Multi-stage remediation method for ion-type rare earth tailing area soil composite pollution

Abstract

The application provides a multistage remediation method for ion-type rare earth tailing area soil composite pollution, aiming at the vertical pollution structure of "surface layer barren-mid-layer heavy metal / radioactivity-deep layer high ammonia nitrogen", and innovatively adopting a "partitioning, different quality, one layer one strategy" strategy. Specifically, the method comprises the following steps: layering and excavating 0-30cm surface layer, 30-50cm mid-layer and >50cm deep layer earthwork; mixing the deep layer earthwork with nitrogen directional conversion coupling material and composite microbial inoculant to construct an ammonia nitrogen targeted removal layer; mixing the mid-layer earthwork with porous matrix coupling stabilizer and functional microbial inoculant to form a heavy metal / radioactivity stable layer; mixing the surface layer earthwork with composite soil conditioner to build a soil improvement layer; and finally planting heavy metal resistant plants. Through the "special material-functional flora-vertical coordination" trinity design, the method realizes accurate, efficient and radical remediation of the whole profile, and avoids resource waste and secondary disturbance.

Description

Technical Field

[0001] This invention relates to the field of environmental remediation, and in particular to a multi-stage remediation method for complex soil pollution in ion-type rare earth tailings areas. Background Technology

[0002] my country is rich in rare earth mineral resources, with ion-adsorption rare earth deposits being particularly prevalent in the south. During mining, the use of large amounts of ammonium sulfate leaching solution to extract rare earth elements through ion exchange also results in the precipitation of heavy metal ions, leading to severe heavy metal and ammonia nitrogen pollution in the tailings soil of ion-adsorption rare earth mines. This pollution is further exacerbated by rainwater runoff, seeping into surrounding soil and water sources. This severely restricts the mining of ion-adsorption rare earth deposits and hinders local economic development. Therefore, in response to the damage to the surrounding ecological environment caused by the mining of ion-adsorption rare earth deposits, there is an urgent need to develop an environmentally friendly ecological restoration solution to revegetate the mining area while mitigating soil pollution or restoring the ecological environment of the mining area.

[0003] Currently, systematic remediation technologies for this type of complex pollution are still immature. Conventional techniques often involve landfilling with imported soil or covering with sludge and then planting vegetation for revegetation; or using microbial agents to restore vegetation in rare earth tailings areas, such as combining arbuscular mycorrhizal fungi solution or other microbial solutions with pioneer plants for ecological restoration; a very few studies have also focused on simple fixation of heavy metals in the surface soil of mining areas. Remediation technologies for single pollutants or single soil layers (such as simple imported soil, chemical passivation, or phytoremediation) only address surface soil improvement, heavy metal remediation, and simple revegetation in ion-type rare earth tailings areas, and often suffer from drawbacks such as high cost, long cycle, unstable effects, and easy secondary pollution. They lack specificity for the "vertically stratified, multi-pollutant complex" characteristics of tailings soil, resulting in the inability to effectively restore the ecological functions of soil at different depths in mining areas, making it difficult to achieve synergistic treatment of soil fertility restoration, heavy metal / radioactive element stabilization, and ammonia nitrogen removal. Summary of the Invention

[0004] The main objective of this invention is to provide a multi-level remediation method for soil composite pollution in ion-type rare earth tailings areas, which involves differentiated and multi-level synergistic remediation based on the dominant pollution type in different soil layers.

[0005] To achieve the above objectives, this invention provides a multi-stage remediation method for complex soil pollution in ion-type rare earth tailings areas, comprising the following steps:

[0006] S1: The ion-adsorption rare earth tailings area is excavated in layers to obtain surface soil with a depth of 0~30cm, middle soil with a depth of 30~50cm and deep soil with a depth of >50cm respectively.

[0007] S2: Mix the deep soil with nitrogen-directed conversion coupling material and composite microbial agent, cure it and backfill it into the original bottom layer to form an ammonia nitrogen targeted removal layer;

[0008] S3: Mix the intermediate soil with a porous matrix coupled with a heavy metal stabilizer and a functional microbial agent, cure it, and then backfill it onto the ammonia nitrogen targeted removal layer to form a heavy metal stabilized layer.

