JP2015229124A - Detoxification treatment method for contaminated soil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a detoxification method for contaminated soil capable of easily and effectively utilizing contaminated soil as purified soil by dry treatment without using a large quantity of water.SOLUTION: Provided is a detoxification method for contaminated soil comprising: an iron powder addition step where iron powder is added to contaminated soil including at least one kind of contaminated substance selected from arsenic, lead, hexavalent chromium, cadmium, selenium, mercury, cyanogen, fluorine and boron; a moisture content regulation step where the moisture content of the contaminated soil before electromagnetic separation is regulated to 36 mass% or lower; and a dry magnetic separation step where the iron powder is recovered and removed from the contaminated soil having a moisture content of 36 mass% or lower by dry magnetic separation.

Description

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

  In recent years, tunnel construction and redevelopment construction have begun to increase with the rise of the economy. Along with this, securing a dumping site for the remaining soil is a problem. The remaining soil includes soil contaminated with naturally occurring pollutants such as arsenic, and there is a problem with how to deal with it. As a characteristic of the contaminated soil, the content of the pollutant is relatively small compared to the content standard stipulated in the Soil Contamination Countermeasures Law, but exceeds the elution standard by several to several tens of times. There is a tendency.

  In the vicinity of such contaminated soil generation place, for example, in the case of road construction, a method of installing a water-blocking containment facility with a water-blocking sheet or concrete on a road body etc., and managing the landfill of the contaminated soil as it is Has been reported (see Non-Patent Document 1). However, this method requires periodic monitoring and facility management, and is not a fundamental measure. Moreover, there is a problem that it is difficult to secure the installation location and obtain the consent of the surrounding residents. Furthermore, there is a problem that the contamination penetrates from the temporary storage site of the contaminated soil and the surrounding groundwater is contaminated.

  In addition, a method is described in which a detoxification process is performed by kneading various insolubilizers in a contaminated soil to insolubilize contaminants (see Non-Patent Document 2 and Patent Document 1). The insolubilized soil is regulated by the Soil Contamination Countermeasures Law so that it can be handled as soil that needs to be managed similarly to the containment measures, and it is an issue to secure a place to backfill and manage the insolubilized soil. .

  In addition, a method for treating contaminated soil as a secondary cement material and a method for landfilling in a management-type disposal site have been reported (see Non-Patent Document 2). However, in these reporting methods, the processing capacity and capacity cannot keep up with the generated amount.

  As a method for obtaining a purified soil by detoxifying the contaminated soil, a method of washing and classifying has been reported (see Non-Patent Document 3). However, since the method of this report is a wet process, it is necessary to increase the capacity of the equipment for slurrying and the capacity of the dewatering equipment, and it is difficult to carry out inexpensive, simple and large-scale processing.

In addition, a method of concentrating and separating contaminants as magnetic deposits using at least one of a wet magnetic separator and a dry magnetic separator has been proposed (see Patent Document 2). However, this proposed method is aimed at reducing the amount of pollutants, and there is a problem that the ability to remove the eluting pollutants is insufficient.
Moreover, after adding iron powder to a contaminated soil slurry and adsorbing contaminants, a method for obtaining purified soil by collecting and removing the iron powder adsorbing the contaminants by magnetic separation has been proposed (Patent Document 3). reference). This proposal also allows for the removal of eluting contaminants. However, since the proposed method is a wet process, it is necessary to increase the capacity of the equipment for slurrying with a large amount of water and the capacity of the dewatering equipment, and it is difficult to carry out a cheap, simple and large-scale treatment. Is the situation.

  Therefore, it is desired to provide a detoxification method for contaminated soil that can be conveniently and effectively used as purified soil by dry treatment without using a large amount of water.

JP 2003-290757 A Japanese Patent Laid-Open No. 10-71387 JP 2000-51835 A

Committee for manuals for responding to sediment containing natural heavy metals in construction work (2010): “Manual for responding to rocks and soils containing natural heavy metals in construction (provisional version)” “Guidelines on Surveys and Measures Based on the Soil Contamination Countermeasures Law (Revised 2nd Edition)” (2012.2.8): Ministry of the Environment “Purification of heavy metal-contaminated soil based on the soil cleaning method”: Resources and Materials Volume: 2000: p. 117-120 (2010)

  An object of the present invention is to solve the above-described problems and achieve the following objects. That is, an object of the present invention is to provide a detoxification method for contaminated soil that can be used simply and effectively as purified soil by dry treatment without using a large amount of water.

