Thermodynamic Modeling of Solubility of Corundum in Water at Supercritical Conditions

The dissolution of corundum, Al2O3, in water is considered as a set of reactions of hydration of the solid phase with the formation of various charge-neutral aqueous Al species. Methods of quantum chemistry (QC) were used to compute the thermodynamic properties of several Al forms in the ideal gas state. The combination of QC results with estimated values of fugacity coefficients of these species in water permits the evaluation of the concentrations of various Al forms over corundum and thus the calculation of the total solubility of solid Al2O3 in supercritical water. It was found that the minimum set of species dominating the Al speciation over corundum in the very wide temperature (from 647 up to 2000 K) and pressure (up to 7000 MPa) ranges consists of at least five species: Al­(OH)3, its hydrates Al­(OH)3 · H2O, Al­(OH)3 · 2H2O, Al­(OH)3 · 3H2O, and the dimer Al2(OH)6. For all species, the values of the entropies and heat capacities in the gaseous state are taken from QC computations; for two out of five forms (Al­(OH)3 · H2O and Al­(OH)3 · 3H2O), the enthalpies of formation at 298.15 K and 0.1 MPa were adjusted to provide agreement of experimental and calculated values of solubility of corundum; for other three species, the QC enthalpy values were accepted. The calculated values of solubility of Al2O3 in water at supercritical temperatures are in satisfactory agreement with experimental data. According to the proposed model, the solubility of corundum increases strongly with temperature. Speciation of Al changes with both the temperature and water density: on an isotherm, an increase in the water density shifts the speciation toward the prevalence of the form with a higher content of water; on an isochore, the increase in temperature leads to a quick dehydration of Al species. As a result, at low supercritical temperatures, the dominating forms are the di- and trihydrate, Al­(OH)3 · 2H2O and Al­(OH)3 · 3H2O, while at temperatures above 1300 K, Al­(OH)3, the dimer Al2(OH)6, and the monohydrate Al­(OH)3 · H2O.