A remediation system and method for hexavalent chromium contaminated soil / fractured rock mass

Abstract

This invention belongs to the field of soil and rock mass pollution remediation technology, and discloses a remediation system and method for hexavalent chromium contaminated soil / fractured rock mass, including a heating unit, a temperature monitoring unit, and a ground barrier unit. The heating unit includes an energy supply system and a heating well, with the energy supply system and the heating well sealed together, using heat conduction heating to heat the soil and fractured rock mass in situ. The temperature monitoring unit includes temperature sensors and monitoring wells, with the temperature sensors spaced along the depth direction of the monitoring wells. The temperature monitoring unit enables real-time temperature monitoring and feeds the data back to the energy supply system. The ground barrier unit is a concrete layer that insulates and seals the surface of the remediation area, preventing heat loss and air ingress into the soil. This invention solves the technical problems of difficult mass transfer of reducing agents, unstable remediation effects, and high difficulty in remediating fractured bedrock in traditional in-situ remediation technologies for Cr(VI) pollution.

Description

Technical Field

[0001] This invention relates to the field of soil and rock mass pollution remediation technology, specifically to a remediation system and method for hexavalent chromium contaminated soil / fractured rock mass. Background Technology

[0002] The chromium-containing waste residue and wastewater generated during the development of the chromium chemical industry can easily cause Cr(VI) pollution in soil and rock masses. This pollution problem has become a key and challenging aspect of soil environmental remediation. Cr(VI) is highly water-soluble, highly mobile, and extremely biotoxic. Its pollution depth often exceeds 10 meters. In mountainous areas with thin bedrock layers, Cr(VI) can easily penetrate into the vadose zone fissures of the bedrock, forming deep pollution and significantly increasing the difficulty of remediation.

[0003] Currently, the mainstream remediation strategy for Cr(VI) contaminated soil is to reduce highly toxic Cr(VI) to less toxic and less mobile Cr(III). In-situ chemical reduction technology is widely studied and applied due to its ease of implementation. This technology reduces Cr(VI) by injecting chemical reducing agents such as zero-valent iron, ferrous sulfate, and calcium polysulfide into the contaminated area. However, in practical engineering applications, the remediation effect of in-situ chemical reduction technology is highly uncertain and fails to meet actual remediation needs. The core problem lies in the low mass transfer efficiency of the reducing agent, specifically manifested in:

[0004] (1) The soil structure of the actual contaminated site is highly heterogeneous. The presence of low-permeability clay will greatly hinder the migration of the reducing agent solution, resulting in the agent not being able to uniformly cover the contaminated area.

[0005] (2) When the pollution depth is large, the reducing agent solution is prone to flow downward due to gravity, making it difficult to achieve horizontal transport in the vadose zone, resulting in insufficient agent supply in the deep pollution area;

[0006] (3) The repair of fractured bedrock is extremely difficult. The bedrock itself has low permeability and the distribution of fractures is heterogeneous and irregular. The reducing agent is difficult to flow through all the contaminated fractures, which can easily form repair blind spots and lead to the problem of rebound in repair effect.

[0007] In addition, for Cr(VI) contamination in fractured bedrock, ex-situ remediation technology is extremely difficult to implement due to problems such as high excavation risk, limited site space, and high excavation cost. Summary of the Invention

[0008] The present invention aims to provide a remediation system and method for hexavalent chromium contaminated soil / fractured rock mass, which solves the technical problems of difficult mass transfer of reducing agent, unstable remediation effect and high difficulty in remediating fractured bedrock in traditional in-situ remediation technology for Cr(VI) contamination.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A remediation system for hexavalent chromium contaminated soil / fractured rock mass includes a heating unit, a temperature monitoring unit, and a ground barrier unit;

[0011] The heating unit includes an energy supply system and a heating well, with the energy supply system and the heating well being sealed together; the heating unit uses heat conduction heating to heat the soil and fractured rock mass in situ.

