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-beta 1 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.