37 Search Results

For this work, 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, CaCO₃·H₂O) and amorphous magnesium carbonate (AMC, MgCO₃·H₂O) 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. The formation of a partial hydration layer and its disruption to the ACC/AMC structure meant the α-quartz–ACC/AMC interfaces weremore » not energetically favored relative to separate α-quartz–water and ACC/AMC–water interfaces and, thus, homogeneous ACC/AMC nucleation was favored over heterogeneous nucleation. The CMD simulations hence provided an atomic-level explanation for a reported nonclassical growth mechanism whereby carbonate minerals grow via homogeneous nucleation and subsequent surface attachment of amorphous intermediates.« less

The thermodynamic and kinetic factors controlling the competitive heterogeneous nucleation and growth of ubiquitous metal carbonate and hydroxide phases are poorly understood. In this work, calcite (CaCO₃) and magnesite (MgCO₃) powders were reacted with NiCl₂ (0–600 μM) for 7 days at 22 °C and 5 °C. The reacted powders were analyzed with X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and energy-dispersive X-ray spectroscopy to characterize the Ni surface precipitates formed. Evidence from these techniques pointed to the formation of mixed Ni carbonate-Ni hydroxide amorphous surface precipitates. On calcite, XPS detected primarily Ni(OH)₂ despite the initial solutions being more supersaturated withmore » respect to gaspéite (NiCO₃) than to theophrastite (Ni(OH)₂) by a factor of 17–18. In contrast, NiCO₃ was the dominant component detected by XPS on magnesite in the same conditions. Decreasing the temperature had the effect of increasing the proportion of NiCO₃ to the detriment of Ni(OH)₂. The experimental observations were consistent with low lattice/cation size mismatch favoring NiCO₃ nucleation and temperature most influencing Ni(OH)₂ nucleation. Comparison to previous work on Co-reacted powders indicated the differences in lattice/cation size mismatch and/or water exchange rate impacted the composition of the surface precipitates more than the relative thermodynamic stabilities of the competing minerals. This work explored the heterogeneous growth regime of Ni carbonate and hydroxide phases on carbonate surfaces and shed light on the factors that control the competition between surface precipitates when mineral surfaces are in contact with aqueous solutions supersaturated with respect to multiple mineral phases. In conclusion, these results contribute to geochemists’ efforts toward interpreting data from geochemical systems with elevated Ni concentrations, improving Ni environmental remediation and recovery strategies, and predicting the fate and transport of Ni in geochemical systems.« less

A synthesis method for nanosized forsterite (Mg₂SiO₄) doped with varying concentrations of Ni and Co has been developed to support studies of carbonation-based extraction and separation of Ni and Co from mafic and ultramafic rocks. The protocol expands upon an existing sol–gel/surfactant method and is demonstrated for doping levels of 5% and 25% of Ni or Co. Variables such as metal reagents, surfactant ratios, and calcination procedures were optimized to achieve high specific surface areas and small particle sizes while minimizing secondary phase formation. Particle sizes ranged from 29 to 83 nm, and specific surface areas were between 11 andmore » 32 m² g⁻¹. Metal oxide impurities were minimal, appearing only in undoped and 25% Ni-doped samples at 0.6 wt% or less. Ni and Co were only detected in the +II oxidation state and partitioned predominantly in the M1 cation site of the forsterite crystal structure. Doped nanosized forsterites prepared with this method will enable in situ experiments that can track, at the molecular scale, the fate of Ni and Co during carbonation reactions and thus provide a knowledge base for improving metal extraction and separation technologies.« less

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. Further, 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. Implications of Zn²⁺ incorporation for ACC stability are discussed.

Cobalt recovery from low-grade mafic and ultramafic ores could be economically viable if combined with CO₂ 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, in situ infrared spectroscopy was used to investigate the carbonation of Co-doped forsterite ((Mg,Co)₂SiO₄) in thin water films in humidified supercritical CO₂ at 50 °C and 90 bar. Rates of carbonation of Co-doped forsterite to Co-rich magnesite ((Mg,Co)CO₃) increased with water film thickness but were at least 10 times smaller than previously measured for pure forsteritemore » at similar conditions. We suggest that the smaller rates are due to thermodynamic drivers that cause water films on Co-doped forsterite to be much less oversaturated with respect to Co-doped magnesite, compared to the pure minerals.« less

Temperature modulation was demonstrated as a novel way to control water partitioning during the reaction of silicate minerals with water-saturated supercritical carbon dioxide.

