JP2003185792A - Method of extracting and separating americium from lanthanoid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an extractant capable of being manufactured at low cost and having the function of separating americium and rare earth elements, which function is equivalent or superior to the functions of conventional extractants, relating to techniques for effectively extracting, separating and removing americium in high-level radioactive solutions. <P>SOLUTION: A compound (dithiocarbamate type 0,0'-decanoyl chitosan) soluble in such an organic solvent as kerosine and obtained by introducing a functional group of dithiocarbamate into a chitosan derivative (0,0'-decanoyl chitosan or the like) is used as the extractant in the selective extraction of americium. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a technique for effectively extracting, separating, and removing americium in a high-level radioactive liquid waste.

[0002]

2. Description of the Related Art Various radioactive elements are contained in so-called high-level radioactive liquid waste after separating and recovering uranium and plutonium by reprocessing spent nuclear fuel. Of these, a particular problem is trans-uranium elements such as americium, which has a very long half-life. If the selective removal of these is possible, the problem of high-level radioactive liquid waste is greatly reduced. For this reason, research on the separation and removal of transuranium elements has been continued for a long time in each country.

The problem here is the presence of a trivalent rare earth element having similar chemical properties to the trivalent transuranium element such as americium. Most preferably, the transuranic element can be directly separated from the high-level radioactive liquid waste as it is,
Since it is very difficult to do so, each country is currently aiming at a method of first separating transuranic elements together with rare earth elements from high-level radioactive waste liquid, and then separating both.

In the United States, as an extractant for extracting and separating transuranium elements and rare earth elements all together, octyl (phenyl) -N, N-diisobutylcarbamoylmethylenephosphine oxide [octyl (phenyl) -N, N-diisobutylcarbam]
oylmethylene phosphine oxide] (CMPO) was developed, and the TRUEX method, which is a separation process using this, was developed.
In France, N, N'-dimethyl-N, N'-dibutyltetradecylmalonamide [N, N'-dimethyl -N, N'-
A diamide type extractant such as dibutyltetradecyl malonamide] (DMDBTDMA) was studied, and a separation process using it, the DIAMEX method, was developed.

In Japan, N, N, N ', N'-tetraoctyl-3-oxapentane-1,5-diamine (which is an extractant having a separation performance far superior to that of DMDBTDMA at the Japan Atomic Energy Research Institute is recently used. N, N, N ', N'-tetraoctyl-3-o
xapentane-1,5-diamide) (TODGA) was developed (Katsuichi Tatemori, Journal of the Atomic Energy Society of Japan, vol. 42, 1124-1129).
(2000)). With these extractants, transuranic elements and rare earth elements are collectively extracted and separated from high-level radioactive waste liquid, which is an aqueous nitric acid solution having a concentration of about 3 M, and back-extracted into dilute nitric acid.

On the other hand, searches for extractants which achieve separation from rare earth elements by selectively extracting transuranic elements from such back extract have been conducted in various countries. Chinese Zh
u et al. are bis (2,4,4'-trimethylpentyl) dithiophosphini
[C acid], Cyanex 301, is suitable for this purpose (Solvent Extraction Ion Exchang
e, Vol. 14, 61-68 (1996)). The separation factor of americium from europium is reported to be 6000 or more. However, this Cyanex 301 is easily susceptible to oxidation by air, and in some cases it is oxidized in about a week, and loses the separation function between the two.

On the other hand, Kolarik et al. Of Germany et al. Are 2,6-di (5,6-dipropyl-1,2,4-triazine-3-
Ill) -pyridine [2,6-di (5,6-dipropyl-1,2,4-triazi
n-3-yl) -pyridine] (DPTP) was found to be an extractant suitable for such purpose (Solvent Extraction Ion E
xchange, Vol. 17, 1155-1170 (1999)).
However, this extractant has problems that the synthesis is complicated and a precipitate is generated during the extraction.

[0008]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
It is an object of the present invention to provide an extractant that is less likely to be oxidized than DPTP, can be produced at a low cost, and has a separation function of americium and rare earth elements equal to or higher than these.

