Treatment method for selenium-contaminated soil and embankment structure

Abstract

To provide a method for treating selenium-contaminated soil that suppresses the elution of selenium in the soil, and also suppress the elution of barium ions contained in a barium compound added to the soil for insolubilization of selenium, and a banking structure.SOLUTION: A method for treating selenium-contaminated soil includes the step of mixing selenium-containing soil with a barium compound and a solidification material for insolubilization of the selenium.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for treating selenium-contaminated soil and an embankment structure. [Background technology]

[0002] Construction work and other projects generate large amounts of contaminated soil containing naturally occurring heavy metals (hereinafter referred to as "heavy metal-contaminated soil"). Therefore, there is a need for simpler, cheaper, and safer methods for treating contaminated soil. The above-mentioned heavy metal-contaminated soil is mixed with insolubilizing materials such as cement or magnesium oxide to prevent the heavy metals from dissolving in water (insolubilization), thereby suppressing the amount of heavy metal elution. Heavy metal-contaminated soil with the amount of heavy metal elution suppressed is then sealed in a water barrier layer containing clay such as bentonite to prevent contact with rainwater.

[0003] Among the heavy metals contained in heavy metal-contaminated soil, those that exist in the form of cations such as lead and chromium are easily adsorbed by the soil and can be relatively easily insolubilized. On the other hand, those that exist in the form of anions such as arsenic and selenium are less likely to be adsorbed by the soil and are difficult to insolubilize. Among these, selenium is mainly found in the form of hexavalent selenium acid (SeO4 2- ) and tetravalent selenious acid (SeO3 2-Selenium exists in the form of selenium (selenium nitrate). Selenium nitrate, in its oxidized form, has high solubility and is extremely difficult to insolubilize. To insolubilize selenium, a large amount of insolubilizing agent must be added to heavy-metal-contaminated soil. However, adding a large amount of insolubilizing agent increases the volume of the heavy-metal-contaminated soil or causes it to solidify more than necessary. The increase in the volume of heavy-metal-contaminated soil puts strain on the capacity of disposal sites. Excessive solidification of heavy-metal-contaminated soil makes it difficult to transport and work on the soil. Furthermore, when selenium nitrate is reduced, it becomes selenite, which has a lower solubility, thereby suppressing the amount of selenium leaching. Therefore, insolubilizing agents containing reducing agents such as iron oxide or sulfite are considered effective for selenium leaching. By reducing selenate to selenite using a reducing agent, the amount of selenium leaching is efficiently suppressed. However, when selenious acid is exposed to the atmosphere for a long period of time, it is oxidized to selenic acid upon contact with oxygen, increasing the amount of selenium eluted. Therefore, even if selenium is effectively insolubilized during treatment, there is a concern that selenium may be re-eluted over time.

[0004] As a method for insolubilizing selenium, for example, a method is known in which barium ions and sulfate ions (only barium ions in selenium-containing wastewater that also contains sulfate ions) are mixed with a selenium-containing substance to produce barium sulfate, thereby trapping barium selenate in crystals and insolubilizing the selenium (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-234306 Summary of the Invention [Problem to be solved by the invention]

[0006] However, soluble barium compounds are toxic, and barium compounds other than barium sulfate and barium 4-(5-chloro-4-methyl-2-sulfonatophenylazo)-3-hydroxy-2-naphthate are designated as hazardous substances. To insolubilize heavy metals, a sufficient amount of barium must be added to the contaminated soil. This raises the risk of incompletely crystallized barium leaching from the contaminated soil, resulting in the leakage of barium ions. Furthermore, adding barium sulfate to contaminated soil does not have the effect of insolubilizing cerium, because barium sulfate is poorly soluble.

