Method for extracting microplastics in municipal sludge

Abstract

The application provides a method for extracting microplastics in municipal sludge, and belongs to the field of environmental health risk assessment. The municipal sludge is mixed with a persulfate solution, and then first digestion and solid-liquid separation are performed to obtain first digestion solids. The first digestion solids are mixed with a hydrogen peroxide solution, and then second digestion and solid-liquid separation are performed to obtain second digestion solids. The second digestion solids are mixed with an alkaline solution, and then third digestion and solid-liquid separation are performed to obtain third digestion solids. The third digestion solids are subjected to flotation to obtain supernatant after flotation. The supernatant is mixed and concentrated to obtain a microplastic extract. The method can extract microplastics from municipal sludge under conditions of minimizing the influence of microplastics, and is convenient and fast, so that the types and abundance of microplastics in municipal sludge can be accurately analyzed, and strong technical support and technical support are provided for the study of the enrichment, migration and conversion rules of microplastics in environmental media, as well as the sludge return to the field and the biological health risk assessment.

Description

Technical Field

[0001] This invention relates to the field of environmental health risk assessment technology, and in particular to a method for extracting microplastics from municipal sewage sludge. Background Technology

[0002] Microplastics (MPs) refer to plastic particles with a diameter of less than 5 mm. They primarily originate from direct sources such as industrial raw materials, household products (facial cleansers, toothpaste, etc.), and laundry detergents, as well as indirect sources where plastics are transformed from larger particles into smaller ones through natural processes like water, wind, or biomass. Sludge, a collection of flocculants and microorganisms, contains over 90% of the microplastics released from daily life and are absorbed, encapsulated, and settled in municipal wastewater treatment. Some regions are promoting the land use of sludge as fertilizer or soil conditioner for landscaping and greening projects, which may lead to the introduction of large amounts of microplastics into the environment. The long-term presence of microplastics in various environmental media can have adverse ecological impacts, but the extent of microplastic pollution in these media remains unclear. Although there are numerous reports on the pollution and occurrence of microplastics in the environment, the lack of authoritative standards for microplastic separation and extraction methods results in low reliability of quantitative and qualitative data. The types and amounts of microplastics in municipal sewage sludge have a significant impact on the medium itself and its subsequent treatment. Currently, there is a lack of rapid, efficient methods for separating and extracting microplastics that do not have an adverse effect on them, which seriously restricts relevant research on the health risk assessment of microplastics.

[0003] Existing microplastic extraction methods are mainly for biological or soil and sediment samples. They use strong acids, strong bases or strong oxidants to digest and remove organic matter in the sample that would interfere with the extraction and detection of microplastics. Then, they use saturated density salt solutions to collect and detect the microplastics by flotation. Biological samples are composed of high molecular weight organic matter such as fats and proteins, while soil and sediments are mainly composed of natural organic matter such as humus and inorganic matter such as silt. Sludge is a flocculent body mainly composed of active organic matter produced by the metabolic activities of a large number of microorganisms. The types of organic matter are more complex than those of biological samples, and the content is greater than that of soil and sediments. Therefore, existing microplastic extraction methods for biological, soil and sediment samples are not suitable for sludge, mainly due to the following shortcomings: (1) Sludge samples cannot be completely degraded by KOH digestion like biological samples; (2) Unlike soil samples with low organic matter content, microplastics cannot be extracted by simple digestion followed by density separation; (3) Most existing digestion schemes are too drastic and cause significant damage to microplastics, which is not conducive to the accurate assessment of the environmental abundance of microplastics.

[0004] Therefore, there is an urgent need to develop a method for extracting microplastics from municipal sludge that can remove organic matter interference without significantly affecting the microplastics themselves. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a method for extracting microplastics from municipal sewage sludge. The extraction method of this invention can accurately analyze the types and abundance of microplastics in municipal sewage sludge in a simple, mild, and efficient manner, with minimal impact on the properties of the microplastics themselves, facilitating subsequent detection of microplastic types and content.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for extracting microplastics from municipal sewage sludge, comprising the following steps:

[0008] The municipal sludge is mixed with a persulfate solution and then subjected to a first digestion and solid-liquid separation to obtain a first digested solid and a first supernatant; the organic matter content in the municipal sludge is 20-70% on a dry basis.

[0009] The first digested solid was mixed with hydrogen peroxide solution and then subjected to second digestion and solid-liquid separation in sequence to obtain the second digested solid and the second supernatant.

[0010] The second digested solid was mixed with an alkaline solution and then subjected to a third digestion and solid-liquid separation to obtain a third digested solid and a third supernatant.

