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

摘要: Abstract The incorporation of radionuclides into low-temperature mineral hosts may strongly influence the concentration and migration radioactive contaminants in subsurface. One difficulty evaluating thermodynamics is that experiments are often performed at high supersaturations typically do not reach equilibrium. An alternative way to obtain equilibrium quantum–mechanical analysis host incorporated species before after incorporation. In this contribution, density functional theory used calculate energetics, resulting structures, electronic configuration uranyl (UO22+) neptunyl (NpO2+) sulfate carbonate minerals. each mineral, gypsum (CaSO4·2H2O), anhydrite (CaSO4), anglesite (PbSO4), celestine (SrSO4), barite (BaSO4), calcite (CaCO3), aragonite cerussite (PbCO3), strontianite (SrCO3), witherite (BaCO3), a divalent cation replaced with either UO22+ or NpO2+ (in case neptunyl, charge balance maintained an additional hydrogen ion). source actinyl ion sink for modeled as both solid aqueous phases, latter which requires expansion previous descriptions By combining periodic cluster computational methods, newly-developed approach enables simulation reactions between charged, molecular phases. Among minerals considered, most favorable uptake ( Δ E gyp,aq U  = 0.19 eV arag,aq  = 0.27 eV from compared gyp,solid  = 1.88 eV  = 1.94 eV if sources sinks used; incorporation, Np  = 0.36 eV,  = 3.29 eV;  = 0.10 eV, arag,solid  = 3.02 eV). Incorporation vacancy site, example by filling cation–anion uranyl-(CO32−, SO42−) neptunyl-(Cl−, HCO3−, HSO4−) pair, energetically more than substitution, mainly due thermodynamic instability defect site. Uranyl decreases band gap on order 3–5 eV creating mid-bandgap states. without 4.35 eV; 1.12 eV 1.08 eV. For anhydrite, 6.39 eV (no incorporation) 2.24 eV 1.04 eV Vibrational entropy changes during were calculated selected examples; however, contribution does significantly lower reaction Gibbs free energy relative enthalpy TΔS ranges 0.01 eV 0.04 eV room temperature. This correction relatively small other error, particular variations different approaches hydration energies. While presented still include uncertainty, especially regard hydration, methodology has promise valuable complement determine energetics reactions.