Extraction method and detection method of micro-plastics in saline-alkali soil

Abstract

The invention relates to the technical field of soil detection, in particular to an extraction method and a detection method of micro-plastics in saline-alkali soil. The extraction method comprises the following steps: (1) mixing a soil sample with a flotation solution, standing for 4-6 hours, and taking an upper solution to obtain a flotation product; (2) mixing the flotation product with a flotation solution, and carrying out flotation again for 1-2 times according to the method in the step (1) to obtain a solution carrying the micro-plastics; (3) filtering the solution obtained in the step (2), mixing substances intercepted on the filter membrane with a hydrogen peroxide solution, and standing for 2-3 days to obtain a digestion solution; and (4) filtering the digestion solution, taking substances intercepted on the filter membrane, and extracting to obtain the microplastics. The extracted micro-plastics are comprehensively analyzed, the distribution characteristics, the type composition and the content level of the micro-plastics can be obtained, and important information is provided for accurately mastering the pollution degree of the micro-plastics in farmland soil and potential influences of the micro-plastics.

Description

Technical Field

[0001] The present invention relates to the technical field of soil detection, and in particular to a method for extracting and detecting microplastics in saline-alkali soil. Background Art

[0002] The accumulation of microplastics in soil has been shown to change soil properties in multiple dimensions, including soil pH, electrical conductivity, organic matter content, and nutrient availability. Experimental observations show that the addition of high-density polyethylene (HDPE) tends to lower soil pH, while polylactic acid (PLA) has the opposite effect, increasing pH and reducing electrical conductivity. At the same time, the effect of polystyrene (PS) on soil pH is less obvious. High concentrations of microplastics may inhibit the decomposition of dissolved organic matter while increasing the nutrient content in the soil. However, the interaction between microplastics and pesticides such as glyphosate may cause the loss of dissolved organic carbon and phosphorus, thereby reducing nutrients in the soil.

[0003] The type and concentration of microplastics significantly affected the physical properties of the soil. Larger microplastic particles affected soil structure and compactness through adsorption. Specifically, the addition of ultrafine polyester fibers promoted the aggregation of soil particles, thereby increasing soil porosity, reducing its water holding capacity, and promoting water loss. In contrast, the incorporation of polypropylene and polyester fibers was found to reduce the water-stable aggregates of the soil, thereby deteriorating the physical structure of the soil. These changes may affect the composition of soil microorganisms, including bacteria, fungi, and protists, and have a chain effect on the interaction between soil and plants. Therefore, microplastics have a certain impact on the ecological balance of the soil.

[0004] Based on this, the present invention is proposed. Summary of the invention

[0005] The purpose of the present invention is to provide a method for extracting and detecting microplastics in saline-alkali soil.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for extracting microplastics from saline-alkali soil, comprising the following steps:

[0008] (1) Mix the soil sample with the flotation solution, let it stand for 4 to 6 hours, and take the upper solution to obtain the flotation product;

[0009] (2) mixing the flotation product with a flotation solution, and flotating again for 1 to 2 times according to the method described in step (1) to obtain a solution carrying microplastics;

[0010] (3) filtering the solution obtained in step (2), taking the substance retained on the filter membrane and mixing it with the hydrogen peroxide solution, and letting it stand for 2 to 3 days to obtain a digestion solution;

[0011] (4) Filter the digestion solution, take the substances retained on the filter membrane, and extract the microplastics.

[0012] Preferably, the mass volume ratio of the soil sample and the flotation solution mixed in step (1) is 1 g: 2-4 mL;

[0013] The flotation solution is a sodium chloride solution or a zinc chloride solution;

[0014] The sodium chloride solution is a saturated sodium chloride solution;

[0015] The initial concentration of the zinc chloride solution is 1.7-1.8 g / mL.

[0016] Preferably, the filter membrane used in step (3) is a steel membrane or a polytetrafluoroethylene membrane, and the pore size of the filter membrane is ≤20 μm;

[0017] The initial concentration of the hydrogen peroxide solution in step (3) is 28-32 vt%;

[0018] The mass volume ratio of the soil sample and the hydrogen peroxide solution is 1 g: 0.5-1.0 mL.

