Investigating Np(VI) Nitrate Speciation Control through Temperature and Optical Spectroscopy

Numerous areas of nuclear processing, such as radioisotope production, nuclear waste remediation, and separations, depend upon the speciation of f-elements in the solution state. However, fundamental knowledge of the actinidesparticularly Nplags behind most of the elements on the periodic table. Despite the importance of Np-(VI) chemistry in separations and nuclear processing, its speciation in HNO₃ remains uncertain. This work addresses some of these gaps by investigating the effect of HNO₃ concentration and temperature on the spectra and speciation of the Np-(VI) nitrate system. Six samples of Np in 1 to 10 M HNO₃ were prepared and treated with (NH₄)₂[Ce-(NO₃)₆] to stabilize the hexavalent oxidation state. The absorbance spectrum of the 10 M HNO₃ is drastically different from the spectra of the other samples; the 10 M spectrum indicates coordination of nitrate ligands to the Np-(VI)-O₂ ²⁺ ion to form a complex with high symmetry. Additionally, absorbance spectra were recorded over the temperature range of 15 to 40 °C, and systematic increases and decreases in certain spectral features of the ultraviolet-visible (UV-vis)-near-infrared (NIR) spectra are present in the spectra. The difference in spectral features suggests that Np-(VI) nitrate speciation depends on temperature. Dilution studies of this sample were monitored with UV-vis-NIR and Raman spectroscopies. Spectral data indicate the presence of multiple complexes with different symmetries, including a high-symmetry complex with an inversion center. As samples are diluted and aquo ligands replace nitrate ligands bound to the Np-(VI) neptunyl cation, changes in spectroscopic signals indicate a decrease in the high-symmetry nitrate complex and an increase in the lower-symmetry aquo complex. Additionally, the Raman band associated with the Np-(VI)-O₂ ²⁺ symmetric stretch broadens with dilution and is fitted with two peaks. These peaks are assigned to Np-(VI) aquo and nitrato complexes, indicating that this vibrational mode is sensitive to the coordination environment of the Np-(VI) neptunyl cation and that contributions from these two Np-(VI) complexes can be distinguished. This unique experimental data could be used to advance computational models describing the electronic transitions of complex actinyl ions.