JP4509708B2 - Purification method for heavy metal contaminated soil - Google Patents

Description

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

Measures for heavy metal-containing agricultural land include the land-and-soil method, the soil-discharging land-and-soil method, the alkaline material application method, the cultivation method under reducing conditions, and the method of washing with various chemicals (for example, Patent Document 1). Are known.
In particular, in a cadmium-containing paddy field soil, even if the cadmium concentration in the soil is low (for example, about 2 ppm), there is a risk that the cadmium concentration in brown rice will be 1 ppm or more (distribution prohibited by the Food Sanitation Law). These paddy fields that produce brown rice with a cadmium content of 1 ppm or more have been subsidized by special land improvement projects such as special land improvement projects under the assistance of the Ministry of Agriculture, Forestry and Fisheries.

  On the other hand, CODEX (Food Standard Committee of WHO and FAO) is considering a cadmium standard value for food, and 0.4 mg / kg or 0.2 mg / kg is proposed for rice. When 2 mg / kg is adopted, it is assumed that about 10 times as much cadmium-contaminated agricultural land will appear. If this huge amount of area cleanup measures are carried out with conventional soil removal and soil removal, it will be necessary to prepare a large amount of soil removal and soil suitable for paddy soil. It may take nearly 10 years to recover productivity, and even if measures are taken, pollution will recur after 20 to 30 years. Rather, there is a need for more efficient methods for soil purification.

Further, as a method for washing with a chemical, for example, Patent Document 2 discloses that contaminated soil is mixed with calcium or iron sulfate, chloride or a mixture thereof, and the soil is present in the presence of moisture. A method is described in which the heavy metal contained therein is made water-soluble and removed by elution. Furthermore, Non-Patent Document 1 describes that when cadmium-containing paddy soil is treated with calcium chloride, acetic acid, etc., the cadmium concentration in the soil decreases, and Non-Patent Document 2 similarly uses EDTA. An example is given.
However, these documents do not show how cadmium can be efficiently removed and cleaned in an actual cadmium-containing paddy soil. In particular, depending on the environment of the soil to be purified, an expected purification effect may not be obtained, and it is difficult to efficiently purify heavy metal contaminated soil.
JP 2002-355562 A Japanese Patent Laid-Open No. 54-13466 Fumio Ogawa, “Study on the Actual Condition of Cadmium Contamination of Rice in Akita Prefecture and Mitigation of the Damage”, Akita Prefectural Agricultural Experiment Station Research Report, 1994, No. 35, p31-38 Seijiro Nakajima, Sueta Ono, “Survey on Heavy Metal Contamination in Tsushima”, Nagasaki Prefectural Agricultural and Forestry Laboratory, 1979, No. 7, p359-364

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

  As a result of various investigations in view of such circumstances, the present inventors have efficiently purified the soil by extracting and washing heavy metals in the soil using chemicals after the contaminated soil is oxidized or dried. The present invention has been completed by finding out what can be done.

That is, the present invention provides a method for purifying heavy metal-contaminated soil, characterized in that the heavy metal-contaminated soil is made into an oxidized state and / or dry state and then washed with ferric chloride .

  According to the present invention, heavy metal-contaminated soil can be efficiently purified.

The heavy metal-contaminated soil to be purified in the present invention is an urban area, a forest, a factory site, an agricultural land, a swamp, or even a soil, and a soil containing a simple substance, compound or ion of lead, cadmium, arsenic, etc. Is mentioned. For example, soil with heavy metal elution measured by the method stipulated in Notification No. 46 of the Environment Agency in 1991, soil containing lead that contains 400 mg or more of lead per kg of soil, 1 kg of soil The present invention can be suitably applied to soil such as cadmium-containing soil containing 2 mg or more of cadmium by weight per cadmium and arsenic-containing soil containing arsenic of 30 mg or more per kg of soil.
In particular, it is suitable for purification of cadmium-containing paddy soil and paddy soil having a cadmium concentration of 0.1 to 5 ppm.

  In the present invention, first, such soil to be purified is brought into an oxidized state and / or dried. Examples of the oxidation or drying method include a method in which soil and air are brought into contact with a tiller at the current position to oxidize, a method in which soil is excavated and removed, and a soil and air is brought into contact with a stirrer to oxidize. .

  As the oxidation state of the soil, the redox potential (Eh) is preferably 100 mV or more, particularly 200 mV or more. The oxidation-reduction potential of the soil was determined by Mitsuko Komada, “Oxidation-reduction potential (Eh) platinum electrode method” Hirotosha, p. It can be measured by the method described in 197-199. That is, the oxidation-reduction potential is, for example, when measured outdoors, after inserting a platinum electrode, which is a non-reactive electrode, in the soil layer of paddy soil under flooded conditions, and after maintaining a sufficient contact time, the reference electrode By measuring the potential difference between the electrode and the sweet potato electrode, it means a certain potential difference generated between the electrode and the soil solution as electrons move from the electrode in the solution direction or in the opposite direction.

