JP2017177039A - Selenium absorber, manufacturing method therefor, processing method of selenium using the absorber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a selenium absorber having excellent selenium processing performance capable of absorbing hexavalent selenium as hexavalent without reducing the same compared to conventional selenium absorbers and a manufacturing method therefor, and a selenium absorbing method using the selenium absorber.SOLUTION: The selenium absorber contains iron (Fe) and magnesium (Mg) derived from iron (Fe) and magnesium (Mg)-containing solution with pH of 9.5 to 10.5 as essential constituent elements and having molar ratio of Mg/Fe of over 0.2 and 59 or less, preferably 0.5 to 59.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a selenium adsorbent, a method for producing the same, and a method for treating selenium, and in particular, a selenium adsorbent capable of treating selenium and selenium-containing wastewater eluted from coal ash and soil, a method for producing the same, and a method for treating selenium. It is about.

Since selenium has an adverse effect on the human body and may cause skin disorders, vomiting, systemic silicosis, hypersensitivity, anemia, gastrointestinal disorders, etc., the concentration in wastewater is regulated by the Water Pollution Control Law.
On the other hand, selenium is widely used in the manufacture of electrical materials such as photoconductors for photocopiers, catalysts, etc. using the coloring, decoloring agent and photoconductivity of glass, so it is contained in wastewater from glass factories and semiconductor factories. And may leach out from the soil of these factory sites.

Furthermore, according to “Examination of leaching characteristics of arsenic and selenium in coal ash” (Seiji Inoba, Hisashi Shimogaki, Research Report of Central Research Institute of Electric Power (2004)), some coal types have a higher concentration than ordinary soil. Coal ash containing selenium is discharged from thermal power plants.
According to the “National Fact-finding Report (2013)” (Coal Energy Center), the proportion of coal-fired power generation in Japan's total power generation increased after the Fukushima Daiichi nuclear power plant accident. There has been a report that an increase in ash emission has been made, and the emission of coal ash containing selenium at a high concentration tends to increase.

  In view of such circumstances, a method and a selenium adsorbent capable of treating selenium eluted from coal ash and soil and selenium-containing wastewater are expected.

In a normal environment, selenium exists mainly in two chemical forms: a tetravalent selenite ion and a hexavalent selenate ion.
Here, the tetravalent selenite ion has a planar structure and has a structure in which the reaction with various compounds easily proceeds, whereas the hexavalent selenite ion has a stable tetrahedral structure. It has a structure and poor reactivity with various compounds.

  Further, the treatment method that is widely used at present as a treatment method for selenium is coprecipitation treatment with an iron salt or an aluminum salt. Such a method has a sufficient effect on tetravalent selenium, but 6 There is a problem that the effect on selenium valence is low.

Therefore, as an effective treatment method for hexavalent selenium, Japanese Patent No. 395978 (Patent Document 1) discloses that hexavalent selenate ions are reduced with divalent iron salts, metal salts, etc. Subsequently, a method by a coprecipitation method is disclosed. Specifically, a step of adding a reducing iron compound to selenium-containing water [iron compound addition step], and a reaction of selenium-containing water to which a reducing iron compound has been added. The process of introducing precipitates into the tank [precipitation process], the process of solid-liquid separation of the generated precipitate (sludge) [solid-liquid separation process], and making all or part of the separated sludge alkaline and returning it to the reaction tank A process for removing selenium by adding a reductive iron compound to selenium-containing water to precipitate selenium, and solid-liquid separating the precipitate, As ferrous compound Use a sealed reaction tank in the precipitation step, adjust the sludge to be returned to the reaction tank to pH 11-13, further adjust the inside of the reaction tank mixed with this alkaline sludge to pH 8.5-11 alkaline, Reducing iron comprising a mixture of green last and iron ferrite and having a ratio of divalent iron ions to total iron ions [Fe ²⁺ / Fe (T)] of 0.4 to 0.8. A method of treating water containing selenium or the like is described, wherein selenium is incorporated into the precipitate and removed from the system by forming a compound precipitate.

  JP-A-2004-89924 (Patent Document 2) discloses a water-soluble selenium removing agent for removing water-soluble selenium from a substance contaminated with water-soluble selenium by using amorphous iron hydroxide (III). Or a substance obtained as a precipitate by neutralizing an aqueous iron (III) nitrate solution with an alkali metal hydroxide, and bringing the water-soluble selenium remover into contact with a substance contaminated with water-soluble selenium, A selenium wastewater treatment method using amorphous iron (III), in which water-soluble selenium is adsorbed and insolubilized by a water-soluble selenium removing agent is disclosed.

