Preparation and application of slow-release dripping pills for acid soil remediation

Abstract

The application provides preparation and application of slow-release dripping pills for acid soil remediation, and belongs to the technical field of soil remediation. The slow-release dripping pills are composed of an inner core layer and a coating layer. The inner core layer comprises anaerobic digestion biogas residue, fly ash, limestone powder, potassium dihydrogen phosphate, humic acid and sulfate-reducing bacteria agent. The coating layer comprises biogas slurry, sodium alginate, polyvinyl alcohol, sodium bicarbonate and polyglutamic acid. The slow-release dripping pills take the anaerobic digestion biogas residue as the inner core and the functionalized biogas slurry as the coating. By accurately controlling the proportion of limestone powder, humic acid, sodium alginate and polyvinyl alcohol and embedding the sulfate-reducing bacteria, the slow-release dripping pills realize multi-dimensional and collaborative remediation of acid mine soil. The application realizes sustainable utilization of solid waste resources and soil remediation function, effectively improves the pH value of acid soil, and has a long-lasting remediation effect.

Description

Technical Field

[0001] This invention relates to the field of soil remediation technology, specifically to the preparation and application of a slow-release pellet for acidic soil remediation, which is particularly suitable for long-term remediation of acidic mining soils. Background Technology

[0002] Acidic soils are a general term for soils with a low pH value, widely found in the natural environment, and typically formed over a long period by the interaction of climate, parent material, and biological processes. Acid mine-drainage-affected soils (AMD soils), however, are a special and extreme type of acidic soil, formed by the oxidation of sulfide minerals (such as pyrite and pyrrhotite) exposed during mining activities, resulting in strongly acidic soils. The acidification mechanism of these soils primarily stems from the large amount of sulfur ions (S) in the minerals. 2- In the presence of oxygen and moisture, it is gradually converted into sulfuric acid through chemical and biological oxidation, releasing a large amount of hydrogen ions (H+). + This leads to a significant drop in soil pH, typically below 4.0, and in severely affected areas even below 2.5. Acidophilic thiobacilli proliferate in this acidic environment, accelerating the oxidation of sulfur ions and further promoting the production of acidic substances, creating a vicious cycle. Such soils are not only highly acidic but also contain high concentrations of soluble heavy metal ions such as manganese, copper, zinc, and cadmium, causing severe damage to the soil ecosystem. This results in vegetation degradation, a monotonous microbial community structure, and loss of soil fertility. Furthermore, the acidic mine runoff (AMD) it produces continuously pollutes surrounding water bodies and soil, with far-reaching environmental impacts and significant challenges to remediation.

[0003] Currently, the main remediation solutions for this type of soil include: (1) Traditional lime neutralization can raise the pH in the short term, but it cannot block the source of sulfide oxidation, and the material is easily leached, requiring repeated application, which is costly and easily leads to soil compaction; (2) Importing uncontaminated soil for covering or mixing to dilute acidic substances; however, it is costly and only treats the symptoms, not the root cause; (3) Stabilization / solidification technology: adding materials such as phosphates, zeolites, and organic matter to fix heavy metals. However, most materials lack sustained release capacity and have limited long-term pH stability. Patent CN120173619A discloses a soil conditioner for soil acidification control, its preparation method, and its application. It mainly includes red mud, humic acid-sodium alginate complex, zeolite, silicon-calcium-magnesium fertilizer, and acid-resistant bacteria. It exhibits good pH regulation and nutrient retention capabilities, but lacks the ability to inhibit the continuous acid production mechanism. It cannot effectively block the sulfur oxidation pathway and cannot curb acid regeneration in AMD soils from the source. It uses a common granulation process, and the particles have no coating structure, making them easily washed away under heavy rainfall or sloping conditions. At the same time, it lacks a slow-release and controlled-release design, and cannot achieve long-term, on-demand release of alkaline substances and functional components.

