Ecological method for combined earthworm-plant remediation of microplastic contaminated soil

Abstract

The present application relates to the technical field of contaminated soil bioremediation, and particularly relates to an ecological method for earthworm-plant combined remediation of microplastic contaminated soil, which comprises the following steps: soil pretreatment and pollution assessment; earthworm inoculation and initial remediation stage, inoculation density is 5-10 pieces per kg of dry soil, no plant is planted for 14-42 days, physical crushing, intestinal microbial degradation and surface modification of microplastics are implemented by using earthworms; after the initial remediation stage ends, plants such as Solamum nigrum, ryegrass or Chinese milk vetch with microplastic absorption and transport capacity are planted; under the coexistence condition of earthworms and plants, continuous remediation is carried out for 30-120 days, the aboveground part of the plants is harvested to realize the net removal of microplastics from the soil; and the earthworms are recycled for reuse. Through phased and time-sequential design, the present application constructs a trinity remediation system of earthworm-microorganism-plant, completely adopts biological remediation means, does not need to add chemicals, and has the advantages of high remediation efficiency, ecological safety, in-situ large-scale application and the like.

Description

Technical Field

[0001] This invention relates to the field of bioremediation technology for contaminated soil, and in particular to an ecological method for the combined remediation of microplastic-contaminated soil by earthworms and plants. Background Technology

[0002] Global annual plastic production has exceeded 400 million tons, with a large amount of plastic waste entering the soil ecosystem through agricultural mulch residue, sewage sludge application, and atmospheric deposition. This results in microplastic concentrations in agricultural soils reaching up to 10,000 particles / kg dry soil. Microplastics such as polypropylene (PP), polyethylene (PE), polystyrene (PS), and polyvinyl chloride (PVC) accumulate in the soil over a long period due to their high chemical stability and long natural degradation cycles of decades, seriously threatening soil health and crop safety.

[0003] The harm caused by microplastics to the soil ecosystem is mainly reflected in the following aspects: (1) It changes the physical structure of the soil, increasing the soil volume density by 15% and reducing the water holding capacity by 20%; (2) It induces oxidative stress in plants, destroys the photosynthetic system, and leads to a significant decrease in crop biomass. Studies have shown that 1% mass concentration of PP microplastics can reduce the height of milkvetch by 27.83%; (3) It accumulates through the food chain and harms soil organisms and human health; (4) Microplastics can also adsorb coexisting pollutants such as heavy metals and pesticides, producing a compound toxic effect.

[0004] Existing technologies for remediating microplastic-contaminated soil have the following main shortcomings: First, physical separation methods (such as density separation and sieving) are energy-intensive and costly, and it is difficult to completely remove small microplastic particles (especially particles <100μm) from the soil, which seriously damages the soil structure.

[0005] Secondly, chemical degradation methods (such as advanced oxidation) require large amounts of reagents, which may introduce secondary pollution, and the reaction efficiency in the soil system is limited by the mass transfer process.

[0006] Third, although microbial degradation is environmentally friendly, it faces bottlenecks such as low degradation efficiency, difficulty in colonizing microbial agents in the soil, and complex operation.

[0007] Fourth, existing bioremediation technologies mostly employ a single remediation agent. For example, patent number 202511671116.2 describes the application and method of earthworms combined with iron-based nanomaterials in the remediation of soil microplastics. This method promotes microplastic degradation through the synergistic effect of earthworms and iron-based nanomaterials, but still requires the addition of nanomaterials and does not involve phytoremediation. Earthworm remediation relies mainly on intestinal microbial degradation and bio-disturbance to promote the physical migration and fragmentation of microplastics. However, the degradation efficiency of earthworms for microplastics is limited by their food intake and retention time in the body, and there is a potential risk that microplastics will be converted into smaller nanoplastics (NPs) and absorbed by plant roots. Although studies have shown that some plants, such as Centipede Grass, can absorb microplastics through their roots and transport them to aboveground tissues, the absorption efficiency of plants for microplastics is limited, and microplastics can cause oxidative stress in plants, inhibiting normal plant growth.

