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• Heat-activated persulfate can effectively remove iohexol, but mineralization was almost negligible. • Raising temperature can significantly improve the degradation rate of iohexol, especially in the beginning 5 min. • Hydroxyl radical contributed more to the iohexol degradation than sulfate radical. • Higher concentrations of chloride (≥1 mM), bicarbonate (≥5 mM), and humic acid (≥5 mg/L) inhibited iohexol degradation. • Iodide was generated during iohexol degradation. In this study, we investigated the degradation kinetics of a commonly used iodinated contrast medium, iohexol, by using heat-activated persulfate. The results indicate that the iohexol concentration can be effectively reduced with a removal percentage higher than 85% despite the long reaction time (2–3 h). The iohexol disappearance fitted well with the pseudo-first-order kinetic model. The persulfate consumption almost reached the maximum value during the initial 5 min of the reaction. Subsequently, persulfate was no longer consumed, which resulted in a decrease in the reaction rate. The pseudo-first-order rate constant of iohexol degradation significantly increased with temperature but decreased with an increase in pH from 5 to 9. The results of the quenching experiments performed using ethanol and tert-butanol indicate that the role of hydroxyl radicals was more important than that of sulfate radicals in iohexol degradation during heat-activated persulfate oxidation. However, without heat activation, persulfate did not react with iohexol. Therefore, the effect of pH on the HO· concentration dominated the effect of pH on iohexol degradation. High concentrations of bicarbonate (≥5 mM), chloride (≥1 mM), and dissolved natural organic matter (≥5 mg/L) inhibited the degradation of iohexol due to the consumption of sulfate and hydroxyl radicals. The mineralization percentage was only 3.8% (no considerable improvement) when the temperature was increased to 80 °C. The molecular structures of the intermediates contained iodine with molecular weights higher than 377 Da. Moreover, the generation of iodide was detected. The experimental results suggest that special attention should be paid to the risk of the formation of disinfection by-products in the subsequent chlorine disinfection process when treating natural water containing iohexol through heat-activated persulfate oxidation. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Dual-wavelength synergy can significantly accelerate iopamidol degradation during UV-LED and UV-LED/chlorine processes. • Dual-wavelength irradiation can promote the photolysis of chlorine. • Maximum k obs for iopamidol degradation occurred at pH 7.0 during UV-LED/chlorine process. • HO· and RCSs are the major radical species for iopamidol degradation during UV-LED/chlorine process. • I-THM formation decreased in Milli-Q water while increased in real water with increasing irradiation time. Abstract This study revealed the iopamidol degradation performance and the synergistic effect of dual-wavelength ultraviolet light-emitting diodes (UV-LEDs). It was found that the dual-wavelength irradiation (265 and 280 nm) can promote iopamidol degradation during both UV-LED alone and UV-LED/chlorine processes (p < 0.05). The effects of key experimental parameters including pH, chlorine dosage and water matrix during UV-LED/chlorine process were evaluated. Peak degradation rate was observed at pH 7.0 due to the combined effects of the quantum yield of aquatic chlorine photolysis (Φ chlorine) and absorptivity of chlorine. The destruction pathways of iopamidol in the UV-LED/chlorine process were proposed by Ultra Performance Liquid Chromatography-Electrospray Ionization-mass spectrometry. Additionally, an opposite trend of iodinated trihalomethane (I-THM) formation during subsequent chlorination between Milli-Q and real waters with prolonging irradiation time was observed, while other detected disinfection by-product (DBP) formation showed the same variation in Milli-Q and real waters. The UV-LED/chlorine treatment using dual-wavelength can enhance iopamidol degradation significantly, but it also promoted I-THM formation in real water, indicating the need for evaluating DBPs before the application of UV-LED/chlorine process in drinking water treatment. [ABSTRACT FROM AUTHOR]

This study investigated the effect of several factors on the formation of disinfection byproducts (DBPs) including trihalomethanes (THMs), haloacetonitriles (HANs) and halonitromethanes (HNMs) during UV-chlor(am)ination of the water collected after filtration unit in a drinking water treatment plant. In general, THM formation was higher during UV-chlorination while HAN and HNM formation were higher during UV-chloramination. Higher Medium pressure (MP) UV dose resulted in more DBP formation. The value of pH affected DBP formation differently. THMs decreased with increasing pH during UV-chlorination but remained stable during UV-chloramination. HNMs increased as pH increased from 5 to 7 but decreased as pH further increased to 9 during UV-chlorination. On the other hand, HNM formation decreased with increasing pH during UV-chloramination. However, pH had little impact on HAN formation. The nitrate concentration had negligible impact on the formation of THM, HAN and HNM during both UV-chlor(am)ination. Increasing bromide concentration improved THM and HAN formation, decreased HNM formation, and shifted DBPs to more brominated species in the three categories. MP UV irradiation enhanced the formation of DBPs compared to low pressure UV during both UV-chlor(am)ination, especially for nitrogenated DBPs. [ABSTRACT FROM AUTHOR]