Molecular Dynamics Simulations of Kinetic Isotope Fractionation During the Diffusion of Ionic Solutes and Noble Gases in Liquid Water

Interpretation of isotope ratios, a powerful tool in geochemical investigations of fluid-rock systems, requires an understanding of all relevant processes that fractionate isotopes. One such process, diffusion in liquid water, has remained problematic despite its potential significance as a major cause of kinetic isotope fractionation. Recent laboratory experiments have shown clearly for the first time that lithium and chloride isotopes are fractionated by diffusion in liquid water, whereas magnesium isotopes are not. We present the results of molecular dynamics simulations of chloride, magnesium, alkali metal cations (Li+, Na+, K+, Cs+) and noble gases (He, Ne, Ar, Xe) in liquid water that were designed to determine the isotopic mass dependence of solute diffusion coefficients. Our results indicate that the self-diffusion coefficients of all solutes follow an inverse power-law dependence on isotopic mass (Di \propto {mi}-β, where Di is the self-diffusion coefficient of a solute with isotopic mass {mi}). The power-law exponents (β) deduced for lithium, chloride, and magnesium from recent diffusivity data are consistent with the mass dependencies found in our simulations. The isotopic mass-dependence of noble gas diffusion coefficients in liquid water found in our simulations is much smaller than assumed in recent groundwater hydrology and paleoclimate reconstruction studies.