Effects of dry density and porewater salinity on Eu(III) diffusion in compacted GMZ bentonite: A perspective from triple pore structure

Compacted bentonite is microscopically characterized by a triple pore structure composed of inter-aggregate pores, inter-particle pores and interlayer pores. These three types of pores are available for diffusive transport of cations through different mechanisms: free water diffusion, surface diffusion, and interlayer diffusion. This study investigated the effects of dry density and porewater salinity on Eu(III) diffusion in compacted GMZ bentonite from the perspective of triple pore structure. A series of through-diffusion (TD) experiments were conducted as a function of the density and porewater salinity. Then the triple pore structure was characterized using SEM and MIP tests. On this basis, a mathematical expression about the dependence of Eu(III) diffusion on the triple pore structure was developed. Results indicated that increasing dry density inhibited the Eu(III) diffusion, as evidenced by decreases in the effective diffusion coefficient De, capacity factor alpha, and apparent diffusion coefficient Da. This inhibition is attributed to the reduction in inter-aggregate pores, inter-particle pores, and interlayer pores with increased density, thereby suppressing free water diffusion, surface diffusion, and interlayer diffusion. Conversely, increasing porewater salinity facilitated the Eu(III) diffusion. This is likely because the relatively high concentration of Eu(III) in the TD tests makes free water diffusion as the dominant pathway. Although surface diffusion and interlayer diffusion were inhibited owning to the decrease of interparticle pores and interlayer pores, their negative effects on total diffusion flux were less than the positive effects of enhanced free water diffusion resulting from the increase of inter-aggregate pores. Finally, by comparing the calculated total diffusion fluxes and experimental values, the applicability of the proposed expression for predicting Eu(III) diffusion in compacted bentonite with varying dry densities and salinities was discussed.