Biochar-enhanced microbial mineralization-improved compacted clay covering barrier layer and construction method thereof

Abstract

The invention relates to a biochar enhanced microbial mineralization improved compacted clay covering barrier layer and a construction method thereof.The barrier layer is prepared from a urease bacterium excitation solution, a calcium acetate solution, biochar and site in-situ soil, the total dry mass of all components in the urease bacterium excitation solution accounts for 4.5%-8.5% of the mass of the site in-situ soil, and the total dry mass of all components in the calcium acetate solution accounts for 4.5%-8.5% of the mass of the site in-situ soil. The dry mass of the solute of the calcium acetate solution accounts for 4.0%-10.0% of the mass of the site in-situ soil material, and the dry mass of the biochar accounts for 4.0%-8.0% of the dry mass of the site in-situ soil material. Compared with the prior art, the method has the advantages that site in-situ soil materials are treated through an in-situ excitation microbial mineralization method, the seepage reduction and gas barrier performance of a compacted clay covering barrier layer can be remarkably enhanced, and meanwhile, the binding force enhanced by mineralization among soil particles plays an important role in inhibiting water loss and cracking of a soil body.

Description

Technical Field

[0001] The invention belongs to the technical field of pollution barriers and relates to a biochar enhanced microbial mineralization improved compacted clay covering barrier layer and a construction method thereof. Background Art

[0002] There are a large number of volatile organic compounds (VOCs) in the contaminated sites left behind by the relocation of industrial enterprises, which pose a great threat to people's health and public safety. The barrier layer in the covering barrier technology is an important control measure to block the migration of VOC gases to the ground and reduce rainwater infiltration. In engineering, compacted clay barrier layers are more common. However, the traditional compacted clay layer has poor water retention performance, and it still loses water and cracks at room temperature, which reduces its permeability and gas barrier performance. In addition, the dry-wet cycle caused by seasonal climate changes can easily lead to permanent cracks in the compacted clay layer, and the barrier performance is significantly weakened. In addition, it is usually recommended to select high-quality clay with low permeability for engineering soil, but its material cost and transportation cost are high. Therefore, the research and development of economical improved compacted clay layers that have both anti-seepage, gas-barrier and water-retention properties is the current development trend.

[0003] In recent years, microbial mineralization technology based on urea hydrolysis has been favored by environmental geotechnical engineers and is suitable for the improvement of sand, silt and low-plasticity clay. This technology uses carbonate ions generated by urease bacteria hydrolyzing urea to combine with exogenous cementing materials in an alkaline environment to form carbonate crystals, which are then filled in the pores of the soil or cemented soil particles to improve the engineering properties of the soil. Among these properties, the filling effect is conducive to improving the anti-seepage and gas barrier properties of the soil, and the cementation effect between particles can inhibit the cracking of the soil caused by dry-wet cycles. In terms of implementation methods, microbial mineralization technology can be achieved through bio-enhancement and in-situ stimulation methods. The bio-enhancement method requires the standard urease bacteria to be cultured in a laboratory environment first, but its operation steps are cumbersome. The in-situ stimulation method directly stimulates the urease bacteria in the in-situ soil, eliminating the processes of bacterial activation, expansion and deployment in the bio-enhancement method, and is more suitable for practical engineering applications. Therefore, it is theoretically feasible to improve the compacted clay cover barrier layer using in-situ stimulation microbial mineralization technology, and it is convenient for practical engineering applications. In engineering applications, the in-situ soil materials (mainly sand, silt and low-plasticity clay) with poor engineering performance can be directly used as the treatment object to reduce the material cost and transportation cost of high-quality clay. However, the improvement effect of microbial mineralization technology on fine-grained soil is poor, and there is a problem of handling the by-product ammonium ions, which needs to be solved by increasing the production of calcium carbonate, adjusting the mineralization process or introducing additives. This needs to be considered in the research and development of microbial mineralization to improve the compacted clay cover barrier layer.

