JP2009226348A - Permeable reaction wall and underground water purifying method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the clogging of a permeable reaction wall containing a powder-like metal reducing agent. <P>SOLUTION: The permeable reaction wall 1, which includes a first reaction layer 11 as an upstream reaction layer and a second reaction layer 12 as a downstream reaction layer 12, is embedded in the ground. The reaction wall 1 is installed so that the first reaction layer 11 is arranged on an upstream side with respect to the flow W of underground water, and the second reaction layer 12 is arranged on a downstream side. The first reaction layer 11 is constituted, for example, by mixing an iron powder having a large particle size as the metal reducing agent with a sand-like water permeable material so that a pore having a large diameter is formed to the second reaction layer 12. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a wall-shaped permeation reaction wall installed in the ground and used for purifying groundwater in its original position, and a method for purifying contaminated groundwater using the same. The present invention particularly relates to a multi-layer type permeation reaction wall configured by arranging a plurality of liquid-permeable reaction layers each including a granular metal reducing agent along the flow of groundwater, and the use thereof. It relates to a method for purifying contaminated groundwater.

  There have been cases where leachate from waste disposal sites and illegal dumping waste, factory effluent, etc. penetrated underground and contaminated groundwater. The contaminated groundwater spreads the contaminated area by its flow and causes various environmental pollution. For this reason, various purification techniques have been proposed in order to prevent contamination of surrounding areas with such contaminated groundwater.

  Conventionally, as a method for purifying contaminated groundwater, there is known a purification method by a permeation reaction wall method in which a water-permeable wall-like structure (“permeable reaction wall”: PRB, Permeable Reactive Barrier) responsible for the purification reaction is created in the ground. It has been. In this method, pollutants such as organic halogen compounds contained in groundwater are decomposed in situ through groundwater through the reaction wall.

  FIG. 3 is a schematic plan view showing a state in which the reaction wall 21 according to the conventional example is buried in the ground. In the figure, the arrow indicates the groundwater flow W, and the reaction wall 21 is disposed downstream of the groundwater flow W from the region (contaminated region) G where the aquifer is contaminated. As shown in this example, a pair of water blocking walls 22 are provided on both sides of the reaction wall 21 as necessary, and the contaminated ground water flows toward the reaction wall 21 by hitting the water blocking wall 22. Has been.

  The reaction wall 21 includes a granular metal reducing agent, and is configured to have water permeability as a whole by mixing a granular material such as sand and a metal reducing agent. When contaminated groundwater passes through such a reaction wall 21 and comes into contact with the metal reducing agent, contaminants such as organic halogen compounds are decomposed by the metal reducing agent, and the groundwater is purified.

  The permeation reaction wall method requires almost no maintenance once the reaction wall is constructed. Therefore, the cost is lower than that of the pumping treatment that pumps contaminated groundwater to the ground and purifies it, and there is an advantage that the land being purified can be used effectively.

  Examples of the metal reducing agent used for such a permeation reaction wall include metallic iron such as iron powder having reducing power. For example, Patent Document 1 and Patent Document 2 disclose the use of fine powder, sliced or fibrous iron, or a composite of these iron and activated carbon as a metal reducing agent constituting the permeation reaction wall. Patent Document 3 and Patent Document 4 disclose reaction conditions and reaction promotion methods for decomposing an organic halogen compound using metallic iron as a metal reducing agent.

As the permeation reaction wall, there is also a so-called multilayer permeation reaction wall in which a plurality of reaction layers having different properties are arranged side by side. For example, Patent Document 5 discloses a multi-layer permeation reaction wall including two reaction layers. The permeation reaction wall disclosed in Patent Document 5 includes two layers: a portion containing iron powder (first reaction layer) and a portion containing copper-containing iron powder (second reaction layer).
Japanese Patent Publication No. 5-501520 JP-T 6-506631 Publication Japanese Examined Patent Publication No. 2-49158 Japanese Patent Publication No. 2-49798 JP 2001-9475 A

  By the way, generally groundwater contains hardness components, such as calcium and magnesium. These hardness components may produce compounds that are insoluble in groundwater. In addition, when iron powder is used as a metal reducing agent when forming the reaction wall, divalent iron ions are generated by the reduction reaction of the iron powder. Like the hardness component, divalent iron ions are also insoluble in groundwater. May be generated. For this reason, when the reaction wall is used for a long period of time, the gap between the permeation reaction walls is gradually filled with insoluble compounds generated by hardness components and divalent iron ions, and in the future, the permeability of the permeation reaction walls will be localized. There is a risk of deteriorating the target or the whole.

