There is still an evident need for selective and stable ligands able to separate actinide(III) from lanthanide(III) metal ions in view of the treatment of the accumulated radioactive waste and of the recycling of minor actinides. We have herein demonstrated that hydrophilic 2,6-bis-triazolyl-pyridines are able to strip all actinides in all the different oxidation states from a diglycolamide-containing kerosene solution into an acidic aqueous phase. The ascertained high actinide selectivity, efficiency, extraction kinetics, and chemical/radiolytic stability spotlight this hydrophilic class of ligands as exceptional candidates for advanced separation processes fundamental for closing the nuclear fuel cycle and solving the environmental issues related to the management of existing nuclear waste.
The complexation of Cm(III) and Eu(III) with the novel i-SANEX complexing agent 2,6-bis[1-(propan-1-ol)-1,2,3-triazol-4-yl]pyridine (PTD) was studied by time-resolved laser fluorescence spectroscopy (TRLFS). The formation of 1:3, 1:2, and 1:1 metal/ligand complexes was identified upon increasing PTD concentration in 10 mol/L HClO and in 0.44 mol/L HNO solutions. For all these complexes, stability constants were determined at different acid concentrations. Though under the extraction conditions proposed for an An/Ln separation process, that is, for 0.08 mol/L PTD in 0.44 mol/L HNO, 1:3 complexes represent the major species, a significant fraction of 1:2 complexes was found. This is caused by ligand protonation, and results in lower Eu(III)/Am(III) separation factors compared to SO-Ph-BTP, until now considered the i-SANEX reference ligand. Focused extraction studies performed at lower proton concentration, where the 1:3 complex is formed exclusively, confirm this assumption.
Four lipophilic 1,10-phenanthroline di(thio)amide, diester or diketone derivatives were studied as ligands for Am(III)/Eu(III) separation from acidic media. The synthesis of these compounds is reported together with the extraction tests in different solvents (kerosene, octanol and o-nitrophenyl hexyl ether), HNO3 concentrations and ratios between the ligand and the synergistic agent (Br-Cosan). The promising results obtained from the large number of solvent extraction tests carried out show that it might be possible to apply this class of ligands to advanced reprocessing of spent nuclear fuel. The experimental data indicate that, under the conditions that simulate the real radioactive waste, the extraction efficiency and Am/Eu separation factors are particularly high, thus suggesting that the combination of soft heterocyclic N-donor atoms and hard carbonyl groups of ester and amides affords a tetradentate donor set of atoms (ONNO) that gives rise to remarkable selectivities. ESI-MS studies and DFT calculations shed light on the possible structure of the Eu(3+) complexes indicating that the 1 : 1 : 2 (cation : ligand : anion) complex is slightly more stable than the 1 : 2 : 1 species.
The applicability of 2,6-bis[1-(propan-1-ol)-1,2,3-triazol-4-yl)]pyridine (PyTri-Diol) in i-SANEX process as water-soluble complexing agent was studied. Preliminary batch experiments were aimed at identifying the optimal formulation of the PyTri-Diol solution and at preparing the ground for single-stage centrifugal contactor experiments. A TODGA-based solvent loaded using a spiked synthetic PUREX raffinate and the optimized PyTri-Diol aqueous phase were contacted in a single-stage annular centrifugal contactor setup with three different flow-rate conditions. No hydrodynamic problems were encountered and promising minor actinides separation from other cations was achieved with satisfactory kinetics and stage efficiency. The flow-sheet of a TODGA-PyTri-Diol based i-SANEX process was designed exploiting batch and single-stage data, promoting the CHON compliant PyTri-Diol as excellent alternative to the formerly used SO 3-Ph-BTP.
The challenging issue of spent nuclear fuel (SNF) management
is
being tackled by developing advanced technologies that point to reduce
environmental footprint, long-term radiotoxicity, volumes and residual
heat of the final waste, and to increase the proliferation resistance.
The advanced recycling strategy provides several promising processes
for a safer reprocessing of SNF. Advanced hydrometallurgical processes
can extract minor actinides directly from Plutonium and Uranium Reduction
Extraction raffinate by using selective hydrophilic and lipophilic
ligands. This research is focused on a recently developed N-heterocyclic selective lipophilic ligand for actinides
separation to be exploited in advanced Selective ActiNide EXtraction
(SANEX)-like processes: 2,6-bis(1-(2-ethylhexyl)-1H-1,2,3-triazol-4-yl)pyridine
(PyTri-Ethyl-Hexyl-PTEH). The formation and stability of metal–ligand
complexes have been investigated by different techniques. Preliminary
studies carried out by electrospray ionization mass spectrometry (ESI–MS)
analysis enabled to qualitatively explore the PTEH complexes with
La(III) and Eu(III) ions as representatives of lanthanides. Time-resolved
laser fluorescence spectroscopy (TRLFS) experiments have been carried
out to determine the ligand stability constants with Cm(III) and Eu(III)
and to better investigate the ligand complexes involved in the extraction
process. The contribution of a 1:3 M/L complex, barely identified
by ESI–MS analyses, was confirmed as the dominant species by
TRLFS experiments. To shed light on ligand selectivity toward actinides
over lanthanides, NMR investigations have been performed on PTEH complexes
with Lu(III) and Am(III) ions, thereby showing significant differences
in chemical shifts of the coordinating nitrogen atoms providing proof
of a different bond nature between actinides and lanthanides. These
scientific achievements encourage consideration of this PyTri ligand
for a potential large-scale implementation.
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