Method for guest soil-free remediation of coal gangue areas using plant-microbe synergy

Abstract

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Description

TECHNICAL FIELD

[0001] The present disclosure belongs to the technical field of mine ecological restoration, and in particular relates to an ecological remediation method for coal gangue areas via plant-microbe synergy without the introduction of guest soil, which method is suitable for the in-situ remediation of coal gangue areas with high acidity, heavy metal enrichment, and poor water retention. BACKGROUND

[0002] Coal gangue areas consist of solid waste accumulated over long periods during coal mining and washing processes, and posing hazards in many aspects, including environment, ecology, health, and social economy. Coal gangue typically contains various heavy metals and sulfides, which may leach into soil or groundwater through rainwater infiltration, leading to water source contamination. Due to their loose structure, coal gangue areas are highly susceptibleto collapse during heavy rainstorms or earthquakes, potentially triggering geological disasters. The coal gangue exhibits poor physical and chemical properties (e.g., high salinity, nutrient deficiency, and toxic substances), which inhibits the plant growth. This leads to extremely low vegetation coverage, a decline in flora and fauna populations, reduced biodiversity, and eventual degradation of ecosystem functions.

[0003] Due to long-term accumulation, coal gangue areas often develop acidic environments with high heavy metal content, loose surface structure, and poor nutrients. The traditional “guest soil method” requires transporting large volumes of soil from other locations, which not only is costly but also damages the ecological environment of the source areas. The prior arts face the following challenges: (1) Limited adaptability of microbial communities. Although the guest soil-free technologies centered on microbial consortia (such as iron-reducing bacteria and sulfate-reducing bacteria) can suppress spontaneous combustion of gangue and promote revegetation, the biological activity is highly sensitive to the extreme environment of coal gangue areas (e.g., high acidity, high temperatures, and heavy metal pollution), leading to inconsistent remediation efficiency; (2) Insufficient capacity for extreme environment mitigation: the oxidation of sulfides in coal gangue areas frequently results in acidic environment (with a low pH) and significant heavy metal leaching; however, the prior art exhibits limited long-term effectiveness in neutralizing extreme pH levels or immobilizing high-concentration pollutants. Consequently, these approaches frequently rely on chemical auxiliaries, which increases both costs and complexity. (3) Incomplete improvement of soil structure and nutrition: the coarse texture and low porosity of coal gangue particles result in poor water and nutrient retention, whether used alone or with minimal substrates. This deficiency often leads to growth decline in the later stages of plant development, making it difficult to establish a sustainable ecological community. The present disclosure achieves efficient in-situ remediation of coal gangue areas by leveraging the synergistic effects between plants and microbes and integrating with substrate amendment and post-remediation management technology. SUMMARY Technical Problem

[0004] An object of the present disclosure is to provide a method for the guest soil-free remediation of coal gangue areas using plant-microbe synergy, which involves a comprehensive management method for guest soil-free coal gangue areas by utilizing plants with significantly different biological characteristics, such as Juncao and climbing plants (English ivy, Boston ivy, clematis, wintercreeper, wisteria, etc.), in combination with microbial fertilizers. Technical Solutions

[0005] In order to achieve the object described above, the present disclosure adopts the following technical solutions. Technical solutions

[0006] 1.1 Pioneer plants

[0007] Deep-rooted Juncao, which is acid-tolerant and nutrient-poor soil-tolerant, is selected (the Juncao includes one of Giant Juncao (Pennisetum giganteum), purple elephant grass, Arund donax cv. lvzhou No. 1, Arund donax cv. lvzhou No. 3, Arund donax cv. lvzhou No. 6, Fucao No. 2, and Fucao No. 6). The root exudates of such Juncao can promote microbial proliferation. Juncao can thrive in the harsh soil environments of coal gangue areas. Despite low fertility and severe heavy metal contamination, it survives and forms dense vegetation within just 3 months of planting. This effectively reduces fugitive dust in coal gangue areas and improves the local microclimate. Dense vegetation cover reduces direct sunlight and lowers the surface temperature of the gangue piles. Furthermore, the root systems and litter layers limits penetration of air into the interior of the coal gangue areas, inhibiting oxidation reactions and reducing the risk of spontaneous combustion in coal gangue areas. The rapid growth and substantial biomass of Juncao produces a large amount of litter that can increase soil organic matter. This facilitates the formation of a soil-like substrate, improving the poor soil structure of coal gangue areas and creating favorable conditions for subsequent plant community succession.

