JP2023044341A - Contaminated soil purification method - Google Patents

Abstract

To provide a method that efficiently purifies contaminated soil containing arsenic.SOLUTION: A contaminated soil purification method includes the steps of: mixing contaminated soil containing arsenic, hypochlorite, and water; blending the resultant liquid mixture with a polymer coagulant to mix them, for liquid-solid separation into the sedimented solid part and the liquid part containing arsenic; and recovering the solid part to give the purified soil.SELECTED DRAWING: Figure 1

Description

Embodiments of the present invention relate to methods for remediation of contaminated soil.

Conventionally, soil contamination by heavy metals such as arsenic and lead has been regarded as a problem. In particular, arsenic is carcinogenic and a harmful substance that affects human health, so it is regulated by the Soil Contamination Countermeasures Law. Therefore, various methods for remediation of soil contaminated with arsenic have been investigated.

An insolubilization method and a washing method are known as representative methods for remediation of contaminated soil. The insolubilization method is a method in which contaminated soil containing contaminants such as heavy metals is mixed with chemicals to modify the properties of the contaminated soil so that the contaminants such as heavy metals are not eluted into water. Therefore, pollutants will continue to be retained in the soil, and there will continue to be concerns about their impact on human health.

On the other hand, the washing method is a method of eluting and removing contaminants by washing contaminated soil containing contaminants such as heavy metals with water or chemicals. According to the washing method, purified soil from which contaminants are removed is obtained. However, in general, in order to obtain purified soil with a low contaminant content by the washing method, it is necessary to wash the contaminated soil multiple times and to wash away the chemicals used during washing. Therefore, cleaning methods generally require large equipment and large amounts of water.

In contaminated _soil containing arsenic, arsenic exists in various forms such as arsenic anhydride ( _As2O3 ), arsenic anhydride ( _As2O5 ), and alkali metal or other _metal arsenates. Arsenic anhydride and arsenic salts have high solubility in water, but arsenous anhydride has low solubility. Therefore, arsenous anhydride is not sufficiently removed by washing and tends to remain in the soil.

On the other hand, Patent Literature 1 discloses a method for remediation of arsenic-contaminated soil in which the arsenic-contaminated soil is washed sequentially or simultaneously with a washing liquid containing an oxidizing agent and a washing liquid containing an alkali. According to this cleaning method, the arsenic anhydride, which has a low solubility in water, is converted into arsenic acid, which has a high solubility, by the oxidizing agent, thereby enhancing the cleaning effect of the cleaning liquid containing alkali.

Japanese Patent Application Laid-Open No. 2002-119950

However, the above cleaning method requires a specific device for repeatedly carrying out the cleaning process while transporting the contaminated soil. In addition, since an alkali-containing washing liquid is used to elute arsenic, a process of neutralizing the soil after alkali washing with an acid solution is required. Therefore, there is a demand for a method that can be carried out more simply and that can purify contaminated soil efficiently.
Therefore, in view of the above situation, embodiments of the present invention provide a method for remediation of contaminated soil containing arsenic simply and efficiently. Further, another embodiment of the present invention provides a remediation method capable of recycling and reusing water used for remediation of contaminated soil containing arsenic.

The present inventors have made intensive studies on methods for remediation of contaminated soil by washing, and have found that contaminated soil can be efficiently remedied by using specific components during washing, and have completed the present invention. That is, the present invention relates to the embodiments described below. However, the present invention is not limited to the embodiments described below, and includes various embodiments.

Embodiments of the present invention include mixing contaminated soil containing arsenic, hypochlorite, and water;
adding a polymer flocculant to the resulting mixed solution and mixing to separate the precipitated solids and the arsenic-containing liquid from solid-liquid;
The present invention relates to a method for remediation of contaminated soil including recovering the above solids and obtaining remedied soil.

In one embodiment, the purification method further comprises contacting the liquid containing arsenic with the layered double hydroxide, followed by solid-liquid separation into the solid containing arsenic and the purified liquid. It is preferred to include

In one embodiment, the purification method preferably further includes using the purified liquid to purify the contaminated soil.

In one embodiment, the hypochlorite preferably comprises sodium hypochlorite.

In one embodiment, the polymer flocculant preferably contains an anionic polymer flocculant.

