Formation of mono- and binuclear neodymium(iii)–gluconate complexes in aqueous solutions in the pH range of 2–8

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 NdGlucH 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 NdGlucH 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 H andC 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.

Formation of mono- and binuclear complexes of Nd3+ with d-gluconate ions in hyperalkaline solutions – Composition, equilibria and structure

“…Giroux et al reported that given the same conditions, precipitate also appears in the case of La 3+ , Eu 3+ , Dy 3+ , Er 3+ , and Lu 3+ ions around pH = 8 or at slightly higher pH when Gluc À is in excess [7]. In their study, seven complexes were described to form with all the five studied lanthanide cations: ML [26], and a precipitate appears at pH = 8, which was proven to have the composition of NdGlucH À1 OH in our previous work [27]. However, precipitation does not occur in the Nd 3+ -ÀGluc À system at pH > 12, which raises concerns of increased solubility since the complex formation was scarcely studied at pH higher than twelve [9,28], although this hyperalkaline medium [10] and high ionic strength (I = 5.3-7.4 M) [11] is inherent to radioactive depositories.…”

“…However, precipitation does not occur in the Nd 3+ -ÀGluc À system at pH > 12, which raises concerns of increased solubility since the complex formation was scarcely studied at pH higher than twelve [9,28], although this hyperalkaline medium [10] and high ionic strength (I = 5.3-7.4 M) [11] is inherent to radioactive depositories. Considering that Kutus et al proved the formation of polynuclear Nd 3+ ÀGluc À complexes as pH increases [26] and Giroux et al reported that lanthanides tend to form M 2 L 2 -H À5 À in alkaline media, the redissolving precipitate implicates that dimer or polynuclear complexes can be present in hyperalkaline medium which is also foreshadowed by the characteristic of Nd 3+ to dimerize even as hydroxide in Nd 2 (OH) 2 4+ form [29].…”

Impacts of Phosphorous Source on Organic Acid Production and Heterotrophic Bioleaching of Rare Earth Elements and Base Metals from Spent Nickel-Metal-Hydride Batteries

“…In theory, high organic acid concentration also leads to increased availability of the complexation agent. However, any occurring complexation mediated contribution to the overall leaching process will be diminished with decreasing pH and might not be compensated by the increased concentration of the complexing agent [5,27]. In order to compare the effect of different phosphorus sources on organic acid production, the PVK medium was modified by substitution of calcium phosphate (= 32.2 mM PO 4 3− ) with either KH 2 PO 4 or K 2 HPO 4 (= 2.7 mM PO 4 3− ).…”

Unexpected Deprotonation from a Chemically Inert OH Group Promoted by Metal Ions in Lanthanide–Erythritol Complexes

“…They suggested that deprotonation occurred in saccharide moieties according to the results of potentiometric measurements and 13 C NMR and Mössbauer spectroscopic methods. Rubini and co-workers , and Kutus et al prepared a series of d -gluconate complexes of several lanthanide ions. Bertani and co-workers prepared an aluminum complex of d -gluconate.…”

“…Burger and co-workers prepared a copper complex of d -gluconate . As a matter of fact, some complexes were obtained in acidic environments, and deprotonations on the OH of the d -gluconate moiety were proposed to occur in these complexes on the basis of the various experimental results such as pH potentiometry, ESI mass spectrometry, and NMR spectroscopy. − Kiss and co-workers prepared some vanadium (IV, V) complexes using two aldaric acids ( d -saccharic acid, mucic acid) as ligands in acidic solutions. They suggested that deprotonation took place on the OH group of the ligand according to the experimental results of pH potentiometry and EPR, NMR, and UV–vis spectroscopy .…”