Fe3O4 catalytic ozonation of iohexol degradation in the presence of 1-hydroxybenzotriazole: Performance, transformation mechanism, and pathways

• Iohexol degradation in Fe 3 O 4 /O 3 system was higher than in O 3 only system. • Trace HBT could enhance iohexol degradation efficiency in Fe 3 O 4 /O 3 system. • 1 O 2 and radicals of O H ∙ , and O 2 - ∙ were identified in the Fe 3 O 4 /O 3 /HBT system. • Iohexol degradation mechanisms and pathways in the Fe 3 O 4 /O 3 /HBT system were proposed. This study explored the degradation of an iodine X-ray contrast media, iohexol, under heterogeneous ozone-catalyzed oxidation by adding Fe 3 O 4 and 1-hydroxybenzotriazole (HBT). Results show that increasing the dosages of Fe 3 O 4 catalyst and ozone as well as solution pH were beneficial to the degradation of iohexol. The presence of low background humic acid (HA) concentration (≤10 mg/L) promoted iohexol degradation, but high HA concentration (≥20 mg/L) inhibited it. Adding HBT exhibited a similar effect on iohexol degradation as HA. The degradation rate constant of iohexol was calculated as 0.07108 min −1 in the Fe 3 O 4 /O 3 /HBT process ([HBT] = 5 μM) at pH 7.0, which was approximately 1.5 times higher than that in the Fe 3 O 4 /O 3 process (0.04533 min −1 ), and 4 times higher than that in the O 3 process (0.01742 min −1 ). Fe 3 O 4 /O 3 /HBT process was pH-dependent with high degradation efficiency of iohexol at pH 5 or 7. Singlet oxygen ( 1 O 2 ), hydroxyl radical ( OH ∙ ), and superoxide radical ( O 2 - ∙ ) have been identified in the Fe 3 O 4 /O 3 /HBT process, and the conversion of Fe 2+ to Fe 3+ was more obvious on the surface of Fe 3 O 4 compared to that in the Fe 3 O 4 /O 3 process. Transformation intermediates were identified, and the degradation pathways of iohexol were proposed. Overall, Fe 3 O 4 /O 3 could enhance iohexol degradation compared with ozone-only system, and adding trace HBT in Fe 3 O 4 /O 3 system can further enhance heterogeneous ozone-catalyzed process for rapid organic matter degradation.