Stabilization of uranyl(v) by dipicolinic acid in aqueous medium.

Preparation of a stable U(v) complex in an aqueous medium is a challenging task owing to its disproportionation nature (conversion into more stable U(vi) and U(iv) species) and sensitivity to atmospheric oxygen. The stable uranyl (UO)/dipicolinic acid (DPA) complex ([UO(DPA)(OH)(HO)]) was formed at pH 10.5-12.0, which was confirmed by potentiometric and spectrophotometric titrations, and NMR, ESI-MS and EXAFS spectroscopy. The complex [UO(DPA)(OH)(HO)] can be electrochemically reduced on the Pt electrode at -0.9 eV (vs. Ag/AgCl) to [UO(DPA)(OH)(HO)] in aqueous medium under an anaerobic environment. According to cyclic voltammetric analysis, a pair of oxidation and reduction waves at E' = -0.592 V corresponds to the [UO(DPA)(OH)(HO)]/[UO(DPA)(OH)(HO)] redox couple and the formation of [UO(DPA)(OH)(HO)] was confirmed by the electron stoichiometry (n = 0.97 ± 0.05) of the reduction reaction of [UO(DPA)(OH)(HO)]. The pentavalent uranyl complex [UO(DPA)(OH)(HO)] was further characterized via UV-vis-NIR absorption spectrophotometry and X-ray absorption (XANES and EXAFS) spectroscopy. The [UO(DPA)(OH)(HO)] complex is stable at pH 10.5-12.0 in anaerobic water for a few days. DFT calculation shows the strong complexing ability of DPA stabilizing the unstable oxidation state U(v) in aqueous medium.