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

Abstract The incorporation of radionuclides into low-temperature mineral hosts may strongly influence the concentration and migration of radioactive contaminants in the subsurface. One difficulty in evaluating the thermodynamics of incorporation is that experiments are often performed at high supersaturations and typically do not reach equilibrium. An alternative way to obtain the equilibrium thermodynamics is the quantum–mechanical analysis of the mineral host and the incorporated species before and after incorporation. In this contribution, density functional theory is used to calculate the energetics, resulting structures, and electronic configuration of uranyl (UO22+) and neptunyl (NpO2+) incorporation into sulfate and carbonate minerals. In each host mineral, gypsum (CaSO4·2H2O), anhydrite (CaSO4), anglesite (PbSO4), celestine (SrSO4), barite (BaSO4), calcite (CaCO3), aragonite (CaCO3), cerussite (PbCO3), strontianite (SrCO3), and witherite (BaCO3), a divalent cation is replaced with either UO22+ or NpO2+ (in the case of neptunyl, charge balance is maintained with an additional hydrogen ion). The source of the actinyl ion and the sink for the host cation are modeled as both solid and aqueous phases, the latter of which requires an expansion of previous descriptions of incorporation. By combining periodic and cluster computational methods, this newly-developed approach enables the quantum–mechanical simulation of reactions between charged, aqueous molecular species and solid mineral phases. Among the host minerals considered, gypsum and aragonite are the most favorable hosts for both uranyl and neptunyl uptake ( Δ E gyp,aq U = 0.19 eV and Δ E arag,aq U = 0.27 eV for incorporation from aqueous species compared with Δ E gyp,solid U = 1.88 eV and Δ E gyp,solid U = 1.94 eV if solid sources and sinks are used; for neptunyl incorporation, Δ E gyp,aq Np = 0.36 eV, Δ E gyp,solid Np = 3.29 eV; Δ E arag,aq Np = 0.10 eV, and Δ E arag,solid Np = 3.02 eV). Incorporation into a vacancy site, for example by filling a cation–anion vacancy with a uranyl-(CO32−, SO42−) or neptunyl-(Cl−, HCO3−, HSO4−) pair, is energetically more favorable than cation substitution, mainly due to the thermodynamic instability of the defect site. 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.