JPH11221553A - Purifying method of heavy metal contamination soil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an economical purification method of heavy metal contamination soil by which the heavy metal is removed from the soil contaminated with the heavy metals which are hardly soluble or insoluble, with high removal efficiency and in a short time. SOLUTION: This process comprises a first step of mixing the contaminated soil A with an acidic solvent and extracting heavy metals with the solvent and a second step by which a cathode part 6 and an anode part 7 separated electrodes 6a, 7a from the contamination soil A with a water-permeable member 8 are arranged in the contaminated soil A after the solvent-extraction treatment, and direct current voltage is impressed between the electrodes 6a, 7a to move ions of the heavy metals, etc., to recover the metal. In the second step, the water in the cathode part 6 is discharged and also clean water is fed, and pH in the cathode part 6 is maintained at <=7.5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for removing heavy metals from soil contaminated with heavy metals which are hardly soluble or insoluble in water,
The present invention relates to a method for purifying heavy metal contaminated soil.

[0002]

2. Description of the Related Art In recent years, there has been an increasing number of cases in which heavy metal contamination on factory sites, waste disposal sites, and the like has been found through investigations associated with urban redevelopment. As a countermeasure for the purification treatment of the contaminated soil by such heavy metals, a method of shielding the contaminated soil from the surrounding environment such as a process of insolubilizing the contaminant, a water shielding work, and a soil covering work is generally used. However, in these methods, since the heavy metals themselves remain in the soil at the site, there is a limitation on land use after the treatment.

[0003] In recent years, therefore, measures have been taken to remove soil contaminated by heavy metals at a high concentration from the site and discard it, and replace it with new soil. However, it is clear that there will be a shortage of final disposal sites for industrial waste in the near future. Therefore, it has been studied to remove and contaminate heavy metals in contaminated soil without discarding the contaminated soil.

[0004] As a method of purifying such contaminated soil,
Soil cleaning methods and electrochemical treatment methods are known. The soil washing method is a method of physically and chemically extracting and separating contaminants from contaminated soil using water or a suitable solvent. Specifically, the contaminated soil is mixed and washed with a solvent to remove particles containing a high concentration of heavy metals, and the obtained clean soil is used for backfilling the contaminated site.

In this soil washing method, the volume of contaminated soil that becomes waste is reduced. Therefore, the higher the ratio of the obtained clean soil amount to the supplied contaminated soil amount, the more effective the purification method. . In addition, when heavy metals that are hardly soluble or insoluble in water are the main contaminants, they are almost impossible to remove even using a salt solution such as water or calcium chloride as an extraction solvent. An acidic solution such as nitric acid or hydrochloric acid is used.

[0006] On the other hand, in the electrochemical treatment method, a cathode and an anode are arranged in contaminated soil, and a direct current voltage is applied between the electricity to cause an iontophoretic action and a voltage osmotic action in the soil to cause pollutants. Is concentrated and recovered.

[0007]

In the above-mentioned soil washing method, an acidic solvent is used as a solvent for removing hardly soluble or insoluble heavy metals, and the soil after washing is separated from the acidic solvent from which contaminants are extracted. Requires filtration and washing. However, in this filtration / washing step, the heavy metal ions once dissolved in the acidic solvent are adsorbed to the soil again,
There is a problem that the removal efficiency is greatly reduced. Also,
When nitric acid or the like is used as the acidic solvent, harmful substances such as nitrate ions remain in the soil. Therefore, more steps and energy are required to wash and remove the harmful substances.

[0008] In the electrochemical treatment method, hydroxide ions are generated by the electrolysis of water occurring at the cathode, and the pH of soil near the cathode rises, causing precipitation of heavy metals such as lead. For this reason, there has been a problem that the moving force of the heavy metal near the cathode gradually decreases, and the applied voltage increases with the elapse of time, the contaminant removal efficiency decreases, and the purification period becomes longer.

