Method for extracting and analyzing microplastics in activated sludge

Abstract

The invention discloses a method for extracting and analyzing microplastics in activated sludge, and belongs to the field of pretreatment and detection of environmental pollutant analysis. According to the technical scheme, the method comprises the steps of screening a micro-plastic extracting solution in an activated sludge sample and extracting and separating the micro-plastic, wherein the steps comprise density flotation extraction, centrifugation, supernate filtration, organic matter oxidation digestion, micro-plastic refiltration, reflotation extraction and collection and micro-plastic analysis. The pH value of the extracting solution is alkaline, the extracting solution does not react with active sludge components, and the problems that a large number of bubbles generated during flotation of a conventional flotation solution are difficult to eliminate for a long time, exothermic reaction occurs, the temperature of the solution is rapidly increased, and micro-plastic deforms and deteriorates are solved; the used filter membrane can be used for directly performing spectrum-mass spectrum qualitative and quantitative analysis without spectrum absorption, so that the miss detection phenomenon of incomplete transfer possibly caused by transfer of the micro plastic particles is avoided; the wet activated sludge sample is directly extracted and separated, so that the efficiency is high.

Description

Technical Field

[0001] The invention discloses a method for extracting and analyzing microplastics in activated sludge, and belongs to the field of pre-analysis treatment and detection of environmental pollutants. Background Art

[0002] At present, a large number of studies on microplastics have been carried out at home and abroad, mainly focusing on the situation of microplastic pollution in natural environments such as oceans, freshwater environments and agricultural land ecosystems. For liquid samples, the main reference is to the research on microplastics in oceans or freshwater, while the research on solid sample microplastics mainly refers to the research on microplastics in marine sediments or soil. This technology is based on the use of separation solutions to extract relatively light artificial particles from heavier sediment particles. At present, the flotation agents used to separate and extract microplastics are mainly: NaCl, ZnCl2, NaI, CaCl2, etc. NaCl is cheap and non-toxic, and its saturated solution (density 1.2 g / cm 3 ) is the most commonly used density flotation agent. However, some high-density microplastics such as PET (1.3~1.4 g / cm 3 ) cannot be floated out by saturated NaCl solution, resulting in a low total count of microplastics. NaI (density 1.6-1.8 g / cm 3 ) and ZnCl2 (1.5~1.7 g / cm 3 ) saturated solution is a density flotation agent that has been widely used in recent years. Its density is higher than that of saturated NaCl solution, and it has a better flotation effect on microplastics. In addition, CaCl2 solution and ZnBr2 solution have also been proposed as density flotation agents. Among the above four solutions, NaI, ZnCl2 and ZnBr2 are all toxic to a certain extent, and CaCl2 is toxic to microplastics. 2+ This can cause organic materials to aggregate, interfering with microplastic detection, raising safety concerns and potential environmental pollution. The National Oceanic and Atmospheric Administration (NOAA) laboratory's analytical method for extracting and characterizing microplastics from seawater and seafloor sediment samples in marine environments uses lithium metatungstate as a flotation agent. However, lithium metatungstate is relatively expensive. Therefore, developing a flotation agent with sufficient density, low toxicity, and low cost is key to microplastic density flotation.

