JPH0742106B2 - Method for recovering cesium in aqueous solution - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently and easily recovering cesium from an aqueous solution containing cesium. More specifically, the present invention relates to the separation and recovery of radioactive cesium in waste liquid containing nitric acid or sodium nitrate as a main component, which is generated from a facility related to nuclear power utilization such as a spent nuclear fuel reprocessing facility. The present invention relates to a suitable cesium recovery method.

[0002]

2. Description of the Related Art Waste liquid generated in nuclear facilities contains various radioactive substances, which must be removed prior to discharge. For example, the high-level waste liquid generated from the reprocessing process of spent nuclear fuel (Burex method) contains a relatively high concentration of nitric acid or sodium nitrate as a main component and contains radioactive cesium and other nuclides.

A method using an insoluble ferrocyanide as an adsorbent has been proposed as a method for selectively removing cesium from such a high-level radioactive liquid waste (Japanese Patent Application No. 240240/1990). Further, at this time, in a high-concentration nitric acid solution, the insoluble ferrocyanide is oxidized and loses its adsorption ability. Therefore, it is also proposed to use a certain kind of antioxidant together to suppress the decrease in adsorption ability. (Japanese Patent Application No. 3-142272).

However, since the insoluble ferrocyanide used in these methods has a large adsorptivity for cesium, once cesium is adsorbed, it is difficult to desorb the cesium with an electrolyte solution, and it can be recycled and reused. It is not possible to do so, but it is inevitable that the disposable method will generate a large amount of new high-level radioactive waste with a high content of radioactive cesium so that the disposable method consumes a large amount of expensive adsorbent. It has drawbacks and poor practicality.

Therefore, it has been desired to develop a method for efficiently desorbing radioactive cesium from the adsorbent, reusing the separated and recovered radioactive cesium, and recycling and reusing the regenerated adsorbent.

[0006]

DISCLOSURE OF THE INVENTION The present invention uses an adsorbent capable of adsorbing cesium in an aqueous solution with a high selectivity and easily desorbing the cesium, and continuously separating and recovering cesium. The purpose is to provide a method of recovering cesium that can be carried out.

[0007]

The present inventor has conducted various studies on an adsorbent which can efficiently recover cesium in an aqueous solution and can be repeatedly used. As a result, insoluble ferrocyanide is generally insoluble. Based on the fact that the adsorption capacity for cesium is greater than that for ferricyanide, and that after cesium is adsorbed as a ferrocyanide and then converted to the corresponding ferricyanide, cesium is desorbed. Invented.

That is, according to the present invention, an aqueous solution containing cesium is brought into contact with an oxidized-reduced insoluble iron cyano complex salt in a reduced state, and cesium is adsorbed on the complex to separate it from the aqueous solution. The present invention provides a method for recovering cesium in an aqueous solution, which comprises treating a cyano complex salt with an oxidizing agent to convert it into an oxidized state and desorbing cesium.

In the reduced state of the oxidation-reduction insoluble iron cyano complex salt used for adsorbing cesium in the method of the present invention, the general formula M ₂ [Fe (CN) ₆ ] (where M is Cu, Co, Ni , Zn, Cd, Mn, Fe and other divalent transition metals),
Alternatively, some of these M are substituted with monovalent cations, and the positions of transition metals are Mo, T
Examples of ferrocyanide substituted by oxides such as i and W, which are sparingly soluble in water, include copper ferrocyanide and a part of the copper substituted by a monovalent cation. It is a thing. Moreover, as an oxidation state, the ferricyanide corresponding to the above-mentioned ferrocyanide can be mentioned.

These oxidized-reduced insoluble iron cyano complex salts may be used as a crystal powder as they are, or may be used by supporting them on an inorganic or organic porous carrier. As the amount of this used, relative to the cesium present in the aqueous solution,
Usually, it is used in an amount of at least equivalent, preferably double equivalent or more.

When the cesium-containing aqueous solution contains a high concentration of nitric acid, it is necessary to add a substance for preventing the insoluble ferrocyanide from being oxidized to ferricyanide by nitric acid. In addition, nitric acid is involved in the oxidation of insoluble ferrocyanide by nitric acid, and when nitrite at a certain concentration or higher coexists, the oxidation by nitric acid is promoted in a chain reaction, so the concentration should be below the concentration that causes this chain reaction. A substance that reduces the nitrite concentration may be added. The critical value of this nitrous acid concentration varies slightly depending on factors such as nitric acid concentration, temperature, and component composition, but it is around 2 × 10 ⁻⁵ M in a 3M room temperature nitric acid solution, and decreases as the nitric acid concentration and temperature increase. There is a tendency to do.

