JPH07236879A - How to capture pollutants - Google Patents

Abstract

PURPOSE:To forcibly catch a chromium ion dissolved in underground water. CONSTITUTION:In the foundation wherein chromium is dissolved in underground water, CB plies 6, 7 based on cement and bentonite are constructed and one of them is set to an anode and the other one of them is set to a cathode. A DC current is applied across both electrodes to gather the chromium ion dissolved in underground water to the CP pile 6 side being the anode to catch the same by the porous substance of the CB pile 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to pollutants, especially in areas where heavy metals are contaminated or at risk of contamination.
Particularly, the present invention relates to a pollutant trapping method for effectively and positively trapping harmful chromium to prevent adverse effects on the surrounding environment.

[0002]

2. Description of the Related Art In recent years, the problems of industrial waste and municipal solid waste have been highlighted with the development of industry and the diversification of urban life. In particular, in addition to increasing the amount of waste processed, hazardous substances contained in industrial waste, such as lead,
Heavy metals such as mercury, cadmium, and chromium are in the realm of life,
For example, runoff through groundwater is a major problem.

A suitable solution to this problem is to construct a barrier around the contaminated or suspected contaminated area to prevent the migration of contaminants from that area to its surroundings. . Actually, conventionally, it has been practiced to construct a concrete wall or the like around it and to construct a water impermeable bottom wall on the bottom.

As a fixing method for preventing heavy metals from flowing into the environment, a chemical method or a method using clay minerals has been conventionally adopted. Among them, the former method is that for heavy metals except chromium, if the pH is adjusted to the alkali side according to each metal ion, a hydroxide is formed to form a sparingly soluble or insoluble precipitate. You can Under appropriate pH conditions, carbonic acid (HCO ₃
^- ) Reacts with heavy metals to form sparingly soluble and insoluble carbonates.

However, among heavy metals, chromium, especially hexavalent chromium (Cr ⁶⁺ ) is highly toxic and cannot form hydroxides and carbonates. Therefore, conventionally, hexavalent chromium has been used with sodium bisulfite. A method of reducing to trivalent chromium (Cr ³⁺ ) and then performing coagulation separation is adopted.

In some cases, a method of reacting heavy metals with sulfur to immobilize them as insoluble matter such as cadmium sulfide (CdS) may be adopted.

On the other hand, in Japanese Patent Laid-Open No. 61-105500, conventionally, as a composition of a porous material used for a barrier, cement bentonite is used as a basic material, and a heavy metal trapping material, clays, silica Disclosed is a barrier wall containing a mixture of materials, sodium carbonate, alkali metal phosphates and tartrates.

[0008]

However, the barrier wall disclosed in Japanese Patent Laid-Open No. 61-105500 has a certain degree of flexibility and an appropriate hardness after curing, and has an advantage of excellent water barrier property. However, even if it is relatively easy to capture or retain cadmium, lead, etc., it was extremely difficult to capture trivalent or hexavalent chromium.

Therefore, the applicant of the present application has filed the above-mentioned Japanese Patent Application No. 4-3.
In 17432, sodium bentonite,
Proposing a blocking wall containing a certain amount of B type or C type of blast furnace cement or a mixture thereof, an admixture for producing ettringite, and an admixture containing metallic aluminum, and forming ultrafine micropores (pores) Proposed a barrier that traps heavy metal ions such as chromium ions.

Certainly, according to the blocking wall described in Japanese Patent Application No. 4-317432, it is possible to trap chromium ions to some extent. However, in this method, when the chromium ions existing in the blocking wall try to flow out to the outside, they are captured by the blocking wall. Therefore, toxic chromium that has not been trapped is constantly present in the area surrounded by the barrier wall, and after the barrier wall reaches the chromium capture limit amount, unless the barrier wall is newly rebuilt. However, after that, it becomes difficult to capture chromium.

