Spectroscopic investigations of Np(V/VI) redox speciation in hyperalkaline TMA-(OH, Cl) solutions

Abstract

Neptunium(V/VI) / TMA-OH / XANES reference spectra / EXAFSSummary. 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 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• 1,3 and log * β • 1,4 ) similar to those available for U(VI).

Solubility and hydrolysis of Np(V) in dilute to concentrated alkaline NaCl solutions: formation of Na–Np(V)–OH solid phases at 22 °C

“…Figure 5 shows the XANES spectra of the solid phases described in Table 1. The comparison of these spectra with Np(IV), Np(V) and Np(VI) reference spectra reported elsewhere [67] clearly indicates that neptunium is predominantly found as +V in all solid phases characterized in this study.…”

“…The distance determined for Np-O eq is in average longer than the distance recently reported for freshly precipitated NpO 2 OH(am) in TMA-OH solutions (2.35 ± 0.03 Å) [67]. Cu Kα radiation NaNpO 2 (OH) 2 (cr) -Almond et al [37] Figure 3: XRD spectra of selected solid phases from Np(V) solubility experiments in 0.1, 1.0, 3.0 and 5.0 M NaCl (m NaCl = 0.10, 1.02, 3.20 and 5.61 m).…”

“…This difference likely reflects slight modifications in the solid crystallinity between a solid phase aged 7 days [67] and ~2 years (present work).…”

Influence of the redox state on the neptunium sorption under alkaline conditions: Batch sorption studies on titanium dioxide and calcium silicate hydrates

“…However, the possible formation of anionic Np(VI) species (NpO 2 (OH) 3 − and NpO 2 (OH) 4 2− ) under oxidizing alkaline conditions, in analogy to U(VI), may significantly limit the stability field of Np(V) in favour of Np(VI) species. Recently, Np redox speciation studies have confirmed the analogy between the uranium and neptunium speciation under alkaline, oxidizing conditions [5]. A predominance diagramme for Np in the pH range 10 < pH < 14 including the anionic Np(VI) species (NpO 2 (OH) 3 − and NpO 2 (OH) 4 2− ), shows that all three redox states may occur under the redox conditions expected in a L/ILW repository (gray-coloured regions in Figure 1a).…”

“…Titanium dioxide (TiO 2 ) is a solid phase known to be stable under alkaline conditions and ambient temperature with low solubility and slow recrystallization [4]. Therefore, radionuclide incorporation into the , d) calculated using the NEA thermodynamic database [9] completed with stability constants for the hexavalent NpO 2 (OH) 2 (aq) [5], NpO 2 (OH) 3 − and NpO 2 (OH) gray-coloured fields in the predominance diagram represent the redox potential ranges expected in the cementitious near-field of a L/ILW repository [11,12].…”