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 Am and Eu 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 Am and Eu ions. The structure of the extracted complex was determined by EXAFS demonstrating the three amidic 'O' atoms of the HONTA complex with the Eu ion. In the case of the Am ion, the pivotal 'N' atom is suggested to bond to the metal ion, which may explain the significantly more favourable extraction of Amvis-à-vis Eu. The absence of HO molecules in the inner coordination sphere of the Eu-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 Nd ions. The results are explained on the basis of DFT calculations using HMNTA, the corresponding hexamethyl analogue of HONTA.

Combined Experimental and Density Functional Theoretical Studies of Am3+ and Eu3+ Extraction and Complexation with Different Nitrilotriacetamide (NTA) Derivatives

“…In the case of all the complexes, each of the two ligands bonded in tetradentate fashion having three‐coordination from the carbonyl oxygen atoms of the amide groups and one from the central ‘N’ atom. In all HRNTA derivatives, the ‘Am−N’ bond distances were found to be shorter than the ‘Eu−N’ bond distances (Figure ), which was reported earlier in case of the methyl derivative . The selectivity of this class of ligands can be explained on the basis of higher covalence in the ‘Am−N’ bonds as compared to that in the ‘Eu−N’ bonds as reflected in the shorter ‘Am‐ N ’ bond distances.…”

“…Am 3+ and Eu 3+ were extracted as 1 : 2 and 1 : 1 complex, respectively, from 0.2 M HNO 3 medium, whereas both were extracted as 1 : 1 complex from 1 M HNO 3 medium. Our previous study on the complexation of Eu 3+ with HONTA in methanol medium, however, indicated the formation of both 1 : 1 and 1 : 2 complexes and the corresponding solvent extraction studies in n ‐dodecane medium revealed that a 1 : 2 complex was extracted for both Am 3+ and Eu 3+ from 0.1 M HNO 3 medium, which is significantly different than the complexes reported by Wang et al. in the MIBK and methanol‐water (1 : 1) mixture.…”

“…In the case of Eu 3+ , however, a slope value of 1.71 was observed in the log‐log plot of the distribution ratio vs the H2EHNTA concentration, indicating some contribution of a 1 : 1 complex along with the 1 : 2 complex in the extract. In our previous studies using the n ‐octyl derivative (HONTA), we observed that both Am 3+ and Eu 3+ were extracted as 1 : 2 complexes from 0.1 M HNO 3 . Sasaki et al., however, reported that Am 3+ was extracted as 1 : 2 complex, whereas Eu 3+ was extracted as a 1 : 1 complex from 0.2 M HNO 3 and upon increasing the aqueous phase nitric acid concentration to 1 M, both the metal ions were extracted as 1 : 1 complex which could be due to the stronger competition from the nitrate ion at higher nitrate concentration.…”

Carrier mediated transport of actinides using hexa–n-hexylnitrilotriacetamide (HHNTA)

“…The same authors reported a reversal in this selectivity using a tripodal amide of nitrilotriacetic acid, termed as NTA amide [17]. This unique feature of this class of extractants has fascinated many researchers and there are a flurry of research activities using these ligands, of late [18][19][20]. We have recently studied the extraction of actinide ions such as UO 2 2+ , Np(IV), Pu(IV), etc.…”

HMNTA Complexes of Tetravalent Metal Ions: On the Roles of Carbonyl Oxygen and Amine Nitrogen in the Stabilization of Gas-Phase M(HMNTA)24+Complexes

“…Nitrilotriacetamide is regarded as a promising family of amides that are capable of extracting various metal ions, and the structures of metal complexes with nitrilotriacetamides − or nitrilotriacetic acids − have been substantially studied by single-crystal X-ray diffraction. It is generally accepted that trivalent/tetravalent metal cations and nitrilotriacetic acid/nitrilotriacetamide ligands tend to form 1:2 complexes, in which the ligands chelate with the metal center through at least three carbonyl oxygen atoms while the distances between the metal center and amine nitrogen are longer than usual.…”