Plutonium sorption and desorption behavior on bentonite

Understanding plutonium (Pu) sorption to, and desorption from, mineral phases is key to understanding its subsurface transport. In this work we study Pu(IV) sorption to industrial grade FEBEX bentonite over the concentration range 10−7-10−16 M to determine if sorption at typical environmental concentrations (≤10−12 M) is the same as sorption at Pu concentrations used in most laboratory experiments (10−7-10−11 M). Pu(IV) sorption was broadly linear over the 10−7-10−16 M concentration range during the 120 d experimental period; however, it took up to 100 d to reach sorption equilibrium. At concentrations ≥10−8 M, sorption was likely affected by additional Pu(IV) precipitation/polymerization reactions. The extent of sorption was similar to that previously reported for Pu(IV) sorption to SWy-1 Na-montmorillonite over a narrower range of Pu concentrations (10−11-10−7 M). Sorption experiments with FEBEX bentonite and Pu(V) were also performed across a concentration range of 10−11-10−7 M and over a 10 month period which allowed us to estimate the slow apparent rates of Pu(V) reduction on a smectite-rich clay. Finally, a flow cell experiment with Pu(IV) loaded on FEBEX bentonite demonstrated continued desorption of Pu over a 12 day flow period. Comparison with a desorption experiment performed with SWy-1 montmorillonite showed a strong similarity and suggested the importance of montorillonite phases in controlling Pu sorption/desorption reactions on FEBEX bentonite.