Dalton Transactions COMMUNICATION Cite this: Dalton Trans., 2013, 42, 15275 Received 20th June 2013, Accepted 29th August 2013 DOI: 10.1039/c3dt51650d www.rsc.org/dalton Structure and spectroscopy of hydrated neptunyl(VI) nitrate complexes† Patric Lindqvist-Reis,* a Christos Apostolidis, b Olaf Walter, b,c Remi Marsac, a Nidhu Lal Banik, a Mikhail Yu. Skripkin, d Jörg Rothe a and Alfred Morgenstern b Complexation between hexavalent neptunium and nitrate was studied in aqueous nitric acid solution using optical absorption, vibrational and X-ray absorption spectroscopies. Distributions of aqueous [NpO 2 ] 2+ , [NpO 2 (NO 3 )] + and [NpO 2 (NO 3 ) 2 ] species were obtained as a function of nitric acid concentration between 0 and 14 M. The crystal structure of [NpO 2 (NO 3 ) 2 (H 2 O) 2 ]·H 2 O was determined. Neptunium (Np) is a radioactive element situated between uranium and plutonium in the actinide series. Similar to its neighboring actinides, the aqueous chemistry of neptunium is rich and complex owing to its accessible oxidation states ranging from +3 to +7, corresponding to the ionic species Np 3+ , Np 4+ , NpO 2 + , NpO 2 2+ , and NpO 4 - . 1 Although neptunium is not primeval in nature, large quantities of 237 Np (2.14 million years half-life) have been generated as a bypro- duct in the nuclear industry. This isotope is a major consti- tuent of high-level radioactive liquid waste. In the PUREX process, spent nuclear fuel is dissolved in nitric acid, followed by liquid–liquid extraction for separation and recovery of usable uranium and plutonium, while neptunium is dis- carded. However, since the oxidation state of neptunium depends on the chemical environment in the process solu- tions, its behavior is not entirely controlled in the PUREX process, where it distributes in different fractions. Basic knowl- edge about neptunium speciation in nitrate-rich solutions, including thermodynamic and structural characteristics, is therefore needed for the optimization of this as well as advanced reprocessing processes. 2 Another reason for studying the solution chemistry of hexavalent neptunium is related to the fact that the concept of oxidation state analogy along the actinide series is frequently used in the field of nuclear waste disposal in order to estimate complexation behavior in aqueous systems when no experimental data is available. Against this conceptual background, fundamental studies allowing a critical evaluation of the chemical analogy between hexavalent actinides are merited. In the literature, there are numerous spectroscopic studies on U(VI)-nitrate complexation in aqueous solution, but few studies on the corresponding Np(VI) and Pu(VI) systems. In addition, the agreement between the studies regarding the stoichiometry of a solution species at a given nitrate concentration is poor. 3–5 For example, using EXAFS Ikeda-Ohno et al. found that U(VI) was coordinated by three nitrate ions in 15 M HNO 3 and Np(VI) by two nitrate ions, 3a,4a whereas Gaunt et al. concluded from their vis-NIR absorption and Raman studies that only one nitrate ion was bound to Pu(VI) in 15 M HNO 3 . 5a These findings are in contrast to two earlier vis-NIR absorption spectroscopic studies by Vasil’ev et al. on Np(VI) and Pu(VI) in nitric acid, where it was concluded that the major complex in 15 M HNO 3 was a dinitrate complex. 4b,5b Actinide(VI)-nitrate complexes crystallizing from aqueous solu- tion may have very different structures and stoichiometries depending on the nitrate concentration and the water activity of the solution. How these parameters control the crystal chemistry of Pu(VI)-nitrate complexes was discussed in a doc- toral thesis by Böhm. 6 We have modified and extended the synthesis schemes in Böhm’s thesis to obtain crystals of [AnO 2 (NO 3 ) 2 (H 2 O) 2 ]·xH 2 O (An = Np, Pu; x = 0, 1), of which the synthesis and structure of [NpO 2 (NO 3 ) 2 (H 2 O) 2 ]·H 2 O (1) is presented here. To the best of our knowledge, there are only two single-crystal structures with a neptunium(VI)-nitrate complex, K 2 ((NpO 2 ) 3 B 10 O 16 (OH) 2 (NO 3 ) 2 ) and Rb[NpO 2 (NO 3 ) 3 ], 7a,b although powder diffraction methods were used to determine the structures of M[NpO 2 (NO 3 ) 3 ] (M = K, NH 4 ). 7c Also, the cell † Electronic supplementary information (ESI) available: Sample preparation, crystallography, EXAFS, infrared, Raman, and vis-NIR spectroscopy, figures and tables. CSD 426318. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt51650d a Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany. E-mail: patric.lindqvist@kit.edu; Fax: +49 721 608 24308; Tel: +49 721 608 22389 b European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany c Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany d Department of Chemistry, St. Petersburg State University, Universitetsky pr., 26, 198904 St. Petersburg, Russia This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 15275–15279 | 15275 Published on 30 August 2013. Downloaded by KIT on 22/10/2013 09:51:33. View Article Online View Journal | View Issue