Analysis of ¹³⁴Cs and ¹³⁷Cs distribution in soil of Fukushima prefecture and their specific adsorption on clay minerals

We collected surface soil samples within 60 km from the Fukushima Daiichi Nuclear Power Plants (FDNPP) and analyzed spatial and temporal radiocesium distributions. No large change in vertical distribution pattern of radiocesium has been observed on the samples collected in April 2011 and April 2012 at the same sampling points, suggesting strong adsorption of ¹³⁴Cs and ¹³⁷Cs on soil. Cesium is known to be adsorbed specifically on clay minerals in soil. To confirm the specific adsorption of Cs on clay minerals in these samples, we divided the soil into different particle sizes and measured the activity in each size fraction. The activity was highest in the clay fraction (<2 μm) and tended to decrease as the particle size increases. The radiocesium interception potential (RIP), which is defined by the product of the frayed edge sites (FES) capacity and Cs-K selectivity coefficient, was measured using carrier-free ¹³⁷Cs. No correlation between the value of RIP and the penetration depth of radiocesium was observed, probably suggesting other important physicochemical factors.