Ab Initio Investigation of Dissolution Mechanisms in Aluminosilicate Minerals

Abstract

The reactions of aluminosilicate clusters with water are investigated using ab initio calculations. There are several reaction sites on a mineral surface, and, in the case of aluminosilicates, the dissolution chemistry is dictated by chemically distinct surface termination sites: Al and Si. Environmental factors such as pH determine the protonation state and configuration around these terminal sites. The dissolution mechanisms for Al- and Si-terminated sites in protonated, neutral, and deprotonated states are determined using density functional theory calculations. In all protonation states, Si are tetra-coordinated; however, the ability of Al to exist in tetra-, penta-, and hexa-coordination states makes the dissolution mechanisms for the two types of terminal sites fundamentally different. The calculated barrier heights for Al-terminated sites are predicted to be lower than those for Si-terminated sites, a trend that has been observed in experimental studies. The sensitivity of the calculations on the choice of density functionals and basis sets is tested using three functionals: B3LYP, PBE1PBE, and M05-2X, in combination with the 6-311+G(d,p) and MG3S basis sets. For all these calculations, the geometries of the stationary points along the reaction path and the barrier heights are presented.