A system and process method for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum rinsing

Abstract

The present application relates to a kind of freeze-thaw pretreatment combined bottom vacuum rinsing system and process method of remediation heavy metal contaminated soft soil.For the current various heavy metal contaminated soft soil remediation method cannot permanently repair soil, or the repair cost is too high, repair is slow, repair effect is poor and many other problems, the present application combines freeze-thaw pretreatment, leaching, bottom vacuum filtration, utilize the mutual superposition and promotion between three, efficiently, quickly remove the pollutants in these contaminated soft soil, reduce pollution level, and can be near migration into agriculture after the repair of soil.Compared with prior art, the present application introduces freeze-thaw pretreatment and bottom vacuum leaching concept, with the function of permanently repairing contaminated soil.Different from most existing methods, the present method is more easily popularized to engineering practice, and has the advantages of high efficiency, low cost and the like.

Description

TECHNICAL FIELD

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

[0002] Soft soil is widely distributed, but at the same time, the continuous development of industry exacerbates the generation of heavy metal pollutants. Due to the shortcomings in the emission and resource utilization process of heavy metal pollutants, heavy metal pollutants inevitably flow to soft soil areas. The heavy metal contaminated soft soil has the following characteristics: 1) fine particles and large specific surface area, which increases the adsorption capacity of heavy metals, thereby significantly increasing the removal difficulty. 2) High biological availability of heavy metals, heavy metals are easily absorbed by plants and organisms, which may accumulate through the food chain, posing a threat to the ecosystem and human health. 3) Extremely low permeability, the permeability coefficient is generally in the order of 10 -14 m 2 -6, and water or solution is difficult to penetrate the soil to flush out the pollutants.

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

[0004] In-situ fixation is to add an amendment to the soil, and to fix the heavy metals in the soil by adsorption, precipitation or complexation, so as to reduce their mobility and bioavailability. It has the advantages of high cost-effectiveness, suitable for large-area application, and can utilize the natural adsorption capacity in soft soil. However, it does not remove heavy metals, and the long-term effect may be weakened due to environmental changes.

[0005] Phytoremediation utilizes plants to absorb heavy metals through root systems, stabilize metals in soil or remove them through volatilization to achieve remediation. Phytoremediation has the advantages of environmental protection, low cost, aesthetics and soil ecological improvement, and is particularly suitable for long-term remediation. However, the process is slow, usually taking several years, and the removal efficiency for high concentration pollution is low. In addition, the disposal of contaminated plant biomass may cause secondary pollution. For soft soil environment, plants that can tolerate flooding or dense conditions need to be selected, and plant adaptability is a key factor.

[0006] Bioremediation utilizes microorganisms to change the form of heavy metals through biotransformation, precipitation or adsorption, thereby reducing their toxicity or mobility. This technology has the advantages of natural process, little interference and low cost, and is suitable for long-term application and can be combined with phytoremediation. However, the remediation efficiency is affected by microbial activity and soil conditions (such as pH and humidity), and the process may be slow, which is not suitable for sites that require urgent remediation. In soft soil, anaerobic conditions are common, so suitable microorganisms need to be selected.

[0007] Soil washing removes heavy metals by excavating contaminated soil and washing with water or chemical solution. It is suitable for high concentration contaminated areas. This technique has high removal efficiency and controllability, and is suitable for local areas with severe pollution. However, it is costly, requires excavation and off-site disposal, and for soft soil, high water content can increase the difficulty of dewatering, and is not suitable for large area application. For soft soil, the washing process may require an additional dewatering step, increasing the complexity of the operation.

[0008] Landfilling completely isolates the pollution source by removing contaminated soil and landfilling to designated sites. This technique removes pollution from the site, is suitable for areas with severe pollution, and has strong controllability. However, it is extremely costly, involves excavation, transportation and landfilling costs, and the problem transfer can cause new environmental problems, which is not sustainable.

[0009] Electrokinetic remediation removes charged particles by applying an electric field to move them to the electrode by electroosmosis and electromigration, and then removing them after collection. Electrokinetic remediation is effective for soft soil, as its low permeability limits other methods, and in-situ application can reduce site disturbance. However, the technology is not yet mature, needs to improve efficiency, has high energy consumption, may require pH control, and long-term effectiveness needs to be verified.

[0010] Current in-situ fixation and landfilling remediation methods are most widely used in practical engineering due to their economic efficiency and short-term effectiveness, but they do not truly repair contaminated soil in nature, and the treated soil becomes a hidden bomb that harms the environment. Plant remediation and bioremediation have practical applications under certain conditions, but the scale is limited. Technologies such as electrokinetic extraction are still in the research stage.

