99TcO4- diffusion and sorption in compacted GMZ bentonite studied by capillary method

Sorption and diffusion of 99TcO4- in compacted bentonite from Gaomiaozi county (Inner Mongolia, China) (denoted as GMZ bentonite) was studied by capillary method. The distribution coefficient (Kd) and diffusion coefficient (Da) were calculated from diffusion curves of 99TcO4- in compacted GMZ bentonite. The Da and Kd values decreased with the increasing of the bulk dry density of the compacted bentonite. With increasing pH values, the Da values increased whereas the Kd values decreased, which is attributed to the ion exchange and electrostatic repulsion between the negative surface charged bentonite and negative charged 99TcO4- ions at high pH values. The diffusion and relative concentration distributions of 99TcO4- in compacted bentonite after long aging time were simulated from the distribution and diffusion coefficients. Effect of aging time on the diffusion of 99TcO4- in compacted bentonite was also calculated using the theoretical equation. The results indicated that 99TcO4- was an easily mobile radionuclide in environment and the release of 99TcO4- from the radioactive waste repository was dangerous to the environmental pollution. (author)