JP2008194544A - Method for neutralizing groundwater in acid soil containing heavy metal - Google Patents

Abstract

Neutralizing groundwater in contaminated soil by neutralizing acidic soil and groundwater contaminated with heavy metals by simple treatment operation with only pH control to prevent elution of heavy metals such as lead from the contaminated soil after treatment Provide a method.
SOLUTION: Weakly alkaline water or calcium carbonate or magnesium oxide solid is put into acidic soil containing heavy metal by boring holes or wells, and the direction and flow velocity of groundwater are controlled by pumping or pouring water or pumping and pouring water. In addition, weak alkaline water is injected into the ground by injecting fresh water into a borehole or well containing an alkaline solid, thereby neutralizing the groundwater.
[Selection] Figure 1

Description

  The present invention relates to a decontamination measure for acidic soil contaminated with heavy metals, and more particularly to a method for neutralizing heavy metal-containing acidic soil and groundwater.

  Contaminated soil and groundwater contaminated with heavy metals such as lead often have low mobility in the ground, and since the depth of contamination is relatively shallow, the contaminated soil is excavated and removed from the ground and filled with healthy soil. There are many examples where measures are taken by returning. In addition, excavation of contaminated soil or insolubilizing agent (for example, sodium sulfide) is added in situ to form a hardly soluble substance (Patent Document 1), or solidification is achieved by mixing cement with contaminated soil (Patent) There are known solidification / insolubilization techniques described in Document 2), techniques for cleaning with water after excavation, and techniques for classifying particle sizes in which contaminants are present in high concentrations.

JP 2002-35733 A JP 10-258263 A

  In the above prior art, excavation removal and backfilling of contaminated soil require a large amount of work, and cleaning and classification with water after excavation are complicated. The formation of a hardly soluble substance by the insolubilizing agent requires an insolubilizing agent and a combination treatment agent, and solidification of contaminated soil using cement increases the material cost due to the use of a large amount of cement. There was a possibility that heavy metals would elute again from the contaminated soil if contacted with acidic groundwater after the treatment.

  Moreover, although the presence or absence of soil contamination is judged by a soil elution test and a content test, the numerical value of the soil elution test greatly depends on the pH of the soil, and on the acidic side, many heavy metals tend to dissolve easily. Similarly, groundwater contamination is also determined by the groundwater concentration. However, when the pH of the groundwater is acidic, most of the heavy metals easily dissolve because the heavy metals that have precipitated in the ground dissolve.

  For this reason, when the groundwater environment is acidic (less than pH 6), even if only a small amount of harmful heavy metals are contained, the soil dissolution test value increases because it dissolves in a larger amount than neutral pH, In the case of the same content, the amount of contaminated soil increases more than in the case of neutral pH. As a result, the groundwater contamination concentration also increases. When the conventional technology is applied to such contamination, there is a problem that the contamination range is widened due to the fact that the environment is easy to elute and the eluted pollutants are diffused by the groundwater flow.

  The present invention solves the problems of the prior art as described above, and treats acid-contaminated soil and groundwater contaminated with heavy metals such as lead by pH control by an economical and simple treatment operation. It is an object of the present invention to provide a method for neutralizing groundwater in contaminated soil so as to prevent elution of heavy metals such as lead from the contaminated soil.

  According to the groundwater neutralization method for heavy metal-containing acidic soil of the present invention, solid or weakly alkaline water that shows weak alkalinity when placed in water is placed below the acidic groundwater surface from a borehole or well.

  In addition, according to the present invention, calcium carbonate and magnesium oxide having a diameter of 75 μm or more or an average particle diameter of the target soil are added as a solid that exhibits weak alkalinity when placed in water.

  Furthermore, according to the present invention, the boring holes or wells are arranged in a closed shape such as a lattice shape, a linear shape, or a circular shape.

  And according to this invention, a groundwater flow direction and flow velocity are controlled by performing pumping or water injection or pumping and water injection.

  Moreover, according to this invention, weak alkaline water is inject | poured in a ground by pouring fresh water into the boring hole or well containing the solid which shows alkalinity.

  In carrying out the present invention, a water injection well 2 is provided in the ground where the acidic soil 1 contaminated with heavy metals such as lead is present, and pH = 7-10 (preferably pH = 7.5-9) from the water injection well 2. It is preferable to neutralize the contaminated acidic soil 1 by pouring the weak alkaline water into the contaminated acidic soil 1.