[0011] The third digested solid is subjected to flotation to obtain the supernatant after flotation.

[0012] The first supernatant, the second supernatant, the third supernatant, and the supernatant after flotation were mixed and concentrated to obtain a microplastic extract.

[0013] Preferably, the persulfate in the persulfate solution includes K2S2O8.

[0014] Preferably, the ratio of municipal sludge to persulfate solution is 5g:10-70mL, and the concentration of persulfate solution is 0.1-0.3mol / L.

[0015] Preferably, the municipal sludge has a moisture content of 80-90 wt%.

[0016] Preferably, the first digestion temperature is 65-80°C and the time is 3-10 hours.

[0017] Preferably, based on the first digested solids produced from 5g of the municipal sludge, the amount of hydrogen peroxide solution used is 10-30mL, and the mass percentage concentration of the hydrogen peroxide solution is 10%-30%.

[0018] Preferably, the second digestion temperature is 30–60°C and the time is 3–22 h.

[0019] Preferably, based on the second digested solids generated from 5g of the municipal sludge, the amount of alkaline solution used is 10-30mL, and the mass percentage concentration of the alkaline solution is 1%-10%.

[0020] Preferably, the third digestion temperature is 35–60°C and the time is 3–8 hours.

[0021] Preferably, the alkaline solution includes a KOH solution or a NaOH solution.

[0022] This invention provides a method for extracting microplastics from municipal sewage sludge, comprising the following steps: mixing municipal sewage sludge with a persulfate solution and then sequentially performing a first digestion and solid-liquid separation to obtain a first digested solid and a first supernatant; the organic matter content in the municipal sewage sludge is 20-70% on a dry basis; mixing the first digested solid with a hydrogen peroxide solution and then sequentially performing a second digestion and solid-liquid separation to obtain a second digested solid and a second supernatant; mixing the second digested solid with an alkaline solution and then sequentially performing a third digestion and solid-liquid separation to obtain a third digested solid and a third supernatant; subjecting the third digested solid to flotation to obtain a flotation supernatant; and mixing and concentrating the first supernatant, second supernatant, third supernatant, and flotation supernatant to obtain a microplastic extract.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] The main reason for using persulfate as the first digestion agent in this invention is that persulfate has a smaller impact on microplastics at high temperatures (65-75°C) compared to hydrogen peroxide and alkaline solutions. If hydrogen peroxide or alkaline solutions are used first in the digestion, some hydrogen peroxide or alkaline solution will remain in the sludge. Introducing persulfate in subsequent digestions will raise the digestion temperature to 65-75°C, resulting in excessively high digestion temperatures for the remaining peroxide or alkaline solutions, which will have a significant impact on microplastics. The main reason for using hydrogen peroxide solution and alkaline solution as the second and third digestion agents is to avoid the generation of a large amount of foam caused by direct contact between hydrogen peroxide and alkaline solution, which would lead to experimental failure. If alkaline solution is used as the digestion agent in the second digestion, the hydrogen peroxide added in the third digestion will be catalyzed by the remaining unreacted alkaline solution to generate a large number of bubbles, resulting in the loss of microplastics and thus experimental failure. This invention can gently digest complex organic matter in sludge, solving the problems of frequent operations and large microplastic loss in existing technologies. It reduces the adverse effects of digestion on microplastics and improves the accuracy of microplastic extraction, enabling subsequent quantitative detection and research of microplastics in municipal sludge. Through comparative analysis of microplastic extraction and separation methods in municipal sludge, and by purposefully selecting and optimizing the type of digestion reagent and digestion steps, the microplastic method based on digestion of organic matter interference in municipal sludge can efficiently separate and extract microplastics.

[0025] Experimental results demonstrate that, compared to existing technologies, the extraction method of this invention is more effective in separating microplastics from sludge. Furthermore, the addition of the digestion reagent affects the extraction and separation effect, providing relevant reference and theoretical support for the management and research of microplastic pollution in municipal sludge and other municipal sludge rich in organic matter. Attached Figure Description

[0026] Figure 1 This is a flowchart of the method for extracting microplastics from municipal sludge in Embodiment 1 of the present invention;

[0027] Figure 2 The images show the PE standard samples before and after spiking in Example 1.

[0028] Figure 3 In Figure a, the digestion and removal rate of each digestion step in Example 1 is shown, and in Figure b, the PE recovery rate of Example 1 and Comparative Examples 1 to 3 is shown.

[0029] Figure 4 The graph shows a comparison of the microplastic content obtained in Examples 29-30, Comparative Examples 1 and 4.

