Effect of alkyl chain length on the extraction properties of U and Th using novel CnmimNTf2/isophthalamide systems

Abstract

An attempt was made to investigate the structural modification of CnmimNTf2 ionic liquid in combination with a novel ligand, 5-bromo-N1,N3-bis(1-methyl-1H-pyrazol-3-yl)isophthalamide on the extraction properties of tetra and hexavalent actinides. In C4mimNTf2, C6mimNTf2, the extraction processes predominantly followed the ‘cation exchange’ mechanism involving the species: [UO2(NO3)L]+ and [Th(NO3)2L]2+, whereas, in C8mimNTf2, the ‘solvation’ mechanism was found to be predominant involving neutral species UO2(NO3)2L2 and Th(NO3)4L3. The speciation was further confirmed by investigating the participation of imidazolium cation in extraction. In ionic liquid, the extraction processes were slower compared to that in the molecular diluent. The time required to attain the equilibrium D values followed the trend: C4mimNTf2 < C6mimNTf2 < C8mimNTf2. This was attributed to the viscosity induced slow mass transfer. At 1 M HNO3, the D values for uranyl and thorium follow the trend C4mimNTf2 > C6mimNTf2 > C8mimNTf2, which was ascribed to the enhancement in organophilicity of the cation on increasing the length of the side chain and hence the effect on their dissolution in water. Three cycles of 1 mM aqueous sodium carbonate solution and four cycles of 1 mM oxalic acid solution were sufficient for almost quantitative back extraction of U and Th, respectively.C8mimNTf2 based systems were better radio-resistant compared to the lower homologue.