  By neutralizing with weak alkaline alkaline water, the groundwater environment does not become extremely alkaline, so even if it is an amphoteric metal such as lead that easily elutes on the alkali side, elution can be reduced.

  The weak alkaline water is preferably prepared using one or more selected from the group consisting of sodium carbonate, caustic soda and sodium hydrogen carbonate and water.

  In this way, sodium ions and carbonate ions, which are ions that are generally present in groundwater, are injected to prevent secondary effects of the neutralizing substance.

  Furthermore, an observation hole 7 is provided at a distance from the water injection well 2, and the weak alkaline water is injected until the groundwater pH by the water injection is observed in the observation hole 7 at a value of 4.5 to 7.5. It is preferable to continue the injection.

  If it does in this way, the soil which exceeds an environmental standard value can be reduced to below a standard value, as explained in the below-mentioned test example.

  Moreover, it is preferable that groundwater is pumped from a pumping well 10 provided separately from the water injection well 2.

  If it does in this way, the neutralization effect can be heightened by collect | recovering acidic groundwater.

  Furthermore, it is preferable that the pumped acidic groundwater is decontaminated on the ground and adjusted to weak alkali, and then condensed into the groundwater through the water injection well 2.

  In this way, the amount of water used can be saved by reusing groundwater pumped from a pumping well.

  Further, it is preferable that the target contamination range is surrounded by the water blocking wall 12 to prevent the inflow of groundwater from the outside, and the purification is performed inside the water blocking wall.

  In this way, it is possible to prevent the inflow of groundwater from outside the target range and the outflow of alkaline water poured into the target range, and the neutralization can proceed efficiently. Moreover, even the heavy metal-containing acidic soil above the groundwater surface (unsaturated layer) can be neutralized by contact with the acidic soil due to accumulation of the poured alkaline water.

  Further, a water injection well 2 for injecting weak alkaline water into the ground where the contaminated acidic soil 1 contaminated with heavy metals such as lead exists, a water injection device 5 for supplying weak alkaline water to the water injection well 2, and the water injection well 2 and a pumping well 10 for pumping up groundwater provided between the contaminated contaminated acidic soil 1 and an observation hole 7 provided separately from the water injection well 2 and the pumping well 10 for measuring the pH of the groundwater Preferably a system consisting of:

The present invention has the following excellent effects.
(A) By adding a solid which shows weak alkalinity when water is added, that is, an alkali eluent, alkali is gradually eluted from the solid, and the neutralization reaction can be continued for a long time.
(B) By filling the alkaline elution material with particles larger than the original ground, it becomes easier for acidic groundwater to pass through, and the amount of neutralization per unit time increases by increasing the contact amount between the acidic groundwater and the alkaline elution material.
(C) By using an alkaline elution material, it is not necessary to create weak alkaline water by installing a ground plant.
(D) By neutralizing with weak alkaline alkaline water, it is prevented from becoming extremely alkaline, so that even if it is an amphoteric heavy metal such as lead, it can be made into a form that is hardly soluble in groundwater.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 shows a first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a heavy metal-containing acidic soil contaminated with heavy metals such as lead, and at least a part thereof is present in the groundwater layer below the surface layer E. A water injection well 2 that is neutralized by injecting solid or weak alkaline water showing weak alkalinity when put into the heavy metal-containing soil is dug from the surface until reaching the groundwater layer 3. Reference numeral 4 denotes a poorly permeable layer below the groundwater layer 3.
In the practice of the present invention, a solid which shows weak alkalinity when placed in water, that is, an alkaline eluent may be used, or weak alkaline water may be prepared in advance, but hereinafter, a case where weak alkaline water is used as an example. explain.

  A water injection device 5 is provided on the ground above the water injection well 2. The water injection device 5 has a tank 6 for storing weak alkaline water, a valve for supplying weak alkaline water from the tank 6 to the water injection well 2, a pump (not shown), and the like. An observation hole 7 for observing the pH of groundwater is provided on the downstream side (right side in FIG. 1) of the groundwater away from the heavy metal-containing soil 1.

  In the present invention, weak alkaline water having a pH of 8 to 9 is adjusted by adding caustic soda (NaOH) to water such as distilled water and stored in the tank 6. Any other alkaline liquid can be used.

