COLLOIDAL BEHAVIOR OF ACTINIDES IN AN OLIGOTROPHIC LAKE

Understanding the speciation of low levels of actinides from fallout and from local sources in freshwater systems is important if we are to predict their distributions in the environment. Since these materials make excellent tracers for determining sedimentation rates and other environmental parameters, it is important to determine their physical and chemical properties in relatively pristine systems. Reported here are the results of actinide analyses in an artificial, oligotrophic lake in northwest Wales, United Kingdom, which is used as a source of cooling water for a nuclear power plant. The concentrations of the actinide elements plutonium, americium, thorium, and curium, and their distributions among different colloidal sizes were determined. Actinide concentrations in the dissolved fraction (≤0.45 μm) were as follows: 239, 240Pu, 6.4-12.5 fCi/L;241 Am, 2.5-18.2 fCi/L; 232Th, 0.11-1.09 fCi/L; and 244Cm, 0.3-1.4 fCi/L. The majority of the actinides in the lake were retained by hollow-fiber ultrafilters of 5-nm (nominal 100000 MW) or 100-nm pore sizes; the actinides appeared to be bound reversibly to colloidal material of unknown composition. The two environmentally stable oxidation states of plutonium, IV and V, could be separated by ultrafiltration. These results indicate that submicron colloidal material can dominate the aqueous properties of actinides. © 1990, American Chemical Society. All rights reserved.