TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 52, 2018

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

Kazuya Tanaka1,2*, Naoko Watanabe2, Shinya Yamasaki3, Aya Sakaguchi3,4, Qiaohui Fan4,5,6 and Yoshio Takahashi4,5

1Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
2Institute for Sustainable Sciences and Development, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8530 Japan
3Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan
4Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526 Japan
5Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan
6Key Laboratory of Petroleum Resource Research, Northwest Institute of Eco-Environment & Resources, CAS 382 West Donggang Road, Lanzhou, Gansu, 730000, China

(Received March 10, 2017; Accepted August 13, 2017)

Abstract: 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.
Key words: radiocesium, stable Cs, particle size, mineralogy, sediment

*Corresponding author E-mail: tanaka.kazuya@jaea.go.jp

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