PNNL

Abstract

Plutonium (Pu) and americium (Am) mobility in unsaturated subsurface environments is controlled by complex geochemical processes, including precipitation, adsorption, complexation, and colloid formation. Pu can occur in multiple oxidation states (+3, +4, +5, and +6 in environmental conditions) and undergoes radioactive decay to hazardous americium (241Pu to 241Am). Am occurs in the +3-oxidation state under environmental conditions and participates in different complexation and sorption reactions that affect its mobility. To identify the role of waste constituents, initial Pu/Am speciation, and sediment interactions in controlling Pu and Am mobility; the solution concentrations of Pu and Am in contaminated sediments from beneath the Hanford 216-Z-9 (Z-9) Trench were compared to the solubility of pure PuO2 materials synthesized by processes similar to those that produced the wastes disposed to the trench (i.e., calcination of Pu oxalate and oxidation of Pu metal). The results of this study show that the solubilities of three PuO2 materials synthesized by different methods and with varying particle sizes, were comparable to results from previous studies that identified solubilities under acidic conditions. The solubilities agree with hydrous PuO2 present as less crystalline coatings on PuO2, although it took longer to reach equilibrium for the PuO2 synthesized from Pu metal likely due to its larger size and lower surface area for reaction. PuO2 has been identified previously in the solid phase in these sediments and this research showed that PuO2 is likely controlling Pu release under conditions where phosphate concentrations are low. However, saturation index calculations suggest that both Pu-phosphate and Am-phosphate phases play a role in controlling release at low pH, higher phosphate concentrations in the sediments from near the trench bottom. The elevated phosphate is likely due to decomposition of tributyl phosphate over time. Deeper sediments are less acidic and contain less phosphate than those from the trench bottom. In this case, the Pu concentration is likely controlled by hydrated PuO2 precipitated following neutralization of the acidic waste stream. Solubility controls on Am in deeper sediments are unclear and potentially involve sediment sorption and/or PuO2 controlled release. The concentrations of both Pu and Am were elevated in the colloidal fraction with trench bottom sediments but not in pure PuO2 experiments, suggesting the potential presence of Pu/Am pseudocolloids (e.g., Pu/Am associated with fine mineral particles). However, there was not a significant fraction of Pu and Am associated with the colloidal size fraction in deeper sediments, suggesting transport of Pu and Am to these depths was not due to colloidal transport.