The extraction of U(VI) and selected M(III) cations by bis n-octyl phosphoric acid in two different hydrocarbon diluents

Abstract

The extraction of Ce 3+, Pm 3+, Eu 3+, Tm 3+, Y 3+, Am 3+, Cm 3+ and UO 2 2+ from an aqueous chloride phase into a solution of ( n-C 8H 17O) 2PO(OH), HDOP, in benzene or in n-heptane has been studied as a function of concentration of extractant in the organic phase and of hydrogen ion in the aqueous phase. From these data the stoichiometries of extraction have been deduced as: U(VI), benzene or n-heptane: UO 2A 2++2 HY 2O⇌ UO 2( HY 2) 2O+2 H A + M(III), benzene: M A 3++3( HY) 2O⇌M( HY 2) 3O+3 H A + M(III), n-heptane: M A 3++2·5( HY) 2O⇌M Y( HY 2) 2O+3 H A + where the subscripts A and O refer, respectively, to mutually equilibrated aqueous and organic phases. (The formulation of extracted entities is based solely upon established ratios and is in no sense to be interpreted in terms of established structure). Corresponding to the stoichiometries, values of K, F and [H +] obtained from the hydrogen ion dependency plots were inserted in the equation: K= K sF a [ H +] b to obtain values of K s . (K is the observed distribution ratio, F the concentration of HDOP in the organic phase and [H +] the concentration of hydrogen ion in the aqueous phase following equilibration, and a and b the respective extractant and hydrogen ion dependencies. The equation is applied specifically to a system embodying 1·0 F (NaCl + HCl) at 22 ± 2°C). The K s ( n-heptane) value is higher than the K s (benzene) value for each of the cations studied. However, although the ratio of the two K s values is large for each of the seven M(III) cations, ranging from 50 to 200, it is only 10 for U(VI). From these observations, it is apparent that the term carrier diluent is preferable to the term inert diluent. Comparisons with systems employing [C 4H 9CH(C 2H 5)CH 2O] 2PO(OH) and ( n-C 8H 17) 2PO(OH), respectively symbolized as HDEHP and H[DOP], as extractants are made in terms of: (1) acidity of the extractant; (2) structure of extractant, that is CP or COP bonds; (3) steric effects within the extractant molecule; and (4) effect of diluent (aromatic or saturated hydrocarbon).