Therefore, in the case of containing a high concentration of nitric acid, a nitrite decomposing agent such as hydrazine, amidosulfuric acid,
Dihydric phenols such as hydrogen peroxide, resorcin, hydroquinone and methylhydroquinone, organic hydrazines such as methylhydrazine, phenylhydrazine and diphenylhydrazine, thioglycolic acid, sulfanilic acid, sulfanilamide, hydroiodic acid and urea alone Or 2
It is necessary to add cesium or more in a sufficient amount to make the concentration of nitrous acid equal to or lower than the above-mentioned critical value, and then perform cesium adsorption treatment.

Next, in the present invention, in order to desorb cesium adsorbed on the insoluble iron cyano complex salt in a reduced state, it is necessary to treat it with an oxidizing agent to convert it into an oxidized state. As the oxidizing agent used at this time, an aqueous nitric acid solution containing nitrous acid having a critical value or more and having a concentration of 1 mol or more is suitable.

[0014] Nitrite in this case, usually nitrogen oxides such one nitrogen, dinitrogen trioxide, nitrogen dioxide, as such dinitrogen tetroxide or nitrite such as sodium nitrite, potassium nitrite, as such ammonium nitrite,
It is added to the nitric acid aqueous solution. Industrial nitric acid is usually sufficient
Since it contains a concentration of nitrous acid, it is used at a concentration of 3M or more.
If so, it is not necessary to add nitrous acid. But,
When residual nitrous acid decomposer used in the adsorption process is observed
In this case, it is necessary to add nitrous acid appropriately.

Other than the above, peroxides, persulfuric acid and the like can be used as the oxidizing agent.

Cesium adsorbed on the insoluble iron-cyano complex salt in the reduced state is rapidly desorbed at the same time as the insoluble iron-cyano complex salt is oxidized and converted to the oxidized state, and elutes in the desorbent solution. This can be recovered by evaporation, concentration or the like.

At this time, it is advantageous to use a volatile aqueous acid solution such as a nitric acid aqueous solution because the desorbent and the cesium salt can be recovered only by heating and evaporating the desorption treatment solution.

The hydrogen ion concentration during the adsorption of cesium is preferably 6 M or less at which the nitrite decomposing agent effectively acts. on the other hand,
The hydrogen ion concentration during desorption is preferably as high as possible in order to increase the desorption rate, but considering the stability of the iron cyano complex salt-based adsorbent to be used against acid and the influence on the equipment, 12 M or less, preferably 7 M or less is suitable. Is. When the acid concentration of the oxidizing agent solution is relatively low, a part of the desorbed cesium may be occluded in the crystal lattice of the oxidized insoluble iron cyano complex salt. In particular, when the carrier is used by being supported on a porous carrier, since the carrier is remarkably occluded in the pores, it is necessary to carefully perform cleaning after desorption and solid-liquid separation in order to increase the desorption rate.

In the method of the present invention, the insoluble iron cyano complex salt in the oxidized state after desorption can be reduced and regenerated to be used again as an adsorbent for cesium.

As the reducing agent used in this regeneration, hydrazine, organic hydrazines having a hydrazine group, dihydric phenols, thioglycolic acid, hydroxylamine, hydrogen peroxide, dihydrochloride, thiosulfate, and An aqueous solution obtained by adding a single substance or a mixture of two or more substances in an acidic solution such as sulfurous acid gas, sulfite salt, bisulfite salt, and dithionite salt is used. Since many of the above reducing agents have the property of significantly increasing the reduction rate of ferricyan by a trace amount of copper ions, the insoluble iron cyano complex salt in the method of the present invention is an insoluble ferrocyanide containing copper as a component, such as ferrocyanide. It is advantageous to use copper oxide or a part of which is replaced by a monovalent cation.

