Factors controlling radiocesium distribution in river sediments: Field and laboratory studies after the Fukushima Dai-ichi Nuclear Power Plant accident

•Kd of 137Cs were not only related to particle size but also to clay mineral content.•137Cs can be extracted from sediments in water when salinity becomes high.•RCs in sediments will be equilibrated with stable Cs during its transportation.

This study used laboratorial estimations and determined the distribution coefficient (Kd) of 137Cs in river sediments sampled from the Abukuma River and the Kuchibuto River to investigate the particle size dependence of RCs distribution. Results show that the Kd patterns of 137Cs (particle size-dependence of Kd) were not only related to the particle size of large particles (low clay mineral content) but also to the clay mineral content of small particles (high clay mineral content) and particularly cation concentration in aqueous phase. By contrast, the Kd patterns of stable Cs (133Cs) exhibited no obvious particle size dependence. Adsorption species of Cs that was added to the river sediments at various particle sizes was almost the same at the molecular scale as determined by the extended X-ray absorption fine structure.Our findings indicate that river sediments have high fixation ability to 137Cs. Nevertheless, adsorbed 137Cs can be extracted from sediments in the water phase when salinity becomes high, such as that in seawater. The distribution patterns of 137Cs at various particle sizes can slowly down the equilibrium of the adsorption of 137Cs during the transportation of river sediments, and then the behavior of 137Cs should eventually be similar to that of stable Cs in the river system. Therefore, the particle size of sediments is an important factor in the distribution of RCs at the early stage of its deposition. In the end, RCs adsorbed on the sediments will be equilibrated with the stable Cs during its transportation in the river system. Controlling factors such as the mineralogy of sediments could possibly make the distribution patterns of RCs similar to that of stable Cs in the long run.