JP3136044B2 - Iodine ion adsorbent - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an iodine ion adsorbent, and more particularly to a method for disposing radioactive waste in geological disposal.
The present invention relates to an iodine ion adsorbent mainly intended to adsorb ¹²⁹ I ions having a long half-life among nuclides eluted in groundwater.

[0002]

2. Description of the Related Art ¹²⁹ I generated in nuclear fuel reprocessing
Has an extremely long half-life of 1.7 × 10 ⁷ years, and the development of a sound treatment and disposal method is required. The method currently considered is to fix the generated radioactive iodine as silver iodide with an iodine adsorbent such as silver-added silica or silver-added alumina, solidify the adsorbent, cement it into drums, and dispose it in geological disposal. .

However, in the geological disposal of radioactive waste, ¹²⁹ I has a low specific activity, but has a long half-life as described above, and therefore the stability of the solidified body in the geological disposal site becomes a problem. For example, when ground water comes into contact with a solidified body and silver iodide is dissolved, it is one of the nuclides that require special attention because it is present as I ⁻ that is hardly adsorbed in a groundwater environment.

[0004] Therefore, such a I ^- adsorbent which effectively adsorbs is demanded. In particular, underground water Cl ^-, etc., since various other ions are contained, I ^- adsorbent capable of selectively adsorbing is demanded.

[0005]

The object of the present invention is to solve the above is primarily, I out of the water ^- is to provide a can be selectively and efficiently adsorbing iodine ion adsorbent.

Secondly, particularly in geological disposal of radioactive waste, radioisotopes of I, which are to be contained in groundwater due to contact between the solidified material and groundwater, particularly ¹²⁹ I ions having a long half-life, are used. An object of the present invention is to provide an iodine ion adsorbent that can selectively and efficiently adsorb.

[0007]

This and other objects are achieved by the following constitutions (1) and (2). (1) An iodine ion adsorbent which is obtained by impregnating a montmorillonite with a silver thiourea complex and selectively adsorbs iodine ions in water. (2) The iodine ion adsorbent according to (1), wherein the iodine ion is derived from radioactive waste.

[0008]

According to the present invention, an iodine ion adsorbent is obtained by impregnating montmorillonite with a silver thiourea complex. It is considered that the silver thiourea complex was intercalated between the layers of montmorillonite.

Therefore, the silver thiourea complex in the montmorillonite reacts with the iodine ion to adsorb the iodine ion. Such a reaction is, for example, a reaction in which iodine is coordinated with a silver thiourea complex, and a reaction product generated by such a reaction is a compound that is hardly soluble in water. Furthermore, Cl ^- even if other halide ions coexist, such as the reaction product formed by the reaction of these halide ions do not form a high precipitate solubility in water. Therefore, a precipitate is formed only by reaction with iodine ions, and it is considered that selective adsorption of iodine ions occurs. Furthermore, since the reaction between the iodide ion and the silver thiourea complex can take place at a stoichiometry of not 1: 1 but higher, the montmorillonite can be exchanged with the iodine ion-reactive site or the iodine ion by intercalation. It is presumed that sites (for example, OH groups) are formed, and it is considered that iodine ions are also adsorbed by these sites.

Further, the adsorption rate of the above-mentioned iodine ion adsorbent is high and efficient.

Therefore, the present invention can be applied to the adsorption of radioactive isotopes of iodine. In the geological disposal of radioactive waste, it is possible to adsorb ¹²⁹ I, which has a long half-life and is hardly adsorbed in a groundwater environment. it can.

[0012]

[Specific Configuration] Hereinafter, a specific configuration of the present invention will be described in detail.

The iodine ion adsorbent of the present invention (hereinafter also referred to as “adsorbent”) is obtained by impregnating montmorillonite with a silver thiourea complex.

The montmorillonite in the present invention is a clay mineral containing montmorillonite as a main component, and is a concept including a clay mineral called montmorillonite, bentonite and the like.

Montmorillonite is one of so-called smectite group clay minerals and has a layered structure. The composition formula is approximately (Na, Ca) _1/3 (Al, Mg) ₂ [(OH)
₂ | Si ₄ O ₁₀ ] · nH ₂ O.

In the present invention, montmorillonite is impregnated with a silver thiourea complex by the method of J. Pleysier, A. Cremer.
s, J. Chem. Soc. Faraday Trans. 1, 71, 256 (1975)
May be impregnated based on the method described in (1).

