CN111426545A - Method for quickly separating micro-plastic in soil - Google Patents

Abstract

The invention belongs to the technical field of pretreatment of environmental pollutants, and in particular relates to a method for rapidly separating microplastics in soil. The method for separating microplastics in the soil includes: pre-digesting the soil sample, density separation flotation, and oxidative digestion; the pre-digesting takes 30% H ₂ O ₂ by mass as the oxidant, under the condition of 65-75° C. The soil samples were digested until the soil samples turned white or gray in color. According to the characteristics of the soil, the present invention proposes to increase the pre-digestion treatment on the basis of the existing method for separating microplastics in the soil, and optimize the operation conditions of the pre-digestion and the subsequent oxidative digestion. The specific pollution status of microplastics, and the extraction time is greatly shortened, the operation is convenient, and manpower and material resources are saved; more importantly, this method does not affect the content of adsorbed pollutants on the surface of microplastics, which is more conducive to the subsequent analysis of the adsorption laws on the surface of microplastics. Research.

Description

A method for rapid separation of microplastics in soil

technical field

The invention belongs to the technical field of pretreatment of environmental pollutants, and in particular relates to a method for rapidly separating microplastics in soil.

Background technique

Plastic products have been widely used in daily life due to their advantages of low cost, good plasticity and good durability. In the past 50 years, the global plastic production is about 9.1 billion tons, with an average annual growth rate of 8.7%. The problem of plastic waste is undoubtedly a serious environmental challenge due to huge production and efficiency mismanagement.

Although recycling rates of plastic products are increasing, most of the plastic is still released into the environment. In the environment, plastic waste can be degraded by physical, chemical and biological driving forces (such as ultraviolet radiation, wind or water erosion, etc.) and become smaller plastic waste.

Microplastics (MPs) are small plastic fragments less than 5mm in size. While microplastics are also commonly found in marine and freshwater systems, soil may be a larger reservoir for them than the ocean. On the one hand, microplastics entering the soil can be stored, transferred, eroded, degraded, and infiltrated into groundwater, threatening organisms and further affecting human health; on the other hand, soil biota can affect the accumulation and fate of microplastics.

Moreover, many types of additives (such as plasticizers, stabilizers, flame retardants, monomers, etc.) used in plastic production to improve the performance of plastic products will leach during the life cycle of plastic products, especially in the soil environment. At the same time, plastics can absorb many toxic substances (such as metals, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs, organochlorine pesticides DDT, HCH, etc.) due to their hydrophobic surface properties, resulting in new complex pollution problems. Therefore, the recovery and analysis of microplastics in soil is particularly important.

However, soil is a solid-liquid mixture, which contains a very high proportion of organic matter, clay, minerals, etc.; in these components, some organic matter will be embedded in microplastics, affecting the effectiveness of microplastics flotation and separation, and also Some organic matter will be adsorbed on the surface of microplastics and interfere with the signal of infrared microscopy to identify microplastics; although there are some reliable analytical methods for separating microplastics from complex, organic-rich soil matrices, such as CN109855930A, CN1108637533A, etc. Separation devices for microplastics, but these devices are too complicated, which limits their widespread use; and some relatively simple devices and methods for analyzing soil microplastics, such as CN110369117A, due to incomplete digestion, resulting in the later qualitative and quantitative analysis of microplastics accuracy decreased.

SUMMARY OF THE INVENTION

In order to solve the above technical problems, the present invention provides a method for rapidly separating microplastics in soil. The method not only has obvious separation effect, high sample recovery rate, and can reflect the specific pollution status of microplastics in the soil, but also greatly shortens the extraction time, is convenient to operate, and saves manpower and material resources; more importantly, the method does not affect the surface of microplastics The qualitative and quantitative analysis of adsorbed pollutants is more conducive to the subsequent study of the adsorption laws on the surface of microplastics.

The method for separating microplastics in soil according to the present invention includes: pre-digesting soil samples, density separation flotation, and oxidative digestion;

The pre-digestion is to digest the soil sample at 65-75° C. with a mass concentration of 30% H ₂ O ₂ as an oxidant until the color of the soil sample turns white or gray.

