Role of soil organic matter in the mobility of radiocesium in agricultural soils common in Japan

Cesium-137 is an important radionuclide because of its long radiological half-life (30 years) and high fission yield from U-235 and (PU)-P-211. In order to evaluate the radiation dose from (CS)-C-137, it is important to know its mobility in different soils. Soil organic matter (SOM) might affect the (CS)-C-137 mobility because SOM is a reactive colloid. Therefore, tracer studies were carried out to clarify the effect of SOM on Cs mobility in different soils. Soil-soil solution distribution coefficients (Kds) of (CS)-C-137 were measured for 87 Japanese agricultural soil samples to evaluate the adsorbability of Cs-137. The soil samples were selected from three common and one miscellaneous soil groups in Japan. The percentage of SOM-bound Cs-137 and the exchangeability of SOM-bound Cs were evaluated by selective extraction. To evaluate the effect of dissolved organic matter (DOM), which is a portion of the SOM fraction, on Cs mobility, the amount of Cs-137 bound to DOM was also determined. The observed K-d values ranged from 215 to 43 400L/kg with a geometric mean of 2210L/kg. Among the four soil groups, relatively low Kd values were observed for Andosol samples. Their high SOM content would possibly affect (CS)-C-137 mobility. Then the effect of SOM was verified for the four soil groups. The selective extraction showed that 3-17% of soil-adsorbed (CS)-C-137 was in an SOM-bound form. Moreover, it was observed that the 27-100% (average: 63%) of Cs-137 adsorbed by the soil humic acid (HA) fraction was exchangeable. The effect of DOM in soil solution was also verified. About 5-82% (average: 45%) of (CS)-C-137 in soil solution was in HA-bound form. These results suggested that SOM could increase the mobility of (CS)-C-137 in soils by binding it in exchangeable and DOM-bound forms. (C) 2007 Elsevier B.V. All rights reserved.