JPH0947748A - Polluted soil purifying method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To recover metal material from soil polluted with the metal material by arranging a pair of electrodes consisting of an anode and a cathode in polluted soil and applying DC voltage to between the electrodes and also feeding acidic solution to around the anode and then recovering the acidic solution from the cathode or from around it. SOLUTION: An anode 2 and a cathode 3 arranged opposite to each other in polluted soil 1 containing heavy metals M each are constituted of hollow pipes 5a, 5b formed of conductive material in which a lot of water passing holes 4 are made. A feeding pipe 6a and recovery pipe 6b are inserted into the hollow pipe 5a and into the hollow pipe 5b respectively. When DC voltage is applied between the anode 2a and the cathode 2b, metals M<2+> , M<3+> in the shape of a cation in soil start moving from the anode 2a to the cathode 2b side by electrophoresis. At the same time, acidic solution is fed into the hollow pipe 5a of the anode 2a through the feeding pipe 6a to move the metals to the cathode 2b as cations M<2+> , M<3+> , and is collected and recovered in the hollow pipe 5b through the passing holes 4, and is transferred to a water treatment facility to remove the heavy metals therein.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to cadmium, lead,
The present invention relates to a method for removing heavy metals from a contaminated soil containing heavy metals such as copper, zinc, nickel and chromium to purify the soil, and in particular, a method for purifying a large amount of contaminated soil in-situ without carrying it out. Regarding

[0002]

2. Description of the Related Art Soil contaminated with industrial wastewater, industrial waste, mining wastewater, etc. may contain heavy metals such as cadmium, lead, copper, zinc, nickel and chromium. If left untouched, there is a risk that heavy metals contained in the soil will diffuse into the environment through groundwater and biological cycles.

Therefore, the contaminated soil is excavated and removed, subjected to a predetermined treatment, and then disposed of at a management-type or blocking-type disposal site, while normal soil is excavated in the excavated hole. It is common to return to the original state after returning to the soil.

However, in such a method, there is a possibility that the contaminated soil may be disturbed during excavation to cause secondary pollution, and a large amount of the contaminated soil must be carried out and transported, and the proximity of existing buildings. There is a problem that the excavation and removal itself becomes difficult directly below. Therefore, recently, in-situ purification technology has begun to be researched, and one of them is a method of collecting pollutants by energization.
No. 6,086,045.

In the method, first, a water blocking wall is constructed in the ground area to be treated, and then an anode and a cathode consisting of hollow tubes having a large number of water passage holes are inserted in the ground area, and then, A DC voltage is applied between the electrodes after water is appropriately sprayed on the ground area, and then the water collected on the cathode side by the electroosmosis phenomenon is drained and recovered through the hollow tube.

According to such a method, the predetermined pollutant is
Since the water flows due to the electroosmosis phenomenon and flows into the cathode side, the pollutant can be removed by collecting the waste water.

[0007]

However, according to the detailed experiments conducted by the present applicant, the pollutants which do not cause a chemical change can be recovered by the above-mentioned method.
Heavy metals such as cadmium, lead, copper, zinc, nickel, and chromium lose their charge and precipitate as hydroxides when the soil pH changes from neutral to alkaline, and then supply water from the anode side. However, it was not possible to reach the cathode by electric attraction, and it was therefore impossible to collect these heavy metals on the cathode side.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for purifying a contaminated soil capable of recovering the metal substance from the soil contaminated with the metal substance.

[0009]

In order to achieve the above object, the method for purifying contaminated soil according to the present invention comprises, as described in claim 1, a pair of electrodes consisting of an anode and a cathode in contaminated soil containing metal. Then, a direct current voltage is applied between the electrodes and a predetermined acidic solution is supplied near the cathode, and then the acidic solution is recovered from the cathode or its vicinity.

Further, in the method for purifying contaminated soil according to the present invention, the cathode is arranged near the ground surface, and the anode is arranged at a predetermined depth position.

According to the third aspect of the present invention, there is provided a method for cleaning contaminated soil, wherein a pair of electrodes consisting of an anode and a cathode are arranged in a contaminated soil containing a metal, and then a direct current is applied between the electrodes. While applying a voltage, a predetermined acidic solution is supplied to the anode or its vicinity, and then the acidic solution is recovered from the cathode or its vicinity.

