Structure and Dynamics of Water and Ions at Quartz (101) and (001) Surfaces under Applied Electric Fields from Molecular Simulations.

Electrical double layer (EDL) models are commonly adopted as a framework for understanding electrokinetic properties at mineral-fluid interfaces but the dynamics of ion and water mobilities are typically not well known. Extending the previous work performed at equilibrium conditions, here it is examined how applied electric fields induce mobilities of monovalent and divalent ions at hydroxylated quartz (001) and (101) interfaces with various electrolyte solutions (NaCl, KCl, and CaCl). The simulations reveal how the diffusion coefficients depend on the orientation and magnitude of the applied electric field, with a particularly strong effect for fields applied parallel to the quartz surfaces.

Decreasing Hygroscopicity Slows Forsterite Carbonation under Low-Water Conditions.

A fundamental understanding of processes that slow divalent metal silicate carbonation is important for developing effective strategies to durably store carbon dioxide and mitigate atmospheric CO. This study presents a detailed investigation of a passivation effect unique to low-water conditions during the carbonation of forsterite (MgSiO) and highlights the importance of hygroscopicity in influencing metal silicate carbonation. Integrated in situ and ex situ experimental results showed that the decrease in the carbonation rate of forsterite observed after ∼10 h in humid supercritical CO (50 °C, 90 bar) correlates with a reduction in water film thickness, and in particular, weakly hydrogen bonded adsorbed water that facilitates ion transport.

Impact of Interfacial Structure on Heterogeneous Nucleation of Amorphous Carbonates.

Classical molecular dynamics simulations were performed to provide physical insight into the impact of interfacial structure on the heterogeneous nucleation of amorphous calcium carbonate (ACC, CaCO·HO) and amorphous magnesium carbonate (AMC, MgCO·HO) by using α-quartz as a model substrate. Interfacial structure and energies were computed for ACC and AMC in contact with the (100), (001), and (101) α-quartz surfaces. The simulations showed α-quartz surfaces drew water molecules out of the carbonate nuclei to form a partial hydration layer.

Affinity for OH Produces Four-Coordinated Zn Impurities in Hydrated Amorphous Calcium Carbonate.

Using ab initio based molecular dynamics and electronic structure calculations, we show that Zn impurities in hydrated amorphous calcium carbonate (ACC) have a much lower coordination number than other divalent impurities due to covalent interactions between the 3d Zn shell and the oxygen atoms of the carbonate and water groups. The local structure around Zn in ACC, including the predicted low coordination number, is confirmed by X-ray absorption spectroscopy of synthetic Zn-bearing ACC. The strong Zn-O chemical interaction leads to substantial water dissociation and slightly disrupts the hydrogen bonding network.

Cobalt substitution slows forsterite carbonation in low-water supercritical carbon dioxide.

Cobalt recovery from low-grade mafic and ultramafic ores could be economically viable if combined with CO storage under low-water conditions, but the impact of Co on metal silicate carbonation and the fate of Co during the carbonation reaction must be understood. In this study, infrared spectroscopy was used to investigate the carbonation of Co-doped forsterite ((Mg,Co)SiO) in thin water films in humidified supercritical CO at 50 °C and 90 bar. Rates of carbonation of Co-doped forsterite to Co-rich magnesite ((Mg,Co)CO) increased with water film thickness but were at least 10 times smaller than previously measured for pure forsterite at similar conditions.

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 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: 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 MCO·HO systems, where M 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 molecular dynamics simulation, across a range of experimentally relevant cations (M = Ca, Mg, Sr) and hydration levels (0 ≤ ≤ 2).

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

Injecting supercritical CO (scCO) into basalt formations for long-term storage is a promising strategy for mitigating CO emissions. Mineral carbonation can result in permanent entrapment of CO; however, carbonation kinetics in thin HO films in humidified scCO is not well understood. We investigated forsterite (MgSiO) carbonation to magnesite (MgCO) via amorphous magnesium carbonate (AMC; MgCO·HO, 0.

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 (TcO) 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 [CaAl(SO)(OH)·26(HO)], a common mineral formed in cementitious waste forms, may be used to directly immobilize TcO.

Low temperature and limited water activity reveal a pathway to magnesite 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.

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

In geologic carbon sequestration, CO is injected into geologic reservoirs as a supercritical fluid (scCO). The carbonation of divalent silicates exposed to humidified scCO occurs in angstroms to nanometers thick adsorbed HO films. A threshold HO film thickness is required for carbonate precipitation, but a mechanistic understanding is lacking.

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)), boehmite (AlOOH), and alumina (AlO) 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.

Incorporation Modes of Iodate in Calcite.

Iodate (IO) incorporation in calcite (CaCO) 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.

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.

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.