[0004] Chinese patent application CN202311212930.9 provides a semi-rigid vertical barrier reinforced by microbial mineralization and carbide slag and its preparation method, wherein the raw material components include calcium-based bentonite, in-situ soil, urease bacteria stimulant and carbide slag, wherein the dry mass of the calcium-based bentonite accounts for 15.0% to 25.0% of the dry mass of the in-situ soil, the total dry mass of each component in the urease bacteria stimulant accounts for 2.0% to 4.0% of the dry mass of the in-situ soil, and the dry mass of the carbide slag accounts for 3.0% to 6.0% of the dry mass of the in-situ soil. This patent mainly utilizes the ammonium ions, a byproduct of microbial mineralization, to exchange cations with calcium ions between bentonite layers to modify the calcium-based bentonite, thereby improving the expansibility of the calcium-based bentonite. The added carbide slag significantly improves the strength of the vertical barrier material. However, the barrier material may not be sufficient to significantly reduce the emission of ammonium ions only through cation exchange. The high alkaline environment caused by carbide slag will inhibit the in-situ activation of urease bacteria, resulting in a slower rate of urea hydrolysis. In addition, when the groundwater level changes, the barrier material may suffer from severe dehydration cracking under the influence of dry-wet cycles. Summary of the invention

[0005] The purpose of the present invention is to provide a biochar enhanced microbial mineralization improved compacted clay cover barrier layer and its construction method. By treating the in-situ soil materials of the site by in-situ microbial mineralization method, the permeability reduction and gas barrier performance of the compacted clay cover barrier layer can be significantly enhanced. At the same time, the bonding force between soil particles strengthened by mineralization will play an important role in inhibiting soil dehydration and cracking. On the other hand, the introduction of biochar with a high specific surface area in the microbial mineralization process can increase the additional nucleation points of mineralization to increase the production of calcium carbonate, and the biochar can also be used as an adsorbent to remove by-product ammonium. In addition, biochar improves the water retention performance of the compacted clay cover barrier layer by improving the pore structure of the soil.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] On the one hand, the present invention provides a biochar enhanced microbial mineralization improved compacted clay cover barrier layer, the raw material components of which include urease bacteria stimulation solution, calcium acetate solution, biochar and on-site soil materials, wherein the total dry mass of each component in the urease bacteria stimulation solution accounts for 4.5% to 8.5% of the mass of the on-site soil materials, the dry mass of the solute of the calcium acetate solution accounts for 4.0% to 10.0% of the mass of the on-site soil materials, and the dry mass of the biochar accounts for 4.0% to 8.0% of the dry mass of the on-site soil materials.

[0008] Furthermore, the mass of the urease bacteria stimulating solution accounts for 5.0% to 15.0% of the mass of the in-situ soil material on the site.

[0009] Furthermore, the mass of the calcium acetate solution accounts for 5.0% to 15.0% of the mass of the in-situ soil material on the site.

[0010] Furthermore, the urease bacteria stimulating solution includes peptone and selected substrate urea, wherein the dry mass of peptone accounts for 2.5% to 3.5% of the mass of the in-situ soil material on the site, and the dry mass of the selected substrate urea accounts for 2.0% to 5.0% of the mass of the in-situ soil material on the site.

[0011] Furthermore, the purity of the calcium acetate is greater than 95%.

[0012] Furthermore, the type of biochar is lignocellulosic biochar, which is in the form of powder and / or fiber, wherein the total carbon content is greater than 28.0%, the ash content is between 10.0% and 20.0%, and the specific surface area is greater than 38m 2 / g, pH value is between 8.2 and 9.7, conductivity EC is controlled below 1500μs / cm, and cation exchange capacity CEC is greater than 21.5meq / 100g.

[0013] Furthermore, the particle size of the in-situ soil material on the site is less than 60 mm, the dry weight of soil particles with a particle size greater than 5 mm does not exceed 20% of the total dry weight of the in-situ soil material on the site, and the soil organic matter content does not exceed 5wt%.

