Speciation of iodine in solid environmental samples by iodine K-edge XANES: Application to soils and ferromanganese oxides

A method was developed for speciation of iodine in solid materials using X-ray absorption near-edge structure (XANES). This method was used to identify the iodine species (mainly inorganic iodine) in environmental samples. It was shown that the XANES spectra of iodide and iodate sorbed within solid materials can be simulated by the linear combination of the spectra of iodide and iodate ions in water. The distribution coefficient (Kd) between soil and water was obtained independently for iodide and iodate, based on iodine speciation both in the solid phase, by XANES, and in the aqueous phase, by HPLC-ICP-MS. It was found that the Kd of iodate is larger than that of iodide by a factor of more than six, showing the more soluble nature of iodide. It was suggested that iodate can form in soil even when iodide is injected into the soil–water system under conditions within the iodide-stable field of the Eh–pH diagram of iodine. This is caused by the much higher affinity of iodate for solid surfaces than iodide. In soil samples under various water saturation conditions, or various Eh conditions, the iodide fraction in water increases with decreasing Eh, which results in an increase in the dissolved total iodine fraction in soil water. The speciation method using XANES was also applied to iodine in a natural soil sample and marine ferromanganese oxides. It is suggested that iodine K-edge XANES is a promising tool for determining the iodide / iodate ratio in natural solid samples, which contributes to better understanding of the behavior of iodine at the Earthʹs surface.