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Description
Isolating ruthenium from spent nuclear fuel is essential both for reducing the environmental and technological burdens of high-level liquid waste management, particularly during vitrification, and for recovering a highly valuable platinum-group metal. Ruthenium poses a considerable challenge during its separation from spent nuclear fuel. That can be attributed to its high yield, wide range of oxidation states, and tendency to form complex compounds. Our research focuses on developing a simple, promising, and well-characterized method for the recovery of ruthenium from nitric acid solutions simulating high-level liquid waste generated during the reprocessing of alternative spent nuclear fuel. The proposed procedure follows a two-step approach. First, the ruthenium, present as nitrosyl complexes, is oxidized with sodium periodate (NaIO4). Different compounds are formed – some are volatile, others are absorbed onto the walls of plastic ampules. After that, upon addition of the organic phase, the subsequent processes transport previously oxidised Ru forms into it. Through reactions with the diluent, Ru species can be transported to the interphase or even back to the aqueous phase.
The influence of trichloromethane or n-dodecane on distribution coefficients D-values is discussed. In addition to the proposed oxidation–reduction mechanism of ruthenium, the influence of key experimental parameters on the D and extraction efficiency (%E) was studied. Particularly, the concentration of the oxidizing agent, the oxidation time, the nitric acid concentration, and the phase contact time. The optimal NaIO4 concentration was determined to be 75 mmol·L-1. With increasing oxidation time, D values increased significantly, with higher values observed in the n-dodecane system (D up to ~800) than in trichloromethane (D ~60). The nitric acid concentration significantly affected the extraction; the highest distribution coefficients were generally observed at lower HNO3 concentrations. Kinetic experiments showed that equilibrium is reached within hours.
In contrast, in the n-dodecane system, D decreased with prolonged contact time, likely due to the reduction of RuO4 to RuO2 at the phase interface. Separation factors against Pd and Rh were also determined, yielding β(Ru/Rh) ~ 1300 and β(Ru/Pd) ~ 600 for n-dodecane, confirming the high selectivity of the method for ruthenium extraction from light platinum metals. Rather than proposing a direct alternative for industrial separation, this work provides a deeper understanding of the physicochemical behaviour of ruthenium in these complex systems. Elucidating these fundamental interactions is crucial for optimising existing separation technologies, which ultimately contributes to the future recovery of platinum-group metals and the reduction of the environmental burden associated with nuclear waste.
We gratefully acknowledge the HORIZON-EURATOM-2021-funded FREDMANS project (GA No. 101060800), SGS project (SGS24/148/OHK4/3T/14) funded by the Czech Technical University in Prague, and the EU and Czech MEYS funded project CROP (CZ.02.01.01/00/22_011/0008569) for supporting this research.