Metal extraction by alkyl substituted diphosphonic acids.: Part 4.: P,P′-di(2-ethylhexyl) butanediphosphonic acid

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-level concentration by o-xylene solutions of P,P'-di(2-ethylhexyl) butanediphosphonic acid, H2DEH[BuDP]. The extractant and acid dependencies of these metal ions exhibited significant differences from those of the previously investigated analogous extractants in which the two phosphonate groups are separated by a methylene or an ethylene bridge. The aggregation of H2DEH[BuDP] was investigated in toluene at 25 degrees C by vapor pressure osmometry. H2DEH[BuDP] was found to exist predominantly as a trimeric species in the 0.1-0.005 molal concentration range. Osmometric measurements and infrared spectra indicate that Ca(II) is extracted into H2DEH[BuDP] solutions with little disruption of the structure of the extractant. Iron(IIE) causes significant deprotonation of the ligand and dramatically changes the apparent aggregation number. A comparison of the extraction of Ca(II), Am(III) and Fe(III) by H2DEH[BuDP] with data obtained using bis(2-ethylhexyl) phosphoric acid (HDEHP) or 2-ethylhexyl 2-ethylhexylphosphonic acid (HEH[EHP]) as the extractant indicates that H2DEH[BuDP] has characteristics similar to these monofunctional analogs. Infrared spectra of the Ca(II) and Fe(III) salts of H2DEH[BuDP] show a shift of both V-asym(POO-) and v(sym)(POO-) to lower frequencies relative to their values in the sodium salt. This indicates a symmetrical interaction between the metal ion and the phosphonate groups through chelate and/or bridging interactions.