Solution and Interface Structure and Dynamics in Geochemistry: Gateway to Link Elementary Processes to Mineral Nucleation and Growth

Abstract

Our understanding of the process of mineral nucleation is starting to evolve from the solely thermodynamical aspects in classical nucleation theory toward a mechanistic study of reactions influenced by solvation structures of ions and their dynamics. A knowledge of how these atomic and molecular interactions respond to changing solution composition and/or the nature of an interface will form a fundamental foundation to understand rate-limiting reactions and processes for nucleation and growth. Here, we review recent advances from multidisciplinary experimental and computational approaches of the structure and dynamics of aqueous solutions and mineral–liquid interfaces that will influence rates of nucleation (and growth behavior) both homogeneously in aqueous solutions and heterogeneously at mineral–liquid interfaces. These processes include structure (and extent) of solute clustering, solvent degrees of freedom, and the presence of noninnocent charge-balancing ions, all of which can drive the selectivity and exert control over nucleation mechanisms. Similar controlling reactions can also occur at interfaces with the added complexities of how solute ions interact with and assemble on surface sites and use different mechanisms from bulk solutes, such as lateral solvent exchanges between distinct surface sites. To gain a predictive understanding of nucleation chemistry, we emphasize several key knowledge gaps that will need to be addressed in this field and highlight the importance of combining computations and experiments.

Nanopore facilitated monohydrocalcitic amorphous calcium carbonate precipitation

“…28 This surprising result may be qualitatively rationalized using classical nucleation theory applied to heterogeneous nucleation, in which the overall thermodynamic stability of a nucleus is expressed in terms of bulk and surface free energies. 4,55 The latter is a function of three component interfacial energies: substrate-precipitate, precipitate-solution, and substratesolution. When the ratio of the substrate surface area to solution volume is high, the substrate-precipitate interfacial energy may drive ACC formation and potentially phase selection.…”

“…Some of these phases are ubiquitous in the subsurface and play a central role in energy-relevant applications such as scale formation during oil and gas production, carbon sequestration, and transport of toxic metal contaminants. Amorphous calcium carbonate, ACC (CaCO 3 ÁnH 2 O) is significant in biomineral formation, and is recognized as one of the possible precursors in the formation of crystalline carbonate minerals based on the high energy barrier for homogeneous calcite nucleation in accordance with classical nucleation theory [1][2][3][4] (e.g., approximately 169 kJ mol À1 energy barrier 4 for formation of B0.8 nm critical radii using saturation index of 2 and interfacial energy of 117 mJ m À2 ). At the same time, there is significant evidence that biogenic ACC exists in a variety of polyamorphic forms; that is, various forms exhibit short-range order matching local atomic bonding of specific crystalline polymorphs.…”

Synergistic Enhancement of Lead and Selenate Uptake at the Barite (001)–Water Interface

“…To quantify the energetics of impurity incorporation at the molecular scale, a multistage free energy perturbation (MFEP) technique , was applied to the existing barite–Pb/SeO 4 system using the USER-FEP package in LAMMPS. This technique has been shown to be effective in sampling reaction pathways in condensed phase solvents with equilibrated structures. − It evaluates the ensemble average water structures that are critical to account for any reactions occurring in aqueous solutions where the hydration exchange rate is relatively rapid (e.g., aqueous barium ions have a water residence time of 208 ps, validated against experimental neutron scattering data), which is difficult or at least impractical to achieve by other methods that are currently available. Briefly, the surface barium and sulfate ions were progressively perturbed/morphed by linearly changing the potential energy Hamiltonian using a coupling parameter, η, to represent the transition from the initial state (no impurity incorporated) to the final state (impurity being fully incorporated) at intervals of η = 0.1 over 10 ns of the CMD simulation (Figure S2).…”

In Situ Observations of Barium Sulfate Nucleation in Nanopores

“…That is, based on analysis of the relative rates of nucleation and growth, such a rapid rate of nucleation of barite phase near the entrances of the nanopores has been shown to lead to solute depletion near the nuclei, in this case, within the interiors of the nanopores. Moreover, a potential rationalization of this phenomenon could be made using classical nucleation theory adapted for heterogeneous nucleation. , The nanoporous silica–water interface should be a higher interfacial energy substrate than a planar one, reducing the effective interfacial energy for nucleation and increasing heterogeneous nucleation rate. An alternate possibility might be that the nonbulk barite phase nucleates rapidly on the nanopore walls because of a templating effect from the silica, but is not stable for growth past a thin film of material due to strain buildup as the layer reaches a critical thickness.…”