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.