Solvent extraction of lanthanum(III) from acidic nitrate-acetato medium by Cyanex 272 in toluene

The extraction behavior of La(III) from acidic nitrate-acetato medium by bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272, H(2)A(2)) in toluene either alone or in combination with trioctylphospine oxide (TOPO, B) has been investigated as a function of contact time, aqueous phase concentration of La3+, H+, NO3- and Ac- (acetate), Cyanex 272 and TOPO concentration in the organic phase and temperature. The distribution coefficient is found to decrease with increasing La3+ concentration up to 7.34 mM La3+ in the aqueous phase, after which it increases up to 9.79 mM La3+ concentration and again decreases. The pH and log extractant concentration dependencies suggest that [La(Ac)(2)A.3HA] and [La(Ac)A(2).B] are extracted in the system without and with TOPO, respectively. The pH dependence of 1 indicates that one hydrogen ion is liberated in forming the complex for both the extraction system with TOPO and without TOPO. The distribution coefficient decreases with increasing acetate ion concentration at higher acetate ion concentrations with and without TOPO but is less dependent at low acetate concentrations. The synergistic effect of TOPO is found only at higher TOPO concentrations. These results suggest that the [La(Ac)(2)A.3HA]((o)) and [La(Ac)A(2).B]((o)) species are extracted in the system without and with TOPO, respectively. The temperature dependence data give the enthalpy of extraction, DeltaH, of 60 kJ/mol. The equilibrium constant, K-ex ,is 10(-2.16) with a standard deviation of log K-ex of 0.096 in the system without TOPO and 10(-2.58) with a standard deviation of log K-ex of 0.21 for the extraction system with TOPO. The loading capacity of Cyanex 272 is found to be 15.97 g La(III)/100 g Cyanex 272. The phosphorus to lanthanum ratio of the solid complex obtained is 3. (C) 2002 Published by Elsevier Science B.V.