Studies of the Complexation of Gluconate with Th(IV) in Acidic Solutions: Stability Constant Determination and Coordination Mode Analysis

Gluconate is a multidentate ligand and its complexation with actinides has received increasing attention because of its existence in high level nuclear wastes as well as in nuclear waste repositories. In this work, the complexation of gluconate with Th­(IV) was studied in deuterated water (D₂O) by pD titrations and nuclear magnetic resonance (NMR) spectroscopy. In the pC_D range 2.0–4.6, gluconate (GD₄^–) forms two 1:1 complexes with Th­(IV), Th­(GD₃)²⁺ and Th­(GD₂)⁺. Their stability constants were determined to be log β^(D)_101(−1) = 1.04 ± 0.12 for Th⁴⁺ + GD₄^– = Th­(GD₃)³⁺ + D⁺ and log β^(D)_101(−2) = −(1.31 ± 0.09) for Th⁴⁺ + GD₄^– = Th­(GD₂)⁺ + 2D⁺ at I = 1.0 mol·L^–1 NaClO₄ and t = 22 °C. The coordination modes of these two complexes were also analyzed. In both complexes, the tridentate chelation forms through the binding of Th­(IV) to one oxygen from the carboxylate group and two oxygens from α- and γ-hydroxyl groups. The difference is that in Th­(GD₂)⁺, both α- and γ-hydroxyl groups deprotonate, and in Th­(GD₃)²⁺, only the α-hydroxyl group deprotonates.