JP2010221105A - Method for purifying soil containing heavy metal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently purifying soil containing heavy metals. <P>SOLUTION: In the method for purifying soil containing heavy metals by extracting the heavy metals in a process of cleaning them with an aqueous solution of chemicals or water in a cleaning bath, the soil and the aqueous solution of chemicals or water are mixed and stirred in a mass ratio of 1:1.8-1:4, the mixture is left to stand for sedimentation of the soil, and wastewater is discharged from the bottom or the side bottom of the cleaning bath. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for efficiently purifying heavy metal-containing soil.

  It is known that heavy metals contained in foods affect the human body when ingested continuously, and there are increasing opportunities for heavy metal content in various foods to be taken up as a social problem. In recent years, the FAO / WHO Joint Food Standards Committee (Codex) has been developing international standards for cadmium content in foods. Cultivate these standards because the standards for agricultural products are even stricter than the existing domestic standards, and new standard values have been established for crops such as cereals and beans. Countermeasures against cadmium contamination in agricultural land will be important in the future.

Known methods for improving heavy metal-containing soils include the custom soil method, phytoremediation, and application of heavy metal absorption control materials. Particularly in heavy metal-containing paddy soil, a method for purifying contaminated soil in situ by using various chemicals and eluting and removing heavy metals from the soil has been studied. For example, as a chemical, a method of washing with one or more aqueous solutions selected from calcium salts, organic acids, inorganic acids, and aminocarboxylic acids (Patent Document 1), or hydrolysis under a soil pH (H ₂ O) or lower. A method of washing with a metal salt compound that coordinates a hydroxide ion to produce a metal hydroxide (Patent Document 2) has been proposed.

  However, field soils are generally coarser than paddy fields and do not have an impervious cultivator. Therefore, when washing is performed in situ, there is no penetration of washing water or water leakage to adjacent fields. There is a risk that heavy metals that are generated and eluted in the wash water will diffuse. For this reason, in the field cleaning, it is necessary to install a washing tank for washing the soil, but the amount of soil that can be processed per washing process depends greatly on the number and volume of washing tanks installed, If these are limited by conditions, the number of cleaning steps increases, so the construction period becomes longer and the cost increases significantly.

JP 2004-283743 A Japanese Patent Application Laid-Open No. 2005-163981

  Accordingly, an object of the present invention is to provide a method for efficiently purifying heavy metal-containing soil.

  As a result of various studies in view of such circumstances, the present inventors have mixed and stirred soil and washing water at a specific ratio in a washing tank, washed, and then drained from the lower part of the washing tank. It discovered that it could drain efficiently and can purify | clean a heavy metal containing soil efficiently, and completed this invention.

  That is, the present invention is a method for washing heavy metal-containing soil with a chemical aqueous solution or water in a washing tank and extracting and purifying heavy metal, wherein the soil and the chemical aqueous solution or water are 1: 1.8. The present invention provides a method for purifying heavy metal-containing soil, characterized by mixing and stirring at a mass ratio of ˜1: 4, allowing the soil to settle and allowing the soil to settle, and then draining from the bottom of the washing tank or the bottom side. .

  According to the present invention, heavy metal-containing soil can be efficiently purified. In particular, it is suitable for purification of soil having a clay content of 25% by mass or less, such as upland soil. Moreover, the washing water can be drained efficiently.

It is a figure which shows an example of the washing tank used by this invention.

  Examples of the heavy metal-containing soil to be purified in the present invention include soil containing a simple substance, compound or ion of heavy metal elements such as cadmium, lead and arsenic. In particular, it is suitable for purification of cadmium-containing soil.

The soil to be purified is preferably soil having a clay content of 25% by mass or less. Clay refers to inorganic particles having a particle size of less than 0.002 mm according to the International Soil Society method among soil components.
The soil is classified according to the International Soil Society Law. Sandy soil (S), loamy sand (LS), sandy loam (SL), loam (L), silty loam (SiL), sandy loam (SCL) ), Loam soil (CL), silty loam soil (SiCL), and sand and silt containing 75% by mass or more are preferable. Sand is an inorganic particle of soil constituents, and refers to coarse sand with a particle size of 2 to 0.2 mm and fine sand of 0.2 to 0.02 mm according to the International Soil Society Law. It has a particle diameter of ˜0.002 mm.
Such soil is suitable for the purification method of the present invention because the soil particles are classified by standing after washing, and a sufficient thickness is obtained for the obtained sand filtration layer. Such soil is mainly distributed in the field and can be purified as it is by the purification method of the present invention.