Further, according to the method for purifying contaminated soil of the present invention, as described in claim 4, a pair of electrodes consisting of an anode and a cathode are arranged in the contaminated soil containing metal, and then a direct current is applied between the electrodes. A voltage is applied and a predetermined acidic solution is supplied between the anode and the cathode, and then the acidic solution is recovered from the cathode or the vicinity thereof.

Further, the method for purifying contaminated soil according to the present invention comprises a hollow conductive tube having water passage holes for supplying or recovering the acidic solution through the hollow tube. Is.

In the method for cleaning contaminated soil according to the present invention,
First, certain metals, especially cadmium, lead, copper, zinc,
A pair of electrodes consisting of an anode and a cathode are arranged in a contaminated soil containing heavy metals such as nickel and chromium, and then a direct current voltage is applied between the electrodes and a predetermined acidic solution is supplied near the cathode. Then, the metal existing in the soil in the form of cations starts to move from the anode to the cathode side by electrophoresis, but since the acidic solution is neutralized with OH ⁻ near the cathode, it reacts with OH ^−. It moves to the cathode as cations without becoming hydroxide. Then, when the acidic solution is recovered from the cathode or its vicinity, the heavy metal is recovered in the form contained in the solution.

Here, when the cathode is arranged near the ground surface and the anode is arranged at a predetermined depth position, the heavy metal ions move upward and are collected together with the acidic solution near the ground surface. It Therefore, even if the soil cannot be completely recovered and remains in the soil and the soil needs to be excavated and removed, the range to be excavated is relatively shallow.

Further, in the method for cleaning contaminated soil according to the present invention, a pair of electrodes consisting of an anode and a cathode are arranged in the contaminated soil containing the metal, and then a DC voltage is applied between the electrodes. , A predetermined acidic solution is supplied to the anode or its vicinity. Then, the metal present in the soil in the form of cations starts to move from the anode to the cathode side by electrophoresis together with the acidic solution, but the acidic solution reaching the vicinity of the cathode neutralizes OH ⁻ . Therefore, the metal is
The cation remains as a cation without reacting with OH ⁻ to form a hydroxide, or the metal that has become a hydroxide again becomes a cation and moves to the cathode. Then, when the acidic solution is recovered from the cathode or its vicinity, the heavy metal is recovered in the form contained in the solution.

Further, in the method for purifying contaminated soil of the present invention, a pair of electrodes consisting of an anode and a cathode are arranged in the contaminated soil containing the metal described above, and then a DC voltage is applied between the electrodes. A predetermined acidic solution is supplied between the anode and the cathode. Then, the acid was supplied between the metal and the electrode present in the soil in the form of a cation solution is an anode by electrophoresis starts to move to the cathode side, an acidic solution that has reached the cathode vicinity, OH ^- medium to Harmonize for that reason,
The metal remains as a cation without reacting with OH ⁻ to form a hydroxide, or the metal that becomes a hydroxide becomes a cation again and moves to the cathode. Then, when the acidic solution is recovered from the cathode or its vicinity, the heavy metal is recovered in the form contained in the solution.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a method for purifying contaminated soil according to the present invention will be described below with reference to the accompanying drawings.

(First Embodiment) FIG. 1 (a) is a schematic view showing a situation in which the method for purifying contaminated soil according to the first embodiment is carried out. As can be seen in the figure, in the method for purifying contaminated soil according to the present embodiment, first, a predetermined metal, in particular, a heavy metal such as cadmium, lead, copper, zinc, nickel and chromium (indicated by M in the figure) is used. A pair of electrodes 2 in the contaminated soil 1 containing
Insert a and 2b and arrange them so as to face each other.

As shown in FIG. 2 (b), the electrode 2a is constructed by forming a large number of water passage holes 4 in a hollow tube 5a formed of a conductive material such as iron, and the electrode 2b is also the same. In addition, a large number of water passage holes 4 are formed in a hollow tube 5b made of a conductive material such as iron. A supply pipe 6a is inserted into the hollow pipe 5a so that the acidic solution can be supplied into the hollow pipe 5a through the supply pipe 6a. On the other hand, hollow tube 5
A recovery pipe 6b is inserted in b, and the acidic solution in the hollow pipe 5b can be recovered through the recovery pipe 6b.

Here, the acidic solution is, for example, pH.
It is possible to use hydrochloric acid, sulfuric acid, organic acid or the like having a pH of about 2, and these may be stored in a tank (not shown) installed on the ground.