The present invention has been made in view of such circumstances.
Highly efficient removal of heavy metals as contaminants from soil contaminated with hardly soluble or insoluble heavy metals in a short time.
It is an object of the present invention to provide an economical purification method of heavy metal contaminated soil.

[0010]

In order to achieve the above object, the present invention provides a method for purifying heavy metal-contaminated soil, comprising the steps of: mixing an excavated contaminated soil with an acidic solvent to extract heavy metals by solvent; And a cathode part and an anode part, in which an electrode is separated from the contaminated soil by a water-permeable member on the contaminated soil after the solvent extraction treatment, and a DC voltage is applied between the electrodes, and heavy metals or the like are applied by electrochemical action. A second step of transferring and recovering the ions of step (a). In the second step, water in the cathode section is drained and clean water is supplied into the cathode section to maintain the pH in the cathode section at 7.5 or less. It is characterized by doing.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, soil contaminated with heavy metals is first treated by a soil washing method using an acidic solvent, and the contaminated soil after the washing treatment is further treated by an electrochemical treatment method. Therefore, the heavy metal ions of the contaminants dissolved and extracted in the acidic solvent by the soil washing method in the first step move to the cathode part by the iontophoretic action in the second step and are concentrated, and thus are adsorbed on the soil surface again. Can be prevented. As a result, the heavy metals of the contaminants can be efficiently collected and removed from the cathode part, and the filtration step and the washing step of the contaminated soil after the washing treatment can be omitted, or the treatment time is greatly reduced even when these steps are required. It becomes possible.

The method of the present invention will be specifically described with reference to FIG. In the first step, the contaminated soil A excavated with a shovel or the like from the contaminated site is transferred to a washing tank 1 equipped with a stirrer 2 and a pH meter 3, and an acidic solvent is added from a solvent tank 4 and stirred.
The pH of the contaminated soil A is lowered to provide a soil atmosphere in which heavy metals can be easily eluted. By this washing treatment, heavy metals in the contaminated soil A are extracted into the acidic solvent, so that the time of the electrochemical treatment in the second step can be reduced.

The type and concentration of the acidic solvent added to the contaminated soil A depend on the contaminants in the contaminated soil and their concentrations. Normally, nitric acid, hydrochloric acid, or the like can be used as the acidic solvent, and it may be added so that the pH of the contaminated soil A in the washing tank 1 becomes 2 to 3 or less. Further, the solid-liquid ratio between the contaminated soil A and the acidic solvent in the washing tank 1 also depends on the state of the contaminated soil A. If the amount of the acidic solvent is large, the processing efficiency per one time in the electrochemical treatment in the second step is reduced. Therefore, it is necessary to adjust the water content by dehydration or the like before moving to the second step. Conversely, when the amount of the acidic solvent is small, the load on the stirrer 2 increases, which hinders the stirring process. In the normal case, if the contaminated soil A is treated with an acidic solvent about 1 to 1.5 times, the subsequent dehydration treatment can be omitted.

After completing the washing treatment with the acidic solvent in the first step, the contaminated soil A in the washing tank 1 is removed together with the acidic solvent in the second step.
It is supplied to the electrolytic cell 5 in the process. Cathode 6 and anode 7 in which electrodes 6a and 7a are separated from contaminated soil by a water-permeable member 8 are disposed in electrolytic bath 5, and a direct current voltage is applied between the electrodes to form cations such as heavy metals. Gather at the cathode section 6, and at the same time, the anions move to the anode section 7. The soil water in the contaminated soil A generally moves to the cathode 6 side by electroosmosis since the soil surface is negatively charged. Therefore,
By draining the water in the cathode section 6, contaminants such as concentrated heavy metals can be collected and removed, and harmful anions can be similarly collected from the anode section 7.