Therefore, the main problems of the present invention are as follows.
Provided is a method capable of forcibly capturing and eliminating chromium ions dissolved in groundwater in a contaminated region surrounded by a barrier wall, independently of trapping by the barrier wall. A second object is to provide a method for improving the water blocking performance of the barrier wall by early adhering another insoluble heavy metal other than chromium to the barrier wall surrounding the contaminated region.

[0012]

[Means for Solving the Problems] The first problem is to construct a pile or an independent wall made of a porous material composed mainly of cement bentonite as an anode in the ground in which chromium is dissolved in groundwater. In addition, a cathode member serving as a counter electrode is provided in the ground, a direct current is applied between these electrodes, and chromium ions dissolved in groundwater are porous substances mainly composed of cement bentonite serving as the anode. The problem can be solved by collecting them on the side of the pile or the independent wall and capturing them in the porous material.

In this case, if the cathode member is a pile or an independent wall made of a porous material mainly composed of cement bentonite, heavy metal other than chromium, such as lead, cadmium, nickel, etc., is used as the cathode. It can be captured on piles or independent walls constructed from bentonite based materials.

The method of trapping pollutants is performed in the chromium elution area, but mainly, the migration of the pollutants from the area to the periphery is performed around the area that is contaminated or at risk of contamination. It is effective when it is performed in the area where the blocking wall for blocking is constructed.

Next, the second problem is to construct a barrier wall around a contaminated or suspected contaminated region to prevent migration of pollutants from the region to the periphery, and to be surrounded by the barrier wall. In the closed area, a pile or an independent wall composed of a porous material mainly composed of cement bentonite is used as an anode, and the blocking wall is used as a cathode, and a direct current is applied between these electrodes to apply groundwater. The problem can be solved by collecting the chromium ions dissolved therein in the pile or the independent wall side made of the porous material mainly composed of cement bentonite serving as the anode, and trapped in the porous material.

In these cases, the porous material mainly composed of cement bentonite contains at least one admixture for producing ettringite as an additive, gypsum, clay mineral, charcoal and activated carbon, so that it is trapped once. Makes it difficult to redissolve chromium that has formed.

Further, after a hollow holder having a plurality of through holes is installed in the ground in an embedded state, the hollow holder is filled with a mass of porous material mainly composed of cement bentonite for capturing chromium, By making this an anode,
When the cement bentonite-based material reaches the chromium capture limit amount, the cement bentonite-based material filled in the hollow holder can be dug out and replaced with a new cement bentonite-based material to perform chromium capture. .

In the case of the pollutant trapping method according to the present invention,
Since the capture rate of chromium is proportional to the electrical conductivity of the groundwater that is the electrolyte, by introducing non-toxic ions to reduce the electrical resistance in the contaminated area surrounded by the barrier, The capture speed can be improved.

[0019]

In the present invention, in the ground to be captured, a pile or an independent wall made of a porous material mainly composed of cement bentonite is constructed and used as an anode, and a cathode member serving as a counter electrode is provided in the ground. A direct current is applied between both electrodes, and chromium ions dissolved in groundwater are collected on the side of a pile or an independent wall made of a porous material mainly composed of cement bentonite, which serves as the anode, in the porous material. To capture. Chromium, which is eluted in groundwater, is separated in a negatively charged ionized state. In the present invention, focusing on this, by applying the electrolysis method using the pile or the independent wall made of cement bentonite-based material as the anode, the chromium ions are forcibly collected on the anode side, and the adsorption function of the cement bentonite-based material is obtained. Since this is captured by chromium, it is possible to actively capture and recover chromium in the ground instead of waiting for chromium ions to be adsorbed on the barrier wall as in the conventional case.

By constructing a pile or an independent wall of the cement bentonite material in the area where the barrier wall surrounding the contaminated area is constructed and using this as an anode, and using the barrier wall as a cathode to capture chromium, Chromium is trapped in the cement bentonite-based material on the anode side, while cations such as Pb ²⁺ , Cd ²⁺ and Ni ²⁺ are trapped on the inner wall of the barrier wall to form an insoluble heavy metal layer. By doing so, it is possible to improve the water blocking property of the barrier wall at the same time as capturing chromium.

