JP7790948B2 - Manufacturing method of anion adsorbent - Google Patents

Description

The present invention relates to a method for producing an anion adsorbent.

Contaminated soil containing naturally occurring heavy metals is generated in huge quantities during construction work and other activities. Therefore, simpler, less costly methods of treating contaminated soil are needed. Large amounts of contaminated soil generated during excavation work and other activities are insolubilized by mixing it with insolubilizing agents 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.

In the above-mentioned 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 exists mainly in the form of hexavalent selenate SeO ₄ ^2- and tetravalent selenite SeO ₃ ^2- . The oxidized form, hexavalent selenate SeO ₄ ^2- , has a higher solubility and is a substance that is very difficult to insolubilize.

In order to insolubilize selenium, an excessive amount of insolubilizing agent must be added to the contaminated soil, which increases the volume of 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 treat the contaminated soil.

Furthermore, when selenate (SeO ₄ ^2-) is reduced, it becomes selenite (SeO ₃ ^2-) , which has a lower solubility, and thus elution is suppressed. The amount of elution 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, raising concerns that it may be re-eluted.

As an adsorption treatment for heavy metals present 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 containing dissolved selenium (see, for example, Patent Document 1).
Furthermore, a soil washing method using an ion adsorbent containing akaganeite is known, which involves treating selenium-containing soil and rocks by eluting selenium from contaminated soil with an oxidizing agent (see, for example, Patent Document 2). However, this method requires first eluting the selenium present in the soil and then adsorbing and recovering the eluted 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).

Japanese Patent Application Laid-Open No. 2018-15703 JP 2019-37929 A Japanese Patent Application Laid-Open No. 2021-137773

However, when producing an ion adsorbent, if the pH is adjusted to the acidic side, iron becomes easily dissolved, and when the ion adsorbent is recovered by centrifugation, the recovery rate is low, which is an issue.

The present invention was made in consideration of the above circumstances, and aims to provide a method for producing an anion adsorbent with excellent recovery rates.

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.

The present invention provides a method for producing an anion adsorbent with excellent recovery rates.

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.

[Method of manufacturing anion adsorbent]
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.

The anion adsorbent obtained by the anion adsorbent manufacturing method of this embodiment has akaganeite as the main component of the adsorbent that adsorbs anions of inorganic compounds. Here, "main component" refers to the component that adsorbs the largest amount of the target anion when comparing the adsorption amounts of each component of the adsorbent.

Examples of silicon-containing compounds include sodium silicate (Na ₂ O.2SiO ₂ ), calcium silicate (CaSiO ₃ ), magnesium silicate (Mg ₂ Si ₃ O ₈ ), etc. Among these, sodium silicate (Na ₂ O.2SiO ₂ ) is preferred because it is less likely to form unwanted precipitates.

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.

When producing akaganite, the pH of the aqueous solution 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 produced in the presence of chloride ions. When the pH of the aqueous solution is less than 4, akaganite can be produced 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 produced may aggregate and incorporate unreacted iron(III) chloride or silicon-containing compounds. On the other hand, when the pH is alkaline, there is a high possibility that iron oxide minerals with different structures (e.g., goethite, sucmetite, etc.) will be produced.

When producing akaganite, the pH of the aqueous solution 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.

The preferred method for adjusting and maintaining the pH of the aqueous solution is to add hydrochloric acid dropwise. Using hydrochloric acid prevents the introduction of unnecessary anions (such as sulfate ions) other than chloride ions, which are useful for producing akaganite, into the aqueous solution, preventing these unnecessary anions from being adsorbed onto the akaganite. It is also preferable to adjust and maintain the pH of the aqueous solution using sodium hydroxide.

By dissolving iron (III) chloride and a silicon-containing compound in water, the ions ionized in the aqueous solution react with each other 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, for example, to about 40°C to 100°C.

In the aqueous solution, the molar ratio of Fe3 ⁺ generated by iron(III) chloride to OH- generated by the silicon ^- containing compound 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 amount of positive charge on the Fe ³⁺ and the amount of negative charge on the OH ⁻ in the aqueous solution are in a balance suitable for producing akaganate, and almost all of the Fe ³⁺ derived from iron(III) chloride is consumed in the reaction, making it possible to easily produce akaganate in a high yield.

Taking all of the above into consideration, it is preferable that the molar ratio of iron (III) chloride to the silicon-containing compound in the aqueous solution for producing akaganate is 1:1.8 to 1:2.2.
Within the above molar ratio range, the charge balance between Fe ³⁺ and OH ⁻ in the aqueous solution is good, and akaganeite can be easily produced in high yield.

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.

After akaganate is produced, the akaganate molecules can be aggregated by adjusting the pH of the aqueous solution to 4 or more and 6 or less. This is preferably carried out within a temperature range that does not hinder 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 method of adjusting the pH of the aqueous solution to a range of 4 to 6 is preferably 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.

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.

