SOLVATED AB INITIO AND DENSITY FUNCTIONAL THEORY (DFT) MODELING OF MINERAL-FLUID SURFACE REACTIONS: TOWARDS A FUNDAMENTAL UNDERSTANDING OF ALUMINOSILICATE DISSOLUTION MECHANISMS

Ab initio modeling has been shown to be useful to improve our understanding of mineral-surface reactions. In particular, it can be used to probe detailed molecular mechanisms, including types of bonding and reaction pathways. In this study, we conducted a hierarchical approach to treat the solvation effect in mineral-fluid surface reactions. Our focus was to calculate the energies required for water to adsorb and hydrolyze the key Si-O-Si and Si-O-Al linkages in aluminosilicate with a combination of high-level solvated ab initio and DFT calculations.

We started by refining earlier gas phase calculations of mineral-fluid surface reactions (e.g., Xiao & Lasaga 1994, 1996) using several larger and more realistic molecular clusters. These calculations served as the basis for subsequent hydration calculations. We then immersed the model in a self-consistent reaction field (SCRF), which treats the solvent as a dielectric continuum. These methods, such as IPCM and SCIPCM, account for free energy changes due to hydration. We also placed the model in an explicit hydration sphere and studied how these nearest neighboring water molecules affect the dynamics of the dissolution reactions. Finally, we combined the best features from the above approaches by surrounding the model with an explicit sphere of water molecules, then immerse the whole in a SCRF. This method provides the most accurate hydration enthalpy. We explored how different solvation treatments would affect the results of the kinetics and mechanisms of mineral dissolution. The calculated results, in particular the transition state structures and kinetic parameters, are consistent with the results from our earlier studies; they confirm that water adsorption and hydrolysis is preferred by the Si-O-Al bonds over the Si-O-Si bonds. In the future, we will include H⁺, OH^-, and Na⁺, Ca²⁺ and repeat the above calculations to understand how pH and mineral composition variations affect the aluminosilicate dissolution behavior.