Solvent Extraction Studies with Substituted Malonamides and Oligopyridines Influence of Structure and Chemical Properties on the Extraction Ability of Trivalent Actinides and Lanthanides
Separation and transmutation of spent nuclear fuel has been considered as a complement to direct disposal in a deep geological repository. The time needed for the waste to decay to natural background levels can be drastically decreased if the long-lived radionuclides, mainly the actinides, in the spent nuclear fuel are separated and transmuted to short-lived or stable nuclides. Critical reactors or subcritical accelerator-driven systems have been considered for the transmutation. An efficient chemical separation of long-lived actinides from fission products is necessary to achieve an efficient transmutation process. Solvent extraction techniques have been suggested as the separation method used prior to each transmutation cycle. Malonamides have been suggested as co-extracting agents for trivalent actinides and lanthanides from high acidity in a first step in such a separation process and nitrogen-donor extractants, such as oligopyridines in synergy with carboxylic acids, have shown potential to be able to separate the trivalent actinides from the lanthanides from low acidity in a second step.
The extractive behaviour and chemical properties of several substituted malonamides and oligopyridines have been studied in this work and have been related to the structure and the basicity of the ligands. It was found that the basicity of the malonamide was strongly related to its molecular structure, and a malonamide with lower basicity was shown to give a better metal extraction, owing to the less severe competition between protons and metal cations for the binding site in the malonamides. Malonamides with aromatic groups attached on the nitrogens were shown to have the lowest basicity of all studied ligands.
The basicity of the oligopyridines was also shown to be dependent on the molecular structure and extractants with low basicity generally resulted in a higher metal extraction. A synergistic mixtures with a carboxylic acid and terpyridine showed good selectivity for trivalent actinides over lanthanides. The main problem with terpyridine is that it is transferred to the aqueous phase when protonated. It was shown, however, that terpyridine present in the aqueous phase has a buffering effect which counteracts the transfer of extractant to the aqueous phase.