Influence of Oxidation States on Plutonium Mobility during Long-Term Transport through an Unsaturated Subsurface Environment

Abstract

Lysimeter and laboratory studies were conducted to identify the controlling chemical processes influencing Pu(IV) mobility through the vadose zone. A 52-L lysimeter containing sediment from the Savannah River Site, South Carolina and solid PuIV(NO3)4 was left exposed to natural wetting and drying cycles for 11 years before the lysimeter sediment was sampled. Pu had traveled 10 cm, with >95% of the Pu remaining within 1.25 cm of the source. Laboratory studies showed that the sediment quickly reduced Pu(V) to Pu(IV) (the pseudo-first-order reduction rate constant, Kobs, was 0.11 h(-1)). Of particular interest was that this same sediment could be induced to release very low concentrations of sorbed Pu under oxidizing conditions, presumably by oxidation of sorbed Pu(IV) to the more mobile Pu(V) species. Transport modeling supported the postulation that Pu oxidation occurred in the lysimeter sediment; the inclusion of an oxidation term in the model produced simulations that capture the Pu depth profile data. By not including the oxidation process in the model, Pu mobility was grossly underestimated by a factor of 3.5. It is concluded that both oxidation and reduction mechanisms can play an important role in Pu transportthrough the vadose zone and should be considered when evaluating disposal of Pu-bearing wastes.

Developments in Bioremediation of Soils and Sediments Polluted with Metals and Radionuclides. 3. Influence of Chemical Speciation and Bioavailability on Contaminants Immobilization/Mobilization Bio-processes

“…The fate of metal contaminants in soils and sediments is controlled by interdependent influences of transport and biogeochemical reactions (Tokunaga et al 2001(Tokunaga et al , 2003Kaplan et al 2004). Laboratory studies of biogeochemical processes are commonly conducted in well-mixed suspensions and solution cultures (Fjeld et al 2003;Kaplan et al 2004).…”

“…Laboratory studies of biogeochemical processes are commonly conducted in well-mixed suspensions and solution cultures (Fjeld et al 2003;Kaplan et al 2004). Limitations in applying results of laboratory studies to field environments often relate to the fact that the subsurface contains a broad spectrum of interconnected microenvironments, while individual laboratory batch systems represent specific isolated microenvironments.…”

Molecular interactions of plutonium(VI) with synthetic manganese-substituted goethite

“…HClO 4 . The initial pH (pH i ) was recorded and a given amount of solid (5 g/L) was added to each solution.…”

Variability of Kd Values in Cementitious Materials and Sediments

“…This minimum of two days of contact time was selected based on careful oxidation state characterization experimentation and approach to equilibration experimentation conducted on SRS sediments (Kaplan et al, 2004;Powell, 2002b). In short, once Pu(V) comes in contact with SRS sediments, even under bench top conditions, it undergoes near complete reduction to Pu ( Furthermore, to estimate the range and distribution of K d values in a cementitious leachate passing through sediment the above selection criteria were used, except:…”