Abstract:
Monitoring and restoration activities at low-level radioactive waste disposal sites have identified complicated mixtures of inorganic and organic contaminants in soil and groundwater. Metallic contaminants are generally complexed with various chelating agents and organic acids which alter the geochemical behavior of the contaminants in subsurface media. The objective of this study was to provide an improved understanding of the geochemical processes controlling the subsurface transport of radioactive 60Co complexed with ethylenediaminetetraacetic acid (EDTA). Specifically, we investigated the kinetics and mechanisms of Co(II)EDTA2- oxidation to Co(III)EDTA- by the soil mineral pyrolusite (β-MnO2). A column displacement technique was utilized to investigate Co(II)EDTA2- reactivity and oxidation rates through packed beds of pyrolusite-coated SiO2. The interaction of Co(II)EDTA2- with the porous media was characterized by a MnO2-induced oxidation of the Co(II)EDTA2- to Co(III)EDTA-. The oxidation of Co(II)EDTA2- appeared to involve the reduction of Mn(IV) to both an aqueous Mn2+ species and a theorized Mn(III)-oxide solid phase. The redox reaction was catalytic since the reduction products were gradually reoxidized in the presence of dissolved O2 to form a Mn(IV)-oxide phase. Oxidation of surface-bound Mn2+ and the theorized Mn(III)-oxide was slow relative to Co(II)EDTA2- oxidation, and a reversible loss in the oxidative ability of the β-MnO2 occurred when exposed to Co(II)EDTA2-. The reduction in catalytic activity of the MnO2 was not the result of direct surface poisoning by Mn2+, but rather was believed to result from the formation of an intermediate Mn(III)-oxide solid phase whose oxidative potential was significantly less than MnO2. Thus, the kinetics of Co(II)EDTA2- oxidation to Co(III)EDTA- by MnO2 was dependent on the rate of MnO2 surface regeneration. The environmental implications of this redox reaction are pronounced, since any Co(III)EDTA- produced is extremely stable, and this enhances the persistence and transport of 60Co in subsurface environments.