Application of Hard X-Ray Microprobe Methods to Clay-Rich Materials
Modern synchrotron-based hard X-ray microprobes allow clay and soil scientists to characterize clay mineralogy and the abundance, distribution and speciation of elements that are both essential structural constituents in clays and adsorbed or incorporated within the clay structure. The instruments can be used to evaluate minute samples that are heterogeneous at the sub-micrometer scale, often in an as-collected state. Synchrotron radiation sources are ideal for developing high-intensity, highly focused X-ray probes and these instruments offer distinct advantages over other analytical techniques by allowing analyses to be done in situ with little or no chemical pretreatment and with low detection limits. The current generation of hard X-ray microprobes provides attogram detection levels for transition and heavy metals, allows for coupled molecular speciation analysis using X-ray absorption fine structure (XAFS) analysis at parts per million concentration, and can be used to evaluate mineralogy at spatial resolutions of <1 μm using X-ray microdiffraction. Microfocused XAFS provides researchers with the ability to evaluate micrometer-scale heterogeneities in elemental valence state and molecular speciation. Synchrotron X-ray microdiffraction now allows mineralogists to obtain diffraction patterns from sample masses on the order of 1 μg or less, with data quality (signal to noise ratio) considerably better than the best diffraction patterns obtainable from commercial laboratory-based diffractometers. Advances in X-ray optics and the construction of new synchrotron sources with low emittance have led to the recent availability of hard X-ray microprobe beamlines with spatial resolutions as small as 30 nm. Examples of the application of these techniques to the study of the mineralogy and geochemistry of clays include the mobility and adsorption of potentially toxic elements in natural clays and geosynthetic clay liners, evaluation of how clays impact the valence state of actinides in soils and in the subsurface, and in visualizing clay gels in a wet state.