High Molecular-Weight Organics as Precursors for Toxic Iodinated Disinfection Byproducts during Chloramination

Iodinated DBPs (I-DBPs), many more toxic than regulated chlorinated and/or brominated DBPs, are a major challenge in the supply of safe drinking water. While over 800 DBPs have been identified, the occurrence and precursors of toxic I-DBPs remain poorly understood. Herein, natural organic matter from two raw drinking waters was fractionated using ultrafiltration membranes into different groups based on molecular weight (MW). The <1 kDa fraction exhibited the highest levels of cytotoxicity and genotoxicity after disinfection. However, in the presence of iodide, the 10-100 kDa fraction had the highest toxicity/dissolved organic carbon values. FT-ICR MS analysis showed high reactivity between hypoiodous acid and the 10-100 kDa fraction, and a shift from aromatic and unsaturated molecules to more saturated and oxidized molecules during chloramination of 10-100 kDa fraction. Based on nontarget analysis using orbitrap MS, four I-DBPs with high peak intensities during chloramination of 10-100 kDa fraction were identified and then confirmed using chemical standards: 4-hydroxy-3-iodo-5-nitrobenzonitrile, 2,4,5-triiodoimidazole, 3,5-diiodo-4-hydroxybenzonitrile, and 2,6-diiodo-4-nitrophenol. The first two have been first identified in drinking water. The concentrations of the four DBPs ranged from <LOD to 20.7 ng/L in the finished waters and tap waters. Moreover, the four I-DBPs were 2-3 orders of magnitude more toxic than regulated tribromomethane and dichloroacetic acid. Overall, our study highlights the 10-100 kDa fraction of high molecular weight organics as the primary precursor group for toxic I-DBPs during chloramination and provides insights into key research challenges for this important category of DBPs.