Specifically, first, a silver thiourea complex ion solution may be prepared by mixing a 0.05 to 0.3 M aqueous solution of AgNO ₃ and a 5 to 15 times the concentration of a thiourea aqueous solution. Thereby, Ag (tu) ₃ ⁺ ion [here, tu
Represents thiourea. ] Is mainly formed. Thereafter, montmorillonite is added to the solution.
The liquid-solid ratio at this time is not particularly limited, but the solution 100 ml
On the other hand, the amount of montmorillonite is preferably 2 to 7 g. After adding the montmorillonite, it is preferable to stir and disperse the mixture and leave it for a certain period of time. The leaving time is not particularly limited, but may be 10 to 100 hours, practically 10 to 30 hours. Note that the above operation may be performed at room temperature (20 to 25 ° C.). The montmorillonite to be dispersed is usually amorphous, and the size thereof is not particularly limited. However, it is preferable to use a montmorillonite having a projected area of 74 μm or less in terms of diameter when converted into a circle. The lower limit at this time is not particularly limited, but is usually about 10 μm. Thereafter, solid-liquid separation is performed by filtration, and the obtained adsorbent is washed as necessary, and further,
Dry if necessary. The washing may be performed several times (about three to four times) using distilled water, and the amount of distilled water at that time may be about 5 to 10 times the total volume of the adsorbent. By washing with distilled water, excess thiourea can be removed. Thiourea, I ^- because reducing the adsorption capacity. In fact, when the washing with thiourea solution I ^- adsorption capacity is confirmed to be reduced. Drying performed as necessary may be vacuum drying at a temperature of 45 to 65 ° C. for about 12 to 24 hours.

Thus, a montmorillonite impregnated with a silver thiourea complex and impregnated with a silver thiourea complex is obtained. Its shape and size are the same as those of the used montmorillonite.

The montmorillonite impregnated with the silver thiourea complex is considered to be an insertion compound in which the silver thiourea complex is intercalated between layers of montmorillonite having a layer structure, and is extremely stable.

This intercalation can be confirmed from the result of X-ray diffraction analysis (XRD) of montmorillonite and montmorillonite impregnated with a silver thiourea complex. That is, the peak position, which is a measure of the spacing between the bottom surfaces of the layers, is different between the two compounds, and the bottom spacing is about 9 to 10 A for montmorillonite, whereas it increases to about 16 to 18 A for montmorillonite impregnated with a silver thiourea complex. I have.

The silver content of the montmorillonite impregnated with the silver thiourea complex is preferably at least 1 wt%, particularly preferably at least 1.8 wt%. In this case, it is considered that the larger the silver content is, the more the function as the iodine ion adsorbent is improved.
It is about 0 wt%. On the other hand, if the silver content is too small, the effect of the present invention cannot be obtained.

The silver content can be determined by ICP (high frequency inductively coupled plasma) emission spectroscopy.

The adsorbent of the present invention is used to adsorb iodine ions in water. The iodine ion concentration in the water at this time may be any, and it is effective for any of high to low concentrations. The concentration ranges over a wide range of about 10 ^-6 to 10 ^-3 M.

The adsorbent of the present invention can adsorb iodine ions in an aqueous solution as described above by selecting the amount of Ag in the adsorbent and controlling the amount of the adsorbent used. Become.

Although the structure of the adsorbent of the present invention after adsorbing iodine ions is not specified, a new absorption different from that of the adsorbent is observed in the XRD pattern. It is believed that new compounds are formed.

The adsorbent of the present invention, Cl present in large quantities, such as the ground water ^- a not completely adsorbed, Cl rather montmorillonite ^- tends to elute. Therefore, the adsorbent of the present invention selectively adsorbs iodine ions.

[0027] Thus, the adsorbent of the present invention, Cl ^- even in the presence of, since the selective adsorption of iodine ions can be used for adsorption of ions of radioactive isotopes of iodine.

Specifically, in geological disposal of radioactive waste in which radioactive iodine is fixed as hardly soluble iodide (for example, silver iodide), iodide in the solidified body comes into contact with groundwater, and iodide is converted into groundwater. ¹²⁹ I, especially when it has a long half-life (t _1/2 ) (t _1/2 = 1.7 × 10 ⁷ years).
However, it becomes a problem that iodine ions migrate with the groundwater. In such a case, by using the adsorbent of the present invention as an artificial barrier material around the solidified body, it is possible to delay the transfer of radioactive iodine to a biological system. Also, since the adsorbent of the present invention is a stable compound,
Use as such a barrier material is effective. further,
The manufacturing method is simple and advantageous in terms of cost.

