Understanding the Stability of the Unusual Oxidation States of Americium in Different Mineral Acids

The oxidation of Am³⁺ and stabilization of its higher oxidation states are critical for Am/Cm separations and for understanding its fundamental chemistry. While most researchers focused on Am oxidation in HNO₃. Other reports indicate stabilization of AmO₂²⁺ and AmO₂⁺ in H₃PO₄ and HCl media, respectively, using NaBiO₃. However, the formation and stabilization of Am higher oxidation states in these ternary heterogeneous systems (NaBiO_3(s) + mineral acid_(aq) + Am_(aq)) remain unclear. In the present work, we investigated the behavior of NaBiO₃ in different acidic solutions (binary system) and extended it to a ternary system (NaBiO₃ + Am³⁺ in HCl). Although different mineral acids react uniquely with NaBiO₃, an increase in the initial pH after NaBiO₃ contact was observed in all cases, but to a different extent. The variations in the equilibrium pH and mineralogy of acid-treated NaBiO₃ suggest that H⁺ ions plays key role in Bi⁵⁺ → Bi³⁺ conversion. Additionaly the specific anion of the mineral acid appears to dictate the formation of additional mineral phases. Our findings suggest that the higher stability of AmO₂⁺ in HCl is primarily due to its elevated pH following NaBiO₃ contact, which prevents its reduction/disproportionation. Conversely, H₃PO_4, being a polyprotic acid, behaves differently and stabilizes AmO₂²⁺ mainly through phosphate complexation.