Modeling absorption and segregation of magnesium and cadmium ions to calcite surfaces:: Introducing MgCO3 and CdCO3 potential models

We introduce a potential model for MgCO3 and CdCO3 which reproduces experimental structural parameters and solution energies. We employed atomistic simulation techniques to model the absorption and segregation of magnesium and cadmium ions to the low-index surfaces of calcite. Both magnesium and cadmium ions absorb at the surfaces from solution. The absorption energies are surface dependent, due to distinct relaxations of the different surfaces, and ion dependent with the smaller sized magnesium ion showing larger relaxations. The absorption energies are larger for magnesium than for cadmium, mainly due to the closer coordination of the water molecules to the magnesium substituted surfaces. The cadmium ions segregate to the bulk crystal more easily than the magnesium ions which prefer to remain at the surface. However, segregation energies for both cations in second and further layers of some surfaces are positive, indicating that once a calcium carbonate layer has overgrown the substituted surface layer, segregation to the bulk is energetically possible. (C) 2000 American Institute of Physics. [S0021-9606(00)70608-8].