Redox reaction of iodine in paddy soil investigated by field observation and the I K-Edge XANES fingerprinting method

In order to elucidate the cause for the leaching of iodine in a flooded paddy field, we investigated the transformation of an iodine species affected by the water management of the paddy field. The increased concentration of iodide (I−) in soil solution of a flooded paddy field suggested that I− was leached from the soil under anaerobic conditions. The post-edge feature of X-ray absorption near-edge structure (XANES) for iodate (IO3−) spiked to soil totally disappeared after anaerobic incubation of the soils, and I− was dissolved in the solution. On the other hand, I− in contact with the soil was not likely to be oxidized to IO3− under aerobic incubation. Iodine was leached out in soil solution as I− under anaerobic conditions, whereas part of the iodine species was retained by soil as I2 or organoiodine both under anaerobic and aerobic conditions.