A method for quantitatively detecting microplastics

Abstract

The present application relates to a kind of quantitative detection microplastics method, comprising the following steps: (1) dissolution: the sample to be detected is dissolved using dissolving agent;(2) filtration;(3) precipitation: after dissolving, precipitant is added to the product and precipitated;(4) filtration;(5) redissolution: the precipitate obtained after filtration is dissolved again using dissolving agent;(6) Wijs reaction, titration, calculate iodine value according to the titration result, to determine the mass of microplastics.The detection method provided by the present application can realize quantitative detection of microplastics using chemical reactions between substances, with the advantages of accurate results, high recovery rate, good precision, good stability and reproducibility, suitable for quantitative detection of microplastics in different environments, not limited by particle size, simple pretreatment steps, low cost, etc.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of detection, and particularly relates to a method for quantitatively detecting microplastics. BACKGROUND

[0002] Large-size plastics exposed in the natural environment are broken into smaller microplastics (<5 mm) through long-term physical, chemical and biological effects. Common types of microplastics include PS-MPs (polystyrene microplastics), PE-MPs (polyethylene microplastics), PP-MPs (polypropylene microplastics) and PVC-MPs (polyvinyl chloride microplastics). Microplastics can affect the growth and development of animals and plants, and affect the physical and chemical properties of soil. Studies have shown that microplastics can be continuously transmitted to higher levels through the food chain, enter the human body and accumulate therein, affecting people's health. The damage caused by microplastics to the ecosystem is immeasurable. Due to the small size of microplastics, the detection of microplastics is difficult, and the quantitative detection method of microplastics in complex environmental media still needs to be supplemented.

[0003] The existing microplastic detection methods have certain universality for environmental media such as water, sediments and soil. The identification of microplastics is mainly through visual inspection method, microscopic method and chemical analysis method according to their shape, color, particle size and the like for classification and identification. When the size of microplastics is large (1-5 mm), it can usually be picked out by hand or under a microscope with tweezers for classification and identification; when the size of microplastics is small, other identification methods need to be used. The existing quantitative detection methods of microplastics mainly include spectroscopic method and thermal analysis method. The spectroscopic method is represented by infrared spectrometer, which detects the type, particle size and number of microplastics by identifying the functional groups of microplastics. The thermal analysis method is represented by pyrolysis gas chromatograph mass spectrometer, which analyzes the gas composition after cracking microplastics into gaseous substances.

[0004] (1) Spectroscopic method

[0005] Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy (Raman) are the most common methods in microplastic analysis (Song et al., 2015), which can accurately identify the type, abundance, shape and size of microplastics (Xu et al., 2019), and can usually detect particle sizes in the micron level (Hidalgo et al., 2012). However, it cannot quantify the mass of microplastics, and the detection process is time-consuming and cannot be performed simultaneously on different samples.

[0006] Fourier transform infrared spectrometer can record the specific chemical bonds of chemical substances, obtain the spectrum of the target polymer by scanning, and compare it with the standard spectrum in the spectrum library to identify the type of specific microplastics. Infrared spectrometer can effectively quantify microplastics above 20 pm (Carr et al., 2016). However, the detection results of infrared spectrum are interfered by the shape, size and color of the measured particles. For example, due to the high absorption of infrared radiation, it is usually impossible to identify the infrared spectrum of black microplastic particles (Zhang et al., 2020). At the same time, the presence of multiple polymers in the sample at the same time will produce a complex absorption spectrum, which will hinder the identification of microplastics. In addition, the application effect of infrared spectrum technology also depends on whether the interference of impurities such as organic matter in the environmental medium is effectively removed in the pretreatment process.

[0007] Raman spectrometer is a photon scattering technology that produces different frequency backscattered light on the laser beam falling on the object according to the different molecular structure and atoms of different samples (Araujo et al., 2018), thereby obtaining a spectrum image unique to different polymers. Raman spectrometer can effectively perform qualitative and quantitative analysis on microplastics above 1 pm, and its running efficiency is lower than that of infrared spectrum, and it is not affected by the shape, size or thickness of the measured particles. The existing surface-enhanced Raman spectrum technology mixes microplastics with Ag ions, and then uses a Raman spectrometer to detect the mixture. The scattering light of metal Ag ions is stronger, and the instrument recognition is higher, so that microplastics above 50 nm can be detected. This method provides a new idea for the detection of small size plastics by spectrometer, but due to the problems of incomplete mixing of microplastics and metal ions or excessive mixing, there is still some controversy about its universality. And it is easy to be interfered by fluorescence, not easy to detect low concentration substances, easy to be affected by particle color, needs to be refined before sample analysis, time-consuming and other defects.

[0008] Both Raman spectrum and Fourier transform infrared spectrum technologies require sufficient pretreatment process of the sample. Since the microplastic pretreatment digestion process needs to be carried out at a relatively low temperature, only the impurities with relatively simple structure in the environmental medium can be removed, and the complex substances containing benzene ring structure still cannot be removed, and the residual organic matter in the sample will directly affect the detection efficiency of the spectrum method. Therefore, both of the above two spectrum technologies are time-consuming, and it is more difficult to process environmental samples with high organic matter content.

[0009] (2) Thermal analysis method

[0010] The thermal analysis method is to measure the changes in the physical and chemical properties of the polymer according to its thermal stability. After selecting the marker of the tested substance, the thermal degradation products of the polymer are analyzed according to the peak area of the marker characteristic peak in the detection result, and the qualitative and quantitative detection of different types of microplastics can be performed by determining the different characteristic fragments at different mass-to-charge ratios (m / z) and according to the mass-to-charge ratio values and the peak area sizes of the characteristic fragments.

[0011] The thermal analysis method instrument mainly includes pyrolysis-gas chromatography-mass spectrometry (py-GC / MS), Fourier infrared spectroscopy-thermal gravimetric analysis instrument (FTIR-TGA), thermal gravimetric analysis-chromatography-mass spectrometry (TGA-GC / MS), and extraction-thermal desorption-gas chromatography-mass spectrometry (ATD-GC / MS) combined instruments (Rocha et al., 2015; Zacharias et al., 2020).

[0012] The thermal analysis method is a destructive technology. First, the solid sample is decomposed into gaseous substances at high temperature under an inert atmosphere; second, the gaseous substances after pyrolysis are separated by a chromatograph (GC); and finally, the components are qualitatively and quantitatively analyzed by a mass spectrometer (MS). The biggest advantage of this method is that it does not require the particle size of the plastic sample, and both micron-sized plastics and nanometer-sized plastics can be detected by this method, and the mass recovery rate of microplastics before and after detection can also be obtained. However, it cannot count the number (number) of microplastics, size, and shape information; there is a risk of misjudgment, different polymers may produce similar pyrolysis products; the detection cost is high, and the selection of microplastic markers needs to be based on experiments to explore for subsequent identification. For example, pyrolysis-gas chromatography / mass spectrometry (py-GC / MS) has the disadvantages of causing damage to the detected sample, being unable to obtain the number of particles, particle size distribution, and morphological characteristics. Extraction-thermal desorption-gas chromatography / mass spectrometry (ATD-GC / MS) has the disadvantages of causing damage to the detected sample, being unable to obtain the number of particles and particle size distribution, and being only suitable for certain polymer types.

[0013] Both the spectroscopic method and the thermal analysis method need to rely on professional equipment to complete, and the purchase cost of the instrument is high, and professional technical personnel need to be technically trained to operate. Both from the equipment and personnel aspects, it requires a high level and is not suitable for large-scale popularization and application.

[0014] Wijs solution (iodine in glacial acetic acid) method is a method for determining the iodine value of high molecular organic matter by chemical titration. The iodine value is an index indicating the degree of unsaturation in an organic compound, referring to the number of grams of iodine that can be absorbed per 100g of the substance. The Wijs solution can be used to detect the content of vinyl ether, unsaturated fatty acid, allyl ester, allyl ether, vinyl double bond and St double bond in the sample to be measured. Taking the reaction principle of Wijs solution and vinyl double bond as an example, the reaction principle is shown as follows.

[0015]

[0016] However, the soil composition is relatively complex, and there are many interfering substances. It is unknown whether the Wijs solution method is suitable for the quantitative detection of microplastics in soil. It is very difficult to obtain a quantitative detection method suitable for microplastics in soil. SUMMARY

[0017] The present application provides a new method for quantitative detection of microplastics and optimizes the experimental method to solve the problems of particle size limitation, complicated pretreatment steps, high cost, the need for professional equipment detection, and high technical requirements for personnel in the existing detection methods.

[0018] The technical solution of the present application to solve the above technical problems is as follows:

[0019] The present application provides a method for quantitative detection of microplastics, comprising the following steps:

[0020] (1) Dissolution: dissolving the sample to be detected with a dissolving agent;

[0021] (2) Filtration;

[0022] (3) Precipitation: adding a precipitating agent to the dissolved product for precipitation;

[0023] (4) Filtration;

[0024] (5) Resolubilization: resolubilizing the precipitate obtained after filtration with a dissolving agent;

[0025] (6) Wijs reaction, titration, calculating the iodine value according to the titration result, and determining the mass of the microplastics.

[0026] The beneficial effects of the above technical solution include:

[0027] The present application applies the Wijs method to the quantitative detection of microplastics. The greater the mass of the microplastics, the more unsaturated bonds it contains, and the greater the iodine value. The mass of the microplastics is obtained by making a microplastic mass-iodine value standard curve.

[0028] Since the Wijs solution method does not have detection specificity, it can react with the unsaturated bonds of various high molecular organic substances, and there are often interfering substances in the environmental samples, in order to make the Wijs method practical and accurate, anhydrous ethanol precipitation is introduced as a step of removing impurities to remove the interfering substances in the environmental samples, and it is found through research that there is still a linear relationship between the mass of the microplastics and the iodine value after the impurity removal step. It is proved that the recovery rate of the detection method provided by the application is higher than 91.33%, the precision is less than 0.88%, and when the microplastics in different textured soils are detected, the minimum detection limit is 0.82ug / g.

[0029] It is proved that the recovery rate of the detection method provided by the application is higher than 91.33%, the precision is less than 0.88%, and when the microplastics in different textured soils are detected, the minimum detection limit is 0.82ug / g.

[0030] The detection method provided by the application does not need to use complex detection instruments, nor does it need to train personnel to be familiar with the operation of the instruments. The detection method provided by the application can realize quantitative detection of microplastics by chemical reactions between substances, has the advantages of accurate results, high recovery rate, good precision, good stability and reproducibility, is suitable for quantitative detection of microplastics in different environments, is not limited by particle size, has a simple pretreatment step, and has low cost.

[0031] Further, the microplastics are selected from polystyrene microplastics.

[0032] The beneficial effects of the above scheme include: the above method can detect polystyrene microplastics, has the advantages of accurate results, high recovery rate, good precision, good stability and reproducibility, is suitable for quantitative detection of microplastics in different environments, is not limited by particle size, has a simple pretreatment step, and has low cost.

[0033] Further, in step (1), the dissolving agent is carbon tetrachloride.

[0034] Preferably, the mass-volume ratio of microplastics in the sample to be tested to the dissolving agent can be ≤1g:50mL.

