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

Abstract

To provide an anion adsorbent capable of adsorbing a large quantity of anions including selenium in a solution containing dissolved selenium at approximately neutral pH, a method for producing the anion adsorbent, and an anion adsorption method using the anion adsorbent.SOLUTION: The present invention provides an anion adsorbent that serves as an agent for adsorbing anions in inorganic compounds, the adsorbent comprising an iron-aluminum composite hydroxide.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Construction work and other projects generate large amounts of contaminated soil containing naturally occurring heavy metals (hereinafter referred to as "heavy metal-contaminated soil"). Therefore, a simpler and less expensive method for treating heavy metal-contaminated soil is required. Heavy metal-contaminated soil, which is generated in large quantities during excavation work and other projects, is mixed with insolubilizers such as cement and magnesium oxide to suppress the amount of heavy metal leaching, and then sealed with a water barrier such as bentonite. In this way, heavy metal-contaminated soil is treated to prevent contact with rainwater.

[0003] In the insolubilization treatment of heavy metals, heavy metals that exist in the form of cations such as lead are easily adsorbed by soil and are relatively easy to insolubilize. On the other hand, heavy metals that exist in the form of anions such as arsenic and selenium are less likely to be adsorbed by soil and are difficult to insolubilize. Among them, selenium is mainly present in the form of hexavalent selenate ions, SeO4 2- and tetravalent selenite ion SeO3 2- The former, being an oxidized form, has a higher solubility and is extremely difficult to insolubilize. In order to insolubilize selenium, an excessive amount of insolubilizer must be added to the heavy metal contaminated soil. As a result, the volume of the heavy metal contaminated soil after insolubilization treatment increases and it solidifies more than necessary. The increase in the volume of heavy metal contaminated soil puts pressure on the capacity of disposal sites. Furthermore, the solidification of heavy metal contaminated soil makes transportation and construction difficult. Furthermore, the selenate ion SeO4 2- When reduced, the selenite ion SeO3 becomes less soluble. 2- This prevents leaching, but the amount of leaching of iron oxides, sulfites, etc. is controlled by using a reducing agent to convert selenite to selenate, but there is a concern that if exposed to the environment for a long period of time, they will come into contact with oxygen and be oxidized, causing re-leaching.

[0004] As a method for adsorbing heavy metals that exist in the form of anions, for example, a method is known in which a solution containing anions of inorganic compounds is brought into contact with the adsorbent akaganite, thereby adsorbing the anions onto akaganite (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-15703 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the method using the above adsorbent has a problem in that the amount of selenium adsorbed is small in a solution having a near-neutral pH (pH 6.0 to 8.5) in which selenium is dissolved.

[0007] The present invention has been made in view of the above circumstances, and aims to provide an anion adsorbent capable of adsorbing a large amount of anions such as selenium in a solution having a pH close to neutral and containing dissolved selenium, a method for producing the anion adsorbent, and a method for adsorbing anions using the anion adsorbent. [Means for solving the problem]

[0008] The present invention has the following aspects. [1] An anion adsorbent that contains iron-aluminum double hydroxide as an adsorbent for adsorbing anions of inorganic compounds. [2] A step of dissolving an aluminum salt and iron (III) chloride in water to prepare an iron-aluminum mixed solution; and adding at least one salt selected from the group consisting of hydrogen carbonates, carbonates, and hydroxides of alkali metals, and hydrogen carbonates, carbonates, and hydroxides of alkaline earth metals to the iron-aluminum mixed solution to obtain a precipitate. [3] A method for adsorbing anions, comprising contacting a solution containing anions of an inorganic compound with an anion adsorbent having an iron-aluminum double hydroxide, thereby adsorbing the anions onto the iron-aluminum double hydroxide. [4] The anion adsorption method according to [3], wherein the anion is an oxoacid ion of an inorganic compound. [5] The anion adsorption method according to [4], wherein the anion is a selenate ion or a selenite ion. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide an anion adsorbent capable of adsorbing a large amount of anions such as selenium in a solution having a pH close to neutral and containing dissolved selenium, a method for producing an anion adsorbent, and an anion adsorption method using an anion adsorbent. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing an iron-aluminum double hydroxide in an anion adsorbent according to one embodiment of the present invention. [Figure 2] FIG. 1 shows the results of measuring the pH and selenium concentration of each solution in an example. [Figure 3] FIG. 10 is a diagram showing the results of measuring the pH and selenium concentration of each solution in a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Anion adsorbent] An anion adsorbent according to one embodiment of the present invention contains an iron-aluminum double hydroxide as an adsorbent for adsorbing anions of inorganic compounds.

