JPH08197028A - Method of catching pollutant - Google Patents

Abstract

PURPOSE: To lower the coefficient of permeability of an isolation wall and also to provide the isolation wall showing the excellent isolation property of hexavalent chromium. CONSTITUTION: In this method, to a region A to be polluted which has been polluted or has the fear of being polluted, an isolation wall 2 essentially consisting of cement or bentnite is constructed to prevent pollutant from flowing out to the outside of the isolation wall 2, and magnesium chloride is incorporated inside the isolation wall 2 or in the region A to be polluted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to environmental protection, in particular, by constructing a barrier wall around a contaminated or suspected contaminated area to prevent migration of pollutants from the area to the environment. The present invention relates to a method for capturing pollutants when preventing environmental impact and promoting environmental protection.

[0002]

2. Description of the Related Art With the development of industry and diversification of urban life, the problems of industrial waste and municipal solid waste have been greatly highlighted. In particular, in addition to increasing the amount of waste treated, it has become a big problem that harmful substances contained in the waste, for example, heavy metals such as mercury, cadmium, and chromium, flow out into the living area, for example, through groundwater.

To this end, the preferred solution is to construct a barrier around the contaminated or suspected contaminated area, which blocks the migration of contaminants from the area to the surroundings. In fact, it has been practiced in the past to construct concrete walls around that area, or even concrete impermeable walls at the bottom.

However, a concrete wall has a high possibility of penetrating heavy metals, and if cracks occur, they will intensively flow out from there.

On the other hand, in Japanese Patent Application Laid-Open No. 61-105500, bentonant-cement is used as a basic material in which clay, siliceous material, sodium carbonate, pyrophosphate or tartrate of alkali metal, etc. are contained. It is proposed that a barrier wall with high barrier properties against heavy metals be constructed by including.

[0006]

However, in the barrier wall mainly composed of bentonite and cement as a material constituting the barrier wall, the barrier wall has an excellent barrier property against pollutants, but still has a sufficiently low water permeability. Does not indicate.

As a countermeasure against this, it is conceivable to combine a shielding sheet made of a rubber or a plastic material to block the flow of groundwater.
It is difficult to say that it is an appropriate means because it breaks or is damaged by deterioration due to unexpected external force or long-term exposure.

Therefore, under the present circumstances, it is the optimum method to achieve the interruption only by the interruption wall mainly composed of bentonant and cement. However, although this barrier has excellent barrier properties for most pollutants, it has an exceptional 6
The barrier property of valent chromium (Cr ⁶⁺ ) is not sufficient.

Therefore, the object of the present invention is to reduce the water permeability of the barrier wall and to obtain excellent hexavalent chromium (Cr).
⁶⁺ ) to provide a barrier.

[0010]

[Means for Solving the Problems] The above-mentioned problem is to construct a barrier wall mainly composed of cement and bentonite in the contaminated area to be polluted which is contaminated or is likely to be contaminated, and the outside of the barrier wall is constructed. In the method of preventing the outflow of pollutants, it can be solved by incorporating magnesium chloride in the blocking wall or in the area to be polluted.

On the other hand, in this case, it is desirable that magnesium chloride be present in a concentration of 250 to 1000 mg in 1 liter of ground water in the peripheral portion of the wall surface of the barrier wall.

[0012]

In the present invention, since the barrier wall composed mainly of cement and bentonite (more preferably sodium bentonite) -based solidifying material is composed of fine calcium silicate hydrates intertwined with each other, It is rich. In addition, the hydration reaction proceeds with time, the crystals of the hydrate become tighter (strengthening), and the water permeability decreases.

However, as described above, there is a limit to the reduction of the water permeability of the barrier wall. Therefore, the present inventors
As a result of the following experiment, it was found that it is effective to contain magnesium chloride in the inside of the barrier wall or in the area to be contaminated, and the present invention has been completed.

The outline of this experiment is as follows.
A columnar specimen with a height of 10 cm was prepared, and distilled water (control), groundwater from Noda City, Chiba Prefecture (hereinafter referred to as pure groundwater for convenience), and the same groundwater in which hexavalent chromium and magnesium chloride were dissolved Water was passed through (pseudo-improved groundwater), and the water permeability, pH, and outflow amount of hexavalent chromium of the test body were examined over time, and the results shown in FIGS. 2 to 5 were obtained.

