THERMODYNAMICS OF THE EXTRACTION OF METAL IONS BY DIALKYL-SUBSTITUTED DIPHOSPHONIC ACIDS. II. THE U(VI) AND Sr(II) CASE

Abstract

The thermodynamics of extraction of U(VI) and Sr(II) from aqueous HNO3 solutions 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 by measuring metal distribution ratios in the 25.0 to 60.0°C temperature range. The extractant aggregation and extraction stoichiometries for both metal ions did not change with temperature. The extraction of U(VI) by all three extractants is strongly driven by both enthalpy and entropy variations. The extraction of Sr(II) by H2DEH[MDP] is enthalpy 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 thermodynamic treatment of previously reported Am(III) data has been performed. The absolute values of the free energy and entropy changes of the extraction are affected. However, the extraction of Am(III) by H2DEH[MDP] is still strongly driven by both enthalpy and entropy variations, whereas Am(III) extraction by H2DEH[EDP] and H2DEH[BuDP] is mainly driven by entropy changes. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under contract W-31-109-ENG-38. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.