Fluorine adsorbent, method for manufacturing purifying substance, and method for inhibiting diffusion of contaminated underground water

Abstract

To provide a fluorine adsorbent capable of reducing an amount of fluorine elution particularly immediately after adsorption initiation.SOLUTION: An adsorbent 10 includes a first adsorbent 11 composed of a layered double hydroxide component, and a second adsorbent 12 that adsorbs fluorine derived from the first adsorbent 11. The second adsorbent 12 includes a red ball soil. The second adsorbent 12 is the red ball soil. A content of the second adsorbent 12 is 50-80 pts.mass in a total of 100 pts.mass of the first adsorbent 11 and the second adsorbent 12.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a fluorine adsorbent, a method for producing a purifying body, and a method for suppressing the diffusion of contaminated groundwater. [Background technology]

[0002] When groundwater contaminated with fluoride (contaminated groundwater) is pumped up and treated as industrial wastewater using water treatment facilities, wastewater standards are applied. The wastewater standard for fluoride concentration is 8 mg / L or less. To reduce the fluoride concentration to 8 mg / L or less, this can be achieved by adding chemicals such as calcium hydroxide and aluminum sulfate to the wastewater in multiple stages.

[0003] On the other hand, for some reasons, standards stricter than the above wastewater standards may be set. Specifically, for example, when groundwater purifiers (permeable groundwater purification walls, groundwater purification piles, etc.) are used to adsorb and remove the pollutants underground without pumping them, when there is an additional charge based on ordinances, etc., or when there is a regional agreement, etc., the treatment performance may be required to be 0.8 mg / L or less, which is the groundwater environmental standard value or water quality environmental standard value. To reduce the concentration to 0.8 mg / L or less, for example, layered double hydroxides (synthetic or natural hydrotalcite, etc.) can be used.

[0004] Patent Document 1 describes that "magnesium oxide and aluminum chloride are used as raw materials, and water is added and mixed so that the sum of the mass of the water and the mass of the water of crystallization in the raw materials is 1 or less when the sum of the masses of the solids in the raw materials excluding the water of crystallization is taken as 1, and then the mixture is cured to obtain an adsorbent having excellent adsorption performance, which contains fine hydrotalcite. In addition, iron powder and magnetite powder are mixed to obtain a granular adsorbent having magnetic properties." [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-136703 A Summary of the Invention [Problem to be solved by the invention]

[0006] Since fluorine has a very large electronegativity, it easily reacts with other elements. Therefore, fluorine in the form of fluoride ions or the like is easily incorporated into layered double hydroxides having an anion exchange function, both in the process of producing natural layered double hydroxides and in the process of producing synthetic layered double hydroxides. Therefore, when fluorine adsorption is performed using the fluorine adsorbent described in Patent Document 1, fluorine derived from the layered double hydroxide may be eluted immediately after water starts passing through the fluorine adsorbent (immediately after adsorption starts). As a result, the amount of fluorine eluted, especially immediately after adsorption starts, may exceed environmental standards, and there may be a period during which the water treatment performance using the adsorbent is not achieved. The problem to be solved by the present disclosure is to provide a fluorine adsorbent capable of reducing the amount of fluorine eluted, particularly immediately after the start of adsorption, a method for producing a purifying body, and a method for suppressing the diffusion of contaminated groundwater. [Means for solving the problem]

[0007] The fluorine adsorbent of the present disclosure includes a first adsorbent composed of a layered double hydroxide component and a second adsorbent that adsorbs fluorine derived from the first adsorbent. Other solutions will be described later in the description of the embodiment of the present invention. Effect of the Invention

[0008] According to the present disclosure, it is possible to provide a fluorine adsorbent capable of reducing the amount of fluorine eluted, particularly immediately after the start of adsorption, a method for producing a purifying body, and a method for suppressing the diffusion of contaminated groundwater. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram of a fluorine adsorbent according to the present disclosure. [Diagram 2] 1 is a graph showing the results of a column test using a conventional fluorine adsorbent. [Diagram 3] FIG. 2 is a diagram illustrating an installation form of a groundwater purifier using the fluorine adsorbent of the present disclosure. [Figure 4] FIG. 2 is a diagram illustrating an underground installation form of a groundwater purifier using the fluorine adsorbent of the present disclosure. [Diagram 5] FIG. 11 is a diagram illustrating an underground installation form of a groundwater purifier according to another embodiment. [Figure 6] 1 is a flowchart showing a method for producing a groundwater purifier according to the present disclosure. [Figure 7] 1 is a graph showing the amount of fluorine elution for each mixing ratio of akadama soil when a first layered double hydroxide and akadama soil are used in combination. [Figure 8] 1 is a graph showing the amount of fluorine elution for each mixing ratio of akadama soil when a second layered double hydroxide and akadama soil are used in combination. [Figure 9] 1 is a graph showing the amount of fluorine elution for each mixing ratio of Akadama soil when a first layered double hydroxide and Kanuma soil are used in combination. [Figure 10] 1 is a graph showing changes in dust concentration in a building over time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, a form for carrying out the present disclosure (referred to as an embodiment) will be described with reference to the drawings. In the following description of one embodiment, other embodiments applicable to the one embodiment will also be described as appropriate. The present disclosure is not limited to the following one embodiment, and different embodiments can be combined with each other or modified as desired without significantly impairing the effects of the present disclosure. In addition, the same symbols will be used for the same members, and duplicate descriptions will be omitted. Furthermore, the same names will be used for members having the same functions. The contents shown are merely schematic, and for the sake of illustration, changes may be made from the actual configuration within a range that does not significantly impair the effects of the present disclosure, and some members may be omitted or modified between drawings. In addition, the same embodiment does not necessarily need to have all the configurations.

