Anion adsorbent, method for producing anion adsorbent, and anion adsorption method

Abstract

To provide an anion adsorbent and a method for producing the anion adsorbent that achieve high efficiency in adsorption of anions.SOLUTION: An anion adsorbent comprises a porous body and akaganeite supported on the porous body. A method for producing the anion adsorbent, the method including generating akaganeite in an aqueous solution in which ferric chloride (III) and at least one salt (S) selected from the group consisting of hydrogen carbonates, carbonates, and hydroxides of alkali metals and hydrogen carbonates, carbonates, and hydroxides of alkaline earth metals are dissolved, in the presence of the porous body.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an anion adsorbent, a method for producing the same, and a method for adsorbing anions. [Background technology]

[0002] Contaminated soil containing naturally occurring heavy metals generated during construction work and other activities is insolubilized by mixing insolubilizing agents such as cement and magnesium oxide to reduce the amount of heavy metals leaching, and then sealing it in a water-blocking layer such as bentonite to prevent contact with rainwater.

[0003] Of the heavy metals contained in contaminated soil, those that exist in the form of cations such as lead and chromium are easily adsorbed by soil, making them relatively easy to insolubilize. On the other hand, those that exist in the form of anions such as arsenic and selenium are difficult to adsorb by soil, making them difficult to insolubilize. Selenium, in particular, is mainly found in the form of hexavalent selenium, SeO4. 2- and tetravalent selenite SeO3 2- The former, which is an oxidized form, has a higher solubility and is a substance that is very difficult to insolubilize.

[0004] To make selenium insoluble, a sufficient amount of insolubilizing agent must be added to the contaminated soil, and sometimes excessive amounts are added for safety reasons. This results in an increase in the volume of contaminated soil, or the contaminated soil after treatment becomes more solid than necessary. If the volume of contaminated soil increases, the capacity of the disposal site becomes strained. If the contaminated soil after treatment becomes more solid than necessary, transportation and treatment become difficult. Also, selenate SeO4 2- is reduced to a less soluble form of selenite, SeO3 2- Therefore, the amount of leaching is suppressed by converting selenite to selenate using reducing agents such as iron oxide and sulfurous acid. However, if selenite is exposed to the environment for a long period of time, it is oxidized to selenate by contact with oxygen, and there is a concern that it may be releasable.

[0005] It has been reported that akaganite exhibits an adsorption effect on anions such as selenate. Patent Document 1 discloses an anion adsorbent containing akaganite. It also discloses a method of dissolving iron(III) chloride and one or more salts selected from the group consisting of hydrogen carbonates, carbonates, and hydroxides of alkali metals and hydrogen carbonates, carbonates, and hydroxides of alkaline earth metals in water, and producing akaganite in the resulting aqueous solution. Patent Document 2 discloses a method for producing a heavy metal absorbing material by recovering and desalting akaganeite produced by the method of Patent Document 1. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-15703 [Patent Document 2] Japanese Patent Publication No. 2021-137773 Summary of the Invention [Problem to be solved by the invention]

[0007] From the perspective of reducing processing costs, it is necessary to further improve the anion adsorption efficiency of akaganate so that it can adsorb more anions with less usage.

[0008] An object of the present invention is to provide an anion adsorbent having excellent anion adsorption efficiency, an anion adsorption method, and a production method for obtaining an anion adsorbent having excellent anion adsorption efficiency. [Means for solving the problem]

[0009] The present invention has the following aspects. [1] An anion adsorbent comprising a porous body and akaganite supported on the porous body. [2] The anion adsorbent according to [1], wherein the porous body is a carbonized organic porous body. [3] A method for producing the anion adsorbent according to [1] or [2], A method for producing an anion adsorbent, comprising: generating akaganite in an aqueous solution in the presence of a porous body, in which iron (III) chloride and one or more salts (S) selected from the group consisting of hydrogen carbonates, carbonates, and hydroxides of alkali metals, and hydrogen carbonates, carbonates, and hydroxides of alkaline earth metals are dissolved in water. [4] A method for adsorbing anions, comprising contacting a material to be treated containing water and anions of an inorganic compound with the anion adsorbent according to [1] or [2]. [5] The anion adsorption method according to [4], wherein the anion includes at least one selected from the group consisting of selenate ions and selenite ions. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an anion adsorbent having excellent anion adsorption efficiency, an anion adsorption method, and a manufacturing method for obtaining an anion adsorbent having excellent anion adsorption efficiency. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a graph showing the evaluation results of Example 1 and Comparative Example 1. [Figure 2] FIG. 1 is a schematic diagram showing the tunnel structure of akaganeite. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the embodiments described below, and various modifications are possible without departing from the gist of the present invention. In this specification, the use of "to" to indicate a range of values ​​means that the values ​​before and after it are included as the lower and upper limits.

