A method for degrading microplastics in soil

Abstract

This invention belongs to the field of soil pollution control, specifically a method for degrading microplastics in soil. This invention provides a technical solution that accelerates the aging of microplastics by utilizing free radicals generated in hydrated iron through the interaction between natural iron minerals and microplastics mediated by soil microorganisms. This effectively promotes the degradation of microplastics, improving their environmental friendliness and degradability.

Description

Technical Field

[0001] The present invention belongs to the field of soil pollution control, and specifically relates to a method for degrading microplastics in soil. Background Art

[0002] Microplastics (MPs), emerging pollutants, are widely distributed in water, soil, and sediments. Due to their resistance to degradation and the resulting long-term ecological risks, they have become a widespread environmental concern in recent years. The degradation of microplastics has been a research focus in environmental science, particularly through the mediation of natural minerals such as ferrihydrite. Interactions between microplastics and minerals have been found to potentially influence the migration, degradation, and stability of microplastics.

[0003] Ferrihydrite is a weakly crystalline iron mineral that is also widely present in water, soil, and sediments. It is a major phosphorus sink. Due to its large specific surface area and adsorption capacity, it adsorbs phosphorus, limiting its bioavailability and affecting microbial function. In addition to phosphorus, ferrihydrite also often binds to organic carbon, preventing its further mineralization. Microplastics are considered to be a hidden source and reservoir of carbon. Their surface functional groups act as "electron shuttles," attracting microorganisms to use them as electron acceptors or donors during their metabolism. In aquatic environments, the interaction between MPs and hydrated iron significantly affects the migration and fate of MPs. During the formation of ferrihydrite flocs, polyethylene plastics can be rapidly incorporated and buried. The binding of MPs to flocs or aggregates can alter their fate and bioavailability. However, there are few reports on the environmental behavior and fate of ferrihydrite-encapsulated microplastics in the environment. Summary of the Invention

[0004] To solve the above problems, the present invention provides a method for degrading microplastics in soil, comprising the following steps:

[0005] (1) Extraction of soil microorganisms: Sterile water and soil were mixed at a ratio of 2.5:1, shaken and allowed to stand to obtain a supernatant containing soil microorganisms;

[0006] (2) Synthesis of ferrihydrite co-precipitated with PLA MPs: First, 40 g of Fe(NO3)3·9H2O was dissolved in 500 mL of deionized water; 4 g of PLA microplastics was added, followed by 310 mL of 1 M KOH to adjust the pH; the mixture was vigorously stirred for 1 h using a magnetic stirrer and then centrifuged; after discarding the supernatant, the resulting colloid was repeatedly washed with deionized water five times; the colloid was dried in a freeze dryer for 24 h and stored at -20°C until use;

[0007] (3) Phosphorus attachment of ferrihydrite: 1.2 g of ferrihydrite was added to 500 mL of 20 mM K2HPO4, the pH was adjusted to 6, incubated for 48 h, and then freeze-dried;

[0008] (4) The ferrihydrite co-precipitated with PLAMPs after phosphorus attachment is placed in the supernatant containing soil microorganisms for cultivation to degrade microplastic particles.

[0009] Furthermore, the shaking mixing condition in step (1) is 150 rpm / min for 30 min.

[0010] Furthermore, the standing time in step (1) is 6 hours.

[0011] Furthermore, the centrifugation condition in step (2) is 12000 rpm for 12 minutes.

[0012] Furthermore, the pH range of the step (2) is adjusted to pH 7.4-7.6.

[0013] Furthermore, the incubation conditions in step (3) are 28° C. and 150 r / min.

[0014] Furthermore, the culture conditions in step (4) are 28° C., 150 rpm / min, and culture for 30 days.

[0015] Furthermore, the culture condition in step (4) is dark culture.

[0016] The present invention has the following beneficial effects:

[0017] This invention proposes that the interaction between ferrihydrite and microplastics, mediated by soil microorganisms, specifically ferrihydrite encapsulation of microplastics, further accelerates microplastic aging and produces less carbon dioxide than direct interaction between microplastics and ferrihydrite. This effectively addresses the issues of microplastics being difficult to degrade in the environment and the carbon dioxide they produce that impacts the climate, demonstrating high environmental adaptability and potential for practical application. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0019] Figure 1 SEM electron micrographs of microplastics before and after aging;

[0020] Figure 2 XRD and FTIR images of microplastics before and after aging;

[0021] Figure 3 SEM electron microscope image and specific surface area of ​​ferrihydrite;

[0022] Figure 4 SEM-EDS characterization of ferrihydrite after phosphorus adsorption;

[0023] Figure 5 SEM-EDS characterization of microplastics in the co-precipitation group after phosphorus adsorption;

[0024] Figure 6 FTIR images of microplastics after incubation;

[0025] Figure 7 Changes in microplastic carbonyl index after incubation;

[0026] Figure 8 2D-COS analysis of microplastics after culture;

[0027] Figure 9 Carbon dioxide gas concentration in each group;

[0028] Figure 10 Absolute quantitative stacking diagram of different functional genes in each group of microorganisms. DETAILED DESCRIPTION

[0029] Various exemplary embodiments of the present invention are now described in detail. Unless otherwise specified, the methods in the examples are conventional methods. Unless otherwise specified, the reagents used are conventional commercially available reagents or reagents prepared using conventional methods. This detailed description should not be considered a limitation of the present invention, but rather should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0030] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0031] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0032] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be illustrative only.