In particular, in the second step, it is important to maintain the pH of the water in the cathode portion 6 at 7.5 or less by draining the water in the cathode portion 6 and supplying clean water to replace the water. It is. This is because, when a voltage is applied between the electrodes, the pH in the vicinity of the cathode 6 increases due to hydroxide ions generated by the electrolysis of water, and as a result, much of the heavy metal ions that have migrated to the cathode 6 precipitate. This is to prevent the removal efficiency from significantly decreasing. At the start of the electrolysis, the pH of the contaminated soil A after the washing treatment with the acidic solvent is in a low state. good.

The clean water supplied into the cathode section 6 may be ordinary water having a pH of neutral or less and not containing a large amount of heavy metals and hydroxide ions, for example, tap water. Further, wastewater containing heavy metals and the like collected from the inside of the cathode section 6 is treated by a water purification device using an ion exchange resin or the like, and purified water after the treatment can be supplied into the cathode section 6. In this case, the water drained from the cathode section 6 can be used repeatedly through the water purification device, which is economically advantageous.
The speed at which water is supplied to and drained from the cathode portion 6 (circulation speed) depends on the applied current value, the size of the electrode, and the like. For example, when the current value is 50 mA, a speed of about 20 to 30 ml / min is sufficient. is there.

In the electrochemical treatment in the second step, soil water moves from the anode 7 to the cathode 6, and the water content in the soil near the anode 7 decreases. If the water content in the soil is low, the processing speed for removing heavy metals is reduced. Therefore, it is preferable to supply a solvent to the anode 7 as necessary. Also, by supplying water to the soil of the anode section 7, p
It also has the effect of accelerating the reduction of H and promoting the ionization of heavy metals in the soil. In this case, the solvent to be supplied may be clean water such as tap water, or an aqueous solution of an acid, an electrolyte, or a chelating agent.

The cathode section 6 and the anode section 7 are respectively composed of a cathode electrode 6a and an anode electrode 7a, and these electrodes 6a and 7a.
a from the contaminated soil A. The material of the cathode electrode 6a is not particularly limited as long as it is a conductive material, and can be made of, for example, iron or stainless steel. Further, as the anode electrode 7a, an inert electrode material such as carbon or titanium is preferable. The water-permeable member 8 that separates these electrodes from the soil is a water-permeable material through which water and heavy metal ions can pass, and is preferably made of an electrically insulating material that is easily electrically insulated from the electrodes. Nonwoven fabric, porous plastic, or the like can be used.

The shapes of the anode electrode and the cathode electrode are not particularly limited, and may be, for example, a plate, a rod, a tube, or the like. For example, if these electrodes are formed of a hollow body having a plurality of through holes, water supplied to the inside of the hollow body can be supplied from the through holes to the inside of the water-permeable member, and the water permeability of the electrodes can be improved. Water can be drained from between the members. The shape of the water-permeable member may be any shape as long as the electrode and the contaminated soil can be separated.

[0020]

EXAMPLE A main contaminant was lead oxide, and a contaminated soil A having a lead concentration of about 2000 ppm was prepared, and 0.1 m ^{3 of the} contaminated soil A was filled in a stirring tank 1 shown in FIG. Next, 1 mol of nitric acid was added as an acidic solvent to 1 kg of the contaminated soil from the solvent tank 4 to make the solid-liquid ratio 1: 1. In this state, the pH of the contaminated soil was 2.8, and the contaminated soil A was stirred by the stirrer 10 for 10 minutes to extract the solvent. Thereafter, the contaminated soil A after the above-mentioned cleaning treatment was supplied to the electrolytic treatment apparatus shown in FIG. 2 without dehydration treatment.

