Accessibility and Reactivity of Pentavalent Plutonium Under Alkaline Conditions

Plutonium exhibits a particularly complex and rich aqueous chemistry due to a dynamic redox equilibrium that results in the coexistence of four different oxidation states in aqueous solution. Within this equilibrium, pentavalent plutonium, Pu(V)O2+, 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 high-level radioactive wastes. Despite this, the fundamental chemistry of Pu(V) under such conditions remains poorly understood. Here we demonstrate 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 precipitating crystalline Na2[Pu(V)O2(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[Pu(V)O2(OH)3] • 2H2O synthetically, as well as isolate two additional novel Pu(V)-bearing hydroxide phases Na[Pu(V)O2(OH)2(H2O)] • 2.5H2O and K2[Pu(V)O2(OH)3] • 2H2O as single crystals from alkaline reactions under mild hydrothermal conditions (<200°C) using Na2O2 and KO2 as reductants as rationalized and informed by our electrochemical measurements. The properties of the Pu(V) solids were further characterized using vibrational and optical spectroscopy in the solid state.