Complex Formation and Molecular Structure of Np(VI) and Np(V) Acetates

Neptunium is one of the most problematic actinides for long-term storage of the radioactive waste. Acetate (AcO-) is one of the important organic ligands occurring in natural environment. To study complexation reactions of Np(V) and Np(VI) with AcO-, we performed titration experiments based on UV-Vis-NIR spectroscopy. Furthermore, we used cyclic voltammetry to obtain information on the stability field of the individual solution species. However, it is difficult to extract structural information solely from these techniques. Therefore, we additionally adopted X-ray absorption fine structure (XAFS) spectroscopy to detect complexation reaction and molecular structure of Np(VI)- and Np(V)-AcO- complexes in aqueous solution.
The UV-Vis-NIR absorption spectra of the Np(VI)-AcO- system as a function of pH do not show any isosbestic points, indicating the presence of more than two equilibria. As a result of stability constant refinement, the spectral change is indicated as a 3-step complexation reaction of NpVIO2(AcO)n2-n (n = 1, 2, 3). The evaluated stability constants are log K1 = 2.98 ± 0.01, log b2 = 4.60 ± 0.01, and log b3 = 6.34 ± 0.01, which are comparable with the values reported previously. For comparison reasons the complex formation of Np(VI)-AcO- was investigated by XAFS spectroscopy (XANES and EXAFS). The refinement analysis of the XAFS spectra revealed similar stability constants (log K1 = 2.87 ± 0.03, log b2 = 4.20 ± 0.06, and log b3 = 6.00 ± 0.01). The k3-weighted EXAFS spectra of NpVIO2(AcO)n2-n (n = 0, 1, 3) provided structural data consistent with the complex stoichiometry, being well-explained by bidentate coordination of AcO- (Np-Oax: 1.76-1.77 Å, Np-Oeq: 2.43-2.47 Å, Np-Ccarbo: 2.87 Å, Np-Cterm: 4.38 Å) which are consistent with the crystal structure of NaNpVIO2(AcO)3.
Also in the UV-Vis-NIR spectrum of a pH series of the Np(V)-AcO- system, no isosbestic points were detected. The refinement analysis resulted also in 3-step complexation equilibrium of NpVO2(AcO)n1-n (n = 1, 2, 3) with the stability constants log K1 = 1.93 ± 0.01, log b2 = 3.11 ± 0.01, and log b3 = 3.56 ± 0.01. This result is corroborated by the structural data obtained from EXAFS (Np-Oax: 1.83-1.85 Å, Np-Oeq: 2.51 Å, Np-Ccarbo: 2.90-2.93 Å) which are comparable with the crystal structure of BaNpVO2(AcO)3.
The observed stability constants were used to calculate a Paurbaix diagram. The validity of the assumed complexation reactions was confirmed by comparing the boundary of the stability field between Np(V) and Np(VI) species with experimental Np(V/VI) redox potentials.