WO2012020449A1 - Method for purifying cadmium-containing paddy soil - Google Patents

Abstract

A method for purifying cadmium-containing paddy soil characterized by gathering a detergent drain containing cadmium at one place in a paddy to be washed, subjecting the drain to neutralization or alkali treatment at the place, and accumulating cadmium, in a in-place purification method including washing cadmium-containing paddy soil by a drug solution to extract cadmium, separating the resultant into soil and supernatant by standing, draining the extracted cadmium as supernatant, and further performing a diluent washing with water. The cadmium-containing paddy soil can be efficiently purified.

Description

Purification method of paddy soil containing cadmium

The present invention relates to a method for efficiently purifying cadmium-containing paddy soil.

In CODEX (Joint Food Standards Committee by WHO and FAO), the standard value of Cd concentration in agricultural products was discussed, and in July 2006, a proposal of 0.4 mg / kg for rice was adopted (Non-patent Document 1). . In response to this, in Japan on April 8, 2010, the Cd ingredient standard for rice based on the Food Sanitation Law was revised from less than 1.0 mg / kg to less than 0.4 mg / kg. Furthermore, the designated requirements of the agricultural land and soil pollution control area for Cd based on the Agricultural Land Soil Contamination Prevention Act on June 11, 2010 are areas of 0.4 mg or more from areas of 1 mg or more per 1 kg of brown rice Or it was revised to the area where the fear is remarkable.
In the future, there is a possibility that a considerable area will be designated as Cd-contaminated agricultural land in Japan, and restoration will be required. In addition, 50% of the average Japanese Cd intake is derived from rice. From the viewpoint of food safety, restoration of Cd-contaminated paddy fields is desired worldwide, especially in Japan.

In the current soil improvement project, the land visit method is mainly adopted, but it is not only expensive, but in recent years it is difficult to collect the mountain soil used for the land. There is a need to increase soil fertility so that it is suitable for large volumes of soil disposal and paddy soil, and there is a need for a physically and cost-effective soil remediation method.

For repairing heavy metal contaminated soil, in addition to soil, solidification, thermal treatment, electroreclamation (electrophoresis, etc.), plant extraction (phytoextraction), and plants were used. There are transpiration (phytovolatilization), soil washing (soil washing, soil flushing) and the like (Non-Patent Document 2, Non-Patent Document 3). Many companies are researching soil remediation using the cleaning method, but most of them are intended to purify contaminated soil by transporting it to a treatment plant, such as the site of a factory. In many cases, the amount of contaminated soil is reduced and the concentration is reduced (Non-patent Document 4), and there is a problem in applying it to paddy fields.

In addition, methods for purifying contaminated soil by eluting and removing heavy metals from heavy metal-containing soil using various chemicals have 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.
These chemical soil cleaning methods are restoration techniques in which cleaning materials are added to contaminated soil, mixed in liquid form to leach and remove heavy metals, and processed by a purification system. Since it is a chemical method, it has the advantages of high removal efficiency and quick repair.

However, in such a method of washing soil using a chemical aqueous solution and water, a large amount of waste liquid containing heavy metals is generated, but a waste water treatment device and a dehydration device are required for the treatment. There is a problem that installation of such a device is very expensive and expensive. Moreover, depending on the location of the soil to be purified, there may be cases where facilities such as a wastewater treatment device cannot be installed.

JP 2004-283743 A Japanese Patent Application Laid-Open No. 2005-169311

Therefore, an object of the present invention is to provide a method for efficiently purifying cadmium-containing paddy soil.

As a result of various studies in view of such circumstances, the present inventors have collected cleaning effluent containing cadmium in a part of the paddy field to be cleaned (accumulated field), and neutralized or alkali-treated the effluent at this position. The present inventors have found that since it is not necessary to install a wastewater treatment device or a dewatering device, the cost can be reduced and the paddy soil can be purified efficiently.

That is, the present invention is to wash cadmium-containing paddy soil with an aqueous chemical solution to extract cadmium, separate it into soil and supernatant water, discharge the extracted cadmium as supernatant water, and further dilute and wash with water. In the in-situ purification method, cleaning wastewater containing cadmium is collected in a part of the paddy field to be cleaned, and the wastewater is neutralized or alkali-treated at that position to collect cadmium. A method is provided.

According to the present invention, since it is not necessary to install a wastewater treatment device or a dehydration device, the cost can be reduced and the cadmium-containing paddy soil can be efficiently purified. In addition, it is possible to purify even in paddy fields in places where facilities such as waste water treatment equipment cannot be installed.

The cadmium-containing paddy field soil to be purified in the present invention is a paddy field soil having a cadmium concentration of 0.1 to 10 ppm, and is suitable for in-situ purification of such paddy field soil.

The agent used in the present invention is not particularly limited as long as it is used for washing heavy metal-containing soil, and examples thereof include calcium salts, organic acids, inorganic acids, and aminocarboxylic acids. 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.

