Chemical species of radiocesium in the contaminated soil, and contribution ofnon-mica minerals to tight association

N. Kozai1), T. Ohnuki1), F. Sakamoto1)
1) R.G. for Bio-actinides chemistry, ASRC, JAEA

The accident at the Fukushima Daiichi Nuclear Power
Plant (FDNPP) occurred as a consequence of the massive
earthquake and associated tsunami that struck the Tohoku
and north Kanto regions of Japan on 11 March 2011. A
series of hydrogen explosion was occurred from 13March to 15 March at the units 1, 2, and 3. The release
rate of 137Cs on 15 March is estimated between 1012 and1015 Bq/h [1]. This fallout radiocesium were dispersed
from FDNPP to the ocean [1,2] and land [1]. Some of thereleased radiocesium were deposited on the ground of thearea located north-west direction from FDNPP. The
spatial concentration distribution and depth profiles ofradiocesium were measured to estimate a dose rate andthe fate in the terrestrial environment.
For the estimation of migration of the fallout
radiocesium, sorption behavior of the depositedradiocesium should be clarified. Cesium at low
concentration is tightly sorbed on mica like minerals. Thisknowledge suggests that some amounts of fallout
radiocesium are tightly sorbed with the contaminatedsoils when the soils contain mica like minerals. However,effects of the other minerals than mica like minerals on
the sorption of radiocesium are scarcely known.
We have conducted the analyses of chemical species ofradiocesium in the contaminated soils sampled in
Fukushima by the desorption experiments using
appropriate reagents solutions and size fractionation of
the contaminated soils [3]. Three different soil samples
were collected in Iitate, Fukushima on May, 2011. Thesoil samples were treated by a 1 mole L-1 NH4Cl solution
and a 1 mole L-1 CH3COOH solution to dissolve mobile
fraction of radiocesium. After the treatment the soil
samples were analyzed by XRD to determine minerals
and clay minerals in soil.
More than 65% of radiocesium were remained in theresidual fraction of the soil samples after treatment of a 1mole L-1 NH4Cl solution and a 1 mole L-1 CH3COOH
solution, indicating that 35% of radiocesium was mobile
and the residual fraction was tightly associated with thesoil. The XRD analysis of the soil showed that two soil
samples contained mica like minerals, and one soil
sample did not. These results strongly suggest that inthose Iitate soils the radiocesium was tightly associatednot only with mica like minerals but also with non-mica
like minerals.
In order to clarify the minerals that tightly associate
radiocesium, we have conducted the sorption anddesorption experiments of radiocesium on 17 minerals,
and have found that some fractions of radiocesium weretightly associated with natural kaolinite and manganeseoxides other than illite [4]. There are several associationsites in minerals for radiocesium. In natural environment,
minerals are altered by weathering to generate the tightassociation sites in the altered minerals. Unfortunately thenumber of the tight association sites is very limited in theminerals of kaolinite and manganese oxides. Since thechemical concentration of radiocesium was extremely
low, most of radiocesium are easily bound in the tightassociation sites (Fig. 2).
This finding of tight association of radiocesium on the
non-mica minerals inform us the migration behaviour ofradiocesium in environments.
Birnessite

Kaolinite

Illite11111

Residual0.8

0.8
0.8
0.6

Desorbed
0.6
by 1 M HCl

0.4
0.4

0.4

0.6

0.8
0.8

0.6
0.6
0.4
0.4

Desorbed by
1 M KCl0.2

0.2
0.400

0.2
0.400

0

10-9 M(Radiotracer)Fig.1

10-5 M

10-9 M(Radiotracer)

10-5 M

10-9 M
10-5 M(Radiotracer)

Desorbed fraction of radiocesium from minerals atdifferent cesium concentrations.When Cs concentrationis very low of 10-9 M, higher amounts of Cs wasresided after the treatment with desorption reagents
solution.

CsMineral

10-9 M(Radiotracer)Cs does not
Tightpenetrate
association
into an intact
site generated
siteby weathering
10-5 M

Fig.2 Schematic diagram of adsorption of radiocesium onweathered minerals. Tight association sites forradiocesium were generated by weathering. Even
though the amounts of the tight association sites werelimited, higher amounts of raiocesium was adsorbed on
the tight association sites because of low concentrationof radiotracer.

References[1] M. Chino, et al., J. Nucl. Sci. Technol. 48, 1129(2011).
[2] H. Kawamura, et al., J. Nucl. Sci. Technol. 48,1349 (2011).
[3] N. Kozai et al., J. Nucl. Sci. Technol. 49, 473(2012).
[4] T. Ohnuki, N. Kozai, J. Nucl. Sci. Technol. 50, 369(2013).