JP3490345B2 - How to clean the soil - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a technique for purifying contaminated soil and groundwater, and more particularly to a technique for purifying soil and groundwater contaminated with organic halogen compounds.

[0002]

2. Description of the Related Art Organic halogen compounds such as trichlorethylene, which have been used as degreasing solvents in semiconductor factories and metal processing factories in large quantities since the past, were discharged after use, or were discarded, accumulated in soil or groundwater. As a result, the reuse of the former factory site and the land development in the surrounding area are hindered, and the pollution of groundwater by the accumulated organic halogen compounds becomes a hindrance to the groundwater use, which is a big social problem.

The following are conventional methods for treating such contaminated soil and groundwater to decompose pollutants to render them harmless. In other words, by sucking the air in the soil to accelerate the vaporization of organic pollutants present in the soil and recovering and removing the pollutants as soil gas (soil gas suction method), pollutants dissolved in groundwater For collecting and removing pollutants by pumping water (pumping aeration method), decomposing and removing pollutants in soil by utilizing the ability of microorganisms to decompose pollutants (microbial decomposition method), and contamination by organic halogen compounds On the other hand, a method of decomposing and removing pollutants by using the dehalogenation and dehydrohalogenation action of metallic iron (iron powder method), a method of excavating and removing soil containing pollutants, and a method of purification by heat treatment (excavation and removal Law) etc.

However, the above method has the following problems. In the case of the soil gas suction method, the pollutant can be collected and removed only if it is contained in the unsaturated zone soil and can be vaporized in the soil air. Also, when targeting a wide range of soil, multiple suction wells and above-ground equipment are required,
Secondary waste is generated by recovery. In the pumped aeration method, only pollutants contained in saturated zone soil and soluble in water can be collected and removed. Also, as in the case of the soil gas suction method, when a wide range of soil is targeted, a plurality of suction wells and above-ground equipment are required, and secondary waste is generated by recovery. In addition, ground subsidence may cause subsidence. In the case of microbial decomposition method,
Although it may be advantageous in terms of purification cost, it requires a longer treatment time than other methods, and it is also susceptible to geological conditions, difficult to control, unsuitable for high concentration pollution, etc. There are many problems. In the case of the excavation and removal method, there are problems that the heat treatment of the excavated soil is expensive and that if pollutants remain around the excavated and removed area, the soil will be re-contaminated.

One of the methods that have recently been in the limelight for overcoming these problems is a method for treating an organic halogen compound using iron powder, that is, an iron powder method. This is a method of decomposing and purifying organic halogen compounds using the dehalogenation and dehydrohalogenation effects of metallic iron.
The following method is disclosed. That is, (A) a method of using iron powder for decomposing organic chlorine compounds (Japanese Patent Publication No. 02-
No. 49158, Japanese Patent Publication No. 02-49798, Japanese Patent No. 2636171), (B) A method of constructing a reaction wall made of iron powder and using this for purification of groundwater (Japanese Patent Publication No.
No. 501520, Japanese Patent Publication No. 06-506631), and (C) a method using the synergistic effect of a microorganism and metallic iron (Japanese Patent Laid-Open No. 10-216694, Japanese Patent Laid-Open No. 11-70).
No. 379) and (D) a method of using a metal body for the permeable purification wall (“46. Demonstration test of permeable purification wall at polluted site” 6th Lecture Meeting on Groundwater / Soil Pollution and its Prevention Measures, 1998/9 / 17-18).

[0006]

However, all of the above methods (A) are purification methods of contaminated water using metallic iron in water, and no specific method for adapting to soil is disclosed. The above method (B) is an in-situ purification method for groundwater flowing underground, but it is not a method for positively purifying the pollution source, and since the purification target is limited to groundwater, it is included in unsaturated soil. Pollutants are not subject to purification. In order to use the method (C), it is necessary to prepare the soil in an anaerobic atmosphere in order to grow anaerobic microorganisms. There is also a problem that the growth of microorganisms is easily affected by the soil environment and the growth of anaerobic microorganisms is difficult. Also, additives such as nutrient salts are required, which is more costly than the method using only iron powder. Further (D) above
Does not include a statement that exceeds the scope of the in-situ groundwater purification method disclosed by the method (B) above.

Therefore, the applicant provided in Japanese Patent Application No. 9-367177 an efficient method for detoxifying organic halogen using sponge-like iron powder. (EN) A purification method capable of preventing recontamination afterwards and decomposing even a small amount of pollutants remaining around the excavation. That is, the present invention, in the purification of soil and groundwater contaminated with organohalogen compounds, by combining the iron powder method and excavation removal method, effectively purify the area where the soil containing pollutants is present, It is an object of the present invention to provide a treatment method for easily and inexpensively purifying soil excavated and removed.

