A method for improving the removal rate of heavy metals in clay based on freeze-thaw method

Abstract

The application provides a method for improving the removal rate of heavy metals in clay based on a freeze-thaw method, which comprises the following steps: pretreatment, water saturation, freezing, temperature rising, collection of leachate, repeated leaching, etc. The pretreatment step comprises loading red soil on a compacted clay column and layering and compacting to form a mixed soil column, i.e. layering and loading red soil and clay. The application uses red soil with strong frost heaving property as the mixed soil, and the structure of the red soil is severely broken and the number of pores is increased after freeze-thaw cycles, which is beneficial to the migration and storage of water. At the same time, the adsorption capacity of red soil for heavy metal elements such as lead and cadmium is weak, and the red soil can be repeatedly used for the repair of heavy metal contaminated clay. Therefore, the above method can effectively improve the intake of leaching agent, improve the removal rate of heavy metal elements, shorten the soil repair period, and is beneficial to large-scale outdoor popularization and application.

Description

Technical Field

[0001] This invention relates to the field of environmental protection technology, and in particular to a method for improving the removal rate of heavy metals in cohesive soil based on the freeze-thaw method. Background Technology

[0002] Heavy metal pollution in my country's soil is severe. Due to rapid industrialization, large areas of farmland have inevitably suffered from heavy metal contamination, seriously damaging the ecological environment of China's land. Traditional methods for remediating heavy metal-contaminated soil include physical, chemical, and biological methods. Among these, chemical leaching, a chemical method, can effectively remove heavy metals. Through the physicochemical processes of desorption, chelation, and dissolution in the leaching solution, pollutants migrate from the soil. The leaching wastewater is then safely disposed of to achieve the remediation goal. However, this method is easily limited by soil texture. When the clay content exceeds 30%, leaching technology is unsuitable because the large specific surface area of ​​clay particles strongly adsorbs heavy metals, and their low permeability hinders the sufficient contact and reaction between the leaching solution and the pollutants, resulting in poor leaching effectiveness.

[0003] To address this, Rui Dahu et al. proposed a method for the synergistic remediation of heavy metal Cd and Pb contaminated clay soil using freeze-thaw-chemical leaching. This method utilizes the characteristics of water migration from the unfrozen soil side to the freezing front and increased permeability caused by freeze-thaw cycles during soil freezing under certain temperature gradients and moisture conditions. Combined with chemical leaching, this method remediates and leaches heavy metal contaminated clay soil. A traditional chelating agent, disodium ethylenediaminetetraacetate (Na2 EDTA) solution, is used instead of the water required for soil frost heave. The frost heave and water absorption of the soil promotes full contact and interaction between the leaching solution and the pollutants adsorbed on the clay particles, forming dissolved metal ions or complexes from the heavy metals adsorbed on the soil particles. Then, through thaw sedimentation and drainage, the leaching solution is collected and chemically treated to recover the heavy metals. After seven freeze-thaw cycles, the removal rates of Cd and Pb reached 77.24% and 37.78%, respectively (Rui Dahu, Wu Zhipeng, Wu Yingfei, et al. Synergistic remediation of heavy metal Cd and Pb contaminated clay soil using freeze-thaw-chemical leaching [J]). Transactions of the Chinese Society of Agricultural Engineering. 2018,(23). P200-205).

[0004] While the aforementioned freeze-thaw synergistic chemical leaching method can effectively improve the removal rate of heavy metals by single chemical leaching, the amount of leaching fluid migrating during soil freezing is small, resulting in low efficiency of single leaching. Chen Xing proposed a model test study on freeze-thaw synergistic leaching for the remediation of heavy metal-contaminated clay soil. Taking Pb and Cd-contaminated clay soil in a certain area as the research object, a continuous freeze-thaw leaching test was conducted using double-layer water replenishment, gradient cooling, and alternating EDTA and clean water to improve the single heavy metal removal rate. Based on the test results, the freeze-thaw scheme was adjusted by adding a heating layer in the middle to achieve repeated condensation into ice, thereby improving the mixing and mass transfer efficiency of the leaching fluid and contaminated soil and further improving the heavy metal removal rate (Chen Xing, Model Test Study on Freeze-Thaw Synergistic Leaching for the Remediation of Heavy Metal-Contaminated Clay Soil [M] Henan Polytechnic University, 2022). Although this method can effectively improve the single-time removal rate of heavy metals, it requires an additional heating layer, making the treatment process cumbersome. Moreover, its single freeze-thaw cycle is as long as 69-93 days, resulting in a long soil remediation cycle. Therefore, the above-mentioned method for remediating heavy metal-contaminated clay soil proposed by Chen Xing is not suitable for large-scale outdoor application. Summary of the Invention