Means for solving the problems are as follows. That is,
<1> An iron powder addition step of adding iron powder to contaminated soil containing at least one pollutant selected from arsenic, lead, hexavalent chromium, cadmium, selenium, mercury, cyanide, fluorine and boron; ,
A moisture content adjusting step for adjusting the moisture content of the contaminated soil before magnetic separation to 36% by mass or less;
And a dry magnetic separation step of recovering and removing iron powder from the contaminated soil having a water content of 36% by mass or less by dry magnetic separation.
<2> The detoxification method for contaminated soil according to <1>, wherein 0.05% by mass or more and 10% by mass or less of iron powder is added to the contaminated soil.
<3> The method for detoxifying contaminated soil according to any one of <1> to <2>, wherein the moisture content of the contaminated soil before addition of iron powder is 60% by mass or less.
<4> The method for detoxifying contaminated soil according to any one of <1> to <3>, wherein in the iron powder addition step, either sulfuric acid or hydrochloric acid is added.

  According to the present invention, it is possible to provide a method for detoxifying contaminated soil, which can solve conventional problems and can simply and effectively use treated soil as purified soil by dry treatment without using a large amount of water. it can.

FIG. 1 is a flowchart of the detoxification method for contaminated soil of the present invention. FIG. 2 is an equipment flow diagram for performing the detoxification method for contaminated soil of the present invention. FIG. 3 is an equipment flow diagram for performing the detoxification method for contaminated soil described in Japanese Patent Application Laid-Open No. 2000-51835 (Patent Document 3). FIG. 4 is a graph showing the relationship between the moisture content before magnetic separation in the soil A, the recovery rate of magnetic deposits and the As recovery rate [a)], and the relationship between the moisture content before magnetic separation and the As elution amount [b)]. FIG. 5 is a graph showing the relationship [a)] between the moisture content before magnetic separation in the soil B, the recovery rate of magnetic deposits, and the F recovery rate, and the relationship [b)] between the moisture content before magnetic separation and the F elution amount. FIG. 6 shows the relationship between the curing time after addition of iron powder and the recovery rate of magnetic deposits and As recovery rate in soil A [a)], and the relationship between the curing time after addition of iron powder and As elution amount [b)]. It is a graph. FIG. 7 shows the relationship between the curing time after the iron powder addition in the soil C, the recovery rate of magnetic deposits and the Se recovery rate [a)], and the relationship between the curing time after the iron powder addition and the Se elution amount [b)]. It is a graph. FIG. 8 shows the relationship between the curing time after the iron powder addition in the soil D and the recovery rate of magnetic deposits, the Pb and Cr recovery rate [a)], the relationship between the curing time after the iron powder addition and the Pb and Cr ⁶⁺ elution amount [ b)]. FIG. 9 is a graph showing the relationship [a)] between the iron powder addition amount in the soil A, the recovery rate of magnetic deposits, and the As recovery rate, and the relationship [b)] between the iron powder addition amount and the As elution amount. FIG. 10 is a graph showing the relationship [a)] between the iron powder addition amount in the soil C, the recovery rate of magnetic deposits and the Se recovery rate, and the relationship [b)] between the iron powder addition amount and the Se elution amount. FIG. 11 is a graph showing the relationship [a)] between the iron powder addition amount in the soil D, the recovery rate of magnetic deposits, and the Pb and Cr recovery rate, and the relationship [b)] between the iron powder addition amount and the Pb and Cr elution amounts. is there. FIG. 12 is a graph showing the relationship [a)] between the addition amount of sulfuric acid, the recovery rate of magnetic deposits and the As recovery rate in soil A, and the relationship [b)] between the addition amount of acid and the As elution amount. FIG. 13 is a graph showing the relationship [a)] between the addition amount of sulfuric acid and the recovery rate of magnetic deposits and Se recovery rate in soil C, and the relationship [b)] between the addition amount of acid and the Se elution amount. FIG. 14 is a graph showing the relationship [a)] between the acid addition amount in the soil D and the recovery rate of magnetic deposits, Pb and Cr recovery rate, and the relationship [b)] between the acid addition amount and Pb and Cr ⁶⁺ elution amounts. . FIG. 15 is a graph showing the relationship between the moisture content before magnetic separation and the magnetized material recovery mass. FIG. 16 is a photograph showing differences in soil properties due to changes in the water content of contaminated soil before magnetic separation in Examples 6, 8, 11 and Comparative Example 1.