[0012] The temperature monitoring unit includes temperature sensors and monitoring wells. The temperature sensors are spaced apart along the depth direction of the monitoring wells. The monitoring wells include heating wells, injection wells, and extraction wells. The temperature monitoring unit realizes real-time temperature monitoring and feeds the data back to the power supply system.

[0013] The ground barrier unit is a concrete layer used to insulate and seal the surface of the repair area, preventing heat loss and air from entering the soil.

[0014] Furthermore, the depth of the heating well exceeds the Cr(VI) contamination depth by 1 meter; the heat conduction heating method of the energy supply system includes electric heat conduction heating and gas heat conduction heating, and the energy supply system is equipped with a control terminal, which is used to dynamically adjust the output power according to the monitoring data of the temperature sensor.

[0015] Furthermore, one temperature sensor is installed every 1 to 2 meters along the depth direction of the monitoring well, and the temperature sensor is used to transmit monitoring data to the power supply system control terminal in real time.

[0016] Furthermore, the area of ​​the ground barrier unit is 1-2m larger than the perimeter of the repair area, the thickness of the concrete layer is 10-60cm, and the barrier unit controls the surface temperature to not exceed 60℃.

[0017] An in-situ remediation method for Cr(VI) contaminated soil / fractured rock mass based on the above-described remediation system includes the following steps:

[0018] S1. Site survey and engineering layout:

[0019] Based on the site survey results, the remediation area and depth were determined, and heating wells and monitoring wells were installed. The depth of the heating wells exceeded the Cr(VI) contamination depth by 1 meter. Temperature sensors were installed in the monitoring wells and connected to the power supply system. A concrete layer was laid on the ground of the remediation area.

[0020] S2, thermal conduction heating and Cr(VI) self-decomposition and reduction

[0021] The power supply system heats the Cr(VI) contaminated soil / fractured bedrock through heating wells. The output power of the power supply system is adjusted according to the data fed back by the temperature monitoring unit to ensure that the temperature of the remediation area is not lower than 500℃ and to maintain this heat treatment temperature for not less than 2 hours, thereby triggering the self-decomposition and reduction of Cr(VI) to Cr(III) by the soil and mineral catalysis.

[0022] S3, Cooling and Post-Monitoring

[0023] After heat treatment, the power supply system is turned off, and the area is allowed to cool naturally to ambient temperature. Samples are then taken from the repaired area for testing. If the samples meet the standards, the well is sealed to complete the repair. If the samples do not meet the standards, step S2 is repeated.

[0024] Furthermore, in step S1, the site investigation includes hydrogeological investigation, geotechnical engineering investigation and pollution status investigation to determine the range, depth, concentration of Cr(VI) pollution and the thermal conductivity and fissure distribution characteristics of the soil and rock.

[0025] The principles and beneficial effects of the technical solution are as follows:

[0026] This invention fundamentally solves the problem of mass transfer of reducing agents: it abandons the use of chemical reducing agents and uses thermal conduction to transfer heat. The heat transfer effect in the soil and rock media is good and is not affected by the heterogeneity of the soil layer, low-permeability clay, or strong heterogeneity of the fractured bedrock. It can uniformly cover deep contaminated areas and fractured blind areas, with no dead corners in the repair, and the repair effect is stable and thorough.

[0027] High repair efficiency and short cycle: In an oxygen-deficient environment above 500℃, rock and soil minerals catalyze the rapid self-decomposition and reduction reaction of Cr(VI), and only more than 2 hours of heat treatment is required to achieve efficient repair. Compared with the long-term chemical reaction of traditional chemical reduction technology, the repair cycle is greatly shortened and the repair efficiency is significantly improved.

[0028] The system is highly controllable and easy to operate: the temperature monitoring unit enables real-time, multi-point, and all-round monitoring of the temperature in the repair area, and the power supply system can dynamically adjust the power according to the monitoring data to ensure precise control of the reaction temperature; each unit has a reasonable structural design, flexible layout, and clear operation process, requiring no complicated construction technology and facilitating practical engineering applications.

