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

The recently emerged actinide (An) M-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 M-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 M-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 M-edge HR-XANES spectroscopy at low metal ion concentrations, which are of specific relevance for contaminated sites and nuclear waste disposal studies.