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 x 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- (logβ₁₀₁=17.9±0.3, 25.0±0.1 C) and a protonated complex TcO(H)(EDTA)^- (logβ₁₁₁=20.5±0.1, 25.0±0.1 C). The associated thermodynamic parameters Δ_rG₁₀₁=-101.7±0.4 kJ.mol^-1, Δ_rH₁₀₁=-47±9 kJ.mol^-1, Δ_rS₁₀₁=179±36 J.mol^-1.K^-1, Δ_rG₁₁₁=-117.2±0.3 kJ.mol^-1, Δ_rH₁₁₁=-23±5 kJ.mol^-1, and Δ_rS₁₁₁=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.