The reduction of the copper-based insoluble ferricyanide after use proceeds almost instantaneously when it is treated with the above reducing agent under neutral to slightly alkaline conditions. Especially hydrazine and its salts, sulfurous acid gas, sulfite, bisulfite, dithionite, thiosulfate, stannous salt, hydroquinone, dihydric phenols such as methylhydroquinone,
Since rapidly proceeds even under nitric acid acidity about 0.1M With the phenyl hydrazine and organic hydrazines, or thioglycolic acid can be washed with water after the cesium desorption performed efficiently subsequent reduction treatment even insufficient. further,
Among these reducing agents, hydrazine and its salts,
Divalent pheno such as hydroquinone and methylhydroquinone
Organic hydrazines such as phenylhydrazine and phenylhydrazine
Ogolic acid has its strong reducing power,
Can be mixed as an acid decomposer in the adsorption process, recycled waste liquid
Is especially preferable due to burning and other reasons.
It is suitable.

Next, an example of a preferred embodiment of the method of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows a method 1 of the present invention.
It is a flow sheet showing an example. In the adsorption step 1, a cesium-containing aqueous solution is brought into contact with an insoluble ferrocyanide to adsorb cesium, and solid-liquid separation is performed to discharge the cesium-free solution. The solid portion is then sent to desorption step 2, where it is treated with an oxidant and converted to insoluble ferricyanide to desorb cesium. Next, the solution containing cesium is sent to the evaporation step 3 where the solvent is evaporated to recover the cesium salt.

On the other hand, the insoluble ferricyanide produced in the desorption step 2 is sent to the regeneration step 4, where it is brought into contact with a regenerant solution having a reducing power to be converted into the insoluble ferrocyanide again, and the adsorption step 1 Is circulated to. In this way, the insoluble ferrocyanide is used repeatedly,
At this time, the cesium adsorption rate is only decreased by several% even after being reused 6 times.

The method of the present invention may be either a continuous method using a column or a batch method.

[0024]

EXAMPLES Next, the present invention will be described in more detail by way of examples. The amount of cesium and the distribution coefficient (K
d) and iron concentration were determined by the following methods.

(1) Amount of cesium: The amount of cesium was measured by atomic absorption photometry (air-acetylene flame) or atomic emission photometry (air-acetylene flame) using 0.1 M potassium chloride as a sensitizer.

(2) Distribution coefficient (Kd): Based on the measurement result of the residual cesium concentration, the distribution coefficient was calculated according to the following formula as an index of the cesium adsorption force of each adsorbent. In the formula, C ₁ is a cesium initial concentration (M), C ₂ is a cesium residual concentration (M), L is a liquid amount (milliliter), and W is an adsorbent weight (gram).

[0027]

[Equation 1]

(3) Iron concentration; Fe
A 0.1 M hydrochloric acid solution of Cl ₃ was used as a standard solution and measured by an atomic absorption photometry.

Reference Example 1 An aqueous solution of sodium ferrocyanide was added to a copper chloride aqueous solution so that the molar ratio of copper to ferrocyanine was 2 or more, and the resulting precipitate was filtered off. Next, this precipitate was washed with water and air-dried to prepare a copper ferrocyanide adsorbent (hereinafter abbreviated as CuFC).

Further, in an aqueous potassium ferrocyanide solution,
A stoichiometrically calculated amount of each copper sulfate aqueous solution was added, and the resulting precipitate was separated, washed with water, and air-dried to give various double salt forms of potassium potassium ferrocyanide adsorbent (K ₂ Cu ₃ F).
C, K ₂ Cu ₅ FC, K ₂ Cu ₁₁ FC) were prepared.

Further, an aqueous solution of sodium ferricyanide is added to an aqueous solution of copper nitrate or zinc nitrate so that the molar ratio of copper or zinc to ferricyan is 1.5 or more, and the resulting precipitate is separated, washed with water and air-dried. Thus, a copper ferricyanide adsorbent and a zinc ferricyanide adsorbent were prepared.

Reference Example 2 Macroporous anion exchanger (Amberlite IRA
904) was brought into contact with an aqueous sodium ferrocyanide solution to convert it into [Fe (CN) ₆ ] ^4- form, and then treated with an aqueous copper chloride solution to precipitate copper ferrocyanide in the pores of the resin. After this operation was repeated twice, CuFC (hereinafter referred to as I
RA904-CuFC) was prepared. The weight increase of this product on the basis of the sample dried at 60 ° C. was 0.452 g / g of IRA904.

