Mineral associations and average oxidation states of sorbed Pu on tuff

Subsurface transport of groundwater contaminants is greatly influenced by chemical speciation, precipitation, and sorption processes. The transport of Pu potentially released from spent nuclear fuel disposal and storage sites will be dependent on its interaction with mineral surfaces and speciation in the subsurface. The sorption of dissolved Pu(V) on a natural zeolitic tuff that was equilibrated with synthetic groundwater was examined using synchrotron-based microanalytical techniques. The tuff contained trace quantities of smectites and iron and manganese oxides, which are present as fracture fill and pore space materials. Synchrotron-based micro-X-ray fluorescence (SXRF) showed that Pu is predominately associated with manganese oxides (rancieite) and smectites but not with iron oxides (hematite). In situ micro-X-ray absorption near-edge structure (XANES) spectroscopy measurements on two highly enriched regions (similar to 10 x 15 mu m(2)) of Pu indicated that the average oxidation state of sorbed Pu was (+V) in one region and (+VI) in the other. The observed heterogeneous speciation of the sorbed Pu demonstrates the complex nature of this process. Thermodynamic equilibrium calculations indicated that the solution was dominated by negatively charged Pu species (such as PuO2CO3-), suggesting that sorption to the negatively charged manganese oxide surfaces would be energetically prohibited. Subsequent speciation changes upon sorption to manganese oxide surfaces are discussed.