JP3214600B2 - How to clean contaminated soil - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to cadmium, lead,
The present invention relates to a method for removing contaminated soil containing heavy metals such as copper, zinc, nickel, and chromium by removing these heavy metals, and in particular, a method for purifying a large amount of contaminated soil in situ without carrying it out. About.

[0002]

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

For this reason, the contaminated soil is excavated and removed and subjected to a predetermined treatment. Thereafter, the contaminated soil is disposed of in a management type or cut-off type disposal site. It is common to return to the original state on the land.

[0004] However, such a method has a problem that contaminated soil is disturbed during excavation, which may cause secondary pollution. In addition, a large amount of contaminated soil must be carried out and transported. The problem that excavation removal itself becomes difficult directly underneath occurs. Therefore, recently, in-situ purification technology has begun to be studied, and as one of the methods, a method of recovering contaminants by energization has been disclosed in Japanese Patent Laid-Open No. 5-59716.
No. 6,086,045.

[0005] In the method, first, a water blocking wall is constructed in a ground area to be treated, and then an anode and a cathode each formed of a hollow tube having a large number of water passage holes are inserted into the ground area, A DC voltage is applied between the electrodes after water is appropriately sprayed on the ground area, and then water collected on the cathode side by an electroosmosis phenomenon is drained and collected through a hollow tube.

According to such a method, the predetermined contaminants are:
The contaminants can be removed by collecting the waste water by flowing into the cathode side by riding on the flow of water caused by the electroosmosis phenomenon.

[0007]

However, according to a detailed experiment conducted by the present applicant, pollutants that do not cause a chemical change can be recovered by the above-described 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 water is supplied from the anode side However, it was found that it was impossible to reach the cathode by electric attraction, and it was impossible to recover these heavy metals on the cathode side.

The present invention has been made in view of the above circumstances, and has as its object to provide a method of purifying contaminated soil in which the metal substance can be recovered in situ as much as possible from the soil contaminated with the metal substance. Aim.

[0009]

In order to achieve the above object, a method for purifying contaminated soil according to the present invention comprises a step of inserting a pair of electrodes into contaminated soil containing metal, as described in claim 1. Applying a DC voltage between the electrodes to move the metal to one of the electrodes; and precipitating and concentrating the metal in the form of a hydroxide in a metal accumulation region located between the electrodes. And after a predetermined period has passed, including a step of excavating and removing the metal accumulation region,
An electrode on the side for moving the metal is disposed near the ground surface,
The other electrode is disposed underground.

In the method for purifying contaminated soil according to the present invention, the metal is moved to the cathode side of the electrode in the moving step.

According to a third aspect of the present invention, there is provided a method for purifying contaminated soil, comprising the steps of: inserting a pair of electrodes into contaminated soil containing metal; and connecting a predetermined DC power source to the electrodes. Applying a DC voltage to move the metal to one of the electrodes; precipitating and concentrating the metal in the form of a hydroxide in a metal accumulation region located between the electrodes; Thereafter, a step of excavating and removing the metal accumulation region, wherein the electrode is relocated while the formation position of the metal accumulation region is kept constant, and the connection of the DC power supply is performed. The moving step and the concentration step are repeated in the opposite direction, and the metal contained in the soil between the electrodes is accumulated in the metal accumulation area.

Further, in the method for purifying contaminated soil according to the present invention, in the moving step, the metal is moved to a cathode side of the electrode.

Further, the method for purifying contaminated soil according to the present invention includes a step of supplying water to the contaminated soil before or during the application of a DC voltage to the electrode.

Further, in the method for purifying contaminated soil according to the present invention, the water supply step is performed through a water passage hole of a hollow tube constituting the electrode.

Further, in the method for purifying contaminated soil according to the present invention, the water supply step is performed by using water collected through a water passage hole of a hollow tube constituting the electrode.

Further, in the method for purifying contaminated soil according to the present invention, the water supply step is performed using a predetermined conductive aqueous solution.

The method for purifying contaminated soil according to the present invention includes a step of constructing a predetermined impermeable wall before the step of supplying water.