[0029]

The present invention will be specifically described below with reference to examples.

Example 1 50 ml of 0.1 M AgNO ₃ aqueous solution was added to 1.0 M (NH ₂ )
_It was added to 50 ml of 2CS aqueous solution with stirring to prepare a silver thiourea complex ion solution. 5 g of montmorillonite (particle size: about 10 to 74 μm) was added to this solution, and the mixture was allowed to stand in a dark place for 24 hours while sufficiently stirring and dispersing to immerse. Thereafter, solid-liquid separation was performed with a glass filter, and the obtained adsorbent was washed several times with 20 ml of distilled water, and vacuum-dried at 50 to 60 ° C. for about 24 hours.

The amount of Ag in the adsorbent thus obtained was determined to be 2 wt% by IPC emission analysis after dissolving the adsorbent.
Was estimated.

Further, X-ray diffraction analysis of montmorillonite and the adsorbent (montmorillonite impregnated with a silver thiourea complex) was performed. The XRD pattern is shown in FIG.

As can be seen from FIG. 1, the peak at 2θ = 9.26 °, which is a measure of the bottom surface interval, is shifted to 5.44 °. That is, the bottom interval is 9.54A to 16.2A.
It is expanded to 3A, and it is considered that the silver complex was intercalated in the montmorillonite.

Next, an aqueous solution having an anion concentration as shown in Table 1 was prepared using NaCl or NaI, and 20 ml of the aqueous solution was added to 0.2 g of the adsorbent (liquid-solid ratio = 100). ), Immersed in shaking at 25 ° C. for 36 days, separated by solid-liquid separation with a 0.45 μm filter, and
^- and Cl ^- concentration was measured. The concentration was measured by ion chromatography, and the amount of adsorption was calculated from the amount of decrease in the concentration in the solution (see Example 5). Table 1 shows the results.

[0035]

[Table 1]

[0036] From Table 1, Cl possibly present in large quantities in groundwater ^- is not at all adsorbed, tends to increase the concentration in solution rather eluted from montmorillonite, I
^It can be seen that only-is selectively adsorbed.

The XRD of the montmorillonite impregnated with the silver thiourea complex to which iodine was adsorbed was 10.2 °
A sharp absorption was observed at around 19.7 ° and 20.5 °, suggesting that a new compound was generated by the adsorption.

In order to confirm the anion adsorption of montmorillonite, montmorillonite was immersed in distilled water or an aqueous solution shown in Table 2 under the same conditions as described above (immersion time = 36).
Day, liquid-solid ratio = 100), similarly, I ⁻ and Cl after immersion.
^- examine its concentration. Table 2 shows the results.

[0039]

[Table 2]

[0040] From Table 2, the montmorillonite I ^- and Cl ^- no ion adsorption capacity, Cl conversely ^- that tend to release was confirmed.

Example 2 An adsorbent was produced in the same manner as in Example 1. However, 2.
0.2 M AgNO in 50 ml of 0 M (NH ₂ ) ₂ CS aqueous solution
₍₃₎ 50 ml of an aqueous solution was added, 5.0 g of montmorillonite was added to the solution thus prepared, and the mixture was allowed to stand for 24 hours, washed with 100 ml of distilled water, and dried at 50 ° C. under vacuum.

The Ag content of this adsorbent was 2.0% by weight.

Next, an aqueous solution having an anion concentration as shown in Table 3 or Table 4 was prepared in the same manner as in Example 1, and an anion adsorption test was performed in the same manner as in Example 1. However, in Table 3, the immersion time of the adsorbent was set to 7 days.
The one for 36 days. The liquid-solid ratio (L /
S) 100 was made under the same conditions as in Example 1, and the liquid-solid ratio (L
/ S) 200 has the same liquid volume and uses 0.1% adsorbent.
1 g. The results are shown in Tables 3 and 4. The pH of the solution after immersion is also described.

[0044]

[Table 3]

[0045]

[Table 4]

Further, Cl ⁻ and I ⁻ by the above adsorbent are used.
The amount of adsorbed [the amount adsorbed per 1 g of adsorbent (number of moles)], the ratio of adsorbed ions (adsorption rate,%), and the silver thiourea complex in the adsorbent reacted with anions The ratio of the complex (reaction rate,%) was estimated. The results are shown in Table 5 (immersed for 7 days) and Table 6 (immersed for 36 days). The reaction rate was determined by the following equation. Also,
The anion concentration in Tables 5 and 6 is higher than the anion concentration when it is assumed that the anion reacts with the complex at a ratio of 1: 1. Assuming that it is filled, the concentration condition is such that all the adsorption sites are filled.

