Molecular simulations of water and ion diffusion in nanosized mineral fractures

Molecular dynamics simulations were carried out to investigate the effects of confinement and of the presence of the mineral surface on the diffusion of water and electrolyte ions in nanosized mineral fractures. Feldspar was used as a representative mineral because recent studies found that it is an important mineral that hosts contaminants within its intragrain fractures at the U.S. Department of Energy Hanford site. Several properties of the mineral-water interface were varied, such as the fracture width, the ionic strength of the contacting solution, and the surface charge,to provide atomic-level insights into the diffusion of ions and contaminants within intragrain regions. In each case, the self-diffusion coefficient of water and that of the electrolyte ions were computed as a function of distance from the mineral surface. Our calculations reveal a 2.0-2.5 nm interfacial region within which the self-diffusion coefficient of water and that of the electrolyte ions decrease asthe diffusing species approach the surface. As a result of the extent of the interfacial region, water and electrolyte ions are predicted to never reach bulk-like diffusion in fractures narrower than approximately 5 nm. The average diffusion coefficient along the mineral fracture was computed as a function of fracture width and indicated that the surface effects only become negligible for fractures several tens of nanometers wide. The molecular dynamics results improve our conceptual models of ion transport in nanoscale pore regions surrounded by mineral surfaces in porous media.