[0029] Wide range of applications and adaptable to complex sites: Heating wells can be flexibly arranged vertically or horizontally, and are suitable for the remediation of Cr(VI) pollution of different depths and site conditions. They are especially suitable for complex Cr(VI) contaminated sites that are difficult to remediate by traditional technologies, such as bedrock mountainous areas, fractured bedrock, and deep vadose zones. The application prospects are broad.

[0030] The project has controllable costs and good economic efficiency: it does not require the investment of a large amount of chemical reducing agents, which greatly reduces the cost of reagents; in-situ repair does not require excavation, avoiding the high excavation and transportation costs of ex-situ repair; the heating equipment is all conventional engineering equipment, which is easy to obtain and maintain, and the overall project cost is controllable. Attached Figure Description

[0031] Figure 1 A schematic diagram of an in-situ remediation system for Cr(VI) contaminated soil / fractured rock mass provided by the present invention;

[0032] Figure 2 The reduction effect of Cr(VI) in contaminated soil under different temperature treatment conditions;

[0033] Figure 3 The reduction effect of Cr(VI) in organic-contaminated soil under different temperature treatment conditions was investigated.

[0034] The names of the corresponding labels in the attached diagram are:

[0035] 1. Power supply system; 2. Heating well; 3. Monitoring well; 4. Ground isolation unit; 5. Temperature sensor. Detailed Implementation

[0036] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments:

[0037] Currently, chromium-containing waste residues and wastewater generated during the development of the chromium chemical industry easily cause Cr(VI) pollution in soil and rock masses, making related pollution issues a key and challenging aspect of soil environmental remediation. Cr(VI) is highly water-soluble, highly mobile, and extremely biotoxic, with its pollution depth often exceeding 10 meters. In mountainous areas with thin bedrock layers, Cr(VI) easily penetrates into the vadose zone fissures of the bedrock, forming deep pollution and significantly increasing the difficulty of remediation. During research on in-situ thermal remediation of contaminated soil, a novel Cr(VI) reduction phenomenon was discovered: when Cr(VI) is adsorbed on the surface of clay minerals, under an oxygen-deficient environment of around 500℃, Cr(VI) can undergo a self-decomposition and reduction reaction to generate Cr(III). Unlike the mass transfer of chemical reducing agents, heat transfer in soil and rock media is effective and is not affected by soil heterogeneity or bedrock fissure distribution. It can uniformly cover deep and fissure contaminated areas. Moreover, the deep underground layer itself is an oxygen-deficient environment. This discovery provides a new approach for the in-situ remediation of Cr(VI) contaminated soil / fissure rock mass.

[0038] The core principle of this invention is the high-temperature, anaerobic self-decomposition and reduction of Cr(VI) under the catalysis of soil and rock minerals: In an anaerobic environment above 500℃, Cr(VI) adsorbed on the surface of clay minerals and soil and rock minerals undergoes a self-redox decomposition reaction under the catalysis of soil and rock minerals. The highly toxic Cr(VI) is reduced to the less toxic and less mobile Cr(III) (Equation 1). The reaction process does not require the participation of external chemical reducing agents, fundamentally solving the problem of mass transfer of reducing agents. At the same time, the heat conduction heating method can achieve uniform and efficient heat transfer in the soil and rock medium, unaffected by the heterogeneity of soil layers and the strong heterogeneity of fractured bedrock, and can cover deep pollution and fractured blind areas.

[0039]

[0040] Based on the above principles, this invention provides an in-situ remediation system for Cr(VI) contaminated soil / fractured rock mass, as described above. Figure 1 The system is an integrated collaborative system consisting of a heating unit, a temperature monitoring unit, and a ground barrier unit. Each unit complements the other's functions and works together to ensure that the repair process is efficient, stable, and controllable.

[0041] The heating unit includes an energy supply system 1 and a heating well 2. The energy supply system 1 and the heating well 2 are connected by a sealed pipeline to provide a stable heat source for the contaminated area. The energy supply system 1 adopts a heat conduction heating method, specifically including two forms: electric heat conduction heating and gas heat conduction heating, which can be flexibly selected according to the actual site conditions. The heating well 2 can be arranged vertically or horizontally, and the depth of the well exceeds the Cr (VI) contamination depth by 1 meter to ensure that the heat covers the entire contaminated area. The layout density of the heating well 2 is adjusted according to the thermal conductivity of the site's soil and rock. The layout spacing is appropriately reduced in areas with low thermal conductivity to ensure uniform heat distribution.

