Method for synergistically flushing heavy metals in soil by activating free radicals and assisting chelating agent

Abstract

The invention is applicable to the technical field of soil remediation, and provides a method for synergistically flushing heavy metals in soil by activating free radicals and assisting a chelating agent, which comprises the following steps: pretreating the soil; determining the heavy metal form of the polluted soil; determining the content of organic matters in the polluted soil: determining the content of the organic matters in the soil by adopting a potassium dichromate-sulfuric acid heating method; judging which activating agent and oxidizing agent are selected for activation and oxidation, and judging the type of chelating agent used for showering to obtain a judgment result; adding an activating agent; adding a chelating agent: adding the chelating agent, adjusting the solid-to-liquid ratio and the pH value of the system, and oscillating at normal temperature until the reaction is complete; and carrying out solid-liquid separation. According to the method, organic matters in the soil are destroyed, heavy metals in an organic combination form are released, generated free radicals can destroy a mineral structure in the soil, so that the tightly combined heavy metals are released from a solid phase, and then a chelating agent solution is used for flushing the soil, so that the synergistic elution effect on the heavy metals in the soil is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of soil remediation, and in particular to a method for activating free radicals to assist a chelating agent in enhancing the flushing of heavy metals in soil. Background Art

[0002] Soil scouring technology achieves the migration of pollutants from the solid phase to the liquid phase by mixing the scouring fluid with the soil. Its advantages of volume reduction and high efficiency have gradually become a primary method for treating heavy metal-contaminated soil. This technology involves adding scouring agents (such as acids, bases, surfactants, and chelating agents) to interact with pollutants in the soil through physicochemical processes such as desorption, complexation, dissolution / solubilization, and oxidation / reduction, achieving mass transfer and separation of pollutants from the soil to the scouring agent. Chelating agents are commonly used as scouring agents for the remediation of heavy metal-contaminated soils, offering high efficiency and excellent results. Among them, ethylenediaminetetraacetate (EDTA), ethylenediamine disuccinic acid (EDDS), and tetrasodium glutamate diacetate (GLDA) are highly effective in removing heavy metals and are widely used. Subsequently discovered low-molecular-weight organic acids, such as oxalic acid, lactic acid, and citric acid, are biodegradable chelating agents that are less likely to cause secondary contamination. However, the use of chelating agents for heavy metal removal in soils remains problematic. Heavy metal contaminants are affected by organic matter in the soil. Some heavy metals easily form heavy metal-organic chelates. Other heavy metals exist in the soil in the form of iron-manganese oxidation, carbonate-bound, and residual forms. These strong bonds make them difficult to separate from chelating agents, significantly reducing the effectiveness of leaching. Therefore, a method for activating free radicals to assist chelating agents in enhancing the efficiencies of heavy metal leaching in soil is needed to address these issues. Summary of the Invention

[0003] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for activating free radicals to assist chelating agents in enhancing the leaching of heavy metals in soil, so as to solve the problems existing in the above-mentioned background technology.

[0004] The present invention is achieved by providing a method for activating free radicals to assist chelating agents in enhancing the efficiencies of heavy metals in soil, the method comprising the following steps: Carry out soil pretreatment: screen the contaminated soil and remove solid waste; Determination of heavy metal forms in contaminated soil: A four-step chemical method is used to determine the distribution of heavy metals in soil in the form of water-soluble ion exchange, carbonate and oxide bound, sulfide bound, organic bound, and residual forms; Determination of organic matter content in contaminated soil: The potassium dichromate-sulfuric acid heating method was used to determine the organic matter content in the soil; Conduct enhanced shower feasibility assessment: determine which activator and oxidant to choose for activation and oxidation, determine the type of chelating agent to use for showering, and obtain the judgment results; Adding activator: Add activator according to the judgment result, adjust the solid-liquid ratio and pH value of the system, and shake at room temperature for a period of time until the reaction is complete; Add chelating agent: Add chelating agent, adjust the solid-liquid ratio and pH value of the system, and shake at room temperature for a period of time until the reaction is complete; Perform solid-liquid separation: Mechanically separate the soil and the flushing liquid.