[0008]

[Means for Solving the Problems] The inventors of the present invention have conducted diligent research to solve the above problems. As a result, the soil contaminated with an organic halogen compound is excavated and iron powder is mixed, and the iron powder is mixed. When soil was backfilled underground or piled up above ground, and when restoration after excavation was performed, backfilling a mixture of non-polluted soil and iron powder. Dehalogenation reaction from pollutants and dehydrohalogenation It was found that a reaction occurred and the pollutants could be decomposed at room temperature, and the present invention could be submitted.

That is, according to the present invention, firstly, after excavating at least a part of an area where a soil containing a pollutant is present, a substance having an action of decomposing the pollutant is mixed with the excavated soil to be grounded. A soil purification method characterized by stacking in piles and leaving the piles to stand still for several days to several months; second, at least one of the areas where soil containing pollutants is present. After excavating the part, the substance that decomposes the pollutant is mixed with uncontaminated soil (non-contaminated soil) and backfilled in the excavation mark to prevent the excavation range from being contaminated again. A characteristic soil purification method; Thirdly, the main component of the substance having an action of decomposing the pollutant is metallic iron.
Fourthly, the present invention provides the soil purification method described in the third aspect, characterized in that the metallic iron is spongy iron powder.

[0010]

The method of the present invention is intended for the purification of contaminated soil and groundwater, in particular dichloromethane, carbon tetrachloride,
1,2-dichloroethane, 1,1-dichloroethylene,
Volatile organochlorine compounds such as cis-1,2-dichloroethylene, 1,1,1-trichloroethane, 1,1,2-trichloroethane, trichloroethylene and tetrachloroethylene, and organochlorine compounds such as PCB and dioxins, and other organic compounds It is aimed at the purification of soil and groundwater contaminated by halogen compounds, and is characterized by decomposing these organic halogen compounds by dehalogenation or dehydrohalogenation action to render them harmless. Iron powder is used as a substance that decomposes pollutants. The chemical reaction between the iron powder and the organic halogen-based compound can be represented by Me + H ₂ O + RX → RH + Me ⁺⁺ + OH ⁻ + X ⁻ . Here, Me is a single metal such as iron powder,
RX represents an organic halogen-based compound, R represents a hydrocarbon, and X represents a halogen element. When the organic halogen-based compound has a plurality of halogen elements, such a reaction is repeated a plurality of times and is finally decomposed into harmless hydrocarbons containing no halogen such as ethane and ethylene. Also, as in the above reaction formula,
Water is involved in the decomposition of organic halogen compounds by iron powder. Therefore, it is desirable that the soil to be purified has a water content of 5% by weight or more.

As the iron powder, an iron powder containing 0.1% by weight or more of carbon is used. When the carbon content of the iron powder is lower than 0.1% by weight, the decomposition rate of pollutants becomes slow, which is not practical. Also,
The particle size of the iron powder is adjusted so that 50% by weight of the whole of the iron powder passes through a sieve having openings of 150 μm. If iron powder having a coarser particle size is used, the decomposition rate of pollutants decreases, the utilization efficiency of iron powder decreases, and a larger amount of iron powder is added, which is economically disadvantageous.

The amount of iron powder added to the soil is in the range of 0.1 to 10% by weight based on the wet weight of the soil. Addition amount is 0.
When it is less than 1% by weight, the decomposition rate is remarkably reduced. A large amount of addition of 10% by weight or more is economically disadvantageous.

The shape of the iron powder added to the soil is SE shown in FIG.
As shown in the M photograph (a is × 100, b is × 700), the surface is spongy (sponge-like) with many protrusions and holes, and its specific surface area is 500 cm ² / g or more, so the reactivity is high. Remarkably high. Further, those having a low degree of crystal growth are desirable, and those having a pearlite structure as a crystal structure are desirable.

No new oxygen source should be supplied to the soil after the iron powder has been mixed, in order to help the iron powder to decompose pollutants. In the post-excavation process, it is desirable to prevent the soil surface from coming into direct contact with outside air.

By such an operation, environmental standards concerning soil pollution within 2 to 3 months (August 23, 1991 notification, amendment, Hei 5 Announcement 19, Hei 6 Announcement 5, Announcement 25, 1995) Annual Announcement 1
9) The soil can be detoxified to the following levels.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments and examples of the present invention will be described below. In addition, these embodiments and examples do not limit the scope of rights of the present invention. Excavation of contaminated soil is performed as follows. First, surround the area where soil is confirmed to be contaminated with water-impermeable walls such as steel sheet piles to the depth of the impermeable layer to prevent groundwater from flowing in from the surrounding area and excavate contaminated soil. A backhoe or the like is used for excavation.
At the time of excavation, it is desirable to classify the soil according to the pollution concentration based on the preliminary survey data and the measurement results during the excavation. Store the excavated soil in a concrete pavement yard to prevent the spread of pollution.

