Mechanisms of Dissolution from Gibbsite Step Edges Elucidated by Ab Initio Molecular Dynamics with Enhanced Sampling

Many predictive models of geochemical processes (e.g., fate and transport of metals) and industrial utilization of minerals rely upon a detailed understanding of mineral dissolution. Yet atomistic details are rarely known due to the complex mineral/fluid interfacial environment. Here, ab initio molecular dynamics simulations with enhanced sampling has been used to explore the detailed process of the detachment of an aluminate monomer from two types of step edges at the gibbsite aqueous interface. Gibbsite is a primary source of Al in soils and mineral within the industrial processing of aluminum. Surface pit models in conjunction with changes to solution composition that mimic pH effects are employed to create realistic starting points for the simulation. The results indicate two potential pathways for detachment that are differentiated based upon the extent of water hydration. The energy profiles of the elementary bond-breaking events indicate the scission of the first or the second hydroxo bridge is the rate-limiting step for the monomer dissociation. The heights of the energy barriers depend upon the local morphology which influence the number of bridges (quasi-)simultaneously broken (1 or 2) or the Al-O coordination of the neighboring aluminum atoms (5 or 6) at the armchair edge.