[0009] S4: The topsoil is mixed with the composite soil conditioner and then backfilled onto the heavy metal stabilizing layer to form a soil improvement layer;

[0010] S5: Plant heavy metal-tolerant plants on the soil amendment layer to complete the remediation;

[0011] The composite soil conditioner includes: soil amendment materials and microbial agents;

[0012] The soil amendment materials include fermented manure, coal gangue-based biomimetic peat, biochar, and water-retaining agents;

[0013] The microbial inoculants include phosphorus-solubilizing and potassium-solubilizing bacteria and organic matter-degrading bacteria.

[0014] According to the first aspect of the present invention, at least the following beneficial effects are achieved:

[0015] (1) Precise targeting, high efficiency and controllability: In response to the vertical composite pollution characteristics of tailings soil, namely "top layer barrenness, middle layer heavy metals / radioactive elements, and deep layer ammonia nitrogen", this invention adopts an engineering strategy of "differentiated and graded remediation". Specialized remediation materials and functional microbial agents are applied to soil at different depths to ensure that the agents and pollutants are highly matched and mixed evenly, which significantly improves the remediation efficiency and the repeatability and reliability of the effect.

[0016] (2) Material-microbe synergistic effect: During the maintenance stage after backfilling each layer of soil, by controlling environmental parameters such as water content and aeration conditions, a suitable living and metabolic environment is provided for functional microorganisms, giving full play to the synergistic effect between nitrogen-directed conversion materials and denitrifying bacteria, heavy metal stabilizers and heavy metal immobilizing bacteria, so as to achieve efficient removal of ammonia nitrogen and long-term stabilization of heavy metals / radioactive elements.

[0017] (3) Standardized process, easy to promote: This invention transforms the complex multi-pollutant synergistic remediation process into a clear and operable standardized process of “layered excavation – directional mixing – layered backfilling – maintenance management”. The parameters of each link are clear and the operation is simple, which facilitates quality control and large-scale engineering implementation, and significantly reduces the technical threshold and remediation cost.

[0018] (4) Balancing ecological safety and resource recycling: All contaminated soil is treated in situ before backfilling, effectively blocking the risk of pollutants migrating downwards or being released into the environment; the stress-resistant pioneer plants planted on the surface not only accelerate ecological restoration, but can also be used as biomass energy or feed resources, promoting the formation of an integrated green circular economy model of "pollution control - ecological reconstruction - resource recycling".

[0019] According to an embodiment of the present invention, in step S2, the nitrogen-directed conversion coupling material comprises at least one of zeolite, ball-milled modified coal gangue, and coal gangue-based Na-X molecular sieve.

[0020] In an embodiment of the present invention, the nitrogen-directed conversion coupling material mentioned in step S2 preferably comprises at least one of zeolite, ball-milled modified coal gangue, and coal gangue-based Na-X molecular sieve, wherein:

[0021] Zeolite possesses a high specific surface area and strong cation exchange capacity, enabling efficient adsorption of ammonium ions in deep soil layers, preventing their migration or volatilization loss with water, and providing a stable substrate for subsequent microbial transformation. Ball-milled modified coal gangue, through mechanical activation, exposes more active sites, enhancing not only its physical adsorption capacity for ammonia nitrogen but also serving as a slow-release carrier to promote electron transfer and synergistically strengthen microbial metabolic activity. Coal gangue-based Na-X molecular sieves, with their regular microporous structure and highly selective ion exchange performance, specifically capture ammonium ions and suppress interference from other coexisting ions, improving the efficiency of nitrogen-directed conversion. Used alone or in combination, they avoid the toxic inhibition of functional microorganisms by high concentrations of ammonia nitrogen and maintain the stable activity of denitrifying bacteria (such as nitrifying bacteria, denitrifying bacteria, and anaerobic ammonia oxidizing bacteria) through the continuous release of ammonium ions, thus achieving efficient and thorough removal of ammonia nitrogen during the backfilling and curing period. Furthermore, all of the above materials use industrial solid waste coal gangue as raw material, making them low-cost, environmentally friendly, and possessing dual value in pollution control and resource utilization.

[0022] According to an embodiment of the present invention, in step S2, the amount of nitrogen-directed conversion coupling material added is 5%-20% of the soil weight.

