Manufacturing method of anion adsorbent

Abstract

To provide a method for producing an anion adsorbent that is excellent in recovery rate.SOLUTION: Provided is a method for producing an anion adsorbent in which iron (III) chloride and a silicon-containing compound are dissolved in water to prepare an aqueous solution, and the iron (III) chloride and the silicon-containing compound are reacted in the aqueous solution to produce an anion adsorbent in the aqueous solution.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing an anion adsorbent. [Background technology]

[0002] The amount of contaminated soil containing naturally occurring heavy metals generated during construction work and other activities is enormous. Therefore, simpler and cheaper methods for treating contaminated soil are needed. Large amounts of contaminated soil generated during excavation work and other activities are insolubilized by mixing it with insolubilizers such as cement and magnesium oxide to reduce the amount of leaching, and then sealing it in a water barrier such as bentonite to prevent contact with rainwater.

[0003] In the above insolubilization treatment, heavy metals such as lead and chromium that exist in the form of cations are easily adsorbed by soil and can be treated relatively easily. On the other hand, heavy metals such as arsenic and selenium that exist in the form of anions are difficult to adsorb by soil and are therefore difficult to insolubilize. Among heavy metals, selenium is mainly found in the form of hexavalent selenium, SeO4 2- and tetravalent selenite SeO3 2- It exists in the oxidized form, hexavalent selenium, SeO4 2- is a substance that is more soluble and very difficult to insolubilize.

[0004] To insolubilize selenium, an excessive amount of insolubilizer must be added to the contaminated soil, which increases the volume of the contaminated soil to be treated and causes the treated soil to solidify more than necessary. If the volume of contaminated soil increases, the capacity of the disposal site may be insufficient. If the contaminated soil solidifies more than necessary, it becomes difficult to transport and process the contaminated soil.

[0005] Also, selenate SeO4 2- is reduced to the less soluble selenite SeO3 2-Therefore, leaching is suppressed. The amount of leaching of many iron oxides, sulfites, etc. is suppressed by using a reducing agent to convert selenate to selenite. However, when selenite is exposed to the environment for a long period of time, it is oxidized to selenate when it comes into contact with oxygen, which raises the concern that it may be releasable.

[0006] As an adsorption treatment for heavy metals that exist in the anionic state, an ion adsorption method is known in which an ion adsorbent containing akaganeite is used to efficiently adsorb selenium from water in which selenium is dissolved (see, for example, Patent Document 1). Furthermore, a known soil washing method using an ion adsorbent containing akaganeite involves treating selenium-containing soil and rocks by using an oxidizing agent to dissolve selenium from contaminated soil (see, for example, Patent Document 2). However, this method requires first dissolving the selenium present in the soil and then adsorbing and recovering the dissolved selenium. Therefore, an insolubilization method and insolubilization material have been devised that can be used for soil improvement by washing the ion adsorbent produced by the method described in Patent Document 1 and mixing it with cement or the like (see, for example, Patent Document 3). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2018-15703 [Patent Document 2] Japanese Patent Application Publication No. 2019-37929 [Patent Document 3] Japanese Patent Publication No. 2021-137773 Summary of the Invention [Problem to be solved by the invention]

[0008] However, when the pH is adjusted to the acidic side during production of the ion adsorbent, iron is easily dissolved, and there is a problem that when the ion adsorbent is recovered by centrifugation, the recovery rate is low.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing an anion adsorbent with an excellent recovery rate. [Means for solving the problem]

[0010] The present invention has the following aspects. [1] A method for producing an anion adsorbent, comprising dissolving iron (III) chloride and a silicon-containing compound in water to prepare an aqueous solution, and reacting the iron (III) chloride with the silicon-containing compound in the aqueous solution to produce an anion adsorbent in the aqueous solution. [2] The method for producing an anion adsorbent according to [1], wherein the pH of the aqueous solution is less than 7. [3] The method for producing an anion adsorbent according to [1] or [2], wherein the silicon-containing compound is sodium silicate. [Effects of the Invention]

[0011] According to the present invention, a method for producing an anion adsorbent with an excellent recovery rate can be provided. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, a method for producing an anion adsorbent according to an embodiment of the present invention will be described. It should be noted that the present embodiment is specifically described to allow a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[0013] [Method of manufacturing anion adsorbents] The method for producing an anion adsorbent according to this embodiment involves dissolving iron (III) chloride and a silicon-containing compound in water to prepare an aqueous solution, and then reacting the iron (III) chloride with the silicon-containing compound in the aqueous solution to produce an anion adsorbent in the aqueous solution.

