Actinide and Lanthanide Species in Ionic Liquid and in Supercritical Carbon Dioxide: Dissolution, Coordination, and Separation

Due to global warming and dwindling petroleum reserves, seeking alternative energy sources has become an important issue in recent years. Nuclear energy is free of carbon emission and perhaps is the most efficient alternative energy source for our modern world. However, the wastes generated from nuclear power production are a big environmental problem. The traditional P&barbelow;lutonium U&barbelow;ranium R&barbelow;efining by E&barbelow;xtraction (PUREX) process is a well-established industry process for recycling uranium and plutonium from spent nuclear fuel. However, the problem of PUREX process is the generation of large quantities of radioactive aqueous and organic wastes. The development of green techniques for nuclear waste management using supercritical carbon dioxide (sc-CO2) and ionic liquid (IL) as solvents has become an active research area in recent years. To minimize the liquid waste generation, uranium dioxide (UO2 ) can be directly dissolved in a hydrophobic ionic liquid, 1-buty1-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][T2N]), with TBP(HNO3)1.8(H2O)0.6 as an complexing agent and the resulting uranyl complex can be extracted into sc-CO2. The ionic liquid in this dissolution/extraction process can be recycled for repeated use. The uranyl species extracted into sc-CO2 are identified to be UO2(NO3)2(TBP)2 and UO 2(NO3)2(TBP)2·HNO3. The dissolution rates of UO2 and neodymium oxide (Nd2O 3) powders in [Bmim][Tf2N] containing TBP(HNO3) 1.8(H2O)0.6 increase with increasing temperature or decreasing the viscosity of the IL. The dissolution rate constant of Nd 2O3 is faster than that of UO2 because dissolution of the latter requires oxidation of UO2 to (UO2) 2+. The activation energy (Ea), enthalpy (DeltaH ‡), and entropy (DeltaS‡) of the dissolution process are estimated using Arrhenius and Erying equations.;When UO2 and Nd2O3 are dissolved in [Bmim][Tf2N] containing TBP(HNO3)1.8(H 2O)0.6, the major uranyl and neodymium species are identified to be UO2(NO3)2(TBP)2 and Nd(NO 3)3(TBP)3 in the IL phase, respectively. The solubilities and distribution ratios of UO2(NO3)2(TBP) 2 and Nd(NO3)3(TBP)3 in sc-CO 2 phase are also measured by a high-pressure fiber-optic cell connected to a CCD array detector. The separation of uranyl and neodymium species from IL phase to sc-CO2 phase can be achieved by ligand exchange with a N,N,N'N'-tetrabutyldiglycolamide (TBDGA) in the IL solution. Complexes of (UO2)2+ and Nd3+ with TBDGA are not soluble in sc-CO2 and Nd3+ forms a stronger complex with TBDGA than (UO2)2+ in [Bmim][Tf2N]. Competition between (UO2)2+ and Nd3+ in this ligand exchange process provides a means for separation of UO 2(NO3)2(TBP)2 and Nd(NO3) 3(TBP)3 in the IL/sc-CO2 coupled extraction process. With a proper ratio of TBDGA to uranium and neodymium, the separation factor of [UO22+]/[Nd3+] can be as large as about 50. The IL/sc-CO2 hyphenated extraction techniques described in this dissertation may have potential applications in dissolution/extraction/separation of lanthanides and actinides existed in spent nuclear fuel and other nuclear wastes.