[0021]

EXAMPLES The present invention will be described in detail below with reference to examples. Figure 1
In, for example, the blocking wall 2 is constructed so as to surround the waste disposal site. The barrier walls 2 and 2 are provided around a contaminated or suspected contaminated area in order to prevent the migration of pollutants from the area to the surroundings, and preferably the impermeable layer 3 at the bottom. Is present,
It is constructed so that the lower end of the barrier wall 2 reaches the impermeable layer 3. In addition, if the impermeable layer 3 is not provided or if there is a risk of permeation due to riding on groundwater below, a bottom wall similar to the blocking wall can be constructed below the contaminated region by appropriate means. In that case, the bottom wall is preferably continuous with the blocking wall 2. Further, if necessary, a liquid-impermeable sheet can be arranged inside the blocking wall 2 or the bottom wall.

In the groundwater in the area surrounded by the barrier wall 2, lead, cadmium, nickel, and chromium molecules are ionized, and Pb ²⁺ , Cd ²⁺ , Ni ²⁺ , CrO ₄ ^2-, etc. Elute in the positive or anionic state of. In the present invention, cement bentonite piles 6 and 7 (hereinafter, also simply referred to as CB piles) made of a porous material mainly composed of cement bentonite are constructed in the regions of the barrier walls 2 and 2. As shown in FIG. 3, the CB piles 6 and 7 are respectively connected to a DC power source with respect to the electrode cores 4 and 5 embedded therein, and the CB pile 6 is an anode and the CB pile 7 is a cathode, Moreover, an electrolysis model using groundwater as an electrolyte is formed. Instead of the CB piles 6 and 7, an independent wall may be constructed.

When a DC power source is applied in the above-mentioned state,
As shown in Fig. 2, chromate ions (Cr
O ₄ ²⁻ ) and sulfate ions (SO ₄ ²⁻ ) move to the CB pile 6 side which is the anode, while each cation such as Pb ²⁺ , Cd ²⁺ and Ni ²⁺ is the cathode side CB pile 7 side. Move towards each. After that, chromate ions (CrO ₄ ²⁻ ) and the like that have moved toward the CB pile 6 side are captured in the porous structure of the cement bentonite-based material of the CB pile 6, and Pb ²⁺ , Cd ²⁺ , Ni ²⁺
Etc. cations are trapped in the porous structure of the CB pile 7.

The composition of the cement vent CB piles 6 and 7 is not limited to the case where it is constructed from a material that is purely cement bentonite, and one of admixtures that produce ettringite, gypsum, clay minerals, charcoal and activated carbon. Materials containing the above, for example, bentonite-cement as a basic material as shown in JP-A-61-105500, in which heavy metals are trapped, clays, siliceous materials, sodium carbonate, alkali A material mixed with a metal phosphate or a gallium salt, and further, the composition disclosed by the present applicant in Japanese Patent Application No. 4-317432, sodium bentonite, B type or C type of blast furnace cement, or a combination thereof. By containing a predetermined amount of a mixture, an admixture that produces ettringite, and an admixture containing metallic aluminum, 6 It is possible to use a material having an improved ability to capture valent chromium.

For the purpose of only capturing chromium, it is possible to simply insert I-type steel, H-type steel or the like into the ground to form the cathode without providing the CB pile 7 serving as the cathode. .

Next, in the second method, as shown in FIG. 4, CB piles 6, 6, ... Which serve as anodes are formed in the inner regions of the barrier walls 2, 2 surrounding the contaminated region, and the barrier walls 2, 6. A method in which a direct current is applied with 2 as a cathode can be mentioned. In the case of this method, while the chromium is captured by the CB piles 6, 6, ..., Each cation such as Pb ²⁺ , Cd ²⁺ , Ni ²⁺ is trapped on the inner walls of the blocking walls 2 and 2, By forming an insoluble heavy metal layer on the inner wall of the barrier wall 2, the water blocking property of the barrier wall can be improved.

