Thermodynamics of the extraction of metal ions by dialkyl-substituted diphosphonic acids. II. The U(VI) and Sr(II) case

The thermodynamics of extraction of U(VI) and Sr(II) from aqueous HNO3 solu tions by o-xylene solutions of P,P ' -di(2-ethylhexyl) methylene- (H2DEH-[E DP]), ethylene-(H2DEH[EDP]), and butylene- (H2DEH[BuDP]) diphosphonic acids has been investigated by measuring metal distribution ratios in the 25.0 t o 60.0 degreesC temperature range. The extractant aggregation and extractio n stoichiometries for both metal ions did not change with temperature. The extraction of U(VI) by all three extractants is strongly driven by both ent halpy and entropy variations. The extraction of Sr(II) by H2DEH[MDP] is ent halpy driven. Unfavorable entropy changes characterize the extraction of Sr (II) by H2DEH[EDP] and H2DEH[BuDP]. This is not indicative of the micellar- type extraction mechanism exhibited by Am(III). A more rigorous thermodynam ic treatment of previously reported Ani(III) data has been performed. The a bsolute values of the free energy and entropy changes of the extraction are affected. However, the extraction of Ani(III) by H2DEH[MDP] is still stron gly driven by both enthalpy and entropy variations, whereas Ani(III) extrac tion by H2DEH[EDP] and H2DEH[BuDP] is mainly driven by entropy changes.