System and method for in-situ remediation of hexavalent chromium polluted low-permeability clay layer through heat treatment reinforced butanediol

Abstract

The invention discloses a system and method for in-situ remediation of a hexavalent chromium polluted low-permeability clay layer through heat treatment reinforced butanediol, the system comprises a resistance heating unit, a butanediol mixed steam injection unit, a monitoring unit and a ground barrier unit, and through the synergistic effect of in-situ resistance heating and high-temperature and high-pressure butanediol mixed steam injection, the hexavalent chromium polluted low-permeability clay layer is subjected to in-situ remediation; the permeability of the low-permeability clay and the mass transfer efficiency of the reducing agent are improved, the kinetic level of the reduction reaction is improved, efficient and thorough in-situ remediation of the Cr (VI polluted low-permeability clay layer is achieved, and the remediation problem of the traditional technology is solved. The method is especially suitable for in-situ treatment of sites polluted by fine-grained soil Cr (VI, such as clay, silty clay and the like, with pollution depth exceeding 10 meters and strong soil layer heterogeneity.

Description

Technical Field

[0001] This invention relates to the field of contaminated soil remediation technology, specifically to a system and method for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using heat-treated and enhanced butanediol. Background Technology

[0002] Hexavalent chromium (Cr(VI)) possesses extremely high biotoxicity and environmental mobility, and is classified as a human carcinogen. Soil contaminated with Cr(VI) poses a serious threat to the ecological environment and human health. As the world's largest producer of chromium chemicals and a core region for the leather and electroplating industries, China faces particularly prominent Cr(VI) contamination problems in its soil sites. Furthermore, chromium-contaminated sites generally exhibit wide-ranging and deep contamination, with most exceeding 10 meters in depth. For deep soil contamination, ex-situ remediation is extremely difficult due to the large amount of excavation work, high costs, and significant environmental risks. Therefore, in-situ remediation technology has become the mainstream choice for treating Cr(VI) contaminated soil.

[0003] In-situ chemical reduction is currently the most commonly used remediation technology for Cr(VI) contaminated soil. It involves injecting reducing agents such as zero-valent iron, ferrous sulfate, and calcium polysulfide into the contaminated area to reduce highly toxic Cr(VI) to less toxic and less mobile Cr(III). However, this technology faces insurmountable technical bottlenecks when applied to low-permeability clay layers, often resulting in remediation effects that fall short of expectations. The core issues are as follows:

[0004] (1) The mass transfer efficiency of the reducing agent is extremely low: Low-permeability clay has the characteristics of small soil particles, large specific surface area, low porosity and slow pore water seepage speed. Liquid or solid reducing agents have high resistance to transmission in clay pores, making it difficult to uniformly cover the contaminated area and easily forming remediation blind spots.

[0005] (2) Pollution “tailing” and “rebound” are prone to occur: Unrepaired Cr (VI) in low-permeability clay layers will continue to dissolve and diffuse to adjacent aquifers, causing the pollutant concentration to decrease slowly in the later stage of remediation but failing to meet the standard (tailing), or the pollutant concentration will rise again after remediation is stopped (rebound), making it impossible to achieve complete remediation.

[0006] (3) Traditional reducing agents are difficult to gasify: Existing commonly used reducing agents (inorganic materials, humic acid, organic acid, etc.) have high melting and boiling points and cannot be injected in gaseous form. The mass transfer resistance of gaseous reducing agents is much lower than that of liquid / solid, which is the optimal form to improve the mass transfer efficiency of low-permeability formations. At present, there is a lack of suitable low-boiling-point, high-reducing gaseous reducing agents.

[0007] (4) Low kinetic efficiency of reduction reaction: Although low-boiling-point small molecule organic compounds such as alcohols can be gasified, their reducing power is weak. Under normal conditions, catalysts or photocatalysis are required to enhance them, but such enhancement methods cannot be achieved in the soil in-situ remediation scenario.

