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

The oxidation of Am 3+ 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 HNO 3 . Other reports indicate stabilization of AmO 2 2+ and AmO 2 + in H 3 PO 4 and HCl media, respectively, using NaBiO 3 . 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 NaBiO 3 in different acidic solutions (binary system) and extended it to a ternary system (NaBiO 3 + Am 3+ in HCl). Although different mineral acids react uniquely with NaBiO 3, an increase in the initial pH after NaBiO 3 contact was observed in all cases, but to a different extent. The variations in the equilibrium pH and mineralogy of acid-treated NaBiO 3 suggest that H + ions plays key role in Bi 5+ → Bi 3+ 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 AmO 2 + in HCl is primarily due to its elevated pH following NaBiO 3 contact, which prevents its reduction/disproportionation. Conversely, H 3 PO 4, being a polyprotic acid, behaves differently and stabilizes AmO 2 2+ mainly through phosphate complexation.