[0023] In this embodiment of the invention, the amount of nitrogen-directed conversion coupling material added in step S2 is controlled to be 5%–20% of the weight of the deeply contaminated soil. On the one hand, an addition amount of not less than 5% ensures that the material provides sufficient cation exchange sites and adsorption capacity to effectively capture high concentrations of ammonia nitrogen, avoiding excessively rapid adsorption saturation, ammonia nitrogen leakage, or toxic inhibition of functional microorganisms due to insufficient dosage, thereby ensuring the stable progress of subsequent nitrification-denitrification and anaerobic ammonia oxidation processes. On the other hand, an addition amount not exceeding 20% ​​avoids resource waste and increased costs caused by excessive material, and also prevents damage to the original soil structure, reduced porosity and permeability due to excessive solid additives, thus affecting the colonization and metabolic activity of microorganisms after backfilling. This addition ratio achieves the best balance between remediation efficiency, economic cost, and maintenance of soil physical properties.

[0024] In an embodiment of the present invention, step S2 further includes adding biochar.

[0025] In an embodiment of the present invention, in step S2, the nitrogen-directed conversion coupling material includes coal gangue and zeolite.

[0026] In an embodiment of the present invention, the mass ratio of the coal gangue, the zeolite, and the biochar is coal gangue:zeolite:biochar = 1.5~2.5:0.8~1.2:0.3~0.7.

[0027] According to an embodiment of the present invention, in step S2, the compound microbial agent includes at least one of iron ammonia oxidizing bacteria, anaerobic ammonia oxidizing bacteria, nitrifying bacteria and denitrifying bacteria.

[0028] In the embodiments of the present invention, firstly, nitrifying bacteria can oxidize ammonia nitrogen to nitrite / nitrate, while denitrifying bacteria further reduce nitrate to nitrogen gas, thereby achieving complete removal of ammonia nitrogen; secondly, anaerobic ammonia oxidizing bacteria can directly generate nitrogen gas under anaerobic conditions, requiring no organic carbon source, consuming little energy, and producing little sludge, making them particularly suitable for deep anoxic environments; furthermore, iron ammonia oxidizing bacteria can couple the Fe³⁺ reduction and ammonia oxidation processes, and in rare earth tailings environments rich in iron minerals, they can utilize in-situ iron oxides as electron acceptors, expanding the denitrification pathway and enhancing system stability.

[0029] According to an embodiment of the present invention, in step S3, the porous matrix coupled heavy metal stabilizer includes at least one of biochar, iron-based phosphorus-containing material, and DTC-modified porous biochar.

[0030] In this invention, biochar (magnesium-modified grapefruit peel biochar) possesses a high specific surface area and abundant oxygen-containing functional groups. After modification with magnesium ions, its adsorption and co-precipitation capacity for heavy metals is enhanced. Simultaneously, its natural porous structure facilitates the attachment and colonization of functional microorganisms, promoting bio-chemical synergistic fixation. Iron-based phosphorus-containing materials can release phosphate ions to form insoluble phosphate precipitates with heavy metals. Furthermore, zero-valent or divalent iron can reduce high-valence radioactive or toxic metals such as U(VI) and Cr(VI) to low-toxicity, low-valence states and co-precipitate, achieving dual stabilization of heavy metals and radioactive elements. DTC-modified porous biochar contains strong chelating groups that form highly stable five-membered ring chelates with various heavy metal ions, significantly reducing their mobility and bioavailability. This pollutant fixation network of the above-mentioned raw materials not only significantly reduces the content of heavy metals and radioactive elements but also provides a favorable habitat for subsequent inoculation of functional microorganisms due to its porous carrier characteristics, further enhancing long-term stability.

[0031] In an embodiment of the present invention, the biochar comprises magnesium-modified grapefruit peel biochar.

[0032] In an embodiment of the present invention, the porous matrix coupled heavy metal stabilizer is composed of biochar and iron-based phosphorus-containing material, with a mass ratio of biochar: iron-based phosphorus-containing material of 3~5:0.8~1.2.

[0033] According to an embodiment of the present invention, in step S3, the amount of the porous matrix coupled heavy metal stabilizer added is 5%-15% of the soil weight.

[0034] In this invention, on the one hand, the addition amount is not less than 5%, which can ensure that the stabilizer provides sufficient active sites and reaction interfaces to effectively achieve the adsorption, precipitation or chelation fixation of heavy metal elements, and avoid incomplete stabilization of pollutants due to insufficient dosage, which would affect long-term environmental safety; on the other hand, the addition amount does not exceed 15%, which can prevent excessive material from causing soil pore blockage and reduced air permeability, thus affecting the colonization and metabolic activity of subsequent functional microorganisms.