[0030] Figure 5 Comparison chart of microplastic types obtained in Examples 29-30, and Comparative Examples 1 and 4;

[0031] Figure 6 This is a comparison chart of the particle size distribution of microplastics obtained in Examples 29-30, Comparative Examples 1 and 4;

[0032] Figure 7 Comparison diagrams of microplastic particle size distribution obtained in Examples 29-30, and Comparative Examples 1 and 4;

[0033] Figure 8 Microscopic images of 25 types of microplastics in their original form;

[0034] Figure 9 Micrographs of 25 types of microplastics after S1 digestion;

[0035] Figure 10 Micrographs of 25 types of microplastics after digestion using S1+S2;

[0036] Figure 11 Micrographs of 25 types of microplastics after digestion using S1+S2+S3 methods;

[0037] Figures 12-17 Infrared spectra of 25 microplastics before and after digestion by S1+S2+S3. Detailed Implementation

[0038] This invention provides a method for extracting microplastics from municipal sewage sludge, comprising the following steps:

[0039] The municipal sludge is mixed with a persulfate solution and then subjected to a first digestion and solid-liquid separation to obtain a first digested solid and a first supernatant; the organic matter content in the municipal sludge is 20-70% on a dry basis.

[0040] The first digested solid was mixed with hydrogen peroxide solution and then subjected to second digestion and solid-liquid separation in sequence to obtain the second digested solid and the second supernatant.

[0041] The second digested solid was mixed with an alkaline solution and then subjected to a third digestion and solid-liquid separation to obtain a third digested solid and a third supernatant.

[0042] The third digested solid is subjected to flotation to obtain the supernatant after flotation.

[0043] The first supernatant, the second supernatant, the third supernatant, and the supernatant after flotation were mixed and concentrated to obtain a microplastic extract.

[0044] The present invention involves mixing municipal sludge with a persulfate solution and then sequentially performing a first digestion and solid-liquid separation to obtain a first digested solid and a first supernatant; the organic matter content in the municipal sludge is 20-70% on a dry basis.

[0045] In this invention, the organic matter content of the municipal sludge is preferably 50% to 60% by mass; the higher the organic matter content in the municipal sludge, the greater the difficulty in digestion and extraction.

[0046] In this invention, the municipal sludge typically contains various components, such as various organic matter, microorganisms, inorganic particles, and colloidal substances. These components can affect the extraction and detection of microplastics during the digestion process. Furthermore, flocculants, minerals, and organic matter may encapsulate and flocculate with microplastics, making them difficult to separate and collect. Therefore, this invention sets a specific digestion sequence to eliminate interference from organic matter in the separation and extraction of microplastics, avoiding adverse effects on the microplastics. This invention does not specifically limit the source of the municipal sludge; sources well-known to those skilled in the art can be used.

[0047] In this invention, the moisture content of the municipal sludge is preferably 80-90 wt%, specifically 80 wt%, 83 wt%, 85 wt%, or 90 wt%.

[0048] In this invention, the persulfate in the persulfate solution preferably comprises K2S2O8; the persulfate generates sulfate free SO4· - The large organic molecules in oxidized sludge, especially the humic substances and aliphatic compounds that are difficult to biodegrade, can transform complex organic molecules into simple organic or inorganic substances, thereby promoting the sludge decomposition process.

[0049] In this invention, the preferred ratio of municipal sludge to persulfate solution is 5g:10-70mL, specifically 5g:10mL, 5g:15mL, 5g:20mL, 5g:25mL, 5g:30mL, 5g:40mL, 5g:50mL, 5g:60mL, or 5g:70mL. The preferred concentration of the persulfate solution is 0.1-0.3mol / L, specifically 0.1, 0.15, 0.2, 0.25, or 0.3mol / L.

[0050] In this invention, the temperature of the first digestion is preferably 65-80°C, specifically 65, 70, 75 or 80°C. Within the temperature range of the first digestion, the persulfate undergoes thermal activation and has strong oxidizing properties. The time is preferably 3-10 hours, specifically 3, 4, 5, 6, 7, 8, 9 or 10 hours.

[0051] In this invention, the temperature of the first digestion is preferably provided by a water bath.

[0052] In this invention, the first digestion is preferably carried out under ultrasonic conditions, and the power of the ultrasonic conditions is preferably 200-300W, specifically 200, 250 or 300W.

[0053] After the first digestion is completed, the present invention preferably allows the obtained digestion product to stand for 5 to 8 hours, then performs solid-liquid separation and collects the first supernatant. The remaining sludge at the bottom is the first digested solid, which is then used for the next digestion step.

[0054] The present invention does not impose any particular limitation on the specific method of solid-liquid separation; any method known to those skilled in the art can be used.