[0011] CN108526208A discloses a system and method for in-situ remediation of contaminated soil, which uses artificial freezing and vertical well leaching to in-situ remediate clay heavy metal contaminated soil, but the removal rates of heavy metals copper and zinc after vertical drainage plate (vertical well) leaching are very low, and high secondary pollution is easily generated.

[0012] In view of the above status, the present application proposes a method for remediation of heavy metal contaminated soft soil by combining freeze-thaw pretreatment with bottom vacuum leaching. The freeze-thaw pretreatment reduces the adsorption capacity of heavy metal contaminated soil and increases the permeability, and then the contaminated soil is treated by the method of combining bottom vacuum leaching, which greatly improves the efficiency of soil remediation. This method is green and energy-saving. SUMMARY

[0013] The present application aims at overcoming the defects of the prior art, and provides a system and a process method for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching.

[0014] The object of the present application can be achieved by the following technical solutions.

[0015] The first object of the present application is to provide a process method for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching, which comprises the following steps:

[0016] 1) indoor test analysis is performed on the heavy metal contaminated soft soil to be remediated, the pollution degree, the permeability, the grading parameters are obtained, a finite element calculation model is constructed, and the vacuum pressure, the reagent concentration and the treatment thickness are selected;

[0017] 2) the leaching site is selected according to the soil properties;

[0018] 3) the foundation pit is excavated in the selected site, and the impermeable barrier wall is arranged to form a water-impermeable contaminated soft soil storage site;

[0019] 4) the drainage board is laid in the foundation pit, the drainage board is arranged in a horizontal manner, the drainage board is arranged in a single layer or multiple layers, the filter membrane or the geotextile is arranged above the drainage board, and the drainage board is connected with the vacuum equipment;

[0020] 5) the metal column perpendicular to the bottom of the foundation pit is arranged in the foundation pit, the metal column is used for fixing the temperature, the humidity and the sensor probe, and the soil state is monitored in real time;

[0021] 6) the heavy metal contaminated soft soil to be remediated is crushed and sieved for pretreatment, the pretreated contaminated soft soil is mixed with the leaching liquid after the impurities are removed, the initial water content is increased, and the pretreated contaminated soft soil is obtained;

[0022] 7) the leaching liquid is injected into the foundation pit, and the pretreated contaminated soft soil is filled layer by layer, after each layer of soil is filled, the contaminated soft soil is placed to a stable state, and the leaching liquid is supplemented to a liquid surface 2-3cm away from the surface of the contaminated soft soil after leveling;

[0023] 8) the contaminated soft soil is subjected to freeze-thaw cycle treatment, and the freeze-thaw cycle treatment comprises natural freeze-thaw cycle treatment and / or artificial freeze-thaw cycle treatment;

[0024] 9) Start the vacuum equipment to extract vacuum, monitor the settlement, pore water pressure and deep displacement changes, and extract the leaching solution through the bottom drainage plate;

[0025] 10) Continue to extract vacuum and supplement the leaching solution according to the solid-liquid ratio of 1:2 until the amount of leaching solution reaches the preset value, specifically, when the solid-liquid ratio is 1:(2-3), the amount of leaching solution reaches 2-3 times (preferably 2 times) of the weight of the soil, and the leaching is terminated;

[0026] 11) Determine whether the content of heavy metals in the soil reaches the standard value or not, if not, return to step 8), if yes, obtain the repaired soil.

[0027] Further, the heavy metal contaminated soil includes one or more of ordinary soft soil, super soft soil, dredged soil, etc.

[0028] Further, after step 11), the following step 12) is performed:

[0029] 12) Perform mineral composition and pollutant residue analysis on the repaired soil obtained in step 11), assess the risk and transfer to agricultural or engineering land, and clean up the site.

[0030] Further, in step 1), specifically including the following process: indoor test analysis is performed on the contaminated soft soil to be repaired to determine the pollution degree, permeability, and grading parameters of the contaminated soft soil to be repaired, and a finite element calculation model is constructed based on Darcy's equation, convection-diffusion equation, elastic consolidation theory, and reaction kinetics, etc. The vacuum pressure, reagent concentration and treatment thickness are selected.

[0031] Further, 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.

[0032] Further, in step 3), the depth of the foundation pit is about 1m.

[0033] Further, in step 3), the impermeable barrier wall is arranged at the bottom and around the foundation pit, the impermeable barrier wall is higher than the ground surface of the site, a cofferdam is arranged around, and a waterproof lining is laid at the bottom and the sidewall.

