Solubility and spectroscopic study of AnIII/LnIII in dilute to concentrated Na-Mg-Ca-Cl-NO3 solutions

Abstract

The complexation of AnIII/LnIII with nitrate was investigated in dilute to concentrated Na-Mg- Ca-Cl-NO3 solutions at 2.94 ≤pHm ≤3.2 and T =22 ±2 °C. Comprehensive solubility experiments with Nd(OH)3(s) were performed from undersaturation conditions and complemented by extensive spectroscopic studies using Cm(III)-TRLFS (time resolved laser fluorescence spectroscopy) and Nd-LIIIEXAFS (extended X-ray absorption fine structure) under analogous pHm, Im and mNO3 - conditions. Solid phases recovered from batch solubility experiments were characterized by XRD and SEM-EDS. A significant influence of nitrate on the solubility of Nd(OH)3(s) is observed in concentrated weakly alkaline MgCl2-Mg(NO3)2 solutions with total salt concentration ≥2.83 m and mNO3 - ≥1.13 m. Nitrate has no effect in any of the studied NaCl-NaNO3 and CaCl2-Ca(NO3)2mixtures, thus highlighting the key role of Mg2+in the Nd(III)-NO3 interaction. The formation of inner-sphere complexes with the participation of Mg2+ is further confirmed by Cm(III)-TRLFS and Nd-LIIIEXAFS. Based on these experimental evidences, the proposed chemical model includes the definition of two new aqueous species Mg[AnIII/LnIIINO3OH]3+ and Mg[AnIII/LnIIINO3(OH)2]2+in equilibrium with solid Nd(OH)3(s) and allows to derive the thermodynamic and activity models (Pitzer) for the system Nd3+/Cm3+-H+-Mg2+-OH-Cl-NO3 -H2O.