JP7532896B2 - Methods for purifying contaminated soil - Google Patents

Description

The present invention relates to a method for purifying contaminated soil.

As a method for purifying soil contaminated with the organic pollutant 1,4-dioxane, a method has been proposed in which an oxidizing agent, sodium persulfate, and an oxidation reaction accelerator, quicklime, are mixed and added to the soil (see Patent Document 1).

As a method for oxidizing organic compounds including 1,4-dioxane, a method has been proposed in which the organic compounds are brought into contact with a composition containing a water-soluble peroxygen compound and a pH adjuster (see Patent Document 2). Water-soluble peroxygen compounds include various persulfates, and pH adjusters include alkaline hydroxides (e.g., calcium hydroxide, sodium hydroxide).

JP 2019-048267 A Special Publication No. 2008-506511

1,4-dioxane is highly water-soluble, so it easily penetrates deep into the soil. When 1,4-dioxane that has penetrated deep into the soil is decomposed with an oxidizing agent, a method of in-situ treatment is adopted, using deep mixing and mixing machinery or a power blender. However, when treating in-situ, the liquid limit of the soil increases as the soil is mixed with the oxidizing agent, etc., and the strength of the soil decreases. As a result, the ground may become weak.

The method of Patent Document 1 uses quicklime, which is a powder. Therefore, a dedicated powder conveying device must be used to deliver the powder deep into the soil, which is cumbersome and undesirable in terms of cost. Furthermore, if the method of Patent Document 2 is used to treat the soil in situ, the strength of the soil may decrease, causing the ground to become weak.

The present invention aims to provide a method for oxidatively decomposing 1,4-dioxane contained in soil and suppressing a decrease in soil strength.

One invention for achieving the above-mentioned object is a method for purifying soil contaminated by 1,4-dioxane, which comprises adding to the soil (A) persulfate, (B) blast-furnace cement type B or calcium hydroxide, (C) sodium hydroxide, and (D) gypsum.
The (A) persulfate is preferably at least one selected from the group consisting of peroxomonosulfate and peroxodisulfate, and more preferably sodium peroxodisulfate.
The gypsum (D) is preferably at least one selected from the group consisting of anhydrous calcium sulfate, calcium sulfate hemihydrate, and calcium sulfate dihydrate. Furthermore, the gypsum (D) is more preferably calcium sulfate dihydrate.

According to the present invention, it is possible to oxidize and decompose 1,4-dioxane contained in soil and prevent the deterioration of soil strength.

1 is a graph showing the amount of 1,4-dioxane eluted in Example 1. 1 is a graph showing the cone index in Example 1. 1 is a graph showing the amount of 1,4-dioxane eluted in Example 2. 1 is a graph showing the cone index in Example 2.

The following describes the embodiments and examples of the present invention, but the scope of the present invention is not limited to these descriptions. In the following description, "%" simply means "% by weight."

==Contaminated soil==
Contaminated soil is soil contaminated with at least 1,4-dioxane. As described above, 1,4-dioxane is highly water-soluble and therefore easily penetrates deep into the soil. In other words, soil contaminated with 1,4-dioxane is highly likely to exist not only on the soil surface but also deep in the soil. It is preferable to carry out purification treatment of contaminated soil present deep in situ using a deep layer mixing heavy machine or the like.

The type of soil is not particularly limited, but an example is clay soil. Generally, clay soil is easily permeated with highly water-soluble substances such as 1,4-dioxane, making it difficult to remove the substances. Clay soil is, for example, soil with a hydraulic conductivity of 1×10 ^-6 m/sec or less.

==(A) Persulfate==
The persulfate functions as an oxidizing agent for oxidatively decomposing 1,4-dioxane. As the persulfate, at least one selected from the group consisting of peroxomonosulfate and peroxodisulfate can be used. The persulfate is, for example, a sodium salt, a potassium salt, or an ammonium salt. Here, in terms of reactivity with 1,4-dioxane, it is preferable to use a peroxodisulfate, and it is more preferable to use sodium peroxodisulfate.

