Trivalent actinide and lanthanide separations by dialkyl-substituted diphosphonic acids

The dialkyl-substituted diphosphonic acids are powerful solvent extraction reagents for actinide separation and preconcentration from biological, environmental, and nuclear waste samples. In spite of the intensive scrutiny of the solvent extraction properties of these compounds, only the extraction of Eu(III) and Am(III), among the lanthanides and trivalent actinides, has been investigated thus far. In this work, we report the extraction of all members of the lanthanide series, plus lanthanum, yttrium, Am(III), Cm(III), and Cf(III), by two representative diphosphonic acids [i.e., P,P'-di(2-ethylhexyl) methylenediphosphonic acid (H2DEH[MDP]) and the P,P'-di(2-ethylhexyl) ethylenediphosphonic acid (H2DEH[EDP])]. H2DEH[MDP] shows very efficient extraction of all ions investigated, but little selectivity, except for the heaviest lanthanides. H2DEH[EDP], on the other hand, exhibits less efficient extraction, but much higher selectivity, with an average separation factor between contiguous elements of 1.8, and extraction equilibrium constants spanning about three orders of magnitude along the lanthanide series. The extraction data for H2DEH[EDP] also exhibit a significant tetrad effect. The differences in affinity and selectivity for the lanthanides and trivalent actinides observed with the two extractants are discussed in terms of chelating ring size, monofunctional character, extractant aggregation, and metal extraction stoichiometries.