Reference Example 3 Reduction of ferricyanide to ferrocyanide can be easily visually recognized by discoloration. Using copper ferricyanide and zinc ferricyanide, an acidic solution (0.1M HNO ₃ ) and a neutral solution (0 The reducing effect of various reducing agents in 15 M sodium acetate buffer) and pure water was tested. As the test method, 2 ml of an acidic solution, a neutral solution, or a solution (10 ⁻² M) of each reducing agent in pure water was placed in a test tube, and about 2 mg of the test adsorbent was added to observe the discoloring effect. Hydrazine salt (hydrochloride, nitrate, sulfate), thiosulfate (sodium salt, ammonium salt), sulfite (sodium salt, potassium salt), hydroquinone, phenylhydrazine hydrochloride,
Methylhydroquinone, sodium thioglycolate,
Copper ferricyanide was rapidly reduced in any of the acidic solution, the neutral solution, and the pure water to change from yellowish brown to reddish brown. Hydroxylamine hydrochloride rapidly reduced and discolored in neutral solutions and pure water, but was ineffective in acidic solutions.

On the other hand, for zinc ferricyanide, among the above reducing agents, hydroquinone, phenylhydrazine hydrochloride and methylhydroquinone are effective not only in a neutral solution but also in an acidic solution. Discolored. It was observed that sulfite, hydroxylamine hydrochloride, and hydrazine salt did not show a reduction reaction in an acidic solution, but reductively changed color in a neutral solution rather rapidly and gradually changed in pure water. The difficulty in proceeding in pure water was due to the decrease in pH accompanying reduction. Sodium thioglycolate and thiosulfate showed almost no reducing action in acidic solution, but gradually reduced and discolored in neutral solution.

Stannous chloride was effective against copper ferricyanide and zinc ferricyanide only in an acidic solution. Hydrogen peroxide did not show a reduction reaction for both ferricyanides in acidic solution or pure water (which shows a slight acidity in the presence of hydrogen peroxide), but it can reductively change color in a neutral solution fairly quickly. Was observed.

Example 1 0.02 g of K ₂ Cu ₃ FC air-dried material (calculated as an anhydride) was weighed in an Erlenmeyer flask equipped with a screw cap, and 10 wt.
2 × 10 ⁻³ M CsCl / 3 containing ⁻⁴ M hydrazine
Add 10 ml of MHNO ₃ solution and incubate in a constant temperature shaker (2
The mixture was shaken in (5 ° C.) and subjected to adsorption treatment for 24 hours. Then.
Filtration (hereinafter simply referred to as filtration) was performed with a vacuum filtration device equipped with a tetrafluoroethylene resin filter, and the residual cesium and iron concentrations in the filtrate were analyzed. The adsorbent was washed with water until the pH of the washed filtrate was 3 or less. Then as desorbent solution 1
The adsorbent was transferred to an Erlenmeyer flask with a screw lid as much as possible using 10 ml of 3 M nitric acid containing 0 ⁻² M nitrous acid, and desorbed in a constant temperature shaking tank (25 ° C.). Filter after 24 hours,
The amount of the filtrate was weighed and the concentration of residual cesium and iron in the filtrate was analyzed. The adsorbent is washed with water until the pH of the washed filtrate is below pH 3,
The amount of the filtrate washed with water was weighed and the residual cesium concentration in the filtrate washed with water was analyzed.

Next, as a regenerant solution, 1.44 × 10
^-Add 10 ml of 0.1 M nitric acid solution containing ² M hydrazine until the generation of bubbles almost stops (about 30 minutes)
After the reduction treatment, the product was washed with water until the pH became 3 or less. Then, 2 × 1 containing 10 ⁻⁴ M hydrazine as a nitrous acid decomposer
0 ^-3 M CsCl / 3M HNO ₃ solution 10ml was transferred as much as possible screw cap Erlenmeyer flask adsorbent used,
The second adsorption treatment was performed in the same manner as above. The above adsorption and desorption operations were repeated 6 times while sandwiching the regeneration operation after desorption. Fig. 2 shows the adsorption rate, desorption rate (apparent desorption rate expressed by the ratio of the amount of desorbed cesium in the filtrate to the new amount of cesium adsorbed from the second time onward) and the iron concentration in the adsorption treatment liquid or desorption treatment. Indicated. From the figure, the adsorption and desorption of cesium were carried out quite efficiently, and K ₂ Cu ₃ F
It can be seen that the decomposition of C is also extremely slight.

From the analysis of cesium in the washing filtrate after desorption filtration, it was confirmed that about 10% of cesium was stored in the crystal lattice of the adsorbent and eluted by washing with water.

