JP2005169381A - Method for cleaning heavy metal-contaminated soil - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently cleaning heavy metal-contaminated soil. <P>SOLUTION: The method for cleaning the heavy metal-contaminated soil is characterized by applying a metal salt compound which forms a metal hydroxide as a result of coordination with hydroxy ions upon being hydrolyzed at a pH (H<SB>2</SB>O) equal to or lower than that of soil to the heavy metal-contaminated soil, and extracting and cleaning the heavy metals from the soil. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

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

  In order to purify heavy metal-contaminated soil, the soil covering method and the soil removal method are generally used. However, the soil covering method only prevents the spread of the top soil of the heavy metal-containing soil, and the soil removal customer soil method only moves the soil, and is not a fundamental solution.

  Moreover, the method (patent document 1) which insolubilizes the heavy metal in heavy metal containing soil using calcium chloride aqueous solution is proposed. However, simply insolubilizing heavy metals cannot be said to be fundamental purification, and secondary use of the soil is also difficult.

  Furthermore, a method for purifying contaminated soil by eluting and removing heavy metals from heavy metal-containing soil using various chemicals has been studied. As a cleaning agent, saponin (Patent Document 2) and pectinic acid are simply used. Mer or oligo form (Patent Document 3), EDTA (Patent Document 4), L-aspartic acid-N, N-diacetic acid, glutamic acid diacetic acid, hydroxyethyliminodiacetic acid, carboxymethyloxysuccinic acid, 3-hydroxy- 2,2′-iminodisuccinic acid, polyaspartic acid (Patent Document 5) and the like are used. These compounds promote the leaching of heavy metals by the chelate effect, but even if these compounds are used, it has been difficult to efficiently remove heavy metals in the soil. In addition, these compounds are expensive and are not suitable for actual use.

  It is also known that calcium chloride is effective for removing cadmium in the soil (Non-patent Document 1). However, Non-Patent Document 1 only describes the cadmium removal rate when the cadmium-containing soil is packed in a column and the calcium chloride aqueous solution is continued to flow in this column. In an actual cadmium-containing soil, There is no indication as to whether it can be cleaned efficiently.

  Among the cadmium-containing soils, especially in paddy field soil, there is a risk that the cadmium concentration in brown rice will be 1 ppm or more (distribution prohibited by the Food Sanitation Law) even if the cadmium concentration in the soil is low (eg, about 2 ppm). 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. Moreover, 0.4 ppm or more and less than 1.0 ppm of rice (brown rice) is purchased from farmers and processed as non-edible (industrial use paste) raw materials.

  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 become apparent. 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.

In addition, as a method for washing with a chemical, for example, Non-Patent Document 2 describes that when cadmium-containing paddy soil is treated with calcium chloride, acetic acid, etc., the cadmium concentration in the soil is reduced. 3 also describes an example using EDTA.
However, these documents only describe the cadmium concentration when the soil is put in a cultivation pot or the like, and the cadmium concentration can be efficiently removed by washing in an actual cadmium-containing paddy soil. And it is not shown whether it can be purified.

Furthermore, the cleaning waste liquid contains chemicals used as a cleaning agent and heavy metals such as cadmium extracted and removed from the soil by cleaning, but this cleaning waste liquid treatment is not specifically shown, and the soil cleaning waste liquid Development of a wastewater treatment method that can purify the wastewater to below the wastewater standard value at the current location has been desired.
JP-A-6-39055 JP-A-10-130623 JP 2002-45839 A JP-A-4-263874 JP 2002-52375 A Wang W., Artiola JF, Brusseau ML, Miller RM, “Use of calcium and fulvic acid to facilitate transport of cadmium in soil”, Am. Chem. Soc. Natl. Meet. Div. Environ. Chem., 1994, 34, No. 1, p208-210 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 studies in view of such a situation, the present inventors have found that if heavy metals in the soil are extracted and washed using a specific metal salt compound, the soil can be efficiently purified, and the present invention has been completed.