[0034] Further, in step 4), a perforated steel protection device is further arranged above the drainage plate.

[0035] Further, in step 4), the width and spacing of the drainage plate are both 100-500mm, and the size of the drainage plate is determined according to the vacuum pressure through experiments and numerical models.

[0036] Further, the drainage plate can also be used for water supply.

[0037] Further, in step 4), when the soil layer is thick, a plurality of drainage plates can be arranged in the soil layer, and the time of alternately extracting the leaching solution is determined by experimental tests.

[0038] The present application provides a low-cost and large-scale heavy metal contaminated soft soil remediation technology, including in-situ and ex-situ remediation methods. Freezing and thawing can cause the destruction of the cementation between soil particles, increase the porosity and permeability, and change the chemical form of heavy metals, making them from stable forms (such as residual state) to more mobile forms (such as exchangeable state, carbonate-bound state). This provides favorable conditions for subsequent leaching treatment. The method of bottom horizontal vacuum leaching greatly solves the problems of low permeability of soft soil and site leaching liquid injection. The bottom vacuum action can make the leaching liquid flow uniformly through the entire soil area while controlling the extraction rate of the leaching liquid without the need for other complex injection equipment, which has the advantage of low cost. The use of horizontal drainage plates can improve the overall heavy metal removal rate of contaminated soil. In addition, the freezing and thawing pretreatment and bottom vacuum leaching have less environmental impact and are more environmentally friendly, and the heavy metals collected in the filtrate can be recycled, achieving multiple effects at once.

[0039] Further, in step 8), the freezing temperature and thawing time need to be determined by laboratory experiments, and the freezing and thawing time needs to be determined according to the size of the soil leaching unit. The temperature of the freezing and thawing cycle is -15~-5℃ (preferably -10℃), and the number of freezing and thawing cycles is 3-5 times.

[0040] Further, in step 8), when artificial freezing and thawing cycle treatment is used, the distance between the end of the vertical freezing pipe and the bottom drainage plate is 20-30 cm.

[0041] Further, in step 9), draw the time history curves of settlement, pore water pressure and deep displacement when vacuumizing, and adjust the vacuum pressure according to the curve changes.

[0042] Further, in step 10), the leaching liquid is recycled, and the recovered liquid is used for subsequent separation of heavy metals for resource recycling and reuse.

[0043] Further, in step 11), the source of the standard value of the content of heavy metals in soil is: Soil Environmental Quality Construction Land Soil Pollution Risk Control Standard (Trial), GB 36600-2018, applicable to residential, commercial, industrial and other construction land. According to the "first type of land" and "second type of land", the screening values and control values of arsenic, cadmium, lead, mercury, nickel and other heavy metals are given.

[0044] Further, in step 12), it also includes the following process: detecting the gradation, particle morphology and pH value of the remediated soil obtained in step 11) to evaluate the risk.

[0045] A second object of the present application is to provide a system for remediation of soft soil contaminated by heavy metals by freeze-thaw pretreatment combined with bottom vacuum leaching, which is used to implement the process method, and comprises a vacuum device, an impermeable barrier wall, a drain board and a foundation pit; the foundation pit is arranged in a site; the vacuum device is connected with the drain board; the impermeable barrier wall is arranged at the bottom and around the foundation pit, and is higher than the ground surface of the site; and the drain board is horizontally arranged in the foundation pit.

[0046] Further, a cofferdam is arranged around the impermeable barrier wall, and a waterproof lining is arranged on the bottom and sidewall of the cofferdam.

[0047] Further, the system further comprises a drain pipe; the vacuum device is connected with the head of the drain board through the drain pipe.

[0048] Further, the system further comprises a filter membrane or geotextile; the filter membrane or geotextile is arranged above the drain board.

[0049] Further, the system further comprises a vertical freezing pipe; the vertical freezing pipe is arranged above the drain pipe.

[0050] Further, the vacuum device is an air compressor.

[0051] Compared with the prior art, the present application has the following beneficial effects:

[0052] 1) The system and process method for remediation of soft soil contaminated by heavy metals by freeze-thaw pretreatment combined with bottom vacuum leaching provided by the technical solution have the function of permanently repairing contaminated soil compared with in-situ remediation methods such as solidification and stabilization landfill. At the same time, unlike most existing methods, the method is more easily popularized into engineering practice, and has the advantages of high efficiency, low cost and the like.

