JP6692136B2 - Decontamination method for contaminated soil - Google Patents

Description

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

  In recent years, the start of tunnel construction and redevelopment construction has been increasing with the economic buoyancy. Along with this, securing a dump site for the remaining soil has become a problem. The remaining soil also contains polluted soil due to naturally-derived pollutants such as arsenic, and a method of coping with it is an issue. As a characteristic of the contaminated soil, the content of pollutants is relatively small compared with the content standard stipulated in the Soil Contamination Countermeasures Law, while the elution standard exceeds several times to several tens of times. There is a tendency.

  Therefore, the applicant has adopted an iron powder addition step of adding iron powder to the contaminated soil, a water content adjustment step of adjusting the water content of the contaminated soil before magnetic separation to 36 mass% or less, and a water content. The method for detoxifying contaminated soil, which comprises a dry magnetic separation step of collecting and removing iron powder from the contaminated soil whose content is 36% by mass or less (see Patent Document 1). According to this proposal, the contaminated soil can be simply and effectively used as the purified soil by the dry treatment.

  However, in this proposal, in the water content adjusting step, the water content of the contaminated soil is adjusted by appropriately selecting it depending on the soil quality and mixing a hygroscopic material (water absorbing agent) such as a neutral solidifying material. In particular, for the contaminated soil containing a large amount of silt and clay, the neutral solidifying material is added to the contaminated soil in an amount of 10% by mass or more and 30% by mass or less. Could not be done. For this reason, the weight of the soil after the treatment becomes large, and there are problems that the supply equipment becomes excessively large and the transportation cost to the use place increases. Further, the contaminated soil after adding the neutral solidifying material tends to have large lumps (aggregates) of soil, and when the dry magnetic separation is performed in a state where the aggregates remain large, the effect of removing pollutants is increased. There is a problem of decrease in the size, and in order to carry out the dry magnetic separation more effectively, there has been a demand for a method of making the aggregates into a smaller state.

  Furthermore, regarding the purified soil obtained by the method of detoxifying contaminated soil, not only the safety but also the properties required for the purified soil (for example, the volume expansion coefficient is low so that the place of backfilling is not limited). It was required to backfill.

  Therefore, it is possible to reduce the aggregate size of the contaminated soil when performing the dry magnetic separation, and further, the weight of the soil after the treatment does not increase, and the volume expansion coefficient when water is contained becomes low. It is desired to provide a method for detoxifying contaminated soil in which the backfilling place of purified soil is not restricted.

Japanese Patent No. 5647371

  An object of the present invention is to solve the above-mentioned problems in the related art and achieve the following object. That is, the present invention can reduce the aggregate of contaminated soil when performing the dry magnetic separation, and further, the weight of the soil after treatment does not increase, and the volume expansion coefficient when water is contained is low. Therefore, it is an object of the present invention to provide a method for detoxifying contaminated soil, in which the backfilling place of the purified soil is not restricted.

The means for solving the above problems are as follows. That is,
<1> 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, cyan, fluorine and boron ,
A water-absorbent polymer addition step of adding a water-absorbent polymer to the contaminated soil before magnetic separation,
A dry magnetic separation step of collecting and removing iron powder from the contaminated soil to which the water-absorbing polymer has been added by dry magnetic separation,
A method for detoxifying contaminated soil, comprising the step of adding an inorganic compound to the non-magnetic substance obtained in the dry magnetic separation step.
<2> The method for detoxifying contaminated soil according to <1>, wherein 0.1% by mass or more and 1.0% by mass or less of the water-absorbent polymer is added to the contaminated soil.
<3> Detoxification of the contaminated soil according to <1> or <2>, wherein the addition amount of the inorganic compound is 0.5 parts by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the non-magnetic substance. It is a processing method.
<4> The inorganic compound is at least one of hemihydrate gypsum, anhydrous gypsum, calcium sulfate, calcium chloride, calcium carbonate, iron sulfate, iron chloride, aluminum sulfate, aluminum chloride, magnesium sulfate, and incinerated ash. The method for detoxifying contaminated soil according to any one of 1> to <3>.
<5> The contaminated soil according to any one of <1> to <4>, wherein the inorganic compound is added when the water-absorbent polymer is added to the contaminated soil in an amount of 0.1% by mass or more. It is a detoxification method.
<6> Cumulative 50% particle size D on a mass basis of 9.5 mm or less obtained by sieving the contaminated soil before the dry magnetic separation to which the water-absorbing polymer was added with a 9.5 mm sieve in advance. ₅₀ is the method for detoxifying contaminated soil according to any one of <1> to <5>, which is 4 mm or less.
<7> The method for detoxifying contaminated soil according to any one of <1> to <6>, wherein the water content of the contaminated soil before addition of iron powder is 60% by mass or less.
<8> The method for detoxifying contaminated soil according to <1> to <7>, wherein the nonmagnetic material added with the inorganic compound has a volume expansion coefficient of 130% or less.

