JP2018020273A - Negative ion adsorption method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative ion adsorption method using an iron oxide mineral capable of adsorbing negative ion easily.SOLUTION: [1] In a negative ion adsorption method, a solution containing negative ion of an inorganic compound and having pH of 2-10 is brought into contact with akaganeite, and thereby the negative ion is adsorbed to akaganeite. [2] In the negative ion adsorption method described in [1], the negative ion is oxo acid ion of an inorganic compound. [3] In the negative ion adsorption method described in [2], the negative ion is selenic acid ion or selenite ion.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an anion adsorption method for adsorbing anions contained in an aqueous solution.

Oxide ions such as selenium, arsenic, and chromium may be contained in wastewater from chemical establishments and construction sites. These anions are highly soluble, depending on inorganic flocculants such as sulfuric acid bands (aluminum sulfate) and PAC (polyaluminum chloride) used in conventional general wastewater treatment, and organic flocculants containing polymer polymers. It is difficult to precipitate and remove. Therefore, in Patent Document 1, selenium, arsenic, and chromium are added to an iron oxide mineral called _{Schwertmannite} [composition formula: Fe ₈ O ₈ (OH) _8-2x (SO ₄ ) _x ; 1 ≦ x ≦ 1.75]. Adsorption methods have been proposed.

JP 2005-95732 A

As a result of intensive studies by the present inventors, the Schwertmannite described in Patent Document 1 inherently contains sulfate ions, so that it is necessary to replace the sulfate ions in order to sufficiently adsorb the target anions. It was thought that there was. In addition, since the binding force of sulfate ions is relatively strong, it has been found that the efficiency of adsorbing the target anion to Schwertmannite is not necessarily high.
Therefore, the present inventors have examined various minerals exhibiting superior adsorption efficiency, and found that Akaganeite exhibits excellent adsorption efficiency for the target anion.

  The present invention provides an anion adsorption method using akaganeate.

[1] An anion adsorption method characterized by adsorbing the anion to the akaganeate by bringing the solution containing an anion of an inorganic compound and having a pH of 2 to 10 into contact with the akaganeate.
[2] The anion adsorption method according to [1], wherein the anion is an oxo acid ion of an inorganic compound.
[3] The anion adsorption method according to [2], wherein the anion is a selenate ion or a selenite ion.

  According to the anion adsorption method of the present invention, anions such as oxo acid of selenium contained in the liquid to be treated can be easily adsorbed.

It is an adsorption isotherm of the selenate ion in three types of iron oxide minerals. It is a schematic diagram showing the tunnel structure of an akaganate. It is a graph which shows the relationship of the dissolved selenium density | concentration after the adsorption | suction process of the selenate ion by akaganate, and pH. It is an experimental result showing that desorption of chloride ions occurs with the adsorption of sulfate ions in akaganeate. It is an experimental result which shows the relationship between pH at the time of iron oxide mineral synthesis | combination, and the adsorption | suction power of the selenate ion with respect to the obtained iron oxide mineral.

<Anion adsorption method>
The anion adsorption method according to the first aspect of the present invention comprises contacting an akaganeite with a solution containing an anion of an inorganic compound and having a pH of 2 to 10 (hereinafter sometimes referred to as a liquid to be treated). In this method, anions are adsorbed on the akaganeate.

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

The inorganic compound is preferably an oxo acid, more preferably a monovalent or divalent inorganic oxo acid containing the inorganic element, from the viewpoint of exhibiting a high adsorptive power to akaganate.
Here, the oxo acid is an inorganic compound in which a hydroxyl group (—OH) and an oxo group (═O) are bonded to one inorganic atom, and a proton of the hydroxyl group can be eliminated. Oxo acid can be an oxo acid ion from which the proton is eliminated in water.

Examples of the oxo acid, from the viewpoint of showing a high adsorption force Akaganeito, oxoacids of selenium is preferred as the oxoacid ions of selenium, selenium ion (SeO 4 ^_2-), selenate hydrogen ions (HSeO ₄ ^-) , Selenite ions (SeO ₃ ²⁻ ), and hydrogen selenite ions (HSeO ₃ ⁻ ).

  One type of anion of the inorganic compound contained in the treatment target liquid may be used, or two or more types may be used.

