Understanding Humic Acid Metal Ion Interaction - A Spectromicroscopy Approach

摘要

Humic acids (HA) play an important role in transport and bioavailability of toxic substances in the geosphere. Due to their chemical heterogeneity and polydispersity, HA exhibit a manifold of possible interactions in the environment. This may include complexation of actinide ions (e.g., Cm(lll) ions) released from an underground repository by spent nuclear fuel dissolution, interaction with actinide 'eigencolloids' (e.g., Pu(lV) oxo/hydroxo species) or coating of inorganic colloids like clays. In-situ spectroscopy and microscopy techniques allowing the characterization of these aquatic colloid species in their hydrated state have become recently available. STXM (Scanning Transmission X-ray Microscopy) at the carbon K-edge directly probes the chemical functional groups involved in HA reactions with a suborn resolution. C 1s-NEXAFS (Near Edge X-ray Absorption Fine Structure) provides - at the same time - direct speciation of carbon containing macromolecules from their characteristic 1s -> pi, sigma* transition resonances. It has been demonstrated by comparison to model substances that modifications of these transition patterns following metal ion interaction give direct hints towards the underlying complexation mechanisms (1). Laser scanning luminescence microscopy (LSLM) reveals the spatial distribution of luminescent trace metal cations in aquatic colloids and their aggregates at a lateral resolution similar to STXM. In the present study STXM and LSLM are applied to investigate the Eu(lll)-HA interaction by correlating the HA carbon density and NEXAFS pattern with the Eu(lll) distribution.