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 I(2), 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.

Superiority of K-edge XANES over LIII-edge XANES in the speciation of iodine in natural soils.

Environmental behavior of iodine is of great importance especially related to the release of radioiodine from the processing of nuclear fuel, nuclear accidents, etc. To understand the fate of radioactive iodine in soil-water systems, it is necessary to establish a speciation method of iodine in soil. XANES is one of the most important candidates and we compared the performance of L(III)-edge and K-edge XANES for this purpose. In particular, fluorescence XANES with a multi-element semiconductor detector is essential for the measurement of XANES spectra for trace amounts of iodine in natural soil samples. When comparing L(III)- and K-edges, L(III)-edge XANES can be useful for the speciation due to its ability to distinguish various iodine species in their XANES spectra. However, at L(III)-edge measuring iodine L(alpha) emission, the proximity of its energy to those of Ca K(alpha) and K(beta1) causes a large contribution of background X-rays in the XANES spectra, since Ca is a major element in soil. Thus, it was concluded that K-edge XANES is more useful than L(III)-edge for the speciation of iodine in natural soils owing to its lower detection limit. The K-edge XANES was successfully applied to the speciation of natural iodine in a soil sample (iodine concentration: 55.8 mg/kg), showing that iodine is present in the sample as organo-iodine species incorporated in humic substances.