CN115307987B - A method for batch separation and extraction of microplastics in film-covered farmland soil - Google Patents

Abstract

The present invention belongs to the technical field of separation and extraction of microplastics, and specifically relates to a method for batch separation and extraction of microplastics in film-covered farmland soil, which can complete the extraction of microplastics in soil more quickly and efficiently by ultrasound, centrifugation, stirring and filtering in sequence. The present invention utilizes the cavitation effect of ultrasonic waves to quickly and efficiently peel off, disperse and fall off the impurities attached to microplastics, and the extracted microplastics are purer; olive oil is added to the solution for separating microplastics, and the lipophilicity of plastics is utilized to avoid the influence of the density of microplastics on the extraction effect; the centrifugal process replaces the traditional static sedimentation process, shortening the extraction time; the separation and extraction method of the present invention is simple to operate and environmentally friendly, and experiments have shown that the method can separate and extract film-like PE microplastics with a particle size of 20-5000μm in batches, and the recovery rate reaches 96%±5%, and the time for completing the separation and extraction of microplastics from a single soil sample is about 30min.

Description

Method for separating and extracting film-covered farmland soil microplastic in batches

Technical Field

The invention relates to the technical field of microplastic separation, in particular to a method for separating and extracting film-covered farmland soil microplastic in batches.

Background

Since the invention and application of the mulch film in the 50 th century of the 20 th century, the problem of pollution to soil caused by microplastic (plastic with the diameter smaller than 5 mm) further decomposed from the mulch film becomes a research hot spot gradually due to the increasingly accumulated mulch film residue in the mulch film covered farmland while the global agriculture is high-yielding and stable-yielding. How to efficiently and rapidly separate and extract microplastic from the soil of a covered farmland becomes the first problem to be solved.

The existing separation and extraction method of the microplastic in the soil is mainly based on the density flotation principle, after the soil is stirred and stood in the saline solution, the soil and impurities are naturally precipitated in the saline solution due to the density difference, the microplastic floating in the supernatant is collected by filtration (patent application numbers 201910379997.9, 202010218711.1, 202110312146.X and 202111196997.9), and the method only needs 4-48 hours or even longer in the standing process, and the period of separating and extracting the microplastic is longer. Lu Xuejiang, he Defu and the like are based on the density flotation principle, microplastic is extracted in a circulating device by using an aeration and overflow method (patent application numbers 201810604532.4, 201910028203.4 and 201920050533.9), tiny impurities participate in circulation during aeration, a filter membrane is easily blocked, so that experiments are interrupted, and the separation and extraction efficiency is low. The main component of the microplastic in the soil of the covered farmland is Polyethylene (PE), the microplastic is in a film shape, and because of long-term abrasion, the surface of the microplastic is rough and porous and is irregularly curled, a large amount of impurities such as soil and organic matters can be adsorbed, the effect of stripping the impurities after the treatment by the method is not ideal, and part of microplastic can be precipitated at the bottom of the solution, so that the microplastic recovery efficiency is low.

Disclosure of Invention

In order to overcome the defects in the existing soil microplastic separation and extraction technology, the invention provides a method for separating and extracting the film-covered farmland soil microplastic in batches. Aims at solving the problems of long time consumption, low extraction efficiency and difficult PE microplastic separation in the process of separating the soil microplastic of the covered farmland.

In order to achieve the above purpose, the technical scheme of the invention is as follows:

A method for separating and extracting film-covered farmland soil micro-plastics in batches comprises the following steps:

(1) Adding saturated sodium chloride solution into a centrifuge tube with a weighed soil sample, mixing uniformly, and then placing into an ultrasonic cleaner for ultrasonic treatment;

(2) Adding olive oil or normal hexane solution into the centrifuge tube besides saturated sodium chloride solution, shaking uniformly, centrifuging, collecting liquid in the centrifuge tube into a glass container, and repeating the steps for three times;

(3) Continuously adding normal hexane solution into a glass container filled with the mixed solution, and magnetically stirring until the olive oil is fully dissolved;

(4) Filtering the solution in the glass container, collecting the microplastic in the soil on the filter membrane, and completing the separation and extraction of the microplastic.

Further, in the step (1), 2-3 mL, preferably 2.5mL of saturated sodium chloride solution is added to each gram of soil.

In the step (1), the centrifuge tube is 50mL in size and made of polytetrafluoroethylene.

Further, in the step (1), the ultrasonic frequency is 40KHz.

Further, in the step (1), the temperature of the ultrasound is 20-30 ℃, preferably 25 ℃.

Further, in the step (1), the time of the ultrasonic treatment is 15-25min, preferably 20min, and the centrifuge tube is taken out in the middle of the ultrasonic treatment to shake the mixed solution once;

further, in the step (2), the centrifugation time is 10 to 15min, preferably 10min.

Further, in the step (2), the centrifugal speed is 5000-5500r/min, preferably 5500r/min.

