JP7600746B2 - Strontium adsorbent and its manufacturing method - Google Patents

Description

The present invention relates to a strontium adsorbent and a method for producing the same, and more specifically, to a strontium adsorbent containing layered manganese oxide and a method for producing the same. The strontium adsorbent of the present invention is useful for efficiently removing strontium from a treatment solution.

Layered manganese oxide is known as an adsorbent that can remove harmful ions from aqueous solutions.

Patent Document 1 discloses a layered manganese oxide, which is synthesized by baking manganese and sodium raw materials at 400°C or higher and has sodium ions present between the layers, as an adsorbent for strontium ions in seawater.

On the other hand, Patent Document 2 discloses a layered manganese oxide in which potassium ions exist between layers, which is synthesized by firing manganese raw materials and potassium raw materials at 400°C or higher.

Patent No. 6679000 JP 2020-15042 A

The layered manganese oxide in Patent Document 1 is a sodium-type layered manganese oxide with an interlayer distance of about 5.5 Å, known as dehydrated birnessite. However, the interlayer distance of dehydrated birnessite increases significantly in an aqueous solution due to hydration, which causes the adsorbent to swell and collapse easily.

The layered manganese oxide in Patent Document 2 is a potassium-type layered manganese oxide with an interlayer distance of 7.0 to 7.2 Å, which is close to that of hydrated birnessite, and is therefore unlikely to collapse due to swelling, but its strontium adsorption capacity is insufficient.

The present inventors have made extensive studies to solve the above-mentioned problems, and have now completed the present invention. That is, the present invention relates to a strontium adsorbent characterized in that the strontium adsorbent is a powder that is composed of a layered manganese oxide containing at least sodium, potassium, and manganese, is represented by the composition formula Na _x K _y MnO ₂ (wherein 0.05≦x, 0.05≦y, and 0.31≦x+y≦0.60 are satisfied), contains a layered manganese oxide having a crystallite diameter of 20 Å to 250 Å, a BET specific surface area of 1 m ² /g to 10 m ² /g, and an interlayer distance of 7.0 Å to 7.6 Å, and has an average particle diameter D ₅₀ of 50 μm to less than 200 μm.

The present invention has the effect of providing an adsorbent that has excellent shape retention because the interlayer distance is unlikely to increase, and also has excellent strontium adsorption performance.

The present invention will be described in detail below.

The strontium adsorbent of the present invention is composed of a layered manganese oxide containing at least sodium, potassium and _manganese , and is represented by the composition _{formula NaxKyMnO2} (wherein 0.05≦x, 0.05≦y, and 0.31≦x+y≦0.60 are satisfied), and is characterized in that it contains a layered manganese oxide having a crystallite diameter of 20 Å or more and 250 Å or less, a BET specific surface area of 1 ^m2 /g or more and 10 ^m2 /g or less, and an interlayer distance of 7.0 Å or more and 7.6 Å or less, and is in the form of a powder having an average particle diameter _D50 of 50 μm or more and less than 200 μm.

The layered manganese oxide contained in the strontium adsorbent of the present invention is composed of layered manganese oxide containing at least sodium, potassium, and manganese, and is characterized by being represented by the composition formula Na _x K _y MnO ₂ (wherein 0.05≦x, 0.05≦y, and 0.31≦x+y≦0.60 are satisfied). Strontium is adsorbed to the layered manganese oxide by ion exchange between the strontium ions of the treatment liquid and the cations between the layers. Therefore, the greater the content of sodium and potassium, the more adsorption sites there are, leading to improved strontium adsorption performance. Therefore, in the present invention, the range is 0.31≦x+y≦0.60, but preferably the range is 0.32≦x+y≦0.60, making it possible to achieve both the layered structure and the adsorption performance. The inclusion of both sodium and potassium as cations between the layers leads to the achievement of both strontium adsorption performance and shape retention. The inclusion of sodium improves the strontium adsorption performance, but the layered manganese oxide in which only sodium is an interlayer cation has a large variation in the interlayer distance between dry and wet, so there is a concern that the adsorbent powder will disintegrate finely in the liquid, making it difficult to recover the material after strontium adsorption. The layered manganese oxide containing potassium has an almost constant interlayer distance between dry and wet, improving shape retention. By containing both sodium and potassium, the layered manganese oxide can achieve both strontium adsorption performance and shape retention. The sodium and potassium contents are preferably 0.05≦x and 0.05≦y, and more preferably 0.1≦x and 0.1≦y.