[0012] The anion adsorbent of this embodiment has iron-aluminum double hydroxide 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 the target anion among the components of the adsorbent. The anion adsorbent of this embodiment may further include a support member that holds the adsorbent. The structure of the iron-aluminum double hydroxide is as shown in Figure 1.

[0013] The iron-aluminum double hydroxide used as an adsorbent can be in any form that is easy to handle, such as powder, gravel, block, or plate. Chemically synthesized powdered iron-aluminum double hydroxide can be used as an adsorbent directly, or the powder can be bound and molded into a larger shape. Methods for binding powdered iron-aluminum double hydroxide include known methods used to bind carbon particles with polymers to form porous bodies (e.g., electrodes and deodorants). Furthermore, lumps obtained by compacting or sintering can be used as is, or they can be crushed or cut into appropriate sizes and molded. Iron-aluminum double hydroxide suspensions, in which iron-aluminum double hydroxide in any of these forms is dispersed in a solvent such as water, can also be used as adsorbents.

[0014] Examples of the holding member include a container for holding the iron-aluminum double hydroxide therein, a column (cylinder), a sieve, a mesh, etc. Also usable are holding members capable of immobilizing the iron-aluminum double hydroxide on their surfaces, such as a plate on which the iron-aluminum double hydroxide is immobilized.

[0015] The anion adsorbent of this embodiment can adsorb a large amount of anions such as selenium in a solution having a pH close to neutral and containing dissolved selenium.

[0016] [Method of manufacturing anion adsorbent] A method for producing an anion adsorbent according to one embodiment of the present invention comprises the steps of: preparing an iron-aluminum mixed solution by dissolving an aluminum salt and iron(III) chloride in water (hereinafter referred to as the "first step"); and adding at least one salt selected from alkali metal bicarbonates, carbonates, and hydroxides, and alkaline earth metal bicarbonates, carbonates, and hydroxides, to the iron-aluminum mixed solution to obtain a precipitate (hereinafter referred to as the "second step").

[0017] "First step" Examples of aluminum salts include aluminum chloride (III) and aluminum nitrate.

[0018] The mixing ratio of aluminum salt to iron (III) chloride is preferably 1:9 to 9:1, more preferably 5:5 to 1:9, in molar concentration. When the mixing ratio is close to 1:9, it is difficult to adsorb anions near neutral. When the mixing ratio is close to 5:5, the amount of iron contained is reduced, making it difficult to adsorb anions.

[0019] The total content of aluminum salt and iron(III) chloride in the iron-aluminum mixed solution is preferably 0.15% by mass or more and 16% by mass or less, more preferably 3% by mass or more and 10% by mass or less, and even more preferably 5% by mass or more and 8% by mass or less. When the content is equal to or greater than the lower limit, a larger amount of adsorbent can be obtained. When the content is equal to or less than the upper limit, stirring becomes easier, and the adsorbent can be easily synthesized.

[0020] "Second step" In the second step, at least one salt selected from the group consisting of alkali metal bicarbonates, carbonates, and hydroxides, and alkaline earth metal bicarbonates, carbonates, and hydroxides, is dissolved in the iron-aluminum mixed solution obtained in the first step, and the desired iron-aluminum double hydroxide is produced by reacting the two salts in the resulting aqueous solution. From the viewpoint of synthesizing the iron-aluminum double hydroxide in high yield, the one or more salts are preferably readily soluble in water, and for example, salts containing the following cations are preferred. The alkali metal is a Group 1 element 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.

[0021] By dissolving one or more of the above salts in water with an aluminum salt and iron(III) chloride, the ions ionized in the aqueous solution react spontaneously to produce iron-aluminum double hydroxide. More specifically, hydroxide ions are produced when one or more of the above salts are dissolved in water. These hydroxide ions react with aluminum ions and iron ions in an acidic aqueous solution containing a large amount of dissolved chloride ions to produce iron-aluminum double hydroxide. In order to promote the reaction for producing the iron-aluminum double hydroxide, the aqueous solution (reaction liquid) may be heated to, for example, about 40°C to 100°C.

[0022] The pH of the aqueous solution when producing the iron-aluminum double hydroxide is preferably 2 or more and 5 or less, more preferably 3.5 or more and 4.5 or less. If the pH is above the lower limit, a large amount of iron precipitates. If the pH is below the upper limit, aluminum can precipitate.

[0023] The pH of the aqueous solution used to produce the iron-aluminum double hydroxide may be adjusted either before or after adding the one or more salts to the iron-aluminum mixed solution.