[0042] The temperature monitoring unit includes a temperature sensor 5 and a monitoring well 3. The temperature sensor 5 is a high-precision temperature sensor, which is deployed at multiple points along the depth direction of the monitoring well 3 (one sensor every 1 to 2 meters) to realize real-time temperature monitoring of polluted areas at different depths. The monitoring well 3 covers the heating well 2, the injection well, and the extraction well, realizing comprehensive and blind-spot-free temperature monitoring of the remediation area. The monitoring data of the temperature sensor 5 is transmitted to the control terminal of the power supply system 1 in real time, providing data basis for the power adjustment of the power supply system 1.

[0043] Ground barrier unit 4 is a concrete layer. The area of ​​the barrier layer is 1-2m larger than the perimeter of the repair area. It has good heat insulation, seepage prevention and sealing performance, preventing outside air from entering the polluted area and preventing groundwater from seeping in. The thickness of the concrete layer is 10-60cm, which effectively prevents heat loss during the repair process and controls the surface temperature to not exceed 60℃, so as to avoid impacting the surrounding environment and people.

[0044] Based on the above remediation system, the present invention also provides an in-situ remediation method for Cr(VI) contaminated soil / fractured rock mass, specifically including the following steps:

[0045] Step S1: Site survey and engineering layout

[0046] Conduct site hydrogeological, geotechnical engineering and pollution status investigations, and determine the extent, depth and concentration of Cr(VI) pollution area, as well as parameters such as thermal conductivity and fracture distribution characteristics of soil and rock through borehole sampling and testing analysis;

[0047] Based on the investigation results, the remediation area was delineated, and the layout of heating well 2 and monitoring well 3 was optimized. The depth of all wells exceeded the Cr (VI) contamination depth by 1 meter to ensure coverage of the entire contaminated area.

[0048] Temperature sensors 5 are installed at multiple points along the depth direction inside monitoring well 3, and the temperature sensors 5 are connected to the control terminal of power supply system 1.

[0049] In the repair area, an impermeable layer and a concrete layer are laid sequentially from bottom to top on the ground surface to complete the layout of ground barrier unit 4 and ensure a sealing effect.

[0050] Step S2: Heat conduction heating and Cr(VI) self-decomposition reduction

[0051] Start the power supply system 1 and heat the Cr(VI) contaminated soil / fractured rock mass in situ through heating well 2. Select the electric heat conduction or gas heat conduction method according to the site soil and rock type.

[0052] The temperature monitoring unit collects temperature data at different depths in the repair area in real time and feeds the data back to the control terminal of the power supply system 1 to dynamically adjust the output power of the power supply system 1 to ensure that the temperature of the repair area is not lower than 500℃.

[0053] Maintaining a heat treatment temperature above 500℃ for no less than 2 hours, under these conditions, Cr(VI) adsorbed on the surface of rock and soil minerals undergoes a self-decomposition and reduction reaction under the catalysis of minerals, transforming into Cr(III), thus achieving pollution remediation.

[0054] Step S3: Cooling and Post-Monitoring

[0055] After heat treatment, power supply system 1 is turned off, and borehole sampling is performed on the repaired area to analyze the concentration of Cr(VI) and Cr(III) and verify the repair effect; if some areas do not meet the repair standard, steps S2 and S3 are repeated for secondary repair.

[0056] After the repair is completed and the required standards are met, allow the repaired area to cool down naturally to ambient temperature, remove ground barrier unit 4, seal all types of wells, and complete the repair project.