The proportion of persulfate added to contaminated soil varies depending on the type of contaminated soil, the valence of the persulfate, etc. For example, in the case of sodium peroxodisulfate, it is preferable to add at least 2% to the contaminated soil in terms of reactivity with 1,4-dioxane. If the proportion of sodium peroxodisulfate added to the contaminated soil is less than 2%, the reaction with 1,4-dioxane will be insufficient, making it difficult to carry out oxidative decomposition. It is preferable to add persulfate in the form of an aqueous solution.

== (B) Blast-furnace cement type B or calcium hydroxide ==
Blast-furnace cement type B and calcium hydroxide function as alkalis that promote the oxidative decomposition by persulfate. Blast-furnace cement type B is a cement specified by JIS standard R5211.

The proportion of blast-furnace cement type B or calcium hydroxide added to contaminated soil varies depending on the type of contaminated soil and the amount of persulfate added. For example, in terms of promoting oxidative decomposition, it is preferable to add at least 3% of blast-furnace cement type B to the contaminated soil, and it is preferable to add at least 1% of calcium hydroxide to the contaminated soil. If the proportion of blast-furnace cement type B added to the contaminated soil is less than 3%, or if the proportion of calcium hydroxide added to the contaminated soil is less than 1%, it becomes difficult to promote oxidative decomposition by persulfate. It is preferable to add blast-furnace cement type B or calcium hydroxide in the form of milk (a mixture of liquid and powder).

In addition, in terms of promoting oxidative decomposition, in the case of blast-furnace cement Type B, it is more preferable to add 150% or more of the amount of persulfate. In the case of calcium hydroxide, it is preferable to add 50% or more of the amount of persulfate, and more preferably 100% or more. If the amount of blast-furnace cement Type B added is less than 150% of the amount of persulfate, or if the amount of calcium hydroxide added is less than 50%, the oxidative decomposition by persulfate may be insufficient, and 1,4-dioxane may not be decomposed reliably.

==(C) Sodium Hydroxide==
Sodium hydroxide functions as an alkali that promotes the oxidative decomposition by persulfate.

The proportion of sodium hydroxide added to the contaminated soil varies depending on the type of contaminated soil, the amount of persulfate added, etc. For example, in order to promote oxidative decomposition, it is preferable to add at least 1% sodium hydroxide to the contaminated soil. If the proportion of sodium hydroxide added to the contaminated soil is less than 1%, it becomes difficult to promote oxidative decomposition by persulfate. It is preferable to add sodium hydroxide in the form of an aqueous solution.

In order to promote oxidative decomposition, it is preferable to add sodium hydroxide in an amount of 50% or more of the amount of persulfate added, and more preferably 100% or more. On the other hand, if the amount of sodium hydroxide added is less than 50% of the amount of persulfate added, the oxidative decomposition by the persulfate may be insufficient, and 1,4-dioxane may not be decomposed reliably.

==(D) Gypsum==
Gypsum acts as an agent to inhibit the loss of soil strength.

The gypsum can be at least one selected from the group consisting of anhydrous calcium sulfate, calcium sulfate hemihydrate, and calcium sulfate dihydrate.

The ratio of gypsum added to contaminated soil varies depending on the type of contaminated soil. For example, in the case of calcium sulfate dihydrate, it is preferable to add at least 1% to the contaminated soil in terms of its soil strengthening properties. If less than 1% of gypsum is added to the contaminated soil, the soil strength may decrease and the ground may become weak. It is preferable to add gypsum in the form of milk.

In terms of soil strengthening performance, it is preferable to add gypsum in an amount of 50% or more of the amount of persulfate added, and more preferably 100% or more. On the other hand, if the amount of gypsum added is less than 50% of the amount of persulfate added, the strength of the soil may decrease and the ground may become weak. Furthermore, it is more preferable that the amount of gypsum added is equal to or greater than the amount of sodium hydroxide added.

==Other==
Any component may be added as long as it does not affect the functions of (A) persulfate, (B) blast-furnace cement type B or calcium hydroxide, (C) sodium hydroxide, and (D) gypsum.

