Geochemical reactions governing the fate of Co-NTA in contact with natural subsurface materials

Subsurface codisposal of toxic metals and radionuclides with organic chelating agents has created vast areas of contaminated soils and groundwater. The fate of the metal/radionuclide and ligand are inexorably linked in their interaction with soil minerals and aquifer solids. The present study was conducted to investigate the geochemical reactions (sorption, dissolution, dissociation, oxidation) that govern the fate of Co^IINTA complexes in contact with natural subsurface materials that are typical of materials underlying some waste disposal areas. Equilibrium measurements indicated that at low pH (4) and in the presence of abundant surface exchangeable A1, the adsorption of Co and NTA was independent of the presence of the other component. By contrast, at higher pH (6 and 7.4) the sorption of both Co and NTA decreased in the presence of the other moiety. Solution phase analyses indicated that the decrease in sorption was driven by the formation of stable aqueous complexes of Co(II) and Co(III) with NTA. The time-dependent loss of Co^IINTA from solution was accounted for by sorption, complex dissociation, and the oxidation of Co(II) to Co(III). Biodegradation of NTA was not an important process over the 21-day incubation period. Formation of Co(III) complexes has broad implications in these systems as these species are kinetically and thermodynamically stable, exhibit lower adsorption onto solids, and are resistant to biodegradation. Thus, with the exception of relatively extreme conditions (low pH, abundant readily available Al), NTA decreased Co partitioning to surfaces through the formation of stable aqueous complexes. This behavior may contribute to the undesirable transport of ⁶⁰Co through the subsurface.