Electron Transfer Between Neptunium and Sodium Chlorite in Acidic Chloride Media

Controlling aqueous 5f-element electron transfer chemistry is critical for processing efforts associated with actinide technologies. Often, redox agents are added during actinide processing steps to control actinide redox chemistry and manipulate the actinide oxidation states for the separation. Sodium chlorite, NaClO2(aq), represents one of these useful redox agents. For example, NaClO2(aq) finds widespread application in the processing of plutonium and americium. Surprisingly, however, redox reactivity between NaClO2(aq) and other actinides, like neptunium, has been largely ignored. That knowledge gap is addressed herein. We characterized some redox reactivity between NaClO2(aq) and Np4+(aq) and identified experimental conditions that held neptunium in the +4 oxidation state or converted Np4+(aq) to NpO22+(aq) or NpO21+(aq). This was achieved by carefully adjusting four variables: ingoing concentrations of (1) Np4+(aq), (2) NaClO2(aq), (3) Cl1-(aq), and (4) H1+(aq). We discovered that three neptunium oxidation states (+4, +5, and +6) could be accessed using one ubiquitous redox agent, NaClO2(aq). These results highlight the diverse electron transfer chemistry available to neptunium in aqueous solutions, provide new insight on how neptunium reacts with NaClO2(aq), and are discussed within the context of their importance to plutonium and americium processing. Redox chemistry between Np4+(aq) and NaClO2(aq) can be controlled as a function of neptunium vs. NaClO2(aq), Cl1-(aq), and H1+(aq) concentrations. Certain chemical environments held Np4+(aq) in the +4 oxidation state. Other chemical environments generated NpO21+(aq) and/or NpO22+(aq).