[0024] 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, which are useful for producing iron-aluminum double hydroxide, into the aqueous solution, thereby suppressing the adsorption of these unnecessary anions onto the iron-aluminum double hydroxide. It is also preferable to adjust and maintain the pH of the aqueous solution using sodium hydroxide.

[0025] The content of iron (III) chloride in the aqueous solution is not particularly limited, and can be, for example, 0.005 mol / L or more and 0.9 mol / L or less.

[0026] The content of the aluminum salt in the aqueous solution is not particularly limited, and can be, for example, from 0.001 mol / L to 0.5 mol / L.

[0027] The content of the one or more salts in the aqueous solution is not particularly limited, and can be, for example, 1 mmol / L or more and 10 mol / L or less.

[0028] In the aqueous solution, Fe produced by iron(III) chloride 3+ and Al produced by aluminum salts 3+ and OH generated by the one or more salts. - The molar ratio of the hydroxybenzoate to the hydroxybenzoate is preferably 9:1:15 to 1:1:6, more preferably 9:1:20 to 7:3:20. Theoretically, a molar ratio of 7:3:20 is most preferable. When the molar ratio is in the above range close to 7:3:20, the Fe content in the aqueous solution is 3+ and Al 3+ The amount of positive charge possessed by OH - The negative charge on the Fe(III) chloride is well balanced to form iron-aluminum double hydroxide. 3+ and Al produced by aluminum salts 3+ Almost all of the iron and aluminum double hydroxides can be easily produced in high yields by consuming them in the reaction.

[0029] The completion of the reaction for producing the iron-aluminum double hydroxide in the aqueous solution (reaction solution) can be determined by the change in color of the reaction solution from yellowish brown to dark brown and the start of aggregation of the synthesized product. The time required for the reaction to settle after the start of the iron-aluminum double hydroxide formation reaction is, for example, about 1 to 60 minutes at 5 to 40°C, depending on the concentration of the iron-aluminum double hydroxide formed.

[0030] After the iron-aluminum double hydroxide is produced, the iron-aluminum double hydroxide can be aggregated by adjusting the pH of the aqueous solution to 3 or more and 6 or less. This is preferably carried out at a temperature range that does not interfere with aggregation, for example, 5°C to 40°C. The time required for the iron-aluminum double hydroxide to aggregate after adjusting the pH is, for example, about 1 to 60 minutes at 5°C to 40°C.

[0031] Here, a method of adjusting the pH of the aqueous solution to between 3 and 6 is preferably a method of adding one or more salts to the aqueous solution. The use of one or more salts can prevent unnecessary anions (e.g., sulfate ions) from being mixed into the aqueous solution and adsorbed onto the iron-aluminum double hydroxide.

[0032] Methods for recovering the iron-aluminum double hydroxide include, for example, known precipitation methods, filtration methods, etc. It is preferable to pre-aggregate the iron-aluminum double hydroxide, as this facilitates recovery. The recovered iron-aluminum double hydroxide can be dried and stored until use. The temperature for drying the iron-aluminum double hydroxide is preferably room temperature (25°C) to 110°C. The amount of salt in the iron-aluminum double hydroxide can be reduced by adding purified water to the dried iron-aluminum double hydroxide to dissolve the salt, and then recovering the precipitate by centrifugation or filtration. The resulting iron-aluminum double hydroxide after drying is usually in the form of a clay-like mass, which can be pulverized into powder form in a mortar or the like.

[0033] According to the above-described method for producing iron-aluminum double hydroxide, iron-aluminum double hydroxide can be recovered and obtained in a yield of, for example, 90% to 99%, assuming that the yield when all of the aluminum added as an aluminum salt and all of the iron ions added as iron(III) chloride ions are converted into iron-aluminum double hydroxide is 100% on a molar basis.

[0034] According to the method for producing an anion adsorbent of this embodiment, an anion adsorbent capable of adsorbing a large amount of anions such as selenium in a solution having a pH close to neutral and containing dissolved selenium can be obtained.

[0035] [Anion adsorption method] An anion adsorption method according to one embodiment of the present invention is a method in which a solution containing anions of an inorganic compound is brought into contact with an anion adsorbent having an iron-aluminum double hydroxide, thereby adsorbing the anions onto the iron-aluminum double hydroxide.

[0036] Examples of inorganic compounds 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.