  The method of bringing akaganate into contact with the treatment target liquid is not particularly limited, for example, a method of adding and stirring the akaganate powder to the treatment target liquid, a method of adding and stirring a suspension containing akaganate to the treatment target liquid, For example, a method may be used in which the liquid to be treated is poured over the akaganate held by the holding member.

Akaganeite (Akaganeite) used as an anion adsorbent in the present invention is an iron oxide mineral represented by the chemical composition β-Fe ³⁺ (O (OH, Cl)). The crystal system is monoclinic, and the crystallographic data of space group I2 / m, unit cell: a = 10.600, b = 3.0339, c = 10.513, β = 90.24 ° are academic. It is described in a paper “Post JE, Buchwald VF, American Mineralogist, 76 (1991) p.272-277, Crystal structure refinement of akaganeite”. It is also described that the crystal structure of akaganeate clarified in this paper has a tunnel structure that retains chloride ions, and a hydroxyl group is pushed out from the tunnel wall toward the center.
FIG. 2 is a diagram schematically showing the tunnel structure. In the figure, gray circles represent oxygen atoms, white circles represent hydrogen atoms, the circle at the center of the octahedron represents iron atoms, and the black circles in the tunnel represent the same occupation ratio (50 :)).

  In the present invention, when the liquid to be treated is brought into contact with akaganate, it is considered that the anion contained in the liquid to be treated is trapped and adsorbed in the tunnel structure of the akaganate. As a result of this adsorption, chloride ions pre-existing in the tunnel structure are replaced with the anions and desorbed (see Example 3).

When akaganeate is added to purified water, the pH of the purified water tends to be acidic. Therefore, even when akaganeate is added to the processing target liquid, the pH of the processing target liquid tends to be low.
The pH of the treatment target liquid (akaganeate dispersion) during the treatment when akaganeate is added to the treatment target liquid and the target anion is adsorbed on the akaganeate is preferably 2 or more and 10 or less, more preferably 3 or more and 7 or less. More preferably 4 or more and 6 or less.
When the pH of the solution to be treated during treatment is 9 or less, the decomposition of akaganate can be prevented, and the adsorption power of the target anion by akaganate can be increased.
The lower the pH of the solution to be treated during treatment, the more protons that bind to the hydroxyl groups facing the center of the tunnel structure of akaganeate. As a result, the tunnel structure can be prevented from being negatively charged, and the target anion can be more easily adsorbed in the tunnel structure. Therefore, from the viewpoint of increasing the target anion adsorptive power, the pH of the liquid to be treated during treatment is preferably pH 2 to 5, more preferably pH 2 to 4, and still more preferably pH 2 to 3.
It is preferable that the pH of the solution to be treated during treatment is 4 or more and 6 or less from the viewpoint of easy aggregation of akaganates and easy recovery of akaganates.
The method for adjusting the pH of the liquid to be treated is not particularly limited, and examples thereof include a method of adding hydrochloric acid, sodium hydroxide, and one or more salts (S) described later.

The temperature of the liquid to be treated when the liquid to be treated and akaganate are brought into contact with each other is not particularly limited, and is preferably, for example, 4 to 60 ° C, more preferably 15 to 50 ° C, and further preferably 30 to 40 ° C.
When the temperature is within the above range, the adsorption force of the target anion by akaganate can be enhanced. When the temperature is at least the lower limit of the above temperature range, the diffusion rate of the target anion in the liquid to be treated is increased, and the efficiency of adsorbing in contact with the akaganeate is further increased. When the temperature is not more than the upper limit of the above temperature range, it is possible to further reduce the desorption of the anion once adsorbed from the akaganeate.

There is no particular limitation on the amount of akaganeate in contact with the treatment target liquid relative to the content of the target anion contained in the treatment target liquid, and the target anion can be sufficiently adsorbed empirically by conducting a preliminary experiment. Can be set to the confirmed amount.
Usually, if the amount of akaganeate added is increased, the amount of anions that can be adsorbed also increases. For example, the amount of inorganic oxoacid ions adsorbed by akaganeate is 0.3 to 0.5 mol / kg.

