  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. To characterize the role of dissolved hydrogen
sulfide in the dehydration of surface Mg 2+ 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)
surfaces from solution. 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). However, aqueous hydrogen
sulfide adsorbed on the surface increases the Mg2+-H2O distances on surrounding
surface sites. Two possible mechanisms were proposed for the catalytic effects
of adsorbed hydrogen sulfide on the anhydrous Ca-Mg-carbonate crystallization
at low temperature.
