Method for mutual separation of radium, actinium and thorium

Abstract

The present method entails separating thorium by anion exchange to obtain a solution of a mixture of radium and actinium radionuclides, desorbing the thorium from the anion-exchange resin, separating the actinium and radium by cation exchange, and purifying the actinium of iron salt impurities and organic impurities. The thorium is desorbed using a solution containing a mixture of ammonium acetate and acetic acid. The actinium is purified of iron salts and organic impurities in a column containing two sorbents: a bottom layer comprised of a hydrophilic macroporous resin based on a polymethyl methacrylic acid ether; and a top layer comprised of a sorbent based on di(2-ethylhexyl)phosphoric acid applied to an inert carrier. The total concentration of ammonium acetate and acetic acid used in the desorption of thorium lies in a range of from 0.5 to 4.0 mol / L, and the concentration of hydrogen ions lies in a pH range of from 3.5 to 5.0. For added purification, the column is washed with a solution of hydrochloric acid having a concentration in a range of from 0.015 to 0.025 mol / L. The actinium is eluted using a solution of hydrochloric acid. The method makes it possible to isolate radionuclidically and chemically pure actinium-225 from a mixture containing radium-224, radium-225, thorium-228 and thorium-229 radionuclides.

Description

[0001] Method for the mutual separation of radium, actinium and thorium

[0002] The invention relates to the technology of producing preparations of radioactive elements and can be used in analytical chemistry.

[0003] The actinium-225 radionuclide possesses the necessary chemical and nuclear properties for use in targeted cancer therapy. Actinium-225-based drugs are being studied for their potential use in the treatment of prostate cancer, melanoma, stomach cancer, and leukemia. Furthermore, actinium-225 is used as a parent radionuclide for the isolation of bismuth-213, which is promising for use in radionuclide therapy.

[0004] The primary method for producing actinium-225 is its periodic radiochemical separation from its long-lived parent radionuclide, thorium-229. Thorium-229, in turn, can be isolated either from the mixture of radioactive decay products of uranium-233 or from radium-226 irradiated in a nuclear reactor. Reactor irradiation of radium-226 always produces a mixture of thorium-229 and thorium-228 radionuclides. If the original uranium-233 contains uranium-232, the thorium isolated from this material is also a mixture of thorium-229 and thorium-228 radionuclides.

[0005] Thus, when isolating actinium-225 from the above-mentioned source materials, it is necessary to ensure its purification from radionuclides of both thorium-228 and thorium-229, as well as from radium-224 and radium-225, which are daughter products of the decay of the corresponding thorium isotopes.

[0006] Most known methods for producing actinium-225 initially involve anion-exchange separation of thorium isotopes from their mixture with radium and actinium, followed by separation of actinium and radium by cation-exchange chromatography. The resulting actinium sample is then purified to remove iron impurities by anion-exchange chromatography in concentrated hydrochloric acid. To increase the degree of purification of actinium from thorium and radium impurities, the anion-exchange and cation-exchange purification processes can be repeated.

[0007] Based on the set of essential features, the closest (prototype) to the claimed method is the method for obtaining a purified actinium-225 preparation described in patent RU 2781190C1. This method consists of anion-exchange separation of thorium radionuclides by means of their sorption on the first column with a strongly basic anion exchanger from a nitric acid solution with a concentration of 8 to 10 mol / l, separation of actinium-225 and radium radionuclides on the second column with a strongly acidic cation exchanger in a nitric acid medium of variable concentration, purification of actinium-225 from iron salt impurities on the third column with a strongly basic anion exchanger in a hydrochloric acid medium with a concentration of 8 to 10 mol / l and subsequent purification of actinium-225 from organic impurities on the fourth column with a hydrophobic porous resin with a polyvinylbenzene structure in a hydrochloric acid medium with a concentration of 8 to 10 mol / l.Regeneration of the parent isotope thorium-229 for the purpose of re-accumulating actinium-225 is carried out by desorption from the first column with a strongly basic anion exchanger with a nitric acid solution with a concentration of 0.01 to 0.05 mol / l.

[0008] The disadvantages of the prototype are:

[0009] 1) Using dilute nitric acid (0.01 to 0.05 mol / L) to desorb thorium-229 from the first column with a strongly basic anion exchanger results in large volumes of desorbate and incomplete desorption of thorium-229. An increase in the volume of desorbate leads to an increase in the duration of the process of preparing the thorium-229 solution for subsequent technological operations, and incomplete desorption of thorium from the anion exchanger leads to an increase in the irreversible loss of the valuable parent isotope.

