Interactions of perrhenate (ReO4-) and pertechnetate (TcO4-) with the uranyl ion (UO22+) in acetonitrile solution: spectroscopy and theoretical calculations

The co-extraction of pertechnetate (TcO4-) along with the uranyl ion (UO22+) is a problematic issue that complicates the Plutonium Uranium Reduction EXtraction (PUREX) process for reprocessing spent nuclear fuel. Unfortunately, research on the interactions of TcO4- with UO22+ is very scarce. This work quantitatively investigated the interactions of TcO4- and its analog ReO4- with UO22+ in acetonitrile with 0.5% water content by spectrophotometric titrations including UV-Vis absorption and Raman spectroscopies. UO22+ and TcO4-/ReO4- form complexes of 1 : 1 to 1 : 3 molar ratios, in which the stability constants of the UO22+-TcO4- complexes are slightly higher than those of the UO22+-ReO4- complexes. The structures of the complexes of UO22+ with TcO4-/ReO4- are modeled by DFT calculations. The DFT calculations indicate that the TcO4-/ReO4- anions are monodentate coordinated with UO22+. The UV-Vis and Raman spectra of the complexes are simulated, and it is found that the second solvation shell needs to be explicitly taken into account in the DFT calculations in order to obtain Raman spectra comparable to the experiments. The results in this work help understand the co-extraction behavior of TcO4- with UO22+ in spent nuclear fuel reprocessing.