The physicochemical distribution of 131 I in a municipal wastewater treatment plant.

As a consequence of therapeutic and diagnostic treatment of patients with thyroid diseases,131 I is introduced into the sewage system on a regular basis. This presents an opportunity to use the131 I as a tracer to study its partitioning and transport within a wastewater treatment plant (WWTP). In the case of nuclear accidents where131 I is one of the most prominent nuclides, an understanding of iodine partitioning and transport will be valuable for developing models that may prognosticate the activity concentrations in sludge and outflow, especially after an accidental input. In this study, samples from various locations inside a municipal WWTP were taken and for each sample, three different fractions were separated by a chemical extraction process. These fractions were analysed for their131 I activity concentrations by gamma-ray spectroscopy. While about 30% of the radioiodine activity in the inflow is associated with organic molecules, this amounts to about 90% after biological treatment. This is caused by the accumulation of131 I bound to organic matter in the return sludge and by a transfer of131 I from the inorganic to the organic fractions, most likely mediated by microbial action. In the outflow, inorganic and low-molecular131 I is dominant, but the overall activity concentration is reduced to about 50-75%.