Mineralogical control of the size distribution of stable Cs and radiocesium in riverbed sediments

抄録

Particle size and mineralogy are important factors controlling the distribution of radiocesium in soils and sediments contaminated by nuclear weapon testing and nuclear power plant accidents. However, it is often difficult to distinguish the influence of particle size and mineralogical composition on the size distribution of radiocesium because they are closely related. The objective of this study was to elucidate the influence of mineralogical composition on the distribution of stable Cs and radiocesium in river sediments. We analyzed size-fractioned samples of riverbed sediments collected at two sites, a pasture and Kuroiwa in the Abukuma River system in Fukushima after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The size distributions of K, Rb, and ¹³³Cs reflected the mineralogy of sediments, where primary host minerals for these alkali elements would be biotite, K-feldspar, and clay minerals. Silt-size fractions contained high ¹³³Cs and ¹³⁷Cs concentrations possibly due to adsorption on clay minerals. Their concentrations decreased with increasing particle size at the pasture site. In contrast, coarse and very coarse sand fractions from the Kuroiwa site showed higher ¹³³Cs and ¹³⁷Cs concentrations in comparison to fine-medium sand fractions. The coarse sand fractions contained many weathered biotite grains, whereas hornblende was the major constituent in the fine-medium sand fractions. Overall, the size distributions of ¹³³Cs and ¹³⁷Cs were similar in the sediments, suggesting that the FDNPP-derived radiocesium was distributed into each particle size fraction in response to the distribution of the stable Cs that was controlled by mineralogical composition. Leaching experiments using 1 M CH₃COONH₄ indicated that ¹³⁷Cs was fixed more strongly in silt-size fractions than in sand-size fractions. However, it should be noted that sand-size fractions retained more than 90% of the ¹³⁷Cs in the leaching experiments. Illite and weathered biotite are likely the hosts of ¹³⁷Cs in silt- and sand-size fractions, respectively. In this study, we demonstrated the strong retention of the FDNPP-derived radiocesium in wide range of particle size fractions of sediments.