[0014] The anion adsorbent obtained by the method for producing an anion adsorbent of this embodiment has akaganite as a main component of the adsorbent that adsorbs anions of inorganic compounds. Here, the "main component" refers to the component that adsorbs the largest amount of the target anion when comparing the adsorption amounts of the respective components of the adsorbent.

[0015] Examples of silicon-containing compounds include sodium silicate (Na2O·2SiO2), calcium silicate (CaSiO3), magnesium silicate (Mg2Si3O8), etc. Among these, sodium silicate (Na2O·2SiO2) is preferred because it is less likely to form unwanted precipitates.

[0016] The amount of iron(III) chloride dissolved in the aqueous solution is not particularly limited and may be, for example, 0.01 mol / L to 3 mol / L. Similarly, the amount of silicon-containing compound dissolved in the aqueous solution is not particularly limited and may be, for example, 0.01 mol / L to 3 mol / L. When preparing the aqueous solution, the order in which iron(III) chloride and the silicon-containing compound are dissolved is not particularly limited, but it is preferable to dissolve iron(III) chloride first in order to maintain the pH of the aqueous solution at an acidic level.

[0017] The pH of the aqueous solution used to generate akaganite is preferably less than 7, more preferably less than 4, and even more preferably between 1 and 3. When the pH of the aqueous solution is less than 7, akaganite is easily generated in the presence of chloride ions. When the pH of the aqueous solution is less than 4, akaganite can be generated in high yield in the presence of chloride ions. Akaganite is also easily formed at a pH of 4 to 6, but within this pH range, the akaganite particles that are being generated may aggregate and incorporate unreacted iron(III) chloride or silicon-containing compounds. On the other hand, when the pH is alkaline, iron oxide minerals with different structures (e.g., goethite, sucmetite, etc.) are likely to be generated.

[0018] The pH of the aqueous solution used to produce akaganite may be adjusted before adding iron(III) chloride and the silicon-containing compound to the water, or after dissolving at least one of iron(III) chloride and the silicon-containing compound in water. However, if the aqueous solution containing iron(III) chloride and the silicon-containing compound is left in an alkaline 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 iron(III) chloride and the silicon-containing compound, or before or during dissolution of at least one of iron(III) chloride and the silicon-containing compound, and maintain the acidic pH.

[0019] 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.

[0020] By dissolving iron (III) chloride and a silicon-containing compound in water, the ionized ions in the aqueous solution react spontaneously to produce akaganite. More specifically, when the silicon-containing compound is dissolved in water, hydroxide ions are produced. Akaganite is produced by the reaction of these hydroxide ions with iron ions in an acidic aqueous solution containing a large amount of dissolved chloride ions. In order to promote the reaction for producing akaganeite and the gelation of the silicate compound, the aqueous solution (reaction liquid) may be heated to, for example, about 40°C to 100°C.

[0021] In the above aqueous solution, Fe produced by iron(III) chloride 3+ and OH generated by silicon-containing compounds -The molar ratio of the hydroxybenzoate to the hydroxybenzoate 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 content in the aqueous solution is 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.

[0022] Taking all of the above into consideration, the molar ratio of iron (III) chloride to the silicon-containing compound in the aqueous solution for producing akaganate is preferably 1:1.8 to 1:2.2. When the molar ratio is within the above 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.

[0023] The completion of the akaganate production reaction in the aqueous solution (reaction liquid) can be empirically determined by the change of the reaction liquid from dark brown to reddish brown, or by the viscosity of the reaction liquid. The time required for the reaction to settle after the start of the akaganate production reaction depends on the concentration of the akaganate produced, but is, for example, about 3 to 5 minutes at 10 to 25°C.

[0024] After akaganate is produced, the akaganate can be aggregated by adjusting the pH of the aqueous solution to 4 or more and 6 or less. This is preferably done at a temperature range that does not interfere with aggregation, for example, 10°C to 40°C. The time required for akaganate to aggregate after adjusting the pH is, for example, about 5 to 10 minutes at 10°C to 25°C. Here, a preferred method for adjusting the pH of the aqueous solution to between 4 and 6 is to add a silicon-containing compound to the aqueous solution. The use of a silicon-containing compound can prevent excess anions (e.g., sulfate ions) from being mixed into the aqueous solution and adsorbed onto the akaganeite.

[0025] Examples of methods for recovering akaganate include known precipitation methods, filtration methods, centrifugation methods, etc. It is preferable to previously aggregate akaganate, as this facilitates recovery. The recovered akaganite can be dried and stored until use. The akaganeite obtained by filtration and drying is usually in the form of a clay-like (slurry-like) mass, which can be pulverized in a mortar or the like to form a powder.

