Description of Mg2+ Release from Forsterite Using Ab Initio Methods

Abstract

Molecular clusters representative of protonated, neutral, and deprotonated sites on a forsterite (Mg2SiO4) surface were employed to facilitate examination of Mg−Obr bond-breaking via density functional theory (DFT) calculations with the B3LYP/6-31+G(d,p) methodology. Hydrolysis reactions of the molecular clusters with a H2O molecule yielded barrier heights of 21, 54, and 39 kJ/mol for protonated, neutral, and deprotonated sites in the gas-phase, respectively, and the rate constants calculated using these barrier heights were 5.7 × 108, 2.7 × 104, and 2.2 × 106 s−1, respectively. Aqueous-phase calculations on the gas-phase structures were also performed, and the barrier heights were 33, 40, and 21 kJ/mol for the protonated, neutral, and deprotonated models. Rate constants were 4.3 × 106, 6.1 × 105, and 6.0 × 108 s−1. For models energy-minimized in the aqueous-phase, the barrier heights were 37, 44, and 40 kJ/mol, and the rate constants were 1.7 × 106, 3.0 × 104, and 9.9 × 105 s−1, respectively. These differences highlight the importance of modeling structures with inclusion of solvent effects. Rates of Mg2+ release from the forsterite surface were predicted using these rate constants and models of the reactive site density and the H+ or OH− surface speciation. These calculations are consistent with a more rapid rate of Mg2+ release under acidic conditions even though the activation energy barriers are equivalent within computational uncertainty. A comparison of these results to previous data shows that the predicted rates are much faster than experimentally measured Mg2+ release rates, suggesting that breaking the Mg−Obr bond is a rapid process which is a component of Mg2+ release from the surface consistent with previous experimental observation of preferential Mg2+ leaching from forsterite. A dissolution mechanism involving polymerization and hydrolysis of Si−Obr−Si linkages is discussed that is consistent with the discrepancy between Mg2+ release rates and dissolution rates of forsterite.