In the method for purifying contaminated soil according to the present invention,
First, certain metals, especially cadmium, lead, copper, zinc,
A pair of electrodes is inserted into contaminated soil containing heavy metals such as nickel and chromium. Next, a DC voltage is applied between the electrodes.

Then, for example, metals existing in the soil in the form of cations start to move from the anode to the cathode by electrophoresis.

Next, the metal is precipitated in the form of a hydroxide in a predetermined metal accumulation region located between the electrodes, and the metal is concentrated. Experiments have shown that the metal accumulation region is formed when the soil pH changes from acidic to neutral and changes to alkaline, so the desired soil position to be excavated and removed later is described above. so that the pH value
The voltage, the electrode insertion interval, the energizing time, and the like are appropriately adjusted.

Next, after a lapse of a predetermined period, the metal accumulation region is excavated and removed. It is preferable that a predetermined soil is put in the excavated and removed hole to return to the original state.

Here, when the electrodes are relocated while maintaining the formation position of the metal accumulation region constant, a wide range of contaminated soil can be purified.

When one of the electrodes is arranged near the ground surface for moving the metal and the other electrode is arranged at a predetermined depth position, the metal is located near the ground surface. Therefore, it is easy to perform excavation and removal.

Further, when water is appropriately supplied to the contaminated soil before or during the application of the DC voltage to the electrode, for example, in a place where the groundwater level is low and the soil cannot be maintained as it is. Also, a predetermined electrical conductivity can be secured.

When the supply of the water is performed through the water passage hole of the hollow tube constituting the electrode, the water can be efficiently replenished.

When the above-mentioned water is supplied by using the water collected through the water holes of the hollow tube constituting the electrode, the water can be recycled.

When the above-mentioned water supply is performed using a predetermined conductive aqueous solution, the conductivity of the soil can be reliably maintained and the recovery efficiency of heavy metals can be increased.

Further, when a predetermined impermeable wall is constructed before the above-described water supply step, it is possible to prevent the supplied water from dispersing to the surrounding ground and lowering the conductivity of the soil on the target ground. Can be prevented.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for purifying contaminated soil according to the present invention will be described below with reference to the accompanying drawings. Note that the first embodiment is suitable for a case where sufficient electrical conductivity can be ensured even without performing water supply, watering, etc., such as soil below the groundwater level. It is suitable when water supply or the like is necessary to secure electrical conductivity as in the case of the soil above.

(First Embodiment) FIG. 1 is a flowchart showing a procedure of a method for purifying contaminated soil according to a first embodiment, and FIGS. 2 to 4 are views showing the state inside the contaminated soil in each step. FIG. In the method for purifying contaminated soil according to the present embodiment, first, as shown in FIG. 2 (a), a predetermined metal, in particular, a heavy metal such as cadmium, lead, copper, zinc, nickel, and chromium (M in FIG. (See FIG. 1, step 101).

The electrodes 2 and 3 are preferably made of, for example, a carbon material, and may have any shape such as a rod shape and a plate shape. The insertion depth of the electrodes 2 and 3 is appropriately adjusted according to the depth of the contaminated soil.

Next, as shown in FIG. 2B, the plus side of the DC power supply is connected to the electrode 2 and the minus side is connected to the electrode 3, and a DC voltage is applied between the electrodes (step 10 in FIG. 1).
2).

Then, on the anode side by water electrolysis,
H ⁺ and OH ⁻ ions are generated on the cathode side, so that the soil near the electrode 2 becomes acidic and the soil near the electrode 3 becomes alkaline. In the vicinity of the electrode 2 which has become acidic, most of the heavy metal M
Dissolves as cations M ²⁺ or M ³⁺ and starts to move toward the electrode 3 by electrophoresis. FIG. 5 is a graph showing the relationship between the solubility of heavy metals and pH, and shows that the solubility of heavy metals is very high in an acidic environment.

Next, such a metal ion is formed as shown in FIG.
As shown in FIG. 3, the metal integrated region 4 located between the electrode 2 and the electrode 3 has a hydroxide form, ie, M (OH) ₂ or M (O
H) Precipitate as ₃ and concentrate the metal (FIG. 1, step 103).

