Sorption of Np(V) onto TiO2, SiO2, and ZnO: An in Situ ATR FT-IR Spectroscopic Study

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Abstract
The migration of hazardous neptunium is strongly affected by sorption processes at the solid-water interface. Up to now, almost no spectroscopic data are available to characterize Np(V) species on a molecular level. For the first time, at a micromolar concentration level the Np(V) speciation in aqueous solution and the sorption of Np(V) onto metal oxides were studied using NIR and in situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. Reference data for future investigations of neptunyl(V) sorption processes on natural mineral phases under environmental conditions are provided. The obtained spectra of aqueous Np(V) solutions confirmed the predominance of the fully hydrated species NpO2(+) up to pH 7.7, predicted by the updated NEA thermochemical database. From the Np(V) sorption studies on TiO2, stable surface species of NpO2(+) are derived. The type of the sorbed species can be elucidated by a spectral shift (approximately 30 cm(-1)) to lower wavenumbers of the antisymmetric stretching vibration v3(NpO2(+)) compared to the aqueous species suggesting an inner-sphere complexation. Outer-sphere complexation is found to play a minor role due to the pH independence of the sorption species throughout the pH range 4-7.6. The comparative spectroscopic experiments of Np(V) sorption onto TiO2, SiO2 and ZnO indicate structurally similar bidentate surface complexes.