Processes at the magnesium-bearing carbonates/solution interface. I. A surface speciation model for magnesite
The surface charge of magnesite (MgCO 3) was measured at 25°C as a function of pH (4.6 to 11) and ionic strength (0.01, 0.1 and 0.5 M NaCl) under pCO 2 from 10 -3.5 to 0.96 atm. The acid-base titrations were performed in a limited residence time reactor following the approach developed by Charlet et al. (1990) for rhodochrosite and siderite. Magnesite zeta potential (ζ) was determined as a function of pH (1.5 to 12), ionic strength (0.001 to 0.1 M) and Mg 2+ and HCO 3- concentrations. Based on these results and spectoscopic data on the calcite and dolomite/water inerfaces, a surface complexation model is presented which postulates the formation of the primary hydration species >CO 3H° and >MgOH°. Within this scheme, magnesite surface speciation is governed by the following species: >CO 3H°, >CO 3-, >CO 3Mg +, >MgOH°, >MgO -, >MgOH 2+, >MgHCO °3, and >MgCO 3-. The intrinsic stability constants of these species were determined by fitting the pH dependent surface charge over a wide range of solution composition. For the conditions investigated in this study, the measured ζ-potential values of magnesite are in good agreement with the sign of the surface potential and its dependence on solution composition calculated using the surface complexation model generated in this study.