  However, the treatment method described in Patent Document 1 has a problem in that the treatment process is complicated because a reduction treatment step is required, the efficiency of the reduction treatment is poor, and a large amount of reducing agent is required. The treatment method described in Patent Document 2 is insufficient in treating high-concentration selenium wastewater, and further improvement has been desired.

Japanese Patent Laying-Open No. 2015-80774 (Patent Document 3) contains iron (Fe), silicon (Si), and oxygen (O) as constituent elements, and the molar ratio of Fe / Si is 5 or more and 300 or less. Is a treatment method in which the selenium adsorbent is brought into contact with a selenium-containing solution containing hexavalent selenate ions, but the recommended reaction system conditions for the adsorbent are set to pH 5 or lower. .
In the treatment method described in Patent Document 3, a Se (VI) solution is used. However, since the reduction reaction of Se is likely to proceed under acidic conditions, Fe or the like acts as a catalyst and acts on Se (IV). It can be inferred that the technology is reduced and adsorbed.

Se (IV) is more easily adsorbed to metal hydroxides and the like than Se (VI), and is in an oxidized state in the atmospheric environment. Therefore, when it is adsorbed to the adsorbent in the form of Se (IV), There is a possibility of desorption from the adsorbent when it is exposed to the atmosphere and changed from Se (IV) to Se (VI).
Accordingly, there is a need for a selenium adsorbent that can adsorb selenium in the form of Se (VI).

Japanese Patent No. 395978 JP 2004-89924 A Japanese Patent Laying-Open No. 2015-80774

The object of the present invention is to solve the above-mentioned problems and to have a selenium adsorbent having an excellent selenium treatment capacity capable of adsorbing hexavalent selenium without reducing it as compared with conventional selenium adsorbents, and its It is to provide a manufacturing method.
Another object of the present invention is to provide a treatment method for effectively adsorbing selenium in a hexavalent state from soil or wastewater containing selenium.

  The present invention has been found that an adsorbent having a specific component at a specific molar ratio can efficiently adsorb hexavalent selenium in a hexavalent state, and has arrived at the present invention.

(1) The selenium adsorbent contains iron (Fe) and magnesium (Mg) -containing solution having a pH of 9.5 to 10.5 as essential constituent elements, and contains Mg / Fe moles. The selenium adsorbent characterized in that the ratio is more than 0.2 and 59 or less.
(2) The selenium adsorbent according to (1) is characterized in that the molar ratio of Mg / Fe is 0.5 to 59.

(3) In the method for producing a selenium adsorbent according to (1) or (2), a solution containing magnesium ions is dropped into a solution containing ferric ions so that the molar ratio of Mg / Fe becomes a predetermined molar ratio. Then, an alkaline solution is added so that the pH of the resulting solution becomes neutral to alkaline, then left to stand at 0 to 60 ° C. to form a precipitate, and solid-liquid separation is performed. It is a manufacturing method of a selenium adsorbent characterized by manufacturing by drying a residue.

(4) In the method for producing a selenium adsorbent of (3), an alkaline solution is added so that the pH of the solution containing ferric ions and magnesium ions after addition of the alkaline solution is 9.5 to 10.5. It is characterized by that.

(4) In the selenium treatment method, the selenium adsorbent of (1) or (2) is mixed with hexavalent selenium-containing wastewater or hexavalent selenium-containing contaminated soil, and in the state of hexavalent selenium, It is characterized by being treated by adsorbing to a selenium adsorbent.

In the past, it was difficult to adsorb hexavalent selenium ions, and the adsorption treatment was carried out after reducing the tetravalent selenite ions to tetravalent selenite ions. Hexavalent selenium can be adsorbed in the hexavalent state without requiring reduction treatment to acid ions.
Therefore, since it is in an oxidized state in the atmospheric environment, when it is adsorbed to the adsorbent in the form of Se (IV), the adsorbent is exposed to the atmosphere and changed from Se (IV) to Se (VI). Although there was a possibility of desorption, according to the present invention, since selenium can be adsorbed in a hexavalent state, it is not desorbed from the adsorbent and has excellent adsorption performance.
In addition, the method for producing a selenium adsorbent of the present invention can efficiently prepare a selenium adsorbent capable of adsorbing hexavalent selenium effectively while remaining in the hexavalent state.
Furthermore, the selenium treatment method using the selenium adsorbent of the present invention can adsorb hexavalent selenium in a hexavalent state by mixing it with wastewater or contaminated soil containing hexavalent selenium. The removal process can be performed effectively and simply.