In one embodiment, the layered double hydroxide is preferably represented by the following formula (I).
[M ²⁺ _1−X M ³⁺ _x (OH) ₂ ] [A ⁿ⁻ _x/n ·yH ₂ O] (I)
(wherein M ²⁺ is a divalent metal ion selected from the group consisting of Cu ²⁺ , Mn ²⁺ , Mg ²⁺ , Fe ²⁺ , Ca ²⁺ , Ni ²⁺ , Zn ²⁺ , Co ²⁺ and Cd ²⁺ M ³⁺ is a trivalent metal ion selected from the group consisting of Al ³⁺ , Cr ³⁺ , Fe ³⁺ , Co ³⁺ , In ³⁺ , Mn ³⁺ and V ³⁺ , ^{A n−} is CO ₃ ²⁻ , SO ₄ ²⁻ , Cl ⁻ , OH ⁻ , SiO ₄ ⁴⁻ , SO ₄ ²⁻ and NO ₃ ⁻ are n-valent anions, where x is 0.20 to 0.33 and n is 1 to 4, and y is 1-3x/2.)

In one embodiment, the layered double hydroxide is preferably an Mg—Al-based layered double hydroxide containing Mg ²⁺ and Al ³⁺ as constituent metals.

In one embodiment, the particle size of the contaminated soil containing the contaminant containing arsenic is preferably 10 mm or less.

Another embodiment of the invention comprises:
mixing contaminated soil containing arsenic, hypochlorite, and water;
adding a polymer flocculant to the resulting mixed solution and mixing to separate the precipitated solids and the arsenic-containing liquid from solid-liquid;
After contacting the arsenic-containing liquid with the layered double hydroxide, solid-liquid separation into the arsenic-containing solid and the purified liquid;
The present invention relates to a method for remediation of contaminated soil, comprising circulating and reusing the purified liquid component as water to be mixed with the contaminated soil containing arsenic.

ADVANTAGE OF THE INVENTION According to embodiment of this invention, the method of remediate|cleaning the contaminated soil containing arsenic efficiently can be provided. Further, according to another embodiment of the present invention, it is possible to provide a purification method capable of recycling and reusing the water used for the purification of contaminated soil containing arsenic.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flow diagram illustrating a contaminated soil remediation method according to an embodiment of the present invention; BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a conceptual diagram schematically explaining the state of soil particles after adding a polymer flocculant in the contaminated soil remediation method of one embodiment of the present invention.

Embodiments of the present invention will be described below. However, the present invention is not limited to the embodiments described below, and includes various embodiments.

A method for purifying contaminated soil, which is one embodiment of the present invention, will be described below with reference to FIG. In the method for remediation of contaminated soil, which is one embodiment of the present invention, first, contaminated soil containing arsenic, hypochlorite, and water are mixed (hereinafter referred to as step 1. See S1 in FIG. 1). ). The contaminated soil, hypochlorite, and water can be mixed, for example, by adding the hypochlorite and water to the contaminated soil and shaking or stirring.

In one embodiment, the contaminated soil used in the purification method may be soil containing a single element, compound, or ion of a heavy metal element such as arsenic. Here, the soil may be general soil collected from forests, urban areas, agricultural lands, factory sites, and the like. Soil contains silica as a major component and contains arsenic of natural or anthropogenic origin. Arsenic exists in soil in various forms such as arsenous anhydride ( _As2O3 ), arsenic anhydride ( _As2O5 ) _, and arsenates of _alkaline earth metals or other metals. Alkali metal or other metal arsenates such as _{Ca3 ( AsO4} ) ₂ , _CaHAsO4 , _{Mg3 ( AsO4} ) _2.10H2O , _MgNH4 ( _AsO4 ), _FeAsO4 , and _Pb3AsO4 etc.

In one embodiment, the contaminated soil may be excavation muck produced by excavation in construction such as tunnels and dams. Drilling muck often contains naturally occurring arsenic. Therefore, the method for remediation of contaminated soil according to the embodiment of the present invention can also be suitably applied to excavated muck.

In contaminated soil, arsenic exists in various forms as described above. Among them, arsenous anhydride has a low solubility, so it tends to remain in contaminated soil. However, according to the purification method of the above embodiment, by adding hypochlorite and water to contaminated soil and mixing, the solubility of arsenic in water is improved, and arsenic is efficiently eluted (removed). can be done. Hypochlorite and water may be added separately or together. When added separately, hypochlorite may be added to the mixed solution of contaminated soil and water.