Examples 2 to 4 K ₂ Cu ₅ FC, K ₂ Cu ₁₁ FC and 0.02 g of CuFC air-dried material (anhydrous equivalent) were subjected to a repeated treatment 6 times in the same manner as in Example 1. Table 1 shows the average adsorption rate of cesium and the average desorption rate (for cesium in the filtrate) of each adsorbent.

Example 5 0.02 g of copper ferricyanide air-dried product (calculated as anhydrous product) was weighed in an Erlenmeyer flask with a screw cap, and a regenerant solution of 1.4 was prepared.
0.1M nitric acid solution 10 containing 4 × 10 ⁻² M hydrazine
Add ml until almost no bubbles are generated (about 3
It was reduced for 0 minutes and filtered. The pH of the adsorbent is pH 3
After washing with water until
2 × 10 ⁻³ M CsCl / containing ⁻⁴ M hydrazine
The adsorbent was transferred to an Erlenmeyer flask with a screw lid as much as possible using 10 ml of a 3M HNO ₃ solution, and the first adsorption treatment was performed in the same manner as in Example 1. Subsequently, as in Example 1, the processing was performed in the order of desorption and regeneration. In this way, the adsorption and desorption operations were repeated 6 times while sandwiching the regeneration operation after desorption. Table 1 shows the average adsorption rate and the average desorption rate of cesium (for cesium in the filtrate).

Example 6 0.2 g of IRA904-CuFC air-dried matter (anhydrous equivalent)
The concentration of hydrazine added during the adsorption treatment was 10 ⁻³
Adsorption and desorption operations were repeated 6 times under the same conditions as in Example 1 except that the desorption agent solution was M and the desorption agent solution had a concentration of 5 M nitric acid containing 10 ⁻² M nitrous acid.
In the solid-liquid separation by filtration after the desorption process, desorbed cesium is stored in the pores of the ion exchange resin,
It was found that a considerable amount of cesium was eluted during the washing operation. Table 1 shows the average adsorption rate and the average desorption rate of cesium (including cesium eluted by washing with water).

Example 7 0.2 g of IRA904-CuFC air-dried matter (anhydrous equivalent)
Regarding, the solution at the time of adsorption treatment does not contain hydrazine 2
× 10 ⁻³ M CsCl / 3M NaNO ₃ solution 10 m
The adsorption and desorption operations were repeated 6 times while sandwiching the regeneration operation after desorption under the same conditions as in Example 6 except that the desorption agent solution was 1 and the desorption agent solution was a 3 M nitric acid solution containing 10 ⁻² M nitrous acid.

Table 1 shows the average adsorption rate and the average desorption rate of cesium (including cesium eluted by washing with water).

[0044]

[Table 1]

Example 8 About 0.02 g of K ₂ Cu ₃ FC air-dried matter (calculated as an anhydride), adsorption was carried out under the same conditions as in Example 1 except that the regenerant solution was a 0.1M nitric acid solution containing 0.02M sulfite. The desorption operation was repeated twice. As a result, the first and second cesium adsorption rates were 99.8 and 99.8%, respectively, and the first and second cesium desorption rates (for cesium in the filtrate).
Was 63.6, 79.3% (desorption rate including cesium eluted by washing with water, 71.8, 90.1%, respectively).

Example 9 1.82 × 10 ⁻³ M CsNO ₃ / 3M NaNO for 0.02 g of K ₂ Cu ₃ FC air-dried matter (anhydrous equivalent)
_Three solutions (10 ml) were added, and the mixture was shaken in a constant temperature shaking tank (25 ° C.) and subjected to adsorption treatment for 24 hours, then filtered, and the residual cesium and iron concentrations in the filtrate were analyzed. Then, 10 ml of 10 M nitric acid containing 1.07 × 10 ⁻⁴ M nitrous acid was used as the desorbent solution, and the adsorbent was transferred to an Erlenmeyer flask with a screw lid as much as possible, and desorbed in a constant temperature shaking tank (25 ° C.). did. After 24 hours, the mixture was filtered, the amount of the filtrate was weighed, and the residual cesium and iron concentrations in the filtrate were analyzed. For the adsorbent, the washed filtrate is p
It was washed with water until H3 or less, the amount of the filtrate washed with water was weighed, and the residual cesium concentration in the filtrate washed with water was analyzed. Next, 0.1 M containing 1.44 × 10 ⁻² M hydrazine as a regenerant solution.
A nitric acid solution (10 ml) was added, and the mixture was reduced until the generation of air bubbles almost stopped (about 30 minutes) and then washed with water until the pH became 3 or less. Then 1.81 × 10 ⁻³ MCsCl / 3
The adsorbent was transferred to an Erlenmeyer flask with a screw cap as much as possible using 10 ml of the M NaNO ₃ solution, and the second adsorption treatment and desorption treatment were performed in the same manner as above.

