JP2019010610A - Storage structure of heavy metal polluted soil - Google Patents

Abstract

To effectively enhance contact frequency between an adsorbent and a heavy metal in an adsorption layer while making reworkability easy.SOLUTION: A storage structure has a banking body part 1A in which at least a water shielding layer 2 and a polluted soil layer 4 containing a heavy metal are laminated from a foundation side G, and a leachate flowing down on an upper surface of the water shielding layer 3 from the polluted soil layer 4 is introduced to a side surface 2A of the water shielding layer 2, and an adsorption layer 6 which is arranged in contact with at least the side surface 2A of the water shielding layer 2, and in which the leachate leaching from the banking body part 1A is distributed and an adsorbent adsorbing the heavy metal contained in the leachate is mixed.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a storage structure for storing contaminated soil containing heavy metals.

  The shear generated by excavation may contain heavy metals (for example, cadmium, lead, hexavalent chromium, mercury, arsenic, selenium, fluorine, boron) specified by soil environmental standards. If it does not comply with the soil elution standard or content standard, it is necessary to take measures such as containment of heavy metals.

  An example of this countermeasure is an adsorption layer method. The adsorption layer method is the method of leaching the heavy metal in the contaminated soil layer into the permeated water that has penetrated into the embankment due to rainfall, etc. and leaching it into the adsorption layer, and adsorbs the heavy metal to the adsorbent while the leachate passes through the adsorption layer. This is a method of discharging by reducing the concentration of heavy metals in the leachate (hereinafter also simply referred to as contamination concentration) to a level that does not affect the environment. As a general embankment structure used for this kind of adsorption layer construction method, for example, in Patent Documents 1 to 3, an adsorption layer in which a base material such as crushed stone or sand and an adsorbent are mixed is provided below the contaminated soil layer. A structure provided over substantially the entire surface is disclosed.

JP 2011-240289A JP 2012-110852 A JP 2014-226589 A

  By the way, in order to effectively reduce the contamination concentration of the leachate, the contact frequency between the adsorbent mixed in the adsorbent layer and the heavy metal contained in the leachate (the chance that the heavy metal is adsorbed by contacting the adsorbent) is increased. It is desirable to increase.

  In the conventional general embankment structure, since the adsorption layer is provided over a wide range below the contaminated soil layer, the volume of the adsorption layer inevitably increases. For this reason, if the frequency of contact between the adsorbent and the heavy metal is increased by simply increasing the amount of adsorbent mixed, the amount of adsorbent becomes excessive with respect to the contamination concentration, resulting in a wasteful cost. There is a problem. On the other hand, if an adsorbent suitable for the contamination concentration is mixed in an adsorption layer with a large volume, the amount of adsorbent per unit volume will decrease, leading to a decrease in contact frequency, making it impossible to effectively reduce the contamination concentration. There are also challenges.

  In addition, when the leaching of heavy metals from the contaminated soil layer is prolonged and the adsorption capacity of the adsorption layer is deteriorated along with this, it is necessary to reconstruct the embankment to excavate and remove the adsorption layer and replace it with a new one. In the conventional embankment structure, since the adsorption layer is laid across the entire lower surface of the contaminated soil layer, exchanging and removing the entire embankment including the upper contaminated soil layer is necessary when replacing the adsorption layer, There is also a problem that the construction period required for reconstruction is prolonged, and further, the construction cost is increased.

  An object of the technology of the present disclosure is to provide a heavy metal-contaminated soil storage structure capable of effectively increasing the contact frequency between an adsorbent and a heavy metal in an adsorption layer while facilitating re-workability.

  In the technology of the present disclosure, at least a water-impervious layer and a contaminated soil layer containing heavy metals are laminated from the ground side, and leachate flowing from the contaminated soil layer to the upper surface of the water-impervious layer is directed to the side surface of the water-impervious layer. The embankment main body portion guided and the outer side of the embankment main body portion are provided in contact with at least the side surface of the water shielding layer, and the leachate leached from the embankment main body portion is circulated and included in the leachate water. And an adsorbing layer mixed with an adsorbing material that adsorbs the heavy metal.

  Further, a drainage layer having a higher permeability than the impermeable layer is provided between the impermeable layer and the contaminated soil layer of the embankment main body, and the leachate flowing down from the contaminated soil layer is the drainage layer. It is preferable that the flow guides toward the side surface of the water shielding layer.

  Moreover, it is preferable to further include a water shielding member that is embedded in the adsorption layer and lengthens the leachate flow path in the adsorption layer by partially blocking the flow of the leachate in the adsorption layer.

  Moreover, the said water-impervious member may be a plurality of water-impervious sheets that are alternately arranged in the adsorbing layer at intervals in the vertical direction and meander the leachate flow path.

  Moreover, the said water-impervious sheet may be embedded in the said adsorption layer, making the sheet | seat surface incline so that it may become an upward gradient toward the downstream from the flow direction upstream of the leaching water.

  In addition, the water-impervious member is a water-impervious sheet embedded in the adsorption layer in the vertical direction, and leachate is temporarily between the sheet surface of the impermeable sheet and the side surface of the impermeable layer. It may be retained and retained.

  In addition, a slope reinforcing layer may be formed on at least a portion of the embankment main body that is in contact with the adsorption layer.