[0014] In another aspect, the present invention also provides a method for constructing a biochar-enhanced microbial mineralization-modified compacted clay cover barrier layer, comprising the following steps:

[0015] (1) Clean up the construction site, remove obstacles, vegetation and garbage, plow and disperse the original soil, screen out the coarse soil with a particle size greater than 60 mm, and place the gravel soil with a particle size between 5 mm and 60 mm on one side of the site for standby use;

[0016] (2) In accordance with the Engineering Classification Standard for Soil GB / T 50145-2007, fine-grained sand (SC, SM) or fine-grained soil with low organic matter content (organic matter content less than 5%) is selected as the main part of the in-situ soil material of the site, and gravel soil with a particle size of 5 mm to 60 mm is selected as the remaining part of the in-situ soil material of the site. The two parts of soil materials are mixed evenly to obtain the in-situ soil material of the site;

[0017] (3) After the soil material is screened, the site is leveled and gravel soil with a particle size of 5 mm to 60 mm is laid as a coarse-grained soil cushion layer;

[0018] (4) After the biochar is evenly mixed with the in-situ soil material on site, it is laid on top of the coarse-grained soil cushion layer, and then urease stimulation solution is added for in-situ stimulation. Then, calcium acetate solution is added and mixed evenly. Finally, the laid soil material is compacted to obtain a modified compacted clay cover barrier layer, and its thickness is controlled between 300 mm and 600 mm.

[0019] Furthermore, when the modified compacted clay cover barrier layer is laid in layers, the biochar of the top modified compacted clay barrier layer is fibrous in shape, and the dry mass of the biochar accounts for 8.0% of the mass of the in-situ soil of the layer;

[0020] The biochar in the bottom modified compacted clay barrier layer is in powder form, and the dry mass of the biochar accounts for 8.0% of the in-situ soil mass of the layer;

[0021] The biochar of the remaining layers of the improved compacted clay barrier layer is in powder form, and the dry mass of the biochar accounts for 4.0 to 8.0% of the mass of the in-situ soil of the layer site.

[0022] Furthermore, the total mass of the urease bacteria stimulating solution and the calcium acetate solution = the dry mass of the on-site soil material × (optimum moisture content + 2%) - the mass of natural water in the on-site soil material. Preferably, the mass ratio between the urease bacteria stimulating solution and the calcium acetate solution is 1:1. If the on-site soil material is fine-grained sand or fine-grained soil with a coarse-grained content greater than 25%, the total dry mass of the components of the urease bacteria stimulating solution accounts for 8.5% of the mass of the on-site soil material, and the dry mass of the calcium acetate solute accounts for 10.0% of the mass of the on-site soil material.

[0023] Furthermore, when laying in layers, the contact surfaces between the layers need to be roughened, rolled 5 to 10 times, and the thickness of the single layer of fill after final compaction shall not exceed 150 mm. The compaction degree of the improved compacted clay barrier layer needs to reach more than 95%, and the moisture content needs to be adjusted in time according to the compaction curve of the soil during the construction process. Each layer of soil should be kept as horizontal as possible during laying to avoid local depressions or elevations. When the compacted clay layer contacts the concrete structure or other types of barrier structures (such as bedrock), it needs to be sealed with sealing materials or bentonite cushions to ensure that no penetration channels are generated at the joints. After completing all the compacted layers, the surface of the last layer should be kept flat to avoid rainwater or groundwater accumulation in low-lying areas.

[0024] Furthermore, the thickness of the coarse-grained soil cushion layer is controlled to be 180 mm to 240 mm, and the relative density is greater than 67%.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] (1) The biochar-enhanced microbial mineralization-improved compacted clay cover barrier layer and its construction process proposed in the present invention adopt biologically mediated environmental geotechnical technology, which has the characteristics of low carbon, low consumption, sustainability and environmental friendliness. Among them, biochar belongs to biomass materials, which are usually carbon-rich solid substances generated by high-temperature cracking of biomass raw materials such as agricultural and forestry wastes, livestock and poultry manure and some urban garbage under oxygen-limited or anaerobic conditions. Microbial mineralization technology is a biological process, which induces mineralization reactions through the metabolism of microorganisms. The in-situ excitation method adopted has the advantages of convenient operation and green ecology compared with traditional bio-enhancement methods. Therefore, through the mutual synergy of biochar and in-situ excitation microbial mineralization technology, the improved compacted clay cover barrier layer is an eco-friendly material.