[0005] As a further embodiment of the present invention: the water-soluble ion exchange form F1 is a heavy metal form that can be extracted by 1 mol / L CH3COONH4; the carbonate and oxide combined form F2 is a heavy metal form that can be extracted by 3 mol / L HCl after removing F1; the sulfide combined form F3 is a heavy metal form that can be extracted by 0.5% KCN after removing F2; the organic combined form F4 is a heavy metal form that can be extracted by 2 mol / L HNO3+5%H2O2 after removing F3; the residual form F5 is a heavy metal form that can be extracted by 70% HNO3+40% HF after removing F4.

[0006] As a further solution of the present invention: the basis for determining which activator and oxidant to select for activation and oxidation is the proportion of heavy metals in organically bound form and the soil organic matter content; when the proportion of heavy metals in organically bound form is higher than the proportion threshold and the soil organic matter content is higher than the content threshold, the activator is used for activation.

[0007] As a further solution of the present invention: the basis for determining the type of chelating agent used for showering is the type and concentration of heavy metals.

[0008] As a further embodiment of the present invention: the activator is a mixture of a reducing agent and an oxidizing agent, the reducing agent is sodium thiosulfate or hydroxylamine hydrochloride; the oxidizing agent is hydrogen peroxide or peroxodisulfate, which can produce ·OH and ·SO4 - Activate free radicals.

[0009] As a further solution of the present invention: the solid-liquid ratio of the system is 1:2 to 1:10.

[0010] As a further solution of the present invention: the pH value ranges from 2 to 5, and the pH value is adjusted by hydrochloric acid and sodium hydroxide or lime.

[0011] As a further embodiment of the present invention: the chelating agent includes EDTA, EDDS, GLDA, oxalic acid, lactic acid and citric acid.

[0012] As a further solution of the present invention: the conditions for the room temperature oscillation are 25°C and 180 r / min.

[0013] Compared with the prior art, the present invention has the following beneficial effects: The present invention measures the distribution of different forms of heavy metals in the soil, selects a suitable flushing agent for the heavy metal form with a high proportion and strong mobility, and measures the content of organic matter in the soil. When the proportion of organically bound heavy metals is high, the organic matter in the soil is destroyed by free radicals, releasing the heavy metals in the organically bound form. The generated free radicals will destroy the mineral structure in the soil, causing the tightly bound heavy metals to be released from the solid phase. The soil is then flushed with a chelating agent solution to achieve a synergistic elution effect on the heavy metals in the soil. After treatment, the removal rate of heavy metals in the soil can reach 80%-97%. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The present invention is a flow chart of a method for activating free radicals to assist chelating agents in enhancing the leaching of heavy metals in soil. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0016] The specific implementation of the present invention is described in detail below with reference to specific embodiments.

[0017] like Figure 1 As shown, an embodiment of the present invention provides a method for activating free radicals to assist chelating agents in enhancing the leaching of heavy metals in soil, the method comprising the following steps: S100, soil pretreatment: contaminated soil is screened to remove solid waste such as stones and glass; S200, determination of heavy metal forms in contaminated soil: a four-step chemical method is used to determine the distribution of heavy metals in soil in the form of water-soluble ion exchange form F1, carbonate and oxide bound form F2, sulfide bound form F3, organic bound form F4 and residual form F5; S300, determination of organic matter content in contaminated soil: the organic matter content in soil is determined using the potassium dichromate-sulfuric acid heating method; S400, conduct enhanced shower feasibility assessment: determine which activator and oxidant to select for activation and oxidation, determine the type of chelating agent to use for showering, and obtain the judgment result; S500, adding an activator: adding a certain amount of activator according to the judgment result, adjusting the solid-liquid ratio and pH value of the system, and shaking at room temperature for a period of time until the reaction is complete; S600, adding chelating agent: adding a certain amount of chelating agent, adjusting the solid-liquid ratio and pH value of the system, and shaking at room temperature for a period of time until the reaction is complete; S700, solid-liquid separation: mechanically separates soil and flushing liquid. After treatment, the removal rate of heavy metals in the soil can reach 80%-97%.

