Ab Initio Investigation of Dissolution Mechanisms in Aluminosilicate Minerals

Abstract

The reactions of aluminosilicate clusters with water are investigated using ab initio calculations. There are several reaction sites on a mineral surface, and, in the case of aluminosilicates, the dissolution chemistry is dictated by chemically distinct surface termination sites: Al and Si. Environmental factors such as pH determine the protonation state and configuration around these terminal sites. The dissolution mechanisms for Al- and Si-terminated sites in protonated, neutral, and deprotonated states are determined using density functional theory calculations. In all protonation states, Si are tetra-coordinated; however, the ability of Al to exist in tetra-, penta-, and hexa-coordination states makes the dissolution mechanisms for the two types of terminal sites fundamentally different. The calculated barrier heights for Al-terminated sites are predicted to be lower than those for Si-terminated sites, a trend that has been observed in experimental studies. The sensitivity of the calculations on the choice of density functionals and basis sets is tested using three functionals: B3LYP, PBE1PBE, and M05-2X, in combination with the 6-311+G(d,p) and MG3S basis sets. For all these calculations, the geometries of the stationary points along the reaction path and the barrier heights are presented.

Pressure–induced crystallization of biogenic hydrous amorphous silica

“…A disiloxane molecule (Si 2 O 7 H 6 , dimer structure) has been extensively applied in theoretical investigations of crystalline and amorphous silicates (e.g., Xiao and Lasaga, 1994;Al Derzi et al, 2008;Morrow et al, 2009;Noritake and Kawamura, 2015;Mitani and Kyono, 2017). We assumed the disiloxane molecule to be the simplest interior structural unit of the amorphous silica in this study.…”

Nuclear Energy Advanced Modeling and Simulation (NEAMS) Waste Integrated Performance and Safety Codes (IPSC) : FY10 development and integration.

“…The formation of the five-coordinate Si complex was predicted as suggested in a number of earlier modeling studies of silica-water interactions. One difference from previous is that instead of a (OH) 4 Nangia and Garrison (2008) and Morrow et al (2009) calculated the activation energy for the hydrolysis of Si-O and Al-O bonds from quantum mechanical calculations on small silicate and aluminosilicate clusters, using a constrained optimization approach similar to Xiao and Lasaga (1994; and Criscenti et al (2005;. Nangia and Garrison (2008) examined the hydrolysis of Si-O-Si bonds for protonated, neutral and deprotonated silica clusters according to the following reactions: For both the protonated and deprotonated systems, the activation energy barrier associated with the formation of the penta-coordinated silica intermediate is larger than the energy involved in breaking the Si-O br bond, suggesting that this is the rate-limiting step in the reaction.…”

“…The net charge on the cluster is shown on the top right of each structure (after Morrow et al 2009) ) on the surface. These reaction rates were compared to the experimental results of Dove and Elston (1992), Knauss and Wolery (1988), Schwartzentruber et al (1987), and Wollast and Chou (1988).…”

A mechanistic model for long-term nuclear waste glass dissolution integrating chemical affinity and interfacial diffusion barrier

“…The hydrolysis reaction has been observed to accelerate in both acid and basic conditions, with a lowest rate in neutral environment [26,27]. Based on the first principles, it has been found [28][29][30][31][32][33] that the activation energy of the hydrolysis of T m -O-T n (where T=Si, Al; m, n = number of bridge oxygen atoms and 0<m, n<4) bonds can be rather variable: lower in acid and basic conditions but higher in neutral conditions.…”