[0011] FIG. 1 is a schematic diagram of a fluorine adsorbent (hereinafter, simply referred to as "adsorbent 10") according to the present disclosure. In FIG. 1, the sizes of the first adsorbent 11 and the second adsorbent 12 are for convenience, and the relative sizes of the first adsorbent 11 and the second adsorbent 12 are not limited to the example shown in FIG. 1. The adsorbent 10 includes the first adsorbent 11 and the second adsorbent 12. The first adsorbent 11 is composed of a layered double hydroxide component. The second adsorbent 12 adsorbs fluorine derived from the first adsorbent 11. The adsorbent 10 may include components other than the first adsorbent 11 and the second adsorbent 12.

[0012] The layered double hydroxide component constituting the first adsorbent 11 contains water-soluble fluorine that cannot be completely removed during the manufacturing process of the layered double hydroxide component. The fluorine referred to here is usually in the form of fluoride ions (F - Fluoride ion) and fluorine exists particularly in groundwater as fluoride ion. However, fluorine may be fluorine alone, a fluorine compound, etc. In the following, unless otherwise specified, the term "fluorine" includes fluoride ion, fluorine alone, fluorine compound, etc.

[0013] Fig. 2 is a graph showing the results of a column test using a conventional fluorine adsorbent (comparative example). First, the test method for obtaining the results of Fig. 2 will be described. In this test, a cylindrical glass column with a diameter of 4 cm and a length of 30 cm was filled with a mixture of a fluorine adsorbent made of layered double hydroxide (hydrotalcite) and sand (permeable base material (parent material)). Therefore, the adsorbent (purifying body) described in the above Patent Document 1 was used, and the second adsorbent 12 (Fig. 1) was not used.

[0014] An aqueous solution with a fluorine concentration (fluoride ion concentration) of 1.5 mg / L was passed from the bottom to the top of this column, and the fluorine concentration (amount of fluorine eluted) at the column outlet (top) was measured over time. The results are shown in Figure 2. In Figure 2, the vertical axis is the fluorine concentration (mg / L) at the column outlet, and the vertical axis is the number of gap replacements (PV=Pore Volume). The number of gap replacements indicates the number of times the pore volume of the adsorbent packed inside the column has been replaced, and is a value obtained by dividing the cumulative amount of water passed by the pore volume. The larger the horizontal value, the more the water has passed.

[0015] As mentioned above, the environmental standard value for fluoride concentration in groundwater is 0.8 mg / L. Therefore, it is preferable that the fluoride concentration at the column outlet is also 0.8 mg / L or less. However, as shown in Figure 2, in the initial stage from the start of the test (PV=0) to about PV=3, the fluoride concentration does not decrease sufficiently, and in this initial stage, the fluoride concentration exceeds 0.8 mg / L. This is thought to be due to the fact that the fluorine present inside the layered double hydroxide is eluted in the form of fluoride ions when water starts to pass through the column, causing the fluoride concentration to momentarily increase in the initial stage.

[0016] For reference, the inventors measured the amount of fluorine elution from a commercially available synthetic layered double hydroxide and its calcined product (neither of which contained the second adsorbent 12 (FIG. 1)). The measurements were carried out based on the elution amount test pursuant to Environmental Notification No. 18 of the Soil Contamination Countermeasures Act. The results are shown in Table 1 below.

[0017] [Table 1]

[0018] For samples 4 and 5, layered double hydroxide A and layered double hydroxide A' are the same brand (type) except for the different production lots. As shown in Table 1, there is variation in the amount of fluorine elution, and there are samples with almost no elution (samples No. 1, 3, and 4). On the other hand, there is also a sample with an amount of elution that greatly exceeds the environmental standard value (sample No. 2). However, for example, in sample No. 2, the large amount of fluorine elution may be due to variation in the production lot, since there is no case where production control was performed from the viewpoint of controlling the amount of fluoride ion elution. For example, when comparing samples No. 4 and 5, there is a two-fold difference due to the difference between the lots. In any case, this may affect the increase in fluoride ion concentration in the early stage described with reference to Figure 2 above, and such variation is a phenomenon that cannot be ignored.

[0019] As shown in Figure 2, once water has passed through to a certain extent (i.e., the number of pore replacement events has increased) and the fluorine in the layered double hydroxide has been dissolved to a certain extent, the fluorine concentration in the wastewater will fall below the environmental standard. For a column of approximately the size of the interior space in this test, it will not take long for the fluorine concentration to fall below the environmental standard. However, because groundwater flows slowly underground, there is a possibility that the fluoride ion concentration will exceed the environmental standard over a certain period of time (e.g., several months) in the early stages, just as in the trend shown in Figure 2. In other words, there may be a period when the adsorption performance cannot be demonstrated.

[0020] Therefore, the adsorbent 10 of the present disclosure contains, in addition to the first adsorbent 11 composed of a layered double hydroxide component, a second adsorbent 12 that adsorbs fluorine derived from the first adsorbent 11. By containing the second adsorbent 12, the second adsorbent 12 can adsorb fluorine derived from the first adsorbent 11 that is generated during the passage of water, and an excessive increase in the fluorine concentration in the wastewater can be suppressed. This makes it possible to reduce the amount of fluorine eluted, particularly immediately after the start of adsorption.

