Transformation of Iodine Species in Submerged Paddy Soil.
Noriko Yamaguchi, National Institute for Agro-Environmental Sciences, 3-1-3, Kan-nondai, Tsukuba Ibaraki, 305-8604, Japan, Masashi Nakano, Research Institute of Soil Science & Technology, 1-6-5, Nishi-shinbashi, Minato-ku, Tokyo, 105-0003, Japan, and Hajime Tanida, Japan Synchrotoron Radiation Research Institute, 1-1-1, Kouto, Sayo, Hyogo, 679-5198, Japan.
Radioactive I-129 is one of the largest contributors to the calculated health risk associated with the reprocessing of nuclear fuel due to its very long period of half decay, 15.9 million years. Environmental behavior of I-129 should be basically similar to that of natural I-127. For the sake of food safety and security, the behavior of I-127 as well as I-129 in the agricultural environment is an urgent issue that must be addressed. Major iodine species in soil include iodide (I-), iodate (IO3-), and organically bound iodine. The reduced form of inorganic iodine, I-, is the most mobile iodine species in soil. On the other hand, IO3- is relatively immobile in soil since IO3- can be sorbed on the soil components. Organically bound iodine is considered as a sink of iodine in soil. The iodine concentration in paddy field soil was substantially lower than that in upland fields. The drop of the redox potential during the flooded period of paddy soil may cause reduction of IO3- to I-, and, as a result, the increased mobility of iodine. The objective of this study is to investigate the cause of iodine eluviations in paddy soil using direct analysis of iodine oxidation state by X-ray absorption near-edge structure (XANES). Andisols were taken from the surface layer of an experimental paddy field in Tsukuba, Japan. Potassium iodate (KIO3) dissolved in irrigation water was added to non-sterilized and γ ray sterilized soils to achieve an iodine content of 2 Ámol g-1. The soil/iodine mixtures were then incubated under submerged condition for 15 and 30 d. After the incubation, the solution phase was separated by centrifugation, and soil paste for XANES analysis was taken from the central portion to avoid the effect of oxidizing layer. Iodine K-edge XANES data acquisition (33.2 keV) was conducted at BL01B1 at SPring-8. Iodine speciation in solution phase was determined by IC-ICPMS. Based on the post-edge feature of iodine species spiked to soil, we found evidence that the decreased concentration of iodine in paddy soil was the result of the reductive reaction of IO3- when the paddy field was submerged conditions. The reduced products would be I- and I2 or organoiodine. After incubation of paddy soils with IO3- for up to 30 d under submerged conditions, the post-edge feature of the XANES spectra of IO3- disappeared, while it was similar to that of I2 or organoiodine but different from that of I-. The concentration of I- in the solution phase increased after incubation under flooded conditions. Therefore, the formed I- was not retained by soil and dissolved into the solution phase, whereas I2 or organically bound iodine remained in the solid phase of the soil. The sterilization of the paddy soils inhibited the reduction of IO3-. It indicated that the biological consumption of oxygen in soil and subsequent drop of the redox potential was critical to the reduced reaction of IO3- in paddy soil systems. Rice plants absorb iodine species as I2 and the excess of iodine causes Akagare reclamation disease. It was suggested that the roots of rice plants oxidized I- to form molecular iodine (I2) which was absorbed more selectively, compared to ionic absorption. Our results showed that reductive reaction of IO3- when paddy soil is flooded may also cause increased concentration of I2 in soil in addition to oxidation of I- in soil solution. Since I2 is highly reactive with organic compounds, the formed I2 may be associated with soil organic matter as organoiodine. These I2 or organoiodine could be a potential source of iodine to be absorbed by rice plants, as well as dissolved I- in soil solution.