That is, the present invention applies to a heavy metal contaminated soil a metal salt compound that generates a metal hydroxide by coordinating hydroxide ions by hydrolysis at a soil pH (H ₂ O) or lower, and removes heavy metals in the soil. An object of the present invention is to provide a method for purifying heavy metal-contaminated soil characterized by extraction and washing.
Moreover, the present invention applies a metal salt compound in which the solubility product of a metal hydroxide produced by coordination of hydroxide ions by hydrolysis is less than 10 ⁻¹⁰ to heavy metal-contaminated soil, and removes heavy metals in the soil. An object of the present invention is to provide a method for purifying heavy metal-contaminated soil characterized by extraction and washing.
The present invention also relates to a metal salt compound that forms a metal hydroxide by coordinating hydroxide ions by hydrolysis at a pH of 9 (H ₂ O) or less with respect to paddy soil having a cadmium concentration of 0.1 to 5 ppm. An aqueous solution or an aqueous solution of a metal salt compound in which the solubility product of a metal hydroxide produced by coordination of hydroxide ions by hydrolysis is less than 10 ⁻¹⁰ is washed using 1 to 5 times the weight of the soil to be purified. The present invention provides a method for purifying cadmium-containing paddy soil.

  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 includes urban areas, mountain forests, factory sites, agricultural land, swamps, and soils that contain heavy metal elements such as lead, cadmium, and arsenic, compounds, or ions. 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.

The metal salt compound used in the present invention coordinates hydroxide ions by hydrolysis below the pH (H ₂ O) of the soil to be purified. 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 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 ⁺
(Wherein M represents a metal ion, X represents a monovalent anion, and n represents the valence of the metal ion)

  When these metal salt compounds are applied to heavy metal contaminated soil, 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 this, 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. In particular, soil cleaning at the site (in situ) is preferable.

Further, in the present invention, the extraction washing means a method of adding a metal salt compound and water separately to the soil, mixing and washing, in addition to directly mixing the soil and the metal salt compound aqueous solution, and metal salt in the soil containing water. Also included are methods of mixing and washing the compounds.
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 a metal salt compound 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 active dehydration, separated and removed from the soil, and detoxified by waste liquid treatment described later.

On the other hand, in the treated soil, a part of the metal salt compound in the used aqueous solution may remain or a part of the cadmium extracted by washing may remain, so the soil is further washed with water. It is preferable to remove them.
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 metal compound used in the aqueous solution washing are below the soil environmental standard, at least once. In particular, it is 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 metal salt compound 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.

  When soil is washed in-situ with an aqueous solution, for example, a metal salt compound can be dissolved in water using a tank and mixed to a predetermined concentration and then applied, or a solution with a concentration higher than the predetermined concentration After preparing and applying, water may be added so as to have a predetermined concentration, and further, it may be applied to a previously flooded paddy field. Moreover, you may apply with the apparatus which can throw in a metal salt compound continuously at the time of water conveyance.

  Further, the soil can be washed by separately adding the metal salt compound and water. In this case, for example, a fertilizer spreading machine can be used for applying the metal salt compound, and the soil is cultivated using a field cultivator or the like. In addition, the metal salt compound and the soil can be stirred and mixed. Water is usually introduced into a paddy field by an amount corresponding to the determined solid-liquid ratio, and then a cleaning operation can be performed in the manner of scraping using a rotary harrow or the like.

  After washing, cadmium in the soil is extracted into the aqueous solution, and then the soil is settled in situ to collect the supernatant waste liquid. Supernatant wastewater can be drained by opening the drain opening that normally opens when falling in a paddy field, temporarily stored in a pit, and then put into a wastewater treatment facility with a pump, or drained directly from a paddy field with a pump. good.

