A method for remediating organically contaminated soil by low-temperature thermal desorption combined with low-dose persulfate

Abstract

The present invention belongs to the field of soil remediation, and specifically relates to a method for remediating organically contaminated soil using low-temperature thermal desorption in conjunction with low-dose persulfate, comprising: S1, soil survey to determine the target range for remediation; S2, clarifying the physical and chemical properties of the organic pollutants detected in S1; S3, determining process parameters; S4, arranging heating wells, extraction wells, and injection wells; S5, starting the heating wells and operating the in-situ low-temperature thermal desorption process; S6, injecting persulfate to degrade pollutants; and S7, detecting the effect of pollutant removal. The present invention, through the design of process modes and process parameters and the continuation of operating time, cleverly utilizes heating in a relatively low-temperature range to first remove volatile organic pollutants in the soil, while simultaneously rendering the soil's own organic active components inert. The residual heat is then used to activate persulfate and subsequently remove less volatile organic pollutants, effectively coupling physical-chemical remediation technologies and reducing remediation costs. The present invention is highly operational and has a high removal efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of contaminated soil remediation, and specifically relates to a method for remediating organic contaminated soil using low-temperature thermal desorption in conjunction with low-dose persulfate. Background Art

[0002] Currently, physical, chemical, and biological remediation technologies are available for organically contaminated soils. However, because microbial degradation is difficult to control, its efficiency is significantly affected by environmental conditions, and the remediation cycle is long, physical and chemical methods are the most commonly used in practical projects. Physical remediation, based on the principles of separation and extraction, uses heating to achieve efficient desorption, and has been widely used in large-scale remediation sites in recent years. As a non-combustion technology, thermal desorption offers advantages such as a short treatment cycle, compatibility with a wide range of pollutant types, and high safety. Especially for treating chlorinated organic pollutants, its non-oxidative combustion method avoids dioxin formation, making it widely used for the remediation of organically contaminated soils. However, thermal desorption also has significant drawbacks. Existing processes fail to account for the variability of different pollutant molecules and only target the most difficult-to-volatilize substances with the highest desorption temperatures. Common processes often require high temperatures of 400°C to 600°C, resulting in high costs and significant carbon emissions. Some companies reduce temperatures to reduce costs, but the result is incomplete remediation, requiring secondary rework and further losses. Furthermore, excessively high temperatures can cause irreversible damage to soil structure, making it difficult to restore a normal ecosystem.

[0003] Chemical remediation technologies include Fenton oxidation, persulfate oxidation, and ozone oxidation. However, the Fenton reagent has a narrow pH range, limiting its practical application in on-site remediation. Ozone, while a strong oxidizing agent, is also costly, rapidly decomposes in soil, has a short vertical transport distance, and soil porosity directly affects its effectiveness. The persulfate method activates persulfates, such as sodium persulfate, by heating to generate highly oxidizing free radicals such as hydroxyl radicals and sulfate radicals, which degrade organic pollutants in the soil. Activated sodium persulfate, due to its high redox potential, low cost, and relatively low environmental toxicity, has become a widely used chemical oxidant for remediation of organically contaminated soils. However, in practice, due to the broad spectrum of active substances, the abundant reducing substances in the soil will initially consume the oxidant. Consequently, to achieve effective remediation, excessive addition of sodium persulfate is necessary. This not only results in high costs but also leaves a large amount of sulfur-containing substances in the soil, altering soil properties and functions (such as pH, conductivity, and cation exchange capacity).

[0004] In summary, current thermal desorption and chemical remediation methods present multiple challenges, including high costs, high carbon emissions, and soil damage. In practical applications, researchers have primarily focused on exploring different persulfate activation methods or thermal desorption devices to improve remediation efficiency. Alternatively, they have mechanically combined multiple remediation techniques, using separate treatment systems to achieve higher removal efficiencies. These processes often require additional equipment or procedures, resulting in higher costs, greater operational complexity, and difficulty replicating them across diverse soil types.

[0005] Therefore, it is necessary to provide an improved technical solution to the above-mentioned deficiencies in the prior art. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a method for remediating organically contaminated soil by combining low-temperature thermal desorption with low-dose persulfate.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] The method for remediating organically contaminated soil by low-temperature thermal desorption and low-dose persulfate comprises the following steps:

[0009] S1. Soil survey: Soil samples will be collected and tested at the contaminated site to identify the types and concentrations of organic pollutants and their distribution in the vertical and horizontal sections of the contaminated site.

