JP3178581B2 - 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 for purifying contaminated soil that can recover the metal substance from the soil contaminated with the metal substance.

[0009]

In order to achieve the above object, a method for purifying contaminated soil according to the present invention comprises, as described in claim 1, forming a pair of electrodes consisting of an anode and a cathode in contaminated soil containing metal. Disposing, then applying a DC voltage between the electrodes and supplying a predetermined acidic solution to the vicinity of the cathode, and then recovering the acidic solution from the cathode or the vicinity thereof, the method for purifying contaminated soil. The cathode is disposed near the ground surface and the anode is disposed underground so that the metal present in the soil in the form of cations rises by electrophoresis.

[0010] In the method for purifying contaminated soil according to the present invention, the anode is provided at a position further lower than the lower limit of the range to be actually treated.

In the method for purifying contaminated soil according to the present invention, the anode is attached to the tip of a pile, the cathode is attached to the head of the pile, and the pile is buried in the contaminated soil in this state. The cathode has a hollow structure in which water holes are formed, and a collection tube is inserted into the hollow.

[0012]

Further, the method for purifying contaminated soil according to the present invention comprises the above-mentioned conductive hollow tube having a water passage hole for supplying or recovering the acidic solution via the hollow tube. It is.

In the method for purifying 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 is provided in contaminated soil containing heavy metals such as nickel and chromium, and then a DC voltage is applied between the electrodes and a predetermined acidic solution is supplied near the cathode. Then, the metal present in the soil in the form of cations, but starts to move from the anode to the cathode side by electrophoresis, near the cathode acidic solutions OH ^- because of the neutralization, OH ^- reacts with The ions move to the cathode as cations without becoming hydroxides. When the acidic solution is recovered from the cathode or its vicinity, heavy metals are recovered in a form contained in the solution.

When the cathode is disposed near the ground surface and the anode is disposed at a predetermined depth, the heavy metal ions move upward and are collected together with the acidic solution near the ground surface. You. Therefore, even if it remains in the soil without being completely collected and the soil needs to be excavated and removed, the area to be excavated can be a relatively shallow area.

In the method for purifying contaminated soil according to the present invention, a pair of electrodes comprising an anode and a cathode are disposed in the above-mentioned contaminated soil containing a metal, and then a DC voltage is applied between the electrodes. A predetermined acidic solution is supplied to or near the anode. Then, the metal present in the soil in the form of cations, but with an acidic solution starts to move from the anode by electrophoresis on the cathode side, an acidic solution that has reached the cathode vicinity, OH ^- to neutralize. Therefore, metal
OH ^- Metal reacted remains cation without a hydroxide, or with a hydroxide and also moves to the cathode again a cation. When the acidic solution is recovered from the cathode or its vicinity, heavy metals are recovered in a form contained in the solution.

In the method for purifying contaminated soil according to the present invention, a pair of electrodes including an anode and a cathode are provided 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 Sum up. for that reason,
Metals, OH ^- metal reacted remains cation without a hydroxide, or with a hydroxide and also moves to the cathode again a cation. When the acidic solution is recovered from the cathode or its vicinity, heavy metals are recovered in a form contained in the solution.

[0018]

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.

(First Embodiment) FIG. 1 (a) is a schematic diagram showing a situation in which a method for purifying contaminated soil according to a first embodiment is performed. As can be seen from the figure, in the method of 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). A pair of electrodes 2 in contaminated soil 1 containing
a and 2b are inserted, and these are arranged to face each other.

As shown in FIG. 2 (b), the electrode 2a has a hollow tube 5a made of a conductive material such as iron and a plurality of water passage holes 4 formed in the hollow tube 5a. Further, 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 via the supply pipe 6a. On the other hand, the hollow tube 5
A collecting tube 6b is inserted into the tube b, so that the acidic solution in the hollow tube 5b can be collected through the collecting tube 6b.

Here, as the acidic solution, for example, pH
However, about 2 hydrochloric acid, sulfuric acid, organic acid and the like can be used, and these may be stored in a tank (not shown) installed on the ground.

Next, as shown in the figure, the plus side of the DC power supply is connected to the electrode 2a (anode) and the minus 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⁺ existing in the soil in the form of a cation is removed by electrophoresis as shown in FIG.
Start moving from a to the cathode 2b side.

At the same time as 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 holes 4 of the hollow tube 5a, and moves to the cathode 2b side together with metal ions by electrophoresis.