[0035] According to an embodiment of the present invention, in step S3, the functional microbial agent includes at least one of sulfate-reducing bacteria, heavy metal adsorbing strains, and radioactive resistant strains.

[0036] First, sulfate-reducing bacteria can reduce sulfate to sulfides under anaerobic conditions, forming extremely insoluble metal sulfide precipitates with heavy metal ions, achieving efficient and long-lasting chemical fixation. Second, heavy metal adsorption strains bioadsorb and complex heavy metal ions through cell wall functional groups or extracellular polymers, further reducing their mobility and bioavailability. Radioresistant strains not only survive in radioactive environments but also reduce highly toxic and highly mobile U(VI) to less toxic and insoluble U(IV) via extracellular electron transfer, precipitating it. Simultaneously, they utilize iron-based stabilizers as electron acceptors to enhance the co-reduction fixation of heavy metals and radioactive elements. These functional strains can be used alone or in combination. On the one hand, the material provides an attachment carrier and reaction interface for microorganisms; on the other hand, microbial metabolites can activate or regenerate the active sites of the material, significantly improving the efficiency of pollutant fixation.

[0037] According to an embodiment of the present invention, the heavy metal adsorption strain includes at least one of Bacillus subtilis, white-rot fungi, and sulfate-reducing bacteria.

[0038] According to an embodiment of the present invention, the radioresistant strain includes Geobacterium. Geobacter sulfurreducens .

[0039] According to an embodiment of the present invention, the inoculation amount of the functional microbial agent ensures an effective viable count ≥ 1 × 10⁻⁶. - 6 CFU / g soil.

[0040] According to an embodiment of the present invention, in step S3, the backfilling and curing time is 14 to 21 days.

[0041] According to an embodiment of the present invention, the fermented manure, the coal gangue-based biomimetic peat, the biochar, the water-retaining agent, and the microbial agent are present in the following proportions by weight: 1-2.5: 1-2.5: 0.25-1.5: 0.02-0.04: 0.05-0.2.

[0042] This formulation system ensures that the components complement each other and work synergistically. Topsoil adjusted using this ratio can maintain a stable pH of 6.0–7.0, increase available phosphorus, and improve vegetation cover.

[0043] According to an embodiment of the present invention, the soil conditioner is applied at a rate of 10-20 kg / mu to adjust the soil pH to 6.0-7.0.

[0044] According to an embodiment of the present invention, the heavy metal-resistant plants include at least two of the following: Napier grass, giant Napier grass, sweet sorghum, ramie, paper mulberry, lespedeza, Chinese pistache, Amorpha fruticosa, and oxtail grass. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0046] Figure 1 This is a schematic diagram of the technical route for the multi-stage remediation method for complex soil pollution in ion-type rare earth tailings areas according to the present invention.

[0047] Figure 2 This is a diagram showing the effect of tailings area restoration in Embodiment 1 of the present invention.

[0048] The realization of the objective, functional characteristics and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0049] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0050] It should be noted that all directional indicators (such as up, down, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indicator will also change accordingly.

[0051] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature.

[0052] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0053] Example 1

[0054] This embodiment selects a typical ion-adsorption rare earth mine contaminated site. The initial pH of the soil is generally below 4.5, which is a strongly acidic soil. The soil organic matter content (5.6-19.1%), especially the available phosphorus content (1.91-9.36%), is low, and the soil nutrient rating is poor. The content of heavy metals such as lead and cadmium in the middle soil (30-50cm) (Cd: 2.94-4.24mg / kg, Pb: 49.6-61.9mg / kg) is significantly higher than the risk control value for agricultural land pollution (Pb: 70, Cd: 0.3; GB15618-2018). The ammonia nitrogen content in the deep soil (>50cm) is as high as 90.6mg / kg.

[0055] S1: The ion-adsorption rare earth tailings area is excavated in layers to obtain surface soil with a depth of 0~30cm, middle soil with a depth of 30~50cm and deep soil with a depth of >50cm respectively.