[0010] S2. Clarify the physical and chemical properties of the organic pollutants detected in S1;

[0011] S3. Determine process parameters: Based on the physical and chemical properties of the organic pollutants, set the maximum temperature of the heating well for the low-temperature thermal desorption process within the range of 30°C-300°C, and the maximum soil temperature that can be achieved by the low-temperature thermal desorption process within the range of 100°C-200°C; and set the concentration and dosage of the persulfate solution to be injected into the contaminated site; the dosage of persulfate should be 0.5%-3% of the mass of the soil to be remediated at the contaminated site;

[0012] S4. Heating wells are arranged at intervals in the contaminated site, and extraction wells and injection wells are arranged around each heating well;

[0013] S5. Start the heating well and run the in-situ low-temperature thermal desorption process. During the thermal desorption process, soil samples are collected to detect the types of residual pollutants. Thermal desorption is terminated after most of the highly volatile organic pollutants are removed.

[0014] S6. After the thermal desorption process is completed, when the soil temperature is below 100°C, a persulfate solution is immediately injected to activate the persulfate using the residual heat energy of the thermal desorption process and the soil, and further oxidatively degrade the pollutants that are difficult to volatilize and difficult to thermally degrade using the residual heat energy of the thermal desorption process;

[0015] S7. Test the remediated soil. If it meets the remediation standards (GB36600-2018), the remediation of the current contaminated soil is considered complete. If it does not meet the remediation requirements, repeat steps S5-S7.

[0016] Furthermore, the organic pollutants are wide-boiling-point complex organic pollutants, which generally include one or more of polycyclic aromatic hydrocarbons, petroleum hydrocarbons, chlorinated hydrocarbons, benzene series, and the like.

[0017] Furthermore, in S3, the organic pollutants actually detected in the soil are divided into a group with a volatilization temperature below 200°C and a group with a volatilization temperature above 200°C; the minimum temperature of the thermal desorption process is determined based on the maximum volatilization temperature of the organic pollutants in the group below 200°C, and the amount of persulfate solution is determined based on the type and concentration analysis of the molecules of the organic pollutants in the group above 200°C.

[0018] Furthermore, in S3, the persulfate is sodium persulfate, and the amount of sodium persulfate added is 0.5%-2% of the weight of the soil to be repaired.

[0019] Furthermore, in S3, the concentration of the persulfate solution is determined based on the initial moisture content of the soil to be repaired and the amount of persulfate added, so that the soil moisture content can reach the target moisture content range of 10%-15% after the persulfate solution is added.

[0020] Furthermore, the thermal desorption temperature is 100° C.-200° C., and the time is more than 8 hours; and the degradation time of the persulfate solution is 4 days-8 days.

[0021] Furthermore, in S4, the center distance between two adjacent heating wells is 1.5m-5m; the extraction well / injection boundary is 0.5m-1m away from the heating well boundary.

[0022] Furthermore, in S4, the maximum depth of the heating well does not exceed 6m, and the heating well uses natural gas heating rods or electric heating rods for thermal desorption.

[0023] Furthermore, in S5, thermal desorption is terminated after more than 70% of the highly volatile organic pollutants are removed.

[0024] Furthermore, in S6, before persulfate was added, the initial soil moisture content was adjusted to 4.5%-5.2% and the pH was adjusted to 7.5-8.5.

[0025] The working principle of the present invention is to utilize the thermal activation characteristic of sodium persulfate and use the residual heat energy from thermal desorption to activate sodium persulfate, thereby achieving the goals of saving energy and reducing carbon emissions. For complex organic pollutants of different properties, only volatile and thermally unstable organic pollutants need to be removed by low-temperature thermal desorption. During the thermal desorption process, the reactivity of the soil's own organic matter decreases, weakening its ability to consume the free radicals generated by the activated persulfate. After the thermal desorption is completed, the remaining non-volatile pollutants can be removed by free radicals, thereby achieving the effect of reducing the oxidant dosage. Low temperature and low dosage further achieve the goal of reducing costs.

