Sorption of Np(V) onto metal oxide surfaces: An in situ ATR FT-IR spectroscopic study.

Neptunium (Np) is one of the most important components of nuclear waste to consider for the long-term safety assessment of nuclear waste repositories and hence, great attention is paid to its geochemistry and migration behavior [1]. Among the various geochemical processes, the migration of actinides in the environment is strongly affected by molecular reactions at the solid-water interface, e.g. sorption onto mineral phases, surface precipitation, and colloid formation [2]. In aqueous solution the pentavalent form dominates the neptunium speciation under a wide range of environmental conditions [1]. Thus, reliable results of Np(V) sorption data and its molecular speciation at the water-mineral interface are crucial to allow an improved modeling of Np migration in the environment. Vibrational spectroscopy is a useful tool for the in situ identification of molecular species in aqueous solution and sorbed onto mineral surfaces [3, 4]. Using attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy, the type of the sorbed species, e.g. inner- and outer-sphere complex, can be elucidated by shifts of the antisymmetric stretching vibration ν3 of the Np=O bond compared to the aqueous species.

In this study, we investigated the Np(V) speciation in aqueous solution and upon sorption onto oxides of titanium, silicon, and zinc at a micromolar concentration level by application of NIR and ATR FT-IR spectroscopy.

The obtained spectra of aqueous 50 µM Np(V) solution confirmed the predominance of the fully hydrated neptunyl(V) species NpO2+ up to pH 7.7 at inert gas atmosphere, excluding atmospheric CO2, as was predicted by the updated NEA thermodynamic database [5]. Upon sorption of 50 µM Np(V) on TiO2 at pH 7.6 at inert gas atmosphere, NpO2+ forms stable surface species. The formation of an inner-sphere complex can be derived from the significant shift of the band representing the antisymmetric stretching vibration ν3 of the NpO2+ ion. Furthermore, since the spectra of the sorption processes show no significant deviations within the pH range from 4 to 7.6, and no indications of aqueous NpO2+ species are obtained, the formation of outer-sphere complexes can be neglected. Additionally, from the on-line monitored measurements it is obvious that only one Np(V) surface species is formed during the sorption experiments.

A comparative in situ ATR FT-IR investigation of Np(V) sorption onto TiO2, SiO2 and ZnO indicates the formation of structurally similar inner-sphere surface complexation. From minor spectral deviations of the ν3 band representing the Np(V) surface species bidentate complexes can be suggested.

For future spectroscopic investigations of neptunyl sorption at the water-mineral interface the present work may provide vibrational reference data for the interpretation of more complex systems relevant for environment.

1.Kaszuba, J. P.; Runde, W. H., Environmental Science & Technology 1999, 33, (24), 4427-4433.
2.O'Day, P. A., Reviews of Geophysics 1999, 37, (2), 249-274.
3.Lefevre, G., Advances in Colloid and Interface Science 2004, 107, (2-3), 109-123.
4.Müller, K.; Brendler, V.; Foerstendorf, H., Inorganic Chemistry 2008, 47, (21), 10127-10134.
5.Guillaumont, R.; Fanghänel, T.; Fuger, J.; Grenthe, I.; Neck, V.; Palmer, D. A.; Rand, M. H., Update on the Chemical Thermodynamics of U, Np, Pu, Am and Tc. Elsevier: Amsterdam, 2003.