Direct spectroscopic characterization of aqueous actinyl(VI) species: A Comparative Study of Np and U

For the first time the hydrolysis reactions of submillimoar aqueous Np(VI) and U(VI) solutions was investigated comparatively, applying Attenuated Total Reflection Fourier-transform Infrared (ATR FT-IR) and NIR absorption spectroscopy. Actinyl(VI) concentration was set to 500 µM in a pH range from 2 to 5.3 under ambient conditions. Both spectroscopic methods give evidence that three different Np(VI) species contribute to the speciation. Using the structural information obtained from the FT-IR spectra these species are the fully hydrated NpO₂ ²⁺ ion at pH ≤ 3, a monomeric complex probably formed by hydrolysis reactions at pH 3 − 5 and a carbonate containing complex at pH ≥ 5. A comparison of the obtained results to the updated NEA thermodynamic data confirms the presence of the free neptunyl(VI) ion and neptunyl(VI) carbonate complexation within the investigated pH range. The monomeric hydrolysis complex is not predicted to be relevant by the current thermodynamic data. In comparison to U(VI) forms structural similar species at pH ≤ 4, namely the fully hydrated AnO₂ ²⁺ and monomeric hydroxo species. In contrast to Np(VI), the infrared spectroscopic data of U(VI) does not evidence carbonate complexation at higher pH.