Complexation of Nd(III)/Cm(III) with gluconate in alkaline NaCl and CaCl2 solutions: Solubility, TRLFS and DFT studies

Highlights: • The solubility of Nd increases in dilute, alkaline CaCl₂ systems containing gluconate. • Formation of stable ternary complexes Ca–Nd/Cm-gluconate confirmed by TRLFS and DFT. • TRLFS supports also the formation of weaker binary complexes Cm-gluconate. • Complexation of Nd/Cm with gluconate outcompeted by Ca in concentrated CaCl₂ systems. The effect of gluconate on the solubility and aqueous speciation of An(III) and Ln(III) was studied using a combination of Nd(III) solubility experiments, Cm(III) time-resolved laser fluorescence spectroscopy (TRLFS) and density functional theory (DFT) calculations. Solubility experiments were performed under an Ar-atmosphere using a well-defined Nd(OH)₃(s) solid phase equilibrated in NaCl (0.1–5.0 M) and CaCl₂ (0.1–3.5 M) solutions with 9 ≤ pH_c ≤ 13 and 10⁻⁶ M ≤ [GLU]_tot ≤ 10⁻² M. The solubility of Nd(OH)₃(s) remains mostly unaffected in NaCl solutions with [GLU]_tot = 10⁻³ M, whereas a clear increase in solubility is observed in dilute CaCl₂ solutions with the same [GLU]_tot and pH_c ≥ 11. In concentrated CaCl₂ solutions, gluconate does not affect the solubility of Nd(III) due to the competition with Ca–GLU complexes. Cm(III) TRLFS spectra collected in NaCl solutions with pH_c ≈ 12 confirm the formation of weak Cm(III)–GLU complexes. The very strong red shift observed in dilute CaCl₂ solutions in connection with high fluorescence intensities supports the formation of ternary Ca–Cm(III)–GLU complexes. The speciation of Cm(III) in 3.5 M CaCl₂ solutions is mostly dominated by the complex Ca₃[Cm(OH)₆]³⁺, although the formation of ternary Ca–Cm(III)–GLU species is hinted at high gluconate concentrations. DFT calculations provide additional support to the formation of stable ternary Ca–Nd(III)–GLU aqueous complexes. This work provides key information to understand the chemical speciation and relevant equilibrium processes of An(III) and Ln(III) in the presence of gluconate under conditions relevant for nuclear waste disposal.