Towards a closed nuclear fuel cycle: ligands for the actinide-lanthanide separation

The present PhD thesis mainly focused on the design, synthesis and characterization of novel lipophilic and hydrophilic ligands for actinide and lanthanides separation from nuclear waste. The properties of these ligands in terms of extraction efficiency and metal selectivity are extensively studied with liquid-liquid extraction tests and with Time-resolved Laser-induced Fluorescence Spectroscopy. Most of the research work was focused on the development of An selective ligands, either hydrophilic or lipophilic, based on the 2,6-bis(1,2,3-triazol-4-yl)pyridine, PyTri, chelating unit. The extractants were obtained exploiting the Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction between the 2,6-diethynyl pyridine (para functionalized as needed) and a proper azide. After a first introductory chapter on the state of the art of spent nuclear fuel reprocessing, chapter 2 deals with studies aimed at the industrial scale implementation of the hydrophilic PTD stripping agent consisting in its scale-up synthesis and investigations on its degradation products under processing conditions. In chapter 3, in order to enhance PTD complexation properties, a revised ligand is presented, PTD-OMe, and the influence of the presence of the electron-donating methoxy group in para position of the pyridine ring on the metal ion complexation and extraction properties is studied. In chapter 4, seven different lipophilic ligands based on the PyTri chelating moiety were synthetized and their properties exhaustively described. Among all, the PTEH is the most promising lipophilic extracting agent. For this reason, a preorganized calixarene-based ligand was prepared bearing three PTEH moieties at the lower rim. Preliminary studies on its extraction properties and on the cooperation effect of the PyTri units are also reported. The last part of the work, described in chapter 5, was carried out at the University of Twente during a three months secondment. The work is focused on the design and synthesis of novel An and Ln lipophilic ligands based on diglycolamide (DGA) chelating unit, prepared via the Schotten-Baumann reaction, and exploiting a novel stereochemical control on metal ion binding.