System and process method for repairing heavy metal polluted soft soil through combination of freeze-thaw pretreatment and bottom vacuum leaching

Abstract

The invention relates to a system and a process method for repairing heavy metal polluted soft soil through combination of freeze-thaw pretreatment and bottom vacuum leaching. In order to solve the problems that various existing heavy metal polluted soft soil remediation methods cannot permanently remedy soil, or the remediation cost is too high, remediation is slow, the remediation effect is poor and the like, freeze-thaw pretreatment, leaching and bottom vacuum filtration are jointly applied, and by means of the mutual superposition and promotion effect of the three, the soil remediation effect is improved. Pollutants of the polluted soft soil are efficiently and rapidly removed, the pollution level is reduced, and the repaired soil can be transported nearby to be put into agriculture. Compared with the prior art, the concepts of freezing and thawing pretreatment and bottom vacuum leaching are introduced, and the function of permanently repairing the polluted soil is achieved. Meanwhile, different from most existing methods, the method is easier to popularize to engineering practice and has the advantages of high efficiency, low cost and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of contaminated soil remediation, and in particular to a system and process method for remediating heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching. Background Art

[0002] Soft soil is widely distributed, but at the same time, the continuous development of industry has intensified the generation of heavy metal pollutants. Due to the inadequacy of heavy metal pollutant discharge and resource utilization, heavy metal pollutants inevitably flow to soft soil areas. Heavy metal contaminated soft soil has the following characteristics: 1) The particles are small and the specific surface area is large, resulting in enhanced adsorption capacity for heavy metals, which significantly increases the difficulty of removal. 2) The bioavailability of heavy metals is high, and heavy metals are easily absorbed by plants and organisms. They may accumulate through the food chain, posing a threat to the ecosystem and human health. 3) The permeability is extremely low, and the permeability coefficient is generally around 10 -14 m 2 At this level, it is difficult for water or solutions to penetrate the soil and wash out pollutants.

[0003] At present, the remediation methods for heavy metal contaminated soil mainly include in situ fixation, phytoremediation, bioremediation, soil cleaning, landfill and electroremediation.

[0004] In situ immobilization involves adding amendments to the soil, which then immobilizes heavy metals through adsorption, precipitation, or complexation, reducing their mobility and bioavailability. This method is cost-effective, suitable for large-scale applications, and can leverage the natural adsorption capacity of soft soils. However, if heavy metals are not removed, long-term effects may be diminished by environmental changes.

[0005] Phytoremediation uses plants to absorb heavy metals through their roots, stabilize metals in the soil, or remove them through volatilization. Phytoremediation offers advantages such as environmental friendliness, low cost, aesthetic appeal, and improved soil ecology, making it particularly suitable for long-term remediation. However, the process is slow, typically taking several years, and its removal efficiency for high-concentration pollution is low. Furthermore, the treatment of contaminated plant biomass may trigger secondary pollution. For soft soil environments, plants that are resistant to flooding or compacted conditions must be selected, and plant adaptability is a key factor.

[0006] Bioremediation uses microorganisms to modify the form of heavy metals through biotransformation, precipitation, or adsorption, thereby reducing their toxicity or mobility. This technology offers advantages such as a natural process, minimal disruption, and low cost, making it suitable for long-term applications and potentially integrated with phytoremediation. However, remediation efficiency is affected by microbial activity and soil conditions (such as pH and moisture), resulting in a slow process and making it unsuitable for sites in urgent need of remediation. In soft soils, anaerobic conditions are common, necessitating the selection of appropriate microorganisms.

[0007] Soil cleaning involves excavating contaminated soil and washing it with water or chemical solutions to remove heavy metals. It is suitable for areas with high levels of contamination. This technology offers high removal efficiency and controllability, making it suitable for severely contaminated localized areas. However, it is costly, requires excavation and off-site treatment, and, for soft soils, the high water content can make dewatering more difficult, making it unsuitable for large-scale application. For soft soils, the cleaning process may require an additional dewatering step, adding operational complexity.

[0008] Landfilling completely isolates the source of contamination by removing contaminated soil and burying it at a designated site. This technique removes contamination on-site, making it suitable for heavily contaminated areas and highly controllable. However, it is extremely costly due to the excavation, transportation, and landfill costs involved. Furthermore, transferring the problem can lead to new environmental problems, making it unsustainable.

[0009] Electrokinetic remediation involves applying an electric field to induce charged particles to migrate toward electrodes through electroosmosis and electromigration, where they are collected and removed. Electrokinetic remediation is effective in soft soils, where low permeability limits other methods, and in situ application minimizes site disturbance. However, the technology is still immature and requires efficiency improvements. It is energy-intensive and may require pH control. Long-term effectiveness remains to be verified.