The hypochlorite may be sodium hypochlorite or potassium hypochlorite, preferably containing at least sodium hypochlorite. In one embodiment, an aqueous solution of hypochlorite can be suitably used. The concentration of hypochlorite in the aqueous solution can be adjusted as appropriate. For example, the concentration of hypochlorite in the aqueous solution may be 0.01-0.15 g/ml, preferably 0.07-0.14 g/ml. In one embodiment, an aqueous solution of hypochlorite with a concentration of 0.01 g/ml can be suitably used.

In one embodiment, the amount of water or hypochlorite aqueous solution used may be preferably 10 times or more by mass based on the mass of contaminated soil (dry state) to be purified. From the viewpoint of the arsenic elution (extraction) effect of hypochlorite, the amount used is preferably 10 to 130 times by mass, more preferably 10 to 100 times by mass.

In one embodiment, the mixture obtained in step 1 above may be in the form of a slurry. If necessary, the mixture may be filtered, such as when the mixture contains obvious foreign matter. The soil particles can also be allowed to settle by allowing the slurry of the mixture to stand. However, in the contaminated soil remediation method of the above embodiment, a polymer flocculant is added to the mixed solution obtained in the above step 1 and mixed, and the precipitated solid content (1) and the liquid content (1) containing arsenic are separated. (hereinafter referred to as step 2, see S2 in FIG. 1). By adding a polymer flocculant to the mixed solution obtained in the above step 1 and then mixing them, the flocculating effect can be further enhanced. The use of a polymer flocculant promotes flocculation of soil particles in the slurry mixture, and separates the mixture into two layers, solid and liquid, in a short period of time.

In the above embodiment, from the viewpoint of efficiently removing arsenic, it is desirable that the polymer flocculant aggregates and settles solid substances such as soil particles, but does not act on arsenic. When the polymer flocculant also acts on arsenic, arsenic is also taken in when soil particles are aggregated, and arsenic tends to remain in the collected soil particles. From such a viewpoint, an anionic polymer flocculant, a cationic polymer flocculant, or a nonionic polymer flocculant can be used as the polymer flocculant. In one embodiment, anionic polymeric flocculants or cationic polymeric flocculants are preferred, and anionic polymeric flocculants are more preferred.

In one embodiment, the polymeric flocculant may be a polyacrylic acid ester-based, polymethacrylic acid ester-based, polyacrylamide-based, or polyamidine-based polymeric flocculant. Among them, polyacrylic acid ester-based or polyacrylamide-based polymer flocculants can be preferably used. In one embodiment, the polymeric flocculant may be used in the form of an aqueous solution.

It is preferable that the polyacrylic acid ester-based polymer flocculant contains a sodium acrylate/acrylamide copolymer. A polymer flocculant containing a sodium acrylate-acrylamide copolymer is also available as a commercial product. For example, Himolock SS series manufactured by Hymo Co., Ltd., and the like can be mentioned. Although not particularly limited, Himolock SS-100, SS-120, SS-200 and the like can be preferably used.

The polyacrylamide-based polymer flocculant preferably contains an acrylamide/[2-(acryloyloxy)ethyl]trimethylammonium chloride copolymer. Polymer flocculants containing acrylamide/[2-(acryloyloxy)ethyl]trimethylammonium chloride copolymers are also commercially available. For example, the Hymolock MP series manufactured by Hymo Co., Ltd., and the like can be mentioned. Although not particularly limited, Himolock MP-384, MP-684 and the like can be preferably used.

The amount of the polymer flocculant used is not particularly limited, but if the amount used is too small, a sufficient flocculation effect cannot be obtained, and the time required for separation by flocculation tends to be long. On the other hand, if the amount used is too large, the viscosity of the solution tends to increase and the handleability tends to deteriorate. From this point of view, it is preferable to appropriately adjust the amount of the polymer flocculant used. In one embodiment, the amount of polymer flocculant used may be preferably 3 g or more, more preferably 5 g or more, per 100 kg of contaminated soil (dry state) to be purified. On the other hand, the amount used may be 14 g or less, more preferably 13 g or less.
In one embodiment, in the case of anionic polymer flocculants and nonionic polymer flocculants, the amount used is preferably 3 to 10 g per 100 kg of contaminated soil (dry state) to be purified. , more preferably 5 to 8 g. In one embodiment, in the case of a cationic polymer flocculant, the amount used is preferably 3.3 to 13.3 g, more preferably 3.3 to 13.3 g, per 100 kg of contaminated soil (dry state) to be purified. may be 8-12 g.