[0026] The beneficial effects of the present invention are:

[0027] This invention proposes a method for remediating composite organic contaminated soil by coupling low-temperature thermal desorption with low-dose persulfate. Through the design of process modes and process parameters, as well as the timing of operation, this method cleverly utilizes relatively low-temperature heating to first remove volatile organic pollutants from the soil. Simultaneously, the active organic components of the soil itself are rendered inert, preventing them from consuming the oxidant free radicals in the second stage. Subsequently, the oxidant persulfate is injected, and the residual heat energy is used to activate the persulfate, subsequently removing the less volatile organic pollutants. This method effectively couples physical and chemical remediation techniques and reduces remediation costs. Conventional high-temperature thermal desorption or high-dose oxidant remediation of contaminated soil inevitably leads to secondary pollution and damage to soil structure. This invention utilizes a low-temperature, low-dose process to mitigate these negative effects.

[0028] The present invention utilizes waste heat to activate persulfate remediation, efficiently coupling two remediation technologies, thereby lowering both the desorption temperature and the amount of oxidant used, thereby reducing economic costs. The present invention has strong operability, high feasibility, strong environmental adaptability, high removal efficiency, and good purification effect on contaminated sites. The purified contaminated sites can be used as Class I construction land. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings and the accompanying drawings, which constitute part of this application, are intended to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to explain the present invention and do not constitute an undue limitation of the present invention. Among them:

[0030] Figure 1 This is a schematic diagram of the heating well arrangement according to an embodiment of the present invention.

[0031] Figure 2 Schematic diagram of the arrangement of extraction wells and injection wells according to an embodiment of the present invention.

[0032] Figure 3 1 and 2 are the pollutant detection results of the embodiment of the present invention; wherein (a), (b), and (c) are the pollutant detection results of the control group, treatment group 1, and treatment group 2, respectively.

[0033] Figure 4 These are the pollutant detection results of the experimental examples of the present invention; among them, (a) is the detection result of pollutants after thermal desorption at 60°C for 8 hours and then treatment with sodium persulfate concentrations of 0, 1%, and 5% for 4 hours and 8 hours; (b) is the detection result of pollutants after thermal desorption at 90°C for 8 hours and then treatment with sodium persulfate concentrations of 0, 1%, and 5% for 4 hours and 8 hours; (c) is the detection result of pollutants after direct injection of sodium persulfate concentrations of 0, 1%, and 5% for 4 hours and 8 hours without thermal desorption.

[0034] Figure 5 The removal rates of the pollutants in the experimental examples of the present invention after thermal desorption at 60°C and 90°C for 8 hours respectively.

[0035] In the figure: 1-heating well, 2-extraction well, 3-injection well, 100-contaminated site. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field are within the scope of protection of the present invention.

[0037] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0038] The present invention provides a method for remediating organically contaminated soil by low-temperature thermal desorption in conjunction with low-dose persulfate, comprising the following steps:

[0039] S1. Detailed investigation of soil pollution status: 100 soil samples were collected and tested at densely distributed locations at the contaminated site to clarify the boundaries of the contaminated site, the types and concentrations of pollutants, and the distribution of different organic pollutants in the vertical and horizontal profiles of the soil;

[0040] S2. Classify the organic pollutants detected in S1 according to their volatility and degradability, and list and indicate their boiling point, vapor pressure, molecular weight, main functional groups and other information;

[0041] S3. Based on the boiling point and stability of volatile pollutants, and taking into account the heat conductivity of the soil, the maximum temperature of the heating well for the low-temperature thermal desorption process is set within the range of 30°C-300°C, and the maximum soil temperature that can be achieved by the low-temperature thermal desorption process is set within the range of 100°C-200°C. Based on the degradation and redox characteristics of non-volatile pollutants, the concentration and addition amount of the injected persulfate solution are set within the range of 0.5%-3% of the soil mass;

[0042] S4, such as Figure 1 、 Figure 2As shown in the figure, heating well 1, extraction well 2 and injection well 3 are set; heating well 1 is arranged at intervals in the polluted area, and extraction well 2 and injection well 3 are arranged around each heating well 1; the distribution of heating well 1 is consistent with the distribution of pollutants, and can be appropriately increased in heavily polluted areas. The center distance D1 between two adjacent heating wells is 1.5m-5m; extraction well 2 and injection well 3 are arranged around heating well 1, and their boundaries are 0.5m-1m away from the boundary of heating well 1 ( Figure 1-2 In the figure, D2 represents the distance from the extraction well / injection well boundary to the heating well boundary); the depth of the thermal desorption zone is the contaminated area within 6 m of the soil surface, and the layout depth of heating well 1 is consistent with the vertical depth of the contamination;

