Understanding the extraction mechanism, radiolytic stability and stripping behavior of thorium by ionic liquid based solvent systems: evidence of 'ion-exchange' and 'solvation' mechanism

Application of room temperature ionic liquid as potential 'green' solvent have been explored in nuclear fuel cycle. Thorium is the most important element in Indian scenario. In view of this, an attempt was made to understand the extraction mechanism of thorium using TBP, Cyanex 923 (tri-n-alkyl phosphine oxide), Cyanex 272 (Bis(2,4,4-trimethyl) pentyl phosphinic acid) and DHOA in C₈mimNTf₂. In case of TBP the extraction was found to proceed via 'solvation' mechanism through Th(NO₃)₄.2L species while for Cyanex 923 and Cyanex 272 it proceed via 'ion exchange' mechanism through (Th(NO₃)₂. 2L)²⁺. In case of Cyanex 923 and Cyanex 272, the C₈mim⁺ ion was found to be partitioned from ionic liquid phase to aqueous phase while no such participation was observed for TBP and DHOA based systems. NTf₂ - ion was found to be not participating in the overall extraction process revealing the non existence of 'anion exchange' mechanism. The extraction efficiency was found to follow the trend: Cyanex 923 > Cyanex 272 > DHOA > TBP. The extraction process was found to follow slower kinetics while the change in Gibb's energy was found to decrease in extraction indicating the spontaneity of the process. These ionic liquid based systems were found to be radiolytically stable, highly efficient and selective for Th while processing the simulated high level waste solutions of Fast Breeder reactor and Research Reactor origins. Oxalic acid was found to be suitable for almost quantitative stripping of Th from extracted ionic liquid phase