  According to the heavy metal contaminated soil storage structure of the present disclosure, it is possible to effectively increase the contact frequency between the adsorbent and the heavy metal in the adsorption layer while facilitating the re-workability.

It is a typical longitudinal section showing embankment concerning this embodiment. It is a typical longitudinal section showing the important section of embankment concerning this embodiment. It is a typical perspective view showing the water collection structure concerning this embodiment. It is a mimetic diagram explaining the construction procedure of embankment concerning this embodiment. It is a schematic diagram explaining the re-construction procedure of the 2nd embankment part which concerns on this embodiment. (A) is a longitudinal cross-sectional view schematically illustrating the flow of leachate in the embankment according to the present embodiment. (B) is a schematic longitudinal cross-sectional view which shows a general embankment. It is a typical longitudinal section showing various modifications of an adsorption layer concerning this embodiment. It is a figure which illustrates typically the test device for a water flow test. (A) is a figure explaining the material used for a water flow test. (B) is a figure explaining the test case of a water flow test. It is a line graph explaining the test result of a water flow test. It is a typical longitudinal section showing embankment concerning other embodiments.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embankment 1 shown in FIG. 1 shows an example of a heavy metal contaminated soil storage structure according to the present invention. The embankment 1 is formed in a substantially trapezoidal cross section having a slope on the side surface, and is provided adjacent to the first embankment portion 1A as the embankment main body portion and the lower slope of the first embankment portion 1A. 2 fill sections 1B.

[First embankment]
The first embankment portion 1 </ b> A is configured by laminating a water shielding layer (hardly permeable layer) 2, a drainage layer 3, a contaminated soil layer 4, and a first cover soil layer 5 in order from the ground G.

  The water shielding layer 2 blocks the permeation of the leachate leached from the contaminated soil layer 4 into the drainage layer 3 into the ground G, and is laid on the bottom layer of the first embankment 1A, specifically, the surface of the ground G. A drainage layer 3 is laminated on the upper surface. The upper surface of the water-impervious layer 2 is formed in a horizontal plane shape, or is formed in an inclined plane shape slightly lowering toward the outer peripheral edge (the head), and from the contaminated soil layer 4 to the drainage layer 3. The leached water that has leached is guided in the outer circumferential direction of the first embankment 1A. Although the thickness of the water shielding layer 2 is not specifically limited, For example, it is about 30 cm-50 cm.

  The water-impervious layer 2 is laid more widely on the surface of the ground G than the outer periphery of the bottom surface of the drainage layer 3 (the buttock) in plan view, and a slope 2A inclined at a predetermined gradient angle is provided on the side surface. ing. The slope of the slope 2A is not particularly limited, but is, for example, about 1: 0.5. In the present embodiment, the slope 2A is preferably provided with a slope reinforcement layer 2B formed of Terracel (registered trademark) having a honeycomb lattice structure in a plan view, or a sheet-like geotextile or geogrid.

The water permeability coefficient P1 of the water shielding layer 2 is set to 1.0 × 10 ⁻⁷ m / s or less, which is a water shielding standard for the pond. In this embodiment, the water-impervious layer 2 having a hydraulic conductivity P1 of 9.20 × 10 ⁻⁸ m / s is formed by compacting a mixed soil of 70% mountain sand and 30% viscous soil. As the cohesive soil, for example, trade name “Tochikure” of Kanto Kasei Co., Ltd. can be used.

  The drainage layer 3 guides the leachate leached from the contaminated soil layer 4 along the upper surface of the impermeable layer 2 toward the outer periphery of the first embankment portion 1A. It is formed between the layers. That is, the lower surface of the drainage layer 3 is in contact with the upper surface of the water shielding layer 2, and the upper surface of the drainage layer 3 is in contact with the lower surface of the contaminated soil layer 4. In addition, the drainage layer 3 is laid on the upper surface of the impermeable layer 2 on the inner side of the upper surface outer periphery (head) of the impermeable layer 2 in a plan view, and the side surface is inclined at a predetermined gradient angle. A surface 3A is provided. The slope of the slope 3A is not particularly limited, but is preferably about 1: 0.5, which is equivalent to the slope of the slope 2A. The slope 3A is preferably provided with a slope reinforcement layer 3B formed of Terracel (registered trademark), geotextile, geogrid or the like.

In the present embodiment, the drainage layer 3 is formed by compacting a mixed material obtained by mixing crushed stone and sand materials such as mountain sand and river sand. The drainage layer 3 is formed by, for example, spreading and compacting the upper surface of the water-impervious layer 2 with a mixed material in which 90% of No. 7 crushed stone and 10% of mountain sand are mixed. In the drainage layer 3 formed in this way, for example, the water permeability coefficient P2 is 1.76 × 10 ⁻³ m / s, which is 1.0 × 10 ² m / s or more higher than the water permeability coefficient P1 of the water shielding layer 2. (P2> P1).