Next, the third method shown in FIG.
This is an example in which the cement bentonite-based material can be exchanged when the pile B reaches the chromium capture limit amount. Hereinafter, this procedure will be described in detail with reference to FIGS.
First, as shown in FIG. 6, a drilling hole 11 is formed by a method such as an earth drill method and a reverse circulation method while using a stabilizing solution 12 such as bentonite according to a known construction procedure for a site-casting pile. Then, FIG.
As shown in FIG. 2, a hollow holder having a large number of through holes 13 a, for example, a hollow pipe 13 is inserted into the excavation hole 11. In this case, when the steady-state spacer 14 is fixed around the hollow tube 13 and inserted, the centering is easy. The spacer 14 can be removed or buried afterwards.

Thereafter, as shown in FIG. 8, a hollow space forming tube 15 with a bottom is inserted into the hollow tube 13. In this case, the space forming pipe 15 may be inserted into the excavation hole 11 together with the hollow pipe 13. The space forming tube 15 is preferably provided with a bottom so that the stabilizing liquid 12 does not enter from the bottom. Instead of the space forming tube 15, a solid tube may be used.

When the insertion of the space forming tube 15 is completed,
As shown in FIG. 9, the solidifying material 16 is solidified by adding a hardening additive to the stabilizing solution 12 and stirring it to cure it, or by replacing the stabilizing solution 12 with cement, for example.

After the fluidity of the solidifying material 16 disappears or solidifies, the space forming tube 15 is pulled out and the electrode core 14 is inserted, as shown in FIG. Then, Figure 1
As shown in FIG. 1, the hollow tube 13 is filled with a porous substance mass 18 (hereinafter, referred to as CB mass pellet) mainly composed of a cement bentonite material for capturing chromium, and the exchange shown in FIG. Build type CB piles 8, 9. In the above example, the hollow tube 13 and the space forming tube 15 may be a metal tube or a plastic tube such as vinyl chloride. In order to facilitate the removal of the space forming pipe 15,
An oil-based mold release agent may be applied to the inner surface of the hollow tube 13 or the outer surface of the space forming tube 15, or both of them.

Thereafter, a direct current is passed through the CB pile 8 as an anode and the CB pile 9 as a cathode, and the CB pellet 1
When 8 and 18 respectively adsorb chromium and other heavy metals and reach the trapping limit amount, the CB lump pellets 18 filled in the hollow tube 13 are dug out by a bucket or the like to obtain new CB lump pellets 18. Refill and re-capture contaminants.

As another method for constructing the exchangeable CB pile, as shown in FIG. 12, after forming the excavation hole 12, the hollow tube 13 with a hole is installed and the electrode core 14 is inserted, and the cement is prepared. Injecting bentonite material, as shown in FIG.
A CB pile 19 that fills the entire drill hole 12 is formed. After that, a direct current is applied, and when the CB pile 19 adsorbs chromium and reaches its capture limit amount, the cement bentonite-based material in the hollow tube 13 is dug out with an auger or the like, A CB pile 19 is newly filled with cement bentonite material.

Next, the effects of the present invention will be clarified by experiments. (Example 1) Blast furnace type B cement-250 k per 1 m ^{3 of} water
g, bentonite # 250-60kg added cement bentonite material manufactured by test pieces 20, 2
1 (φ5 cm × 10 cm) was prepared, and as shown in FIG. 14, DC 12 V was applied to the Cr ⁶⁺ solution while immersed in a 15 mg / L concentration solution of Cr ⁶⁺ and a 100 mg / L concentration solution. The change in concentration was observed. In addition, as a comparative example, changes in the concentration of the Cr ⁶⁺ solution during non-energization were observed and compared with the case of applying a direct current. The results are shown in FIG. 15 and FIG.
Shown in.

From the experimental results, simply replace the CB test piece with C
It is confirmed that Cr ⁶⁺ is trapped in the CB test piece and the Cr ⁶⁺ concentration is gradually reduced even when immersed in the r ⁶⁺ solution, but when a direct current is applied according to the present invention, It is clear that the Cr ⁶⁺ trapping rate is increased and the final solution concentration is significantly reduced.

