Extraction of uranyl ion from nitric acid medium using solvent containing TOPO and its mixture with D2EHPA in room temperature ionic liquids

• Uranium(VI) extraction was studied using TOPO/D2EHPA/RTIL.
• The results are first of its kind involving actinide ions.
• The extraction mechanism follows solvation mechanism.
• The extraction trend is Pu(IV) > U(VI) > Am(III).

Studies on the extraction of uranyl ion were carried out from nitric acid feed solutions using solutions of tri-n-octyl phosphine oxide (TOPO) in several room temperature ionic liquids (RTIL), viz. [C4mim][PF6], [C6mim][PF6], [C8mim][PF6], [C4mim][NTf2], [C6mim][NTf2], [C8mim][NTf2]. All solvent extraction studies used 0.05 M TOPO in the respective RTILs. Out of the RTILs studied, those containing C8mim+ as the cationic part showed significantly higher extraction efficiency. The ionic liquids with PF6− as the counter anion showed faster extraction kinetics as well as higher extraction efficiency as compared to those containing NTf2− as the counter anion. The extraction studies carried out at varying concentrations of nitric acid indicated solvation mechanism of extraction similar to that displayed with molecular diluents such as n-dodecane. The extracted species were ascertained from the slope analysis method which indicated 1:2 (metal:ligand) ratio in the complex which displayed variations with the feed nitric acid concentration. UV–visible and fluorescence spectroscopic studies were also carried out to understand the nature of the extracted species.Solutions of D2EHPA and its mixture with TOPO in the RTILs were also used for the extraction of U(VI) from both HNO3 as well as H3PO4 medium. While the solvent system containing D2EHPA + TOPO in RTILs do not show appreciable variation with HNO3 concentration, a sharp decrease was seen with increasing H3PO4 concentration.