Kinetics and model development of iohexol degradation during UV/H 2 O 2 and UV/S 2 O 8 2- oxidation.

The degradation rates and kinetics of one commonly used iodinated contrast medium, iohexol, were investigated and compared during ultraviolet (UV) photolysis, UV/H ₂ O ₂ and UV/S ₂ O ₈ ^2- advanced oxidation processes (AOPs). Results indicate that the iohexol degradation rate increased in the order of UV/H ₂ O ₂  < UV irradiation < UV/S ₂ O ₈ ^2- and followed pseudo-first-order kinetics. Increasing persulfate concentration significantly increased iohexol degradation rate, whereas increasing H ₂ O ₂ 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/H ₂ O ₂ and was consumed by excess H ₂ O ₂ . 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) × 10 ⁸ and 3.91 (±0.01) × 10 ¹⁰  M ^-1  s ^-1 , respectively. Iohexol degradation rate remained stable at pH 5-9 during UV irradiation and UV/H ₂ O ₂ , 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 ^- ≫ SO 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 CO ₂ but promoted the generation of toxic iodoform (CHI ₃ ), and the subsequent chlorination had the potential to reduce the content of disinfection by-products; therefore, further evaluation of possible environmental hazards is warranted.
(Copyright © 2019 Elsevier Ltd. All rights reserved.)