[0008] 1.2 Companion plants

[0009] Climbing plants (English ivy, Boston ivy, clematis, wintercreeper, wisteria, etc.) areplanted simultaneously, which can withstand extreme environments to some extent. Due to the loose structure of coal gangue, the area is highly susceptible to rainwater erosion. The climbing characteristics of climbing plants enable rapid surface coverage, and their dense foliage effectively lowers ground temperatures, thereby mitigating the risk of spontaneous combustion in coal gangue areas. Climbing plants are evergreen throughout the year, enabling the rapid formation of green coverage. This mitigates visual pollution in coal gangue areas and enhances the overall landscape of the mining sites.

[0010] The Juncao nitrogen-fixing bacterial fertilizer includes two microbial species: the nitrogen-fixing Klebsiella oxytoca and Bacillus mucilaginosus with heavy metal passivation capability, with a mass ratio of 1 : 1. The nitrogen-fixing Klebsiella oxytoca can convert atmospheric nitrogen (N2) into plant-available ammonium nitrogen (NH4+) by nitrogenase. This process enhances soil fertility and reduces dependency on chemical nitrogen fertilizers. The Bacillus mucilaginosus secretes extracellular polysaccharides to immobilize heavy metals.

[0011] The Juncao nitrogen-fixing bacterial fertilizer is prepared according to the literature “Effects of chemical fertilizer reduction and co-application with a JUNCAO nitrogen-fixing biofertilizer on growth and nutritional quality of Pennisetum giganteum and soil nutrient status” (JIA Yulei, Liao Zhen, Wang Lifang, et al., ACTA PRATACULTURAE SINICA, 2021, 30(3): 215-223). The specific method includes: inoculating a mixed liquid bacterial culture of the activated Klebsiella oxytoca strain and Bacillus mucilaginosus strain in a mass ratio of 1 : 1 into an LB liquid medium (5 g of yeast extract, 10 g of peptone, 10 g of sodium chloride, and distilled water made up to 1 L), and incubating the liquid bacterial culture at 30oC with shaking at 150-180 r min-1 until the exponential growth phase is reached.

[0012] Step 1: improvement of substrate formulation and nursing of seedlings

[0013] A mixture of loose, breathable, and well-drained humus soil and a Juncao nitrogen-fixing bacterial fertilizer (with a mass ratio of 1 : 1) is used as a seedling substrate. Degradable nursery bags with a diameter of 5-40 cm are selected. The degradable nursery bags are filled with the seedling substrate, then compacted, and regularly arranged in a nursery shed. Juncao stems aged 8 months or older with robust buds and no pests or diseases are selected and then cut into seed cuttings with a single bud each, with a straight cut 2-3 cm above the bud at the top, and a horse-ear shaped cut 3-5 cm below the bud at the cuttage end. Before cuttage, the seedling substrate is watered until thoroughly soaked, and cuttage is performed once the soil is dried slightly and is no longer sticky, where the cuttage depth should be sufficient to cover the upper axillary bud by 1 cm. Watering is performed immediately after cuttage. The Juncao seedlings are transplanted when 4 to 6 tillers are developed. For a climbing plant, the seed thereof is sowed and lightly covered with the seedling substrate (about 0.5 centimeters), and the seedlings are transplanted upon reaching 20-30 cm in height.

[0014] Step 2: planting

[0015] The Juncao and the climbing plant are intercropped at a row ratio of N : 1, where 1 < N < 20, i.e., N rows of the Juncao are planted followed by 1 row of the climbing plant, and this pattern is repeated. The specific steps include: 1) planting in holes along fish-scale contours, with a row and plant spacing of 200 cm x 100 cm, a hole depth of 30 cm, and a hole diameter of 25 cm; 2) placing the seedlings vertically into the holes (without removing the bag), and then covering same with coal gangue residue and compacting same; and 3) applying sufficient root-settling water after planting.