In the present invention, soil having a clay content exceeding 25% by mass can also be purified. The soil properties with clay content exceeding 25% by mass are classified according to the International Soil Society Law, in sandy clay (SC), light clay (LiC), silty clay (SiC), and heavy clay (HC). Yes, soil with sand and silt content of 75% by mass or less.
Such soil is mainly distributed in paddy fields, but soil particles are classified by standing after washing, and sufficient thickness cannot be obtained in the obtained sand filtration layer. Therefore, sand and silt content is 75 mass%. The purification method of the present invention can be applied by adding sand and silt so that the clay content is 25% by mass or less.
The sand and silt to be added may be any inorganic particles having a particle diameter of 0.002 to 2 mm, and use a crushed rock particle size adjusted material such as river sand, sea sand, volcanic sand, silica sand, etc. Can do.

First, the soil is cultivated several times by a tractor equipped with a rotary or the like with a soil thickness for purification purposes, usually about 20 to 30 cm in thickness. At this time, it is preferable to repeat the tilling several times at a low speed because the soil block can be crushed as much as possible.
Next, the crushed soil is scraped using a wheel loader, a bull tozer having a similar mechanism, a hydraulic excavator, or the like. The scraped soil is loaded into a dump truck with a wheel loader and transported to the vicinity of the cleaning facility. The soil can be transported by using a belt conveyor or the like in addition to the wheel loader.

  The transported soil is preferably classified to about 5 mm by a classifier, and the soil having a particle size of 5 mm or less is preferably washed. Those exceeding 5 mm are preferably further crushed, further classified and washed after removing foreign substances and the like. As the classifier, a trommel classifier, a vibration sieve, or the like can be used.

  The soil thus classified is put into a washing tank using a hydraulic excavator, a wheel loader, a belt conveyor or the like, and washed with a chemical aqueous solution and water.

The cleaning tank may have any shape such as a cylindrical shape or a rectangular shape, but the bottom portion is inclined at an angle of 20 to 60 degrees and preferably has a cone structure. Furthermore, a hole (aeration port) for sending compressed air for mixing the soil and the cleaning chemical solution and a hole for drainage (drainage port) are installed at the bottom. The size of these holes varies depending on the size of the washing tank, the particle size of the soil to be purified, etc., and is not particularly limited, but usually a diameter of about 2 to 5 cm is appropriate.
As such a cleaning tank, for example, the one shown in FIG. 1 can be used.

  Although it will not restrict | limit especially if it is used for washing | cleaning of heavy metal containing soil as a chemical | medical agent used by this invention, For example, a calcium salt, an organic acid, an inorganic acid, and aminocarboxylic acid are mentioned. Examples of calcium salts include calcium chloride, calcium nitrate, calcium acetate, and calcium iodide; examples of organic acids include citric acid, succinic acid, acetic acid, tartaric acid, lactic acid, butyric acid, malic acid, itaconic acid, and gluconic acid. And propionic acid; and inorganic acids include hydrochloric acid, nitric acid, sulfuric acid and the like.

  As aminocarboxylic acid, it forms a complex with cadmium. For example, amino acids such as alanine, glutamic acid, glycine, cysteine, ethylenediaminetetraacetic acid (EDTA), ethyleneglycolbis (2-aminoethylether) tetraacetic acid (EGTA). ), 1,2-diaminocyclohexanetetraacetic acid (DCTA), diethylenetriaminepentaacetic acid (DTPA), 2-hydroxyethyldiaminetriacetic acid (HEDTA), nitrilotriacetic acid (NTA), glutamic acid diacetic acid 4 soda, aspartic acid diacetic acid 4 Soda (ASDA), methylglycine diacetic acid 3 soda (MGDA), S, S-ethylenediamine succinic acid (EDDS4H), S, S-ethylenediamine disuccinic acid 3 soda (EDDS3Na) can be mentioned. Of these, biodegradable chelating agents such as glutamic acid diacetate 4 soda, aspartate diacetate 4 soda (ASDA), methylglycine diacetate 3 soda (MGDA), S, S-ethylenediamine succinic acid (EDDS4H), S , S-ethylenediamine disuccinic acid 3 soda (EDDS3Na) is preferred.

In addition, a metal salt compound that generates a metal hydroxide by coordinating a hydroxide ion by hydrolysis at a soil pH (H ₂ O) or lower can also be used as a drug. The pH of the soil to be purified is approximately pH 9 or lower, and at this pH or lower, hydroxide ions are coordinated to the metal salt to generate metal hydroxide.

