The use of XAFS spectroscopy and related synchrotron radiation techniques for the molecular-level speciation of environmental contaminants including actinides has led to an improved understanding of the fundamental chemical and biological processes determining their behavior in complex systems. Several recent applications of XAFS spectroscopy to actinides in model systems and more complex environmental samples are reviewed to highlight the impact these studies have on our knowledge about the bioavailability of actinides, the development of remediation strategies, and predictive models for risk assessment. XAFS studies of actinide ion sorption at solid/aqueous solution interfaces are presented in greater detail. Representative examples include XAFS studies in combination with batch-type experiments of U(VI), Np(V), Pu(III), and Pu(IV) sorption on kaolinite.

Antimony­(III) mobility in natural aquatic environments is generally enhanced by dissolved organic matter. Tartaric acid is often used to form complexes with and stabilize dissolved Sb­(III) in adsorption studies. However, competition between such low-molecular-weight organic acid complexation and adsorption of Sb­(III) has received little attention, which prompted us to measure Sb­(III) adsorption by iron oxyhydroxide adsorbents commonly used in water treatment plants. Sb K-edge X-ray absorption fine structure (EXAFS) spectra gave Sb–O and Sb–Fe distances and coordinations compatible with a bidentate binuclear inner-sphere complex with trigonal Sb­(O,OH)3 polyhedra sharing corners with Fe­(O,OH)6 octahedra and a bidentate mononuclear inner-sphere complex but with Sb­(O,OH)4 tetrahedra at alkaline pH. Experimental batch titration data were fitted using the charge-distribution multisite surface complexation (CD-MUSIC) model, constrained by the EXAFS molecular level information and taking competitive effects by the organic ligand into consideration. The proportion adsorbed at acid to neutral pH decreased as the Sb­(III) concentration increased. The CD-MUSIC adsorption model indicates that this was solely caused by strong competition from tartrate complexation in solution, which leads to adsorption constants higher than those derived without taking this competition into account. The adsorption model results allow for calculating a pe–pH predominance diagram using the USGS PhreePlot code. The study provides consistent surface complexation stability constants, allowing the new database to be used also to reliably model adsorption of toxic oxyanions in anoxic aqueous environments: for example, to accurately simulate competition between Sb­(III) and As­(III).

As an alternative to the analysis of EXAFS spectra by conventional shell fitting, the Tikhonov regularization method has been proposed [1]. An improved algorithm that utilizes a priori information about the sample has been developed and applied to the analysis of U L 3-edge spectra of soddyite, (UO 2) 2 SiO 4 •2H 2 O, and of U(VI) sorbed onto kaolinite. The partial radial distribution functions g 1 (UU), g 2 (USi), and g 3 (UO) of soddyite agree with crystallographic values and previous EXAFS results.

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