[0005] In view of this, the main objective of this invention is to provide a method for improving the removal rate of heavy metals in cohesive soil based on the freeze-thaw method. This method mainly uses Akadama soil with strong frost heave as admixture soil to increase the migration amount and leaching efficiency of the leaching agent, shorten the soil remediation cycle, and facilitate large-scale outdoor application.

[0006] Specifically, the technical solution provided by this invention is as follows: A method for improving the removal rate of heavy metals in cohesive soil based on freeze-thaw reaction includes the following steps: Pretreatment: Akadama soil is filled into the compacted cohesive soil column and compacted in layers to form a mixed soil column; Water replenishment saturation: Add leaching agent to the mixed soil column until the soil-water potential in the mixed soil column reaches equilibrium, forming a saturated soil mass; Freezing and freezing: The saturated soil is cooled and frozen using a one-way freeze-thaw method, while continuously adding leaching agent to the low-temperature part of the saturated soil until the leaching agent in the saturated soil is converted into solid and frozen together with the Akadama soil and clay soil therein to form frozen soil. Heating and Melting: Instantly raising the temperature at both ends of the frozen soil body, causing the absorbed leaching agent to completely melt with the Akadama soil and clay soil, forming a molten mixed soil body; Leachate collection: Collect the leachate from the melted and mixed soil. Repeated rinsing: Repeat the freezing and leaching process described above, and collect the leaching liquid from each rinsing to obtain a finished clay product with heavy metals removed.

[0007] The cohesive soil contains heavy metals such as Pb, Cd, and Hg. In one embodiment, the cohesive soil contains Pb and Cd elements, with the Pb content exceeding 1000 mg / Kg and the Cd content exceeding 4.0 mg / Kg.

[0008] Based on the above, the pretreatment steps include: first, drying and crushing the raw materials Akadama soil and clay to obtain Akadama soil particles and clay particles with a particle size not exceeding 2 mm; then, mixing the Akadama soil particles and clay particles with water at the optimal moisture content, placing them in a sealed bag and letting them stand for 20-32 hours to obtain Akadama soil slurry and clay soil slurry respectively; compacting the clay soil slurry in layers to form a compacted clay column, and filling the filling column with Akadama soil slurry and compacting it in layers to form a compacted Akadama soil column, thereby producing the mixed soil column.

[0009] Preferably, the mass percentage of Akadama clay in the mixed soil column is 10%-32%.

[0010] The leaching agent used in this invention mainly consists of heavy metal chelating agents, such as one or more combinations of disodium ethylenediaminetetraacetate, citric acid, tartaric acid, and oxalic acid. Preferably, the leaching agent has a concentration of 0.025–0.1 mol / L, such as 0.025 mol / L, 0.05 mol / L, 0.075 mol / L, or 0.1 mol / L.

[0011] The main purpose of the water replenishment and saturation step is to avoid the formation of a vacuum layer in the mixed soil column during the subsequent freezing process, which would affect the frost heave effect of the soil column and ensure that only solid and liquid phases exist in the soil column, thus saturating the soil column.

[0012] Based on the above, the freezing and thawing steps include: freezing and thawing the saturated soil using a one-way freeze-thaw method to create a temperature difference between the upper and lower surfaces of the saturated mixture, controlling the cooling rate to ensure the speed of the freezing front's advance, and continuously replenishing the leaching agent from the unfrozen area of ​​the saturated mixture to the frozen area until the entire saturated soil is frozen, forming the frozen soil.

