Eleven-Year Field Study of Pu Migration from Pu III, IV, and VI Sources

Abstract

Understanding the processes controlling Pu mobility in the subsurface environment is important for estimating the amount of Pu waste that can be safely disposed in vadose zone burial sites. To study long-term Pu mobility, four 52-L lysimeters filled with sediment collected from the Savannah River Site near Aiken, South Carolina were amended with well-characterized solid Pu sources (PuIIICl3, PuIV(NO3)4, PuIV(C2O4)2, and PuVIO2(NO3)2) and left exposed to natural precipitation for 2-11 years. Pu oxidation state distribution in the Pu(III) and Pu(IV) lysimeters sediments (a red clayey sediment, pH = 6.3) were similar, consisting of 0% Pu(III), >92% Pu(IV), 1% Pu(V), 1% Pu(VI), and the remainder was a Pu polymer. These three lysimeters also had near identical sediment Pu concentration profiles, where >95% of the Pu remained within 1.25 cm of the source after 11 years; the other 5% of Pu moved at an overall rate of 0.9 cm yr(-1). As expected, Pu moved more rapidly through the Pu(VI) lysimeter, at an overall rate of 12.5 cm yr(-1). Solute transport modeling of the sediment Pu concentration profile data in the Pu(VI) lysimeter indicated that some transformation of Pu into a much less mobile form, presumably Pu(IV), had occurred during the course of the two-year study. This modeling also supported previous laboratory measurements showing that Pu(V) or Pu(VI) reduction was 5 orders of magnitude faster than corresponding Pu(III) or Pu(IV) oxidation. The slow oxidation rate (1 x 10(-8) hr(-1); t1/2 = 8000 yr) was not discernible from the Pu(VI) lysimeter data that reflected only two years of transport butwas readily discernible from the Pu(III) and Pu(IV) lysimeter data that reflected 11 years of transport.

Uptake, distribution, and velocity of organically complexed plutonium in corn (Zea mays)

“…Most of the Pu was observed near the sources, however for all sources significant transport was observed below and above the source burial depth. Native grasses (primarily, crabgrass (Digitaria sanguinalis), bahiagrass (Paspalum notatum), and broomsadge (Andropogon virginicus)) invaded the open lysimeters and were periodically cut and deposited on the soil (Demirkanli et al, 2008(Demirkanli et al, , 2009Kaplan et al, 2006). Over long time periods in the field, plant transpiration accounts for a large water flux, so relatively small Pu uptake by plants may affect its distribution in the root zone (Kaplan et al, 2010).…”

Going extreme for small solutions to big environmental challenges.

“…Briefly, an open top lysimeter gallery was operated at the U.S. Department of Energy's Savannah River Site (located in Aiken, SC USA) for 11 years (1981 -1991) to study the affects of natural environmental conditions and soil biogeochemical processes on Pu mobility and speciation (Kaplan et al, 2004(Kaplan et al, , 2006. Lysimeters were backfilled with native vadose soil (kaolinite -Fe-oxide composition, 0.01% OM, 0.1 ppm TOC, low ionic strength, pH 5.5) and characterized sources of weapons grade 238,239,240 Pu (III, IV, VI); leachate samples were collected regularly for Pu speciation.…”

Neptunium Iv and v Sorptin to End-Member Subsurface Sediments to the Savannah River Site

“…By chemical analogy, Pu, which also exists primarily in the +5 oxidation state in the aqueous phase, PuO 2 + , and which has only one atomic number more than Np (Pu = 94), readily reduces to PuO 2(s) in a matter of minutes when it comes in contact with vadose zone SRS sediments (Kaplan et al 2004). Once the PuO 2(s) forms, Pu solubility decreases sharply to 10 -9 M (K d values of 10,000 L kg -1 ; Kaplan et al 2006). It was this observation that motivated us to expect that some NpO 2 + might be reduced by SRS sediments.…”