DIALKYLPHOSPHORIC ACIDS AS CARRIERS IN SEPARATION OF LANTHANIDES AND THORIUM ON SUPPORTED LIQUID MEMBRANES

Permeation of seven lanthanides (Ln) and thorium through a supported l iquid membrane containing di-(n-octyl)phosphoric (DOPA) or di-(n-penty l)phosphoric (DPPA) acid as a carrier has been studied as a function o f the chemical composition of the system. The results have been compar ed with a previous study in which di-(2-ethyl-hexyl)phosphoric acid wa s used. Metal cations were transported from feed solutions of pH 1.1-4 .8 (HNO3) into strip solutions of 0.015-0.1 mol l(-1) nitric acid. The ionic strength was kept constant at 0.1 mol l(-1) (HNO3, KNO3). The i nitial lanthanide concentration and carrier concentration in the liqui d membrane were varied from 0.5 to 500 mu mol l(-1) and from 0.01 to 0 .5 mol l(-1) respectively. To describe the mass transfer of metal cati ons, permeability coefficients have been determined by inductively-cou pled plasma atomic emission spectroscopy or by on-line flow-injection analysis of metal concentrations in strip or feed solution. Probably a s the result of a higher solubility of the carrier in aqueous media, t ransport of Ln with DPPA was not observed. By using DOPA, La, Ce, Pr, and Nd permeated through the membrane while transport of heavier Ln wa s partly or totally suppressed. This enables these four Ln (separation factor alpha = 3.0 for Nd and Sm) to be separated from the others. Fu rthermore, at a very low acidity gradient, only La (III) is transporte d over the membrane (alpha greater than or equal to 3.4 for La, Ce and next Ln). The seven elements from La to Tb can be separated from Th(I V) because no evidence of its permeation through the membrane was foun d under the conditions of Ln transport. In contrast to previous studie s on Ln transport with dialkylphosphoric acid carrier, the possibility of participation of species other than Ln(AHA)(3) in the transport ha s been discussed. The decrease of permeability observed at higher Ln c oncentrations and higher pH of the feed solution has been explained as the result of formation of species, e.g. polymeric ones, that are una ble to permeate through the membrane.