[0013] Based on the above, during the freezing process, the temperature of the upper surface of the saturated soil gradually decreases from -2℃ to 2℃ to -12℃ to -8℃ and then remains constant for 4 to 6 hours; the temperature of the lower surface of the saturated soil first gradually decreases from 8℃ to 12℃ to -2℃ to 2℃, and then is instantly lowered to -12℃ to -8℃ and held constant for 4 to 6 hours, so that the saturated soil is completely frozen. Since a too slow cooling rate will result in a large frost heave but a slow change in permeability coefficient, while a too fast cooling rate will result in a small frost heave but a drastic change in permeability coefficient, the cooling rate should be moderate to ensure both frost heave and permeability performance. Therefore, the preferred cooling rate during the freezing process is 0.1-0.3℃ / h.

[0014] Preferably, the rinsing agent in the freezing step is replenished without pressure by the freezing suction force generated by the freeze heave.

[0015] Based on the above, the heating and melting step includes: simultaneously and instantaneously raising the temperature of the upper and lower surfaces of the frozen soil to 8℃~12℃, and continuing for 8~12 hours to melt the leaching agent, Akadama soil and clay in the frozen soil, forming the melted mixed soil.

[0016] In the repeated rinsing step, the number of cycles of freezing and collecting the leachate is determined according to the actual situation. In one embodiment, the rinsing is repeated 0 to 2 times, that is, the entire treatment process is continuously implemented 1 to 3 times for the above freeze-thaw rinsing cycle. In another embodiment, the number of cycles continues until the heavy metal content in the leachate from two consecutive cycles meets the national standard value for heavy metal contaminated soil.

[0017] Based on the above, the method also includes component detection: taking a sample from the finished clay sample after heavy metal removal, digesting the obtained clay sample to obtain a digestion solution, and using ICP-MS to detect the heavy metal content in the digestion solution.

[0018] Because clayey soil contains large aggregates and has flocculent deposits on its surface, resulting in a dense overall structure, and because Akadama soil exhibits strong frost heave, its structure is severely fragmented and the number of pores increases after freeze-thaw cycles, which facilitates water migration and storage. Simultaneously, Akadama soil has a weak adsorption capacity for heavy metals such as lead and cadmium, allowing for repeated use in the remediation of heavy metal-contaminated clayey soil. Therefore, the method for improving the removal rate of heavy metals in clayey soil based on the freeze-thaw method provided in this invention is suitable for clayey soils with heavy metal content exceeding national standards. Using Akadama soil as a filler can effectively increase the intake of leaching agents and improve the removal rate of heavy metal elements. Experimental verification shows that the method provided in this invention can effectively improve the removal rate of heavy metal elements and shorten the soil remediation cycle. Since this invention mainly achieves effective removal of heavy metal elements from clayey soil by setting a compacted Akadama soil layer on top of a compacted clayey soil layer, it has a high removal rate, is simple and easy to operate, and is conducive to large-scale outdoor application. Attached Figure Description

[0019] Figure 1 The following is a unidirectional freeze-thaw device used in an embodiment of the present invention. The components in the figure are: cylinder 1, lower low temperature circulation unit 2, upper low temperature circulation unit 3, sample chamber 4, permeable stone 5, temperature sensor 6, leaching liquid collection unit 7, water replenishment container 81, electronic balance 82, load unit 9, and displacement sensor 10. Figure 2 The temperature control curve is shown in the method for improving the removal rate of heavy metals in cohesive soil based on the freeze-thaw method provided in the embodiments of the present invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions protected by this invention will be further explained and illustrated below through specific embodiments.

[0021] Unless otherwise specified, the amount, content and removal rate of each raw material in this invention refer to "weight". All terms used are common terms in the relevant field. Unless otherwise specified, the preparation process and testing methods used in each embodiment are conventional methods well known to those skilled in the art. All raw materials and equipment used can be obtained from publicly available commercial channels.

[0022] In the method for improving the removal rate of heavy metals in cohesive soil based on freeze-thaw leaching provided by this invention, the selection, dosage, and filling method of Akadama soil, as well as the number of freeze-thaw cycles, have a significant impact on the heavy metal removal rate. The following examples and comparative examples further illustrate the influence of these factors.

