Complexation of Trivalent Lanthanides (Eu) and Actinides (Cm) with Aqueous Phosphates at Elevated Temperatures

In this study, the complexation of Eu(III) and Cm(III) with aqueous phosphates was investigated using laser-induced luminescence spectroscopy. Experiments at 25 °C and different ionic strengths (0.6-3.1 mol·L^-1 NaClO₄) established the formation of EuH₂PO₄²⁺ and CmH₂PO₄²⁺. From the conditional stability constants, the respective values at infinite dilution as well as the ε(Me(H₂PO₄)²⁺;ClO₄^-) (Me = Eu or Cm) ion interaction coefficients (using the specific ion interaction theory - SIT) were derived. Further experiments (at constant ionic strength of 1.1 mol·L^-1) showed that upon increasing the temperature (25-80 °C), the formation of both EuH₂PO₄²⁺ and CmH₂PO₄²⁺ was favored. Using the van't Hoff equation, the molal enthalpy Δ _R H _m^° and molal entropy Δ _R S _m^° of these reactions were derived, corroborating an endothermic and entropy driven complexation process. This work contributes to a better understanding of the coordination chemistry of both trivalent lanthanides and actinides with phosphate ions.