Iodine Species Quantification and Transformation during Chlorination in the Presence of Ammonia.

Iodinated disinfection byproducts (I-DBPs) are increasingly detected in drinking water due to the widespread use of chlorination and the presence of iodide (I) in source waters. However, understanding the transformation of iodine species during disinfection remains challenging, particularly under varying ammonia concentrations. In this study, a method based on spectrophotometry and HPLC was applied to quantify I, hypoiodous acid (HOI), and iodate (IO) in various water matrices. With this method, the transformation of iodide during breakpoint chlorination was elucidated for the first time. Results revealed that ammonia inhibited IO formation by scavenging free chlorine and promoted the accumulation of HOI, a key intermediate in I-DBPs formation. A comprehensive kinetic model was established, which accurately reproduced the experimental trends and helped elucidate the transformation mechanisms of iodine species. Additionally, the fate of organic iodine compounds, including 4-iodobenzoic acid and diatrizoate (DTZ, a widely used iodinated contrast medium), was examined during breakpoint chlorination. Although typically considered resistant to chlorination, both compounds underwent partial degradation under Cl/N = 2 conditions, leading to iodide release and enhanced I-DBPs formation. This highlights the risk of converting stable iodine-containing organics into reactive species under high chlorine doses in ammonia-containing water. These findings highlight the importance of precisely controlling ammonia and disinfectant concentrations to reduce water toxicity and ensure safer water disinfection practices.