Multinuclear complex formation between Ca(II) and gluconate ions in hyperalkaline solutions

Impact of selected cement additives and model compounds on the solubility of Nd(III), Th(IV) and U(VI): screening experiments in alkaline NaCl, MgCl2 and CaCl2 solutions at elevated ionic strength

The solubility of Nd(III), Th(IV) and U(VI) was studied from undersaturation conditions in the presence of selected organic cement additives and model compounds: adipic acid, methyl acrylate, citric acid, melamine, ethylene glycol, phthalic acid and gluconic acid. Experiments were performed under Ar atmosphere in NaCl (2.5 and 5.0 M), MgCl2 (1.0 and 3.5 M) and CaCl2 (1.0 and 3.5 M) solutions with 9 ≤ pHm ≤ 13 (pHm = −log[H+]). Initial concentrations of organic ligands in solution were set constant in all systems to [L]0 = 0.025 M, except in specific cases (e.g. adipic acid, melamine and phthalic acid) where the ligand concentration in the matrix solutions was lower and controlled by solubility. Adipic acid, methyl acrylate, melamine, ethylene glycol and phthalic acid do not impact the solubility of Nd(III), Th(IV) and U(VI) in the investigated NaCl, MgCl2 and CaCl2 systems. Citrate significantly enhances the solubility of Nd(III), Th(IV) and U(VI) in NaCl systems. A similar effect was observed for Th(IV) and U(VI) in the presence of gluconate in NaCl systems. The impact of pH on the stability of the complexes is different for both ligands. Because of the larger number of alcohol groups in the gluconate molecule, this ligand is prone to form more stable complexes under hyperalkaline conditions that likely involve the deprotonation of several alcohol groups. The complexation of gluconate with U(VI) at pHm ≈ 13 is however weaker than at pHm ≈ 9 due to the competition with the highly hydrolysed moiety prevailing at pHm ≈ 13, i.e. UO2(OH)4 2−. The impact of citrate and gluconate in MgCl2 and CaCl2 systems is generally weaker than in NaCl systems, expectedly due to the competition with binary Mg-L and Ca-L complexes. However, the possible formation of ternary complexes further enhancing the solubility is hinted for the systems Mg/Ca-Th(IV)-GLU and Ca-U(VI)-GLU. These observations reflect again the differences in the complexation properties of citrate and gluconate, the key role of the alcohol groups present in the latter ligand, and the importances of interacting matrix cations. The screening experiments conducted within this study contribute to the identification of organic cement additives and model compounds potentially impacting the solution chemistry of An(III)/Ln(III), An(IV) and An(VI) under intermediate to high ionic strength conditions (2.5 ≤ I ≤ 10.5 M). This shows evident differences with respect to investigations conducted in dilute systems, and thus represents a very relevant input in the safety assessment of repositories for radioactive waste disposal where such elevated ionic strength conditions are expected.

Colorimetric and SERS dual-mode detection of lead Ions based on Au-Ag core-shell nanospheres: featuring quick screening with ultra-high sensitivity

A novel strategy for colorimetric and surface-enhanced Raman scattering (SERS) dual-mode sensing of the lead ions (Pb²⁺) was established based on gluconate ion (Gluc) modified and 2-Naphthalenethiol (2-NT) tagged Au-Ag core-shell nanoparticles (NPs). Due to the complex formation between adsorbed Gluc and Pb²⁺, the addition of Pb²⁺ can induce the aggregation of Gluc/2-NT@Au@Ag NPs. Correspondingly, the aggregated Gluc/2-NT@Au@Ag NPs caused a significant difference in the color and SERS intensity. As a result, such Gluc/2-NT@Au@Ag NPs can achieve the sensing of Pb²⁺ using both colorimetric and SERS signals as the indicator, which features with wide response range from 10^-11 to 10^-5 M, rapid screening and high sensitivity (with a limit of detection (LOD) of 0.185 pM). Furthermore, such dual-mode sensor was demonstrated not to be responsive to other cations, and facilitate the sensing of real samples in practical environment. With rapid screening ability and outstanding sensitivity, we anticipate that this method would holding great potential for the applications in environmental monitoring.

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

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.

Comparison of the Ca2+ complexing properties of isosaccharinate and gluconate – is gluconate a reliable structural and functional model of isosaccharinate?

During the interactions of α-d-isosaccharinate and d-gluconate with Ca²⁺ in aqueous solution, differences rather than similarities prevail.

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

The structure and stability constants of four mononuclear and two, so far unknown and highly stable binuclear complexes have been determined.

ML and ML2 complex formation between Ca(ii) and d-glucose derivatives in aqueous solutions

Besides the well-known ML species, Ca(ii) forms ML₂ complexes with carbohydrates having at least one carboxylate group and conformational flexibility.

Solubility constraints affecting the migration of selenium through the cementitious backfill of a geological disposal facility

This work presents the study of the solubility of selenium under cementitious conditions and its diffusion, as SeO3(2-), through monolithic cement samples. The solubility studies were carried out under alkaline conditions similar to those anticipated in the near-field of a cement-based repository for low- and intermediate-level radioactive waste. Experiments were conducted in NaOH solution, 95%-saturated Ca(OH)2, water equilibrated with a potential backfill material (Nirex reference vault backfill, NRVB) and in solutions containing cellulose degradation products, with and without reducing agents. The highest selenium concentrations were found in NaOH solution. In the calcium-containing solutions, analysis of the precipitates suggests that the solubility controlling phase is Ca2SeO3(OH)2·2H2O, which appears as euhedral rhombic crystals. The presence of cellulose degradation products caused an increase in selenium concentration, possibly due to competitive complexation, thereby, limiting the amount of calcium available for precipitation. Iron coupons had a minor effect on selenium solubility in contrast to Na2S2O4, suggesting that effective reduction of Se(IV) occurs only at Eh values below -300mV. Radial through-diffusion experiments on NRVB and in a fly ash cement showed no evidence of selenium breakthrough after one year. However, autoradiography of the exposed surfaces indicated that some migration had occurred and that selenium was more mobile in the higher porosity backfill than in the fly ash cement.

A comprehensive study on the dominant formation of the dissolved Ca(OH)2(aq) in strongly alkaline solutions saturated by Ca(ii)

In hyperalkaline aqueous solutions the calcium(ii) concentration is at least 2.3 × 10⁻⁴ M due to the formation of Ca(OH)_2(aq).

Calcium complexation and acid–base properties of l-gulonate, a diastereomer of d-gluconate

The structures of Ca-l-gulonate and Ca-d-gluconate complexes are slightly different due to the differences in the configuration of their ligands.

Calcium l-tartrate complex formation in neutral and in hyperalkaline aqueous solutions

In hyper-alkaline aqueous solutions, Ca²⁺ and l-tartrate (Tar²⁻) ions form CaTarH₋₁⁻_(aq) and CaTarH₋₂²⁻_(aq) complexes containing deprotonated alcoholate group(s).

Speciation and structure of tin(ii) in hyper-alkaline aqueous solution

In hyper-alkaline aqueous solutions, the three-legged stool-like [Sn(OH)₃]⁻ is the only hydroxido complex with a very short (2.078 Å) Sn–O distance.