FIG. 2 is a conceptual diagram schematically explaining the state of soil particles after adding a polymer flocculant. As shown in FIG. 2( a ), soil particles 10 and arsenic 20 are dispersed in the mixed liquid A containing arsenic-containing contaminant particles, hypochlorite, and water obtained in step 1 . When the polymer flocculant 30 is added to the mixed solution A, the action of the polymer flocculant 30 causes the soil particles 10 to flocculate, and the polymer It settles together with flocculant 30 to form solids 40 . For example, by selecting the polymer flocculant 30 that easily forms a complex with the soil particles 10, a higher flocculation effect can be obtained.
On the other hand, the arsenic 20 is retained in the liquid component A1 while remaining dispersed. Solid-liquid separation between the settled solids 40 and the arsenic-containing liquids 50 can be performed by methods well known in the art, such as, for example, filtration.

As shown in FIG. 1, arsenic contained in contaminated soil can be removed as a liquid component (1) by the solid-liquid separation. On the other hand, the solid content (1) can be recovered as clarified soil and reused. The solid content may be reused after further separation into gravel, sand, silt, sludge, etc., if necessary.

In one embodiment, a step of pulverizing and classifying contaminated soil may be provided before step 1 described above. In the soil, soil particles of different particle sizes such as gravel, sand, silt, and clay are mixed, and adhere to each other to form a lump of soil. For this reason, the soil mass is finely crushed by pulverization, and the size of the soil particles is adjusted by classification, thereby facilitating purification treatment. Therefore, in one embodiment, the contaminated soil used for remediation is preferably in the form of soil particles.

The particle size of the soil particles may be 10 mm or less. From the viewpoint of easily obtaining an excellent cleaning effect, the particle size is preferably 5 mm or less, more preferably 3 mm or less, and even more preferably 2 mm or less. In particular, when the particle size of the soil particles is 2 mm or less, a more excellent cleaning effect can be easily obtained.

In one embodiment, the purifying method further includes purifying the arsenic-containing liquid (1) (hereinafter also referred to as step 3, see S3 in FIG. 1) following step 2 described above. It's okay. For example, it preferably includes solid-liquid separation into the arsenic-containing solid content (2) and the purified liquid content (2) after contacting the liquid component (1) with the layered double hydroxide. .

The layered double hydroxide (LDH) used for purifying the arsenic-containing liquid is preferably represented by the following formula (I).
[M ²⁺ _1−X M ³⁺ _x (OH) ₂ ] [A ⁿ⁻ _x/n ·yH ₂ O] (I)
(wherein M ²⁺ is a divalent metal ion selected from the group consisting of Cu ²⁺ , Mn ²⁺ , Mg ²⁺ , Fe ²⁺ , Ca ²⁺ , Ni ²⁺ , Zn ²⁺ , Co ²⁺ and Cd ²⁺ M ³⁺ is a trivalent metal ion selected from the group consisting of Al ³⁺ , Cr ³⁺ , Fe ³⁺ , Co ³⁺ , In ³⁺ , Mn ³⁺ and V ³⁺ , ^{A n−} is CO ₃ ²⁻ , SO ₄ ²⁻ , Cl ⁻ , OH ⁻ , SiO ₄ ⁴⁻ , SO ₄ ²⁻ and NO ₃ ⁻ are n-valent anions, where x is 0.20 to 0.33 and n is 1 to 4, and y is 1-3x/2.)

LDH consists of an octahedral host layer that is positively charged by substituting M ³⁺ for a portion of M ²⁺ in the divalent metal M(OH) ₂ , an anion that compensates for the positive charge of the host layer, and It is composed of a guest layer consisting of inter-layer water and has excellent adsorption capacity. LDH can be produced by methods well known in the art. For example, LDH can be produced by preparing a mixed aqueous solution containing divalent metal ions and trivalent metal ions, adding it dropwise to the guest anion-containing aqueous solution while adjusting the pH, and hydrolyzing the metal ions. .

