JPH11253924A - Purification of soil polluted with heavy metal and electrolytic bath for purification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To purify/restore polluted soil by a method in which soil polluted with heavy metals which was subjected to solvent extraction is directly subjected to electrochemical treatment without being dewatered, and the heavy metals are removed economically at high treatment efficiency. SOLUTION: Polluted soil which was subjected to extraction with an acidic solvent is placed into a filter medium 2 arranged in an electrolytic cell 1, a cathode 3a arranged on the lower side of the electrolytic cell 1 is separated from the polluted soil to form a cathode part, soil particles in the polluted soil is precipitated, water is introduced from the filter medium 2 into the cathode part 3, and while water in the cathode part 3 being discharged, direct current voltage is applied between the cathode 3a and an anode 4a to perform electrochemical treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention 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, in particular, from heavy metal contaminated soil in fine-grained parts having a high buffer capacity, and a purification method used in the method. It relates to an electrolytic cell.

[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.

Further, in the above-mentioned electrochemical treatment method, especially in the case of contaminated soil contaminated with heavy metals which are hardly soluble or insoluble in water, the soil is degraded due to the high buffering power and low conductivity of the soil. There was a problem that the time required for restoration was prolonged.

Therefore, it is conceivable that after performing the solvent extraction treatment by the soil washing method, the contaminated soil after the solvent extraction treatment is further treated by the electrochemical treatment method. According to this method, the solvent extraction treatment in the first step allows the p
It is possible to lower H and increase the conductivity of the soil. Therefore, when the electrochemical treatment is performed as the second step, the heavy metal ions dissolved and extracted in the contaminated soil move toward the cathode by the iontophoretic action, so that the adsorption of the heavy metal ions to the soil surface can be prevented. .

However, in the solvent extraction treatment, an acidic solvent is added to the contaminated soil and the mixture is stirred and mixed. Therefore, the contaminated soil after the treatment is muddy having a large water content of 100% or more.
If the electrochemical treatment is performed as it is, the soil weight per volume is extremely small, and the amount of soil treated per unit time is reduced, resulting in a disadvantage that the treatment efficiency is extremely poor. Further, a mechanical dehydration step can be provided between the extraction solvent treatment and the electrochemical treatment. However, in that case, extra steps and energy consumption are increased, and thus there is a problem that economic efficiency is impaired.

The present invention has been made in view of such a conventional situation,
The heavy metal-contaminated soil that has been subjected to solvent extraction can be subjected to electrochemical treatment without performing extra dehydration treatment, increasing the amount of soil treatment per unit time and increasing the efficiency of economical heavy metal-contaminated soil. It is an object of the present invention to provide a purification method and an apparatus therefor.

[0012]

In order to achieve the above object, a method for purifying heavy metal contaminated soil provided by the present invention comprises a first step of mixing an excavated contaminated soil with an acidic solvent and extracting heavy metal with a solvent. A second step of applying a DC voltage between electrodes arranged in an electrolytic cell containing the contaminated soil after the solvent extraction treatment, and moving and collecting ions such as heavy metals by electrochemical action. A filter material for putting the contaminated soil after the solvent extraction treatment is arranged in the electrolytic cell of the second step, and the cathode electrode disposed below the electrolytic cell is separated from the contaminated soil by the filter material to form a cathode portion. A method in which a DC voltage is applied between a cathode electrode and an anode electrode while sedimenting soil particles in the contaminated soil and allowing water to flow from the filter medium into the cathode portion, and discharging water from the cathode portion. I do.

The electrolytic cell for purifying heavy metal-contaminated soil used in the above-described method for purifying heavy metal-contaminated soil is provided with a filter material for containing the contaminated soil after the solvent extraction treatment. Forming a cathode portion separated from the contaminated soil, comprising a cathode electrode disposed in the cathode portion, an anode electrode disposed to face the cathode electrode, and a drainage channel for draining water in the cathode portion. I do.

In a preferred form of the purifying electrolytic cell, a cathode electrode is arranged along a side wall portion below the electrolytic cell,
An anode electrode is disposed in the center of the lower part of the electrolytic cell so as to face the cathode electrode, and a second filtering material is provided for separating the anode electrode from the contaminated soil to form an anode portion. In this electrolytic cell, a water supply channel for supplying clean water into the cathode portion,
It is preferable to provide a water supply / drainage channel for supplying a solvent into the anode section or draining water in the anode section.

