Auto-Inhibition of Oxide Mineral Reductive Capacity Toward Co(II)EDTA

Subsurface migration of ⁶⁰Co has been attributed to organic chelating agents and oxidation of Co(II)EDTA^2- to Co(III)EDTA^- by mineral surfaces. Although the oxidized product (Co(III)EDTA^-) has been detected in solution, a reduced species has not been measured. As a result, fate and transport mechanisms involving ⁶⁰Co remain ill-defined. Accordingly, the objective of this research was to determine redox changes in the solid-phase oxidants, β-MnO₂ and Fe(OH)₃·nH₂O, during reaction with Co(II)EDTA^2-. Time-resolved changes in the surface composition of β-MnO₂ was accomplished in hydrodynamic systems using XANES spectroscopy. The transport of Co(II)EDTA^2- through packed beds of β -MnO₂ resulted in a decrease in structural Mn(IV) and an increase in Mn(III) as a Mn₂O₃-like phase. As the quantity of Mn₂O₃ increased, the production of Co(III)EDTA^- decreased. Thus, it appears that the surface association of Mn₂O₃ produced from the oxidation of Co(II)EDTA^2- impedes the redox reaction. We were unable to detect surface structural alterations on Fe(OH)₃·nH₂O upon reacting with Co(II)EDTA^2-.