Insight into Speciation and Electrochemistry of Uranyl Ions in Deep Eutectic Solvents

Understanding the speciation of metal ions in heterogeneous hydrogen-bonded deep eutectic solvents (DES) has immense importance for their wide range of applications in green technology, environmental remediation, and nuclear industry. Unfortunately, the fundamental nature of the interaction between DES and actinide ions is almost completely unknown. In the present work, we outline the speciation, solvation mechanism, and redox chemistry of uranyl ion (UO₂²⁺) in DES consisting of choline chloride (ChCl) and urea as the hydrogen-bond donor. Electrochemical and spectroscopic techniques along with molecular dynamics (MD) simulations have provided a microscopic insight into the solvation and speciation of the UO₂²⁺ ion in DES and also on associated changes in physical composition of the DES. The hydrogen-bonded structure of DES plays an important role in the redox behavior of the UO₂²⁺ ion because of its strong complexation with DES components. X-ray absorption spectroscopy and MD simulations showed strong covalent interactions of uranyl ions with the constituents of DES, which led to rearrangement of the hydrogen-bonding network in it without formation of any clusters or aggregations. This, in turn, stabilizes the most unstable pentavalent uranium (UO₂⁺) in the DES. MD analysis also highlights the fact that the number of H-bonds is reduced in the presence of uranyl nitrate irrespective of the presence of water with respect to pristine reline, which suggests high stability of the formed complexed species. The effect of added water up to 20 v/v % on speciation is insignificant for DES, but the presence of water influences the redox chemistry of UO₂²⁺ ions considerably. The fundamental findings of the present work would have far reaching consequences on understanding DES, particularly for application in the field of nuclear fuel reprocessing.