[0056] S2: Using a mixing device, the deep soil is mixed with a nitrogen-directed conversion coupling material (coal gangue: zeolite: biochar in a mass ratio of 2:1:0.5) and a compound microbial agent (nitrosomonas). Nitrosomonas europaea Pseudomonas Pseudomonas hunanensis Mix thoroughly and cure for 18 days, turning the pile every 3 days during this period to alternate between aerobic and anaerobic environments;

[0057] S3: Using soil stabilization mixing equipment, soil is coupled with 5% by mass of porous matrix, along with heavy metal stabilizer (biochar: iron-based phosphorus-containing material (iron-based carbon-supported phosphorus-containing material nZVI@C / P) at a ratio of 4:1) and compound microbial agent (sulfate-reducing bacteria). Desulfovibrio desulfuricans with Geyperibrium Geobacter sulfurreducens Mix thoroughly and evenly, then backfill and cure for 10 days, maintaining the moisture content at 60%-70% of field capacity.

[0058] The preparation method of the iron-based carbon-supported phosphorus-containing material nZVI@C / P is as follows: using anhydrous ferric chloride (FeCl3) as the iron source, potassium dihydrogen phosphate (KH2PO4) as the phosphorus source, and adding glucose as the carbon source, nZVI@C / P is synthesized by hydrothermal-carbonization in one step.

[0059] S4: Use an excavator to deeply till and break up the topsoil (0-30cm) in situ. Mix the compound soil conditioner (fermented sheep manure: coal gangue-based biomimetic peat: biochar: water-retaining agent: phosphorus-solubilizing bacteria agent in a ratio of 1.2:1:1:0.025:0.2) with the excavated soil at a mass ratio of 1:50. After leveling the land, sow Napier grass and sweet sorghum seeds evenly at a seeding rate of 3 kg / mu.

[0060] Repair results:

[0061] Six months after remediation, the soil pH stabilized at around 6, the vegetation coverage was >95%, the available Cd extracted by DTPA in the middle backfill soil was not detected, the available Pb was reduced by more than 90%, and the ammonia nitrogen content in the deep backfill soil was reduced by more than 94%. The soil environmental quality of the entire tailings site was significantly improved, and the ecological risk index was significantly reduced.

[0062] Figure 1 It is a technical roadmap diagram, illustrating the multi-level repair strategy of this invention, which is based on "differentiated and qualitative approaches, with one policy per layer".

[0063] The physical and chemical properties of the remediated soil are shown in Table 1. The remediation effect of the tailings area is as follows: Figure 2 .

[0064] Table 1 Comparison of ammonia nitrogen and heavy metal concentrations in soil before and after remediation in rare earth mining areas

[0065]

[0066] This invention is the first to clearly identify the typical vertical composite pollution structure of soil in ion-adsorption rare earth tailings areas: "a barren surface layer, heavy metal and radioactive element pollution in the middle layer, and high concentration of ammonia nitrogen in the deep layer." Based on this, it innovatively proposes a "regional and qualitative, three-layer synergistic" remediation strategy, combining the synergistic mechanism of "composite functional materials and functional microorganisms" to achieve systematic treatment of the entire pollution profile. This method scientifically divides the tailings area soil into three functional units: surface, middle, and deep layers. For each layer's dominant pollutant, a specific remediation plan is designed—the surface layer focuses on soil fertility reconstruction and ecological restoration; the middle layer emphasizes the solidification and stabilization of heavy metals and radioactive elements; and the deep layer specializes in the targeted transformation and removal of ammonia nitrogen—precisely addressing the core problem while also considering other potential pollution risks. This model effectively avoids the waste of reagents, low efficiency, and secondary disturbance to non-target soil layers caused by the "one-size-fits-all" approach in traditional remediation methods, truly achieving precise, efficient, thorough, and reliable radical remediation with a "one-layer-one-policy" approach.

[0067] Example 2

[0068] The difference between this embodiment and Embodiment 1 is that S2: 5% nitrogen-directed conversion coupling material (coal gangue: zeolite: biochar in a ratio of 2:1:0.5) + composite microbial agent (nitrosomonas). Nitrosomonas europaea Pseudomonas Pseudomonas hunanensis ), maintenance for 18 days;

[0069] S3: 15% porous matrix coupled with heavy metal stabilizer (biochar: iron-based phosphorus-containing material at a ratio of 4:1) + functional microbial agent (sulfate-reducing bacteria) Desulfovibrio desulfuricans Geyperibract Geobacter sulfurreducens ), maintenance for 10 days;

[0070] The remaining steps (S1, S4) are exactly the same as in Example 1.