In one embodiment, the layered double hydroxide is preferably an Mg—Al-based layered double hydroxide containing Mg ²⁺ and Al ³⁺ as constituent metals. Such LDH can be produced according to a known method, and can also be obtained as a commercial product.

The contact between the arsenic-containing liquid component (2) and LDH can be achieved, for example, by mixing the arsenic-containing liquid component and LDH. Alternatively, the arsenic-containing liquid may be passed through a column filled with LDH. Furthermore, LDH may be molded as a filter material, and the liquid component containing arsenic may be filtered using this filter material.
Arsenic in the liquid can be adsorbed on the LDH by the contact. The adsorption is due to the exchange of arsenate anions (eg, AsO ₄ ³⁻ , HAsO ₄ ²⁻ ) with interlayer anions of LDH, and arsenic is insolubilized by adsorption. As a result, the liquid portion (washing wastewater) obtained by solid-liquid separation is clarified, and the load of wastewater treatment can be reduced. In one embodiment, the collected liquid can be circulated and reused as water in step 1 above (see S4 in FIG. 1).

The solid content (2) recovered by the solid-liquid separation is LDH that adsorbs arsenic and can be treated as industrial waste. The arsenic adsorption capacity of LDH is very high, and the maximum adsorption capacity is, for example, 142.86 mg/g (when Mg/Al=2). Thus, for example, contaminated soil with an arsenic content of 1.93 mg/kg can theoretically be concentrated about 74,020 times. From such a point of view, according to the purification method of the above embodiment, the amount of industrial waste that finally needs to be treated can be easily reduced.

EXAMPLES The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples, and includes various embodiments. In addition, description of "part" in an Example and a comparative example represents a "mass part."

(reference experiment)
Contaminated soil was collected, pulverized and classified to prepare soil particles with particle sizes of <75 μm and 75 μm to 2.0 mm, respectively. Table 1 shows the results of analyzing the metal components contained in the soil particles using an inductively coupled plasma (ICP) emission spectrometer (iCAP 6500 Duo manufactured by Thermo Fisher Scientific).
Next, 2.0 g of the soil particles were placed in a 100 mL container, 2.0 g of sodium hypochlorite and 20 mL of water were added, and the mixture was stirred at 150 rpm using a stirrer to obtain a mixed solution. Stirring was carried out at room temperature (30° C.) for 2 hours. This mixed solution was filtered using a 0.45 mm membrane filter to obtain a filtrate. Then, the filtrate was analyzed by an ICP emission spectrometer (iCAP 6500 Duo manufactured by Thermo Fisher Scientific) to determine the amount of metal components in the filtrate.
On the other hand, the solid content was obtained by separating the filtrate from the liquid mixture in the above treatment. After drying the solid content, the mass was measured. As a result, the mass of the solid content (soil particles after the treatment) was almost the same as the mass of the soil particles before the treatment. From this, it is considered that the elution of silica, which is the main component of the soil particles, into the filtrate is extremely small in the above treatment. Therefore, based on the amounts of the components in the soil particles shown in Table 1, the elution rate of the metal components eluted from the soil particles into the filtrate was calculated from the values of the amounts of the metal components in the filtrate. Table 2 shows the results.

From the results shown in Tables 1 and 2, for arsenic among the metal components contained in the soil particles, hypochlorite The elution rate by mixing and stirring the sodium sulfate and water is 100%, indicating that the extraction efficiency is very excellent.

(Example 1)
Contaminated soil was collected, pulverized and classified to prepare soil particles with a particle size of <75 μm. 3.0 g of the soil particles were placed in a 100 mL container, 0.3 g of sodium hypochlorite and 30 mL of water were added, and the mixture was stirred using a shaker to obtain a mixed solution. Stirring was carried out at room temperature (25° C.) for 120 minutes. The mixed liquid was analyzed by an ICP emission spectrometer (Agilent 7900 ICP-MS manufactured by Agilent Technologies, Inc.) to determine the arsenic concentration in the mixed liquid. Table 3 shows the results.
Next, 100 μL of a polymer flocculant (an aqueous solution of Hymo Lock SS-100, a product name of Hymo Co., Ltd.) was added to 30 mL of the mixed solution, and the mixture was gently stirred. The amount of polymer flocculant used (in terms of solid content) was 0.0067% with respect to the mass of the dry soil particles. Stirring was performed manually for 1 minute at room temperature. Sedimentation started immediately after the stirring was stopped, and it was confirmed that the liquid had become transparent after 10 minutes. 10 minutes after the stirring was stopped, solid-liquid separation was performed using a filter. The liquid (1) collected after the solid-liquid separation was analyzed by an ICP emission spectrometer to determine the arsenic concentration. The flocculation effect was evaluated in terms of arsenic concentration. Table 3 shows the evaluation of the flocculation effect and the results of the arsenic concentration.