[0023] (1) The methods for improving the removal rate of heavy metals in cohesive soil based on the freeze-thaw method provided in the following embodiments and comparative examples are mainly based on Figure 1The unidirectional freeze-thaw test device shown is implemented. The unidirectional freeze-thaw test device includes an acrylic cylinder 1, a lower low-temperature circulation unit 2, an upper low-temperature circulation unit 3, permeable stones 5, temperature sensors 6, a leachate collection unit 7, an automatic water replenishment unit, a load unit 9, and a displacement sensor 10. The cylinder 1 contains a sample chamber 4 for loading the sample. The lower end of the sample chamber 4 is connected to the lower low-temperature circulation unit 2, and the upper end of the sample chamber 4 is connected to the upper low-temperature circulation unit 3. A permeable stone 5 is installed at each of the upper and lower ends of the sample chamber 4, and the permeable stone 5 is respectively attached to the corresponding low-temperature circulation unit. Temperature sensors 6 are installed on the permeable stones 5. The lower end of the sample chamber 4 is connected to the leachate collection unit 7 via a pipe. The leachate collection unit 7 includes a leachate collection bottle connected to the sample chamber 4 and a balance for weighing the leachate collection bottle and its contents. A removable insulation plate is installed on the outer surface of the cylinder 1. The lower low-temperature circulation unit 2 and the upper low-temperature circulation unit 3 each include a coolant and a cooling container for filling the coolant. The cooling containers are connected to corresponding permeable stones 5. The temperature of the coolant in the upper low-temperature circulation unit 3 is lower than the temperature of the coolant in the lower low-temperature circulation unit 2; that is, the lower low-temperature circulation unit 2 is the warm end, and the upper low-temperature circulation unit 3 is the cold end. The temperature control curves used in the following embodiments and comparative examples are as follows: Figure 2 The freeze-thaw process was carried out as shown, with a total duration of 75 hours.

[0024] The lower end of the sample chamber 4 is also connected to an automatic water replenishment unit via a pipe. The automatic water replenishment unit includes a water replenishment container 81 and an electronic balance 82 for weighing the water replenishment container 81 and its contents. The load unit 9 is located above the sample chamber 4 and connected to the upper low-temperature circulation unit 3. It is used to apply a load to the sample filled in the sample chamber 4 to prevent the sample from expanding due to water absorption, which could create a vacuum layer inside the sample during freezing and affect the overall freezing performance of the sample. By applying pressure through the load unit 9, water can fully penetrate into the sample, expel air, and prevent the formation of a vacuum layer. The displacement sensor 10 is mounted on the load unit 9 and is used to measure the change in height of the sample in the sample chamber 4.

[0025] (2) The basic physical properties of the soil samples used in the following examples and comparative examples are shown in Table 1 below.

[0026] Table 1 Basic characteristics of soil samples

[0027] The original Pb content in the cohesive soil sample was 5237.56 mg / Kg, and the original Cd content was 764.51 mg / Kg.

[0028] Rinsing agent: Prepared from analytical grade disodium ethylenediaminetetraacetate solution at 0.05 mol / L.

[0029] (3) Sampling method of soil sample obtained after freeze-thaw treatment: After the freeze-thaw test, the corresponding soil column was sampled in layers at 1cm intervals. The corresponding soil sample was air-dried and crushed, then ground with an agate grinding bowl, and sieved through a sieve to obtain a 200-mesh soil sample for pretreatment.

[0030] (4) The soil sample digestion and testing methods in the following embodiments include: Soil sample digestion method: Weigh 0.2 g of soil sample (with an error margin of 0.02%) into a polytetrafluoroethylene beaker (crucible); add 5 ml of hydrofluoric acid, then add 10 ml of mixed acid (HCl: HNO3: HClO4 = 3:2:1); place the crucible on a hot plate, set the temperature to 170 ℃, and heat for 1 h; raise the hot plate temperature to 230 ℃ and heat to dissolve for 1 h (or dissolve at 170 ℃ for 2 h, then directly raise the temperature to 320 ℃); raise the temperature to 320 ℃ and heat until only solid remains in the crucible and no white smoke is emitted; after the white smoke dissipates, cool down to 170 ℃, first add 3 ml of aqua regia (HCl: HNO3 = 1:3) to dissolve, gently heat and remove, then add 20 ml of distilled water and let stand overnight (or you can do the following: remove the crucible from the hot plate when no white smoke is emitted, add 3 ml of aqua regia after 1 min, and repeat after 2 minutes). Add 20ml of distilled water after 1 minute, let stand overnight (the solution will then be ready for use), and you will get the digestion solution.

