Transformation of Iodide in Natural and Wastewater Systems by Fixation on Humic Substances

Separation of different fractions of humic substances (HS) by their molecular weight was carried out by size exclusion chromatography (SEC). The fixation of inorganic iodide by HS of these fractions was determined by on-line coupling of SEC with inductively coupled plasma mass spectrometry (ICP-MS). Possible changes of HS during this transformation process of HS/iodine species could be followed by UV absorption. A 129I-labeled sodium iodide solution was used to determine the transformation of iodide into iodine fixed on humic substances. In the case of natural samples, iodide was exclusively fixed by HS fractions where natural HS/iodine species have also been observed in the original sample. In wastewater samples from sewage disposal plants organoiodine compounds were also identified which were not affected by the transfer of iodide. A strong influence of the transformation process was seen on the microbiological activity of the sample and a complicated transfer mechanism by the formation of different intermediate HS/iodine species was observed. In general, the results contribute to a better understanding of the biogeochemical cycle of the important trace element iodine, but they are also important for the possible fixation of radioactive iodine, e.g. emitted from nuclear plants, in natural aquatic systems. On the other hand, it must be assumed that substantial amounts of iodine are removed as HS/iodine species during water processing, where humic substances are usually separated. This reduces the iodine content in drinking and mineral waters with respect to the natural level of this essential trace element.