  The contaminated soil layer 4 is formed by raising contaminated soil generated by excavation or the like. The contaminated soil of this embodiment is arsenic-containing shear. The contaminated soil layer 4 is laid on the upper surface of the drainage layer 3 on the inner side of the upper surface outer periphery (the head) of the drainage layer 3 in a plan view, and the lower slope is inclined at a predetermined gradient angle on the side surface. 4A and the upper slope 4B are provided. The slope of the lower slope 4A is preferably about 1: 0.5, which is equivalent to the slope of each slope 2A, 3A, and the slope of the upper slope 4B is preferably each slope 2A, 3A, It is about 1: 1.8 which is smaller than the gradient of 4A. The hydraulic conductivity P3 of the contaminated soil layer 4 is at least a value (P3> P1) higher than the hydraulic conductivity P1 of the water shielding layer 2.

  The first cover layer 5 covers the outer surface of the contaminated soil layer 4 to prevent the contaminated soil from scattering or flowing out. The first soil covering layer 5 can be crushed stone or earth and sand. The first soil covering layer 5 of this embodiment is formed using the crushed stone 7 and covers the lower slope 4A, the upper slope 4B, and the upper surface 4C of the contaminated soil layer 4 in series. A slope reinforcing layer 5B formed of Terracel (registered trademark), geotextile, geogrid, or the like is provided at least in a portion corresponding to the lower slope 4A of the first covering layer 5.

[Second fill section]
The 2nd embankment part 1B is comprised by laminating | stacking the adsorption layer 6 and the 2nd covering soil layer 7 in an order from the ground G. As shown in FIG.

  The adsorption layer 6 adsorbs heavy metals contained in the leachate leached from the contaminated soil layer 4 through the drainage layer 3 and is provided along the slopes 2A and 3A of the water shielding layer 2 and the drainage layer 3. It has been. Specifically, as shown in FIG. 2, the adsorbing layer 6 has a vertical cross section of a substantially trapezoidal shape or a substantially triangular shape, and an inner side surface 6B of each of the slopes 2A and 3A of the water shielding layer 2 and the drainage layer 3. Is formed from the head of the drainage layer 3 (the upper end of the slope reinforcing layer 3B) to the slope of the water shielding layer 2 (the lower end of the slope reinforcing layer 2B). The gradient of the outer surface (slope) 6A of the adsorption layer 6 is not particularly limited, but is preferably about 1: 1.8 which is equivalent to the gradient of the upper slope 4B of the contaminated soil layer 4.

The adsorption layer 6 is formed by compacting a mixed material obtained by mixing crushed stone, sand material such as mountain sand and river sand, and heavy metal adsorbent. In the present embodiment, arsenic is used as the heavy metal, so an arsenic adsorbent is used. As the arsenic adsorbent, for example, trade name “Fixall (registered trademark) FB” of Ishihara Sangyo Co., Ltd. can be used. This Fixall FB contains iron oxide and calcium sulfate as main components, and the specific surface area of the iron oxide particles is 180 m ² / g or more.

The adsorption layer 6 is obtained by adding a necessary amount (for example, 30 to 50 kg / m ³ ) of an arsenic adsorbent to a base material in which 90% of No. 7 crushed stone and 10% of mountain sand are mixed and mixed. It is formed by spreading and compacting so as to cover the surface of the ground G and the slopes 2A and 3A of the water shielding layer 2 and the drainage layer 3. In the adsorption layer 6 formed in this way, for example, the permeability coefficient P4 is 1.76 × 10 ⁻³ m / s, which is equivalent to the permeability coefficient P2 of the drainage layer 3, and is 1. The value is higher than 0 × 10 ² m / s (P4≈P2> P1).

  The second soil covering layer 7 functions as a slope reinforcing layer of the adsorption layer 6 by covering the slope 6 </ b> A of the adsorption layer 6. For the second soil covering layer 7, crushed stone, earth and sand, etc. similar to the first soil covering layer 5 can be used. In the present embodiment, the second covering layer 7 is preferably formed of Terracell (registered trademark), geotextile, geogrid, or the like.

[Water collection structure]
Next, based on FIG.2, 3, the detail of the water collection structure 20 is demonstrated. The water collecting structure 20 is for monitoring the adsorption capacity (arsenic concentration of leachate) of the adsorption layer 6, and includes a plurality of perforated pipe members 21A to 21C and a plurality of perforated pipe members 21A to 21C. Individual primary catchment basins 22A-C, drainage grooves 23 connecting each primary catchment basin 22A-C, and final catchment basin 24 for collecting and storing leachate flowing out from each primary catchment basin 22A-C. (Shown only in FIG. 3). In FIG. 2, reference numeral 8 denotes a water shielding sheet that is laid between the ground G and the adsorption layer 6 and prevents permeation of the leachate into the ground G. The water-impervious sheet 8 is preferably laid between the slope reinforcing layers 2B and 3B from the vicinity of the upper edge of the primary catchment basins 22A to 22C.

  The perforated pipe members 21 </ b> A to 21 </ b> C take out at least a part of the leachate flowing in the adsorption layer 6, and the base end side in which a plurality of through holes are formed in the side surface is embedded in the adsorption layer 6. ing. In addition, the perforated pipe members 21 </ b> A to 21 </ b> C have a tip side that does not have a through-hole on the side surface protruding outward from the second soil covering layer 7. In this embodiment, as shown in FIG. 3, each perforated pipe member 21A-C is arrange | positioned by the space | interval (for example, 50-100m space | interval) suitably in the longitudinal direction of the method bottom of the 2nd covering soil layer 7, The adsorption layer 6 is divided into a plurality of monitoring sections A to C by these perforated pipe members 21A to 21C.