[0031] Test method for Cd and Pb content: ICP-MS was used to detect the Cd and Pb content in the digestion solution.

[0032] (5) Formula for calculating heavy metal removal rate: Q = C·V (1-1) M hm = M s ·B (1-2) W = Q / M hm ×100% (1-3) In the above calculation formula: Q The amount of Cd and Pb removed from the leaching solution, in mg; C The concentrations of Pb and Cd in the leachate are in mg / L. V The volume of the filtrate is in liters (L). M hm The total amount of heavy metals in the soil sample, in mg; M s The mass of dry soil in the soil sample, in kg or mg. ；BThe mass ratio of heavy metals in the soil, in mg / kg; W The rinsing removal rate is %.

[0033] (6) Evaluation methods related to frost heave performance The frost heave of the soil is measured by the displacement sensor 10; the water absorption during freeze-thaw cycles is automatically recorded by the electronic balance 82; the frost heave rate is calculated using the following formula: Freeze-thaw rate = (V2 - V1) / V1 × 100% (1-4) In the above calculation formula: V1 is the soil freezing depth (excluding frost heave), mm; V2 is the frost heave of the soil after freezing, mm.

[0034] Example 1 This embodiment provides a method for improving the removal rate of heavy metals in cohesive soil based on freeze-thaw leaching, including the following steps: Pretreatment: Akadama clay and clayey soil raw materials were naturally air-dried and crushed to obtain Akadama clay particles with a particle size not exceeding 2 mm and clayey soil particles with a particle size not exceeding 2 mm. The Akadama clay particles and clayey soil particles were mixed with water at the optimal moisture content and placed in sealed bags and left to stand for 24 hours. First, the clayey soil sample was filled into sample cavity 4 and compacted in layers to form a clayey soil column. Then, the Akadama clay sample was filled into the clayey soil column in sample cavity 4 and compacted in layers to form an Akadama clay column, thus producing a cylindrical mixed soil column sample with a diameter of φ100 mm × 50 mm × 50 mm. In this embodiment, the Akadama clay content in the mixed soil column is 13.75%.

[0035] Water replenishment saturation: The automatic water replenishment unit continuously replenishes the leaching agent from the lower surface (warm end) to the upper surface (cold end) of the mixed soil column until the soil-water potential in the mixed soil column reaches equilibrium, forming a saturated soil body; Freezing: according to Figure 2 The temperature control curves shown employ a lower low-temperature circulation unit 2 and an upper low-temperature circulation unit 3 to simultaneously maintain a constant temperature for 10 hours on both the lower and upper surfaces of the saturated soil. Specifically, the lower surface of the cohesive soil column is maintained at 10°C, while the upper surface of the Akadama soil column is maintained at 0°C. Next, the saturated soil is cooled. The upper surface of the saturated soil is cooled from 0°C to -10°C at a rate of -0.2°C / h and then maintained at that temperature for 5 hours. The lower surface is cooled from 10°C to 0°C at a rate of -0.2°C / h and then instantly cooled to -10°C and maintained at that temperature for 5 hours. Throughout the cooling process, a leaching agent is continuously added to the low-temperature portion of the saturated soil. The leaching agent in the saturated soil solidifies and freezes together with the Akadama and cohesive soil within it, forming frozen soil. Heating and thawing: The frozen soil is heated to 10°C at both ends at a rate of 1°C / min and held at that temperature for 10 hours, so that the leaching agent absorbed therein can completely melt with the Akadama soil and clay soil to form a molten mixed soil. Leachate collection: Turn on the leaching collection unit 7 and collect the leaching from the melted mixed soil through the leaching collection unit 7 until the collection is completed. Then turn off the leaching collection unit 7 to obtain a cohesive soil sample with heavy metals removed.

[0036] Example 2 This embodiment provides a method for improving the removal rate of heavy metals in cohesive soil based on freeze-thaw leaching. This method is basically the same as the method provided in Embodiment 1, except that the amount of Akadama soil in the mixed soil column in this embodiment is 29.79%, while the other steps and parameters are the same.

[0037] Example 3 This embodiment provides a method for improving the removal rate of heavy metals in cohesive soil based on freeze-thaw leaching. This method is basically the same as the method provided in Embodiment 2. The main difference is that in this embodiment, the freezing-collection of leaching solution is repeated twice, that is, three freeze-thaw cycles are performed continuously in this embodiment. Other steps and parameters are the same.