(Examples 2-5)
Contaminated soil was purified in the same manner as in Example 1, except that the polymer flocculant used in Example 1 was changed to the polymer flocculant shown in Table 3. The polymer flocculants used in Examples 2 and 3 are the product names of Hymo Co., Ltd., "Himolock SS-120" and "Himolock SS-200," respectively. Each was used in the form of an aqueous solution in the same manner as in Example 1, and the amount used was also the same as in Example 1 (0.0067% relative to the mass of dry soil particles). In addition, the polymer flocculants used in Examples 4 and 5 are the product names of Hymo Co., Ltd., "Hymolock MP-384" and "Hymolock MP-684", respectively. Each was used in the form of an aqueous solution in the same manner as in Example 1, and the amount of polymer flocculant used (in terms of solid content) was 0.010% relative to the mass of the dry soil particles.
Next, in the same manner as in Example 1, the flocculation effect by addition of the polymer flocculant was evaluated, and the arsenic concentration in the liquid (1) recovered after the solid-liquid separation was analyzed. These results are shown in Table 3.

(Comparative example 1)
In the same manner as in Example 1, a mixture of soil particles, an aqueous solution of sodium hypochlorite, and water was obtained. Next, without adding the polymer flocculant, the mixture was stirred manually at room temperature for 1 minute. The aggregation effect was visually evaluated when the mixture was allowed to stand for 10 minutes after stirring. Since no phase separation occurred when the mixture was left for 10 minutes, it was left as it is. Phase separation was confirmed after 2 hours. Liquid (1) collected after solid-liquid separation in the same manner as in Example 1 was analyzed by an ICP emission spectrometer. Table 3 shows the analysis results of the arsenic concentration in the recovered liquid.

(Comparative Examples 2-5)
Contaminated soil was purified in the same manner as in Example 1, except that the polymer flocculant 1 used in Example 1 was changed to the flocculant shown in Table 3. Also, in the same manner as in Example 1, the flocculation effect by addition of the polymer flocculant was evaluated, and the arsenic concentration in the liquid (1) recovered after the solid-liquid separation was analyzed. These results are shown in Table 3.

In Table 3, the evaluation criteria for the aggregation effect are as follows.
A: The arsenic concentration in the liquid (1) collected after solid-liquid separation is almost 100% of the arsenic concentration in the mixed liquid before solid-liquid separation, which is extremely good.
B: The arsenic concentration in the liquid (1) collected after solid-liquid separation is 80% or more of the arsenic concentration in the mixed liquid before solid-liquid separation, which is good.
C: The arsenic concentration in the liquid (1) recovered after the solid-liquid separation is 50% or more of the arsenic concentration in the mixed liquid before the solid-liquid separation, which is within a practically acceptable range.
D: The arsenic concentration in the liquid (1) recovered after solid-liquid separation is less than 50% of the arsenic concentration in the mixed liquid before solid-liquid separation, which is practically impossible.

As can be seen from Table 3, in Examples 1 to 5, the arsenic concentration in the liquid portion (1) recovered after solid-liquid separation is almost the same as the arsenic concentration in the mixed liquid before solid-liquid separation. Therefore, it can be seen that arsenic is efficiently removed from contaminated soil by washing, the arsenic concentration in the solid content recovered after solid-liquid separation is reduced, and the contaminated soil can be reused as purified soil.
On the other hand, in Comparative Examples 2 to 5, the arsenic concentration in the liquid collected after solid-liquid separation is significantly lower than the arsenic concentration in the mixed liquid before solid-liquid separation. This is presumably because when the soil particles in the mixed solution are flocculated by the flocculant, arsenic is also taken in and precipitated together.
In Comparative Example 1, the arsenic concentration in the liquid portion (1) recovered after solid-liquid separation is substantially the same as the arsenic concentration in the soil particles before washing. However, in Comparative Example 1, the time required for phase separation of the mixed liquid is very long, and it is clear that the efficiency is significantly lowered.