[0013] [Anion adsorbent] The anion adsorbent of this embodiment includes a porous body and akaganite supported on the porous body.

[0014] <Akaganeite> Akaganeite (akaganeite) has the chemical composition β-Fe 3+ It is an iron oxide mineral represented by the formula (O(OH,Cl)). Its crystal system is monoclinic, with a space group of I2 / m and unit cell: a = 10.600, b = 3.0339, c = 10.513, β = 90.24°. This crystallographic data is described in the academic paper "Post JE, Buchwald VF, American Mineralogist, 76 (1991), pp. 272-277, Crystal structure refinement of akaganeite." The paper also describes that the crystal structure of akaganeite has tunnel structures that hold chloride ions, with hydroxyl groups projecting from the walls of these tunnels toward the center. Figure 2 is a schematic diagram of the tunnel structure, in which gray circles represent oxygen atoms, white circles represent hydrogen atoms, the circle in the center of the octahedron represents an iron atom, and black circles within the tunnel indicate that chloride ions and hydrogen ions exist in equal occupancy (50:50).

[0015] It is believed that when the anion adsorbent of this embodiment is brought into contact with anions, the anions are adsorbed into the tunnel structure of akaganate. The mechanism of adsorption is thought to be that in the case of anions containing heavy metal atoms, chloride ions already present in the tunnel structure are replaced by the anions and desorbed, while in the case of cations containing heavy metal atoms, they are exchanged with hydrogen ions of hydroxyl groups projecting from the tunnel walls toward the center.

[0016] <Porous body> When akaganite is supported on a porous body, the anion adsorption efficiency increases, and the amount of anions adsorbed per Fe-equivalent mass of akaganite increases, even in cases where the porous body alone does not have the ability to adsorb anions.

[0017] The porous material is not particularly limited, and examples thereof include organic porous materials such as biomass and synthetic resin porous materials; inorganic porous materials such as carbides of organic porous materials, porous metal oxide materials, ceramics, volcanic rock, clay, and other ground materials. Examples of biomass include wood, bamboo, rice husks, and sewage sludge carbides. Examples of metal oxides include silica and alumina.

[0018] Among the above porous bodies, a carbonized organic porous body is preferred. The carbonized organic porous body can be burned after use as an anion adsorbent. This reduces the amount of waste and also makes it easy to recover akaganeite and metal components contained in the adsorbed anions. Among carbonized organic porous materials, carbonized biomass (biochar) is preferred from the viewpoint of carbon neutrality. The type of biochar is not particularly limited, and examples thereof include wood charcoal, bamboo charcoal, and rice husk charcoal.

[0019] The shape of the porous body is not particularly limited, but granular shape is preferred. The size of the porous body is not particularly limited, but the smaller the size, the better. For example, a size that can pass through a sieve with 2 mm openings is preferred, and a size that can pass through a sieve with 0.25 mm openings is more preferred.

[0020] The porosity of the porous body is, for example, 0.3 to 0.95 and is calculated from the ratio of the bulk density to the true density. The pore diameter of the porous body is, for example, 1 to 300 Å. The pore diameter is measured according to JIS Z8831. The specific surface area of ​​the porous body is, for example, 300 to 3000 m 2 / g. The specific surface area is measured according to JIS Z8830.

[0021] In the anion adsorbent, the ratio of the mass of the porous body to the Fe-equivalent mass of akaganate (mass of porous body / mass of akaganate in Fe equivalent) is preferably from 1 to 100, more preferably from 1 to 10. When this ratio is within the above range, the anion adsorption efficiency per unit mass of the anion adsorbent is superior.