As a result, the first and second cesium adsorption rates were 99.2 and 98.9%, respectively, and the first and second desorption rates (for cesium in the filtrate) were 90. 2,
It was 97.4% (desorption rate including cesium eluted by washing with water was almost the same value). The iron concentrations in the first and second desorption treatment solutions were 1.8 and 0.4 ppm, respectively.

Example 10 With respect to 0.02 g of K ₂ Cu ₃ FC air-dried matter (calculated as an anhydride), the desorbing agent solution was 1.28 × 10 ⁻⁴ M containing nitrous acid.
Under the same conditions as in Example 9 except that the 2M nitric acid solution was used, the adsorption and desorption operations were repeated twice while sandwiching the regeneration operation after desorption. As a result, the first and second cesium adsorption rates were 99.3 and 98.3%, respectively, and the first and second desorption rates (for cesium in the filtrate) were 90.0 and 9 respectively.
It was 7.1% (desorption rate including cesium eluted by washing with water was almost the same value). The iron concentrations in the first and second desorption treatment solutions were 1.4 and 0.2 ppm, respectively.
Example 11 Example of adsorbent in which copper ferrocyanide is supported on inorganic porous material
Then, a composite adsorbent of silica gel and copper ferrocyanide
(Silica gel-CuFC) was prepared as follows.
It was Spherical silica gel (MB-manufactured by Fuji Devison Chemical Co., Ltd.
4B) is impregnated with an aqueous potassium ferrocyanide solution and dried.
Repeat as much as possible to put potassium ferrocyanide into the pores.
After supporting, wet with saturated copper nitrate solution with stirring,
Repeated drying to fully precipitate copper ferrocyanide in the pores
Let Floating ferro by adding water and decanting
The copper cyanide precipitate was removed and air dried. 1g of dried silica gel
The supported amount of copper ferrocyanide per unit was 0.136 g.
It was 0.111 g of the silica gel-CuFC air dried material
Therefore, the concentration of hydrazine added during the adsorption treatment was set to 10 ⁻³ M.
Then, the desorbent solution was replaced with a 5 M nitric acid solution containing 10 ⁻³ M nitrous acid.
Regeneration operation after desorption under the same conditions as in Example 1 except that
The adsorption and desorption operations were repeated 6 times while sandwiching. Cess
Average adsorption rate and desorption rate of um (elute by washing with water)
(Including cesium) was 85.3% and 96.5%, respectively.
It was. The average iron concentration in the adsorption treatment solution and desorption treatment solution is
The values were 0.48 ppm and 3.36 ppm, respectively.

Reference Example 4 With respect to 0.02 g (dry matter equivalent) of K ₂ Cu ₃ FC air-dried matter, the same as Example 9 except that the nitric acid concentration of the desorbent solution was changed under the addition of a certain amount of sodium nitrite. Under the same conditions,
Adsorption and desorption operations were performed. FIG. 3 shows the cesium desorption rate (for cesium in the filtrate) when the nitric acid concentration of the desorbent solution was adjusted with commercially available high-purity nitric acid (nitrous acid concentration at the time of diluting 3M: 2-3 × 10 ⁻⁵ M). ) Is the result of measurement under some constant addition amount of sodium nitrite. From this result, a certain concentration (1.
^{5 to} 2.3 × 10 ⁻⁵ M) or more is required in the presence of nitrous acid, but in the region where desorption is remarkable, the desorption rate does not depend on the nitrite concentration, but the effect of nitric acid concentration is large. Was given. Further, the increase in the desorption rate at a nitric acid concentration of 7 M or more was slight.

Reference Example 5 With respect to 0.02 g of K ₂ Cu ₃ FC air-dried matter (anhydrous equivalent), a desorbent solution containing 3M of sodium nitrite was not added.
HCl solution, except for using ^{10 -2} M sodium nitrite 3M HCl solution was added, or ^{10 -2} 2 M HCI was added sodium nitrite M / 1M HNO ₃ mixed solution of the adsorption under the same conditions as in Example 9, desorption The operation was performed. As a result, the cesium desorption rate was 0, 13.5, and 47.8%, respectively.
(For cesium in the filtrate).