FIG. 4 is a schematic diagram showing changes associated with the use of the reaction wall 21 of FIG. 4A is a schematic cross-sectional view of the reaction wall 21 taken along the line Z ₁ -Z ₂ at the start of purification, and FIG. 4B is a schematic plan view showing the flow of water with respect to the reaction wall 21 of FIG. FIG. 4C is a schematic cross-sectional view along the Z ₁ -Z ₂ line of the reaction wall 21 in a state where the water permeability of the permeation reaction wall is locally reduced as a result of continuing use, and FIG. It is a plane schematic diagram which shows the flow of the water with respect to the reaction wall 21.

  Regarding the reaction wall 21 according to the conventional example, the particle size of the metal reducing agent and the water-permeable material such as sand constituting the reaction wall 21 is substantially constant along the groundwater flow direction (that is, the thickness direction D of the reaction wall 21). It is. For this reason, when the purification by the reaction wall 21 is started, as shown in FIG. 4A, the gap of the reaction wall 21 is substantially constant in the thickness D direction, and the water permeability of the groundwater is also substantially equal in the thickness D direction.

  However, as the purification reaction proceeds, insoluble compounds generated by hardness components and iron ions accumulate in the reaction wall 21 and reduce the size of the gap between the reaction walls 21, so that the permeability of groundwater decreases. When the water permeability decreases, the groundwater containing the contaminants bypasses the reaction wall 21 as shown in FIG. As this detour amount increases, the organic halogen compound cannot be treated efficiently.

  This invention is made | formed in view of the said subject, and it aims at providing the purification | cleaning method of the contaminated groundwater using the structure of the permeation | transmission reaction wall in which obstruction | occlusion is prevented, and a permeation | transmission reaction wall.

  Conventionally, the particle size of iron powder used as a metal reducing agent has not been sufficiently studied, and the reaction layer is large enough to ensure the permeability of groundwater and is necessary to maintain high reaction efficiency. It was only recognized that it is preferable that the surface area be as small as possible. On the other hand, as a result of examining the above problems, it has been found that the decrease in the permeability of the permeation reaction wall occurs from the inlet side of the permeation reaction wall, that is, the upstream side of the groundwater flow. The present invention has been completed on the basis of such findings, and increases the porosity on the inlet side of the permeation reaction wall, and prevents processing defects due to a decrease in water permeability of the reaction wall even if precipitates are generated. Specifically, the present invention provides the following.

(1) A wall-shaped permeation reaction wall provided in the ground,
An upstream reaction layer comprising a particulate metal reducing agent provided upstream of a flow of contaminated groundwater flowing through the ground;
A downstream reaction layer provided on the downstream side of the groundwater flow with respect to the upstream reaction layer and containing a particulate metal reducing agent,
A permeation reaction wall in which the metal reducing agent in the upstream reaction layer has an average particle size larger than that of the metal reducing agent in the downstream reaction layer.
(2) The upstream reaction layer and the downstream reaction layer are configured by mixing the metal reducing agent and a granular water permeable material,
The permeable reaction wall according to (1), wherein the water permeable material of the upstream reaction layer has an average particle size larger than that of the water permeable material of the downstream reaction layer.
(3) The metal reducing agent is iron powder,
The permeation reaction wall according to (1) or (2), wherein the metal reducing agent and the water permeable material of the upstream reaction layer have an average particle diameter of 0.5 mm to 10.0 mm.
(4) The permeation reaction wall according to any one of (1) to (3), wherein the thickness of the upstream reaction layer is 10 to 50% of the thickness of the entire permeation reaction wall.
(5) A groundwater purification method for purifying groundwater containing an organic halogen compound through the permeation reaction wall according to any one of claims 1 to 4.

