JP4069019B2 - Soil restoration method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for detoxifying harmful substances in soil by mixing particles containing iron into the soil.
For example, in order to repair soil contaminated with chlorinated organic substances such as trichlorethylene, dichloroethylene, dioxins and hexavalent chromium, contaminated soil remediation agents composed of fine iron particles are used to remove soil contaminants. It relates to a detoxifying method.
[0002]
[Prior art]
In the past, organic halogen compounds such as trichlorethylene have been used for cleaning machinery and semiconductors. Since these organic halogen compounds are carcinogenic, they are recognized as harmful substances. In recent years, their use has been suppressed and emission management has become strict. However, contaminated soil due to past use of organohalogen compounds remains, which is an environmental problem. Moreover, it may be contaminated with hexavalent chromium due to the drainage of the plating factory.
[0003]
Known methods for purifying soil contaminated with volatile organic compounds such as organic halides include a soil gas suction method, a groundwater pumping method, and a soil excavation method. The soil gas suction method forcibly sucks the target substances present in the unsaturated zone. A suction well is installed in the ground by boring, and the inside of the suction well is depressurized by a vacuum pump, and the vaporized organic The compounds are collected in a suction well, guided to the underground, and treated by a method such as adsorption of organic compounds in soil gas onto activated carbon. When the contamination by the organic compound extends to the aquifer, a method is adopted in which a submersible pump is installed in the suction well and the water is pumped up simultaneously with the soil gas. The underground pumping method is a method of setting up a pumping well in soil and pumping up contaminated groundwater. Furthermore, the soil excavation method is a method in which contaminated soil is excavated, and the excavated soil is subjected to wind drying and heat treatment to remove and collect organic compounds.
[0004]
As a method of purifying contaminated water such as collected water or ground water as described above, for example, in Japanese Patent No. 2636171, after removing dissolved oxygen in contaminated water, the contaminated water is applied to a metal surface such as iron. A method for reducing and removing organic halide contained in contaminated water by contacting is disclosed. Such a method for purifying contaminated water using the iron reducing action is also described in JP-A-3-106496, JP-A-8-257570, JP-A-10-263522, and the like. In any of these methods, the contaminated water is treated by passing it through a layer containing iron or a filter.
[0005]
Further, a method of using micron-sized fine iron-containing particles as a soil repairing agent exhibiting a strong reducing action has been invented. In particular, Japanese Patent Application Laid-Open No. 2001-198567 slurries fine iron powder, Techniques for injecting into are shown.
[0006]
[Patent Document 1]
Japanese Patent No. 2636171 [Patent Document 2]
JP-A-3-106496 [Patent Document 3]
JP-A-8-257570 [Patent Document 4]
Japanese Patent Laid-Open No. 10-263522 [Patent Document 5]
Japanese Patent Laid-Open No. 2001-198567
[Problems to be solved by the invention]
Among the methods using iron-containing particles as a soil restoration agent, the method described in JP-A-2001-198567 uses extremely fine iron particles, and therefore has a very high detoxification reaction rate. However, since particles having a large specific surface area of about 0.5 to 5 microns are used, there is a problem that the oxidation rate of iron-containing particles in the soil is high and the reaction activity rapidly decreases. In the experiments by the present inventors, the reaction activity of such fine iron-containing particles was ½ or less in several days to several weeks. Therefore, when the concentration of pollutants such as trichlorethylene in groundwater is as high as about 1000 mg / liter or more, there is a problem that the reaction activity is lowered before all the pollutants are rendered harmless. Therefore, this technique could not be applied when the removal of contaminants was incomplete and the groundwater contained such high concentrations of contaminants.
[0008]
On the other hand, in the method of treating contaminated water described in JP-A-10-263522 by passing it through a layer containing iron in the soil, a filter or the like, the reaction period takes from several months to one year. Iron-containing particles were used whose reaction activity did not decrease over time. Such particles have a relatively large size of 20 to 100 microns, and therefore have a small specific surface area and a relatively low reaction activity. As a result, when the concentration of contaminants such as trichlorethylene in the groundwater is high, there is a problem that removal of these contaminants is not completed within the time when the groundwater flows through the layer containing iron in the soil. As a result, in order to remove the pollutants in the groundwater, a plurality of treatments are required, and there has been a problem that the cost is greatly increased.
