Understanding the limits of binary diffusion for enhanced clay barrier design

Abstract

Waste containment and isolation strategies often utilize bentonite as a buffer material due to its swelling capacity, sealing efficiency, low permeability and limited diffusive transport. However, previous experimental studies of ionic diffusion through bentonite have shown discrepancies with binary diffusion assumptions. Meticulous experiments and complementary analyses reveal that the migration of preexisting ions in the medium enables the differential flux of diffusing anions and cations, while maintaining local electro-neutrality in all cases. The separation between the cationic and anionic fronts is electrically tied to the motion of the preexisting ions, and reflects the interplay between valence, concentration, and self-diffusion coefficients of the ions involved. Imposing binary diffusion conditions forces the faster anions to diffuse at the same rate as cations. Therefore, effective barriers to mitigate both cation and anion transport should have low surface charge and low excess salts to minimize the preexisting ionic concentration.