It is an XRD chart figure of the selenium adsorbent of an example of the present invention. It is a figure which shows the adsorption isotherm of hexavalent selenium using the selenium adsorbent of an example of this invention. An adsorbent obtained by adsorbing selenium from a hexavalent selenium-containing solution and a tetravalent selenium-containing solution, and a sodium selenite solution (Se: IV) and a sodium selenate solution (Se: It is a XANES spectrum diagram of VI).

The present invention is illustrated by the following embodiments, but is not limited thereto.
The selenium adsorbent of the present invention contains iron (Fe) and magnesium (Mg) derived from an iron (Fe) and magnesium (Mg) solution having a pH of 9.5 to 10.5 as essential constituent elements, and has a Mg / Fe mole. A selenium adsorbent having a ratio exceeding 0.2 and not more than 59, preferably 0.5 to 59.
With this configuration, hexavalent selenium can be adsorbed and removed in a hexavalent state.
When the Mg / Fe molar ratio in the selenium adsorbent of the present invention is 0.2 or less, or when the Mg / Fe molar ratio exceeds 59, the adsorption capacity of selenium decreases.

  The selenium adsorbent of the present invention may be any one containing iron (Fe) and magnesium (Mg) as essential constituent elements as long as the solution containing iron (Fe) and magnesium (Mg) compounds has a pH of 9.5 to 10.5. Examples of the compound containing iron include iron oxide, iron oxyhydroxide, ferric chloride, ferric sulfate and the like, and as the compound containing magnesium, Examples include magnesium chloride and magnesium sulfate.

In addition, iron (Fe) and magnesium (Mg), which are essential constituent elements of the selenium adsorbent of the present invention, are (Fe) derived from a solution containing iron (Fe) and magnesium (Mg) at pH 9.5 to 10.5. Magnesium (Mg).
By using such iron (Fe) and magnesium (Mg) as essential constituent elements, it is possible to exhibit excellent adsorption performance for selenium.

  The selenium adsorbent of the present invention is not particularly limited, whether it is a crystalline material, an amorphous material, or a mixture thereof. In particular, amorphous iron hydroxide has a higher selenium adsorption. From the standpoint of performance, it is preferable that the crystalline material is small and the material is an amorphous material.

  The selenium adsorbent of the present invention may contain other components as long as the effects of the present invention are not hindered. For example, a pH adjuster, a flocculant, and the like can be contained as necessary.

  The production method of the selenium adsorbent of the present invention is not particularly limited. For example, a solution containing magnesium ions is added to a solution containing ferric ions, and the molar ratio of Mg / Fe is 0. .2 to 59 or less, preferably 0.5 to 59, and an alkaline solution is added so that the pH of the resulting solution is preferably neutral to alkaline, preferably 0 to 60 ° C. The adsorbent of the present invention can be prepared by standing and aging under the above conditions to produce a precipitate.

  The alkaline solution is preferably added so that the pH of the solution containing ferric ions and magnesium ions after addition of the alkaline solution is 9.5 to 10.5, and the selenium adsorbent obtained thereby Can have excellent adsorption performance.

  Next, the obtained precipitated product is subjected to solid-liquid separation, preferably solid-liquid separation by means such as centrifugation, and then dried, desirably dried at 60 to 120 ° C., and the obtained dried product is obtained. The selenium adsorbent of the present invention is preferably pulverized appropriately.

  The ferric ion source used for producing the selenium adsorbent of the present invention is not particularly limited as long as it is a compound that dissolves in water to form ferric ions, and various compounds can be used. And ferric sulfate, ferric nitrate, ferric chloride and the like.

The magnesium ion source used for producing the selenium adsorbent of the present invention is not particularly limited as long as it is a compound that dissolves in water to form magnesium ions, and various compounds can be used.
For example, magnesium chloride, magnesium sulfate and the like can be exemplified, and magnesium chloride can be particularly preferably used.

The pH of the solution containing iron and magnesium is preferably neutral to alkaline from the viewpoint of the yield of the adsorbent, and an alkaline solution is added to the solution containing iron and magnesium as necessary.
Examples of the alkaline solution include an aqueous sodium hydroxide solution and an aqueous potassium hydroxide solution.