[0021] Returning to FIG. 1, the layered double hydroxide component constituting the first adsorbent 11 may be either the layered double hydroxide itself or a component derived from the layered double hydroxide (a component derived from the layered double hydroxide). The layered double hydroxide is a clay mineral, of which hydrotalcite is a representative natural mineral. The layered double hydroxide is represented by the general formula [M 2+ (1-x) M 3+ x (OH)2] x+ [A n- x / n ·yH2O] x- It is expressed as M 2+ is a divalent cation, M 3+ is a trivalent cation, A is an n-valent anion, x is a number greater than 0 and less than 1, and y is a real number greater than or equal to 0. Examples of divalent cations include Mg, Fe, Cu, Ni, and Ca. Examples of trivalent cations include Al, Fe, Cr, and Co.

[0022] In layered double hydroxides, octahedra are formed in which 6 OH groups are coordinated to divalent cations. Layered double hydroxides have a structure in which some of the hydroxide layers in which the octahedra are arranged in a plane are replaced with trivalent cations. Anions exist between the layers to offset (neutralize) the positive charge according to the amount of replaced trivalent cations. These anions are exchangeable and can be replaced by various anions (CO3 2- , Cl - , SO4 2- , F - In addition, anions can be adsorbed not only between the layers but also on the edge surfaces of the hydroxide layers.

[0023] The layered double hydroxide preferably contains an Mg-Al double hydroxide. The Mg-Al double hydroxide contains Mg as a divalent cation and Al as a trivalent cation. By containing an Mg-Al double hydroxide, it is possible to easily adsorb anions such as fluoride ions.

[0024] The layered double hydroxide component preferably contains a calcined layered double hydroxide. 2-) can be removed, so by using the fired product, it is possible to use a layered double hydroxide in which the presence of carbonate ions between the layers is suppressed. Carbonate ions have low exchangeability with other anions such as fluoride ions. Therefore, by using a layered double hydroxide in which the presence of carbonate ions is suppressed, it is possible to promote the adsorption of fluoride ions.

[0025] However, the amount of fluorine eluted tends to increase by firing, as shown in Samples Nos. 1, 4 and 5 in Table 1. The reason for this is not necessarily clear, and is not limited to the following reasons, but according to the inventors' investigation, it is believed to be due to the following reasons.

[0026] First, fluorine is a halogen like chlorine, and is therefore contained in the layered double hydroxide so as to substitute for chlorine in magnesium chloride, aluminum chloride, etc. In particular, when producing layered double hydroxide by wet synthesis, a solid (white precipitate) is obtained by dissolving the raw material (e.g., magnesium hydroxide, aluminum hydroxide, etc.) and adjusting the pH to alkaline in the presence of carbonate. This solid is uncalcined Mg-Al layered peroxide, and the ions present between the layers are mainly carbonate ions, with some fluoride ions.

[0027] Because these ions are fixed between the layers, the amount of fluorine eluted is small even when water is passed through. However, when calcined under conditions such as at 500°C or higher, the carbonate ions and the like are removed by gasification, but the fluoride ions remain without being gasified, and some of them are converted to a water-soluble form such as magnesium fluoride. The fluorine that has been converted to a water-soluble form remains in the layered double hydroxide and is easily eluted from the layered double hydroxide when water is passed through. For this reason, the amount of fluorine eluted from a calcined layered double hydroxide is greater than that from an uncalcined (uncalcined) layered double hydroxide.

[0028] However, as described above, the adsorbent 10 of the present disclosure can reduce the amount of fluorine eluted immediately after the start of adsorption, effectively suppressing the increase in the amount of fluorine eluted that occurs with the improvement in exchangeability with anions due to firing.

[0029] The firing conditions are not particularly limited, but the firing can be performed, for example, at 400° C. to 800° C. The firing time can be, for example, 1 hour to 12 hours.

[0030] The BET specific surface area of ​​the first adsorbent 11 is, for example, 50 m 2 / g or more, preferably 100m 2 / g or more, for example, 250m 2 / g or less, preferably 200m 2 / g or less.

[0031] The shape of the first adsorbent 11 is preferably, for example, powder (particulate). The particle size of the first adsorbent 11 is, for example, 0.1 μm or more, preferably 1 μm or more, more preferably 5 μm or more, and the upper limit is, for example, 50 μm or less, preferably 20 μm or less, more preferably 10 μm or less. The particle size can be measured as the average particle size D50 using, for example, a laser diffraction particle size distribution measuring device.

[0032] The second adsorbent 12 preferably contains an adsorbent that adsorbs fluorine by anion exchange. The fluorine referred to here is usually a fluoride ion, but the fluoride ion generated from the above-mentioned elemental fluorine or a fluorine compound may be adsorbed by anion exchange. By using such an adsorbent, it is possible to easily adsorb fluoride ions by utilizing the anion exchange effect.

[0033] The second adsorbent 12 preferably contains volcanic ash clay (volcanic ash soil). By using volcanic ash clay, fluorine can be adsorbed by anion exchange. In addition, volcanic ash clay is porous, and can adsorb fine powder when mixed with a water-permeable base material such as gravel or sand, as described in detail below. This makes it possible to suppress the generation of dust during mixing and subsequent handling. As a result, it is possible to improve the handling of the fluorine adsorbent during the manufacture of the groundwater purifier 1 at the construction site.

[0034] The volcanic ash clay soil preferably contains at least one of Akadama soil and Kanuma soil. In particular, the second adsorbent 12 preferably contains Akadama soil as the volcanic ash clay soil.

[0035] Akadama soil is a gardening material made by drying red soil from the Kanto Loam layer and then crushing and classifying it. It is a natural soil-based material, and water that comes into contact with Akadama soil is weakly acidic (e.g., pH 4.5 to 6.5). Here, for example, fluorine in groundwater is Al(OH)3F - In some cases, fluorine may be ionized in a complex form such as [Al(OH)3]nF. This type of fluorine may change to a form such as [Al(OH)3]nF due to the weakly acidic pH in the vicinity of Akadama soil. As a result, fluorine can be coprecipitated, improving the fluorine adsorption performance.