[0035] The beneficial effects of the above scheme include: the present application uses carbon tetrachloride to dissolve the sample to be detected, which does not affect the subsequent Wijs reaction, and carbon tetrachloride has strong solubility for polystyrene microplastics of different particle sizes in the sample, which can effectively dissolve polystyrene.

[0036] Through experimental exploration, 50mL of carbon tetrachloride can dissolve at least 1g of polystyrene microplastics, because microplastics are trace substances in the environment, generally not reaching 1g, so in the experiment, the mass-volume ratio of microplastics to carbon tetrachloride can be 1g:50mL, and the volume of carbon tetrachloride added can be 50mL.

[0037] Using the above ratio, it can be ensured that the microplastics are completely dissolved.

[0038] Further, in step (3), the precipitant is anhydrous ethanol, and the volume ratio of the precipitant to the dissolving agent is 1:1 to 3:1, preferably, the volume ratio of the precipitant to the dissolving agent is 2:1 to 3:1, most preferably, the volume ratio of the precipitant to the dissolving agent is 2:1.

[0039] The beneficial effects of the above scheme include: the use of the above ratio is conducive to complete precipitation and separation of the precipitate. If too little anhydrous ethanol is added, it can lead to incomplete precipitation, affect the test results, and make it difficult to separate the precipitate. If too much anhydrous ethanol is added, it can lead to reagent waste.

[0040] Further, in step (3), the precipitation time is ≥24 h.

[0041] The beneficial effects of the above scheme include: the use of the above time can ensure complete precipitation of polystyrene.

[0042] Further, in step (5), the dissolving agent is carbon tetrachloride, and the volume ratio of the dissolving agent added for re-dissolution to the precipitant is 1:(2-3). Preferably, the volume ratio of the dissolving agent added for re-dissolution to the precipitant is 1:2.

[0043] The beneficial effects of the above scheme include: the use of the above ratio is conducive to forming polystyrene plastic precipitate with hard but not loose texture. If too much dissolving agent is added, it can lead to too loose precipitate, which can be easily lost during transfer between containers. If too little dissolving agent is added, it can lead to incomplete dissolution and inaccurate test results.

[0044] Further, in step (6), the Wijs reaction includes the following steps: adding the re-dissolved solution to the Wijs solution, placing it in the dark at 15-20°C for 30 min, and then adding potassium iodide solution and distilled water.

[0045] The Wijs solution can be prepared as follows: take iodine trichloride 7.90 g and iodine 8.70 g, respectively, dissolve in glacial acetic acid, combine the two liquids after complete dissolution, dilute with glacial acetic acid to 1000 mL, store in a brown bottle, and use within 30 days.

[0046] Further, the volume ratio of the Wijs solution, the potassium iodide solution, and the distilled water is 10:15:100, and the content of potassium iodide in the potassium iodide solution is 10%.

[0047] Further, in step (6), titration includes the following steps: titration with sodium thiosulfate standard solution and recording the titration results.

[0048] Further, the concentration of sodium thiosulfate in the sodium thiosulfate standard solution is 0.05 mol / L.

[0049] Further, in step (6), the calculation includes the following method: calculating the iodine value of the sample to be detected according to the amount of standard solution consumed, and calculating the mass of microplastics contained in the sample to be detected according to the standard curve of microplastic mass and solution iodine value.

[0050] The beneficial effects of the above scheme include: the iodine value of the sample to be detected can be calculated by titration, and the mass of microplastics contained in the sample to be detected can be calculated according to the standard curve of microplastic mass and solution iodine value, so as to realize quantitative detection of microplastics. The detection method provided by the present application can realize quantitative detection of microplastics by chemical reaction between substances, has the advantages of accurate results, high recovery rate, good precision, good stability and reproducibility, being suitable for quantitative detection of microplastics in different environments, not being limited by particle size, simple pretreatment steps, low cost, etc. BRIEF DESCRIPTION OF DRAWINGS

[0051] Figure 1 Fig. 1 is a schematic diagram of three particle sizes of PS-MPs (from left to right, the particle sizes are 20, 200 and 2000 μm). The PS-MPs with a particle size of 20 μm and the PS-MPs with a particle size of 200 μm are ground by a machine, and the PS-MPs with a particle size of 2000 μm are foamed polystyrene plastics cut by hand.

[0052] Figure 2 Fig. 4 is a schematic diagram of color change during titration.

[0053] Figure 3 Fig. 5 is a standard curve of mass of PS-MPs with three particle sizes and solution iodine value (I).

[0054] Figure 4 Fig. 8 is a color change diagram of the solution at different precipitation times.

[0055] Figure 5 Fig. 11 is a precipitation form diagram of the PS-MPs with a particle size of 200 μm and different volumes of ethanol.

[0056] Figure 6 Fig. 14 is a characteristic fragment spectrum when the volume ratio of ethanol to carbon tetrachloride is different (a, b, c and d are the spectra when the volume ratio of ethanol to carbon tetrachloride is 3, 2, 1 and 0.5, respectively).

[0057] Figure 7 Fig. 17 is a standard curve of mass of PS-MPs and solution iodine value (II).

[0058] Figure 8 Fig. 20 is a standard curve for quantitative detection of PS-MPs in soil (III). DETAILED DESCRIPTION

[0059] The principles and characteristics of the present application are described below in conjunction with the accompanying drawings, and the examples are only used to explain the present application and are not intended to limit the scope of the present application.

[0060] Since the traditional microplastic quantitative detection method has the problems of damaging microplastics in pretreatment and high detection cost, the present application explores a method of converting microplastics into a solution form by using chemical principles for detection, thereby realizing the ingenious separation of microplastics from soil and avoiding the problem of high cost of traditional methods.

[0061] The present application provides a method for quantitatively detecting microplastics, comprising the following steps:

[0062] (1) Dissolution: dissolving the sample to be detected with a dissolving agent;

[0063] (2) Filtration;

[0064] (3) Precipitation: adding a precipitating agent to the dissolved product for precipitation;

[0065] (4) Filtration;

[0066] (5) Resolubilization: resolubilizing the precipitate obtained after filtration with a dissolving agent;

[0067] (6) Wijs reaction, titration, and calculating the mass of microplastics in the sample to be detected according to the titration result.

[0068] In the above method, the dissolving agent can be carbon tetrachloride; the precipitating agent can be anhydrous ethanol, and the volume ratio of the precipitating agent to the dissolving agent is 2:1 to 3:1, preferably, the volume ratio of the precipitating agent to the dissolving agent is 2:1.

[0069] The Wijs solution method utilizes the principle of similar dissolves similar, first dissolving the high molecular organic matter in an organic solvent similar in structure, then the iodine in the Wijs solution will react with the unsaturated bond in the high molecular organic matter dissolved in the organic solvent, and the remaining iodine is titrated with a standard sodium thiosulfate solution, so that the unsaturation of the high molecular organic matter can be calculated according to the amount of the standard sodium thiosulfate solution consumed, and the more unsaturated bonds contained in the organic solvent that dissolves the high molecular organic matter, the less standard solution is needed for titration.

[0070] The styrene double bond in polystyrene microplastics can participate in the Wijs reaction, and the known mass of polystyrene microplastics (PS-MPs) is quantitatively detected by using this relationship, i.e. the greater the mass of PS-MPs, the more unsaturated bonds it has, and the less standard solution is needed for titration. The selection of organic solvent is related to the solubility parameter and the toxicity of the solution, and the commonly used organic solvents are carbon tetrachloride, chloroform and chloroform, etc. The present application selects relatively low toxicity carbon tetrachloride as the organic solvent to dissolve PS-MPs, and the subsequent experiments are carried out according to the original method of Wijs solution.

[0071] But since the Wijs solution is not specific to the chemical bond of polystyrene, it has detection ability for allyl ether, allyl ester and unsaturated fatty acids. The soil in nature contains unsaturated fatty acid substances, which will react with the chemical bond of polystyrene in the Wijs solution. Therefore, the unsaturated fatty acid substances in the environmental medium should be removed first, and then the subsequent experimental analysis is carried out. According to the principle that ethanol can dissolve unsaturated fatty acid substances but cannot dissolve polystyrene, the step of "ethanol impurity removal" is introduced in the present application. Research has found that this step can realize the effective separation of PS-MPs and unsaturated fatty acid substances in soil, thereby avoiding the influence of impurities in soil on the quantitative results.

[0072] In the examples, three kinds of soil with different textures are selected as environmental media, and 20 μm and 200 μm machine ground PS-MPs and 2000 μm hand cut PS-MPs are selected as representatives to study the new method for quantitative detection of PS-MPs in soil.

[0073] The present application first verifies the possibility of quantitative detection of PS-MPs by Wijs solution method, and explores the experimental conditions to determine the solubility range and dissolution time of the required dissolving agent in the quantitative detection process of Wijs solution method. Then, according to the detection characteristics of Wijs solution method, a quantitative detection method suitable for PS-MPs in soil environment is established, and this method is applied to different texture soils. The applicability of this method for quantitative detection of PS-MPs in soil is analyzed through three indicators of PS-MPs recovery rate in soil, method precision and minimum detection limit. The research results can provide a new way for quantitative detection of PS-MPs, and provide method guidance and technical support for further exploration of the accumulation, pollution degree and toxic effect of PS-MPs in environmental medium. The main contents are as follows:

[0074] (1) Exploring the applicability of Wijs solution method for quantitative detection of PS-MPs

[0075] The solubility range and dissolution time of PS-MPs with different mass and different particle size in carbon tetrachloride are explored, the influence of carbon tetrachloride on Wijs reaction is excluded, and the feasibility of Wijs solution method for quantitative detection of PS-MPs is determined.

[0076] (2) Establishment of quantitative detection method of PS-MPs in soil

[0077] To verify the necessity of removing interfering substances in soil for PS-MPs detection; to explore the precipitation performance of ethanol precipitator and determine the appropriate amount, and to explore the effect of precipitation impurity removal step. After dissolution-precipitation-redissolution-Wijs titration, establish the standard curve of PS-MPs mass and iodine value, and determine the applicability of the method in the detection of PS-MPs in soil according to the determination coefficient of the standard curve.

[0078] (3) Application of the method to the quantitative detection of PS-MPs in different soil textures

[0079] According to the PS-MPs mass recovery rate, the minimum detection limit and the method precision, the applicability of the new method in different soil textures was analyzed.

[0080] The following will be introduced by specific examples. The methods used in each example, if not specified, are conventional methods in the art. The materials, reagents and instruments used, if not specified, are conventional materials, reagents and instruments in the art, and can be obtained through commercial channels.

[0081] Experimental instruments: laser infrared imaging spectrometer (model 8700LDIR), produced by Agilent. Pyrolysis gas chromatography mass spectrometer (Py-GC-MS), pyrolysis instrument model PY-303D, produced by Frontier, gas chromatography mass spectrometer model GCMS-QP2020, produced by Shimadzu.

[0082] Experimental reagents and experimental materials:

[0083] Carbon tetrachloride (analytical pure), ethanol (if not specified, all are anhydrous ethanol), iodine (super pure), iodine trichloride (analytical pure), glacial acetic acid (analytical pure), potassium iodide (analytical pure), sodium thiosulfate (Na2SO3·5H2O) (analytical pure), soluble starch (analytical pure), all can be obtained through commercial channels.

