Lipophilic 2,6-bis-triazolyl-pyridines for minor actinide recycling: a comparative study

In the recent years, interest in the spent nuclear fuel partitioning and transmutation (P&T) is blooming, aiming to minimize the environmental footprint of nuclear energy. [1] In this endeavor, the separation of minor actinides from the high activity raffinate (HAR) of a PUREXlike process occupies a prominent position. In Europe, this goal has been addressed by different variants of a Selective ActiNide EXtraction (SANEX) process. The 1-cycle SANEX variant consists in the selective extraction of trivalent actinides from the HAR feed, using lipophilic extractants to selectively complex actinides(III), over lanthanides(III) and fission products. Nowadays, the ligand of reference for this type of process is CyMe4-BTBP, albeit suffering various problems. [2] In an attempt to overcome this problem and optimize the process, a new type of ligands has been proposed, boasting the promising PyTri core (2,6-bis(1H-1,2,3-triazol- 4-yl)-pyridine). The high performance of this N3 chelating unit in SANEX-like processes has been demonstrated, through the hydrophilic derivative PyTri-Diol. [3] Functionalization of the 1-position of the triazoles with alkyl chains has produced various lipophilic ligands. In the present work, two novel lipophilic ligands were studied and compared to 2,6-bis[1-(2- ethylhexyl)-1H-1,2,3-triazol-4-yl]-pyridine already investigated by the research group. [4] Their solubility in various organic diluents was studied, along with their efficiency and selectivity properties towards actinides(III), through liquid-liquid extraction testing. Nitric acid solutions spiked with 241Am and 152Eu, representing the trivalent actinides and lanthanides, respectively, were mixed with organic solutions containing the PyTri ligands. All three ligands showed comparable performance as regards selectivity. However, differences in extraction efficiency were observed, despite having the same complexing core. To better understand the potential role of the lateral chains, further testing was pursued. The effect of protonation was investigated by focused extraction tests. Furthermore, the ligand-cation speciation mechanism was explored through electrospray ionization mass spectrometry and UV-Vis spectrophotometric titration. For the purpose of comprehending the complex species involved in the extraction process, ad hoc monophasic solutions containing the ligands and the cations (stable La3+ and Eu3+) at different ratios as well as organic phases coming from extraction experiments were analyzed. The results for all ligands demonstrated a prevalence of the complex species with M:L 1:2 stoichiometry. The experimental work reported in the present study intends to contribute to a better understanding of the behavior of the PyTri class of ligands.