[0053] 2) The system and process method for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching provided by the technical solution can be applied to soft soil with extremely low permeability, and the permeability of the soft soil is increased and the heavy metal adsorption capacity in the soil is reduced by freeze-thaw pretreatment. And by the action of the bottom vacuum, the leaching rate is further accelerated, so that the purpose of quickly and efficiently remediating the soil is achieved; compared with the traditional off-site leaching, the present application does not need large-scale cleaning equipment, only needs to place the leaching and soil in the leaching pool paved with drainage plates, uses natural temperature freeze-thaw or artificial freeze-thaw, injects leaching liquid after freeze-thaw cycle, and then uses an air compressor to filter, so that the leaching is completed. The process of treating the contaminated soft soil is greatly simplified, and the contaminated soft soil site with a larger scale can be treated, and the effect is better. It is verified by experiments that when the permeability of the soft soil treated by the method is 1.0E-15 (the order of magnitude of the permeability coefficient is 1E-9 m / s), the removal rate of heavy metals is more than 70%, which is better than the traditional leaching method or single bottom vacuum leaching, and the remediation period is greatly shortened. In addition, the method significantly reduces the risk of pollutant leakage.

[0054] 4) The system and process method for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching provided by the technical solution, increasing the number of freeze-thaw cycles can significantly improve the uniformity of the soil, improve the overall remediation effect of the leaching remediation on the soil, and avoid the formation of preferential flow due to cracks, which leads to the local remediation effect of the soil being better than the overall effect of the leaching unit.

[0055] 5) The system and process method for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching provided by the technical solution, the drainage plate arrangement mode is bottom horizontal arrangement or multi-layer horizontal arrangement in the soil body, which can make the soil and the medicament contact more fully in combination with the good transverse permeability of water in the soil and the action of gravity, and the liquid is injected from the top of the soil body and extracted from the bottom. Under the action of vacuum suction and gravity, the seepage rate of the leaching liquid and the liquid flow direction are more stable. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 The process flow chart of the process method for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching in the embodiment of the present application.

[0057] Figure 2 The longitudinal section view of the site where the system for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching in the embodiment of the present application is located.

[0058] Figure 3 The drainage plate arrangement form schematic diagram of the system for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching in the embodiment of the present application.

[0059] Figure 4 The arrangement point of the vertical freezing pipe of the system for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching in the embodiment of the present application is shown in the schematic diagram.

[0060] Figure 5 The heavy metal concentration result simulated by the finite element model is shown in the example.

[0061] Figure 6 The effect comparison schematic diagram of the vertical laid drainage plate and the horizontal laid drainage plate is shown in the example.

[0062] Figure 7 The simulation schematic diagram of the heavy metal residue after the bottom vacuum leaching is shown in the example, wherein (a) is Cu and (b) is Zn.

[0063] The figure reference:

[0064] 1, air compressor, 2, drain pipe, 3, leaching liquid, 4, contaminated soft soil, 5, cofferdam, 6, drainage plate, 7, filter membrane or geotextile, 8, vertical freezing pipe. DETAILED DESCRIPTION

[0065] The present application will be described in detail below in conjunction with the drawings and specific embodiments. In the technical solution, the component model, material name, connection structure, control method, algorithm and other features not explicitly described are considered as common technical features disclosed in the prior art.

[0066] In the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connecting", "fixing" and other terms should be understood in a broad sense. For example, it can be fixed connection, or detachable connection, or integrated; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through an intermediate medium; it can be the internal communication of two elements or the interaction relationship between two elements. For ordinary skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0067] It is to be noted that the terms such as first and second, etc. are used herein merely to differentiate one entity or operation from another entity or operation, and do not necessarily require or imply there is any such actual relationship or order between these entities or operations. Also, the terms "comprising", "containing" or any other variant thereof are intended to cover non-exclusive inclusions, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements in the list, but can also include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without more limitations, an element defined by the phrase "comprising a..." does not exclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0068] The content of the present application will be further described in detail below in connection with specific embodiments.

[0069] It should be noted that similar reference numerals and letters refer to similar items in the following drawings, and thus, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0070] Some embodiments of the present application will be described in detail below in connection with the accompanying drawings. The following embodiments and features in the embodiments can be combined with each other without conflict.

[0071] In the following embodiments, unless otherwise specified, the raw materials or processing techniques are all conventional commercially available raw material products or conventional processing techniques in the art. Unless otherwise specified, the functional components or structures are all conventional components or conventional structures adopted in the art to achieve the corresponding functions.

