| AVS 58th Annual International Symposium and Exhibition | |
| Actinides and Rare Earths Focus Topic | Thursday Sessions |
| Session AC+TF-ThA |
| Session: | The Structure, Properties and Chemistry of Thin Films of Actinides and Rare Earths |
| Presenter: | Moritz Schmidt, Argonne National Laboratory |
| Authors: | M. Schmidt, Argonne National Laboratory P.A. Fenter, Argonne National Laboratory S.S. Lee, Argonne National Laboratory R.E. Wilson, Argonne National Laboratory L. Soderholm, Argonne National Laboratory |
| Correspondent: | Click to Email |
Reliable long-term predictions about the safety of a potential nuclear waste repository must be based on a sound, molecular-level comprehension of the geochemical behavior of the radionuclides. We apply in situ crystal truncation rod (CTR) measurements and resonant-anomalous X-ray reflectivity (RAXR) in combination with alpha-spectrometry to elucidate the sorption behavior of tetravalent actinides on muscovite under varying solution conditions.
Key retention mechanisms, particularly in clay formations, are surface mediated processes. In order to be able to understand these processes analytical techniques that allow selectively probing the mineral/water interface and elucidating processes at the interface under in situ conditions are required. X-ray reflectivity techniques (CTR, RAXR) have proven to be valuable tools for geochemical studies concerning the sorption behavior of metal ions [1] . More recently they have also been applied to study the sorption behavior of actinides [2] .
From CTR measurements the complete adsorption structure, consisting of adsorbed water and the ions adsorbed as inner sphere, outer sphere or extended outer sphere complex can be derived. RAXR extends this approach by providing elemental specificity to the CTR measurements, thus characterizing the contribution of a particular element to the structure.
Using a purpose-built sample cell for radiological experiments, X-ray reflectivity data was collected in situ from muscovite (KAl2[(OH,F)2|AlSi3O10]) in contact with the actinide-bearing solutions varying in composition (ionic strength, actinide concentration and speciation, background electrolyte). The data is complemented by precise quantitative analysis by means of alpha-counting experiments. The results clearly show the strong influence of the actinides’ aqueous chemistry on their sorption behavior. It will be shown that a full description of the sorption behavior requires parameters such as the hydration enthalpy, complexation constants, hydrolysis constants, and polymer formation constants. At low actinide concentrations the formation of hydrated surface complexes is observed in good agreement with the large hydration enthalpies of the highly charged ions. At large excess of background electrolyte sorption occurs under preservation of the complexation by the anion. When a threshold metal ion concentration is exceeded sorption of polymers is observed which grow up to 150Å in size, while covering only small fractions of the surface.
1. Fenter, P., Reviews in Mineralogy and Geochemistry 2002, 49, 149-220.
2. Fenter, P.; Lee, S. S.; Park, C.; Soderholm, L.; Wilson, R. E.; Schwindt, O., GCA, 2010, 74, 6984-6995.