  Moreover, as a dry state of soil, it is preferable that the water potential is -1 Mpa or less, particularly -5 Mpa or less. The water potential of the soil is described by Masayoshi Nakano, Satoshi Miyazaki, Choshi Shiozawa, “Cyclometer Method, Soil Physical Environment Measurement Method”, University of Tokyo Press, p. It is measured by the method described in 82-87. Soil moisture potential refers to the chemical potential of water contained in soil, and consists of a matrix potential component due to the surface tension of water and the adsorption power of soil particles, an osmotic potential component due to the inclusion of solutes, and a gravity potential component. . When soil contains a lot of water, it is expressed as having a high soil water potential, and when it is low, it has a low potential. To measure soil moisture potential, the relative humidity of air in equilibrium with the water contained in the soil is measured. Specifically, the water potential of the soil sample and the water vapor pressure of the air are balanced in a small chamber, and the relative humidity is measured as the humidity difference between the wet bulb and the dry bulb.

  Thus, after making heavy metal contamination soil into an oxidation state and / or a dry state, it wash | cleans using a chemical | medical agent. It does not restrict | limit especially as a chemical | medical agent, The thing conventionally used for soil washing | cleaning can be used. For example, calcium salts such as calcium chloride, calcium nitrate, calcium acetate, calcium iodide; organic acids such as citric acid, succinic acid, acetic acid, tartaric acid, lactic acid, butyric acid, malic acid, itaconic acid, gluconic acid, propionic acid; hydrochloric acid Inorganic acids such as nitric acid and sulfuric acid.

  Further, aminocarboxylic acids that form complexes with cadmium, such as amino acids such as alanine, glutamic acid, glycine, and cysteine, ethylenediaminetetraacetic acid (EDTA), ethylenecricolbis (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 Examples include soda (ASDA), methylglycine diacetic acid 3 soda (MGDA), S, S-ethylenediamine succinic acid (EDDS4H), S, S-ethylenediamine disuccinic acid 3 soda (EDDS3Na), and the like.

In addition, agents as may be used metal salt compound coordinated to hydroxyl ions by hydrolysis in soil pH (H ₂ O) or less to produce a metal hydroxide. 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.

These metal salt compounds generate metal hydroxides (M (OH) _n ) by coordinating hydroxide ions by hydrolysis, for example, as shown in the following formula. In the present invention, the use of a metal salt compound having a solubility product of the produced metal hydroxide of less than 10 ⁻¹⁰ , particularly less than 10 ⁻¹⁵ , lowers the pH at which the hydroxide ions are coordinated, and removes heavy metals. Since an effect improves, it is preferable. In particular, ferric chloride is preferable because the precipitation pH of the iron hydroxide Fe (OH) ₃ produced is as low as 2.3 or more, and the solubility product of the produced precipitate is as low as 10 ⁻³³ .

MX _n + nH ₂ O → M (OH) _n + nX ⁻ + nH ⁺
(In the formula, M represents a metal ion, X represents a monovalent anion, and n represents the valence of the metal ion)

In particular, when a metal salt compound that coordinates a hydroxide ion by hydrolysis and generates a metal hydroxide at a soil pH (H ₂ O) or lower is used as a chemical, when the soil is in an oxidized / dry state, As shown in the following formula, the proton emission amount is 1.5 times that of the reduced state, and the trivalent metal hydroxide produced has a lower solubility product than the divalent metal hydroxide, and the coordination of hydroxide ions When the pH to be reduced is low and the pH reduction effect is enhanced, the heavy metal purification effect is further enhanced.

(Reaction in oxidized / dry soil)
MX ₃ + 3H ₂ O → M (OH) ₃ + 3X ⁻ + 3H ⁺
(Reaction in reduced soil)
MX ₂ + X ⁻ + 2H ₂ O → M (OH) ₂ + 3X ⁻ + 2H ⁺

  In addition, when the coagulation sedimentation method is adopted in the wastewater treatment of soil washing wastewater, the metal salt compound of the drug applied in the reduced soil does not generate hydroxide, and a large amount of metal salt compound is mixed in the wastewater. In addition to increasing the amount of sludge generated, it is necessary to oxidize wastewater by aeration for a long time, so it is very important to make the soil oxidized and dry.

  The method for washing the soil with such a chemical is not particularly limited, and can be performed by a known method. For example, when extracting and washing heavy metals in soil using a chemical, it is preferably used as an aqueous solution, and the concentration thereof is 1 to 200 mM, particularly 3 to 100 mM. Since there is little residue to soil, it is preferable.

  The method for extracting and washing heavy metals in the soil using such an aqueous solution is not particularly limited, and any of a method for washing on-site, a method for excavating and washing the soil, and then refilling the purified soil, etc. good.

Further, in the present invention, extraction washing means not only mixing the soil and the chemical aqueous solution directly, but also a method of adding the chemical and water separately to the soil and washing them by mixing, and washing the soil containing water by mixing the chemicals. The method of doing is also included.
As an example of a method for washing soil containing water, there is a method in which bottom soil of rivers and lakes is dredged with water, put into a mixer, and drug powder is added to a predetermined concentration and mixed.