[0072] Embodiments

[0073] As shown in the following embodiments, unless otherwise specified, the raw materials or processing techniques are all conventional commercially available raw material products or conventional processing techniques in the art. Unless otherwise specified, the functional components or structures are all conventional components or conventional structures adopted in the art to achieve the corresponding functions. Figures 1-4 The present embodiment provides a process for remediating heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum leaching, and the main steps include:

[0074] (1) On-site sampling of heavy metal contaminated soft soil, determination and analysis of its physical and chemical properties such as pollution degree, gradation composition, mineral composition, etc. Based on Darcy quantification, convection-diffusion equation, elastic consolidation theory, reaction kinetics, etc. to construct a finite element calculation model, and combined with the information of soil, heavy metal concentration, etc. obtained from indoor experiments to determine the experimental scheme, i.e. to confirm the required vacuum pressure, reagent concentration, and corresponding contaminated soil thickness.

[0075] (2) According to the analysis results of step (1) and the location of the contaminated land, select the leaching site, determine the type, dosage and concentration of the leaching agent, and the spacing of the drainage plate 6, select the leaching site, and the site can be selected according to the actual situation, or choose the address nearby, and the conditions are better and shady and humid.

[0076] (3) According to the actual situation of the construction site such as the size of the site, labor, mechanical shift, etc., design the foundation pit, vacuum equipment, and filling method, etc., and according to the construction method of excavating the foundation pit, a soil pit (as a leaching pool) for treating contaminated soft soil is dug in the selected site, and the pit depth is about 1m. After excavation, support well, and build a seepage prevention barrier wall at the bottom and around, forming a water-impermeable soil storage site. The seepage prevention barrier wall is higher than the site, and a cofferdam 5 is built around the higher part. In order to prevent the risk of leakage of leachate, waterproof lining or bentonite material can be laid at the bottom and around to enhance the impermeability.

[0077] (4) According to the area and thickness of the soil, lay the drainage plate 6 horizontally, which can be laid horizontally in single layer or in multiple layers. The width and length of the drainage plate 6 are determined according to the vacuum pressure generated by the air compressor 1, which can be determined by laboratory experiments. In order to reduce the extrusion deformation of the drainage plate 6, a protective tool made of perforated steel can be fixed above the drainage plate 6. After the drainage plate 6 is laid, the plate head of the drainage plate 6 is connected to the air compressor 1 of the vacuum equipment, and a filter membrane or geotextile 7 is laid above the drainage plate 6 area. Specifically, the drainage plate 6 is arranged at the bottom with a certain spacing, and the spacing of the drainage plate 6 is as shown in Figure 3 , usually with a width and spacing of 100mm-500mm. After laying, the plate head of the drainage plate 6 is connected to the vacuum equipment (i.e. air compressor 1) through the drainage pipe 2, as shown in Figure 2 . After arrangement, a layer of filter membrane or geotextile 7 (geotextile in this embodiment) is laid above the drainage plate 6.

[0078] This embodiment adopts horizontal laying of drainage plate 6, as shown in Figure 6 , which is a comparison diagram of vertical and horizontal laying of drainage plate, and uses the device in Figure 6 . The removal rate of heavy metals obtained by experiment (without freeze-thaw) is shown in Table 1.

[0079] Table 1 Removal rate of heavy metals of vertical and horizontal laying of drainage plate

[0080]

[0081] As can be seen from Table 1, the heavy metal removal rate under the action of the bottom horizontal drainage plate 6 is higher than that under the action of the vertical drainage plate (the experimental drainage plate specification is horizontal d = 8 cm, vertical 5.5 cm x 10 cm, and the thickness is 3 mm). The experiment found that the vertical drainage plate has a faster drainage rate, which is not conducive to the full contact of the reagent with the soil.

[0082] In addition, the bottom vacuum leaching can reduce the downward migration of soil layer pollutants and avoid causing secondary pollution.

[0083] (5) Some metal columns perpendicular to the bottom of the foundation pit are arranged to arrange temperature, humidity, pH, and conductivity sensors. The sensors are used to monitor the changes of various parameters in the leaching process in real time, so as to facilitate timely adjustment.

[0084] (6) The contaminated soil is excavated by the excavating equipment, and then the contaminated soft soil is subjected to coarse screening, crushing, and removal of large impurities, etc. In the treatment process, the leaching reagent is mixed with the contaminated soft soil and filled into the foundation pit pre-injected with a certain amount of leaching liquid.

[0085] (7) Artificial freezing has the advantages of controllable freezing temperature, time, and frequency. When artificial freezing is used, artificial freezing pipes should be arranged. The arrangement points of the artificial vertical freezing pipes 8 are as shown in Figure 4 .