[0026] The collected slurried akaganite is dried at room temperature (25°C) to 110°C.

[0027] The obtained akaganite contains salts, so it is preferable to purify it. Specifically, the dried akaganite is placed in purified water to dissolve the salt contained in the akaganite. The akaganite is then recovered by filtration or centrifugation. This reduces the amount of salt contained in the akaganite.

[0028] According to the method for producing an anion adsorbent of this embodiment, it is possible to efficiently produce an anion adsorbent that has poor recovery efficiency through washing operations. As an example, it is possible to efficiently produce an anion adsorbent that can efficiently suppress the elution amount of anions, particularly selenium, contained in soil. By using an anion adsorbent containing akaganite obtained by the method for producing an anion adsorbent of this embodiment, it is no longer necessary to add an excess insolubilizer to the contaminated soil, making application easier and preventing an increase in the volume of the target soil. Since the elution of selenic acid is not suppressed by reduction to selenious acid, but rather by the adsorbent adsorbing both selenic acid and selenious acid, it is believed that the elution amount does not change due to oxidation. Furthermore, according to the method for producing an anion adsorbent of the present embodiment, it is possible to remove salt contained in the anion adsorbent. By removing the salt, the anion adsorbent can be used to insolubilize soil without inhibiting solidification by cement or an insolubilizing agent.

[0029] [Anion adsorbent] The anion adsorbent according to the present embodiment is obtained by the method for producing an anion adsorbent according to the present embodiment, and as described above, contains akaganite as a main component of the adsorbent that adsorbs anions of inorganic compounds. The anion adsorbent according to the present embodiment may further include a holding member that holds the adsorbent.

[0030] 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 contains tunnel structures that hold chloride ions, with hydroxyl groups projecting from the walls of these tunnels toward the center.

[0031] Akaganate as an anion adsorbent can be in any form that is easy to handle, such as powder, gravel, block, or plate. Powdered akaganate can be used as an adsorbent as is, or the powder can be bound and molded into a larger shape. For example, known methods used to bind carbon particles with a polymer to form porous bodies (e.g., electrodes, deodorants) can be used as methods for binding powdered akaganate. Furthermore, lumps obtained by compacting or sintering can be used as is, or the lumps can be crushed or cut into appropriate sizes and molded. Akaganate suspensions in these forms, prepared by dispersing akaganate in a solvent such as water, can also be used as adsorbents.

[0032] The anion adsorbent of this embodiment preferably has a salt content of 10,000 ppm or less, more preferably 5,000 ppm or less. When the salt content is equal to or less than the upper limit, the anion adsorbent can be used to insolubilize soil without inhibiting solidification by cement or an insolubilizing agent.

[0033] Examples of the holding member include a container that holds akaganite inside, a column (cylinder), a sieve, a net, etc. Also, a holding member that can fix akaganite to its surface can be used, for example, a form in which akaganite is fixed to the surface of a plate material.

[0034] [Method of contacting soil with an anion adsorbent] As a method for contacting the soil with the anion adsorbent, for example, a method in which the powder of the anion adsorbent is directly mixed with the soil and stirred and mixed can be mentioned.

[0035] The above-mentioned soil contains heavy metals and the like. The heavy metals and the like may be in the form of simple heavy metal elements or compounds containing heavy metal atoms adsorbed to the soil, or in the form of compounds containing heavy metal atoms dissolved in minerals constituting the soil, or in the form of elements contained in minerals. The compounds may be ionic compounds or nonionic compounds. Ionic compounds may be ionized. The solid solution of heavy metals in minerals constituting the soil can be confirmed by elemental analysis using EDS (energy dispersive X-ray spectroscopy) or XPS (X-ray photoelectron spectroscopy).

[0036] The amount of heavy metals contained in the soil is measured using the measurement method in the attached table of the Notice on the Determination of Measurement Methods for Soil Leaching Amount Survey (Ministry of the Environment Notification No. 18, March 6, 2003), and it is preferable that it is equal to or greater than the value listed in the attached table of the Environmental Standards for Soil Contamination (Environment Agency Notification No. 46, August 23, 1991). For example, the selenium content is 0.01 mg or less per 1 L of test solution, and the arsenic content is 0.01 mg or less per 1 L of test solution.

[0037] The mixing ratio of soil and anion adsorbent was 1 m 3 It is preferable to use 10 kg to 200 kg of anion adsorbent per 1 m of soil. 3 It is more preferable that the amount of anion adsorbent is 20 kg to 100 kg.

