Molecular Dynamics Simulations of Solvation and Kink Site Formation at the {001} Barite−Water Interface

Solvation and kink site formation on step edges are known to be controlling parameters in crystal growth and dissolution. However, links from classical crystal growth models to specific reactions at the mineral−water interface have remained elusive. Molecular dynamics is used here to examine the water structure on barium surface sites and kink site formation enthalpies for material adsorbed to and removed from the step parallel to the [120] direction on the {001} barite-water interface. The bariums at the interface are shown to be coordinatively unsaturated with respect to water, and it is suggested that this is due to a steric hindrance from the nature of the interface. Kink site detachment energies that include hydration energies are endothermic for barium and exothermic for sulfate. The implications and problems of using these parameters in a crystal growth model are discussed

Growth Rate of Calcite Steps As a Function of Aqueous Calcium-to-Carbonate Ratio: Independent Attachment and Detachment of Calcium and Carbonate Ions

Growth rates of monolayer-height steps on the {101̅4} calcite surface have been measured as a function of the aqueous calcium-to-carbonate ratio. The maximum growth rates of the two common crystallographic orientations were found to deviate from the ideal stoichiometric ratio of 1:1, and dissolution features were observed under supersaturated solutions containing high calcium-to-carbonate ratios. To explain these phenomena, a theory is applied that treats the rates of attachment and detachment of aqueous calcium and carbonate ions separately. The resultant attachment rate constants are 1−3 orders of magnitude smaller than the water exchange rate of the constituent aqueous ions, suggesting that ligand-exchange processes may directly drive attachment. The broader implication is that the saturation state alone is not adequate to fully describe the rates of the multiple, independent reactions that occur on mineral surfaces under these conditions

Growth Kinetics and Morphology of Barite Crystals Derived from Face-Specific Growth Rates

We investigate the growth kinetics and morphology of barite (BaSO₄) crystals by measuring the growth rates of the (001), (210), (010), and (100) surfaces using vertical scanning interferometry. Solutions with saturation indices 1.1, 2.1, and 3.0 without additional electrolyte, in 0.7 M NaCl, or in 1.3 mM SrCl₂ are investigated. Face-specific growth rates are inhibited in the SrCl₂ solution relative to a solution without electrolyte, except for (100). Contrarily, growth of all faces is promoted in the NaCl solution. The variation of face-specific rates is solution-specific, which leads to a change of the crystal morphology and overall growth rate of crystals. The measured face-specific growth rates are used to model the growth of single crystals. Modeled crystals have a morphology and size similar to those grown from solution. Based on the model the time dependence of surface area and growth rates is analyzed. Growth rates change with time due to surface area normalization for small crystals and large growth intervals. By extrapolating rates to crystals with large surfaces areas, time-independent growth rates are 0.783, 2.96, and 0.513 mmol·m^–2·h^–1, for saturation index 2.1 solutions without additional electrolyte, NaCl, and SrCl₂, respectively

Growth Kinetics and Morphology of Barite Crystals Derived from Face-Specific Growth Rates

We investigate the growth kinetics and morphology of barite (BaSO₄) crystals by measuring the growth rates of the (001), (210), (010), and (100) surfaces using vertical scanning interferometry. Solutions with saturation indices 1.1, 2.1, and 3.0 without additional electrolyte, in 0.7 M NaCl, or in 1.3 mM SrCl₂ are investigated. Face-specific growth rates are inhibited in the SrCl₂ solution relative to a solution without electrolyte, except for (100). Contrarily, growth of all faces is promoted in the NaCl solution. The variation of face-specific rates is solution-specific, which leads to a change of the crystal morphology and overall growth rate of crystals. The measured face-specific growth rates are used to model the growth of single crystals. Modeled crystals have a morphology and size similar to those grown from solution. Based on the model the time dependence of surface area and growth rates is analyzed. Growth rates change with time due to surface area normalization for small crystals and large growth intervals. By extrapolating rates to crystals with large surfaces areas, time-independent growth rates are 0.783, 2.96, and 0.513 mmol·m^–2·h^–1, for saturation index 2.1 solutions without additional electrolyte, NaCl, and SrCl₂, respectively