  Next, the adjusted weak alkaline water is supplied from the tank 6 to the water injection well 2, and the weak alkaline water is injected from the water injection well 6 to the heavy metal-containing acidic soil 1. Groundwater mixed with weak alkaline water for neutralizing heavy metal-containing acidic soil 1 is extracted from the observation hole 7 and measured for pH. When the pH of the groundwater reaches 4.5 or more, the injection of alkaline water is stopped and heavy metal is stopped. The neutralization purification of the contained acidic soil 1 is finished. In this case, neutralization and purification can be performed by continuing to inject until the pH is increased by 2 or more with respect to the original pH.

The amount of elution of lead, which is an example of a contaminated heavy metal, tends to be minimal when the pH is about neutral, and increases from the neutral range to the alkaline side or acidic side.
Therefore, elution of lead can be suppressed by neutralizing the acid-contaminated soil 1 or groundwater with weak alkaline water having a pH of 7 to 10.

The state of the contaminated soil before and after the purification treatment will be described with reference to FIG. 5 and FIG. 6 after the purification. As shown in FIG. 5, when the pH is on the acidic side, the amount of heavy metal 1-A and groundwater contaminated portion 1-B dissolved from heavy metal-containing soil 1 is large. In addition, the arrow in the groundwater 3 shows the flow of groundwater.
On the other hand, after the purification treatment according to the present invention, elution of heavy metals from the heavy metal-containing soil 1 is suppressed, and as shown in FIG. 6, the amounts of dissolved heavy metal 1-A and groundwater contaminated portion 1-B are as described above. The amount is very small compared to the amount shown in FIG.

FIG. 2 shows a second embodiment of the present invention. The same members as those in FIG. In the embodiment of FIG. 2, the contaminated soil layer 1 to be purified is disposed so as to be sandwiched between the water injection well 2 and the pumping well 10, and the observation hole 7 is disposed between the water injection well 2 and the pumping well 10.
The in-situ purification system for the heavy metal-containing soil 1 is constituted by the water injection well 2, the pumping well 10, the water injection device 5, and the observation hole 7.

  By performing water injection and water pumping simultaneously with the water injection well 2 and the pumping well 10, the contaminated soil 1 between the two wells 2 and 10 is neutralized and purified. The water injection method from the water injection well 2 is the same as that of the embodiment of FIG. Further, the alkaline water injection is continued between the water injection well 2 and the pumping well 10 until the observation hole 7 confirms that the groundwater pH becomes 4.5 or more or rises 2 or more from the original pH.

  FIG. 3 shows a third embodiment of the present invention. The same members as those in FIG. In the embodiment of FIG. 3, the water pumped from the pumping well 10 is sent to the reuse plant 11, the water is decontaminated on the ground and adjusted to weak alkalinity, and then returned to the tank 6 to be reused for alkaline water for neutralization. Use. The water injection method from the water injection well 2 is the same as that of the embodiment of FIG. Further, the alkaline water injection is continued between the water injection well 2 and the pumping well 10 until the observation hole 7 confirms that the groundwater pH becomes 4.5 or more or rises 2 or more from the original pH.

FIG. 4 shows a fourth embodiment of the present invention. The same members as those in FIG.
In the embodiment of FIG. 4, the target contamination range is provided by being surrounded by the water blocking wall 12, and is embedded to the hardly water-permeable layer 4. And the water injection well 2, the pumping well 10, and the observation hole 7 are arrange | positioned in the inside enclosed by the water stop wall 12, water is injected, and the inside is filled with alkali water.

  The water injection method from the water injection well 2 is the same as that of the embodiment of FIG. Alkaline water injection is continued until the observation hole 7 confirms that the groundwater pH becomes 4.5 or higher or rises by 2 or more from the original pH, but if the inside is filled with water, The water is pumped by 10 and the pumped water is reused as needed as in the embodiment of FIG.

According to the test of the present inventor, it is considered effective if the elution rate is suppressed to 10 ⁻³ or less, and the pH of the soil for that purpose may be 4-7. Therefore, the pH of weak alkaline water for the contaminated soil in each of the above embodiments is adjusted to 7 to 10 and injected from the injection well, and the groundwater pH from the observation hole 7 is 4.5 to 7.5, or the original pH. As a result, the ratio of lead elution / lead content in the contaminated soil was reduced to about 10 ⁻³ .

