Kinetics and model development of iohexol degradation during UV/H2O2 and UV/S2O82− 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 − ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$ 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 − ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$ 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 − ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$ , 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 − ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$ . The presence of anions displayed inhibitory effects on iohexol degradation during UV/ S 2 O 8 2 − ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$ in the order of Cl − ${\text{Cl}}^{-}$ > HCO 3 − ${\text{HCO}}_3^{-}$ ≫ S O 4 2 − $\text{S}{\text{O}}_4^{2-}$ . UV/ S 2 O 8 2 − ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$ 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 − ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$ 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.