When an adsorption method in which akaganate powder is added to the treatment target solution and stirred, the anion is adsorbed and can be recovered from the treatment target solution.
Examples of the method for recovering the akaganeate powder from the liquid to be treated include a precipitation method and a filtration method. Examples of the precipitation method include, for example, a method in which the liquid to be treated is allowed to settle, a method in which a sulfuric acid band, PAC, a polymer flocculant, etc. are added to the liquid to be treated for aggregation, and a pH of the liquid to be treated. For example, a method of agglomerating akaganeates by adjusting to 4-6.

  It is also possible to employ an adsorption method in which akaganeate powder is packed in a column and a liquid to be treated containing a target anion flows into this column. In this case, it is possible to obtain the processing target liquid from which the target anion is adsorbed and the target anion is removed from the column.

《Anion adsorbent》
The anion adsorbent of the second aspect of the present invention has akaganeate as a main component of the adsorbent that adsorbs the anion of the inorganic compound. Here, “main component” means a component having the largest adsorption amount when the adsorption amount of the target anion between the components of the adsorbent is compared. The adsorbent may further include a holding member that holds the adsorbent.

  As the shape of the akaganeate as the adsorbent, for example, a shape such as powder, gravel, lump, or plate can be easily used. The powdered akaganate obtained by chemical synthesis may be used as an adsorbent as it is, or may be formed into a larger shape by binding this powder. As a method for binding powdered akaganate, for example, a known method used when forming a porous body (for example, electrode, deodorant) by binding carbon particles with a polymer is adopted. can do. In addition, a lump obtained by pressing or sintering may be used as it is, or the lump may be formed by crushing or cutting the lump to an appropriate size. The akaganeate suspension in which akaganeates of these shapes are dispersed in a solvent such as water can be used as the adsorbent.

  Examples of the holding member include containers, columns (cylinders), baskets, nets, and the like that hold akaganate inside. Also, a holding member capable of fixing akaganate to the surface can be adopted, and for example, a form in which akaganate is fixed to the surface of the plate material can be mentioned.

《Synthesis of akaganate》
The akaganate used in the present invention may be chemically synthesized by a known method or may be naturally produced, but the akaganate synthesized by the method described below is of high quality, It is preferable because of its excellent adsorption power.

One or more salts (S) selected from alkali metal hydrogen carbonates, carbonates and hydroxide salts, and alkaline earth metal hydrogen carbonates, carbonates and hydroxide salts, and iron chloride (III) ) Can be dissolved in water and reacted in the resulting aqueous solution to produce the desired akaganeate.
From the viewpoint of synthesizing akaganeate in high yield, the one or more salts (S) 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 preferable.
The alkaline earth metal is a Group 2 element of the periodic table, and magnesium, calcium, and barium are preferable.

By dissolving iron (III) chloride and the one or more salts (S) in water, ions ionized in an aqueous solution react spontaneously to produce akaganate. More specifically, hydroxide ions are generated when the one or more salts (S) are dissolved in water. The hydroxide ions and iron ions react in an acidic aqueous solution in which a large amount of chloride ions are dissolved, thereby producing akaganeate.
The aqueous solution (reaction solution) may be heated to, for example, about 40 to 100 ° C. in order to promote the production reaction of akaganeate.

The pH of the aqueous solution when producing akaganeate is preferably less than 7, more preferably less than 4, and still more preferably 1 to 3.
If the pH is less than 7, akaganeate is easily produced in the presence of chloride ions.
When the pH is less than 4, particularly when the pH is 3 or less, akaganeate can be produced in a high yield in the presence of chloride ions. In addition, although akaganate is easily formed even at pH 4-6, if it is within this pH range, the akaganate being produced may aggregate and unreacted iron chloride (III) or salt (S) may be taken in. is there. On the other hand, when the pH is alkaline, there is a high possibility that iron oxide minerals having different structures (for example, goethite, scumite, etc.) are generated.

  Adjustment of the pH of the aqueous solution when producing akaganeate may be performed before adding at least one of iron (III) chloride and the one or more salts (S) to the water, You may carry out after melt | dissolving in water. However, if the pH of the aqueous solution in which both are dissolved is left in an alkaline state, iron oxide minerals other than akaganate may be generated. Therefore, it is preferable that the pH of the aqueous solution is adjusted to be acidic and maintained at an acidic pH immediately after dissolving both, or before or during dissolution of the at least one.

