Experimental study of 60Co behaviour in Tejo River sediments

The objective of this study was to obtain information on the transfer of radiocobalt in freshwater environments that can be used to predict its environmental distribution. The sediment-water behaviour of ⁶⁰Co in freshwater systems was studied through adsorption and desorption experiments undertaken using sediments and water from Fratel Reservoir in the Tejo River. The suspended sediment concentrations (C_s: 500–2000 mg 1^–1) and Co distribution coefficient (K_d) were inversely related: K_d = 2211–2001 ln C_s]; K_d ranged from 4000 to 8000 ml g^–1. With a suspended sediment concentration of 1000 mg 1^–1, the ⁶⁰Co concentration remaining in solution (C_t) was given by: C_t = 49.4 e^–0.584t + 46.3 e^–0.014t; where t is the time in days and the half-life periods are 1.2 and 50 days. In a closed system, desorption of ⁶⁰Co could be described by a one-component relation with a half-life of 104 days, and a two component relation (half-life 5 hours and 45 days) in an open system. In river water the ⁶⁰Co was found to be almost 100% in cationic forms, however, in the presence of sediment there was a decrease in the proportion of cationic forms (to 50%), with some anionic forms appearing.