[0016] Step 3: maintenance and care

[0017] A supporting drip irrigation system is installed; and after planting, the Juncao is mowed regularly and spread relatively evenly over the coal gangue.

[0018] Synergistic effect: The Klebsiella oxytoca and Bacillus mucilaginosus in the Juncao nitrogen-fixing bacterial fertilizer form a synergistic symbiotic relationship with Juncao root exudates through metabolic cross-feeding. Substances such as sugars, organic acids, and amino acids secreted by Juncao roots serve as carbon and energy sources for the two types of microbes, thereby stimulating their metabolic activity. The Klebsiella oxytoca converts atmospheric nitrogen into ammonium nitrogen through nitrogen fixation, while the Bacillus mucilaginosus releases insoluble nutrients from the soil via phosphorus and potassium solubilization. Together, the two microbes act synergistically to enhance the rhizosphere nutrient availability. At the same time, phytohormones produced via microbial metabolism, such as auxins and cytokinin, together with organic acids, further stimulate Juncao root development and amplify the intensity of root exudate release. This establishes a positive feedback loop of “root exudate-microbial activation-nutrient supply-plant growth”, significantly enhancing the soil fertility and plant stress resistance.

[0019] Ecological benefits: within 6 months, the volume of leachate is reduced by 60%-85%, the available forms of heavy metals decrease by 50%-65%, and the biodiversity increases by 100% or more.

[0020] Low cost: without the need for guest soil, the remediation costs are reduced by 50% or more compared to conventional methods.

[0021] Plant-microbe feedback mechanism: through the root exudate-microbial metabolism cycle, a self-sustaining remediation system is formed.

[0022] Improved substrate formulation: Tailored to the environmental characteristics of coal gangue areas, functional strains are added to prepare a Juncao nitrogen-fixing bacterial fertilizer, which creates a specialized substrate that enhances water and nutrient retention while improving the ability to passivate heavy metals.

[0023] Management innovation: Leveraging the rapid growth and substantial biomass of Juncao, a large amount of litter is produced through multiple mowing and surface-spreading cycles. This promotes the formation of a soil-like substrate within the coal gangue areas, improving the poor soil structure of coal gangue areas and creating favorable conditions for subsequent plant community succession. Beneficial effects

[0024] In the present disclosure, the guest soil-free remediation of coal gangue areas using plant-microbe synergy is achieved. This planting model can rapidly increase the vegetation coverage in coal gangue areas, effectively enhancing water and nutrient retention; furthermore, it reduces soil erosion and the leaching of pollutants. Simultaneously, the management technique of the present disclosure enables the production of a large amount of litter through multiple mowing and surface-spreading of Juncao, forming a soil-like substrate in the coal gangue area, thereby optimizing the soil structure, and minimizing rainwater leaching of coal gangue. This, in turn, mitigates the adverse impact of leachate on the natural environment.

[0025] Experimental results demonstrated that, compared with the blank control group, the leaching volume of the experimental group is reduced by 65.0%, showing a significant difference (P < 0.05). The total hardness and sodium content of the leachate in the experimental group are the lowest, with significant differences (P < 0.05). The manganese content in the leachate of the experimental group is significantly different from that of the control group (P < 0.05). These results demonstrate that the plant-microbe combined remediation technology utilized in the present disclosure facilitates the remediation of coal gangue areas without guest soil. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 shows the remediation effects of Application Example 1; and

[0027] FIG. 2 shows the remediation effects of Application Example 2. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] To make the above-mentioned features and advantages of the present disclosure more apparent and understandable, specific embodiments are described in detail below. Unless otherwise specified, the methods used in the present disclosure are all conventional methods in the art.

