Colloid-associated plutonium transport in the vadose zone sediments at Lop Nor

A framework to describe the characteristics of pore water in unsaturated media was established in order to study transport of colloid-associated 239Pu (i.e., colloidal Pu) through the vadose sediments. Effluent concentrations and recoveries of Pu were found to decrease with increasing ionic strength. However, they would remain approximately constant at a critical value of 0.0289 M (Na+) though ionic strengths were further increased. Fast deposition rate coefficient (kfast) was thus experimentally determined. To our knowledge, this relationship between the mobility of colloidal Pu and the critical ionic strength was the first time observed. On the other hand, significant detachment of colloidal Pu once retained in the sediments was not observed during the subsequent chemical and physical perturbations. But slow release and transport could persist as long as flow continued. The threshold infiltration intensity (0.166 cm/min) revealed a nonmonotonic dependence of the cumulative amount of detached colloidal Pu on the intensity.

► A relationship between colloidal Pu mobility and critical Na+ strength was found.
► Fast deposition rate coefficient for colloidal Pu was experimentally determined.
► Slowly sustained release of retained colloidal Pu occurred in unsaturated flow.
► Mechanisms governing the release of retained colloidal Pu were revealed.