caa a22 4500 37887702X CHVBK 20180305123423.0 cr unu---uuuuu 161128e20030201xx s 000 0 eng 10.1524/ract.91.2.87.19985 doi (NATIONALLICENCE)gruyter-10.1524/ract.91.2.87.19985 Precipitation of crystalline neptunium dioxide from near-neutral aqueous solution [Elektronische Daten] [K. E. Roberts, T. J. Wolery, C. E. Atkins-Duffin, T. G. Prussin, P.G. Allen, J. J. Bucher, D. K. Shuh, R. J. Finch, S. G. Prussin] We report experimental evidence that crystalline neptunium dioxide, NpO2(cr), will precipitate from near-neutral aqueous solutions that initially contain NpO2+(aq), even under mildly oxidizing conditions. Observed decreases in aqueous Np concentrations with time are balanced by corresponding decreases in pH and accompanied by formation of high-purity crystals of NpO2. The crystalline NpO2 was characterized by X-ray powder diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy, and scanning electron microscopy (SEM). Thermodynamic data in the literature suggest that NpO2(cr) is possibly the stable solid phase in aqueous systems containing dissolved NpO2+(aq); however, the precipitation of NpO2(cr) from solutions containing initially NpO2+(aq) had not been observed experimentally until now, possibly because of slow reaction kinetics. Our experiments were conducted at 200°C in order to overcome slow reduction reactions and/or precipitation kinetics. Because long time scales may render slow reaction kinetics irrelevant, thermodynamically stable NpO2(cr) may keep dissolved Np concentrations well below the relatively high Np concentrations (10-3M) associated with meta-stable Np(V) solids observed in previous laboratory experiments. Projects investigating the suitability of sites for an underground repository for high-level nuclear waste disposal may not need be so concerned with high dose rates due to migration of Np over long times, because these may not apply to repository-relevant time scales over which NpO2(cr) is likely to be stable. © 2003 Oldenbourg Wissenschaftsverlag GmbH Laboratory techniques, experiments nationallicence Analytical chemistry nationallicence Inorganic chemistry nationallicence Roberts K. E. aut Wolery T. J. aut Atkins-Duffin C. E. aut Prussin T. G. aut Allen P.G. aut Bucher J. J. aut Shuh D. K. aut Finch R. J. aut Prussin S. G. aut Radiochimica Acta/International journal for chemical aspects of nuclear science and technology Oldenbourg Wissenschaftsverlag GmbH 91/2/2003(2003-02-01), 87-92 0033-8230 91:2/2003<87 2003 91 ract https://doi.org/10.1524/ract.91.2.87.19985 text/html Onlinezugriff via DOI 1 research article jats NATIONALLICENCE 856 40 https://doi.org/10.1524/ract.91.2.87.19985 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Roberts K. E. aut NATIONALLICENCE 700 1- Wolery T. J. aut NATIONALLICENCE 700 1- Atkins-Duffin C. E. aut NATIONALLICENCE 700 1- Prussin T. G. aut NATIONALLICENCE 700 1- Allen P.G. aut NATIONALLICENCE 700 1- Bucher J. J. aut NATIONALLICENCE 700 1- Shuh D. K. aut NATIONALLICENCE 700 1- Finch R. J. aut NATIONALLICENCE 700 1- Prussin S. G. aut NATIONALLICENCE 773 0- Radiochimica Acta/International journal for chemical aspects of nuclear science and technology Oldenbourg Wissenschaftsverlag GmbH 91/2/2003(2003-02-01), 87-92 0033-8230 91:2/2003<87 2003 91 ract CC0 http://creativecommons.org/publicdomain/zero/1.0 nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-gruyter