Using EXAFS to identify sorbed uranium(VI) species on chlorite and secondary Fe phases

The aim of this study was to obtain information about the sorption mechanism of uranium(VI) in chlorite systems. Chlorite is a main mineralogical constituent of phyllite and therefore found in tailings related to the former uranium mining activity in Saxony and Thuringia/Germany. It is also a common subsolidus alteration product in granites and therefore important for risk assessment of high level waste in granitic environments. Alteration of the ferrous chlorite leads to a release of iron, followed by precipitation as ferrihydrite, goethite or hematite. The altered chlorite as well as the ferric iron precipitates affect the uranium migration by sorption processes.

Samples for Extended X-ray Absorption Fine Structure (EXAFS) analyses were prepared under ambient conditions using 200 mg of a ferrous iron chlorite (grain size 2 to 6.3 µm, [Mg_5,5Al_2,48Fe²⁺ _3,02Fe³⁺ _0,94)[(Si_5,33Al_2,66)O₂₀(OH)₁₆], from Flagstaff Hill, California), dispersed in 1000 ml of 0.01 N NaClO₄ solution. The initial uranium(VI) concentrations were set to 1x10^-5 M at approximately pH 6.5. The uranium(VI) was added to the first sample immediately after pH adjustment, allowing no time for alteration reactions. Uranium(VI) was added to the second and third sample allowing an alteration period of 2 and 6 months, respectively. The pH was not adjusted during the alteration. After a sorption period of 6 hours for the first and approx. 60 hours for the second and third sample, the suspensions were centrifuged and the sorption samples were prepared as wet pastes for EXAFS measurements.

Extended X-ray Absorption Fine Structure (EXAFS) is a well-established spectroscopic method to identify the local coordination of uranium. EXAFS spectra were recorded at room temperature in fluorescence mode at the Rossendorf beamline (ROBL) at the ESRF in Grenoble. The measured EXAFS oscillations were fitted using the EXAFSPAK program. The theoretical phase shifts and backscattering amplitudes were calculated with FEFF8.
Two axial oxygens at a distance of 1.79 Å to the uranium atom were found, suggesting that uranium(VI) reduction to uranium(IV) is not the dominant sorption mechanism. The low mean distance of the equatorial oxygen's of 2.32 to 2.36 Å and the high Debye-Waller factors are typical for an inner-sphere surface complexation. No magnesium, aluminum, silicon, or iron backscatterers at distances of 3 to 4 Å , which would indicate a bidentate or monodentate surface complexation, were observed. On the basis of these results it was not possible to attibute the adsorbed uranium to different crystallographic sites (edges versus basal plane sites) including secondary iron sites.
However, a simulation of EXAFS spectra shows a destructive interference of aluminum (or magnesium) and iron EXAFS contributions in bidentate coordination of uranium(VI) on metal octahedras.