[0010] 2) To remove iron salts and organic impurities, concentrated hydrochloric acid (8 to 10 mol / L) is used in the third and fourth columns. Using hydrochloric acid at this concentration leads to intense corrosion of the stainless steel used in radiation protection and process equipment, thereby creating a high risk of contamination of the target radionuclide with corrosion product impurities (iron, chromium, and nickel salts).

[0011] To eliminate these shortcomings, the following technical solution is proposed:

[0012] 1) Desorption of thorium radionuclides from a column with a strongly basic anion exchanger is carried out with a solution containing a mixture of ammonium acetate and acetic acid.

[0013] 2) Purification from impurities of iron salts and organic impurities is carried out without the use of concentrated solutions of hydrochloric acid, while purification from the said impurities is carried out simultaneously, on one column containing two sorbents: the lower layer is a hydrophilic macroporous resin based on a simple ester of polymethylmethacrylic acid, the upper layer is a sorbent based on di(2-ethylhexyl)phosphoric acid (D2EHPA), applied to an inert carrier.

[0014] Using solutions containing a mixture of ammonium acetate and acetic acid to desorb thorium from a column with a strongly basic anion exchanger reduces the volume of desorbate obtained, increases the degree of thorium desorption, and thereby reduces the level of irreversible loss of the parent radionuclide thorium-229. The total concentration of ammonium acetate and acetic acid in the claimed method ranges from 0.5 to 4.0 mol / L, and the hydrogen ion concentration ranges from pH 3.5 to 5.0.

[0015] The minimum value of the total concentration of ammonium acetate and acetic acid is determined by the decrease in the completeness of thorium desorption from the anion exchange resin in more dilute solutions. Increasing the total concentration of ammonium acetate and acetic acid above 4.0 mol / L does not lead to an additional reduction in the desorbate volume and is therefore impractical. The specified pH range of ammonium acetate and acetic acid ensures the highest degree of thorium desorption from the anion exchange resin into a given desorbate volume. Outside this range, increasing the degree of thorium desorption requires a significant increase in the volume of the desorbent solution.

[0016] The use of a column containing the two specified sorbents for purifying actinium-225 from iron salt impurities and organic impurities allows both to eliminate the use of concentrated hydrochloric acid solutions and to reduce the duration of the technological cycle for obtaining the actinium-225 preparation by combining the operations of purifying from the specified impurities, performed sequentially in the prototype, on a single column, on the third and fourth columns.

[0017] The sorption of actinium-225 on a column containing the two specified sorbents (the lower layer is a hydrophilic macroporous resin based on polymethylmethacrylic acid ether, the upper layer is a sorbent based on D2EHPhK applied to an inert carrier), as well as subsequent column washing, should be carried out with hydrochloric acid solutions with a concentration in the range of 0.015 to 0.025 mol / L. Using hydrochloric acid with a concentration outside the specified range reduces the degree of purification of actinium-225 from radium radionuclides.

[0018] Elution of actinium-225 from a two-layer sorbent column should be performed with hydrochloric acid solutions with a concentration ranging from 0.5 to 1.0 mol / L. Using hydrochloric acid with a concentration of less than 0.5 mol / L increases the volume of actinium-225 eluate, while increasing the concentration above 1.0 mol / L reduces the degree of purification of actinium-225 from iron salt impurities and thorium radionuclides.

[0019] Example 1. Using a solution containing a mixture of ammonium acetate and acetic acid for desorption of thorium from an anion exchanger.

[0020] Thorium was isolated from a model mixture containing 10 mg of natural isotopic thorium and a thorium-227 radioactive indicator. Chromatographic columns containing 1 cm were used. 3BioRad AG 1x8 strongly basic anion exchange resin (100-200 mesh, NO3' form) was used. Thorium was adsorbed from 2 ml of 8 mol / L nitric acid, after which the columns were washed with 8 ml of 8 mol / L nitric acid. Thorium was desorbed using solutions of both dilute nitric acid and a mixture of ammonium acetate and acetic acid. The results are presented in Table 1.

[0021] As the table shows, using a solution containing a mixture of ammonium acetate and acetic acid significantly reduces the volume of thorium desorbate compared to the prior art. With equal desorbate volumes, the degree of thorium desorption using the claimed method exceeds that of the prior art. The best technical result for the claimed method is achieved with a total concentration of ammonium acetate and acetic acid ranging from 0.5 to 4.0 mol / L and a desorbing solution pH of 3.5 to 5.0.