(Method for detoxifying contaminated soil)
The method for detoxifying contaminated soil of the present invention includes an iron powder addition step, a moisture content adjustment step, and a dry magnetic separation step, and further includes other steps as necessary.

<Iron powder addition process>
The iron powder adding step is a step of adding iron powder to contaminated soil containing at least one pollutant selected from arsenic, lead, hexavalent chromium, cadmium, selenium, mercury, cyan, fluorine and boron. It is.

The contaminated soil is, for example, residual soil generated in connection with various construction works such as road construction, tunnel construction work, and redevelopment construction, and means soil containing the naturally-occurring pollutants.
Examples of the pollutants include arsenic (As), lead (Pb), hexavalent chromium (Cr (VI)), cadmium (Cd), selenium (Se), mercury (Hg), cyanogen (CN), fluorine ( F), boron (B) and the like. These pollutants are substances that are naturally derived and present in rocks and soil among the environmental standards related to soil contamination.

  The moisture content of the contaminated soil before adding iron powder is preferably 60% by mass or less, preferably 0% by mass to 45% by mass, and more preferably 10% by mass to 35% by mass. If it is within the range of the moisture content of the contaminated soil, the moisture adjustment in the moisture content adjustment process described later is less (may be omitted), and depending on the soil, it can be liquefied by loading it onto a dump truck or the like. This is advantageous in terms of transportability.

The addition amount of the iron powder is preferably 0.05% by mass or more and 10% by mass or less, and more preferably 0.05% by mass or more and 1% by mass or less with respect to the contaminated soil. Contaminants can be efficiently recovered and removed by dry magnetic separation within the range of the iron powder addition amount.
There is no restriction | limiting in particular as a kind of said iron powder, According to the objective, it can select suitably, For example, reduced iron powder, Dariko iron powder (made from scrap iron as a raw material), atomized iron powder, etc. are mentioned. Among these, reduced iron powder is preferable.

It is preferable to add an acid together with the addition of the iron powder to the contaminated soil. The acid is added to facilitate the migration of the contaminant.
As the acid, either hydrochloric acid or sulfuric acid is preferable.
There is no restriction | limiting in particular in the addition amount of the said acid, Although it can select suitably according to the objective, 0 mass% or more and 1 mass% or less are preferable with respect to the said contaminated soil.
The pH of the contaminated soil after the acid treatment is preferably 4.0 to 9.0, and more preferably 6.0 to 8.0. When the pH is within the above range, the contaminant does not change to elution and is safe. In addition, when the treated soil is used as the purified soil, it is preferably in the pH range because normal soil is in a neutral region.
In addition, when the acid is used, it is preferable not to perform dilution with water because a water content adjusting step is performed later. This is because it is necessary to increase any of the added amount of the dehydrating agent, the drying time, and the drying temperature for adjusting the water content.

  At least one of iron powder and acid is introduced into a mixer and kneaded with respect to the contaminated soil excavated with water content before the iron powder is added. In this case, when coarse gravel or the like is contained in the contaminated soil, pretreatment such as sieving and crushing in advance is preferably performed in order to hinder the kneading.

  The kneading method is not particularly limited and may be appropriately selected depending on the intended purpose. However, in consideration of the subdivided effect of the agglomerates, a shearing type mixer is preferable to an impact type mixer. Examples of the shear mixer include a biaxial paddle mixer.