[0084] Polystyrene microplastics (PS-MPs), such as Figure 1 shown, specifications are 20 μm, 200 μm, 2000 μm, plastic microspheres, all purchased from Guangdong Tesilang Chemical Co., Ltd.

[0085] Soil texture: sandy soil (organic matter content 5.2 g / kg), loam soil (organic matter content 21.5 g / kg), clay soil (organic matter content 40.3 g / kg).

[0086] Solution preparation:

[0087] (1) Preparation of Wijs solution: take iodine trichloride 7.90 g and iodine 8.70 g, respectively, dissolve in glacial acetic acid, after all dissolved, combine the two liquids, then dilute to 1000 mL with glacial acetic acid, store in a brown bottle for future use, can be used within 30 days.

[0088] (2) Preparation of 10% potassium iodide solution: Take potassium iodide 10.00 g, dissolve in 90 g distilled water, and the potassium iodide solution needs to be prepared and used immediately.

[0089] (3) Preparation of 0.01 mol / L sodium thiosulfate standard solution:

[0090] a. Configuration: Take anhydrous sodium thiosulfate 16 g, dissolve in 1000 mL distilled water, and boil for 10 min before cooling.

[0091] b. Calibration: Take potassium dichromate dried at 120°C to constant weight 0.18 g, place in an iodine flask, dissolve in 25 mL distilled water, add 2 g potassium iodide and 20% sulfuric acid solution, shake well, place in the dark for 10 min, then add 150 mL distilled water, titrate with the prepared sodium thiosulfate solution, near the end point, add 2 mL 10 g / L starch indicator, continue titration until the solution changes from blue to green, and perform a blank experiment.

[0092] c. Calculation: The concentration (mol / L) of sodium thiosulfate standard titration solution is calculated as follows:

[0093] C(Na2S2O3) = 1000m / (V1-V2)M (1-1)

[0094] Where: m - the exact value of the mass of potassium dichromate, g

[0095] V1 - the volume of sodium thiosulfate solution, mL

[0096] V2 - the volume of sodium thiosulfate solution for the blank experiment, mL

[0097] M - the molar mass of potassium dichromate M(1 / 6K2CrO7) = 49.0311, g / mol

[0098] (4) Preparation of 0.5% starch indicator: Take 0.50 g soluble starch and place it in a small beaker, add 10.0 mL distilled water, stir gently to form a paste, then introduce 90.0 mL boiling water under stirring and boil for 2 min, cool and transfer to a 100 mL reagent bottle.

[0099] Example 1 Investigation of Wijs solution method for quantitative determination of PS-MPs

[0100] The Wijs solution method is a method for determining the degree of unsaturation of a specific unsaturated bond in a high molecular organic compound, and the index of unsaturation of the organic compound is obtained. Wijs solution is iodine in glacial acetic acid, and the amount of iodine consumed by the organic compound is used to calculate the degree of unsaturation of the organic compound.

[0101] Wijs solution method first dissolves high molecular organic matter in organic solvent similar in structure to the high molecular organic matter according to the principle of similar dissolves similar, after the organic matter is completely dissolved, Wijs solution and potassium iodide solution are added to make the measured substance consume iodine in the reagent, and finally sodium thiosulfate solution is titrated, and the iodine value of the organic matter is calculated according to the amount of titration solution consumed.

[0102] The present application carries out screening experiment of dissolving agent in early experimental stage. The selection of dissolving agent is related to the solubility parameter of PS-MPs, wherein the closer the solubility parameter of the dissolving agent to that of PS-MPs, the stronger the dissolving power, and the solubility parameters of common organic matters are shown in Table 1, the solubility parameter of polystyrene is 9.1, and the solubility parameters of chloroform, carbon tetrachloride, benzene and toluene are similar to that of polystyrene, but due to the high toxicity of chloroform, benzene and toluene, the present application selects carbon tetrachloride as the dissolving agent of Wijs solution method. After PS-MPs are completely dissolved in carbon tetrachloride, the relationship between the iodine value of the organic matter and the content of the unsaturated bond in the sample is used to quantitatively analyze PS-MPs, that is, the greater the mass of PS-MPs sample, the more unsaturated bonds it contains, and the more iodine in Wijs solution is consumed, and finally the relationship between the mass of PS-MPs and the iodine value is established.

[0103] Table 1 solubility parameters of organic matter

[0104]

[0105] Confirmation of the type and particle size of the sample to be detected:

[0106] Qualitative analysis and particle size determination of the sample are carried out by using a laser infrared imaging spectrometer, the sample to be measured is uniformly mixed with ethanol, and is added dropwise on a high-reflective glass, after the ethanol is completely volatilized, the sample is tested on the machine. The particle analysis mode is selected, the microplastic spectrum library is established, and the automatic detection method is set to test. The particle size range detected by the equipment is 20-500 μm, so the qualitative detection of PS-MPs with particle sizes of 20, 200 and 2000 μm is carried out, the particle sizes of PS-MPs with particle sizes of 20 and 200 μm are measured, and the particle size of 2000 μm PS-MPs is measured by using a ruler.

[0107] The sample to be tested is compared with the polystyrene (PS) spectrum in the spectral library according to the laser infrared imaging spectrometer, and if the comparison result is greater than 0.65, the sample is determined to be polystyrene, and if the comparison result is greater than 0.8, it is called high matching degree. The three parameters of width (Width), height (Height) and diameter (Diameter) are determined as the maximum value of the three. According to the comparison result, the qualitative results of the three kinds of particle size particles are all polystyrene, and the particle size detection results of 20 μm and 200 μm PS-MPs are consistent with the expected results. After the ruler detection, the detection result of the particle size of 2000 μm PS-MPs is consistent with the expected result. The confirmed sample is used for subsequent experiments.

[0108] 1.1 Dissolution of PS-MPs with different particle sizes in the dissolving agent

[0109] The experimental method includes the following steps:

[0110] 0.10 g, 0.20 g, 0.30 g, 0.40 g, and 0.50 g of PS-MPs with particle sizes of 20 μm, 200 μm, and 2000 μm were respectively placed in 50 mL beakers, and 10.0 mL of carbon tetrachloride was added to dissolve the PS-MPs (at this time, the laboratory temperature was 19±℃). At the beginning of dissolution, the beaker was kept stationary to avoid the adhesion of incompletely dissolved PS-MPs to the beaker wall and affect the test results.

[0111] The dissolution of PS-MPs was observed, and the test groups with no obvious PS-MPs particles were filtered under naked eye observation. After washing the beaker with distilled water for 3 times, the filtration was continued. If there were no residual particles on the filter paper, it was determined that the dissolution was complete, and the dissolution time was recorded. For the test groups with obvious undissolved PS-MPs particles, the amount of PS-MPs added was determined as the amount of solute dissolved when the PS-MPs of this particle size reached the saturation state. According to this, the solubility range of PS-MPs with three particle sizes (g / 10 mL of carbon tetrachloride) and the time required for dissolving different amounts of PS-MPs were determined. According to the test results, the minimum amount of carbon tetrachloride and the time required for dissolving PS-MPs of different particle sizes at room temperature were determined, which provided a theoretical basis for subsequent experiments.

[0112] Experimental results and analysis:

[0113] (1) Dissolution time of PS-MPs in carbon tetrachloride

[0114] After 0.10 g, 0.20 g, 0.30 g, 0.40 g, and 0.50 g of PS-MPs with particle sizes of 20 μm, 200 μm, and 2000 μm were dissolved in 10 mL of carbon tetrachloride, the dissolution time was as shown in Table 2.

[0115] Table 2 Dissolution time of PS-MPs with three particle sizes in carbon tetrachloride

[0116]

[0117] It was found that PS-MPs were completely dissolved in carbon tetrachloride to form a colorless solution. When PS-MPs were not completely dissolved, PS-MPs particles floated on the surface of carbon tetrachloride and the solution was turbid. As shown in Table 2, the dissolution time of PS-MPs with three different particle sizes in carbon tetrachloride was different. Among them, the dissolution rate of 20 μm PS-MPs was the slowest, followed by 200 μm PS-MPs, and 2000 μm PS-MPs was the fastest.

[0118] In theory, the smaller the particle size of PS-MPs, the larger the specific surface area, the more sufficient the contact with carbon tetrachloride, and the faster the dissolution. However, it was found during the experiment that the volume of 20 μm PS-MPs was larger than that of 200 μm PS-MPs under the same mass. Therefore, it was guessed that the reason why 20 μm PS-MPs dissolved more slowly than 200 μm PS-MPs was that the content of polystyrene in 20 μm PS-MPs was higher under the same mass. The dissolution rate of 2000 μm PS-MPs was much higher than that of 200 μm PS-MPs, which was due to the fact that the 2000 μm PS-MPs used were foamed plastics, and the content of polystyrene was relatively low.

[0119] (2) Range of solubility of PS-MPs

[0120] Solubility definition: the highest amount of a substance dissolved in a certain amount of solvent under certain temperature and pressure. Generally, it is expressed in grams of substance dissolved in 100 grams of solvent. However, in this case, the solubility is expressed in the mass (g) of PS-MPs dissolved in 10 mL of carbon tetrachloride.

[0121] According to the dissolution time in Table 2, the range of solubility of PS-MPs with three different particle sizes can be obtained, as shown in Table 3.

[0122] Table 3 Range of solubility of PS-MPs in carbon tetrachloride

[0123]

[0124] From Table 3, it can be seen that when the laboratory temperature is 19℃, the solubility of 20, 200, 2000 pm PS-MPs in carbon tetrachloride is respectively in the range of 0.20-0.30 g / 10 mL, 0.40-0.50 g / 10 mL and > 0.50 g / 10 mL. From Table 3, it can be seen that 0.50 g of 2000 pm PS-MPs can still be dissolved in 10 mL of carbon tetrachloride at a relatively fast speed. Since PS-MPs are trace substances in the real environment, the upper limit of the solubility of 2000 pm PS-MPs is not further explored, and the solubility is much greater than 0.50 g / 10 mL. From the table, the minimum amount of carbon tetrachloride required to dissolve PS-MPs can be determined, for example, for 20 pm microplastics, about 0.2 g is needed, which requires 10 mL of carbon tetrachloride; if there are 0.4 g of microplastics, 20 mL of carbon tetrachloride is required, which provides a theoretical basis for subsequent experiments.

[0125] 1.2 Excluding the effect of dissolving agent on Wijs reaction

[0126] (1) The experimental method includes the following steps:

[0127] Wijs reaction: 25, 50, 100 mL of carbon tetrachloride were respectively taken into 500 mL conical flasks, 10.0 mL of Wijs solution was added to each, and the stopper was plugged after being placed in the dark (15-20℃) for 30 min;

[0128] Titration: Then 15.0 mL of 10% potassium iodide solution and 100 mL of distilled water were added, and the obtained two-phase mixture of inorganic and organic phases was titrated with 0.05 mol / L sodium thiosulfate standard solution until the color in the inorganic phase completely disappeared, followed by adding 5 mL of 0.5% starch solution, and then continuing to titrate until the iodine in the organic phase completely disappeared, and the color change was as shown in Figure 2 Figure 2 The results from left to right in the figure are the addition of Wijs solution, the addition of KI and distilled water, the addition of starch indicator, the result before the titration end point and the titration end point. It can be seen that the color from left to right is brown, yellow, blue-violet, light pink and colorless, respectively, indicating that the iodine in the solution is consumed, and finally Na2S4O6 and NaI are generated without color.

