372632 Electrodeposition of Fission Platinoids in Ionic Liquids for Nuclear Waste Management
More than 57,000 metric tonnes of spent nuclear fuel (SNF) are stored on-site at the nuclear power plants in USA. The popular SNF reprocessing process PUREX (Plutonium Uranium Extraction) is based on dissolving the SNF in highly concentrated nitric acid followed by extraction of U and Pu with tributyl phosphate (TBP) diluted in volatile organic compounds (VOCs) such as kerosene and dodecane. The remaining fission products dissolved in nitric acid is called high level liquid waste (HLLW). Valuable metals such as fission platinoids (Pd, Rh, and Ru) could be obtained from the HLLW. In addition, recovery of the fission platinoids will reduce the volume of the waste to be managed. Ionic liquids (ILs) are suitable media for the extraction and recovery of the fission platinoids from the HLLW. Since ILs have negligible vapor pressure, they are much safer than the VOCs. ILs also have complexing ability; therefore, additional complexing agents are not needed. Also, due to its wide electrochemical window, metals having reduction potential more negative than that of the hydrogen reduction could be recovered through electroreduction. However, study of ILs for the recovery of the fission platinoids from the HLLW is limited. Due to numerous elements present in the HLLW, selective extraction and recovery of fission platinoids present a huge challenge.
In this work, Pd electrodeposition in suitable ILs was explored. Different ILs were investigated for the dissolution of representative Pd salts, PdCl2 and Pd(NO3)2. PdCl2 was used for benchmarking while Pd(NO3)2 is the form of Pd likely to be present in the HLLW. This was followed by study of electrochemical behavior of the Pd species in the ILs. In addition to the ILs used, the electrochemical behavior of Pd depends on its speciation. The speciation of the palladium complex is determined by the anions in the ILs. The speciation affects the potentials, kinetics and mass-transfer during the electroreduction. The insights gained from the study of the electrochemical behavior of palladium complexes in the ILs were used to understand and optimize electroreduction of the Pd onto a suitable substrate.