[0055] In this invention, the first supernatant preferably contains microplastics with low density, specifically including polystyrene (PS), polypropylene (PP), polyethylene (PE), polymethyl methacrylate (PMMA), and polycarbonate (PC), which are plastic polymers that can float in water.

[0056] After obtaining the first digested solid, the present invention mixes the first digested solid with hydrogen peroxide solution and then performs a second digestion and solid-liquid separation in sequence to obtain a second digested solid and a second supernatant.

[0057] In this invention, based on the first digested solids produced from 5g of the municipal sludge, the amount of hydrogen peroxide solution used is preferably 10-30mL, specifically 10, 15, 20, 25, or 30mL. The mass percentage concentration of the hydrogen peroxide solution is preferably 10%-30%, specifically 10%, 15%, 20%, 25%, or 30%. Hydrogen peroxide is typically used in sludge treatment with Fe... 2+ The combination forms Fenton's reagent. The ·OH and singlet oxygen in Fenton's reagent have high oxidizing power, which can effectively destroy the macromolecular organic matter in sludge, especially humic substances and fats. The hydroxyl radicals can attack and break the chemical bonds of organic molecules, transforming complex organic molecules into simple organic or inorganic substances, thereby promoting the sludge decomposition process.

[0058] In this invention, the temperature of the second digestion is preferably 30 to 60°C, specifically 30, 35, 40, 45, 50, 55 or 60°C, and the time is preferably 3 to 22 hours, specifically 3, 4, 5, 6, 10, 20 or 22 hours.

[0059] In this invention, the temperature for the second digestion is preferably provided by a water bath.

[0060] In this invention, the second digestion is preferably carried out under ultrasonic conditions, and the power of the ultrasonic conditions is preferably 200 to 300W, specifically 200, 250 or 300W.

[0061] After the second digestion is completed, the present invention preferably allows the obtained digestion product to stand for 5 to 8 hours, then performs solid-liquid separation and collects the second supernatant. The remaining sludge at the bottom is the second digested solid, which is then used for the next digestion step.

[0062] After obtaining the second digested solid, the present invention mixes the second digested solid with an alkaline solution and then performs a third digestion and solid-liquid separation in sequence to obtain a third digested solid and a third supernatant.

[0063] In this invention, based on the second digested solids produced from 5g of the municipal sludge, the amount of alkaline solution used is preferably 10-30mL, specifically 10, 15, 20, 25 or 30mL, and the mass percentage concentration of the alkaline solution is preferably 1%-10%, specifically 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0064] In this invention, the alkaline solution preferably includes KOH solution or NaOH solution; KOH in the KOH solution and NaOH in the NaOH solution are strong bases, and their main function in sludge treatment is to adjust the pH value, which can destroy the cell structure in the sludge, release the intracellular contents, improve the solubility and biodegradability of macromolecular organic matter in the sludge, thereby improving the sludge decomposition effect.

[0065] In this invention, the temperature of the third digestion is preferably 35-60°C, specifically 35, 40, 45, 50, 55 or 60°C, and the time is preferably 3-8 hours, specifically 3, 4, 5, 6 or 8 hours.

[0066] In this invention, the temperature of the third digestion is preferably provided by a water bath.

[0067] In this invention, the third digestion is preferably carried out under ultrasonic conditions, and the power of the ultrasonic conditions is preferably 200-300W, specifically 200, 250 or 300W.

[0068] After the third digestion is completed, the present invention preferably allows the obtained digestion product to stand for 5 to 8 hours, then performs solid-liquid separation and collects the third supernatant. The remaining solid at the bottom is the third digestion solid, which is then used for the next flotation step.