In another preferred embodiment of the purifying electrolytic cell, a cathode electrode is arranged along a bottom wall portion below the electrolytic cell, and an anode electrode having a plurality of through holes is provided substantially in the vertical direction of the electrolytic cell. It is arranged at the center so as to face the cathode electrode. In this electrolytic cell, it is preferable to provide a water supply means for supplying clean water or a solvent above the electrolytic cell.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION In the solvent extraction treatment of the first step, soil contaminated with heavy metals is excavated with a shovel or the like, transferred to a solvent extraction tank, added with an acidic solvent, and stirred and washed.
H is lowered to provide a soil atmosphere in which heavy metals are easily eluted. By this treatment, heavy metals in the contaminated soil are dissolved and extracted in the acidic solvent, so that the time required for the electrochemical treatment in the subsequent second step can be reduced. The type and concentration of the acidic solvent added to the contaminated soil depend on the contaminant in the contaminated soil and its concentration. Usually, nitric acid or hydrochloric acid can be used as the acidic solvent, and the pH of the contaminated soil in the solvent extraction tank is 2 or less.
What is necessary is just to add so that it may be set to ~ 3 or less.

The contaminated soil subjected to the solvent extraction treatment is transferred from the solvent extraction tank to the electrolytic cell in the second step as it is. In the electrolytic cell, a filtering material is arranged at least along the side wall portion and, if necessary, along the bottom wall portion, and the contaminated soil after the solvent extraction treatment is put in the filtering material. . In addition, a cathode electrode and an anode electrode are arranged opposite to each other below the electrolytic cell (substantially from the center in the vertical direction), and at least the cathode electrode is arranged in a cathode portion formed separately from the contaminated soil by a filtering material. Have been.

In the electrochemical treatment of the second step, a dc voltage is applied between the anode electrode and the cathode electrode to energize, and cations such as heavy metals are removed by the electroosmotic action and the iontophoretic action. And anions move to the anode part at the same time. Soil water in contaminated soil generally moves to the cathode side due to electroosmosis because the soil surface is negatively charged. Therefore, by draining the water in the cathode part, contaminants such as concentrated heavy metals can be recovered and removed, and harmful anions can be similarly recovered from the anode part. The drainage from the anode section need not always be performed continuously, but may be performed as needed or at regular intervals.

In the present invention, since the contaminated soil after the solvent extraction treatment is directly put into the filter medium of the electrolytic cell without dehydration, the soil particles in the muddy contaminated soil naturally settle due to gravity difference, It gradually separates into a solid phase and a liquid phase. At the same time, excess water in the muddy contaminated soil flows into the cathode portion through the filter medium due to gravity. Therefore, since the liquid phase gradually lowers due to drainage from the cathode part, a constant liquid surface is maintained at least in the soil electrolysis part below the electrolytic tank by supplying water as necessary.

In particular, by disposing no electrode in the solid-liquid separation section where the liquid phase is formed on the upper side of the electrolytic cell, but disposing the electrode only in the lower soil electrolytic section, Since an electrode reaction occurs only in the electrolytic section, the applied current can be minimized. In addition, since excess water in the contaminated soil is naturally dehydrated in the cathode portion, there is no need to dehydrate the contaminated soil to be subjected to the second step and adjust the water content. Furthermore,
Since the step of adjusting the water content can be omitted, the degree of freedom of the solid-liquid ratio that can be employed in the solvent extraction treatment of the first step increases, the type of applicable soil increases, the viscosity of the soil decreases, and The load can be reduced.

Moreover, in the electrochemical treatment of the second step,
Moisture is required for ions and the like to move in the soil by electroosmosis, but the excess water in the contaminated soil after the solvent extraction treatment in the first step can be effectively used without dehydrating. . If excess water in the contaminated soil is drained through the filter medium and the water in the contaminated soil is insufficient due to drainage from the cathode, replenish clean water or solvent sequentially from the cathode and anode. do it. In this case, an acid, an electrolyte, or an aqueous solution of a chelating agent can be used as a solvent to be replenished in this case.

When a voltage is applied between the electrodes, hydroxide ions are generated by the electrolysis of water, and the pH around the cathode is reduced.
As a result, the heavy metal ions that have migrated to the cathode part precipitate and precipitate, and the removal efficiency tends to decrease. On the other hand, precipitation of heavy metal ions can be prevented by absorbing the clean water simultaneously with the drainage in the cathode part and maintaining the pH of the water in the cathode part at 7.5 or less.

As the anode electrode, an inert electrode material such as carbon or titanium is preferable. The cathode electrode may be any conductive material, and usually iron or stainless steel can be used. Further, the filtering material is preferably a water-permeable material through which water or heavy metal ions can pass, and is preferably made of an insulating material that is easily electrically insulated from the electrode. For example, a nonwoven fabric or a porous plastic is preferably used. can give.