[0018] In the embodiment of the present invention, it is proposed to use activated free radicals to enhance the chelating agent flushing and remediation effect. This technology uses different activators to activate free radicals to attack heavy metal-organic chelates, thereby realizing the transformation of the organically bound form of heavy metal pollutants in the soil into a readily soluble form, thereby improving the chelating agent flushing effect. The key to achieving the above effect is: (1) analyzing the distribution of different heavy metal forms in the soil and selecting appropriate flushing agents for heavy metal forms with a high proportion and strong mobility; (2) determining the content of organic matter in the soil. When the proportion of organically bound heavy metals is high, the organic matter in the soil is first destroyed by oxidation to release the heavy metals in the organically bound form. The free radicals generated will destroy the mineral structure in the soil, causing the tightly bound heavy metals to be released from the solid phase. The soil is then flushed with a chelating agent solution to achieve a synergistic elution effect on the heavy metals in the soil. Among them, activated free radical enhancement technology is a technology that uses strongly oxidizing free radicals to degrade organic pollutants. It generally refers to a chemical oxidation technology in which a large number of hydroxyl radicals participate in the reaction process. Its basis is to use an activator in combination with an oxidant to produce highly active free radicals (such as hydroxyl radicals, sulfate radicals, superoxide radicals, singlet oxygen radicals, etc.) in the reaction, and then through the addition, substitution, electron transfer, etc. between the free radicals and the pollutants, the pollutants are completely or nearly completely mineralized.

[0019] In the embodiment of the present invention, the heavy metals in the contaminated soil include heavy metals in soil from mining areas, smelting areas, and industrial areas. The water-soluble ion exchange form F1 is a form of heavy metals that can be extracted by 1 mol / L CH3COONH4; the carbonate and oxide-bound form F2 is a form of heavy metals that can be extracted by 3 mol / L HCl after F1 is removed; the sulfide-bound form F3 is a form of heavy metals that can be extracted by 0.5% KCN after F2 is removed; the organic-bound form F4 is a form of heavy metals that can be extracted by 2 mol / L HNO3+5% H2O2 after F3 is removed; and the residual form F5 is a form of heavy metals that can be extracted by 70% HNO3+40% HF after F4 is removed.

[0020] In an embodiment of the present invention, the basis for determining which activator and oxidant to select for activation and oxidation is the proportion of heavy metals in organically bound form and the soil organic matter content; when the proportion of heavy metals in organically bound form is higher than the proportion threshold and the soil organic matter content is higher than the content threshold, the activator is used for activation, and the proportion threshold and content threshold are customized and set in advance.

[0021] In the embodiment of the present invention, the type of chelating agent used for showering is determined based on the type and concentration of heavy metals and the characteristics of the chelating agent, which can be analyzed through experiments.

[0022] In the embodiment of the present invention, the activator is a mixture of a reducing agent and an oxidizing agent, wherein the reducing agent is sodium thiosulfate or hydroxylamine hydrochloride, and the oxidizing agent is hydrogen peroxide or peroxodisulfate, which can generate ·OH and ·SO4 - Activate free radicals.

[0023] In the embodiment of the present invention, the solid-liquid ratio of the system is 1:2 to 1:10. The pH value ranges from 2 to 5, and the pH value is adjusted by hydrochloric acid and sodium hydroxide or lime.

[0024] In the embodiments of the present invention, the chelating agent includes EDTA, EDDS, GLDA, oxalic acid, lactic acid, and citric acid. The dosage of the activator, the dosage of the chelating agent, and the shaking time vary in different contaminated soils and can be analyzed experimentally. The shaking conditions are 25°C and 180 rpm. Mechanical separation of soil and flushing solution can be performed using conventional soil flushing technology and mud-water separation equipment.

