Soil purification method

Abstract

To provide a soil purification method that decomposes contaminants in soil in-situ, in which persistence of a Fenton reaction in a neutral region is heightened.SOLUTION: A chelating agent and hydrogen peroxide are injected into the soil. The chelating agent is an aminocarboxylic acid compound or its salt that has one hydroxyl group and two or more coordination sites that can coordinate to an iron atom besides that one hydroxyl group. The iron complex formed by the chelating agent is strong, and trivalent iron ions are less likely to dissociate. For this reason, a decrease in permeability due to precipitation of iron (III) hydroxide is unlikely to occur.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for remediating soil. [Background technology]

[0002] A technique using the Fenton reaction has been known for in-situ decomposition of contaminants in soil to purify the soil (see, for example, Patent Document 1). When hydrogen peroxide is injected into the soil, divalent iron ions present in the soil or added as a chemical are oxidized to trivalent iron ions, and the hydrogen peroxide converts to hydroxy ions and hydroxy radicals. The highly active hydroxy radicals react with contaminants to decompose them. On the other hand, trivalent iron ions precipitate as iron(III) hydroxide if left untreated, but this precipitation can be prevented by capturing the trivalent iron ions with a chelating agent. The trivalent iron ions eventually return to divalent iron ions in the presence of hydrogen peroxide or other chemicals. In this example, the use of a chelating agent allows the process to continue even in the neutral pH range, where iron(III) hydroxide is likely to form. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 4785067 Summary of the Invention [Problem to be solved by the invention]

[0004] It has since become clear that this technology can have problems with the sustainability of remediation. For example, when hydrogen peroxide and a chelating agent (collectively referred to as "chemicals") are injected from above ground to underground using an injection pipe inserted into the soil, iron (III) hydroxide precipitates before the chemicals can sufficiently diffuse around the injection pipe, even though a chelating agent is used, reducing the soil's permeability and hindering the chemicals' diffusion. When chemicals are injected using a well, iron (III) hydroxide can also clog the strainer. In these cases, the area of chemical remediation is limited to the immediate vicinity of the injection pipe or well.

[0005] Therefore, an object of the present invention is to provide a soil remediation method for decomposing contaminants in soil in situ, in which the continuity of the Fenton reaction in the neutral range is improved. [Means for solving the problem]

[0006] The inventors investigated a purification method using the Fenton reaction in the neutral pH range and found that the organic compounds used as chelating agents, or complexes of chelating agents and trivalent iron ions, may be decomposed by hydroxyl radicals, resulting in the precipitation of iron(III) hydroxide. Through extensive research, the inventors found that the use of certain chelating agents can retain trivalent iron as iron ions even after the chelating agents are decomposed to some extent by hydroxyl radicals, thereby maintaining soil purification. Furthermore, even if the decomposition of the chelating agent proceeds further and iron(III) hydroxide is produced, the resulting iron(III) hydroxide is highly dispersible and can be retained in water for long periods without precipitating, thereby suppressing soil clogging.

[0007] The present invention provides a soil remediation method for decomposing contaminants in soil in situ, which comprises injecting a chelating agent and hydrogen peroxide into the soil, the chelating agent being an aminocarboxylic acid compound or a salt thereof having one hydroxyl group and two or more coordination sites other than the one hydroxyl group that can coordinate to an iron atom.

[0008] The chelating agent used here is thought to form a chelate with trivalent iron ions through the two or more coordination sites of the aminocarboxylic acid compound, and also to be able to coordinate hydroxyl groups to the iron atom. This stabilizes the complex, making it difficult for trivalent iron ions to be liberated. As a result, soil into which the agent has been injected is less susceptible to a decrease in permeability due to the precipitation of iron(III) hydroxide, and the range of penetration of the agent is expanded, allowing for the purification of a wide area of soil.

[0009] One hydroxyl group of the chelating agent may be bonded to the α-carbon of the carbonyl group.

