Pressure induced speciation changes in the aqueous Al3+ system

We have developed a simple model for incorporating the influence of external pressure and solution pH into a cluster based (i.e. comprising the central Al3+ cation and nearest neighbor coordinating H2O and OH− ligands) 1st principles approach to investigate the hydrolysis equilibria of aqueous Al3+ monomeric species in high pressure environments such as are found in the Earth's mantle. Our model is demonstrated to reproduce the well documented bulk chemistry of the aqueous Al3+ system under ambient conditions, namely the system is dominated at low and high pH by the 6-coordinated aqua species and 4 coordinated hydroxide species, respectively, while all remaining species occupy a narrow intermediate pH range. Coupling this model to changes in solution pH is achieved by using [H3O+] as a parameter in the definition of the formation equilibrium constants used; the influence of external pressure is evaluated using Planck's equation. This approach predicts that changes in external pressure will induce drastic changes in the aqueous solubility of these species under high pressure conditions and moderate changes at as low as 5 GPa. Finally, some industrial and geochemical implications of this result are discussed.