Extraction of Am(iii) using novel solvent systems containing a tripodal diglycolamide ligand in room temperature ionic liquids: a ‘green’ approach for radioactive waste processing

Abstract

Extraction of Am3+ from acidic feed solutions was investigated using novel solvent systems containing a tripodal diglycolamide (T-DGA) in three room temperature ionic liquids (RTIL), viz. [C4mim][NTf2], [C6mim][NTf2] and [C8mim][NTf2]. Compared to the results obtained with N,N,N′,N′-tetra-n-octyl diglycolamide (TODGA), T-DGA gave significantly higher distribution coefficients in these RTILs. The DAm values decreased with increasing carbon chain length in the RTILs, which was related to the solubility of the RTIL in the aqueous phase. The distribution studies included the effect of equilibration time, aqueous phase acid concentration variation and T-DGA concentration variation. In general, significantly higher equilibration times were observed for the extraction systems, which was partly due to the viscous RTIL phase and partly due to the slow conformational changes of the T-DGA ligand during complexation. Apart from Am3+, extraction of Pu4+, UO22+, Eu3+, Sr2+ and Cs+ was also investigated, since they have significant implications in radioactive waste processing. Stripping studies indicated >99% stripping in three stages using 0.5 M EDTA or DTPA in 1 M guanidine carbonate. Slope analysis indicated the extraction of 1 : 1 complexed species of Am(III) with T-DGA. Time resolved laser fluorescence spectroscopy (TRLFS) studies showed a strong complexation with no inner-sphere water molecules in the Eu(III)–T-DGA complexes for [C4mim][NTf2] as the diluent. Radiolytic degradation studies of the solvent systems containing T-DGA in the three RTILs were also carried out and while the DAm values decreased marginally when the solvents were exposed to 500 kGy absorbed dose, a relatively sharp decrease (60%) was seen when the solvents were exposed further to 1000 kGy absorbed dose, suggesting the possibility of recycling. Extraction studies were also carried out at varying temperatures and the thermodynamic parameters were calculated.