The heavy metal contaminants to be removed by the present invention include lead, cadmium, chromium, and the like. In particular, the present invention is suitable for treating contamination by substances that are hardly soluble or insoluble in water. In addition, the particle size of the contaminated soil is not limited, but is particularly effective for fine granularity of 75 μm or less, which has a low mechanical dehydration effect, is concentrated in contaminants, and has a high soil buffering capacity, and is viscous. is there.

Next, a preferred embodiment of the electrolytic cell used for the electrochemical treatment in the second step will be described. The electrolytic cell 1 of FIG. 1 is entirely composed of two parts, a lower soil electrolytic part 1a and an upper solid-liquid separation part 1b, and a filter medium 2 for putting contaminated soil therein is disposed inside. A cathode electrode 3 is provided along the side wall of the electrolytic cell 1 on the soil electrolytic section 1a below the electrolytic cell 1.
a, and an anode electrode 4a is vertically arranged at the center of the electrolytic cell 1 so as to face the cathode electrode 3a.

The cathode electrode 3a is plate-shaped, and a cathode portion 3 is formed between the filtering material 2 and the side wall of the electrolytic cell 1 so as to include the cathode electrode 3a, and the cathode portion 3 drains water inside. Connected to the drainage channel 5. The water drained from the drainage channel 5 is supplied to a separately provided water purification device to remove contained pollutants such as heavy metals. In addition, a water supply channel (not shown) for supplying clean water or a solvent to the inside of the cathode portion 3 can be provided in the cathode portion 3 separately from the drainage channel 5. In this case, a plurality of through holes are provided in the plate-shaped cathode electrode 3a, and a water supply passage is provided between the cathode electrode 3a and the side wall of the electrolytic cell 1, and a water supply passage is provided between the cathode electrode 3a and the filter medium 2. Can eliminate the effect of water supply on the flow of water in the soil.

The anode electrode 4a is formed of a hollow tube, and a second filter material 6 for separating the anode electrode 4a from the contaminated soil.
Thereby, the anode part 4 is formed inside. The anode electrode 4a may be disposed only below the electrolytic cell 1, but is provided so as to protrude above the electrolytic cell 1 as shown in FIG. The hollow tube 4 of the anode electrode 4a can be used as a water supply / drainage channel by connecting to a water supply / drainage channel for draining water. In this case, the entirety of the anode electrode 4a may be a current-carrying portion, but the upper portion located at the solid-liquid separation portion 1b is preferably electrically insulative.

As another preferred form of the purifying electrolytic cell,
There is one shown in FIG. In this electrolytic cell 11, a filter material 12 for putting contaminated soil inside is disposed horizontally along the side wall and the bottom wall of the electrolytic cell 11, and is disposed in a soil electrolytic section 11 a below the electrolytic cell 11. The anode electrode 13a and the cathode electrode 14a are configured to be vertically opposed. That is,
A cathode portion 13 is formed between a bottom wall portion of the electrolytic cell 11 and a filter medium 12 along the bottom wall portion, and a plate-like cathode electrode 3a having a plurality of through holes in the cathode portion 13 is substantially formed. It is arranged horizontally. An anode electrode 14a provided with a plurality of through-holes so that soil particles can pass therethrough is disposed substantially horizontally at a substantially vertical center of the electrolytic cell 11 so as to face the cathode electrode 13a.

A drain 15 for draining water in the cathode 13 is connected to the bottom of the electrolytic cell 11. The drained water is supplied to a separately provided water purification device to remove contained contaminants such as heavy metals. In addition, a water supply channel (not shown) for supplying clean water or a solvent to the cathode portion 3 can be provided in the cathode portion 13 separately from the drainage channel 15. Further, a water supply means (not shown) for replenishing the contaminated soil with clean water or a solvent is provided above the electrolytic cell 11.

When a cathode electrode and an anode electrode are vertically arranged in an electrolytic cell as shown in FIG. 2, the upper and lower relation of each electrode is not particularly limited, but contaminants such as heavy metals moved to the cathode electrode together with water. In order to form a drainage channel for draining water, it is preferable to dispose it near the bottom wall of the electrode tank with the cathode electrode 13a facing down.