[0043] S5. Start the heating well and run the in-situ low-temperature thermal desorption process. The in-situ low-temperature thermal desorption process can first remove most of the volatile and thermally degradable organic pollutants, which are usually pollutants with smaller molecular weights. The heating temperature of the heating well does not exceed 300°C, and the soil temperature is maintained at approximately 100°C-200°C. Thermocouple arrays (such as K-type thermocouples) are installed in injection wells at different distances from the heating well (such as 0.5m, 0.6m, 0.75m, 0.8m, 0.9m, and 1.0m) to monitor the temperature changes of the soil around the heating well. During the desorption process, soil samples are collected to detect the types and contents of residual pollutants. Based on experience and the rate of change of the contents, soil samples are collected and tested every half a day, one day, two days or three days until most of the highly volatile organic pollutants are removed and the thermal desorption is terminated. The end of the thermal desorption process mainly depends on the volatile pollutants. In actual projects, the boiling point of volatile pollutants is very low. For example, at 87.2°C (standard conditions), the removal rate of trichloroethylene is required to be at least 80%. If it is a nitrobenzene pollutant at 210°C (standard conditions), the removal rate is required to be 60%-70%. In summary, for complex organic pollution, the standard for the end of the thermal desorption process is a removal rate of more than 70% for highly volatile pollutants. According to preliminary tests, this standard can be fully achieved after 8 hours of thermal desorption process. Therefore, in the present invention, the process parameters of thermal desorption for 8 hours are unified to reduce variables.

[0044] S6. After the thermal desorption process is completed, when the soil temperature drops below 100° C., a persulfate solution is immediately injected to activate the persulfate using the thermal desorption process and the residual heat energy in the soil, thereby further oxidizing and degrading macromolecular pollutants that are difficult to volatilize and difficult to thermally degrade. Preferably, the amount of persulfate added is 0.5%-2.0% of the soil mass. The persulfate solution is used to remove non-volatile organic pollutants and pollutants with strong thermal stability.

[0045] S7. After the two-stage treatment process is complete, soil samples are collected for pollutant concentration testing to determine treatment effectiveness. Soil sampling and testing is conducted at multiple points, both horizontally and vertically. Compliance with soil remediation standards is determined according to the "Soil Environmental Quality Standard for Soil Pollution Risk Control in Construction Land" (Trial Implementation) (GB36600-2018). If the remediated soil meets the remediation standards, remediation is considered complete. If not, repeat S5-S7 until the soil remediation standards are met.

[0046] Furthermore, in S1, when determining the concentration, properties and distribution of relevant organic pollutants, the horizontal and vertical sampling method is adopted, that is, multi-point sampling in the horizontal and vertical directions, so as to more accurately determine the type, concentration and distribution of pollutants.

[0047] Furthermore, in S2, the steps of classifying organic pollutants and determining treatment process parameters include: classifying the detected pollutants according to boiling point, thermal stability, and reactivity with chemical agents, determining the maximum temperature for the thermal desorption process (heating well temperature not exceeding 300°C, soil temperature between 100°C and 200°C) and the maximum amount of chemical agents used (not exceeding 3%). The heating well is heated to a maximum of 300°C using natural gas or electric heating, and the soil temperature is maintained between 100°C and 200°C. The temperature is measured using a thermocouple device in the injection well, and the soil mass is estimated based on the volume of the treatment area, thereby determining the amount of oxidant used (calculated based on the soil mass percentage). Specifically, assuming the boiling points of the organic pollutants are 180°C, 200°C, and 500°C, respectively, the required soil temperature should be the highest boiling point temperature (200°C) that does not exceed 300°C in the organic pollutant category. The primary purpose of this measure is to remove low-boiling-point pollutants, which typically have boiling points ranging from tens to over 200 degrees Celsius (under standard conditions). To maintain a soil temperature of 100°C-200°C, the heating equipment can be directly heated to 300°C for rapid soil heating. However, the temperature should not exceed 300°C, as the boundary between high- and low-temperature thermal desorption is generally 300°C. Maintaining a soil temperature of 100°C-200°C does not damage the soil structure and can passivate the active components of soil organic matter, rendering them inert. This reduces subsequent persulfate consumption and increases soil humification. High-boiling-point organic pollutants partially volatilize during thermal desorption at 200°C, allowing persulfate degradation to continue. The persulfate concentration is primarily determined by the mass of the soil to be treated, with the oxidant concentration not exceeding 3% of the soil mass, preferably 0.5%-2%.

