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

Abstract

Abstract:The solubility of Np(V) was investigated at T = 22 ± 2 °C in alkaline NaCl solutions of different ionic strength (0.1-5.0 M). The solid phases controlling the solubility at different -log 10 m H +(pH m ) and NaCl concentration were characterized by XRD, quantitative chemical analysis, SEM-EDS and XAFS (both XANES and EXAFS). Aqueous phases in equilibrium with Np(V) solids were investigated for selected samples within 8.9 ≤ pH m ≤ 10.3 by UV-vis/NIR absorption spectroscopy. In 0.1 M NaCl, the experimental solubility of the initial greenish NpO 2 OH(am) solid phase is in good agreement with previous results obtained in NaClO 4 solutions, and is consistent with model calculations for fresh NpO 2 OH(am) using the thermodynamic data selection in NEA-TDB. Below pH m ~ 11.5 and for all NaCl concentrations studied, Np concentration in equilibrium with the solid phase remained constant during the timeframe of this study (~2 years). This observation is in contrast to the aging of the initial NpO 2 OH(am) into a more crystalline modification with the same stoichiometry, NpO 2 OH(am, aged), as reported in previous studies for concentrated NaClO 4 and NaCl. Instead, the greenish NpO 2 OH(am) transforms into a white solid phase in those systems with [NaCl] ≥ 1.0 M and pH m ≥ 11.5, and into two different pinkish phases above pH m ~ 13.2. The solid phase transformation is accompanied by a drop in Np solubility of 0.5-2 log 10 -units (depending upon NaCl concentration). XANES analyses of green, white and pink phases confirm the predominance of Np(V) in all cases. Quantitative chemical analysis shows the incorporation of Na + in the original NpO 2 OH(am) material, with Na:Np ≤ 0.3 for the greenish solids and 0.8 ≤ Na:Np ≤ 1.6 for the white and pinkish phases. XRD data confirms the amorphous character of the greenish phase, whereas white and pink solids show well-defined but discrepant XRD patterns. Furthermore, the XRD pattern collected for one of the pink solid phases match the data recently reported for NaNpO 2 (OH) 2 (cr). UV-vis/NIR spectra collected in 0.1-5.0 M NaCl solutions show the predominance of NpO 2 + ( ≥ 80%) at pH m ≤ 10.3. This observation is consistent with the Np(V) hydrolysis scheme currently selected in the NEA-TDB. This work provides sound evidences on the formation of ternary Na-Np(V)-OH solid phases in Na-rich hyperalkaline solutions and ambient temperature conditions. Given the unexpectedly high complexity of the system, further experimental efforts dedicated to assess the thermodynamic properties of these solid phases are needed, especially in view of their likely relevance as solubility controlling Np(V) solid phases in Na-rich systems such as saline and cementbased environments in the context of the safety assessment for nuclear waste disposal.

Interaction of Np(v) with borate in alkaline, dilute-to-concentrated, NaCl and MgCl2solutions

“…Solubility of Np(V) in the presence of borate In the latter case either NpO 2 OH(am) or ternary Na/Mg-Np(V)-borate(cr) solid phases were used as the "starting material". Np(V) solubility data reported in the literature for borate-free systems under analogous pH m and ionic strength conditions are appended to the figure for comparison purposes, [38][39][40] as well as the solubility curves of NpO 2 OH(am,fresh) calculated with the thermodynamic data selected in the NEA-TDB. 16 As shown in Fig.…”

Single-Hydroxide Bridged Dimers of U and Np Actinyls: A Density Functional Study on Their Existence and Structure in Aqueous Solution

“…It is also interesting to compare the length of characteristic bonds of mononuclear aqua and monohydroxo complexes, as such species are present in the pH > 9 regime 52 where [(NpO 2 ) 2 (OH)] + might exist, because this comparison could be helpful for an experimental identification of the Np(V) diactinyl monohydroxo complex. The structures of the Np(V)−pe dimers are distinguished from those of the favorable An(VI) diactinyl monohydroxo complexes of the pa type by different Np�O actinyl bond lengths of about 184 and 187 pm in the essentially linear neptunyl units (Table 2).…”

Crystal Structure and Stability in Aqueous Solutions of Na0.5[NpO2(OH)1.5]·0.5H2O and Na[NpO2(OH)2]

“…This fact gave cause to some debates as recently summarized in Petrov et al and is reflected by the large uncertainty values of the current NEA-TDB data. 19 The current NEA-TDB database provides recommended solubility constants for only one Np(V) hydroxide compound that is relevant for the H−Na−Cl−OH−H 2 O system: NpO 2 OH(am). Due to the great thermodynamic stability of the redox neighbors Np(IV) and Np(VI), the modeled predominance field of NpO 2 OH(am) and, hence, of solid Np(V) ends at about pH m ≈ 11 as indicated by the red dotted line in Figure 3b.…”

“…Recent solubility investigations performed with Np­(V) hydroxides set another priority. They showed that (poly)­crystalline M–Np­(V)–OH solid phases (M = Na and Ca) significantly enhance the thermodynamic stability of Np­(V) in alkaline NaCl and CaCl 2 solutions but lacked detailed structural models of the solubility controlling solid phases. − Herein, we report on the successful combination of a comprehensive structural and thermodynamic investigation of two different Na–Np­(V)–OH solid phases. Synthesis, crystal structures, and the results of systematic solubility studies of the hitherto unknown Na 0.5 [NpO 2 (OH) 1.5 ]·0.5H 2 O (I) and the previously reported Na­[NpO 2 (OH) 2 ] (II) are discussed.…”

“…In contrast, log *β′ n constants recommended by the NEA-TDB were derived from experimental solubility studies performed with an uncharacterized Np­(V) solid phase considered to be NpO 2 OH­(am). This fact gave cause to some debates as recently summarized in Petrov et al and is reflected by the large uncertainty values of the current NEA-TDB data …”