The layered manganese oxide contained in the strontium adsorbent of the present invention has a crystallite diameter of 20 Å or more and 250 Å or less. From the viewpoint of material transfer within the particle, the smaller the crystallite diameter, the greater the strontium adsorption performance. However, if the crystallite diameter is too small, the crystallite becomes close to amorphous, and it is thought that it becomes difficult to retain strontium inside the crystal. Therefore, by making the crystallite diameter 20 Å or more and 250 Å or less, good strontium adsorption performance can be obtained. The crystallite diameter is preferably 20 Å or more and 200 Å or less, and more preferably 30 Å or more and 100 Å or less. The crystallite diameter mentioned here is the crystallite diameter obtained by powder X-ray diffraction using CuKα as a radiation source, using the Scherrer formula (constant K = 0.9) from the peak of the 001 plane.

The layered manganese oxide contained in the strontium adsorbent of the present invention has a BET specific surface area of 1 ^m2 /g or more and 10 ^m2 /g or less. The contact area between the layered manganese oxide and the treatment liquid has a large effect on the strontium adsorption performance and the strength of the layered manganese oxide. If the BET specific surface area is less than 1 ^m2 /g, the adsorption performance is low, and if it exceeds 10 ^m2 /g, there is a high possibility that trimanganese tetroxide is formed as a by-product phase. Since trimanganese tetroxide does not have strontium adsorption performance, if it is formed, the strontium adsorption performance of the powder decreases. The BET specific surface area is preferably 1 ^m2 /g or more and 5 ^m2 /g or less.

The strontium adsorbent of the present invention is a powder having an average particle diameter D ₅₀ of 50 μm or more and less than 200 μm. If the average particle diameter D _{50 is smaller than 50 μm, it is considered that the filterability when separating the adsorbent from the liquid is deteriorated. If the average particle diameter D 50 is} 200 μm or more, it takes time for the liquid to penetrate into the adsorbent particles, so it is not suitable for use as an adsorbent powder. The average particle diameter D ₅₀ is preferably 50 μm or more and 100 μm or less, more preferably 50 μm or more and 80 μm or less. In this specification, the "average particle diameter D ₅₀ " means the particle diameter at 50% of the cumulative value on a volume basis in the particle size distribution obtained by using a spherical approximation by a laser diffraction/scattering method.

Strontium is adsorbed to the layered manganese oxide by ion exchange between the strontium ions in the treatment solution and the cations between the layers. This is said to be because there is an interlayer distance suitable for the ionic radius or hydration radius of the exchanged strontium. From the viewpoint of strontium adsorption performance, the interlayer distance of the potassium-type layered manganese oxide contained in the strontium adsorbent of the present invention is 7.0 Å or more and 7.6 Å or less. The interlayer distance mentioned here is represented by the distance between the basal planes of the opposing manganese layers, and can be determined from the peak representing the 001 plane of the XRD diffraction peak. The layered manganese oxide with an interlayer distance of 7.0 Å or more and 7.6 Å or less has diffraction peaks at 2θ = 12.2 ± 0.6 °, 2θ = 24.6 ± 1.5 °, and 2θ = 36.5 ± 2.0 ° in powder X-ray diffraction using CuKα as a radiation source. If the interlayer distance is smaller than 7.0 Å, when the layered manganese oxide is introduced into an aqueous solution, the interlayer distance will fluctuate significantly when water is inserted between the layers or when strontium is adsorbed, raising concerns about the collapse of the adsorbent particles, and if the interlayer distance is larger than 7.6 Å, there is concern about a decrease in strontium adsorption performance. It is preferable that the interlayer distance is 7.1 Å or more and 7.5 Å or less.

