Mechanistic Approach to Reveal Interaction of Uranyl Ions in Alkyltriphenylphosphonium Bromide-Based Deep Eutectic...

Speciation is known to control fundamental aspects of metal processing and electrochemical behavior such as solubility and redox potentials. Deep eutectic solvents (DESs) are an emerging class of green, low-cost and designer solvents and are being explored as alternatives for recycling nuclear fuel and critical materials. However, there is a lack of knowledge about the behavior of metals in them. Here, for the first time, we synthesized three new DESs based on alkyltriphenylphosphonium bromide (CPPh3Br), with varied alkyl chain lengths (n), as the hydrogen-bond acceptor along with decanoic acid (DA) as the hydrogen-bond donor and explored the redox speciation of uranyl nitrate. The changes in the Fourier transform infrared and NMR spectra helped elucidate the formation of hydrogen bonds in DES. The absorption maxima of uranyl in DES was red-shifted by 10 nm compared to the free uranyl, with concomitant increase in intensity and luminescence lifetime, which suggested a strong interaction of uranyl nitrate with DES. Cyclic voltammetry was probed to understand the redox thermodynamics, transport properties, and heterogeneous electron transfer kinetics of the irreversible electron transfer of uranyl ions in the three DESs. Electrochemical and spectroscopic techniques together with density functional theory calculations unlocked microscopic insights into the solvation and speciation of UO₂²⁺ ions in three DESs and also the associated unusual trends observed in the physical properties of the DESs. The hydrogen-bonded structure of DES plays a crucial role in the redox behavior of the UO₂²⁺ ion due to its strong potent complexation with its components. The basic findings of the present work can have far-reaching consequences for the extraction, electrochemical separation, and future development of redox-based separation processes in the nuclear fuel cycle.