[0031]

EXAMPLE A main contaminant was lead oxide, a contaminated soil having a lead concentration of about 2000 ppm was prepared, and 0.1 m ³ of the contaminated soil was filled in a solvent extraction tank. Next, 1 mol of nitric acid was added as an acidic solvent to 1 kg of the contaminated soil, and the solid-liquid ratio was set to 1: 2. In this state, the pH of the contaminated soil was 2.8, and the solvent was extracted by stirring the contaminated soil for 20 minutes using a stirrer. The contaminated soil subjected to the solvent extraction treatment was directly transferred to the electrolytic cell 1 shown in FIG. 1 without dehydration treatment.

In the electrolytic cell 1 of FIG. 1, the distance between the cathode electrode 3a and the anode electrode 4a was 1 m, and a DC voltage was applied between the electrodes 3a, 4a to supply a current of 1A. During this electrolytic treatment, the soil particles of the contaminated soil gradually settled downward, and excess water gradually flowed into the cathode part 3 and the anode part 4 through the filter medium 2. Pump the water in the cathode part 3 and the anode part 4 through the drainage channel 5 etc., and supply the clean water into the cathode part 3 in accordance with the drainage after the excess water in the contaminated soil is eliminated. Thus, the water in the cathode section 3 was replaced at a circulation rate of 0.5 liter / min to maintain the pH in the cathode section 3 at about 6.8 or less. The anode 4 was not supplied with water or solvent.

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.

[0034]

According to the present invention, a heavy metal-contaminated soil is subjected to a solvent extraction treatment with an acidic solvent and then to 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. Further, the contaminated soil after the solvent extraction treatment can be transferred to the electrolytic cell as it is, and the electrochemical treatment can be performed, the step of dehydrating excess water can be omitted, and the excess water can be effectively used. Therefore, the purification treatment of the heavy metal contaminated soil can be performed more efficiently and economically.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing a specific example of an electrolytic cell according to the present invention.

FIG. 2 is a schematic sectional view showing another specific example of the electrolytic cell according to the present invention.

[Explanation of symbols]

 1, 11 Electrolyzer 1a, 11a Soil electrode part 1b, 11b Solid-liquid separation part 2, 12 Filter material 3, 13 Cathode part 3a, 13a Cathode electrode 4, 14 Anode part 4a, 14a Anode electrode 5, 15 Drainage channel 6 2 filter media

Claims (6)

[Claims] 1. A first step of mixing an excavated contaminated soil with an acidic solvent to extract heavy metals as a solvent, and a step between a cathode electrode and an anode electrode arranged in an electrolytic cell containing the contaminated soil after the solvent extraction treatment. A second step of applying a DC voltage to the cells to move and collect ions such as heavy metals by an electrochemical action, and filtering the contaminated soil after the solvent extraction treatment into the electrolytic cell of the second step. The cathode material is disposed, and the cathode electrode disposed below the electrolytic cell is separated from the contaminated soil by the filtration material to form a cathode portion, the soil particles in the contaminated soil are settled, and moisture is removed from the filtration material inside the cathode portion. A method for purifying heavy metal contaminated soil, comprising applying a DC voltage between a cathode electrode and an anode electrode while allowing water to flow into the cathode and discharging water from the cathode. 2. The electrode tank used in the second step according to claim 1, further comprising a filter material for containing the contaminated soil after the solvent extraction treatment, wherein the filter material is separated from the contaminated soil below the electrolytic cell by the filter material. Forming a cathode portion, comprising a cathode electrode disposed in the cathode portion, an anode electrode disposed on the lower side of the electrolytic tank opposite to the cathode electrode, and a drainage channel for draining water in the cathode portion. Electrolyzer for purification of soil contaminated with heavy metals. 3. The method according to claim 1, wherein the cathode electrode is disposed along a side wall portion below the electrolytic cell, and an anode electrode is disposed at a central portion below the electrolytic cell so as to face the cathode electrode. The electrolytic cell for purifying soil contaminated with heavy metals according to claim 2, further comprising a second filter medium that separates from the soil to form an anode portion. 4. The water supply system according to claim 3, further comprising a water supply passage for supplying clean water into the cathode portion, and a water supply and drainage passage for supplying a solvent into the anode portion or draining water from the anode portion. Electrolyzer for purification of soil contaminated with heavy metals. 5. The method according to claim 1, wherein the cathode electrode is disposed along a bottom wall portion below the electrolytic cell, and an anode electrode having a plurality of through holes is disposed substantially at the center in the vertical direction of the electrolytic cell so as to face the cathode electrode. The electrolytic cell for purifying heavy metal contaminated soil according to claim 2, wherein: 6. The electrolytic cell for purifying heavy metal contaminated soil according to claim 5, further comprising a water supply means for supplying clean water or a solvent above the electrolytic cell.