[0010] The chelating agent may be a compound represented by the following formula (10):

[0011] [ka] (In the formula, R is a substituent having 1 to 8 carbon atoms, and X is hydrogen or an alkali metal.)

[0012] The chelating agent may be a compound represented by the following formula (20):

[0013] [ka] (wherein X is hydrogen or an alkali metal.)

[0014] In addition, considering that the chelating agent will be injected into the ground, it is preferable that the chelating agent be biodegradable.

[0015] The purification method of the present invention may be carried out by injecting a chelating agent and hydrogen peroxide into a well. Injection using a well does not require a large site, so it can be carried out even on a small site. In this case, clogging of the well strainer is also prevented.

[0016] In the purification method of the present invention, the chelating agent may be injected into the soil first, and then hydrogen peroxide may be injected into the soil after a predetermined time has elapsed. Alternatively, the chelating agent and hydrogen peroxide may be injected into the soil simultaneously. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a soil remediation method for decomposing contaminants in soil in situ, in which the continuity of the Fenton reaction in a neutral range is improved. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a cross-sectional view showing soil and a well according to an embodiment of the present invention. [Figure 2] (A) is a graph showing the decomposition status of pollutants, and (B) is a graph showing the presence of dissolved iron. [Figure 3] (A) is a photograph showing the state before adding hydrogen peroxide, and (B) is a photograph showing the state after adding hydrogen peroxide. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

[0020] The remediation method of this embodiment is a technology for in-situ remediation of underground soil, in which chemicals are injected into the soil to decompose contaminants. The contaminants are assumed to be volatile organic compounds (VOCs). In this specification, "soil" is a concept that includes not only soil present underground but also groundwater.

[0021] <Drugs> The chemicals to be injected into the soil are explained below. The chemicals are a chelating agent and hydrogen peroxide. The chelating agent is an aminocarboxylic acid compound or its salt having one hydroxyl group and two or more coordination points other than the one hydroxyl group that can coordinate to an iron atom. The number of coordination points possessed by the chelating agent may be three or more, or may be four or more. The upper limit of the coordination points is, for example, ten, nine, or eight. When the aminocarboxylic acid compound is in the form of a salt, it is preferably in the form in which the carboxylic acid moiety is a sodium salt or a potassium salt.

[0022] The aminocarboxylic acid compound may have 12 or less, 11 or less, or 10 or less carbon atoms. The aminocarboxylic acid compound is preferably highly water-soluble. The aminocarboxylic acid compound is preferably a non-natural compound.

[0023] The "one hydroxyl group" may be located anywhere in the overall chemical structure of the aminocarboxylic acid compound or its salt, but it is particularly preferred that it be bonded to the α-carbon of the carbonyl group. This positions the hydroxyl group near the iron ion when a chelate is formed with the iron ion, so that interaction between the hydroxyl group and the iron ion can be expected. The aminocarboxylic acid compound or its salt may have multiple hydroxyl groups in the overall chemical structure.

[0024] The chelating agent may be a compound represented by the following formula (10):

[0025] [ka] (In the formula, R is a substituent having 1 to 8 carbon atoms, and X is hydrogen or an alkali metal.)

[0026] In formula (10), the number of carbon atoms constituting the moiety of the substituent R may be 7, 6, 5, 4, or 2. The carbon skeleton of the substituent R may be linear or branched. The substituent R may have 1 to 4 functional groups such as a hydroxyl group, an amino group, or an acidic group. Examples of the acidic group include a carboxyl group or a sulfo group. The acidic group may be in the form of a salt (e.g., an alkali metal salt).

[0027] The chelating agent may be a compound represented by the following formula (20), or may be a compound represented by the following formula (4) or a salt thereof.

[0028] [ka] (wherein X is hydrogen or an alkali metal.)

[0029] [ka]

[0030] Preferably, the chelating agent is biodegradable.

