Reactive solute transport simulation of multicomponent cation exchange and diffusion processes in column experiments with clay-rich rock

Clays and clay-rich rocks play often an important role in nuclear waste disposal due to their low permeability and high sorption capacity, acting as natural barriers to fluid movement and contaminant migration. Understanding the transport and sorption behaviours of hazardous elements in clay-rich environments is therefore essential for long-term simulations with validated models of experimental data. This study investigates the reactive transport of 17 ionic compounds in the Woodford claystone using both experimental and modelling approaches. The experiment was conducted by injecting a multi-tracer solution into a column filled with crushed claystone, employing a flow-interruption method for examining kinetic behaviour during diffusion-dominated mass transfer. TOUGHREACT V4.0 OMP reactive transport code was applied to replicate the tests, using an advective-diffusive single porosity flow model that considers mineral dissolution/precipitation and cation exchange. The modelling results demonstrated that cation exchange and diffusion, along with advection, were the primary processes influencing ionic concentrations in the experiment. The primary mineral dissolution reactions were pyrite oxidation and silicate weathering, releasing Si, Al, and Fe that reprecipitated or contributed to cation exchange. The findings indicated that the claystone sample effectively sorbs Cs, Pb, and Eu through cation exchange. While the model showed good agreement with the experimental data, an excessive diffusion effect was simulated using the single-porosity model, which would likely be less if employing a dual-porosity model and accounting for immobile water.