Reference Example 6 0.01 g of K ₂ Cu ₃ FC air-dried material (calculated as anhydrous product) was weighed in an Erlenmeyer flask equipped with a screw cap, and 10 ⁻³ M sodium nitrite and various concentrations of hydrazine were added thereto. Done 10
^-3 M CsCl / 3M HNO ₃ solution 10 ml or 10 ^-3 M CsCl / 3M HNO ₃ solution 1 with various concentrations of hydrazine added without the addition of sodium nitrite
0 ml was added, and the mixture was shaken in a constant temperature shaking tank (25 ° C.), subjected to adsorption treatment for 24 hours and then filtered. Cesium in the filtrate was analyzed and the distribution coefficient (Kd) was calculated. FIG. 4 compares changes in the distribution coefficient of cesium depending on the concentration of hydrazine added, with and without addition of sodium nitrite. This result indicates that it is necessary to add an equivalent amount or more of hydrazine to coexisting nitrous acid in order to prevent the decrease of the cesium adsorption capacity due to the oxidation of K ₂ Cu ₃ FC.

Reference Example 7 With respect to 0.01 g of K ₂ Cu ₃ FC air-dried material (calculated as an anhydride), hydrazine was changed to amido sulfuric acid and the same treatment as in Reference Example 6 was performed. FIG. 5 compares changes in the distribution coefficient of cesium depending on the concentration of amide-sulfuric acid added, with and without the addition of sodium nitrite. This result indicates that addition of an equivalent amount or more of amido-sulfuric acid to coexisting nitrous acid is necessary to prevent the decrease of cesium adsorption capacity due to the oxidation of K ₂ Cu ₃ FC.

[0053]

According to the method of the present invention, by repeatedly using the insoluble iron cyano complex salt-based adsorbent, it is possible to achieve efficient separation and recovery of cesium in an aqueous solution containing nitric acid or sodium nitrate as a main component. It is clear that less waste is generated. If the method of the present invention is applied to the separation and recovery of radioactive cesium in high-level waste liquid containing nitric acid or sodium nitrate as a main component, it will be extremely useful in developing the efficient treatment technology.

[Brief description of drawings]

1 is a flow sheet of the method of the present invention.

FIG. 2 is a graph showing the relationship between the number of repeated uses of an iron cyano complex salt-based adsorbent and the cesium adsorption rate and desorption rate.

FIG. 3 is a graph showing the relationship between nitric acid and nitrite concentration of the iron cyano complex salt-based adsorbent and the desorption rate of tocesium.

FIG. 4 is a graph showing the relationship between the amount added and the cesium distribution coefficient when hydrazine is used as the nitrous acid decomposer.

FIG. 5 is a graph showing the relationship between the addition amount and the cesium distribution coefficient when using amido-sulfuric acid as a nitrous acid decomposer.

Claims (7)

[Claims] 1. A cesium-containing aqueous solution is brought into contact with an oxidized-reduced insoluble iron cyano complex salt in a reduced state, and cesium is adsorbed on this to separate it from the aqueous solution. A method for recovering cesium in an aqueous solution, which is characterized in that the cesium is desorbed by converting into an oxidized state by treatment with cesium. 2. The method according to claim 1, wherein the oxidizing agent is an aqueous nitric acid solution containing nitrous acid. 3. The method according to claim 1, wherein the insoluble iron cyano complex salt after desorption of cesium is subjected to a reduction treatment to be regenerated and repeatedly used. 4. The method according to claim 1, wherein the oxidizing agent and cesium are recovered by subjecting the desorbed cesium-containing nitric acid aqueous solution to heat evaporation treatment. 5. A nitrous acid decomposing agent is added to a cesium-containing aqueous solution containing a high concentration of nitric acid, which is then brought into contact with an oxidized-reduced insoluble iron cyano complex salt in a reduced state to adsorb cesium on the complex. After separation from the aqueous solution, the oxidation-
A method for recovering cesium in an aqueous solution, which comprises treating a reduced insoluble iron cyano complex salt with an oxidizing agent to convert it into an oxidized state to desorb cesium. 6. The method according to claim 5, wherein the oxidizing agent is an aqueous nitric acid solution containing nitrous acid. 7. The method according to claim 5, wherein the oxidizing agent and cesium are recovered by subjecting the desorbed cesium-containing nitric acid aqueous solution to heat evaporation treatment.