Next, as shown in the figure, the positive side of the DC power source is connected to the electrode 2a (anode) and the negative side is connected to the electrode 2b (cathode), and a DC voltage is applied between the electrodes. Then, the metal M ²⁺ or M ³⁺ present in the soil in the form of cations is electrophoresed on the anode 2 as shown in FIG. 2 (a).
Starting to move from a to the cathode 2b side.

Simultaneously with the energizing operation, the acidic solution is supplied into the hollow tube 5a of the electrode 2a via the supply tube 6a. Then, the acidic solution diffuses into the contaminated soil 1 through the water passage holes 4 of the hollow tube 5a and moves to the cathode 2b side together with the metal ions by electrophoresis.

On the other hand, OH ⁻ is generated on the side of the cathode 2b, and when such OH ⁻ reacts with the metal M ²⁺ or M ³⁺ , it becomes hydroxide and stays in the soil.
It will not move to the b side. However, since the acidic solution that has moved to the vicinity of the cathode 2b with cations neutralizes OH ⁻ , the vicinity of the cathode 2b changes from an alkaline environment to a neutral or acidic environment. Therefore, the metal is the cation M ²⁺
Or, M ³⁺ as it is, or even hydroxide that has already become ionized again moves to the cathode 2 b, and the water passage hole 4
And gather in the hollow tube 5b through. FIG. 2 (b) is a graph showing the relationship between the solubility of heavy metals and pH, and shows that the solubility of heavy metals is extremely high in an acidic environment.

Next, the acidic solution in the hollow tube 5b is recovered by using a pump or the like (not shown), and then sent to a water treatment facility to remove heavy metals in the solution.

The supply of such an acidic solution and the energization are continuously performed for a predetermined time, and the heavy metals in the contaminated soil 1 are recovered. If the heavy metal that cannot be collected remains in the vicinity of the cathode 2b, it may be appropriately excavated and removed with a backhoe or a shovel.

Next, the pH distribution in the soil, the accumulation state of heavy metals, and the like in the case where the acidic solution was not supplied were examined by experiments, which will be described below.

FIG. 3 is a perspective view of the experimental apparatus 11. As can be seen from the figure, the experimental apparatus 11 has a container 15 having a width of 15 cm and a length of about 100 cm, and the contaminated soil 1 containing heavy metal
6 and put electrodes 13 and 14 on both ends of the contaminated soil 16.
Is provided and the electrodes 13 and 14 are connected to the positive side and the negative side of the DC power source 12, respectively, so that a DC voltage of about 25 V can be applied. In addition, in order to ensure the conductivity of the soil, water is appropriately sprayed on the contaminated soil 16,
A drain port 18 for draining the sprinkled water is attached to the side of the container 15. Further, silica sand 17 is put in the vicinity where the electrodes 13 and 14 are inserted.

FIG. 4 is one of the experimental results obtained by the experimental apparatus 11, and is a graph showing how the pH of the soil changes depending on the distance from the anode 13 with the energization time as a parameter.

As can be seen from the figure, the pH of the soil is almost constant at any position until the energization time is up to about 2 days. On the other hand, when the energization time is 7 days, the acidity tends to be strong at a position close to the anode, but immediately after a short distance from the anode, it changes to neutrality, and conversely, it rapidly changes to alkalinity near the cathode. When the energization time is 15 days, the acidified region near the anode expands more than that when the energization time is 7 days,
It changes to neutral from around cm. And 8
Around 0 cm, it rapidly changes to alkaline. Even if the energization time is extended to 30 days, the overall tendency is not much different from that of 15 days, but the acidified region near the anode is further expanded. From the experimental results, the energization time is 30
In the case of a day, it is suggested that up to about 40 cm from the anode, it is in an acidic state, that is, the heavy metal is dissolved in the form of a cation and easily moved by electrophoresis.

FIG. 5 is a graph showing the amount of heavy metals contained in the contaminated soil 16 with copper as an index. From this figure, in the state where current is not applied yet (dotted line), copper is almost evenly distributed in the soil regardless of the position from the anode, but when the current is applied for 30 days (solid line), the anode is From 50
In the range up to cm, particularly in the range up to about 20 cm from the anode, the content is about one-tenth of that in the case of not energizing, and 60 c from the anode.
On the contrary, it can be seen that the content is about three times as large as m.

This is because the copper initially present in the vicinity of the anode moved to the cathode side by energization, and the pH of the soil gradually changed from acidic to neutral and further changed to alkaline, and gradually precipitated as hydroxide. , Can be considered to be concentrated in the area. If the energizing time is further increased,
The accumulation position moves a little further to the cathode side, the purification range on the anode side further expands, and the rise of the curve becomes sharper.