[0036] Akadama soil also has a pH buffering effect because it contains a clay mineral called allophane and soil humus. Akadama soil is a soil that is rich in silica (Si), aluminum (Al), iron (Fe) and carbon (C). Akadama soil has a relatively similar moisture content (for example, 15% to 30% by mass) regardless of the area where it is collected or the production lot. Therefore, the use of Akadama soil makes it easier to suppress dust generation. For the same reason, dust generation can also be suppressed even with soil with a similar moisture content, such as Kanuma soil.

[0037] Powdered (particulate) akadama soil can be manufactured, for example, by the following procedure. First, akadama soil is excavated from a mining site, such as the Kanto Loam layer, and the excavated akadama soil is transported to a factory. Next, the akadama soil is dried in the sun, for example, outdoors under a roof, crushed and classified, and finally bagged according to particle size. The bagged akadama soil can be generally sold, for example, as a gardening material. On the other hand, powdered akadama soil that has passed through a sieve with the smallest openings during classification is surplus powder and cannot be used for sale as a gardening material. Therefore, such akadama soil can be preferably used as the second adsorbent 12.

[0038] The size of the second adsorbent 12 is preferably a size that passes through a sieve with an opening of, for example, 1 cm or less, preferably 5 mm or less, more preferably 2 mm or less. There is no particular lower limit, but it is, for example, 10 μm or more.

[0039] The BET specific surface area of ​​the second adsorbent 12 is, for example, 50 m 2 / g or more, preferably 100m 2 / g or more, for example, 300m 2 / g or less, preferably 250m 2 / g or less.

[0040] As for the relative amounts of the first adsorbent 11 and the second adsorbent 12 used, it is preferable that the content of the second adsorbent 12 is 50 parts by mass or more and 80 parts by mass or less, in a total of 100 parts by mass of the first adsorbent 11 and the second adsorbent 12. By making the content of the second adsorbent 12 50 parts by mass or more, the fluorine adsorption effect due to the addition of the second adsorbent 12 can be fully exerted. On the other hand, by making the content of the second adsorbent 12 80 parts by mass or less, the dust generation suppression effect due to the use of the second adsorbent 12 can be enhanced.

[0041] The second adsorbent 12 is preferably in a powder form. The powdered second adsorbent 12 has a large specific surface area, and therefore has an increased fluorine adsorption capacity and dust adsorption capacity, and can particularly suppress an increase in the amount of fluorine eluted and dust generation.

[0042] However, the second adsorbent 12 does not need to be in powder form, but it is preferable that the particle size of the second adsorbent 12 is relatively small. In particular, if the particle size is, for example, 1 cm or less as measured using a tape measure or the like, the dust generation suppression effect can be particularly effectively exhibited. The second adsorbent 12 having a particle size of, for example, 1 cm or less as an actual measurement value can be produced, for example, by crushing a large second adsorbent 12 collected from the collection site.

[0043] The second adsorbent 12 may be molded into a powder or the like when the adsorbent 10 is used. For example, at a construction site, a bagged adsorbent 10 (premix) may be unpacked and spread on a water-permeable base material (mainly crushed stone, etc.) that has been laid in a steel water tank or the like, and mixed for, for example, about 10 to 30 minutes with a heavy machine such as a backhoe. By this mixing, even if the size of the second adsorbent 12 before mixing exceeds, for example, 1 cm, the second adsorbent 12 may be molded into a size of, for example, 1 cm or less (preferably powdered) due to, for example, the pressure of a bucket, friction with the water-permeable base material, and the like.

[0044] FIG. 3 is a diagram for explaining a use form of a groundwater purifier 1 using the adsorbent 10 of the present disclosure. In the example of FIG. 1, the groundwater purifier 1 is, for example, a groundwater purification wall having water permeability and a wall shape. However, the groundwater purifier 1 may also be a groundwater purification pile having a pile shape. The purifier that purifies water by adsorbing fluorine in water is not limited to the groundwater purifier 1 and the groundwater purification pile. In addition, the adsorbent 10 (FIG. 1) usually adsorbs fluoride ions contained in groundwater. However, the fluorine adsorbent of the present disclosure may adsorb fluorine such as fluorine (element) and fluorine compounds.

[0045] In FIG. 3, a pollution source 2 that generates fluoride ions is present underground within a site S of a factory. Contaminated groundwater 3 that is generated from the pollution source 2 or that is contaminated by passing near the pollution source 2 flows underground. The groundwater purifier 1 is installed, for example, at a right angle to the direction of the flow of the contaminated groundwater 3 and in the shape of a wall so as to hinder the flow of the contaminated groundwater 3. In FIG. 3, the groundwater purifier 1 is provided with groundwater purifiers 1a, 1b, and 1c by bending any two points toward the pollution source 2 at a predetermined angle. The groundwater purifiers 1b and 1c, which are also rectangular plates, are connected to both ends of the rectangular plate-shaped groundwater purifier 1a. The pair of groundwater purifiers 1b and 1c are bent toward the pollution source 2 so that the contaminated groundwater 3 that comes into contact with the surfaces of the groundwater purifiers 1b and 1c flows along the surfaces into the groundwater purifier 1a.