  According to the present invention, it is possible to solve the problems in the prior art, it is possible to reduce the aggregate of contaminated soil when performing the dry magnetic separation, further, without increasing the weight of the soil after treatment, moisture It is possible to provide a method for detoxifying contaminated soil in which the backfilling place of the purified soil is not limited because the volume expansion coefficient when it is included is low.

FIG. 1 is a flow chart of a method for detoxifying contaminated soil according to the present invention. FIG. 2 is a graph showing the relationship between the mass-based cumulative 50% particle diameter D ₅₀ and the magnetic substance recovery rate. FIG. 3 is a graph showing the relationship between the added amount of the water-absorbing polymer of Comparative Example 1 and the volume expansion coefficient.

(Method of detoxifying contaminated soil)
The method for detoxifying contaminated soil according to the present invention includes an iron powder addition step, a water-absorbing polymer addition step, a dry magnetic separation step, and an inorganic compound addition step, and further includes other steps as necessary. ..
In addition, detoxification of contaminated soil means that the pollutants contained in the contaminated soil are treated in accordance with the provisions of the Enforcement Regulations of the Soil Contamination Countermeasures Act (Ministry of the Environment Ordinance No. 29 of 2002) Article 5 Paragraph 3 Item 4 The value should be below the standard (Ministry of the Environment Notification No. 18) related to the soil elution amount survey established by the Minister.

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

The contaminated soil is, for example, residual soil generated by various construction works such as road construction, tunnel construction work, redevelopment work, etc., and means soil containing the pollutant derived from nature.
Examples of the pollutants include arsenic (As), lead (Pb), hexavalent chromium (Cr (VI)), cadmium (Cd), selenium (Se), mercury (Hg), cyanide (CN), fluorine ( F), boron (B) and the like. These pollutants are substances that are of natural origin and are present in rocks and soil among the substances that are subject to environmental standards for soil pollution.

  The water content of the contaminated soil before the addition of iron powder is preferably 60% by mass or less, more preferably 0% by mass or more and 45% by mass or less, still more preferably 10% by mass or more and 35% by mass or less. When the water content of the contaminated soil is in the range of 60% by mass or less, it is advantageous in that the amount of the water-absorbent polymer added in the water-absorbent polymer addition step described later can be reduced. Further, it is advantageous because it can be transported from the construction site to the treatment facility by a transportation means such as a truck.

The amount of the iron powder added 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. When the amount of the iron powder added is 0.05% by mass or more and 10% by mass or less, contaminants can be efficiently collected and removed by dry magnetic separation.
The type of the iron powder is not particularly limited and may be appropriately selected depending on the purpose, and examples thereof include reduced iron powder, Daliko iron powder (using scrap iron as a raw material), atomized iron powder and the like. 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 migration of the contaminant.
The acid is preferably hydrochloric acid or sulfuric acid.
The amount of the acid added is not particularly limited and may be appropriately selected depending on the intended purpose, but is preferably 0% by mass or more and 1% by mass or less based on the contaminated soil.
The pH of the contaminated soil after the acid treatment is preferably 4.0 to 9.0, more preferably 6.0 to 8.0. When the pH is within the above range, contaminants are not changed in dissolution property, which is safe. Further, when the treated soil is used as the purified soil, the normal pH of the soil is in the neutral range, so that the pH is preferably within the above range.
In addition, when the above-mentioned acid is used, it is preferable not to dilute it with water, because a water-absorbing polymer addition step is performed later. This is because it is necessary to increase the addition amount of the water absorbing polymer in the water absorbing polymer adding step.

  At least one of the iron powder and the acid is put into a mixer and kneaded well with respect to the excavated contaminated soil having the water content before the addition of the iron powder. In this case, if coarse gravel or the like is contained in the contaminated soil, it hinders the kneading, and thus it is preferable to perform pretreatment such as sieving and crushing in advance.

