Photon Factory Activity Report 2014 #32 (2015) B
BL-4A, BL-9A, BL-12C/2013G052

Factors controlling radiocesium distribution in river sediments: Field and laboratory
studies after the Fukushima Dai-ichi Nuclear Power Plant accident
Yoshio Takahashi1,2* and Fan Qiaohui1,2
1
Department of Earth and Planetary Science, Graduate School of Science,
The University of Tokyo, Tokyo 113-0033, Japan2
Department of Earth and Planetary Systems Science, Graduate School of Science,
Hiroshima University, Hiroshima 739-8526, Japan
After the accident of the Fukushima Dai-ichi nuclear
power plants, the distribution of radiocesium (RCs) inriver system has been of important issue related to its
transportation in environment. In this study, thedistribution coefficient (Kd) of 137Cs in river sediment
sampled from the Abukuma and Kuchibuto Rivers wasdetermined to explore particle-size dependence of RCs
distribution combined with laboratorial estimations. The
results showed that the Kd patterns of 137Cs (= particle
size dependence of Kd) were not only related to theparticle size for larger particles (with small content ofclay mineral), but also dependent on the content of claymineral for smaller particles (with high content of clay
mineral), sampling site, and especially cation
concentration in the aqueous phase. In contrast, there wasno obvious particle-size dependence in the Kd patterns ofstable cesium (= 133Cs).
In order to eliminate the contradiction of natural andlaboratorial distributions of 137Cs on sediments withvarious particle sizes, the adsorption species of Cs wasexplored by EXAFS. Based on the analysis recorded at
BL-9A and BL-12C in Photon Factory, k3χ(k) function
and radial structure functions (RSFs) for Cs adsorbed onriver sediments with various particle size were obtained.
Here, CsNO3 solution and vermiculite were also used as
two end members, i.e., outer-sphere (OS) and innersphere (IS) complexes, because Fan et al. (2014) haveconfirmed almost pure IS complex for Cs adsorption on
vermiculite. The k3χ(k) function of Cs adsorbed on
sediments exhibited some obvious differences inoscillatory frequencies and amplitude from CsNO3solution, which testified that a new local atomic structureof Cs after adsorbed on sediment. A small beat wasobserved at 4.2 Å-1 in k space for Cs adsorbed sedimentswith different particle size, and very similar tovermiculite which has characteristic oscillation of IS
complex. It is interesting that no obvious difference wasobserved between the particle sizes of 63-125 μm and
500-850 μm, which suggested that similar adsorption
species were formed for Cs in sediments with different
particle size.
Correspondingly, radial structural function showed that
two shells were observed for Cs adsorbed on river
sediments and vermiculite in the R ranges of 2.2-2.8 Åand 3.6-3.8 Å, respectively. In reality, the characteristic

shells can be attributed to OS and IS complexes,respectively (Qin et al., 2012; Fan et al., 2014). Similarintensities of OS and IS complex shells suggested thesimilar composition of OS and IS complexes as Cs
species adsorbed on the sediments. In fact, thecoordination ratio of CNIS/CNOS is more sensitive andreliable to the variation in the adsorption species of Cs.
The largest CNIS/CNOS for Cs adsorption on vermiculite(~4.6) suggested that IS complex was dominant species. Asmaller ratio of CNIS/CNOS was observed for Cs
adsorption on sediment at 63-125 μm and 500-850 μm,respectively. The ratios of CN were around 1.0 and 2.0
for sites A and C, respectively. This fact suggested thatboth IS and OS complexes contributed to Cs species
adsorbed on river sediments. More importantly, there isno obvious particle-size dependence for Cs species in
sediments with different particle sizes, which is consistent
with the distribution profile of stable cesium and of Kd
without having clear particle-size dependence.
Our findings indicated that river sediment has highfixation ability to 137Cs. However, the adsorbed 137Cs canbe extracted from sediments to water phase when salinity
becomes high in water such as in seawater. In particular,
the distribution patterns of 137Cs at various particle sizes
can suffer a slow equilibrium process of adsorption of137
Cs during the transportation of river sediments, andshould eventually behave similarly to stable cesium in theriver system, if the contribution of cesium within 3D
crystal structure of some minerals is minimal. Therefore,
the particle size of sediments is a primarily importantfactor to RCs distribution at the early stage of its
deposition, but finally Cs adsorbed on sediment will beequilibrated with the stable cesium during its
transportation in the river system. The controlling factor
such as the mineralogy in sediments will possibly makethe distribution patterns of RCs similar to that of stable
cesium at longer-term.
References[1] Q. H. Fan et al. Geochim. Cosmochim. Acta 135, 49(2014).
[2] H. Qin et al. Geochem. J. 46, 297 (2012).
* ytakaha@eps.s.u-tokyo.ac.jp