[0010] Currently, in-situ fixation and landfill remediation methods are the most widely used in practical projects due to their cost-effectiveness and short-term effectiveness. However, these methods do not truly remediate contaminated soil, and the treated soil becomes a hidden environmental threat. Phytoremediation and bioremediation have practical applications under specific conditions, but on a limited scale. Technologies such as electrokinetic extraction are still in the research stage.

[0011] CN108526208A discloses a system and application method for in-situ remediation of contaminated soil. The system uses a combination of artificial freezing and vertical shaft leaching to in-situ remediate clayey heavy metal contaminated soil. However, the removal rate of heavy metals copper and zinc after leaching through vertical drainage boards (vertical shafts) is very low, and it is easy to produce high secondary pollution.

[0012] To address this situation, the present invention proposes a method for remediating heavy metal-contaminated soft soil by combining freeze-thaw pretreatment with bottom vacuum leaching. This method reduces the adsorption capacity of heavy metal-contaminated soil and increases infiltration through freeze-thaw pretreatment. This combined method with bottom vacuum leaching significantly improves soil remediation efficiency. This method is environmentally friendly and energy-efficient. Summary of the Invention

[0013] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a system and process method for repairing heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching. In view of the many problems that various current heavy metal contaminated soft soil repair methods cannot permanently repair the soil, or the repair cost is too high, the repair is slow, and the repair effect is poor, the present invention combines the application of freeze-thaw pretreatment, leaching, and bottom vacuum filtration, and utilizes the mutual superposition and promotion effect between the three to efficiently and quickly remove pollutants from these contaminated soft soils, reduce the pollution level, and the repaired soil can be transported nearby for agricultural use.

[0014] The purpose of the present invention can be achieved by the following technical solutions: The first object of the present invention is to provide a process for remediating heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching, the process comprising the following steps: 1) Conduct indoor test analysis on the heavy metal contaminated soft soil to be repaired to obtain the pollution degree, permeability, and gradation parameters, build a finite element calculation model, and select the vacuum pressure, reagent concentration, and treatment thickness; 2) Select the leaching site according to soil characteristics; 3) Select a site to excavate a foundation pit and set up an anti-seepage barrier wall to form an impermeable storage area for contaminated soft soil; 4) Laying drainage boards in the foundation pit. The drainage boards are arranged horizontally in a single layer or multiple layers. A filter membrane or geotextile is placed above the drainage boards. The drainage boards are connected to the vacuum equipment. 5) A metal column perpendicular to the bottom of the foundation pit is set up in the foundation pit to fix temperature, humidity, and sensor probes to monitor the soil status in real time; 6) crushing and screening the heavy metal contaminated soft soil to be repaired, removing impurities and mixing it with the elution solution to increase the initial water content to obtain the pretreated contaminated soft soil; 7) Inject leaching liquid into the foundation pit and fill the pre-treated contaminated soft soil in layers. After each layer of filling, let the contaminated soft soil stand until it stabilizes. After leveling, add leaching liquid until the liquid level is 2-3 cm above the contaminated soft soil surface; 8) performing freeze-thaw cycle treatment on the contaminated soft soil, wherein the freeze-thaw cycle treatment includes natural freeze-thaw cycle treatment and / or artificial freeze-thaw cycle treatment; 9) Start the vacuum equipment to draw out the vacuum, monitor the changes in settlement, pore water pressure and deep displacement, and extract the eluent through the bottom drainage plate; 10) Continue to vacuum and add eluent at a solid-liquid ratio of 1:2 until the eluent volume reaches the preset value. Specifically, when the solid-liquid ratio is 1:(2-3), the eluent output reaches 2-3 times (preferably 2 times) the weight of the soil, and the leaching is terminated; 11) Determine whether the heavy metal content in the soil reaches or is below the standard value. If not, return to step 8). If yes, obtain the remediated soil.

[0015] Furthermore, the heavy metal contaminated soil includes one or more of ordinary soft soil, super soft soil, dredged mud, etc.

[0016] Furthermore, after step 11), the following step 12) is performed: 12) Analyzing the mineral composition and residual contaminants of the remediated soil obtained in step 11), and after risk assessment, transferring the remediated soil to agricultural or engineering land and cleaning the treatment site.

