ACIDS. PART 1. P,P"-DI(2-ETHYLHEXYL) METHANEDIPHOSPHONIC ACID

Abstract

Two novel extractants, P,P'-di(2-ethylhexyl) methanediphosphonic acid (H2DEH[MDP]) and P.P'-dioctyl methanediphosphonic acid (H2DO[MDP]) have been synthesized at high purity and yield. H₂DEH[MDP] was selected for metal extraction studies because of its better physical properties. An investigation of the extraction of alkaline earth cations, Fe(III) and representative tri-, tetra- and hexavalent actinide ions from nitric acid solutions into o-xylene solutions of H₂DEH[MDP] at different concentrations was performed. With a few exceptions, the acid dependencies of the extraction of the above metal species strongly resembles those measured in the uptake of the same metals by the chelating ion exchange resin Diphonix®, which contains gem-diphosphonic acid groups chemically attached to a polymeric matrix. H₂DEH[MDP] exhibits practically no selectivity among the alkaline earth cations. The almost lack of acid dependency observed with Fe(lll) and tetra- and hexavalent actinides indicates that these ions are chelated by H₂DEH[MDP] mostly through the P=0 groups of the extractant. With Fe(lll) and the actinides, variable slopes of the extractant dependencies were measured, their values being strongly dependent on the acidity of the aqueous phase. Typically, the slope of the extractant dependency has a value of about two if the metal is extracted from high acidity solutions. This value becomes progressively lower as the aqueous acidity is reduced, tending to almost zero for acidities around 0.1 M. This phenomenon has been attributed to the formation of metal complexes having different stoichiometrics, ligand protonation and solubility in the aqueous phase. H₂DEH[MDP] possesses an extraordinary affinity for actinides and Fe(III). In analogy with the Diphonix resin, this property can be exploited for actinide preconcentration and separation from complex matrices of analytical interest and from mixed waste solutions.