  Moreover, it is also possible to mix | blend an aggregating agent as needed in the range which does not prevent the said effect of this invention.

Next, the solution containing iron and magnesium is allowed to stand and mature at 0 to 60 ° C. to produce a precipitate, and the resulting precipitate is subjected to solid-liquid separation, etc., thereby preparing the adsorbent of the present invention. Is possible.
Specifically, it is preferable that the solid residue obtained after the solid-liquid separation is dried, and the obtained dried product is appropriately pulverized to form the selenium adsorbent of the present invention in powder form.
As the drying means, for example, any solid-liquid separation means such as freeze-drying or centrifugation can be used, and preferably drying at 60 to 120 ° C.

  The selenium adsorbent of the present invention is brought into contact with coal ash from which hexavalent selenate ions elute, soil, and waste liquid containing hexavalent selenate ions, thereby allowing the hexavalent selenate ions to remain as they are. Selenium can be treated by adsorbing in the state and insolubilizing.

By removing the selenium adsorbent of the present invention in contact with contaminated soil or contaminated wastewater, the selenium contained in the contaminated soil or contaminated wastewater can be adsorbed and removed.
Any known method can be applied as the contact method, and examples thereof include mixing of the selenium adsorbent of the present invention with soil, and a method of stirring into waste water.
In addition, for example, when it is put into the contaminated waste water, it is possible to add a flocculant and collect it by a solid-liquid separation method.
Furthermore, for example, a method of filling the adsorbent of the present invention into a column or the like and passing a selenium-containing waste liquid into the column or the like can be used.

  The temperature at which the selenium adsorbent of the present invention is brought into contact with coal ash or soil from which hexavalent selenate ions are eluted and waste liquid containing hexavalent selenate ions is not particularly limited. It is sufficient to make it contact in the range of 0-50 degreeC, Preferably it is room temperature etc., The pH at the time of making it contact is not specifically limited, For example, about pH 5-11 is preferable.

Further, for example, it is possible to combine the selenium adsorbent of the present invention with a known heavy metal treating agent to form a treatment method for removing not only selenium but also other heavy metals.
For example, selenium and other harmful substances can be highly removed by using together with a semi-fired dolomite, lightly burned dolomite, a heavy metal treating agent such as magnesium oxide, and the like.

The invention is illustrated by the following examples and comparative examples.
(Examples 1-4 and Comparative Examples 1-8)
(Preparation of adsorbent)
In a 1000 ml poly beaker, magnesium chloride hexahydrate and iron chloride hexahydrate are in a predetermined molar ratio (total 1.5M) shown in Table 1 so that magnesium chloride hexahydrate and iron chloride hexahydrate are present. The Japanese product was added, and 700 ml of ultrapure water was poured and mixed to prepare a mixed solution of magnesium chloride and iron chloride.

  An 8N aqueous sodium hydroxide solution was appropriately added so that the resulting magnesium chloride and iron chloride mixed solution had a pH of 9.5 to 10.5. Then, it left still for 12 hours and solid-liquid-separated the produced | generated deposit by centrifugation (3000 rpm, 10 minutes).

The supernatant was removed and 350 ml of ultrapure water was added to disperse the precipitate, followed by solid-liquid separation by centrifugation. This operation was repeated four times to remove salts in the precipitate.
Subsequently, after drying a deposit for 12 hours with a dryer (85 degreeC), the obtained solid was grind | pulverized to 75 micrometers or less with the mortar, and it was set as the selenium adsorption agent.

The powder X-ray diffraction Rietveld method (XRD) under the following conditions was performed on the selenium adsorbent of Example 3, and the obtained XRD chart is shown in FIG.
Measuring condition use device: PANalytical X'Pert Pro MPD
Measurement conditions: Tube Cu-Kα
Tube voltage: 45 kV
Current: 40 mA
Divergence slit: variable (12 mm)
Anti-scatter slit (incident side): None Solar slit (incident side): 0.04 rad.
Receiving slit: None Anti-scatter slit (receiving side): Variable (12 mm)
Solar slit (light receiving side): 0.04 rad
Scanning range: 2θ = 5-50 °
Scanning step: 0.008 °
Counting time: 0.0004 ° / sec.

  From FIG. 1, it can be seen that a halo peak is observed at 24.5 to 30.5 °, and that other peaks are also broad and thus are amorphous or have low crystallinity.