[0017] Furthermore, step 1) specifically includes the following processes: conducting indoor test analysis on the contaminated soft soil to be repaired to determine the contamination degree, permeability, and grading parameters of the contaminated soft soil to be repaired, and based on this and in combination with theories such as Darcy quantification, convection-diffusion equation, elastic consolidation theory, and reaction kinetics, constructing a finite element calculation model to select the vacuum pressure, agent concentration, and treatment thickness.

[0018] Furthermore, in step 2), the leaching site is selected to be an in-situ site and / or an off-site site, and the site is a naturally frozen site or an artificially selected cool and humid area.

[0019] Furthermore, in step 3), the depth of the foundation pit is about 1m.

[0020] Furthermore, in step 3), the anti-seepage barrier wall is set at the bottom and around the foundation pit, the anti-seepage barrier wall is higher than the site ground, cofferdams are set around it, and waterproof pads are laid on the bottom and side walls.

[0021] Furthermore, in step 4), a porous steel protective device is provided above the drainage board.

[0022] Furthermore, in step 4), the width and spacing of the drain plates are both 100-500 mm, and the size of the drain plates is determined by experiments and numerical models according to the vacuum pressure.

[0023] Furthermore, the drainage board can also be used for water supply.

[0024] Furthermore, in step 4), when the soil layer is thick, multiple layers of drainage boards can be arranged in the soil layer, and the time for alternately extracting the eluent can be tested experimentally.

[0025] This invention proposes a low-cost, large-scale remediation technology for heavy metal-contaminated soft soil, including both in situ and ex situ remediation methods. Freeze-thaw cycles disrupt the bonds between soil particles, increasing porosity and permeability. Furthermore, freeze-thaw cycles alter the chemical form of heavy metals, transforming them from stable forms (such as residual forms) to more mobile forms (such as exchangeable forms or carbonate-bound forms). This provides favorable conditions for subsequent leaching treatment. The bottom horizontal vacuum leaching method significantly addresses the issues of low soft soil permeability and the need for on-site leaching solution injection. The bottom vacuum effect allows the leaching solution to flow evenly throughout the soil area while controlling its extraction rate, eliminating the need for complex injection equipment. This offers the advantage of low cost, and the use of horizontal drainage boards can improve the overall heavy metal removal rate of contaminated soil. Furthermore, freeze-thaw pretreatment and bottom vacuum leaching have minimal environmental impact and are more environmentally friendly. Heavy metals collected in the filtrate can also be recovered, achieving multiple goals at one stroke.

[0026] Furthermore, in step 8), the freezing temperature and thawing time need to be determined by laboratory experiments, and the freeze-thaw time needs to be determined according to the scale of the soil leaching unit. The temperature of the freeze-thaw cycle is -15~-5°C (preferably -10°C), and the number of freeze-thaw cycles is 3-5 times.

[0027] Furthermore, in step 8), when artificial freeze-thaw cycle treatment is adopted, the end of the vertical freezing pipe is 20 cm to 30 cm away from the bottom drainage board.

[0028] Furthermore, in step 9), a time course curve of settlement, pore water pressure and deep displacement is drawn during vacuuming, and the vacuum pressure is adjusted according to the change of the curve.

[0029] Furthermore, in step 10), the elution liquid is recovered, and the recovered liquid is used for subsequent separation of heavy metals for resource recovery and reuse.

[0030] Furthermore, in step 11), the standard values ​​for soil heavy metal content are derived from the "Soil Environmental Quality - Construction Land Soil Pollution Risk Control Standard (Trial)" (GB 36600-2018), applicable to residential, commercial, and industrial construction land. Screening and control values ​​for heavy metals such as arsenic, cadmium, lead, mercury, and nickel are provided for "Class I" and "Class II" land, respectively.

[0031] Furthermore, step 12) also includes the following process: testing the gradation, particle morphology and pH value of the repaired soil obtained in step 11) to assess the risk.

[0032] The second object of the present invention is to provide a system for freeze-thaw pretreatment combined with bottom vacuum leaching to repair heavy metal contaminated soft soil. The system is used to implement the process method. The system includes vacuum equipment, an anti-seepage barrier wall, a drainage board, and a foundation pit; the foundation pit is arranged on the site; the vacuum equipment is connected to the drainage board; the anti-seepage barrier wall is arranged at the bottom and around the foundation pit, and the anti-seepage barrier wall is higher than the ground of the site; the drainage board is arranged horizontally in the foundation pit.

[0033] Furthermore, cofferdams are arranged around the anti-seepage barrier wall, and waterproof pads are laid on the bottom and side walls.

[0034] Furthermore, the system also includes a drainage pipe; the vacuum device is connected to the board head of the drainage board through the drainage pipe.