These experimental results do not directly support the effect of supplying the acidic solution, but if the current is supplied while the acidic solution is supplied, the heavy metal does not accumulate as a hydroxide and accumulate. It can be a sufficient basis for deciding that it will move to the cathode.

As described above, according to the method for purifying contaminated soil of the present embodiment, the electrodes are arranged in the contaminated soil to conduct electricity, and the acidic solution is supplied from the anode and recovered from the cathode. Therefore, while preventing the heavy metal contained in the contaminated soil from becoming a hydroxide in the vicinity of the cathode, the heavy metal can be moved to the cathode by energization and recovered together with the acidic solution.

Therefore, it is possible to recover only heavy metals from the contaminated soil without excavating and removing the contaminated soil, and not only excavation work becomes unnecessary, but also the trouble of carrying out the excavated earth and sand by a dump or the like and after that. Can be omitted.

Further, since the conductive hollow tube having the water passage hole is used as the electrode and the acidic solution is supplied and recovered through the water passage hole, the electrode and the hollow tube are separated from each other. The installation work in the soil becomes easier than the case.

Further, since the acidic solution is supplied from the anode side, when the vicinity of the cathode is reached, the O near the cathode is discharged.
It not only plays a role of neutralizing H ⁻ , but also plays a role of securing conductivity in the contaminated soil when it is not secured sufficiently.

In this embodiment, heavy metals such as cadmium, lead, copper, zinc, nickel and chromium are used as the metals, but aluminum, magnesium,
It goes without saying that metals such as calcium, titanium, manganese, iron, cobalt, gallium, molybdenum, silver, tin and bismuth can also be recovered and removed by using the cleaning method of the present embodiment.

In this embodiment, the acidic solution is supplied from the anode side. However, as can be seen from the above experiment, the anode side is in an environment where the metal is likely to exist in an ionic state. Therefore, as shown in FIG. 6 (a), the anode 2a
From the above, only water for ensuring conductivity in the soil may be supplied, and the acidic solution may be supplied to the vicinity of the cathode 2b by a method of spraying water from the ground, for example. According to this configuration,
The acidic solution effectively contributes to neutralization of OH ⁻ without diffusing to the surroundings, and therefore, the supply amount of the acidic solution can be saved.

If the contaminated soil 1 is, for example, below the groundwater level and electricity can be ensured without any treatment such as water supply or water sprinkling, the anode will be as shown in FIG.
As shown in (b), the structure may be made of a conductive bar member 21 and the water supply function may be omitted.

The acidic solution may be supplied between the anode and the cathode. In such a configuration, it is positioned as an intermediate use form between the case of supplying the acidic solution to the cathode and the case of supplying it to the anode, and it is possible to secure electric conductivity in the soil and save the amount of the acidic solution used to some extent. Can be achieved.

Although not particularly shown, the hollow tube for supply or recovery does not necessarily have to serve as the electrode, and the hollow tube is made of a non-conductive material, and the electrode is separately made of iron or carbon. You may make it comprised with a stick.

(Second Embodiment) FIG. 7A shows an electrode arrangement state according to this embodiment. As can be seen from the figure, also in the present embodiment, similarly to the first embodiment, the electrodes 32, 33 are arranged in the contaminated soil 1 containing the heavy metal M, and the positive side of the DC power source is the electrode 32 and the negative side is the same. Electrode 3
3, but the electrode 33 is arranged near the ground surface, and the electrode 32 is arranged at a predetermined depth position. Also, the electrode 32
Is made of iron, carbon rod, etc., and the electrode 33 is the electrode 2b.
Similarly to the above, the hollow pipe 5b made of a conductive material is formed with a large number of water passage holes 4, and the recovery pipe 6b is provided therein.
Has been inserted.

It should be noted here that the current density varies depending on the position in the contaminated soil 1. That is,
The current density at the point a shown in FIG. 7 (b) is smaller than that at the point b, and therefore, it is considered that the migration effect by electrophoresis cannot be expected so much for the heavy metal existing at the point a. Therefore, it is preferable to install the electrode 32 at a position which is lower than the lower limit of the range to be actually processed by a certain degree. Specifically, in consideration of these things, the electrode 3
2 are arranged at a depth of about 1 to 2 m from the ground surface, for example, and are arranged at a pitch of about 0.5 to 1 m in the horizontal direction.

