Complexation of Hexavalent Neptunium(VI) with Oxydiacetic Acid and Its Amide Derivatives in Aqueous Solution: Spectrophotometry and DFT Calculations

Unfolding the solution coordination chemistry of high-valent transuranium elements with the "CHON"-type ligands is important to understand the fundamental chemistry of actinides and to design more efficient extractants for partitioning of transuranium elements in advanced nuclear fuel cycles. Here, the complexation of a hexavalent neptunyl ion (NpO22+ or Np(VI)) with oxydiacetic acid (ODA) has been systematically investigated in comparison with its amide analogues N,N-dimethyl-3-oxa-glutaramic acid (DMOGA) and N,N,N',N'-tetramethyl-3-oxa-glutaramide (TMOGA) both experimentally and computationally. The formation of both 1:1 and 1:2 complexes between Np(VI) and the three ligands was identified by spectrophotometry, and their stability constants were obtained and compared with those of hexavalent U(VI) and Pu(VI). The corresponding bonding nature is elucidated by using energy decomposition analysis (EDA), electrostatic potential (ESP), ELF contours, and natural orbitals for chemical valence (NOCV) methods, which shows that the Np-O bonds are essentially ionic in character and the unoccupied 6d orbitals of Np play a key role in enhancing the covalent interactions between Np(VI) and the three ligands.

Extraction of Nitric Acid and Uranium with DEHiBA under High Loading Conditions

The mechanism by which high concentrations (1.5 M in n-dodecane) of N,N-di-2-ethylhexyl-isobutyramide (DEHiBA) extracts HNO₃ and UO₂(NO₃)₂ is under examination. Most prior studies have examined the extractant and the mechanism at a concentration of 1.0 M in n-dodecane; however, under the higher loading conditions that can be achieved by a higher concentration of extractant, this mechanism could change. Increased extraction of both nitric acid and uranium is observed with an increased concentration of DEHiBA. The mechanisms are examined by thermodynamic modeling of distribution ratios, ¹⁵N nuclear magnetic resonance (NMR) spectroscopy, and Fourier transform infrared (FTIR) spectroscopy coupled with principal component analysis (PCA). Speciation diagrams produced through thermodynamic modeling have been qualitatively reproduced through PCA of the FTIR spectra. The predominant extracted species of HNO₃(DEHiBA), HNO₃(DEHiBA)₂, and UO₂(NO₃)₂(DEHiBA)₂ are in good agreement with prior literature reports for 1.0 M DEHiBA systems. Evidence for an additional species of either UO₂(NO₃)₂(DEHiBA) or UO₂(NO₃)₂(DEHiBA)₂(HNO₃) also contributing to the extraction of uranium species is given.