[0035] Furthermore, the system also includes a filter membrane or geotextile; the filter membrane or geotextile is arranged above the drainage board.

[0036] Furthermore, the system also includes a vertical freezing pipe, which is arranged above the drain pipe.

[0037] Furthermore, the vacuum equipment is an air compressor.

[0038] Compared with the prior art, the present invention has the following beneficial effects: 1) This technical solution provides a system and process for remediating heavy metal-contaminated soft soil using freeze-thaw pretreatment combined with bottom vacuum leaching. Compared to in-situ remediation methods such as solidification, stabilization, and landfill, this method incorporates the concepts of freeze-thaw pretreatment and bottom vacuum leaching, enabling permanent remediation of contaminated soil. Furthermore, unlike most existing methods, this method is more easily applicable to engineering practice and offers advantages such as high efficiency and low cost.

[0039] 2) This technical solution provides a system and process for remediating heavy metal-contaminated soft soil using freeze-thaw pretreatment combined with bottom vacuum leaching. This method is suitable for remediating soft soil contaminated with extremely low permeability. Freeze-thaw pretreatment increases the permeability of the soft soil and reduces its heavy metal adsorption capacity. The bottom vacuum further accelerates the leaching rate, achieving rapid and efficient soil remediation. Compared to traditional off-site leaching, this method requires no large-scale cleaning equipment. Instead, the soil and the leaching agent are placed in a leaching tank equipped with drainage boards. Freeze-thaw cycles are performed naturally or artificially. Following the freeze-thaw cycle, the leaching solution is injected, and leaching is completed using an air compressor for filtration. This significantly simplifies the contaminated soft soil treatment process and can be used on larger contaminated soft soil sites with improved results. Experimental results show that this method achieves a heavy metal removal rate exceeding 70% for soft soil with a permeability of 1.0E-15 (a permeability coefficient on the order of 1E-9 m / s), outperforming traditional leaching methods or single bottom vacuum leaching, while significantly shortening the remediation cycle. Furthermore, this method significantly reduces the risk of contaminant leakage.

[0040] 4) This technical solution provides a system and process method for remediating heavy metal-contaminated soft soil by combining freeze-thaw pretreatment with bottom vacuum leaching. Increasing the number of freeze-thaw cycles can significantly improve soil uniformity and enhance the overall remediation effect of leaching on the soil, thereby avoiding the formation of dominant flow due to cracks, which results in the local soil remediation effect being better than the overall effect of the leaching unit.

[0041] 5) This technical solution provides a system and process for remediating heavy metal-contaminated soft soils using freeze-thaw pretreatment combined with bottom vacuum leaching. The drainage panels are arranged horizontally at the bottom or in multiple layers within the soil. The good lateral permeability of water in soil, combined with gravity, ensures more complete contact between the soil and the reagent. Liquid is injected from above the soil and withdrawn from below, flowing out of the bottom of the leaching unit. The combination of vacuum suction and gravity stabilizes the seepage rate and flow direction of the leaching liquid. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 This is a process flow chart of a process method for repairing heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching in an embodiment of the present invention.

[0043] Figure 2 This is a longitudinal cross-sectional view of the site where the freeze-thaw pretreatment combined with bottom vacuum leaching system for remediating heavy metal contaminated soft soil in an embodiment of the present invention is located.

[0044] Figure 3 This is a schematic diagram of the drainage board arrangement of a system for freeze-thaw pretreatment combined with bottom vacuum leaching to repair heavy metal contaminated soft soil in an embodiment of the present invention.

[0045] Figure 4 This is a schematic diagram of the arrangement points of vertical freezing pipes in a system for freeze-thaw pretreatment combined with bottom vacuum leaching to repair heavy metal contaminated soft soil in an embodiment of the present invention.

[0046] Figure 5 Example of heavy metal concentration results simulated by a finite element model.

[0047] Figure 6 This is a schematic diagram comparing the effects of vertically laying drainage boards and horizontally laying drainage boards.

[0048] Figure 7 Schematic diagram of the simulation of heavy metal residues after bottom vacuum elution, where (a) is Cu and (b) is Zn.

[0049] Numbers in the figure: 1. Air compressor, 2. Drain pipe, 3. Leaching fluid, 4. Contaminated soft soil, 5. Cofferdam, 6. Drain board, 7. Filter membrane or geotextile, 8. Vertical freezing pipe. DETAILED DESCRIPTION

[0050] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Component models, material names, connection structures, control methods, algorithms, and other features not explicitly described in this technical solution are considered common technical features disclosed in the prior art.

