Synthesis and evaluation of variously substituted 18-membered azamacrocycles for selective americium separation

Reprocessing spent nuclear fuel is of great interest for reducing the size and footprint of deep geological repositories. The selective separation of americium is particularly beneficial for decreasing the long-term radiotoxicity and heat load of nuclear waste. However, this separation remains extremely challenging due to the presence of trivalent lanthanides and curium. In this study, twelve substituted 18-membered azamacrocycles, six of which have not been reported earlier, were successfully synthesized in moderate to high yields. All ligands, except for the tetrapicolinic acid-substituted py₂-hexaazacrown ligand which exhibited insufficient solubility, were evaluated in solvent extraction experiments and compared with macropa. The ligands were tested in a modified TALSPEAK process using 0.05 M HDEHP in o-xylene as the organic phase and an aqueous phase containing 1 mM ligand and 1 M NaNO₃ in 0.05 M lactate buffer at pH 4. The tetrapicolinic acid-substituted tetraaza-18-crown-6 ether showed high selectivity for americium, yielding a Eu/Am separation factor of 911 ± 80, a Ln/Am separation factor greater than 10, and a Cm/Am separation factor of 2.0 ± 0.3 in combination with HDEHP. However, a contact time of at least 15 minutes was required for americium to reach equilibrium. Slope analysis indicated predominantly 1 : 1 complex formation with americium, curium, and the lanthanides. The other investigated ligands exhibited poor stripping efficiency and/or selectivity for americium.