[0008] In-situ thermal treatment (in-situ thermal desorption) is a mature remediation technology for organically contaminated sites. It achieves remediation goals by heating the soil layer in situ. Resistance heating technology, due to the high conductivity of clay, can achieve precise and efficient heating of the clay layer. The applicant's research found that when low-permeability clay is heated, larger pores are formed inside the clay, and adsorbed water is converted into pore water. Increased water flow significantly improves clay permeability. Simultaneously, increasing the temperature effectively improves the reduction kinetic efficiency of alcohol reducing agents, achieving efficient reduction without a catalyst. Currently, there are no reports on combining in-situ thermal treatment technology with gaseous alcohol reducing agents for the remediation of Cr(VI) contaminated low-permeability clay layers. There is an urgent need to develop a synergistic technology to overcome existing remediation bottlenecks. Summary of the Invention

[0009] To address the aforementioned technical problems, the present invention aims to provide a system and method for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using heat-treated and enhanced butanediol. Through the synergistic effect of in-situ resistance heating and high-temperature, high-pressure butanediol mixed steam injection, the permeability of the low-permeability clay and the mass transfer efficiency of the reducing agent are improved, enhancing the kinetics of the reduction reaction. This achieves efficient and thorough in-situ remediation of Cr(VI)-contaminated low-permeability clay layers, solving the remediation challenges of traditional techniques. It is particularly suitable for in-situ remediation of Cr(VI)-contaminated sites in fine-grained soils such as clay and silty clay with contamination depths exceeding 10 meters and high soil heterogeneity.

[0010] To achieve the above objectives, the present invention provides a system for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using heat-treated and enhanced butanediol, characterized in that it comprises a resistance heating unit, a butanediol mixed vapor injection unit, a monitoring unit, and a ground barrier unit.

[0011] The resistance heating unit includes a heating resistor, a resistance heating well dug underground, and a three-phase AC power system that provides power to the resistor. The heating resistor is located inside the heating resistor well and is sealed and electrically connected to the three-phase AC power system. The clay layer is heated by resistance heating.

[0012] The butanediol steam injection unit includes a butanediol solution tank and an injection well. The butanediol solution tank contains butanediol solution and is connected to the inlet of a steam boiler. The steam outlet pipeline of the steam boiler is connected to a superheater, and the outlet pipeline of the superheater is connected to the injection well. The injection well opening is sealed.

[0013] The monitoring unit includes a monitoring well, which is equipped with a thermocouple and a pressure detector.

[0014] The ground barrier unit covers the soil surface and includes an equipotential layer and a concrete layer arranged sequentially from bottom to top.

[0015] The three-phase AC power supply system is connected to the heating resistor well. This is existing technology and will not be elaborated here. The three-phase AC current generates Joule heat through the clay, which can accurately and uniformly heat the contaminated clay layer. The heating temperature is controllable, which creates conditions for improving the clay permeability and subsequent reduction reaction.

[0016] The injection well is located in the middle of the adjacent resistance heating well, with the same depth as the resistance heating well. The resistance heating well and the injection well cover the entire contaminated clay layer, ensuring that the gaseous butanediol vapor can be evenly diffused into the heated clay area.

[0017] Monitoring wells are deployed between the resistance heating wells and injection wells in the remediation area, with the well depth covering the contaminated clay layer. Thermocouples and pressure detectors are deployed at multiple points along the depth direction of the monitoring wells to monitor the temperature of the contaminated clay layer and the pressure of the injected steam in real time. The monitoring data is transmitted to the system control terminal in real time, providing accurate data support for the temperature regulation of resistance heating, the injection rate of butanediol mixed steam, and the pressure regulation, ensuring that the parameters of the remediation process are controllable.

[0018] The monitoring data from the thermocouples and pressure detectors are transmitted to the system control terminal in real time, providing data support for the regulation of resistance heating power and steam injection rate. This is an existing method.

[0019] An equipotential bonding layer and a concrete layer are laid sequentially from bottom to top on the surface of the repair area. The equipotential bonding layer is used to eliminate stray currents on the ground, prevent the risk of electric shock, and ensure the safety of construction personnel and equipment. The concrete layer has good heat insulation and sealing properties, which can effectively prevent heat loss from the repair area, maintain the stability of the clay temperature, and control the surface temperature to not exceed 60°C, avoiding thermal impact on the surrounding environment.

