Redox speciation of uranyl in citrate medium: kinetics and reduction mechanism with in situ spectroelectrochemical investigation

Abstract

Electrochemical methodology, viz. cyclic and square wave voltammetry, was used to investigate the dominant species of UO₂²⁺–citrate at particular physiochemical conditions on the basis of their redox behavior. The UO₂²⁺ in citrate media exists as UO₂²⁺, monomer [UO₂Cit]⁻, dimer [(UO₂)₂Cit₂]²⁻ and polymeric species. [UO₂Cit]⁻ and [(UO₂)₂Cit₂]²⁻ were found to be reduced in two irreversible one-electron steps, with a chemical reaction coupled between them (ICI reduction mechanism), to a stable [U^IVCit]⁺ species at pH ≤ 5 and via a single reduction step with two-electron transfers at pH > 5. Chronopotentiometry and chronoamperometry techniques were applied to corroborate the finding of an ICI reduction mechanism, and to explore the kinetics of reduction by evaluating heterogeneous electron-transfer kinetic (k⁰_f,h and α) parameters. Using the Deford–Hume formula, the stability constant (log β) of monomeric species was calculated to be 6.8 ± 0.11, which is in agreement with the literature. Spectroelectrochemical measurements were carried out to validate the chemical reaction interposed between the two-electron transfer steps. Additionally, evidence of a new U(V) species, i.e., [U^VO₂Cit]²⁻, in an aqueous system was obtained by in situ spectroelectrochemical measurements during electrolysis, and its stability constant was obtained by cyclic voltammetry. ESI-MS studies on solutions of varying UO₂²⁺ : citrate ratios and pH were used to confirm complex stoichiometries.