Micro-plastic separation and enrichment method based on combination of density flotation and electrostatic adsorption

Abstract

The invention discloses a micro-plastic separation and enrichment method based on combination of density flotation and electrostatic adsorption. The micro-plastic separation and enrichment method comprises the following steps: pretreating a sample containing micro-plastic; the pretreated sample is mixed with a high-density salt solution for density flotation, and supernate containing micro-plastics is obtained; arranging at least one pair of electrodes in the supernate, and applying direct-current voltage, so that the micro-plastics are adsorbed and enriched on the surfaces of the electrodes; and taking out the electrode, carrying out surface elution, and collecting an eluent containing the micro-plastics. According to the method, an active capture mechanism of electrostatic adsorption enrichment is creatively introduced, and high-density micro-plastics (such as PVC and PET) which are difficult to separate by a traditional flotation method can be effectively captured, so that the separation bottleneck of only depending on density is broken through, and the total recovery rate of the micro-plastics is greatly improved; and the required core equipment is only a set of direct-current power supply and a pair of electrodes, so that the device is low in cost, simple in structure, convenient to operate and easy to popularize and apply in a conventional environment monitoring laboratory.

Description

Technical Field

[0001] This invention relates to the field of microplastic separation technology, and in particular to a method for the separation and enrichment of microplastics based on the combined use of density flotation and electrostatic adsorption. Background Technology

[0002] Microplastic pollution, as a new global environmental problem, has attracted widespread attention from the scientific community and the public. Accurately and efficiently separating and enriching microplastics from complex environmental samples (such as seawater, river water, soil, sediments, etc.) is a core prerequisite and key step for conducting abundance statistics, composition analysis, source tracing, and ecological risk assessment.

[0003] Currently, the mainstream international microplastic separation technology is density flotation. This method utilizes the density difference between microplastics and the sample matrix (such as silt, organic debris). By adding high-density salt solutions such as saturated sodium chloride, zinc chloride, and sodium iodide, low-density microplastics such as polyethylene and polypropylene float to the surface, while high-density inorganic particles settle, achieving preliminary separation. However, this technology has significant limitations and is difficult to meet the analytical needs of samples from complex environments. (1) The separation efficiency of high-density microplastics is extremely low: Since the density of high-density microplastics such as polyvinyl chloride (PVC) and polyethylene terephthalate (PET) is close to or higher than that of commonly used salt solutions, the separation effect of density flotation is poor. High-density microplastics are easy to settle together with sediments, resulting in a serious underestimation of the detection results.

[0004] (2) Significant interference from the physical and chemical properties of particles: Microplastics are prone to aging in the environment, and biofilms will form on the surface, adsorbing organic matter or inorganic minerals, leading to an increase in their density and changes in surface properties, which in turn affects their buoyancy behavior; at the same time, for microplastics with a particle size of less than 100 μm, the difference between gravity and buoyancy is slight, and the separation efficiency drops significantly.

[0005] (3) The subsequent processing is cumbersome and the risk of error is high: the supernatant after flotation needs to be filtered through a filter membrane and then manually picked and identified by an optical microscope. This process is not only time-consuming and labor-intensive, but also prone to missed detection and misjudgment of transparent, colorless or irregularly shaped particles. It is highly subjective and seriously affects the accuracy and comparability of the results.

[0006] Therefore, developing a microplastic separation and enrichment technology with higher separation efficiency, wider applicability, and simpler operation is of great practical significance for promoting research on the environmental behavior of microplastics and pollution control. Summary of the Invention

[0007] The purpose of this invention is to provide a method for the separation and enrichment of microplastics based on the combined use of density flotation and electrostatic adsorption, thereby solving the problems mentioned in the background art. To achieve the above objective, this invention provides the following technical solution: A method for separating and enriching microplastics based on density flotation and electrostatic adsorption includes the following steps: pretreating a sample containing microplastics; mixing the pretreated sample with a high-density salt solution and performing density flotation to obtain a supernatant containing microplastics; setting at least one pair of electrodes in the supernatant and applying a DC voltage to adsorb and enrich the microplastics onto the electrode surface; removing the electrodes and performing surface elution to collect the eluent containing microplastics.

[0008] Preferably, the pretreatment step includes chemical digestion using an oxidant.

[0009] Preferably, the high-density salt solution comprises a saturated sodium chloride solution.

[0010] Preferably, the density flotation operation is performed at least twice, and the supernatant obtained each time is combined for use in the next step.

[0011] Preferably, the DC voltage range is 1V to 30V.

[0012] More preferably, the DC voltage range is 3V to 15V.

[0013] Preferably, the duration of the applied DC voltage is 10 to 120 minutes.

[0014] Preferably, the electrode is made of any one of graphite, stainless steel, platinum, titanium, and conductive glass.

[0015] Preferably, the supernatant is stirred during the microplastic adsorption and enrichment process.

[0016] Preferably, the surface elution operation includes at least one of liquid rinsing, ultrasonic treatment, or mechanical scraping.