[0008] Therefore, there is an urgent need to develop a method for remediating microplastic-contaminated soil that is not dependent on external chemical reagents, has high remediation efficiency, is eco-friendly, and sustainable. Summary of the Invention

[0009] In order to overcome the above-mentioned defects of the prior art, the present invention provides an ecological method for earthworm-plant co-remediation of microplastic-contaminated soil, so as to solve the problems existing in the background art.

[0010] This invention provides the following technical solution: an ecological method for the co-remediation of microplastic-contaminated soil using earthworms and plants, comprising the following steps: Step 1, Soil Pretreatment and Pollution Assessment: Assess the microplastic pollution level of the soil to be remediated, and determine the polymer type, particle size distribution, and mass concentration of microplastics in the soil; Step 2, Earthworm Inoculation and Initial Repair Stage: Inoculate earthworms into the soil to be repaired at a density of 5-10 earthworms / kg dry soil. Maintain the soil moisture content at 55%-75% of the field capacity and the temperature at 15-28℃. Allow the earthworms to feed, burrow, and reproduce in the soil for 14-42 days. Do not plant any plants during this stage. Step 3, Planting of remediation plants: After the initial remediation phase of earthworm inoculation is completed, remediation plants with microplastic absorption and translocation capabilities are sown or transplanted into the soil. Step 4, Collaborative Restoration Management: Continue restoration for 30 to 120 days under the condition of coexistence of earthworms and plants, during which the soil moisture content is maintained at 55% to 75% of the field capacity, and organic fertilizer is supplemented in a timely manner according to the plant growth needs; Step 5, Plant Harvesting and Microplastic Removal: After the co-remediation period, the aboveground parts of the plants and / or whole plants are harvested and removed from the soil to achieve net removal of microplastics and degradation intermediates from the soil. Step six: Earthworm recycling and subsequent soil utilization.

[0011] Furthermore, in step one, when the mass concentration of microplastics in the soil is 0.5–5 g / kg dry soil, it is determined to be within the pollution level range applicable to the method described above; the microplastic polymer types include one or more of polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polyamide (PA).

[0012] Furthermore, in step two, the earthworm is selected from one or more of Eisenia fetida, Pheretima guillelmi, and / or Eisenia rubida.

[0013] Furthermore, in step two, before inoculating earthworms, well-rotted organic material with a mass concentration of 0.5% to 3% of the dry weight of the soil is pre-mixed into the soil as an initial food source and microbial inoculation source for earthworms; the well-rotted organic material is selected from one or more of well-rotted cow dung, earthworm castings, and humic acid.

[0014] Furthermore, in step two, the preferred earthworm inoculation density is 7-8 earthworms / kg dry soil, and the preferred initial repair stage is 21-28 days.

[0015] Furthermore, the remediation plants are selected from plant species that have the ability to absorb microplastics and / or nanoplastics through their roots and translocate them through their aboveground parts, including one or more of Pteris vittata L., Leymus chinensis, Lolium perenne L., and Astragalus sinicus L.

[0016] Furthermore, in step three, the remediation plants are planted using a rotation or intercropping method. Specifically, the first round of planting is of Pteris vittata to absorb and transport microplastics, and after harvesting, Acer clover is sown to restore soil fertility and further degrade residual microplastics.

[0017] Furthermore, in step four, the amount of organic fertilizer applied is 0.5 to 5 g of organic fertilizer (dry weight) per kilogram of soil, and it is replenished every 15 to 30 days; no chemically synthesized pesticides are applied during the remediation period.

[0018] Furthermore, in step five, the plants are harvested before they enter the reproductive growth stage to prevent microplastics from entering the reproductive organs and spreading through the seeds.

