The dynamic role of natural colloids in enhancing plutonium transport through porous media

Unexpectedly fast migration of Pu in aquifers at some nuclear sites was known as the result of enhanced transport by natural colloids. Nevertheless, transport experiments still need to provide direct evidence for the role of colloids in promoting Pu transport. On the other hand, systematic laboratory studies should be carried out to comprehensively evaluate Pu mobilization as affected by colloids that are varied in concentrations, since naturally occurring colloids have greatly different concentrations in aquatic environments. We isolated mineral colloids with the diameter of 1 nm–1 μm from sandy soils and prepared the colloidal suspensions with the concentrations ranged from c = 0 (solution) to c = 2017.80 mg/L. The results of transport experiments show that the mobile fraction of 239Pu (RPu = 8.62%) associated with the colloids of an extremely low concentration (c = 0.5 mg/L) is much larger than that not associated with the colloids (RPu = 1.31%, c = 0). The mobile fraction is defined by the percent recovery of Pu (RPu). This is strong evidence for the enhanced transport of Pu by the colloids. Plutonium mobility (reflected by RPu) continually increases with the colloid concentrations of c < 375.37 mg/L and then declines at c > 375.37 mg/L, which is demonstrated as a dynamic role of colloids in Pu transport. The highest mobility (RPu = 52.48%), occurring at c = 375.37 mg/L, is referred to as the critical transport-enhanced concentration. Plutonium transport mechanism with a focus on its dynamic deposition onto the surfaces of porous media is explored.