[0020] In the above scheme: injection wells and heating resistor wells are arranged at intervals of 3-5m, with the injection wells located between adjacent heating resistor wells, and monitoring wells are arranged at intervals of 6-10m. The layout of all wells covers the entire contaminated clay layer.

[0021] In the above scheme, the depth of the resistance heating well, injection well and monitoring well all exceeds the Cr(VI) contamination depth by 1 meter.

[0022] In the above scheme: butanediol is 1,2-butanediol, the mass concentration of 1,2-butanediol aqueous solution in the butanediol solution tank is 20%-30%, and the superheater heats the butanediol mixed vapor to above 150°C.

[0023] In the above scheme: 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℃.

[0024] A method for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using heat-treated butanediol, comprising the following steps:

[0025] (1) Based on the site investigation results, determine the remediation area and depth of Cr(VI) contaminated clay. According to the system for in-situ remediation of hexavalent chromium contaminated low-permeability clay layer with heat treatment enhanced butanediol, install monitoring wells, resistance heating wells and injection wells under the ground, and install heating resistors, thermocouples and pressure detectors. Lay an equipotential layer and a concrete layer on the ground of the remediation area in sequence.

[0026] (2) Turn on the three-phase AC power supply system, use the resistance heating unit to heat the contaminated clay layer, and adjust the heating temperature through the monitoring unit to heat the contaminated clay layer evenly to 100°C and then stop heating.

[0027] (3) Start the butanediol steam injection unit, vaporize the butanediol aqueous solution through the steam boiler and heat it up through the superheater to form high temperature and high pressure butanediol mixed steam and inject it into the contaminated clay layer; control the injection pressure to be no more than 11 kPa / m during the injection process, and stop the butanediol mixed steam injection when the volume of butanediol solution used is twice the volume of soil pores in the remediation area.

[0028] (4) After standing for 24~48 hours, take samples from the repaired area for testing. If the Cr (VI) concentration reaches the standard, seal the well to complete the repair. If it does not reach the standard, repeat steps (2) and (3).

[0029] Step (1) Site investigation includes hydrogeological investigation, geotechnical engineering investigation and pollution status investigation. Through borehole sampling and testing analysis, the extent, depth and concentration of Cr(VI) contaminated low-permeability clay layer, as well as parameters such as clay permeability coefficient and conductivity, are determined.

[0030] In the above scheme, a flow regulating valve is installed on the outlet pipeline of the superheater. By controlling the injection rate of butanediol mixed vapor, precise control of the injection pressure is achieved, avoiding excessive pressure that could damage the clay structure.

[0031] The core principle of this invention is to enhance the synergistic effect of mass transfer and reduction of gaseous butanediol through in-situ resistance heating, thereby achieving efficient remediation of Cr(VI) in low-permeability clay layers through a two-step synergistic process.

[0032] Resistance heating enhances the permeability and reactivity of clay. Utilizing the high conductivity of clay, Joule heating is generated through current conduction between electrodes to precisely heat low-permeability clay layers to 100°C. This expands the pores between clay particles and converts adsorbed water into free-flowing pore water, significantly reducing mass transfer resistance and improving permeability. Furthermore, the increased temperature enhances the kinetic efficiency of the subsequent reduction reaction of butanediol and Cr(VI), compensating for the weak reducibility of butanediol and achieving efficient reduction without the need for additional catalysts.

[0033] High-temperature and high-pressure gaseous butanediol efficient mass transfer and reduction: 1,2-Butanediol with a low boiling point is selected as the reducing agent and prepared into an aqueous solution with a mass concentration of 20%~30%. This solution is vaporized in a steam boiler and heated to 150℃ or above by a superheater to form high-temperature and high-pressure butanediol mixed steam. The mass transfer resistance of gaseous butanediol is much lower than that of liquid / solid. Under the enhancement of high-pressure steam, it can be rapidly and uniformly transported in the heated and permeable clay pores, and fully contacted with Cr(VI). The high-temperature and high-pressure environment further accelerates electron transfer, enabling butanediol to efficiently reduce Cr(VI) to low-toxicity and stable Cr(III), thus achieving complete remediation of pollution.