Studies have shown that the existing method for separating microplastics in soil adopts the process of flotation and then digestion, but some organic matter is embedded in the microplastics, which affects the flotation effect and reduces the recovery rate. In order to improve the flotation effect, technicians try the pre-digestion-flotation-post-digestion process, but it is often difficult to grasp the scale, which is easy to cause insufficient digestion, unsatisfactory flotation effect, or excessive digestion, resulting in the adsorption of pollutants on the surface of microplastics content decline.

In response to this problem, according to the characteristics of organic matter and impurities in the soil, the technical personnel of the present application propose to control the temperature and degree of digestion before the digestion to ensure the effect of the pre-digestion, which can effectively digest most of the organic matter in the soil and prevent it from affecting the microplastics. The flotation effect can improve the recovery rate of microplastics, and also provide a better environment for the subsequent oxidation and digestion of organic matter on the surface of microplastics; more importantly, it can avoid the influence of excessive digestion on the content of adsorbed pollutants on the surface of microplastics and ensure subsequent Accuracy of adsorption studies on microplastic surfaces.

According to some embodiments of the present invention, the temperature of the pre-digestion is preferably 69-71°C; studies have shown that under this temperature condition, the comprehensive effect of the digestion is better.

According to some embodiments of the present invention, the oxidant is slowly added to the soil sample at 10-20° C. to avoid affecting the digestion effect due to a rapid temperature rise; some cooling devices, such as a cold water bath, can be selected according to actual needs .

The added amount of the oxidant (30% by mass H ₂ O ₂ ) is not easy to be added too much. According to some embodiments of the present invention, the volume-to-mass ratio of the oxidant to the soil sample is (180-250) ml:50 g. Studies have shown that this ratio can not only ensure sufficient digestion of organic matter, help improve the recovery rate of microplastics, but also avoid excessive digestion and affect the content of adsorbed pollutants on the surface of microplastics.

According to some embodiments of the present invention, n-butanol is also added during the pre-digestion process; studies have shown that adding n-butanol during the digestion process can inhibit the generation of a large amount of foaming and help improve the digestion effect. The addition amount of the n-butanol is not easy to be too much, usually just a few drops are added.

According to some embodiments of the present invention, after the pre-digestion is completed, the sample is dried and cooled for use.

In order to improve the recovery rate of microplastics, the supernatant obtained by the density separation flotation must be further oxidized and digested. The oxidant used in the oxidative digestion is H ₂ O ₂ with a mass concentration of 30%; preferably, the temperature of the oxidative digestion is 65-75 ° C; through further oxidative digestion, the adsorption and the organic matter on the surface of the microplastics and other influences can be detected The impurities are removed, usually digestion for 48-50h. After the oxidative digestion is completed, the obtained digestion solution is filtered to obtain a microplastic solid material, which can be subjected to subsequent detection. The pore size of the filter membrane is 20um.

According to some embodiments of the present invention, the process of collecting soil samples includes: air-drying and sieving the collected soil samples. Preferably, after sieving, the particle size of the obtained soil sample is less than 2 mm for better digestion and recovery.

Preferably, when the collection is performed, the setting of sampling points should reflect the overall or average level of on-site (micro)plastic pollution, so that subsequent analysis and quantification can accurately represent the state of soil microplastic pollution.

According to some embodiments of the present invention, the flotation agent used in the density separation flotation is zinc chloride.

According to some embodiments of the present invention, the process of density separation and flotation includes: using a flotation solution to flotate the pre-digested and dried samples, let stand, and separate a supernatant containing microplastics, and then The supernatant was separated by a vacuum sand filter to collect the microplastic solid material. In order to improve the recovery rate, flotation can be performed several times, and the supernatants are combined and separated.

