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

Abstract

Alkaline solutions containing polyhydroxy carboxylates and Ca(II) are typical in cementitious radioactive waste repositories. Gluconate (Gluc(-)) 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(2+) and gluconate have been studied. It was found that Gluc(-) significantly increases the solubility of portlandite (Ca(OH)2(s)) under these conditions and Ca(2+) complexes of unexpectedly high stability are formed. The mononuclear (CaGluc(+) and [CaGlucOH](0)) complexes were found to be minor species, and predominant multinuclear complexes were identified. The formation of the neutral [Ca2Gluc(OH)3](0) (log β213 = 8.03) and [Ca3Gluc2(OH)4](0) (log β324 = 12.39) has been proven via H2/Pt-electrode potentiometric measurements and was confirmed via XAS, (1)H NMR, ESI-MS, conductometry, and freezing-point depression experiments. The binding sites of Gluc(-) 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(2+) 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(2+) complex, which is a precursor of various high-stability ternary complexes with actinides, is demonstrated.

Complex formation and lactonization reactions in aqueous solutions containing Ca2+ or Nd3+ ions and sugar-type ligands

“…Concerning Gluc -, a surprisingly high log Ka, -11.18 was determined (T = 37 °C, I = 0.15 M) employing glass electrode [24]. Using hydrogen electrode, values of -13.66 [61] and -13.68 [62] were deduced (T = 25 °C, I = 1 M), whilst -13 was proposed on the basis of 13 C NMR experiments (T = 22 °C, I = 0.1 M) [36]. Interestingly, it was found that gluconate ion could be deprotonated twofold; the log Ka of GlucH-1 2was found to be -14.02 [61].…”

“…For the most frequently studied Gluc - (Figure 7), the thermodynamic association constant, log β11 0 , was found to be 1.7-1.9 [37,[84][85][86]. At varying ionic strength, log β11 varies in the range of 0.37-1.36 obtained from Ca-ISE potentiometric [22,62,75,[87][88][89][90], solubility [82], ion-exchange [91] and 13 C NMR spectroscopic [37] measurements. The value of 0.37 [62] is probably underestimated, since it was obtained indirectly from pH-potentiometric titrations performed in alkaline solutions.…”

“…At varying ionic strength, log β11 varies in the range of 0.37-1.36 obtained from Ca-ISE potentiometric [22,62,75,[87][88][89][90], solubility [82], ion-exchange [91] and 13 C NMR spectroscopic [37] measurements. The value of 0.37 [62] is probably underestimated, since it was obtained indirectly from pH-potentiometric titrations performed in alkaline solutions. At higher [Gluc − ]T/[Ca 2+ ]T ratios, the formation of the 1:2 calcium complex, CaGluc2 0 , was also observed [87,92].…”

“…For Gluc -, the proton displacement from the CaGluc + complex was observed polarimetrically in the pH range of 10-11 [92]. Later, pHpotentiometric measurements revealed the formation of the CaGlucH-1 0 [62], CaHpglH-1 0 [41] as well as the CaGlucurH-1 0 [98] species. The respective log β11-1 values were found to be -10.94 [62], -10.35 [41] as well as -10.40 [98]; each was determined at 25 °C and 1 M ionic strength.…”

“…Surprisingly, Sipes [92] found a Ca(II) gluconate complex of 2:1 metal:ligand stoichiometry at higher pH indicating that such species is likely to form with Ca 2+ as well. Later, the Ca2GlucH-3 0 (log β21-3 = -33.24), the Ca3Gluc2H-4 0 (log β32-4 = -42.60) [41] and the Ca3Hpgl2H-4 0 (log β32-4 = -41.28) [62] complexes were identified through potentiometric titrations (T = 25 °C, I = 1 M). Additionally, the existence of the bi-and trinuclear gluconate-containing species was supported by ESI-MS and EXAFS [62].…”