[0086] (8) Before formal filling, a certain amount of leaching liquid is continuously injected into the soil pit, and then the soft soil is slowly filled. When the soil layer is thick, the soft soil is filled in layers. When the contaminated soft soil of the upper layer flows to a stable state, the second layer of drainage plates 6 can be selected to be laid. After the laying is completed, the remaining contaminated soft soil is continuously injected.

[0087] (9) The contaminated soft soil is static for a period of time to reach stability, and then a certain amount of leaching liquid is added on the surface of the contaminated soft soil after being appropriately leveled. The distance between the liquid surface of the leaching liquid and the surface of the contaminated soft soil is kept within the range of 2-3 cm.

[0088] (10) Soil freezing. Soil freezing can be performed by using the natural low temperature of the northern site. If the freezing temperature cannot be reached, artificial freezing method can also be used. Appropriate low-temperature weather can be selected for soil remediation to reduce the energy consumption required for freeze-thaw. After a period of static state, the contaminated soft soil is subjected to freeze-thaw cycle treatment by using natural or artificial freezing methods. The natural freezing method has certain uncontrollable factors. If artificial freeze-thaw cycle is used (such as Figure 4 ), the vertical freezing pipe 8 has certain requirements for the arrangement depth. The pipe end of the vertical freezing pipe 8 should be 20-30 cm away from the bottom drainage plate 6. When multiple layers of drainage plates 6 are used, the spacing between the drainage plates 6 except the bottom layer should be reserved for the arrangement of the freezing pipe. The number of times, time length, and temperature parameters are obtained by experimental research. For example, Figure 7As shown, the remediation process of heavy metal contaminated soil under freeze-thaw cycles and bottom vacuum leaching was simulated, 10 cm soil column was used, A1, A2, A3, A4 were freeze-thawed for 0, 2, 4, 6 times, and the initial heavy metal residue (Cu, Zn) was 1000 mg / kg. The remediation effect of heavy metal contaminated soil under freeze-thaw cycles and bottom vacuum leaching was significant, but it did not increase continuously with the increase of freeze-thaw times.

[0089] At the same time, the information parameters of the soil samples after freeze-thaw were combined with the finite element model to guide the engineering practice, including the following processes: considering the reaction of the reagent, the migration of each component, the deformation of the soil body and the seepage rate at the same time. The freeze-thaw times and freeze-thaw temperature will affect the soil permeability and the adsorption and desorption capacity of heavy metals, which can be quantified as model parameters for coupling. Freeze-thaw cycle times, etc. can also be optimized and adjusted according to the model, and the preliminary calculation results need to be verified in combination with the engineering practice experience in similar Shanghai soft soil remediation projects: experience shows that for thicker contaminated soil layer, in order to prevent incomplete freezing, 1-2 times of safety cycle need to be added based on the calculated value. The freezing site, the number of drainage boards and the spacing form will affect the seepage of the liquid, and then affect the entire leaching process. Similarly, the reagent concentration and other parameters will affect the reaction rate of heavy metal pollutants. The model considers the attenuation effect of freeze-thaw times N and soil deformation on the permeability K of the soil body in the subsequent leaching stage (through the N-K relationship curve established by the previous test), which is ignored by the conventional leaching design model. Engineering practice (in the Shanghai soft soil leaching model experiment) shows that relying solely on theoretical calculation may lead to uneven distribution of leaching liquid in actual operation. Therefore, it is necessary to combine experience and adopt a segmented leaching strategy (such as low-flow infiltration first, and then high-flow flushing), and dynamically adjust according to the real-time monitoring of the leaching liquid pollutant concentration. This 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 actually using the model, small-scale model experiments can be used to determine the parameters, and artificial freezing numerical simulation is also needed to predict the freezing site, freezing temperature and freezing time. An example of heavy metal concentration results simulated by the finite element model is shown in Figure 5 .

[0090] (11) After the freeze-thaw cycle treatment is completed, the leaching liquid is added, and the air compressor 1 is used to perform vacuum extraction. During the vacuum extraction process, the appropriate vacuum pressure should be controlled, which is generally above 60 kPa. The settlement, pore water pressure, deep lateral displacement and other data should be monitored, and the corresponding time history curve should be drawn to monitor the change law of each index. The leaching liquid can be extracted from the bottom drainage board 6.

[0091] (12) If multiple layers of drainage plates 6 are used for leaching, the order and duration of vacuum extraction of each layer need to be adjusted to ensure that the leaching liquid can fully wash out the heavy metal pollutants. The specific order is obtained through indoor experiments and verified through laboratory model tests. Using the alternating leaching method of "first bottom, then middle" and "first middle, then bottom", the leaching time is reduced by 20% and 40% respectively, and the heavy metal removal rate is increased by 16.2% and 26.4% respectively.