Washing with an aqueous solution is performed at least once, preferably 1 to 3 times. Washing is preferably repeated until the cadmium concentration in the waste liquid after washing is 0.5 ppm or less. The cadmium concentration is determined by the measurement method (Heisei 8 declaration 55) defined by the Director-General of the Environment Agency based on the provisions of Article 9-4 of JIS K 0102-93 “Factory drainage test method” or the Water Pollution Control Law enforcement regulations Article 9-4. Measured.
Furthermore, among the cadmium content of the soil, the cadmium content that does not elute into the washing aqueous solution, that is, the cadmium content in the soil after the washing treatment is repeatedly reduced to 1 mg / kg or less, particularly 0.35 mg / kg or less. Is preferred. If the cadmium content in the soil is 1 mg / kg or less, it can be expected that the cadmium content in the rice cultivated in the soil after washing treatment can be reduced to 0.4 mg / kg or less, and the cadmium content in the soil. If it is 0.35 mg / kg or less, it can be expected that the cadmium content in the rice grown in the soil after the washing treatment can be reduced to 0.2 mg / kg or less.
In addition, the cadmium content in paddy soil is determined by the method specified in “Ministerial Ordinance for Deciding Method of Cadmium Examination Related to Specified Requirements for Agricultural Land Soil Contamination Countermeasure Areas” It is measured by an analytical instrument such as an atomic absorption method, an ICP emission method, and an ICP mass spectrometry.

Since a part of the aqueous solution in which cadmium is dissolved remains in the soil washed with the aqueous solution, the soil is then washed with water to remove it.
Washing with water may be performed in the same manner as washing with an aqueous solution, and the amount of water to be used is preferably 1 to 5 times by weight, particularly 1 to 2.5 times by weight with respect to the soil to be purified. Washing with water is preferably repeated until the cadmium concentration in the waste liquid after washing is 0.5 ppm or less, particularly 0.1 ppm or less.

Furthermore, when paddy soil after washing is used as agricultural soil, washing with water is repeated until the chlorine concentration in the soil is 500 ppm or less and the electrical conductivity is 2 mS / cm or less, particularly 1 mS / cm or less. preferable.
In-situ cadmium and metal salt compound concentrations, chlorine concentration, and EC may be analyzed individually to confirm the effect of water washing, or the most easily analyzed EC may be analyzed. These analysis items may also be calculated on the assumption that they are diluted in proportion.

In this way, the heavy metal-containing waste liquid discharged after washing the paddy soil may remove the dissolved heavy metal after separating the solid suspended matter in the waste liquid, or the solid suspended matter and the dissolved heavy metal in the waste liquid. May be removed simultaneously.
Examples of the method for separating the solid suspended matter in the waste liquid include a screen, a micro strainer, a mopac / auto strainer, a sand settling device, vacuum filtration, pressure filtration, a rotating wire mesh, and centrifugal separation. When removing the colloid, after the aggregation with an inorganic flocculant (aluminum sulfate, polyaluminum chloride, ferrous sulfate, ferric chloride) or an organic polymer flocculant, the above separation method or precipitation separation What is necessary is just to process by the law.

  In order to remove dissolved heavy metals, waste liquid can be treated by ion exchange resin method, chelate resin method, electrodialysis method, reverse osmotic pressure method, ion flotation / foam separation method, evaporation concentration method, activated carbon adsorption method, etc. it can.

  Furthermore, in order to simultaneously remove solid suspended solids and dissolved heavy metals in the waste liquid, the above methods may be combined as appropriate.

  The cadmium-containing waste liquid containing heavy metals discharged by washing the paddy soil is preferably treated by a coagulation precipitation method. Specifically, the cadmium-containing waste liquid is treated in a waste liquid treatment step by aeration treatment, neutralization treatment, chelate solution treatment, polymer flocculant addition treatment, aggregation precipitation treatment, and dehydration treatment. When coarse impurities such as rice straw are mixed in the waste liquid, it is preferably removed with a screen or a cyclone.

