Temperature effects in the extraction of metal ions by dialkyl-substituted diphosphonic acids. I. The Am(III) case

The extraction of Am(IH) 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 degreesC 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 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 H2DEH[EDP] and possibly for H2DEH[BuDP] a micellar-like type of extraction process.