(Example 2) In Example 2, the CB test piece was made of pure water, cement and bentonite, and mixed with various additives.
The difference in r ⁶⁺ capture ability was investigated. As additives, sodium carbonate (Na ₂ CO ₃ ), Denka CSA, gypsum, clay mineral, charcoal, and activated carbon were added. The amount of each additive added is 24 kg per 1 m ³ of water. The result is shown in FIG.

As is clear from FIG. 17, pure CB,
No significant difference was observed in the Cr ⁶⁺ trapping ability between the case of adding Denka CSA and the case of adding gypsum. Also, the addition of activated carbon, although some is in Cr ⁶⁺
Although the results are inferior to the capture ability, no significant difference is seen in this case either. However, it has been proved that when sodium carbonate is added to these, the Cr ⁶⁺ trapping ability obviously decreases.

When DENKA CSA, gypsum, clay mineral, charcoal, and activated carbon are added, the internal structure is densified, which makes it difficult to re-separate the chromium once captured. Unless added, it is desirable to add these additives.

However, when adding sodium carbonate,
It seems that this is a result of the cement bentonite structure being highly densified too much, but since the chromium capturing ability will be extremely reduced, an additive material whose internal structure is not densified more than necessary when selecting an additive Is preferred.

By the way, in the experiments in Examples 1 and 2, the current value between the CB electrodes changed with time as shown in FIG. The conductivity of CrO ₄ ^2- is as small as 40 μS / cm (microsiemens / cm) at a concentration of 10 mg / L, but Ca ²⁺ and Na are better than those of the CB test piece.
As a result of the elution of ions such as ⁺ and SO ₄ ^2- , the solution conductivity is 2
It has been changing between ~ 4mS / cm (Millisiemens / cm). The solution conductivity decreases as the number of days increases. The reason is that ions such as Ca ²⁺ , Na ⁺ , SO ₄ ²⁻ once dissolved in the solution are adsorbed on the CB test piece on the cathode side and the solution This is because the number of ions decreases.

Example 3 Next, the relationship between the applied voltage and the Cr ⁶⁺ trapping ability was investigated. As shown in FIG. 19, when the CB test pieces 20 and 21 were embedded side by side in the simulated sandy ground and DC currents of 12 V and 24 V were applied, changes in the inter-electrode current and changes in the Cr ⁶⁺ concentration were measured. I investigated the difference. The results are shown in FIGS. 20 and 21.

As shown in FIG. 20, the inter-electrode current shows the maximum value on the 1st to 2nd days when the transition after energization is observed, and thereafter tends to decrease. As a matter of course, when comparing the lines for the applied voltages of 24 V and 12 V, the lines of the applied voltages of 24 V and 12 V are almost parallel lines, and the higher the applied voltage is, the higher the inter-electrode current is. . C
As shown in FIG. 21, the change in the r ⁶⁺ concentration was significantly different between the applied voltage of 12V and 24V. It is found that the applied voltage of 24 V has much higher Cr ⁶⁺ trapping ability.

From the above, it was found that the trapping rate of Cr ^{6+ on} the CB electrode is proportional to the magnitude of the current between the electrodes. Therefore, increasing the applied voltage is effective for improving the chromium trapping ability. On the contrary, by introducing non-toxic ions such as saline or seawater to the chromium-contaminated area to reduce the electric resistance, the conductivity of groundwater, which is the electrolyte, is improved and the current flow is increased. By improving the property, it is also possible to improve the chromium capturing ability.

[0043]

As described above in detail, according to the present invention, chromium ions dissolved in groundwater are forcibly irrespective of trapping by the barrier wall, particularly in a contaminated area surrounded by the barrier wall. Can be captured and removed.
Further, by using the blocking wall as a cathode, other insoluble heavy metals other than chromium can be attached to the blocking wall surrounding the contaminated region to improve the water blocking performance of the blocking wall.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a first example of a pollutant trapping method according to the present invention.

