J Lloyd

K Tagami

M O’Leary

N Ishii

P Ciais

S Kaneko

Nobuyoshi Ishii

Shinichi Ogiyama

Shinji Sakurai

Keiko Tagami

Shigeo Uchida

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Environmental Transfer of Carbon-14 in Japanese Paddy Fields

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Chapter 26
Environmental Transfer of Carbon-14
in Japanese Paddy Fields
Nobuyoshi Ishii, Shinichi Ogiyama, Shinji Sakurai, Keiko Tagami,
and Shigeo Uchida
Abstract It has been recognized that carbon-14 (14C) is one of the dominant
radionuclides affecting dose from transuranic (TRU) wastes. This radionuclide
has a decay half-life of 5,730 years, and 14C organic materials have very low
sorption properties to clay and rock in the environment, which raises some concerns
about the releases of 14C to the biosphere from radioactive waste repositories. For
the safety assessment of TRU waste disposal, we studied the behavior of 14C in rice
paddy field soils. We also determined key parameters such as soil–soil solution
distribution coefficients (Kds) and soil-to-rice plant transfer factors (TFs) of
14C in
the field soils. The TFs were obtained in laboratory and field experiments. In our
laboratory experiments, we used [1,2-14C] sodium acetate as a source of 14C
because it has been suggested that low molecular weight organic-14C compounds
are released from metallic TRU wastes. The results showed that 14C-bearing
sodium acetate in irrigated paddy soils was rapidly decomposed by indigenous
bacteria. Although some of the 14C was assimilated into the bacterial cells, most of
the 14C was released into the air as gaseous compounds. The main chemical species
of 14C gases was 14CO2, and a part of the released
14CO2 gas was used by rice plants
during photosynthesis. Only a negligible amount of 14C was absorbed through the
roots. Therefore, the contamination of rice plants is mainly caused by gasification of
14C, and microorganisms are responsible for driving this process. The activity of
microorganisms is a key issue in the behavior of 14C in paddy fields.
Keywords Bacteria • Behavior • Degradation • Radiocarbon • Rice paddy fields
• Safety assessment • TRU wastes
26.1 Introduction
Transuranic (TRU) wastes contain a variety of radionuclides, for example, Np, Pu,
and long-lived radionuclides such as 14C and 129I. In Japan these wastes are
categorized into four groups in accordance with their physical properties and the
N. Ishii (*) • S. Ogiyama • S. Sakurai • K. Tagami • S. Uchida
Research Center for Radiation Protection, National Institute of Radiological Sciences,
4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
e-mail: nobu@nirs.go.jp
© The Author(s) 2015
K. Nakajima (ed.), Nuclear Back-end and Transmutation Technology for Waste
Disposal, DOI 10.1007/978-4-431-55111-9_26
303
concentration of radioactive materials. Group two waste includes hull and end piece
wastes with relatively high amounts of 14C, and leaching of low molecular weight
14C organic materials from simulated hull wastes has been reported [1]. The 14C
organic materials have very few sorption properties to clay and rock, and 14C has a
relatively long half-life of 5,730 years. These properties raise concerns about
releases of 14C to the biosphere from radioactive waste repositories.
Rice is a major agricultural crop throughout Asia, and thus human exposure to
14C through rice intake must be considered. To reduce the risk of the internal
radiation dose from 14C, it is important to clarify the behavior of 14C in rice
paddy fields. In this study, we determined transfer pathways of 14C through the
rice paddy fields to rice grains. Environmental parameters such as soil–soil solution
distribution coefficients (Kds) and soil-to-rice plant transfer factors (TFs) of
14C
were also determined, because these parameters are often used in transfer models to
predict the behavior of radionuclides in the environment. From a series of our
experimental results, we describe the behavior of 14C in rice paddy field soils and
the importance of microbial activity.
26.2 Partitioning of 14C into Solid, Liquid, and Gas Phases
We carried out batch sorption experiments using 63 Japanese rice paddy soil
samples to clarify the transfer pathways of 14C in rice paddy fields. The soil samples
were collected throughout Japan and taken to our laboratory where they were air
dried and sieved (<2 mm). These sieved soils were mixed with a [1,2-14C] sodium
acetate solution at the ratio of soil : solution¼ 0.5 g : 5 ml, and the flooded soil
samples were incubated at 25 C for 7 days [2]. During the incubation period, the
14C atoms of the sodium acetate were partitioned into solid, liquid, and gas phases.
