Comparison of two micro-analytical methods for detecting the spatial distribution of sorbed Pu on geologic materials

Subsurface transport of groundwater contaminants is greatly influenced by chemical speciation, precipitation and sorption processes at the mineral–water interface. The retardation of contaminants is often greatest at boundaries between minerals and in fractures and pore spaces. The investigation of the spatial distribution of sorbed contaminants along these boundaries requires micro-analytical techniques. The sorption of dissolved Pu(V) on a natural zeolitic tuff from Yucca Mountain (NV, USA) was examined using microautoradiography (MAR), X-ray diffraction (XRD), electron microprobe (EM) techniques, and synchrotron-based micro-X-ray fluorescence (micro-SXRF). The tuff contained a heterogeneous distribution of zeolites and trace quantities of smectites, Fe oxides (hematite), and Mn oxides (rancieite), which are present as fracture fill and pore space materials. Micro-SXRF studies showed that Pu is mostly associated with bodies of smectite plus Mn oxides, which were typically elevated in Ce, Ga, Nb, Pb, Y, Ca, Ti, and Zn. Sorbed Pu was not associated with Fe-rich bodies, which were enriched in Cl and Rb. Results of the MAR studies were complementary to that of the micro-SXRF studies in that Pu was associated with similar elements in the tuff. Indirect detection of Pu by EM or micro-SXRF (by analyzing Ag developed on the MAR photoemulsion) was a more sensitive method for detecting lower levels of sorbed Pu than the direct detection of sorbed Pu via micro-SXRF in the absence of the photoemulsion.