Midterm Report on Removal of Radioactive Iodine and Cesium from Rainwater Contaminated by Fukushima Daiichi Nuclear Accident

摘要

The removal rate of radioactive materials from contaminated water was experimentally obtained using various materials and commercial water purifiers with the aim of enabling the public to easily remove radioactive materials from rainwater and tap water contaminated by radioactive ~(131)I, ~(134)Cs, and ~(137)Cs released by the Fukushima Daiichi Nuclear Power Plant accident using readily obtainable instruments and materials at home. Since it was difficult to obtain contaminated tap water, contaminated rainwater was used as samples in our experiments. In the rainwater, ~(131)I, ~(132)I, ~(134)Cs, ~(137)Cs, ~(132)Te, ~(129m)Te, and ~(129)Te were detected. The abundance ratio of the isotopes depended on the location and date of collection, and the abundance of ~(131)I was 12 to 26 times higher than that of ~(137)Cs. Most of the radioactivity in the rainwater originated from ~(131)I. The removal rate was obtained in the case of using readily available materials and water purifiers at home and in a university laboratory. The results of model experiments using nonradioactive I_(3)~(-) and radioactive ~(125)I instead of ~(131)I suggested that activated carbon was effective. On the basis of these results, we investigated the removal rates of radioactive iodine and cesium using five different pot-type water purifiers with activated carbon as the basic adsorbent, to which ion-exchange resin, a hollow fiber membrane, or a ceramic was added. Approximately 90 to 99% of ~(131)I and 94 to 100% of ~(137)Cs were removed by consecutive purifications using the pot-type water purifiers. These results indicated that these water purifiers can be easily used to remove ~(131)I and ~(137)Cs at home, although special care is required when boiling using an electric kettle because it causes the concentration of ~(131)I. Faucet-mounted- and countertop-type water purifiers with activated carbon as the basic material are expected to have a similar performance to pot-type water purifiers, although this requires future experimental verification. We found that 1-9% of the radioactive ~(131)I remained in the rainwater samples after repeated treatment with the pot-type commercial water purifiers, depending on the sampling location and time, and that some of the residual isotopes were adsorbed on silica nanoparticles. Furthermore, to improve the removal rate, it is necessary to develop a removal method for the remaining components.