  The primary water collecting basins 22A to 22C are, for example, substantially casing-shaped concrete water collecting basins that are open at the top, and are embedded in the ground G corresponding to the lower end portions of the perforated pipe members 21A to 21C, respectively. Has been. In each primary catchment 22A-C, leachate discharged from the corresponding perforated pipe members 21A-C is stored.

  The drainage groove 23 is, for example, a concrete U-shaped groove whose upper part is open, and is embedded in the ground G adjacent to the method bottom of the second covering soil layer 7. Moreover, the drainage groove 23 is extended substantially in parallel with the method bottom longitudinal direction of the 2nd covering soil layer 7 so that each primary catchment 22A-C may be connected (refer FIG. 3). The depth of the drainage groove 23 is formed shallower than that of the primary catchment basins 22A to 22C, and when the level of leachate stored in the primary catchment basin 22A to 22C rises above the bottom surface of the drainage trench 23, The leachate flows out into the drainage groove 23 from the primary catchment basins 22A-C. Further, the bottom surface of the drainage groove 23 is formed in an inclined surface shape that descends from one end side in the longitudinal direction (left side in FIG. 3) to the other end side (right side in FIG. 3). The leachate flowing out from C into the drainage groove 23 is guided to the other end of the drainage groove 23.

  The final catchment 24 shown in FIG. 3 is, for example, a substantially case-like concrete catchment with an open top, embedded in the ground G where the other end (downstream end) of the drainage groove 23 is located. Has been. The final catchment basin 24 is preferably formed with a volume larger than that of each primary catchment basin 22A-C. In the final catchment basin 24, leachate flowing out from the primary catchment basins 22A to 22C and flowing in the drainage groove 23 is stored.

  In the water collecting structure 20 configured as described above, the adsorption capacity (arsenic concentration of leachate) of the adsorption layer 6 can be monitored according to the following procedures (1) to (3).

  (1) First, the arsenic concentration of leachate stored in the final catchment basin 24 is measured, and when a change in the measured value is confirmed (for example, when the arsenic concentration exceeds a predetermined control value) Measures the arsenic concentration of the leachate stored in each primary catchment 22A-C individually.

  (2) If there is a primary catchment 22A-C in which the arsenic concentration of the leachate exceeds the control value among the primary catchments 22A-C, the leachate in the primary catchment 22A-C Continue to monitor the arsenic concentration.

  (3) If the arsenic concentration of the leachate exceeds the control value as a result of monitoring, the monitoring sections A to C (for example, the primary collection of the perforated pipe members 21A to C corresponding to the primary catchment 22A to C) If the arsenic concentration of the leachate in the water tank 22B exceeds the control value, it is estimated that the adsorption capacity of the adsorption layer 6 in the monitoring section B) corresponding to the perforated pipe member 21B has deteriorated. In such a case, the adsorption layer 6 and the second soil covering layer 7 in the corresponding monitoring section B are excavated and removed, and the second embankment portion 1B is newly reconstructed to replace the adsorption layer 6 anew. The detailed procedure for reconstructing will be described later.

[Construction procedure, re-construction procedure]
Next, based on FIG. 4, the construction procedure of the embankment 1 of this embodiment is demonstrated.

  First, as shown in FIG. 4 (A), the mixed soil to be the water-impervious layer 2 is spread and compacted on the surface of the ground G, and the slope reinforcing layer 2B is provided on the side surface, thereby providing the water-impervious layer. 2 is formed.

  Next, as shown in FIG. 4 (B), on the upper surface of the impermeable layer 2, the mixed material that will become the drainage layer 3 is spread on the inner side of the head of the impermeable layer 2 and compacted. The drainage layer 3 is formed by providing the slope reinforcing layer 3B.

  Next, as shown in FIG. 4 (C), on the upper surface of the drainage layer 3, the contaminated soil that becomes the contaminated soil layer 4 is spread inside the head of the drainage layer 3, and the contaminated soil is covered. A crushed stone or the like to be the first soil covering layer 5 is spread and compacted, and a slope reinforcing layer 5B is provided on the lower side surface. Thereby, the 1st embankment part 1A which laminated | stacked the water-impervious layer 2, the drainage layer 3, the contaminated soil layer 4, and the 1st cover soil layer 5 in order from the ground G is completed.

  Finally, as shown in FIG. 4 (D), the adsorbing layer 6 is formed by spreading and compacting the mixed material to be the adsorbing layer 6 on the surface of the ground G and the side surfaces of the respective slope reinforcing layers 2B and 3B. 6 is formed, and further, the second embankment portion 1B is completed by laying and compacting and reinforcing the crushed stone or the like to be the second covering soil layer 7 so as to cover the side surface of the adsorption layer 6.

  The bank 1 thus completed is configured such that the first bank 1A and the second bank 1B can be separated. For this reason, for example, when the leaching of the pollutant from the contaminated soil layer 4 is prolonged and the adsorption capacity of the adsorption layer 6 is deteriorated accordingly, the second embankment is not excavated and removed. Only the part 1B is excavated and removed, and re-construction for newly replacing the adsorption layer 6 can be performed. Hereinafter, based on FIG. 5, the re-construction procedure of the 2nd embankment part 1B is demonstrated.