[0046] The groundwater purifier 1a includes an adsorbent 10 (FIG. 1) and is configured to have water permeability. The specific form in which the groundwater purifier 1a includes the adsorbent 10 will be described later. Meanwhile, the groundwater purifiers 1b and 1c are configured with water-impermeable structures such as steel sheet pile water barriers or soil cement diaphragm walls. This allows the contaminated groundwater 3 to be efficiently guided to the groundwater purifier 1a. Also, the amount of adsorbent 10 used can be reduced compared to when the adsorbent 10 is used throughout the groundwater purifier 1.

[0047] Contaminated groundwater 3 flowing into the groundwater purifier 1a has the fluorine contained therein adsorbed and removed by the adsorbent 10 as it passes through the groundwater purifier 1a, and the resulting purified groundwater 4 flows out on the opposite side to the pollution source 2.

[0048] Fig. 4 is a diagram illustrating an underground installation form of a groundwater purifier 1 using the adsorbent 10 (Fig. 1) of the present disclosure. The groundwater purifier 1a can be installed by installing a columnar or wall-shaped body containing the adsorbent 10 underground. The groundwater purifier 1a may be installed in a state in which the groundwater purifiers 1b and 1c are connected. There are no particular limitations on the specific installation method, and any method capable of installing the purifier underground may be adopted.

[0049] When adopting the columnar embodiment, it is preferable to use a plurality of columnar bodies 1d and arrange them in a row so that they are in contact with each other. This allows almost all of the contaminated groundwater 3 introduced into the groundwater purifier 1 to come into contact with the adsorbent 10, achieving a higher degree of purification.

[0050] The columnar body 1d usually includes, for example, a water-permeable base material. The columnar body 1d can be manufactured, for example, by molding a mixture of the adsorbent 10 and the water-permeable base material into, for example, a columnar shape. Examples of the water-permeable base material include ground materials such as sand, gravel, crushed stone, and gravel, plant materials such as activated carbon, and artificial structures such as porous concrete. By including the water-permeable base material, the permeability of the contaminated groundwater 3 can be improved, and the adsorption efficiency of the groundwater purifier 1 for fluoride ions can be improved. In addition, clogging in the groundwater purifier 1 can be suppressed, and the adsorption performance can be maintained for a long period of time.

[0051] Fig. 5 is a diagram illustrating an underground installation form of a groundwater purifier 1 according to another embodiment. A plurality of pillars 1d may be intermittently arranged in a row. This can reduce the amount of adsorbent 10 used and the installation cost. The installation form shown in Fig. 5 is preferably adopted when the fluoride ion concentration in the contaminated groundwater 3 is relatively low. In addition, the interval between adjacent pillars 1d is preferably an interval (density) that can sufficiently purify the contaminated groundwater 3.

[0052] The groundwater purifier 1 is not limited to the examples shown in Fig. 4 and Fig. 5. For example, the groundwater purifier 1 may be configured as a straight wall without bending. The side groundwater purifiers 1b and 1c may be formed of columnar bodies 1d or wall bodies using the adsorbent 10, similar to the central groundwater purifier 1a, instead of steel sheet piles.

[0053] According to the groundwater purifier 1 shown in the above-mentioned Figs. 3 to 5, the diffusion of the contaminated groundwater 3 can be suppressed. That is, the method for suppressing the diffusion of the contaminated groundwater 3 of the present disclosure is a method for suppressing the diffusion of the contaminated groundwater 3 containing fluorine from the area A1 shielded by the permeable groundwater purifier 1 to the area A2 outside the area A1. As described above, the permeable groundwater purifier 1 is configured to include the adsorbent 10 and a base material such as a permeable base material. According to such a method, the purified groundwater 4 flows out to the area A2 by the action of the adsorbent 10, so that the diffusion of the contaminated groundwater 3 into the area A2 can be suppressed.

[0054] Fig. 6 is a flow chart showing a manufacturing method of the groundwater purifier 1 of the present disclosure (hereinafter referred to as the manufacturing method of the present disclosure). The manufacturing method of the present disclosure will be described below by taking as an example the groundwater purifier 1 (Fig. 3) that purifies contaminated groundwater 3 (Fig. 3; an example of an object to be treated) by adsorbing fluorine in the contaminated groundwater 3. The groundwater purifier 1 is a purifier that purifies groundwater.

[0055] However, the manufacturing method of the present disclosure may be a method for manufacturing a purifier that purifies a treatment object by adsorbing fluorine in the treatment object, and the treatment object is not limited to the contaminated groundwater 3, and the purifier is not limited to the groundwater purifier 1. Therefore, the treatment object may be, for example, water other than the contaminated groundwater 3, a liquid other than water, a solid, etc. The purifier may be, for example, a purifier that purifies water other than groundwater, a purifier installed on the ground, etc. The fluorine adsorption to the purifier can be performed, for example, by contacting the treatment object containing fluorine with the purifier. The contact here refers to, for example, passing a fluorine-containing liquid through the inside of the purifier, or physical contact between a fluorine-containing solid and the purifier. In the physical contact between the fluorine-containing solid and the purifier, the liquid may be passed through the solid and the purifier continuously in the contacted state, so that fluorine flows out from the solid to the purifier, and the fluorine may be adsorbed by the purifier.

[0056] The manufacturing method of the present disclosure includes a mixing step S1 and a manufacturing step S2. The mixing step S1 is a step of mixing an adsorbent 10 (FIG. 1) with a water-permeable substrate (an example of a substrate) to obtain a mixture. The adsorbent 10 is as described above with reference to FIG. 1 and the like. The water-permeable substrate is as described above with reference to FIG. 4 and the like.

