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

• Published in: Geochimica et cosmochimica acta 2015-07, Vol.161 (C), p.19-35
• Main Authors: Walker, Sarah M., Becker, Udo
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2015.03.002

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 (ΔEgyp,aqU=0.19eV and ΔEarag,aqU=0.27eV for incorporation from aqueous species compared with ΔEgyp,solidU=1.88eV and ΔEgyp,solidU=1.94eV if solid sources and sinks are used; for neptunyl incorporation, ΔEgyp,aqNp=0.36eV, ΔEgyp,solidNp=3.29eV; ΔEarag,aqNp=0.10eV, and ΔEarag,solidNp=3.02eV). 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 3–5eV by creating mid-bandgap states. The band gap for aragonite without actinyl incorporation is 4.35eV; with UO22+ incorporation the band gap decreases to 1.12eV and with NpO2+ incorporation to 1.08eV. For anhydrite, the band gap d