[0003] Furthermore, conventional flotation methods and extraction procedures cannot achieve rapid and effective extraction and separation of microplastics in complex matrix systems such as activated sludge, and also increase the difficulty of subsequent analysis of microplastics. Therefore, the present invention has developed a method for the extraction and analysis of microplastics in activated sludge. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides an extract of microplastics in activated sludge and an extraction, separation and analysis method, which can extract and analyze microplastics in activated sludge simply, quickly, efficiently and accurately. To achieve the purpose of the present invention, the technical solution of the present invention is: the screening of microplastic extracts in activated sludge samples and the extraction and separation of microplastics steps include density flotation extraction, centrifugation, supernatant filtration, organic matter oxidation digestion, re-filtration of microplastics, re-flotation extraction and collection, and analysis of microplastics. The specific steps are as follows: (1) Density flotation extraction, separation and analysis of microplastics: The activated sludge mixture is filtered onto a 500-mesh stainless steel filter membrane for preliminary dehydration to obtain an activated sludge wet sample. One portion is dried in an oven at 60°C to constant weight and the moisture content of the activated sludge wet sample is calculated. The other portion is directly used for the extraction, separation and analysis of microplastics. If the activated sludge wet sample has been preliminarily dehydrated by an activated sludge filter press, it can be sampled directly. One portion is dried in an oven at 60°C to constant weight and the moisture content of the activated sludge wet sample is calculated. The other portion is directly used for the extraction and separation of microplastics. Weigh an appropriate amount of activated sludge wet sample, add the extract according to a certain volume ratio, stir with a magnetic stirrer for 30 minutes, and then centrifuge in a centrifuge. Filter the supernatant after centrifugation onto a glass cellulose filter membrane. Repeat the aforementioned extraction steps, and combine the supernatants after the second centrifugation and filter them onto the aforementioned glass cellulose filter membrane.

[0005] (2) Oxidative digestion of organic matter: Place the filter membrane that collected microplastics in step (1) in a beaker and add Fenton's reagent or tetramethylammonium hydroxide solution for digestion. The digestion temperature is 30~60℃ and the digestion time is 2~4 h.

[0006] (3) Re-flotation centrifugal filtration: The solution after the oxidation and digestion of organic matter in step (2) is re-filtered onto the original glass cellulose filter membrane, the filter membrane is soaked and rinsed with the extract, the microplastics on the membrane are transferred to a centrifuge tube for centrifugation, and the supernatant is collected on a ceramic filter membrane or a gold filter membrane after centrifugation. The ceramic filter membrane or the gold filter membrane is then placed in a clean and dry culture dish and dried at room temperature to obtain the separated and extracted microplastics.

[0007] (4) Qualitative and quantitative analysis of microplastics: According to the moisture content of activated sludge, the content of microplastics in activated sludge on a dry weight basis is calculated. The microplastic content w is calculated as follows: , where n is the number of microplastics, in pieces; m is the mass of the activated sludge wet sample, in g; w2 is the moisture content of the activated sludge wet sample, in mass %.

[0008] Preferably, in step (1), the extract is a saturated solution prepared by mixing one or more of potassium pyrophosphate, potassium tripolyphosphate, and dipotassium hydrogen phosphate, and the density of the extract is in the range of 1.54-1.65 g / cm 3 .

[0009] Preferably, in step (1), the activated sludge wet sample is weighed in an amount of 10 to 100 g.

[0010] Preferably, in step (1), the volume ratio of the activated sludge wet sample to the extract is 1:1 to 1:2.

[0011] Preferably, in step (1), the pore size of the glass cellulose filter membrane is 0.45 µm.

[0012] Preferably, a set of blank controls is set up for each experiment to verify the interference of the experimental operation process and possible microplastic pollution in the air on the measurement results.

[0013] The beneficial effects of the present invention are: (1) The extracting solution for microplastics in activated sludge proposed by the present invention is a saturated solution prepared by mixing one or more of potassium pyrophosphate, potassium tripolyphosphate, and dipotassium hydrogen phosphate. The density of the solution is greater than the density of most plastics, and the microplastics can be completely extracted. In view of the large amount of carbonate contained in activated sludge, the pH value of the extracting solution of the present invention is alkaline, and the extracting solution does not react with the activated sludge components, thus avoiding the large amount of bubbles generated during flotation with conventional flotation solutions, which are difficult to eliminate for a long time. Figure 1 As shown, an exothermic reaction occurs, causing the solution temperature to rise rapidly, causing the microplastics to deform and deteriorate.