[0025] In a more specific embodiment, two soil heavy metal activators and six chelating agents are provided as soil leaching agents. Specifically, activated free radicals are used to assist chelating agents to elute Cu, Zn, Pb and Cr in contaminated soil in steps. The effects of heavy metal form distribution, soil organic matter content, activator type and concentration, chelating agent type and concentration, soil pH, solid-liquid ratio and oscillation time on the leaching effect in contaminated soil were investigated. A four-step chemical method was used to analyze the distribution of Cu, Zn, Pb and Cr in the original soil and treated soil. The soil organic matter content was determined by the potassium dichromate-sulfuric acid heating method. The activator and oxidant selected for activation and oxidation were determined based on the heavy metal form distribution and soil organic matter content.

[0026] In the examples of the present invention, experiments were conducted using two oxidants, hydrogen peroxide and peroxodisulfate, to determine the type and optimal ratio of activators. The same volume fraction of activators was added to contaminated soil at a solid-to-liquid ratio of 1:5. The mixture was shaken at room temperature until the reaction was complete. An equal amount of 0.1 mol / L EDTA was then added and shaken at room temperature until the reaction was complete. The activator's effect was then analyzed. To determine the optimal volume fraction of the activator, reaction kinetics experiments were conducted at a gradient of 0.5%, 1%, 2%, 5%, 10%, and 20%.

[0027] In this example, experiments were conducted using six artificial and natural chelating agents, including EDTA, EDDS, GLDA, oxalic acid, lactic acid, and citric acid, to determine the type and optimal ratio of chelating agents. First, single-agent experiments were conducted, with chelating agent concentrations of 0 mol / L, 0.01 mol / L, 0.03 mol / L, 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.7 mol / L, and 1 mol / L. The artificial chelating agent (A) and natural chelating agent (B) with the highest removal rates were selected, and then experiments with composite chelating agents were conducted. A:B concentration ratios of 1:0.5, 1:1, 1:2, 1:5, and 1:10 were added to a contaminated soil solution with a solid-to-liquid ratio of 1:5 to determine the optimal chelating agent ratio.

[0028] In the embodiment of the present invention, in order to determine the optimal solid-liquid ratio, the solid-liquid ratio is set to 1:2, 1:5, 1:10, 1:20, 1:40, 1:60 and 1:80, and experiments are carried out on the activator and the chelating agent respectively.

[0029] In the examples of the present invention, in order to determine the optimal pH, the treatment pH was set to 2, 3, 4 and 5, and experiments were carried out for the activator and the chelating agent respectively.

[0030] In the embodiment of the present invention, in order to determine the optimal shaking time, the treatment time was set to 0.1h, 0.5h, 1h, 6h, 12h, 24h, 36h and 72h, and experiments were carried out for the activator and the chelating agent respectively.

[0031] In the present embodiment, soil leaching is performed according to the obtained experimental parameters, the optimal activator ratio, the chelating agent ratio, and the reaction conditions. After treatment, the soil and the leaching solution are mechanically separated, and the heavy metal leaching effect is analyzed by leaching.

[0032] The present invention is further described below with reference to specific scenarios and examples. A four-step chemical method was used to determine the distribution of heavy metal species in untreated and activator-treated contaminated soil from an industrial area. Table 1 shows the distribution of these species, while Table 2 shows the total metal content and organic matter content of the contaminated soil. This indicates that the organic matter content of soil samples from the industrial area was significantly higher than that of soil samples from other areas, and that the proportion of organically bound forms (F4) among the heavy metal species was higher. Subsequent experiments using hydrogen peroxide and peroxodisulfate as activators for free radical activation significantly reduced the proportion of organically bound forms (F4). For the heavy metal Cr, after activating the contaminated soil from an industrial area with 5% peroxodisulfate, the soil was rinsed with 1 mol / L GLDA and 1 mol / L citric acid at a solid-to-liquid ratio of 1:10, achieving an optimal removal rate of 79.1%. This was significantly higher than the 21.6% removal rate without peroxodisulfate and the 72.3% removal rate with hydrogen peroxide activation.

