Metal extraction by sulfur-containing symmetrically-substituted bisphosphonic acids. Part I. P,P '-di(2-ethylhexyl) methylenebisthio-phosphonic acid

P,P'-dialkyl methylenebisphosphonic acids are powerful metal extraction reagents exhibiting strong affinity for a variety of metal ions, especially lanthanides and actinides. While the affinity of gem -bisphosphonic acids is generally high for most metal ions because of their relative high acidity and ability to form six-member chelate rings, the selectivity often is low. Thus, a strategy of incorporating soft-donor atoms such as sulfur into gem-bisphosphonic acids has been adopted to obtain enhanced metal selectivity while retaining high extraction efficiency. To this end a new class of sulfur-containing gem -bisphosphonic acid solvent extraction reagents was designed, synthesized, and evaluated for heavy element separations. Specifically, the novel sulfur-containing P,P'-di(2-ethylhexyl) methylenebisthiophosphonic acid, H2DEH[MBTP], was synthesized, characterized and its aggregation, metal extraction and acid-base behavior assessed. Vapor phase osmometry measurements indicate that H 2 DEH[MBTP] is less aggregated than its P,P'-di(2-ethylhexyl) methylenebisphosphonic acid analogue, H2DEH[MBP], existing in toluene primarily as an equilibrium mixture of monomer and dimer in the concentration range studied. The acid dependency data for the extraction of Am3+ and Eu3+ from aqueous perchlorate solutions by H-2 DEH[MBTP] in o-xylene exhibit slopes close to -3 at low acidity, consistent with extraction of a trivalent metal ion. The extractant dependency data exhibit pH dependent slopes, suggesting different stoichiometry of metal extraction under different acidities.