Photon Factory Activity Report 2017 #35 (2018)

BL-27B/2016G637

EXAFS analysis of cesium in the sediment at Akagi Onuma
Takamasa Nagahashi1,* Haruaki Matsuura1, Shun Imagawa1,
Takafumi Uchiyama1, Kyuma Suzuki2, Shun Watanabe2, Hajime Arai2,
Kin-ichi Tsunoda3, Masanobu Mori4 and Yukiko Okada1
1
Cooperative Major in Nuclear Energy, Tokyo City University,
1-28-1 Tamazutsumi, Setagaya-ku, Tokyo, 158-8557, Japan
2
Gunma Prefectural Fisheries Experimental Station,
13, Shikishima-cho, Maebashi, Gunma, 371-0036, Japan
3
Gumma University, 1-5-1, Tenjin-cho, Kiryu, Gunma, 376-8515, Japan
4Kochi University, 2-5-1, Akebono-cho, Kochi, Kochi, 780-8520, Japan

2 Experiment
We have collected the lake sediments in each depth
fraction by the core sampler at central points of Akagi
Onuma. Samples were freeze-dried, cut every 1 cm or 2cm and milled with a mortar.
Samples in the environment can not be used as such for
EXAFS experiments, thus 1M CsCl solution was added tolake sediment and stirred for 1 hour, precipitated for 1hour to 24 hours to adsorb cesium, filtrated from theliquid and dried. EXAFS measurement was carried out at
High Energy Accelerator Research Organization Photon
Factory BL-27B, in transmission mode, using CsLIIIabsorption edge.
3 Results and Discussion
Radioactivity of cesium in the lake sediment tends togradually decrease until the depth of 20 cm from thesurface of the bottom, but radioactivity of that in deeper
than 20 cm has been scattered. Cesium dissolved in water
is bonded via oxygen of 8 water molecules. In addition,
cesium ion specifically adsorbed by clay minerals such as
layered silicates forms much stabilized as an inner-spherecomplex than hydrated state in solution.
Figure 1 shows that structure functions of Cs adsorbed
sediments in each fraction, compared with that of Cs
adsorbed in one of clay minerals. The peak of the firstneighbor is a bond with oxygen of hydrated water, i.e.,2.8

Å. The peak of the second neighbor is considered to beoxygen of the clay minerals, i.e., 4.1Å. The coordination
number of second neighbor of sediments with highradioactivity was higher than that of sediments with low
radioactivity. It is found that the identification of second
coordination is closely related to radioactivity.

clay mineral

FT |x(k)・ k 3 |[arb unit]

1 Introduction
In the Lake Akagi Onuma, exceeding radioactivity ofcesium over the standard value had been still detectedfrom the lake water and aquatic lives due to theFukushima Daiichi Nuclear Power Plant accident.
Currently radioactivity of water decreased under thestandard value, but a decreasing trend of the radioactivityof cesium of the lake was being slowly after 2015.
Since the continuous survey of the lake shows thatdissolved Cs is dominant of radioactivity at a bottom oflake, we think that radioactive cesium might be eluted
from bottom of lake sediment. Thus, in order to elucidate
releasing mechanism of cesium from the sediment, wehave investigated the local structure of cesium insediment by EXAFS.

sed2-4sed18-20
sed30-32

01

2

3

4

5

6

R[Å]

Fig. 1: Structure functions of Cs-adsorbed sediments andclay mineral in wet sample.
Acknowledgement
This work was financially supported by the JSPS
KAKENHI Grant Number JP26292100, JP18K04995.
References[1] Y. Okamoto et al., Transactions of the Atomic Energy
Society of Japan, vol.13, No.3, 113-118, (2014)[2] K. Suzuki et al, The Japanese Society of Fisheries
Science, 82, 774-776（2016)* g1781013@tcu.ac.jp