[0017] The beneficial effects of this invention are as follows: This invention creatively introduces an "active capture" mechanism of electrostatic adsorption enrichment, which can effectively capture high-density microplastics (such as PVC and PET) that are difficult to separate by traditional flotation methods, improve the efficiency and reliability of analysis, break through the separation bottleneck that relies solely on density, and greatly improve the overall recovery rate of microplastics; moreover, the core equipment required is only a set of DC power supply and a pair of electrodes, which is low in cost, simple in structure, convenient to operate, and easy to promote and apply in conventional environmental monitoring laboratories. Attached Figure Description

[0018] Figure 1 This is a flowchart of the microplastic separation and enrichment method based on the combined use of density flotation and electrostatic adsorption according to the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] like Figure 1 As shown, this invention discloses a method for the separation and enrichment of microplastics based on the combined use of density flotation and electrostatic adsorption, comprising the following steps: S1. Sample pretreatment: Pretreatment of samples containing microplastics.

[0021] It should be noted that the environmental samples to be tested are usually water or sediment. For water, the samples are first concentrated by filtration and then pretreated. For sediment, the samples are first dried and sieved and then pretreated.

[0022] In the pretreatment process, an oxidant is added for chemical digestion to remove organic matter (such as biodebris). A 20%-30% (v / v) hydrogen peroxide solution is used as the oxidant.

[0023] S2. Density flotation: The pretreated sample is thoroughly mixed with a high-density salt solution (such as saturated sodium chloride solution, zinc chloride solution or sodium iodide solution), and allowed to stand for density flotation. The density difference causes most of the microplastics and some suspended matter to float to the surface, forming a supernatant, and a supernatant containing microplastics is obtained.

[0024] To improve the initial recovery rate of microplastics, the density flotation operation should be performed at least twice (2 to 4 times depending on the sample conditions). The supernatant obtained each time should be combined and then used for the next step of electrostatic adsorption enrichment.

[0025] If the obtained supernatant contains large non-target suspended matter, it is best to filter the supernatant to further improve its purity.

[0026] S3. Electrostatic Adsorption Enrichment: At least one pair of electrodes is placed in the supernatant, with an electrode spacing of 1 cm to 5 cm. A DC voltage is applied to form a stable electric field, causing the microplastics to be adsorbed and enriched onto the electrode surface. During the electrostatic adsorption enrichment process described above, the supernatant can also be stirred.

[0027] Understandably, since microplastic particles typically carry a negative charge on their surface in aqueous solutions, they will migrate directionally under the influence of an electric field and be adsorbed onto the surface of the anode electrode. This process is maintained for a predetermined period of time to achieve the active capture and enrichment of microplastics.

[0028] In this step, the electrode material is selected from materials with good conductivity and chemical stability, such as graphite, stainless steel, platinum, titanium, and ITO conductive glass; preferably, graphite or stainless steel electrodes are used to reduce costs. The DC voltage range is 1V to 30V, preferably 3V to 15V. The duration of applying the DC voltage is 10 minutes to 120 minutes, preferably 30 minutes to 60 minutes.

[0029] S4. Elution and Collection: Remove the anode electrode with adsorbed microplastics from the solution and elute the microplastic particles from the electrode surface with a small amount of pure liquid (such as ultrapure water or ethanol). Collect the eluent. This eluent is a high-purity, high-concentration microplastic sample, which can be directly used for subsequent microscopic observation, spectroscopic analysis, or chemical characterization.

[0030] In this step, the electrode surface elution operation can be achieved by at least one of the following methods: liquid rinsing (repeatedly blowing and rinsing the electrode surface with a pipette), ultrasonic treatment of the electrode, or mechanical scraping with a soft tool (such as a silicone scraper) (gently scraping the electrode surface).

[0031] The microplastic separation and enrichment method based on the present invention will be further described in detail below with reference to embodiments.

[0032] Example 1: Separation and enrichment of microplastics from seawater A1. Take 20L of surface water sample from a sea area in Maoming City, Guangdong Province, filter it through a 200-mesh (approximately 75μm) stainless steel sieve, and rinse the retained material with deionized water into a beaker.

[0033] A2. Add 30% (v / v) hydrogen peroxide solution to a beaker and digest for 24 hours at room temperature.

[0034] A3. Add sufficient saturated sodium chloride solution to the digested sample, stir thoroughly, and let stand. After standing for 24 hours, carefully transfer the supernatant (approximately 500 mL) to a clean electrolytic cell.

[0035] A4. Use two polished graphite plates, each 5cm x 5cm in size, as electrodes. Insert them parallel to each other into the electrolytic cell with a distance of 3cm between the electrodes. Connect them to a DC regulated power supply and apply a voltage of 10V.

[0036] A5. Turn on the electric field and simultaneously start the magnetic stirring (low speed, 50-100 rpm) for 60 minutes. During this time, fine particles will gradually adhere to the surface of the anode graphite plate.

