Interactions of Tc(IV) with citrate in NaCl media

Technetium Complexation with Multidentate Carboxylate-Containing Ligands: Trends in Redox and Solubility Phenomena

The chemistry of technetium (t_1/2(⁹⁹Tc) = 2.11 × 10⁵ years) is of particular importance in the context of nuclear waste disposal and historic contaminated sites. Polycarboxylate ligands may be present in some sites and are potentially capable of strong complexing interactions, thus increasing the solubility and mobility of ⁹⁹Tc under environmentally relevant conditions. This work aimed to determine the impact of five organic complexing ligands [L = oxalate, phthalate, citrate, nitrilotriacetate (NTA), and ethylenediaminetetraacetate (EDTA)] under anoxic, alkaline conditions (pH ≈ 9-13) on the solubility of technetium. X-ray absorption spectroscopy confirmed that TcO₂(am,hyd) remained the solubility-controlling solid phase in undersaturation solubility experiments. Ligands with maximum coordination numbers (CN) ≥ 3 (EDTA, NTA, and citrate) exhibited an increase in solubility from pH 9 to 11, while ligands with CN ≤ 2 (oxalate and phthalate) at all investigated pH and CN ≥ 3 at pH ≈ 13 were outcompeted by hydrolysis reactions. Though most available thermodynamic values were determined under acidic conditions, these models satisfactorily explained high-pH undersaturation solubility of technetium for citrate and NTA, whereas experimental data for Tc(IV)-EDTA were highly overestimated. This work illustrates the predominance of hydrolysis under hyperalkaline conditions and provides experimental support for existing thermodynamic models of Tc-L except Tc-EDTA, which requires further research regarding aqueous speciation and solubility.

Thermodynamic parameters for the complexation of Tc(IV) with bromide under aqueous conditions

Technetium-99 is a long-lived fission product present in nuclear wastes, found mainly as Tc(VII) and Tc(IV) in the environment. The quantification of the equilibrium constants for the formation of Tc(IV) aqueous complexes has been limited to carboxylate ligands and interactions with the halides is mostly unknown. This work reports equilibrium constants of the formation of the TcO(OH)+ complexes with Br−, in a 3 M NaClO4 solution of pcH 2 and varied temperature, using a liquid-liquid extraction system. Neutron activation confirmed the suitability of the extraction technique for this work. Under the working conditions, Br− forms a weak exothermic TcO(OH)Br complex, with a Gibbs free energy (ΔGr) of 3 ± 3 kJ · mol−1 at a temperature of 273.15 K. The values for ΔHr (−32 ± 3 kJ · mol−1) and ΔSr (106 ± 9 J · mol−1 · K−1) of the complexation reaction were quantified using a van’t Hoff analysis. This work also showed that bromide addition does not displace the hydroxide from TcO(OH)+, as the equilibrium constant of bromide addition is much weaker than the first hydrolysis constant of the metal.

Thermodynamic parameters for the complexation of technetium(IV) with EDTA

Technetium-99 is a high yield (~6% fission yield) fission product and long-lived (2.13×105 year half-life) component of nuclear waste that will be disposed of in a geological repository. Some 99Tc has been released into the environment due to nuclear fuel and weapon production activities at sites such as Hanford, WA. Strongly complexing ligands such as ethylenediamine-N,N,N′,N′-tetraacetic acid (EDTA) are known to increase Tc(IV) solubility and mobility in environmental systems and an accurate quantification of the complexation of Tc(IV) with EDTA is important for predicting its behavior in a geological repository. A liquid–liquid extraction system utilizing 0.2 M TOPO in dodecane was used to measure the stability constants of Tc(IV)-EDTA in 0.50 m NaNO3 at variable temperatures (14.0±0.1, 25.0±0.1, and 32.0±0.1°C). The acid dependence of the apparent stability constants in the pC H range of 2.00–2.70 indicated the formation of TcO(EDTA)2− (logβ 101=17.9±0.3, 25.0±0.1°C) and a protonated complex TcO(H)(EDTA)− (logβ 111=20.5±0.1, 25.0±0.1°C). The associated thermodynamic parameters Δr G 101=−101.7±0.4 kJ·mol−1, Δr H 101=−47±9 kJ·mol−1, Δr S 101=179±36 J·mol−1·K−1, Δr G 111=−117.2±0.3 kJ·mol−1, Δr H 111=−23±5 kJ·mol−1, and Δr S 111=315±63 J·mol−1·K−1 (0.50 m NaNO3, 25.0±0.1°C) were determined by van’t Hoff analysis. The formation of each Tc(IV)–EDTA complex is exothermic and present favorable entropy terms.

Carboxyl groups of citric acid in the process of complex formation with bivalent and trivalent metal ions in biological systems

Binary complexes of citric acid (H₃L - protonated form, H₂L and HL - partly protonated forms, L - fully deprotonated) with d- and f-electron metal ions were investigated. The studies have been performed in aqueous solution using the potentiometric method with computer analysis of the data, electron paramagnetic resonance, infrared, visible as well as luminescence spectroscopies. The overall stability constants of the complexes were determined. Analysis of the equilibrium constants of the reactions and spectroscopic data has allowed determination of the type of coordination and effectiveness of the carboxyl groups in the process of complex formation. On the basis of potentiometric titration for d-electron were found dimeric and monomeric type of complexes and for f-electron four type of complexes: MHL, ML, ML(OH) and ML(OH)₂.

Stability constant determinations for technetium (IV) complexation with selected amino carboxylate ligands in high nitrate solutions

The stability constants for Tc(IV) complexation with the ligands IDA, NTA, HEDTA, and DTPA were determined in varied nitrate concentrations using liquid-liquid extraction methods. The determined log β101 stability constants at 0.5 M NaNO3 were found to be 9.2±0.3, 10.3±0.3, and 15.3±0.3 for IDA, NTA, and HEDTA, respectively. The log β111 stability constant for DTPA was determined to be 22.0±0.6. These determined stability constants show a slight decrease in magnitude as a function of increasing NaNO3 concentration. These stability constants were used to model the total dissolution of Tc(IV) in acidic aqueous solutions in the presence of each ligand. The results of these predictive models indicate that amino carboxylic ligands have a high potential for increasing the aqueous dissolution of Tc(IV); at pH 2.3, 0.01 M ligand yield dissolved Tc(IV) concentrations of 1.42·10−5 M, 1.33·10−5 M, 6.07·10−6 M, 9.65·10−7 M, for DTPA, HEDTA, NTA, and IDA, respectively.