Dawsonite synthesis and reevaluation of its thermodynamic properties from solubility measurements: Implications for mineral trapping of CO2
Résumé
Over the last decade, a significant research effort has focused on determining the feasibility of sequestering large amounts of CO2 in deep, permeable geologic formations to reduce carbon dioxide emissions to the atmosphere. Most models indicate that injection of CO2 into deep sedimentary formations will lead to the formation of various carbonate minerals, including the common phases calcite (CaCO3), dolomite (CaMg(CO3)2), magnesite (MgCO3), siderite (FeCO3), as well as the far less common mineral, dawsonite (NaAlCO3(OH)2). Nevertheless, the equilibrium and kinetics that control the precipitation of stable carbonate minerals are poorly understood and few experiments have been performed to validate computer codes that model CO2 sequestration. In order to reduce this uncertainty we measured the solubility of synthetic dawsonite according to the equilibrium: NaAlCO(OH)+2HO?Al(OH)4-+HCO3-+Na+H, from under- and oversaturated solutions at 50 200 °C in basic media at 1.0 mol · kg?1 NaCl. The solubility products (Qs) obtained were extrapolated to infinite dilution to obtain the solubility constants (Kso). Combining the fit of these logKso values and fixing ?Cp,roat?185.5J·mol·K at 25 °C, which was derived from the calorimetric data of Ferrante et al. [Ferrante, M.J., Stuve, J.M., and Richardson, D.W., 1976. Thermodynamic data for synthetic dawsonite. U.S. Bureau of Mines Report Investigation, 8129, Washington, D.C., 13p.], the following thermodynamic parameters for the dissolution of dawsonite were calculated at 25 °C: ?Gro=102.1kJ·mol, ?Hro=97.0kJ·mol and ?Sro=-17.1J·mol·K. Subsequently, we were able to derive values for the Gibbs energy of formation (?fG298.15o=-1782±2kJ·mol), enthalpy of formation (?fH298.15o=-1960±7kJ·mol) and entropy (S298.15o=131±2J·mol·K) of dawsonite. These results are within the combined experimental uncertainties of the values reported by Ferrante et al. (1976). Predominance diagrams are presented for the dawsonite/boehmite and dawsonite/bayerite equilibria at 100 °C in the presence of a saline solution with and without silica-containing minerals.