Reaction rate (%) = [Amount of anions adsorbed (m
ole)] × 100 / [Amount of silver thiourea complex in adsorbent (mol
e)] Here, the amount (mol) of the silver thiourea complex in the adsorbent is (the amount of adsorbent used) × (silver content (wt%)) / 107.87.

The results are shown in Tables 5 and 6.

[0049]

[Table 5]

[0050]

[Table 6]

FIG. 2 shows a plot of the amount of adsorption versus initial I ^- concentration.

Consider these results.

From FIG. 2, it can be seen that the adsorption amount was almost 7 days and 36 days.
Since they are almost the same, adsorption equilibrium is reached within 7 days
It is thought that it is. Also, it can be seen from Tables 5 and 6.
Like, Cl^- Even in excessive conditions, I^- Is adsorbed
Therefore, the adsorbent has I^- Has the property of selectively adsorbing
It can be said. Furthermore, I^- In high concentration solution systems
The response rate is over 100%. This is Montmorillo
Silver thiourea complex in knight and I^- Reaction with ions is 1:
Suggests that it is happening with more than one stoichiometry
ing. This is an unexpected result and the silver thiourea complex
Intercalating between layers of montmorillonite
Than I^- Sites that react with or exchange ions with
It is thought that it was done. Because montmorillonite
Things are I ^- This is because there is no ion adsorption ability. Possibilities and
Therefore, montmorillonite is intercalated with silver thiourea complex.
Calling for I^- OH easily ion-exchanged with
Groups are formed and they are^- Mechanism for ion exchange only with
Is proposed. This indicates that the adsorbent and distilled water
Sex, but I^- PH increases as the amount of adsorption increases
It will be supported by becoming.

Example 3 In the preparation of the adsorbent of Example 2, 0.27 M AgNO
_An adsorbent was prepared in the same manner except that a 3M aqueous solution and a 2.1M (NH ₂ ) ₂ CS aqueous solution were used, and the amount of montmorillonite was 3.0 g.

The Ag content was 9.7% by weight.

Next, an aqueous solution having an anion concentration as shown in Table 7 was prepared in the same manner as in Example 1, and an anion adsorption test was performed in the same manner as in Example 2. Table 7 shows the results. The indications in Table 7 are the same as those in Tables 3 and 4.

Further, the amount of adsorption, the adsorption rate and the reaction rate were determined in the same manner as in Example 2. Table 8 shows the results.

[0058]

[Table 7]

[0059]

[Table 8]

As shown in Table 8, in system 2 and system 3, Cl ⁻
Is also adsorbed. However, in both systems the Cl ^- concentration is I
^- concentration and an equal amount or excess is despite I ^- is more adsorbed. This silver thiourea-impregnated montmorillonite I ^- have been shown to have a selective adsorption property for. Also, this adsorbent may have been insufficiently cleaned of thiourea during its preparation. Therefore, during the adsorption test, the silver complex is eluted to the solution side, and partly exists as silver ions from the relation of the stability constant. Then, the silver ions and Cl ^- are reacted with Ag
It is considered that this precipitated as Cl, and as a result, Cl 2 ^- was also adsorbed.

In any case, even in this adsorbent, I
^- is selectively adsorbed, and the reaction rate does not exceed 100% by a large margin, pH is higher with increasing adsorption. This indicates the existence of an interaction between the easily exchangeable OH group formed by the silver thiourea complex and its intercalate and the I ^- ion, as described above.

Example 4 50 ml of 0.2 M AgNO ₃ aqueous solution was added to an equal amount of (NH ₂ ) ₂
It was added to 50 ml of an aqueous CS solution (2.0 M) with stirring to prepare a silver thiourea complex ion solution. 5 g of montmorillonite is added to this solution, and while stirring and dispersing well,
It was immersed in a dark place for 24 hours. Thereafter, solid-liquid separation was performed with a glass filter to obtain an adsorbent. This is designated as Sample No. 1.

Sample No. 1 was washed twice with 100 ml of distilled water after solid-liquid separation.
Let it be 2.

In sample No. 2, the immersion time was 24
Sample No.
3 was produced.

In sample No. 2, the immersion time was 81
Sample No. 4 was prepared in the same manner except that the washing was repeated twice with 100 ml of a 0.1 M thiourea solution.