[0014] (2) After digesting the organic matter, the filter membrane that collected the microplastics is re-extracted and centrifuged, and the microplastics are collected on a ceramic filter membrane or a gold filter membrane. This method can effectively remove the precipitation and residual carbon generated during the digestion process, and obtain a microplastic filter membrane sample with a relatively pure base. This greatly reduces the base interference during microscopic examination of microplastics and subsequent qualitative and quantitative analysis by spectroscopy and mass spectrometry. The ceramic filter membrane and gold filter membrane used can be directly used for qualitative and quantitative analysis by spectroscopy and mass spectrometry, and there is no spectral absorption, which avoids the phenomenon of incomplete transfer and missed detection that may occur due to the transfer of microplastic particles.

[0015] (3) The method for extracting and separating microplastics from activated sludge described in the present invention uses wet activated sludge samples for direct extraction and separation. Compared with the conventional method of air-drying the soil or sediment before flotation, it effectively avoids the problem of severe solidification after air-drying due to the high organic matter content of the activated sludge and the inability to disperse it after long-term soaking and stirring. It greatly shortens the analysis time and significantly improves the efficiency of microplastic extraction and separation. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Flotation extraction pictures of conventional flotation solution and the extracting solution of the present invention Figure 2 Coordinate microscopic images of microplastic particles at 40x and 100x magnification Figure 3 Typical microplastic Fourier transform microinfrared spectrum Figure 4 Typical microplastic pyrolysis-gas chromatography / mass spectrometry DETAILED DESCRIPTION

[0017] The present invention will be further described in detail below with reference to specific examples, but the present invention is not limited thereto. Example 1

[0018] The activated sludge from the biochemical section of the landfill leachate treatment plant in a certain city was selected to extract and analyze the microplastics. The specific steps are as follows: (1) Sampling A landfill leachate was selected to treat the activated sludge in the biochemical section, and a stainless steel barrel was used for sampling. The activated sludge mixture was filtered onto a 500-mesh stainless steel filter membrane for preliminary dehydration to obtain a wet sample of activated sludge. The sample was sealed in an aluminum foil bag and transported back to the laboratory for storage at low temperature in a refrigerator.

[0019] (2) Extraction and separation The above activated sludge wet sample was placed in an oven at 60°C and dried to constant weight, and the moisture content of the activated sludge wet sample was calculated. The other sample was directly used for the extraction and separation of microplastics. Weigh 50 g of activated sludge wet sample, add 100 mL of saturated potassium pyrophosphate solution as the extract, stir with a magnetic stirrer for 30 minutes, and centrifuge at 5000 rpm for 5 minutes on a centrifuge. The supernatant after centrifugation was filtered onto a glass cellulose filter membrane with a pore size of 0.45 µm. Repeat the above extraction steps and extract once more. The supernatant after the second centrifugation was combined and filtered onto the above glass cellulose filter membrane.

[0020] (3) Oxidation and digestion of organic matter The filter membrane that collected the microplastics was placed in a beaker and Fenton's reagent was added for digestion at a temperature of 50°C and a digestion time of 4 h.

[0021] (4) Re-extraction The solution after the above organic matter oxidation digestion step was re-filtered onto the original glass cellulose filter membrane, and the filter membrane was soaked and rinsed with a saturated potassium pyrophosphate solution as the extracting solution. The microplastics on the membrane were transferred to a centrifuge tube for centrifugation. After centrifugation, the supernatant was collected on a ceramic filter membrane, and the filter membrane was placed in a clean and dry culture dish and dried at room temperature to obtain the separated and extracted microplastics. Microplastics were observed, identified, photographed, and counted using a stereo microscope. Figure 2 , the number of microplastics was 32. Combined with the experimental measurement, the moisture content of activated sludge was 75.5%. The microplastic content w was calculated as follows: , where n is the number of microplastics, in pieces; m is the mass of the wet sample of activated sludge, in g; w2 is the moisture content of the wet sample of activated sludge, in mass %. The abundance of microplastics in activated sludge is 2612 pieces / kg dry weight.

