Uranium(VI) complexation with trans-1,2-cyclohexanediaminetetraacetic acid in solution: thermodynamic and structural studies

Interaction between uranium(VI) and aminopolycarboxylic acids could significantly influence the chemical behavior of uranium(VI) in the processing and disposal of nuclear wastes. Trans-1,2-cyclohexanediaminetetraacetic acid (CDTA) is a commonly used aminopolycarboxylic acid, but its complexation with uranium(VI) has not been sufficiently studied. In this study, stability constants of CDTA protonation and U(VI)-CDTA complexes at different temperatures were measured by potentiometry. Their enthalpies and entropies were obtained from van’t Hoff plots. As a result, four U(VI)-CDTA species, UO2(H2CDTA), UO2(HCDTA)-, UO2CDTA2- and UO2(OH)CDTA3-, were identified at pH 3-6 and their coordination modes in solution were demonstrated by thermodynamic studies. The proposed coordination modes of UO2(H2CDTA) and UO2(HCDTA)- in solution differ from the reported crystals; the former is bidentate bonding with one carboxylate and the latter is tridentate bonding with iminodicarboxylate.