Method for extracting plastics from sediments

Abstract

The present application provides a method for extracting plastics from sediments, comprising the following steps: (1) freeze-drying the sediments; (2) sieving the particles larger than a first size from the freeze-dried sediments; (3) mixing the sieved sediments with a zinc chloride solution to form a mixed solution; (4) collecting the upper suspension after the mixed solution is stratified; (5) adding hydrogen peroxide solution to the collected suspension until the solution is clear, and if there are plant tissue residues, supplementing potassium hydroxide solution to remove interference and retain degradable plastics; (6) sieving the particles larger than a second size from the clear solution, wherein the second size is smaller than the first size.

Description

Technical Field

[0001] This invention relates to a method for extracting plastics from sediments, applicable (but not limited to) methods for extracting microplastics and biodegradable plastics from root-dense sediments. Background Technology

[0002] In recent years, microplastic pollution has become a focal point of global environmental issues. Numerous studies have shown that microplastics are not only widely present in oceans and lakes, but also accumulate in estuaries, mangrove forests, and wetlands—areas where land and water meet. These areas, due to their unique ecological characteristics, typically feature well-developed root systems and high organic matter content, and the complex sediment matrix presents significant challenges to the separation and identification of microplastics. Traditional microplastic extraction methods largely rely on single-pass density separation and conventional filtration membranes, such as glass fiber membranes or polytetrafluoroethylene (PTFE) membranes. These methods often struggle to guarantee high recovery rates and accurate detection results when dealing with sediments characterized by dense root systems and significant organic matter interference. Furthermore, existing methods do not adequately address high-density or biodegradable plastics (such as polylactic acid, PLA), failing to fully meet the current requirements for precision and comprehensiveness in microplastic analysis in environmental monitoring and pollution assessment. Summary of the Invention

[0003] The present invention aims to provide a method for extracting plastics from sediments, comprising the following steps: (1) freeze-drying the sediments; (2) sieving particles larger than a first size from the freeze-dried sediments; (3) mixing the sieved sediments with a zinc chloride solution to form a mixture; (4) collecting the upper suspended solids after the mixture has separated into layers; (5) adding hydrogen peroxide solution to the collected suspended solids until the solution is clear; and (6) sieving particles larger than a second size from the clear solution, wherein the second size is smaller than the first size. Attached Figure Description

[0004] Embodiments of the present invention will be described by way of example only with reference to the accompanying drawings, wherein:

[0005] Figure 1 A flowchart of a method for extracting plastics from sediments according to the present invention;

[0006] Figure 2 These are mangrove sediments from the estuary of the Dasha River in Nanshan District, Shenzhen. The left image shows large particles (greater than 5 mm) and plant matter, while the right image shows freeze-dried particles and particles that have passed through a 5 mm sieve.

[0007] Figure 3A and 3B The image shows particulate matter collected by the filter membrane, among which... Figure 3AThe results showed that a large amount of white plant tissue sample remained even after the first digestion. Figure 3B This shows the microplastic particles obtained after the second digestion with potassium hydroxide solution. Detailed Implementation

[0008] Figure 1 A flowchart of a method for extracting plastics from sediments according to the present invention is shown. The specific steps of this method are as follows.

[0009] (1) Freeze-drying and coarse screening pretreatment S102

[0010] The collected sediments were freeze-dried and then sieved using a stainless steel sieve with a 5mm aperture to remove particles larger than 5mm, thus retaining microplastics smaller than 5mm.

[0011] (2) Weigh the sample and mix it with the solution.

[0012] Weigh 20 grams of the sieved sediment and place it in a 300 mL tall beaker. Add 100 mL of saturated zinc chloride (ZnCl2) solution, with a density of approximately 1.9 g / cm³. 3 Use magnetic stirring at 650 rpm for 10 minutes to thoroughly mix the deposit with the solution and ensure that the plastic particles are evenly dispersed.

[0013] (3) Preliminary density separation

[0014] After standing for 24 hours, the mixture was allowed to separate into distinct layers: the lower layer consisted mainly of heavier matrix particles, while the upper layer was a suspension containing plant debris and microplastics. The upper suspension was collected in a beaker using a stainless steel spoon and a glass dropper.

[0015] (4) Initial filtration separation

[0016] A portion of the collected supernatant (e.g., 1 / 3 to 1 / 2 volume) is vacuum filtered, and suspended particles are separated using a stainless steel filter membrane with a 50-micron pore size.

[0017] (5) Density flotation operation S104

[0018] After adding a certain amount of saturated zinc chloride solution, the mixture is transferred to two 100mL glass colorimetric tubes. Taking advantage of the height of the containers to facilitate stratification, the tubes are allowed to stand for 24 hours. The upper layer of particles is then aspirated with a glass dropper, and the filtration (step 4) is repeated. To ensure the recovery rate, this density flotation step is repeated one or more times.

