Electrochemical reduction mechanism of EDTA complexes of U 4+ and UO 2+2 at HMDE in aqueous solution

Abstract

The electroreduction of uranium-EDTA (ethylenediaminetetra acetic acid, H 4Y) complexes at the HMDE (hanging mercury drop electrode) was studied in an aqueous 0.5 M NaClO 4 solution. From the cyclic voltammogram and differential pulse polarogram of U 4+-EDTA solution, it was determined that the one electron reversible reduction of UOHY − to UOHY 2− occurs at −1.6V vs. SSCE (sodium chloride-saturated calomel electrode) in the pH range 6.0–7.0.The scan rate and pH dependence suggest that the electroreduction of the UO 2+ 2 -EDTA complex occurs by two different mechanisms depending on the pH of the solution. At the relatively lower pH of 5.5-7.5, the UO 2+ 2 -EDTA complex in the form of UO 2HY − is reduced reversibly to UO 2H 2Y − and then UO 2H 2Y − is further reduced irreversibly to UO 2. The UO 2H 2Y − formed at the first step turns also into UO 2HY − and UY by disproportionation. UO 2 formed at the second step dissolves to become UY, which undergoes a further reduction at −1.6 V. In higher pH range of 7.5–9.0, the reversible reduction of UO 2+ 2 -EDTA complex, as UO 2Y 2−, to UO 2Y 3− is followed by the disproportionation of UO 2Y 3− to UO 2Y 2− and UOY 2−. UOY 2− is further reduced to a U 3+ state at potentials more negative than −1.45 V.