[0009]
Thus, in the case of purifying soil containing a high concentration of pollutants, it is necessary to increase the reaction activity and to maintain the reaction activity for a long time. It was difficult to achieve at the same time. Accordingly, there has been a demand for a new technique for continuing a soil remediation reaction over a long period of one month to several months using a soil repair agent having a high reaction activity.
[0010]
[Means for Solving the Problems]
The present invention has been made to solve the problems of the prior art as described above, and the gist of the present invention is the following (1) to (3) as described in the claims. It is the contents to show.
(1) A water-permeable layer with high water permeability is constructed in the soil, the water-permeable layer has a specific surface area of 0.1 to 4 square meters per gram, and is composed of metallic iron, wustite, and magnetite. a particle, the metallic iron and ferrous ions by repeated injections what comprising 60 wt% or more, to form a hazardous substance removing layer, when passing the ground water containing harmful substances in the hazardous substance removing layer The product ( A * T ) of the period ( T : day) and the specific surface area ( A : square meter / gram) in which iron-containing particles are repeatedly mixed into the soil is set to an appropriate value in the range of 20-60. A soil remediation method characterized.
(2) A water-permeable layer with high water permeability is constructed in the soil, the water-permeable layer has a specific surface area of 0.1 to 4 square meters per gram, and is composed of metallic iron, wustite, and magnetite. When particles containing 25 to 60 % by mass of metallic iron and ferrous ions are repeatedly injected to form a harmful substance removal layer and groundwater containing the harmful substance is passed through the harmful substance removal layer In addition, the product ( A * T ) of the period ( T : day) and the specific surface area ( A : square meter / gram) of mixing iron-containing particles repeatedly into the soil should be an appropriate value in the range of 10-40. A soil remediation method characterized by
(3) When the converter gas generated from the steelmaking converter is processed by the non-combustion converter gas recovery device, particles containing a large amount of iron obtained by collecting with a wet dust collector are used ( The soil repair method as described in 1) or (2) .
[0011]
DETAILED DESCRIPTION OF THE INVENTION
In this invention, the particle | grains containing metal iron and ferrous ion with high reaction activity are used as a soil purifier. Here, the particles having a high reaction activity are particles having a large specific surface area and a high ratio of metallic iron to ferrous ions. The conditions of the soil repairing agent of the present invention are particles having a specific surface area of 0.1 to 4 square meters per gram and containing 25% by mass or more of metallic iron and ferrous ions. The particle diameter of the particles having the specific surface area is about 0.2 to 5 microns in the case of a general shape close to a sphere. In addition, when using particles with many pores, the particle diameter is about 0.8 to 20 microns. The specific surface area is measured by the BET method or the like, and in the case of a shape close to a sphere, it is within the range of the particle diameter.
[0012]
The mechanism by which iron-containing particles react with soil pollutants is that divalent iron ions (ferrous ions) in which the metal iron in the particles is in the form of oxides or hydroxides, and divalent iron ions This is due to the reduction reaction when changing to trivalent iron ions (ferric ion). In our study, if the total amount of metallic iron and divalent iron ions is 25% by mass or more, even if a substance such as trichlorethylene has a high concentration of 100 mg / liter or more, it can be removed to a sufficiently low concentration. It was elucidated to have a reaction capacity of Moreover, the study of the present inventors, in particular, metallic iron reaction in the changes to the ferrous ions were found to be effective in soil purification reaction. Therefore, in particular, in order to fully exhibit the effects of the present invention, the ratio of metallic iron is more preferably 25% by mass or more. In such particles with a large specific surface area, it is important that the wustite or magnetite layer covering the metallic iron or ferrous ion that lowers the reaction activity is thin. It is also important that the initial wustite or magnetite layer does not exceed about 0.1 to 0.3 microns when mixed into the soil. In addition, since both wustite and magnetite contain ferric ions, the effect of the present invention is significant when the ratio of initial ferric ions when mixed into soil is 25% by mass or less.
[0013]
In the present invention, the purification rate of soil pollutants is increased by increasing the specific surface area of particles having high reaction activity. The present inventors have clarified that the specific surface area capable of exhibiting sufficient reaction activity is 0.1 square meter or more per gram of iron-containing particles. However, if the specific surface area is 4 square meters / gram or more, there is a problem that the oxidation of the particles proceeds during the transportation and construction, and the reaction activity decreases.