On the other hand, OH ^- is generated on the side of the cathode 2b. When the OH-reacts with the metal M ²⁺ or M ³⁺ , the OH ^- becomes hydroxide and remains in the soil.
It does not move to the b side. However, acidic solution which has moved to the vicinity of the cathode 2b with cations OH ^- Since neutralize, near the cathode 2b is changed from alkaline environment neutral or acidic environment. Therefore, the metal is a cation M ²⁺
Alternatively, M ³⁺ , or already converted to hydroxide, is ionized again and moves to the cathode 2 b, and the water passage hole 4
And collects in the hollow tube 5b. 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 using a pump (not shown) or the like, and then sent to a water treatment facility to remove heavy metals in the solution.

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

Next, the pH distribution in the soil when the acidic solution was not supplied, the accumulation state of heavy metals, and the like were examined by experiments, and 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 about 15 cm and a length of about 100 cm in a contaminated soil 1 containing heavy metals.
6 and the electrodes 13 and 14 at both ends of the contaminated soil 16.
And the electrodes 13 and 14 are connected to the positive side and the negative side of the DC power supply 12, respectively, so that a DC voltage of about 25 volts can be applied. In addition, water is appropriately sprinkled on the contaminated soil 16 in order to secure electrical conductivity of the soil.
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 a graph showing one of the experimental results obtained by the experimental apparatus 11 and showing how the pH of the soil changes depending on the distance from the anode 13 using the energizing time as a parameter.

As can be seen from the figure, the soil pH is almost constant at any position until the energization 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. 5 is a graph showing the amount of heavy metals 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.

[0032] This is because the copper initially present in the vicinity of the anode moves to the cathode side by energization, and gradually precipitates as hydroxide as the soil pH changes from acidic to neutral and further 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.

The results of these experiments do not directly support the effect of supplying an acidic solution. However, if electricity is supplied while supplying an acidic solution, heavy metals will not be accumulated as hydroxides. This can be a sufficient basis to judge that it will move to the cathode.

As described above, according to the method for purifying contaminated soil according to the present embodiment, the electrodes are arranged in the contaminated soil to energize, and the acidic solution is supplied from the anode and collected 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, which not only eliminates the need for excavation work, but also takes time and effort to carry out the excavated soil with a dump or the like. Can be omitted.

Further, since the conductive hollow tube provided with the water hole is used as an electrode and the supply and recovery of the acidic solution are performed through the water hole, the electrode and the hollow tube are separated. Installation work in the soil is easier than in the case of

Further, since the acidic solution is supplied from the anode side, when the acid solution reaches the vicinity of the cathode, the oxygen solution near the cathode is supplied.
H ^- not only serves to neutralize, if the conductivity of the contaminated soil is not sufficiently ensured, also serves of ensuring this.

In this 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.

In this embodiment, the acidic solution is supplied from the anode side. However, as can be seen from the above-described experiment, the environment is such that the metal tends to exist in the ion state on the anode side. Therefore, as shown in FIG.
May supply only water for ensuring conductivity in the soil, and the acidic solution may be supplied to the vicinity of the cathode 2b by, for example, spraying water from the ground. According to such a configuration,
Acidic solution, OH without diffusing around ^- efficiently contribute neutralization of the, thus, it is possible to save the supply amount of the acid solution.

If the contaminated soil 1 is below the groundwater level, for example, and if it is possible to secure the power supply without taking measures such as water supply and water sprinkling, the anode will be replaced with the one shown in FIG.
As shown in (b), the structure may be made of a conductive rod 21 without the water supply function.

Further, an acidic solution may be supplied between the anode and the cathode. In such a configuration, it is positioned as an intermediate usage between the case where the acidic solution is supplied to the cathode and the case where the acidic solution is supplied to the anode. Can be achieved.

Although not particularly shown, the supply or recovery hollow tube and the electrode do not necessarily need to be shared, and the hollow tube is formed of a non-conductive material, and the electrode is separately made of iron or carbon. It may be constituted by a rod.

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

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. 7B 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 electrophoretic transfer effect. 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 consideration, 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.

In arranging the electrodes 32 and 33,
As shown in FIG. 8, 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, the metal M ²⁺ or M ³⁺ existing in the soil in the form of a cation starts to rise from the anode 32 to the cathode 33 by electrophoresis as shown in FIG.

At the same time as the energizing operation, the acidic solution described in the first embodiment is supplied from the ground by watering or the like. Then, the acidic solution becomes contaminated soil 1 near the cathode 33.
Neutralize ^- diffuse into, OH occurring at the cathode 33 side. Then, as described in the first embodiment, the cathode 33
The vicinity changes from an alkaline environment to a neutral or acidic environment, and the metal that has 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. Is ionized again, reaches the cathode 33, and collects in the hollow tube 5b through the water passage hole 4.