[0007] The present invention was devised in consideration of the above circumstances, and aims to provide a method for treating selenium-contaminated soil and an embankment structure that suppresses the leaching of selenium in the soil and the leaching of barium ions contained in barium compounds added to the soil to insolubilize selenium. [Means for solving the problem]

[0008] The present invention has the following aspects. [1] A method for treating soil containing selenium by mixing a barium compound and a solidifying agent, and then insolubilizing the selenium. death, the solidification material is at least one selected from the group consisting of cement, magnesium oxide, and an insolubilizing material; The amount of the barium compound added to the soil is 0.1 g / 100 g-dry soil or more and 10 g / 100 g-dry soil or less, The amount of the solidification material added to the soil is 0.5 g / 100 g-dry soil or more and 40 g / 100 g-dry soil or less. A method for treating selenium-contaminated soil. [2] The method for treating selenium-contaminated soil according to [1], further comprising the step of forming an embankment structure having an embankment layer made of the soil that has been insolubilized. [3] The method for treating selenium-contaminated soil according to [2], wherein in the step of forming the embankment structure, a structure having at least one of physical and chemical properties that suppress the elution of barium ions is provided below the embankment layer. 。 [4] The barium compound is barium chloride, [1] to [ 3 10. The method for treating selenium-contaminated soil according to claim 9, wherein the selenium-contaminated soil is a selenium-containing soil. [5 ] S Soil containing lent and a barium compound and a solidifying material mixed with the soil. Equipped with an embankment layer 、 the solidification material is at least one selected from the group consisting of cement, magnesium oxide, and an insolubilizing material; The amount of the barium compound added to the soil is 0.1 g / 100 g-dry soil or more and 10 g / 100 g-dry soil or less, The amount of the solidification material added to the soil is 0.5 g / 100 g-dry soil or more and 40 g / 100 g-dry soil or less. An embankment structure. [ 6 ] a structure provided below the embankment layer and having a mechanism for suppressing the elution of barium ions; 5 ] Embankment structure described in. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a method for treating selenium-contaminated soil and an embankment structure that suppresses the leaching of selenium in the soil and the leaching of barium ions contained in barium compounds added to the soil to insolubilize selenium. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic diagram showing an example of a method for treating selenium-contaminated soil and an embankment structure according to an embodiment of the present invention. [Figure 2] 1 is a schematic diagram showing an example of a method for treating selenium-contaminated soil and an embankment structure according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] A method for treating selenium-contaminated soil and an embankment structure according to an embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a schematic diagram showing an example of a method for treating selenium-contaminated soil and an embankment structure according to an embodiment of the present invention. It should be noted that the present embodiment is specifically described to allow a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[0012] (First embodiment) [Methods for treating selenium-contaminated soil, embankment structure] The method for treating selenium-contaminated soil according to this embodiment includes a step of mixing selenium-containing soil with a barium compound and a solidifying agent and insolubilizing the selenium (hereinafter referred to as "Step A"). The method for treating selenium-contaminated soil according to this embodiment may further include a step of forming an embankment structure including an embankment layer made of insolubilized soil (hereinafter referred to as "Step B").

[0013] (Step A (selenium insolubilization treatment step)) In step A, a barium compound and a solidifying agent are mixed with soil containing selenium (selenium-contaminated soil).

[0014] Examples of soil containing selenium include excavated soil generated during ground excavation or mountain excavation. The selenium-containing soil may also contain other heavy metals such as arsenic, lead, etc. Heavy metals such as selenium in the soil may exist as simple elements or as compounds. The selenium content in the soil may be similar to the selenium content in soil that has conventionally been subject to containment using an adsorption layer method or the like (soil requiring countermeasures).

[0015] The barium compound is not particularly limited as long as it can insolubilize selenium, and examples thereof include barium chloride (BaCl), barium carbonate (BaCO), barium acetate ((CHCOO)Ba), barium phosphate (BaPO), barium hydroxide (Ba(OH)), metallic barium (Ba), barium oxide (BaO), etc. Among these barium compounds, barium chloride is preferred because of its excellent ability to insolubilize selenium. Insolubilizing selenium with a barium compound means that the amount of selenium compounds contained in soil that leach out is less than before insolubilization, even when soil and water are present.