  The method of adjusting and maintaining the pH of the aqueous solution is preferably a method of dropping hydrochloric acid. If hydrochloric acid is used, it is possible to prevent the addition of extra anions (for example, sulfate ions) other than chloride ions useful for the production of akaganate to the aqueous solution, and to prevent the extra anions from adsorbing to the akaganate. It is also preferable to adjust and maintain the pH of the aqueous solution using sodium hydroxide.

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. Similarly, the total amount of the one or more salts (S) that are dissolved when the aqueous solution is prepared is not particularly limited, and may be, for example, 0.01 to 3 mol / L.
In preparing the aqueous solution, the order of dissolving iron (III) chloride and the one or more salts (S) is not particularly limited, but in order to maintain the pH of the aqueous solution acidic, iron (III) chloride is used. It is desirable to dissolve it first.

In the aqueous solution, the molar ratio of Fe ³⁺ produced by iron (III) chloride and OH ⁻ produced by the one or more salts (S) is 1: 1 to 1: 3. Preferably, it is 1: 1.5 to 1: 2.5, more preferably 1: 1.8 to 1: 2.2. Theoretically, a molar ratio of 1: 2 is most preferred.
When the molar ratio is in the above range close to 1: 2, the amount of positive charge possessed by Fe ³⁺ in the aqueous solution and the amount of negative charge possessed by OH ⁻ become a balance suitable for the production of akaganeate, and iron chloride (III Almost all of Fe ³⁺ derived from) can be consumed in the reaction, and akaganate can be easily produced in high yield.

Specifically, for example, in a 1 L aqueous solution in which 0.1 mol of sodium hydrogen carbonate is dissolved, an apparent acid dissociation constant pKa ₁ = 6.3 (influence of equilibrium with carbon dioxide) of carbonic acid. When the solution pH is 1 or more lower than pKa ₁ and pH 5.3 or less, the hydroxide ion concentration (carbonic acid molecule concentration) generated in the aqueous solution is 0.09 to 0.1 mol / L. It is thought to be about. Based on this, the concentration of iron (III) chloride is preferably 1/3 to 1 times the concentration of 0.09 to 0.1 mol / L, and more preferably 1/2 to 1 times the concentration.
In addition, for example, in a 1 L aqueous solution in which 0.1 mol of sodium carbonate is dissolved, in consideration of the acid dissociation constant pKa ₂ = 10.3 of carbonic acid and the apparent acid dissociation constant pKa ₁ = 6.3, When the solution pH is 5.3 or lower, the hydroxide ion concentration (carbonic acid molecule concentration) generated in the aqueous solution is considered 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 the concentration of 0.18 to 0.2 mol / L, and more preferably 1/2 to 1 times the concentration.
In addition, for example, in a 1 L aqueous solution in which 0.1 mol of sodium hydroxide is dissolved, water generated in the aqueous solution in an acidic region where the solution pH is 7 or less in consideration of the acid dissociation constant pKa = 13. The oxide ion concentration is considered to be approximately 0.1 mol / L. Based on this, the concentration of iron (III) chloride is preferably 1/3 to 1 times the concentration of 0.1 mol / L, and more preferably 1/2 to 1 times the concentration.

When using any bicarbonate, carbonate, or hydroxide salt, if the pH of the aqueous solution is 1 or more lower than the pKa _{1 of} the salt, it is about 0.9 to 2 times the molar concentration of the dissolved salt. Of hydroxide ions are formed. Therefore, iron (III) chloride is about 0.3 to 2 times the molar concentration of the dissolved one or more salts (S) in the above pH range (1/3 of the hydroxide ion concentration to be generated). It is preferable to dissolve at a concentration of 1 time), and more preferable to dissolve at a concentration of 0.45 to 1 time (1/2 times the concentration of hydroxide ions to be generated).

Moreover, considering the above comprehensively, in the aqueous solution producing akaganeate, the molar ratio of iron (III) chloride to the one or more salts (S) is 2: 1 to 1: 3. preferable.
When the molar ratio is within the above range, the charge balance between Fe ³⁺ and OH ^{− in} the aqueous solution becomes good, and akaganate can be easily produced in a high yield.

The completion of the akaganeate formation reaction in the aqueous solution (reaction solution) can be determined empirically based on the fact that the reaction solution has changed from dark brown to reddish brown.
The time required for the reaction to complete once after the start of the reaction for producing akaganate is about 3 to 5 minutes at 10 to 25 ° C., depending on the concentration of the produced akaganate.