[0004] In summary, existing technologies generally suffer from problems such as high cost, short-lasting effects, complex construction, easy generation of secondary pollution, and failure to achieve resource utilization. Therefore, there is an urgent need to develop a new type of soil remediation material specifically for acidic soils, which integrates functions such as intelligent slow release, source acid suppression, multiple passivation, and solid waste resource utilization. Summary of the Invention

[0005] The main objective of this invention is to provide a method for preparing and applying slow-release pellets for acidic soil remediation, thereby addressing the problems mentioned in the background art. This invention utilizes a composite granulation process involving dried sludge anaerobic digestion residue, fly ash, limestone powder, potassium dihydrogen phosphate, humic acid, and sulfate-reducing bacteria (SRB) to form the core. A fluidized bed coating is then applied using a coating solution composed of concentrated digestate, sodium alginate, polyvinyl alcohol, sodium bicarbonate, and polyglutamic acid to obtain functional slow-release pellets with excellent slow-release properties and heavy metal passivation capabilities.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing sustained-release pellets for acidic soil remediation includes the following steps: (1) After mixing biogas residue, fly ash, limestone powder, potassium dihydrogen phosphate and humic acid evenly, granulate them and control the particle size to 2-5 mm to obtain core particles. (2) Stir the biogas slurry, sodium alginate, polyvinyl alcohol, sodium bicarbonate and polyglutamic acid until completely dissolved to form a uniform coating solution; (3) Place the core particles in a fluidized bed coating equipment and spray the coating liquid evenly in the airflow suspension state until the coating thickness is 0.2-0.5 mm. Then dry them with hot air at 40-60℃ until solidified to obtain the sustained-release droplets.

[0008] Preferably, the biogas residue is dried anaerobic digestion biogas residue with a moisture content ≤30%; the biogas slurry is concentrated anaerobic digestion biogas slurry with a solid content ≥15%; and the limestone powder contains ≥90% CaCO3.

[0009] Preferably, the anaerobic digestion residue of the sludge has an organic matter dry weight ratio of ≥30%, a pH of 7.5-8.5, and a total nutrient (N, P2O5 and K2O) dry weight ratio of >1%.

[0010] Preferably, the mass ratio of the biogas residue, fly ash, limestone powder, potassium dihydrogen phosphate, and humic acid is 50-70:10-20:5-15:5-10:3-8.

[0011] Preferably, the mass ratio of the biogas slurry, sodium alginate, polyvinyl alcohol, sodium bicarbonate, and polyglutamic acid is 30-50:5-10:3-5:2-5:1-2.

[0012] Preferably, the mass ratio of limestone powder to humic acid is 2-4:1. Limestone powder continuously neutralizes soil acidity, while humic acid, through its active functional groups, complexes and fixes free heavy metal ions in the soil, and synergistically inhibits the metabolic activity of acidophilic sulfur-oxidizing bacteria. If there is too much humic acid (ratio <2:1), due to insufficient alkalinity, it cannot effectively neutralize the strongly acidic environment, resulting in a slow increase in soil pH. Humic acid protonates at low pH, reducing its complexing ability, causing heavy metals to remain in a highly reactive ionic state, leading to poor remediation effects. If there is too much limestone powder (ratio >4:1), excessive alkalinity may cause a sudden increase in local pH, disrupting the microbial community (including embedded sulfate-reducing bacteria SRB), lacking biological inhibition of sulfur-oxidizing bacteria. Simultaneously, due to insufficient humic acid, it cannot effectively complex heavy metals, especially after pH recovery, which may lead to the re-release of heavy metals due to the redissolution of carbonate / hydroxide colloids. Therefore, this invention achieves a balance between acid neutralization and heavy metal passivation capabilities by controlling the ratio of limestone powder to humic acid.

[0013] The mass ratio of sodium alginate to polyvinyl alcohol is 1-2:1. Under the influence of calcium ions and hydrogen bonds, they can form a cross-linked network structure, regulating the release of the core material while their degradation products replenish soil organic matter. If the mass ratio is <1:1, the coating is too dense, resulting in slow degradation, hindering the release of the core material, delaying the initiation of remediation, and making it difficult to degrade even under strong acid conditions, failing to achieve the intelligent response of "the more acid, the faster the release." If the mass ratio is >2:1, the coating disintegrates prematurely in humid or weakly acidic environments, causing the core components to be released all at once, losing its slow-release function, and easily being washed away by rainwater, resulting in a short-lived remediation effect. A suitable coating structure initially forms a semi-permeable barrier, allowing the exchange of water and small molecules but restricting the large-scale entry of oxygen, maintaining an anaerobic microenvironment for sulfate-reducing bacteria (SRB) in the core. This invention achieves intelligent remediation by strictly controlling the mass ratio of sodium alginate to polyvinyl alcohol.

[0014] Preferably, the bulk density of the core particles is 0.8-1.2 g / cm³. 3 The compressive strength is ≥1.5MPa, and the moisture content of the particles is 12%-18%; the particle size of the sustained-release pellets is 2.2-5.5mm.

