The impact of atmospheric carbonate on the sorption of actinyl(V/VI) ions onto gibbsite studied by in situ ATR FT IR spectroscopy

The migration behavior of heavy metal contaminants like actinyl ions (U, Np) is mainly controlled by sorption processes at water-mineral interfaces [1]. Hence, the investigation of the interactions of actinides with metal oxides such as Al(OH)3, Fe(OOH)x, TiO2, or SiO2, serving as model phases for more complex, naturally occurring minerals in aqueous solution, becomes essential for the safety assessment in the near and far field of nuclear repositories.

In this study, gibbsite is used as a mineral model system because it the most common crystalline aluminum hydroxide and an ubiquitous weathering product of aluminosilicate minerals. Furthermore, the structure of gibbsite, Al(OH)6 octahedrons, occurs as parts of the structure of important clays like kaolin [2]. Gibbsite is very stable under environmental conditions and is capable to sorb anions and metal cations as well on its surface [3].

Spectroscopic data of surface complexes of uranium(VI) on gibbsite were obtained from batch samples by by time-resolved laser-induced fluorescence spectroscopy [4] and X-ray absorption spectroscopy [5]. From these studies, the formation of a bidendate mononuclear inner-sphere surface complex was derived. In case of Np(V), no spectroscopic data of surface complexes on gibbsite are available up to now. However, the formation of inner-sphere complexes of NpO2+ ion was suggested from batch experiments [6].

Attenuated total reflection Fourier-transform (ATR FT-IR) spectroscopy is a powerful technique for in situ investigations of U(VI) and Np(V) sorption processes in a micro molar actinyl concentration range [7]. Structural information of the molecular complexes occurring during the sorption processes of actinide ions on the solid-liquid interface of mineral phases can be obtained [8]. In this work we focus on the sorption behavior and the formed complexes of uranium(VI) on synthetic gibbsite. The experiments were performed in the presence and absence of atmospheric carbonate in order to illustrate the impact of carbonate ions on the sorption processes. In the absence of carbonate, only one inner-sphere complexes are formed at the mineral surface. In addition, surface precipitation was observed after prolonged sorption which can be derived from a characteristic absorption band at 942 cm−1 (Fig. 1). In the presence of carbonate, two different surface species were derived from the spectra: an inner-sphere U(VI) complex and a ternary carbonate containing uranyl surface complex. The inner-sphere complex is suggested from the significantly shifted frequency of the antisymmetric stretching vibration ν3(UO2) (~ 913 cm−1) compared to the aqueous U(VI)-species (~ 923 cm−1). A much more red-shifted absorption band (903 cm−1) is observed in ambient atmosphere which is obviously due to the formation of a carbonate containing uranyl complex (Fig. 1).

The sorption of a pentavalent actinyl ion, that is Np(V), onto gibbsite was investigated to gain information of the migration behavior of pentavalent actinide ions. The results obtained from first sorption experiments performed at pH 7.6 in the absence of atmospheric carbonate suggest the formation of stable surface species, most probably an inner-sphere complex, which can be derived from the significant shift of the band representing the antisymmetric stretching vibration ν3 of the NpO2+ ion to lower frequencies.

This study provides a first comparative insight into the course of the surface complex formation of U(VI) and Np(V) onto gibbsite on a molecular level.

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