  Paddy soil often has a reducing atmosphere due to stagnant water, and a large amount of divalent iron is eluted by soil washing treatment. This is converted into an oxidized state by aeration treatment to obtain trivalent iron having a low solubility product. Aeration can be performed, for example, by blowing air into the cleaning waste liquid using a blower or the like, and it is preferable to promote oxygen absorption efficiency by installing a diffuser at the discharge port. Furthermore, the oxidation treatment by aeration can be performed by adding ozone, chlorine, or hypochlorous acid.

  The neutralization treatment can be performed by adding an alkaline material and stirring with a stirrer, and the pH is adjusted to 6 or more and less than 12, preferably 8 or more and less than 10. Thereby, trivalent iron forms a floc of iron hydroxide, and most heavy metals containing cadmium also form a floc. As the alkaline material, sodium hydroxide, potassium hydroxide, slaked lime, or the like can be used.

If only aeration is performed first, it takes a long time to bring the waste liquid into an oxidized state. However, if it is performed in parallel with the neutralization treatment, the aeration treatment is completed in a short time, and the aeration and neutralization treatment is performed in the same tank. This is preferable because the treatment tank can be omitted.
In addition, if iron does not elute into the washing waste liquid in non-paddy soil, etc., or iron does not elute into the washing waste liquid in the oxidized paddy soil or water washing process, the generated heavy metal flocs become lighter. In the subsequent aggregation and precipitation treatment, the sedimentation property is deteriorated. For this reason, when ferric chloride is added in an amount of about 20 to 50 mg / kg in iron concentration, the floc becomes heavy and the pH at which heavy metal hydroxide precipitates containing cadmium are generated is lowered. Therefore, it is preferable.

  Chelate solution treatment can be performed by adding liquid heavy metal scavengers such as pyrrolidine, imine, carbamic acid, etc. and stirring vigorously. Cadmium that was not precipitated as hydroxide by aeration treatment and neutralization treatment The heavy metal ion containing is used as a floc to form an insoluble precipitate.

  Polymer aggregating agent addition treatment includes sodium alginate, sodium CMC, sodium polyacrylate, polyacrylamide partial hydrolysis salt, maleic acid copolymer, water-soluble aniline resin, polythiourea, polyethyleneimine, quaternary ammonium salt, Polyvinylpyridines, polyacrylamide, polyoxyethylene, causticized starch and the like can be added and gently stirred to grow and coarsen flocs and improve sedimentation.

  In the coagulation sedimentation treatment, flocs aggregated in the treatment tank are precipitated, and the treated water in which the supernatant is cleaned is drained by overflow.

  In the dehydration process, the sludge-like floc accumulated in the lower layer in the coagulation sedimentation tank is dehydrated and heavy metals containing cadmium are recovered. For dehydration, the sludge-like floc is extracted from the coagulation sedimentation tank, dehydrated by vacuum dehydration, centrifugal dehydration, filter press, belt press, screw press, etc., and the dehydrated cake is recovered. The squeezed water is again subjected to wastewater treatment.

  The soil after washing is acidic, and heavy metals containing cadmium remain as an aqueous solution, making the soil suitable for plant cultivation and insolubilizing heavy metals containing cadmium dissolved in the soil. It is preferable to apply a neutralization treatment. As the alkaline material, sodium hydroxide, potassium hydroxide, slaked lime, quicklime, calcium carbonate, and the like 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, so that the growth of the plant becomes healthy and most heavy metals are insolubilized. The application amount of the alkaline material for neutralization may be applied by shaking an alkaline material in an amount to be applied per unit soil, obtaining a buffer curve, and applying an amount to achieve the target pH.