  The kneading method is not particularly limited and may be appropriately selected depending on the intended purpose. However, in consideration of the effect of subdividing the aggregates, the shear mixer is preferable to the impact mixer. Examples of the shear mixer include a twin-screw paddle mixer.

<Water absorbing polymer addition step>
The water absorbent polymer adding step is a step of adding a water absorbent polymer to the contaminated soil before magnetic separation. Regarding the contaminated soil to which the water-absorbing polymer has been added before the magnetic separation, a mass-based cumulative 50% particle diameter D ₅₀ in a 9.5 mm or less classification obtained by sieving with a 9.5 mm sieve in advance is 4 mm or less. Is preferred. If the D ₅₀ exceeds 4 mm, the effect of removing contaminants may be reduced.

Here, the mass-based cumulative 50% particle diameter D ₅₀ can be calculated as follows.
A predetermined amount of an additive (for example, the iron powder, the water-absorbent polymer, etc.) is added to 100 g of soil having a size of 9.5 mm or less, which has been sieved in advance with a standard sieve having an opening of 9.5 mm, and mixed. Next, the mixed soil is measured for a particle size distribution on a mass basis using a standard sieve while keeping the aggregated particles of the soil, and a cumulative particle diameter (D ₅₀ ) of 50% on a mass basis is obtained. Ask.

The water-absorbent polymer is not particularly limited and may be appropriately selected depending on the intended purpose. Examples thereof include polyacrylate (for example, sodium polyacrylate), polysulfonate, maleic anhydride, polyacrylamide, Examples thereof include polyvinyl alcohol, polyethylene oxide, polyaspartate, polyglutamate, polyalginate, starch-based, cellulose water-absorbing polymers, and derivatives thereof. Among these, polyacrylic acid salts, which are industrially superior in productivity, are preferable, and sodium polyacrylate is more preferable.
The average particle size of the water-absorbent polymer is not particularly limited and may be appropriately selected depending on the purpose, but the average particle size is preferably small. This is because the water absorbing polymer has a large specific surface area in the process of absorbing water, and is easily mixed evenly with soil.
The amount of the water-absorbent polymer added is not particularly limited and can be appropriately selected according to the water content of the contaminated soil. For example, 0.1% by mass or more and 1.0% by mass relative to the contaminated soil. % Or less is preferable. If the addition amount is less than 0.1% by mass, the effect of adding the water-absorbing polymer may not be obtained, and if it exceeds 1.0% by mass, the effect may be saturated.
Further, the amount of the water-absorbent polymer added may be determined according to the water content of the contaminated soil.
The water absorbing polymer adding step may be added to the contaminated soil at any time before the dry magnetic separation step, and may be performed at the same time as the iron powder adding step.
Further, the addition of the water-absorbent polymer is advantageous in that the D _{50 of the} contaminated soil before magnetic separation is reduced, and the recovery rate of magnetic substances is improved in the dry magnetic separation process.

The water content of the contaminated soil before the magnetic separation is preferably 36% by mass or less, more preferably 22% by mass or less, and particularly preferably 14% by mass or less. When the water content of the contaminated soil before the magnetic separation is 36% by mass or less, the aggregates become the soil particles alone, and the magnetic separation is facilitated, and the dry magnetic separation can be efficiently performed.
For the water content of the contaminated soil, for example, after measuring the mass of the contaminated soil (wet soil mass w1), the contaminated soil is dried using a drying oven or the like, and then the soil mass is regenerated (dry soil mass w2). Can be measured and calculated by the following formula.
Water content (%) = [1- (dry soil mass w2 / wet soil mass w1)] x 100

<Dry magnetic separation process>
The dry magnetic separation step is a step of collecting and removing the iron powder from the contaminated soil to which the water-absorbing polymer has been added by dry magnetic separation.

  In the present invention, since the water-absorbing polymer is added to the contaminated soil before the magnetic separation, the dry magnetic separation can be performed even if the water content of the contaminated soil is relatively large.

