Chemical speciation of Am, Cm and Eu with EDTA at high ionic strength: thermodynamics and laser fluorescence spectroscopy studies

Summary

The complexation of Am³⁺, Cm³⁺ and Eu³⁺ with EDTA has been studied at an ionic strength of 6.60 mol kg⁻¹ (NaClO₄) and pcH 3.60 in the temperature range of 0 to 60 °C with a solvent extraction technique. The complex M(EDTA)⁻ was formed with stability constant values (log β₁₀₁) at 25 °C between 15.9 and 16.1 which increased with increasing temperature. The complex formation enthalpies have been obtained from the temperature dependence of the stability constants. The large positive enthalpy and entropy changes are attributed to the considerably reduced water activity at such high ionic strength resulting in large positive contributions from the dehydration of the trivalent metal ions as compared to the smaller negative contributions from the M³⁺–EDTA⁴⁻ bonding interaction. The nature of the species of the EuEDTA complex formed at 6.60 mol kg⁻¹ (NaClO₄) and pcH ≥3.60 was studied by time-resolved laser fluorescence spectroscopy and lifetime measurements. Between pcH 3.60 and 7.0, Eu(EDTA)(H₂O)₃⁻ was formed while at pcH 9.0, Eu(OH)(EDTA)(H₂O)₂²⁻ was present.