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 (UO22+)/dipicolinic acid (DPA) complex ([U(VI)O2(DPA)(OH)(H2O)]-) 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 [U(VI)O2(DPA)(OH)(H2O)]- can be electrochemically reduced on the Pt electrode at -0.9 eV (vs. Ag/AgCl) to [U(V)O2(DPA)(OH)(H2O)]2- in aqueous medium under an anaerobic environment. According to cyclic voltammetric analysis, a pair of oxidation and reduction waves at E'0 = -0.592 V corresponds to the [U(VI)O2(DPA)(OH)(H2O)]-/[U(V)O2(DPA)(OH)(H2O)]2- redox couple and the formation of [U(V)O2(DPA)(OH)(H2O)]2- was confirmed by the electron stoichiometry (n = 0.97 ± 0.05) of the reduction reaction of [U(VI)O2(DPA)(OH)(H2O)]-. The pentavalent uranyl complex [U(V)O2(DPA)(OH)(H2O)]2- was further characterized via UV-vis-NIR absorption spectrophotometry and X-ray absorption (XANES and EXAFS) spectroscopy. The [U(V)O2(DPA)(OH)(H2O)]2- 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.