Each partitioning ratio is shown in Fig. 26.1. Approximately 63 % of the total 14C
on average was released into the air as gaseous compounds. Partitioning ratios into
solid and liquid phases were 34 % and 3 %, respectively. These results suggest that
gasification is an important pathway in the environmental transfer of 14C in
Japanese rice paddy fields.
When 14C is released into the air, 14C-bearing gases must pass through the soil
solution. Because soil solution pH affects chemical reactions such as hydrolysis and
degassing of CO2, chemical forms of
14C-bearing gases may change in the soil
solution. We, therefore, investigated relationships between pH and partitioning
ratios of 14C into the liquid phase at day 7 of incubation (Fig. 26.2). The partitioning
ratio increased with increasing in pH, and a significant correlation (r¼ 0.7) was
found. These data fit well with the solubility curve of total carbonic acid in water,
which refers to the sum of dissolved carbon dioxide and the carbonic acid. This
observation suggested that the dominant chemical species of 14C in gas forms was
carbon dioxide. To confirm the effect of pH on the partitioning of 14C into the liquid
phase, a soil sample was suspended in MES [2-(N-morpholino)ethanesulfonic acid]
buffers with the initial pH value adjusted to 5.5, 6.5, and 7.5 (Fig. 26.3). A control
304 N. Ishii et al.
sample was prepared consisting of the soil and deionized water (pH unadjusted).
The partitioning ratio also increased with increasing pH, suggesting that the
partitioning ratio of 14C into the liquid phase depended on the pH of the soil
solution.
Soil–soil solution distribution coefficient (Kd) is a commonly used parameter to
evaluate behaviors of radionuclides in the environment. In our study, the Kd values
were calculated from activities of the 14C in the solid and liquid phases at the end of
incubation, and the obtained Kd value was 139 77 ml g1 on average. Negatively
charged anions generally have low Kd values because of simple electrostatic
interaction. Our value, however, was higher than expected from the chemical
form of 14CH3
14COO. For example, Kaneko et al. [1] obtained the Kd value of
9.5 ml g1 for the sorption test of acetic acid using cement materials. The reason for
our high Kd value is explained next.
26.3 Involvement of Microorganisms in the 14C Behavior
Many microorganisms inhabit rice paddy fields, and they are responsible for
nutrient cycling. We studied the involvement of microorganisms in environmental
transfer of 14C. Microorganisms in batch cultures were treated with autoclaving
(121 C, 15 min), mixing with glutaraldehyde [final concentration of 2.5 %
(vol/vol)], and mixing with cycloheximide (final concentration, 250 μg ml1).
Autoclaving and expose to glutaraldehyde inactivate bacteria and fungi, but expo-
sure to cycloheximide only inhibits fungi. The partitioning ratios of 14C into solid,
liquid, and gas phases for each treatment sample are listed in Table 26.1. When
microorganisms were treated by autoclaving and exposing to glutaraldehyde,
almost all the 14C added remained in the liquid phase; that is, negligible transfor-
mation of 14C occurred. On the other hand, the 14C atoms in the control and the
cycloheximide-treated sample were partitioned into solid, liquid, and gas phases at
certain ratios, and these ratios were similar between the control and the cyclohex-
imide samples. We confirmed fungi made no contribution to partitioning of 14C
Fig. 26.1 Box plots for
each partitioning ratio of
14C into solid, liquid, and
gas phases
26 Environmental Transfer of Carbon-14 in Japanese Paddy Fields 305
based on these results. We concluded that environmental transfer of 14C in rice
paddy fields was driven by bacteria, not by fungi.
To confirm incorporation of 14C into bacteria cells, bacteria that were isolated
from a flooding water of a paddy soil sample were cultivated on agar plates
containing [1,2-14C] sodium acetate [3]. After cultivation, bacterial colonies were
formed, and their autoradiography images showed that all colonies had the ability to
take up 14C (Fig. 26.4). In our experimental procedure, bacterial cells were conse-
quently partitioned into the solid phase, and thus the solid phase contains the 14C
incorporated by bacteria, which could be one of the reasons for the relatively high
Kd values.
26.4 Transfer of 14C from Soil to Rice Plants
Soil-to-rice plant transfer factors (TFs) of 14C, which was defined as 14C concen-
tration in rice grains (Bq/kg-dry) divided by that in soil (Bq/kg-dry), were deter-
mined by laboratory and field experiments. In the laboratory experiment using a
