Magnesium(II) d-Gluconate Complexes Relevant to Radioactive Waste Disposals: Metal-Ion-Induced Ligand Deprotonation or Ligand-Promoted Metal-Ion Hydrolysis?

Bence Kutus,Amr A A Attia,Alexandru Lupan,István Pálinkó,Eszter Orbán,Csilla Dudás,Gábor Peintler,Pál Sipos

doi:10.1021/acs.inorgchem.9b00289

Abstract

The complexation equilibria between Mg2+ and d-gluconate (Gluc–) ions are of particular importance in modeling the chemical speciation in low- and intermediate-level radioactive waste repositories. NMR measurements and potentiometric titrations conducted at 25 °C and 4 M ionic strength revealed the formation of the MgGluc+, MgGlucOH0, MgGluc(OH)2–, and Mg3Gluc2(OH)40 complexes. The trinuclear species provides indirect evidence for the existence of multinuclear magnesium(II) hydroxido complexes, whose formation was proposed earlier but has not been confirmed yet. Additionally, speciation calculations demonstrated that MgCl2 can markedly decrease the solubility of thorium(IV) at low ligand concentrations. Regarding the structure of MgGluc+, both IR spectra and density functional theory (DFT) calculations indicate the monodentate coordination of Gluc–. By the potentiometric data, the acidity of the water molecules is higher in the MgGluc+ and MgGlucOH0 species than in the Mg(H2O)62+ aqua ion. On the basis of DFT calculations, this ligand-promoted hydrolysis is caused by strong hydrogen bonds forming between Gluc– and Mg(H2O)62+. Conversely, metal-ion-induced ligand deprotonation takes place in the case of calcium(II) complexes, giving rise to salient variations on the NMR spectra in a strongly alkaline medium.

Highlights

The chemical behavior of actinides in aqueous solutions is of general importance for the disposal of nuclear waste in underground salt mines
Stable binary complexes forming between Ca2+ or Mg2+ and Gluc− ions may suppress the formation of actinide gluconate species, resulting in a decrease of the solubility
In this Article, we report on the solution equilibria of magnesium(II) gluconate complexes forming in a neutral-toalkaline medium

Summary

■ INTRODUCTION

The chemical behavior of actinides in aqueous solutions is of general importance for the disposal of nuclear waste in underground salt mines. MgCl2; both salts are expected to be present at high concentrations.[3] First, stable binary complexes forming between Ca2+ or Mg2+ and Gluc− ions may suppress the formation of actinide gluconate species, resulting in a decrease of the solubility. The same downfield shift is seen on the H2 signal, indicating that the structures of the MgGlucOH0 and the newly-formed MgGluc(OH)2− species are similar This conclusion is corroborated by the 13C NMR spectra as well (Figure S13), on which the same trend is seen. From pH* = 6 to 10, only small spectral variations are discernible for both metal-ion-containing solutions compared to the spectra of free Gluc− These changes are caused by formation of the mononuclear species, discussed in detail in the previous sections as well as in ref 46.

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES