Comparison of Potentials of Higher Plants for Phytoremediation of Radioactive Cesium from Contaminated Soil

In 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident occurred following the Great East Japan Earthquake at March 2011. As a result, a huge amount of radionuclides, such as I, cesium (Cs) and Cs, were discharged to the surrounding environment from the site of accident (Tokyo Electric Power Company, 2012). Agricultural lands and forests were heavily contaminated with these radionuclides, consequently, commercial distribution of many crops cultivated (and to be cultivated) in severely contaminated area, chiefly in Fukushima prefecture, have been restricted since agricultural products from these area often exceeded the regulatory level of radioactivity defined by laws in Japan (100 Bq kg ). Among the soil-contaminating radionuclides, C is the major radionuclide to be concerned since its half-life is relatively long (30.1 years; Chino et al., 2011) in comparison to other radionuclides released from the FDNPP. In addition, Cs can emit gamma ( )-ray, hence long-term biological effects by the -ray released from Cs in biota including wild animals might also be concerned. Indeed, following adverse effects in ecosystem in Fukushima have been reported; increased frequency of morphological abnormality in the pale grass blue butterfly (Hiyama et al., 2012), reduced abundance in common birds (Møller et al., 2012), and decrease in blood cell number in wild Japanese monkeys (Ochiai et al., 2014). Therefore, clean-up of radionuclides from the contaminated soil becomes a big issue in reconstruction and revitalization of Fukushima area. Previous studies relevant to Chernobyl accident showed that soil-penetrated Cs binds strongly to the clay and showed low mobility. For example, Kirk and Staunton (1989) showed that migration rate of Cs was less than 1 cm per year. Thus, the major portion of the Cs must be distributed in the upper 10 cm-thick surface layer of the soil column in most types of soils even seven years after of the accident (Arapis et al., 1997). In the FDNPP accident, contamination of Cs occurred in vast land area especially in Fukushima prefecture. Because of the high cost, decontamination of the large area polluted with Cs by conventional engineering methods remains an intractable problem. Furthermore, these methods often encompass negative effects for physical properties of the soil and also wreak ecosystem and landscape. Phytoremediation, the use of higher plants to clean-up the contaminants from soil through accumulation of contaminants in plants, is an alternative technology to conventional methods (Pilon-Smits and Freeman, 2006). The behavior of Cs in soil and plants is similar to that of potas-