The layered manganese oxide contained in the strontium adsorbent of the present invention can be produced by mixing an aqueous solution of a metal salt containing manganese, an aqueous solution of an alkali metal and an oxidizing agent, and drying the precipitated matter. The production method is characterized in that at least an aqueous solution of a metal salt containing manganese, an aqueous solution of potassium hydroxide and an aqueous solution of sodium peroxodisulfate are mixed at a potassium/manganese molar ratio of 3 to 10 and a sodium peroxodisulfate/manganese molar ratio of 0.5 to 4 at a temperature of 0°C to 100°C, the precipitated matter is dried and then pulverized into a powder.

The manganese-containing metal salt aqueous solution can be prepared by dissolving a manganese-containing metal salt in water. Examples of manganese-containing metal salts include water-soluble manganese sulfate (II), manganese chloride (II), manganese nitrate (II), manganese acetate (II), etc., and may be anhydrous or hydrated. Of these, manganese sulfate (II) is the most suitable when considering waste liquid treatment, corrosiveness, and raw material costs.

The potassium hydroxide aqueous solution may be of any concentration, but is preferably 1 mol/L or more from the viewpoint of productivity. The potassium/manganese molar ratio is 3 to 10. If it is less than 3, trimanganese tetroxide (Mn ₃ O ₄ ) is by-produced, and if it exceeds 10, trimanganese tetroxide (Mn ₃ O ₄ ) is also by-produced. From the viewpoint of strontium adsorption performance, it is preferably 4 to 8.

The sodium peroxodisulfate aqueous solution can be used at any concentration, but from the viewpoint of productivity, 5% or more is preferable. The sodium peroxodisulfate/manganese molar ratio is 0.5 to 4. If it is less than 0.5, layered manganese oxide is not produced, and if it exceeds 4, manganese is excessively oxidized to hexavalent manganese ions, which are released into the liquid. From the viewpoint of productivity, 0.7 to 2 is preferable, and 0.7 to 1.2 is more preferable.

When mixing the metal salt aqueous solution, potassium hydroxide aqueous solution, and sodium peroxodisulfate aqueous solution, either a batch or continuous reaction may be used. As for the method of adding the aqueous solutions, it is preferable to drop them at a constant rate at the same time while keeping the alkali metal/manganese molar ratio and the sodium peroxodisulfate/manganese molar ratio constant, as this leads to a uniform reaction.

After the reaction, the precipitated substance can be separated by filtration or centrifugation. Any method that allows solid-liquid separation may be used. At this time, it is preferable to wash the precipitated substance to remove by-products such as potassium sulfate and other potassium salts, and unreacted potassium hydroxide.

There are no particular limitations on the method for drying the precipitate after solid-liquid separation, but drying at temperatures above 600°C is not possible because manganese oxide changes from a layered structure to a hollandite-type tunnel structure at temperatures above 600°C.

The dried precipitate can be pulverized by any method, such as using a mortar, ball mill, or jaw crusher.

The crushed powder can be used as is as a strontium adsorbent.

The present invention will be described in more detail below with reference to examples, but is not limited to these.

<Measurement of sodium, potassium, and manganese in a sample>
The composition of the obtained sample was analyzed by inductively coupled plasma emission spectrometry (ICP method). That is, 50 mg of sample powder was dissolved in 1 ml of hydrogen peroxide water and 5 ml of hydrochloric acid, and diluted to 1 L with ion-exchanged water to prepare a measurement solution. The chemical composition of the obtained sample was analyzed by measuring the obtained measurement solution using a general inductively coupled plasma emission spectrometry device (trade name: OPTIMA3000DV, manufactured by PERKIN ELMER). At this time, the molar concentration of potassium in the obtained measurement solution was C _Na [mol/L], the molar concentration of potassium was C _K [mol/L], and the molar concentration of manganese was C _Mn [mol/L], and the molar ratio x of sodium to manganese and the molar ratio y of potassium to manganese in the sample were calculated.

x=C _Na /C _Mn
y = C _K / C _Mn
<Powder X-ray diffraction measurement>
The obtained sample was subjected to powder X-ray diffraction measurement using an X-ray diffractometer (product name: Ultima 4, manufactured by Rigaku). The radiation source used was CuKα radiation (λ = 1.5405 Å), and the measurement mode was The step scan was performed under the following scanning conditions: 0.04° per second, measurement time was 0.25 seconds, and the measurement range was 2θ from 5° to 70°.