Experiments have revealed that there is a close relationship between the position where the metal accumulation region 4 is formed and the pH distribution of the soil 1.
The metal accumulation region 4 can be formed at a desired position by appropriately setting the energization time and the like and adjusting the pH value or the pH distribution.

Next, as shown in FIG. 3 (a), the electrode 2 is relocated (FIG. 1, step 105), and as shown in FIG. 3 (b), the area on the right side of the already purified area is described above. Purification is performed in the same procedure as described above (FIG. 1, steps 102 to 103).

Next, as shown in FIG. 3 (c), the same procedure is repeated with the connection of the DC power supply reversed so that the electrode 2 becomes the cathode and the electrode 3 becomes the anode, and FIG. As shown in (2), the right region of the metal accumulation region 4 is purified.

If the heavy metal contained in the soil between the electrodes 2 and 3 can be accumulated in the metal accumulation area 4 to purify the soil (step 104, YE
S), as shown in FIG. 4B, the electrodes 2 and 3 are removed, and the metal accumulation region 4 is excavated and removed with a backhoe or shovel (step 106). During excavation,
In order to prevent the backflow and diffusion of the heavy metal from the metal accumulation region 4, it is preferable to continue the energization.

Next, as shown in FIG. 4 (c), a predetermined soil that is not contaminated is buried in the excavated and removed hole, and the soil is restored to its original state (step 107). On the other hand, the excavated and removed soil is subjected to processing such as solidification of concrete and vaporization and recovery of heavy metal by heating, and is disposed at a predetermined place.

FIG. 6 is a perspective view showing an apparatus in which an experiment for confirming the effect of the method for purifying contaminated soil according to the present embodiment was performed. As can be seen from FIG.
A contaminated soil 16 containing heavy metals is placed in a container 15 having a length of about 100 cm and a length of about 100 cm.
3. An electrode 14 is provided, and the electrodes 13 and 14 are connected to the plus side and the minus side of the DC power supply 12, respectively, to apply a DC voltage of about 25 volts. In addition, water is appropriately sprinkled on the contaminated soil 16 in order to ensure electrical conductivity of the soil, but a drain port 18 for draining the sprinkled water is provided.
Is attached to the side of the container 15. Also, the electrodes 13,
A quartz sand 17 is put in the vicinity of the insertion of 14.

FIG. 7 shows one of the experimental results obtained by the experimental apparatus 11 shown in FIG. 6, and shows how the soil pH changes depending on the distance from the anode 13 using the energizing time as a parameter. It is a graph.

As can be seen from the drawing, the pH of the soil is almost constant at any position until the energizing time is about two days. On the other hand, when the energization time is 7 days, the acidity tends to be strong at the position near the anode, but changes to neutral as soon as it is slightly away from the anode, and conversely changes to alkaline near the cathode. When the energization time reaches 15 days, the acidified region near the anode expands more than when the energization time is 7 days, and the acidification region extends from the anode by 30 days.
It changes to neutral from around cm away. And 8
From around 0 cm, it rapidly changes to alkaline. Even if the energization time is extended for 30 days, the overall tendency is not so different from that for 15 days, but the acidified region near the anode is further expanded. Based on the results of this experiment, the energization time was set to 30
In the case of about a day, it is suggested that an area of about 40 cm from the anode is in an acidic state, that is, a state in which the heavy metal is dissolved in the form of a cation and easily moved by electrophoresis.

FIG. 8 is a graph depicting the amount of heavy metal contained in the contaminated soil 16 using copper as an index. From this figure, in the state where the power has not been supplied yet (dotted line), copper is almost evenly distributed in the soil regardless of the position from the anode. From 50
cm, in particular, in the range from the anode to about 20 cm, the content is about 1/10 smaller than when no current is supplied,
Conversely, it can be seen that the content is about three times higher than m.

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

As described above, according to the method for purifying contaminated soil according to the present embodiment, heavy metals contained in contaminated soil are moved by energization and concentrated in a predetermined metal accumulation area. Excavation and removal of the metal accumulation area makes it possible to purify a wide range of contaminated soil by excavating and removing a small amount of soil, and it is possible to remove heavy metals virtually in situ It can be said that it became. In addition, since the amount to be disposed is reduced, the trouble of solidifying with concrete or the like is greatly reduced.