[0016] In this embodiment, the amount of barium compound added to soil is expressed as the mass (g) of barium compound per 100 g of dry soil. The unit used to express the amount of barium compound added to soil is "g / 100 g-dry soil." The soil used here is crushed to a particle size of 2 mm or less and then dried for one week at room temperature (25°C).

[0017] The amount of barium compound added to soil is preferably 0.1 g to 10 g / 100 g dry soil, and more preferably 0.5 g to 5 g / 100 g dry soil. If the amount of barium compound added is equal to or greater than the lower limit, selenium can be effectively insolubilized by mixing with selenium-containing soil. On the other hand, if the amount of barium compound added is equal to or less than the upper limit, excess barium compound can be prevented from leaching without crystallizing and leaking into the environment.

[0018] The solidifying agent is not particularly limited as long as it can insolubilize the barium compound, and examples thereof include cement, magnesium oxide (MgO), an insolubilizing agent, and the like. Examples of cement include portlite cement, blast furnace cement, fly ash cement, and silica cement. Examples of insolubilizing agents include magnesium oxide-based insolubilizing agents, hydrotalcite-based insolubilizing agents, and schwertmannite-based insolubilizing agents. These solidifying materials may be used alone or in combination of two or more.

[0019] In this embodiment, the amount of solidification material added to soil is expressed in the same manner as the amount of barium compound added to soil. The amount of solidification material added to soil is preferably 0.5 g to 40 g / 100 g dry soil, and more preferably 1 g to 10 g / 100 g dry soil. If the amount of solidification material added is equal to or greater than the lower limit, selenium can be effectively insolubilized by mixing with selenium-containing soil. On the other hand, if the amount of solidification material added is equal to or less than the upper limit, excessive increase in the volume of heavy metal-contaminated soil and excessive solidification of the heavy metal-contaminated soil can be prevented.

[0020] The method for mixing the barium compound and the solidifying material into the soil is not particularly limited, but examples thereof include mixing with a backhoe or a self-propelled mixer.

[0021] (Process B (Process for forming embankment structure)) In step B, an embankment structure is formed having an embankment layer made of the soil that has been insolubilized in step A. In step B, specifically, an embankment structure 10 shown in FIG. 1 is formed. The embankment structure 10 comprises an embankment layer 11, a protective embankment layer 12, and a water-impermeable layer 13.

[0022] The embankment layer 11 is provided on the ground G. The protective embankment layer 12 is provided so as to cover the side and top surfaces of the embankment layer 11. The water shielding layer 13 is provided so as to cover the bottom surface of the embankment layer 11 and the side and top surfaces of the protective embankment layer 12. That is, from the bottom of the embankment structure 10, the water shielding layer 13, the embankment layer 11, the protective embankment layer 12, and the water shielding layer 13 are layered in this order.

[0023] The widthwise cross section of each of the embankment layer 11, the protective embankment layer 12, and the water-impermeable layer 13 is trapezoidal, becoming wider as it goes downward. At the underside of the embankment structure 10, the upper surface of the water-impermeable layer 13 and the lower surface of the embankment layer 11 are in contact, and the width of the upper surface of the water-impermeable layer 13 is wider than the width of the lower surface of the embankment layer 11. The cross section of the entire embankment structure 10 is also trapezoidal.

[0024] A through-hole 14 is formed below the embankment structure 10, penetrating the water-impermeable layer 13 in the thickness direction (height direction of the embankment structure 10). Moisture contained in the embankment layer 11 is discharged to the outside of the embankment structure 10 through the through-hole 14.

[0025] The banking layer 11 is formed from soil that has been subjected to insolubilization treatment in step A. The thickness of the embankment layer 11 is preferably 0.5 m or more and 200 m or less, and more preferably 1 m or more and 100 m or less.

[0026] The protective embankment layer 12 has a structure that is provided so as not to damage the embankment layer 11 during construction, and has water permeability (the property of passing through the moisture contained in the embankment layer 11). Because the protective embankment layer 12 is water permeable, it does not have a structure that swells due to water absorption, such as bentonite. The thickness of the protective embankment layer 12 is preferably 0.1 m or more and 5 m or less, and more preferably 0.3 m or more and 1 m or less.

