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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)

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

The solubility and mobility of actinides (An), like uranium, neptunium, and plutonium, in the environment largely depends on their oxidation states. Actinyls (AnV,VIO2+/2+(aq)) form strong complexes with available ligands, like carbonate (CO32−), which may inhibit reduction to relatively insoluble AnIVO2(s). Here we use quantum-mechanical calculations to explore the kinetics of aqueous homogeneous reaction paths of actinyl tricarbonate complexes ([AnO2(CO3)3]5−/4−) with two different reductants, [Fe(OH)2(H2O)4]0 and [H2S(H2O)6]0. Energetically-favorable outer-sphere complexes (OSC) are found to form rapidly, on the order of milliseconds to seconds over a wide actinyl concentration range (pM to mM). The systems then encounter energy barriers (Ea), some of which are prohibitively high (>100 kJ/mol for some neptunyl and plutonyl reactions with Fe2+ and H2S), that define the transition from outer- to inner-sphere complex (ISC; for example, calculated Ea of ISC formation between UO2+ and UO22+ with Fe2+ are 35 and 74 kJ/mol, respectively). In some reactions, multiple OSCs are observed that represent different hydrogen bonding networks between solvent molecules and carbonate. Even when forming ISCs, electron transfer to reduce An6+ and An5+ is not observed (no change in atomic spin values or lengthening of An–Oax bond distances). Proton transfer from bicarbonate and water to actinyl O was tested as a mechanism for electron transfer from Fe2+ to U6+ and Pu6+. Not all proton transfer reactions yielded reduction of An6+ to An5+ and only a few pathways were energetically-favorable (e. g. H+ transfer from H2O to drive Pu6+ reduction to Pu5+ with ΔE = −5 kJ/mol). The results suggest that the tricarbonate complex serves as an effective shield against actinide reduction in the tested reactions and will maintain actinyl solubility at elevated pH conditions. The results highlight reaction steps, such as inner-sphere complex formation and electron transfer, which may be rate-limiting. Thus, this study may serve as the basis for future research on how they can be catalyzed by a mineral surface in a heterogeneous process.

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