pH-dependent fate and transport of NTA-complexed cobalt through undisturbed cores of fractured shale saprolite

The codisposal of toxic metals and radionuclides with organic chelating agents has been implicated in the facilitated transport of the inorganic contaminants away from primary waste disposal areas. We investigated the transport of Co^IINTA through undisturbed cores of fractured shale saprolite. Experiments were conducted across the pH range 4 to 8 by collecting cores from different locations within the weathering profile. Aqueous complexation, adsorption, dissociation and oxidation reactions influenced Co^IINTA transport. The suite of reaction products identified in column effluent varied with experimental pH. At low pH and in the presence of abundant exchangeable aluminum, Co transport occurred predominantly as the Co²⁺ ion. At higher pH, Co was transported primarily as Co^IINTA and the Co(III) species Co^III(HNTA)₂ and Co^III(IDA)₂. The formation of the geochemical oxidation products (Co(III) species) has far reaching implications as these compounds are kinetically and thermodynamically stable, are transported more rapidly than Co^IINTA, and are resistant to biodegradation. These results demonstrate that natural minerals, in the physical structure encountered naturally, can be more important in the formation of mobile, stable contaminant forms than they can be for the retardation and dissociation of the contaminants.