[0069] In this invention, persulfate is used for the first digestion because persulfate requires a high temperature (above 60°C) to activate its oxidizing ability and achieve the effect of digesting organic matter. However, high temperatures with H2O2 and alkaline substances (such as KOH) can severely damage microplastics (MPs), leading to significant interference with the extracted microplastics and resulting in large deviations in the detection data. If H2O2 is used for the first digestion, although the supernatant is transferred after H2O2 digestion, some H2O2 will remain in the system. Subsequent addition of persulfate requires high temperatures, which will still cause H2O2 to react at high temperatures. Using alkaline substances as the first digestion method also presents the same problem as with H2O2. Therefore, using persulfate as the first digestion method is more suitable. As for using H2O2 for the second digestion and alkaline substances as… The reason for the third digestion is that H2O2 reacts with a small amount of KOH to produce a large number of bubbles, ultimately leading to experimental failure. If an alkaline substance is added first, although the supernatant will be transferred after digestion, some KOH will still remain. When an excess of H2O2 is added at this point, the alkaline substance will catalyze the extensive decomposition of H2O2, producing a large number of bubbles, leading to experimental failure. However, using H2O2 as the second digestion method is problematic because prolonged digestion leads to the extensive decomposition of H2O2, resulting in a very low residual H2O2 content in the system. After transferring the supernatant, the residual H2O2 content is extremely low. Even if an excess of alkaline substance is added at this point, it will accelerate the decomposition of the remaining H2O2 and produce bubbles, but the amount of bubbles will not be sufficient to cause experimental failure. Therefore, H2O2 is used as the second digestion method, and KOH is used as the third digestion method. This invention can remove organic matter that interferes with the separation of microplastics in municipal sludge using a relatively mild digestion scheme, and can selectively remove different types of organic matter in municipal sludge in batches, improving the efficiency of organic matter digestion. Meanwhile, the extraction method of the present invention can avoid the problem in the prior art where the persulfate, hydrogen peroxide and alkaline solution react violently during the digestion process, resulting in the inability to exert their respective effects; the order of adding digestion reagents can ensure that while removing the organic matter interference of municipal sludge, it will not have a significant adverse effect on microplastics, so as to facilitate subsequent extraction and analysis.

[0070] After obtaining the third digested solid, the present invention performs flotation (density separation) on the third digested solid to obtain the supernatant after flotation.

[0071] In this invention, the flotation preferably uses a density salt solution, which is preferably a saturated ZnCl2 solution.

[0072] In this invention, based on the third digestion solids produced by 5g of the municipal sludge, the amount of the density salt solution is preferably 50-100mL, specifically 50, 60, 70, 80, 90 or 100mL.

[0073] In this invention, after flotation, the obtained product is preferably allowed to stand for 5 to 8 hours, and then solid-liquid separation is performed to collect the supernatant after flotation. The solid-liquid separation operation is repeated 3 to 5 times.

[0074] In this invention, the flotation is preferably carried out under ultrasonic conditions, and the power of the ultrasonic conditions is preferably 200-300W, specifically 200, 250 or 300W.

[0075] After obtaining the first supernatant, the second supernatant, the third supernatant and the supernatant after flotation, the present invention mixes the first supernatant, the second supernatant, the third supernatant and the supernatant after flotation and concentrates them to obtain the microplastic extract.

[0076] In this invention, the mixture obtained by mixing is preferably filtered onto a stainless steel filter membrane, then transferred by ultrasound to an evaporating dish for concentration, and finally enriched into a 1.5 mL sample bottle to obtain the microplastic extract.

[0077] In this invention, the pore size of the stainless steel filter membrane is preferably 5 μm, and the diameter is preferably 47 mm.

[0078] In this invention, the microplastics in the microplastic extract preferably include terpolymers of acrylonitrile (A), butadiene (B), and styrene (S) (ABS), acrylonitrile-styrene copolymer (AS), cyclic olefin copolymer (COC), epoxy-polypropylene copolymer (COP), ethylene-vinyl acetate copolymer (EVA), high-density polyethylene (HDPE), styrene-butadiene copolymer (K ​​glue), low-density polyethylene (LDPE), polyamide (PA), copolymer of butylene adipate and butylene terephthalate (PBAT), polybutylene succinate (PBS), and poly... One or more of the following: butylene terephthalate (PBT), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyethylene terephthalate-1,4-cyclohexanediol (PETG), polyhydroxyalkanoate (PHA), polylactic acid (PLA), polymethyl methacrylate (PMMA), thermoplastic elastomers (POE) polymerized in situ with ethylene and α-olefins using metallocene catalysts, polyoxymethylene (POM), polypropylene (PP), polyphenylene sulfide (PPS), polystyrene (PS), polyethylene (PE), and syndiotactic polystyrene (SPS).

[0079] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0080] In the embodiments and comparative examples of the present invention

[0081] The steps for photographing and counting microplastics are as follows: After filtration, the polycarbonate filter membrane is photographed using a low-power microscope or a close-focus camera. Based on the photographs, manual counting is performed using ImageJ to calculate the PE recovery rate.

[0082] PE recovery rate = Microplastics obtained by photographing and counting after digestion / Microplastics obtained by photographing and counting before digestion was added.

[0083] Example 1

[0084] Figure 1 This is a flowchart of the method for extracting microplastics from municipal sludge in Embodiment 1 of the present invention, which includes the following steps:

[0085] (1) PE count before spiking: Transfer the PE standard to a glass petri dish, take a picture using a low-power microscope or macro camera, and count the red PE based on the picture. The count result is the number of particles before spiking, n1.

