Chlorination experiments with iodinated contrast media in typical German raw waters for drinking water supply

Iodinated contrast media (ICM) are pharmaceuticals widely used in X-ray diagnostics to improve image contrast. Due to their high dosage and minimal metabolism, ICM are excreted almost unchanged and can enter aquatic environments, appearing in surface water used for drinking water production. Although ICM are chemically stable, under certain conditions, they can release iodine and be transformed in iodinated by-products (I-DBPs) during the disinfection processes occurred in water treatment plants. Previous studies on this phenomenon have relied on experimental conditions that poorly reflect real-world drinking water treatment practices, in terms of ICM and disinfectants concentrations and contact times. Rather than challenging established literature, this study aimed to investigate whether chlorination can lead to the formation of iodinated trihalomethanes (I-THMs) from ICM dissolved in water samples, when disinfection is performed under real-world conditions. The evaluation focused on Germany, where high attention is posed on the environmental impact of ICM and their potential role in the formation of I-DBPs. Therefore, the experimental setup was designed to be aligned with the German Drinking Water Directive (TrinkwV 2023), reflecting realistic conditions for disinfectant dosage (chlorine < 1.2 mg/L) and contact time. Four ICM (iohexol, iomeprol, iopamidol, iopromide) were tested in two different raw waters, commonly used in Germany for drinking water production. Concentrations of ICM 10 µg/L (typical environmental level) and 100 µg/L (worst-case scenario) were applied, with additional tests for iopamidol at a maximum concentration of 3900 µg/L. The study also examined the role of free iodide in I-THMs formation. Results indicated no correlation between I-DBPs formation and the presence of ICM, even at the highest tested concentrations. I-DBPs were detected only in one raw water type, specifically the water samples with the highest dissolved organic carbon (DOC), suggesting that the composition of water, rather than ICM, modulate the formation of I-DBPs. Conversely, free iodide was confirmed as a significant contributor to I-DBPs generation when chlorinated. Under standard German drinking water treatment conditions, ICM do not promote I-DBP formation. Instead, water matrix characteristics and iodides play a decisive role in I-DBPs formation.