Permeable groundwater purification wall

Abstract

To provide a permeable underground water purification wall capable of adsorbing an organic fluorine compound (PFAS).SOLUTION: The permeable ground water purification wall 1 is a ground water purification wall including a purification material, wherein the purification material includes at least one of active carbon, ion exchange resins, and an iron-based adsorbent, the active carbon is at least one of granular (diameters: 1 to 2mm) (GAC) and powdery (diameters <1. 0mm) (PAC), and the iron-based adsorbent includes akaganate as a main component that adsorbs anions.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a permeable groundwater purification wall. [Background technology]

[0002] PFAS (fluorocarbons) is a general term for fluorocarbon compounds with a basic structure in which all hydrogen atoms in hydrocarbons have been replaced with fluorine atoms, and more than 10,000 types have been identified to date. PFAS are also known as "forever chemicals" because they rarely decompose in nature. PFAS have water- and oil-repellent properties, so they have been used in a wide range of applications, including preventing cookware from burning, waterproofing clothing, and firefighting foam.

[0003] However, recent research has pointed to a link between PFAS and health problems such as cancer. Restrictions on the production and use of PFAS have been moving forward, primarily in Europe and the United States, and this trend is accelerating in Japan as well. Against this backdrop, provisional guideline values ​​for water quality management targets for public water bodies were set in Japan in 2020 (50 ng / L for the combined value of PFOS (perfluorooctane sulfonate) and PFOA (perfluorooctanoic acid)). However, PFAS levels exceeding these provisional guideline values ​​have been detected around airports and bases where large amounts of firefighting foam have been used, raising calls for measures to prevent the spread of groundwater contamination.

[0004] On the other hand, permeable groundwater purification walls that purify groundwater are known (see Patent Documents 1 and 2 below). As measures to prevent the spread of groundwater contamination, the "Guidelines for Investigations and Measures Based on the Soil Contamination Countermeasures Act (Revised Edition 3.1) (hereinafter referred to as the "Guidelines")" specifies two measures: "Measures to Prevent the Spread of Groundwater Contamination by Pumping Facilities" and "Measures to Prevent the Spread of Groundwater Contamination by Permeable Groundwater Purification Walls."

[0005] The principles of the purification wall in the guidelines include decomposition and adsorption as follows.

[0006] Decomposition of volatile organic compounds such as trichloroethylene prevents the spread of groundwater contamination. Decomposition involves artificially creating conditions underground for the continuous decomposition of specific hazardous substances while maintaining the ground's permeability at or above that of the surrounding aquifer. Specific hazardous substances are introduced by diffusion or replacement techniques, and the specific hazardous substances in the groundwater that flows down are steadily decomposed and purified. For example, reduced iron powder, which has the ability to decompose organic chlorine compounds, is known to exert a long-term chemical decomposition effect when mixed and stirred in an aquifer. Methods for installing permeable groundwater purification walls include mixing and stirring or injecting the necessary decomposing agent into the aquifer, or replacing part of the aquifer with the purification wall.

[0007] Adsorption prevents the spread of groundwater contamination for all hazardous substances, including heavy metals such as fluorine and boron. Adsorption involves injecting, stirring, or replacing substances into the ground that have the ability to adsorb specific hazardous substances while maintaining the ground's permeability at or above that of the surrounding aquifer. This steadily adsorbs the specific hazardous substances in the groundwater that flows down, allowing the purified groundwater to flow downstream. Examples of adsorbent materials used include activated carbon and adsorbent metals. Methods for installing permeable groundwater purification walls include mixing, stirring, or injecting adsorbent materials into the aquifer, or replacing the aquifer with a purification wall pre-mixed with adsorbent materials. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-263068 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-200756 Summary of the Invention [Problem to be solved by the invention]

[0009] However, because PFAS are not yet subject to the Soil Contamination Countermeasures Act, they are not listed in the guidelines. However, if this is applied, pumping from barrier wells would be the first option. Currently, activated carbon adsorption is the primary method of water treatment for PFAS in water supply facilities, and it can be positioned as an effective measure. However, because the maintenance of equipment such as pumping wells, pumps, piping, wiring, and water treatment equipment is semi-permanent, there is a need to develop a nearly maintenance-free permeable purification barrier for PFAS, just as there is for current substances.