[0086] (2) Sample digestion: 5g of anaerobic municipal wet sludge sample (on a dry basis, the organic matter content of the anaerobic municipal wet sludge sample is 60% and the water content of the anaerobic municipal wet sludge sample is 83wt%) and PE standard were transferred to a beaker. S1: 70mL of K2S2O8 solution with a concentration of 0.2mol / L was added to it. The reaction temperature was controlled at 65℃ and the reaction was continued for 6h. After standing for 8h, the supernatant was collected after solid-liquid separation.

[0087] S2: Add 50 mL of 15% H2O2 solution to the deposited sludge, control the temperature at 40℃ and continue the reaction for 18 h, let it stand for 8 h to allow solid-liquid separation and collect the supernatant; S3: Add 40 mL of 2% KOH solution to the deposited sludge, control the reaction temperature at 40℃ and continue the reaction for 4 h, let it stand for 8 h to allow solid-liquid separation and collect the supernatant; 250W ultrasonic-assisted digestion is used during the process.

[0088] (3) Flotation: Add 200 mL of saturated ZnCl2 solution to the solid sludge after digestion in step (2), shake thoroughly and let stand for 8 hours until solid-liquid separation, collect the supernatant, add another 200 mL of saturated ZnCl2 solution, repeat this operation 5 times to collect the supernatant, and use 250W ultrasonic assistance during the flotation process.

[0089] (4) Filtration, enrichment and detection: The supernatant collected in steps (2) and (3) was filtered onto a PC filter membrane (pore size 10 μm, diameter 25 mm). The PE adhering to the filter cup was transferred to a culture dish. The filter membrane and culture dish were photographed using a low magnification microscope. The red PE on the filter membrane was counted based on the photograph. The count result of the filter membrane was n2, and the count result of the culture dish was n3.

[0090] (5) Recovery rate calculation: (n2+n3) / n1

[0091] The standard PE is a red, round PE standard with a diameter of 125–150 μm.

[0092] The PE on the filter cup is eluted into the petri dish because during filtration, PE forms coffee rings under the action of aqueous and ethanol solutions, causing a large amount of PE to diffuse to the edge of the PC filter membrane. Under capillary action, a large amount of PE is adsorbed at the bottom of the filter cup. If it is not eluted and counted, the data will be lower.

[0093] Example 2

[0094] This embodiment is the same as Embodiment 1, except that the concentration of K2S2O8 is different. A K2S2O8 solution with a concentration of 0.1M is added to municipal sludge containing microplastics.

[0095] Example 3

[0096] This embodiment is the same as Embodiment 1, except that the concentration of K2S2O8 is different. A K2S2O8 solution with a concentration of 0.3M is added to municipal sludge containing microplastics.

[0097] Example 4

[0098] This embodiment is the same as Embodiment 1, except that the water bath temperature is reduced to 50°C in S1.

[0099] Example 5

[0100] This embodiment is the same as Embodiment 1, except that the water bath temperature is reduced to 60°C in S1.

[0101] Example 6

[0102] This embodiment is the same as Embodiment 1, except that the water bath temperature is increased to 70°C in S1.

[0103] Example 7

[0104] This embodiment is the same as Embodiment 1, except that the water bath temperature is increased to 80°C in S1.

[0105] Example 8

[0106] This embodiment is the same as Embodiment 1, except that in S1, the water bath is heated for 4 hours.

[0107] Example 9

[0108] This embodiment is the same as Embodiment 1, except that in S1, the water bath is heated for 8 hours.

[0109] Example 10

[0110] This embodiment is the same as Embodiment 1, except that a 10% H2O2 solution is added to the municipal sludge containing microplastics.

[0111] Example 11

[0112] This embodiment is the same as Embodiment 1, except that a 20% H2O2 solution is added to the municipal sludge containing microplastics.

[0113] Example 12

[0114] This embodiment is the same as Embodiment 1, except that a 30% H2O2 solution is added to the municipal sludge containing microplastics.

[0115] Example 13

[0116] This embodiment is the same as embodiment 1, except that the water bath temperature is reduced to 30°C in S2.

[0117] Example 14

[0118] This embodiment is the same as Embodiment 1, except that the water bath temperature is increased to 50°C in S2.

[0119] Example 15

[0120] This embodiment is the same as Embodiment 1, except that the water bath temperature is increased to 60°C in S2.

[0121] Example 16

[0122] This embodiment is the same as Embodiment 1, except that in S2, the water bath is heated for 10 hours.

[0123] Example 17

[0124] This embodiment is the same as Embodiment 1, except that in S2, the water bath is heated for 14 hours.

[0125] Example 18

[0126] This embodiment is the same as Embodiment 1, except that water bath heating is performed for 22 hours in S2.