[0009]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that a compound in which a functional group of dithiocarbamate is introduced into a chitosan derivative soluble in an organic solvent such as kerosene is trivalent. The inventors have found that Americium of A. is selectively extracted, and completed the present invention.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Chitosan is a powdered polysaccharide obtained by alkaline hydrolysis of chitin, which is a main component of shells of crustaceans such as shrimp and crab. Since chitosan has many hydroxyl groups like chitin, it has high hydrophilicity and cannot be dissolved in an organic solvent as it is. But for example
Nishimura et al. (Chemistry Letters, 243-246 (1990)
)), A chitosan derivative soluble in various organic solvents such as chloroform can be obtained by acylating the hydroxyl group of chitosan with an acid chloride having a long-chain alkyl group (for example, O , O'-decanoyl chitosan) can be produced.

In this case, the acid chloride used for acylation has 7 to 25 carbon atoms, preferably 9 to 15 carbon atoms. When the number of carbon atoms is 6 or less, most of the products do not dissolve in the organic solvent, and when the number of carbons is 26 or more, the product becomes very viscous and the subsequent solvent extraction operation becomes difficult.

Furthermore, by immobilizing various functional groups on the primary amino group of such a chitosan derivative, chitosan derivatives having various functional groups and soluble in organic solvents can be prepared. For example, a chitosan derivative having a dithiocarbamate functional group (eg, dithiocarbamate type O, O′-decanoyl chitosan) can be prepared by reacting with carbon disulfide. Hereinafter, embodiments of the present invention will be described in more detail.

[0013]

Example 1 Example 1 Sensory function of dithiocarbamate
Induced chitosan soluble in organic solvents such as kerosene having a group
Method for Preparing Conductor Chitosan derivative having a functional group of dithiocarbamate and soluble in an organic solvent such as kerosene is reported in, for example, the following 19th Japan Solvent Extraction Discussion Meeting held on October 19, 2000. Is adjusted by the method of (1st
9th Solvent Extraction Debate, Proceedings, page 21). However, the method for preparing a chitosan derivative having a dithiocarbamate functional group and soluble in an organic solvent such as kerosene used in the present invention is not limited to the following method.

1) First, in order to solubilize chitosan in an organic solvent, the hydroxyl group of chitosan is acylated with an acid chloride. In this case, when the reaction is carried out directly, not only the hydroxyl group of chitosan but also the primary amino group reacts with the acid chloride. Therefore, the amino group is protected by previously reacting with phthalic anhydride as follows.

In 450 ml of dimethylformamide, 20 g of chitosan and 52.5 g of phthalic anhydride were added,
Heat and stir in an oil bath at ℃ for 5 hours. The brown viscous liquid obtained is poured little by little into ice water and stirred for 1 hour to obtain a brown precipitate. After filtration, the precipitate is washed with distilled water and further stirred in hot ethanol for 30 minutes. This is filtered, washed with ethanol and then with diethyl ether, and then dried under reduced pressure. This gives 40.8 g of a dark brown solid which is N-phthaloyl chitosan.

2) Then, the hydroxyl group of the obtained N-phthaloyl chitosan is reacted with an acid anhydride having a long-chain alkyl group to carry out an acylation reaction as follows. To 240 ml of methanesulfonic acid, add 30.0 g of N-phthaloyl chitosan and stir until completely dissolved. Thereafter, 126 g of decanoic acid chloride is slowly added in an ice bath at 10 ° C. under a nitrogen atmosphere. After stirring for 3 hours, the mixture is left at -20 ° C for 12 hours. The black liquid obtained is poured little by little into ice water to form a precipitate. The precipitate is filtered, washed with ice water, put again in ice water, neutralized with diluted ammonia water, and then filtered again. Wash with ice water again, dissolve in chloroform, and wash with distilled water until neutral. The organic phase is dried over anhydrous magnesium sulfate and chloroform is removed by distillation under reduced pressure to obtain a black liquid. When this black liquid is put in hexane and left to stand, a black precipitate of 20.5 g of O, O'-decanoyl N-phthaloyl chitosan is obtained.

