Local density changes and carbonate rotation enable Ba incorporation in amorphous calcium carbonate.

Incorporation of a Ba impurity in amorphous calcium carbonate (ACC) is shown with ab initio molecular dynamics simulations to have a long-range effect on its atomic-level structure and to be energetically favoured relative to incorporation in crystalline calcium carbonate polymorphs. The ability of carbonate ions to rotate and of ACC to undergo local density changes explain ACC's propensity for incorporating divalent metal impurities with a wide range of ionic radii. These findings provide an atomic-level basis for understanding the significant effects of low concentrations of impurities on the structure of ACC.

Structural water in amorphous carbonate minerals: ab initio molecular dynamics simulations of X-ray pair distribution experiments.

Water is known to play a controlling role in directing mineralization pathways and stabilizing metastable amorphous intermediates in hydrous carbonate mineral MCO3·nH2O systems, where M2+ is a divalent metal cation. Despite this recognition, the nature of the controls on crystallization is poorly understood, largely owing to the difficulty in characterizing the dynamically disordered structures of amorphous intermediates at the atomic scale. Here, we present a series of atomistic models, derived from ab initio molecular dynamics simulation, across a range of experimentally relevant cations (M = Ca, Mg, Sr) and hydration levels (0 ≤ n ≤ 2). Theoretical simulations of the dependence of the X-ray pair distribution function on the hydration level n show good agreement with available experimental data and thus provide further evidence for a lack of significant nanoscale structure in amorphous carbonates. Upon dehydration, the metal coordination number does not change significantly, but the relative extent of water dissociation increases, indicating that a thermodynamic driving force exists for water dissociation to accompany dehydration. Mg strongly favors monodentate conformation of carbonate ligands and shows a marked preference to exchange monodentate carbonate O for water O upon hydration, whereas Ca and Sr exchange mono- and bidentate carbonate ligands with comparable frequency. Water forms an extensive hydrogen bond network among both water and carbonate groups that exhibits frequent proton transfers for all three cations considered suggesting that proton mobility is likely predominantly due to water dissociation and proton transfer reactions rather than molecular water diffusion.

Thin Water Films Enable Low-Temperature Magnesite Growth Under Conditions Relevant to Geologic Carbon Sequestration.

Injecting supercritical CO2 (scCO2) into basalt formations for long-term storage is a promising strategy for mitigating CO2 emissions. Mineral carbonation can result in permanent entrapment of CO2; however, carbonation kinetics in thin H2O films in humidified scCO2 is not well understood. We investigated forsterite (Mg2SiO4) carbonation to magnesite (MgCO3) via amorphous magnesium carbonate (AMC; MgCO3·xH2O, 0.5 < x < 1), with the goal to establish the fundamental controls on magnesite growth rates at low H2O activity and temperature. Experiments were conducted at 25, 40, and 50 °C in 90 bar CO2 with a H2O film thickness on forsterite that averaged 1.78 ± 0.05 monolayers. In situ infrared spectroscopy was used to monitor forsterite dissolution and the growth of AMC, magnesite, and amorphous SiO2 as a function of time. Geochemical kinetic modeling showed that magnesite was supersaturated by 2 to 3 orders of magnitude and grew according to a zero-order rate law. The results indicate that the main drivers for magnesite growth are sustained high supersaturation coupled with low H2O activity, a combination of thermodynamic conditions not attainable in bulk aqueous solution. This improved understanding of reaction kinetics can inform subsurface reactive transport models for better predictions of CO2 fate and transport.

Low temperature and limited water activity reveal a pathway to magnesite via amorphous magnesium carbonate.

Forsterite carbonated in thin H2O films to magnesite via amorphous magnesium carbonate during reaction with H2O-bearing liquid CO2 at 25 °C. This novel reaction pathway contrasts with previous studies that were carried out at higher H2O activity and temperature, where more highly hydrated nesquehonite was the metastable intermediate.

Immobilizing Pertechnetate in Ettringite via Sulfate Substitution.

Technetium-99 immobilization in low-temperature nuclear waste forms often relies on additives that reduce environmentally mobile pertechnetate (TcO4-) to insoluble Tc(IV) species. However, this is a short-lived solution unless reducing conditions are maintained over the hazardous life cycle of radioactive wastes (some ∼10,000 years). Considering recent experimental observations, this work explores how rapid formation of ettringite [Ca6Al2(SO4)3(OH)12·26(H2O)], a common mineral formed in cementitious waste forms, may be used to directly immobilize TcO4-. Results from ab initio molecular dynamics (AIMD) simulations and solid-phase characterization techniques, including synchrotron X-ray absorption, fluorescence, and diffraction methods, support successful incorporation of TcO4- into the ettringite crystal structure via sulfate substitution when synthesized by aqueous precipitation methods. One sulfate and one water are replaced with one TcO4- and one OH- during substitution, where Ca2+-coordinated water near the substitution site is deprotonated to form OH- for charge compensation upon TcO4- substitution. Furthermore, AIMD calculations support favorable TcO4- substitution at the SO42- site in ettringite rather than gypsum (CaSO4·2H2O, formed as a secondary mineral phase) by at least 0.76 eV at 298 K. These results are the first of their kind to suggest that ettringite may contribute to TcO4- immobilization and the overall lifetime performance of cementitious waste forms.

Critical Water Coverage during Forsterite Carbonation in Thin Water Films: Activating Dissolution and Mass Transport.