[0027] The water-blocking layer 13 contains an alkaline water-blocking clay. Examples of alkaline waterproofing agents include clay and cement-based waterproofing agents. The clay has a permeability of 1 x 10 when it is made into a layer of 0.5 m thick. -8 Clay having a viscosity of not more than cm / s is used. Examples of clay include bentonite. The alkalinization inhibitor contained in the water shielding layer 13 may be one type or two or more types. The water-blocking layer 14 may be made of only an alkaline water-blocking agent, or may contain other materials in addition to the alkaline water-blocking agent. The water-blocking performance of the water-blocking layer 13 may be the same as that of the water-blocking layer in the conventional adsorption layer construction method or the like. The thickness of the water shielding layer 13 is preferably 0.1 m or more and 3 m or less, and more preferably 0.3 m or more and 1 m or less.

[0028] The embankment structure 10 can be constructed, for example, by the following procedure. On the ground G, a waterproof layer 13, which is the bottom layer of the embankment structure 10, is formed using alkaline waterproof clay. Next, the soil that has been insolubilized in step A is piled up on the water-impermeable layer 13 to form the banking layer 11. The sides and top of the fill layer 11 are then covered with a protective fill layer 12 . Next, the sides and top of the protective embankment layer 12 are covered with alkaline waterproof clay to form a waterproof layer 13.

[0029] According to the method for treating contaminated soil of this embodiment, a barium compound and a solidifying material are mixed with soil containing selenium, and the selenium contained in the soil is insolubilized, thereby suppressing the leaching of selenium from the soil.

[0030] According to the embankment structure 10 of this embodiment, the embankment layer 11 is made of soil containing selenium that has been insolubilized, and therefore it is possible to suppress the dissolution of barium ions contained in the barium compound added to the soil to insolubilize the selenium, and the dissolution of selenium contained in the soil.

[0031] (Second embodiment) [Methods for treating selenium-contaminated soil, embankment structure] The method for treating selenium-contaminated soil according to this embodiment includes the above-mentioned step B, in which a structure having at least one of physical and chemical properties that suppress the elution of barium ions is provided below the fill layer.

[0032] (Process B (process of setting up structures)) In step B, a structure having at least one of physical and chemical properties that inhibit the elution of barium ions is provided below the fill layer made of soil that has been insolubilized in step A. In step B, specifically, an embankment structure 20 shown in Fig. 2 is formed. In the embankment structure 20 according to this embodiment, the same components as those in the previous embodiment are denoted by the same reference numerals, and their explanation will be omitted, with only the differences being explained. The embankment structure 20 comprises an embankment layer 11, a protective embankment layer 12, a water shielding layer 13, and a structure 21.

[0033] The structure 21 is provided below the embankment layer 11. The protective embankment layer 12 is provided so as to cover a portion of the upper surface (outer edge) of the structure 21 and the side and upper surface of the embankment layer 11. The waterproof layer 13 is provided so as to cover the side and lower surface of the structure 21 and the side and upper surface of the protective embankment layer 12. That is, from the bottom of the embankment structure 10, the waterproof layer 13, the structure 21, the embankment layer 11, the protective embankment layer 12, and the waterproof layer 13 are layered in this order.

[0034] The cross section in the width direction of each of the waterproof layer 13, structure 21, and embankment layer 11 is trapezoidal in shape, becoming wider as it goes downward. At the underside of the embankment structure 20, the upper surface of the waterproof layer 13 and the lower surface of the structure 21 are in contact, and the width of the upper surface of the waterproof layer 13 and the width of the lower surface of the structure 21 are the same. In addition, the upper surface of the structure 21 and the lower surface of the embankment layer 11 are in contact, and the width of the upper surface of the structure 21 is larger than the width of the lower surface of the embankment layer 11. The cross section of the entire embankment structure 20 is also trapezoidal.