[0127] Example 19

[0128] This embodiment is the same as Embodiment 1, except that a 1% KOH solution is added to the municipal sludge containing microplastics.

[0129] Example 20

[0130] This embodiment is the same as Embodiment 1, except that a 3% KOH solution is added to the municipal sludge containing microplastics.

[0131] Example 21

[0132] This embodiment is the same as Embodiment 1, except that a 10% KOH solution is added to the municipal sludge containing microplastics.

[0133] Example 22

[0134] This embodiment is the same as Embodiment 1, except that the water bath temperature is reduced to 30°C in S3.

[0135] Example 23

[0136] This embodiment is the same as Embodiment 1, except that the water bath temperature is increased to 50°C in S3.

[0137] Example 24

[0138] This embodiment is the same as Embodiment 1, except that the water bath temperature is increased to 60°C in S3.

[0139] Example 25

[0140] This embodiment is the same as Embodiment 1, except that the water bath heating in S3 is carried out for 2 hours.

[0141] Example 26

[0142] This embodiment is the same as Embodiment 1, except that water bath heating is performed for 6 hours in S3.

[0143] Example 27

[0144] This embodiment is the same as Embodiment 1, except that water bath heating is performed for 8 hours in S3.

[0145] Example 28

[0146] This embodiment is the same as Embodiment 1, except that anaerobic municipal sludge is replaced with aerobic municipal sludge.

[0147] Example 29

[0148] This example is a blank group. This example is the same as Example 1, except that no PE standard is added and a stainless steel filter membrane (5μm pore size, 47mm diameter) is used.

[0149] Example 30

[0150] This embodiment is a blank group. This embodiment is the same as embodiment 29, except that the anaerobic municipal sludge is replaced with aerobic municipal sludge.

[0151] Comparative Example 1 (Direct Density Separation of Anaerobic Municipal Sludge)

[0152] Anaerobic municipal sludge was used as the experimental sample, with three replicate control groups for each experiment. 2g of wet municipal sludge was added to a beaker, along with PE (125-150μm) standard and saturated ZnCl2 solution. The beaker was shaken and agitated to mix thoroughly with the saturated ZnCl2 solution. After standing for 8 hours to allow solid-liquid separation, the supernatant was collected. This process was repeated five times. A 250W ultrasonic-assisted flotation was used during the process. The collected supernatant was filtered onto a stainless steel filter membrane (5μm, 47mm). The beaker and vacuum-filtered glassware were rinsed with ethanol solution to prevent microplastic adhesion and loss during the experimental process.

[0153] The microplastic content in the municipal sludge used in this comparative example was calculated using the method of Comparative Example 1.

[0154] Comparative Example 2 (S1+ density separation)

[0155] Same as in Example 1, except that S2+S3 will be omitted.

[0156] Comparative Example 3 (S1+S2+Density Separation)

[0157] Same as in Example 1, except that S3 will be omitted.

[0158] Comparative Example 4 (Direct Density Separation of Aerobic Municipal Sludge)

[0159] Similar to Comparative Example 1, except that anaerobic municipal sludge was replaced with aerobic municipal sludge.

[0160] Photos were taken of the PE standard samples before and after spiking in Example 1. The results are shown below. Figure 2 As can be seen, the plastic added in this invention is a standard red circular plastic, which helps to distinguish it from the interference of the original microplastics in the sludge on the experimental results.

[0161] The digestion and removal rates of each digestion step in Example 1 are shown in the figure. Figure 3 The PE recovery rates of Example a, Example 1, and Comparative Examples 1-3 are shown in Figure 1. Figure 3 b, by Figure 3It is known that multi-stage digestion technology can increase the efficiency of organic matter removal in sludge. As the efficiency of organic matter removal increases, it will also promote the release of microplastics from organic matter and flocs.

[0162] Examples 29-30 and Comparative Examples 1 and 4 were tested for the content, type, and particle size distribution of microplastics. Figure 4 This is a comparison chart of microplastic content. Figure 5 This is a comparison chart of different types of microplastics. Figure 6 This is a comparison chart of the proportion of microplastic particle sizes. Figure 7 This is a comparison chart of microplastic particle size distribution. Figures 4-7 It can be seen that, compared with undigested municipal sludge samples, microplastics were released and detected in large quantities after municipal sludge underwent multi-stage digestion treatment.

[0163] The PE recovery rates of Examples 1, 28, and 29 are shown in Table 1. It can be seen that the method of the present invention has a high recovery rate of microplastics in municipal sludge.

