EXTRACTION IN SYSTEMS WITH 2 IMMISCIBLE AQUEOUS PHASES BASED ON POLYETHYLENEGLYCOL (PEG) AND A SALT SOLUTION AS AN EXAMPLE OF EQUILIBRIA WITH DISSOCIATION IN BOTH PHASES

Phase equilibria and extraction of HNO3 and uranyl, Li, Na, Ce(III), E u(III), and Am(III) nitrates are studied in systems with two immiscibl e phases, one of which contains 20-30% of the water-soluble polymer PE G (polyethyleneglycol) with a mass of 1500-6000; the other of which is a 20-40% salt solution [(NH4)2SO4]. The phase equilibria are treated as a manifestation of the limited solubility of PEG in the salt soluti on and of the salt in the PEG phase. The concentration of the salt and the mass of the PEG have a strong influence on the solubility of PEG in the salt solution. ne concentration and mass of the PEG have little effect on the solubility of the salt in the PEG phase. The data are d escribed by the Sechenov equation. The salts effective in phase format ion are those that interact poorly with water (sulfates, carbonates, p hosphates). The free energy of hydration is positive. The temperature of the solution usually decreases as the salt dissolves. The system ca n be kept heterogeneous up to high [HNO3] (6 M) by increasing the salt concentration. The distribution of metals and acid between the phases was found to be determined by the small difference of the hydration e nergy of the extracted substance (its decrease in the PEG phase; extra ction with a diluent demonstrated that the coordination to PEG is very small). Thus, the distribution coefficient of HNO3 is close to 1 (alp ha congruent-to 0.7). ne distribution coefficients of the metals are l ow and decrease with increasing salt concentration owing to complexati on by the anion (for actinides, alpha is approximately [SO42-]4-). The equilibria have a number of unusual properties. The metal (and HNO3) distribution coefficients weakly depend on their concentration (and do not revert to zero as the concentration does). The distribution coeff icients of tracer Ce (III) and Am (III) are independent of the salt co ncentration (Li and Na nitrates) but are greater with LiNO3. These fea tures of the equilibria are explained by the dissociation of the compo unds in both phases. This was proved by measuring the electric conduct ivity (that the extraction was best from LiNO3 solutiOns was explained by the fact that this salt is extracted more poorly than NaNO3). The distribution coefficients in these systems can be increased by decreas ing the salt concentration (to 15% so that it remains heterogeneous), by approaching the critical mixing point, and by increasing [HNO3]. Ne ar the layering limit the distribution coefficients of Ce (III) rose f rom 10(-2) to 0.5. The distribution coefficients significantly increas e on adding complexants to near-neutral solutions.