[0022] Eliminate pollution interference Without adding the activated sludge wet sample, the above steps from (2) to (4) were repeated to obtain the microplastic abundance under blank conditions. This can eliminate the interference of the experimental operation process and possible microplastic pollution in the air on the measurement results. Example 2

[0023] Select the activated sludge from the biochemical section of a municipal sewage treatment plant and extract and analyze the microplastics therein. The specific steps are as follows: (1) Sampling Activated sludge from the biochemical section of a municipal sewage treatment plant was selected. A stainless steel shovel was used to collect wet samples of activated sludge that had been filtered through an activated sludge filter press. The samples were sealed in aluminum foil bags and transported back to the laboratory for low-temperature storage in a refrigerator.

[0024] (2) Flotation extraction The activated sludge sample was divided into two parts. One part was dried in an oven at 60°C to constant weight and the moisture content of the activated sludge wet sample was calculated. The other part was directly used for the extraction and separation of microplastics. Weigh 25 g of activated sludge wet sample, add 40 mL of a saturated solution of potassium pyrophosphate and potassium tripolyphosphate in a mass ratio of 2:1, stir with a magnetic stirrer for 30 minutes, and centrifuge at 5000 rpm for 5 minutes. The supernatant after centrifugation was filtered onto a glass cellulose filter with a pore size of 0.45 µm. Repeat the above extraction steps and extract once more. The supernatant after the second centrifugation was combined and filtered onto the above glass cellulose filter.

[0025] (3) Oxidation and digestion of organic matter The filter membrane that collected the microplastics was placed in a beaker and Fenton's reagent was added for digestion at a temperature of 60°C and a digestion time of 3 h.

[0026] (4) Re-extraction The solution after the organic matter oxidation and digestion step was re-filtered onto the original glass cellulose filter membrane, which was then soaked and rinsed with the extract. The microplastics on the membrane were transferred to a centrifuge tube and centrifuged. After centrifugation, the supernatant was collected onto a gold filter membrane. The filter membrane was then placed in a clean, dry petri dish and air-dried at room temperature to obtain the separated and extracted microplastics. The microplastics were observed, identified, photographed, and counted using a stereo microscope. The number of microplastics was 43. Combined with the experimental measurement, the moisture content of the activated sludge was 67.8%. The microplastic content w was calculated as follows: , where n is the number of microplastics, in units of pieces; m is the mass of the wet activated sludge sample, in g; and w2 is the moisture content of the wet activated sludge sample, in mass %. The abundance of microplastics in the activated sludge was 5342 pieces / kg dry weight. Fourier transform infrared spectroscopy was used to characterize the microplastics, and the main types of microplastics were polyethylene, polypropylene, and their copolymers. The corresponding typical microplastic infrared spectra are shown in Figure 3 . Example 3

[0027] Select the activated sludge from the biochemical section of a municipal sewage treatment plant and extract and analyze the microplastics therein. The specific steps are as follows: (1) Sampling Activated sludge from the biochemical section of a municipal sewage treatment plant was selected. A stainless steel shovel was used to collect wet samples of activated sludge that had been filtered through an activated sludge filter press. The samples were sealed in aluminum foil bags and transported back to the laboratory for low-temperature storage in a refrigerator.

[0028] (2) Flotation extraction The activated sludge sample was divided into two parts. One part was dried in an oven at 60°C to constant weight and the moisture content of the activated sludge wet sample was calculated. The other part was directly used for the extraction and separation of microplastics. Weigh 10 g of activated sludge wet sample, add 40 mL of a saturated solution of potassium pyrophosphate, potassium tripolyphosphate, and dipotassium hydrogen phosphate in a mass ratio of 1:1:1, stir with a magnetic stirrer for 30 minutes, and centrifuge at 5000 rpm for 5 minutes. The supernatant after centrifugation was filtered onto a glass cellulose filter with a pore size of 0.45 μm. Repeat the above extraction steps and extract once more. The supernatant after the second centrifugation was combined and filtered onto the above glass cellulose filter.

