• Aiming for elucidating the mechanism of selective separation of lanthanides and minor actinides from radioactive waste, we developed a novel infrared spectroscopy system adjusted for radioactive samples.
• We successfully obtained infrared spectra of complex ions formed between model ligands and a variety of lanthanoids as well as a minor actinide (americium, Am³⁺).
• For complexes involving the representative extraction ligand, DGA, an inverse correlation was found between the frequency of the C=O stretching vibration and the extraction efficiency.

  In the context of treating radioactive waste at nuclear power plants, the mutual separation of lanthanides (Ln) and minor actinides (MA), which have generally similar chemical properties, is a critical process. To achieve this, solvent extraction methods using organic ligands are commonly employed. However, the fundamental principles governing this mutual separation remain unclear. Elucidating these principles from a molecular scientific perspective is highly desirable, as it could lead to further improvements in separation efficiency through the rational design of ligands.

  Infrared spectroscopy stands out as a promising approach to address this challenge. However, the use of radioactive MAs is limited to radiation-controlled areas with restricted amounts for experiment, posing challenges for spectroscopic measurements of these compounds. To overcome this, we focused on surface-enhanced infrared absorption spectroscopy (SEIRA) and developed a novel SEIRA system capable of minimal sample measurements (Patent application No. 2021-161781). Utilizing this system, we successfully obtained infrared spectra of complex ions formed between model ligands and a variety of lanthanoids as well as a minor actinide (americium, Am³⁺). Notably, for complexes involving the representative extraction ligand, DGA, we made a significant discovery—an inverse correlation between the vibrational frequency of the C=O stretching mode in the spectra and the extraction efficiency obtained from solvent extraction experiments. This finding represents a promising step towards understanding the underlying principles of selective separation, and our ongoing project involves investigating the origins of this correlation.

Paper Info
Sakiko Hirata, Ryoji Kusaka, Shogo Meiji, Seita Tamekuni, Kosuke Okudera, Shoken Hamada, Chihiro Sakamoto, Takumi Honda, Kosuke Matsushita, Satoru Muramatsu, Takayuki Ebata, Daisuke Kajiya, Ken-ichi Saitow, Toshiaki Ikeda, Takehiro Hirao, Takeharu Haino, Masayuki Watanabe, Yoshiya Inokuchi: “Lanthanide and Actinide Ion Complexes Containing Organic Ligands Investigated by Surface-Enhanced Infrared Absorption Spectroscopy” Inorganic Chemistry, 62, 474-486 (2023).
https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c03618