[0129] The volume of the consumed sodium thiosulfate standard solution (0.05 mol / L) under different carbon tetrachloride dosage conditions of each group was recorded. If the volume of the consumed titration solution of each group is constant, it indicates that carbon tetrachloride does not participate in the Wijs titration reaction, and the amount of carbon tetrachloride added can be changed according to the actual situation without affecting the titration result. If the volume of the consumed sodium thiosulfate standard solution is different due to the different amount of carbon tetrachloride, it indicates that carbon tetrachloride will interfere with the detection method. ​

[0130] (2) Results and analysis:

[0131] Wijs solution method titration was performed after dissolving PS-MPs in carbon tetrachloride. In order to verify whether carbon tetrachloride would react with the Wijs solution, potassium iodide solution and sodium thiosulfate solution used in the Wijs solution method, the present application compared the volume of sodium thiosulfate titrant consumed with different amounts of carbon tetrachloride to determine whether carbon tetrachloride participated in the reactions in each step of the Wijs method, and the experimental results are shown in Table 4.

[0132] Table 4 Relationship between the amount of carbon tetrachloride added and the volume of titrant consumed

[0133]

[0134] According to Table 4, after adding different amounts of carbon tetrachloride as a dissolving agent and reacting with the Wijs solution, the volume of sodium thiosulfate titrant consumed was 33.70 mL, which proves that the dissolving agent does not participate in the reactions in each step of the Wijs method. Therefore, it can be concluded that when dissolving PS-MPs in carbon tetrachloride, the amount of carbon tetrachloride does not need to be determined to a certain fixed value.

[0135] Based on the range of solubility of PS-MPs in carbon tetrachloride in Example 1.1, it can be known that in subsequent experiments, the amount of dissolving agent carbon tetrachloride can be adjusted as long as it can completely dissolve PS-MPs. In the following, the amount of carbon tetrachloride is uniformly 50 mL. At 19±1℃, 50 mL of carbon tetrachloride can dissolve 1.00-1.50 g of 20 μm PS-MPs, or 2.00-2.50 g of 200 μm PS-MPs, or at least 2.50 g of 20 μm PS-MPs.

[0136] Since the experimental method is expected to be applied to the quantitative detection of PS-MPs in soil, if carbon tetrachloride is added to the soil using the Wijs method to dissolve the PS-MPs contained therein, the dissolution of PS-MPs cannot be directly observed. Therefore, based on the range of solubility of PS-MPs in carbon tetrachloride and leaving a certain amount of excess, 50 mL of carbon tetrachloride is used to dissolve PS-MPs.

[0137] 1.3 Establishing the relationship between the Wijs solution method and the detection of PS-MPs

[0138] 1.3.1 Exploring the applicability of the Wijs solution method to the detection of PS-MPs with different particle sizes

[0139] (1) The experimental method includes the following steps:

[0140] Dissolution: Weigh 0, 0.10, 0.20, 0.30, 0.40 and 0.50 g of PS-MPs with particle sizes of 20, 200 and 2000 μm respectively and place them in 500 mL Erlenmeyer flasks. Add 50.0 mL of carbon tetrachloride to each flask and then stopper the flasks.

[0141] Wijs reaction: After the PS-MPs are completely dissolved, add 10.0 mL of Wijs solution and place in the dark (15-20℃) for 30 min;

[0142] Titration: Add 15.0 mL of 10% potassium iodide solution and 100 mL of distilled water, and titrate the two-phase mixture with 0.05 mol / L sodium thiosulfate standard solution until the color in the inorganic phase completely disappears. Then add 5 mL of 0.5% starch solution, shake, and continue titrating until the iodine in the organic phase completely disappears.

[0143] Calculation: Calculate the iodine value of different masses of PS-MPs based on the amount of standard solution consumed.

[0144] Iodine value is an indicator of the degree of unsaturation in organic compounds, referring to the number of grams of iodine that can be absorbed (added) in 100g of a substance.

[0145] The calculation formula is as shown in 2-2, and a standard curve of PS-MPs mass versus solution iodine value is plotted. The coefficient of determination R of the curves plotted for PS-MPs of three particle sizes is used. 2 The study determined whether there was a linear relationship between the mass of PS-MPs and the iodine value of the solution; and analyzed the slope k of the particle size curves to determine the applicability of the Wijs solution method to PS-MPs particles of different sizes.

[0146]

[0147] Where: V—volume of Na2S2O3 consumed in the titration of the sample, mL

[0148] V0 — Volume of Na2S2O3 consumed in blank process, mL

[0149] C – Standardized concentration of Na₂S₂O₃, mol / L

[0150] m — Sample mass, g

[0151] (2) Experimental Results and Analysis:

[0152] Based on the investigation of solvent dosage, PS-MPs were dissolved in carbon tetrachloride, and the solution was subjected to Wijs titration. A PS-MPs mass-iodine value standard curve was plotted to establish the relationship between the two. The coefficient of determination R of the standard curve was used to determine the relationship. 2The applicability of the Wijs solution method for quantitative detection of PS-MPs was determined. The mass of PS-MPs with particle sizes of 20, 200, and 2000 μm and the iodine value standard curve are shown in FIG. 1. Figure 3 Figure 3 A is the mass of PS-MPs with a particle size of 20 μm and the iodine value standard curve, B is the mass of PS-MPs with a particle size of 200 μm and the iodine value standard curve, and C is the mass of PS-MPs with a particle size of 2000 μm and the iodine value standard curve.

[0153] 1.3.2 Minimum detection limit

[0154] After verification, the method in Example 1.3.1 is feasible. According to the nature of titration, the theoretical minimum detection limit of the method is calculated. That is, when the blank group consumes a sodium thiosulfate standard solution volume of X mL, according to the minimum scale value of 0.01 mL that can be read during titration, the theoretical minimum detection limit is the mass of PS-MPs corresponding to the volume of sodium thiosulfate consumed by X-0.01 mL. By combining Formula 2-2 in Example 1.3.1 and the curve equation (I) of the standard curve in Example 1.3.1, the minimum detection limit can be calculated.

[0155] Table 5 PS-MPs-iodine value standard curve parameters

[0156]

[0157] From Figure 3 and Table 5, it can be seen that the mass of PS-MPs with particle sizes of 20, 200, and 2000 μm and the iodine value all show a linear relationship, and the determination coefficient R 2 is greater than 0.99, and the linearity is good; by comparing the slopes k of the three curve equations, it is found that the smaller the PS-MPs particle size, the smaller the slope k. The slope k reflects the content of styrene double bonds in PS-MPs, so it can be known that the smaller the volume of PS-MPs with the same mass, the more the content of styrene double bonds, which is consistent with the result guessed in Example 1.1.

[0158] Since the Wijs method shows good linearity for the quantitative detection of PS-MPs with three particle sizes, it is confirmed that the Wijs method has good applicability for PS-MPs with different particle sizes. For PS-MPs samples with unknown mass, the iodine value of the sample can be detected, the detection result can be substituted into the standard curve, and the mass of the PS-MPs sample can be obtained, so as to realize the quantitative detection of the mass of PS-MPs.

[0159] ​When titrating the blank group, 33.70 mL of sodium thiosulfate standard solution (0.05 mol / L) is consumed. According to the titration reading rules, the minimum scale value 0.01 mL can be read during the titration process, and the theoretical minimum detection limit of the Wijs solution method is calculated. Therefore, the theoretical minimum detection limit is the mass of PS-MPs corresponding to the volume of sodium thiosulfate standard solution consumed by 33.70-0.01 mL during titration. By combining Formula 2-2 in Example 1.3.1 and the curve equation of the standard curve in Example 1.3.1, the minimum detection limit is calculated to be 0.032 g.

[0160] The Wijs solution method is originally used to determine the iodine value of large-size high molecular organic matter in production and life, which refers to the number of grams of iodine (Wijs solution) that can be absorbed by 100 g of matter. According to this principle, the larger the mass of the test substance, the more grams of iodine are absorbed, so the experimental conditions of this process are explored. The first step of the Wijs solution method is to dissolve the organic matter in an organic solvent using the principle of similar compatibility. To determine the appropriate amount of dissolving agent, the solubility of PS-MPs with different particle sizes in carbon tetrachloride (19°C) is first determined. The solubility of 20 μm PS-MPs is between 0.20 g-0.30 g / 10 mL, the solubility of 200 μm PS-MPs is between 0.40 g-0.50 g / 10 mL, and the solubility of 2000 μm foamed polystyrene PS-MPs is much greater than 0.50 g / 10 mL. Unlike our usual understanding, this result shows that the smaller the particle size of PS-MPs, the greater the specific surface area, and the longer the dissolution time, which is contrary to common sense. After observing the experimental process, it is hypothesized that the smaller the particle size of PS-MPs with the same mass, the larger the volume, and the more the number of styrene double bonds, so the dissolution time is longer.

[0161] After dissolving PS-MPs in carbon tetrachloride, Wijs titration is performed on the solution. To determine the amount of carbon tetrachloride, only the mass of PS-MPs is related, and it does not participate in the Wijs reaction. The solution with different volumes of carbon tetrachloride is titrated, and the experimental results show that the volume of sodium thiosulfate standard solution consumed is equal when different volumes of carbon tetrachloride are added for Wijs titration, indicating that carbon tetrachloride does not participate in the Wijs reaction. Therefore, when dissolving PS-MPs, the amount of carbon tetrachloride only needs to ensure that it can completely dissolve PS-MPs without causing reagent waste. Therefore, 50 mL of carbon tetrachloride is used to dissolve the organic matter. According to the above research results, 50 mL of carbon tetrachloride is sufficient to dissolve trace amounts of PS-MPs in the environment, so in the subsequent experiments, the amount of carbon tetrachloride used to dissolve PS-MPs is 50 mL.

[0162] After determining the experimental conditions, the possibility of Wijs method for quantitative detection of PS-MPs was explored. The experimental results showed that the mass of PS-MPs with particle sizes of 20, 200 and 2000 μm had a linear relationship with iodine value, and the linear relationship was good, and the determination coefficients R 2 were all greater than 0.99. It was also found that the slope of the standard curve became smaller as the particle size increased. The iodine value of the slope reflected the content of styrene in the sample, which indicated that the smaller the particle size of PS-MPs with the same mass, the more the number of styrene double bonds, which was consistent with the above guess. Wijs method could realize quantitative detection of pure PS-MPs samples with different particle sizes, and showed good performance in the detection of PS-MPs with different particle sizes, which laid a foundation for the quantitative detection of PS-MPs in soil.

[0163] Example 2: Establishment of PS-MPs quantitative detection method

[0164] This example verified the influence of the step of removing interfering substances in soil on the PS-MPs detection method; the precipitation performance of the ethanol precipitant was explored and the appropriate amount was determined, and a method suitable for quantitative detection of PS-MPs in soil was established.

