Synchrotron-Based X-Ray Spectromicroscopy of Organic Nanoparticles Complexing Actinides

Abstract

Humic acids (HA) have a high binding capacity toward traces of toxic metal cations, including actinides, and have a propensity for forming nano- to micron-sized colloids. They can therefore act as transport vehicles for actinides in aquatic systems, which can have detrimental consequences for the safety of nuclear waste disposal. Modern scanning transmission X-ray microscopy (STXM) at the carbon K(1s)-edge is an excellent tool for investigations of such and other processes. STXM studies, combined with other sophisticated techniques and quantum chemical calculations, provide the necessary spatial resolution in the sub-μm range to resolve characteristic aggregate morphologies and identify molecular processes involved in HA–actinide interaction. Systematic experimental and theoretical investigations of reference systems allow successful interpretation of C 1s near edge X-ray absorption fine structure (NEXAFS) extracted from STXM image stacks of HA–metal aggregates. The results show that different HA domains (of likely structurally supramolecular HA associations) exhibit different interactions with metal cations. Metal cations bound to HA are enriched in a minority fraction containing higher densities of complexing carboxylic sites. This fraction is likely to play a dominant role in HA colloid-mediated transport of actinides and other toxic trace metals in the hydrosphere.