Investigation into the radiolytic stability of lanthanide-complexed extraction ligands

The recycling of nuclear fuels to extract the remaining useful actinides is one of the most important strategies required for viable nuclear power in the future, as geologic waste disposal of high-level radioactive waste has considerable negative impacts on the environment . As many of these strategies are based on solvent extraction processes, our aim is to establish the radiolytic stability of the extraction ligands. The oxidative stability of the lanthanide metal-complexed extraction ligand diethylenetriaminepentaacetic acid (DTPA) has been investigated through its reaction rate constant with the hydroxyl radical under TALSPEAK acidic pH conditions for Eu ³⁺. In aqueous solution at pH 3.00 and 2.00 these reaction rate constants were determined using thiocyanate competition kinetics, with values of (5.75 ± 0.38) × 10 ⁹ and (3.68 ± 0.06) × 10 ⁹ M ^-1 s ^-1, respectively. These values are slightly greater than the corresponding rate constants determined for only the DTPA protonated ligands at these pH's, determined by the same methodology. Based on literature pKa values and composite rate constant measurements at multiple pH values, individual rate constants for the species DTPA-H ₅, DTPA-H ₄ ^- and DTPA-H ₃ ^2- of 2.69 × 10 ⁹, 3.40 × 10 ⁹ and 3.26 × 10 ⁹ M ^-1 s ^-1 have been determined.