Kinetics and model development of iohexol degradation during UV/HO and UV/SO oxidation.

The degradation rates and kinetics of one commonly used iodinated contrast medium, iohexol, were investigated and compared during ultraviolet (UV) photolysis, UV/HO and UV/SO advanced oxidation processes (AOPs). Results indicate that the iohexol degradation rate increased in the order of UV/HO < UV irradiation < UV/SO and followed pseudo-first-order kinetics. Increasing persulfate concentration significantly increased iohexol degradation rate, whereas increasing HO concentration caused reverse effect. Radical scavenging test results show that UV photolysis, OH and radicals all contributed to iohexol degradation during UV/SO, but OH was the main contributor during UV/HO and was consumed by excess HO. 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 and 3.91 (±0.01) × 10 M s, respectively. Iohexol degradation rate remained stable at pH 5-9 during UV irradiation and UV/HO, but gradually decreased at pH 5-7 and remained stable at pH 7-9 during UV/SO. The presence of anions displayed inhibitory effects on iohexol degradation during UV/SO in the order of Cl >HCO ≫ SO. UV/SO 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/SO AOP can effectively mineralize iohexol to CO but promoted the generation of toxic iodoform (CHI), and the subsequent chlorination had the potential to reduce the content of disinfection by-products; therefore, further evaluation of possible environmental hazards is warranted.