When the pH of the weak alkaline water exceeds 10, the cost-effectiveness for the purification of the contaminated soil with the weak alkaline water is reduced, and when the pH is less than 7, the time required for the pH of the contaminated soil to reach 4-7. Therefore, the pH of weak alkaline water was selected to be 7-10. Moreover, the pH of the contaminated soil could be adjusted to 4 to 7 by raising the pH of the groundwater from the observation hole 7 by 4.5 or more or by 2 or more from the original pH.
If the pH of the observation hole 7 is 4.5 or less, the amount of elution is large and inappropriate, and if it is 7.5 or more, the alkaline solution is wasted.

  Although the embodiment of FIGS. 1 to 4 has been described with an example of an acid-contaminated soil layer contaminated with a heavy metal of lead, the present invention can also be applied to a staying groundwater layer contaminated with a heavy metal. It can be applied to cadmium, iron, manganese, and copper contamination other than lead.

For the adjustment of the weak alkaline water, any of sodium carbonate (Na ₂ CO ₃ ), caustic soda (NaOH), and sodium hydrogen carbonate (NaHCO ₃ ) may be used alone or in combination.
1 to 4, the heavy metal-containing soil layer illustrated on one side of the water injection well 2 may be on the other side or both sides of the water injection well 2.

Next, an example in which calcium carbonate is used as a solid exhibiting weak alkalinity when placed in water will be described. In addition to calcium carbonate, magnesium oxide and magnesium carbonate can also be used.
FIG. 7 is a view showing the arrangement of the calcium carbonate filling holes A drilled for carrying out the present invention. As shown in the figure, these filling holes A are provided in a grid at intervals of 5 m.

FIG. 8 shows an example in which the calcium carbonate filling holes A are arranged in two lines as indicated by reference characters X and Y. In this example, the interval between the calcium carbonate filling holes A is 2 m.
In the figure, black circle B indicates a water injection well and white circle C indicates a pumping well.

  FIG. 9 shows an example in which a large number of calcium carbonate filling holes A are arranged in the form of a lattice by arranging them vertically and horizontally, and a water injection well B and a pumping well C are provided in the center of the calcium carbonate filling holes A.

  Thus, suitable neutralization can be obtained over a long period of time by the arrangement shown in FIG. 7, FIG. 8, and FIG.

The figure which shows 1st Embodiment which concerns on the groundwater neutralization method of the heavy metal containing acidic soil of this invention. The figure which shows 2nd Embodiment which concerns on the groundwater neutralization method of the heavy metal containing acidic soil of this invention. The figure which shows 3rd Embodiment which concerns on the groundwater neutralization method of the heavy metal containing acidic soil of this invention. The figure which shows 4th Embodiment which concerns on the groundwater neutralization method of the heavy metal containing acidic soil of this invention. The schematic diagram which shows the state before neutralization of the ground water of heavy metal containing acidic soil. The schematic diagram which shows the state after neutralization of the groundwater of heavy metal containing acidic soil. The figure which shows an example of arrangement | positioning of the boring hole or well for implementing this invention. The figure which shows the other example of arrangement | positioning of the boring hole or well for implementing this invention. The figure which shows another example of arrangement | positioning of the boring hole or well for implementing this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Heavy metal containing soil 2 ... Water injection well 3 ... Ground water layer 5 ... Water injection equipment 6 ... Tank 7 ... Observation hole 10 ... Pumping well 12 ... Water stop wall

Claims (5)

A groundwater neutralization method for heavy metal-containing acidic soil, characterized in that solid or weakly alkaline water, which is weakly alkaline when placed in water, is placed below the acidic groundwater surface from a borehole or well. The groundwater of heavy metal-containing acidic soil according to claim 1, wherein calcium carbonate or magnesium oxide having a diameter of 75 µm or more or an average particle diameter of the target soil is added as a solid that exhibits weak alkalinity when placed in water. Sum method. 3. The method for neutralizing groundwater of heavy metal-containing acidic soil according to claim 1, wherein the boreholes or wells are arranged in a closed shape such as a lattice shape, a linear shape, or a circular shape. The groundwater neutralization method for heavy metal-containing acidic soil according to any one of claims 1 to 3, wherein the direction and flow velocity of groundwater are controlled by pumping or pouring water or pumping and pouring water. 4. Groundwater of heavy metal-containing acidic soil according to any one of claims 1 to 3, wherein weak alkaline water is injected into the ground by pouring fresh water into a borehole or well containing a solid exhibiting alkalinity. Sum method.

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