3) Thereafter, the N-phthaloyl group protecting the primary amino group is deprotected by the following reaction to obtain the desired product O, O'-decanoyl chitosan. 250 ml
6.5 g of the O, O'-decanoyl N-phthaloyl chitosan obtained above was put into the chloroform of and completely dissolved. Subsequently, 12 g of hydrazine is slowly added, and the mixture is stirred at room temperature for 5 hours. The brown substance that has precipitated is filtered off,
Wash the filtrate with plenty of water until neutral. This solution is dried over anhydrous magnesium sulfate, and chloroform is distilled off under reduced pressure. Hexane is further added to obtain 1.0 g of a target white solid of O, O'-decanoylchitosan. This solid dissolves in chloroform and toluene, but not in aliphatic hydrocarbon diluents such as hexane and kerosene.

4) Next, O, O'- prepared by the above method
The functional group of dithiocarbamate is introduced into decanoyl chitosan by the following method. 1 g of O, O'-decanoyl chitosan obtained by the above method in 200 ml of ethanol
, Stirred until completely dissolved, added with 5 g of carbon disulfide and stirred for 1 hour, and then slowly added 3 ml of concentrated aqueous ammonia. After stirring at 37 ° C. for 24 hours, 5 g of carbon disulfide and 3 ml of concentrated aqueous ammonia are added again. After stirring for 24 hours, the solution is neutralized with 0.1 M hydrochloric acid. After removing ethanol and carbon disulfide by distillation under reduced pressure, chloroform is added and the organic phase is washed with water until it becomes neutral. After drying over magnesium sulfate, the chloroform was distilled off under reduced pressure to obtain 1.02 g of a brown viscous solid of the desired product, dithiocarbamate type O, O'-decanoylchitosan.

This solid is, in addition to chloroform and toluene,
It also dissolved in hexane and kerosene. The content of sulfur in this solid was 11.64% (% by weight), which corresponds to 96.0% when converted into the amount of the functional group of dithiocarbamate introduced into the monomer unit of the sugar chain.

Example 2 Dithiocarbamate Official
Chitosa, which has a functional group and is soluble in organic solvents such as kerosene
Trivalent Americium Rare Earth Element
Extraction and separation of dithiocarbamate type O, O'-decanoyl chitosan at a concentration of 2325 ppm, 2.5 ml and 50 pp of kerosene solution
pH of 3 to 4 containing americium at a concentration of b and trivalent lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium at a concentration of 1 ppm each. 2.5 ml of the above aqueous solution was placed in a flask and shaken at 26 ° C. for 15 minutes.

The concentration of americium in the aqueous and organic phases after extraction was determined using a high purity germanium spectrometer system.
It was determined by measuring the γ dose of each phase. The concentration of each rare earth element in the aqueous solution before and after extraction was analyzed by an ICP mass spectrometer. From these changes in concentration, the concentration of each element in the organic phase after extraction and the distribution ratio were determined. The partition ratio in this case is the ratio of the concentration of the element existing in the organic phase after extraction to the concentration of the element remaining in the aqueous solution. PH = 3 and 3.55 and 4 in Figure 1.
The logarithmic value of the partition ratio of extraction of americium and rare earth elements to dithiocarbamate type O, O'-decanoylchitosan in. It can be seen that the value of americium is much larger than the values of all rare earth elements.

The same extraction experiment was performed again after 7 days, but the same value was obtained. That is, it was proved that the extractant was not deteriorated even after 7 days. Europium was selected as a representative rare earth element, and the separation coefficient of americium and europium at each pH was obtained. Here, the separation coefficient is the ratio of the distribution ratio of the two. As a result, the following values were obtained.

