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- (H₂DEHMDP]), ethylene- (H₂DEHEDP]), and butylene- (H₂DEHBuDP]) 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 H₂DEHMDP] is favored by both enthalpy and entropy variations. When Am(III) is extracted by either H₂DEHEDP] or H₂DEHBuDP] the process is entropy controlled. The enthalpy variation for Am(III) extraction exhibited a trend along the series H₂DEHMDP], H₂DEHEDP], 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 formation of progressively less stable chelate rings along the diphosphonic acids series. The zero or positive enthalpy variation with a concomitant favorable entropy variation confirm for H₂DEHEDP] and possibly for H₂DEHBuDP] a micellar-like type of extraction process.