Uranyl (VI) and neptunyl (V) incorporation in carbonate and sulfate minerals: Insight from first-principles

Uranyl and neptunyl incorporation decreases the band gap on the order of 3-5 eV by creating mid-bandgap states. The band gap for aragonite without actinyl incorporation is 4.35 eV; with UO22+ incorporation the band gap decreases to 1.12 eV and with NpO2+ incorporation to 1.08 eV. For anhydrite, the band gap decreases from 6.39 eV (no incorporation) to 2.24 eV for UO22+ incorporation and to 1.04 eV for NpO2+ incorporation. Vibrational entropy changes during incorporation from solid sources were calculated for selected examples; however, the entropy contribution does not significantly lower the reaction Gibbs free energy relative to the enthalpy of incorporation. TΔS ranges from 0.01 eV for calcite to 0.04 eV for anglesite at room temperature. This correction is relatively small compared with other sources of error, in particular variations between different approaches to calculate hydration energies. While the presented approach may still include sources of uncertainty, especially with regard to changes in entropy and hydration, this methodology has promise as a valuable complement to determine the energetics of incorporation reactions.