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In the geological disposal system, if colloids can stably exist in groundwater, radionuclides (RNs) might be sorbed onto the colloids and migrate long distance without retardation such as the sorption onto minerals and the matrix diffusion. The distribution coefficient ( R  d ), which is a measure for sorption onto colloids, is one of the sensitive parameters in evaluation of the effect of colloids on the RN migration. In this study, the  R  d s of Am onto bentonite colloids were evaluated by batch sorption experiments under weakly basic and low ionic strength conditions. Colloidal species of Am seems to be generated even in the absence of bentonite colloids, however, it has less contribution to  R  d . The obtained  R  d  values are larger than those obtained for larger montmorillonite particles reported in literatures. The difference of  R  d  is considered to be due to a difference of composition of experimental solution and a difference of reactive site capacity affected by particle size. Site capacities of bentonite colloids are evaluated by acid titration and it is observed that bentonite colloids have about 8 times larger site capacities than bentonite sediments. A relatively simple and generalized mechanistic sorption model, 2SPNE SC/CE model, is applied to estimate the  R  d  for Am sorption onto bentonite colloids using reaction constants reported in literatures and site capacities obtained in this study. Calculated  R  d s mostly agree well with corresponding experimental data at pH 8, while,  R  d s are underestimated at pH 10. Based on the speciation plots, it is probably due to the underestimation of surface complexation reaction.

Sorption behavior of americium onto bentonite colloid