[0034] Meanwhile, heating the clay to 100°C can prevent the subsequent injection of high-temperature butanediol mixed vapor from condensing, ensuring that butanediol is always transported in gaseous form in the clay, maintaining high mass transfer efficiency, eliminating repair blind spots from the root, and avoiding "tailing" and "rebound" phenomena.

[0035] The injection pressure of butanediol is controlled to be no greater than 11 kPa / m. Precise pressure control is achieved by adjusting the injection rate to avoid excessive pressure from damaging the clay structure.

[0036] After the repairs are completed and the standards are met, all system equipment will be shut down, the resistance heating well, injection well, and monitoring well will be sealed, and excess surface facilities will be removed to complete the repair project.

[0037] Beneficial effects

[0038] Compared with traditional in-situ remediation technologies for Cr(VI) contaminated low-permeability clay layers, this invention has the following significant advantages:

[0039] (1) Synergistic process to break through the mass transfer bottleneck of low-permeability soil layer: In-situ resistance heating is combined with high-temperature and high-pressure gaseous butanediol vapor injection. Heating significantly improves the permeability of clay. The mass transfer resistance of gaseous butanediol is much lower than that of liquid / solid reducing agent. Steam enhancement further realizes efficient and uniform mass transfer in clay pores, and completely solves the mass transfer problem of low-permeability clay layer.

[0040] (2) Eliminate “tailing” and “rebound” from the root: Gaseous butanediol can fully cover the contaminated clay area, leaving no blind spots for remediation. It can completely restore the adsorbed and dissolved Cr(VI) in the clay, avoiding the continuous diffusion of residual Cr(VI), thus solving the “tailing” and “rebound” phenomenon of traditional remediation from the root, and the remediation effect is long-lasting and stable.

[0041] (3) High reduction efficiency without the need for additional enhancement methods: resistance heating and hot steam improve the kinetic efficiency of the reduction reaction, making up for the weak reduction of butanediol. Without the need for additional enhancement methods such as catalysts and photocatalysis, the efficient reduction of Cr(VI) can be achieved, which is suitable for the field conditions of soil in-situ remediation.

[0042] (4) Green and environmentally friendly, with no secondary pollution: 1,2-Butanediol is selected as a reducing agent, which is easily degraded by environmental microorganisms and leaves no residue; no external harmful chemical substances are introduced during the remediation process, and the reaction product is low-toxic Cr(III), which has low migration and will not cause secondary pollution to the surrounding soil and groundwater; there is no harmful gas emission during the resistance heating and steam injection process, which meets the requirements of green and environmentally friendly remediation.

[0043] (5) Wide adaptability and controllable cost: It is suitable for Cr (VI) contaminated sites of fine-grained soil such as low-permeability clay and silty clay with a contamination depth of more than 10 meters and a wide range, and is not affected by the heterogeneity of the soil layer; the dosage of butanediol agent can be accurately calculated according to the soil pore volume, without the need for excessive injection, the energy consumption of resistance heating is controllable, and in-situ remediation does not require excavation, which greatly reduces the engineering cost and has good economic efficiency.

[0044] (6) This invention applies in-situ heat treatment technology to the remediation of chromium-contaminated sites, filling the gap in the application of this technology in the field of chromium pollution remediation, and providing new technical ideas and methods for the remediation of heavy metal pollution in low-permeability strata. Attached Figure Description

[0045] Figure 1 This is a diagram illustrating the reducing power of different alcohols on Cr(VI).

[0046] Figure 2 The graph shows the effect of butanediol concentration on the reduction of Cr(VI).

[0047] Figure 3 An in-situ heat treatment enhanced mixed steam injection experimental device was built for the laboratory to construct a clay column.

[0048] Figure 4 The changes in temperature and pressure at the inlet, middle, and outlet of the soil column are as follows: (a) Butanediol mixed steam was injected into the clay in the soil column without preheating; (b) Butanediol mixed steam was injected into the clay in the soil column after it was heated to 100°C by a heating belt.

[0049] Figure 5 The effect of different steams on the reduction of Cr(VI) in clay.

[0050] Figure 6 This is a schematic diagram of a system for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using butanediol enhanced by heat treatment, as provided by the present invention.