<Measurement of BET specific surface area>
The BET specific surface area of the obtained sample was measured using a BET specific surface area measurement device (product name: FlowSorbIII, manufactured by Micromeritics). In the BET specific surface area measurement, the sample was heated in a nitrogen gas flow at 120° C. for 1 hour as a pretreatment using a pretreatment device attached to the BET specific surface area measurement device to remove moisture.

<Measurement of average particle diameter _D50 >
A particle size distribution measuring device (product name: MT3300 EXII, manufactured by Microtrack Bell) was used to measure the average particle size _D50 (particle size of the sample at 50% cumulative volume in the particle size distribution obtained by the laser diffraction/scattering method using spherical approximation). At this time, the sample was dispersed in pure water using a standard sample circulator and the measurement was performed. The particle size was calculated using a function associated with the measurement software attached to the device, and the calculation mode was set to HRA, the measurement time to 30 seconds, and the refractive index of the particles to 2.5.

<Measurement of strontium adsorption amount>
Strontium chloride was dissolved in ion-exchanged water so that the concentration of strontium was 10 mg/L, to prepare an aqueous strontium solution.

0.05 g of the adsorbent sample was added to 750 mL of the strontium aqueous solution. This was stirred for 1 hour at 25°C, and the adsorption test was performed by suction filtration to obtain a filtrate (hereinafter, the treated liquid) from which the adsorbent sample had been removed.

The strontium concentration in the sample aqueous solution was measured using inductively coupled plasma emission spectrometry (ICP). A general inductively coupled plasma emission spectrometry device (product name: OPTIMA3000DV, manufactured by PERKIN ELMER) was used for the measurement, and the strontium concentration was determined by measuring the obtained measurement solution.

The strontium concentration in the aqueous strontium solution before the adsorption test was defined as C ₀ (mg/L), and the strontium concentration in the treated solution was defined as C (mg/L), and the strontium removal rate was calculated by the following formula.

Strontium removal rate (%)=(C ₀ −C)/C ₀ ×100
Example 1
130g of pure water was placed in a reaction vessel with an internal volume of 1L, and the temperature was raised to 40°C and maintained. Separately, manganese sulfate hexahydrate (special grade reagent, manufactured by Kishida Chemical) was dissolved in pure water to prepare a 2.0 mol/L manganese sulfate aqueous solution. In addition, a 40% potassium hydroxide aqueous solution (special grade reagent, manufactured by Kanto Chemical) was diluted with pure water in a separate vessel to prepare a 5 mol/L potassium hydroxide aqueous solution. Furthermore, sodium peroxodisulfate (first grade reagent, manufactured by Kishida Chemical) was dissolved in pure water in a separate vessel to prepare a 21% sodium peroxodisulfate aqueous solution. A four-blade paddle blade with an outer diameter of 80 mm was inserted into the reaction vessel, and the manganese sulfate aqueous solution, potassium hydroxide aqueous solution, and sodium peroxodisulfate aqueous solution were added to the reaction vessel at the same speed over 20 minutes while stirring by rotating the paddle blade at 600 ppm. The addition rate of the manganese sulfate aqueous solution was 5.0 g/min, the addition rate of the potassium hydroxide aqueous solution was 7.7 g/min, and the addition rate of the sodium peroxodisulfate aqueous solution was 9.0 g/min. The potassium/manganese molar ratio was 4, and the sodium peroxodisulfate/manganese molar ratio was 1. After the addition of the aqueous solution was completed, stirring was continued for another 10 minutes while keeping the container temperature constant. The above operation resulted in a slurry in which potassium-type layered manganese oxide was precipitated. The obtained slurry was filtered and washed with pure water, and the washed wet cake was dried at 60°C for 16 hours to obtain an aggregate of potassium-type layered manganese oxide. The aggregate was ground with an alumina mortar and pestle to obtain a powder of potassium-type layered manganese oxide (strontium adsorbent). The average particle size D ₅₀ of the powder was 108 μm.