[0015] Preferably, the core particles further contain 1-5 parts of sulfate-reducing bacterial agent, with an effective viable count ≥ 2.0 × 10⁻⁶. 9 CFU / g, this bacterial agent can reduce SO42- in the anaerobic environment inside the pellet. 2- Restore to S 2- This leads to the formation of stable sulfide precipitates with heavy metal ions.

[0016] Preferably, the molecular weight of polyglutamic acid (PGA) in the coating solution is 100-200 kDa, thereby increasing the saturated water absorption rate of the pellets to 200%-350%, thus maintaining the continuous release of the core material even when the soil moisture content is below 15%.

[0017] The second aspect of the present invention provides a slow-release pellet for acidic soil remediation obtained by the preparation method described above. The slow-release pellet consists of a core layer and a coating layer. The core layer is mainly made of anaerobic digestion residue of sludge, providing alkaline substances, organic matter and nutrients. The coating layer is based on modified biogas slurry, forming a biodegradable semi-permeable barrier to regulate the release rate of the core substances.

[0018] Preferably, the coating layer degrades in a simulated acidic soil solution at pH 4.0 and 25°C for 90-150 days; the degradation rate of the coating layer is positively correlated with the hydrogen ion concentration in the environment, thus achieving slow-release regulation of alkaline substances and nutrients and avoiding rapid loss due to rainfall.

[0019] The third aspect of the present invention provides the application of slow-release pellets for acid soil remediation, wherein the slow-release pellets are evenly spread on the surface of acid soil at a ratio of 50-100 kg / mu, and then rotary tilled.

[0020] Preferably, the pH of the acidic soil is <4.5, and after 3-6 months of application, the soil pH is 6.0-7.0; and the content of available Cu, Zn and Cd in the soil is reduced by 30%-80%, and the remediation effect can be maintained for more than 12 months.

[0021] Preferably, the slow-release pellets can be used alone as a soil pretreatment material before vegetation reconstruction, or combined with tolerant plants to construct a comprehensive remediation system, applicable to scenarios such as mine tailings, spoil heaps, and abandoned mine pits with severe heavy metal pollution.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention utilizes dried anaerobic digestion residue and high-purity limestone powder to synergistically neutralize acidity, while sodium bicarbonate provides an initial rapid buffer. Within 3-6 months, strongly acidic soil (pH < 4.5) is adjusted to a neutral range (pH = 6.0-7.0), and the remediation effect lasts for more than 12 months, effectively eliminating the toxicity of acidic ions to plants and microorganisms, creating a favorable environmental foundation for ecological restoration.

[0023] 2. In this invention, the humic acid in the core of the slow-release pellet fixes free heavy metal ions through complexation; potassium dihydrogen phosphate generates insoluble phosphate precipitate; simultaneously, sulfate-reducing bacteria (SRB) reduce sulfate ions to sulfide ions in an anaerobic microenvironment, further reacting with Cu... 2+Zn 2+ Cd 2+ The formation of extremely insoluble metal sulfides, and the synergistic effect of these substances, significantly reduces the bioavailability of available heavy metals in the soil, achieving efficient, stable passivation and long-term stabilization.

[0024] 3. The coating layer of the sustained-release droplets of this invention adopts a composite system of sodium alginate, polyvinyl alcohol and polyglutamic acid, which has an intelligent responsive sustained-release function. In an acidic environment, the coating degrades as needed, and the higher the acidity, the faster the degradation. This realizes the release of alkaline substances and nutrients as needed, effectively avoiding the problem of rapid loss of traditional repair materials due to rain erosion, and significantly improving repair efficiency and resource utilization.

[0025] 4. The slow-release pellets of this invention are based on biogas residue with an organic matter content of ≥30% in the core, providing nutrients such as nitrogen, phosphorus, and potassium (total nutrients >1%). Combined with humic acid and coating degradation products, they significantly increase soil organic matter content and cation exchange capacity, promote the formation of aggregate structure, and lay a good foundation for subsequent vegetation restoration. Moreover, humic acid can inhibit the metabolic activity of acidophilic sulfur-oxidizing bacteria, while the embedded sulfate-reducing bacteria consume sulfate ions and generate alkaline sulfides. This dual mechanism effectively blocks the pyrite oxidation acid production pathway, achieving long-term ecological restoration.

[0026] 5. This invention uses anaerobic digestion residue and concentrated biogas slurry as the main raw materials, turning waste into treasure. It not only reduces remediation costs but also realizes the recycling of organic waste. All components of this invention are biodegradable or environmentally friendly materials, with no risk of secondary pollution and good ecological safety. Detailed Implementation

[0027] The technical solution of the present invention will be further explained below with reference to specific embodiments. It should be noted that the following embodiments are only preferred embodiments of the present invention and should not be construed as limiting the present invention.

