding a sodium carbonate aqueous solution to a water sample of a surface water body containing microplastics to adjust the pH value to 8.0-8.5, and then adding an adsorbent to perform an adsorption treatment so that the adsorbent is in full contact with the microplastics, adsorbing the negatively charged microplastics to form multiple aggregate centers; S202, filtering the surface water sample containing the aggregates with a microporous membrane to obtain the aggregates, thereby completing the enrichment and separation of the microplastics; S203, dissolve the microplastics on the eluted aggregates with a digestion solution, collect the washed liquid, concentrate it to 1 mL, blow it dry with a nitrogen blower, add chromatography-grade methanol, transfer it to a brown injection bottle, and make up to 2 mL; determine the content of microplastics using a liquid chromatography-mass spectrometer; The digestion solution is a mixed solution obtained by mixing tetrahydrofuran and dichloromethane in a mass ratio of 1:3; the mass ratio of the adsorbent to the water sample of the surface water body containing microplastics is 0.1 to 2:100; The adsorbent is a carbon quantum dot active site microporous adsorbent, which is prepared from carbon quantum dot active site powder and modified montmorillonite.

2. The method for enriching and detecting microplastics in surface water according to claim 1, characterized in that: The carbon quantum dot active site powder is prepared by the following steps: S001, stirring and heating chitosan and deionized water at 100°C for 80-120 min until completely dissolved, to obtain a chitosan aqueous solution; The mass concentration of the chitosan aqueous solution is 10-15%; S002, dissolving sodium hydroxide and deionized water evenly, adding lignin, stirring for 30 to 50 minutes until the lignin is completely dissolved; adding chitosan aqueous solution, and then adding polyethyleneimine, stirring evenly, reacting at 50 to 60° C. for 2 to 3 hours to allow lignin, chitosan and polyethyleneimine to undergo a cross-linking reaction, and obtaining a solution containing lignin grafted with amine groups; the mass concentration of the sodium hydroxide aqueous solution is 25%; S003, dissolving monochloroacetic acid in deionized water, adding the monochloroacetic acid solution dropwise to the lignin solution finally obtained in S002, heating to 80-90°C for reaction for 4-5 hours, cooling after the reaction, pouring out the reaction solution, adjusting the solution pH to 5.0±0.1 with 2 mol / L hydrochloric acid, filtering the reaction product with a 0.5 μm microporous membrane to remove impurities, and then dialyzing it in deionized water with a 1000 da dialysis bag for 24 hours, and freeze-drying it for 72 hours to obtain a carbon quantum dot active site powder; wherein the mass concentration of the monochloroacetic acid solution is 60%, and the mass ratio of monochloroacetic acid to lignin is 6:50-60.

3. The method for enriching and detecting microplastics in surface water according to claim 2, characterized in that: The mass ratio of polyethyleneimine, chitosan and lignin is 3-4:5-8:50-60.

4. The method for enriching and detecting microplastics in surface water according to claim 1, characterized in that: The carbon quantum dot active site microporous adsorbent is prepared by: S101, selecting calcium-based montmorillonite, adding the calcium-based montmorillonite to deionized water, adding dilute hydrochloric acid and stirring evenly, so that the calcium-based montmorillonite is dispersed in the dilute hydrochloric acid solution, heating to 60° C., adding dimethyldiallyl ammonium chloride, stirring fully, reacting for 1 to 2 hours, and obtaining an aminated montmorillonite suspension, adjusting the pH value to 5.0 to 5.5, adding histidine, stirring fully, and reacting for 1 to 2 hours to obtain an aminated montmorillonite microsphere suspension with a negative charge on the outside and a high density of cations on the inside; wherein the mass ratio of dimethyldiallyl ammonium chloride to calcium-based montmorillonite is 1:20; the mass ratio of the histidine to calcium-based montmorillonite is 1:4 to 5; S102, dissolving the carbon quantum dot active site powder in a citric acid aqueous solution with a mass concentration of 8% to activate the active sites on the carbon quantum dot active site powder, and then adding the suspension of aminated montmorillonite microspheres with negative charge on the outside and high density of cations on the inside obtained in S101, reacting at 50-60°C for 2-3 hours to allow the carbon quantum dot active site powder and montmorillonite microspheres to fully react and cross-link, filtering, and low-temperature drying to obtain a carbon quantum dot active site microporous adsorbent; the mass ratio of the carbon quantum dot active site powder to the calcium-based montmorillonite is 1:

5.

5. The method for enriching and detecting microplastics in surface water according to claim 1, characterized in that: The microplastics include at least one of polystyrene, polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, and polymethyl methacrylate.

6. The method for enriching and detecting microplastics in surface water according to claim 1, characterized in that: The adsorption treatment time is 40 to 50 minutes; the microporous membrane is a glass fiber membrane, and the pore size of the microporous membrane is 0.5 to 1 μm.

7. Use of the method for enrichment and detection of microplastics in surface water according to claim 1 in monitoring environmental pollution of surface water caused by microplastics.

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