Abstract
Cesium adsorption/desorption experiments for various clay minerals, considering actual contamination conditions in Fukushima, were conducted using the 137Cs radioisotope and an autoradiography using imaging plates (IPs). A 50 μl solution containing 0.185 ~ 1.85 Bq of 137Cs (10−11 ~ 10−9 molL−1 of 137Cs) was dropped onto a substrate where various mineral particles were arranged. It was found that partially-vermiculitized biotite, which is termed “weathered biotite” (WB) in this study, from Fukushima sorbed 137Cs far more than the other clay minerals (fresh biotite, illite, smectite, kaolinite, halloysite, allophane, imogolite) on the same substrate. When WB was absent on the substrate, the amount of 137Cs sorbed to the other clay minerals was considerably increased, implying that selective sorption to WB caused depletion of radiocesium in the solution and less sorption to the coexisting minerals. Cs-sorption to WB continued for about one day, whereas that to ferruginous smectite was completed within one hour. The sorbed 137Cs in WB was hardly leached with hydrochloric acid at pH 1, particularly in samples with a longer sorption time. The presence/absence of WB sorbing radiocesium is a key factor affecting the dynamics and fate of radiocesium in Fukushima.
Highlights
Experiments[21] were performed using a solution with a cesium concentration as low as sub-ppm (~10−5 molL−1)
In order to discuss the cesium contamination event in the Fukushima soil, the sorption experiments should be performed using a similar low concentration of Cs. This is close to or below the detection limit of the most sensitive analytical instruments. This problem can be solved if radioactive cesium itself is used as the cesium source and the sorption/desorption process is estimated by measuring the radiation from the radioisotope, as was done in previous works[6,13,22]
By using imaging plates (IPs) autoradiography, we can investigate kinetic aspects of the reaction between a solution and multi-minerals. This approach is considered to reproduce better the reaction between the raindrops and soil that consists of various mineral species occurring in Fukushima, than the reaction between a solution and mono-mineral. The results of such unique adsorption/desorption experiments for clay minerals possibly present in Fukushima soils are shown
Summary
Our recent investigation[25] indicated that the radioactive soil particles in the field generally have radioactivity of about the order of 10−3 ~ 10−2 Bq, which roughly corresponded to the results in Fig. 1 with a concentration ranging from 0.185 Bq/50 μ L to 1.85 Bq/50 μ L It is not certain where the actual sorption sites of cesium are in the structure of WB at such a low concentration level. Our results clearly support the view that such a phenomenon occurred in Fukushima
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