nai

Project Progress

The key kinetic barrier to dolomite formation is related to the surface Mg²⁺-H₂O complex, which hinders binding of surface Mg²⁺ ions to the CO₃^2- ions in solution. It has been proposed that this reaction can be catalyzed by dissolved hydrogen sulfide. We find that water is thermodynamically more stable on the surface with the difference in adsorption energy of -13.6 kJ/mol (in vacuum) and -12.8 kJ/mol (in aqueous solution).

Although water is more energetically stable on dolomite surface than aqueous hydrogen sulfide, our two-phase adsorption model predicts a small amount of H₂S adsorbed on the surface depending on the adsorption energy difference and pH of solution (affects the concentration of aqueous H₂S). At pH of 7.0~8.2 where the modern dolomite is precipitating, 6%~50% of dissolved sulfide are in aqueous H₂S phase. Usually more than 5mM dissolved sulfide was measured in pore water from some modern dolomite site. In order to test the effect of the surface H₂S on the Mg²⁺-water complex, a mixed layer of 50% H₂S and 50% H₂O molecules was placed above the dolomite surface in the presence of bulk water. We tested multiple configurations by using the same method as described in previous section and the lowest energy structure was obtained by inserting an extra water molecule between the bulk water and the adsorbed first layer. In the adsorbed mixed layer, the Mg²⁺-O_w distance increases from ~2.18 Å to ~2.23 Å compared to the respective case of pure layers (Table 1). Therefore, the adsorbed hydrogen sulfide will enhance surface water removal and dolomite crystallization. The surface water and adsorbed hydrogen sulfide on dolomite (104) surface, and a surface with mixed water and hydrogen sulfide are illustrated in figures below. Similar works on the effect of adsorbed polysaccharides on surface water removal is in progress.

Table 1. Energies and distances between surface Mg and O of adsorbed water for adsorption of a) 1 mono-layer (ML) of H₂O in solution; b) 1 ML of H₂S in solution; and c) a mixed H₂O and H₂S layer in solution.