Actinide dioxides such as NpO2 are key solid phases for prediction of environmental transport of actinides because of their extremely low solubility but potential for extensive dissolution and environmental mobility if oxidized to more the mobile actinyl species. Significant variability in actinide dioxide solubility data is often attributed to poorly characterized solid phases. Differences in particle and grain size, morphology, and crystallinity can affect not only solubility, but dissolution rate and mechanisms. Polycrystalline NpO2, produced on the HB-line at Savannah River Site, is dissolved under oxidizing conditions in the laboratory to determine dissolution kinetics over a 10-month period. While typical conceptual models would predict surface oxidation of neptunium and subsequent release of Np(V) as the dissolution mechanism, aqueous and solid phase data indicate a more complicated reaction mechanism. A two-step dissolution mechanism is proposed from kinetic data, with a hydration and subsequent oxidation step. Particular emphasis is placed on solid phase characterization before, during, and after dissolution using electron microscopy to illustrate dissolution mechanisms and surface alteration. Dissolution occurs preferentially at grain boundaries of the polycrystalline solid, leading to eventual disaggregation of solid matrix. Laboratory dissolution studies mirror existing field studies of HB-line NpO2 dissolution in the vadose zone and support observed mobility of a particulate Np phase. While dissolution mechanisms will certainly vary based on morphology, grain size, etc., the dissolution studies of polycrystalline NpO2 highlight the complexity of dissolution mechanisms for the actinide dioxides and suggest the need for more thorough characterization of solid phases used in dissolution and solubility studies.

Dissolution mechanisms of polycrystalline NpO2

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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