Direct Spectroscopic Characterization of Aqueous Actinyl(VI) Species: A Comparative Study of Np and U Katharina Mu ¨ ller,* Harald Foerstendorf, Satoru Tsushima, Vinzenz Brendler, and Gert Bernhard Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf e.V., P.O. Box 510119, D-01314 Dresden, Germany ReceiVed: January 30, 2009; ReVised Manuscript ReceiVed: May 5, 2009 The hydrolysis reactions of Np(VI) were investigated under an ambient atmosphere by attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy, NIR absorption spectroscopy, and speciation modeling applying the updated NEA thermodynamic database. For the first time, spectroscopic results of Np(VI) hydrolysis reactions are provided in the submillimolar concentration range and at pH values up to 5.3. The calculated speciation pattern and the results from FT-IR spectroscopy are comparatively discussed with results obtained from the U(VI) system under identical conditions. For both actinides, the formation of similar species can be derived from infrared spectroscopic results at pH values e4, namely, the free cation AnO 2 2+ (An ) U, Np) and monomeric hydrolysis products. At higher pH, the infrared spectra evidence structurally different species contributing to the speciation of both actinides. At pH 5, the formation of a carbonate-containing dimeric complex, that is, (NpO 2 ) 2 CO 3 (OH) 3 - , probably occurs during the hydrolysis reactions of neptunium, which is supported by the calculated speciation and results from NIR spectroscopy. For uranium, the presence of additional hydroxo complexes is assumed in this pH range. However, an unequivocal assignment of the spectral features to distinct species remains difficult. In particular, in the concentration range (0.5 mM) that constitutes the lower limit for the spectroscopic investigations of Np(VI) in the present work, monomeric and polymeric species obviously contribute to the U(VI) speciation considerably increasing the complexity of the spectral data. Introduction For the long-term storage of nuclear waste, the assessment of water contamination, depending on retention and migration processes of radionuclides in the geosphere, is of primary environmental concern. The migration behavior of (radioactive) contaminants, that is, their mobility and bioavailability in the environment, is strongly affected by molecular reactions oc- curring in and among solid, aqueous, and gas phases. 1 Nep- tunium (Np) is one of the most important actinide components of nuclear waste. Although the neptunium concentration in spent nuclear fuel is relatively low (0.03%), its concentration increases with time because of the radioactive decay of 241 Am (half-life of 432.7 years). The isotope 237 Np will become a major contributor to the radiation inventory of nuclear waste reposi- tories after about 100 000 years because of its long half-life (2.14 × 10 6 years). 2,3 Therefore, in the long term safety assessment of underground disposals, great attention must be paid to its geochemistry and migration behavior. 4 Geochemical reactions, such as complexation in solution, sorption onto mineral and biological phases, and colloids formation, are primarily defined by the oxidation state and the distribution of appropriate aqueous species of Np. 4-6 The variety of possible oxidation states, ranging from +III to +VII is one of the peculiarities of the Np chemistry. Although, in aqueous solution, neptunium mainly occurs as the neptunyl ions NpO 2 n+ , where n ) 1 for Np(V) and 2 for Np(VI). 7 The pentavalent ion dominates the Np speciation under a wide range of environ- mental conditions. 8,9 However, the solution chemistry of the hexavalent form is predicted to be relevant under oxidizing conditions of near-surface groundwater. 10-12 The speciation of Np(VI) in aqueous solution under an ambient atmosphere is basically controlled by hydrolysis reactions and complexation with dissolved atmospheric carbon dioxide, with a strong dependence on the concentration level and pH range. 5,13 In acidic Np(VI) solutions, the uncomplexed cation NpO 2 2+ is known to be the predominant species. Under higher pH conditions, hydroxo and mixed hydroxo carbonate species may form. 14,15 Up until now, very few studies dealt with the speciation of neptunium, and most of them focused on Np(V). 6,9,16-22 The current knowledge of thermodynamic constants of Np(VI) species is mainly based on data exclusively resulting from potentiometry and solubility measurements performed at Np(VI) concentrations in the millimolar range. 10,14,15,23 But a structural characterization of the found species by spectroscopic techniques is still insufficient. Furthermore, modeling the commonly accepted thermodynamic data at a reduced concentration level that is adequate to environmental conditions requires extrapola- tion, risking inaccuracies. Therefore, a verification of the Np(VI) species at low concentrations is of urgent need for trustworthy data evaluation. Vibrational spectroscopy and spectrophotometry are useful tools for the identification of aqueous molecular species. Regarding vibrational spectroscopy, mainly Raman studies were carried out to investigate Np(VI) hydrolysis in the past. 24-27 However, they were restricted to Np(VI) concentrations in the millimolar range (∼0.1 M) because of the detection limit of the applied instrumentation. Consequently, the experiments had to be performed in acidic or in carbonate solutions to avoid * Corresponding author. Tel: ++49 351 260 2438. Fax: ++49 351 260 3553. E-mail: k.mueller@fzd.de. J. Phys. Chem. A 2009, 113, 6626–6632 6626 10.1021/jp9008948 CCC: $40.75  2009 American Chemical Society Published on Web 05/26/2009