Activation energy of diffusivities for deuterated water in compacted sodium-bentonite. Through-diffusion experiment and molecular dynamics simulation

In order to quantify effect of temperature on diffusivity of deuterated water (HDO) in compacted sodium-bentonite, through-diffusion experiments were conducted at elevated temperature from 298 to 333 K. Kunipia F (Na-montmorillonite content > 98 wt. %; Kunimine Industry Co.) was compacted to a dry density of 0.9 and 1.35 Mg/m3. Since smectite flakes were perpendicularly oriented to a direction of compaction, anisotropy of diffusivity was investigated parallel and normal to the preferred orientation of smectite. Effective diffusion coefficient De of HDO was larger for a diffusional direction parallel to the preferred orientation than normal to that for both dry densities. These results well agreed to the previously reported ones for tritiated water. Activation energies of De in compacted bentonite increased with increasing dry density in the range of 19-25 kJ/mol which was slightly larger than that in bulk water (18 kJ/mol). This relationship can be considered to be due to both the pore structure development and high activation energy of water (18-23 kJ/mol) in the vicinity of smectite surface (within 2 nm) on the basis of molecular dynamics simulations. (author)