Further, in the step (2), when the operation is performed three times, the first time of adding the olive oil and the third time of adding the n-hexane solution except the saturated sodium chloride solution in the centrifuge tube, and the volume ratio of the saturated sodium chloride solution to the olive oil or the saturated sodium chloride solution to the n-hexane solution is 25:3.

Further, in the step (3), the volume of the added n-hexane solution is 7-15 times, preferably 10 times, the volume of the olive oil in the step (2).

Further, in the step (3), the magnetic stirring speed is 1400-1600 r/min, preferably 1500r/min.

Further, in the step (3), the magnetic stirring time is 2-5min, preferably 3min.

The method also comprises a pretreatment step before the step (1) of the method, wherein the soil sample is naturally aired, is loosened by simple rolling, and is removed from large-particle impurities such as stones and straws which are visible to naked eyes, and the soil sample is screened by a screen with the aperture of 5 mm.

The solvent for separating and extracting the microplastic is generally a salt solution, and the lipophilicity of the plastic can better adsorb most of the microplastic (0.05-1.45 g/cm ³), so the invention utilizes the characteristic, adds the olive oil into the solution for extracting the microplastic, assists in density flotation, uses a low-cost and environment-friendly saturated sodium chloride solution, can efficiently separate and extract the microplastic in the soil of a coated farmland, and has environment-friendly extraction process.

In one embodiment, the method of the present invention comprises the following steps:

The pretreatment process comprises preparing saturated sodium chloride solution, weighing 10g of naturally dried soil sample by a quartering method.

The treatment process comprises the steps of adding soil samples and the solution into a centrifuge tube for ultrasonic and centrifugation, collecting the centrifuged liquid in a glass container, adding the solution and fully magnetically stirring, filtering the mixed solution in the glass container by using a vacuum filter flask, collecting microplastic, condensing and recovering normal hexane in the filtered solution. When the mixed solution is filtered, in order to prevent the residual microplastic from adhering to the inner wall of the glass container, a small amount of n-hexane solution is continuously added into the mixed solution to wash the inner wall of the glass container, and when the suction filtration is about to finish, ultrapure water is used for washing the bottom edge of the funnel.

Compared with the prior art, the invention has the beneficial effects that:

1) The cavitation effect of ultrasonic waves is utilized to enable impurities attached to the microplastic to be rapidly and efficiently stripped, dispersed and shed, and the extracted microplastic is purer;

2) The lipophilicity of plastics is utilized, and the olive oil is added into the solution, so that the influence of the density of the microplastic on the extraction effect is avoided;

3) The centrifugation process replaces the traditional standing precipitation process, so that the extraction time is shortened;

4) The equipment and the instrument used in the invention are common, the soil microplastic can be separated and extracted in batches, taking only 6 soil samples as an example in one group of experiments, the average time is shorter when each micro plastic in the soil sample is separated and extracted for about 30 minutes, and a plurality of groups of experiments are continuously carried out.

Drawings

FIG. 1 is a roadmap of the microplastic extraction technique of the invention;

FIG. 2 is a view showing the extraction effect of PE microplastic in the form of film according to the method of the present invention;

FIG. 3 is a microscopic image of PE microplastic in the form of a film extracted according to the present invention

Detailed Description

The present invention will be further described in detail with reference to the following specific examples. The procedures, conditions, experimental methods, etc. for carrying out the present invention are common knowledge and common knowledge in the art, except for the following specific references, and the present invention is not particularly limited.

As shown in fig. 1-3:

Example 1

A method for separating and extracting soil microplastic of covered farmland in batches is taken as an example for evaluating the separation and extraction efficiency of PE microplastic in soil

The specific process comprises the following steps:

① Making film PE microplastic, continuously grinding, shearing and sieving to obtain film PE microplastic with particle diameter of 20-5000 μm

② Weighing 10g of soil sample by a quartering method (the soil sample is collected in a Xinjiang tectorial membrane cotton field, is heated in advance at 500 ℃ in a muffle furnace for 7 hours, organic matters in the soil and the existing microplastic are removed), adding 100mg of the microplastic, adding the microplastic into a PTFE centrifuge tube with 50mL specification, uniformly distributing the microplastic in the soil, adding 25mL of saturated NaCl solution, and uniformly mixing and shaking.

③ Placing the centrifuge tube into an ultrasonic cleaner, carrying out ultrasonic treatment for 20min at 20 ℃ for about 10min in the middle, taking out the centrifuge tube once, and shaking the mixed solution uniformly.

④ Adding the solution into a centrifuge tube, mixing, shaking uniformly, putting into a centrifuge 5500r/min, centrifuging for 10min, collecting the liquid in a glass container, wherein the step is required to be carried out for three times, the solution in the centrifuge tube is slightly different, the first and second times are 25mL of saturated NaCl and 3mL of olive oil, and the third time is 25mL of saturated NaCl and 3mL of normal hexane solution.

⑤ 30ML of n-hexane solution was continuously added to the glass vessel containing the mixture, and the mixture was placed on a magnetic stirrer at 1500r/min and stirred for 3min.

