Determination of liquid-solid partition coefficients (K_d) of radionuclide anionic species from a contaminated aquifer

摘要

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 (~(60)Co, ~(106)Ru, ~(137)Cs, ~(238)Pu, ~(239/240)Pu, ~(241)Am and ~(244)Cm). The extracts were then mixed with native uncontaminated soil material to determine their liquid-solid partitioning coefficients (K_d). Separate K_d values of the same elements were also obtained with the usual short-term "batch" technique for comparison, but using different radioisotopes as tracers (~(57)Co and ~(134)Cs), spiked to the extracts. The comparison included separate K_d determinations with ~(239/240)Pu and ~(241) Am as tracers, added to uncontaminated groundwater. In all cases, the radionuclides originally present in the contaminated groundwater extracts exhibited low K_d values, compared to the values obtained in the "batch" method with tracers (~(57)Co, ~(134)Cs, ~(239/240)Pu and ~(241)Am). 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, ~(57)Co and ~(134)Cs 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 K_d values obtained using the "batch" method with tracers are used in transport models, radionuclide transport could be underestimated.