[0048] Furthermore, the present invention is applicable to the in-situ thermal desorption of wide-boiling-point complex organic pollutants, which include one or more organic pollutants such as polycyclic aromatic hydrocarbons, petroleum hydrocarbons, chlorinated hydrocarbons, and benzene series. Specifically, n-undecane (C 11), n-eicosane (C 20 ), benzo[a]pyrene (BAP) as characteristic pollutants. The properties of the pollutants are: C 11 (Boiling point 196℃) Highly volatile, C 20 (boiling point 343°C) and BAP (boiling point 495°C) are less volatile. PAHs are highly reactive with persulfates, while linear alkanes are less reactive with persulfates. This leads to the determination of a thermal desorption process temperature of 196°C-200°C.

[0049] Furthermore, in S3, a traditional natural gas heating rod or an electric heating rod may be used for thermal desorption.

[0050] Furthermore, when the thermal desorption process is adopted, processes such as gas phase extraction through the extraction well and treatment of the extracted tail gas should be carried out simultaneously.

[0051] Furthermore, in S5, after the thermal desorption process is completed, when the soil temperature drops below 100°C, a persulfate solution is immediately added, wherein the persulfate is sodium persulfate and is injected through an injection well.

[0052] Furthermore, after the thermal desorption process and before the persulfate solution is added, the contaminated soil moisture content is measured. Based on a target soil moisture content of 10%-15% and a persulfate dosage of 0.5%-3.0% of the contaminated soil weight, the persulfate solution concentration is designed to ensure that the soil moisture content reaches the target moisture range after the persulfate solution is added through the injection well. Rather than calculating the soil moisture content of the entire contaminated site, measurements can be taken at gradient points at different depths within the injection well, and the soil moisture content at the injection well can then be calculated. Soil moisture can be measured using a capacitive soil moisture sensor.

[0053] In order to enhance the application effect of contaminated soil remediation technology and reduce process costs, the present invention proposes a method of physical and chemical coupling technology based on the different types and properties of site pollutants. Organic pollutants that are volatile or have weak thermal stability can be removed through a thermal desorption process, and the remaining pollutants that are difficult to volatilize are subjected to persulfate oxidation treatment. On the other hand, due to the low temperature, it is difficult to promote the breakage of the peroxide bond of persulfate to generate sulfate radicals. Excessive temperature will quench the generated free radicals. The normal activation temperature of persulfate is 50°C-90°C. It is entirely possible to utilize the residual heat energy of thermal desorption and the heat preservation effect of the soil itself to thermally activate persulfate. In addition, during the thermal desorption process, the reactivity of soil organic matter decreases, and the ability of soil organic matter to consume persulfate becomes weaker, which reduces the loss of persulfate, saves costs, and improves the degradation efficiency.

[0054] In the design of the wellhead, the present invention only requires three types of wellheads: extraction wells, heating wells, and injection wells. Conventional vertical drilling can meet the use requirements. After the in-situ heating and desorption process, the persulfate solution can be injected through the injection well and diffused deep into the soil. There is no need to cover the surface with a surface barrier layer (such as an insulating cotton layer, HDPE film, etc.). The maximum depth of the heating well does not exceed 6m, and the maximum heating temperature does not exceed 300°C. The construction difficulty is small, the construction energy consumption is less, and the operability is strong.

[0055] Example

[0056] A chemical contaminated site, the soil is sandy, the pollutants are petrochemical pollutants, the contamination depth is 5m, the pollutants include petroleum hydrocarbons and polycyclic aromatic hydrocarbons, select C 11 (boiling point 196°C), C 20 (boiling point 343℃) was selected as a representative of petroleum hydrocarbons, and BAP (benzo[a]pyrene, boiling point 495℃) was selected as a representative of polycyclic aromatic hydrocarbons; the initial soil moisture content was 4.9%, pH 8.3, C 11 The content is 743.61mg / kg, C 20 The content is 1463.64mk / kg and the BAP content is 122.61mg / kg.

[0057] In-situ heating wells, extraction wells, and injection wells are all located within 6 meters of the surface and are evenly distributed across the contaminated area. The extraction and injection wells are located within 0.5-1 meter of the heating wells. The heating device temperature is controlled at 200°C, and the average soil temperature is monitored at 120°C via thermocouples in the injection wells. Soil samples are regularly taken during thermal desorption to monitor contaminant types.