<Moisture content adjustment process>
The moisture content adjusting step is a step of adjusting the moisture content of the contaminated soil before magnetic separation to 36% by mass or less.
In addition, when the moisture content of the contaminated soil before magnetic separation is already 36% by mass or less, the dry magnetic separation step described later can be performed without performing the moisture content adjusting step.
The moisture content of the contaminated soil before magnetic separation is 36% by mass or less, preferably 22% by mass or less, and more preferably 14% by mass or less. When the moisture content of the contaminated soil before the magnetic separation is 36% by mass or less, the aggregate becomes almost a single soil particle, and magnetic separation is facilitated, and dry magnetic separation can be performed efficiently.
The moisture content of the contaminated soil is, for example, after measuring the mass of the contaminated soil (wet soil mass w1), after drying the contaminated soil using a drying furnace or the like, the soil mass (dry soil mass w2). Can be calculated by the following formula.
Water content (%) = [(1−dry soil mass w2) / wet soil mass w1] × 100

There is no restriction | limiting in particular as the adjustment method of the water content in the said water content adjustment process, According to the objective, it can select suitably, For example, the method of adding a hygroscopic agent, the drying using a dryer, etc. are mentioned. It is done.
The hygroscopic agent is not particularly limited, and materials such as quick lime or cement are also conceivable. However, since these are strongly alkaline materials, it is necessary to control the pH range, and thus a neutral solidifying material is more preferable. .
Examples of the neutral solidifying material include a material mainly composed of hemihydrate gypsum and a material mainly composed of magnesium oxide. Among these, the material which has hemihydrate gypsum as a main component is preferable from an economical point.
In addition, when the moisture content of the contaminated soil after addition of iron powder is 36% by mass or less, no aggregate is formed, it is in a subdivided state, and there is no problem in dry magnetic separation, the addition of the moisture absorbent Can be omitted.

  There is no restriction | limiting in particular as a dryer used for the said drying, According to the objective, it can select suitably, For example, a twin-screw kneading-type drying furnace, a rotary dryer, a tunnel furnace etc. are mentioned. Among these, a biaxial kneading type drying furnace and a rotary dryer are preferable from the viewpoint of the effect of subdividing the aggregate.

<Dry magnetic selection process>
The dry magnetic separation step is a step of recovering and removing iron powder from the contaminated soil in which the moisture content of the contaminated soil before magnetic separation is adjusted to 36% by mass or less.

Here, as shown in FIG. 15, there are two regions in which the iron powder can be magnetically selected from the contaminated soil according to the moisture content of the contaminated soil before the magnetic separation.
One is a low moisture content region in which the moisture content before magnetic separation is 36% by mass or less, and the other is a high moisture content region in which the moisture content before magnetic separation is 45% by mass or more.
The method for detoxifying contaminated soil according to the present invention performs dry magnetic separation in the low moisture content region.
On the other hand, in the method described in Japanese Patent Laid-Open No. 2000-51835 (Patent Document 3), wet magnetic separation is performed in the high moisture content region. In this method, iron powder can be magnetically selected from the contaminated soil, but in order to make the contaminated soil into a slurry, a large amount of water is required, and its drainage is also necessary. It is difficult.
When the moisture content of the contaminated soil before magnetic separation is in the range of 38% by mass to 43% by mass, both the dry magnetic separation and the wet magnetic separation are regions where separation efficiency is extremely poor (magnetic separation difficult region).

The dry magnetic separation is not particularly limited and can be appropriately selected according to the purpose. For example, the contaminated soil having a moisture content of 36 mass% or less before the magnetic separation is put into a magnetic separator. The magnet is separated into a magnetized product and a non-magnetized product. There is no restriction | limiting in particular as magnetic force of the said magnetic separator, Although it can select suitably according to the objective, 1,500G or more and 12,000G or less are preferable, and 1,500G or more and 7,000G or less are more preferable.
The purpose of the dry magnetic separation process is to reduce the amount of harmful substance elution from the magnetic adherend by separating and removing the iron powder adsorbing the pollutants, and the magnetic force of 1,500G to 7,000G is sufficient. Separated and recovered. If the magnetic force is higher than this, weak magnetic soil particles previously present in the contaminated soil are also collected, and the amount of magnetic deposits increases. Since the magnetic deposits need to be disposed of separately as contaminated and concentrated soil, it is not economical to unnecessarily collect a large amount of them.

