Revealing atomistic mechanism of lithium diffusion in montmorillonite structure: A molecular simulation study

Abstract

The Li-bearing clays have attracted increasing attention as an important resource for lithium (Li) recovery. But the mobility of Li in clay lattice structure remains unclear, which hinders understanding of Li enrichment in Li deposits and technical development of Li extraction from Li-bearing claystones. In this study, we employed molecular simulations to investigate Li+ diffusion between different sites in the montmorillonite lattice. The calculation indicates that Li+ diffusion from the montmorillonite interlayer space to the vacant octahedral site occurs on a time scale of hours at 200 °C, which well agrees with the available Hofmann-Klemen effect. Our results also reveal that isomorphic substitution plays an important role in the Hofmann-Klemen effect. At temperatures up to 200 °C, the structure of montmorillonite with octahedral substitutions collapses due to the diffusion of almost all of the interlayer Li+ into the octahedral sheets, whereas montmorillonite bearing tetrahedral substitutions could maintain 28 % Li+ in the interlayer and thus the interlayer space can be well kept. Heating the montmorillonite to 300 °C in a water vapor environment causes Li+ to reversibly diffuse out of the octahedral sheets. Besides, the dehydroxylation of montmorillonite significantly lowers the activation energy barrier of Li+ diffusion between the octahedral layer and the interlayer. The disclosed diffusion of Li in montmorillonite may partially explain the Li enrichment in Li-bearing claystone, such as swinfordite. The revealed Li+ diffusion mechanism in montmorillonite may also help design and improve the technology of extracting Li from Li-bearing clays.