[0051] In the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," "connect," "fixed," etc. should be understood broadly. For example, they may refer to fixed connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0052] It should be noted that, in the present invention, relational terms such as first and second, etc. are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply the existence of any such actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or device. In the absence of further limitations, an element defined by the phrase "comprising a..." does not exclude the presence of other identical elements in the process, method, article, or device comprising the element.

[0053] The present invention will be further described in detail below with reference to specific embodiments.

[0054] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not require further definition or explanation in subsequent drawings.

[0055] The following describes some embodiments of the present invention in detail with reference to the accompanying drawings. In the absence of conflict, the following embodiments and features in the embodiments may be combined with each other.

[0056] In the following embodiments, unless otherwise specified, raw materials or processing techniques are conventionally available in the market or conventional processing techniques are used. Unless otherwise specified, functional components or structures are conventional components or structures used in the art to achieve the corresponding functions.

[0057] Example

[0058] like Figures 1 to 4 As shown, this embodiment provides a process for repairing heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching, the main steps of which include: (1) Heavy metal contaminated soft soil samples were collected on site, and their physical and chemical properties, such as contamination degree, gradation composition, and mineral composition, were measured and analyzed. A finite element calculation model was constructed based on Darcy quantification, convection-diffusion equations, elastic consolidation theory, and reaction kinetics. At the same time, information such as soil and heavy metal concentrations obtained from indoor experiments was combined to determine the experimental plan, that is, to confirm the required vacuum pressure, reagent concentration, and corresponding contaminated soil thickness.

[0059] (2) Select a leaching site based on the analysis results of step (1) and the geographical location of the contaminated soil, determine the type, dosage, concentration of the leaching agent, and the spacing of the drainage board 6, and select an off-site leaching site. The site can be a natural frozen site or a nearby address with good conditions and a cool and humid place.

[0060] (3) Design the foundation pit, vacuum equipment, and backfill method based on the actual conditions of the construction site, such as site size, labor, and machine shifts. At the selected site, dig a pit (as a leaching pool) for treating contaminated soft soil according to the construction method of excavating the foundation pit. The pit depth should be about 1m. After the excavation is completed, provide support and build an anti-seepage barrier wall at the bottom and around the perimeter to form an impermeable soil storage area. The anti-seepage barrier wall is higher than the site, and a cofferdam is built around the raised part. To ensure that the risk of filtrate leakage is eliminated, waterproof padding or bentonite material can be laid at the bottom and around the perimeter to enhance the anti-seepage performance.

[0061] (4) Drainage boards 6 are laid horizontally according to the soil area and thickness. Drainage boards 6 can be laid horizontally in a single layer or in multiple layers. The width and length of the drainage boards 6 depend on the vacuum pressure generated by the air compressor 1, and the appropriate size can be determined by laboratory experiments. In order to reduce the extrusion deformation of the drainage boards 6, protective equipment made of porous steel can be fixed above the drainage boards 6. After the drainage boards 6 are laid, the board heads of the drainage boards 6 are connected to the vacuum equipment air compressor 1, and a filter membrane or geotextile 7 is laid above the drainage board 6 area. Specifically: the drainage boards 6 are arranged at a certain distance at the bottom, and the spacing between the drainage boards 6 is as follows: Figure 3 As shown, the width and spacing of the drainage board 6 are usually 100mm~500mm. After laying, the board head of the drainage board 6 is connected to the vacuum equipment (i.e., the air compressor 1) through the drainage pipe 2. Figure 2 After the arrangement is completed, a filter membrane or geotextile 7 (geotextile in this embodiment) is laid on top of the drainage board 6.

[0062] This embodiment adopts horizontal laying of drainage board 6, such as Figure 6 The following is a schematic diagram comparing the effects of vertically laying drainage boards and horizontally laying drainage boards. Figure 6 The heavy metal removal rates obtained from the experiment (without freeze-thaw) are shown in Table 1.

[0063] Table 1 Heavy metal removal rates of vertically laid drainage boards and horizontally laid drainage boards.

[0064]

[0065] Table 1 shows that the heavy metal removal rate was higher with the horizontal drain board 6 than with the vertical drain board (the experimental drain board specifications were horizontal d = 8 cm, vertical 5.5 cm × 10 cm, and both were 3 mm thick). This experiment found that the vertical drain board had a faster drainage rate, which was not conducive to sufficient contact between the agent and the soil.

[0066] In addition, bottom vacuum leaching can reduce the downward migration of soil pollutants and avoid secondary pollution.