Here, FIG. 1 is a flowchart of the detoxification method for contaminated soil of the present invention.
First, at least one of iron powder and acid is put into a mixer and mixed well with excavated contaminated soil. At this time, when coarse gravel or the like is contained in the contaminated soil, it is preferable to perform pretreatment such as sieving and crushing in advance in order to hinder mixing.
Next, a predetermined amount of iron powder is added to the contaminated soil (iron powder addition step). After the soil contaminated with iron powder and acid added and mixed is cured for 0 minutes or more, preferably for about 10 minutes, a neutral solidifying material is added to the mixer together with 0 kg or more and 200 kg or less as a moisture absorbent as necessary. The moisture content of the contaminated soil before magnetic separation is adjusted to 36% by mass or less by mixing well or removing moisture with a dryer (moisture content adjustment step).
Next, the iron powder having adsorbed contaminants is recovered and removed by dry magnetic separation with a magnetic force of 1,500 G or more and 7,000 G or less (dry magnetic separation step).

FIG. 2 shows a facility flow diagram of the detoxification method for contaminated soil by dry magnetic separation according to the present invention. As a comparison, FIG. 3 shows an equipment flow diagram showing an example of a method for detoxifying contaminated soil by wet magnetic separation described in Japanese Patent Application Laid-Open No. 2000-51835 (Patent Document 3).
When FIG. 2 is compared with FIG. 3, the harmless treatment method described in Japanese Patent Laid-Open No. 2000-51835 has a more complicated process flow, and operation management becomes difficult. For example, the kneading machine in FIG. 2 can be substituted with a soil improvement machine, which is a general-purpose rental machine, and can be detoxified with a simple equipment flow. Moreover, since the flow of FIG. 3 is wet, it needs to be hydrated with water or spray water.

  Examples of the present invention will be described below, but the present invention is not limited to these examples.

<Sample used for testing>
Five types of soil A to soil E shown in Table 1 below were used as samples. The soil D is an artificially contaminated soil and the other is a naturally contaminated soil.

* Regarding the content of pollutants in soil A to soil E in Table 1, the sediment survey method and the amount of pollutants eluted from soil A to soil E were carried out in Notification No. 18 of the Ministry of the Environment.

(Examples 1-11 and Comparative Example 1)
-Effect of moisture content-
Using soil A and soil B, the effect of water content was examined.
First, soil A and soil B were air-dried in advance, and then an arbitrary amount of ion-exchanged water was added to adjust the soil with the water content shown in Tables 2 and 3.
Next, soil A or soil B with adjusted water content was weighed into a 100 g polypropylene cup, both of which were 1 g of reduced iron powder (special iron powder E200, manufactured by DOWAIP Creation Co., Ltd.) and concentrated sulfuric acid for soil A. 0.3 mL (0.1 N / kg as the acid concentration) and 5 mL of 10 mass% hydrochloric acid aqueous solution (0.1 N / kg as the acid concentration) were added to the soil B, and mixed with a spoon for 1 minute. After curing for 10 minutes, a neutral solidifying material mainly composed of gypsum (Gypsander C, manufactured by Ishihara Sangyo Co., Ltd.) was added in the range of 0 to 20 g and mixed for about 1 minute.
After that, the soil is opened with a bat and spread thinly, and the surface of the soil with a surface magnetic force of 1,500 G is scanned to magnetically separate the magnetized particles and non-magnetized particles (purified soil). Contaminant content in the soil (Sediment A for the bottom sediment survey method, soil B for the Ministry of the Environment Notification No. 19), and elution amount analysis (Ministry of the Environment Notification No. 18) for the non-magnetized particles.
In addition, the moisture content of the contaminated soil before the addition of iron powder and after the treatment (before magnetic separation) is measured after drying the contaminated soil using a drying furnace after measuring the mass of the contaminated soil (wet soil mass w1). Then, the soil mass (dry soil mass w2) was again measured and calculated by the following formula.
Water content (%) = [(1−dry soil mass w2) / wet soil mass w1] × 100
The test results of soil A and soil B are shown in Table 2 and Table 3, respectively.
Moreover, about the soil B, about Example 6, 8, 11, and the comparative example 1 whose moisture content before magnetic separation is 10.4 mass%, 20.0 mass%, 35.5 mass%, and 41.7 mass% The properties of the soil are shown in FIG. From the results of FIG. 16, it was found that the soil had no fluidity and was solid up to a moisture content before magnetic separation of 35.5% by mass.