[0051] In the diagram: 1-Butanediol solution tank; 2-Steam boiler; 3-Superheater; 4-Resistant heating well; 5-Butanediol mixed steam injection well; 6-Monitoring well; 7-Resistant heating functional system; 8-Ground barrier unit; 9-Cr (VI) contaminated clay layer. Detailed Implementation

[0052] The present invention will be further described below with reference to embodiments.

[0053] Example 1

[0054] The system for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using heat treatment-enhanced butanediol consists of a resistance heating unit, a butanediol mixed vapor injection unit, a monitoring unit, and a ground barrier unit.

[0055] The resistance heating unit includes a heating resistor, a resistance heating well 4 dug underground, and a three-phase AC power system 7 that provides electrical energy to the resistor. This three-phase AC power system 7 includes a control system to control the heating temperature and heating time of the heating resistor; this is a conventional method and will not be elaborated upon here. The heating resistor is located inside the heating resistor well 4, meaning the wellhead-sealed heating resistor is electrically connected to the three-phase AC power system. It heats the clay layer through resistance heating. The three-phase AC power supply system is connected to the heating resistor well; this is existing technology and will not be elaborated upon here. The three-phase AC current generates Joule heat through the clay, achieving precise and uniform heating of the contaminated clay layer. The heating temperature is controllable, creating conditions for improving clay permeability and subsequent reduction reactions.

[0056] The butanediol vapor injection unit includes a butanediol solution tank 1 and an injection well 5. The butanediol solution tank 1 contains a butanediol solution and is connected to the inlet of a steam boiler 2. It can be connected via a pump to pump the butanediol solution into the steam boiler 2 to generate steam. The steam outlet pipeline of the steam boiler 2 is connected to a superheater 3, and a flow regulating valve is installed on the outlet pipeline of the superheater. The mixed steam of butanediol and water is heated to above 150°C by the superheater. The outlet pipeline of the superheater 3 is connected to the injection well 5, and the wellhead of the injection well 5 is sealed. The butanediol is 1,2-butanediol, and the 1,2-butanediol aqueous solution in the butanediol solution tank has a mass concentration of 20%-30%. The superheater raises the temperature of the butanediol mixed steam to above 150°C.

[0057] The monitoring unit includes monitoring well 6, which is equipped with thermocouples and pressure detectors (not shown in the figure). These thermocouples and pressure detectors are deployed at multiple points along the depth of the monitoring well to monitor the temperature of the contaminated clay layer and the pressure of the injected steam in real time. The monitoring data is transmitted to the system control terminal in real time, providing precise data support for temperature control of resistance heating, injection rate of butanediol mixed steam, and pressure control, ensuring that the parameters of the remediation process are controllable. This control method is existing technology and will not be described in detail here.

[0058] Injection wells and heating resistor wells are arranged at intervals of 3-5m, with the injection wells located between adjacent heating resistor wells, and monitoring wells are arranged at intervals of 6-10m.

[0059] The depth of the resistance heating well, injection well, and monitoring well all exceeds the Cr(VI) contamination depth by 1 meter. All three wells are located underground, with a width of approximately 10cm, ensuring convenient installation of heating resistors, thermocouples, etc.

[0060] Ground barrier unit 8 covers the soil surface and includes an equipotential bonding layer and a concrete layer, arranged sequentially from bottom to top. The area of ​​the ground barrier unit is 1-2m larger than the perimeter of the repair area, and the concrete layer is 10-60cm thick. The equipotential bonding layer and the concrete layer are laid sequentially on the surface of the repair area from bottom to top. The equipotential bonding layer is used to eliminate stray currents on the ground, prevent the risk of electric shock, and ensure the safety of construction personnel and equipment; this is existing technology and a conventional method. The concrete layer has good heat insulation and sealing properties, which can effectively prevent heat loss from the repair area, maintain stable clay temperature, and control the surface temperature to not exceed 60℃, avoiding thermal impact on the surrounding environment.

[0061] The repair includes the following steps:

[0062] (1) Based on the site investigation results, determine the remediation area and depth of Cr(VI) contaminated clay. According to the system for in-situ remediation of hexavalent chromium contaminated low-permeability clay layer with heat treatment enhanced butanediol, install monitoring wells, resistance heating wells and injection wells under the ground, and install heating resistors, thermocouples and pressure detectors. Then, lay an equipotential layer and a concrete layer on the ground of the remediation area in sequence.