[0057] In the mixing step S1, it is preferable to use a premix in which the first adsorbent 11 and the second adsorbent 12 are premixed as the adsorbent 10. By using a premixed mixture (premix) in this way, a mixture of the adsorbent 10 and the water-permeable base material can be produced in an open system by mixing with heavy machinery such as a backhoe. Note that the "open system" refers to, for example, outdoors, the inside of a mixer without a lid, the inside of a mixer that is in communication with the outside air, etc.

[0058] For example, when the adsorbent 10 is manufactured by mixing a powder with a high specific gravity, such as metallic iron powder, with a water-permeable base material such as crushed stone or sand, the columnar body 1d (FIG. 4, an example of a purifying body) can be manufactured while suppressing dust generation by simply stirring and mixing with heavy machinery such as a backhoe in a large open tank and appropriately spraying water. On the other hand, when a fine powder material with a low specific gravity, such as a calcined product of a layered double hydroxide, is used as the first adsorbent 11, a different method is used from the viewpoint of suppressing dust generation. Specifically, for example, a so-called "two-stage manufacturing" can be used in which water and the layered double hydroxide are mixed in a small closed mixer to obtain a slurry, and then the slurry is mixed with a water-permeable base material again.

[0059] However, as disclosed herein, for example, when the adsorbent 10 is shipped from the factory, a premix (advance mixture) in which the first adsorbent 11 and the second adsorbent 12 are mixed in advance is packed (e.g., stored in a flexible container pack) in advance in fixed amounts, and the premix is ​​used as the adsorbent 10. This makes it possible to produce the material used in the groundwater purifier 1 while suppressing dust, even if the adsorbent 10 and the water-permeable base material are mixed using heavy machinery in an open system.

[0060] By using such a premix of the adsorbent 10, a dust suppression effect is obtained when the adsorbent 10 is mixed with the base material. Therefore, the mixing step S1 can be performed in an open system. By performing the mixing step in an open system, no large-scale equipment is required to suppress scattering of dust into the surroundings, which simplifies the equipment configuration and improves the degree of freedom in the location of use of the adsorbent 10.

[0061] The reason why the dust suppression effect occurs is not necessarily clear, and is not limited to the following reasons. However, according to the inventors' investigations, it is presumed to be, for example, the following reasons.

[0062] For example, particles of volcanic ash clay such as Akadama soil and Kanuma soil have a large specific surface area. For this reason, it is believed that dust is adsorbed to the surface of the porous volcanic ash clay. Furthermore, the moisture content of Akadama soil, Kanuma soil, etc. is more or less the same (for example, 15 parts by mass to 30 parts by mass). Moisture penetrates into the pores, and despite having a moisture content in this range, the volcanic ash clay actually appears dry. Therefore, there is no excess moisture, and it is in a moderately wet state, which is believed to further improve the dust generation suppression effect.

[0063] The manufacturing step S2 is a step of manufacturing a groundwater purifier 1 (FIG. 3, an example of a purifier) ​​using the mixture obtained in the mixing step S1. Specifically, for example, the mixture is molded into a columnar body 1d (FIG. 4, etc.), and the columnar body 1d is placed underground, whereby the groundwater purifier 1 can be manufactured underground. EXAMPLES

[0064] The present disclosure will now be further explained with reference to specific examples.

[0065] <Material> As the first adsorbent 11 (FIG. 1), two types of Mg-Al-based layered double hydroxides having the physical properties shown in Table 2 below were used. Both the first layered double hydroxide and the second layered double hydroxide are Mg-Al-based double hydroxides, and both are fired products. The Mg salt indicates the type of magnesium salt contained, and the average particle size is a value measured using a laser diffraction particle size distribution measuring device. The amount of elution is the amount of fluorine elution measured based on the soil elution amount test described later.

[0066] [Table 2]

[0067] Akadama soil from Tochigi Prefecture was used as the second adsorbent 12 (Fig. 1) as the volcanic ash clay. Akadama soil was used that was suspended in 10 times the volume of the soil in water so that the pH of the water became 5.8. The Akadama soil used was Akadama soil that passed through a sieve with a mesh size of 2 mm. For the Akadama soil, multiple sieves with finer mesh were used, and the amount of the soil that passed through each sieve was calculated. The average particle size of the Akadama soil used was calculated from the size of the mesh size of each sieve and the amount of the soil that passed through. As a result, the average particle size was 350 μm. The volcanic ash clay was obtained by premixing the surplus powder generated during sieving (surplus powder when Akadama soil for gardening was used. Powder that passed through a sieve with a mesh size of 2 mm) with the first layered double hydroxide to obtain the adsorbent 10 (Fig. 1). The mixing ratio of the first adsorbent 11 and the second adsorbent 12 was 100 parts by mass in total, and the content of the second adsorbent 12 was 50 parts by mass, 60 parts by mass, 70 parts by mass, 80 parts by mass, and 90 parts by mass.

[0068] The above-mentioned operation was carried out for each of the cases where the first layered double hydroxide and the second layered double hydroxide were used as the first adsorbent 11. Thus, a total of 10 types of adsorbents 10 were produced.

[0069] <Test 1 (Evaluation of adsorption performance using the first layered double hydroxide)> First, for five types of adsorbents 10 made using the first layered double hydroxide, distilled water was added in an amount 10 times the mass of the adsorbent 10 in accordance with the soil elution amount test (Ministry of the Environment Notification No. 18), and the amount of eluted fluorine was measured. The results are shown in FIG.

