Nd(III)/Cm(III) complexation with gluconate in NaCl and CaCl₂ alkaline solutions: solubility and TRLFS studies

Gluconic acid (GLU) is a poly-hydroxo-carboxylic acid expected in repositories for low and intermediate-level radioactive waste as a component of cementitious materials, added into the cement as water reducer and plasticiser. This work aims at assessing the solubility and aqueous speciation of An(III) and Ln(III) in the presence of GLU and dilute NaCl and CaCl₂ solutions, as background electrolytes. Under-saturation solubility studies with Nd(OH)₃(am) were conducted in inert gas (Ar) gloveboxes at (22±2) C. degrees. Samples were prepared in dilute NaCl (0.1 M) and CaCl₂ (0.1 and 0.25 M) solutions as background electrolytes. Parallel experimental series were prepared with pH_c = constant ≅ 12 and 10^-6 M ≤ [GLU] ≤ 10^-2 M, and with [GLU] = constant = 10^-3 M and 9 ≤ pH_c ≤ 13. TRLFS measurements were performed with [Cm(III)] ∼ 10^-7 M per sample, and NaCl (0.1 M) and CaCl₂ (0.1 and 0.25 M) as background electrolytes. Three different concentration levels of Ca²⁺ were considered in the NaCl systems: 0, 10^-3 M and 10^-2 M. The initial concentration of GLU in all samples (10^-6 M) was increased to 3.10^-3 M by step-wise additions of NaCl-NaGLU or CaCl₂-CaGLU₂ solutions of appropriate ionic strength. The solubility of Nd(OH)₃(am) remained unaffected by GLU in 0.1 M NaCl solutions. On the other hand, the solubility of Nd(OH)₃(am) in 0.1 and 0.25 M CaCl₂ solutions was clearly increased by GLU under hyper-alkaline conditions. It was observed that the species forming are pH-dependent and unequivocally involve the participation of Ca²⁺, with the likely formation of a Nd-GLU complex with stoichiometry 1:2. No further increase of the Nd(III) concentration was observed at [GLU]tot ≥ 10^-3 M, resulting in an upper solubility limit at [Nd] ∼ 10^-6.5 M. This suggests the formation of a new Ca-Nd-GLU solubility limiting solid phase. Consistently with Nd(III) solubility data, TRLFS confirmed the key role of Ca²⁺ in the complexation process, with the likely formation of a Ca-Cm(III)-GLU complex with Ca:GLU ratio 1:1. Two Ca-Cm(III)-GLU species were further identified in 0.1 M and 0.25 M CaCl₂ solutions. These results indicate that gluconate has a clear but limited effect on the solubility of An(III) under repository-relevant conditions. (authors)