  First, as shown in FIG. 5A, the second embankment portion 1 </ b> B where the adsorption capacity has deteriorated is excavated and removed in the order of the second covering layer 7 and the adsorption layer 6. At this time, the slope of the first embankment portion 1A is strongly reinforced by the slope reinforcement layers 2B, 3B, and 5B. Therefore, even if the second embankment portion 1B is removed, the slope does not collapse and cracks are generated. The shape can be maintained without any occurrence.

  Next, as shown in FIG. 5 (B), a new admixture containing adsorbent is spread on the surface of the ground G and the side surfaces of the respective slope reinforcement layers 2B and 3B and compacted. An adsorbing layer 6 is formed. At this time, if the water-impervious sheet 8 or the perforated pipe members 21A to 21C are deteriorated or damaged, they are also replaced with new ones.

  Finally, as shown in FIG. 5 (C), the second embankment portion is formed by laying and compacting and reinforcing the crushed stone or the like to become the second covering layer 7 so as to cover the side surface of the new adsorption layer 6. The re-construction of 1B is finished. As described above, when the second embankment portion 1B is reconstructed, only the adsorbing layer 6 of the portion where the adsorbing capacity is deteriorated and the second covering soil layer 7 covering the adsorbing layer 6 while leaving the first embankment portion 1A. The excavation and removal can be done, and a new adsorption layer 6 can be replaced at the removed location. Therefore, the construction period can be shortened and the construction cost can be effectively reduced compared to the conventional structure that requires the entire embankment to be reconstructed. can do. Further, every time the adsorption capacity of the adsorption layer 6 deteriorates, it is only necessary to reconstruct the second embankment portion 1B while leaving the first embankment portion 1A including the contaminated soil layer 4, so that the diffusion of the pollutants is prevented. It becomes possible to store the contaminated soil continuously over a long period of time.

[Adsorption]
Next, the heavy metal adsorption action by the embankment 1 of the present embodiment will be described.

  As shown in FIG. 6, the permeated water that has permeated into the contaminated soil layer 4 due to rain or the like flows down inside the contaminated soil layer 4 as indicated by an arrow F1. At that time, arsenic contained in the contaminated soil is eluted into the permeated water. For this reason, the arsenic concentration in the permeated water becomes higher as it goes below the contaminated soil layer 4. The permeated water is leached into the drainage layer 3 from the lower surface of the contaminated soil layer 4. Here, since the lower surface of the drainage layer 3 and the upper surface of the impermeable layer 2 are in contact with each other, the leachate flowing down to the drainage layer 3 is drained along the upper surface of the impermeable layer 2 as indicated by the arrow F2. It flows in the layer 3 toward the outer periphery of the embankment 1. When reaching the slope 3A of the drainage layer 3, the leachate flows downward through the adsorption layer 6 and is discharged from the adsorption layer 6 to the ground G as indicated by the arrow F3.

Here, arsenic eluted in the leachate is adsorbed by the adsorbent while flowing through the adsorption layer 6. In the present embodiment, the adsorption layer 6 is provided on the side surfaces of the water shielding layer 2 and the drainage layer 3 without being laid on the entire lower surface of the contaminated soil layer 4. That is, as shown in FIG. 6B, in this embodiment, the adsorbing layer 600 has an adsorbing layer 600 as compared with the conventional embankment 100 in which the adsorbing layer 600 is laid over a wide range between the water shielding layer 200 and the contaminated soil layer 400. The volume of 6 is kept relatively small, and when approximately the same amount of adsorbent is mixed, a large amount of adsorbent is contained per unit volume (adsorbent concentrates). As described above, when the adsorbent mixing amount per unit volume increases, the contact frequency (contact opportunity) between arsenic and the adsorbent in the adsorbent layer 6 is reliably increased, and the adsorbent 6 is discharged from the adsorbent layer 6. The arsenic concentration of the leachate can be effectively reduced. Further, in order to uniformly mix the adsorbent with the adsorbing layer 6 in general civil engineering work, it is necessary to secure a predetermined amount or more (for example, 30 to 50 kg / m ³ or more) of the adsorbent per unit volume. preferable. In this embodiment, since the mixing amount of the adsorbent per unit volume is increased, it becomes easier to mix the adsorbent more uniformly than the conventional embankment 100, and the workability can be improved reliably.

[Modification]
7A to 7C show the adsorption layer 6 in which the leaching water flow path in the adsorption layer 6 is lengthened to effectively increase the contact frequency between the arsenic contained in the leachate and the adsorbent. It is a typical longitudinal cross-sectional view which shows various modifications.

