Diffusive transport properties of seawater in calcite nanopores: A molecular dynamics study

Abstract

Classical molecular dynamics (MD) simulations were used to explore the diffusive transport properties of seawater through porous media of different calcite structures. Two different calcite surfaces constituted the pore slit models with a size of 1.5 nm; seawater with a 3.5 wt% of salinity occupied the free volume between calcite surfaces at the corresponding average seawater density. The SPC/E water model was employed to describe the water molecules' interactions, while a Buckingham-type potential modeled the calcite. All cross-interactions were modeled using the Lennard-Jones potential and Ewald sums electrostatics. The simulations demonstrated that calcite surfaces reduce the diffusivity of Na⁺ and Cl^{\protect \relax \special {t4ht=-}} ions, and the magnitude of the diffusivity reduction depends on the structure of the calcite surface. The topology and charge distribution features of the energetically most stable calcite (104) surface led to a slight reduction in the electrolyte diffusivity. On the contrary, density profiles evidenced that the least stable calcite surface (100) favored preferential adsorption of Na⁺, leading to significant differences in ionic diffusion coefficients. Finally, the ionic-specific diffusion coefficients obtained from molecular dynamics simulations were loaded into an advection-dispersion model to simulate a seawater intrusion into coastal aquifers scenario, with parameters associated with a predominantly diffusive transport through the most stable calcite pore. The concentration profiles showed that minor differences between Na⁺ and Cl^{\protect \relax \special {t4ht=-}} diffusivities result in Na⁺/Cl^{\protect \relax \special {t4ht=-}} concentration disparities a few meters away from the coastline after years of seawater shifting the dispersion zone.

Keywords: Calcite pores; Molecular dynamics; Seawater intrusion.