The removal of the minor actinides americium(III) and curium(III) from spent nuclear fuel is currently being studied worldwide as part of an ongoing strategy to reduce its long-term heat load and radiotoxicity. This strategy aims to close the back end of the nuclear fuel cycle and ultimately increase the safety and sustainability of civil nuclear energy. Within the framework of several EU research programs, promising solvent extraction processes have been developed that involve the selective extraction of the trivalent minor actinides from the lanthanides from aqueous nitric acid into an organic diluent containing a hydrophobic bis-triazinyl pyridine (BTP), bis-triazinyl bipyridine (BTBP) or bis-triazinyl phenanthroline (BTPhen) ligand. Recent studies have focused on modified ligands containing electron donating or withdrawing substituents attached to the aromatic rings of these ligands, and their effects on minor actinide extraction performance and actinide/lanthanide selectivity.
In contrast, less emphasis has been placed on modifying the aliphatic part of these ligands. Here we present our recent studies on modified bis-triazine ligands (BTP, BTBP and BTPhen) containing 5-membered rings appended to the outer triazine rings, instead of 6-membered rings, and the effects of this modification on ligand solubility, minor actinide extraction performance and actinide/lanthanide selectivity. Using a combination of solvent extraction experiments, NMR titrations and time-resolved laser fluorescence spectroscopy, it has been shown that changing the ring size of the aliphatic part of these ligands has subtle effects on metal ion speciation, complex stability and minor actinide extraction performance and selectivity.