[0067] (5) Set up some metal columns perpendicular to the bottom of the pit to install temperature, humidity, pH, and conductivity sensors. They are used to monitor the changes of various parameters during the elution process in real time and facilitate timely adjustments.

[0068] (6) Use excavation equipment to excavate the contaminated soil, and then perform coarse screening, crushing, and removal of large debris on the contaminated soft soil. During the treatment process, the leaching agent is mixed with the contaminated soft soil and filled into the foundation pit that has been pre-injected with a certain amount of leaching liquid.

[0069] (7) Artificial freezing has the advantage of controllable freezing temperature, time and number of times. When artificial freezing is used, artificial freezing pipes should be arranged. The layout points of artificial vertical freezing pipes 8 are as follows: Figure 4 shown.

[0070] (8) Before filling, continuously inject a certain amount of leaching liquid into the pit, and then slowly fill in the soft soil. If the soil layer is thick, use a layered filling method. When the upper layer of contaminated soft soil flows to a stable state, you can choose to lay the second layer of drainage board 6. After laying is completed, continue to inject the remaining contaminated soft soil.

[0071] (9) Allow the contaminated soft soil to stabilize for a period of time. After it is properly leveled, add a certain amount of leaching liquid to the surface of the contaminated soft soil. Ensure that the leaching liquid level is within 2-3 cm from the surface of the contaminated soft soil.

[0072] (10) Soil freezing. Soil freezing can be achieved by utilizing the natural low temperature of northern sites. If the freezing temperature cannot be reached, artificial freezing can also be used. Low temperature weather can be appropriately selected for soil remediation to reduce the energy consumption required for freezing and thawing. After standing for a period of time, natural or artificial freezing methods are used to perform freeze-thaw cycles on the contaminated soft soil. The natural freezing method has certain uncontrollable factors. If artificial freeze-thaw cycles (such as Figure 4 ), there are certain requirements for the depth of the vertical freezing pipe 8. It is required that the end of the vertical freezing pipe 8 should be 20cm-30cm away from the bottom drainage board 6. When using multiple layers of drainage boards 6, the spacing between the other layers of drainage boards 6 should be set except for the bottom layer to reserve gap space for the layout of the freezing pipe. The number, duration and temperature parameters are obtained from experimental research. Figure 7 As shown in the figure, the remediation process of heavy metal contaminated soil by bottom vacuum leaching under freeze-thaw cycles was simulated. A 10 cm soil column was used, and A1, A2, A3, and A4 were frozen and thawed 0, 2, 4, and 6 times respectively. The initial heavy metal residue (Cu, Zn) was 1000 mg / kg. The remediation effect of heavy metal contaminated soil by bottom vacuum leaching under freeze-thaw cycles was significant, but it did not increase continuously with the increase of freeze-thaw times.

[0073] At the same time, soil sample parameters collected after freeze-thaw cycles were combined with finite element models to guide engineering practice. This process includes the following: simultaneous consideration of chemical reaction, component migration, soil deformation, and seepage rate. The number of freeze-thaw cycles and freeze-thaw temperature affect soil permeability and heavy metal adsorption and desorption capacity, and can be quantified as model parameters for coupling. The number of freeze-thaw cycles and other parameters can also be optimized based on the model. Preliminary calculation results need to be verified against engineering experience from projects such as soft soil remediation projects in Shanghai. Experience has shown that for thick contaminated soil layers, one to two safety cycles should be added to the calculated values ​​to prevent incomplete freezing. The freezing site, the number of drain panels, and their spacing affect liquid seepage, and thus the overall leaching process. Similarly, parameters such as chemical concentration can influence the reaction rate of heavy metal contaminants. The model considers the attenuation effect of the freeze-thaw cycle N and soil deformation on soil permeability K in the subsequent leaching phase (using the NK relationship curve established through preliminary experiments), which is neglected in conventional leaching design models. Engineering practice (in the Shanghai soft soil leaching model experiment that has been carried out) shows that relying solely on theoretical calculations may lead to uneven distribution of leaching liquid in actual operation. Therefore, it is necessary to adopt a segmented leaching strategy based on experience (such as low-flow infiltration first, followed by high-flow flushing), and dynamically adjust according to the changes in the concentration of pollutants in the leaching liquid monitored in real time on site. The model simultaneously couples the four fields of reaction-deformation-flow-component migration, which can better reflect the actual leaching situation and provide engineering guidance for bottom vacuum leaching under freeze-thaw pretreatment conditions. When the model is actually used, small-scale model experiments can be used to determine the parameters, and artificial freezing numerical simulations need to be supplemented to predict the freezing site, freezing temperature and freezing time. Examples of heavy metal concentration results simulated by finite element models are as follows: Figure 5 shown.