To add the iron powder to the excavated soil, a device for mixing the soil and the chemical, such as a soil improvement device, can be used. Also, a heavy machine such as a backhoe can be used. When mixing, contaminants may volatilize from the soil, so it is desirable to take measures such as installing an activated carbon adsorption device.

The soil excavated and mixed with iron powder is left standing on the ground or backfilled for a period required for decomposition of pollutants. When standing on the ground, stack them in piles and leave them for several days to several months. Since the reaction between the iron powder and the organohalogen compound proceeds in a reducing atmosphere, it is desirable to roll the pile to reduce the air in the soil as much as possible and then to cover the air with a sheet or the like. It is also possible to dismantle the pile and reuse the soil once the cleaning is complete.

When the soil mixed with the iron powder is backfilled underground by excavation, it may be backfilled in the excavation trace or buried underground at another place. In the case of backfilling in the excavation trace, there are cases in which the impermeable wall that surrounds the excavation area is removed after the backfilling, and cases in which it is not removed as it is. When steel sheet piles are used for impermeable walls, they are often removed for economic reasons.

If the impermeable wall is removed after backfilling the excavation trace, pollutants remaining around the excavation area will flow into the excavation area in the form of groundwater or soil interstitial gas, and soil pollution will occur again. There is. This is due to the excavation removal method
This is because it is difficult to completely remove the pollutants in the area.

In order to prevent the recurrence of such pollution,
When backfilling excavation traces, iron powder is mixed even if the soil used for backfill is uncontaminated soil. In this case, the iron powder mixing method does not necessarily have to be homogeneous mixing, and the iron powder can be sandwiched in a layered manner as a simpler method. The reason for this is that the soil to be backfilled does not form a layered structure with a different permeability like the natural strata, so the inflowing pollutants are easy to move in the vertical direction, and it is high in the iron powder sandwiched in layers. This is because it is considered that there is a probability of contact. The iron powder layer is pinched every 0.2 to 2 m of soil,
The amount of iron powder should be in the range of 0.1 to 10% by weight of the soil on average. The amount of iron powder added may be approximately the same as in homogeneous mixing. If the spacing between the iron powder layers is smaller than 0.2, it is equivalent to homogeneous mixing in construction, and if it is larger than 2 m, the probability of contact between the contaminant and the iron powder is significantly reduced. When the backfilled soil is below the water table and groundwater flows constantly, the vertical movement of pollutants decreases, so
It is desirable to reduce the gap between the iron powder layers or to perform homogeneous mixing.

An example in which non-contaminated soil mixed with iron powder was backfilled in the excavation trace to prevent recontamination by contaminants remaining around the excavation trace, and the contaminated soil mixed with iron powder was piled up. An example in which they are piled up and cleaned up will be described in detail below with reference to the drawings.

[0023]

[Embodiment 1] An embodiment of a purifying method by backfilling will be described with reference to FIG. FIG. 1 is a peripheral view (plan view) of the excavation and removal site. 1,1,1 in the place as shown in FIG.
-Some soils were contaminated with trichloroethane and 1,1-dichloroethylene above environmental standards. According to the preliminary survey, it was predicted that the horizontal distribution would have a horizontal distribution as shown in Fig. 1, and the vertical distribution would be continuously distributed from just below the surface to 5.5 m below the surface where the impervious layer exists. . The water table was about 3 m below the surface. As a measure against pollution, the soil in the contaminated area has a maximum depth of 5.5 m, and the maximum depth is about 200.
Excavated and removed 0 tons. This drilling removed most of the pollutants.

As the backfill soil, uncontaminated mountain sand was used, and iron powder was mixed in a layered form. Iron powder has a depth of about 0.
Spraying was performed using a heavy machine one by one every 5 m. The amount of iron powder mixed is about 1% by weight of the soil as an average value, that is, 2
It was 0 ton. As the iron powder, "E-200" manufactured by Dowa Iron Powder Co., Ltd., which is a spongy iron powder, was used.

After the above construction was completed, a monitoring well was set up within the excavation area, and groundwater was collected and analyzed. It was possible to confirm the effect that the pollutants that remained slightly from the outside of the excavation area flowed in, and the pollutants were decomposed by the iron powder. The results obtained are shown in Table 1.