[0014]
In the present invention, first, the iron-containing particles are mixed into the soil as the first step. As a method of mixing, there are a method of digging up soil and mixing it with iron-containing particles, and then backfilling, a method of mixing iron-containing particles and water to form a slurry, and injecting this into the soil. Moreover, the method of removing a pollutant by mixing iron-containing particle | grains and contaminated soil substantially uniformly using said method is common. On the other hand, iron-containing particles and particles with high water permeability (such as sand) are mixed to form a pile in the soil, thereby forming a water-permeable layer through which contaminated water easily passes. By passing the contaminated water through the water permeable layer, the contaminant is removed.
[0015]
Subsequently, after a predetermined period has elapsed, the iron-containing particles are reinjected into the soil or the water permeable layer. This is because the iron-containing particles used in the method of the present invention have a high reaction activity but a short period during which the reaction activity can be maintained. The period of reinjection is determined by the specific surface area and chemical composition of the iron-containing particles. The present inventors have clarified that the specific surface area and the period of maintaining the reaction activity are almost inversely proportional. In the experiments by the present inventors, the reaction activity of iron-containing particles having a particle diameter of about 1 micron, a specific surface area of about 0.8 square meter / gram, and metal iron and ferrous ions of 38% by mass was investigated. However, after 15 days, the reaction rate had decreased to 20% of the initial value. Further, in the case of iron-containing particles having almost the same chemical composition and a specific surface area of 1.4 square meters / gram, 20% of the initial reaction activity was reduced to 9 days, but 9 days. It was also clarified that the coefficient of inverse proportion is almost determined by the chemical composition of the iron-containing particles. When iron-containing particles contain a large amount of metallic iron and ferrous ions (total of 60% by mass or more), the product (A * T) of the specific surface area (A: square meter / gram) and the period until re-injection (T: days) Is 20 to 60, the reaction activity can be continuously maintained. Moreover, when there are few metal iron and ferrous ion of iron containing particle | grains (25-60 mass% in total), the product of a specific surface area and the period until reinjection, A * T should just be 10-40.
[0016]
The reinjection method of the present invention has several construction methods, but it is best to drive the injection pipe into the soil or water permeable layer and press-fit the iron-containing particles in a slurry state. A construction example of the present invention is shown using the apparatus of FIG. This apparatus includes a slurry tank 1, a slurry pump 2, a slurry pipe 3, and an injection pipe 4. First, a mixed slurry of iron-containing particles and water is produced in the slurry tank 1. The iron-containing particle concentration of the slurry is preferably about 10 to 100 g / liter. This slurry is sent to the injection pipe 4 via the slurry pipe 3 by the power of the slurry pump 2. The injection tube 4 has a structure having a plurality of holes on the side surface of the pipe, and is inserted into the soil. From the hole on the side of the injection tube 4, the slurry is dispersed in the soil. When dealing with a wide area, a plurality of injection tubes 4 are used. The interval between the injection tubes 4 at this time is about 2 to 10 m. Moreover, when inject | pouring a slurry into a water-permeable layer, it is good to install the injection pipe 4 in the center of each sandpile.
[0017]
If the conditions of chemical composition and specific surface area are satisfied, the iron-containing particles used in the method of the present invention may be produced by any method, but it is economical to use the following production method. First, the method of hydrogen reduction of fine iron oxide particles produced by the atomization method, the method of producing fine iron-containing particles by blowing oxygen to molten iron, evaporating the iron, and cooling it, etc. The iron-containing particles used are produced. In addition, when the converter gas generated from a steelmaking converter is processed by a non-combustion converter gas recovery device, the method using the iron-rich particles recovered by a wet dust collector is the most economical iron-containing method. A method for producing particles. The iron-containing particles produced by this method have a particle diameter of about 1 micron and the total mass of metallic iron and ferrous ions is 50 to 65%, and are iron-containing particles suitable for the present invention.
[0018]
【Example】
The method of the present invention was used to repair soil contaminated with trichlorethylene.