[0063] (2) Turn on the three-phase AC power supply system, use the resistance heating unit to heat the contaminated clay layer, and adjust the heating temperature through the monitoring unit to heat the contaminated clay layer evenly to 100°C and then stop heating.

[0064] (3) Start the butanediol steam injection unit, vaporize the butanediol aqueous solution through the steam boiler and heat it up through the superheater to form high temperature and high pressure butanediol mixed steam and inject it into the contaminated clay layer; control the injection pressure to be no more than 11 kPa / m during the injection process, and stop the butanediol mixed steam injection when the volume of butanediol solution used is twice the volume of soil pores in the remediation area.

[0065] (4) After standing for 24~48 hours, take samples from the repaired area for testing. If the Cr (VI) concentration reaches the standard, seal the well to complete the repair. If it does not reach the standard, repeat steps (2) and (3).

[0066] Experimental Example 1

[0067] Laboratory research

[0068] First, the reducing power of different alcohol solutions on Cr(VI) at 95°C was investigated. 4 ml of different alcohol solutions (methanol, ethanol, n-butanol, ethylene glycol, 1,2-propanediol, and 1,2-butanediol) were added to 20 ml of a 20 mg / L Cr(VI) solution. After heating at 95°C in the dark for 2 hours, the solutions were cooled to room temperature, and the residual Cr(VI) concentration was measured. The results are as follows: Figure 1 As shown, monohydric alcohols have a weak reducing ability for hexavalent chromium, and the longer the carbon chain, the better the reduction effect. n-Propanol reduced 10% of Cr(VI). Dihydric alcohols have a significantly higher reducing ability than monohydric alcohols, and the longer the carbon chain, the better the reduction effect. More than 90% of Cr(VI) was reduced by 1,2-butanediol.

[0069] Furthermore, we investigated the effect of 1,2-butanediol concentration on the reduction of 1,2-butanediol. The results are as follows... Figure 2 As shown, the reducing power increases significantly with increasing 1,2-butanediol concentration. At a butanediol concentration of 0%, the reduction of hexavalent chromium is almost zero, while at a butanediol concentration of 20 wt%, more than 85% of the hexavalent chromium is reduced.

[0070] Experimental Example 2

[0071] Laboratory research results:

[0072] Laboratory equipment such as Figure 3As shown, a soil column made of polytetrafluoroethylene (PTFE) with an inner diameter of 5 cm and a length of 15 cm was set up, containing 300 g of soil at a time. A heating element was wrapped around the soil column to simulate resistance heating, and the temperature of the heating element was controlled by a temperature controller. A mixture of alcohol and water (placed in the conical flask shown in the diagram) was pumped to a steam generator. The alcohol solution generated steam through the steam generator and entered the soil column from the top. Three thermocouples were inserted at the top, middle, and bottom of the soil column to monitor the temperature. Two pressure gauges were used at the top and bottom of the soil column to monitor pressure changes. The steam outlet at the tail (bottom) of the soil column was connected to a condensate collection beaker via a condenser tube, which was circulated with cooling water. The steam and water exiting the tail of the soil column were cooled and then entered the collection beaker. The concentration of residual hexavalent chromium in the soil was measured after the reaction.

[0073] like Figure 4 As shown in Figure a, when butanediol mixed steam was directly injected into the soil column, the pressure and temperature at the inlet end of the soil column rose rapidly, reaching 2 kPa and 140°C respectively after 60 minutes. For safety reasons, the injection of butanediol mixed steam was stopped. At this point, the temperatures at the middle and outlet of the soil column were only 80°C and 40°C respectively, indicating that under these conditions, butanediol mixed steam was difficult to transport in the clay, and the accumulation of mixed steam at the injection port increased the inlet pressure and temperature. Figure 4 As shown in Figure b, after heating the clay to 100°C using a heating belt, and then injecting butanediol mixed steam, the pressure at the inlet only rose to 0.1 kPa, a 20-fold decrease compared to the pressure before preheating to 100°C. Simultaneously, the temperature in the middle of the soil column began to rise significantly after maintaining 100°C for 70 minutes, indicating that the PG mixed steam had reached the middle section. Since the clay had already been heated to 100°C, the temperature at the tail of the soil column reached 100°C 5 minutes after the injection of the PG mixed steam. This demonstrates that heating the clay to 100°C significantly improves its permeability, thereby enhancing the transport capacity of the butanediol mixed steam within the clay, which plays a crucial role in improving its contaminant removal capabilities.