==Method of purifying contaminated soil==
In the purification method according to this embodiment, (A) persulfate, (B) blast-furnace cement type B or calcium hydroxide, (C) sodium hydroxide, and (D) gypsum are added to soil contaminated with 1,4-dioxane.

The timing of adding each component is arbitrary. For example, components (A) to (D) may be added in any order, or components (A) to (D) may be mixed in advance and then added. By stirring the soil to which components (A) to (D) have been added, 1,4-dioxane is oxidized and decomposed. As a result, the contaminated soil is purified. The method of adding each component and the method of stirring the soil can be any known method.

== Example 1 (Component (B) is Blast-furnace cement type B) ==
<Creating simulated contaminated soil>
Air-dried loam soil (moisture content: 9.7%) was sieved through a sieve with 4.75 mm openings, and the soil that passed through was used as the test soil. An aqueous solution of 1,4-dioxane was added to the test soil to prepare simulated contaminated soil. The aqueous solution of 1,4-dioxane was prepared by diluting 1,4-dioxane reagent with deionized water (concentration: 300 mg/L). The proportion of 1,4-dioxane in the simulated contaminated soil was approximately 100 mg/kg.

Test specimens were then created by adding each component to the simulated contaminated soil. As component (A), an aqueous solution of sodium peroxodisulfate (mass concentration 0.3 g/mL) was used. As component (B), milk of blast-furnace cement type B (mass fraction 50%) was used. As component (C), an aqueous solution of sodium hydroxide (mass fraction 33%) was used. As component (D), milk of gypsum dihydrate (calcium sulfate dihydrate) (mass fraction 50%) was used.

Each specimen was covered with plastic wrap and left to cure at room temperature (22°C) for five days.

The amount of simulated contaminated soil and the amount of each component added for each test specimen are as shown in Table 1. Test specimen No. 1 is an untreated area where none of components (A) to (D) were added. In addition, "SPS" in Table 1 stands for sodium peroxodisulfate. The "%" values shown in parentheses for each component are the proportion of each component added to the simulated contaminated soil.

<Analysis method>
Each of the cured specimens was then tested for the amount of 1,4-dioxane eluted and the soil strength.

Specifically, the amount of 1,4-dioxane leaching was tested in accordance with "Measurement methods for soil leaching surveys (Ministry of the Environment Notification No. 18, March 6, 2003)." The amount of 1,4-dioxane leaching was then determined by subjecting the eluate to a gas chromatograph mass spectrometer (GCMS-2010Ultra, manufactured by Shimadzu Corporation).

The strength of the soil was judged by the cone index. A fall cone penetration test (see "Portable Cone Penetration Test" in the Geotechnical Society Standards (JIS 1431-1995)) was conducted on each specimen to measure the fall cone penetration. The cone index was calculated by applying the measured fall cone penetration to the previously calculated relationship between the fall cone penetration and the cone index (see Figure 1 in Patent No. 5448380).

<Experimental Results>
Figure 1 is a graph showing the amount of 1,4-dioxane eluted from each test specimen. As is clear from Figure 1, test specimens No. 8 to No. 10, which contained the four components of sodium peroxodisulfate, blast-furnace cement type B, sodium hydroxide, and gypsum dihydrate, showed low amounts of 1,4-dioxane eluted. In other words, it was made clear that adding the above four components to contaminated soil ensures the oxidative decomposition of 1,4-dioxane.

Figure 2 is a graph showing the cone index for each test specimen. As is clear from Figure 2, test specimens No. 8 to No. 10, which were added with four components, sodium peroxodisulfate, blast-furnace cement type B, sodium hydroxide, and gypsum dihydrate, showed high cone index values. In other words, it was revealed that adding the above four components to contaminated soil can suppress the decrease in soil strength in addition to the oxidative decomposition of 1,4-dioxane. Among test specimens No. 8 to No. 10, test specimen No. 10, which had a high ratio of sodium hydroxide added to sodium peroxodisulfate, showed a higher cone index. Furthermore, test specimen No. 10 showed a higher cone index than test specimen No. 1, which was not added. In other words, it was revealed that adding the four components in the ratio of test specimen No. 10 not only suppressed the decrease in soil strength, but also strengthened the soil itself.