[0029] A method for the guest soil-free remediation of coal gangue areas using plant-microbe synergy includes the following steps:

[0030] step 1: improvement of substrate formulation and nursing of seedlings

[0031] using a mixture of loose, breathable and well-drained humus soil and a Juncao nitrogen-fixing bacterial fertilizer as a seedling substrate, charging the seedling substrate into degradable nursery bags, then compacting the seedling substrate, and arranging the seedling substrate under a nursery shed; selecting Juncao stems aged 8 months or older with robust buds and having no pests or diseases, and then cutting Juncao stems into seed cuttings with a single bud each, with a straight cut 2-3 cm above the bud at the top, and a horse-ear shaped cut 3-5 cm below the bud at the cuttage end; before cuttage, watering the seedling substrate until thoroughly soaked, and beginning cuttage once the soil is dried slightly and is no longer sticky, where the cuttage depth should be sufficient to cover the upper axillary bud by 1 cm; and watering immediately after the cuttage, and transplanting the Juncao seedlings when 4 to 6 tillers are developed; and for a climbing plant, sowing seeds of the climbing plant and lightly covering the seeds with a 0.5 cm layer of the seedling substrate, and transplanting seedlings upon reaching 20-30 cm in height;

[0032] step 2: planting

[0033] intercropping the Juncao and the climbing plant at a row ratio of 5 : 1, i.e., planting five rows of Juncao followed by 1 row of climbing plant, and repeating this pattern; specifically: 1) planting in holes along fish-scale contours, with a row and plant spacing of 200 cm x 100 cm, a hole depth of 30 cm, and a hole diameter of 25 cm; 2) placing the seedlings vertically into the holes without removing the bag, and then covering seedlings with coal gangue residue and compacting coal gangue residue; and 3) applying sufficient root-settling water after planting; and

[0034] step 3: maintenance and care

[0035] installing a supporting drip irrigation system; and mowing the Juncao regularly after planting, and spreading the mowedJuncao evenly over the coal gangue.

[0036] Preferably, the mass ratio of the humus soil to the Juncao nitrogen-fixing bacterial fertilizer in step 1 is 1 : 1. The degradable nursery bags have a diameter of 20 cm. Example

[0037] 1.1 Test materials

[0038] Giant Juncao, provided by the China National Engineering Research Center of Juncao Technology at Fujian Agriculture and Forestry University, was selected for this study, and English ivy was chosen as the climbing plant.

[0039] 1.2 Overview of the experimental area

[0040] In March 2023, in Xibei Village, Gaobei Town, Yongding District, Longyan City, Fujian Province, a coal gangue area was selected to conduct the experiment. Yongding District features a subtropical marine monsoon climate with mild temperatures all year round. The annual average temperature is 20.1°C, with a frost-free period of 300 days. The annual average precipitation is approximately 1647 mm, with relatively concentrated rainfall occurring throughout the year. The experimental site was a derelict coal mine characterized by massive accumulations of coal gangue that formed gangue piles. The site was devoid of soil, andheavy rainfall generated significant flow-out leachate due to the steep slopes, leading to environmental pollution.

[0041] 1.3 Experimental design

[0042] Nine plots, each with a projection area of 30 m2 (10 m x 3 m), were constructed at the experimental site. The plots had a slope of approximately 25°. Each plot was separated by cement boards and connected at the downslope end to a 3 m x 1 m x 1 m catchment basin designed to collect runoff from the plot. There were 3 treatments tested (in the experimental groups Juncao and climbing plants were intercropped with a row ratio of 5 : 1): (1) experimental group A (improved seedling substrate, the present disclosure); (2) experimental group B (common seedling substrate, only humus soil); and (3) blank control. Each treatment was set up in 3 replicates, with a row and plant spacing of 200 cm x 100 cm. The observation period spanned from June 2023 to August 2023. At the end of each month, the leachate volume from each plot was recorded and quantified.

[0043] 1.4 Measurement of indicators

[0044] (1) Measurement of leachate: The catchment basins were monitored at the end of each month. The leachate volumes were determined via water level gauges, and the volumes were recorded before the basins were completely emptied.

[0045] (2) Measurement of leachate quality: At the end of August 2023, the leachate samples were collected to determine the total hardness, concentrations of manganese and sodium, and pH in the leachate.

[0046] 1.5 Data analysis

[0047] Data analysis was performed using the Excel 2016 data processing system and Grafhpad Prism software.

[0048] 2.1 Effect of different treatments on leachate volume

[0049] The measurement and analysis results of leachate volume for different treatments at the experimental site from June to August 2023 are presented in Table 1.