[0028] In the following examples, the dried sludge anaerobic digestion residue and slurry of Example 1 came from Leiyang Sludge Anaerobic Digestion Plant, and the residue and slurry of Examples 2 and 3 came from Lu'an Sludge Anaerobic Digestion Plant.

[0029] Example 1: (1) Mix 60 parts of dried sludge anaerobic digestion residue, 15 parts of fly ash, 12 parts of limestone powder, 7 parts of potassium dihydrogen phosphate, 4 parts of humic acid and 3 parts of sulfate reducing bacteria evenly, and then granulate using disc granulation to obtain core particles with a particle size of 2-5 mm. (2) Stir 40 parts of concentrated sludge anaerobic digestion liquid, 6 parts of sodium alginate, 4 parts of polyvinyl alcohol, 4 parts of sodium bicarbonate and 1.5 parts of polyglutamic acid until completely dissolved to form a uniform coating liquid. (3) The core particles are placed in a fluidized bed coating equipment and coated with coating liquid uniformly in the airflow suspension state until the coating thickness is 0.3-0.4 mm. Then, they are dried with hot air at 50°C until solidified to obtain the slow-release droplets with a particle size between 2.2-5.5 mm and an average particle size of 4.1 mm.

[0030] Example 2: (1) Mix 50 parts of dried sludge anaerobic digestion residue, 10 parts of fly ash, 6 parts of limestone powder, 5 parts of potassium dihydrogen phosphate, 3 parts of humic acid and 1 part of sulfate reducing bacteria evenly, and then granulate using disc granulation to obtain core particles with a particle size of 2-5 mm. (2) Stir 30 parts of concentrated sludge anaerobic digestion liquid, 5 parts of sodium alginate, 4 parts of polyvinyl alcohol, 2 parts of sodium bicarbonate and 1 part of polyglutamic acid until completely dissolved to form a uniform coating liquid. (3) Place the core particles in a fluidized bed coating equipment and spray the coating liquid evenly in the airflow suspension state until the coating thickness is 0.3-0.4 mm. Then dry them with hot air at 50°C until solidified to obtain the slow-release droplets with a particle size between 2.2-5.5 mm and an average particle size of 4.7 mm.

[0031] Example 3: (1) Mix 70 parts of dried sludge anaerobic digestion residue, 20 parts of fly ash, 15 parts of limestone powder, 10 parts of potassium dihydrogen phosphate, 4.5 parts of humic acid and 5 parts of sulfate reducing bacteria evenly, and then granulate using disc granulation to obtain core particles with a particle size of 2-5 mm. (2) Stir 50 parts of concentrated sludge anaerobic digestion liquid, 10 parts of sodium alginate, 5 parts of polyvinyl alcohol, 5 parts of sodium bicarbonate and 2 parts of polyglutamic acid until completely dissolved to form a uniform coating liquid. (3) Place the core particles in a fluidized bed coating equipment and spray the coating liquid evenly in the airflow suspension state until the coating thickness is 0.3-0.4 mm. Then dry them with hot air at 50°C until solidified to obtain the slow-release droplets with a particle size between 2.2-5.5 mm and an average particle size of 3.1 mm.

[0032] Comparative Example 1: Same as Example 3, except that the amount of biogas residue added in step (1) is 10 parts.

[0033] Comparative Example 2: Same as Example 3, except that sulfate-reducing bacteria (SRB) is not added in step (1).

[0034] Comparative Example 3: Same as Example 3, except that in step (2), sodium alginate is 15 parts and polyvinyl alcohol is 4 parts.

[0035] Comparative Example 4: Same as Example 3, except that in step (2), sodium alginate is 5 parts and polyvinyl alcohol is 8 parts.

[0036] Comparative Example 5: Same as Example 3, except that only the core particles were prepared.

[0037] Performance testing: The slow-release pellets prepared in the examples and comparative examples were used to remediate acidified soil (80 kg / mu). The pH value and the content of available heavy metals (Cu, Zn, Cd) in the acidified soil before remediation were tested. After one remediation cycle, the pH value and heavy metal content of the remediated soil were tested. The remediation results of the soil pH and available heavy metal pollution before and after remediation are shown in Table 1.

[0038] Table 1. Soil pH and heavy metal content (mg / kg) before and after remediation in the examples and comparative cases.