[0033] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0034] 1. Extraction of Soil Microorganisms

[0035] Sterile water and soil were mixed at a ratio of 2.5:1 and shaken at 150 rpm for 30 minutes. The supernatant (SE) containing soil microorganisms was obtained after standing for 6 hours. Soil was collected from Fujian, China, air-dried, and then ground through a 2 mm soil sieve.

[0036] 2. Aging of polylactic acid microplastics

[0037] Because microplastics in the environment are typically aged, this method uses a combination of laboratory chemical aging and photoaging. The specific method is as follows: 100 g of PLA MPs were added to a 1 L glass beaker and then immersed in 500 mL of a 10% (mass fraction) hydrogen peroxide solution. The beaker was then placed in a photochemical reaction apparatus (BILON, Shanghai, BL-GHX-V) equipped with a UV mercury lamp (28°C, light / dark = 12 / 12 h, 500 W, 120 rpm) for 7 days. During this period, hydrogen peroxide was replenished every morning and evening. After aging, the samples were rinsed three times with deionized water and then dried at 40°C. The crystal structures of the pristine and aged PLA MPs were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR).

[0038] like Figure 1 and 2 As shown in the figure, compared with the original microplastics, the surface fragments of the aged microplastics increased, the crystallinity decreased, and the functional group peak intensity weakened. This indicates that the functional groups in the non-crystalline area of ​​the microplastic surface have degraded after aging.

[0039] 3. Synthesis of ferrihydrite and phosphorus adsorption:

[0040] Pure ferrihydrite (Fh) and phosphohydrite were synthesized using the method of Darcy et al. [5]. Briefly, 40 g of Fe(NO3)3·9H2O was first dissolved in 500 mL of deionized water. 310 mL of 1 M KOH was added and titrated to pH 7.4-7.6. The mixture was vigorously stirred for 1 hour using a magnetic stirrer and then centrifuged at 12,000 rpm for 12 minutes. After discarding the supernatant, the resulting colloid was repeatedly washed with deionized water five times. The colloid was dried in a freeze dryer for 24 hours and stored at -20°C until use. In addition, ferrihydrite (Fh-MP) co-precipitated with PLA MPs was synthesized by adding 4 g of polylactic acid microplastics (PLA MPs) (in the form of spheres with a diameter of 2 mm, purchased from Huakong Plastic Co., Ltd., Dongguan, China) after the addition of Fe(NO3)3·9H2O during the ferrihydrite synthesis process. Phosphorus adsorption on ferrihydrite was determined by adding 1.2 g of ferrihydrite to 500 mL of 20 mM K₂HPO₄ (adjusted to pH 6) and incubating for 48 hours (28°C, 150 rpm) followed by freeze-drying. The surface morphology, specific surface area, and phosphorus adsorption of the ferrihydrite after phosphorus adsorption were analyzed using scanning electron microscopy (SEM), the Brunauer-Emmett-Teller (BET) method, and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS).

[0041] like Figure 3 As shown in the results, the co-precipitated ferrihydrite (Fh-MP) has a larger pore size and smaller specific surface area than pure ferrihydrite (Fh), and has a lower phosphorus adsorption capacity (0.97 mM P / g and 1.17 mM P / g, respectively). Figure 4 SEM-EDS showed that both ferrihydrite surfaces successfully adsorbed phosphorus.

[0042] like Figure 5 As shown, after co-precipitation, the surface of the microplastics is coated with ferrihydrite in the form of scales. The coated ferrihydrite also adsorbs phosphorus, which is relatively evenly distributed on the micron scale on the surface of the microplastics. During the co-precipitation process of ferrihydrite and microplastics, active functional groups are coated by ferrihydrite. This direct contact may enhance the redox reaction of iron, promote the release of phosphorus, and promote the degradation of microplastics.

[0043] 4. Ferrihydrite mediated by soil microorganisms accelerates microplastic degradation

[0044] Soil microorganisms, ferrihydrite, and microplastics were added as shown in Table 1. A microorganism-free group was also used as a control. The different groups were incubated at 28°C, 150 rpm / min, and darkness for 30 days.

[0045] Table 1 Composition of each group

[0046]

[0047]

[0048] 5. Characterization of microplastic degradation after culture

[0049] After the 30-day incubation period, the microplastics were separated and washed with deionized water and air-dried. The crystal structures of the original and aged PLA MPs were characterized using Fourier transform infrared spectroscopy (FTIR), and the changes in the carbonyl index of the microplastics were analyzed in combination with FTIR. The carbonyl index is usually used to quantify the degradation of microplastic functional groups. The carbonyl index of polylactic acid microplastics is calculated using a wavelength of 1746 cm -1 and 1452cm -1 In addition, the order of changes in functional groups of microplastics in different groups was analyzed based on the FTIR results combined with two-dimensional correlation spectroscopy (2D-COS).