[0022] <Method of manufacturing anion adsorbent> The following method is preferred as a method for producing the anion adsorbent of this embodiment. Akaganate produced by the following method is of higher quality and has superior adsorption power than akaganate synthesized by other methods or naturally occurring akaganate. A method for producing akaganite in the presence of a porous body in an aqueous solution in which iron (III) chloride and one or more salts (S) selected from the group consisting of hydrogen carbonates, carbonates, and hydroxides of alkali metals, and hydrogen carbonates, carbonates, and hydroxides of alkaline earth metals are dissolved in water.

[0023] When iron(III) chloride is dissolved in water, chloride ions and iron(III) ions are produced. When salt(S) is dissolved in water, hydroxide ions are produced. When iron(III) chloride and salt(S) are dissolved in water to prepare an aqueous solution, these ions in the aqueous solution spontaneously react with each other to produce akaganite. More specifically, akaganite is produced by the reaction of iron(III) ions and hydroxide ions in an acidic aqueous solution containing a large amount of dissolved chloride ions. In order to promote the reaction for producing akaganeite, the aqueous solution (reaction liquid) may be heated to, for example, about 40 to 100°C.

[0024] When preparing an aqueous solution by dissolving iron (III) chloride and salt (S) in water, the order in which iron (III) chloride and salt (S) are dissolved is not particularly limited. However, it is desirable to dissolve iron (III) chloride first in order to maintain the pH of the aqueous solution at an acidic level. Although the porous body may be contacted with an aqueous solution in which both iron chloride (III) and salt (S) are dissolved, it is preferable to contact the porous body with a first aqueous solution in which iron chloride (III) (or salt (S)) is dissolved in water, and to dissolve salt (S) (or iron chloride (III)) in the first aqueous solution. This allows akaganate to be produced in a state in which iron (III) ions have penetrated the interior of the porous body, making it easier to retain akaganate in the porous body. An example of a method for contacting the aqueous solution with the porous body is to pour the porous body into the aqueous solution. At this time, a decompression treatment or the like may be performed to allow the aqueous solution to penetrate into the pores of the porous body.

[0025] The porous body may be an existing one or one produced by a known production method. For example, biochar is produced by heating biomass at a temperature above 350°C under an oxygen concentration controlled to a level that does not cause combustion. After heating, the resulting charcoal may be crushed. An existing porous body may be crushed and used.

[0026] From the viewpoint of synthesizing akaganeite in high yield, the salt (S) is preferably readily soluble in water, and for example, salts containing the following cations are preferred. The alkali metal is an element of Group 1 of the periodic table, and sodium and potassium are preferred. The alkaline earth metals are elements of Group 2 of the periodic table, and magnesium, calcium and barium are preferred.

[0027] The amount of iron(III) chloride dissolved when preparing the aqueous solution is not particularly limited, and can be, for example, 0.01 to 3 mol / L. The total amount of salt (S) dissolved when preparing the aqueous solution is not particularly limited, and can be, for example, 0.01 to 3 mol / L.

[0028] In aqueous solution, Fe is produced by iron(III) chloride. 3+ and OH generated by salt (S) - Molar ratio of (OH - :Fe 3+) is preferably 1:1 to 1:3, more preferably 1:1.5 to 1:2.5, and even more preferably 1:1.8 to 1:2.2. Theoretically, a molar ratio of 1:2 is most preferable. When the molar ratio is in the above range close to 1:2, the Fe in the aqueous solution 3+ The amount of positive charge possessed by OH - The negative charge of iron(III) chloride and the Fe(III) chloride provide a suitable balance for the formation of akaganeite. 3+ Almost all of the is consumed in the reaction, and akaganeite can be easily produced in high yield.

