Metal extraction by alkyl substituted diphosphonic acids .3. P,P'-di(2-ethylhexyl) ethanediphosphonic acid solvent extraction study

As part of an ongoing investigation of the properties of dialkyl substituted diphosphonic acids as solvent extraction reagents for metal cations, we have studied the extraction of alkaline earth cations, Fe(III) and representative actinides (Am(III), U(VI) and Th(IV)) at tracer concentration levels by o-xylene solutions of P,P'-di(2-ethylhexyl) ethanediphosphonic acid, H2DEH[EDP]. The extractant and acid dependencies of these metal ions were determined by measuring their distribution ratio as a function of the concentrations of H2DEH[EDP] and aqueous nitric acid. From an analysis of the extractant dependencies, a micellar extraction process has been proposed for the alkaline earth cations, Am(III), and, under some conditions, for U(VI). The hydrated metal cations are transferred into the hydrophilic internal cavity of the hexameric extractant aggregate with simultaneous release of H+ ions to compensate for the cation charge. With Fe(III), variable slopes were measured for the extractant dependencies, the values being strongly dependent upon the acidity of the aqueous phase. The slope of the extractant dependency has a value of about two if the metal ion is extracted from highly acidic solutions. This value becomes progressively lower as the aqueous acidity is reduced, approaching zero at pH 1. This phenomenon has been attributed to the formation of metal complexes having different stoichiometries and ligand protonation through strong interaction of the metal with the phosphonate groups of the ligand.