Extraction, Characterization, and Stability Studies of Bistriazinyl-Derived Carboxylic Acids

Separation of An-(III) and Ln-(III) ions will benefit the recycling of used nuclear fuel (UNF). For this purpose, many ligands have been tested over the years, and several separation processes have been successfully demonstrated on the laboratory scale. Current research aims at the development of new ligands that are built only with carbon, hydrogen, oxygen, and nitrogen (CHON), as they can be incinerated completely without secondary waste production. Here, we tested a new class of water-soluble ligands, the bistriazinyl-octa-carboxylic acids. One member, in particular, 2,6-bis-[5,6-di-(3,4-dicarboxyphenyl)-1,2,4-triazin-3-yl]-pyridine (BTPOA), was found to be suitable for the selective separation of Am-(III) and Cm-(III) ions from Ln-(III) ions and may act as a CHON alternative to its sulfonated analogue (SO₃-Ph-BTP). BTPOA exhibited good extraction results and a high selectivity for Am-(III) over Eu-(III) ions. This ligand's complexation of metal ions was further studied using potentiometric spectroscopy, as well as time-resolved laser-induced spectroscopy with Cm-(III) and Eu-(III) in aqueous HClO₄ and HNO₃ media. Conditional stability constants of each formed species were determined. In the HClO₄ system, Cm-(III) formed three species (1:1, 1:2, and 1:3) through the stepwise addition of a single BTPOA molecule. On the other hand, in HNO₃, Cm-(III) formed two 1:2 complexes and one 1:3 complex, while the stepwise formation of three species was observed for Eu-(III). The stability constants are comparable to the values for SO₃-Ph-BTP. The radiolytic behavior of BTPOA was also investigated using electron pulse irradiation measurements to determine absolute rate coefficients (<i>k</i>) under ambient temperature conditions for the reaction of BTPOA with typical UNF reprocessing radical radiolysis productsthe hydrated electron (e_aq ^-, <i>k</i> = (1.60 ± 0.02) × 10¹⁰ M^-1 s^-1), the hydrogen atom (H^•, <i>k</i> = (2.17 ± 0.03) × 10⁹ M^-1 s^-1), and hydroxyl (^•OH, <i>k</i> = (6.95 ± 0.06) × 10⁹ M^-1 s^-1) and nitrate (NO₃ ^•, <i>k</i> = (0.37 ± 0.02) × 10⁷ M^-1 s^-1) radicals. These rate coefficients indicate that the radiolytic longevity of BTPOA should increase with HNO₃ concentration, owing to the consumption of e_aq ^-/H^•, by nitrate anions, and the replacement of ^•OH by the less reactive NO₃ ^•.