Interaction of Pu with Opalinus Clay Studied by μ-XRF, μ-XRD, and μ-XANES

A combination of spatially-resolved synchrotron based techniques (μ-XRF, μ-XRD, μ-XANES) was used to study the distribution and speciation of Pu on Opalinus Clay (OPA) after sorption and diffusion processes. μ-XRF maps showed Ca, Fe, and Pu hot spots on the OPA surface of all investigated thin sections. Pu LIII-edge μ-XANES spectra on Pu hot spots confirmed that Pu(IV) is the dominating species on OPA, i.e., the highly soluble Pu(VI) was retained by OPA in the reduced and less mobile tetravalent oxidation state of Pu. Preliminary μ-XRD results indicate that Pu is localized on or in the close vicinity of the Fe(II) mineral siderite and the clay mineral kaolinite. For all samples fluorescence maps with good signal intensity were collected for Pu, Ca, and Fe. In the regions where Pu localizes, the correlation between Pu and Fe was analyzed using the program XY-MAP DISPLAY. The resulting correlation plots do not immediately point towards an unequivocally correlation between the local concentrations of Pu and Fe. However, in all cases where elevated levels of Pu were detected, a moderately enhanced Fe concentration could be established as well. Areas of highest Fe concentrations, however, turned out to be free of Pu. Similar correlated detection of Pu and Fe was observed also in the diffusion sample. Pu L_III-edge μ-XANES spectra of Pu hot spots show a complete reduction of Pu(VI) to Pu(IV) in all samples. Pu(IV) is the dominating oxidation state after sorption and diffusion processes. Preliminary results of μ-XRD measurements indicate that Pu potentially localizes on siderite and kaolinite mineral phases contained in OPA