An experimental approach to assess the post-depositional mobility of ¹³⁴Cs

The partitioning coefficient, K_d, which is defined by the reversible sorption processes between a solid and an aqueous phase at equilibrium, is one of the most important parameters to assess environmental transport and risk. In this study, a series of simple laboratory experiments were conducted to investigate sorption properties of ¹³⁴Cs on a model sediment under two treatments (shaken vs non-shaken) and with three (small: <75 μm, large: > 75 μm and bulk i.e., composite) particle size fractions. Vertical transport of ¹³⁴Cs across the water-sediment interface and into sediment was also evaluated. As expected, grain size had the strongest influence on ¹³⁴Cs K_d values, with the small particle size fraction yielding significantly higher K_d values than the large and bulk fractions. The mean K_d values obtained from the various experiments ranged from 89 ± 13-130 ± 5 L kg^-1 (small), 44 ± 10-91 ± 13 L kg^-1 (large), 73 ± 3-112 ± 11 L kg^-1 (bulk, shaken) and 73 ± 5-110 ± 4 L kg^-1 (bulk, non-shaken). Most of the ¹³⁴Cs partitioning processes occurred rapidly (<2 h) into the experiment. Physical mixing (shaken) did not appear to significantly affect the ¹³⁴Cs K_d values. In complement, a separate experiment on the vertical penetration of ¹³⁴Cs into a bulk sediment column showed that ¹³⁴Cs was able to penetrate up to 5 cm into the sediment column after 88 days (∼0.6 mm d^-1) and this flux rate is comparable to natural settings. Adsorption and contact time were found to be key for the ¹³⁴Cs penetration process. Results from these experiments add to the literature on post-event radionuclide transport studies in marine settings and provide an experimental perspective that can be built upon to complement field observations.