The composition formula of the obtained sample _{was Na0.13K0.23MnO2} . The interlayer distance was 7.45 Å, the crystallite diameter was 30 Å, and the BET specific surface area was 4.65 ^m2 _/ g.

The strontium removal rate of the obtained sample was 70.3%.

Example 2
The synthesis was carried out in the same manner as in Example 1, except that the addition rate of the potassium hydroxide aqueous solution during synthesis was changed to 12 g/min. The potassium/manganese molar ratio during this synthesis was 6. A potassium-type layered manganese oxide powder (strontium adsorbent) was obtained by this synthesis. The average particle size _D50 of the powder was 56.4 μm.

The composition formula of the obtained sample _{was Na0.11K0.25MnO2} . The interlayer distance was 7.31 Å, the crystallite diameter was 53 Å, and the BET specific surface area was 4.17 ^m2 _/ g.

The strontium removal rate of the obtained sample was 77.9%.

Example 3
800 g of pure water was placed in a reaction vessel with an internal volume of 1 L having an outlet at the top of the vessel, and the temperature was raised to 60° C. and maintained. Separately, manganese sulfate was dissolved in pure water to prepare a 0.90 mol/L manganese sulfate aqueous solution. In addition, a 40% potassium hydroxide aqueous solution was diluted with pure water in a separate vessel to prepare a 2.3 mol/L potassium hydroxide aqueous solution. Furthermore, sodium peroxodisulfate was dissolved in pure water in a separate vessel to prepare a 10% sodium peroxodisulfate aqueous solution. A four-blade paddle blade with an outer diameter of 80 mm was inserted into the reaction vessel, and the manganese sulfate aqueous solution, potassium hydroxide aqueous solution, and sodium peroxodisulfate aqueous solution were added to the reaction vessel at a constant speed at the same time while stirring by rotating the paddle blade at 600 ppm. The addition rate of the manganese sulfate aqueous solution was 10 g/min, the addition rate of the potassium hydroxide aqueous solution was 19 g/min, and the addition rate of the sodium peroxodisulfate aqueous solution was 18 g/min. The potassium/manganese molar ratio at this time was 5, and the sodium peroxodisulfate/manganese molar ratio was 1. The above operation was carried out continuously for 4 hours. The slurry from which the layered manganese oxide was precipitated continuously flowed out from the outlet at the top of the vessel. The slurry that flowed out between 120 and 160 minutes after the start of synthesis was collected, filtered, and washed with pure water. The washed wet cake was dried at 60°C for 16 hours to obtain an aggregate of potassium-type layered manganese oxide. The aggregate was ground with an alumina mortar and pestle to obtain a powder of potassium-type layered manganese oxide (strontium adsorbent). The average particle size _D50 of the powder was 73.5 μm.

The composition formula of the obtained sample _{was Na0.10K0.21MnO2} . The interlayer distance was 7.26 Å, the crystallite diameter was 31 Å, and the BET specific surface area was 1.44 ^m2 _/ g.

The strontium removal rate of the obtained sample was 80.8%.

Comparative Example 1
200 g of pure water was placed in a reaction vessel with an internal volume of 1 L, and the temperature was raised to 40° C. and maintained. Separately, manganese sulfate hexahydrate (special grade reagent, manufactured by Kishida Chemical) was dissolved in pure water to prepare a 2.0 mol/L manganese sulfate aqueous solution. In addition, a 40% potassium hydroxide aqueous solution (special grade reagent, manufactured by Kanto Chemical) was diluted with pure water in a separate vessel to prepare a 5 mol/L potassium hydroxide aqueous solution. Furthermore, a 35% hydrogen peroxide aqueous solution (first grade reagent, Kishida Chemical) was prepared in a separate vessel. A four-blade paddle blade with an outer diameter of 80 mm was inserted into the reaction vessel, and the manganese sulfate aqueous solution, potassium hydroxide aqueous solution, and hydrogen peroxide aqueous solution were added to the reaction vessel at a constant speed over 30 minutes while stirring by rotating the paddle blade at 600 ppm. The addition rate of the manganese sulfate aqueous solution was 5.0 g/min, the addition rate of the potassium hydroxide aqueous solution was 5.8 g/min, and the addition rate of the hydrogen peroxide aqueous solution was 1.6 g/min. The potassium/manganese molar ratio at this time was 3, and the hydrogen peroxide/manganese molar ratio was 2. After the addition of the aqueous solution was completed, stirring was continued for another 10 minutes while maintaining the container temperature constant. The above operation resulted in a slurry in which potassium-type layered manganese oxide was precipitated. The obtained slurry was filtered and washed with pure water, and the washed wet cake was dried at 60°C for 16 hours to obtain an aggregate of potassium-type layered manganese oxide. The aggregate was ground with an alumina mortar and pestle to obtain a powder of potassium-type layered manganese oxide (strontium adsorbent). The average particle size _D50 of the powder was 77.0 μm.