[0165] The above examples have shown that Wijs solution method can be applied to the quantitative detection of PS-MPs, but this method can also detect vinyl ether, unsaturated fatty acid, allyl ester, allyl ether and St double bond while detecting polystyrene double bond. In order to improve the practical value of Wijs solution method, the possible interfering substances in soil should be separated first, and then Wijs solution method is used for quantitative detection.

[0166] During the research, the inventors screened different types of alcohol and unexpectedly found that the detection effect was better when ethanol was used. The inventors guessed that it might be due to the following reasons: PS-MPs can be dissolved in organic solvents such as carbon tetrachloride based on the principle of similar solubility, but PS-MPs cannot be dissolved in chemical solvents that are not similar in structure. Alcohols are strong polar organic solvents, while polystyrene is weakly polar, so PS-MPs cannot be dissolved in alcohol organic solvents. Alcohol organic solvents can make polystyrene dissolved in carbon tetrachloride precipitate again and change the original form of polystyrene, which is related to factors such as the amount of alcohol added and the stirring intensity. And because alcohol substances can remove unsaturated fatty acid substances that may exist in the environment, after PS-MPs are dissolved in carbon tetrachloride, alcohol organic solvents are added for impurity removal, and then Wijs method is used for quantitative detection, which can improve the detection effect.

[0167] Both methanol and ethanol can make PS-MPs dissolved in carbon tetrachloride re-precipitate, but due to the fact that methanol is extremely volatile and toxic, it is more appropriate to use ethanol as the precipitant, which can make PS-MPs dissolved in carbon tetrachloride re-precipitate while separating the possible unsaturated fatty acid substances in the sample, so as to realize the quantitative detection of Wijs solution method in PS-MPs.

[0168] 2.1 Precipitant dosage and performance exploration

[0169] 2.1.1 Determination of the time required for complete precipitation

[0170] (1) The experimental method includes the following steps:

[0171] Dissolution: 0.10 g of 20, 200, and 2000 μm PS-MPs was respectively placed in a 500 mL conical flask, and 50.0 mL of carbon tetrachloride was added to make the PS-MPs completely dissolved;

[0172] Precipitation: After dissolution, 25.0, 50.0, 100.0, and 150.0 mL of anhydrous ethanol was respectively added, i.e. the volume of anhydrous ethanol was 0.5, 1, 2, and 3 times the volume of carbon tetrachloride, so as to make PS-MPs dissolved in carbon tetrachloride re-precipitate.

[0173] After adding anhydrous ethanol for 0.5, 8, 15, and 24 h, the precipitation was recorded by taking photos, and the supernatant of the suspension of the precipitate and anhydrous ethanol was titrated by Wijs solution method at each time node. According to the content of polystyrene in the supernatant at each precipitation time point, it was analyzed whether the polystyrene was completely re-precipitated. That is, after precipitation for 0.5, 8, 15, and 24 h, the sample in the conical flask was stirred, and then about 15.0 mL of supernatant was filtered after stirring, 10 mL of filtrate was collected, 10.0 mL of Wijs solution was added, and the mixture was placed in the dark (15-20℃) for 30 min, then 15.0 mL of 10% potassium iodide solution and 100 mL of distilled water were added, and the two-phase mixture was titrated with 0.05 mol / L sodium thiosulfate standard solution. The titration results were recorded, and the time required for complete precipitation of polystyrene was determined according to the difference between the titration results.

[0174] (2) Experimental results and analysis:

[0175] Color change of the solution: After adding anhydrous ethanol for 0.5, 8, 15, and 24 h, the polystyrene precipitation was recorded by taking photos, and the color change of the solution was as shown in Figure 4 Figure 4 ​It can be observed that the solution is milky white when the precipitation time is 0.5 h, and the color of the solution gradually becomes clear as the precipitation time increases, and the solution is most clear after 24 h of precipitation. The PS-MPs with three particle sizes of 20, 200 and 2000 μm all show this color change rule. The polystyrene dissolved in carbon tetrachloride is re-precipitated into polystyrene solids under the action of the ethanol precipitant and deposited at the bottom of the conical flask. The precipitate is colorless paste when 25 mL of ethanol is added, and is milky white solid when 50, 100 and 150 mL of ethanol is added.

[0176] Iodine value of the solution at each precipitation period:

[0177] From Figure 4 It can be observed that the solution gradually changes from milky white to colorless as the precipitation time increases. In order to determine the time when the polystyrene is completely precipitated, the solution in the group with 100 mL of ethanol is titrated by the Wijs method (the precipitate is most suitable for subsequent experiments when the volume ratio of ethanol to carbon tetrachloride is 2), and the iodine value of the solution is calculated by using formula 2-2. The iodine value of the solution at each precipitation time is shown in Table 6.1.

[0178] Table 6.1 Polystyrene content in the supernatant of the precipitation

[0179]

[0180] It can be known from Table 6.1 that the iodine value of the solution decreases as the precipitation time increases. According to formula 2-2, the fewer the number of styrene double bonds in the solution, the smaller the iodine value, which proves that the PS-MPs dissolved in carbon tetrachloride gradually produce polystyrene solids under the action of the ethanol precipitant, so that the number of styrene double bonds in the solution gradually decreases.

[0181] Since the iodine values of the solutions at the precipitation times of 15 h and 24 h are not the same, it is not possible to determine when the polystyrene is completely precipitated. Therefore, the iodine value of the solution is calculated again at the precipitation time of 30 h by using the same method. It can be known from Table 6.1 that the iodine values of the solutions at the precipitation times of 24 h and 30 h are the same, which proves that the polystyrene is completely precipitated at 24 h, and the complete precipitation time is between 15 h and 24 h. Therefore, in order to ensure complete precipitation, the precipitation time in the subsequent experiments should be greater than or equal to 24 h.

[0182] According to the above experimental conclusion, when the PS-MPs are dissolved in carbon tetrachloride and the ethanol precipitant is added to precipitate the polystyrene, the precipitation time is 24 h.

[0183] 2.1.2 Determination of the suitable volume ratio of ethanol to carbon tetrachloride

[0184] Since the morphology and hardness of the re-precipitated polystyrene plastic are related to the volume ratio of anhydrous ethanol / carbon tetrachloride, granular, mucous and blocky morphologies may appear when different volume ratios are used, so the volume ratio of anhydrous ethanol / carbon tetrachloride corresponding to the polystyrene morphology conducive to the subsequent experiment is explored.

[0185] At the same time as the steps in 2.1.1, 20, 200, 2000 μm of PS-MPs were weighed 0.10 g respectively and put into 500 mL conical flasks, 50.0 mL of carbon tetrachloride was added to completely dissolve the PS-MPs, and after dissolution, 25.0, 50.0, 100.0, 150.0 mL of anhydrous ethanol was added respectively. After 24 h, the mixed solution containing polystyrene precipitate, carbon tetrachloride and anhydrous ethanol was filtered, the filter paper was washed with distilled water for 3 times after the first filtration and then filtered again to obtain the re-precipitated polystyrene plastic. The suitable volume ratio of anhydrous ethanol / carbon tetrachloride was selected according to the morphology of the polystyrene plastic and applied to the subsequent experiment.

[0186] Experimental results and analysis:

[0187] Morphology of the precipitate with different ethanol / carbon tetrachloride volume ratios: After the PS-MPs were dissolved in carbon tetrachloride, 25.0, 50.0, 100.0, 150.0 mL of anhydrous ethanol was added for precipitation, i.e. the volume of anhydrous ethanol was 0.5, 1, 2, 3 times of the volume of carbon tetrachloride, and the polystyrene precipitate was obtained by filtration. The morphology of the precipitate is shown in Table 6.2. Figure 5 The morphology of the precipitate with different volumes of ethanol for 200 μm of PS-MPs. The same volume of ethanol was used for precipitation, and the morphology of the precipitate obtained from the three particle sizes was consistent.

[0188] Table 6.2 Description of the morphology of the precipitate

[0189]

[0190] From Figure 5 and Table 6.2, when the volume ratio of ethanol to carbon tetrachloride is 0.5, the precipitate polystyrene is mucous, which is not easy to separate from the filter paper before drying, and the volume is the smallest; when the volume ratio of ethanol to carbon tetrachloride is 3, the structure of the precipitate is loose and can be easily crushed, and the volume is the largest. With the increase of the volume ratio of ethanol / carbon tetrachloride, the volume of the precipitate polystyrene increases, and the structure becomes more loose. The three particle sizes of 20, 200, 2000 μm of PS-MPs all show this rule.

[0191] 2.1.3 Qualitative and quantitative analysis of the precipitate

[0192] The precipitates obtained by filtering the ethanol / carbon tetrachloride volume ratio of 0.5, 1, 2, and 3 in 2.1.2 were weighed and recorded, i.e. preliminary quantitative detection, if the weighing results are not equal, further quantitative detection is required to determine whether the polystyrene is completely precipitated after adding different amounts of ethanol. The pyrolysis gas chromatography mass spectrometry (Py-GC / MS) can be used for quantitative analysis of polystyrene. The sample is first passed through a high-temperature pyrolysis furnace, and the polystyrene is rapidly cracked into styrene gas at high temperature, and then enters the chromatographic column for separation and detection in the mass spectrometer.

[0193] The precipitates obtained by filtering the ethanol / carbon tetrachloride volume ratio of 0.5, 1, 2, and 3 were weighed, and the weighing results are shown in Table 7. As can be seen from Table 7, after adding different amounts of ethanol, the mass of the precipitate is different, and it is not possible to determine whether the polystyrene is completely precipitated. Therefore, Py-GC / MS is used for accurate qualitative and quantitative detection of the precipitate.

[0194] Table 7 Mass of precipitate at different ethanol / carbon tetrachloride volumes

[0195]

[0196] Because the mass of the precipitate is not equal after adding different volumes of ethanol precipitant, further quantitative analysis of the precipitate is carried out using Py-GC / MS. Take part of the sample to be tested, dilute it twice with chloroform to obtain a diluent; take part of the diluent and add it to the sample crucible of Py-GCMS, and after the solvent in the crucible is completely volatilized, test it on the machine, and at the mass spectrometry analysis stage, take the styrene monomer with m / z of 51, 78 and 104 as the characteristic fragments, and record the peak area of the characteristic fragments in the spectrum at this time. Py-GC / MS requires that the sample amount is less than 50 mg, so 1 / 20 of the mass of each precipitate is detected to ensure that the sample amount is less than 50 mg. According to the proportion of the peak area of the characteristic spectrum in the detection results, the relative content of styrene double bond in polystyrene of the same mass is calculated, and whether the PS-MPs are completely precipitated after adding different amounts of ethanol is analyzed.

[0197] The Py-GC / MS test conditions are shown in Table 8. At the mass spectrometry analysis stage, the styrene monomer with m / z of 51, 78 and 104 is taken as the characteristic fragment, and when the sample enters the GC / MS after the cracker, the characteristic fragment peak value appears at about 5.310 min, and the spectrum and characteristic peak integral area are recorded.

[0198] Due to the small injection port of Py-GC / MS, it is impossible to ensure that all microplastic samples enter the instrument, but the instrument can monitor the mass of microplastic injection. According to the integral area of the characteristic fragment of different sample injection mass, it can be known that after adding different volumes of ethanol precipitator, the properties and styrene content of the precipitate. The mass of microplastic injection is uniformly recorded as 1 μg, and the integral area of the characteristic fragment is converted, as shown in Table 9, and the characteristic fragment spectrum is shown in Figure 6 .