After the akaganate is produced, the akaganate can be aggregated by adjusting the pH of the aqueous solution to 4 or more and pH 6 or less. At this time, it is preferable to carry out at a temperature range that does not prevent aggregation, for example, at 10 to 40 ° C. The time required for adjusting the pH to aggregate the akaganeate is, for example, about 5 to 10 minutes at 10 to 25 ° C.
Here, as a method of adjusting the pH of the aqueous solution to 4 or more and pH 6 or less, a method of adding the one or more salts (S) in addition to the aqueous solution is preferable. By using the one or more salts (S), it is possible to prevent extra anions (for example, sulfate ions) from being mixed into the aqueous solution and adsorbed on the akaganate.

Examples of the method for recovering akaganeate include known precipitation methods and filtration methods. It is preferable to flocculate the akaganeate in advance because recovery becomes easy.
The collected red akaganate can be dried and stored until use.
The form of the dried akaganeate obtained by filtration is usually a clay-like lump, and can be pulverized into a powder by a mortar or the like.

  According to the above-mentioned method for synthesizing akaganeate, when the yield when all of the iron ions introduced as iron (III) chloride become akaganeate is 100% on a molar basis, for example, the yield is 90 to 99%. You can collect and obtain akaganate.

(Synthesis of akaganate)
To 1 L of 0.2 mol / L iron (III) chloride aqueous solution, 0.4 L / L sodium hydroxide (1 L) was added, and while stirring gently for 5 minutes, about pH 2 aqueous solution (Fe ³⁺ : OH ⁻ = about 1: 2) Akaganate was produced. Next, sodium hydroxide was further added to the suspension containing the produced akaganeate, adjusted to pH 4 to 5, and agglomerated with each other while gently stirring for 5 minutes. Aggregated akaganeate was collected by filtration to obtain a dried clay-like akaganeto lump. The akaganeate, which was crushed in a mortar and made into powder, was used in the following experiment.
When the yield when all of the iron ions added as iron (III) chloride became akaganate was assumed to be 100% on a molar basis, the akaganate was recovered and obtained at a yield of 95%.
When the synthesized akaganate was analyzed by XRD, a peak indicating akaganate was confirmed.

[Example 1]
A sodium selenate aqueous solution (pH 9) containing 10 mg / L of selenium was prepared. The following experimental procedure was performed using the akaganeate obtained by the above synthesis.
(1) 0.015, 0.025, 0.05, 0.1, 0.2, 0.5, 1.0 (unit: w) is added to the above-described aqueous solution containing selenate ions. / W%) at each concentration. After the aqueous solution adjusted to pH 6 was stirred at 20 ° C. for 1 hour, akaganeate was precipitated, the supernatant was recovered, and the selenate ion concentration was determined according to “67. Method ".
(2) Green last was added to the aqueous solution containing selenate ions so as to have a weight ratio of 0.15 w / w% to 1.0 w / w%. After the aqueous solution at pH 6 was stirred at 20 ° C. for 1 hour, green last was precipitated, the supernatant was collected, and the selenate ion concentration was measured by the above method.
(3) To the above aqueous solution containing selenate ions, Schwertmannite is added to 0.015, 0.025, 0.05, 0.1, 0.2, 0.5, 1.0 (unit: w / w). %) At each concentration. After the aqueous solution adjusted to pH 6 was stirred at 20 ° C. for 1 hour, Schwertmannite was precipitated, the supernatant was collected, and the selenate ion concentration was measured by the above method.
By the above experiment, adsorption isotherms for selenate ions in iron oxide minerals of akaganeate, green last, and schbertmanite were obtained (FIG. 1).
From the results shown in FIG. 1, it is clear that the iron oxide mineral whose dissolved selenate ion equilibrium concentration is not more than the environmental standard (0.01 mg / L) is only akaganate, and its adsorption amount is the highest.