[0026]

[Table 1]

The elapsed days indicate the number of days after the steel sheet pile surrounding the excavation area has been pulled out. With the passage of days,
1,1,1-Trichloroethane is gradually reduced, and 1,1-dichloroethane which is a decomposition product is detected. However, 1,1-dichloroethane is also gradually reduced. Also,
The amount of 1,1-dichloroethylene increased initially and exceeded the environmental standard value, but then turned to decrease. Other intermediate products and final decomposition products such as ethane and ethylene were not detected in groundwater, which is considered to be due to their extremely low solubility. As described above, it was possible to prevent recurrence of pollution due to pollutants flowing in from the periphery of the excavation area by mixing the iron powder in the backfilled soil in layers.

[0028]

[Embodiment 2] An embodiment of a cleaning method using an iron powder mixing pile will be described with reference to FIGS. Construction of piles (koyama) as shown in Fig. 2 was carried out on 53 tons of soil (loamy soil: water content 45% per wet soil) contaminated with 1,1,1-trichloroethane and 1,1-dichloroethylene. did. Half of the constructed piles were iron powder mixed piles, the other half were reference piles, and a partition was provided between the two. The pile was covered with a sheet to reduce contact with the air. The pollution status of the target soil is as follows, and it has been confirmed that the soil is almost uniformly contaminated. Contaminant concentration (dissolution test value according to Environment Agency Notification No. 46) 1,1,1-trichloroethane: 1.67 mg / L 1,1-dichloroethylene: 0.072 mg / L

Table 2 shows the construction conditions of each pile.

[Table 2]

The evaluation of each pile was performed by the following procedure. Boring was performed from the upper surface of the pile to a depth of 60 cm at the sampling position shown in FIG. 3 according to the elapsed days, and a soil sample was collected. A soil elution test was carried out for each soil sample in accordance with the Environmental Agency Notification No. 46. GC-MS (H
P-5973) was used to quantitatively analyze the pollutant concentration in the obtained test solution, and the change with time of the pollutant concentration in the soil was evaluated. The changes over time in the 1,1,1-trichloroethane concentration and the 1,1-dichloroethylene concentration are shown in Table 3 and Table 4, respectively.

[0031]

[Table 3]

[0032]

[Table 4]

From Tables 3 and 4, the 1,1,1-trichloroethane concentration and 1,1-
It was confirmed that the dichloroethylene concentration was significantly decreased in the iron powder mixed pile. In addition, when soil void gas was analyzed, in the iron powder mixed pile, 1,1,
Ethylene and ethane gas, which are decomposition products of 1-trichloroethane and 1,1-dichloroethylene, were detected, but intermediate products such as vinyl chloride and monochloroethane were not detected. On the other hand, in the reference pile without addition of iron powder, although there were variations due to sampling, no reduction in pollutants was observed. From these, it was confirmed that the pollutants were sufficiently decomposed and purified by the iron powder. Environmental standard values related to soil pollution are 1,1,1
Considering that trichloroethane is 1 mg / L and 1,1-dichloroethylene is 0.02 mg / L, it means that the target soil of this example was completely cleaned in just 5 days by the construction of the iron powder mixed pile. Become.

[0034]

As described above, according to the method of the present invention,
Regarding soil and groundwater contaminated with organohalogen compounds, it is possible to effectively purify the area where soil containing pollutants is present to prevent recurrence of pollution, and excavate and remove the soil to easily and inexpensively purify it. It was also confirmed that the pollutants were eventually converted to halogen-free hydrocarbons such as ethane and ethylene.

[Brief description of drawings]

FIG. 1 is a plan view showing the periphery of a site for excavation and removal according to a first embodiment.

FIG. 2 is a perspective view showing an outline of an iron powder pile according to a second embodiment.

FIG. 3 is a plan view showing sampling positions from an iron powder pile according to a second embodiment.

FIG. 4 is an X-ray photograph showing the particle shape of spongy iron powder that can be conveniently used in the practice of the present invention, in which a is taken at × 100 and b is taken at × 700.

Continuation of the front page (56) Reference JP-A-10-113679 (JP, A) JP-A-10-244248 (JP, A) JP-A-10-263522 (JP, A) (58) Fields investigated (Int .Cl. ⁷ , DB name) B09C 1/00-1/10 A62D 3/00

Claims (4)

(57) [Claims] 1. After excavating at least a part of an area where soil containing pollutants is present, a substance having an action of decomposing the pollutants is mixed with the excavated soil and piled up on the ground in a pile shape. A method for soil purification, which comprises leaving the pile (pile) still for several days to several months. 2. After excavating at least a part of an area where soil containing pollutants is present, a substance having an action of decomposing the pollutants is mixed with uncontaminated soil (non-polluted soil) to form excavation traces. A method for remediating soil, which comprises backfilling to prevent re-contamination of an excavated area. 3. Also claim 1 main component of a substance of action decomposing said contaminants comprises a metal iron
Is the soil purification method described in 2 . 4. The soil purification method according to claim 3, wherein the metallic iron is spongy iron powder.