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(II)NTA 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(II)NTA 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(II)NTA 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(II)NTA, 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.
Brooks
Scott C
SC
Environmental Sciences Division, Oak Ridge National Laboratory, TN 37831-6038, USA. brookssc@ornl.gov
Carroll
Sue L
SL
D002614:Chelating Agents / Q000737:chemistry*; D002852:Chromatography, Ion Exchange; D003037:Cobalt Radioisotopes / Q000737:chemistry*; D004058:Diffusion; D005618:Fresh Water; D006863:Hydrogen-Ion Concentration; D009571:Nitrilotriacetic Acid / Q000737:chemistry*; D014874:Water Pollutants, Chemical; D014875:Water Pollutants, Radioactive
D002614:Chelating Agents; D003037:Cobalt Radioisotopes; D014874:Water Pollutants, Chemical; D014875:Water Pollutants, Radioactive; D009571:Nitrilotriacetic Acid