Effect of redox potential on fixation of 137Cs in lake sediment

Fixation of 137Cs was determined in lake sediment suspensions under controlled redox potentials in the laboratory. The activity of previously added 137Cs on various clay sites was determined by time-series selective extractions. Monovalent cations, particularly NH+4, were much more effective at displacing 137Cs than divalent cations or Na+. Ammonium ion (NH+4) and Na+ were used to extract 137Cs from selective and non-selective 137Cs-binding sites, respectively. The activity of water-soluble 137Cs and Na-extractable 137Cs was significantly higher under anaerobic redox conditions (-200 mV), when soluble NH+4 concentrations in the anaerobic suspensions were 1000 microM or higher. Activities of 137Cs were highest (initially 40-50% of the 137Cs added) on the NH+4-extractable site. Over the long term, activities of NH+4-extractable 137Cs decreased linearly to below 10% as 137Cs was fixed on inter-lattice sites. Water-soluble 137Cs was significantly correlated with Na-extractable 137Cs in short-term experiments, suggesting the existence of an equilibrium between the different clay sites. High concentrations of NH+4 under anaerobic redox conditions could shift this equilibrium, resulting in increases in water-soluble 137Cs and increases in the activities of 137Cs bound on non-selective clay sites. Additional studies are necessary to evaluate the interaction of 137Cs with sulfides, iron oxides, and other reactive chemical species which may attenuate 137Cs in sediment.