  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
5 g of cadmium-containing paddy field soil of A, B, C, D and E is weighed in a 50 mL centrifuge tube, and ferric chloride aqueous solution of each concentration 1.5 times the soil weight is added, The mixture was shaken for 1 minute, allowed to stand for 10 minutes, further shaken for 1 minute, and then allowed to stand for 10 minutes. The supernatant was filtered through a 0.45 μm membrane filter, and the cadmium concentration in the solution was quantified with an IPC-MS apparatus. The results are shown in Table 1. In addition, pH of the used paddy field soil is A: 4.8, B: 4.7, C: 6.3, D: 4.3, E: 5.5.

Comparative Example 1
The same procedure as in Example 1 was performed except that distilled water was used instead of the ferric chloride aqueous solution. The results are shown in Table 1.

Comparative Example 2
The same procedure as in Example 1 was performed except that a 100 mM calcium chloride aqueous solution was used instead of the ferric chloride aqueous solution. The results are shown in Table 1.

  From the results in Table 1, it was found that the extraction concentration of cadmium increased with increasing ferric chloride concentration in all the soils used. In particular, when the concentration of ferric chloride added was 3 mM or more, cadmium could be extracted efficiently.

Example 2 (soil cleaning treatment)
In paddy field (pH 6.0) with 0.1N hydrochloric acid extracted cadmium 0.63 mg / kg, area 108.36 m ² , soil layer thickness 17.2 cm, soil capacity 18.64 m ³ , dry soil weight 18.64 t. The soil to be purified was subjected to soil washing treatment at the current position by the following method.

(1) 15 mM aqueous solution of ferric chloride, so as to be 28 m ³ 1.5 times the dry soil weight of target soil, water conveying and 40 Baumé 38% industrial to a depth of 32.2cm from tillage Release Ferric chloride was applied.
(2) After stirring and mixing with a cultivator for 1 hour so that the soil and the cleaning aqueous solution became a slurry, the mixture was allowed to stand for 30 minutes and further stirred and mixed with a cultivator for 30 minutes.
(3) The mixture was allowed to stand, and the soil was naturally settled to separate into solid and liquid, and the supernatant was treated with 10 cm and 10.36 m ³ of waste water.
(4) further water conveyance to a depth of 32.2cm from plowing machine as the waste water treated 10.836M ³ minutes of water, stirred for 1 hour at tillers, and allowed to stand, the soil was separated solid was allowed to settle naturally, 10 cm of supernatant and 10.36 m ³ were treated with waste water.
(5) The above (4) was repeated three times.
(6) The soil pH after the washing treatment was 3.93. From the pH neutralization curve obtained in advance, it is known that applying 2.5 g of calcium carbonate to 1 kg of soil results in a soil pH of 5.6, applying 46.6 kg of calcium carbonate, And stirred.

  After the above soil washing treatment, the 0.1N hydrochloric acid-extracted cadmium content in the soil was 0.35 mg / kg, and 44% of the cadmium in the soil was removed. Moreover, the chlorine concentration in the soil after the soil washing treatment was 494 mg / L, pH 5.6, which was a level that did not cause phytotoxicity to paddy rice.

Example 3 (waste water treatment)
The cadmium-containing waste liquid generated in the soil washing treatment of Example 2 was treated with waste water by the following method using a coagulation sedimentation method. The waste liquid treatment system process flow used is shown in FIG.
The soil washing wastewater drained from the paddy field includes trivalent iron applied as a cleaning agent, divalent iron eluted from the reduced soil, cadmium leached by washing, and other heavy metals.

