Np(V) double carbonate with magnesium: synthesis, solubility and transformation

In this study, neptunium(V) double carbonate with magnesium was synthesized by the cation exchange synthesis method. Synchrotron powder X-ray diffraction, X-ray absorption spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were employed for comprehensive solid-phase analysis. Solubility experiments were carried out in 0.01 M and 0.05 M NaClO₄ solutions in the pH range 6-10 for the evaluation of the thermodynamic stability of solid magnesium-neptunium(V) double carbonate. Based on these data, the solubility product constant was estimated from the experimental data and established to be log K_sp = -15.9. During the solubility experiments, the initial Np(V)-Mg double carbonate underwent a phase transformation. This transformation led to the formation of a Np(V)-K double carbonate, caused by increasing potassium concentrations in solution primarily from the KCl-filled pH electrode. The solubility constant for the Np(V) double carbonate with potassium was determined to be log K_sp = -16.53 ± 0.10, which is significantly lower than that reported in the literature, confirming the high thermodynamic stability of the KNpO₂CO₃ solid phase.