Understanding the extraction behaviour of UO22+ and Th4+ using novel picolinamide/N-oxo picolinamide in ionic liquid: A comparative evaluation with molecular diluent

Abstract

The manuscript deals with efficient separation of hexavalent UO22+ and tetravalent Th4+ from aqueous acidic waste solution in ‘green way’ using ionic liquid with novel picolinamide (L I) and N-oxo picolinamide (L II) based ligands. A comparative evaluation was carried out to understand the extraction mechanism, kinetics, thermodynamics, speciation, radiolytic stability and stripping behaviour of UO22+ and Th4+ in ionic liquid vis-a-vis molecular diluent. The investigation demonstrates the predominance of ‘cation exchange’ mechanism in ionic liquid and ‘solvation' mechanism in n-dodecane based systems. The slower extraction kinetics in ionic liquid was attributed to the ‘viscosity effect’. The extractive mass transfer processes were found to be spontaneous, endothermic and entropically driven in nature. The picolinamide and N-oxo picolinamide ligands were found to form inner-sphere complexes in ionic liquid as well as n-dodecane. In molecular diluent, the species involved for separation of UO22+ were UO2(NO3)2. 2 LI and UO2(NO3)2. L II, respectively, whereas in ionic liquid thet were [UO2(NO3). 2 L]+ and [UO2(NO3). L]+. For Th4+, in molecular diluent, the most plaussible extracted species were Th(NO3)4. 2 L I and Th(NO3)4. L II, while in ionic liquid they were [Th(NO3)2. 2 L]2+ and [Th(NO3)2. L]2+, respectively. Ionic liquid based solvent systems exhibited more radiolytic stability compared to that in molecular diluent. The CO32− was more effective aqueous phase complexing agent to back extract UO22+, while C2O42− exhibited the same for Th4+.