[0074] Experimental Example 3

[0075] In a laboratory setting, clay contaminated with hexavalent chromium (Cr(VI)) at a concentration of 200 mg / kg was artificially prepared. After aging for 14 days, 300 g of the contaminated soil was placed into a soil column. The removal capacity of pure steam, ethanol, and a mixed steam generated from 1,2-butanediol solution for Cr(VI) in the clay was investigated. Alcohol solutions with a concentration (volume ratio) of 20% were prepared. After the clay was heated to 100°C, a metering pump injected the solutions into a steam generator at a rate of 1.5 ml / min. The resulting mixed steam was then injected into the soil column. The experiment ran for 3 hours, with the injected volume of alcohol solution approximately twice the pore volume of the clay. After the soil cooled to room temperature, samples were taken to analyze the Cr(VI) reduction rate. Figure 4 As shown, pure steam (water steam) and ethanol mixed steam have weak reduction capabilities for Cr(VI), while 1,2-butanediol mixed steam has a reduction rate of over 95% for Cr(VI). These results demonstrate that in-situ heat treatment enhanced with 1,2-butanediol can efficiently remove Cr(VI) from soil, showing promising prospects for engineering applications.

[0076] Example 4

[0077] Actual project implementation plan:

[0078] The site surrounding an electroplating plant is contaminated with Cr(VI). The soil layer is mainly low-permeability clay with a permeability coefficient of 1.2 × 10⁻⁻⁻⁶. 7 The soil concentration of Cr(VI) was 98 mg / kg, with a Cr(VI) contamination depth of 12 meters and a Cr(VI) concentration of 98 mg / kg. Traditional ferrous sulfate reduction remediation techniques resulted in severe "rebound" phenomena. The remediation system and method of this invention were used for in-situ remediation, with the following specific steps:

[0079] Project Layout: The designated remediation area is approximately 600㎡. Resistance heating wells, 13 meters deep, exceeding the Cr(VI) contamination depth by 1 meter, are installed at 3.5-meter intervals. Injection wells, 13 meters deep, are installed between adjacent resistance heating wells at 3.5-meter intervals. Monitoring wells, 13 meters deep, are installed at 8-meter intervals. An equipotential bonding layer plus a 40cm thick concrete layer is laid on the ground surface. A thermocouple and pressure detector are installed every 2 meters from top to bottom in each monitoring well.

[0080] In-situ resistance heating: The three-phase AC power supply system is turned on to heat the contaminated clay layer. The power is adjusted by monitoring data to heat the clay evenly to 100°C and then heating is stopped.

[0081] Steam injection: Prepare a 25% mass concentration 1,2-butanediol aqueous solution, vaporize it in a steam boiler, and then heat it to 150°C through a superheater to form high-temperature and high-pressure mixed steam. Inject it at a rate of 0.8 m³ / h, controlling the injection pressure at 8 kPa / m. When the volume of butanediol solution used reaches twice the volume of soil pores (a total of 120 m³ is injected), the injection is stopped.

[0082] Effect test: After standing for 36 hours, samples were taken for testing. The results showed that the Cr(VI) concentration in the clay decreased to below 5 mg / kg, which met the screening value of Class II land use in the "Soil Environmental Quality Construction Land Soil Pollution Risk Control Standard (Trial)" (GB 36600-2018), and the remediation efficiency reached more than 95%. After 12 months of follow-up monitoring, the Cr(VI) concentration did not rebound, and the remediation effect was stable.