  ADVANTAGE OF THE INVENTION According to this invention, the fall of the water permeability of the reaction wall by the deposit produced | generated when groundwater passes a reaction wall can be prevented. Therefore, according to the present invention, it is possible to prevent purification failure due to a decrease in the water permeability of the reaction wall and to prolong the life of the reaction wall.

  Hereinafter, the present invention will be described in detail with reference to the drawings. Hereinafter, the same members are denoted by the same reference numerals, and description thereof is omitted or simplified.

FIG. 1 is a schematic plan view showing a state in which a multilayer permeation reaction wall 1 according to the first embodiment of the present invention is buried in the ground. FIG. 2 is a schematic diagram showing a change in the reaction wall 1 of FIG. 1 with use. FIG. 2A is a schematic cross-sectional view of the reaction wall 1 taken along line X ₁ -X ₂ at the start of purification, and FIG. 2B is a schematic plan view showing the flow of water with respect to the reaction wall 1 of FIG. is there. FIG. 2C is a schematic cross-sectional view of the reaction wall 1 that continues to be used along the Z ₁ -Z ₂ line, and FIG. 2D is a schematic plan view showing the flow of water with respect to the reaction wall 1 of FIG. is there.

  The reaction wall 1 is a wall body standing in the ground, and includes two reaction layers, a first reaction layer 11 as an upstream reaction layer and a second reaction layer 12 as a downstream reaction layer. The first reaction layer 11 and the second reaction layer 12 are arranged in the thickness direction D, and the first reaction layer 11 is disposed upstream of the second reaction layer 12 with respect to the groundwater flow W. The reaction wall 1 is arranged so that the wall surface is substantially perpendicular to the flow of contaminated groundwater, and the edge (lower end) located at the deepest part reaches the impermeable layer 73 below the aquifer 72 through which the groundwater flows. Yes. The upper end (edge facing the lower end) of the reaction wall 1 is on the ground side, about 0 to 100 cm from the groundwater level, and in this example, is located in a saturated layer 71 composed of surface soil. In this example, a pair of water blocking walls 2 are disposed in contact with a pair of edges (side edges) connecting the lower end and the upper end.

  The water blocking wall 2 is constituted by a water-impervious plate, for example, a sheet pile, and is installed so that the wall surface stands vertically in the ground so that the wall surface blocks the flow of groundwater, like the reaction wall 1. The pair of water blocking walls 2 are arranged so that the distance between them becomes narrower from the upstream side to the downstream side, and the reaction wall 1 is sandwiched between the downstream edges so that the ground water is stopped. Water is collected by the water wall 2 and passes through the reaction wall 1.

  The reaction layers 11 and 12 are both configured by mixing iron powder as a metal reducing agent with a granular water-permeable material. Sand or crushed stone is used as the water permeable material. As the metal reducing agent, granular aluminum, zinc, etc. can be used in addition to iron powder, but iron powder can be suitably used in terms of cost, and iron waste such as pig iron (cast iron) scraps is used. May be. The iron powder is preferably reduced iron.

  Both the iron powder and the water permeable material are substantially granular, and the reaction layers 11 and 12 configured by being filled with a mixture of the iron powder and the water permeable material have many particles generated between the particles. A void is formed. In the present invention, the first reaction layer 11 having a larger gap than the second reaction layer 12 is disposed on the upstream side of the groundwater flow W with respect to the second reaction layer 12.

  In order to adjust the size of the gap between the reaction layers 11 and 12, the particle size of the particulate metal reducing agent (iron powder) and / or the water-permeable material is adjusted. The ratio which a metal reducing agent accounts in the material which comprises a reaction layer is 10 to 100% by mass ratio, Preferably it is 20 to 100%. When mixing a metal reducing agent and a water-permeable material to form a reaction layer (that is, when the proportion of the metal reducing agent in the material constituting the reaction layer is less than 100%), consider the mixing ratio of the two However, for one or both of the metal reducing agent and the water permeable material, the average particle size of the metal reducing agent (and / or the water permeable material) of the first reaction layer 11 is set to the metal reducing agent (and / or the second reaction layer 12). Alternatively, the average particle size of the water-permeable material) is made larger.

