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 : solution ¼ 0.5 g : 5 ml, and the flooded soil samples were incubated at 25 oC 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 (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 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

Fig. 26.1 Box plots for each partitioning ratio of 14C into solid, liquid, and gas phases

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 g-1 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 14CH314COO-. For example, Kaneko et al. [1] obtained the Kd value of 9.5 ml g-1 for the sorption test of acetic acid using cement materials. The reason for our high Kd value is explained next.

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 oC, 15 min), mixing with glutaraldehyde [final concentration of 2.5 % (vol/vol)], and mixing with cycloheximide (final concentration, 250 μg ml-1). Autoclaving and expose to glutaraldehyde inactivate bacteria and fungi, but exposure 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 transformation 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 cycloheximide samples. We confirmed fungi made no contribution to partitioning of 14C

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

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.

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