When disposing the electrodes 32 and 33,
As shown in FIG. 8, first, the contaminated soil 1 is excavated by a backhoe or the like to form a trench 41, then the electrode 32 is dropped into the trench and covered with soil 42, and then the electrode 33 is placed near the ground surface. It may be arranged and covered with soil 43.

When the electrodes 32 and 33 are arranged, a DC voltage is applied between these electrodes. Then, the metal M ²⁺ or M ³⁺ existing in the soil in the form of cations starts to rise from the anode 32 to the cathode 33 side by electrophoresis as shown in FIG. 7.

Simultaneously with the energizing work, the acidic solution described in the first embodiment is supplied from the ground by a method such as watering. Then, the acidic solution becomes the contaminated soil 1 near the cathode 33.
It diffuses in and neutralizes OH ⁻ generated on the cathode 33 side. Then, similarly to the case described in the first embodiment, the cathode 33
The metal that has changed from an alkaline environment to a neutral or acidic environment in the vicinity and moved to the vicinity of the cathode 33 remains as a cation M ²⁺ or M ³⁺ without becoming a hydroxide, or is already a hydroxide. Those that have become ionized again reach the cathode 33 and collect in the hollow tube 5b through the water passage hole 4.

Next, the acidic solution in the hollow tube 5b is recovered by using a pump or the like (not shown), and then sent to a water treatment facility to remove heavy metals in the solution.

The supply of such an acidic solution and the energization are continuously performed for a predetermined time, and the heavy metal in the contaminated soil 1 is recovered. If the heavy metal that cannot be collected remains in the vicinity of the cathode 33, it may be appropriately excavated and removed by a backhoe or a shovel.

As described above, according to the method for cleaning contaminated soil of the present embodiment, it is possible to recover only heavy metals from the contaminated soil without excavating and removing the contaminated soil, as in the first embodiment. In addition to this, even if heavy metal that could not be collected remains, it is possible to excavate and remove it easily because the remaining place is limited to the shallow part near the ground surface. Moreover, the amount of excavated soil is very small.

Although not particularly mentioned in the present embodiment, a wide variety of applicable metals are the same as in the first embodiment.

Further, in this embodiment, the acidic solution is supplied from the ground by a method such as water sprinkling. However, as shown in FIG. 9, a hollow tube 51 similar to the hollow tube 5b is provided near the ground surface. It may be embedded and supplied to the vicinity of the cathode 33 through the hollow tube 51. Even in this configuration, the cathode 3
OH ⁻ in the vicinity of 3 is neutralized, and the metal that has moved to the vicinity of the cathode 33 remains as the cation M ²⁺ or M ³⁺ , or the metal that has already become a hydroxide is ionized again and the cathode 3
3 through the water passage hole 4 and collect in the hollow tube 5b of the cathode 33.

(Third Embodiment) In the second embodiment, the long electrodes 32 and 33 are arranged horizontally, but the present invention is not limited to such a configuration, and the point is that
The cathode may be arranged near the ground surface and the anode may be arranged in the ground.

FIG. 10 shows the arrangement of electrodes in this embodiment. In the present embodiment, as can be seen in the figure, an electrode 62 is attached to the tip of a pile 61 formed of concrete or the like, and an electrode 63 is attached to the head, and these are arranged in a grid pattern in the contaminated soil 1 at a predetermined pitch. The electrodes 62 and 63 are both embedded and connected to the positive and negative sides of a DC power source, respectively. Further, the cathode 63 is
As shown in FIG. 11, a water passage hole 4 is bored to form a hollow structure, and a recovery pipe 6b is inserted inside the hollow structure.

In this embodiment, as in the second embodiment, a DC voltage is applied between the electrodes 62 and 63 to raise the metal M ²⁺ or M ³⁺ from the anode 62 to the cathode 63 side by electrophoresis. The acidic solution is supplied from the ground to the vicinity of the cathode 63 by a method such as sprinkling water. Then, the metal that has moved to the vicinity of the cathode 63 remains as the cation M ²⁺ or M ³⁺ , or the metal that has already become a hydroxide is ionized again to reach the cathode 63 and pass through the water passage hole 4. It is possible to collect the acid solution inside the hollow together with the heavy metal by using a pump (not shown) or the like.

In this embodiment as well, substantially the same effect as in the second embodiment is obtained, and in addition, the electrodes are automatically arranged by driving the piles, and the electrode arrangement work is facilitated.