[0029] (3) Oxidation and digestion of organic matter The filter membrane that collected the microplastics was placed in a beaker and tetramethylammonium hydroxide solution was added for digestion. The digestion temperature was 60°C and the digestion time was 4 h.

[0030] (4) Re-extraction The solution after the above organic matter oxidation digestion step is re-filtered onto the original glass cellulose filter membrane, and the filter membrane is soaked and rinsed with the extraction solution. The microplastics on the membrane are transferred to a centrifuge tube for centrifugation. After centrifugation, the supernatant is collected on the gold filter membrane, and the filter membrane is placed in a clean and dry culture dish and dried at room temperature to obtain the separated and extracted microplastics. The microplastics were analyzed using pyrolysis-gas chromatography / mass spectrometry, and the main types of microplastics were polyethylene, polymethyl acrylate, and polypropylene. The corresponding typical microplastic pyrolysis-gas chromatography / mass spectrometry is shown in Figure 4 .

Claims

1. A method for extracting and analyzing microplastics in activated sludge, characterized in that Here are the steps: (1) Density flotation extraction and separation analysis of microplastics: Take one portion of the wet activated sludge sample and dry it in an oven at 60°C to constant weight to calculate the moisture content of the wet activated sludge sample. The other portion is directly used for the extraction and separation of microplastics. Weigh an appropriate amount of the wet activated sludge sample, add the extract according to a certain volume ratio, stir it with a magnetic stirrer for 30 minutes, and then centrifuge it in a centrifuge. Filter the supernatant after centrifugation onto a glass cellulose filter membrane. Repeat the aforementioned extraction steps, and combine the supernatants after the second centrifugation and filter them onto the glass cellulose filter membrane. (2) Oxidative digestion of organic matter: Place the glass cellulose filter membrane that collected the microplastics in step (1) in a beaker, add Fenton's reagent or tetramethylammonium hydroxide solution for digestion, the digestion temperature is 30-60°C, and the digestion time is 2-4 hours; (3) Re-flotation centrifugal filtration: The solution after the oxidative digestion of organic matter in step (2) is re-filtered onto the glass cellulose filter membrane, the filter membrane is soaked and rinsed with the extracting solution, the microplastics on the membrane are transferred to a centrifuge tube for centrifugation, and the supernatant is collected onto a ceramic filter membrane or a gold filter membrane after centrifugation. The ceramic filter membrane or the gold filter membrane is then placed in a clean and dry culture dish and dried at room temperature to obtain the separated and extracted microplastics; (4) Qualitative and quantitative analysis of microplastics: According to the moisture content of activated sludge, the content of microplastics in activated sludge on a dry weight basis is calculated. The microplastic content w is calculated according to the following formula: , where n is the number of microplastics, in pieces; m is the weight of the activated sludge wet sample, in g; w2 is the moisture content of activated sludge wet sample, expressed in mass %.

2. The method for extracting and analyzing microplastics in activated sludge according to claim 1, characterized in that: In step (1), the extract is a saturated solution prepared by mixing one or more of potassium pyrophosphate, potassium tripolyphosphate, and dipotassium hydrogen phosphate.

3. The method for extracting and analyzing microplastics in activated sludge according to claim 2, characterized in that: The density of the extract is in the range of 1.54-1.65 g / cm 3 .

4. The method for extracting and analyzing microplastics in activated sludge according to claim 1, characterized in that: In step (1), the activated sludge wet sample is weighed in an amount of 10 to 100 g.

5. The method for extracting and analyzing microplastics in activated sludge according to claim 1, characterized in that: In step (1), the volume ratio of the activated sludge wet sample to the extract is 1:1 to 1:

2.

6. The method for extracting and analyzing microplastics in activated sludge according to claim 1, characterized in that: In step (1), the pore size of the glass cellulose filter membrane is 0.45 µm.

7. The method for extracting and analyzing microplastics in activated sludge according to claim 1, characterized in that: A set of blank controls is set up for each experiment to verify the interference of the experimental operation process and possible microplastic pollution in the air on the measurement results.

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