As shown in Tables 1 and 2 above, arsenic can be efficiently eluted from soil particles by mixing arsenic-containing soil particles, hypochlorite, and water. Therefore, as shown in Table 3, according to the embodiments of the present invention (Examples 1 to 5), elution (purification) using hypochlorite and separation using a specific polymer flocculant can efficiently remove arsenic from arsenic-containing soil particles. In addition, it can be seen that the solid content recovered by solid-liquid separation after addition of the polymer flocculant can be reused as purified soil particles with reduced arsenic content.
Furthermore, contacting an arsenic-containing aqueous solution with a layered double hydroxide (LDH) can significantly reduce the arsenic concentration in the aqueous solution. Since LDH has a high adsorption capacity for arsenic, it can reduce the arsenic concentration in an aqueous solution to below the measurement limit. From these facts, the arsenic concentration in the liquid can be efficiently reduced by bringing the liquid (arsenic-containing liquid) obtained after solid-liquid separation after the addition of the polymer flocculant into contact with LDH. . Therefore, the arsenic-containing liquid collected after contact with LDH is purified and can be recycled for washing soil particles.

10 soil particles 20 arsenic 30 polymer flocculant 40 solid content (1)
50 liquids (1)
A mixed liquid A1 liquid

Claims (9)

mixing contaminated soil containing arsenic, hypochlorite, and water;
adding a polymer flocculant to the resulting mixed solution and mixing to separate the precipitated solids and the arsenic-containing liquid from solid-liquid;
A method for remediation of contaminated soil, comprising collecting the solids and obtaining remedied soil. 2. The pollution according to claim 1, further comprising solid-liquid separation into the arsenic-containing solid content and the purified liquid content after contacting the arsenic-containing liquid content with the layered double hydroxide. Soil remediation method. 3. The method for remediation of contaminated soil according to claim 2, further comprising using the purified liquid for remediation of contaminated soil. The method for remediation of contaminated soil according to any one of claims 1 to 3, wherein the hypochlorite contains sodium hypochlorite. The method for remediation of contaminated soil according to any one of claims 1 to 4, wherein the polymer flocculant contains an anionic polymer flocculant. The method for remediation of contaminated soil according to any one of claims 2 to 5, wherein the layered double hydroxide is represented by the following formula (I).
[M ²⁺ _1−X M ³⁺ _x (OH) ₂ ] [A ⁿ⁻ _x/n ·yH ₂ O] (I)
(wherein M ²⁺ is a divalent metal ion selected from the group consisting of Cu ²⁺ , Mn ²⁺ , Mg ²⁺ , Fe ²⁺ , Ca ²⁺ , Ni ²⁺ , Zn ²⁺ , Co ²⁺ and Cd ²⁺ M ³⁺ is a trivalent metal ion selected from the group consisting of Al ³⁺ , Cr ³⁺ , Fe ³⁺ , Co ³⁺ , In ³⁺ , Mn ³⁺ and V ³⁺ , ^{A n−} is CO ₃ ²⁻ , SO ₄ ²⁻ , Cl ⁻ , OH ⁻ , SiO ₄ ⁴⁻ , SO ₄ ²⁻ and NO ₃ ⁻ are n-valent anions, where x is 0.20 to 0.33 and n is 1 to 4, and y is 1-3x/2.) The method for remediation of contaminated soil according to any one of claims 2 to 6, wherein the layered double hydroxide is an Mg—Al-based layered double hydroxide containing Mg ²⁺ and Al ³⁺ as constituent metals. The method for remediation of contaminated soil according to any one of claims 1 to 7, wherein the particle size of the contaminated soil containing the contaminant containing arsenic is 10 mm or less. mixing contaminated soil containing arsenic, hypochlorite, and water;
adding a polymer flocculant to the resulting mixed solution and mixing to separate the precipitated solids and the arsenic-containing liquid from solid-liquid;
After contacting the arsenic-containing liquid with the layered double hydroxide, solid-liquid separation into the arsenic-containing solid and the purified liquid;
A method for remediation of contaminated soil, comprising circulating and reusing the purified liquid component as water to be mixed with the contaminated soil containing arsenic.

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