  In an example shown in FIG. 7A, a plurality of (two in the illustrated example) water-impervious sheets 9A and B extending in the substantially horizontal direction in the adsorption layer 6 are alternately arranged opposite to each other with an interval therebetween. The leachate flow path is formed in a substantially meandering shape. Specifically, the upper first water-impervious sheet 9 </ b> A substantially extends in the adsorption layer 6 from the head of the water-impervious layer 2 (the upper end of the side surface of the slope reinforcing layer 2 </ b> B) toward the inner surface of the second soil covering layer 7. It extends in the horizontal direction and is embedded in the adsorption layer 6 while ensuring a flow path for allowing the leachate to flow down between its tip and the inner side surface of the second soil covering layer 7. The lower second water-impervious sheet 9B extends in a substantially horizontal direction in the adsorption layer 6 below the first water-impervious sheet 9A from the inner surface of the second cover layer 7 toward the slope 2A of the water-impervious layer 2. It is embedded in the adsorption layer 6 while extending and securing a flow path for allowing the leachate to flow between the front end portion and the slope 2 </ b> A of the water shielding layer 2.

  That is, the leachate leached from the drainage layer 3 into the adsorption layer 6 is caused to flow along the upper surface of the first impermeable sheet 9A, as indicated by the arrow F1 in the figure, and from the tip of the first impermeable sheet 9A. It flows down to the upper surface of the second water-impervious sheet 9B. Further, the leachate flowing down to the second water-impervious sheet 9B is caused to flow along the upper surface of the second water-impervious sheet 9B as shown by the arrow F2 in the figure, and from the tip of the second water-impervious sheet 9B to the ground G. By flowing down to the surface, leachate flows in the adsorbing layer 6 in a meandering manner. In this way, when the leachate flow path in the adsorption layer 6 is formed long in a meandering manner, the contact frequency between the arsenic contained in the leachate and the adsorbent is effectively increased, and the arsenic concentration of the leachate is increased. It becomes possible to reduce it reliably.

  In the example shown in FIG. 7B, the first and second water shielding sheets 9A and B in FIG. 7A are inclined. Specifically, the upper first water-impervious sheet 9 </ b> A has its sheet surface inclined at a predetermined angle so as to rise upward from the water-impervious layer 2 side toward the second cover soil layer 7 side. The lower second water-impervious sheet 9 </ b> B is inclined at a predetermined angle so that the second water-impervious sheet 9 </ b> B has an upward slope from the second soil covering layer 7 side toward the water-impervious layer 2 side.

  That is, the leachate leached from the drainage layer 3 into the adsorption layer 6 goes up the gradient surface of the first water-impervious sheet 9A as indicated by an arrow F1 in the figure, and the second leachate from the tip of the first water-impervious sheet 9A. 2 Flowed down to the impermeable sheet 9B. Further, the leachate flowing down to the second water-impervious sheet 9B goes up the slope surface of the second water-impervious sheet 9B as indicated by an arrow F2 in the figure, and the surface of the ground G from the tip of the second water-impervious sheet 9B. By flowing down, the leachate flows in a meandering manner while rising obliquely in the adsorption layer 6. Thus, when the leachate flow path in the adsorption layer 6 is formed in a meandering shape having an upward gradient, the flow rate of the leachate is kept low, and the contact frequency between the arsenic contained in the leachate and the adsorbent is effective. Thus, the arsenic concentration of the leachate can be reliably reduced.

  In an example shown in FIG. 7C, the flow of leachate in the adsorption layer 6 is temporarily dammed by embedding a water-impervious sheet 9C in the adsorption layer 6 so that the sheet surface is substantially vertical. A flow path that stops and stays is formed. Specifically, the water-impervious sheet 9 </ b> C extends obliquely upward in the adsorption layer 6 from the surface of the ground G substantially parallel to the slope 2 </ b> A of the water-impervious layer 2, and has an upper end portion and an inner portion of the second soil covering layer 7. It is embedded in the adsorption layer 6 while ensuring a flow path for the leachate to flow between the side surfaces.

  That is, the leachate leached from the drainage layer 3 into the adsorption layer 6 is retained and retained between the slope 2A of the water shielding layer 2 and the water shielding sheet 9C, as indicated by an arrow F1 in the figure. When the water level rises from the upper end portion of the water-impervious sheet 9C, the water level flows over the water-impervious sheet 9C as indicated by an arrow F2 in the figure. As described above, when the leachate is temporarily retained in the adsorption layer 6, the contact frequency between the arsenic contained in the leachate and the adsorbent can be effectively increased, and the arsenic concentration of the leachate can be reliably ensured. It becomes possible to reduce.

  In any of the modifications shown in FIGS. 7A to 7C, the members that form the leachate flow path have been described as the water shielding sheets 9 </ b> A to 9 </ b> C. However, the flow path can be partitioned in the adsorption layer 6. As long as it is a member, it may be a plate member having water shielding properties.

[Water flow test]
It was confirmed by a water flow test that the adsorption efficiency of arsenic increases by increasing the contact frequency between arsenic and adsorbent. Hereinafter, the water flow test will be described. The water flow test was performed using a test apparatus 50 shown in FIG. The test apparatus 50 includes a water flow column 51, a liquid feeding tube 52, a storage container 53, a liquid feeding pump 54, a three-way valve 55, and a collection container 56.

  The water flow column 51 is a member for allowing the simulated contaminated water 57 to flow through the simulated adsorption layer 58, and includes a cylindrical column 61, a first plug member 62, a second plug member 63, and a first filter material 64. The second filter material 65 is provided. The cylindrical column 61 has an inner diameter of 100 mm and a height (length in the vertical direction) of 150 mm. Then, with the lower end of the cylindrical column 61 closed by the first plug member 62 and the first filter material 64 disposed, the simulated adsorption layer 58 is filled until the height reaches 60 mm. After the filling, the upper end of the cylindrical column 61 is closed by the second plug member 63 in a state where the second filter material 65 is disposed on the upper surface of the simulated adsorption layer 58.

