Neptunium Carbonato Complexes in Aqueous Solution: An Electrochemical, Spectroscopic, and Quantum Chemical Study

The electrochemical behavior and complex structure of Np carbonato complexes, which are of major concern for the geological disposal of radioactive wastes, have been investigated in aqueous Na2CO3 and Na2CO3/NaOH solutions at different oxidation states by using cyclic voltammetry, X-ray absorption spectroscopy, and density functional theory calculations. The end-member complexes of penta- and hexavalent Np in 15 M Na2CO3 with pH = 11.7 have been determined as a transdioxo neptunyl tricarbonato complex. [NpO2(CO3)(3)](n-) (n=5 for Np-V, and 4 for Np-VI). Hence, the electrochemical reaction of the Np-V/VI redox couple merely results in the shortening/lengthening of bond distances mainly because of the change of the cationic charge of Np, without any structural rearrangement. This explains the observed reversible-like feature on their cyclic voltammograms. In contrast, the electrochemical oxidation of Np-V in a highly basic carbonate solution of 2.0 M Na2CO3/1.0 M NaOH (pH > 13) yielded a stable heptavalent Np complex of [(NpO4)-O-VII(OH)(2)](3-), indicating that the oxidation reaction from Np-V to Np-VII in the carbonate solution involves a drastic structural rearrangement from the transdioxo configuration to a square-planar-tetraoxo configuration, as well as exchanging the coordinating anions from carbonate ions (CO32-) to hydroxide ions (OH-).