The dry magnetic separation is not particularly limited and may be appropriately selected depending on the purpose.For example, the contaminated soil before the magnetic separation to which the water-absorbing polymer is added is put into a magnetic separator, and a magnetic substance is attached by a magnet. Separated from non-magnetic material. The magnetic force of the magnetic force sorter is not particularly limited and may be appropriately selected depending on the intended purpose, but is preferably 1,500 G or more and 12,000 G or less, more preferably 1,500 G or more and 7,000 G or less.
The magnet is appropriately selected depending on the intended purpose without any limitation.
The dry magnetic separation process is intended to reduce the amount of harmful substances elution of non-magnetic substances by separating and removing iron powder that has adsorbed contaminants, and a magnetic force of 1,500 G or more and 7,000 G or less is sufficient. Can be separated and collected. If the magnetic force is higher than this, weakly magnetic soil particles that were previously present in the contaminated soil will also be collected, and the amount of magnetic material will increase. Since the magnetic substance needs to be separately disposed as contaminated concentrated soil, it is not economical to recover a large amount of it.

<Inorganic compound addition step>
The inorganic compound adding step is a step of adding the inorganic compound to the non-magnetic substance obtained in the dry magnetic separation step.
The addition amount of the inorganic compound is not particularly limited and may be appropriately selected depending on the intended purpose, but is preferably 0.5 parts by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the non-magnetic substance. It is more preferably 1.0 part by mass or more and 5.0 parts by mass or less.
The inorganic compound is not particularly limited and may be appropriately selected depending on the purpose, for example, hemihydrate gypsum, anhydrous gypsum, calcium sulfate, calcium chloride, calcium carbonate, iron sulfate, iron chloride, aluminum sulfate, chloride. Aluminum, magnesium sulfate, incineration ash, etc. may be mentioned. The incineration ash is neutral, and examples thereof include incineration ash of sewage sludge. Among these, the hemihydrate gypsum, the anhydrous gypsum, and the incinerated ash are preferable from the viewpoint of reducing the volume expansion coefficient.
The inorganic compound is preferably added when the water absorption capacity of the water-absorbent polymer is added to the contaminated soil more than saturated, the inorganic compound is preferably added to the contaminated soil from the viewpoint of the effect. On the other hand, it is more preferable to add the inorganic compound when the water-absorbent polymer is added in an amount of 0.1% by mass or more.

Conventionally, when the contaminated soil and the non-magnetic substance containing the water-absorbent polymer are treated to obtain purified soil, by using the water-absorbent polymer to utilize the soil for backfilling, rainwater When water or groundwater penetrates the purified soil, the water-absorbent polymer absorbs water, and the purified soil as a whole expands into a gel. Therefore, there is a problem that the place for backfilling is limited. However, including the water-absorbent polymer, with respect to the contaminated soil and the non-magnetic substance, by mixing the inorganic compound, even when the water-absorbent polymer was contained in the purified soil when backfilling , Even if the rainwater or the groundwater penetrates the purified soil, due to the principle of osmotic pressure, unnecessary water is not absorbed by the water-absorbent polymer, and the purified soil undergoes the expansion and gelation. The volume expansion coefficient can be reduced to such an extent that the above can be prevented. Therefore, there is an advantageous effect that there is no limitation on the backfilling place of the purified soil. Although the soil after treatment is in a state where the elution amount is reduced, it is still in a state where it contains a poorly soluble low-concentration heavy metal. If it is determined that this may elute over time, mix iron sulfate, iron chloride, aluminum sulfate, aluminum chloride, or magnesium sulfate with each hydroxide in the soil. It is also possible to form the adsorbent for these heavy metals. In addition, when there is a risk that these heavy metals will elute due to acidification of the soil due to acid rain, etc., it is optimal to add calcium carbonate having acid buffering capacity, incineration ash, etc. .. In some cases, it is considered that advantageous effects can be obtained by combining a plurality of these materials and mixing them.
The volume expansion coefficient is preferably 130% or less, and is preferably as close to 100% as possible. If it is 130% or less, as described above, the expansion and gelation of the purified soil can be prevented.
As the measurement of the volume expansion coefficient, for example, with respect to the purified soil (non-magnetic substance), a beaker of 200 mL is filled with 50 mL of the purified soil while compacting the purified soil, and the volume capacity (V ₁ ) thereof is measured. Next, 50 mL of water is injected, the mixture is allowed to stand for 24 hours, its volume capacity (V ₂ ) is measured, and the volume expansion coefficient of soil can be obtained from the following formula.
Volume expansion rate (%) = (V ₂ / V ₁ ) × 100

<Other processes>
The other steps are appropriately selected depending on the intended purpose without any limitation, and examples thereof include a transportation step, which can be performed by a transportation means.
The carrying step is not particularly limited and may be appropriately selected depending on the purpose. For example, the step of carrying the magnetically-adsorbed material after the magnetic separation to a treatment facility, the non-magnetically-adhered material after the magnetic separation is backfilled. Examples include the process of transporting to a place.
The transportation means is not particularly limited and may be appropriately selected depending on the purpose, and examples thereof include transportation by an automobile such as a truck.

