In:
Radiochimica Acta, Walter de Gruyter GmbH, Vol. 100, No. 10 ( 2012-10), p. 759-770
Abstract:
The redox chemistry of Np(V/VI) was investigated in ∼0.6 M tetramethylammonium hydroxide/chloride (TMA-(OH, Cl)) solutions with 9 ≤ −log [H + ] ≤ 13.5. Redox conditions were defined by the absence or presence of ClO − as oxidizing agent (Na-salt, 5 × 10 −3 M and 5 × 10 −2 M). The high total Np concentration ([Np] tot ∼ 2 × 10 −3 M) led to the precipitation of solid phases in some of the samples. The carbonate concentration (as impurity of TMA-OH) was 2–3 × 10 −3 M. UV-vis/NIR spectra obtained from the supernatant in TMA-(OH, Cl) solutions and absence of ClO − showed clear Np(V) features, identified as NpO 2 + , NpO 2 CO 3 − and (NpO 2 ) x (CO 3 ) y (OH) z x−2 y − z . No NIR features were observed within 800 nm ≤ λ ≤ 1300 nm for samples with ClO − . XANES edge energies and features of these samples confirmed the predominance of Np(V) in the absence of ClO − and Np(VI) in the presence of ClO − , by comparison to XANES reference spectra of Np(III/IV/V/VI) prepared within the present work by in-situ electrolysis. A similar Np redox distribution was observed for the solid phases based on XANES and EXAFS measurements. EXAFS spectra indicative of Np V O 2 OH(s) and Np VI O 3 · x H 2 O(s) were obtained for samples in absence and presence of ClO − , respectively. The formation of a Na-Np(VI) phase in 5 × 10 −2 M ClO − and −log [H + ] ∼ 12 was also indicated from the EXAFS, chemical analysis and SEM-EDS. These results indicate that Np(VI) aqueous species and solid compounds prevail far below the oxidation border of water in alkaline solutions and also far below the E H border calculated with the current NEA data selection [1]. These observations are further supported by correlations of literature thermodynamic data for actinides (U, Np, Pu and Am), which predict the formation of NpO 2 (OH) 3 − and NpO 2 (OH) 4 2− aqueous species with stability constants (log * β º 1,3 and log * β º 1,4 ) similar to those available for U(VI).
Type of Medium:
Online Resource
ISSN:
2193-3405
DOI:
10.1524/ract.2012.1948
Language:
English
Publisher:
Walter de Gruyter GmbH
Publication Date:
2012
