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 –log10 mH+(pHm) 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≤pHm≤10.3 by UV-vis/NIR absorption spectroscopy. In 0.1 M NaCl, the experimental solubility of the initial greenish NpO2OH(am) solid phase is in good agreement with previous results obtained in NaClO4 solutions, and is consistent with model calculations for fresh NpO2OH(am) using the thermodynamic data selection in NEA–TDB. Below pHm~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 NpO2OH(am) into a more crystalline modification with the same stoichiometry, NpO2OH(am, aged), as reported in previous studies for concentrated NaClO4 and NaCl. Instead, the greenish NpO2OH(am) transforms into a white solid phase in those systems with [NaCl]≥1.0 M and pHm≥11.5, and into two different pinkish phases above pHm~13.2. The solid phase transformation is accompanied by a drop in Np solubility of 0.5–2 log10-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 NpO2OH(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 NaNpO2(OH)2(cr). UV-vis/NIR spectra collected in 0.1–5.0 M NaCl solutions show the predominance of NpO2 + (≥80%) at pHm≤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 cement-based environments in the context of the safety assessment for nuclear waste disposal.