As a concrete experimental facility, as shown in FIG. 1, a material having a ratio of 250 kg of cement and 60 kg of Na bentonite was cured in water at a water temperature of 20 ° C. for 7 days.
The sample is finished to the above dimensions, placed in a water permeation test mold, and adjusted to a hexavalent chromium concentration of 5 mg / 1 liter and magnesium chloride of 2705 mg / 1 liter using a potassium dichromate reagent. The pseudo-improved groundwater described above was passed through the specimen at a pressure of 0.5 kgf / cm ² . Also, considering the pressure of groundwater in the ground, a lateral pressure of 1.0 kgf / cm ² was applied to the test piece. After passing water, water was collected in a plastic bag, and each water was tested for the above measurement items. The experiment was conducted for 28 days for each condition.

Looking at the results, as shown in FIG.
Although the coefficient of permeability decreases with time, the rate of decrease is slow in distilled water and groundwater, whereas in pseudo-improved groundwater in which hexavalent chromium and magnesium chloride are dissolved, the coefficient of permeability decreases in about 7 days. However, it is understood that it has a lower hydraulic conductivity than distilled water and groundwater.

As shown in FIG. 3, in the case of the pseudo-improved groundwater, there is a slight tendency to decrease depending on the number of days elapsed, but there is practically no change. Shows a tendency to decrease. Also,
For pseudo-polluted water and pseudo-improved groundwater that do not contain magnesium chloride, the amount of leaching water is changed by changing the amount of water passing through the specimen, and Cr for the leaching rate is changed.
^The results shown in FIGS. 4 and 5 showing the results of examining the ⁶⁺ concentration show that the addition of magnesium chloride can significantly reduce the leaching of hexavalent chromium. Note that FIG. 5 shows the theoretical value of Cr ⁶⁺ amount accompanying water flow. Moreover, when considering an actual barrier wall, it was found that the one having the above-mentioned low permeability can be constructed. Therefore, it is considered that the leachate amount is 5 liters or less in FIG. It can be inferred that the leaching amount of hexavalent chromium is infinitely close to zero. Even if it is assumed that a water passage is formed on the barrier wall due to an unexpected accident, according to the results shown in the figure, even if the leachate amount is larger,
In the case of pseudo-improved groundwater, the leaching amount of hexavalent chromium is lower than that of pseudo-polluted water, so that the leaching of hexavalent chromium can be suppressed to a low level.

On the other hand, separately, 5 mg / 1 liter of Cr ⁺⁶ was added to distilled water and a magnesium chloride solution.
It was immersed in the solution in a beaker containing the resin and visually observed after 18 days. No white turbidity was observed at the bottom of the beaker, and there was no color change in the sample. On the other hand, when it was similarly immersed in groundwater, white turbidity occurred at the bottom of the beaker, and the specimen exhibited a light green color. In addition, about 10 in groundwater
It contains mg / 1 liter of Mg ⁶⁺ .

From these facts, it is inferred that, according to the present invention, the addition of magnesium chloride to ground water causes the following reaction.

The potassium dichromate K ₂ Cr ₂ O ₇ under ^{_{alkaline, Cr 2 O 7 2- + 2OH}} - → 2CrO 4 2- + 2H 2 O ... (1) Furthermore, sulfur, such as slag cement (CaS) Acts, and Cr is reduced to trivalent as described below. 2CrO ₄ ²⁻ + 4H ₂ O + 3e ⁻ → Cr (OH) ₃ + 5OH ⁻ (2) Cr (OH) ₃ → Cr ₂ O ₄ · nH ₂ O (3) Moreover, when becoming alkaline, Cr ₂ O ₄ · nH ₂ O → spinel type complex oxide (chromite) M ^II (CrO ₂ ) ₂ : sparingly soluble in water (4) Cr ₂ O ₄ · nH ₂ O → Spinel type other than MgO · 2CrO ₃ … ( 5) Thus, as shown in the equations (4) and (5), chromite is generated, clogging occurs in the barrier wall, and C
^Detain r ⁶⁺ in the barrier.

