Redox behavior of Ce(IV)/Ce(III) in the presence of nitrilotriacetic acid: A surrogate study for An(IV)/An(III) redox behavior

Abstract

AbstractUsing cyclic voltammetry, we investigated the redox behavior of Ce(IV)/Ce(III), which is a surrogate for An(IV)/An(III) (An=actinides), in a solution of nitrilotriacetic acid (NTA) at 25 °C. The cyclic voltammogram of Ce in a 0.1 M NTA solution at pH 6 showed a reversible one-electron redox reaction for Ce(IV)/Ce(III) at 0.51 Vvs.Ag/AgCl. This redox potential was much lower than that obtained in 1 M nitric acid, indicating that Ce(IV) was preferentially stabilized by complexation with NTA. The redox potential in the NTA solution was independent of the Ce concentration from 2 to 20 mM, NTA concentration from 5 to 200 mM and pH between 3 and 7. These results indicated that no polymerization and no additional coordination of NTA and OH−to the Ce(III)-NTA complex took place during the redox reaction. As the speciation calculation of Ce(III) in the NTA solution showed that the predominant species was CeIII(nta)23−(H3nta=NTA), the redox reaction of the Ce-NTA complex was expressed by the following: CeIV(nta)22−+e−⇋CeIII(nta)23−. The logarithm of the stability constant of CeIV(nta)22−was calculated to be 38.6±0.8 forI=0 from the redox potential shift of Ce(IV)/Ce(III) in the NTA solution. The value was in good accordance with the stability constant of the NpIV(nta)22−complex, demonstrating that the aqueous coordination chemistry of Ce(IV) with NTA is quite similar to that of An(IV). These results strongly suggest that a negative shift of the Pu(IV)/Pu(III) redox potential in the NTA solution should make Pu(IV) more stable than Pu(III) even in a reducing environment.