0.1 g of each of these sample Nos. 1 to 4 was used, and simulated groundwater or distilled water (in each case, 10 ml) containing a tracer amount of ¹²⁶ I was added thereto and shaken for 1 hour. After immersion, the solid and liquid phases were separated by ultrafiltration. By comparing the radioactivity of the filtrate with the radioactivity of the tracer solution initially added, the adsorption rate of radioactive iodine was determined.

The simulated groundwater was prepared by adding the following compound to 10 liters of distilled water, leaving it to stand for a certain time,
This is a solution obtained by filtration through a filter.

Additive compound NaHCO ₃ 5.415 g / l CaCO ₃ 499 mg / l MgCl ₂ .6H ₂ O 418 mg / l FeCl ₃ .6H ₂ O 145 mg / l SiO ₂ 100 mg / l KCl 33.5 mg / l NaF 22.1 mg / 1 Na ₂ SO ₄ 14.8 mg / l KBr 7.45 mg / l

Table 9 shows the results. In the table, the ratio of the radioactivity C in the filtrate to the initial radioactivity Co Rd = C /
The log value (log Rd) of Co is also described.

[0070]

[Table 9]

Table 9 shows that the sample which has not been washed or washed with distilled water has a higher adsorbability than the sample washed with an aqueous thiourea solution. In addition, it can be used in groundwater from tests in simulated groundwater.

Example 5 A sample was prepared in the same manner as in Sample 4 of Example 4 except that the immersion time was changed to 20 hours. This is designated as Sample No. 5. This was washed with an aqueous thiourea solution.

This sample No. 5 was used.
1g of the 10 ^-3 M I ^- the aqueous solution 10ml containing added 20
As shown in Table 10, after shaking (immersion) for a certain period of time in a thermostat kept at ℃, the solution was filtered through a 0.45 μm filter, and the I ^- concentration in the filtrate was measured by liquid chromatography. . Then, the adsorption rate was determined by the following equation.

Adsorption rate (%) = [(I ^- concentration in solution) −
(I ^- concentration in filtrate)] × 100 / (I ^- concentration in solution)

Table 10 shows the results.

[0076]

[Table 10]

This sample No. 5 was washed with an aqueous thiourea solution, and is considered to have poorer adsorptivity than that washed with distilled water, but shows that it exhibits I ^- adsorbability. When the time reaches a certain time, the effect tends to be saturated or reduced.

In addition to the results of Example 4, the reason that the adsorptivity is poor when washed with a thiourea solution is that the intercalated silver thiourea complex is eluted once into montmorillonite and the number of I ^- adsorption sites decreases. It is thought to be.

[0079]

According to the present invention, I ^- can be selectively adsorbed. It is also efficient. Therefore, in geological disposal of radioactive waste, it is effective when applied to an adsorbent that adsorbs ¹²⁹ I ions having a long half-life and eluted from the solidified substance into groundwater.

[Brief description of the drawings]

FIG. 1 is a graph showing XRD patterns of montmorillonite and montmorillonite impregnated with silver thiourea.

FIG. 2 is a graph showing a plot of the amount of I ⁻ adsorbed versus the initial I ⁻ concentration of the adsorbent of the present invention.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Mikio Kumagai 1201 Takada, Kashiwa-shi, Chiba Prefecture Kashiwa Research Institute, Industrial Creation Research Institute (72) Inventor Yoichi Takashima 1201, Takada, Kashiwa-shi, Chiba Research Institute for Kashiwa Research, Kashiwa Research Institute In-house (72) Inventor Miwa Yui 4-33 Muramatsu, Oji, Tokai-mura, Naka-gun, Ibaraki Pref. Inside the Tokai Works, Power Reactor and Nuclear Fuel Development Corporation (72) Hideki Yoshikawa 4-33, Muramatsu, Tokai-mura, Naka-gun, Ibaraki (56) References JP-A-4-254408 (JP, A) JP-A-58-55332 (JP, A) JP-A-3-293598 (JP, A) JP JP-A-4-143700 (JP, A) JP-A-56-108532 (JP, A) JP-A-60-200822 (JP, A) JP-A-2-68140 (JP, A) JP-A-63-12345 (JP) , A) 4-4890 (JP, A) JP-A-63-162040 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01J 20/00-20/34

Claims (2)

(57) [Claims] 1. An iodine ion adsorbent which is obtained by impregnating a montmorillonite with a silver thiourea complex and selectively adsorbs iodide ions in water. 2. The iodine ion adsorbent according to claim 1, wherein said iodine ions are derived from radioactive waste.