[0037] As the inorganic compound, an oxoacid is preferred from the viewpoint of exhibiting a high adsorptive power to the iron-aluminum double hydroxide, and a monovalent or divalent inorganic oxoacid containing an 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 a single inorganic atom, and the proton of the hydroxyl group can be eliminated. In water, an oxoacid can become an oxoacid ion by eliminating the proton.

[0038] As the oxoacid, selenium oxoacid is preferred from the viewpoint of its high adsorption ability to iron-aluminum double hydroxides. As the oxoacid ion of selenium, selenate ion (SeO4 2- ), hydrogen selenate ion (HSeO4 -), selenite ion (SeO3 2- ), hydrogen selenite ion (HSeO3 - ) are listed.

[0039] The anions of the inorganic compound contained in the liquid to be treated may be of one type or two or more types.

[0040] The method for bringing the iron-aluminum double hydroxide into contact with the liquid to be treated is not particularly limited, and examples thereof include a method in which iron-aluminum double hydroxide powder is added to the liquid to be treated and stirred, and a method in which the liquid to be treated is poured over the iron-aluminum double hydroxide held on a holding member.

[0041] In this embodiment, it is believed that when the liquid to be treated is brought into contact with the iron-aluminum double hydroxide, the anions contained in the liquid to be treated are trapped and adsorbed by the iron-aluminum double hydroxide.

[0042] When iron-aluminum double hydroxide is added to purified water, the pH of the purified water becomes acidic. Therefore, when iron-aluminum double hydroxide is added to the liquid to be treated, the pH of the liquid to be treated also tends to decrease. When iron-aluminum double hydroxide is added to the liquid to be treated and the target anions are adsorbed onto the iron-aluminum double hydroxide, the pH of the liquid to be treated (iron-aluminum double hydroxide dispersion) during the treatment is preferably 3 or more and 8 or less, more preferably 4 or more and 7 or less, and even more preferably 5 or more and 6 or less. If the pH of the solution to be treated during treatment is kept below 7, decomposition of the iron-aluminum double hydroxide can be prevented, and the adsorption capacity of the iron-aluminum double hydroxide for the target anions can be increased. The lower the pH of the solution being treated during treatment, the more protons will bind to the hydroxyl groups facing the center of the iron-aluminum double hydroxide. This prevents the iron-aluminum double hydroxide from becoming negatively charged, making it easier for the iron-aluminum double hydroxide to adsorb the target anions. Therefore, from the perspective of increasing the adsorption capacity of the target anions, the pH of the solution being treated during treatment is preferably between 3 and 8, more preferably between 4 and 7, and even more preferably between 5 and 6. It is preferable that the pH of the solution to be treated during treatment is between 5 and 6, since this facilitates aggregation of the iron-aluminum double hydroxides, making it easier to recover the iron-aluminum double hydroxides. The method for adjusting the pH of the liquid to be treated is not particularly limited, and examples thereof include adding hydrochloric acid, sodium hydroxide, or one or more salts of the above.

[0043] The temperature of the liquid to be treated when it is brought into contact with the iron-aluminum double hydroxide is not particularly limited, and is preferably 4°C to 40°C, more preferably 10°C to 30°C, for example. Within this temperature range, the adsorption capacity of the target anions by the iron-aluminum double hydroxide can be increased. At or above the lower limit of the above temperature range, the diffusion rate of the target anions in the treatment solution increases, further increasing the efficiency of their contact with the iron-aluminum double hydroxide and their adsorption. At or below the upper limit of the above temperature range, the desorption of the anions from the iron-aluminum double hydroxide once adsorbed can be further reduced.

[0044] There are no particular restrictions on the amount of iron-aluminum double hydroxide that comes into contact with the liquid to be treated relative to the content of the target anions contained in the liquid, and it is sufficient to set the amount based on empirical results obtained through preliminary experiments to ensure that the amount can adequately adsorb the target anions. Generally, the more iron-aluminum double hydroxide is added, the greater the amount of anions that can be adsorbed. For example, the adsorption capacity of inorganic oxoacid ions by iron-aluminum double hydroxide is 0.013 mol / kg to 0.26 mol / kg.

[0045] When an adsorption method is adopted in which iron-aluminum double hydroxide powder is added to the liquid to be treated and stirred, iron-aluminum double hydroxide that has adsorbed anions can be recovered from the liquid to be treated. Methods for recovering iron-aluminum double hydroxide powder from the liquid to be treated include, for example, precipitation, filtration, etc. Precipitation methods include, for example, a method in which the liquid to be treated is allowed to stand to cause precipitation, a method in which aluminum sulfate, PAC, a high molecular weight polymer flocculant, etc. is added to the liquid to be treated to cause flocculation and precipitation, and a method in which the pH of the liquid to be treated is adjusted to 5 to 6 to cause flocculation of the iron-aluminum double hydroxides.

