The hydrolysis and carbonate complexation of strontium and calcium in aqueous solution. Use of molecular modeling calculations in the development of aqueous thermodynamic models

Abstract

The hydrolysis and carbonate complexation of the alkaline earth cations Ca2+and Sr2+were investigated by using a combined experimental and modeling approach. The modeling approach uses both macroscopic thermodynamic models and molecular modeling at the density functional theory (DFT) level. The molecular modeling calculations identify possible speciation schemes in the thermodynamic modeling and provide molecular level insight into the macroscopically observed thermodynamic measurements. In order to develop accurate thermodynamic models valid to high electrolyte concentration and to test for the possible existence of species suggested by the molecular models, experimental measurements were made on the solubility of Ca(OH)2, Sr(OH)2·8H2O, and carbonate compounds extending to high base molality (≈5 mol<@TBLEND>kg−1), or carbonate molality (≈2 mol<@TBLEND>kg−1). A thermodynamic model is developed that satisfactorily explains the macroscopic aqueous thermodynamic data and correlates with the molecular modeling results. The first published values of the equilibrium constants for the formation of Sr(CO3)2−2(aq) and Ca(CO3)2−2(aq), and of the solubility product of Sr(OH)2·8H2O are also provided. In certain cases, specifically when the DFT calculations suggest that the hydroxyl groups are more closely associated with the first hydration layer of water molecules than directly with the central alkaline earth cation, the thermodynamic relations for these cation–hydroxyl interactions are described by means of Pitzer ion-interaction parameters, rather than by the explicit introduction of an aqueous hydrolysis species.