MODELING THE EFFECT OF DISSOLVED HYDROGEN SULFIDE ON Mg2+-WATER COMPLEX ON DOLOMITE {104} SURFACE

It has been realized that the dolomite problem lie in the kinetic barriers at low temperature that hinder the incorporation of Mg²⁺ ions into the calcite structure because of strong surface Mg²⁺-water bond. Ca-rich dolomite has been synthesized at low temperature under laboratory conditions by using sulfate-reducing bacteria (Vasconcelos et al., 1995) and dissolved hydrogen sulfide (Zhang et al., 2010). However, the exact role of dissolved sulfide in overcoming the kinetic barrier is uncertain. On one hand, it has been proposed that dissolved hydrogen sulfide could be more strongly bonded to the dolomite surface than water, thus repulsing the water molecules from the surface. Our experiment result also shows that hydrogen sulfide can be adsorbed on the calcite and dolomite surface. On the other hand, it is possible that dissolved hydrogen sulfide with a low dielectric constant on dolomite surface could lower the dielectric constant of the surface solution and thus weakens the hydration bond between water and surface Mg²⁺.

In order to test the hypothesis that dissolved hydrogen sulfide lowers the energy barrier of dehydration of surface Mg²⁺-water complex, quantum mechanical calculations based on the density functional theory ( DFT) rather than empirically derived rules were carried out. This approach is efficiently implemented in the Vienna Ab Initio Simulation Package (VASP) which was used in this study. The system was relaxed using both the static energy minimization scheme and ab initio molecular simulations performed at 10K. At the first step, the adsorptions of a monolayer of water, HS^-, H₂S on dolomite {104} surface that is the main cleavage plane and crystal surfaces of dolomite were calculated respectively. Our results show that H₂S is more favorably bound to the surface than water. The binding between HS^- itself in vacuum is much stronger than the HS^-–surface interaction. The effect of HS^- and H₂S on the interfacial water structure and on the bond strength was calculated in the liquid water-hydrogen sulfide solution, in which HS^- ions or H₂S molecules are in the vicinities of Mg²⁺-water complex. The bulk water structure was calculated in molecular simulations using LAMMPS package initially and was relaxed in VASP eventually.