Formation of toxic iodinated by-products during the oxidation process of iohexol by catalytic ozonation in water

The formation of iodinated by-products during the chemical oxidation process for organic compounds contaminated water has gained increasing attention. Iodinated by-products are more genotoxic, cytotoxic and mutagenic than those of chlorine and bromine analogs. It was reported that the oxidation process of iodinated X-ray contrast media (ICM) could generate iodinated by-products. This study investigated the generation of iodinated by-products during catalytic ozonation of iohexol (one of wide detected ICM). The formation of iodoform (IF) and monoiodoacetic acid (MIAA), and the degradation kinetics of iohexol were compared in different oxidation systems. The effect of various factors, including the iohexol concentration, ozone concentration, HA concentration and solution pH on iodinated by-products generation was evaluated. Additionally, the fate of iodine was investigated by the mass balance calculation. The generated concentrations of I-THMs species with one iodide atom were far higher than those of species with more iodine atoms in the catalytic system at the coexistence of Cl⁻ and Br⁻. The generation of I-THMs in actual water was relatively low and the normalized value of I-THMs/DOC (μg/mg) had no significant difference. The formation mechanism of high molecular weight iodine-containing by-products included cleavage of the side chain, H-abstraction, substitution iodine for chlorine or bromine and amide oxidation.