[0050] Table 1 Leachate volume of different treatment plots from June to August 2023 Treatment      - Leachate volume (m3) June July August Average Experimental group A 0.136±0.001 a 0.142±0.002 a 0.148±0.002 a 0.142±0.001 a Experimental group B 0.291±0.005 b 0.286±0.004 b 0.301±0.002 b 0.293±0.002 b Control group 0.875±0.014 c 0.883±0.012 c 0.921±0.011 c 0.889±0.066 c

[0051] As shown in Table 1, the leachate volumes of experimental groups A and B were reduced by 84.0% and 67.0%, respectively, compared to the control group, representing significant differences (P < 0.05). This indicated that the rapid growth and well-developed root systems of Juncao and climbing plants could increase the vegetation coverage in coal gangue areas, playing a crucial role in water retention. Compared with experimental group B, experimental group A showed a 51.5% reduction in leachate volume, representing a significant difference (P < 0.05). This indicated that a positive feedback loop was established between the Juncao nitrogen-fixing bacterial fertilizer and the plants, which effectively enhanced the water retention effects.

[0052] 2.2 Effect of different treatments on leachate quality

[0053] The measurement and analysis results of leachate quality for different treatments at the experimental site in August 2023 are presented in Table 2.

[0054] Table 2. Determination of leachate quality in August 2023 Treatment Total hardness / (mg L-1) Manganese / (mg L-1) Sodium / (mg L-1) pH Experimental group A 64±3.4 a 0.019±0.003 a 1.3±0.1 a 6.55±0.10 a Experimental group B 99±6.5 b 0.022±0.004 b 1.8±0.2 b 6.18±0.13 b Control group 181±11.7 c 0.037±0.005 c 2.0±0.1 c 5.97±0.12 c

[0055] As shown in Table 2, compared with that in the control group, significant differences (P < 0.05) were observed in the total hardness, sodium content, pH, and manganese content of the leachate in experimental groups A and B. This indicated that the intercropping model of Juncao and climbing plants could mitigate the impact of rainfall on coal gangue areas through their tall and dense above-ground biomass. Furthermore, the well-developed root systems of Juncao could absorb more water, thereby reducing the leaching effect of rainfall on coal gangue. Additionally, Juncao could uptake pollutants from the water, further minimizing the environmental hazards posed by the leachate. Significant differences (P < 0.05) were also observed in the total hardness, sodium content, pH, and manganese content of the leachate between experimental group A and experimental group B. This indicated that a positive feedback loop was established between the Juncao nitrogen-fixing bacterial fertilizer and the plants, which significantly enhanced the overall remediation effect.

[0056] This application example provided a method for the guest soil-free remediation of coal gangue areas using plant-microbe synergy, exemplified by the Banzhai Coal Mine in Xibei Village, Gaobei Town, Yongding District, Longyan City, Fujian Province.

[0057] Yongding District in Longyan City, Fujian Province, is characterized by its abundant mineral resources. However, historical large-scale mining activities have led to significant ecological degradation in the region. Coal mining in the Yongding District is characterized by the historical prevalence of small, scattered, and unregulated traditional southern mines. These have left behind a large number of abandoned mines, resulting in ecological issues that have hindered local socio-economic development and the progress of ecological civilization construction.

[0058] In 2022, the research team from the China National Engineering Research Center of Juncao Technology at Fujian Agriculture and Forestry University selected the Banzhai Coal Mine to conduct guest soil-free remediation trials in coal gangue areas. English ivy was selected as the climbing plant. The detailed implementation methods and operational procedures were consistent with those described in the specific embodiments mentioned above.

[0059] Six months after the implementation of the aforementioned steps, the plant height of Giant Juncao reached 2.0 m-2.5 m, while the English ivy extended to 10 m-12 m. The vegetation coverage of the coal gangue area increased to 75%-80%, demonstrating an excellent vegetation establishment effect (as shown in FIG. 1). Furthermore, the leachate volume was reduced by 84%, and the content of available manganese in the leachate decreased by 49%.

[0060] This application example provides a method for the guest soil-free remediation of coal gangue areas using plant-microbe synergy, exemplified by Wangjiazhai Village, Panjiazhuang Town, Xingren County, Qianxinan Buyi and Miao Autonomous Prefecture, Guizhou Province.

