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) M-4,M-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 acti-nides. In this work, for the first time, Np M-5-edge HR-XANES was applied to study Np sorption on illite. By improving the experimental conditions, notably by operation of the spectrometer under He atmo-sphere, it was possible to measure Np M-5-edge HR-XANES spectra of a sample with similar to 1 mg Np/g illite (1 ppm). This is 30-200 0 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-4,M-5-edge HR-XANES spectroscopy at low metal ion concentrations, which are of specific relevance for contaminated sites and nuclear waste disposal studies.(c) 2022 The Authors. Published by Elsevier B.V.

Automated summary

The recently developed high-energy resolution X-ray absorption near-edge structure (HR-XANES) technique has proven to be a powerful tool for studying the oxidation states of actinide elements. In this study, Np M-5-edge HR-XANES was applied for the first time to investigate the sorption of Np on illite. By optimizing the experimental conditions and employing a cryogenic configuration, the researchers were able to obtain Np M-5-edge HR-XANES spectra of a sample with very low Np loadings, which opens up new possibilities for examining redox reactions and solid-liquid interface reactions of actinide ions.