Method for improving removal rate of heavy metals in cohesive soil based on freeze-thaw method

Abstract

The invention provides a method for improving the removal rate of heavy metals in cohesive soil based on a freeze-thaw method. The method comprises the steps of pretreatment, water replenishing saturation, freezing and freezing, heating thaw settlement, leachate collection, repeated leaching and the like. Wherein the pretreatment step comprises the steps of filling a compacted cohesive soil column with akadama soil and compacting the akadama soil in a layered manner to prepare a mixed soil column, that is, the akadama soil and the cohesive soil are filled in a layered manner, the akadama soil with strong frost heaving property is used as admixture soil, the akadama soil structure is seriously broken after freezing and thawing circulation, the number of pores is increased, and migration and storage of water are facilitated; meanwhile, the adsorption capacity of the akadama soil to lead, cadmium and other heavy metal elements is weak, and the akadama soil can be repeatedly used for repairing the heavy metal polluted cohesive soil, so that the method provided by the invention can effectively improve the intake of the eluting agent, improve the removal rate of the heavy metal elements and shorten the soil repairing period, and is beneficial to outdoor large-scale popularization and application.

Description

Technical Field

[0001] The invention relates to the technical field of environmental protection, and in particular to a method for improving the removal rate of heavy metals in clay soil based on a freeze-thaw method. Background Art

[0002] my country's soil is seriously polluted by heavy metals. Due to the rapid development of China's industrialization, a large number of farmlands are inevitably harmed by heavy metals, which has seriously damaged the ecological environment of China's land. Traditional methods for remediating heavy metal-contaminated soil include physical methods, chemical methods and biological methods. Among them, chemical leaching in chemical methods can effectively remove heavy metals. Through the physical and chemical effects of desorption, chelation, dissolution, etc. of the leaching liquid, the pollutants are migrated out of the soil, and then the leaching waste liquid is safely disposed of to achieve the purpose of remediation. However, this method is easily restricted by soil texture. If the clay content exceeds 30%, it is not suitable for the application of leaching technology. The reason is that the large specific surface area of ​​clay soil particles strongly adsorbs heavy metals and their low permeability hinders the leaching liquid from fully contacting and reacting with pollutants, resulting in poor leaching effect.

[0003] Therefore, Rui Dahu et al. proposed a freeze-thaw-chemical leaching method to synergistically repair heavy metal Cd and Pb contaminated clay soil. The method used the characteristics of the migration of water on the unfrozen soil side to the freezing front and the increase of permeability caused by freeze-thaw during the freezing process of the soil under certain temperature gradient and moisture conditions. Combined with chemical leaching, the heavy metal contaminated clay soil was repaired and leached. The traditional chelating agent disodium ethylenediaminetetraacetic acid (Na2 EDTA, hereinafter referred to as EDTA) solution was used to replace the water required for soil frost heave. The frost heave-water absorption of the soil was used to promote the leaching liquid to fully contact and interact with the pollutants adsorbed on the clay particles, and the heavy metals adsorbed on the soil particles were formed into soluble metal ions or complexes. Then, the leaching liquid was collected by thawing-drainage for chemical treatment to recover the heavy metals. In this way, after 7 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 repair of heavy metal Cd and Pb contaminated clay soil by freeze-thaw-chemical leaching method [J]. Transactions of the Chinese Society of Agricultural Engineering. 2018, (23). P200-205).

[0004] Although the freeze-thaw synergistic chemical leaching method can effectively improve the removal rate of heavy metals by single chemical leaching, the migration amount of the elution solution during the freezing process of the soil is small, and there is a problem of low efficiency of single leaching. Chen Xing proposed a model test study on freeze-thaw synergistic leaching to repair heavy metal contaminated clay soil. The clay soil contaminated by Pb and Cd in a certain place was used as the research object. A continuous freeze-thaw leaching test with double-layer water replenishment, gradient cooling, EDTA and clean water alternation was used to improve the single heavy metal removal rate; based on the test results, the freeze-thaw scheme was adjusted, that is, by adding a heating layer in the middle, repeated condensation into ice was achieved, the mixed mass transfer efficiency of the elution solution and the contaminated soil was improved, and the heavy metal removal rate was further improved (Chen Xing, Model Test Study on Freeze-thaw Synergistic Leaching to Repair Heavy Metal Contaminated Clay Soil [M] Henan Polytechnic University, 2022). Although this method can effectively improve the single removal rate of heavy metals, it requires an additional heating layer, making the treatment method more complicated; and its single freeze-thaw time is as long as 69-93 days, and the soil remediation cycle is relatively long. Therefore, the above-mentioned method of repairing heavy metal-contaminated clay soil proposed by Chen Xing is not conducive to large-scale promotion and application outdoors. Summary of the invention

