Thermodynamic study of metal silicate complexation in perchlorate media

The thermodynamic parameters of formation of the o-silicate complexes of UO22+, Cm3+ and Eu3+ have been measured in the temperature range 5-45 C in an aqueous medium of 0.20 M (NaClO4) ionic strength. Solvent extraction with bis(2-ethylhexyl) phosphoric acid (HDEHP) was used to measure the stability constant (log β1) values of the silicate complexes as a function of temperature to obtain the enthalpies and entropies of complexation. The complexation reactions are entropy driven and opposed by the enthalpy. These thermodynamic values are consistent with the formation of an inner sphere complex. (orig.)