Paving the way for examination of coupled redox/solid-liquid interface reactions: 1 ppm Np adsorbed on clay studied by Np M5-edge HR-XANES spectroscopy

The recently emerged actinide (An) M4,5-edge high-energy resolution X-ray absorption near-edge structure (HR-XANES) technique has proven to be very powerful for oxidation state studies of actinides. In this work, for the first time, Np M5-edge HR-XANES was applied to study Np sorption on illite. By improving the experimental conditions, notably by operation of the spectrometer under He atmosphere, it was possible to measure Np M5-edge HR-XANES spectra of a sample with ≈ 1 μg Np/g illite (1 ppm). This is 30–2000 times lower than Np loadings on mineral surfaces usually investigated by X-ray absorption spectroscopy. A newly designed cryogenic configuration enabled sample temperatures of 141.2 ± 1.5 K and successfully prevented beam-induced changes of the Np oxidation state. The described approach paves the way for the examination of coupled redox/solid-liquid interface reactions of actinide ions via An M4,5-edge HR-XANES spectroscopy at low metal ion concentrations, which are of specific relevance for contaminated sites and nuclear waste disposal studies. [Display omitted] •Paving the way for in situ coupled redox/solid-liquid interface reactions analysis.•Actinide (An) M4,5-edge HR-XANES spectroscopy at 1 μg An/g solid achieved.•Estimated detection limit for speciation studies on mineral surface adsorbed Ans.•Robust method for exclusion of X-ray beam-induced changes in environmental samples.•Essential analytical advance for contaminated sites and nuclear waste disposal studies.