Diffusion behaviour of Se(-II) and Sm(III) in compacted sodium bentonite

Apparent diffusion coefficients (Da) for Se(-II) and Sm(III) in a compacted sodium bentonite, Kunigel-V1 trademark, were obtained for a range of dry densities from 800-1800 kg . m-3. The Da values for Se were obtained under reducing conditions and those for Sm were obtained over a pH range of 8-11. The Da values for Se were between 6.1 x 10-11 and 4.3 x 10-10 m2 . s-1 and showed a tendency to slightly decrease with increasing dry density of bentonite. The dominant Se species in the porewater is predicted to be HSe-, and the Da values were approximately the same as those for TcO4-, which has the same charge. The Da values obtained for Sm were between 8.0 x 10-15 and 1.0 x 10-13 m2 . s-1 and decreased with increasing dry density of bentonite. The dominant species of Sm in the porewater are predicted to be SmCO3+ and Sm(CO3)2- at pH8 and 11, respectively. However, all Da values for the same bentonite density were approximately the same and no influence of the porewater pH on diffusivity was found. The Da values for Sm were similar to those for Eu, Am and Cm in the same range of bentonite density. Therefore, it is concluded that these trivalent elements show similar diffusion behaviour. (orig.)