[0164] Table 1 PE Recovery Rate

[0165]

[0166] Using the same digestion scheme as in Example 1, 25 MPs (ABS, AS, COC, COP, EVA, HDPE, K-resin, LDPE, PA66, PBAT, PBS, PBT, PC, PVC, PET, PETG, PHA, PLA, PMMA, POE, POM, PP, PPS, PS, SPS) were tested. Phenomorphological photographs before and after each digestion step are shown below. Figures 8-11 , Figure 8 Micrographs of 25 different microplastics as is. Figure 9 These are micrographs of 25 microplastics after S1 digestion. Figure 10 These are micrographs of 25 microplastics after digestion using S1+S2. Figure 11 These are micrographs of 25 microplastics after digestion using the S1+S2+S3 method. Figures 8-11 The scale bars in the text are all 1 mm, which shows that the multi-stage digestion treatment does not have a significant impact on the surface structure of microplastics. Figures 12-17 The infrared spectra of 25 microplastics before and after digestion by S1+S2+S3 show that the functional groups on the surface of the microplastics did not change significantly after digestion.

[0167] Table 2 shows the percentage change in mass of 25 microplastics after different digestion steps. It can be seen that the digestion treatment has a relatively low impact on the weight of microplastics. Overall, the multi-stage digestion technology for municipal sludge does not have a significant impact on microplastics and does not interfere with the qualitative and quantitative results of microplastic analysis.

[0168] Table 225 Percentage Mass Changes of MPs After Different Digestion Steps (Mass Change Percentage)

[0169]

[0170]

[0171] Table 3 shows the mass removal rate (organic matter removal rate) of municipal sludge in Examples 1-28. It can be seen that the reaction conditions in Example 1 were optimal and can be considered a preferred case. 。

[0172] Table 3. Mass removal rate of municipal sludge in Examples 1-28

[0173] Example Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Quality removal rate / % 63.62 59.71 63.65 56.18 59.35 63.98 Example Example 7 Example 8 Example 9 Example 10 Example 11 Example 12 Quality removal rate / % 63.58 60.21 63.52 55.36 60.25 64.13 Example Example 13 Example 14 Example 15 Example 16 Example 17 Example 18 Quality removal rate / % 52.36 65.21 64.53 58.31 61.85 64.18 Example Example 19 Example 20 Example 21 Example 22 Example 23 Example 24 Quality removal rate / % 56.11 63.88 61.21 57.52 63.96 64.12 Example Example 25 Example 26 Example 27 Example 28 Quality removal rate / % 57.35 63.21 63.82 69.23

[0174] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for the extraction of microplastics in municipal sludge, characterized in that, The method comprises the following steps: mixing wet municipal sludge with a persulfate solution, and then sequentially performing first digestion and solid-liquid separation to obtain first digestion solid and first supernatant; the mass content of organic matter in the municipal sludge is 20-70% on a dry basis; mixing the first digestion solid with a hydrogen peroxide solution, and then sequentially performing second digestion and solid-liquid separation to obtain second digestion solid and second supernatant; mixing the second digestion solid with an alkaline solution, and then sequentially performing third digestion and solid-liquid separation to obtain third digestion solid and third supernatant; floating the third digestion solid to obtain supernatant after floating; mixing the first supernatant, the second supernatant, the third supernatant and the supernatant after floating, and then concentrating to obtain a microplastic extract.

2. The extraction method according to claim 1, characterized in that, The persulfate in the persulfate solution comprises K2S2O8.

3. The extraction method according to claim 1 or 2, characterized in that, The use amount ratio of the municipal sludge to the persulfate solution is 5g:10-70mL, and the concentration of the persulfate solution is 0.1-0.3mol / L.

4. The extraction method according to claim 3, characterized in that, The water content of the municipal sludge is 80-90wt%.

5. The extraction method of claim 1, wherein, The temperature of the first digestion is 65-80℃, and the time is 3-10h.

6. The extraction method of claim 1, wherein, The use amount of the hydrogen peroxide solution is 10-30mL, and the mass percentage concentration of the hydrogen peroxide solution is 10%-30% based on 5g of the first digestion solid generated from the municipal sludge.

7. The extraction method according to claim 1 or 6, characterized in that, The temperature of the second digestion is 30-60℃, and the time is 3-22h.

8. The extraction method of claim 1, wherein, The use amount of the alkaline solution is 10-30mL, and the mass percentage concentration of the alkaline solution is 1%-10% based on 5g of the second digestion solid generated from the municipal sludge.

9. The extraction method according to claim 1 or 6, characterized in that, The temperature of the third digestion is 35-60℃, and the time is 3-8h.

10. The extraction method of claim 1, wherein, The alkaline solution comprises KOH solution or NaOH solution.

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