  Specifically, it is preferable that the metal reducing agent of the first reaction layer 11 has an average particle size of 0.5 mm or more. In this specification, if 50% by mass or more of the metal reducing agent contained in the first reaction layer 11 has a particle size of the above value or more, the average particle size of the metal reducing agent of the first reaction layer 11 is not less than the above value. Treat as there is. Moreover, when the metal reducing agent is contained in the constituent material in an amount of 50% by mass or more, the reaction layers 11 and 12 are treated as reaction layers having a purification action by the metal reducing agent.

  As for the metal reducing agent, if the average particle size is too large, the reactivity may be insufficient, so the upper limit is preferably 10.0 mm. On the other hand, the water-permeable material of the first reaction layer 11 preferably has an average particle size of 0.5 mm or more in order to ensure water permeability. However, if the average particle diameter of the water-permeable material of the first reaction layer 11 is too large, the metal reducing agent enters the gaps that should be formed between the water-permeable materials, and the gaps are blocked, and instead the water permeability of the entire reaction wall. May decrease. For this reason, it is preferable that the average particle diameter of a water-permeable material is 10 mm or less.

  The metal reducing agent and the water-permeable material constituting the reaction layers 11 and 12 may have different average particle diameters for each reaction layer, but it is preferable that the particle diameters of the same reaction layer are uniform. Specifically, classification is performed using a sieve having a predetermined size so that 50% by mass or more of the whole is a granule having a particle size of about ± 20% of the average particle size (weight average). It is preferable that Moreover, in the same reaction layer, it is preferable that the particle diameters of the metal reducing agent and the water permeable material are also approximated. In particular, for the first reaction layer 11 that should have a large gap, the water permeable material and the metal reducing agent are The ratio of the average particle diameter is preferably about 1: 0.8 to 1: 1.2.

  About the metal reducing agent of the 2nd reaction layer 12, an average particle diameter should just be 0.25 mm or more, and should just be about 0.25 mm-2.5 mm. About the water permeable material of the 2nd reaction layer 12, an average particle diameter should just be 0.25 mm or more, and should just be about 0.25 mm-2.5 mm. Regarding the second reaction layer 12, the metal reducing agent and the water-permeable material need not have the same particle diameter, and the ratio of the average particle diameter of the metal reducing agent and the water-permeable material may be arbitrary.

  The total thickness D of the reaction wall 1 including the first reaction layer 11 and the second reaction layer 12 is about 10 cm to 5 m, and the thickness of the first reaction layer 11 is 10 to 50 with respect to the entire reaction wall 1. It is preferable to set to be%.

  Next, a method for creating the reaction wall 1 will be described. Prior to the formation of the reaction wall 1, the direction of the groundwater flow W is grasped in advance with respect to the aquifer 72 through which the contaminated groundwater flows, so that the contaminated groundwater flows toward the reaction wall 1. Determine the installation location. The reaction wall 1 is installed so that the extending direction of the wall surface and the direction of the groundwater flow W are substantially perpendicular so that the wall surface blocks the groundwater flow W.

  Although the method of burying the reaction wall 1 in the ground is not limited, a groove (trench) reaching the impermeable layer 73 is formed by excavating the ground where the reaction wall 1 is to be installed, and a metal reducing agent is formed in the trench. And a method of backfilling with a mixture of water permeable material. In the trench, the first reaction layer 11 is formed on the upstream side with respect to the groundwater flow W, and the second reaction layer 12 is formed on the downstream side. It is sufficient to embed a mixture of metal reducing agents having different particle diameters and / or water-permeable materials. Specifically, a wall such as a sheet pile is installed at the boundary between the first reaction layer 11 and the second reaction layer 12 in the trench, and each compartment is filled with a mixture constituting each reaction layer, and then the compartment Just pull through the wall separating the two.

  Next, a method for treating contaminated groundwater using the reaction wall 1 will be described. In the present invention, groundwater containing an organic halogen compound as a pollutant is treated. Examples of the organic halogen compound include tetrachloroethylene, trichloroethylene, dichloroethylene, trichloroethane, carbon tetrachloride, and dichloromethane. In the present invention, since a metal reducing agent such as iron powder is used for purification, a hardly decomposable organic halogen compound such as dioxin or PCB is not treated.