This electrolytic processing apparatus comprises an electrolytic cell 5 and a cathode 6
And an anode section 7. The cathode part 6 is composed of a plate-shaped cathode electrode 6a and a water-permeable member 8, and the inside of the cathode part 6 separated from the contaminated soil A by the water-permeable member 8 has a plate-shaped cathode electrode 6a.
On one side is a drainage channel 9a, and on the other side is a clean water supply channel 10a.
Connected to each other. In this way, by arranging the water supply channel 10a of the cathode unit 6 with the contaminated soil A separated from the cathode 6a and the drainage channel 9a, the flow of water supplied from the water supply channel 10a minimizes the influence of the flow of soil water. Can be The plate-like cathode electrode 6a is provided with a plurality of through holes 6b that communicate between the drain passage 9a and the water supply passage 10a.

On the other hand, the anode part 7 is a hollow cylindrical anode electrode 7.
a and a water-permeable member 8, and a hollow cylindrical anode electrode 7.
The inside of a is connected to the solvent water supply passage 10b, and the outside of the anode electrode 7a and the water permeable member 8 are connected to the drainage passage 9b. It is not necessary to constantly perform drainage and water supply in the anode part, and it is sufficient to perform the drainage and water supply at regular intervals. For example, in the case of the anode part 7 of FIG. 2, either the inside or the outside of the hollow cylindrical anode electrode 7a is
b and the water supply path 10b.

The distance between the cathode section 6 and the anode section 7 was set to 1 m, and water was filled between the cathode electrode 6a, the anode electrode 7a and each of the water-permeable members 8, and the liquid level was controlled to be constant. .
In this state, a DC voltage was applied between the cathode electrode 6a and the anode electrode 7a to supply a current of 1A. During this electrolytic treatment,
The water in the cathode part 6 is drained by a pump from the drainage channel 9a and supplied to the water purification device 11 while the purified water is supplied from the water purification device 11 through the water supply channel 10a, so that the water in the cathode part 6 is 0.5. Replace with a liter / min circulation rate,
The pH of the water in the cathode part 6 was maintained at about 6.8.

Further, inside the anode section 7, a 0.01N sodium chloride solution as a solvent was successively supplied from the solvent tank 12 through a water supply passage 10b. Further, in accordance with the replenishment of the solvent, the water between the anode electrode 7a and the water-permeable member 8 was drained from the drain passage 9b by a pump and supplied to a water purification device (not shown).

When the above electrolytic treatment was performed for 15 days, the lead concentration in the soil was reduced to less than 300 mg / kg. Also, when the nitrate ion concentration in the soil was measured,
Nitrate ions were not detected.

[0026]

According to the present invention, a heavy metal-contaminated soil is subjected to a soil washing treatment with an acidic solvent and then an electrochemical treatment to remove hardly soluble or insoluble heavy metals and harmful ions in the contaminated soil. Thus, it is possible to efficiently remove the particles in a short time. In addition, in the electrochemical treatment, the pH of the cathode part is maintained at 7.5 or less, and the supply of the solvent from the anode part further reduces the treatment efficiency due to the precipitation of heavy metal ions and the decrease in soil moisture content. Prevention and economical purification of contaminated soil in even less time.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing first and second steps of the method of the present invention.

FIG. 2 is a schematic sectional view showing a specific example of an electrolytic treatment apparatus used in the method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Washing tank 2 Stirrer 3 pH meter 4 Solvent tank 5 Electrolysis tank 6 Cathode part 6a Cathode electrode 7 Anode part 7a Anode electrode 8 Water-permeable member 9a, 9b Drainage channel 10a, 10b Water supply channel 11 Water purification device 12 Solvent tank

Claims (2)

[Claims] 1. A first step of mixing an acidic solvent with an excavated contaminated soil to extract a heavy metal as a solvent, and a cathode section in which an electrode is separated from the contaminated soil by a water-permeable member on the contaminated soil after the solvent extraction treatment. A second step of arranging an anode part, applying a DC voltage between the electrodes, and moving and collecting ions such as heavy metals by electrochemical action. In the second step, water in the cathode part is removed. Draining water and supplying clean water into the cathode section to maintain the pH in the cathode section at 7.5 or less. 2. The method according to claim 1, wherein a solvent is supplied into the anode in the second step.