Nanoscale spatial analysis of clay minerals containing cesium by synchrotron radiation photoemission electron microscopy

摘要

A synchrotron radiation photoemission electron microscope (SR-PEEM) was applied to demonstrate the pinpoint analysis of micrometer-sized weathered biotite clay particles with artificially adsorbed cesium (Cs) atoms. Despite the insulating properties of the clay, we observed the spatial distributions of constituent elements (Si, Al, Cs, Mg, and Fe) without charging issues and clarified reciprocal site-correlations among these elements with nanometer resolution. We found that Cs atoms were likely to be adsorbed evenly over the entire particle; however, we identified an occupational conflict between Cs and Mg atoms, implying that Cs sorption involves ion exchange processes. Spatially resolved X-ray absorption spectra (XAS) of the Cs_(4,5) M-edge region showed Cs to be present in a monocation state (Cs~+) as typically observed for Cs compounds. Further pinpoint XAS measurements were also performed at the Fe L_(2,3)-edge to determine the chemical valence of the Fe atoms. The shapes of the spectra were similar to those for Fe_2O_3, indicating that Fe in the clay was in a 3+ oxidation state. From these observations, we infer that charge compensation facilitates Cs adsorption in the vicinity of a substitution site where Si~(4+) ions are replaced by Fe~(3+) ions in SiO_4 tetrahedral sheets. Our results demonstrate the utility of SR-PEEM as a tool for spatially resolved chemical analyses of various environmental substances, which is not limited by the poor conductivity of samples.