The distribution coefficient for cesium in different clay fractions in soils developed from granite and Paleozoic shales in Japan

The distribution coefficient of cesium (Cs) (Kd Cˢ) was investigated for clays which were collected from upland forest soils developed from granites and Paleozoic shales in Japan. They were divided into three size fractions, coarse (2.0–1.0 µm), medium (1.0–0.2 µm), and fine-clay fractions (<0.2 µm), abbreviated CC, MC, and FC, respectively. The Kd Cˢ value increased in the fractions as follows: FC < MC < CC in all the horizon samples. The amount of potassium (K) in the clay fractions also followed this same pattern. As the amount of K in the clay fraction can be a quantitative indicator of illitic minerals, the positive relationship between the Kd Cˢ and the amount of K indicated that the coarser clay fraction had a higher ¹³⁷Cs adsorption potential due to containing higher amounts of illitic minerals. The soil clay fraction from the Paleozoic shales both contained a large amount of K and showed greater Kd Cˢ values than that of the granite when comparing the same particle sizes. Hence, the type of parent material can be a significant factor in determining the ¹³⁷Cs adsorption potential of the soils. In all particle fractions, those collected from deeper in the profile had smaller Kd Cˢ values than those from closer to the soil surface in the same profile, although amount of K present was similar. The X-ray diffraction (XRD) analysis showed that the hydroxy-aluminum (Al) polymer was present in the 2:1 interlayer sites to a greater extent in the clay fractions from deeper in the soil profile than in those from upper in the same soil profile. This result suggests that the hydroxy-Al polymers may be fixed at the expanded weathering front of the illitic minerals, blocking Cs adsorption on the frayed edge sites of these minerals. Thus, the Kd Cˢ value of the soil clays is determined by the degree of illitic minerals and hydroxy-Al interlayering present, with the larger the amount of illitic minerals the greater the Cs adsorption potential (with any hydroxy-Al interlayering reducing this effect).