[0019] An earthworm-plant co-remediation ecosystem for microplastic-contaminated soil, characterized in that it is constructed using any one of the methods described above, and the system comprises: Microplastic-contaminated soil awaiting remediation; The earthworm population inoculated in the soil has undergone an initial remediation activity for 14 to 42 days before plant planting, which physically breaks down microplastics in the soil, degrades them through gut microbes, and modifies their surfaces. The remediation plants are planted in soil that has undergone initial earthworm remediation, wherein the remediation plants absorb microplastics and / or nanoplastics and their degradation products that have been pretreated by earthworms through their roots and at least partially transport them to the aboveground tissues. The earthworms continuously secrete mucus, excrete worm castings, and regulate the rhizosphere microbial community during coexistence with the plant, thereby alleviating the growth stress of microplastics on the repaired plant and promoting plant growth.

[0020] The technical effects and advantages of this invention are as follows: This invention employs a completely bioremediation approach, requiring no external chemical synthesis or nanomaterials, thus eliminating the risk of secondary pollution at the source. It is a truly green in-situ soil remediation technology. In the initial remediation stage, earthworms physically break down and modify the surface of soil microplastics through feeding, gizzard grinding, and intestinal microbial degradation. This significantly improves the absorption efficiency of microplastics and nanoplastics by subsequent remediation plants. Furthermore, earthworm intestinal microorganisms can achieve a degradation rate of over 50% for microplastics such as polystyrene within 28 days, creating favorable conditions for plant absorption. The remediation plants further absorb microplastics and their degradation intermediates through their roots and transport them to the aboveground parts. Harvesting the aboveground parts achieves net removal of microplastics from the soil, solving the fundamental difficulty of completely removing microplastics from the soil system through earthworm remediation alone. The topic is that earthworms, during the synergistic remediation period, systematically alleviate the oxidative stress and growth inhibition of remediation plants caused by microplastics by improving soil nutrient status, increasing the activity of soil enzymes such as acid phosphatase and urease, regulating the structure of rhizosphere microbial communities, and upregulating the expression of plant root ribosomal protein genes and sugar metabolism pathway genes. This results in a biomass increase of remediation plants under polluted conditions of more than 30%, thus overcoming the bottleneck of poor plant growth and low remediation efficiency caused by microplastic toxicity in single-plant remediation. The remediation process simultaneously achieves the improvement of soil organic carbon, restoration of nutrient cycling function, and improvement of soil physical structure. The remediated soil can be directly restored for agricultural or ecological use. Earthworms can be recycled and reused, and the aboveground parts of the remediation plants are easy to harvest. The overall operating cost is low, making it suitable for in-situ remediation of large-area microplastic-contaminated soil. Attached Figure Description

[0021] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0022] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. These embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0023] Example 1: Remediation of Polyethylene (PE) Microplastic Contaminated Soil in Farmland Soil from a greenhouse vegetable growing base contaminated with residual microplastics from PE mulch film was selected. Testing revealed a PE microplastic concentration of 2.5 g / kg dry soil, with microplastic particle sizes primarily ranging from 50 to 500 μm. A remediation experiment was conducted on an area of ​​20 m². 2 The experiments were conducted in field test ponds with a depth of 30 cm.

[0024] Step 1: Soil Pretreatment and Pollution Assessment: Soil samples were collected, and the type, particle size distribution, and mass concentration of microplastic polymers were determined using density separation-Fourier transform infrared spectroscopy. The test results confirmed that the main pollutant was PE microplastics, with a mass concentration of 2.5 g / kg, which was determined to be the pollution level suitable for this method.

[0025] Step Two: Earthworm Inoculation and Initial Remediation: Mix well-rotted cow manure into the soil at a dry weight ratio of 2% as initial food for earthworms and a source of microbial inoculation, ensuring thorough mixing. Inoculate with a mixed population of Eisenia fetida and Pheretima guillelmi (mass ratio 1:1) at a density of 7 earthworms / kg dry soil. Maintain soil moisture content at 60%–70% of field capacity, and keep soil temperature between 18–26°C using shade netting and intermittent spraying. This stage lasts for 28 days, during which no plants are planted. During this period, active earthworm activity and a large accumulation of vermicompost on the soil surface are observed.

[0026] Step 3: Planting Restorative Plants: After the initial restoration phase, sow ryegrass (Lolium perenne L.) evenly in the soil at a rate of 20 g / m². 2 Sow at a depth of 1-2 cm, and lightly press the soil over the seeds after sowing.