Growth Kinetics and Morphology of Barite Crystals Derived from Face-Specific Growth Rates

We investigate the growth kinetics and morphology of barite (BaSO₄) crystals by measuring the growth rates of the (001), (210), (010), and (100) surfaces using vertical scanning interferometry. Solutions with saturation indices 1.1, 2.1, and 3.0 without additional electrolyte, in 0.7 M NaCl, or in 1.3 mM SrCl₂ are investigated. Face-specific growth rates are inhibited in the SrCl₂ solution relative to a solution without electrolyte, except for (100). Contrarily, growth of all faces is promoted in the NaCl solution. The variation of face-specific rates is solution-specific, which leads to a change of the crystal morphology and overall growth rate of crystals. The measured face-specific growth rates are used to model the growth of single crystals. Modeled crystals have a morphology and size similar to those grown from solution. Based on the model the time dependence of surface area and growth rates is analyzed. Growth rates change with time due to surface area normalization for small crystals and large growth intervals. By extrapolating rates to crystals with large surfaces areas, time-independent growth rates are 0.783, 2.96, and 0.513 mmol·m^–2·h^–1, for saturation index 2.1 solutions without additional electrolyte, NaCl, and SrCl₂, respectively

Magnesite Step Growth Rates as a Function of the Aqueous Magnesium:Carbonate Ratio

Step velocities of monolayer-height steps on the (101̅4) magnesite surface have been measured as functions of the aqueous magnesium:carbonate ratio and saturation index (SI) using a hydrothermal atomic force microscope. At SI ≤ 1.9 and 80–90 °C, step velocities were found to be invariant with changes in the magnesium:carbonate ratio, an observation in contrast with standard models for growth and dissolution of ionically bonded, multicomponent crystals. However, at high saturation indices (SI = 2.15), step velocities displayed a ratio dependence, maximized at magnesium:carbonate ratios slightly greater than 1:1. Traditional affinity-based models could not describe growth rates at the higher saturation index. Step velocities also could not be modeled solely through nucleation of kink sites, in contrast to other minerals whose bonding between constituent ions is also dominantly ionic in nature, such as calcite and barite. Instead, they could be described only by a model that incorporates both kink nucleation and propagation. On the basis of observed step morphological changes at these higher saturation indices, the step velocity maximum at SI = 2.15 is likely due to the rate of attachment to propagating kink sites overcoming the rate of detachment from kink sites as the latter becomes less significant under far from equilibrium conditions

Calcite Growth Rates As a Function of Aqueous Calcium-to-Carbonate Ratio, Saturation Index, and Inhibitor Concentration: Insight into the Mechanism of Reaction and Poisoning by Strontium

Using in situ atomic force microscopy, the growth rates of the obtuse and acute step orientations on the {101̅4} calcite surface were measured at two saturation indices as a function of the aqueous calcium-to-carbonate ratio and aqueous strontium concentration. The amount of strontium required to inhibit growth was found to correlate with the aqueous calcium concentration, but did not correlate with carbonate, suggesting that strontium inhibits attachment of calcium ions to reactive sites on the calcite surface. Strontium/calcium cation exchange selectivity coefficients, Kex, are estimated at 1.09 ± 0.09 and 1.44 ± 0.19 for reactive sites on the obtuse and acute step orientations, respectively. The implication of this work is that, to avoid poisoning calcite growth, the concentration of calcium should be higher than the quotient of the strontium concentration and Kex, regardless of the saturation index. Previous analytical models of nucleation of kink sites on steps are expanded to include growth rates at multiple saturation indices and the effect of strontium. The rate constants for calcium attachment are found to be similar for the two step orientations, but those of carbonate vary significantly. This work will have implications for natural or engineered calcite growth, such as to sequester subsurface strontium contamination

