concentration of radioactive materials. Group two waste includes hull and end piece
wastes with relatively high amounts of
14 C, and leaching of low molecular weight
14 C organic materials from simulated hull wastes has been reported [1]. The
14 C
organic materials have very few sorption properties to clay and rock, and
14 C has a
relatively long half-life of 5,730 years. These properties raise concerns about
releases of
14
C to the biosphere from radioactive waste repositories.
Rice is a major agricultural crop throughout Asia, and thus human exposure to
14 C through rice intake must be considered. To reduce the risk of the internal
radiation dose from
14 C, it is important to clarify the behavior of
14 C in rice
paddy fields. In this study, we determined transfer pathways of
14 C through the
rice paddy fields to rice grains. Environmental parameters such as soil–soil solution
distribution coefficients (K d s) and soil-to-rice plant transfer factors (TFs) of
14 C
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
14 C in rice paddy field soils and
the importance of microbial activity.
26.2 Partitioning of
14
C 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
14 C 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 [1,214 C] sodium
acetate solution at the ratio of soil : solution ¼ 0.5 g : 5 ml, and the flooded soil
samples were incubated at 25
C for 7 days [2]. During the incubation period, the
14 C 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
14 C
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
14 C in
Japanese rice paddy fields.
When
14 C is released into the air,
14 C-bearing gases must pass through the soil
solution. Because soil solution pH affects chemical reactions such as hydrolysis and
degassing of CO 2 , chemical forms of
14 C-bearing gases may change in the soil
solution. We, therefore, investigated relationships between pH and partitioning
ratios of
14
C into the liquid phase at day 7 of incubation (Fig. 26.2). The partitioning
ratio increased with increasing in pH, and a significant correlation (r ¼ 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
14 C in gas forms was
carbon dioxide. To confirm the effect of pH on the partitioning of
14 C 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.
wastes with relatively high amounts of
14 C, and leaching of low molecular weight
14 C organic materials from simulated hull wastes has been reported [1]. The
14 C
organic materials have very few sorption properties to clay and rock, and
14 C has a
relatively long half-life of 5,730 years. These properties raise concerns about
releases of
14
C to the biosphere from radioactive waste repositories.
Rice is a major agricultural crop throughout Asia, and thus human exposure to
14 C through rice intake must be considered. To reduce the risk of the internal
radiation dose from
14 C, it is important to clarify the behavior of
14 C in rice
paddy fields. In this study, we determined transfer pathways of
14 C through the
rice paddy fields to rice grains. Environmental parameters such as soil–soil solution
distribution coefficients (K d s) and soil-to-rice plant transfer factors (TFs) of
14 C
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
14 C in rice paddy field soils and
the importance of microbial activity.
26.2 Partitioning of
14
C 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
14 C 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 [1,214 C] sodium
acetate solution at the ratio of soil : solution ¼ 0.5 g : 5 ml, and the flooded soil
samples were incubated at 25
C for 7 days [2]. During the incubation period, the
14 C 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
14 C
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
14 C in
Japanese rice paddy fields.
When
14 C is released into the air,
14 C-bearing gases must pass through the soil
solution. Because soil solution pH affects chemical reactions such as hydrolysis and
degassing of CO 2 , chemical forms of
14 C-bearing gases may change in the soil
solution. We, therefore, investigated relationships between pH and partitioning
ratios of
14
C into the liquid phase at day 7 of incubation (Fig. 26.2). The partitioning
ratio increased with increasing in pH, and a significant correlation (r ¼ 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
14 C in gas forms was
carbon dioxide. To confirm the effect of pH on the partitioning of
14 C 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.