[0071] Example 3

[0072] The difference between this embodiment and embodiment 1 is in S2: the nitrogen directional conversion coupling material is changed to coal gangue-based Na-X molecular sieve (single component, 15% addition) + compound microbial agent (nitrosomonas, pseudomonas), and cured for 18 days;

[0073] S3: The porous matrix coupled with the heavy metal stabilizer is replaced with magnesium-modified grapefruit peel biochar (single component, 10% addition) + functional microbial agent (Bacillus mucilaginosus). Bacillus mucilaginosus +Geobacterium);

[0074] S4: The compound soil conditioner was adjusted to a ratio of fermented cow manure: coal gangue-based biomimetic peat: biochar: water-retaining agent: potassium-solubilizing bacteria of 2.0:2.0:0.5:0.03:0.1. The remaining operations (S1) were the same as in Example 1.

[0075] Example 4

[0076] The difference between this embodiment and embodiment 1 is that in S4: the composite soil conditioner removes the coal gangue-based biomimetic peat and is adjusted to fermented sheep manure: biochar: water-retaining agent: phosphate-solubilizing bacteria agent = 1.2:1:1:0.025:0.2. The remaining operations (S1, S2, S3) are the same as in embodiment 1.

[0077] Comparative Example 1:

[0078] Compared with Example 1, this comparative example uses a conventional passivating agent instead of the "nitrogen-directed conversion coupling material + denitrifying bacteria" system.

[0079] The difference between this comparative example and Example 1 is that in step S2, the nitrogen-directed conversion coupling material (coal gangue / zeolite / molecular sieve) + compound denitrification bacteria agent is replaced with single lime (CaO) to adjust the pH + conventional organic fertilizer (simulating the traditional ammonia nitrogen control method).

[0080] Comparative Example 2:

[0081] Replace the "porous matrix coupled stabilizer + functional bacteria" system with a general stabilizer.

[0082] (Verifying the necessity of S3 layer material – microbial synergy)

[0083] The difference between this comparative example and Example 1 is that in step S3, the iron-based phosphorus-containing material / modified biochar + sulfate-reducing bacteria / Geobacterium is replaced with a commercially available general soil heavy metal passivating agent (calcium dihydrogen phosphate + ordinary biochar, non-functional bacteria).

[0084] Specific procedure: Mix the middle layer of soil with 5% calcium dihydrogen phosphate and 5% ordinary rice husk biochar, backfill and cure for 10 days (no sulfate-reducing bacteria, no radioactive bacteria).

[0085] Comparative Example 3:

[0086] Compared with Example 1, this comparative example uses conventional organic fertilizer instead of the "compound soil conditioner" system.

[0087] Replacement: In step S4, replace the compound soil conditioner with a single conventional organic fertilizer.

[0088] Specific steps: Mix conventional organic fertilizer with excavated soil at a mass ratio of 1:50. After leveling the land, sow Napier grass and sweet sorghum seeds evenly at a sowing rate of 3 kg / mu.

[0089] Test case

[0090] The key indicator test results of this test case, example, and comparative example one month after remediation include three core parameters: surface soil organic matter content, middle layer lead content, and deep layer ammonia nitrogen concentration. These are used to quantitatively evaluate the treatment effect of different remediation schemes on the complex pollution of the vertical profile of tailings area in the early stage.

[0091] Table 2 Analysis of the effects one month after repair

[0092]

[0093] Comparative Example 1 used lime (CaO) to adjust the pH and supplemented with conventional organic fertilizer to replace the "nitrogen-directed transformation coupling material + composite denitrifying microbial agent" system in step S2 of this invention. This traditional method relies solely on high pH to promote ammonia nitrogen volatilization, lacking a mechanism for targeted adsorption and biotransformation of ammonium nitrogen, resulting in low ammonia nitrogen removal efficiency and leaving a high concentration of NH4 in deep soil layers. + -N; Due to the lack of functional microorganisms such as nitrifying bacteria, denitrifying bacteria, or anaerobic ammonia-oxidizing bacteria, complete mineralization and removal of nitrogen from ammonium nitrogen to nitrogen gas cannot be achieved, making it difficult to achieve the goal of radical remediation; Although Comparative Example 2 can fix some heavy metals to a certain extent through phosphate precipitation, it cannot reduce highly mobile, highly toxic high-valence radioactive or toxic metals to low-toxicity, insoluble low-valence forms; At the same time, due to the lack of sulfate-reducing bacteria, there is no endogenous S²⁻ generated in the system, and thermodynamically stable metal sulfide precipitates cannot be formed, resulting in limited heavy metal fixation effect; In addition, this passivation method mainly relies on surface adsorption or simple chemical precipitation, which is prone to desorption or dissolution when encountering acidic environments such as acid rain, causing heavy metals to be released again, resulting in significantly insufficient long-term stability; Comparative Example 3 has a single conditioner component, with only 48.37% organic matter on the surface, which is significantly lower than Example 1 (83.34%), indicating that the composite conditioner is irreplaceable for soil fertility reconstruction.