[0027] Step 4, Collaborative Restoration Management: Continue restoration for 90 days under conditions of coexistence of earthworms and ryegrass. Every 20 days, supplement with 2 g of organic fertilizer (well-rotted cow manure, dry weight) per kilogram of soil. Maintain soil moisture content at 55%–75% of field capacity through regular sprinkler irrigation.

[0028] Step 5: Plant Harvesting: 90 days after co-remediation, harvest the above-ground parts of the ryegrass at ground level before it enters the heading stage and remove it from the remediation area. The harvested biomass can be safely disposed of after drying (e.g., incineration for power generation or landfill) to prevent secondary microplastic diffusion.

[0029] Step Six: Earthworm Recycling and Effectiveness Evaluation: Earthworms were recycled using a zoned trapping method (laying a layer of moist, well-rotted cow dung on the soil surface). The recycling rate was approximately 78%. The recycled earthworms were preserved for the next round of remediation. Microplastic content was measured in soil samples collected after remediation.

[0030] Remediation Results: After 90 days of remediation, the total mass concentration of PE microplastics in the soil decreased from 2.5 g / kg to 1.1 g / kg, with a removal rate of approximately 56.0%. The aboveground dry biomass of ryegrass increased by approximately 38% compared to the microplastic-contaminated soil without earthworms. Soil organic carbon content increased from 8.2 g / kg before remediation to 11.5 g / kg, and alkaline phosphatase activity increased by approximately 65%. The remediated soil is suitable for subsequent vegetable cultivation.

[0031] Example 2: Remediation of soil contaminated with polystyrene (PS) and polypropylene (PP) composite microplastics Degraded soil from an industrial area surrounding a site contaminated with various microplastics was selected. Testing revealed a PS microplastic concentration of 1.8 g / kg and a PP microplastic concentration of 1.2 g / kg, totaling 3.0 g / kg. Remediation experiments were conducted in 50L plastic basins, each containing 30 kg of dry soil.

[0032] Step 1, Soil Pretreatment and Pollution Assessment: Same as Example 1, confirming that PS and PP are combined pollutants with a total mass concentration of 3.0 g / kg dry soil, and that this method is applicable.

[0033] Step 2: Earthworm Inoculation and Initial Remediation: Mix humic acid and earthworm castings at a ratio of 1.5% (dry weight) (mass ratio 1:2). Inoculate with Eisenia fetuses at a density of 8 earthworms / kg dry soil. Maintain soil moisture content at 60%–75% of field capacity and temperature at 18–25℃. This stage lasts for 21 days. During this period, the degradation of PS microplastics by earthworm gut microbiota was observed, and the PS content in the soil showed an initial decrease.

[0034] Step 3: Planting the repaired plants: Transplant the centipede grass seedlings that have been pre-cultivated for 30 days, planting 3 seedlings per pot with a spacing of about 15cm between plants.

[0035] Step 4, Collaborative Restoration Management: Continue restoration for 120 days. Replenish with 3 g of organic fertilizer (well-rotted cow manure) per kilogram of soil every 25 days. Maintenance conditions are the same as in Example 1.

[0036] Step 5, Plant Harvesting: When harvesting, cut the above-ground parts (leaves) of the centipede grass at the base and dig out the roots as well, removing the entire plant.

[0037] Step 6, Earthworm Recycling and Effect Evaluation: Same as Example 1.

[0038] Remediation Results: After 120 days of remediation, the concentration of PS microplastics in the soil decreased by approximately 62% (compared to the 56.67% PS degradation rate reported by gut microbiota in previous studies after 28 days, this embodiment extended the remediation period and increased the continuous inoculation effect of earthworm gut bacteria via vermicompost, resulting in a cumulatively enhanced degradation effect), and the concentration of PP microplastics decreased by approximately 48%. Enrichment of PE and PP microplastics was detected in the upper part of the centipede grass, confirming the absorption and translocation of microplastics by plants after earthworm pretreatment. Soil urease activity increased by approximately 55%, and microbial biomass carbon increased by approximately 42%.