[0022] Example 2. Purification of actinium-225 from iron salts and organic impurities. Purification was carried out from a model mixture containing radioactive indicators actinium-225, thorium-227, radium-223, and iron-59. Chromatographic columns containing 0.5 cm of thorium-227 as the upper layer were used. 3 Ln-resin (sorbent based on D2EGFK applied to an inert carrier) and as a bottom layer of 0.1 cm 3 Prefilter sorbent (hydrophilic macroporous resin based on polymethylmethacrylic acid ether). The volume of the model solution was 2 ml, the hydrochloric acid concentration was 0.015 mol / L. After the adsorption of the mixture components, the column was washed with 8 ml of a 0.015 mol / L hydrochloric acid solution, then with 5 ml of a 1.0 mol / L hydrochloric acid solution and 5 ml of deionized water. Thorium desorption was carried out with 10 ml of an ammonium carbonate solution with a concentration of 1.0 mol / L.

[0023] Radiometric analysis of the column eluate revealed that the radium-223 impurity yield in the fraction with a hydrochloric acid concentration of 0.015 mol / L was 99.6% of its initial amount. Actinium-225 breakthrough in this fraction was 0.04% of its initial amount, and the thorium-227 and iron-59 radionuclide content in this fraction was below the detection limits. The actinium-225 yield in the fraction with a hydrochloric acid concentration of 1.0 mol / L was 99.7% of its initial amount, and the iron-59 and radium-223 impurity content in this fraction was 0.25% and 0.01% of their initial amounts, respectively. The thorium-227 radionuclide content in this fraction was below the detection limits. The yield of radionuclides in the combined fraction of water wash and thorium desorbate was: iron-59 - 99.58%, thorium-227 - 99.9%, radium-223 - 0.39% and actinium-225 - 0.001% of their initial quantities.Thus, the purification factors for actinium-225 were: 400 from iron salts, 250 from radium radionuclides, and over 1000 from thorium radionuclides. Using 0.025 mol / L hydrochloric acid as the initial and washing solution, it was found that the yield of actinium-225 in the 1.0 mol / L hydrochloric acid eluate fraction was over 99.9% of its initial amount, while the content of iron-59 and radium-223 impurities in this fraction was 0.18% and 0.29% of their initial amounts, respectively. The content of thorium-227 radionuclide in this fraction was below its detection limits. Thus, the purification factors for actinium-225 were: from iron salts - 550, from radium radionuclides - 345, from thorium radionuclides - more than 1000.

[0024] When using 0.025 mol / L hydrochloric acid and 0.5 mol / L hydrochloric acid as the initial and washing solutions for eluting actinium-225, it was found that the yield of actinium-225 in this fraction was more than 99.9% of its initial amount, while the content of iron-59 and radium-223 impurities in this fraction was 0.30% and 0.37% of their initial amounts, respectively. The content of thorium-227 radionuclide in this fraction was below its detection limits. Thus, the purification factors for actinium-225 were: 330 from iron salts, 270 from radium radionuclides, and more than 1000 from thorium radionuclides.

[0025] The degree of actinium-225 purification from organic impurities was monitored visually by the color of the solution and by the presence of a dry residue after evaporation. The initial actinium-225 solution was obtained using a cation exchange method similar to that described in the prototype. After evaporation of this solution, a yellow-brown dry residue was detected, indicating the presence of organic impurities. After performing the actinium-225 purification process under the conditions described above, a colorless eluate solution was obtained, yielding no colored dry residue after evaporation to dryness. In all cases, a darkening of the lower layer of the sorbent in the column was observed, indicating the absorption of organic impurities.

[0026] Thus, the claimed method not only ensures the effective purification of actinium-225 from iron salts and organic impurities, but also allows for the simultaneous additional deep purification of actinium-225 from impurities of radium and thorium radionuclides.

[0027] Example 3. Mutual separation of actinium-225, radium-224,225 and thorium-228,229.