The composition formula of the obtained sample _{was K0.30MnO2} . The interlayer distance was 7.24 Å, the crystallite diameter was 40 Å, and the BET specific surface area was 0.17 ^m2 /g.

The strontium removal rate of the obtained sample was 63.8%.

Comparative Example 2
A potassium-type layered manganese oxide (strontium adsorbent) was obtained in the same manner as in Comparative Example 1, except that the synthesis temperature was changed to 60° C. The average particle size _D50 of the powder was 139 μm.

The composition formula of the obtained sample _{was K0.30MnO2} . The interlayer distance was 7.14 Å, the crystallite diameter was 78 Å, and the BET specific surface area was 0.49 ^m2 /g.

The strontium removal rate of the obtained sample was 44.9%.

Comparative Example 3
A potassium-type layered manganese oxide (strontium adsorbent) was obtained in the same manner as in Comparative Example 1, except that the synthesis temperature was changed to 80° C. and the addition rate of the potassium hydroxide aqueous solution was changed to 12 g/min. The average particle size _D50 of the powder was 95.5 μm.

The composition formula of the obtained sample _{was K0.22MnO2} . The interlayer distance was 7.15 Å, the crystallite diameter was 76 Å, and the BET specific surface area was 44.1 ^m2 /g.

The strontium removal rate of the obtained sample was 61.2%.

The strontium adsorbent of the present invention, which contains layered manganese oxide, can be used as an adsorbent for strontium.

Claims (8)

A strontium adsorbent comprising a layered manganese oxide containing at least sodium, potassium and manganese, and represented by the composition _{formula NaxKyMnO2} (wherein _0.05 ≦x, 0.05≦y, and 0.31≦x+y≦0.60 are satisfied), a crystallite diameter of 20 Å to 250 Å, a BET specific surface area of 1 ^m2 /g to 10 ^m2 /g, and an interlayer distance of 7.0 Å to 7.6 Å, and a powder having an average particle diameter _D50 of 50 μm or more and less than 200 μm. The strontium adsorbent according to claim 1, characterized in that the composition formula satisfies 0.1≦x, 0.1≦y, and 0.31≦x+y≦0.60. 3. The strontium adsorbent according to claim 1, wherein the BET specific surface area is from 1 m ² /g to 5 m ² /g. The strontium adsorbent according to any one of claims 1 to 3, characterized in that the crystallite diameter is 30 Å or more and 100 Å or less. A strontium adsorbent according to any one of claims 1 to 4, characterized in that the interlayer distance is 7.1 Å or more and 7.5 Å or less. The strontium adsorbent according to any one of claims 1 to 5, characterized in that the average particle diameter _D50 is 50 µm or more and 100 µm or less. A method for producing a strontium adsorbent according to any one of claims 1 to 6, characterized in that an aqueous solution of a metal salt containing at least manganese, an aqueous solution of potassium hydroxide, and an aqueous solution of sodium peroxodisulfate are mixed at a potassium/manganese molar ratio of 3 to 10 and a sodium peroxodisulfate/manganese molar ratio of 0.5 to 4 at a temperature of 0°C to 100°C, the precipitate is dried, and then pulverized into a powder form. The method for producing a strontium adsorbent according to claim 7, characterized in that the metal salt aqueous solution is an aqueous manganese sulfate or manganese nitrate solution.