We know that when a mixture of materials of different densities (such as soil and microplastics) is put into a medium density liquid (water or salt solution), materials with a lower density than the liquid (such as microplastics) will float on top of the liquid, Whereas materials with a higher density than liquid such as (soil matrix) will sink; thus floating microplastics can be collected by sucking up the liquid (supernatant) above the sediment layer. In the present invention, preferably, the density of the flotation solution is 1.4-1.6 g/cm ³ . The flotation effect is better in this range.

Preferably, in the density separation flotation, the stirring speed is 200-250r/min, and the stirring time is 1-1.5h. In order to fully guarantee the flotation effect, multiple flotation can be carried out.

The vacuum sand core filter device used in the present invention is the existing separation filter device. Before use, a filter membrane can be placed on the standard port sand core filter, the aperture is preferably 5mm, and washed with a small amount of zinc chloride solution , for better collection of solid materials.

The beneficial effects obtained by the present invention are as follows:

(1) The separation method proposed by the present invention utilizes pre-digestion to remove the organic matter embedded in the microplastics, significantly improves the flotation effect, and significantly improves the recovery rate of the microplastics; at the same time, reasonably controls the pre-digestion operating conditions and the degree of digestion to avoid surface adsorption on the microplastics The reduction of pollutant content is more conducive to the subsequent research on the adsorption performance of microplastic surfaces.

(2) Compared with the existing separation method, the separation method of the present invention greatly shortens the operation time, from 3-4 days of the existing separation to 2 days, and improves the separation efficiency.

(3) The separation method of the present invention has a wide range of applications, and can be used to separate various types of microplastics, such as high-density plastics PET, PVC, and the like.

(4) The separation method of the present invention is environmentally friendly, the selected flotation liquid and other solutions have less pollution to the environment, and the flotation liquid can be reused.

Description of drawings

FIG. 1 is a process flow diagram of the method for rapidly separating microplastics in soil according to Example 1.

FIG. 2 is a vacuum sand filter device for separating microplastics according to Example 1. FIG.

Detailed ways

The following examples are intended to illustrate the present invention, but not to limit the scope of the present invention.

Example 1

The present embodiment provides a method for separating microplastics in soil, as shown in Figure 1, and the specific steps are as follows:

1) Collection and pretreatment of soil samples:

Clean all laboratory surfaces and adhere to the microplastic contamination prevention protocol, collect a mixed soil sample of about 1 kg and pack it into a woven bag; thoroughly mix the bulk soil sample with a metal spatula for 2 minutes, taking care to avoid contamination, air dry naturally in a tray, and then After sieving, the aperture of the plug is 2mm;

2) Pre-digestion process:

_A 30% mass concentration H2O2 solution was used as the oxidant; 50 _g of soil sample was added to a ₂₅₀ mL Erlenmeyer flask, and ₂₀₀ mL of 30% H2O2 solution was slowly added incrementally. To avoid the temperature rising too quickly, place the Erlenmeyer flask in a cold water bath.

When the temperature remains stable, heat on a digestion plate at 70°C with frequent stirring to avoid bumping. Add a few drops of n-butanol when the foam is too high.

When the color of the sample changes to white or gray, the sample is continued to be heated to near dryness and then cooled for the next process.

3) Density separation flotation:

Prepare zinc chloride (ZnCl ₂ ) solution (density 1.5g/cm ³ ) as flotation solution;

After pre-digestion and drying, approximately 200 mL of the flotation solution was added to the Erlenmeyer flask. Stir on a magnetic stirrer at 200 r/min for 1 hour. After standing for 48 hours, absorb about 100ml of the supernatant; add 100ml of zinc chloride salt solution to the conical flask, stir repeatedly for 30min, and absorb 100ml of the supernatant; combine the two supernatants; the microplastics in the supernatant The separation is carried out by a vacuum sand core filter device as shown in Figure 2.

Set up the vacuum sand filter device to ensure sealing, turn on the vacuum pump and place a filter membrane on the standard sand filter with a pore size of 5mm. Pour a small amount of zinc chloride solution into a cylindrical glass funnel to wet the filter and flush the system.