[0005] In view of this, the main purpose of the present invention is to provide a method for improving the removal rate of heavy metals in clay soil based on the freeze-thaw method. The method mainly uses Akadama soil with strong frost heaving properties as the admixture soil to increase the migration amount and elution efficiency of the elution agent, shorten the soil remediation cycle, and facilitate large-scale promotion and application outdoors.

[0006] Specifically, the technical solution provided by the present invention is as follows: A method for improving the removal rate of heavy metals in clay soil based on freeze-thaw method, comprising the steps of: Pretreatment: Fill the compacted clay column with Akadama soil and compact it layer by layer to form a mixed soil column; Water replenishment and saturation: adding the eluent to the mixed soil column until the soil water potential in the mixed soil column reaches equilibrium to form a saturated soil body; Freezing and freezing: using a one-way freeze-thaw method to cool and freeze the saturated soil, while continuously adding an eluent to the low-temperature part of the saturated soil, until the eluent in the saturated soil is converted into a solid state and is frozen as a whole with the red jade soil and clay therein to form a frozen soil; Heating and melting: Instantly raising the temperature at both ends of the frozen soil body, so that the absorbed eluent, red jade soil and clay soil are completely melted to form a melted mixed soil body; Collecting leachate: collecting leachate from the melted mixed soil; Repeat elution: Repeat the steps of freezing and collecting the leaching solution, and collect the leaching solution of each elution to obtain a finished clay soil product with heavy metals removed.

[0007] The clay soil contains heavy metals, such as Pb, Cd, Hg, etc. In one embodiment, the clay soil contains Pb and Cd elements, and the content of Pb exceeds 1000 mg / Kg, and the content of Cd exceeds 4.0 mg / Kg.

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

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

[0010] The main component of the eluent used in the present invention is a heavy metal chelating agent, such as a combination of one or more of disodium ethylenediaminetetraacetic acid, citric acid, tartaric acid, oxalic acid, etc. Preferably, the eluent has a concentration of 0.025 to 0.1 mol / L, such as 0.025 mol / L, 0.05 mol / L, 0.075 mol / L, 0.1 mol / L, etc.

[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 affects the frost heave effect of the soil column, and to ensure that only solid and liquid phases exist in the soil column to saturate the soil column.

[0012] Based on the above, the freezing step includes: using a one-way freeze-thaw method to freeze the saturated soil, so that a temperature difference is formed between the upper surface and the lower surface of the saturated mixture, controlling the cooling rate, ensuring the movement speed of the freezing front, and continuously adding eluent from the unfrozen area of ​​the saturated mixture to the frozen area until the saturated soil is frozen as a whole to form the frozen soil.

[0013] Based on the above, during the freezing process, the upper surface temperature of the saturated soil gradually decreases from -2℃ to 2℃ to -12℃ to -8℃ and then keeps constant temperature for 4 to 6 hours; the lower surface temperature of the saturated soil first gradually decreases from 8℃ to 12℃ to -2℃ to 2℃, then instantly cools to -12℃ to -8℃ and keeps constant temperature for 4 to 6 hours, so that the saturated soil is completely frozen. Since a too low cooling rate will cause a large amount of frost heave of the sample, but the permeability coefficient changes slowly, while a too high cooling rate will cause a small amount of frost heave of the sample, but the permeability coefficient changes violently, in order to ensure its frost heave and permeability, the cooling rate should be moderate, not too large or too small, so the cooling rate during the freezing process is preferably 0.1-0.3℃ / h.

[0014] Preferably, the eluent in the freezing step is supplemented by pressureless supplementation through the freezing suction force generated by the frost heave effect.

[0015] Based on the above, the step of heating and thawing includes: instantly raising the temperature of the upper and lower surfaces of the frozen soil body to 8°C to 12°C at the same time, and continuing for 8 to 12 hours to melt the eluent, red jade and clay in the frozen soil body to form the thawed mixed soil body.

