In
industrial nuclear fuel reprocessing, small amounts of ruthenium
are extracted by tri-n-butyl-phosphate (TBP) at the
same time as uranium and plutonium. This behavior increases solvent
radiolysis and requires secondary extraction cycles to minimize the
residual ruthenium content in uranium and plutonium products. However,
the solvent ruthenium extraction mechanism remains largely unexplored.
This study addresses the speciation of ruthenium in solvent extraction
conditions by complementary infrared and X-ray absorption spectroscopy.
First, spectroscopic result interpretation is supported by a single
crystal X-ray diffraction study on reference compounds to unambiguously
demonstrate that the ruthenium extraction mechanism is driven by a
weak outer-sphere Ru–TBP interaction. Second, the ruthenium
coordination sphere is quantitatively characterized. Ruthenium speciation
in the organic phase depends on the initial aqueous phase, and both
monomeric ruthenium nitrosyl trinitrate complexes and a hydrolyzed
dimeric ruthenium nitrosyl complex are shown. Average coordination
numbers for nitrate, hydroxide, and aquo ligands are accurately determined
in both phases, by applying a constrained EXAFS fit approach.