Gibbsite is an important aluminum mineral, a key Bayer process precipitate, and a component of radioactive tank waste at the Hanford site, Washington, USA. Gibbsite is composed of basal edge-sharing octahedral Al sheets perpendicular to [001] that are furthermore closely related to the octahedral sheets in many aluminosilicate clays and micas.  Although the dissolution of gibbsite at low pH is well studied, it is much less so at the extreme high pH values associated with aluminum and nuclear waste processing.  At high pH, dissolving Al takes the form of the aluminate hydroxyanion that dominates the speciation in solution.  We hypothesize that although the importance of higher edge surface reactivity may still pertain at high pH, the mechanism of dissolution may be different than at low pH. We used a combination of classical molecular dynamics simulations and ab initio methods to study the dissolution of Al monomers from surfaces/steps and the transformation to aluminate in solution. The two most stable steps parallel to &lt;110&gt; and &lt;010&gt; on gibbsite (001) surface were examined based on previous experimental precedent. The surface speciation and deprotonation on gibbsite edges at high pH, important to the dissolution mechanism, were examined by using a hexamer cluster model for gibbsite surface/edges and ab initio MD simulations.  A combination of 3-d metadynamics (to explore the dissolution pathway) and potential of mean force (PMF) calculations was then used to obtain the detachment free energies and the kinetics of elementary steps of dissolution.  These calculated free energies and rates for corresponding step edge surfaces will be directly compared to complementary in-situ AFM studies of synthetic gibbsite dissolution at high pH.<br/>

Theoretical study of gibbsite dissolution at high pH

253rd National Meeting (2017)

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