[0016] In the repeated leaching step, the number of cycles of the freezing and freezing to collecting the leaching solution is determined according to the actual situation. In one embodiment, the leaching is repeated 0 to 2 times, that is, the entire treatment process continues to implement the above-mentioned freeze-thaw-leaching cycle 1 to 3 times. In another embodiment, the number of cycles is until the heavy metal content in two adjacent leaching solutions meets the national heavy metal contaminated soil standard value.

[0017] Based on the above, the method further comprises the step of component detection: sampling from the finished product of heavy metal-removed clay soil, digesting the obtained clay soil sample to obtain a digestion test solution, and using ICP-MS to detect the heavy metal content in the digestion test solution.

[0018] Since the aggregates in the clay soil are large and floccules are attached to the surface, the overall structure is relatively dense, and Akadama has a strong frost heave. After the freeze-thaw cycle, the Akadama structure is severely broken and the number of pores increases, which is conducive to the migration and storage of water; at the same time, Akadama has a weak adsorption capacity for heavy metals such as lead and cadmium, and can be reused for the repair of heavy metal-contaminated clay soil. Therefore, the above-mentioned method for improving the removal rate of heavy metals in clay soil based on the freeze-thaw method provided by the present invention is suitable for clay soil with heavy metal content exceeding the national standard. Using Akadama as admixture soil can effectively increase the intake of the eluent and improve the removal rate of heavy metal elements. It has been verified by experiments that the method provided by the present invention can effectively improve the removal rate of heavy metal elements, and the soil repair cycle. Since the present invention mainly realizes the effective removal of heavy metal elements in clay soil by setting a compacted Akadama soil layer on a compacted clay soil layer, the removal rate is high, the method is simple, and it is easy to operate, which is conducive to large-scale promotion and application outdoors. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a one-way freeze-thaw device used in an embodiment of the present invention, wherein the component symbols 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, leachate collection unit 7, water replenishment container 81, electronic balance 82, load unit 9, displacement sensor 10; Figure 2 A temperature control curve diagram used in the method for improving the removal rate of heavy metals in clay soil based on the freeze-thaw method provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solutions and advantages of the present invention more clear, the technical solutions protected by the present invention are further explained below through specific embodiments.

[0021] Unless otherwise specified in the present invention, the amount, content and removal rate of each raw material all refer to "weight", and the terms used are all commonly used terms in the relevant field. The preparation processes, testing methods, etc. used in each embodiment are conventional means well known to those skilled in the art unless otherwise specified, and the raw materials and equipment used can be obtained from public commercial channels.

[0022] In the method for improving the removal rate of heavy metals in clay soil based on freeze-thaw leaching provided by the present invention, the selection, dosage and filling method of Akadama soil and the number of freeze-thaw cycles have an important influence on the removal rate of heavy metals. The influence of the above factors is further explained by the following examples and comparative examples.

[0023] (1) The methods for improving the removal rate of heavy metals in clay 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 in the figure is realized. 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, a permeable stone 5, a temperature sensor 6, a leachate collection unit 7, an automatic water replenishment unit, a load unit 9 and a displacement sensor 10. The cylinder 1 is provided with a sample cavity 4 for filling the sample, the lower end of the sample cavity 4 is connected to the lower low-temperature circulation unit 2, and the upper end of the sample cavity 4 is connected to the upper low-temperature circulation unit 3. A permeable stone 5 is provided at each of the upper and lower ends of the sample cavity 4, and the permeable stone 5 is respectively arranged in accordance with the corresponding low-temperature circulation unit. The permeable stone 5 is respectively installed with a temperature sensor 6. The lower end of the sample cavity 4 is connected to the leachate collection unit 7 through a pipeline, and the leachate collection unit 7 includes a leachate collection bottle connected to the sample cavity 4 and a Tianzi balance for weighing the leachate collection bottle and its contents. A detachable insulation board 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 respectively include a coolant and a cooling container for filling the coolant, and the cooling containers are respectively connected to the corresponding permeable stones 5, and the temperature of the refrigerant in the upper low temperature circulation unit 3 is lower than the temperature of the refrigerant 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. Among them, the temperature control curves used in the following embodiments and comparative examples are as follows Figure 2 Freeze-thaw treatments were performed as indicated for a total of 75 h.