[0019] Preferably, the filter membrane used to filter the digestion solution in step (4) is a steel membrane or a polytetrafluoroethylene membrane, and the pore size of the filter membrane is 10 to 500 μm.

[0020] The present invention also provides application of the extraction method in preparing a product for detecting microplastics in saline-alkali soil.

[0021] The present invention also provides a method for detecting microplastics in saline-alkali soil, comprising the following steps: extracting microplastics in the soil according to the extraction method; detecting the morphological characteristics of the microplastics by a scanning electron microscope and detecting the chemical functional groups of the microplastics by a Fourier transform infrared spectrometer.

[0022] The present invention also provides the application of the method in detecting the morphological characteristics and chemical functional groups of microplastics in saline-alkali soil.

[0023] The present invention provides a method for extracting and detecting microplastics in saline-alkali soil. The extraction method of the present invention comprises the following steps: (1) mixing a soil sample with a flotation solution, standing for 4 to 6 hours, taking an upper solution, and obtaining a flotation product; (2) mixing the flotation product with the flotation solution, and flotating again for 1 to 2 times according to the method described in step (1) to obtain a solution carrying microplastics; (3) filtering the solution obtained in step (2), taking the substance retained on the filter membrane and mixing it with a hydrogen peroxide solution, standing for 2 to 3 days, and obtaining a digestion solution; (4) filtering the digestion solution, taking the substance retained on the filter membrane, and extracting microplastics. A comprehensive analysis of the extracted microplastics can obtain the distribution characteristics, type composition, and content level of the microplastics, and a comprehensive assessment of the current status of microplastic pollution can be made through experimental data. This method will provide important information for accurately grasping the extent of microplastic pollution in farmland soil and its potential impact, and thus provide a solid theoretical basis for specifying targeted governance strategies and scientific decision-making. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the extraction process of microplastics;

[0025] Figure 2 is the horizontal abundance distribution of microplastics;

[0026] Figure 3 is the abundance distribution of microplastic profiles;

[0027] Figure 4 is the average size distribution of microplastic levels;

[0028] Figure 5 is the average size distribution of microplastic profile;

[0029] Figure 6 This is the size distribution of soil microplastics in the Hetao irrigation area;

[0030] Figure 7 This is the distribution of soil microplastic profiles in the Hetao irrigation area;

[0031] Figure 8 This is the color distribution of soil microplastic profiles in the Hetao irrigation area;

[0032] Fig. 9 Distribution of microplastic profile types for six rows. DETAILED DESCRIPTION

[0033] In the present invention, when filtering the digestion solution, filter membranes with different pore sizes are used for filtration. First, the filtrate is filtered through a 500 μm filter membrane, and then the filtrate is filtered through a 10 μm filter membrane. In this way, microplastics of different particle sizes can be obtained.

[0034] After filtering with a 10 μm filter membrane, use a saturated NaCl solution to rinse the retained substances on the filter membrane into a conical flask, add saturated NaCl solution close to the mouth of the conical flask, let it stand for 12 hours, use a glass funnel filter device (purchased from Beijing Boyuan Hongda Biotechnology Co., Ltd.), and let the supernatant flow from the end of the funnel into a new beaker. Use ultrapure water to rinse the device, and perform three flotation operations in total. The liquid obtained after three flotations is pure small-particle microplastics.

[0035] The solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be understood as limiting the protection scope of the present invention.

[0036] In the embodiment of the present invention, Dengkou County, Wuyuan County and Urad Front Banner in the Hetao Irrigation District of Inner Mongolia Autonomous Region (106°33′—109°84′E, 40°08′—41°34′N) were selected. The soil in this area is saline-alkali land, and the first, second, third, fifth, sixth, eighth, ninth and tenth drainage areas of the main drainage canals in the Hetao Irrigation District were used as sampling areas.