[0027] (2) The in-situ stimulated microbial mineralization technology used in the present invention can generate a large amount of calcium carbonate crystals to fill the soil pores, thereby significantly enhancing the permeability reduction and gas barrier properties of the compacted clay layer. In addition, the additional nucleation sites provided by biochar for calcium carbonate mineralization can also increase the yield of calcium carbonate crystals. On the other hand, the in-situ soil material selected for construction is a mixture of coarse-grained soil and fine-grained soil with good gradation. By mixing biochar particles with smaller particle sizes, the particle composition of the in-situ soil material on site is further improved, which is beneficial to improving the mineralization efficiency of microorganisms and reducing the permeability coefficient. In particular, the use of powdered biochar can greatly improve the mineralization efficiency.

[0028] (3) The improved compacted clay cover barrier layer in the present invention has a strong water holding capacity. Under the action of microbial mineralization, a large number of calcite clusters generated in the soil pores reduce the volume of large pores, improve the water retention capacity of the soil, and increase the resistance to capillary water migration. In particular, the formation of a dense cementing body can significantly reduce the evaporation rate of soil moisture. The addition of biochar can increase the volume and capillary porosity of extremely small pores in the compacted clay layer, further improving the water holding capacity. On the other hand, the bonding force between soil particles in the compacted clay, which is strengthened by mineralization, will significantly inhibit water loss and cracking of the soil. At the same time, the fibrous biochar used in the top layer of the compacted clay cover barrier layer can also help improve the ability to resist dry-wet cycles.

[0029] (4) The on-site soil material screening and utilization proposed in the present invention can reduce the material cost and transportation cost of high-quality clay, and reduce the cost of handling waste soil in the traditional compacted clay barrier layer during site cleaning. The improved technology of enhancing microbial mineralization through biochar can make the compacted clay covering barrier layer meet the engineering anti-seepage requirements. If the degree of pollution is high, the thickness of the covering barrier layer can be increased to improve the long-term effectiveness. In addition, gravel soil with a particle size of 5mm to 60mm in the site can be used as a cushion layer to block the rise of capillary water, which can avoid the increase in the moisture content of the compacted clay layer caused by the rise of groundwater level and the invasion of liquid pollutants. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the effect of modified compacted clay cover barrier for biochar enhanced microbial mineralization;

[0031] Figure 2 This is a construction process flow chart of Example 1 of the present invention;

[0032] Figure 3 The effects of biochar dosage on calcium carbonate production and hydraulic permeability of microbially mineralized modified compacted clay cover barrier layers were investigated. DETAILED DESCRIPTION

[0033] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0034] In the following embodiments, the peptone used was purchased from Hongrun Baoshun industrial-grade peptone, and the total nitrogen content (on a dry basis) was not less than 13.0%; urea was agricultural urea (superior quality); calcium acetate was purchased from Jinan Yihewang Chemical Technology Co., Ltd. Industrial-grade calcium acetate with a purity of 99%; biochar can be purchased from Hainan Blue Arrow Agricultural Science and Technology Development Co., Ltd. Coconut fiber biochar and rice husk biochar are provided, and the in-situ soil material of the site comes from the soil of a contaminated site to be controlled.

[0035] The above raw materials are only used to introduce the embodiments of the present invention, and are not designated products. The raw materials are all conventional commercially available raw materials in the field. The equipment or technology involved in the examples are all conventional engineering technologies.

[0036] Embodiment 1:

[0037] (1) This embodiment provides a biochar-enhanced microbial mineralization-modified compacted clay cover barrier layer and a construction process, the raw materials of which include: urease bacteria stimulating solution, calcium acetate solution, biochar and site in-situ soil material.

[0038] (2) Site condition investigation. The area of ​​a contaminated site to be controlled is 10,000 m 2 After investigation, the main organic pollution source of a polluted site to be controlled was located in the saturated water zone at a depth of 10m, posing a threat of VOC invasion.

[0039] (3) Site cleaning. Before laying the compacted clay cover barrier layer, obstacles such as boulders, concrete blocks, weeds and garbage on the construction site are removed through the cooperation of excavators and manual labor. To prevent water accumulation during construction, a drainage system is set up with reference to the surface drainage design of the highway pavement to ensure that the site is dry and controllable before laying the compacted clay barrier layer.

[0040] (4) Preliminary screening of the on-site soil. First, use an excavator equipped with a tiller to plow the on-site soil to a depth of 1m. Then use an excavator equipped with a screening bucket to screen out the coarse particles of soil with a particle size greater than 60mm, and place the gravel particles with a particle size between 5mm and 60mm on one side of the site for use.