[0199] Table 8 Py-GC / MS test conditions

[0200]

[0201] Table 9 Peak area of spectrum when the volume ratio of ethanol / tetrachloride is different

[0202]

[0203]

[0204] From Table 9 and Figure 6 , it can be known that when PS-MPs are dissolved in tetrachloride carbon and anhydrous ethanol is added to make polystyrene re-precipitate, the precipitate is polystyrene; after conversion, the integral area of the characteristic fragment is roughly equal, which proves that when the volume ratio of ethanol / tetrachloride is 3, 2, 1 and 0.5, polystyrene can be completely precipitated. However, as the results in 2.1.2 show that when the volume ratio of anhydrous ethanol / tetrachloride is 0.5 and 1, the precipitate is sticky, and there is a possibility that the precipitate and filter paper cannot be completely separated, therefore, in order to ensure the accuracy of the subsequent test results, the volume ratio of ethanol / tetrachloride should be 2 or 3, and the same experimental purpose should be achieved without causing waste of ethanol reagent. In the subsequent experiment, the volume ratio of ethanol / tetrachloride is 2 times.

[0205] 2.2 Necessity of precipitation step

[0206] (1) The experimental method includes the following steps:

[0207] Since the main purpose of the precipitation step is to remove the interfering substances in the soil to affect the test results, the ethanol removal effect is tested. Three kinds of soil samples with different textures, sand, loam and clay, are selected, and the soil samples are sieved through a 5 mm sieve for use. The soil samples are divided into two groups for experiment.

[0208] a. Dissolution-titration: Take 50.0 g of sieved soil sample into a 250 mL beaker, add 100 mL of tetrachloride carbon and stir with a glass rod to make the tetrachloride carbon fully wet the soil sample. After standing, filter, take 50 mL of filtrate for Wijs solution method titration, record the volume of sodium thiosulfate standard solution (0.05 mol / L) consumed, and at the same time, do a blank experiment;

[0209] b. Dissolution-precipitation-redissolution-titration: 50.0 g of the sieved soil sample was placed in a 250 mL beaker, 100 mL of carbon tetrachloride was added and stirred with a glass rod. The method took the soil sample as the sample to be tested, and the mass-volume ratio of the sample to be tested to carbon tetrachloride was 50.0 g: 100 mL. After standing, filtration was performed and 50.0 mL of filtrate was taken, 100 mL of anhydrous ethanol was added for precipitation, and the precipitate was redissolved with 50 mL of carbon tetrachloride. The Wijs solution method was used for titration, and the volume of sodium thiosulfate standard solution (0.05 mol / L) consumed was recorded. At the same time, a blank experiment was performed.

[0210] Comparing the experimental results of groups a and b, the experimental results do not need to be converted by iodine value. If the volume of sodium thiosulfate standard solution consumed in the experiment of group b is basically equal to that of the blank experiment, and forms a numerical difference with the results of group a, it is proved that the impurities in the soil that may participate in the Wijs solution reaction are dissolved in anhydrous ethanol in the experiment of group b, and the effective separation of the interfering substances in the soil and PS-MPs can be achieved, solving the non-specificity of the Wijs solution method detection, so that it can be applied to the quantitative detection of PS-MPs in soil.

[0211] Experimental results and analysis:

[0212] Carbon tetrachloride was added to sandy soil, loamy soil and clay, respectively, so that the substances that may exist in the soil and interfere with Wijs detection were dissolved in carbon tetrachloride. After the dissolution-Wijs titration and dissolution-precipitation-redissolution-Wijs titration steps, the volume (unit: mL) of sodium thiosulfate standard titration solution consumed was as shown in Table 10.

[0213] Table 10 Effect of precipitation step on impurity removal

[0214]

[0215] As can be seen from Table 10, in the experiment of group a, since the precipitation step was not set, the titration results of the three types of soil were different from each other and less than that of the blank experiment group, proving that there were interfering substances participating in the Wijs reaction in each type of soil sample. Among them, the sandy soil sample had the titration value closest to the blank group, and the clay consumed the least sodium thiosulfate solution. According to formula 2-2, it can be inferred that the clay contains the most interfering substances, followed by loamy soil, and sandy soil contains the least.

[0216] After the removal of interfering substances by the precipitation step in group b, the amount of titration solution consumed by sandy soil, loamy soil and clay was the same as that of the blank group, proving that the addition of the "precipitation impurity removal" step can better remove the interfering substances originally present in the soil. If the precipitation-redissolution step is not performed, it will cause the inaccuracy of the Wijs titration result, proving that the precipitation step is necessary.

[0217] 2.3 Establishing the standard curve of PS-MPs mass-iodine value of different particle sizes

[0218] The above test results show that after introducing the ethanol precipitant impurity removal step in the Wijs solution method, the interference of impurities in the soil on the quantitative detection of PS-MPs can be effectively eliminated. If this method is applied to the detection of PS-MPs in soil, the relationship between the mass of PS-MPs and the iodine value based on the Wijs solution method needs to be established, i.e., a standard curve, and the influence of interfering substances in the soil needs to be eliminated, so that the application of the Wijs solution method in the detection of PS-MPs in soil can be realized.

[0219] (1) The experimental method includes the following steps:

[0220] Dissolution: 0, 0.10, 0.20, 0.30, 0.40, and 0.50 g of PS-MPs of 20, 200, and 2000 μm particle sizes were respectively placed in 500 mL conical flasks, and 50.0 mL of carbon tetrachloride was added to each flask, and then the flasks were stoppered;

[0221] Precipitation: After the PS-MPs were completely dissolved, 100.0 mL of anhydrous ethanol was added (the amount of anhydrous ethanol added was determined according to the conclusion of 2.1.1), and the PS-MPs were precipitated;

[0222] Filtration: After the precipitation was complete, the filtrate was filtered, and the filter paper was washed with anhydrous ethanol three times before being filtered again. Then the precipitate was transferred to a 500 mL conical flask;

[0223] Dissolution: 50.0 mL of carbon tetrachloride was added again to dissolve the re-precipitated polystyrene;

[0224] Wijs reaction: 10.0 mL of Wijs solution was added, and the flask was placed in the dark (15-20°C) for 30 min, then 15.0 mL of 10% potassium iodide solution and 100 mL of distilled water were added;

[0225] Titration: The titration was performed with 0.05 mol / L sodium thiosulfate standard solution, and the titration results were recorded.

[0226] Calculation: The iodine value of PS-MPs of different masses was calculated according to the amount of standard solution consumed, and the calculation formula (2-2) was used to establish the standard curve of PS-MPs mass-iodine value, and the determination coefficients R of the standard curves of PS-MPs of three different particle sizes were compared 2 with the slope k, and the universality of this method for PS-MPs of different particle sizes was analyzed.

[0227] (2) Experimental results and analysis:

[0228] PS-MPs mass-iodine value standard curve: after dissolving PS-MPs in carbon tetrachloride, precipitating and redissolving, the solution was titrated by Wijs and the PS-MPs mass-iodine value standard curve was drawn to establish the relationship between them, and the determination coefficient R of the standard curve was determined 2 The applicability of Wijs solution method for quantitative detection of PS-MPs was determined, and the mass of PS-MPs with particle sizes of 20, 200 and 2000 μm was plotted against the iodine value standard curve (II) as shown in Figure 7

[0229] Table 11 PS-MPs-iodine value standard curve parameters

[0230]

[0231] From Figure 7 and Table 11, it can be seen that after introducing the step of "removing impurities by precipitation", the mass of PS-MPs with particle sizes of 20, 200 and 2000 μm has a linear relationship with the iodine value, and the determination coefficient R 2 is greater than 0.99, and the linearity is good; by comparing the slopes k of the three curve equations, it is found that as the particle size of PS-MPs increases, the slope k becomes smaller, which is consistent with the analysis in Example 1.3, that is, under the same mass, the volume of PS-MPs with small particle size is large, and the number of styrene double bonds is more, and the iodine value is greater.

[0232] Therefore, it is confirmed that the detection method of dissolving-precipitating-redissolving-Wijs titration can be applied to the quantitative detection of PS-MPs in actual samples. The above process is applied to environmental samples containing PS-MPs with unknown mass, and the Wijs titration results are brought into the standard curve, which can realize the quantitative detection of the mass of PS-MPs, and is suitable for PS-MPs with different particle sizes.

[0233] 2.4 Effect of precipitation step on quantitative detection of PS-MPs by Wijs method

[0234] Since the precipitation-redissolving process is introduced into the original Wijs method, the change of the standard curve before and after the introduction of this process is compared, and the change of the slope k is used as the standard, and the slope reflects the mass of PS-MPs, so the mass recovery rate of PS-MPs is calculated, and the slopes k of the mass-iodine value standard curves of PS-MPs with different particle sizes in Example 1.3.1 and Example 2.1.3 are compared, and it is calculated that after the addition of the ethanol impurity removal step, the mass recovery rate of PS-MPs is

[0235] Formula 3-1:

[0236]

[0237] K - slope of the standard curve made by dissolving and titration ​

[0238] K' – The slope of the standard curve obtained from dissolution-precipitation-dissolution-titration.

[0239] Experimental results and analysis of method recovery rate:

[0240] In Example 1.3, the correlation coefficient R of the PS-MPs mass-iodine value standard curve (Ⅰ) is... 2 This verified the applicability and accuracy of the original Wijs method for the quantitative detection of PS-MPs; while the detection method for PS-MPs in soil established in Example 2.3 yielded a standard curve (II). Since the detection method for obtaining standard curve (I) did not include a "precipitation and impurity removal" step, while the detection method for obtaining standard curve (II) did, the parameters of standard curves I and II are inconsistent.

[0241] By comparing the slope k of standard curve (Ⅰ) and standard curve (Ⅱ), the reasons were analyzed and the curve and experimental steps applicable to the detection of PS-MPs in actual samples were determined. The slope k of standard curve (Ⅰ) and (Ⅱ) are shown in Table 12.

[0242] Table 12 MPS recovery rate after dissolution-precipitation-dissolution treatment

[0243]

[0244] Calculations showed that K1 / K1', K2 / K2', and K3 / K3' were 83.70%, 83.40%, and 83.05%, respectively. Although the slopes of curves I and II were different, the ratios of their slopes were quite similar. As shown in Formula 2-2, the main reason affecting the slope of the curves was the difference in iodine value. The iodine value reflects the content of double bonds in polystyrene; the higher the content, the higher the iodine value. Therefore, the reason for the inconsistency between the slopes of standard curves I and II was that the precipitation-redissolution step caused a mass loss of polystyrene, approximately 16%-17%. The mass loss was relatively stable, so the Wijs method combined with the precipitation and impurity removal step still has a good ability to detect PS-MPs.

[0245] discuss

[0246] To make the Wijs method practically applicable, a precipitation step was introduced. In this embodiment, the precipitation time and the amount of ethanol precipitant were investigated, and it was determined that the Wijs method still has good detection capability after adding this step.