[Example 2]
An aqueous solution (about pH 7) containing sodium selenate at 1.152 mg / L was prepared. The following experimental procedure was performed using the akaganeate obtained by the above synthesis.
The prepared aqueous solution of sodium selenate was divided into a plurality of parts, and the akaganeate synthesized above was added to each aqueous solution at 0.2 (w / w)%. Subsequently, the pH of each aqueous solution was adjusted to 2 to 10 using sodium hydroxide or hydrochloric acid, and selenate ions were adsorbed on the akaganeate. After gently stirring at about 20 ° C. for several minutes, the dissolved selenium concentration in the supernatant of each aqueous solution was measured by ICP emission spectroscopy as described above.
The result of the experiment is shown in the graph of FIG. In FIG. 3, the broken line represents the initial concentration (1.152 mg / L) of dissolved selenium in the aqueous solution, the vertical axis represents the dissolved selenium concentration in the aqueous solution, and the horizontal axis represents the pH of each aqueous solution after pH adjustment.
From the results of FIG. 3, it is clear that the adsorption of selenate ions by akaganate is excellent in the range of p4 to 7, and that the amount of adsorption decreases when the pH is 3.5 or lower and the pH is higher than 8.

[Example 3]
0.1, 0.5, 1.0, 2.0, 3.0 (units) of the akaganate synthesized above in an aqueous solution (pH 10) containing about 1200 mg / L (about 12.5 mmol / L) of sulfuric acid. : W / w%) at each concentration. After the aqueous solution adjusted to pH 6 was stirred at 20 ° C. for 1 hour, akaganeate was precipitated, the supernatant was collected, and the concentrations of sulfate ion and chloride ion were measured by ion chromatography.
As a result, as shown in the graph of FIG. 4, the chloride ion concentration in the aqueous solution increased in proportion to the amount of akaganeate added, and the sulfate ion concentration decreased accordingly. The increased chloride ion concentration was approximately twice the reduced sulfate ion concentration. This result means that the charge amount of the chloride ion desorbed from the akaganeate and the charge amount of the sulfate ion adsorbed on the akaganeate are almost the same.
From the above results, it is considered that the chloride ion constituting the akaganeate is replaced when another anion is adsorbed.

[Experiment 3]
(Relationship between pH at the time of synthesizing akaganeate (iron oxide mineral) and adsorption capacity)
The pH of the reaction solution for synthesizing akaganeate (iron oxide mineral) was changed in increments of 2 to 10 and then allowed to stand overnight to obtain a suspension containing iron oxide mineral.
As a result of XRD analysis of the iron oxide mineral obtained by the above synthesis, a peak indicating that the iron oxide mineral synthesized at pH 2 and pH 3 is akaganate was confirmed. On the other hand, the iron oxide mineral synthesized at pH 4 or higher did not show a clear peak, and was an amorphous iron oxide mineral.
Even when synthesized at any pH, the yield of akaganeate or other iron oxide minerals was 90% or more.

A suspension of each iron oxide mineral synthesized above in an aqueous sodium selenate solution (pH 9) with an initial concentration of 0.90 mg / L of selenium prepared separately as water to be treated is 0.1 w / w% in terms of dry matter. And stirred gently for 5 minutes. After stirring, the Se concentration of the supernatant liquid excluding the iron oxide mineral was measured in the same manner as described above. Each iron oxide mineral whose pH at the time of synthesis was changed was tested with the number of samples = 2 (N = 2). The result is shown in FIG.
In FIG. 5, the “◯” plot indicates that the akaganate synthesized at the above pH 2-3 was used as the adsorbent, and the “◇” plot adsorbs the other iron oxide minerals synthesized at the above pH 4-10. It shows that it was used as an agent.
From the results of FIG. 5, it is understood that the lower the pH during synthesis, the higher the adsorption power of selenium (selenate ion). That is, it is clear that akaganeates synthesized at pH 2 to 3 exhibit excellent anion adsorption power.

  The configurations and combinations thereof in the embodiments described above are examples, and additions, omissions, substitutions, and other modifications of known configurations are possible without departing from the spirit of the present invention.

  The present invention can be widely applied to uses for purifying contaminated water containing heavy metals such as selenium, arsenic, and chromium.

Claims (3)

  An anion adsorption method comprising adsorbing the anion to the akaganate by bringing the solution containing an anion of an inorganic compound and having a pH of 2 to 10 into contact with the akaganate.   The anion adsorption method according to claim 1, wherein the anion is an oxo acid ion of an inorganic compound.   The anion adsorption method according to claim 2, wherein the anion is a selenate ion or a selenite ion.