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 EuH2PO4 2+ and CmH2PO4 2+. 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 EuH2PO4 2+ and CmH2PO4 2+ 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.