[0031] <Injection method> The method for injecting the chemical into the soil is not particularly limited as long as it can deliver the chemical to the contaminated area. For example, as shown in Figure 1, a well 3 is drilled toward the predicted contaminated area 2 in the soil 1, and an appropriate injection pipe 4 is inserted to inject the chemical. The chemicals may be injected simultaneously as a chelating agent and hydrogen peroxide, or, to minimize the self-decomposition of hydrogen peroxide, the chelating agent may be injected first, followed by the hydrogen peroxide after a predetermined time has elapsed. The injected chemical diffuses into the surrounding area of the well by natural diffusion or via groundwater. Injection using a well 3 does not require a large site, so it can be carried out even on a small site.

[0032] Upon injection, the concentration of the chelating agent alone is preferably 1.0 mmol / L to 100 mmol / L, more preferably 10 mmol / L to 50 mmol / L, and the concentration of hydrogen peroxide alone is preferably 100 mmol / L to 10,000 mmol / L, more preferably 500 mmol / L to 5,000 mmol / L.

[0033] When the chelating agent and hydrogen peroxide are mixed before injection, the concentration of the chelating agent is preferably 0.5 mmol / L to 50 mmol / L, more preferably 5 mmol / L to 25 mmol / L, and the concentration of the hydrogen peroxide is preferably 50 mmol / L to 5000 mmol / L, more preferably 250 mmol / L to 2500 mmol / L.

[0034] For larger sites, drilling and injection can be performed using a boring machine, followed by mixing within the soil. Although not shown, a borehole is excavated using a boring machine. A double-tube injection rod with an injection mechanism at its tip is then inserted into the borehole. A solution containing a chelating agent is injected horizontally from the inner tube of the double-tube, and compressed air is passed through the outer tube, both of which are injected horizontally from the injection mechanism to cut the contaminated area. By raising and lowering the injection rod while injecting, a wide area can be cut in the depth direction. This allows the contaminated area to be mixed with the chelating agent over a wide area while cutting. Hydrogen peroxide is then injected from the inner tube. If a triple-tube injection rod is used, the chelating agent, hydrogen peroxide, and compressed air can be injected simultaneously using separate systems.

[0035] <Action and effect> In the remediation method of this embodiment, the Fenton reaction proceeds in the soil. Specifically, a chelating agent injected into the soil forms a chelate with divalent iron ions present in the soil. The injected hydrogen peroxide oxidizes the divalent iron ions to trivalent iron ions, and the hydrogen peroxide converts them to hydroxy ions and hydroxy radicals. The highly active hydroxy radicals react with contaminants (volatile organic compounds) and decompose them. Meanwhile, the chelate-forming iron trivalent ions are prevented from precipitating as iron (III) hydroxide and are eventually converted back to divalent iron ions by hydrogen peroxide or other chemicals. Because the chelating agent captures the iron ions, these reactions can continue even in a neutral pH range where iron (III) hydroxide is likely to form. If these reactions proceed smoothly, iron (III) hydroxide is less likely to form, allowing the chemical to diffuse over a wide area without reducing the soil's permeability.

[0036] In the past, despite the use of chelating agents, their effectiveness was not fully demonstrated, resulting in reduced water permeability (i.e., precipitation of iron(III) hydroxide). However, the purification method of the present embodiment enhances the sustainability of the Fenton reaction in the neutral range. The reason why the chelating agent of the present embodiment enhances the sustainability of the Fenton reaction is presumably because the hydroxyl groups present in the chelating agent molecule of the present embodiment interact with (e.g., coordinate with) the iron atom, making the chelate more stable, or inhibiting the coordination of hydroxide ions with the iron atom, thereby suppressing the formation of iron(III) hydroxide. Furthermore, if the chelating agent has many coordination sites available for coordination with the iron atom, it is presumed that the capture / recapture of the iron atom will not be significantly affected even if the chelating agent is attacked by hydroxyl radicals.