== Example 2 (Component (B) is Calcium Hydroxide) ==
<Creating simulated contaminated soil>
Kaolin clay (moisture content: 1.1%) and mountain sand (from Ibaraki Prefecture, moisture content: 10.4%) were mixed at a wet weight ratio of 1:1 to prepare the test soil. 900 mL of 1,4-dioxane aqueous solution was added to 1.8 kg of the test soil to prepare simulated contaminated soil. The 1,4-dioxane aqueous solution was prepared by diluting 1,4-dioxane reagent with deionized water (concentration: 180 mg/L).

Test specimens were then created by adding each component to the simulated contaminated soil. As component (A), an aqueous solution of sodium peroxodisulfate (mass concentration 0.3 g/mL) was used. As component (B), calcium hydroxide milk (mass fraction 50%) was used. As component (C), an aqueous solution of sodium hydroxide (mass fraction 33%) was used. As component (D), gypsum dihydrate (calcium sulfate dihydrate) milk (mass fraction 50%) was used.

The amount of simulated contaminated soil and the amount of each component added for each test specimen are as shown in Table 2. Test specimen No. 1 is an untreated area where none of components (A) to (D) were added. In addition, "SPS" in Table 2 stands for sodium peroxodisulfate. The "%" values shown in parentheses for each component are the proportion of each component added to the simulated contaminated soil.

Each specimen was covered with plastic wrap and left to cure at room temperature (22°C) for five days.

<Analysis method>
The cured specimens were then subjected to tests for the amount of 1,4-dioxane eluted and the soil strength.

<Experimental Results>
Figure 3 is a graph showing the amount of 1,4-dioxane eluted from each test specimen. As is clear from Figure 3, test specimens No. 7 and No. 8, to which the four components of sodium peroxodisulfate, calcium hydroxide, sodium hydroxide, and gypsum dihydrate were added, showed low amounts of 1,4-dioxane eluted. In other words, it became clear that by adding the above four components to contaminated soil, oxidative decomposition of 1,4-dioxane can be reliably carried out.

Figure 4 is a graph showing the cone index for each test specimen. As is clear from Figure 4, test specimens No. 7 and No. 8, to which the four components of sodium peroxodisulfate, calcium hydroxide, sodium hydroxide, and gypsum dihydrate were added, showed high cone index values. In other words, it was revealed that adding the above four components to contaminated soil, in addition to oxidative decomposition of 1,4-dioxane, strengthens the soil and prevents the weakening of the ground. Among test specimens No. 7 and No. 8, test specimen No. 8, which had a higher ratio of sodium hydroxide added to sodium peroxodisulfate, showed a higher cone index. In addition, test specimen No. 8 showed a higher cone index than test specimen No. 1, which was not added. In other words, it was revealed that adding the four components in the ratio of test specimen No. 8 not only suppresses the decrease in soil strength, but also strengthens the soil itself.

Claims (5)

1. A method for remediating soil contaminated with 1,4-dioxane, comprising:
(A) persulfate, (B) blast-furnace cement type B or calcium hydroxide, (C) sodium hydroxide, and (D) gypsum are added to the soil ;
A method for purifying contaminated soil , wherein the amount of gypsum (D) added is 1 to 2 times the weight of the sodium hydroxide (C) . The method for purifying contaminated soil according to claim 1, characterized in that the (A) persulfate is at least one selected from the group consisting of peroxomonosulfate and peroxodisulfate. The method for purifying contaminated soil according to claim 2, characterized in that the persulfate (A) is sodium peroxodisulfate. The method for purifying contaminated soil according to any one of claims 1 to 3, characterized in that the (D) gypsum is at least one selected from the group consisting of anhydrous calcium sulfate, calcium sulfate hemihydrate, and calcium sulfate dihydrate. The method for purifying contaminated soil according to claim 4, characterized in that the gypsum (D) is calcium sulfate dihydrate.