[0094] The above technical solutions of the present invention are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent structural transformations made under the technical concept of the present invention using the contents of the present invention specification and drawings, or direct / indirect applications in other related technical fields, are included in the patent protection scope of the present invention.

Claims

1. A multi-stage remediation method for complex soil pollution in ion-adsorption rare earth tailings areas, characterized in that, Includes the following steps: S1: The ion-adsorption rare earth tailings area is excavated in layers to obtain surface soil with a depth of 0~30cm, middle soil with a depth of 30~50cm and deep soil with a depth of >50cm respectively. S2: Mix the deep soil with nitrogen-directed conversion coupling material and composite microbial agent, cure it and backfill it into the original bottom layer to form an ammonia nitrogen targeted removal layer; S3: Mix the intermediate soil with a porous matrix coupled with a heavy metal stabilizer and a functional microbial agent, cure it, and then backfill it onto the ammonia nitrogen targeted removal layer to form a heavy metal stabilized layer. S4: The topsoil is mixed with the composite soil conditioner and then backfilled onto the heavy metal stabilizing layer to form a soil improvement layer; S5: Plant heavy metal-tolerant plants on the soil amendment layer to complete the remediation; In step S2, the nitrogen-directed conversion coupling material comprises at least one of zeolite, ball-milled modified coal gangue, and coal gangue-based Na-X molecular sieve; In step S2, the compound microbial agent includes at least one of iron ammonia oxidizing bacteria, anaerobic ammonia oxidizing bacteria, nitrifying bacteria, and denitrifying bacteria; In step S3, the functional microbial agent includes at least one of sulfate-reducing bacteria, heavy metal adsorbing strains, and radioactive resistant strains. In step S3, the porous matrix coupled heavy metal stabilizer includes at least one of biochar, iron-based phosphorus-containing material, and DTC-modified porous biochar. In step S4, the composite soil conditioner includes: soil amendment materials and microbial agents; The soil amendment materials include fermented manure, coal gangue-based biomimetic peat, biochar, and water-retaining agents; The microbial inoculants include phosphorus-solubilizing and potassium-solubilizing bacteria and organic matter-degrading bacteria.

2. The multi-stage remediation method for composite soil pollution in ion-adsorption rare earth tailings areas according to claim 1, characterized in that, In step S2, the amount of nitrogen-directed conversion coupling material added is 5%-20% of the soil weight.

3. The multi-stage remediation method for composite soil pollution in ion-adsorption rare earth tailings areas according to claim 1, characterized in that, In step S3, the amount of the porous matrix coupled heavy metal stabilizer added is 5%-15% of the soil weight.

4. The multi-stage remediation method for composite soil pollution in ion-adsorption rare earth tailings areas according to claim 1, characterized in that, By weight, the fermented manure, the coal gangue-based biomimetic peat, the biochar, the water-retaining agent, and the microbial agent are in the following proportions: 1-2.5: 1-2.5: 0.25-1.5: 0.02-0.04: 0.05-0.

2.

5. The multi-stage remediation method for composite soil pollution in ion-adsorption rare earth tailings areas according to claim 1, characterized in that, The dosage of soil conditioner is 10-20 kg / mu, which adjusts the soil pH to 6.0-7.

0.

6. The multi-stage remediation method for composite soil pollution in ion-adsorption rare earth tailings areas according to claim 1, characterized in that, The heavy metal-resistant plants include at least two of the following: Napier grass, giant reed grass, sweet sorghum, ramie, paper mulberry, lespedeza, Chinese pistache, Amorpha fruticosa, and oxtail grass.

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