Effect of water saturation on the diffusive transport of water and solutes in compacted kaolinite, illite and vermiculite

Abstract

Through-diffusion experiments were carried out under partially water saturated conditions and using uncharged tracers (HTO and HDO; water molecule with one tritium and deuterium atom, respectively), negatively charged tracers (¹²⁵I⁻ and ³⁶Cl⁻) and positively charged tracer (²²Na⁺) in two reference clayey materials: (i) illite, a negatively-charged clay mineral with only interparticle porosity and (ii) vermiculite, a negatively-charged clay mineral with both inter-particle and interlayer porosities. For both types of porous media, a sharp decrease in diffusive flux was observed for all three types of tracer when water saturation (S_w) was reduced from S_w = 0.95 to 0.88. The data were compared to those obtained previously with porous media made of kaolinite, a weakly-charged clay mineral having only inter-particle porosity, to quantify the relative influences of surface charge and pore geometry/pore size distribution on diffusion properties as a function of water saturation. The latter parameter had a prominent role on the diffusional properties in the investigated range of S_w, regardless of the type of clayey material. Then, a relationship predicting diffusivities of cation under partially water saturated conditions as a function of diffusivities of anion at the same level of water saturation is proposed for the reference clayey materials investigated.