  Examples of such metal salt compounds include iron salts such as ferrous chloride, ferric chloride, ferrous sulfate, ferric sulfate, ferrous nitrate, ferric nitrate, and polyiron sulfate; aluminum sulfate, chloride Aluminum salts such as aluminum and polyaluminum chloride; manganese salts such as manganese chloride, manganese nitrate and manganese sulfate; cobalt salts such as cobalt chloride, cobalt nitrate and cobalt sulfate; copper salts such as copper chloride, copper nitrate and copper sulfate Can be mentioned.

Of these drugs, calcium chloride, ferric chloride, and hydrochloric acid are particularly preferable.
In addition, the drug can be used alone or in combination of two or more, and the concentration of the drug aqueous solution is 5 mM to 1 M, particularly 10 mM to 0.1 M. This is preferable in terms of reducing the load of cleaning waste liquid treatment.

  For washing, mixing and stirring the soil and the aqueous chemical solution or water at a mass ratio of 1: 1.8 to 1: 4, particularly 1: 2 to 1: 3, increases the extraction efficiency of heavy metals, and washing. It is preferable because soil particles are classified in the tank and a sand filtration layer is easily formed, and drainage efficiency can be improved.

  In the present invention, washing with an aqueous solution of medicine means, in addition to mixing the soil and the aqueous solution directly, a method of adding and washing the medicine and water separately to the soil, and washing the soil containing water by mixing the medicine. A method is also included. What is necessary is just to use so that the density | concentration and usage-amount of aqueous solution may become in the said range.

The washing is preferably performed with stirring using a stirring device until the soil and the drug are sufficiently mixed. The extraction time is preferably secured for 1 hour or more. During this time, stirring may be continued, or after sufficient mixing, the mixture may be allowed to stand and further stirred after 1 hour. Stirring can be performed by using a stabilizer in which a rotor stirrer is attached to an attachment of a hydraulic excavator, and can also be performed by stirring with compressed air, stirring by compression circulation of slurry, or the like.
Compressed air is generated by a roots blower, a compressor, or the like, and can be supplied in an amount of the same amount to 5 mass times / hour per soil and chemical slurry volume. Moreover, stirring by the compression circulation of slurry can circulate the quantity of 0.5-2 mass times / hour per soil and chemical | medical solution slurry volume with a mono pump, a tube pump, a diaphragm pump, a submersible pump, etc. These stirring operations may be performed independently, but it is preferable to perform the stirring operations in combination, because high stirring efficiency is obtained.

After completion of the stirring, the mixture is allowed to stand and separated into solid and liquid by natural sedimentation in a washing tank. Due to the sedimentation effect of the washing slurry, it settles from the gravel, so that a sand filtration layer is formed and drained from a drain outlet installed at the bottom of the washing tank or the bottom side by the principle of sand filtration. The drainage may be either natural drainage due to gravity or forced drainage using a suction pump or the like. The size of the drain outlet is not particularly limited, but it is preferable that the opening at the bottom of the cleaning tank is covered with a mesh of 1 mm or less so that gravel does not pass through.
In addition, the supernatant water obtained by solid-liquid separation may be drained from the top by a drain pump or the like.

Since the eluted heavy metals and cleaning chemicals remain in the dehydrated soil, it is diluted and washed with water. Dilution washing with water is carried out in the same process as washing with chemicals, and is preferably repeated until the chlorine concentration in the free water of the soil becomes 700 ppm or less after washing, and is usually achieved sufficiently if carried out once.
In addition, water is gently introduced to the surface of dehydrated soil with a thickness of 10 to 20 cm, and the eluted heavy metals and cleaning chemicals remaining in the dehydrated soil can be replaced by suction dehydration from the lower drainage port without stirring. Is possible.

  On the other hand, the washed waste water discharged by either method is stored in a storage tank, and by carrying out alkali precipitation treatment with a waste water treatment device, heavy metals are precipitated as hydroxides, and solid-liquid separation is performed with a solid-liquid separator Thereafter, the supernatant is neutralized, and the suspended matter is removed with a back filter, and then the water is discharged. The sediment is dehydrated with a filter press and disposed of as industrial waste.

  The purified soil is discharged from the washing tank with a backhoe or the like, and the discharged soil is loaded into a dump truck with a backhoe or a wheel loader and returned to the original field. The soil can be transported using a wheel loader, a belt conveyor or the like.

  Since the pH of the soil returned to the field is acidic, it is preferable to perform neutralization using an alkaline agent. As the alkali agent, sodium hydroxide, potassium hydroxide, slaked lime, quicklime, calcium carbonate, calcium carbonate, etc. can be used, and calcium carbonate is particularly preferable. In the neutralization treatment, it is preferable to adjust the pH of the soil to 5 to 8, particularly about pH 5.5 to 6.5 because the growth of the plant becomes healthy and most heavy metals are insolubilized.

