Impact of Natural Organic Matter on Plutonium Vadose Zone Migration from an NH₄Pu(V)O₂CO₃(s) Source

We investigated the influence of natural organic matter (NOM) on the behavior of Pu­(V) in the vadose zone through a combination of the field lysimeter and laboratory studies. Well-defined solid sources of NH₄Pu­(V)­O₂CO₃(s) were placed in two 5-L lysimeters containing NOM-amended soil collected from the Savannah River Site (SRS) or unamended vadose zone soil and exposed to 3 years of natural South Carolina, USA, meteorological conditions. Lysimeter soil cores were removed from the field, used in desorption experiments, and characterized using wet chemistry methods and X-ray absorption spectroscopy. For both lysimeters, Pu migrated slowly with the majority (>95%) remaining within 2 cm of the source. However, without the NOM amendment, Pu was transported significantly farther than in the presence of NOM. Downward Pu migration appears to be influenced by the initial source oxidation state and composition. These Pu­(V) sources exhibited significantly greater migration than previous studies using Pu­(IV) or Pu­(III) sources. However, batch laboratory experiments demonstrated that Pu­(V) is reduced by the lysimeter soil in the order of hours, indicating that downward migration of Pu may be due to cycling between Pu­(V) and Pu­(IV). Under the conditions of these experiments, NOM appeared to both enhance reduction of the Pu­(V) source as well as Pu sorption to soils. This indicates that NOM will tend to have a stabilizing effect on Pu migration under SRS vadose zone field conditions.