Kinetics of the adsorption and desorption of radionuclides of Co, Mn, Cs, Fe, Ag and Cd in freshwater systems: experimental and modelling approaches

The sorption and release kinetics of Mn-54, Co-58, Fe-59, Cd-109, Ag-110m a nd Cs-134 by freshwater suspended particles were investigated to better ide ntify the biogeochemical processes involved and to obtain suitable data for improving models describing radionuclide migration in freshwater streams. In order to observe any seasonal variability in the interaction of radionuc lides with natural particles, experiments were performed both in winter and in summer during a phytoplanktonic bloom. Two kinetic models are compared in this paper: the "one-step reversible" model, based on the hypothesis tha t the transfer of radionuclides between water and suspended solids is gover ned by a reversible reaction, and the "two-successive-step reversible" mode l, which assumes two distinct types of sites or reactions on the solid phas e. The "one-step reversible" model is generally unable to describe properly the exchange kinetics; this result shows that at least two processes are g enerally involved in radionuclide exchange between water and suspended part icles. On the contrary, a model involving the existence of two successive r eversible reactions properly simulates both the sorption and release kineti cs. The determination of the kinetic coefficients allows quantitative asses sment of the relative importance and kinetics of the processes. In particul ar, it has been shown that, except for Cs and Cd, major fractions of the ra dionuclides are associated at equilibrium with particulate sites involving strong interactions. The kinetics to reach this equilibrium depend on seaso nal conditions, especially for Co and Mn: the transfer of Co and Mn to part iculate sites involving strong interactions is much slower in winter. The d istribution of the radionuclides between water and particulate sites involv ing weak interactions also shows seasonal variations for Co, Mn, Fe and Ag: the capacity to associate radionuclides is much higher in summer for Co an d Mn, while the inverse tendency is observed for Fe and Ag. For Cs and Cd, no significant seasonal differences were observed. (C) 2001 Elsevier Scienc e Ltd. All rights reserved.