Mechanisms of Plutonium Redox Reactions in Nitric Acid Solutions

Abstract

Plutonium (Pu) exhibits a complex redox behavior in aqueous solutions. This is due to the ability of the element to adapt a wide range of oxidation states typically from +3 to +6 and the tendency for dynamic interconversion between the oxidation states that primarily depend upon acid concentration and presence of coordinating ligands. This work interrogates the Pu redox behavior in aqueous nitric acid via a combination of voltammetry and in situ vis–NIR spectroelectrochemistry under controlled potentials to map the interconversion between the various Pu oxidation states. The NIR-spectroelectrochemistry studies used to complement the visible spectroscopy bring a new and more complete perspective into the plutonium redox transformations. This allows elucidation of the mechanisms of the involved redox reactions facilitating an in-depth understanding of the relative stability of the Pu oxidation states as a function of redox potentials and nitric acid concentrations. It is observed that oxidation of Pu­(III) results in generation of Pu­(IV) and Pu­(VI) (the latter as PuO2 2+), bypassing the Pu­(V) oxidation state. Further, with increasing acid concentrations, the formation of the Pu­(VI) species progressively decreases so that the dynamic equilibrium between the Pu­(III) and Pu­(IV) oxidation states dominates. These findings have significant implications for developing separation processes for used nuclear fuel reprocessing and treatment.

Highly Efficient Extraction Chromatography Resin Containing Hexa-n-Octyl Nitrilotriacetamide (HONTA) for Selective Recovery of Plutonium from Acidic Feeds

“…This will reduce the free ligand concentration on the resin and consequently will eventually reduce the K d value. Second, Pu 4+ is known to form an anionic complex in nitric acid medium, which is dominating at higher acidity (>6 M HNO 3 ) . This may also cause a reduction of the K d value, especially at higher nitric acid concentrations.…”

“…Second, Pu 4+ is known to form an anionic complex in nitric acid medium, which is dominating at higher acidity (>6 M HNO 3 ). 25 This may also cause a reduction of the K d value, especially at higher nitric acid concentrations. The separation factors of Pu 4+ over Am 3+ and UO 2 2+ are very large, namely, in the range of a few thousands.…”

Light‐Driven Oxidation of Pu(IV) to Pu(VI) Enables Green and Efficient Pu Recovery

“…[ 2 ] To date, Pu is the only element in the periodic table that was found to exist in four different oxidation states (III, IV, V, VI) simultaneously in aqueous solution. [ 3‐5 ] To effectively recover Pu from nuclear wastes and to control its fate in environment, the oxidation states of Pu must be precisely controlled. For example, in the well‐known PUREX process for nuclear spent fuel reprocessing, Pu is controlled back and forth between the IV and III oxidation states to ensure its co‐extraction with U and its subsequent separation from U.…”

“…Besides, electrochemistry has long be exploited as a “green” method in regulating the oxidation state of Pu. [ 4,12‐13 ] Nevertheless, the quantitative electrochemical oxidation of Pu(IV) to Pu(VI) in nitric acid solution encounters great challenge, [ 4 ] and the direct contact of the electrode with the highly corrosive and radioactive solution requires high stability of the electrode material and may generate additional waste. Therefore, searching for more efficient and environmentally benign methods to generate high valent Pu is still of great interest to help establish greener approaches for the recovery of Pu.…”

Quantifying neptunium oxidation states in nitric acid through spectroelectrochemistry and chemometrics

“…A controlled spectropotentiometric approach has been leveraged previously to study the mechanisms for Np and Pu redox reactions in HNO 3 solutions but has not been applied to the development of regression models to measure their relative concentrations or applied to systems with Np concentrations >10 mM. (Edelstein, 2015;Lines et al, 2017;Chatterjee et al, 2020). In this work, thin-layer, in situ spectropotentiometry was used to isolate Np species in pure oxidation states without compromising solution optical properties from the additional redox reagents to generate spectral data for the development of supervised and unsupervised methods for quantifying Np in various oxidation states.…”