FIG. 2 is a trap conceptual diagram of a pollutant trapping method according to the present invention.

FIG. 3 is a plan view of a CB pile.

FIG. 4 is a schematic diagram showing a second example of a pollutant trapping method according to the present invention.

FIG. 5 is a schematic diagram showing a third example of a pollutant trapping method according to the present invention.

FIG. 6 is a schematic sectional view of a first step of the construction procedure of the third example.

FIG. 7 is a schematic sectional view of a second step of the construction procedure of the third example.

FIG. 8 is a schematic sectional view of a third step of the construction procedure of the third example.

FIG. 9 is a schematic sectional view of a fourth step of the construction procedure of the third example.

FIG. 10 is a schematic sectional view of a fifth step of the construction procedure of the third example.

FIG. 11 is a schematic sectional view of a sixth step of the construction procedure of the third example.

FIG. 12 is a schematic cross-sectional view of a construction procedure of another exchanging CB pile construction method.

FIG. 13 is a schematic cross-sectional view of a construction procedure of another method of forming a replaceable CB pile.

FIG. 14 is a schematic diagram showing an experimental procedure of Example 1.

FIG. 15 is a diagram showing the results of Example 1.

16 is a diagram showing the results of Example 1. FIG.

17 is a diagram showing the results of Example 2. FIG.

FIG. 18 is a diagram showing a change in inter-electrode current and a change in electric conductivity in Examples 1 and 2.

FIG. 19 is a schematic diagram showing an experimental procedure of Example 3.

20 is a diagram showing the results of Example 3. FIG.

21 is a diagram showing the results of Example 3. FIG.

[Explanation of symbols]

2 ... Barrier wall, 3 ... Impermeable layer, 4/5 ... Electrode core, 6.7 ...
CB pile, 13 ... Hollow tube, 15 ... Space forming tube, 18 ... CB
Lump pellet, 20 ・ 21 ... CB test piece

Claims (7)

[Claims] 1. In the ground in which chromium is dissolved in groundwater, a pile or an independent wall made of a porous material mainly composed of cement bentonite is constructed and used as an anode, and a cathode member serving as a counter electrode is formed in the ground. , A direct current is applied between these electrodes to collect chromium ions dissolved in groundwater on the side of a pile or an independent wall made of a porous material mainly composed of cement bentonite serving as the anode, A method for trapping a pollutant characterized by trapping it in a porous substance. 2. The pollutant trapping method according to claim 1, wherein the cathode member is a pile or an independent wall made of a porous material mainly composed of cement bentonite. 3. A barrier wall is constructed around a contaminated or suspected contaminated region to prevent migration of contaminants from the region to the periphery, and the barrier wall is surrounded by the barrier wall. The method for trapping a contaminant according to claim 1. 4. A barrier wall is constructed around a contaminated or suspected contaminated region to prevent migration of contaminants from the region to the periphery, and within the region surrounded by the barrier wall. Cement bentonite is a pile or an independent wall made of a porous material as a main body and this is used as an anode, and the blocking wall is used as a cathode, and a direct current is applied between these electrodes, and it is dissolved in groundwater. A method for trapping a pollutant, characterized in that chromium ions are collected on a pile or an independent wall side made of a porous material mainly composed of cement bentonite as the anode and are trapped in the porous material. 5. The pollution according to any one of claims 1 to 4, wherein the porous material mainly composed of cement bentonite contains at least one admixture for producing ettringite, gypsum, clay mineral, charcoal and activated carbon as an additive. How to capture substances. 6. A hollow holder having a plurality of through holes is installed in the ground in an embedded state, and then the hollow holder is filled with a lump of a porous material mainly composed of cement bentonite for capturing chromium, The method for trapping pollutants according to claims 1 to 5, wherein this is used as an anode. 7. The method for trapping pollutants according to claim 1, wherein non-toxic ions for reducing electric resistance are introduced into a contaminated region surrounded by the barrier wall.