[0021]

[Examples] The present invention will be further described based on the results of other experiments. When the changes due to the pH of the pseudo-improved groundwater were examined, the results shown in FIGS. 6 and 7 were obtained. That is, it has been found that a pH of about 9 is particularly excellent in the reduction of hydraulic conductivity and the blocking property of chromium. Therefore, in view of practical use and alkali pollution, the pH is preferably 7.0 to 9.5.

The concentration of magnesium chloride is preferably 250 to 1000 mg per liter of groundwater. The Mg ²⁺ ion concentration in groundwater is 50-30
It is 0 mg / 1 liter (usually 50 to 200 mg). Therefore, in order to reduce the water permeability and prevent the hexavalent chromium from leaching, it is necessary to increase the Mg ²⁺ ion concentration at least higher than that of general groundwater. On the other hand, if magnesium chloride is excessively contained, as shown in FIG.
If it is added in excess of 1000 mg, the water permeability will increase, and especially in the amount of about 5000 mg, the cement-bentonite wall will be softly broken.

Next, when actually constructing a barrier wall and capturing contaminants, for example, the following method can be adopted.

FIG. 9 and FIG. 10 are conceptual views showing the construction target area of the present invention, in which the shielding wall 2 is constructed so as to surround the waste 1 disposal site. Around the area A contaminated or suspected to be contaminated by waste, the barrier wall 2 for preventing migration of contaminants from the area A to the surrounding area is preferably provided with the impermeable layer 3 at the bottom. Then, the impermeable layer 3 is reached and it is constructed. Also, it does not have the impermeable layer 3,
Alternatively, if there is a risk of infiltration by getting on the groundwater downward, a bottom wall similar to the blocking wall 2 can be constructed below the contaminated region by appropriate means. In this case, the bottom wall is preferably continuous with the blocking wall 2.

As a material for constructing the blocking wall 2,
A cement-bentonite-based material is used. In the barrier wall 2, a trapping core material consisting of particles for adsorbing and holding the pollutants in the waste 1 or pellets formed by granulating the particles, or a film for preventing direct contact with the pollutants on the surface thereof It is possible to include the trapping material that has formed.

In constructing the barrier wall 2, when a continuous underground wall is used in the construction method, a guide wall is formed,
The excavation groove is excavated while being filled with a bentonite stabilizing solution, and after the excavation, the solidifying material such as cement and the above-mentioned trapping material are added and stirred, or substitution stirring is performed to solidify the bentonite stabilizing solution.

The barrier wall 2 thus constructed generally has a large water permeability coefficient at the beginning, and therefore a liquid containing a contaminant permeates through the barrier wall 2 at a high rate. At this time, the coating of the trapping material is partially or wholly dissolved by contact with the liquid, and the trapping core material is exposed. Contaminants are adsorbed and held on the exposed trapping core material. Eventually, with the passage of time after the construction of the barrier wall 2, the coefficient of water permeability becomes small and a high liquid barrier property is exhibited. Even in this case, the trapping core material adsorbs and holds the pollutant with respect to the liquid that enters the blocking wall 2 through the crack or the like. As a result, it is possible to prevent the outflow of contaminants around the blocking wall 2. Further, it is possible to control the coating so that the coating is dissolved by the time when the solidification is completed, by selecting the material or the thickness of the coating.

In the case where the excavation groove 12 is not filled with the stabilizing liquid, the trapping material can be premixed together with the constituent material of the blocking wall 2 and the mixture can be placed in the excavation groove.

In the above example, the trapping material adsorbs or retains pollutants, such as activated carbon, coal, charcoal, zeolite, vermiculite, kaolin,
In addition to sodium bentonite, silica, blast furnace slag,
Other clay minerals can be used alone or in a mixed state. This type can be used because it has an ion exchange ability, an adsorption ability, or a retention ability in its pores due to its porous nature.

Further, as the coating film for coating the surface of the trapping core material, a material which can be dissolved by water or a slurry constituting the barrier wall may be used, and for example, gelatin, cellulose material, vinyl acetate material can be used. .

As a concrete composition, 20 to 250 kg, especially 40 to 80 kg of Na bentonite per 250 kg of cement (more preferably blast furnace cement), if necessary, an expanding agent such as "Denka CSA" 15 to 4 manufactured by Denki Kagaku Kogyo Co., Ltd.
0 kg, if necessary a water-reducing agent such as 1-5 kg of "Pozoris series" manufactured by Pozoris Bussan Co., Ltd., and further, if necessary, an appropriate amount of the above-mentioned pollutant trapping material and water, for example 100
0 kg can be blended and used.

