Aging Effects on Cesium-137 (137Cs) Sorption and Transport in Association with Clay Colloids

Abstract 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 hours. 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.