Amino carboxylic acids form complexes with cadmium. For example, amino acids such as alanine, glutamic acid, glycine and 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 Examples include soda (ASDA), methylglycine diacetate 3 soda (MGDA), S, S-ethylenediamine succinic acid (EDDS4H), and S, S-ethylenediamine disuccinate 3 soda (EDDS3Na). Among these, biodegradable chelating agents such as glutamic acid diacetic acid 4 soda, aspartic acid diacetic acid 4 soda (ASDA), methylglycine diacetic acid 3 soda (MGDA), S, S-ethylenediaminesuccinic 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.

In the present invention, first, a chemical aqueous solution is applied to cadmium-containing paddy soil, the soil and the aqueous solution are mixed and stirred, washed, and then left to stand to extract cadmium in the soil.

The amount of the chemical aqueous solution used for cleaning is 1.3 to 3 times, especially 1.5 to 2 times the amount of the soil to be purified, which increases the extraction efficiency of cadmium and the removal efficiency of cadmium and cleaning agent by drainage. Therefore, it is preferable.

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.

When the soil is washed by adding the drug and water separately in situ, for example, if it is a solid or powdered drug, it can be sprayed by a fertilizer spreader, a drug spreader, or a tractor equipped with these. . Moreover, if it is a liquid chemical | medical agent, it can apply directly with a pump etc. Water is added in an amount corresponding to the determined solid-liquid ratio by a normal water introduction method or a pump.

When washing the soil by adding the drug as an aqueous solution, for example, the drug can be dissolved in water using a tank and mixed to a predetermined concentration, and then applied using a pump. After preparing and applying a high-concentration solution, it can be applied using a pump, and after preparing and applying a solution with a concentration higher than a predetermined concentration, water may be added to a predetermined concentration. It may be applied to flooded paddy fields. Moreover, you may apply with the apparatus which can throw in a chemical | medical agent continuously at the time of water conveyance.

For mixing and stirring the soil and the aqueous chemical solution, for example, a tractor equipped with a work machine such as a rotary can be used.

After allowing the soil particles to settle by standing, it is separated into soil and supernatant water, and the extracted cadmium is drained as supernatant water. In order to remove the supernatant water, it is possible to drain the water by opening the drain opening that normally opens when falling in a paddy field, temporarily storing it in the pit, and then putting it into a wastewater treatment facility with a pump, or draining it from the paddy field as it is. You may do it.

By treating in this way, the soil is washed with the aqueous chemical solution, and the cadmium in the soil is eluted into the aqueous solution. Washing with an aqueous solution is performed at least once, preferably 1 to 3 times.

Since a part of the aqueous solution in which cadmium is dissolved remains in the soil washed with the aqueous chemical solution, the soil is then diluted with water and removed.
The washing with water may be performed in the same manner as the washing with the aqueous chemical solution, and the amount of water used is preferably the same as in the case of the aqueous chemical solution.

In addition, the washing with water is preferably repeated until the cadmium and drug concentrations to be eluted do not affect the growth of rice.

In the present invention, cleaning wastewater containing cadmium discharged by the aqueous chemical solution and water cleaning is collected in a part of the paddy field to be purified (accumulated field). It is preferable to install the paddy field at the end of the paddy field to be purified. Especially, the paddy field should be installed at the end in contact with the road from the viewpoint of the soil removal of the paddy field soil and the workability of the soil to be carried out after the cadmium is accumulated. preferable.

The washing wastewater collected in the accumulation field is subjected to neutralization or alkali treatment at the position. Neutralization or alkali treatment is preferably carried out at pH 5.8 to 8.6, particularly pH 7 to 8.6, because neutralization retreatment is not necessary for the cadmium insolubilizing effect and pH drainage standard adaptation.
Neutralization or alkali treatment can be used without particular limitation as long as the aqueous solution exhibits alkalinity, such as sodium hydroxide, potassium hydroxide, ammonia, calcareous material, and the like. Of these, calcareous materials are particularly preferable from the viewpoints of safety of the spraying work, cost, and environmental load. Examples of the calcareous material include calcium carbonate, calcium carbonate, calcium silicate, quick lime, hard lime, slaked lime, bitter slaked lime, by-product lime, shell fossil lime, mixed lime, etc. It is preferable because neutralization or alkali treatment can be performed with a small application amount. Further, the calcareous material can be used in any of powdery products, granular products, and the like.
Neutralization or alkali treatment can be performed by adding these alkali materials and the like and stirring the slurry in the slurry. Agitation of the soil can be carried out in the same manner as in soil washing.

In the case of neutralization or alkali treatment, it is preferable to mix adsorbing materials with the soil (accumulated field) because cadmium in the waste water is easily adsorbed.
As the adsorbing material, peat, zeolite, humic acid material, vermiculite, bentonite, diatomaceous earth calcined product, etc. having a large cation exchange capacity can be used among the soil improving materials specified by the geopower enhancement method. In addition, materials such as Kanuma soil, dolomite, iron-containing materials, manganese materials, and alumina materials can also be used. As the adsorbing material, Kanuma earth, dolomite, zeolite, bentonite, vermiculite, iron-containing material, manganese material, and alumina material are particularly preferable.
The adsorbing material is preferably applied in an amount of 1 to 20% by mass, particularly 5 to 15% by mass, based on the dry soil in the accumulated field.