[0057]

As described above, the method for purifying contaminated soil according to the present invention comprises disposing a pair of electrodes consisting of an anode and a cathode in contaminated soil containing metal, and then applying a DC voltage between the electrodes. Since a predetermined acidic solution is supplied to the vicinity of the cathode while being applied and then the acidic solution is recovered from the cathode or the vicinity thereof, it is possible to recover only the metal without excavating and removing the contaminated soil. In addition, the amount of the acidic solution used can be greatly saved.

In the method for cleaning contaminated soil according to the present invention, a pair of electrodes consisting of an anode and a cathode are arranged in contaminated soil containing a metal, and then a DC voltage is applied between the electrodes and the anode or Since a predetermined acidic solution is supplied to the vicinity of the cathode and then the acidic solution is recovered from the cathode or the vicinity thereof, only the metal can be recovered without excavating and removing the contaminated soil, and the conductivity in the soil can be improved. There is also an effect that it is possible to secure.

In the method for purifying contaminated soil according to the present invention, a pair of electrodes consisting of an anode and a cathode are arranged in a contaminated soil containing a metal, and then a direct current voltage is applied between the electrodes and the anode and the anode. A predetermined acidic solution is supplied between the cathode and the cathode, and then the acidic solution is recovered from the cathode or the vicinity thereof, so that only metal can be recovered without excavating and removing contaminated soil. It is possible to achieve a certain amount of saving of the amount of use and ensuring of electrical conductivity.

[0060]

[Brief description of drawings]

1A and 1B are diagrams showing an implementation situation of a method for purifying contaminated soil according to the present embodiment, in which FIG. 1A is an overall schematic view, and FIG. 1B is a side view showing in detail an electrode structure.

FIG. 2 is an explanatory view showing the operation of the present embodiment, (a)
Is a diagram showing the movement of metal ions, and (b) is a graph showing the relationship between the solubility of metal ions and pH.

FIG. 3 is a perspective view showing an apparatus on which an experiment related to the present embodiment was conducted.

FIG. 4 is a graph showing the experimental results obtained by the experimental apparatus shown in FIG. 3 and showing how the pH of the soil changes depending on the distance from the anode, with the energization time as a parameter.

FIG. 5 is a graph in which the amount of heavy metals contained in contaminated soil is drawn using copper as an index.

FIG. 6 is a schematic diagram showing a modified example of the first embodiment.

FIG. 7 is a cross-sectional view showing how electrodes are arranged in the second embodiment.

FIG. 8 is a cross-sectional view showing a construction procedure for arranging electrodes in the second embodiment.

FIG. 9 is a sectional view showing a modified example of the second embodiment and its operation.

10A and 10B are diagrams showing an electrode arrangement state in a third embodiment, FIG. 10A is a plan view, and FIG. 10B is a sectional view taken along line AA.

FIG. 11 is a detailed sectional view of an electrode used in the third embodiment.

[Explanation of symbols]

 1 Contaminated soil 2a, 2b Electrode 4 Water passage hole 5a, 5b Hollow tube 21 Electrode 32, 33 Electrode 51 Hollow tube 62, 63 Electrode

Claims (5)

[Claims] 1. A pair of electrodes consisting of an anode and a cathode are arranged in a contaminated soil containing a metal, and then a direct current voltage is applied between the electrodes and a predetermined acidic solution is supplied in the vicinity of the cathode. A method for purifying contaminated soil, comprising recovering the acidic solution from the cathode or the vicinity thereof. 2. The method for purifying contaminated soil according to claim 1, wherein the cathode is provided near the ground surface, and the anode is provided at a predetermined depth position. 3. A pair of electrodes consisting of an anode and a cathode are arranged in a contaminated soil containing metal, and then a direct current voltage is applied between the electrodes and a predetermined acidic solution is supplied to the anode or its vicinity. Then, the method for purifying contaminated soil, which comprises recovering the acidic solution from the cathode or its vicinity. 4. A pair of electrodes consisting of an anode and a cathode are arranged in a contaminated soil containing a metal, and then a direct current voltage is applied between the electrodes and a predetermined acidic solution is provided between the anode and the cathode. And then recovering the acidic solution from the cathode or the vicinity thereof, the method for purifying contaminated soil. 5. The electrode according to claim 1, wherein the electrode is formed of a conductive hollow tube having a water passage hole, and the acidic solution is supplied or recovered through the hollow tube. The method for cleaning contaminated soil according to.