Seeming Steady-State Uphill Diffusion of ²²Na⁺ in Compacted Montmorillonite

Whereas the transport of solutes in nonreactive porous media can mostly be described by diffusion driven by the concentration gradients in the external bulk water phase, the situation for dense clays and clay rocks has been less clear for a long time. The presence of fixed negative surface charges complicates the application of Fick’s laws in the case of ionic species. Here we report the seeming uphill diffusion of a 22Na+ tracer in compacted...