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 = 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 (r = 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.