EXAFS studies of pentavalent neptunium carbonato complexes. Structural elucidation of the principal constituents of neptunium in groundwater environments

Journal Article · Wed Feb 28 04:00:00 EST 1996 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/ja9516423· OSTI ID:232411

Clark, D L; Runde, W ^[1]; Conradson, S D; Ekberg, S A; Neu, M P; Palmer, P D; Tait, C D ^[1]; Hess, N J ^[1]

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1. Los Alamos National Lab., NM (United States)

Neptunium is the most problematic actinide element with respect to environmental migration because its solubility under typical groundwater conditions is expected to be high enough to be radiological concern, and its sorption on common minerals is expected to be relatively low. Neptunium solubility in carbonate solutions is controlled by the formation of solids of general formula M{sub 2n-1}NpO{sub 2}(CO{sub 3}){sub n}, whose features have been reviewed. The limited solubilities (10{sup -4}-10{sup -7} M) of these phases make the solution species in equilibrium with the solids very difficult to study using common structural methods. We find that alkali free synthesis employing the tetrabutylammonium (TBA) cation provides 10{sup -3} M solutions of NpO{sub 2}(CO{sub 3}){sup -} and NpO{sub 2}(CO{sub 3}){sub 2}{sup 3-}. These relatively high solubilities are unprecedented and afford the opportunity to determine the molecular structures of these environmentally important complexes employing EXAFS spectroscopy. The near-IR absorption spectra of the neptunium electrolyte solutions are nearly identical, indicating that the same species are present in both solutions. Known thermodynamic constants were used to prepare single-component solutions, and near- IR spectra were measured both before and after EXAFS analysis to confirm that each solution sample contained a single, pure neptunyl carbonato species. 19 refs., 1 fig., 1 tab.

• DOE Contract Number: W-7405-ENG-36
• OSTI ID: 232411
• Journal Information: Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 8 Vol. 118; ISSN JACSAT; ISSN 0002-7863
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
54 ENVIRONMENTAL SCIENCES
ABSORPTION SPECTRA
CARBONATES
GROUND WATER
INFRARED SPECTRA
MOLECULAR STRUCTURE
NEPTUNIUM COMPLEXES
STRUCTURAL CHEMICAL ANALYSIS
X-RAY SPECTRA