[0037] The selection of each point took into account many factors, including geological conditions, years of film covering, geographical location, crop types, and salinity. They include the middle reaches of the first row of main streams (numbered 1-2), the lower reaches of the first row of main streams (numbered 1-3), the middle reaches of the second row of main streams (numbered 2-2), the upper reaches of the third row of main streams (numbered 3-1), the lower reaches of the fifth row of main streams (numbered 5-3), the lower reaches of the sixth row of main streams (numbered 6-3), the middle reaches of the eighth row of main streams (numbered 8-2), the upper reaches of the ninth row of main streams (numbered 9-1), the lower reaches of the ninth row of main streams (numbered 9-3), and the lower reaches of the tenth row of main streams (numbered 10-3). Additional profile samples were collected from four points, namely the middle reaches of the first row of main streams, the lower reaches of the fifth row of main streams, the lower reaches of the sixth row of main streams, and the lower reaches of the ninth row of main streams.

[0038] The sampling depth was 1m, and it was divided into three layers vertically, with a surface layer of 0-20cm, a middle layer of 20-60cm, and a deep layer of 60-100cm. Three replicate samples were collected at each sampling point, and the samples were placed in an aluminum box, quickly transported back to the laboratory, and stored at low temperatures. The soil samples transported back to the laboratory were placed in a 60°C oven to dry to constant weight, ground in a mortar to loosen the soil, and then passed through a 5mm stainless steel sieve to obtain soil samples.

[0039] Example 1

[0040] Take 20g of soil sample and place it in a beaker. After adding 80mL of saturated NaCl solution, stir it with a magnetic stirrer for 30min, then let it stand for 5h, and filter the upper solution into a new beaker to obtain the flotation product. Add 80mL of saturated NaCl solution to the flotation product again, stir it with a magnetic stirrer for 30min, then let it stand for 5h, and filter the upper solution into a new beaker to obtain a solution carrying microplastics. Use a 10μm steel membrane to filter the solution carrying microplastics, take the material retained on the filter membrane and mix it with 15mL of 30vt% hydrogen peroxide solution, let it stand for 3d to digest, remove organic impurities, and obtain the digestion solution after digestion. First filter the digestion solution with a 500μm steel membrane, remove the material on the 500μm filter membrane, and obtain microplastics. The filtrate obtained by membrane filtration is filtered again using a 10μm steel membrane, the substance on the filter membrane is taken, and it is rinsed into a triangular flask with a saturated NaCl solution, and the saturated NaCl solution is added to 80mL. After magnetic stirring for 30min, it is allowed to stand for 12h, and the supernatant is filtered into a new beaker using a glass funnel filter device, and the device is rinsed with ultrapure water to complete the flotation process of small-particle microplastics. The flotation step is repeated twice to obtain a pure solution of small-particle microplastics. The pure small-particle microplastic solution is passed through a 10μm steel membrane, the filter membrane is placed in a coarse test tube, anhydrous ethanol is added for ultrasonication for 30min, the filter membrane is removed, and the filter membrane is washed with anhydrous ethanol to obtain an alcohol solution containing the target substance, and the alcohol solution containing the target substance is concentrated to 100μL on a heating plate to obtain an alcohol solution of small-particle microplastics.

[0041] The specific flow chart is as follows Figure 1 shown.

[0042] Example 2

[0043] Analysis of soil microplastic abundance

[0044] Under a microscope, toothless stainless steel tweezers and dissecting needles were used to select microplastics, and they were placed on a mixed cellulose mesh filter membrane, marked, and the abundance of microplastics was counted. The average abundance of microplastics in the surface soil of each drainage in the Hetao Irrigation District was 323.64 pieces / kg, among which the surface soil of the fifth drainage had the highest microplastic content, which was 403.73 pieces / kg. The surface soil of the ninth and tenth drainages had significantly lower microplastic abundance, among which the surface soil of the tenth drainage without agricultural film had the lowest microplastic abundance, which was 140.00 pieces / kg ( Figure 2 ). It is speculated that the abundance of soil microplastics is related to whether there is agricultural film covering.

