Processes at the magnesium-bearing carbonates solution interface. I. A surface speciation model for magnesite

The surface charge of magnesite (MgCO3) was measured at 25 degrees C as a f unction of pH (4.6 to 11) and ionic strength (0.01, 0.1 and 0.5 M NaCl) und er pCO(2) from 10(-3.5) to 0.96 atm. The acid-base titrations were performe d in a limited residence time reactor following the approach developed by C harlet et al. (1990) for rhodochrosite and siderite. Magnesite zeta potenti al (zeta) was determined as a function of pH (1.5 to 12), ionic strength (0 .001 to 0.1 M) and Mg2+ and HCO3- concentrations. Based on these results an d spectoscopic data on the calcite and dolomite/water inerfaces, a surface complexation model is presented which postulates the formation of the prima ry hydration species >CO3H degrees and >MgOH degrees. Within this scheme, m agnesite surface speciation is governed by the following species: >CO3H deg rees, >CO3-, >CO3Mg+, >MgOH degrees, >MgO-, >MgOH2+, >MgHCO(3)degrees, and >MgCO3-. The intrinsic stability constants of these species were determined by fitting the pH dependent surface charge over a wide range of solution c omposition. For the conditions investigated in this study, the measured zet a-potential values of magnesite are in good agreement with the sign of the surface potential and its dependence on solution composition calculated usi ng the surface complexation model generated in this study. Copyright (C) 19 99 Elsevier Science Ltd.