<Soil A test results>
* The lower limit of quantification of the As elution amount is 0.001 mg / L, but a value below that is shown for reference.

<Soil B test results>
* Analysis of F content is based on the sediment survey method.
* Analysis of F content is based on the sediment survey method.

By applying the method for detoxifying contaminated soil according to the present invention, in both soil A and soil B, the amount of pollutant elution in the purified soil is the same as that before treatment regardless of the moisture content before magnetic separation in the tested range. It was shown to be lower than soil.
For soil A, if the moisture content of the contaminated soil before addition of iron powder is 10% by mass or less, magnetic separation is relatively easy without forming aggregates, and the addition of a neutral solidifying material is unnecessary. It was. When the water content of the contaminated soil before the addition of iron powder is 20% by mass or more, the contaminated soil forms aggregates, and thus it is necessary to add a neutral solidifying material. When the moisture content before iron powder addition was 30% by mass, the contaminated soil before magnetic separation was in a fluidized state, but magnetic separation was possible by increasing the amount of neutral solidified material. The moisture content before magnetic separation at this time was 25.0% by mass.

  As for the soil B, as in the case of the soil A, if the water content of the contaminated soil before the addition of iron powder is 24.1% by mass or less, magnetic separation is easy, and the addition of a neutral solidifying material is unnecessary. When the water content before iron powder addition was 34.8% by mass or more, it was necessary to add a neutral solidifying material. When the water content before iron powder addition was 41.7% by mass, the addition of 15% by mass of the neutral solidified material resulted in aggregates capable of magnetic separation, but the moisture content before magnetic separation at this time was 35 It was 5% by mass. When the water content before iron powder addition was 41.7% by mass and no neutral solidifying material was added, magnetic separation could not be performed (Comparative Example 1).

FIG. 4 shows the relationship [a)] between the moisture content before magnetic separation in the soil A, the recovery rate of magnetic deposits, and the As recovery rate, and the relationship [b)] between the moisture content before magnetic separation and the As elution amount. FIG. 5 shows the relationship between the moisture content before magnetic separation in the soil B, the recovery rate of magnetic deposits and the F recovery rate [a)], and the relationship between the moisture content before magnetic separation and the F elution amount [b)]. From the results of FIG. 5, it was shown that the amount of magnetic deposits recovered and the amount of contaminants recovered increased as the moisture content before magnetic separation increased. This is considered to be because the aggregate of soil particles slightly increases when the water content increases. On the other hand, the elution amount tended to decrease as the water content before magnetic separation increased. It was considered that the adsorption of As to the iron powder facilitated when the moisture content before magnetic separation was large. In addition, in the range tested, in any moisture content range before magnetic separation, it was found that the elution amount of the pollutant was sufficiently reduced and effective.

(Examples 12 to 39)
<Effect of curing time after iron powder addition, iron powder addition amount and acid addition amount>
Using soil A, soil C, and soil D, the effects of curing time, iron powder addition amount, and acid addition amount after iron powder addition were confirmed.
100 g of each soil sample was weighed into a polypropylene cup, and the same amount of reduced iron powder as in Example 1 and an arbitrary amount of concentrated sulfuric acid were added and mixed with a spoon for 1 minute. After curing for an arbitrary time, 10 g of a neutral solidifying material (Gypsander C, manufactured by Ishihara Sangyo Co., Ltd.) mainly composed of gypsum was added and mixed for about 1 minute. After that, the soil is opened in a bat and spread thinly. Using a magnet with a surface magnetic force of 1,500G, magnetically separated particles and non-magnetically adhered particles (purified soil) are magnetically separated, and each contains contaminants in the soil. Elution amount analysis (Ministry of the Environment Notification No. 18) was conducted on the amount (bottom quality survey method) and non-magnetic deposits. The results of soil A, soil C, and soil D are shown in Tables 4 to 6 and FIGS.

<Result of soil A>
* In Table 4-1, “curing time” means the curing time after the addition of iron powder.

<Result of soil C>
* In Table 5-1, “curing time” means the curing time after the addition of iron powder.