[0041] (5) Classification of in-situ soil on site. The plasticity index, particle composition and organic matter content of the in-situ soil on site after initial screening shall be determined with reference to the Standard for Geotechnical Test Methods GB / T 50123-2019, and classified in accordance with the Standard for Engineering Classification of Soil GB / T 50145-2007.

[0042] (6) Selection of in-situ soil. After the initial screening, the in-situ soil material of the site is selected as the fine-grained soil with low organic matter content as the main soil material for the compacted clay cover barrier layer. Its dry mass accounts for 85% of the total dry weight of the soil particles, and the coarse particle group content is 20%. The remaining 15% of the soil material is gravel soil with a particle size between 5mm and 60mm. The natural moisture content of the two types of soil samples after mixing evenly was measured to be 5%, the optimal moisture content was 20%, and the maximum dry density was 2.05kg / m 3 According to the project plan, the total thickness of the compacted clay cover barrier layer is 400mm, the thickness of a single layer is 100mm, and the compaction degree is controlled to be 96%. The total weight of the soil required is 96%×2.05kg / m 3 ×(1+5%)×10000m 2 ×0.4m≈8266kg. The dry mass of the original soil material on site is 8266kg / (1+5%)=7872kg, and the dry mass of a single layer is 7872kg / 4=1968kg.

[0043] (7) Laying of coarse-grained soil cushion layer. After the soil material is screened, the site is leveled with a bulldozer, and then rolled back and forth with a roller roller. After the site is prepared, the maximum dry density of the gravel soil with a particle size of 5mm to 60mm is measured to be 2.20kg / m 3 , minimum dry density is 1.60kg / m 3 , the natural moisture content is 1%. According to the designed relative density of 70%, the dry density of the gravel soil is calculated to be 1.98kg / m 3 , wet density is 2.00kg / m 3 The total mass of gravel soil required to lay a 200mm cushion layer in this site is 2.00kg / m 3 ×10000m 2 ×0.2m=4000kg, that is, 4 tons of gravel soil is needed as the coarse-grained soil cushion. During construction, a roller roller is used to control the construction requirements and level the site.

[0044] (8) Preparation of microbial mineralization solution. The total mass of urease-stimulating solution and calcium acetate solution = dry mass of the on-site soil × (optimal moisture content + 2%) - mass of natural water in the on-site soil. The total mass of urease-stimulating solution and calcium acetate solution is calculated to be 7872kg × (20% + 2%) - 8266kg × 5% ≈ 1319kg; the mass ratio between urease-stimulating solution and calcium acetate solution is 1:1, and its mass is 660kg, which accounts for 8.0% of the mass of the on-site soil. Among them, the dry mass of peptone accounts for 2.5% of the mass of the on-site soil, the dry mass of the selected substrate urea accounts for 3.0% of the mass of the on-site soil, and the dry mass of the calcium acetate solute accounts for 6.0% of the mass of the on-site soil. The dry mass of peptone is calculated to be 207kg, the dry mass of the selected substrate urea is 248kg, and the dry mass of the calcium acetate solute is 496kg. After calculating the required raw material mass, add peptone, select substrate urea and tap water in stirring tank 1 in sequence, stir evenly, cover with rainproof and dustproof tarpaulin and set aside; add calcium acetate and tap water to stirring tank 2, stir evenly, cover with rainproof and dustproof tarpaulin and set aside.

[0045] (9) Biochar mixed with soil. The compacted clay cover barrier layer is planned to be compacted in 4 layers, with each layer compacted to a thickness of 100 mm. The top layer of biochar is coconut fiber biochar, and its dry mass accounts for 8.0% of the dry mass of the single-layer in-situ soil, i.e. 1968 kg × 8.0% = 157 kg; the remaining layers of biochar are powdered rice husk biochar, and its dry mass accounts for 8.0% of the dry mass of the single-layer in-situ soil, i.e. 157 kg. The biochar and soil are evenly mixed by the cooperation of the shovel loader and the excavator for later use. The coconut fiber biochar used has a total carbon content of 30.2%, an ash content of 10.1%, and a specific surface area of ​​39 m 2 / g, pH value 8.4, electrical conductivity (EC) 924μs / cm, cation exchange capacity (CEC) 25.4meq / 100g. The total carbon content of the rice husk biochar used is 38%, the ash content is 18%, and the specific surface area is 60m 2 / g, pH 9.6, electrical conductivity (EC) 1240μs / cm, cation exchange capacity (CEC) 23.2meq / 100g.

