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

Abstract

Many low-level radioactive wastes, historically disposed in shallow land trenches, are illdefined 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 CaCl 2 was passed through saturated columns that were packed with ferrihydrite (Fe(OH) 3)-coated Si0 2. 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(H)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)EDTP 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.