Separation of Am3+ and Eu3+ using hexa-n-octylnitrilo triacetamide (HONTA): complexation, extraction, luminescence, EXAFS and DFT studies

Abstract

This paper reports the solvent extraction of Am³⁺ and Eu³⁺ using N,N,N',N',N'',N''-hexa-n-octylnitrilotriacetamide (HONTA) as the extractant in n-dodecane. The results are in variance with those reported previously with respect to the nature of the extracted species. The solvent extraction data were entirely different from those reported previously as the extracted species conformed to 1 : 2 (M : L) species for both Am³⁺ and Eu³⁺ ions. The structure of the extracted complex was determined by EXAFS demonstrating the three amidic 'O' atoms of the HONTA complex with the Eu³⁺ ion. In the case of the Am³⁺ ion, the pivotal 'N' atom is suggested to bond to the metal ion, which may explain the significantly more favourable extraction of Am³⁺vis-à-vis Eu³⁺. The absence of H₂O molecules in the inner coordination sphere of the Eu³⁺-HONTA extract was confirmed by luminescence spectroscopic measurements. Complexation studies in MeOH and EtOH indicated the formation of both 1 : 1 and 1 : 2 complexes with Nd³⁺ ions. The results are explained on the basis of DFT calculations using HMNTA, the corresponding hexamethyl analogue of HONTA.