[0045] Analysis of the soil in the drainage section of the Hetao Irrigation Area found that the distribution of microplastics varies with soil depth. In the first drainage, the abundance of microplastics was highest in the top layer of soil at 0 to 20 cm (344.85 pieces / kg), and gradually decreased with the increase of soil depth. The abundance of microplastics reached the lowest in the soil at 60 to 80 cm (191.62 pieces / kg), and in the soil at 80 to 100 cm, the abundance of microplastics rebounded to a certain extent (243.97 pieces / kg). The abundance of microplastics showed a trend of "first decreasing and then increasing" with depth. The soil in the fifth and sixth rows also conformed to this trend, reaching the lowest value of microplastic abundance in the soil at 40-60 cm (277.19 pieces / kg and 136.85 pieces / kg). The highest value of microplastic abundance in the fifth row appeared at 0-20 cm (403.63 pieces / kg), and the highest value of microplastic abundance in the sixth row appeared at 80-100 cm (419.22 pieces / kg). The distribution of microplastic abundance in the ninth row showed another pattern, with the highest abundance of microplastics in the soil at 20-40 cm (478.69 pieces / kg) and the lowest abundance of microplastics in the soil at 40-60 cm (182.06 pieces / kg), showing no obvious trend of change ( Figure 3 ).

[0046] It can be observed that the abundance of microplastics in the surface soil is relatively high, and it is speculated that the use of agricultural film is the cause of this phenomenon; in addition, it is observed that the abundance of microplastics at some depths changes suddenly, especially between 20-40cm and 40-60cm in the ninth row of the river, where the concentration of microplastics drops by 62%, which may be caused by the absorption of microplastics by plant roots. The abundance distribution of microplastics in the profile is also closely related to local agricultural activities, and the situation is relatively complicated.

[0047] Example 3

[0048] Analysis of physical characteristics of soil microplastics

[0049] The microplastic samples are evenly distributed on a single layer plane, and the longest diameter is determined by taking photos. The Gilent 8700LDIR laser infrared imaging system is used to accurately measure and record the size of the microplastics.

[0050] The average size of microplastics in the surface soil of each drainage in the Hetao Irrigation District is 0.456 mm. The average size of microplastics in the middle reaches of the eighth drainage and the upper reaches of the ninth drainage is higher. The average size of microplastics in the surface soil of the upper reaches of the ninth drainage is the highest, which is 0.646 mm, and the average size of microplastics in the soil of the lower reaches of the ninth drainage is the lowest, which is 0.266 mm. The large difference in the average size of microplastics in the upstream and downstream of the same drainage may indicate that the source of microplastics in farmland has nothing to do with the adjacent river, resulting in huge differences in the size of microplastics in farmland adjacent to the same river ( Figure 4 ).

[0051] Analysis of the soil in the drainage profile of the Hetao Irrigation District found that the average particle size of microplastics in the soil of the first drainage was relatively low at 0-20 cm (0.486 mm), then gradually increased, reaching a maximum value (0.624 mm) at 40-60 cm, and then gradually decreased, reaching a minimum value (0.462 mm) at 80-100 cm, showing an overall trend of "first increasing and then decreasing"; the ninth drainage showed the same trend, reaching a minimum value (0.266 mm) at 0-20 cm and a maximum value (0.646 mm) at 40-60 cm. However, the average particle size distribution of microplastics in the fifth and sixth rows of soil showed an irregular distribution. The average particle size of microplastics in the fifth row of soil reached the minimum value (0.248mm) at 60-80cm, which was also the highest average particle size of microplastics in all samples, and reached the highest value (0.490mm) at 40-60cm; the average particle size of microplastics in the sixth row of soil reached the minimum value (0.389mm) at 80-100cm and the highest value (0.600mm) at 20-40cm ( Figure 5 ). This complex distribution pattern may mean that the average particle size of microplastics is determined by multiple factors, rather than a simple linear change caused by a single determining factor.