[0037] A6. Turn off the power and carefully remove the anode graphite plate. Use a wash bottle containing 20mL of ultrapure water to thoroughly rinse the surface of the anode plate, washing away all adhering particles into a clean glass petri dish.

[0038] A7. Place the petri dish in a 40℃ oven to dry, and obtain the enriched microplastic sample. Then, count the microplastic sample under a microscope.

[0039] Example 2: Separation and enrichment of microplastics from marine sediments B1. Take 50g of air-dried marine sediment sample and pass it through a 10-mesh sieve to remove large debris.

[0040] B2. Place the sample in a beaker, add a saturated sodium chloride solution, stir vigorously for 10 minutes, and let stand for 2 hours. Transfer the supernatant to an electrolytic cell. Repeat this flotation process three times, combining all the supernatants.

[0041] B3. Perform electrostatic adsorption on the combined supernatant, with the same parameters as steps A4 and A5 in Example 1.

[0042] B4. Remove the anode, elute and collect it to obtain the final microplastic sample, and count the microplastic sample under a microscope.

[0043] Comparative example (filter-sorting method): The supernatant was obtained by following steps A1, A2 and A3 of Example 1. Instead of electrostatic adsorption enrichment, the supernatant was directly filtered through a glass fiber membrane with a pore size of 0.45 μm, and then the membrane was manually picked and counted under a microscope.

[0044] Comparison of experimental results: In Example 1, the number of microplastics obtained using the method of the present invention was 1523, while the number of microplastics obtained using the conventional filtration-sorting method was 609. The recovery rate of the present invention was 40%. Furthermore, FTIR identification showed that the method of the present invention detected 21% PET fibers and 16% PVC fragments, while the corresponding proportions of the conventional method were 7% and 4%, respectively. The present invention significantly detected more PET fibers and PVC fragments. This demonstrates the superiority of the method of the present invention in improving the recovery rate and broadening the detection types.

[0045] In summary, compared with the prior art, the present invention has the following significant advantages and beneficial effects: (1) Significantly improves separation and recovery rate, especially for high-density microplastics: This invention creatively introduces the "active capture" mechanism of electrostatic adsorption, which can capture microplastics of different particle sizes (including <100μm); for high-density microplastics (such as PVC and PET) that are difficult to separate by traditional flotation methods, as long as their surface is charged, they can be effectively captured by electric field force, thereby breaking through the separation bottleneck that depends only on density and greatly improving the overall recovery rate of microplastics; (2) Wide applicability reduces dependence on particle physical properties: Electrostatic adsorption mainly depends on the surface charge of particles. This method is still effective for microplastics whose density increases due to aging or biofilm adhesion, or microplastics whose buoyancy is insufficient due to their small size, which greatly broadens the scope of application of the technology. (3) Simplify subsequent operations and improve analytical efficiency and accuracy: After electrostatic adsorption, microplastics are enriched on a small area of ​​electrode surface. Afterwards, only the electrode needs to be eluted to obtain a relatively pure concentrated sample. This greatly reduces the amount of manual picking on the filter membrane in the traditional method, avoids subjective errors, and improves the efficiency and reliability of the analysis. (4) The method is low cost and easy to implement: The core equipment required by this method is only a DC power supply and a pair of electrodes. No special expensive instruments are required. It is low cost, simple structure, easy to operate, and easy to promote and apply in conventional environmental monitoring laboratories.

[0046] Regarding the embodiments of the present invention, it should also be noted that, without conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other to obtain new embodiments.

[0047] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. The scope of protection of the present invention should be determined by the scope of the claims. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for separating and enriching microplastics based on the combined use of density flotation and electrostatic adsorption, characterized in that, Includes the following steps: Pretreatment of samples containing microplastics; The pretreated sample was mixed with a high-density salt solution and subjected to density flotation to obtain a supernatant containing microplastics. At least one pair of electrodes are placed in the supernatant and a DC voltage is applied to cause microplastics to be adsorbed and enriched on the electrode surface; The electrode was removed and surface eluted, and the eluent containing microplastics was collected.

2. The method according to claim 1, characterized in that, The pretreatment step includes chemical digestion using an oxidant.

3. The method according to claim 1, characterized in that, The high-density salt solution includes a saturated sodium chloride solution.

4. The method according to claim 1, characterized in that, The density flotation operation is performed at least twice, and the supernatant obtained each time is combined for use in the next step.

5. The method according to claim 1, characterized in that, The DC voltage range is 1V to 30V.

6. The method according to claim 5, characterized in that, The DC voltage range is 3V to 15V.

7. The method according to claim 1, characterized in that, The duration of the applied DC voltage is from 10 minutes to 120 minutes.

8. The method according to claim 1, characterized in that, The electrode is made of any one of graphite, stainless steel, platinum, titanium, and conductive glass.

9. The method according to claim 1, characterized in that, During the microplastic adsorption and enrichment process, the supernatant is stirred.

10. The method according to claim 1, characterized in that, The surface washing operation includes at least one of liquid rinsing, ultrasonic treatment, or mechanical scraping.

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