  In the present invention, the reaction wall including the first reaction layer and the second reaction layer described above is buried in the ground through which contaminated groundwater flows, and the contaminated groundwater passes through the reaction wall. When contaminated groundwater passes through the reaction wall, it comes into contact with a metal reducing agent contained in the reaction wall, whereby the organic halogen compound is reduced to metallic iron, dechlorinated, and purified. The action when decomposing an organic halogen compound using iron powder as the metal reducing agent is exemplified below.

When the organohalogen compound is reduced to metallic iron and dechlorinated, the iron contained in the reaction wall changes from zero-valent metallic iron to divalent iron ions and elutes into the groundwater. For example, when the organic halogen compound is trichlorethylene (TCE), the reaction is a reaction represented by Chemical Formula 1.
(Chemical formula 1)
CCl ₂ CCHCl + 3Fe + 3H ₂ O → CH ₂ CH ₂ + 3Fe ²⁺ + 3OH ⁻ + 3Cl ⁻

In addition, when groundwater passes through the reaction wall, in addition to the reduction reaction of the organic halogen compound of Chemical Formula 1, reactions of Chemical Formula 2 and Chemical Formula 3 in which metallic iron is oxidized to water also occur.
(Chemical formula 2)
Fe + 2H ₂ O → Fe ²⁺ + H _2aq + 2OH ⁻
(Chemical formula 3)
4Fe + 3O _2aq + 6H ₂ O → 4Fe ³⁺ + 12OH ⁻

The divalent iron ions generated by the reaction of Chemical Formula 1 and Chemical Formula 2 and the trivalent iron ions generated by the reaction of Chemical Formula 3 are eluted into the ground water. Further, the hydroxide ions (OH ⁻ ) generated by the reactions shown in Chemical Formulas 1 to 3 increase the pH of the water passing through the reaction wall. Divalent iron ions form insoluble iron hydroxide (Fe (OH) ₂ ) as the pH increases. As a result, iron hydroxide precipitates in the reaction wall, and the water permeability becomes worse by reducing the voids in the reaction wall.

  According to the knowledge of the present inventor, such a reduction in the gap of the reaction wall occurs more remarkably in the most upstream portion of the reaction wall. This is considered to be due to the fact that the reaction of Formula 1 in which the organic halogen compound is reductively dechlorinated occurs more actively in the most upstream portion of the reaction wall. On the other hand, as the groundwater permeates through the reaction wall, the organic halogen compound in the groundwater is decomposed and the concentration thereof decreases, so that the reaction of Formula 1 is less likely to occur toward the downstream side of the reaction wall. Therefore, elution of iron ions and increase in pH due to generation of hydroxide ions hardly occur on the downstream side, and it is considered that an environment in which precipitates due to insolubilization of iron ions are difficult to be generated is obtained.

  As described above, the reduction of the void in the reaction wall due to the precipitation of the insoluble iron compound is most likely to occur in the vicinity of the most upstream part. When the water permeability in the vicinity of the uppermost stream of the reaction wall decreases, the downstream flow is hindered, the water permeability of the entire reaction wall is deteriorated, and the purification performance of the permeation reaction wall is deteriorated.

Furthermore, the groundwater also contains hardness components such as calcium and magnesium (Ca ²⁺ , Mg ²⁺ ), and these hardness components precipitate on the surface of the reaction wall components, block the reaction wall and reduce its water permeability. There is a possibility of causing it. This tendency becomes stronger near the entrance.

  In order to solve such a problem, in the present invention, the reaction wall is a multi-layer type, and a portion corresponding to the uppermost stream side of the reaction wall with respect to the groundwater flow is a first reaction layer in which the water permeability is increased by increasing the gap (Upstream reaction layer). As a result, even if an insoluble compound is generated and deposited in the reaction wall, the water permeability of the upstream portion of the reaction wall is secured, so that the groundwater flows without bypassing the reaction wall 1 as shown in FIG. Problems that impair the overall water permeability can be avoided.

