Advanced EXAFS analysis: Examples from mineral/water interface redox reactions

Depending on their oxidation state, actinides and other metal/metalloid radionuclides show a great variety of hydrolysis constants, solubility, complexation behavior, and sorption/precipitation reactions. Redox conditions have therefore a great impact on the migration behavior of radionuclides and the safety of nuclear waste repositories. The redox conditions of such repositories are often determined by Fe(II) and mixed Fe(II,III) (hydr)oxides, which are naturally widespread in host rocks or form as corrosion products on steel containers. Furthermore, they catalyze metal/metalloid reduction processes at their surfaces, thereby speeding up reaction kinetics which would otherwise be extremely slow. In spite of their relevance, such interfacial redox processes are still poorly understood, since they proceed through complex reaction schemes, and are commonly far from thermodynamic equilibrium.
X-ray absorption spectroscopy encompassing the methods XANES and EXAFS is in principle ideally suited to elucidate such processes by deciphering in situ the oxidation state and short range structure of radionuclides at reactive surfaces. However, the inherent shortcomings of the two methods, like limited energy resolution for XANES, and distal and angular resolution, non-Gaussian disorder as well as limited elemental resolution of backscattering atoms for EXAFS, make the data interpretation with conventional analysis methods often difficult and do not allow the derivation of unequivocal conclusions. I will therefore demonstrate the usefulness of a range of advanced XAFS analysis methods, including (1) Iterative transformation factor analysis (2), (2) Monte-Carlo analysis (see Fig. 1) (3), (3) wavelet analysis (4,5), and (4) Landweber-iteration to derive the pair distribution function from EXAFS spectra.

KEYWORDS: XAFS modeling, wavelet, factor analysis, PDF, Landweber

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