[0019] (6) Oxidative digestion treatment of S106

[0020] Add 50 mL of 30% hydrogen peroxide (H2O2) solution to the floating matter collected in the beaker, and use a dropper to backwash the residual particles on the filter membrane. Then place the beaker in a shaker and heat it at 100 rpm and 50°C for one day. The next day, add the corresponding volume of 30% hydrogen peroxide solution and continue shaking until the solution becomes clear to further decompose plant organic matter and other interfering substances.

[0021] (7) Particle screening and diversion treatment

[0022] The treated solution was separated by passing it through a 0.5 mm stainless steel sieve. Particles larger than 0.5 mm were manually picked up using stainless steel tweezers and air-dried before being identified as microplastics (S108) using Fourier transform infrared spectroscopy (FTIR). Microplastics smaller than 0.5 mm were retained in the solution and filtered through a 50-micron stainless steel filter membrane. The aforementioned density flotation steps (2-5) were then repeated to further recover S110.

[0023] (8) Potassium hydroxide (KOH) assisted digestion (optional step)

[0024] When a large amount of white plant tissue still remains on the filter membrane (S112), the filter membrane can be immersed in a 1% potassium hydroxide solution and reacted at 50°C for 5 hours, followed by 19 hours at room temperature to effectively dissolve the residual plant tissue. Simultaneously, due to the mild conditions selected, easily degradable plastics such as PLA can be well preserved. Subsequently, microplastic particles are collected by vacuum filtration through a 10-micron stainless steel filter membrane (S114).

[0025] (9) Ultrasonic and Laser Infrared (LDIR) Detection

[0026] Finally, the collected filter membranes were placed in an ethanol solution and sonicated for 30 minutes (S116). After concentration and air drying, the morphology and type of microplastic particles were automatically detected using a laser infrared imager (LDIR) to achieve rapid and accurate statistical identification.

[0027] Practice verification

[0028] In the mangrove area at the mouth of the Dasha River in Nanshan District, Shenzhen, 50cm×50cm quadrats were set up, and 100mg of polylactic acid (PLA) microplastics with a size of 100-250μm were buried in the quadrats. After one month, approximately 1 kg of surface sediment (0-10 cm) was collected from five sample plots and their surrounding areas. The samples were processed using the method described above. Microplastic particles were successfully identified within the 20-500 μm range. Within the sample plots, common polyethylene (PE) plastics (average abundance 3451.7 particles / kg dry weight sediment), polyethylene terephthalate (PET) (191.1 particles / kg), polypropylene (PP) (15590.3 particles / kg), polystyrene (PS) (1376.4 particles / kg), and biodegradable plastic PLA (2323.1 particles / kg) were detected. Sediments outside the sample plots also showed the presence of microplastics PE (597.7 particles / kg), PET (106.6 particles / kg), PP (2630.0 particles / kg), PS (51.0 particles / kg), and biodegradable plastic PLA (29.3±58.6 particles / kg). Practical results have demonstrated the feasibility and accuracy of this invention in efficiently extracting microplastics and biodegradable plastics from complex sediment matrices, and also indicated that microplastics accumulate in plant-growing areas such as mangroves, posing a potential environmental risk.

[0029] The present invention has the following innovative features:

[0030] (1) Adaptable to complex matrix samples

[0031] This invention designs a complete pretreatment and density separation process for sediments from areas with high organic matter and root content, such as mangroves, wetlands, and estuaries, which significantly reduces the impact of plant tissues and other interfering substances on microplastic identification.

[0032] (2) Application of stainless steel filter membranes

[0033] This invention uses a stainless steel filter membrane with higher mechanical strength to replace the traditional glass fiber membrane and polytetrafluoroethylene (PTFE) membrane. It not only withstands ultrasonic treatment and is easy to reuse, but also reduces the risk of sample loss due to membrane breakage.

[0034] (3) Multiple density flotation operations

[0035] Using a saturated ZnCl2 solution (density approximately 1.9 g / cm³) 3 Density separation is performed, and by repeating the flotation operation several times, the full recovery of high-density plastics in complex sediments is ensured.

[0036] (4) Dual-technology joint detection

[0037] Depending on the size of the microplastics, FTIR technology is used to manually select and accurately identify particles of 0.5-5mm, while LDIR is used to achieve automatic and rapid detection for microplastics smaller than 0.5mm. The two technologies complement each other, ensuring both accuracy and improving detection efficiency.

[0038] (5) Mild chemical digestion conditions

[0039] The sample was digested with 1% KOH solution, which effectively removed interfering plant tissues while avoiding excessive decomposition of biodegradable plastics (such as PLA), thus balancing the purity of the sample with the integrity of the plastic.

