Investigation of iohexol degradation kinetics by using heat-activated persulfate

Abstract

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.