Effect of salinity on diffusivities for strontium ion, iodite and deuterated water in compacted sodium bentonite

Suzuki, Satoru; Sato, Haruo

For a better understanding of the relationship between groundwater salinity and diffusivity of radionuclides in compacted bentonite, salinity dependences of effective diffusion coefficients De for I, Sr and HDO were investigated using a through-diffusion method. A sodium-bentonite was purified from Kunipia F (Na-montmorillonite content 99 wt.%), in order to eliminate unknown influences of impurities (e.g. quartz and organic substances), MX80 bentonite also used included 25% of impurities. The powdered sample was compacted to a dry bulk density () of 0.9 Mg/m or 1.6 Mg/m for the purified bentonite and 1.8 Mg/m for MX80 bentonite. These samples were then soaked in NaCl solutions of 0.01 to 0.5 M. Effective diffusion coefficients for I and Sr and increased with increasing salinity as represented by the power law, while De for HDO was almost constant. A salinity dependence of De in compacted sodium-bentonite was well expressed by the power n that was practically independent of impurities or dry densities. The Power n was evaluated to be 0.5 to 0.7 for anions (I and Cl), about 1.5 for Sr and 0.0 to 0.1 for HDO. Deuterated water may migrate through both interstitial pore water and interlamellar water, whereas iodine (I) may prefer interstitial pore water as major diffusion pathways as interpreted by the anion exclusion effect. On the other hand, a positive salinity dependence of De for Sr conflicts with the previously reported tendency explained by the surface diffusion theory. Discrepancies can be considered to be due to the diffusion cell design on the basis of rock capacity factor and boundary conditions for Sr.