[0038] Comparative Example 1 This comparative example provides a method for removing heavy metals based on freeze-thaw leaching, which is basically the same as the method provided in Example 1. The main difference is that a pure Akadama soil column is used in this comparative example instead of the mixed soil column in Example 1, while the other steps and parameters are the same.

[0039] Comparative Example 2 This comparative example provides a method for removing heavy metals based on freeze-thaw leaching, which is basically the same as the method provided in Example 1. The main difference is that pure peat moss columns are used in this comparative example instead of mixed soil columns in Example 1. Other steps and parameters are the same. The raw material of the pure peat moss columns consists of peat and peat, which are products of the swamp development process. They are formed by the accumulation of swamp plant remains under anaerobic conditions with high water content, which cannot be completely decomposed.

[0040] Comparative Example 3 This comparative example provides a method for removing heavy metals based on freeze-thaw leaching, which is basically the same as the method provided in Example 1. The main difference is that a pure cohesive soil column is used in this comparative example instead of the mixed soil column in Example 1, while the other steps and parameters are the same.

[0041] Comparative Example 4 This comparative example provides a method for removing heavy metals based on freeze-thaw leaching, which is basically the same as the method provided in Example 1. The main difference is that in the pretreatment step, the Akadama soil particles and clay particles are mixed with water according to their respective optimal moisture content, placed in a sealed bag and left to stand for 24 hours. Then, the two are mixed evenly and then filled into the sample cavity 4 and compacted in layers to form a soil column with uniformly mixed soil, which replaces the mixed soil column with two layers in Example 1. Other steps and parameters are the same.

[0042] Comparative Example 5 This comparative example provides a method for removing heavy metals based on freeze-thaw leaching, which is basically the same as the method provided in Example 2. The main difference is that in the pretreatment step, the Akadama soil particles and clay particles are mixed with water according to their respective optimal moisture content, placed in a sealed bag and left to stand for 24 hours. Then, the two are mixed evenly and then filled into the sample cavity 4 and compacted in layers to form a soil column with uniformly mixed soil, which replaces the mixed soil column with two layers in Example 2. Other steps and parameters are the same.

[0043] Comparative Example 6 This comparative example provides a method for removing heavy metals based on freeze-thaw leaching, which is basically the same as the method provided in Example 3. The main difference is that a cohesive soil column is used in this comparative example instead of the mixed soil column in Example 3, while the other steps and parameters are the same.

[0044] The removal rates of lead and cadmium elements after treatment with the methods provided in Examples 1-3 and Comparative Examples 1-6, as well as the frost heave properties of the corresponding mixed soil columns or pure soil columns, were tested, and the results are shown in Table 2.

[0045] Table 2 Results of freeze-thaw performance and heavy metal removal rate

[0046] Based on Table 2 and the frost heave performance data of Comparative Examples 1-3, it can be seen that: clay soil exhibits some frost heave, peat soil has virtually no frost heave, but akadama soil exhibits strong frost heave, with a frost heave rate reaching 124.48%, far exceeding that of clay soil and peat soil. Although both akadama soil and peat soil possess permeability and air permeability, akadama soil has better water retention, higher surface energy, and a higher liquid limit water content. Therefore, during freezing, akadama soil can promote the migration of a large amount of water, thus giving it better frost heave performance. In addition, akadama soil has a weaker adsorption capacity for Pd and Cd elements, resulting in less adsorption of Pd and Cd during the leaching process. This allows Pd and Cd in the clay soil to be leached out as much as possible into the leachate, thereby improving the recovery rate of Pd and Cd.

[0047] The comparative data from Examples 1-3 and Comparative Examples 3-6 further demonstrate that Akadama soil has strong frost heave properties and that the heavy metal removal rate can be improved by increasing the amount of leaching agent.

[0048] Comparing the removal rate and frost heave performance data of Example 1 and Comparative Example 4, and Example 2 and Comparative Example 5, it can be seen that, under the same Akadama clay content, the layered filling method has a higher frost heave and water absorption than the mixed filling method, and also has a greater removal rate of Pd and Cd elements.