[0046] An adsorption method can also be used, in which iron-aluminum double hydroxide powder is packed into a column and the liquid to be treated containing the target anions is passed through the column. In this case, the iron-aluminum double hydroxide adsorbs the target anions, and the liquid to be treated from which the target anions have been removed is obtained by flowing out the column.

[0047] According to the anion adsorption method of this embodiment, it is possible to adsorb a large amount of anions such as selenium in a solution having a pH close to neutral and containing dissolved selenium.

[0048] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as set forth in the claims. [Example]

[0049] The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0050] [Example] Iron(III) chloride and aluminum(III) chloride were mixed to a molar ratio of iron to aluminum of 7:3 to prepare an iron-aluminum mixed solution. To the resulting iron-aluminum mixed solution, 10N aqueous sodium hydroxide was added dropwise to adjust the pH to 4.0, to prepare a suspension of iron-aluminum double hydroxide. The resulting suspension was centrifuged at 10,000 rpm for 10 minutes, and the precipitate was collected. The collected precipitate was dried at 110°C. The dried precipitate was mixed with the same amount of pure water as the iron-aluminum mixed solution and stirred thoroughly to dissolve the salts. The pH of the suspension was then adjusted back to 4, and the suspension was centrifuged at 10,000 rpm for 10 minutes to precipitate the precipitate. The supernatant was discarded, and the precipitate was dried at 110°C. The dried precipitate was crushed and used as an adsorbent. The obtained adsorbent was subjected to a selenium adsorption experiment. 100 mL of a 0.3 mg / L selenium solution and 0.1 g of the adsorbent were placed in a 100 mL beaker and stirred for 30 minutes using a magnetic stirrer. Thereafter, the pH of the solution was adjusted to 3.0, 5.0, 7.0, 8.0, and 9.0 using an aqueous sodium hydroxide solution, and the solution was stirred for an additional 30 minutes while maintaining each pH. In addition, a test group was also set up in which the pH was not adjusted. After stirring, the solutions were filtered through a 0.45 μm membrane filter, and the pH and selenium concentration (mg / L) of each solution were measured. The results are shown in Figure 2.

[0051] [Comparative Example] 1 L of 0.4 mol / L sodium hydroxide was added to 1 L of 0.2 mol / L iron(III) chloride aqueous solution, and the solution was adjusted to a pH of approximately 2 (Fe 3+ :OH - Akaganeite was produced in a mixture of 1:2. Next, sodium hydroxide was further added to the suspension containing the produced akaganite, the pH was adjusted to 4 to 5, and the akaganite particles were aggregated together while gently stirring for 5 minutes. The aggregated akaganite was collected by filtration to obtain a dry clay-like mass of akaganite. The resulting akaganeite was subjected to a selenium adsorption experiment in the same manner as in Example 1. The results are shown in Figure 3.

[0052] From the results shown in FIG. 3, it can be seen that in the adsorbent using akaganite of the comparative example, the selenium concentration was 0.16 mg / L at pH 7, which exceeded the reference value (0.01 mg / L). The results shown in Figure 2 indicate that the selenium concentration of the adsorbent using the iron-aluminum double hydroxide of the example was below the standard value. Even at a pH of 9, the adsorbent using the iron-aluminum double hydroxide had an average selenium concentration of 0.13 mg / L, exceeding the standard value, but was still able to adsorb selenium to a concentration less than half the 0.28 mg / L achieved by the adsorbent of the comparative example.

Claims

1. An anion adsorbent having iron-aluminum double hydroxide as an adsorbent for adsorbing anions of inorganic compounds.

2. A step of dissolving an aluminum salt and iron (III) chloride in water to prepare an iron / aluminum mixed solution; and adding at least one salt selected from the group consisting of alkali metal bicarbonates, carbonates, and hydroxides, and alkaline earth metal bicarbonates, carbonates, and hydroxides, to the iron / aluminum mixed solution to obtain a precipitate.

3. A method for adsorbing anions, comprising contacting a solution containing anions of an inorganic compound with an anion adsorbent having an iron-aluminum double hydroxide, thereby adsorbing the anions onto the iron-aluminum double hydroxide.

4. The anion adsorption method according to claim 3 , wherein the anion is an oxoacid ion of an inorganic compound.

5. The anion adsorption method according to claim 4 , wherein the anion is a selenate ion or a selenite ion.

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