(Test Example 1: Adsorption test of hexavalent selenium)
Sodium selenate (special grade reagent: Kanto Chemical Co., Inc.) was dissolved in water to prepare Se (VI) solutions having concentrations of 1 mg / l and 5 mg / l.
0.3 g of each selenium adsorbent obtained in Examples 1 to 6 above was weighed into a 50 ml conical tube, and the 1 mg / l (Ci) Se (VI) solution or 5 mg / l (Ci) concentration was measured. 30 ml of each Se (VI) solution was added and shaken for 24 hours to mix uniformly.
After 24 hours, suction filtration was performed using a 0.45 μm membrane filter to perform solid-liquid separation, and the selenium concentration in the filtrate was determined by “hydrogen compound generation ICP emission spectroscopic analysis” (JIS K 0102: 2013). Measured and quantified (Cf), and the adsorption removal rate was calculated from the following formula.
Adsorption removal rate (%) = (Ci−Cf) / Ci × 100
In the above formula, Ci = initial concentration (mg / l) in the adsorption test, Cf = final concentration (mg / l) in the adsorption test.

  Further, the pH and oxidation-reduction potential (ORP) of the filtrate were measured with a desktop pH meter: F-73 (pH electrode: 9615S-10D, ORP electrode: 9300-10D) manufactured by Horiba, Ltd. The results are also shown in Table 1.

(Hexavalent selenium adsorption isotherm)
The hexavalent selenium adsorption isotherm for the adsorbent of Example 1 (Mg / Fe molar ratio: 2) is shown in FIG.

Specifically, in the adsorption test of hexavalent selenium of Test Example 1 above, Ci (initial concentration in the adsorption test) was changed between 1 and 1000 mg / l, and the same adsorption test was performed. At that time, Cf (final concentration in the adsorption test) was set to the Se (VI) equilibrium concentration, and the Se (VI) adsorption amount was calculated from the following formula.
The Se (VI) adsorption amount of the adsorbent was calculated on the assumption that Se (VI) adsorption to the LDPE container used in the adsorption test was zero.

From the obtained results, the Se (VI) adsorption amount was plotted on the vertical axis, and the Se (VI) equilibrium concentration was plotted on the horizontal axis to obtain an adsorption isotherm.
Adsorption amount (mg / g) = (Ci-Cf) / Ad Ad = Adsorbent addition amount (g / l)
FIG. 2 shows that the adsorbent of Example 1 has a high hexavalent selenium adsorption capacity and has an excellent adsorption performance even for a high concentration of hexavalent selenium.

(Valence analysis of selenium in adsorbent residue by XANES)
In order to confirm that the selenium adsorbed on the adsorbent of the present invention is adsorbed in the state of hexavalent selenium, the adsorbent residue by XANES is obtained using the measurement samples A and B prepared as follows. The valence analysis of selenium in it was conducted.
Specifically, using a hexavalent selenium solution or a tetravalent selenium solution, selenium is adsorbed to the adsorbent of Example 1, and valence analysis of selenium contained in the adsorbent is performed by Se-K-edge XANES. It was measured.
The result is shown in FIG.

Measurement condition:
Facility: Aichi SR Center, Beam line used: BL5S1
Spectrometer: Cam-type double crystal spectrometer using two Si (111) crystals Energy calibration: Correct the value of the spectrometer encoder so that the pre-edge peak of the copper foil is 8980.3 eV Measurement method: Transmission method ( QXAFS), detector: ion chamber for transmission measurement Target: Se-k-edge, measurement range: 12351-13749 eV, data analysis software: Athens

Preparation of measurement sample:
A 100 mg / l solution of hexavalent selenium was prepared, and the adsorbent of Example 1 was added to the hexavalent selenium solution at a solid-liquid mass ratio of 1: 100, and then shaken for 24 hours to add selenium to the adsorbent. Adsorbed.

Thereafter, filtration is performed using a 0.45 μm membrane filter, and the obtained residue is freeze-dried. The freeze-dried residue is pelletized and measured for valence analysis of selenium in the adsorbent residue by XANES. As prepared.
The residual powder was pelletized as follows.
Pelletization of powder sample: The sample amount was adjusted so that the edge jump at the Se-K absorption edge was about 1, and the pellet was formed into a 7 mm diameter pellet using a manually pressurized tablet molding machine.

For comparison, a 100-mg / l solution of tetravalent selenium was used, and in the same manner as described above, selenium in the tetravalent selenium solution was adsorbed on the adsorbent of Example 1, freeze-dried and pelletized. B was prepared, and the valence analysis measurement of selenium in the adsorbent residue by XANES was carried out in the same manner as the measurement sample A of hexavalent selenium.
The result is shown in FIG.