[0049] Comparing the data from Examples 2-3, Comparative Examples 3 and 6, it can be seen that when Akadama soil is used as an admixture in layered filling, after one freeze-thaw leaching cycle, the removal rates of Pb and Cd are increased by 130.91% and 113.39% respectively compared to cohesive soil. After three freeze-thaw leaching cycles, the removal rates of Pb and Cd are 57.51% and 66.77% respectively, which are increased by 173.73% and 113.94% respectively compared to cohesive soil, demonstrating significant removal effects of heavy metals such as Pb and Cd. Therefore, the heavy metal removal method provided in Example 3 of this invention is optimal, with the highest removal rates of Pb and Cd.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the present invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A method for improving the removal rate of heavy metals in cohesive soil based on freeze-thaw interaction, comprising the following steps: Pretreatment: Akadama soil is layered onto a compacted cohesive soil column, and each layer is compacted to create a mixed soil column with a layered structure of Akadama and cohesive soil. The mass percentage of Akadama soil in the mixed soil column is 10%–32%. The cohesive soil column contains heavy metals; Water replenishment saturation: Add leaching agent to the mixed soil column until the soil-water potential in the mixed soil column reaches equilibrium, forming a saturated soil mass; Freezing and freezing: The saturated soil is subjected to cooling and freezing treatment using a one-way freeze-thaw method at a cooling rate of 0.1-0.3℃ / h. At the same time, the leaching agent is continuously replenished to the low-temperature part of the saturated soil without pressure using the frost heave effect until the leaching agent in the saturated soil is converted into a solid and frozen together with the Akadama soil and clay soil therein to form a frozen soil. Heating and thawing: The temperature of the upper and lower surfaces of the frozen soil is simultaneously and instantaneously raised to 8℃~12℃ and maintained for 8~12 hours, so that the absorbed leaching agent and Akadama soil and clay soil are completely melted to form a molten mixed soil. Leachate collection: Collect the leachate from the melted and mixed soil. Repeated rinsing: Repeat the freezing and leaching process until the leaching liquid is collected, and collect the leaching liquid from each rinsing to obtain the finished clay product with heavy metals removed.

2. The method according to claim 1, characterized in that, The pretreatment steps include: first, drying and crushing the raw materials Akadama clay and clay to obtain Akadama clay particles and clay particles with a particle size not exceeding 2 mm; then, mixing the Akadama clay particles and clay particles with water at the optimal moisture content, placing them in a sealed bag and letting them stand for 20-32 hours to obtain Akadama clay slurry and clay slurry respectively; compacting the clay slurry in layers to form a compacted clay column, filling the clay column with Akadama clay slurry and compacting it in layers to form a compacted Akadama clay column, thereby producing the mixed soil column.

3. The method according to claim 2, characterized in that, The cohesive soil contains Pb and Cd elements, with the Pb content exceeding 1000 mg / Kg and the Cd content exceeding 4.0 mg / Kg.

4. The method according to any one of claims 1 to 3, characterized in that, The main components of the rinsing agent are one or more of the following: disodium ethylenediaminetetraacetate, citric acid, tartaric acid, and oxalic acid.

5. The method according to any one of claims 1 to 3, characterized in that, The freezing process includes: freezing the saturated soil using a one-way freeze-thaw method to create a temperature difference between the upper and lower surfaces of the saturated soil, controlling the cooling rate to ensure the speed of the freezing front, and continuously replenishing the leaching agent from the unfrozen area of ​​the saturated soil to the frozen area until the entire saturated soil is frozen, forming the frozen soil.

6. The method according to claim 5, characterized in that, During the freezing process, the temperature of the upper surface of the saturated soil gradually decreases from -2℃ to 2℃ to -12℃ to -8℃ and then remains constant for 4 to 6 hours; the temperature of the lower surface of the saturated soil first gradually decreases from 8℃ to 12℃ to -2℃ to 2℃, and then instantly drops to -12℃ to -8℃ and remains constant for 4 to 6 hours, so that the saturated soil is completely frozen.

7. The method according to any one of claims 1 to 3, characterized in that, The process also includes component detection: taking a sample from the finished clay sample after heavy metal removal, digesting the obtained clay sample to obtain a digestion solution, and using ICP-MS to detect the heavy metal content in the digestion solution.

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