[0037] As a result, the purification method of this embodiment is less likely to cause a decrease in permeability due to the precipitation of iron (III) hydroxide in the soil into which the chemical has been injected, compared to conventional purification methods using the Fenton reaction, and the range of penetration of the chemical is expanded, making it possible to purify a wide area of soil. In particular, when a well 3 is used to inject the chemical, as shown in Figure 1, the strainer is less likely to become clogged with iron (III) hydroxide, making it possible to carry out the method even in narrow spaces.

[0038] Although preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments. For example, the above embodiments illustrate the case where the target soil originally contains a sufficiently high concentration of divalent iron ions and utilizes these. However, if the soil does not contain a sufficiently high concentration of divalent iron ions (or heavy metals that, like iron, promote the generation of hydroxyl radicals), divalent iron ions may be injected into the soil. In this case, the chelating agent may be injected in a state where it has formed a complex with the iron ions. The concentration of the injected iron ions is preferably set so that the total concentration of the iron ions contained in the soil in the area to be purified is 0.2 to 2.0 times (molar ratio) that of the chelating agent. [Example]

[0039] The present invention will be described in more detail below with reference to experimental examples, but the present invention is not limited to the following examples.

[0040] <Experiment 1: Test simulating mixing chemicals on the ground and injecting them into the soil> 1,4-Dioxane was used as a pollutant to simulate its decomposition and the behavior of dissolved iron ions.

[0041] [Chelating agent] Three types of chelating agents were prepared. A chelating agent that is an aminocarboxylic acid compound with a hydroxyl group bonded to the alpha carbon of the carbonyl group (formula (1) below: also known as "HIDS"; manufactured by Nippon Shokubai Co., Ltd.) A chelating agent that is an aminocarboxylic acid compound and does not have a hydroxyl group (formula (2) below: also known as "MGDA", manufactured by BASF) A chelating agent that is an aminocarboxylic acid compound and does not have a hydroxyl group (formula (3) below: also known as "(S,S)-EDDS", manufactured by Cherest Co., Ltd.)

[0042] [ka]

[0043] [reagent] Using water as the solvent, the reagents were prepared as follows: Iron(III) ions = 10 mmol / L Each chelating agent = 10mmol / L Hydrogen peroxide = 10,000 mmol / L 1,4-dioxane = 1000 mg / L

[0044] [Implementation Procedure] Aqueous solutions of each chelating agent were mixed with solutions of iron(III) ions and hydrogen peroxide. The post-mixing concentrations were 1.0 mmol / L for each chelating agent and iron ion, and 75 mmol / L for hydrogen peroxide. A blank sample containing no chelating agent was also prepared. After each sample was allowed to stand for 30 minutes, 1,4-dioxane aqueous solution was added to a concentration of 5 mg / L. The concentrations of 1,4-dioxane and dissolved iron were then measured at predetermined intervals using gas chromatography and colorimetry, respectively. The pH of the mixed solutions during the test was approximately 6.0.

[0045] [result] The change in 1,4-dioxane concentration over time is shown in Figure 2(A). When HIDS was used as the chelating agent, 1,4-dioxane decomposition continued well even 5 hours after the addition of 1,4-dioxane. In contrast, when MGDA and (S,S)-EDDS were used, 1,4-dioxane decomposition was good for the first 30 minutes after the addition of 1,4-dioxane, but after that, decomposition of 1,4-dioxane only progressed to the same extent as when no chelating agent was added (blank).

[0046] The time course of dissolved iron concentration is shown in Figure 2(B). When HIDS was used as the chelating agent, chelate formation occurred within 30 minutes before the addition of 1,4-dioxane. Approximately 80% of the iron remained dissolved even 5 hours after the addition of 1,4-dioxane. In contrast, when MGDA and (S,S)-EDDS were used, the chelating ability of most of the chelating agent was lost due to hydrogen peroxide within 30 minutes before the addition of 1,4-dioxane. Finally, 1.5 hours after the addition of 1,4-dioxane, almost all of the iron precipitated.

[0047] <Experiment 2: Test simulating sequential injection of chemicals into soil> The interaction between chelating agents, iron ions, and hydrogen peroxide was simulated.