[0083] The present invention relates to a system and method for in-situ heat treatment to enhance the in-situ remediation of hexavalent chromium contaminated low-permeability clay layers with butanediol. This system and method overcomes the mass transfer bottleneck in the remediation of Cr(VI) contamination in low-permeability clay layers, eliminates the "tailing" and "rebound" phenomena at the source, and has significant advantages such as high remediation efficiency, stable effect, green and environmentally friendly, simple operation, and controllable cost.

[0084] This technology is applicable to Cr(VI) contaminated sites in fine-grained soils such as low-permeability clay and silty clay produced by industries such as chromium chemical industry, electroplating, and leather making. It is especially suitable for complex sites with contamination depths exceeding 10 meters and strong soil heterogeneity. The electrodes, steam boilers, superheaters, and other equipment used are all conventional engineering equipment, which are easy to obtain and maintain. The engineering layout is flexible, the construction process is simple, and it is easy to promote and apply on a large scale in industry.

[0085] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A system for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using heat-treated and enhanced butanediol, characterized in that: It includes a resistance heating unit, a butanediol mixed vapor injection unit, a monitoring unit, and a ground barrier unit. The resistance heating unit includes a heating resistor, a resistance heating well dug underground, and a three-phase AC power system that provides power to the resistor. The heating resistor is located inside the heating resistor well and is sealed and electrically connected to the three-phase AC power system. The clay layer is heated by resistance heating. The butanediol steam injection unit includes a butanediol solution tank and an injection well. The butanediol solution tank contains butanediol solution and is connected to the inlet of a steam boiler. The steam outlet pipeline of the steam boiler is connected to a superheater, and the outlet pipeline of the superheater is connected to the injection well. The injection well opening is sealed. The monitoring unit includes a monitoring well, which is equipped with a thermocouple and a pressure detector. The ground barrier unit covers the soil surface and includes an equipotential layer and a concrete layer arranged sequentially from bottom to top.

2. The system for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using heat-treated and enhanced butanediol as described in claim 1, characterized in that: Injection wells and heating resistor wells are arranged at intervals of 3-5m, with the injection wells located between adjacent heating resistor wells, and monitoring wells are arranged at intervals of 6-10m.

3. The system for heat-treated enhanced butanediol in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers according to claim 1 or 2, characterized in that: The depth of the resistance heating well, injection well, and monitoring well all exceeds the Cr(VI) contamination depth by 1 meter.

4. The system for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using heat-treated and enhanced butanediol as described in claim 3, characterized in that: Butanediol is 1,2-butanediol. The mass concentration of the 1,2-butanediol aqueous solution in the butanediol solution tank is 20%-30%. The superheater heats the butanediol mixed vapor to above 150°C.

5. The system for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using heat-treated and enhanced butanediol as described in claim 4, characterized in that: 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℃.

6. A method for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using heat-treated and enhanced butanediol, comprising the following steps: (1) Based on the site investigation results, determine the remediation area and depth of Cr(VI) contaminated clay, and install monitoring wells, resistance heating wells and injection wells under the ground according to the heat treatment enhanced butanediol in-situ remediation system for hexavalent chromium contaminated low-permeability clay layer as described in any one of claims 1-5, and install heating resistors, thermocouples and pressure detectors, and lay equipotential layers and concrete layers on the ground of the remediation area in sequence. (2) Turn on the three-phase AC power supply system and use the resistance heating unit to heat the contaminated clay layer. Stop heating after the contaminated clay layer is heated to 100°C. (3) Start the butanediol steam injection unit, vaporize the butanediol aqueous solution through the steam boiler and heat it up through the superheater to form high temperature and high pressure butanediol mixed steam and inject it into the contaminated clay layer; control the injection pressure to be no more than 11 kPa / m during the injection process, and stop the butanediol mixed steam injection when the volume of butanediol solution used is twice the volume of soil pores in the remediation area. (4) After standing for 24~48 hours, take samples from the repaired area for testing. If the Cr (VI) concentration reaches the standard, seal the well to complete the repair. If it does not reach the standard, repeat steps (2) and (3).

7. The method for in-situ remediation of hexavalent chromium-contaminated low-permeability clay layers using heat-treated and enhanced butanediol as described in claim 6, characterized in that... A flow regulating valve is installed on the outlet pipeline of the superheater.

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