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Project Progress

The key kinetic barrier in dolomite crystal formation is believed to be the surface Mg²⁺-H₂O complex which hinder the accessibility of surface Mg²⁺ ions to the CO₃^2- ions in the solution (Lippmann, 1973). It has been demonstrate that dissolved hydrogen sulfide acting as a catalyst can promote dolomite nucleation and growth (Zhang, 2012). It is a consensus that the place where catalysts such as dissolved hydrogen sulfides promote dolomite formation is the surface of dolomite or solid precursor phase (Lippmann, 1973; and Yang et al., 2012). In order to understand the role of dissolved hydrogen sulfide in the dehydration of surface Mg²⁺ ions, ab initio simulations based on density functional theory (DFT) were carried out to study the thermodynamics of competitive adsorption of hydrogen sulfide and water on dolomite (104) surface from solution. Aqueous hydrogen sulfide among all the species of dissolved hydrogen sulfide was used in this modeling for it is believed to be concentrated on the dolomite surface. According to our simulation results, an aqueous hydrogen sulfide is energetically more strongly bonded to the dolomite surface with adsorption energy -57.26 kJ/mol (in vacuum modeling environment) or -20.77 kJ/mol (in solution modeling environment) lower than that of a water molecule. Thus, aqueous H₂S can be more strongly bonded to the dolomite surface than water, thus repulsing the water molecules from the surface.

Since even a small amount of hydrogen sulfide in the solution could promote the Mg²⁺ incorporation into the calcite structure (Zhang et al., 2012), the effect of adsorbed H₂S on the surface magnesium hydration bond is also important to the comprehensive understanding of the role of dissolved hydrogen sulfide. According to our study, the adsorbed aqueous H₂S on the surface does not chemically affect the Mg²⁺-H₂O bond but mechanically influences the bond distance. The aqueous hydrogen sulfide adsorbed on the surface can increase the surrounding surface Mg²⁺-H₂O distances and thus weaken the electrostatic interactions between surface Mg²⁺ and water molecules.

Since dissolved hydrogen sulfides are able to replace the adsorbed water molecules and weaken the surface Mg²⁺ hydration bond, the question is how carbonate groups manage to be attracted to the surface Mg²⁺ with the presence of hydrogen sulfides. One speculation is, similar to the H₂S effect on Mg²⁺-H₂O bond, that there is room for direct interaction of Mg²⁺ and CO₃^2- due to the large size and geometry of H₂S and larger space between H₂S and dolomite surface. It is also possible that the weakened Mg²⁺-H₂O interaction increases the competence of CO₃^2- to bond to the Mg²⁺. Detailed mechanisms will be explored in our future simulations.