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

Abstract

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.