In this paper, we examined a new method for decontamination of soils by magnetic separation to reduce the volume of contaminated soils. In this method, we separate paramagnetic 2:1 type clay minerals with strong adsorbility of Cs ions from diamagnetic 1:1 type clay minerals with weak adsorbility of Cs ions. In addition, Cs ions desorbed from 1:1 types by washing are migrated to 2:1 type clay minerals. As a result, the doses of 1:1 type clay minerals can be reduced. First, it became apparent by the particle trajectory simulation that vermiculite, which is 2:1 type clay mineral, could be selectively separated by high gradient magnetic separation using superconducting magnet. Second, we confirmed the separation of vermiculite coexisting with kaolinite, which is 1:1 type clay mineral, by the experiments based on the simulation. Moreover, the experiments using the actual contaminated soils were conducted to examine the migration of Cs ions from 1:1 to 2:1 type clay minerals. As a result, it was observed that Cs ions desorbed from 1:1 types could be adsorbed to 2:1 type clay minerals when using 0.1 mol/L KI as wash solution. It showed the possibility of decreasing the dose of 1:1 type clay minerals.
This study intends to develop a new volume reduction method of the soil that is contaminated by radioactive cesium. The volume reduction process by the magnetic separation method is described as follows. First, the soil constituent particles with different particle sizes such as gravel, sand and clay are separated by classification. In this method, the clay, which mainly adsorbs cesium, is separated from the soil. Then, the magneto-Archimedes method is applied to the clay, that utilizes the difference of magnetic susceptibility of substances. The clay minerals consist of two principle types, 1 : 1 and 2 : 1 type. The 1 : 1 type clay mineral is diamagnetic and weakly adsorbs small amount of cesium, whereas the 2 : 1 type clay mineral is paramagnetic and strongly adsorbs a large amount of cesium. The recovery of paramagnetic 2 : 1 type clay mineral by superconducting magnet enables selective separation of high-dose soil. It will reduce the volume of contaminated soil. In this study, the separation of 1 : 1 and 2 : 1 type clay mineral by the magneto-Archimedes method was investigated. It was confirmed that it was possible to separate between 1 : 1 and 2 : 1 type clay mineral effectively. Based on the result, the magneto-Archimedes method was applied to the actual contaminated soil in Fukushima. It was shown that there was a possibility that the volume reduction of contaminated soil could be reduced.