[0033] Table 1: Distribution of heavy metal forms in soil

[0034] Table 2: Physical and chemical properties of untreated soil (unit: mg / kg)

[0035] The above is only a detailed description of the preferred embodiments of the present invention, which is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0036] Those skilled in the art will readily appreciate other embodiments of the present disclosure after considering the disclosure in the specification and examples. This application is intended to cover any variations, uses, or adaptations of the present disclosure that follow the general principles of the present disclosure and include common knowledge or customary techniques in the art not disclosed herein. The description and examples are to be considered merely as exemplary, and the true scope and spirit of the present disclosure are indicated by the claims.

Claims

1. A method for activating free radicals to assist chelating agents in enhancing the efficiencies of heavy metals in soil, characterized in that: The method comprises the following steps: Carry out soil pretreatment: screen the contaminated soil and remove solid waste; Determination of heavy metal forms in contaminated soil: A four-step chemical method is used to determine the distribution of heavy metals in soil in water-soluble ion exchange forms, carbonate and oxide bound forms, sulfide bound forms, organic bound forms, and residual forms; Determination of organic matter content in contaminated soil: Potassium dichromate-sulfuric acid heating method was used to determine the organic matter content in soil; Conduct enhanced shower feasibility assessment: determine which activator and oxidant to choose for activation and oxidation, determine the type of chelating agent used for showering, and obtain the judgment result; Adding activator: adding activator according to the judgment result, adjusting the solid-liquid ratio and pH value of the system, and oscillating at room temperature for a period of time until the reaction is complete; Add chelating agent: Add chelating agent, adjust the solid-liquid ratio and pH value of the system, and oscillate at room temperature for a period of time until the reaction is complete; Solid-liquid separation: Mechanical separation of soil and flushing liquid.

2. The method for activating free radicals to assist chelating agents in enhancing the efficiencies of heavy metals in soil according to claim 1, characterized in that: The water-soluble ion exchange form F1 is a heavy metal form that can be extracted by 1mol / L CH3COONH4; the carbonate and oxide combined form F2 is a heavy metal form that can be extracted by 3mol / L HCl after removing F1; the sulfide combined form F3 is a heavy metal form that can be extracted by 0.5%KCN after removing F2; the organic combined form F4 is a heavy metal form that can be extracted by 2mol / L HNO3+5%H2O2 after removing F3; the residual form F5 is a heavy metal form that can be extracted by 70%HNO3+40%HF after removing F4.

3. The method for activating free radicals to assist chelating agents in enhancing the efficiencies of heavy metals in soil according to claim 1, characterized in that: The basis for determining which activator and oxidant to select for activation and oxidation is the proportion of heavy metals in organic combination form and the content of soil organic matter; when the proportion of heavy metals in organic combination form is higher than the proportion threshold and the soil organic matter content is higher than the content threshold, the activator is used for activation.

4. The method for activating free radicals to assist chelating agents in enhancing the efficiencies of heavy metals in soil according to claim 1, characterized in that: The basis for determining the type of chelating agent used for showering is the type and concentration of heavy metals.

5. The method for activating free radicals to assist chelating agents in enhancing the efficiencies of heavy metals in soil according to claim 1, characterized in that: The activator is a mixture of a reducing agent and an oxidizing agent, wherein the reducing agent is sodium thiosulfate or hydroxylamine hydrochloride; the oxidizing agent is hydrogen peroxide or peroxodisulfate, which can produce ·OH and ·SO4 - Activate free radicals.

6. The method for activating free radicals to assist chelating agents in enhancing the efficiencies of heavy metals in soil according to claim 1, characterized in that: The solid-liquid ratio of the system is 1:2 to 1:

10.

7. The method of activating free radicals to assist chelating agents in enhancing the efficiencies of heavy metals in soil according to claim 1, characterized in that: The pH value ranges from 2 to 5, and the pH value is adjusted by hydrochloric acid and sodium hydroxide or lime.

8. The method of activating free radicals to assist chelating agents in enhancing the efficiencies of heavy metals in soil according to claim 1, characterized in that: The chelating agents include EDTA, EDDS, GLDA, oxalic acid, lactic acid and citric acid.

9. The method of activating free radicals to assist chelating agents in enhancing the efficiencies of heavy metals in soil according to claim 1, characterized in that: The conditions of the room temperature oscillation are 25° C. and 180 r / min.

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