[0024] The lower end of the sample chamber 4 is also connected to an automatic water replenishment unit through a pipeline. The automatic water replenishment unit includes a water replenishment container 81 and an electronic balance 82 for weighing the weight of the water replenishment container 81 and its contents. The load unit 9 is arranged above the sample chamber 4 and is connected to the upper low-temperature circulation unit 3, and is used to apply a load to the sample loaded in the sample chamber 4, so as to avoid the sample from expanding due to absorbing water, so that a vacuum layer is generated in the sample during the freezing process, affecting the overall freezing performance of the sample; applying pressure through the load unit 9 can enable water to fully penetrate into the sample, expel air, and avoid the generation of a vacuum layer. The displacement sensor 10 is installed on the load unit 9, and is used to measure the changing 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.

[0026] Table 1 Basic characteristics of soil samples

[0027] Among them, the original content of Pb in the clay soil sample was 5237.56 mg / Kg, and the original content of Cd was 764.51 mg / Kg.

[0028] Eluent: Analytical grade disodium ethylenediaminetetraacetic acid solution prepared into 0.05 mol / L solution.

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

[0030] (4) The soil sample digestion and detection methods in the following embodiments include: Soil sample digestion method: weigh 0.2 g soil sample (error 0.02%) into a polytetraethylene beaker (crucible); add 5 ml hydrofluoric acid, then add 10 ml mixed acid (HCl: HNO 3 : HClO 4 =3:2:1); place the crucible on a hot plate, set the temperature to 170 ℃, and heat for 1 h; raise the temperature of the hot plate to 230 ℃, heat and dissolve for 1 h (or dissolve at 170 ℃ for 2 h, then directly raise to 320 ℃); raise the temperature to 320 ℃, and heat until there is only solid in the crucible and no white smoke; wait until the white smoke dissipates, lower the temperature to 170 ℃, first add 3 ml of reverse aqua regia (HCl: HNO 3 =1:3) to dissolve, remove it after slight heat, and then add 20 ml of distilled water and let it stand overnight (you can also do this: remove the crucible without white smoke from the heating plate, add 3 ml of aqua regia after 1 min, add 20 ml of distilled water after an interval of 2 min, and let it stand overnight for use), and you can get the digestion solution.

[0031] Testing method for Cd and Pb content: ICP-MS is used to detect the content of Cd and Pb in the digestion test solution.

[0032] (5) Calculation formula for 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 is the elution removal amount of Cd and Pb in the leaching solution, mg; C is the concentration of Pb and Cd in the leachate, mg / L; V is the volume of leachate, L; M hm is the total amount of heavy metals in the soil sample, mg; M sis the mass of dry soil in the soil sample, kg, mg ；B is the mass ratio of heavy metals in soil, mg / kg; W is the elution removal rate, %.

[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 freezing and thawing is automatically recorded by the electronic balance 82; wherein, the frost heave rate calculation formula is: Frost heave rate = (V 2 -V 1 ) / V 1 ×100% (1-4) In the above calculation formula: V 1 V is the soil freezing depth (excluding frost heave), mm; 2 is the frost heave of soil after freezing, mm.

[0034] Example 1 This embodiment provides a method for improving the removal rate of heavy metals in clay soil based on freeze-thaw leaching method, comprising the following steps: Pretreatment: The Akadama soil and clay raw materials are naturally air-dried and crushed to obtain Akadama soil particles with a particle size not exceeding 2 mm and clay soil particles with a particle size not exceeding 2 mm; the Akadama soil particles and clay soil particles are respectively mixed with water according to the optimal moisture content, placed in a sealed bag and left to stand for 24 hours, and then the clay soil sample is first filled into the sample cavity 4 and compacted in layers to form a clay soil column, and then the Akadama soil sample is filled into the clay soil column in the sample cavity 4 and compacted in layers to form an Akadama soil column, thereby making a φ100 mm×50 mm×50 mm cylindrical mixed soil column sample; wherein, in this embodiment, the amount of Akadama soil in the mixed soil column is 13.75%.