⑥ Placing quantitative filter paper on a Buchner funnel, pouring the mixed solution in a glass container into the funnel for vacuum filtration, continuously adding 10mL of n-hexane solution into the glass container to wash the inner wall of the glass container, pouring the mixed solution into the funnel, adding about 100 mL of ultrapure water twice when the liquid in the funnel is about 1/3 of the liquid, diluting NaCl, washing the bottom edge of the funnel for 1-2 times by using a bottle washing with ultrapure water when the suction filtration is about to be finished, preventing the inner wall of the funnel from being attached with microplastic, and separating and extracting the microplastic.

⑦ The experiment was repeated in parallel for 3 groups, and after the experiment was completed, n-hexane was recovered by condensation.

⑧ And (5) after drying the microplastic, calculating the recovery rate.

The microplastic after separation and extraction is shown in figure 2, and the result of multiple experiments shows that the recovery rate of the film PE microplastic in the soil is 96% +/-5% which is far higher than that in the prior patent (see table 1).

Table 1 comparative film microplastic recovery

Example 2

Taking field film-covered farmland soil sample collection as an example, separating and extracting microplastic in soil in batches

The specific process comprises the following steps:

① After 6 parts of soil samples of different farmlands are naturally dried, the soil samples are simply rolled to be loose, large-particle sundries such as stones, straws and the like which are visible to naked eyes are removed, and the soil samples are filtered through a screen with the aperture of 5mm.

② 500ML of saturated NaCl solution is prepared, and 6 parts of 10g soil sample are weighed by a quartering method, and the numbers are #1 to #6.

③ And adding 6 parts of soil samples one by one, continuously adding 25mL of saturated NaCl solution, and uniformly mixing and shaking, wherein the specification of the PTFE centrifuge tube is 50mL, and the marks are #1 to # 6.

④ The steps of the ultrasonic treatment, the centrifugation, the magnetic stirring and the suction filtration are carried out, and the specific operation steps are the same as those of the example 1 of the invention.

The average time of 6 soil samples is 28min after the experiment, the PE microplastic of the film-covered farmland is detected to be mainly film-shaped (figure 3), with little fiber, the film-covered farmland is photographed by a microscope of a split microscope (20X-100X), and the microplastic obtained by counting 6 soil samples by combining with Nano software is 3661 (54-3756 mu m), 2879 (43-2033 mu m), 4560 (32-2353 mu m), 2466 (45-4011 mu m), 1558 (53-102 mu m) and 3413 (67-3056 mu m) respectively.

The foregoing is merely illustrative of specific embodiments of the present invention, and the scope of the invention is not limited thereto, but any changes or substitutions that do not undergo the inventive effort should be construed as falling within the scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope defined by the claims.

Claims (4)

1. The method for separating and extracting the film-covered farmland soil micro-plastics in batches is characterized by comprising the following steps of:

(1) Adding saturated sodium chloride solution into a centrifuge tube with a weighed sample, mixing uniformly, and then placing into an ultrasonic cleaner for ultrasonic treatment;

(2) Adding olive oil or normal hexane solution into the centrifuge tube, shaking uniformly, centrifuging, collecting liquid in the centrifuge tube into a glass container, and repeating the steps for three times;

(3) Continuously adding normal hexane solution into a glass container filled with the mixed solution, and magnetically stirring until the olive oil is fully dissolved;

(4) Filtering the solution in the glass container, collecting the microplastic in the soil on a filter membrane, and completing the separation and extraction of the microplastic;

2-3 mL saturated sodium chloride solution is added to each gram of soil in the step (1);

in the step (1), the ultrasonic frequency is 40KHz, the ultrasonic temperature is 20-30 ℃, the ultrasonic time is 15-25 min, and the centrifuge tube is taken out during ultrasonic process to shake the mixed solution uniformly once;

The centrifugation time in the step (2) is 10-15 min, and the centrifugation speed is 5000-5500 r/min;

and (3) when the centrifugal operation in the step (2) is carried out for three times, adding the olive oil into the centrifugal tube for the first time and adding the normal hexane solution for the third time except for the saturated sodium chloride solution, wherein the volume ratio of the saturated sodium chloride solution to the olive oil or the saturated sodium chloride solution to the normal hexane solution is 25:3.

2. The method for batch separation and extraction of the film-covered farmland soil micro-plastic according to claim 1, wherein the volume of the normal hexane solution added in the step (3) is 7-15 times of that of the olive oil in the step (2), the magnetic stirring speed is 1400-1600 r/min, and the stirring time is 2-5 min.

3. The method for separating and extracting the film-covered farmland soil micro-plastics in batches according to claim 1, wherein the method is characterized by further comprising a pretreatment step of naturally airing a soil sample, simply rolling to loosen the soil sample, removing macroscopic stone and straw large-particle impurities, and sieving the soil sample with a sieve with the aperture of 5 mm.

4. The method for batch separation and extraction of film-covered farmland soil micro-plastic according to claim 1, wherein the olive oil and the normal hexane are of uniform AR grade purity.