[0057] To better understand the above repair methods and systems, the following will provide a detailed explanation of the solutions in conjunction with laboratory research and specific implementation examples:

[0058] Example 1

[0059] Laboratory research results:

[0060] A study was conducted in the laboratory on the heat treatment of Cr(VI) contaminated soil under anoxic and aerated conditions. The actual concentration of Cr(VI) contaminated soil was 169 mg / kg. 10 g of Cr(VI) contaminated soil was placed in a tubular furnace, and nitrogen gas was introduced to create an anoxic environment. The heat treatment time was 2 hours. Figure 2 As shown, the reduction rate of Cr(VI) increases with increasing heat treatment temperature. The reduction rate of Cr(VI) under anoxic conditions is significantly higher than that under air conditions. When the temperature is 500 degrees, the reduction rate of Cr(VI) under anoxic conditions reaches 95%, and the residual Cr(VI) concentration after treatment reaches the screening value for Class II land use, while the reduction rate of Cr(VI) under air conditions is only 40%.

[0061] Considering that organic matter in the soil can reduce Cr(VI) under high temperature conditions, clean soil was first treated at 800 degrees Celsius for 2 hours to completely remove the reducing organic matter. Further heat treatment experiments were conducted using artificially prepared Cr(VI) contaminated soil with a concentration of 150 mg / kg. Figure 3 As shown, after removing organic matter, the reduction rate of Cr(VI) remained at 80% under anaerobic conditions at 500 degrees Celsius; however, the reduction rate decreased significantly under air conditions, and the Cr(VI) concentration in the soil actually increased when the temperature reached 500 degrees Celsius. This may be because under high-temperature air conditions, Cr(III) is oxidized to Cr(VI). The experimental results indicate that soil minerals efficiently catalyze the decomposition and reduction of Cr(VI) under high-temperature and anaerobic conditions.

[0062] Implement column 2

[0063] Actual project implementation plan:

[0064] A Cr(VI) contaminated site is located near an electroplating plant in a bedrock mountainous area. The soil layer is approximately 5m thick, beneath which lies fractured limestone bedrock. The Cr(VI) contamination depth reaches 12m. The average Cr(VI) concentration in the soil and fractured bedrock is 85mg / kg, and the thermal conductivity of the soil and rock is 1.2W / (m²). Traditional chemical reduction techniques cannot achieve effective repair due to difficulties in mass transfer within fractured bedrock. The repair system and method of this invention are used for in-situ repair, with the following specific steps:

[0065] Project layout: The remediation area is designated to be approximately 500㎡. 20 heating wells are vertically installed, with a depth of 13m (1m beyond the contamination depth) and a spacing of 4m between them. 10 monitoring wells are installed, with one temperature sensor installed every 2 meters in each well. A 30cm thick concrete layer is laid on the ground surface.

[0066] Heat conduction heating: Gas-fired heat conduction heating is used. The power supply system is started and the power is adjusted according to the temperature sensor monitoring data to ensure that the temperature of the repair area is stable at 500℃ and maintained at this temperature for 2.5 hours.

[0067] Post-treatment testing: The power supply system was shut off and the temperature was allowed to drop naturally to ambient temperature. Samples were taken from boreholes at different depths in the remediation area. The test results showed that the Cr(VI) concentration in the soil and fractured bedrock was reduced to below 5 mg / kg, which met the screening value for Class II land use in the "Soil Environmental Quality Standard for Construction Land Soil Pollution Risk Control (Trial)" (GB 36600-2018). The remediation efficiency was over 95%, the aquifer permeability did not change significantly, and there was no secondary pollution.

[0068] The in-situ remediation system and method for Cr(VI) contaminated soil / fractured rock mass of this invention eliminates the use of traditional chemical reducing agents, fundamentally solving the problem of mass transfer of reducing agents under complex site conditions. It boasts significant advantages such as high remediation efficiency, stable effects, environmental friendliness, wide applicability, system controllability, and ease of operation. This technology is adaptable to the remediation of Cr(VI) contamination at different depths and site types, and is particularly suitable for complex Cr(VI) contaminated sites that are difficult to remediate using traditional technologies, such as bedrock mountainous areas, fractured bedrock, and low-permeability clay areas. It can be widely applied to Cr(VI) contaminated soil / fractured rock mass remediation projects in industries such as chromium salts, electroplating, and leather tanning.