The recovered akaganite slurry is dried at room temperature (25°C) to 110°C.

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.

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.

[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.

Akaganeite (akaganeite) is an iron oxide mineral with the chemical composition β-Fe ³ ++ (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°. Crystallographic data is described in the academic paper "Crystal structure refinement of akaganeite" by Post JE and Buchwald VF in American Mineralologist, 76 (1991) pp. 272-277. The paper also describes the existence of tunnel structures that hold chloride ions in the crystal structure of akaganeite, with hydroxyl groups projecting from the walls of these tunnels toward the center.

Akaganate as an anion adsorbent can be in any form that is easy to handle, such as powder, gravel, blocks, or plates. Powdered akaganate can be used as an adsorbent as is, or the powder can be bound and molded into a larger shape. Methods for binding powdered akaganate include known methods used to bind carbon particles with a polymer to form porous bodies (e.g., electrodes, deodorants). 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 which akaganate in these forms is dispersed in a solvent such as water, can also be used as adsorbents.

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

Examples of the holding member include a container, column (cylinder), strainer, or mesh that holds the akaganite inside. Holding members that can fix the akaganite to their surfaces can also be used, such as a plate with the akaganite fixed to its surface.

[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.

The above-mentioned soil contains heavy metals, etc. The heavy metals may be present in the soil in the form of simple heavy metal elements or compounds containing heavy metal atoms adsorbed to the soil, in the form of compounds containing heavy metal atoms dissolved in minerals that make up the soil, or in the form of elements that make up the minerals. The compounds may be ionic or non-ionic compounds. Ionic compounds may be ionized. The presence of heavy metals in solid solutions in minerals that make up the soil can be confirmed by elemental analysis using EDS (energy dispersive X-ray spectroscopy) or XPS (X-ray photoelectron spectroscopy).

The amount of heavy metals, etc. contained in the soil is measured using the measurement method in the appendix to the Notice on Establishing Measurement Methods for Soil Leaching Surveys (Ministry of the Environment Notification No. 18, March 6, 2003), and is preferably equal to or greater than the values listed in the appendix to the Environmental Standards for Soil Contamination (Environment Agency Notification No. 46, August 23, 1991). For example, the amount of selenium is 0.01 mg or less per 1 L of test solution, and the amount of arsenic is 0.01 mg or less per 1 L of test solution.

The mixing ratio of the soil and the anion adsorbent is preferably 10 kg to 200 kg of anion adsorbent per 1 m ³ of soil, and more preferably 20 kg to 100 kg of anion adsorbent per 1 m ³ of soil.

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

The present invention has been described above using embodiments, but the present invention is not limited to these embodiments. The configurations and combinations thereof in the above embodiments are merely examples, and additions, omissions, substitutions, and other modifications to the configurations are possible without departing from the spirit of the present invention.

The present invention will be explained in more detail below using examples, but the present invention is not limited to the following examples.

[Example 1]
An akaganate suspension was prepared by adding 27.5% aqueous sodium silicate solution ( _Na2O.2SiO2 ) dropwise to 400 _mL of 0.2 mol/L aqueous iron(III) chloride solution to adjust 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 aqueous solution, and the pure water and precipitate were thoroughly stirred to dissolve the salt contained in the precipitate in the pure water. This aqueous 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 operation 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 contained in the supernatant after washing the precipitate was measured. The results are shown in Table 1.

[Comparative Example 1]
Akaganeite was obtained in the same manner as in Example 1, except that an akaganeite suspension was prepared by adding 10 N aqueous sodium hydroxide solution (NaOH) dropwise to 400 mL of a 0.2 mol/L aqueous solution of iron (III) chloride 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.

The results shown in Table 1 show that the recovery rate of akaganate was improved in the akaganate produced by neutralization with aqueous sodium silicate solution (Na ₂ O.2SiO ₂ ) in Example 1 compared to the akaganate produced by neutralization with sodium hydroxide (NaOH) in Comparative Example 1. Furthermore, because the iron concentration in the supernatant during washing of the precipitate in Example 1 was lower than that in Comparative Example 1, it was found that in Example 1, the iron ions derived from the raw material iron(III) chloride were converted to akaganate more efficiently.

[Comparative Example 2]
Aqueous sodium hydroxide solution (NaOH) was added dropwise to a 0.4 mol/L aqueous solution of iron (III) chloride, and samples (akaganeite suspensions) were collected when the pH reached 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, and 5.5. These 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 akaganate recovered by this operation was measured. The mass was calculated from the concentration of the original aqueous solution of iron (III) chloride, 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 akaganate. The results are shown in Table 2.

The results shown in Table 2 indicate that when recovering akaganite by neutralization with an aqueous sodium hydroxide solution, it is not possible to recover all of it if the pH is below 4. If it is desired to produce akaganite while maintaining a pH below 4, it is believed that akaganite can be recovered efficiently by recovering it by neutralizing with an aqueous sodium silicate solution.

Claims (2)

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