[0039] Following the aforementioned 120-day remediation period, a further restoration phase involving milkvetch rotation was implemented: after harvesting centipede grass, milkvetch (Astragalus sinicus L.) was sown in the same soil still containing earthworms, and continued to grow synergistically for 60 days. The aboveground dry weight of milkvetch increased by approximately 45% compared to the control without earthworms, and the expression of its root ribosomal protein genes was significantly upregulated (consistent with previous research results). Simultaneously, the concentration of residual microplastics in the soil further decreased by approximately 15%. Returning milkvetch to the field as green manure resulted in a comprehensive restoration of soil fertility.

[0040] Example 3: Remediation of PET microplastic pollution in urban green space soil Soil from a 50m² urban park was selected that was contaminated with PET microplastics (mainly from the degradation of synthetic textile debris and discarded beverage bottle fragments). The PET microplastic concentration was 0.8 g / kg dry soil, which is considered low-concentration contamination. Remediation was carried out on this area. 2 The activity will be conducted in natural green spaces within the community.

[0041] Step 1, Soil pretreatment and pollution assessment: Same as in Example 1, confirming that PET microplastics are the main component, with a concentration of 0.8 g / kg, which is within the applicable range (0.5-5 g / kg).

[0042] Step 2, Earthworm Inoculation and Initial Remediation: Mix humic acid at a ratio of 1% by dry weight. Inoculate a mixed population of Eisenia fetida and Eisenia purpurea (mass ratio 2:1) at a density of 5 earthworms / kg dry soil. Maintain natural soil moisture conditions and a temperature range of 15–28℃. This stage lasts for 14 days (the initial remediation period is shortened due to the low initial concentration).

[0043] Step 3: Planting of repaired plants: Transplant the pre-cultivated centipede grass at a planting density of 4 plants per square meter.

[0044] Step 4, Collaborative Remediation Period Management: Continue remediation for 60 days (due to low pollution concentration), with management measures the same as in Example 1.

[0045] Steps five and six: Same as in Example 1.

[0046] Remediation Results: After 60 days of remediation, the concentration of PET microplastics in the soil decreased to 0.35 g / kg, with a removal rate of approximately 56.3%. PET microplastics were detected in both the upper part and roots of the *Pteris vittata* plant, verifying its ability to absorb and translocate PET microplastics. No chemical agents were used during the remediation process, and there was no negative disturbance to the urban green space ecosystem. The earthworm population increased from an initial 250 to approximately 410, achieving self-reproduction.

[0047] Comparative Example: Single Plant Remediation A comparative example was set up: The soil was contaminated with the same PE microplastics as in Example 1, but only ryegrass was sown without inoculating earthworms, while other conditions remained the same. After 90 days, the aboveground biomass of the ryegrass was only about 62% of that in Example 1, and the soil PE microplastic removal rate was only about 12% (mainly through plant root absorption and rhizosphere microbial degradation), far lower than the 56.0% in Example 1 of this invention. The ryegrass leaves showed obvious yellowing and stunted growth, and the activities of catalase and peroxidase were significantly increased, indicating that the plants were under microplastic-induced oxidative stress.

[0048] Comparison table of repair effects between embodiments and comparative examples of the present invention: The above are merely specific embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. An ecological method for the co-remediation of microplastic-contaminated soil using earthworms and plants, characterized in that, Includes the following steps: Step 1, Soil Pretreatment and Pollution Assessment: Assess the microplastic pollution level of the soil to be remediated, and determine the polymer type, particle size distribution, and mass concentration of microplastics in the soil; Step 2, Earthworm Inoculation and Initial Repair Stage: Inoculate earthworms into the soil to be repaired at a density of 5-10 earthworms / kg dry soil. Maintain the soil moisture content at 55%-75% of the field capacity and the temperature at 15-28℃. Allow the earthworms to feed, burrow, and reproduce in the soil for 14-42 days. Do not plant any plants during this stage. Step 3, Planting of remediation plants: After the initial remediation phase of earthworm inoculation is completed, remediation plants with microplastic absorption and translocation capabilities are sown or transplanted into the soil. Step 4, Collaborative Restoration Management: Continue restoration for 30 to 120 days under the condition of coexistence of earthworms and plants, during which the soil moisture content is maintained at 55% to 75% of the field capacity, and organic fertilizer is supplemented in a timely manner according to the plant growth needs; Step 5, Plant Harvesting and Microplastic Removal: After the co-remediation period, the aboveground parts of the plants and / or whole plants are harvested and removed from the soil to achieve net removal of microplastics and degradation intermediates from the soil. Step six: Earthworm recycling and subsequent soil utilization.