[0070] Fig. 7 is a graph (Example) showing the amount of fluorine eluted for each blend ratio of akadama soil when the first layered double hydroxide is used in combination with akadama soil. The numerical values ​​written above each bar graph are the amount of fluorine eluted, and this is the same in Figs. 8 and 9 described later. Fig. 7 also shows, as a comparative example, a bar graph showing the amount of elution (values ​​in Table 2 above) when only the first layered double hydroxide without akadama soil is used. This is the same in Fig. 9 described later. On the vertical axis, 0.6 mg / L is the amount of elution when only the first layered double hydroxide without akadama soil is used. Also, 0.8 mg / L is the environmental standard value, and this is the same in Figs. 8 and 9 described later.

[0071] In the case of the first layered double hydroxide alone, which does not contain akadama soil (Comparative Example), the amount of fluorine elution (0.6 mg / L) was slightly below the environmental standard value (0.8 mg / L), with almost no margin above the environmental standard value. On the other hand, when 50 parts by mass of akadama soil was mixed, the amount of elution was reduced to 0.16 mg / L. This value was a reduction of more than 70% from the amount of fluorine elution (0.6 mg / L) in the Comparative Example. Furthermore, the amount of elution increased as the amount of akadama soil used increased. In particular, by using 70 parts by mass or more of akadama soil, the amount of elution was reduced by more than 95% compared to the Comparative Example.

[0072] From this result, it was found that the fluorine adsorption effect can be sufficiently exhibited by setting the content of the second adsorbent 12 to 50 parts by mass or more with the total of the first adsorbent 11 and the second adsorbent 12 being 100 parts by mass.

[0073] <Test 2 (Evaluation of adsorption performance using second layered double hydroxide)> Except for using the second layered double hydroxide instead of the first layered double hydroxide, the amount of fluorine eluted was measured in the same manner as in Test 1. The results are shown in FIG.

[0074] Figure 8 is a graph showing the amount of fluorine eluted for each mixing ratio of akadama soil when the second layered double hydroxide is used in combination with akadama soil. Figure 8 also shows a bar graph showing the amount of elution (values ​​in Table 2 above) when only the second layered double hydroxide without akadama soil is used as a comparative example. On the vertical axis, 0.2 mg / L is the amount of elution for the comparative example when only the second layered double hydroxide without akadama soil is used.

[0075] Unlike the case where only the first layered double hydroxide was used, even when only the second layered double hydroxide was used without Akadama soil, the amount of leaching was significantly below the environmental standard value. Furthermore, when Akadama soil was added to the second layered double hydroxide, the amount of leaching could be reduced to almost zero. In particular, when the amount of Akadama soil used was 60 parts by mass or more and 90 parts by mass or less, the amount of leaching could be reduced to 90% or less.

[0076] Thus, in this test, the use of the second layered double hydroxide kept the environmental standard below the standard without using akadama soil. However, as mentioned above, the amount of fluorine elution can vary depending on the production lot. Therefore, even if another lot approaches or exceeds the environmental standard, akadama soil can prevent it from exceeding it.

[0077] <Test 3 (Evaluation of adsorption performance using Kanuma soil)> The amount of fluorine elution was measured in the same manner as in Test 1 above, except that Kanuma soil (average particle size 510 μm, measured in the same manner as for Akadama soil) was used instead of Akadama soil. The results are shown in Figure 9.

[0078] Figure 9 is a graph showing the amount of fluorine elution for each mixing ratio of Akadama soil when the first layered double hydroxide and Kanuma soil are used in combination. When Kanuma soil was used, the amount of fluorine elution was reduced, just like Akadama soil (Figure 7). In particular, by using 80 parts by mass or more of Kanuma soil, the amount of elution was reduced to almost 0 mg / L.

[0079] From the test results shown in Figures 7 to 9, it was confirmed that the amount of fluorine elution can be reduced by using volcanic ash clayey soil such as Akadama soil or Kanuma soil in combination with layered double hydroxide in a ratio of 50 parts by mass or more.

[0080] <Test 4 (dust suppression evaluation)> The purpose of the dust suppression evaluation was to simulate the working conditions at current construction sites and evaluate the dust generation conditions. The dust suppression evaluation was carried out using the double layered double hydroxide and Akadama soil described above in <Materials>. The tests were carried out in the factory building in the four cases described below.

[0081] Case 0: Background measurement Before the start of the test, the dust concentration in the building was measured using a digital dust meter (LD-3K2; manufactured by Shibata Scientific Co., Ltd.). The digital dust meter was fixed at a height of 1 m above the ground surface. Measurements were taken every minute for 10 minutes from the start of the test.

[0082] Case 1: Simulation of on-site work using existing materials (comparison example) When using only the second layered double hydroxide without using Akadama soil, the second layered double hydroxide is mixed with the base material such as sand after being delivered to the site. Therefore, silica sand No. 4 (permeable base material) and the second layered double hydroxide were mixed in a 70L mixer. The mass mixing ratio was 95% silica sand and 5% second layered double hydroxide. At this point, dust had already been generated inside the mixer, so the material was discharged again onto a large square tray, and water was sprinkled at a rate of approximately 3% of the total mass, and the dust concentration was measured while stirring manually.

[0083] Case 2: Use of second layered double hydroxide and Akadama soil in a mass ratio of 50:50 (Example) The second layered double hydroxide and Akadama soil were used in a mass ratio of 50:50 and mixed in advance (premixed) at a factory in a different location. Therefore, in Case 2, the relative amounts of the first adsorbent 11 and the second adsorbent 12 used were 50 parts by mass out of a total of 100 parts by mass of the first adsorbent 11 and the second adsorbent 12. The premix was bagged and opened at the construction site (mixing location). Then, the dust concentration was measured in the same manner as in Case 1, except that the premix was used instead of only the second layered double hydroxide. Therefore, silica sand No. 4 and the premix were mixed in a mixer.