[0046] (10) In-situ stimulation of microbial mineralization improvement. The prepared urease-stimulating solution was fully mixed with the biochar-mixed soil material and laid on the coarse-grained soil cushion layer. Then, after 2 days of in-situ stimulation, calcium acetate solution was added to the soil material and mixed evenly. The thickness of the loose soil of the single-layer compacted clay cover barrier layer did not exceed 300 mm, and the mixing equipment was a bucket excavator.

[0047] (11) Improved compacted clay barrier layer is laid in layers. After laying, the soil is compacted by a roller roller or a vibration-free sheep's foot roller. Among them, the roller roller is often used for preliminary compaction, while the sheep's foot roller is more suitable for clay with higher plasticity, and the tamping machine is used in small areas or corners. When laying in layers, the contact surface between layers needs to be roughened with a roughening depth of 25mm. The number of rolling times is 8 times, and the thickness of the single layer of fill after final compaction is 100mm, and the total thickness of the compacted clay covering barrier layer is 400mm.

[0048] (12) Construction quality control. Every 500m 2 Take 3 points for compaction test, which is required to reach more than 95%. During the construction process, the moisture content needs to be adjusted in time according to the compaction curve of the soil. If the moisture content is too high, it can be reduced by drying. Each layer of soil should be kept as level as possible during laying to avoid local depressions or elevations. When the compacted clay layer comes into contact with a concrete structure or other types of obstacle structures (such as bedrock), it needs to be sealed with sealing materials or bentonite cushions to ensure that no penetration channels are generated at the joints. After completing all the compacted layers, the surface of the last layer should be kept flat to avoid rainwater or groundwater accumulation in low-lying areas.

[0049] Figure 1 The principle diagram of biochar enhancing microbial mineralization to improve the compacted clay cover barrier is given. Figure 1 As shown in the figure, the in-situ microbial mineralization method is used to enrich the indigenous urease bacteria in the in-situ soil materials of the site and induce the mineralization reaction of calcium carbonate. Calcium carbonate crystals will use bacteria and biochar as nucleation sites, gradually forming larger crystal clusters to fill the soil pores and enhance the permeability reduction and gas barrier properties of compacted clay. The reduction in the size of large pores and the formation of dense cemented bodies caused by microbial mineralization in compacted clay can increase the resistance to capillary water migration and reduce the evaporation rate of soil moisture, thereby improving the water holding capacity of the compacted clay cover barrier layer. In addition, the addition of biochar increases the volume and capillary porosity of extremely small pores in compacted clay, further improving its water holding capacity. On the other hand, the cementation force between soil particles in compacted clay due to mineralization will significantly inhibit the dehydration and cracking of the soil. At the same time, the fibrous biochar used in the top layer of the compacted clay cover barrier layer can also help improve the resistance to dry-wet cycles. In the treatment of by-product ammonium radicals, the strong adsorption of biochar and the weak acidity of calcium acetate can be used to remove ammonium radicals.

[0050] Figure 2 A construction process flow chart of a biochar-enhanced microbial mineralization-improved compacted clay cover barrier layer is provided in this embodiment.

[0051] Figure 3The effects of biochar dosage on calcium carbonate production and the effects of biochar dosage on the hydraulic permeability coefficient of microbial mineralized improved compacted clay cover barrier are given. The total dry mass of each component in the urease bacteria stimulation solution used in the experiment accounts for 8.5% of the mass of the in-situ soil material on the site, and the dry mass of the solute of the calcium acetate solution used accounts for 10.0% of the mass of the in-situ soil material on the site; the in-situ soil material on the site can be divided into fine-grained soil sand (fine-grained content is 45%) according to the "Engineering Classification Standard for Soil" GB / T 50145-2007; the biochar uses powdered rice husk biochar, and the other conditions and parameters are the same as Example 1.