<Result of soil D>
* “Curing time” in Table 6-1 means the curing time after addition of iron powder.

<Effect of curing time after iron powder addition>
From the results of Tables 4 to 6, in any tested range, there is no correlation between the magnetic deposit recovery rate, the contaminant recovery rate, and the pollutant elution amount with respect to the curing time after the addition of iron powder in any soil. It was. From this, it was found that about 1 minute is sufficient as the iron powder mixing time.

<Influence of iron powder addition amount>
From the results of Tables 4 to 6, in the tested range, an increase in the magnetic deposit recovery rate and the contaminant recovery rate was observed with an increase in the amount of iron powder added in any soil. Moreover, the elution amount of the pollutant showed a tendency to decrease as the amount of iron powder added increased except for Cr ⁶⁺ in soil D. At any level, the elution amount was significantly reduced with respect to the elution amount before the treatment, and it was judged that there was an effect of removing the eluted components. In the range tested, the smallest iron powder addition amount was 0.05 mass% in the soil A, but a sufficient reduction effect was recognized even with this iron powder addition amount.

<Effect of acid addition amount>
From the results of Tables 4 to 6, in any tested range, no correlation was found between the magnetic deposit recovery rate and the contaminant recovery rate with respect to the acid addition amount in any soil. Further, except for soil D, no correlation between the amount of acid added and the amount of pollutant elution was observed. In soil D, as the amount of acid added increased, the Pb elution amount showed a slight increase trend, and Cr ⁶⁺ showed a slight decrease trend. At any level, the elution amount before treatment was significantly increased. The amount was decreasing.
It should be noted that in all soils, the sufficient amount of elution-polluting components could be removed with an acid addition amount of 0 N / kg, and it was considered that it was not necessary to add acid in consideration of economic effects.

(Comparative Example 2)
<When water content is not adjusted>
Other than using soil B whose water content before iron powder addition was adjusted to 41.3 mass%, not using a neutral solidifying material, and not adjusting the water content (moisture content before magnetic separation 41.3 mass%) Tried magnetic separation in the same manner as in Example 1. As a result, it was not possible to obtain a magnetic deposit due to the high viscosity of the soil and large aggregates. Furthermore, when magnetic separation was attempted using a magnet having a surface magnetic force of 12,000 G, all the soil adhered to the magnet, and separation of magnetic and non-magnetic substances was not possible.

(Examples 40 to 42)
<Comparison of magnetic separation with different iron powder>
100 g of Se-contaminated soil (Se content 0.75 mg / kg to 0.86 mg / kg, Se elution amount 0.013 mg / L) at the same site as soil C was taken in a polypropylene cup, and three of them were prepared.
To each cup, 1 g of the same reduced iron powder, dariko iron powder (CC-1004, manufactured by Connelly), or atomized iron powder (-180 μm, manufactured by Wako Pure Chemical Industries, Ltd.) as in Example 1 was added, and hydrochloric acid was 0. 0.07 N / kg was added and mixed for 1 minute. After curing for 30 minutes, 10 g of a neutral solidifying material (Gypsander C, manufactured by Ishihara Sangyo Co., Ltd.) mainly composed of gypsum was added and mixed for about 1 minute.
After that, the soil is opened on a bat and spread thinly. Using a magnet with a surface magnetic force of 7,000 G, magnetically separated particles and non-magnetically adhered particles (purified soil) are magnetically separated, and the content of contaminants in the soil ( The amount of elution (Ministry of the Environment Notification No. 18) was measured for the bottom sediment investigation method) and non-magnetic deposits. The results are shown in Table 7.

<Results of magnetic separation with different iron powder>
From the results of Table 7, when a treatment test was carried out with different iron powders, the reduced magnetic powder collection rate and Se recovery rate were the highest for the reduced iron powder, followed by Daliko powder and atomized powder. The difference in the recovery rate of magnetic deposits is considered to be a result of an increase in the Se recovery rate because of the relatively small particle size of the reduced iron powder that formed aggregates and a large amount of attached soil. On the other hand, when comparing the elution amount of non-magnetic deposits, there was a slight difference.

