This research demonstrates the stability and reactivity of pentavalent plutonium in alkaline media, suggesting implications for managing radioactive wastes.
Plutonium exhibits a particularly complex and rich aqueous chemistry due to a dynamic redox equilibrium that can result in the coexistence of four different oxidation states in aqueous solution. Within this equilibrium, pentavalent plutonium is among the most soluble and dominant oxidation states of Pu in neutral to alkaline aqueous solutions of relevance to environmental chemistries and the extreme chemical environments encountered in high-level radioactive wastes. Despite this, the fundamental chemistry of Pu(V) under such conditions remains poorly understood. Here, we demonstrate the redox accessibility of Pu(V) in alkaline media based on the observations of an electrochemically reversible Pu(VI/V) couple at E1/2 = 0.19 V vs Hg/HgO (0.29 V vs NHE), allowing us to electrolytically generate Pu(V) in alkaline solution. These Pu(V) solutions remained stable under ambient conditions for at least 9 days before precipitation of crystalline Na₂[PuVO₂(OH)₃]·2H₂O, the first structurally characterized Pu(V) hydroxide. The modest potential of the Pu(VI/V) redox couple allowed us to reproduce Na₂[PuVO₂(OH)₃]·2H₂O synthetically as well as isolate two additional novel Pu(V)-bearing hydroxide phases Na[PuVO₂(OH)₂(H₂O)]·2.5H₂O and K₂[PuVO₂(OH)₃]·2H₂O as single crystals from alkaline reactions under mild hydrothermal conditions (<200 °C) using Na₂O₂ and KO₂ as reductants. This reactivity is rationalized and informed by our electrochemical measurements. The properties of the Pu(V) solids were further characterized by using vibrational and optical spectroscopy.
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Kravchuk et al. (2026) studied this question.
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