[0247] After PS-MPs were dissolved in carbon tetrachloride, polystyrene was re-precipitated by adding anhydrous ethanol. The supernatant was taken at 0.5, 8, 15 and 24 hours after precipitation for Wijs titration. The experimental results showed that polystyrene was completely precipitated after 24 hours, and the solution gradually became clear as the precipitation time increased. During this period, polystyrene continued to precipitate, so in the subsequent experiments, the precipitation time was 24 hours to ensure complete precipitation. Our experimental results showed that when the volume ratio of ethanol to carbon tetrachloride was 2, the morphology of the precipitate polystyrene was most conducive to the subsequent experiments. When the volume ratio of ethanol to carbon tetrachloride was 0.5 and 1, the precipitate was prone to adhere to the filter paper during filtration, resulting in inaccurate test results. When the volume ratio of ethanol to carbon tetrachloride was 3, the structure of the precipitate was relatively loose.

[0248] The precipitates obtained under different volume ratios of ethanol to carbon tetrachloride were subjected to qualitative and quantitative analysis. First, the weight analysis showed that the masses of the precipitates in each group were not equal, so the precipitates were further analyzed. According to the integral area of the py-GC / MS spectrum generated by the characteristic fragments of styrene with mass-to-charge ratios of 51, 78 and 104, it was found that when the volume ratio of ethanol to carbon tetrachloride was 0.5, 1, 2 and 3, the integral areas of the characteristic fragments were roughly equal, polystyrene was completely precipitated, and the precipitate was indeed polystyrene. Combined with the morphology of polystyrene after adding different amounts of ethanol, the volume ratio of ethanol to carbon tetrachloride was 2, which was most suitable.

[0249] After determining the experimental conditions, the necessity of the precipitation and impurity removal step was explored. It was found that after the a group treatment step, the titration results of sand, loam and clay were different and less than the blank group, indicating that the interfering substances in the soil samples consumed iodine in the Wijs solution. The results showed that the most interfering substances were in clay, followed by loam, and the least in sand. After the b group treatment step, the titration results of sand, loam and clay were equal and equal to the blank group, indicating that after introducing the precipitation and impurity removal step, the interfering substances in the soil samples were removed and no longer affected the quantitative detection results.

[0250] To ensure that after introducing the precipitation and impurity removal step, the Wijs method still has good quantitative detection ability for PS-MPs, the PS-MPs mass-iodine value standard curve was again prepared. The experimental results showed that PS-MPs with particle sizes of 20, 200 and 2000 μm still had a linear relationship between mass and iodine value, and the linear relationship was good, with a determination coefficient R 2All are greater than 0.99. With the increase of particle size, the curve slope decreases, and the reason is the same. The smaller the particle size of the same mass PS-MPs, the more styrene content, and the larger the iodine value of the solution. Compared with the standard curve of each particle size PS-MPs in Chapter 2, the curve slope of PS-MPs with the same particle size after different treatments is not the same, but it is found that the ratio of the two curve slopes is about 83%. Due to the difference of the curve slope, the ordinate iodine value is different, that is, the content of styrene double bond in the measured sample is different. According to the analysis, after adding the precipitation-redissolution step, the loss of polystyrene is about 16%-17%. But because of the good linear relationship of the curve and the certain rule, it is proved that the introduction of the precipitation step does not affect the quantitative detection of PS-MPs in different particle size PS-MPs. This method can be applied to the quantitative detection of PS-MPs in actual soil samples.

[0251] Example 3 Quantitative detection of PS-MPs in different texture soils

[0252] According to the mass recovery rate, the minimum detection limit and the precision of the method, the applicability of the new method in different texture soils is analyzed.

[0253] According to the research results of Example 1.3 and Example 2.1.3, the Wijs solution method is not limited by the particle size of PS-MPs, and has good detection ability for PS-MPs with three particle sizes. In order to better meet the real situation of soil environment, that is, each particle size of PS-MPs may exist, therefore, in this example, three particle sizes of PS-MPs are mixed into the soil sample, and the applicability of Wijs solution method in quantitative detection of PS-MPs in soil is analyzed.

[0254] 3.1 Establishment of mixed particle size PS-MPs mass-iodine value standard curve

[0255] (1) The experimental method includes the following steps:

[0256] Mix 20 μm particle size PS-MPs, 200 μm particle size PS-MPs and 2000 μm particle size PS-MPs in a mass ratio of 1:1:1 to obtain mixed particle size PS-MPs.

[0257] Dissolution: 0, 0.10, 0.20, 0.30, 0.40, 0.50 g of mixed particle size PS-MPs were weighed into 500 mL conical flask, and 50.0 mL of carbon tetrachloride was added respectively, and the bottle stopper was plugged;

[0258] Precipitation: after the PS-MPs were completely dissolved, 100.0 mL of anhydrous ethanol was added to precipitate the PS-MPs;

[0259] Filtration: After the precipitation is completed, filtration is performed, and the filter paper is rinsed with anhydrous ethanol three times before filtration is performed again. Then, the precipitate is moved to a 500 mL conical flask;

[0260] Redissolution: 50.0 mL of carbon tetrachloride is added again to dissolve the re-precipitated polystyrene;

[0261] Wijs reaction: 10.0 mL of Wijs solution is added, and the solution is placed in the dark (15-20°C) for 30 min. Then, 15.0 mL of a 10% potassium iodide solution and 100 mL of distilled water are added;

[0262] Titration: Titration is performed with a 0.05 mol / L sodium thiosulfate standard solution, and the titration results are recorded.

[0263] Calculation: The iodine value of different masses of PS-MPs is calculated according to the amount of standard solution consumed (the iodine value is calculated according to formula 2-2), and a standard curve of PS-MPs mass-solution iodine value is drawn.

[0264] (2) Experimental results and analysis

[0265] The mixed particle size PS-MPs are titrated by the Wijs solution method, and a PS-MPs mass-iodine value standard curve suitable for quantitative detection of PS-MPs in soil is drawn. The curve graph and curve parameters are shown in Figure 8 and Table 13. In the curve equation, y is the iodine value, and x is the PS-MPs mass (unit: g).

[0266] Table 13 PS-MPs mass-iodine value standard curve parameters

[0267]

[0268] As Figure 8 and Table 13 show, there is a linear relationship between the mass of PS-MPs containing three particle sizes and the iodine value, which is recorded as the total PS-MPs mass-iodine value standard curve in soil. The determination coefficient R 2 is greater than 0.99, and the linearity is good. When quantitative detection is performed on environmental samples containing unknown mass PS-MPs, the Wijs titration results are converted into iodine values and brought into the standard curve equation, which can realize quantitative detection of the mass of PS-MPs, and is suitable for particle size PS-MPs.

[0269] 3.2 Recovery rate calculation

[0270] (1) The experimental method includes the following steps:

[0271] Mixing and pretreatment:

[0272] The 20 pm particle size PS-MPs, 200 pm particle size PS-MPs and 2000 pm particle size PS-MPs were mixed in a mass ratio of 1:1:1 to obtain mixed particle size PS-MPs.

[0273] 0.25 g and 0.45 g of the mixed particle size PS-MPs were respectively taken and mixed with 50.0 g of each of the three types of soil samples of sandy soil, loamy soil and clay soil, and then filtered after mixing. The filtrate was titrated by the Wijs solution method, and six groups of repeated experiments were performed.

[0274] Dissolution: After the pretreatment was completed, the soil mixed with PS-MPs was moved to a 250 mL beaker, 100 mL of carbon tetrachloride was added and stirred with a glass rod, and then covered with a sealing film after the soil was completely soaked. The time required for complete dissolution of PS-MPs was determined according to the conclusion in Example 1.

[0275] In the above method, the soil mixed with PS-MPs was used as the sample to be detected, and the mass-volume ratio of the sample to be detected to carbon tetrachloride was 50.25 g:100 mL and 50.45 g:100 mL, respectively.

[0276] Precipitation: After stirring with a glass rod, filtration was performed, 50.0 mL of filtrate was collected in a beaker, 100 mL of anhydrous ethanol was added to precipitate PS-MPs (the volume ratio of anhydrous ethanol to carbon tetrachloride was 1:1), and after 24 hours (determined according to the previous titration result), the filtrate was again filtered, and the filter paper was washed with ethanol to obtain the re-precipitated polystyrene plastic.

[0277] Quantitative detection: The precipitate was placed in a 500 mL conical flask, and 50 mL of carbon tetrachloride was again added to dissolve it (the volume ratio of anhydrous ethanol to carbon tetrachloride was 2:1). After dissolution, the Wijs solution method was used for titration to obtain the amount of sodium thiosulfate standard solution consumed, and the iodine value was calculated. The test was performed in six parallel groups.

[0278] Recovery rate calculation: The recovery rate when 0.25 g and 0.45 g of PS-MPs were added to different types of soil was calculated according to formula 3-1. Since only 50.0 mL of filtrate was collected in the above precipitation process, and the PS-MPs were dissolved in 100 mL of carbon tetrachloride in the first dissolution process, according to the titration result, the calculated mass of PS-MPs was multiplied by 2 before being compared with the original mass for calculation.

[0279] (2) Experimental results and analysis

[0280] Different types of soil were mixed with 0.25 g and 0.45 g of mixed particle size PS-MPs after filtration, and the filtrate was titrated by the Wijs solution method, and six groups of repeated experiments were performed.

[0281] The average detection result of the sample of the sandy soil containing 0.25 g of the mixed particle size PS-MPs is 0.229 g. The average detection result of the sample of the loam soil containing 0.25 g of the mixed particle size PS-MPs is 0.230 g. The average detection result of the sample of the clay soil containing 0.25 g of the mixed particle size PS-MPs is 0.231 g. The average sample detection result of the sandy soil containing 0.45 g of the mixed particle size PS-MPs is 0.413 g. The average sample detection result of the loam soil containing 0.45 g of the mixed particle size PS-MPs is 0.415 g. The average sample detection result of the clay soil containing 0.45 g of the mixed particle size PS-MPs is 0.415 g.

[0282] The recovery rate of the PS-MPs in the different texture soils is calculated by using formula 3-1, and the average value of the recovery rate is calculated, and the calculation result is shown in Table 14.

[0283] Table 14 Recovery rate of PS-MPs in different texture soils

[0284]

[0285]

[0286] As shown in Table 14, the recovery rate of the Wijs solution method for 0.25 g and 0.45 g of PS-MPs is greater than 90%, which can well meet the detection requirements. The average value of the PS-MPs recovery rate of each group of sandy soil, loam soil and clay soil increases in turn, and the soil texture is more viscous, and the filtration time increases. We analyze the reason why the recovery rate of PS-MPs in different texture soils increases in turn. When the carbon tetrachloride in which polystyrene is dissolved is filtered and separated from the soil, part of the carbon tetrachloride volatilizes after a long time of filtration in the clay, and the air flow rate in the fume hood is relatively fast, so the same volume of carbon tetrachloride contains more styrene double bonds, and the recovery rate of polystyrene increases. The reason why the recovery rate of the 0.25 g experimental group is slightly lower than that of the 0.45 g experimental group is that the more polystyrene dissolved in the same volume (50 mL) of carbon tetrachloride, the greater the viscosity of the solution, the longer the filtration time, the more volatile the carbon tetrachloride, and the greater the recovery rate of polystyrene.

