Abstract:Incorporation reactions play an important role in dictating immobilization and release pathways for chemical species in low-temperature geologic environments. Quantum-mechanical investigations of incorporation seek to characterize the stability and geometry of incorporated structures, as well as the thermodynamics and kinetics of the reactions themselves. For a thermodynamic treatment of incorporation reactions, a source of the incorporated ion and a sink for the released ion is necessary. These sources/sinks in a real geochemical system can be solids, but more commonly, they are charged aqueous species. In this contribution, we review the current methods for ab initio calculations of incorporation reactions, many of which do not consider incorporation from aqueous species. We detail a recently-developed approach for the calculation of incorporation reactions and expand on the part that is modeling the interaction of periodic solids with aqueous source and sink phases and present new research using this approach. To model these interactions, a systematic series of calculations must be done to transform periodic solid source and sink phases to aqueous-phase clusters. Examples of this process are provided for three case studies: (1) neptunyl incorporation into studtite and boltwoodite: for the layered boltwoodite, the incorporation energies are smaller (more favorable) for reactions using environmentally relevant source and sink phases (i.e., ΔE rxn (oxides) > ΔE rxn (silicates) > ΔE rxn (aqueous)). Estimates of the solid-solution behavior of Np to predict the limit of Np-incorporation into boltwoodite (172 and 768 ppm at 300 °C, respectively); (2) uranyl and neptunyl incorporation into carbonates and sulfates: for both carbonates and sulfates, it was found that actinyl incorporation into a defect site is more favorable than incorporation into defect-free periodic structures. In addition, actinyl incorporation into carbonates with aragonite structure is more favorable than into carbonates with calcite structure; and (3)

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 (UO 2 2+) and neptunyl (NpO 2 +) incorporation into sulfate and carbonate minerals. In each host mineral, gypsum (CaSO 4 2H 2 O), anhydrite (CaSO 4), anglesite (PbSO 4), celestine (SrSO 4), barite (BaSO 4), calcite (CaCO 3), aragonite (CaCO 3), cerussite (PbCO 3), strontianite (SrCO 3), and witherite (BaCO 3), a divalent cation is replaced with either UO 2 2+ or NpO 2 + (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 (∆E ୷୮,ୟ୯

Mineral wettability and wettability alteration are important factors that determine the distribution and mobility of oil during the recovery process. Because wettability is dependent on many factors (e.g., hydrocarbon composition, mineralogy, and pH), predicting mineral wettability is often difficult. The goal of this study is to look at changes that occur on the mineral itself, specifically changes in the surface structure and surface potential, using experimental methods complemented by quantum-mechanical calculations to better understand the molecular-level processes involved in wettability alteration. Nanoscale surface imaging is combined with Kelvin probe force microscopy (KPFM) to characterize changes in topography and surface potential for water-wet (hydrophilic) and oil-wet (hydrophobic) calcite surfaces, using the surfactant hexamethyl­disilazane (NHSi2(CH3)6, HMDS) to render the calcite surface oil-wet. KPFM measurements show that HMDS adsorbs preferentially on step edges of the calcite surface and is coupled by an increase in surface potential, which suggests a decrease in electron density in the valence band wherever HMDS is adsorbed. Density functional theory (DFT) calculations of HMDS adsorption on calcite confirm an increase in the surface potential of oil-wet calcite and show that Ca corner sites are associated with the most favorable HMDS adsorption energies. Coadsorption of H+ and OH– with HMDS is more likely to occur at edges and Ca kink sites and indicates that this surfactant may be an effective wettability modifier at a range of pH conditions. This study is the first application of KPFM to mineral wettability and demonstrates that with further development KPFM can be a powerful tool to study interactions between specific functional groups and surface sites modifying the surface’s electronic structure and wettability.

Hexavalent chromium is a highly toxic and readily mobile metal contaminant introduced to the environment through a variety of industrial operations. In the presence of reductants, such as Fe(II), and catalytic mineral surfaces, such as iron oxides surfaces, Cr(VI) may be reduced to a less toxic and relatively insoluble form Cr(III). In this study, we investigate the interaction between Cr(VI) and the surface of the Fe(II)-bearing mineral magnetite, Fe(II)Fe(III) 2 O 4 , as an example catalyst, using electrochemical atomic force microscopy (EC-AFM). With this method, the redox potential is controlled by an electrode, and Cr deposition on the magnetite surface is imaged over time as a function of redox potential and pH of the solution. Quantitative analyses of volumetric growth and surface coverage reveal that more precipitation occurs over time at very negative (-500 mV at pH 3,-750 mV at pH 7, and-1000 mV at pH 11) and very positive (+1000 mV at pH 3 and +500 mV at pH 11) electrochemical potentials. Up to 70% of the surface is covered with precipitates at pH 7, while less coverage is observed at pH 11 (< 8%) and pH 3 (< 2%). Particle growth at pH 3 is predominantly lateral in nature with a tendency to form a higher number of smaller adsorbate particles. At pH 11, growth is primarily vertical (perpendicular to the surface), and smaller particles tend to aggregate into larger clusters on the surface with increasingly negative redox polarization. These