Dissociation Rates of Various Heavy Rare Earth Metal-Diethylenetriaminepentaacetic Acid Complexes

Abstract

In relation to the separation of rare earth metal ions (M³⁺) by solvent extraction in the presence of diethylenetriaminepentaacetic acid (DTPA) in an aqueous phase, the rate of dissociation of rare earth metal-DTPA complex, i.e., MDTPA^2– + 2H⁺ ↔ M³⁺ + H₂DTPA^3–, was determined by the analysis of the rate of exchange reaction between MDTPA^2– and Eu³⁺, which is described by MDTPA^2– + EU³⁺ ↔ EuDTPA^2– + M³⁺ (MDTPA^2–; DyDTPA^2–, HoDTPA^2–, TmDTPA^2–). The exchange rate could be expressed by the following equation and the parameters α, β and γ in the equation were determined.

Rate = α[MDTPA^2–][H⁺]² + β[MDTPA^2–] [EU³⁺] + γ[MDTPA^2–][H⁺][EU³⁺]

A method for estimating approximate values of α, that is, the rate constants of proton-catalyzed dissociation of MDTPA^2– for various rare earth metals was proposed. Based on the dissociation rate constants of DTPA complexes obtained, selectivities in solvent extraction in the presence of DTPA were estimated for various cases.