Solubility and spectroscopic study of An III /Ln III in dilute to concentrated Na–Mg–Ca–Cl–NO 3 solutions

The complexation of An III /Ln III with nitrate was investigated in dilute to concentrated Na–Mg–Ca–Cl–NO 3 solutions at 2.94 ≤ pH m ≤ 13.2 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–L III EXAFS (extended X-ray absorption fine structure) under analogous pH m , I m and m NO 3 − 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 MgCl 2 –Mg(NO 3 ) 2 solutions with total salt concentration ≥ 2.83 m and m NO 3 − ≥ 1.13 m. Nitrate has no effect in any of the studied NaCl–NaNO 3 and CaCl 2 –Ca(NO 3 ) 2 mixtures, thus highlighting the key role of Mg 2+ in the Nd(III)–NO 3 interaction. The formation of inner-sphere complexes with the participation of Mg 2+ is further confirmed by Cm(III)–TRLFS and Nd–L III EXAFS. Based on these experimental evidences, the proposed chemical model includes the definition of two new aqueous species Mg[An III /Ln III NO 3 OH] 3+ and Mg[An III /Ln III NO 3 (OH) 2 ] 2+ in equilibrium with solid Nd(OH) 3 (s) and allows to derive the thermodynamic and activity models (Pitzer) for the system Nd 3+ /Cm 3+ –H + –Mg 2+ –OH − –Cl − –NO 3 − –H 2 O.