Sequential extraction techniques have been developed to assess partitioning of anthropogenic radio-nuclides (Pu-240, Pu-239, and Np-237), originating from a variety of sources, as identified by using bulk sediment isotopic composition. Sediments were leached sequentially with a series of six chemical treatments designed to approximate different environmental processes that may occur or to selectively extract trace metals contained in different solid phases of the sediments (i.e., exchangeable, reducible, carbonate, organic, acid leachable, and refractory). Results indicate the majority of Pu and Np is similarly distributed within many of the extracted fractions, with the largest percentage (66-97%) of both elements being observed in sediments treated with buffered citrate dithionite (CDB), which targets easily reduced constituents such as Mn and Fe hydrous oxides. While these results do indicate an association of Pu and Np with redox sensitive elements, the environmental implications are unclear given that the CDB treatment is more extreme than naturally occurring conditions. Minor amounts of Np partition differently from Pu in sediments. The NH4-acetate treatment, which is designed to liberate trace metals that are loosely adsorbed onto the surfaces of sedimentary materials such as hydrated iron oxides and humic substances, or present at exchangeable sites in clay minerals, mobilized similar to 12% of the total Np while Pu levels were below detection. The H2O2 treatment, which is designed to liberate trace metals bound to organic matter, mobilized similar to 8 and similar to 1% of Np and Pu, respectively. These results indicate that a minor portion of the total Np may be affected by environmental conditions that have little or no effect on Pu. Between 7 and 24% of the Pu was observed in treatments designed to liberate Pu and Np that are tightly bound to lithogenic phases or refractory silicates. The Pu-241/Pu-219 observed in accessible and refractory fractions ranged between 0.11 and 0.18. With the exception of one sample, the Pu-240/Pu-239 isotope ratios measured in the refractory fractions are essentially identical having a mean ratio value of 0.123 +/- 0.001 (1 sigma). The Pu-140/Pu-131 ratios provide isotopic evidence that suggests a portion of non-fallout contamination has a refractory nature. The presence of similarly low Pu-240/Pu-239 ratios in refractory fractions of sediments from the Ob and Irtysh Rivers suggests the existence of a source of refractory Pu which is consistent with refractory "hot particles" derived from surface tests at the Semipalitinsk test site. (c) 2009 Elsevier Ltd. All rights reserved.
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