  Each of the first plug member 62 and the second plug member 63 is formed with a through-hole penetrating in the thickness direction of the plug. The first plug member 62 is inserted with the downstream end of the first liquid supply tube 52A, and the second plug member 63 is inserted with the upstream end of the second liquid supply tube 52B.

  In the storage container 53, the simulated contaminated water 57 is stored, and the upstream end of the first liquid feeding tube 52A is inserted. The liquid feed pump 54 is provided in the middle of the first liquid feed tube 52A, and supplies the simulated contaminated water 57 stored in the storage container 53 to the water flow column 51 through the first liquid feed tube 52A at a predetermined speed. . The three-way valve 55 is provided in the middle of the second liquid feeding tube 52B, and the simulated contaminated water 57 'discharged from the water flow column 51 is allowed to flow downstream of the second liquid feeding tube 52B, or a waste liquid container ( Select whether to discharge (not shown). The collection container 56 is inserted with the downstream end of the second liquid feeding tube 52b, and collects leachate 57 'discharged from the second liquid feeding tube 52B.

  As shown in FIG. 9A, an arsenic solution having a concentration of 0.1 mg / L was used as simulated contaminated water 57 in the water flow test. Further, No. 4 silica sand or No. 6 silica sand was used as the soil of the simulated adsorption layer 58, and the above-mentioned Fixall FB was used as the arsenic adsorbent. And the mixed soil used as the base of the simulated adsorption layer 58 was produced by mixing an arsenic adsorbent with soil at a predetermined ratio.

  In the water flow test, six types of test cases A to F shown in FIG. In test case A, a simulated adsorption layer 58 was prepared by adding and mixing an arsenic adsorbent at a rate of 5 kg / t with respect to the soil. Then, the simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 0.2 mL / min. In test case B, a simulated adsorption layer 58 was prepared by adding and mixing an arsenic adsorbent at a rate of 5 kg / t to the soil. The simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 2 mL / min. In test case C, an arsenic adsorbent was added and mixed at a rate of 5 kg / t to the soil to produce a simulated adsorption layer 58. The simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 20 mL / min. In test case D, a simulated adsorption layer 58 was produced by adding and mixing an arsenic adsorbent at a rate of 20 kg / t with respect to the soil. Then, the simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 0.2 mL / min. In test case E, an arsenic adsorbent was added and mixed at a rate of 20 kg / t with respect to the soil to produce a simulated adsorption layer 58. The simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 2 mL / min. In test case F, a simulated adsorption layer 58 was produced by adding and mixing an arsenic adsorbent at a rate of 20 kg / t with respect to the soil. The simulated contaminated water 57 was passed through the simulated adsorption layer 58 at a water flow rate of 20 mL / min.

  The test results of the water flow test are shown in the graphs of FIGS. 10 (A) and 10 (B). The vertical axis of each graph indicates the arsenic concentration contained in the leachate 57 ′ collected by the collection container 56, that is, the arsenic concentration in the leachate 57 ′ after passing through the simulated adsorption layer 58. In addition, the horizontal axis of each graph indicates the amount of water passing through the simulated adsorption layer 58.

  In the water flow test, for each of the test cases A to F, the arsenic concentration of the leachate 57 ′ was measured every time a predetermined amount of water passed through the simulated adsorption layer 58, and the relationship between the water flow rate and the arsenic concentration was obtained. Regarding the arsenic concentration, the Environmental Agency Notification No. 46 sets the environmental standard value in soil to 0.01 mg / L. For this reason, in the water flow test, the amount of water flow until the arsenic concentration reached 0.01 mg / L was compared in each test case.

  As shown in FIG. 10 (A), in test cases A to C in which the arsenic adsorbent was added at a rate of 5 kg / t, the water flow rate was about 80 L and the arsenic concentration exceeded 0.01 mg / L.

  As shown in FIG. 10 (B), in all of the test cases D to F to which the arsenic adsorbent was added at a rate of 20 kg / t, the arsenic concentration was almost constant over the period until the water flow reached about 200 L. In general, the level was in the vicinity of 0 mg / L. When the water flow rate exceeded about 300 L, the arsenic concentration increased and reached about 0.01 mg / L at about 400 L.

  In test cases A to C in which the arsenic adsorbent was added at a rate of 5 kg / t, the water flow rate until the arsenic concentration reached 0.01 mg / L was about 80 L, and the arsenic adsorbent was added at a rate of 20 kg / t. In the test cases D to F, the amount of water flow until the arsenic concentration reaches 0.01 mg / L is about 400 L. Therefore, the larger the amount of arsenic adsorbent mixed per unit volume, the greater the amount of arsenic and arsenic adsorbent. It was confirmed that the arsenic treatment capacity was improved by increasing the contact frequency of arsenic. Like the embankment 1 of this embodiment, if the volume of the adsorption layer 6 is kept small and the amount of the arsenic adsorbent mixed per unit volume is increased, the contact frequency between the arsenic and the arsenic adsorbent in the adsorption layer 6 can be increased. be able to. For this reason, in the embankment 1, arsenic can be effectively adsorbed to the adsorption layer 6.