[0010] Therefore, the present invention has been made in consideration of the above circumstances, and provides a permeable groundwater purification wall capable of adsorbing PFAS. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention employs the following means. That is, the permeable groundwater purification wall according to the present invention is a groundwater purification wall having a purification material, and the purification material has at least one of activated carbon, ion exchange resin, and iron-based adsorbent.

[0012] In the permeable groundwater purification wall according to the present invention, the activated carbon may be in at least one of granular and powder form.

[0013] In the permeable groundwater purification wall according to the present invention, the iron-based adsorbent may include akaganite.

[0014] A permeable groundwater purification wall constructed in this way can adsorb PFAS (fluorinated organic compounds). [Effects of the Invention]

[0015] The permeable groundwater purification wall according to the present invention is capable of adsorbing PFAS (fluorinated organic compounds). [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram of a permeable groundwater purification wall according to the present invention. [Figure 2] 1 is a diagram showing a construction method of a permeable groundwater purification wall according to the present invention. [Figure 3] 2 is a diagram showing a method for constructing a permeable groundwater purification wall according to the present invention, showing a step subsequent to that shown in FIG. [Figure 4] 4 is a diagram showing a method for constructing a permeable groundwater purification wall according to the present invention, showing a step subsequent to that shown in FIG. 3. FIG. [Figure 5] 5 is a diagram showing a method for constructing a permeable groundwater purification wall according to the present invention, showing a step subsequent to that shown in FIG. 4. [Figure 6] 5A and 5B are diagrams showing a method for constructing a permeable groundwater purification wall according to the present invention, and show a subsequent step to that shown in FIG. [Figure 7] FIG. 1 is a diagram showing the relationship between the amount of adsorbent added and the equilibrium concentration when the purification material (adsorbent) is activated carbon. [Figure 8] FIG. 1 is a diagram showing the relationship between the amount of adsorbent added and the equilibrium concentration when the purification material (adsorbent) is an ion exchange resin. [Figure 9] FIG. 1 is a diagram showing the relationship between the amount of adsorbent added and the equilibrium concentration when the purification material (adsorbent) is an iron-based adsorbent. DETAILED DESCRIPTION OF THE INVENTION

[0017] A permeable groundwater purification wall according to one embodiment of the present invention will be described below with reference to the drawings. As shown in Fig. 1, a permeable groundwater purification wall (hereinafter simply referred to as a "purification wall") 1 according to this embodiment is buried above an impermeable layer G1 in the ground G so as to straddle a groundwater level G2 from top to bottom. The purification wall 1 contains a purification material. The purification material contains at least one of activated carbon, an ion exchange resin, and an iron-based adsorbent.

[0018] (activated carbon) Activated carbon can be used in water treatment systems in either granular (1-2 mm diameter) (GAC) or powdered (<1.0 mm diameter) (PAC) form. When activated carbon is applied to a purification barrier (1), it must be highly permeable to prevent groundwater flow from bypassing the barrier. For this reason, the use of GAC is preferred for purification barrier (1). GAC and PAC have been shown to be effective in removing PFAS from water.

[0019] (ion exchange resin) For common anionic PFASs, anion exchange resins can be used to remove PFASs, especially high molecular weight PFASs, and can do so more effectively than GAC.

[0020] (Iron-based adsorbent) The iron-based adsorbent contains akaganeite as the main component that adsorbs anions, and can be in an easy-to-handle form such as powder, gravel, block, or plate.

[0021] (Management method) An observation well is installed within the purification wall 1, and water samples are periodically collected and analyzed to confirm the purification effect. If an increase in concentration is confirmed in the observation well, re-construction is considered. Note that if the target substance is a regulated substance under the current Soil Contamination Countermeasures Act, the analysis period is approximately one week, but for PFAS, this currently takes approximately five to six weeks. In other words, compared to a typical purification wall, the purification wall 1 of this embodiment will be approximately one month late in detecting an increase in concentration. In light of this situation, it is necessary to devise a way to make the distance from the installation location of the observation well to the downstream end of the purification wall longer by approximately groundwater flow rate x 1 month in the case of the purification wall 1 of this embodiment than in the case of a typical purification wall.

[0022] (Construction method) The construction method of the purification wall 1 will be described.

