Computational Insights into Mg²⁺ Dehydration in the Presence of Carbonate

Abstract

Water exchange around a free magnesium ion and magnesium paired with carbonate in aqueous solution was studied using free energy methods. Both a rigid-ion and a polarizable force field based on the AMOEBA model were examined. The parameters were adjusted to accurately reproduce the hydration structures of magnesium and carbonate in aqueous solution. The magnesium carbonate ion pairing free energies calculated with both force fields were found to be in excellent agreement with experimental data. Metadynamics simulations of the water exchange conducted with both models revealed that the formation of a contact magnesium carbonate ion pair significantly decreases the energy barrier for water exchange relative to the free magnesium ion in solution. This finding suggests that the presence of carbonate could accelerate the water exchange around magnesium and constitutes a first step toward a better understanding of the atomic-scale mechanisms involved in the nucleation of magnesium-bearing carbonate minerals.

Probing pH-Dependent Dehydration Dynamics of Mg and Ca Cations in Aqueous Solutions with Multi-Level Quantum Mechanics/Molecular Dynamics Simulations

“…Decreasing the pH decreases the number of hydroxides available to stabilize the lower-coordinate geometries, leading to their destabilization relative to the six-coordinate equilibrium geometry. These results are in line with classical FF-MD observations of the acceleration of water exchange when the symmetry of the hexa-aquo shell is broken by a negatively charged ligand. , …”

“…Aqueous calcium (Ca) and magnesium (Mg) cations are abundant in both geochemical as well as biological systems, , where their respective hydration dynamics are theorized to give rise to the observed differences in their chemical behavior. − On the geochemical side, dehydration of aqueous Ca 2+ and Mg 2+ is a key step in the formation of Ca and Mg carbonates, − which hold significant potential as carbon sinks for the capture of CO 2 from the Earth’s atmosphere. − Applications range from small-scale projects, where carbon capture and storage (CCS) by the go-to methodinjection of fluid CO 2 into geological formations for long-term storageis not feasible and instead mineralization to carbonates provides a valuable alternative, − to large-scale projects such as CO 2 mineralization to Ca and Mg carbonates in the direct capture of solvated CO 2 from seawater. , Here, proposed reactor designs such as the single-step carbon sequestration and storage (sCS2) approach put forth the use of mesh electrodes to generate a locally alkaline environment using electrolysis, inducing formation of CO 3 2– and mineralization to Ca and Mg carbonates . This approach has several advantages over traditional carbon removal solutions that utilize the injection of CO 2 into geological reservoirs as a storage mechanism, namely that seawater contains carbon dioxide at nearly 150 times its concentration in air, while thr Earth’s oceans are oversaturated with respect to Ca and Mg carbonates .…”

“…Despite the fundamental importance of Mg 2+ and Ca 2+ ion dehydration to such a wide range of processes of both biological and chemical importance, our understanding of the elementary atomic-level processes involved is rather limited. In particular, high-level theoretical and computational work aimed at resolving the dehydration dynamics of the aqueous cations has been sparse, with just a few studies utilizing density functional theory (DFT) molecular dynamics (MD) simulations, − while several have been carried out using classical force-field MD simulations, ,,− which can suffer from significant uncertainty in their description of quantum mechanically governed processes, such as bond breaking and forming. Here, we carry out multi-level electronic structure theory and dynamics calculations, combining rare-event sampling via DFT-MD simulations with high-level embedded correlated wavefunction (ECW) theory calculations. − This recently validated multi-level approach allows us to elucidate both dynamic and electron correlation effects on the dehydration of these aqueous cations with high accuracy.…”

Formation and Dissolution of Surface Metal Carbonate Complexes: Implications for Interfacial Carbon Mineralization in Metal Silicates

“…This barrierless growth is in contrast to the attachment events to the basal calcite surface, which is found to be a limiting growth step . Although the water exchange rate around magnesium ions is orders of magnitude lower than that around calcium, recent metadynamics simulations show that it becomes significantly higher when magnesium is present in the form of the Mg–CO 3 contact ion pair . Interestingly, Mergelsberg and Kerisit et al observe the formation of amorphous magnesium carbonate (AMC) nanoparticles when carbonating forsterite with wet supercritical CO 2 fluids.…”

“…77 Although the water exchange rate around magnesium ions is orders of magnitude lower than that around calcium, recent metadynamics simulations show that it becomes significantly higher when magnesium is present in the form of the Mg−CO 3 contact ion pair. 78 Interestingly, Mergelsberg and Kerisit et al 12 observe the formation of amorphous magnesium carbonate (AMC) nanoparticles when carbonating forsterite with wet supercritical CO 2 fluids. Akin to the calcium carbonate system, 79 this amorphous phase is a precursor to the formation of thermodynamically stable crystalline magnesium carbonate phases.…”