Further, when the electrodes are relocated, the position of the electrodes is adjusted so that the metal accumulation area is maintained at a fixed location, so that heavy metals contained in a wide range of contaminated soil can be collected with a very limited metal accumulation. Can be concentrated in the area.

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

When the metal is present as an anion, for example, when chromium is present in the form of Cr ₂ O ₇ ²⁻ (bichromic acid), it is also possible to adopt a configuration in which the metal is accumulated on the anode side. It is.

Further, in this embodiment, the non-contaminated soil is put in the excavated and removed hole to return to the original state. However, the drainage channel may be constructed using the excavation hole without using the original state. Alternatively, a concrete substructure may be constructed.

Further, the present embodiment is suitable for a case where the electrical conductivity of the soil can be sufficiently secured without sprinkling or supplying water, as in the case of soil below the groundwater level. When it is necessary to improve the electric conductivity of the soil, the following embodiment may be applied.

(Second Embodiment) In this embodiment,
Instead of the electrodes 2 and 3 described in the first embodiment, electrodes 21 and 24 made of a hollow tube provided with a large number of water passage holes 22 and 25 as shown in FIG. 9 are used. And the electrodes 21, 2
Before or during the application of the DC voltage to the electrode 4, water is supplied into the soil through a water supply pipe 23 disposed in the electrode 21, and a drain pipe 26 disposed in the electrode 24.
Through the soil through the water. Note that other configurations and operations are almost the same as those of the first embodiment, and a detailed description thereof will be omitted.

According to this configuration, in addition to the effects described in the first embodiment, even in the case of soil having a groundwater level or higher, moisture in the soil moves to the cathode side due to prolonged energization, and the water near the anode is removed. Even when the soil is dried and the electrical conductivity decreases, the electrical conductivity of the soil can be secured, the recovery efficiency of heavy metals can be increased, and the contaminated soil can be purified.

A hollow tube provided with a water passage hole for the anode is provided.
Since water is supplied through the water holes, water required for maintaining the electrical conductivity can be efficiently supplied into the soil. Also, since the cathode is a hollow tube provided with a water hole, and water supplied through the water hole is collected,
Water collected near the cathode can be efficiently collected.

In the above-described embodiment, the hollow tube provided with the water passage hole for the cathode is used to collect the water supplied through the water passage hole. The water accumulated in the tank may be collected by a drain pump or the like. In addition, heavy metals concentrated in the metal accumulation area do not settle in the area and diffuse into groundwater, etc., so if the supplied water does not overflow into the ground surface and interfere with work, Need not be collected.

As described above, heavy metals in soil precipitate on the way and do not reach the cathode side, and the water recovered at the cathode does not contain heavy metals. Therefore, the water collected on the cathode side may be subjected to a pH treatment, and this may be recycled as water for water supply on the anode side.

Further, in the present embodiment, the anode is formed as a hollow tube provided with a water passage hole, and water is supplied through the water passage hole. Watering may be performed from the surface.

Further, by using a conductive aqueous solution in which salt or the like is dissolved in place of ordinary water for water supply and water sprinkling, the conductivity of soil may be improved and the recovery efficiency of heavy metals may be increased. If the concentration of salt or the like is too high, electricity flows too much and electrolysis is dominant, and the transfer efficiency of heavy metals decreases.
Therefore, the electric conductivity of the aqueous solution is in the range of several mS / cm to several tens mS / cm (from 1/10 to 100% of the seawater concentration).
(Approximately 1 /).

If soil conductivity cannot be ensured even by water supply or watering as described above, a water impervious wall may be constructed around the soil.

(Third Embodiment) FIG. 10A shows an electrode arrangement according to the third embodiment. As can be seen from the figure, also in this embodiment, as in the first embodiment,
The electrodes 32 and 33 are disposed in the contaminated soil 1 containing the heavy metal M, and the positive side of the DC power supply is connected to the electrode 32 and the negative side is connected to the electrode 33. In this embodiment, the electrode 33 is located near the ground surface. The electrode 32 is arranged at a predetermined depth position.
Since the materials of the electrodes and the like are substantially the same as those in the first embodiment, the description thereof is omitted here.