Upscaling Calcite Growth Rates from the Mesoscale to the Macroscale

Quantitative prediction of mineral reaction rates in the subsurface remains a daunting task partly because a key parameter for macroscopic models, the reactive site density, is poorly constrained. Here we report atomic force microscopy (AFM) measurements on the {101̅4} calcite surface of monomolecular step densities, treated as equivalent to the reactive site density, as a function of aqueous calcium-to-carbonate ratio and saturation index. Data for the obtuse step orientation are combined with existing step velocity measurements to generate a model that predicts overall macroscopic calcite growth rates. The model is quantitatively consistent with several published macroscopic rates under a range of alkaline solution conditions, particularly for two of the most comprehensive data sets, without the need for additional fit parameters. The model reproduces peak growth rates, and its functional form is simple enough to be incorporated into reactive transport or other macroscopic models designed for predictions in porous media. However, it currently cannot model equilibrium or pH effects and it may overestimate rates at high aqueous calcium-to-carbonate ratios. The discrepancies in rates at high calcium-to-carbonate ratios may be due to differences in pretreatment, such as exposing the seed material to SI ≥ 1.0 to generate/develop growth hillocks, or other factors

Nucleation Rate Theory for Coordination Number: Elucidating Water-Mediated Formation of a Zigzag Na₂SO₄ Morphology

Predicting and controlling nanostructure formation during nucleation can pave the way to synthesizing novel energy materials via crystallization. However, such control over nucleation and crystallization remains challenging due to an inadequate understanding of critical factors that govern evolving atomistic structures and dynamics. Herein, we utilize coordination number as a reaction coordinate and rate theory to investigate how sodium sulfate, commonly known as a phase-change energy material, nucleates in a supersaturated aqueous solution. In conjunction with ab initio and force field-based molecular dynamics simulation, the rate theoretical analysis reveals that sodium sulfate from an initially dissolved metastable state transits to a heterogeneous mixture of prenucleated clusters and finally to a large cylindrical zigzag morphology. Measurements of Raman spectra and their ab initio modeling confirm that this nucleated morphology contains a few waters for every sulfate. Rate processes such as solvent exchange and desolvation exhibit high sensitivity to the evolving prenucleation/nucleation structures, providing a means to distinguish between critical nucleation precursors. Desolvation and forming the first-shell interionic coordination structure via monomer-by-monomer addition around sulfates are found to explain the formation of large nuclei. Thus, a detailed understanding of the step-by-step structure formation across scales has been achieved. This can be leveraged to predict nucleation-related structures and dynamics and potentially control the synthesis of novel phase-change materials for energy applications

Adhesion of <i>Shewanella oneidensis</i> MR-1 to Iron (Oxy)(Hydr)Oxides: Microcolony Formation and Isotherm

The adhesion of dissimilatory metal reducing bacteria (DMRB) to iron (oxy)(hydr)oxides may play an important role in their respiration on ferric iron-containing minerals, but few quantitative surface cell density measurements have been made thus far. We used confocal microscopy to examine the adhesion of a common DMRB species, Shewanella oneidensis MR-1, onto iron (oxy)(hydr)oxide particulate-coated glass slides across a broad range of bulk (i.e., solution phase) cell densities from 105 cells/mL to 2 × 109 cells/mL. At bulk cell densities less than 1 × 107 cells/mL, cells adhered to the slide surface formed an evenly distributed, homogeneous monolayer, while at the bulk cell densities higher than 2 × 108 cells/mL the adhered cells formed distinct microcolonies. As a result of this complex adhesion behavior, simple Langmuir or Freundlich adsorption isotherms do not capture the relationship between the surface cell density and the bulk cell density over the entire range of bulk cell densities. Thus a new, two-step isotherm was developed that incorporated both isolated attached cells at low cell densities as well as microcolonies at higher cell densities