Determination of liquid-solid partition coefficients (Kd) of radionuclide anionic species from a contaminated aquifer
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Determination of liquid-solid partition coefficients (Kd) of radionuclide anionic species from a contaminated aquifer François Caron*, Michael K. Haas, Edward L. Cooper and David E. Robertson** Atomic Energy of Canada Limited, Environmental Technologies Branch, Chalk River, ON, Canada, K0J 1J0; *Current address: Dept. of Chemistry & Biochemistry, Laurentian University, Sudbury, ON, Canada, P3E 2C6; **Pacific Northwest National Laboratories, Richland, WA; ABSTRACT Contaminated groundwater from a former liquid waste discharge area at the Chalk River Laboratories was treated with ion exchange resins to isolate aqueous extracts of selected radionuclides (60Co, 106Ru, 137Cs, 238Pu, 239/240Pu, 241Am and 244Cm). The extracts were then mixed with native uncontaminated soil material to determine their liquid-solid partitioning coefficients (Kd). Separate Kd values of the same elements were also obtained with the usual short-term “batch” technique for comparison, but using different radioisotopes as tracers (57Co and 134Cs), spiked to the extracts. The comparison included separate Kd determinations with 239/240 Pu and 241Am as tracers, added to uncontaminated groundwater. In all cases, the radionuclides originally present in the contaminated groundwater extracts exhibited low Kd values, compared to the values obtained in the “batch” method with tracers (57Co, 134Cs, 239/240Pu and 241Am). The difference was up to two orders of magnitude. This was attributed to differences in aqueous speciation of the nuclides in the contaminated groundwater, allowing limited interactions of complexed radionuclide species with soil material. Our results indicated that all the radionuclides were predominantly in anionic form in the groundwater, whereas in the tracer “batch” experiment, 57Co and 134Cs were predominantly cationic, Pu and Am were predominantly anionic. Hydrolysis partially explains the dominant anionic character of Am and Pu in the tracer experiments, however dissolved organics are suspected to dominate the speciation of the radionuclides in contaminated groundwater. Our experiment implies that in some cases, if Kd values obtained using the “batch” method with tracers are used in transport models, radionuclide transport could be underestimated. INTRODUCTION Many contaminant transport models use a retardation factor based on the solid-liquid partition coefficient Kd1, where Kd is usually defined as the ratio of the concentration of a contaminant on a solid to the concentration of the contaminant in a contacting solution, at equilibrium. A common way to obtain Kd is to add a known amount of the contaminant to a mixture of soil material and solution. After equilibration, the solution is analyzed for the contaminant, and losses from the solution are attributed to sorption to the soil material. This measurement, although useful and simple, suffers from limitations, e.g, the types of sorption sites in the soil constituents, the aqueous speciation of the contaminant. The current work is a scoping study aiming to determine the difference i
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