The wastewater treatment in the integrated paddy field will be installed by driving a corrugated plate up to the cultivator on the shore of the paddy field, so that the water depth of 30 to 50 cm can be submerged from the cultivator. Next, the adsorbent material is applied to the accumulation field, and mixed and stirred by a tractor equipped with a rotary. In addition, the washing drainage containing cadmium generated by soil washing is introduced from the cultivator to the paddy field so that the water depth is 30 to 50 cm.
A neutralization or alkali treatment agent is applied thereto, and the mixture is stirred and mixed with a tractor equipped with a rotary to perform neutralization or alkali treatment. Neutralized or alkali-treated cadmium and heavy metals such as iron contained in the washing wastewater are converted to hydroxides by neutralization or alkali treatment, resulting in precipitation.

In the integrated field, by further standing after such neutralization or alkali treatment, the sediment containing cadmium settles together with the soil particles, and a treated wastewater that is clear and does not contain cadmium is obtained as the rice field water. Since this treated water conforms to the drainage standards, it can be drained as it is, but it can also be drained through a filtration step such as pH adjustment treatment, bag filter, sand filtration or the like.
Furthermore, the paddy soil to be purified other than the accumulation field is washed, and the washing wastewater is repeatedly introduced into the accumulation field, but the application of the adsorbent material is not necessary after the second time, neutralization or alkali treatment, standing, settling After the treatment, wastewater treatment is repeated, and the cadmium in the wastewater is accumulated in the accumulation field.

Lastly, only the accumulated paddy soil with a high concentration of cadmium is excavated and removed, and the soil not contaminated with cadmium is removed, so that all soil in the paddy field to be purified can be efficiently and low-cost. Can be purified.

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

Example 1
2000 g of cadmium-containing air-dried paddy field soil was weighed into a 5000 mL tank, 4000 mL of 20 mM ferric chloride solution was added, stirred for 1 hour with a three-one motor, and allowed to stand, and the supernatant was recovered. This was designated as “soil washing drainage”. “Soil wash drainage” had a pH of 3.3 and a cadmium concentration of 1.55 mg / L.
50 g of cadmium-containing air-dried paddy field soil was weighed into a 200 mL beaker, 100 mL of “soil washing drainage” was added, and an aqueous sodium hydroxide solution was added in several stages. The mixture was further stirred for 10 minutes with a magnetic stirrer and then allowed to stand to settle the soil, the supernatant water was collected, and the pH and cadmium concentration were measured. In addition, the case where not using sodium hydroxide aqueous solution but treating only cadmium containing air-dried paddy field soil and soil washing drainage is shown as a comparison. These results are shown in Table 1.

Example 2
Several 50 g of air-dried paddy field soil was weighed into a 200 mL beaker, and 100 mL of “soil washing drainage” prepared in Example 1 was added. 50 to 250 mg of various alkaline materials were added to each beaker. The mixture was shaken and stirred for 10 minutes, allowed to stand to settle the soil, and the pH and cadmium concentration of the supernatant water were measured. The results are shown in Table 2.

Example 3
Several 50 g of air-dried paddy field soil was weighed in a 200 mL beaker, 2.5 g of bentonite (5% by mass with respect to the air-dried paddy field soil) was added, and the mixture was stirred and mixed with a glass rod. To this, 100 mL of “soil washing wastewater” prepared in Example 1 was added, and the mixture was shaken and stirred for 10 minutes while adding an appropriate amount of an aqueous sodium hydroxide solution, and adjusted to pH 7 or pH 8. After this, the soil was allowed to settle and the cadmium concentration of the supernatant water was measured. The results are shown in Table 3.

Claims (7)

1. In the in-situ purification method where cadmium-containing paddy soil is washed with a chemical aqueous solution to extract cadmium, separated into soil and supernatant water, the extracted cadmium is discharged as supernatant water, and further diluted and washed with water. A method for purifying cadmium-containing paddy soil, wherein the waste water containing cadmium is collected in a part of the paddy field to be cleaned, the waste water is neutralized or alkali-treated at the position, and cadmium is accumulated.
2. The purification method according to claim 1, wherein the neutralization or alkali treatment is performed at a pH of 5.8 to 8.6.
3. 3. The purification method according to claim 1 or 2, wherein the neutralization or alkali treatment is performed with a calcareous material.
4. The calcareous material is selected from calcium carbonate, calcium carbonate, calcium silicate, quick lime, quick lime, slaked lime, white lime slaked lime, byproduct lime, shell fossil lime, and mixed lime. Purification method.
5. The purification method according to any one of claims 1 to 4, wherein an adsorption material selected from Kanuma soil, dolomite, zeolite, bentonite, vermiculite, iron-containing material, manganese material, and alumina material is applied to paddy soil accumulating cadmium. .
6. 6. The purification method according to claim 5, wherein the application amount of the adsorbing material is 1 to 20% by mass of the paddy soil dry soil accumulating cadmium.
7. The purification method according to any one of claims 1 to 6, wherein the paddy soil accumulated with cadmium is discharged and removed, and the soil not contaminated with cadmium is removed.