The incorporation of radionuclides into mineral hosts may strongly influence the concentration and migration of radioactive contaminants in the subsurface. Density functional theory is used to calculate the energetics, optimized structure, and electronic configuration of uranyl (UO22+) and neptunyl (NpO2+) incorporation into sulfate (gypsum, anhydrite, anglesite, celestine, barite) and carbonate minerals (calcite, aragonite, cerussite, strontianite, witherite). Periodic and cluster computational methods are combined as separate chemical equations in which each species is calculated under identical computational parameters. With this newly-developed approach, reactions between charged, aqueous molecular species and solid mineral phases can be simulated quantum mechanically. Incorporation energy is calculated by replacing a host mineral cation with either a uranyl or a neptunyl ion. In the case of neptunyl(V) incorporation, charge balance is maintained with an additional hydrogen ion. Gypsum and aragonite are the most favorable hosts for both uranyl (?E = 0.19 eV for gypsum and 0.27 eV for aragonite) and neptunyl uptake (?E = 0.17 eV and -0.37 eV, respectively). Incorporation into a vacancy site is energetically more favorable than cation substitution. The vibrational entropy of solids was also calculated, although the entropy contribution does not significantly (< 0.2 eV) lower the reaction free energy relative to other sources of error, including hydration energy.

First-principles insight into uranyl (VI) and neptunyl (V) incorporation in carbonate and sulfate minerals

248th National Meeting (2014)

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