TEMPERATURE EFFECTS IN THE EXTRACTION OF METAL IONS BY DIALKYL-SUBSTITUTED DIPHOSPHONIC ACIDS. I. THE Am(III) CASE

The extraction of Am(III) from HNO3 by o-xylene solutions of P,P′-di(2-ethylhexyl) methylene- (H2DEH[MDP]), ethylene- (H2DEH[EDP]), and butylene- (H2DEH[BuDP]) diphosphonic acids has been investigated. Thermodynamic parameters of the extraction process were determined by the temperature coefficient of the metal distribution ratio in the 25.0 to 60.0°C temperature range. The extractant aggregation and the extraction stoichiometries with the different diphosphonic acids did not exhibit any significant temperature dependence. Extraction of Am(III) by H2DEH[MDP] is favored by both enthalpy and entropy variations. When Am(III) is extracted by either H2DEH[EDP] or H2DEH[BuDP] the process is entropy controlled. The enthalpy variation for Am(III) extraction exhibited a trend along the series H2DEH[MDP], H2DEH[EDP], H2DEH [BuDP], with the reaction being exothermic for the first, thermoneutral for the second, and endothermic for the third extractant, respectively. This trend was explained as arising from the format...