[0084] Case 3: The dust concentration was measured in the same manner as in Case 2, except that the mass ratio was changed from 50:50 to 20:80. Therefore, in Case 3, the relative amounts of the first adsorbent 11 and the second adsorbent 12 used were such that the content of the second adsorbent 12 was 80 parts by mass out of a total of 100 parts by mass of the first adsorbent 11 and the second adsorbent 12.

[0085] FIG. 10 is a graph showing the change over time in dust concentration in a building. The thick solid line is Case 2 (Example), the thin solid line is Case 3 (Example), the dashed line is Case 1 (Comparative Example), and the dashed line is Case 0 (Background). The horizontal axis is the elapsed time, and the vertical axis is the logarithmic value of the dust counts per minute (CPM; Counts per minute). In Cases 2 and 3 using Akadama soil, the dust concentration remained below 200 cpm, which was significantly reduced from the dust concentration (6000 cpm) after 4 minutes, which was the peak time in Case 1. Therefore, by using the second adsorbent 12, the dust concentration was reduced by up to 97%. In particular, as shown in Case 3, it was confirmed that the dust generation suppression effect could be improved by setting the content of the second adsorbent 12 to 80 parts or less.

[0086] As described above, some layered double hydroxides are produced by wet synthesis (mixing raw material solutions, drying and pulverizing the crystallized product). The primary particles of layered double hydroxides produced by such a method are fine (on the order of 100 nm), and the product particles obtained by granulation are also fine, such as a few μm. Moreover, they have a low bulk density. For this reason, when applying purifiers such as groundwater purifiers, there is a tendency for dust generation to be noticeable, for example, when mixing raw materials.

[0087] For example, when using materials with a high specific gravity such as iron powder, dust generation during mixing with heavy machinery can be suppressed by sprinkling a small amount of water. However, when using materials with a low specific gravity such as layered double hydroxide, it is difficult to suppress dust generation. For this reason, special measures have been taken at the construction site, such as mixing the layered double hydroxide into a slurry in small amounts before mixing it with a water-permeable base material.

[0088] However, by using the adsorbent 10 of the present disclosure, it is possible to suppress dust generation during mixing, for example, due to the second adsorbent 12. This makes it possible to improve the handleability of materials used in the manufacture of purifiers such as groundwater purifiers at the construction site.

[0089] In addition, since dust generation can be suppressed, for example, by packing a mixture (premixture) of the first adsorbent 11 and the second adsorbent 12 in fixed amounts (using bags such as flexible container packs), the material for the purifier can be manufactured in an open system by mixing with heavy machinery. Also, if bags with different mixing ratios are prepared, it is possible to select a bag with the required mixing ratio for each construction site, eliminating the need to mix the first adsorbent 11 and the second adsorbent 12 for each adsorbent 10.

[0090] In addition, it is expected that the mixing time of the base material such as silica sand and the adsorbent 10 (FIG. 1) can be made uniform and shortened. That is, in general, when mixing with heavy machinery, the time required for uniform mixing can be shortened if a certain amount is used. For example, when layered double hydroxide is mixed into the base material at a ratio of 2% by mass (for example, Case 2 above), if an equal amount of Akadama soil or the like (second adsorbent 12) is further used, the total amount of the base material will be 4% by mass, which is greater than the total amount of the material to be mixed when not used. This makes it easier to mix using heavy machinery, and allows for uniform mixing and shortening of the time required.

[0091] In particular, this effect becomes more pronounced as the amount of second adsorbent 12 used increases. For example, in the above-mentioned Case 3, the amount of adsorbent 10 used is a total of 10% on a mass basis relative to the substrate, which makes it possible to achieve further uniform mixing and reduce the time required. [Explanation of symbols]

[0092] 1. Groundwater purifier (purifier) 10 Adsorbents (fluorine adsorbents) 11 First adsorbent 12 Second adsorbent 1a Groundwater purifier 1a Groundwater purification 1b Groundwater purifier 1c Groundwater purifier 1d columnar body 11 First adsorbent 12 Second adsorbent 2. Sources of Contamination 3. Contaminated groundwater (subject to treatment) 4. Purified groundwater A1 area A2 area S1 Mixing process S2 manufacturing process

Claims

1. The present invention includes a first adsorbent composed of a layered double hydroxide component, and a second adsorbent that adsorbs fluorine derived from the first adsorbent. Fluorine adsorbent characterized by:

2. The second adsorbent contains Akadama soil 2. The fluorine adsorbent according to claim 1 .

3. The second adsorbent is in powder form.

2. The fluorine adsorbent according to claim 1 .

4. The content of the second adsorbent is 50 parts by mass or more and 80 parts by mass or less in a total of 100 parts by mass of the first adsorbent and the second adsorbent.

2. The fluorine adsorbent according to claim 1 .

5. a mixing step of mixing the fluorine adsorbent and the base material to obtain a mixture; and a manufacturing step of manufacturing a purifying body using the mixture, the purifying body purifying the object to be treated by adsorbing fluorine in the object to be treated, The fluorine adsorbent includes a first adsorbent constituted by a layered double hydroxide component, and a second adsorbent that adsorbs fluorine derived from the first adsorbent. The method for producing a purifying body comprising the steps of:

6. A method for suppressing diffusion of contaminated groundwater, the method comprising: suppressing diffusion of contaminated groundwater containing fluorine from a region shielded by a permeable groundwater purifying body comprising a fluorine adsorbent and a substrate to the outside of the region, the method comprising the steps of: The fluorine adsorbent includes a first adsorbent constituted by a layered double hydroxide component, and a second adsorbent that adsorbs fluorine derived from the first adsorbent. A method for suppressing the diffusion of contaminated groundwater.

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