  As described above, the reaction wall can be composed of a granular water-permeable material such as sand and a granular purification material such as iron powder, and the water permeability can be adjusted by increasing the particle size of these particles. Here, according to Creager's equation, the water permeability coefficient is proportional to the square of the 10% particle diameter, and the larger the particle diameter, the higher the water permeability. On the other hand, the more the reaction wall is made of a granular material having a larger particle size, the more the reduction reaction proceeds because the specific surface area decreases. Therefore, when the particle size of the metal reducing agent constituting the entire reaction wall is increased, the ability to decompose the organic halogen compound is lowered. In contrast, in the present invention, the reaction wall is a multi-layer type and a metal reducing agent having a small particle diameter is used on the downstream side. Thereby, the fall of the water permeability of a permeation | transmission reaction wall can be prevented, without impairing the resolution | decomposability of the whole permeation | transmission reaction wall.

The reaction wall according to the present invention can be variously modified in addition to the reaction wall 1 according to the first embodiment. For example, a layer other than the first reaction layer (upstream reaction layer) and the second reaction layer (downstream reaction layer) may be provided. Specifically, with respect to the groundwater flow, a layer filled with a hardness component removing agent is disposed further upstream than the first reaction layer (see Japanese Patent Application Laid-Open No. 2004-255314), and downstream from the first reaction layer. For example, disposing an adsorption layer that adsorbs fluorine at any position (see JP-A-2007-260525). The former is suitable when the hardness of groundwater is high, and the latter is suitable when the groundwater contains fluorine in addition to the organic halogen compound. In addition, since metallic iron itself will be consumed by corrosion of metallic iron, the porosity will become high correspondingly. However, the true specific gravity of metallic iron is 8 g / cm ³ , whereas the true specific gravity of Fe (OH) ₂ is 3.4 g / cm ^3. Compared with the molar volume of metallic iron, it is 7 cm ³ / mol. , Fe (OH) ₂ is 26 cm ³ / mol, and when compared at the same molar concentration, metallic iron has a volume three times or more that of iron hydroxide. Therefore, it is considered that the amount of void reduction due to iron hydroxide generation is greater than the amount of void increase due to metal iron consumption during corrosion of metallic iron. Moreover, the reaction wall of this invention is not limited to the type which uses a water stop wall as shown in FIG. 1, For example, you may be comprised only with a reaction wall.

[Comparative Example 1]
A cylindrical column made of a vinyl chloride resin (inner diameter 10 cmφ × height 1 m) was filled with spherical iron powder having an average particle diameter of 1 mm, and a pan was placed at the upper and lower ends to form an experimental column C1, and a water flow device was attached. An experimental device was created. Water was passed through the column C1 from the tank T through the pump P at a flow rate of 0.2 mL / min. In order to promote the corrosion of the iron powder in the column C1, tap water in which NaCl was dissolved at 5 g / L was adjusted to prepare an adjustment liquid for an acceleration test, and the upward circulating water was supplied to the column C1.

Further, in order to follow the daily change in the water permeability coefficient of the reaction layer formed in the column C1, the column C1 is removed from the water flow apparatus, and water tanks are arranged at the upper end and the lower end of the column C1, respectively (FIG. 5B). The water permeability coefficient was measured using the apparatus shown in FIG. The hydraulic conductivity was measured once every 10 days. The hydraulic conductivity is calculated by Formula 2, which is a modification of Darcy's law expressed by Formula 1. In Equation 1, q is the volume flux of water (cm / s), ΔH is the head differential (cmH ₂ O), Δx is the distance in the direction along the flow (cm), and k is the saturated hydraulic conductivity (cm / s). ). Further, the column gradient of Equation 2 is obtained by ΔH / Δx (Equation 1).
q = −k (ΔH / Δx)
(Formula 2)
Hydraulic conductivity of the entire column = flow rate x column cross-sectional area / column gradient

  The measurement result of the daily change of the water permeability of the reaction layer in the column of Comparative Example 1 is shown in FIG. Here, the corrosion rate of metallic iron in contact with the NaCl aqueous solution having a concentration of 5 g / L is 100 times the corrosion rate with respect to water. As estimated from this, the permeability coefficient when the adjustment liquid containing NaCl is passed is reduced by about 50% after 60 days from the start of the test. Therefore, when water in the natural environment is passed, It is considered that the hydraulic conductivity decreases by about 50% after 6000 days (about 16 years).

