Aging effects on Cesium-137 (¹³⁷Cs) sorption and transport in association with clay colloids

Migration of radionuclides via colloid-facilitated transport is an important component of nuclear repository performance models. ¹³⁷Cs sorption to bentonite colloids follows multi-site behavior, with sorption to weak sites being a rapid process and sorption to strong sites having slow kinetics. Experiments in this study targeted desorption of ¹³⁷Cs from strong sites on the colloids by placing the ¹³⁷Cs-bearing colloids in contact with a strongly-sorbing zeolite material that competes with the colloids for ¹³⁷Cs sorption. Batch and column experiments were conducted to examine the effects of aging (i.e., increased contact time between ¹³⁷Cs and colloids) on colloid-facilitated transport of ¹³⁷Cs through crushed analcime columns. A larger proportion of ¹³⁷Cs-bearing colloids eluted through a series of columns when the colloids were aged for 1200 days prior to injection in comparison to unaged colloids. Aging the colloids increased the partitioning of ¹³⁷Cs to the colloids by nearly 20% after 1200 h. Slow desorption (0.27 hr^-1) from the strong sites resulted in an increase of the Cs fraction bound to the strong sites from 0.365 to 0.87 by the second column injection, resulting in increased colloid-facilitated transport of Cs through strongly-sorbing zeolites from 0 in the second unaged column to 10% in the second aged column.