[0035] The structure 21 has at least one of physical and chemical properties that inhibit the elution of barium ions contained in the barium compound in the embankment layer 11. The structure 21 is not particularly limited as long as it has the above-mentioned properties. For example, the physical property may be to retain barium ions within the structure by mixing an adsorbent that adsorbs cations, and the chemical property may be to precipitate barium ions in the seepage water contained in the embankment layer 11 as barium sulfate within the structure. Examples of chemical properties include a method of insolubilizing the barium ions to prevent them from dissolving in the wastewater discharged from the embankment structure 20 through the through-holes 14, and a function of converting barium ions into harmless compounds (e.g., barium sulfate, barium = 4-(5-chloro-4-methyl-2-sulfonatophenylazo)-3-hydroxy-2-naphthalate). Furthermore, the structure 21 has water permeability (the property of passing through the water contained in the embankment layer 11). Because the structure 21 has water permeability, it does not have a structure such as bentonite that swells due to water absorption. The structure 21 may also serve as the protective embankment layer 12, or the protective embankment layer 12 may also serve as the structure 21. The thickness of the structure 21 is preferably 0.1 m or more and 20 m or less, and more preferably 0.3 m or more and 10 m or less.

[0036] The embankment structure 20 can be constructed, for example, by the following procedure. On the ground G, a waterproof layer 13, which is the bottom layer of the embankment structure 10, is formed using alkaline waterproof clay. Next, the structure 21 is placed on the water impermeable layer 13 . Next, the soil that has been insolubilized in step A is piled up on the structure 21 to form the piled up soil layer 11. Next, the portion (outer edge) of the upper surface of the structure 21 that is not covered with the embankment layer 11 and the sides and upper surface of the embankment layer 11 are covered with the protective embankment layer 12. Next, the sides of the structure 21 and the sides and top of the protective embankment layer 12 are covered with alkaline waterproof clay to form the waterproof layer 13.

[0037] According to the method for treating contaminated soil of this embodiment, by providing a structure 21 having a mechanism for suppressing the elution of barium ions below an embankment layer 11 consisting of soil that has been insolubilized, it is possible to suppress the elution of barium ions contained in the barium compound added to the soil to insolubilize selenium.

[0038] The embankment structure 20 of this embodiment comprises an embankment layer 11 made of soil containing insolubilized selenium, and a structure 21 located below the embankment layer 11 and having a mechanism for suppressing the leaching of barium ions, thereby making it possible to suppress the leaching of barium ions contained in the barium compound added to the soil to insolubilize the selenium.

[0039] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. Each configuration and combination thereof in the above embodiments is an example, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope of the present invention. [Example]

[0040] The present invention will be explained in more detail below by way of experimental examples, but the present invention is not limited to the following experimental examples.

[0041] [Experimental Example] Rocks containing naturally occurring arsenic and selenium were crushed to particles of 2 mm or less and then dried at room temperature. The amount of barium eluted, the concentration of selenium eluted, and the concentration of arsenic eluted were compared between the cases where a barium compound or a solidifying agent was added to the crushed rock (hereinafter referred to as "sample") and the cases where no barium compound or a solidifying agent was added. Barium chloride was used as the barium compound, and was added to the sample in an amount of 0.5 parts by mass per 100 parts by mass of the sample. The solidification materials used were cement (blast furnace cement type B), an insolubilizer (magnesium oxide (MgO)-based insolubilizer), and magnesium oxide (MgO). The solidification material was added to the sample so that the amount of solidification material added was 4 parts by mass per 100 parts by mass of the sample. After thoroughly mixing the sample to which barium chloride and solidifying agent had been added, water equivalent to 15% of the sample's mass was added and the mixture was further mixed. The mixed sample was packed in a polypropylene pack, sealed, and cured at 20°C for 7 days. In order to measure the amount of heavy metals eluted from the cured samples, test solutions were prepared in accordance with the Ministry of the Environment Notification No. 46 of 2008 (Environment Notification No. 46), and the barium concentration (barium elution concentration), selenium concentration (selenium elution concentration), and arsenic concentration (arsenic elution concentration) were measured. This procedure was repeated three times, and the residue after preparation of the test solution was used to conduct the elution test again according to Environmental Notification No. 46 Test, and the cumulative amount of eluted barium per unit mass of rock was calculated. The results are shown in Table 1.