Ni and Co are critical elements for the world economy and modern technologies. Mafic and ultramafic deposits represent low-grade yet abundant alternatives to traditional Ni and Co ores. Here, in this work, density functional theory (DFT) with the Hubbard U correction (DFT+U) was used to simulate the incorporation of Ni and Co in forsterite (Mg₂SiO₄), the Mg endmember of olivine, a common mineral in mafic and ultramafic rocks. Hubbard U terms for Ni and Co were parametrized using a series of oxide, hydroxide, carbonate, silicate, and sulfide minerals relevant to extraction and recovery of Ni and Co from mafic andmore » ultramafic deposits. Electronic, energetic, magnetic, and structural properties were considered in the parametrization. For each of Ni and Co, an effective Hubbard correction (U_eff) value that optimized agreement with either experimental data or a hybrid exchange-correlation functional for all of the minerals considered is reported. DFT+U ab initio molecular dynamics (AIMD) simulations of Ni and Co incorporated into the M1 and M2 octahedral sites of forsterite were then performed. Ni and Co substitution in the M1 site was more energetically favorable than substitution in the M2 site, in agreement with published partition coefficients. AIMD trajectories were used to compute extended X-ray absorption fine structure (EXAFS) spectra of Ni in the M1 and M2 sites for direct fitting to a published experimental spectrum of Ni in a natural San Carlos olivine sample. The results of the fit indicated that ordering of Ni in the M1 site was not as strong at the low Ni concentrations relevant to mafic and ultramafic silicate minerals as that at the higher concentrations of the Ni-Mg olivine solid solutions studied to date.« less

Here, understanding and describing reactivity at mineral-water interfaces such as ion adsorption, the kinetics of dissolution, or surface charge development depends on our ability to improve the accuracy of electrical double layer (EDL) models. While molecular dynamics (MD) simulations are routinely used to investigate the structure and energetics of adsorbed ions comprising the EDL, less attention is paid to their self-diffusion dynamics, which can uniquely inform on coupling to interfacial reactions. Here we use MD to investigate both the organization and diffusion dynamics of water and electrolyte ions (NaCl, KCl, CaCl₂) at hydroxylated quartz (001) and (101) surfaces, a comparisonmore » which allowed us to assess surface structural effects of corrugation and silanol density. We found that inner- versus outer-sphere complex formation depends on cation size and charge but not necessarily hydration energies. Participation of surface silanols in the hydration spheres of Na⁺ and K⁺ generally indicated their preference for inner-sphere complexation, but this depends strongly on the orientation of the surface considered through its influence over the organization and dynamics of adsorbed water layers. In particular, surface orientation substantially affects the diffusive behavior of the near-surface water. Na⁺ was found to decrease the mobility of water in the first layer, consistent with an increasing frequency of hydrolysis implied by faster quartz dissolution rates observed in experiments via the well known salt effect. Our results are also in good agreement with the observed dissolution rate of quartz vs. surface adsorption strength measure by Dove and Nix. This study sets the stage for a forthcoming paper examining how the dynamics at quartz/electrolyte interfaces are influenced by externally applied electric fields.« less

The development of predictive models of minor element incorporation in crystalline carbonate end products requires an understanding of the fundamental controls on metastable intermediate phase composition. In this study, we used small-angle X-ray scattering (SAXS), X-ray pair distribution function (PDF), thermogravimetric analysis (TGA), inductively coupled plasma mass spectrometry (ICP-MS), and transmission electron microscopy (TEM) to determine the composition-dependent density of an amorphous calcium–strontium carbonate (ACSC) solid solution. The amorphous calcium carbonate (ACC) and strontium carbonate (ASC) measured densities were ρ_ACC = 2.19 ± 0.04 g/cm³ and ρ_ASC = 2.97 ± 0.05 g/cm³. Throughout our work, the experiments showed a dependencemore » of the water content of the amorphous solid solution on the Sr mole fraction. An equation that relates the molar volume to the average cation radius, the carbonate ion radius, and the water volume was parameterized for hydrated crystalline carbonates and predicted well the molar volume of the ACSC solid solution. This finding indicates that, as for hydrated crystalline carbonates, the molar volumes of amorphous carbonates are additive and that water is a structural component of ACSC. Ab initio molecular dynamics (AIMD) simulations of the ACSC solid solutions showed strong linear correlations between calculated molar volumes and Sr and H₂O contents, thus supporting the experimental results. In conclusion, our findings highlight the need to consider the full CaCO₃–MeCO₃–H₂O ternary when quantifying metal cation incorporation in ACC.« less

Developing an understanding of the response of mineral/water interfaces to applied electric fields is central to detecting and interpreting signatures of interfacial processes in the subsurface. Here we focus density functional theory calculations on understanding K⁺ cation binding and diffusion across the (001) surface of orthoclase feldspar under various applied electric fields with and without surface hydration. The calculations reveal how water ligands labilize surface K⁺ cations for diffusion while also increasing their sensitivity to electric field effects on the binding energy at different surface sites. The calculations also show how the direction and strength of the electric field systematicallymore » affect surface cation mobility, sorption, and hydration behavior. Specifically, electric fields directed toward the surface increase the potassium binding energies, facilitate K⁺ diffusion across multiple binding sites, and increase the propensity of surface K⁺ ions to lose their hydration shells. Furthermore, the findings help fill a major knowledge gap into the impact of electric fields on mineral/water interface structure and dynamics more generally, featuring in this case a commonly found type of feldspar involved in a multitude of atmospheric and geochemical processes.« less