Fig. 26.2 Relationships
between pH and
partitioning ratios of 14C
into the liquid phase (scatter
plots). Solid line shows the
solubility curve of total
carbonic acid in water
Fig. 26.3 Effect of pH on
the partitioning of 14C into
the liquid phase
306 N. Ishii et al.
growth chamber, we grew rice plants with addition of [1,2-14C] sodium acetate.
This 14C compound was supplied once to rice plants in the flooding water just
before blooming, and TF of 6.8 2.4 on average was obtained. In these tracer
experiments, rice plants were also cultivated without [1,2-14C] sodium acetate as
negative controls in the same growth chamber as the 14C-treated rice. Interestingly
14C was detected even from the rice grains of negative control samples. These
results suggested that the 14C-bearing gas, which was released from bacterial cells
in rice paddy soils, was fixed by the rice plants in the negative controls through
photosynthesis.
We also examined the possibility of root uptake of 14C by stable isotope
techniques under field conditions [4]. If plant carbon originates from the atmo-
spheric CO2, the δ13C values in crops can be calculated using the δ13C value,8‰
in air [5], and the 13C fractionation ratio in photosynthesis by rice plants of 18 to
20‰ [6, 7]. The calculated δ13C values in our study ranged from 28‰ to
26‰, and the results implied that no soil carbon contribution occurred for white
rice; however, by setting some conditions, for example, 13C fractionation ratio of
19‰, we obtained the average TF value of 0.11 0.04 for white rice. To compare
these TF values obtained in laboratory and field experiments, it is necessary to pay
attention to the difference between [1,2-14C] sodium acetate and the actual organic
compounds present in the natural soil.
Table 26.1 The partitioning ratios of 14C into solid, liquid, and gas phases for each treatment.
Treatment
Partitioning ratio (%)
Solid phase Liquid phase Gas phase
Control 27.9 4.5 67.5
Autoclaving 0 98.0 2.0
Glutaraldehyde exposure 0 96.8 3.2
Cycloheximide exposure 29.3 4.8 65.9
a b
Fig. 26.4 Colonies of bacteria (a) and their autoradiography image (b). Heterotrophic bacteria
have the ability to uptake 14C from an agar medium
26 Environmental Transfer of Carbon-14 in Japanese Paddy Fields 307
26.5 Behavior of 14C in Rice Paddy Fields
From the aforementioned results, the behavior of 14C in rice paddy fields could be
considered as follows (a conceptual diagram appears in Fig. 26.5). When irrigation
water is contaminated by 14C-bearing sodium acetate, the 14C compound is taken up
and metabolized by indigenous bacteria. A part of the 14C is assimilated by the
bacterial cells, and the rest of the 14C is released as gaseous compounds from the
cells as a result of dissimilation. The dominant chemical species of 14C in gas forms
is carbon dioxide, and thus some of the released 14CO2 is dissolved in soil solution
depending on pH. For example, when the pH of the soil solution is less than 6.5,
most of 14C in gas forms is released into the air. The released 14CO2 is eventually
taken up by rice plants during photosynthesis. When the pH of the soil solution is
between 6.5 and 10.5, 14C-bearing bicarbonate ion dominates in the soil solution. In
addition, once 14CO2 has been released into the air, a part of the
14CO2 gas may be
redissolved in the soil solution again as bicarbonate ion. When the pH of the soil
solution is greater than 10.5, although this is not probable in paddy fields, 14C-
bearing carbonate ion dominates in the soil solution. Carbonate ion is thermally
unstable and thus precipitates as carbonate minerals such as CaCO3. In these
alkaline situations, the ratio of 14C in the solid phase may increase as a result of
the precipitation of 14C. Because the root uptake of 14C by rice plants is negligible,
gasification of 14C is an important environmental transfer pathway for the safety
assessment of TRU wastes, and bacteria are responsible for driving this pathway.
Acknowledgments This work has been partially supported by the Agency for Natural Resources
and Energy, the Ministry of Economy, Trade and Industry (METI), Japan.
Open Access This chapter is distributed under the terms of the Creative Commons Attribution
Noncommercial License, which permits any noncommercial use, distribution, and reproduction in
any medium, provided the original author(s) and source are credited.
Fig. 26.5 A conceptual diagram for the behavior of 14C in rice paddy fields
308 N. Ishii et al.
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26 Environmental Transfer of Carbon-14 in Japanese Paddy Fields 309


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