The amount of the aqueous solution used for the extraction washing is preferably 1 to 5 times by weight, particularly 1 to 2.5 times by weight of the soil to be purified.
By treating in this way, heavy metals in the soil are extracted into an aqueous solution.
Washing is preferably repeated until the heavy metal content of the soil that does not elute in the washing aqueous solution falls below the content reference value stipulated in the Soil Contamination Countermeasures Law (Act No. 53, No. 53). Furthermore, it is preferable to wash until the concentration of heavy metals eluted from the soil after the washing treatment is equal to or less than the elution standard value stipulated in “Environmental standards for soil contamination” (Health 3, 8, 23 notification No. 40). .

  In addition, the heavy metal content in the soil is determined according to the “Procedure for measuring methods related to soil content survey” (Heisei 15, 3, 6 notification 19), and the concentration of heavy metals eluted from the soil It is measured by an atomic absorption method, an ICP emission method, and an ICP mass spectrometry analyzer according to the method defined in “Environmental standards for pollution” (Heisei 3, 8, 23 Circular 40)

  The aqueous solution from which heavy metals have been extracted is solid-liquid separated by natural sedimentation or aggressive dehydration, and separated and removed from the soil. The discharged heavy metal-containing water is removed by ion exchange, electrolysis, insolubilized coagulation precipitation, etc. What is necessary is just to process.

On the other hand, in the treated soil, some of the chemicals in the aqueous solution used may remain or some of the cadmium extracted by washing may remain, so by further washing the soil with water These are preferably removed.
Washing with water may be performed in the same manner as washing with an aqueous solution, and is preferably repeated until the concentration of heavy metals in the soil and the residual amount of the chemical used in the aqueous solution washing are below the soil environmental standard, at least once. It is particularly preferable to wash with water 1 to 6 times.

The method of the present invention is suitable for the purification of cadmium-containing paddy soil, particularly paddy soil having a cadmium concentration of 0.1 to 5 ppm.
In this case, it is preferable to wash with an aqueous solution of the drug as described above using 1 to 5 times by weight, particularly 1 to 2.5 times by weight of the soil to be purified, and then wash with water.

  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
Paddy soil with a cadmium content (solid-liquid ratio of 1: 5, cadmium extracted with 0.1 M hydrochloric acid) of 0.67 mg / kg is passed through a 5 mm sieve to remove impurities, and a moisture content of 26. Air-dried to 24% in a greenhouse and filled into 100 L containers. At this time, the oxidation-reduction potential (Eh) of the soil was 300 mV.
After filling the container, the soil was quickly washed. Washing is performed using a 15 mM ferric chloride aqueous solution at a solid-liquid ratio (amount of the washing aqueous solution with respect to the soil) of 1: 1.5, stirred uniformly for 20 seconds with a hand mixer for concrete kneading, and then allowed to stand for 20 minutes. And repeated this three times. Thereafter, the mixture was allowed to stand for 10 hours, and 10 cm of the supernatant was drained as washing waste. Further, the drained amount of water was poured into the container, and the mixture was left to stir in the same manner as in the case of the cleaning aqueous solution, and 10 cm of the supernatant liquid was drained as a water cleaning waste liquid, and this was repeated three times.
Table 1 shows the pH, iron concentration, and cadmium concentration of the cleaning waste liquid and the water cleaning waste liquid. The cadmium content in the soil after washing was 0.33 mg / kg, and 51% of the cadmium contained was removed.

Comparative Example 1
The soil similar to Example 1 was not air-dried, filled in a container, and maintained in a flooded state for 2 months. At this time, the oxidation-reduction potential (Eh) of the soil was −100 mV. About this soil, it carried out similarly to Example 1, and performed the soil washing process.
Table 1 shows the pH, iron concentration, and cadmium concentration of the cleaning waste liquid and the water cleaning waste liquid. The cadmium content in the soil after washing was 0.52 mg / kg, and 23% of the cadmium contained was removed.

Example 2
The paddy soil collected in Ibaraki Prefecture was passed through a 5 mm sieve to remove impurities, and a raw soil having a soil water potential of -0.75 Mpa was obtained as a soil material. The raw soil was air-dried in six stages, and the amount of cadmium extracted with a solid-liquid ratio of 1:10 and 1M ammonium nitrate was quantified. The amount of cadmium extracted from each soil subjected to air drying treatment was shown as a relative value, with the amount of cadmium extracted from the raw soil being 100. The results are shown in Table 2.

Claims (3)

A method for purifying heavy metal-contaminated soil, characterized by washing heavy metal-contaminated soil with oxidized and / or dried state and then using ferric chloride .   The purification method according to claim 1, wherein the oxidation state of the soil is an oxidation-reduction potential (Eh) of 100 mV or more.   The purification method according to claim 1 or 2, wherein the dry state of the soil is a water potential of -1 Mpa or less.