Formation of Organic Iodine Supplied as Iodide in a Soil-Water System in Chiba, Japan

Speciation of iodine in a soil water system was investigated to understand the mechanism of iodine mobility in surface environments. Iodine speciation in soil and pore water was determined by K-edge XANES and HPLC-ICP-MS, respectively, for samples collected at a depth of 0-12 cm in the Yoro area, Chiba, Japan. Pore water collected at a 0-6 cm depth contained 50%-60% of organic iodine bound to dissolved organic matter, with the other portion being I(-). At a 9-12 cm depth, 98% of iodine was in the form of dissolved I(-). In contrast, XANES analysis revealed that iodine in soil exists as organic iodine at all depths. Iodine mapping of soil grains was obtained using micro-XRF analysis, which also indicated that iodine is bound to organic matter. The activity of laccase, which has the ability to oxidize I(-) to 12, was high at the surface of the soil water layer, suggesting that iodide oxidizing enzymes can promote iodine organification. The distribution coefficient of organic iodine in the soil water system was more than 10-fold greater than that of iodide. Transformation of inorganic iodine to organic iodine plays an important role in iodine immobilization, especially in a surface soil water system.