Insights into the Mechanism of Neptunium Oxidation to the Heptavalent State

Abstract

Neptunium can exist in multiple oxidation states, including the rare and poorly understood heptavalent form. In this work, we monitored the formation of heptavalent neptunium [Np(VII)O₄(OH)₂]^3- during ozonolysis of aqueous MOH (M=Li, Na, K) solutions using a combined experimental and theoretical approach. All experimental reactions were closely monitored via absorption and vibrational spectroscopy to follow both the oxidation state and the speciation of neptunium guided by the calculated vibrational frequencies for various neptunium species. The mechanism of the reaction partly involves oxidative dissolution of transient Np(VI) oxide/hydroxide solid phases, the identity of which are dependent on the co-precipitating counter-cation Li⁺/Na⁺/K⁺. Additional calculations suggest that the most favorable energetic pathway occurs through the reaction of a [Np(V)O₂(OH)₄]^3- with the hydroxide radical to form [Np(VI)O₂(OH)₄]^2-, followed by an additional oxidation with HO⋅ to create [Np(VII)O₄(OH)₂]^3-.