[0052] The soil of the drainage section of the Hetao Irrigation Area was analyzed and counted. Microplastics with a size of 0.5-1mm were the most abundant, accounting for 33.4%; followed by microplastics of 0.10-0.30mm, accounting for 30.8%; microplastics of 0.30-0.50mm and 0.005-0.010mm were less identified, accounting for 19.3% and 11.5%; microplastics <0.005mm were even less, accounting for 3.9%; the smallest were 2.00-1.00mm and >2.00mm, accounting for 0.6% and 0.5% ( Figure 6 ). It can be seen that the vast majority of microplastics are between 0.005 and 1.00 mm in size (95%), and there are far fewer microplastics of extreme sizes. It can be seen that small-sized microplastics account for a relatively higher proportion in deeper soils, which may be because microplastics in deeper soils tend to undergo a longer and more complete fragmentation and decomposition process. In general, the size distribution of microplastics is relatively stable at all depths, and the composition ratio has not shown drastic changes, which may mean that the decomposition process of microplastics in the soil is slow.

[0053] Example 4

[0054] Shape distribution of soil microplastics

[0055] The soil in the drainage section of the Hetao Irrigation Area was analyzed and counted. The most fibrous microplastics accounted for more than half, accounting for 53.0%, followed by film microplastics, accounting for 21.6%; fragmented, oval and rod-shaped microplastics accounted for less than 10%, respectively 8.2%, 6.7% and 6.5%; foam microplastics accounted for only 3.4%, and microbead microplastics accounted for the least, accounting for 0.6%. At different depths, the shape distribution of microplastics did not change significantly, and still maintained a similar proportion. ( Figure 7 ).

[0056] Example 5

[0057] Color distribution of soil microplastics

[0058] The soil in the drainage section of the Hetao Irrigation Area was analyzed and counted. The content of blue microplastics was the highest, reaching 28.0%; white microplastics were slightly less, accounting for 26.9%; transparent microplastics and red microplastics were next, accounting for 14.0% and 12.9%; black microplastics, green microplastics, and yellow microplastics all accounted for less than 10%, accounting for 9.7%, 5.8%, and 2.6% respectively ( Figure 8 The color distribution of microplastics on the cross-section did not show an obvious trend of change, but the color proportion fluctuated greatly. This may be because the color proportion of microplastics is easily affected by accidental factors. A piece of plastic waste of a specific color can significantly change the color proportion of microplastics in the area.

[0059] Example 6

[0060] Analysis of soil microplastic types

[0061] Fourier transform infrared spectroscopy (FTIR) was performed in reflection / transmission mode with a mercury cadmium telluride (MCT) detector. The wavelength range was set between 4000 and 750 cm -1 , with 4.00cm -1 The measurement was performed with a total of 16 scans. The size of the grating and the number of scans were adjusted appropriately according to the size of the sample.

[0062] For particles with a diameter less than or equal to 500μm, microscopic infrared spectroscopy (μ-FTIR) was used in transmission mode for detailed chemical analysis. For particles with a diameter of more than 500μm and up to 5000μm, Fourier transform attenuated total reflection infrared spectroscopy (ATR-FTIR) was used to identify their polymer type and functional groups. By matching the collected spectral data with a known standard spectral database, a standard was set: only when the match exceeded 60% can the sample be confirmed as microplastic, thereby determining its specific chemical composition.

