Determination of radionuclide exchangeability in freshwater systems

Two freshwater sediments were spiked with ⁵⁷Co, ⁸⁵Sr and ¹³⁴Cs and left for adsorption times ranging from 1 day to over 60 days. Following adsorption, the 'exchangeable' pool of each radionuclide was measured using ammonium acetate extractions and a sequential leach procedure (Tessier et al., 1979), and the results were compared. Exchangeability was found to depend upon the sediment, radionuclide, sorption time and the identity of extracting agent. All three radionuclides showed a shift with increasing adsorption time from regular exchange sites to sites which are sterically hindered, but a fixation within the sediment was only observed for ⁵⁷Co and ¹³⁴Cs, with similar ammonium acetate extraction yields for both radionuclides. Misleading results were obtained during the sequential leach procedure due to redistribution and, therefore, the inclusion of a NH₄/⁺ leach after the MgCl₂ extraction step was suggested. A mathematical model of element speciation was fairly successful at defining the 'exchangeable' fraction, suggesting that the chemically- and mathematically-defined fractions were similar.