(1) Soil washing wastewater was taken by overflow from a movable water intake that was devised so that only the supernatant water could be taken into a raw water pit installed 20 cm deeper than the ground level.
(2) The soil washing wastewater taken into the raw water pit was sent to the reaction tank 1 by a submersible pump.
(3) Caustic soda is added in the reaction tank 1 to make the pH neutral to weakly alkaline, and at the same time, air is fed, iron is converted into ferric hydroxide, and some cadmium and some heavy metals are also converted into hydroxides. A floc was formed, and water was sent to the reaction vessel 2.
(4) Addition of alkali metal salt of polymer having dithiocarbamic acid group and thiol group and liquid chelate mainly composed of sodium hydrogen sulfide in reaction tank 2, agitation, complete dissolution of cadmium and heavy metals to floc Water was sent to the tank.
(5) In the flocculation tank, an anionic polymer flocculant mainly composed of an acrylamide / acrylic acid copolymer was added, and the generated floc was grown and coarsened to improve the sedimentation property, and water was fed to the precipitation tank.
(6) The floc was precipitated in the settling tank, and the purified supernatant water was discharged into the agricultural waterway.
(7) The precipitated sludge-like floc was sent to a filter press by a drawing pump and recovered as a dehydrated cake. The squeezed water was returned to the raw water pit.
Table 2 shows the concentrations of heavy metals before and after the treatment of the waste liquid subjected to the above treatment.

It is a figure which shows the waste liquid processing system process flow used in Example 3. FIG.

Claims (15)

It is characterized by applying metal salt compounds that coordinate hydroxide ions by hydrolysis to form metal hydroxides below soil pH (H ₂ O) to heavy metal contaminated soil and extract and wash heavy metals in the soil. Purification method for heavy metal contaminated soil. The purification method according to claim 1, wherein the metal salt compound is a compound that forms a metal hydroxide by coordinating a hydroxide ion by hydrolysis at a pH of 9 (H ₂ O) or lower. A metal salt compound in which the solubility product of a metal hydroxide produced by coordination of hydroxide ions by hydrolysis is less than 10 ⁻¹⁰ is applied to heavy metal contaminated soil, and heavy metals in the soil are extracted and washed. Purification method for heavy metal contaminated soil.   The purification | cleaning method of any one of Claims 1-3 which wash | cleans soil using 1-200 mM aqueous solution of a metal salt compound.   The purification method according to any one of claims 1 to 4, wherein heavy metal in the soil is extracted and washed with a metal salt compound aqueous solution 1 to 5 times the weight of the soil to be purified.   The purification method according to any one of claims 1 to 5, wherein the soil to be purified is paddy soil. For paddy soil with a cadmium concentration of 0.1 to 5 ppm, an aqueous solution of a metal salt compound that coordinates a hydroxide ion by hydrolysis at pH 9 (H ₂ O) or lower, or generates a metal hydroxide, or by hydrolysis. Cadmium is characterized in that an aqueous solution of a metal salt compound in which the solubility product of a metal hydroxide produced by coordination of hydroxide ions is less than 10 ⁻¹⁰ is washed using 1 to 5 times by weight of the soil to be purified. Purification method of contained paddy soil.   The purification method according to claim 7, further comprising washing with water after washing with an aqueous solution of a metal salt compound.   The purification method according to claim 7 or 8, wherein the soil is neutralized by applying an alkaline material after washing the soil.   The purification method according to claim 7 or 8, wherein after washing the soil, an alkaline material is applied to insolubilize heavy metals dissolved in the soil.   The purification method according to claim 9 or 10, wherein the soil pH is adjusted to 5-8.   After extracting cadmium in paddy soil into an aqueous solution of metal salt compound, the soil is settled in situ, the supernatant waste liquid is collected, the solid suspended matter in the waste liquid is separated, and the dissolved cadmium is removed. The purification method according to claim 7 or 8.   The cadmium in the paddy soil is extracted into an aqueous solution of a metal salt compound, and then the soil is settled in situ to collect the supernatant waste liquid, and the solid suspended matter and the dissolved cadmium in the waste liquid are simultaneously removed. Or the purification method of 8.   The cadmium-containing waste liquid discharged after washing the paddy soil is treated in a waste liquid treatment step by aeration treatment, neutralization treatment, chelate solution treatment, polymer flocculant addition treatment, coagulation precipitation treatment and dehydration treatment. The purification method according to any one of the above.   The purification method according to claim 14, wherein the aeration treatment and the neutralization treatment are performed in parallel.

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