  Further, since the soil is made into a single grain by the purification treatment, it is preferable to add organic materials such as compost and supplement the trace elements as appropriate. Furthermore, it is preferable to perform mixed leveling by a tractor equipped with a working machine such as a rotary for homogenizing the soil classified by the washing treatment and mixing and leveling the applied materials.

  EXAMPLES Next, although an Example is given and this invention is demonstrated further in detail, this invention is not restrict | limited to these at all.

Example 1
A cadmium-containing air-dried soil (collected from Niigata Prefecture, gray lowland soil, soil properties: sandy loam soil (SL), clay content) 10.8% by weight) 666 g, 37% industrial ferric chloride 6.58 g and water 1664 g. Ferric chloride is equivalent to a 15 mM aqueous solution, and the mass ratio of soil: aqueous solution is 1: 2.5.
Air is sent to the aeration port with an air pump and agitated by aeration for 5 minutes. The height of the interface between the slurry slurry and the supernatant water (slurry height) was measured over time with the end of aeration as the sedimentation start time. In the case of lower drainage, drainage was started from the lower part 5 minutes after the start of sedimentation, and the lower drainage was continued until no water was completely discharged. Supernatural water is discharged after 2 hours from the start of sedimentation and after 1 hour from suction water. In addition, the suction drainage attached the hose to the drain outlet, sucked with the vacuum pump through the trap, and stored the drainage in the trap.
The same amount of water as the amount of supernatant water drained and the amount of lower drainage was added, and water washing was performed in the same manner.
The finally remaining soil was dried at 105 ° C. for 1 day, and the moisture content was measured.
Moreover, pH of the obtained waste_water | drain was measured using pH meter F-22 (made by HORIBA). Furthermore, the cadmium density | concentration in soil and waste_water | drain was measured using ICP emission spectrometer ULTIMA2 (made by HORIBA). The results are shown in Table 1.

Example 2
The same apparatus as in Example 1 was filled with 666 g of cadmium-containing air-dried soil (Fukuoka Prefecture collected soil, Gly lowland soil, soil property: light clay (LiC) clay content 30.7% by mass). Here, 220 g of No. 6 silica sand was added (the amount of soil was 886 g), and the clay content was added to 23 mass%. No. 6 silica sand is adjusted so that the particle size is distributed to 0.105 to 0.5 mm. To this, 1644 g of 30 mM industrial ferric chloride solution was added so that the amount of the soil: aqueous solution was 1: 2.5.
Air was sent to the aeration port with an air pump and agitated by aeration for 5 minutes. After leaving still for 1 hour, it stirred by aeration for further 5 minutes and left still. As a result, cadmium in the soil is washed and extracted into free water.
After standing for 2 hours, the supernatant water separated into solid and liquid was collected with a spoid. Furthermore, a hose was attached to the aeration port, and it was sucked with a vacuum pump through the trap, dehydrated until free water in the soil was not collected, and collected in the trap. The collected wastewater was weighed and the Cd concentration in the wastewater was measured with an ICP emission analyzer ULTIMA2 (manufactured by HORIBA).
The same amount of drained water was further added to the graduated cylinder test apparatus, and the soil was washed with water by the same treatment. As a result, cadmium in the free water remaining in the soil is diluted and washed. This water washing treatment was performed twice.
The amount of cadmium removed from the soil was determined from the amount of collected wastewater and Cd concentration, and the removal rate was calculated. The results are shown in Table 2.

Comparative Example 1
In Example 2, the same treatment was performed except that silica sand was not added and suction dehydration was not performed. The results are shown in Table 2.

Claims (6)

  A method of cleaning heavy metal-containing soil with a chemical aqueous solution or water in a washing tank and extracting and purifying heavy metal, wherein the mass of the soil and the chemical aqueous solution or water is 1: 1.8 to 1: 4. A method for purifying heavy metal-containing soil, comprising mixing and stirring at a ratio, allowing the soil to settle and allowing the soil to settle, and then draining from the bottom or bottom side of the washing tank.   The purification method according to claim 1, wherein the heavy metal-containing soil is cadmium-containing soil.   The purification method according to claim 1 or 2, wherein the aqueous chemical solution is calcium chloride, ferric chloride or hydrochloric acid aqueous solution.   The purification method according to any one of claims 1 to 3, wherein forced drainage is performed using a suction pump from the bottom or bottom side of the washing tank.   The purification method according to any one of claims 1 to 4, wherein after the soil is settled, the supernatant is drained from above.   The purification method according to any one of claims 1 to 5, wherein the heavy metal-containing soil is soil having a clay content of 25% by mass or less.

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