Interlayer confinement mediated oxidation of americium by sodium bismuthate and stability of its higher redox states in acidic solution

Abstract

Mutual separation of trans-plutonium actinides (Ans) from lanthanides (Lns) or adjacent Ans is challenging as they exhibit great chemical similarity. Selective Am oxidation-based Am–Lns or Am–Cm separation strategies give very high separation factors. There exist several reports on different aspects of Am³⁺ oxidation by NaBiO₃·xH₂O (x = 2–3), which suggests that the oxidation mechanism is still not well understood. We, therefore, carefully performed several experiments taking advantage of the difference in the redox chemistry of Eu³⁺ and Am³⁺. The ion exchange of Eu³⁺ with the interlayer Na⁺ in NaBiO₃ was confirmed by an ionic strength variation experiment. The shift in the 001 peak of NaBiO₃ in XRD and the appearance of peaks corresponding to the trivalent Ln ions in XRF after Ln³⁺ loading give direct evidence for Na⁺ exchange with Ln³⁺. The stability of the higher oxidation state of Am was monitored spectrophotometrically. The experimental investigations also suggested the role of Am³⁺ → AmO₂²⁺ transformation in NaBiO₃·xH₂O (x = 2–3) dissolution even at pH ∼ 1, which otherwise is difficult to achieve even with >1 M HNO₃. The role of colloidal NaBiO₃ particles and dissolved BiO₃⁻ in controlling or stabilizing different oxidation states of Am was also discussed.