[0287] 3.3 Calculation of the lowest detection limit

[0288] (1) The experimental method comprises the following steps:

[0289] According to the amount of titration blank group consumption titration solution, and the minimum graduation value of titration method is 0.01, the theoretical minimum detection limit of PS-MPs quantitative detection can be calculated, but due to the difference of different texture soil porosity, particle composition and other properties, the volume of filtrate obtained in the above precipitation process is not the same, which will make the Wijs solution detection method have different minimum detection limit in different texture soil, so for the soil sample used in this experiment, the minimum detection limit of different texture soil is determined.

[0290] (2) Experimental results and analysis

[0291] According to the experimental results of titration blank group, the theoretical minimum detection limit value of Wijs solution method can be calculated, but when applying Wijs solution method to the process of quantitative detection of PS-MPs in soil, only part of the filtrate can be filtered out when separating the carbon tetrachloride dissolved with PS-MPs from the soil sample, and the initial added carbon tetrachloride cannot be filtered out completely. The amount of filtrate is related to the degree of soil weight. According to the amount of filtrate that can be filtered out, the minimum detection limit of Wijs solution method for PS-MPs in different texture soil is calculated, and the results are shown in table 15 and table 16.

[0292] Table 15 Average amount of filtrate that can be collected from three kinds of texture soil

[0293]

[0294] Table 16 Minimum detection limit of Wijs method in three kinds of texture soil

[0295]

[0296] From table 15, it can be seen that the average volume of filtrate that can be filtered out from sandy soil, loamy soil and clay soil decreases in turn. It can be known that the higher the content of organic matter in soil, the smaller the soil particles, and the less the filtrate collected. When filtering and separating the carbon tetrachloride dissolved with PS-MPs from the soil, there is a difference in the minimum detection limit of different texture soil. According to the amount of filtrate that can be collected in different texture soil, the corresponding minimum detection limit value is calculated.

[0297] The minimum detection limit of Wijs solution method for quantitative detection of PS-MPs in different texture soil is shown in table 16. Since 74.8, 65.0 and 53.3 mL of filtrate can be filtered out from sandy soil, loamy soil and clay soil respectively, and the initial amount of carbon tetrachloride is 100 mL, the minimum detection limit in three kinds of soil sample is 1.32, 1.51 and 1.87 times the theoretical minimum detection limit value of blank group respectively.

[0298] 3.4 Precision calculation

[0299] (1) The experimental method includes the following steps:

[0300] Each sample was tested in 6 parallel experiments, and the precision of the method was expressed by the relative standard deviation (RSD), which was calculated according to the following formula, and the good precision of the Wijs solution method applied to different textured soils was analyzed according to the results.

[0301]

[0302]

[0303] Wherein:

[0304] X i - each measurement value

[0305] - Xi arithmetic mean value

[0306] n - number of measurements

[0307] SD - standard deviation

[0308] (2) Experimental results and analysis

[0309] The Wijs solution method was used to quantitatively detect PS-MPs with different masses in different textured soil samples, and 6 groups of parallel experiments were performed, and the precision was calculated according to the above formula, and the results are shown in Table 17.

[0310] Table 17 Precision when applied to three soil textures

[0311]

[0312] As shown in Table 17, the Wijs solution method applied to the quantitative detection of PS-MPs in different textured soils has a precision value of less than 0.88%, and has good precision, and the detection results have good repeatability and reproducibility, and the Wijs solution method is suitable for the quantitative detection of PS-MPs in soil.

[0313] Discussion

[0314] After the above examples are verified, the Wijs solution method combined with impurity removal by precipitation can realize the quantitative detection of PS-MPs with different particle sizes, and the method is now applied to actual soil samples for detection and analysis of its applicability. Since PS-MPs with various particle sizes may exist in the real soil environment, a mass-iodine value standard curve of 20, 200 and 2000 μm mixed particle size PS-MPs was made, and the curve was linear, and the determination coefficient R 20.99. 0.25 g and 0.45 g of PS-MPs (one third of each of the three particle sizes) were mixed into the sandy soil, loamy soil and clay soil samples respectively, and after a dissolution-filtration-precipitation-redissolution-Wijs titration process, the titration results were brought into the standard curve to calculate the PS-MPs recovery rate.

[0315] The experimental results show that the recovery rate of PS-MPs of different qualities in different soil is higher than 91.33%, and the recovery rate is relatively high. We also found that the average recovery rate of clay is the highest, followed by loamy soil, and the lowest is sandy soil. We guess that the reason for this phenomenon may be that the clay soil is dense, and the filtration time is longer than that of loamy soil and sandy soil. Carbon tetrachloride has volatility, and in the environment with fast air flow in the fume hood, the increase of filtration time increases the volatilization of carbon tetrachloride, so that the content of styrene double bond in the solution is more, which leads to the larger recovery rate. In addition, the recovery rate of the group with 0.25 g of PS-MPs is lower than that of the group with 0.45 g of PS-MPs. The reason is consistent with the reason described above. The solution of the group with 0.45 g of PS-MPs is more viscous than that of the group with 0.25 g, so the filtration time is increased, which causes the volatilization of carbon tetrachloride. For the volatilization loss of the solution during the filtration process, the volatilization of carbon tetrachloride can be reduced by using a short-necked funnel during filtration and sealing the container containing the solution in time during the filtration process. The py-GC / MS method, which is also a mass quantitative method, has a recovery rate of about 86.6-94.6% when detecting PS-MPs, while the Wijs method has a recovery rate of about 91.33-93.05%, which shows that this method can basically meet the requirements of quantitative detection for recovery rate. However, the py-GC / MS method needs gas chromatography and mass spectrometer equipment to realize detection, which has a high purchase cost of equipment and needs professional personnel to operate. The Wijs method provided by the present application can realize quantitative detection of microplastics by chemical reaction between substances, which does not need complex professional equipment and has a low cost.

[0316] The minimum detection limit of the Wijs method provided by the present application for different soil textures is different, and the main reason is that the particle composition of different soil textures is different, and the soil weight is different. When the carbon tetrachloride solution dissolved with PS-MPs is filtered and separated from the soil, the maximum amount of filtrate obtained is different. The average amount of filtrate of sandy soil, loamy soil and clay is 74.8, 65.0 and 53.3 mL respectively. With the decrease of soil particles and the increase of soil weight, the maximum amount of filtrate decreases. Therefore, the minimum detection limit changes, and the minimum detection limit of PS-MPs in the three types of soil is 0.82, 0.94 and 1.2 μg / g respectively.

[0317] The precision (RSD) of the Wijs method for quantitatively detecting different quality PS-MPs in different soils by using the average recovery rate is less than 0.88%, and the precision of the py-GC / MS for quantitatively detecting PS-MPs is about 6.6%, and the smaller the precision value is, the better the precision is, and the detection method provided by the application has good detection stability and reproducibility.

[0318] It can be found through the test of the three indexes of recovery rate, minimum detection limit and precision that the Wijs method for detecting PS-MPs in soil provided by the application is feasible, and has good applicability to different soil textures.

[0319] Polystyrene (PS) is one of the common plastic types, and the global annual output of polystyrene has exceeded 2000 tons, and polystyrene is applied in many fields, such as biomedical, nanometer instruments and disposable foam lunch boxes. The polystyrene plastic flowing into the environment is subjected to physical, chemical and biological effects of nature or human, and is broken into polystyrene microplastics smaller than 5 mm, and the microplastics can cause certain damage to the organism after being taken by the organism, and in addition, the microplastics have the property of adsorbing heavy metals and organic pollutants, and the influence of the microplastics on the ecological environment is self-evident. The existing researches are mostly for the influence and phenomenon of the microplastics in the organism, and lack of quantitative detection methods and standards. Based on a more accurate and convenient quantitative method, follow-up researches in the field of microplastics can be better carried out. Due to the characteristics of the microplastics, there is a certain difficulty in the quantitative detection of the microplastics, and there is no recognized detection method for the microplastics in the industry at present.

[0320] The application provides a new method for quantitatively detecting microplastics, optimizes the detection method, and verifies the feasibility of the method in the quantitative detection of microplastics in environmental media. It is proved through experiments that the detection method provided by the application does not need to use complex detection instruments, and does not need to train personnel to be familiar with the operation of the instruments. The detection method provided by the application can realize the quantitative detection of microplastics by using chemical reactions between substances, and has the advantages of accurate results, high recovery rate, good precision, good stability and reproducibility, suitable for quantitative detection of microplastics in different environments, not limited by particle size, simple pretreatment steps, low cost and the like.

[0321] The above only describes the preferred embodiments of the application, and is not used to limit the application, and any modification, equivalent replacement, improvement and the like made within the spirit and principle of the application should be included in the protection scope of the application.

Claims

1. A method for quantitatively detecting microplastics, characterized in that, The method comprises the following steps: (1) dissolving: dissolving the sample to be detected with a dissolving agent; the sample to be detected is a soil sample, and the soil is selected from any one or any combination of sandy soil, loamy soil and clay; the microplastics are selected from polystyrene microplastics, and the particle size of the polystyrene microplastics is 20, 200 or 2000 μm; the dissolving agent is carbon tetrachloride; the mass-volume ratio of the microplastics in the sample to be detected to the dissolving agent is ≤1 g:50 mL; (2) filtering; (3) precipitating: adding a precipitating agent to the dissolved product for precipitation; the precipitating agent is anhydrous ethanol; the volume ratio of the precipitating agent to the dissolving agent is 2:1 to 3:1; the precipitation time is ≥24 h; (4) filtering; (5) redissolving: redissolving the precipitate obtained after filtering with a dissolving agent; the dissolving agent is carbon tetrachloride; the volume ratio of the dissolving agent added for redissolving to the precipitating agent is 1:(2-3); (6) Wijs reaction, titration, calculating the iodine value according to the titration result to determine the mass of the microplastics.

2. The method for quantitative detection of microplastics according to claim 1, characterized in that, The volume ratio of the precipitating agent to the dissolving agent is 2:

1.

3. The method for quantitative detection of microplastics according to claim 1 or 2, characterized in that, In step (6), the Wijs reaction comprises the following steps: adding the redissolved solution to a Wijs solution, placing in the dark at 15-20°C for 30 min, and then adding a potassium iodide solution and distilled water.

4. The method for quantitative detection of microplastics according to claim 3, characterized in that, The volume ratio of the Wijs solution, the potassium iodide solution and the distilled water is 10:15:100, and the content of potassium iodide in the potassium iodide solution is 10%.

5. The method for quantitative detection of microplastics according to claim 1 or 2, characterized in that, In step (6), the titration comprises the following steps: titrating with a sodium thiosulfate standard solution and recording the titration result.

6. The method for quantitative detection of microplastics according to claim 5, characterized in that, The concentration of sodium thiosulfate in the sodium thiosulfate standard solution is 0.05 mol / L.

7. The method for quantitative detection of microplastics according to claim 1 or 2, characterized in that, In step (6), the iodine value of the sample to be detected is calculated according to the amount of the standard solution consumed, and the mass of the microplastics is calculated according to a standard curve of the mass of the microplastics to the iodine value of the solution.

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