Distribution of Cs and Am in the solution-bentonite colloids-granite ternary system: effect of addition order and sorption reversibility

Bentonite colloids may be generated from bentonite in the geological disposal system planned in Japan, and sorb radionuclides (RNs) thus facilitating migration. In this context, distribution coefficients and reversibility of the sorption of RNs onto colloids are sensitive factors in safety assessment. In this study, distribution behavior of Cs and Am in the synthetic groundwater-bentonite colloids-granite ternary system was investigated by batch methods. To evaluate the sorption reversibility, three series with different addition order of components were prepared, such as CR+G corresponding to the series beginning with a colloid (C)-RN (R) solution binary system followed by addition of granite (G) after around 100 d, GR+C and CG + R. Distribution coefficients for the bentonite colloids (R d,col ) interaction with Cs obtained in CR+G and CG+R are similar to calculated value (6.7-15 m 3 kg -1 ) based on a previously reported sorption model, while that of GR + C is higher. Considering the contribution of micaceous colloids generated from granite, high R d,col in GR + C can be explained. The experimental R d,col data for Am in each series show similar values and agree well with calculation results (7.6 x 10 3 -8.7 x 10 3 m 3 kg -1 ) by applying the sorption models. Therefore, it is concluded that the sorption behavior of Cs and Am onto the bentonite colloids are reversible and the sorption models developed in the binary system are applicable to the ternary system. Interaction of Cs with granite shows slow kinetics both in sorption and desorption process. The K d,gra values for Am in CR+G are lower than those of other series just after addition of granite, however, they gradually increase with time and finally congruent values are attained. The concentration of free Am in CR + G is one order of magnitude higher than that in GR + C in spite of the ternary system under similar chemical conditions of the solution, suggesting temporal generation of species such as colloidal Am which is not sorbed on granite in CR+G.