Next, the acidic solution in the hollow tube 5b is recovered using a pump (not shown) or the like, 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 collected. If heavy metals that cannot be recovered remain in the vicinity of the cathode 33, they may be appropriately excavated and removed with a backhoe or shovel.

As described above, according to the method for purifying contaminated soil of the present embodiment, it is possible to recover only heavy metals from contaminated soil without excavating and removing the contaminated soil, as in the first embodiment. In addition, in addition to this, even if heavy metals that could not be recovered remain, such residual parts are limited to shallow parts near the ground surface, so it is possible to easily excavate and remove them. Moreover, the amount of excavated soil is very small.

Although not particularly mentioned in the present embodiment, the applicable metal types are various as in the first embodiment.

In the present embodiment, the acidic solution is supplied from the ground by a method such as watering. 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 buried and supplied near the cathode 33 via the hollow tube 51. In such a configuration, the cathode 3
3 near the OH ^- to neutralize the cathode 3 to be ionized again those metals that have moved to the vicinity remains cations M ²⁺ or M ^3+, or already a hydroxide of the cathode 33
3 and can be collected in the hollow tube 5b of the cathode 33 through the water hole 4.

(Third Embodiment) In the second embodiment, the elongated electrodes 32 and 33 are arranged horizontally. However, the present invention is not limited to such a configuration.
The cathode may be provided near the ground surface and the anode may be provided underground.

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 made of concrete or the like, and an electrode 63 is attached to the head. In addition to being buried, the positive and negative sides of the DC power supply are connected to the electrodes 62 and 63, respectively. The cathode 63 is
As shown in FIG. 11, the water passage hole 4 has a hollow structure, and a recovery pipe 6b is inserted into the hollow.

In this embodiment, as in the second embodiment, a direct current 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 by electrophoresis. The acidic solution is supplied to the vicinity of the cathode 63 from the ground by watering or the like. 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 again ionized to reach the cathode 63, and is passed through the water hole 4. And the acidic solution inside the hollow can be collected together with heavy metals using a pump (not shown) or the like.

In this embodiment, substantially the same effects as those of the second embodiment can be obtained. In addition, the electrodes are automatically arranged by driving the stakes, so that the operation of arranging the electrodes becomes easy.

[0057]

As described above, according to the method for purifying contaminated soil according to the present invention, it is possible to recover only metal without excavating and removing contaminated soil. In addition, there is an effect that the amount of the acidic solution used can be greatly reduced.

[0058]

[0059]

[0060]

[Brief description of the drawings]

FIG. 1 is a view showing an implementation state of a method for purifying contaminated soil according to the present embodiment, where (a) is an overall schematic view and (b) is a side view showing an electrode structure in detail.

FIG. 2 is an explanatory view showing the operation of the present embodiment, and FIG.
FIG. 3 is a diagram showing the movement of metal ions, and FIG. 4B is a graph showing the relationship between the solubility of metal ions and pH.

FIG. 3 is a perspective view showing an apparatus in which an experiment according to the embodiment is performed.

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

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

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

FIG. 7 is a sectional view showing the arrangement of electrodes in the second embodiment.

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

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

FIGS. 10A and 10B are diagrams showing the arrangement of electrodes in the third embodiment, where FIG. 10A is a plan view and FIG. 10B is a cross-sectional view along line AA.

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

[Explanation of symbols]

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

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-218355 (JP, A) JP-A-5-59716 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B09B 3/00 304-5/00 E02D 3/11 A62D 3/00

Claims (4)

(57) [Claims] 1. A pair of electrodes consisting of an anode and a cathode are disposed in contaminated soil containing a metal, and then a DC voltage is applied between the electrodes and a predetermined acidic solution is supplied near the cathode. A method for purifying contaminated soil by recovering the acidic solution from the cathode or the vicinity thereof, wherein the metal is present in the soil in the form of cations and rises by electrophoresis so that the cathode is located near the ground surface. And disposing the anode in the ground. 2. The method for purifying contaminated soil according to claim 1, wherein the anode is provided at a position lower than a lower limit of a range to be actually treated. 3. Attaching the anode to the tip of a stake, attaching the cathode to the head of the stake, burying the stake in the contaminated soil in this state,
2. The method for purifying contaminated soil according to claim 1, wherein the water passage hole has a hollow structure, and a collection pipe is inserted inside the hollow. 4. The method for purifying contaminated soil according to claim 1, wherein the electrode is constituted by a conductive hollow tube having a water passage hole, and the acidic solution is supplied or recovered through the hollow tube.