[0029] Specifically, for example, in a 1 L aqueous solution containing 0.1 mol of sodium bicarbonate, if the solution pH is 5.3 or lower, which is 1 or more lower than pKa1, taking into account the apparent acid dissociation constant (affected by equilibrium with carbon dioxide) of carbon dioxide, pKa1 = 6.3, the hydroxide ion concentration (carbonate molecule concentration) generated in the aqueous solution is thought to be approximately 0.09 to 0.1 mol / L. Based on this, the concentration of iron(III) chloride is preferably 1 / 3 to 1 times, and more preferably 1 / 2 to 1 times, 0.09 to 0.1 mol / L. Furthermore, for example, in a 1 L aqueous solution containing 0.1 mol of sodium carbonate, taking into consideration the acid dissociation constant pKa2 of carbonic acid = 10.3 and the apparent acid dissociation constant pKa1 = 6.3, the hydroxide ion concentration (carbonate molecule concentration) generated in the aqueous solution at a solution pH of 5.3 or less is thought to be about 0.18 to 0.2 mol / L. Based on this, the concentration of iron(III) chloride is preferably 1 / 3 to 1 times, and more preferably 1 / 2 to 1 times, 0.18 to 0.2 mol / L. Furthermore, for example, in a 1 L aqueous solution containing 0.1 mol of sodium hydroxide, the concentration of hydroxide ions generated in the aqueous solution is considered to be approximately 0.1 mol / L, taking into consideration its acid dissociation constant pKa = 13, in an acidic range where the solution pH is less than 7. Based on this, the concentration of iron(III) chloride is preferably 1 / 3 to 1 time, and more preferably 1 / 2 to 1 time, of 0.1 mol / L.

[0030] When any bicarbonate, carbonate, or hydroxide salt is used as the salt (S), hydroxide ions are generated at a molar concentration of about 0.9 to 2 times the molar concentration of the dissolved salt, provided that the pH of the aqueous solution is lower than the pKa1 of the salt by at least 1. Therefore, iron(III) chloride is preferably dissolved at a concentration of about 0.3 to 2 times the molar concentration of the dissolved salt (S) (1 / 3 to 1 times the concentration of hydroxide ions generated) in the above pH range, and more preferably at a concentration of 0.45 to 1 times (1 / 2 times the concentration of hydroxide ions generated).

[0031] In addition, taking the above into consideration comprehensively, the molar ratio of iron (III) chloride to salt (S) in the aqueous solution (iron (III) chloride: salt (S)) is preferably 2:1 to 1:3. When the molar ratio is within this range, the Fe in the aqueous solution 3+ and OH - The charge balance of the hydroxyl group is improved, and akaganeite can be easily produced in high yield.

[0032] The pH of the aqueous solution used to produce akaganeite is preferably less than 7, more preferably 6 or less, even more preferably less than 4, and particularly preferably 1 to 3. When the pH of the aqueous solution is less than 7, particularly 6 or less, akaganeite is easily produced in the presence of chloride ions. Akaganate can be produced in higher yield in the presence of chloride ions when the pH of the aqueous solution is less than 4, particularly 3 or less. Akaganate is easily formed at a pH of 4 to 6, but within this pH range, the akaganate particles that are being produced may aggregate together, resulting in the incorporation of unreacted iron(III) chloride or salt (S). On the other hand, when the pH of the aqueous solution is alkaline, iron oxide minerals with different structures (e.g., goethite, sucmetite, etc.) may be produced.

[0033] The pH of the aqueous solution used to produce akaganite may be adjusted before adding at least one of iron chloride (III) and salt (S) to water, or after dissolving both in water. However, if the aqueous solution containing both dissolved therein is left in an alkaline pH state, iron oxide minerals other than akaganite may be produced. Therefore, it is preferable to adjust the pH of the aqueous solution to an acidic level immediately after dissolving both compounds, or before or during dissolving at least one of the compounds in water, and maintain the acidic pH.

[0034] The method for adjusting and maintaining the pH of the aqueous solution is preferably a method of adding hydrochloric acid dropwise. The use of hydrochloric acid prevents the introduction of unnecessary anions (e.g., sulfate ions) other than chloride ions useful for producing akaganite into the aqueous solution, thereby preventing the unnecessary anions from being adsorbed onto akaganite. It is also preferable to adjust and maintain the pH of the aqueous solution using sodium hydroxide.