[0092] (13) Under stable pressure conditions, continuously extract a certain amount of leaching liquid as a reference. For example, when the solid-liquid ratio is 1:2, 1 kg of contaminated soft soil (dry soil) requires 2 L of leaching liquid. During the leaching process, leaching liquid needs to be continuously supplemented above the contaminated soft soil, but not higher than the cofferdam 5.

[0093] (14) After leaching, sample and measure the heavy metal content of the contaminated soil to determine whether it meets the safety indicators for the reuse of soil, i.e., whether the soil heavy metal content meets the standard value and below. The source of the soil heavy metal content standard value is: "Soil Environmental Quality, Construction Site Soil Pollution Risk Control Standards (Trial)", GB 36600-2018, applicable to residential, commercial, industrial and other construction sites. According to "first type of land" and "second type of land", the screening values and control values of heavy metals such as arsenic, cadmium, lead, mercury and nickel are given. If the indicators meet the requirements, the leaching is completed, and the treated contaminated soft soil is obtained. If not, repeat the freeze-thaw cycle and leaching steps, i.e., repeat steps (9)~(13).

[0094] (15) After completion, remove the vacuum drainage system (frozen-thaw pretreatment combined with bottom vacuum leaching remediation of heavy metal contaminated soft soil system), and detect the characteristics of the treated contaminated soft soil, such as gradation, particle change, pH, and mineral composition change, to analyze the effect of contaminated soil remediation.

[0095] (16) Evaluate the soil remediation, risk of recontamination, and put the remediated soil back into use. Clean up the soil remediation site.

[0096] For example, Figures 1-4As shown, the embodiment further provides a system for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum rinse, which is based on site setting and comprises a vacuum device, a drain pipe 2, an impermeable 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 arranged in the site. The vacuum device is connected with the drain board 6. The impermeable barrier wall is arranged at the bottom and around the foundation pit, and is higher than the ground of the site. The drain board 6 is horizontally arranged in the foundation pit. A cofferdam 5 is arranged around the impermeable barrier wall, and a waterproof lining is arranged at the bottom and sidewall of the cofferdam. The vacuum device is connected with the board head of the drain board 6 through the drain pipe 2. The filter membrane or geotextile 7 is arranged above the drain board 6. The vertical freezing pipe 8 is arranged above the drain pipe 2.

[0097] The vacuum device is an air compressor 1.

[0098] The above description of the embodiments is for the purpose of enabling a person of ordinary skill in the art to understand and use the present application. Those skilled in the art can easily make various modifications to the embodiments, and apply the general principles described herein to other embodiments without creative labor. Therefore, the present application is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present application without departing from the scope of the present application should be within the scope of the present application.

Claims

1. A process for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum rinse, characterized in that, The process method comprises the following steps: 1) indoor test analysis is performed on the heavy metal contaminated soft soil to be repaired, the pollution degree, permeability, grading parameters are obtained, a finite element calculation model is constructed, and the vacuum pressure, reagent concentration and treatment thickness are selected; 2) select a leaching site according to the soil properties; 3) excavate a foundation pit in the selected site, set up an impermeable barrier wall, and form a water-impermeable contaminated soft soil storage site; 4) lay drainage boards in the foundation pit, the drainage boards are arranged horizontally, the drainage boards are arranged in single or multiple layers, a filter membrane or a geotextile is arranged above the drainage boards, and the drainage boards are connected with a vacuum device; 5) set up metal columns in the foundation pit which are perpendicular to the bottom of the foundation pit, the metal columns are used to fix temperature, humidity, and sensor probes to monitor the soil state in real time; 6) the heavy metal contaminated soft soil to be repaired is pretreated by crushing and screening, and after removing impurities, the pretreated contaminated soft soil is mixed with the leaching solution to increase the initial moisture content, and the pretreated contaminated soft soil is obtained; 7) inject the leaching solution into the foundation pit, fill the pretreated contaminated soft soil layer by layer, after filling each layer of soil, the contaminated soft soil is left to stand until it reaches a stable state, and then the leaching solution is supplemented to a liquid level of 2-3 cm above the surface of the contaminated soft soil; 8) freeze-thaw cycle treatment is performed on the contaminated soft soil, and the freeze-thaw cycle treatment comprises natural freeze-thaw cycle treatment and / or artificial freeze-thaw cycle treatment; 9) start the vacuum device to create a vacuum, monitor the settlement, pore water pressure and deep displacement changes, and extract the leaching solution through the drainage boards; 10) continue to create a vacuum and supplement the leaching solution, and terminate the leaching when the leaching solution reaches 2-3 times the weight of the soil; 11) determine whether the heavy metal content of the soil reaches or is below the standard value, if not, return to step 8), and if yes, obtain the repaired soil; In step 1), the following process is specifically included: indoor test analysis is performed on the contaminated soft soil to be repaired, the pollution degree, permeability, grading parameters of the contaminated soft soil to be repaired are determined, and a finite element calculation model is constructed in combination with Darcy's law, convection-diffusion equation, elastic consolidation theory and reaction kinetics theory, and the vacuum pressure, reagent concentration and treatment thickness are selected; The double-layer drainage boards are alternately leached, and the "bottom first and then middle" and "middle first and then bottom" alternating leaching methods are used to shorten the leaching time by 20% and 40% respectively, and the heavy metal removal rate is increased by 16.2% and 26.4%.