[0048] [Chelating agent] Four types of chelating agents were prepared. A chelating agent that is an aminocarboxylic acid compound and does not have a hydroxyl group (above formula (3): also called "(S,S)-EDDS") A chelating agent that is an aminocarboxylic acid compound and does not have a hydroxyl group (above formula (2): also called "MGDA") A chelating agent that is an aminocarboxylic acid compound and has a hydroxyl group on a moiety other than the α-carbon of the carbonyl group (formula (4) below: also known as "DHEG") Chelating agents that are aminocarboxylic acid compounds with a hydroxyl group bonded to the alpha carbon of the carbonyl group (formula (1) above: also known as "HIDS")

[0049] [ka]

[0050] [reagent] Using water as the solvent, the reagents were prepared as follows: Iron(III) ions = 200mmol / L Each chelating agent = 200mmol / L

[0051] [Implementation Procedure] 2 mL of each chelating agent solution was measured into a colorless, transparent centrifuge tube. 28 mL of water was added to each tube. Next, 2 mL of the above aqueous solution of trivalent iron ions was added. The pH was adjusted to 6-7 using aqueous sodium hydroxide, and the solution was then diluted to 40 mL. This resulted in a 1:1 molar ratio of chelating agent to iron, and a concentration of 10 mmol / L for each. Each centrifuge tube was left to stand for 2 hours, and the appearance of each solution was observed (Figure 3(A)).

[0052] After that, hydrogen peroxide was added to each centrifuge tube to a concentration of 750 mmol / L and allowed to stand for 30 minutes. 0.4 mL of methanol was added to stop the activity of hydrogen peroxide. After allowing to stand for 2 hours, the appearance of each solution was observed (Figure 3(B)).

[0053] [result] Before the addition of hydrogen peroxide, no precipitation occurred with any of the chelating agents, as shown in Figure 3(A). After the addition of hydrogen peroxide, as shown in Figure 3(B), when compounds without a hydroxyl group ((S,S)-EDDS and MGDA) were used as chelating agents, a large amount of precipitation was observed. When a compound with a hydroxyl group at a position other than the α-carbon of the carbonyl group (DHEG) was used as a chelating agent, a small amount of precipitation was observed, but most of the precipitation was dissolved or dispersed. When a compound with a hydroxyl group bonded to the α-carbon of the carbonyl group (HIDS) was used as a chelating agent, no precipitation was observed at all. [Industrial Applicability]

[0054] The present invention can be used for soil remediation. [Explanation of symbols]

[0055] 1...soil, 2...contaminated area, 3...well, 4...injection pipe.

Claims

1. A method for remediating soil by decomposing contaminants in soil in situ, comprising: injecting a chelating agent and hydrogen peroxide into the soil; The chelating agent is an aminocarboxylic acid compound or a salt thereof having one hydroxyl group and two or more coordination sites other than the one hydroxyl group that can coordinate to an iron atom.

2. The purification method according to claim 1 , wherein the one hydroxyl group is bonded to the α-carbon of a carbonyl group.

3. 2. The purification method according to claim 1, wherein the chelating agent is a compound represented by the following formula (10): 【Chemical 1】 (wherein R is a substituent having 1 to 8 carbon atoms, and X is hydrogen or an alkali metal.)

4. 2. The purification method according to claim 1, wherein the chelating agent is a compound represented by the following formula (20): 【Chemistry 2】 (wherein X is hydrogen or an alkali metal.)

5. The method of claim 1 , wherein the chelating agent is biodegradable.

6. 2. The method of claim 1, wherein the chelating agent and the hydrogen peroxide are injected into a well.

7. 2. The method of claim 1, wherein the chelating agent is first injected into the soil, and then the hydrogen peroxide is injected into the soil after a predetermined time has elapsed.

8. 2. The method of claim 1, wherein the chelating agent and the hydrogen peroxide are injected into the soil simultaneously.

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