[0058] After 8 days of thermal desorption, the volatile pollutants C in the soil 11 The removal rate was 74.33%, C 20 The removal rate was 33.27%, and the BAP removal rate was 55.22%. After the highly volatile pollutants were desorbed and removed significantly (more than 70% removal), the heating was stopped, and the temperature was monitored. When the soil temperature dropped below 100°C, sodium persulfate solution was injected through the injection well. The amount of sodium persulfate used in treatment group 1 was 1% of the soil mass, and the amount of sodium persulfate used in treatment group 2 was 3% of the soil mass. After the sodium persulfate solution was injected into treatment groups 1 and 2, the soil moisture content was 15%. After sodium persulfate was added to the soil in treatment groups 1 and 2, the removal of various boiling point pollutants in the soil is shown in Table 1. Figure 3 The removal rates in Table 1 are the total removal rates (thermal desorption + persulfate oxidation) calculated based on the initial contamination concentration. The control group underwent thermal desorption for 8 days without sodium persulfate addition. Treatment Group 1, Treatment Group 2, and Control Group are designated P1-120°C, P3-120°C, and CK-120°C, respectively.

[0059] Table 1 Residual amount and removal rate of pollutants at each boiling point in treatment group 1, treatment group 2 and control group

[0060]

[0061] like Figure 3 As shown in the figure, after thermal desorption at 120℃ (soil temperature), only 1% sodium persulfate and 4 days of oxidation are needed to remove the difficult-to-remove C 20 The residual amount was reduced to 662.3 mg / kg, which is lower than the screening value standard for Class I construction land (TPHs (total petroleum hydrocarbons): 826 mg / kg; PAHs (polycyclic aromatic hydrocarbons): 0.55 mg / kg). 11 and BAP were completely removed; after 8 days of oxidation, C 20 The residual amount is further reduced. When the dosage of 3% sodium persulfate is used, C 20 The removal rate was as low as 304.27 mg / kg, with a total removal rate of 79.21%. The results show that after thermal desorption at 120°C (soil temperature), adding 1% sodium persulfate to the soil for four days of oxidation can remediate the contaminated site to meet the standards for Class I construction land. For better remediation results, increasing the sodium persulfate dosage or the persulfate oxidation time can be used.

[0062] Benzo[a]pyrene (BAP) is a polycyclic aromatic hydrocarbon (five benzene rings) with a conjugated π electron system, forming electron-rich regions, especially high electron cloud density on some carbon atoms. Sulfate radical (SO4 - ) as a strong electrophile, preferentially attacks these electron-rich sites, initiating oxidation reactions. In contrast, n-eicosane (C 20 ) is a saturated straight-chain alkane containing only σ bonds, with a uniform electron cloud distribution and low density, making it difficult for free radicals to attack effectively. 11 , which exists in the soil mainly as non-aqueous phase liquid (NAPL). Most of it was removed in the early thermal desorption process, and the residual concentration was not high. It can also be quickly oxidized and removed by oxidants. Compared with the control group (not treated with thermal desorption), the large amount of organic matter active components in the soil will consume the free radicals produced by activated sodium persulfate, thereby reducing the ability to remove pollutants. The pollutant removal rate of the non-thermal desorption group is lower than that of the thermal desorption group. C of treatment group 1 20 The removal rate does not increase much over time, which may be due to the limitation of the amount of oxidant used. In order to improve the removal efficiency, the amount of oxidant can be appropriately increased.

[0063] Experimental example

[0064] In addition to on-site repairs, the inventors also conducted indoor simulation experiments.

[0065] Preparation of simulated contaminated soil: 2 kg of clean soil was used. The original soil was sandy soil and ground through a 2 mm sieve. The pollutant n-undecane (C 11 boiling point 196℃), n-eicosane (C 20 boiling point 343℃), benzo[a]pyrene (BAP boiling point 495℃) as characteristic pollutants, 150mg / kg (calculated by soil dry weight) of BAP, 1000mg / kg of C 11 and 1500mg / kg of C 20 Dissolve in a 1:1 mixture of acetone and dichloromethane, then mix into the soil and stir evenly. Age in a well-ventilated area for one month. Before the experiment, spread the soil to a thickness of 15 cm, adjust the initial moisture content to 5%, and adjust the initial pH to 8.3.