2. The ecological method for earthworm-plant co-remediation of microplastic-contaminated soil according to claim 1, characterized in that, In step one, when the mass concentration of microplastics in the soil is 0.5 to 5 g / kg dry soil, it is determined to be within the pollution level range applicable to the method described above; the microplastic polymer types include one or more of polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyethylene terephthalate, and polyamide.

3. The ecological method for earthworm-plant co-remediation of microplastic-contaminated soil according to claim 1, characterized in that, In step two, the earthworms are selected from one or more of Eisenia fetida, Vertrilia William, and / or Eisenia purpureus.

4. The ecological method for earthworm-plant co-remediation of microplastic-contaminated soil according to claim 1, characterized in that, In step two, before inoculating earthworms, well-rotted organic material with a mass concentration of 0.5% to 3% of the dry weight of the soil is pre-mixed into the soil as an initial food source and microbial inoculation source for earthworms; the well-rotted organic material is selected from one or more of well-rotted cow dung, earthworm castings, and humic acid.

5. An ecological method for the co-remediation of microplastic-contaminated soil by earthworms and plants according to claim 1 or 3, characterized in that, In step two, the preferred earthworm inoculation density is 7-8 earthworms / kg dry soil, and the preferred initial repair stage is 21-28 days.

6. The ecological method for earthworm-plant co-remediation of microplastic-contaminated soil according to claim 1, characterized in that, In step three, the remediation plants are selected from plant species that have the ability to absorb microplastics and / or nanoplastics through their roots and translocate them through their aboveground parts, including one or more of centipede grass, sheep grass, ryegrass, and milkvetch.

7. An ecological method for the co-remediation of microplastic-contaminated soil by earthworms and plants according to claim 6, characterized in that, In step three, the remediation plants are planted using a rotation or intercropping method. Specifically, the first round of planting is of centipede grass to absorb and transport microplastics. After harvesting, milkvetch is sown to restore soil fertility and further degrade residual microplastics.

8. An ecological method for the co-remediation of microplastic-contaminated soil by earthworms and plants according to claim 1, characterized in that, In step four, the amount of organic fertilizer applied is 0.5 to 5g per kilogram of soil, and it is replenished every 15 to 30 days; no chemically synthesized pesticides are applied during the remediation period.

9. An ecological method for the co-remediation of microplastic-contaminated soil by earthworms and plants according to claim 1, characterized in that, In step five, the plants are harvested before they enter the reproductive growth stage to prevent microplastics from entering the reproductive organs and spreading through the seeds.

10. An ecosystem for the co-remediation of microplastic-contaminated soil by earthworms and plants, characterized in that, The system, constructed using the method of any one of claims 1 to 9, comprises: Microplastic-contaminated soil awaiting remediation; The earthworm population inoculated in the soil has undergone an initial remediation activity for 14 to 42 days before plant planting, which physically breaks down microplastics in the soil, degrades them through gut microbes, and modifies their surfaces. The remediation plants are planted in soil that has undergone initial earthworm remediation, wherein the remediation plants absorb microplastics and / or nanoplastics and their degradation products that have been pretreated by earthworms through their roots and at least partially transport them to the aboveground tissues. The earthworms continuously secrete mucus, excrete worm castings, and regulate the rhizosphere microbial community during coexistence with the plant, thereby alleviating the growth stress of microplastics on the repaired plant and promoting plant growth.

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