In geologic carbon sequestration, CO2 is injected into geologic reservoirs as a supercritical fluid (scCO2). The carbonation of divalent silicates exposed to humidified scCO2 occurs in angstroms to nanometers thick adsorbed H2O films. A threshold H2O film thickness is required for carbonate precipitation, but a mechanistic understanding is lacking. In this study, we investigated carbonation of forsterite (Mg2SiO4) in humidified scCO2 (50 °C and 90 bar), which serves as a model system for understanding subsurface divalent silicate carbonation reactivity. Attenuated total reflection infrared spectroscopy pinpointed that magnesium carbonate precipitation begins at 1.5 monolayers of adsorbed H2O. At about this same H2O coverage, transmission infrared spectroscopy showed that forsterite dissolution begins and electrical impedance spectroscopy demonstrated that diffusive transport accelerates. Molecular dynamics simulations indicated that the onset of diffusion is due to an abrupt decrease in the free-energy barriers for lateral mobility of outer-spherically adsorbed Mg2+. The dissolution and mass transport controls on divalent silicate reactivity in wet scCO2 could be advantageous for maximizing permeability near the wellbore and minimize leakage through the caprock.

Electronic response of aluminum-bearing minerals.

Aluminum-bearing minerals show different hydrogen evolution and dissolution properties when subjected to radiation, but the complicated sequence of events following interaction with high-energy radiation is not understood. To gain insight into the possible mechanisms of hydrogen production in nanoparticulate minerals, we study the electronic response and determine the bandgap energies of three common aluminum-bearing minerals with varying hydrogen content: gibbsite (Al(OH)3), boehmite (AlOOH), and alumina (Al2O3) using electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and first-principles electronic structure calculations employing hybrid density functionals. We find that the amount of hydrogen has only a small effect on the number and spectrum of photoexcitations in this class of materials. Electronic structure calculations demonstrate that low energy electrons are isotropically mobile, while holes in the valence band are likely constrained to move in layers. Furthermore, holes in the valence band of boehmite are found to be significantly more mobile than those in gibbsite, suggesting that the differences in radiolytic and dissolution behavior are related to hole transport.

Incorporation Modes of Iodate in Calcite.

Iodate (IO3-) incorporation in calcite (CaCO3) is a potential sequestration pathway for environmental remediation of radioiodine-contaminated sites (e.g., Hanford Site, WA), but the incorporation mechanisms have not been fully elucidated. Ab initio molecular dynamics (AIMD) simulations and extended X-ray absorption fine structure spectroscopy (EXAFS) were combined to determine the local coordination environment of iodate in calcite, the associated charge compensation schemes (CCS), and any tendency for surface segregation. IO3- substituted for CO32- and charge compensation was achieved by substitution of Ca2+ by Na+ or H+. CCS that minimized the I-Na/H distance or placed IO3- at the surface were predicted by density functional theory to be energetically favored, with the exception of HIO3, which was found to be metastable relative to the formation of HCO3-. Iodine K-edge EXAFS spectra were calculated from AIMD trajectories and used to fit the experimental spectrum. The best-fit combination consisted of a significant proportion of surface-segregated IO3- and charge compensation was predominantly by H+. Important implications are therefore that pH should strongly affect the extent of IO3- incorporation and that IO3- accumulated at the surface of CaCO3 particles may undergo mobilization under conditions that promote calcite dissolution. These impacts need to be considered in calcite-based iodate remediation strategies.

Free-Energy Landscape of the Dissolution of Gibbsite at High pH.

The individual elementary reactions involved in the dissolution of a solid into solution remain mostly speculative due to a lack of direct experimental probes. In this regard, we have applied atomistic simulations to map the free-energy landscape of the dissolution of gibbsite from a step edge as a model of metal hydroxide dissolution. The overall reaction combines kink formation and kink propagation. Two individual reactions were found to be rate-limiting for kink formation, that is, the displacement of Al from a step site to a ledge adatom site and its detachment from ledge/terrace adatom sites into the solution. As a result, a pool of mobile and labile adsorbed species, or adatoms, exists before the release of Al into solution. Because of the quasi-hexagonal symmetry of gibbsite, kink site propagation can occur in multiple directions. Overall, our results will enable the development of microscopic mechanistic models of metal oxide dissolution.

The roles of outer membrane cytochromes of Shewanella and Geobacter in extracellular electron transfer.

As key components of the electron transfer (ET) pathways used for dissimilatory reduction of solid iron [Fe(III)] (hydr)oxides, outer membrane multihaem c-type cytochromes MtrC and OmcA of Shewanella oneidensis MR-1 and OmcE and OmcS of Geobacter sulfurreducens mediate ET reactions extracellularly. Both MtrC and OmcA are at least partially exposed to the extracellular side of the outer membrane and their translocation across the outer membrane is mediated by bacterial type II secretion system. Purified MtrC and OmcA can bind Fe(III) oxides, such as haematite (α-Fe2 O3 ), and directly transfer electrons to the haematite surface. Bindings of MtrC and OmcA to haematite are probably facilitated by their putative haematite-binding motifs whose conserved sequence is Thr-Pro-Ser/Thr. Purified MtrC and OmcA also exhibit broad operating potential ranges that make it thermodynamically feasible to transfer electrons directly not only to Fe(III) oxides but also to other extracellular substrates with different redox potentials. OmcE and OmcS are proposed to be located on the Geobacter cell surface where they are believed to function as intermediates to relay electrons to type IV pili, which are hypothesized to transfer electrons directly to the metal oxides. Cell surface-localized cytochromes thus are key components mediating extracellular ET reactions in both Shewanella and Geobacter for extracellular reduction of Fe(III) oxides.