[0038] When mixing the soil with the anion adsorbent, water may be added to the target soil. The amount of water to be added may be selected depending on the properties of the soil.

[0039] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. Each configuration and combination thereof in the above embodiments is an example, and additions, omissions, substitutions, and other modifications of the configurations are possible within the scope of the present invention. [Example]

[0040] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0041] [Example 1] To prepare an akaganite suspension, 27.5% sodium silicate solution (Na2O·2SiO2) was added dropwise to 400 mL of 0.2 mol / L iron(III) chloride solution, adjusting the pH to 3.0. This suspension was centrifuged at 10,000 rpm for 10 minutes to precipitate the precipitate, and after removing the supernatant, the precipitate was dried at 105°C. The dried precipitate was added to the same amount of pure water as the original solution, and the pure water and precipitate were thoroughly stirred to dissolve the salt contained in the precipitate into the pure water. This solution was again centrifuged to settle the precipitate, and after removing the supernatant, the precipitate was dried at 105°C to obtain akaganeite. The mass of akaganite recovered by this procedure was measured. Based on the amount of added compound, all iron was calculated as FeOOH, all silicon as SiO2, and the recovery rate of akaganite was calculated assuming that all Na and Cl had been removed. Furthermore, the concentration of iron in the supernatant after washing the precipitate was measured. The results are shown in Table 1.

[0042] [Comparative Example 1] Akaganeite was obtained in the same manner as in Example 1, except that an akaganeite suspension was prepared by adding 10N sodium hydroxide solution (NaOH) dropwise to 400 mL of 0.2 mol / L aqueous iron (III) chloride solution to adjust the pH to 3.0. The mass of the recovered akaganite was measured in the same manner as in Example 1. The recovery rate of the recovered akaganite was calculated in the same manner as in Example 1. Furthermore, the concentration of iron contained in the supernatant after washing was measured. The results are shown in Table 1.

[0043] [Table 1]

[0044] The results shown in Table 1 indicate that the recovery rate of akaganate produced by neutralization with an aqueous sodium silicate solution (Na2O·2SiO2) in Example 1 was higher than that of akaganate produced by neutralization with sodium hydroxide (NaOH) in Comparative Example 1. Furthermore, because the iron concentration in the supernatant obtained when washing the precipitate in Example 1 was lower than that in Comparative Example 1, it was found that Example 1 was more efficient at converting iron ions derived from the raw material iron (III) chloride into akaganate.

[0045] Comparative Example 2 Aqueous sodium hydroxide (NaOH) solution was added dropwise to a 0.4 mol / L aqueous solution of iron (III) chloride, and samples (akaganeite suspensions) were collected at pH values ​​of 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, and 5.5. The samples were centrifuged at 10,000 rpm for 10 minutes, and the precipitate was collected. The dried precipitate was added to the same amount of pure water as the sample, and the pure water and precipitate were thoroughly stirred to dissolve the components contained in the precipitate other than akaganate in the pure water. This aqueous solution was centrifuged again to settle the precipitate, and after removing the supernatant, the precipitate was dried at 105°C to obtain akaganate. The mass of the akaganite recovered by this procedure was measured. The mass was calculated from the concentration of the original iron (III) chloride aqueous solution, assuming that all of the iron had precipitated as FeOOH. This calculated value was compared with the measured mass to determine the recovery rate of akaganite. The results are shown in Table 2.

[0046] [Table 2]

[0047] The results shown in Table 2 indicate that when akaganate is recovered by neutralization with a sodium hydroxide aqueous solution, it cannot be recovered completely if the pH is below 4. If it is desired to produce akaganate while maintaining the pH at 4 or below, it is believed that akaganate can be recovered efficiently by recovering it by neutralization with a sodium silicate aqueous solution.

Claims

1. A method for producing an anion adsorbent, comprising dissolving iron (III) chloride and a silicon-containing compound in water to prepare an aqueous solution, and reacting the iron (III) chloride with the silicon-containing compound in the aqueous solution to produce an anion adsorbent containing akaganite as a main component in the aqueous solution, When reacting the iron (III) chloride with the silicon-containing compound, hydrochloric acid is added dropwise to the aqueous solution to adjust the pH of the aqueous solution to less than 7; After the akaganite is produced, the silicon-containing compound is further added to the aqueous solution, and the pH of the aqueous solution is adjusted to 4 or more and 6 or less, thereby flocculating the akaganite particles together.

2. 2. The method for producing an anion adsorbent according to claim 1, wherein the silicon-containing compound is sodium silicate.

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