Lithium partition and isotopic fractionation for cation exchange in clay: Insights from molecular simulations

Chemical weathering is one of the most important processes on the Earth’s surface, and lithium isotopes have been used as a promising tracer of silicate weathering, as the secondary clay minerals precipitated during silicate weathering are thought to preferentially incorporate the lighter 6Li isotope. However, to utilize lithium isotopes as a geochemical tracer, it is essential to quantitatively understand the partitioning of Li in clay minerals, including structurally bound Li and exchangeable interlayer ions, as well as the associated isotope fractionation. With classical molecular dynamic simulations and thermodynamic calculations, the cation exchange between Na-/K-montmorillonite and solvated Li in environmental solution are disclosed under different aqueous activity conditions. The Li+ ion, whose hydration ability is stronger than Na+ and K+ ions, is easier to swell the montmorillonite and shows less affinity for the clay. And the calculated cationic selectivity coefficients between Na+/K+ and Li+ ions are Kc (Li→Na) = 2.0, Kc (Li→K) = 10.0 for Arizona-type montmorillonite and Kc (Li→Na) = 1.0, Kc (Li→K) = 4.0 for Wyoming-type montmorillonite, which are comparable to reported experiments. The cationic partition of Li in the clay interlayer decreases when the hydration states of montmorillonite transform from triple-layer to bilayer and then to monolayer. Based on the density functional theory, the Li isotope fractionations for Li in clay interlayer sites and structural octahedral sites are further derived by thermodynamic calculations with path integral molecular dynamic simulation, which deals with the quantum anharmonicity. The behaviors of Li isotope in the bilayer hydration state closely resembles that in the aqueous solution, and only a slight Li isotope fractionation is found between the monolayer hydrated octahedral-substituted clay and solution phase (Δ7Limonolayer-aq = -2.0‰). While Li isotope fractionation between aqueous solution and mica structural sites is significant (Δ7Listructural-aq = -10.4‰) and shows an obvious quantum anharmonicity, which indicates the importance of the anharmonic treatment for Li isotope fractionation calculations. The newly calculated Li equilibrium isotope fractionation between clay and aqueous solution deviates from the experimentally determined fractionation factor, which can be attributed to the kinetic effects. Moreover, on the basis of the surface kinetic reaction model, the Li isotope fractionation becomes more pronounced with higher clay growth rate. By integrating the cationic selectivity coefficient and Li isotope fractionation, it is possible to constrain the fluid composition during Li-clay deposit and diagenesis, and to evaluate the intensity of chemical weathering processes as well as the related carbon cycle.