Abstract
The complex formation between Nd(iii) and d-gluconate (Gluc-) is of relevance in modelling the chemical equilibria of radioactive waste repositories. In the present work, the formation of NdpGlucqH-r complexes at 25 °C and pH = 2-8 was studied via spectrophotometry, potentiometry, freezing point depression, conductometry and NMR spectroscopy. In addition to the four mononuclear complexes (pq-r = 110, 120, 130 and 11-2), the formation of two binuclear, so far unknown complexes (pq-r = 23-2 and 24-2) was revealed. Between pH = 5.5 and 7, with the increasing metal ion and ligand concentrations, the Nd2Gluc3H-2+ species becomes progressively predominant. Under the conditions characteristic of waste repositories, however, the formation of these complexes can be neglected. Regarding the binding sites of Gluc-, C2-OH and C3-OH groups, in addition to the carboxylate ion, were identified from 1H and 13C spectroscopic measurements. Above pH = 6, the metal-ligand interactions became stronger implying the formation of deprotonated complexes involving the C2-OH group, while the displacement of the second proton at the C3-OH is also possible. The metal ion induced deprotonation of the ligand was confirmed by DFT calculations.
Highlights
IntroductionNd(III) is a good model for studying the behaviour of trivalent actinides. This replacement is justified by its stable +3 oxidation state, availability and safe usage
In the case of deep geological sites, which are considered as possible nuclear waste repositories, salt-rock formations consisting of NaCl and MgCl2 are likely to be present
The interaction of strongly saline aqueous solutions with the backfilling material of waste containers would promote the mobilization of actinides aDepartment of Inorganic and Analytical Chemistry, University of Szeged, 7 Dóm tér, H-6720 Szeged, Hungary
Summary
Nd(III) is a good model for studying the behaviour of trivalent actinides. This replacement is justified by its stable +3 oxidation state, availability and safe usage. Along with NdGlucH−1+, NdGlucH−20, NdGluc2H−10 and NdGluc2H−2−,25–29 NdGlucH−3− is formed at high pH.[25] These previous measurements in the Nd(III)–Gluc− system were conducted under different experimental conditions (i.e., temperature, ionic strength, pH-range and ligand to metal ratio), the results cannot be compared directly. To minimize the effect of lactonization, the measurements were started from pH = 7–8 instead of the acidic range[23] and the titrant was 1.026 M HCl solution, except for (a), where 0.103 M was used This simple method using very simple means was found to be useful to support the results obtained via other methods.[24] For relatively dilute solutions, the theoretical freezing point depression (ΔTf,theo) is proportional to the free concentration of the species X (Blagden’s law): ΔT f;theo 1⁄4 T f;water À T f;solution 1⁄4 K f;water Á 1⁄2X.
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