Here, FIG. 1 is a flow chart of the method for detoxifying contaminated soil according to the present invention.
First, iron powder and, if necessary, acid are put into the excavated contaminated soil in a mixer and mixed well. At this time, if coarse gravel or the like is contained in the contaminated soil, mixing is hindered, and therefore pretreatment such as sieving and crushing is preferably performed in advance.
Next, a predetermined amount of iron powder is added to the contaminated soil (iron powder addition step). The contaminated soil, to which iron powder and, if necessary, an acid have been added and mixed, is cured for 0 minutes or more, preferably for 10 minutes, and then the water-absorbing polymer is added (water-absorbing polymer adding step).
Next, the iron powder that has adsorbed the 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 process).
Next, an inorganic compound is added to the non-magnetic substance after the dry magnetic separation (inorganic compound adding step).

  Examples of the present invention will be described below, but the present invention is not limited to these examples. In addition, although the present example is the experimental data of the contaminated soil containing arsenic (As), it can be similarly measured for the contaminated soil by other substances.

(Relationship between the amount of collected magnetic substances and D ₅₀ )
After adding 1% by mass of iron powder to 100 g of soil A (silt mixed sand) having a water content of 23.4% by mass, which has been sieved in advance to a mesh size of 9.5 mm or less, and mixing 0.1 mL of 10% by mass sulfuric acid. As the water absorbing agent, hemihydrate gypsum (Gypsander C, Ishihara Sangyo Co., Ltd.) was added and mixed in place of the water absorbing polymer in the predetermined amounts shown in Table 1. Then, the particle size distribution of the non-magnetic material after collecting the magnetic material with a 1,500 G magnet was measured using a standard sieve in a tangible state (while keeping the soil aggregates), and a cumulative mass-based 50% was obtained. The particle size (D ₅₀ ) of was determined. Table 1 and FIG. 2 show the relationship between the D ₅₀ and the recovery rate of magnetic substances.
Further, 1 mass% of iron powder was added to 100 g of soil B (sand mixed with clay) having a water content of 21.5 mass% and sieved to an opening of 9.5 mm or less, and 10 mass% of sulfuric acid was mixed with 0.1 mL. After that, water-absorbent polymer (sodium polyacrylate, super water-absorbent resin CP-1, manufactured by Chemical Technos Co., Ltd.) was added in a predetermined amount shown in Table 2 and mixed. Then, the particle size distribution of the non-magnetic material after collecting the magnetic material with a 1,500 G magnet was measured using a standard sieve in a tangible state (while keeping the soil aggregates), and a cumulative mass-based 50% was obtained. The particle size (D ₅₀ ) of was determined. The relationship between the D ₅₀ and the recovery rate of magnetic substances is similarly shown in Table 2 and FIG.

As shown in FIG. 2, Table 1 and Table 2, the recovery rate of the magnetic deposits tended to increase as the D ₅₀ decreased, indicating that the magnetic deposits could be efficiently recovered. The magnetic substance recovery rate is higher than the amount of the iron powder added, because magnetic minerals are also present in the soil.

(Example 1)
- Dry magnetic separation measurement of the particle diameter D ₅₀ of the previous contaminated soil -
1 mass% of iron powder was added to 100 g of soil having a water content of 23.4 mass%, which had been sieved in advance to a mesh size of 9.5 mm or less, and a water-absorbing polymer (sodium polyacrylate, superabsorbent resin CP- 1, manufactured by Chemical Technos Co., Ltd.) was added and mixed in a predetermined amount shown in Table 3. Then, after collecting the magnetic substance with a 1,500 G magnet, the particle size distribution of the non-magnetic substance was measured using a standard sieve in the tangible state (while keeping the aggregates of the soil), and the mass-based accumulation The 50% particle size (D ₅₀ ) was determined. The results are shown in Table 3.