Pour the collected supernatant slowly into a cylindrical glass funnel to collect any solid material. Rinse the beaker carefully with deionized water and pour the rinse into a cylindrical glass funnel to ensure that all material in the beaker is collected. Remove the filter from the cylindrical glass funnel, making sure that no material is lost on the filter.

4) Post digestion:

After vacuum filtration, rinse the solid material _on the filter membrane into a 50 ml digestion tube with 30% _H2O2 . A 30% membrane solution was used for digestion at 70 °C for 2 d to completely remove the organic matter attached to the MPs and the organic matter with other properties retained by the filtration.

Then, the digested solution was filtered, the pore size of the filter membrane was 20um, and it was observed on a stereo microscope (Nikon SMZ 745T).

Effect verification

1. Carry out the verification test of recovery effect with analytical grade polymers (PVC, polyethylene, polypropylene, polystyrene and polyamide, diameter 500um to 2mm).

Specific steps are as follows:

30 polymer plastic particles were mixed with 50 _g of relatively clean soil, the mixture containing the final microplastics was washed from the filter using the method described in Example ₁ , and 30% H2O2 was added to digest organic matter. Finally, the digested solution was filtered through a 20 μm membrane filter.

The recovered microplastic particles were counted under a stereo microscope, and the results are shown in Table 1.

Table 1

From the results in Table 1, it can be seen that 30% H ₂ O ₂ pre-digestion treatment can significantly improve the extraction rate of MPs in soil and sludge, especially for soil samples with high organic matter content.

2. The effect verification of the optimization conditions for the specific operation of the pre-digestion

Taking polyethylene PE fibrous microplastics as an example, the method described in Example 1 was used to separate and extract, and at the same time, the same amount of PE fibrous microplastic soil samples were separated and recovered by changing the pre-digestion conditions. The comparison results are shown in Table 2.

Table 2

It can be seen from Table 2 that the recovery rate of microplastics can be significantly improved by using the pre-digestion conditions of the present invention. At the same time, it was verified that the increase of pre-digestion treatment did not affect the change of the content of adsorbed pollutants on the surface of microplastics according to the results of the subsequent microplastics detection test.

Although the present invention has been described in detail above with general description and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements can be made on the basis of the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention fall within the scope of the claimed protection of the present invention.

Claims (10)

1. A method for separating soil micropolastics, comprising: carrying out digestion, density separation flotation and oxidation digestion treatment on a soil sample;

the pre-digestion is 30% H by mass concentration₂O₂Digesting the soil sample at 65-75 ℃ for an oxidant until the color of the soil sample is changed into white or gray.

2. The method for separating micro-plastic in soil according to claim 1, wherein the temperature of the pre-digestion is 69-71 ℃.

3. The method for separating micro-plastic in soil according to claim 1 or 2, wherein the oxidant is slowly added to the soil sample at 10-20 ℃.

4. The method for separating the micro-plastic in the soil as claimed in claim 3, wherein the volume mass ratio of the oxidant to the soil sample is (180) ml:50 g.

5. The method for separating the micro-plastics in the soil according to any one of claims 1 to 4, wherein n-butanol is further added in the pre-digestion process.

6. The method for separating the micro-plastics in the soil as claimed in any one of claims 1 to 5, wherein the oxidant adopted for oxidative digestion is H with the mass concentration of 30%₂O₂。

7. The method for separating microplastic in soil according to claim 6, wherein the temperature of oxidative digestion is 65-75 ℃.

8. The method of separating microplastics in soil of any of claims 1-7, characterized in that said soil sample has a particle size of less than 2 mm.

9. The method for separating micro plastic in soil according to any one of claims 1 to 8, wherein the flotation agent used for density separation flotation is zinc chloride.

10. The method for separating micro plastic in soil according to claim 8, wherein the density of the flotation solution used in the density separation flotation is 1.4-1.6g/cm³；

Preferably, in the density separation floating, the stirring speed is 200-250r/min, and the stirring time is 1-1.5 h.

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