Comparing in situ distribution coefficients and exchangeability of radiocaesium in freshwater sediments with laboratory predictions

Two short-term (24 h) laboratory procedures, based on measurement of the affinity and capacity of the specific binding sites for radiocaesium on illitic clay minerals, are used to predict the in situ 137Cs distribution coefficients (KD) in two Dutch freshwater sediments. Differences in the measured amount of specific binding sites between the sediments are shown to be related to differences in the particle size and illite content. A procedure based on measurement of the combined parameter KD[NH4+] predicts the measured in situ 137Cs distribution in both sediments very well. This is remarkable because the in situ data are based on 137Cs/sediment contact times of > 1 year, while previous experiments have shown that increasing contact times between Cs+ and sediments decreases the short-term exchangeability of caesium, as measured by extraction with a high ammonium concentration. The good predictions of the 37Cs solid/water distribution by short-term ion-exchange experiments, therefore, indicate that more 137Cs in the sediments is exchangeable than is measured by the commonly used extraction with high ammonium concentrations.