-Measurement of volume expansion coefficient-
Hemihydrate gypsum (Gypsander, manufactured by Ishihara Sangyo Co., Ltd.) was added to and mixed with the non-magnetic substance used in the measurement of D ₅₀ . Then, the volume expansion coefficient due to water absorption was measured as follows.
Into a 200 mL beaker, 50 mL of the non-magnetic substance was compacted while being compressed, and the volume capacity (V ₁ ) was measured. Next, 50 mL of water was injected, the mixture was allowed to stand for 24 hours, its volume capacity (V ₂ ) was measured, and the volume expansion coefficient of soil was determined from the following formula. The results are shown in Table 3.
Volume expansion rate (%) = (V ₂ / V ₁ ) × 100

-Method for detoxifying contaminated soil of Example 1-
After mixing 1.0 g of iron powder and 0.1 mL of 10 mass% sulfuric acid with 100 g of arsenic-contaminated soil, No. 3 in Table 3 was used. 1-No. The water-absorbing polymer shown in 12 and hemihydrate gypsum were mixed.
Next, magnetic separation was carried out with a 1,500 G magnet to separate into a magnetic substance and a non-magnetic substance. The As content of each product and the elution amount of As of the non-magnetic substance were analyzed.

  The obtained non-magnetized material and magnetic material were subjected to content analysis based on the bottom sediment investigation method and elution amount analysis based on Ministry of the Environment Notification No. 18.

  In addition, "As content of untreated soil" and "As balance in magnetic material" in Tables 4 to 15 and Tables 17 to 21 below are obtained from the following equations.

-As content (mg / kg) of soil before treatment = (As content of soil after treatment x dry mass distribution of soil after treatment + As content of magnetic substance x dry mass distribution of magnetic substance) / (soil after treatment) (Dry mass distribution + magnetic material dry mass distribution)

-As balance (%) in magnetic material = (As content of magnetic material x dry mass distribution of magnetic material) / (As content of untreated soil x dry mass distribution of untreated soil) x 100

  No. 1 of the first embodiment. 1-No. The results of No. 12 are shown in Tables 4 to 15.

<No. 1>

<No. 2>

<No. 3>

<No. 4>

<No. 5>

<No. 6>

<No. 7>

<No. 8>

<No. 9>

<No. 10>

<No. 11>

<No. 12>

(Comparative Example 1)
- Dry magnetic separation measurement of the particle diameter D ₅₀ of the previous contaminated soil -
To 100 g of the soil used in Example 1, 1% by mass of iron powder was added, and the same water-absorbing polymer as in Example 1 was added in a predetermined amount shown in Table 16 and mixed. Then, after collecting the magnetic substance with a 1,500 G magnet, the particle size distribution of the non-magnetic substance was measured using a standard sieve in the tangible state (while keeping the aggregates of the soil), and the mass-based accumulation The particle diameter (D ₅₀ ) of 50% was measured, and the D ₅₀ was obtained in the same manner as in Example 1. The results are shown in Table 16.

-Measurement of volume expansion coefficient-
To 100 g of the same soil as in Example 1, 1% by mass of iron powder was added and 0.1 mL of 10% by mass sulfuric acid was mixed, and then a predetermined amount of a water-absorbent polymer shown in Table 16 was added and mixed. Thereafter, the volume expansion coefficient of the non-magnetized material after the magnetic material was collected with a 1,500 G magnet was determined in the same manner as in Example 1. The results are shown in Table 16 and FIG.

-Method for detoxifying contaminated soil in Comparative Example 1-
No hemihydrate gypsum was added, and no. 13-No. The As content of each product and the elution amount of As of the non-magnetic substance were analyzed in the same manner as in Example except that the water-absorbing polymer shown in 17 was mixed.

  Comparative Example 1 No. 13-No. The results of No. 17 are shown in Tables 17-21.

<No. 13>

<No. 14>

<No. 15>

<No. 16>

<No. 17>

  In Example 1, as shown in Table 3, the volume expansion coefficient tends to increase as the addition amount of the water-absorbing polymer increases, but the volume expansion coefficient can be decreased by increasing the addition amount of hemihydrate gypsum. did it. On the other hand, in Comparative Example 1, as shown in Table 16 and FIG. 3, since hemihydrate gypsum was not added, the volume expansion coefficient also increased as the amount of the water-absorbent polymer added increased.