[0023]

[Table 1]

It became a large value of 1227 at pH = 4. Although it is not as high as the Cyanex 301 value mentioned above, it is an extremely large value. (Comparative Example 1) Preparation of dithiocarbamate chitosan
Method It is possible to introduce the functional group of dithiocarbamate directly into chitosan and use it as a solid adsorbent. Here, such a dithiocarbamate type chitosan can be obtained, for example, by the method of Riccard et al. (Carbohydrate Researc) described below.
h, 104, pp. 235-243 (1984)).

5.0 g of chitosan was dissolved in 100 ml of a 10% by volume aqueous acetic acid solution, and this was dissolved in methanol for 4-5 times.
Dilute twice. To this, 10 g of carbon disulfide was added, stirred with a stirrer for a while, and 90 ml of ammonia water was added little by little. Then, after stirring at room temperature for 24 hours, the gel-like substance produced is filtered, washed with methanol several times, and further stirred in methanol for one day. Then, it is filtered and washed with methanol until the filtrate becomes colorless. Then 10
Stir for 1 day in mM hydrochloric acid and wash with distilled water until the filtrate is neutral. This is vacuum dried and then ground in a mortar to obtain a pale yellow powder.

When the functional group of dithiocarbamate introduced per sugar chain unit of chitosan was evaluated in the same manner as in Example 1, it was 25%. (Comparative Example 2) Using dithiocarbamate chitosan
Adsorption and separation of trivalent americium from rare earth elements 20 mg dithiocarbamate chitosan and 50 ppb
PH of 3 to 4 including americium at a concentration of 1 ppm and trivalent lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium at concentrations of 1 ppm each. 15 ml of an aqueous solution was placed in a flask and shaken at 26 ° C. for 15 minutes.

The concentration of americium in the aqueous solution before and after the adsorption was determined by measuring the γ dose using a high purity germanium spectrometer system. In addition, the concentration of each rare earth element in the aqueous solution before and after adsorption was analyzed by an ICP mass spectrometer. The adsorption amount of each element was determined from the change in concentration before and after the adsorption, and the distribution ratio was determined. The partition ratio here is the ratio of the number of moles of the element adsorbed by 1 kg of the adsorbent to the molar concentration of the element remaining in the aqueous solution.

FIG. 2 shows logarithmic values of adsorption distribution ratios of americium and rare earth elements to dithiocarbamate type chitosan at pH = 3 and 4 and 5. The distribution ratio of americium at each pH is only slightly higher than the distribution ratio of rare earth elements.

[0029]

INDUSTRIAL APPLICABILITY The solvent extraction method using a chitosan derivative soluble in an organic solvent, which has a functional group of dithiocarbamate proposed in the present invention by the above Examples and Comparative Examples, is used to extract trivalent americium from rare earth elements It became clear that it is effective for selective extraction and separation.

[Brief description of drawings]

FIG. 1 shows the logarithmic values of the distribution ratio of americium and rare earth elements at pH = 3 and 3.55 and 4 (it can be seen that the value of americium is significantly larger than the values of all rare earth elements).

FIG. 2 shows logarithmic values of distribution ratios of americium and rare earth elements at pH = 3 and 4 and 5 (at each pH, the distribution ratio of americium is slightly larger than the distribution ratio of rare earth elements). There is only).

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shoichi Tatemori             4 of 2 Shirane, Shikata, Tokai-mura, Naka-gun, Ibaraki Prefecture               Japan Atomic Energy Research Institute Tokai Research Center (72) Inventor Hironaga Naganawa             4 of 2 Shirane, Shikata, Tokai-mura, Naka-gun, Ibaraki Prefecture               Japan Atomic Energy Research Institute Tokai Research Center F-term (reference) 4D056 AB03 AB10 AC01 BA03 CA01                       CA39 DA10

Claims (2)

[Claims] 1. Having a dithiocarbamate functional group,
A method for separating americium from rare earth elements by selective solvent extraction of americium from an aqueous solution in which americium and a rare earth element coexist, characterized by using a chitosan derivative soluble in an organic solvent. 2. The pH of the aqueous solution is 2 to 6, preferably 3 to 5.
A method for separating americium from a rare earth element according to the method of claim 1 under the conditions of.