[0028] The initial product, a mixture of thorium-228 and thorium-229 radionuclides, was obtained by irradiating a target containing radium-226 in the SM-3 nuclear reactor (JSC SRC RIAR, Dimitrovgrad). After holding the product for the time required to accumulate the daughter actinium-225, the product was dissolved in 100 ml of nitric acid at a concentration of 8 mol / L and passed through a 50 cm column. 3BioRad AG 1x8 anion exchange resin (NO3', 100-200 mesh). The column was washed with 300 ml of nitric acid with a concentration of 8 mol / L. A mixture of thorium-228 and thorium-229 radionuclides was desorbed with 500 ml of a solution containing a mixture of ammonium acetate and acetic acid with a total concentration of 1.0 mol / L, having a pH of 4.0.

[0029] The resulting mixture of radium and actinium-225 radionuclides was evaporated to dryness, the residue was dissolved in 30 ml of nitric acid with a concentration of 8 mol / l and this solution was passed through a second column with 10 cm 3anion exchange resin BioRad AG 1x8 (NO3', 100-200 mesh). The column was washed with 60 ml of nitric acid with a concentration of 8 mol / L. A mixture of thorium-228 and thorium-229 radionuclides was desorbed with 50 ml of a solution containing a mixture of ammonium acetate and acetic acid with a total concentration of 1.0 mol / L, having a pH of 4.0. The solution of the mixture of radium and actinium-225 radionuclides purified on the second column was evaporated to dryness, the residue was dissolved in 20 ml of nitric acid with a concentration of 1.5 mol / L and this solution was passed through a column with 3 cm 3 BioRad AG 50x8 cation exchange resin (H*, 100-200 mesh). The column was washed with 130 ml of 1.5 mol / L nitric acid solution. Actinium-225 was eluted with 40 ml of 8 mol / L nitric acid.

[0030] The resulting actinium-225 solution was evaporated to dryness, the residue was dissolved in 2 ml of hydrochloric acid with a concentration of 0.015 mol / l and this solution was passed through a column containing a mixed layer of sorbents: 0.1 cm 3 Prefilter (bottom layer) and 0.5 cm 3Ln-resin (upper layer). The column was washed with 8 ml of 0.015 mol / L hydrochloric acid, and actinium-225 was then eluted with 5 ml of 1.0 mol / L hydrochloric acid. The actinium-225 eluate was evaporated to dryness. The dry residue was free of colored organic compounds, and after its dissolution in 4 ml of 0.1 mol / L hydrochloric acid, a colorless, transparent solution of the final actinium-225 preparation was obtained. The results of radiometric and spectrographic analysis of the obtained actinium-225 preparation are presented in Table 2.

[0031] The purification coefficients of the target actinium-225 from the radionuclides thorium-228, thorium-229, radium-224, radium-225 amounted to more than 3.0-10 7 , 1 ,6 • 10 7 , 1.5 • 10 7 , 2, 1 • 10 6 respectively.

[0032] Table 1. Dependence of the degree of thorium desorption on the composition of the solution used

[0033] Table 2. Characteristics of the obtained actinium-225 preparation

Claims

Invention formula 1. A method for the mutual separation of radium, actinium and thorium radionuclides, comprising anion-exchange separation of thorium to obtain a solution of a mixture of radium and actinium radionuclides, desorption of thorium from an anion exchanger, cation-exchange separation of actinium and radium, purification of actinium from iron salt impurities and organic impurities, characterized in that the desorption of thorium from a column with anion exchanger is carried out with a solution containing a mixture of ammonium acetate and acetic acid, and the purification of actinium from iron salt impurities and organic impurities is carried out on a column containing two sorbents: the lower layer is a hydrophilic macroporous resin based on a simple ester of polymethylmethacrylic acid, the upper layer is a sorbent based on di(2-ethylhexyl)phosphoric acid (D2EHPA) applied to an inert carrier.

2. The method according to claim 1, characterized in that the total concentration of ammonium acetate and acetic acid during the desorption of thorium is in the range from 0.5 to 4.0 mol / l, and the concentration of hydrogen ions is in the pH range from 3.5 to 5.

0.

3. The method according to paragraph 1, characterized in that the purification of actinium from impurities of iron salts and organic impurities is carried out on a column containing a Prefilter sorbent as the bottom layer and a Ln-resin sorbent as the top layer.

4. The method according to paragraph 3, characterized in that, for the purpose of additional purification of actinium from impurities of radium and thorium radionuclides, the column is washed with a solution of hydrochloric acid with a concentration in the range from 0.015 to 0.025 mol / l and then actinium is eluted from the column with a solution of hydrochloric acid with a concentration in the range from 0.5 to 1.0 mol / l.

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