(Comparative Examples 3-4)
-Results of magnetic separation only without adding iron powder (method described in JP-A-10-71837)-
For the air-dried soil A and the soil samples used in Examples 40 to 42, each of them was spread thinly on a 100 g vat and magnetized particles and non-magnetized particles (purified soil) using a magnet having a surface magnetic force of 7,000 G. ) Were magnetically separated, and the content of pollutants in the soil (sediment survey method) and elution amount analysis (Ministry of the Environment Notification No. 18) were conducted for non-magnetic deposits. The results are shown in Tables 8 and 9.

<Magnetic selection result of soil A>

<Magnetic separation results of soil samples used in Examples 40 to 42>

  From the results of Tables 8 and 9, as a result of magnetic separation of soil A, 10.4% by mass of magnetic deposits were obtained, and 10.3% by mass was recovered as As, but no decrease in As elution amount was observed. . From this, it was judged that the elution As was recovered by adsorbing to the iron powder. On the other hand, in the soil samples used in Examples 40 to 42, magnetic deposits due to magnetic separation were hardly obtained. From this, it was found that the method for detoxifying contaminated soil according to the present invention is more effective in the recovery and removal of eluting pollutants than the method described in JP-A-10-71837.

(Comparative Example 5)
100 g of Se-contaminated soil (Se content of 0.75 to 0.86 mg / kg, Se elution amount of 0.013 mg / L) at the same site as the soil C used in Examples 40 to 42 was taken in a 1.0 L wide-mouthed polybin, The same reduced iron powder 1 g as in Example 1, 400 mL of ion exchange water, and 0.5 mL of concentrated sulfuric acid were added and sealed, and the mixture was shaken with a shaker at 200 reciprocations / min for 30 minutes to slurry Se-contaminated soil. .
After shaking, the slurry in the polybin was opened in a polypropylene vat, and a surface magnetic force 0.7 T bar magnet was scanned in the slurry to separate the magnetic deposits. In addition, when transferring a slurry to another container etc., in order to collect | recover the soil particles which adhere, 100 mL of ion-exchange water was used.
The non-magnetized product slurry remaining in the polypropylene vat was suction filtered using 5C filter paper to obtain a non-magnetized product and treated water.
The obtained non-magnetized material was subjected to Se content analysis based on the sediment survey method and Se elution analysis based on Ministry of the Environment Notification No. 18. The magnetized material was subjected to Se content analysis based on the bottom sediment investigation method. The treated water was subjected to Se content analysis based on the bottom sediment investigation method. The results are shown in Table 10. An extract of Example 40 in Table 7 is shown in Table 11.

<Excerpt from Table 7>
From the results in Table 10, the treated soil (non-magnetized material) decreased to the same extent as the results of Example 40 in Table 11 with respect to the soil before addition of iron powder. However, in the contaminated soil of Comparative Example 5 having a high water content, the balance of Se in the magnetic deposit is much lower than that in Example 40 in Table 11, and rather, Se is distributed to the treated water. Results different from Example 40 were obtained. From this fact, it was found that when wet magnetic separation is performed on Se-contaminated soil, Se is transferred to the treated water, and the amount of Se that can be recovered as a magnetic deposit is reduced compared to when dry magnetic separation is performed. .
The Se content of the treated water was 0.022 mg / kg, which exceeded the environmental standard, and therefore a separate wastewater treatment was required.

Claims (4)

An iron powder addition step of adding iron powder to contaminated soil containing at least one pollutant selected from arsenic, lead, hexavalent chromium, cadmium, selenium, mercury, cyanide, fluorine and boron;
A moisture content adjusting step for adjusting the moisture content of the contaminated soil before magnetic separation to 36% by mass or less;
And a dry magnetic separation step of recovering and removing iron powder from the contaminated soil having a moisture content of 36% by mass or less by dry magnetic separation.   The method for detoxifying contaminated soil according to claim 1, wherein 0.05 to 10 mass% of iron powder is added to the contaminated soil.   The method for detoxifying contaminated soil according to any one of claims 1 to 2, wherein the moisture content of the contaminated soil before addition of iron powder is 60% by mass or less.   The method for detoxifying contaminated soil according to any one of claims 1 to 3, wherein either sulfuric acid or hydrochloric acid is added in the iron powder addition step.