Reactive transport of EDTA-complexed cobalt in the presence of ferrihydrite

Many low-level radioactive wastes, historically disposed in shallow land trenches, are ill-defined mixtures of radionuclides and organic chelating agents. The observed migration of nuclides, such as ⁶⁰Co, away from burial sites has been attributed, in part, to the formation of aqueous complexes with ethylenediaminetetraacetic acid (EDTA). The stability of Co-EDTA complexes, and thus the fate and transport of ⁶⁰Co in the subsurface, is strongly dependent on the oxidation state of cobalt (log K_Co(II)EDTA = 18.3; log K_Co(III)EDTA = 43.9). The factors that control the oxidation of Co(II) to Co(III) in subsurface environments are not well understood. We conducted a series of column flow experiments to provide an improved understanding of the geochemical processes that control the reactive transport of cobalt in the subsurface. A solution of 0.2 mM Co(II)EDTA^2- in 5 mM CaCl₂ was passed through saturated columns that were packed with ferrihydrite (Fe(OH)₃)-coated SiO₂. During transport through the column, a portion of the Co(II)EDTA^2- was oxidized to Co(III)EDTA^-; the amount of oxidation reached a steady-state under oxic conditions. Transport of the oxidized species, Co(III)EDTA^-, was substantially more rapid than the transport of Co(II)EDTA^2-. The retardation of both Co-EDTA species and the extent of cobalt oxidation increased as the pH decreased. These results are consistent with the hypothesis that the association of Co(II)EDTA^2- with the ferrihydrite surface is essential for the charge-transfer involved in the oxidation reaction. Co(III)EDTA^- exhibited less retardation because this monovalent anion had a lower affinity for the surface than the divalent Co(II)EDTA^2-. At faster flow rate, the retardation of Co(II)EDTA^2- decreased whereas Co(III)EDTA^- breakthrough occurred later; the amount of Co(III)EDTA^- formed decreased with increasing flow rate. Under anoxic conditions, the oxidation of Co(II)EDTA^2- was decreased, but was not eliminated, suggesting that ferric iron may serve as an oxidant in the system. The loss of oxidative sites under continuous exposure to Co(II)EDTA^2- and the blocking of oxidative sites by ions residing on the ferrihydrite surface resulted in a slow decline in the amount of oxidation under anoxic conditions. The oxidation of Co(II)EDTA^2- effectively competed with other geochemical reactions such as the Fe(III)-induced dissociation of Co(II)EDTA^2- complexes under oxic and anoxic conditions. These results indicate that an iron mineral can be more important for the formation of Co(III)EDTA^2- in the subsurface than the mineral is important for the dissociation of Co(II)EDTA^- and the concomitant formation of Fe(III)EDTA^-. The results suggest that conditions of pH and flow rate that inhibit the formation of the very stable Co(III)EDTA^- also promote the undesirable rapid transport of Co(II)EDTA^2-, posing a challenge to the selection of future waste sites and the development of remedial strategies for existing sites impacted by EDTA-complexed ⁶⁰Co.