[0039] As shown in Table 1, after remediation in Examples 1-3, the soil pH increased from 4.16-4.31 to 6.15-6.86; simultaneously, the contents of available Cu, Zn, and Cd decreased significantly (removal rates were 50-70%, 45-65%, and 50-70%, respectively). This indicates that the slow-release pellets of the present invention can effectively remediate acidified soil and reduce heavy metal activity. In Comparative Example 1, the soil remediation effect was not as good as in Example 3 due to the low amount of biogas residue. This was because the low biogas residue content resulted in insufficient buffering capacity and nutrient supply, leading to limited pH increase and decreased heavy metal fixation capacity. In Comparative Example 2, the lack of sulfate-reducing bacteria, although having a basic alkaline adjustment effect from limestone powder, resulted in a significant decrease in heavy metal removal efficiency due to the lack of biochemical synergy. In Comparative Examples 3-4, excessive sodium alginate made the membrane too soft and easily damaged, affecting the slow-release time. Excessive PVA may hinder nutrient release or reduce hydrophilicity, indicating that the ratio of sodium alginate to PVA must be strictly controlled to achieve stable slow-release control and high remediation efficiency. In Comparative Example 5, the lack of a coating layer resulted in low utilization of the effective ingredients, preventing them from exerting a sustained effect, indicating that the coating layer is an important structure for achieving slow release and long-term stable remediation. The above results show that the slow-release pellets of this invention, with biogas residue as the core, SRB bio-enhanced, and optimized coating slow release, achieve rapid pH increase and long-term passivation of heavy metals in acidified soils.

[0040] The above embodiments are merely preferred technical solutions of the present invention and should not be considered as limitations on the present invention. The scope of protection of the present invention should be limited to the technical solutions described in the claims, including equivalent substitutions of the technical features described in the claims. That is, equivalent substitutions and improvements within this scope are also within the scope of protection of the present invention.

Claims

1. A method for preparing sustained-release pellets for acidic soil remediation, characterized in that, Includes the following steps: (1) After mixing biogas residue, fly ash, limestone powder, potassium dihydrogen phosphate and humic acid evenly, granulate them and control the particle size to 2-5 mm to obtain core particles. (2) Stir the biogas slurry, sodium alginate, polyvinyl alcohol, sodium bicarbonate and polyglutamic acid until completely dissolved to form a uniform coating solution; (3) Place the core particles in a fluidized bed coating equipment and spray the coating liquid evenly in the airflow suspension state until the coating thickness is 0.2-0.5 mm. Then dry it with hot air at 40-60℃ until it solidifies to form complete slow-release droplets.

2. The preparation method according to claim 1, characterized in that, The biogas residue is dried anaerobic digestion residue of sludge with a moisture content of ≤30%; the biogas slurry is concentrated anaerobic digestion slurry of sludge with a solid content of ≥15%; and the limestone powder contains ≥90% CaCO3.

3. The preparation method according to claim 1, characterized in that, The mass ratio of biogas residue, fly ash, limestone powder, potassium dihydrogen phosphate, and humic acid is 50-70:10-20:5-15:5-10:3-8.

4. The preparation method according to claim 1, characterized in that, The mass ratio of the biogas slurry, sodium alginate, polyvinyl alcohol, sodium bicarbonate, and polyglutamic acid is 30-50:5-10:3-5:2-5:1-2.

5. The preparation method according to claim 1, characterized in that, The mass ratio of limestone powder to humic acid is 2-4:1, and the mass ratio of sodium alginate to polyvinyl alcohol is 1-2:

1.

6. The preparation method according to claim 1, characterized in that, The bulk density of the core particles is 0.8-1.2 g / cm³. 3 Compressive strength ≥1.5MPa, particle moisture content 12%-18%.

7. The preparation method according to claim 1, characterized in that, The core particles also contain 1-5 parts of sulfate-reducing bacterial agent, with an effective viable count ≥2.0 × 10⁻⁶. 9 CFU / g; the molecular weight of polyglutamic acid (PGA) in the coating solution is 100-200 kDa.

8. The slow-release pellets for acidic soil remediation obtained by the preparation method according to any one of claims 1-7.

9. The application of the slow-release pellets for acidic soil remediation as described in claim 8, characterized in that, The slow-release pellets are evenly spread on the surface of acidic soil at a ratio of 50-100 kg / mu, and then rotary tilled.

10. The application according to claim 9, characterized in that, The acidic soil has a pH < 4.

5. After application, the soil pH is 6.0-7.0 3-6 months later; and the content of available Cu, Zn and Cd in the soil decreases by 30%-80%.

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