Now, according to the present invention, as a concrete means for containing magnesium chloride in the inside of the barrier wall 2 or in the area to be polluted, the outlet pipe 10 is installed on the barrier wall 2.

As shown in a concrete example in FIG. 12, as the spouting pipe 10, a plurality of injection ports 11 are formed in the wall surface of the outer pipe 10A in a high direction, and the spouting pipe 10 is concentrically inserted and installed therein. The magnesium chloride aqueous solution 12 can be discharged from the pipe 10B, and can be injected into the blocking wall 2 and the contamination target area while expanding the flexible short tubular sleeve 15 from the injection port 11 of the outer pipe 10A.

In this case, in particular, since the magnesium chloride aqueous solution 12 cannot be expected to permeate the wide range of the barrier wall 2 in particular, the barrier wall 2 is expanded and contracted by a fluid pressure such as air or water with the inlet 13 of the inner tube 10B interposed therebetween. Packer 14, 1
4, the packers 14 and 14 are inflated from the ground and in contact with the inner wall surface of the outer tube 10A, the magnesium chloride aqueous solution 12 is injected, and then stepped up for a predetermined length, for example. It is preferable to repeat the same injection at the stage of.

The time of this injection is the time when the hardening of the barrier wall 2 is started, more specifically, 24 hours after the barrier wall is formed.
It is preferred to inject after ~ 48 hours. As a result, water lines can be formed in the direction of the contamination target area of the blocking wall 2,
This is because hexavalent chromium can be captured later when contaminated groundwater tries to pass through the blocking wall 2 through this water passage.

In addition to the injection at a low pressure by an injection pump, the magnesium chloride aqueous solution 12 may be stored in an elevated tank 16 and gradually injected by using a head head as shown in FIG. .

If necessary, the aqueous solution of magnesium chloride may be allowed to be present in the area to be contaminated by an appropriate means such as mere pouring in addition to the above-mentioned injection form. Requires a large amount of magnesium chloride, and since it is realistically or ultimately intended to be captured by the blocking wall, a position close to the inside of the blocking wall 2, particularly the side of the contamination target area is defined as the injection position. It is desirable to do.

[0038]

As described above, according to the present invention, it is possible to reduce the water permeability of the barrier wall and to obtain an excellent 6
It is possible to construct a material that exhibits a barrier property to valent chromium (Cr ⁶⁺ ).

[Brief description of drawings]

FIG. 1 is a schematic diagram of experimental equipment that is the basis of the present invention.

FIG. 2 is a graph showing experimental results.

FIG. 3 is a graph showing experimental results.

FIG. 4 is a graph showing experimental results.

FIG. 5 is a graph showing experimental results.

FIG. 6 is a graph showing experimental results.

FIG. 7 is a graph showing experimental results.

FIG. 8 is a graph showing experimental results.

FIG. 9 is a schematic cross-sectional view of a construction example of the present invention.

FIG. 10 is a plan view of a construction example.

FIG. 11 is a schematic sectional view of an installation example of an injection tube.

FIG. 12 is a schematic sectional view of an example of an injection tube.

[Explanation of symbols]

1 ... Waste, 2 ... Barrier wall, 3 ... Impermeable layer, 10 ... Outflow pipe, 10A ... Outer pipe, 10B ... Inner pipe, A ... Contamination area.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location B09C 1/02 1/08 (72) Inventor Masatoshi Iio 4-2 35, 9th dan, Chiyoda-ku, Tokyo No. Light Industrial Co., Ltd.

Claims (2)

[Claims] 1. A barrier wall composed mainly of cement and bentonite is constructed for a polluted target polluted region which is contaminated or is likely to be polluted, and a contaminant is prevented from flowing out of the barrier wall. A method for trapping a pollutant, characterized in that magnesium chloride is contained in the inside of the barrier wall or in an area to be polluted. 2. Magnesium chloride of 250 to 1000 m in 1 liter of groundwater around the wall surface of the barrier wall.
The method for trapping a pollutant according to claim 1, which is present at a concentration of g.