  Prior to measuring the valence analysis of selenium in the adsorbent residue by XANES for the measurement samples A and B, the measurement samples A and B It was confirmed that the drying of B did not affect the redox and XANES spectra of selenium.

  Further, as standard samples, standard sample (STD) A: sodium selenite (Se (IV)) solution (concentration 0.1 M) and standard sample (STD) B: sodium selenate (Se (VI) solution (concentration 0) 1M), the valence analysis of selenium by XANES was performed, and the results are also shown in FIG.

The Se-K-edge XANES spectra of the measurement sample A in which selenium was adsorbed on the adsorbent of Example 1 using a hexavalent selenium solution and the standard sample A (sodium selenate solution) matched.
In addition, the Se-K-edge XANES spectra of the measurement sample B obtained by adsorbing selenium using the tetravalent selenium solution to the adsorbent of Example 1 and the standard sample B (sodium selenite solution) matched. .
From these results, it can be seen that the adsorbent of the present invention adsorbs selenium without changing the valence of selenium.

(Comparative Example 1 and Comparative Example 7)
A selenium adsorbent was prepared in the same manner as in Example 1 using only ferric chloride hexahydrate (Comparative Example 1) and magnesium chloride hexahydrate (Comparative Example 7) as the selenium adsorbent. The selenium adsorption test was conducted.
The results are shown in Table 2.

(Comparative Examples 2-6)
Magnesium chloride hexahydrate and ferric chloride hexahydrate were added respectively so that the molar ratios of magnesium chloride hexahydrate and ferric chloride hexahydrate are as shown in Table 2, and 700 ml A selenium adsorbent was prepared in the same manner as in Example 1 except that a mixed solution of magnesium chloride and iron chloride was prepared by pouring and mixing the ultrapure water.
The results are shown in Table 2.

(Comparative Example 8)
Magnesium chloride hexahydrate and ferric chloride hexahydrate were added respectively so that the molar ratios of magnesium chloride hexahydrate and ferric chloride hexahydrate are as shown in Table 2, and 700 ml A selenium adsorbent was prepared in the same manner as in Example 1 except that 8N sodium hydroxide aqueous solution was added so that the pH of the mixed solution of magnesium chloride and iron chloride would be 13. The selenium adsorption test was conducted.
The results are shown in Table 2.

  From Table 1 and Table 2 above, the selenium adsorbent of the present invention has excellent selenium adsorption removal ability without requiring pretreatment such as reduction, and selenium re-elution and desorption with the addition of a small amount of adsorbent. It is possible to treat hexavalent selenium without concern.

Since the adsorbent of the present invention can perform adsorption treatment without reducing hexavalent selenium to tetravalent, the harmful hexavalent selenium contained in waste water or soil can be efficiently adsorbed and removed. For example, it can be effectively applied to the treatment of selenium-contaminated soil generated in large quantities due to excavation work and construction work such as tunnels and dams, and the wastewater treatment of factories that manufacture products using selenium as a raw material. it can.

Claims (5)

  Contains iron (Fe) and magnesium (Mg) derived from iron (Fe) and magnesium (Mg) containing pH 9.5 to 10.5 as essential constituent elements, and the molar ratio of Mg / Fe exceeds 0.2. A selenium adsorbent characterized by being 59 or less.   The selenium adsorbent according to claim 1, wherein the molar ratio of Mg / Fe is 0.5 to 59.   When producing the selenium adsorbent according to claim 1 or 2, a solution containing magnesium ions is dropped into a solution containing ferric ions so that the molar ratio of Mg / Fe becomes a predetermined molar ratio. An alkaline solution is added so that the pH of the solution becomes neutral to alkaline, and then left to stand at 0 to 60 ° C. to form a precipitate, which is separated into solid and liquid, and the resulting solid residue is dried. A method for producing a selenium adsorbent, characterized by being produced by   4. The method for producing a selenium adsorbent according to claim 3, wherein the alkaline solution is added so that the pH of the solution containing ferric ions and magnesium ions after addition of the alkaline solution is 9.5 to 10.5. A method for producing a selenium adsorbent, which is characterized. The selenium adsorbent according to claim 1 or 2 is mixed with hexavalent selenium-containing wastewater or hexavalent selenium-containing contaminated soil and adsorbed to the selenium adsorbent in a hexavalent selenium state for treatment. A selenium treatment method, which is characterized.

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