Plutonium exhibits a variety of oxidation states and has a strong affinity for complexation with organic ligands. Isosaccharinic acid (ISA) is a major degradation product of cellulose materials present in the low to intermediate radioactive wastes. The interaction between trivalent plutonium and ISA can significantly impact the migration and containment of plutonium in the repository environment. In this study, formation of Pu(III) and ISA complexes was investigated at an ionic strength of 1 M of NaClO4 using UV-Vis absorption spectrophotometry. To exclude the effect of the Pu(III) oxidation, absorption spectra were measured within 10 min after adding ISA into Pu(III) solution and processed using HYPSPEC software for deconvolution after baseline correction. Several previous studies showed that the presence of ligands accelerates the oxidation of Pu(III) to Pu(IV). To investigate whether ISA complexation can also accelerate the Pu(III) oxidation, UV-Vis absorption spectra changes over 24 hours were analyzed as a function of the ratio of ISA to plutonium concentration.

Elucidating the redox behavior of actinide elements in aqueous solution is important for the safety assessments of nuclear waste disposal. Despite ongoing endeavors for decades, some points of uranium and plutonium redox mechanism are ambiguous and unclear. In this study, the electrochemical redox behaviors of U(VI) and Pu(III and VI) ions in perchloric acid media were investigated by using a gold (Au) working electrode via cyclic voltammetry (CV) and cyclic square wave voltammetry (CSWV) with the temperature control (10–55°C). The cyclic voltammograms of U(V/VI), Pu(III/IV) and Pu(V/VI) redox couple were transformed to semi-integral form to calculate the diffusion coefficient and formal potential in the electrochemical quasi-reversibility prevailed system. The CSWV was additionally used for a more precise interpretation of the redox mechanism. From the investigation of the redox chemistry of U(VI) ions, a clear U(V/VI) redox peak and one unidentified oxidation peak appear around pH 2. With the temperature control and CSWV, the relevance of the oxidation peak and U(IV) was confirmed. In the case of voltammetry of Pu(VI) solution, Pu(V/VI) redox peak and an additional reduction peak appear. The redox behavior resposible for this additional reduction peak are also examined. The cyclic voltammograms of Pu(III) solution show a clear reversible redox reaction of Pu(III/IV) couple. With the temperature control, using the change of formal potential at ionic strength 1 M (ClO4 −), thermodynamic parameters of conditional molar enthalpy and entropy change were evaluated in this system.