[0035] Water replenishment and saturation: the eluent is continuously replenished from the lower surface (warm end) of the mixed soil column to its upper surface (cold end) through the automatic water replenishment unit until the soil water potential in the mixed soil column reaches equilibrium to form a saturated soil body; Freeze: According to Figure 2The temperature control curve shown in the figure, respectively, uses the lower low temperature circulation unit 2 and the upper low temperature circulation unit 3 to simultaneously perform constant temperature treatment on the lower surface and the upper surface of the saturated soil for 10 hours, wherein the lower surface of the clay column is kept at a constant temperature of 10°C, and the upper surface of the red jade column is kept at a constant temperature of 0°C; then 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 kept at a constant temperature for 5 hours, and 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 kept at a constant temperature for 5 hours; during the entire cooling process, the eluent is continuously added to the low temperature part of the saturated soil, and the eluent in the saturated soil is converted into a solid state and is frozen as a whole with the red jade and clay therein to form a frozen soil; Heating and thawing: The two ends of the frozen soil are instantly heated to 10°C at a heating rate of 1°C / min and kept at a constant temperature for 10 hours, so that the absorbed eluent, red jade soil and clay soil are completely melted to form a molten mixed soil; Collecting the leachate: opening the leachate collecting unit 7, collecting the leachate in the melted mixed soil through the leachate collecting unit 7, and closing the leachate collecting unit 7 after the collection is completed, to obtain a clay soil sample with heavy metals removed.

[0036] Example 2 This embodiment provides a method for improving the removal rate of heavy metals in clay soil based on freeze-thaw leaching method. This method is basically the same as the method provided in Example 1, with the main difference being that the amount of Akadama soil in the mixed soil column in this embodiment is 29.79%, and 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 clay soil based on freeze-thaw leaching method. This method is basically the same as the method provided in Example 2, with the main difference being that: in this embodiment, the steps of freezing and collecting leaching liquid are repeated twice, that is, three freeze-thaw cycles are continued in this embodiment, and 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 elution, which is basically the same as the method provided in Example 1, with the main difference being that pure Akadama soil columns are used in this comparative example instead of the mixed soil columns in Example 1, and 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, with the main difference being that pure peat soil columns are used in this comparative example instead of the mixed soil columns in Example 1, and the other steps and parameters are the same. The raw materials of the pure peat soil columns are composed of peat and peat, which are the products of the swamp development process and are formed by the residues of swamp plants that cannot be completely decomposed and accumulated under water-rich anaerobic conditions.

[0040] Comparative Example 3 This comparative example provides a method for removing heavy metals based on freeze-thaw elution, which is basically the same as the method provided in Example 1, with the main difference being that a pure clay column is used in this comparative example instead of the mixed soil column in Example 1, and 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 elution method, which is basically the same as the method provided in Example 1, with the main difference being that in the pretreatment step of this comparative example, the Akadama soil particles and the clay soil particles are respectively mixed with water according to the corresponding optimal moisture content, placed in a sealed bag and allowed to stand for 24 hours, and then the two are first evenly mixed and then loaded into the sample cavity 4 and compacted in layers to form a soil column with the two soils evenly mixed, instead of the mixed soil column with the two soils in layers in Example 1. The 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 elution method, which is basically the same as the method provided in Example 2, with the main difference being that in the pretreatment step of this comparative example, the Akadama soil particles and the clay soil particles are respectively mixed with water according to the corresponding optimal moisture content, placed in a sealed bag and left to stand for 24 hours, and then the two are evenly mixed and then loaded into the sample cavity 4 and compacted in layers to form a soil column with the two soils evenly mixed, instead of the mixed soil column with the two soils in layers in Example 2. The 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 elution, which is basically the same as the method provided in Example 3, with the main difference being that a clay column is used in this comparative example instead of the mixed soil column in Example 3, and the other steps and parameters are the same.

[0044] The removal rates of lead and cadmium elements treated by the methods provided in Examples 1 to 3 and Comparative Examples 1 to 6 and the frost heave performance 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 frost heave performance and heavy metal removal rate

[0046] Combined with Table 2, according to the frost heave performance data of Comparative Examples 1 to 3, it can be seen that: clay soil has a certain frost heave, peat soil basically has no frost heave, but Akadama soil has a strong frost heave, and its frost heave rate reaches 124.48%, which is much higher than clay soil and peat soil. Although Akadama soil and peat soil both have permeability and air permeability, Akadama soil has good water retention, large surface energy and high liquid limit water content, so during the freezing process, Akadama soil can promote the migration of a large amount of water, so that it has good frost heave. In addition, Akadama soil has a weak adsorption capacity for Pd and Cd elements, thereby reducing the adsorption of Pd and Cd by Akadama soil during the leaching process, so that Pd and Cd in clay soil are leached out as much as possible and enter the leaching liquid, which is conducive to improving the recovery rate of Pd and Cd.