[0023] In the device installation step, as shown in Fig. 2, a CD machine (rotary casing driver machine) 2 is installed on the ground surface at the position where the purification wall 1 is to be installed.

[0024] The casing rotation press-fitting step is carried out. As shown in Fig. 3, the casings 31 are repeatedly connected vertically to each other, and the casings 31 are rotated and pressed-fit to the target depth. As shown in Fig. 4, a casing body 3 in which multiple casings 31 are connected is completed.

[0025] A soil and obstacle removal process is carried out. The contaminated soil and obstacles inside the casing body 3 are pulled up by the hammer grab 4. The work of rotating and pressing the casing 31 and the work of removing the soil and obstacles may be carried out simultaneously in parallel, or the work of removing the soil and obstacles may be carried out after the work of rotating and pressing the casing 31 is completed.

[0026] 5, the inside of the casing body 3 is backfilled with a mixture 11 of the purification material and sand.

[0027] The casing removal process is carried out. As shown in FIG. 6, the casing 31 is removed. The mixture 11, which is long in the vertical direction, is placed underground G. The above process is repeated so that the mixture 11 is arranged in a row. In this way, a purification wall 1 is constructed in which the mixture 11 is arranged in a row. The same procedure is also used when re-constructing the purification wall 1.

[0028] (Experimental example) The present invention will be explained in more detail below by way of experimental examples, but the present invention is not limited to the following experimental examples.

[0029] Adsorption experiments for perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) were conducted, and the adsorption efficiency and characteristics were compared between the above-mentioned activated carbon, a commercially available ion exchange resin, and an iron-based adsorbent. The details are as follows.

[0030] PFOS / PFOA-contaminated water was prepared by diluting firefighting foam. The initial PFOS concentration in the foam-contaminated water was 0.89–1.7 μg / L, and the initial PFOA concentration was 1.0 μg / L. Each solution was placed in a 2-L plastic container, a predetermined amount of adsorbent added, and shaken for 1 hour. The amount of adsorbent added was set based on known adsorption performance and ranged from 0.001 to 10 g / L. After shaking, the entire solution was vacuum filtered through a 0.45 μm cellulose acetate filter, and the filtrate was used as the test solution. PFOS / PFOA contained in the test solution was adsorbed onto a solid-phase extraction polymer and eluted with a 0.1% ammonia / methanol solution. The solution was then concentrated by spraying nitrogen gas, and the concentrations were measured by LC-MS / MS.

[0031] The results are shown in Figures 7 to 9. Figure 7 shows activated carbon, Figure 8 shows ion exchange resin, and Figure 9 shows iron-based adsorbents. Figures 7 to 9 show the relationship between the amount of adsorbent added and the PFOS / PFOA equilibrium concentration obtained from adsorption tests using fire extinguishing agent-contaminated water for each adsorbent. The dotted line in the figures indicates the provisional target value of 50 ng / L for the combined PFOS / PFOA concentration.

[0032] For PFOS, the equilibrium concentration was lower with iron-based adsorbents than with other adsorbents, and the equilibrium concentration was below the provisional target value at addition of 5g / L or more. On the other hand, for PFOA, the equilibrium concentration was below the provisional target value at addition of 1g / L or more with iron-based adsorbents, and 5g / L or more with activated carbon and ion exchange resin. It was found that the equilibrium concentration of both PFOS and PFOA could be reduced to below the provisional target value of 50ng / L with the smallest addition amount of iron-based adsorbents.

[0033] The permeable groundwater purification wall constructed in this way can adsorb PFAS, which is effective in preventing the spread of contaminated groundwater. It can also be installed in a nearly maintenance-free manner.

[0034] The shapes and combinations of the components shown in the above-described embodiment are merely examples, and various modifications can be made based on design requirements, etc., within the scope of the present invention. [Explanation of symbols]

[0035] 1. Permeable groundwater purification wall

Claims

1. 1. A permeable groundwater purification wall having a purification material, comprising: The purification material is a permeable groundwater purification wall having at least one of activated carbon, ion exchange resin, and iron-based adsorbent.

2. 2. The permeable groundwater purification wall according to claim 1, wherein the activated carbon is in at least one of granular and powder form.

3. 3. The permeable groundwater purification wall according to claim 1, wherein the iron-based adsorbent comprises akaganite.

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