Resolving Organic Iodine Speciation in Natural Waters and Engineered Wastewaters

Organic iodine compounds (Org-I) have been widely reported in atmospheric and marine systems, yet their occurrence, speciation, and cycling in freshwater drinking water sources and wastewater systems remain comparatively poorly understood because of their chemical complexity and associated analytical challenges. Herein, complementary ion chromatography–inductively coupled plasma mass spectrometry (IC-ICP-MS) and liquid chromatography–tandem mass spectrometry (LC-MS/MS) workflow were used to characterize inorganic and unresolved organic iodine species in lakes, rivers, groundwater, and treated wastewater effluents across the United States. IC-ICP-MS chromatograms revealed abundant iodine-containing peaks distinct from iodate (IO 3 – ) and iodide (I – ), indicating that Org-I represents a substantial fraction of total iodine, particularly in surface waters and wastewater effluents. Concentrations of IO 3 – and I – measured by IC-ICP-MS were systematically higher than those by LC-MS/MS, suggesting coelution of unresolved Org-I species with inorganic iodine species. Retention behavior and mass balance analysis indicate that aqueous Org-I is predominantly anionic and becomes increasingly diverse from groundwater to wastewater effluents. Multivariate analysis further showed strong associations between iodine speciation patterns and water chemistry. These results demonstrate that Org-I is ubiquitous, chemically diverse, and analytically under-resolved in environmentally relevant water samples, highlighting the need for improved characterization of Org-I during water and wastewater treatment.