[Others]
In addition, this invention is not limited to the above-mentioned embodiment, In the range which does not deviate from the meaning of this invention, it can change suitably and can implement.

  In the said embodiment, although the adsorption layer 6 demonstrated as what was provided so that each slope 2A, 3A of the water-impervious layer 2 and the drainage layer 3 might be provided, for example, as shown in FIG. The height may be approximately the same as the height of the water shielding layer 2, and the drainage layer 3 may be extended to the upper surface of the adsorption layer 6. Also in this case, the volume of the adsorbing layer 6 is kept small, and a large amount of adsorbing material per unit volume is ensured, so that the contact frequency between arsenic and the adsorbing material in the adsorbing layer 6 can be reliably increased. .

  In the above embodiment, the drainage layer 3 is provided between the water-impervious layer 2 and the contaminated soil layer 4. However, the drainage layer 3 is omitted and the upper surface of the impermeable layer 2 is contaminated. The soil layer 4 may be directly laminated. In this case, it is preferable to provide a horizontal gutter for guiding leachate in the width direction of the embankment 1 between the impermeable layer 2 and the contaminated soil layer 4.

  Further, the heavy metal to be adsorbed has been described by taking arsenic as an example in the above embodiment, but is not limited to arsenic. In addition to arsenic, cadmium, lead, hexavalent chromium, mercury, selenium, fluorine, boron, and other heavy metals specified by soil environmental standards are heavy metals to be adsorbed.

DESCRIPTION OF SYMBOLS 1 ... Embankment, 1A ... 1st embankment part (embankment main-body part), 1B ... 2nd embankment part, 2 ... Water-impervious layer (hard-permeable layer), 2A ... The slope of a water-impervious layer, 2B ... The method of a water-impervious layer Surface reinforcement layer, 3 ... Drainage layer, 3A ... Drainage layer slope, 3B ... Drainage layer slope reinforcement layer, 4 ... Contaminated soil layer, 4A ... Bottom slope of contaminated soil layer, 4B ... Contaminated soil layer Upper slope, 4C ... upper surface of contaminated soil layer, 5 ... first soil cover layer, 5B ... slope reinforcement layer of first soil cover layer, 6 ... adsorption layer, 6A ... slope of adsorption layer, 7 ... second soil cover layer , 9A to C ... Water shielding sheet (water shielding member), 20 ... Catchment structure, 21A to C ... Perforated pipe member, 22A to C ... Primary catchment, 23 ... Drain, 24 ... Final catchment, DESCRIPTION OF SYMBOLS 50 ... Test apparatus of water flow test, 51 ... Water flow column, 52 ... Liquid feed tube, 52A ... 1st liquid feed tube, 52B ... 2nd liquid feed tube, 53 ... Storage container, 54 ... Liquid feed pump, 55 ... Three-way valve 56 ... collection container, 57 ... simulated contaminated water, 57 '... leachate, 58 ... simulated adsorption layer, 61 ... cylindrical column, 62 ... first plug member, 63 ... second plug member, 64 ... first filter material, 65 ... second filter material, G ... ground

Claims (7)

An embankment main body in which at least a water shielding layer and a contaminated soil layer containing heavy metals are laminated from the ground side, and leachate flowing from the contaminated soil layer to the upper surface of the water shielding layer is guided toward the side surface of the water shielding layer. And
An adsorbent that is provided on the outer side of the embankment main body portion so as to be in contact with at least the side surface of the water shielding layer and that circulates the leachate leached from the embankment main body portion and adsorbs the heavy metal contained in the leachate water. And a mixed adsorbing layer. A storage structure for heavy metal contaminated soil. A drainage layer having a higher permeability than the impermeable layer is provided between the impermeable layer and the contaminated soil layer of the embankment main body, and the leachate flowing down from the contaminated soil layer passes through the drainage layer. The heavy metal contaminated soil storage structure according to claim 1, wherein the heavy metal contaminated soil storage structure is guided toward the side surface of the water shielding layer. 3. The heavy metal according to claim 1, further comprising a water shielding member that is embedded in the adsorption layer and lengthens the leachate flow path in the adsorption layer by partially blocking the flow of the leachate in the adsorption layer. Contaminated soil storage structure. The heavy metal-contaminated soil storage structure according to claim 3, wherein the water-impervious member is a plurality of water-impervious sheets that are alternately arranged in the adsorbing layer at intervals in the vertical direction to meander the leachate flow path. . 5. The heavy metal contaminated soil according to claim 4, wherein the water shielding sheet is embedded in the adsorption layer with the sheet surface inclined so as to rise upward from the upstream side in the flow direction of the leachate. Storage structure. The water-impervious member is a water-impervious sheet embedded in the adsorption layer in the vertical direction, and leachate is temporarily blocked between the sheet surface of the water-impervious sheet and the side surface of the water-impervious layer. The heavy metal contaminated soil storage structure according to claim 3. The storage structure for heavy metal-contaminated soil according to any one of claims 1 to 6, wherein a slope reinforcing layer is formed at least on a portion of the embankment main body that is in contact with the adsorption layer.

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