2. The process for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum rinse according to claim 1, characterized in that, After step 11), the following step 12) is performed: 12) analyze the mineral composition and pollutant residue of the repaired soil obtained in step 11), assess the risk, and transfer the repaired soil to agricultural or engineering land, and clean up the site.

3. The process of claim 1, wherein the freeze-thaw pretreatment combined with bottom vacuum rinse-out remediation of heavy metal contaminated soft soil is characterized by, In step 2), the leaching site is selected from in-situ sites and / or off-site sites, 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 1 m; In step 3), the impermeable barrier wall is arranged at the bottom and around the foundation pit, the impermeable barrier wall is higher than the ground surface of the site, a cofferdam is arranged around, and a waterproof lining is laid on the bottom and side wall.

4. The process of claim 1, wherein the freeze-thaw pretreatment combined with bottom vacuum rinse-out remediation of heavy metal contaminated soft soil is characterized by, In step 4), a porous steel protection device is further arranged above the drainage boards; In step 4), the width and spacing of the drainage boards are both 100-500 mm.

5. The process of claim 1, wherein the freeze-thaw pretreatment combined with bottom vacuum rinse-out remediation of heavy metal contaminated soft soil is characterized by, In step 8), the temperature of the freeze-thaw cycle is -15~ -5℃, and the number of freeze-thaw cycles is 3-5 times. In step 8), when the artificial freeze-thaw cycle is used, the vertical freezing pipe is 20-30 cm away from the bottom of the drainage plate.

6. The process of claim 1, wherein the freeze-thaw pretreatment combined with bottom vacuum rinse-out remediation of heavy metal contaminated soft soil is characterized by, In step 9), draw the time history curves of settlement, pore water pressure and deep displacement during vacuum extraction, and adjust the vacuum pressure according to the curve changes.

7. The process of claim 1, wherein the freeze-thaw pretreatment combined with bottom vacuum rinse-out remediation of heavy metal contaminated soft soil is characterized by, In step 10), the leaching solution is recovered, and the recovered liquid is used for subsequent separation of heavy metals for resource recycling.

8. The process of claim 1, wherein the freeze-thaw pretreatment combined with bottom vacuum rinse-out remediation of heavy metal contaminated soft soil is characterized by, In step 12), the following process is further included: detecting the grading, particle morphology and pH value of the repaired soil obtained in step 11) to evaluate the risk.

9. A system for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum rinse, said system being used for carrying out the process method according to any one of claims 1 to 8, characterized in that, The system comprises a vacuum device, an impermeable barrier wall, a drainage plate (6), and a foundation pit; The foundation pit is arranged in the site; The vacuum device is connected with the drainage plate (6); The impermeable barrier wall is arranged at the bottom and around the foundation pit, and the height of the impermeable barrier wall is higher than that of the ground of the site; The drainage plate (6) is horizontally arranged in the foundation pit.

10. The system for remediation of heavy metal contaminated soft soil by freeze-thaw pretreatment combined with bottom vacuum rinse according to claim 9, characterized in that, The impermeable barrier wall is provided with a cofferdam (5) around, and a waterproof lining is laid at the bottom and the side wall; The system further comprises a drainage pipe (2); the head of the drainage plate (6) is connected with the vacuum device through the drainage pipe (2); The system further comprises a filter membrane or geotextile (7); the filter membrane or geotextile (7) is arranged above the drainage plate (6); The system further comprises a vertical freezing pipe (8), which is arranged above the drainage pipe (2); The vacuum device is an air compressor (1).

Images