[0035] The completion of the akaganate production reaction in the aqueous solution (reaction liquid) can be empirically determined by checking when the reaction liquid changes from dark brown to reddish brown. After the start of the akaganate production reaction, the time required for the reaction to settle down is, for example, about 3 to 5 minutes at 10 to 25°C, although this depends on the concentration of the akaganate produced.

[0036] When akaganate is produced by adjusting the pH of the aqueous solution to less than 4, after the completion of the akaganate production reaction, the akaganate molecules can be aggregated by adjusting the pH of the aqueous solution (reaction liquid) to 4 to 6. This is preferably carried out within a temperature range that does not hinder aggregation, for example, 10 to 40° C. The time required for akaganate to aggregate after adjusting the pH to 4 to 6 is, for example, about 5 to 10 minutes at 10 to 25° C. A method of adding additional salt (S) to the aqueous solution is preferred as a method of adjusting the pH of the aqueous solution to 4 to 6. The use of salt (S) can prevent excess anions (such as sulfate ions) from being mixed into the aqueous solution and adsorbed onto the akaganeite.

[0037] Thereafter, if necessary, the anion adsorbent is recovered from the aqueous solution by known methods such as precipitation and filtration. If necessary, the recovered anion adsorbent is dried, preferably at a drying temperature of room temperature (25°C) to 110°C.

[0038] If necessary, the recovered anion adsorbent is subjected to a desalination treatment. The anion adsorbent obtained as described above contains a large amount of salts such as sodium chloride. By reducing the salt content through desalination treatment, the salt content in the wastewater after treatment can be reduced. Desalination can be carried out, for example, by mixing the dried anion adsorbent with deionized water to dissolve the salt in the water, and then performing solid-liquid separation to recover the solid. Higher-grade water, such as distilled water or ultrafiltered water (RO water), may be used instead of deionized water. A preferred solid-liquid separation method involves precipitating the anion adsorbent by centrifugation or the like, and then separating the solid from the liquid by filtration, decantation, or other means.

[0039] The salt content of the anion adsorbent after desalting treatment is preferably 1% by mass or less, more preferably 0.8% by mass or less, and even more preferably 0.5% by mass or less, as the sodium chloride content of the akaganeite in the anion adsorbent after desalting treatment.

[0040] The anion adsorbent obtained as described above may be used for anion adsorption as is, or may be further shaped and used for anion adsorption. For example, granular anion adsorbents may be bound and shaped into larger shapes (e.g., pellets, gravels, blocks, or plates). Methods for binding granular anion adsorbents include known methods used to form porous bodies (e.g., electrodes, deodorants) by binding carbon particles with a polymer. Alternatively, the granular anion adsorbent may be compressed or sintered to form blocks, or the blocks may be crushed or cut to an appropriate size. The granular anion adsorbent may also be dispersed in a dispersion medium such as water to form a dispersion liquid.

[0041] [Anion adsorption method] By contacting a material to be treated containing water and anions of inorganic compounds with the anion adsorbent of this embodiment, the anions contained in the material to be treated can be adsorbed onto the anion adsorbent.

[0042] Examples of the inorganic compound include inorganic compounds containing inorganic elements such as selenium, arsenic, chromium, fluorine, sulfur, and phosphorus. Specific examples include oxoacids of selenium, arsenic, and chromium, hydrofluoric acid (hydrofluoric acid), sulfuric acid, and phosphoric acid.

[0043] As the inorganic compound, an oxoacid is preferred from the viewpoint of exhibiting a high adsorptivity to akaganite, and a monovalent or divalent inorganic oxoacid containing the inorganic element is more preferred. Here, an oxoacid is an inorganic compound in which a hydroxyl group (-OH) and an oxo group (=O) are bonded to one inorganic atom, and the proton of the hydroxyl group can be eliminated. In water, an oxoacid can become an oxoacid ion by elimination of the proton.

[0044] As the oxoacid, a selenium oxoacid is preferred from the viewpoint of exhibiting a high adsorption power to akaganite. As the selenium oxoacid ion, a selenate ion (SeO4 2- ), hydrogen selenate ion (HSeO4 - ), selenite ion (SeO32- ), hydrogen selenite ion (HSeO3 - Among these, it is preferable that the ion contains at least one selected from the group consisting of selenate ions and selenite ions, and it is particularly preferable that the ion contains selenate ions.