[0066] Degradation reaction experiments: Thermal desorption utilizes heating rods, evenly spaced with 10 cm center-to-center spacing. The rods are inserted 10 cm below the soil surface, and the heating time is 8 hours. Extraction and injection wells are arranged around the rods, maintaining the same depth. The distance between the extraction and injection wells and the heating wells is 3-10 cm. Exhaust gas is treated by absorption with a dichloromethane solution. The oxidant solution is sodium persulfate, injected via syringe at concentrations of 0, 1%, and 5% soil dry weight. The soil moisture content is maintained at approximately 15%, and the degradation time is 4 and 8 hours. Pollutant concentrations during thermal desorption and chemical oxidation are determined qualitatively and quantitatively using gas chromatography-mass spectrometry (GC-MS). Pretreatment, including extraction, cleanup, and concentration, is required according to the national standard method (HJ 805-2016).

[0067] The thermal desorption temperatures were 0, 60°C, and 90°C, respectively; the addition amounts of sodium persulfate were 0, 1%, and 5%; Figure 5 The thermal desorption effects of various boiling point pollutants at 60℃ and 90℃ for 8h; Figure 4 (a) After thermal desorption at 60℃ for 8h, the pollutants were treated with sodium persulfate at concentrations of 0, 1%, and 5% for 4h and 8h, respectively. The three treatments are denoted as CK-60℃, P1-60℃, and P5-60℃. Figure 4 (b) After thermal desorption at 90℃ for 8h, the pollutants were treated with sodium persulfate at concentrations of 0, 1%, and 5% for 4h and 8h, respectively, and recorded as CK-90℃, P1-90℃, and P5-90℃; Figure 4 (c) shows the detection results of pollutants that were directly injected with sodium persulfate concentrations of 0, 1%, and 5% for 4 hours and 8 hours without thermal desorption, and are recorded as CK-0, P1-0, and P5-0, respectively.

[0068] Table 2 Residual amount and removal rate of pollutants at various boiling points (unit: mg / kg)

[0069]

[0070] like Figure 5 As shown, after thermal desorption at 60℃, C 11 The removal rate of PTFE was the highest, reaching 57.52%, followed by BAP, with a removal rate of 14.08%. The most difficult pollutant to desorb was C 20 , the removal rate was 8.39%. As the thermal desorption temperature increased to 90℃, the removal rates of all pollutants increased. At 90℃, 11 The removal rate was the highest, reaching 63.44%, BAP reached 24.94%, and C 20 When the temperature is low, except C 11 The removal rates of the other two pollutants were relatively low. This suggests that the difficulty of thermal desorption of various pollutants is related to their boiling point and thermal stability. Similar to the results in the examples, low-boiling-point pollutants are more easily desorbed. Therefore, in traditional thermal desorption, high temperatures are often used to desorb high-boiling-point, less volatile pollutants. However, this can severely damage the soil structure and increase economic costs.

[0071] After thermal desorption, the residual pollutants in the group without oxidant were high and could not meet the screening value standard for the first type of construction land (TPHs (total petroleum hydrocarbons): 826 mg / kg; PAHs (polycyclic aromatic hydrocarbons): 0.55 mg / kg). After adding oxidant, a low dose of oxidant (1% soil dry weight) can remove BAP and C 11 falls below the screening value, but for C 20 It is necessary to increase the amount of oxidant or extend the oxidation time to reduce it below the screening value; Figure 4 As shown in (a), after thermal desorption at 60℃, the oxidant dosage is 5%, the oxidation time is 4h, and C 20 The residual amount is 767.28 mg / kg; the oxidant dosage is 1%, the oxidation time is 4h, C 20 The residual amount is 1297.79 mg / kg. With the increase of the thermal desorption temperature in the early stage, under the same oxidant dosage conditions, the oxidation time can be shortened and the pollutants can be reduced to below the screening value, for example Figure 4 As shown in (b), after thermal desorption at 90℃, 5% sodium persulfate and oxidation time of 4h can remove the difficult-to-remove C 20 The residual amount dropped to 576.39 mg / kg.

