Characterization of Gorleben groundwater colloids by scanning transmission X-ray microscopy

The role of groundwater colloids influencing mobilization or immobilization of actinides in natural aquatic systems is addressed in the context of safety aspects of nuclear waste disposal. Ground-water colloids from the Gorleben site (Lower Saxony, northwest Germany) are characterized in the presence of Eu(III) as a homologue for trivalent actinides. Scanning transmission X-ray microscopy (STXM) measurements are performed with samples in contact with aqueous solution to ensure that colloids and their aggregates are in the native hydrated state. At least five regions with different C K near-edge X-ray absorption fine structure (NEXAFS) spectral features can be detected in the STXM micrographs of the Eu(III)-saturated humic aggregates. The C K NEXAFS measurements indicate segregation of carbon species into regions with different contents of carboxylic, aromatic, phenolic or aliphatic chemical functional groups.