[0050] like Figure 6 and 7 As shown in the figure, the functional group degradation degree of polylactic acid microplastics in the ferrihydrite and microplastics co-precipitation group (SE+Fh-MP) mediated by soil microorganisms is the greatest, and the absorbance and carbonyl index of carbonyl groups are significantly reduced. Figure 8 As shown in the figure, according to the Node rule, the degradation order of each functional group is different, 1042, 1080, 1127, 1180, and 1746 cm -1 The main automatic peaks were observed at , which represent COC, -OC=O, CO, CO and C=O functional groups of polylactic acid plastic. The order of functional group changes in different groups is as follows: SE+Fh+MP group, 1746>1180>1127>1080>1042cm -1 SE+Fh-MP group, 1080>1180>1746>1042>1127cm -1 ;

[0051] Fh+MP group, 1746>1042>1127>1180>1080cm -1 ; Fh-MP group,

[0052] 1080>1746>1042>1127>1180cm -1. These indicate that soil microorganism-mediated ferrihydrite significantly promotes the degradation of microplastics. In addition, the co-precipitation of ferrihydrite and microplastics also promotes the degradation of microplastic functional groups and changes the degradation order of microplastic functional groups compared with the group with direct addition of microplastics. In the SE+Fh+MP and Fh+MP groups, C=O is preferentially degraded, while -OC=O is preferentially degraded in the SE+Fh-MP and Fh-MP groups. This suggests that when ferrihydrite and microplastics are directly mixed, the free radicals generated by ferrihydrite preferentially attack C=O, causing functional group degradation. In the group co-precipitated with ferrihydrite and microplastics, -OC=O in the microplastics is preferentially degraded, causing the long chains of polylactic acid molecules to break into short chains.

[0053] In addition, carbon dioxide will be produced during the complete degradation of microplastics. Although microbial respiration also produces carbon dioxide, Figure 9 As shown in the results, the carbon dioxide concentration produced by the co-precipitation of ferrihydrite and microplastics was lower than that produced by the direct interaction between ferrihydrite and microplastics. In addition, the direct interaction between ferrihydrite and microplastics significantly promoted the functional metabolism of soil microorganisms, and the abundance of functional genes in different metabolic pathways ( Figure 10 ).

[0054] Through the above technical solution, the soil microbial-mediated ferrihydrite in the present invention can effectively accelerate the degradation process of microplastics. In particular, microplastics may be wrapped in the process of ferrihydrite formation in the natural environment, and the present invention verifies that the co-precipitation of ferrihydrite and microplastics will further accelerate the degradation of microplastics and produce lower concentrations of carbon dioxide. In addition, the functional genes of microorganisms are also significantly increased due to the interaction between ferrihydrite and microplastics, which may promote different metabolic pathways of microorganisms in the environment. However, incomplete degradation of microplastics may produce more microplastics or even nanoplastics, changing the environmental behavior and fate of microplastics. Therefore, on the other hand, the present invention provides a theoretical basis for studying the interaction and environmental impact of microplastics and ferrihydrite in the environment.

[0055] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for degrading microplastics in soil, characterized in that: The following steps are involved: (1) Extraction of soil microorganisms: Sterile water and soil were mixed in a ratio of 2.5:1, shaken to mix, and allowed to stand to obtain a supernatant containing soil microorganisms; (2) Synthesis of ferrihydrite co-precipitated with PLA MPs: First, 40 g of Fe(NO3)3·9H2O was dissolved in 500 mL of deionized water; 4 g of PLA microplastics was added, and then 310 mL of 1 M KOH was added to adjust the pH; the mixture was vigorously stirred for 1 h using a magnetic stirrer and then centrifuged; after discarding the supernatant, the resulting colloid was repeatedly washed with deionized water 5 times; the colloid was dried in a freeze dryer for 24 h and stored at -20°C until use; (3) Phosphorus attachment of ferrihydrite: 1.2 g of ferrihydrite was added to 500 mL of 20 mM K2HPO4, the pH was adjusted to 6, incubated for 48 hours, and then freeze-dried; the incubation conditions were 28°C and 150 rpm; (4) The ferrihydrite co-precipitated with PLA MPs after phosphorus attachment is placed in a supernatant containing soil microorganisms for cultivation to degrade microplastic particles; the culture conditions are 28 °C, 150 rpm / min, and dark cultivation for 30 days.

2. The method according to claim 1, characterized in that The shaking mixing condition in step (1) is 150 rpm / min for 30 min.

3. The method according to claim 1, characterized in that The standing time in step (1) is 6 hours.

4. The method according to claim 1, wherein The centrifugation condition in step (2) is 12000 rpm for 12 minutes.

5. The method according to claim 1, wherein The pH range of the step (2) is adjusted to pH 7.4-7.6.

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