[0047] The comparative data of Examples 1 to 3 and Comparative Examples 3 to 6 further prove that Akadama has strong frost heaving properties and the heavy metal removal rate can be improved by increasing the intake of the eluent.

[0048] By comparing the removal rates 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 content, the frost heave and water absorption of the layered filling method are higher than those of the mixed filling method, and the removal rate of Pd and Cd elements is also greater.

[0049] By comparing the data of Examples 2 to 3, Comparative Example 3 and Comparative Example 6, it can be seen that when Akadama is used as the admixture soil and loaded in layers, the removal rates of Pb and Cd are increased by 130.91% and 113.39% respectively compared with clay soil after one freeze-thaw-elution; after three freeze-thaw-elutions, 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 with clay soil, and the removal effect of heavy metals such as Pb and Cd is significant. Therefore, the heavy metal removal method provided by Example 3 of the present invention is the best, and the removal rates of Pb and Cd are the highest.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or some technical features can be replaced by equivalents without departing from the spirit of the technical solution of the present invention, which should be included in the scope of the technical solution for protection of the present invention.

Claims

1. A method for improving the removal rate of heavy metals in clay soil based on freeze-thaw method, comprising the steps of: Pretreatment: Fill the compacted clay column with red jade soil and compact it layer by layer to form a mixed soil column. The clay soil column contains heavy metals; Water replenishment and saturation: adding the eluent to the mixed soil column until the soil water potential in the mixed soil column reaches equilibrium to form a saturated soil body; Freezing and freezing: using a one-way freeze-thaw method to cool and freeze the saturated soil, while continuously adding an eluent to the low-temperature part of the saturated soil, until the eluent in the saturated soil is converted into a solid state and is frozen as a whole with the red jade soil and clay therein to form a frozen soil; Heating and melting: Instantly raising the temperature at both ends of the frozen soil body, so that the absorbed eluent, red jade soil and clay soil are completely melted to form a melted mixed soil body; Collecting leachate: collecting leachate from the melted mixed soil; Repeat elution: Repeat the steps of freezing and collecting the leaching solution, and collect the leaching solution of each elution to obtain a finished clay soil 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 and clay to obtain Akadama particles and clay particles with a particle size not exceeding 2 mm, respectively; then adding water to the Akadama particles and clay particles according to the optimal moisture content and mixing them evenly, putting them into a sealed bag and letting them stand for 20-32 hours to obtain Akadama mud and clay mud, respectively; compacting the clay mud in layers to form a compacted clay column, adding the Akadama mud on the fill column and compacting it in layers to form a compacted Akadama column, thereby making the mixed soil column.

3. The method according to claim 2, characterized in that The clay contains Pb and Cd elements, wherein the Pb content exceeds 1000 mg / Kg and the Cd content exceeds 4.0 mg / Kg.

4. The method according to any one of claims 1 to 3, characterized in that: In the mixed soil column, the mass percentage of red jade soil is 10% to 32%.

5. The method according to claim 4, characterized in that The main components of the eluent include one or more combinations of disodium ethylenediaminetetraacetate, citric acid, tartaric acid, oxalic acid, etc.

6. The method according to claim 4, characterized in that The freezing step includes: using a one-way freeze-thaw method to freeze the saturated soil to form a temperature difference between the upper and lower surfaces of the saturated mixture, controlling the cooling rate, ensuring the movement speed of the freezing front, and continuously adding eluent from the unfrozen area of ​​the saturated mixture to the frozen area until the saturated soil is frozen as a whole to form the frozen soil.

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

8. The method according to claim 7, characterized in that The step of heating and thawing comprises: instantly raising the temperature of the upper and lower surfaces of the frozen soil body to 8°C to 12°C simultaneously and continuously for 8 to 12 hours to melt the eluent, red jade and clay in the frozen soil body to form the thawed mixed soil body.

9. The method according to any one of claims 1 to 3, characterized in that: The method also includes the step of component detection: sampling from the finished clay soil product from which heavy metals have been removed, digesting the obtained clay soil sample to obtain a digestion test solution, and using ICP-MS to detect the heavy metal content in the digestion test solution.

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