[0052] Depend on Figure 3 (a) It can be seen that the addition of 4% biochar can increase the calcium carbonate production by 1.18 times, while the addition of 8% biochar can increase the calcium carbonate production by 1.41 times. The increase in calcium carbonate production is attributed to the additional nucleation sites provided by biochar for calcium carbonate mineralization. The three biochar-enhanced microbial mineralization-modified compacted clay cover barrier samples were tested in a flexible wall permeability test. Figure 3 (b) It can be seen that the hydraulic permeability coefficients of the three samples are significantly higher than the permeability of the traditional compacted clay cover barrier layer (1×10 -9 The sample with 8% biochar had the largest reduction, with the permeability coefficient reaching 1.4×10 -10 m / s, which is about one order of magnitude lower than that of the traditional compacted clay cover barrier. This is the beneficial effect of biochar-enhanced microbial mineralization. The increase in calcium carbonate production and the incorporation of biochar with smaller particle size significantly reduced the porosity of the soil sample, thereby reducing the hydraulic permeability coefficient of the compacted clay cover barrier layer.

[0053] In terms of water retention and gas diffusion coefficient, the higher calcium carbonate yield and lower hydraulic permeability coefficient brought about by the addition of biochar will make the microbial mineralized improved compacted clay cover barrier layer of the present invention have excellent water retention and gas diffusion coefficient.

[0054] Comparative Example 1:

[0055] Compared with Example 1, most of them are the same, except that the addition of calcium acetate is omitted. In Comparative Example 1, since no calcium source required for microbial mineralization is added, calcium carbonate crystals cannot be formed to fill or cement the soil. Although relying solely on biochar to improve the compacted clay cover barrier layer can improve water retention performance, the effect of improving the anti-seepage and gas barrier performance is minimal.

[0056] Comparative Example 2:

[0057] Compared with Example 1, most of the contents are the same, except that calcium acetate is replaced by an equimolar amount of calcium chloride. Although the replacement of calcium acetate with calcium chloride in Comparative Example 2 does not significantly affect the yield of calcium carbonate, the alkaline environment caused by microbial mineralization easily causes the byproduct ammonium radical to be converted into ammonia gas, which is then discharged into the external environment. If the calcium source used is calcium acetate, the acetic acid generated by the acetate radical can react with ammonia gas to reduce the emission of ammonia gas.

[0058] Embodiment 2:

[0059] Compared with Example 1, most of them are the same, except that in this example, the mass of the urease bacteria stimulation solution accounts for 15% of the mass of the in-situ soil material on the site, the dry mass of peptone accounts for 3% of the mass of the in-situ soil material on the site, the dry mass of the selected substrate urea accounts for 4% of the mass of the in-situ soil material on the site; the mass of the calcium acetate solution accounts for 10% of the mass of the in-situ soil material on the site, the dry mass of the calcium acetate solute accounts for 15% of the mass of the in-situ soil material on the site; the dry mass of the biochar accounts for 4.0% of the dry mass of the in-situ soil material on the site.

[0060] Embodiment 3:

[0061] The in-situ soil material of the site is fine-grained sand or fine-grained soil with a coarse-grained content greater than 25%. The method is similar to that of Example 1, except that in this embodiment, the mass of the urease bacteria stimulating solution accounts for 10% of the mass of the in-situ soil material of the site, the dry mass of peptone accounts for 3.5% of the mass of the in-situ soil material of the site, the dry mass of the selected substrate urea accounts for 5% of the mass of the in-situ soil material of the site; the mass of the calcium acetate solution accounts for 10% of the mass of the in-situ soil material of the site, the dry mass of the calcium acetate solute accounts for 10% of the mass of the in-situ soil material of the site; and the dry mass of the biochar accounts for 6.0% of the dry mass of the in-situ soil material of the site.

[0062] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A biochar enhanced microbial mineralization improved compacted clay cover barrier layer, characterized in that: The raw material components include urease bacteria stimulating solution, calcium acetate solution, biochar and in-situ soil materials on site, wherein the total dry mass of each component in the urease bacteria stimulating solution accounts for 4.5% to 8.5% of the mass of the in-situ soil materials on site, the dry mass of the solute of the calcium acetate solution accounts for 4.0% to 10.0% of the mass of the in-situ soil materials on site, and the dry mass of the biochar accounts for 4.0% to 8.0% of the dry mass of the in-situ soil materials on site.

