Role of Electrochemical Surface Potential and Irradiation on Garnet-type Almandine's Dissolution Kinetics

Nanoscale resolved quantifications of almandine's (Fe3Al2(SiO4)3) dissolution rates across a range of pH's (1 ≤ pH ≤ 13) - established using vertical scanning interferometry (VSI) - reveal that its dissolution rate achieves a minimum around pH 5. This minimum coincides with almandine's point of zero charge (PZC). These trends in almandine's dissolution can be estimated using the Butler-Volmer (BV) equation that reveals linkages between surface potentials and dissolution rates, demonstrating proton- and hydroxyl-promoted breakage of Si-O bonds. In contrast with well-polymerized silicates, the dissolution of almandine can also occur through the rupture of its cationic bonds. This behavior is reflected in the observed influences of irradiation on its dissolution kinetics. Molecular dynamics simulations highlight that irradiation induces alterations in the atomic structure of almandine by reducing the coordination state of the cations (Fe2+ and Al3+); thereby enhancing its reactivity by a factor of two. This is consistent with the minor change induced in the structure of almandine's silicate backbone, whose surface charge densities produce the observed pH dependence (and rate control) of dissolution rates. These findings reveal the influential roles of surface potential arising from solution pH, and atomic scale alterations on affecting the reactivity of garnet-type silicates.