Kinetics of interlayer ion migration in non-swelling clays: An atomic-scale study

Abstract:

Clay-rich geologic repositories serve as hosts for fossil methane reserves and as traps for contaminant radionuclides and sequestered CO₂. Despite the abundance of non-swelling clay minerals in sedimentary formations, the mechanisms of ion exchange and mass transport mediated by these minerals are not well understood. Ion exchange kinetics in collapsed clays are characterized by a long tail of slow exchange, which suggests that interlayer ions can exchange with the bulk solution. Recent High-Resolution TEM evidence suggests that Cs⁺ ion exchange K⁺ in collapsed interlayers leads to interstratified structures, where entire interlayers are completely exchanged while others remain pristine [Okamura T et al., (2005) Microscopy 6365-72]. This phenomenon could be explained by kinetic feedbacks arising when a larger ion substitutes for a smaller one, although the details of this exchange mechanism are currently unknown.

We investigated the kinetics and mechanisms of interlayer cation migration in illite (K_0.7Al₂[Al_0.7Si_3.3O₁₀](OH)₂) using molecular simulations. A Monte Carlo scheme was used to distribute interlayer K ions, and these ions were found to prefer sites neighboring two or more Al³⁺ substitutions in the tetrahedral sheets. Interlayer K⁺ ion migration between stable ditrigonal cavity sites was observed directly in molecular dynamics simulations performed at temperatures ranging from 500 K to 900 K and at constant volume. The Climbing Image Nudged Elastic Band method was used to determine the activation energy barrier on 660 K⁺ ion migration paths. Interlayer ions were observed to migrate between stable lattice sites with migration barriers of 2.35 ± 1.06 eV. Only about 20% of this variation is statistically explained by the distribution of charge deficit sites in the layer caused by Al³⁺ substitution for Si⁴⁺. Remarkably, we find that migration barriers decrease as we increase interlayer spacing. These results suggest that frayed edge sites – local regions with larger interlayer spacing commonly caused by weathering – can help accelerate ion mobility by lowering the migration barrier in their vicinity, possibly facilitating ion interstratification in collapsed clays. We discuss the limitations of our molecular simulations and how we may overcome them using electronic structure methods.