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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.

The sorption and diffusion behavior of 8 x 10(-6) M Np(V) in Opalinus Clay (OPA) with synthetic pore water (pH 7.6) as mobile phase was studied under ambient conditions by batch and diffusion experiments, respectively. The Kd value determined by batch experiments with OPA suspensions is equal to 0.025 +/- 0.005 m3/kg. The diffusion-accessible porosity epsilon of intact OPA as determined by through- and out-diffusion experiments with tritiated water (HTO) is equal to 0.15 +/- 0.01. The diffusion coefficient De and the rock capacity factor alpha of 22Na+ in OPA were measured by through-, out-, and in-diffusion experiments and asserted the reliability of these diffusion techniques. For the diffusion of Np(V) in synthetic pore water, the capillary method gave Dw = (6.0 +/- 1.0) x 10(-10) m/s. Due to the strong sorption of Np(V) on CPA, the diffusion of Np(V) was investigated bythe in-diffusiontechnique.The diffusion parameters for Np(V) in OPA are De = (6.9 +/- 1.1) x 10(-12) m2/s and alpha = 243 +/- 4. This corresponds to Kd = 0.10 +/- 0.01 m3/kg for the sorption of Np(V) in intact OPA.

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