[0061] Xingren County is abundant in coal resources; however, solid wastes such as coal gangue generated during mining can produce leachate containing heavy metals and harmful substances under the processes of rainwater scouring, leaching, and immersion. This poses a serious pollution threat to the surrounding environment.

[0062] In April 2023, the research team from the China National Engineering Research Center of Juncao Technology at Fujian Agriculture and Forestry University selected the Xingli Coal Mine to conduct guest soil-free remediation trials in coal gangue areas. Boston ivy was selected as the climbing plant. The remaining detailed implementation methods and operational procedures were consistent with those described in the specific embodiments mentioned above.

[0063] Ten months after the implementation of the aforementioned steps, the plant height of Giant Juncao reached 3.5 m-3.7 m, while the Boston ivy extended to 12 m-15 m. The vegetation coverage of the coal gangue area increased to 85% or more, demonstrating an excellent vegetation establishment effect (as shown in FIG. 2). Furthermore, the leachate volume was reduced by 73%, and the content of available manganese in the leachate decreased by 61%.

[0064] The above descriptions are only preferred embodiments of the present disclosure. Any equivalent changes and modifications made in accordance with the scope of the patent application of the present disclosure shall fall within the coverage of the present disclosure.

Claims

1. A guest soil-free method for remediating coal gangue areas using plant-microbe synergy, comprising the following steps:step 1: improving substrate formulation and nursing seedlings, comprising:using a mixture of loose, breathable and well-drained humus soil and a Juncao nitrogen-fixing bacterial fertilizer as a seedling substrate, charging the seedling substrate into degradable nursery bags, then compacting same, and arranging same within a nursery shed; selecting Juncao stems aged 8 months or older with robust buds and no pests or diseases, and then cutting same into seed cuttings with a single bud each, with a straight cut 2-3 cm above the bud at the top, and a horse-ear shaped cut 3-5 cm below the bud at the cuttage end; before cuttage, watering the seedling substrate until thoroughly soaked, and beginning cuttage once the soil is dried slightly and is no longer sticky, where the cuttage depth should be sufficient to cover the upper axillary bud by 1 cm; and watering immediately after the cuttage, and transplanting the Juncao seedlings when 4 to 6 tillers are developed; and for a climbing plant, sowing the seed thereof and lightly covering the seed with a 0.5 cm layer of the seedling substrate, and transplanting seedlings upon reaching 20-30 cm in height;step 2: planting a Juncao and a climbing plant, comprising:intercropping the Juncao and the climbing plant at a row ratio of N : 1, where 1 < N < 20, i.e., planting N rows of the Juncao followed by 1 row of the climbing plant, and repeating this pattern; specifically: 1) planting in holes along fish-scale contours, with a row and plant spacing of 200 cm x 100 cm, a hole depth of 30 cm, and a hole diameter of 25 cm; 2) placing the seedlings vertically into the holes without removing the bag, and then covering same with coal gangue residue and compacting same; and 3) applying sufficient root-settling water after planting; andstep 3: performing maintenance and care, comprising:installing a supporting drip irrigation system; and after planting, mowing the Juncao regularly and spreading same evenly over the coal gangue;wherein the Juncao comprises one selected from the group consisting of Giant Juncao, purple elephant grass, Arund donax cv. lvzhou No. 1, Arund donax cv. lvzhou No. 3, Arund donax cv. lvzhou No. 6, Fucao No. 2, and Fucao No. 6;wherein the Juncao nitrogen-fixing bacterial fertilizer comprises two microbial2025352952   04 Aug 2026species: Klebsiella oxytoca and Bacillus mucilaginosus.

2. The guest soil-free method as claimed in claim 1, wherein a mass ratio of the humus soil to the Juncao nitrogen-fixing bacterial fertilizer in step 1 is 1 : 1.

3. The guest soil-free method as claimed in claim 1 or 2, wherein a mass ratio of Klebsiella oxytoca and Bacillus mucilaginosus is 1 : 1.

4. The guest soil-free method as claimed in claim 1, wherein the degradable nursery bags have a diameter of 5-40 cm.

5. The guest soil-free method as claimed in claim 1, wherein the climbing plant comprises one selected from the group consisting of English ivy, Boston ivy, clematis, wintercreeper, and wisteria.

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