[0072] Compared with direct chemical oxidation remediation technology without thermal desorption treatment to passivate the active components of soil organic matter, it is more difficult for complex pollutants to reach the remediation screening value. Figure 4 (c) shows that C 20Regardless of the conditions of oxidant dosage of 1% and repair time of 8h, or oxidant dosage of 5% and repair time of 8h (in this case, C 20 The residual concentration is still as high as 954.6mg / kg), and it cannot be reduced below the screening value for Class I construction land. The BAP concentration can only be removed below the screening value when the oxidant dosage is 5% and the oxidation time is 8h. 11 Only when the oxidant dosage is 5% and the oxidation time is 8h can it be completely removed. Through the examples and experimental examples, it can be seen that the method of the present invention is applicable to the degradation of high-concentration complex organic pollutants in soil, such as common petrochemical pollutants such as petroleum hydrocarbons and polycyclic aromatic hydrocarbons, with petroleum hydrocarbons C 11 (boiling point 196°C), C 20 (boiling point 343℃) and polycyclic aromatic hydrocarbon benzo[a]pyrene (boiling point 495℃) are representative organic pollutants with a wide boiling point. It also has a high efficiency in degrading polycyclic aromatic hydrocarbons with a concentration of more than 200mg / kg in soil and more than 8000mg / kg of petroleum hydrocarbons. After treating such contaminated soil with appropriate thermal desorption temperature, sodium persulfate dosage and degradation time, it can reach the C 11 , BAP is completely removed, and C 20 Degrade to the level of first-class construction land standards.

[0073] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are within the scope of protection of the pending claims of the present invention.

Claims

1. A method for remediating organically contaminated soil by low-temperature thermal desorption in combination with low-dose persulfate, characterized in that: The following steps are involved: S1. Soil survey: Soil samples will be collected and tested at the contaminated site to determine the types and concentrations of organic pollutants, as well as their distribution in the vertical and horizontal sections of the contaminated site. The organic pollutants are complex organic pollutants with wide boiling points. S2. Clarify the physical and chemical properties of the organic pollutants detected in S1; S3. Determine process parameters: According to the physical and chemical properties of organic pollutants, they are divided into o C's group sum is higher than 200 o C grouping; set the maximum temperature of the heating well for low temperature thermal desorption process in the range of 30℃-300℃; based on the o The highest volatilization temperature of the organic pollutants in the group of C determines the lowest temperature of the thermal desorption process, and the highest soil temperature that can be reached by the low-temperature thermal desorption process is set within 100℃-200℃; and the concentration and addition amount of the persulfate solution injected into the contaminated site are set, based on the temperature above 200℃. o The amount of persulfate solution to be used is determined by analyzing the type and concentration of the molecules of the organic pollutants in the group C. The amount of persulfate added is 0.5%-3% of the mass of the soil to be remediated at the contaminated site; S4. Heating wells are arranged at intervals in the contaminated site, and extraction wells and injection wells are arranged around each heating well; S5. Start the heating well and run the in-situ low-temperature thermal desorption process; the thermal desorption temperature is 100°C-200°C and the time is more than 8 hours; during the thermal desorption process, soil samples are collected to detect the types of residual pollutants, and the thermal desorption stage is terminated after more than 70% of highly volatile organic pollutants are removed; S6. After the thermal desorption process is completed, when the soil temperature is below 100°C, a persulfate solution is immediately injected to activate the persulfate using the thermal desorption process and the residual heat energy in the soil, further oxidizing and degrading the organic pollutants that are difficult to volatilize and thermally degrade; The degradation time of persulfate solution is 4-8 days; S7. After testing the remediated soil, if it meets the remediation standards, the remediation of the currently contaminated soil is considered complete.

2. The method for remediating organically contaminated soil by low-temperature thermal desorption in conjunction with low-dose persulfate according to claim 1, characterized in that: The persulfate used is sodium persulfate, and the amount of sodium persulfate added is 0.5%-2% of the weight of the soil to be repaired.

3. The method of low-temperature thermal desorption in conjunction with low-dose persulfate for remediation of organically contaminated soil according to claim 1, characterized in that: The concentration of the persulfate solution is determined based on the initial moisture content of the soil to be repaired and the amount of persulfate added, so that the soil moisture content can reach the target moisture content range of 10%-15% after adding the persulfate solution.

4. The method of low-temperature thermal desorption in conjunction with low-dose persulfate for remediation of organically contaminated soil according to claim 1, characterized in that: Before persulfate addition, the initial soil moisture content was adjusted to 4.5%-5.2% and the pH was adjusted to 7.5-8.

5.

5. The method of low-temperature thermal desorption in conjunction with low-dose persulfate for remediation of organically contaminated soil according to claim 1, characterized in that: The center distance between two adjacent heating wells is 1.5m-5m; the extraction well / injection boundary is 0.5m-1m away from the heating well boundary respectively.

6. The method of low-temperature thermal desorption in conjunction with low-dose persulfate for remediation of organically contaminated soil according to claim 1, characterized in that: The maximum depth of the heating well does not exceed 6m, and the heating well uses natural gas heating rods or electric heating rods for thermal desorption.

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