[0074] (11) After the freeze-thaw cycle is completed, the eluent is added and the air compressor 1 is used for vacuuming. During the vacuuming process, the appropriate vacuum pressure should be controlled, generally above 60 kPa. The data such as settlement, pore water pressure, and deep lateral displacement are monitored, and the corresponding time curve is drawn to monitor the change pattern of various indicators. The eluent can be extracted from the bottom drainage board 6.

[0075] (12) If multiple layers of drainage boards 6 are used for leaching, the order of vacuuming and the duration of vacuuming of each layer need to be adjusted to ensure that the leaching liquid can fully wash out the heavy metal pollutants. The specific order was obtained from indoor experiments and verified by laboratory model tests. Using double-layer drainage boards for alternating leaching, the alternating leaching methods of "bottom first, then middle" and "middle first, then bottom" can shorten the leaching time by 20% and 40%, respectively, and increase the heavy metal removal rate by 16.2% and 26.4%.

[0076] (13) Continuously apply vacuum under constant pressure to extract a certain amount of eluent as a reference. For example, at a solid-to-liquid ratio of 1:2, 1 kg of contaminated soft soil (dry soil) requires 2 L of eluent. During the elution process, the eluent needs to be continuously replenished above the contaminated soft soil, but it must not exceed the cofferdam 5.

[0077] (14) After leaching, take samples to measure the heavy metal content of the contaminated soil to determine whether it meets the safety indicators for soil reuse, that is, whether the heavy metal content in the soil reaches the standard value or below the standard value. The source of the standard value of the heavy metal content in the soil is: "Soil Environmental Quality Construction Land Soil Pollution Risk Control Standard (Trial)", No.: GB 36600-2018, Scope of application: residential, commercial, industrial and other construction land. According to "First Class Land" and "Second Class Land", the screening values ​​and control values ​​of heavy metals such as arsenic, cadmium, lead, mercury, and nickel are given respectively. If the index requirements are met, the leaching is terminated and the treated contaminated soft soil is obtained. If not, the freeze-thaw cycle and leaching steps are repeated, that is, steps (9) to (13) are repeated.

[0078] (15) After completion, the vacuum drainage system (a system combining freeze-thaw pretreatment with bottom vacuum leaching to repair heavy metal contaminated soft soil) is dismantled, and the gradation, particle size changes, pH, mineral composition changes, and other characteristics of the treated contaminated soft soil are tested to analyze the effect of contaminated soil remediation.

[0079] (16) Assess soil remediation status and recontamination risks, and return remediated soil to use. Clean up the soil remediation treatment site.

[0080] like Figures 1 to 4 As shown, this embodiment further provides a system for freeze-thaw pretreatment combined with bottom vacuum leaching to repair heavy metal contaminated soft soil. The system is set up based on the site, and the system includes vacuum equipment, a drain pipe 2, an anti-seepage barrier wall, a drain board 6, a filter membrane or geotextile 7, a vertical freezing pipe 8, and a foundation pit; the foundation pit is set in the site; the vacuum equipment is connected to the drain board 6; the anti-seepage barrier wall is set at the bottom and around the foundation pit, and the anti-seepage barrier wall is higher than the site ground; the drain board 6 is arranged horizontally in the foundation pit. A cofferdam 5 is set around the anti-seepage barrier wall, and a waterproof liner is laid on the bottom and side walls. The vacuum equipment is connected to the head of the drain board 6 through the drain pipe 2. The filter membrane or geotextile 7 is set above the drain board 6. The vertical freezing pipe 8 is set above the drain pipe 2.

[0081] The vacuum equipment is an air compressor 1.

