OOS 47-2
From the micro- to the nanoscale – studying soil microsites using X-ray spectromicroscopy
X-ray spectromicroscopy is extremely suitable to probing soil microsites with high spectral and high spatial resolution. Routinely, X-ray microprobes reach sub-micrometer spatial resolutions, whereas nanoprobes image even sub-100nm sized structures. Clear images without scattering background are obtained even when studying thick and inhomogeneous samples. Therefore, X-ray microscopy images can be used for tomographic reconstructions of thick samples. By tuning the X-ray energy appropriately, it is possible to perform spectromicroscopy studies. At X-ray energies of several keV, X-ray fluorescence can be used as a highly sensitive method to identify trace elements. Comprising, the combination of microscopy and spectroscopy is a powerful way for addressing key questions in many scientific areas, e.g. to study structures in the environment showing dimensions on the nanoscale. Electron storage rings, being X-ray light sources of extreme brightness, are the site of installation for X-ray spectromicroscopy stations. The upcoming NSLS-II with its ultra high brightness and exceptional beam stability, promises to be the world’s leading light source for an X-ray spectromicroscopy station suited for these studies. A combination of x-ray microscopy, elemental mapping and X-ray absorption near edge structure (XANES) spectroscopy at the K-absorption edges of sulfur (S) and iron (Fe) was used to analyze the elemental and particulate composition of microsites in soils. Two examples will be presented.
Results/Conclusions
Spatial patterns of S and Fe speciation as well as relationships between the speciation of S and Fe in soil colloids or aggregates at the micrometer and sub-micrometer level have been studied, using an anoxically prepared dissected soil aggregate from the BC horizon of a clayey Cambisol located near Balingen (SW Germany) that had formed by weathering of Jurassic shale. The study includes elemental mapping at the S and Fe K-edges, acquisition of images of regions of interest with several X-ray energies, as well as acquisition of S and Fe XANES spectra at different positions, where image analysis suggests the dominance of reduced and oxidized S and Fe, respectively. The elemental and particulate composition of an urban soil loaded with debris from WWII, exemplarily from Berlin, Germany, has been analyzed using X-ray spectromicroscopy. The goal has been to specify and analyze the sulfur pool of soils with major anthropogenic impact, i.e. the dumping of war debris. This impact obviously influences soil composition and soil formation processes, but may, due to sulfate leaching, also be a substantial risk to urban water quality.