III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Ionic liquid extraction of astatine
for a nuclear medical utilization
Y. Nagai,∗1 K. Aoi,∗1 S. Maruyama,∗1 A. Yokoyama,∗2,∗3 K. Washiyama,∗4 I. Nishinaka,∗5 Y. Wang,∗3 X. Yin,∗3A. Nambu,∗3 Y. Shigekawa,∗3 and H. Haba∗3
Astatine-211 is a nuclide with potential applicationsin α-ray internal therapy. Solvent extraction with an organic solvent has been used as a conventional method
for separating 211 At from the 209 Bi target irradiatedwith an α beam from an accelerator. Although organic
solvents are inexpensive and readily available, they arevolatile and ﬂammable, and therefore, they raise concerns about environmental pollution and handling of
dangerous substances.
In recent years, ionic liquids have attracted attention
as an alternative to organic solvents from the viewpointof green chemistry. Ionic liquids are in the liquid phaseat room temperature, have low volatility, and are ﬂameretardant: therefore, there is little environmental loador risk of accidents. Further, the amount of radioactive
waste can be reduced compared to that using organic
solvents because they can be used repeatedly. In addition, ionic liquids are considered promising solventsfor extracting 211 At given the reports on radiation resistance.1) However, few studies on the extraction of211
At with ionic liquids have been reported in the literature.2) Therefore, in this study, we investigated thesuitability of the HCl system, which has been reported
to show excellent performance for 211 At solvent extraction,2) depending on the type of ionic liquid.
The nuclide of 211 At was produced via the209Bi(α, 2n) reaction at the RIKEN AVF cyclotron
and delivered to Kanazawa University. The irradiated Bi target was dissolved in 3 mL of 6 M HNO3and mixed with an appropriate amount of H2 O toprepare a 1 M HNO3 solution, 7 mL of which wasused to extract the 211 At nuclide into 7 mL of dodecane solvent. After extracting 211 At to dodecanefrom HNO3 solution, it was further extracted to 0.1 M,1 M, and 3 M HCl solutions. Then, 211 At was extracted into ﬁve kinds of ionic liquids: [C8 mim]+[PF6 ]− , [C8 mim]+ [BF4 ]− , and [C4,6,8 mim]+ [Tf2 N]− ,where [Cn mim]+ and [Tf2 N]− stand for alkylimidazolium ion and bis(triﬂuoromethanesulfonyl)imide ion,
respectively. Finally, it was back extracted into 0.1, 1,and 3 M NaOH solutions. The radioactivity of 211 At
was measured with a liquid scintillation counter for theHCl solution samples before and after extraction and∗1∗2∗3∗4∗5Graduate School of Natural Science and Technology,Kanazawa University
Institute of Science and Engineering, Kanazawa University
RIKEN Nishina Center
Fukushima Global Medical Science Center, Fukushima Medical University
Quantum Beam Science Research Directorate, National Institutes for Quantum Science and Technology

Fig. 1. At extraction rates of ionic liquids from HCl solutions.

the NaOH solution samples after back extraction. The
extraction rate of 211 At from the HCl solution to theionic liquid and the back extraction rate from the ionicliquid to the NaOH solution were determined. Further,
the At chemical species in the HCl solution were inferred by the TLC analysis of dodecane before and afterextraction.
Figure 1 shows the extraction rates of 211 At fromthe HCl solutions for ﬁve ionic liquids of ion pairs
from three cations and three anions. When comparing cations, the larger the carbon chain, the higher isthe extraction rate. When comparing anions, the larger
the hydrophobicity, the smaller is the extraction rate.
The hydrophobicity of cations and anions in the ionicliquid aﬀects the ion exchange with At ions in the aqueous phase. In addition, the rates for [C8 mim] [Tf2 N]
have values exceeding 90% for both extraction and backextraction while [C8 mim] [PF6 ] and [C8 mim] [BF4 ] exhibit back extraction rates of less than 20%. Therefore,
one of the ion liquids studied is considered promisingfor 211 At solvent extraction in practical applications.
References
1) L. Yuan et al., J. Phys. Chem. B 113, 8948 (2009).
2) K. Kawasaki, Master thesis, Graduate School of Natural
Science and Technology, Kanazawa University (2020).

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