KINETICS AND MECHANISMS OF CO(II)EDTA OXIDATION BY PYROLUSITE

Monitoring and restoration activities at low-level radioactive waste d isposal sites have identified complicated mixtures of inorganic and or ganic contaminants in soil and groundwater. Metallic contaminants are generally complexed with various chelating agents and organic acids wh ich alter the geochemical behavior of the contaminants in subsurface m edia. The objective of this study was to provide an improved understan ding of the geochemical processes controlling the subsurface transport of radioactive Co-60 complexed with ethylenediaminetetraacetic acid ( EDTA). Specifically, we investigated the kinetics and mechanisms of Co (II)EDTA(2-) oxidation to Co(III)EDTA(-) by the soil mineral pyrolusit e (beta-MnO2). A column displacement technique was utilized to investi gate Co(II)EDTA(2-) reactivity and oxidation rates through packed beds of pyrolusite-coated SiO2. The interaction of Co(II)EDTA(2-) with the porous media was characterized by a MnO2-induced oxidation of the Co( II)EDTA(2-) to Co(II)EDTA(-). The oxidation of Co(II)EDTA(2-) appeared to involve the reduction of Mn(IV) to both an aqueous Mn2+ species an d a theorized Mn(III)-oxide solid phase. The redox reaction was cataly tic since the reduction products were gradually reoxidized in the pres ence of dissolved O-2 to form a Mn(IV)-oxide phase. Oxidation of surfa ce-bound Mn2+ and the theorized Mn(III)-oxide was slow relative to Co( II)EDTA(2-) oxidation, and a reversible loss in the oxidative ability of the beta-MnO2 occurred when exposed to Co(II)EDTAZ-.The reduction i n catalytic activity of the MnO2 was not the result of direct surface poisoning by Mn2+, but rather was believed to result from the formatio n of an intermediate Mn(III)-oxide solid phase whose oxidative potenti al was significantly less than MnO2. Thus, the kinetics of Co(II)EDTA( 2-) oxidation to Co(III)EDTA(-) by MnO2 was dependent on the rate of M nO2 surface regeneration. The environmental implications of this redox reaction are pronounced, since any Co(III)EDTA(-) produced is extreme ly stable,and this enhances the persistence and transport of Co-60 in subsurface environments.