[0045] The anions of the inorganic compound contained in the object to be treated may be of one type or two or more types.

[0046] The method for contacting the material to be treated with the anion adsorbent is not particularly limited, and examples include a method in which a powdered anion adsorbent is added to the liquid material to be treated and stirred, and a method in which the liquid material to be treated is passed through the anion adsorbent held by a holding member. Examples of the holding member include a container that holds the anion adsorbent therein, a column (cylinder), a sieve, a mesh, etc. Also, a holding member that can fix the anion adsorbent to its surface can be used, for example, a form in which the anion adsorbent is fixed to the surface of a plate material. [Example]

[0047] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to the examples described below.

[0048] Example 1 Iron (III) chloride was added to 200 mL of deionized water to a concentration of 0.05 mol / L (2.8 g of iron equivalent), and 10 g of biochar was added. The total mass of iron and biochar at this time was 3.57. Charcoal that passed through a 2 mm mesh sieve was used as the biochar. This liquid was mixed under reduced pressure to remove air trapped in the pores of the biochar, and then a 10 N sodium hydroxide solution was added to adjust the pH to 4, producing akaganeite. The solid was then precipitated by centrifugation, the supernatant was removed, and an equal amount of deionized water was added and stirred thoroughly. The mixture was then centrifuged again, and the supernatant was removed, completing the desalination process. This yielded the anion adsorbent of Example 1.

[0049] <Comparative Example 1> Desalted akaganite was obtained by the method described in Example 1 of JP 2021-137773 A. This was used as the anion adsorbent of Comparative Example 1.

[0050] <Evaluation> Using each anion adsorbent, a selenium adsorption experiment was carried out according to the following procedure. A 100 mL solution with a selenium (Se) concentration of 0.3 mg / L was prepared using sodium selenate. 0.1 g of an anion adsorbent was added, and the pH was adjusted to 5, 7, or 9 by adding 0.1 N NaOH. The mixture was then stirred for 30 minutes or more to adsorb selenium onto the akaganeite. The anion adsorbent was then removed by filtering the mixture with a 0.25 μm membrane filter, and the selenium concentration remaining in the solution was measured by inductively coupled plasma mass spectrometry (ICP-MS). From the measurement results, the amount of Se adsorbed per mass of Fe in akaganate contained in the anion adsorbent was calculated. The mass of Fe in akaganate was a theoretical value calculated assuming that all of the iron added as iron(III) chloride was converted to akaganate.

[0051] The evaluation results are shown in Figure 1. The selenium concentration is shown as a bar graph, and the pH is plotted as a circle. As shown in FIG. 1, when the results of Example 1 and Comparative Example 1 are compared at the same pH, the anion adsorbent of Example 1 adsorbed more selenium per mass of Fe than the anion adsorbent of Comparative Example 1 at all pH values, and the anion adsorption efficiency was improved. [Industrial Applicability]

[0052] The anion adsorbent of this embodiment has akaganate supported on a porous body, and therefore has a higher anion adsorption efficiency than akaganate alone, and can exhibit excellent anion adsorption effects with a smaller amount of adsorbent than conventional methods. Reducing the amount of adsorbent used is useful for reducing the processing costs of the materials to be processed.

Claims

1. An anion adsorbent comprising a porous body and akaganite supported on the porous body.

2. 2. The anion adsorbent according to claim 1, wherein the porous body is a carbonized organic porous body.

3. A method for producing the anion adsorbent according to claim 1 or 2, A method for producing an anion adsorbent includes generating akaganite in an aqueous solution in the presence of a porous body, in which iron (III) chloride and one or more salts (S) selected from the group consisting of hydrogen carbonates, carbonates, and hydroxides of alkali metals, and hydrogen carbonates, carbonates, and hydroxides of alkaline earth metals are dissolved in water.

4. A method for adsorbing anions, comprising contacting a material to be treated containing water and anions of an inorganic compound with the anion adsorbent according to claim 1 or 2.

5. 5. The anion adsorption method according to claim 4, wherein the anions include at least one selected from the group consisting of selenate ions and selenite ions.

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