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Speciation Mapping of Environmental Samples Using XANES Imaging
Citation:
Etschmann, B., E. Donner, J. Brugger, D. Howard, M. de Jonge, D. Paterson, R. Naidu, K. G. SCHECKEL, C. Ryan, AND E. Lombi. Speciation Mapping of Environmental Samples Using XANES Imaging. K. Francesconi (ed.), ENVIRONMENTAL CHEMISTRY. CSIRO Publishing, Collingwood Victoria, Australia, 11(3):341-350, (2014).
Impact/Purpose:
In this study, we further investigate Cu speciation in fresh and aged biosolids at the microscale using a novel XANES-imaging approach. This approach to investigate metal speciation in environmental matrices combines the advantages of bulk and µ-XANES while at the same time offering the potential to overcome their respective limitations. In fact, while it can be argued that bulk XANES provides the most representative speciation of a sample, interpretation of a spectrum is not trivial as it is the weighted sum of the spectral signal of all the species present in the analysed volume. Consequently, species contributing to 5-10% of the overall spectra are not usually resolved unless characterised by unique and intense spectral features. To overcome this issue and to gain an understanding of species distribution, µ-XANES is often employed. Using this approach, XANES spectra are collected at points of interest (POIs). However, probing only a few POIs cannot provide an accurate assessment of the proportion of each species in the whole sample. Furthermore, an ‘operator bias’ may be involved in the selection of POIs.
Description:
Fast X-ray detectors with large solid angles and high dynamic ranges open the door to XANES imaging, in which millions of spectra are collected to image the speciation of metals at micrometre resolution, over areas up to several square centimetres. This paper explores how such multispectral datasets can be analysed in order to provide further insights into the distribution of Cu species in fresh and stockpiled biosolids. The approach demonstrated uses Principal Components Analysis to extract the ‘significant’ spectral information from the XANES maps, followed by cluster analysis to locate regions of contrasting spectral signatures. Following this model-free analysis, pixel-by-pixel linear combination fits are used to provide a direct link between bulk and imaging XANES spectroscopy. The results indicate that both the speciation and distribution of Cu species are significantly affected by ageing. The majority of heterogeneously distributed micrometre-sized Cu sulfide particles present in fresh biosolids disappear during the oxidative stockpiling process. In aged biosolids most of the Cu is homogeneously redistributed on organic matter suggesting that Cu mobility is temporarily increased during this redistribution process. This manuscript demonstrates how large XANES imaging datasets could be analysed and used to gain a deep understanding of metal speciation in environmental samples.
