Kinetics and model development of iohexol degradation during UV/H₂O₂ and UV/S₂O₈^2- oxidation

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/S₂O₈^2- advanced oxidation processes (AOPs). Results indicate that the iohexol degradation rate increased in the order of UV/H₂O₂ < UV irradiation < UV/S₂O₈^2- 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/S₂O₈^2-, 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/S₂O₈^2-. The presence of anions displayed inhibitory effects on iohexol degradation during UV/S₂O₈^2- in the order of Cl^- >HCO₃^- ≫ SO₄^2-. UV/S₂O₈^2- 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/S₂O₈^2- 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.