Crossing Scales with Computational Tools: Applications to Divalent Silicate Dissolution
Open Access
- Author:
- Morrow, Christin Palombo
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 02, 2010
- Committee Members:
- Karl T. Mueller, Committee Chair/Co-Chair
James D. Kubicki, Committee Chair/Co-Chair
Sharon Hammes-Schiffer, Committee Member
William Noid, Committee Member
Vincent Crespi, Committee Member - Keywords:
- density functional theory
molecular dynamics
mineral dissolution
silicate
computational
kinetics
geochemistry - Abstract:
- Computational tools are used to cross spatial scales during investigations of geochemical reactions and mineral surfaces. Density functional theory (DFT) calculations are employed to investigate M–O (M = Mg<sup>2+</sup>, Ca<sup>2+</sup>, and Ni<sup>2+</sup>) bond breaking and H<sub>2</sub>O exchange using a H<sub>2</sub>O molecule and molecular sized clusters analogous to sites on silicate mineral surfaces. The barrier heights for hydrolysis of protonated, neutral, and deprotonated Mg–O–Si sites on the forsterite surface were determined. These barrier heights were used to calculate the rate constants, and in turn, a rate for the release of Mg<sup>2+</sup> due to the breaking of the Mg–O bond. In a second set of calculations, hydrolysis of protonated M–O–Si (M = Mg<sup>2+</sup>, Ca<sup>2+</sup>, and Ni<sup>2+</sup>) sites was investigated to determine whether H<sub>2</sub>O exchange or bond breaking occurred for Ni<sup>2+</sup>–, Mg<sup>2+</sup>–, and Ca<sup>2+</sup>–silicate molecular clusters. Here again, the barrier heights are used to calculate rate constants for release of these metals from protonated sites on silicate surfaces. A comparison with experimental data is given, and experimental trends are replicated. Density functional theory molecular dynamics (DFT-MD) simulations enable the use of a unit cell sized system and allow for an investigation of several reaction sites on the mineral surface. The forsterite (100) and (010) surfaces were investigated to determine the most stable structures for these surfaces when initially covered with all H<sub>2</sub>O molecules or OH groups. The surfaces yielded similar structures comprised of H<sub>2</sub>O, OH, O<sup>–</sup>, and O<sub>br</sub> sites, and a true forsterite surface likely has a distribution of all of these sites. These surfaces were simulated in the presence of bulk water to investigate the surface structure at the aqueous-mineral interface over time, and H<sup>+</sup> and H<sub>2</sub>O transfers between groups throughout the surface and between the solution and the surface were observed.