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 (IO₃^-) 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 IO₃^- 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 Cl₂/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.