Search

The degradation rates and kinetics of one commonly used iodinated contrast medium, iohexol, were investigated and compared during ultraviolet (UV) photolysis, UV/H2O2 and UV/ S 2 O 8 2 − advanced oxidation processes (AOPs). Results indicate that the iohexol degradation rate increased in the order of UV/H2O2 S 2 O 8 2 − and followed pseudo-first-order kinetics. Increasing persulfate concentration significantly increased iohexol degradation rate, whereas increasing H2O2 concentration caused reverse effect. Radical scavenging test results show that UV photolysis, OH and radicals all contributed to iohexol degradation during UV/ S 2 O 8 2 − , but OH was the main contributor during UV/H2O2 and was consumed by excess H2O2. The kinetic models of iohexol degradation by both AOPs were developed, and the reaction rate constants with OH and were calculated as 5.73 (±0.02) × 108 and 3.91 (±0.01) × 1010 M−1 s−1, respectively. Iohexol degradation rate remained stable at pH 5–9 during UV irradiation and UV/H2O2, but gradually decreased at pH 5–7 and remained stable at pH 7–9 during UV/ S 2 O 8 2 − . The presence of anions displayed inhibitory effects on iohexol degradation during UV/ S 2 O 8 2 − in the order of Cl − > HCO 3 − ≫ S O 4 2 − . UV/ S 2 O 8 2 − AOP exhibited high degradation efficiency and stability on the basis of UV irradiation, which can be applied as a promising degradation method for iohexol. UV/ S 2 O 8 2 − AOP can effectively mineralize iohexol to CO2 but promoted the generation of toxic iodoform (CHI3), and the subsequent chlorination had the potential to reduce the content of disinfection by-products; therefore, further evaluation of possible environmental hazards is warranted.

In this study, the degradation kinetics, and the formation of disinfection byproducts (DBPs) during iohexol chlor(am)ination were studied. The chlorination kinetics can be described using a second-order model. As the pH of the solution increased, the apparent second-order rate constant of iohexol chlorination increased considerably. The reaction rate constants of iohexol with HOCl and OCl− were calculated as ( 3.75 ± 0.70 ) × 10 - 4 and ( 1.39 ± 1.19 ) × 10 - 4 M−1 s−1, respectively. In addition, the chloramination of iohexol followed tertiary kinetics, and the maximum apparent rate constant of 3.78 × 10 - 6 M−1 s−1 occurred at pH 7. NH4+ can promote the degradation of iohexol during chlorination, whereas Br− inhibits iohexol degradation during chloramination. The formation of conventional carbonated DBPs and emerging nitrogenated DBPs during chlor(am)ination of iohexol was measured. However, in the presence of bromide, the conversion of iohexol to toxic iodinated DBPs increased significantly during chlorination, and chlorination resulted in higher formation of toxic I-DBP compared to chloramination. Therefore, chloramine disinfection is preferred for treating raw water containing high bromide concentrations in terms of DBP formation control.

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.