Here, it should be noted that there is a difference in the current density depending on the position in the contaminated soil 1. That is,
The current density at the point a shown in FIG. 10B is smaller than that at the point b. Therefore, it is considered that the heavy metal present at the point a cannot be expected to have much transfer effect by electrophoresis. For this reason, it is preferable that the electrode 32 is provided at a position which is lower to some extent than the lower limit of the range to be actually processed. Specifically, taking these factors into account, for example, the electrodes 32 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.

In arranging the electrodes 32 and 33,
As shown in FIG. 11, first, the contaminated soil 1 is excavated with a backhoe or the like to form a trench 41, then the electrode 32 is dropped into the trench and covered with the soil 42, and then the electrode 33 is placed near the ground surface. What is necessary is just to arrange | position and cover it with the soil 43.

After the electrodes 32 and 33 are arranged, a DC voltage is applied between these electrodes.

Then, as shown in FIG. 12A, the heavy metal M existing in the form of M ²⁺ or M ^{3+ in} the contaminated soil 1 moves toward the electrode 33 by electrophoresis as in the first embodiment. I do. Then, as shown in FIG. 4B, the form of hydroxide, that is, M is added to the metal accumulation region 51 located near the ground surface.
Precipitate and concentrate with (OH) ₂ or M (OH) ₃ . Note that the relationship between the solubility of heavy metals and pH, the relationship between the position where the metal accumulation region is formed and the pH, and the like are substantially the same as those in the first embodiment, and a description thereof will not be repeated.

When the heavy metal M included in the processing range 52 is accumulated in the metal accumulation region 51 in this manner, the electrode 33 is removed as necessary, and the metal accumulation region 51 is removed with a backhoe or shovel. Drill and remove. Then, the predetermined soil not contaminated by the excavated and removed portion is taken as soil, and the soil is returned to its original state and excavated and removed.
Processing such as solidification of concrete, vaporization and recovery of heavy metals by heating, etc., is disposed at a predetermined place.

As described above, according to the method for purifying contaminated soil according to the present embodiment, as in the first embodiment, it is possible to purify a wide range of contaminated soil only by excavating and carrying out a small amount of soil. In addition to this, in addition to this, the metal accumulation region is formed near the ground surface, so that a simple excavating machine is sufficient, and the excavating and removing operation is facilitated, and the workability is improved. Further, since the excavation depth is small, the excavated soil amount can be further reduced as compared with the case of the first embodiment.

Although not particularly mentioned in the present embodiment, the types of metals that can be applied are various, and if the polarity of the electrode is reversed, the present invention can be applied to the case where an anion exists. This is the same as the first embodiment.

The present embodiment is suitable for the case where the electrical conductivity of the soil can be sufficiently ensured without sprinkling or supplying water as in the case of the soil below the groundwater level. In this case, the electrodes 21 and 24 as described in the second embodiment are arranged horizontally instead of the electrodes 32 and 33, and the water holes 2 of the electrodes 21 and 24 are provided.
Water supply and drainage may be appropriately carried out via 2 and 25 to improve the electrical conductivity of the soil.

In this embodiment, the long electrode 3
Although the two and 33 are arranged horizontally, the present invention is not limited to such a configuration. In short, one of the pair of electrodes is arranged near the ground surface and the other is arranged underground. do it.

FIG. 13 shows a modification of the arrangement of the electrodes. In this modified example, as can be seen from the figure, an electrode 6 is attached to the tip of a pile 61 made of concrete or the like.
The electrodes 63 are mounted on the head and embedded in the contaminated soil 1 in a grid at a predetermined pitch.
2, 63 are connected to the positive and negative sides of the DC power supply, respectively.

In this configuration, substantially the same effects as those of the present embodiment can be obtained, and the electrodes are automatically arranged by driving the piles, so that the electrode arrangement work is facilitated.

[0071]

As described above, according to the method for purifying contaminated soil according to the present invention, the metal can be recovered in situ from the soil contaminated with the metal.

[0072]

[Brief description of the drawings]

FIG. 1 is a flowchart showing a procedure of a method for purifying contaminated soil according to the present embodiment.