[Example 1]
As Example 1, the column C1 used in Comparative Example 1 was filled with spherical iron powder having an average particle diameter of 4 mm in the part up to 10 cm on the inlet side of the column C1, and the remaining 90 cm part was spherical with an average particle diameter of 1 mm. Filled with iron powder. An experiment similar to the comparative example was performed except that the configuration of the reaction layer inside the column C1 was changed in this way. The results are shown in FIG.

  As shown in FIG. 7, the water permeability in the case where the NaCl adjustment liquid after 60 days passed is reduced by about 5%. In this case, the permeability coefficient is considered to decrease by about 5% after 100 times, that is, 6000 days (about 16 years).

  Thus, it was shown that the lowering of the hydraulic conductivity due to the reduction of the voids can be delayed by configuring the 10 cm portion on the inlet side of the column with iron powder having a larger particle size than the remaining 90 cm portion.

  Further, in order to confirm that the void is prevented from being reduced by the present invention, the columns of Comparative Example 1 and Example 1 were opened, iron powder was taken out every 10 cm from the inlet side, and the porosity at each position was measured. . Further, in order to reproduce the porosity before water flow in Comparative Example 1 and Example 1, the same method was used using a column newly filled with spherical iron powder having an average particle diameter of 1 mm in the apparatus of FIG. The value measured in was used as a blank. The results are shown in FIG. 8 and FIG.

  As shown in FIG. 8, it can be seen that in Comparative Example 1, the porosity in the vicinity of the column inlet is reduced by passing water for 60 days. On the other hand, according to FIG. 9, in Example 1, there is a slight decreasing tendency of the porosity in the vicinity of the column inlet due to water flow for 60 days, but since the porosity was originally high, it was the same level as the porosity of the entire column. It can be seen that the state is maintained at a high level.

  From these results, by increasing the porosity in the vicinity of the column inlet beforehand, even if the iron powder corrodes near the inlet and becomes iron hydroxide and deposits in the column, it adheres to the entire column. It was shown that the hydraulic conductivity of the can be prevented from decreasing.

  INDUSTRIAL APPLICABILITY The present invention can be used for construction work of structures such as in-situ purification of contaminated groundwater.

The top view of the permeation | transmission reaction wall which concerns on the 1st embodiment of this invention. The schematic diagram which shows the change accompanying use of the said permeation | transmission reaction wall. The top view of the permeation | transmission reaction wall which concerns on a prior art example. The schematic diagram which shows the change accompanying use of the said permeation | transmission reaction wall. The schematic diagram which shows the apparatus used for the test. The graph which shows the result of the comparative example 1. FIG. FIG. 3 is a graph showing the results of Example 1. The graph which shows the result of the comparative example 1. FIG. FIG. 3 is a graph showing the results of Example 1.

Explanation of symbols

1 Permeation reaction wall 2 Water blocking wall 11 First reaction layer (upstream reaction layer)
12 Second reaction layer (downstream reaction layer)
G Pollution area W Groundwater flow

Claims (5)

A wall-shaped permeation reaction wall provided in the ground,
An upstream reaction layer comprising a particulate metal reducing agent provided upstream of a flow of contaminated groundwater flowing through the ground;
A downstream reaction layer provided on the downstream side of the groundwater flow with respect to the upstream reaction layer and containing a particulate metal reducing agent,
A permeation reaction wall in which the metal reducing agent in the upstream reaction layer has an average particle size larger than that of the metal reducing agent in the downstream reaction layer. The upstream reaction layer and the downstream reaction layer are configured by mixing the metal reducing agent and a granular water permeable material,
The permeable reaction wall according to claim 1, wherein the water permeable material of the upstream reaction layer has an average particle size larger than that of the water permeable material of the downstream reaction layer. The metal reducing agent is iron powder,
The permeation reaction wall according to claim 1 or 2, wherein the metal reducing agent and the water-permeable material in the upstream reaction layer have an average particle diameter of 0.5 mm to 10.0 mm.   The permeation reaction wall according to any one of claims 1 to 3, wherein a thickness of the upstream reaction layer is 10 to 50% of a thickness of the entire permeation reaction wall.   A method for purifying groundwater, wherein groundwater containing an organic halogen compound is passed through the permeation reaction wall according to any one of claims 1 to 4.

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