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

The extraction of Am(IH) from HNO3 by o-xylene solutions of P,P ' -di(2-eth ylhexyl) methylene- (H2DEH[MDP]), ethylene(H2DEH[EDP]), and butylene- (H2DE H[BuDP]) diphosphonic acids has been investigated. Thermodynamic parameters of the extraction process were determined by the temperature coefficient o f the metal distribution ratio in the 25.0 to 60.0 degreesC temperature ran ge. The extractant aggregation and the extraction stoichiometries with the different diphosphonic acids did not exhibit any significant temperature de pendence. Extraction of Am(III) by H2DEH[MDP] is favored by both enthalpy a nd entropy variations. When Am(III) is extracted by either H2DEH[EDP] or H2 DEH[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, thermoneutr al for the second, and endothermic for the third extractant, respectively. This trend was explained as arising from the formation of progressively les s stable chelate rings along the diphosphonic acids series. The zero or pos itive enthalpy variation with a concomitant favorable entropy variation con firm for H2DEH[EDP] and possibly for H2DEH[BuDP] a micellar-like type of ex traction process.