Formation dynamics of inorganic iodine species during UV-based advanced oxidation of iopamidol and iohexol and their correlation with iodinated disinfection by-product yields†

Iodinated contrast media (ICM) are iodine-containing micropollutants detected in both natural and engineered water systems. These compounds serve not only as contaminants but also as precursors to iodinated disinfection by-products (I-DBPs) during water and wastewater treatment. This study comparatively investigates the treatment of iopamidol and iohexol, the most widely used ICM in clinical practice, via advanced oxidation processes utilizing 254 nm UV (UV-AOPs) and different oxidants, peroxymonosulfate (PMS), peroxydisulfate (PDS), hypochlorite (NaOCl), and hydrogen peroxide (H₂O₂). This study is the first to directly compare the formation dynamics and kinetic profiles of inorganic iodine species – iodide (I⁻), reactive iodine species (RIS), and iodate (IO₃⁻) – across different UV AOPs and identify their correlation with the yields of I-DBPs such as iodinated trihalomethanes (I-THMs) and haloacetic acids (I-HAAs). Our results showed that UV AOPs with NaOCl achieved the highest degradation rate (based on measured pseudo-1st order rate constants) for iopamidol, degrading it 14 times faster than the UV-only process, while AOPs with PMS or PDS achieved the highest degradation rate for iohexol with a three times faster rate than UV alone. During the treatment, UV AOPs with NaOCl and PMS efficiently transformed I⁻ released from ICM to IO₃⁻ without forming RIS, whereas UV AOPs with PDS or H₂O₂ did not fully oxidize I⁻ to IO₃⁻, leading to RIS formation. Formation of I-THMs and I-HAAs after chlorination showed a positive correlation with the concentrations of I⁻ and RIS across ICM treated via different UV AOPs (e.g., UV AOPs with PMS and NaOCl resulted in minimal I-DBP formation). These findings will provide a foundation for developing and optimizing UV AOP strategies to achieve effective ICM degradation while minimizing I-DBP yields.