  Further, as shown in Tables 4 to 15 and Tables 17 to 21, the treated soils (non-magnetic substances) of Example 1 and Comparative Example 1 had pollutants (As) reduced to the same extent. The As distribution ratio to the magnetic substance showing the effect of removing contaminants increased slightly as the amount of the water-absorbent polymer added increased in the range tested. On the other hand, the changes in the amount of As eluted were all below the lower limit of quantification in the treated soil, and the results were also below the lower limit of quantification even after the addition of hemihydrate gypsum.

(Example 2)
The volume expansion coefficient was obtained in the same manner as in Example 2 except that hemihydrate gypsum was changed to the inorganic compound shown in Table 22. In addition, the elution amount analysis was performed on the treated soil. The results are shown in Table 22.

* Note that sewage sludge incineration ash in the table is constituted major component _{P 2 O 5, SiO 2,} Al 2 O 3, CaO, Fe 2 O 3, MgO, and _{K 2} O in approximately 90 wt% or more It is an inorganic powder.
(Reference: "Effects of various component ratios and melting conditions in the production of molten phosphate fertilizer using sewage sludge incineration ash as a raw material" Yoshihiro Iwai Vol. 20, No. 3, 2009)

  In Example 2, as shown in Table 22, the volume expansion coefficient varied depending on the type of the inorganic compound, but the expansion was substantially suppressed, which was a good result. On the other hand, in Comparative Example 1, as shown in Table 16, since no inorganic compound was added, the volume expansion rate also increased as the amount of the water-absorbent polymer added increased.

  Further, as shown in Table 22, the arsenic elution amount of the treated soil (non-magnetic substance) decreased to the same extent as the results of Example 2 and Comparative Example 1, and even when various inorganic compounds were added, It turns out that it has little effect on the value. Regarding the soil pH, anhydrous gypsum and sewage sludge incineration ash tended to increase slightly as the addition amount increased, and others tended to decrease. It may be possible to promote the elution of heavy metals. It is important to appropriately determine the addition amount of these so that the soil pH is maintained in the neutral range.

Claims (6)

An iron powder addition step of adding iron powder to a contaminated soil containing at least one contaminant selected from arsenic, lead, hexavalent chromium, cadmium, selenium, mercury, cyan, fluorine and boron;
A water-absorbing polymer addition step of adding a water-absorbing polymer to the contaminated soil before magnetic separation in an amount of 0.1% by mass or more and 1.0% by mass or less with respect to the contaminated soil,
A dry magnetic separation step of collecting and removing iron powder from the contaminated soil to which the water-absorbing polymer has been added by dry magnetic separation,
See contains an inorganic compound addition step of adding an inorganic compound to a non-magnetically attracted material obtained in the dry magnetic separation step,
The inorganic compound is characterized by being at least one of hemihydrate gypsum, anhydrous gypsum, calcium sulfate, calcium chloride, calcium carbonate, iron sulfate, iron chloride, aluminum sulfate, aluminum chloride, magnesium sulfate, and incineration ash. Method for detoxifying contaminated soil.   The method for detoxifying contaminated soil according to claim 1, wherein the addition amount of the inorganic compound is 0.5 parts by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the non-magnetic substance. The contaminated soil according to claim 1, wherein the inorganic compound is added when the water-absorbent polymer is added to the contaminated soil in an amount of 0.1% by mass or more and 1.0% by mass or less. Detoxification treatment method. The contaminated soil before the dry magnetic separation to which the water-absorbing polymer was added was previously sieved with a 9.5 mm sieve to obtain a mass-based cumulative 50% particle diameter D of 9.5 mm or less. _Fifty Is 4 mm or less, and the method for detoxifying contaminated soil according to any one of claims 1 to 3. The method for detoxifying contaminated soil according to any one of claims 1 to 4, wherein the water content of the contaminated soil before the addition of iron powder is 60% by mass or less. The method of detoxifying contaminated soil according to any one of claims 1 to 5, wherein the volume expansion coefficient of the non-magnetic substance to which the inorganic compound is added is 130% or less as determined by the following method.
<Measurement method>
Into a 200 mL beaker, 50 mL of the non-magnetic substance was compressed while being compressed, and the volume capacity (V ₁ ). Next, 50 mL of water was injected, and the mixture was allowed to stand for 24 hours, and its volume capacity (V _Two ) Is measured and calculated from the following formula.
Volume expansion rate (%) = (V _Two / V ₁ ) X 100