Multinuclear Complex Formation between Ca(II) and Gluconate Ions in Hyperalkaline Solutions

Alkaline solutions containing polyhydroxy carboxylates and Ca­(II) are typical in cementitious radioactive waste repositories. Gluconate (Gluc<sup>–</sup>) is a structural and functional representative of these sugar carboxylates. In the current study, the structure and equilibria of complexes forming in such strongly alkaline solutions containing Ca<sup>2+</sup> and gluconate have been studied. It was found that Gluc<sup>–</sup> significantly increases the solubility of portlandite (Ca­(OH)<sub>2</sub>(s)) under these conditions and Ca<sup>2+</sup> complexes of unexpectedly high stability are formed. The mononuclear (CaGluc<sup>+</sup> and [CaGlucOH]<sup>0</sup>) complexes were found to be minor species, and predominant multinuclear complexes were identified. The formation of the neutral [Ca<sub>2</sub>Gluc­(OH)<sub>3</sub>]<sup>0</sup> (log β<sub>213</sub> = 8.03) and [Ca<sub>3</sub>Gluc<sub>2</sub>(OH)<sub>4</sub>]<sup>0</sup> (log β<sub>324</sub> = 12.39) has been proven via H<sub>2</sub>/Pt-electrode potentiometric measurements and was confirmed via XAS, <sup>1</sup>H NMR, ESI-MS, conductometry, and freezing-point depression experiments. The binding sites of Gluc<sup>–</sup> were identified from multinuclear NMR measurements. Besides the carboxylate group, the O atoms on the second and third carbon atoms were proved to be the most probable sites for Ca<sup>2+</sup> binding. The suggested structure of the trinuclear complex was deduced from ab initio calculations. These observations are of relevance in the thermodynamic modeling of radioactive waste repositories, where the predominance of the binuclear Ca<sup>2+</sup> complex, which is a precursor of various high-stability ternary complexes with actinides, is demonstrated.