FIG. 2 is an explanatory diagram showing a state in a contaminated soil in each step.

FIG. 3 is an explanatory view showing a state in a contaminated soil in each step.

FIG. 4 is an explanatory diagram showing a state in the contaminated soil in each step.

FIG. 5 is a graph showing the relationship between the solubility of heavy metals and pH.

FIG. 6 is a perspective view showing an apparatus in which an experiment supporting the effect of the method for purifying contaminated soil according to the embodiment is performed.

FIG. 7 is a graph showing one of the experimental results obtained by the experimental apparatus 11 shown in FIG. 6, illustrating how the pH of the soil changes depending on the distance from the anode 13 using the energizing time as a parameter.

FIG. 8 is a graph illustrating the amount of heavy metal contained in the contaminated soil 16 using copper as an index.

FIG. 9 is a side view showing an electrode structure according to a second embodiment.

FIGS. 10A and 10B are views showing an electrode arrangement state according to the third embodiment, wherein FIG. 10A is an overall perspective view, and FIG. 10B is a vertical sectional view.

FIG. 11 is a sectional view showing a procedure for arranging electrodes in the third embodiment.

FIG. 12 is an explanatory view showing a state in which a metal is concentrated as a hydroxide in an accumulation region in the third embodiment.

FIGS. 13A and 13B are diagrams showing an electrode structure according to a modification of the third embodiment, wherein FIG. 13A is a plan view and FIG. 13B is a cross-sectional view taken along line AA.

[Explanation of symbols]

 Reference Signs List 101 electrode insertion step 102 metal transfer step 103 metal concentration step 105 electrode transfer step 106 excavation removal step 1 contaminated soil 2, 3 electrode 4 metal accumulation area 21, 24 electrode 32, 33 electrode 51 metal accumulation area 62, 63 electrode

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Jun Komoto 2-3-2 Kandajicho, Chiyoda-ku, Tokyo Obayashi Corporation Tokyo Head Office (56) References JP-A-6-218355 (JP, A) JP Hei 6-158635 (JP, A) (58) Fields surveyed (Int. Cl. ⁷ , DB name) B09C 1/00-1/10 E02D 3/11

Claims (9)

(57) [Claims] A step of inserting a pair of electrodes into contaminated soil containing metal; a step of applying a DC voltage between the electrodes to move the metal to one of the electrodes; A step of precipitating and concentrating the metal in the form of a hydroxide in the located metal accumulation region, and a step of excavating and removing the metal accumulation region after a predetermined period has elapsed; A method for purifying contaminated soil, comprising disposing an electrode on the side to be moved near the ground surface and disposing the other electrode underground. 2. The method according to claim 1, wherein in the moving step, the metal is moved to a cathode side of the electrode. 3. A step of inserting a pair of electrodes into contaminated soil containing a metal, and a step of connecting a predetermined DC power source to the electrodes and applying a DC voltage to move the metal to one of the electrodes. Contaminating soil, comprising: a step of precipitating and concentrating the metal in the form of a hydroxide in a metal accumulation region located between the electrodes, and a step of excavating and removing the metal accumulation region after a predetermined period of time. A method, wherein the electrode is relocated while maintaining the formation position of the metal integrated region constant, and the moving step and the enrichment step are repeated with the connection of the DC power supply in the opposite direction, and between the electrodes. A method for purifying contaminated soil, wherein metal contained in soil is accumulated in the metal accumulation region. 4. The method according to claim 3, wherein in the moving step, the metal is moved to a cathode side of the electrode. 5. The method for purifying contaminated soil according to claim 1, further comprising a step of supplying water to the contaminated soil before or during the application of a DC voltage to the electrode. 6. The method for purifying contaminated soil according to claim 5, wherein the water supply step is performed through a water passage hole of a hollow tube constituting the electrode. 7. The method for purifying contaminated soil according to claim 5, wherein the water supply step is performed using water recovered through a water passage hole of a hollow tube constituting the electrode. 8. The method for purifying contaminated soil according to claim 5, wherein the water supply step is performed using a predetermined conductive aqueous solution. 9. The method for purifying contaminated soil according to claim 5, comprising a step of constructing a predetermined impermeable wall before the step of supplying water.