[0042] [Table 1]

[0043] The results shown in Table 1 show that when only barium chloride was added, the cumulative amount of barium leaching was 27 mg / kg, whereas when barium chloride and a solidification agent were added, the cumulative amount of barium leaching was suppressed to 18 mg / kg to 20 mg / kg.

[0044] In addition, the selenium concentration (eluted selenium concentration) and the arsenic concentration (eluted arsenic concentration) were measured in the same manner as in the measurement of the cumulative eluted amount of barium. The eluted selenium concentration is shown in Table 2, and the eluted arsenic concentration is shown in Table 3.

[0045] [Table 2]

[0046] [Table 3]

[0047] The results shown in Tables 2 and 3 indicate that the selenium insolubilization effect of barium chloride and the arsenic insolubilization effect of the solidification material are not impaired by mixing barium chloride with the solidification material. Therefore, it was confirmed that the method of this experimental example is also effective for soil containing heavy metals such as arsenic and selenium. Furthermore, the results shown in Table 2 indicate that when only barium chloride was added, the selenium elution concentration increased in the second test compared to the first, whereas adding a solidifying agent did not increase the selenium elution concentration. This is thought to be because the elution of barium was suppressed, allowing the suppression of selenium elution to be maintained even after multiple elution tests. In other words, when barium chloride is mixed with a solidifying agent, selenium elution can be suppressed for a longer period of time than when a barium compound is used alone. Furthermore, the results shown in Table 3 indicate that when only barium chloride was added, the arsenic elution concentration increased in the second test compared to the first, whereas adding a solidification material prevented this increase. This is thought to be because the elution of barium was suppressed, allowing the suppression of arsenic elution to be maintained even after multiple elution tests. In other words, when barium chloride is mixed with a solidification material, it is thought that arsenic elution can be suppressed for a longer period of time than when barium chloride is used alone. [Explanation of symbols]

[0048] 10,20 Embankment structure 11 Fill layer 12 Protective fill layer 13 Water impermeable layer 14 through holes 21 Structures

Claims

1. The method includes the steps of: mixing a barium compound and a solidifying material with soil containing selenium; and performing an insolubilization treatment of the selenium. The solidification material is at least one selected from the group consisting of cement, magnesium oxide, and an insolubilizing material; The amount of the barium compound added to the soil is 0.1 g / 100 g-dry soil or more and 10 g / 100 g-dry soil or less, A method for treating selenium-contaminated soil, wherein the amount of the solidification material added to the soil is 0.5 g / 100 g-dry soil or more and 40 g / 100 g-dry soil or less.

2. 2. The method for treating selenium-contaminated soil according to claim 1, further comprising the step of forming an embankment structure having an embankment layer made of the soil that has been subjected to the insolubilization treatment.

3. 3. The method for treating selenium-contaminated soil according to claim 2, wherein in the step of forming the embankment structure, a structure having at least one of physical properties and chemical properties for suppressing the elution of barium ions is provided below the embankment layer.

4. 4. The method for treating selenium-contaminated soil according to claim 1, wherein the barium compound is barium chloride.

5. A method for producing a selenium-containing soil, comprising: a fill layer containing soil containing selenium; and a barium compound and a solidification material mixed with the soil; The solidification material is at least one selected from the group consisting of cement, magnesium oxide, and an insolubilizing material; The amount of the barium compound added to the soil is 0.1 g / 100 g-dry soil or more and 10 g / 100 g-dry soil or less, The amount of the solidification material added to the soil is 0.5 g / 100 g-dry soil or more and 40 g / 100 g-dry soil or less.

6. The embankment structure according to claim 5, further comprising: a structure provided below the embankment layer and having a mechanism for suppressing the elution of barium ions.

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