2. The biochar-enhanced microbial mineralization-improved compacted clay cover barrier layer according to claim 1, characterized in that: The mass of the urease bacteria stimulating solution accounts for 5.0% to 15.0% of the mass of the in-situ soil material of the site.

3. The biochar-enhanced microbial mineralization-improved compacted clay cover barrier layer according to claim 1, characterized in that: The mass of the calcium acetate solution accounts for 5.0% to 15.0% of the mass of the in-situ soil material of the site.

4. The biochar-enhanced microbial mineralization-improved compacted clay cover barrier layer according to claim 1, characterized in that: The urease bacteria stimulating solution comprises peptone and selected substrate urea, wherein the dry mass of peptone accounts for 2.5% to 3.5% of the mass of the original soil material on the site, and the dry mass of the selected substrate urea accounts for 2.0% to 5.0% of the mass of the original soil material on the site.

5. The biochar-enhanced microbial mineralization-improved compacted clay cover barrier layer according to claim 1, characterized in that: The purity of the calcium acetate is greater than 95%.

6. The biochar-enhanced microbial mineralization-improved compacted clay cover barrier layer according to claim 1, characterized in that: The biochar is lignocellulose biochar in the form of powder and / or fiber, wherein the total carbon content is greater than 28.0%, the ash content is between 10.0% and 20.0%, and the specific surface area is greater than 38m 2 / g, pH value is between 8.2 and 9.7, conductivity EC is controlled below 1500μs / cm, and cation exchange capacity CEC is greater than 21.5meq / 100g.

7. The biochar-enhanced microbial mineralization-improved compacted clay cover barrier layer according to claim 1, characterized in that: The particle size of the on-site soil material is less than 60 mm, the dry weight of soil particles with a particle size greater than 5 mm does not exceed 20% of the total dry weight of the on-site soil material, and the soil organic matter content does not exceed 5wt%.

8. A construction method for biochar enhanced microbial mineralization improved compacted clay cover barrier layer, characterized in that: The following steps are involved: (1) Clean up the construction site, remove obstacles, vegetation and garbage, plow and disperse the original soil, screen out the coarse soil with a particle size greater than 60 mm, and place the gravel soil with a particle size between 5 mm and 60 mm on one side of the site for standby use; (2) In accordance with the Engineering Classification Standard for Soil GB / T 50145-2007, fine-grained sand or fine-grained soil with low organic matter content is selected as the main part of the in-situ soil material of the site, and gravel soil with a particle size of 5 mm to 60 mm is selected as the remaining part of the in-situ soil material of the site. The two parts of soil material are mixed evenly to obtain the in-situ soil material of the site; (3) After the soil material is screened, the site is leveled and gravel soil with a particle size of 5 mm to 60 mm is laid as a coarse-grained soil cushion layer; (4) After the biochar is evenly mixed with the in-situ soil material on site, it is laid on top of the coarse-grained soil cushion layer, and then urease stimulation solution is added for in-situ stimulation. Then, calcium acetate solution is added and mixed evenly. Finally, the laid soil material is compacted to obtain a modified compacted clay cover barrier layer, and its thickness is controlled between 300 mm and 600 mm.

9. The construction method of a biochar enhanced microbial mineralization improved compacted clay cover barrier layer according to claim 8, characterized in that: When the modified compacted clay cover barrier layer is laid in layers, the biochar in the top modified compacted clay barrier layer is fibrous in shape, and the dry mass of the biochar accounts for 8.0% of the in-situ soil mass of the layer; The biochar in the bottom modified compacted clay barrier layer is in powder form, and the dry mass of the biochar accounts for 8.0% of the in-situ soil mass of the layer; The biochar of the remaining layers of the improved compacted clay barrier layer is in powder form, and the dry mass of the biochar accounts for 4.0 to 8.0% of the mass of the in-situ soil of the layer site.

10. The construction method of a biochar enhanced microbial mineralization improved compacted clay cover barrier layer according to claim 8, characterized in that: The total mass of the urease bacteria stimulating solution and the calcium acetate solution = the dry mass of the on-site soil material × (optimum moisture content + 2%) - the mass of natural water in the on-site soil material.

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