The extraction of U(VI) and selected M(III) cations by bis neo-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 [C 4H 9C(CH 3) 2CH 2O] 2PO(OH), HDNOP, in benzene or in n-heptane has been studied as a function of the 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⇌ MY( 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 HDNOP 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 (benzene) value for each of the cations studied. However, although the ratio of the K s values is large for each of the seven M(III) cations (in the 50–250 range), it is only 6 for U(VI). Comparison with extraction systems employing two other di octyl phosphoric acids, ( n-C 8H 17O) 2PO(OH) and [C 4H 9CH(C 2H 5)CH 2O] 2PO(OH), respectively symbolized as HDOP and HDEHP, as extractants are made in terms of: (1) acidity of the extractant, (2) steric effects within the extractant molecule, and (3) effect of diluent (aromatic or saturated hydrocarbon.) In a recently reported study [1] of the extraction of U(VI) and selected M(III) elements from an aqueous chloride phase by bis n-octyl phosphoric acid, ( n-C 8H 17O) 2PO(OH), in a diluent it was shown that under comparable conditions the distribution coefficient, K, for an M(III) element is much higher in the system employing n-heptane as diluent than in that employing benzene. The stoichiometry of M(III)-extraction also differed in the two diluents. The stoichiometry of extraction of U(VI) was shown to be the same in the two diluents, and under comparable conditions the K for U(VI) was found to be only slightly higher in n-heptane than in benzene. The extractant was considered to have no significant steric hindrance. The present study is similar in scope but employs a bis octyl phosphoric acid which is moderately sterically hindered, bis neo-octyl phosphoric acid, [C 4H 9C(CH 3) 2CH 2O] 2PO(OH), symbolized as HDNOP. The neo-octyl group, 2,2-dimethyl hexyl, is an isomer of both the n-octyl group referred to above and the 2-ethyl hexyl group of the widely studied extractant [C 4H 9CH(C 2H 5)CH 2O] 2PO(OH), HDEHP.