[0040] This invention has the following main functions:

[0041] (1) High-efficiency extraction of microplastics

[0042] A method for efficiently extracting microplastics (including biodegradable plastics) from sediments with well-developed plant root systems is provided. Through steps such as freeze-drying, grading and sieving, density separation, and organic matter digestion, the extraction efficiency and accuracy of microplastics are significantly improved.

[0043] (2) Remove organic matter interference

[0044] Organic matter is digested using hydrogen peroxide solution (e.g., 30% hydrogen peroxide solution), and plant tissue interference is selectively removed using potassium hydroxide solution (e.g., 1% potassium hydroxide solution) to ensure the reliability of the extraction results.

[0045] (3) Protecting the integrity of biodegradable plastics

[0046] In the process of digesting organic matter and plant tissues, the loss or degradation of biodegradable plastics can be avoided by optimizing reagent concentration and treatment conditions.

[0047] (4) Grading of microplastics of different particle sizes

[0048] Through graded sieving and repeated processing steps, optimized extraction is performed for microplastic particles larger than 0.5 mm and smaller than 0.5 mm, respectively. The corresponding microplastic particles are then detected using Fourier transform infrared spectroscopy and laser infrared imaging, leveraging the advantages of different detection equipment. This invention has the following main applications:

[0049] (1) Environmental monitoring and assessment

[0050] It is suitable for monitoring and analyzing microplastic pollution in sediments of aquatic bodies with well-developed plant root systems, such as mangroves, rivers, lakes, and wetlands.

[0051] (2) Scientific research field

[0052] This provides reliable technical support for research on the environmental behavior, migration patterns, and ecological risks of microplastics.

[0053] (3) Standardization Analysis

[0054] It can serve as a standardized method for extracting microplastics from densely packed plant root deposits, and promote the development of technical specifications in related fields.

[0055] (4) Ecological restoration assessment

[0056] Used to assess the microplastic pollution status of sediments in ecological restoration projects, providing data support for restoration effectiveness.

[0057] In summary, this invention has significant practical value in environmental monitoring, scientific research analysis, and standardized applications. It is particularly suitable for the efficient extraction and analysis of microplastics in sediments with well-developed plant root systems, and provides a suitable analytical scheme in the context of large-scale biodegradable plastic production. Of course, the above descriptions are merely specific embodiments of this invention and are not intended to limit the scope of the invention. All equivalent changes or modifications made to the structure, features, and principles described in the claims of this patent application should be included within the scope of this invention. Furthermore, it should be understood that the above are merely examples of how this invention can be implemented, and various modifications and / or substitutions can be made without departing from the spirit of the invention. It should also be understood that, for the sake of brevity, the various features of the invention described in the present embodiment can also be provided individually or in any suitable combination of sub-examples.

Claims

1. A method for extracting plastics from sediments, comprising the following steps: (1) Freeze-dry the sediments; (2) Screening particles larger than the first size from the freeze-dried sediment; (3) The sieved sediment is mixed with zinc chloride solution to form a mixture; (4) After the mixture separates into layers, collect the upper suspended matter; (5) Add hydrogen peroxide solution to the collected suspension until the solution becomes clear; (6) Screening particles larger than the second size from the clarified solution, wherein the second size is smaller than the first size.

2. The method according to claim 1, characterized in that, The density of the zinc chloride solution is approximately 1.9 g / cm³. 3 .

3. The method according to claim 1, characterized in that, The sieving process in step (2) is performed using a sieve with a 5 mm aperture.

4. The method according to claim 1, characterized in that, After step (4), at least a portion of the collected upper suspended matter is vacuum filtered to separate particles larger than a third size through a filter membrane, wherein the third size is smaller than the first size and the second size.

5. The method according to claim 1, characterized in that, Step (5) further includes heating the solution after adding hydrogen peroxide solution at 50°C.

6. The method according to claim 1, characterized in that, The particles larger than the second size that were screened out in step (6) were air-dried and identified using Fourier transform infrared spectroscopy.

7. The method according to claim 1, characterized in that, After completing step (6), the solution is filtered through the filter membrane, and steps (3) to (4) are repeated at least twice.

8. The method according to claim 7, characterized in that, Potassium hydroxide solution was added to the particles on the filter membrane, and microplastics were detected using a laser infrared imager.

9. The method according to claim 8, characterized in that, The concentration of the potassium hydroxide solution is approximately 1%.

10. The method according to claim 7, characterized in that, The filter membrane is a reusable stainless steel filter membrane.

11. The method according to claim 6, characterized in that, Microplastics with a size of 0.5-5 mm were identified by a combination of manual selection and Fourier transform infrared spectroscopy.

12. The method according to claim 8, characterized in that, For microplastics smaller than 0.5 mm, an automatic and rapid detection method using a laser infrared imager is employed.

13. The method according to claim 4, characterized in that, The first dimension is 5 mm, the second dimension is 0.5 mm, and the third dimension is 50 micrometers.

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