[0082] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. 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 process for repairing heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching, characterized in that: The process comprises the following steps: 1) Conduct indoor test analysis on the heavy metal contaminated soft soil to be repaired to obtain the pollution degree, permeability, and gradation parameters, build a finite element calculation model, and select the vacuum pressure, reagent concentration, and treatment thickness; 2) Select the leaching site according to soil characteristics; 3) Select a site to excavate a foundation pit and set up an anti-seepage barrier wall to form an impermeable storage area for contaminated soft soil; 4) Laying drainage boards in the foundation pit. The drainage boards are arranged horizontally in a single layer or multiple layers. A filter membrane or geotextile is placed above the drainage boards. The drainage boards are connected to the vacuum equipment. 5) A metal column perpendicular to the bottom of the foundation pit is set up in the foundation pit to fix temperature, humidity, and sensor probes to monitor the soil status in real time; 6) crushing and screening the heavy metal contaminated soft soil to be repaired, removing impurities and mixing it with the elution solution to increase the initial water content to obtain the pretreated contaminated soft soil; 7) Inject leaching liquid into the foundation pit and fill the pre-treated contaminated soft soil in layers. After each layer of filling, let the contaminated soft soil stand until it stabilizes. After leveling, add leaching liquid until the liquid level is 2-3 cm above the contaminated soft soil surface; 8) performing freeze-thaw cycle treatment on the contaminated soft soil, wherein the freeze-thaw cycle treatment includes natural freeze-thaw cycle treatment and / or artificial freeze-thaw cycle treatment; 9) Start the vacuum equipment to draw out the vacuum, monitor the changes in settlement, pore water pressure and deep displacement, and extract the eluent through the drainage board; 10) Continue to vacuum and add eluent. Stop leaching when the eluent output reaches 2 to 3 times the weight of the soil. 11) Determine whether the heavy metal content in the soil reaches or is below the standard value. If not, return to step 8). If yes, obtain the remediated soil.

2. The process for repairing heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching according to claim 1 is characterized in that: After step 11), perform the following step 12): 12) Analyzing the mineral composition and residual pollutants of the remediated soil obtained in step 11), and after risk assessment, transferring the remediated soil to agricultural or engineering land and cleaning the treatment site.

3. The process for repairing heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching according to claim 1 is characterized in that: Step 1) specifically includes the following processes: conducting indoor test analysis on the contaminated soft soil to be repaired to determine the contamination degree, permeability, and gradation parameters of the contaminated soft soil to be repaired, and constructing a finite element calculation model in combination with Darcy quantification, convection-diffusion equation, elastic consolidation theory, and reaction kinetics theory to select the vacuum pressure, agent concentration, and treatment thickness; In step 2), the leaching site is selected as an in-situ site and / or an off-site site, and the site is a naturally frozen site or an artificially selected cool and humid area; In step 3), the depth of the foundation pit is 1m; In step 3), the anti-seepage barrier wall is set at the bottom and around the foundation pit. The anti-seepage barrier wall is higher than the site ground, cofferdams are set around it, and waterproof pads are laid on the bottom and side walls.

4. The process for repairing heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching according to claim 1 is characterized in that: In step 4), a porous steel protective device is also provided above the drainage board; In step 4), the width and spacing of the drainage boards are both 100-500 mm.

5. The process for repairing heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching according to claim 1 is characterized in that: In step 8), the temperature of the freeze-thaw cycle is -15 to -5°C, and the number of freeze-thaw cycles is 3 to 5 times; In step 8), when artificial freeze-thaw cycle treatment is adopted, the end of the vertical freezing pipe is 20 cm to 30 cm away from the bottom drainage board.

6. The process for repairing heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching according to claim 1 is characterized in that: In step 9), a time course curve of settlement, pore water pressure and deep displacement is drawn during vacuuming, and the vacuum pressure is adjusted according to the curve changes.

7. The process for repairing heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching according to claim 1 is characterized in that: In step 10), the eluent is recovered, and the recovered liquid is used for subsequent separation of heavy metals for resource recovery and reuse.

8. The process for repairing heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching according to claim 1 is characterized in that: Step 12) further includes the following process: testing the gradation, particle morphology and pH value of the repaired soil obtained in step 11) to assess the risk.

9. A system for remediating heavy metal-contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching, the system being used to implement the process according to any one of claims 1 to 8, characterized in that: The system includes vacuum equipment, an anti-seepage barrier wall, a drainage board (6), and a foundation pit; The foundation pit is located within the site; The vacuum device is connected to the drainage board (6); The anti-seepage barrier wall is arranged at the bottom and around the foundation pit, and the anti-seepage barrier wall is higher than the ground surface of the site; The drainage board (6) is arranged horizontally in the foundation pit.

10. The freeze-thaw pretreatment combined with bottom vacuum leaching system for repairing heavy metal contaminated soft soil according to claim 9, characterized in that: The anti-seepage barrier wall is surrounded by a cofferdam (5), and a waterproof liner is laid on the bottom and side walls; The system further comprises a drainage pipe (2); the vacuum device is connected to the head of the drainage board (6) via the drainage pipe (2); The system further comprises a filter membrane or a geotextile (7); the filter membrane or the geotextile (7) is arranged above the drainage board (6); The system further comprises a vertical freezing pipe (8), wherein the vertical freezing pipe (8) is arranged above the drainage pipe (2); The vacuum equipment is an air compressor (1).

Images