[0063] The distribution types of microplastics in the farmland soil of the Hetao Irrigation District reveal the diversity of plastic pollution. Among them, PET (polyethylene terephthalate) microparticles dominate with a proportion of 45.0%. This material is widely used in the manufacture of beverage bottles and food containers due to its light weight, transparency and heat resistance. PA (polyamide) microparticles account for 24.4%, which are commonly used in the production of textiles and wear-resistant products such as nylon nets and ropes, and may come from the wear of these materials used in agricultural activities. PVC (polyvinyl chloride) and PE (polyethylene) microparticles account for 9.8% and 6.4% respectively. These two materials are often used for pipe, wire insulation and packaging materials. Their distribution in the soil may be related to agricultural irrigation systems and improper disposal of packaging waste. PP (polypropylene) and PS (polystyrene) microparticles account for 2.2% and 0.6% respectively. PP is often used in food containers and automotive parts due to its heat and chemical corrosion resistance, while PS is used in disposable tableware and foam packaging materials. The remaining 11.5% of microplastics include other types of plastics or plastic mixtures, and their sources may be more complex, involving many aspects such as daily life and industrial activities. These microplastics may enter the soil through weathering, water flow, and biological activities, which may have a potential impact on soil quality and ecological balance ( Fig. 9 ).

[0064] As can be seen from the above embodiments, the present invention provides a method for extracting and detecting microplastics in saline-alkali soil. The extraction method of the present invention comprises the following steps: (1) mixing a soil sample with a flotation solution, standing for 8 to 12 hours, taking an upper solution, and obtaining a flotation product; (2) mixing the flotation product with the flotation solution, and flotating again 1 to 2 times according to the method described in step (1) to obtain a solution carrying microplastics; (3) filtering the solution obtained in step (2), taking the substance retained on the filter membrane and mixing it with a hydrogen peroxide solution, standing for 1.8 to 2.2 hours, and obtaining a digestion solution; (4) filtering the digestion solution, taking the substance retained on the filter membrane, and extracting microplastics. A comprehensive analysis of the extracted microplastics can be performed to obtain the distribution characteristics, type composition, and content level of microplastics, and a comprehensive assessment of the current status of microplastic pollution can be performed through experimental data. This method will provide important information for accurately grasping the extent of microplastic pollution in farmland soil and its potential impact, and thus provide a solid theoretical basis for specifying targeted governance strategies and scientific decision-making.

[0065] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for extracting microplastics from saline-alkali soil, characterized in that: The steps include: (1) Mix the soil sample with the flotation solution, let it stand for 4 to 6 hours, and take the upper solution to obtain the flotation product; (2) mixing the flotation product with a flotation solution, and flotating again for 1 to 2 times according to the method described in step (1) to obtain a solution carrying microplastics; (3) filtering the solution obtained in step (2), taking the substance retained on the filter membrane and mixing it with the hydrogen peroxide solution, and letting it stand for 2 to 3 days to obtain a digestion solution; (4) Filter the digestion solution, take the substances retained on the filter membrane, and extract the microplastics.

2. The extraction method according to claim 1, characterized in that The mass volume ratio of the soil sample and the flotation solution in step (1) is 1 g: 2-4 mL; The flotation solution is a sodium chloride solution or a zinc chloride solution; The sodium chloride solution is a saturated sodium chloride solution; The initial concentration of the zinc chloride solution is 1.7-1.8 g / mL.

3. The extraction method according to claim 2, characterized in that The filter membrane used in step (3) is a steel membrane or a polytetrafluoroethylene membrane, and the pore size of the filter membrane is ≤20 μm; The initial concentration of the hydrogen peroxide solution in step (3) is 28-32 vt%; The mass volume ratio of the soil sample and the hydrogen peroxide solution is 1 g: 0.5-1.0 mL.

4. The extraction method according to claim 3, characterized in that The filter membrane used to filter the digestion solution in step (4) is a steel membrane or a polytetrafluoroethylene membrane, and the pore size of the filter membrane is 10 to 500 μm.

5. Use of the extraction method according to any one of claims 1 to 4 in the preparation of products for detecting microplastics in saline-alkali soils.

6. A method for detecting microplastics in saline-alkali soil, characterized in that: The steps include: Extract microplastics from soil according to the extraction method described in any one of claims 1 to 4; detect the morphological characteristics of microplastics by scanning electron microscopy, and detect the chemical functional groups of microplastics by Fourier transform infrared spectrometer.

7. Use of the method described in claim 6 in detecting the morphological characteristics and chemical functional groups of microplastics in saline-alkali soil.

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