[0069] The equipment used in this invention is all conventional engineering equipment, which is easy to obtain and maintain, allows for flexible engineering deployment, simple construction processes, and controllable overall engineering costs, facilitating large-scale industrial promotion and application. This invention provides a technically feasible, efficient, and sustainable new approach for the in-situ remediation of Cr(VI) contaminated sites, and has significant practical implications and application value for soil environmental remediation, ecological environmental protection, and the green development of the chromium chemical industry in my country.

[0070] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific technical solutions or characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A hexavalent chromium contaminated soil / fissured rock mass remediation system, characterized in that, Includes heating unit, temperature monitoring unit, and ground barrier unit (4); The heating unit includes an energy supply system (1) and a heating well (2), with the energy supply system (1) and the heating well (2) being sealed together; the heating unit uses heat conduction heating to heat the soil and fractured rock mass in situ; The temperature monitoring unit includes a temperature sensor (5) and a monitoring well (3). The temperature sensor (5) is arranged at intervals along the depth direction of the monitoring well (3). The temperature monitoring unit realizes real-time temperature monitoring and feeds the data back to the power supply system (1). The ground barrier unit (4) is a concrete layer used to insulate and seal the surface of the repair area to prevent heat loss and air from entering the soil layer.

2. A hexavalent chromium contaminated soil / fractured rock remediation system according to claim 1, characterized in that: The depth of the heating well (2) exceeds the Cr (VI) contamination depth by 1 meter; the heat conduction heating method of the power supply system (1) includes electric heat conduction heating and gas heat conduction heating, and the power supply system (1) is equipped with a control terminal, which is used to dynamically adjust the output power according to the monitoring data of the temperature sensor (5).

3. The hexavalent chromium contaminated soil / fractured bedrock remediation system of claim 1, wherein: The temperature sensor (5) is installed every 1 to 2 meters along the depth direction of the monitoring well (3). The temperature sensor (5) is used to monitor data and transmit it to the control terminal of the power supply system (1) in real time.

4. The remediation system for hexavalent chromium contaminated soil / fractured rock mass according to claim 1, characterized in that: The area of ​​the ground barrier unit (4) is 1-2m larger than the periphery of the repair area, the thickness of the concrete layer is 10-60cm, and the barrier unit controls the surface temperature to be no higher than 60℃.

5. A method for in-situ remediation of Cr(VI) contaminated soil / fractured rock mass based on the remediation system according to any one of claims 1-4, characterized in that, Includes the following steps: S1. Site survey and engineering layout: Based on the site survey results, the remediation area and depth were determined, and heating wells (2) and monitoring wells (3) were installed. The depth of heating well (2) exceeded the Cr (VI) contamination depth by 1 meter. Temperature sensor (5) was installed in monitoring well (3) and connected to power supply system (1). A concrete layer was laid on the ground of the remediation area. S2, thermal conduction heating and Cr(VI) self-decomposition and reduction The power supply system (1) heats the Cr(VI) contaminated soil / fractured bedrock through the heating well (2). The output power of the power supply system (1) is adjusted by the data fed back by the temperature monitoring unit to ensure that the temperature of the remediation area is not lower than 500℃ and to maintain the heat treatment temperature for not less than 2 hours, thereby triggering the self-decomposition and reduction of Cr(VI) to Cr(III) by the soil and mineral catalysis. S3, Cooling and Post-Monitoring After heat treatment, shut down the power supply system (1), allow it to cool naturally to ambient temperature, and then take samples from the repaired area for testing. If the sample meets the standard, seal the well to complete the repair. If the sample does not meet the standard, repeat step S2.

6. The in-situ remediation method for Cr(VI) contaminated soil / fractured rock mass according to claim 5, characterized in that, In step S1, the site investigation includes hydrogeological investigation, geotechnical engineering investigation and pollution status investigation to determine the range, depth and concentration of Cr (VI) pollution, as well as the thermal conductivity of the soil and rock and the characteristics of crack distribution.

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