Adsorption and desorption kinetics of 60 Co and 137 Cs in fresh water rivers Fabricio Fiengo P  erez a, b, * , Lieve Sweeck b , Willy Bauwens a , May Van Hees b , Marc Elskens c a Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050, Brussels, Belgium b Biosphere Impact Studies, Belgian Nuclear Research Centre SCKCEN, Boeretang 200, BE-2400, Mol, Belgium c Laboratory of Analytical and Environmental Chemistry, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050, Brussels, Belgium article info Article history: Received 13 April 2015 Received in revised form 10 July 2015 Accepted 13 July 2015 Available online xxx Keywords: Radionuclides Distribution coefficient Adsorption Desorption Kinetics Uncertainty analysis abstract Radionuclides released in water systems e as well as heavy metals and organic toxicants e sorb to both the suspended solid particles and the bed sediments. Sorption is usually represented mathematically by the distribution coefficient. This approach implies equilibrium between phases and instantaneous fixa- tion (release) of the pollutant onto (from) the surface of the soil particle. However, empirical evidence suggests that for some radionuclides the fixation is not achieved instantaneously and that the revers- ibility of the process can be slow. Here the adsorption/desorption kinetics of 60 Co and 137 Cs in fresh water environments were simulated experimentally and later on modelled mathematically, while the influence of the most relevant factors affecting the sorption were taken into account. The experimental results suggest that for adsorption and the desorption more than 24 h are needed to reach equilibrium, moreover, It was observed that the desorption rate constants for 60 Co and 137 Cs lie within ranges which are of two to three orders of magnitude lower than the adsorption rate constants. © 2015 Published by Elsevier Ltd. 1. Introduction Radionuclides released in surface water systems can follow three different pathways. They can be (1) transported in solution, also called dissolved phase, (2) in suspension attached to solid particles, known as suspended solid phase, or (3) accumulated in the bed sediments. In the dissolved phase, the radionuclides follow the water flow dynamics and their residence time in the system is the same as that of the water, while in the other two phases the sediment transport processes determine the radionuclide migra- tion, and hence the residence time is longer. The interaction between the radionuclides and the sediments is mainly governed by adsorption and desorption. Adsorption repre- sents the transfer of a substance from the dissolved to the solid phase and desorption is the release of the substance from a particle into the water. Both are often represented by the distribution co- efficient K d (Eq. (1)) (also known as partition coefficient) under the assumption of instantaneous equilibriums and reversibility. K d  ml g  ¼ C s C w (1) where C w , C s are the concentrations of the pollutant in the dissolved phase [Bq/ml] and in the suspended solid phase [Bq/g], respec- tively. Here Bq represents Becquerel the quantity of radioactive material in which one nucleus decays per second. Values for K d vary greatly as a function of aqueous and solid phase chemistry, implying that a constant single K d value is hardly ever acceptable for an entire study site and/or period. Then, the K d value should be modified as the chemically important environ- mental conditions (i.e. water ionic composition, sediment charac- teristics, pH, redox potential, temperature, etc.) change (Chapra, 1997; IAEA, 2009; Radovanovic and Koelmans, 1998) in order to reduce the uncertainty ranges, and improve the reliability of the predictions. Moreover, the assumptions of fast equilibrium and reversibility are not necessarily valid for all radionuclides. For some of them, the equilibrium between phases is not instantaneously achieved, and the process is hardly reversible. In these cases, besides the distri- bution coefficient, knowledge about the kinetics of the adsorption and desorption processes should be included. Studies related to the modelling transport of contaminants in * Corresponding author. Boeretang 200, BE-2400, Mol, Belgium. E-mail address: ffperez@sckcen.be (F. Fiengo P erez). Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad http://dx.doi.org/10.1016/j.jenvrad.2015.07.010 0265-931X/© 2015 Published by Elsevier Ltd. Journal of Environmental Radioactivity 149 (2015) 81e89