Example 1
Treatment was performed by injecting iron-containing particles into soil containing 160 mg / liter of trichlorethylene in groundwater. In the apparatus of FIG. 1, a slurry containing 30 g / liter of iron-containing particles was poured into the soil. The injection ratio of iron-containing particles in the earth and sand was 6 kg / cubic meter. The specific surface area of the iron-containing particles was 1.4 square meters / gram, and the ratio of metallic iron to ferrous ions was 37% by mass.
[0019]
As a result, after 10 days, the trichlorethylene concentration in the groundwater decreased to 22 mg / liter. However, the reaction rate decreased after this. Therefore, after 12 days, iron-containing particles were reinjected at a rate of 3 kg / cubic meter. As a result, the reaction activity increased, and the concentration of trichlorethylene in groundwater decreased to 4 mg / liter 21 days after the first construction. Here, iron-containing particles were once again injected. The injection rate was 3 kg / cubic meter. As a result, the trichlorethylene concentration finally decreased to 0.5 mg / liter.
[0020]
(Example 2)
A sandpile with high water permeability was applied around the soil containing 75 mg / liter of trichlorethylene in the groundwater to contain iron-containing particles. The groundwater was pumped outside the sandpile, and the contaminated groundwater passed through the permeable layer composed of sandpile to remove trichlorethylene. Again, a slurry containing 55 g / liter of iron-containing particles was poured into the sandpile with the apparatus of FIG. The iron-containing particles had a specific surface area of 0.8 square meters / gram and a ratio of metallic iron to ferrous ion of 45% by mass. The ratio of iron-containing particles in the sand pile was 22 kg / cubic meter.
[0021]
In this treatment, on the first day of treatment, the concentration of trichlorethylene in the groundwater that passed through the permeable layer decreased to 8 mg / liter. That is, the trichlorethylene removal rate was about 85%. After 5 days, the removal rate became 75%, and after 15 days, it decreased to 55%. At this point, the iron-containing particles were reinjected at a rate of 7 kg / cubic meter. As a result, the trichlorethylene removal rate returned to about 80%. Thereafter, by repeating reinjection every 15 days, the trichlorethylene removal rate of groundwater passing through the permeable layer could be maintained at 60 to 80%. This treatment was repeated for 6 months. The groundwater after the treatment was returned to the contaminated area and recirculated to finally reduce the concentration to 1 mg / liter or less.
[0022]
[Effect of the present invention]
ADVANTAGE OF THE INVENTION According to this invention, the soil repair agent which consists of iron particles with high reaction activity can be made to react over a long period of time. As a result, there are significant industrially useful effects such as efficient reduction of hexavalent chromium and decomposition of organic halogen compounds.
[Brief description of the drawings]
FIG. 1 is a view showing an apparatus for injecting a slurry of iron-containing particles into soil, which is used in the present invention.
[Explanation of symbols]
1 ... slurry tank,
2 ... slurry pump,
3 ... slurry pipe,
4 ... Injection tube

Claims (3)

A water-permeable layer with high water permeability is constructed in the soil, the specific surface area of the water-permeable layer is 0.1 to 4 square meters per gram, and the particles are composed of metallic iron, wustite, and magnetite. Then, by repeatedly injecting metal iron and ferrous ion containing 60 % by mass or more to form a harmful substance removal layer, when passing groundwater containing harmful substances through the harmful substance removal layer, iron is added. The product ( A * T ) of the period ( T : day) and the specific surface area ( A : square meter / gram) in which the contained particles are repeatedly mixed in the soil is set to an appropriate value in the range of 20-60. Soil restoration method. A water-permeable layer with high water permeability is constructed in the soil, the specific surface area of the water-permeable layer is 0.1 to 4 square meters per gram, and the particles are composed of metallic iron, wustite, and magnetite. Then, by repeatedly injecting particles containing 25-60 mass% of metallic iron and ferrous ions to form a harmful substance removal layer, and passing groundwater containing harmful substances through the harmful substance removal layer, It is characterized in that the product ( A * T ) of the period ( T : days) and the specific surface area ( A : square meter / gram) in which the particles containing the particles are repeatedly mixed into the soil is an appropriate value in the range of 10-40. How to repair soil. When processing converter gas generated from the steelmaking converter in a non-combustion type converter gas recovery system, according to claim 1 or, characterized by using particles containing a large amount of iron obtained by recovering a wet dust collector The soil restoration method according to claim 2 .

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